Friction Evolution of Granitic Faults: Heating Controlled Transition From Powder Lubrication to Frictional Melt

The experimental shearing of granitic faults revealed complex evolution of strength and microstructure. We use a rotary shear apparatus to shear three types of granitic rocks at moderate slip velocity of 0.01–0.11 m/s, normal stress of 1.0–6.8 MPa, and slip distance up to 60 m. Three stages of strength evolution with slip distance were systematically observed. In Stage I, faults exhibited initial weakening in which the friction coefficient dropped to a steady state level of μ ≈ 0.3. This weakening is associated with powder lubrication and the development of cohesive gouge layers. Stage II included strengthening to μ ≈ 0.5 associated with volumetric expansion, melting of fault patches, and viscous braking at these patches. In Stage III, fault weakening is due to melt lubrication when the melted patches reach a critical fraction of the fault surface area. We show that this complex weakening-strengthening-weakening evolution is controlled by thermally activated deformation processes that can explain the friction behavior of igneous rocks at seismic velocity range.