~eefo~u~~~s~c~,181 (1990) 179-205 179 Elsevier Science Publishers B.V., Amsterdam

Accreted oceanic materials in Japan

Y. Isozaki I, S. Maruyama 2 and F. Furuoka 3 ’ Department of Geology and Mineralogical Sciences, Faculty of Science, Yamaguchi University, Yamaguchi 753 (Japan) 2 Departmen! of Earth Sciences and Astronomy, College of Arts and Sciences, University of Tokyo at Komaba, Tokyo 153 (Japan) ’ Department of Earth Sciences, Faculty of Education, Toyama University, Toyama 930 (Japan) (Received June 61989; revision accepted September 4,1989)

ABSTRACT

Isozaki, Y., Maruyama, S. and Furuoka, IF., 1990. Accreted oceanic materials in Japan. In: M. Kono and B.C. Burchfiel (Editors), Tectonics of Eastern Asia and Western Pacific Continental Margin. Tectonophysics, 181: 179-205.

The Phanerozoic &cum-Pacific erogenic belts contain numerous ocean-derived materials accreted through plate converg- ing processes. Japanese Islands, in particular, display various kinds of oceanic materials of different origins including fragments of seamounts, oceanic reef limestone, MORB-like rocks and oceanic mantle, and pelagic sediments. The compila- tion of these rocks in many subduction complexes of Late Permian to the present, led to following conclusions. Accretion processes work effectively only for materials primarily composing the upper portion of subducting oceanic crust, i.e. Layer 1 and Layer 2. Many fragments of seamount with alkali basalt (600), hot-spot seamount (26), oceanic reef limestone (291), MORB-like basalt (200), and numerous cherts (more than 1000) are recognized as ancient oceanic materials accreted to the Japanese Islands. However, gabbros and mantle materials of Layer 3 and lower parts of the oceanic lithosphere, scarcely occur in subduction-accretion complexes except for a few examples of back-arc basin or fore-arc origin. Accretion occurs episodically. In Southwest Japan, oceanic materials were accreted intermittently in (a) end-Permian, (b) Middle-Late Jurassic, (c) Late Cretaceous times, (d) at ea. 50 Ma, and (e) in Miocene times, while in Northeast Japan and Hokkaido this ocwrred in (b) Middle-Late Jurassic, (cf Late Cretaceous, and (f) Early Cretaceous times. In wntrast to the general belief on accretion of younger oceanic plates, the majority of Japanese subduction-accretion complexes were formed during the subduction of plates, up to 160 Ma old. The accretionary events in end-Permian and Middle-Late Jurassic times coincide with northward collision of ancient island arcs, oceanic rises or seamount chains (of hot-spot origin) with the Asian continent. Accretion relevant to subduction of older plates may be controlled by the collision-subduction process of these topographic reliefs on an oceanic plate. In addition, the chronological coincidence with the continent collision-amalgamation between the Sino-Korean and Siberian platforms and between the Sino-Korean and Yangtze blocks, also implies collision-induced voluminous supply of elastics from back-arc regions and its contribution to the formation of huge accretionary complexes. Accreted fragments of ancient seamounts are much smaller than the average size of modem seamounts. This impties that most parts of a colliding seamount are not accreted but subducted together with the underlying oceanic crust to much deeper levels. With respect to the metamorphic grades for Japanese subduction complexes, oceanic materials have less than 1 vol.% in the zeolite facies, 15-20% in the prehnite-pumpellyite metagraywacke facies, and ca. 30% in the greenschist/glaucophane schist facies and albite-epidote amphibolite facies. This relationship indicates that the major process for landward accretion of oceanic materials is not off-scraping or sedimentary mixing at the trench, but underplating (subcretion) at much deeper levels of a subduction zone.

Introduction been interpreted as products of tectono-sedimen- tary processes active at ancient subduction zones. Since the pioneering studies by Dietz (1964) Studying these complexes is important for under- Hamilton (1969) and Dewey and Bird (1970) un- standing accretionary processes once active at an- der the breaking-through paradigm of plate cient convergent margins, and complements stud- tectonics (cf. Miyashiro et al., 1982), accretionary ies of modern systems (e.g. Dickinson, 1971; complexes in Cord~~er~-tee erogenic belts have Maxwell, 1974). The recognition of oceanic

0040-1951/90/$03.50 0 1990 - Elsevier Science Publishers B.V. 180 Y. 1SOZAKl ET AL materials in ancient erogenic belts, in particular, is critical as these materials provide the only affor- dable source of information on extinct oceanic plates that have interacted with the continents to form orogens. Such understanding was once culminated in the simple analogy between on-land ophiolites and modern oceanic crust-upper man- tle materials (cf. Coleman, 1971). Furthermore, under the influence of the “allochthonous terrane” concept since late 1970s (e.g. Coney et al., 1980), strong emphasis has been given on landward ter- rane-accretion by collision of topographic reliefs on the ocean floor, such as seamounts, oceanic rises, plateaus and aseismic ridges (e.g. Nur and Ben-Avraham, 1982). However, such over-sim- plified ideas are now faced with criticism (e.g. SengBr, 1988). The Japanese Islands, a segment of circum-

Pacific erogenic chain, record a history of accre- Fig. 1. Distribution of modem topographic reliefs such as tion that has been active since the Late Paleozoic seamounts, plateaus, rises and ridges on the western Pacific and still continues at present. Adjacent to the seafloor near the Japanese Islands. Oceanic features: solid Japanese Islands on the western Pacific seafloor black = se~ount, blank = hot-spot seamount, striped = rise are abundant topographic reliefs including and ridge. Broken tines show predicted outer limit of the disappearing part of the ocean floor in the coming 10, 20, and seamounts of hot-spot origin such as the Hawai- 30 Ma. See Table 1 with the predicted numbers and categories ian-Emperor seamount chain, oceanic rises such of toppographic reliefs, which are now on the westernmost as the Shatsky and Hess Rise and other minor Pacific seafloor but will collide against the Japanese Islands in seamounts and/or aseismic ridges. These topo- 10, 20 and 30 Ma after the present. graphic features will eventually travel to the Japan-Izu-Ogasaw~a Trench in the near future. Given the current convergence rate of the Pacific accretion from topographic highs, episodic rhythm plate along the Japan Trench (10.4 cm y-l; Ad- in accretion, and the intimate relations between dicott and Richards, 1981), it is estimated that accretion depth and volume of oceanic rocks. In more than 300 topographic highs will approach addition, we will criticize some previous ideas and collide against the active arc-trench system of conveming subduction-accretion processes, in- Japan in the next 30 Ma (Fig. 1, Table 1). Ex- cluding the relationship between age of subduct- trapolating this back into time, this estimate im- ing plate and its ability in forming an accretionary plies that several thousands of oceanic features complex (Uyeda and Kanamori, 1979). may have been consumed since the Late Paleozoic to form the Japanese Islands. Geologic overview of pre-Miocene Japan In this paper, we compile all accreted oceanic materials exposed in late Paleozoic to Tertiary The pre-Miocene geology of Japan consists of subduction-accretion (S-A) complexes through- an amalgamation of ancient S-A complexes, re- out the Japanese Islands. Through the examina- mnant arc complexes, and ~cr~ontinent~ frag- tion of late Paleozoic to Tertiary subduction- ments. These complexes and landmass fragments accretion processes accompanied by numerous are distributed in belts that are separated from collision-subduction of oceanic reliefs, some es- one another by low-angle thrusts and/or vertical sential tectonic constraints that controlled build- strike-slip faults with unknown, but probably large, ing S-A complexes are clarified, such as selective amounts of displacement (Fig. 2A). ACCRETED OCEANIC MATERIALS IN JAPAN 181

