LOW EFFECTIVE FAULT STRENGTH DUE TO FRICTIONAL-VISCOUS FLOW IN PHYLLONITES, KARAKORAM FAULT ZONE, NW INDIA

Phyllosilicate-rich fault rocks are common in the cores of large-scale fault zones and can dramatically impact fault rheology. Multi-mechanism frictional-viscous flow (FVF) has been inferred to operate in such fault rocks based on experimental evidence and microphysical modelling. A growing body of field evidence suggests also that FVF may be an important mechanism for weakening the cores of large-scale natural faults. We report microstructures indicative of FVF in exhumed phyllonites of the central portion of the 800 km long dextral Karakoram Fault Zone (KFZ), NW India. These include highly 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 parameters from the natural fault rock combined with reported experimentally-derived material properties, predicts low peak shear strengths of < 20 MPa within the frictional-viscous transition zone of the KFZ. Chlorite geothermometry indicates that synkinematic chlorites grew at 351 ± 34 °C (c. 10 km depth) during FVF, just above the transition to quartz crystal plasticity. The velocity strengthening nature of FVF promotes stable creep in these lithologies at low strain rates. However, velocity weakening behaviour and dynamic weakening induced by high slip-rates imposed by earthquake rupture propagation suggests that phyllosilicate-rich fault rocks do not act as barriers to seismic slip. The deformation processes and low frictional strength of the exhumed KFZ fault rocks provide analogues for processes operating at depth in other active faults of similar scale.