“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 calcitedolomite
gouges deformed experimentally at slip velocities <1.13 m s−1 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.
The most significant microstructural changes in the bulk gouge occurred before and during dynamic weakening
(<0.08 m displacement). Strain was localized to a bounding slip surface by the end of dynamic weakening, and
thus microstructural evolution in the bulk gouge ceased. Our experiments suggest that certain types of foliated
gouge and cataclasite can form by distributed brittle “flow” as strain localizes to a bounding slip surface during
coseismic shearing.