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doi:10.14379/iodp.sp.352.2013

Background

The Izu-Bonin-Mariana system

The IBM system is, as already noted, the type locality for studying oceanic crustal accretion immediately following subduction initiation. It is sufficiently old that it carries a full record of the evolution of crustal accretion from the start of subduction to the start of normal arc volcanism (a ~7 m.y. period) but sufficiently young that the key features have not been excessively disturbed by subsequent erosion or deformation. Intra-oceanic arcs are built on oceanic crust and are sites of formation of juvenile continental crust (Rudnick, 1995; Tatsumi and Stern, 2006). Most active intra-oceanic arcs are located in the western Pacific. Among these, the IBM system stands out as a natural scientific target. This predominantly submarine convergent plate boundary is the result of ~50 m.y. of subduction of the Pacific plate beneath the eastern margin of the Philippine Sea plate. Stretching for 2800 km from the Izu Peninsula, Japan, to Guam, USA (Fig. F1), the IBM system (summarized in Stern et al., 2003) has been extensively surveyed and is a very suitable natural laboratory for IODP expeditions aimed at understanding subduction initiation, arc evolution, and continental crust formation. The scientific advantages of studying the IBM were recognized by the U.S. National Science Foundation MARGINS-Subduction Factory experiment as the intra-oceanic arc focus site (the other focus site being the quasicontinental arc of Central America). Most importantly, the IBM fore arc is likely the best site on the planet for studying the initial magmatic products of a subduction zone. We know when subduction and arc construction began (~51–52 Ma; Ishizuka et al., 2011; Reagan et al., 2013), even if the precise paleogeography is controversial, and there is a good time-space record of crustal development.

Petrologic evolution

The petrologic evolution of early stage magmatism in the IBM arc has been reconstructed mainly based on volcanic sections that are exposed on the fore-arc islands (Bonin Islands and Mariana Islands) and that have been recovered from DSDP and ODP fore-arc drill sites. Recent dredging and submersible studies provide additional information (see “Geology of the fore arc around proposed Sites BON-1A and BON-2A”). Consequently, we can predict the sequence of magmas likely to characterize the drill site and its surrounding region, which developed prior to establishment of a stable magmatic arc ~150 km west of the trench by the Oligocene. This compositional evolution reflects the reorganization of mantle convection and slab-derived fluid flows in response to the changing behavior of the sinking Pacific plate: from sinking without downdip mantle motion to establishment of true subduction with downdip motion (see “Tectonic evolution”). This evolution, from (1) initial seafloor spreading and eruption of mid-ocean-ridge basalt (MORB)-like tholeiites to (2) eruption of boninites to (3) fixing of the magmatic arc ~150 km west of the trench (separated by a broad, dead fore arc) took ~7 m.y. (Ishizuka et al., 2011). The process is reflected in the succession of igneous rocks of the Bonin Ridge, which is described in greater detail below and depicted in the time-space diagram (Fig. F2).

Subduction initiation volcanism

Basaltic rocks have been recovered in the IBM fore arc from stratigraphic levels below boninite as described in “Geology of the fore arc around proposed Sites BON-1A and BON-2A.” These basalts have chemical compositions that are similar to those of normal mid-ocean-ridge basalt (N-MORB), and the term “fore-arc basalt” (FAB) was coined by Reagan et al. (2010) to distinguish them from MORB. Most of the reliable ⁴⁰Ar/³⁹Ar ages and U-Pb zircon ages of FAB from the fore-arc slope east of the Bonin Ridge and south of Guam are identical within error and indicate that FAB magmatism occurred from ~50 to 52 Ma, preceding boninite eruption by at least 2–3 m.y. (Ishizuka et al., 2011). U-Pb zircon ages from gabbros below the FAB indicate that these are contemporaneous (Ishizuka et al., 2011; Reagan et al., 2013) and probably comagmatic. Lavas with compositions transitional between FAB and boninites from Site 458 were dated at 49 Ma (Cosca et al., 1998). FAB and related gabbros are thought to relate to the first magmas produced as the IBM subduction zone began to form (Reagan et al., 2010).