Southwest Japan, west of the Itoigawa- lel to the general trend of Southwest Japan and Shizuoka Tectonic Line (Fig. 2B), is underlain the Nankai Trough (Fig. 2A). From the inner mainly by several late Paleozoic to Tertiary S-A continent side to the outer ocean side, these com- complexes which are oriented parallel to subparal- plexes include: the Late Permian complex of the

3 P -ii

m Granittc intrusives

tf Miocene and yo”“gW Sedl~tS

0 Pre-Miocene BCCretIon-COIIiwll complexes

% % %

ltoi@ewa-Shizuoka Tectrmlc Line x ..& .x

% % ‘-

Eurasia Plate

Pacific Plate

PhlllpplnoSea Plate

(arc) 3WJkm UC comprx I , - A Fig. 2. Geologic overview of Japan. A. Tectonic framework of the Japanese Islands, showing distribution of su~uction-a~retion complexes and collided microcontinental blocks. Ak = Akiyoshi belt; Sn = Sangun belt; Mr = Maizuru belt; Ry = Ryoke belt; $6 = Sanbagawa belt; Ch = Chichibu belt; Kr = Kurosegawa belt; Sh = Shimanto belt, Ng = Nagasaki belt; Jo = Joetsu belt; As = Ashio belt; Ab = Abukuma belt; Eu = Eastern Abukuma belt; SK = Southern Kitakami belt; Kt-Iw = Kitakami-Iwaizumi belt; Kk = Kamuikotan belt; Hd-Tk = Hidaka-Tokoro belt; Nm = Nemuro belt; MTL = Median Tectonic Line; TTL = Tanakura Tectonic Line; ATL = Abashiri Tectonic Line, m = triple junction. B. Distribution of Miocene and younger cover sediments and grtitic intrusives. Note the extensive areas in Northeast Japan where subduction-accretion complexes are concealed by thick sedimentary cover. 182 Y. ISOZAKI ET AL

TABLE 1 Kitakami massif towards the eastern part of Number of oceanic topographic reliefs on western Pacific Northeast Japan, a belted arrangement of units seafloor which will collide and/or subduct along the Japan similar to that in Southwest Japan occurs. How- and Mariana trenches in the coming 10, 20 and 30 Ma ever, the belts have an oblique trend to the ad- Arc Ma Rise Seamount Seamount jacent Japan Trench (Fig. 2A) suggesting tectonic AP of hot-spot erosion or t~ncation at the Japan Trench during origin the opening of the Japan Sea at about 15 Ma B.P. (Maruyama and Seno, 1986). The western half of Hokkaido is an extension of Northeast Japan, and consists of sporadic ex- Northeast Japan 10 posures of the Cretaceous paired belt composed of the Rebun-Kabato granitic belt and the Kamui- kotan bluesc~st/se~enti~te belt (Figs. 2A). The Izu-Bonin 10 0 31 I 0 eastern half of Hokkaido is an allochthonous ter- 20 0 28 109 0 rane which has migrated from the east, collided, 30 0 41 0 and amalgamated with Japan in the Miocene, Mariana although the site of terminal suture is still a matter of debate (Maruyama, 1984). The Late Cretaceous - Hidaka-Tokoro S-A complex extends north- Total 30 s 301 13 south and is truncated by the E-W-trending Kurile Trench. The 17-30 Ma Hidaka low-P/T granite- gneiss complex was uplifted by the Miocene colli- Akiyoshi belt, the Jurassic complex of the Mino- sion of the Kuril fore-arc sliver with west Hok- Tanba belt, the Jurassic complex of the Chichibu kaido (Kimura, 1985). The western limit of the belt (Northern Chichibu belt and Sanbosan belt) Kuril back-arc basin is a N-S-trending strike slip and the Cretaceous-Tertiary complexes of the fault that extends to the Abashiri Tectonic Line Shimanto belt. These complexes were more or less separating the Hidaka-Tokoro belt from the metamorphosed, partly forming major high-grade Nemuro belt composed of Late Cretaceous- metamorphic belts such as the high-P/T Sangun Paleogene fore-arc sediments (probably with an schists adjacent to the Akiyoshi belt, the low-P/T underlying S-A complex). Ryoke metamorphic rocks south of the Mino- In the next section, the structure and field Tanba belt, and the high-P/T Sanbagawa schists relationships of oceanic rocks are described in structurally above the Shimanto belt. In addition, order to document their accretion-related feature. intervened between these S-A complexes are Then these accreted oceanic materials are classi- minor amount of collided allochthonous land- fied and described with respect to their prove- masses of remnant arc complex or microcontinent nance. Compared with Northeast Japan, South- origin such as the Kurosegawa belt (Maruyama et west Japan is rather free from thick and extensive al., 1984; Isozaki, 1987) and the Mairzuru belt Cenozoic volcano-sedimentary cover (Fig. 2B), (Maruyama and Seno, 1986). therefore, our description will concentrate on The bed rock geology of Northeast Japan is Southwest Japan. fundament~ly regarded as a lateral extension of Southwest Japan, with the exception of the alloch- Accreted oceanic rocks thonous Southern Kitakami-Abukuma Block, a microcontinental fragment which collided and Structure and field relationships accreted to Northeast Japan in the Early Creta- ceous (Saito and Hashimoto, 1982). Although the Late Paleozoic, Mesozoic and Cenozoic S-A extensive thick Cenozoic cover limits the exposure complexes in Southwest Japan are mostly com- of Mesozoic S-A complexes and the Southern posed of imbricated thrust sheets, melanges ACCRETED OCEANIC MATERIALS IN JAPAN 183 and/or olistostromal complexes bearing numerous chert units (Early Triassic-Early Jurassic) that are irregularly shaped bodies and blocks of various folded into a well developed synformal structure lithologies and sizes. In all of the complexes, (Fig. 3). Sedimentary structures and microfossil terrigenous siliciclastic rocks, such as mudstone data indicate that all these units are facing right- and sandstone, dominate over exotic components side-up. Unfolding the synform produces a config- derived from oceanic crust. uration for these units analogous with the internal The ocean-derived rocks occur, primarily, in structure of modem accretionary prisms such as in one of two structural modes. Either as decametric the Barbados ridge complex (Watkins et al., 1981); and larger-scale sheet-like bodies tectonically in- Moore and Biju-Duval, 1984; Westbrook et al., terlayered with elastic rocks, or as metric scale 1988) and in the trench inner wall of the Nankai blocks and/or lenses surrounded by argillaceous Trough (Okuda et al., 1979; Le Pichon et al., matrices. Figures 3 and 4 display some examples 1987), which is composed of an imbricate pile of from the Inuyama area, Mino belt, central Japan thrust sheets. (Matsuda et al., 1980, 1981; Yao et al., 1980; In addtition to this large-scale structural geom- Matsuda and Isozaki, in press). The Inuyama area etry, at outcrop scale, the second structural mode is underlain by tectonically interlayered siliciclas- also locally occurs in the Inuyama area (see inset tic sandstone (Middle-Late Jurassic) and bedded of Fig. 4). Metric scale blocks of oceanic rock are

