NEW EVIDENCE FOR WESTWARD TILTING IN THE SOUTHERN COLORADO FRONT RANGE

We use river profile analysis and river terraces in the Upper Arkansas River drainage basin in southern Colorado to explore drivers of transient landscape evolution and differential incision from the High Plains, across the Front Range and into the Rockies. The Upper Arkansas River Basin exhibits evidence of transient landscape evolution in the form of large fluvial knickpoints in main tributaries within the eastern Rockies and differential river incision as recorded by fluvial terraces along the western High Plains. The drivers of such landscape dynamics, particularly over Quaternary timescales, are strongly debated leading to the development of competing hypotheses of (1) increased late Cenozoic rock uplift associated with dynamic topography and waning activity of the Rio Grande Rift versus (2) enhanced erosional efficiency brought on by late Cenozoic climate change. To contribute to this discussion, we present new evidence using river profile analysis, hydraulic geometry and rock strength measurements in the eastern Rockies and terrace mapping along the western High Plains. River profile analysis shows a series of low elevation slope-break knickpoints in the Arkansas River tributaries upstream of the Front Range that exhibit an east-to-west decreasing elevation pattern. Downstream of the Front Range in the High Plains, preliminary analysis of terrace incision patterns suggests a progressive eastward increase in incision magnitude through time. Collectively, the tributary knickpoint and terrace incision patterns imply a general westward tilting of the Upper Arkansas basin during the Quaternary. These new findings are best explained by a model where an external driver (e.g., climate change, dynamic topography, or both) ignites accelerated incision in the highly erodible High Plains rock units generating a flexural-isostatic response and west-directed back tilting of the Upper Arkansas River basin. Future work will focus on refining terrace mapping and dating and modeling signals of landscape transience and the ensuring of a flexural-isostatic response associated with climatic and geodynamic drivers. Outcomes from this research will shed light on how climate-lithology-tectonics interactions conspire to generate topographic changes in mountain belts at various spatial and temporal scales.