A NUMERICAL SKETCH OF LATE CENOZOIC EVOLUTION OF THE FRONT RANGE, COLORADO

The Laramide Ranges formed 50-80 Ma ago by deep-seated thrusting which placed crystalline basement against easily eroded Mesozoic sediments. Since the end of the Laramide Orogeny, the topography of these ranges has evolved largely in the absence of strong tectonic forcing. We seek to explain Late Cenozoic topographic change in the Colorado Front Range, a classic Laramide Range, using numerical models of stream profile and hillslope evolution. Landforms that serve as targets for these models include high, smooth summit and sub-summit surfaces, glacial troughs, deeply incised bedrock river canyons that ornament the edges of the range, and exhumed, sediment-filled basins. We argue that the high surfaces exhibit essentially steady state morphology, that they are lowering at rates of order 1 m/Myr, and that they are decoupled from the incision of the adjacent canyons. Glacial erosion in the headwaters not only produces the classic U-shaped valley cross-sections but also generates steps and flats in the long valley profiles. Because glacial erosion rates in the valleys are 2-3 orders of magnitude greater than on the interfluves, local relief is rapidly generated. The fluvial system within the range responds both to baselevel lowering as the adjacent basin is eroded and to sediment periodically produced by glacial erosion in the headwaters. The fluvial profiles are presently in a state of transient response to the baselevel lowering: strong convexities of the channels within the range separate deeply incised bedrock canyons below from low relief valleys above. Our modeling is consistent with basin exhumation starting within the last few million years. The basin exhumation that drives this Late Cenozoic range evolution likely reflects a regional response to both climate change, fluvial network rearrangement, and local tectonic tilting, each of which drives a wave of incision up the fluvial network.