Subdivision of the hida metamorphic complex, central Japan, and its bearing on the geology of the Far East in pre-Sea of Japan time

Tectonophysics, 16 (1981) 317-333 Elsevier Scientific Publishing Company,

317 Amsterdam

- Printed

in The Netherlands

SUBDIVISION OF THE HIDA METAMORPHIC COMPLEX, CENTRAL JAPAN, AND ITS BEARING ON THE GEOLOGY OF THE FAR EAST IN PRESEA OF JAPAN TIME

YOSHIKUNI

HIROI

Institute of Earth Science,

(Japan)

(Received

September

Faculty of Educntion,

2, 1980; revised version

Kanazawa University, Kanazawa, 920

accepted

December

4, 1980)

ABSTRACT Hiroi, Y., 1981. Subdivision of the Hida metamorphic complex, central Japan, and its bearing on the geology of the Far East in pre-Sea of Japan time. Tectonophysics, 76: 317-333. Petrological and structural characteristics of the regional metamorphic rocks in southwest Japan and in the Korean Peninsula make it possible to speculate on the geological correlation between Japan and the Asian continent prior to the opening of the Sea of Japan, a typical marginal sea. The Hida metamorphic complex, situated on the Sea of Japan side of southwest Japan, is subdivided into two distinct geological units, the Hida gneisses and the Unazuki schists. The Hida gneisses are polymetamorphosed Precambrian rocks, while the Unazuki schists occurring on its eastern and southern sides are low- to medium-grade metamorphic rocks originating from Upper Paleozoic deposits. Sedimentary facies and other geological features suggest that the Hida gneisses were the basement of the Unazuki schists. Consequently, the geotectonic framework of southwest Japan is revised from north to south as follows: Hida gneiss region (Precambrian massif) Unazuki zone (late Permian intermed. P/T metamorphic belt) Circum-Hida tectonic zone (Mid- to Late Paleozoic melange zone) Mine-Tan ba terrain (Late Paleozoic to Mesozoic geosynclinal region) and so on. The Unazuki zone is similar to the Okcheon zone in the Korean Peninsula in respect of age, lithology and biofacies of sedimentary rocks as well as the age and type of regional metamorphism. Furthermore, the Hida gneisses and the Gyeonggi polymetamorphosed Precambrian gneisses in the Korean Peninsula are similar in the apparent baric type of metamorphism, radiometric ages and the relationship with the overlying metamorphosed formations. The similarity of the geotectonic frameworks of southwest Japan and the Korean Peninsula suggests that the Unazuki zone and the Okcheon zone once formed a continuous geotectonic unit. Thus we have a new geological coordinate in reconstructing the paleogeography prior to the opening of the Sea of Japan.

INTRODUCTION

The Hida metamorphic complex is located on the Sea of Japan side of southwest Japan, the southern border of which is the Hida marginal tectonic OU40-1951/81/0000~000/$

02.50 @ 1981 Elsevier

Scientific

Publishing

Company

318

zone (Figs. 1 and 2). In the 1961 scheme of Miyashiro (1961) for the development of the metamorphic belts of the Japanese Islands, the Hida metamorphic complex was considered to form a paired metamorphic belt of Late Paleozoic age with the “Sangun belt”, which included the Hida marginal tectonic zone. Progress since 1961, mainly radiometric dating (Ohmoto, 1964; Sato et al., 1967; Hayase and Ishizaka, 1967; Nozawa, 1968; Ishizaka and Yamaguchi, 1969; Yamaguchi and Yanagi, 1970; Shibata et al., 1970; Noza-

2000

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SEA OF JAPAN

v--

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)

PACIFIC

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Fig. 1. The geotectonic frameworks of the northern part of Southwest Japan and the Korean Peninsula. Radiometric ages of several significant geotectonic provinces are also shown. KB = K-Ar biotite age, KM = K-Ar museovite age, KH = K-Ar hornblende age, KW = K-Ar whole rock age, RB = RbSr biotite age, RM = Rb-Sr muscovite age, RW = RbSr whole rock age, UZ = U-Th-Pb zircon age.

