Colorado Plateau: Geoid and Means of Isostatic Support

Abstract

Abstract The Colorado Plateau underwent 1.5 to 2 km of surface uplift from near sea level during Late Cretaceous time. Neither the mechanism nor the timing of the uplift are well constrained. Knowledge of how the current topography of the Plateau is isostatically compensated could provide direct constraints on the mechanism, and perhaps indirect constraints on the timing, of uplift. Crustal thickening and/or decreases in mantle density must provide the buoyancy required for the uplift. These two modes have very different tectonic implications, and result in quite different average depths of the compensating mass deficits, potentially detectable with geoid anomalies. Geoid anomalies caused by long-wavelength continental topography are proportional to the elevation multiplied by the mean depth of compensation. Thus, for a particular elevation, the greater the average depth of the isostatic "root," the larger the geoid anomaly. The main complication in analyzing geoid data for this kind of problem is separating lithospheric geoid anomalies from those with deeper sources, especially those that arise in the lower mantle. The anomalies shown here were obtained by high-pass spherical harmonic filtering of the order-360 EGM96 geoid, with a cosine taper between orders 7 and 11. Taking the point-by-point ratio of filtered geoid to topography at 0.25° resolution, the Colorado Plateau (and the rest of the western United States, except for Yellowstone and the Snake River Plain) shows a moderate geoid/topography ratio of 6-8 m/km. The small amplitude of these geoid anomalies favors isostatic compensation of Colorado Plateau topography at depths of around 50 km, matching approximate Moho depths, and certainly shallower than asthenospheric depths of 80 km or greater. Mechanical or thermal lithospheric thickness changes may also contribute to the buoyancy that drove the uplift, but models involving strictly thermal thinning of the lithosphere still require crustal thickening. Some models that involve crustal thickening acting alone succeed in matching the elevation and geoid anomalies over the Colorado Plateau. Most plausibly, such thickening would have been Laramide in age. The only viable alternative to shallow compensation is mechanical loss of an ancient dense, deep lithospheric root in Late Cretaceous time, because the Plateau's elevation was near sea level for all of the Phanerozoic before the end of the Mesozoic.