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doi:10.2204/iodp.sp.350.2013

Background

The IBM subduction zone began as part of a hemisphere-scale foundering of old, dense lithosphere in the western Pacific at ~50 Ma (Bloomer et al., 1995; Cosca et al., 1998; Stern, 2004), perhaps aided by reorganization of plate boundaries throughout the western Pacific (Okino et al., 2004; Hall et al., 2003; Whittaker et al., 2007). The latter is consistent with the initiation of the Hawaiian-Emperor bend near Kimmei Seamount, suggesting a major change in Pacific plate motion at 50 Ma (Sharp and Clague, 2006). During this stage, the fore arc was the site of prodigious igneous activity (Fig. F3). Magmatic products consist of boninite, low-K tholeiite, and subordinate low-K rhyodacite everywhere the fore arc has been sampled, implying a dramatic episode of asthenospheric upwelling and melting associated with seafloor spreading over a zone that was hundreds of kilometers broad and thousands of kilometers long.

After ~5 m.y., the active magmatic front localized ~20 km east of the present front, building the first mature arc from 42 to 25 Ma (Taylor, 1992; Ishizuka et al., 2006). The rear-arc crust of this age is one of our targets (Layer L5, described below). This retreat of magmatism allowed fore-arc lithosphere to cool. Arc volcanism was accompanied until at least 33 Ma by spreading along the west-northwest–east-southeast (present coordinates)-trending Central Basin Fault in the western Philippine Sea (Deschamps et al., 2002; Deschamps and Lallemand, 2002; Taylor and Goodliffe, 2004). Eocene–Oligocene arc rocks have been found both at the frontal arc highs (Taylor, 1992), one of which was drilled at ODP Site 792 and is the target for drilling to the middle crust at proposed Site IBM-4 (Proposal 698 Full2), and at the Kyushu-Palau Ridge (Malyarenko and Lelikov, 1995; Mizuno et al., 1977; Shibata and Okuda, 1975; Ishizuka et al., 2011). In addition, Yamazaki and Yuasa (1998) reported three conspicuous north–south rows of long-wavelength magnetic anomalies in the Izu-Bonin arc that are slightly oblique to the present volcanic front. The eastern row correlates with the frontal-arc highs, the western row coincides with the KPR (the remnant arc), and the middle row lies at 139°E at our proposed site. Yamazaki and Yuasa (1998) attributed all three to loci of Oligocene magmatic centers.

At ~30 Ma, the IBM arc began to form its back-arc basins, the Shikoku Basin and Parece Vela Basin spreading systems, which met at ~20 Ma, stranding the KPR as a remnant arc (Fig. F3). This back-arc basin spreading stopped at ~15 Ma, simultaneous with the opening of the Japan Sea. This also caused the northernmost IBM arc to collide with Honshu beginning at ~15 Ma. Izu arc magmatism was minimal or even absent from 25 to 15 Ma during the opening of the Shikoku Basin, and when it resumed, the volcanic front was ~20 km west of its Oligocene position and has remained there ever since (Taylor, 1992). A new episode of rifting of the southern IBM arc began at ~7 Ma, with seafloor spreading to form the Mariana Trough back-arc basin beginning ~3–4 Ma (Yamazaki and Stern, 1997).

Neogene volcanism along the rear-arc seamount chains and at adjacent isolated seamounts began at ~17 Ma, slightly before the Shikoku Basin ceased spreading, and continued until ~3 Ma (Fig. F4) (Ishizuka et al., 1998, 2003b). The most obvious features of the Izu rear arc are the several ~50 km long en echelon chains of large seamounts striking N60°E (Figs. F4, F5). Basalts to dacites (Fig. F6) with ages ranging from 17 to 3 Ma have been dredged from many of the seamounts. Volcanism along these chains occurred sporadically along their total length, but lavas dredged from the top of seamounts in the western part of the chains are generally older than those to the east (Fig. F5); rear arc–type volcanism ceased altogether at the initiation of rifting behind the volcanic front at ~2.8 Ma (Ishizuka et al. 2002). The eastern end of the chains lies above the middle row of Yamazaki and Yuasa’s magnetic anomalies, and the western end lies on Shikoku Basin crust. In some cases (e.g., Manji and Genroku), the seamount chains seem aligned with large volcanoes on the volcanic front (e.g., Aogashima and Sumisu-jima, respectively) and with areas of thickened middle and total crust, but the association is imperfect.

