Background and objectives

International Ocean Discovery Program (IODP) Expedition 350 was one of three closely related IODP expeditions carried out in sequence in the Izu-Bonin-Mariana (IBM) arc system in 2014 (Figs. F1, F2, F3). Expedition 350 was the first expedition to be drilled in the Izu rear arc; all previous Integrated Ocean Drilling Program sites were drilled in or near the Izu-Bonin arc front or fore arc (Fig. F1B), leading to an incomplete view of Izu arc magmatism. Thus, the main objective of Expedition 350 was to reveal the history of “the missing half” of the subduction factory (Tamura et al., 2013). The second expedition (351) will focus on IBM arc origins by drilling west of the Kyushu-Palau Ridge (Fig. F3), where it is inferred that the foundation, origin, and early evolution of the IBM arc are recorded (Arculus et al., 2013). The third expedition (352) will examine the processes of subduction initiation, by drilling the outer IBM fore arc (Pearce et al., 2013).

The goal of Expedition 350 was to core and log one site on the Izu rear arc, Site U1437 (proposed Site IBM-3C). This site was chosen to provide a temporal record of rear-arc magma compositions, ideally from Eocene to Neogene time, allowing comparison with the previously drilled fore-arc magmatic record and determination of across-arc geochemical variations throughout the history of the arc system. Rear-arc magmatic evolution is important to understand because the chemistry of the samples dredged from the tops of rear-arc volcanoes are more similar to the average composition of continental crust than those of frontal volcanoes and seismic crustal structure suggests that the rear arc overlies the majority of “continental type” crust in the Izu arc system (Tamura et al., 2013). The Izu rear arc is therefore important for understanding how arc magmas and intracrustal differentiation produces crust that is similar in composition to the “averaged continental crust.”

A secondary goal of Expedition 350 was to obtain a geotechnical core for a potential future deep (5500 mbsf) drilling program at Site U1436 (proposed Site IBM-4) with the D/V Chikyu. Although this operation took only 1 day of the 60 day expedition, it yielded a rich, relatively complete record of Late Pleistocene fore-arc sedimentation that is strongly influenced by frontal arc explosive volcanism. This is highly complementary to the main objective of Expedition 350, the rear-arc subduction factory (Site U1437), and will enable us to further understand the formation and the evolution of the Izu arc system.

Before presenting our scientific results, we first review general information on the evolution of the IBM arc system and then provide background on three topics related to our scientific results in the Izu arc: the evidence that Paleogene crust lies beneath both drilled sites, the Quaternary arc-front volcanic record, and the Neogene rear-arc volcanic record.

