The Plate-Tectonics Interpretations of the Japanese Paired Metamorphic

地学雑誌 84, 4 (1974)

The Plate-Tectonics Interpretations of

the Japanese Paired Metamorphic

Belts•\An Armchair Appraisal

Chao-Siang WANG*

ABSTRACT The Japanese Akiyoshi and Sakawa paired metamorphic belts recognized and documented by MIYASHIRO appear most likely apparent pairs consisting of high pressure belts survived from Palaeozoic origin and low pressure belts created due to Mesozoic convergence. In general, the age of regional metamorphism is younger than that of the original sedimentaries and igneous rocks. And in plate tectonics considerations, both members of the metamorphic pair are coeval but their age of formation can't be younger than that of their non-metamorphic equivalent sedimentaries which were formed as an arc-trench-gap or a convergence-relaxation/cessation deposit derived from the same continent. Among the six Japanese stratigraphic- structural zones (Table 1) originated from the Chichibu geosyncline , the non-metamorphic Yamaguchi in the Sangun-Hida pair and Chichibu in the Sambagawa-Ryoke combination are considered as such ;the Sangun and Sambagawa high pressure belts are therefore regarded as being formedin the trench-subduction zones not younger than Permo-Carboniferous. Such an original age of origin for the paired metamorphic belts is also supported from the stratigraphic evidence as well as fromthe radiometric dates. The apparent Mesozoic ages are believed to be resulted from overprinting by thermal events due to Akiyoshi-Sakawa plate convergence with concurrent uplifting and subsidence.

Mainly through the works of KOBAYASHI (1941) and others (KIMURA et al., 1971 TAKAI et al., 1963 ; MATSUMOTO, 1967), the Japanese Islands are known to have developed out of the Palaeozoic-Early Mesozoic Chichibu geosyncline, the Mesozoic-Early Tertiary Shimanto (and Nakamura) geosyncline, and the Neogene geosyncline successively accreted along the Pacific margin. Within the Chichibu geosyncline especially in southwesternHonshu, Shikoku, and Kyushu, prominent stratigraphic-structural zones (Fig. 1) in the order successively oceanwardknown as Hida, Sangun, Yamaguchi, Ryoke, Sambagawa, and Chichibu (Sambosan) have been distinguished (TAKAI et al., 1963, Figs. 1, 2). Among them the Hida-Sangun and the Ryoke-Sambagawa complexes were considered as axial metamorphic zones corresponding to the Akiyoshi and Sakawa orogenies respectively whereas the Yama- guchi was regarded as the non-metamorphic equivalent of the former and the Chichibu of the latter, according to KOBAYASHI and others (Fig. 1).

* Department of Geology , National Taiwan University, Taipei, Taiwan

28 The Plate-Tectonics Interpretations 233

29 284 Chao-Siang WANG

Fig. 1. Map showing the active Japanese arc with inner volcanic belt and outer trench belt superposed on Mesozoic inactive Japanese arc with paired metamorphic belts. Hd : Hida belt, Sg : Sangun belt, M: Maizuru belt, Yg : Yamaguchi belt, R: Ryoke belt, Sm : Sambagawa belt (with Mikabu zone on the southern border), C: Chichibu belt (with Kurosegawa zone close to southern border), Sh : Shimanto belt (including Nakamura zone), K: Kamuikotan belt, Hk : Hidaka belt. 1-1 : Median Tectonic Line, 2-2 : Mikabu Line, 3-3 : Butsuzo Line, 4-4 : Itoigawa- Shizuoka Line (Fossa Magna). (modified after MIYA- SHIRO, 1972 ; MATSUDA and UYEDA, 1970 ; TAKAI et al., 1963)

