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Crustal structure of the eastern Basin-Range province and its transition to the Colorado Plateau-Rocky Mountain provinces has been influenced by several tectonic events: Precambrian margin rifting, Paleozoic sedimentation, late Mesozoic and early Cenozoic crustal compression and related thrust faulting, and middle to late Cenozoic extension and normal faulting. As a result of this complex history, the crust and upper mantle of this region is laterally heterogeneous across its 500-km east-west breadth.

The crust is ~30 km thick beneath the central Basin-Range and increases eastward across the transition to more than 40 km beneath the Colorado Plateau and middle Rocky Mountains. Despite the lateral variation in crustal thickness, the upper mantle has a generally uniform P-wave velocity of 7.9 km/sec. On the basis of surface-wave analyses, the lithosphere is estimated to be 65 km thick beneath the central Basin-Range and increases eastward to 80 km beneath the Colorado Plateau. An anomalously low, apparent velocity layer (~7.5 km/sec), at a depth of ~25 km was identified in the Basin and Range-Colorado Plateau transition from unreversed refraction profiles. This layer has been interpreted as a high-velocity zone at the top of a mantle bulge; it may, however, represent an ~10-km mantle upwarp of 7.9-km/sec upper-mantle material with the low apparent velocities resulting from down-dip ray propagation.

Seismic reflection profiling in the Basin and Range-Colorado Plateau-Rocky Mountain transition has been used principally to assess the geometry and structural style of Cenozoic basins and upper crustal faults. These seismic data reveal several asymmetric, eastward-tilted basins that are bounded by low- to high-angle (30° to 60°), planar to listric normal faults. An unusually widespread, 10° to 15° west-dipping reflection, identified as the Sevier Desert detachment, has been detected across a 190-km east-west width from near the surface in central Utah, westward to a depth of ~15 km near the Utah-Nevada border. This major structure extends over an area of 20,000+ km2 and may have accommodated as much as 60 km of late Cenozoic crustal extension.

The Basin and Range-Colorado Plateau-Rocky Mountain transition is also coincident with the southern Intermountain seismic belt where diffusely distributed epicenters occur along a 100- to 200-km-wide, north-south-trending zone. Where precise hypocenters have been mapped with detailed seismograph networks, background seismicity does not in general correlate with mapped faults. Source studies of three historic M7+ earthquakes in the Basin-Range have provided a hypothetical working model for large normal-faulting earthquakes in the region. These large events occurred on 40° to 60°

planar normal faults and nucleated at midcrustal depths of ~15 km, near the maximum depth of background seismicity. However, there is an intriguing paradox between the geometries of the seismogenic faults, associated with large M7+ events, and the attitudes of some of the shallow normal faults identified from the seismic reflection data. Several faults identified on the reflection profiles reveal Quaternary planar, low-angle to listric normal faults that extend to depths of <6 km, unlike the deeper penetrating and steeper planar faults inferred from the large, M7+, normal-faulting earthquakes. Rheologic models for this region of active extension show that the M7+ earthquakes occurred near the transition between the brittle upper crust and a quasi-plastic middle crust, suggesting that the larger events require large stress drops to relieve strain in a more ductile medium.

Crustal extension rates have been derived from seismic moment tensors of historic earthquakes along the transition that vary from <1 to 5 mm/yr. An integrated east-west extension rate for the entire Basin-Range of ~10 mm/yr is similar to Quaternary extension rates determined from geological and other geophysical data. The deformation rates of the transition area, however, are significantly smaller than those along the San Andreas fault system, where contemporary deformation may exceed 50 mm/yr.

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