Temperature affects the frictional stability of experimental clay-bearing fault gouges

Field observations have shown that mature fault zones are rich in clay minerals (e.g. MTL in Japan, Punchbowl Fault in USA, and Alpine Fault Zone in New Zealand). Most mature fault zones are also seismogenic, which is at odds with the velocity strengthening behaviour observed for clay-bearing material in rock deformation experiments. The measurements of rate and state friction in clay-bearing material show that most clay-bearing material would favour aseismic creep when the experiments are conducted at room temperature. To address this disparity between experimental and field observations, a set of controlled friction experiments were devised to investigate the effect of varying temperature conditions on the frictional properties of clay-bearing fault gouges.
The velocity-step friction experiments were conducted in a triaxial deformation apparatus at an effective normal stress of 90MPa and ambient temperatures that increased from room temperature (23°C) to 180°C in increments of 40°C. In order to measure the rate and state frictional properties of the fault gouges, the imposed slip velocity was stepped between 0.3-3 μm/s. The simulated quartz-clay fault gouges had controlled clay (kaolinite) contents in increments of 25wt% from 0-100wt%. Preliminary results show that by increasing the ambient temperature during fault slip, the rate and state friction parameter [a–b] consistently decreases significantly in clay-bearing fault gouges, often from a velocity strengthening [a–b] value to a weakening [a–b] value. This is consistent with the previous, limited studies of clay-bearing material at elevated temperatures. In the clay-poor gouges, the velocity weakening [a–b] parameter is accompanied by dynamic stick-slip behaviour, whereas in clay-rich gouges the velocity weakening [a–b] parameter shows initially unstable slip that is dampened and arrests to aseismic slip. The elevated temperatures in fault zones at depths up to ~6km, as investigated in this study, can therefore lead to unstable fault slip in clay-rich material that is velocity strengthening at room temperature. It is proposed that elevated temperatures are an important component of seismogenic slip occurring in clay-rich material, as is observed in natural faults.