Koen De Jong

Chikao Kurimoto

Gilles Ruffet

Triassic 40 Ar/ 39 Ar ages from the Sakaigawa unit, Kii Peninsula, Japan: implications for possible merger of the Central Asian Orogenic Belt with large-scale tectonic systems of the East Asian margin

Keywords:

The 218.4 ± 0.4, 228.8 ± 0.9 and 231.9 ± 0.7 Ma 40 Ar/ 39 Ar laser probe pseudoplateau ages (2σ; 49-63% 39 Ar-release) of very low-grade meta-pelitic whole-rocks from the Sakaigawa unit date high-P/T metamorphism. We argue that this event occurred in a subduction-accretion complex, not along the East Asian continental margin, but on the Pacific side of the proto-Japan superterrane. Proto-Japan was a Permian magmatic arc, presently dispersed in the Japanese islands, which also contained older subduction-accretion complexes. The arc system was fringing but not yet part of the Eurasian continent. The Middle to Late Triassic high-P/T tectono-metamorphic event was partly coeval with proto-Japan's collision with proto-Eurasia along the southward extension of the Central Asian Orogenic Belt, causing the main metamorphism in the Hida-Oki terrane. It is possible that this system continued via the Cathaysia block (China) to Indochina. The Late Permian to Middle Triassic Indosinian event might stem from docking of Pacific-derived terranes with Southeast Asia's continental margin. The concept of the proto-Japan superterrane implies that the Qinling-Dabie-Sulu suture zone joined the Central Asian Orogenic Belt to the east of the North China craton and did not continue to Japan, as commonly assumed.