Middle- Late Jurassic elastic rocks

Mid. Triassic - Early Jurassic bedded chert

/ fault

syncline

3km

Fig. 3. Block diagram showing geometry of imbricated thrust sheets of bedded chert and terrigenous elastic rocks in the Inuyama area, central Japan (modified from Kondo and Ada&i, 1975; Yao et al., 1980). 184 Y. ISOZAKI ET AL found surrounded along sharp contacts by silici- younger terrigenous melange matrix is ubiquitous elastics. Microfossil data indicate that the cherts in other S-A complexes throughout the Japanese of Early Jurassic age are contained in Middle Islands (Akiyoshi belt: Kanmera and Nishi, 1983; Jurassic sandstone/ mudstone. Many additional Kanmera et al., 1989; Chichibu belt: Yamato ~crop~eontolo~cal studies, in particular those Omine Research Group, 1981; Yao, 1984; Isoz&i, on radiolarian biostrati~aphy, have revealed that 1987; Shimanto belt: Taira et al., 1988). and all blocks and lenses of limestone, bedded chert, elsewhere in other Cordilleran-type orogens (e.g. and greenstone are apparently older than their Hsti, 1974; Cowan, 1985). surrounding elastic rocks (e.g. Isozaki and Matsuda, 1980; Matsuda et al., 1980,1981; Yao et Major categories of accreted oceanic materials al., 1980; Mizutani et al., 1981; Matsuoka, 1984). Accordingly, these ocean-derived rocks are alloc- The lithologies of oceanic materials in the S-A hthonous in origin, even though some of them complexes in Southwest Japan can be divisible retain an internal coherent stratigraphy. Such oc- into two groups: (1) igneous-metamorphic rocks, currences of older blocks of oceanic rock in including basaltic-gabbroic greenstones of various

n elastic rocks (Mid.- Late Jurassic)

3 Early Jurassic

2 Late Triassic ~ bedded chert 1 1 Middle Triassic

datsuda et al. I1991 )

Fig. 4. Geologic sketch map of the Inuyama area (modified from Yao et al., 1980), central Japan, showing mode of Occurrence of a~~h~onous bedded chert units (Matsuda et al., 1981; Mizutani and Koike, 1982). ACCRETED OCEANIC MATERIALS IN JAPAN 1x5 geochemical affinity and ultrabasic rocks; and (2) Shikoku, are intimately associated with reefal oceanic sedimentary rocks of non-terrigenous na- limestone, suggesting their seamount origin. ture. The former is subdivided into: (a) metaba- MORB-like tholeiitic basalts are also common but sites with origins of dominantly effusive volcani- relatively minor than alkali basalts. elastic rocks, and (b) metabasites with serpentin- Although the occurrence of hot-spot basalts is ized ultramafic rocks referred to as ophiolite. The rather rare in Japanese S-A complexes, a signifi- sedimentary rock types include bioclastic reefal cant example is found in a greenstone belt called limestone and pelagic bedded radiolarian chert. the Mikabu greenstone complex in Southwest In the following parts of this section, we will Japan that extends laterally for about 800 km give classification criteria for each rock category, along the southwestern margin of the latest and summarize their occurrences in Japan. Jurassic-Early Cretaceous S-A complex. The Mikabu greenstone complex is composed of gab- bro, dolerite, pillow basalt, hyaloclastite, and ra- Oceanic greensfones diolarian chert, with minor ultramafic rocks (U&da, 1967, 1981; Iwasaki, 1969). Minor alkali Based on modal mineralogy, major, trace, REE, basalts were also described from this belt (Takeda, and isotopic geochemistry, oceanic basalts are di- 1984). The following petrographic aspects of asso- vided into (1) seamount basalt, (2) mid-ocean ridge ciated cummulate gabbro, dolerite and basalt- basalt (MORB), (3) hot-spot, rise or plateau picrite (Nakamura, 1971; Suzuki et al., 1971, 1972; basalts, and (4) back-arc basin basalt (BABB). Takeda, 1984) suggest that the Mikabu greenstone MORB differs from other oceanic basalts by the complex is petrogenetically similar to Hawaiian absence of augite phenocryst, by having less than hot-spot basalts (Wright, 1971, 1973) or oceanic 0.2 wt.% K,O, in the flat pattern of chondrite-nor- plateaus or rises such as the Shatsky or Hess Rise malized REE abundances, and by having specific in the northern Pacific Ocean. The crystallization Ti-Zr-Y ratios. BABB is intermediate between sequence from olivine to clinopyroxene followed island arc tholeiite and MORB, and contains by plagioclase is quite different from that of higher incompatible elements, lower MgO at a abyssal tholeiite and asociated peridotite in the given FeO*/MgO ratio Seamount volcanics are present-day ocean floor (Shido and Miyashiro, mainly composed of alkali basalt and related 1971; Dick et al., 1984; Ozawa, 1986). Olivine fractionated rocks, although MORB or BABB-like ranges in composition from Fo,, to Fo,,, which rocks were recovered from seamounts (Batiza et indicates cumulates of picritic-basaltic magma al., 1984; Tokuyama, 1985). Alkali basalt can be with tholeiitic affinity (Nakamura, 1971). Inverted distinguished from tholeiites by the composition pigeonite has been found in this zone (Nakayama of titaniferous augite, kaersutite and titan biotite, et al., 1973). These, togeter with major and trace by the presence of alkali feldspar, and by bulk element chemistries (Suzuki et al., 1971, 1972; chemical compositions. Hot-spot, rise or plateau Tanaka and Sugisaki, 1973; Kawabe et al., 1979), volcanics are composed of tholeiite and alkali rock indicate higher degrees of partial melting of man- series: tholeiites differ from MORB by earlier tle peridotite than that of MORB (Ishiwatari, augite crystallization than plagioclase, and by 1985a), and thereby that the Mikabu greenstone higher TiO, and MgO at a given FeO*/MgO for differs from rocks of the oceanic crust and un- the rock composition. This reflects the higher de- derlying mantle in major oceans. gree of partial melting of mantle peridotite than Back-arc basin basalts will be discussed with that of MORB or BABB. ophiolite below. Most of the major greenstone masses in Japanese S-A complexes are classified in alkali Ophiolite basalt of seamount affinity as mentioned later. Some of them, such as the Sawadani greenstone Metabasites and associated serpentinized peri- complex (Maruyama and Yamasaki, 1978) in dotite occur along narrow zones within and/or 186 Y. ISOZAKI ET AL between S-A complexes. These on-land ophiolites (1) Most mantle peridotites are highly depleted are mostly dismembered without showing the typi- in basaltic components, some contain high-Mg cal conference ophiolite stratigraphy. These andesites (Ohashi and Shiraki, 1981) or island arc mafic-ultramafic rocks are classified based on volcanics (Ishiwatari et al., 1988). comparison with the following modem analogues, (2) the crystallization sequence of minerals in and the sedimentolo~cal aspects of associated mafic-ultram~ic cumulates indicates a higher pelagic sediments. degree of partial melting than that of MORB. Petrologic aspects of abyssal peridotites dredged (3) The ocean-floor metamorphism recorded or drilled in the Atlantic, Indian and Pacific within the ophiolites occurred at lower geothermal Oceans are characterized by their modal mineral- gradients than that of present-day ocean-floor ogy (e.g. diopside present, thus not depleted in rocks, as shown by their common occurrence of basaltic component), by major and trace bulk pre~te-pumpell~te facies (Ma~yama and Liou, chemistry, mineral chemistry (Al in pyroxene, Y,, 1988). in spinel, and XiMsin olivine), and isotope ratios (4) The thickness of pelagic sediments (time (Dick et al., 1984; Ozawa, 1986). length of pelagic environment) indicates that the Serpentinized peridotite and associated meta- ophiolites were incorporated into orogens within gabbros were dredged from Mariana, Tonga and 30 Ma after their formation (Maruyama and off-Guatemala fore-arc regions (Fisher and Engel, Tatsumi, 1988). 1969; Hussong et al., 1981; Bloomer and Hawkins, These characteristics strongly suggest that most 1983; Hawkins et al., 1984; Fryer et al., 1985; of the Japanese ophiolites are of back-arc basin, Ishii, 1985; Von Huene et al., 1985). These are fore-arc, or island arc origin (Maruyama and quite different from abyssal peridotite and associ- Tatsumi, 1988; Ishiwatari, 1989). Another possible ated with metagabbros from major oceans by the explanation is the assumption that their formation more depleted nature of mantle peridotite. occurred near the RTT triple junction where these Seven dismembered ophiolite belts occur in the ophiolites were emplaced within S-A complexes Japanese erogenic belts. They are the Late Perm- immediately after their birth at the mid-oceanic ian Maizuru ophiolite belt (Mz in Fig. 2A; ridge. This may explain the Late Cretaceous west- Ishiwatari, 1985a, b; Kurokawa, 1985; Koide et ern Hidaka ophiloite belt (Miyashita, 1983) which al., 1987) the Kurosegawa serpentinite melange possesses similar petrologic characteristics to those belt, Southwest Japan (Maruyama, 1981; of MORB and related mafic-ultramafic rocks. Yokoyama, 1987) the Miocene Mineoka ophio- litic belt, Southwest Japan (Tazaki, 1975; Oceanic sediments Takasawa, 1976; Arai and U&da, 1978; Tazaki and Inomata, 1980; Ogawa and Taniguchi, 19X7), Those sedimentary rocks classified as oceanic the Ordovician Miyamori ophiolite, Northeast in this paper, are completely free from terrigenous Japan (Ozawa, 1984, 1988) the Late Jurassic detritus derived from sialic provenances such as Horokanai ophiolite, Hokkaido (Asahina and continents or arcs. They are interpreted as having Komatsu, 1979; Ishizuka, 1981, 1985), the Late been deposited in open-ocean environments re- Cretaceous Hidaka western ophiolite belt, Hok- mote from land areas. In other words, they were kaido (Miyashita and Niida, 1981; Miyashita, deposited upon oceanic crust before arrival at the 1983; Miyas~ta and Yoshida, 1988), and the subduction zone. The rocks include two lithoiogic Mesozoic Horoman ophiolite, Holckaido (Niida, groups : 1984). (1) Reefal limestone-white to light-gray Except for the Hidaka western ophiolite belt, sparry limestone associated with greenstones is Japanese ophiolites are not comparable to MORB, common in late Paleozoic and Mesozoic S-A gabbro and peridotite in the Atlantic, Indian and complexes in Japan. Abundant and various kind Pacific Oceans (Ishiwatari, 1989), and have the of frame-building fossils (e.g. corals, bryozoans) following characteristics. with associated faunas (fusu~~ds, etc.) are pre- ACCRETED OCEANIC MATFXIALS IN JAPAN 187

