MICROSTRUCTURAL ANALYSIS OF METASEDIMENTARY AND QUARTZOFELDSPATHIC ROCKS WITHIN THE WILDHORSE DETACHMENT FAULT IN THE PIONEER MOUNTAINS METAMORPHIC CORE COMPLEX, IDAHO

Detachment faults in extensional systems record km-scale extension and preserve evidence of ductile strain at mid-crustal depths. In this study, strain partitioning is assessed through microstructural analyses of differing lithologies within the mylonitic footwall of the late Eocene Wildhorse detachment in the Pioneer metamorphic core complex, Idaho. We collected samples along a 350 m vertical transect from the top of the detachment footwall. The stratigraphic sequence consists of migmatitic gneiss, an overlying forsterite-bearing dolomitic marble interlayered with minor quartzite, and an overlying quartzite. An Eocene granodiorite sill intruded along the gneiss-marble contact and granodiorite forms additional sills and boudins within the marble and the base of the quartzite.

Meter-scale shear bands and asymmetric microstructures, such as mica fish, feldspar sigma clasts, and S-C structures, record normal sense shear in the quartzite. The quartzite microstructure is dominated by quartz ribbons and smaller, dynamically recrystallized quartz grains. Quartzites near the top of the detachment contain prevalent C’ shear bands with abundant, small recrystallized quartz. In the granodiorite, porphyroclastic feldspar displays deformation twinning and quartz-rich zones show abundant subgrains. In the upper portion of the marble unit, recrystallized calcite displays deformation twins, whereas directly above the granodiorite sill and migmatitic gneisses, marbles contain larger, more equant calcite grains. The migmatitic gneiss displays sutured quartz grain boundaries, quartz neoblasts, and deformation lamellae in plagioclase feldspar. Crystallographic preferred orientations (CPO) of samples from the quartzite, marble, and granodiorite were analyzed by scanning electron microscopy with electron backscatter diffraction (EBSD). Quartz analyzed from the quartzite unit and a granodiorite boudin show rhomb <a> slip; calcite from the top of the marble unit also show rhomb <a> slip. At the base of the marble unit an interlayered quartzite displays prism <a> slip and a marble sample preserves no CPO. Heat from the migmatitic gneiss and granodiorite sill may have influenced the change in active slip systems and microstructure toward the base of the interlayered marble and quartzite.