We report inversely correlated Os and Pb isotopic data from mantle-derived continental rift basalts from Death Valley, California. The data project toward a crustal end member that is extremely radiogenic with respect to Os and nonradiogenic with respect to Pb, and a mantle end member that is unradiogenic with respect to Os and moderately radiogenic with respect to Pb. The data favor ancient mafic lower crust as the source of the radiogenic Os. A mafic lower crust composition for the Basin and Range leads to rheological coupling between strong lower crust and lithospheric mantle. This may explain the flat Moho topography seen across extended and stable crustal blocks, necessitating upper crustal and lower crustal strain decoupling, leaving the more fluid middle crust across which upper crustal mass compensation must have occurred.