GEOPHYSICAL EVIDENCE FOR PROCESSES OF CORDILLERAN UPLIFT

The processes responsible for post-Laramide uplift of the Rocky Mountains (RM) and Colorado Plateau (CP) remain as controversial as the timing. Crustal thickening, warming of the lithosphere and/or asthenosphere, and mass changes following the cessation of Laramide subduction probably all contribute to some degree. Investigation of the regional scale uplift is complicated by localized uplift processes, particularly footwall uplift on normal faults, rift flank uplift at the boundaries adjoining extensional provinces, and residual Laramide thrust topography. However, these essentially surficial features can be removed via flexural analysis of isostatic compensation. Combined effects of the removal of subducted slab material and the associated reduction of the geoid could contribute in excess of 1000 m of the 1800-2000 m of CP-RM elevation change since Cretaceous, and stagnation of slab material at the 660 km phase boundary can delay post-subduction uplift by tens of millions of years. Isostatic mass estimates derived from seismic and other data sets suggest that about 1000 m of the current CP-RM elevation derives from anomalous crustal buoyancy, with the remainder coming from anomalous mantle buoyancy. However, uncertainties in these estimates at 95% confidence can be nearly as large as the estimates themselves. I will review the current state of understanding of CP-RM elevation based on geophysical constraints, and discuss new insights that can be derived from combined analysis of seismic and gravity data.