TESTING THE IMPACT OF LATE CENOZOIC ROCK UPLIFT ON THE TOPOGRAPHY OF THE ROCKY MOUNTAINS

The dramatic high relief of the modern Rocky Mountain landscape formed in the late Cenozoic due to downcutting of a fluvial network that links a series of easily eroded sedimentary basins across relatively resistant crystalline cores. Previous researchers have tried to correlate relief production directly with regional episodes of post-Laramide geophysically induced rock uplift or of late Cenozoic climate change, implicitly assuming the landscape response time to a given forcing should be relatively short. We use a two-dimensional numerical model of stream power-based fluvial erosion and the associated flexural isostatic response to calculate the expected response pattern and pace of exhumation derived from each proposed forcing mechanism. In tectonic uplift calculations, stream incision is driven by rock uplift associated with migration of the Yellowstone hotspot and with growth of the northern portion of the Rio Grande Rift. In climate change calculations, increasing discharge drives stream incision. We find that four factors dominate the spatial and temporal pattern of regional landscape evolution: 1) the time since the forcing was imposed, 2) the forcing pattern, 3) the distribution of relatively resistant bedrock within the region, and 4) the regionalized pattern of isostatic response to sequential basin exhumation. In particular, we show that each type of forcing initially produces a predictable, non-uniform pattern of erosion that gradually relaxes into steady erosion rates that balance the local rates of rock uplift over >100-kyr timescales. Our results suggest that proposed rates of tectonic uplift cannot explain documented rates of stream incision >0.1 mm/yr, and that proposed amounts of tectonic uplift cannot explain documented amounts of basin exhumation >500 m. Because our model predicts spatial patterns of exhumation for each forcing scenario, further constraints on the ages of landforms formed during exhumation should allow better discrimination between the proposed forcing mechanisms.