CALC-MYLONITES IN THE BUCKSKIN-RAWHIDE METAMORPHIC CORE COMPLEX, ARIZONA: LOCALIZATION OF DETACHMENT FAULT STRAIN IN RHEOLOGICALLY WEAK ZONES INHERITED FROM CRETACEOUS THRUST FAULTING

Although crystalline rocks dominate the footwall of the Buckskin-Rawhide detachment fault in west-central Arizona, we estimate that thin (<1 m- to 100 m-thick) calcite-rich metasedimentary mylonites were present along 25-35% of the detachment fault, and in parts of the footwall they were continuous for ~30 km in the slip direction. New field observations, geochronology, and detailed microstructural data provide insight into the origin of these metasedimentary rocks and their role in the structural evolution of the detachment fault system. We propose that calc-mylonites along the Ives Peak footwall corrugation were derived from locally overturned Pennsylvanian-Permian strata that were buried to mid-crustal depths beneath a SE-vergent Cretaceous thrust fault, which was subsequently reactivated in the Miocene by the Buckskin detachment fault shear zone. These laterally-persistent calc-mylonites were locally smeared out along the detachment fault during footwall incisement, forming a carapace of rheologically weak rocks. Metasedimentary mylonites consistently record top-to-the-NE simple shear parallel to the detachment fault slip direction. Strain, synmylonitic veins, and paleostress recorded in these mylonites increase towards the detachment fault. Marble mylonites <1 m below the detachment fault preserve strong calcite crystallographic preferred orientations and lack cataclastic deformation that characterizes quartz-rich rocks along the detachment fault. In addition, unlike quartzo-feldspathic mylonites, calc-mylonites typically lack extension via postmylonitic normal faults and associated horizontal axis rotation. Paleopiezometery and rheological modelling of the metasedimentary mylonites suggest that when quartzite layers were being sheared at ~100 MPa and 10^-13 to 10^-14 s^-1 near the brittle-plastic transition, marble layers could have been strained ~100x faster at ~20 MPa. Detachment fault strain localized within the metasedimentary rocks, and the calcite marbles exerted significant control on the rheology of the footwall shear zone. This study highlights the important role that pre-existing weak zones may play in influencing the location, geometry, rheology, and style of deformation associated with detachment fault systems.