SPATIALLY VARIABLE COSEISMIC TEMPERATURE RISE AND TRANSIENT RHEOLOGY ALONG HEMATITE FAULT MIRRORS IN THE WASATCH FAULT ZONE, UTAH, USA

Earthquakes dissipate mechanical energy as frictional heat. Coseismic temperature rise may vary spatially with fault properties such as shear zone thickness and slip surface topography, influencing thermally activated weakening and transient rheology. We present microtextural observations, electron backscatter diffraction data, and hematite (U-Th)/He (HeHe) dates from small (~0.3 m²) hematite-coated fault mirrors (FMs) hosted in the seismogenic Wasatch fault zone to evaluate the interplay between fault structure, strain localization, and weakening mechanisms during paleoearthquakes. Samples consist of mm- to cm-thick hematite-cemented breccia veins, cut by an overlying 25-300 µm-thick hematite cataclastic layer that forms the FM (FM volume; FMV). The FMV exhibits inter- and intra-sample textural heterogeneity. Cataclasite grades to ultracataclasite where total FMV thickness is greatest. Where the FMV is thinnest and(or) at mm-scale geometric asperities on the FM surface, hematite particles having subgrains and serrated boundaries coexist with low-strain polygonal grains whose diameter increases with distance from the FM. These morphologies imply crystal plastic deformation and annealing, respectively. Spatial relations between FM microtextures and comparison to hematite deformation experiments indicate temperatures ranging from <300 ⁰C (ultracataclasite) to ≥600-1100 ⁰C (polygonal grains) over mm scales during seismic slip. The hottest temperatures may be attained by flash heating at asperities, but mm-scale likely displacements preserve textural evidence of concomitant brittle and plastic deformation. Complementary HeHe dates from isolated breccia matrix and the FMV are ~70-20 Ma and ~40-13 Ma. Comparison of the HeHe closure temperature and other thermochronometry data support Late Cretaceous brecciation and Paleocene partial exhumation, followed by Miocene fault reactivation and FM development at ≤4 km depth. We propose a model of FM development where repeated hematite alteration weakens host rock, facilitating subsequent seismic slip. Strain localization and fault geometry promote heterogenous temperature rise and transient plastic deformation during slip, even at depths and ambient temperatures dominated by brittle deformation in classic models of fault rheology.