Accretion tectionics of the Japanese islands and evolution of continental crust

0 Acadbmle des sciences / Elsevier, Tectonics / JecfMque Geodynamics / Gbodynamique

Paris

Note remix i I’wwtation du Comitk de lecture Nore remise le 6 septembre 1997, acceptee aprh r&sion

C. R. Acad. Sci. Paris, Sciences 1997. 325,467478

de la terre

le 23 septembre 1997

et des planetes

/ Earth & Planefury

Sciences

A. Taira et a

VERSION ABR~G~E Depuis une vingtaine d’anrkes, une interpretation nouvelle de la geologic du socle des arcs insulaires japonais montre que ces derniers sont constituk de deux ensembles principaux : une ceinture c~volcanique - granitique )p (uolcanics-greenstonegranitoid-belt VGB) et une autre u turbiditique - granitique m (turbiditegranitoid beZtTGB) (Taira et al , 1989, figures 1 et 2). La ceinture du type VGB est constituie d’kpaisses kcailles tectoniquer., imbriqukes, de croOte moyenne ou supkieure des arcs insulaires, incluant beaucoup de materiel volcanique andksitique ou metabasaltique, intrudees tardivement par des plutons granitiques. L’environnement gkdynamique est celui d’une collision arc-arc. La ceinture du type TGB est composke d’unit6s de turbidites, de complexes d’accktion et de mklanges incluant du matitriel oceanique et a et6 formee par l’accumulation progressive de primes d’accrktion. Elle est 6galement intrudke par des plutons granitiques. Ces deux wpes lithologiques, identifies dans les arcs insulaires japonais, correspondent g des constituants majeurs de la croiite supkrieure continentale et sont caractkiitiques de l’accretion crustale dans les marges convergentes. Ces concepts ont kti: appliquks 5 l’analyze et g l’interprktation de plusieurs ceintures orogCniques. Un premier exemple etudik est celui de la zone de collision d’Izu entre l’arc d’Izu-Bonin et celui d’HDnshu (figure 3) pr& de la triple jonction u Eurasie-Pacifique-Philippines 21au large du Japon central. Le taux d’accr&ion crustale obtenu pour la partie nord de l’arc d’Izu-Bonin, dont on connait bien la structure crustale

1. Introduction The Japanese Islands are composed af a complex framework of accretionary belts and volcano-plutonic complexes. Although there are still outstanlling questions to be addressed to the origin of their geotectonic framework, the works produced in the last 20 years or so strongly suggest that they evolved mostly through subduction of oceanic plates from the early phase of a contiilental arc setting to the later phase of an island arc setting (Taira, 1983; Taira et al., 1983; Taira, 1985; Taira et al., 1989). Through these subduction phases, the Japanese arc crust grew outboard of the older continental margin of the :Southern China and Sino-Korea blocks (Taira and Tashiro, 1987; Isozaki, 1996). The crust, at least in its upper part, is composed of basically two suites of rocks; one suite is accretionary complex of turbidite and granitoid intrusives and the other is accretionary complex of volcanics and associated granitoids (Taira et al., 1989) (see figures 1 and 2). There are minor components such as granulite-gneiss rocks which compose either former Asian continental margin (Hida

468

C. R. Acad.

(figure 41, suggkre que le magmatisme d’art pourrait @tre un constituant important de la croissance de la croQte supkrieure continentale. Un autre exemple est celui de la ceinture archeenne qui affleure en bordure du craton de Pilbara, dans l’ouest de l’ilustralie et qui est t&s similaire au VGB japonais. On y reconnait plusieurs empilements de co&es rhyolitiques ou basaltiques, associ&s P des sediments volcanoclastiques ou chimiques, sans terrigene, trk similaires 2 l‘kaillage crustal d’un arc insulaire comme celui qui est implique clans la zone de collision d’Izu. Nous proposons enfin une classification de la gkologie de la croGte supkieure continentale, avec la distinction de cinq ensembles lithologiques et structuraux : (1) les ceintures de type VGB et (2) de type TGB ; (3) les nappes de socle et de couverture ; (4) les ceintures granulite-gneiss et (5) les sequences de couverture et les plutons intrusifs. II semble que de vastes assemblages de VGB et TGB aient &? incorpor& lors des episodes majeurs d’accr&ion crustale, telle la cratonisation de 1’Archiren terminal - debut du Protkrozo’ique, l’orogen?se panafricaine, les orogenkes pharkrozdiques d’Asie centrale et orientale, etc. Dans ces ceintures orogkques, l’implication de plusieurs arcs insulaires et la formation de VGB semblent avoir & un mkcanisme important d’addition de crofite continentale. La formation de TGB a jouC un r6le majeur dans le remaniement et le recyclage de la croOte continentale, avec comme conskquence un changement important dans la composition de la croiite supkrieure. La comprkhension des processus d’accrktion des arcs insulaires japonais est l’une des clefs de l’ktude de l’i-volution de la croiite continentale.

