Seismic Refraction Study of Crustal Structure in the Western United States
A network of 64 seismic-refraction profiles recorded by the U.S. Geological Survey in California and Nevada and adjacent areas of Idaho, Wyoming, Utah, and Arizona from 1961 to 1963 was re-interpreted. From record sections compiled for all profiles, a basic travel-time diagram can be derived. In addition to the first arrivals on profiles in the Snake River Plain, the northern Basin and Range province, and the middle Rocky Mountains, two dominant phases can be correlated in secondary arrivals, whereas the profiles in other areas show only one dominant phase in later arrivals. Based on velocity-depth functions calculated for each profile after the method of Giese, the crustal structure of the western United States is presented on contour maps and on a fence diagram that is composed of 15 crustal cross sections. Crustal thickness reaches maxima under the Sierra Nevada (42 km), the Transverse Ranges of southern California (37 km), and in southwestern Nevada (36 km), whereas the crust is relatively thin under the Coast Ranges of California (24–26 km), under the Mojave Desert (28 km), and under parts of the central Basin and Range province in Nevada and Utah (29–30 km). The base of the crust dips generally from the Basin and Range province toward greater depths in the Colorado Plateau (43 km), the middle Rocky Mountains (45 km), and the Snake River Plain (44 km). The upper-mantle velocity is less than 8.0 kmps under the Great Basin of the Basin and Range province, the Sierra Nevada, and the Colorado Plateau, but it is equal to or greater than 8.0 kmps under the Coast Ranges of California, the Mojave Desert, and the middle Rocky Mountains. Velocity inversions within the upper crust are indicated under the southern Cascade Mountains and the middle Rocky Mountains, but not under the Sierra Nevada. The average velocity of the upper crust beneath the Basin and Range province is 6.1 to 6.2 kmps to a depth of 15 to 20 km. Only beneath the middle Rocky Mountains, the Snake River Plain, and the northern part of the Basin and Range province can a boundary zone between upper and lower crust be determined confidently.