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 disc harges from the western back-are Tyrrhenian Sea across the Apennine thrust- belt to the Po Valley and Adriatic coast foredeep basins in the Northern Ap ennines (central-northern Italy) reveal a large-scale fluid motion in the u pper crust, both vertically and horizontally. On the basis of gas compositi ons, two different domains of rising fluids have been distinguished: (1) CO 2-rich, He-poor, He-3/He-4-high domain in the western peri-Tyrrhenian exten sional sector; (2) CH4-rich, He-rich, He-3/He-4-low domain in the eastern A driatic compressional sector. Such gases, rising from various depths, are c rossed 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 aqui fer hosted in a quite thick Mesozoic carbonate series. Morphologically, the CO2 vents consist of mud basins with high gas-rate emi ssion, 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 extru sions, and the methane is mostly carried to the surface by the associated m ud. The two rising mechanisms described locate southwest and northeast of t he 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 pointi ng to a different rheological behavior of the terrains with respect to the more internal peri-Tyrrhenian area. Owing to the quite high thermal gradien t of the latter, the boundary of the brittle-ductile crust in the peri-Tryr rhenian sector can be located at a <10 km shallow depth. Although the presence of several post-orogenic basins would suggest widespr ead extensional tectonics in all areas west of the Apennine watershed, thos e 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 ar e not due to tensive stress, as suggested by their morphological shape. The y are piggy-back (thrust-top) basins developed and evolved in a still actin g compressive tectonic regime. (C) 2000 Elsevier Science B.V. All rights re served.