Collaborative Research: The Cenozoic History of the Western US – A Data Integration Strategy for Earthscope

We propose to develop a model for the physical structure of the crust and mantle beneath the continental western US and to use this model to improve our understanding of the geologic events responsible for its formation. To do so, we will simultaneously model available seismic, gravimetric and topographic data in order to constrain the isotropic and anisotropic seismic and density structure of the mantle.

There are two parallel themes in this proposal. The first, of course, is to understand more fully the structure of the mantle and its relation to process. The second, which defines an important new direction for geophysics, is the simultaneous inversion of geophysical data of very different signature. Both topics serve as much needed developments to prepare us for the voluminous flow and integration of new and diverse data to be expected from the EarthScope observatory.

The upper mantle seismic contrast between the stable craton and the tectonically active and elevated western US is among the largest to be found (e.g., Grand, 1994. This contrast is actually more highly localized than global tomograms suggest, and is reflected in abrupt contrasts in crustal structure as well as mantle structure. The western US represents an area of continental growth and modification across the truncated margin of Precambrian lithosphere that was created about 500 Ma by continental rifting (Karlstrom and Humphreys, 1998). Continental construction was dominated by subduction processes, which culminated with the Laramide orogeny about50 Ma, and the intensity of Laramide orogeny generally is attributed to rapid subduction of the Farallon slab beneath North America. Starting with the Laramide and continuing through the Cenozoic, the US portion of the Cordilleran orogen has remained unusual for its width, nature of uplift, and style of tectonic and magmatic activity. Rocky Mountain tectonism, widespread vigorous post-Laramide volcanism, uplift and extension of the western US are important geologic events occurring within the interior of North America and with no obvious causes. Yet these processes clearly have controlled the strength, fertility and density structure of this highly modified lithosphere.