belt) or collision related uplift zone (Hidaka metamorphic belt). Also allochthonous nappes or tectonic slivers are present (e.g. Kurosegawa-Southern Kitakami belt). Their distributions, however, are quite limited. The first suite is composed of accretionary prisms of trench-fill or ocean floor turbidites intermixed with oceanic materials including hemipelagic-pelagic sediments, pillowed basalt of oceanic crust and volcanicssedimentary (often carbonate) caps of seamounts or oceanic plateaus. The entire complex grew oceanward with progressive deposition of cover sequence (slope basin/forearc basin sequences) and later intruded by granitic intrusives. This suite is hereafter called a turbiditegranitoid belt (TGB). The TGB occupies about 80% of surface basement geology of Japan (figure 1). Among this, the distribution of granitoids occupies about 30% of the surface (figure 2). The second suit is composed of imbricated uppermiddle crust of oceanic island arcs including greenish colored meta-basaltic and andesitic volcanics (greenstone: green color is due to chlorite and epidote) with

Sci. Paris, Sciences

de la terre

et des planQtes

/ Earth & Planetary Sciences 1997. 325,467.478

Accretion

tectonics

of the

Japanese

islands

-

Shikoku

Figure 1. Basement geologic complexes of turbidite-granitoid :,, ,,

map

of Japan. Granitic belts and volcanic-granitoid

Carte g&ologique du socle du/apon, de ceintures turbiditiques-granitiques C. R. Acad. Sc:i. Paris, 1997. 325,467.-478

Sciences

roches grartitiques et de ceirrtures de

la terre

et des

rocks

are not belts.

/ Eaftb

evolution

0

included

here.

& Plonefory

Sciences

Japanese

sent

islands

composhes

of continental

F&7.4

basin

non incluses. Les i/es japonaises volcaniques-granitiques. plar&tes

and

are

pour

mostly

/a plupart

100

2OOkm

composed

de complexes

of accretionary

d’accr&ion

crust

A. Taira et al duction related magmatic materials. The collision occurred after the opening of the Shikoku Basin in the Philippine Sea plate and was succeeded by episodic phases of peak collision in about 8-5 Ma and 2 Ma-present (Niitsuma, 1989; Amano, 1991). The result is the formation of thrust-bound tectonic crustal segments up to 120 km wide, of volcanic and volcaniclastic successions from the separated

Izu-Bonin into four

collision stratigraphic segments volcanics basin fill granitic cover

arc (figure 3). The crustal segments main blocks. Overall, theTanzawa-lzu

zone resulted in the following tectonoand igneous assemblages: 1) thrust-bound of oceanic island arc crust (mostly metaand a tonalite pluton); 2) foredeep terrigenous sequences along the boundary thrusts; 3) later

plutons rocks.

intruded

into

Each crustal segment (block) submarine volcanic/volcaniclastic intercalated with hemipelagic

Figure 2. Distribution of granitic rocks in Japan. Granitoids are mostly Cretaceous and Cenozoic age. limited distribution in Hokkaido suggests that this part is mostly composed of collisional belts of forearc assemblage. ‘&, :i : ‘, R&partition plupart, d’Hokkaido, consfit&e

des roches granitiques au Japon. Cc//es-ci sont, pour /a d’sge Cr&acC et Gnozoique. Leu,’ rkpartition, dans I’ile suggPre que cette partie du Japon est essentiellement de ceintures collisionnelles d’assemblage avant-arc.

