Fault Mirror Formation and Destruction Correlates With Slip Rates on Carbonate Faults

Abstract:

Glossy, highly smooth surfaces, recently termed Fault Mirrors (FMs), are commonly observed on exposed fault-zones. At least for carbonate faults, TEM investigation of FMs indicates that they are composed of a thin layer of ultra-fine, nano-size grains (Siman-Tov et al., 2013). Recent carbonate shearing experiments suggest FMs form during seismic slip and may serve as seismic indicators. To explore the formation and destruction of FMs we performed rotary shear experiments on experimental fault surfaces in three types of limestone. The experiments were conducted at slip-velocities ranging between V=0.001-0.7 m/s, and normal stress up to 1.5 MPa. FMs started to develop when the slip velocity exceeded a few cm/s, and fault coverage by FMs increased systematically with velocity, reaching ~ 50% of fault surface at V=0.6 m/s. No FMs developed at V~<3 cm/s, and importantly, pre-existing FMs were destroyed at these low velocities. The measured friction coefficient, μ, was found to be inversely correlated with the FM coverage: μ~0.8 for no-FM, low velocity, and μ~0.4 for 50% FMs coverage at high velocity. The analysis of the experimental thermal conditions and SEM FM images suggests that the FMs form by sintering of gouge nano-grains in a process similar to industrial 'hot pressing'. We propose that formation and destruction of FMs is indicative of a thermally and mechanically controlled brittle–ductile transition: destruction of the highly smoothed fault mirrors is caused by brittle wear at relatively low temperature, while their formation is controlled by ductile deformation at higher temperature. We further suggest that the observed velocity weakening at high velocity is caused by formation of an extremely localized and ductile nano-slip zone that reduces friction. Based on these results we conclude that the presence of fault mirrors along natural carbonate faults indicates that the fault slipped at seismic velocities and had weakened during the slip event, and also in some cases may indicate that the fault has not crept since the earthquake.