A microphysical model for strong velocity weakening in phyllosilicate-bearing fault gouges

[1] Previous rotary shear experiments, performed on a halite-muscovite fault gouge analogue system have shown that the presence of phyllosilicates, under conditions favoring the operation of cataclasis and pressure solution in the matrix phase, can have major effects on the frictional behavior of gouges. While 100% halite and 100% muscovite samples exhibit rate-independent frictional/brittle behavior, the strength of mixtures containing 10-30% muscovite is both normal stress and sliding velocity-dependent. At high sliding velocities (> 1 mu m s(-1)), such mixtures show unusually marked velocity weakening, along with the development of a structureless, cataclastic microstructure. In the present paper, a micromechanical model is developed in an attempt to explain this behavior. The model assumes a granular flow process involving competition between intergranular dilatation and compaction by pressure solution. The predictions of the model agree favorably with the experimental results. Extension of the model to quartz-mica systems implies that the presence of phyllosilicates plus the operation of pressure solution can strongly promote ( unstable) velocity-weakening behavior at rapid slip rates on natural faults, under midcrustal conditions. Static stress drop predictions based on the model agree reasonably well with estimates from seismic observations. Our results may help explain the discrepancy between laboratory-derived rate-and-state friction parameter values, obtained for dry, low-strain and/ or single-phase rock systems, and the values for natural fault rocks inferred from seismological data.