INVERSION AND REINVERSION OF THE NORTHERN RIO GRANDE RIFT, SAN LUIS BASIN, SOUTHERN COLORADO

The Rio Grande Rift is made up of several sub-basins that vary in geometry and polarity. The San Luis Basin is located in the southern Colorado part of the Rift and is a half-graben, bounded on the east by the Sangre de Cristo Normal Fault. Well, and geophysical data have been used to interpret the Neogene structural architecture. The interpretation and structural restoration of the Neogene rift tectonics provides a means to peer through the veil of Tertiary extension and view the earlier, Late Cretaceous-Paleogene (Laramide) structural fabric of the region. The Sangre de Cristo Mountains to the east of the San Luis Basin are a stack of Laramide, east directed, thrust sheets that carry Precambrian, Pennsylvanian and Cretaceous rocks. The present basin was part of the uplifted core of an earlier, Laramide structure called the Sangre de Cristo Uplift. Structural restorations of the Laramide thrust structures permit another layer of the veil to be removed and show the geometry of the Late Paleozoic Central Colorado Trough that formed during the development of intraplate deformation that produced the Ancestral Rocky Mountains. The Central Colorado Trough was asymmetric and approximately 5 kilometers deep on its western side, adjacent to the San Luis Uplift. The western side of the Central Colorado Trough was an uplift called the San Luis Highland that is partly preserved in the hanging walls of the thrust stack in the present Sangre de Cristo Range. The San Luis Highland has been interpreted as a simple southward continuation of the Uncompaghre Uplift to the northwest, but the data suggests that the relationship is more complex. The insights provided by the geophysical data, and the structural restorations indicate that the San Luis Basin is the site of two episodes of structural inversion. The deep, narrow Central Colorado Trough that accumulated coarse arkosic synorogenic sediments during the Pennsylvanian was completely inverted during the Laramide shortening and eastward thrusting that formed the San Luis Uplift. The basement rocks in the core of the Uplift subsided on the hanging wall of the Sangre de Cristo Normal Fault and are now approximately 10 kilometers below the correlative rocks in the footwall, which are located on the crest of the Sangre de Cristo Range.