Several explanations of the seamount chains have been proposed. They may be related to compression caused by collision between the southwest Japan and Izu arcs associated with the Japan Sea opening (Karig and Moore, 1975; Bandy and Hilde, 1983). Alternatively, they may have formed along Shikoku Basin transform faults (Yamazaki and Yuasa, 1998). A third hypothesis, presented in Figure F7, is that they overlie diapirs in the mantle wedge, such as the “hot fingers” proposed for northeast Japan (Tamura et al., 2002).

A less obvious aspect of the Izu rear arc is the 100 km wide extensional zone that lies between the Quaternary volcanic front and the eastern end of the seamount chains (Fig. F4). This is where all <3 Ma rear-arc rift-type volcanism has occurred, mostly on small cones or volcanic ridges associated with several kilometer–deep rift grabens that lie just behind large volcanoes on the volcanic front. These bimodal volcanic rocks differ in composition from those of the rear-arc seamount chains, which predate them (they are mostly mafic but range to dacite; Fig. F6). Post–3 Ma volcanism behind the volcanic front has been “rift type,” which is bimodal in silica and distinguishable in trace element and isotope ratios from both the volcanic front and the rear-arc chains. This volcanism is not simply intermediate in composition, as it is in location (Hochstaedter et al., 2001; Ishizuka et al., 2003a). The differences have been attributed to some combination of a transition from flux to decompression mantle melting as arc rifting commences, a change in the character of the slab-derived flux, or a change in the mantle (Hochstaedter et al., 1990a, 1990b, 2001; Ishizuka et al., 2003a, 2006; Tollstrup et al., 2010). Thus, two different magmatic suites occur in the Izu rear arc: “rear-arc type” from 17 to 3 Ma and “rift type” from 3 to 0 Ma. Both lie in the rear arc; neither formed in a back-arc basin.

Large basalt-dominated volcanoes are spaced at ~100 km intervals along the Quaternary volcanic front and correlate with thickened portions of arc middle crust and perhaps total crust (Kodaira et al., 2007a, 2007b). Rhyolite-dominated calderas lie between the large volcanoes of the volcanic front north of 31°N, with gaps of 50–75 km that have no volcanic edifices (Tamura et al., 2009). Similar wavelength along-strike variations in the thickness of middle and total crust also have been imaged in the rear arc from 28° to 32°N (Kodaira et al., 2008). Crustal development in the rear arc appears similar to the volcanic front, although no Quaternary volcanoes exist in the rear arc and Neogene chemical compositions show clear across-arc variations. Thus, the magmatic evolution of the rear arc is vital to understanding the history and composition of Izu arc crust.

The proposed sites for Expedition 350 lie directly above the middle row of north–south magnetic anomalies and near two large seamounts of the Manji seamount chain. Therefore, this row of anomalies may coincide with an area of thickened crust behind Aogashima. It is also within 10–15 km of several <3 Ma cones east of the Enpo chain and therefore should have received volcaniclastic sediment both from the small rift-type cones that were active in the Pliocene–Pleistocene and the rear-arc seamounts that were active in the Miocene. Finally, the proposed sites should overlie Oligocene arc rocks.

Primary proposed Site IBM-3C is at 31°47.38′N, 139°01.58′E and 2114 meters below sea level (mbsl) in the eastern half of the Izu-Bonin rear-arc seamount chain, ~90 km west of the arc volcano Myojin-sho (Fig. F5). The site is located between the Manji and Enpo rear-arc seamount chains where Neogene rear-arc sediments lap onto a Paleogene basement (Fig. F5). Based on site survey multichannel seismic (MCS) reflection data, the site location has been adjusted to be largely isolated from the volcanic front topographically by having a large edifice or trough or both between them. Because complete isolation from the volcanic front is not possible, we will use a combination of rock and mineral chemistry, clast morphology, and general sediment characteristics to identify and exclude volcanic front–sourced material (Fig. F6). There may be an inherent problem if volcanic front–type magmas were erupted in the rear arc between the cross-chains (i.e., if the hypothesis of inherent chemical asymmetry is not completely correct) or if in the past the volcanic front was located in the present rear arc. However, there is no evidence that the volcanic front was ever that far west, and Bednarz and Schmincke (1994) successfully used volcaniclast morphology to distinguish proximal from distal sources. We have chosen the best site based on