Geochemical data show the similarity of these basalts to MORB, with no (or minor) slab signature. FAB has light rare earth element–depleted rare earth element (REE) patterns, indicating derivation from a moderately depleted lherzolitic upper mantle, similar to that responsible for generating MORB (Fig. F3). FAB has low Ti/V ratios (14–16), which distinguish FAB from subducting Pacific MORB (26–32) and from Philippine Sea MORB (17–25) (Fig. F4). Chemically and petrographically, Bonin Ridge FAB is indistinguishable from Mariana FAB, which is also considered to be related to subduction initiation and which also predates boninitic volcanism in that region (Ishizuka et al., 2011). This strongly implies that FAB tholeiitic magmatism was associated with fore-arc spreading along the length of the Izu-Bonin-Mariana arc. Like the overlying boninites, the probable source of Mariana FAB was Indian Ocean–type mantle. Low concentrations of incompatible elements and low trace element ratios such as Nb/Yb imply that FAB magmas were derived from depleted mantle and/or were larger degree mantle melts compared to typical Philippine Sea MORB.

Pb isotopic compositions of FAB from the Bonin fore arc show that, like other IBM magmas, they are derived from a mantle with Indian Ocean characteristics, as demonstrated by high Δ8/4 Pb compared to Pacific MORB. Isotopic characteristics indicate some differences between the mantle sources of Philippine Sea MORB and FAB, including distinctly higher ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb (Fig. F5), which may imply the presence of lithospheric mantle with ancient enrichment (Parkinson et al., 1998). Most significantly, there is no evidence that subducted sediments (with elevated ²⁰⁷Pb/²⁰⁴Pb) affected the source region of these basalts, although some FAB lavas from the Mariana fore arc have Pb isotopic compositions consistent with a weak subduction influence (Reagan et al., 2010). Differences in isotopic and trace element characteristics between IBM FAB and Philippine Sea MORB strongly imply that FAB does not represent the preexisting ocean crust of the West Philippine Basin, trapped prior to subduction initiation, as originally concluded by DeBari et al. (1999) for MORB-like tholeiites recovered from the Izu inner trench wall.

Lavas with compositions that transition upward between FAB and boninite were recovered at DSDP Leg 60 Sites 458 and 459 (the alternate site) and illustrate that FAB and boninite are genetically linked (Reagan et al., 2010). The oldest of these lavas have REE patterns similar to those of MORB but are more enriched in silica and have higher concentrations of “fluid-soluble” elements such as K, Rb, U, and Pb than FAB. These lavas also have Pb isotopic compositions that are more similar to lavas from the Pacific than those of the Indian plate, supporting the contention that subducted fluids were involved in their genesis. The youngest lavas at Site 458 are strongly depleted in REE, somewhat resembling boninites but less magnesian and more calcic.

Boninitic and high-Mg andesitic volcanism

Boninite volcanism follows FAB volcanism as an integral part of the evolution of the nascent subduction zone. The type locality of boninite is in the Bonin (Ogasawara) Islands, an uplifted segment of the IBM fore arc. Boninites and other early arc lavas are better exposed on the Bonin Islands than anywhere else in the world particularly on the two islands of Chichijima and Hahajima. This is the most important reason that these islands became a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site in 2011 (whc.unesco.org/en/list/1362/). ⁴⁰Ar/³⁹Ar dating indicates that boninitic volcanism on Chichijima took place briefly during the Eocene, between 46 and 48 Ma (Ishizuka et al., 2006). A slightly younger volcanic succession is exposed along the Bonin Ridge, including 44.74 ± 0.23 Ma high-Mg andesites (HMA) from the Mikazukiyama Formation, the youngest volcanic sequence on Chichijima, and 44.0 ± 0.3 Ma tholeiitic to calc-alkaline andesites from Hahajima. Four submersible Shinkai 6500 dives on the Bonin Ridge Escarpment mapped an elongate constructional volcanic ridge atop the escarpment and recovered fresh andesitic clasts from debris flows along the northern segment of the ridge; they also recovered HMA lava blocks from the escarpment northwest of Chichijima. Three samples of andesite collected from the Bonin Ridge Escarpment range in age from 41.84 ± 0.14 to 43.88 ± 0.21 Ma (Ishizuka et al., 2006).

