LITHOSPHERIC STRUCTURE AND EVOLUTION OF THE ROCKY MOUNTAINS-SUMMARY OF RESULTS OF THE CD-ROM (CONTINENTAL DYNAMICS OF THE ROCKY MOUNTAINS) EXPERIMENT

The CD-ROM experiment is a multidisciplinary geological/geophysical study focused on a NNW-trending transect from Wyoming to New Mexico. Our goal is to integrate a series of coordinated seismic experiments and companion geological studies to delineate crust and mantle structure and decipher lithospheric evolution and geodynamical processes. Results indicate that the mantle, to depths of > 200 km, contains distinct velocity anomalies with dipping elements that can be traced to overlying Proterozoic crustal provinces and boundaries, and hence are interpreted to have initially developed in the Proterozoic. Crustal studies identify Proterozoic paleosubduction zones at the Cheyenne Belt, the Lester-Farwell Mountain area of the Park Range, the Colorado Mineral Belt (?), and the Jemez lineament that are related to these dipping mantle domains. This thick Proterozoic lithospheric mantle was part of the North American continent by 1.6 Ga, and has remained both fertile and weak as shown by repeated intracratonic deformational and magmatic reactivations from 1.4 Ga to present. Also, topography of the Rocky Mountains changes across Proterozoic province boundaries indicating that this architecture continues to control reactivation. Throughout much of the southern Rocky Mountains, seismic refraction data has delineated a ~10 km thick 7.xx km/sec lower crustal layer. The base of this layer (Moho) varies from ~ 40-55 km in depth. We interpret it to have formed diachronously and by a combination of processes, including original arc development and subsequent underplating of basalt. Episodic addition of basaltic magmas thickened the crust, provided thermal energy and mass for episodic crustal differentiation, and simultaneously helped stabilize the lithospheric mantle. The Proterozoic lithosphere of Colorado and New Mexico differs from lithosphere under the Archean core of the continent, possibly in thickness, but most importantly by its strongly segmented nature, long-term fertility for magmatism, and its relative weakness, expressed as a tendency to be reactivated. We suggest that this type of weak, yet fairly stable, Proterozoic lithosphere is the product of rapid subduction-accretion processes and mantle hydration during Indonesian-style crustal growth by accretion of dominantly oceanic tectonic elements.