Figure F1. Bathymetric features of the eastern Philippine Sea, including the Izu-Bonin-Mariana (IBM) arc system. Old seafloor (135–180 Ma) of the western Pacific plate subducts beneath the active IBM arc at the Izu-Bonin-Mariana Trenches. Spreading centers are active in the Mariana Trough (7–0 Ma) and relict in the Shikoku and Parece Vela Basins (30–15 Ma) and West Philippine Basin (50–35 Ma). The Ogasawara and Amami Plateaus and Daito and Oki-Daito Ridges are Cretaceous–Eocene features. The Kyushu-Palau Ridge (KPR) marks the rifted western edge of the initial IBM arc system (50–30 Ma), subsequently separated by the back-arc spreading into the Shikoku and Parece Vela Basins. Black dashed lines = locations of wide-angle seismic profiles (1) along the present-day volcanic front (Kodaira et al., 2007b), (2) along the rear arc ~150 km west of the volcanic front (Kodaira et al., 2008), and (3) across the arc. IBM-1, IBM-2, IBM-3, and IBM-4 = proposed drill sites. Basalt-dominant Quaternary volcanoes on the volcanic front: Mi = Miyakejima, Ha = Hachijojima, Ao = Aogashima, Su = Sumisu, To = Torishima; andesite Oligocene volcano east of the front: Om = Omachi Seamount. Numbered circles = sites drilled during Ocean Drilling Program Legs 125 and 126. The 6.0–6.3, 7.1–7.3, and 7.8 km/s layers correspond to parts of middle crust, lower crust, and upper mantle, respectively.
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