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The frictional velocity dependence and healing behavior of subduction fault zones play key roles in the nucleation of stick‐slip instabilities at convergent margins. Diagenetic to low‐grade metamorphic processes such as pressure solution are proposed to be responsible for the change in frictional properties of fault materials along plate interfaces; pressure solution also likely contributes to the acceleration of healing according to previous studies. Here, we report friction studies for temperatures of 20–100°C and normal stresses from 20 to 125 MPa on samples collected from ancient subduction fault zones, the Lower Mugi and Makimine mélanges of the Cretaceous Shimanto belt. The two mélanges correspond to the updip and downdip limits of the seismogenic zone and include deformation features that indicate lower and higher degrees of pressure solution. Our data show that the Lower Mugi mélange exhibits velocity‐weakening to velocity‐neutral frictional behavior under low normal stress and that the Makimine mélange sample shows velocity‐strengthening behavior under high normal stress. We suggest that mineralogical changes due to diagenesis and metamorphism influence fault slip behavior. We measure frictional healing in slide‐hold‐slide experiments for the Lower Mugi mélange sample and document the role of pressure solution in fault healing. Our results show that frictional healing increases at higher temperatures. The microstructures related to pressure solution found in the post‐experimental gouges support the idea that the enhanced healing is related to pressure solution. Plain Language Summary: Investigating the mechanical properties of rock materials in plate boundary faults is crucial for the understanding of subduction zone earthquakes. The nucleation of earthquakes along the plate boundary occurs in a zone where friction drops during slip and fault strength rebuilds after an earthquake. The mechanical properties of fault materials in this zone are likely influenced by local diffusion of mobile elements during the interseismic period. Here, we perform friction experiments using two samples of ancient plate boundary faults that were near the updip and downdip limits of the zone, respectively. The first set of experiments reveals a variation in frictional response to slip under an experimental setup analogous to subduction zone conditions. Variation in the magnitude of interseismic pressure solution strain preserved in the samples is likely the cause of varying responses that lead to the nucleation and cessation of earthquakes. We conduct another series of experiments under varying hydrothermal conditions that reveal a faster restrengthening of the fault material at elevated temperatures. This is likely related to the temperature dependence of pressure solution, which is found to be an operative deformation mechanism based on the examination of post‐experimental materials. Key Points: Mélanges bounding the upper and lower limits of the seismogenic zone show changes in frictional velocity‐dependence with normal stressMicrostructures of the sheared samples suggest that temperature dependence of healing occurs via diffusive mass transferThermally‐activated deformation mechanisms can modulate frictional velocity‐dependence and healing behavior of plate boundary faults [ABSTRACT FROM AUTHOR]

Pore fluids are ubiquitous throughout the lithosphere and are commonly invoked as the cause of induced seismicity and slow earthquakes. We perform lab experiments to address these questions for drained fault conditions and low pore pressure. We shear simulated faults at effective normal stress σn′ $\left({\sigma }_{n}^{\prime }\right)$ of 20 MPa and pore pressures Pp from 1 to 4 MPa. We document the full range of lab earthquake behaviors from slow slip to elasto‐dynamic rupture and show that slow slip can be explained by the slip rate dependence of the critical rheologic stiffness without dilatancy hardening or other fluid effects. Our fault permeabilities ranges from 10−18 to 10−17 m2 with an initial porosity of 0.1 and estimated fluid diffusion time ≈1 s. Slow slip and quasi‐dynamic fault motion may arise from high Pp at higher pressures but dilatancy strengthening is not a general requirement. Plain Language Summary: Earthquakes begin and propagate within the fluid‐saturated rocks of Earth's crust. Many investigators have suggested that high pore fluid pressure (Pp) is essential for slow earthquakes and tremor. These studies rely on the idea that changes in Pp can impact rupture propagation speed by dilatant volume increase during faulting with concurrent increases in fault effective normal stress. Thus, understanding the processes that produce slow‐slip versus dynamically propagating rupture is integral to seismic hazard forecasting. Here, we describe experiments on granular faults that produce the full spectrum of slip observed in nature. We measure the mechanical and hydraulic behavior of the faults and determine that frictional and fluid‐driven processes occur in conjunction. Importantly, we demonstrate that frictional processes are sufficient to explain slow‐slip when fluid migration is not inhibited. We demonstrate that for low pore fluid pressures, the full transition from slow slip to dynamic rupture events can be explained as a frictional effect via the critical rheologic stiffness. Key Points: The frictional stability transition does not require dilatant hardening for granular fault zones sheared at low pore pressuresSlow earthquakes and quasi‐dynamic fault slip can be explained by the strain‐rate dependence of the critical fault stiffness (Kc)For the effective normal stresses studied, pore pressure has a negligible impact on frictional stability and the mode of fault slip [ABSTRACT FROM AUTHOR]