Boninites from the Bonin Islands are characterized by high MgO at given SiO₂ concentrations, low high-field-strength elements, low Sm/Zr, low REE, and a U-shaped REE pattern (Fig. F3). These are “low-Ca boninites” (Crawford et al., 1989) and can be explained by low-pressure melting of depleted harzburgite that was massively affected by slab flux. These boninites are isotopically characterized by high Δ7/4 Pb, high ⁸⁷Sr/⁸⁶Sr, and low ¹⁴³Nd/¹⁴⁴Nd relative to local MORB and FAB sources (Fig. F5). In contrast to the FAB mantle source, which was not much affected by subduction-related fluids or melts, the boninite magma source manifests a major contribution from subducted pelagic sediment and oceanic crust. The boninites are also distinct from ~44 Ma lavas exposed on Hahajima Island and recovered by Shinkai 6500 diving on the Bonin Ridge Escarpment (Ishizuka et al., 2006). HMA from Chichijima and the Bonin Ridge Escarpment are more similar to relatively enriched boninitic lavas from Site 786 (Pearce et al., 1999) and Guam, including having higher Sm/Zr at a given Zr content and higher REE and Ti concentrations compared to Chichijima boninites (cf. Taylor and Nesbitt, 1994). The HMA are isotopically distinct from the boninites (Fig. F5) and were derived from a source mantle that was less affected by fluids or melts derived from the subducted plate.

Post-45 Ma, tholeiitic to calc-alkaline andesites from the Bonin Ridge and ~45 Ma tholeiites from Saipan (Reagan et al., 2008) exhibit strong characteristics of arc magmas: they are depleted in Nb and enriched in fluid-mobile elements such as Sr, Ba, U, and Pb. These characteristics indicate that, by 45 Ma, near-normal configurations of mantle flow and melting, as well as subduction-related fluid formation and metasomatism, were established for this part of the IBM arc-trench system. The Bonin Ridge Escarpment, Mikazukiyama Formation, and Hahajima andesites thus represent a transitional stage from the waning stages of fore-arc spreading (represented by FAB and perhaps boninites) and the stable, mature arc that developed in the late Eocene to early Oligocene. These orthopyroxene-bearing, high-Mg, tholeiitic to calc-alkaline andesites erupted along the Bonin Ridge Escarpment as the arc magmatic axis localized and retreated from the trench. Post-45 Ma andesites (and basalts), unlike Chichijima boninite and HMA, do not show the influence of pelagic sediment melt from the slab (Fig. F5); instead, the mantle source seems to have only been affected by hydrous fluid derived mainly from subducted altered oceanic crust. Post-44 Ma lavas are isotopically similar to HMA (Fig. F5) and were derived from a mantle source that was less affected by fluids or melts derived from subducted sediments.

Overall, modeling of these data (not shown) indicates that the geochemical and isotopic characteristics of the IBM arc along its entire length evolved in tandem with the formation of a new subduction zone and a new mantle flow regime by (1) initial decompression melting without significant slab flux producing MORB-like basalt and fore-arc spreading (49–52 Ma), (2) mixing of fluids or melts from subducted sediments and oceanic crust into an extremely depleted (harzburgitic) mantle to generate boninites (48–45 Ma), and (3) continued influx of hydrous fluid input into increasingly fertile lherzolitic mantle to generate tholeiitic and calc-alkaline magma (post-45 Ma), marking the time when a mature, stable arc magmatic system was finally established (Ishizuka et al., 2006, 2011).

The observations above, together with the geochronological data summarized earlier, imply that shallow melting of depleted mantle with the aid of hydrous fluids from the newly subducted slab produced boninitic volcanism nearly simultaneously along the entire length of the IBM arc system during the earliest stage of arc evolution. Casey and Dewey (2009) argued that continued spreading, in what is now the West Philippine Basin, requires that the infant arc was lengthening throughout the Paleogene, so that subduction initiation may have started at different times along the IBM arc system. This is an important consideration for understanding how and when the entire IBM convergent plate margin formed but does not diminish the importance of understanding how a new subduction zone began along the Bonin Ridge. Note also that, although we have established a general volcanic stratigraphy, it is evident from Figure F2 that this is a composite stratigraphy based on dredging, submersible grab sampling, and coring at widely spaced localities. There is no reference stratigraphic section to check this subduction initiation stratigraphy and, in particular, identify the nature of the boundaries between the units and demonstrate that units have not been missed. Defining this stratigraphic section is the aim of this expedition.

