1035—　　—

Ontong - Java Plateau, the World's largest Oceanic Plateau,

Has Been Subducted 50％, with the Remaining 50％

on the Surface, and with a ＜ 1％ Accretion on the

Hanging Wall of the Solomon Islands

Shigenori MARUYAMA＊, Atsushi UTSUNOMIYA＊＊＋ and Akira ISHIKAWA＊＊＊

Abstract　　The Ontong - Java plateau （OJP）, which is ca. 2000 km across, was formed at ca. 122 Ma and 90 Ma directly above the Pacific Superplume region in the central Pacific. It migrated westward after being formed, and has subducted southward due to the rapid northward movement of the Australian plate since 65 Ma. Subduction of OJP started at 20-25 Ma under the Solomon Islands to form an accretionary complex that is at least 1500 km in length along the trench, and about half of it has been taken deep into the lower mantle, with the remaining 1％ on the hanging wall of the overriding plate as fragments. 　　The Solomon Islands, an EW-trending narrow chain of accretionary complexes, are composed mainly of a pile of massive lava flows, auto-brecciated lavas with minor dikes and dominant pillowed basalts, which are a few kilometers thick, overlain by calcareous pelagic sediments, all derived from the subducted Ontong - Java plateau. The representative outcrops of those effusive rocks are introduced here in detail. 　　CA volcano-plutonism is recorded to have stopped in the overlying cover sediments on the Solo-mon Islands at 25-20 Ma, indicating the arrival of a buoyant OJP mass to change the tectonic regime of the hanging wall plate together with extensive tectonic erosion of the frontal portion of Solomon-Australia plate.　　Subduction of most of the OJP together with Pacific slab into the deep mantle, with the remaining minor fragments accounting for ＜ 1％ volume seems to be consistent with Jurassic-Cretaceous examples in Japan and other orogenic belts around the world.

Key words： Ontong - Java plateau （OJP）, Cretaceous pulse, Pacific superplume, tectonic erosion, Solomon Islands

　　＊ Department of Earth and Planetary Sciences, Tokyo Institute of Technology, Tokyo, 152-8551, Japan　＊＊ Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan＊＊＊ Department of Earth Science and Astronomy, The University of Tokyo, Tokyo, 153-8902, Japan　　＋ Present address: Japan Geocommunications Co. Ltd., Tokyo, 160-0004, Japan

地学雑誌（Chigaku Zasshi）Journal of Geography120（6）1035⊖1044 2011

The 100s: Significant Exposures of the World （No. 5）

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I．Introduction

　The Ontong-Java plateau is the world's largest oceanic plateau at ca. 2000 km across with a 40-50 km thick oceanic crust. It is located in the western Pacific to the north of the Australia continent （Fig. 1）. The great bulk of the OJP is located off the Solomon trench to the north, much of it at a general elevation of ca. 2000 m above the surrounding ocean-floor at a depth of 4000 m （Kroenke, 1989）. Ar-Ar dating of drilled basalts by ODP and DSDP boreholes into the OJP （e.g., Ishikawa, 2011, this issue） and of basalts from Isabel and Malaita Islands （Fig. 2）suggests a bimodal distribution of age cluster around 122 Ma and 90 Ma as a second population （Mahoney et al., 1993; Tejada et al., 1996）.　In the Cretaceous pulse period of the Pacific superplume, the OJP was formed as an oceanic plateau with a volume of 36-101.3 × 106 km3, which cannot be explained by the mantle plume within the upper mantle. The plume is larger than the diameter of 660 km （thickness of upper mantle, Coffin and Eldholm, 1994）, hence the CMB origin of the superplume （Fig. 3）.　Most previous research considers the OJP to have just been trapped within the hanging wall plate at 2 Ma. The authors, however, propose that subduction began at 20-25 Ma, when the buoyant OJP docked, choked, and stopped arc magmatism by reducing the angle of subduction with the re-sultant cessation of steady-state mantle upwelling in the wedge mantle. The extensive modification of the hanging wall region began after the docking of OJP, and triggered the Miocene opening of the Woodlark basin, which is the major conclusion of this summary review.　Another view is the tectonic implication as to whether or not it can be subducted against its buoyancy with respect to mantle peridotite such as is the case of the Indian collision. OJP collision-subduction-accretion is an excellent on-going example. As a result, the geology of OJP of the Solomon Islands indicates it is subducted entirely together with the underlying slab, with ＜ 1％ remaining on the surface of the overriding plate. This is consistent with Archean and older

analogs such as komatiite and associated OIB rocks in the Archean.

