Contrasting two of orogen in Permo-Triassic Japan...

The Island Arc (1997) 6, 2-24

Thematic Article Contrasting two types of orogen in Permo-Triassic Japan:

Accretionary versus collisional

YUKIO ISOZAKI Department of E a r t h and Planetary Sciences, Tokyo Inst i tute of Technology, 0-okayama, Meguro,

152 Tokyo, Japan

Abstract Proto-Japan originated from a continental margin of the Neoproterozoic Yangtze (South China) craton. It represents a unique Permo-Triassic tectonic setting in western Panthalassa, where two distinct types of orogenic belt occurred side by side. There was an accretionary orogen between the Yangtze craton and the Proto-Pacific (Farallon) Plate and a collisional orogen between the Sino-Korean (North China) and Yangtze cratons. This article reviews results of the latest on-land geological studies concerning Permo-Triassic tectonics in Japan and proposes a new plate tectonic interpretation as well as a paleogeographic reconstruction of this particularly unique geotectonic regime. Special emphases are given to (i) the accretion processes and products derived by collision-subduction of the Permian Akiyoshi paleoseamount and Maizuru paleo-oceanic plateau; (ii) the field occurrence of 220-Ma Sangun high-P/T schists and its implication for the exhumation process and ‘tectonic sandwich’ structure; (iii) the extensive development of a subhorizontal nappe of the pre-Jurassic rocks and their bearing on the orogenic edifice; and (iv) the restricted occurrence of the 250-Ma collision complex in the Hida and Oki belts and the relevant connection to the Precambrian cratons and collision suture in East Asia. The newly proposed paleogeographic reconstruction is also tested by faunal provinciality of Permo-Triassic fossils from shallow-water sediments.

Key words: accretion, collision, high-PIT schists, Japan, nappe, ophiolite, Permo-Triassic, seamount, Sino-Korea, Yangtze.

INTRODUCTION

The predominance of accretionary complexes (AC) and the association of detached continental fragments in Japan suggest tha t the Japanese Islands have fundamentally developed through conver- gence between oceanic and continental plates along active margins. The origin of Japan goes back to the Neoproterozoic era (ca 750-700 Ma) when the proto-Pacific basin was formed by the break-up of the supercontinent Rodinia (Hoffman 1991; Dalziel 1992; Powell et al. 1993; Park et al. 1995). After the conversion from passive to active continental margin in the Early Paleozoic, Pacific-ward orogenic growth of Japan was achieved successively by subduction-accretion processes from the Late Paleozoic to the present (Isozaki & Maruyama

Accepted for publication July 1996

1991; Isozaki 1996). With regard to the evolution of the Japanese Islands, the Permo-Triassic tectonics are particularly important because the fundamental framework of the Mesozoic-Cenozoic orogenic belt of Japan was established and stabilized a t that time by virtue of the generation of two orogenic belts of distinct type side by side; tha t is, the subduction-accretion type and the continent- continent collision type.

The Permo-Triassic tectonics of the Japanese Islands also provide a vital piece of information for reconstructing the paleogeography of the western Panthalassa (proto-Pacific) margin, because this period corresponds to the zenith of the supercontinent Pangea (Klein (ed.) 1994; Embry et al. (eds) 1994). Compared with the Tethyan peripheries (or paleo-Tethyan [Sengor 1989]), information has

Accretion and collision in Permo-Triassic Japan 3

supercontinent, engulfing a closed ocean basin called Tethys (or paleo-Tethys). Both the Sino- Korean and Yangtze Blocks are composed of Pre- cambrian rocks including Archean cratonic core (Jahn 1990; Zheng et al. 1991; Zhou 1994), and are regarded as having detached originally from the supercontinent Rodinia before 700 Ma. The polar wander history of these two continental blocks has been partly documented (Lin et al. 1985; Powell et al. 1993), and a t ca 250 Ma they collided against each other to form a larger continental piece which nearly corresponds to the major part of the present China.

The collisional boundary between these two blocks is known as the Qinling-Dabie suture in central China marked by a 230-Ma ultrahigh- pressure metamorphic belt (Wang et al. 1989; Okay et al. 1989; Maruyama et al. 1994; Cong & Wang 1995). The eastern location of the suture is well established up to the Shandong Peninsula, Northeast China (Yang & Smith 1989; Hirajima et al. 1990; Enami & Zang 1990), but further recognition to the east remains debatable. Judging from the similarity in protolith lithology and age, Sohma et al. (1990) regarded the Hida belt (including the Unazuki schist belt) in central Japan as an eastern extension of this collision suture. In addition, following the comment by Ernst et al. (1988) and Cluzel et al. (1992), Sohma and Kunugiza (1993) and Isozaki (1996) nominated two alternative can-

been limited or almost absent regarding tectonics as well as the paleoenvironment of the western proto-Pacific domains.

In this article, I describe the Permo-Triassic orogenic units of the Japanese Islands and discuss the nature of two distinct orogenic belts developed in proto-Japan, highlighting some diagnostic aspects, such as the subduction-accretion of the reef-capped Akiyoshi paleoseamount and the Mai- zuru paleo-oceanic plateau, exhumation of the 220-Ma Sangun high-P/T meta-AC, the formation of a large-scale Kurosegawa nappe of pre-Jurassic rocks, and generation of 250-Ma medium-pressure type Hida gneiss through continent-continent collision along the western Panthalassa margin.

TECTONIC OUTLINE OF PERMO-TRIASSIC PROTO-JAPAN

A brief summary is given here of the large-scale tectonic background of the western Panthalassic margin where proto-Japan was situated during the Permo-Triassic. Two isolated continental pieces named the Sino-Korean (or North China) block and Yangtze (or South China) block were then located in the eastern margin of the supercontinent Pan- gea, as shown in the reconstructed paleogeographic map (Fig. I ) . Together with other smaller continental pieces, these two blocks almost entirely bridged the northern and southern ends of the

Fig. 1 Tectonic framework of the western Panthalassa at the latest Permian time, ca 255 Ma (modified from Scotese & Langford 1995). Note the location of proto-Japan at the northeastern edge of the Yangtze craton on the Pacific side

4 Y. Isoxaki

didates for the suture in the Korean Peninsula; the Imjingan belt in North Korea, or the Ogchon zone in South Korea.

The Permo-Triassic orogenic complexes occur mostly in Southwest Japan, extending 2000 km along the arc from central Japan to the southern Ryukyu Islands close to Taiwan (Fig. 2). They always occupy the Pacific side of the putative location of the YangtzelSino-Korean suture which is in China and Korea. This suggests that proto- Japan was attached to the margin of the Yangtze Block (Isozaki & Maruyama 1991; Isozaki 1996), rather than the Sino-Korean block, otherwise an unrealistic large-scale strike-slip fault is needed with along-arc displacement of more than 2000 km within Japan. Accordingly, the Japan side of the Yangtze craton is regarded to have faced the proto-Pacific, not directly to paleo-Tethys (Fig. 1). After a collision betvveen the Yangtze and Sino- Korean blocks, proto-Japan kept growing around the amalgamated blocks, and started receiving

I

I A$? Ishigaki Island /

materials also from the Sino-Korean craton. Other parts of Asia, such as Siberia, Tarim, and Burya (Amuria), were scattered along the periphery of Pangea in the Permo-Triassic period.

PERMO-TRIASSIC OROGENIC UNITS IN JAPAN

Distribution of the Permo-Triassic orogenic units in Japan is depicted in Fig. 2. It is remarkable that the occurrence of Permo-Triassic orogenic units is mostly limited to Southwest Japan and the Ryukyu Islands. There is no mappable-sized example of the Permo-Triassic orogenic unit recognized in North- east Japan and Hokkaido, excepting the Hitachi- Takanuki Belt a t the southern tip of Northeast Japan. Although minor amounts of Permo-Triassic sedimentary rock occur also in Northeast Japan, they represent overlapping continental shelf-slope sequences that accumulated upon the Early-Middle Paleozoic complexes (older continental crust and

b B Japan Sea

Permo- Triassic Orogenic Units in Japan

Taishaku area

500 km I

Continental block

Permiam AC

High-PTT metamorphosed Permian AC (with Triassic age)

Tectonic outliei of Permo-Triassic units (Kurosegawa klippe)

Fig. 2 Distribution of the Permo-Triassic orogenic units in Japan (modified from lsozaki 1996) and localities of referred units and areas mentioned in the text. Ak, Akiyoshi belt, Mz, Maizuru belt; Sn, Sangun belt; HT, Hitachi-Takanuki belt; SK, Southern Kitakami belt; MTL, Median Tectonic Line; I-KTL, Ishigaki-Kuga Tectonic Line; TTL, Tanakura Tectonic Line.


southwestern extension of the Permo-Triassic accretionary orogen in Southwest Japan. There is no known occurrence of the collision-related Permo- Triassic unit to date either in the Outer Zone of Southwest Japan or in the Ryukyus.

The Permo-Triassic AC in the Inner Zone of Southwest Japan comprises three units distributed in three distinct belts; the Akiyoshi, Maizuru and Sangun belts (Fig. 2). The AC in the Sangun belt was metamorphosed a t high-P/T blueschist facies. As demonstrated schematically in Fig. 3, all three units occur as subhorizontal nappes, forming large- scale imbricated bodies. These units are sandwiched tectonically between the older and overlying 300-Ma blueschists of the Renge belt and the younger and underlying Jurassic AC of the Mino- Tanba belt, respectively (Hayasaka 1985; Isozaki & Itaya 1991; Kabashima et al. 1993). The basal boundary fault (called the Ishigaki-Kuga tectonic line) can be traced for -2000 km along Southwest Japan and the Ryukyus (Isozaki & Nishimura 1989).

The non-accretionary units occur in the Hida and

accretionary complexes) or olistostromal fragments incorporated in younger Jurassic-Cretaceous accretionary complexes. These units are not dealt with here because of their lesser tectonic significance to the present study.

Southwest Japan is subdivided into the Inner Zone, on the Japan Sea side, and Outer Zone, on the Pacific side, and the boundary is presently demarcated by a distinct fault called the Median Tectonic Line (MTL). As shown in Fig. 2, Permo- Triassic orogenic complexes have a clear double- belted distribution pattern; namely, one large area on the Japan Sea side of the Inner Zone and the other discontinuous narrow belt in the middle of the Outer Zone. The major components of the Permo-Triassic orogen in Southwest Japan are, on one hand, ancient accretionary complexes (AC) and their high-P/T metamorphosed equivalents that were generated by oceanic subduction from the Pacific side, and on the other hand, continent- continent collision-related orogenic units. In the Ryukyu Islands, high-P/T metamorphosed AC on Ishigaki Island, close to Taiwan, represent the

+- Pre-Jurassic complexes - of the Inner Zone

piGa] I Kuroseaawa I Tectonic Outlier I (Klipie, I

Nagato-Hida Marginal T . L .

