Japan's continental shelf survey has revealed that the Ogasawara Plateau is colliding with the Izu–Ogasawara Arc and accreting to it (Ohara et al., 2015). The Japan Arc is considered to have developed through the collision and accretion of bathymetric highs (e.g., Taira, 2001); here, an ongoing continental growth process is observed. The collision between the Ogasawara Plateau and the Izu–Ogasawara forearc causes shoaling of the axis of the Izu–Ogasawara Trench. The depth of the axis at the collision point is 3450 m, 〜6000 m shallower than the deepest point within the Izu–Ogasawara Trench. This collision process caused numerous lineaments interpreted as faults in the western part of the Ogasawara Plateau, and tectonic emplacement of the Hahajima Seamount (HS) at the forearc just in front of the collision point. The Commission on the Limits of the Continental Shelf (CLCS) recognized the accretion of the Ogasawara Plateau to the Izu–Ogasawara Arc, and concluded in the recommendation to Japan that the western part of the Ogasawara Plateau is considered to be a “submarine elevation” in the sense of Article 76, paragraph 6 of the United Nations Convention on the Law of the Sea (UNCLOS). In other words, the western part of the Ogasawara Plateau is considered to be a natural prolongation of Japan's landmass on the Izu–Ogasawara Arc, including Chichi-jima and Haha-jima Islands.
(Yasuhiko OHARA, Yukihiro KATO and Akira NISHIMURA)
