A microphysical model for fault gouge friction applied to subduction megathrusts

A microphysical model is developed for the steady state frictional behavior of illite-quartz fault gouge and applied to subduction megathrust P-T conditions. The model assumes a foliated, phyllosilicate-supported microstructure which shears by rate-independent frictional slip on the aligned phyllosilicates plus thermally activated deformation of the intervening quartz clasts. At low slip rates or high temperatures, the deformation of the clasts is easy, accommodating slip on the foliation without dilatation. With increasing velocity or decreasing temperature, the shear of the clasts becomes more difficult, increasing bulk shear strength, until slip is activated on inclined portions of the phyllosilicate foliation, where it anastomoses around the clasts. Slip at these sites leads to dilation involving clast/matrix debonding, balanced, at steady state, by compaction through thermally activated clast deformation. Model predictions, taking pressure solution as the thermally activated mechanism, show three regimes of velocity-dependent frictional behavior at temperatures in the range of 200-500 degrees C, with velocity weakening occurring at 300-400 degrees C, in broad agreement with previous experiments on illite-quartz gouge. Effects of slip rate, normal stress, and quartz fraction predicted by the model also resemble those seen experimentally. Extrapolation of the model to earthquake nucleation slip rates successfully predicts the onset of velocity-weakening behavior at the updip seismogenic limit on subduction megathrusts. The model further implies that the onset of seismogenesis is controlled by the thermally activated initiation of fault rock compaction through pressure solution of quartz, which counteracts dilatation due to slip on the fault rock foliation.