DYNAMIC UPLIFT OF THE COLORADO PLATEAU- ROCKY MOUNTAIN REGION DRIVEN BY ASTHENOSPHERE-LITHOSPHERE INTERACTIONS

Cenozoic surface uplift of the Colorado Plateau and Rocky Mountain region involved an unresolved mixture of: A) Laramide-age uplift driven by hydration of lithosphere above the low angle subducting Farallon slab, B) mid-Tertiary age uplift driven by replacement of the Farallon slab with upwelling asthenosphere, and C) post- 10 Ma and ongoing uplift driven by small scale mantle convection. Each involved advection of melt and fluids across the lithosphere-asthenosphere boundary, not simple heating of lithosphere. Geologic evidence supports models for polyphase uplift (A+B+C) of the western U.S., with surface response in different subregions reflecting different components (A,B,C) superimposed on inherited structures. At subregional scale: 1) the Aspen low velocity anomaly of central Colorado is mainly the result of B+C acting on the 1.4 Ga Colorado Mineral belt structure; 2) the Rio Grande rift is mainly the result of B+C imposed on an earlier (1.0- 0.8 Ga) structure and is producing topographic structure similar to mid-ocean ridges (Alvarado Ridge); 3) uplift of the southwestern Colorado Plateau reflects A (about 1 km) plus ongoing differential uplift driven by ongoing edge-driven convection across a sharp mantle velocity boundary (C; producing an additional ~0.5 km of uplift. Our geodynamic models introduce small scale mantle convection, formation of lithospheric drips, and delamination of lithosphere and translate modeled temperature structures into synthetic seismic wave velocities that match well with available tomography. Geologic evidence is used to parse contributions of A,B, and C in each subregion. Focusing on ongoing dynamic components (C ): 1) slope-area comparisons of the Green and Colorado River systems indicate that the Colorado is steeper than the Green for a given drainage basin area, with the Colorado having higher discharge, compatible with ongoing surface uplift above of the Colorado headwaters (Colorado Rockies) relative to the Green; 2) low mantle velocities associated with the Rio Grande rift and Jemez lineament produce the modern high heat flow, magmatism, and upwarping of stream profiles, 3) differential incision of Grand Canyon, active fault slip across the CP- BR boundary, and the observed eastwards sweep of magmatism reflect ongoing small scale mantle convection and surface uplift of the western edge of the Colorado Plateau.