The boundary between the Paleoproterozoic Mojave and Yavapai crustal provinces in the southwestern United States is a 75-km-wide zone characterized by complex, isotopically mixed (Pb) crust, similar to other transitional crustal and lithospheric boundaries worldwide. Mojave crust is more isotopically evolved than the juvenile crust of the Yavapai province, indicating an older crustal component. Several areas of isotopically juvenile rocks within and near the boundary zone correspond to exposures of coeval >1.73 Ga supracrustal rocks that contain pillow basalts and bimodal metavolcanic sequences. We suggest that regions of juvenile crust within the Mojave province and its boundary zone represent rift basins, produced by extension or transtension near the eastern margin of the province, into which primitive basaltic lavas and bimodal volcanic sequences were erupted. The largest region of juvenile material within the boundary zone is bound to the west by a series of coincident, N-trending isotopic, geochemical, metamorphic, and geophysical discontinuities that may represent a fundamental, buried crustal structure, perhaps a rift margin. We propose that the distinct isotopic signature of the boundary zone persisted through subsequent deformational events, including the one that juxtaposed the two provinces. If our model for the formation of the boundary zone is correct, its isotopically mixed character is at least partly an artifact of the precollision history of the eastern Mojave province, and may not solely reflect the processes that juxtaposed the two provinces. Formation of new crust by rifting may represent an underappreciated form of crustal growth during the Paleoproterozoic that is distinct from arc accretion. Models for the formation of complex crustal boundary zones must consider the early, prejuxtaposition history of terranes in interpreting their geochemical and geophysical characteristics.