Low effective fault strength due to frictional-viscous flow in phyllonites, Karakoram Fault Zone, NW India

AbstractPhyllosilicate-rich fault rocks are common in large-scale fault zones and can dramatically impact fault rheology. Experimental evidence suggests that multi-mechanism frictional-viscous flow (FVF) may operate in such lithologies, potentially significantly weakening mature fault cores. We report microstructures indicative of FVF in exhumed phyllonites of the Karakoram Fault Zone (KFZ), NW India. These include interconnected muscovite foliae, lack of quartz/feldspar crystal preferred orientations, and sutured grains and overgrowths indicative of fluid-assisted diffusive mass transfer. FVF microphysical modelling, using microstructural observations from the natural fault rock and experimentally-derived friction and diffusion coefficients, predicts low peak shear strengths of