tonalitic pluton and later intruded This is hereafter called a volcanics belt (VGB). It has been demonstrated

by granitic intrusives. (greenstone)-granitoid that this complex

essentially composed of the same kind structure as Precambrian greenstone-granitoid rane) (Taira et al., 1992).

of lithology belt(or

is and ter-

tion of accretion tectonics VGB at the Izu-collision

of Japan with the zone, central Japan.

formation

of

470

considered as a part

4)

volcanic

is composed of deformed rocks. The upper part is sediments and reefal lime-

to be a part of tectonic slice, therefore of Izu-Bonin oceanic island arc.

Short-lived

foredeep

basins,

filled

by

coarsening-upward elastic sequence, have along the boundary thrust between segments 1991). The basin-fill sediments tend to become age from north to south, ranging from Middle Pleistocene suggesting progradation stacking slices

(Aoike

volcano inside

the

and

Taira,

existed an

overall

developed (Soh et al., younger in Miocene to of crustal

1997).

the collision involved traverse across the formation on the crust (Takahashi,

active collision

surface 1989).

and

island zone

arcs, igneresulted in

granitic

plutons

The results of the France-Japanese “KAIKO” project (including various phases) showed that this mechanism of crustal growth is still active between the Izu-Bonin and Honshu arcs with the development of an incipient boundary behind the Zenisu Ridge into the Shikoku (Le Pichon et al., 1987b; Lallemant et al., 1987).

ated

basin-fill

sequence

In this collision zone, juvenile crust addition,

An example of a collision zone betwleen two active arcs occurs near the trench triple junction offshore from central Honshu (figure 3). An oceanic island Izu-Bonin (Ogasawara) arc is in the process of collision against a more matured Honshu arc, which is (composed of continental crust (Taira et al., 1989). An oceanic island arc is within an by juvenile,

slices;

plate Basin The

present plate boundary marked by the eastern end of the Nankai, Suruga and Sagami Troughs will become deactivated and a part of the Izu-Bonin arc will be incorporated into the next phase of the crustal segment with the associ-

II. Formation of volcanics (greenstone)granitoid belt (VGB) in the llzu-collision zone

here defined to be an arc which is built oceanic environment almost exclusively

crustal

stones mostly Middle Miocene age. The volcanic complex is virtually devoid of terrigenous elastics. Metamorphism ranges from actinolite facies in the Tanzawa block and zeolite facies in the Izu block. Within the Tanzawa block, there is a tonalite pluton, about 26 x 8 km. This pluton is

Because ous activity

In this paper, we would like to demonstrate that the above two lithologic assemblages identified in the japanese Islands represent a major constituent of the upper part of continents. Understanding the accretion processes of these belts thus provides a key to the study of the evolution of continental crust. Let us begin our explana-

are

intrasub-

C. R. Acad.

taking

III.

plate

boundary.

therefore, an important to form a new continental

process crust,

of is

place.

Nature

Then, a question been originally recently acquired

Sci. Paris, Sciences

of the present-day

de la terre

of Izu-Bonin naturally formed P-wave

arc crust

raised is what kind of crust has before the collision process? A seismic velocity structure by our

et des planetes

/ Earth & Planetary Sciences 1997. 325,467.478

Accretion

tectonics

of the Japanese

islands

----

Carte gkotectonique

and

et coupe

C, R, Acad. Sci. Para, Sciences 1997 325,467-478

cross

dans

section

la zone

de la terre

of Izu-collision de collision

zone. d’lzu.

et des plan&es

L’active

Active

arc-arc

tectmique

/ Earth & Planeby

.

collision

tectonics

has

de collision

arc-arc

a pris

Sciences

cru$t

here since

15 Ma.

Thrust fwlt .

map

of continental

Tactonic line

-

Figure 3. Ceotectonic >~II :. ~ ‘b

and evalution

tncipiant thrud fault

been taking place

dam

place cefte

mne

il y a 15 Ma.

