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

Abstract

[1] 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 μm/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.