Frictional strength and wear-rate of carbonate faults during high-velocity, steady-state sliding

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We ran an extensive series of shear experiments to test the effect of shear velocity and normal stress on wear-rate and frictional strength. The experiments were conducted on three types of carbonate samples with a rotary shear apparatus on solid, ring-shaped rock samples that slipped for displacements up to tens of meters at slip velocity of V=0.002–0.96 m/s, and normal stress σn=0.25–6.9 MPa.The analysis reveals that during steady-state stage, the values of wear-rate and frictional strength depend on both slip velocity and normal stress. The wear-rates at low slip velocity show linear relations to the normal stress (Archardʼs model), however, at high velocity, V>0.5 m/s, the wear-rates are independent of the normal stress, and may vanish at the highest velocity and normal stress of the present experiments. The steady-state friction coefficient, μ, correlates best with the experimental power-density (= shear stress ⋅ slip velocity). We recognized three friction regimes: high μ>0.8 at low power-density, low μ∼0.3 at high power-density, and a transition regime of fast drop of friction coefficient as the power-density increases from 0.03 to 0.3 MW/m2. Runs of low power-density (high friction) displayed fault surfaces covered with fine-grained gouge, whereas runs of high power-density (low friction) displayed shiny, smooth fault surfaces. We interpret the observed intensity variations of wear-rate and frictional strength as indicating a brittle to ductile transition associated with frictional heating.