From seal to pathway: carbonate veins reveal fluid flow in the clay-rich strike-slip Yangsan Fault, South Korea

• Clay-rich Yangsan Fault core shows dynamic permeability over seismic cycles. • Meteoric fluids infiltrated damage zone, heated, and crossed into fault core. • Calcite veins reveal alternating slow interseismic and rapid co-seismic flow. • Overpressured fluids caused transient permeability and hydrofracturing events. Clay-rich fault cores are commonly considered low-permeability barriers to across-fault fluid flow. However, increasing field-based evidence revealed syn-tectonic mineralizations within such fault cores, raising questions about their long-term hydraulic behavior, particularly during seismic events. This study investigates calcite veins and slickenfibres from the clay-rich fault core and damage zone of the 200-km-long strike-slip Yangsan Fault in South Korea, the major intraplate active fault in this region, by integrating structural and microstructural analyses with stable and clumped isotope geochemistry. Our data reveal that meteoric fluids downward infiltrated the fault damage zone over long time periods (possibly since late Cretaceous time), heated with depth according to the geothermal gradient, mixed with local formation water, and laterally infiltrated (i.e. across-fault) through a network of foliation planes into the clay-rich fault core, possibly affecting fault strength. We also propose meteoric water mixing with hydrothermal fluids derived by Late Cretaceous and Paleogene granite emplacement. We infer that mineralizations precipitated during different stages of the seismic cycle. Slickenfibres along fault-parallel foliation and shear planes probably precipitated during post-seismic to interseismic creep. Precipitation of foliation parallel extensional veins and stockwork veins probably occurred during the co-seismic phase due to earthquake fracturing. We speculate that transient pressurized fluid release may have occurred during the co-seismic phase. These findings challenge the prevailing view of clay-rich fault cores as static fluid barriers, instead pointing to a dynamic sealing behavior that evolves with the seismic cycles and over long-term geological timescales. Both slow, long-term interseismic fluid infiltration and rapid, short-term co-seismic fluid flow must be accounted for modeling the permeability of clay-rich faults in risk assessments for CO 2 and H 2 storage sites or for resource (hydrocarbons, geothermal fluids) exploration, especially in seismically active areas.