I S N , I

Crel -Paleog Shimanto complexes

L A INNER ZONE - - - ourm ZONE

1 Mid-Cretaceous] Pre- Jurassic

Pre-Japan Sea complexes N voIc. (Aklyoshi Orogen) S

I Jurassic complex - f i -- I A S a k a w a Orogen)

- .a_--

high-PIT met

./-----:, '/A/ ' 1' / I==> ,'

/ oceanward orogenic growth

i v 9 oo2

P

Fig. 3 Fundamental structure of Southwest Japan (Isozaki & ltaya 1991) (a) Schematized geotectonic profile of Southwest Japan at present Ch, Chichibu belt, Ry, Ryoke belt (b) Schematized profile of Cretaceous Southwest Japan prior to the klippe formation, (c) Schematized map view of Cretaceous Southwest Japan

6 Y. Isoxaki

Oki belts in Southwest Japan, and these are composed of polymetamorphosed gneisdschist complexes with a signature of 230-Ma regional metamorphism of the medium-pressure type facies series (Komatsu 1990). In the Outer Zone, a narrow domain called the Kurosegawa belt (or tectonic zone, terrane etc.; Yoshikura et a,l. 1990; Isozaki et al. 1992) represents a tectonic outlier of the equiv- alent pre-Jurassic units in the Inner Zone and will be described later. Mesozoic cover sediments unconformably resting on these orogenic rocks are minor in volume but they provide excellent age constraints for the Permo-Triassic tectonics. In the following sections, the characteristics and mode of occurrence of these Permo-Triassic orogenic units are briefly described.

PERMO-TRIASSIC ACCRETIONARY COMPLEXES

Permo-Triassic AC in Southwest Japan consist of three subhorizontal nappe units; the weakly metamorphosed Permian AC of the Akiyoshi belt, the 220-Ma high-P/T metamorphosed AC of the San- gun belt, and the weakly metamorphosed Permian AC of the Maizuru belt, from top to bottom. The Akiyoshi AC and the Maizuru AC experienced regional metamorphism of lower greenschists facies or less, and yield fossils useful for dating and for assigning their accretion setting a t an ancient trench, through oceanic plate stratigraphy (OPS) analysis (Isozaki 1996). Each AC is nowhere unique. The Akiyoshi AC is characterized by kilometric-sized reef limestone blocks of Car- boniferous-Permian age. The Maizuru AC contains a dismembered ophiolite suite, known as the Yakuno Complex. The Sangun AC, on the other hand, was metamorphosed into blueschists with strong deformation but retain protolith assemblage suggesting an AC origin.

AKlYOSHl AC

The Permian Akiyoshi AC occurs as a subhorizontal veneer-like nappe of approximately 2000 m thick, occupying the highest structural horizon among the Permo-Triassic orogen, above the San- gun belt (Fig. 3). The Akiyoshi AC consists of rocks derived both from continental and oceanic crusts; that is, terrigenous clastics such as gray- wacke sandstone, mudstone, conglomerate, and acidic tuff, as well as bedded radiolarian and/or spicular chert, basaltic greenstones, and reef limestone. Most parts are occupied by chaotically mixed

units characterized by a block-in-matrix texture (Kanmera & Nishi 1983). Some parts are repre- sented by a series of tectonically imbricated thrust sheets composed mainly of coherent terrigenous clastics (Hara & Kiminami 1989).

One of the best examples of this AC occurs in the Akiyoshi area (Fig. 4), where large slabs or blocks of Carboniferous-mid-Permian limestone are enveloped in a matrix of coarse-grained elastic rocks of Late Permian age. The limestone blocks vary from pebble-sized pieces to the biggest block of -3 x 5 km. Field occurrence and fossil ages indicate that the older limestone blocks are allochthonous clasts secondarily incorporated into the argillaceous matrix of younger age.

Bedded cherts and limestone in the Akiyoshi AC are regarded as ancient pelagic sediments and shallow-water organic reef complexes developed

0 terrigenous clastics chert

a limestone (isolated small blocks)

limestone (large block) E v - J basaltic greenstones

7 limestone clastics

Y

I CI-7, P1-4: Carboniferous and Permian fossil zone

Fig. 4 Geologic map of the Akiyoshi area, Southwest Japan (a, simplified from Kanmera & Nishi 1983) and schematic cross-section of a large limestone body (b, modified from Sano & Kanmera 1991). The Permian Akiyoshi AC consists of the Late Permian terrigenous clastics with numerous allochthonous blocks of the Carboniferous-middle Permian limestones and cherts plus basaltic greenstones. (a) Note the scattered limestone blocks within the younger terrigenous clastics in the southwestern part of the area. (b) Appar- ently large and thick limestone bodies are, in fact, comprised of numerous smaller fragments that occurred in stratigraphically random order.


1979). Litho- and biofacies analyses of the limestones indicate a shallow-water reef environment for their origin (Ota 1968). In particular, a capping reef on top of a basaltic seamount that is isolated from continental influence is most likely because of the absence of coarse-grained terrigenous clastics. Lithology, texture and field occurrence indicate that these rocks represent remnants of a collapsed ancient mid-oceanic seamount complex (Kanmera & Nishi 1983; Sano & Kanmera 1991). Faunas from the Akiyoshi limestone are generally of Tethyan type, however, Early Carboniferous corals show a strong affinity with those found in Australia (Kato 1990), suggesting its potential proximity to Gondwana.

In contrast, bedded chert from this AC represents a sedimentary facies of deep-water origin (Kanmera & Nishi 1983; Uchiyama et al. 1986; Goto 1988). The age of the bedded chert almost completely overlaps with that of the reef limestone (Fig. 5), indicating that these two distinct oceanic lithologies are regarded as lateral equivalents in the same ocean basin. The limestone-greenstone complex represents rocks of an ancient seamount, while the cherts represent sediments accumulated in the deep sea below the carbonate compensation depth (CCD) around the seamount (Fig. 6). Older limestone clasts are sporadically contained within younger cherts, and in addition, allodapic limestone (calcareous turbidite) are interbedded with these

upon basaltic seamounts, respectively. A tectonic setting of a convergent plate boundary between a continent and an ocean, namely, a trench, is indi- cated by this lithologic assemblage and field occurrence (Kanmera & Nishi 1983; Kanmera et al. 1990). Using microfossils such as fusulinids, cono- donts and radiolarians, a detailed age determina- tion of the primary OPS has been reconstructed (Fig. 5). This stratigraphic reconstruction indicates that the Akiyoshi AC was formed a t ca 260 Ma by the subduction of -80-million-year-old oceanic plate (probably the Farallon Plate according to Engebretson e t al. 1985 and Maruyama & Sen0 1986) along a continental margin of Yangtze. The Akiyoshi AC is unconformably covered by Late Triassic and Early Jurassic shallow-water clastics.

COLLAPSE OF THE AKlYOSHl PALEOSEAMOUNT

Compared with other AC units in Japan, the Aki- yoshi AC is unique in having huge reef limestone blocks of mid-Carboniferous-Permian age that are associated with basaltic greenstones (Fig. 4). The fusulinid and rugose coral biostratigraphy indicates that the primary thickness of the Akiyoshi limestone was -700 m (Toriyania 1958; Hase et al. 1974). The base of the limestone (Visean) rests conformably upon pillowed alkali basalts (less than 100 m thick), which are distinct from typical mid- ocean ridge basalt (MORB) (Hase & Nishimura

260 Mi

Fig. 5 Oceanic plate stratigraphy (OPS) of the 295 M: Permian Akiyoshi accretionary complex ( A C ) demonstrated in three isolated areas (Yasuba- Shirakidani, Taishaku and Akiyoshi areas) in Southwest Japan (Isozaki 1987) h, OPS for seamount (topographic high), d, OPS for deep-sea floor, m, OPS for marginal flank of seamount Note the coeval development of three distinct facies as lateral equivalents

-aii columns not to scale

, d , ~ m , h

U I

, d , siliceous mudstone I s conglomerate

4 , a acidic tuff l imestone I ,

chert greenstones

\ I : # , ! , I

4 YASUBA~SHlRAKlDANl 3 TAISHAKU

: d !

8 Y. Isozaki

reef limestone + OIB into AC as

lensoids/blocks ' I I I ^._ I

deep-sea pelagic chert

?"w.&am

. . v y v v v v v v v v "'Bl..2

d v v v v v v v v v v v

' paleo-seamount v v v v v v v M O R B v v v v not to scale - deep-sea facies I u. marginal facies- lopo-high facres marginal facies - - -

Fig. 6 Topographic reconstruction of the paleo-seamount with a reef caD and deep-sea floor (modified from lsozaki 1987) Refer to Fig 5 for the OPS of three distinct facies

cherts (Hase e t al. 1974; Isozaki 1987). These indicate a lateral lithofacies change between shallow-water limestone and coeval deep-water chert, according to paleotopographic relief maps.

The chaotic occurrence in the field of these limestone-greenstone complexes within terrigenous clmtics (Fig. 4) indicates a collapse of the primary topographic relief of the seamount prior to the subduction (Sano & Kanmera 1991). Figure 7 illustrates a possible process for seamount collapse and successive accretion a t the trench. An exten- sional tectonic regime usually appears a t the hinge of the subducting plate just in front of a trench, triggering the gravitational collapse that is related to normal faulting. The accreted reef limestone in the Akiyoshi AC is a fault-bounded lensoid body in which ill-sorted, debris-flow-like megabreccia occurs. A modern analog for the collapse and accretion of oceanic seamounts can be observed in the present western Pacific, off Northeast Japan, where the Daiichi-Kashima and Erimo seamounts are currently entering the active trench (Cadet et nl. 1987).

THE MAIZURU AC: A PALEO-OCEANIC PLATEAU ACCRETION

The Permian Maizuru AC occupies the lowest structural level in the Permo-Triassic orogenic units (Hayasaka 1987). Fossils from this unit are not so abundant as those in the Akiyoshi AC, but OPS analysis indicates that the Maizuru AC was formed in the Late Permian, more or less the same time as the Akiyoshi AC. The main difference between the Maizuru and the Akiyoshi AC lies in lithologic assemblage: (i) the occurrence of 280-Ma ophiolitic rocks; (ii) the absence of chert; and (iii) the presence of minor amounts of limestone. The Maizuru AC in this article includes a unit some- times called the Ultra-Tanba belt (Ut in Fig. 3a; Caridroit e t al. 1982; Ishiga 1990).

The ophiolitic rocks in the Maizuru AC are -8 km thick, and are collectively called the Yakuno

2

3

4

' - 1

5

* .' m c o l l a p s e products m r e d e p o s l t c d collapse products

Fig. 7 A model for collapsing paleoseamount at trench (Sano & Kanmera 1991). Note the prE-accretion brecciation of limestone due to the gravitational collapse of the seamount at the trench.

