Highly reflective, continuous smooth surfaces, known as "mirror-like surfaces" (MSs), have been observed in experimental carbonate-hosted faults, which were sheared at both seismic and aseismic velocities. MSs produced during high-velocity friction experiments (> 0.1 m s(-1)) are typically interpreted to be frictional principal slip surfaces, where weakening mechanisms are activated by shear heating. We re-examined this model by performing friction experiments in a rotary shear apparatus on calcite gouge, at seismic velocities up to v = 1.4 m s(-1) and normal stress sigma(n) = 25 MPa, to analyze the evolution of microstructures as displacement increases. After the onset of dynamic weakening, when the friction coefficients are low (mu << 0.6), sheared gouges consistently develop a well-defined, porosity-free principal slip zone (PSZ) of constant finite thickness (a few tens of micrometers) composed of nanometric material, which displays polygonal grain shapes. MSs occur at both boundaries of the PSZ, where they mark a sharp contrast in grain size with the sintered, much coarser material on either side of the PSZ. Our observations suggest that, with the onset of dynamic weakening, MSs partition the deformation by separating strong, sintered wall rocks from a central weak, actively deforming viscous PSZ. Therefore, the MSs do not correspond to frictional slip surfaces in the classical sense, but constitute sharp rheological boundaries, while, in the PSZ, shear is enhanced by thermal and grain-size-dependent mechanisms.
