The Frictional-Viscous Transition in Experimentally Deformed Granitoid Fault Gouge

In crustal faults dominated by granitoid gouges, the frictional-viscous transition marks a significant change in strength constraining the lower depth limit of the seismogenic zone. Dissolution-precipitation creep (DPC) may play an important role in initiating this transition, especially within polymineralic materials. Yet, it remains unclear to what extent DPC contributes to the weakening of granitoid gouge materials at the transition. Here we conducted sliding experiments on wet granitoid gouges to large displacement (15 mm), at an effective normal stress and pore fluid pressure of 100 MPa, at temperatures of 20–650°C, and at sliding velocities of 0.1–100 μm/s, which are relevant for earthquake nucleation. Gouge shear strengths were generally ∼75 MPa even at temperatures up to 650°C and at velocities >1 μm/s. At velocities ≤1 μm/s, strengths decreased at temperatures ≥450°C, reaching a minimum of 37 MPa at the highest temperature and lowest velocity condition. Microstructural observations showed that, as the gouges weakened, the strain localized into thin, dense, and ultrafine-grained (≤1 μm) principal slip zones, where nanopores were located along grain contacts and contained minute biotite-quartz-feldspar precipitates. The stress sensitivity exponent n decreased from a large number at 20°C to ∼2.2 at 650°C at the lowest velocities. These findings suggest that high temperature, slow velocity and small grain sizes promote DPC-accommodated granular flow over cataclastic frictional granular flow, leading to the observed weakening and strain localization. Field observations together with extrapolation suggest that DPC-induced weakening occurs at depths of 7–20 km depending on geothermal gradient.