Late Miocene Erosion and Evolution of Topography along the Western Slope of the Colorado Rockies

Open Access
Author:
Rosenberg, Russell Harris
Graduate Program:
Geosciences
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
February 13, 2013
Committee Members:
  • Dr Eric Kirby, Thesis Advisor
Keywords:
  • Rocky Mountains
  • Colorado
  • incision
  • uplift
  • dynamic topography
  • mantle
  • Late Miocene
  • Yampa River
  • White River
Abstract:
It is increasingly apparent that dynamic effects associated with changes in mantle flow and buoyancy can influence the evolution of surface topography. In the Rocky Mountain province of the western United States, recent high-resolution seismic imaging of the crust and upper mantle reveals intriguing correlations between mantle velocity anomalies and regions of high topography. To determine whether these regions of low-seismic wavespeed are associated with recent changes in buoyancy structure of the lithosphere, I explore the relationship between the longitudinal profiles of tributaries draining the western slope of the northern Colorado Rockies and the history of Late Cenozoic fluvial incision and exhumation across this region. Major tributaries of the upper Colorado River, including the Gunnison and Dolores Rivers, that drain high topography in central and western Colorado overlie upper mantle with slow seismic wave velocities; these drainages exhibit relatively steep longitudinal profiles (normalized for differences in drainage area and discharge) and are associated with ~1000-1500 m of incision over the past 10 Ma. In contrast, tributaries of the Green River that drain the western slope in northern Colorado (White, Yampa, and Little Snake Rivers) overlie mantle of progressively lower seismic wave velocities to the north. River profiles in northern Colorado are two to three times less steep along reaches within comparable bedrock lithologies. New 40Ar/39Ar ages on basalt flows capping the Tertiary Browns Park Formation in this region range in age from ~11-6 Ma, and provide local datums from which I reconstruct ~500-900 m of incision along tributaries of the Green River. The correspondence of steep river profiles in regions of greater incision and lower gradient profiles in regions of less incision suggests that the fluvial systems are dynamically adjusting to an external forcing. Moreover, spatial differences in the pattern and magnitude of incision are not readily explained by a putative increase in erosivity associated with late Cenozoic climate change. Rather, fluvial incision appears to reflect relative base level fall along the western slope. Given the correspondence of steep channels, deep incision and regions of low seismic velocity mantle, I suggest that differential rock uplift driven, in part, by differences in the buoyancy and/or convective flow of the mantle beneath western Colorado is the likely driver for Neogene incision.