Frictional behaviour of megathrust fault gouges under in-situ subduction zone conditions

Abstract

Subduction zone megathrusts generate the largest earthquakes and tsunamis known. Understanding and modelling “seismogenesis” on such faults requires an understanding of the frictional processes that control nucleation and propagation of seismic slip. However, experimental data on the frictional behaviour of megathrust fault rocks is limited and almost no experiments have been performed on compositionally realistic materials under relevant in-situ conditions. Data that do exist are usually described empirically, using the Rate and State Friction (RSF) model, with little physical basis to constrain extrapolation to nature. I report the results of double-direct and rotary shear experiments performed at (near) in-situ megathrust P-T conditions. The double-direct shear experiments were performed wet at effective normal stresses (sneff) of 5-30 MPa, sliding velocities (V) of 0.16-18um/s, at room temperature. The rotary shear experiments were performed at sneff = 25-200 MPa, pore fluid pressures (Pf) of 50-200 MPa, V = 1-100 um/s, at 140-600°C. Compositionally realistic megathrust fault gouges were used, consisting of smectite-rich Nankai ODP material, illite-quartz mixtures of varying illite:quartz ratio, 65:35 muscovite-quartz mixtures and pure muscovite. The double-direct shear experiments showed velocity-strengthening for 65:35 illite-quartz gouge and ODP material. These experiments also showed increasing slip hardening rate with increasing sneff. At sneff = 170 MPa and Pf = 100 MPa (ring shear experiments), 65:35 illite-quartz gouge showed a decrease in the RSF parameter (a-b) with temperature, followed by an increase, defining three regimes of velocity-dependence, with velocity-strengthening at ~150-250°C, velocity-weakening at ~250-400°C and velocity-strengthening at ~400-500°C. These regimes shifted towards lower temperature (T) with decreasing slip rate and towards higher temperatures with decreasing sneff. Increasing pore fluid pressure and quartz content caused an increase and decrease in (a-b), respectively. Alongside these trends in (a-b) versus T, an increase in friction coefficient with increasing temperature and quartz content was found in the ring shear experiments. Like the illite-quartz gouge, muscovite-quartz gouge showed three regimes of velocity dependence at sneff = 170 MPa and Pf = 100 MPa, but with velocity-weakening at ~350-500°C. Pure muscovite showed predominantly velocity strengthening behaviour. Based on microstructural observations, a microphysical model was developed to explain the illite-quartz results. This model assumes a phyllosilicate-rich, matrix-supported microstructure with rate-independent frictional slip occuring on aligned phyllosilicates and thermally activated deformation of the intervening quartz clasts occurring by pressure solution. At low V or high T, easy, velocity-strengthening pressure solution of the quartz clasts accommodates slip on the phyllosilicate foliation. With increasing velocity, pressure solution becomes more difficult, increasing the shear stress and activating dilatant slip on phyllosilicates anastomosing around the clasts. Dilation is balanced at steady state by compaction through pressure solution, leading together to velocity-weakening. The three-regime slip stability behaviour observed in both phyllosilicate-quartz gouges bears a striking parallel to the sequence of aseismic-seismic-aseismic behaviour seen on megathrusts. This, combined with the microphysical model developed, implies that seismogenesis is caused by a key brittle-ductile transitional process, namely competition between dilation, due to slip on an anastomosing phyllosilicate foliation, and thermally activated compaction, involving pressure solution of quartz clasts.