THE SEISMIC STRUCTURE OF THE SOUTHERN ROCKY MOUNTAINS AND THE COLORADO PLATEAU AND ITS TECTONIC SIGNIFICANCE

The tectonic history of the southwestern United States dates to formation of this part of North America in the Paleoproterozoic, ~1.8-1.9 Ga, when a succession of island arcs accreted to the southern margin of the Archean Wyoming province, leaving a pronounced NE-SW structural grain in the entire lithosphere. Phanerozoic tectonism has largely occurred along a N-S axis, paralleling the western margin of the continent, and has included formation of the Paleozoic ancestral Rocky Mountains, the Mesozoic Sevier fold and thrust belt, and the Cenozoic thick-skinned Laramide uplifts, Basin and Range and Rio Grande Rift extension, and episodes of voluminous volcanism. The mid-Mesozoic through modern history is largely attributed to processes associated with subduction of the Farallon plate beneath North America, with the Laramide uplifts resulting from flat slab subduction, and the vast regions of extension due to subsequent Farallon slab removal and development of the San Andreas transform system.

Seismic investigations are beginning to unravel a complicated crust and upper mantle structure that is a composite of Archean, Proterozoic, and Phanerozoic influences. The structure of the lithosphere and upper mantle of the Southern Rockies and Colorado Plateau, as determined by recent experiments and regional and global tomography, is highly heterogeneous as a result of ancient and modern tectonism: The Colorado Plateau, which escaped most of the Mesozoic-Cenozoic mountain building around it, has an old lithosphere at least 100 km thick. Its ~2 km uplift is attributed to asthenospheric replacement of the sinking Farallon slab, and is therefore thermally driven. Rocky Mountain upper mantle velocity anomalies along NE-SW trends represent a combination of chemically buoyant Proterozoic slab fragments and upwelling Cenozoic upper mantle, with isostatic support of the Rockies thus provided in the upper mantle both thermally and chemically. The Wyoming craton has a shallow, to ~100km depth, lithosphere that is tectospheric in nature, with its elevations supported by upper mantle chemical buoyancy. The deeper structure is not tectospheric, implying decratonization processes whose ages are uncertain. Long term lithospheric heterogeneity as well as Cenozoic plate behavior appear to control the location of much Cenozoic volcanism.