Complex. They include basalts, gabbros, ultramafic rocks, and minor amounts of cover sediments and felsic intrusives (the Maizuru granite). These rocks


1990). Under the circumstances a multiple origin of the Yakuno complex was proposed that assumes a complicated paleogeographic setting including arc, hotspot and spreading ridge features in a small area just like the present southwest Pacific.

However, metamorphic petrology of the suite provides an important constraint on its origin. The pyroxenite member near the mafic/ultramafic boundary (regarded as an ancient Moho) in the suite reached the granulite facies, suggesting a high pressure between 5-10 kbar. This requires unusually thick (15-30 km) oceanic crust, which is nearly three to six times the average thickness of modern examples (Ishiwatari 1985). In addition, scarcity of capping carbonates indicates the crust surface was beneath the CCD at that time. Given the depth of the CCD of -3000 m deep, this suggests that the topographic relief from the deep- sea floor was less than 2 km. On the basis of these observations, presented here is another possible explanation for the origin of the Yakuno Complex. Oceanic plateaus, such as the Ontong-Java, are the biggest topographic reliefs in the oceanic domain except for the mid-oceanic ridge, although its internal structure has not yet been revealed in detail. Judging from their external configuration, however, an extraordinarily thick oceanic crust, that is, more than 15 km as estimated for the Yakuno Complex, can be expected for plateaus. Abnormally thick oceanic plateau crust appears favorable to compensate the extra-overburden of a huge relief of up to 2 km high. The Yakuno ophiolite complex probably represents a fragment of a paleo-oceanic plateau located on the Farallon Plate. The initiation of a plume is suggested as giving rise to an oceanic plateau (Maruyama unpubl. data).

On the other hand, the dismembered feature of the ophiolite was secondarily given through the accretion process of an oceanic plateau a t a subduction zone. Telescoping of thick oceanic crust into a dismembered ophiolite suite was not likely to be achieved by the commonly accepted ‘obduction’ process for ophiolite but probably by subduction- related underplating, because the structural horizon of the Yakuno Complex beneath the Akiyoshi AC is consistent with the downward-younging growth polarity in piled AC nappes in Southwest Japan (Isozaki 1996).

are sliced and chaotically mixed in part, not showing the typical Troodos-style ophiolite stratigraphy (Ishiwatari 1985; Fig. 8). In particular, the basaltic unit (-4 km thick) appears abnormally thick in comparison with other ophiolites. The igneous age of the basalt-gabbro suite is dated to be ca 280 Ma, while the age of the sedimentary cover is ca 260 Ma. Basalts of this suite have been analyzed in terms of petrography, bulk and trace element chemistry and isotopic composition. However, results cannot pinpoint any unique tectonic setting because petrochemical signatures suggesting affinity to a mid-oceanic ridge, arc, back-arc, and/or hotspot were obtained a t the same time from various parts of the ophiolite (Ishiwatari et al.

Ophiolite Lithology Metamorphism (km) Succession

+mudstone _r

l~ &

basalt

Y

lli 3 k. u u C

J

metabasall/ +

e, Qz-diorite

.-

arnphibolite

transition

rnetagabbro Y

paleo-Moho 3

prehnite-pumpellyite f.

(greenvchistf.)

epidote-amphiboliteJ

amphiholite f.

+---

granulite f .

Fig. 8 Reconstructed ophiolite stratigraphy of the Yakuno complex in the Permian Maizuru AC, Southwest Japan (simplified from Ishiwatari 1985) This complex represents a dismembered ophiolite, however the mafic/ultramafic transition horizon IS correlated with the paleo-Moho surface on the basis of the metamorphic aspects This suggests that the original thickness of the oceanic crust may have attained more than 15 km

220-MA HIGH-P/T METAMORPHOSED SANGUN AC

Occupying the middle structural horizon of the Permo-Triassic orogenic complex in the Inner

10 Y. Isoxaki

Zone, the Sangun AC occurs as a subhorizontal nappe beneath the Akiyoshi AC nappe and above the Maizuru AC nappe (Nishimura et al. 1989; Isozaki & Maruyama 1991; Fig. 3). Its along-arc extent is -1500 km in Southwest Japan and the Ryukyu Islands, and its thickness is less than 2 km. This unit is composed mostly of pelitic to mafic schists with minor amounts of siliceous and psammitic schists and rarely with metagabbro and marble. The radiometric ages of the Sangun meta-AC concentrate in the 230-200-Ma range, and its metamorphic peak is suggested to be ca 220 Ma as established by Rb-Sr whole-rock and K-Ar white-mica measurements (Nishimura 1990). The grade of the regional metamorphism reaches the greenschist to glaucophane schist facies, suggesting subduction-related high-P/T metamorphism in a Triassic subduction zone with a maximum pressure of -5-6 kbar. Due to its strong recrystalization, OPS analysis using microfossils is not available for this unit. Lithologic associations, however, indicate that this unit represents an AC formed a t an ancient trench prior to the high-P/T metamorphism, and thus the age of accretion must be older than 220 Ma. Nishimura et al. (1989) reported Middle-Late Permian (ca 260 Ma) microfossils from a chlorite-pumpellyite-bearing phyllite unit, which has a 220-Ma K-Ar age for metamorphic white mica. This unit is closely associated with the Sangun blueschists that include 225-240-Ma metagabbroic blocks. On the basis of these rela- tions, the protolith age of blueschists, namely the pre-metamorphic accretion age of the main high- PIT Sangun meta-AC a t the trench, is regarded as being ca 250-230 Ma.

The Sangun meta-AC is tectonically sandwiched between the over- and underlying nappes of non- high-P/T AC by sharp fault planes (Fig. 3). The pressure difference across the faults between the Sangun meta-AC and adjacent non-high-PIT AC suggest an -3-4-kb difference corresponding to an overburden of -9-12 km of crustal material. This relation imposes the following two constraints, that is, (i) crustal materials nearly 10 km thick must have been removed to juxtapose the Sangun meta- AC and the overlying Akiyoshi AC; and (ii) the Sangun meta-AC must have gone across the same crustal thickness to overlie the Maizuru AC. Devel- opment of a normal fault with displacement of - 10 km across-crust is therefore suggested along the upper surface of the Sangun meta-AC, while a reverse fault of the same order of magnitude along the lower surface. Accordingly, from the primary high-PIT metamorphic domain along the Benioff

zone a t -20 km deep to the near-surface domain, a thin nappe of the Sangun meta-AC was tectonically uplifted (or exhumated) by activation of the paired faults of opposite nature, as suggested by Maruyama (1990). As the nappe of the Sangun meta-AC occurs extensively along arc for 1500 km, the mechanism of the exhumation was not of a local extent but regional, as well as the high-P/T metamorphism per se.

The timing of the final surface exposure is poorly constrained. However, because the preservation of blueschists requires a quick exhumation from the deep metamorphic domain to avoid thermal anneal- ing, the emplacement of the Sangun meta-AC into a shallower crustal level must have occurred immediately after the peak metamorphism (220 Ma), thus probably in the Late Triassic-Early Jurassic. The oldest evidence for their surface erosion is the presence of 200-Ma schist clasts in the Late Cre- taceous (87-83 Ma) intra-arc sediments, marking the youngest age limit for the exhumation (Isozaki & Itaya 1989).

On the other hand, the emplacement of the Sangun schist nappe over the weakly metamorphosed Jurassic AC (160 Ma) by thrusting constrains the exhumation to shallow crustal levels and to having occurred no earlier than 160 Ma. In addition, the regional intrusion of 85-Ma I-type granitoids into the Sangun nappe also roughly constrains the timing of emplacement into shallow crustal levels. A tectono-metamorphic history for the Sangun meta-AC is summarized in Fig. 9, showing a possible path in burial depth versus time for the Sangun meta-AC unit. This trajectory is unique to the Sangun unit, and quite distinct from those of the older 400-300-Ma Renge high-P/T unit and the younger 100-Ma Sanbagawa high-P/T unit in Southwest Japan (Isozaki 1996).

KUROSEGAWA KLIPPE: A TECTONIC OUTLIER OF PRE-JURASSIC COMPLEXES

The Kurosegawa belt in the Outer Zone of South- west Japan has long been a key geotectonic unit for understanding the orogenic structure of the Japanese Islands. This belt features a great variety of rock types including the Permian AC, the 220-Ma high-P/T schists, fragments of 400-Ma granite-gneisses, the 400-300-Ma high-PIT schists, a serpentinite, plus weakly to non-metamorphosed Siluro-Devonian and Mesozoic sediments of continental shelf origin (Maruyama et al. 1984; Yoshikura et al. 1990; Isozaki et al. 1992). These


’ Age carbon! Permian Triassic Jurassic Cretaceous (Ma) ’ 3y0 250 200 150 100

I I I I I I ~ I I I I , , ,

20 -

25 -

(km)

Tectonic

various components iusually occur as lensoid bodies, and range in size from kilometers to meters. They are randomly distribluted and commonly separated from each other by serpentinite. Given these features, the belt has been often described as a serpentinite mklange zone. In contrast to such variety in rock types, the distribution is highly restricted to an extremely narrow zone (Fig. 2), usually less than 3 km in width, and the belt is discontinuous along its length.

As the Outer Zone of Southwest Japan is domi- nated by Jurasso-Cretaceous to Tertiary AC and their metamorphic equivalents, the pre-Jurassic orogenic complexes of the Kurosegawa belt appear as a unique area in the Outer Zone. These pre- Jurassic units are isolated from the main distribution of the those in the Inner Zone (Fig. 2). Various plate tectonic models and interpretations for the origin of the Kurosegawa belt have been proposed; for example, a remnant of an ancient Benioff zone, a rifted continental margin, a collided microcontinent or arc, a strike-slip fault zone etc. (Maruyama et al. 1984; Taira & Tashiro 1987). None of them, however, can explain all aspects of this belt. A klippe model was recently proposed for the Kuro- segawa belt, in which the belt is regarded as a tectonic outlier of the pre-Jurassic rocks in the Inner Zone (Isozaki & Itaya 1991; Fig. 3) on the basis of following lines of evidence.

