Abstract

Travel times and amplitude interpretation of crustal phases on the seismic refraction profiles Delta-west, Utah, in the northern Basin and Range and Lamar-west, Colorado, in the Southern Rocky Mountains-Great Plains provinces yielded new crustal P velocity-depth functions in the Western United States. The new velocity-depth functions not only confirm the broad features of the crustal structure inferred from earlier studies, but they also resolve more details of the crustal velocity variations in the region. The Basin and Range province is characterized by a thin crust (crustal thickness: 22 to 26 km), low-average crustal P velocity (∼5.8 km/sec), and a low Pn velocity (∼7.6 km/sec), with the presence of a very prominent upper crustal low-velocity layer (LVL) between 6 to 13 km depth with a minimum velocity of 5.30 km/sec. The top and bottom of this upper crustal LVL are transitional boundaries of 6.0 to 6.9 km and 9.4 to 12.9 km, respectively. There is also another LVL in the lower crust, between 20 km depth to the Moho discontinuity, with a minimum velocity of 6.05 km/sec in this region. The bottom of this lower crustal LVL is the Moho discontinuity, which is a sharp boundary in this region. There is also an indication for the presence of a discontinuity with a rather small velocity jump (7.60 to 7.65 km/sec) at a depth of 47 km in the upper mantle in the Basin and Range province. The Southern Rocky Mountains and the Great Plains provinces, on the other hand, are characterized by a much thicker crust (crustal thickness: 40 to 50 km), higher average crustal P velocity (∼6.1 km/sec), and a higher Pn velocity (8.2 to 8.4 km/sec), with the absence of an upper crustal LVL. Instead, there are prominent velocity discontinuities at depths of 10 km (6.0 to 6.2 km/sec) and 21 to 23 km (6.2 to 6.6 km/sec). A lower crustal LVL, not well-constrained by the available data, is inferred in these regions also from about 30 km depth to the Moho discontinuity, with a minimum velocity of 6.15 to 6.20 km/sec. The bottom of this lower crustal LVL is the Moho discontinuity, which may be a 1-km-thick transitional boundary. The prominent features of the crustal P velocity-depth functions derived in the present study are qualitatively consistent with those of the new model of the continental crust proposed by Mueller (1977).

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