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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 . |
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