Results of the CD-ROM project seismic refraction/wide-angle reflection experiment: The upper and middle crust

Abstract

During the field experiments of the Continental Dynamics Rocky Mountains Project (CD-ROM) in 1999, a 950 km long refraction / wide-angle reflection seismic profile was acquired in the southern Rocky Mountains. This 950 km long seismic line extended from northern New Mexico to central Wyoming. The sedimentary thickness along the line varies between 3 km in the North Park basin, Colorado. The Precambrian basement in New Mexico is ∼2 km deep. In Colorado and Wyoming, portions of the line are located on basement exposures intervened by basins up to 2.5 km in depth. The velocities at the top of the upper crust vary between 5.75 km/s and 6.15 km/s and increase grad-ually to (v max = 6.35 km/s) at 25 km depth. The data suggest that the uppermost Precambrian basement consists of late Proterozoic volcanic and sedimentary rocks or felsic intrusions. The northern Wet Mountains are underlain by high velocities in the upper crust, and Cambrian mafic rocks crop out in these mountains. The Colorado Mineral belt correlates with a broad zone of relatively low velocities in the Precambrian crust indicating the presence of felsic intrusions. The average thickness of the upper crust is 20 km in the Great Plains and 25 km in the Rocky Mountains region. Strong lateral velocity variations occur in the upper crust. Major vertical velocity steps are prominent at the upper to middle crust. The middle crust velocities increase from 6.60 to 6.75 km/s and its average thickness is ∼10 km.