Volcanism at the western margin of the Pacific Ocean has developed above subducting ocean crust, generating island arcs and spreading basins; however, the controls on magmatism at and before the initiation of subduction are less clear. We present new 40Ar/39Ar data from a 1000 km northwest-southeast line of oceanic plateaus on the Philippine Sea plate demonstrating that they formed as a time-progressive volcanic chain mirrored on either side of the West Philippine Basin backarc spreading center. In the north this chain is bounded by a Mesozoic remnant arc, and in the south by subduction beneath the Philippine arc. Geochemically these oceanic plateaus have an EM-2 (enriched mantle 2) ocean island basalt signature matching the older 45–51 Ma volcanic edifices discovered overlapping the remnant arc and intervening basins. The wide distribution of these edifices could mark the first arrival of upwelling mantle, which also mixed with local mid-oceanic-ridge basalt during contemporaneous West Philippine Basin spreading. These features are consistent with the extension and splitting of the Mesozoic arc terrane, driven by regional upwelling centered on the impact of the Oki-Daito mantle plume at 51 Ma.