Figure F5. Wide-angle seismic profiles (Kodaira et al., 2008) showing middle-crust thickness variation beneath volcanic front/arc front and along rear-arc Nishi-shichito Ridge used to infer that rear-arc crust was obliquely rifted off the arc front (Fig. F4), probably during the opening of the Shikoku and Parece Vela Basins (~25 Ma; Fig. F2C). The 6.0–6.8, 7.1–7.3, and 7.8 km/s layers correspond to middle crust, lower crust, and upper mantle, respectively. Basalt-dominant Quaternary volcanoes on volcanic front: Mi = Miyake, Ha = Hachijo, Ao = Aogashima, Su = Sumisu, To = Torishima; andesite Oligocene volcano east of front: Om = Omachi Seamount. ODP Sites 787, 792, and 793 also shown.
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