The recognition by MIYASHIRO (1961) of both the Akiyoshi and Sakawa axial metamorphic zones as comprising a pair of low- and high-pressure metamorphic belts is a significant advancement in Japanese metamorphic tectonic geology within the last decade. MIYASHIRO further correlated the Sakawa metamorphic pair with analogous metamorphic belts in California and other circum-Pacific regions. His attribution of their formation to underthrusting of the ocean floor along the Benioff zone (MIYAsHiRo, 1967) subscribes without reservation to DIETZ's earlier proposal (DIETz, 1963) of ocean floor spreading and collapse of the continental rise as the principal mechanism of the geosynclinal mountain evolution which leads eventually to the introduction of the theory of plate tectonics. However, MIYASHIRO's discus-

30 The Plate-Tectonics Interpretations 235 sion (MIYASHIRO, 1961, 1967, 1972) did not elaborate on the spatial-temporal rela- tions of the young and old pairs during their development out of the Chichibu geosyncline. This left an area with discrepancies of opinions among geologists from the point of view of plate tectonics. MATSUDA and UYEDA (1970) among others are the foremost advocates of paired metamorphic belts as the uniquefeature of the so-called Pacific type orogeny. According to the authors, during such an orogeny, as exemplified by Japan, abelt no less than 1500 km wide is differentially deformed. On the continent side, the inner belt* is block faulted with acid-intermediate volcanism, basin sedimentation, and plutonism accompanied by low pressure metamorphism while on the ocean side, the outer belt** is down sinking with eugeosyncline sedimentation, ophiolitemagma- tism***, and high pressure metamorphism. In between these two belts, there is usually a medial geanticlinal uplift of no magmatic activity but with positive gravity anomalies. Specifically in the case ofthe eastern Japanese active arc system since the Miocene (Fig. 1) : the inner belt is represented by the Japanese volcanic-plutonic arc of the green tuff region includingthe Japan Sea and extending into the Asiatic continent the outer belt is represented by the trench zone of low heat flow and negative gravity anomalies not more than 200 km in width, and the medial geanticline uplift, the non-volcanic Kitakami-Abukuma region. However, MTSUDA and UYEDA's consideration (MATSUDA and UYEDA, 1970, p. 12-13) of the medial geanticline as the non-volcanic arc and consequently the present Japanese arc as a double arc (MIYASHIRO, 1967, p. 438-439) is inappropriate because the uplift's positive gravity anomaly is incompatible with UMBGROVE's original definition (UmBGRovE, 1947). For the Late Mesozoic-Early Tertiary Sakawa pair, MATSUDA and UYEDA's interpretation of the Ryoke belt and area further northwest as the inner volcanic- plutonic are is the same as most authors, but their contention of the Shimanto instead of the Sambagawa belt as the corresponding outer trench zone is very much different from what MIYASHIRO has originally formulated (MIYASHIRO, 1961, 1967). The Sambagawa-Chichibu belt was regarded as the medial geanticline uplift during Sakawa convergence as according to MATSUDA and UYEDA, whereas MIYASHIRO considered it as trench subduction zone. KIMURA and TOKUYAMA (1971) expressed almost the same idea as to the pre- Miocene Japanese arc development during Sakawa Pacific-type orogenesis.To quote their own words (KIMURA et al . , 1971, p. 16-17) : 'when the Shimanto zone was in the eugeosynclinal stage in the Cretaceous time, the Hida and Sangun-Yamaguchi zones were mainly terrains of erosion or terrestrial deposition and in the stage of

* The inner belt in this paper should be tectonically called the external belt . ** The outer belt in this paper shouldbe tectonically called the internal or proximal belt . *** Probably merely detached blocks ofthe consuming oceanic crust .