Evolution of the IBM arc system

The IBM arc formed in response to subduction of the Pacific plate over the past 52 My (Stern et al., 2003). Subduction began as part of a hemisphere-scale foundering of old, dense lithosphere in the western Pacific (Bloomer et al., 1995; Cosca et al., 1998). During the subduction initiation stage (~52–47 Ma), igneous activity successively produced low-K mid-ocean-ridge basalt (MORB)-like tholeiite, boninite, and subordinate low-K rhyolite across the region that now lies in the fore arc. This suggests that sinking of the downgoing plate was rapidly followed by a dramatic episode of asthenospheric upwelling and melting, sometimes enhanced by solute-bearing water fluxes released from the downgoing plate, over a zone that was thousands of kilometers long and as wide as 200 km (Reagan et al., 2010). As subduction proceeded, hydrous mantle melting overprinted decompression mantle melting, establishing the first mature arc in Eocene to Oligocene time (Taylor, 1992; Ishizuka et al., 2006a, 2006b, 2011). This mature arc is labeled the “Kyushu-Palau arc” on Figure F2A and F2B. Eocene arc-front lava, dated at ~40–42 Ma by 40Ar/39Ar by Ishizuka et al. (2011), has been drilled in what is now the Izu fore-arc basement. However, no Oligocene arc rocks (lava or intrusions) have been recovered in the Izu arc, although Oligocene turbidites with andesitic clasts rest upon Eocene lava in what is now the fore arc (Taylor, Fujioka, et al., 1990; Gill et al., 1994), indicating that Oligocene arc volcanism occurred somewhere west in the region. By ~25 Ma, rifting began along the length of the Kyushu-Palau arc (Fig. F2C), and opening of the Shikoku Basin isolated the rear-arc volcanoes from the arc-front volcanoes (Fig. F2D), producing the Kyushu-Palau Ridge remnant arc, which has Eocene and Oligocene rear-arc rocks. However, Oligocene Kyushu-Palau arc-front rocks have not been found, although they are inferred to underlie the Neogene IBM arc on Figure F2B and F2C. Moreover, they may lie beneath the rear arc, as inferred by Kodaira et al. (2008) and discussed further below (see Figs. F4, F5). Seafloor spreading of the Shikoku and Parece Vela Basins at ~25–17 Ma was likely accompanied by a hiatus in arc magmatism (Fig. F2D), but the fore-arc sedimentary record shows that arc-front volcanism resumed by ~17 Ma (Stern et al., 2003), referred to as the Neogene arc, and shown as the IBM arc on Figure F2D and F2E, where the Neogene IBM arc front is shown in nearly the same position as the Paleogene arc front. However, pre-Quaternary rocks have not been recovered from the IBM arc front, perhaps because they are buried or could be partly remelted and/or remobilized during the Quaternary. In contrast, the Izu rear arc (Fig. F2E) has not been extensively buried or modified by Quaternary magmatic processes, so Neogene rocks are well preserved; these are dominated by ~17 to 3 Ma northeast-trending rear-arc seamount chains (Fig. F6), described in “Neogene rear arc volcanism, Izu arc.” The Marianas segment of the IBM arc (Fig. F1A) differs from the Izu segment by lacking the rear-arc seamount chains; instead, a new episode of arc rifting began at ~7 Ma, resulting in opening of the Mariana Trough back-arc basin by seafloor spreading at ~3–4 Ma (Fig. F3) (Yamazaki and Stern, 1997). Rifting of the Izu arc began at ~3 Ma, behind the arc front, described in “Neogene rear arc volcanism, Izu arc.”

We know more about the Neogene history of the IBM arc than we do about its Paleogene history; yet it is thought that most of the IBM crust was generated in the Paleogene (Eocene–Oligocene; Kodaira et al., 2008). Furthermore, silicic volcanoes of the Quaternary arc front and Miocene granitic rocks in the Izu collision zone on Honshu are inferred to have formed by melting of Eocene–Oligocene arc crust (Tamura et al., 2009, 2010). As discussed in “Scientific results,” Neogene rhyolite volcanism may be more important in the Izu rear-arc seamount chain than previously thought and could have resulted from melting of Paleogene “arc basement.” For this reason, we will now review the evidence for Paleogene arc basement highs in the Izu arc and discuss constraints on their age and origin.

Paleogene arc basement highs in the Izu arc

Magnetic and seismic surveys, summarized in this section, indicate that both IODP Sites U1436 and U1437 lie along buried north–south ridges that consist of magmatic crystalline rocks, which are inferred to be Oligocene–Eocene (Paleogene) in age. However, no Oligocene lava or crystalline rocks have been found in the Izu arc, as summarized here.

