New constraints on megathrust slip stability under subduction zone P-T conditions

To understand and model subduction zone seismogenesis, data are needed on the frictional properties of (meta)pelitic subduction zone materials under in situ megathrust conditions. Here, we report the results of rotary shear friction experiments on simulated illite-quartz fault gouge at an effective normal stress of 170 MPa, a pore fluid pressure of 100 MPa, at 150-500 degrees C and sliding velocities of 1-100 mu m/s. The results show three temperature-dependent regimes, characterized by velocity-strengthening at 150-250 degrees C, velocity-weakening at 250-400 degrees C and velocity-strengthening at 400-500 degrees C. The regimes are defined by a decrease in the rate and state friction (RSF) parameter (a-b) at 150-300 degrees C followed by an increase at 300-500 degrees C. These trends correlate with systematic changes in other RSF parameters, as well as an increase in friction coefficient above 300 degrees C. We explain the effects of increasing temperature in terms of a transition from frictional granular flow (150-250 degrees C), through granular flow whereby dilatation is balanced by thermally activated compaction involving stress corrosion cracking (SCC) of quartz clasts (250-400 degrees C), to non-dilatant slip on the phyllosilicates with accommodation at quartz clasts by SCC (400-500 degrees C). Taking into account the effects of sliding velocity, the observed velocity-weakening regime broadly explains the extent of the seismogenic zone within subduction megathrusts. (C) 2012 Elsevier B.V. All rights reserved.