31 236 Chao-Siang WANG

(superficial) thrusting and folding.., the Ryoke and Sambagawa-Chichibu zones were the terrains of metamorphism, plutonism, and intense folding '. Evidently KIMURA and TOKUYAMA considered all the zones north of Shimanto inclusive as the inner volcanic-plutonic arc while MATSUDA and UYEDA, merely the Ryoke and zones further to the continent side were regarded as such ; both these views are different from MIYASHIRO's original consideration of the Sambagawa zone as the outer trench belt with respect to the coeval Ryoke volcanic-plutonic zone. Besides this disparity in the disposition of the Sakawa pair, there is also no unanimous interpretation regarding them during the older Akiyoshi orogenic cycle. KIMURA and TOKUYAMA considered the Chichibu geosyncline as eugeosyncline from Silurian or Devonian to Permian ; in Permo-Triassic time, when the Sambosan zone was in the eugeosynclinal stage, the Hida and the Sangun-Yamaguchi zones were in the stage of metamorphism, plutonism, and intense folding. ' No mention was made pertaining to the Ryoke and Sambagawa-Chichibu zones as to whether they were under conditions of deformation or continuous sedimentation, although their figure 1 (KIMURA et al . , 1971, p. 10) has shown as implying that these zones were under geosynclinal sedimentation. In such case, KIMURA and TOKUYAMA are not consistent in their own pattern of paired metamorphic-belt tectonics. Because, like their interpretation of the Sakawa pair, the Ryoke and Sambagawa-Chichibu zones immediately adjacent to their Sambosan eugeosyncline zone should also be in a stage of plutonism-volcanism instead of under geosyncline sedimentation during Akiyoshi orogeny. Throughout the whole course of evolution of the Japanese Islands, especially the southwest portion in Mesozoic-Tertiary times, the general trend of progressive accretive formation of continental crust was accomplished by the oceanward migration of geosynclines (KIMURA et al , 1971 ; MATSUMOTO, 1967 ; KOBAYASHI, 1956). Just as what MATSUMOTO (1967, p. 604) has described as when a zone of an older cycle was subjected to intense orogeny, another adjacent zone of younger cycle was generated or in an eugeosynclinal stage of deep subsidence and submarinemagma- tism ', the Chichibu geosyncline was deformed and finally gave way successively to the formation of the Sambosan (Permo-Trias), Shimanto (Mesozoic), Nakamura (Palaeogene), Neogene eugeosynclines and their respective continent-side ororgens ' characterized by plutonism, volcanism,and intense folding (KIMURA et al . , 1971). As a matter of fact, it was employing such a tectonic model in addition to temporal matches of the paired zones that led KIMURA and TOKUYAMA to correlate the Akiyoshi orogen ' of Hida and Sangun-Yamaguchi with the Sambosan eugeosyncline zone, and the Sakawa orogen ' of the Ryoke and Sambagawa-Chichibu with the Shimanto zone. However, MATSUDA and UYEDA (1970) have matched the Ryoke and Shimanto forming their pair with the Sambagawa-Chichibu zone as a

32 The Plate-Tectonics Interpretations 237 medial geanticline uplift of no magmatic activity in between. Nevertheless in either way of accomodating the pair zones, contrary to the MIYASHIRO's original cusposi- tion, the Sambagawa high pressure zonehas to be considered as already existing during the Sakawa deformation, and sois the Sangun as a survival high pressure zone in the Akiyoshi cycle from the earlier convergence.

Such inference of the age discrepancies of the Sambagawa and Sangun high pressure zone from what MIYASHIRO has originally designated found out byregroup- ing of the appropriate paired metamorphic belts pushes really an important step towards understanding of the true nature of the evolution of the paired metamorphics out of the Chichibu geosyncline.Although KIMURA and TOKUYAMA did not specifically and emphatically point out such an age inference, MATSUDA andUYEDA

(1970, p. 12) did definitely claim the80 •~ 106 years Sambagawa K-A age asrepre- senting its time of upheaval instead of its principal time of metamorphism,implying its formation as due to earlier convergence. Actually this age interpretation of the paired metamorphic belts had already been made, for example by MINATO et al.,

