Evolution of backarc basins in the Philippine Sea

The Hydrographic Department of Japan ini-tiated a long-term geological and geophysical survey of the Philippine Sea in 1983. We here summarize the bathymetry and magnetic anomalies of this area, focusing on the structure and the evolu-tionary process of the marginal basins. The Shikoku and Parece Vela Basins developed as one backarc system in the later phase of their formation; the significant difference be-tween the two basins, however, is clearest near the extinct spreading centers. The steep Oki-Daito Escarpment, located in the northern West Philippine Basin, was mapped for the first time during this survey. The N-S trending spreading fabric north of the escarpment is in striking contrast to the NW-SE to E-W lineament in the south. Our data reveal a more compli-cated history of the western Philippine Sea than has been pro-posed in previous studies.