Three conspicuous, approximately north–south rows of long-wavelength magnetic anomalies were identified by Yamazaki and Yuasa (1998) in the Izu-Bonin arc system and attributed to loci of middle- to lower-crustal magmatic bodies (Fig. F3):

  • The western north–south anomaly corresponds to the Kyushu-Palau Ridge, where Eocene and Oligocene lava was dredged; these have been geochemically characterized as rear-arc magmas (Ishizuka et al., 2011), rifted off the Paleogene arc during the opening of the Shikoku Basin (Kodaira et al., 2008).
  • The eastern north–south anomaly lies in the modern fore arc near the arc front and corresponds to the Shin-Kurose Ridge (Fig. F3) (Yamazaki and Yuasa, 1998), also referred to as the Izu fore-arc high (Taylor, Fujioka, et al., 1990). The Shin-Kurose Ridge/fore-arc high forms a bathymetric high in the northern Izu arc and is buried beneath Oligocene to Quaternary volcaniclastic and sedimentary rocks in the southern Izu arc, at Ocean Drilling Program (ODP) Site 792 and Site U1436. Andesite lava in the lowest 82 m at Site 792 was referred to as “Oligocene basement,” on the basis of K/Ar ages (Taylor, Fujioka, et al., 1990; Taylor, 1992). However, 40Ar/39Ar dating on the lava from two different depths gave consistent and well-defined plateaus of 40.4 ± 0.8 Ma and 40.6 ± 0.3 Ma, or Eocene ages (Ishizuka et al., 2011) (Fig. F7). Similarly, andesite lava “basement rocks” drilled in the modern fore-arc basin outboard of this, at ODP Site 793, are Eocene (41 Ma; Ishizuka et al., 2011). Farther outboard, in the outer arc high drilled at ODP Site 786, the basement consists of boninite lava (45.3–46.7 Ma) overlain by andesite lava (44.7 Ma; all by 40Ar/39Ar; Ishizuka et al., 2006a), all Eocene in age. Thus, Oligocene basement has not been found in the fore arc.
  • The central north–south magnetic anomaly lies buried in the Izu rear arc (Fig. F3) and is referred to as the Nishi-shichito Ridge (Figs. F4, F5) (Yamazaki and Yuasa, 1998). This basement high has not been drilled and was one of the objectives of Expedition 350. Kodaira et al. (2008) ran a wide-angle seismic profile along the length of the rear-arc Nishi-shichito Ridge and compared it to a wide-angle seismic profile made along the length of the arc front by Kodaira et al. (2007a, 2007b) (Fig. F5). They divided the arc front into segments based on variations in the thickness of middle crust and did the same for the rear-arc Nishi-shichito Ridge. They concluded that although the thickness of the middle crust for each rear-arc segment is smaller than the frontal arc, the bulk compositions of the crust are almost identical. Furthermore, they used the match on middle crustal thicknesses to infer that the Nishi-shichito Ridge is a “paleo-arc” that obliquely rifted off the volcanic front in an extension direction parallel to the northeast–southwest Sofugan Tectonic Line (Fig. F4). The Sofugan Tectonic Line is the boundary between the Izu and Bonin arc segments (Fig. F3); south of it lies the prominent Bonin Ridge and the deep fault-bounded Ogasawara Trough to the west, produced by Eocene to early Oligocene arc magmatism and back-arc extension, respectively. This prominent arc ridge and fault-controlled back-arc basin are not present north of the Sofugan Tectonic Line, so we speculate that the Sofugan Tectonic Line originated as an accommodation fault between a region of high extension to the south and little or no extension to the north. Kodaira et al. (2008) propose that oblique rifting of the Nishi-shichito Ridge paleo-arc off the arc front occurred during the opening of the Shikoku Basin, sometime after ~30 Ma. If the oblique rifting model is correct, the crystalline basement beneath Site U1437, not reached during Expedition 350, may represent rear-arc crust but formed in a position much closer to the arc front than it is now; alternatively, it may represent arc-front crust that has become stranded in the rear arc by rifting. New seismic surveys undertaken in preparation for drilling at Site U1437, described briefly in “Scientific results,” also support the interpretation that the rear arc is underlain by Paleogene arc basement rocks.

Quaternary arc-front volcanic record, Izu arc

A brief overview of Quaternary arc-front volcanism is provided as background for discussion of Pleistocene tephra encountered at Sites U1436 and U1437.