471

A. Taira

et al.

group (Suyehiro et al., 1996) preserted one of the best images of crustal structure of an oceanic island arc (figure 4). The Northern Izu-Ogasawara arc is considered to be a typical oceanic island arc (Taylor,. 1992). The result of the seismic survey shows the following characteristics of the arc crust (Suyehiro et al., 1996). The crust can be classified into four layers based on the P-wave velocity (figure 4): Layer A: 2-4 km/set layer, Layer B: 4.5-5.5 km/set layer, Layer C: 6.0-6.3 km/set layer, Layer D: 7.1-7.3 km/set layer with overlying thin 6.8-6.9 km/set layer. Moho is about 20 km deep under the rift zone. Figure 4 shows the crustal structure and lithologic interpretation based on the above classification (Taira et al., in press). LayerA: Direct evidence for the lithology of this layer can be obtained from the nearby ODP drilling results (e.g. Fryer, Pearce, Stokking et al., 1992; Taylor, Fujioka et al., 1992). The results show that this layer is mostly composed of intercalntion of volcaniclastics, lava flows and biogenic hemipelagites.

Crustal Structure C-f 0

1

Layer B: A part of Layer B, so called the arc basement in seismic reflection profiles (Cooper et al.,l992) of the forearc region has been penetrated by ODP Sites 786 and 793 (Fryer, Pearce, Stokking et al., 1992; Taylor, Fujioka et al., 1992; Arculus et al., 1992). The resulting lithology shows volcaniclastics, lavas and intrusives of boninitic composition of Oligo-Eocene age. Under the rift zone, the basement is probably intruded by a sheeted dike complex (Morita, 1994). layer C: This is a layer of unique velocity. The velocity of 6.1-6.3 km/set at a depth of 5-l 5 km suggests a felsic rock composition. It is noteworthy that, using modern seismic techniques, a layer, at least 5 km thick, with this velocity range has not yet been detected in thick mafic crustal regions such as the Ontong Java Plateau (Miura et al., 1996), Iceland hot spot (Bjarnason et al., 1993) and North Atlantic volcanic margin (Barton and White, 1995). Another line of supporting evidence for felsic rocks comes from the land geology and dredged data. A tonalite pluton of the Tanzawa block is an obvious candidate. Tonalites

of Northern

Rift

.

Izu-Bonin Arc

Forearc

r\ .\' /

.

> Layer A (2-4 Sediments

krtvsec)

cl.

I

Sefpentinite

-25 50

0

loo

150

200

250

300

400

350

450

Distance (km) @

@ Bcninite (8%) 1 Volcanic Cl Sediments (3%) Figure 4. Izu-Bonin See figure 1 for the .‘:y,i: E, Profil crustal Voir /a figure

472

crustal location.

profile. Figure

Izu-Bonin. Interpdtation 1 pour /a localisation.

Geological also shows @olo,:ique

interpretation proportion

Basalt (14%)

of recently of various

de la structure

C. R. Acad.

rock

crustale

Sci. Paris,

acquired types. nkemment

Sciences

@ Tonalite (29%)

P-wave

acquise

de

structure

crustal

grace

la terre

aux

et des

ondes

plan&es

(3

Gabbro (46%)

(after

Suyehiro

P (d’apr6s

Suyehiro

/ Earth

& Planetary

et al., 1996).

et al.,

1996).