Introduction

Eurasia is a composite of many small continents separated by broad belts of Palaeozoic, Mesozoic and Early Cainozoic magmatic, deformed, and in part metamorphosed rocks [START_REF] Maruyama | Orogeny and relative plate motions: Example of the Japanese Islands[END_REF][START_REF] Zonenshain | Geology of the USSR: a Plate Tectonic Synthesis[END_REF]Şengör and Natal'in 1996;[START_REF] Maruyama | Pacific-type orogeny revisited: Miyashiro-type orogeny proposed[END_REF][START_REF] Wakita | Ocean plate stratigraphy in East and Southeast Asia[END_REF][START_REF] Cocks | Siberia, the wandering northern terrane, and its changing geography through the Palaeozoic[END_REF]. Siberia, Tarim, North and South China, as well as Indochina are such continental pieces, and the Central Asian Orogenic Belt is one of the principal large-scale tectonic systems of the composite continent (Figs. [START_REF] Masago | Exhumation of the Sanbagawa high-pressure metamorphic belt, SW Japan-constraints from the upper and lower boundary faults[END_REF]2). Japan separated from mainland Asia by back-arc spreading in the Middle Miocene (see [START_REF] Kaneoka | Isotope geochemistry of igneous rocks in the Japan Sea[END_REF]van der Werff 2000, for reviews). Because they are the nearest to Asia's mainland, medium and high-grade metamorphic rocks and granitoids of central Japan and on the island of Oki-Dogo, which is part of the northern continental margin of Honshu in the Sea of Japan (Fig. 3), have been viewed as its reworked Precambrian crust [START_REF] Faure | Late Permian/early Triassic orogeny in Japan: piling up of nappes, transverse lineation and continental subduction of the Honshu block[END_REF][START_REF] Charvet | Accretion and collision during East-Asiatic margin building-a new insight on the peri-Pacific orogenies[END_REF][START_REF] Banno | Metamorphic belts of Japanse Islands[END_REF]. These constitute the Hida-Oki terrane. Parts of the submerged continental crust of the Yamato Bank in the Sea of Japan (Fig. 2) have been compared to this terrane [START_REF] Kaneoka | Isotope geochemistry of igneous rocks in the Japan Sea[END_REF]. Commonly, the metamorphic rocks of the main Hida belt of central Japan are regarded as belonging to the North China craton, and the Oki metamorphics as part of the South China craton [START_REF] Sohma | Hida metamorphic belt[END_REF][START_REF] Banno | Metamorphic belts of Japanse Islands[END_REF]Isozaki 1997a;[START_REF] Maruyama | Paleogeographic maps of the Japanese Islands: plate tectonic synthesis from 750 Ma to the present[END_REF][START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF][START_REF] Ernst | Permo-Triassic collision, subduction-zone metamorphism, and tectonic exhumation along the East Asian continental margin[END_REF]. Consequently, the Middle-Late Triassic Qinling-Dabie-Sulu suture between both the cratons was considered to continue into the Japanese region [START_REF] Maruyama | Orogeny and relative plate motions: Example of the Japanese Islands[END_REF]Isozaki 1997a;[START_REF] Maruyama | Pacific-type orogeny revisited: Miyashiro-type orogeny proposed[END_REF][START_REF] Maruyama | Paleogeographic maps of the Japanese Islands: plate tectonic synthesis from 750 Ma to the present[END_REF][START_REF] Ernst | Permo-Triassic collision, subduction-zone metamorphism, and tectonic exhumation along the East Asian continental margin[END_REF]. Accordingly, various efforts have been made to correlate rocks, structures and belts in both cratons with elements of the Japanese islands [START_REF] Suzuki | 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[END_REF][START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF]; [START_REF] Ishiwatari | Paleozoic ophiolites and blueschists in Japan and Russian Primorye in the tectonic framework of East Asia: a synthesis[END_REF][START_REF] Oh | A new concept on tectonic correlation between Korea, China and Japan: Histories from the late Proterozoic to Cretaceous[END_REF][START_REF] Osanai | The Higo metamorphic complex in Kyushu, Japan as the fragment of Permo-Triassic metamorphic complexes in East Asia[END_REF][START_REF] Tsujimori | Triassic paragonite-and garnet-bearing epidote-amphibolite from the Hida Mountains, Japan[END_REF][START_REF] Ernst | Permo-Triassic collision, subduction-zone metamorphism, and tectonic exhumation along the East Asian continental margin[END_REF][START_REF] Oh | The Late Permian to Triassic Hongseong-Odesan collision belt in South Korea, and its tectonic correlation with China and Japan[END_REF]. Recent studies have suggested that the Qinling-Dabie-Sulu suture could continue to the Permo-Triassic Hida-Oki terrane [START_REF] Oh | A new concept on tectonic correlation between Korea, China and Japan: Histories from the late Proterozoic to Cretaceous[END_REF], or to the Higo metamorphic complex (Fig. 3), which is correlated with this terrane [START_REF] Osanai | The Higo metamorphic complex in Kyushu, Japan as the fragment of Permo-Triassic metamorphic complexes in East Asia[END_REF][START_REF] Oh | The Late Permian to Triassic Hongseong-Odesan collision belt in South Korea, and its tectonic correlation with China and Japan[END_REF]. The suture belt would thus wrap around the eastern margin of the North China craton [START_REF] Oh | A new concept on tectonic correlation between Korea, China and Japan: Histories from the late Proterozoic to Cretaceous[END_REF][START_REF] Ernst | Permo-Triassic collision, subduction-zone metamorphism, and tectonic exhumation along the East Asian continental margin[END_REF][START_REF] Oh | The Late Permian to Triassic Hongseong-Odesan collision belt in South Korea, and its tectonic correlation with China and Japan[END_REF]. However, the correlation of the suture with the Higo metamorphic complex is compromised by the results of SHRIMP dating of zircons from metamorphic and associated magmatic rocks by [START_REF] Ruffet | Large-scale displacement along the Median Tectonic Line, Japan: evidence from SHRIMP zircon U-Pb dating of granites and gneisses from the South Kitakami and paleo-Ryoke belts[END_REF]. The latter authors concluded that protoliths of a high-grade paragneiss in the Higo complex were in fact early Mesozoic sediments that were metamorphosed in Early Cretaceous time, as will be discussed later on in our paper Isozaki (1997a) and [START_REF] Maruyama | Pacific-type orogeny revisited: Miyashiro-type orogeny proposed[END_REF] argued that erosion of the Qinling-Dabie-Sulu belt would have caused a huge sedimentary discharge into the western part of the Palaeo-Pacific ocean, nourishing the turbidite sandstones and intercalated conglomerates in trench fill deposits in the Jurassic subduction-accretion complexes with detritus of metamorphic and granitic rocks. Yet, equivalents of the sedimentary basins that record the exhumation and deep erosion of the suture zone in China, like the thick synorogenic Triassic-Jurassic foreland sedimentary series surrounding the Dabieshan (e.g. [START_REF] Grimmer | When did the ultrahigh-pressure rocks reach the surface? A 207 Pb/ 206 Pb zircon, 40 Ar/ 39 Ar white mica, Si-in-white mica, single-grain provenance study of Dabie Shan synorogenic foreland sediments[END_REF][START_REF] Li | Jurassic depositional records and sandstone provenances in Hefei Basin, central China: implication for Dabie orogenesis[END_REF]), seem to be lacking in Japan. This makes an extension of this suture to Japan unlikely.

