Shear localization, velocity weakening behavior, and development of cataclastic foliation in experimental granite gouge

Microstructural aspects of room-temperature deformation in experimental Westerly granite gouge were studied by a set of velocity stepping rotary-shear experiments at 25 MPa normal stress. The experiments were terminated at: (a) 44 mm, (b) 79 mm, and (c) 387 mm of sliding, all involving variable-amplitude fluctuations in friction. Microstructural attributes of the gouge were studied using scanning (SEM) and scanning transmission electron microscopy (STEM), image processing, and energy dispersive X-ray (EDX) analyses. The gouge was velocity weakening at sliding distances >10 mm as a core of cataclasites along a through-going shear zone developed within a mantle of less deformed gouge in all experiments. Unlike in experiment (a), the cataclasites in experiments (b) and (c) progressively developed a foliation defined by stacks of shear bands. The individual bands showed an asymmetric particle-size grading normal to shearing direction. These microstructures were subsequently disrupted and reworked by high-angle Riedel shears. While the microstructural evolution affected the effective thickness and frictional strength of the gouge, it did not affect its overall velocity dependence behavior. We suggest that the foliation resulted from competing shear localization and frictional slip hardening and that the velocity dependence of natural fault gouge depends upon compositional as well as microstructural evolution of the gouge.