Sciences

1997. 325,467-478

Accretion

were repeatedly dredged from the eastern scarp of Komabashi Daini Seamount of Kyushu-Palau ridge, a rifted counter part of Northern Izu-Bonin arc (dredge data: lshii and Haraguc:hi, 1997, personal comma nication). A preliminary processing of newly acquired s,eismic refraction data along th’e dredge site (Shinohara,l997, personal communication) !jhows the possible presence of a 6.0-6.3 km/ set layer. The age obtained from these rocks ranges from 50-30 Ma which is comparable to the initial emplacement of Izu-Ogasawara arc magma. Tonalite samples were also dredged from the Mariana forearc re,gion (Fryer, 1997, personal communication). From these, Suyehiro et al. (1996) and Taira et al. (in press) suggested that Layer C represented a tonalitic plutonic layer. Layer D: This layer shows a higher-velocity lower crust compared with other arc examples. For example, the NE Honshu arc lower crust is composed of a 6.8 to 7.3 km/set layer. The hiigh speed lower crust is a characteristic of mafic magmatic underplating such as in the rift zone and volcanic rifted margin (Fountain, 1989). Taira et al. (in press) showed that Layer D is composed of gabbros and mafic and partly ultramafic amphibolite facies rocks. The important implication of this crusial structure study is that oceanic island arcs are more felsic in composition than previously estimated (basaltic composition: e.g., Arculus, 1981). This further suggests that assemblage of oceanic island arcs can produce a continental crust which is intermediate in composition (Taira et al., in press). The tonalitic middle layer can provide extra buoyancy to make

Figure 5. Cross section of southwest See figure 1 for the location. ‘::,‘,” Coupe du .%d.-Ouest turbiditique-granitique.

Japan.

du lapon. Cette Voir /a figure

C. R. Acad. Sci. Paris, Sciences 1997. 325.467-478

coupe I pour

de la terre

This cross

section

montre de man&e /a localisation.

et des plan&es

provides

typique

/ E&h

tectonics

of the Japanese

islands

and evolution

of continental

the arc unsubductable further reinforcing the by Cloos (1993). The origin of tonalitic layers assessed in a future study but, we note that the this layer requires a major revision of island genesis.

crust

suggestion have to be presence of arc magma

IV. Formation of the turbidite-granitoid belt (TGB) in the Nankai-Shimanto acrretionary prism The present plate boundary at the Izu collision zone extends to the Suruga and Nankai Troughs to the west, and the Sagami Trough to a trench triple junction to the east. At the Nankai Trough and landward, a process of formation of a secondary crust by TCB can be observed (Taira, 1985). The Nankai Trough extends 700 km from the Suruga Trough to the northern tip of the Kyushu-Palau Ridge. The trough shows water depths between 4 000 to 4 900 m deepening from east to west. The floor is shallow and flat compared with normal trenches becauseof large sediment accumulation there. The stratigraphy of the sediment-fill in the Nankai Trough is revealed by several deep-sea drill holes of DSDP Legs 31,87 and ODP Leg 131 (see summary by Taira, Hill et al., 1992). The lithology is basically composed of two units: upper turbidite unit and lower hemipelagic unit (figure 5). The turbidite sediments have been transported along the trough axis from the Suruga Trough and Izu-

a typical

outward

/a croissance

& Planetary

growth

d’une

Sciences

crolite

of continental

crust

continentale

vers

by turbidite-granitoid

/‘ext&ieur

par

belt.

