Abstract

The Rocky Mountains have fascinated the geological community for over 100 years, but crustal-scale geophysical studies are relatively rare in this region. However, a knowledge of crustal structure is essential if we are to fully understand the region's tectonic history. Thus, we have compiled and synthesized existing information on crustal structure in order to provide as complete a picture as possible at this time. We have focused on Wyoming, Colorado, and New Mexico where there are enough data to make useful correlations with geologic features. The Rocky Mountain region includes the crest of a broad uplift on which the Southern Rocky Mountains are located. In turn, the Southern Rocky Mountains are bisected by the Rio Grande rift. In the Rio Grande rift, distinct crustal thinning (at least 5 km relative to adjacent areas) can be documented from Albuquerque, New Mexico southward. The area of thinned crust widens southward, as does the physiographic expression of the rift. The thinnest crust documented to date (about 28 km) is found west of El Paso, Texas. In contrast with East Africa, the crustal thinning is gradual from the rift valley to the shoulders, perhaps reflecting the back-arc thermal regime that existed prior to rifting. The thickest crust in the region (about 53 km) appears to be associated with both the Southern Rockies in Colorado and the topographically lower Great Plains in Colorado and New Mexico. This lack of correlation between topography and crustal thickness implies that the mantle is playing a major role in the attainment of isostatic balance in this area. Magmatic modification of the crust during rifting appears to have been minor. However, the modification due to the voluminous mid-Tertiary magmatism in the Datil–Mogollon volcanic field (southwestern New Mexico) and San Juan volcanic field (southwestern Colorado) is substantial. In the Datil–Mogollon field, a batholith that accounts for about one fifth of the crustal thickness has been detected in the upper crust, and a feature of similar dimensions is indicated in the San Juan region. There is evidence of the crust thinning northward from Colorado into Wyoming, which could be a relic of Archean rifting of the southern margin of the Wyoming craton.

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