Tectonic evolution

It has been generally accepted (Bloomer et al., 1995; Pearce et al., 1999; Stern, 2004; Hall et al., 2003) that the IBM subduction zone began as part of a hemispheric-scale foundering of old, dense lithosphere in the western Pacific (Fig. F6). The beginning of large-scale lithospheric subsidence, not true subduction but its precursor, is constrained by the age of igneous basement of the IBM fore arc to have begun in the Eocene, just before 50 Ma (Bloomer et al., 1995; Cosca et al., 1998; Ishizuka et al., 2006). The sequence of initial magmatic products is similar everywhere the fore arc has been sampled, implying a dramatic episode of asthenospheric upwelling and melting associated with arc magmatism and seafloor spreading over a zone that was hundreds of kilometers broad and thousands of kilometers long. It is clear from the extensive geochronology for IBM fore-arc rocks that this episode took place ~45–52 m.y. ago. It is this part of the tectonic history of the IBM arc that Expedition 352 drilling intends to sample.

Interestingly, these time-space trends in IBM fore-arc composition can be found in many ophiolite terranes. The world’s largest ophiolite, the Semail ophiolite of Oman/United Arab Emirates has long been known to exhibit a stratigraphy of FAB-like tholeiites overlain by depleted arc tholeiites (e.g., Alabaster et al., 1982), and recent discoveries of boninites in the upper part of the sequence (Ishikawa et al., 2002) confirm the full trend from tholeiite to boninites. Other large, complete ophiolites with complex fore-arc-type stratigraphies involving tholeiites and boninites include the Troodos Massif of Cyprus, the Pindos Mountains in Greece, and the Bay of Islands ophiolite in Newfoundland (Canada), and there are numerous others distributed through most of the world’s mountain belts (e.g., Pearce et al., 1984; Dilek and Flower, 2003). Many of these are economically significant, with associated volcanogenic massive deposits and/or podiform chromite mineralization. Dredging and diving along the inner trench wall sampled parts of the SSZ oceanic crust, but the complete lava section at Sites BON-1A and BON-2A is needed to explain the transition from ocean crust to arc volcanism seen in many SSZ ophiolites.

The presence of boninites is in itself an important tectonic indicator, requiring a combination of shallow melting, high water content, and depleted mantle. Boninites are defined by the International Union of Geological Sciences (IUGS) to have >52 wt% silica, <0.5 wt% TiO₂, and >8 wt% MgO. They can usefully be distinguished from basalts on a diagram of Ti8 versus Si8 where Ti8 and Si8 refer to the oxide concentrations at 8 wt% MgO (Pearce and Robinson, 2010). On this projection (Fig. F7), the earliest lavas are basalts (FAB) that plot in the MORB field. Later lavas (from ~48 to 44 Ma) plot as boninites before compositions eventually become basaltic again with eruptions at, for example, Hahajima. This appears to be a characteristic of subduction initiation, but to properly interpret its tectonic significance we need the full lava stratigraphy to know whether the basalt–boninite transition is gradational or episodic or has both magma sources available simultaneously. Drill core would also enhance the opportunity to obtain glass samples that can be analyzed for volatile and fluid-mobile element concentrations.

After a brief period of spreading, magmatic activity began to retreat from the trench, at the same time changing composition, perhaps first from FAB to boninite and then from boninite to calc-alkaline and tholeiitic arc magmas. Magma evolution was accompanied by migration of the magmatic locus away from the trench. Rare 40–43 Ma adakites were recovered from a Bonin fore-arc seamount. Eventually, perhaps some 10 m.y. after subduction initiation, the locus of magmatism reached the equivalent location of the present magmatic arc. This left vast tracts of infant arc crust “stranded” to form the IBM fore arc, so it cooled and experienced only minor tectonic activity while the arc-basin system continued to evolve magmatically to its present crustal structure (Taylor, 1992). Thus the fore arc was “frozen” in a primitive state and did not evolve into the more complex arc with tonalitic middle crust (Suyehiro et al., 1996). Understanding the formation of fore-arc crust is clearly critical for understanding the formation of subduction zones (and the magmatic responses), growth of arcs, evolution of continental crust, and origins of ophiolite.

