No abstract

Grain size reduction due to cataclasis is a key process controlling fault frictional properties during the seismic cycle. We investigated the role of cleavage planes on fracturing and microstructural evolution during cataclasis in wet and dry carbonate fault gouges (50 wt.% calcite, 50 wt.% dolomite) deformed in a rotary-shear apparatus over a wide range of slip rates (30 µms -1 to 1 ms -1 ) and displacements (0.05 to 0.4 m). During shearing, progressive strain localization forms a narrow slip zone that undergoes significant frictional heating (at high slip rates), but the bulk gouge always accommodates low finite shear strains and deforms at low temperatures. Microstructural analysis of the bulk gouges indicates that deformation occurred by brittle fracturing and twinning.Microfractures in calcite are closely spaced, often exploit cleavage r-rhomb planes, and { 1014 } occur mainly subparallel to the expected principal stress orientation (σ 1 ). Instead, twin planes typically occur sub-perpendicular to σ 1 . Electron backscatter diffraction analysis of the bulk gouges shows that calcite develops a well-defined crystallographic preferred orientation (CPO) at all investigated deformation conditions. The CPO is defined by a clustering of the calcite c-axes around an orientation sub-parallel to σ 1 . The calcite CPO is interpreted to result from grain rotation during granular flow, followed by brittle fracturing that occurred preferentially along calcite cleavage planes. This interpretation is supported by measurements of calcite grain shape-preferred orientations that show a population of elongate calcite grains oriented with their long axes subparallel to σ 1 . Our experimental results indicate that well-defined CPOs can form at low temperature in cataclastic fault rocks, and that mineral cleavage can strongly influence the evolution of grain sizes and shapes during comminution.

Nanograins (≪1 μm) are common in the principal slip zones of natural and experimental faults, but their formation and influence on fault mechanical behavior are poorly understood. We performed transmission Kikuchi diffraction (spatial resolution 20-50 nm) on the principal slip zone of an experimental carbonate gouge (50 wt% calcite, 50 wt% dolomite) that was deformed at a maximum slip rate of 1.2 m/s for 0.4 m displacement. The principal slip zone (PSZ) consists of nanogranular aggregates of calcite, Mg-calcite, dolomite and periclase, dominated by grain sizes in the range of 100-300 nm. Nanograins in the ultrafine (< 800 nm) PSZ matrix have negligible internal lattice distortion, while grains > 800 nm in size contain subgrains. A weak crystallographic preferred orientation is observed as a clustering of calcite c-axes within the PSZ. The high-resolution microstructural observations from transmission Kikuchi diffraction, in combination with published flow laws for calcite, are compatible with high-velocity slip in the PSZ having been accommodated by a combination of grain size sensitive creep in the ultrafine matrix, and grain size insensitive creep in the larger grains, with the former process likely controlling the bulk rheology of the PSZ after dynamic weakening. If the activation energy for creep is lowered by the nanogranular nature of the aggregates, this could facilitate grain size sensitive creep at high (coseismic) strain rates and only moderate bulk temperatures of approximately 600°C, although temperatures up to 1000°C could be locally achieved due to processes such as flash heating.