PATTERN OF LATE CENOZOIC INCISION OF THE ROCKY MOUNTAINS AND IMPLICATIONS FOR EVALUATING TECTONIC AND CLIMATIC CONTROLS

The Rocky Mountain orogenic plateau is characterized by high elevations (>2 km) and deep post-Laramide incision (up to 1.2 km). While it is not clear when modern elevations were first attained, most studies indicate that incision began during the past 10 m.y. and may coincide with elevation gain. The ultimate cause of downcutting reflects the interplay between regional tectonic uplift and climate change. We evaluate the relative roles of these driving mechanisms by mapping the distribution of, and incision into, a variety of paleodatums. These datums include high-level subsummit erosion surfaces, the maximum elevation of once continuous remnants of post-Laramide basin deposits, young volcanic flows and pedimented terraces. These surfaces are not contemporaneous, however they are all post-Laramide in age and so provide an envelope for the magnitude of incision.

Tentative results indicate that: 1) the incision pattern is broadly domal, paralleling the trend of the Rio Grande Rift in Colorado and the Bighorn Mountains in Wyoming, and decaying to the north and east over distances of several hundred kilometers; 2) locally maximum incision is best preserved where streams enter or exit bedrock canyons or where volcanic flows cap the Tertiary deposits; 3) in several places, most notably the Cheyenne Tablelands of western Nebraska and eastern Wyoming, incision is associated with surfaces that have been post-depositionally tilted, and 4) the turnaround from net aggradation to incision took place ~6±1 Ma.

The distribution of incision suggests that tectonic uplift exerts major control. The broad wavelength of downcutting suggests upper mantle involvement in the origin of uplift. The role of climate seems less important because: 1) the incision pattern parallels known young tectonic features; 2) does not strongly correlate with drainage area; 3) is greatest where bedrock is involved but does not correlate with paleoelevation; 4) climate change should not result in short-wavelength tilting; and 5) the duration of incision is longer than expected for the response time to climate change. Our results appear to differ with those derived from published paleobotanical estimates of elevation change since the end of the Laramide orogeny. However the uncertainties in those estimates (up to ±1.5 km) are not inconsistent with our results.