Structure and evolution of the lithosphere beneath the rocky mountains: Initial results from the CD-ROM experiment

An integration of new seismic reflection, seismic refraction, teleseismic, and geological data provides insights into the nature and evolution of the lithosphere along a transect extending from Wyoming to New Mexico. Perhaps the major issue in interpreting the seismic data is distinguishing lithospheric structures that formed during Precambrian growth and stabilization of the continent from those that record Cenozoic tectonism. Tomographic data show that the upper mantle, to depths of >200 km, contains several dipping velocity anomalies that project up to overlying Proterozoic crustal boundaries. Our integrated studies define crustal sutures that are congruent with the dipping mantle domains, and we interpret these crust and mantle features as the signatures of Proterozoic paleosubduction zones. Proposed sutures are the Cheyenne belt, Lester-Farwell Mountain area of northern Colorado, and Jemez lineament. The resulting thick Proterozoic lithosphere was part of North America by 1.6 Ga, and has remained both fertile and weak as shown by repeated deformational and magmatic reactivations from 1.4 Ga to present. Proterozoic lithosphere of Colorado and New Mexico differs from lithosphere beneath the Archean core of the continent, possibly in thickness but most important by its strongly segmented nature, its longterm fertility for magmatism, and its relative weakness, expressed as a tendency to be reactivated. Throughout much of the southern Rocky Mountains, seismic refraction data have delineated a 10-15 km thick, 7.0-7.5 km/s mafic lower crustal layer. The base of this layer (Moho) varies from 40 to 55 km in depth. We interpret it to have formed diachronously and by a combination of processes, including original arc development and subsequent magmatic underplating, and to be the product of progressive evolution of the lithosphere.