MICROSTRUCTURAL STUDY OF DEFORMED DIAMICTITES ALONG THE WILLARD THRUST FAULT, WITH IMPLICATIONS FOR FLUID INTERACTION ALONG A FAULT

Microstructural assemblages and geochemical variations in a deformed diamictite across a major thrust fault (Willard thrust of the Sevier fold-thrust belt, Utah) record relationships between deformation and fluid-rock interaction. Three sampling areas were selected: Antelope Island in the footwall (low to high strain gradient); and Fremont Island and Little Mountain areas in the hanging wall (medium to low strain). Clasts of granite orthogneiss, paragneiss, and quartzite in diamictite display varying styles. Quartzite clasts have lower strain than other types. Granite orthogneiss clasts contain quartz and feldspar, which is mostly altered to fine grained mica at high strain. Paragneiss clasts contain quartz, variable amounts of feldspar, and primary coarse mica, which is partly altered to fine grained mica, and tend to have higher strain.

Microstructurally, quartzite clasts have microcracks and limited crystal plastic deformation at low strain; bulged boundaries and fluid inclusions increase at higher strain. At low strain, orthogneiss clasts contain quartz grains with microcracks and fluid inclusions; feldspar grains have microcracks that concentrate alteration. With increased strain clasts have kinked micas and extensively fractured and altered feldspars. At high strain elongate quartz grains, strain shadows, and selvage seams develop. Paragneiss clasts at low strain also exhibit fluid inclusions in quartz and microcracked feldspar, but mica is more widespread. At medium strain, clasts have kinked micas, selvage seams, and fractured feldspar. At high strain, quartz ribbons, strain shadows, and selvage seams are well developed.

Deformation mechanisms vary based on mineral and clast type, strain, and position with respect to the fault. Quartz deforms by dislocation creep in quartzite clasts, but transitions from dominant deformation via microfracturing to dislocation creep in gneissic clasts with increasing strain. Feldspar microfractures throughout, but alters more extensively to micas in the footwall with increasing strain. Diffusive mass transfer is more prevalent at higher strain. Deformation was approximately isovolumetric in the footwall where quartz dissolution and precipitation were nearly balanced, whereas the hanging wall experienced net volume loss and quartz dissolution.