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

The microstructures developed during a late stage of inhomogeneous ductile deformation in the Sesia Zone (Western Alps, lower Aosta valley, Italy) sug gest an exceptionally high flow stress. Quartz is recrystallized with a gra in size down to ca. 5 mu m, jadeite and omphacite are deformed by mechanica l twinning, calcite reveals very high twin densities up to 400 mm(-1), and garnet in mylonites deformed by cataclastic flow. Based on available paleop iezometers these microstructures indicate a differential stress on the orde r of 300 +/- 100 MPa. Plastic flow under these conditions requires a high e ffective confining pressure and is incompatible with a lithostatic pore flu id pressure, as commonly assumed for the ductile regime. The Goetze criteri on predicts fully plastic flow for sigma(1) - sigma(3) < P-eff. The composi tion 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 degrees C. Temperatures in excess of ca. 350 degree s 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 pressu re of only 150 to 300 MPa at 300 degrees to 350 degrees C. This is interpre ted 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 gradien t (lambda approximate to 0.4) being permitted by the data. For a differenti al stress of 300 MPa and a temperature of 300 degrees to 350 degrees C, ava ilable 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 shor t-term post-seismic creep in the uppermost plastosphere after failure of th e schizosphere in a major seismic event. This may have created a transient high permeability or caused sufficient dilatancy giving rise to a sublithos tatic pore fluid pressure in the ductile regime. (C) 1999 Elsevier Science B.V. All rights reserved.