Enhanced Dehydration Weakening of Antigorite Driven by Slow Shear Heating: Insights From High‐Pressure Experiments With a Modified Apparatus Stiffness

Abstract

AbstractDeformation experiments were conducted to investigate the influence of apparatus stiffness on frictional stability during dehydration of antigorite. These experiments also provided a novel way to constrain relationships between fault slip rate and reaction weakening. Experiments were conducted at a confining pressure of 1 GPa using a Griggs apparatus with a modified loading column. During experiments, the temperature was ramped from 400°C to 700°C over 600 seconds, while deformation was imposed with varying load‐point displacement rates. No stick‐slip behavior was observed during the experiments, despite an order of magnitude lower apparatus stiffness, suggesting there are inherent differences in the frictional stability between low pressure (where stick‐slip is observed during dehydration) and high pressure experiments. Fault slip at high pressure may be stabilized by higher dehydration temperatures and higher shear stresses prior to the onset of dehydration reactions. In comparison to reference experiments conducted with an unmodified stiffness, the weakening rate during dehydration increases by a factor of two. In addition, the time (and therefore imposed temperature) at which the maximum weakening rate is observed decreases linearly with increasing maximum weakening rate. Microstructures in the deformed samples illustrate much greater reaction extents (locally >50%) relative to the reference experiments (<1%), with decreasing reaction extent away from the primary slip surfaces. These observations indicate that the dehydration reaction is enhanced by additional shear dissipation resulting from a tenfold increase in displacement per increment of weakening. The shear‐enhanced reaction mechanism could potentially generate runaway slip in subduction zone settings, leading to intermediate‐depth earthquakes.