Flow-to-Friction Transition in Simulated Calcite Gouge: Experiments and Microphysical Modeling

A (micro)physical understanding of the transition from frictional sliding to plastic or viscous flow has long been a challenge for earthquake cycle modeling. We have conducted ring-shear deformation experiments on layers of simulated calcite fault gouge under conditions close to the frictional-to-viscous transition previously established in this material. Constant velocity (v) and v-stepping tests were performed, at 550 degrees C, employing slip rates covering almost 6 orders of magnitude (0.001-300 mu m/s). Steady-state sliding transitioned from (strong) v-strengthening, flow-like behavior to v-weakening, frictional behavior, at an apparent critical velocity (v(cr)) of similar to 0.1 mu m/s. Velocity-stepping tests using v v(cr) showed semi-brittle flow behavior, characterized by high stress sensitivity (n-value) and a transient response resembling classical frictional deformation. For v >= v(cr), gouge deformation is localized in a boundary shear band, while for v v(cr), the gouge is well-compacted, displaying a progressively homogeneous structure as the slip rate decreases. Using mechanical data and post-mortem microstructural observations as a basis, we deduced the controlling shear deformation mechanisms and quantitatively reproduced the steady-state shear strength-velocity profile using an existing micromechanical model. The same model also reproduces the observed transient responses to v-steps within both the flow-like and frictional deformation regimes. We suggest that the flow-to-friction transition strongly relies on fault (micro)structure and constitutes a net opening of transient microporosity with increasing shear strain rate at v v(cr), under normal stress-dependent or semi-brittle flow conditions. Our findings shed new insights into the microphysics of earthquake rupture nucleation and dynamic propagation in the brittle-to-ductile transition zone.