  1. Existing information from dredges and bathymetry (Fig. F5),

  2. Maximum protection from volcanic front mass wasting,

  3. Likelihood of receiving sediment from as much rear-arc diversity as possible (rear-arc seamount chains and rift-type magmas),

  4. Location east of the eastern extent of the Shikoku Basin as defined by magnetic lineations (K. Okino, pers. comm., 2013), and

  5. Overlying seismically “typical” middle crust with a low velocity gradient (Fig. F2).

The tectonic setting of different magmas: arc front (enriched and depleted), rear arc, and rift type

Basalts and andesites of the rear-arc seamount chains are enriched in alkalis, high–field strength elements (HFSE; e.g., Nb and Zr), and other incompatible elements but have less enriched Sr, Nd, Hf, and Pb isotopes compared to the volcanic front (Hochstaedter et al., 2000; Ishizuka et al., 2003a) (Fig. F6). Thus, we can clearly identify different magmatic sources (front arc versus rear arc versus rift type) using geochemical criteria such as these.

Figure F6A shows K2O and rare-earth element (REE) differences between the arc-front and rear-arc areas. A striking characteristic of orogenic andesites and associated rocks within many volcanic arcs of modest width is the consistent increase of their incompatible element concentrations, notably K2O, away from the arc front (Gill, 1981). Basalts and andesites along the Izu-Bonin volcanic front have significantly less K, U, and Th and lower Th/U than those from the rear of the arc (Fig. F6), which can be monitored using the gamma radiation logging tool. Rocks from the frontal volcanoes are low-K as defined by Gill (1981), but the rear arc–type lavas are medium- and high-K. Basalt and andesite magmas at the front of the Izu-Bonin arc are so depleted in K2O and other incompatible elements that they are dissimilar to the “average continental crust” of Rudnick and Gao (2004).

Figure F6B shows a chondrite-normalized REE plot for the Izu-Bonin basalt and andesites. All basalts from arc-front volcanoes are strongly depleted in the more incompatible LREEs compared with the middle and heavy REE (MREE and HREE). In contrast, basalts and andesites from rear-arc sites are enriched in LREE and MREE compared with HREE (Fig. F6B). Thus, rear-arc compositions are closer approximations to the average continental crust of Rudnick and Gao (2004).

Detailed descriptions of seismic stratigraphy

Six low-fold MCS profiles intersecting in the area of the possible drilling target (proposed Site IBM-3C) were obtained during Japan Agency for Marine-Earth Science and Technology (JAMSTEC) Cruise KY06-14 in December 2006 and JAMSTEC Cruise KR07-09 in June and July 2007 (Figs. F8, F9, F10, F11, F12, F13, F14). Our proposed Site IBM-3C is at the intersection of Lines IBM3d (Fig. F10), IBr5 (Fig. F11), and IBM3e (Fig. F12) (see composite in Fig. F14). These profiles between the Manji and Enpo seamount chains enabled us to determine 3-D structural images of sedimentary deposits in the targeted area. This seismic information, along with ages of the rear-arc seamounts, allowed us to determine the age and thickness of each layer and the best drilling site. Many sediment layers are laterally discontinuous. This lateral heterogeneity suggests a proximal nature of these deposits, which are similar to those of the uplifted Izu rear arc (i.e., in the Miocene–Pliocene Shirahama Group of the Izu Peninsula) (Tamura et al., 1991; Cashman and Fiske, 1990).

The reflector sequence can be divided into five layers (L1– L5; Table T1). We can estimate the age of the units by combining onlapping relationships and Ar-Ar ages reported by Ishizuka et al. (2003) from dredge samples of nearby seamounts (Fig. F9). We describe these characteristics from top to bottom as follows.