(1965). According to these authors, the metamorphic belts are polymetamorphic in origin resulting mainly from the Abean (Variscan) and Honshu (Akiyoshi)tectogenic movements as recorded from stratigraphic evidence as well as fromthe radiometric dating. MINATO et al. (1965, p. 86-89)said : at all events , the age of metamorphism in which a greater part of the Sangun metamorphic complex was generated cannot be viewed to be as young as the intra-Triassic.... The K-A age forthe Hida gneiss complex and the Funatsu granites around 200 to 180 •~ 106 years merely in- dicates the time when these rocks were rapidly upheaved from the depth ... The

Sambagawa complex seems to partly involve the Upper Permian as its original rocks ; accordingly, the final metamorphism must be viewed at least as post-Permian, but perhaps prior to intra-Triassic. The K-A age as young as some 90 •~ 106 years only demonstrates the age of rapid upheaval. In regard to the Ryoke metamorphic complex, there are many difficulties in concluding the age of its metamorphism as late as the Middle or Late Cretaceous... So far as is known , mostpetrologists are now of the opinion that the Ryokemetamorphic complex may possibly involve a certain part of the Upper Permian as their original rocks. Hence, this belt must be concluded to have been formed between the post-Permian and the pre-Norian. ' Age inferences like these agree well with those from plate-tectonics synthesis. It thus appears that both the Sambagawa and Sangun high pressure zones are originally product of the pre-Upper Visean Abean subduction (YAMA-

GUCHI et al., 1970 ; MINATO, 1968) which were first modified by Permo-Triassic

Akiyoshi or Honshu convergence with Sambosan as trench subduction zone and later by the Sakawa convergence (see Table 1) with Shimanto as subduction zone giving rise to their apparent Mesozoic dates. Up to this point the writer

33 238 Chao-Siang WANG should like to quote KOJIMA (1963) from ICHIKAWA (1964, p. 92) concerningthe formation and metamorphism of the Sambagawa schists. He remarked that fundamental structure of the Sambagawacrystalline schist zone is the construction of the zone of a particular type of synclinorium, accompanied by the zonesof flexure and anticlinorium and that the tectonic movement essentially responsible for the formation of the crystalline schist was the downward movement which is mani- fested in the zone of synclinorium. This downward movement was regarded to have set in during the sedimentation of the original geosyncline, probably at the phase of extrusion and intrusion of ophiolitic rocks. Several stages of deformation and mineralization are discriminated in the process of the metamorphism the age of which should not necessarily beconfined to a short span of time. ' He further went on to state that the mainsynclinal body of the original Sambagawa segment (must have been) concealed under the Ryoke belt' (ICHIKAWA, 1964, p. 94) separated by the Median Tectonic Line.Could it be a mere coincidence that in the formation of the Sambagawa, the condition inferred in KOJIMA's particular type of synclinorium ' is so much similar to the plate tectonics subduction zone ? One further point the writer wished to elaborate on is the tectonic setting of the Yamaguchi and the Chichibu slightly or unmetamorphosed belts amidst the paired metamorphics which appears to have been completely overlooked by the Japanese geologists concerned with plate-tectonics interpretions. In the writer's opinion this is a point bearing important clues to the age interpretation of the high pressure zone. As already pointed out in a previous paper by the writer (WANG, 1972 b), no matter whether the high pressure Sangun is exactly paired withlow pressure Hida or the high pressure Sambagawa exactly with the low pressure Ryoke, in Yamaguchi-Sangun and the Sambagawa-Chichibu relation, the unmetamorphosed equivalent, Yamaguchi or Chichibu, appears most likely to be either an arc-trench gap deposit (DICKINSON, 1971) or a deposit left after the convergence diminished or ceased (MITCHELL et al.,1971). In the former case, the age ofthe high pressure metamorphism is coeval with the unmetamorphosed equivalent while in the latter, prior to the unmetamorphosed equivalent. At all events, in either case the original age of metamorphism of the high pressure zones is compatible with the age inferred from the above consideration : i.e. Upper Palaeozoic for both pairs. From what has been discussed, the Japanese paired metamorphic belts advocat- ed and so well described by MIYASHIRO (1961, 1967, 1972) appear thus most likely to be apparent pairs consisting of high pressure belts survived from Palaeozoic origin and low pressure belts created due to Mesozoic convergence. However, the physicochemical condition for the formation of the paired metamorphics are so-well established, no matter how the pairs should have been combined, it does notdeprive

