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Continental extension in the Basin and Range province of the western United States has long been a subject of extensive research and debate. Many aspects of the evolution of the Basin and Range remain a mystery, including its detailed crustal structure, its fault geometries, and the nature of its highly reflective lower crust. In this chapter we address these questions with a reanalysis of existing refraction, reflection, and gravity data, supplemented with new seismic reflection data from representative regions within the province.

Our reanalysis of a seismic refraction profile, recorded between Fallon and Eureka, Nevada, in the early 1960s, leads to a preferred model in which the western Basin and Range is underlain by a lens of material transitional between crust and mantle with its base at an approximately constant depth of 34.5 km below our datum (1 km above sea level). In this model, the lens is centered beneath the Carson Sink in western Nevada, where it attains a maximum thickness of about 9 km and is characterized by an estimated refraction velocity of 7.5 km/sec, intermediate between usual lower crustal and upper mantle velocities. Gravity modeling along this Fallon to Eureka transect is improved by introducing moderate thinning (about 12 km) of the mantle lithosphere. On seismic reflection profiles, the base of the crust-mantle transition zone correlates roughly with an abrupt change from a strongly laminated lower crust into a seismically transparent upper mantle. We suggest that this transition zone represents an interlayering of maficultramafic rocks formed during repeated episodes of partial melting, underplating of the crust by gabbroic magma, crystal settling, and subhorizontal shearing near the crust-mantle boundary. An alternate model, without the lens of material transitional between crust and mantle, satisfies the gravity data with a major (about 34 km) thinning of the mantle lithosphere.

We also address the geometry of faulting in the upper crust within the province with a review of representative seismic reflection profiles, and geodetic and earthquake studies. Although geodetic and earthquake data indicate that the Basin and Range is characterized by high-angle planar faults that penetrate to a depth of about 15 km, seismic reflection data suggest that listric faults and low-angle detachment surfaces are also important in the extension of the brittle upper crust. In contrast, the deeper (>15 km) crust is aseismic and is thought to deform ductilely. The transition from an upper crust that is comparatively transparent in seismic reflection records, to a middle crust (about 12 to 25 km) that in many places is highly laminated and reflective may indicate a rheologic change from brittle to ductile deformation with depth. As much as 10 km or more of upper crust has been removed tectonically and erosionally in places (the metamorphic core complexes), and therefore what was once midcrust is now at or near the surface. We address the prominent reflectivity in the midcrust by analyzing reflection and borehole data in conjunction with synthetic seismogram modeling. Discontinuous, high-amplitude reflections in the midcrust are attributed to water-filled fracture zones and compositional layering on a scale of tens of meters. Geologically, primary lithologic variations (e.g., quartzofeldspathic to micaceous rocks) are enhanced by extension and subhorizontal shearing. Alignment of anisotropic minerals, especially micas, within zones of concentrated deformation may enhance velocity contrasts, producing the high-amplitude reflections observed.

Partial melting and inflation of the crust by mantle-derived basalts were probably important in generating the strongly reflective Moho at the base of the crust. Because this reflective boundary is subhorizontal across the province, we believe that it is a relatively young feature, indicative of the repeated reworking of the crust and upper mantle of this region of high heat flow and Cenozoic extension.

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