Techniques directly dating fault slip are few, limiting the ability to interpret 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. Glossy hematite-coated fault surfaces in the Wasatch fault footwall damage zone, Utah (USA), exhibit evidence of hematite cataclasis and preserve Pliocene hematite (U-Th)/He dates. Apatite (UTh)/ He a d fission track data from the host gneiss indicate footwall unroofing through ~ 2 km by ca. 4.5 Ma. Internally reproducible but disparate hematite (U-Th)/He dates 4.5 Ma and younger from isolated locations on a single fault surface do not reflect ambient cooling. We hypothesize that these dates, and associated iridescence and annealed crystal texture, document rapid cooling from friction-generated heat during small seismic slip events between ca. 4.5 and 2.5 Ma. Thus, hematite(U-Th)/He dating offers the potential to decipher thermal anomalies in the rock record associated with slip on 105-106 yr time scales.
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