First, almost all of these sedimentary, igneous, and metamorphic components of the Kurosegawa Belt can be identified and correlated with those in the pre-Jurassic be1t.s of the Inner Zone in terms of

sssz peak metamorphism

(220 Ma) max. pressure: 5-6 !d~

I I I I surface of arc crust

ocean floor trench 1 Benioff zone I fore-arc prism/crust I (open ocean) (near continent) I I i

I I

lithology, age, and even faunal provinciality. In addition, the timing of regional tectonics including high-P/T metamorphism, formation of an accretionary complex and major unconformity-forming tectono-sedimentation are identical strictly within the Inner Zone/Kurosegawa belt and nowhere else in East Asia. Correlative components and events of pre-Jurassic belts in the Inner Zone and the Kurosegawa belt are listed in Table 1. Age and lithology of components, tectono-sedimentary events, and faunal provinciality all support the consanguinity of the pre-Jurassic orogenic units presently distributed in two separated zones in Southwest Japan. Concerning the pre-Cretaceous tectonics in Southwest Japan, the previously accepted distinction of the Inner and Outer zones is unnecessary.

Second, all these pre-Jurassic rocks overlie the Jurassic AC, both in the Inner and Outer Zones. A remarkable example of a klippe of the Kurosegawa rocks was first found in central Shikoku (Isozaki & Itaya 1991; Fig. 10). In this area, 230-Ma high- PIT schists sit tectonically upon the Jurassic AC, separated by a subhorizontal fault. The size of this klippe is 5 km east-west, 3 km north-south and 0.5 km in thickness. In addition, there are several supplementary examples to support the klippe- style occurrence of the Kurosegawa rocks else- where in Southwest Japan (Suzuki et al. 1990; Isozaki et al. 1992; Suzuki & Itaya 1994).

Occupying the same structural horizon and shar- ing common geologic history, the pre-Jurassic rocks of the Inner Zone and the Kurosegawa Belt

12 Y. Isoxaki

Table 1 Comparison of components between the Kurosegawa Belt of the Outer Zone and pre-Jurassic belts in the Inner Zone, Southwest Japan (Isozaki & Itaya 1991)

Compared items

Rocks and strata 400-Ma granltes Siluro-Devonian Ultramafic rocks

Paleozoic calc-alkali volcanic rocks

Schists 280-350 Ma

170-230 Ma

End-Permian AC

Faunal endemism L. kumaeiisis As. (fusulinid)

l? bipartitus-F. charveti As. (radiolaria)

Tectonic events PermiadTriassic unconformity

(Akiyoshi orogeny)

Pre-Jurassics of the Inner Zone

Dai orthogneiess (Ng) Yoshiki and Fukuji F. (Hm) Oeyama and Yakuno ophiolties (Mz)

Higashihirano F. (Ng) Sorayama F. (Hm) Nishiki G.

Ise schists (Hm) Renge schists ‘Sangun’ schists Nishiki G. (Ak)

Perman formations in the Akiyoshi and Taishaku areas

Maizuru G. (Mz)

Kozuki and Oi F. (Ut)

Mine unconformity (Tsunemori/Mine)

Kurosegawa components ~~~~

Mitaki granites Okanaro G. Serpentinite at Ino Gokasho-Arashima T. L. Okanaro G. Shingal F.

Ino F. Sawadani schists Ino F. Agekura F., Agawa Unit, Kamikatsu phyllites Shingai F. ‘Shirakidani G.’ Sawadani G.

Kuma F. Shingai F. Kuma F., Shingai F. Takano F.

Sakashu unconformity (Hisone/Kochigatani)

AC, accretionary complex; Ng, Nagato tectonic zone along the Oki belt; Hm, marginal part of the Hida belt.

I I

Fig. 10 Example of the Kurosegawa klippe in the Nakatsu area, Southwest Japan (Isozaki & ltaya 1991) A component of the Kurosegawa belt (180-230-Ma semi-schists unit) occurs as a kl ippe tectonically resting on the Jurassic accretionary complex (AC) unit (nappe) of the Chichibu belt

are regarded as having once formed different parts unit. The erosion level of the nappe of the pre- of the same nappe, although they are a t present Jurassic rocks is not deep enough in northern isolated by 100-150 km in a north-south direction Kyushu to expose the underlying Jurassic nappe in central Japan. In this model, the Jurassic AC in nor to form an isolated klippe/window (Fig. 11) so the Mino-Tanba belt and Chichibu belt, on both that we can observe the merging of the two belts sides of the Kurosegawa belt (Fig. 3a), are accord- into one with the greatest width of the Pre-Jurassic ingly regarded as a structurally underlying nappe rocks in Kyushu (Fig. 2). The long-term exposure


essary, because the Zone-bounding Median Tec- tonic Line (MTL) initially became activated after the Late Cretaceous (Isozaki 1996). urosegawa belt

~ y u ~ h '

Sh,kOku

Fig. 11 Model showing 3-D configuration of the Kurosegawa klippe and the relation between the erosion levi:l and the surface trajectory of the base of the klippe (Isozaki et a/ 1992) Compare the widest distribution area of the Kurosegawa belt in Kyushu by shallow erosion and its absence in !he central Kii Peninsula by deep erosion

of the pre-Jurassic nappe a t the surface (occupying the top horizon of piled nappe) is supported by the occurrence of non-metamorphosed Mesozoic shallow-water clastics that unconformably cover the pre-Jurassic rocks both in the Inner Zone and the Kurosegawa belt. The chaotically mixed nature of the Kurosegawa. belt may have developed through the gravitational collapse of the nappe along an advancing nappe front.

The biggest advantage of the current klippe model lies in the following aspects. This model allows us to reconstruct paleogeography with only one subduction zone (probably along the Yangtze continental margin), while previous models, in particular collision models, require more than two subduction zones (i.e. a t least one orogenic complex for the Inner Zone and another identical orogenic complex for the Outer Zone a t the same time). I t is unlikely to assume that two or more subduction systems generated identical AC and high-PIT metamorphosed AC simultaneously in the same region. Furthermore, there is no reason to assume a hypothetical island arc nor microcontinent, because sialic rocks in the Kurosegawa belt are present in quite minor amounts. Judging from the OPS analysis, the Permian AC and 220-Ma high-PIT schists are products of the subduction of a major oceanic plate of -100 million years old (probably the Farallon Plate). Short-lived marginal basins cannot produce such an OPS. The recognition of the Kurosegawa klippe has brought about a new model of subhorizontally layered nappes for the whole Late Paleozoic-Mesozoic orogenic history of Japan. Accordingly, the distinction of the Inner and Outer Zones of Southwest Japan in pre-Cretaceous tectonics was revealed to be unnec-

PERMO-TRIASSIC COLLISION COMPLEXES IN JAPAN

In contrast to the abovementioned AC units, development of the medium-pressure-type gne idschis t units represents a quite distinct facet in the tectonic history of Permo-Triassic proto-Japan. These units are regarded as a product of the collision between the Yangtze and Sino-Korean cratons, instead of the AC. The metamorphic grades of these units reached the amphibolite to granulite facies, as evidenced by kyanite, staurolite, and sillimanite. Such a metamorphic aspect is rare in other units. The protoliths include terrigenous clastic sediments, mafic to acidic volcanic rocks of an alkaline nature, and bedded impure carbonates, lacking bedded chert. Such a lithologic assemblage is unique and remarkably different to those of the AC units in Japan. In order to emphasize the contrast with the AC units, I briefly describe the Permo-Triassic orogenic units of the Hida and Oki Belts in Southwest Japan.

HlDA BELT

Facing the Japan Sea to the north, the Hida belt (Fig. 2) occurs in the most continent-sided position in Japan. The rocks of the Hida belt occupy as a nappe the highest structural horizon in Southwest Japan (Komatsu 1990; Fig. 12), which thrust ob- liquely upon the Permian and Jurassic AC around the end of the Jurassic (Sohma & Kunugiza 1993). The main components of the Hida belt are high- grade gneisses associated with granitic rocks. In addition, fragments of Paleozoic shelf sediments, Paleozoic high-PIT schists, Permian AC and serpentinite occur along its eastern and southern margin. Polymetamorphism is detected in terms of petrography and a trident clustering in radiometric ages a t 330, 240 and 170 Ma (Sohma et al. 1990; Suzuki & Adachi 1991). The 240-Ma event corresponds to the timing of amphibolite facies metamorphism and accompanied migmatite formation. In addition to the regional development of a medium-pressure-type schist belt (Hiroi 1983), localized occurrences of kyanite, staurolite and sillimanite grains were recently reported from river sands exclusively in the central gneiss domain (Kano et al. 1993). This medium-pressure-type metamorphism is regarded as a consequence of a

14 Y. Isoxaki

230 Ma medium pressure-type metamorphic rocks

a 200-170 Ma granile

Fig. 12 Geologic framework of the Hida belt in Southwest Japan. (a) Geotectonic sketch map of central Japan showing the overlapping relationship of the Hida belt (nappe) to the other belts of Southwest Japan (modified from Komatsu 1990). Ak, Akiyoshi belt; Sn, Sangun belt; Mz, Maizuru belt; MT, Mino-Tanba belt; Ry, Ryoke belt. Note a linear magnetic anomaly zone (hatched area) beneath the Hida belt suggesting an easterly extension of the Maizuru belt. (b) Geologic map of the Hida belt (modified from Sohma 8, Kunugiza 1993) Note the marginal thrust zone composed of fragmentary bodies of geotectonic units in Southwest Japan including fragments of the Oeyama belt (Oe), Renge belt (Rn), Akiyoshi belt (Ak), and Maizuru belt (Mz)

continent-continent collision between the Sino- Korean and Yangtze blocks (Sohma et al. 1990; Sohma & Kunugiza 1993).

The gneiss protoliths contain thick impure carbonates intercalated with mafic to acidic volcanic rocks of an alkali rock series and subordinate peraluminous argillaceous sedimentary rocks yield- ing chloritoid (Sohma et al. 1990). Late Carbonif- erous fossils (foraminifera and bryozoa) were reported from less-metamorphosed marble (Hiroi et al. 1978). The absence of chert, the occurrence of peraluminous pelitic rocks, and the coexistence of bimodal volcanic rocks suggest that the protoliths were likely to be derived from the Middle-Late

Paleozoic continental shelf-platform sediments accumulated on a rifted continental margin (Sohma et aL. 1990). The Permian brachiopod fauna from the marginal part of the Hida belt contain mostly cold-water (Boreal) elements (Nakamura & Tazawa 1990), probably indicating its northerly position with respect to other Tethyan fauna from the low-latitude mid-Tethyan realm.

The Hitachi-Takanuki belt (Fig. 2) is the only known area of kyanite-bearing 250-Ma metamorphic rocks in Northeast Japan (Tagiri 1973; Tagiri et aL. 1992). Its protolith assemblage, fossil ages and U-Pb zircon age (200-280 Ma; Hiroi et al. 1994) all suggest that this unit probably represents not only an eastern extension of the Hida Belt in Japan but also a fragment of the 250-Ma collision complex in central China and Korea.