The Hida-Oki terrane's range of isotopic ages is virtually identical to the 245-225 Ma age assigned to the ultrahigh-pressure metamorphism in the Qinling-Dabie-Sulu suture (see [START_REF] Hacker | Subduction, collision, and exhumation in the Qinling-Dabie Orogen[END_REF][START_REF] Ernst | Permo-Triassic collision, subduction-zone metamorphism, and tectonic exhumation along the East Asian continental margin[END_REF] for reviews). However, the east-and southward extension of this suture to Korea is still controversial and a number of highly different tectonic scenarios have been proposed [START_REF] Ishiwatari | Paleozoic ophiolites and blueschists in Japan and Russian Primorye in the tectonic framework of East Asia: a synthesis[END_REF][START_REF] Oh | A new concept on tectonic correlation between Korea, China and Japan: Histories from the late Proterozoic to Cretaceous[END_REF][START_REF] Osanai | The Higo metamorphic complex in Kyushu, Japan as the fragment of Permo-Triassic metamorphic complexes in East Asia[END_REF][START_REF] Tsujimori | Triassic paragonite-and garnet-bearing epidote-amphibolite from the Hida Mountains, Japan[END_REF][START_REF] Ernst | Permo-Triassic collision, subduction-zone metamorphism, and tectonic exhumation along the East Asian continental margin[END_REF][START_REF] Oh | The Late Permian to Triassic Hongseong-Odesan collision belt in South Korea, and its tectonic correlation with China and Japan[END_REF]. But the widespread occurrence of 290-240 Ma isotopic ages in the polymetamorphosed Gyeonggi granulitic gneiss terrane and the Okcheon and Imjingang metamorphic belts along its northern and southern margins, points to a major tectono-metamorphic event in the Permian-Triassic period, such as a terrane collision (see reviews by [START_REF] Ernst | Permo-Triassic collision, subduction-zone metamorphism, and tectonic exhumation along the East Asian continental margin[END_REF][START_REF] Oh | A new concept on tectonic correlation between Korea, China and Japan: Histories from the late Proterozoic to Cretaceous[END_REF][START_REF] Oh | The Late Permian to Triassic Hongseong-Odesan collision belt in South Korea, and its tectonic correlation with China and Japan[END_REF]. Yet, Late Permian to Early Triassic events are not limited to the collision zone between the North and South China cratons, but also affect the East Asian margin farther to the south. A wide area of the southeastern part of South China craton is affected by magmatism, metamorphism and deformation in this period (e.g. [START_REF] Faure | Doming in the southern foreland of the Dabieshan (Yangtze block, China)[END_REF][START_REF] Li | Tectonic history of the major East Asian lithospheric blocks since the mid-Proterozoic-a synthesis[END_REF][START_REF] Xiao | Early Mesozoic thrust tectonics of the NW Zhejiang region (Southeast China)[END_REF][START_REF] Li | Formation of the 1300-km-wide intracontinental orogen and postorogenic magmatic province in Mesozoic South China: a flat-slab subduction model[END_REF]. Also in Indochina magmatism, (ultra) high-temperature metamorphism and ductile deformation occurred in the 260-240 Ma time span [START_REF] Lepvrier | The Early Triassic Indosinian orogeny in Vietnam (Truong Son Belt and Kontum Massif); implications for the geodynamic evolution of Indochina[END_REF]).

In the current contribution, we present 218-232 Ma 40 Ar/ 39 Ar laser probe ages of very lowgrade metapelite whole-rocks from the Sakaigawa subduction-accretion complex in the western Kii Peninsula, Japan (Figs. 3,4). We explore the meaning of these late Middle to early Late Triassic dates in the context of the possible interactions of orogenic belts in Central Asia with the palaeo-Pacific subduction-accretion systems of East and Southeast Asia, as well as the role of island arcs and micro-continents along this margin, like the proto-Japan superterrane that formed a Permian arc system.

Eastward extension of the Central Asian Orogenic Belt

The Central Asian Orogenic Belt is situated between the Siberian craton to the north and Tarim and North China cratons to the south (Figs. [START_REF] Masago | Exhumation of the Sanbagawa high-pressure metamorphic belt, SW Japan-constraints from the upper and lower boundary faults[END_REF]2). The system extends from the Urals in the west to Sikhote-Alin' in the Russian Far East, where it is truncated by subductionaccretion complexes and continental margin arc systems associated with subduction of Pacific oceanic lithosphere in Mesozoic time (Fig. 1; [START_REF] Kojima | A correlation of accretionary complexes of southern Sikhote-Alin of Russia and the Inner Zone of Southwest Japan[END_REF][START_REF] Nokleberg | Phanerozoic tectonic evolution of the Circum-North Pacific[END_REF][START_REF] Nokleberg | Digital files for Northeast Asia geodynamics, mineral deposit location, and metallogenic belt maps, stratigraphic columns, descriptions of map units, and descriptions of metallogenic belts[END_REF][START_REF] Ernst | Permo-Triassic collision, subduction-zone metamorphism, and tectonic exhumation along the East Asian continental margin[END_REF]. The vast Central Asian orogenic system formed in the Palaeozoic in a southwestern Pacific-like setting, by prolonged accretion of oceanic plate sediments, oceanic crust, including oceanic islands, forearc and backarc basins, and magmatic arcs, as well as by amalgamation of terranes including Gondwana-and Siberia-derived micro-continents with their passive margins [START_REF] De Jong | Ordovician 40 Ar/ 39 Ar phengite ages from the blueschist-facies Ondor Sum subduction-accretion complex (Inner Mongolia) and implications for the early Paleozoic history of continental blocks in China and adjacent areas[END_REF][START_REF] Windley | Tectonic models for accretion of the Central Asian Orogenic Belt[END_REF]andreferences in de Jong et al. 2008 andXiao et al. 2008, this issue). