Structural Evolution of the Sangre De Cristo Uplift, South-Central Colorado

Geologic mapping and structural analysis of the Sangre de Cristo uplift in south-central Colorado at 37N was conducted to better understand the geometry and kinematics of the first order structures as well as its relationship with the adjacent Late Cretaceous-Tertiary basin fill of the Raton basin. Three north-trending east-vergent asymmetric folds characterize the eastern side of the uplift. Folds involve Pennsylvanian and early Permian rock units and have wavelengths that range from 3.5 to 2 km. Axial planes strike north and dip approximately 60 degrees to the west. The magnitude of structural relief decreases towards the east. The frontal most (easternmost) fold involves Mesozoic strata as well as the western margin of the Raton basin fill indicating deformation was ongoing during late Laramide time. Deformation is concentrated in the eastern steeply dipping limbs of all three folds. The primary deformation mechanisms are layer parallel slip and penetrative shear fractures which parallel the axial planes. Measured slickenlines on layer parallel slip surfaces show slip towards the NE indicating moderately oblique convergence. We interpret these folds result from fault-propagation folding. Preliminary geometric modeling of the fault-fold relationship was performed using trishear modeling techniques and suggests the tips of buried reverse faults are at 1.5 to 0.5 km depth. Involvement of the basin fill in the frontal most fold indicates that basin development occurred prior to development of a significant portion of the Sangre de Cristo uplift. This suggests that much of the Raton basin fill extended westward over much of the region and that the source of these rocks lies west of the crest of the Sangre de Cristos. This is supported by a predominance of Paleoproterozoic(?) crystalline basement clasts in the basin fill.