une

ceinture

473

A. Taira et 01.

collision zone (Taira and Niitsuma, 1986). The history of sedimentation can be interpreted as follows. In the newly formed Shikoku Basin (25-l 5 Ma), fine-grained terrigenous sediments, volcanic ash and biogenic sediments started to accumulate as hemipelagic sediment cover. The collision of the Izu-Bonin arc against the Honshu arc produced a major mountain range in central Honshu. Eroded debris shifted at the plate boundary foredeep (presently the mouth of Fuji River) forming a fan-delta system. Occasional mega-earthquakes destroyed the fan-delta front and generated turbidity currents which flowed along the trough from east to west filling the Nankai trough floor. A sequence of hemipelagite and turbidite sediments has been deformed and scraped off to fovm an accretionary prism with a typical fold-thrust belt geometry (Moore et al., 1990; Ashi and Taira, 1992). The decollement has been developed within the hemipelagic unit. The rapid deposition of turbidite on an impermeable hemipelagic unit resulted in overpressuring (Le Pichon et al., 1993). This is considered to be the main cause of the initial decollement development. This linkage of arc collision tectonics and the formation of an accretionary prism indicates a change in crustal composition from juvenile to secondary nature. The Cretaceous to early Miocene accretionary prism developed landward of the Nankai accretionary prism is called the Shimanto Belt (figures 1 and 5). Extending from the central Honshu to the Ryukyu Islands, the belt is about 1 800 km along the strike and has a Imaximum width of 100 km. Its equivalent extends from Hokkaido to Sakhalin. This belt, therefore, comprises one of the major geological elements of the Eastern Asia Pacific rim. The Shimanto belt is composed of two main lithologic units: a relatively coherent turbidite unit and a highly deformed melange unit (Taira, 1981; Taira et al., 1982; Taira et al.., 1988; Taira et al., 1992). These lithologies occur in a repeated fashion, similar to the structure of a fold-thrust belt with systematic oceanward younging. The melange unit includes tectonic slivers of basaltic pillow lavas, red pelagic shales and varicolored hemipelagic shaleswith volcanic ash layers; all ofwhich are in a highly sheared argillaceous matrix. The entire package is then subjected to a series of out-of-sequence thrusts (Ohomori et al., 1997). The best documented example of a reconstructed oceanic plate stratigraphy has been obtained from the Cretaceous Belt of Shikoku (Taira et al., 1988; Taira et al., 1992). Dating of the melange lithologies by microfossils indicates that lower to mid-Cretaceous (130-90 Ma) oceanic materials (nanno-plankton bearing limestone, chert and hemipelagite) is mixed with Campanian (70 Ma) argillaceous matrix and the entire melange zone is juxtaposed with Campanian turbidite sequence. Paleomagnetic measurement suggests that the 130 Ma pillow lava and limestone show equatorial paleolatitude while the turbidite sequence shows more or less the present position (30°N) (Kodama et al., 1983). These data led to the interpretation

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C. R. Acad.

that an ocean floor originating at equatorial latitude at about 130 Ma moved north at least 3 000 km, and then subducted at 70 Ma. Within the Shimanto Belt, middle Miocene granitic rocks are present. These unusual near-trench igneous rocks represent an episodic thermal event probably related to the subduction of the young Shikoku Basin oceanic lithosphere (Takahashi, 1980). The Shimanto Belt occupied a forearc region of a Cretaceous and Tertiary continental arc at the eastern margin of Asia before the opening of the Sea of Japan (Tamaki, 1995). A time equivalent volcanic arc now occupies the Honshu side of SW Japan where Cretaceous and Tertiary batholith and felsic volcanics with continental deposits are exposed. The Cretaceous granites intruded into older accretionary prisms of Jurassic and Permian age (figure 5). These accretionary prisms are composed of basaltic rocks of various origins (Ogawa and Taniguchi, 1989; Kimura et al., 1994), reef limestones (Sano and Kanmera, 1988), extensive deposits of red to green bedded radiolarian cherts and varicolored shales, and volumetrically overwhelming turbidite sequence (Ichikawa et al., 1990; isozaki, 1996). Overall, the geology of the main part of Japanese Island arcs is characterized by the progressive growth of turbidite-rich accretionary prisms and the intrusion of granitic rocks. This is a consequence of prolonged subduction, accretion of turbidites and later intrusion granites due to progressive retreat of trench. Here, we call this geological terrain as a turbidite-granitoid belt.

V. Archean greenstone-granitoid and Izu collision zone

belt

From the mid 80’ s, the accretion tectonics of SW Japan were taken as an analogy to some Archean-Proterozoic erogenic belts (e.g. Card, 1990). This drew our attention to the Precambrian geology (Taira et al., 1992) and in the last seven years or so, we started to map a greenstone-granite belt along the coastline of Pilbara Craton in Western Australia. Our field mapping and lithological analysis revealed that the belt is composed of a thrust belt of about 20 km thick, 3.1 Ga supracrustal and crustal units (Kiyokawa, 1994; Kiyokawa and Taira, in press). The supra crustal unit exhibits well-preserved low-grade metamorphosed pillow basalt, massive rhyolite, felsic tuff, black chert and bedded varicolored chert and banded-ironformation (BIF). The metamorphic units are composed of metabasite, felsic rocks, low-K granite, granitoid gneiss, gabbro and peridotite. The units experienced granulite to amphibolite facies metamorphism. The inspection of the supra-crustal unit suggests that this is devoid of terrigenous elastics; and three megacycles of basaltic to rhyolitic lava emplacement and successive volcaniclastic/chemical sedimentation are observed. From these characteristics, Kiyokawa and Taira (1997 in

