Two distinct volcanic assemblages characterize the central Cascades of Oregon. The earlier, larger, and longer lived of the two is part of a calc-alkaline, continental margin andesite belt exposed in the Western Cascades. The younger assemblage, conventionally known as the ''High Cascades,'' overlies the eastern part of the older assemblage and extends eastward to the Deschutes Valley. The High Cascades are also calc-alkaline, but mafic components of a broad lava flow platform surround isolated silicic center. A short-lived episode of subsidence produced a graben within the central High Cascades. Subsequent mafic eruptions largely filled the graben, while the silicic centers continued to be active. This twofold volcanic development followed by extreme localized crustal extension is ascribed to the influence of slow circulating currents in the sub-Cascade mantle, induced by drag against the subducted slab. Circulation of the lithosphere was enhanced by prolonged metasomatism and loss of strength in the mantle wedge; similar circulation may be a common or even essential component of volcanism above other subduction systems. The rates of convergence and subduction have decreased in the Cascade system to a degree that has permitted conductive heating of the slab and adjacent mantle. Circulation has migrated to higher levels within the mantle wedge, causing a more direct interaction with the crust, development of a new mafic-dominated volcanic assemblage, and graben subsidence. ¿ American Geophysical Union 1990