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The generally accepted model of rifting (the McKenzie model) has certain geometric and spatial constraints that seem to preclude its operation in the earliest stage of rifting. It may be a more advanced stage of the rifting process, if it is correctly described. An aborted rift system can be studied in the subsurface of the Permian basin. The Delaware, Val
The earth's outer shell rifts continuously, stretching and splitting both on the ocean's floor and on continents. Every 30 million years or so the rifting becomes cataclysmic, releasing continent-size floods of magma. This paper explains that the same mechanism is at work in both cases, the difference being in the slightly hotter temperature of the parent mantle for spectacular volcanic outbursts. Two kinds of evidence are described: quantitative descriptions of rock melting and a wide range of observations made on the rifted edges of continents and in the oceans that have opened between them.
White, R.S.; McKenzie, D.P.
Evolution of Oblique Rifting on the Main Ethiopian Rift
NSDL National Science Digital Library
Movie showing the evolution of oblique rifting in analogue models (from Corti, 2008, Nature Geosc). Obliquity in this model is 30° (angle between the normal to the rift axis and the direction of extension). Note the two-phase rift evolution with a first phase of boundary fault activity and basin subsidence, followed by activation of en-echelon arranged internal faults obliquely cutting the rift floor.
Corti, Giacomo
ERIC Educational Resources Information Center
Investigates the nature of catastrophic volcanism and the rifting process. Describes two kinds of evidence: quantitative descriptions of rock melting and a wide range of observations. Discusses examples of continent growth in the North Atlantic, India and the Seychelles islands, and the South Atlantic. (YP)
White, Robert S.; McKenzie, Dan P.
Continental rifts and mineral resources
Continental rifts are widespread and range in age from the present to 3 b.y. Individual rifts may form parts of complex systems as in E. Africa and the Basin and Range. Rifts have originated in diverse environments such as arc-crests, sites of continental collision, collapsing mountain belts and on continents at rest over the mantle circulation pattern. Continental rift resources can be classified by depth of origin: For example, in the Great Dike, Norilsk and Mwadui magma from the mantle is the host. At shallower depths continental crust partly melted above mafic magma hosts ore (Climax, Henderson). Rift volcanics are linked to local hydrothermal systems and to extensive zeolite deposits (Basin and Range, East Africa). Copper (Zambia, Belt), zinc (Red Dog) and lead ores (Benue) are related to hydrothermal systems which involve hot rock and water flow through both pre-rift basement and sedimentary and volcanic rift fill. Economically significant sediments in rifts include coals (the Gondwana of Inida), marine evaporites (Lou Ann of the Gulf of Mexico) and non-marine evaporites (East Africa). Oil and gas in rifts relate to a variety of source, reservoir and trap relations (North Sea, Libya), but rift-lake sediment sources are important (Sung Liao, Bo Hai, Mina, Cabinda). Some ancient iron ores (Hammersley) may have formed in rift lakes but Algoman ores and greenstone belt mineral deposits in general are linked to oceanic and island arc environments. To the extent that continental environments are represented in such areas as the Archean of the Superior and Slave they are Andean Arc environments which today have locally rifted crests (Ecuador, N. Peru). The Pongola, on Kaapvaal craton may, on the other hand represent the world's oldest preserved, little deformed, continental rift.
Burke, K. (Univ. of Houston, TX (United States). Geosciences Dept.)
NASA Astrophysics Data System (ADS)
The Lower Permian Rotliegendes troughs in Central Europe are directly comparable to the Upper Cenozoic Basin and Range province in the western United States in most tectonostratigraphic aspects. These intracratonic rifts were formed by extensional tectonics in the foreland area of major Cordilleran-type orogenies beginning 20-25 Ma after mountain building. The main characteristics of the basins include: crustal thinning from 40-45 to 30 km, change from calc-alkaline magmatic arc suites to basaltic or bimodal (basalt-rhyolite) igneous suites, normal to listric block faulting, high heat flow and hydrothermal activity, regional uplift, and immature clastic and evaporite sediments rapidly deposited within the subsiding rifts in a semi-desert environment. The genetic model proposed for these "foreland rifts" assumes that an extensional force under the continental foreland results from mantle material being drawn back towards the subducting plate by viscous drag. This force is counteracted by a compressional force produced at the trench by subducting a young, relatively buoyant plate until subduction is ended, at which time the plate breaks off and continues to sink. Tension in the foreland, possibly produced by the sinking slab, continues for some time later, causing stretching and thinning of the continental crust, extensional tectonics and higher regional heat flow and uplift. The decrease in overburden pressure and higher heat flow induces partial melting in the lower lithosphere and bimodal volcanism. Hotter asthenosphere rising up behind the trailing edge of the sinking plate causes thermal expansion and further uplift. The heat flow anomaly rapidly decays by hydrothermal activity within 15-20 Ma after last volcanism. Block faulting and rift basin infilling follow the main period of volcanism and continue until the sinking slab is decoupled from the overriding plate and no longer influences the continental foreland.
Jowett, E. Craig; Jarvis, Gary T.
Summary Rift Valley fever (RVF) is an arthropod-borne viral disease of ruminants, camels and humans. It is also a significant zoonosis which may be encountered as an uncomplicated influenza-like illness, but may also present as a haemorrhagic disease with liver involvement; there may also be ocular or neurological lesions. In animals, RVF may be inapparent in non-pregnant adults, but outbreaks
G. H. Gerdes
NASA Astrophysics Data System (ADS)
Sedimentary basins of late Precambrian age have been identified beneath Lake Superior using seismic reflection profiles leased by Argonne National Laboratory, Argonne, Ill., from Grant Norpac, Inc. [McGinnis et al., 1989]. These data, along with 650 km of Great Lakes International Multidisciplinary Program for Crustal Evolution (GLIMPCE) profiles [Behrendt et al., 1988], are being used to develop an understanding of failed rift processes, from initial plate separation, through basin evolution, to final quiescence.
McGinnis, L. D.
East African Rift Valley, Kenya
NASA Technical Reports Server (NTRS)
This rare, cloud free view of the East African Rift Valley, Kenya (1.5N, 35.5E) shows a clear view of the Turkwell River Valley, an offshoot of the African REift System. The East African Rift is part of a vast plate fracture which extends from southern Turkey, through the Red Sea, East Africa and into Mozambique. Dark green patches of forests are seen along the rift margin and tea plantations occupy the cooler higher ground.
Parga Chasma: Coronae and Rifting on Venus.
National Technical Information Service (NTIS)
The majority of coronae (quasicircular volcano-tectonic features) are found along rifts or fracture belts, and the majority of rifts have coronae. However, the relationship between coronae and rifts remains unclear. There is evidence that coronae can form...
S. E. Smrekar E. R. Stofan W. R. Buck P. Martin
Rifting of old oceanic lithosphere
Geophysical data from five regions in the Pacific and Indian oceans reveal that long distance (>400 km) spreading center jumps have occurred in the past. The present-day seafloor morphology is used to develop a scenario for a spreading center jump. The major events are (1) thinning and weakening of the lithosphere at the future rifting site, (2) rifting of the
J. Mammerickx; D. Sandwell
NASA Astrophysics Data System (ADS)
The Rio Grande rift of the southwestern United States is one of the world's principal continental rift systems. It extends as a series of asymmetrical grabens from central Colorado, through New Mexico, to Presidio, Texas, and Chihuahua, Mexico�a distance of more than 1000 km. Although the Rio Grande rift is closely related in timing and structural style to the contiguous Basin and Range extensional province, the two can be distinguished by a variety of geological and geophysical signatures. Rifts (both oceanic and continental) can be defined as elongate depressions overlying places where the entire lithosphere has ruptured in extension. The lithosphere of the Rio Grande rift conforms to this definition, in that: (1) the crust is moderately thinned�Moho depths range from about 45 km under the flanks to about 33 km beneath the rift axis. (2) anomalously low P n velocities (7.6-7.8 km s -1) beneath the rift and a long wavelength gravity low suggest that the asthenosphere is in contact with the base of the crust. The P-velocity is abnormally low (6.4-6.5 km s -1) in the lower half of the crust beneath the rift, suggesting high crustal temperatures. However, associated seismic and volcanologic data indicate the sub-rift lower crust is not dominated by a massive composite mafic intrusion such as is sometimes inferred for the East African rifts. Seismic and magnetotelluric data suggest the presence of a thin (< 1 km) sill-like contemporary midcrustal magma body which may perhaps extend intermittently along much of the length of the rift. Seismic and structural studies indicate a dominant horizontal fabric in the upper and middle crust. The brittle-ductile transition is at depths -15 km except for the major volcanic fields, where it rises to 2-3 km. Structural development of the rift occurred mainly during two time intervals: the early phase beginning at -30 Ma. and lasting 10-12 m.y., and the late phase extending from -10 to 3 Ma. The early phase involved extensive low-angle normal faulting throughout the rift region which was subsequently offset by high-angle normal faulting during the later deformational event. Volcanism of the Rio Grande rift is minor compared to some other continental rifts. Most of the volcanism is basaltic and occurred less than about 5 m.y. ago. Compositions range from alkalic to tholeiitic, with no unique spatial or temporal pattern. Magmas were probably derived from a variety of depths, indicating an unintegrated heat source with only local melting. Basaltic andesites and related calc-alkaline rocks erupted in the southern rift between about 30 and 18 m.y. ago were not uniquely related to the rifting process. Rather, the thermal pulse which generated these magmas was part of the previous, subduction-related event. Our interpretation of existing data concerning the evolution of the Rio Grande rift does not fit either simple active or passive "end-member" models. In particular, there is no compelling evidence for a major thermal event in the mantle uniquely associated with rifting. Yet heat�inherited from the immediately-preceding deformational regime�was certainly a critical factor in, and was probably a necessary condition for, rifting.
Olsen, Kenneth H.; Scott Baldridge, W.; Callender, Jonathan F.
Kinematics of Rift-Parallel Deformation Along the Rukwa Rift, Western Branch, and Main Ethiopian Rift
NASA Astrophysics Data System (ADS)
The East African Rift System spans N-S ~5000 km and currently experiences E-W extension. Previous kinematic studies of the EARS delineated 3 relatively rigid sub-plates (Victoria, Rovuma, and Lwandle) between the Nubian and Somalian plates. GPS observations of these block interiors confirm the rigid plate model, but we also detect a systematic along-rift deformation pattern at GPS stations located within rift zones bounding the western Victoria block and continuing north between the Nubian and Somalian plates. Here we present a kinematic model of present-day rift-parallel deformation along the Western branch, Rukwa Rift, and Main Ethiopian Rift constrained by a new GPS solution, earthquake slip vectors, and mapped active fault structures. We test the roles of block rotation, elastic deformation, and anelastic deformation by varying block geometry, fault slip distribution parameters, estimating permanent strain rate, and scoring each model with GPS observations. We also explore how the present-day deformation patterns relate to longer-term paleostress indicators. Observations of slickensides and offsets in seismic reflection profiles in the northern Western branch (Albertine rift) indicate a change from ~NNE trending normal faulting to include strike-slip motion within the past 7 My that may be related to previously studied stress changes in the Turkana rift. Preliminary results from the kinematic modeling demonstrate simple elastic strain accumulation on major border faults cannot explain an observed systematic northward component in GPS velocities relative to the Victoria block and the Nubian plate.
Stamps, D.; Koehn, D.; Burke, K. C.; d'Oreye, N.; Saria, E.; Xu, R.
Molten core model for Hawaiian rift zones
Kilauea volcano's East Rift Zone (ERZ) is extraordinary in that abundant lateral dike intrusions and rift zone widening associated with seaward slip of the south flank over a basal fault may have allowed an extensive molten core to develop. The rift zones of Mauna Loa and the Southwest Rift Zone (SWRZ) of Kilauea do not appear to have such extensive
Daniel J. Johnson
Rift basins - Origin, history, and distribution
NASA Technical Reports Server (NTRS)
Rifts are elongate depressions overlying places where the lithosphere has ruptured in extension. Where filled with sediment they may contain exploitable quantities of oil and gas. Because rits form in a variety of tectonic settings, it is helpful to define the particular tectonic environment in which a specific rift or set of rifts has developed. A useful approach has been to relate that environment to the Wilson Cycle of the opening and the closing of oceans. This appreciation of tectonic setting can help in better understanding of the depositional, structural and thermal history of individual rift systems. The global distribution of rifts can also be related to tectonic environment. For example, rifts associated with continental rupture at a temporary still-stand of a continent over the mantle convective system (rifts like those active in East Africa today) can be distinguished from those associated with continental collision (rifts like the Cenozoic rifts of China).
Burke, K. C.
Rift Valley Fever (RVF) is a zoonotic arbovirosis. Among animals, it mainly affects ruminants, causing abortions in gravid females and mortality among young animals. In humans, RVF virus infection is usually asymptomatic or characterized by a moderate fever. However, in 1 to 3% of cases, more severe forms of the disease (hepatitis, encephalitis, retinitis, hemorrhagic fever) can lead to the death of infected individuals or to major sequels. The RVF virus (Bunyaviridae, genus Phlebovirus) was identified for the first time in the 1930s in Kenya. It then spread over almost all African countries, sometimes causing major epizootics/epidemics. In 2000, the virus was carried out of Africa, in the Middle East Arabian Peninsula. In 2007-2008, Eastern-African countries, including Madagascar, reported significant episodes of RVF virus, this was also the case for the Comoros archipelago and the French island of Mayotte. This ability to spread associated with many vectors, including in Europe, and high viral loads in infected animals led the health authorities worldwide to warn about the potential emergence of RVF virus in areas with a temperate climate. The awareness has increased in recent years with climate changes, which may possibly modify the vector distribution and competence, and prompted many RVF virus-free countries to better prepare for a potential implantation of RVF. PMID:21295425
P�pin, M
The geometry of propagating rifts
NASA Astrophysics Data System (ADS)
The kinematics of two different processes are investigated, both of which have been described as rift propagation. Courtillot uses this term to describe the change from distributed to localised extension which occurs during the early development of an ocean basin. The term localisation is instead used here to describe this process, to distinguish it from Hey's type of propagation. Localisation generally leads to rotation of the direction of magnetisation. To Hey propagation means the extension of a rift into the undeformed plate beyond a transform fault. Detail surveys of the Galapagos rift have shown that the propagating and failing rifts are not connected by a single transform fault, but by a zone which is undergoing shear. The principal deformation is simple shear, and the kinematics of this deformation are investigated in some detail. The strike of most of the lineations observed in the area can be produced by such deformation. The mode of extension on the propagating rift appears to be localised for some periods but to be distributed for others. Neither simple kinematic arguments nor stretching of the lithosphere with conservation of crust can account for the observed variations in water depth.
McKenzie, Dan
Relationships between pre-rift structure and rift architecture in Lakes Tanganyika and Malawi, East Africa
Continental rift systems are rips in plates caused by focusing of extensional stresses along some zone. In the same way that tensile cracks in the side of a brick building generally follow the mortar between bricks, rifts initially follow the weakest pathways in the pre-rift materials. There has even been a suggestion that the occurrence of rifts is controlled by
J. Versfelt; B. R. Rosendahl
NASA Astrophysics Data System (ADS)
This overview paper considers the East African rift system (EARS) as an intra-continental ridge system, comprising an axial rift. It describes the structural organization in three branches, the overall morphology, lithospheric cross-sections, the morphotectonics, the main tectonic features�with emphasis on the tension fractures�and volcanism in its relationships with the tectonics. The most characteristic features in the EARS are narrow elongate zones of thinned continental lithosphere related to asthenospheric intrusions in the upper mantle. This hidden part of the rift structure is expressed on the surface by thermal uplift of the rift shoulders. The graben valleys and basins are organized over a major failure in the lithospheric mantle, and in the crust comprise a major border fault, linked in depth to a low angle detachment fault, inducing asymmetric roll-over pattern, eventually accompanied by smaller normal faulting and tilted blocks. Considering the kinematics, divergent movements caused the continent to split along lines of preexisting lithospheric weaknesses marked by ancient tectonic patterns that focus the extensional strain. The hypothesis favored here is SE-ward relative divergent drifting of a not yet well individualized Somalian plate, a model in agreement with the existence of NW-striking transform and transfer zones. The East African rift system comprises a unique succession of graben basins linked and segmented by intracontinental transform, transfer and accommodation zones. In an attempt to make a point on the rift system evolution through time and space, it is clear that the role of plume impacts is determinant. The main phenomenon is formation of domes related to plume effect, weakening the lithosphere and, long after, failure inducing focused upper mantle thinning, asthenospheric intrusion and related thermal uplift of shoulders. The plume that had formed first at around 30 Ma was not in the Afar but likely in Lake Tana region (Ethiopia), its almost 1000 km diameter panache weakening the lithosphere and preparing the later first rifting episode along a preexisting weak zone, a Pan-African suture zone bordering the future Afar region. From the Afar, the rift propagated afterward from north to south on the whole, with steps of local lithospheric failure nucleations along preexisting weak zones. These predisposed lines are mainly suture zones, in which partial activation of low angle detachment faults reworked former thrust faults verging in opposite directions, belonging to double verging ancient belts. This is responsible for eventual reversal in rift asymmetry from one basin to the next. Supposing the plume migrated southward, or other plumes emplaced, the rift could propagate following former weaknesses, even outside areas influenced by plumes. This view of rift formation reconciles the classical models: active plume effect triggered the first ruptures; passive propagations of failure along lithospheric scale weak zones were responsible for the onset of the main rift segments. Various other aspects are shortly considered, such as tectonics and sedimentation, and relationships of the 'cradle of Mankind' with human evolution. By its size, structure and occurrence of oceanic lithosphere in the Afar, the EARS can be taken as a model of the prelude of oceanic opening inside a continent.
Chorowicz, Jean
Current rifting episode in north Iceland
A major rifting episode is now occurring in north Iceland. This started on 20 December 1975, with a basaltic eruption, an exceptionally intense earthquake swarm and movement on an 80-km segment of the plate boundary. Inflation and deflation of the Krafla caldera indicate upwelling of magma and injection into the rift zone. Historical records show that similar episodic rifting occurs
Axel Bj�rnsson; Kristj�n Saemundsson; P�ll Einarsson; Eysteinn Tryggvason; Karl Gr�nvold
Off-rift volcanism in rift zones determined by crustal unloading
NASA Astrophysics Data System (ADS)
When continents are stretched over a long period of time, deep elongated rift valleys form at Earth's surface and zones of ponded magma, centred beneath the rift, form at the crust-mantle boundary. Ascending magma sometimes erupts within the rift valley or, counterintuitively, at volcanic fields away from the rift valley that are offset by tens of kilometres from the source of magma at depth. The controls on the distribution of this off-rift volcanism are unclear. Here we use a numerical model of magmatic dyke propagation during rifting to investigate why some dykes reach the surface outside the rift valley, whereas others are confined to the valley. We find that the location of magmatism is governed by the competition between tectonic stretching and gravitational unloading pressure, caused by crustal thinning and faulting along the rift borders. When gravitational unloading dominates over tectonic stretching forces, dykes ascending from the ponded magma are steered towards the rift sides, eventually causing off-rift eruptions. Our model also predicts the formation of stacked magma sills in the lower crust above the magma-ponding zone, as well as the along-rift propagation of shallow dykes during rifting events, consistent with observations of magmatism and volcanism in rift zones globally. We conclude that rift topography-induced stress changes provide a fundamental control on the transfer of magma from depth to the surface.
Maccaferri, Francesco; Rivalta, Eleonora; Keir, Derek; Acocella, Valerio
Parga Chasma: Coronae and Rifting on Venus
NASA Technical Reports Server (NTRS)
The majority of coronae (quasicircular volcano-tectonic features) are found along rifts or fracture belts, and the majority of rifts have coronae [e.g. 1,2]. However, the relationship between coronae and rifts remains unclear [3-6]. There is evidence that coronae can form before, after, or synchronously with rifts [3,4]. The extensional fractures in the rift zones have been proposed to be a result of broad scale upwelling and traction on the lower lithosphere [7]. However, not all rift systems have a significant positive geoid anomaly, as would be expected for an upwelling site [8]. This could be explained if the rifts lacking anomalies are no longer active. Coronae are generally accepted to be sites of local upwelling [e.g. 1], but the observed rifting is frequently not radial to the coronae and extends well beyond the coronae into the surrounding plains. Thus the question remains as to whether the rifts represent regional extension, perhaps driven by mantle tractions, or if the coronae themselves create local thinning and extension of the lithosphere. In the first case, a regional extension model should be consistent with the observed characteristics of the rifts. In the latter case, a model of lithospheric loading and fracturing would be more appropriate. A good analogy may be the propagation of oceanic intraplate volcanoes [9].
Smrekar, S. E.; Stofan, E. R.; Buck, W. R.; Martin, P.
Magmatism in a Cambrian Laurentian Plate Rift
NASA Astrophysics Data System (ADS)
Evidences of the Cambrian Southern Oklahoma Aulacogen extend over 1000km from about Dallas out to the Uncompahgre Plateau in SW Colorado. The signature of this originally extensional feature can be traced geophysically, and in some places at the present surface, petrologically and temporally, by the presence of mafic rock. It appears to have been the intracontinental third arm of a plume-generated? triple junction which helped to dismember the southern part of Laurentia on the final break-up of a Neoproterozoic supercontinent. Other parts of Laurentia rifted away and are now found in the Precordillera of Argentina. Rift magmatism appears to have been concentrated nearer the plate edge during the breakup. Perhaps as much as 40,000 km3 of mostly subaerial silicic volcanics and shallow-seated granites overlay and filled the top of the rift in the area of SW Oklahoma. The rift fill below the silicic rocks is large, layered mafic complexes and smaller, layered, hydrous gabbros, the whole set appearing as a shallow AMCG complex. Unusually, direct rift sediments are not obvious. Furthermore, silicic and mafic rocks have identical Nd signatures. Finally, about 20 Ma after rifting ceased and later into the Paleozoic during sea incursion, overlying sediments are thickened 4X compared to equivalent units 100's of kms to the rift sides. This rift appears distinct from most modern rifts. Conclusions are 1) This was a hot, narrow rift; 2) Basaltic magmatism , not sedimentation, filled the rift; 3) Magmatic intensity varied along the rift strike; 4) Silicic rocks were generated mostly directly from new mantle-derived basalt liquids through fractionation, not melting of older crustal rocks; 5) Laurentian lithosphere was weak allowing centering of the Early/Middle Paleozoic large "Oklahoma" basin (pre-Anadarko) over the rift.
Gilbert, M. C.
Magmatism in rifting and basin formation
NASA Astrophysics Data System (ADS)
Whether heating and magmatism cause rifting or rifting processes cause magmatic activity is highly debated. The stretching factor in rift zones can be estimated as the relation between the initial and the final crustal thickness provided that the magmatic addition to the crust is insignificant. Recent research demonstrates substantial magmatic intrusion into the crust in the form of sill like structures in the lowest crust in the presently active Kenya and Baikal rift zones and the DonBas palaeo-rift zone in Ukraine. This result may be surprising as the Kenya Rift is associated with large amounts of volcanic products, whereas the Baikal Rift shows very little volcanism. Identification of large amounts of magmatic intrusion into the crust has strong implications for estimation of stretching factor, which in the case of Baikal Rift Zone is around 1.7 but direct estimation gives a value of 1.3-1.4 if the magmatic addition is not taken into account. This may indicate that much more stretching has taken place on rift systems than hitherto believed. Wide sedimentary basins may form around aborted rifts due to loading of the lithosphere by sedimentary and volcanic in-fill of the rift. This type of subsidence will create wide basins without faulting. The Norwegian- Danish basin in the North Sea area also has subsided gradually during the Triassic without faulting, but only few rift structures have been identified below the Triassic sequences. We have identified several mafic intrusions in the form of large batholiths, typically more than 100 km long, 20-40 km wide and 20 km thick. The associated heating would have lifted the surface by about 2 km, which may have been eroded before cooling. The subsequent contraction due to solidification and cooling would create subsidence in a geometry similar to basins that developed by loading. These new aspects of magmatism will be discussed with regard to rifting and basin formation.
Thybo, H.
Structural Evolution of the Incipient Okavango Rift Zone, NW Botswana
Studies of the East African Rift System (EARS) and other continental rifts have significantly improved our understanding of rifting processes; however, we particularly lack studies of the embryonic stages of rift creation. The Okavango Rift Zone (ORZ), NW Botswana is one of few places worldwide where one can study the early stages of continental extension prior to the accumulation of
E. A. Atekwana; B. D. Kinabo; M. P. Modisi; J. P. Hogan; D. D. Wheaton
Continental Rifts: Evolution, Structure and Tectonics
NASA Astrophysics Data System (ADS)
Twenty one �friends of continental rifts� wrote Continental Rifts: Evolution, Structureand Tectonics. They define the object of their passion as elongate tectonic depressions along which the entire lithosphere has been modified by extension. Strictly speaking, passive margins and highly extended terranes such as the Basin and Range are not included in this definition, but the authors consider them to be related to continental rifts. The authors hail from academia and set as their main goal �an improved understanding of the fundamental lithospheric processes of rifting, with primary focus on deep structures and processes associated with rifting.� Consequently, many well-known extensional systems, for example, the North Sea grabens, the Suez Basin, onshore and offshore eastern China, and large areas of southeast Asia, are barely considered. Rift aficionados from the petroleum industry will find very little to interest them in this book.
Bally, A. W.
Continental rift systems and anorogenic magmatism
Precambrian Laurentia and Mesozoic Gondwana both rifted along geometric patterns that closely approximate truncated-icosahedral tessellations of the lithosphere. These large-scale, quasi-hexagonal rift patterns manifest a least-work configuration. For both Laurentia and Gondwana, continental rifting coincided with drift stagnation, and may have been driven by lithospheric extension above an insulated and thermally expanded mantle. Anorogenic magmatism, including flood basalts, dike swarms,
James W. Sears; Gregory M. St. George; J. Chris Winne
Rift Valley fever outbreak, southern Mauritania, 2012.
After a period of heavy rainfall, an outbreak of Rift Valley fever occurred in southern Mauritania during September-November 2012. A total of 41 human cases were confirmed, including 13 deaths, and 12 Rift Valley fever virus strains were isolated. Moudjeria and Temchecket Departments were the most affected areas. PMID:24447334
Sow, Abdourahmane; Faye, Ousmane; Ba, Yamar; Ba, Hampath�; Diallo, Diawo; Faye, Oumar; Loucoubar, Cheikh; Boushab, Mohamed; Barry, Yahya; Diallo, Mawlouth; Sall, Amadou Alpha
Rift Valley fever in Namibia, 2010.
During May-July 2010 in Namibia, outbreaks of Rift Valley fever were reported to the National Veterinary Service. Analysis of animal specimens confirmed virus circulation on 7 farms. Molecular characterization showed that all outbreaks were caused by a strain of Rift Valley fever virus closely related to virus strains responsible for outbreaks in South Africa during 2009-2010. PMID:24274469
Monaco, Federica; Pinoni, Chiara; Cosseddu, Gian Mario; Khaiseb, Siegfried; Calistri, Paolo; Molini, Umberto; Bishi, Alec; Conte, Annamaria; Scacchia, Massimo; Lelli, Rossella
Dynamics of crustal rifting in NE Iceland
Magnetotelluric measurements have revealed a crustal thickness of 8-10 km in the axial rift zone of NE Iceland and above the proposed hot spot in central east and north Iceland. The crust thickens with age and is 20-30 km thick in the older Tertiary areas to the east and west of the axial rift zone. It also thickens toward north
Axel Bj�rnsson
Rifting in iceland: new geodetic data.
Small but measurable lengthening of several survey lines within the eastern rift zone of Iceland occurred between 1967 and 1970. The changes can be interpreted as a widening of the rift by 6 to 7 centimeters, possibly during the 1970 eruption of Hekla volcano. PMID:17738437
Decker, R W; Einarsson, P; Mohr, P A
Rift Valley Fever in Namibia, 2010
During May�July 2010 in Namibia, outbreaks of Rift Valley fever were reported to the National Veterinary Service. Analysis of animal specimens confirmed virus circulation on 7 farms. Molecular characterization showed that all outbreaks were caused by a strain of Rift Valley fever virus closely related to virus strains responsible for outbreaks in South Africa during 2009�2010.
Monaco, Federica; Pinoni, Chiara; Khaiseb, Siegfried; Calistri, Paolo; Molini, Umberto; Bishi, Alec; Conte, Annamaria; Scacchia, Massimo; Lelli, Rossella
Elliptical caldera formation throughout the Kenyan Rift
NASA Astrophysics Data System (ADS)
Many of the world's calderas are elliptical in shape, and their orientation is often used as a proxy for the local stress regime. However, in some rift settings, pre-existing structural trends have been shown to control caldera orientation. We test these competing hypotheses in the Kenyan Rift, which consists of two distinct rift segments, with different tectonic and magmatic characteristics. Of the fourteen Quaternary volcanoes lying along the central rift axis, seven have undergone caldera collapse and six are highly elliptical. We present a remote-sensing study that investigates the structural and tectonic control on caldera ellipticity and orientation within the Kenyan Rift. Satellite-based mapping using ArcGIS on imagery derived from ASTER and GDEM data to identify the orientations of the main East African Rift border faults, intra-rift faults and the geometry of Kenyan calderas to determine the extensional setting, horizontal compressive stress orientations and the pre-existing rift fabric direction. Other data sources included the GPS-derived plate-kinematic model of East Africa and information from the literature. We find that deformation in the Kenyan Rift is characterised by orthogonal extension in the north and oblique opening in the south, suggesting that both tectonic stresses and magmatic pressures drive intra-rift fault formation. The long axis elongation of calderas are orientated NW-SE in the north, aligned with pre-existing structures and perpendicular to recent rift-faults. In contrast, the long axes are aligned NE-SW in the southern group of volcanoes, at an angle which is highly oblique to the recent rift faults, but aligned with pre-existing structures. Thus we conclude that in oblique continental rifts, pre-existing structures play a dominant role in the rise of magma through the crust. Understanding the geometry of caldera systems gives us important information as to the structural controls on magmatic and tectonic behaviour in extensional settings and the mechanisms by which continental rifts evolve from fault-controlled basins into mid-ocean ridges.
Robertson, Elspeth; Biggs, Juliet; Cashman, Katharine; Floyd, Michael; Vye-Brown, Charlotte
The Effect of Continental Rifting on Lithospheric Fabric: Evidence From the Mid-Continent Rift
NASA Astrophysics Data System (ADS)
The Mid-Continent Rift (MCR) is a major feature of the North American continent: a 1.1 Ga rift that failed to develop into an ocean basin. Though the crustal expression of the rift is preserved, it is impossible to determine from crustal evidence the nature of the lithospheric contribution to the rifting process. The installation of teleseismic instrumentation through the Superior Province Rifting Earthscope Experiment (SPREE) is allowing investigation of the lithosphere beneath the MCR, which will help in addressing questions about the initiation, propagation, and failure of the rift structure. We focus on observing the strength and orientation of lithospheric fabric through measurements of the splitting of teleseismic SK(K)S waves at instruments in and near the rift axis, using the method of Silver and Chan (1991) to find the set of parameters that optimally restores linear particle motion. Our results show that the fast direction varies only subtly across the study area, with the exception of localized outliers. The fast direction is close to the direction of absolute plate motion, but shows greater scatter within the MCR itself. Split times show strong variations (from near-zero to 1.5 s), with lower values within the rift; the Nipigon Embayment stands out as a particularly low-anisotropy region. These measurements suggest that the rifting process thinned the lithosphere or reset its fabric, indicating significant lithospheric participation in the rifting process.
Ola, O. B.; Frederiksen, A. W.
Cenozoic rift tectonics of the Japan Sea
The Japan Sea is one of the back-arc basins in trench-arc systems bordering the western Pacific. Recent paleomagnetic works suggest the Japan Sea opened during early to middle Miocene. Radiometric and microfossil ages of the Cenozoic onland sequences in the Japanese Islands elucidate the rift tectonics of the Japan Sea. The rifting history is summarized as follows: nonmarine volcanic formations of prerift stage before 50 Ma, rift-onset unconformity at 40 Ma, nonmarine volcanic formations of synrift stage 20-33 Ma, breakup unconformity 19 Ma showing the opening of the Japan Sea, marine volcanic and sedimentary formations of synrift stage 14.5-18 Ma, beginning of regional subsidence 14.5 Ma corresponding to the end of the Japan Sea opening, marine sedimentary formations of postdrift stage after 14.5 Ma. Rifting is not limited to the synrift stage but is continued to the syndrift stage. Rifting led to a horst-and-graben structure. Thus, the Cenozoic onland sequences in the Japanese Islands are suited for a study of rift tectonics because the sequences were subaerially exposed by the late Miocene-Holocene island-arc tectonics. Rift tectonics cannot be studied as easily in most Atlantic-type passive margins.
Kimura, K.
Investigation of rifting processes in the Rio Grande Rift using data from unusually large earthquake swarms
San Acacia Swarm in the Rio Grande Rift. Because the Rio Grande rift is one of the best seismically instrumented rift zones in the world, studying its seismicity provides an exceptional opportunity to explore the active tectonic processes within continental rifts. We have been studying earthquake swarms recorded near Socorro in an effort to link seismicity directly to the rifting process. For FY94, our research has focused on the San Acacia swarm, which occurred 25 km north of Socorro, New Mexico, along the accommodation zone between the Albuquerque-Belen and Socorro basins of the central Rio Grande rift. The swarm commenced on 25 February 1983, had a magnitude 4.2 main shock on 2 March and ended on 17 March, 1983.
Sanford, A.; Balch, R. [New Mexico Inst. of Mining and Technology, Socorro, NM (United States); House, L.; Hartse, H. [Los Alamos National Lab., NM (United States)
Animation of Volcanism Along a Rift Zone
NSDL National Science Digital Library
This animation illustrates the processes that occur at a rift zone that can result in volcanic eruptions. Users can stop, play, fast forward and rewind the animation at any time to stress important points.
Loomis, Jen
The Shear along the Dead Sea Rift
Recent surface and subsurface geological investigations in Israel and Jordan provide new data for the re-examination of Dubertret's (1932) hypothesis of the left-hand shear along the Dead Sea rift. It is found that while none of the pre-Tertiary sedimentary or igneous rock units extend right across the rift, all of them resume a reasonable palaeographical configuration once the east side
R. Freund; Z. Garfunkel; I. Zak; M. Goldberg; T. Weissbrod; B. Derin; F. Bender; F. E. Wellings; R. W. Girdler
East African Rift Valley Links for Learning
NSDL National Science Digital Library
This web page contains links to a collection of resources devoted to the East African Valley Rift, with emphasis on environmental issues and concerns. It lists web addresses by subtopics, in outline form, with a short description to assist the viewer in searching for information. Topics include The Rift Valley, Stromboli online, African volcanoes, the Alid page, paleontology, and many more. The creator of this collection has checked uRLs provided for content and guarantees them to be quality web pages.
Moyra/mysticpc; Geolor.com
The 1974 Ethiopian rift geodimeter survey
NASA Technical Reports Server (NTRS)
The field techniques and methods of data reduction for five successive geodimeter surveys in the Ethiopian rift valley are enlarged upon, with the considered conclusion that there is progressive accumulation of upper crustal strain, consonant with on-going rift extension. The extension is restricted to the Quaternary volcanotectonic axis of the rift, namely the Wonji fault belt, and is occurring at rates of 3 to 6 mm/yr in the northern sector of the rift valley. Although this concurs with the predictions of platetectonic analysis of the Afar triple junction, it is considered premature to endorse such a concurrence on the basis of only 5 years of observations. This is underlined by the detection of local tectonic contractions and expansions associated with geothermal and gravity anomalies in the central sector of the rift valley. There is a hint of a component of dextral slip along some of the rift-floor fault zones, both from geological evidence and from the strain patterns detected in the present geodetic surveys.
Mohr, P.
Surface deformation in volcanic rift zones
The principal conduits for magma transport within rift zones of basaltic volcanoes are steeply dipping dikes, some of which feed fissure eruptions. Elastic displacements accompanying a single dike emplacement elevate the flanks of the rift relative to a central depression. Concomitant normal faulting may transform the depression into a graben thus accentuating the topographic features of the rift. If eruption occurs the characteristic ridge-trough-ridge displacement profile changes to a single ridge, centered at the fissure, and the erupted lava alters the local topography. A well-developed rift zone owes its structure and topography to the integrated effects of many magmatic rifting events. To investigate this process we compute the elastic displacements and stresses in a homogeneous, two-dimensional half-space driven by a pressurized crack that may breach the surface. A derivative graphical method permits one to estimate the three geometric parameters of the dike (height, inclination, and depth-to-center) and the mechanical parameter (driving pressure/rock stiffness) from a smoothly varying displacement profile. Direct comparison of measured and theoretical profiles may be used to estimate these parameters even if inelastic deformation, notably normal faulting, creates discontinuities in the profile. Geological structures (open cracks, normal faults, buckles, and thrust faults) form because of stresses induced by dike emplacement and fissure eruption. Theoretical stress states associated with dilation of a pressurized crack are used to interpret the distribution and orientation of these structures and their role in rift formation. ?? 1983.
Pollard, D. D.; Delaney, P. T.; Duffield, W. A.; Endo, E. T.; Okamura, A. T.
Incipient continental rifting: Insights from the Okavango Rift Zone, northwestern Botswana
NASA Astrophysics Data System (ADS)
In this dissertation aeromagnetic, gravity, and Shuttle Radar Topography Mission Digital Elevation Model (SRTM DEM) data from the Okavango Rift Zone in northwest Botswana are used to map the distribution of rift and basement structures. The distribution of these structures provide useful insights into the early stages of continental rifting. The objectives of this study are (1) assessing the role of pre-existing structures on rift basin development, (2) characterizing the geometry of the nascent rift basins, (3) documenting fault growth and propagation patterns, and (4) investigating the border fault development. Potential field data especially aeromagnetic data are used to map out structures in the sediment covered basement, whereas SRTM DEM data express the surface morphology of the structures. The azimuth of rift faults parallel the orientation of the fold axes and the prominent foliation directions of the basement rocks. This indicates that pre-existing structures in the basement influenced the development of the rift structures. NE dipping faults consistently exhibit greater displacements than SE dipping faults, suggesting a developing half-graben geometry. Individual faults grow by along axis linkage of small segments that develop from soft linkage (under lapping to overlapping segments) to hard linkage (hooking, fused segments). Major rifts faults are also linking through transfer zones by the process of "fault piracy" to establish an immature border fault system. The relationships between scam heights and vertical throws reveal that the young and active faults are located outside the rift while the faults with no recent activities are in the middle suggesting that the rift is also growing in width. This study demonstrates the utility of potential field data and SRTM DEM to provide a 3-D view of incipient continental rifting processes such as fault growth and propagation.
Kinabo, Baraka Damas
Dynamics of continental rift propagation: the end-member modes
NASA Astrophysics Data System (ADS)
An important aspect of continental rifting is the progressive variation of deformation style along the rift axis during rift propagation. In regions of rift propagation, specifically transition zones from continental rifting to seafloor spreading, it has been observed that contrasting styles of deformation along the axis of rift propagation are bounded by shear zones. The focus of this numerical modeling study is to look at dynamic processes near the tip of a weak zone in continental lithosphere. More specifically, this study explores how modeled rift behavior depends on the value of rheological parameters of the crust. A three-dimensional finite element model is used to simulate lithosphere deformation in an extensional regime. The chosen approach emphasizes understanding the tectonic forces involved in rift propagation. Dependent on plate strength, two end-member modes are distinguished. The stalled rift phase is characterized by absence of rift propagation for a certain amount of time. Extension beyond the edge of the rift tip is no longer localized but occurs over a very wide zone, which requires a buildup of shear stresses near the rift tip and significant intra-plate deformation. This stage represents a situation in which a rift meets a locked zone. Localized deformation changes to distributed deformation in the locked zone, and the two different deformation styles are balanced by a shear zone oriented perpendicular to the trend. In the alternative rift propagation mode, rift propagation is a continuous process when the initial crust is weak. The extension style does not change significantly along the rift axis and lengthening of the rift zone is not accompanied by a buildup of shear stresses. Model predictions address aspects of previously unexplained rift evolution in the Laptev Sea, and its contrast with the tectonic evolution of, for example, the Gulf of Aden and Woodlark Basin.
Van Wijk, J. W.; Blackman, D. K.
Off-axis volcanism in the Gregory rift, east Africa: implications for models of continental rifting
The largest volcanic centers of the Gregory rift occur in two belts located 100 to 150 km east and west of the axis of the rift valley. These off-axis volcanic belts include the highest peaks on the continent of Africa and are interpreted to lie above the intersection of low-angle detachment systems with the base of a regionally thinned lithosphere. These detachment systems are manifested at the surface as a series of breakaway zones and regional bounding faults that produce subbasins with half-graben form. The asymmetry of subbasins alternates along the rift axis, indicating that the polarity of the underlying active detachment systems also reverses. The detachments are separated laterally by regional oblique-slip accommodation zones typified by wrench-style tectonism. Off-axis from the rift, the detachments are inferred to merge along strike as they cut to the base of the lithosphere. This results in irregular but persistent paired zones of volcanism and lithospheric thinning off-axis from the rift proper. The development of major volcanic cones such as Mount Kilimanjaro may be controlled by the interaction of leaky accommodation zones with the regions of structurally thinned lithosphere. The central Kenya hot spot has produced the anomalous quantities of volcanic material that fills the Gregory rift and probably enhances the off-axis volcanism but does not directly control its location. The model proposed here for tectonic controls of volcanism in the Gregory rift may be applicable to Phanerozoic continental rifts in general.
Bosworth, W.
InSAR observations of post-rifting deformation around the Dabbahu rift segment, Afar, Ethiopia
NASA Astrophysics Data System (ADS)
Increased displacement rates have been observed following manylarge earthquakes and magmatic events. Although an order of magnitude smaller than the displacements associated with the main event, the post-seismic or post-rifting deformation may continue for years to decades after the initial earthquake or dyke intrusion. Due to the rare occurrence of subaerial rifting events, there are very few observations to constrain models of post-rifting deformation. In 2005 September, a 60-km-long dyke was intruded along the Dabbahu segment of the Nubia-Arabia Plate boundary (Afar, Ethiopia), marking the beginning of an ongoing rifting episode. Continued activity has been monitored using satellite radar interferometry and data from global positioning system instruments deployed around the rift in response to the initial intrusion. Using multiple satellite passes, we are able to separate the rift perpendicular and vertical displacement fields around the Dabbahu segment. Rift perpendicular and vertical rates of up to 180 and 240 mm yr-1, respectively. Here, we show that models of viscoelastic relaxation alone are insufficient to reproduce the observed deformation field and that a large portion of the observed signal is related to the movement of magma within the rift segment. Our models suggest upper mantle viscosities of 1018-19 Pa s overlain by an elastic crust of between 15 and 30 km. To fit the observations, inflation and deflation of magma chambers in the centre of the rift and to the south east of the rift axis is required at rates of �0.13 and -0.08 km3 yr-1.
Hamling, Ian J.; Wright, Tim J.; Calais, Eric; Lewi, Elias; Fukahata, Yukitoshi
Off rift and on rift volcanism along the southern most extremity of the Reykjanes Ridge.
NASA Astrophysics Data System (ADS)
In August-September 2013 R/V Marcus G Langseth conducted a geophysical survey of the southern Reykjanes Ridge and flanks to the Bight transform fault including the first orthogonally spreading segment to the south. The objectives were to better understand how the Reykjanes Ridge replaced the earlier transform fault-dominated structure. The survey acquired full-coverage multibeam bathymetry of some 90,000 km2 and acoustic backscatter imagery and coincident gravity and magnetic profiles. The Rift axis of the RR is defined by a rift valley, striking 36� NE, and deepens from N to S towards the Bight transform fault. Volcanism along the rift axis is characterized by en-echelon volcanic ridges striking 14�NE and rising some 400-1000 m above the valley floor, single circular volcanic sea mounts 400-600 m high, lava flow sheets and craters. Fissures and faults are not very prominent with in the rift valley. However, at both sides bounding the rift valley, fissure, faults and uplifting of the crust is a dominant feature. Surprisingly numerous volcanic edifices are observed on the faulted crust drifting away from the plate boundary. Further these volcanic edifices do not all show any faulting and have cone shape forms, indicating more explosive activity than within the rift. The volcanic edifices range in size from 2-3 km at the base to some hundreds of meters. Backscatter analysis shows that in general the volcanic edifices have higher values than the surrounding basement. These vents are observed as far as 100 km from the rifting center. High backscatter along with little or no faulting indicates that these off rift volcanic vents are younger than the basement they are resting on, thus manifesting that volcanism is not solely confined to the active rift boundary in the area. The segment south of Bight transform fault is highly dotted by these off rift volcanic vents
Hoskuldsson, Armann; Martinez, Fernando; Hey, Richard
Stratigraphic and structural constraints on mechanisms of active rifting in the Gregory Rift, Kenya
NASA Astrophysics Data System (ADS)
A review of the stratigraphie and structural events associated with the early evolution of the Gregory Rift indicates that they are consistent with the active model of rifting. At present, there is no geochemical evidence to indicate unequivocally the presence of magma derived from asthenospheric material. However, the evidence for limited pre-rift uplift and the morphologic and geochemical features of the early basalts and phonolite flood lavas support the intrusion of an asthenospheric plume into the lithospheric upper mantle during mid-Miocene times. The scale of uplift (< 1 km) and the limited volumes of lava produced indicate that a small convective cell, with an initial diameter of perhaps only 100-150 km, rather than a major asthenospheric plume underlies the Gregory Rift. The subsequent rise of this small plume through the lithosphere to the base of the crust is documented by Late Miocene and Pliocene magmatic and rifting events. The presence of a relatively thick mechanical boundary layer beneath the Gregory Rift prior to rifting, indicated by the general absence of tholeiitic volcanism and low extension rates, is supported by geophysical and geochemical evidence for igneous underplating and significant magma fractionation within the lithosphere. There is a strong coincidence of location of magmatic activity and rifting with pre-existing zones of crustal weakness beneath the Gregory Rift. The rising plume was focused beneath a weak zone marking the contact between the reworked and buried margin of the Tanzanian craton and the adjacent Proterozoic mobile belt. Within this zone, major crustal-scale shear zones and thrusts accommodated limited lithospheric stretching and influenced the location and development of half-graben basins and transfer zones. Early basins were nucleated on NW-SE- and N-S-trending weaknesses. As the plume ascended and spread laterally, rifting propagated away from this zone and younger basins developed across both mobile belt and cratonic crust.
Smith, Martin
Evidence of Melt-Induced Seismic Anisotropy and Magma Assisted Rifting in the North Ethiopian Rift
NASA Astrophysics Data System (ADS)
The complex process of the transition from continental to oceanic rifting remains poorly understood. The Northern Ethiopian Rift, being free from interference from other tectonic processes, is an ideal place to study the continental breakup process. With this in mind, the recent EAGLE experiment saw the deployment of 79 broadband seismometers over an area 250-350km centred on the Northern Ethiopian Rift. We investigate the signature of crustal and upper-mantle anisotropy in these data as it provides insights into rifting processes. Recent studies employing shear-wave splitting techniques provide strong and consistent evidence for a rift-parallel (NNE-SSW) fast anisotropic direction beneath the rift. The detailed characteristics of these observations imply a single anisotropic layer confined to the upper 100km. Surface-wave tomography shows that the fast rift-parallel directions persist to a depth of 400km beneath a broader area surrounding the rift. These observations eliminate a number of plausible causes of anisotropy including plate motion drag, radial mantle-flow in the Afar plume head, mantle flow perpendicular to the rift induced by the rifting process, or pre-existing frozen-in crystallographic fabric. The observed anisotropy is more likely to be caused by either channeled horizontal mantle-flow along the rift axis, which would cause the lattice preferred orientation (LPO) of olivine with the fast ? -axes paralleling the rift, or the presence of rift-aligned melt-filled pockets (MFP) in the mantle. % SKS-splitting results show that the distribution of the fast anisotropic orientation mimics closely the distribution of strain and magmatism in the rift, implying MFP-induced anisotropy. % However, the techniques employed by the studies to date do not provide a means of conclusively separating the two candidate causes of anisotropy. The speeds of horizontally propagating S v and S h waves vary in similar fashions with azimuth for LPO- and MFP-induced anisotropy ( S v ? cos({? }), S h ? cos({2? }), where ? is the azimuth measured from the rift axis). However, the the relative change in the two shear-wave velocities is distinctive for LPO- and MFP-induced anisotropy. This provides a powerful tool for distinguishing between the two candidate causes of anisotropy. % We present strong evidence for MFP-induced anisotropy beneath the rift in the depth range of 20--60~km, by showing that the azimuthal variation of the speeds of S v and S h waves propagating horizontally through the rift area is in good agreement with predicted anisotropy models for vertical rift-parallel melt-filled dykes.We obtain shear-velocity models by inverting the group and phase velocity dispersion experienced by a number of local and teleseismic Rayleigh and Love waves with total or inter-station propagation paths crossing the rift area with a variety of azimuths. By only using highly coherent waves for the phase-velocity measurements and by applying a phase-matching filtering technique in the group velocity extraction procedure we ensure that our measurements are free from bias introduced by scattering and noise and that our results are reliable and robust.
Pilidou, S.; Kendall, J.; Stuart, G.; Bastow, I.
Anisotropy beneath a highly extended continental rift
NASA Astrophysics Data System (ADS)
have employed shear wave splitting techniques to image anisotropy beneath the D'Entrecasteaux Islands, in southeastern Papua New Guinea. Our results provide a detailed picture of the extending continent that lies immediately ahead of a propagating mid-ocean ridge tip; we image the transition from continental to oceanic extension. A dense shear wave splitting data set from a 2010 to 2011 passive-source seismic deployment is analyzed using single and multichannel methods. Splitting delay times of 1-1.5 s are observed and fast axes of anisotropy trending N-S, parallel to rifting direction, predominate the results. This trend is linked to lattice-preferred orientation of olivine, primarily in the shallow convecting mantle, driven by up to 200 km of N-S continental extension ahead of the westward-propagating Woodlark Rift. This pattern differs from several other continental rifts that evince rift-strike-parallel fast axes and is evident despite the complex recent tectonic history. We contend that across most of this rift, the unusually high rate and magnitude of extension has been sufficient to produce a regime change to a mid-ocean-ridge-like mantle fabric. Stations in the south of our array show more complex splitting that might be related to melt or to complex inherited structure at the edge of the extended region.
Eilon, Zachary; Abers, Geoffrey A.; Jin, Ge; Gaherty, James B.
Earthquake clusters in Corinth Rift
NASA Astrophysics Data System (ADS)
Clusters commonly occur as main shock-aftershock (MS-AS) sequences but also as earthquake swarms, which are empirically defined as an increase in seismicity rate above the background rate without a clear triggering main shock earthquake. Earthquake swarms occur in a variety of different environments and might have a diversity of origins, characterized by a high b-value in their magnitude distribution. The Corinth Rift, which was selected as our target area, appears to be the most recent extensional structure, with a likely rate of fault slip of about 1cm/yr and opening of 7mm/yr. High seismic activity accommodates the active deformation with frequent strong (M?6.0) events and several seismic excitations without a main shock with clearly discriminative magnitude. Identification of earthquake clusters that occurred in this area in last years and investigation of their spatio-temporal distribution is attempted, with the application of known declustering algorithms, aiming to associate their occurrence with certain patterns in seismicity behavior. The earthquake catalog of the National Hellenic Seismological Network is used, and a certain number of clusters were extracted from the dataset, with the MS-AS sequences being distinguished from earthquake swarms. Spatio-temporal properties of each subset were analyzed in detail, after determining the respective completeness magnitude. This work was supported in part by the THALES Program of the Ministry of Education of Greece and the European Union in the framework of the project entitled "Integrated understanding of Seismicity, using innovative Methodologies of Fracture mechanics along with Earthquake and non-extensive statistical physics - Application to the geodynamic system of the Hellenic Arc, SEISMO FEAR HELLARC".
Mesimeri, Maria; Papadimitriou, Eleftheria; Karakostas, Vasilios; Tsaklidis, George
Rift System Architecture on Venus and Implications for Lithospheric Structure
NASA Astrophysics Data System (ADS)
Terrestrial continental rifts are half graben, with a master boundary fault on only 1 side of the rift basin. Devana Chasma on Venus has long segments with full graben morphologies (2 boundary faults), indicating differences in lithosphere structure.
Kiefer, W. S.
Young rift kinematics in the Tadjoura rift, western Gulf of Aden, Republic of Djibouti
NASA Astrophysics Data System (ADS)
The Tadjoura rift forms the westernmost edge of the westerly propagating Sheba ridge, between Arabia and Somalia, as it enters into the Afar depression. From structural and remote sensing data sets, the Tadjoura rift is interpreted as an asymmetrical south facing half-graben, about 40 km wide, dominated by a large boundary fault zone to the north. It is partially filled up by the 1-3 Myr old Gulf Basalts which onlapped the older Somali Basalts along its shallower southern flexural margin. The major and trace element analysis of 78 young onshore lavas allows us to distinguish and map four distinct basaltic types, namely the Gulf, Somali, Goumarre, and Hayyabley Basalts. These results, together with radiometric age data, lead us to propose a revised volcano-stratigraphic sketch of the two exposed Tadjoura rift margins and to discriminate and date several distinct fault networks of this oblique rift. Morphological and statistical analyses of onshore extensional fault populations show marked changes in structural styles along-strike, in a direction parallel to the rift axis. These major fault disturbances are assigned to the arrest of axial fault tip propagation against preexisting discontinuities in the NS-oriented Arta transverse zone. According to our model, the sinistral jump of rifting into the Asal-Ghoubbet rift segment results from structural inheritance, in contrast with the en �chelon or transform mechanism of propagation that prevailed along the entire length of the Gulf of Aden extensional system.
Daoud, Mohamed A.; Le Gall, Bernard; Maury, Ren� C.; Rolet, Jo�L.; Huchon, Philippe; Guillou, Herv�
Seismic anisotropy and mantle flow beneath the Baikal rift zone
SEISMIC studies have shown that continental rifts such as Lake Baikal and the Great Rift Valley of East Africa are like mid-ocean rifts in that they lie above broad regions of asthenospheric upvvarp of much greater extent than the surface expression of rifting1-4. The direction of mantle flow in such regions can be investigated using the seismic anisotropy created by
S. Gao; P. M. Davis; H. Liu; P. D. Slack; Yu. A. Zorin; V. V. Mordvinova; V. M. Kozhevnikov; R. P. Meyer
Early structural development of the Okavango rift zone, NW Botswana
NASA Astrophysics Data System (ADS)
Aeromagnetic and gravity data collected across the Okavango rift zone, northwest Botswana are used to map the distribution of faults, provide insights into the two-dimensional shallow subsurface geometry of the rift, and evaluate models for basin formation as well as the role of pre-existing basement fabric on the development of this nascent continental rift. The structural fabric (fold axes and foliation) of the Proterozoic basement terrane is clearly imaged on both gravity and magnetic maps. The strike of rift-related faults (030-050� in the north and 060-070� in the south) parallels fold axes and the prominent foliation directions of the basement rocks. These pre-existing fabrics and structures represent a significant strength anisotropy that controlled the orientation of younger brittle faults within the stress regime present during initiation of this rift. Northwest dipping faults consistently exhibit greater displacements than southeast dipping faults, suggesting a developing half-graben geometry for this rift zone. However, the absence of fully developed half-grabens along this rift zone suggests that the border fault system is not fully developed consistent with the infancy of rifting. Three en-echelon northeast trending depocenters coincide with structural grabens that define the Okavango rift zone. Along the southeastern boundary of the rift, developing border faults define a 50 km wide zone of subsidence within a larger 150 km wide zone of extension forming a rift-in-rift structure. We infer from this observation that the localization of strain resulting from extension is occurring mostly along the southeastern boundary where the border fault system is being initiated, underscoring the important role of border faults in accommodating strain even during this early stage of rift development. We conclude that incipient rift zones may provide critical insights into the development of rift basins during the earliest stages of continental rifting.
Kinabo, B. D.; Atekwana, E. A.; Hogan, J. P.; Modisi, M. P.; Wheaton, D. D.; Kampunzu, A. B.
New Geophysical Results About the Relationship Between the Reelfoot Rift and the Rifted Margin of Laurentia
NASA Astrophysics Data System (ADS)
The Reelfoot rift beneath the northern Mississippi embayment is an intracratonic graben system, which formed Early Cambrian time as a result of continental breakup, and has been subsequently reactivated by compressional or tensional stresses related to plate tectonic interactions. It strikes northeastward into the continent, and is approximately perpendicular to the rifted margin of the Laurentia that is shaped by the southeast-striking Alabama-Oklahoma transform fault. The northern section of the rift near the town of New Madrid, Missouri, was the site of three great 1811-1812 earthquakes, and it remains the most seismically active area east of the Rocky Mountains. However, the southern end of the rift is obscure, and the relationship between the Reelfoot rift and the rifted margin of Laurentia remains disputed. We analyzed the gravity and magnetic database for the region using new data enhancement techniques to shed some light on this relationship. We analyzed a large area to assess the regional geological structure. Complete Bouguer gravity data and and total magnetic intensity (TMI) data were assembled and gridded on a regular grid with spacing of 2km, the TMI data were then reduced to the magnetic pole. Then the data were processed with standard techniques to attenuate the high-frequency noise, and we analyzed the regional and residual anomalies. Specially, we calculated the tilt-angle derivatives of the data. We then calculated the directional horizontal derivatives of the tilt-angle derivatives both along and perpendicular to the strike of the rift. The maps of these derivatives clearly delineate the boundaries of the edges of the Reelfoot rift, the leading edge of the Ouachita thrust belt and the margin of Laurentia. The results of the preliminary processing indicate that the southern end of the rift is near the leading edge of the Ouachita thrust belt, which produces a more curvilinear shape for the Laurentian margin than the very linear Alabama-Oklahoma transform fault suggesting its structure is more complex than usually depicted.
Guo, L.; Keller, G. R.
Rifting Attractor Structures in the Baikal Rift System: Location and Effects
NASA Astrophysics Data System (ADS)
The current geodynamics and tectonophysics of the Baikal rift system (BRS) as recorded in lithospheric stress and strain are discussed in the context of self organization of nonlinear dissipative dynamic systems and nonlinear media. The regional strain field inferred from instrumental seismic moment and fault radius data for almost 70,000 MLH ? 2.0 events of 1968 through 1994 shows a complex pattern with zones of high strain anisotropy in the central part and both flanks of the rift system (the South Baikal, Hovsg�l, and Muya rift basins, respectively). The three zones of local strain anisotropy highs coincide with domains of predominantly vertical stress where earthquakes of different magnitudes are mostly of normal slip geometry. Pulse-like reversals of principal stresses in the high-strain domains appear to be nonlinear responses of the system to subcrustal processes. In this respect, the BRS lithosphere is interpreted in terms of the self organization theory as a geological dissipative system. Correspondingly, the domains of high strain anisotropy and stress change, called rifting attractor structures (RAS), are the driving forces of its evolution. The location and nonlinear dynamics of the rifting attractors have controlled lithospheric stress and strain of the rift system over the period of observations, and the same scenario may have been valid also in the Mesozoic-Cenozoic rifting history. The suggested model of a positive-feedback (fire-like) evolution of nonlinear dynamical systems with rifting attractors opens a new perspective on the current geodynamics and tectonophysics of the Baikal rift system.