served in these bioclastic limestones. Sedimento- upon bedded chert are in turn covered by coarse- logical studies indicate that these limestones were grained trench-fill trubidites. deposited in a strong water-agitation environment, receiving no coarse-grained terrigenous elastics. Summary and statistics of accreted oceanic materi- Basaltic greenstones, mostly alkalic, with pillow als in Japan structure form the basement of these limestones. Most of these limestones are regarded to have In this section, the oceanic materials found in been reef complexes developed on top of S-A complexes in Japan are assigned to one of seamounts (e.g. Hase et al., 1974; Ota, 1977; the above-mentioned Ethologic group and this Yokoyama et al., 1979; Kanmera and Nishi, 1983; compilation is summarized in Table 2. Most of the Isozaki, 1987; Sano, 1988; Kanmera et al., 1990). complexes possess a staple set of oceanic frag- (2) Bedded radiolarian chert-siliceous mud- ments, such as basaltic greenstone, chert, and stone-bedded chert is one of the most abundant limestone; however, the amount varies consider- oceanic constituents in S-A complexes, both in ably from one complex to another, and reflects volume and in number of occurrences. Rhythmic not only differences in the volume ratio between bedding exhibited by alternating vitreous chert accreed oceanic materials and terrigenous elastics and argillaceous layer is one of the most striking but also in the relative proportions of oceanic features. This lithology is usually less than 100 m materials. For example, bedded chert is dominant in thickness and yields conodonts and radio- among the oceanic constituents in the Middle to larians. Isozaki and Matsuda (1982b) and Matsuda Late Jurassic complex of the Mino-Tanba belt, and Isozaki (in press) summarized depositional whereas reefal limestone with greenstone is the constraints for Mesozoic bedded radiolarian cherts most voluminous material in the Akiyoshi belt. In in Southwest Japan. They found that these sedi- the Shimanto belt, on the other hand, oceanic ments were characterized by (a) being exclusively materials are very scarce; in particular, the south- fine-grained, (b) being genuinely biogenic and ern complex of this belt is composed mainly of non-terrigenous, (c) having very slow average sedi- terrigenous elastic rocks with very minor accreted mentation rate at less than 10 mm ky- ’ (Matsuda oceanic materials. et al., 1980), (d) representing continuous sedimen- Figure 5 illustrates the distribution of accreted tation for more than several tens of millions of oceanic materials within the S-A complexes in years, and (e) representing extensive lateral dis- Japan. Northeast Japan and Hokkaido and some tances of more than several thousand kilometers parts of Southwest Japan have thick and extensive across an ocean basin. These constraints suggest a Cenozoic covers (Fig. 2B), hence leave large blank pelagic open-ocean environment for chert deposi- areas in Fig. 5. On the other hand, Fig. 6 shows an tion. Except for the occurrence of sponge spicuies enlargement of Shikoku Island, where the Meso- in some Paleozoic bedded cherts, they have almost zoic S-A complexes are extensively developed on the same characteristics (Uc~y~a et al., 1986; the surface. Isozaki, 1987) as the Mesozoic bedded cherts. About 500 publications, mainly written in Commonly associated with bedded chert is Japanese, are used in the compilation * . The num- light-gray siliceous mudstone that lithologically ber of accreted oceanic materials was counted as resembles the argillaceous portion of bedded follows: each tectonic block has a large mappable cherts; it occurs immediately above thick bedded size at a scale of 1: 25,000, and is completely chert. Judging from the gradual lithologic change enclosed by mat~x-for~ng terrigenous elastics. If and fossil ages, these siliceous mudstones were a block is separated from the adjacent one, it is probably deposited in a sedimentary environment counted as one block, even though the two blocks adjacent to that of bedded chert, but more in- fluenced by elastic supplies from land areas

