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The examination of the crustal structure and ODP deep-sea drilling results introduced the following opening model of the Japan Sea. The opening of the Japan Sea was initiated by the extension and thinning of the proto-Japan island arc, which was situated on the margin of the Eurasian continent at ca. 30 Ma. During the extension process a large strike-slip fault was generated at the eastern margin of the proto-Japan Sea. The strike-slip movement cut through the entire lithosphere and caused it to split, triggering development of a seafloor spreading system at ca. 28 Ma. The seafloor spreading system propagated to the WSW direction into the thinned crustal zone and formed the eastern part of the Japan Basin that is presently underlain by oceanic crust. In the meantime, the southwestern part of the Japan Sea suffered continuing extension and thinning of the arc crust and formed basins and rises. The basins are composed of thinned lower arc crust overlain by volcanic layers, and the rises are composed of fragmented upper arc crusts. The propagation ceased at ca. 18 Ma, leaving the contrasting topography of the present northern and the southern Japan Sea. The extension and thinning of arc or continental crusts and subsequent development of a propagating spreading system, which is initiated at a basin margin strike-slip zone, are common and fundamental processes of most backarc basins.

The Bonin Arc is a well-developed island arc, associated with a deep trench, an active volcanic chain, and a back-arc basin. The Bonin Arc is approximately 1100 km long, extending north—south from latitude 35°N to 25°N within longitude 139°E and 145°E, facing the Pacific plate in front and the Shikoku Basin behind (Fig. 1). At its northern limit, the Bonin Arc forms part of a triple junction of trench—trench—transform fault or trench—trench—trench with the Tohoku and the Seinan (southwest) Japan arcs. The Mariana Arc continues the line of the Bonin Arc to the south. There is no marked morphological boundary between the Bonin and the Tohoku arcs, but one is inferred by the depression along the Sagami Trough (see Fig. 13). The southern boundary of the Bonin Arc is even less well defined than the northern margin. The Bonin Arc extends to the Mariana Arc as part of the same morphological sequence from north to south. These three arcs face the Pacific plate.