Geophysical Investigations of the Origins and Effects of Density Variations in the Crust and Upper Mantle Beneath the Western and Central United States

Variations in density within the earth are the dominant cause of both surface topography--generating mountains, valleys, and plateaux--and convection, leading to plate tectonics. Density varies as a function of chemistry, mineralogy and temperature, containing information about physical state and history. I develop methods to estimate the density of the crust and upper mantle from seismic, gravity, heat flow, and topographic data. Decomposing density variations into thermal and compositional components provides insight into the origin of topography, tectonic history, and active processes. These techniques are applied to the inspiring landscapes of the western United States. The modern density structure of the Sierra Nevada, California, suggests that post-Miocene range uplift occurred in response to removal of dense mantle lithosphere. A numerical model of the flexural response of the surface to mantle loads shows that this material is likely now found in the upper mantle just west of the range, where it has created the Tulare basin. A broader density model of the entire western U.S. highlights a dichotomy in upper mantle buoyancy between the low-relief Great Plains and regions modified in the Cenozoic. Relief within the Cordillera is generated by varying degrees of crustal thermal and compositional buoyancy. A targeted thermal modeling study of the Colorado Plateau shows that ~2 km of Cenozoic uplift--in the absence of crustal shortening--can be ascribed to removal of tens of km of mantle lithosphere and related hydration of the lower crust. Overall, these four studies highlight the utility of density as a window into tectonic processes and a record of lithospheric history