The IBM volcanic arc system is an excellent example of an intraoceanic convergent margin where the effects of crustal anatexis and assimilation are considered to be minimal (Stern et al., 2003; Tatsumi and Stern, 2006). Nonetheless, volume estimates of rock types from the Quaternary Izu arc suggest that dacite and/or rhyolite form a major mode, although basalt and basaltic andesite (<57 wt% SiO2) are clearly the predominant eruptive products (Tamura and Tatsumi, 2002). About half of the edifices at the Quaternary volcanic front are calderas dominated by rhyolite (Figs. F8, F9) (Yuasa and Kano, 2003). Turbidites sampled during ODP Leg 126 in the Izu arc, which range in age from 0.1 to 31 Ma, are similarly bimodal (Gill et al., 1994).

The Quaternary volcanic front of the Izu arc shows along-strike correlations between crustal structure and the average composition of volcanic front magmas, shown by an active source wide-angle seismic study along the northernmost 550 km of the Izu arc front (Fig. F5). As illustrated in Figure F8, there is an along-arc periodic variation in average crustal thickness with a wavelength of ~80–100 km, reflecting variations in the thickness of the middle crust (whereas the lower crust has uniform thickness). These periodic variations correlate well with the average chemical composition of the overlying arc volcanoes; that is, the thicker middle crust underlies the basaltic island volcanoes, whereas the thinner middle crust underlies the rhyolitic submarine volcanoes (mainly calderas). Thus, the velocity structure of this part of the Izu arc crust, which has a complex 50 My history, appears to correlate well with the chemical composition of the Quaternary volcanoes. Contrary to the situation common for continental arcs, the basaltic volcanoes overlie lower average velocity (more continental-like) crust compared with the silicic volcanoes, which overlie thinner middle crust. This may indicate that the silicic volcanoes formed by melting of Eocene–Oligocene arc crust (Tamura et al., 2009) and that this process thinned the middle crust beneath them in some way. If this model is correct, the rear arc may show similar variation (i.e., silicic volcanoes on thinner middle crust and more mafic volcanoes on thicker middle crust). However, rear-arc Miocene volcanoes (white stars in Fig. F5) and underlying crustal structure (possibly Oligocene–Eocene) do not correlate each other like the volcanic front.

Neogene rear-arc volcanism, Izu arc

We refer to all Neogene volcanic rocks behind the Izu arc front as rear-arc volcanic rocks. Rear-arc volcanic rocks (Fig. F10) include (1) the ~17–3 Ma east northeast–trending basaltic to rhyolitic rear-arc seamount chains, (2) the <3 Ma bimodal back-arc knolls of the broad extensional zone, and (3) the <1.5 Ma bimodal volcanic rocks of the active rift immediately behind the arc front. Thus, Izu rear-arc volcanism falls into two magmatic suites: the <3 Ma bimodal rift-type magmas and the ~17–3 Ma basalt to rhyolite rear-arc seamount-type magmas. Both types lie within the rear part of the arc (i.e., behind the arc front) and lie on arc crust, although the westernmost end of the rear-arc seamount chains lies on Shikoku Basin oceanic crust. The bimodal rift-type magmas differ from both the volcanic front and the rear-arc seamount chains in trace element and radiogenic isotopic ratios; this has been variably attributed to (1) a transition from flux to decompression mantle melting as arc rifting commences, (2) a change in the character of slab-derived flux, or (3) a change in the magma source mantle through mantle wedge convection (Hochstaedter et al., 1990a, 1990b, 2001; Ishizuka et al., 2003a, 2006b; Tollstrup et al., 2010).