Sci. Paris, Sciences

de la terre

et des planetes

/ Earth

& Planetary Sciences 1997. 325,467-478

Accretion press) concluded that sents a crustal stacking the lzu collision zone.

this greenstane-gl*anite of an oceanic island

VI. Lithological interpretation continental crust Recent erogenic

advances in the understanding belts (e.g., Hoffman, 1988;

win, 1991; Windley,l995) vast area of P~hanerozoic Tashiro, 1987; Maruyama

belt reprearc similar to

islands we

of upper

of the Precambrian Condie, 1989; Good-

and the previously unexplored orogens in central Asia (Taira and et al., 1992; Sengor et al., 1993)

to the interpretation propose a simple

of global five-hold

erogenic belts. lithological-

structural classification of upper crust of continent. They are: 1) VGB: volcanics (greenstone)-granitoid belt; 2) TGB: turbidite-granitoid belt; 3) sedimentarybasement fold-thrust belt; 4) granulite-gneiss belt; 5) cover sequences and intrusives. Recent

descriptions

of

various

cratons

and

orogens

(e.g., Camp, 1984; Percival and Williams, 1989; Card, 1990; Monger, 1993; Windley, 1995) clearly show that the majority of thle upper continental crust is composed of the assemblage of the above components in various proportions. lithologic components and TGB are already fold-thrust belt

typically represents the deformed passive continental margin sequence overthrust onto the host continent due to collision tectonics. Thrusting often involves basement and ophiolites. The granulite-gneiss belt is part of the crust (including various of igneous and seclimentary metamorphosed into a lower crust P-T condition exhumed to the surface. The exhumation process complex but thrust-related crustal duplexing gravitational-extensional gen are proposed

collapse (Wernicke,

1992;

origin) then can be and

of overthickened Percival and

oroWest,

1994). There belts are covered by sediments (foreland, rift margin, strike-slip and intra-plate basins) and also covered and intrudecl by various igneous rocks, including flood basalt swarms.

genie belt indicates sedimentary-basement this period.

(large

igneous

provinces:LIP)

In older Archean cratons such as and Yulgarn), North America (Slave, Africa before 2.5 Ga, the geology is volcanics(greenstone)-granite belt

and

giant

dike

in Australia (Pilbara Superior) and South cornposed mostly of and granulite-gneiss

belt with volcano-chemical-elastic sedirnentary cover sequences. After 2.5 Ga or so, the orogens become dominated by alternating complex or collage of volcanics (greenstone)turbidite-granite belts ant granulite-gneiss belt. In some erogenic belts, a thick pile of sedimentary sequences which are thrust seen in the Labrador section C. R. Acad. Sc:i. Paris, Sciences I 997. 325.40-478

over onto the basement of the Trans-Hudsonian de la terre

et Ides plan&es

islands

and evolution

that the fold-thrust

of continental

crust

formation of the linear belt started to form from

1994) and the Ural-Altai-Mongol-Eastern Asia orogens since the Permian period, the volcanics (greenstone)granite belt become a major component of the orogens (Taira and Tashiro, 1987; Sengor et al., 1993). Also, in the Cordilleran orogen of North America, the Cascadian accretionary prism and the Insular and Coastal Belts compose a complex of a turbidite-volcanics-granite belt (Cowan, 1990). We suggest here that the episodes of major crustal growth in the geologic history involves formation of a turbidite-volcanics-granite belt. On the contrary, Alpine-Himalayan orogens are dominated by pre-existed continental crust (sediment-basement belt) (e.g., Schmid intense deformation

as oro-

and basin-fill/platform sediments thrust-fold belt and granulite-gneiss et al., 1996). These orogens display and crustal reworking, but involve-

ment of juvenile crust is subordinate orogens mentioned above. Therefore, growth, the Alpine-Himalayan erogenic role.

VII.

