History and causes of post-Laramide relief in the Rocky Mountain orogenic plateau

Abstract

The Rocky Mountain orogenic plateau has the highest mean elevation and topographic relief in the contiguous United States. The mean altitude exceeds 2 km above sea level and relief increases from 30 m in the river valleys of the Great Plains to more than 1.6 km deep in the canyons and basins of the Rocky Mountains and Colorado Plateau. Despite over a century of study, the timing and causes of elevation gain and incision in the region are unclear. Post-Laramide development of relief is thought to either result from tectonic activity or climatic change. Interpretation of which of these causes dominated is based upon reconstruction of datums developed from, and supported by, paleoelevation proxies and interpretations of landscape incision. Here we reconstruct a datum surface against which regional incision can be measured in order to evaluate late Cenozoic tectonic and climatic infl uences. The distribution, magnitude, and timing of post-Laramide basin fi lling and subsequent erosion are constrained by depositional remnants, topographic markers, and other indicators across the region. We suggest that post-Laramide basin fi lling resulted from slow subsidence during Oligocene to Miocene time. Incision into this basin fi ll surface began in late Miocene time and continues today. The pattern of incision is consistent with control by localized extensional tectonism superimposed upon regional domal surface uplift. Localized extension is associated with the projection of the Rio Grande Rift into the central Rockies, and the domal uplift generally coincides with the position of buoyant mantle anomalies interpreted at depth. If the magnitudes of incision directly refl ect magnitudes of surface elevation gain, they are less than can be resolved by existing paleoelevation proxy methods. In addition, the combination of post-Laramide subsidence followed by regional surface uplift reduces the net magnitude of surface elevation change since Laramide time.