VORTICITY AND MICROSTRUCTURAL ANALYSIS OF THE WILDHORSE DETACHMENT IN THE PIONEER METAMORPHIC CORE COMPLEX, IDAHO

Eocene extensional strain and exhumation of ductile mid-crustal rocks occurred along the Wildhorse detachment within the Pioneer metamorphic core complex (MCC), but the kinematic history within the detachment is not fully clarified. To quantify the proportions of pure and simple during deformation, we conducted microstructural analysis along two, 300 m transects across the mylonitic sections of the detachment footwall. The transects preserve different levels of the Pioneer MCC footwall along the northern segment of the Wildhorse detachment. The structurally deeper Boulder Lake transect comprises northwest dipping mylonitic quartzofeldspathic gneisses. The structurally shallower Kane Creek transect consists of north-dipping mylonitic quartzites and marbles of the middle plate, underlain by lower plate quartzofeldspathic gneiss. A sill of Eocene granodiorite at the contact between the middle plate and the lower plate displays mylonitic textures, interfoliation with the quartzites and marbles, boudinaged lenses, as well as rare mylonitic quartzite xenoliths.

Mylonitic rocks preserve normal sense asymmetric structures such as S-C’ structures and asymmetric feldspar porphyroclasts. Quartz microstructure display either intensely deformed, small, recrystallized grains or stretched ribbons containing subgrains and small, recrystallized grains at their margins. EBSD analyses of the mylonitic gneisses show strong quartz crystallographic preferred orientation (CPO) dominated by prism <a> slip, with minor rhomb <a> slip. The CPO in the mylonitic quartzites is dominated by rhomb <a> slip with lesser amounts of prism <a> and basal <a> slip. Based on analysis of oblique foliation, rigid grain methods, and C’-shear bands, kinematic vorticity ranges from 0.50­–0.90 for both transects, indicative of general shear. Rigid grain analyses produced lower vorticity estimates for samples from the Boulder Lake transect and no critical ratio for some of the Kane Creek transect quartzites. The complex rigid grain results may represent a transition from pure to simple shear as the Wildhorse detachment exhumed or they could support a rolling hinge model in which during exhumation the Wildhorse detachment captured an earlier period of mid-crustal detachment that included the middle plate-lower plate contact.