NEW INSIGHTS INTO THE TECTONIC HISTORY OF GREAT SAND DUNES NATIONAL PARK AND PRESERVE IN SOUTHERN COLORADO FROM DETAILED GEOLOGIC MAPPING

Great Sand Dunes National Park and Preserve (GSDNP) in southern Colorado records evidence of polycyclic, crustal-scale tectonic events from the Paleoproterozoic to the Neogene. New detailed geologic mapping and petrographic analysis in the Sangre de Cristo Range in GSDNP has revealed a significant thermal event in the Oligocene, characterized by intrusive magmatism, high-grade metamorphism, and localized penetrative deformation. This event precedes and overlaps the initiation of extension in the Rio Grande Rift. Within the preserve, Proterozoic crystalline rocks are exhumed in the hanging walls of range-parallel, W-dipping Laramide thrust faults. These are truncated by the range-bounding, more steeply W-dipping Sangre de Cristo normal fault, responsible for uplift of the range in the Miocene. East of the GSDNP Visitor Center, a 9.5 km² erosional window into the Mosca thrust reveals pervasive intrusions into Pennsylvanian Minturn Formation strata and magmatically modulated deformation in the footwall. Leucocratic sills cut the thrust, and E-W trending F2 folds spatially associated with intrusions refold N-S trending, F1 Laramide folds. Metamorphic mineral assemblages and Raman spectroscopy of carbonaceous material within the aureole indicate contact metamorphism occurred at ~2-3 kbar and peak temperatures of 340-640° C. U-Pb dating of monazite grains elongate parallel to metamorphic foliation yield dates of 33-31 Ma, placing metamorphism and coeval deformation in the early Oligocene. North of Mosca Creek, the Deadman Creek thrust juxtaposes Proterozoic crystalline rocks over Paleozoic sedimentary rocks. The thrust is folded into a NE-vergent antiform and along the range front hosts gently-dipping mylonite zones in its footwall that predominantly record top-SW normal sense shear. These zones are locally 10s of meters thick and are characterized by plastic deformation of quartz, brittle deformation of feldspar, and extensive chloritization, indicating greenschist facies deformation conditions (~300-500°C). The thickest mylonite zones are generally found proximal to Oligocene gabbroic intrusions and are not concentrated near the mapped contact of the Deadman Creek thrust, suggesting high-temperature deformation is associated with gabbro emplacement.