Development of crystallographic preferred orientation during cataclasis in low-temperature carbonate fault gouge

Abstract

Grain size reduction due to cataclasis is a key process controlling fault frictional properties during the seismiccycle. We investigated the role of cleavage planes on fracturing and microstructural evolution during cataclasisin wet and dry carbonate fault gouges (50 wt% calcite, 50 wt% dolomite) deformed in a rotary-shear apparatusover a wide range of slip rates (30μms−1to 1 ms−1) and displacements (0.05–0.4 m). During shearing, pro-gressive strain localization forms a narrow slip zone that undergoes significant frictional heating (at high sliprates), but the bulk gouge always accommodates lowfinite shear strains and deforms at low temperatures.Microstructural analysis of the bulk gouges indicates that deformation occurred by brittle fracturing andtwinning. Microfractures in calcite are closely spaced, often exploit {101 ̄4} cleavager-rhomb planes, and occurmainly subparallel to the expected principal stress orientation (σ1). Instead, twin planes typically occur sub-perpendicular toσ1. Electron backscatter diffraction analysis of the bulk gouges shows that calcite develops awell-defined crystallographic preferred orientation (CPO) at all investigated deformation conditions. The CPO isdefined by a clustering of the calcite c-axes around an orientation sub-parallel toσ1. The calcite CPO is inter-preted to result from grain rotation during granularflow, followed by brittle fracturing that occurred pre-ferentially along calcite cleavage planes. This interpretation is supported by measurements of calcite grainshape-preferred orientations that show a population of elongate calcite grains oriented with their long axes sub-parallel toσ1. Our experimental results indicate that well-defined CPOs can form at low temperature in cata-clastic fault rocks, and that mineral cleavage can strongly influence the evolution of grain sizes and shapesduring comminution.