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 () and -stepping tests were performed, at 550°C, employing slip rates covering almost 6 orders of magnitude (0.001-300 μm/s). Steady-state sliding transitioned from (strong) -strengthening, flow-like behavior to -weakening, frictional behavior, at an apparent "critical" velocity ( ) of ~0.1 μm/s. Velocity-stepping tests using  <  showed "semi-brittle" flow behavior, characterized by high stress sensitivity ("-value") and a transient response resembling classical frictional deformation. For  ≥  , gouge deformation is localized in a boundary shear band, while for  <  , 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 -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  <  , 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.