Late Cenozoic deformation in the U.S. southern Colorado Front Range revealed by river profile analysis and fluvial terraces

Abstract

Many post-orogenic settings exhibit a rugged topography, but the underlying mechanisms driving topographic rejuvenation are poorly understood. For example, the U.S. southern Colorado Front Range, a widely recognized and studied post-orogenic setting, contains deep canyons and steep channels even though the crustal deformation that built the range during the Laramide Orogeny ended at ca. 40 Ma. Two prevailing hypotheses are typically used to explain these topographically youthful features in the Colorado Front Range: (1) mantle dynamics and active tectonics during the late Cenozoic; and (2) enhanced erosional efficiency associated with Quaternary climatic changes. Here, we evaluate these end-member hypotheses through a tectonic geomorphological study of the upper Arkansas River Basin in southern Colorado. We perform river profile analysis on bedrock channels in the eastern Rockies and map and analyze fluvial terraces in the western High Plains. In the eastern Rockies, river knickpoints record a one- to two-stage increase in base-level fall rate downstream of the Colorado Front Range mountain front and an eastward increase in the magnitude of incision. In the western High Plains, Quaternary fluvial terraces also show an eastward increase in the total magnitude of incision. Supported by flexural and supplemental geomorphic analyses, these results suggest a previously undetected regional-scale, west-directed back-tilting associated with differential rock uplift. Based on the average timing of deformation, locations of major faults, and seismic activity, seismic tomographic data, and existing geodynamic models, we infer that this newly recorded westward tilting in the upper Arkansas Basin is the result of unsteady and potentially migrating dynamic topography that developed ca. 4 Ma.