OK1 BELT

Isolated physiographically and geologically from the Hida Belt, the Oki Belt exposes another non-AC unit in Southwest Japan (Fig. 2). The distribution of component rocks is limited to the Oki Islands in the southern Japan Sea where a high-grade gneiss complex occurs (Hoshino 1979). The complex consists mostly of psammitic and pelitic gneisses with minor amounts of amphibolite and marble. Regarding protolith composition, this lithologic assemblage differs considerably from that of the Hida Belt. Metamorphism reached the amphibolite facies and went partly into the granulite facies. The age of this metamorphic event is dated a t 250 Ma (Suzuki & Adachi 1994; Fig. 13). In addition, inherited zircon grains with ages of 3.0, 2.0, 1.7, and 1.25 Ga and 350 Ma have been identified, indicating that protoliths formed adjacent to an Archean-Proterozoic continental block.

Like the Hida gneiss, the lithologic assemblage of the Oki gneiss suggests the protoliths were derived from a rifted continental margin, but the latter lacks an extensive carbonate platform. Al- though the Hida and Oki Belts share the same metamorphic age of 250 Ma, differences in protolith lithology and faunal provinciality suggest that these two belts may have been derived from different continental margins, and that these two margins were primarily separated from each other.

The Southern Kitakami belt in Northeast Japan (Fig. 2) is regarded as an eastern extension of the Yangtze block, on the basis of stratigraphy of Middle-Late Paleozoic continental shelf facies sediments with an affinity to that of the Yangtze platform rather than Sino-Korea (Isozaki &


0.2-

0 . 1 -

- -0

" ' 0 1

M01 from sample 1404A M71 from sample 0304 250+_20 Ma 250-120 Ma

MSWD=O.IO MSWD=0.26

1 , I I , , I

Maruyama 1991). The previously accepted correlation between the Southern Kitakami belt and the Sino-Korean block is refuted by the total absence of Siluro-Devonian strata in the latter. The faunal analysis on Permian bivalves positively supports this correlation (Nakazawa 1991), on the other hand, those of Permian corals or brachiopods (Kato 1990; Nakamura & Tazawa 1990) appear equivo- cal. Judging from geological and paleontological similarities, the Oki belt is correlated not only to the Yangtze block but also to the Southern Kitakami belt (Isozalii & Maruyama 1991).

e 0.2- M30 from sample 1404C 0 .2 - 25OSO Ma

MSWD=0.44

0 1 - 0.1- Fig. 13 Th-U-total Pb isochron of monazite grains from the Oki gneiss (Suzuki & Adachi 1994) (a-c) lsochrons for a single monazite grain; (d) isochron for 35 monazite grains from the same rock sample. Note the 0 1 I I I I I I I Ot

250-MA GRANITE

35 grains from sample 1502 250i10 Ma

MSWD=0.54

I , I I , I I I I

There are several scattered occurrences of 250-Ma granites in Japan, although minor in volume: they are usually less than a kilometer in diameter. Besides the Maizuru granite contained in the Yakuno ophiolite, most of them occur along the MTL on its Outer Zone side, forming a nappel klippe upon the younger Jurassic-Cretaceous AC and/or their metarnorphosed equivalents (On0 1983; Takagi & Fujiimori 1989). Their field occurrence suggests subhorizontal transport through nappe tectonics from the Inner Zone, however, the root zone of these granite nappes has not been identified. From the viewpoint of regional tectonics, there are two alternatives for the origin of the 250-Ma granite: one related to subduction as being responsible for forming the Akiyoshi-Maizuru AC

and Sangun meta-AC, and the other related to the collision between the Yangtze and Sino-Korean cratons. No critical evidence has yet been obtained, however, the latter option appears likely a t present because 250-Ma granites are found in the Hida and Southern Kitakami belts (Suzuki et al. 1992), in particular, associated with high-grade gneisses in the Hida belt, while no large granite batholith of ca 250 Ma has been found in Japan.

DISCUSSION

On the basis of the above descriptions, this section summarizes significant tectonic aspects of Permo- Triassic Japan. First, tectonic implications of subduction-accretion of oceanic topographic highs a t trenches and the subhorizontal nature of accretion-generated orogens are discussed. Then the unique tectonic setting featuring two 250-Ma orogens of distinct characteristics are discussed with respect to the paleogeographic reconstruction and faunal provinciality of western Panthalassa a t that time.

ACCRETING EFFICIENCY OF SEAMOUNTS AND OCEANIC PLATEAUS

Compared with the Jurassic and Cretaceous AC in Japan (Isozaki 1997; Kimura 1997), the Permian AC in Southwest Japan are unique in having

16 Y. Isoxaki

abundant oceanic materials; that is, igneous and sedimentary rocks derived primarily from oceanic crust such as basaltic/gabbroic greenstones, reef limestone, ophiolitic complexes including ultramafic rocks, and deep-sea chert. In particular, the predominance of large slabs/blocks of reef limestone and ophiolitic rocks is remarkable when compared with other ancient AC exposed in Japan. Despite the considerable variation in mutual volume ratio, oceanic materials usually do not exceed 30% volume of all components of an AC (Isozaki et al. 1990). The Permian AC in Southwest Japan as a whole also follow this empirical rule but the localized occurrence of considerably huge oceanic blocks appears exceptional; for example, the Akiyoshi reef limestone is 10 x 10 km wide and 1 km thick, and the Yakuno ophiolite is 100 km long and 8 km thick.

Lithostratigraphic and petro-chemical analyses suggest that such blocks are allochthonous fragments of paleoseamounts and paleo-oceanic plateaus. Given the average bathymetry of the modern ocean, the height of the reef-capped Akiyoshi paleoseamount is estimated a t 4 km, even allowing for reef-drowning after the over 700 m-thick limestone accumulation under photosynthetic shallow water (Fig. 14) . The scarcity of basaltic green-

Trench A”;” ,!>.,- --

stones underlying reef limestone indicates that a basaltic seamount pedestal more than 3 km thick per se has vanished to the subduction zone without accretion, just leaving its reef cap in the trench.

Concerning the Maizuru paleo-oceanic plateau, the original thickness of total crustal materials is estimated to be a t least 15 km. On the other hand, the on-land exposed Yakuno ophiolite is -8 km thick, indicating that it is highly dismembered or telescoped. This mismatch in thickness suggests that more than 7 km of crustal materials have been separated from the original oceanic crust. These examples of the Permian AC support the sugges- tion that large oceanic reliefs (seamount, rise, or plateau) can subduct a t a trench, leaving thin fragments in AC that are thinner and smaller than the primary thickness and volume (Isozaki et al. 1990). What appears contradictory is the scarcity of accretion of basaltic greenstones from a highly elevated seamount while there is abundant accretion from a rather low-lying oceanic plateau. This contrast may have been due to the difference in total volume of topographic relief that entered the ancient trench, or to differences in rigidity because the oceanic plateau is larger in volume than isolated seamounts in general (Fig. 14).

Mid-Oceanic Ridge

201 II II

Fig. 14 Scheniatic diagram showing a crustal section of an ancient oceanic plate reconstructed on the basis of the stratigraphy and metamorphic constraints from the Maizuru ophiolite and Akiyoshi limestone within the Permian accretionary complex (AC) in Southwest Japan Note that this reconstruction is highly speculative and needs further testing with information from modern examples when it is available Prominent oceanic topographic-highs show the Akiyoshi paleoseamount with an over-700 m-thick reef carbonates above the carbonate compensation depth (CCD) and the Maizuru paleo-oceanic plateau with an over-1 5 km-thick crust below the CCD


On the other hand, collision-subduction of a seamount is regarded as the main mechanism of subduction erosion by the destruction of previously accreted materials in the trench inner wall (von Huene & Scholle 1991). Accordingly, the total volume of the originally formed Permian AC was probably much greater than what we see today, and a considerable amount of AC material was subducted along with collided seamounts. The chaotically mixed units in the Permian AC probably represent re-accreted AC material after the primary accretion and collision-induced collapse, and their predominance a.lso supports the tectonic erosion event.

The abovementioned abundance of material derived from paleoseainounts and plateaus implies that the subducting oceanic plate may have carried numerous topographic highs on it. Furthermore, it indirectly suggests that an ancient plume (or su- perplume) somewhere in the Carboniferous-Early Permian Pacific/Panl,halassa Ocean produced numerous topographic highs, like the one in the Cretaceous mid-Pacific. This can be tested in coeval AC along the eastern Panthalassic margin by future work.

SUBHORIZONTAL STRUClURE OF ACCRETIONARY OROGEN

Another important contribution from the latest studies on the Permo-Triassic AC orogen in South- west Japan is the documenting of a predominant piled nappe structure, including isolated klippes and windows. The Jurassic AC also contributed to the piled nappe structure, and these suggest that the subhorizontal structure governs the entire Paleozoic-Mesozoic orogens of Southwest and Northeast Japan and Ryukyus (Isozaki 1996, 1997). With respect to the piled nappe structure, the downward-younging growth polarity is observed particularly in Southwest Japan (Fig. 3). The uni-directional-younging polarity implies that ancient AC have accumulated downward, keeping essentially the same growth direction in accordance with the initial accretion polarity a t the trench on a smaller scale. Occurrence of similar AC have been reported sporadically from various areas in the world but there has been no better documented example of such a subhorizontal structure in an AC orogen as in Japan, which stretches more than 2000 km in length and 200 km in width.

Concerning the emplacement mechanism of each AC nappe into the piled nappe edifice, active movement of the lowermost nappe is emphasized here. With respect to the continent, it is not an overlying

nappe but an underlying nappe that moves horizon- tally to make piled nappes because nappe-forming layer-parallel shortening is driven by oceanic subduction. This suggests that the pre-Jurassic complex stayed mostly in the same position relative to the continent, and without long-distance transpor- tation, they turned into an ‘autochthonous’ nappe when the younger Jurassic AC nappe was em- placed underneath it. In other words, the Kuroseg- awa klippe (or nappe) per se did not travel for more than 100 km across-arc, all the way from the present Inner Zone of Southwest Japan.

Subhorizontal piled nappe structures have been previously emphasized for continent-continent collision-related orogens such as the Himalayas and Alps, as well as foreland fold and thrust belt such as the Canadian Rockies. On the other hand, concerning the subduction-related orogens classi- fied as Cordilleran-type by Dewey & Bird (1970), vertical structures have been emphasized in the central or aged part of an orogen. In the 1980s, the ‘exotic terrane’ concept also over-emphasized vertical faults which bounded terranes by second- ary strike-slip movement. Analyses of other AC- dominant orogens will test whether or not a subhorizontal nature is essential to oceanic subduction-related orogens in general.

