Fault mechanics at the base of the continental seismogenic zone: Insights from geochemical and mechanical analyses of a crustal-scale transpressional fault from the Argentera crystalline massif, French-Italian Alps

Faulting mechanics is examined by combining geochemical and mechanical analyses from a key exposure of a major fault zone crossing the Argentera crystalline massif (French Italian Alps). The analyses are carried out on co-genetic and syn-tectonic quartz and chlorite extension veins and shear veins associated with late stage deformation in the fault zone. Paleothermometry based on chlorite compositions gives formation temperatures of 300 degrees C +/- 20 degrees C, that is near the base of the seismogenic zone. delta O-18 values of quartz and delta D and delta O-18 values of chlorite indicate that veins crystallized from a metamorphic fluid. In parallel, a mechanical analysis based on the Mohr-Coulomb theory shows that the pore fluid factor lambda(v) required to simultaneously reactivate the studied fault and to open the extension veins was close to a lithostatic value (lambda(v) similar to 1). Comparisons with the 2003-2004 Ubaye seismic swarm, having occurred in the continuity of the studied fault zone, suggest that the base of the seismogenic zone may act as a limit separating an upper permeable reservoir saturated with meteoric waters under hydrostatic to supra-hydrostatic pressures from a lower low-permeability reservoir containing metamorphic waters under lithostatic pressure. This study suggests that overpressured fluids can be released upwards in the brittle crust by shear-enhanced permeability and can trigger earthquakes. (C) 2014 Elsevier Ltd. All rights reserved.