EOCENE BASIN RECORD OF METAMORPHIC CORE COMPLEX EXHUMATION IN THE WESTERN UNITED STATES CORDILLERA

Extension is a critical tectonic process in our understanding of the growth and collapse of major orogenic belts. The timing, rates, and styles of extension in the U.S. Cordillera following Sevier and Laramide compressional deformation vary widely and likely depend on multiple tectonic drivers. In southwest Montana, Sevier- and Laramide-style uplifts are overprinted by later extension and sedimentary basin formation. Eocene-Oligocene basin formation in the region is associated with metamorphic core complex (MCC) exhumation, brittle upper-crustal extension, late-stage shortening, and paleo-valley incision and filling. We aim to determine the mechanisms and timing of basin formation and MCC exhumation through detailed stratigraphy, ⁴⁰Ar/³⁹Ar geochronology, and detrital zircon (U-Th)/(He-Pb) double dating. Here we present initial results from stratigraphic sections and U-Pb analyses of syntectonic basin strata adjacent to the Anaconda MCC, as well as the Salmon and Muddy Creek Basins.

Salmon Basin samples contain detrital zircon grain age populations of 1200-2000 Ma and 2200-2800 Ma, both likely sourced from Jurassic-Cretaceous strata in the Sevier fold-thrust belt and local Belt Supergroup rocks. The youngest grain age population provides a late Eocene maximum depositional age. These grains were likely sourced from volcanic centers in northeastern Nevada. An Anaconda Basin sample contains a dominant detrital zircon grain age population of 65-80 Ma and multiple older populations (n<5). Grain ages of 65-80 Ma are likely sourced from the nearby Tobacco Root and Pioneer Batholiths. Grain ages of 95-970 Ma could be sourced from the Early Cretaceous Blackleaf Formation, exposed in the Pioneer Range, or other Jurassic-Cretaceous sedimentary strata. Grain age populations >1000 Ma are likely sourced from the Proterozoic Belt Supergroup or local Cretaceous strata. Anaconda Basin stratigraphy shows a cyclic progression of coarse-grained, traction-structured, channel-fill-dominated strata fining upward to organic-rich floodplain deposits and paleosols, while the Muddy Creek Basin is dominated by fine-grained, organic-rich and tuffaceous strata. Detrital zircon U-Pb ages from both basins show primarily local sediment-source regions and provide a late Eocene maximum depositional age for basal Salmon Basin strata.