The petrographic, chemical, and isotopic characteristics of synextensional Miocene-Pliocene volcanic rocks of the Nova Formation, Death Valley area, record interaction of three components. Two of these are ancient crust and enriched mantle; they have been recognized previously in the Basin and Range province. The third component is isotopicaily similar to rocks of the Sierra Nevada batholith exposed to the west of the study area. We interpret this source to be the mantle lithosphere and/or lower crust of the Sierra Nevada batholith that was emplaced beneath the Death Valley area by extensional deformation. This is consistent with 150 to 250 km of extension across the southern Basin and Range province inferred from other evidence, and supports the conclusions of workers who suggest that the dominant mode of crustal extension in this area is simple shear with large amounts of lateral lithospheric transport.