The intrusion of 85-Ma I-type granitoids into the 220-Ma high-P/T Sangun nappe provides a refer- ence for estimating the rate of oceanward growth of the subhorizontal AC-dominant orogen in Japan. The granite penetrating the 220-Ma schists represents a part of a large Late Cretaceous granitic belt called the Ryoke belt in Southwest Japan, which is regarded as the basement of a Late Cretaceous volcanic arc. In general, the volcanic arc occurs -100-200 km inland from the coeval trench. As the oceanward margin of the 220-Ma Sangun nappe approximately represents a paleoposition of the 220-Ma trench, the oceanward shift of the trench position within the AC-dominant orogen is suggested to be 100-200 km during an approximately 135-million-year (220-85 Ma) inter- val, which appears consistent with the average horizontal growth rate of the Japan orogen, that is, 400 km in 400 million years (Isozaki 1996).

COLLISION SUTURE AND LATERAL VARIATION

The occurrence of an ultrahigh-pressure metamorphic belt in East Asia is restricted to the Dabie- Sulu zone (Cong & Wang 1995), particularly to the central bending part of this collision suture between the Yangtze and Sino-Korean blocks

18 Y. Isoxaki

(Fig. 15). The metamorphic grade of the collision- related units apparently decreases westward, and probably eastward judging from the absence of an ultrahigh-pressure unit to the east of the Shangdon Peninsula, Northeast China. The 250-Ma medium- pressure-type metamorphic units in the Oki and Hida belts in Southwest Japan are the best candi- date for the easterly extension of the suture, as the rest of Southwest Japan is occupied by non- collisional orogenic units. The Hitachi-Takanuki belt and Southern Kitakami belt in Northeast Japan can be viewed as the probable easternmost tract of the suture. Restoring their strike-slip displacement induced by the Miocene Japan Sea opening (Otofuji 1996), the pre-Miocene paleoposition of these fragments is expected to be a t a higher latitude than their present position.

The decrease in metamorphic grade toward both directions along the suture may demonstrate that a considerable lateral variation on a scale of more than 1000 km existed within the same collisional orogen. Major compressional stress by collision probably had been localized in the central portion

;ion orogen

subduction zone

terrigenous cIastics

/

Fig. 15 Triassic-Jurassic Japan The Qinling-Dabie zone in China represents the collision suture between the Sino-Korean (North China) and Yangtze (South China) continental blocks (modified from lsozaki & Maruyama 1991 ) Note the concentrated occurrence of coesite/diamond-bearing ultrahigh- pressure metamorphic rocks (shown by open squares after Cong & Wang 1995) exclusively in the bending part of the suture in the Dabie-Shandon areas See text for information regarding autocannibalism of the Yangtze Block related to the unique double-edged orogenic setting

enough to bring buoyant continental materials down to -100 km deep, while other parts suffered less stress and metamorphism. This localization of maximum stress may have been driven by the collision of two continental margins with heteroge- neous geometry. Continental collision usually starts from the promontory-promontory collision stage as perceived earlier by Dewey & Burke (1974). Ac- cordingly, the ultrahigh-pressure part within a collision suture may indicate the ancient site of promontory-promontory (or promontory on one side) collision (Fig. 16). Concerning the detailed exhumation history of this ultrahigh-pressure unit, Maruyama et al. (1994) proposed a model including subduction of microcontinent and subsequent delamination of a subducted slab. Their microcontinent can be virtually replaced by the detached continental fragment of a collided promontory.

However, lesser metamorphic grades in lateral extensions probably suggest that the compressional stress was a t a maximum around the indentor (promontory), while less stress was experienced on both sides of the indentor, a t least not enough stress to bring buoyant protoliths down to such deep levels. Concerning this along-suture metamorphic variation, another explanation is possible; namely, that collision-induced escape tectonics may have occurred to extrude excess materials laterally to free spaces of oceanic domain and to disrupt primary metamorphic zoning (Fig. 16b), just like the case of the Indochina block induced by the Indian collision against Asia (Tapponnier et al. 1982). The 230-Ma right-lateral shearing detected in the Korean Peninsula (Yanai et al. 1985; Cluzel et al. 1991) and in the Hida belt (Nagase et al. 1990; Takagi & Hara 1994) may represent a part of a conjugate fault system related to escape tectonics in the eastern marginal part of the collision suture. The present nappe occurrence of the Hida Belt (Komatsu 1990; Fig. 12) and its formation process is not discussed here because it is a result of the endJurassic across-arc contraction (Sohma & Kunugiza 1993) which occurred long after the 250-Ma collision event.

The correlation between the two cratons in China and the geotectonic units in Japan presented here is in remarkable contrast to previous theories. The HidaISouthern Kitakami belts have been commonly correlated to the Sino-Korean block simply because they show a physiological proximity a t present (Kobayashi 1941; Minato et al. 1965; Taira & Tashiro 1987). The latest tectonic evalua- tion of the suture between the twa-cratons and the search for its easterly extension in Japan, however,


tinct types side by side; the Akiyoshi-Sangun- Maizuru accretionary orogen and the Hida-Oki collisional orogen, and both had extensive dimen- sions with lengths of some 1000 km. From the viewpoint of the regional tectonics of Asia, what appears more important is the collision-related orogenic belt between the Yangtze and Sino- Korean continental blocks because these two form the core of present East Asia, which grew much bigger through the collision/amalgamation with Siberia, Bureya (Amuria) and other continental pieces originally detached from Rodinia a t 750- 700 Ma.

The AC-related orogen facing the proto-Pacific, however, has nearly the same weight in geotectonic significance as well, because this involved the subduction of a major oceanic plate in the proto- Pacific, probably the Farallon Plate of some thousands of kilometers wide. This implies that the subduction of a major plate continued actively along the western margin of Panthalassa, a t least along the Yangtze margin, even a t the time of the Pangea assembly when most of the subduction zones between continents were terminated by collision. In short, the Yangtze craton was double- edged in the Permo-Triassic, featuring two active fronts of completely different natures a t the same time (Fig. 16b).

Such a double-edged situation brought another unique geologic setting around Japan in the following Jurassic time. The collision-related exhumation of ultrahigh-pressure rocks probably triggered a regional upheaval along the suture and it resulted in the provision of abundant terrigenous clastics along the suture, away from the center to the marginal delta. The collision-derived clastic materials are delivered eastward along the suture to build a delta (Fig. 15). By transport further along the trench axis, they consequently returned to the opposite side (southeastern margin) of the Yangtze Block and again accreted to the continent (Isozaki & Maruyama 1991; Isozaki 1997). Thus semi- autocannibalistic-material recycling occurred around the Yangtze Block.

The paleogeographic reconstruction of Permo- Triassic proto-Japan and its vicinity (Fig. 16) can be tested by the faunal provinciality of Permo- Triassic taxa such as bivalves, fusulinids and brachiopods. In general, distinction of faunal provinces is apparent during the Permian (Ross & Ross 1983; Ishii 1990), while it is ambiguous in the Triassic due to its monotonous nature. According to Naka- zawa (1991), Permian faunas from clastic cover sequences in the Maizuru and Kurosegawa Belts

Dabie Promontory

Q S Kitakami A l l

1 Akiyoshi A paleo-seamounts -

Panrhalassa

Farallon Plate

Maizuru 0 paieo-oceanic plateau .%... ,?$$

Farallon Plate

Izanagf Plate

Fig. 16 Paleogeographic reconstruction of proto-Japan and its vicinity (a) Middle Permian ca 270 M a klote the paleopositions of the Hida belt on the Sino-Korean margin, Oki and Southern Kitakami belts on the Yangtze margin and those of the Akiyoshi paleoseamount and Maizuru paleo-oceanic plateau in Panthalassa (b) Early-Middle Triassic, 250-230 M a Note the double- edged orogenic nature of the Triassic Yangtze craton featuring a continental collision against the Sino-Korean block on the north and subduction by the proto-Pacific (Farallon) oceanic plate on the south The sinuous part of the suture in the Dabie-Sulu area I:, characterized by an ultrahigh-pressure metamorphic belt probably reflectinij the site of a promontory collision between the two continental blocks Dextral strike-slip deformation in the Hida belt and in the Korean Peninsula suggests eastward escape tectonics driven by the collision

suggested a strong link between the Yangtze block and most of the Japanese units.

JUXTAPOSITION OF CONTRASTING OROGENS AND PALEOGEOGRAPHY

In the Permo-Triassic of proto-Japan and its sur- roundings, there were two active orogens of dis-

20 Y. Isoxaki

show a strong affinity with the warm-water Yangtze fauna rather than those from the Sino- Korean Block, while the Hida and Southern Kita- kami faunas partly accompany Boreal representa- tives (Nakamura & Tazawa 1990; Tazawa 1992). These observations, including the monotonous aspect in Triassic fauna, appear concordant with the present paleogeographic reconstruction. The incor- poration of some Boreal elements in the Hida and Southern Kitakami fauna may indicate a stronger influence of cold currents along the northwestern Panthalassa according to their relatively northerly paleoposition to the rest of the Southwest Japan fauna. The Hida, Oki, and Southern Kitakami belts have previously been correlated with the Sino- Korean block (Kobayashi 1941; Minato e t al . 1965; Tazawa 1992), however, this correlation faces dif- ficulty in identifying the suture between the Sino- Korea and Yangtze blocks in and around Japan unless an arc-parallel strike-slip displacement (of an unrealistically large scale) is assumed.

On the other hand, the Akiyoshi reef limestone is characterized by a peculiar faunal succession, distinct from those from the Hida and Southern Kitakami belt. The Early Carboniferous fauna from the Akiyoshi limestone is similar to Australian fauna in the southern hemisphere (Kato 1990). The OPS analysis on the Akiyoshi AC suggests that the Carboniferous paleoposition of the Akiyoshi paleoseamount with reef was some thousands of kilometers away from the Yangtze coast, that is, somewhere in the open ocean within western Panthalassa. The Tethyan aspects of the Akiyoshi limestone, particularly in the Permian part, were secondarily overprinted when the paleoseamount entered the paleo-equatorial domain immediately before the accretion (Fig. 16b). Thus its peculiar faunal succession and distinction from those in the Hida and Southern Kitakami Belts is consistent with the paleogeographic reconstruction based on accretion tectonics.

CONCLUSION

The latest views on the Permo-Triassic tectonics of the Japanese Islands are summarized as follows.

(1) The Japanese Islands record two coeval and contrasting types of convergent orogeny in the Permo-Triassic time: an accretionary orogeny between the Yangtze craton and subducted paleo- Pacific seafloor (probably the Farallon Plate) and a collisional orogeny between the Yangtze and Sino- Korean cratons.

(2) The Permo-Triassic accretionary orogeny in Japan involved collision-subduction of a paleoseamount and paleo-oceanic plateau, and left smaller/ thinner fragments in the accretionary complex.

(3) A subhorizontal piled nappe structure governs the Permo-Triassic orogen in Japan, showing several klippes and a peculiar ‘sandwich structure’ of high-P/T schist nappe.

