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

• Published in: Journal of structural geology 2014-09, Vol.66, p.115-128
• Main Authors: Leclère, Henri, Lacroix, Brice, Fabbri, Olivier
• Format: Article
• Language: English
• Subjects: Brittle-plastic; Earth Sciences; Environmental Sciences; Fault; Global Changes; Isotopes; Mohr-Coulomb; Reactivation; Sciences of the Universe; Tectonics; Veins
• ISSN: 0191-8141; 1873-1201
• DOI: 10.1016/j.jsg.2014.05.009

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 °C ± 20 °C, that is near the base of the seismogenic zone. δ18O values of quartz and δD and δ18O 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 λv required to simultaneously reactivate the studied fault and to open the extension veins was close to a lithostatic value (λv ∼ 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. [Display omitted] •Quartz-chlorite veins are analysed combining mechanical and geochemical analyses.•Faulting occurred at the top of the brittle–plastic transition.•Fluid has a metamorphic origin and lithostatic pressure.•Brittle-plastic transition separates two reservoirs with different fluid pressures.