Localized Slip and Associated Fluidized Structures Record Seismic Slip in Clay‐Rich Fault Gouge
French, Melodie E.
Chester, Judith S.
Fault rocks can weaken dramatically with increasing slip rate, which results in localization of slip and earthquakes. Exhumed fault zones and fault rocks deformed at seismic rates in the laboratory both show that deformation can become extremely localized to zones less than or equal to millimeters thick. However, localization can occur during aseismic slip, so evidence of localization cannot necessarily be interpreted as having occurred coseismically. Dynamic weakening that occurs during earthquakes is the result of processes that are unique to seismic slip rates, and previous results from carbonates show that these processes produce unique microstructures. We evaluate whether coseismic deformation at low normal stress produces unique structures within the localized slip zones and adjacent gouge that develop in clay‐rich gouge from the Central Deforming Zone of the San Andreas fault. We measured the thickness and orientations of localized slip zones and their internal lamina, particle orientations, and particle size distributions of gouge sheared from 0.35 to 1.3 m/s velocity, up to 25‐m displacement, and 1‐MPa normal stress, under water‐wet and room‐dry conditions. We find that the thicknesses of localized slip zones and their internal laminae are consistent with numerical formulations for thermal pressurization in wet gouge and both thermal decomposition and cataclastic deformation in dry gouge. Localized zones form coincident with a zone of fluidized gouge that accommodates at most 10% of the shear strain. We conclude that the combined occurrence of foliated localized shear zones with a zone of fluidized gouge may provide a record of seismicity in clay‐rich gouges.