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

Abstract

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 ∼25–200 μm‐thick, texturally heterogeneous veins that form the hematite FM volume (FMV). Grain morphologies and textures vary with FMV thickness over μm 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°C to ≥800–1100°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.

Evaluating the Compatibility of Hematite (U‐Th)/He Data and Hematite‐Carried Secondary Magnetizations: An Example From the Colorado Front Range

“…Depending on the magnitude of grain rotations from cataclastic flow, any vein material that shows signs of fault movement and deformation would likely pass a micro-conglomerate test, if possible to perform, and provide further evidence that the CRM was acquired before fault initiation. This may not hold true for fault surfaces developed on secondary hematite from other localities that display textural and geochemical evidence of flash heating and/or recrystallization (e.g., Ault et al, 2019;McDermott et al, 2017McDermott et al, , 2023, as these surfaces are more likely to acquire new localized magnetizations related to the timing of fault slip as opposed to initial vein formation.…”

“…We do not consider shear heating in our models (cf. Armstrong et al, 2022) because hematite aliquots do not exhibit grain morphologies diagnostic of temperatures that might induce partial to complete He loss (Ault et al, 2015(Ault et al, , 2019McDermott et al, 2017McDermott et al, , 2023Odlum et al, 2022). Details of our thermal history model setup are included in Table S4 in Supporting Information S2.…”

Hematite Frictional Behavior and He Loss From Comminution During Deformation Experiments at Slow Slip Rates

“…Experimental cataclasis and gouge production resemble textures formed by brittle deformation in upper-crustal fault zones that developed in coarse-grained specularite veins. In nature, comminution of up to 10s of μm-thick hematite plates that originally crystalize in random to semi-aligned orientations facilitates slip localization and development of thin slip surfaces (Ault et al, 2015(Ault et al, , 2019McDermott et al, 2017McDermott et al, , 2023Odlum et al, 2022). However, in most examples studied to date, gouge particles produced via grain size reduction are subsequently transformed into equant "polygonal" grains with sintered grain boundaries at and below the slip interface (Ault et al, 2015(Ault et al, , 2019McDermott et al, 2017McDermott et al, , 2023Odlum et al, 2022).…”

“…In nature, comminution of up to 10s of μm-thick hematite plates that originally crystalize in random to semi-aligned orientations facilitates slip localization and development of thin slip surfaces (Ault et al, 2015(Ault et al, , 2019McDermott et al, 2017McDermott et al, , 2023Odlum et al, 2022). However, in most examples studied to date, gouge particles produced via grain size reduction are subsequently transformed into equant "polygonal" grains with sintered grain boundaries at and below the slip interface (Ault et al, 2015(Ault et al, , 2019McDermott et al, 2017McDermott et al, , 2023Odlum et al, 2022). This transformation reflects transient temperature rise that accompanies seismic slip, but grain size reduction appears to be a pre-requisite of this process.…”

“…Integration of textural observations with hematite (U-Th)/He (hematite He) thermochronometry can inform the timing, temperatures, depths, and rates of fault slip (Ault, 2020). For example, hematite fault mirrors comprise ultracataclasite with regions of euhedral, "polygonal" grain morphologies at the slip interface, reflecting localized, transient amorphization and subsequent grain growth or rapid dynamic recrystallization that accompany high temperatures during seismic slip (Ault et al, 2015(Ault et al, , 2019McDermott et al, 2017McDermott et al, , 2023Odlum et al, 2022). Coseismic temperature rise induces He loss in hematite via recrystallization or thermally activated volume diffusion, resulting in hematite He dates that postdate hematite formation (Ault et al, 2015;McDermott et al, 2017).…”