Friction and instability of glaucophane gouges at blueschist temperatures support abundance of intermediate-depth earthquakes

Fluid release from the hydraulically-confined dehydration of blueschist minerals may significantly elevate fluid pressures and concomitantly reduce effective stresses. This is a viable mechanism to explain brittle failure and frictional instability, and it is a possible explanation for intermediate-depth earthquakes in cold subduction zones. We examine this hypothesis for glaucophane – a key index mineral for blueschist facies – at lower confining stresses where behavior is poorly understood. We conduct laboratory shear tests on glaucophane gouge at temperatures of 100–500℃ and effective normal stresses of 50–200 MPa, to explore the controls of temperature, stresses and excess pore fluid pressures on fault friction. Frictional strength of glaucophane gouge at representative temperatures and stresses is ~0.70 and insensitive to temperature with a slight increase in friction coefficient at lower effective stresses. Elevating temperature promotes a transition from velocity-strengthening to mild velocity-weakening behavior, indicating the destabilizing effect of high-temperature downdip in subduction zones. Reducing effective normal stress or concomitantly elevating pore fluid pressure, potentially sourced from dehydration reactions, further strengthens the velocity-weakening response and would be manifest as moderate-sized earthquakes. This observed instability at higher temperatures or lower stresses is indexed with and accompanied by denser distributions of strongly- to moderately-localized shears in the microstructures – congruent with the observed mechanical response. Our results support the potential for enhanced unstable sliding of glaucophane gouges at lower effective stresses and at blueschist facies temperatures - and have significant implications for understanding the abundance of intermediate-depth earthquakes apparent in cold subduction zones.