Abstract The Izu-Bonin-Mariana (IBM) arc system extends 2800 km from the Izu Peninsula to Guam Island (Fig. 2f.1) and provides an excellent example of an intra-oceanic convergent margin where the effects of crustal anatexis and assimilation are considered to be minimal (Stern et al. 2003; Tatsumi Stern 2006). The current IBM activity is caused by subduction of the Pacific Plate beneath the Philippine Sea Plate, which dips at 35° at the northern tip of this system and ia nearly vertical at Mariana. Evolution of the IBM Arc since 50 Ma has strongly in?uenced the present architecture of the Japanese archipelago. One important event during this arc system’s evolution is back-arc rifting from 15-25 Ma that separated the IBM Arc system from the remnant Kyushu-Palau Ridge by c. 500 km and created the young oceanic lithosphere of the Shikoku Basin that is now being subducted beneath the SW Japan Arc. It should be further stressed that recent geophysical and geological survey results suggest this intra-oceanic arc is an active site of both creation and growth of the continental crust. This chapter will highlight the tectonic and structural evolution of the IBM arc system.

Abstract Three submarine Diamante cross-chain volcanoes in the southern Mariana arc mark a magma-healed zone of along-arc (north–south) extension that allows either mafic mantle-derived basalts or felsic magmas from the middle of thickened arc crust to erupt. The largest volcano is East Diamante, with a well-developed (5×10 km) caldera that formed via violent felsic submarine eruptions beginning nearly 0.5 Ma. One or more of these eruptions also formed a giant submarine dune field extending 30 km to the NW of the volcano. Felsic igneous activity continues at least as recently as c. 20 000 years ago, with emplacement of resurgent dacite domes, some hot enough to power the only black smoker hydrothermal system known in the Mariana arc. In contrast, felsic eruptions do not occur on the two volcanoes to the west, implying that the mid-crustal felsic zone does not underlie the thinner crust of the Mariana Trough back-arc basin. Diamante cross-chain lavas define a medium K suite; mafic lava phenocryst assemblages show arc-like associations of anorthite-rich plagioclase with Fe-rich olivine. Magmatic temperatures for a basaltic andesite and three dacites are c. 1100 °C and c. 800 °C, respectively, typical for cool, wet, subduction-related felsic magmas. Felsic magmas formed under low-P crustal conditions. The Diamante cross-chain is the southernmost of at least seven and perhaps eight Mariana arc volcanoes in a c. 115 km long arc segment characterized by felsic eruptions. This is the ‘Anatahan Felsic Province’, which may have formed above a mid-crustal tonalite body that formed by fractionation or was re-melted when heated by c. 1200 °C mafic, mantle-derived magmas. Across- and along-arc variations suggest that felsic eruptions and dome emplacement occurred when midcrustal tonalite was remobilized by intrusions of mafic magma, while north–south extension facilitated the development of conduits to the surface. Supplementary material: Detailed Hyperdolphin ROV dive tracks, Cook 7 dredge locations, 40 Ar/ 39 Ar analytical data, analytical methods, major and selected trace element analyses of whole rock samples, and compositional data for minerals are available at http://www.geolsoc.org.uk/SUP18611

Abstract The laser-heating 40 Ar/ 39 Ar dating method was applied to volcanic rocks systematically collected from the back-arc region of the central part of the Izu-Bonin arc. Dating results combined with whole-rock chemistry and other geological information reveal the volcanic history of the back-arc region of the Izu-Bonin arc. In the back-arc seamount chains area, andesitic-basaltic volcanism initiated at c. 17 Ma, slightly before the Shikoku Basin ceased spreading, and continued until c. 3 Ma. Relatively old volcanism (>8 Ma) has been found only from the western part of the seamount chains, and younger volcanism mainly occurs in the eastern part of the chains, indicating the western margin of the active volcanic zone of the Izu-Bonin arc has migrated eastward with time. At around 2.8 Ma, volcanism initiated in the western part of the back-arc knolls zone. This volcanism is characterized by eruption of clinopyroxene-olivine basalt. In the first stage of rifting, this type of basalt erupted from N-S-trending fissures and/or vents aligned in this direction and formed N-S-trending ridges. Between 2.5 and 1 Ma, many small knolls were formed by eruption of basalt and minor felsic rocks. Volcanism younger than 1 Ma occurred only in the currently active rift zone and its adjacent area. The active volcanic zone in the back-arc seamount chains area converged to the volcanic front with time from 17 to 3 Ma. Active rifting and rifting-related volcanism also migrated or converged eastward after 1 Ma. The observed temporal variation of locus of volcanism may be explained by rapid retreat of the Philippine Sea Plate relative to the Pacific Plate and resulting steepening of the subducting slab.