II．Geology of Malaita, Isabel, and related regions 　

　The tectonic position of the Solomon Islands is shown in Fig. 2. The Solomon Islands are located 5-6 km above the Northern Solomon Trench （NST）, running along the trench and extending 1500 km in the NE-SW direction. The subducting Pacific together with the OJP can be traced down to at least 200 km and presumably is much deeper than 400 km because of the presence of deep seismicity （Mann and Taira, 2004）.　As shown in Fig. 2 below, the newly opened Miocene Woodlark basin （300 × 800 km） is being subducted northeastward under the Solomon Islands to form CA volcanism very close to the NBT-SCT trench （Fig. 2）. Therefore, the Solomon Islands are underlain by subduction at both sides: from the NE by the Pacific and from the SW by the Solomon Sea microplate. CA volcanism with Pacific subduction stopped at ca. 25-20 Ma, having continued for a long time （Peterson et al., 1997）. A fault named KKKF runs along the southern part of Isabel Island and divides the basement rocks of the Cretaceous MORB with a south-dipping high-angle reverse fault （Fig. 4）. The KKKF fault is a dividing fault between OJP and the hanging wall Solomon Sea Plate. The overlying unit is composed largely of 60 Ma San Jorge CA volcanics and contemporaneous CA Kolose'eru gabbros, either of which intruded or covered the Cretaceous MORB crust due to Pacific subduction during the Eocene to Miocene （Fig.

GMT

Fig. 1　Locality map of the Ontong Java plateau in the western Pacific.

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Fig. 2　Tectonic outline of OPJ to the NE of New Guinea and Australia （Mann and Taira, 2004） （top） and enlarged map of the Solomon Islands and Solomon Sea （below）. Note at the back （SW） of the Solomon Islands there is an active spreading of Woodlark basin. It is being subducted northeastward to cause near-trench CA volcanism shown by yellow dots （site of active volcano）. Also shown is the distribution of seismicity dominantly down to 200 km with minor deep earthquakes at around 400 km. The relative plate motion between the Pacific and Australia （Louisiade Plateau） 12.6 cm/yr in the NE-SW direction. The Solomon Islands run parallel to the active trench to the Pacific and form an uplifted portion in front of the subducting OJP. The trench with subducting OJP contact extends over 1500 km.

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4 cross-section）. Along the boundary between the subducted OJP and overlying Cretaceous arc crust, a blueschist-bearing serpentinite mélange belt is seen （Fig. 4; Ota anzd Kaneko, 2010）.　The structural and stratigraphical relationships are well observed on Malaita Island between the OJP and overlying units （Fig. 5）. The tectonically sliced fragments of OJP form domes on the struc-tural bottom along the central high-mountain range with a general strike of NW-SE （Fig. 5）. The OJP is covered with Kware mudstone, Alite Limestone, and Saufa Chalk Formations in ascending order （see inset stratigraphic column of Fig. 5）. The oceanic kimberlite in a broad sense or alnöite in a strict sense intruded or erupted at 34 Ma （for more details see Ishikawa （2011）, this issue）.　The Saufa Chalk Formation, which is in turn overlain by the Tomba Siltostone Formation spreads over a wide area of the Island. Because of the recent uplifting of the Solomon Islands, macroscopic calcareous sedimentary rocks seem to be similar in stratigraphy established by OJP bor-ings in the Pacific floor （e.g., Peterson et al., 1997）, and it is difficult to determine the exact timing of OPJ subduction. For this reason, the initiation of subduction has been debated. Most previous researchers consider it occurred at 2 Ma or is much younger （Mann and Taira, 2004; Phinney et al., 2004; Cowley et al., 2004）.