319

Funatsu gr,anites (Jurassic) Unazuki

l*

Unazuki

v

zone

schists

Hida gneisses Sedimentary metamorphic Ultramafic

& rocks rocks

/

SEA OF JAPAN

36” 138”

>

Mino-Tanba

terrain

Fig. 2. The distribution of the predurassic rocks and the Jurassic Funatsu granites in the Hida Mountains and adjacent area (after Nozawa, 1977). See text for detail.

wa, 1971) and a few petrological data (Sato, 1968; Suzuki and Kojima, 1970), made it necessary to reconsider the significance of the Hida metamorphic complex in the geology of Japan. Thus, in his textbook in 1973, Miyashiro (1973) distinguished four events of igneous activity and metamorphism, which he named Hida I (1640 m.y.), II (500 m.y.), III (320 m.y.) and IV (180-250 m.y.), respectively. Recent geological and petrological works on the Hida metamorphic complex (Kano, 1973; Suzuki, 1977; Hiroi, 1978; Hiroi et al., 1978; Shibata and Nozawa, 1980) have established that its most northeasterly part represents a different geological unit from the major part, distinct both geologically and petrologically. This northeastern part, which will be called the Unazuki schists, is composed of Upper Paleozoic rocks regionally metamorphosed in late Permian time and re-metamorphosed thermally in early Jurassic time (Hiroi, 1978; Hiroi et al., 1978). The regional metamorphism belongs to the kyanite-sillimanite type of metamorphic facies series. In this paper, recent progress in geology and petrology of the Hida metamorphic complex will be briefly reviewed, and its significance in reconstructing the geology of the Far East prior to the opening of the Sea of Japan will be discussed.

320

OUTLINE OF GEOLOGY ROUNDING REGION

OF THE HIDA METAMORPHIC

The Hida metamorphic

complex

COMPLEX

AND THE SUR-

The Hida metamorphic complex consists of various kinds of schists, gneisses and granitic rocks. The major outcrops are in the Hida Mountains (Figs. 1 and 2), but it also constitutes the basement of the Jurasso-Cretaceous and Tertiary formations in the Hokuriku province including the Noto Peninsula, and in the Oki-Dogo Island in the Sea of Japan off the San-in coast (Fig. 1.). The constituent rocks of the Hida complex are divided into three geological units, the Funatsu granites, the Hida gneisses and the Unazuki schists. Funatsu

granites

This is a collective name for the plutonic rocks ranging from diorite to granite with radiometric ages of about 180 m.y. (Ishizaki and Yamaguchi, 1969; Shibata et al., 1970). They intruded both the Hida gneisses and the Unazuki schists along with the Permo-Carboniferous rocks developed on the south of the Hida gneisses. Contact aureoles, sometimes represented by migmatitic rocks and/or augen gneisses, are common (Kano, 1973; Hiroi, 1978). Hida gneisses

It is the major geological unit of the Hida complex and is composed mainly of quartzo-feldspathicgneiss and marble with subordinate amphibolite and minor pelitic gneiss in the Hida Mountains (Hashimoto et al., 1970; Suzuki, 1977), while it consists dominantly of pelitic gneiss and quartzofeldspathic gneiss (migmatitic gneiss) with subsidiary amphibolite and minor calcareous gneiss in the Oki-Dogo Island (Hoshino, 1979). The gneisses and amphibolite are intruded by vein, dyke and pool of granitic rocks, customarily called the grey granite by local researchers. Radiometric ages of the Hida gneisses show two peaks at about 180 m.y. (K-Ar and Rb-Sr ages of mica and amphibole) and about 240 m.y. (K-Ar and Rb-Sr ages of amphibole and U-Th-Pb ages of zircon and sphene). The age of 180 m.y. reflects the contact metamorphic effect of the Funatsu granites (Shibata et al., 1970; Shibata and Nozawa, 1978; Hiroi, 1978). The age of 240 m.y. represents a regional metamorphism contemporaneous with that of the Unazuki schists. Recently Shibata and Nozawa (1980) reported a Rb-Sr whole rock age of about 600 m.y. for the grey granite, confirming the Precambrian metamorphism of the Hida gneisses. The greatest age so far obtained from in situ Hida gneisses is about 1500 m.y. for detrital zircon in gneiss (Ishizaka and Yamaguchi, 1969). Shibata et al. (1971) and Shibata and Adachi (1972) reported K-Ar and Rb-Sr mica ages of about 1600 m.y. for gneiss cobbles in the Kamiaso conglomerate in the Mino terrain, situated about 80 km south of the Hida metamorphic complex. On the basis of paleocurrent data, which indicate transportation of the cobbles from north to

