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 °C and sliding velocities of 1–100 μm/s. The results show three temperature-dependent regimes, characterized by velocity-strengthening at 150–250 °C, velocity-weakening at 250–400 °C and velocity-strengthening at 400–500 °C. The regimes are defined by a decrease in the rate and state friction (RSF) parameter (a–b) at 150–300 °C followed by an increase at 300–500 °C. These trends correlate with systematic changes in other RSF parameters, as well as an increase in friction coefficient above 300 °C. We explain the effects of increasing temperature in terms of a transition from frictional granular flow (150–250 °C), through granular flow whereby dilatation is balanced by thermally activated compaction involving stress corrosion cracking (SCC) of quartz clasts (250–400 °C), to non-dilatant slip on the phyllosilicates with accommodation at quartz clasts by SCC (400–500 °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.