(Matsuoka, 1984; Isozaki, 1987). In particular, * Only major referencesare listed in Table 2. A complete list those ancient hemipelagic sediments lying just of references is available on request from the authors. TABLE 2

Catalogue of accreted oceanic materials in Japan

Southwest Japan

S-A complex: Akiyoshi (Ak) Mino-Tanba + Ashio (As) Chichibu (Ch) Shimanto (Sh) (North Chicbibu + Sanbosan)

end Permian Middle Jurassic (Type II) Early-MIddle Jurassic (North Late Cretaceous (north subbelt) Late Jurassic-earliest Cretaceous Chichibu) Late Jurassic-Early Paleogene (south subbelt) (Type I) Cretaceous (Sanbosan)

Total volume of oceanic 15-208 ca. 15% 15% in volume less than 1% rocks

Volume ratio limestone > chert > greenstone greenstone > chert * limestone greenstone > chert X= limestone greenstone > chert > limestone

Category of greenstones mostly alkali basalt (major ele- mostly alkali basalt + few MORB mostly alkali basalt + few MORB MORB (major and trace elements, (criteria) ments, relict cpx mineralogy) (major and trace elements, relict (major and trace elements, relict and relict cpx mineralogy) cpx mineraloyg. rare earth abun- cpx mineralogy, rare earth abun- dance, Sr isotope ratios) dance, Sr isotope ratio)

Major references Kanmera and Nishi (1983). Kan- Mizutani (1964), Mizutani et al. Kanmera (1968). Maruyama and Kishu Shimanto Research Group mera et al. (1990), Ota (1977), (1981). Kondo and Ada&i (1975), Yamasaki (1978), Murata (1982) (1974, 1986), Taira et al. (1980, Hase et al. (1974), Hase and Nishi- Yao et al. (1980) Ada&i and Yamakita (1986), Isozaki (1987), 1988). Sakai and Kanmera (1981), mura (1979) Nishimura (1984), Kojima (1983). Hattori (1984), Hashimoto et al. (1970), Hashimo- Sakai (1981, 1988), Tsucbiya et al. Nishimura et al. (1979, 1989) Wakita (1985, 1988) Otsuka to (1972), Kawabe et al. (1979), (1979) Naka (1985), Yanai (1984), Uchiyama et al. (1986). Naka et al. (1988) Sano (1988), Tanba Belt Sugisaki and Tanaka (1971), Sugisaki et al. (1979). Suzuki and (1986), Tanaka et al. (1987) I&ii Research Group (1974), Isozaki Tanaka (1975), Tanaka (1977), Hada (1979), Sano et al. (1979) et al. (1985), Tazaki et al., (1989) and Matsuda (1980), Tanaka Tanaka et al. (1979), Sano and (1980), Ishiga (1983, 1985) Tazaki (1989). Kanmera and Furukawa (1964), Ishida (1977). Yamato Omine Research Group (1981) Murata (1982), Hisada (1983). Yao (1984). Matsuoka (1984)

Metamorphic equivalents

S-A complex: Hida marginal Sangun (Sg) Sanbagawa (Sb) ( + its western extension)

Age Carboniferous, Permian end Permian Early Cretaceous Total volume of oceanic rocks ca. 30% ca. 36% 30%

Voiume ratio greenstone X fimestone = chert greenstone JI>limestone = chert greenstone > chert z limestone

Category of greenstones (criteria) mostly alkali basalts t few MORB (major alkali basalt + few MORB (major ele- dominantly alkali basalt + minor MORB elements, relict cpx mineralogy) ments, relict cpx mineralogy) (major and trace elements, rare earth abundances, Sr isotopes ratio, relict cpx mineralogy)

Major references Banno (19S8), Matsumoto et al. (198% Hashimoto (1968) Nishimura (1971), Seki (1958), Iwasaki (1963), Iwasaki et al. Komatsu et al. (1985), Nakamizu et al. Nishimura et al. (1989), Shibata and (1984), Banno (1964), Ernst (1972), (1989), Murakami and Nishimura (1979), Nishimura (1989), Hase and Nishimura Watanabe (1974), Toriumi (1975), Matsu- Nishimura et al. (1983), Shibata and (1979). Watanabe et al. (1987) da (197Q Suyari et al. (1980), Xsozaki Nisbimura (1989) (1988), Isa&i and ltaya (1990), Sawada (1973), Tanaka (1971), Sugisaki et al. (19?2), Kawabe (1974), Tanaka (19743, Tanaka and Sugisaki (1973), Tanaka et ai. (1979). Suzuki et al. (1971), Takeda, (1984), Kawachi et al. (1973), Miyashiro (1975}, Kenzan Research Group (1984). Uchida (1981), Hara et al. (1977), Faure (1983)

Northeast Japan S-A complex: Northern Ki takami-iwaizumi Sarachi-Hidaka-Tokoro Kamuikotan {Kk) (Kt - Iw) + Oshima-Kabato ( Hd-Tk)

A8e Middle Jurassice-Early Cretaceous Farly Cretaceous-Paleogene Early Cretaceous

Total volume of oceanic rocks 10% 1.5-201 30%

Volume ratio greenstone > chert = limestone greenstone >‘ chert > limestone greenstone :, chert > limestone

Category of fqeenstones (criteria) few data available MORB + minor alkali basal& (major and alkali basalts + minor MORB (major and trace elements, relict cpx mineralogy) trace elements, relict CPX ineralogy)

Major references Onuki (1969). Sugimoto (1979) Minoura Komatsu et al. (1982). Komatsu (1985), fmaizumi (1984), Gouchi (1983), Herve (1985), Kawamura et al, (1988). Oho and Kiminami et al. (1985), Kito et al. (1986), (1975), Maekawa (1983), Nakano (1981) Iwamatsu (1986), Tazawa (1988), Okami Nakano, (1981), Nakano and Komatsu and Ehiro (1988) (1979), Research Group of Tokoro Belt (1984), Sakakibara et al. (1986), Niida and Kito (1986) 190 Y. ISOZAKI ET AL

TABLE 3 Based on this areal (two-dimensional) compila- Category of accreted oceanic materials contained in subduc- tion, the volume ratio (in three dimensions) of tion-accretion complexes in Southwest Japan and their accreted oceanic materials was estimated by as- frequency of occurrence suming the same density distribution below the Seamount Seamount MORB Lime- Chert surface (Table 2). Table 3 lists the estimated num- (alkali of hot-spot stone ber of occurrences of accreted oceanic materials in basalt) origin each tectonic belt in Japan. The following discus- ca. 600 26 ca. 200 291 numerous sion on the characteristics and related tectonic ( ‘l@W processes in the accretion of various oceanic materials is based on Figs. 5, 6 and Table 2.

