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doi:10.2204/iodp.sp.351.2013

Background

Geological setting

The northwestern part of the Philippine Sea plate, where proposed Site IBM-1 is located, is composed of the following geologic features: (1) Mesozoic remnant arcs (Daito Ridge group), (2) the West Philippine Basin (WPB), (3) the Eocene–Oligocene KPR, and (4) the ASB.

Mesozoic remnant arcs (Daito Ridge group)

The Daito Ridge group is a complex array of ridges and basins. This comprises three remnant arcs: the Amami Plateau, the Daito and Oki-Daito Ridges, and two ocean basins neighboring these ridges (the Kita-Daito and Minami-Daito Basins) (Fig. F2). Granites and arc volcanics of Cretaceous age (e.g., Matsuda et al., 1975; Hickey-Vargas, 2005) are exposed on the Amami Plateau, which has a crustal thickness of up to 19 km (Nishizawa et al., 2011). Geochemical characteristics of the volcanic rocks are consistent with formation of the plateau by Cretaceous-aged subduction zone magmatism (Hickey-Vargas, 2005). These remnant arcs predate the inception of IBM arc magmatism initiated at 52 Ma (Ishizuka et al., 2006a, 2011a).

The Daito Ridge is generally east–west trending and intersects the KPR at its eastern end (Fig. F2). Low-grade metamorphic rock, sedimentary rock, and some volcanic rock were recovered by dredging, apparently from beneath Eocene sedimentary rock, (Mizuno et al., 1975, 1978; Yuasa and Watanabe, 1977), whereas recent shallow drilling recovered fresh volcanic rock from the eastern part of the Daito Ridge. Two of these drilled samples (andesites), with the distinctive trace element characteristics of arc magmas, yielded ⁴⁰Ar/³⁹Ar ages of 116.9 and 118.9 Ma (Ishizuka et al., 2011b). Both of these ages are significantly older than the KPR volcanism. These results revealed that the Daito Ridge comprises Mesozoic arc rock overlain by middle Eocene sedimentary rock (e.g., Mizuno et al., 1978).

The Oki-Daito Ridge is WNW–ESE trending and ~600 km long. This ridge is characterized by crust ranging from 20 to 25 km in thickness, based on its seismic velocity structure (Nishizawa et al., 2011). A wide bathymetric high west of the Oki-Daito Ridge is the Oki-Daito Rise, which occupies an area of ~200 km². The eastern margin of the rise appears to overlap the western part of the Oki-Daito Ridge. The rise is characterized by much thinner crust (10–15 km) compared to the neighboring Oki-Daito Ridge.

The Kita-Daito Basin separates the Amami Plateau and the Daito Ridge. There are irregularly shaped seamounts and ridges in the basin. The Minami-Daito Basin is located between the Daito and Oki-Daito Ridges. This basin has many more bathymetric highs compared to the Kita-Daito Basin, including conical seamounts. Drilling at Deep Sea Drilling Project (DSDP) Site 446 in the western part of the basin recovered basalt sills with ⁴⁰Ar/³⁹Ar plateau ages of 51.3 and 42.8 Ma (Hickey-Vargas, 1998). These basalts are geochemically varied with tholeiitic and alkaline compositions and clearly have ocean-island basalt (OIB)–like geochemical characteristics (Hickey-Vargas, 1998).

West Philippine Basin

The WPB is an ocean basin occupying the western half of the Philippine Sea plate (Fig. F2). The Ryukyu and Philippine Trenches mostly bound the western margin of the basin. Between these trenches, the Gagua Ridge separates the WPB from the Huatung Basin, which is a Cretaceous-aged ocean basin (Deschamps et al., 1998, 2000).

The prominent bathymetric features in the WPB include the broad highs of the Benham Rise and the Urdaneta Plateau (Fig. F2). These features are located approximately equidistant from the Central Basin Fault, which is an extinct spreading center. The Benham Rise was drilled on its southeastern flank at DSDP Site 292. A thick plagioclase-porphyritic basalt layer was recovered from beneath Eocene–early Oligocene sediment (Karig, Ingle, et al., 1975). Hickey-Vargas (1998) reported ⁴⁰Ar/³⁹Ar ages of 35.6 and 36.2 Ma for this basalt and OIB-like geochemical characteristics. The Urdaneta Plateau has dimensions of about 300 km × 200 km and consists of two bathymetric highs, with seafloor fabrics similar to overlapping spreading centers or dueling propagators (Deschamps et al., 2008) and ⁴⁰Ar/³⁹Ar ages between 35.87 and 39.79 Ma (Ishizuka et al., in press). These oceanic plateaus within and north of the WPB form age-progressive volcanic chains, which are hypothesized to have been produced by a mantle plume that remained fixed at the spreading center for ~10 m.y. (Ishizuka et al., in press).

