MICROSTRUCTURAL ANALYSES OF CLASTS IN A TECTONICALLY DEFORMED DIAMICTITE FROM THE WILLARD THRUST, UT

Quantitative microstructural analysis of clasts in a tectonically deformed diamictite reveals varying patterns of deformation mechanisms, fluid-rock interaction, and softening processes along the Willard thrust zone in northern Utah. Sampling areas were chosen on both the hanging wall and footwall to represent a range of strain values. Clasts of granite orthogneiss, paragneiss, and quartzite in diamictite display varying strain magnitudes, deformation styles, and alteration patterns at individual sites, along strain gradients, and across the Willard thrust, based on petrographic point counting and SEM-cathodoluminescence (CL) imaging. Optically, quartz grains display microcracks related to cataclastic deformation that decrease in abundance with increasing strain, whereas CL images reveal increasing prevalence of fracturing and healing processes with increasing strain. Undulose extinction, deformation lamellae, and subgrains related to crystal plastic deformation increase in abundance with strain, with recrystallization at high strains. CL images of quartz grains also reveal dark-light lamellae, likely related to deformation. Selvage seams enriched in mica and strain shadows enriched in quartz present in some samples show that diffusive mass transfer processes also increased with strain. Feldspar displays microcracks at lower strains, along with limited undulose extinction in the hanging wall. Feldspar is often altered to phengitic muscovite in the footwall and to more paragonitic muscovite and biotite in the hanging wall. Abundant microstructures such as fluid inclusion planes, veins, and selvage seams provide optical petrographic evidence for fluid-rock interaction along the fault. Dark quartz present along fracture zones, rimming more luminescent quartz grains, and in strain shadows in CL images indicates the reprecipitation of quartz from a fluid. The greatest degree of fluid related deformation occurs in the rocks of highest strains on the footwall. Differences in alteration patterns and microstructural assemblages on either side of the thrust suggest differing fluid flow systems in the footwall and hanging wall.