Dynamic weakening of gouge layers in high-speed shear experiments: Assessment of temperature-dependent friction, thermal pressurization, and flash heating

The rotary shear of gouge demonstrates dramatic weakening at coseismic slip rates, but the inherent variation in shear-rate with radius of the rotary shear configuration prohibits determining friction constitutive properties from measures of whole-sample response. Here, representative results of rotary-shear high-speed experiments including both constant-velocity tests (0.1-1.3 m/s) and constant-acceleration tests (0-1.3 m/s, 0.05, and 0.1 m/s(2)) are analyzed using a thermal, mechanical and hydrologic finite element method (FEM) model to constrain friction constitutive properties and test hypotheses of dynamic weakening by thermal pressurization and flash heating. The observed frictional behavior of room-dry gouge can be explained by using a state-variable friction constitutive relation in which the friction coefficient is inversely proportional to temperature, and by employing a two-mechanism constitutive formulation in which the friction coefficient increases with temperature (temperature-strengthening) at low temperatures and decreases with temperature (temperature-weakening) at higher temperatures. Water-dampened gouge displays a transient weakening during the early stages of constant-acceleration tests. FEM analysis indicates the weakening could reflect thermal pressurization of pore water provided both the permeability of the gouge layer and the sealing capacity of the Teflon sleeve, used to contain the gouge during shear, contribute to restricting fluid flow. Microstructural observations indicate that the dynamic weakening coincides with slip localization and temperature increase by frictional heating, which are conditions that favor weakening by flash heating. At steady state, the relationship between slip-rate and coefficient of friction by the FEM model analysis is consistent with predictions of micromechanical models for weakening by flash heating.