The origin of the WPB has long been debated. Hilde and Lee (1984) published magnetic lineation data for this area (Fig. F3A). Based on these data, they proposed that spreading from the Central Basin Fault formed the WPB. A spreading direction for the WPB was determined to be northeast–southwest between 60 and 45 Ma at a rate of 44 mm/y. After 45 Ma, the spreading direction changed to a more north–south direction associated with a reconfiguration of the Central Basin spreading center, and the spreading rate decreased to 18 mm/y. The major phase of spreading is inferred to have finished at ~35 Ma.

Other suggestions for the origin of the WPB include that of Lewis et al. (1982), who proposed the basin formed as a back-arc basin behind the east Mindanao-Samar arc, and Seno and Maruyama (1984), who suggested the WPB formed behind the KPR. The hypothesis that the WPB is of back arc origin has been further developed by recent studies (Fig. F3B, F3C) (e.g., Fujioka, et al., 1999; Deschamps and Lallemand, 2002, 2003; Okino and Fujioka, 2003), based on new detailed bathymetric and geomagnetic data, mainly acquired around the Central Basin Fault and the northern part of the basin. Combined with the notion that subduction initiation of the IBM arc was contemporaneous to or preceded the opening of the WPB, Hall et al. (1995), Hall (2002), and Deschamps and Lallemand (2002) proposed models assuming the WPB opened between the two subduction zones of the East Philippine and IBM arcs. Deschamps and Lallemand (2002) further proposed rifting started at 55 Ma and spreading ended at 33–30 Ma. The spreading axis was parallel to the East Philippine arc but became inactive when a new spreading ridge propagated from the eastern part of the basin. Spreading of the basin occurred mainly from this new axis, which rotated counterclockwise during its existence.

Another hypothesis for the origin of the WPB is the so-called “trapped basin” model (Fig. F3D) (e.g., Uyeda and Ben-Avraham, 1972). Le Pichon et al. (1985) proposed the extinct spreading center of the WPB was a remnant of the North New Guinea-Kula spreading ridge that was captured at 43 Ma. Jolivet et al. (1989) presented a similar, but modified, model, arguing the WPB is a remnant of the Pacific-North New Guinea spreading ridge captured at 43 Ma by inception of a new subduction zone (i.e., the IBM) along a transform fault.

Kyushu-Palau Ridge

The KPR forms the eastern margin of the WPB. It is now a remnant arc separating the WPB from the Shikoku and Parece Vela back-arc basins (Fig. F2). The KPR was an active arc in the Eocene and Oligocene (e.g., Mizuno et al., 1977; Shibata et al., 1977; Ishizuka et al., 2011b) and became isolated from the volcanic front of the IBM arc at ~25 Ma through the initiation of rifting and seafloor spreading in the Shikoku and Parece Vela Basins (Ishizuka et al., 2011b). The extinct spreading center of the WPB (Central Basin Fault) is truncated by the KPR at ~15°N. Radiometric ages for samples collected from the northern to central KPR range in age between 43 and 25 Ma but are mostly between 27 and 25 Ma, indicating arc volcanism ended on the KPR at about this time (Fig. F4). In other words, back-arc rifting (or spreading) of the Shikoku and Parece Vela Basins initiated at ~25 Ma (Ishizuka et al., 2011b). This interpretation is generally consistent with the estimated timing of rifting and spreading of the Shikoku Basin based on magnetic anomaly data and seafloor fabric observations. Okino et al. (1994) identified a magnetic lineation corresponding to Anomaly 7 in the westernmost (oldest) margin of the basin and suggested spreading started at 26 Ma.

The lack of systematic age variations of volcanic rock along the KPR indicates that rifting was initiated almost concurrently along the entire ridge between 30°N and 11°N. By extension, this means that initiation of the Shikoku and Parece Vela Basins and isolation of the KPR as a remnant arc occurred at about the same time.