III．When did Ontong Java Plateau begin subduction?　　　　

　However, the presence of a series of layer-parallel thrusts not only within accreted OJP fragments but also overlying the Saufa Chalk Formation, and 34 Ma strongly alkaline volcanic rocks, presumably not within some Saufa Chalk horizons and the Tomba siltstone Formation above, suggest the time that subduction was initiated was much earlier. One indication is the cessation of CA volcanism at 25-20 Ma and there-after until the present （Peterson et al., 1997）. Moreover, it should be stressed that extensive tectonic erosion is present between the Cretaceous MORB basement intruded by Eocene CA volcano-plutonism and the accreted OJP seen on Isabel Island （See Fig. 4）.　The sudden tectonic erosion and direct contact of the ca. 50 Ma arc basement with the OJP fragment, and associated intrusion of serpentinite with Miocene blueschists along the KKKF thrust plane, suggest that buoyant OJP subduction began at 25-20 Ma, which prevented further high-angle subduction of the Pacific lithosphere due to its buoyancy. The OPJ was choked at the trench, and shallow angle subduction thereafter caused the stop of steady-state mantle upwelling in the mantle wedge to supply material for continuous CA volcanism （Fig. 6）.　Moreover, the OJP buoyant subduction forced the frontal destruction of the Cretaceous-Eocene volcanic arc, and delivered tectonically eroded materials into the deep mantle by subduction. Through this tectonic erosion, the accretionary complex formed by the subduction of the precursor Pacific slab before the OJP arrived at trench has been completely eroded, having direct contact been made OJP with the Eocene arc basement called San Jorge Volcanics and Kolose'eru Gabbro （Fig. 4）.

IV．Origin and history of OJP

　OJP was formed directly above the Pacific superplume during the Cretaceous pulse period, ca. 125-80 Ma （Larson, 1991）. The Cretaceous pulse can be observed only in the Pacific domain

Fig. 3　Sizes of plumes of the major oceanic plateaus and large igneous provinces （LIPs, after Coffin and Eldholm, 1994）.

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and not in both Indian and Atlantic regions. This can be explained by the model showing the collapse of circum-Pacific stagnant slabs into the lower mantle in the Cretaceous with resultant mantle upwelling, which has been selectively accelerated through a majorite window with a positive Clapeyron slope directly above the Pacific superplume （Maruyama et al., 2007）. OJP and related huge plateaus or seamounts such as Manihiki Plateau, Marcus Wake Seamounts, East Mariana Basin, Magellan Seamounts, Marshall Gilbert Seamounts, Magellan Rise, Mid-Pacific Seamounts, Pigafetta Basin, Line Islands, Shatsky Rise, Hess Rise, and Caribbean LIPs, as well as accreted on-land ophiolitic rocks such as Sorachi and Mikabu Ophiolites in Japan and East

Sulawesi Ophiolite in Indonesia （Kadarusman et al., 2004） （see a summary by Utusnomiya et al., 2007）, which were all formed directly above the Pacific superplume （Maruyama, 1994） in the late Jurassic to Cretaceous, and migrated around the Pacific rim to be trapped in Pacific-type orogenic belts. Mostly minor fragments are observed within the latest Jurassic to Cretaceous accretion-ary foldbelts around the Pacific Ocean （Fig. 7）.

V．Field Exposures on Malaita Island

　Along a steep river with a NS trend at the central part of Malaita Island （see lines 1, 2, and 3 in Fig. 5 right） a series of excellent exposures of fragments of OJP are observed. In the following, we explain representative exposures. Fig. 8a

Fig. 6　A graphic showing the tectonic evolution of the Solomon Islands before and after OJP subduction.

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Fig. 7　History of OJP together with related plateaus and seamounts in the Central Pacific （Utusnomiya et al., 2008）.