Structure and thickness of fore-arc crust

The most detailed trench-orthogonal published images of IBM fore-arc crustal structure in the region of interest come from a seismic refraction/reflection study by Kamimura et al. (2002). This survey was accomplished with two 130 km long, orthogonal arrays of ocean-bottom seismometers (23 in total, in which 106 × 20 kg chemical explosions and 1835 pulses from 2 × 17 L air guns were used as seismic sources) in a region some distance north of the section from Sites BON-1A and BON-2A. In Figure F8, the approximate relative position of proposed drill Sites BON-1A and BON-2A are projected onto the east–west line between ODP Leg 125 Hole 786B and the trench, but it must be recognized that their actual crustal structure may be slightly different than that shown.

With that caveat, we infer from the study of Kamimura et al. (2002) that the crust beneath the proposed drill sites is 6–8 km thick—slightly thicker than normal oceanic crust. In detail, the crust beneath this part of the fore arc can be divided into five identifiable layers (Fig. F5). The first layer (V_P = 1.8–2.0 km/s) is mostly composed of thin sediments; this layer is actually very variable and both Sites BON-1A and BON-2A are chosen to have at least 100 m of sediment in order to facilitate drilling and casing operations of the uppermost part of the holes. The second layer (V_P = 2.6–3.3 km/s) is 1–2 km thick and probably consists of fractured volcanic rocks and dikes; this information contributes to our estimate of 1.25 ± 0.25 km as the likely lava thickness that we will need to drill in order to reach the sheeted dikes.

The third layer (V_P = 4.3–6.1 km/s) varies considerably in thickness, from 2 to 5 km. The velocities of the third layer correspond to those for the “tonalitic” layer in the arc farther west (Suyehiro et al., 1996; Takahashi et al., 1998), with which continuity may exist, though here they are more likely to represent sheeted dikes with perhaps some tonalites. The velocities of the fourth layer vary from 5.8 to 6.4 km/s, indicative of altered gabbroic rocks. The fifth layer, with a velocity of 7.0 km/s, possibly olivine gabbros and troctolites, thins and velocities decrease from west to east. This layer pinches out west of the proposed drill site. The sixth layer comprises the mantle wedge in the west and the plate boundary layer in the east. The velocity of the mantle wedge is 8.0 km/s in the westernmost part of the survey and decreases in velocity toward the trench, with a velocity of ~6.8 km/s immediately beneath the proposed drill site. The velocity of 6.8 km/s is not typical for the mantle and is taken to indicate that the mantle beneath the proposed drill site is pervasively serpentinized.

The best evidence for trench-parallel variations in seismic structure comes from a recent wide-angle seismic experiment along the Bonin fore arc (using densely deployed ocean-bottom seismometers), at a longitude ~20–30 km west of the proposed sites. Figure F9 shows the seismic velocity and reflectivity images of this profile (Kodaira et al., 2010). For ease of description, the model is divided into Units A–E, mainly on the basis of seismic velocity, and laterally continuous reflectors aligned subparallel to iso-velocity contours are partly used for defining the layer boundaries.

The structure in the northern half of the model is relevant to Sites BON-1A and BON-2A, which are located ~150 km along the section. It is characterized by thin crust, of similar thickness to that imaged by Kamimura et al. (2002) (Fig. F8). The total thickness of the units with crustal seismic velocity (<7.6 km/s; Units A–C) is less than ~10 km. In particular, the crustal units between Mukojima and Chichijima (230–290 km on the profile) are remarkably thin (<7 km). Reflections from the base of Unit C, which we interpret as the Mohorovicic Discontinuity (Moho), are not remarkably strong in this part of the profile. Another characteristic structure in this part of the profile is layering of the uppermost mantle.

