MICROSTRUCTURAL AND LUMINESCENCE EVIDENCE OF FAULT MATERIAL TRANSFORMATION AND COSEISMIC HEATING ALONG THE HURRICANE FAULT, UTAH

Quantifying coseismic temperature rise from exhumed brittle faults can identify past seismicity, with implications for modern seismic hazards. We apply low temperature thermochronometry to the seismically active Hurricane fault, Utah, locally characterized by large silica and iron-oxide-rich fault mirrors (FMs) that cut the Rock Canyon Conglomerate of the Moenkopi Formation. Prior (U-Th)/He (He) thermochronometry from hematite patches along the FM provide evidence of up dip propagation of earthquake ruptures at ~300 m depth ~0.65-0.4 Ma (Taylor et al., 2021). New scanning and transmission electron microscopy of the silica FMs indicate highly localized deformation and partially to fully amorphous silica particles within the first ~5 μm of the slip surface exhibit polygonal, recrystallized textures. Detrital zircon He dates range from ~150-350 Ma and show no difference between grains from <1 cm and >4 cm away from the FM surface. Apatite He dates >4 cm away from the FM are ~8-12 Ma, consistent with exhumation related cooling.

Data collectively motivate development of quartz optically stimulated luminescence (OSL) and thermal luminescence (TL) to fingerprint temperature rise associated with shallow seismicity. We isolated quartz from thin (~2 mm) slabs parallel to a mirrored slip surface that has never been exposed to sunlight. Pulsed annealing linearly modulated OSL and TL measurements indicate that trap depths and lifetimes and TL sensitivity are lowest within 4 mm of the fault plane and increase away from the fault plane. The physical transformations of the quartz assisted by mechanical and fluid processes during slip along the fault plane are likely contributing to these differences in luminescence properties. The sensitivity corrected natural OSL and TL signals exhibit patterns consistent with signal loss at the fault plane, providing evidence of coseismic temperature rise. Collectively, micro to nanoscale observations and luminescence data illustrate that in natural fault rocks mechanical processes, fluids, and/or elevated temperatures during seismicity work constructively to transform fault materials and affect the quartz luminescence properties and signals. These processes may facilitate the updip propagation of potentially large earthquakes along thin slip surfaces in the shallow crust.