Topography, the geoid, and compensation mechanisms for the southern Rocky Mountains

The southern Rockies of Colorado are anomalously high (elevations greater than 2800 m), topographically rough (implying active uplift), and underlain by significant low-velocity anomalies in the upper mantle that suggest an intimate relationship between mantle geodynamic processes and the surface topography. The region is in isostatic equilibrium (i.e., near-zero free-air gravity anomaly); however, the poor correlation between the high topography and crustal thickness makes the application of simple compensation models (e.g., pure Heiskanen or Pratt-Hayford) problematic. Knowledge of how the current topography of the Rockies is isostatically compensated could provide constraints on the relative role of sublithospheric buoyancy versus lithospheric support. Here we evaluate the geoid and its relationship to the topography (using the geoid-to-elevation ratio (GTR) in the spatial domain and the admittance in the frequency domain) to constrain the mechanism of compensation. We separate the upper mantle geoid anomalies from those with deeper sources through the use of spherical harmonic filtering of the EGM2008 geoid. We exploit the fact that at wavelengths greater than the flexural wavelength where features are isostatically compensated, the geoid/topography ratio can be used to estimate the depth of compensation and the elastic thickness of the lithosphere. The results presented below indicate that the main tectonic provinces of the western United States have moderate geoid/topography ratios between 3.5 and 5.5 m/km (similar to 3.9 for the southern Rockies, similar to 4.25 for the Colorado Plateau, and similar to 5.2 for the Northern Basin and Range) suggesting shallow levels of isostatic compensation. In terms of the elastic thickness of the lithosphere, our results indicate an elastic thickness of less than 20 km. These value support the notion that a major portion of the buoyancy that has driven uplift resides at depths less than 100 km and that upper mantle processes such as small-scale convection may play a significant role in the buoyant uplift of the southern Rockies (as well as other actively uplifting areas of the western United States). Further support for this hypothesis is provided by high coherence for the geoid-topography relationship for nearly all wavelengths between 50 and 1000 km.