Fluid environment controls along-strike variation in slip style: Midcrustal geological signatures from the Red River fault, China

The slip style of continental midcrustal shear zones plays a crucial role in determining the seismogenic potential of faults, but it remains poorly understood because geological observations that can be directly tied to seismic behavior are scarce. We describe frictional-viscous shear zones in the Red River fault, China, which consists of two segments with distinct seismic behaviors and fluid availabilities. The northern segment hosts moderate to large earthquakes, and midcrustal fault slip is localized into mylonitized pseudotachylyte-bearing layers where dynamically recrystallized quartz records flow stresses exceeding 100 MPa and accelerated viscous creep. The southern segment is dominantly aseismic but active microseismically. Fault slip is accommodated in several mylonitized cataclasite layers, comprising interconnected biotite and intervening fractured clasts, with evidence for pervasive dissolution-precipitation creep. Microstructures, paleopiezometry, and microphysical modeling suggest transient aseismic slip in response to increased strain rates during viscous creep at <50 MPa. We interpret that along-strike variations in fluid environment control fault slip styles and seismic behaviors. The dry and strong northern segment is capable of nucleating large earthquakes, while greater fluid availability in the southern segment activates dissolution-precipitation creep at low driving stresses, which limits interseismic elastic strain accumulation at frictional-viscous transition depths. In this model, compaction-driven fluid pressurization and dilatant hardening are invoked to explain the aseismic slip transients in the southern segment.