EARLY EOCENE SUPRA-DETACHMENT BASIN DEVELOPMENT IN RESPONSE TO PLATEAU SUBDUCTION AND EXTENSIONAL COLLAPSE IN THE SOUTHERN MOJAVE REGION, CA

Recent studies have suggested that subduction, and subsequent rollback, of large igneous provinces beneath southwestern North America in Late Cretaceous–Eocene time led to widespread extension and collapse of the Cordilleran arc. We suggest that the Maniobra Formation, located in southern CA along the northern margin of the Orocopia Mountains, developed as a supra-detachment basin in response to this process in the early Eocene. New stratigraphic descriptions, mapping, magnetostratigraphy, and detrital zircon provenance analysis provide evidence of rapid deposition within a deforming basin. Maniobra stratigraphy consists of ~1000m of conglomerate, sandstone, and shale interpreted as marine turbidite and submarine-canyon deposits. The base of the section comprises boulder conglomerate resting nonconformably above grussified granite of the Hayfield Mountains. The section preserves at least three NW-SE trending normal faults that create domino-style blocks - characteristic of supra-detachment basin architecture - cutting the base of the section. Six magnetozones within the upper part of the Maniobra strata are defined from 17 sites and are consistent with deposition between 55-50 Ma, as previously inferred from probable Early Eocene mollusk fossils. These preliminary magnetostratigraphic results suggest that deposition, and likely extension, was rapid and short-lived. Detrital zircon spectra from the base of the section are characterized by two peaks: one at ca. 1.7 Ga, and another broad peak in the Late Jurassic. The top of the section yields those same populations, but also contains a large ca. 1.1 Ga peak and a cluster of Late Triassic grains. We interpret these data to suggest an up-section change in provenance from hanging-wall-dominated derivation at the base to increasing contributions from the footwall block (ie. Orocopia Mts) as it was unroofed. Taken together, these data are most easily explained by rapid deposition of the Maniobra strata within a supra-detachment basin formed above the Orocopia and/or Clemens-Well detachment structures. We suggest that this extensional event likely occurred as a transient tectonic response to subduction and passage of a large igneous province beneath the Mojave sector of the Cordilleran margin.