Deep seismic reflection profiling in the Mojave and Death Valley regions of the southern Great Basin provides a basis for comparing the seismic reflection signatures of two adjacent, but distinct, extensional terranes. Those data show contrasting reflection characteristics that may indicate significant variation in upper crustal structure between the two areas. The Mojave data show predominantly low-angle (less than 20°) reflectors in the upper crust, with offsets indicative of a late stage of high-angle (more than 40°) faulting, whereas the Death Valley data show numerous moderately dipping reflectors above a subhorizontal, midcrust reflecting horizon. The Death Valley structures are interpreted to be extensional features related to the Pliocene to Recent tectonic activity in that area. The interpretation of the Mojave data includes identification of structures ranging in age from Mesozoic to Recent. In contrast to the apparent record of faulting in the seismic data from the upper crust, the lower crustal reflectors appear to be similar throughout the two regions. In particular the reflection Moho is prominent at a depth of approximately 30 km in all of the seismic profiles. A midcrustal reflecting zone in Death Valley is interpreted to continue into the northern Mojave. That zone dips from the midcrust to the reflection Moho beneath the surface position of the Garlock fault zone and the Nopah Range, suggesting that the lower crust has the configuration of a flat-topped dome beneath the area of active upper crustal extension. That dome and the apparently continuous reflection Moho are here suggested to be primarily the product of magmatic activity in the extending lower crust and upper mantle.