Uppermost seismic Layer L1 parallels the seafloor and may represent silicic channel deposits derived from Myojin Knoll. It is 32 m thick and onlaps seismic Layer L2. Layer L1 corresponds to the sediment layer most likely to be derived from the volcanic front.

The top of seismic Layer L2 has strong amplitude and is subparallel to Layer L1. Layer L2 dips southwestward and crops out in Line IBM3a. The lower part of this layer is commonly interrupted and deformed by faulting. The layer is well bedded with high-amplitude reflectors and a transparent portion, and its thickness is almost constant (>0.5 s) along each MCS line. The layer onlaps Manji Seamount (6.5–6.9 Ma) to the north, but sediment from a 1.96 Ma seamount overlies the top of Layer L2 in Line IBM3b to the south. In Line IBM3d (Fig. F10), the boundary between Layers L1 and L2 lies on the basement of a 2.77 Ma seamount. Consequently, seismic Layer L2 accumulated after ~3 Ma and coincides with back-arc extension and eruption of rift-type magmas to the east and south of proposed Site IBM-3C.

Like Layer L2, L3 is well bedded and laps onto seamounts of the Manji chain (Fig. F10). The interface between seismic Layers L2 and L3 crops out on Lines IBM3a and IBr5 (Fig. F11). Thus, Layer L3 seems to be 3–6.5 Ma and coincides with the development of the Manji and Enpo chain seamounts in the vicinity of the proposed site.

Seismic Layer L4 also is well bedded and subparallel to Layer L3, and its upper part laps onto nearby Manji chain seamounts in Lines IBM3d, IBM3e, and IBr5 (Figs. F9, F10, F11, F12, F13). The layer’s lower part is less clear and may onlap or be intruded by the seamounts. Overall, Layer L4 is more strongly faulted than the overlying units and is characterized by inhomogeneous, discontinuous reflectors of low frequency. From the seismic profile of Line IBr5 (Fig. F11), the boundary between Layers L4 and L5 could be as young as 9 Ma.

The age of seismic Layer L5 is important but uncertain. The boundary between Layers L4 and L5 has a high relief that may be erosional. Layer L5 uniformly lacks the well-bedded character of the overlying units, and its chaotic, discontinuous reflectors have low to medium amplitude. We attribute these features to greater lithification or the presence of lava. The simplest interpretation of all these features is that Layer L5 is Oligocene arc volcaniclastic rocks (with lavas?), and the boundary above it represents the unconformity developed during the Shikoku back-arc basin formation. The relationships of Layer L5 with younger units are uncertain; it appears intruded by seamounts of both chains on Lines IBM3d and IBM3e. Because western seamount volcanism becomes younger toward the east, we hypothesize that the unconformity does also (Fig. F5). This suggests that Oligocene basement may be uplifted beneath the Miocene rear-arc volcanoes. These hypotheses will be tested by drilling.

In Line IBr5, layers below Layer L1 are deformed by contraction east of proposed Site IBM-3C, closer to the Enpo chain. These structures are imaged as simple folds in other profiles. They may indicate the presence of a transcurrent fault at a high angle to Line IBr5, with some faults propagated beneath Layer L2. An eastward-dipping reflector (X) is recognized beneath Layer L5 in Line IBr5. This boundary coincides with a velocity gradient in the rear-arc crustal structure (T. Kodaira, unpubl. data) and may be the Eocene basement, but this is too deep to reach by drilling.

New, higher resolution MCS profiles for proposed Site IBM-3C (not shown) confirm the previous seismic interpretation, which is that the Neogene to Oligocene–Eocene transition occurs within volcaniclastic sediment at ~1200 meters below seafloor (mbsf) (Table T1). The boundary between Oligocene–Eocene sediment and igneous basement (crystalline rock), which we hope to reach by drilling, is deeper (~2100 mbsf; Table T1). Figure F14 shows seismic images and the velocity structure of the upper 10 km in this rear-arc region along Line IBr5. Generally, the velocity transition to >5 km/s is thought to represent the transition to igneous crystalline rocks. As shown in Figure F14, the 5 km/s iso-velocity contour lies at 2080 mbsf at proposed Site IBM-3C.

Supporting site survey data for Expedition 350 are archived at the IODP Site Survey Data Bank.

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