321

south (Adachi and Mizutani, 1971), they considered that the provenance of the cobbles was situated in the area where the Hida complex is now exposed. Andalusite, sillimanite and cordierite together with garnet are the common constituent minerals of the Hida pelitic gneiss (Hashimoto et al., 1970; Fujiyoshi, 1973; Asami and Adachi, 1976), suggesting low pressure conditions for the last metamorphism. Suzuki (1977) considered that the Hida gneisses suffered repeated metamorphism, the earlier amphibolite- to granulite-facies metamorphism and later metamorphism of amphibolite facies. Hoshino (1979) reported two-pyroxene amphibolite from the Oki-Dogo Island. It may be recapitulated that most, if not all, of the Hida gneisses are the Precambrian metamorphic rocks, the mineralogy of which was determined by the metamorphic events of 180 m.y. and 240 m.y. ago. Unazuki schists It has long been known that kyanite and staurolite with or without andalusite and sillimanite occur in the most northeasterly part of the Hida metamorphic complex (Ishioka, 1949; Kobayashi and Kobayashi, 1951; Ishioka and Suwa, 1956). Recent work by the present author revealed that the rocks in the most northeasterly part represent a different geological unit from the Hida gneisses, having the following features: (1) It is a low- to medium-grade metamorphic terrain showing kyanite-sillimanite type facies series (Fig. 3). (2) The metamorphics (Unazuki schists) consist of bedded limestones, pelitic rocks rich in Al and Fe, quartzo-feldspathic rocks originating from acidic volcanics and alternation of pelitic-psammitic rocks and basic rocks. The first two rock types are characteristic of the Unazuki schists, and have never been known in the main Hida gneisses. Fossil bryozoa and foraminifera of late Carboniferous age occur in the bedded limestones (Hiroi et al., 1978). (3) The Unazuki schists are over-thrust by the Hida gneisses from the west (Fig. 3). (4) Muscovite and biotite separates from pelitic quartzo-feldspathic rocks yield Rb-Sr ages of 210-250 m.y. (Yamaguchi and Yanagi, 1970;‘Shibata et al., 1970), while a K-Ar biotite age of 175 m.y. is obtained from a sample whose muscovite gives a Rb-Sr age of 214 m.y. (Shibata et al., 1970). The age of 175 m.y. corresponds to the contact metamorphism by the Funatsu granites, and the ages ranging from 210 to 250 m.y. may represent the regional metamorphism of kyanite-sillimanite type. (5) When the contact metamorphic effect of the Funatsu granites has been allowed for, the thermal structure of the regional metamorphism is independent of the distribution of the Hida gneisses (Fig. 3). (6) The Unazuki schists together with the Hida gneisses suffered intense deformation after the peak of the regional metamorphism and before the intrusion of the Funatsu granites. Kano (1975) emphasized the significance of the Jurassic deformation accompanied with the emplacement of the Funatsu granites.

322

1 km,

i +

Sedimentary & vokzanic rockstertiary) Funatsu granites (Jurassic) Meta -gabbro (age unknown) Unatuki Hida

schists

gneisses

Fault Thrust Antic~jne

tsograd

323

Correlatives of the Unazuki schists in regard to age, lithology and biofacies of the original rocks, and age and type of the regional metamorphism occur In the Fukuji area (Igo, 1961), Makido area (T. Soma and T. Nozawa, personal communication, 1978), Itoshiro area (Konishi, 1954) and Asahi area (Asami, 1970; Hiroi, in press) in the eastern and southern border of the Hida gneiss region (Fig. 2). The upper Carboniferous deposits at Fukuji characteristically contain freshwater sediments and have faunal assemblages similar to those in Korea and northeastern China, while most, if not all, of the upper Carboniferous deposits in southwest Japan are composed of marine sediments perhaps formed within the marginal sea (Igo, 1961). The Hida marginal tectonic

