The Rocky Mountain system constitutes the eastern side of the broad Cordilleran orogen of central North America. This chapter summarizes seismic and other geophysical data that constrain crustal thickness and structure in the region. The crustal thickness ranges from less than 40 to more than 50 km. It is thickest in southern Montana and in Colorado, and is 10 to 15 km thinner in the intervening middle Rocky Mountains. Crustal thickness along the Rio Grande rift, essentially a prong of basin-and-range structure that projects northward into the southern Rocky Mountains, is less than 35 km, and transitions from adjacent thicker crust may be abrupt. Velocities in the upper crust are generally 6 km/sec or less to depths of at least 20 km; lower crustal velocities are 6.4 to 7.4 km/sec. Minor inversions in velocity structure occur only in the upper crust. The lower crust is separated from the mantle by a transition zone several kilometers thick, except possibly in the Yellowstone Plateau and the Rio Grande rift. Upper-mantle velocities beneath the Moho are less than 8 km/sec through most of the region.
With the exception of the Basin and Range province to the west, the surrounding areas have similar crustal thickness; thus, no distinct crustal root is concentrated below the Rocky Mountains. The major crustal contrast with the Great Plains and Colorado Plateau provinces is a higher mean velocity of 6.2 km/sec of the upper crust and an upper-mantle velocity of greater than 8 km/sec east of the Rocky Mountain front. The longitudinal variation in total crustal thickness may be partly a relict from cratonization in Archean time, as suggested by the lack of change in crustal thickness across the Rocky Mountain front. Late Mesozoic to early Tertiary Laramide compressional uplifts and magmatism, widespread in the Rocky Mountain region, can be correlated with decreased crustal velocities and densities in the upper crust. Decreased rigidity of the lithosphere and permanently modified Moho structure, evident in the Rio Grande rift, possibly encroached gradually northward into the southern Rocky Mountains. The geologically diverse provinces of the Rocky Mountain region offer especially favorable opportunities to evaluate the evolution of Cordilleran crustal structure through time: some features appear to represent little-modified Precambrian craton, whereas others record overprinting and crustal modification by successive Phanerozoic tectonic and magmatic events.