A (micro)physical understanding of the transition from frictional sliding to plastic or viscous flow has long been a challenge for earthquake cycle modelling. 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 ˚C, employing slip rates covering almost six 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 micro-porosity 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.