zone

This is a kind of melange zone where glaucophane schist, garnet amphibolite, metagabbro and Middle to Upper Paleozoic deposits occur as blocks in serpentinites (Banno, 1958; Seki, 1959; Fujimoto et al., 1962; Ito, 1966,1975; Hashimoto, 1978; K. Matsumoto and S. Maruyama, personal communication, 1979). Glaucophane schist and associated rocks yield mica Rb-Sr and K-Ar ages of 310-360 m.y. (Shibata and Nozawa, 1968; Shibata et al., 1970; Shibata and Ito, 1978). Hornblende separated from meta-gabbro give K-Ar ages of about 400 m.y. (K. Shibata, personal communication, 1980). The Mino-Tanba

terrain

The Mino-Tanba terrain is a Paleozoic-Mesozoic geosynclinal region where sandstone, shale, chert, mafic volcanics and limestone with subsidiary conglomerate were accumulated (Adachi, 1976). Recently it became evident that most of the constituent rocks of the terrain belong to the TriassicJurassic from the conodont biostratigraphy (Igo, 1972; Igo and Koike, 1975) and other paleontological criteria (Nishida et al., 1974; Sato, 1974). It is noteworthy that rock fragments such as sillimanite-bearing gneiss of Precambrian age, orthoquartzite, chloritoid-bearing phyllite and bedded limestone containing late Carboniferous fossils occur in the sandstone of Triassic-Jurassic age (Adachi, 1976). Regional paleocurrent data and the distribution of sedimentary facies indicate that major source of sediment were exposed to the north and to the south of an E-W trending elongated basin of the MinoTanba terrain (Adachi, 1976). The northern source terrain could have included the region where the Unazuki schists and the correlatives were exposed, because chloritoid-bearing phyllite and late Carboniferous bedded limestone are the major constituents of the Unazuki schists. Furthermore, the uplift of the Unazuki schists during Triassic time to supply the elastic Fig. 3. Generalized phic complex. The also shown.

geological map of the northeastern kyanite-sillimanite type metamorphic

most part of the Hida metamorzones in the Unazuki schists are

324

materials to the Mino-Tanba terrain is possible, because kyanite-bearing metamorphics were re-metamorphosed thermally in early Jurassic times by the Funatsu granite to produce low pressure minerals such as andalusite and Fe-rich cordierite, indicating that they were uplifted before the Jurassic (Hiroi, 1978). The Precambrian gneiss cobbles could have come from the Hida gneiss region, but they could also come from the area where the Unazuki schists and the correlatives were exposed, because metamorphosed conglomerate with cobbles of gneissic and granitic rocks were discovered in the Unazuki schists (Suwa, 1966; Hiroi, 1978). They could be reworked cobbles. The western border of the Mino-Tanba terrain is the Maizuru zone where Late Paleozoic ophiolite occurs (Ishiwatari, 1978) (Fig. 1). THE RELATIONSHIP

BETWEEN

THE HIDA GNEISSES

AND THE UNAZUKI

SCHISTS

The Unazuki schists are metamorphosed Upper Paleozoic deposits, while most, if not alI, of the Hida gneisses are Precambrian metamorphics re-equilibrated in late Permian and early Jurassic times. The Unazuki schists and the Hida gneisses differ in metamorphic grade, apparent baric type of metamorphism and Ethology, but have common radiometric mineral ages of 240 m.y. and 180 m.y. In the most northeastern part of the Hida complex the Unazuki schists were over-thrust by the Hida gneisses from the west during the Triassic deformation, while in the other part of the Hida complex they are not in contact. We have no direct evidence in hand to relate these two metamorphic areas but we can speculate that the Hida gneisses were the basement of the Late Paleozoic sedimentation of the original rocks of the Unazuki schists, because the sedimentary facies of the Unazuki schists are continental shelf and continental deposits, in which rhyolite and related pyroclastic rocks are present and metamorphosed. conglomerate with cobbles of gneissic and granitic rocks occur. The amphibolite to granulite facies mineralogy of the Hida gneisses may not have changed during the metamorphic events of 240 m.y. and 180 m.y. ago, only radiometric ages being changed, or else the present mineralogy of the Hida gneisses is the superimposition of metamorphic events of 240 m.y. and 180 m.y. ago. The latter possibility cannot be excluded, because andalusite-sillimanite type contact metamorphism by the Funatsu granites is detected in the Unazuki schists, and if the contact metamorphic effect of the Funatsu granites on the Hida gneisses is allowed for, there is no reason to deny that they suffered kyanite-sillimanite type metamorphism. In either case, the common radiometric age of 240 m.y. of the two groups of metamorphic rocks strongly suggests that they were located side by side in Permo-Triassic times. DISCUSSION