Even though the KPR is a remnant half of the IBM arc, the geochemical characteristics of the KPR magmatic lithologies are distinct compared to those of Quaternary age at the IBM arc volcanic front. Whereas the Quaternary IBM arc has clear along-arc geochemical variations (e.g., more enriched isotopic and trace element signatures) (Fig. F5) both toward Honshu and the “alkalic volcano province,” typified by Iwo Jima at the Izu-Bonin/Mariana intersection in the volcanic front and the rear arc (Bloomer et al., 1989; Ishizuka et al., 2003, 2006b, 2007), the KPR does not show any systematic along-arc variations (Ishizuka et al., 2011b). This observation suggests the magmatic record to be obtained at this expedition’s primary site (IBM-1) will be representative of adjacent along-strike KPR arc magmatism and provide us with representative magmatic evolution of this Paleogene stage of the IBM system.

However, there are a couple of locations where distinct geochemical signatures have been obtained. One of these is where the Daito Ridge intersects the KPR. High-K andesite only occurs in this area. These Eocene volcanic rocks from the KPR/Daito Ridge intersection have a distinctly enriched trace element and isotopic character relative to the surrounding KPR samples. In particular, they have higher ²⁰⁶Pb/²⁰⁴Pb and light versus heavy rare earth element (REE) ratios in combination with small deviations of ²⁰⁸Pb/²⁰⁴Pb from the Northern Hemisphere Reference Line (i.e., Δ²⁰⁸Pb/²⁰⁴Pb) relative to the KPR (Fig. F6). Arc magmatism at the KPR/Daito Ridge intersection is thought to have been established on preexisting Daito Ridge crust, which is a Mesozoic remnant arc. Therefore, it is possible the distinct geochemical characteristics of the KPR/Daito Ridge intersection are related to the involvement of sub-Daito Ridge lithospheric mantle or subarc crust, which was metasomatized by a Cretaceous-aged subduction event. This observation demonstrates the critical importance of understanding the geochemical character of the arc basement, part of which will be drilled during Expedition 351 at Site IBM-1.

Amami Sankaku Basin

The initial products of the IBM system are preserved today in two longitudinal belts: (1) one forming the eastern margin of the WPB and abandoned as a remnant arc (the KPR) (Fig. F2) when the Parece Vela-Shikoku Basin opened, and (2) a second belt preserved in the IBM fore arc that is mostly submarine but emerges sporadically as islands, such as Chichi-jima and Guam.

The ASB is bordered to the west across a major north-south–striking fault scarp (Minami-Amami Escarpment [MAE]) by the Amami Plateau, to the south by the Daito Ridge group, and to the northeast by the KPR (Fig. F2). It is important to note the KPR is not oriented parallel to either the Amami Plateau-Daito Ridge or the MAE. Instead, the strike of the KPR is at a high angle to the trends of these features, apparently constructed independently of any preexisting basement fabric. If the MAE represents a fossil transform fault adjacent to an ASB basement formed through seafloor spreading, it appears the initiation of the KPR was independent of preexisting transform fault control.

The basement of the east Daito Ridge south of the ASB has a ⁴⁰Ar/³⁹Ar date of 118 Ma (Ishizuka et al., 2011b). Thus, the early ASB sediment and basement are likely to be Early Cretaceous or older (i.e., Neotethyan). A grid of Japanese multichannel seismic (MCS) profiles across the ASB (Japan National Oil Corporation, 1998; Higuchi et al., 2007) reveals a sedimentary section 1–2 km thick. The sediment is underlain by igneous basement, with a Mohorovicic Discontinuity (Moho) reflection a further 2 s two-way traveltime below that is typical of normal oceanic crust.

Reconnaissance of the subseafloor crust along the western margin of the ASB began during a Shinkai 6500 dive (337), conducted at the MAE (Fig. F7) in 1996. The dive started at the foot of a 1 km high steep cliff and ascended to the top of the escarpment. Lithologies identified on the dive transect from shallower to deeper parts along the submersible track line were the following: ash turbidite with burrows, altered tuffs, calcareous chalk, scoria, and basalt breccia with calcareous matrixes, all covered with pelagic mud and manganese sediments. Occasional pumice blocks were scattered on the sediment surface. Intact basaltic basement (pillows or sheet flows) was not observed.

