GSA Connects 2024 Meeting in Anaheim, California

Paper No. 118-9
Presentation Time: 8:00 AM-5:30 PM

GEOLOGIC MAPPING OF THE MOSCA CREEK WINDOW AT GREAT SAND DUNES NATIONAL PARK, SOUTHERN COLORADO: NEW INSIGHTS INTO LARAMIDE CONTRACTION, OLIGOCENE CONTACT METAMORPHISM AND RIO GRANDE RIFT EXTENSION


MALAVARCA, Samantha1, SINGLETON, John2, BROEDER, Hunter1, HOLM-DENOMA, Christopher3 and RAHL, Jeffrey4, (1)Department of Geosciences, Colorado State University, Warner College of Natural Resources, 1401 Campus Delivery, Fort Collins, CO 80523, (2)Department of Geosciences, Colorado State University, Fort Collins, CO 80523, (3)U.S. Geological Survey, Geology, Geophysics, and Geochemistry Science Center, Denver Federal Center, Denver, CO 80225, (4)Department of Earth and Environmental Geoscience, Washington and Lee University, 204 W Washington St, Lexington, VA 24450-2116

Within Great Sand Dunes National Park, Colorado, the Laramide Mosca Creek thrust contains an ~9.5 km2 erosional window. The Pennsylvanian Minturn Formation is exposed in the thrust footwall beneath Proterozoic crystalline rock in the hanging wall, with exhumation of the thrust surface occurring along the Sangre de Cristo normal fault system in the Neogene. Strata of the Minturn show evidence of superposed Cenozoic tectonics including Laramide orogenesis, Oligocene magmatism and contact metamorphism, and Rio Grande rift (RGR) extension. We present new 1:10,000-scale geologic mapping within the Mosca Creek window paired with petrographic, kinematic and geochronologic analyses and Raman spectroscopy of carbonaceous material (RSCM) to refine the spatial and temporal interplay of these events. Foliated cataclasites and mylonites of the Minturn Formation along the Mosca Creek thrust record mixed brittle-plastic deformation, with fine-grained metapelitic intervals recording S-C-C’ fabrics, and quartz recording cataclastic deformation and dislocation creep via basal <a> slip. Kinematic indicators record top-NE shear consistent with Laramide contraction overprinted by subsequent top-SW shear. Both are locally overprinted by syn-to-post-kinematic metamorphic mineral assemblages whose spatial association with magmatic intrusions suggests contact metamorphism at ~2-3 kbar and peak temperatures >~610° C. Mineral assemblages are consistent with maximal reheating RSCM temperatures ranging from ~340 to 640 ± 30° C, defining steep lateral thermal gradients across the thrust window. LA-ICP-MS U-Pb dating of monazite grains elongate parallel to metamorphic foliation and proximal to high-grade assemblages yielded a date of 31.1 ± 1.0 Ma, placing metamorphism in the Early Oligocene. Localized patterns in bedding orientations and a structural contour of the thrust surface also confirm polyphase deformation within the window. N-S trending, upright F1 folds that likely record Laramide shortening are subsequently deformed by E-W trending, upright F2 folds consistent with N-S shortening, potentially related to intrusive magmatism. Younger brittle faults with quartz slickenfibers record dominantly NE-SW extension consistent with regional-scale RGR extension, and minor quartz plasticity in the slickenfibers consistent with brittle-plastic deformation.