COMPOSITIONAL CONTROL OF WESTERN U.S. MAGMATISM, TECTONISM AND UPLIFT

Tectonic and magmatic activity across the western US during the last 100 m.y., including its high (and rising) elevations, inland Laramide orogeny and post orogenic collapse, ignimbritic flareup and hotspot initiation and propagation, have acted on a variable thickness lithosphere (up to 200 km thick) composed of Archean, Proterozoic and Phanerozoic domains. This recent and ongoing activity affords exceptional opportunity to study the processes of continental creation and fabrication and to discover what is fundamental about these processes. I will summarize some of this history from a geodynamical perspective with an emphasis on lithospheric composition. Composition is both a record of lithospheric magmatic and tectonic activity and a primary control on its behavior. For instance, the mantle west of the Colorado Plateau, created relatively fertile during post-rift cooling about 500 Ma, was the magmatically productive mantle of the ignimbrite flareup and now that being extended across the Basin and Range; it is depleted and strong Archean lithosphere and thick high-velocity lower crust that underlies the dispersed Laramide uplifts in Wyoming, whereas Proterozoic lithosphere accommodated a more confined uplift pattern in Colorado and New Mexico; Laramide-age hydration of the uplifted continental interior may have been essential to its Cenozoic magmatic and tectonic activity; and, Yellowstone initiated along the section of rifted continental margin that has the greatest lateral contrast between Precambrian North America and accreted terrain, and the intense Columbia River basalt eruptions may have been initiated by eclogite-driven delamination of the lithosphere beneath the Willowa plutonic system. Thus composition appears to have played a key role in controlling the magmatic, thermal and mechanical behavior the western US interior during the Cordilleran orogeny.