"Coseismic foliations" in gouge and cataclasite: experimental observations and consequences for interpreting the fault rock record

Foliated gouges and cataclasites are commonly interpreted as the product of distributed (aseismic) fault creep. However, foliated fault rocks are often associated with localized slip surfaces, the latter indicating potentially unstable (seismic) behavior. One possibility is that such fault zones preserve the effects of both seismic slip and slower aseismic creep. An alternative possibility explored here is that some foliated fault rocks and localized slip surfaces develop contemporaneously during seismic slip. We studied the microstructural evolution of calcite- dolomite gouges deformed experimentally at slip velocities <1.13 m/s and for total displacements of 0.03 - 1 m, in the range expected for the average coseismic slip during earthquakes of Mw 3-7. As strain progressively localized in the gouge layers at the onset of high-velocity shearing, an initial mixed assemblage of calcite and dolomite grains evolved quickly to an organized, foliated fabric. The foliation was defined mainly by compositional layering and grain size variations that formed by cataclasis and shearing of individual foliation domains. Quantitative image analysis (e.g. grain size, strain) showed that the most significant microstructural changes in the bulk gouge occurred before and during dynamic weakening (