Existing trace element and isotopic data, as well as new U-series data for Nicaragua and Costa Rica, are examined in light of the recent suggestion that asthenospheric mantle is flowing northward behind the subducting Nazca plate around the northwest corner of South America. Lavas from central Nicaragua have trace element and isotopic compositions suggesting that they are derived from relatively depleted mantle with a long history of interaction with the subducting Cocos plate. Lavas from northern and central Costa Rica have compositions indicating a more enriched mantle source that has been influenced little by the subducting slab. These compositional traits vary irregularly between the two regions, suggesting that mantle compositions and subduction influences intermingle in a complex manner beneath northern Costa Rica. The dip of the subducting slab decreases abruptly southward, and accretion of sediment begins in this same vicinity. These changes in composition and physical parameters may all be induced or influenced by an influx of asthenospheric material from behind the retreating Nazca plate.