AND CONCLUSIONS

As explained above, the Hida metamorphic complex is subdivided into two distinct geological units, the Hida gneisses and the Unazuki schists. The

325

Unazuki schists and the correlatives form the Unazuki zone on the eastern and southern sides of the Hida gneiss region (Fig. 2). Consequently the geotectonic framework of central Japan is revised, from what has been accepted among Japanese geologists, from north to south as follows: Hida gneiss region Unazuki zone Hida marginal tectonic zone Mino-Tanba terrain, and so on. The Korean Peninsula is divided into six geotectonic provinces, the Nangrim massif, the Pyeongan basin, the Gyeonggi massif, the Okcheon zone, the Sobaeksan (Ryeongan) massif and the Gyeongsang basin from north to south (Lee, 19’72; Reedman and Urn, 1975; OCKE and KIGAM, 1977; Kobayashi, 1977; Na and Lee, 1978) (Fig. 1). The Nangrim, Gyeonggi and Sobaeksan massifs are composed of Precambrian metamorphic rocks (op. cit.). The Pyeongan basin and the Okcheon zone are the Late Precambrian to Late Paleozoic or Early Mesozoic geosynclinal regions, where no outcrop of SiluroDevonian or lower Carboniferous formations has been found (op. cit.). Late Triassic to early Jurassic granites are known to occur widely in the abovementioned provinces. The Gyeongsang basin is a late Cretaceous to Tertiary basin where sedimentary and volcanic rocks accumulated (op. cit.). Late Cretaceous granites are intruded in this basin (Lee, 1980). Recently, Na (1978) demonstrated that the Gyeonggi massif can be divided into two distinct geological units, the Gyeonggi gneiss complex and the Yeoncheon group. The former is Precambrian metamorphic rocks composed mainly of grey granitic gneiss, whose apparent metamorphic facies series is of andalusite-sillimanite type. The latter is a series of metamorphosed formations overlying the Gyeonggi gneiss complex. The Yeoncheon group suffered a regional me~morphism of ky~i~-s~~rn~i~ type. Yamaguchi (1951) once reported some Silurian fossils such as ~ono~~~t~s from the so-called Sangwon system, which is a part of the Yeoncheon group of Na (1978). Hurley et al. (1973) reported a Rb-Sr biotite age of 180 m.y. for a sample from the Gyeonggi massif, and Na (1978) reported that biotite and hornblende separates from the Gyeonggi gneiss complex show metamorphic ages of 180 m.y. and 240 m.y., respectively. Lee (1980) reported a K-Ar whole-rock age of 220 m.y. for a sample of granitic gneiss from the basement of the Gyeong~ng basin, suggesting late Permian or early Triassic metamorphism of the Sobaeksan massif which is located across the Okcheon zone from the Gyeonggi massif. The Late Paleozoic rocks together with the older rocks in the Okcheon zone were metamorphosed partly into phyllites and schists of kyanite-sillimanite type just like the Yeoncheon group of Na (1978) (Lee, 1972; Reedman and Urn, 1975; OCKE and KIGAM, 1977). Na (1979) demonstrated that the Yeoncheon group and the Okcheon me~mo~hic rocks show similar structural characteristics, suggesting that both groups of me~morphic rocks were formed during the same orogeny. Reedman and Urn (1975) reported