Klyuchevskii, Anatoly V.
Crustal extension in the Baikal rift zone
Analysis of the gravity field along four profiles crossing the Baikal rift zone permits an estimate of the amount of anomalous mass produced by 1. (1) graben-fill sediments, 2. (2) Moho uplift and intrusion of mantle sills and dikes, 3. (3) an asthenospheric bulge. Crustal extension is evaluated based on the idea of mass and volume balance of material introduced into and removed from the initial volume of the crust. Extension in the Baikal rift increases southwestward from 0.9 km in the Chara depression to 19.3 km in the South Baikal depression. These values generally agree with the position of the Euler pole determined from seismic data (fault plane solutions). Average rotation velocity for the lithospheric plates separated by the rift zone is estimated to be 5.93 ?? 10-4 rad/m.y. over about 30 m.y. ?? 1991.
Zorin, Yu.; Cordell, L.
Submarine thermal sprirngs on the galapagos rift.
The submarine hydrothermal activity on and near the Gal�pagos Rift has been explored with the aid of the deep submersible Alvin. Analyses of water samples from hydrothermal vents reveal that hydrothermal activity provides significant or dominant sources and sinks for several components of seawater; studies of conductive and convective heat transfer suggest that two-thirds of the heat lost from new oceanic lithosphere at the Gal�pagos Rift in the first million years may be vented from thermal springs, predominantly along the axial ridge within the rift valley. The vent areas are populated by animal communities. They appear to utilize chemosynthesis by sulfur-oxidizing bacteria to derive their entire energy supply from reactions between the seawater and the rocks at high temperatures, rather than photosynthesis. PMID:17776033
Corliss, J B; Dymond, J; Gordon, L I; Edmond, J M; von Herzen, R P; Ballard, R D; Green, K; Williams, D; Bainbridge, A; Crane, K; van Andel, T H
Thermomechanical models of the Rio Grande rift
Fully two-dimensional, coupled thermochemical solutions of a continental rift and platform are used to model the crust and mantle structure of a hot, buoyant mantle diapir beneath the Rio Grande rift. The thermomechanical model includes both linear and nonlinear laws of the Weertman type relating shear stress and creep strain rate, viscosity which depends on temperature and pressure, and activation energy, temperature-dependent thermal conductivity, temperature-dependent coefficient of thermal expansion, the Boussinesq approximation for thermal bouyancy, material convection using a stress rate that is invariant to rigid rotations, an elastically deformable crust, and a free surface. The model determines the free surface velocities, solid state flow field in the mantle, and viscosity structure of lithosphere and asthenosphere. Regional topography and crustal heat flow are simulated. A suite of symmetric models, assumes continental geotherms on the right and the successively increasing rift geotherms on the left. These models predict an asthenospheric flow field which transfers cold material laterally toward the rift at > 300 km, hot, buoyant material approx. 200 km wide which ascends vertically at rates of 1 km/my between 175 to 325 km, and spreads laterally away from the rift at the base of the lithosphere. Crustal spreading rates are similar to uplift rates. The lithosphere acts as stiff, elastic cap, damping upward motion through decreased velocities of 1 km/10 my and spreading uplift laterally. A parameter study varying material coefficients for the Weertman flow law suggests asthenospheric viscosities of approx. 10/sup 22/ to 10/sup 23/ poise. Similar studies predict crustal viscosities of approx. 10/sup 25/ poise. The buoyant process of mantle flow narrows and concentrates heat transport beneath the rift, increases upward velocity, and broadly arches the lithosphere. 10 figures, 1 table.
Bridwell, R.J.; Anderson, C.A.
Longitudinal evolution of the Suez rift structure (Egypt)
A three-dimensional study of the structure of the Suez Rift has been carried out using field and subsurface data in an attempt to determine the role of transverse faults and the longitudinal evolution of the rift. As in most intracontinental rifts, the structure of the Gulf of Suez area is governed by normal faults and tilted blocks, whose crests constitute
B. Colletta; P. Le Quellec; J. Letouzey; I. Moretti
A Multidisciplinary Investigation of Rio Grande Rift Deformation
The Rio Grande rift is the easternmost actively deforming province of the western margin of North America. Geologic observations suggest the character of rifting changes from north to south, with a narrow rift marked by linear topographic depressions in Colorado and northern New Mexico grading to a broad ``basin and range'' expression in south-central New Mexico. Reasons behind the variable
A. R. Lowry; A. F. Sheehan; M. Roy; E. Jones; S. Nerem
Two-stage rifting of Pangea by two different mechanisms
Two types of rifting may have characterized the two-stage breakup of Pangea---passive rifting during Early Jurassic rapid supercontinental motion and active rifting during Cretaceous supercontinental stillstand. This suggestion is based primarily on the observation that global continental angular momentum in the hotspot reference frame was very high from 180 to 150 Ma and very low from 140 to 90 Ma.
Andrew Hynes
Geophysical studies of the West Antarctic Rift System
The West Antarctic rift system extends over a 3000 � 750 km, largely ice covered area from the Ross Sea to the base of the Antarctic Peninsula, comparable in area to the Basin and Range and the East African rift system. A spectacular rift shoulder scarp along which peaks reach 4�5 km maximum elevation marks one flank and extends from
J. C. Behrendt; W. E. LeMasurier; A. K. Cooper; F. Tessensohn; A. Tr�hu; D. Damaske
Kinematics of the South Atlantic rift
NASA Astrophysics Data System (ADS)
The South Atlantic rift basin evolved as branch of a large Jurassic-Cretaceous intraplate rift zone between the African and South American plates during the final breakup of western Gondwana. While the relative motions between South America and Africa for post-breakup times are well resolved, many issues pertaining to the fit reconstruction and particular the relation between kinematics and lithosphere dynamics during pre-breakup remain unclear in currently published plate models. We have compiled and assimilated data from these intraplated rifts and constructed a revised plate kinematic model for the pre-breakup evolution of the South Atlantic. Based on structural restoration of the conjugate South Atlantic margins and intracontinental rift basins in Africa and South America, we achieve a tight fit reconstruction which eliminates the need for previously inferred large intracontinental shear zones, in particular in Patagonian South America. By quantitatively accounting for crustal deformation in the Central and West African rift zone, we have been able to indirectly construct the kinematic history of the pre-breakup evolution of the conjugate West African-Brazilian margins. Our model suggests a causal link between changes in extension direction and velocity during continental extension and the generation of marginal structures such as the enigmatic Pre-salt sag basin and the S�o Paulo High. We model an initial E-W directed extension between South America and Africa (fixed in present-day position) at very low extensional velocities until Upper Hauterivian times (?126 Ma) when rift activity along in the equatorial Atlantic domain started to increase significantly. During this initial ?17 Myr-long stretching episode the Pre-salt basin width on the conjugate Brazilian and West African margins is generated. An intermediate stage between 126.57 Ma and Base Aptian is characterised by strain localisation, rapid lithospheric weakening in the equatorial Atlantic domain, resulting in both progressively increasing extensional velocities as well as a significant rotation of the extension direction to NE-SW. From Base Aptian onwards diachronous lithospheric breakup occurred along the central South Atlantic rift, first in the Sergipe-Alagoas/Rio Muni margin segment in the northernmost South Atlantic. Final breakup between South America and Africa occurred in the conjugate Santos-Benguela margin segment at around 113 Ma and in the Equatorial Atlantic domain between the Ghanaian Ridge and the Piau�-Cear� margin at 103 Ma. We conclude that such a multi-velocity, multi-directional rift history exerts primary control on the evolution of this conjugate passive margins systems and can explain the first order tectonic structures along the South Atlantic and possibly other passive margins.
Heine, C.; Zoethout, J.; M�ller, R. D.
Seismicity, deformation and seismic hazard in the western rift of Corinth: New insights from the Corinth Rift Laboratory (CRL)
This paper presents the main recent results obtained by the seismological and geophysical monitoring arrays in operation in the rift of Corinth, Greece. The Corinth Rift Laboratory (CRL) is set up near the western end of the rift, where instrumental seismicity and strain rate is highest. The seismicity is clustered between 5 and 10�km, defining an active layer, gently dipping
P. Bernard; H. Lyon-Caen; P. Briole; A. Deschamps; F. Boudin; K. Makropoulos; P. Papadimitriou; F. Lemeille; G. Patau; H. Billiris; D. Paradissis; K. Papazissi; H. Castar�de; O. Charade; A. Nercessian; A. Avallone; F. Pacchiani; J. Zahradnik; S. Sacks; A. Linde
Finite element modelling of crustal deformation in the Baikal rift zone: new insights into the active�passive rifting debate
The origin of the forces responsible for crustal extension in the Baikal rift zone, Siberia, is the object of a debate between `passive rifting' models, where crustal extension is primarily caused by horizontal forces related to the kinematics of Asia (India�Eurasia collision), and `active rifting' models, where crustal extension is primarily caused by a diapiric mantle upwelling. In this work,
O. Lesne; E Calais; J Deverch�re
Implications of new gravity data for Baikal Rift zone structure
NASA Technical Reports Server (NTRS)
Newly available, 2D Bouguer gravity anomaly data from the Baikal Rift zone, Siberia, indicate that this discrete, intracontinental rift system is regionally compensated by an elastic plate about 50 km thick. However, spectral and spatial domain analyses and isostatic anomaly calculations show that simple elastic plate theory does not offer an adequate explanation for compensation in the rift zone, probably because of significant lateral variations in plate strength and the presence of subsurface loads. Our results and other geophysical observations support the interpretation that the Baikal Rift zone is colder than either the East African or Rio Grande rift.
Ruppel, C.; Kogan, M. G.; Mcnutt, M. K.
Stratigraphy and rifting history of the Mesozoic-Cenozoic Anza rift, Kenya
Lithological and compositional relationships, thicknesses, and palynological data from drilling cuttings from five wells in the Anza rift, Kenya, indicate active rifting during the Late Cretaceous and Eocene-Oligocene. The earlier rifting possibly started in the Santonian-Coniacian, primarily occurred in the Campanian, and probably extended into the Maastrichtian. Anza rift sedimentation was in lacustrine, lacustrine-deltaic, fluvial, and flood-basin environments. Inferred synrift intervals in wells are shalier, thicker, more compositionally immature, and more poorly sorted than Lower Cretaceous ( )-lower Upper Cretaceous and upper Oligocene( )-Miocene interrift deposits. Synrift sandstone is mostly feldspathic or arkosic wacke. Sandstone deposited in the Anza basin during nonrift periods is mostly quartz arenite, and is coarser and has a high proportion of probable fluvial deposits relative to other facies. Volcanic debris is absent in sedimentary strata older than Pliocene-Holocene, although small Cretaceous intrusions are present in the basin. Cretaceous sandstone is cemented in places by laumontite, possibly recording Campanian extension. Early Cretaceous history of the Anza basin is poorly known because of the limited strata sampled; Jurassic units were not reached. Cretaceous rifting in the Anza basin was synchronous with rifting in Sudan and with the breakup and separation of South America and Africa; these events likely were related. Eocene-Oligocene extension in the Anza basin reflects different stresses. The transition from active rifting to passive subsidence in the Anza basin at the end of the Neogene, in turn, records a reconfigured response of east African plates to stresses and is correlated with formation of the East Africa rift.
Winn, R.D. Jr.; Steinmetz, J.C. (Marathon Oil Co., Littleton, CO (United States)); Kerekgyarto, W.L. (Marathon Oil Co., Houston, TX (United States))
Venus: Geology of beta Regio Rift System.
National Technical Information Service (NTIS)
Beta Regio is characterized by the existence of rift structures. We compiled new geologic maps of Beta Regio according to Magellan data. There are many large uplifted tesserae on beta upland. These tesserae are partly buried by younger volcanic cover. We ...
A. M. Nikishin V. K. Borozdin N. N. Bobina
Longitudinal evolution of Suez rift structure, Egypt
A three-dimensional study of the structure of the Suez Rift has been carried out using field and subsurface data in an attempt to determine the role of transverse faults and the longitudinal evolution of the rift. As in most intracontinental rifts, the structure of the Gulf of Suez area is governed by normal faults and tilted blocks, whose crests constitute the main target of exploratory wells. The fault pattern consists of two major sets of trends: (1) longitudinal faults parallel with the rift axis and created in an extensional regime, trending east-northeast-west-southwest, and (2) transverse faults with north-south to north-northeast-south-southwest dominant trend. The transverse faults are inherited passive discontinuities, whereas most of the longitudinal faults were created during the Neogene in a purely extensional regime. Both sets were simultaneously active, producing a zigzag pattern and rhombic-shaped blocks. The transverse faults can show horizontal strike-slip components and act as relays between major normal faults.
Colletta, B.; Le Quellec, P.; Letouzey, J.; Moretti, I.
Reemergence of Rift Valley Fever, Mauritania, 2010
A Rift Valley fever (RVF) outbreak in humans and animals occurred in Mauritania in 2010. Thirty cases of RVF in humans and 3 deaths were identified. RVFV isolates were recovered from humans, camels, sheep, goats, and Culex antennatus mosquitoes. Phylogenetic analysis of isolates indicated a virus origin from western Africa.
Faye, Ousmane; Ba, Hampathe; Ba, Yamar; Freire, Caio C.M.; Faye, Oumar; Ndiaye, Oumar; Elgady, Isselmou O.; Zanotto, Paolo M.A.; Diallo, Mawlouth
Reemergence of Rift Valley fever, Mauritania, 2010.
A Rift Valley fever (RVF) outbreak in humans and animals occurred in Mauritania in 2010. Thirty cases of RVF in humans and 3 deaths were identified. RVFV isolates were recovered from humans, camels, sheep, goats, and Culex antennatus mosquitoes. Phylogenetic analysis of isolates indicated a virus origin from western Africa. PMID:24447381
Faye, Ousmane; Ba, Hampath�; Ba, Yamar; Freire, Caio C M; Faye, Oumar; Ndiaye, Oumar; Elgady, Isselmou O; Zanotto, Paolo M A; Diallo, Mawlouth; Sall, Amadou A
Unexpected Rift Valley Fever Outbreak, Northern Mauritania
During September�October 2010, an unprecedented outbreak of Rift Valley fever was reported in the northern Sahelian region of Mauritania after exceptionally heavy rainfall. Camels probably played a central role in the local amplification of the virus. We describe the main clinical signs (hemorrhagic fever, icterus, and nervous symptoms) observed during the outbreak.
El Mamy, Ahmed B. Ould; Baba, Mohamed Ould; Barry, Yahya; Isselmou, Katia; Dia, Mamadou L.; Hampate, Ba; Diallo, Mamadou Y.; El Kory, Mohamed Ould Brahim; Diop, Mariam; Lo, Modou Moustapha; Thiongane, Yaya; Bengoumi, Mohammed; Puech, Lilian; Plee, Ludovic; Claes, Filip; Doumbia, Baba
The 1973 Ethiopian-Rift geodimeter survey
NASA Technical Reports Server (NTRS)
Remeasurement of the Adama, Lake Langana, and Arba Minch (Lake Margherita) geodimeter networks in 1973 has enabled Mohr's interpretation concerning possible surface ground deformation in the Ethiopian rift to be considerably developed. Extension appears to have occurred across the Mojjo-Adama horst at a rate of about 1 cm yr/1. The opposing rims of the Adama graben have not moved significantly relative to one another (between 1969 and 1973), but stations on the sliced graben floor show possible movement with a large rift-trend component. In the Wolenchiti quadrilateral, significant movement of station RABBIT is confirmed, but the radical change of vector (that of 1970-1971 to that of 1971-1973) casts doubt on a tectonic cause and seems to indicate that stations on steep hillslopes are liable to be unstable. South of the quadrilateral and east of the Adama graben, alternating rift-trend zones of extension and shortening appear to coexist. In the Lake Langana network, significant movements of the order of 0.5 cm yr/1 are directed perpendicular to the rift floor faulting.
Mohr, P. A.
Submarine Thermal Springs on the Galapagos Rift
The submarine hydrothermal activity on and near the Galapagos Rift has been explored with the aid of the deep submersible Alvin. Analyses of water samples from hydrothermal vents reveal that hydrothermal activity provides significant or dominant sources and sinks for several components of seawater; studies of conductive and convective heat transfer suggest that two-thirds of the heat lost from new
John B. Corliss; Jack Dymond; Louis I. Gordon; John M. Edmond; Richard P. von Herzen; Robert D. Ballard; Kenneth Green; David Williams; Arnold Bainbridge; Kathy Crane; Tjeerd H. van Andel
Magma-assisted rifting in Ethiopia.
The rifting of continents and evolution of ocean basins is a fundamental component of plate tectonics, yet the process of continental break-up remains controversial. Plate driving forces have been estimated to be as much as an order of magnitude smaller than those required to rupture thick continental lithosphere. However, Buck has proposed that lithospheric heating by mantle upwelling and related magma production could promote lithospheric rupture at much lower stresses. Such models of mechanical versus magma-assisted extension can be tested, because they predict different temporal and spatial patterns of crustal and upper-mantle structure. Changes in plate deformation produce strain-enhanced crystal alignment and increased melt production within the upper mantle, both of which can cause seismic anisotropy. The Northern Ethiopian Rift is an ideal place to test break-up models because it formed in cratonic lithosphere with minor far-field plate stresses. Here we present evidence of seismic anisotropy in the upper mantle of this rift zone using observations of shear-wave splitting. Our observations, together with recent geological data, indicate a strong component of melt-induced anisotropy with only minor crustal stretching, supporting the magma-assisted rifting model in this area of initially cold, thick continental lithosphere. PMID:15650736
Kendall, J-M; Stuart, G W; Ebinger, C J; Bastow, I D; Keir, D
Airborne Survival of Rift Valley Fever Virus.
National Technical Information Service (NTIS)
The aerosol stability characteristics of an Egyptian isolate of Rift/Valley fever virus (ZH-501 strain) were determined in a static aerosol chamber. Aerosolized particles had a mass median diameter of 4.0 micrometers. At 30, 55, 80% relative humidity (RH)...
J. L. Brown J. W. Dominik E. W. Larson
Hydrothermal mineralization along submarine rift zones, Hawaii
Describes mineralization of midplate submarine rift zones and hydrothermal manganese oxide mineralization of midplate volcanic edifices. Hydrothermal Mn oxides were recovered from submarine extensions of two Hawaiian rift zones, along Haleakala and Puna Ridges. These Mn oxides form two types of deposits, metallic stratiform layers in volcaniclastic rocks and cement for clastic rocks; both deposit types are composed of todorokite and birnessite. Unlike most other hydrothermal Mn oxide deposits, those from Hawaiian rift zones are enriched in the trace metals Zn, Co, Ba, Mo, Sr, V, and especially Ni. Metals are derived from three sources: mafic and ultramafic rocks leached by circulating hydrothermal fluids, clastic material (in Mn-cemented sandstone), and seawater that mixed with the hydrothermal fluids. Precipitation of Mn oxide below the seafloor is indicated by its occurrence as cement, growth textures that show mineralizing fluids were introduced from below, and pervasive replacement of original matrix of clastic rocks.Hydrothermal Mn oxides were recovered from submarine extensions of two Hawaiian rift zones, along Haleakala and Puna Ridges. These Mn oxides form two types of deposits, metallic stratiform layers in volcaniclastic rocks and cement for clastic rocks. Both deposit types are composed of todorokite and birnessite. This article describes in detail the specific characteristics of these Mn oxides.
Hein, J. R.; Gibbs, A. E.; Clague, D. A.; Torresan, M.
Desarrollo de un framework b�sico para la construcci�n de aplicaciones basadas en Internet utilizando el protocolo XMPP
El desarrollo de aplicaciones que se ejecutan a trav�s de Internet ha crecido exponencialmente durante los �ltimos a�os. Sin embargo, la construcci�n de aplicaciones Web que se ejecutan sobre el protocolo HTTP y se visualizan en los navegadores Web, generan problemas de usabilidad, ya que estos interpretan de manera diferente algunas etiquetas, y la propia naturaleza de la actualizaci�n de
L I Herman; Geovany Ayala Z��iga; MSIA Juan Francisco; Algara Norzagaray; �ngel Flores; Juan de Dios Batiz
Development regimes of rifted basins and criteria of their petroleum potential
The majority of great sedimentary basins were formed primarily by stretching and usually initiated by rifting. The evolution and development of intercontinental and passive margin rifted basins are discussed. Each basin type described (in the paper) is associated with either single or branched rift zones. The basin types are (1) unburied rift, i.e., recent rifts, or ancient rifts with post-rift stage without significant subsidence such as East Africa rifts, and Reconcavo basin in Brazil; and (2) buried rift zones where the post-rift stage is characterized by active subsidence and sediment accumulation. The basins often form by repeated rifting, and a rift zone of a different age may be completely or partially superimposed or separated from other rifting events.
Larchenkov, E. (Odessa State Univ. (Ukraine))
Heat flow in the Keweenawan rift system
NASA Astrophysics Data System (ADS)
The emplacement of large volumes of mafic volcanic rocks during the Keweenawan rifting has modified the average crustal composition and affects the present steady state heat flux in the region. We have combined new heat flux measurements in the Superior Province of the Canadian Shield and previously published data to characterize the heat flux field around the Keweenawan rift system. For the Nipigon embayment, North of lake Superior in Ontario, mafic intrusions associated with the Keweenawan rifting have resulted in an increase in the volume of mafic rocks in the crust and caused a very small <3mW m-2 decrease in the mean heat flux. There is a very marked decrease in the heat flux (? Q ? 20mW m-2) beneath the western half of Lake Superior and to the west. The very low values of the surface heat flux (? 22mW m-2 correlate with the maximum Bouguer gravity anomaly. The heat flux at the base of the crust in the Canadian Shield has been determined from surface heat flux, heat production, and crustal stucture to be ? 15 mW m-2. In the Keweenawan rift, the surface heat flux is only a few mW m-2 higher than the mantle heat flux, which implies that the contribution of the entire crustal column to the surface heat flux is small and that the crust is exclusively made up of depleted mafic volcanic rocks. In the eastern part and northeast of Lake Superior, there is a marked increase in heat flux that correlates with a lower Bouguer anomaly. Local high heat flux anomalies due to intrusions by felsic rocks are superposed with a long wavelength trend of higher heat flow suggesting a more felsic crustal composition in the eastern part of the Keweenawan rift. Simple models suggest that such a thick dense volcanic pile as accumulated in the Keweenawan rift is almost invariably unstable and that very particular conditions were required for it to stabilize in the crust.
Perry, C.; Mareschal, J.; Jaupart, C. P.
Controls on (anomalous) topography in rifted margin settings
NASA Astrophysics Data System (ADS)
Contrasting end members of volcanic and non-volcanic passive margin formation show a large variability in basin shape and structure, subsidence history, and associated topographic evolution of the onshore rifted margins. The large range of structural style and associated topography of these systems imply a strong variability in the underlying thermo-mechanical conditions at the time of rifting. Rift - passive margin styles ranging from narrow to ultra wide are explained using forward numerical models with varying rheological structure, with strong crust lithosphere leading to narrow rift formation associated with highly elevated rift shoulders and conversely weak crust lithosphere resulting in highly stretched wide rifted conjugate margins and little flank morphology. In some cases rifted margins appear to indicate the formation of anomalous post rift topography. A number of mechanisms including small-scale convective removal of the lower lithosphere, lithosphere counter-flow, and dynamic topography, have been invoked to explain the anomalous topography. Forward numerical models are used to predict the magnitude and characteristic topography associated with each of these mechanisms and to evaluate their potential for explaining these apparent anomalous characteristics of rifts and rifted margins.
Huismans, Ritske S.
Structure of continental rifts: Role of older features and magmatism
Recent geological and geophysical studies in several continental rifts have begun to shed light on the details of the processes which govern the structural evolution of these important exploration targets. In Kenya and Tanzania, the classic East African rift has been the object of several investigations which reveal that its location follows the boundary (suture ) between the Tanzanian craton (Archean) and Mozambiquan belt (Proterozoic), The Baikal rift also follows a similar boundary, and the Mid-continent rift of North America appears to do the same. Rifts themselves often act as zones of weakness which are reactivated by younger tectonic regimes. The classic North American example of this effect is the Eocambrian Southern Oklahoma aulacogen which was deformed to create the Anadarko basin and Wichita uplift in the late Paleozoic. The Central basin platform has a similar history although the original rift formed at [approximately]1,100Ma. Integration of geophysical data with petrologic and geochemical data from several rift zones has also provided a new picture of the nature and extent of magmatic modification of the crust. An interesting contradiction is that Phanerozoic rifts, except the Afar region, show little evidence for major magmatic modification of the crust whereas, at least in North America, many Precambrian rifts are associated with very large mafic bodies in the crust. The Kenya rift displays evidence for modification of the lower crust in a two-phase magmatic history, but upper crustal magmatic features are limited to local intrusions associated with volcanoes. In this rift, complex basement structure plays a much more important role than previously realized, and the geophysical signatures of basement structure and magmatism are easy to confuse. If this is also the case in other rifts, additional rift basins remain to be discovered.
Keller, G.R. (Univ. of Texas, El Paso, TX (United States))
Geochemical evidence of lithospheric thinning in the southern Main Ethiopian Rift
Lithospheric thinning is a fundamental process associated with the transition from continental to oceanic regimes during continental rifting. Precisely how and when this lithospheric thinning proceeds are first order controls on rift basin evolution. The Main Ethiopian Rift, part of the ?2000km long East African Rift System, is the archetypical modern example of continental rifting, and a key location in
Tyrone O. Rooney
How do volcanic rift zones relate to flank instability? Evidence from collapsing rifts at Etna
NASA Astrophysics Data System (ADS)
Volcanic rift zones, characterized by repeated dike emplacements, are expected to delimit the upper portion of unstable flanks at basaltic edifices. However, the relationship between flank instability and rift zone behavior, activity, and location on longer, inter-diking timescales is poorly understood. To characterize the relationship between rift kinematics, dike emplacement and flank instability at Etna, we used long-term (1992-2010) InSAR ground deformation time series, giving unprecedented coverage of the volcano's summit. As snow coverage prevents coherence of the backscattered radar signal, long-term InSAR time series on the summit of seasonally snow-capped volcanoes, such as Mount Etna, are affected by decorrelation, which hinders detection of any summit deformation. To increase the summit coverage, we removed from our Etna dataset all the SAR images possibly affected by snow-related decorrelation phenomena. We then jointly processed ERS data between 1992 and 2010 and ENVISAT data between 2003 and 2010, acquired from both ascending and descending orbits, and computed 204 ascending and 194 descending interferograms. These were inverted by applying the Small BAseline Subset (SBAS) technique (e.g., Pepe et al., 2005) to produce combined ERS-ENVISAT velocity maps and deformation time series. Ascending and descending maps and time series were used to separate vertical and East-West components of on-going displacements. This approach greatly improved the spatial coverage of elevated areas. A spatial comparison with a similar dataset used by Solaro et al. (2010), but limited to 1994-2008, shows an increase of the summit coverage of about 14 km2 in our dataset. The results highlight a general eastward shift of the volcano summit, including the northeast and south rifts. This steady-state eastward movement (1-2 cm/yr) is interrupted or even reversed during transient dike injections. Detailed analysis of the northeast rift shows that only during phases of dike injection, as in 2002, the rift transiently becomes the upper border of the unstable flank. The flank's steady-state eastward movement is inferred to result from the interplay between magmatic activity, asymmetric topographic unbuttressing, the gravitational load of the intrusive complex, and eastward dipping detachments. This study documents the first evidence of steady-state volcano rift instability interrupted by transient dike injection at basaltic edifices. More broadly, this study shows that volcanic rift zones may not be the upper boundaries of unstable volcano flanks, as other non-magmatic processes may control the evolution of volcanic rift zones.
Pepe, Susi; Ruch, Joel; Casu, Francesco; Acocella, Valerio; Neri, Marco; Solaro, Giuseppe; Sansosti, Eugenio
The Main Ethiopian Rift: a Narrow Rift in a Hot Craton?
NASA Astrophysics Data System (ADS)
The Main Ethiopian Rift (MER) is a classic example of a narrow rift, but a synthesis of our results from the EAGLE (Ethiopia-Afar Geoscientific Lithospheric Experiment Phase I broadband experiment) and from the EBSE experiment (Ethiopia Broadband Seismic Experiment) suggests the MER formed in thin, hot, weak continental lithosphere, in strong contrast with predictions of the Buck model of modes of continental lithospheric extension. Our joint inversion of receiver functions and Rayleigh-wave group velocities yields shear-wave velocities of the lowermost crust and uppermost mantle across the MER and the Ethiopian Plateau that are significantly lower than the equivalent velocities in the Eastern and Western branches of the East African Rift System. The very low shear-wave velocities, high electrical conductivity in the lower-crust, and high shear-wave splitting delay times beneath a very broad region of the MER and the Ethiopian Plateau indicate that the lower-crust is hot and likely contains partial melt. Our S-receiver function data demonstrate shallowing of the lithosphere-asthenosphere boundary from 90 km beneath the northwestern Ethiopian Plateau to 60 km beneath the MER. Although we lack good spatial resolution on the lithosphere-asthenosphere boundary, the region of thinned lithosphere may be intermediate in width between the narrow surface rift (< 100 km) and the broader zone of strain in the lower crust (~ 300 km). The MER developed as a narrow rift at the surface, localized along the Neoproterozoic suture that joined East and West Gondwana. However, a far broader of lower crust and uppermost mantle remains thermally weakened since the Oligocene formation of the flood basalts by the Afar plume head. If the lithosphere- asthenosphere boundary is indeed a strain marker then lithospheric mantle deformation is localized beneath the surface rift. The development of both the Eastern/Western branches of the East African Rift System to the south and of the MER in the north as narrow rifts, despite vastly different lithospheric strength profiles, indicates that inherited structure, rather than rheological stratification, is the primary control on the mode of extension in these continental rifts.
Gashawbeza, E.; Keranen, K.; Klemperer, S.; Lawrence, J.
Evidence for a Nascent Rift in South Sudan: Westward Extension of the East African Rift System?
NASA Astrophysics Data System (ADS)
Joint inversion of seismic and gravity data of eastern Africa reveals a low seismic wave velocity arm stretching from the southern Main Ethiopian rift westward in an east-west direction that has not been noticed in earlier work. The zone of low velocities is located in the upper mantle and is not overlain by a known structural rift expression. We analyzed the local pattern of seismicity and the stresses in the African plate to interpret this low velocity arm. The zone of low velocities is located within the Central African Fold Belt, which dissects the northern and southern portions of the African continent. It is seismically active with small to intermediate sized earthquakes occurring in the crust. Seven earthquake solutions indicate (oblique) normal faulting and low-angle normal faulting with a NS to NNW-SSE opening direction, as well as strike-slip faulting. This pattern of deformation is typically associated with rifting. The present day stress field in northeastern Africa reveals a tensional state of stress at the location of the low velocity arm with an opening direction that corresponds to the earthquake data. We propose that the South Sudan low velocity zone and seismic center are part of an undeveloped, nascent rift arm. The arm stretches from the East African Rift system westward.
Maceira, M.; Van Wijk, J. W.; Coblentz, D. D.; Modrak, R. T.
The Pathogenesis of Rift Valley Fever
Rift Valley fever (RVF) is an emerging zoonotic disease distributed in sub-Saharan African countries and the Arabian Peninsula. The disease is caused by the Rift Valley fever virus (RVFV) of the family Bunyaviridae and the genus Phlebovirus. The virus is transmitted by mosquitoes, and virus replication in domestic ruminant results in high rates of mortality and abortion. RVFV infection in humans usually causes a self-limiting, acute and febrile illness; however, a small number of cases progress to neurological disorders, partial or complete blindness, hemorrhagic fever, or thrombosis. This review describes the pathology of RVF in human patients and several animal models, and summarizes the role of viral virulence factors and host factors that affect RVFV pathogenesis.
Ikegami, Tetsuro; Makino, Shinji
Kinematics of the South Atlantic rift
NASA Astrophysics Data System (ADS)
The South Atlantic rift basin evolved as a branch of a large Jurassic-Cretaceous intraplate rift zone between the African and South American plates during the final break-up of western Gondwana. While the relative motions between South America and Africa for post-break-up times are well resolved, many issues pertaining to the fit reconstruction and particularly the relation between kinematics and lithosphere dynamics during pre-break-up remain unclear in currently published plate models. We have compiled and assimilated data from these intraplated rifts and constructed a revised plate kinematic model for the pre-break-up evolution of the South Atlantic. Based on structural restoration of the conjugate South Atlantic margins and intracontinental rift basins in Africa and South America, we achieve a tight-fit reconstruction which eliminates the need for previously inferred large intracontinental shear zones, in particular in Patagonian South America. By quantitatively accounting for crustal deformation in the Central and West African Rift Zones, we have been able to indirectly construct the kinematic history of the pre-break-up evolution of the conjugate west African-Brazilian margins. Our model suggests a causal link between changes in extension direction and velocity during continental extension and the generation of marginal structures such as the enigmatic pre-salt sag basin and the S�o Paulo High. We model an initial E-W-directed extension between South America and Africa (fixed in present-day position) at very low extensional velocities from 140 Ma until late Hauterivian times (?126 Ma) when rift activity along in the equatorial Atlantic domain started to increase significantly. During this initial ?14 Myr-long stretching episode the pre-salt basin width on the conjugate Brazilian and west African margins is generated. An intermediate stage between ?126 Ma and base Aptian is characterised by strain localisation, rapid lithospheric weakening in the equatorial Atlantic domain, resulting in both progressively increasing extensional velocities as well as a significant rotation of the extension direction to NE-SW. From base Aptian onwards diachronous lithospheric break-up occurred along the central South Atlantic rift, first in the Sergipe-Alagoas/Rio Muni margin segment in the northernmost South Atlantic. Final break-up between South America and Africa occurred in the conjugate Santos-Benguela margin segment at around 113 Ma and in the equatorial Atlantic domain between the Ghanaian Ridge and the Piau�-Cear� margin at 103 Ma. We conclude that such a multi-velocity, multi-directional rift history exerts primary control on the evolution of these conjugate passive-margin systems and can explain the first-order tectonic structures along the South Atlantic and possibly other passive margins.
Heine, C.; Zoethout, J.; M�ller, R. D.
Experimental Rift Valley fever in rhesus macaques
Summary Rift Valley fever (RVF) is a major cause of human morbidity and mortality in endemic areas of sub-Saharan Africa and has the potential to cause epidemic disease in receptive areas world-wide. In this study, a RVF viral isolate from the 1977 Egyptian epidemic (ZH-501) inoculated intravenously into rhesus macaques caused a benign viremic infection in most, but resulted in
C. J. Peters; D. Jones; R. Trotter; J. Donaldson; J. White; E. Stephen; T. W. Slone
Volcano Deformation in the Main Ethiopian Rift
NASA Astrophysics Data System (ADS)
Magmatism strongly influences continental rift development yet the mechanism, distribution and timescales on which melt is emplaced and erupted through the shallow crust are not well characterized. The Main Ethiopian Rift (MER) has experienced significant volcanism and the mantle beneath is characterized by high temperatures and partial melt. Despite its magma-rich geological record, only one eruption has been historically recorded and no dedicated monitoring networks exist. Consequently, the present-day magmatic processes in the region remain poorly documented, and the associated hazard neglected. We use satellite-based InSAR observations to demonstrate that significant deformation has occurring at 4 volcanic edifices in the MER (Alutu, Corbetti, Bora and Haledebi) from 1993-2010. This raises the number of volcanoes known to be deforming in East Africa beyond 12, comparable to many subduction arcs despite the smaller number of recorded eruptions. The largest displacements are at Alutu volcano, the site of a geothermal plant, which showed two pulses of rapid inflation (10-15 cm) in 2004 and 2008 separated by gradual subsidence. Our observations indicate a shallow (<10 km), frequently replenished zone of magma storage associated with volcanic edifices and add to the growing body of observations that indicate shallow magmatic processes operating on a decadal timescale are ubiquitous throughout the East African Rift. In the absence of detailed historical records of volcanic activity, satellite-based observations of monitoring parameters, such as deformation, could play an important role in assessing volcanic hazard.
Biggs, J.; Bastow, I.; Keir, D.; Hutchison, W.
Crustal deformation in the Baikal rift from GPS measurements
Three years and four campaigns of Global Positioning System (GPS) measurements (1994-1997) in the Baikal rift zone, largest active continental rift system in Eurasia, show crustal extension at a rate of 4.5+\\/-1.2mm\\/yr in a WNW-ESE direction. A comparison with moment release of large historical earthquakes suggests that elastic strain is currently accumulating in the Baikal rift zone along active faults
Eric Calais; Olivia Lesne; Jacques D�verch�re; Vladimir San'kov; Andrei Lukhnev; Andrei Miroshnichenko; Vladimir Buddo; Kirill Levi; Vjacheslav Zalutzky; Yuri Bashkuev
The South China sea margins: Implications for rifting contrasts
Implications regarding spatially complex continental rifting, crustal extension, and the subsequent evolution to seafloor spreading are re-examined for the northern and southern-rifted margins of the South China Sea. Previous seismic studies have shown dramatic differences in the present-day crustal thicknesses as the manifestations of the strain experienced during the rifting of the margin of south China. Although the total crustal
Dennis E. Hayes; Susan S. Nissen
Tertiary arc rifting in northern Luzon, Philippines
NASA Astrophysics Data System (ADS)
The North Luzon terrane (NLT), comprising the section of Luzon north of the Philippine Fault, is one of the largest arc terranes in the Philippine Archipelago. Numerous features suggest that the NLT is a late Oligocene to early Miocene analogue for the processes in the modern intra-arc rift zone at the northern end of the Mariana Trough. First, the NLT has bifurcating magmatic arcs sharing similar magmatic histories. These include the Northern Sierra Madre (NSM) and Cordillera Central (CC) magmatic arcs, which are separated by the Cagayan basin but which are linked in the Caraballo Range to the south. The rock record indicates that the NSM, CC, and Caraballo Ranges were active arcs in late Eocene to late Oligocene time. Second, seismic reflection and well data indicate that the Cagayan basin formed by extensional faulting in late Oligocene to early Miocene time. Third, alkalic arc magmatism, recognized to be a precursor of intra-arc rifting in modern settings, occurred at the juncture of the NSM and CC arcs in late Oligocene to early Miocene time. Fourth, oceanic crust, represented by the Itogon ophiolite, formed at the southwestern end of the Cagayan basin in late Oligocene to early Miocene time. Major and trace element chemistry show that the Itogon sheeted dikes have tholeiitic arc and backarc basin basalt affinities. The rock record and geophysical offshore data suggest that the NLT was developing in an island arc system above the subducting West Philippine plate in late Eocene time. Rifting occurred in the island arc from late Oligocene to early Miocene time but did not mature into backarc spreading, most likely because of the collision of the Benham Rise, a basaltic rise in the West Philippine basin, with the NLT. The arc rifting in the NLT may be another manifestation of the extensional tectonism that affected most of Southeast Asia in late Oligocene to early Miocene time, during which the South China and Southeast Sulu basins formed. Subsequent to arc rifting, the history of the NLT has been linked to the subduction of the South China plate along the Manila Trench. The structural history of the Cagayan basin and magmatic history of the southern CC suggest that the subduction in the Manila Trench at the latitude of the NLT began about 15 Ma.
Florendo, Federico F.
The MOZART Project - MOZAmbique Rift Tomography
NASA Astrophysics Data System (ADS)
Project MOZART (MOZAmbique Rift Tomography) is an ongoing joint effort of Portuguese, Mozambican and British research groups to investigate the geological structure and current tectonic activity of the southernmost tip of the East African Rift System (EARS) through the deployment of a network of 30 broad band seismic stations in Central and Southern Mozambique. In contrast with other stretches of the EARS to the North and with the Kapvaal craton to the West and South, the lithosphere of Mozambique was not previously studied with a dense seismographic deployment on account of past political instability, and many questions remain unanswered with respect to the location and characteristics of the EARS to the south of Tanzania. In recent years, space geodesy revealed the existence of three microplates in and off Mozambique - Victoria, Rovuma, Lwandle - whose borders provide a connection of the EARS to the South West Indian Ridge as required by plate tectonics. However, the picture is still coarse concerning the location of the rift structures. The 2006 M7 Machaze earthquake in Central Mozambique highlighted the current tectonic activity of the region and added a further clue to the location of the continental rift, prompting the MOZART deployment. Besides helping unravel the current tectonics, the project is expected to shed light on the poorly known Mesoproterozoic structure described by Arthur Holmes in 1951 as the Mozambique Belt, and on the mechanisms of transition from stable craton to rifted continental crust, through the development of a tomographic model for the lithosphere. The MOZART network is distributed South of the Zambezi river at average inter-station spaces of the order of 100 km and includes four stations across the border in South Africa. Data exchange was agreed with AfricaArray. The deployment proceeded in two phases in March 2011, and November and December 2011. Decommissioning is foreseen for August 2013. We report preliminary results for this previously unexplored region concerning the seismicity and ambient noise (see also Domingues et al, this conference), receiver function analysis, surface wave dispersion and SEM forward modelling. These preliminary results will pave the way for a tomographic model of the lithosphere, to be developed in the next stage of the project.
Fonseca, J. F.; Chamussa, J. R.; Domingues, A.; Helffrich, G. R.; Fishwick, S.; Ferreira, A. M.; Custodio, S.; Brisbourne, A. M.; Grobbelaar, M.
Physical characteristics and evolutionary trends of continental rifts
NASA Technical Reports Server (NTRS)
Rifts may be defined as zones beneath which the entire lithosphere has ruptured in extension. They are widespread and occur in a variety of tectonic settings, and range up to 2,600 m.y. in age. The object of this review is to highlight characteristic features of modern and ancient rifts, to emphasize differences and similarities in order to help characterize evolutionary trends, to identify physical conditions favorable for initiation as well as termination of rifting, and to provide constraints for future modeling studies of rifting. Rifts are characterized on the basis of their structural, geomorphic, magmatic and geophysical features and the diverse character of these features and their evolutionary trends through time are discussed. Mechanisms of rifting are critically examined in terms of the physical characteristics and evolutionary trends of rifts, and it is concluded that while simple models can give valuable insight into specific processes of rifting, individual rifts can rarely, if ever, be characterized by well defined trends predicted by these models. More data are required to clearly define evolutionary trends, and the models require development to incorporate the effects of lithospheric heterogeneities and complex geologic histories.
Ramberg, I. B.; Morgan, P.
Structure of Mid-Continent rift beneath Lake Superior
The 1.1 Ga Mid-Continent rift system extends from Kansas through the Lake Superior region and into southern Michigan. The rift is filled with thick sequences of basalt and clastic sedimentary rocks, which are now mostly buried beneath Paleozoic rocks. Rocks of the rift system are exposed only in the Lake Superior region and comprise the Keweenawan supergroup. Seismic reflection surveys by GLIMPCE in 1986 imaged much of the deep structure of the rift beneath the lake in detail. Reflection profiles reveal a deep asymmetrical central graben whose existence and magnitude was not previously documented. Volcanic and sedimentary rocks, in places greater than 30 km thick, fill the central graben, which is bounded by normal growth faults. Thinner volcanic and sedimentary units lie on broad flanks of the rift outside of the graben. Near the rift axis, the pre-rift crust is thinned to about one-fourth of its original thickness, apparently by low-angle extensional faulting and ductile stretching or distributed shear. The sense of asymmetry of the central graben changes along the trend of the rift, documenting the segmented nature of the structure and suggesting the existence of accommodation zones between the segments. The location of the accommodation zones is inferred from abrupt disruptions in the Bouguer gravity anomaly associated with the rift. Late uplift of the central graben transposed graben-bounding normal faults into high-angle reverse faults with throws of 5 km or more.
Cannon, W.F.
Lake Baikal - A Touchstone for Global Change and Rift Studies
NSDL National Science Digital Library
This is a United States Geological Survey (USGS) fact sheet about the Lake Baikal rift system. This site provides a good general overview of this rift system, illustrating its importance to the overall study of plate tectonics. The Lake Baikal rift system is a modern analogue for formation of ancient Atlantic-type continental margins. It tells us the first chapter in the story of how continents separate and ultimately develop into ocean basins like the Atlantic Ocean. Continental rifting is an important component of plate tectonics theory.
How Is Lower Crust Modified As A Neo-Rift Becomes A Paleo-Rift and Part Of The Craton?
NASA Astrophysics Data System (ADS)
The Southern Oklahoma Aulacogen (SOA), at the southern end of Laurentia (present coordinates), if behaving as neo-rifts, such as the Rio Grande Rift, presumably possessed a rift structure in the Cambrian with a continental thickness of about 28km. Seismic data, though sparse, suggest a present thickness of the SOA is about 45km, indistinguishable from adjacent rifted Proterozoic crust. By what process do we add 15km to the original SOA crust: underplating, eclogite-gabbro transformation, or deformation? This question has bearing on how we understand and interpret all paleo-rifts now a part of continental cores. Geology of the southern Midcontinent of North America does not show evidence of significant thermal events in the Phanerozoic. This effectively rules out underplating and phase transformation as a cause of change in M-discontinuity depth. Present SOA outcrops are in the Wichita Mountains of southwestern Oklahoma, part of the easternmost Ancestral Rockies. These outcrops are in the Wichita-Amarillo crustal block uplifted about 7km in the Pennsylvanian. The Anadarko Basin to the north went down about 7km. Large Pennsylvanian thrust faults in the upper brittle crust are documented. Thus it appears that compressive deformation may be able to account for the change in crustal thickness from neo-rift type to paleo-rift and craton type. However, the accommodation made in the lower crust may be more dramatic than deformation in the upper crust because shortening, and thickening of the order of 2X, is probably required. Comparisons with other paleo-rifts in North America, such as the Middle Proterozoic Midcontinent Rift and the NeoProterozoic Reelfoot Rift, show that their crustal thicknesses now also match their previously rifted margins. Can the same sequence, as seems to be the case with the SOA, apply to other paleo-rifts?
Gilbert, M. C.
Numerical modelling of quaternary deformation and post-rifting displacement in the Asal�Ghoubbet rift (Djibouti, Africa)
Over the last three decades a host of information on rifting process relating to the geological and thermal structure, long-time scale deformation (Quaternary and Holocene) and rifting cycle displacement across the Asal�Ghoubbet rift has been made available. These data are interpreted with a two-dimensional thermo-mechanical model that incorporates rheological layering of the lithosphere, dyke inflation and faulting. Active fault locations
Rodolphe Cattin; C�cile Doubre; Jean-Bernard de Chabalier; Geoffrey King; Christophe Vigny; Jean-Philippe Avouac; Jean-Claude Ruegg
Rifting and breakup in the South China Sea
NASA Astrophysics Data System (ADS)
The magma-poor or intermediate magmatic South China Sea is a natural laboratory for studying rifting and breakup. The basin shows an irregular triangular shape with a SW pointing apex, which manifests a preceding propagating rift. The earliest phase of rifting started in the Early Paleocene when a Mesozoic convergent margin changed to extension. After about 30 Million years of rifting, breakup in the major eastern subbasin of the SCS occurred in the Early Oligocene but rifting continued and subsequent breakup of the southwest subbasin took place in the Late Oligocene. The wide Early Cenozoic South China Sea rift preserves the initial rift architecture at the distal margins. Seismic reflection data imaging conjugate crustal sections at the South China Sea margins result in a conceptual model for rift-evolution at conjugate magma-poor margins in time and space. Most distinct are regular undulations in the crust-mantle boundary. Individual rift basins are bounded to crustal blocks by listric normal faults on either side. Moho uplifts are distinct beneath major rift basins, while the Moho is downbended beneath crustal blocks, with a wavelength of undulations in the crust-mantle boundary that approximately equals the thickness of the continental crust. Most of the basin-bounding faults sole out within the middle crust. At the distal margins, detachment faults are located at a mid-crustal level where a weak zone decouples crust and mantle lithosphere during rifting. The lower crust in contrast is interpreted as being strong. Only in the region within about 50 km from the continent-ocean transition (COT) we suggest that normal faults reach the mantle, enabling potentially a coupling between the crust and the mantle. Here, at the proximal margins detachment fault dip either seaward or landward. This may indicate the presence of exhumed mantle bordering the continental margins. Post-rift shallow-water platform carbonates indicate a delay in subsidence during rifting in the South China Sea. We propose that this is an inherent process in highly-extended continental margins and a common origin may be the influx of warm asthenospheric material into initially cool sub-lithospheric mantle. On a crustal-scale largely symmetric process predominate in the initial rifting stage. At the future COT either of the rift basin bounding faults subsequently penetrates the entire crust, resulting in asymmetry at this location. However, asymmetric deformation which is controlled by large scale detachment faulting is confined to narrow areas and does not result in a margin-wide simple-shear model. Rather considerable along-margin variations are suggested resulting in alternating "upper and lower plate" margins.
Franke, Dieter; Savva, Dimitri; Pubellier, Manuel; Steuer, Stephan; Mouly, Benoit; Auxietre, Jean-Luc; Meresse, Florian; Chamot-Rooke, Nicolas
Analog models of oblique rifting in a cold lithosphere
NASA Astrophysics Data System (ADS)
New lithospheric analog models of oblique rifting presented here capture the main characteristics of natural oblique rifts and provide insights into the fault evolution, basin segmentation, and mantle exhumation occurring during rift localization. We present two models: one with a preexisting oblique lithospheric weakness (model B) and another with no weakness zone (model A). Both oblique rifts have an obliquity of about 40�. The main results are as follows. (1) The fault populations, especially during the early stages of deformation, are composed of faults that in strike are largely intermediate between rift-parallel and perpendicular to displacement. This fault population is characteristic of oblique rifts observed in previous studies. (2) In later stages, faults parallel to the rift become numerous in both models. Buoyancy forces related to thickness variations in the lithosphere during rift localization play a significant role and control the initiation of rift-parallel faults. (3) During the final stages of extension, in model B the crust is deformed by rift-parallel faults, while in the basins the small-scale deformation pattern is composed of displacement-normal faults. However, in model A, displacement-normal faults tend to accommodate most of the extension, controlling its final stages. They probably also control the formation of the ocean-continent transition, any possible mantle exhumation, as well as the geometry of oceanic accretion centers. These results provide an insight into the possible evolution of the Gulf of Aden conjugate margins, which developed in an oblique context and most probably without any preexisting rift-parallel localizing heterogeneity in the lithosphere.
Autin, Julia; Bellahsen, Nicolas; Husson, Laurent; Beslier, Marie-Odile; Leroy, Sylvie; D'Acremont, Elia
Understanding the Transition From Continental to Oceanic Rifting in the Northern Ethiopian Rift - the EAGLE Project
NASA Astrophysics Data System (ADS)
A consortium of UK (Leeds, Leicester, Royal Holloway, Edinburgh,), US (Stanford, UTEP, Penn State,) and Ethiopian (Addis Ababa) universities are exploring the kinematics and dynamics of continental breakup through the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE), which aims to probe the crust and upper mantle structure between the Main Ethiopian (continental) and Afar (ocean spreading) rifts. EAGLE is a multi-disciplinary study centered around a set of passive and controlled-source seismic experiments, and incorporates additional magnetotelluric, gravity, GPS and petrological studies. The initial Phase I seismic experiment consisted of a deployment of 30 broadband seismometers for a period of 16 months (Oct. 2001 to Jan. 2003) over a 250 km x 250 km area of the rift valley and its uplifted flanks. P- and S-wave tompography from teleseismic traveltime residuals, SKS splitting analyses and receiver functions provide images of crust and deep earth structure. The Phase II seismic experiment consisted of a further 50 broadband instruments for a period of 4 months over a 200 km x 100 km area encompassing 4 magmatic segments in the Main Ethiopian Rift. These recordings have furthered our understanding of the location of active seismicity, fault plane mechanisms and segmentation of rift crustal structure. Phase III consisted of the deployment of a further 1100 seismic instruments during a controlled source seismic project involving 20 shots being fired into one 450 km cross-rift profile (Profile 1), one 450 km axial profile (Profile 2), and a dense 2D array of instruments in a 150 km diameter circle around the profiles1 intersection (Profile 3), all centered on the magmatically active Nazret region. The crust and upper mantle velocity models derived provide estimates of total crustal thinning across the rift, assess the role of basement in the location of major faults and magmatic segments, and determine whether significant underplating takes place. An 18 sounding magnetotelluric profile along the cross rift profile I provides additional constraints on the nature and distribution of crustal heterogeneity, and images melt accumulation zones. New gravity and geodetic information have also been acquired.
Stuart, G.
Oldest Homo and Pliocene biogeography of the Malawi Rift
THE Malawi Rift and Pliocene palaeofaunas, which include a hominid mandible attributed to Homo rudolfensis, provide a biogeographical link between the better known Plio-Pleistocene faunal records of East and Southern Africa. The Malawi Rift is in a latitudinal position suitable for recording any hominid and faunal dispersion towards the Equator that was brought on by increased aridity of the Late
Friedemann Schrenk; Timothy G. Bromage; Christian G. Betzler; Uwe Ring; Yusuf M. Juwayeyi
New Vectors of Rift Valley Fever in West Africa
After an outbreak of Rift Valley fever in Southern Mauritania in 1987, entomologic studies were conducted in a bordering region in S�n�gal from 1991 to 1996 to identify the sylvatic vectors of Rift Valley fever virus. The virus was isolated from the floodwater mosquitoes Aedes vexans and Ae. ochraceus. In 1974 and 1983, the virus had been isolated from Ae.
D. Fontenille; M. Traore-Lamizana; M. Diallo; J. Thonnon; J. P. Digoutte; H. G. Zeller
Dynamics of rifting and modes of extension on icy satellites
A simple numerical model of extension in icy satellite shells is developed. Thinning of the ice weakens the shell, promoting further extension. If lateral flow in the lower part of the shell is unimportant, extension is opposed and wide rifts are generated; if lateral flow is rapid, localized extension is favored and narrow rifts are produced. Thick shells or high
F. Nimmo
Palinspastic restoration suggesting late paleozoic north atlantic rifting.
Palinspastic restoration of sinistral wrench faults in Britain and of dextral wrenches in Canada, starting with the palinspastic pre-continental-drift map, implies the possibility of as much as 424 kilometers of rifting between Newfoundland and Ireland. The wrench-faulting and postulated resultant rifting are of Devonian and Carboniferous age. PMID:17768977
Webb, G W
Ethiopian Rift and Plateaus: Some Volcanic Petrochemical Differences
Volcanism on the Arabo-Ethiopian swell has accompanied the development of the three traversing spreading zones conjoining at Afar: the Red Sea, Gulf of Aden, and African rift systems. The Red Sea and Gulf of Aden floors are formed by oceanic tholerites, but Afar and the main Ethiopian rift show a wider range of more alkaline volcanics, related to slower crustal
P. A. Mohr
Anatomy of lithosphere necking during orthogonal rifting
NASA Astrophysics Data System (ADS)
The evolution of lithosphere necking is a fundamental parameter controlling the structural architecture and thermal-state of rifted margin. The necking shape depends on several parameters, including the extensional strain-rate and thermal layering of the lithosphere. Despite a large number of analogue and numerical modelling studies on lithosphere extension, a quantitative description of the evolution of necking through time is still lacking. We used analogue modelling to simulate in three-dimension the progression of lithosphere thinning and necking during orthogonal rifting. In our models we simulated a typical "cold and young" 4-layer lithosphere stratigraphy: brittle upper crust (loose quartz sand), ductile lower crust (silicon-barite mixture), brittle upper mantle (loose quartz sand), and ductile lower mantle (silicon-barite mixture). The experimental lithosphere rested on a glucose syrup asthenosphere. We monitored model evolution by periodic and coeval laser scanning of both the surface topography and the lithosphere base. After model completion, each of the four layers was removed and the top of the underlying layer was scanned. This technical approach allowed us to quantify the evolution in space and time of the thinning factors for both the whole lithosphere (?z) and the crust (?). The area of incremental effective stretching (?y) parallel to the extensional direction was obtained from the ?z maps.
Nestola, Yago; Cavozzi, Cristian; Storti, Fabrizio
Extension in the Rio Grande rift.
A positive gravity anomaly along the axis of the Rio Grande rift reflects a volume of anomalous mass added at the base of the crust and intruded into the crust. Part of this volume can be associated with vertical uplift of the crust. The remainder of this anomalous volume, plus the volume of surficial graben fill, can be associated with horizontal crustal extension. The volume of crustal uplift in the Rio Grande rift is unknown, but this term can be eliminated by means of an independent equation provided by assumption of generalized isostatic equilibrium. The volume and mass equations combined provide a solution for extension of the crust in terms of the following parameters: total anomalous mass deficiency in the mantle lithosphere, total anomalous mass excess in the crust and its density contrast, total anomalous mass deficiency of surficial graben fill and its density contrast, and the volume of material eroded from the uplift. Using standard density estimates and masses determined by equivalent-source modeling of gravity profiles, I obtained 1-km extension at 37oN (Colorado-New Mexico border), 13- km extension at 35oN (Albuquerque, New Mexico), and 24-km extension at 33oN in S New Mexico.-Author
Cordell, L.
Magmatic lithospheric heating and weakening during continental rifting: A simple scaling law, a 2-D thermomechanical rifting model and the East African Rift System
NASA Astrophysics Data System (ADS)
Continental rifting is accompanied by lithospheric thinning and decompressional melting. After extraction, melt is intruded at shallower depth thereby heating and weakening the lithosphere. In a feedback mechanism this weakening may assist rifting and melt production. A one-dimensional kinematic lithospheric thinning model is developed including decompressional melting and intrusional magma deposition. The intrusional heating effect is determined as a function of thinning rate and amount, melting parameters, potential temperature, and the depth range of emplacement. The temperature increases approximately proportionally to the square root of the thinning rate and to the square of the supersolidus potential temperature. Simple scaling laws are derived allowing predicting these effects and the surface heat flux for arbitrary scenarios. Two-dimensional thermomechanical extension models are carried out for a multicomponent (crust-mantle) two-phase (melt-matrix) system with a rheology based on laboratory data including magmatic weakening. In good agreement with the 1-D kinematic models it is found that the lithosphere may heat up by several 100 K. This heating enhances viscous weakening by one order of magnitude or more. In a feedback mechanism rifting is dynamically enforced, leading to a significant increase of rift induced melt generation. Including the effect of lateral focusing of magma toward the rift axis the laws are applied to different segments of the East African Rift System. The amount of intrusional heating increases with maturity of the rift from O(10 K) to up to 200 K or 400 K at the Afar Rift depending on the depth range of the magmatic emplacement.
