EXHUMATION OF THE BOEHLS BUTTE-CLEARWATER METAMORPHIC CORE COMPLEX, NORTH CENTRAL IDAHO: HETEROGENEOUS STRAIN DISTRIBUTION AND COMPOSITIONALLY CONTROLLED MYLONITIZATION

New geologic mapping, quantitative thermobarometry, and garnet geochronology across the eastern boundary of the Boehls Butte Metamorphic Core Complex (BBCC) provide insights into the processes of exhumation at mid crustal depths along a basement involved extensional strike-slip fault relay system associated with the northwest trending Lewis and Clark fault zone. The north trending, east dipping, ~0.5 km wide Jug Rock mylonite zone (JRMZ) marks the eastern boundary of the BBCC and is characterized by protomylonitic schist and anorthositic gneiss with local 0.5 m thick ultramylonite bands. Kinematic indicators (C-S surfaces, C’ shear bands, asymmetric mantled porphyroclasts) are heterogeneously distributed throughout the zone and indicate a down-dip, top to the east (090° - 100°) sense of motion. The JRMZ trends north south for 7-8 km and then abruptly turns east west to the south near Cedar Creek. Subvertical mylonitic foliatons, subhorizontal lineations, and well-developed extensional crenulations are consistent with a dextral sense of strike-slip motion along this portion of the shear zone. The JRMZ differs from typical Eocene detachment faults in that there is significant heterogeneity in the distribution of mylonites and chloritic breccias, or a well-defined upper boundary is absent. The transition from footwall mylonites to less deformed hanging wall rocks is gradational; quartzites that overly the mylonitized anorthosite/aluminous schist border zone are only locally mylonitized. The BBCC appears to have been a deep-seated mylonite zone that underwent uplift but not to a depth sufficient to cause brittle fabrics. The abrupt change in the strike of the JRMZ appears to have been controlled by proximity to the anorthosite and degree of mylonite development was a function of bulk rock composition (i.e. rocks lacking abundant quartz are protomylonitic with coarse (up to 10 cm) rounded aluminosilicate porphyroclasts with MS + BT pressure shadows, whereas quartz saturated rocks are more highly strained), implying that rocks were deep, but still quartz controlled strain. Ongoing quantitative thermobarometry and garnet geochronology across the zone could constrain the local peak thermal event and strengthen the hypothesis for Eocene evolution of the BBCC as a basement involved extensional shear zone.