Timing of accretion and age of subducting slab may have formed originally as a single entity. The total number of accreted oceanic materials in Reconstruction of “oceanic plate stratigraphy” Japan could be twice as many if we remove the granitic intrusives and the Miocene and younger Oceanic material delivered to the subduction cover sediments on the S-A complex. zone on the downgoing slab is fragmented and

Eurasia Plate

Plate

s Philimina SOI ~~~~~7

Fig. 5. Ancient oceanic materials found in on-land subduction-accretion complexes in the Japanese Islands. ACCRETED OCEANIC MATERIALS IN JAPAN 191 incorporated within S-A complexes. Althou~ strati~aphy indicates ~nt~uous deposition of only a small portion of the downgoing slab and its bedded cherts from the Early Triassic to Early sedimentary cover are incorporated into the com- Jurassic (Yao et al., 1980). This is succeeded by a plexes, these accreted oceanic materials provide gradual lithologic change from pelagic bedded the only record of extinct oceanic crust. By virtue chert to hemipelagic siliceous mudstone in the of detailed research of microfossils in the accreted Early Jurassic. And this is capped by Middle oceanic materials, the initial spatial relationship, Jurassic coarse-grained terrigenous elastics, prob- both lateral and vertical, of these oceanic materi- ably of trench-fill turbiditic origin. Similar re- als was reconstructed in analogy with the frame- stored oceanic plate stratigraphies have been re- work of modem oceanic crust. In particular, the ported from other S-A complexes in Japan (e.g. primary vertical succession of oceanic compo- Akiyoshi belt: Uchiyama et al., 1986; Chichibu nents, i.e. what we call reconstructed “oceanic belt: Matsuoka, 1984, Isozaki, 1987; Shimanto plate stratigraphy”, is very critical for understand- belt: Taira et al., I980), however, each S-A com- ing the travel history of the subducted slab. plex is characterized by its own age spectrum as The best documented example of such an oc- represented in the oceanic plate stratigraphy. eanic plate stratigraphy was first reported from the Inuyama area (Matsuda et al., 1980; Yao et Travel time and age of subducting plate al., 1980; Matsuda and Isozaki, in press; Figs. 3, 4). Although the base of the stratigraphy is mis- The oceanic plate stratigraphy is theoretically sing, presumably subducted, the rest of the stratig- best represented by a columnar section of the raphy is well preserved within the thrust sheets in oceanic crust at the trench (Fig. 7). This strati- the area. The well documented radiolarian bio- graphic column records the history of sedimenta-

Median Tectonic Line--- _ _ , SOtolntrnd Sea

1 0 30km wI - Fig. 6. Ancient oceani c materials found in subduction-accretion complexes in Shikoku Island, Southwest Japan (partial blow-up of Fig. 5). Note the relation between depth of burial (metamorphic grade) and volume of accreted oceanic materials (cf. Table 4). 192 Y. ISOZAKI ET AL tion on the oceanic crust from its formation at a fore, the age assignment of the base of pelagic mid-oceanic ridge to its arrival at the subduction sediments and the top of hemipelagic sediments is zone. Each section consists of a basal basalt unit the most critical in deciphering the age of extinct of MORB affinity, overlain by pelagic-hemipe- oceanic plate. In the Inuyama area, for example, lagic sediments, which in turn is overlain by the age of the base of bedded chert is Early trench-fill terrigenous elastics (e.g. Chipping, 1971; Triassic (240 Ma) or older, while the top of sili- Von Huene et al., 1972; Lash, 1985). The oceanic ceous mudstone is of late Early Jurassic age (170 plate stratigraphy provides two significant time Ma). The time span between these two horizons is constraints; the arrival time of the accretion-re- 70 million years suggesting that the age of the lated oceanic crust at trench and the time span of subducted oceanic crust was at least 70 Ma old at sedimentation on the oceanic crust. the trench. This method to estimate the age of The arrival time of the oceanic crust at a sub- subducted plate is particularly useful when dealing duction zone is represented by the age at which with Mesozoic and older S-A complexes, because genuinely pelagic sediments change upward into pre-Jurassic oceanic plates have already disap- terrigenous ones. In particular, the bedding plane peared completely from the present Earth’s between hemipelagic siliceous mudstone and surface. overlying terrigenous elastics is the best horizon to determine the arrival time. This horizon also indi- Discussion cates the age when the S-A complex is formed. For example, in the Inuyama area, as Middle Origin of accreted oceanic materials Jurassic sandstone/ mudstone covers late Early Jurassic siliceous mudstone, the S-A complex was According to the component catalogue made formed at ca. 170 Ma. for Japanese S-A complexes (Tables 2, 3), the The age of the oceanic plate at the trench is accreted oceanic materials since Late Permian in- approximated by the duration of pelagi-hemipe- cludes seamounts (about 600), hot-spot seamounts lagic sedimentation that should have begun soon (26) reefal limestones (291), MORBs (about 200), after formation of the underlying ocean-floor and cherts (more than 1000). The exposed area of basalts at the mid-oceanic ridge (Fig. 7). There- oceanic materials covers, in average, less than 5%

Duration of pelagic (+hemipelagic) sedimentation - travel time from MOR to trench - age of subducting slab

Fig. 7. Framework of typical ridge-subduction system and idealized oceanic plate stratigraphy. Note that the travel time of an oceanic plate from mid-oceani c ridge to trench corresponds to the age of the subducting slab. ACCRETED OCEANIC MATERIALS IN JAPAN 193