Schmeling, Harro; Wallner, Herbert
New insights into continental rifting from a damage rheology modeling
NASA Astrophysics Data System (ADS)
Previous studies have discussed how tectonic processes could produce relative tension to initiate and propagate rift zones and estimated the magnitude of the rift-driving forces. Both analytic and semi-analytic models as well as numerical simulations assume that the tectonic force required to initiate rifting is available. However, Buck (2004, 2006) estimated the minimum tectonic force to allow passive rifting and concluded that the available forces are probably not large enough for rifting of thick and strong lithosphere in the absence of basaltic magmatism (the "Tectonic Force" Paradox). The integral of the yielding stress needed for rifting over the thickness of the normal or thicker continental lithosphere are well above the available tectonic forces and tectonic rifting cannot happen (Buck, 2006). This conclusion is based on the assumption that the tectonic stress has to overcome simultaneously the yielding stress over the whole lithosphere thickness and ignore gradual weakening of the brittle rocks under long-term loading. In this study we demonstrate that the rifting process under moderate tectonic stretching is feasible due to gradual weakening and "long-term memory" of the heavily fractured brittle rocks, which makes it significantly weaker than the surrounding intact rock. This process provides a possible solution for the tectonic force paradox. We address these questions utilizing 3-D lithosphere-scale numerical simulations of the plate motion and faulting process base on the damage mechanics. The 3-D modeled volume consists of three main lithospheric layers: an upper layer of weak sediments, middle layer of crystalline crust and lower layer of the lithosphere mantle. Results of the modeling demonstrate gradual formation of the rift zone in the continental lithosphere with the flat layered structure. Successive formation of the rift system and associated seismicity pattern strongly depend not only on the applied tectonic force, but also on the healing parameters of the crustal rocks. Results of the modeling also demonstrate how the lithosphere structure and especially depth to the Moho interface affects the geometry of the propagating rift system. With the same boundary conditions and physical properties of rocks as in the case of the flat continental structure, a rift terminates above the passive continental margin and a new fault system is created normal to the direction of the rift propagation. These results demonstrate that the local lithosphere structure is one of the major key factors controlling the geometry of the evolving rift system, faulting and seismicity pattern. Results of simulations suggest that under wide range of conditions a rift propagating through a continental lithosphere might cease before it reaches the margin where transition to oceanic lithosphere occurs. Close to the margin different tectonic styles might take over the propagation. This behavior has been suggested for the NW continuation of the active Red Sea-Suez rift system and initiation of the Dead Sea Transform (Steckler and ten Brink, 1986). With the onset of the Red Sea opening (about Oligocene) the sub-parallel Azraq-Sirhan rift was also activated and propagated in a NW direction from the Arabian continent toward the Levant basin oceanic crust. By applying our 3-D lithosphere-scale numerical simulations on the Azraq-Sirhan rift system, we conclude that thinning of the crystalline crust and strengthening of the Arabian lithosphere led to a decrease or even termination of the rate of rift propagation next to the continental margin.
Lyakhovsky, Vladimir; Segev, Amit; Weinberger, Ram; Schattner, Uri
Structural Evolution of the Incipient Okavango Rift Zone, NW Botswana
NASA Astrophysics Data System (ADS)
Studies of the East African Rift System (EARS) and other continental rifts have significantly improved our understanding of rifting processes; however, we particularly lack studies of the embryonic stages of rift creation. The Okavango Rift Zone (ORZ), NW Botswana is one of few places worldwide where one can study the early stages of continental extension prior to the accumulation of significant amounts of sediments, volcanism, and multiphase deformation that obscure the investigation of these early time processes in more evolved continental rift zones. In this study, gravity and aeromagnetic data have been used to examine the initiation and development of the nascent ORZ. The Okavango basin in NW Botswana is located at the southern tip of the southwestern branch of the EARS. The rift is hosted within the Proterozoic fold and thrust belt of the Ghanzi-Chobe formation. Our objectives include (1) assessing the role of pre-existing structures on the development of rift faults and basin architecture, (2) Examining fault linkage patterns and boarder fault development, and (3) determining the shallow subsurface basin geometry. Aeromagnetic data from the ORZ suggest two main structural trends: 1) northeast-southwest (030- 070o) and 2) northwest - southeast (290 - 320o). The 030- 070o structures occur within the rift zone and throughout the surrounding basement. They form the main bounding fault system of this incipient rift. The NE - SW orientations of rift faults mirror the fold axes and foliation of the basement rocks, suggesting that the basement fabric played an important role in localizing the development of faults within the stress regime present during the initiation of this rift. Additionally, the greatest throw (~400- ~700 m) occurs along the Kunyere (NW dipping) and Tsau faults (SE dipping), defining a full graben as observed on gravity models. This differs from the half-graben model typical of most continental rift zones. Thus, it appears the basin geometry was strongly influenced by the position of these pre-existing faults. Evidence of fault linkage is seen along some of the faults. Linked segments of faults are well defined and some are > 200 km long. We suggest from this result that fault linkage and propagation occurred very early and prior to significant basin development. We conclude that basement fabric seems to be a controlling factor at least in the early stages of basin architecture and structural evolution of ORZ.
Atekwana, E. A.; Kinabo, B. D.; Modisi, M. P.; Hogan, J. P.; Wheaton, D. D.
Rapid onset of narrowing and along-strike propagation of an intra-arc rift: The Taupo Rift, New Zealand
NASA Astrophysics Data System (ADS)
Intra-arc active continental rifting occurs within the Taupo Rift in the North Island, New Zealand. Based on geological and geophysical evidence, we show that the Taupo rift has narrowed via inward and eastward migration of faulting (asymmetric narrowing) and propagated southwards along its axis. This evolution has occurred at relatively high rates of ~25 km/Ma (narrowing), ~ 7 to 15 km/Ma (eastward migration), and ~ < 200 to 275 km/Ma (southward propagation; rates only for the last ~ 340 kyr). The initial onshore narrow rift width is likely to be an effect of a narrow propagating rift from offshore. While several process are likely to influence rapid evolution, we propose that the main control on further rapid narrowing appears to be the presence of large heterogeneities in the crust that enable concentration of deformation, such as large magma bodies of the volcanic arc of Hikurangi subduction margin. The presence of these magma bodies localises faulting. Once faulting is localised it propagates along strike from the heterogeneity into non volcanic segments of the rift, which causes generalised narrowing. Temporal and spatial correlation between voluminous volcanic eruptions and major active faulting migration supports this model. Eastward migration of faulting also follows the eastward migration of the volcanic arc and is likely related to slab rollback. Finally, we show that southward propagation of rifting is linked to southward migration of the Hikurangi plateau and occurs episodically aided by voluminous local volcanism. The detailed recent spatial and temporal evolution of continental rifting in the Taupo Rift reveals the early stages of continental break-up and demonstrates fast evolution of rifting when aided by large scale volcanic processes such as rhyolitic supereruptions.
Villamor, P.; Berryman, K. R.; Ellis, S. M.; Schreurs, G.; Wallace, L. M.; Leonard, G.; Langridge, R. M.; Nairn, I. A.
Lithospheric thickness beneath the southern Kenya Rift: implications from basalt geochemistry
Geochemical data are reported for samples from the flanks and floor of the southern Kenya Rift Valley in the Lake Magadi area, and from two central volcanoes located within the rift valley. Rift lavas include samples of Singaraini and Ol Tepesi basalts on the eastern flank, Kirikiti basalts from the western flank, and plateau trachytes from the rift valley floor.
Anton P. Roex; Andreas Sp�th; Robert E. Zartman
Fault system at the southeastern boundary of the Okavango Rift, Botswana
The seismically active Okavango Rift in northwestern Botswana is probably the southern extension of the East Africa Rift System. Relief is low and many of the geomorphic features of the incipient rift are subtle. The northeast-southwest trending Kunyere and Thamalakane Faults form the southeastern boundary of the rift. Proterozoic structural fabrics of similar trend, belonging to the Ghanzi-Chobe Belt, control
M. P. Modisi
Martian canyons and African rifts: Structural comparisons and implications
NASA Technical Reports Server (NTRS)
The resistant parts of the canyon walls of the Martian rift complex Valled Marineris were used to infer an earlier, less eroded reconstruction of the major roughs. The individual canyons were then compared with individual rifts of East Africa. When measured in units of planetary radius, Martian canyons show a distribution of lengths nearly identical to those in Africa, both for individual rifts and for compound rift systems. A common mechanism which scales with planetary radius is suggested. Martian canyons are significantly wider than African rifts. The overall pattern of the rift systems of Africa and Mars are quite different in that the African systems are composed of numerous small faults with highly variable trend. On Mars the trends are less variable; individual scarps are straighter for longer than on earth. This is probably due to the difference in tectonic histories of the two planets: the complex history of the earth and the resulting complicated basement structures influence the development of new rifts. The basement and lithosphere of Mars are inferred to be simple, reflecting a relatively inactive tectonic history prior to the formation of the canyonlands.
Frey, H. V.
Magma-compensated crustal thinning in continental rift zones.
Continental rift zones are long, narrow tectonic depressions in the Earth's surface where the entire lithosphere has been modified in extension. Rifting can eventually lead to rupture of the continental lithosphere and creation of new oceanic lithosphere or, alternatively, lead to formation of wide sedimentary basins around failed rift zones. Conventional models of rift zones include three characteristic features: surface manifestation as an elongated topographic trough, Moho shallowing due to crustal thinning, and reduced seismic velocity in the uppermost mantle due to decompression melting or heating from the Earth's interior. Here we demonstrate that only the surface manifestation is observed at the Baikal rift zone, whereas the crustal and mantle characteristics can be ruled out by a new seismic profile across southern Lake Baikal in Siberia. Instead we observe a localized zone in the lower crust which has exceptionally high seismic velocity and is highly reflective. We suggest that the expected Moho uplift was compensated by magmatic intrusion into the lower crust, producing the observed high-velocity zone. This finding demonstrates a previously unknown role for magmatism in rifting processes with significant implications for estimation of stretching factors and modelling of sedimentary basins around failed rift structures. PMID:19212408
Thybo, H; Nielsen, C A
Composition of the crust beneath the Kenya rift
We infer the composition of the crust beneath and on the flanks of the Kenya rift based on a comparison of the KRISP-90 crustal velocity structure with laboratory measurements of compressional-wave velocities of rock samples from Kenya. The rock samples studied, which are representative of the major lithologies exposed in Kenya, include volcanic tuffs and flows (primarily basalts and phonolites), and felsic to intermediate composition gneisses. This comparison indicates that the upper crust (5-12 km depth) consists primarily of quartzo-feldspathic gneisses and schists similar to rocks exposed on the flanks of the rift, whereas the middle crust (12-22 km depth) consists of more mafic, hornblende-rich metamorphic rocks, probably intruded by mafic rocks beneath the rift axis. The lower crust on the flanks of the rift may consist of mafic granulite facies rocks. Along the rift axis, the lower crust varies in thickness from 9 km in the southern rift to only 2-3 km in the north, and has a seismic velocity substantially higher than the samples investigated in this study. The lower crust of the rift probably consists of a crust/mantle mix of high-grade metamorphic rocks, mafic intrusives, and an igneous mafic residuum accreted to the base of the crust during differentiation of a melt derived from the upper mantle. ?? 1994.
Mooney, W. D.; Christensen, N. I.
Evolution, distribution, and characteristics of rifting in southern Ethiopia
NASA Astrophysics Data System (ADS)
Ethiopia is a key region to understand the evolution of the East African rift system, since it is the area of interaction between the main Ethiopian rift (MER) and the Kenyan rift. However, geological data constraining rift evolution in this remote area are still relatively sparse. In this study the timing, distribution, and style of rifting in southern Ethiopia are constrained by new structural, geochronological, and geomorphological data. The border faults in the area are roughly parallel to preexisting basement fabrics and are progressively more oblique with respect to the regional Nubia-Somalia motion proceeding southward. Kinematic indicators along these faults are mainly dip slip, pointing to a progressive rotation of the computed direction of extension toward the south. Radiocarbon data indicate post 30 ka faulting at both western and eastern margins of the MER with limited axial deformation. Similarly, geomorphological data suggest recent fault activity along the western margins of the basins composing the Gofa Province and in the Chew Bahir basin. This supports that interaction between the MER and the Kenyan rift in southern Ethiopia occurs in a 200 km wide zone of ongoing deformation. Fault-related exhumation at ~10-12 Ma in the Gofa Province, as constrained by new apatite fission track data, occurred later than the ~20 Ma basement exhumation of the Chew Bahir basin, thus pointing to a northward propagation of the Kenyan rift-related extension in the area.
Philippon, Melody; Corti, Giacomo; Sani, Federico; Bonini, Marco; Balestrieri, Maria-Laura; Molin, Paola; Willingshofer, Ernst; Sokoutis, Dimitrios; Cloetingh, Sierd
Martian canyons and African rifts - Structural comparisons and implications
NASA Technical Reports Server (NTRS)
The resistant parts of the canyon walls of the Martian rift complex Valles Marineris have been used to infer an earlier, less eroded reconstruction of the major troughs. The individual canyons are compared with individual rifts of East Africa. When measured in units of planetary radius, Martian canyons show a distribution of lengths nearly identical to those in Africa, both for individual rifts and for compound rift systems. A common mechanism which scales with planetary radius is suggested. Martian canyons are significantly wider than African rifts. This is consistent with the longstanding idea that rift width is related to crustal thickness: most evidence favors a crust on Mars at least 50% thicker than that of Africa. The overall patterns of the rift systems of Africa and Mars are quite different in that the African systems are composed of numerous small faults with highly variable trend. On Mars the trends are less variable; individual scraps are straighter for longer than on earth. The basement and lithosphere of Mars are inferred to be simple, reflecting a relatively inactive tectonic history prior to the formation of the canyonlands.
Frey, H.
Reassessment of the rifting process in the Western Corinth Rift from relocated seismicity
NASA Astrophysics Data System (ADS)
The seismic activity in the western part of the Corinth Rift (Greece) over the period 2000-2007, monitored by a dense network of three-component stations, is analysed in terms of multiplets and high precision relocation using double difference techniques. This detailed analysis provides new insights into the geometry of faults at depth, the nature and the structure of the active zone at 6-8 km depth previously interpreted as a possible detachment, and more generally into the rifting process. The seismicity exhibits a complex structure, strongly varying along the rift axis. The detailed picture of the seismic zone below the rift indicates that its shallower part (at depths of 6-8 km) is 1-1.5 km thick with a complex microstructure, and that its deeper part (at depths of 9-12 km) gently dipping to the north (10-20�) is 0.1-0.3 km thick with a microstructure consistent with the general slope of the structure. Although the nature of this seismic zone remains an open question, the presence of seismicity beneath the main active area, the strong variability of the structure along the rift over short distances and the complex microstructure of the shallower part revealed by the multiplet analysis are arguments against the hypothesis of a mature detachment under the rift: this active zone more likely represents a layer of diffuse deformation. The geometry of the mapped active faults is not well defined at depth, as no seismicity is observed between 0 and 4 km, except for the Aigion Fault rooting in the seismic layer at 6 km depth with a dip of 60�. A distinct cloud of seismicity may be associated with the antithetic Kalithea Fault, on which the 1909 Fokis earthquake (Ms = 6.3) may have occurred. The link between the 1995 rupture (Ms = 6.2) and the faults known at the surface has been better constrained, as the relocated seismicity favours a rupture on an offshore, blind fault dipping at 30�, rather than on the deeper part of the East Helike Fault. Consequently, the 1995 event is expected to have decreased the Coulomb stress on the East Helike Fault. To explain these seismic observations along with the geodetic observations, a new mechanical model for the rifting process in this region is proposed, involving non-elastic, mostly aseismic uniform NS opening below the rift axis, coupled with the downward and northward growth of a yet immature detachment: the reported GPS rates would mainly result from this deep, silent source, and the seismicity would reveal the detachment position, not yet connected to the ductile lower crust. In such a model, the strong fluctuations of microseismicity would result from small strain instabilities, undetected by continuous GPS and possibly related to pore pressure transients.
Lambotte, S.; Lyon-Caen, H.; Bernard, P.; Deschamps, A.; Patau, G.; Nercessian, A.; Pacchiani, F.; Bourouis, S.; Drilleau, M.; Adamova, P.
Reassessment of the rifting process in the Western Corinth Rift from relocated seismicity
NASA Astrophysics Data System (ADS)
The seismic activity in the western part of the Corinth Rift (Greece) over the period 2000-2007, monitored by a dense network of three-component stations, is analysed in terms of multiplets and high precision relocation using double difference techniques. This detailed analysis provides new insights into the geometry of faults at depth, the nature and the structure of the active zone at 6-8 km depth previously interpreted as a possible detachment, and more generally into the rifting process. The seismicity exhibits a complex structure, strongly varying along the rift axis. The detailed picture of the seismic zone below the rift indicates that its shallower part (at depths of 6-8 km) is 1-1.5 km thick with a complex microstructure, and that its deeper part (at depths of 9-12 km) gently dipping to the north (10-20�) is 0.1-0.3 km thick with a microstructure consistent with the general slope of the structure. Although the nature of this seismic zone remains an open question, the presence of seismicity beneath the main active area, the strong variability of the structure along the rift over short distances and the complex microstructure of the shallower part revealed by the multiplet analysis are arguments against the hypothesis of a mature detachment under the rift: this active zone more likely represents a layer of diffuse deformation. The geometry of the mapped active faults is not well defined at depth, as no seismicity is observed between 0 and 4 km, except for the Aigion Fault rooting in the seismic layer at 6 km depth with a dip of 60�. A distinct cloud of seismicity may be associated with the antithetic Kalithea Fault, on which the 1909 Fokis earthquake (Ms = 6.3) may have occurred. The link between the 1995 rupture (Ms = 6.2) and the faults known at the surface has been better constrained, as the relocated seismicity favours a rupture on an offshore, blind fault dipping at 30�, rather than on the deeper part of the East Helike Fault. Consequently, the 1995 event is expected to have decreased the Coulomb stress on the East Helike Fault. To explain these seismic observations along with the geodetic observations, a new mechanical model for the rifting process in this region is proposed, involving non-elastic, mostly aseismic uniform NS opening below the rift axis, coupled with the downward and northward growth of a yet immature detachment: the reported GPS rates would mainly result from this deep, silent source, and the seismicity would reveal the detachment position, not yet connected to the ductile lower crust. In such a model, the strong fluctuations of microseismicity would result from small strain instabilities, undetected by continuous GPS and possibly related to pore pressure transients.
Lambotte, S.; Lyon-Caen, H.; Bernard, P.; Deschamps, A.; Patau, G.; Nercessian, A.; Pacchiani, F.; Bourouis, S.; Drilleau, M.; Adamova, P.
Biogeochemistry of Kenyan Rift Valley Lake Sediments
NASA Astrophysics Data System (ADS)
The numerous lakes in the Kenyan Rift Valley show strong hydrochemical differences due to their varying geologic settings. There are freshwater lakes with a low alkalinity like Lake Naivasha on the one hand and very salt-rich lakes with high pH values like Lake Logipi on the other. It is known that the underlying lake sediments are influenced by the lake chemistry and by the microorganisms in the sediment. The aim of this work is to provide a biogeochemical characterization of the lake sediments and to use these data to identify the mechanisms that control lake chemistry and to reconstruct the biogeochemical evolution of each lake. The examined rift lakes were Lakes Logipi and Eight in the Suguta Valley, Lakes Baringo and Bogoria south of the valley, as well as Lakes Naivasha, Oloiden, and Sonachi on the Kenyan Dome. The porewater was analysed for different ions and hydrogen sulphide. Additionally, alkalinity and salinity of the lake water were determined as well as the cell numbers in the sediment, using fluorescent microscopy. The results of the porewater analysis show that the overall chemistry differs considerably between the lakes. In some lakes, concentrations of fluoride, chloride, sulphate, and/or hydrogen sulphide show strong concentration gradients with depth, whereas in other lakes the concentrations show only minor variations. Fluoride is present in all lakes; the lowest concentration is found in Lake Oloiden (60 - 90 mg/l), the highest one in Lake Bogoria (1,025 - 1,930 mg/l). The lakes show also large differences in sulphate concentrations. The values vary between 2 mg/l in Lake Baringo and 15,250 mg/l in Lake Eight. In all cores, sulphate concentration does not change significantly with depth; however, there is a distinct peak in each core, raising the question of synchronicity. As expected, chloride concentrations correlate with total salinity. There is no hydrogen sulphide present in the porewater of Lakes Naivasha, Baringo, and Oloiden, whereas in the other lakes, the hydrogen sulphide concentrations are decreasing with depth. Although in close proximity to each other, the Kenyan Rift Valley lakes offer the unique opportunity to study a wide range of geochemical environments and the associated biogeochemical processes.
Grewe, Sina; Kallmeyer, Jens
Melt migration and mantle anisotropy beneath the Ethiopian rift
NASA Astrophysics Data System (ADS)
The EAGLE broadband experiment aims to study the crust and upper-mantle structure in the northern Ethiopian rift, a region of transition from continental to oceanic rifting. 30 broadband seismometers have been deployed for a period of 16 months over an area of 250 km2, centered on the Nazret volcanic zone. These stations are complimented by data from a 3-month deployment of 50 instruments concentrated in the rift valley. We present analysis of upper-mantle anisotropy from measurements of shear-wave splitting in SKS, SKKS and PKS phases. The concentrated station coverage and nearly 500 splitting measurements provide a detailed picture of mantle anisotropy in a rifting environment. In general the results are of high quality with low error estimates. The highest splitting (> 2 secs) occurs on the rift flanks, where the strain partitioning is expected to be highest. There is an asymmetry in results from either side of the rift (i.e., the Ethiopia plateau vs. the Somalian plate). Within the Rift there is a consistent increase in delay times towards the north and Afar, from 1.0 secs in the south to 1.6 secs in the north. Outside the Rift, the polarisations of the fast shear-wave lie in a NE--SW rift--parallel trend and do not align with the direction of absolute plate motion. Within the Rift the orientations swing to more northerly azimuths, following magmatic segments and faulting patterns. The increase in rift splitting times northwards correlates with the amount of magma within the system. Large variations in magnitude of splitting or orientation of the shear-wave over short distances suggests that the source of the anisotropy is quite shallow (<100 km). Cumulatively, these results suggest that the anisotropy is associated with melt migration beneath the rift. To explain these results we appeal to recent deformation experiments of Holtzman et al. (2003) which show anisotropy in partially molten rocks being due to a combination of preferred melt inclusion alignment, layering and olivine crystal alignment.
Kendall, J.; Stuart, G.; Bastow, I.; Ebinger, C.
The rifted margin of Saudi Arabia
NASA Astrophysics Data System (ADS)
The structure of rifted continental margins has always been of great scientific interest, and now, with dwindling economic oil deposits, these complex geological features assume practical importance as well. The ocean-continent transition is, by definition, laterally heterogeneous and likely to be extremely complicated. The southernmost shotpoints (4, 5, and 6) in the U.S. Geological Survey seismic refraction profile in the Kingdom of Saudi Arabia lie within a transition region and thus provide a testing ground for methods that treat wave propagation in laterally heterogeneous media. This portion of the profile runs from the Farasan Islands in the Red Sea across the coast line and the Hijaz-Asir escarpment into the Hijaz-Asir tectonic province. Because the southernmost shotpoint is within the margin of the Saudi sub-continent, the full transition region is not sampled. Furthermore, such an experiment is precluded by the narrowness of the purely oceanic portion of the Red Sea.
McClain, J. S.; Orcutt, J. A.
Prediction of a Rift Valley fever outbreak
El Ni�o/Southern Oscillation related climate anomalies were analyzed by using a combination of satellite measurements of elevated sea-surface temperatures and subsequent elevated rainfall and satellite-derived normalized difference vegetation index data. A Rift Valley fever (RVF) risk mapping model using these climate data predicted areas where outbreaks of RVF in humans and animals were expected and occurred in the Horn of Africa from December 2006 to May 2007. The predictions were subsequently confirmed by entomological and epidemiological field investigations of virus activity in the areas identified as at risk. Accurate spatial and temporal predictions of disease activity, as it occurred first in southern Somalia and then through much of Kenya before affecting northern Tanzania, provided a 2 to 6 week period of warning for the Horn of Africa that facilitated disease outbreak response and mitigation activities. To our knowledge, this is the first prospective prediction of a RVF outbreak.
Anyamba, Assaf; Chretien, Jean-Paul; Small, Jennifer; Tucker, Compton J.; Formenty, Pierre B.; Richardson, Jason H.; Britch, Seth C.; Schnabel, David C.; Erickson, Ralph L.; Linthicum, Kenneth J.
Diagnostic approaches for Rift Valley fever.
Disease outbreaks caused by arthropod-borne animal viruses (arboviruses) resulting in significant livestock and economic losses world-wide appear to be increasing. Rift Valley fever (RVF) virus is an important arbovirus that causes lethal disease in cattle, camels, sheep and goats in Sub-Saharan Africa. There is concern that this virus could spread because of global warming, increased animal trade or through bioterrorism. This paper discusses the current and developing approaches to diagnosis of RVF. Diagnostic assays are available for RVF, but availability can be limited and there is a need for global harmonization. Continued improvement of standard serological and viral genome amplification approaches, including new embedded/syndromic testing, biosensor, emerging virus detection and characterization technologies is needed. PMID:23689885
Wilson, W C; Weingartl, H M; Drolet, B S; Dav�, K; Harpster, M H; Johnson, P A; Faburay, B; Ruder, M G; Richt, J A; McVey, D S
Rift Valley Fever, Sudan, 2007 and 2010
To elucidate whether Rift Valley fever virus (RVFV) diversity in Sudan resulted from multiple introductions or from acquired changes over time from 1 introduction event, we generated complete genome sequences from RVFV strains detected during the 2007 and 2010 outbreaks. Phylogenetic analyses of small, medium, and large RNA segment sequences indicated several genetic RVFV variants were circulating in Sudan, which all grouped into Kenya-1 or Kenya-2 sublineages from the 2006�2008 eastern Africa epizootic. Bayesian analysis of sequence differences estimated that diversity among the 2007 and 2010 Sudan RVFV variants shared a most recent common ancestor circa 1996. The data suggest multiple introductions of RVFV into Sudan as part of sweeping epizootics from eastern Africa. The sequences indicate recent movement of RVFV and support the need for surveillance to recognize when and where RVFV circulates between epidemics, which can make data from prediction tools easier to interpret and preventive measures easier to direct toward high-risk areas.
Aradaib, Imadeldin E.; Erickson, Bobbie R.; Elageb, Rehab M.; Khristova, Marina L.; Carroll, Serena A.; Elkhidir, Isam M.; Karsany, Mubarak E.; Karrar, AbdelRahim E.; Elbashir, Mustafa I.
Regional magnetic anomaly constraints on continental rifting
NASA Technical Reports Server (NTRS)
Radially polarized MAGSAT anomalies of North and South America, Europe, Africa, India, Australia and Antarctica demonstrate remarkably detailed correlation of regional magnetic lithospheric sources across rifted margins when plotted on a reconstruction of Pangea. These major magnetic features apparently preserve their integrity until a superimposed metamorphoric event alters the magnitude and pattern of the anomalies. The longevity of continental scale magnetic anomalies contrasts markedly with that of regional gravity anomalies which tend to reflect predominantly isostatic adjustments associated with neo-tectonism. First observed as a result of NASA's magnetic satellite programs, these anomalies provide new and fundamental constraints on the geologic evolution and dynamics of the continents and oceans. Accordingly, satellite magnetic observations provide a further tool for investigating continental drift to compliment other lines of evidence in paleoclimatology, paleontology, paleomagnetism, and studies of the radiometric ages and geometric fit of the continents.
Vonfrese, R. R. B.; Hinze, W. J.; Olivier, R.; Bentley, C. R.
Gravity analysis of the main Ethiopian rift
NASA Astrophysics Data System (ADS)
An analysis of gravity data collected during the 2003 Ethiopia-Afar Geoscientific Lithospheric Experiment (EAGLE) and previously available data is used in conjunction with EAGLE seismic results to determine a preliminary crustal structure model of the main Ethiopian rift (MER). The Bouguer gravity anomaly is dominated by a regional gravity anomaly that increases in amplitude from the central MER to the southern Afar region, while the residual gravity anomaly indicates gravity maxima occur over magmatic segments that occur within the MER. Gravity modeling using the EAGLE controlled-source axial seismic model as a constraint indicates that the crust thins from �40 km over the central MER to �30 km over the southern Afar region. However, to model the large amplitude gravity gradient in the Nazreth region, a denser lower crust and less dense upper mantle were needed over the southern Afar region. The denser lower crust is interpreted to be caused by mafic material formed during extensional tectonics, while the less dense upper mantle is indicative of a hotter thermal regime. The presence of a thinned crust and higher density lower crust is characteristic of volcanic rifted margins and implies that the southern Afar region may be developing into a continental margin. The gravity maxima occurring over the magmatic segments were modeled as dense, mafic bodies caused by intrusion into the lower and upper crust with tops between 7 and 10 km. These bodies add constraints to recently obtained three-dimensional seismic tomographic models suggest that the lower to lower-upper crust is in a ductile regime and the upper 7 km of the upper crust is in a brittle regime.
Mickus, K.; Tadesse, Ketsela; Keller, G. R.; Oluma, Befekadu
Teleseismic body wave tomography within a highly extended continental rift: the Woodlark Rift, Papua New Guinea
NASA Astrophysics Data System (ADS)
The Woodlark Rift, Papua New Guinea, has been a region of westward-propagating continental extension for 6-8 Ma, grading westward from seafloor spreading to newly thinned continent. The D'Entrecasteaux Islands (DIs) lie immediately to the west of the youngest spreading centres in continental crust that has undergone 140-190 km of extension. These islands are dominated by metamorphic core complexes (MCCs) containing 5-6 Ma ultra-high pressure (UHP) coesite-eclogite exhumed at ~20 mm/yr coeval with extension. An array of 31 PASSCAL broadband seismometers and 8 broadband OBSs was installed around the region from 2010-2011 to investigate the thinned continent close to the onset of seafloor spreading. We present results of a teleseismic P- and S- wave tomography study that images the mantle beneath the rapidly extending continent. Preliminary observations include strong azimuthal dependence of differential travel times, indicating significant lateral velocity variations and inferred thermal gradients. Using Ps receiver functions and SsPmP reflections, we estimate variations in Moho depth to correct for the crustal effect on travel times. We observe large (>1s) travel time delays beneath the DIs in both P and S arrivals, while stations on the Trobriand Islands and Papuan Peninsula exhibit travel time deficits of 1-2 s. This indicates that lithosphere is thinnest beneath the DIs, along the axis of the rift, in agreement with the location of Quaternary volcanism and consistent with results from surface waves [Ge et al., AGU2013 abstract] and a previous, lower-resolution tomographic study nearby. There is also evidence for moderately thinned lithosphere in the basin immediately south of the DIs. We have previously established strong, spreading-parallel anisotropy from SKS splitting caused by mantle olivine fabric beneath the DIs and the Trobriand Platform, inferred to represent asthenospheric flow in response to rifting. Detailed tomography will reveal how thinning of mantle lithosphere varies with depth and distance from the rift axis, and possibly other features related to UHP exhumation. We anticipate an improved understanding of the kinematics of lithospheric thinning, and the transition in deep structure between rifted continent and adjacent spreading centers. Differential teleseismic P-wave travel times from cross-correlation of 192 earthquakes between March 2010 and August 2011, with crustal correction applied assuming Vp_crust = 6.2 km/s. Delay times at each station are plotted by back azimuth.
Eilon, Z.; Abers, G. A.; Jin, G.; Kim, Y.; Gaherty, J. B.
Low lower crustal velocity across Ethiopia: Is the Main Ethiopian Rift a narrow rift in a hot craton?
[1] The Main Ethiopian Rift (MER) is a classic narrow rift that developed in hot, weak lithosphere, not in the initially cold, thick, and strong lithosphere that would be predicted by common models of rift mode formation. Our new 1-D seismic velocity profiles from Rayleigh wave/receiver function joint inversion across the MER and the Ethiopian Plateau indicate that hot lower crust and upper mantle are present throughout the broad region affected by Oligocene flood basalt volcanism, including both the present rift and the adjacent Ethiopian Plateau hundreds of kilometers from the rift valley. The region of hot lithosphere closely corresponds to the region of flood basalt volcanism, and we interpret that the volcanism and thermal perturbation were jointly caused by impingement of the Afar plume head. Across the affected region, Vs is 3.6-3.8 km/s in the lowermost crust and ???4.3 km/s in the uppermost mantle, both ??0.3 km/s lower than in the eastern and western branches of the East African Rift System to the south. We interpret the low Vs in the lower crust and upper mantle as indicative of hot lithosphere with partial melt. Our results lead to a hybrid rift mode, in which the brittle upper crust has developed as a narrow rift along the Neoproterozoic suture between East and West Gondwana, while at depth lithospheric deformation is distributed over the broad region (??400 km wide) thermally perturbed by the broad thermal upwelling associated with the Afar plume head. Development of both the East African Rift System to the south (in cold, strong lithosphere) and the MER to the north (in hot, weak lithosphere) as narrow rifts, despite their vastly different initial thermal states and depth-integrated lithospheric strength, indicates that common models of rift mode formation that focus only on temperature, thickness, and vertical strength profiles do not apply to these classic continental rifts. Instead, inherited structure and associated lithospheric weaknesses are the primary control on the mode of extension. ?? 2009 by the American Geophysical Union.
Keranen, K. M.; Klemperer, S. L.; Julia, J.; Lawrence, J. F.; Nyblade, A. A.
Low lower crustal velocity across Ethiopia: Is the Main Ethiopian Rift a narrow rift in a hot craton?
NASA Astrophysics Data System (ADS)
The Main Ethiopian Rift (MER) is a classic narrow rift that developed in hot, weak lithosphere, not in the initially cold, thick, and strong lithosphere that would be predicted by common models of rift mode formation. Our new 1-D seismic velocity profiles from Rayleigh wave/receiver function joint inversion across the MER and the Ethiopian Plateau indicate that hot lower crust and upper mantle are present throughout the broad region affected by Oligocene flood basalt volcanism, including both the present rift and the adjacent Ethiopian Plateau hundreds of kilometers from the rift valley. The region of hot lithosphere closely corresponds to the region of flood basalt volcanism, and we interpret that the volcanism and thermal perturbation were jointly caused by impingement of the Afar plume head. Across the affected region, Vs is 3.6-3.8 km/s in the lowermost crust and ?4.3 km/s in the uppermost mantle, both �0.3 km/s lower than in the eastern and western branches of the East African Rift System to the south. We interpret the low Vs in the lower crust and upper mantle as indicative of hot lithosphere with partial melt. Our results lead to a hybrid rift mode, in which the brittle upper crust has developed as a narrow rift along the Neoproterozoic suture between East and West Gondwana, while at depth lithospheric deformation is distributed over the broad region (�400 km wide) thermally perturbed by the broad thermal upwelling associated with the Afar plume head. Development of both the East African Rift System to the south (in cold, strong lithosphere) and the MER to the north (in hot, weak lithosphere) as narrow rifts, despite their vastly different initial thermal states and depth-integrated lithospheric strength, indicates that common models of rift mode formation that focus only on temperature, thickness, and vertical strength profiles do not apply to these classic continental rifts. Instead, inherited structure and associated lithospheric weaknesses are the primary control on the mode of extension.
Keranen, Katie M.; Klemperer, Simon L.; Julia, Jordi; Lawrence, Jesse F.; Nyblade, Andy A.
Immunofluorescent Cell-Counting Assay of Rift Valley Fever Virus.
National Technical Information Service (NTIS)
Rift Valley fever (RVF) virus was quantitatively assayed by enumerating the cells containing fluorescent viral antigens after infection of L cell monolayers. Efficiency and speed of virus attachment were markedly enhanced when augmented by centrifugal for...
N. Hahon
Preferential rifting of continents - A source of displaced terranes
NASA Technical Reports Server (NTRS)
Lithospheric rifting, while prevalent in the continents, rarely occurs in oceanic regions. To explain this preferential rifting of continents, the total strength of different lithospheres is compared by integrating the limits of lithospheric stress with depth. Comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of three. Also, a thickened crust can halve the total strength of normal continental lithosphere. Because the weakest area acts as a stress guide, any rifting close to an ocean-continent boundary would prefer a continental pathway. This results in the formation of small continental fragments or microplates that, once accreted back to a continent during subduction, are seen as displaced terranes. In addition, the large crustal thicknesses associated with suture zones would make such areas likely locations for future rifting episodes. This results in the tendency of new oceans to open along the suture where a former ocean had closed.
Vink, G. E.; Morgan, W. J.; Zhao, W.-L.
Serpentized mantle at rifted margins: The Goban Spur example
NASA Astrophysics Data System (ADS)
The crustal structure of rifted continental margins can tell us about the processes that operated from continental extension to eventual break-up and sea floor spreading. Variations between margins may record different processes operating during extension or indicate changes in the external geological controls such as mantle plume influence. Extension between Europe and North America began in the mid Cretaceous, dated at the Goban Spur-Flemish Cap rift as late Hauterivian-early Barremian (126-128 Ma) from deep sea drilling (DSDP leg 80) results on the Goban Spur margin. Marine magnetic anomaly 34 can be identified clearly on both margins and indicates that sea floor spreading began no later than 83 Ma. Syn-rift volcanism is limited to a 20 km basaltic body, with considerable lateral extent, at the foot of the continental slope, emplaced at the end of continental rifting. \
Bullock, A. D.; Minshull, T. A.
Relationship of coronae, regional plains and rift zones on Venus
NASA Astrophysics Data System (ADS)
Coronae and rifts are the most prominent volcano-tectonic features on the surface of Venus. Coronae are large radial-concentric structures with diameters of 100 to over 1000 km. They have varied topographical shapes, radial and concentric fracturing and compressional tectonic structures are common for their annuli. Massive volcanism is also connected with some of the structures. Coronae are interpreted to be the result of updoming and fracturing on the surface due to interaction of mantle diapirs with the lithosphere and its subsequent gravitational relaxation. According to Stofan et al. (2001), two types of coronae are observed: type 1 - coronae that have annuli of concentric ridges and/or fractures (407 structures), and type 2 that have similar characteristics to type 1 but lack a complete annulus of ridges and fractures (107 structures). We analyzed 20% of this coronae population (we chose each fifth structure from the Stofan et al. (2001) catalog; 82 coronae of type 1 and 22 coronae of type 2, in total 104 coronae) for the (1) spatial distribution of rift structures and time relationship of rift zones activity with time of regional volcanic plains emplacement, and (2) tectonics, volcanism, age relative to regional plains and relationship with rifts. Two different age groups of rifts on Venus were mapped at the scale 1:50 000 000: old rifts that predate and young rifts that postdate regional plains. Most of young rifts inherit strikes of old rifts and old rifts are reworked by them. This may be evidence of rift-produced uplift zones that were probably mostly stable during both types of rifts formation. Evolution of distribution of rift systems with time (decreasing of distribution and localization of rift zones) imply thickening of the lithosphere with time. Coronae-producing mantle diapirism and uplift of mantle material in rift zones are not well correlated at least in time in most cases, because majority of coronae (77%) of both types has no genetic association with rifts. Majority of coronae (72%) were mostly active before regional plains formation, and only 3% appear to have begun to form after the plains emplacement, which may be also due to thickening of the lithosphere. According to the relationship with regional plains type 2 coronae are in general older than type 1 coronae. Three types of corona-related volcanic activity were observed: shield volcanoes and their clusters, as well as extensive lobate lava flows and smooth volcanic plains. Shield volcanoes during coronae evolution were mostly active before regional plains emplacement. Most active phase of volcanism of corona may not coincide with the time of the major tectonic activity of corona, as majority of coronae (77%) were most active before regional plains formation, but almost half of all coronae have traces of post regional plains volcanism. Detailed mapping and stratigraphic analysis of seven regions with 34 examples of coronae showed a similarity in the sequence of regional geologic units.
Krassilnikov, A. S.; Kostama, V.-P.; Aittola, M.; Guseva, E. N.; Cherkashina, O. S.
Mapping of the Major Structures of the African Rift System.
National Technical Information Service (NTIS)
The author has identified the following significant results. The new fault map of the main Ethiopian rift, based on aerial photo compilations, generally agrees well with the maps produced from ERTS-1 imagery. Characteristically, the ERTS-1 imagery shows s...
P. Mohr
Lithospheric thinning beneath rifted regions of Southern California.
The stretching and break-up of tectonic plates by rifting control the evolution of continents and oceans, but the processes by which lithosphere deforms and accommodates strain during rifting remain enigmatic. Using scattering of teleseismic shear waves beneath rifted zones and adjacent areas in Southern California, we resolve the lithosphere-asthenosphere boundary and lithospheric thickness variations to directly constrain this deformation. Substantial and laterally abrupt lithospheric thinning beneath rifted regions suggests efficient strain localization. In the Salton Trough, either the mantle lithosphere has experienced more thinning than the crust, or large volumes of new lithosphere have been created. Lack of a systematic offset between surface and deep lithospheric deformation rules out simple shear along throughgoing unidirectional shallow-dipping shear zones, but is consistent with symmetric extension of the lithosphere. PMID:21979933
Lekic, Vedran; French, Scott W; Fischer, Karen M
An updated global earthquake catalogue for stable continental regions: Reassessing the correlation with ancient rifts
We present an updated global earthquake catalogue for stable continental regions (SCRs; i.e. intraplate earthquakes) that is available on the Internet. Our database contains information on location, magnitude, seismic moment and focal mechanisms for over 1300 M (moment magnitude) ??? 4.5 historic and instrumentally recorded crustal events. Using this updated earthquake database in combination with a recently published global catalogue of rifts, we assess the correlation of intraplate seismicity with ancient rifts on a global scale. Each tectonic event is put into one of five categories based on location: (i) interior rifts/taphrogens, (ii) rifted continental margins, (iii) non-rifted crust, (iv) possible interior rifts and (v) possible rifted margins. We find that approximately 27 per cent of all events are classified as interior rifts (i), 25 per cent are rifted continental margins (ii), 36 per cent are within non-rifted crust (iii) and 12 per cent (iv and v) remain uncertain. Thus, over half (52 per cent) of all events are associated with rifted crust, although within the continental interiors (i.e. away from continental margins), non-rifted crust has experienced more earthquakes than interior rifts. No major change in distribution is found if only large (M ??? 6.0) earthquakes are considered. The largest events (M ??? 7.0) however, have occurred predominantly within rifts (50 per cent) and continental margins (43 per cent). Intraplate seismicity is not distributed evenly. Instead several zones of concentrated seismicity seem to exist. This is especially true for interior rifts/taphrogens, where a total of only 12 regions are responsible for 74 per cent of all events and as much as 98 per cent of all seismic moment released in that category. Of the four rifts/taphrogens that have experienced the largest earthquakes, seismicity within the Kutch rift, India, and the East China rift system, may be controlled by diffuse plate boundary deformation more than by the presence of the ancient rifts themselves. The St. Lawrence depression, Canada, besides being an ancient rift, is also the site of a major collisional suture. Thus only at the Reelfoot rift (New Madrid seismic zone, NMSZ, USA), is the presence of features associated with rifting itself the sole candidate for causing seismicity. Our results suggest that on a global scale, the correlation of seismicity within SCRs and ancient rifts has been overestimated in the past. Because the majority of models used to explain intraplate seismicity have focused on seismicity within rifts, we conclude that a shift in attention more towards non-rifted as well as rifted crust is in order. ?? 2005 RAS.
Schulte, S. M.; Mooney, W. D.
The sensitivity of East African rift lakes to climate fluctuations
Sequences of paleo-shorelines and the deposits of rift lakes are used to reconstruct past climate changes in East Africa.\\u000a These recorders of hydrological changes in the Rift Valley indicate extreme lake-level variations on the order of tens to\\u000a hundreds of meters during the last 20,000 years. Lake-balance and climate modeling results, on the other hand, suggest relatively\\u000a moderate changes in
Lydia A. OlakaEric; Eric O. Odada; Martin H. Trauth; Daniel O. Olago
Crustal structure of central Lake Baikal: Insights into intracontinental rifting
The Cenozoic rift system of Baikal, located in the interior of the largest continental mass on Earth, is thought to represent a potential analog of the early stage of breakup of supercontinents. We present a detailed P wave velocity structure of the crust and sediments beneath the Central Basin, the deepest basin in the Baikal rift system. The structure is characterized by a Moho depth of 39-42.5 km; an 8-km-thick, laterally continuous high-velocity (7.05-7.4 km/s) lower crust, normal upper mantle velocity (8 km/s), a sedimentary section reaching maximum depths of 9 km, and a gradual increase of sediment velocity with depth. We interpret the high-velocity lower crust to be part of the Siberian Platform that was not thinned or altered significantly during rifting. In comparison to published results from the Siberian Platform, Moho under the basin is elevated by <3 km. On the basis of these results we propose that the basin was formed by upper crustal extension, possibly reactivating structures in an ancient fold-and-thrust belt. The extent and location of upper mantle extension are not revealed by our data, and it may be offset from the rift. We believe that the Baikal rift structure is similar in many respects to the Mesozoic Atlantic rift system, the precursor to the formation of the North Atlantic Ocean. We also propose that the Central Baikal rift evolved by episodic fault propagation and basin enlargement, rather than by two-stage rift evolution as is commonly assumed.
ten, Brink, U. S.; Taylor, M. H.
Anatomy of the Midcontinent Rift beneath Lake Superior
The structure and geometry of the 1.1-b.y.-old Midcontinent Rift system under Lake Superior is interpreted from 20 seismic reflection profiles recorded during the early and mid-1980s. The seismic data reveal that rift basins under Lake Superior are variable in depth and are partially filled with Keweenawan age sediments to depths of 7 km or more and volcanic flows to depths of 36 km. These rift basins form a continuous and sinuous feature that widens in the Allouez Basin and Marquette Basin in the western and central lake and narrows between White Ridge and the Porcupine Mountains. The rift basin bends southeast around the Keweenaw Peninsula, widens to about 100 km as it extends into the eastern half of Lake Superior, and exists the lake with its axis in the vicinity of Au Sable Point in Pictured Rocks National Lake Shore, about 50 km northeast of Munising, Michigan. The axis of the rift may exit the western end of the lake near Chequamegon Bay in Wisconsin. However, lack of data in that area limits interpretation at this time. Prior to late-stage reverse-faulting, a continuous basin of more uniform thickness was present beneath the lake. Crustal extension during rifting of approximately 50 km was followed by plate convergence and crustal shortening of approximately 30 km, with the major component of thrust from the southeast. Crustal shortening occurred after development of rift grabens and their filling with lava flows, but before deposition of the final sag basin sediments. Integration of information obtained from outcrops with data reported here indicates that the Lake Superior section of the rift is associated with as many as three major boundary faults.
Thompson, M.D.; McGinnis, L.D. [Argonne National Lab., IL (United States); Ervin, C.P. [Northern Illinois Univ., DeKalb, IL (United States); Mudrey, M.G. [Wisconsin Geologic and Natural History Survey, Madison, WI (United States)
Geodynamic Evolution of the Southern Flank of the Corinth Rift
The Gulf of Corinth is the most seismically active area of Europe extending in a N-S direction at a rate of 12mm\\/yr. Many studies have focused on currently active normal faults, which bound the southern flank of the Corinth rift. However, many prominent faults, that are considered no longer active, outcrop over a broader area along with their syn-rift sedimentary
D. Jousselin; S. Bourlange; M. Ford; C. Le Carlier; S. Rohais
TDRS satellite over African Rift Valley, Kenya, Africa
NASA Technical Reports Server (NTRS)
This post deploy view of a TDRS satellite shows a segment of the African Rift Valley near Lake Baringo, Kenya, Africa (3.0S, 36.0E). The African Rift Valley system is a geologic fault having its origins in southern Turkey, through the near east forming the bed of the Jordan River, Gulf of Aqaba, the Red Sea and down through east Africa. The line of lakes and valleys of east Africa are the result of the faulting activity.
Neotectonic activity along the Shanxi rift system, China
NASA Astrophysics Data System (ADS)
The Shanxi rift system is one of the most outstanding Pliocene-Quaternary continental rift systems and strong earthquake belts in China. It extends as a series of en echelon left-stepping asymmetrical half-graben basins on the Shanxi Highlands over a distance of more than 1200 km. It describes a sinous S-shaped curve with a NNE-trending transtensional segment in the middle, and NE-ENE-trending extensional domains on both terminal segments. The latter are characterized by apparently synchronous, high-angle normal faulting, accommodating large vertical and relatively smaller lateral strains (3.5-8.5%), which produces the modern basin and range structure. The rift system has been intermittently active since the Pliocene. Geomorphological, neotectonic and seismic studies indicate that the rift system is at present still developing, as demonstrated by the occurrence of strong destructive historical earthquakes of magnitudes 7-8 and the large slip rates on the NNE-trending transtensional faults in the middle segment. The slip rates of these faults reached 4.9-6.4 mm per year during the Holocene. Geophysical studies show that the rifting occurred in a thickened crust, and no compelling evidence exists for the major thermal event in the mantle uniquely associated with the rifting. The development of the Shanxi rift system is consistent with the regional brittle strain pattern of a right-lateral shear belt and a regional stress field of ENE-WSW compression and NNW-SSE extension of the North China subplate. This structural setting corroborates the hypothesis that the deformation is in response to the escape tectonics caused by the Himalayan indenter from the southwest, and at the same time by the counter-clockwise rotation of the intervening crustal blocks. This provides the mode of formation of the Shanxi rift system.
Xu, Xiwei; Ma, Xingyuan; Deng, Qidong
Are elevated passive margins formed by processes related to rifting?
NASA Astrophysics Data System (ADS)
It is commonly assumed that the morphology of elevated passive continental margins (EPCMs) is directly related to the processes of rifting and crustal separation. The elevated plateau along such a margin is widely believed to have remained high since continental separation and remained flat by continuous erosion to a perched base level (no surface uplift) or alternatively represent a breakup surface (no rock uplift), despite continental-stretching theory predicting deposition of a thick post-rift sequence overlying both the rift and its margins. The absence of a post-rift section from many EPCMs is taken as evidence that it was never deposited, consistent with these margins being permanently elevated since rifting. However recent studies in West Greenland show that typical EPCM topography formed c. 50 Myr after break-up, and that the present-day high-level plateau is the remnant of a post-rift erosion surface that was uplifted in the late Neogene. Since elevated plateaux cut by deeply incised valleys are a characteristic feature of the West Greenland margin and of other EPCMs, this similarity suggests that such topography elsewhere in the world may also be unrelated to the processes of rifting and continental separation. We present geological, geomorphological and thermochronological evidence from EPCMs around the world in support of this hypothesis, and we argue that the absence of a post-rift section is due to its removal by erosion during uplift events that are unrelated to formation of the margin but are related, in some way that is not understood, to the presence of the margin.
Japsen, Peter; Chalmers, James A.; Green, Paul F.; Bonow, Johan M.; Lidmar-Bergstr�m, Karna
Structure of the southern Rio Grande rift from gravity interpretation
NASA Technical Reports Server (NTRS)
Regional Bouguer gravity anomalies in southern New Mexico have been analyzed by two-dimensional wave number filtering and poly-nomial trend surface analysis of the observed gravity field. A prominent, regional oval-shaped positive gravity anomaly was found to be associated with the southern Rio Grande rift. Computer modeling of three regional gravity profiles suggests that this anomaly is due to crustal thinning beneath the southern Rio Grande rift. These models indicate a 25 to 26-km minimum crustal thickness within the rift and suggest that the rift is underlain by a broad zone of anomalously low-density upper mantle. The southern terminus of the anomalous zone is approximately 50 km southwest of El Paso, Texas. A thinning of the rifted crust of 2-3 km relative to the adjacent Basin and Range province indicates an extension of about 9 percent during the formation of the modern southern Rio Grande rift. This extension estimate is consistent with estimates from other data sources. The crustal thinning and anomalous mantle is thought to result from magmatic activity related to surface volcanism and high heat flow in this area.
Daggett, P. H.; Keller, G. R.; Wen, C.-L.; Morgan, P.
Geochemistry of East African Rift basalts: An overview
NASA Astrophysics Data System (ADS)
Mafic lavas erupted along the East African Rift System from the Afar triangle in northern Ethiopia to the Rungwe province in southern Tanzania display a wide range of geochemical and isotopic compositions that reflect heterogeneity in both source and process. In areas with the lowest degree of crustal extension (the Western and Southern Kenya Rifts) primitive lavas record the greatest extent of lithospheric melting, manifest in elevated abundances of incompatible elements and highly radiogenic Sr-Nd-Pb isotopic compositions. Where prolonged extension has removed most or all of the mantle lithosphere (the Turkana and Northern Kenya Rifts), a larger role for sub-lithospheric processes is indicated. At intermediate degrees of extension (the Main Ethiopian Rift) both lithospheric and sub-lithospheric contributions are observed, and crustal assimilation occurs in some cases. Despite the wide compositional range of African Rift basalts, a restricted number of source domains contribute to magmatism throughout the area. These individual domains are: (1) the subcontinental mantle lithosphere; (2) a plume source with high-? Sr-Nd-Pb-He isotopic affinities, present in all areas within and south of the Turkana Depression; and (3) a plume source with isotopic signatures analogous to those observed in some ocean islands, including high 3He/ 4He values, present throughout the Ethiopian Rift and the Afar region. The two plume sources may both be derived from the South African Superplume, which is likely to be a compositionally heterogeneous feature of the lower mantle.
Furman, Tanya
Contribution of Transverse Structures, Magma, and Crustal Fluids to Continental Rift Evolution: The East African Rift in Southern Kenya
NASA Astrophysics Data System (ADS)
The Magadi rift in southern Kenya formed at ~7 Ma within Proterozoic rocks of the Mozambique orogenic belt, parallel to its contact with the Archean Tanzania craton. The rift is bounded to the west by the ~1600-m-high Nguruman border fault. The rift center is intensely dissected by normal faults, most of which offset ~1.4-0.8 Ma lavas. Current E-W extensional velocities are ~2-4 mm/yr. Published crustal tomography models from the rift center show narrow high velocity zones in the upper crust, interpreted as cooled magma intrusions. Local, surface-wave, and SKS-splitting measurements show a rift-parallel anisotropy interpreted to be the result of aligned melt zones in the lithosphere. Our field observations suggest that recent fault activity is concentrated at the rift center, consistent with the location of the 1998 seismic swarm that was associated with an inferred diking event. Fault zones are pervasively mineralized by calcite, likely from CO2-rich fluids. A system of fault-fed springs provides the sole fluid input for Lake Magadi in the deepest part of the basin. Many of these springs emanate from the Kordjya fault, a 50-km-long, NW-SE striking, transverse structure connecting a portion of the border fault system (the NW-oriented Lengitoto fault) to the current locus of strain and magmatism at the rift center. Sampled springs are warm (44.4�C) and alkaline (pH=10). Dissolved gas data (mainly N2-Ar-He) suggests two-component mixing (mantle and air), possibly indicating that fluids are delivered into the fault zone from deep sources, consistent with a dominant role of magmatism to the focusing of strain at the rift center. The Kordjya fault has developed prominent fault scarps (~150 m high) despite being oblique to the dominant ~N-S fault fabric, and has utilized an en echelon alignment of N-S faults to accommodate its motion. These N-S faults show evidence of sinistral-oblique motion and imply a bookshelf style of faulting to accommodate dextral-oblique motion along the Kordjya fault. Fault relationships imply that the NW-SE transverse structures represent recent activity in the rift, and have locally tilted Late Pleistocene sediments. Given the abundance of N-S striking faults in the rift, the tendency for fault activity along transverse features suggests a change in the rifting driving forces that are likely the result of an interplay between strain localization at the rift center, inherited crustal fabric (NW structures in the Mozambique belt), a possible counterclockwise rotation of stress related to interacting rift segments in southern Kenya, and an active hydrothermal fluid regime that facilitates faulting. By connecting the Lengitoto fault to the rift center, the Kordjya fault has effectively caused the Magadi rift to bypass the Nguruman border fault, which has been rendered inactive and thus no longer a contributor to the rifting process.
Kattenhorn, S. A.; Muirhead, J.; Dindi, E.; Fischer, T. P.; Lee, H.; Ebinger, C. J.
Elevated, passive continental margins: Not rift shoulders, but expressions of episodic, post-rift burial and exhumation
NASA Astrophysics Data System (ADS)
Many studies of elevated, passive continental margins (EPCMs) assume that their characteristic, large-scale morphology with high-level plateaux and deeply incised valleys has persisted since rifting and crustal separation, and that the absence of post-rift sediments is evidence of non-deposition. The high mountains in West Greenland, however, expose evidence of km-scale, post-rift subsidence, and recent studies showed that typical EPCM morphology with elevated plateaux formed c. 50 Myr after breakup through a process of uplift and dissection of a regional, post-rift erosion surface. Since the West Greenland margin shares all the morphological characteristics of EPCMs, the results from West Greenland lead us to question the common assumption that EPCMs have remained high since the onset of continental separation. We present published evidence of post-rift burial followed by uplift and exhumation from a number of EPCMs and their adjacent basins to support the notion that EPCMs are not permanent highs and that their morphology is unrelated to rifting and continental breakup. Geodynamic models that explain EPCMs as permanent highs since the time of rifting require either no lithospheric mantle extension below extending crust or effective elastic thicknesses > 100 km. Such models are, however, not consistent with the subsidence history inferred from actual rifts and their margins. Geodynamic models using low elastic thicknesses and a much more uniform distribution of strain within the lithosphere are more consistent with observations of early post-rift behaviour, but some additional process is needed to uplift the margins later. We suggest that EPCMs represent anticlinal, lithospheric folds formed under compression where an abrupt change in crustal or lithospheric thickness occurs between cratons and rift basins. We propose that EPCMs are expressions of episodes of post-rift burial followed by compression-induced uplift and exhumation; one episode of uplift results in erosion of the region to produce a low-relief surface near the level of the adjacent, opening ocean, and a second (or more) episode(s) raises the plateau to its present elevation, after which the plateau is dissected by fluvial and possibly glacial erosion.
Japsen, Peter; Chalmers, James A.; Green, Paul F.; Bonow, Johan M.
