Deep seismic-reflection data collected by COCORP along a 145-km-long line in westernmost Nevada and easternmost California as part of the 40°N latitude transect of the western Cordillera reveal the geometry of three crustal-scale features associated with the Walker Lane and the transition from the Basin and Range extensional province to the Sierra Nevada crustal block. The Walker Lane, a zone of late Cenozoic strike-slip and normal faults crossed in the central third of the line, is underlain by a zone of moderately west-dipping reflections that terminate at a discontinuous mid-crustal zone of subhorizontal reflections extending across most of the line at depths ranging from 14 to 23 km. We interpret the mid-crustal reflections as an active detachment accommodating strike-slip and extensional displacements. It may coincide with the base of the Sierran batholith, a shear zone, and/or the transition from brittle to ductile crustal deformation. The dipping reflections are inferred to be from a moderately dipping (45°) fault that soles in this mid-crustal detachment and accommodates the strike-slip and normal displacement of the northern Walker Lane. Discontinuous reflections at 9.5–10.2 s two-way travel time (28–30 km) on the eastern half of the line are interpreted as the Moho, similar to the Moho imaged by COCORP lines farther east in Nevada. Moho reflections are imaged at 11.0–11.5 s (33–35 km) in the western third of the line, and project westward toward similar reflections as deep as 12–13 s (37–41 km) beneath the eastern Sierra Nevada, and so the crust appears to thin gradually from the Sierra Nevada into the Basin and Range. The lower crust east of the Walker Lane is relatively highly reflective, characteristic of COCORP data from other Nevada lines, but the lower crust west of the Walker Lane is less reflective, similar to COCORP data from the northern Sierra Nevada. The change in reflectivity of the lower crust also appears to occur gradually, with no sharp offsets or boundaries. On the basis of the COCORP data, therefore, the transition between the Basin and Range and Sierra Nevada appears to be a wide (∼100 km) zone of crustal thinning possibly accompanied by a change in deformation fabric that may correspond to changes in lower crustal rheology or degree of deformation. This transition zone is dominated by a moderately dipping fault underlying the Walker Lane and accommodating strike-slip and normal displacement down to a mid-crustal detachment level.