KINEMATICS, STRAIN, AND DEFORMATION CONDITIONS DURING EARLY MIOCENE MYLONITIZATION IN THE BUCKSKIN-RAWHIDE METAMORPHIC CORE COMPLEX, WEST-CENTRAL ARIZONA

Kinematics, vorticity, strain, dynamic recrystallization, and quartz crystallographic preferred orientations associated with mylonitization of early Miocene granitoids in the Buckskin-Rawhide metamorphic core complex, west-central Arizona, define a consistent pattern which constrains the structural and geometric development of the lower plate shear zone. New geologic mapping and U-Pb zircon geochronology confirm the widespread presence of mylonitized early Miocene granites and granodiorites of the Swansea Plutonic Suite (21.3 ± 1.0 Ma and 21.5 ± 1.8 Ma). The abundance of these synkinematic intrusions allows us to separate strain related to extensional unroofing of the core complex from cryptic older deformation. Detailed microstructural analysis indicate that mylonites across the lower plate consistently record non-coaxial-dominated (W_k =0.8-1), top-NE shear (113 out of 115 kinematic indicators are top-NE). Quartz and feldspar deformation/recrystallization mechanisms and quartz crystallographic preferred orientation patterns indicate that peak mylonitization conditions were ~450°C and were relatively uniform across a distance of ~35 km in the extension direction. Strain analysis indicates that most Swansea Plutonic Suite mylonites record finite X:Z strains of 2.2-3.7 and shear strains of ~1-2. Overall, the amount of quartz grain boundary migration, feldspar recovery, feldspar recrystallization, and finite strain increase slightly in the extension direction, but this pattern can be explained by a ≤50 °C increase in deformation temperatures towards the northeast end of the core complex. These data suggest the lower plate shear zone initiated at a subhorizontal (≤4°) angle and was unroofed by a more steeply-dipping detachment fault.