Coseismic dilatancy controlling thermal pressurization efficiency and fluid redistribution in clay-rich fault gouge

Thermal pressurization driven by frictional heating is a potentially important dynamic weakening mechanism in low-permeability clay-rich fault gouge, producing reduced frictional resistance with limited temperature rise. However, geological evidence of high-temperature coseismic processes recorded in such fault rocks suggests that mechanisms capable of reducing the effectiveness of thermal pressurization have also operated. Here, we report coseismic dilatancy in foliated gouge from the Yangsan fault, a large strike-slip fault in southeastern Korea, and discuss its implications for thermal pressurization efficiency and fluid redistribution. Our observations include gouge injection structures (IV type) penetrating into the surrounding rock, foliation opening structures (F type) within the gouge, and calcite aggregates precipitated within these structures. The IV type is interpreted to record coseismic extension fracturing of the surrounding rock under fault-parallel dynamic tension, whereas the F type is inferred to reflect the opening of foliation driven by thermal pressurization. The formation of these structures may have caused a rapid drop in fluid pressure, potentially diminishing the effectiveness of thermal pressurization and promoting fluid redistribution. Our findings suggest that gouge foliation, while acting as a fluid flow barrier that promotes thermal pressurization, may simultaneously serve as a mechanically weak plane that, once opened during seismic slip, reduces its efficiency by providing a pathway for fluid flow. Notably, the postseismically precipitated calcite aggregates within these coseismic opening structures may represent promising targets for directly constraining the timing of past seismic slip using U-Pb or U-series dating.