Cretaceous–Cenozoic Tectonic Evolution of The Southwestern and Northeastern Extents of the North American Cordillera in the Western U.S.A

Abstract

The mid–late Mesozoic era was a time of significant climate, paleogeographic, and tectonic reorganization of western North America. By the Late Jurassic (ca. 155 Ma), subduction of the Farallon oceanic slab beneath the western margin of the North American plate established a classic Cordilleran-type orogenic system extending from southern Mexico to Canada and Alaska. Late Cretaceous––Eocene flat slab subduction during the Laramide event partitioned the continental-scale foreland basin with basement-involved uplifts and intervening intraforeland basins and an inboard migration of magmatic arc activity. While the Cordilleran orogenic belt is typified by the Sevier fold-thrust belt and foreland basin and the Laramide province, important issues remain to be settled regarding the tectonic evolution of these systems and hence the North American Cordillera in the southwestern margin of the U.S.A. This work therefore targets the anomalously thick (3–7 km) McCoy Mountains Formation in southeastern California–western Arizona to reconstruct the paleogeographic and tectonic history of the southwestern U.S.A. and its spatiotemporal relationship to the North American Cordillera. Detailed sedimentologic, detrital zircon, and provenance results from the McCoy Mountains Formation at its type locality in the McCoy Mountains of southeastern California indicate that this ~7 km thick succession was deposited entirely during the Cretaceous between ca. 137 and 70 Ma and was characterized by deep-water turbidite systems. Subsidence analysis suggests that the McCoy Mountains Formation was deposited within a subaqueous flexural foreland basin loaded by the Maria fold-thrust belt to the north-northeast and possibly the Mule Mountains thrust to the southwest (Appendix A). Continuing work in western Arizona builds upon this model by correlating the ~3.5 km thick succession of clastic strata exposed at the Livingston Hills with the type section in California using detailed sedimentology, detrital zircon, and provenance analyses. Results suggest that the Livingston Hills strata were deposited during the Late Cretaceous after ca. 96 Ma, consist of relatively deep water turbidite deposits, and are correlative with the upper portion of the McCoy Mountains Formation in California. Bedding geometries in the lower Livingston Hills strata are typical of syn-contractional growth strata and provide evidence for wedge-top deposition within the McCoy retroarc foreland basin (Appendix B). Thermochronological results from the McCoy Mountains Formation, Maria fold-thrust belt, and Mule Mountains thrust in California suggest that the eastern Mojave transitioned from relatively deep-water basin deposition and regional shortening to regional cooling as early as ca. 70 Ma. We suggest that cooling Late Cretaceous–Eocene cooling was driven by uplift and erosion associated with flat slab subduction and emplacement of the Orocopia Schist. This was coeval with cooling ca. 70–50 Ma farther northeast in the Arizona Transition Zone, supporting that exhumation associated with bulldozing of continental lower crust and mantle lithosphere during Laramide flat-slab subduction was rapid and regional in scale (Appendix C). Expanding the investigation of the Laramide event to the northeast, Appendix D focuses on the topographic development of the Wyoming Laramide province. Apatite fission track thermochronological results from key Laramide uplifts and intraforeland basins indicate that the Wyoming Laramide region experienced rapid cooling between ca. 75 and 50 Ma, contemporaneous with the southwestern U.S. Cordillera. However, in contrast to the southwestern U.S. Cordillera, where cooling was a response to the underplating of trench-derived schists, cooling in the Wyoming Laramide region occurred along discrete basement-cored uplifts. Additionally, these results indicate that Laramide ranges were subsequently buried by Cenozoic basin fill deposited between 50 and 10 Ma. Our results suggest that Wyoming's modern topography is a relatively young feature that began to develop after ∼10 Ma due to widespread incision and sediment evacuation along continental-scale river systems such as the paleo-Mississippi.