ASPERITY FLASH HEATING AND DYNAMIC WEAKENING OF HEMATITE-COATED FAULT SURFACES FROM MICROTEXTURAL ANALYSIS, (U-TH)/HE THERMOCHRONOLOGY, AND THERMOMECHANICAL MODELING

Constraining fault surface temperature evolution in natural faults is challenging, yet critical for testing theoretical inferences and experimental observations of the frictional weakening mechanisms that promote earthquakes. We target high-gloss, hematite-coated fault surfaces in the footwall damage zone of the seismogenic Wasatch Fault zone (WFZ) for coupled microtextural analysis via scanning electron microscopy (SEM) and hematite (U-Th)/He (He) thermochronology to decipher slip surface paleotemperatures. Microtextural observations reveal the presence of comminuted Fe-oxide grains and crystals with lobate and/or polygonal, triple junction-forming grain boundaries. Crystals with lobate and/or polygonal boundaries occur at the fault surface and in spatially isolated clusters ~<1 to >20 μm in diameter. These grain morphologies, when compared with observations from limited, longer-duration heating and deformation experiments, suggest localized recrystallization at the fault surface from temperatures >1000 °C. Hematite He dates from SEM-imaged aliquots range from 5.0 ± 1.1 Ma to 2.4 ± 0.1 Ma, although 40-60% are younger than new and previously published apatite He dates of 4.5 ± 0.6 and 4.5 ± 1.4 Ma from isoelevational host rock samples. This date relationship and calculated closure temperatures of 90-115 °C and 55 °C for hematite and apatite He, respectively, suggest nonmonotonic cooling of fault surface hematite aliquots at depths shallower than the apatite He closure isotherm (≤2 km). Dated aliquots display microtextural evidence for elevated fault surface temperatures, further supporting this interpretation. Thermomechanical model simulations of asperity flash heating show that paleoasperities with dimensions of >20 μm generate local fault surface temperatures >1200 °C, causing ~90-100% fractional He loss from hematite within 200 μm of the fault surface and potentially inducing dynamic weakening of asperities during slip. Temperatures of ~700 °C from smaller asperities result in partial (~20 %) He loss. These collective observations highlight the use of integrated textural analysis and hematite He thermochronology in deciphering fault surface paleotemperatures and suggest hematite-coated fault surfaces preserve a record of seismogenic slip in the WFZ.