The effective stress law at a brittle-plastic transition with a halite gouge layer

Abstract

We investigated the effect of pore pressure Pf near the brittle-plastic transition (BPT) for a halite (NaCl) shear zone. Our series of precut friction experiments with a gas-medium apparatus with temperature T ≤ 200°C, confining gas pressure Pc ≤ 150 MPa and Pf ≤ 140 MPa, revealed that a tanh connection between the brittle and plastic regimes works well even at elevated Pf, with a coefficient for Pf in an effective stress law α being unity. Plastic deformation around the real contacts independent of the mean stress results in α = 1 regardless of the ratio of the real contact area Ar/A. The functional dependency of the shear strength on the effective normal stress may deviate from a linear dependency with increasing Ar/A. The present findings support a smooth transition in a hypothetical steady state strength profile around a BPT, providing new insights in geologically obtained paleostress data in exhumed mylonitic shear zones.