POSTMYLONITIC AND POSTDETACHMENT STRUCTURAL EVOLUTION OF THE BUCKSKIN-RAWHIDE METAMORPHIC CORE COMPLEX, WEST-CENTRAL ARIZONA

Brittle fault data from the Buckskin-Rawhide metamorphic core complex, west-central Arizona, document the transition from large-magnitude, NE-directed extension to distributed E-W extension and dextral faulting. The Buckskin detachment fault locally records a clockwise rotation of the slip direction from dominant top-NE-directed slip to ENE- and E-directed slip during the last stages of activity. Postmylonitic brittle deformation is dominated by normal and dextral slip along moderately to steeply NE-dipping faults. Relative timing relationships indicate that most NE-directed postmylonitic normal faulting was coeval with detachment fault slip (~17-11 Ma), whereas dextral and oblique dextral faulting postdates detachment fault slip (post ~11-10 Ma). Based on the persistent SW-dip of lower plate fabrics and NE-dip of abundant postmylonitic normal faults, the amount of NE-SW brittle extension of the lower plate is probably ~20-30%. This estimate suggests extension was not concentrated in the upper plate, as predicted by most models. Previous thermochronologic studies of metamorphic core complexes neglect postmylonitic extension of the lower plate, potentially resulting in overestimates of detachment fault slip rates.

The end of detachment faulting in the Buckskin-Rawhide metamorphic core complex was marked by a transition to dominantly E-W extension and associated dextral-normal faulting. At least ten NE-dipping postdetachment faults record ~0.1-1 km dextral or oblique dextral displacement. The cumulative amount of dextral 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. Geologic mapping confirms that the misalignment between the lower plate topography and the corrugation axis in the Little Buckskin Mountains is due to dextral faulting. Postdetachment dextral/transtensional faulting across the core complex reflects the increasing influence of the diffuse Pacific-North American transform plate boundary towards the end of the middle Miocene.