Rifting and volcanism: Examples from volcanic rifted and magma-poor margins based on multichannel seismic data
NASA Astrophysics Data System (ADS)
Great efforts in the research of passive rifted margins in the last decades highlighted also that lots of open questions remain. A considerable controversy exists about the role of the mantle during rifting and the subsequent formation of oceanic crust and about the interaction of mantle and surface processes, i.e. the precise nature of volcanism in the rifting process. There are two end-member extremes of passive rifted margins. Volcanic rifted margins evolve by a combination of extension, and extensive extrusive flood volcanism over short time periods during breakup, manifested in reflection seismic data as seaward dipping reflectors. These margins are commonly related to mantle plumes; however, in the past years this has been questioned. Magma-poor rifted margins in contrast show wide extensional features as rotated faults blocks and detachment surfaces near the base of the continental crust, but limited magmatism that in addition seems to be delayed to post-breakup. In this presentation examples from three locations that are less frequently cited in the discussion about (either magma-poor or volcanic) rifted margins will be shown: The Laptev Sea margin in the Arctic Ocean, where the active Arctic mid-oceanic ridge meets continental lithosphere at a high angle, the southernmost South Atlantic with well expressed conjugate volcanic rifted margins in a comparably "simple" configuration, potentially influenced by a mantle plume, the Tristan hot spot, and the South China Sea that may represent an intermediary form of continental extension between the end member extremes. The role of (hot-spot related) volcanism during break-up will be discussed for the three example margins that evolved in the Early Cretaceous, the Paleocene and the Oligocene, respectively.
Franke, D.
Isotopic and geochemical evidence for a heterogeneous mantle plume origin of the Virunga volcanics, Western rift, East African Rift system
Virunga volcanics in the western rift of the East African Rift system (EARS) show silica-undersaturated, ultra-alkaline, alkalic-mafic compositions. The two active Virunga volcanoes, Nyiragongo and Nyamuragira, are 15�km apart. Nyiragongo shows unusual compositions not seen globally and has the lowest recorded viscosity among terrestrial magmas while Nyamuragira is unusually effusive. These volcanoes occur along the fringes of a topographic uplift
Ramananda Chakrabarti; Asish R. Basu; Alba P. Santo; Dario Tedesco; Orlando Vaselli
Rifting, Volcanism, and the Geochemical Character of the Mantle Beneath the West Antarctic Rift System (Invited)
NASA Astrophysics Data System (ADS)
The West Antarctic Rift System (WARS) is one of the largest extensional alkali volcanic provinces on Earth, but the mechanisms responsible for generating the massive amounts of its associated magmatism remain controversial. The failure of both passive and active decompression melting models to adequately explain the observed lava volumes has prompted debate about the relative roles of thermal plume-related melting and ancient subduction-related flux melting. 40Ar/39Ar dating and geochemical analyses of the lavas, as well as volatile and trace-element determinations of olivine-hosted melt inclusions shed light on the relationship between rifting and volcanism, and also improve our understanding of the geochemical character of the mantle beneath the WARS. Results show that the magmatism post-dates the main phase of extension along the Terror Rift within the WARS, which supports a decompression-melting model without the benefit of a significant thermal anomaly. However, the observed large magma volumes seem to require a volatile-fluxed mantle, a notion supported by a long history of subduction (>500 Myr) along the paleo-Pacific margin of Gondwana. In fact, the legacy of that subduction may manifest itself in the high H2O concentrations of olivine-hosted melt inclusions (up to 3 wt% in preliminary results from ion probe measurements). The major oxide compositions of lavas in the WARS are best matched to experimental melts of garnet pyroxenite and carbonated peridotite sources. The Pb and Nd isotopic systems are decoupled from each other, suggesting removal of fluid-mobile elements from the mantle source possibly during the long history of subduction along this Gondwana margin. Extremely unradiogenic 187Os/188Os ranging to as low as 0.1081 � 0.0001 hints at the involvement of lithospheric components in generation of magmas in the WARS.
Mukasa, S. B.; Aviado, K. B.; Rilling-Hall, S.; Bryce, J. G.; Cabato, J.
Tectonic controls on rift basin morphology: Evolution of the northern Malawi (Nyasa) rift
NASA Technical Reports Server (NTRS)
Radiometric (K-Ar and Ar-40/Ar-39) age determinations of volcanic and volcaniclastic rocks, combined with structural, gravity, and seismic reflection data, are used to constrain the age of sedimentary strata contained within the seismically and volcanically active northern Malawi (Nyasa) rift and to characterize changes in basin and flank morphologies with time. Faulting and volcanism within the Tukuyu-Karonga basin began at approximately 8.6 Ma, when sediments were deposited in abroad, initially asymmetric lake basin bounded on its northeastern side by a border fault system with minor topographic relief. Extensions, primarily by a slip along the border fault, and subsequent regional isostatic compensation led to the development of a 5-km-deep basin bounded by broad uplifted flanks. Along the low-relief basin margin opposite border fault, younger stratigraphic sequences commonly onlap older wedge-shaped sequences, although their internal geometry is often progradational. Intrabasinal faulting, flankuplift, and basaltic and felsic volcanism from centers at the northern end of the basin became more important at about 2.5 Ma when cross-rift transfer faults developed to link the Tukuyu-Karonga basin to the Rukwa basin. Local uplift and volcanic construction at the northern end of the basin led to a southeastward shift in the basin's depocenter. Sequence boundaries are commonly erosional along this low-relief (hanging wall) margin and conformable in the deep lake basin. The geometry of stratigraphic sequences and the distribution of the erosion indicate that horizontal and vertical crustal movements both across and along the length of the rift basin led to changes in levels of the lake, irrespective of paleoclimatic fluctuations.
Ebinger, C. J.; Deino, A. L.; Tesha, A. L.; Becker, T.; Ring, U.
[Rift Valley fever virus: evolution in progress].
Several viruses now circulating in tropical zones around the globe are potential threats for ever-increasing human populations even in temperate zones that have long remained unaffected. The mechanisms underlying transport and transmission, which can be enhanced by human activity, can be even stronger in zones where factors needed to support development of these viruses, i.e., hosts, reservoirs and vectors, are already present. This possibility has been illustrated by dengue virus, and now by the rapid spread of the Chikungunya virus on Reunion Island in 2005 and then in Italy in 2007. The spreading of Chikungunya virus despite its mild reputation had a major unexpected impact. It showed that the evolution of the virus, whether a cause or consequence of observed events, could be determinant. The risk of extension of more pathogenic viruses due to similar mechanisms must be considered as a possibility. In this regard the Rift Valley fever virus, that already involves a large area and has a major reservoir, is one of the viruses that deserves close surveillance. PMID:19702138
Tolou, H; Plumet, S; Leparc-Goffart, I; Couissinier-Paris, P
Rift basins of ocean-continent convergent margins
Modern and ancient circum-Pacific convergent margins contain many examples of forearc basins where subsidence, occurring simultaneously with subduction of oceanic lithosphere, is controlled by rifting transverse to the margin. The elongate axes of these deep and narrow basins jut obliquely from the plate margin into the interior of the forearc. Similar to aulacogens, faulting and related subsidence appear greatest at their seaward limits and decreases inland. Examples from eastern Pacific forearcs suggest that localized rifting accommodates margin-parallel extension of forearc blocks that are kinetically linked to motions along major margin-parallel strike-slip fault systems. The most prominent examples of modern forearc rift basins are the Sanak and East Sanak basins of the western Alaska Peninsula subduction zone. In this region, the continental shelf is being rifted apart by a series of northwest- and northeast-trending faults. Basement-activated normal faults bounding the basins have listric geometries. Seismostratigraphic relationships within the basins indicate the protracted, synsedimentary, and active nature of faulting and basin subsidence. Along the Peru-Chile trench, two prominent rifted basins also occur: the Gulf of Guayaquil and the Gulf of Penas-Taitao basin of southern Chile. There, margin-parallel rifting controls subsidence in localized basins at the southern terminus to margin-parallel dextral fault systems. These and other examples suggest that strike-slip motion and transverse rifting of forearcs is a common phenomenon inadequately described by existing two-dimensional models of forearcs. Margin-parallel motions of forearc blocks can be related not only to oblique plate convergence, but also to the geometric and compositional nature of the overriding and subducted plates.
Forsythe, R.D.; Newcomb, K.R.
Crust and Mantle Structure of a Closed Rift System from the Superior Province Rifting Earthscope Experiment (SPREE) (Invited)
NASA Astrophysics Data System (ADS)
The existence of the 1.1 Ga Mid-continent Rift System (MRS) in the Great Lakes region of North America is well known on account of its prominent gravity and magnetic anomalies. These elongated anomalies are associated with dense igneous rocks, which surface in sparse outcrops and are imaged in a handful of active source profiles. Part of the MRS cuts across the Archean Superior Craton while other parts cut through at least three different Proterozoic terranes, though there are indications that offsets between rift segments, such as the Belle Plaine Fault, may follow pre-existing terrane boundaries. The total volume of igneous rock imaged in active source data is consistently estimated as at least one million km3, which is enough for a sea floor of the size of the current Gulf of California, or five times the size of Lake Baikal. However, cessation of rifting and closure of the rift uplifted the igneous rocks along the axes, causing lateral gravity gradients of 150 mgal over 50 km between the gravity high above the uplifted igneous rift axis and the low above the sediment deposits in the original rift flanks. Our seismic experiment (SPREE) covers an area around a one thousand km long segment of the MRS. A long, interrupted line of stations follows the rift axis, another line cuts across this high gravity gradient, yet another line cuts across the Belle Plaine rift axis offset, and a TA-like station group north of Lake Superior complements surrounding Transportable Array coverage. The Superior Province Rifting Earthscope Flexible Array (FA) Experiment (SPREE) has been running for two years with a data return of over 96%. Preliminary SPREE and other analyses show puzzling low velocities along the rift axis and complex Moho structure beneath thickened crust. Other crustal features include a large diversity of sediments, from soggy Quaternary mud through meta-sedimentary Proterozoic rocks. At the time of writing we are quantifying the effects of this complex geological history on our seismic data and attempting to analyze and interpret the residual data in the context of this complex geologic and rich MRS studies history. We will present and discuss constraints from SPREE data on lithospheric structure beneath the MRS from receiver functions, noise analysis, surface waves, and teleseismic travel times.
van der Lee, S.; Wolin, E.; Bollmann, T. A.; Tekverk, K.
Understanding Along-strike Variations in Extension and Magmatism in Active Rifts: Discontinuous Structure Along the Main Ethiopian Rift
NASA Astrophysics Data System (ADS)
A compilation of recent geophysical and geological data reveals a discontinuity in the structure of the Main Ethiopian Rift (MER) at ~8.5�N. Recent wide-angle seismic data (from the 2003 EAGLE project) recorded along the axis of the MER show a rapid increase of crustal thickness from c. 26 km in the NE to c. 40 km in the SW at this latitude, and receiver functions recorded on the northwestern plateau show a change in crustal thickness from over 40 km in the NE to c. 33 km in the SW. The thin crust (c. 26 km) in the NE segment of the rift is markedly thinner than the adjacent rift shoulders (over 40 km), as expected for an active rift. In contrast, the thick rift crust to the SW (c. 40 km) is apparently *thicker* than the crust of the adjacent northern rift shoulder. We consider two hypotheses to explain these observations: 1. The crust within the rift valley in the SW has been thickened by magmatic processes, i.e. a high degree of magmatism (underplating) resulting from the modest extension of unusually hot mantle has led to rift-crust thickening rather than thinning; or 2. The thick crust along the active-source profile in the SW represents pre-rift crustal thickness, which the active MER has as yet barely modified. The former hypothesis is unlikely because crustal structure in the SW appears relatively unmodified by magmatic processes, e.g. there is no observed 7.x km/s layer at the base of the crust and only very slightly elevated velocities are present in the lower or upper crust. In the latter hypothesis, extension of the MER may have hardly affected the location of the wide-angle profile SW of 8.5�N; rather, this latitude represents a discontinuity between the northern MER and a distinct rift segment south of 8.5�N. Seismic tomography from EAGLE active-source and broadband data supports this hypothesis, showing crustal and mantle segmentation (between NE and SW) at this location. Along with surface geological data, these data indicate that the northern MER may originally have turned west along the Yerer-Tullu Wellel volcanotectonic lineament (YTVL), causing off-axis crustal thinning and volcanism.
Keranen, K. M.; Klemperer, S. L.
Decadal variability of rift propagation on the Amery Ice Shelf
NASA Astrophysics Data System (ADS)
The Amery Ice Shelf, East Antarctica, features five prominent rifts within 30 km of its calving front. We produce a time series of changes in rift length for the period 2002-2012 using available MODIS and MISR data. We find that all five are actively propagating, but with a complex spatio-temporal pattern of variability in which some rifts propagate in tandem while others appear to tradeoff. Temporal variability in rift propagation is dominated by large episodic bursts. These bursts, analogous to the much smaller propagation events detected from field observations, are not synchronous across all five rifts nor do the timing of propagation events exhibit any correlation with observed proxies for environmental forcing (e.g., atmospheric temperatures, sea-ice extent). However, we find that several propagation events take place after the predicted arrival from tsunamis originating in the Indian Ocean. This is especially apparent following the December 2004 Sumatra earthquake and three other earthquakes in the Sumatra/W. Indonesia area. This connection is bolstered by the observation of similar effects at other ice shelves, e.g., a large iceberg calving after the sudden propagation of two front-initiated rifts at Larsen C after the December 2004 tsunami. In comparing rift propagation at Amery with 61 rifts on 10 other ice shelves, we find that with the exception of the occasional tsunami triggered propagation event, the extreme variability on the Amery Ice Shelf is highly atypical. We postulate that the pronounced activity on the Amery is due to the fact that it last had a large calving event in 1963/64, and is approaching its pre-calved position. This suggests that the AIS is poised for another major calving event and the highly dynamic propagation we observe is the precursor to such an event. That multiple rifts exist and propagate due to structural heterogeneity and shelf geometry also makes these observations relevant to the highly fractured shells of the icy moons, which exhibit a high number of fractures and active surfaces.
Walker, C. C.; Bassis, J. N.; Czerwinski, R. J.; Fricker, H. A.
East Antarctic rifting triggers uplift of the Gamburtsev Mountains
The Gamburtsev Subglacial Mountains are the least understood tectonic feature on Earth, because they are completely hidden beneath the East Antarctic Ice Sheet. Their high elevation and youthful Alpine topography, combined with their location on the East Antarctic craton, creates a paradox that has puzzled researchers since the mountains were discovered in 1958. The preservation of Alpine topography in the Gamburtsevs may reflect extremely low long-term erosion rates beneath the ice sheet, but the mountains� origin remains problematic. Here we present the first comprehensive view of the crustal architecture and uplift mechanisms for the Gamburtsevs, derived from radar, gravity and magnetic data. The geophysical data define a 2,500-km-long rift system in East Antarctica surrounding the Gamburtsevs, and a thick crustal root beneath the range. We propose that the root formed during the Proterozoic assembly of interior East Antarctica (possibly about 1?Gyr ago), was preserved as in some old orogens and was rejuvenated during much later Permian (roughly 250?Myr ago) and Cretaceous (roughly 100?Myr ago) rifting. Much like East Africa, the interior of East Antarctica is a mosaic of Precambrian provinces affected by rifting processes. Our models show that the combination of rift-flank uplift, root buoyancy and the isostatic response to fluvial and glacial erosion explains the high elevation and relief of the Gamburtsevs. The evolution of the Gamburtsevs demonstrates that rifting and preserved orogenic roots can produce broad regions of high topography in continental interiors without significantly modifying the underlying Precambrian lithosphere.
Ferraccioli, F.; Finn, Carol A.; Jordan, Tom A.; Bell, Robin E.; Anderson, Lester M.; Damaske, Detlef
East Antarctic rifting triggers uplift of the Gamburtsev Mountains.
The Gamburtsev Subglacial Mountains are the least understood tectonic feature on Earth, because they are completely hidden beneath the East Antarctic Ice Sheet. Their high elevation and youthful Alpine topography, combined with their location on the East Antarctic craton, creates a paradox that has puzzled researchers since the mountains were discovered in 1958. The preservation of Alpine topography in the Gamburtsevs may reflect extremely low long-term erosion rates beneath the ice sheet, but the mountains' origin remains problematic. Here we present the first comprehensive view of the crustal architecture and uplift mechanisms for the Gamburtsevs, derived from radar, gravity and magnetic data. The geophysical data define a 2,500-km-long rift system in East Antarctica surrounding the Gamburtsevs, and a thick crustal root beneath the range. We propose that the root formed during the Proterozoic assembly of interior East Antarctica (possibly about 1 Gyr ago), was preserved as in some old orogens and was rejuvenated during much later Permian (roughly 250 Myr ago) and Cretaceous (roughly 100 Myr ago) rifting. Much like East Africa, the interior of East Antarctica is a mosaic of Precambrian provinces affected by rifting processes. Our models show that the combination of rift-flank uplift, root buoyancy and the isostatic response to fluvial and glacial erosion explains the high elevation and relief of the Gamburtsevs. The evolution of the Gamburtsevs demonstrates that rifting and preserved orogenic roots can produce broad regions of high topography in continental interiors without significantly modifying the underlying Precambrian lithosphere. PMID:22094700
Ferraccioli, Fausto; Finn, Carol A; Jordan, Tom A; Bell, Robin E; Anderson, Lester M; Damaske, Detlef
Crustal Velocity Structure Across the Main Ethiopian Rift
NASA Astrophysics Data System (ADS)
As part of the project EAGLE (the Ethiopia Afar Geophysical Lithospheric Experiment) a 400 km NW-SE wide-angle reflection/refraction seismic profile, was recorded across the Main Ethiopian Rift in January 2003. Instruments were deployed at a nominal spacing of 1 km and consisted primarily of single component Reftek "Texan" recorders with 3 component Guralp CMG-6TD seismometers deployed every 4 - 5 km. Eight in-line borehole shots and eleven fan shots of between 1 and 2 tonnes were recorded over a 4-day period. The data show good energy propagation along the entire length of the profile with a wide coverage of Pg, PmP and Pn phases plus at least one intra-crustal reflected phase. Considerable variation is observed in the amplitude of the reflected Moho and intra-crustal phases across the entire length of the profile. We present preliminary 2D velocity models of the lithospheric structure of the rift from the modelling of this data using both tomographic and raytrace modelling methods. These show a low velocity surface layer across the length of the profile with a clear graben-like structure beneath the centre of the Rift. This layer is likely to represent layered basalts and Mesozoic sediments beneath the plateau region and Quaternary sediments and volcanics within the Rift. An approximately 38 km thick crust, thinning slightly beneath the Rift and including at least 1 intra crustal reflector is also modelled.
MacKenzie, G. D.; Thybo, H.; Maguire, P. K.; Mammo, T.; Khan, M. A.
Estimation of Age of Dali-Ganis Rifting and Associated Volcanic Activity, Venus.
National Technical Information Service (NTIS)
This paper deals with the estimation of age for the Dali and Ganis Chasma rift zones and their associated volcanism based on photogeologic analysis of stratigraphic relations of rift-associated features with impact craters which have associated features i...
A. T. Basilevsky
From hyper-extended rifts to orogens: the example of the Maul�on rift basin in the Western Pyrenees (SW France)
NASA Astrophysics Data System (ADS)
An integral part of plate tectonic theory is that the fate of rifted margins is to be accreted into mountain belts. Thus, rift-related inheritance is an essential parameter controlling the evolution and architecture of collisional orogens. Although this link is well accepted, rift inheritance is often ignored. The Pyrenees, located along the Iberian and European plate boundary, can be considered as one of the best places to study the reactivation of former rift structures. In this orogen the Late Cretaceous and Tertiary convergence overprints a Late Jurassic to Lower Cretaceous complex intracontinental rift system related to the opening of the North Atlantic. During the rifting, several strongly subsiding basins developed in the axis of the Pyrenees showing evidence of extreme crustal extension and even locale mantle exhumation to the seafloor. Although the exact age and kinematics of rifting is still debated, these structures have an important impact in the subsequent orogenic overprint. In our presentation we discuss the example of the Maul�on basin, which escaped from the most pervasive deformations because of its specific location at the interface between the western termination of the chain and the Bay of Biscay oceanic realm. Detailed mapping combined with seismic reflection, gravity data and industry wells enabled to determine the 3D rift architecture of the Maul�on basin. Two major diachronous detachment systems can be mapped and followed through space. The Southern Maul�on Detachment (SMD) develops first, starts to thin the crust and floors the Southern Maul�on sub-Basin (SMB). The second, the Northern Maul�on Detachment (SMD) is younger and controls the final crustal thinning and mantle exhumation to the north. Both constitute the whole Maul�on basin. Like at the scale of the overall Pyrenees, the reactivation of the Maul�on Basin increases progressively from west to east, which enables to document the progressive reactivation of an aborted hyper-extended rift system. In our presentation, we discuss the compressional reactivation of the rift structures by the study of dip sections across the basin, from weakly reactivated sections in the west to strongly reactivated sections in the east. Comparing the sections, it results that the compression reactivated the rift structures (mainly the detachment faults) and that this reactivation occurred in 2 steps. It corresponds to the reactivation through time of the NMB before the SMB. This evolution is in line with an early proto-subduction of the hyper-extended domain beneath the European plate whereas the NMB sediments are wedged, folded and thrust onto the Iberia and Europe margins ("thin-skin" tectonics). The second step occurs when the deformation started to migrate southward resulting in the formation of the axial Pyrenees nappe stack (thick-skin tectonics). These results suggest that the inherited rift structures strongly controlled the initial convergence. Future work will revisit the more reactivated Albian basins throughout the chain to investigate how far the results from western Pyrenees can be used to understand the Central and Eastern Pyrenees. Moreover, this field-oriented study can serve as an example of how rift structures may control style and timing of orogenic processes.
Masini, E.; Manatschal, G.; Tugend, J.
Volcanism, tectonism, sedimentation, and the paleoanthropological record in the Ethiopian Rift System
The Ethiopian Rift System consists of basins that are in different stages of evolu- tion. Some of the rift-related basins in southwestern Ethiopia are half-grabens that have not evolved to symmetrical rifts since the initiation of rifting here in the middle Miocene. These basins contain fossiliferous Pliocene-Pleistocene volcaniclastic sedi- ments and volcanic rocks and have been occupied by early hominid
Giday WoldeGabriel; Grant Heiken; Tim D. White; Berhane Asfaw; William K. Hart; Paul R. Renne
Patterns of Rift Valley fever activity in Zambia.
An hypothesis that there was an annual emergence of Rift Valley fever virus in Zambia, during or after the seasonal rains, was examined with the aid of sentinel cattle. Serum samples taken during 1974 and 1978 showed evidence of epizootic Rift Valley fever in Zambia, with more than 80% positive. A sentinel herd exposed from 1982 to 1986 showed that some Rift Valley fever occurred each year. This was usually at a low level, with 3-8% of the susceptible cattle seroconverting. In 1985-6 more than 20% of the animals seroconverted, and this greater activity was associated with vegetational changes--which could be detected by remote-sensing satellite imagery--which have also been associated with greater virus activity in Kenya. PMID:1547835
Davies, F G; Kilelu, E; Linthicum, K J; Pegram, R G
The First Prediction of a Rift Valley Fever Outbreak
NASA Technical Reports Server (NTRS)
El Nino/Southern Oscillation (ENSO) related anomalies were analyzed using a combination of satellite measurements of elevated sea surface temperatures, and subsequent elevated rainfall and satellite derived normalized difference vegetation index data. A Rift Valley fever risk mapping model using these climate data predicted areas where outbreaks of Rift Valley fever in humans and animals were expected and occurred in the Horn of Africa from December 2006 to May 2007. The predictions were subsequently confirmed by entomological and epidemiological field investigations of virus activity in the areas identified as at risk. Accurate spatial and temporal predictions of disease activity, as it occurred first in southern Somalia and then through much of Kenya before affecting northern Tanzania, provided a 2 to 6 week period of warning for the Horn of Africa that facilitated disease outbreak response and mitigation activities. This is the first prospective prediction of a Rift Valley fever outbreak.
Anyamba, Assaf; Chretien, Jean-Paul; Small, Jennifer; Tucker, Compton J.; Formenty, Pierre; Richardson, Jason H.; Britch, Seth C.; Schnabel, David C.; Erickson, Ralph L.; Linthicum, Kenneth J.
Patterns of Rift Valley fever activity in Zambia.
An hypothesis that there was an annual emergence of Rift Valley fever virus in Zambia, during or after the seasonal rains, was examined with the aid of sentinel cattle. Serum samples taken during 1974 and 1978 showed evidence of epizootic Rift Valley fever in Zambia, with more than 80% positive. A sentinel herd exposed from 1982 to 1986 showed that some Rift Valley fever occurred each year. This was usually at a low level, with 3-8% of the susceptible cattle seroconverting. In 1985-6 more than 20% of the animals seroconverted, and this greater activity was associated with vegetational changes--which could be detected by remote-sensing satellite imagery--which have also been associated with greater virus activity in Kenya.
Davies, F. G.; Kilelu, E.; Linthicum, K. J.; Pegram, R. G.
Submarine thermal springs on the Gala??pagos Rift
The submarine hydrothermal activity on and near the Gala??pagos Rift has been explored with the aid of the deep submersible Alvin. Analyses of water samples from hydrothermal vents reveal that hydrothermal activity provides significant or dominant sources and sinks for several components of seawater; studies of conductive and convective heat transfer suggest that two-thirds of the heat lost from new oceanic lithosphere at the Gala??pagos Rift in the first million years may be vented from thermal springs, predominantly along the axial ridge within the rift valley. The vent areas are populated by animal communities. They appear to utilize chemosynthesis by sulfur-oxidizing bacteria to derive their entire energy supply from reactions between the seawater and the rocks at high temperatures, rather than photosynthesis. Copyright ?? 1979 AAAS.
Corliss, J. B.; Dymond, J.; Gordon, L. I.; Edmond, J. M.; Von Herzen, R. P.; Ballard, R. D.; Green, K.; Williams, D.; Bainbridge, A.; Crane, K.; Van Andel, T. H.
Constraints on rift thermal processes from heat flow and uplift
NASA Technical Reports Server (NTRS)
The implications of heat flow data available from five major Cenozoic continental rift systems for the processes of continental rifting are discussed, and simple thermal models of lithospheric thinning which predict uplift are used to further constrain the thermal processes in the lithosphere during rifting. Compilations of the heat flow data are summarized and the salient results of these compilations are briefly discussed. The uplift predictions of the slow and rapid thinning models, in which thinning is assumed to occur at a respectively slower and faster rate than heat can be conducted into the lithosphere, are presented. Comparison of uplift rates with model results indicates that the lithosphere is in a state between the two models. While uplift is predicted to continue after thinning has ceased due to thermal relaxation of the lithosphere, the rapid thinning model is always predicted to apply to surface heat flow, and an anomaly in this flow is not predicted to develop until after thinning has stopped.
Morgan, P.
Petroleum system of the Shelf Rift Basin, East China Sea
The Tertiary section of the Oujioang and Quiontang Depressions of the East China Sea Basin consists of at least eight rift-related depositional sequences identified seismically by regionally significant onlap and truncation surfaces. These sequences are calibrated by several wells including the Wenzhou 6-1-1 permitting extrapolation of petroleum system elements using seismic facies analysis. Gas and condensate correlated to non-marine source rocks and reservoired in sandstone at the Pinghu field to the north of the study area provides an known petroleum system analogue. In the Shelf Rift Basin, synrift high-amplitude parallel reflections within the graben axes correlate with coaly siltstone strata and are interpreted as coastal plain and possibly lacustrine facies with source rock potential. Synrift clinoform seismic facies prograding from the northwest footwall correlate with non-marine to marginal marine conglomerate, sandstone and siltstone, and are interpreted as possible delta or fan-delta facies with reservoir potential although porosity and permeability is low within the Wenzhou 6-1-1 well. Post-rift thermal sag sequences are characterized by parallel and relatively continuous seismic reflections and locally developed clinoform packages. These facies correlate with porous and permeable marine sandstone and siltstone. Shales of potential sealing capacity occur within marine flooding intervals of both the synrift and post-rift sequences. Traps consist of differentially rotated synrift fill, and post-rift inversion anticlines. Major exploration risk factors include migration from the synrift coaly source rocks to the post-rift porous and permeable sandstones, and seismic imaging and drilling problems associated with extensive Tertiary igneous intrusions.
Cunningham, A.C.; Armentrout, J.M.; Prebish, M. (Mobil Oil Corp., Dallas, TX (United States)) (and others)
Fault Architecture Within the Eastern Terror Rift, Western Ross Sea, Antarctica
The Terror Rift is a zone of intraplate deformation within the Antarctica plate. Association with Late Cenozoic volcanic rocks and fault scarps on the seafloor indicates neotectonic to recent activity. Intraplate extension and transtension associated with regional strike-slip deformation have both been proposed for the Terror Rift by previous workers. Marine seismic data across the Terror Rift, western Ross Sea,
J. M. Hall; T. J. Wilson; S. Henrys
Neotectonic Structure of Terror Rift, Western Ross Sea, Antarctica: Initial Interpretations of New Geophysical Data
The Terror Rift in the western Ross Sea is a prominent neotectonic element of the West Antarctic Rift, yet insufficient data have been available to define the geometries of faults and volcanic bodies or to constrain rift timing in any detail. The RVIB Nathaniel B. Palmer completed a geophysical cruise in the western Ross Sea in early 2004 in order
T. J. Wilson; L. A. Lawver; S. Henrys
Significant crustal thinning beneath the Baikal rift zone: New constraints from receiver function analysis
Thinning of the crust of more than 10 km is a major feature of typical continental rifts such as the East African (EAR) and Rio Grande (RGR) rifts. However, numerous previous studies across the Baikal rift zone (BRZ), which has similar surface expressions and tectonic history, and more active seismicity relative to EAR and RGR, have resulted in contradicting amount
Stephen S. Gao; Kelly H. Liu; Chizheng Chen
Oldest Homo and Pliocene biogeography of the Malawi Rift.
The Malawi Rift and Pliocene palaeofaunas, which include a hominid mandible attributed to Homo rudolfensis, provide a biogeographical link between the better known Plio-Pleistocene faunal records of East and Southern Africa. The Malawi Rift is in a latitudinal position suitable for recording any hominid and faunal dispersion towards the Equator that was brought on by increased aridity of the Late Pliocene African landscape. The evidence suggests that Pliocene hominids originated in the eastern African tropical domain and dispersed to southern Africa only during more favourable ecological circumstances. PMID:8413666
Schrenk, F; Bromage, T G; Betzler, C G; Ring, U; Juwayeyi, Y M
Lithosphere Response to Intracratonic Rifting: Examples from Europe and Siberia
NASA Astrophysics Data System (ADS)
Several cratons have experienced a significant modification of their crustal and mantle lithosphere structure during Phanerozoic large-scale lithosphere-mantle interactions. In Eurasia, the most prominent examples include the Dniepre-Donets rift in the East European craton, the Oslo graben in the Baltic shield, the Viluy rift and the Baikal rift in Siberia. Despite some similarities, mostly in the crustal structure, there are also significant differences in the lithospheric structure of these rifts. Besides, a large lithosphere-scale Riphean suture/rift runs across the East European craton. While this suture can be recognized in the crustal structure, it is not clearly seen in the structure of the lithospheric mantle. In contrast, Phanerozoic processes associated with emplacement of large magma volumes had a strong effect on modification of the lithosphere structure, primarily by infiltration of basaltic magmas and consequently in a change in mantle density and seismic velocities. Although kimberlite magmatism is commonly not considered as a rifting events, its deep causes may be similar to the mantle-driven rifting and, as a consequence, modification of mantle density and velocity structure may also be expected. We present a new model for the structure of the crust in an area that encompasses the East European craton, the West Siberian basin, and the Siberian cratons. The region includes a nearly continuous age record for lithosphere evolution over ca. 3.6-3.8 billion years. The crustal model is based on critically assessed results from various seismic studies, including reflection and refraction profiles and receiver function studies. We also use global shear-wave tomography models, gravity constraints based on GOCE data, and thermal models for the lithosphere to speculate on thermo-chemical heterogeneity of the mantle. An analysis of the lithosphere structural heterogeneity is presented in relation to geotectonic setting and mantle geodynamics based on interpretations of geophysical models. The results indicate that there is no simple rule for lithosphere modification by intracontinental rifting. The pattern of is controlled by the pre-existing tectonic setting and the intensity of lithosphere-mantle interaction. The results are summarized in a series of maps of lateral variations in lithosphere structure, including the depth to the LAB and compositional heterogeneity of the lithosphere as reflected in it seismic wave velocity and density structure.
Artemieva, I. M.; Thybo, H.; Herceg, M.; Cherepanova, Y. V.; Chemia, Z.; Cammarano, F.
Sensitivity of the East African rift lakes to climate variability
NASA Astrophysics Data System (ADS)
Lakes in the East African Rift have provided excellent proxies to reconstruct past climate changes in the low latitudes. The lakes occupy volcano-tectonic depressions with highly variable climate and hydrological setting, that present a good opportunity to study the climatic and hydrogeological influences on the lake water budget. Previous studies have used lake floor sediments to establish the sensitivity of the East African rift lakes. This study focuses on geomorphology and climate to offer additional or alternative record of lake history that are key to quantifying sensitivity of these lakes as archives to external and internal climatic forcings. By using the published Holocene lake areas and levels, we analyze twelve lakes on the eastern arm of the East African rift; Ziway, Awassa, Turkana, Suguta, Baringo, Nakuru, Elmenteita, Naivasha, Natron, Manyara and compare with Lake Victoria, that occupies the plateau between the east and the western arms of the rift. Using the SRTM data, Hypsometric (area-altitude) analysis has been used to compare the lake basins between latitude 80 North and 30 South. The mean elevation for the lakes, is between 524 and 2262 meters above sea level, the lakes' hypsometric integrals (HI), a measure of landmass volume above the reference plane, vary from 0.31 to 0.76. The aridity index (Ai), defined as Precipitation/ Evapotranspiration, quantifies the water available to a lake, it encompasses land cover and climatic effects. It is lowest (arid) in the basin between the Ethiopian rift and the Kenyan rift and at the southern termination of the Kenyan Rift in the catchments of lake Turkana, Suguta, Baringo and Manyara with values of 0.55, 0.43, 0.43 and 0.5 respectively. And it is highest (wet) in the catchments of, Ziway, Awassa, Nakuru and Naivasha as 1.33,1.03 and 1.2 respectively, which occupy the highest points of the rift. Lake Victoria has an index of 1.42 the highest of these lakes and receives a high precipitation. We use a simple model written on a Matlab code to illustrate the lake volume and area response to climate of surficialy closed, graben shaped and panshaped lake basins. From preliminary results, lake basins that are sensitive to climate variability have a high HI and high aridity index, which will be presented in this conference
Olaka, L.; Trauth, M. H.
Geophysical studies of the West Antarctic Rift System
NASA Astrophysics Data System (ADS)
The West Antarctic rift system extends over a 3000 � 750 km, largely ice covered area from the Ross Sea to the base of the Antarctic Peninsula, comparable in area to the Basin and Range and the East African rift system. A spectacular rift shoulder scarp along which peaks reach 4-5 km maximum elevation marks one flank and extends from northern Victoria Land-Queen Maud Mountains to the Ellsworth-Whitmore-Horlick Mountains. The rift shoulder has maximum present physiographic relief of 5 km in the Ross Embayment and 7 km in the Ellsworth Mountains-Byrd Subglacial Basin area. The Transantarctic Mountains part of the rift shoulder (and probably the entire shoulder) has been interpreted as rising since about 60 Ma, at episodic rates of �1 km/m.y., most recently since mid-Pliocene time, rather than continuously at the mean rate of 100 m/m.y. The rift system is characterized by bimodal alkaline volcanic rocks ranging from at least Oligocene to the present. These are exposed asymmetrically along the rift flanks and at the south end of the Antarctic Peninsula. The trend of the Jurassic tholeiites (Ferrar dolerites, Kirkpatric basalts) marking the Jurassic Transantarctic rift is coincident with exposures of the late Cenozoic volcanic rocks along the section of the Transantarctic Mountains from northern Victoria Land to the Horlick Mountains. The Cenozoic rift shoulder diverges here from the Jurassic tholeiite trend, and the tholeiites are exposed continuously (including the Dufek intrusion) along the lower- elevation (1-2 km) section of Transantarctic Mountains to the Weddell Sea. Widely spaced aeromagnetic profiles in West Antarctica indicate the absence of Cenozoic volcanic rocks in the ice covered part of the Whitmore-Ellsworth-Mountain block and suggest their widespread occurrence beneath the western part of the ice sheet overlying the Byrd Subglacial Basin. A German Federal Institute for Geosciences and Natural Resources (BGR)-U.S. Geological Survey (USGS) aeromagnetic survey over the Ross Sea continental shelf indicates rift fabric and suggests numerous submarine volcanoes along discrete NNW trending zones. A Bouguer anomaly range of approximately 200 (+50 to -150) mGal having 4-7 mGal/km gradients where measured in places marks the rift shoulder from northern Victoria Land possibly to the Ellsworth Mountains (where data are too sparse to determine maximum amplitude and gradient). The steepest gravity gradients across the rift shoulder require high density (mafic or ultramafic?) rock within the crust as well as at least 12 km of thinner crust beneath the West Antarctic rift system in contrast to East Antarctica. Sparse land seismic data reported along the rift shoulder, where velocities are greater than 7 km/s, and marine data indicating velocities above 7 km/s beneath the Ross Sea continental shelf support this interpretation. The maximum Bouguer gravity range in the Pensacola Mountains area of the Transantarctic Mountains is only about 130 mGal with a maximum 2 mGal/km gradient, which can be explained solely by 8 km of crustal thickening. Large offset seismic profiles over the Ross Sea shelf collected by the German Antarctic North Victoria Land Expedition V (GANOVEX V) combined with earlier USGS and other results indicate 17-21 km thickness for the crust beneath the Ross Sea shelf which we interpret as evidence of extended rifted continental crust. A regional positive Bouguer anomaly (0 to +50 mGal), the width of the rift, extends from the Ross Sea continental shelf throughout the Ross Embayment and Byrd Subglacial Basin area of the West Antarctic rift system and indicates that the Moho is approximately 20 km deep tied to the seismic results (probably coincident with the top of the asthenosphere) rather than the 30 km reported in earlier interpretations. The interpretation of horst and graben structures in the Ross Sea, made from marine seismic reflection data, probably can be extended throughout the rift (i.e., the Ross Ice shelf and the Byrd Subglacial Basin areas). The near absence of earthquakes in the West Antarctic r
Behrendt, J. C.; Lemasurier, W. E.; Cooper, A. K.; Tessensohn, F.; Tr�Hu, A.; Damaske, D.
How Magmatic are Magma-Poor Rifted Margins?
NASA Astrophysics Data System (ADS)
The results of the Ocean Drilling Program (ODP) together with seismic reflection and refraction surveys from the West Iberia and East Greenland rifted margins show a divergent style of margin architecture and evolution in which quantity and distribution of syn-rift magmatism and fault structures are the most variable features. These observations led to an oversimplified classification of rifted margins as either volcanic or non-volcanic. Although this simple concept may lead to the idea that margins evolve either under the presence or absence of magma, the available results show that so called �non-volcanic� margins are not necessarily amagmatic. This leads to the question about nature and timing of magmatic processes in �non�-volcanic or magma-poor rifted margins and their role in controlling the rheology, thermal structure and isostatic evolution of these margins during final rifting and continental breakup. In our presentation, we review results from the Iberia/Newfoundland and Alpine Tethys rifted margins with the aim to discuss their magmatic evolution during final rifting and onset of seafloor spreading. Along these margins the volume of magma increases oceanwards and the emplacement processes are either controlled by infiltration via porous flow, dyking or extrusion at the seafloor. Based on field relationships, direct dating of magmatic rocks or related sediments it can be shown that: 1) infiltration predates mantle exhumation at the seafloor, 2) the emplacement of intrusive and extrusive magmatic rocks of MOR composition post date first mantle exhumation and marks the onset of seafloor spreading, and 3) alkaline magmas are occasionally emplaced in the Ocean Continent Transition even after onset of seafloor spreading. Thus, the magmatic processes observed on these margins are complex and polyphase. The observations suggest that entirely non-magmatic margins do not exist. However, it remains unclear whether decompression melting is the driving force or rather the consequence of extension. In our presentation we will discuss to what extent magma may control the rheology, thermal structure and isostatic evolution of magma-poor rifted margins during continental breakup.
Manatschal, G.; Muntener, O.
Intermittent upwelling of asthenosphere beneath the Gregory Rift, Kenya
K-Ar dates and chemical compositions of basalts in the Gregory Rift, Kenya, demonstrate marked secular variation of lava chemistry. Two magmatic cycles characterized by incompatible element relative depletion are recognized; both occurring immediately after the peak of basaltic volcanism and coeval with both trachyte/phonolite volcanism and domal uplift of the region. These cycles may be attributed to increasing degree of partial melting of mantle source material in association with thinning of the lithosphere by thermal erosion through contact with hot upwelling asthenospheric mantle. Cyclic variation in asthenosphere upwelling may be considered an important controlling process in the evolution of the Gregory Rift.
Tatsumi, Yoshiyuki (Univ. of Tasmania (Australia) Kyoto Univ. (Japan)); Kimura, Nobukazu (Kyoto Univ. (Japan)); Itaya, Tetsumaru (Okayama Univ. of Science (Japan)); Koyaguchi, Takehiro (Kumamoto Univ. (Japan)); Suwa, Kanenori (Nagoya Univ. (Japan))
Depositional and tectonic framework of the rift basins of Lake Baikal from multichannel seismic data
Recent multichannel seismic reflection data from Lake Baikal, located in a large, active, continental rift in central Asia, image three major stratigraphic units totalling 3.5 to 7.5 km thick in four subbasins. A major change in rift deposition and faulting between the oldest and middle-rift units probably corresponds to the change from slow to fast rifting. A brief comparison of the basins of Lake Baikal with those of the East African rift system highlights differences in structural style that can be explained by differences in age and evolution of the surrounding basement rocks. -from Authors
Hutchinson, D. R.; Golmshtok, A. J.; Zonenshain, L. P.; Moore, T. C.; Scholz, C. A.; Klitgord, K. D.
Genetic Reassortment of Rift Valley Fever Virus in Nature
Rift Valley fever virus (RVFV), a phlebovirus of the Bunyaviridae family, is an arthropod-borne virus which emerges periodically throughout Africa, emphasizing that it poses a major threat for animal and human populations. To assess the genetic variability of RVFV, several isolates from diverse localities of Africa were investigated by means of reverse transcription-PCR followed by direct sequencing of a region
Sensitivity of the East African rift lakes to climate variability
Lakes in the East African Rift have provided excellent proxies to reconstruct past climate changes in the low latitudes. The lakes occupy volcano-tectonic depressions with highly variable climate and hydrological setting, that present a good opportunity to study the climatic and hydrogeological influences on the lake water budget. Previous studies have used lake floor sediments to establish the sensitivity of
L. Olaka; M. H. Trauth
Seismicity of the Baikal rift system from regional network observations
NASA Astrophysics Data System (ADS)
In the paper we report the state-of-the-art of seismicity study in the Baikal rift system and the general results obtained. At present, the regional earthquake catalog for fifty years of the permanent instrumental observations consists of over 185,000 events. The spatial distribution of the epicenters, which either gather along well-delineated belts or in discrete swarms is considered in detail for different areas of the rift system. At the same time, the hypocenters are poorly constrained making it difficult to identify the fault geometry. Clustered events like aftershock sequences or earthquake swarms are typical patterns in the region; moreover, aftershocks of M ? 4.7 earthquakes make up a quarter of the whole catalog. The maximum magnitude of earthquakes recorded instrumentally is MLH7.6 for a strike-slip event in the NE part of the Baikal rift system and MLH6.8 for a normal fault earthquake in the central part of the rift system (Lake Baikal basin). Predominant movement type is normal faulting on NE striking faults with a left lateral strike-slip component on W-E planes. In conclusion, some shortcomings of the seismic network and data processing are pointed out.
Radziminovich, N. A.; Gileva, N. A.; Melnikova, V. I.; Ochkovskaya, M. G.
Thermal and Mechanical Development of the East African Rift System.
National Technical Information Service (NTIS)
The deep basins, uplifted flanks, and volcanoes of the Western and Kenya rift systems have developed along the western and eastern margins of the 1300 km-wide East African plateau. Structural patterns deduced from field, Landsat, and geophysical studies i...
C. E. Ebinger
Hydrothermal vents in Lake Tanganyika, East African, Rift system
Sublacustrine hydrothermal vents with associated massive sulfides were discovered during April 1987 at Pemba and Cape Banza on the Zaire side of the northern basin of Lake Tanganyika, East African Rift system. New investigations by a team of ten scuba divers during the multinational (France, Zaire, Germany, and Burundi) TANGANYDRO expedition (August-October 1991) found hydrothermal vents down to a depth
Jean-Jacques Tiercelin; Catherine Pflumio; Maryse Castrec; Jacques Boul�gue; Pascal Gente; Jo�l Rolet; Christophe Coussement; Karl O. Stetter; Robert Huber; Sony Buku; Wafula Mifundu
Helium isotopes in fluids of the Baykal rift zone
The isotopic composition of helium in subterranean fluids from 34 locations in southern Siberia and northern Mongolia has been measured. The ratio [sup 3]He/[sup 4]He ranges from radiogenic ([approximately]10[sup [minus]8]) to mantle ([approximately]10[sup [minus]5]) values. For the most part, the hydrocarbon fluids of the Irkutsk amphitheater contain only radiogenic helium; this distinguishes the amphitheater from ancient platforms. In contrast, fluids from the Baykal rift zone are distinguished by the presence of helium with a mantle component; this component reaches almost 100% in thermal fluids in the Tunka basin. The ratios CH[sub 4]/[sup 3]He and (CO[sub 2] + CH[sub 4])/[sup 3]He suggest that the methane and carbon dioxide in the rift zone fluids may be partially magmatogenic; in the Irkutsk amphitheater, on the other other hand, such gases originated as a result of purely crustal processes. The [sup 3]He/[sup 4]He ratio varies along the length of the Baikal rift zone, with decreasing values correlating with decreasing heat flow, increasing thickness of crust, and decreasing dimensions of rift basins. 39 refs., 4 figs., 3 tabs.
Polyak, B.G.; Prasolov, E.M.; Tolstikhin, I.N.; Kozlovtseva, S.V.; Kononov, V.I.; Khutorskoy, M.D. (Inst. of Geology, Moscow (Russian Federation) All-Union Petroleum Geologic Exploration Research Inst., Saint Petersburg (Russian Federation) Kola Research Center, Apatity (Russian Federation))
Late cretaceous and cenozoic stratigraphy of the Baikal Rift sediments
NASA Astrophysics Data System (ADS)
The data obtained from long-term field studies in the Baikal Rift area are summarized. A new stratigraphic scheme is developed on the basis of previous stratigraphic research of N.A. Logachev. The new elements of the scheme are (1) the use of regional correlation horizons; (2) recognition of pre-Tankhoi (pre-Late Oligocene) sediments correlated with the Maastrichtian-Early Oligocene deposits of the Baikal Fore-deep; (3) elimination from the scheme of the Khalagai and Anosovka formations and distinction on their basis of the Tagai, Sasa, Osinovka, and Shankhaikha formations; (4) recognition of several weathering crust beds and Neogene paleosols. The "lower Eopleistocene (Upper Pliocene)" red-rock formation of Logachev is subdivided into the following stratigraphic units: the Cretaceous-Paleogene unit characterized by a few finds of Early Oligocene fossils, the Upper Miocene-Lower Pliocene red clay bearing numerous fossil remains, and the Upper Pliocene reddish clay with abundant localities of fossils. The sections examined in the land portion of the Baikal Rift are correlated with bottom sediments of the Baikal depression and are subdivided into three instead of the two commonly accepted large tectonic-lithological-stratigraphic complexes. Stratigraphic studies provide a new insight into the history of the Baikal Rift and into some general questions of the continental rift formation.
Mats, V. D.
Clastic rocks associated with the Midcontinent rift system in Iowa
The Middle Proterozoic Midcontinent Rift System (MRS) of North America is a failed rift that formed in response to region-wide stresses about 1,100 Ma. In Iowa, the MRS is buried beneath 2,200?3,500 ft of Paleozoic and Mesozoic sedimentary rocks and Quaternary glaciogenic deposits. An extremely large volume of sediments was deposited within basins associated with the rift at several stages during its development. Although the uplift of a rift-axial horst resulted in the erosional removal of most of these clastic rocks from the central region of the MRS in Iowa, thick sequences are preserved in a series of horst-bounding basins. Recent studies incorporating petrographic analysis, geophysical modeling, and other analytical procedures have led to the establishment of a preliminary stratigraphy for these clastic rocks and interpretations of basin geometries. This information has allowed the refinement of existing theories and history of MRS formation in Iowa. Additionally, drill samples previously interpreted as indicating the existence of early Paleozoic basins overlying the Proterozoic MRS basins were re-examined. Samples previously interpreted as deep-lying Paleozoic rocks are now known to have caved from upper levels of the drillhole and were out of stratigraphic position. No deep Paleozoic basins exist in this area. These investigations led to the development of petrographic parameters useful in differentiating the Proterozoic MRS Red clastics from Paleozoic clastic rocks having similar lithologies.
Anderson, Raymond R.; McKay, Robert M.
Magmatism of the Kenya Rift Valley: a review
Tertiary� Recent magmatism in the Kenya Rift Valley was initiated c. 35Ma, in the northern part of Kenya. Initiation of magmatism then migrated southwards, reaching northern Tanzania by 5� 8Ma. This progression was accompanied by a change in the nature of the lithosphere, from rocks of the Panafrican Mozambique mobile belt through reworked craton margin to rigid, Archaean craton. Magma
Ray Macdonald
Surveillance for Rift Valley Fever in Egypt During 1983.
National Technical Information Service (NTIS)
In 1977, Egypt experienced its first recorded outbreak of Rift Valley Fever (RVF). Since the last isolation of RVF virus in 1981, continued transmission of RVF in Egypt has been disputed. A surveillance system was established by NAMRU-3 in 1982 using acce...
B. A. Botros P. W. Mellick A. W. Salib A. K. Soliman M. T. Dalam
Postspreading rifting in the Adare Basin, Antarctica: Regional tectonic consequences
Extension during the middle Cenozoic (43�26 Ma) in the north end of the West Antarctic rift system (WARS) is well constrained by seafloor magnetic anomalies formed at the extinct Adare spreading axis. Kinematic solutions for this time interval suggest a southward decrease in relative motion between East and West Antarctica. Here we present multichannel seismic reflection and seafloor mapping data
R. Granot; S. C. Cande; J. M. Stock; F. J. Davey; R. W. Clayton
Shear-wave splitting in the Rio Grande Rift
Lag times and fast directions associated with SKS shear-wave splitting are determined for six digital stations in the Rio Grande Rift (RGR). The mean fast direction for three stations in the central RGR is about 45 deg oblique to the axis of the RGR. The pattern of the fast directions along the RGR gives evidence for a rising asthenosphere beneath
Eric Sandvol; James Ni; Serdar Ozalaybey; John Schlue
Masirah Graben, Oman: A hidden Cretaceous rift basin
Reflection seismic data, well data, geochemical data, and surface geology suggest that a Cretaceous rift basin exists beneath the thrusted allochthonous sedimentary sequence of the Masirah graben, Oman. The Masirah graben is located east of the Huqf uplift, parallel to the southern coast of Oman. The eastern side of the northeast-trending Huqf anticlinorium is bounded by an extensional fault system that is downthrown to the southeast, forming the western edge of the Masirah graben. This graben is limited to the east by a large wedge of sea floor sediments and oceanic crust, that is stacked as imbricate thrusts. These sediments/ophiolites were obducted onto the southern margin of the Arabian plate during the collision of the Indian/Afghan plates at the end of the Cretaceous. Most of the Masirah graben is covered by an allochthonous sedimentary sequence, which is complexly folded and deformed above a detachment. This complexly deformed sequence contrasts sharply with what is believed to be a rift sequence below the ophiolites. The sedimentary sequence in the Masirah graben was stable until further rifting of the Arabian Sea/Gulf of Aden in the late Tertiary, resulting in reactivation of earlier rift-associated faults. Wells drilled in the Masirah graben in the south penetrated reservoir quality rocks in the Lower Cretaceous Natih and Shuaiba carbonates. Analyses of oil extracted from Infracambrian sedimentary rocks penetrated by these wells suggest an origin from a Mesozoic source rock.
Beauchamp, W.H. [Cornell Univ., Ithaca, NY (United States); Ries, A.C. [Ries-Coward Associates Ltd., Caversham (United Kingdom); Coward, M.P. [Imperial College, London (United Kingdom)] [and others
Continental rifting across the southern Gulf of California
NASA Astrophysics Data System (ADS)
This study of continental rifting in the Gulf of California has two aims: (1) To better understand the tectonic evolution of the Gulf of California, and (2) To gain insight into continental rifting processes. The combination of seismic reflection and refraction techniques provides a robust method to analyze crustal structure, allowing for the amount, style, and temporal evolution of continental extension over conjugate rifted margins to be assessed. Multi-channel seismic data spanning 600 km image basins created by upper crustal extension. These basins appear to have formed in two phases: an initial phase beginning at or near 14 Ma, forming large basins and a second phase beginning around 6 Ma forming smaller half-grabens. There is a reflective, ropey layer seen mantling basement along much of the transect; modeling of velocity-depth profiles in the upper crust show that this layer has a velocity of 2.5 km/s and is a few hundred meters thick. This layer is interpreted as the arc-related 20-11 Ma Comondu formation, and its continuity across the Tamayo Bank and Trough suggests it was deposited either synchronous or after faulting of the Tamayo Trough. This provides compelling evidence for onset of NW-SE oriented extension in the Gulf before 11 Ma. Refraction data spanning an 881 km profile over conjugate rifted margins between the Baja peninsula and mainland Mexico were used to construct a seismic velocity model. This model shows that the crust has experienced an overall pure shear mode of extension and, on average, has thinned to half its original thickness. Estimated total opening across the southern Gulf of California is 495 km, indicating an alternative tectonic evolution characterized by oblique NW-SE extension and dextral slip in the Gulf since �14 Ma. Average lower crustal velocity of �6.3 km/s on both margins indicates there was no large-scale magmatism during rifting, and significant Moho topography suggests brittle deformation of the whole crust and no lower crustal flow, indicating rifting of cold lithosphere. However, some magmatism is seen at the in the regions of greatest crustal thinning at the continent-ocean transition and allowed for the transition from rifting to seafloor spreading.
Sutherland, Fiona Helen
The Pongola structure of southeastern Africa - The world's oldest preserved rift?
NASA Technical Reports Server (NTRS)
Rocks of the Pongola Supergroup form an elongate belt in the Archean Kaapvaal Craton of southern Africa. Because these rocks exhibit many features that are characteristic of rocks deposited in continental rifts, including rapid lateral variations in thickness and character of sediments, volcanic rocks that are bimodal in silica content, coarse, basement derived conglomerates and thick sequences of shallow water sedimentary facies associations, it is suggested that the Pongola Supergroup was deposited in such a rift. The age of these rocks (approximately 3.0 Ga) makes the Pongola structure the world's oldest well-preserved rift so far recognized, and comparison of the Pongola Rift with other rifts formed more recently in earth history reveals striking similarities, suggesting that the processes that formed this rift were not significantly different from those that form continental rifts today.
Burke, K.; Kidd, W. S. F.; Kusky, T. M.
Hydrothermal vents in Lake Tanganyika, East African, Rift system
NASA Astrophysics Data System (ADS)
Sublacustrine hydrothermal vents with associated massive sulfides were discovered during April 1987 at Pemba and Cape Banza on the Zaire side of the northern basin of Lake Tanganyika, East African Rift system. New investigations by a team of ten scuba divers during the multinational (France, Zaire, Germany, and Burundi) TANGANYDRO expedition (August-October 1991) found hydrothermal vents down to a depth of 46 m along north-trending active faults bounding the Tanganyika rift on the western side. Temperatures from 53 to 103 �C were measured in hydrothermal fluids and sediments. Veins of massive sulfides 1-10 cm thick (pyrite and marcasite banding) were found associated with vents at the Pemba site. At Cape Banza,active vents are characterized by 1-70-cm-high aragonite chimneys, and there are microcrystalline pyrite coatings on the walls of hydrothermal pipes. Hydrothermal fluid end members show distinctive compositions at the two sites. The Pemba end member is a NaHCO3-enriched fluid similar to the NaHCO3 thermal fluids from lakes Magadi and Bogoria in the eastern branch off the rift. The Cape Banza end member is a solution enriched in NaCl. Such brines may have a deep-seated basement origin, as do the Uvinza NaCl brines on the eastern flank of the Tanganyika basin. Geothermometric calculations have yielded temperatures of fluid-rock interaction off 219 and 179 �C in the Pemba and Cape Banza systems, respectively. Abundant white or reddish-brown microbial colonies resembling Beggiatoa mats were found surrounding the active vents. Thermal fluid circulation is permitted by opening of cracks related to 130 �N normal-dextral faults that intersect the north- south major rift trend. The source of heat for such hydrothermal systems may relate to the existence of magmatic bodies under the rift, which is suggested by the isotopic composition of carbon dioxide released at Pemba and Cape Banza.
Tiercelin, Jean-Jacques; Pflumio, Catherine; Castrec, Maryse; Boul�gue, Jacques; Gente, Pascal; Rolet, Jo�l; Coussement, Christophe; Stetter, Karl O.; Huber, Robert; Buku, Sony; Mifundu, Wafula
Crustal structure beneath the Kenya Rift from axial profile data
Modelling of the KRISP 90 axial line data shows that major crustal thinning occurs along the axis of the Kenya Rift from Moho depths of 35 km in the south beneath the Kenya Dome in the vicinity of Lake Naivasha to 20 km in the north beneath Lake Turkana. Low Pn velocities of 7.5-7.7 km/s are found beneath the whole of the axial line. The results indicate that crustal extension increases to the north and that the low Pn velocities are probably caused by magma (partial melt) rising from below and being trapped in the uppermost kilometres of the mantle. Along the axial line, the rift infill consisting of volcanics and a minor amount of sediments varies in thickness from zero where Precambrian crystalline basement highs occur to 5-6 km beneath the lakes Turkana and Naivasha. Analysis of the Pg phase shows that the upper crystalline crust has velocities of 6.1-6.3 km/s. Bearing in mind the Cainozoic volcanism associated with the rift, these velocities most probably represent Precambrian basement intruded by small amounts of igneous material. The boundary between the upper and lower crusts occurs at about 10 km depth beneath the northern part of the rift and 15 km depth beneath the southern part of the rift. The upper part of the lower crust has velocities of 6.4-6.5 km/s. The basal crustal layer which varies in thickness from a maximum of 2 km in the north to around 9 km in the south has a velocity of about 6.8 km/s. ?? 1994.