 1042—　　—

（a）

（b）

（c）

Fig. 8　（a） Sheet-flows of olivine basalts ca. 5-10 m thick, with radial chilled cracks. （b） Glassy skin covers pillowed lava flows. Glassy surface of pillowed lava flows is well preserved with minor alternations. （c） Cut surface of pillow tubes with a hollow is cemented by secondary quartz or successively re-new ed flow. Pillow within a pillow structure. See hammer for scale. Nearly horizontal pillows, forms, and radial cracks are clear.

 1043—　　—

shows the bottom of a 5-10 m-thick massive sheet lava flow of olivine basalt. The bottom boundary with a chilled margin occurs near the water-flow surface. Nearly horizontal parallel layering is seen repeatedly in the sheet flow due to the cumulated difference in the amounts of olivine and plagioclase. Radiating cracks are formed perpendicular to the chilled surface （Fig. 8a）.　The excellent structure of pillow lava flows is seen along the riverside, e.g., Fig. 8b. The surface of the glassy black skin of pillow lava is well preserved. Fresh volcanic glass is observed under a microscope. Fig. 8c is a cross-section of pillow tubes with radial cracks and pillow in a pillow structure. The few millimeter-thick outer margin is a palagonite layer composed of a mixture of secondary clay minerals with or without zeolites. Some of the pillows are occupied by late-stage flows in the open tube at the center, or remain vacant. In some cases, secondary quartz replaces them （see white portion within pillows）.

References

Coffin, M.F. and Eldholm, O. （1994）: Large igneous provinces: Crustal structure, dimensions, and external consequences. Reviews of Geophysics, 32, 1-36.

Cowley, S., Mann, P., Coffin, M. and Shipley, T. （2004）: Oligocene to Recent tectonic history of the Central Solomon intra-arc basins as determined from marine seismic reflection data and compilation of onshore geology. Tectonophysics, 389, 267-307.

Ishikawa, A. （2011）: Spectacular mantle xenoliths derived from “Oceanic Kimberlite”, Malaita, Solomon Islands: A unique window into the Earth's deep interior. Journal of Geography （Chigaku Zasshi）, 120, 1026-1034．

Kadarusman, A., Miyashita, S., Maruyama, S., Parkinson, C.D. and Ishikawa, A. （2004）: Petrology, geochemistry and paleogeographic reconstruction of the East Sulawesi Ophiolite, Indonesa. Tectonophysics, 392, 55-83.

Kroenke, L.W. （1989）: Interpretation of multi-channel seismic reflection profile northeast of the Solomon Islands from the southern flank of the Ontong Java Plateau across the Malaita anticlinorium to the Solomon island arc. in Geology and Offshore Resources of the Pacific Island Arcs-Solomon Islands and Bougainville, Papua New Guinea Regions edited by Vedder, J.G. and Bruns, T.R., Earth Science Series,

Circum-Pacific Council for Energy and Mineral Resources, Houston, TX, 145-148.

Larson, R.L. （1991）: Geological consequences of superplumes. Geology, 19, 963-966.

Mahoney, J.J., Storey, M., Duncan, R.A., Spencer, K.J. and Pringle, M. （1993）: Geochemistry and geochronology of the Ontong Java Plateau. in The Mesozoic Pacific edited by Pringle, M., Sager, W., Silter, W. and Stein, S., AGU Geophysical Monograph, AGU, Washington, D.C., 233-261.

Mann, P. and Taira, A. （2004）: Global tectonic signi fi-cance of the Solomon Islands and Ontong-Java Plateau convergent zone. Tectonophysics, 389, 137-190.

Maruyama, S. （1994）: Plume Tectonics. Journal of the Geological Society of Japan, 100, 24-49.

Maruyama, S., Yuen, D.A. and Windley, B.F. （2007）: Dynamics of plumes and superplumes through time. in Superplumes: Beyond Plate Tectonics edited by Yuen, D.A., Maruyama, S., Karato, S. and Windley, B.F., Springer, Dordrecht, 441-502.

