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 sedimenta- ries 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-meta- morphic 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 metamor- phic 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 cor- responding 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 metamor- phic belts. Hd : Hida belt, Sg : Sangun belt, M: Maizuru belt, Yg : Yamaguchi belt, R: Ryoke belt, Sm : Sambagawa belt (with Mikabu zone on the sou- thern 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 gean- ticline uplift, the non-volcanic Kitakami-Abukuma region. However, MTSUDA and UYEDA's consideration (MATSUDA and UYEDA, 1970, p. 12-13) of the medial gean- ticline 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 in- terpretation 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 migra- tion 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