of the S-A complex, although it varies from one spot basalts, as mentioned above. The lateral complex to another owing to the depth-related length of Hawaiian hot-spot tracks or the Shatsky accretion mechanism by which the complex was or Hess rises (Fig. 1) is comparable with that of formed, as discused later. The accreted oceanic the Mikabu greenstone complex. The timing of materials summarized in Tables 2 and 3 are re- accretion-collision of the Mikabu greenstone stricted to rocks of Layers 1 and 2 of the oceanic complex is constrained to be latest Jurassic to plate, and rarely include rocks from Layer 3 and Early Cretaceous by index radiolarian fossils from from the underly~g mantle. Accretions of materi- tuffaceous mudstone (Iwasaki et al., 1984). The als from topographic highs such as sporadic Mikabu aseismic ridge (presumably hot-spot tracks seamounts together with reef limestone and pelagic or a huge oceanic rise) may have been collided sediments were dominant and those of MORB are with, and accreted obliquely to, the Mesozoic less common. Ophiolites in Japan may be of back- Asian continental margin, although it is not clear arc or small ocean basin origin, and are clearly whether the collision proceeded eastward or west- different from those of mid-oceanic ridge origin. ward due to rare preservation of microfossils by subsequent blueschist facies metamorphism Size comparison with modern seamounts (Iwasaki, 1963; Banno, 1964; Maruyama and Liou, 1985). Although the total number of seamount frag- The Mikabu greenstone complex and its equiv- ments found in Japanese S-A complexes exceeds alents (= Sanbosan belt; Isozaki, 1988) appear to 600 (Table 3), the actual number of collided fringe the southernmost part of the latest Jurassic seamounts would be more if we extrapolate the to Early Cretaceous accretionary complex, sug- predicted number for modem western Pacific gesting that the formation of the accretionary margin (more than 300 during 30 Ma; see Table 1) complex was related to the collision of such topo- back into Permian time. This implies that most of graphic reliefs, This may be one of the possible the collided seamounts, in particular smaller ones, reasons why the voluminous accretionary complex were not necessarily accreted but were subducted was formed in spite of subduction of the relatively with the descending oceanic slab. heavy oceanic slab older than 150 Ma. Likewise, In addition, the sizes of accreted basalts de- collision-subduction of large topographic feature rived from seamounts are usually quite small. The such as large oceanic rises or plateaus may be seamounts are commonly one or two orders of responsible for building a huge accretionary wedge, magnitude smaller than the seamounts in the west- while those of isolated smaller seamounts do not em Pacific today (Fig. 1). For example, modem contribute much but give temporal deformation seamounts such as Kinan off Shikoku, Daiichi- features such as extensional structures on the in- Kashima off Tokyo, and Erimo off Hokkaido, and ner wall of the trench and the landward indenta- the Haw~i-Emperor se~ount chain are actually tion of the trench axis (La~em~d and Chamot- several times or much larger than the largest Rooke, 1986). accreted seamount in Japan, the Sawadani green- stone mass in Shikoku (Maruyama and Yamasaki, Episodic accretionary events 1978). The size comparison indicates that most parts of a seamount body are probably subducted By reconst~~tion of the oceanic plate stratigra- with the downgoing oceanic slab, and that only phy of Japanese S-A complexes, it is clarified that superficial parts are peeled off from the pedestal each S-A complex or tectonic belt is characterized to be incorporated into the S-A complex. by its unique age spectrum (Fig. 8). In addition, a Coilision-accretion of hot-spot tracks of seamounts younging polarity from the inner continent side to or huge oceanic rises the outer ocean side of Southwest Japan is sug- gested. Although the Kurosegawa zone within the The Mikabu greenstone complex in Southwest Chichibu belt and the Sanbagawa belt apparently Japan is petrogenetically similar to Hawaiian hot- disturb this polarity, their bizarre occurrences are 194 Y. ISOZAKI ET AL reasonably explained by tectonics as follows: the Pacific plate during most of the Paleogene, and Kurosegawa zone is an allochthonous fragment of has been attached to the Philippine Sea plate since older (end-Permian) orogen with a S-A complex 30 Ma. These interactions between Southwest which is tectonically intervened between the Japan and the oceanic plates are mostly dominated Jurassic complexes of the Chichibu belt (Maru- by subduction, except for some strike-slip motions yama et al., 1984; Isozaki, 1987); the Sanbagawa due to highly oblique convergence. Formation of belt occurs as a tectonic window, surrounded by the respective S-A complexes in Southwest Japan older S-A complexes at the surface (Isozaki, 1988; corresponds to subduction of the above-men- Isozake and Itaya, 1990). This southward and tioned oceanic plates. Furthermore, the ages of downward younging polarity recognized in the some S-A complexes seem to coincide with the pile nappe structure of the Japanese S-A com- arrival time of the mutual boundary between ad- plexes suggests an oceanward growth of the Asian jacent oceanic plates, probably the mid-oceanic continental margin through subduction-related ridge, to Southwest Japan. For example, the processes, which is, an essential characteristic of Mino-Tanba and Chichibu S-A complexes were Cordilleran-type erogenic belts such as the west- formed when the Farallon/ Izanagi boundary em coast of North America. (ridge) passed by Southwest Japan: the norhtem Formation of the S-A complexes in Japan since Shimanto complex when the Izanagi/ Pacific the end of the Permian, however, has not been boundary (ridge) arrived; and the southern continuous, but highly episodic. For example, from Shimanto complex when the Pacific/ Philippine north to south, the S-A complex of the Akiyoshi Sea plate boundary (arc) arrived. This suggests belt was formed at end-Permian time (1); that of that collision of huge oceanic features such as the Mino-Tanba and Chichibu belts in the Mid- ridges or intra-oceanic arcs is potentially effective dle-Late Jurassic ( + earliest Cretaceous?) (2); that in forming huge S-A complexes. of northern Shimanto belt in the Late Cretaceous Using these data sets, the paleogeographic re- (3); that of southern Shimanto belt at about 50 construction of extinct oceanic plate is worth at- Ma (4); and that of the Mineoka belt at Miocene tempting as discussed later. time (5). Continuous accretion, the so-called “ two-way Accretionary wedges develop also over old subduct- street model” of Suppe (1972), may have occurred ing slab only on a short time-scale ranging from 15 to 20 Ma, corresponding to the growth of a l-2 km The time span of pelagic-hemipelagic sedimen- wide zone, as in Middle-Late Jurassic S-A com- tation in oceanic plate stratigraphy best approxi- plex in Shikoku (Matsuoka, 1984). However, such mates the age of subducted slab, as mentioned example is rather exceptional, and the episodic above. Judging from the oceanic plate stratigra- zoned growth of the S-A complex is apparent on phies demonstrated in Fig. 8, the S-A complexes a gross time-scale in Japan. in the Akiyoshi belt, Mino-Tanba (type-II) and Outward step-wise younging of S-A complexes Chichibu belts, Mino-Tanba (type-I) belt, north- in Southwest Japan, well demonstrated by the em Shimanto belt, and southern Shimanto belt, oceanic plate stratigraphy, implies that a segment respectively, represent the subduction of ocean of the eastern continental margin of Asia corre- plate as old as 80 Ma, 160 Ma, > 100 Ma, and 60 sponding to Southwest Japan has intermittently Ma, > 20 Ma. changed its interacting partner, i.e. the subducting The age of subducted slab varies from 20 to 180 or passing-by oceanic plate, one after another. Ma, consequently, there is no apparent age se- According to the plate paleogeography by En- quence for subducted slab to form accretionary gebretson et al. (1986) and Maruyama and Seno complex. This suggestion is not consistent with the (1986), Southwest Japan was in contact with the simplified explanation by Uyeda and Kanamori Farallon plate until the Jurassic, with the Kula- (1979) who speculated that the Chilean subduc- Izanagi plate during the Cretaceous, with the tion of young oceanic slab would generate a ACCRETED OCEANIC MATERIALS IN JAPAN

xa@ro~ ofseemp pexo4dJo~ele~ a -! i= I -__--_------_- 196 Y. ISOZAKI ET AL voluminous amount of subduction complex due to TABLE 4 the strong coupling of the two plates (by strong Relation between the volume of accreted oceanic materials and frictional force), whereas in the Mariana Trench subduction-related metamorphic grades (depth of burial)-see no S-A complex exists, and even tectonic erosion Fig. 6. occurs due to the weak coupling by old-slab sub- Metamorphic Belt Volume duction. The abundant S-A complexes in Japan, grade percentage however, strongly refute this notion, and there of oceanic material5 seems to be no relation with age of the subducted slab. The formation of S-A complexes, instead, Zeolite facies Shimanto >l% seem to be related with timing of supply of volu- Prehnite-pumpellyite facies Chichibu 15-20% minous terrigenous sediments, because the major Greenschist facies

