FLUID-DRIVEN SHIFTS IN DEFORMATION MECHANISMS IN QUARTZ-RICH ROCKS NEAR A MAJOR FAULT: WILLARD THRUST ZONE, NORTHERN UTAH

Integrated petrographic microstructural and electron backscatter diffraction (EBSD) studies reveal varying deformation mechanisms and fluid interaction within quartz-rich lithologies that span the footwall and hanging wall of the Willard thrust in the Sevier belt of northern Utah. Sampled lithologies include the Tintic Quartzite and siliciclastic beds in the Mineral Fork diamictite within the footwall, and graywackes in the Perry Canyon Formation within the hanging wall. Microstructures, macroscopic veins, and geochemical variations record significant fluid interaction, especially in higher strain samples nearer the fault. Synchrotron-source FTIR maps show that quartz grains in higher strain samples have increased water concentrations and that water entered grains along healed microfractures, resulting in hydrolytic weakening. Fluid interaction also caused widespread alteration of feldspar to mica, resulting in reaction weakening. Crystal-plasticity was dominant in all lithologies. Samples proximal to the fault experienced additional deformation mechanisms that worked in tandem, partly related to increased fluid-rock interaction. Near the fault, diffusive mass transfer (DMT) accompanied crystal plasticity in fine-grained, mica-rich lithologies, and brittle fracturing accompanied crystal plasticity in coarse-grained, quartz-rich lithologies. All rocks display weak quartz crystallographic preferred orientation (CPO) fabrics with misorientation (M) indices <0.09, with strongest CPO in samples close to the fault with high quartz concentrations. Localized high strain zones were achieved by enhanced crystal-plasticity along with enhanced fluid-driven DMT or brittle fracturing, dependent on mineralogy and grain size. Crystal-plasticity and fluid-driven processes may have been asynchronous due to fluid pumping. Single girdle quartz CPO fabrics indicate basal <a> slip was most common, but point maxima in some footwall samples indicate prism <a> slip and mixed <a> slip were also activated near the fault, which may indicate hydrothermal heating from fluids. Channelized fluid flow along the fault may have been continuous, cyclical, or sporadic. Overall, deformation mechanisms were controlled by a combination of fluid-rock interaction, lithology, and structural position.