Frictional weakening of gabbro faults after quasi-stationary contact under hydrothermal conditions

• Quasi-stationary contact under hydrothermal conditions weakens the gabbro fault strength during subsequent slip. • Fault weakening is related to the formation of smectite, driven by hydrothermal alteration at grain contacts. • Short-term fluid-rock interactions influences fault frictional behavior. Quasi-stationary fault contacts are known to enhance static friction. However, its role in modulating dynamic friction remains poorly understood. Here, we investigate how quasi-stationary contacts influence subsequent fault slip. We perform slide-hold-slide experiments on simulated gabbro gouge with distilled water at temperatures ranging from 25 to 400 °C and a pore pressure of 30 MPa. At 300 °C, prolonged holds (≥ 3000 s) induce significant post-hold frictional weakening and enhanced fault stability (i.e., decrease of the magnitude of stress drop). The post-hold frictional weakening correlates with hold duration, fluids and ambient temperature, suggesting the involvement of thermally-activated, time-dependent processes on the contact surfaces. Microanalytical investigations reveal the formation of clay minerals via hydrothermal alteration in the experimental fault. Our findings demonstrate that short-term hydrothermal processes occurring during static holds may leave persistent mineralogical imprints, thereby modifying fault strength and slip behavior.