DEVELOPMENT OF EXTENSION-PARALLEL DETACHMENT FAULT CORRUGATIONS IN THE BUCKSKIN-RAWHIDE METAMORPHIC CORE COMPLEX, WEST-CENTRAL ARIZONA

Extension-parallel detachment fault corrugations are one of the defining features of metamorphic core complexes, yet the origin of these corrugations remains controversial. Structural data from the Buckskin-Rawhide metamorphic core complex in west-central Arizona demonstrate that prominent NE-trending detachment fault corrugations are folds produced by extension-perpendicular (NW-SE) shortening during the middle to late stages of core complex extension (~17-10 Ma). Abundant kinematic data indicate that dominant NE-directed slip on the Buckskin-Rawhide detachment fault was progressively overprinted by NW- and SE-directed slip associated with corrugation folding. The last motion recorded along several NW- and SE-dipping flanks of the detachment fault is reverse slip directed towards the corrugation antiform hinges. 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. Constriction and grain-scale Y-axis shortening were most important during the later stages of mylonitization in the synextensional Swansea Plutonic Suite, which typically form L>S tectonites. Upright m-scale and km-scale lower plate folds parallel the detachment fault corrugations and are best developed in well-layered pre-Tertiary rocks. The geometry and kinematics of these folds indicate that most lower plate folding occurred by postmylonitic flexural slip that was coeval with detachment faulting. New geologic mapping and cross-section analysis indicate that 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 the corrugations in the Buckskin-Rawhide core complex suggest that extension-perpendicular shortening was primarily driven by a reduction of vertical stresses through crustal thinning and tectonic denudation, although the increasing influence of the Pacific-North American transform plate boundary in the mid-Miocene may have contributed to elevated extension-perpendicular horizontal stresses.