TOP-DOWN OR BOTTOM-UP? INTRAPLATE DEFORMATION IN THE WESTERN NORTH AMERICAN INTERIOR RECORDED AND CONTROLLED BY LITHOLOGY

Both the Himalayan system and the western United States feature high topography > 1000 km from the plate margin, ongoing intraplate extension, and broad regions of high elevation/low relief topography lacking extensive crustal deformation. The passage of the Transportable Array across the US provides a tool for understanding such features. Joint analysis of seismic, gravity, topographic, and heat flow data shows that crustal/upper mantle density both influences modern deformation and records tectonic history. An example of each is presented here. Extension at the eastern margins of the Himalaya collocates with several kilometers of topographic relief, but in the western US the topographically highest southern Rockies exhibit minimal geodetic strain while the Northern Basin and Range extends at rates of order 10^-15/s: at least an order of magnitude higher than plausible there from plate boundary stresses. A 3D density model to depths of 150 km shows that the contrast at ~25 to 55 km between denser mantle beneath the Basin and Range and buoyant crust beneath the Rockies leads to a 1-2 TN/m difference in gravitational potential energy, sufficient to explain modern deformation. The second example explores Cenozoic uplift of the Colorado Plateau to ~2 km and Great Plains to >1 km in the absence of pervasive crustal shortening and far from collisional boundaries. Thermal models of the upper mantle show that thinning of the mantle lithosphere beneath the Colorado Plateau is responsible for 1-1.5 km of uplift there, while several km of Cenozoic sediment on the Plains lends ~500 meters of surface uplift. The remaining 0.5-1 km of uplift of both regions can be understood from crustal xenoliths that record hydration-induced retrogression of garnet-bearing assemblages at ~60­­–90 Ma, roughly coincident with emplacement of the Farallon slab beneath the region. Compared to the stable Proterozoic interior of North America, bedrock densities in the upper crust (to ~20 km) in the Plains/Plateau are similar, but a pervasive ~50 kg/m³ difference—less dense in the West—suggests lower crustal hydration as a viable mechanism of widespread uplift. Taken together, these examples highlight that both internally-derived stress and interaction with underlying material are potential causes of intracontinental deformation.