This dissertation investigates the crustal structure of the eastern Basin and Range Province in the western United States and its relationship with the present-day extensional regime governing this region. The use of combined results from different geophysical methods provide a better understanding of the subsurface crustal structure and the processes involved in this extensional deformation. Teleseismic receiver functions were used to create a uniformly sampled map of the crustal thickness variations and stacked images of the crust beneath the majority of the state of Utah, which provide additional constraints on the seismic characteristics of the crust and upper mantle. These results reveal crustal variations characterized by a distinct change in crustal thickness that closely follows the surface trace of the Wasatch fault, with differences in depth of up to 10 km across a distance of less than 55 km. Analysis of seismic reflection profiles, horizontal and vertical crustal velocities from continuous GPS, and surface geology provide new constraints on the relationships between interseismic strain accumulation, subsurface fault geometry, and geologic slip rates on seismogenic faults. Seismic reflection data show recent activity along high-angle normal faults that become listric with depth, sole into preexisting décollements, reactivating them, and appear to be connected at depth with a regionally extensive detachment horizon. GPS data reveal present-day crustal extension of ~3 mm/yr and no net vertical motion between the Colorado Plateau and eastern Basin and Range. Inverse modeling results of the crustal deformation data include a low-angle dislocation (~8-20°) at a locking depth of ~7-10 km, consistent with the interpreted seismic data, and slipping at 3.2±0.2 mm/yr, suggesting an active regionally extensive sub-horizontal surface beneath the eastern Basin and Range. A test of this hypothesis using seismic data interpretation as the basis for a forward strain accumulation model shows that displacement across a deep low-angle detachment imaged seismically is also consistent with geodetic velocities. Seismic and geodetic data support a model for eastern Basin and Range mechanics wherein diffuse permanent strain of the upper crust by multiple discrete faults is facilitated by displacement along a single low-angle detachment at midcrustal depth.