The Izu rear-arc seamount chains are as long as ~80 km and strike N60°E (Fig. F6). The tops of the Izu rear-arc volcanic chains were sampled by dredging, and their compositions range from basalt to rhyolite (Ishizuka et al., 1998, 2003b; Hochstaedter et al., 2000). Three main hypotheses have been proposed for the origin of the seamount chains:

  1. They are related to compression caused by collision between the southwest Japan and Izu arcs, associated with opening of the Japan Sea (Karig and Moore, 1975a; Bandy and Hilde, 1983).
  2. They formed along Shikoku Basin transform faults (Yamazaki and Yuasa, 1998).
  3. They overlie diapirs in the mantle wedge (Fig. F11), such as the “hot fingers” proposed for northeast Japan (Tamura et al., 2002).

In some cases (e.g., Manji and Genroku seamount chains), the seamount chains seem aligned with large volcanoes on the volcanic front (e.g., Aogashima and Sumisu, respectively) (Fig. F6); however, the alignment is imperfect, and it is not clear which hypothesis a perfect alignment would support.

A striking characteristic of volcanic arcs is the asymmetry in geochemical characteristics with distance from the trench, which was known prior to the advent of plate tectonics (Kuno, 1959; Dickinson and Hatherton, 1967). Izu arc-front rocks are low-K, but the rear-arc type lava is medium- to high-K (Gill, 1981) (Fig. F12). Similarly, arc-front volcanic rocks are strongly depleted in incompatible light rare earth elements (REEs) relative to the middle and heavy REEs, whereas lava from rear-arc seamount chains is enriched in light REEs. Gamma radiation logs obtained during Expedition 350 rear-arc drilling should record higher K, U, and Th and lower Th/U than those from drilling at the Izu-Bonin fore-arc sites, where tephra come from the arc front. On both K2O versus SiO2 and REE plots, the composition of the rear-arc seamount chain magmas is more similar to the continental crust composition than the arc-front magmas (Fig. F12). Thus, the Izu rear-arc magmatism and crust formation appears to be a better analog to generate continental crust than the arc front.

Although Site U1437 is in a location chosen to be topographically shielded from arc front–derived sediment gravity flows, arc front–derived ash fall and turbidity current deposits may be present. However, it should be possible to distinguish between arc-front and rear-arc seamount chain sources because the lava of the rear-arc seamount chains is enriched in alkalis, high–field strength elements (HFSE) (e.g., Nb and Zr), and other incompatible elements but has less enriched Sr, Nd, Hf, and Pb isotopes compared to the lava of the volcanic front (Hochstaedter et al., 2001; Ishizuka et al., 2003a; Tamura et al., 2007) (Fig. F12). However, the <3 Ma bimodal volcanic rocks of the broad extensional zone and narrow active rift are not easily distinguished from rear-arc seamount chain or arc-front rocks (Hochstaedter et al., 2001), and Site U1437 may not be shielded from sediment gravity flows from those sources. Therefore, we expect to be able to distinguish rear-arc and arc-front magmas for all rocks ranging from 17 to 3 Ma but not necessarily for rocks <3 Ma.

We do not know if arc geochemical asymmetry was present early in the history of the arc (in the Paleogene) or if it is strictly a Neogene feature. These two options are presented as “from the beginning” and “from the middle” hypotheses in Figure F13. Paleogene rocks were drilled in the fore arc, so their chemistry is known; however, no drilling has been done in the rear arc, so its Paleogene chemistry is unknown.

Site U1437 lies in a ~20 km wide basin in the low area between two major constructional volcanic ridges: the Manji and Enpo rear-arc seamount chains. It is therefore classified as a volcano-bounded intra-arc basin using the criteria elucidated by Smith and Landis (1995) as illustrated in Figure F14. In contrast, the active rift to the east of Site U1437 is a fault-bounded intra-arc basin. For simplicity, the volcano-bounded basin bounded by the Enpo and Manji rear-arc seamount chains is referred to as the Enpo-Manji volcano-bounded basin (Fig. F15). Similarly, we propose that future workers refer to other basins between rear-arc seamount chains by the names of the chains that bound them (e.g., Genroku-Enpo Basin and Manji-Kan’ei Basin, Fig. F6).