326

the contact metamorphism by the Jurassic granites in the Okcheon zone. Because of the scarcity of radiometric age determinations and the contact metamorphic effect by the intruding granites, it is difficult to determine the age of the Okcheon and/or Yeoncheon regional metamorphism of kyanitesillimanite type, but the mineral ages of 240 m.y. and 180 m.y. of the polymetamorphosed Gyeonggi gneiss complex and the K-Ar whole-rock age of 220 m.y. of the polymetamorphosed Sobaeksan massif are important. The age of 180 m.y. perhaps reflects the contact metamorphism by the Jurassic granites and the age of 240 m.y. and 220 m.y. may represent the regional metamorphism contemporaneous with the Okcheon and/or Yeoncheon regional metamorphism of kyanite-sillimanite type. Although many Korean geologists emphasize the significance of the Jurassic Daebo orogeny, the Triassic Songrim disturbance must also be important, because this tectonism radically altered the previously existing pattern of basin development in the Korean Peninsula (Reedman and Urn, 1975; Kobayashi, 1977). The geological development of the northern part of central Japan and the central part of the Korean Peninsula is summarized in Table I. The Unazuki zone and the Okcheon zone and/or “Yeoncheon zone” have several significant geological events in common. Furthermore the Hida gneisses and the Gyeonggi gneiss complex are similar in their radiometric ages of 180 m.y. and 240 m.y., apparent baric type of metamorphism and the relationship with the overlying formations which suffered a Permian regional metamorphism of kyanite-sillimanite type. These facts strongly suggest that the Unazuki zone and the Okcheon zone and/or “Yeoncheon zone” once formed a continuous zone, although the following features apparently contradict this idea: (1) The Late Precambrian to Early Paleozoic formations are present in the Okcheon zone, but do.not outcrop in the Unazuki zone and throughout Japan. (2) A correlative of the Sobaeksan massif is absent in southwest Japan, and correlatives of the Circum-Hida tectonic zone and the Mino-Tanba terrain are absent in the Korean Peninsula. Kobayashi (1977) pointed out that faunas and lithologies of the Lower Paleozoic are considerably different between the Pyeongan basin and the Okcheon zone, while every Late Paleozoic basin formed on the Sino-Korean massif and the Okcheon zone has similar faunas and lithologies. Among the Japanese Upper Paleozoics, those at Fukuji and Itoshiro, that belong to the Unazuki zone, have the same features in common with those on the SinoKorean massif (Igo, 1961), while the other Upper Paleozoic in the northern part of southwest Japan are mainly composed of basic volcanics and massive limestones perhaps formed within the marginal sea. These facts suggest that the pattern of basin development during Late Precambrian to Early Paleozoic times was not so regular as that of Late Paleozoic time, at least, in the Korean Peninsula and the neighbouring region.

sedimentation of the Joseon supergroup and the Okcheon group

sedimentation of the original rocks of the Unazuki schists and the correlatives

???

Late Paleozoic

Late PrecambrianEarly Paleozoic metamorphism

sedimentation of the Pyeongan group

pre-Late Precambrian

regional metamorphism of kyanite-sillimanite

regional metamorphism of kyanite-sillimanite

Late PaleozoicEarly Mesozoic

type

deformation

deformation

Triassic type

deformation

intrusion of the Daebo granites

Okcheon zone and/or “Yeoncheon zone”

intrusion of the Funatsu granites deformation

Hida gneiss region

Korean Peninsula

Jurassic

Unazuki zone

Central Japan

Geological development of the northern part of central Japan and the central part of the Korean Peninsula

TABLE I

metamorphism

Gyeonggi gneiss complex

328

For the second problem, the difference in degree of destruction of geotectonic units between southwest Japan and Korea may have some implications. In contrast with the Okcheon zone, the Unazuki zone is a narrow discontinuous zone just like the C&urn-Hida tectonic zone. Occurrence of glaucophane schist and associated rocks of Middle Paleozoic age in the Circum-Hida tectonic zone and Late Paleozoic ophiolite in the Maizuru zone (Ishiwatari, 1978) suggests the presence of a marginal sea on the southern side of the Unazuki zone during Middle to Late Paleozoic times. In this connection it is si~ific~t that some Silurian fossils such as ~aZys~tes have been reported from the limestone cobbles in the basal conglomerate of Jurassic strata in the vicinity of Guemipo, northern Korea (OCKE and KIGAM, 1977; Kobayashi, 1977). Furthermore sone ‘green rocks” are known to occur near the boundary zone between the Okcheon zone and the Sobaeksan massif (Y.J. Lee and K.C. Na, personal communication, 1980). Adachi (1976) suggested the presence of Precambrian massifs not only to the north of a E-W trending Mino-Tanba geosynclinal region but also to the south of the region during Triassic-Jurassic times. Thus, intimate correlation between the northern part of central Japan and the central part of the Korean Peninsula strongly suggests that the Unazuki zone and the Okcheon zone and/or “Yeoncheon zone” once formed a continuous geotectonic unit. The pattern of zonal arrangement of geotectonic units in southwest Japan and the distribution of the late Triassic-Early Jurassic granites and the late Cretaceous granites are consistent with the origin of the Sea of Japan by the southward drift of Japan (Fig. 4). Before the general theory of plate tectonics achieved wide acceptance, some authors inferred the development of the Japan Sea by southward migration of Japan on the basis of morphological analysis of the bottom of the

