Kinematic and geometric evolution of the Buckskin-Rawhide metamorphic core complex, west-central Arizona

Singleton, John Selwyn
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Reconstructing the structural evolution of metamorphic core complexes is critical to understanding how large-magnitude extension is accommodated in the middle to upper crust. This dissertation focuses on the Miocene geometric and kinematic evolution of the Buckskin-Rawhide metamorphic core complex in west-central Arizona, addressing controversial topics including the geometric development of mid-crustal shear zones, the formation of detachment fault corrugations, and the transition from detachment faulting to more distributed deformation. Detailed microstructural data from mylonites in the lower plate of the Buckskin-Rawhide detachment fault indicate that early Miocene mylonitization was characterized by consistent top-NE-directed shear and ~450-500°C deformation temperatures that varied by [less-than or equal to]50°C across a distance of ~35 km in the extension direction. The relatively uniform deformation conditions and strain recorded in mylonitized ~22-21 Ma granitoids are incompatible with models in which the lower plate shear zone represents the down-dip continuation of a detachment fault. Instead, lower plate mylonites initiated as a subhorizontal shear zone that was captured and rapidly exhumed by a moderately to gently dipping detachment fault system. Structural data and geologic mapping demonstrate that the prominent NE-trending Buckskin-Rawhide detachment fault corrugations are folds produced by extension-perpendicular (NW-SE) shortening during core complex extension. Dominant NE-directed slip on the detachment fault was progressively overprinted by NW- and SE-directed slip associated with corrugation folding. Orientation patterns of upper plate bedding across the corrugations are compatible with folding about a NE-trending axis. Extension-perpendicular shortening in the lower plate is recorded by synmylonitic constriction and folding. Upright m-scale and km-scale lower plate folds parallel the detachment fault corrugations and developed primarily by postmylonitic flexural slip that was coeval with detachment faulting. The total amount of NW-SE shortening across the lower plate is ~10%, but the amount of NW-SE shortening recorded by the younger detachment fault is only ~1%. The relatively late-stage development of corrugations in the Buckskin-Rawhide metamorphic core complex suggests that extension-perpendicular shortening was primarily driven by a reduction of vertical stresses through crustal thinning and tectonic denudation. Brittle fault data document the transition from large-magnitude, NE-directed extension to distributed E-W extension and right-lateral faulting. Following exhumation to brittle conditions, lower plate mylonites were extended up to ~20-30% by NE-dipping, syndetachment normal faults. Towards the end of detachment faulting, the extension direction rotated clockwise, and some portions of the Buckskin detachment fault record a transition from dominant top-NE slip to ENE- and E-directed slip. After detachment faulting ceased, E-W extension was accommodated primarily by steeply NE-dipping, right-lateral and oblique right-lateral-normal faults. The cumulative amount of right-lateral shear across the core complex is probably 7-9 km, which is the amount needed to restore the topographic trend of lower plate corrugations into alignment with the dominant extension direction. Postdetachment right-lateral/transtensional faulting across the Buckskin-Rawhide metamorphic core complex reflects the increasing influence of the Pacific-North American transform plate boundary towards the end of the middle Miocene.