Fluid channeling along thrust zones: the Lagonegro case history, southern Apennines, Italy

A regional study on late burial syntectonic dolomites and syntectonic calcite and quartz veins occurring in the hemipelagic Triassic-Jurassic succession of the Lagonegro Basin (southern Apennines) was carried out by integrating fieldwork with structural, fluid inclusion, and geochemical investigations on both surface and subsurface samples. The main goal was to characterize the parent fluids – partly dolomitizing – channelled along a major, out of sequence thrust zone. Calcite veins from the deeper tectonic mélange zone that occurs at the base of the allochthonous wedge – as evidenced by numerous oil wells – were investigated as well. The results point out that the fluids at peak burial conditions, achieved during the first deformation stage, were characterized by temperatures of 120-140 °C and salinities averaging 2 wt % NaCl eq, whereas those channelled along the late (out of sequence) thrust zone and responsible for partial dolomitization of the succession had slightly lower temperatures (Th=95±10 °C) and salinities in the range of slightly modified to normal marine seawater (mean at 3.7 wt % NaCl eq). Similar low salinities are encountered in late dolomites from other Fold and Thrust Belts of the Italian peninsula. The most probable source for the fluids channelled along the investigated out of sequence thrust zone is represented by Miocene marine pore-waters expelled from the tectonic mélange zone. A role on fluid salinities appears to be played by the different lithologies of the Fold and Thrust Belts décollement surfaces