Deformation processes and origin of fluids during Oligocene-Miocene post-orogenic extension in Alpine Corsica

Alpine Corsica is an accretionary wedge formed during the Alpine orogenesis and exhumed through Oligocene-Miocene lithospheric extension controlled by the eastward migration of Apenninic subduction. Here we integrate field geological surveys with microstructural and carbonate stable isotope (δ18O–δ13C) analyses of fault zone rocks to constrain the evolution of the W-dipping extensional Patrimonio Fault System (PFS). The PFS consists of multiple gouge-bearing fault core strands and splay faults in the footwall damage zone, and exhumed the Schistes Lustrés (e.g., impure quartzites, marbles, calcschists, serpentines) and slices of Hercynian granitoids in the footwall block, accommodating ~6 km of cumulative displacement. We describe a deformation sequence during exhumation consisting of D1 mylonitic shearing, D2 seismogenic faulting and D3 shallow veining events. D1 mylonitic shearing produced a decameter mylonitic zone forming the roots of PFS, coeval with localized brittle-ductile shear zones and quartz ± chlorite vein arrays observed in the footwall metamorphic units. Ductile shearing was accommodated by low-temperature quartz and calcite crystal-plasticity, and pressure-solution mechanisms at greenschist conditions (i.e., 300-400 °C). D2 seismogenic faulting either overprinted or cut the D1 structures. Ancient seismic faulting is attested by occurrence of (i) altered pseudotachylytes and (ii) cockade-bearing fault-veins injecting into the host-rocks and mutually overprinting dolomite-rich veinlet mesh and mirror-like slip surfaces observed in the footwall splay faults. Seismic faulting is also accommodated by dolomite-quartz(-chalcedony) crack-seal veins, which have isotopic compositions similar to those of the carbonate-rich units of the Schistes Lustrés. These structural and geochemical observations indicate that ancient seismicity was cyclically modulated by overpressured fluids which isotopic composition was buffered by the host-rocks. The later D3 shallow (≤ 1 km depth) veining event consists of calcite-bearing veins and concretions filling open fractures, which have distinct isotopic compositions compared to the Schistes Lustrés units, suggesting percolation of meteoric fluids at depths. Based on these observations, we speculate that the D2 faults may represent a fossil analogue of the extensional faults active in the Apennines where seismicity is driven by CO2-rich deep-sourced fluids.