THERMOMECHANICAL EVOLUTION OF THE RUBY MOUNTAINS-EAST HUMBOLDT RANGE MYLONITIC SHEAR ZONE

Mylonitic shear zones are ubiquitous in metamorphic core complexes and offer key insight to the thermo-rheological evolution of the crust during exhumation. Evaluating the conditions of deformation can elucidate the spatiotemporal path rocks followed to the surface, which is critical for evaluating modes of large-magnitude crustal extension. In this work, we investigate the microstructural evolution of the Ruby Mountains-East Humboldt Range mylonitic shear zone to elucidate the extensional style that controlled core complex development. We integrate petrographic and EBSD microstructural observations with Ti-in-quartz thermometry to track the evolution of strain as a function of temperature. Quartz dynamic recrystallization mechanisms provide a first order estimate of deformation temperatures, which decrease from the base of the mylonite zone (chessboard extinction -- grain boundary migration; >600℃) to the shallowest structural levels (subgrain rotation; ~400 ℃). There is a corresponding decrease in recrystallized grain size from deep to shallow levels, however intermediate to high temperature intervals of the shear zone contain lower temperature fabrics, such as thin seams of ultramylonite and cataclasite, suggesting strain localized in weak intervals throughout the mylonite zone during late stage shearing. Characterization of strain through EBSD analysis shows primarily flattening strain and kinematic vorticity numbers indicate an ~60% pure shear component. High kinematic vorticity number correlate with low recrystallized grain size and weak fabric strength. Quartz c-axis girdles are present in two structurally shallow samples for which we calculate temperatures of ~400-475 ℃ using the opening-angle thermometer. Ti-in-quartz thermometry returns deformation temperatures in line with qualitative estimates from dynamic recrystallization textures. We present these observations in the context of a general shear model. Combined pure and simple shear extension in the core complex footwall is consistent with a kinematic model that minimizes extension magnitude. The Ruby Mountains-East Humboldt Range metamorphic core complex likely developed through a rolling-hinge style detachment system, as suggested for the neighboring metamorphic core complexes.