The calderas of the two isotopically distinct volcanoes Sete Cidades and Agua de Pau on São Miguel are only 30 km apart. Sete Cidades lavas generally have ⁸⁷Sr/⁸⁶Sr < 0.7038 while Agua de Pau lavas have ⁸⁷Sr/⁸⁶Sr > 0.7040. Two overlapping rift zones exist between the two volcanoes and the lava compositions show that each rift zone is connected to the neighbouring volcano. Limited mixing between the volcanic systems implies separate ascent paths and magma sources with diameters of ∼20 km. The two volcanic systems follow contrasting directions; the Sete Cidades volcanic rift strikes NW parallel to the extensional Terceira Rift, while the Agua de Pau volcanic rift zone strikes E-W. The covariation of magma sources and lithospheric tectonics may suggest that the tectonic structure of the lithosphere controls the mantle upwelling beneath São Miguel.