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Deformation experiments on hematite characterize its slip‐rate dependent frictional properties and deformation mechanisms. These data inform interpretations of slip behavior from exhumed hematite‐coated faults and present‐day deformation at depth. We used a rotary‐shear apparatus to conduct single‐velocity and velocity‐step experiments on polycrystalline specular hematite rock (∼17 μm average plate thickness) at slip rates of 0.85 μm/s to 320 mm/s, displacements of primarily 1–3 cm and up to 45 cm, and normal stresses of 5 and 8.5 MPa. The average coefficient of friction is 0.70; velocity‐step experiments indicate velocity‐strengthening to velocity‐neutral behavior at rates <1 mm/s. Scanning electron microscopy showed experimentally generated faults develop in a semi‐continuous, thin layer of red hematite gouge. Angular gouge particles have an average diameter of ∼0.7 μm, and grain size reduction during slip yields a factor of 10–100 increase in surface area. Hematite is amenable to (U‐Th)/He thermochronometry, which can quantify fault‐related thermal and mechanical processes. Comparison of hematite (U‐Th)/He dates from the undeformed material and experimentally produced gouge indicates He loss occurs during comminution at slow deformation rates without an associated temperature rise required for diffusive loss. Our results imply that, in natural fault rocks, deformation localizes within coarse‐grained hematite by stable sliding, and that hematite (U‐Th)/He dates acquired from ultracataclasite or highly comminuted gouge reflect minor He loss unrelated to thermal processes. Consequently, the magnitude of temperature rise and associated thermal resetting in hematite‐bearing fault rocks based on (U‐Th)/He thermochronometry may be overestimated if only diffusive loss of He is considered. 

Abstract High‐spatial resolution textural and geochemical data from thin slip surfaces in exhumed fault zones archive thermal and rheological signatures of past fault slip. A network of minor, glossy, iridescent silica fault mirrors (FMs) cut Paleoproterozoic gneiss in the Wasatch fault zone (WFZ), Utah. We report field to nanoscale observations from scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy with energy‐dispersive X‐ray spectroscopy of a silica FM to infer deformation mechanisms during FM development. The FM volume comprises a ∼40–90 μm‐thick basal layer of sintered, µm‐ to nm‐diameter silica particles with polygonal to anhedral morphologies, pervasive crystalline Ti‐bearing phases containing measurable N, and µm‐ to nm‐scale void spaces. Silica particles lack shape and crystallographic preferred orientation and some are predominantly amorphous with internal crystalline domains. The basal layer is overlain by a ∼10–130 nm‐thick, chemically heterogeneous, amorphous film at the FM interface. Mass balance calculations of Ti in the basal layer and host rock indicate the FM volume can be sourced from the underlying gneiss. Multiple textural and geochemical lines of evidence, including N substitution in Ti‐bearing phases, support temperature rise during deformation, associated amorphization of host gneiss, and creation of the FM volume. During thermal decay, interstitial anatase and titanite fully crystallized, silica textures capture their incipient crystallization, and some residual elements are solidified in the nanofilm. Our results support a mechanism of weakening and re‐strengthening of silica FM during fault slip and, together with data from adjacent hematite FMs, record shallow, ancient microseismicity in the WFZ.