CRUSTAL EVOLUTION OF THE SOUTHERN ROCKY MOUNTAIN REGION

The evolution of continents involves modification of the lithosphere by many processes resulting in a crustal column that changes in thickness creating a dynamic crust/mantle boundary (Moho). The southern Rocky Mountain region is a particularly interesting area to study continental evolution because it has experienced 1.8 Ga of crustal growth and modification that is relatively well known geologically, but until recently, modern seismic data were sparse. Analysis of the CD-ROM seismic refraction, deep reflection, and teleseismic data show that the crustal thickness is ~50 km along much of the transect, which is thick compared to the global average continent. Another consideration is that the Precambrian rocks exposed today were once at depths of about 10 km in Colorado and about 20 km in northern, New Mexico. The seismic refraction data show clear evidence of high-velocity lower crust, and this layer is apparently widespread being observed in many areas of the mid-continent region east of the Rocky Mountains and in the Colorado Plateau. Using the word “underplating” to refer to the complex processes whereby mantle material or its derivatives are added to the lowermost portion of the crust, this layer seems best interpreted as representing such material. There was considerable magmatism that was in part arc-related during the period of continental assembly (1.7-1.6 Ga). From the Great Lakes region westward, the period around 1.1 Ga involved considerable extension and magmatism and the upper crust in southeastern New Mexico was extensively intruded by mafic material. However, there is no evidence to suggest that significant magmatic activity extended far beyond the limits of the rifted regions. The final event of the Precambrian lithospheric assembly that led to the formation of Rodinia was the Grenville orogeny. Phanerozoic tectonism produced many examples of significant crustal modification but none seem widespread enough to produce the thick, mafic crust that is observed over an extensive area. We do not doubt that other events added to this layer or modified it, but 1.4 Ga, when magmatism was widespread, appears to be the best time subsequent to the original formation of the crust to form it. Preserving this layer through the subsequent tectonic events may not be unusual based on recent results from Baltica.