Ota, T. and Kaneko, Y. （2010）: Blueschists, eclogites, and subduction zone tectonics: Insights from a review of Late Miocene blueschists and eclogites, and related young high-pressure metamorphic rocks. Gondwana Research, 18, 167-188.

Peterson, M.G., Neal, C.R., Mahoney, J.J., Kroenke, L.W., Saunders, A.D., Babbs, T.L., Duncan, R.A., Tolia, D, and McGrail, B. （1997）: Structure and deformation of north and central Malaita. Solomon Islands: Tectonic implications for Ontong Java Plateau-Solomon arc collision, and for the fate of oceanic plateaus. Tectono-physics, 283, 1-33.

Phinney, E., Mann, P., Coffin, M. and Shipley, T. （2004）: Sequence stratigraphy, structural style, and age of deformation of the Malaita Accretionary prism （Solomon arc-Ontong-Java Plateau convergent zone）. Tectonophysics, 389, 221-246.

Tejada, M.L.G., Mahoney, J.J., Duncan R.A. and Hawkins, M.P. （1996）: Age and geochemistry of base-ment and alkalic rock of Malaita and Santa Isabel, southern margin of the Ontong-Java Plateau. Journal of Petrology, 37, 361-394.

Utsunomiya, A., Ota, T., Windley, B.F., Suzuki, N., Uchio, Y., Munekata, K. and Maruyama, S. （2007）: History of the Pacific Superplume: Implications for Pacific paleogeography since the Late Proterozoic. in Superplumes: Beyond Plate Tectonics edited by Yuen, D.A., Maruyama, S., Karato, S. and Windley, B.F., Springer, Dordrecht, 363-408.

Utsunomiya, A., Suzuki, N. and Ota, T. （2008）: Pre served paleo-oceanic plateaus in accretionary complexes: Implications for the contributions of the Pacific Superplume to global environmental change. Gond-wana Research, 14, 115-125.

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世界最大のサイズのオントンジャワ海台は約 50％すでに

マントルに沈み込み，残りは西太平洋プレートの表層に

残存するが，上盤側プレートに全体の質量の 1％以下しか

ソロモン諸島付加体に付加していない

丸 山 茂 徳＊ 宇 都 宮 敦＊＊＋ 石 川 晃＊＊＊

　オントンジャワ海台は太平洋中央部の太平洋スーパープルームによって白亜紀（122 Maと90 Ma）につくられた巨大な海台である。形成後，西方に移動し，オーストラリアプレートが北上することによってプレート相対運動が変化して，20-25 Maごろから海溝に沈み込みはじめた。その結果，ソロモン諸島には，それらの海台の破片が付加体として露出している。付加しはじめた時期については異論があり，これまでの多くの研究者は 2 Ma以降であると考えてきた。　しかしながら，われわれは地質調査と文献に基づいた証拠，すなわち一連の逆断層の形成と構造浸食，および 20-25 Ma以降の沈み込み帯火山活動の停止から以下の解釈を行った。海台が 25-

20 Maに海溝に沈み込みを開始すると，海台のその浮力のために、沈み込み角度が低角度になり島弧火山活動が停止した。そして始新世以降の付加体を構造浸食して，崩壊物をマントルへと運んだ。また中新世におけるオントンジャワ海台の衝突と沈み込みが背後のウッドラーク海盆の拡大の開始のトリガーになったと思われる。　過去 25-20 Maにわたるオントンジャワ海台の沈み込みは，付加体としては海台のごく一部（＜1％）のみを上盤側のプレートに付加したのみで，主要部のほとんどはマントル深部へ沈み込んだと思われる。これは顕生代の環太平洋造山帯に普通にみられる現象である。

キーワード：オントンジャワ海台，白亜紀のパルス，太平洋スーパープルーム，構造浸食，ソロモン諸島

　　＊ 東京工業大学大学院理工学研究科　＊＊ 台湾科学院地球科学研究科＊＊＊ 東京大学大学院総合文化研究科　　＋ 現所属：株式会社ジオ・コミュニケーションズ