LINKING HEMATITE (U-TH)/HE DATING WITH THE MICROTEXTURAL RECORD OF SEISMICITY IN THE WASATCH FAULT DAMAGE ZONE, UTAH

Fault microtextures combined with geochronologic data can be used to decipher the rock record of seismicity. Hematite is commonly found in fault zones, amenable to (U-Th)/He dating, and slip surface hematite may be reset by shear heating events and/or recrystallization. We investigate glossy, iridescent hematite-coated fault surfaces in the Wasatch fault footwall damage zone, Utah, that exhibit evidence of hematite cataclasis and elevated slip surface temperatures from polygonal Fe-oxide crystals within 2 μm of the fault surface at iridescent areas. This recrystallized texture is analogous to that observed in Fe-oxide annealing experiments with temperatures >800–1000 °C. Fault surfaces yield Pliocene hematite (U-Th)/He dates. Apatite (U-Th)/He and fission-track data from the host gneiss indicate footwall unroofing through ~2 km by ~4.5 Ma. Internally reproducible but disparate hematite (U-Th)/He dates ≤4.5 Ma from three isolated locations on a single fault do not reflect ambient cooling. He loss from hematite aliquots with comminuted microstructure require temperatures of ~550–750 °C for a heat duration of ≤100 s, consistent with temperature estimates due to flash heating at asperities and microtextural observations. We hypothesize that these dates, associated iridescence, and annealed crystal texture, document thermal resetting from friction-generated heat during small seismic slip events between ~4.5-2.5 Ma. Prior slip caused progressive hematite comminution, reduction in the diffusion domain length scale, and a lower closure temperature, making hematite susceptible to this flash heating resetting. Preserved events occurred at ~2 km depth or the upper boundary of the seismogenic zone. Now exhumed damage zone minor faults were repeatedly activated during seismic events, perhaps aftershocks following major earthquakes, although not all major earthquakes necessitate seismicity on every minor fault surface. Hematite (U-Th)/He dating offers promise for resolving transient thermal anomalies in the rock record and directly dating shallow brittle deformation associated with this seismicity.