Friction on subduction megathrust faults: Beyond the illite–muscovite transition

• Friction experiments on simulated megathrust fault gouges under in-situ conditions.
• Muscovite–quartz gouges show velocity-weakening at 350–500 °C.
• Extrapolation of result helps to explain the depth range of seismogenesis.
• Pure muscovite gouge shows velocity-strengthening proving a key role of quartz.
• Seismogenesis ultimately caused by thermally activated deformation of quartz.

Previous experimental studies addressing subduction megathrust friction have focused on shallow fault gouges, dominated by smectite, illite and quartz. Here, we aim to determine the effect of the transition to muscovite-rich fault rock at depths beyond the illite-dominated region of the seismogenic zone. To achieve this, rotary friction experiments were performed on simulated muscovite–quartz (65:35) gouge at 100–600 °C, an effective normal stress of 170 MPa, pore pressures of 100 and 200 MPa, and sliding velocities of 1–100 μm/s. We isolated the effects of muscovite by shearing pure muscovite gouge at 200–600 °C, under otherwise identical conditions. At low shear strains (γ≈1), muscovite–quartz gouge showed a friction coefficient μ of 0.4–0.5, while pure muscovite gouge exhibited μ≈0.3. Both gouges showed an increase in μ with γ and temperature. The muscovite–quartz gouge was characterised by velocity-strengthening or neutral behaviour at 100–350 °C, velocity-weakening at 350–500 °C and velocity-strengthening at 500–600 °C. In contrast, pure muscovite gouge showed predominantly velocity-strengthening or neutral behaviour, demonstrating a key role of quartz in producing velocity-weakening. The velocity-weakening regime in the mixed gouge extends that of 250–400 °C, recently reported for illite–quartz gouge, up to 500 °C. Taking into account the observed effect of sliding velocity on (a-b), the data for illite–quartz and muscovite–quartz together help explain the depth extent of seismogenesis on subduction megathrusts. Interestingly, comparison with previous microphysical models suggests that the mechanism causing velocity-weakening in phyllosilicate–quartz gouges is one of granular flow involving competition between shear-induced dilation and compaction through thermally activated quartz deformation.