Mechie, J.; Keller, G. R.; Prodehl, C.; Gaciri, S.; Braile, L. W.; Mooney, W. D.; Gajewski, D.; Sandmeier, K. -J.
Two-stage magmatism during the evolution of the transitional Ethiopian rift
NASA Astrophysics Data System (ADS)
The Ethiopian rift marks the transition between continental rifting and incipient seafloor spreading. The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) included a 400 km-long cross-rift profile with 97 broadband passive seismometers with the aim to investigate the change from mechanical to magmatic extension by defining the lithospheric structure and extent of magmatism beneath the rift. Complimentary studies of P-wave receiver functions, shear-wave splitting and teleseismic earthquake arrival times show that the lithospheric structure is inherently different beneath the north-western rift flank, rift valley and south- eastern rift flank, with contrasting crustal thickness and composition, upper mantle velocity and lithospheric anisotropy. Two stages of magmatic addition are interpreted: 1) a 6--18 km-thick underplate lens at the base of the crust, which probably formed synchronous with an Oligocene flood basalt event (and therefore pre-dates the adjacent rifting by ~20 Myr); and 2) a 20--30 km-wide zone of intense dyking and partial melt, which most likely pervades the entire crust beneath the rift valley and marks the locus of current rift extension. Furthermore, Precambrian collision-related lithospheric fabric is proposed to be the main source of the strong anisotropy that is observed along the entire cross-rift profile, which may be augmented by magmatism beneath the rift. An active, followed by a passive magma-assisted rifting model that is controlled by a combination of far-field plate stresses, the pre-existing lithospheric framework and magmatism is invoked to explain the rift evolution.
Cornwell, D. G.; England, R. W.; Maguire, P. K.; Kendall, M.; Stuart, G. W.
Rifting and volcanism: The formation of rift-related unconformities and the timing of volcanism in the Laptev Sea rift, the South China Sea, and the southern South Atlantic based on multichannel seismic data
NASA Astrophysics Data System (ADS)
There are two end-member extremes of passive rifted margins. Volcanic rifted margins evolve by a combination of extension, and extensive extrusive flood volcanism over short time periods during breakup, manifested in reflection seismic data as seaward dipping reflectors. These margins are commonly related to mantle plumes; however, in the past years this has been questioned. Magma-poor rifted margins in contrast show wide extensional features as rotated faults blocks and detachment surfaces near the base of the continental crust, but limited magmatism that in addition seems to be delayed to post-breakup. In this presentation examples from three locations that are less frequently cited in the discussion about (either magma-poor or volcanic) rifted margins will be discussed: The Laptev Sea margin in the Arctic Ocean, where the active Arctic mid-oceanic ridge meets continental lithosphere at a high angle, the southernmost South Atlantic with well expressed conjugate volcanic rifted margins in a comparably "simple" configuration, potentially influenced by a mantle plume, the Tristan hot spot, and the South China Sea that may represent an intermediary form of continental extension between the end member extremes. The idea of a controlling role of (hot-spot related) magmatism during breakup on the rift evolution is questioned for the three margins that evolved in the Early Cretaceous, the Paleocene and the Oligocene, respectively. Instead much more top-down control on the rift evolution and rift-related magmatism than previously thought is suggested.
Franke, D.
Mildly peraluminous high-silica granites in a continental rift: the Drammen and Finnemarka batholiths, Oslo Rift, Norway
The peraluminous Drammen batholith (650 km2) is the largest granite complex within the mainly alkaline province of the Permo-Carboniferous Oslo Rift, and peraluminous to metaluminous granites are also present in the southern part of the otherwise alkaline Finnemarka complex (125 km2). The emplacement of the Drammen granite, and probably most of the other biotite granite complexes, predate the alkaline syenites
Reidar G. Tr�nnes; Alan D. Brandon
Geochronological and geochemical assessment of Cenozoic volcanism from the Terror Rift region of the West Antarctic Rift System
NASA Astrophysics Data System (ADS)
The work presented in this dissertation explains results from three different methods to determine the relation between tectonism and rift-related volcanism in the Terror Rift region of the West Antarctic Rift System (WARS). Alkaline lavas from seven submarine features, Beaufort Island and Franklin Islands, and several locations near Mt Melbourne were dated by 40Ar/39Ar geochronology and analyzed for elemental and isotopic chemical signatures. Each chapter addresses a different aspect of the hypothesis that the presence of volatiles, primarily H2O or CO2, in the magma source has led to anomalously high volumes of magmatism after rift-related decompressional melting rather than requiring an active mantle plume source. Chapter 2 provides the temporal framework, illustrating that the sampled features range in age from 6.7 Ma to 89 ka, post-dating the main Miocene age phase of Terror Rift extension. Chapter 3 illustrates the traditional enriched elemental and isotopic chemical signatures to support the overall homogeneity of these lavas and previously analyzed areas of the WARS. This chapter also provides a new model for the generation of the Pb isotopic signatures consistent with a history of metasomatism in the magma source. Chapter 4 provides an entirely new chemical dataset for the WARS. The first platinum group element (PGE) abundances and extremely unradiogenic Os isotopic signatures of Cenozoic lavas from Antarctica provide the strongest evidence of melting contributions from a lithospheric mantle source. The combined results from these three studies consistently support the original hypothesis of this dissertation. New evidence suggests that WARS related lavas are not related to a mantle plume(s) as previously proposed. Instead, they are generated by passive, decompressional melting of a source, likely a combination of the asthenospheric and lithospheric mantle, which has undergone previous melting events and metasomatism.
Rilling, Sarah E.
Colorado Basin Structure and Rifting, Argentine passive margin
NASA Astrophysics Data System (ADS)
The Argentine margin presents a strong segmentation with considerable strike-slip movements along the fracture zones. We focus on the volcanic segment (between the Salado and Colorado transfer zones), which is characterized by seaward dipping reflectors (SDR) all along the ocean-continent transition [e.g. Franke et al., 2006; Gladczenko et al., 1997; Hinz et al., 1999]. The segment is structured by E-W trending basins, which differs from the South African margin basins and cannot be explained by classical models of rifting. Thus the study of the relationship between the basins and the Argentine margin itself will allow the understanding of their contemporary development. Moreover the comparison of the conjugate margins suggests a particular evolution of rifting and break-up. We firstly focus on the Colorado Basin, which is thought to be the conjugate of the well studied Orange Basin [Hirsch et al., 2009] at the South African margin [e.g. Franke et al., 2006]. This work presents results of a combined approach using seismic interpretation and structural, isostatic and thermal modelling highlighting the structure of the crust. The seismic interpretation shows two rift-related discordances: one intra syn-rift and the break-up unconformity. The overlying sediments of the sag phase are less deformed (no sedimentary wedges) and accumulated before the generation of oceanic crust. The axis of the Colorado Basin trends E-W in the western part, where the deepest pre-rift series are preserved. In contrast, the basin axis turns to a NW-SE direction in its eastern part, where mainly post-rift sediments accumulated. The most distal part reaches the margin slope and opens into the oceanic basin. The general basin direction is almost orthogonal to the present-day margin trend. The most frequent hypothesis explaining this geometry is that the Colorado Basin is an aborted rift resulting from a previous RRR triple junction [e.g. Franke et al., 2002]. The structural interpretation partly supports this hypothesis and shows two main directions of faulting: margin-parallel faults (~N30�) and rift-parallel faults (~N125�). A specific distribution of the two fault sets is observed: margin-parallel faults are restrained to the most distal part of the margin. Starting with a 3D structural model of the basin fill based on seismic and well data the deeper structure of the crust beneath the Colorado Basin can be evaluate using isostatic and thermal modelling. Franke, D., et al. (2002), Deep Crustal Structure Of The Argentine Continental Margin From Seismic Wide-Angle And Multichannel Reflection Seismic Data, paper presented at AAPG Hedberg Conference "Hydrocarbon Habitat of Volcanic Rifted Passive Margins", Stavanger, Norway Franke, D., et al. (2006), Crustal structure across the Colorado Basin, offshore Argentina Geophysical Journal International 165, 850-864. Gladczenko, T. P., et al. (1997), South Atlantic volcanic margins Journal of the Geological Society, London 154, 465-470. Hinz, K., et al. (1999), The Argentine continental margin north of 48�S: sedimentary successions, volcanic activity during breakup Marine and Petroleum Geology 16(1-25). Hirsch, K. K., et al. (2009), Tectonic subsidence history and thermal evolution of the Orange Basin, Marine and Petroleum Geology, in press, doi:10.1016/j.marpetgeo.2009.1006.1009
Autin, Julia; Scheck-Wenderoth, Magdalena; Loegering, Markus; Anka, Zahie; Vallejo, Eduardo; Rodriguez, Jorge; Marchal, Denis; Reichert, Christian; di Primio, Rolando
Geochemical Overview of the East African Rift System
NASA Astrophysics Data System (ADS)
Mafic volcanics of the East African Rift System (EARS) record a protracted history of continental extension that is linked to mantle plume activity. The modern EARS traverses two post-Miocene topographic domes separated by a region of polyphase extension in northern Kenya and southern Ethiopia. Basaltic magmatism commenced �45 Ma in this highly extended region, while the onset of plume-related activity took place �30 Ma with eruption of flood basalts in central Ethiopia. A spatial and temporal synthesis of EARS volcanic geochemistry shows progressive lithospheric removal (by erosion and melting) as the degree of rifting increases, with basalts in the most highly extended areas recording melting of depleted asthenosphere. Plume contributions are indicated locally in the northern half of the EARS, but are absent from the southern half. The geochemical signatures are compatible with a physical model in which the entire EARS is fed by a discontinuous plume emanating from the core-mantle boundary as the South African Superswell. Quaternary basaltic lavas erupted in the Afar triangle, Red Sea and Gulf of Aden define the geochemical signature attributed to the Afar plume (87Sr/86Sr 0.7034-0.7037, 143Nd/144Nd 0.5129-0.5130; La/Nb 0.6-0.9; Nb/U 40-50). These suites commonly record mixing with ambient upper mantle having less radiogenic isotopes but generally overlapping incompatible trace element abundances. Within the Ethiopian dome both lithospheric and sub-lithoshperic contributions can be documented clearly; lithospheric contributions are manifest in more radiogenic isotope values (87Sr/86Sr up to 0.7050) and distinctive trace element abundances (e.g., La/Nb <2.0, Nb/U > 10). The degree of lithospheric contribution is lowest within the active Main Ethiopian Rift and increases towards the southern margin of the dome. The estimated depth of melting (65-75 km) is consistent with geophysical observations of lithospheric thickness. In regions of prolonged volcanism the lithospheric contributions and estimated melting depths decrease through time, corresponding to a higher degree of rifting. In the Kenyan dome, including the western rift, the degree of extension is low and lithospheric melting is the dominant source for basaltic magmatism. Mafic lavas from these regions have generally lower MgO but higher contents of alkalis, P2O5 and many incompatible trace elements than are observed in the Ethiopian Rift. High values of 87Sr/86Sr, 207Pb/204Pb and Zr/Hf relative to other parts of the EARS indicate melting of metasomatized lithosphere. Melting in this area occurs at depths up to 100+ km, consistent with the thick crustal section observed seismically. Between the topographic domes, basalts from the Turkana region record melting at shallow levels ( �35 km) consistent with seismic evidence for nearly complete rifting of the crustal section. The geochemistry of these lavas is dominated by asthenospheric source materials, with only minor lithospheric involvement. Temporal evolution of EARS geochemistry reflects progressive rifting of the thick craton. This change is manifest within lavas that are interpreted as plume-derived, as Tb/Yb values decrease from 30 Ma through the present. The modern thermal anomaly associated with Afar volcanism does not appear to extend below the shallow mantle, but may reflect a large blob of deep mantle material that became stuck to Africa 30 Ma and has contributed to regional volcanism ever since. Relative contributions from this deep mantle source, shallow asthenosphere and lithosphere are controlled by the extent of rifting and cannot be predicted solely on the basis of surface topography.
Furman, T.
Albertine Rift, Uganda: Deformation-Sedimentation-Erosion relationships
NASA Astrophysics Data System (ADS)
The Albertine Rift is the northern part of the western branch of the East African Rift that runs over a distance of around 2000 km from Lake Albert in the north to Lake Malawi in the south. Lake Albert Basin is assumed to be a classical half-graben initiated around 12 Ma and oriented NNW-SSW, with a major northwesterly bounding fault - the Bunia fault - located along the western Congolese shoreline (Ebinger, 1989; Pickford & al., 1993). The aim of this study is to understand the relationships between deformation, erosion, and sedimentation of the rift through time by restoring (1) the timing and amplitude of vertical movements (subsidence, uplift), (2) the geometry and paleo-environmental evolution (including climate) of the sedimentary infilling and (3) the geomorphological evolution of the surrounding area and associated erosion budget. Seismic data and outcrops studies suggest a much more complex history than previously described. (1) The age model, mainly based on mammal fossils (Pickford et al., 1993; Van Damme and Pickford, 2003), is debated, but the early stage of the rift is probably Middle Miocene. (2) No half-graben geometry has been characterized: the infilling consists of juxtaposed tabular compartments with sharp thicknesses variations along bounding faults, in response of either low rate extensional or combined strike-slip/extensional movements. The following onshore-offshore evolution is proposed: - Middle Miocene (~ 13 Ma) to Late Miocene (?): rifting 1 - differential subsidence along N60� faults - major deepening from fluvio-deltaic to deep lacustrine environments (maximum flooding at 8 Ma) - uplift, erosion and reworking of weathered profiles - first generation of pediments. - Late Miocene (?) to Late Pliocene (~ 3 Ma): quiescence phase - homogenous subsidence - lacustrine clays interbedded with sandy flood-lobes - uplift, erosion and reworking of ferruginous laterite (iron duricrusts) - second generation of pediments. - Late Pliocene (~ 3Ma) to Early Pleistocene (~ 2 Ma): rifting 2 - major uplift and growth of the Ruwenzori Mountains (5000 m of elevation) - differential subsidence - deltaic to wave-dominated coast (shoreface) sandy deposits - pediments degradation by fluvial erosion. - Middle-Late Pleistocene: late regional uplift and tilting - drainage inversion and present-day scarp formation.
Simon, Brendan; Guillocheau, Fran�ois; Robin, C�cile; Dauteuil, Olivier; Nalpas, Thierry; Bourges, Philippe; Bez, Martine; Lays, Philippe
Strain transfer between disconnected, propagating rifts in Afar
NASA Astrophysics Data System (ADS)
We showed before that both the Aden and Red Sea plate boundaries are currently rifting and propagating along two distinct paths into Afar through the opening of a series of disconnected, propagating rifts. Here we use new geochronological, tectonic, and paleomagnetic data that we acquired mostly in the southeastern part of Afar to examine the geometry, kinematics, and time-space evolution of faulting related to strain transfer processes. It appears that transfer of strain is accommodated by a bookshelf faulting mechanism wherever rifts or plate boundaries happen to overlap without being connected. This mechanism implies the rotation about a vertical axis of small rigid blocks along rift-parallel faults that are shown to slip with a left-lateral component, which is as important as their normal component of slip (rates of �2-3 mm/yr). By contrast, where rifts do not overlap, either a classic transform fault (Maskali) or an oblique transfer zone (Mak'arrasou) kinematically connects them. The length of the Aden-Red Sea overlap has increased in the last �0.9 Myr, as the Aden plate boundary propagated northward into Afar. As a consequence, the first-order blocks that we identify within the overlap did not all rotate during the same time-span nor by the same amounts. Similarly, the major faults that bound them did not necessarily initiate and grow as their neighboring faults did. Despite these variations in strain distribution and kinematics, the overlap kept accommodating a constant amount of strain (7 to 15% of the extension amount imposed by plate driving forces), which remained distributed on a limited number (seven or eight) of major faults, each one having slipped at constant rates (�3 and 2 mm/yr for vertical and lateral rates, respectively). The fault propagation rates and the block rotation rates that we either measure or deduce are so fast (30-130 mm/yr and 15-38�/Myr, respectively) that they imply that strain transfer processes are transient, as has been shown to be the case for the processes of tearing, rift propagation, and strain jumps in Afar.
Manighetti, I.; Tapponnier, P.; Courtillot, V.; Gallet, Y.; Jacques, E.; Gillot, P.-Y.
Deformation and seismicity associated with continental rift zones propagating toward continental margins
NASA Astrophysics Data System (ADS)
We study the propagation of a continental rift and its interaction with a continental margin utilizing a 3-D lithospheric model with a seismogenic crust governed by a damage rheology. A long-standing problem in rift-mechanics, known as thetectonic force paradox, is that the magnitude of the tectonic forces required for rifting are not large enough in the absence of basaltic magmatism. Our modeling results demonstrate that under moderate rift-driving tectonic forces the rift propagation is feasible even in the absence of magmatism. This is due to gradual weakening and "long-term memory" of fractured rocks that lead to a significantly lower yielding stress than that of the surrounding intact rocks. We show that the style, rate and the associated seismicity pattern of the rift zone formation in the continental lithosphere depend not only on the applied tectonic forces, but also on the rate of healing. Accounting for the memory effect provides a feasible solution for thetectonic force paradox. Our modeling results also demonstrate how the lithosphere structure affects the geometry of the propagating rift system toward a continental margin. Thinning of the crystalline crust leads to a decrease in the propagation rate and possibly to rift termination across the margin. In such a case, a new fault system is created perpendicular to the direction of the rift propagation. These results reveal that the local lithosphere structure is one of the key factors controlling the geometry of the evolving rift system and seismicity pattern.
Lyakhovsky, V.; Segev, A.; Schattner, U.; Weinberger, R.
Crustal Structure Variation Across The Northern Yadong-Gulu Rift From Teleseismic Receiver Function Studies
NASA Astrophysics Data System (ADS)
The Yadong-Gulu rift is one of several long north-south oriented rifts iin southern Tibet formed as a result of the east-west extension of the Tibetan lithosphere. We investigated crustal structure variation across the rift north of the Yarlung-Zangpo Suture using available broadband seismic stations in the region. P receiver functions of 61 stations were calculated using teleseismic events larger than 5 since 1991. Crustal thicknesses and Vp/Vs ratios at individual stations were estimated using the H-kappa stacking technique. High resolution images of the crust and uppermost mantle across the rift were obtained using the Common Conversion Point (CCP) stacking method. Receiver functions and surface wave dispersion data derived from ambient noise cross-correlation were combined to determine 1-D S-wave velocity models beneath stations in and outside the rift. The results show a clear and relative Moho at about 70 km depth outside the rift zone. The crustal structure in the rift zone is more complicate. There are two strong interfaces at depths of 15 km and 50 km in addition to a weak Moho at 70 km depth. The rapid variation of lower crustal structure across the rift zone indicates that the rifting is not decoupled at the mid crust level and the whole southern Tibetan crust, probably the lithosphere too, is undergone rifting deformation at several narrow zones.
Zhou, Y.; Zhu, L.; Song, X.
Neoproterozoic rifting in the southern Georgina Basin, central Australia: Implications for reconstructing Australia in Rodinia
NASA Astrophysics Data System (ADS)
A system of northwest striking Neoproterozoic rift basins underlies Paleozoic strata in the southern Georgina Basin of central Australia. Normal faults bounding these rift basins were selectively reactivated during the mid-Paleozoic Alice Springs Orogeny and are now expressed as high-angle reverse faults that invert the preexisting rift basins. Exhumed and eroded rift basin remnants are present in the hanging wall of the Oomoolmilla, Lucy Creek, Tarlton, and Toomba reverse faults, and rift basins may be preserved in the subsurface beneath the Toko Syncline and Burke River Structural Belt. Rift basin fill indicates two periods of extension: a major rift-forming episode between approximately 700 and 650 Ma (coeval with Sturtian glacial deposits) and a second episode of extension at approximately 600 Ma (coeval with Marinoan glacial deposits). This northwest striking rift system in central Australia supports results from other regions, indicating that the Neoproterozoic continental margin of Australia consisted of northwest striking rift segments offset by northeast striking transform faults. Such a configuration is geometrically incompatible with a Laurentian continental margin consisting of northeast striking rift segments and conflicts with reconstructions such as SWEAT and AUSWUS that match Australia with western Laurentia in the Rodinia supercontinent.
Greene, David C.
Contour mapping of relic structures in the Precambrian basement of the Reelfoot rift, North American midcontinent
A new contour map of the basement of the Reelfoot rift constructed from drill hole and seismic reflection data shows the general surface configuration as well as several major and minor structural features. The major features are two asymmetric intrarift basins, bounded by three structural highs, and the rift margins. The basins are oriented normal to the northeast trend of the rift. Two of the highs appear to be ridges of undetermined width that extend across the rift. The third high is an isolated dome or platform located between the basins. The minor features are three linear structures of low relief oriented subparallel to the trend of the rift. Two of these, located within the rift basins, may divide the rift basins into paired subbasins. These mapped features may be the remnants of initial extensional rifting, half graben faulting, and basement subsidence. The rift basins are interpreted as having formed as opposing half graben, and the structural highs are interpreted as having formed as associated accommodation zones. Some of these features appear to be reactivated seismogenic structures within the modem midcontinent compressional stress regime. A detailed knowledge of the geometries of the Reelfoot rift's basement features, therefore, is essential when evaluating their seismic risk potential.
Dart, R. L.; Swolfs, H. S.
Aerosol and Cloud Microphysical Characteristics of Rifts and Gradients in Maritime Stratocumulus Clouds
NASA Technical Reports Server (NTRS)
A cloud rift is characterized as a large-scale, persistent area of broken, low reflectivity stratocumulus clouds usually surrounded by a solid deck of stratocumulus. A rift observed off the coast of Monterey Bay, California on 16 July 1999 was studied to compare the aerosol and cloud microphysical properties in the rift with those of the surrounding solid stratus deck. Variables measured from an instrumented aircraft included temperature, water vapor, and cloud liquid water. These measurements characterized the thermodynamic properties of the solid deck and rift areas. Microphysical measurements made included aerosol, cloud drop and drizzle drop concentrations and cloud condensation nuclei (CCN) concentrations. The microphysical characteristics in a solid stratus deck differ substantially from those of a broken, cellular rift where cloud droplet concentrations are a factor of 2 lower than those in the solid cloud. Further, CCN concentrations were found to be about 3 times greater in the solid cloud area compared with those in the rift and aerosol concentrations showed a similar difference as well. Although drizzle was observed near cloud top in parts of the solid stratus cloud, the largest drizzle rates were associated with the broken clouds within the rift area. In addition to marked differences in particle concentrations, evidence of a mesoscale circulation near the solid cloud rift boundary is presented. This mesoscale circulation provides a mechanism for maintaining a rift, but further study is required to understand the initiation of a rift and the conditions that may cause it to fill.
Sharon, Tarah M.; Albrecht, Bruce A.; Jonsson, Haflidi H.; Minnis, Patrick; Khaiyer, Mandana M.; Van Reken, Timothy; Seinfeld, John; Flagan, Rick
Rifting and Subsidence in the Gulf of Mexico: Implications for Syn-rift, Sag, and Salt Sections, and Subsequent Paleogeography
NASA Astrophysics Data System (ADS)
Thick (up to 5 km), rapid (<3 Ma), salt deposition is problematic for basin modelling because such accommodation cannot be thermal, yet GoM salt deposits (Late Callovian-Early Oxfordian) appear to be post-rift (most salt overlies planar sub-salt unconformities on syn-rift section). One possible solution is that the pre-drift GoM was a deep (~2 km) air-filled rift depression where basement had already subsided tectonically, and thus could receive up to 5 km of salt, roughly the isostatic maximum on exhumed mantle, hyper-thinned continent, or new ocean crust. ION-GXT and other seismic data along W Florida and NW Yucat�n show that (1) mother salt was only 1 km thick in these areas, (2) that these areas were depositionally connected to areas of thicker deposition, and (3) the top of all salt was at global sea level, and hence the sub-salt unconformity along Florida and Yucat�n was only 1 km deep by end of salt deposition. These observations fit the air-filled chasm hypothesis; however, two further observations make that mechanism highly improbable: (1) basinward limits of sub-salt unconformities along Florida/Yucat�n are deeper than top of adjacent ocean crust emplaced at ~2.7 km subsea (shown by backstripping), and (2) deepest abyssal sediments over ocean crust onlap the top of distal salt, demonstrating that the salt itself was rapidly drowned after deposition. Study of global ION datasets demonstrates the process of "rapid outer marginal collapse" at most margins, which we believe is achieved by low-angle detachment on deep, landward-dipping, Moho-equivalent surfaces such that outer rifted margins are hanging walls of crustal scale half-grabens over mantle. The tectonic accommodation space produced (up to 3 km, < 3 Ma) can be filled by ~5 km of sag/salt sequences with little apparent hanging wall rifting. When salt (or other) deposition lags behind, or ends during, outer marginal collapse, deep-water settings result. We suggest that this newly identified, "outer marginal detachment phase", normally separates the traditional "rift" from "drift" stages during continental margin creation. Importantly, this 2-3 km of subsidence presently is neither treated as tectonic nor as thermal in traditional subsidence analysis; thus, Beta estimates may be excessive at many outer margins. Outer marginal collapse was probably eastwardly diachronous with initiation of spreading in the GoM. Additionally, recent paleo-climate studies suggest humid Early/Middle Jurassic conditions in equatorial GoM, hindering air-filled chasm development, but North America's northward flight into middle latitudes initiated Callovian aridity.
Pindell, J. L.; Graham, R.; Horn, B.
Potential for autoimmune pathogenesis of Rift Valley Fever virus retinitis.
Rift Valley Fever (RVF) is a significant threat to human health because it can progress to retinitis, encephalitis, and hemorrhagic fever. The timing of onset of Rift Valley Fever virus (RVFV) retinitis suggests an autoimmune origin. To determine whether RVFV retinitis is associated with increased levels of IgG against retinal tissue, we measured and compared levels of IgG against healthy human eye tissue by immunohistochemical analysis. We found that serum samples from RVFV-exposed Kenyans with retinitis (n = 8) were slightly more likely to have antibodies against retinal tissue than control populations, but the correlation was not statistically significant. Further investigation into the possible immune pathogenesis of RVFV retinitis could lead to improved therapies to prevent or treat this severe complication. PMID:23918215
Newman-Gerhardt, Shoshana; Muiruri, Samuel; Muchiri, Eric; Peters, Clarence J; Morrill, John; Lucas, Alexander H; King, Charles H; Kazura, James; LaBeaud, Angelle Desiree
Deformational models of rifting and folding on Venus
NASA Astrophysics Data System (ADS)
Features of presumed tectonic origin on Venus are reviewed, and lithospheric strength envelopes are derived based on laboratory measurements of the deformational properties of crustal and subcrustal rocks, extrapolated to conditions appropriate to Venus. Models for rifting and folding are developed that use this lithospheric structure and take into account both brittle and ductile yielding as well as finite elastic strength. For both rifting and folding, structures with characteristic widths and spacings are predicted whose size depends on the thickness of the lithosphere, density contrast, and elastic properties of the layer. Finally, the model predictions are compared with the widths and spacings of observed tectonic features, and it is concluded that they are consistent with a relatively strong mantle layer separated from a thin brittle surface layer by a ductile lower crust. These results allow constraints to be placed on the crustal thickness and thermal gradient on Venus.
Banerdt, W. B.; Golombek, M. P.
The East African rift system in the light of KRISP 90
On the basis of a test experiment in 1985 (KRISP 85) an integrated seismic-refraction/teleseismic survey (KRISP 90) was undertaken to study the deep structure beneath the Kenya rift down to depths of 100-150 km. This paper summarizes the highlights of KRISP 90 as reported in this volume and discusses their broad implications as well as the structure of the Kenya rift in the general framework of other continental rifts. Major scientific goals of this phase of KRISP were to reveal the detailed crustal and upper mantle structure under the Kenya rift, to study the relationship between mantle updoming and the development of sedimentary basins and other shallow structures within the rift, to understand the role of the Kenya rift within the Afro-Arabian rift system and within a global perspective and to elucidate fundamental questions such as the mode and mechanism of continental rifting. The KRISP results clearly demonstrate that the Kenya rift is associated with sharply defined lithospheric thinning and very low upper mantle velocities down to depths of over 150 km. In the south-central portion of the rift, the lithospheric mantle has been thinned much more than the crust. To the north, high-velocity layers detected in the upper mantle appear to require the presence of anistropy in the form of the alignment of olivine crystals. Major axial variations in structure were also discovered, which correlate very well with variations in the amount of extension, the physiographic width of the rift valley, the regional topography and the regional gravity anomalies. Similar relationships are particularly well documented in the Rio Grande rift. To the extent that truly comparable data sets are available, the Kenya rift shares many features with other rift zones. For example, crustal structure under the Kenya, Rio Grande and Baikal rifts and the Rhine Graben is generally symmetrically centered on the rift valleys. However, the Kenya rift is distinctive, but not unique, in terms of the amount of volcanism. This volcanic activity would suggest large-scale modification of the crust by magmatism. Although there is evidence of underplating in the form of a relatively high-velocity lower crustal layer, there are no major seismic velocity anomalies in the middle and upper crust which would suggest pervasive magmatism. This apparent lack of major modification is an enigma which requires further study. ?? 1994.
Keller, G. R.; Prodehl, C.; Mechie, J.; Fuchs, K.; Khan, M. A.; Maguire, P. K. H.; Mooney, W. D.; Achauer, U.; Davis, P. M.; Meyer, R. P.; Braile, L. W.; Nyambok, I. O.; Thompson, G. A.
Crustal Structure of the Ethiopian Rift and Adjacent Plateaus: Results of new integrated interpretation
NASA Astrophysics Data System (ADS)
The Ethiopian rift is the large part of the East African Rift system, which represents an incipient divergent plate boundary. This important structure provides excellent opportunities to study the transition from continental to oceanic. As a result, geophysical data are becoming increasingly available but some results are contradictory. We used a newly enhanced gravity database and seismic information to produce an integrated interpretation of the crustal structure beneath the Ethiopian rift and the adjacent plateaus. Wide regions have been covered to assess the regional structures including the Kenyan and Ethiopian rifts and the area covered by the Ethiopian flood basalt. Broad negative Bouguer gravity anomalies are delineated over the Ethiopian Plateaus and the Kenyan dome. Residual gravity anomalies, which parallel the major border faults clearly highlight the segregation between the plateaus and the rift valleys. Results of other filtering techniques have clearly revealed individual volcanic centers within the rift valleys. Positive gravity anomalies outside the rift valleys may be associated with older structures, shield volcanoes, or structures that are related to the initiation and propagation of rifting. A long axial profile from the central part of Kenya to the Afar triple junction has been modeled to investigate along-axis crustal variation of the East African rift system, with emphasis on the Ethiopian rift. This modeling has been constrained using seismic refraction data from the Ethiopian Afar Geoscientific Lithospheric Experiment (EAGLE) and Kenya Rift International Seismic Project (KRISP) results. We are able to see a thin crust (~26 km) in the Afar triangle with a gradual thickening (~40 km) southwards towards the Main Central Ethiopian rift (MER). The crust thickness decreases towards Turkana rift (~22 km), and increases again towards the central eastern rift section in Kenya. Our profile model across the MER has revealed that the eastern rift flank has thicker crust than the western rift flank near the MER. The cross profile modeled across the southern part of the Afar triple junction shows a shallow Moho. This may be the result of the rift being initiated by a plume as suggested elsewhere in many studies.
Tadesse, K.; Keller, G. R.
The Corinth Rift Laboratory (CRL) strainmeters: calibration and data analysis
The Gulf of Corinth (Greece) is one of the most seismic regions in Europe, producing some strong earthquakes in the decades, 1 to 1.5 cm\\/yr of north-south extension, and frequent seismic swarms. This structure is a 110 km long, N110E oriented graben bounded by systems of very recent normal faults. The Corinth Rift Laboratory (CRL) project is concentrated in the
A. Canitano; P. Bernard; A. T. Linde; S. I. Sacks; F. Boudin
A Mathematical Model of Rift Valley Fever with Human Host
Rift Valley Fever is a vector-borne disease mainly transmitted by mosquito. To gain some quantitative insights into its dynamics,\\u000a a deterministic model with mosquito, livestock, and human host is formulated as a system of nonlinear ordinary differential\\u000a equations and analyzed. The disease threshold $$\\\\mathcal{R}_0$$ is computed and used to investigate the local stability of the equilibria. A sensitivity analysis is
Saul C. MpesheHeikki; Heikki Haario; Jean M. Tchuenche
Seismic Evidence for an Active Southern Rio Grande Rift
NASA Astrophysics Data System (ADS)
Competing models exist to explain what caused the Earth�s crust to spread apart 29 million years ago to create a region known today as the Rio Grande Rift (RGR). The RGR extends from central Colorado through New Mexico to northern Mexico, near El Paso. A growing body of evidence shows that geologic activity still occurs in the RGR, with a continuation of faulting, seismicity and a small widening rate. We map of the seismic velocity structure and crustal thickness using data from the Rio Grande Rift Seismic TRAnsect (RISTRA) experiment and the EarthScope Transportable Array (USArray) dataset. In addition to the data we collected from the RISTRA experiment and USArray dataset, we also acquired receiver functions from the EarthScope Automatic Receiver Survey (EARS) website (http://www.earthscope.org/data) and waveform data from the Incorporated Research Institutes for Seismology (IRIS) Data Management Center (DMC). In particular, we requested seismograms from the IRIS DMC database where we acquired teleseismic events from Jan 2000 to Dec 2009. This includes 7,259 seismic events with a minimum magnitude of 5.5 and 106,389 continuous waveforms. This data was preprocessed (merged, rotated) using a program called Standing Order of Data (SOD). We computed receiver functions and receiver function stacks for all data in the Southern Rio Grande Rift (SRGR). We map the crustal thickness, seismic velocity, and mantle structure to better determine the nature of tectonic activity that is presently taking place and further investigate the regional extension of the Southern Rio Grande Rift (SRGR). Here we present results of the crustal and velocity structure using the kriging interpolation scheme and interpret our results in relation to southern RGR deformation and extension.
Thompson, L. E.; Velasco, A. A.
Intracontinental rifting and inversion: Missour basin and Atlas Mountains, Morocco
The intracontinental High and Middle Atlas mountain belts in Morocco intersect to form the southern and western margins of the Missour basin, an intermontane basin formed as a result of the uplift and inversion of the Mesozoic Atlas paleorifts. These rifts were areas where the crust was greatly attenuated and more subject to deformation in response to nearby plate boundary tectonics. Data from observations based on seismic reflection profiles and wells over the Missour basin for hydrocarbon exploration and field mapping were used to understand the basin evolution, structural styles, and inversion timing of the nearby Atlas Mountains. Hercynian and Mesozoic normal faults were reactivated into high-angle reverse and thrust faults in the Mesozoic during the Jurassic, Early Cretaceous (early Alpine phase), and the Paleogene (late Alpine phase). The reactivation of synrift normal faults of the paleo-Atlas rifts inverted previous half grabens into anticlinal structures, with the axis of the half graben centered below the axis of the inverted anticline. The resulting inverted fold geometries are controlled by the geometries of the extensional planar or listric faults. The Atlas paleorift system is one of the largest rift systems in Africa. Little hydrocarbon exploration has occurred within the Atlas Mountains and the margins of the paleo-Atlas rift system. Inversion of synrift structures can lead to both the destruction and preservation of synrift traps and the creation of new hydrocarbon traps. The study of the effects of inversion in the Missour basin may lead to the discovery of footwall subthrust hydrocarbon traps in the Mesozoic sedimentary sequence of the Atlas Mountains.
Beauchamp, W.; Barazangi, M. [Cornell Univ., Ithaca, NY (United States); Demnati, A.; Alji, M.E. [Office National de Recherches et d`Exploitations Petrolieres, Rabat (Morocco)
Inflation along Kilauea's Southwest Rift Zone in 2006
We report on InSAR and GPS results showing the first crustal inflation along the southwest rift zone at Kilauea volcano in over 20�years. Two independent interferograms (May 2�August 2, 2006 and June 22�Nov 7, 2006) from the ALOS PALSAR instrument reveal domal uplift located southwest of the main caldera. The uplift is bounded on the northeast by the caldera and
David Myer; David Sandwell; Benjamin Brooks; James Foster; Masanobu Shimada
Age of volcanism and rifting in southwestern Ethiopia
It has been suggested that volcanism in the Ethiopian region of the Afro-Arabian Rift System has migrated with time, both laterally towards the present axial zone1-3 and longitudinally southwards from the Red Sea4,5. Field data and K-Ar isotopic ages from southwestern Ethiopia, summarised below, indicate that volcanism in this area began earlier than previously suspected, and that Quaternary volcanism was
A. Davidson; D. C. Rex
Structural control of flank volcanism in continental rifts.
Many volcanoes emerge from the flank (footwall) of normal faults in continental rift zones. Because such locations are commonly topographically high and exhibit minor compressional structures, the association is enigmatic. A simple flexing plate model shows that deformation of a flexurally supported upper crust during normal faulting generates a dilational strain field in the footwall at the base of the crust. This strain field allows cracking and tapping of preexisting melt. PMID:17787173
Ellis, M; King, G
Eocene to Miocene geometry of the West Antarctic Rift System
Tectonic models for the Late Cretaceous\\/Tertiary evolution of the West Antarctic Rift System range from hundreds of kilometres of extension to negligible strike-slip displacement and are based on a variety of observations, as well as kinematic and geodynamic models. Most data constraining these models originate from the Ross Sea\\/Adare Trough area and the Transantarctic Mountains. We use a new Antarctic
R. D. M�ller; K. Gohl; S. C. Cande; A. Goncharov; A. V. Golynsky
The rifting to spreading transition in the Red Sea
NASA Astrophysics Data System (ADS)
The transition from continental rifting to seafloor spreading is presently occurring at only a few places on Earth, such as the Red Sea or the Woodlark Basin. Competing theories for how spreading begins (either by quasi-instantaneous formation of a whole spreading segment or by initiation of spreading at multiple discrete �nodes� separated by thinned continental lithosphere) have been put forward. The major evidence for the nodes theory comes from the Red Sea and geophysical surveys carried out there in the �multi-deeps region� during the 1970's and 1980's. We present new high-resolution multibeam bathymetric information over the same region, which, when combined with acoustic backscatter data, seafloor sampling and magmatic geochemical information appears to provide no support for the nodes model. We show that, although the discrete deeps undoubtedly exist, they are not separated from one another by tectonic boundaries but rather represent �windows� onto a continuous spreading axis which is locally inundated and masked by massive slumping of sediments and evaporites from the rift flanks. The geophysical data that was previously used to support the presence of continental crust between the �nodes� can be equally well explained by processes related to the sedimentary blanketing and sub-sedimentary hydrothermal alteration. A single, �quasi-instantaneous segment formation� model would appear to be all that is required to explain observations from present-day rifting/spreading transitions globally.
Augustin, Nico; Devey, Colin W.; van der Zwan, Froukje M.; Feldens, Peter; Tominaga, Masako; Bantan, Rashad A.; Kwasnitschka, Tom
Present-day kinematics of the East African Rift
NASA Astrophysics Data System (ADS)
The East African Rift (EAR) is a type locale for investigating the processes that drive continental rifting and breakup. The current kinematics of this ~5000 km long divergent plate boundary between the Nubia and Somalia plates is starting to be unraveled thanks to a recent augmentation of space geodetic data in Africa. Here we use a new data set combining episodic GPS measurements with continuous measurements on the Nubian, Somalian, and Antarctic plates, together with earthquake slip vector directions and geologic indicators along the Southwest Indian Ridge to update the present-day kinematics of the EAR. We use geological and seismological data to determine the main rift faults and solve for rigid block rotations while accounting for elastic strain accumulation on locked active faults. We find that the data are best fit with a model that includes three microplates embedded within the EAR, between Nubia and Somalia (Victoria, Rovuma, and Lwandle), consistent with previous findings but with slower extension rates. We find that earthquake slip vectors provide information that is consistent with the GPS velocities and helps to significantly reduce uncertainties of plate angular velocity estimates. We also find that 3.16 Myr MORVEL average spreading rates along the Southwest Indian Ridge are systematically faster than prediction from GPS data alone. This likely indicates that outward displacement along the SWIR is larger than the default value used in the MORVEL plate motion model.
Saria, E.; Calais, E.; Stamps, D. S.; Delvaux, D.; Hartnady, C. J. H.
The Afro-Arabian rift system�an overview
NASA Astrophysics Data System (ADS)
The Afro-Arabian rift system is reviewed beginning with the Dead Sea transform and Gulf of Suez in the north, followed by the Gulf of Aden and Red Sea and ending with a brief mention of rifting through eastern and southern Africa. A consistent interpretation is obtained for geophysical data from the Gulf of Aden and Red Sea and geological data for the Dead Sea transform, Suez graben and Red Sea and Gulf of Aden margins. Geophysical data are used to estimate the locations of the ocean-continent boundaries in the northern Red Sea; these are found to coincide when the total 107 km shear along the Dead Sea is restored. The Red Sea is therefore reconstructed in stages beginning with the 45 km post-Miocene shear and 62 km early-Miocene shear along the Dead Sea transform followed by estimates for extension in the early Red Sea-Suez graben. When this is done, there is a remarkable alignment of various features revealed on Landsat TM imagery for both sides of the northern Red Sea. North-south differences in the Red Sea and west-east differences hi the Gulf of Aden are discussed and found to be best explained by a propagating rift model.
Girdler, R. W.
Intraplate stresses, nonthermal subsidence, and fluid regimes in rifted basins
Short-term fluctuations of the level of intraplate stresses modulate the shape of rifted basins and therefore affect the stratigraphic record. We investigated the effect of such stress variations on fluid flow and sedimentation patterns in sedimentary basins using a dynamic numerical model which combines the stretching mechanism for rifted-basin formation with the lithospheric necking concept. The results show that increases in the level of compressive intraplate stresses strongly influence the hydrodynamic regime during the postrift phase of sedimentary basins by causing an increase of meteoric water influx and compactional driven flow. These short-term perturbations in fluid-flow patterns occur simultaneously with subsidence rates far in excess of those predicted by thermal models. The results of the modeling enable us to discriminate the effect of tectonically induced distortions of the basin shape (and associated differential vertical motions in rifted basins) on the fluid-flow regime from perturbations induced by changes in eustatic sea level. We discuss implications of the modeling predictions for maturation and migration of hydrocarbons and diagenesis. Episodic diagenetic and expulsion events can be explained by short-term changes in the magnitude of nonthermal subsidence.
Van Balen, R.T.; Cloetingh, A.P.L. (Vrije Universiteit, Amsterdam (Netherlands))
The Sagatu Ridge dike swarm, Ethiopian rift margin. [tectonic evolution
NASA Technical Reports Server (NTRS)
A swarm of dikes forms the core of the Sagatu Ridge, a 70-km-long topographic feature elevated to more than 4000 m above sea level and 1500 m above the level of the Eastern (Somalian) plateau. The ridge trends NNE and lies about 50 km east of the northeasterly trending rift-valley margin. Intrusion of the dikes and buildup of the flood-lava pile, largely hawaiitic but with trachyte preponderant in the final stages, occurred during the late Pliocene-early Pleistocene and may have been contemporaneous with downwarping of the protorift trough to the west. The ensuing faulting that formed the present rift margin, however, bypassed the ridge. The peculiar situation and orientation of the Sagatu Ridge, and its temporary existence as a line of crustal extension and voluminous magmatism, are considered related to a powerful structural control by a major line of Precambrian crustal weakness, well exposed further south. Transverse rift structures of unknown type appear to have limited the development of the ridge to the north and south.
Mohr, P. A.; Potter, E. C.
Dike intrusions during rifting episodes obey scaling relationships similar to earthquakes
As continental rifts evolve towards mid-ocean ridges, strain is accommodated by repeated episodes of faulting and magmatism. Discrete rifting episodes have been observed along two subaerial divergent plate boundaries, the Krafla segment of the Northern Volcanic Rift Zone in Iceland and the Manda-Hararo segment of the Red Sea Rift in Ethiopia. In both cases, the initial and largest dike intrusion was followed by a series of smaller intrusions. By performing a statistical analysis of these rifting episodes, we demonstrate that dike intrusions obey scaling relationships similar to earthquakes. We find that the dimensions of dike intrusions obey a power law analogous to the Gutenberg-Richter relation, and the long-term release of geodetic moment is governed by a relationship consistent with the Omori law. Due to the effects of magma supply, the timing of secondary dike intrusions differs from that of the aftershocks. This work provides evidence of self-similarity in the rifting process.
L., Passarelli; E., Rivalta; A., Shuler
Was the Mid-Continent Rift part of a successful seafloor-spreading episode?
NASA Astrophysics Data System (ADS)
The ~1.1 Ga Mid-Continent Rift (MCR), the 3000-km long largely-buried feature causing the largest gravity and magnetic anomaly within the North American craton, is traditionally considered a failed rift formed by isolated midplate volcanism and extension. We propose instead that the MCR formed as part of the rifting of Amazonia (Precambrian northeast South America) from Laurentia (Precambrian North America) and became inactive once seafloor spreading was established. A cusp in Laurentia's apparent polar wander path near the onset of MCR volcanism, recorded by the MCR's volcanic rocks, likely reflects the rifting. This scenario is suggested by analogy with younger rifts elsewhere and consistent with the geometry and timing of Precambrian rifting events including the MCR's extension to southwest Alabama along the East Continent Gravity High, southern Appalachian rocks having Amazonian affinities, and recent interpretation of large igneous provinces in Amazonia.
Stein, Carol; Stein, Seth; Merino, Miguel; Keller, G. Randy; Flesch, Lucy; Jurdy, Donna
Faulting Mode Characterization using fault attributes : Example of a nascent oceanic rift the Manda-Hararo rift in Afar (Ethiopia)
NASA Astrophysics Data System (ADS)
The Manda-Hararo rift segment, located in the Afar depression, underwent a major dyke injection of 65 km long in September 2005, that initiated a rifting episode. From June 2006 to May 2010, 13 other successive dykes were intruded and monitored using InSAR and seismic surveys. Aside from its recent activity, the Manda-Hararo rift architecture shows some particularities which distinguish the segment North of the central magma chamber from the rest of the rift. This Northern segment shows a change of strike of the rift axis and of the overlying faults, as well as a marked asymmetry featured by high no-conjugated west-dipping scarps. These observations led to wonder how the Northern part of this rift has been integrated into the long-term evolution of the whole rift, and whether its deformation mode and fault growth processes might be influenced by the Dabbahu volcano. To address such questions, we focus our analysis on the scaling laws applied to the fault attributes such as fault length, fault scarps or spacing between adjacent faults. This study is based on a fault mapping which was done using optical images (SPOT and, QUICKBIRD images) together with SAR interferograms and coherence images. This map is divided into three regions to isolate the different sources of deformation : the Northern segment close to the Dabbahu volcano, the central one where the main magma reservoir is located and dyke intrusions occurred, and finally the southernmost one coinciding with the segment end. A first stage in determining the scaling law, and consequently the growth mode, consists in characterizing the displacement (Dmax) versus length (L) relationship. With our whole dataset and the different groups of segments defined previously, we observe a scattering suggesting no clear evidence for a linear trend associated with self-similar processes. A possible explanation for such observation in addition to the sampling issue would be a distributed mode of deformation (Soliva et al. 2008). Next, for each of these three regions, we determine the distribution law and discuss them in terms of fault growth processes and the possible role of the fragile thickness as a limiting factor. The center and southern regions tend to a gamma law (Davy 1993), unlike the Northern part where an exponential law seems to be more appropriate. Such observation in the Northern part of the rift mean that faulting would be distributed and scale dependent, when the central and southern parts would be characterized by a faulting mode closer from the transition localized-distributed. The analysis of scaling laws applied to fault attributes is also discussed in terms of inward-outward dipping faults and compared to the oceanic ridge models (Carbotte et al. 1990). These observations suggest a similar and more advanced stage of evolution for the Central and Southern part of the segment, unlike the Northern segment, which shows a less localized deformation. Indeed, the preferential zone of intrusion of the Northern segment seems to be able to laterally jump over time, as illustrated by the unexpected path taken by the September 2005 intrusion.
Dumont, St�phanie; Socquet, Anne; Doubre, C�cile; Grandin, Rapha�l; Klinger, Yann; Medynski, Sarah; Jacques, Eric
Stratigraphic and structural evolution of the Selenga Delta Accommodation Zone, Lake Baikal Rift, Siberia
Seismic reflection profiles from the Lake Baikal Rift reveal extensive details about the sediment thickness, structural geometry\\u000a and history of extensional deformation and syn-rift sedimentation in this classic continental rift. The Selenga River is the\\u000a largest single source of terrigenous input into Lake Baikal, and its large delta sits astride the major accommodation zone\\u000a between the Central and South basins
C. A. Scholz; D. R. Hutchinson
Preliminary Structural and Thermochronological Observations from the South Lunggar Rift, Western Tibet
Here we present the results of recent structural and neotectonic mapping and preliminary zircon (U-Th)\\/He thermochronology from the recently discovered South Lunggar Rift, in the western Lhasa terrane, Tibet. The ~N-trending rift is ~50 km long in the N-S direction and up to 12 km wide in the E-W direction. Maximum elevations exceed 6600 m in the rift flanking range
M. H. Taylor; R. H. Styron; D. F. Stockli; K. E. Sundell; L. Ding
Two mantle plumes beneath the East African rift system: Sr, Nd and Pb isotope evidence from Kenya Rift basalts
Major and trace element and radiogenic isotope ratios (Sr, Nd and Pb) are presented for a suite of Neogene to Recent basalts (MgO>4 wt%) from the axial regions of the Kenya Rift. Samples have compositions ranging from hypersthene-normative basalt through alkali basalt to basanite and are a subset of a larger database in which compositions extend to nephelinite. A broadly
Nick Rogers; Ray Macdonald; J. Godfrey Fitton; Rhiannon George; Martin Smith; Barbara Barreiro
Oligocene Miocene formation of the Haifa basin: Qishon Sirhan rifting coeval with the Red Sea Suez rift system
NASA Astrophysics Data System (ADS)
During mid-Oligocene to early-Miocene times the northeastern Afro-Arabian plate underwent changes, from continental breakup along the Red Sea in the south, to continental collision with Eurasia in the north and formation of the N-S trending Dead Sea fault plate boundary. Concurrent uplift and erosion of the entire Levant area led to an incomplete sedimentary record, obscuring reconstructions of the transition between the two tectonic regimes. New well data, obtained on the continental shelf of the central Levant margin (Qishon Yam 1), revealed a uniquely undisturbed sedimentary sequence which covers this time period. Evaporitic facies found in this well have only one comparable location in the entire eastern Mediterranean area (onland and offshore) over the same time frame � the Red Sea-Suez rift system. Analysis of 4150 km of multi and single-channel seismic profiles, offshore central Levant, shows that the sequence was deposited in a narrow basin, restricted to the continental shelf. This basin (the Haifa Basin) evolved as a half graben along the NW trending Carmel fault, which at present is one of the main branches of the Dead Sea fault. Re-evaluation of geological data onland, in view of the new findings offshore, indicates that the Haifa basin is the northwestern-most of a larger series of basins, comprising a failed rift along the Qishon-Sirhan NW-SE trend. This failed rift evolved spatially parallel to the Red Sea-Suez rift system, and at the same time frame. The Carmel fault would therefore seem to be related to processes occurring several million years earlier than previously thought, before the formation of the Dead Sea fault. The development of a series of basins in conjunction with a young spreading center is a known phenomenon in other regions worldwide; however this is the only known example from across the Arabian plate.
Schattner, U.; Ben-Avraham, Z.; Reshef, M.; Bar-Am, G.; Lazar, M.
Hydrothermal vents is Lake Tanganyika, East African Rift system
Sublacustrine hydrothermal vents with associated massive sulfides were discovered during April 1987 at Pemba and Cape Banza on the Zaire side of the northern basin of Lake Tanganyika, East African Rift system. New investigations by a team of ten scuba divers during the multinational (France, Zaire, Germany, and Burundi) TANGANYDRO expedition (August-October 1991) found hydrothermal vents down to a depth of 46 m along north-trending active faults bounding the Tanganyika rift on the western side. Temperatures from 53 to 103 {degrees}C were measured in hydrothermal fluids and sediments. Veins of massive sulfides 1-10 cm thick (pyrite and marcasite banding) were found associated with vents at the Pemba site. At Cape Banza, active vents are characterized by 1-70-cm-high aragonite chimneys, and there are microcrystalline pyrite coatings on the walls of hydrothermal pipes. Hydrothermal fluid end members show distinctive compositions at the two sites. The Pemba end member is a NaHCO{sub 3}-enriched fluid similar to the NaHCO{sub 3} thermal fluids form lakes Magadi and Bogoria in the eastern branch of the rift. The Cape Banza end member is a solution enriched in NaCl. Such brines may have a deep-seated basement origin, as do the Uvinza NaCl brines on the eastern flank of the Tanganyika basin. Geothermometric calculations have yielded temperatures of fluid-rock interaction of 219 and 179 {degrees}C in the Pemba and Cape Banza systems, respectively. Abundant white or reddish-brown microbial colonies resembling Beggiatoa mats were found surrounding the active vents. Thermal fluid circulation is permitted by opening of cracks related to 130{degrees}N normal-dextral faults that intersect the north-south major rift trend. The sources of heat for such hydrothermal systems may relate to the existence of magmatic bodies under the rift, which is suggested by the isotopic composition of carbon dioxide released at Pemba and Cape Banza. 21 refs., 2 figs.
Tiercelin, J.J. [Universite de Bretagne Occidentale, Brest (France)] [Universite de Bretagne Occidentale, Brest (France); Pflumio, C.; Castrec, M. [Universite Paris VI, Paris (France)] [and others] [Universite Paris VI, Paris (France); and others
The Chukchi Borderland: a Sediment-starved Rifted Continental Margin
NASA Astrophysics Data System (ADS)
The origin and geologic structure of the Chukchi Borderland region, approximately 650 by 400 km in size, has been the subject of speculation since the earliest ice island research groups discovered its existence more than 60 years ago. Multichannel seismic reflection and refraction data acquired between 2007 and 2011, together with legacy seismic data show fragments of high-standing basement (continental) horsts. The structure is draped with less than a kilometer of sediment. Between the high-standing blocks are deep grabens with locally tilted but mostly flat-lying deposits generally only 1-2 km thick. Northwind Escarpment, along the eastern boundary of the Borderland, is a 600-km-long fault adjacent to the deeply subsided and hyper-extended crust of the Canada Basin to the east. The long, linear, sub-parallel orientation of the major structures (including Northwind Escarpment) is consistent with transtensional deformation of the Borderland. The general paucity of thick sediments indicates a sediment-starved environment. Both the North Chukchi Basin on the west and an unnamed deeply buried valley east on the Beaufort margin provide sediment-routing conduits through which sediment by-passed the Borderland throughout much of the Cretaceous history of the growing Brooks Range to the south. Canada Basin deposits also show strata thicken towards the southwest, suggesting sediment influx via the deeply buried valley on the Beaufort margin. On the northeastern side of the Canada Basin, the region is underlain by horst and graben structures with orientations similar to the Chukchi Borderland, but the intervening valleys are filled with as much as two km of sediment and the entire feature is buried beneath another 2 km of post-rift sediment. The similarity of structural styles on both sides of the Canada Basin suggests that this style of transtensional rifting could have been widespread during the early extension of this part of the Arctic and perhaps the Chukchi Borderland and parts of the Canadian Arctic Archipelago margin were conjugates prior to rifting. Seismic data also show that volcanism associated with the High-Arctic Large Igneous Province to the north has intruded or flowed over the northern parts of the Borderland. The Chukchi Borderland, because of its lack of sedimentary cover, offers a unique window into the early rifting history of the Canada Basin and the transition from rifted to hyper-extended continental crust.
Hutchinson, D. R.; Houseknecht, D.; Mosher, D. C.; Hart, P. E.; Jackson, H. R.; Lebedeva-Ivanova, N. N.; Shimeld, J.; Chian, D.
Motion in the north Iceland volcanic rift zone accommodated by bookshelf faulting
NASA Astrophysics Data System (ADS)
Along mid-ocean ridges the extending crust is segmented on length scales of 10-1,000km. Where rift segments are offset from one another, motion between segments is accommodated by transform faults that are oriented orthogonally to the main rift axis. Where segments overlap, non-transform offsets with a variety of geometries accommodate shear motions. Here we use micro-seismic data to analyse the geometries of faults at two overlapping rift segments exposed on land in north Iceland. Between the rift segments, we identify a series of faults that are aligned sub-parallel to the orientation of the main rift. These faults slip through left-lateral strike-slip motion. Yet, movement between the overlapping rift segments is through right-lateral motion. Together, these motions induce a clockwise rotation of the faults and intervening crustal blocks in a motion that is consistent with a bookshelf-faulting mechanism, named after its resemblance to a tilting row of books on a shelf. The faults probably reactivated existing crustal weaknesses, such as dyke intrusions, that were originally oriented parallel to the main rift and have since rotated about 15� clockwise. Reactivation of pre-existing, rift-parallel weaknesses contrasts with typical mid-ocean ridge transform faults and is an important illustration of a non-transform offset accommodating shear motion between overlapping rift segments.
Green, Robert G.; White, Robert S.; Greenfield, Tim
The influence of rifting on escarpment migration on high elevation passive continental margins
NASA Astrophysics Data System (ADS)
Using numerical models that couple surface processes, flexural isostasy, faulting and the thermal effects of rifting, we show that fault-bounded escarpments created at rift flanks by mechanical unloading and flexural rebound have little potential to "survive" as retreating escarpments if the lower crust under the rift flank is substantially stretched. In this configuration, a drainage divide that persists through time appears landward of the initial escarpment in a position close to a secondary bulge that is created during the rifting event at a distance that depends on the flexural rigidity of the upper crust. Moreover, the migration of the escarpment to the secondary bulge occurs when the pre-rift topography dips landward, otherwise the evolution of the escarpment is guided by the pre-existing inland drainage divide. To illustrate this new mechanism for the evolution of passive margins, we study the examples of Southeastern Australia and Southeastern Brazil. We propose that a pre-existing inland drainage divide with rift related flank uplift can produce the double drainage divide observed in Southeastern Australia. On the other hand, we conclude that it is possible that the Serra do Mar escarpments on the Southeastern Brazilian margin originated as a secondary flexural bulge during rifting that persisted through time. In both cases, the retreating escarpment scenario is unlikely and the present-day margin morphology can be explained as resulting from rift-related vertical motions alone, without requiring significant post-rift "rejuvenation".
