Crust and upper mantle velocity structure of the southern Rocky Mountains from the Jemez Lineament to the Cheyenne belt

Abstract

We have interpreted the refraction/wide-angle reflection seismic profile acquired as part of the Continental Dynamics of the Rocky Mountains project. The profile extends ∼955km from northern New Mexico to southern Wyoming, crossing the Tertiary-Quaternary volcanics of the Jemez lineament, the Paleoproterozoic Mazatzal and Yavapai terranes, the Cheyenne belt, and the southern Archean Wyoming Province. We inverted the travel-time data from the 10 shot profile with both a layer based inversion method and a tomographic method. The two techniques yield comparable upper and middle crustal velocity structures. Lower crustal velocities are well constrained in the layer based model but are not in the tomographic model. From the layer based model, velocities in the crystalline crust and the upper mantle are lower than typical for continents and for modern orogens. Lower crustal velocities rarely exceed 7.00 km/s, likely due to the regionally high heat flow. We infer that the low upper mantle velocities beneath the Jemez lineament (7.70-7.76 km/s) are indicative of upper mantle partial melt. Crustal thickness increases from south to north, with thinner crust under the Jemez lineament (40-42 km), and thicker crust under northern Colorado, the Cheyenne belt, and southern Wyoming (51-53 km). Although the Cheyenne belt outcrops as a narrow zone separating Paleoproterozoic and Archean terranes, the seismic model shows broad lateral variation in crustal velocity near the suture, and a thick crust in the northern half of the profile. Part or all of the crustal thickening is likely to have occurred subsequent to continental accretion.