Unazuki

zone

m&gi”a,

wxHida

....... Fp$$;

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.~~:&g~

zone

8 ,~ ::::.:::.: d &iX.~ - ---___ ‘1, j; .:.:: ,:,,” .: 1, .‘.

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Late Permian (c.240 m.y.) belt intermed.WT metamorphic Mid Paleozoic (400-300 my.) high P/Tmetamorphic belt

Q

Late Cretaceous (i05-75m.y) granites Late Triassic-Early Jurassic (lgO-160 m.y.) granites

PACIFIC Fig. 4. Reconstruction

OCEAN

‘,/ cl

Cretacepus magnetic pole

of Southwest Japan in pre-Sea of Japan time.

329

Sea of Japan and the regional geology around the Sea of Japan (Terada, 1927, 1934; Kobayashi, 1941, 1956; Carey, 1958). After the establishment of the general theory of plate tectonics, the separation of the Japanese Islands from the Asian continent has been advocated by many authors mainly on the basis of geophysical data such as crustal structure, heat flow, the magnetic pattern of the bottom of the Sea of Japan and paleomagnetism around the Sea of Japan (Kropotkin and Shakhvarstova, 1965; Yasui et al., 1968; Bersenev, 1971; Murauchi, 1971; Isezaki and Uyeda, 1973; Segawa and Oshima, 1975; Yaskawa, 1975; Uyeda and Kanamori, 1979). Ichikawa (1972) and Melankholina and Kovylin (1976) demonstrated the similarity of geological formations and structures on different sides of the Sea of Japan. The exact correlation between Japan and the Asian continent, however, has not been elucidated. If the Unazuki zone and the Okcheon zone and/or “Yeoncheon zone” are the same geotectonic unit as discussed above, we have a new definite geologi-al coordinate in reconstructing the Japanese Islands prior to the opening of the Sea of Japan. Figure 4 gives the reconstruction of southwest Japan on the basis of the paleomagnetic study of Yaskawa (1975) and the geological correlation between southwest Japan and the Korean Peninsula discussed in this paper. ACKNOWLEDGEMENTS

I wish to express my sincere thanks to Professor S. Banno of Kyoto University for his critical and enlightening discussions on many aspects of this work. Special thanks are due to Professor A. Miyashiro of the State University of New York at Albany for critically and constructively reviewing the manuscript. Thanks are also extended to Professor D.S. Lee of Yonsei University and Dr. K.C. Na of Chungbuk National University for their useful information on the geology of Korea, and to Professor T. Soma, Dr. S. Maruyama and Mr. K. Matsumoto of Toyama University and Drs. T. Nozawa and K. Shibata of the Geological Survey of Japan for their information on geological and geochronological data of the Unazuki zone and the Hida marginal tectonic zone. I am grateful to Professor H. Onuki of Hirosaki University, Dr. M. Tagiri of Ibaragi University, Professor N. Fuji, Dr. M. Sugimoto and the other earth scientists of Kanazawa University for their encouragement. I express my sincere thanks to Professor S. Uyeda of the University of Tokyo for correcting the manuscript. This study was partly financed by Grant-in-Aid for Scientific Research of Ministry of Education (Nos. 574328,434045). REFERENCES Adachi, M., 1971. Permian intraformational conglomerate at Kamiaso, Gifu Prefecture, central Japan. J. Geol. Sot. Jpn., 77: 471-482. Adachi, M., 1976. Paleogeographic aspects of the Japanese Paleozoic-Mesozoic geosyncline. J. Earth Sci. Nagoya Univ., 23/24: 13-55.

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