Experimental deformation of clay-rich fault gouges within the rate-and state-dependent friction and flow law frameworks

Clay-rich fault gouges, such as those commonly found in mature fault zones, exhibit complex frictional-plastic behavior. The standard rate- and state-dependent friction law (RSF) can capture the macroscopic frictional behavior of geologic materials in laboratory experiments but offers limited insight into the underlying microphysical processes. Flow laws (FL), i.e. constitutive equations of material behavior with a microphysical basis, have been proposed as a suitable tool to explain the rate and temperature dependence of friction. Here, we use both frameworks, RSF and FL, to analyze the deformation behavior of kaolinite-rich gouges.
We report on the results of velocity stepping and slide-hold-slide friction experiments on dry and water-saturated kaolinite-rich powder (75 % kaolinite, 14 % muscovite/illite, 8 % K-feldspar, 3 % quartz), at a range of temperatures (20 ^oC to 180 ^oC) and load point velocities (0.03 µm/s to 100 µm/s, corresponding to bulk strain rates of ~3*10^-5 s^-1 to 10^-1 s^-1). The experiments were performed using two different experimental devices covering a broad range of normal stress, displacement, and sliding rate conditions: a triaxial direct shear apparatus (effective normal stress values of 60 MPa and 160 MPa) and a rotary shear apparatus (effective normal stress of 60 MPa). In the velocity stepping tests, we observed both velocity weakening and velocity strengthening friction. At 180 ^oC, we found that (a - b) decreased with increasing target velocity. At 20 ^oC, 70 ^oC, and 120 ^oC, there is no clear trend in (a - b) with respect to target velocity or step direction. The results of the slide-hold-slide tests suggest the activation of water-assisted, heat-driven mechanisms at temperatures above 70 ^oC: the healing rate β transitioned from positive values at 20 ^oC and 70 ^oC, to negative values at 120 ^oC and 180 ^oC, leading to net weakening at long hold times. For the saturated samples at 120 ^oC and 180 ^oC, the decrease in β was accompanied by a significant decrease in the stress exponent n to values below 50 with increasing temperature and decreasing strain rate, suggesting a switch in the dominant deformation mechanism as well. Overall, our preliminary findings demonstrate the complementarity of the RSF and FL frameworks in analyzing fault gouge deformation over a wide range of strain rates relevant for earthquake nucleation.