A PLAINS ATTACK ON THE CORDILLERA

The western U.S. is a menagerie of thrust- and normal-fault-bounded mountains, high plateaus, lava plains and sedimentary basins. Underlying all of this is a very wide, fairly high region that seems to call out for a more unified origin than the complex surface geology suggests. Uncertainty in many of the impacts of known geologic events has clouded our ability to discern this broader origin, if indeed such an origin exists. We discuss some results from two different approaches to understanding the origins of topography in this region to try to identify any elements common to uplift in the west: a surface-based approach to removing what we know has happened, and a seismological approach to try and constrain what is there today. The first, discussed more in a separate presentation by Bogolub et al., removes sedimentary and modern flexural loads to see what topography remains. Although results in mountains are very uncertain in our early analysis, the western Plains looks to need 900-1000m of support beyond that of flexure and sedimentation. The second approach converts seismic velocity structures to density variations over a greater range of latitude. For this, we ignore the thrust-bounded mountains and focus on the more uniform and less deformed Plains and Colorado Plateau. The western Plains looks to be fully supported by variations in density within the crust and possibly the very uppermost mantle. Similar to our conclusion from the other approach, some 800-900m of support comes from levels below the surficial sedimentary rocks. We find that the seismological results indicate that support comes from above ~55 km depth; a very similar amount helps support the Colorado Plateau, though the Plateau also benefits from ~400m of thermally supported elevation. Given the similar values, it seems likely that the Southern Rockies in Colorado and New Mexico might also have benefited from a similar source of support within or at the base of the crust, at least up until substantial modification by rifting processes in the past c. 20 Ma. Working in from the least complex parts of the orogeny will, we hope, tighten the bounds on any other mechanisms operating within the interior of the Cordillera.