Crustal thicknesses in the Intermontane system—Colorado Plateau, Basin and Range province, and High Lava Plains of the Cordilleran region—have been reliably determined by an extensive network of seismic-refraction profiles as about 30 km in the Basin and Range and 42 km in both the Snake River Plain and the Colorado Plateau. The upper crust (velocity, 5.2 to 6.3 km/sec) averages about 20 km in thickness in the Basin and Range, 10 km in the Snake River Plain, and 28 km in the Colorado Plateau. The velocity in the lower crust ranges from 6.4 to possibly 7.5 km/sec. Upper-mantle velocities of 7.4 to 8.0 km/sec have been reported, but the lower Pn velocities are probably apparent downdip velocities. The true upper-mantle velocity throughout the Intermontane system is probably 7.8 to 7.9 km/sec.
Moho depths from seismic-reflection studies are about the same as those from refraction studies in the Basin and Range, but they are 5 to 10 km deeper in the Colorado Plateau. The patterns of crustal reflections are distinctly different in the western and eastern parts of the Basin and Range.
Geophysical studies indicate that the lithosphere is 50 to 65 km thick in the Basin and Range, 50 km in the Snake River Plain, and 90 to 100 km in the Colorado Plateau.
Bouguer gravity values range from -50 mGal in southwestern Arizona to -250 mGal in the Uinta basin of northeastern Utah. Gravity correlates inversely with topography in the Basin and Range, indicating isostatic equilibrium, but the Snake River Plain is slightly undercompensated. Long-wavelength gravity anomalies indicate that much of the isostatic compensation results from a mass deficiency in the mantle, probably related to variations in the thickness of the lithosphere. Prominent magnetic-high anomalies in the Snake River Plain and north-central Nevada result from mafic rock formed in Miocene rifts.
Heat flow is high throughout the Intermontane system, ranging from 1.5 heat-flow units (HFU) in central Nevada and the Colorado Plateau to more than 2.5 HFU in northwestern Nevada and the Snake River Plain. Electrical conductivity is anomalously high in the lower crust and upper mantle of the system.
Seismicity is distributed in broad bands in the Nevada and intermountain seismic zones, and epicenters commonly do not coincide with mapped faults; the larger earthquakes (magnitude greater than 6.0), however, occur only on planar, normal faults with dips of 50 to 60°. The stress field is compatible with lateral extension in the Basin and Range.
Examination of evidence for a “double Moho” in Utah suggests that the apparent velocities associated with the Moho propagation paths on which this model is based vary widely, indicating severe relief on the refractor surface; the evidence favors a single Moho, below which the velocity is 7.8 to 7.9 km/sec. Ray tracing and amplitude studies leave unresolved the question of whether a transitional layer of velocity 7.5 km/sec exists in the lower crust of the Basin and Range.
Many geologic and geophysical characteristics are similar throughout the Intermontane system, but lithospheric models are distinctly different in the separate provinces, owing to the different geologic processes that have prevailed. The crust throughout the system prior to Cenozoic extension and volcanism probably consisted of an upper layer of silicic gneisses and schists overlying a lower crust of silicic to intermediate composition in the granulite facies. During extension, the crust of the Basin and Range was thinned and invaded by gabbroic magma from the mantle, resulting in a lower crust consisting of as much as two-thirds mafic material. During crustal rifting or extension in the Snake River Plain, the crust was extensively invaded by mafic magma from the mantle, and a silicic fraction was fused and erupted as rhyolite. The Snake River Plain crust is now predominantly mafic, and the 10-km-thick upper crust consists primarily of sedimentary and volcanic rocks.
Resolution of remaining unsolved problems must await thorough reanalysis of available data and new field experiments based on coincident application of different geophysical methods, combined with geologic studies and deep drilling.