Fault mirrors along carbonate faults: Formation and destruction during shear experiments

•Fault mirror surfaces are formed and destroyed in experiments, above and below a critical slip velocity of 0.07 m/s.•Fault mirrors are formed ductilely by sintering of hot nanograins.•Thermal runaway at dynamic asperities result in extreme slip localization on fault mirror patches.•Dynamic weakening of carbonate faults occurs by thermally-activated plasticity and reduced wear.•At least for shallow faults, natural fault mirrors indicate slip during seismic events.

Glossy, light reflective surfaces are commonly exposed in carbonate fault-zones. It was suggested that such surfaces, recently termed Fault Mirrors (FMs), form during seismic slip. Ultramicroscopic analyses indicate that FMs are highly smooth and composed of a cohesive thin layer of nano-size grains. We explore here mechanisms of formation and destruction of FMs by shear experiments that were conducted on three types of limestone which were sheared at wide range of slip-velocities of v=0.001-0.63m/s, and normal stress up to 1.57 MPa. The experiments showed that FMs started to develop as local patches when the slip velocity exceeded a critical value of 0.07 m/s. The area coverage by FM patches increases systematically with increasing velocity, reaching in a few cases ∼100% coverage. The measured quasi-steady-state friction coefficient, μss, was inversely correlated with the FM coverage: μss∼0.8 for no-FM, at v<0.07m/s, and μss∼0.4 for 50% FM coverage at v∼0.6m/s. Further, in a series of slip-velocity alternation between low and high values, the FMs which formed at a high-velocity stage were destroyed during a subsequent low-velocity stage. Our analyses of the experimental thermal conditions and ultramicroscopy imaging of the FMs suggest that the FMs form by sintering of gouge nanograins during shear. We propose that formation/destruction of FMs in high/low slip-velocity reflects a competition between brittle and ductile processes: FMs form in a ductile mode, and are destroyed by brittle wear. Shear heating during high velocity leads to ductile deformation and sintering so that FM construction rate exceeds brittle FM destruction rate. Based on our results, we suggest that, at least for shallow faults, the presence of extensive FM coverage along natural carbonate faults indicates that the fault segment slipped at seismic velocities and experienced dynamic weakening.