High differential stress and sublithostatic pore fluid pressure in the ductile regime — microstructural evidence for short-term post-seismic creep in the Sesia Zone, Western Alps

Abstract

The microstructures developed during a late stage of inhomogeneous ductile deformation in the Sesia Zone (Western Alps, lower Aosta valley, Italy) suggest an exceptionally high flow stress. Quartz is recrystallized with a grain size down to ca. 5 μm, jadeite and omphacite are deformed by mechanical twinning, calcite reveals very high twin densities up to 400 mm −1, and garnet in mylonites deformed by cataclastic flow. Based on available paleopiezometers these microstructures indicate a differential stress on the order of 300±100 MPa. Plastic flow under these conditions requires a high effective confining pressure and is incompatible with a lithostatic pore fluid pressure, as commonly assumed for the ductile regime. The Goetze criterion predicts fully plastic flow for σ 1− σ 3< P eff. The composition of newly formed phengites and the densities of stretched early-formed fluid inclusions suggest a lithostatic pressure of about 400 to 500 MPa at temperatures of ca. 300°C. Temperatures in excess of ca. 350°C are ruled out by the absence of significant post-kinematic grain growth in quartz. The densities of fluid inclusions in quartz formed along healed fractures that are truncated by mobile grain boundaries, indicate a pressure of only 150 to 300 MPa at 300° to 350°C. This is interpreted to indicate a sublithostatic pore fluid pressure at depths of 16 to 20 km at the time of deformation and crack healing, with a hydrostatic gradient ( λ≈0.4) being permitted by the data. For a differential stress of 300 MPa and a temperature of 300° to 350°C, available flow laws for quartz suggest a high strain rate on the order of 10 −12 to 10 −11 s −1. The magnitude of stress and the strain rates imply an episodic deformation and a very high rate of loading, as expected for short-term post-seismic creep in the uppermost plastosphere after failure of the schizosphere in a major seismic event. This may have created a transient high permeability or caused sufficient dilatancy giving rise to a sublithostatic pore fluid pressure in the ductile regime.