QUANTIFYING POST-LARAMIDE EROSION ON THE COLORADO PLATEAU TO UNDERSTAND RESULTANT ISOSTATIC ROCK UPLIFT

The timing and mechanisms of rock uplift and associated high elevations of the Colorado Plateau are being actively debated, and have been conventionally attributed to either Laramide or later-Cenozoic tectonic uplift. An overlooked third source of major uplift is the isostatic response to post-Laramide and ongoing erosion of the Plateau, which is significant and can be quantified. We must first know the amount of mass removed from the Colorado Plateau since the Laramide Orogeny and its spatial distribution in order to model isostatic response. To do this we reconstruct the Eocene/Oligocene boundary and its corresponding stratigraphy and topography at 68 locations both around and within the perimeter of the Plateau. Several mark locations where Eocene/Oligocene stratigraphy is still preserved. An example is the upper Eocene Duchesne River Formation in the Uinta Basin, the depositional top of which can be used as a datum for post-Laramide topography. Similarly, the San Juan Mountains and the Aquarius Plateau have Oligocene extrusive volcanic rocks, the basal contacts of which can be used to approximate paleo-topography. These known data points aid in estimating section missing elsewhere on the Plateau, and through extrapolation as a TIN both missing section and topography can be reconstructed using GIS software. When subtracted from present-day topography this provides the spatially variable thickness of section removed. The next step is to integrate this data set into a numerical flexural model constraining the amount of post-Laramide rock uplift due to erosion. There is considerable variance in the spatial degree of erosion within the Plateau. 2,700 m of section has been removed from the axis of the Grand Canyon, whereas there are many places where no net erosion has taken place since late- Eocene time. Though the average thickness of section removed is less than might be expected, it can probably account for most post-Laramide rock uplift in the Plateau, suggesting there may be little or no post-Laramide tectonic uplift is needed.