THE CD-ROM EXPERIMENT: 4-D IMAGE OF THE LITHOSPHERE BENEATH THE ROCKY MOUNTAINS AND A MODEL FOR GEODYNAMIC INTERPLAY BETWEEN ANCIENT COMPOSITIONAL HETEROGENEITY AND MODERN ASTHENOSPHERIC PROCESSES

The CD-ROM experiment produced a geophysical/geological cross section of the crust and upper mantle of the southern Rocky Mountain region from Wyoming to New Mexico. This region preserves an interaction between old structures and young driving forces and is a natural laboratory for exploring geodynamic processes that form and modify continents. Integrated geologic studies added the time dimension resulting in better understanding of evolution of the lithospheric plate during processes of assembly and stabilization (1.8- 1.6 Ga), intraplate reactivation (1.4 Ga, 1.1 Ga, 0.8 Ga, 70 Ma), and Cenozoic and ongoing heating and thinning of the plate. Combined seismic reflection, refraction, and teleseismic data reveal Paleoproterozoic paleosuture zones at the Cheyenne belt, Colorado mineral belt, and Jemez lineament, with crustal geometries showing interwedging of terranes, and mantle geometries suggesting long-lived subduction scars in a thick (> 200 km) chemical lithosphere. Regional block diagrams from 2-D teleseismic sections (Deep Probe, RISTRA, CD-ROM, Snake River Plain) suggest that today's mantle velocity domain boundaries are sharp, tabular, and dipping, correspond with ancient suture boundaries, and extend to >200 km depth. These mantle velocity variations are interpreted to represent blocks of Proterozoic lithosphere with original differences in bulk composition, thickness, and hydration that have been activated differently by Laramide hydration and Cenozoic (and ongoing) asthenospheric upwelling. Observed variable thickness mafic high velocity lower crustal, 10-km-scale Moho topography, and xenolith data are interpreted to record upward- transfer of basaltic melt and resulting mafic underplating during lithosphere formation as well as several episodes of intraplate basaltic underplating (1.45-1.35 Ga, ~1.1 Ga, and Cenozoic). Cenozoic changes in lithospheric buoyancy have resulted in epeirogenic uplift such that the highest elevation region of the Rocky Mountains may be directly responding to active mantle tectonism as well as to persistent buoyancy of mantle compositional provinces. As a new geodynamic process, we infer that reactivation of old structures can induce material and energy exchange across the base of the lithosphere and thus influence asthenospheric circulation.