The model of Figure F9 shows the average velocity of the 3 km thick Unit D to be 7.8 km/s. The top and bottom of this unit are not continuously imaged, but reflections from some parts of the boundary are clearly evident, for example at 15 km depth between 50 and 150 km distance. Unit D and its reflectors are interpreted as structural discontinuities within the uppermost mantle because the average velocity of Unit D immediately above the reflectors (7.8 km/s) is too high for crustal material. The petrological significance of this layer could correspond to a pyroxene-rich region inferred to define the crust–mantle transition beneath some arcs (e.g., Tatsumi et al., 2008). Unit E, which is in the deepest part of the well-resolved area, has a velocity higher than 8 km/s, as expected for mantle peridotite.

The model also shows abrupt thickening of the crustal units in the central part of the profile that can be attributed mainly to thickening of Unit C (lower crust). The profile in the southern part is intermediate between the profiles of the northern and central parts. It should be noted that the seismic structure modeled to the north of Chichijima is not fully consistent with a structure recently reported by Takahashi et al. (2009) that crosses the Bonin Ridge. Kodaira et al. (2010) discussed the possibility that this apparent inconsistency is due to the fact that the tomographic modeling of the across-arc profile by Takahashi et al. (2009) did not resolve the abrupt eastward thinning of crust beneath the Bonin Ridge.

Choice of drill sites

The three locations in the IBM fore arc that best enable us to study subduction initiation are (1) the southern Mariana fore arc southeast of Guam, (2) the Mariana fore arc along 18°N (which includes Sites 458 and 459), and (3) the Bonin fore arc along ~29°N. All have their merits. The southern Mariana fore-arc option has the advantage of being the type locality for the work of Reagan et al. (2010), who defined FAB and demonstrated that it underlies boninite, at least at this locality (see also Ohara et al., 2006, 2008). We necessarily rejected this location because of a lack of geophysical site survey information. The Bonin fore-arc option has the advantage of being in the same region as Chichijima (Bonin Island), the type locality for the key boninite rock type. It is part of a complete ophiolite section that has been sampled by dredging and diving (Ishizuka et al., 2011) and has full site survey data (S. Kodaira et al., pers. comm., 2013). The Mariana fore arc at 18°N has the advantage of being drilled during DSDP, so there is already a scientific platform upon which to build. Geophysical surveying including multichannel seismic (MCS) profiling was carried out by Mrosowksi et al. (1982) and Chapp et al. (2008), and further surveying was recently completed by D. Lizarralde et al. (pers. comm., 2013), providing crossing lines at both sites. Both the Bonin and central Mariana fore arcs are therefore good drilling targets.

Two important hypotheses to be tested by drilling are (1) that subduction initiation produces a consistent volcanic stratigraphy (from oldest to youngest): FAB, transitional lavas, low-Ca boninites, enriched HMA and related rocks, and normal arc volcanic rocks (Reagan et al., 2010); and (2) that this sequence was originally stacked vertically before erosion and therefore represents an in situ analog for sections through many supra-subduction zone ophiolites. Final choice of the Bonin locale as the primary drilling target and the central Marianas for contingency was based on the identification of the sheeted dike/FAB contact during Shinkai 6500 diving in 2009 along the inner wall of the Bonin Trench, near a location where the drill can spud into a sediment pond and sample the lower part of the fore-arc volcanic succession. We do not know the position of this contact in the Mariana fore arc at 18°N.

We debated whether Sites 458 and 459 or the Bonin fore arc would be the best place to start drilling through the contact of boninites overlying FAB, and two considerations led us to prefer the Bonin fore-arc site: (1) low-Ca boninites are found there, whereas only high-Ca boninites are found overlying Site 458; and (2) most of the boninite–FAB transition zone has already been sampled at Sites 458 and 459. Note, however, that Site 459 offers the opportunity to continue sampling this transition into true FAB and on into related intrusive rocks. Thus Sites BON-1A and BON-2A are best located to test ophiolite models. Site 459 provides a site survey–ready alternate site of near-comparable scientific significance.

Geology of the fore arc around proposed Sites BON-1A and BON-2A

The Bonin Ridge is an unusually prominent fore-arc massif in the Izu-Bonin arc that exposes early arc volcanic rocks on Chichijima, Hahajima, and smaller islands. These outcrops represent the best preserved and exposed sequence of igneous rocks associated with subduction initiation so far found on Earth. However, only part of the subduction-initiation igneous record is preserved on the islands. Submarine parts of the IBM fore arc (of which this ridge is part) contain a more complete record of subduction initiation, but by necessity, these parts have only been investigated by ocean drilling (e.g., Leg 60: Natland and Tarney, 1982; Leg 125: Arculus et al., 1992; Pearce et al., 1992), dredging (Bloomer and Hawkins, 1983), and diving (Ishizuka et al., 2006, 2011). The Bonin Ridge itself has not been drilled but has been investigated by dredging and manned submersible diving.