Sediment samples obtained during this dive were predominantly pelagic brown mud, indicating deposition below the carbonate compensation depth (CCD). However, calcareous chalk is consistent with a shallower depositional environment for the older lithologies. The topography of the escarpment is a combination of gentler sedimented slopes with steep to occasional overhanging cliffs. A notable slump scar, erosional gulley, and slope failure–induced debris flow and turbidite were seen everywhere along the dive track. These phenomena strongly suggested the occurrence of past slope failure in relation to likely fault movement along the MAE.

Seismic studies/Site survey data

Seismic profiles of Ocean Drilling Program Site 1201

It is useful to examine the seismic and lithologic structures of a recently drilled site in the WPB because of the anticipated shedding of pyroclastic debris and ash from the KPR at the Expedition 351 primary site (IBM-1). One of the objectives of Ocean Drilling Program (ODP) Leg 195 was coring and casing a hole (Site 1201) in the WPB (Fig. F8) for the installation of a broadband seismometer as part of the International Ocean Network seismometer net (Shipboard Scientific Party, 2002). Site 1201 lies ~100 km west of the KPR on 49 m.y. old crust (near Chron 21) formed at the Central Basin spreading center of the WPB. We emphasize that although the results from Site 1201 cannot be used to satisfy the specific objectives for Expedition 351, some aspects of the sedimentological processes at the former location are contextually important. As Site 1201 drifted away from the Central Basin spreading center, volcanism ceased and about 0.5 km of sediment was deposited in three stages: (1) quiescent marine sedimentation in deep water into the late Eocene; (2) pelagic sedimentation mixed with, and finally overwhelmed by, volcaniclastic turbidites from the KPR from the late Eocene through the early Oligocene; and (3) waning turbidite deposition, followed by barren, deep-sea pelagic sedimentation below the CCD from the early Oligocene to the early Pliocene, when sedimentation ceased altogether, as reported by Salisbury et al. (2006).

Subsequent to ODP drilling at Site 1201, a new MCS line (D99-2) (Fig. F8) has been run northwest–southeast through the site and across the KPR into the Parece Vela Basin. This line is reproduced in Figure F7. The most obvious feature of this seismic line is the thickening of the upper part of the sedimentary package toward the prominent topographic high of the KPR and the relative constancy in thickness of the lower parts.

The Japan Oil, Gas, and Metals National Corporation (JOGMEC) acquired extensive MCS reflection data in the northern part of the Philippine Sea plate. These surveys covered a wide area of the IBM-KPR as well as the Amami Plateau and Daito Ridge regions; the major objective of the surveys was to obtain detailed images of the sedimentary and deeper crustal structures. These data provide important information for drilling into the sedimentary and igneous sections, particularly in the ASB.

The interpretation of these profiles coupled with information from DSDP holes (e.g., Holes 296, 445, and 448) and Site 1201 (Fig. F7) have resulted in the description of two layers in the ASB; the uppermost layer contains five stratigraphic units (herein referred to as “intervals”) (Table T1). The top interval (A; ~110 m thick) is estimated to comprise Pliocene–Pleistocene pelagic sediment. The second interval (B; ~160 m) is suggested to be upper Miocene turbidites, which may come from the KPR but is more likely pelagite given the termination of eruptive activity on the KPR by this time. The third interval (C; ~310 m) is suggested to be lower Miocene turbidites, which may be derived from a by-then magmatically extinct KPR. The fourth interval (D; 490 m) is estimated to be Oligocene and Eocene volcaniclastic turbidites from the KPR. The thickness of Interval D increases eastward toward the KPR with a maximum exceeding 1 km, which is consistent with a prominent source on the ridge. The nature of this section is important because it is likely to also be drilled during IODP Expedition 350 at the rear arc of the still-active IBM arc. Interval E (230 m) is suggested to be pelagic sediment of Eocene or older age; the distribution of this layer is discontinuous across the ASB but present at the primary site of Expedition 351 (Site IBM-1).

Site IBM-1 has been selected at the intersection of two MCS profiles (Lines D98-A and D98-8) obtained by JOGMEC (Figs. AF1, AF2) located ~50–80 km southwest of the nearest part of the KPR. The supporting site survey data for Expedition 351 are archived at the IODP Site Survey Data Bank.

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