(4) The Qinling-Dabie suture in central China, a major collisional suture in Asia between the Yangtze and Sino-Korean cratons, extends eastward to the Hida and Oki belts in Southwest Japan and the Hitachi-Takanuki belts in Northeast Japan.

(5) A paleogeographic reconstruction around Permo-Triassic proto-Japan with the double-edged Yangtze Block is consistent with the faunal characteristics of Permo-Triassic units in Japan.

ACKNOWLEDGEMENTS

The present author thanks S. Maruyama for his comment on ophiolite, and W. McDonough, A.S. Jayko and A. Taira who read the manuscript and gave valuable comments.

REFERENCES

CADET J. P., KOBAYASHI K., LALLEMAND S. et al. 1987. Deep scientific dives in the Japan and Kurile trenches. Earth and Planetary Science Letters 83, 313-28.

CARIDROIT M., CHARVET J. & ICHIKAWA K. 1982. The Ultra-Tanba zone, a new unit in the Inner Zone of Southwest Japan - its importance in the nappe structure after the example of the Maizuru area. Chikyu- Kagaku 39, 210-19.

CLUZEL D., LEE B. J. & CADET J. P. 1991. Indosinian dextral ductile fault system and synkinematic pluton- ism in the southwest of the Ogcheon belt (South Korea). Tectonophysics 194, 13 1-5 1.

CONG B. & WANG Q. 1995. Ultra-high-pressure metamorphic rocks in China. Episodes 18, 91-4.

DALZIEL I. W. D. 1992. On the organization of American plates in the Neoproterozoic and the breakout of Laurentia. G S A Today 2, 240-1.

DEWEY J. F. & BIRD J. M. 1970. Mountain belts and the new global tectonics. Journal of Geophysical Re- search 75, 2625-47.

DEWEY J. F. & BURKE K. 1974. Hot spots and continental break-up: Implications for collisional orogeny. Geology 2, 57-60.

EMBRY A. F., BEAUCHAMP B. & GLASS D. J., eds. 1994. Pangea: Global environments and resources. Memoir of Canadian Society of Petroleum Geologists 17.

ENAMI M. & ZANG Q. 1990. Quartz pseudomorphs after


Japanese Association of Mineralogy, Petrology and Economic Geology 74, 87-99.

ISHIGA H. 1990. Ultra-Tanba terrane. I n Ichikawa K., Mizutani S., Hara I., Hada S. & Yao A. eds. Pre- Cretaceous Terranes of Japan, pp. 97-107. Publica- tion of International Geological Correlation Pro- gramme #224, Osaka.

ISHII K. 1990. Provinciality of some fusulinacean faunas of Japan. I n Ichikawa K., Mizutani S., Hara I., Hada S. & Yao A. Pre-Cretaceous Terranes of Japan, pp. 297-305, Publication of International Geological Correlation Programme #224, Osaka.

ISHIWATARI A. 1985. Granulite facies metacumulates of the Yakuno ophiolite, Japan: Evidence for unusually thick oceanic crust. Journal of Petrology 26, 1-30.

ISHIWATARI A., IKEDA Y. & KOIDE Y. 1990. The Yakuno ophiolite, Japan: Fragments of Permian island arc and marginal basin crust with a hotspot. I n Malpas J. G., Moores E. M., Panayiotou A. & Xenophontos C. eds. Oceanic Crust Analogues, pp. 497-506. Geolog- ical Survey Department of Nicosia, Cyprus.

ISOZAKI Y. 1987. End-Permian convergent zone along the northern margin of the Kurosegawa landmass and its products in central Shikoku, Southwest Japan. Journal of Geoscience, Osaka City University 30, 51-131.

ISOZAKI Y. 1996. Anatomy and genesis of a subduction- related orogen: A review of geotectonic subdivision and evolution of the Japanese Islands. The Island

ISOZAKI Y. 1997. Jurassic accretion tectonics of Japan. The Island Arc 6, 25-51.

ISOZAKI Y., HASHICUCHI Y. & ITAYA T. 1992. The Kurosegawa klippe: An examination. Journal of the Geological Society of Japan 98, 917-41 (in Japanese with English abstract).

ISOZAKI Y. & ITAYA T. 1989. Origin of schist clasts of Upper Cretaceous Onogawa Group, Southwest Japan. Journal of the Geological Society of Japan 95,

ISOZAKI Y. & ITAYA T. 1991. Pre-Jurassic klippe in northern Chichibu belt in western-central Shikoku, Southwest Japan - Kurosegawa terrane as a tectonic outlier of the pre-Jurassic rocks of the Inner Zone. Journal of the Geological Society of Japan 97, 431-50 (in Japanese with English abstract).

ISOZAKI Y. & MARUYAMA s. 1991. Studies on orogeny based on plate tectonics in Japan and a new geotectonic subdivision of the Japanese islands. Journal of Geography 100, 697-761 (in Japanese with English abstract).

ISOZAKI Y., MARUYAMA s. & FURUOKA F. 1990. Ac- creted oceanic materials in Japan. Tectonophysics 181, 179-205.

ISOZAKI Y. & NISHIMURA Y. 1989. Fusaki Formation, Jurassic subduction-accretion complex on Ishigaki island, southern Ryukyus and its geologic implication to Late Mesozoic convergent margin of east Asia.

Arc 5, 289-320.

361-8.

coesite in eclogites from Shandong province, east China. American Mineralogist 75, 381-6.

ENCEBRETSON D. C., COX A. & GORDON R. G. 1985. Relative motions between oceanic and continental plates in the Pacific basin. Geological Society of America Special Paper 206, 1-59.

ERNST W. G., CAO R. I,. & JIANG J. Y. 1988. Reconnais- sance study of Precambrian metamorphic rocks, northeastern Sino-Korean shield, People’s Republic of China. Bulletin of Geological Society of America 100, 692-701.

GOTO N. 1988. Stratigraphy of Permian siliceous- argillaceous rocks and their tectonic relation with coarse-grained clasiks in the Taishaku area, South- west Japan. Journul of Geological Society of Japan 94, 501-14 (in Japanese with English abstract).

HARA A. & KIMINAMI K. 1989. Ancient trench-fill and trench-slope basin deposits: An example from the Permian Nishiki Group, Southwest Japan. I n Taira A. & Masuda F. eds. Sedimentary Facies in the Active Plate Margin, pp. 557-75. Terra Sci. Pub., Tokyo.

HASE A. & NISHIMURA Y. 1979. Greenstones in the Chugoku district. Journal of Geological Society of Japan 85, 401-12 (in Japanese with English abstract).

HASE A, OKIMURA Y. & YOKOYAMA T. 1974. The Upper Paleozoic formations in and around Taishaku-dai, Chugoku massif, Southwest Japan, with special ref- erence to the sedimentary facies of limestones. Geo- logical Report of Hiroshima University 19, 1-39 (in Japanese with English abstract).

HAYASAKA Y. 1987. Study on the late Paleozoic to early Mesozoic tectonic development of western half of the Inner Zone of Southwest Japan. Geological Report of Hiroshima University 27, 119-204 (in Japanese with English abstract).

HIRAJIMA T., ISHIWATARI A., CONG B., ZHANG R., BANNO, S. & NOZAKA T. 1990. Coesite from Meng- zhong eclogite at Dhonghai county, northeastern Jiangsu province, China. Mineralogical Magazine 54,

HIROI Y. 1983. Progressive metamorphism of the Una- zuki pelitic schists in the Hida terrane, central Japan. Contributions to Mineralogy and Petrology 82, 334- 50.

HIROI Y., FANNING C:. M., ELLIS D. J. et al. 1994. SHRIMP U-Pb zircon dating of the Abukuma metamorphic rocks and tectonic significance. Abstracts 102nd Annual Meeting of Geological Society of Japan, 177 (in Japanese).

HIROI Y., FUJI N. & OKIMURA Y. 1978. New fossil discovery from the Hida metamorphic rocks in the Unazuki area, central Japan. Proceedings of Japan Academy 54B, 268--71.

HOFFMAN P. F. 1991. Did the breakout of Laurentia turn Gondwana inside-out? Science 252, 1409-12.

HOSHINO M. 1979. Two-pyroxene amphibolites in Dogo, Oki islands, Shimane-ken, Japan. Journal of the

579-83.

22 Y. Isoxaki

Memoir of Geological Society of Japan 33, 259-75 (in Japanese with English abstract).

JAHN B. M. 1990. Early Precambrian basic rocks of China. In Hall R. P. & Hughes D. J. eds. Early Precambrian Basic Magmatism, pp. 294-316. Blackie, Glasgow.

KABASHIMA T., ISOZAKI Y. & NISHIMURA Y. 1993. Find of boundary thrust between 300 Ma high-PIT type schists and weakly metamorphosed Permian accretionary complex in the Nagato tectonic zone, South- west Japan. Journal of the Geological Society of Japan 99, 877-80 (in Japanese with English abstract).

KANMERA K. & NISHI H. 1983. Accreted oceanic reef complex in Southwest Japan. In Hashimoto M. & Uyeda S. eds. Accretion Tectonics in the Circum- PaciJic Regions, pp. 195-206. Terra Sci. Pub., Tokyo.

KANMERA K., SANO H. & ISOZAKI Y. 1990. Akiyoshi terrane. In Ichikawa, K., Mizutani S., Hara I., Xada S. & Yao A. eds. Pre-Cretaceous Terranes of Japan, pp. 49;62. Publication of International Geological Correlation Programme #224, Osaka.

KANO T., TANAKA S. & OKUSHIMA K. 1993. Exploration of staurolite and kyanite in the Hida gneisses by means of Uruno’s river sand method. Abstracts 100th Annual Meeting, Geological Society of Japan, 611 (in Japanese).

KATO M. 1990. Paleozoic corals. In Ichikawa, K., Mizu- tani S., Hara I., Hada S. & Yao A. eds. Pre- Cretaceous Terranes of Japan, pp. 307-12. Publica- tion of International Geological Correlation Programme #224, Osaka.

KIMURA G. 1997. Cretaceous episodic growth of the Japanese Islands. The Island Arc 6, 52-68.

KLEIN G. D. ed. 1994. Pangea: Paleoclimate, tectonics, and sedimentation during accretion, zenith, and breakup of a supercontinent. Geological Society of America, Special Paper, 288, 1-295.

KOBAYASHI T. 1941. The Sakawa orogenic cycle and its bearing on the origin of the Japanese Islands. Jour- nal of the Faculty of Science, Imperial University of Tokyo, Sec. 2 5 , 219-578.

KOMATSU M. 1990. Hida ‘Gaien’ belt and Joetsu belt. In Ichikawa K., Mizutani S., Hara I., Hada S. & Yao A. eds. Pre-Cretaceous Terranes of Japan, pp. 25-40. Publication of International Geological Correlation Programme #224, Osaka.

