Elevated temperatures promote frictional instability in kaolinite-quartz fault gouges

• Clay-bearing fault gouges underwent velocity-step tests at elevated temperatures. • Increasing test temperatures increased the frictional strength and gouge compaction. • Increasing test temperatures decreased the rate and state frictional stability. Large earthquakes nucleate on crustal faults that have accumulated significant slip displacement, and field observations show that these faults are ubiquitously rich in clay minerals. Earthquake nucleation and propagation requires a reduction in shear resistance for instability to develop and then to continue. Room temperature laboratory friction measurements indicate that only stable fault creep should typically occur in clay-rich faults; a result at odds with observations of widespread earthquake behaviour on mature faults in nature. A suite of frictional velocity step experiments was designed to test the notion that temperature may reduce the stability of clay-rich faults, measured by the friction parameter ( a-b ), up to 180 °C. The stability of synthetic kaolinite clay-bearing fault gouges was found to decrease systematically with elevated temperatures commensurate with those found at typical earthquake depths, at an effective pressure of 90 MPa. In materials containing 25–50 % clay, the stability of slip decreased with increasing temperature so that the gouges displayed unstable slip at temperatures between 100 and 180 °C. At room temperature the same materials showed stable slip and velocity strengthening characteristics. The reduction in stability with increasing temperature coincides with a greater degree of shear localization observed in the gouge microstructure and is hypothesized to result from progressive loss of water adsorbed on the clay surfaces. These results indicate that clay-bearing fault rocks can nucleate unstable slip at conditions common to the clay-bearing brittle crust – an outcome in line with the observation that mature clay-bearing faults in nature can nucleate and propagate earthquakes.