MULTIPHASE DEFORMATION ALONG THE DEADMAN CREEK THRUST IN THE SANGRE DE CRISTO MOUNTAINS, COLORADO: SUPERPOSITION OF RIO GRANDE RIFT EXTENSION ON LARAMIDE CONTRACTION

The western flank of the Sangre de Cristo Range in southern Colorado contains exposures of Proterozoic gneisses and granitoids and slivers of Mississippian, Devonian, and Ordovician sedimentary rocks (MDO) that have been complexly folded and faulted in multiple stages of deformation, likely associated with Laramide contraction and later Rio Grande Rift extension. We investigated this multistage deformation by detailed geologic mapping and kinematic analysis of sheared rocks associated with the Deadman Creek thrust near Great Sand Dunes National Park. Previous mapping interprets the Deadman Creek thrust as a NE-vergent structure that is locally folded into an isoclinal, upright NW-SE-trending antiform with Proterozoic rocks in the hanging wall and MDO in the footwall. Based on our preliminary results, we propose a structural model that interprets the Deadman Creek thrust as a Laramide shear zone that has been folded by fault-propagation folds, locally reactivated as an extensional shear zone, and later dissected by Rio Grande Rift-associated brittle-plastic normal faults.

Proterozoic rocks in the hanging wall of the Deadman Creek thrust host gently-SW dipping mylonite zones with SW-plunging lineations and S-C fabrics indicative of 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 mid-greenschist facies deformation conditions. Conversely, steeply-NE dipping zones of sheared MDO in the footwall of the Deadman Creek thrust with top-NE normal-sense shear display lowermost greenschist facies deformation characterized by secondary white mica and chlorite in pelitic intervals, calcite marble mylonites, brittle-plastic strain in quartz, and cataclastic fault cores. These brittle-plastic normal faults locally parallel steeply NE-dipping strata in the MDO, suggesting they may have localized along the steep limbs of Laramide fault-propagation folds. We interpret the top-SW mylonite zones and top-NE brittle-plastic faults to record early stages of Rio Grande Rift extension in the Late Oligocene to Early Miocene. Ongoing work will focus on constraining the age of deformation using U-Pb monazite and zircon geochronology.