NEW EVIDENCE FOR LATE CENOZOIC DEFORMATION IN THE SOUTHERN COLORADO FRONT RANGE

The Colorado Front Range maintains high relief, elevated topography, and evidence of ongoing unsteadiness since its construction during the Laramide Orogeny. Two prevailing hypotheses have been proposed to explain its geologically-recent unsteady landscape evolution: (1) mantle geodynamics and active tectonics during the late Cenozoic; (2) enhanced erosional efficiency associated with Quaternary climate change. Here we evaluate these end-member hypotheses through a case study of tectonic geomorphology of the Upper Arkansas River basin in the southern Colorado Front Range. We perform river profile analysis on bedrock tributaries of the Arkansas River in the eastern Rockies and terrace analysis on fluvial terraces in the Arkansas River valley in the western High Plains. Results from these analyses show an eastward increasing magnitude of steepness and incision below major fluvial knickpoints across the eastern Rockies and a reversed fanning pattern and local deformation in fluvial terraces across the western High Plains. Supported by flexural and geomorphic analyses, these deformation patterns suggest a previously undetected long-wavelength west-directed tilting associated with differential rock uplift across the southern Colorado Front Range and recent active tectonics in the western High Plains. Based on existing geodynamic models, we interpret the deformation patterns in the Upper Arkansas basin as a landscape response to a wave of migrating dynamic topography that swept the southern Colorado Front Range from west to east during the late Cenozoic. Under this geodynamic interpretation, the previously suggested hypotheses can be explained by a northward decaying uplift gradient that resulted in mixed tectonic and climatic signals in different parts of the Front Range.