ISOSTATIC SUPPORT OF THE COLORADO PLATEAU

The Colorado Plateau underwent up to 2 km of rock and surface uplift from near sea level in Late Cretaceous. Neither the mechanism nor the timing of the uplift is well constrained. The buoyancy required for the uplift must be provided by either crustal thickening and/or mantle expansion, which have very different tectonic implications and require in quite different depths of compensation. Seismic and Bouguer gravity studies have not definitively resolved the density structure under the Plateau. However, geoid anomalies are almost linearly sensitive to compensation depth. At long wavelengths, the geoid anomaly is directly proportional to the vertical dipole moment of the density distribution. For example, there is an approximately +8 m geoid offset between zero-elevation continental crust and old oceanic lithosphere. For long-wavelength continental topography, the geoid anomaly is proportional to the elevation times the depth of compensation. Thus, the greater the average depth of the isostatic "root", the larger the geoid anomaly. The main problem with 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. Here spherical harmonic filtering is used to remove wavelengths greater than 2500 km. The Colorado Plateau has an average geoid/topography ratio of 6 m/km. This small value favors isostatic compensation of Colorado Plateau topography at depths around 50 km, matching approximate Moho depths, and certainly shallower than asthenospheric depths of 80 km. This conclusion of shallow compensation allows crustal thickening as the source of the buoyancy that drove uplift of the Colorado Plateau. Mechanical or thermal lithospheric thickness changes may also contribute to the buoyancy that drove the uplift, but successful models including thermal thinning of the lithosphere still require some 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. Because the Plateau's elevation was near sea level for all Phanerozoic time before the end of the Mesozoic, the only viable alternative to crustal thickening is loss of an ancient dense and deep lithospheric root in Late Cretaceous.