LIN J. L., FULLER M. & ZHANG W. Y. 1985. Preliminary Phanerozoic polar wander paths for the North and South China blocks. Nature 313, 444-9.

MARUYAMA S. 1990. Denudation of high-pressure metamorphic belts. Abstracts of 97th Annual Meeting, Geological Society of Japan, 484 (in Japanese).

MARUYAMA s., BANNO s., MATSUDA T. & NAKAJIMA T. 1984. Kurosegawa zone and its bearing on the development of the Japanese islands. Tectonophysics 110,

MARUYAMA s. & SEN0 T. 1986. Orogeny and relative 47-60.

plate motions: Example of the Japanese Islands. Tectonophysics 127, 305-29.

MARUYAMA S., LIOU J. G. & ZHANG R. 1994. Tectonic evolution of the ultrahigh-pressure (UHP) and high- pressure (HP) metamorphic belts from central China. The Island Arc 3, 112-21.

MINATO M., GORAI M. & FUNAHASHI M. (eds) 1965. The Geologic Development of the Japanese Islands. Tsu- kiji Shokan, Tokyo.

NAGASE M., NAITO K., KOMATSU M. & SUGANO T. 1990. Kinematic pictures of Funatsu shear zone of the Hida metamorphic belt. Earth Monthly 12, 396- 401 (in Japanese).

NAKAMURA K. & TAZAWA J. 1990. Faunal provinciality of the Permian brachiopods in Japan. In Ichikawa K., Mizutani S., Hara I., Hada S. & Yao A. eds. Pre- Cretaceous Terranes of Japan, pp. 313-20. Publica- tion of International Geological Correlation Pro- gramme #224, Osaka.

NAKAZAWA K. 1991. Mutual relation of Tethys and Japan during Permian and Triassic time viewed from bivalve fossils. Proceedings of Shallow Tethys 3, Sendai 1990 (Saito Ho-onkai Spec. Publ. no. 3), 3-20.

NISHIMURA Y. 1990. ‘Sangun metamorphic rocks’: Terrane problem. In Ichikawa K., Mizutani S., Hara I., Hada S. & Yao A. eds. Pre-Cretaceous Terranes of Japan, pp. 63-79. Publication of International Geo- logical Correlation Programme #224, Osaka.

NISHIMURA Y., ITAYA T., ISOZAKI Y. & KAMEYA A. 1989. Depositional age and metamorphic history of 220 Ma high PIT type metamorphic rocks: An example of the Nishiki-cho area, Yamaguchi Prefecture, Southwest Japan. Memoir of the Geological Society of Japan 33, 443-66 (in Japanese with English abstract).

ON0 A. 1983. K-Ar age of Kinshozan quartz diorite, Kanto mountains. Journal of the Japanese Associa- tion of Mineralogy, Petrology and Economic Geology 78, 38-9 (in Japanese with English abstract).

OKAY A. I., XU S. & SENGOR A. M. C. 1989. Coesite from the Dabie Shan eclogites, central China. Euro- pean Journal of Mineralogy, 1, 595-8.

OTA M. 1968. Akiyoshi Limestone Group: A geosynclinal oroganic reef complex. Bulletin of Akiyoshi-dai Sci- ence Museum 5, 1-44 (in Japanese with English abstract).

OTOFUJI Y. 1996. Large tectonic movement of the Japan Arc in late Cenozoic times inferred from paleomag- netism: Review and synthesis. The Island Arc 5 , 229-490.

PARK J. K., BUCHAN K. L. & HARLAN S. S. 1995. A proposed giant dyke swarm fragmented by the sepa- ration of Laurentia and Australia based on paleomag- netism of ca 780 Ma mafic intrusions in western North America. Earth and Planetary Science Let- ters 132, 129-39.

POWELL C. MCA., LI Z. X., MCELHINNY M. W., MEERT

Accretion and collision in Permo-Triassic J a p a n 23

Devonian’ elastic rocks in the Southern Kitakami terrane. Journal of the Japanese Association of Mineralogy, Petrology and Economic Geology 87, 330-49 (in Japanese with English abstract).

TAIRA A. & TASHIRO M. eds. 1987. Historical Biogeog- raphy and Plate Tectonic Evolution of Japan and Eastern As ia . Terra Sci. Publ., Tokyo.

TAGIRI M. 1973. Metamorphism of Paleozoic rocks in the Hitachi district, southern Abukuma plateau, Ja- pan. Science Report of the Tohoku University, series 111 12, 1-67.

TAGIRI M., HIROI Y., HIRAJIMA T. & SHIMIZU I. 1992. Abukuma and Sanbagawa metamorphic belts in the kanto district. 29th International Geological Con- gress Field Trip Guidebook.

TAKAGI H. & FUJIMORI H. 1989. Allochthonous granitic bodies in the northern marginal area of the Kanto Mountains. Journal of the Geological Society of Japan 95, 663-85 (in Japanese with English abstract).

TAKAGI H. & HARA T. 1994. Kinematic pictures of the ductile shear zones in the Hida terrane and their tectonic implication. Journal of the Geological Soci- ety of Japan 100, 931-50 (in Japanese with English abstract).

TAPPONNIER P., PELTZER G., LE DAIN Y., ARMIJO R. & COBBOLD P. 1982. Propagating extrusion tectonics in Asia: New insights from simple experiments with plasticine. Geology 10, 611-16.

TAZAWA J. 1992. Middle Permian brachiopod faunas in East Asia and their zoogeographic significance. Jour- nal of the Geological Society of Japan 98, 483-96 (in Japanese with English abstract).

TORIYAMA R. 1958. Geology of Akiyoshi, Part 111, Fusulinids of Akiyoshi. Memoir of the Faculty of Science, Kyushu University, ser. D 7, 1-264.

UCHIYAMA T., SANO H. & KANMERA K. 1986. Deposi- tional and tectonic settings of cherts around the Akiyoshi Limestone Group, Southwest Japan. Mem- oir of the Faculty of Science, Kyushu University, ser. D 26, 51-68.

VON HUENE R. & SCHOLLE D. W. 1991. Observations a t convergent margins concerning sediment subduction, subduction erosion, and the growth of continental crust. Reviews of Geophysics 29, 279-316.

WANG X., LIOU J. G. & MA0 H. K. 1989. Coesite-bearing eclogites from the Dabie mountain in central China. Geology 17, 1085-8.

YANAI S., PARK B. s. & OTOH S. 1985. The Honan shear zone (S. Korea): Deformation and tectonic implication in the Far East. Science Papers of the College of A r t s and Sciences, University of Tokyo 35, 181- 210.

YANC J. & SMITH D. C. 1989. Evidence for a former sanidine-coesite-eclogite a t Lanshantou, eastern China, and recognition of the Chinese ‘Su-Lu coesite- eclogite province’. Abstracts 3rd International Ecologite Conference a t Wurxburg 1, 26.

J. G. & PARK J. K. 1993. Paleomagnetic constraints on timing of the Neoproterozoic breakup of Rodinia and the Cambrian formation of Gondwana. Geology

ROSS C. A. & ROSS J. R. P. 1983. Late Paleozoic accreted terranes of western North America. I n Stevens C. H. ed. Pre-Jurassic Rocks in Western North American Suspect Terranes, Society of Eco- nomic Paleontologists and Mineralogists, Pacific section, 7-22.

SANO H. & KANMERA K. 1991. Collapse of ancient oceanic reef complex - what happened during collision of Akiyoshi reef complex? Sequence of collisional collapse and general ion of collapse products. Journal of the Geological Society of Japan 97, 631-44.

SCOTESE C. R. & LANGFORD R. P. 1995. Pangea and the paleogeography of the Permian. I n Scholle P. A. et al. eds. The Permian of Northern Pangea, Paleo- geography, Paleoclimates, Stratigraphy, vol. 1, pp. 3-19. Springer-Verlag, Berlin.

SENGOR A. M. C. 1989. Cimmeride orogenic system and the tectonics of Eur,ssia. Geological Society of Amer- ica, Special Paper 195, 1-82.

SOHMA T., KUNUGIZA K. & TERABAYASHI M. 1990. Hida metamorphic belt. Excursion Guidebook of 96th Annual Meeting gf Geological Society of Japan, 27-53 (in Japanese).

SOHMA T. & KUNUGIZA K. 1993. The formation of the Hida nappe and the tectonics of Mesozoic sediments: The tectonic evolution of the Hida region, central Japan. Memoir of the Geological Society of Japan 42, 1-20 (in Japanese with English abstract).

SUZUKI H., ISOZAKI Y. & ITAYA T. 1990. Tectonic superposition of the Kurosegawa Terrane upon the Sanbagawa metamorphic belt in eastern Shikoku, Southwest Japan: I<-Ar ages of weakly metamorphosed rocks in northeastern Kamikatsu Town, Tokushima Prefecture. Journal of the Geological Society of Japan 96, 143-53 (in Japanese with English abstract).

SUZUKI H. & ITAYA T. 1994. Accretionary complexes of the Kurosegawa, Northern Chichibu and Sanbagawa belts in the Kamikatsu Town area (Shikoku), South- west Japan. Journlrcl of the Geological Society of Japan 100, 585-99 (in Japanese with English abstract).

SUZUKI K. & ADACHI M. 1991. The chemical Th-U- total Pb isochron ages of zircon and monazite from the Gray granite of the Hida terrane, Japan. Journal of E a r t h Science, hragoya University 38, 11-38.

SUZUKI K. & ADACHI M. 1994. Middle Precambrian detrital monazite and zircon from the Hida gneiss on Oki-Dogo island, Japan: Their origin and implications for the correlation of basement gneiss of Southwest Japan and Korea. Tectonophysics 235, 277-92.

SUZUKI K., ADACHI M., SANG0 K. & CHIBA H. 1992. Chemical Th-U-total Pb isochron ages of monazites and zircons from the Hikami Granite and ‘Siluro-

21, 889-92.

24 Y. Isoxaki

YOSHIKURA s., HADA s. & ISOZAKI Y. 1990. Kuro- t in of the Institute of Geology and Mineral Re- segawa terrane. In Ichikawa K., Mizutani S., Hara I., sources, Chinese Academy of Geological Sciences 16,

Japan, PP. 185-201. Publication of Intei-national ZHOU X. H. 1994. Geochemical constraints on tectonic Geological Correlation Programme #224, Osaka. evolution of South China. In Yu J. S. ed. Isotopic

ZHENG w. z . , LIU G. L. & WANG x. w. 1991. New Geochemical Research oj' China, pp. 98-117, Science information on the Archean Eon for the Kongling Press, Beijing. Group in northern Huangling anticline, Hubei. Bulle-

Hada S. & Yao A. eds. Pre-Cretaceous Terranes of 97-106.

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Contrasting two of orogen in Permo-Triassic Japan...

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