The Buckskin-Rawhide and northern Plomosa Mountains metamorphic core complexes, west-central Arizona, USA

Abstract

ABSTRACT The Buckskin-Rawhide and northern Plomosa Mountains in west-central Arizona are metamorphic core complexes that record NE-directed, large-magnitude extension in the early to middle Miocene. Both core complexes consist of mylonites exposed in the footwall of corrugated, low-angle detachment faults. The Late Cretaceous Orocopia Schist and early Miocene intrusions dominate the mylonitic footwall of the northern Plomosa Mountains. The Orocopia Schist was emplaced during low-angle subduction of the Farallon plate and exhibits the hallmarks of the underplated Laramide subduction complexes, including blocks of metasomatized peridotite encased in quartzofeldspathic schist. In the Buckskin-Rawhide Mountains, carbonate-rich metasedimentary rocks that were buried to midcrustal depths by Mesozoic thrust faults preferentially absorbed Miocene footwall strain and localized the Buckskin detachment fault. A correlation between distinct granodiorite in the footwall of the Buckskin detachment fault in the eastern Bouse Hills and a hanging-wall conglomerate sourced from this granodiorite provides constraints on middle Miocene displacement across the detachment fault and indicates that displacement increased northeastward in the slip direction. The termination of slip on the Buckskin detachment fault also younged northeastward, largely ending by ca. 19 Ma in the western Bouse Hills and ca. 17 Ma in the westernmost Buckskin Mountains, but continuing to <16 Ma in the southern Buckskin Mountains and to ca. 12 Ma in the eastern Buckskin Mountains. Late stages of slip on the detachment fault record minor NW-SE shortening during amplification of corrugation folds. Postdetachment faulting across the region was dominated by dextral and oblique-dextral slip on NW-striking faults influenced by the Pacific–North America plate boundary. Locations visited on this three-day field trip highlight the structural evolution of these metamorphic core complexes.