Figure F10 summarizes the distribution of rocks sampled during three expeditions: YK04-05, the first manned submersible (Shinkai 6500) diving survey of the western escarpment of the Bonin Ridge (Ishizuka et al., 2006); R/V Hakuho-maru KH07-2, which dredged 19 stations along the length of Bonin Ridge; and YK09-06 in the proposed Site BON-1A and BON-2A area (Ishizuka et al., 2011). They show the following, in particular:

1. Overall, there is an ophiolite-like sequence in the inner trench wall of lavas, dikes, gabbros, and peridotites.

2. Of the lavas and dikes, MORB-like tholeiites occupy the deepest part of the trench-side slope of the ridge (i.e., the easternmost part of the ridge). These are chemically indistinguishable from FAB as defined by Reagan et al. (2010).

3. Boninites crop out to the west and upslope of the FAB/MORB outcrops.

4. Younger tholeiitic/calc-alkaline basalt to rhyolite outcrops occupy the westernmost part of the Bonin Ridge and are especially well exposed on the western escarpment.

5. This spatial distribution of rock types is also found around 32°N, where boninitic rocks were drilled at Site 786. MORB-like basalts were also recovered near the trench at that latitude by Shinkai 6500, although these originally were interpreted as trapped crust of the Philippine Sea plate (DeBari et al., 1999). However, the Bonin section provides the better drilling location, having a simpler structure and more detailed sampling.

The 2009 diving survey using the submersible Shinkai 6500 examined, and better established, the igneous fore-arc stratigraphy exposed on the trench-side slope of the Bonin Ridge (YK09-06 cruise: 24 May–10 June 2009; Ishizuka et al., 2011). Two dive areas were located near the proposed drill sites, shown as boxes in Figure F10. The northernmost area near 28°25′N (Area A; see the more detailed map in Fig. F11) contains drill Sites BON-1A and BON-2A, located with the help of four dives (1149, 1150, 1153, and 1154) that examined the lower to upper crustal section formed in the earliest stage of oceanic island arc formation.

The deepest dive (1149) sampled gabbro and basalt/dolerite and appears to have traversed the boundary between the two units. The lower slope traversed during Dive 1149 is composed of fractured gabbro, whereas pillow lavas were observed in the uppermost part of this dive at ~6000 m water depth. Dives 1153 and 1154 surveyed upslope of Dive 1149. These two dives found outcrops of numerous diabase dikes, as well as fractured basalt lava cut by dikes, between 6000 and 5500 m water depth. The shallowest dive (1150) recovered volcanic breccia and conglomerate with boninitic and basaltic clasts. The boundary between boninite and basalt is estimated to lie at ~4800 m water depth because no basalt was recovered shallower than this.

Combined with results from other dives and dredging, a relatively simple fore-arc crustal igneous stratigraphy can be envisaged (Figs. F12, F13). The section, from bottom to top, consists of (1) mantle peridotite, (2) gabbroic rocks, (3) a sheeted dike complex, (4) basaltic lava flows (FAB), (5) volcanic breccia and conglomerate with boninitic and basaltic clasts, and (6) boninite and tholeiitic andesite lava flows and dikes. The uppermost part of this section is exposed in the Bonin Islands. These observations indicate that almost all of the fore-arc crust down to and deeper than the Moho is preserved and exposed in the inner trench wall of the Bonin Ridge.

Site survey data

The MCS data along the north–south profiles (KT06 and KT07) and the east–west profiles (IBr11 and IBr11n) included in “Site summaries” were acquired by JAMSTEC (S. Kodaira, pers. comm., 2013). For MCS data acquisition, a tuned air gun array with a total volume of 7200 inch³ was used, and the seismic waves were recorded by a 444-channel streamer cable ~6000 m long. Supporting site survey data for Expedition 352 are archived at the IODP Site Survey Data Bank.

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