A simplified genetic link of these to plate tectonics follows (VGB explained). The sedimentary-basement

of the Japanese

In Proterozoic and Phanerozoic orogens, the proportion of greenstone-granite belts decreases compared to the Archean orogens. However, in some areas of rapid crustal growth such as the Arabian-Nubia shield during the Late Proterozoic Pan-African orogenesis (Camp, 1984; Stern,

suggest the importance of arc-related geologic units in the formation of crust in erogenic belts. This enables us to apply the concept of the accretion tectonics of the lapanese Here,

tectonics

Bulk

Growth composition

compared to the in terms of crustal belts play a minor

rate of continental of the

continental

crust

crust is thought

to be

similar to andesite. This led to an early proposal by Taylor (1967) that the continental crust was made by arc magmatism. This simple hypothesis however, was challenged by Reymer and Schubert (1984) who suggested that Phanerozoic arc addition rate is too small to account for some certain phases of rapid crustal growth. Although there have been a variety of opinions expressed regarding this arc addition rate (e.g., Dixon and Golombek, 1988), their suggestion seems to be widely accepted. The rate of igneous addition of the Northern Izu-Bonin arc since 80 km3/km/Ma. a reasonable

its

birth (45 Ma) was calculated as Taira et al. (in press) showed that this was estimate for the western Pacific Plate arcs.

Using the above rate, it has been estimated that the global addition rate by arc magmatism is 2.96 km3/yr. This exceeds the average crustal addition rate since the birth of the Earth (1.76 km3/yr). From this, we suggest that the juvenile igneous addition rate by arc magmatism strongly supports the lithologic interpretation of continental upper crust. The recent accumulation of knowledge of plumerelated magmatism (Coffin and Eldholm, 1994) and its influence on continental evolution suggests that the emplacement of LIP can play an important role in crust genesis (Abouchami and Bohier, 1990; Stein and Goldstein, 1996; Davis, 1997). This hypothesis contradicts the long-accumulated information of continental surface ge-

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A. Taira et al

ology as stated in this paper. A large-scale igneous addition, however, to the lower crust could have occurred during LIP emplacement (Rudnick, 1995). Also, at the same time, delamination of mantle and lower crust lithosphere (Turcotte,l989; Kay and Kay, 1993), and flake tectonics (Hoffman and Ranalli,l98~5) can remove the lower crust. It is therefore possible that the lower crust history is very different from that of upper crust (Fountain, 1989; Kay and Kay, 1991). In a simplistic way, we conclude that the upper crust was dominantly formed by arc magmatism and the lower crust might have been largely influenced by mantle plume activity.

VI I I. Conclusion The Japanese Island arcs are composed of two basic lithologic belts: volcanics (greenstone)-granitoid belt and turbidite-granitoid belt. The volcanics-granitoid belt was formed by collision of an oceanic island arc against Honshu arc. The turbidite-granitoid belt was formed by the progressive growth of a trench accretionary prism and later the intrusion of granitoids.

The west Pilbara Archean greenstone-granitoid belt of Australia represents an oceanic island arc collisionaccretion belt. The application of Japanese accretion tectonics suggests that continental upper crust geology can be classified into five lithologic assemblages: (1) volcanics(greenstone)granitoid belt,(2) turbidite-granitoid belt, (3) sedimentarybasement fold-thrust belt, (4) granulite-gneiss belt and (5) cover sequences and intrusives. The arc crustal addition rate of the Northern Izu-Bonin arc suggests that the island arc crust is potentially the most important juvenile material contributor at least to the upper continental crust. The episode of major crustal growth is associated with the volcanics (greenstone)-turbidite-granitoid belt. The episodic development of the western-pacific-type island arc chains in the geological history and later accretion of them formed a vast volcanics (greenstone)granitoid belt. The turbidite-granitoid belt was a major mechanism of continental crust reworking and recycling and probably contributed to the secular compositional change of the continental crust.

Acknowledgements:

We thank Drs K. SlJyehiro and H. Takuyama for their cooperation and discussion throughout our research development. The senior author (A.T.) di’eply appreciates the invitation by the France-Japanese exchange fellowship program. Profs. X. Le Pichon and J.-P. Cadet kindsy hosted A.T. and Prof. J. Dercourt provided the opportunity for this contribution. We are grateful for their support.

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