Sacek, V.; Braun, J.; van der Beek, P.
Modeling suggests that oblique extension facilitates rifting and continental break-up
NASA Astrophysics Data System (ADS)
In many cases the initial stage of continental break-up was and is associated with oblique rifting. That includes break-up in the Southern and Equatorial Atlantic, separation from eastern and western Gondwana as well as many recent rift systems, like Gulf of California, Ethiopia Rift and Dead Sea fault. Using a simple analytic mechanical model and advanced numerical, thermomechanical modeling techniques we investigate the influence of oblique extension on the required tectonic force in a three-dimensional setting. While magmatic processes have been already suggested to affect rift evolution, we show that additional mechanisms emerge due to the three-dimensionality of an extensional system. Focusing on non-magmatic rift settings, we find that oblique extension significantly facilitates the rift process. This is due to the fact that oblique deformation requires less force in order to reach the plastic yield limit than rift-perpendicular extension. The model shows that in the case of two competing non-magmatic rifts, with one perpendicular and one oblique to the direction of extension but otherwise having identical properties, the oblique rift zone is mechanically preferred and thus attracts more strain.
Brune, Sascha; Popov, Anton A.; Sobolev, Stephan V.
Late Paleoproterozoic rift-related magmatic rocks in the North China Craton: Geological records of rifting in the Columbia supercontinent
NASA Astrophysics Data System (ADS)
Late Paleoproterozoic (1.84?1.62 Ga) magmatic rocks including dykes/sills/intrusions and volcanic rocks occur throughout the North China Craton (NCC), which is considered to be part of the Columbia supercontinent by ca. 1.9-1.85 Ga. On the basis of petrogeochemical data, these magmatic rocks can be classified into three major magma types: HN (Nb/La > 0.8, Ce/Nb = 1.7?3, (Th/Nb)N = 0.6?1.2), MN (Nb/La = 0.8-0.5, Ce/Nb = 3?5, (Th/Nb)N = 0.9?3.5) and LN (Nb/La < 0.5, Ce/Nb = 5?80, (Th/Nb)N = 1?60). The geochemical variation of the MN and LN rocks can be explained by lithospheric contamination of asthenosphere- (or plume-) derived magmas, whereas the parental magmas of the HN rocks did not undergo, during their ascent, pronounced lithospheric contamination. These magmatic rocks exhibit at least two characteristics: (1) most displaying a spectrum of compositions from mafic to silicic; (2) forming in an intracontinental rift setting. This Late Paleoproterozoic rift-related magmatism is the most distinguishing feature of the rifting of the Columbia supercontinent.
Xia, Linqi; Xia, Zuchun; Xu, Xueyi; Li, Xiangmin; Ma, Zhongping
High Fluoride and Geothermal Activities In Continental Rift Zones, Ethiopia
NASA Astrophysics Data System (ADS)
The Central Main Ethiopian Rift basin is a continental rift system characterized by volcano-tectonic depression endowed with huge geothermal resource and associated natural geochemical changes on groundwater quality. Chemical composition of groundwater in the study area showed a well defined trend along flow from the highland and escarpment to the rift floor aquifer. The low TDS (< 500mg/l) Ca-Mg-HCO3 dominated water at recharge area in the highlands and escarpments evolve progressively into Ca-Na-HCO3 and Na-Ca-HCO3 type waters along the rift ward groundwater flow paths. These waters finally appear as moderate TDS (mean 960mg/l) Na-HCO3 type and as high TDS (> 1000 mg/l) Na-HCO3-Cl type in volcano-lacustrine aquifers of the rift floor. High concentrations of fluoride (up to 97.2 mg/l) and arsenic (up to 98?g/l) are recognized feature of groundwaters which occur mostly in the vicinity of the geothermal fields and the rift lakes in the basin. Fluoride and arsenic content of dry volcaniclastic sediments close to these areas are in the range 666-2586mg/kg and 10-13mg/kg respectively. The relationship between fluoride and calcium concentrations in groundwaters showed negative correlation. Near-equilibrium state attained between the mineral fluorite (CaF2) and the majority of fluoride-rich (>30mg/l) thermal groundwater and shallow cold groundwater. This indicated that the equilibrium condition control the high concentration of fluoride in the groundwaters. Whereas undersaturation state of fluorite in some relatively low-fluoride (<30mg/l) thermal waters indicated a dilution by cold waters. Laboratory batch leaching experiments showed that fast dissolution of fluoride from the sediment samples suddenly leached into the interacting water at the first one hour and then remain stable throughout the experiment. The concentrations of leached fluoride from the hot spring deposits, the lacustrine sediments, and the pyroclastic rock are usually low (1% of the total or less than the content in the sediment or rock) but strongly correlated with the concentrations in groundwaters in the local vicinity. The readily leachable hot spring deposits and local lacustrine sediments, which were leached easily as high as three fold of other sediments leachability, are considered as the reservoir for the potential fluoride contamination of the rift groundwater. Leaching of fluoride in the sub-surface system is simulated with sediment-packed column leached by flowing water and applying temporary interruption of flow during the experiment. The result indicated that a sharp increase of fluoride concentration (up to 58mg/kg) observed in leachates before one pore-volume of water eluted from the column. The concentration of leached fluoride consequently declined with the increased flowing pore-volume of water and finally the lowest concentrations of leached fluoride occurred in the end of the experiment. Flow interruption during column leaching experiment causes a noticeable fluoride concentration perturbation due to the heterogeneity of the sediment.
Weldesenbet, S. F.; Wohnlich, S.
Fluoride and Geothermal Activities In Continental Rift Zones, Ethiopia
NASA Astrophysics Data System (ADS)
The Central Main Ethiopian Rift basin is a continental rift system characterized by volcano-tectonic depression endowed with huge geothermal resource and associated natural geochemical changes on groundwater quality. Chemical composition of groundwater in the study area showed a well defined trend along flow from the highland and escarpment to the rift floor aquifer. The low TDS (< 500mg/l) Ca-Mg-HCO3 dominated water at recharge area in the highlands and escarpments evolve progressively into Ca-Na-HCO3 and Na-Ca-HCO3 type waters along the rift ward groundwater flow paths. These waters finally appear as moderate TDS (mean 960mg/l) Na-HCO3 type and as high TDS (> 1000 mg/l) Na-HCO3-Cl type in volcano-lacustrine aquifers of the rift floor. High concentrations of fluoride (up to 97.2 mg/l) and arsenic (up to 98?g/l) are recognized feature of groundwaters which occur mostly in the vicinity of the geothermal fields and the rift lakes in the basin. Fluoride and arsenic content of dry volcaniclastic sediments close to these areas are in the range 666-2586mg/kg and 10-13mg/kg respectively. The relationship between fluoride and calcium concentrations in groundwaters showed negative correlation. Near-equilibrium state attained between the mineral fluorite (CaF2) and the majority of fluoride-rich (>30mg/l) thermal groundwater and shallow cold groundwater. This indicated that the equilibrium condition control the high concentration of fluoride in the groundwaters. Whereas undersaturation state of fluorite in some relatively low-fluoride (<30mg/l) thermal waters indicated a dilution by cold waters. Laboratory batch leaching experiments showed that fast dissolution of fluoride from the sediment samples suddenly leached into the interacting water at the first one hour and then remain stable throughout the experiment. The concentrations of leached fluoride from the hot spring deposits, the lacustrine sediments, and the pyroclastic rock are usually low (1% of the total or less than the content in the sediment or rock) but strongly correlated with the concentrations in groundwaters in the local vicinity. The readily leachable hot spring deposits and local lacustrine sediments, which were leached easily as high as three fold of other sediments leachability, are considered as the reservoir for the potential fluoride contamination of the rift groundwater. Leaching of fluoride in the sub-surface system is simulated with sediment-packed column leached by flowing water and applying temporary interruption of flow during the experiment. The result indicated that a sharp increase of fluoride concentration (up to 58mg/kg) observed in leachates before one pore-volume of water eluted from the column. The concentration of leached fluoride consequently declined with the increased flowing pore-volume of water and finally the lowest concentrations of leached fluoride occurred in the end of the experiment. Flow interruption during column leaching experiment causes a noticeable fluoride concentration perturbation due to the heterogeneity of the sediment.
Weldesenbet, S. F.
Comparative sequence stratigraphy of low-latitude versus high-latitude lacustrine rift basins: Seismic data examples from the East African and Baikal rifts
Lakes Baikal, Malawi and Tanganyika are the world's three largest rift valley lakes and are the classic modem examples of lacustrine rift basins. All the rift lakes are segmented into half-graben basins, and seismic reflection datasets reveal how this segmentation controls the filling of the rift basins through time. In the early stages of rifting, basins are fed primarily by flexural margin and axial margin drainage systems. At the climax of syn-rift sedimentation, however, when the basins are deeply subsided, almost all the margins are walled off by rift shoulder uplifts, and sediment flux into the basins is concentrated at accommodation zone and axial margin river deltas. Flexural margin unconformities are commonplace in the tropical lakes but less so in high-latitude Lake Baikal. Lake levels are extremely dynamic in the tropical lakes and in low-latitude systems in general because of the predominance of evaporation in the hydrologic cycle in those systems. Evaporation is minimized in relation to inflow in the high-latitude Lake Baikal and in most high-latitude systems, and consequently, major sequence boundaries tend to be tectonically controlled in that type of system. The acoustic stratigraphies of the tropical lakes are dominated by high-frequency and high-amplitude lake level shifts, whereas in high-latitude Lake Baikal, stratigraphic cycles are dominated by tectonism and sediment-supply variations.
Scholz, C. A.; Moore, Jr. , T. C.; Hutchinson, D. R.; Golmshtok, A. Ja.; Klitgord, K. D.; Kurotchkin, A. G.
Geochemical evidence of lithospheric thinning in the southern Main Ethiopian Rift
NASA Astrophysics Data System (ADS)
Lithospheric thinning is a fundamental process associated with the transition from continental to oceanic regimes during continental rifting. Precisely how and when this lithospheric thinning proceeds are first order controls on rift basin evolution. The Main Ethiopian Rift, part of the � 2000 km long East African Rift System, is the archetypical modern example of continental rifting, and a key location in which to study the evolution of the lithosphere during extension. This study explores lithospheric modification in the interface region between the Main Ethiopian rift and northward propagating rifting from Kenya through a major and trace element study of rift initiation and maturation using the 19 myr magmatic record preserved in this region. Initial rifting in southern Ethiopia is coincident with the eruption of basalts along the rift shoulders that are characterized by deep fractionation trends (0.5 GPa) and poorly developed magmatic pathways. The earliest of these basalts are derived from melting columns where the aluminum phase is garnet-dominated (Tb N/Yb N � 1.8-2) and has geochemical characteristics interpreted as melting of the lithospheric mantle. The transition from initial rift shoulder magmatism to Quaternary magmatic-tectonic fault belts on the modern rift floor at Arba Minch (6�N) is coincident with a shallowing of the melting column (Tb N/Yb N � 1.3-1.7), less significant contributions from the lithospheric mantle, and the establishment of a shallow fractionation regime (0.1 GPa). At Chencha (� 6.3�N) newly dated (12.32 � 0.17 Ma) magmatism on the rift shoulder has similar fractionation paths to contemporaneous magmatism to the south (0.5 GPa), but is derived from a different, shallower mantle source (Tb N/Yb N � 1.3-1.5) that we interpret results from lithospheric thinning associated with the now-inactive Chow Bahir rift. Between 6.5 and 8�N, significant surface faulting and shallow magmatic fractionation paths (0.1 GPa) in the dominant Quaternary structure of the Main Ethiopian Rift (the Wonji Fault Belt and Silti-Debre Zeyit Fault Zone), highlights the strong connection between magmatism and extensional tectonics in these structures. Along the eastern rift margin, Wonji Fault Belt magmas are derived from a dominantly shallow melting column (Tb N/Yb N � 1.4-1.7) that is similar in composition to the older rift shoulder lavas at Chencha. Adjacent to the western rift margin, magmas erupted in the Silti-Debre Zeyit Fault Zone are interpreted to have erupted through a thicker lithosphere as these magmas are derived from a deeper melting column (Tb N/Yb N � 1.7-2.1) that contains some minor apatite. The inferred variations in lithospheric thickness in southern Ethiopia outlined in this study illustrate the interaction between northward and southward rift propagation in addition to lateral variations across the rift floor as extension migrates into zones of focused magmatic intrusion. The results of this investigation show that geochemical techniques can be applied to probe the history of lithospheric modification during rifting and provide new constraints for models of rift development.
Rooney, Tyrone O.
Beta Regio rift system on Venus: Geologic interpretation of Magellan images
NASA Technical Reports Server (NTRS)
Magellan SAR images and altimetric data were used to produce a new geologic map of the Northern part of Beta Regio within the frames of C1-30N279 mapsheet. It was part of our contributions into C1-formate geologic mapping efforts. The original map is at 1:8,000,000 scale. The rift structures are typical for Beta Regio on Venus. There are many large uplifted tessera areas on Beta upland. They occupy areas of higher topography. These tessera are partly burried by younger volcanic cover of plain material. These observations show that Beta upland was formed mainly due to lithospheric tectonical uplifting, and only partly was constructed by volcanic activity. A number of rift valleis traverse Beta upland and spread to the surrounding lowlands. The largest rift crosses Beta N to S. Typical width of rifts is 40 to 160 km. Rift valleis in this region are structurally represented by crustal grabens and half-grabens. There are symmetrical and asymmetrical rifts. A lot of them have shoulder uplifts with the relative high up to 0.5-1 km and width 40 to 60 km. Preliminary analysis of the largest rift valley structural cross-sections leads to the conclusion that it originated due to a 5-10 percent crustal extension. The prominent shield volcano - Theia Mons - is located at the center of Beta rift system. It could be considered as the surface manifestation of the upper mantle hot spot. Most of the rift belts are located radially to Theia Mons. The set of these data leads to conclusion that Beta rift system has an 'active-passive' origin. It was formed due to the regional tectonic lithospheric extension. Rifting was accelerated by the upper mantle hot spot located under the center of passive extension (under Beta Regio).
Nikishin, A. M.; Bobina, N. N.; Borozdin, V. K.; Burba, G. A.
Rapid opening of the Asal rift in Afar observed with radar interferometry
NASA Astrophysics Data System (ADS)
Located at the western end of the Aden ridge, the Asal rift is the first emerged section of the ridge propagating into Afar and the locus of intense volcanic and seismic activity. We use radar interferometry data acquired by the Canadian satellite Radarsat between 1997 and 2003 from both ascending and descending passes to measure the surface deformation in a 100 km wide region centered on the rift. The turbulent atmosphere in this sub-tropical region produces a phase delay error in the data exceeding the tectonic signal we seek. To estimate the deformation rates from the series of interferograms, we solve a least-square problem and derive the vertical and rift-perpendicular, horizontal components of the surface velocity from the series of ascending and descending line of sight observations by the radar. The resulting 2-component surface velocity map of the rift area shows the following features: A ~40 km wide zone centered on the rift is inflating at a rate of ~7 mm/yr. The ~8 km-wide central rift subsides at a rate of ~2mm/yr with respect to the shoulders of the rifts. The horizontal velocity indicates extension across the central rift at a rate of up to 20 mm/yr, gradually decreasing in the far field, the maxima of the horizontal velocity being located on both side of the rift, ~12 km from its axis. This local opening rate exceeds the 13 mm/yr far-field plate motion between the Arabia and Nubia plates, suggesting that magmatic activity is currently controlling the opening of the Asal rift. Preliminary models shows that a 4 km deep dyke system expanding both laterally and upward accounts for the observed velocity field across the Asal rift.
Peltzer, G.; Mignan, A.; King, G.; Manighetti, I.
Stress perturbation associated with the Amazonas and other ancient continental rifts
The state of stress in the vicinity of old continental rifts is examined to investigate the possibility that crustal structure associated with ancient rifts (specifically a dense rift pillow in the lower crust) may modify substantially the regional stress field. Both shallow (2.0-2.6 km depth) breakout data and deep (20-45 km depth) crustal earthquake focal mechanisms indicate a N to NNE maximum horizontal compression in the vicinity of the Paleozoic Amazonas rift in central Brazil. This compressive stress direction is nearly perpendicular to the rift structure and represents a ???75?? rotation relative to a regional E-W compressive stress direction in the South American plate. Elastic two-dimensional finite element models of the density structure associated with the Amazonas rift (as inferred from independent gravity modeling) indicate that elastic support of this dense feature would generate horizontal rift-normal compressional stresses between 60 and 120 MPa, with values of 80-100 MPa probably most representative of the overall structure. The observed ???75?? stress rotation constrains the ratio of the regional horizontal stress difference to the rift-normal compressive stress to be between 0.25 and 1.0, suggesting that this rift-normal stress may be from 1 to 4 times larger than the regional horizontal stress difference. A general expression for the modification of the normalized local horizontal shear stress (relative to the regional horizontal shear stress) shows that the same ratio of the rift-normal compression relative to the regional horizontal stress difference, which controls the amount of stress rotation, also determines whether the superposed stress increases or decreases the local maximum horizontal shear stress. The potential for fault reactivation of ancient continental rifts in general is analyzed considering both the local stress rotation and modification of horizontal shear stress for both thrust and strike-slip stress regimes. In the Amazonas rift case, because the observed stress rotation only weakly constrains the ratio of the regional horizontal stress difference to the rift-normal compression to be between 0.25 and 1.0, our analysis is inconclusive because the resultant normalized horizontal shear stress may be reduced (for ratios >0.5) or enhanced (for ratios <0.5). Additional information is needed on all three stress magnitudes to predict how a change in horizontal shear stress directly influences the likelihood of faulting in the thrust-faulting stress regime in the vicinity of the Amazonas rift. A rift-normal stress associated with the seismically active New Madrid ancient rift may be sufficient to rotate the horizontal stress field consistent with strike-slip faults parallel to the axis of the rift, although this results in a 20-40% reduction in the local horizontal shear stress within the seismic zone. Sparse stress data in the vicinity of the seismically quiescent Midcontinent rift of the central United States suggest a stress state similar to that of New Madrid, with the local horizontal shear stress potentially reduced by as much as 60%. Thus the markedly different levels of seismic activity associated with these two subparallel ancient rifts is probably due to other factors than stress perturbations due to dense rift pillows. The modeling and analysis here demonstrate that rift-normal compressive stresses are a significant source of stress acting on the lithosphere and that in some cases may be a contributing factor to the association of intraplate seismicity with old zones of continental extension.
Zoback, M. L.; Richardson, R. M.
Rifted continental margins: geometric control on crustal architecture and melting
NASA Astrophysics Data System (ADS)
A new model is provided for the distribution of magma-poor and magma-rich rifted margins. The South Atlantic, Central Atlantic, North Atlantic - Arctic (Eurasia Basin), and Red Sea all are magma-rich at their distal ends and magma-poor at their proximal ends (with respect to their poles of rotation). The well-known architectural zonation across fully developed magma-poor margins (limited crustal stretching, hyperextension, exhumed mantle, oceanic crust) is also observed along the lengths of many margins at the super-regional scale. Zones of exhumed mantle, marking magma-poor margin, can be mapped for thousands of kilometers. Likewise can zones of seaward dipping reflectors (SDR) marking magma-rich margins. At this scale, the age of the oceanic crust becomes younger in the direction of the rotation pole, implying that the continents ruptured by rift tip propagation (and rotation pole propagation). Propagation is also manifested by the age of pre-break-up magmatism, break-up unconformity, and margin uplift. Hence, the classic cross-sectional depiction of margin evolution has a third dimension. The degree of melting follows the same pattern. At the distal end of e.g. the South Atlantic, SDR zones are wide and gradually thin toward the rotation pole. Eventually exhumed mantle takes over, marking the transition to the magma-poor margins, which remain to the proximal end of rifting. SDR zones also thin laterally from ca 10-15 km thickness at the continent-ocean boundary (COB) to ca 7 km thick oceanic crust beyond the SDRs. Outcrop data demonstrate that also exhumed mantle contains up to ca 12% melt, infiltrated in the peridotites. Thus, melting is largest at the distal ends near the COB, and decreases both laterally toward the evolving ocean and along strike toward the rift tip. Accepting that continents are rigid to a first order, the linear rate of extension at any given location along an evolving rift and ocean, is governed by the angular rate of opening, the distance to the rotation pole, and the rate of propagation of the pole. For a fixed angular rate, the linear extension rate increases away from the pole. Numerical models reveal that both mantle temperature and rate of extension can govern the degree of melting. However, the above empirical observations suggest that to a first order the rifted margin architecture, including the degree of melting, is governed by the linear rate of extension, which is a direct outcome of geometric rules of plate tectonics. Rapid pole propagation, or a pole jump, will induce a rapid increase in the linear rate. Magma-rich margins seem to form when continents break at a high extension/strain rate caused by rapid propagation; this occurs at the distal end of a rupturing plate. Our testable model questions the common ad hoc introduction of mantle plumes to explain "excess" melting along magma-rich margins. This does not rule out that mantle heterogeneities may exist, but such heterogeneities appear second order when it comes to generating magma-rich margins.
Lundin, Erik; Redfield, Tim; Peron-Pinvidic, Gwenn
North Sinai-Levant rift-transform continental margin
The passive continental margin of northern Egypt and the Levant coast formed during the Early mesozoic as the relatively small Anatolia plate broke away from northern Africa. The oceanic basin of the eastern Mediterranean and the unusual right-angle bend in the North Sinai-Levant shelf margin are both products of plate separation along a rift-transform fracture system, the south arm of Tethys. The north-south trending Levant transform margin is considerably narrower than the east-west trending rift margin of northern Egypt. Both exhibit similar facies and depositional histories through the mid-Tertiary. Analysis of subsurface data and published reports of the regional stratigraphy point to a three-stage tectonic evolution of this passive margin. The Triassic through mid-Cretaceous was marked by crustal breakup followed by rapid rotational subsidence of the shelf margins about hinge lines located just south and east of the present shorelines. Reef carbonates localized on the shelf edge separated a deep marine basin to the north from a deltaic-shallow marine platform to the south and east. In the Late Cretaceous-Early Tertiary, inversion of earlier formed half-grabens produced broad anticlinal upwarps of the Syrian Arc on the shelf margin that locally influenced facies patterns. The episode of inversion corresponds with the onset of northward subduction of the Africa plate beneath southern Asia. Beginning in the Oligocene and continuing to the present, there has been renewed subsidence of the North Sinai shelf margin beneath thick, outward building clastic wedges. The source of this large volume of sediment is the updomed and erosionally stripped margins of the Suez-Red Sea Rift and the redirected Nile River.
Ressetar, R.; Schamel, S.; Travis, C.J.
Present-day Kinematics of the East African Rift
NASA Astrophysics Data System (ADS)
The East African Rift (EAR), a ~5000 km-long series of seismically active structures that mark the divergent boundary between the Somalia and Nubia plates, is often cited as a modern archetype for rifting and continental breakup. Paradoxically, its current kinematics is the least well-known of all major plate boundaries, owing to its tremendous extent, difficult access, and lack of geodetic data. The existence of two "new" tectonic plates within the EAR (Victoria and Rovuma) between the main Nubian and Somalian plates has recently been proposed through combined analysis of earthquake slip vector and (sparse) space-geodetic data (Calais et al., 2006). A third, dominantly oceanic Lwandle plate was recently postulated by a re-analysis of the post 3.2 Ma spreading rates and transform-fault azimuths along the Southwest Indian Ridge (SWIR; Horner-Johnson et al., 2007). Here we use an updated geodetic solution -- a combination of all continuous GPS stations on the Nubia, Somalia, and Antarctic plates, campaign GPS measurements in Africa, and a global DORIS solution -- together with earthquake slip vectors in the EAR and transform azimuth and spreading rates along the SWIR. We show that the space geodetic data and the 3.2 Ma average oceanic data along the SWIR are consistent with each other at the 95% confidence level. The data support a kinematic model with (1) a present-day Nubia-Somalia Euler pole located to the SE of the southern tip of Africa, (2) the existence of three distinct microplates embedded in the EAR: Victoria, Rovuma, and Lwandle. Additional geodetic data in the EAR are now needed to confirm and further refine this model. References Calais, E., C. Hartnady, C. Ebinger, and J.M. Nocquet, Kinematics of the East African Rift from GPS and earthquake slip vector data, In: Yirgu, G., Ebinger, C.J. & Maguire, P.K.H. (eds) Structure and Evolution of the Rift Systems within the Afar volcanic province, Northeast Africa, Geological Society Special Publications, 259, p.9-22, 2006. Horner-Johnson, B., R.G. Gordon, D.F. Argus, Plate kinematic evidence for the existence of a distinct plate between the Nubian and Somalian plates along the Southwest Indian Ridge, Journal of Geophysical Research, 112, 2007
Stamps, D. S.; Calais, E.; Saria, E.; Mbede, E.; Ebinger, C.; Delvaux, D.; Kervyn, F.; Combrinck, L.; Hartnady, C.; Nocquet, J.; Fernandes, R.
Crustal structure of the Columbia Plateau: Evidence for continental rifting
NASA Astrophysics Data System (ADS)
Data from a 260-km-long seismic refraction profile provide a detailed look at the crustal structure beneath the central Columbia Plateau (CCP). The CCP profile, centered on the Pasco Basin in eastern Washington, trends approximately N50� E between the towns of Wasco, Oregon, and Warden, Washington. The upper crust above the crystalline basement consists of between 5 and 12 km of the Columbia River Basalt Group (CRBG) and underlying sediments. The CRBG is laterally variable in thickness, ranging from 3 to 6 km over the length of the CCP seismic profile, but is thickest near the center of the Columbia Plateau. A thick, but laterally varying, accumulation of sediments underlies the CRBG with the thickest accumulation of sediments (5.0 km/s) occurring beneath the Pasco Basin in what we interpret to be a graben. Sediments there extend from the base of the CRBG to basement (6.3 km/s) at depths between 10 and 12 km. We recognize two layers between the basement and the upper mantle with seismic velocities of 6.8 and 7.5 km/s, located at 18- and 25-km depths, respectively. The 6.8-km/s layer thins beneath the graben, where it has apparently been replaced by the 7.5-km/s layer, creating the characteristic geometry and velocity of the "rift pillow" layer previously observed in some other recognized continental rifts. The Mono (8.4 km/s upper mantle) is located at about 40-km depth; an apparent localized, low-velocity layer is observed within the upper mantle at a depth of about 50 km. Thus the deep crustal and upper mantle structure beneath the central Columbia Plateau is complex and is atypical of normal deep continental crust. The presence of an upper crustal graben and lower crustal rift pillow, as inferred from the seismic refraction data, combined with gravity, earthquake, electrical, and geologic data, suggest that Eocene (and possibly later) continental rifting occurred prior to deposition of the CRBG.
Catchings, R. D.; Mooney, W. D.
Seismic investigation of the southern Rio Grande Rift
NASA Astrophysics Data System (ADS)
Competing models exist to explain what caused the Earth's crust to spread apart 29 million years ago to create a region known today as the Rio Grande Rift (RGR). The RGR extends from central Colorado through New Mexico to northern Mexico, near El Paso. The RGR has different geologic features that distinguish it from most other valleys (e.g., the RGR was not cut by a river nor does a river branch upstream). A growing body of evidence shows that geologic activity still occurs in the RGR, with a continuation of faulting, seismicity and widening at a small rate of about 0.3 mm/yr (Woodward , 1977). We map of the seismic velocity structure and crustal thickness using data from the Rio Grande Rift Seismic TRAnsect (RISTRA) experiment and the EarthScope Transportable Array (USArray) dataset. In addition to the data we collected from the RISTRA experiment and USArray dataset, we also acquired receiver functions from the EarthScope Automatic Receiver Survey (EARS) website (http://www.earthscope.org/data) and waveform data from the Incorporated Research Institutes for Seismology (IRIS) Data Management Center (DMC). We requested seismograms from the IRIS DMC database where we acquired teleseismic events from Jan 2000 to Dec 2009. This includes 7,259 seismic events with a minimum magnitude of 5.5 and 106,389 continuous waveforms. This data was preprocessed (merged, rotated) using a program called Standing Order of Data (SOD). The RISTRA experiment and the USArray were designed to image crust and mantle structures by computing receiver functions for all data in the Southern Rio Grande Rift (SRGR). We map the crustal thickness, seismic velocity, and mantle structure for the sole purpose to better determine the nature of tectonic activity that is presently taking place and further investigate the regional extension of the Southern Rio Grande Rift (SRGR). Here we present preliminary results of the crustal and velocity structure using the kriging interpolation scheme seem stable and we are now able to clearly observe certain patterns we can use to interpret the southern RGR deformation and extension.
Thompson, Lennox E.
Probing the age and temperature of rifting in Afar
NASA Astrophysics Data System (ADS)
Rifting along the southern part of the Red Sea margin in NE Africa (leading to formation of Afar) has been closely associated with magmatic activity since the initiation of extension at around ~ 25Ma. Numerous active volcanoes are currently found along rift zones here and magma intrusion into the crust has potentially accommodated significant amounts of extension. This extensive present-day volcanism has been linked to elevated mantle temperature, perhaps due to a thermal plume, or as a consequence of passive flow in the mantle beneath the extending lithosphere. Geochemical evidence for basaltic lavas erupted in Afar have been used to suggest that mantle temperatures are in the range 1370 to 1490�C, and that the region is currently experiencing late stage rifting. Analysis of changes in shear wave seismic velocities and relative travel time tomography suggests mantle temperatures are within a similar range, yet the region has greater similarities to a young spreading centre. The range in potential temperature estimates is however very large, with different implications for the volcanic history of the region and hence timing of break-up. Rather than focusing a single observable, we use a relatively straight forward model of extension and decompression melting to predict the seismic-velocity and attenuation structure of the asthenosphere and lithosphere, synthetic receiver functions as a result of this seismic structure, crustal thickness as a result of decompression and finally the melt composition. From this combined study we find that melt composition and seismic structure are dependent on both temperature and time. If mantle potential temperature is 1350�C then both the seismic structure and melt composition can be matched if the duration of extension is more than 30 Myr. However this is longer than the estimated duration of extension from plate reconstructions, and given the low rate of extension in Afar, this cold model only generates up to 5 km of igneous crust. If mantle potential temperature is 1450�C then both the seismic structure and melt composition can be matched after 22 Myr of extension. Furthermore, igneous crustal thickness is in excess of 10 km. Therefore, the most simple explanation is that Afar is a developing continental rift zone where melting is enhanced by a warm mantle.
Armitage, John; Goes, Saskia; Ferguson, David; Hammond, James; Calais, Eric
Internal structure and deformation of an accommodation zone in the northern part of the Suez rift
NASA Astrophysics Data System (ADS)
Detailed structural study of the eastern (onshore) part of the Gharandal accommodation zone that separates the northern (SW-dipping) and central (NE-dipping) half grabens of the Suez rift helpeddecipher the internal structure and deformation of accommodation zones of continental rifts. This 60 km-wide zone is affected by pure normal faulting. The NE-dipping faults of the northern half graben extend southward into the accommodation zone where they interfinger with SW-dipping faults extending from the central half graben. These two sets of rift-parallel faults form several horsts and grabens in the accommodation zone. Areas dipping parallel to the northern or southern half grabens form several intermixing dip domains in the accommodation zone. Smaller-scale accommodation of dip between these dip domains proceeds by the development of rift-parallel folds (twist zones). In contrast to the southern accommodation zone of the Suez rift, the internal structure of the Gharandal accommodation zone is believed to be representative of accommodation zones in regions (a) unaffected by prerift structures lying at high angles to the rift; and (b) experiencing relatively small extension. Accommodation zones in areas having pre-rift structures lying at high angle to the rift have relatively narrow width and are characterized by transverse, strike-slip faults. Strike-slip movement on these faults is related to the torsional strain resulting from the opposite tilt directions and transport of fault blocks of adjacent half grabens.
Moustafa, Adel R.
Episodic spreading and rift propagation: New paleomagnetic and geochronologic data from the afar Nascent passive margin
A recent model proposes that afar is one of the few areas in the world where the formation of a passive margin by rift propagation can be observed directly. Forty sites spread over approximately 10,000 km2 in the Republic of Djibouti, between the Asal and Abhe rifts, have been sampled for paleomagnetic and geochronologic study. Most basalt samples have a
V. Courtillot; F. Landre; J. Achache; N. Bonhommet; R. Montigny; G. F�raud
Melting during late-stage rifting in Afar is hot and deep.
Investigations of a variety of continental rifts and margins worldwide have revealed that a considerable volume of melt can intrude into the crust during continental breakup, modifying its composition and thermal structure. However, it is unclear whether the cause of voluminous melt production at volcanic rifts is primarily increased mantle temperature or plate thinning. Also disputed is the extent to which plate stretching or thinning is uniform or varies with depth with the entire continental lithospheric mantle potentially being removed before plate rupture. Here we show that the extensive magmatism during rifting along the southern Red Sea rift in Afar, a unique region of sub-aerial transition from continental to oceanic rifting, is driven by deep melting of hotter-than-normal asthenosphere. Petrogenetic modelling shows that melts are predominantly generated at depths greater than 80?kilometres, implying the existence of a thick upper thermo-mechanical boundary layer in a rift system approaching the point of plate rupture. Numerical modelling of rift development shows that when breakup occurs at the slow extension rates observed in Afar, the survival of a thick plate is an inevitable consequence of conductive cooling of the lithosphere, even when the underlying asthenosphere is hot. Sustained magmatic activity during rifting in Afar thus requires persistently high mantle temperatures, which would allow melting at high pressure beneath the thick plate. If extensive plate thinning does occur during breakup it must do so abruptly at a late stage, immediately before the formation of the new ocean basin. PMID:23823795
Ferguson, D J; Maclennan, J; Bastow, I D; Pyle, D M; Jones, S M; Keir, D; Blundy, J D; Plank, T; Yirgu, G
Rifting of the plate boundary in North Iceland 1975-1978
A rifting episode started in 1975 on the accreting plate boundary in North Iceland after 100 years of quiescence. Horizontal extension of some 3 m has been observed in the Krafla caldera and the associated 80 km long fissure swarm. The rifting occurs periodically in short active pulses at a few months intervals. Between these active pulses, continuous inflation of
Axel Bj�rnsson; Gunnar Johnsen; Sven Sigurdsson; Gunnar Thorbergsson; Eysteinn Tryggvason
He and Ne isotopic ratios along the Terceira Rift: implications for the Azores mantle source
Noble gas data (He and Ne) on olivine phenocrysts obtained from Azores' lavas sampled along the Terceira Rift will be presented in this work. The Terceira Rift is considered as one of the slowest spreading system in the world (Vogt & Jung, 2004). Lava samples were collected inland at S. Miguel, Terceira, Graciosa, Pico and Faial Islands as well at
P. Madureira; M. A. Moreira; J. Nunes; N. Lourenco; M. Carvalho; J. Mata; M. Pinto de Abreu
The Effect of Plume Impingement on Lithospheric Preservation Beneath the Kenya Rift, East Africa
The Kenya Rift is located at the transition between Archean Tanzanian craton and Proterozoic mobile belt. Currently, discrepancies exist between geochemical and geophysical interpretations of lithospheric preservation in the Kenya Rift. Seismic data show a sharp vertical boundary between low velocity mantle in the axis and higher velocity mantle on the flanks, which is interpreted to reflect lithospheric erosion from
J. M. Hamblock; E. Y. Anthony; J. T. Chesley; P. A. Omenda
East African Rift System (EARS) Plume Structure: Insights from Quaternary Mafic Lavas of Turkana, Kenya
Quaternary mafic lavas from Lake Turkana (northern Kenya) provide information on processes operating beneath the East African Rift in an area of anomalous lithospheric and crustal thinning. Inferred depths of melting beneath Turkana (15---20km) are shal- lower than those recorded elsewhere along the rift, consistent with the anomalously thin crustal section. The mafic lavas have elevated incompatible trace element contents
Anatomy of a river drainage reversal in the Neogene Kivu Nile Rift
NASA Astrophysics Data System (ADS)
The Neogene geological history of East Africa is characterised by the doming and extension in the course of development of the East African Rift System with its eastern and western branches. In the centre of the Western Rift Rise Rwanda is situated on Proterozoic basement rocks exposed in the strongly uplifted eastern rift shoulder of the Kivu-Nile Rift segment, where clastic sedimentation is largely restricted to the rift axis itself. A small, volcanically and tectonically controlled depository in northwestern Rwanda preserved the only Neogene sediments known from the extremely uplifted rift shoulder. Those (?)Pliocene to Pleistocene/Holocene fluvio-lacustrine muds and sands of the Palaeo-Nyabarongo River record the influence of Virunga volcanism on the major drainage reversal that affected East Africa in the Plio-/Pleistocene, when the originally rift-parallel upper Nile drainage system became diverted to the East in order to enter the Nile system via Lake Victoria. Sedimentary facies development, heavy mineral distributions and palaeobiological controls, including hominid artefacts, signal a short time interval of <300-350 ka to complete this major event for the sediment supply system of the Kivu-Nile Rift segment.
Holzf�rster, F.; Schmidt, U.
Melt-induced seismic anisotropy and magma assisted rifting in Ethiopia: Evidence from surface waves
The East African rift in Ethiopia is unique worldwide because it captures the final stages of transition from continental rifting to seafloor spreading. A recent study there has shown that magma intrusion plays an important role during the final stages of continental breakup, but the mechanism by which it is incorporated into the extending plate remains ambiguous: wide-angle seismic data
I. D. Bastow; S. Pilidou; J.-M. Kendall; G. W. Stuart
Protection of Mice and Lambs Against Pantropic Rift Valley Fever Virus, Using Immune Serum.
National Technical Information Service (NTIS)
Immune serum was used prophylactically and therapeutically in mice and lambs infected with Rift Valley fever virus. One-tenth milliliter of immune serum was effective in protecting mice against challenge with Rift Valley fever virus for a period of 2 week...
D. G. Bennett R. D. Glock P. J. Gerone
Tectonic history of the northern Rio Grande rift derived from apatite fission-track geochronology
This study is concerned with the uplift history of the mountain ranges that bound the eastern margin of the Rio Grande rift in northern New Mexico. The principal objectives of this paper are to present the results of fission-track dating of granitic rocks from three ranges on the eastern margin of the rift, and to use these dates to infer
S. A. Kelley; I. J. Duncan
Connection of the Panama fracture zone with the Galapagos rift zone, eastern tropical Pacific
Magnetic data recently collected in the eastern tropical Pacific confirm that the Galapagos rift zone is connected to the Panama fracture zone by a short north-south fracture zone (the Ecuador fracture zone) and a short east-west center of sea floor spreading (the Costa Rica rift zone). These features were found approximately in the locations predicted by Molnar and Sykes from
Paul J. Grim
Geoscience Methods Lead to Paleo-anthropological Discoveries in Afar Rift, Ethiopia
NASA Astrophysics Data System (ADS)
With few exceptions, most of the hominid evolutionary record in Africa is closely associated with the East African Rift System. The exceptions are the South African and Chadian hominids collected from the southern and west-central parts of the continent, respectively. The Middle Awash region stands alone as the most prolific paleoanthropological area ever discovered (Figure 1). Its paleontological record has yielded over 13,000 vertebrate fossils, including several hominid taxa, ranging in age from 5.8 Ma to the present. The uniqueness of the Middle Awash hominid sites lies in their occurrence within long, > 6 Ma volcanic and sedimentary stratigraphic records. The Middle Awash region has yielded the longest hominid record yet available. The region is characterized by distinct geologic features related to a volcanic and tectonic transition zone between the continental Main Ethiopian and the proto-oceanic Afar Rifts. The rift floor is wider-200 km-than other parts of the East African Rift (Figure 1). Moreover, its Quaternary axial rift zone is wide and asymetrically located close to the western margin. The fossil assemblages and the lithostratigraphic records suggest that volcanic and tectonic activities within the broad rift floor and the adjacent rift margins were intense and episodic during the late Neogene rift evolution.
WoldeGabriel, Giday; Renne, Paul R.; Hart, William K.; Ambrose, Stanley; Asfaw, Berhane; White, Tim D.
Earthquakes along the East African Rift System: A multiscale, system-wide perspective
On the basis of a comprehensive data set of precisely determined depths of 121 large to moderate-sized earthquakes along and near the entire East African Rift System (EARS), there are three distinct patterns in focal depths which seem to correlate with progressive stages in the development of the largest active rift in the world. First, away from both ends of
Zhaohui Yang; Wang-Ping Chen
Earthquakes along the East African Rift System: A multi-scale, continent-wide perspective
Based on a comprehensive dataset of precisely determined depths of 121 large to moderate-sized earthquakes along and near the entire East African Rift system (EARS), there are three distinct patterns in focal depths which seem to reflect progressive stages in the development of the largest active rift in the world. First, away from both ends of the western, younger branch
Z. Yang; W. Chen
Strain partitioning in hyper-extended, strongly segmented rift systems: insights from the Cretaceous Bay of Biscay- Pyrenean rift system and comparison with present-day mature rift-transform margins
NASA Astrophysics Data System (ADS)
Continental margins are often subdivided into transform and volcanic and non-volcanic rifted margins, although, in reality, such end-member type margins do not exist and the distribution of strain and magma leading to lithospheric breakup is more complex. Key questions related to the development of oblique and/or segmented rift-transform margins include the importance of inheritance, the partitioning of deformation in time and space, the interplay between deformation and magmatism, the timing and location of breakup, and the isostatic evolution of these systems during and after final rifting. At present-day continental margins the initial stages associated with the development of highly segmented rift-transform margins are often masked by thick sedimentary sequences and the relation between the rift structures, syn-tectonic sediments and magmatic additions remain poorly understood. Moreover, it looks as if the oceanic transform faults do not develop from transfer or transform faults within continental rifts, suggesting that the continental and oceanic systems are decoupled within the ocean continent transition. In this study we use the Bay of Biscay - Pyrenean system to understand how deformation was distributed in time and space during the evolution of a highly segmented rift-transform system along the Iberian/European plate boundary during Late Jurassic to Mid Cretaceous time. We will show that the rift basins (Parentis, Arzacq-Maul�on, Cantabrian basins) that developed along this embryonic plate boundary record a complex poly-phase deformation history, showing locale evidence for extreme crustal thinning and locally also mantle exhumation. Because these basins are preserved to the west (Bay of Biscay-Parentis), but reactivated and exposed in the east (Pyrenees), the basins and related structures can be studied using geological and geophysical methods. In our presentation we will show new observations and preliminary results that enable discussion about how a segmented rift-transform plate boundary formed in time and space. We will also show that the poly-phase evolution recorded along the European-Iberian plate boundary has important kinematic implications for the pre-breakup evolution that cannot be taken into account by kinematic models based on magnetic anomaly restorations only. These results, combined with those of present day margins, may give some insights on the pre-breakup evolution and processes that are at the origin of highly segmented rift-transform margins as seen in the Equatorial Atlantic.
Manatschal, G.; Tugend, J.; Masini, E.; Kusznir, N. J.
Melt distribution in the Ethiopian rift system: Constraints from seismic observations and modelling
NASA Astrophysics Data System (ADS)
Seismic observations from the EAGLE experiment in the Main Ethiopian Rift have been interpreted in terms of melt-induced anisotropy and support ideas of magma-assisted rifting in continental regions. Following the 2005 Dabbahu rifting event in Afar a further 9 broadband seismometers were installed around the newly active rift segment. These recorded more than one year of continuous data and shear-wave splitting observed in core phases (SKS/SKKS) shows considerable variability across the array. Three stations centred above the Dabbahu rift segment show markedly different splitting characteristics from the other stations. The fast direction is oriented roughly north/south and parallel to the Dabbahu magmatic segment, compared to NNE/SSW orientations at nearby stations. Also the magnitude of splitting is slightly larger at the rift stations compared to those nearby (~1s compared to 0.7-0.9s). These observations supports previous work in the Main Ethiopian Rift (MER), where fast directions change abruptly from being rift parallel on the rift flanks to magmatic-segment parallel in the rift valley. Furthermore, observations of frequency-dependent splitting in the data further suggest that the underlying cause of the anisotropy is related to aligned melt inclusions. Poroelastic modelling support mechanisms for melt-induced anisotropy due to vertically-aligned melt pockets that are on the order of centimetres in length scale. The abrupt change in splitting parameters over small lateral distances (~ 30 � over ~30~km) suggests that the source of anisotropy is shallow. To further constrain the location of the anisotropy and study the influence of the rift transition on shear-wave splitting results, we model finite-frequency waveforms for a suite of model representations of the rift zone. In each model, the orientation of the anisotropic fabric varies laterally (i.e., the symmetry axis of the HTI symmetry rotates between 0� and 30�), but the strength of anisotropy and depth of transition differs. Waveforms are modeled using a narrow-angle one-way elastic vector wave equation to simulate finite-frequency waveform effects for an incident near-planar S-wave. The modelling is used to examine the influence of changing anisotropic symmetry across the rift as a function of wavefront curvature, and strength and depth of transition as well as lateral width of transition zone. The results show how a simple model with two regimes of anisotropy can explain the variability across the rift, in both delay time and shear-wave polarization, over short length scales.
Hammond, J.; Kendall, J.; Angus, D.; Wookey, J.; Keir, D.; Ebinger, C.
Melt distribution in the Ethiopian rift system: Constraints from seismic observations and modelling
NASA Astrophysics Data System (ADS)
Seismic observations from the EAGLE experiment in the Main Ethiopian Rift have been interpreted in terms of melt-induced anisotropy and support ideas of magma-assisted rifting in continental regions. Following the 2005 Dabbahu rifting event in Afar a further 9 broadband seismometers were installed around the newly active rift segment. These recorded more than one year of continuous data and shear-wave splitting observed in core phases (SKS/SKKS) shows considerable variability across the array. Three stations centred above the Dabbahu rift segment show markedly different splitting characteristics from the other stations. The fast direction is oriented roughly north/south and parallel to the Dabbahu magmatic segment, compared to NNE/SSW orientations at nearby stations. Also the magnitude of splitting is slightly larger at the rift stations compared to those nearby (~1s compared to 0.7-0.9s). These observations supports previous work in the Main Ethiopian Rift (MER), where fast directions change abruptly from being rift parallel on the rift flanks to magmatic-segment parallel in the rift valley. Furthermore, observations of frequency-dependent splitting in the data further suggest that the underlying cause of the anisotropy is related to aligned melt inclusions. Poroelastic modelling support mechanisms for melt-induced anisotropy due to vertically-aligned melt pockets that are on the order of centimetres in length scale. The abrupt change in splitting parameters over small lateral distances (~ 30 � over ~30~km) suggests that the source of anisotropy is shallow. To further constrain the location of the anisotropy and study the influence of the rift transition on shear-wave splitting results, we model finite-frequency waveforms for a suite of model representations of the rift zone. In each model, the orientation of the anisotropic fabric varies laterally (i.e., the symmetry axis of the HTI symmetry rotates between 0� and 30�), but the strength of anisotropy and depth of transition differs. Waveforms are modeled using a narrow-angle one-way elastic vector wave equation to simulate finite-frequency waveform effects for an incident near-planar S-wave. The modelling is used to examine the influence of changing anisotropic symmetry across the rift as a function of wavefront curvature, and strength and depth of transition as well as lateral width of transition zone. The results show how a simple model with two regimes of anisotropy can explain the variability across the rift, in both delay time and shear-wave polarization, over short length scales.
Hammond, J.; Kendall, J.; Angus, D.; Wookey, J.; Keir, D.; Ebinger, C.
Shear-Wave Splitting due to Rifting and Precambrian Accretion of Ethiopia
NASA Astrophysics Data System (ADS)
We have utilized a dataset from the broadband seismic experiment of Nyblade and Langston (EOS v.83 p. 405, 2002) for a shear-wave splitting analysis in Ethiopia. A total of twenty-five broadband seismic stations, widely distributed in various physiographic regions in Ethiopia, were used. Six stations were installed on the southeastern plateau, twelve stations on the western plateau, and seven stations inside the Rift Valley, which runs northeast-southwest and separates the western and eastern highlands. The distribution of the broadband stations in the present study spans a broad region and allows us to compare the results of shear-wave splitting analysis inside the rift and on the rift-bounding plateaus. Previous shear-wave splitting results in Kenya, located south of Ethiopia, show fast polarization azimuths sub-parallel to the strike of the rift which are interpreted to be the result of vertical magma-filled cracks in the lithosphere opening in the direction perpendicular to the extension direction (Gao et al., 1997; Barruol and Ismail, 2001). However, this orientation is also perpendicular to the collision direction for the Mozambique belt and thus consistent with the fast azimuth being the result of fossilized anisotropy in the Precambrian lithosphere. Results from an SKS splitting analysis in Ethiopia (Maguire et al., EOS 2003 in press; Kendall et al., this session) show the orientation of the fast polarization azimuth within the Main Ethiopian Rift to be approximately NNE-SSW. Similar to the results from Kenya, this is parallel to the volcanic centers in the rift and perpendicular to the geodetically determined opening direction. Splitting directions on the rift shoulders are closer to NE, parallel to both the rift-bounding faults (perpendicular to the inferred average Neogene opening direction) and to the Precambrian accretionary structures. Our preliminary result from Addis Ababa suggests a fast azimuth of N23E, sub-parallel to the rift orientation in agreement with previous results by other workers. Further south near Arbaminch, results suggest an azimuth of N6E possibly due to the shift of rift orientation from NE-SW to nearly N-S in this region. Stations 250 km NW of the rift axis (Debre Markos) and 250 km SE of the rift axis (Goba) both show more north-easterly trends (N37E and N21E, respectively). Because of the distance of these stations from the rift, we suggest that this NE fast splitting direction is more likely related to fossilized anisotropy in the Precambrian lithosphere than to Neogene rifting.
Gashawbeza, E.; Keranen, K.; Nyblade, A.; Klemperer, S. L.; Walker, K.
Keweenaw hot spot: Geophysical evidence for a 1. 1 Ga mantle plume beneath the Midcontinent Rift System
The Proterozoic Midcontinent Rift System of North America is remarkably similar to Phanerozoic rifted continental margins and flood basalt provinces. Like the younger analogues, the volcanism within this older rift can be explained by decompression melting and rapid extrusion of igneous material during lithospheric extension above a broad, asthenospheric, thermal anomaly which the authors call the Keweenaw hot spot. Great
D. R. Hutchinson; R. S. White; W. F. Cannon; K. J. Schulz
Was the Midcontinent Rift part of a successful seafloor-spreading episode?
NASA Astrophysics Data System (ADS)
~1.1 Ga Midcontinent Rift (MCR), the 3000 km long largely buried feature causing the largest gravity and magnetic anomaly within the North American craton, is traditionally considered a failed rift formed by isolated midplate volcanism and extension. We propose instead that the MCR formed as part of the rifting of Amazonia (Precambrian northeast South America) from Laurentia (Precambrian North America) and became inactive once seafloor spreading was established. A cusp in Laurentia's apparent polar wander path near the onset of MCR volcanism, recorded by the MCR's volcanic rocks, likely reflects the rifting. This scenario is suggested by analogy with younger rifts elsewhere and consistent with the MCR's extension to northwest Alabama along the East Continent Gravity High, southern Appalachian rocks having Amazonian affinities, and recent identification of contemporaneous large igneous provinces in Amazonia.
Stein, Carol A.; Stein, Seth; Merino, Miguel; Randy Keller, G.; Flesch, Lucy M.; Jurdy, Donna M.
Controls of lithospheric structure and plate kinematics on rift architecture and evolution: An experimental modeling of the Baikal rift
NASA Astrophysics Data System (ADS)
Analog models investigate the evolution and architecture of the Baikal rift in relation to the rheology of the extending lithosphere and rift kinematics. The models focus on the development of the narrow, deep, and asymmetric basins composing Lake Baikal and reproduce the extension between the strong Siberian craton and the weaker Sayan-Baikal belt. Model results suggest that the presence of a near-vertical weak suture separating the cratonic keel from the mobile belt represents the more convenient rheological configuration leading to a narrow rift characterized by prominent vertical motions and deep depressions. These depressions are typically asymmetric, and model results suggest that this asymmetry is a consequence of lateral variations in lithospheric rheology, which is in turn related to both the variation in thickness of the strong mantle and, more importantly, the variation in the brittle-ductile transition depth between the craton and the belt. A significant shallowing of the brittle-ductile transition in the crust passing from the craton to the belt is required to fit the asymmetric architecture of the Baikal basins, with a master fault on the craton side and a monocline with no significant faulting on the belt side. Analysis of the model deformation pattern suggests that the overall architecture of the basins hosting Lake Baikal is best fitted for a N140�E directed extension, similar to the current GPS-derived motion and compatible with the stress field inferred on the basis of fault and focal mechanism data. This kinematics (along with the shape of the Siberian craton) exerted the major control on the plan view fault architecture and its along-axis variations.
Corti, Giacomo; Calignano, Elisa; Petit, Carole; Sani, Federico
Spatial variation of primordial 3-He in crustal fluids along the East-African Rift system (the Ethiopian and the Kenya Rift section)
NASA Technical Reports Server (NTRS)
(3)He/(4)He compositions are presented for groundwater samples from the Ethiopian segment of the East-Afrikan Rift and from its northern extension, the adjacent Afar region (Djibuti). Helium isotope data are compared to those obtained previously from the Gregory Rift, south of Ethiopia. The distribution pattern of mantle-derived volatiles along the entire East-African-Rift (-from south Kenya to Djibuti-) is discussed and their sources are identified. Helium isotope ratios (R) for samples from the Ethiopian part of the Rift range from 6.3 to 16.0 times the atmospheric ratio (Ra=1.4 x 10(exp -6) and thus show together with a MOR component a considerable hotspot helium component. These mantle helium concentrations are comparable to those observed in groundwaters and volcanic rocks from the Afar plume region in Djibuti. Here R/Ra values range from 9 to 13 times the atmospheric composition, with mantle-derived helium concentrations being higher than at spreading ocean ridges. R/Ra values from Ethiopia and Djibuti are entirely different from those observed in groundwaters at the southerly extending Gregory Rift in Kenya, where R/Ra values scatter between 0.5 and 6. At the northernmost part of the Gregory Rift, close to Ethiopia mantle helium contents are slightly higher, with R/Ra-values varying between 6.5 and 8.0.
Griesshaber, E.; Weise, S.; Darling, G.
ALVIN investigation of an active propagating rift system, Galapagos 95.5?? W
ALVIN investigations have defined the fine-scale structural and volcanic patterns produced by active rift and spreading center propagation and failure near 95.5?? W on the Galapagos spreading center. Behind the initial lithospheric rifting, which is propagating nearly due west at about 50 km m.y.-1, a triangular block of preexisting lithosphere is being stretched and fractured, with some recent volcanism along curving fissures. A well-organized seafloor spreading center, an extensively faulted and fissured volcanic ridge, develops ~ 10 km (~ 200,000 years) behind the tectonic rift tip. Regional variations in the chemical compositions of the youngest lavas collected during this program contrast with those encompassing the entire 3 m.y. of propagation history for this region. A maximum in degree of magmatic differentiation occurs about 9 km behind the propagating rift tip, in a region of diffuse rifting. The propagating spreading center shows a gentle gradient in magmatic differentiation culminating at the SW-curving spreading center tip. Except for the doomed rift, which is in a constructional phase, tectonic activity also dominates over volcanic activity along the failing spreading system. In contrast to the propagating rift, failing rift lavas show a highly restricted range of compositions consistent with derivation from a declining upwelling zone accompanying rift failure. The lithosphere transferred from the Cocos to the Nazca plate by this propagator is extensively faulted and characterized by ubiquitous talus in one of the most tectonically disrupted areas of seafloor known. The pseudofault scarps, where the preexisting lithosphere was rifted apart, appear to include both normal and propagator lavas and are thus more lithologically complex than previously thought. Biological communities, probably vestimentiferan tubeworms, occur near the top of the outer pseudofault scarp, although no hydrothermal venting was observed. ?? 1992 Kluwer Academic Publishers.
Hey, R. N.; Sinton, J. M.; Kleinrock, M. C.; Yonover, R. N.; Macdonald, K. C.; Miller, S. P.; Searle, R. C.; Christie, D. M.; Atwater, T. M.; Sleep, N. H.; Johnson, H. P.; Neal, C. A.
Case for Upper Miocene continental break-up in western Afar, southern Red Sea rift
NASA Astrophysics Data System (ADS)
We describe the distribution, nature and timing of extension in the youthful Western Afar in order to infer mechanisms for continental break-up. New and existing remote sensing, fieldwork and structural analyses calibrated by geochronology were integrated to define rift stages and the distribution and nature of volcanic units and structures that accommodate extension. Oligo-Miocene continental rift basins of the Southern Red Sea rift, Ethiopia are typical steep-sided continental rift graben, but contain almost exclusively volcanic fill. Rift segmentation was defined by border faults that were the locus of strain, but strain migrated to magmatic centres within the basins in the Upper Miocene. Widespread basaltic volcanism in the Upper Miocene led to extension, burial, riftward rotation and seismic reactivation of Oligo-Miocene rift basins. These basalts were sourced from linear magmatic segments that are independent of the older border fault segmentation. Cycles of subsidence and volcanism gave the basalt successions a concave-down profile similar to seaward-dipping reflectors (SDRs) on buried volcanic margins. Mid-Miocene propagation of the Main Ethiopian rift (MER) arm into the Afar Depression led to overprinting of the early Red Sea rift structures. We suggest that the locus of strain in western Afar jumped from Upper Miocene Southern Red Sea magmatic segments to Plio-Quaternary MER magmatic segments. The distribution of strain across the western Afar margin, and age of SDR-like basalts and the intersecting Southern Red Sea and Main Ethiopian rifts suggests that the Southern Red Sea magmatic segments were abandoned in the Upper Miocene leaving SDRs stranded above Oligo-Miocene rift basins, possibly due to the northward propagation of the seismically and volcanically active MER.
Wolfenden, E.; Ebinger, C.; Yirgu, G.; Kelley, S.; Deino, A.; Renne, P.
