Italian Journal of Geosciences (2005) 124 (2): 475-491.
The geological and lithological features of the Tuscany-Emilia-Romagna Apennines are described in several papers (Bartolini et alii, 1983; Boccaletti & Coli, 1983; Castellarin et alii, 1986; Vai, 1987; Pialli et alii, 1995; Ricci Lucchi, 1986; Barchi et alii, 1998; Costa et alii, 1998; Carminati et alii, 1999; Argnani, 2002; Cassinis, 2002). In this portion of the Apennine chain different belts, structurally homogeneous, may be recognized (Bartolini et alii, 1983); in particular, Boccaletti & Coli (1983) identified three different portions: 1) The Internal Chain is located to south of the Apennine ridge. It includes the peri-Tyrrhenian area and is dominated by Miocene Pliocene-Quaternary sedimentary basins. These are located along the chain and separated one from one another by geological highs consisting of metamorphic rocks of the geological basement as well as formations of the Tuscan Unit. 2) The Main Chain occupies the central belt of the Apennine chain and is characterized by compressive tectonics, which mostly involved the Umbrian-Tuscan units with folding and faulting (Cremonini & Elmi, 1971); these processes locally produced significant crustal. thickening. 3) The north-sited External Chain is also characterized by compressive tectonics and is separated from the Main chain by a flexure and several reverse faults (Boccaletti et alii, 1985); the dominant rocks are marine Miocene-Pliocene formations and Quaternary alluvial sediments of the Po plain. After collision between the African and European plates, intense volcanic activity developed in Central Italy; the geodynamic processes which produced the magmatism were responsible for high heat flow, which is still active in the peri-Tyrrhenian area and is responsible for important geothermal fields (e.g.: Larderello, Latera etc.). The present study is concerned with fluid emissions (water and gas) located in the External and Main chains as well as in the portion of the Internal Chain just at the boundary with the Main Chain. The south-western peri-Tyrrhenian area is not taken into consideration because it has already been investigated in many papers (e.g.: Trevisan, 1951; Calamai et alii, 1970; Baldi et alii, 1973; Zuppi et alii, 1974; Fancelli et alii, 1976; Bencini et alii, 1977; Panichi et alii, 1979; D'Amore et alii, 1980; Barazzuoli & Micheluccini, 1982; Chiodini & Giaquinto, 1982; Panichi, 1982; Duchi & Prati, 1985; Duchi & Paolieri, 1989; Gianelli & Scandiffio, 1989; Duchi et alii, 1992a; Duchi et alii, 1992b; Chiodini et alii, 1995). The chemistry of fluid emissions along the Tuscany-Emilia-Romagna Apennines is dependent on the on the lithology of the formations and the structural geology of the areas. Water: (a) the most common surface and shallow aquifers are Ca- and Na-bicarbonate and exhibit low salinity. (b) In the Internal Chain, where horst and graben are dominant, the water of meteoric origin, infiltrating through the outcropping Mesozoic carbonates, reach the deep-seated anhydrite-bearing Mesozoic formations. The resulting thermal waters are Ca-sulphate-bicarbonate or with Na-Cl character, frequently oversaturated in calcite, and reach the surface along the graben faults by thermo-artesian pressure. On the contrary, waters circulating in formations belonging to the southern-eastern portion of the Internal Chain, are frequently bicarbonate alkaline (Pergine, Arezzo province) or bicarbonate alkaline-earth (Chitignano, Arezzo province) with high salinity. The strong bicarbonate alkaline character of some water is mostly due to dissolution of CO <sub>2</sub> , which comes from depth along deep (also transcurrent) faults. In places, addition of H <sub>2</sub> S and subsequent oxidation, may generate sulphate alkaline or sulphate alkaline-earth (+ or -sulphide) waters. (c) The rock formations outcropping along the central belt of the Tuscany-Emilia-Romagna Apennines (Main Chain) do not allow the meteoric water to infiltrate to great depth; thus, in this area, thermal waters are scarce; only in zones where the rocks are strongly fractured, may water infiltrate down to 2000-3000 m where fracturing decreases (depth estimated on the basis of geochemical geothermometers). The sodium, which is abundant in the bicarbonate alkaline water (e.g.: Bagno di Romagna, Forli-Cesena province; S. Pellegrino, Firenze province), comes from silicates; for instance, albite is usually present in many rock-formations (pelites, sandstones and also carbonates) which in places underwent very low- to low-grade metamorphism. The waters become chlorine sodic when they mix with Cl-rich waters (e.g.: Porretta, Bologna province; Bobbio, Piacenza province), which were trapped during sedimentation or during folding and overthrusting, or when they dissolve halite related to evaporites. The transport to the surface of the strongly saline, chlorine sodic waters occurring as springs and in wells and mud-volcanos of the Emilian piedmont area (between the Main Chain and the External Chain) is probably enhanced by compressive conditions at depth. During their ascent to the surface, these waters may mix with phreatic waters in variable amount. Gases: Along the Internal Chain, the gas emissions are dominated by CO <sub>2</sub> , whereas along the central (Main Chain) and external (External Chain) belt the dominant species is CH <sub>4</sub> probably of both thermogenic and biogenic origin. On the basis of geothermometric estimations, these gases come from great depth (at least 5000 m); during their ascent, they may be contaminated by atmospheric N <sub>2</sub> , O <sub>2</sub> and Ar.