Fluid geochemical transect in the Northern Apennines (central-northern Italy): fluid genesis and migration and tectonic implications

Chemical and isotopic characteristics of natural gas and thermal water discharges from the western back-are Tyrrhenian Sea across the Apennine thrust-belt to the Po Valley and Adriatic coast foredeep basins in the Northern Apennines (central-northern Italy) reveal a large-scale fluid motion in the upper crust, both vertically and horizontally. On the basis of gas compositions, two different domains of rising fluids have been distinguished: (1) CO2-rich, He-poor, He-3/He-4-high domain in the western peri-Tyrrhenian extensional sector; (2) CH4-rich, He-rich, He-3/He-4-low domain in the eastern Adriatic compressional sector. Such gases, rising from various depths, are crossed by a huge lateral N-2-rich water flow, in the peri-Tyrrhenian sector, of Ca-SO4(HCO3) meteoric-derived waters that move in a regional wide aquifer hosted in a quite thick Mesozoic carbonate series. Morphologically, the CO2 vents consist of mud basins with high gas-rate emission, where the rising fluids move upwards through diatremes. On the other hand, CH4 emissions seep out from typical mud volcanoes with a reduced gas flow-rate, where the fluid motion is likely related to saline diapir extrusions, and the methane is mostly carried to the surface by the associated mud. The two rising mechanisms described locate southwest and northeast of the Apennine watershed respectively. From a seismic point of view the CH4 domain in the thrust-belt and foredeep areas is characterized by a large number of earthquakes, indirectly pointing to a different rheological behavior of the terrains with respect to the more internal peri-Tyrrhenian area. Owing to the quite high thermal gradient of the latter, the boundary of the brittle-ductile crust in the peri-Tryrrhenian sector can be located at a <10 km shallow depth. Although the presence of several post-orogenic basins would suggest widespread extensional tectonics in all areas west of the Apennine watershed, those located in the easternmost part display gas vents with typical crustal He-3/He-4 ratios. As this ratio is very sensitive to deep fluids rising from the mantle, we hypothesize that such basins at the foot of the Apennines are not due to tensive stress, as suggested by their morphological shape. They are piggy-back (thrust-top) basins developed and evolved in a still acting compressive tectonic regime. (C) 2000 Elsevier Science B.V. All rights reserved.