SLIP LOCALIZATION IN EPIDOTE: OBSERVATIONS FROM THE WASATCH FRONT, UT

Epidote alteration material and veins are common in diverse basement rocks. Thin, brittle slip surfaces are localized within these veins, but epidote rheology and factors contributing to slip localization are not well understood. Limited published deformation experiments on epidote gouge indicate epidote is frictionally strong and velocity weakening, motivating our investigation into how natural epidote accommodates brittle deformation. Here we characterize epidote slip surfaces from outcrop to microscale in the exhumed footwall of the southern Brigham City segment of the Wasatch fault zone (WFZ), UT. Networks of cm-spaced epidote-coated slip surfaces are observed both within and outside of the footwall damage zone of the WFZ. Striated, planar to curviplanar surfaces are developed in mm- to cm-thick, waxy epidote veins that locally contain quartz, chlorite, and hematite. Multiple, sub-parallel surfaces occur in most epidote veins. Individual faults have slickenlines of variable orientation, though possibly on different horizons within the fault.

Scanning electron microscopy reveals the boundary between epidote vein material and wall rock is irregular, with epidote permeating the wall rock, which also locally exhibits diffuse greenschist facies epidote alteration. Vein epidote grains are µm to mm in diameter, with a blocky, interlocking texture. Approaching the slip surface, grain size dramatically decreases to nm- to µm-diameter, subhedral particles, and just below the slip interface, particles are locally euhedral with sintered, triple-junction grain boundaries. In plan view, portions of slip surfaces appear featureless and aphanitic, with some visible anhedral to angular particles that are <10 nm in diameter. These textures collectively support extreme grain size reduction during cataclastic deformation within epidote veins. Slip surface nanoparticle textures, potentially reflecting rapid dynamic recrystallization and/or grain growth, suggest localized temperature rise achieved during seismic slip. Slip localization in epidote veins is facilitated by comminution of frictionally strong epidote and evolving slip surface temperatures complementing observations from deformation experiments.