CAUSES FOR CENOZOIC UPLIFT OF THE WESTERN U.S

Thermal processes such as lithospheric thinning or plume emplacement often are evoked to explain vertical tectonics. But most western US vertical motion in the past 100 m.y. appears to be related to other processes. Slab flattening prior to the Laramide orogeny would pull the continent down as slab replaces asthenosphere and mid-Tertiary slab removal would cause protracted (ongoing) uplift as slab sinks into the lower mantle. Flat-slab subduction would hydrate the lithosphere, creating low-density minerals beneath the Great Plains to the Colorado Plateau, and magmatism leaves a depleted residuum that below 70 km is buoyant. Delamination of eclogitic plutonic roots is uplifting the southern Sierra Nevada and appears to have been responsible for Willowa Mountains uplift and perhaps the related Columbia River flood basalt eruptions, whereas eclogite roots have stabilized unusually thick crust beneath the Cheyenne Belt suture. Unlike thermal processes, these compositionally-related buoyancy mechanisms are long lived.

There are increasing amounts of evidence for young and ongoing western US uplift. Resolving which buoyancy mechanisms are actually responsible among the many that are possible is one of the primary challenges before us. Currently, we do not know if the primary processes are magmatic or tectonic, or if they have been recently imposed from great depth (i.e., a plume) or are an aftermath of the Laramide orogeny.