fault system consist of a series of en echelon segments within an overall fan shaped zone. From the focal depth of earthquakes along this zone it is clear that these faults are developed within the underlying basement rocks. Persian Gulf The Persian Gulf and the area of lowland occupied by the alluvial plain of the Tigris–Euphrates river system, known as the 'Mesopotamian Basin', together represent the active foreland basin to the Zagros FTB, caused by the loading of the leading edge of the Arabian Plate by the Zagros thrust sheets. The gulf is being progressively infilled by the southeastward prograding delta of the river system. Development of the Zagros FTB Northeastward subduction of Tethyan oceanic crust continued along this portion of the Eurasian Plate until continental crust of the Arabian Plate became involved in this convergent boundary. The exact timing of the onset of the subsequent collision is uncertain although there is evidence of some deformation during the deposition of the Asmari Formation in the Oligocene, or possibly as early as the Late Eocene. Deformation within the Zagros FTB has continued from then until the present day, although unconformities of regional extent suggest that there have been several distinct phases of deformation. Unconformities have been recognised at the base and top of the Asmari Formation (Late Eocene and Early Miocene), at the base of the Agha Jari Formation (late mid-Miocene) at the base of the Bakhtyari Formation (latest Pliocene) and mid Pleistocene, interpreted to date these separate phases. These deformation pulses were associated with southwestward migration of the active deformation front. There is no evidence of continued subduction of oceanic crust beneath the Eurasian Plate along this part of the plate boundary, in contrast to the neighbouring segment along the Makran Trench, an active subduction zone, where a dipping slab is well imaged by seismic tomography. The ending of subduction has been linked to evidence that the Main Zagros Reverse Fault is no longer active, suggesting that further deformation is occurring by distributed deformation of the leading edge of the Arabian Plate. This is consistent with observations of recent seismicity. Economic importance The Zagros FTB contains 49% of the world's hydrocarbon reserves hosted in fold and thrust belts and about 7% of all reserves. The Zagros province includes many giant and supergiant oilfields, such as the Kirkuk Field with over ten billion barrels of remaining oil reserves as of 1998,