BASIN DEVELOPMENT DURING SLIP ON THE EAGLE PASS DETACHMENT FAULT, PINALEÑO METAMORPHIC CORE COMPLEX

Abstract

Metamorphic Core Complexes (MCCs) are fundamental features associated with the redistribution of crustal mass following orogenic thickening. In southern Arizona, the Pinaleño Mountains are the northeastern-most MCC in an extension-parallel belt of three MCCs, a feature which is not observed elsewhere in the North American Cordillera. Despite long-term interest in the evolution of MCC systems in North America, the Pinaleño MCC, and the associated Eagle Pass detachment fault have been relatively under-investigated. In this study, we provide detailed stratigraphic and chronological constraints on a section of growth strata in the hanging wall of the Eagle Pass detachment fault to constrain the timing of core complex-related faulting and exhumation. We present detailed stratigraphic sections and report clast counts from select intervals of the volcaniclastic fanglomerate exposed in Crazy Horse Wash to characterize the depositional history of the growth strata and provide context for geochronologic analyses. Two detrital zircon U-Pb geochronologic age spectra from the volcaniclastic fanglomerate yield age distributions with three main age modes at 1.8-1.6 Ga, 1.5-1.3 Ga, and 28-21 Ma. Based on the immaturity of sediments, older age modes, and inclusion of mylonitic clasts, we interpret the sediment to be locally sourced, with provenance ties to 1.8-1.6 Ga Yavapai/Mazatzal province rocks and 1.5-1.3 Ga granitoids in the footwall of the detachment. The 28 to 21 Ma age mode matches the crystallization ages of volcanic rocks in the breakaway zone to the MCC and is interpreted to approximate true depositional ages for the upper unit of the growth strata, consistent with other known timing constraints for detachment faulting and core complex exhumation in southern Arizona. These data suggest that the three extension-parallel MCCs (the Pinaleños, Catalina-Rincon, and Coyote) were syn-tectonically active, analogous to the modern southern Tibetan Plateau, providing further evidence for a broad region of thick and hot plateau-like crust in southern Arizona before the MCC extension.