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

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 sigma(n) = 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 (Archard's 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, mu, correlates best with the experimental power-density (= shear stress . slip velocity). We recognized three friction regimes: high mu > 0.8 at low power-density, low mu similar to 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 03 MW/m(2). 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. (C) 2013 Elsevier B.V. All rights reserved.