Fluidized Gouge Injections Record Fluid Overpressure Transients and Seismic Slip in Clay-Rich Fault Zones

Abstract Identifying diagnostic signatures of coseismic slip in the geological record is essential for constraining fault rheology and earthquake dynamics, yet unambiguous geological markers of past seismic ruptures are still scarce. Among seismic slip indicators, solidified frictional melt is widely regarded as the most reliable; however, its restrictive formation conditions and poor preservation limit the geological record of seismic rupture. Here, we investigated fluidized fault gouge injections into host cataclasites preserved in the clay-rich fault zones of the Altyn Tagh fault (northern Tibetan Plateau) and Carboneras fault (southeast Spain), two structurally distinct strike-slip systems showing similar injection geometries and microstructures. The injections display elongate tips with pinch-out terminations and internal undulated foliation surfaces with irregular, lobate geometries, indicating flow within relatively more viscous, quartz-rich host cataclasites. The injected material embeds clasts derived from the host rock, and fractal dimensions of the clast size distribution exceeding 3 are consistent with extreme comminution comparable to that documented in faults accommodating coseismic slip. We interpret gouge fluidization and injection as resulting from transient fluid overpressure, a process that may play a key role in dynamic fault weakening. Therefore, such injections may represent reliable geological indicators of past earthquakes and provide constraints on rupture dynamics in structurally and rheologically complex fault zones.