The Laramide orogeny in the Southern Rocky Mountains has been ascribed to exceptionally low-angle subduction of oceanic plates beneath most of the western U.S., and a considerable literature has been developed upon that assumption. But there are difficulties with the geometry, physics, and timing of low angle subduction as currently envisioned. This project tests this current framework against an alternative that proposes that the Laramide orogeny was the result of interaction of a shallowing slab with thick lithosphere under Wyoming. The research focuses on three aspects of the problem: the timing and cause of the thick accumulation of sediments prior to uplift of the Southern Rockies, the timing and origin of igneous activity, and the possible connections of changes in slab geometry to tectonism in the western U.S. The resolution of the temporal development of the Late Cretaceous basin of central and eastern Wyoming and Colorado will be improved through stratigraphic and sedimentological studies. Forces acting on the lithosphere through simple plate flexure calculations will be determined. New, high resolution ages of critical igneous rocks in the region will be acquired in order to better determine the timing of intrusive activity relative to changes at the plate margin and examination of the geochemistry of these rocks to will determine the relative role of hydration (arc-style magmatism) vs. decompression melting (back-arc style magmatism). Numerical experiments will allow quantification of the forces acting on the lithosphere for different kinds of slab geometries and also explore the possible relationship of secondary convection systems on continental deformation and magmatism.
During the period from about 75 to 45 million year ago a former inland sea was transformed into the Rocky Mountains over an area extending from New Mexico to Montana. Among the effects of this mountain building event (the Laramide Orogeny) are the creation of most of the major mineral deposits in Colorado, creation of the large oil-shale deposits in western Colorado, northeastern Utah, and southwestern Wyoming, development of many of the natural gas fields now being exploited in the region, and creation of conditions that continue to deform the region, leading to earthquake hazards in central Colorado and geothermal resources in much of the western U.S. It is one of the finest and most cryptic examples of mountain building far from a tectonic plate boundary. Geologic understanding of the Rocky Mountain region thus depends upon improved knowledge of the nature and origin of this mountain building event. This study focuses on the most unexplained aspects of the event to best constrain its past history and present behavior. Successful conclusion of this work should greatly improve the understanding of similar events worldwide, some of which might have also produced resources that remain undiscovered owing to the unusual nature of such events.
The origin of the Southern Rocky Mountains is a geological enigma widely explained as the result of a shallowly dipping piece of subducting seafloor (a "flat slab") scraping against the bottom of North America. This makes predictions that are not confirmed by observation: for instance, our work has shown that the uppermost mantle under the Rockies that was present before the creation of the Rockies starting about 70 million years ago was still present only 9 million years ago, an observation disagreeing with predictions of some "flat slab" models. Instead we suggest that viscous stresses in the deeper mantle were created as the subducting ocean floor shallowed somewhat and pulled down an unusually thick part of North America: the southern edge of Wyoming, one of the oldest pieces of the continent. This produced a basin filled with great thicknesses of marine muds (the Pierre Shale). By pulling down this area, stresses in the crust developed as the crust tried to flow into the basin. We have shown in this project that the orientation of these stresses is as consistent or more consistent with geologic observation than the "flat slab" model. Numerical simulations of such viscous stresses in the mantle indicate that complex deformation of the continent might extend away from the major basin, a prediction that might help constrain the viscosity of the mantle and deep continental crust. By better understanding the forces creating this mountain belt well in the interior of the continent, we learn more about how other such mountains are created as well as the different ways that continents interact with subducting ocean floor.
