Nanocrystalline slip zones in calcite fault gouge show intense crystallographic preferred orientation: Crystal plasticity at sub-seismic slip rates at 18–150 °C

Abstract

A central aim in fault mechanics is to understand the microphysical mechanisms controlling
\naseismic-seismic transitions in fault gouges, and to identify microstructural indicators for such
\ntransitions. We present new data on the slip stability of calcite fault gouges, and on microstructural
\ndevelopment down to the nanometer scale. Our experiments consisted of direct
\nshear tests performed dry at slip rates of 0.1–10 μm/s, at a constant normal stress of 50 MPa,
\nat 18–150 °C. The results show a transition from stable to (potentially) unstable slip above
\n~80 °C. All samples recovered showed an optical microstructure characterized by narrow,
\n30–40-μm-wide, Riedel and boundary shear bands marked by extreme grain comminution,
\nand a crystallographic preferred orientation (CPO). Boundary shear bands, sectioned using
\nFIB-SEM (focused ion beam scanning electron microscopy), revealed angular grain fragments
\ndecreasing from 10 to 20 μm at the outer margins to ~0.3 μm in the shear band core, where
\ndense aggregates of nanograins also occurred. Transmission electron microscopy, applied to
\nfoils extracted from boundary shears using FIB-SEM, combined with the optical CPO, showed
\nthat these aggregates consist of calcite nanocrystals, 5–20 nm in size, with the (104)[201] dislocation
\nglide system oriented parallel to the shear plane and direction. Our results suggest
\nthat the mechanisms controlling slip include cataclasis and localized crystal plasticity. Because
\ncrystal plasticity is strongly thermally activated, we infer that the transition to velocity-weakening
\nslip is likely due to enhanced crystal plasticity at >80 °C. This implies that tectonically
\nactive limestone terrains will tend to be particularly prone to shallow-focus seismicity.