The EAGLE Broadband Seismic Experiment - A Study of Continental Rifting in the Ethiopia
NASA Astrophysics Data System (ADS)
The EAGLE broadband experiment aims to study the crust and upper-mantle structure in a region of transition from continental to oceanic rifting - the northern Ethiopian Rift. 30 broadband seismometers have been deployed for a period of 16 months over an area of 250 km2, centred on the Nazret volcanic zone of the Ethiopian Rift. Preliminary SKS splitting results show delay times varying from 1.5 -- 1.8 secs on the Ethiopian Plateau in the west, to 1.6 -- 2.1 secs in the east towards the Somalia Plate. Within the Rift there is a consistent increase in delay times towards the north and Afar from 1.0 secs in the south to 2.1 secs in the north. Outside the Rift, the polarisations of the fast shear-wave lie on a NE--SW rift-parallel trend; within the Rift the orientations swing to more northerly azimuths. The increased delay times in the east on the Somalia plate was unexpected and initially interpreted in terms of stretched/fractured lithosphere or alternatively represents different tectonic domain. The increase in Rift splitting times northwards correlates with the amount of magma within the system and may be related to the presence of magma-filled cracks. Teleseismic traveltime residuals are used to further investigate mantle velocity structure. Average residuals show a regional trend (c. 0.5 secs per 100km), upon which the Rift signature is perched; on average there are faster ray paths to the east and slower ones to the west on the uplifted Ethiopian Plateau. The regional trend may represent a plume signature or a change in tectonic domain either side of the Rift. This clear asymmetry in the cross-axis direction is consistent with the SKS patterns. Traveltime tomography will be used to further investigate the detailed velocity structure.
Stuart, G.; Kendall, M.; Bastow, I.; Ayele, A.; Ebinger, C.; Maguire, P.; Fowler, M.
Kinematic problem of oblique rift zones in Iceland
Regional distribution of surficial isochrons on Iceland locally lacks the oceanic-ridge subparallel alignment that is characteristic of the adjoining North Atlantic sea floor. Strikes of Icelandic lava isochrons generally reflect dips imparted by continued lava accumulation, subsidence, and burial. The angle between trends of central volcano-related fissure swarms, dikes, and faults, and regionally-adjacent approximate isochrons, defines the Obliquity Angle (OA). The OA is measured from the isochron to the extensional structures, and may vary from 0/sup 0/ to 90/sup 0/; cw angles are (+), ccw (-). Three large-OA neovolcanic zones are present in Iceland: the WNW-trending zone is central Iceland; the ENE zone along Reykjanes peninsula; and, possibly, the WNW and largely submarine Tjornes Fracture Zone (TFZ). The TFZ shares characteristics of a highly-oblique rift zone and a transform-related shear zone. In north-central Iceland, OA's along 7-11 Ma isochrons vary continuously from +40/sup 0/ to 90/sup 0/ to -50/sup 0/, over 130 km from West to East. As subaerial isochrons on Iceland range up to 90/sup 0/ to spatially-associated extensional structures, isochron trends locally parallel presumed plate-flow lines (N8OW); offshore, isochrons are approximately normal to flow lines. However extension in central Iceland occurs within the long-lived, WNW-trending oblique rift zone, with little or no commensurate extension and volcanism parallel to structural strike to the South and North, constitutes a major kinematic problem.
Jancin, M.
Rift Valley Fever: Recent Insights into Pathogenesis and Prevention?
Rift Valley fever virus (RVFV) is a zoonotic pathogen that primarily affects ruminants but can also be lethal in humans. A negative-stranded RNA virus of the family Bunyaviridae, this pathogen is transmitted mainly via mosquito vectors. RVFV has shown the ability to inflict significant damage to livestock and is also a threat to public health. While outbreaks have traditionally occurred in sub-Saharan Africa, recent outbreaks in the Middle East have raised awareness of the potential of this virus to spread to Europe, Asia, and the Americas. Although the virus was initially characterized almost 80 years ago, the only vaccine approved for widespread veterinary use is an attenuated strain that has been associated with significant pathogenic side effects. However, increased understanding of the molecular biology of the virus over the last few years has led to recent advances in vaccine design and has enabled the development of more-potent prophylactic measures to combat infection. In this review, we discuss several aspects of RVFV, with particular emphasis on the molecular components of the virus and their respective roles in pathogenesis and an overview of current vaccine candidates. Progress in understanding the epidemiology of Rift Valley fever has also enabled prediction of potential outbreaks well in advance, thus providing another tool to combat the physical and economic impact of this disease.
Boshra, Hani; Lorenzo, Gema; Busquets, Nuria; Brun, Alejandro
An Epidemiological Model of Rift Valley Fever with Spatial Dynamics
As a category A agent in the Center for Disease Control bioterrorism list, Rift Valley fever (RVF) is considered a major threat to the United States (USA). Should the pathogen be intentionally or unintentionally introduced to the continental USA, there is tremendous potential for economic damages due to loss of livestock, trade restrictions, and subsequent food supply chain disruptions. We have incorporated the effects of space into a mathematical model of RVF in order to study the dynamics of the pathogen spread as affected by the movement of humans, livestock, and mosquitoes. The model accounts for the horizontal transmission of Rift Valley fever virus (RVFV) between two mosquito and one livestock species, and mother-to-offspring transmission of virus in one of the mosquito species. Space effects are introduced by dividing geographic space into smaller patches and considering the patch-to-patch movement of species. For each patch, a system of ordinary differential equations models fractions of populations susceptible to, incubating, infectious with, or immune to RVFV. The main contribution of this work is a methodology for analyzing the likelihood of pathogen establishment should an introduction occur into an area devoid of RVF. Examples are provided for general and specific cases to illustrate the methodology.
Niu, Tianchan; Gaff, Holly D.; Papelis, Yiannis E.; Hartley, David M.
Young Stellar Object Candidates in the Aquila Rift Region
NASA Astrophysics Data System (ADS)
Using the 2m telescope of the Turingia State Observatory at Tauten-berg (TLS), imaging observations in 3 wavebands (H ?, R and I) are performed in the 16 fields in the Aquila Rift region. The observed fields cover about 7 square degrees. Excluding the 3 fields with unqualified data, the photometrical analysis is made for the remaining 13 fields, from which point sources are identified, and finally 7 H ? emission-line star candidates are identified by color-color diagrams. The 7 candidates are located in five fields. Three of them are located near the Galactic plane, while the galactic latitudes of the rest are greater than 4�. The 2 M ASS counterparts of the point sources are identified, and the properties of the 7 H ? emission-line star candidates are further analyzed by using the two-color diagrams. It is found that the near-infrared radiation from these H ? emission-line star candidates has no obvious infrared excess, one of them even falls on the main-sequence branch. This indicates that the H ?-emissive young stellar objects (YSOs) are not always accompanied with the infrared excess, and that the results of the H ? emission line observation and the infrared excess observation are mutually supplemented. If the 7 H ? emission-line star candidates are regarded as YSO candidates, then the number of YSOs in the Aquila Rift region is quite small. The further confirmation of these candidates needs subsequent spectral observations.
Zhang, Miao-miao; Wang, Hong-chi; Stecklum, B.
Lithospheric cross sections of the European Cenozoic rift system
NASA Astrophysics Data System (ADS)
The lithospheric structure of the European Cenozoic rift system ( ECRIS) is presented in transects through the southern Rhine Graben and the Rhenish Massif/Hessen depression, emphasizing the geophysical structure of the lithosphere based on seismic refraction/reflection investigations, teleseismic tomography, electromagnetic depth-sounding models, and gravity, aeromagnetic, earthquake, uplift/subsidence and heat flow data. The rift is clearly expressed in the Rhine Graben, but is not evident at the surface in the area of the Rhenish Massif where its existence is indicated by seismicity. It is characterized by abnormal crustal and upper-mantle structures which vary considerably in horizontal direction. For example, under the Rhine Graben the crust is thinned to 25 km, but at 40 km depth anomalously high velocities are observed. In contrast, beneath the Rhenish Massif the crust is thickened to 35-37 km and under its eastern part a high-velocity thin upper-mantle slice is seen at 30 km depth within the lower crust which, towards the Hessen depression, is gradually replaced by normal Variscan mantle with the Moho near 30 km depth. Under the western part of the Rhenish Massif P- and S-wave velocities are reduced below 50 km depth which is not seen east of the Rhine river. Under the Rhine Graben the existence of a low-velocity upper mantle above 100 km cannot be generalized, but is restricted to confined regions.
Prodehl, C.; Mueller, St.; Glahn, A.; Gutscher, M.; Haak, V.
Extension on rifted continental margins: Observations vs. models.
NASA Astrophysics Data System (ADS)
Mapping the signature of extensional deformation on rifted margins is often hampered by thick sedimentary or volcanic successions, or because salt tectonics makes sub-salt seismic imaging challenging. Over the past 20 years the literature is witnessing that lack of mapable faults have resulted in a variety of numerical models based on the assumption that the upper crust takes little or no extensional thinning, while the observed reduction of crustal thickness is taken up in the middle and lower crust, as well as in the mantle. In this presentation two case studies are used to highlight the difference that 3D seismic data may have on our understanding. The small patches of 3D resolution data allow us to get a glance of the 'real' signature of extensional faulting, which by analogy can be extrapolate from one margin segment to the next. In the South Atlantic salt tectonics represents a major problem for sub-salt imaging. The conjugate margins of Brazil and Angola are, however, characterized by pronounced crustal thinning as documented by crustal scale 2D reflection and refraction data. Off Angola the 3D 'reality' demonstrates that upper crustal extension by faulting is comparable to the full crustal, as well as lithospheric thinning as derived from refraction data and basin subsidence analysis. The mapped faults are listric low angle faults that seem to detach at mid crustal levels. 2D seismic has in the past been interpreted to indicate that almost no extensional faulting can be mapped towards the base of the so-called 'sag basin'. The whole concept of the 'sag basin', often ascribed to as crustal thinning without upper crustal deformation, is in fact related to this 'lack of observation', and furthermore, have caused the making of different types of dynamic models attempting to account for this. In the NE Atlantic significant Paleocene extensional faulting is locally seen adjacent to the 50 to more than 200 km wide volcanic cover on each side of the breakup axis. The associated amount of lateral motion on these, mainly listric, normal faults represents several tens of km. These observations contrast with the general lack of observed faults along volcanic margins due to the overall problem with sub-basalt imaging. A variety of models with respect to mode and duration of extension, including narrow and fast breakup, melt generation by small scale convection, and different modes of mantle flow have been suggested. The interesting aspect is that it is all based on features we can't see. Both study areas clearly points towards the importance of improved seismic imaging, a need for revised understanding of strain rates and strain partitioning during rift development, and the necessity of moving from 2D cross section modeling to more realistic 3D spatial distribution of rift elements and subsequent break-up processes. One important aspect is that both volcanic and non-volcanic margins are rifted margins formed by a protracted rift development.
Skogseid, Jakob
Comparison of the Tanganyika, Malawi, Rukwa and Turkana Rift zones from analyses of seismic reflection data
NASA Astrophysics Data System (ADS)
Northwest-southeast extension has opened the East African Rift System along two main branches, the Western and Eastern Branches. Rift zones along the Western Branch are marked by narrow lakes floored by thick piles of fluvial clastic and 'pelagic' sediment. Magmatism is restricted to a few small areas in the 'arches' between the lakes. In contrast, rift zones along the Eastern Branch are largely filled with volcanic and volcaniclastic materials and magmatism is generally perceived to be an integral part of the rifting process. In an attempt to sort out the significance and meaning of these and other differences, we have compared multifold seismic data from three Western Branch rift zones (Tanganyika, Rukwa and Malawi) and one Eastern Branch zone (Turkana). The Tanganyika and Malawi Rift Zones are composed of half-graben basins linked in complex ways by accommodation zones which generally trend oblique to the rift axes, and sometimes oblique to the extension direction. Half-grabens alternate basinal polarities where the rift crosses Proterozoic dislocation zones. Complex fault geometries are associated with some accommodation zones; elsewhere faults are almost exclusively planar. Sedimentary fill reaches at least 4-5 km and the section is mostly Cenozoic in age. Patches of Permo-Triassic sedimentary rocks are believed to occur within both rift zones. The Rukwa Rift is a pull-apart zone that connects the northern (Livingstone) basin of Lake Malawi to the Kalemie Basin in central Lake Tanganyika. The entire pull-apart system may be a series of down-to-the-east half-grabens. An accommodation zone develops along a short stretch of the Rukwa Rift, but no full polarity reversal occurs. The break-away faults of the Livingstone, Rukwa and Kalemie basins are essentially coincident with the Proterozoic Rukwa dislocation zone, which sub-parallels the inferred extension direction. Fault geometries in the Rukwa Rift are markedly listric, especially in the pre-Cenozoic section. Sedimentary fill ranges in age from pre-Karroo through Cenozoic and locally exceeds 10 km in thickness. The Turkana Rift is composed of short, linear, NNE-trending normal fault segments that are offset in a left-lateral sense by numerous, NW-SE trending transfer faults, linking facing border fault segments together. The overall trend of the rift zone is oblique to the opening direction, like the Tanganyika and Malawi cases, but the border fault segments are sub-perpendicular. Fault geometries are highly variable, but flower structures associated with transfer faults predominate. Igneous activity is ubiquitous and appears to be localized along the transfer faults. Basin fill reaches 4-5 km in thickness and is dominated by fluvial clastic, volcaniclastic and volcanic materials. The structural differences within the Tanganyika-Rukwa-Malawi system stem mainly from the modifying effects of pre-rift anistropies on strain expressions. Fundamentally, this system is a NW-SE trending series of single-polarity pull-apart basins. At the two ends of the pull-apart zone, the rift is deflected into more N-S trending basins which have a high tendency to alternate polarities along strike. This explanation does not account for the differences in fault forms between the Tanganyika-Malawi (planar) and Rukwa (listric) Rifts. For the time being, we presume these differences arise from systematic differences between Tanganyika-Malawi and Rukwa in the age ranges of the fill and/or the maximum depths of seismic imaging. Rifting in Turkana is profoundly different than in the Tanganyika-Rukwa-Malawi sub-branch and seems to involve a softer, more ductile, more organized style of extension which may be closer to the ideal case. In a thermal sense, rifting has progressed further in Turkana than along the Western Branch zones. This does not preclude original, fundamental difference in the thermal states of two branches.
Rosendahl, Bruce R.; Kilembe, Elias; Kaczmarick, Kurt
Influence of heterogeneities within the lithosphere on the deformation pattern of continental rift systems.
NASA Astrophysics Data System (ADS)
Understanding how heterogeneities within the lithosphere influence the deformation pattern in continental rifts still remains a challenge and is of real importance to constrain continental break-up. We have selected the Main Ethiopian Rift in East Africa and the Rio Grande Rift in the south-western U.S. These two rifts are perfect natural laboratories to investigate the effect of inherited as they share similar structural characteristics but develop above different kinds of lithosphere-scale heterogeneities. From a structural point of view both rifts show similar length (1000km), width (50 to 70 km) and asymmetry. The Main Ethiopian rift is the NE-SW trending plate boundary between the Nubian and Somalian plates that has been developing for the past 11 Ma above a palaeo-Proterozoic lithospheric-scale weak zone re-heated by the Afar hotspot, whereas the Rio Grande Rift is the eastern "boundary" of the Basin & Range system which has been developing for the past 30 Ma in the frame of a westward-retreating Farallon subduction zone. However, the Rio Grande Rift shows evidence of low angle normal faulting whereas the Main Ethiopian Rift shows steeply dipping (with a mean close to 70�) normal faults. The Main Ethiopian Rift shows larger volume of erupted lavas than the Rio Grande Rift. Combined with a structural analyses of both rifts, we present here a series of 2D cross sections numerical models that allow better understanding of the influence of initial heterogeneities such as 1) the rheological state of the crust; 2) the presence of a crustal-scale to lithospheric-scale discrete weak or strong zone, 3) the effects of the presence of magma. We illustrate that rheological boundaries are not reactivated if the rheological contrast it too high, which is the case of the Rio Grande Rift that developed to the east of the North American Craton within thinned lithosphere. We also illustrate that the width of the weak zone do no have any influence on the exhumation of the asthenospheric mantle. The temperature at the base of the lithosphere is the parameter controlling the asthenosphere rising.
Philippon, Melody; Thieulot, Cedric; van Wijk, Jolante; Sokoutis, Dimitrios; Willingshofer, Ernst; Cloetingh, Sierd
Upper Mantle Seismic Structure of the Northern Ethiopian Rift - a Region of Incipient Continental Breakup
NASA Astrophysics Data System (ADS)
The northern Ethiopian rift forms the third arm of the Red Sea, Gulf of Aden triple junction and captures the transition from continental rifting in East Africa to oceanic sea-floor spreading in Afar. As rifting proceeds to sea-floor spreading, it is expected that fault controlled (mechanical) segmentation associated with the continental rift setting will be superseded by asthenospheric and magmatic processes, which are more prevalent in oceanic settings. The EAGLE passive networks consisted of 79 broadband seismometers over an area 250x350km centered on the Boset magmatic segment 70km SE of Addis Ababa, in the center of the rift. Analysis of absolute travel-time residuals from the stations shows that the northern Ethiopian rift is one of the most seismically slow regions on Earth. We have imaged upper-mantle structure between depths of 40-300km depth using teleseismic tomographic inversion of P and S wave travel-time data. Our tomographic inversions image a 75km-wide tabular low velocity zone (Vp=-1.5%, Vs=-4%) between 40-300km depth, interpreted as upwelling in the upper mantle beneath the less evolved southern part of the rift. At depths of ~100km, north of 9 degrees N this anomaly broadens towards the oceanic spreading centre of Afar where it may be connected to deeper low-velocity structure. Along axis, the low velocity upwelling beneath the rift is segmented, with low velocity material in the uppermost 100 km always offset to whichever side of the rift has the highest rift flank topography. Lateral velocity contrasts and comparison of P and S travel-times suggests high temperatures and partial melt in the upper-mantle beneath the rift. Furthermore, the travel-time data indicate that the melt beneath the study area exists in elongate inclusions - consistent with interpretations of SKS shear wave splitting data. We propose, that magma emplacement has occurred within the lithosphere, thereby allowing rifting in an area where large-scale tectonic forces may be too small for amagmatic rifting to occur.
Kendall, J.; Bastow, I. D.; Stuart, G. W.; Ebinger, C. J.; Ayele, A.
Spatial Variations in Lithospheric Structure and Anisotropy Across the Northern Main Ethiopian Rift
NASA Astrophysics Data System (ADS)
The magmatic Northern Main Ethiopian Rift (NMER) is believed to represent the transitional stage between continental and oceanic rifting, with recent studies from the Ethiopia Afar Geophysical Lithospheric Experiment (EAGLE) project supporting models of magma assisted rifting. 97 broadband seismometers were deployed across the northern Main Ethiopian Rift at a nominal spacing of 5 km and the resulting data have been analyzed using shear-wave splitting and receiver function methods. Split shear waves are analysed from SKS and SKKS arrivals in two distinct period bands (2-5 and 5-14 s) along the cross-rift profile. The orientations of the fast shear-wave for both period bands are broadly rift-parallel with a systematic rotation northwards toward the rift valley from each end of the profile. Significant variation in both the fast-to-slow shear-wave lag time and the orientation of the fast shear-wave is present over short length scales in both the short- and long-period bands. Lag times vary from 1.0 to 2.5 s with maximum splitting occurring beneath the rift and beneath the southeast rift flank. Comparison of lag times for the short- and long-period bands reveals an average of ~ 0.3 s less splitting for the short-period band on the northwestern rift flank which we interpret in terms of depth-dependent anisotropy. Up to 2.5 s of lag is observed in both period bands beneath the rift, indicating that the dominant anisotropy mechanism is confined to upper lithosphere. Models of anisotropy are tested to explain the results in terms of the preferred alignment of melt pockets and/or pre-existing lithospheric fabric. The shear-wave splitting results are combined with and compared to receiver function results that provide an estimation of the Moho depth and Vp/Vs ratio at each station along the cross-rift profile. Together, these analyses allow the identification of regions of melt across the rift that in turn enable an assessment of the extent and role of melt in continental break-up that is dominated by magmatic processes.
Cornwell, D. G.; Kendall, J. M.; Maguire, P. K.; England, R. W.
Upper Mantle Seismic Structure of the Northern Ethiopian Rift - a Region of Incipient Continental Breakup
NASA Astrophysics Data System (ADS)
The northern Ethiopian rift forms the third arm of the Red Sea, Gulf of Aden triple junction and captures the transition from continental rifting in East Africa to oceanic sea-floor spreading in Afar. As rifting proceeds to sea-floor spreading, it is expected that fault controlled (mechanical) segmentation associated with the continental rift setting will be superceded by asthenospheric and magmatic processes, which are more prevalent in oceanic settings. The EAGLE passive networks consisted of 79 broadband seismometers over an area 250x250km centred on the Boset magmatic segment 70km SE of Addis Ababa, in the centre of the rift. We image upper-mantle structure between depths of 40-300km depth using teleseismic tomographic inversion of P and S wave travel-time data. Crustal structure, unresolvable in our inversions, is accounted for using an a priori crustal model which we derive from receiver function analyses of Moho P-to-S converted phases and from the results of recent EAGLE refraction experiments in Ethiopia. Analysis of relative arrival-time residuals shows that the rift flanks are asymmetric with arrivals associated with the south-eastern Somalian Plate faster ( �0.65s for the P waves; �2s for the S waves) than the north-western Nubian Plate. Our tomographic inversions image a 75km-wide tabular low velocity zone (Vp=-1.5%, Vs=-4%) between 40-300km depth, interpreted as upwelling in the upper mantle beneath the less evolved southern part of the rift. At depths of �100km, north of 9 degrees N this anomaly broadens towards the oceanic spreading centre of Afar where it appears to be connected to deeper low-velocity structure. Along axis, the low velocity upwelling beneath the rift is segmented, with low velocity material in the uppermost 100 km always offset to whichever side of the rift has the highest rift flank topography. Lateral velocity contrasts and comparison of P and S travel-times suggests high temperatures and partial melt in the upper-mantle beneath the rift. We propose, that magma emplacement has occured within the lithosphere, thereby allowing rifting in an area where large scale tectonic forces may be too small for amagmatic rifting to occur.
Bastow, I. D.; Stuart, G. W.; Kendall, M. J.; Ebinger, C. J.; Ayele, A.
Mode of rifting in magmatic-rich setting: Tectono-magmatic evolution of the Central Afar rift system
NASA Astrophysics Data System (ADS)
Observation of deep structures related to break-up processes at volcanic passive margins (VPM) is often a troublesome exercise: thick pre- to syn-breakup seaward-dipping reflectors (SDR) usually mask the continent-ocean boundary and hide the syn-rift tectonic structures that accommodate crustal stretching and thinning. Some of the current challenges are about clarifying 1) if tectonic stretching fits the observed thinning and 2) what is the effect of continuous magma supply and re-thickening of the crust during extension from a rheological point of view? The Afar region in Ethiopia is an ideal natural laboratory to address those questions, as it is a highly magmatic rift that is probably close enough to breakup to present some characteristics of VPM. Moreover, the structures related to rifting since Oligocene are out-cropping, onshore and well preserved. In this contribution, we present new structural field data and lavas (U-Th/He) datings along a cross-section from the Ethiopian Plateau, through the marginal graben down to the Manda-Hararo active rift axis. We mapped continent-ward normal fault array affecting highly tilted trapp series unconformably overlain by tilted Miocene (25-7 Ma) acid series. The main extensional and necking/thinning event took place during the end of this Miocene magmatic episode. It is itself overlain by flat lying Pliocene series, including the Stratoid. Balanced cross-sections of those areas allow us to constrain a surface stretching factor of about 2.1-2.9. Those findings have the following implications: - High beta factor constrained from field observations is at odd with thinning factor of ~1.3 predicted by seismic and gravimetric studies. We propose that the continental crust in Central Afar has been re-thickened by the emplacement of underplated magma and SDR. - The deformation in Central Afar appears to be largely distributed through space and time. It has been accommodated in a 200-300 km wide strip being a diffuse incipient plate boundary until the formation of present-day magmatic segments. - The difference in tectono-magmatic style between Central Afar (distributed extension and thick crust) and Northern Afar, i.e. Erta Ale segment (narrow graben, thin crust) may be explained by the difference of magma volume (extruded & underplated) brought to the crust during extension. Magma supply in Central Afar allows the crust to be stretched without subsequent thinning despite high degree of extension. - Presence or absence of thinned crust does not necessarily announce break-up. It may occur in both Central and Northern Afar, depending upon a sudden change in magmatic regime. The striking difference between the two tectono-magmatic styles of Central and Northern Afar are probably due to a combination of: 1) magma supply that affects both crustal thickness and rheology, 2) the amount of extension that may be higher in Central Afar, 3) the distance to the magmatic province, and 4) the presence of an early syn-rift transfer/transform between the two segments that might have controlled the distribution of magmatic activity.
Stab, Martin; Bellahsen, Nicolas; Pik, Rapha�l; Leroy, Sylvie; Ayalew, Dereje
Transient cracks and triple junctions induced by Cocos-Nazca propagating rift
NASA Astrophysics Data System (ADS)
The Galapagos triple junction is a ridge-ridge-ridge triple junction where the Cocos, Nazca, and Pacific plates meet around the Galapagos microplate (GMP). On the Cocos plate, north of the large gore that marks the propagating Cocos-Nazca (C-N) Rift, a 250-km-long and 50-km-wide band of NW-SE-trending cracks crosscuts the N-S-trending abyssal hills of the East Pacific Rise (EPR). These appear as a succession of minor rifts, accommodating some NE-SW extension of EPR-generated seafloor. The rifts successively intersected the EPR in triple junctions at distances of 50-100 km north of the tip of the C-N Rift. We proposed a simple crack interaction model to explain the location of the transient rifts and their junction with the EPR. The model predicts that crack locations are controlled by the stress perturbation along the EPR, induced by the dominant C-N Rift, and scaled by the distance of its tip to the EPR (Schouten et al., 2008). The model also predicts that tensile stresses are symmetric about the C-N Rift and thus, similar cracks should have occurred south of the C-N Rift prior to formation of the GMP about 1 Ma. There were no data at the time to test this prediction. In early 2009 (AT 15-41), we mapped an area on the Nazca plate south of the C-N rift out to 4 Ma. The new bathymetric data confirm the existence of a distinctive pattern of cracks south of the southern C-N gore that mirrors the pattern on the Cocos plate until about 1 Ma, and lends support to the crack interaction model. The envelope of the symmetric cracking pattern indicates that the distance between the C-N Rift tip and the EPR varied between 40 and 65 km during this time (1-4 Ma). The breakdown of the symmetry at 1 Ma accurately dates the onset of a southern plate boundary of the GMP, now Dietz Deep Rift. At present, the southern rift boundary of the GMP joins the EPR with a steep-sided, 80 km long ridge. This ridge releases the stress perturbation otherwise induced along the EPR by elastic interaction with the C-N Rift and prevents the formation of minor rifts of the type in the North of the C-N Rift. However, the seafloor displays traces of rifts formed as the Dietz Deep Rift was approaching the EPR. In fact, the present day ridge appears to have originated as one of these minor rifts, probably stabilized by enhanced magma supply from a nearby volcano at the southwestern end of Dietz Deep.
Schouten, H.; Smith, D. K.; Zhu, W.; Montesi, L. G.; Mitchell, G. A.; Cann, J. R.
Elastic Lithosphere Thickness and Heat Flux Estimates from Rift Valley Topography: Coracis Fossae, Mars
NASA Astrophysics Data System (ADS)
The Coracis Fossae in the Thaumasia region on Mars are two several hundred kilometer long and �50 km wide extensional structures. Their complex morphology, fractured graben floors and segmented border faults, which are arranged in en echelon pattern, suggest that they are Martian analogues to terrestrial rift systems. At Coracis Fossae's NE segment rift flank uplift is most pronounced, the rift shoulders having heights of more than 1000 m with respect to the surrounding planes. We model the uplift by fitting a flexed broken plate to the topography data obtained by the Mars Orbiter Laser Altimeter. Thus, the elastic thickness at the time of rifting is constrained to 10.3 - 12.5 km. Assuming a diabase composition of the crust, this corresponds to a thermal gradient of 27 - 33 K km-1. Investigating the key surface units associated with the rifting, the time of rift formation is determined by measuring their crater size-frequency distribution and comparing the results to an impact cratering chronology model. The time of rifting is thus constrained to 3.5 - 3.9 Gyr b.p. Given the fault block topography and elastic thickness, the stresses acting on the bounding faults which support the topography may be calculated. We estimate that at the Coracis Fossae the faults need not withstand stresses in excess of 5 MPa, a value comparable to terrestrial faults. We take this weakness as an indication that the faults are or have been in contact with liquid water below the surface.
Grott, M.; Hauber, E.; Werner, S. C.; Kronberg, P.; Neukum, G.
Radar, electromagnetic, and active seismic investigation of a propagating ice shelf rift tip
NASA Astrophysics Data System (ADS)
A rift system has been growing inward from the seaward edge of the Amery Ice Shelf in East Antarctica for over twenty years. Currently active, the 'Loose Tooth' rift system is in the process of calving off an iceberg that will be at least 30 x 30 km in size. Over the past five field seasons passive seismic and geodetic measurements have been made at one of the propagating tips of the rift system. During the 2006-07 field season pilot Ground Penetrating Radar and Transient Electromagnetic (TEM) data were collected over and near that propagating rift tip, to provide complimentary constraints regarding the internal structure of the ice shelf and to look for evidence of penetration of electrically conductive seawater into the rift from below. In another auxiliary experiment, small charges were set off near the rift tip to provide calibration and velocity structure information for the primary passive seismic data set. Preliminary results are presented in comparison with the passive seismic and geodetic results, as well as a discussion of practical constraints and challenges in collecting these types of data at an ice shelf rift.
Behrens, J.; Bassis, J.; Fricker, H.; Coleman, R.; Darnell, D.
Upper mantle seismic anisotropy in the intra-continental Kachchh rift zone, Gujarat, India
NASA Astrophysics Data System (ADS)
Shear wave splitting study of 411 SKS/SKKS phases covering backazimuth range of 13 � to 305 � recorded by 12 broadband stations in the Kachchh rift has led to estimates of fast axis orientations and splitting times for 118 good measurements. The average vector mean of fast axis orientation (86 � 14 �) corresponds to the E-W axis of the Kachchh rift and the delay time (~ 1.6 s) is attributed to the ~ 184 km-thick upper mantle layer with 4% anisotropy. The anisotropic character observed for the Kachchh rift (KR) is comparable to other continental rifts and these are related to the high-temperature, lattice-preferred orientation fabric of olivine, inherited from the mantle flows. The source of the rift-axis parallel anisotropy is traced to the rift-parallel flows within the 76 � 6 km-thick lithosphere. Additionally, the rift-parallel pockets of partial melts also induce anisotropy within the asthenosphere. Both these are inherited from the plume-lithosphere interaction during the Deccan/Reunion plume episode (~ 65 Ma).
Mandal, Prantik
The northern Nevada rift: regional tectono-magmatic relations and Middle Miocene stress direction
As defined by the most recent aeromagnetic surveys, the north-northwest-trending northern Nevada rift zone extends for at least 500km from southern Nevada to the Oregon-Nevada border. At several places along the rift, the magnetic anomaly is clearly related to north-northwest-trending dikes and flows that erupted between 17 and 14 Ma. The tectonic significance of the rift is dramatized by its length, its coincidence in time and space with the oldest silicic caldera complex along the Yellowstone hotspot trend, and its parallelism with the subduction zone along the North American coast prior to the establishment of the San Andreas fault. The northern Nevada rift is also equivalent in age, trend, and composition to feeder dikes that fed the main eruptive pulse of the Columbia River flood basalts in northern Oregon ~15.6-16.5 Ma. Because of these similarities, both regions are considered to be part of an enormous lithospheric rift that propagated rapidly south-southeast and north-northwest, respectively, from a central mantle plume. The present north-northwest trend of the rift reflects the state of stress in the Basin and Range during middle Miocene time and is consistent with stress indicators of similar age throughout the Basin and Range Rio Grande rift provinces. -from Authors
Zoback, M. L.; McKee, E. H.; Blakely, R. J.; Thompson, G. A.
Structural geology of the African rift system: Summary of new data from ERTS-1 imagery. [Precambrian influence
NASA Technical Reports Server (NTRS)
ERTS imagery reveals for the first time the structural pattern of the African rift system as a whole. The strong influence of Precambrian structures on this pattern is clearly evident, especially along zones of cataclastic deformation, but the rift pattern is seen to be ultimately independent in origin and nature from Precambrian tectonism. Continuity of rift structures from one swell to another is noted. The widening of the Gregory rift as its northern end reflects an underlying Precambrian structural divergence, and is not a consequence of reaching the swell margin. Although the Western Rift is now proven to terminate at the Aswa Mylonite Zone, in southern Sudan, lineaments extend northeastwards from Lake Albert to the Eastern Rift at Lake Stefanie. The importance of en-echelon structures in the African rifts is seen to have been exaggerated.
Mohr, P. A.
Form and growth of an embryonic continental rift: InSAR observations and modelling of the 2009 western Arabia rifting episode
NASA Astrophysics Data System (ADS)
A magma-driven rifting episode occurred at the Harrat Lunayyir (Harrat Al-shaqa) volcanic field, western Arabia, between 2009 April and July. It was accompanied by a swarm of more than 4000 M > 2 earthquakes, the largest ever documented in that region, with a peak Mw 5.7 shock on May 19. We combine Interferometric Synthetic Aperture Radar (InSAR) measurements and elastic modelling with seismic moment calculations to resolve the evolution of surface deformation associated with this event. Phase discontinuities and low-coherence lineaments are used to infer the location of the main active structures during the various deformation stages and descending-track interferograms that span the entire period are inverted to resolve the slip and opening distributions along two graben-bounding normal faults and a dyke, respectively. Assuming negligible rift-parallel displacements, we combine ascending- and descending-track interferograms to derive the vertical and rift-perpendicular deformation, which add up to a maximum surface extension of 1.5 m across the rift and subsidence of 0.8 m. The far-field deformation is dominated by the dyke opening, whereas the near-field displacements are mostly associated with movements along the faults. The cumulative seismic moment released during the entire swarm period accounts for about 14 per cent of the total geodetic moment, compared to about 55 per cent at the 2007 Gelai (Tanzania) and about 8 per cent at the 2005 Manda Hararo-Dabbahu (Afar) events. We propose that the differences in moment partitioning ratios are due to the different crustal and seismogenic layer thicknesses in the three regions and represent different stages in the evolution of a volcanic rift. The Gelai event represents the most juvenile stage of rifting, the Dabbahu event represents the most evolved and the Harrat Lunayyir event represents a rift that is intermediate between the two in its degree of maturity.
Baer, Gidon; Hamiel, Yariv
Which mantle below the active rift segments in Afar?
NASA Astrophysics Data System (ADS)
The evolution of mantle sources beneath the Ethiopian volcanic province has long been discussed and debated with a long-lived controversy in identifying mantle reservoirs and locating them in the mantle. One interpretation of the isotopic composition of erupted lavas considers that the Afar mantle plume composition is best expressed by recent lavas from Afar and Gulf of Aden (e.g. Erta Ale, Manda Inakir and the 45�E torus anomaly on the Gulf of Aden) implying that all other volcanics (including other active segments and the initial flood basalt province) result from mixing of this plume component with additional lithospheric and asthenospheric components. A completely opposite view considers that the initial Oligocene continental flood basalts best represent the isotopic composition of the Afar mantle plume, which is subsequently mixed in various proportions with continental lithospheric mantle for generating some of the specific signature of Miocene and Quaternary volcanics. The precise and correct identification of mantle components involved in the generation of magmas is of particular importance because this is the only way to document the participation of mantle during extension and its potential role in break-up processes. In this contribution we provide new isotopic data for central Afar and we revisit the whole data set of the Ethiopian volcanic province in order to: (i) precisely identify the distinct mantle components implicated and (ii) discuss their location and evolution not only considering geochemical mixings, but also taking into account additional characteristics of erupted magmatic suites (volumes, location and relationships with amount of extension and segmentation). This new interpretation of geochemical data allows reconsidering the evolution of mantle in the course of rift evolution. In terms of mantle sources, two populations of active segments are frontally opposed in the volcanic province: those that share exactly the same composition with plume related CFBs (e.g. the Manda Hararo and the Main Ethiopian rift segments), and those that involve the participation of additional components characterized by more radiogenic lead isotopes (Erta Ale, Manda Inakir, Asal). We show that the material of the Afar mantle plume is not dispersed and attenuated laterally away from the centre of the province, but instead distributed and controlled spatially by rift segmentation.
Pik, Raphael; Stab, Martin; Ancellin, Marie-Anne; Sarah, Medynski; Cloquet, Christophe; Vye-Brown, Charlotte; Ayalew, Dereje; Chazot, Gilles; Bellahsen, Nicolas; Leroy, Sylvie
The mode of rifting of the Tyrrhenian Sea
NASA Astrophysics Data System (ADS)
The back-arc evolution of the Tyrrhenian Sea has mainly been attributed to the roll-back towards the south-east of the subducting Ionian plate that could have provided the bulk of the space required for this extension. The Tyrrhenian Sea is a triangular basin characterized by two large bathyal basins (Vavilov and Marsili basins) that are covered by some hundred meters of sediments, and a number of peri- Tyrrhenian basins filled by thousands of meters of clastic and/or volcaniclastic sediments. The stratigraphic record of these basins offers an opportunity to study the timing and kinematics of the basin-forming faults that are relevant for the creation of a model on the opening of the Tyrrhenian Sea. Basin analysis was performed using interpretation of seismic reflection profiles and well logs. The interpretation of these data was made using seismic and sequence stratigraphy and structural geology in a GIS-dedicated environment. The sequence stratigraphy interpretation of the deepest wells were performed using discontinuities and trends in wireline log pattern. Systems tracts and transgressive-regressive cycles were identified in well log succession and seismic profiles. The sequence stratigraphy approach allows the identification of 4th-order depositional sequences (100 ka). The geologic evolution, in terms of age of basin formation, style of deformation, timing of activity of the fault bounding basins, tectonic subsidence, post-rift infill and volcanic activity, was analyzed for several peri-Tyrrhenian basins. The study reconstructed the three-dimensional architecture of the peri-Tyrrhenian basins and illustrated the link between the bathyal basin and the Tyrrhenian margin. We document that during the evolution of the Tyrrhenian region several basins opened contemporaneously with different direction of extension and a progressive change in rifting direction occurred along the Campania Margin. The mode of rifting of the Tyrrhenian Sea was characterized by different styles of the extensional basins and detachment faulting. Taking into account the published geological data, we propose a kinematic evolution of the Tyrrhenian basin over the last 12 Ma.
Milia, Alfonsa; Torrente, Maurizio M.
Distributed deformation ahead of the Cocos-Nazca Rift at the Galapagos triple junction
NASA Astrophysics Data System (ADS)
The Galapagos triple junction is not a simple ridge-ridge-ridge (RRR) triple junction. The Cocos-Nazca Rift (C-N Rift) tip does not meet the East Pacific Rise (EPR). Instead, two secondary rifts form the link: Incipient Rift at 2�40?N and Dietz Deep volcanic ridge, the southern boundary of the Galapagos microplate (GMP), at 1�10?N. Recently collected bathymetry data are used to investigate the regional tectonics prior to the establishment of the GMP (?1.5 Ma). South of C-N Rift a band of northeast-trending cracks cuts EPR-generated abyssal hills. It is a mirror image of a band of cracks previously identified north of C-N Rift on the same age crust. In both areas, the western ends of the cracks terminate against intact abyssal hills suggesting that each crack initiated at the EPR spreading center and cut eastward into pre-existing topography. Each crack formed a short-lived triple junction until it was abandoned and a new crack and triple junction initiated nearby. Between 2.5 and 1.5 Ma, the pattern of cracking is remarkably symmetric about C-N Rift providing support for a crack interaction model in which crack initiation at the EPR axis is controlled by stresses associated with the tip of the westward-propagating C-N Rift. The model also shows that offsets of the EPR axis may explain times when cracking is not symmetric. South of C-N Rift, cracks are observed on seafloor as old as 10.5 Ma suggesting that this triple junction has not been a simple RRR triple junction during that time.
Smith, Deborah K.; Schouten, Hans; Zhu, Wen-lu; Mont�si, Laurent G. J.; Cann, Johnson R.
EAGLE - Design of 2003 Controlled Source Seismic Profile Across the Ethiopian Rift
NASA Astrophysics Data System (ADS)
The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) is a project to image the crust and upper mantle just prior to break-up in the northern Ethiopian Rift, where the transition from continental rifting to incipient sea-floor spreading is captured. A major component of EAGLE is a 400km cross-rift controlled source profile involving the detonation of 8 borehole shots into a nominal 450 recorders distributed along the line. The principal objective of this part of the project is to provide a cross-rift P-wave velocity model of the crust and upper mantle across a transitional rift segment. The results will be used: (1) to constrain the volume of magmatic material that has been added to the crust across the rift; (2) to determine the distribution of crustal strain; (3) to identify pre-rift variations in lithospheric properties that may have influenced high strain location; and (4) to provide high resolution crustal velocity control to enhance interpretation of teleseismic and local earthquake data recorded on the linked EAGLE passive array projects. A planning visit to Ethiopia in 2001 resulted in 7 of the 8 borehole sites being provisionally identified. Information concerning population distribution and environmental risk, water table depths and restrictions on the use of underwater shots will limit the distance to which seismic energy can be observed from each shotpoint. Results from previous surveys both in Ethiopia and in the Kenya Rift enable expected amplitude-distance relations to be estimated. Using (1) the optimum crustal seismic velocity model consistent with available gravity data, (2) the proposed distribution of shots and recording stations, (3) the range to which energy from each shot should be observed, and(4) field and remote sensing constraints on major rift structures and volcanic centres, 2-D forward modelling of first arrival travel times is being undertaken to enable tighter control on the experiment design in line with the defined experiment objectives.
Maguire, P. K.; Ebinger, C. J.; Asfaw, L. M.; Mackenzie, G.; Khan, M. A.
Thermo-tectonic evolution of the Upper Rhine Graben rift shoulders
NASA Astrophysics Data System (ADS)
The Upper Rhine Graben (URG) is the central segment of the European Cenozoic rift system, extending from the Mediterranean to the North Sea (Ziegler, 1992). The URG extends over a distance of 300 km from Basel (Switzerland) to Frankfurt (Germany) with an average width of 30--40 km. An initial rifting phase is recognised by Middle Eocene (Lutetian) lake deposits. The main rifting phase started at the end of the Eocene (Priabonian) and was followed by prominent uplift in the southern URG area in the Miocene (Schumacher, 2002). Apatite fission track (FT) dating is used to constrain the thermo-tectonic evolution of the rift shoulders. By FT data modelling varying cooling histories are derived for different tectonic blocks of the rift shoulders. In some areas tectonic uplift started in the Upper Cretaceous, a long time before the initial rifting of the URG. In the modelled data no increase in cooling, which might represent rift shoulder uplift and denudation related to the main rifting phase, can be observed. In contrast, in the southern part, the cooling rate increased in Miocene times due to greater uplift and denudation, although the climate changed to cooler and dryer conditions. Contemporaneous with this phase is enormous sediment erosion in the southern graben segment. As a consequence, the driving forces leading to the observed thermo-tectonic evolution of the rift shoulders cannot be explained by URG formation and/or climate changes alone, but must be seen in the context of a European intraplate stress regime. References: Schumacher, M.E. (2002): Tectonics, 21: 1--17. Ziegler, P.A. (1992): Tectonophysics, 208: 91--111.
Link, K.; Rahn, M.; Keller, J.
Crustal structure of the northern mississippi embayment and a comparison with other continental rift zones
Previous geological and geophysical investigations have suggested that the Mississippi Embayment is the site of a Late Precambrian continental rift that was reactivated in the Mesozoic. New information on the deep structure of the northern Mississippi Embayment, gained through an extensive seismic refraction survey, supports a rifting hypothesis. The data indicate that the crust of the Mississippi Embayment may be characterized by six primary layers that correspond geologically to unconsolidated Mesozoic and Tertiary sediments (1.8 km/s), Paleozoic carbonate and clastic sedimentary rocks (5.9 km/s), a low-velocity layer of Early Paleozoic sediments (4.9 km/s), crystalline upper crust (6.2 km/s), lower crust (6.6 km/s), modified lower crust (7.3 km/s), and mantle. Average crustal thickness is approximately 41 km. The presence and configuration of the low-velocity layer provide new evidence for rifting in the Mississippi Embayment. The layer lies within the northeast-trending upper-crustal graben reported by Kane et al. (1981), and probably represents marine shales deposited in the graben after rifting. The confirmation and delineation of a 7.3 km/s layer, identified in previous studies, implies that the lower crust has been altered by injection of mantle material. Our results indicate that this layer reaches a maximum thickness in the north-central Embayment and thins gradually to the southeast and northwest, and more rapidly to the southwest along the axis of the graben. The apparent doming of the 7.3 km/s layer in the north-central Embayment suggests that rifting may be the result of a triple junction located in the Reelfoot Basin area. The crustal structure of the Mississippi Embayment is compared to other continental rifts: the Rhinegraben, Limagnegraben, Rio Grande Rift, Gregory Rift, and the Salton Trough. This comparison suggests that alteration of the lower crust is a ubiquitous feature of continental rifts. ?? 1983.
Mooney, W. D.; Andrews, M. C.; Ginzburg, A.; Peters, D. A.; Hamilton, R. M.
The development of extension and magmatism in the Red Sea rift of Afar
NASA Astrophysics Data System (ADS)
Despite the importance of continental breakup in plate tectonics, precisely how extensional processes such as brittle faulting, ductile plate stretching, and magma intrusion evolve in space and time during the development of new ocean basins remains poorly understood. The rifting of Arabia from Africa in the Afar depression is an ideal natural laboratory to address this problem since the region exposes subaerially the tectonically active transition from continental rifting to incipient seafloor spreading. We review recent constraints on along-axis variations in rift morphology, crustal and mantle structure, the distribution and style of ongoing faulting, subsurface magmatism and surface volcanism in the Red Sea rift of Afar to understand processes ultimately responsible for the formation of magmatic rifted continental margins. Our synthesis shows that there is a fundamental change in rift morphology from central Afar northward into the Danakil depression, spatially coincident with marked thinning of the crust, an increase in the volume of young basalt flows, and subsidence of the land towards and below sea-level. The variations can be attributed to a northward increase in proportion of extension by ductile plate stretching at the expense of magma intrusion. This is likely in response to a longer history of localised heating and weakening in a narrower rift. Thus, although magma intrusion accommodates strain for a protracted period during rift development, the final stages of breakup are dominated by a phase of plate stretching with a shift from intrusive to extrusive magmatism. This late-stage pulse of decompression melting due to plate thinning may be responsible for the formation of seaward dipping reflector sequences of basalts and sediments, which are ubiquitous at magmatic rifted margins worldwide.
Keir, Derek; Bastow, Ian D.; Pagli, Carolina; Chambers, Emma L.
Tectonics and magmatism of the Hess deep region of the Galapagos rift
NASA Astrophysics Data System (ADS)
The joint analysis of data on the anomalous maganetic field, seismicity, and structures of the Hess deep basalts have allowed us to specify the elongation of zone of spreading and to correct the spatial distribution of the neovolcanic zone. The precise petrogeochemical analysis of various types of basalts composing the uneven-aged oceanic crust of the basin showed that the neovolcanic zone magmatics are related to the primitive type in contrast to rift boards of differential basalts. A model of the Galapagos rift�s deep structure in the area of the western Hess deep has been suggested.
Kashintsev, G. L.; Shreider, A. A.
Stress control of deep rift intrusion at Mauna Loa volcano, Hawaii.
Mauna Loa volcano, Hawaii, deforms by a combination of shallow dike intrusions in the rift zones and earthquakes along the base of the volcano, but it is not known how the spreading is accommodated in the lower part of the volcanic edifice. We present evidence from interferometric synthetic aperture radar data for secular inflation of a dike-like magma body at intermediate depth in the southwest rift zone during 2002 to 2005. Magma accumulation occurred in a section of the rift zone that was unclamped by previous dikes and earthquakes, suggesting that stress transfer plays an important role in controlling subsurface magma accumulation. PMID:17510364
Amelung, Falk; Yun, Sang-Ho; Walter, Thomas R; Segall, Paul; Kim, Sang-Wan
Evolution and segmentation of oblique rift in a cold lithosphere: Insights from analogue modeling
NASA Astrophysics Data System (ADS)
New lithospheric analogue models of oblique rifting provide insights into the fault evolution, basin segmentation, and mantle exhumation during rift localization and capture main characteristics of natural oblique rifts. We present two models of oblique rifting: an oblique rift (obliquity about 50�) with a pre-existing lithospheric weakness (heterogeneous model) and one homogeneous. The following evolution is observed: (i) The fault populations, especially in early stages of deformation, are composed of faults that are, in strike, intermediate between the rift-parallel trend and the perpendicular to the opening direction. This fault population is characteristic of oblique rifts as in previous studies. (ii) In later stages, faults parallel to the rift become numerous in both models. In the homogeneous model, displacement-normal faults also play a major role. The rift localization in an oblique direction involves a thinning in an oblique direction and variations of crustal thickness. The induced local (extensional) stresses seems to control the formation of rift-parallel faults. (iii) During final stages of extension, in the heterogeneous model, the crust is deformed by rift-parallel faults, and in the basins, displacement-normal faults compose the small-scale deformation pattern. Other main result is the complete different pattern of deformation of the entire lithosphere with or without pre-existing lithospheric weakness. Indeed, the heterogeneous model shows the start of mantle exhumation in an oblique direction whereas in the homogeneous model the extension is accommodated in en-echelon displacement-normal graben. The probable direction of ocean-continent transition, mantle exhumation if any, and the geometry of oceanic accretion centres would thus be very different according to the presence of a pre-existing lithospheric oblique weakness. Moreover, counterclockwise rotations of horsts are observed mainly in the homogeneous models. They result in the initiation of transfer zones. Such structures are parallel to the extension direction, particularly observed in the brittle mantle layer. We propose that they could represent proto-transfer/transform fracture zones observed in oblique rifts and oceanic basins. Those results may provide insights into the possible evolution of the Gulf of Aden conjugate margins.
Autin, J.; Bellahsen, N.; Husson, L.; Beslier, M.-O.; Leroy, S.; D'Acremont, E.
Rift Valley Fever outbreaks in Mauritania and related environmental conditions.
Four large outbreaks of Rift Valley Fever (RVF) occurred in Mauritania in 1998, 2003, 2010 and 2012 which caused lots of animal and several human deaths. We investigated rainfall and vegetation conditions that might have impacted on RVF transmission over the affected regions. Our results corroborate that RVF transmission generally occurs during the months of September and October in Mauritania, similarly to Senegal. The four outbreaks were preceded by a rainless period lasting at least a week followed by heavy precipitation that took place during the second half of the rainy season. First human infections were generally reported three to five weeks later. By bridging the gap between meteorological forecasting centers and veterinary services, an early warning system might be developed in Senegal and Mauritania to warn decision makers and health services about the upcoming RVF risk. PMID:24413703
Caminade, Cyril; Ndione, Jacques A; Diallo, Mawlouth; MacLeod, Dave A; Faye, Ousmane; Ba, Yamar; Dia, Ibrahima; Morse, Andrew P
Modeling the spatial spread of Rift Valley fever in Egypt.
Rift Valley fever (RVF) is a severe viral zoonosis in Africa and the Middle East that harms both human health and livestock production. It is believed that RVF in Egypt has been repeatedly introduced by the importation of infected animals from Sudan. In this paper, we propose a three-patch model for the process by which animals enter Egypt from Sudan, are moved up the Nile, and then consumed at population centers. The basic reproduction number for each patch is introduced and then the threshold dynamics of the model are established. We simulate an interesting scenario showing a possible explanation of the observed phenomenon of the geographic spread of RVF in Egypt. PMID:23377629
Gao, Daozhou; Cosner, Chris; Cantrell, Robert Stephen; Beier, John C; Ruan, Shigui
Localized crustal deformation in the Godavari failed rift, India
NASA Astrophysics Data System (ADS)
Six years of GPS measurements of crustal deformation in the Godavari failed rift (GFR) of stable India plate suggest very localized deformation. Elsewhere, all along the GFR the deformation is very low (<1.5 mm/yr). Localized deformation (up to 3.3�0.5 mm/yr) at least at two sites, implying compression on steep faults located on the southern margin of the GFR, is coincident with the region characterized by high level low-magnitude seismicity of past six years and implies strain accumulation for future moderate to strong magnitude earthquake in the region. The localized deformation is consistent with the view about deformation in such regions where seismicity migrates and deformation rate changes with time.
Mahesh, P.; Gahalaut, V. K.; Catherine, J. K.; Ambikapathy, A.; Kundu, Bhaskar; Bansal, Amit; Chadha, R. K.; Narsaiah, M.
Arc rifting of the Carolina terrane in northwestern South Carolina
Recent mapping and whole-rock geochemistry studies demonstrate that mafic metavolcanic rocks found along the boundary between the exotic Carolina terrane and the Inner Piedmont formed in a subduction-related volcanic arc and do not represent the Iapetan suture. Mafic metavolcanic rocks are spatially and genetically related to zoned mafic-ultramafic intrusive complexes. These rocks are similar to those found in other ancient and modern volcanic island arcs where ankaramites and picrites are well known, and they are locally associated with zoned complexes, e.g., Sierra Foothills-Klamath Mountains of the western U.S. Cordillera. The authors propose that prior to accretion to Laurentia in the early to middle Paleozoic, the Carolina arc terrane underwent an episode of intra-arc rifting which allowed primitive arc magmas to ascend and erupt without significant crystal fractionation or lithospheric assimilation. This interpretation may help resolve some stratigraphic problems in the eastern part of the Carolina terrane (Caroline slate belt).
Dennis, A.J. (Univ. of South Carolina, Aiken (USA)); Shervais, J.W. (Univ. of South Carolina, Columbia (USA))
Rare Wisconsin wildcat to target midcontinent rift Nonesuch shale
This paper reports on a Michigan operator which is to start drilling a remote Midcontinent rift wildcat in nonproducing Wisconsin early in 1992. Terra Energy Ltd., Traverse City, Mich., is attempting to obtain a state permit to drill a rank 6,000 ft wildcat to Middle Proterozoic Copper Harbor conglomerate. The wildcat will evaluate oil possibilities in Middle Proterozoic Nonesuch shale, just above Copper Harbor, in Bayfield County, Wis., about 10 miles west of Ashland. The company hopes to spud by yearend 1991 using a rig and services from Michigan, says Steve Savoie, Terra geologist. The proposed location is about 280 miles northwest of the nearest production, in the northwestern Michigan basin. The proposed location is about 70 miles west of the White Pine mine in northwestern Michigan, where copper has been mined and noncommercial volumes of oil have leaked from Lower Nonesuch.
Petzet, G.A.
One Health approach to Rift Valley fever vaccine development.
Since its discovery in the 1930s, Rift Valley fever virus (RVFV) spread across the African continent and invaded the Arabian Peninsula and several islands off the coast of Southeast Africa. The virus causes recurrent outbreaks in these regions, and its continued spread is of global concern. Next-generation veterinary vaccines of improved efficacy and safety are being developed that can soon be used for the widespread vaccination of livestock. However, due to regulatory and economic challenges, vaccine manufacturers have been reluctant to develop a human vaccine. Recent innovations in veterinary vaccinology, animal models and licensing strategies can now be used to overcome these hurdles. This paper reviews the historical impact of RVFV on human health and proposes strategies to develop and license a next-generation vaccine for both animals and humans. PMID:24681125
Kortekaas, Jeroen
Metavolcanics trace the development of ocean rifting, SW Svalbard
NASA Astrophysics Data System (ADS)
This study investigates late Neoproterozoic metavolcanics from southwestern Spitsbergen to determine the evolution of rifting and the relationship with continental breakup. A number of exposures of metavolcanics are connected with the regional Torellian unconformity, which is associated with Late Neoproterozoic orogenesis - post 640 Ma. This unconformity divides the older metasedimentary sequence of the Deilegga Group from the younger Sofiebogen metasedimentary Group. Samples have been collected from three areas from south to the north: Wedel Jarlsberg Land including the Werenski�ldbreen area (Jens Erikfjellet Formation) and the Chamberlindalen area as well as pillow lavas from Nordenski�ld Land, all belonging to the Sofiebogen Group. The rocks from the Werenski�ldbreen area are classified as subalkaline basalt to basaltic andesite, while metavolcanics from Chamberlindalen are alkaline basalts, whereas rocks from Nordenski�ld Land are tholeiitic basalts. All of them are affected by metamorphism under a minimum of greenschist facies conditions. The immobile REE are used throughout, because they are least affected by metamorphism. The REE profiles show flat patterns for samples from Nordenski�ld Land, whereas the rest of the samples are enriched in LREE. The trace element geochemistry indicates LREE-enrichment for samples from the Werenski�ldbreen area (Lan/Smn 2.1 - 5.7) and Chamberlindalen (Lan/Smn = 1.8 - 3.2), but LREE-depletion for Nordeski�ld metabasalts (Lan/Smn = 1.0 - 1.5). The Smn/Ybn ratio is low for metavolcanics from the Werenski�ldbreen area and from Nordenski�ld Land, while rocks of Chamberlindalen have elevated Smn/Ybn. The metavolcanics from SW Svalbard indicate an increasingly depleted character from south to north. We believe this reflects increasing degrees of melting as rifting develops, signaling continental break-up. This research is supported by NCN No. 2012/05/N/ST10/03594
Goluchowska, Karolina; Barker, Abigail; Manecki, Maciej; Majka, Jaros?aw; Czerny, Jerzy
Epidemiological study of Rift Valley fever virus in Kigoma, Tanzania.
