On the transient response of serpentine (antigorite) gouge to stepwise changes in slip velocity under high‐temperature conditions

Shear‐sliding tests were conducted on serpentine (antigorite) gouge to understand the rheology of serpentine‐bearing faults. The experiments were carried out using a constant confining pressure (100 MPa), a constant pore water pressure (30 MPa), and a range of temperatures (from room temperature to 600°C). The transient response in frictional behavior following stepwise changes in the slip velocity were documented at each temperature. Slip rates varied between 0.0115 and 11.5 μ m/s. Both the general level of frictional strength and the transient responses changed drastically at around 450°C. As the temperature increased from 400°C to 450°C, the strength of antigorite rose sharply. The transient response also indicated a change in the mode of deformation from flow‐type behavior at temperatures below 400°C to frictional behavior (stick‐slip) at temperatures above 450°C–500°C. Although only a limited volume of serpentine was involved in the dehydration reaction, X‐ray diffraction analyses and scanning electron microscopy observations showed that forsterite had nucleated in the experimental products at the higher temperatures that were associated with frictional behavior. Submicron‐sized, streaky forsterite masses in shear‐localized zones may be evidence of shear‐induced dehydration that caused strengthening and embrittlement of the gouge. Although antigorite rheology is complicated, the subsequent change in friction coefficient per order‐of‐magnitude change in sliding velocity increased with both increasing temperature and decreasing velocity, implying that a possible flow mechanism of intragranular deformation became activated.