34 The Plate-Tectonics Interpretations 239 the status of the high pressure zone as being formed in a trench subduction zone. During the Sambagawa metamorphism, subduction is inferred to have taken place. A recent report (ERNST, 1972) of the occurrence of Permo-Carboniferous ocean crustal fragments represented by the Mikabu Greenstones along the southern border of Sambagawa further supports this interpretation. While much still remains to be clarified before a detailed scheme of development of the Chichibu geosyncline can be established, the broad pattern as outlined by ICHIKAWA et al. (1972) seems to be compatible with the above-mentioned established fact relevant to the development of the paired metamorphics. In a mode much like the writer (WANG, 1972 a) advanced, ICHIKAWA et al. postulated that the Chichibu geosyncline first experienced a period of stretching, then followed by a long-continued stage of compression still without any sign of ending up to the present. As ICHIKAWA et al. (1972) have written, in the early Palaeotectonic stage (Silurian to Mid-Devonian), the Chichibu geosyncline was a ridge-trough complex with ridges of pre-Silurian basement like the Maizuru, Palaeo-Ryoke, and Kurosegawa separating in many cases ocean deep troughs like the Chugoku, Tamba, Sambagawa, Chichibu, and open ocean (outside of the Kurosegawa Ridge). During the late Palaeotectonic stage, the mode of tectonism changed from stretching to compression ; various paroxysms were then felt in the Chichibu geosyncline through the Abean and culminated in the Honshu orogenetic movements. In this later course of development, the writer believes that episodic instead of continuous plate convergence was due to the brake effect of the ridges the most prominent feature recognizable. As a result, progressively younger suites of high pressure subduction metamorphics like Sangun and non-metamorphosed brake sediments, Yamaguchi, and of Sambagawa-Chichibu alike, were being formed. It is hoped the appraisal might have thrown some light on the problems.

Aknowledgements

The writer acknowledges his sincere thanks to Professor Tatsuro MATSUMOTOof Kyushu University and Professor Hakuyu OKADAof Kagoshima University for their beneficial discus- sions.

Rceferences Cited DICKINSON,W. R. (1971) : Clastic sedimentary sequences deposited in shelf, slope, and trough settings between magmatic arcs and associated trenches. Pacific Geology, Vol. 3, pp. 15-30. DIETZ, R. S. (1963): Collapsing continental rises : an actualistic concept of geosynclines and mountain building. Jour. Geol., Vol. 71, pp. 314-333. ERNST, W. G. (1972): Possible Permian oceanic crust and plate junction in central Shikoku, Japan. Tectonophysics, Vol. 15, p. 233. ICHIKAWA,K. (1964): Tectonic status of the Honshu major belt in SW.Japan during

