Schematic diagrams illustrating the potential origin of the crustal rocks discussed in this paper. (A), (C), and (E) represent cross sections along lines illustrated on schematic maps (B), (D), and (F). Brown areas illustrate arc crust; basaltic crust originating in a subduction initiation, backarc, or mid-ocean ridge setting is gray; mantle is illustrated in light green. The dashed line in (F) is the West Santa Rosa boundary fault. AP—Amami Plateau, DR—Daito Ridge, ODR—Oki-Daito Ridge, WPB—West Philippine Basin, MOC—Mesozoic oceanic crust, KPR—Kyushu Palau Ridge, WMR—West Mariana Ridge, PVSB—Parece-Vela and Shikoku basins, Bon—early-arc boninitic crust. The red star illustrates the location of the Shinkai 6500 dive sites 6K1232 and 6K1236, which were on a lithospheric block consisting of Eocene crust underlain by more recently emplaced mantle. The AP-DR-ODR area consists of Mesozoic arc terranes, plume-related ocean islands, and basins of late Mesozoic and Eocene age (see Ishizuka et al., 2013; Arculus et al., 2015). MOC probably consisted of oceanic lithosphere with minor arc and ocean-island terranes. Izu-Bonin-Mariana (IBM) plate rotations over time are not illustrated. (A, B) Early-arc phase shortly after initiation of subduction when upwelling mantle might have triggered delamination of deep segments of a crustal terrane, such as the Oki-Daito Ridge. (C, D) Rifting at 25 Ma to create the Parece-Vela and Shikoku basins, causing disaggregated fragments of delaminated crust to rise to shallow levels in the mantle. The dots with varied colors represent fragments of the crust, which could have varying metamorphic grade. (E, F) The situation today, where mantle with embedded crustal blocks crop out near the Mariana Trench.
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