Friction of Megathrust Gouges at in-Situ Subduction Zone Conditions: Strength, Rate Dependence, and Microphysical Mechanisms

Abstract:

Slip along subduction megathrusts generally ranges from seismogenic, to slow slip events (SSE) and aseismic creep. Although temperature and clay mineralogy are widely believed to play key roles in governing modes of slip, little is known about the microphysical mechanisms controlling this diverse behaviour, and laboratory friction data for realistic materials at in-situ conditions are few. Recently, we have published a series of large strain, rotary shear experiments on synthetic gouges consisting of phyllosilicates and quartz, sheared at in-situ PT conditions and low sliding velocities V (1-100 µm/s) relevant to earthquake nucleation. Here, we report new results for natural megathrust materials exhumed from different depths on Shikoku, SW Japan. These natural gouges were deformed at sample-specific, peak in-situ PT conditions with fluid overpressure ratios λ between 0.4 and 0.9, and V=0.1-100 µm/s. The simulated and natural materials show broadly consistent behaviour with rate strengthening [(a-b)>0] at T<~200-300°C (Regime 1), rate weakening [(a-b)<0] that would potentially allow nucleation of unstable slip at T up to ~400-500°C (Regime 2) and again rate strengthening at T>~500° C (Regime 3). These observations suggest the potential for earthquake nucleation in Regime 2. In addition, (a-b) tends to become more positive with increasing λ and with increasing V up to ~200-300°C, suggesting possible SSE nucleation in Regime 1, where high λ values have a stabilizing effect. These observations can be explained by a quantitative microphysical model for the steady state frictional behaviour of illite-quartz gouges, which includes rate independent slip on phyllosilicates and time dependent pressure solution of quartz clasts.