Rift Valley fever virus (RVFV) is an acute, zoonotic viral disease caused by a �Phlebovirus,�which belongs to the Bunyaviridae family. Among livestock, outbreaks of the disease are�economically devastating. They are often characterised by large, sweeping abortion storms and�have significant mortality in adult livestock. The aim of the current study was to investigate�RVFV infection in the Kigoma region, which is nestled under the hills of the western arm of the�Great Rift Valley on the edge of Lake Tanganyika, Tanzania. A region-wide serosurvey was�conducted on non-vaccinated small ruminants (sheep and goats, n = 411). Sera samples were�tested for the presence of anti-RVFV antibodies and viral antigen, using commercial enzyme-linked immunosorbent assay and reverse transcriptase polymerase chain reaction, respectively.�The overall past infections were detected in 22 of the 411 animals, 5.4% (Confidence Interval�(CI) 95%�= 3.5% - 8.1%). The Kigoma rural area recorded the higher seroprevalence of 12.0%�(CI 95% = 7.3% - 18.3%; p < 0.0001), followed by Kibondo at 2.3% (CI 95%�= 0.5% - 6.5%;�p > 0.05) and the Kasulu district at 0.8% (CI 95%�= 0.0% - 4.2%; p > 0.05). The prevalence�was 12.5% and 4.7% for sheep and goats, respectively. Reverse transcriptase polymerase chain�reaction results indicated that only eight samples were found to be positive (n = 63). This study�has confirmed, for the first time, the presence of the RVFV in the Kigoma region four years�after the 2007 epizootic in Tanzania. The study further suggests that the virus activity exists�during the inter-epizootic period, even in regions with no history of RVFV. PMID:25005550
Kifaro, Emmanuel G; Nkangaga, Japhet; Joshua, Gradson; Sallu, Raphael; Yongolo, Mmeta; Dautu, George; Kasanga, Christopher J
Aedes Mosquito Saliva Modulates Rift Valley Fever Virus Pathogenicity
Background Rift Valley fever (RVF) is a severe mosquito-borne disease affecting humans and domestic ruminants. Mosquito saliva contains compounds that counteract the hemostatic, inflammatory, and immune responses of the host. Modulation of these defensive responses may facilitate virus infection. Indeed, Aedes mosquito saliva played a crucial role in the vector's capacity to effectively transfer arboviruses such as the Cache Valley and West Nile viruses. The role of mosquito saliva in the transmission of Rift Valley fever virus (RVFV) has not been investigated. Objective Using a murine model, we explored the potential for mosquitoes to impact the course of RVF disease by determining whether differences in pathogenesis occurred in the presence or absence of mosquito saliva and salivary gland extract. Methods C57BL/6NRJ male mice were infected with the ZH548 strain of RVFV via intraperitoneal or intradermal route, or via bites from RVFV-exposed mosquitoes. The virus titers in mosquitoes and mouse organs were determined by plaque assays. Findings After intraperitoneal injection, RVFV infection primarily resulted in liver damage. In contrast, RVFV infection via intradermal injection caused both liver and neurological symptoms and this route best mimicked the natural infection by mosquitoes. Co-injections of RVFV with salivary gland extract or saliva via intradermal route increased the mortality rates of mice, as well as the virus titers measured in several organs and in the blood. Furthermore, the blood cell counts of infected mice were altered compared to those of uninfected mice. Interpretation Different routes of infection determine the pattern in which the virus spreads and the organs it targets. Aedes saliva significantly increases the pathogenicity of RVFV.
Le Coupanec, Alain; Babin, Divya; Fiette, Laurence; Jouvion, Gregory; Ave, Patrick; Misse, Dorothee; Bouloy, Michele; Choumet, Valerie
Active Tectonics In The Rukwa Rift (sw Tanzania): A Study of The Potential For Large Earthquakes In A Continental Rift.
NASA Astrophysics Data System (ADS)
The Rukwa rift is a deep sedimentary basin that is considered as a tectonic trans- fer zone between the Tanganyika and the Malawi troughs. The tectonic evolution of the depression is controlled by the reactivation of proterozoic structures and started with the deposition of the permo-triasic Karoo sediments. In the southeast, the rift is divided into two facing half graben separated by a Precambrian horst, whereas its northwestern part has a more symmetrical graben structure. Although most of the vertical displacement is accommodated by the Lupa eastern boundary fault, onshore shallow seismic profiles have confirmed the co-occurrence of intrabasin synthetic- and strike-slip faults within the sub surface sediments. Both normal and dextral strike-slip movement are indeed observed in the basin in response to the E-W to WNW-SSE ex- tension. The region has a moderate seismic activity and the earthquakes magnitude is generally below M 6.5. However, a M 7.4 earthquake occurred in the Rukwa region in 1910 but its exact location remains uncertain. The current research aimed at the identi- fication of active faults within the recent deposits of the basin by the combination in a GIS of radar interferometric data with topographical and geological maps, geophysical data, and field observations. Radar interferometry (InSAR) was found to be especially suitable for DEM computation in low relief areas where available topographic data are limited in accuracy. Numerous topographic lineaments were observed on InSAR DEM, and follow two main directions, both oblique to the main NW-SE trend of the rift. On the one hand, the GIS analysis confirms that the observed lineaments corre- spond to real natural alignment such like the drainage for example, and are therefore not related to atmospheric artefacts. On the other hand, the field observations revealed that in most cases, the topographic lineaments are very subtle and difficult to identify. However, direct correlations with tectonic structures (faults, fissures, ...) were recog- nized in some places. From the satellite imagery, the northwestern part of the basin is marked by the succession of paleo-shorelines evidencing the lake level fluctuations. The highest level was dated to 10.000 yrs BP, when the Rukwa was supposed to flow into lake Tanganyika. The corresponding shoreline is cut by a fault, ~ 40 km long, striking NNW-SSE, evidenced on the InSAR DEM and confirmed by the LANDSAT and ASTER optical imagery. Although a possible strike-slip component, the 5 to 10 1 meters of vertical offset from one to the other side of the fault would yield to a slip rate of 0.5 U 1mm/yr. 2
Kervyn, F.
Formation of hyperextended rifted margins: Insights from flexural isostatic structural-stratigraphic modeling and observations from present-day rifted margins
NASA Astrophysics Data System (ADS)
The understanding of how continental lithosphere extends, thins and ruptures leading to the formation of a new divergent plate boundary represent a fundamental question in Earth Sciences. In particular the mechanisms controlling the extreme pre-breakup stretching and thinning of the continental crust and lithosphere, documented at many present-day rifted margins, are still poorly known. Many questions remain of the fundamental processes controlling the extensional deformation of the continental crust and lithosphere, including fault geometries and their evolution in space and time, the occurrence of decoupling horizons within the continental crust and the importance of depth-dependent lithosphere thinning processes. We investigate the control of these key factors on continental crust and lithosphere thinning processes by combining seismic reflection and drill-hole observations from present-day Iberia-Newfoundland rifted margins with flexural isostatic forward tectonic and stratigraphic modeling. These observations made at present-day rifted margins constrain the input parameters used in the flexural isostatic forward modeling. At the same time, the forward tectonic and stratigraphic modeling provides validation of the interpretation of the seismic reflection data. Through this modeling, we produce isostatically and thermally balanced sections reproducing the geometries observed along the Iberia-Newfoundland rifted margins. Our results suggest that crustal and lithospheric thinning results from the combination of both pure- and simple-shear deformation. The model predicts the critical role of intra-crustal decoupling horizons confirming the importance of depth-dependent thinning through polyphased rifting events.
Mohn, Geoffroy; Kusznir, Nick; Manatschal, Gianreto
Fluids in hyper-extended rifted margins: Examples from the paleomargins in the eastern Alps and west Pyrenees and present-day Iberia rifted continental margin.
NASA Astrophysics Data System (ADS)
The evolution of deep-water rifted margins is intimately linked with complex and poly-phase fault structures. These structures known as detachment faults are responsible for extreme crustal thinning and mantle exhumation. During the evolution of detachment faults fluid-rock interaction play an important role, changing the chemical and physical properties of rocks with major implications for the strain localization and structural evolution of the margin. The change in rock chemistry and rheology is best indicated by the breakdown of feldspars and olivine into clays and serpentine minerals, and the pervasive cementation and precipitation of quartz within the fault rocks and veins along detachment faults. Although the chemistry reaction is well known it is still unclear to what extent those reactions can lead to changes in the rheology of the lithosphere and how they can affect the thermal evolution of deep water, hyper-extended rifted margins. Another important question arise about the origin, timing, pathways and composition of these fluids. Are they mantle-derived fluids and/or of marine origin? In this work we present preliminary results from fossil rift-related detachment faults exposed in the Alps and Pyrenees and discuss the role of fluids during lithospheric thinning. We compare the results obtained from these fossil rifted margins with those from the present-day Iberia rifted continental margin.
Pinto, V. H.; Manatschal, G.; Viana, A.; Karpoff, A. M.; Masini, E.; Szatmari, P.; Penteado, H.; Svartman, A.; Lemarchand, D.
From surface observations to in depth structures and plate kinematics in oblique rifts. Insights from the Main Ethiopian Rift.
NASA Astrophysics Data System (ADS)
The Main Ethiopian Rift (MER) is the oblique NE-SW trending plate boundary between Nubia and Somalia that developed above an inherited lithospheric-scale weak zone, the Mozambique Ocean Suture Zone (MOSZ). In this area, plate's kinematics has been estimated from various sources including: GPS and seismic data, spreading rate estimates, magnetic anomaly & paleostress reconstruction from field and seismic data. These various data sets give a range for the orientation of the stretching direction between N105� and 115�. However, in the MER, it is shown that strain is strongly partitioned between boundary and internal faults. Far field stress is re-oriented along the boundary faults that are activated in pure extension. This observation questions the use of paleostress reconstructions based on fault slip data and focal mechanisms to estimate the direction of plate motion. Detailed analysis of fault orientation and fault kinematics in analogue model illustrates that strain partitioning is triggered by the geometry of the deep-seated weak zone and that fault orientations give a better insight on the direction of stretching than paleostress tensors. Moreover, our model allows the recovering of the far field stress direction and the orientation of the weak zone in depth from surface observation of fault trend data. Applying this model to surface data of the MER give a different stretching direction for the formation of the boundary and the internal fault suggesting a clockwise rotation of Somalia. Also the model gives major constraints on the direction of the deep-seated weak zone. Its orientation is evolving from N62� in the Northern MER, to N18� in the Wide Rifted Zone (passing from N35� and N25� for the Central MER and the Southern MER).
Philippon, Melody; Sokoutis, Dimitrios; Willingshofer, Ernst; Corti, Giacomo; Bonini, Marco; Sani, Federico; Cloetingh, Sierd
Bookshelf faulting and horizontal block rotations between overlapping rifts in southern Afar
Lateral slip on initially rift-parallel normal faults may be a particularly efficient mechanism to accommodate strain between overlapping oceanic rifts. It occurs in southern Afar, where clockwise block rotations result from distributed dextral shear between the overlapping Ghoubbet Asal-Manda Inakir and Manda Hararo-Abhe Bad rifts. Faulting observed during the 1969, Serdo earthquakes and on SPOT images is consistent with the shear being taken up by left-lateral slip on steep NW-SE striking faults, which formed as normal faults before extensional strain became localized in the two rifts. This bookshelf faulting accounts quantitatively for the 14.5{degree} {plus minus}7.5{degree} rotation documented by paleomagnetism in the 1.8 {plus minus}0.4 Ma old Afar stratoid basalts, given the 17.5 {plus minus}5 mm/yr rate of separation between Arabia and Somalia.
Tapponnier, P.; Armijo, R.; Manighetti, I.; Courtillot, V. (Institut de Physique du Globe de Paris (France))
Three-Dimensional (3D) Structure of the Malawi Rift from Remote Sensing and Geophysics Data
NASA Astrophysics Data System (ADS)
The Malawi rift is a Cenozoic aged rift representing the southernmost segment of the Western Branch of the East African Rift System (EARS). This rift extends over 900 km from the Rungwe volcanic province (Tanzania) in the north to the Urema graben (Mozambique) to the south, with an average width of 50km. It traverses a complex array of Proterozoic orogenic belts of different ages and Permo-Triassic (Karoo) and cretaceous graben systems. The rift's depth is between 3 to 5km partitioned between the topographic escarpment and the sediments fill. The basin's subsidence reflects accumulation of sediments and rift flank uplift. Regardless of its importance in understanding rift tectonics, especially in Africa, the three-dimensional (3D) geometry of the rift is not fully understood. This research presents results from detailed analysis of Digital Elevation Model (DEM) extracted from the Shuttle Radar Topography Mission (SRTM) data to map surface morphological expressions of the entire basin. These results are compared with available seismic data to provide along-strike and at depth variation of the geometry of the border fault systems, nature of rift segmentation and alternation of the polarity of half-grabens, and the partitioning of displacement between exposed and sub-surface border faults. Our results show the following: (1) Surface expression of border faults show that, unlike the typical half-graben en-echelon rift model, where half-graben segments with opposite polarity are linked together through accommodation zones indicative of soft linkage, the Malawi rift shows along-strike segmentation by changing geometry from half-graben to full graben geometry. A half-graben with specific polarity passes through a full-graben geometry before giving place to a half-graben with the opposite polarity. The length of half-gaben and graben segments becomes shorter as the rift progresses from north to south, and this is accompanied by a decrease in displacement within border faults. This geometry is indicative of the propagation of border faults through hard linkage. (2) The continuation of border faults at the subsurface show patterns consistent with those observed at the surface. At the sub-surface, the general trend of rift segmentation, formation of full grabens at the end of each segment, and the decreases in the length of the segments from north to south is consistent with observations at the surface. This suggests the homogeneity of strain accommodation throughout the depth of border faults. (3) Zones of segmentation of the Malawi rift coincide with regions where the pre-existing structures (both the Proterozoic basement and the Karoo grabens) are at high angle to the trend of the rift whereas well-developed border faults of the basin coincides with N-trending pre-existing structures sub-parallel to the rift.
Salmi, Haifa S. Al; Abdelsalam, Mohamed G.
Simulated ChemCam Laboratory Investigations of East African Rift Sedimentary Samples
NASA Astrophysics Data System (ADS)
The East African Rift (EAR) system may be a good Earth analogue for martian surface sediments. Seventeen EAR samples were probed with a remote LIBS instrument designed to replicate the ChemCam instrument at a 9 m standoff distance.
Clegg, S. M.; Wiens, R. C.; Barefield, J. E.; Dyar, M. D.; Delaney, J. S.; Ashley, G. M.; Driese, S. G.
Studies on Rift Valley fever in some African murids (Rodentia: Muridae).
Brains, spleens and livers of 2214 murids, 27 shrews and 7 dormice, trapped at 7 sites in Rhodesia, were tested in 277 pools for the presence of Rift Valley Fever virus. There were no isolations of Rift Valley Fever, but 69 isolations of an unidentified virus were obtained. Sixteen out of 867 sera had low-titre haemagglutination-inhibition activity against Rift Valley Fever antigen, but only one out of 1260 sera had neutralizing antibody. The evidence suggests that murids fail to encounter infection in nature and are unlikely to play a role in circulation and dissemination of Rift Valley Fever virus. Four out of seven widely distributed species of muried, Rhabdomys pumilio, Saccostomys campestris, Aethomys chrysophilus and Lemniscomys griselda, were shown to be capable of circulating amounts of virus likely to be infective for mosquitoes. PMID:632561
Swanepoel, R; Blackburn, N K; Efstratiou, S; Condy, J B
Serological Tests for Detecting Rift Valley Fever Viral Antibodies in Sheep from the Nile Delta,
National Technical Information Service (NTIS)
To determine the accuracy of serological methods in detecting Rift Valley fever (RVF) viral antibodies, we examined serum samples obtained from 418 sheep in the Nile Delta by using five tests. The plaque reduction neutralization test (PRNT) was considered...
R. M. Scott F. M. Feinsod I. H. Allam T. G. Ksiazek C. J. Peters
Failure was not an option- the Mid-Continent Rift system succeeded
NASA Astrophysics Data System (ADS)
The 1.1 Ga Mid-Continent Rift (MCR) in North America is often viewed as a failed rift formed by isolated midplate volcanism and extension within the ~1.3-~0.98 Ga Grenville orogeny. An alternative view is suggested by analogy with younger and morphologically similar rift systems, whose plate tectonic settings are more easily understood because their surroundings - including seafloor with magnetic anomalies - have not been deformed or destroyed by subsequent collisions and rifting events. In this view, the MCR was part of a larger plate boundary rifting event that resulted in a successful episode of seafloor spreading. This view is motivated by various pieces of evidence. The MCR rifting looks much like rigid plate block motion, such as associated with the West Central African Rift systems formed during the Mesozoic breakup of Africa and South America and the ongoing rifting in the East African Rift region with seafloor spreading in the Gulf of Aden and the Red Sea. This view explains the affinities of the Grenville-age rocks in the central and southern Appalachians to Amazonia rather than Canadian Grenville-age Appalachian rocks. The MCR extends farther to the south than traditionally assumed along the East Continental Gravity High (a buried feature from Ohio to Alabama). This failed portion of the rift system connected to the rift successfully separating Laurentia and Amazonia. The seafloor spreading separating Amazonia from Laurentia may explain the former's relative motion toward Greenland and Baltica. This model is consistent with some of the ~1.1 Ga geological events in Amazonia. A change in the apparent polar wander path for Laurentia during the period of volcanism of the MCR could be attributed to this plate reconfiguration. The extensional phase on the MCR may have ended because motion was taken up by seafloor spreading between Laurentia and Amazonia rather ending due to another continental collision. Later reverse faulting on the MCR normal faults due to compression, perhaps from collisions around Rodinia's margins, would not be unexpected because the MCR would be a relatively weak intraplate zone due to higher crustal temperatures and faults.
Merino, M.; Stein, C. A.; Stein, S. A.; Keller, G. R.; Flesch, L. M.; Jurdy, D. M.
Imaging the midcontinent rift beneath Lake Superior using large aperture seismic data
The authors present a detailed velocity model across the 1.1 billion year old Midcontinent Rift System (MRS) in central Lake Superior. The model was derived primarily from onshore-offshore large-aperture seismic and gravity data. High velocities obtained within a highly reflective half-graben that was imaged on coincident seismic reflection data demonstrate the dominantly magic composition of the graben fill and constrain its total thickness to be at least 30 km. Strong wide-angle reflections are observed from the lower crust and Moho, indicating that the crust is thickest (55-60 km) beneath the axis of the graben. The total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north. Above the Moho is a high-velocity lower crust interpreted to result from syn-rift basaltic intrusion into and/or underplating beneath the Archean lower crust. The lower crust is thickest beneath the axis of the main rift half-graben. A second region of thick lower crust is found approximately 100 km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting. The crustal model presented here resembles recent models of some passive continental margins and is in marked contrast to many models of both active and extinct Phanerozoic continental rift zones. It demonstrates that the Moho is a dynamic feature, since the pre-rift Moho is probably within or above the high-velocity lower crust, whereas the post-rift Moho is defined as the base of this layer. In the absence of major tectonic activity, however, the Moho is very stable, since the large, abrupt variations in crustal thickness beneath the MRS have been preserved for at least a billion years.
Trehu, A.; Shay, J. (Oregon State Univ., Corvallis (United States)); Morel-a-l'Huissier, P.; Milkereit, B. (Geological Survey of Canada, Ottawa, Ontario (Canada)); Meyer, R.; Jefferson, T.; Shih, X.R. (Univ. of Wisconsin, Madison (United States)); Karl, J. (Univ. of Saskatchewan, Saskatoon (Canada)); Mereu, R.; Epili, D. (Univ. of Wisconsin, Oshkosh (United States)); Sexton, J.; Wendling, S. (Univ. of Western Ontario, London (Canada)); Hajnal, Z.; Chan, W.K. (Southern Illinois Univ., Carbondale (United States)); Hutchison, D. (Geological Survey, Woods Hole, MA (United States))
Local Moho Updoming Beneath The Western Eger Rift, Central Europe ? Results From Teleseismic Receiver Functions
The Eger rift is part of the European Cainozoic rift system. The tectono-magmatic activity in this area started in the uppermost Cretaceous. The Quaternary to active period is associated with CO2-emanations at the surface, alkaline volcanic activity, neotectonic uplift in the Slavskovsky Les area, active basin formation of the Cheb basin and earthquake swarm activity in the German\\/Czech boundary region
W. H. Geissler; R. Kind; H. Kaempf; K. Klinge; T. Plenefisch; J. Zednik; J. Horalek
Geodetic observations of the ongoing Dabbahu rifting episode: new dyke intrusions in 2006 and 2007
A 60-km-long dyke intruded the Dabbahu segment of the Nubia-Arabia Plate boundary (Afar, Ethiopia) in 2005 September, marking the beginning of an ongoing rifting episode. We have monitored the continuing activity using Satellite Radar Interferometry (InSAR) and with data from Global Positioning System (GPS) instruments and seismometers deployed around the rift in response to the initial intrusion. These data show
Ian J. Hamling; Atalay Ayele; Laura Bennati; Eric Calais; Cynthia J. Ebinger; Derek Keir; Elias Lewi; Tim J. Wright; Gezahegn Yirgu
Seismological Evidence of Active Faulting in the Tendaho Rift (Afar Triangle, Ethiopia)
A temporary network has been used to study the seismic activity in the central-northern part of the Tendaho Rift (Afar Triangle, Ethiopia). Small size earthquakes (MB2.5) characterize the seismicity during the time interval 1993-1994 when the seismic array was operating. Shocks are mainly located in the central part of the Tendaho rift and their epicentral distribution highlights a remarkable coincidence
S. Gresta; D. Patan�; A. Daniel; L. Zan; A. Carletti; O. Befekadu
238U� 230Th disequilibrium in recent basalts and dynamic melting beneath the Kenya rift
Trace element and U-series isotope analyses are presented for a suite of recent (<10�ka) basalts from the axial portion of the Kenya rift. Samples from throughout the rift have LREE-enriched patterns with HREE >10�chondrite and the LREE between 60 and 200�chondrite. REE fractionation is consistent with melting a garnet lherzolite source region with between 2% and 6% modal garnet. Other
N. W. Rogers; L. E. Thomas; R. Macdonald; C. J. Hawkesworth; F. Mokadem
Crustal thinning beneath the Rwenzori region, Albertine rift, Uganda, from receiver-function analysis
The Rwenzori mountains in western Uganda, with a maximum elevation of more than 5,000�m, are located within the Albertine\\u000a rift valley. We have deployed a temporary seismic network on the Ugandan side of the mountain range to study the seismic velocity\\u000a structure of the crust and upper mantle beneath this section of the rift. We present results from a receiver-function
Ingo W�lbern; G. R�mpker; A. Schumann; A. Muwanga
Imaging the midcontinent rift beneath Lake Superior using large aperture seismic data
NASA Astrophysics Data System (ADS)
We present a detailed velocity model across the 1.1 billion year old Midcontinent Rift System (MRS) in central Lake Superior. The model was derived primarily from onshore-offshore large-aperture seismic and gravity data. High velocities obtained within a highly reflective half-graben that was imaged on coincident seismic reflection data demonstrate the dominantly mafic composition of the graben fill and constrain its total thickness to be at least 30km. Strong wide-angle reflections are observed from the lower crust and Moho, indicating that the crust is thickest (55-60km) beneath the axis of the graben. The total crustal thickness decreases rapidly to about 40 km beneath the south shore of the lake and decreases more gradually to the north. Above the Moho is a high-velocity lower crust interpreted to result from syn-rift basaltic intrusion into and/or underplating beneath the Archean lower crust. The lower crust is thickest beneath the axis of the main rift half-graben. A second region of thick lower crust is found approximately 100km north of the axis of the rift beneath a smaller half graben that is interpreted to reflect an earlier stage of rifting. The crustal model presented here resembles recent models of some passive continental margins and is in marked contrast to many models of both active and extinct Phanerozoic continental rift zones. It demonstrates that the Moho is a dynamic feature, since the pre-rift Moho is probably within or above the high-velocity lower crust, whereas the post-rift Moho is defined as the base of this layer. In the absence of major tectonic activity, however, the Moho is very stable, since the large, abrupt variations in crustal thickness beneath the MRS have been preserved for at least a billion years.
Tr�hu, A.; Morel-�-l'Huissier, P.; Meyer, R.; Hajnal, Z.; Karl, J.; Mereu, R.; Sexton, J.; Shay, J.; Chan, W.-K.; Epili, D.; Jefferson, T.; Shih, X.-R.; Wendling, S.; Milkereit, B.; Green, A.; Hutchinson, D.
Normal faulting from simple shear rifting in South Tibet, using evidence from passive seismic profiling across the Yadong-Gulu Rift
NASA Astrophysics Data System (ADS)
The Tibetan Plateau is undergoing north-south shortening accompanied by west-east extension, as evidenced by the widespread development of north-south trending normal faults, grabens and rifts. While the mode of the north-south shortening has been the main focus of most international studies, knowledge of the deep structure beneath South Tibet is required for understanding the mechanism of the west-east extension. The onset of the north-south trending normal faulting is commonly taken as an indicator that the Tibetan Plateau was uplifted to a near-maximum elevation before entering a collapsing stage. Here we report on the receiver functions of a seismological experiment across the northern segment of the Yadong-Gulu Rift (YGR), one of the youngest rifts in South Tibet. The migrated receiver function images reveal that the YGR is a high-angle normal fault characterized by a 5-km Moho rise from its western to eastern flank, together with distinct differences in the crustal structure and intracrustal seismic conversion patterns between the two flanks. This highly asymmetric lithospheric structure suggests whole-crustal extension controlled by a simple/general shear rifting mechanism. This simple/general shear rifting in the YGR is attributed to an eastward (horizontal) shear at the base of the upper crust, as evidenced by the observed Tibetan GPS velocity field and our observation of shear wave splitting discrepancy among the upper crust, lower crust and lithospheric mantle. We propose that in the YGR, simple shear rifting accommodates the northward injection of the Indian lithosphere, which may suggest that the onset of the north-south normal faulting does not indicate gravitational collapse of the Tibetan lithosphere.
Zhang, Zhongjie; Chen, Yun; Yuan, Xiaohui; Tian, Xiaobo; Klemperer, Simon L.; Xu, Tao; Bai, Zhiming; Zhang, Hongshuang; Wu, Jing; Teng, Jiwen
Deep structure of the northern Rio Grande rift beneath the San Luis basin (Colorado) from a seismic reflection survey: implications for rift evolution
NASA Astrophysics Data System (ADS)
A seismic reflection survey by Chevron across the San Luis basin (northern Rio Grande rift) and San Juan volcanic field of southern Colorado is reprocessed with extended correlation to search for basement structure. The trace of the main bounding fault of the basin, a high-angle normal fault against the Sangre de Cristo Range, can be correlated to a wide zone of dipping reflection fabric and soles out at lower crustal depths (26-28 km). The deeper reflection fabric represent either broad extensional strain or pre-existing structure, such as a Laramide thrust system. The Sangre de Cristo bounding fault in San Luis basin does not sole out at mid-crustal depths but continues into the lower crust with a shallower dip. The basin architecture in the northern Rio Grande rift (San Luis basin) provides little if any evidence that the Sangre de Cristo bounding fault should flatten in a shallow listric fashion. This fault geometry is quite similar to the high-angle bounding fault in the Espanola basin but contrasts with less deeply-rooted faults in the Albuquerque basin in the central Rio Grande rift. Deeper soling out of the Sangre de Cristo bounding fault could be due to less extension in the northern Rio Grande rift and/or greater strength of the lithosphere compared to the central Rio Grande rift. Unequivocal Moho reflections beneath the San Luis basin cannot be identified, probably due to limited signal penetration or a gradational nature of the Moho. The majority of rift-related movement observed on the Sangre de Cristo bounding fault is post-Eocene. Either the western margin of the basin is marked by a tight monocline or a low-angle normal fault.
Tandon, Kush; Brown, Larry; Hearn, Thomas
Accumulation of fossil fuels and metallic minerals in active and ancient rift lakes
A study of active and ancient rift systems around the world suggests that accumulations of fossil fuels and metallic minerals are related to the interactions of processes that form rift valleys with those that take place in and around rift lakes. The deposition of the precursors of petroleum, gas, oil shale, coal, phosphate, barite, Cu-Pb-Zn sulfides, and uranium begins with erosion of uplifted areas, and the consequent input of abundant nutrients and solute loads into swamps and tectonic lakes. Hot springs and volcanism add other nutrients and solutes. The resulting high biological productivity creates oxidized/reduced interfaces, and anoxic and H2S-rich bottom waters which preserves metal-bearing organic tissues and horizons. In the depositional phases, the fine-grained lake deposits are in contact with coarse-grained beach, delta, river, talus, and alluvial fan deposits. Earthquake-induced turbidites also are common coarse-grained deposits of rift lakes. Postdepositional processes in rifts include high heat flow and a resulting concentration of the organic and metallic components that were dispersed throughout the lakebeds. Postdepositional faulting brings organic- and metal-rich sourcebeds in contact with coarse-grained host and reservoir rocks. A suite of potentially economic deposits is therefore a characteristic of rift valleys. ?? 1983.
Robbins, E. I.
Variation in magma volume along the two arms of the Midcontinent Rift System
NASA Astrophysics Data System (ADS)
The 2000km-long Midcontinent Rift System (MCRS) has two major arms meeting in the Lake Superior region. One extends southwestward at least as far as central Kansas, and the other extends southeastward through Michigan. Gravity and magnetic anomalies delimit the rift zone because the highly magnetic and dense mafic igneous rocks filling the central grabens of the rift system have strong susceptibility and density contrasts with adjacent rock formations. Because the rift lies hidden beneath gently dipping Phanerozoic sedimentary rocks except in the Lake Superior region, most models of rift structure have been extrapolated from the few areas that have seismic reflection data. A fundamental question is how the two arms differ. The west arm is more pronounced than the east arm in the gravity data. Whether this difference is due to the east arm being buried by the Michigan Basin or reflects magma volume has implications for the formation of the MCRS. Existing gravity models are not standardized across the rift and therefore cannot be used to directly compare the arms. We have thus conducted gravity modeling with a uniform approach and find that the west arm has significantly more magma and that the magma volume along the west arm increases toward Lake Superior. These results imply that there was more spreading along the western arm and that the spreading on this arm decreased southward, consistent with the arms being boundaries of a microplate rotating with respect to the Superior province with its rotation pole to the southwest.
Merino, M.; Keller, G. R.; Stein, S. A.
Basaltic volcanism in Ethiopia: Constraints on continental rifting and mantle interactions
Middle to late Cenozoic mafic lavas from the Ethiopian volcanic province exhibit considerable chemical and isotopic diversity that is linked to eruption age and eruption location. These variations provide a geochemical framework in which continental rifting can be examined. Trace element and Sr, Nd, and Pb isotopic data are interpreted to indicate involvement of up to two depleted and two enriched mantle reservoirs throughout Cenozoic rift development in Ethiopia. Superimposed on the characteristics imparted by varying degrees of melting of these distinct reservoirs are the effects of crystal fractionation and, in some instances, crustal contamnation. Initial stages of Oligocene rifting and volcanism, as manifested by the rift-bounding plateau flood basalts, are attributed to asthenospheric upwelling and melting of a heterogeneous, enriched subcontinental lithospheric mantle. Mildly alkaline lavas were produced from an enriched source with characteristics similar to those of the inferred source of other mantle/minus/derived lavas and xenoliths from east Africa (LoNd array, EMI to HIMU). Contemporaneous tholeiitic lavas were derived from a source similar to that producing oceanic basalts from Samoa and the Society Islands (EMII). As lithospheric thinning and rifting continued into the Miocene, upwelling depleted asthenosphere (depleted OIB reservoir, PREMA) interacted with the lithospheric sources producing lavas with hybrid elemental and isotopic characteristics (11-6 Ma plateau and rift margin basalts).
Hart, W. K.; WoldeGabriel, G.; Walter, R. C.; Mertzman, S. A.
Rates of volcanic activity along the southwest rift zone of Mauna Loa volcano, Hawaii.
Flow-by-flow mapping of the 65 km long subaerial part of the southwest rift zone and adjacent flanks of Mauna Loa Volcano, Hawaii, and about 50 new 14C dates on charcoal from beneath these flows permit estimates of rates of lava accumulation and volcanic growth over the past 10 000 years. The sequence of historic eruptions along the southwest rift zone, beginning in 1868, shows a general pattern of uprift migration and increasing eruptive volume, culminating in the great 1950 eruption. No event comparable to 1950, in terms of volume or vent length, is evident for at least the previous 1000 years. Rates of lava accumulation along the zone have been subequal to those of Kilauea Volcano during the historic period but they were much lower in late prehistoric time (unpubl. Kilauea data by R. T. Holcomb). Rates of surface covering and volcanic growth have been markedly asymmetric along Mauna Loa's southwest rift zone. Accumulation rates have been about half again as great on the northwest side of the rift zone in comparison with the southeast side. The difference apparently reflects a westward lateral shift of the rift zone of Mauna Loa away from Kilauea Volcano, which may have acted as a barrier to symmetrical growth of the rift zone. -Author
Lipman, P. W.
Structural evolution of the southern transfer zone of the Gulf of Suez rift, Egypt
NASA Astrophysics Data System (ADS)
We present a detailed study about the initiation and reactivations of Zeit-El Tor transfer zone, south Gulf of Suez rift, and its structural setting and tectonic evolution with respect to the Cretaceous-Cenozoic tectonic movements in North Egyptian margin. NE trending zone of opposed-dipping faults (22 km wide) has transferred the NE and SW rotations of the sub-basins in central and south Gulf of Suez rift, respectively. The evolution of this zone started by reactivation of the NE oriented late Neoproterozoic fractures that controlled the occurrence of Dokhan Volcanics in the rift shoulders. Later, the Syrian Arc contraction reactivated these fractures by a sinistral transpression during the Late Cretaceous-Eocene time. N64�E extension of the Oligo-Miocene rift reactivated the NE fractures by a sinistral transtension. During this rifting, the NE trending faults forming the transfer zone were more active than the rift-bounding faults; the Upper Cretaceous reverse faults in the blocks lying between these NE trending faults were rotated; and drape-related reverse faults and the positive flower structures were formed. Tectonic inversion from contraction to extension controlled the distribution and thickness of the Upper Cretaceous-Miocene rocks.
Abd-Allah, Ali M. A.; Abdel Aal, Mohamed H.; El-Said, Mohamed M.; Abd El-Naby, Ahmed
Along-axis Segmentation of the Main Ethiopian Rift from Tomographic Inversion of Local Earthquakes
NASA Astrophysics Data System (ADS)
Geophysical, structural, and geochemical data indicate that the East African rift system in Ethiopia is in a stage of development transitional between continental and oceanic spreading. In the northern Main Ethiopian rift (MER) the archetypal continental rift stage border fault systems bounding the asymmetric rift basins formed at ~11 Ma. After 3.5 Ma, strain localised to fault systems within the central basin. By 1.8 Ma, faulting and magmatism localised to an en echelon series of 20 km-wide zone, 50-60 km-long 'magmatic segments' comprising felsic shield volcanoes and basaltic fissural flows and dikes. As part of the international EAGLE project, 50 broadband Guralp 6TD seismometers were deployed across 3 magmatic segments, and a sector of the rift without magmatic segments. These instruments nested within the Leeds University broadband array, and the combined net of 80 instruments was used to locate local and regional events. P wave inversion for velocity with a series of synthetic tests will be presented at the meeting along with the best 1d velocity model for locating local earthquakes within the Main Ethiopian rift. Results indicate a series of high velocity zones beneath the magnetic segments at a depth of 10km.
Daly, E.; Ebinger, C.; Stuart, G.; Keir, D.; Ayele, A.; Waltham, D.
Comparisons between the rift systems of East Africa, Earth and Beta Regio, Venus
NASA Astrophysics Data System (ADS)
The rift systems of southern East Africa and Beta Regio, Venus are similar in a number of ways. The rifted East African and Venusian lithospheres have effective elastic thicknesses of � 30 km, suggesting that both lithospheres maintain significant flexural strength during rifting. Both rift systems have maximum fault segment lengths of � 100 km. The effective elastic thickness and maximum fault segment length of both rifts are greater than those seen in many other active extensional regions on Earth, despite the high surface temperatures on Venus. We suggest that the southern East African and Venusian elastic thicknesses and maximum fault segment lengths are due to stronger lithosphere. The rift systems differ in the maximum width of their half graben. East African half grabens are up to � 50 km wide, whilst those on Venus are up to � 150 km wide. To support the topography associated with such half grabens requires shear stresses to act on the bounding faults. In East Africa the greater elastic thickness (compared to most other terrestrial extensional regions) means that wide half grabens can form without requiring the shear stresses acting on the bounding faults to be greater than the � 1-10 MPa (10-100 bars) stress drop typically seen in earthquakes. However, on Venus the absence of sediment infill, greater widths and larger effective topographic steps of the half grabens require shear stresses of up to � 80 MPa (800 bars) to act on the bounding faults. This difference is significant; Venusian faults must be stronger than those on Earth.
Foster, Adrian; Nimmo, Francis
East African rift and northeast lineaments: continental spreading�transform system?
NASA Astrophysics Data System (ADS)
Seafloor spreading and transform faulting processes are also likely to be operative during continental rifting events. Continental lines of old weakness oriented at high angles to the direction of continental rifting may be reactivated by transform faulting. These older continental transform faults, which predate and accomodate the rifting, will continue to propagate as younger oceanic transform faults as the rift develops into seas and oceans. This model is applied to the East African Rift which is postulated to be a continental spreading rift that is accommodated by east-northeast continental transform lineaments that are reactivated older crustal defects of appropriate orientation. At least five continental transform lineaments can be tentatively identified by empirical best fits to oceanic transform directions of the South Atlantic Ocean and to various continental African northeast-trending structures: (1) Cape Town-Maputo (CT-LM); (2) Orange River-Beira (OR-B); (3) Luderitz-Lindi (L-Li); (4) Walvis Bay-Mombasa-Mogadishu (WB-M-Mo); and (5) Luanda-Afar (Lu-Af). As these postulated lineaments are perennial deep seated crustal defects they may also control the development of mineral deposits.
Katz, M. B.
Rift Basin Architecture Near the Collision Zone in the Northern end of Back-arc Rift of the Izu-Bonin Arc, Based on Airgun Profiles
NASA Astrophysics Data System (ADS)
Two concatenated basins, Hachijo and Mikura basins in northward order, are distributed in the northern part of the Izu-Bonin Arc, in collision with the NE Japan Arc. The Izu-Bonin Arc has long but punctuated narrow depression in back-arc area; the `back-arc rift'. The Hachijo basin is accepted as `Hachijo Rift'(e.g. Tamaki et al., 1981), whereas the Mikura basin remains uncertain, because the Hachijo basin is similar to the Sumisu Rift, well-investigated typical rift in the Izu-Bonin Arc, while the Mikura basin shows unclear boundary fault. The Hachijo basin is accordant to bathymetric trends in the Izu-Bonin Arc (Murakami, 1996): Rift basins tend to shallow northward. However, the Mikura basin is deeper (1600m) than the Hachijo basin (1100m). The Hachijo basin is 40�60 km2 NS-elongated basin, bounded the eastern and the northern sides by cliff to 500m deep bank and cliff to the Mikura basin respectively. The Mikura basin is 50km wide square basin, bounded the northern and the eastern sides by steep slope to 300m deep bank and slope to the 500m deep Kitakurose Bank respectively. Profiles show the Hachijo basin is thickly-deposited(?1.2sec in two-way travel time) half-graben, bounded the eastern cliff by NS-strike normal fault. The Mikura basin is also filled with thick deposits(?1.5sec) but bounded the northern slope by EW-strike normal faults partially covered with slump deposits. The reflectors in eastern slope of the basin continue to east of the basin, Kitakurose Bank. In this bank, the NS-strike W-dip normal fault is developed with downthrow offset more than 1.2sec. The deposits on the hanging wall are estimated at Pliocene (Yuasa, 1984). The Mikura basin might have been originally NS-elongated (half-)graben, bounded by this fault, and changed its spreading axis. Concludingly the Mikura basin shows tectonic margin, influenced with the collision to the NE Japan Arc as well as repeated rifting, whereas the Hachijo basin represents as the northern end of typical back-arc rift. The northern slope and EW normal fault of Mikura basin is situated at the boundary between northern shallow area rich in volcanogenic islands, and southern deep area rich in rifts, and might indicate the structural boundary.
Tsujino, T.; Ishizuka, O.
The Historic and Future Roles of Magmatism and Underplate Beneath the Northern Main Ethiopian Rift
NASA Astrophysics Data System (ADS)
The magmatic Northern Main Ethiopian Rift (NMER) is believed to represent the transitional stage between continental and oceanic rifting, with recent studies from the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE) project concluding that magmatic processes dominate the rifting. 133 broadband seismometers were deployed in three separate arrays across the NMER at a spacing of between 5 and 30 km and receiver functions have been analyzed using H-? stacking, forward modelling and inversion techniques. H-? stacking results reveal a consistent V_p/V_s ratio of 1.75--1.80 and Moho depth of 38--40 km along 125 km of the south-eastern rift flank. Beneath the rift valley, the V_p/V_s ratios rise to over 1.90 with the Moho depth thinning to 30--35 km. The average crustal V_p/V_s ratio varies between 1.80 and 2.00 to the north-west of the rift. A lower crustal or upper mantle layer is confirmed beneath the north-western rift flank from the observation of a double P- to S-wave conversion, as identified by seismic refraction and interpreted as underplate. This layer thins from ~15 km to less than 10 km from NW to SE into the rift and is likely to have a V_p/V_s ratio of between 1.75 and 1.95. Beneath this layer, evidence for an upper mantle discontinuity exists, that shallows into the rift. These results, together with further receiver function analyses, indicate that significant magma has been added to the base of the crust and throughout the crust on the north-western rift flank and throughout the crust beneath the rift valley. It is unlikely that magma has been added beneath the south-eastern rift flank. The maximum crustal thinning and surface expression of the present-day rifting occur at the location where the underplate layer is at its thinnest or where there is no evidence for its existence. This may indicate that the underplate emplacement phase occurred and strengthened the crust beneath the Ethiopian Plateau and rifting followed by magmatic processes exploited the relatively thin and weak crust (aided by a pre-existing lithospheric suture or weak zone). The receiver function results are combined with and compared to shear-wave splitting, gravity, seismic tomography and controlled-source seismic results to build a 3D model of the location, volume, nature and emplacement history of magmatic addition to the base of the crust beneath the Ethiopian Plateau. We examine how this underplate layer affects the current magmatically-driven rifting in the NMER in terms of crustal thinning, recent magmatic addition throughout the whole crust and upper mantle partial melting.
Cornwell, D. G.; England, R. W.; Maguire, P. K.; Stuart, G. W.
Active Faulting and Regional Flow in the Corinth Rift
NASA Astrophysics Data System (ADS)
The Corinth Rift, in western Greece, opens at the yearly rate of 1.5 cm/year with an uplift of its southern shore reaching locally 1 mm/year and elevations reaching 2500 m. The fault system on this southern shore involves a set of parallel fault segments with lengths reaching up to 40 to 50 km and offsets of the order of 1000 m. In the context of the European Corinth Rift Laboratory development, a 1000 m deep borehole has been drilled on the seashore, through the smaller Aegion fault, re-activated during a magnitude 6.2 earthquake, in 1995. The upper 490 m of the well intersects a series of quaternary conglomerates interbedded locally with horizons of calcilutite (calcite mud) which become predominant in the lower 250 m of this section. From 490 m to 690 m a strongly tectonized radiolarite series is encountered till the well penetrates a cretaceous series of fractured limestone, locally karstified. The 15 m thick fault zone, encountered between 750 m and 790 m, within the limestone, has been cored through its whole length and shows crystalline calcite grown in multidecimetric open cavities, within the fault breccia. A one-meter thick siliceous fault gouge made up from the radiolarite is intersected at 760 m. Laboratory permeability measurements yield 9 to 15 10-18 m2 for the calcilutite and 0.9 to 2.0 10-18 m2, for the fault gouge. Production tests conducted during the drilling operation yield a hydraulic conductivity of 10-5 to 10-4 m/s for the upper conglomerates (200m depth) with no artesian flow. The limestone encountered just above the fault, below the radiolarite, is artesian with a 0.5 MPa overpressure and a flow rate equal to 0.7 m3/h. Its hydraulic conductivity has been measured to be about 10-7 m/s. Below the fault, an artesian flow is produced through local karsts observed down to 960 m, with a total flow rate of about 50 m3/h and a 1MPa overpressure, in no flow conditions. The down-hole temperature is only 32� C, for an upper-hole temperature of 17� C. The fault is intersected some 250 m below the local sea-floor in the rift. Hence this active normal faults system acts as a strong hydraulic barrier, east-west oriented, while the mountainous terrain of the southern shore produces a natural hydrostatic load that induces downward flow down to depths, which remains to be ascertained. Geological observations, in the nearby Cyclades islands, some 300 km to the east, suggest meteoritic flow may reach the brittle-ductile transition.
Cornet, F. H.
Three-Dimensional Modelling of Rift Basin Evolution
NASA Astrophysics Data System (ADS)
Numerical simulation of faulting is gaining interest, typically utilising the Discrete Element Modelling (DEM) technique. As computational power increases, so does the ability to run larger and more complex experiments. This research aims to explore rift basin evolution, with emphasis placed on the interaction of multiple faults as they evolve. Importance is placed on the technical aspects of implementing a complex and large DEM, specifically parallelism and whether currently popular software libraries known as physics engines can be a useful tool in their development. The technique employed is a variation of Mora and Place�s Lattice-Solid Model (1994). We use a 3D block of spheres, initially packed into a defined volume, interconnected using spring-damper structures. Elemental position is controlled throughout the simulation using Newtonian physics, while underlying plate movement is introduced by applying a specific force to each element. The spring-damper components provide a compressive force in the system, restricting each element to its neighbours, until the point the spring-damper has reached a pre-defined maximum extension, at which point it breaks resulting in a fault. Physical calculations are performed using using a publically available engine. The technical implementation of a physics engine is markedly more advanced than that found in many DEM codes used to date, specifically that of the collision detection system, which is implemented as a broad and then narrow phase process, which is solved as a Linear Complementarity Problem. In order to make use of large, distributed computing resources, a method by which to split the DEM into smaller sub-DEMs and then calculate each on an individual CPU has been defined. Typically results are accurate to within four decimal places compared to idealised values; this is comparable to other integration techniques such as Verlet or RK4. Through splitting the model and using multiple instances of a physics engine, it has been possible to limit the scale of simulation to available computing hardware, rather than software libraries. Extension of the crust results in an initial phase of a large number of small faults over which extensional strain is accommodated. As extension progresses, these faults are noted to either link to become larger structures or become inactive and passively rotated in the hanging-wall and footwalls of neighbouring structures. This does not mean, however, that they cannot be reactivated later during rift development. As deformation within the system reaches the rift climax phase, extension is focused on a small number of large faults within the basin. The timing, location, linkage and strain accommodated by these structures is determined during extension and examined as it progresses in three-dimensions. It can also be concluded that using a physics engine in the design of a geological DEM can be beneficial for development time, speed of execution and ensuring physical correctness. Model splitting provides a neat and scalable method by which to distribute computation of a large DEM over multiple CPUs and memory resources.
Longshaw, S. M.; Finch, E.; Turner, M.; Gawthorpe, R. L.
The Corinth Rift Laboratory (CRL) strainmeters: calibration and data analysis
NASA Astrophysics Data System (ADS)
The Gulf of Corinth (Greece) is one of the most seismic regions in Europe, producing some strong earthquakes in the decades, 1 to 1.5 cm/yr of north-south extension, and frequent seismic swarms. This structure is a 110 km long, N110E oriented graben bounded by systems of very recent normal faults. The Corinth Rift Laboratory (CRL) project is concentrated in the western part of the rift, around the city of Aigion, where instrumental seismicity and strain rate is highest. The CRL Network is made up about fifteen seismic stations as well as tiltmeters, strainmeters or GPS in order to study the local seismicity, and to observe and model the short and long term mechanics of the normal fault system. The instrumental seismicity in the Aigion zone clearly shows a strong concentration of small earthquakes between 5 and 10 km. In order to study slow transient deformation,two borehole strainmeters have been installed in the area (Trizonia, Monasteraki). We focus here on the one installed in the Trizonia island, which is continuously recording the horizontal strain at 150 m depth with a short term resolution better than 10-9. The dominant signal is the earth and sea tidal effects (few 10-7 strain), this one is modulated by the mechanical effects of the free oscillations of the Gulf with periods between 8 and 40 min. The barometric pressure fluctuations acts in combination with the mean sea level variation at longer periods and both effects are not independant. The comparison between the strain data and the two forcing signals exhibits a non zero phase delay of the sea-level which is increasing with period. We estimate a transfer function after few correlation iterations for each forcing signal but the physical interpretation of the sea-level function is still unclear. As the strainmeter is at 150 m depth, below the shoreline, a sea water percolation on land would increase the effect of sea level fluctuation, and be more efficient at longer periods. The dilatometer response to crustal sources, made possible recently through the record of the coseismic strain steps of the 2010, M=5.3 local events, shows a fast relaxation (hours). This suggests poor coupling to the rock, due to cementation problems. The sensor would then act dominantly as a pore pressure sensor. Taking into account the related transfer function, together with the one of the external signals, allow us to study accurately the residual signal in order to find slow transient signatures, especially during the reported seismic swarms. We also present here the first analysis of the Monasteraki strainmeter records after correction of the fast long term drift (10-7/day).
Canitano, A.; Bernard, P.; Linde, A. T.; Sacks, S. I.; Boudin, F.
The Role of Rift Obliquity in Formation of the Gulf of California
NASA Astrophysics Data System (ADS)
The Gulf of California illustrates how highly oblique rift geometries, where transform faults are kinematically linked to large-offset normal faults in adjacent pull-apart basins, enhance the ability of continental lithosphere to rupture and, ultimately, hasten the formation of new oceanic basins. The Gulf of California rift has accommodated oblique divergence of the Pacific and North America tectonic plates in northwestern Mexico since Miocene time. Due to its infancy, the rifted margins of the Gulf of California preserve a rare onshore record of early continental break-up processes from which to investigate the role of rift obliquity in strain localization. Using new high-precision paleomagnetic vectors from tectonically stable sites in north-central Baja California, I compile a paleomagnetic transect of Miocene ignimbrites across northern Baja California and Sonora that reveals the timing and distribution of dextral shear associated with inception of this oblique rift. I integrate detailed geologic mapping, basin analysis, and geochronology of pre-rift and syn-rift volcanic units to determine the timing of fault activity on Isla Tiburon, a proximal onshore exposure of the rifted North America margin, adjacent to the axis of the Gulf of California. The onset of strike-slip faulting on Isla Tiburon, ca. 8 - 7 Ma, was synchronous with the onset of transform faulting along a significant length of the nascent plate boundary within the rift. This tectonic transition coincides with a clockwise azimuthal shift in Pacific-North America relative motion that increased rift obliquity. I constrain the earliest marine conditions on southwest Isla Tiburon to ca. 6.4 - 6.0 Ma, coincident with a regional latest Miocene marine incursion in the northern proto-Gulf of California. This event likely flooded a narrow, incipient topographic depression along a �650 km-long portion of the latest Miocene plate boundary and corresponds in time and space with formation of a newly-constrained �50-100 kilometer-wide transtensional belt of focused strike-slip faulting, basin formation, and rotating crustal blocks. This proto-Gulf of California shear zone, embedded within the wider Mexican Basin and Range extensional province and connected to the San Andreas fault in southern California, hosted subsequent localization of the plate boundary and rupture of the continental lithosphere.
Bennett, Scott Edmund Kelsey
The development of the Midcontinent Rift in the context of rapid paleogeographic change
NASA Astrophysics Data System (ADS)
Despite being active for >20 million years and resulting in the thinning of pre-rift crust by a factor of 3 or more, the 1.1 Ga Midcontinent Rift failed to dismember the Laurentian craton. This failure resulted in the preservation of a thick record of rift-related volcanic and sedimentary rocks that give geoscientists a powerful window into the development of this ancient rift. Most models for the development of the Midcontinent Rift attribute its origin to the upwelling and decompression melting of a mantle plume. On the basis of the great volume of generated magma and interpretation of geochemical data, it is argued that the early stage plateau flood basalts of the rift (~1110-1105 Ma) and the main stage volcanics that erupted into the central basin (~1100-1095 Ma) were both dominated by plume-sourced melts. However, this model needs to be reconciled with paleomagnetic data from rift volcanics that reveal a significant decrease in inclination between the early and main stage volcanics. New data we have developed from 90+ flows of the early stage Osler Volcanic Group bolster evidence from the succession at Mamainse Point that this change in inclination is the result of fast equatorward plate motion during the early stage and into the main stage of rift volcanism. Even with >20� of latitudinal motion from the time of initial volcanism to eruption of the thick main stage volcanics, magmatism was largely confined to the same geographic region in a relatively narrow central basin. If a long-lived plume was in a fixed position relative to Earth's spin axis, the large relative motion of Laurentia would make it unable to continue to be a source of melt to the rift. Two possible explanations to reconcile a plume-contribution in the main stage with this latitudinal change are: (1) That the active contribution from an underlying plume was limited to the early stage of volcanism, but substantial volume of material accreted to the lithosphere that was subsequently sampled during the main stage. While such a scenario could explain the geochemical signature, it removes an active plume as a heat source contributing to voluminous melt generation. (2) That the North American plate and a deep-seated mantle plume traveled in unison to lower latitudes as a result of large-scale true polar wander. An interpretation of an active contribution from a plume throughout rift development implicitly favors this true polar wander hypothesis.
Swanson-Hysell, N.; Vaughn, A. A.; Mustain, M. R.; Feinberg, J. M.
Melt Distribution in the Ethiopian Rift System: Constraints From Seismic Observations and Finite-Frequency Modelling
NASA Astrophysics Data System (ADS)
As part of the Ethiopian Afar Geoscientific Lithospheric Experiment (EAGLE) 79 seismic stations were deployed, for up to 18 months, in the Main Ethiopian Rift (MER). Many indicators of melt were observed leading to the idea that magma was driving the rifting process in this region. Some of the best evidence for melt came from observations of anisotropy in studies of surface waves and shear-wave splitting. The shear- wave splitting shows fast directions which change abruptly from being rift parallel on the rift flanks to magmatic-segment parallel in the rift valley. This was interpreted in terms of melt-induced anisotropy. The abrupt change in splitting parameters over small lateral distances suggests that the source of anisotropy is shallow. To further constrain the location of the anisotropy and study the ability of shear-wave splitting to identify sharp lateral changes in anisotropy, we model finite-frequency waveforms for a suite of model representations of the rift zone. This allows us to determine the lateral and vertical extent of the melt-induced anisotropy. The results show how a simple model with two regimes of anisotropy can explain the variability across the rift, in both delay time and shear-wave polarization, over short length scales of the order 20- 40 km. Our models have enabled us to constrain the anisotropic characteristics beneath the MER. Our best model has a 9% anisotropy on the western rift margin, with fast directions of 30�, a 100 km wide rift zone with fast direction of 20� inside the rift zone and with 9% anisotropy close to the western margin, 7% elsewhere, and 7% anisotropy on the eastern margin with fast directions of 30�. In all regions of the model we constrain anisotropy to begin at a depth of 90 km. The depth of anisotropy co-incides with the proposed depth of melt initiation beneath the region, based on geochemistry. Also the elevated splitting beneath the western margin supports evidence of low velocities and highly conductive bodies from seismic tomography and magneto-tellurics, suggesting melt is more focused along the western margin. This study shows that SKS-wave splitting is a powerful technique that can map sharp lateral changes, and has the potential to constrain the depth of the anisotropy.
Angus, D.; Hammond, J. O.; Kendall, J.; Wookey, J.
Toarcian�Kimmeridgian depositional cycles of the south-western Morondava Basin along the rifted continental margin of Madagascar
After rifting and final breakup of Gondwana along the former East-African-Antarctic Orogen during the Toarcian�Aalenian, passive\\u000a margins formed around the Proto-Indian Ocean. Sedimentological and stratigraphic studies in the southern Morondava Basin contribute\\u000a to an improved reconstruction of palaeoenvironmental changes during the syn-rift and post-rift margin formation. Depositional\\u000a models based on outcrop and literature data in combination with subsurface data sets
Markus Geiger; G�nter Schweigert
Probable rift origin of Canada Basin, Arctic Ocean
Formation of the Canada basin by post-Triassic rifting seems the most workable and logical hypothesis on the basis of available information. Speculated counterclockwise rotation of the Alaska-Chukchi continental edge best rationalizes the complex geology of northern Alaska, whereas the assumption that a single continental block was present before the Jurassic makes the best palinspastic fit for Arctic America. The Arctic Ocean is the focus of present-day spreading and probably was the focus of earlier stages of spreading in which spread of the Canada basin would have been an initial stage. Spread of the Canada basin is probable if the Atlantic formed by sea-floor spreading, because analogies between the Arctic and Atlantic edges indicate a common origin for the ocean basins. Late Cretaceous and younger deflections of the Cordillera in the Arctic and diabasic emplacements in the northern Arctic Islands may reflect later stages of spreading. Pre-Mesozoic plate tectonism may be represented by the widespread Proterozoic diabasic emplacements in the Canadian Arctic and by the Franklinian-lnnuitian tract, where the volcanogenic rocks and deformation resulted not from a classical eugeosyncline-miogeosyncline couple, but from the junction of a mid-Paleozoic continental edge and another plate on closure of a pre-Arctic Ocean.
Tailleur, Irvin L.
Probable rift origin of the Canada basin, Arctic Ocean
Formation of the Canada basin by post-Triassic rifting seems the most workable and logical hypothesis with information available. Speculated counterclockwise rotation of the Alaska-Chukchi continental edge best rationalizes the complex geology of northern Alaska, whereas a single continental block before the Jurassic makes the best palinspastic fit for Arctic America. The Arctic Ocean is the focus of present-day spreading and probably was the focus of earlier stages of spreading in which spread of the Canada basin would be an initial stage. If the Atlantic formed by seafloor spreading, spread of the Canada basin is probable because analogies between the Arctic and Atlantic edges indicate a common origin for the ocean basins. Late Cretaceous and younger deflections of the Cordillera in the Arctic and diabasic emplacements in the northern Arctic Islands may reflect later stages of spreading. Pre-Mesozoic plate tectonism may be represented by the widespread Proterozoic diabasic emplacements in the Canadian Arctic and by the Franklinian-Innuitian tract where the volcanogenic rocks and deformation resulted not from a classical eugeosyncline-miogeosyncline couple but from the junction of a mid-Paleozoic continental edge and another plate on closure of a pre-Arctic Ocean.
Tailleur, Irvin L.
Security Conference in Europe Exposes Rifts between US, Allies
NSDL National Science Digital Library
A high-level security conference in Munich this weekend exposed rifts between the US and its European allies on the issues of a US missile defense system and the deployment of a "rapid reaction" European Union force. The US missile defense system took center stage over the weekend as Secretary of Defense Donald Rumsfeld announced the Bush administration's plan to go ahead with development and deployment, unilaterally if necessary. Deliberately dropping "national" or "US" from his references to the system, Rumsfeld offered to develop it with European allies and extend its protection to their countries as well. Nonetheless, the European response was guarded and cautiously negative. German Chancellor Gerhard Schroeder warned the US against "overly hasty and early determinations" about deploying missile defenses. The reaction of Russia was more pointed, referring to the possible development of a new arms race that could extend even into outer space (the Bush administration is considering a laser defense system) and the system's violation of the 1972 Anti-Ballistic Missile Treaty. Rumsfeld dismissed European fears as unfounded and suggested that the relationship with Russia needed to be renegotiated on the ground of current threats, rather than outdated cold war assumptions. In the meantime, talks at the conference also revealed some US concern over current European Union plans to develop a "rapid reaction" force that could operate independently of NATO. Turkey's opposition to such a force, combined with the US's concerns, may help to ultimately kill the project.
Charbonneau, David D.

subset: commoncrawl