part of S-A complexes is composed of terrigenous Greenschist facies sediments. Presumably, if voluminous sediments Glaucophane schist facies Sanbagawa 30% were available in the present Mariana Trench, a Albite-epidote amphibolite facies S-A complex would have formed. As mentioned above, the subordinate role of rise, plateau, or ridge collision may be emphasized, but details will pumpellyite/ greenschist facies, greenschist/ be given elsewhere. glaucophane (blue-)schist/ albite-epidote amphi- bolite facies grades (Fig. 6, Table 4). The positive mechanism of accreting oceanic ~teriais-meta- correlation of increasing volume of accreted oce- morphic constraint anic materials with increasing depth in subduction zone strongly suggests that underplating (subcre- The mechanism of accreting oceanic materials tion) plays a dominant role in accreting oceanic has been discussed for a long time, accompanied materials, as prognosticated by Karig and Kay with the proposals for several models such as: (1) (1981). Thus we conclude that the deeper the sedimentary mixing at trench (e.g., Taira et al., accreting materials are buried, the more they can 1981), (2) off-scraping at shallow depth (Seely et be accreted landward. al., 1974, and others), and (3) underplating On the basis of our new data set and relevant (Watkins et al., 1981) or subcretion (I&rig and discussion mentioned above, we summarize the Kay, 1981). There is no example of oceanic subduction-accretion process in a schematic di- materials contained in the matrix-forming terrige- agram (Fig. 9). In shallower levels of a subduction nous sandstone in Japan. This suggests that model zone, off-scraping plays a major role in forming (1) may be less significant, because the present-day an accretionary prism, accompanied by sedimen- trench is usually buried with coarse-grained tary mixing on the surface such as debris flows turbidites (Thornburg and Kulm, 1987; Le Pichon induced by local gravimetric instability or mud et al., 1987). If oceanic materials are mixed with diapirism. In these levels, accretion accounts only sediments driven by debris flow along the land- for the very upper portion of the subducting slabs, ward fault escarpment of oceanic slab, they should i.e. thin sedimentary drape capping oceanic floor, be surrounded mostly by a sandstone-dominant in addition to rocks forming topographic reliefs matrix. such as seamounts, rises and plateaus. At depths Through comparison of the volume of accreted sufficient for generating blueschist rnet~o~~srn, oceanic materials with respect to their metamor- on the other hand, underplating (subcretion) seems phic grades, it may be evaluated which model, to be the most important process to accrete abun- either (2) or (3), is more critical to the accretion of dant oceanic materials from descending slabs. oceanic materials. For example in Shikoku (Fig. Modem analogues of accretionary complexes in 6), metamorphic rocks range from less than 1% Barbados and the Nankai Trough suggest that the (volume), through 15-209, to about 30%, corre- development of a decollement zone can help sponding respectively to zeolite facies, prehnite- accrete kilometric-sized tectonic slices of subduct- ACCRETEII OCE.ANfC MATERIALS EN JAPAN 198 Y. ISOZAKI ET AL

At 160 Ma B.P. the Permian oceanic plate was subducting underneath the leading edge of the Eurasian plate. Numerous sporadic seamounts, oceanic plateaus, rises, or hot-spot tracks includ- ing that of the Mikabu greenstone complex, occur on the oceanic plate, presumably Izanagi. At that time, slightly younger oceanic slab was subducting under Northeast Japan and Hokkaido, indicating the possible existence of a mid-oceanic ridge off Farallon Plate Hokkaido (Fig. 10). The paleo-plate geometry by Engebretson et al. (1986) suggests that the Faral- ion-Izanagi ridge arrived at the eastern margin of Eurasia at about 150 Ma B.P. Their independent estimate of the ridge-transform offset is consistent with our geological conclusion. During the Late Jurassic to Early Cretaceous, an extensive volcano-plutonism occurred within the eastern Eurasian continent in addition to a volcanic front. The presence of such ma~atism unrelated with the plate margin suggests a re- gional uplifting of the eastern margin of Eurasia, which may be the source provenance to supply voluminous sediments into the trench. Additional Fig. 10. Reconstruction of paleoplate geometry around Japan at 160 Ma. Symbol V represents area of igneous activity on provenances are the collision zones between the continent. Siberian platform and the Sino-Korean craton to the north, and Qinling suture between the Sino- ing oceanic slab to the sole of an accretions Korean craton and the Yangtze platform to the wedge.

Paleogeographic reconstruction of ancient plates 250Ma Siberian Platform around Japan

It is theoretically impossible to reconstruct the paleo-plate geometry, including the age of sub- ducting slab, before 200 Ma B.P. due to the lack of present-day ocean floor older than 200 Ma B-P. However, if we introduce the above-mentioned method in analyzing the attribution of oceanic materials in ancient S-A complexes and their ‘Farallon Plate oceanic plate stratigraphy, a detailed paleogeo- Hualya graphic reconstruction will be possible. Combined with the relative plate motions based on the hot- Yang* spot framework on a global scale (Engebretson et al., 1986), the distribution of the various topo- graphic features on extinct oceanic plates may be restored. The two paleogeographic maps of Figs. 10, 11 Fig. 11. Reconstruction of paleoplate geometry around Japan are preliminary examples of this application for at 250 Ma. Legends used are common with those in Fig. 10, Japan and its vicinity at 160 Ma and 250 Ma B.P. except the symbol + for area of plutonism. ACCRETED OCEANIC MATERIALS IN JAPAN 199 south (Lin et al., 1985; Maruyama and Sakai, (6) Pre-Mesozoic paleogeo~ap~c reconstruc- 1986). tion of Japan and its vicinity was attempted on At 250 Ma Eurasia was not amalgamated yet; the basis of the attribution of accreted oceanic in the eastern part, the Siberian platform, the materials and their oceanic plate stratigraphy. Sino-Korean craton (North China) and the Yangtze platform (South China) were separated Acknowledgements by oceans and consuming plate boundaries (Bur- rett, 1974; Klimetz, 1983; Ma~y~a and Sakai, We greatly appreciate valuable comments and 1986; Maruyama et al., 1989). Southwest Japan, at critical reading of the manuscript provided by Dr. the eastern tip of the Yangtze platform, was being Andrew Tomlinson, Prof. J.G. Liou, and Dr. subducted by an oceanic plate as old as Middle Tetsuo Matsuda. Carboniferous (ca. 330 Ma) or much older, prob- ably the Izanagi plate (Maruyama and Seno, 1986), on which many seamounts capped by reef lime- References stones were present (Fig. 11). The Maizuru island arc which may have ceased subduction-related Ada&i, M. and Kojima, S., 1983. Geology of the Mt. Hikage- volcano-plutonic activity by 250 Ma was also pre- daira area, east of Takayama, Gifu Prefecture, central Japan. J. Earth Sci. Nagoya Univ., 31: 37-67. sent on the Izanagi plate, and moved toward Addicot, W.O. and Richards, P.W., 1981. Plate tectonic map of Southwest Japan and collided-amalgamated at the circum-Pacific region. American Association of Petro- some time in the Triassic. leum Geology, Tulsa, Okla. Arai, S. and Uchida, T., 1978. Highly magnesian dunnite from Conclusions the Mineoka belt, central Japan. J. Jpn. Mineral. Petrol. Econ. GeoL.73: 176-179. Asahina, T. and Komatsu, M., 1979. The Horokanai ophiolitic Through detailed compilation of many accreted complex in the Kamuikotan tectonic belt, Hokkaido, Japan. oceanic materials since Late Permian in Japan, J. Geol. Sot. Jpn., 85: 317-330. several conclusions on the consuming plate Banno, S., 1958. Glaucophane schists and associated rocks in boundary process are deduced. They are listed as the Omi district, Niigata prefecture, Japan. Jpn. J. Geol. follow: Geogr., 29: 29-44. Banno, S., 1964. Petrological studies on Sanbagawa crystailine (1) Oceanic materials accreted to the subduc- schists in the Besshi-Ino district, central Shikoku, Japan. J. tion complex are mainly rocks from above Layer 2 Fat. Sci. Univ. Tokyo, Sect. 2, 15: 203-319. of the oceanic crust, and rarely from below Layer Batiza, R., Melson, W.G. and Vat&a, D., 1984. Petrology of 3 and the underlying mantle. Dominant oceanic young Pacific seamounts. J. Geophys. 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