LATE QUATERNARY DEFORMATION ACROSS THE SOUTHERN RIO GRANDE RIFT

The Rio Grande rift (RGR) is a localized zone of crustal extension at the eastern edge of the distributed plate boundary deformation zone in the western United States. Overall, the RGR narrows northward from southern New Mexico into central Colorado. The southernmost portion of the rift merges with the Basin and Range province to the west, although it can be distinguished by higher heat flow, Quaternary volcanism, exhumation history, and more rapid late Quaternary extension. To the north, the extent of the rift is defined by the Colorado Plateau to the west. We focused on understanding the distribution of deformation rate along a transect near the latitude of the southern Colorado Plateau margin, where deformation is distributed among three late Quaternary faults that bound two major basins.

We have generated high-resolution topographic datasets at sites along the transect for the Caballo, San Andres Mountains, and Alamogordo faults. Topographic data were collected using GNSS surveying and structure-from-motion photogrammetry applied to images collected by an uncrewed aerial vehicle (UAV). We analyzed the topography of faulted alluvial fan surfaces using a swath strategy to assess local uncertainty in vertical separation and scarp morphology. We are exploring variations in scarp form using geomorphic metrics including slope and curvature to assess scarp morphology as a tool for correlating faulted surfaces across the rift. We use numerical age estimates derived from regional soil studies and correlation with dated Rio Grande terrace sequences to estimate rates of fault throw and heave.

Comparison of the late Quaternary faulting rates with modern rates observed from GPS geodesy and late Cenozoic faulting history from geologic cross-sections yields insight on the temporal evolution of this continental rift. Based on our preliminary analysis, the rates of late Quaternary faulting are broadly consistent with the low strain rates derived from geodetic observations, and suggest most faults have slowed relative to rates averaged over the late Cenozoic history of rifting.