Development of extension-parallel corrugations in the Buckskin-Rawhide metamorphic core complex, west-central Arizona

Abstract

Extension-parallel detachment fault corrugations are one of the defining features of metamorphic core complexes, yet the origin of these corrugations remains disputed. 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 (ca. 18–10 Ma). Kinematic data indicate that dominant NE-directed slip on the Buckskin-Rawhide detachment fault was locally 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 toward 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 meter- and kilometer-scale lower plate folds parallel the detachment fault corrugations and are best developed in well-layered pre-Tertiary rocks. Most lower plate folding occurred 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 the constriction and corrugation folding developed primarily in response to crustal thinning and tectonic denudation.