35 240 Chao-Siang WANG

the early Mesozoic. Jour. Geosciences, Osaka City Univ., Vol. 8, Art. 3, pp. 71-107. ICHIKAWA, K., MATSUMOTO, T. and IWASAKI, M. (1972) : Origin of Japanese Islands (in Japanese). Kagaku, Vol. 42, pp. 181-191. KIMURA, T. and TOKUYAMA, A. (1971) :Geosynclinal prisms and tectonics in Japan. Mem. Geol. Soc. Japan, No. 6, pp. 9-20. KOBAYASHI, T. (1941) : The Sakawa orogenic cycles and its bearing on the origin of the Japanese Islands. Jour. Fac. Sci., Imp. Univ. Tokyo, Vol. 5, pp. 219-578. KOBAYASHI, T. (1956) : The shifting of the chert-bearing facies caused by the migration of geosyncline. Gedekbock H. A. Brouwer Verhand, van het Konink, Nederland, Geol. Mijnb. Genootschap. deel 16, pp. 1-11. KOJIMA, G. (1963) : On the fundamental structure of the Sambagawa crystalline schist zone (in Japanese with English abstract). Geol. Rep. Hiroshima Univ., Vol. 12, pp. 173-182. MATSUDA, T. and UYEDA, S. (1970) : On the Pacific-type orogeny and its model exten- sion of the paired belts concept and possible origin of marginal seas. Tectonophysics, Vol. 11, pp. 5-27. MATSUMOTO, T. (1967) : Fundamental problems in the Circum-Pacific orogenesis. Tectonophysics, Vol. 4, pp. 595-613. MINATO, M., GORAI, M. and HUNAHASHI, M. (eds.) (1965) : The geologic development of the Japanese Islands. Tsukiji Shokan Co., Ltd., Tokyo, 442 p. MINATO, M. (1968) : Basement complex and Palaeozoic orogeny in Japan. Pacific Geology, No. 1, pp. 85-95. MITCHELL, A. and READING, H. G. (1971) : Continental margins, geosynclines, and ocean floor spreading. Jour. Geol., Vol. 77, pp. 629-646. MIYASHIRO, A. (1961) : Evolution of metamorphic belts. Petrology, Vol. 2, pp. 277-311. MIYASHIRO, A. (1967) : Orogeny, regional metamorphism, and magmatism in the Jap- anese Islands. Dansk Geol. For. Medd., Vol. 17, pp. 390-446. MIYASHIRO, A. (1972) : Metamorphism and related magmatism in plate tectonics . Am. Jour. Science, Vol. 272, pp. 629-656. TAKAI, F., MATSUMOTO, T. and TORIYAMA, R (eds.) (1963) : Geology of Japan. Univ. of Tokyo Press, Tokyo, 279 p. UMBGROVE, J. H. F. (1947) : The pulse of the earth. Martinus Nijhoff, The Hague, 358 p. WANG, C. S. (1972a) : Geosynclines in the new global tectonics. Bull. Geol. Soc. Amer., Vol. 83, pp. 2105-2110. WANG, C. S. (1972b) : Structural evolution of the Chinese continental margin and oil prospect of the Yellow Sea and East China Sea. Petroleum Geol. Taiwan, No.10, pp. 11-33. YAMAGUCHI, M. and YANAGI, T. (1970) : Geochronology of some metamorphic rocks in Japan. Eclogae Geol. Helv., Vol. 63, pp. 371-388.

36 The Plate-Tectonics Interpretations 241

太師椅上談日本成双変成帯

王超翔

要旨都城によつて明確にされた秋吉・佐川成双変成帯は古生界起源の高圧型変成帯と中生代圧縮運動で生成した低圧型変成帯とが見掛上の対をなしている。一般に広域変成作用の時期は原堆積岩・火成岩の時代よりは新しい。プレートテクトニクス概念によると,成双変成帯の両構成員は同時成を意味するが,その生成時期は火山弧一海溝間隙(arc-trenchgap)堆積物ないしは圧縮運動休止期堆積物として形成された非変成層より若くはならない。すなわち,秩父地向斜から発達した6構造帯(第1表)のうち,三郡― 飛騨成双帯における非変成山口相と三波川― 領家帯における秩父相がそれに当たる.したがつて,三郡と三波川高圧変成帯は石炭～ペルム紀より若くはない海溝沈み込帯に形成されたとみられる。かかる成双変成帯源岩の時代はまた層位的にも放射年代的にも支持される。見掛上の中生代という時代は秋吉一佐川プレート圧縮運動による熱事件の重複を意味する。 (1974年5月28日受理)