Microscale Spatial Variations in Coseismic Temperature Rise on Hematite Fault Mirrors in the Wasatch Fault Damage Zone

Coseismic temperature rise activates fault dynamic weakening that promotes earthquake rupture propagation. The spatial scales over which peak temperatures vary on slip surfaces are challenging to identify in the rock record. We present microstructural observations and electron backscatter diffraction data from three small-displacement hematite-coated fault mirrors (FMs) in the Wasatch fault damage zone, Utah, to evaluate relations between fault properties, strain localization, temperature rise, and weakening mechanisms during FM development. Millimeter- to cm-thick, matrix-supported, hematite-cemented breccia is cut by similar to 25-200 mu m-thick, texturally heterogeneous veins that form the hematite FM volume (FMV). Grain morphologies and textures vary with FMV thickness over mu m to mm lengthscales. Cataclasite grades to ultracataclasite where FMV thickness is greatest. Thinner FMVs and geometric asperities are characterized by particles with subgrains, serrated grain boundaries, and(or) low-strain polygonal grains that increase in size with proximity to the FM surface. Comparison to prior hematite deformation experiments suggests FM temperatures broadly range from >= 400 degrees C to >= 800-1100 degrees C, compatible with observed coeval brittle and plastic deformation mechanisms, over sub-mm scales on individual slip surfaces during seismic slip. We present a model of FM development by episodic hematite precipitation, fault reactivation, and strain localization, where the thickness of hematite veins controls the width of the deforming zones during subsequent fault slip, facilitating temperature rise and thermally activated weakening. Our data document intrasample coseismic temperatures, resultant deformation and dynamic weakening mechanisms, and the length scales over which these vary on slip surfaces. Plain Language Summary Earthquakes produce friction-generated heat pulses that in turn influence their mechanics. Fault surfaces produced or modified by past earthquakes may carry a signature of this heat and are geological archives of the mechanisms that facilitate earthquake-producing slip. Identifying and interpreting earthquake-related thermal signatures, how and why they may vary on a fault surface, and their mechanical influence on earthquakes is challenging. We characterize three, thin, small hematite-coated fault surfaces from the Wasatch fault in northern Utah with microscopy techniques that illuminate hematite structure and the physical mechanisms of deformation at micro- to nano-scale. Microstructures diagnostic of transient, hot temperatures vary on a single fault surface over sub-mm scales and are linked to multiple hypothesized deformation mechanisms that facilitate earthquake slip. Fault properties such as thickness of the deforming zone and surface roughness influence variable temperature rise. Our results provide insight into how temperatures and the physical mechanisms that facilitate earthquakes vary on a single fault surface, and the factors and geological history that promote these variations.