QUANTIFYING HETEROGENEOUS STRAIN, FLUID-ROCK INTERACTION, AND DEFORMATION MECHANISMS ALONG THE WILLARD THRUST, UTAH

Detailed structural and petrographic analyses of deformed diamictite along the Willard thrust in northern Utah reveal variations in strain, fluid-rock interaction, and deformation mechanisms. Strains were quantified for different clast types including: cobble-size gneissic, granitic, and quartzite clasts in outcrop; and granule-size micaceous and quartz clasts in thin section. In the Antelope Island area, located on the footwall of the Willard, deformation intensity increases overall to the NW, with X/Z strain ratios increasing from 2:1 to 10:1 in micaceous clasts. At a given outcrop, different clast types behaved differently; clasts containing more mica deformed the most, whereas quartzite clasts deformed the least. Three-dimensional strain ellipsoids are triaxial and record near plane strain with limited volume loss. In the Fremont Island area, located on the hanging wall of the Willard, deformation intensity is overall lower, with X/Z strain ratios ranging from 2:1 to 3:1. Strain ellipsoids have oblate shapes with minor extension in the Y direction, consistent with 10-30% volume loss. Larger clasts with limited mica are less deformed than smaller micaceous clasts. Deformed diamictite in both areas shows evidence for fluid influx during deformation with alteration along fractures, complex veining in higher strained outcrops, and reaction rims around some clasts. The detailed nature of alteration, however, varies between the hanging wall and footwall. Microstructures for lower strain clasts in the Antelope Island area include microfractures, patchy undulose extinction, and deformation lamellae. Microstructures in higher strain areas include subgrains and local recrystallization of quartz, and widespread precipitation of quartz fibers in strain shadows. Microstructures in the Fremont Island area include widespread undulatory extinction, subgrains and recrystallization of quartz, limited alteration of feldspar to mica along microcracks, and selvages and strain caps enriched in immobile material. Overall, there is less alteration of feldspar to mica, more widespread plastic deformation of quartz, and greater volume loss in the hanging wall. These observations are consistent with strain and fluid partitioning across the fault.