Structure of the crust and upper mantle in the western United States

Abstract

Seismic waves generated by underground nuclear and chemical explosions have been recorded in a network of nearly 2000 stations in the western United States as a part of the Vela Uniform program. The network extends from eastern Colorado to the California coastline and from central Idaho to the border of the United States and Mexico. The velocity of P waves in the upper mantle ranges from 7.7 km/sec in the southern part of the Basin and Range province to 8.2 km/sec in the Great Plains province, and, in general, the velocity tends to be nearly the same over large areas within individual geologic provinces. Measured crustal thickness ranges from less than 20 km in the Central Valley of California to 50 km in the Great Plains province. Changes in crustal thickness across provincial boundaries are not controlled by regional altitude above sea level unless the properties of the upper mantle are the same across those boundaries. The thickness of the crust tends to be large or small as the P wave velocity (and presumably the density) in the upper mantle is large or small. Within the Basin and Range province, the thickness of the crust seems to vary directly with regional altitude above sea level. Evidence that rocks of intermediate P wave velocity exist in the lower part of the crust has been accumulated from seismic waves that have traveled least-time paths, as well as from secondary arrivals (particularly reflections). On a scale that includes many geologic provinces, isostatic compensation is related largely to variations in the density of the upper-mantle rocks. Within geologic provinces or adjacent provinces, isostatic compensation may be related to variations in the thickness of crustal layers. Regions of thick crust and dense upper mantle have been relatively stable in Cenozoic time. Regions of thinner crust and low-density upper mantle have had a Cenozoic history of intense diastrophism and silicic volcanism.