THE REAL MCCOY: A NEW RECORD OF MARINE BASIN DEPOSITION IN SOUTHWESTERN NORTH AMERICA DURING THE CRETACEOUS

The southwestern U.S.A. has experienced several tectonic events during the Mesozoic including development of the N.A. Cordilleran system and continental rifting along the Late Jurassic Mexican border rift system. The spaciotemporal interactions between these events still challenge our understanding of the tectonic and paleogeographic evolution of the southwestern U.S.A., particularly in SE California­­ and SW Arizona. The McCoy Mountains Formation is exposed across this region and represents an anomalously thick (>7 km) record of Mesozoic sedimentation originally interpreted as alluvial-fluvial deposits that were low-grade metamorphosed, perhaps due to tectonic burial via the Late Cretaceous Mule Mountains thrust. The timing and nature of deposition in the McCoy basin remain controversial with workers proposing two contrasting models to explain its tectonic significance: (1) deposition in a transtensional basin associated with the Late Jurassic supposed Mojave-Sonora megashear and (2) deposition in a composite basin that initially developed due to extension along the Late Jurassic Mexican border rift system, and continued to develop during the Cretaceous, possibly as a retroarc foreland basin developed due loading from the Maria fold and thrust belt. We present new sedimentologic, isotopic, and geo-thermochronologic results that test these models and suggest that the McCoy Mountains Formation consists primarily of turbidites that were deposited between ca. 128–74 Ma. New detrital zircon U-Pb ages suggest that the McCoy Basin was supplied by Paleozoic–early Mesozoic strata shed from the south advancing Maria fold and thrust belt. Based on these results, we propose an alternative model that suggests the McCoy Mountains Formation is entirely Cretaceous in age and consists of turbidites deposited in a retroarc foreland basin associated with flexural loading from the Maria fold and thrust belt. Preliminary thermochronological data show basin exhumation at ~55 Ma, which we interpret to be related to flat-slab subduction processes (e.g. underplating and exhumation of the Pelona-Orocopia-Rand schists). Our results call for a re-evaluation of the Cretaceous paleogeography of the southwestern U.S.A. to gain a comprehensive understanding of the evolution of western North America during the Mesozoic.