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Accreted Oceanic Materials in Japan ~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.
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