U-Pb calcite dating and isotopic fluid signatures of extensional fault gouges affecting the Penninic Frontal Thrust : implications for exhumation of the seismogenic zone.

Coupled fluid characterization and absolute dating of fracture networks may provide insights into the understanding of critical stages of evolution of a growing orogen. As a result of the collision between the European and Apulian plates, the Alps have experienced several evolutionary stages comprising continental subduction, nappe stacking, thick- to thin-skin tectonics in relation with the frontal propagation of a fold and thrust belt, and extensional reactivation of the major Penninic Frontal Thrust (PFT). Current evolution of the orogen (Tricart et al., 2001, 2007 and Sue et al., 2007) shows an ongoing extensional seismic activity along PFT while borders of the orogenic system remain in compression. The transition from compression to extension along the PFT remains unconstrained.

This study aims to constrain the time of the PFT inversion and provide a characterization of the tectonic structures through time during the formation of upper Durance normal fault system. For this, we applied several novel dating techniques (in-situ U-Pb calcite and (U-Th)/He hematite dating techniques). In addition, we determined the geochemical signature of the fluids trapped (calcite crystallization) deformation by δ¹³C and δ¹⁸O stable isotope analysis of calcites to constrain the fluid reservoirs, and thus the size of the involved tectonic structures. Stable isotopes show that the fluids associated with the early extensive structures bear isotopic signatures close to those of their host rocks, indicating a fluid at equilibrium and thus a close system in agreement with the small (mm-cm) size of mostly ductile structures. In a second stage, connection of veins and fractures lead to major fault formation (metric to kilometric scale structures) show isotopic signatures in agreement with ascending metamorphic fluids, featuring an open system along the PFT.

U-Pb dating on calcite was successful on several samples despite high common lead concentrations. Two fault gouge samples associated with kilometric scale faults gave ages between 3.5 Ma and 2.5 Ma. These structures are a signature of the paleoseismic activity wich occured some 2.5-3.5 Ma ago when the wall domain of PFT was few km depth. Moreover, (U-Th)/He hematite dating was used on slickensides of the same fault system. Preliminary ages of 2.5, 1.5 and 15 Ma were obtained. The 15 Ma age is interpreted as a minimum age inversion of the PFT, while other ages overlap with the U-Pb calcite ages. This multidisciplinary inverstigation in the Western Alps helps to constrain the exhumation history of the paleo-seismogenic zone related to the inversion of the PFT.