Late Cretaceous‐Early Paleogene Extensional Ancestry of the Harcuvar and Buckskin‐Rawhide Metamorphic Core Complexes, Western Arizona

Metamorphic core complexes in the western North American Cordillera are commonly interpreted as result of a single phase large-magnitude extension during middle to late Cenozoic. We present evidence that mylonitic shear zones Harcuvar and Buckskin-Rawhide west-central Arizona also accommodated an earlier Late Cretaceous early Paleocene. Microstructural data indicate substantial top-NE mylonitization occurred at amphibolite-facies, 40Ar/39Ar thermochronology documents post-tectonic footwall cooling <500°C by Paleocene mid-Eocene. Amphibolite-facies mylonites spatially associated with voluminous variably deformed leucogranites were emplaced from ca. 74–64 Ma, kinematic 63 Ma dike indicates this had waned Reconstruction architecture latest Cretaceous—early deformation within NE-dipping extensional zone. The likely crustal melting due orogenic thickening, consistent model whereby heating triggered gravitational collapse overthickened crust. Other tectonic processes, such Laramide underplating Orocopia Schist or mantle delamination, may have contributed episode extension. Miocene was superimposed on older zone similar kinematics, suggesting location geometry strongly influenced inheritance. speculate other Cordilleran experienced more complex polyphase history than previously recognized, but many cases be obscured later overprinting.