Mechanical Amorphization of Synthetic Fault Gouges During Rotary‐Shear Friction Experiments at Subseismic to Seismic Slip Velocities

Abstract Although the effects of mechanical amorphization by fault motion on fault rocks have been investigated both in nature and experiments, the relationship between slip processes and the amount of amorphous materials produced remains unclear. We performed rotary‐shear friction experiments on synthetic quartz and kaolinite gouges at room temperature, normal stresses of 1 or 3 MPa, slip velocities of 0.001 or 1 m s −1 , and displacements of 1–101 m. X‐ray diffraction and microscopic observation data revealed that mechanical amorphization in both materials was accompanied by grain‐size reduction, particle rounding, and the formation of ultrafine particles. The amount of amorphous materials produced was strongly dependent on mineralogy and total frictional work regardless of slip velocity. Therefore, mechanical amorphization can occur during both coseismic and subseismic slip. Amorphization of ~7 wt% of quartz gouge required 30.33 MJ kg −1 of frictional work, whereas for kaolinite gouge, only 0.77 MJ kg −1 was required, which is consistent with observed preferential amorphization of clay minerals in faults. For kaolinite, up to 6% of frictional work can be used for mechanical amorphization, indicating its potential importance for faulting energetics. Because some kinds of amorphous phyllosilicates release water at lower temperature than crystalline, mechanical amorphization of fault rocks may influence thermochemical pressurization. Gradual weakening of quartz gouge at 0.001 m s −1 slip velocity suggested that <4 wt% of amorphous silica can weaken a fault by amorphous silica‐related lubrication. Mechanical amorphization may accelerate pressure solution healing of faults if dissolution can occur under high porous conditions.