Frictional Properties of Natural Granite Fault Gouge Under Hydrothermal Conditions: A Case Study of Strike-Slip Fault From Anninghe Fault Zone, Southeastern Tibetan Plateau

The Anninghe Fault (ANHF) is a major left-lateral strike-slip fault in southwestern China and one of the main seismogenic fault zones with a history of strong earthquakes. To understand the frictional properties of natural granitic gouges from the principal slip zone, we conducted hydrothermal friction experiments using both saw-cut and ring shear methods. These experiments were performed at temperatures (T) of 25–600°C, pore pressures (P_f) of zero (dry), 30 and 100 MPa, sliding velocities (V) of 0.01–100 μm/s and effective normal stresses ((Formula presented.)) of 68, 100, and 200 MPa. The (apparent) friction coefficient is low (μ < 0.5) at high T (600°C), high P_f (100 MPa) and low V (<1 μm/s); but high (μ > 0.6) under all other T, P_f and V conditions. Under high P_f, the velocity dependence of friction, (a-b), displays three regimes with increasing temperature, from positive below ∼100°C to negative at 100–300°C (at V = 1–3 μm/s) or else 100–450°C (at V = 30–100 μm/s), becoming positive again above 300–450°C. At low P_f, the negative (a-b) expands to the range ∼300–600°C. Microstructural observations and microphysical interpretation imply that the frictional weakening and transitions in (a-b) are related to competition between dilatant granular flow and deformation of the fine-grained gouge by intergranular pressure solution accompanied by healing phenomena (leading to cavitation-creep-like behavior). Our results provide a possible explanation for the distribution of earthquakes at different depths in the continental crust, in particular for the depth range of the seismogenic zone between 4 and 24 km along the ANHF.