Relationship of evolution of slip-surface structure in Indian sandstone to changes in friction coefficient for a wide range of slip rates

Although many experimental studies have investigated the frictional properties of sandstone, few of them have considered the evolution of slip-surface structures with changes of friction coefficient over a wide range of slip rates. Here we report the results of rock-on-rock rotary shear experiments on specimens of Indian sandstone at slip rates from 2 × 10⁻⁴ to 1 m s⁻¹. The resultant slip behaviors and microscale surface structures indicate two types of dependency on slip rate. At high slip rates (≥5 × 10⁻¹ m s⁻¹), the friction between the sandstone blocks fluctuated markedly and the surfaces of the blocks were intensively worn. At low slip rates (≤1 × 10⁻¹ m s⁻¹), the friction decreased gradually with increasing slip distance and the slip surfaces became reflective. The high slip-rate experiments produced abundant rock fragments (∼10–100 μm diameter), whereas the slip surfaces after low slip-rate experiments were highly polished fault mirrors accompanied by abundant rounded ultrafine grains (∼100 nm diameter). Such slip-rate-dependent evolution of slip-surface structures in Indian sandstone may arise from the relatively low cohesiveness of that rock. This insight may further our understanding of faulting and sliding mechanisms in sandstone near the ground surface.