The West Philippine Basin and the initiation of subduction, revisited

New swath bathymetry plus existing geophysical data reveal that the direction of West Philippine Basin (WPB) seafloor spreading rotated 100degrees counter-clockwise between 49 Ma and 33 Ma. The curvilinear and multi-stranded Mindanao Fracture Zone separates the WPB from the Palau Basin to the south. WPB opening was contemporaneous with, and behind, early Izu-Bonin-Mariana (IBM) subduction, whose arc volcanism began by 50 Ma. This produced over 1000 km of arc parallel spreading ( or stretching plus magmatism) in the Mariana segment of the Eocene IBM arc/forearc. The initial IBM subduction cut across, rather than followed, pre-existing structures (remnant arcs, fracture zones and spreading fabric). New models of subduction initiation are required to reproduce such characteristics.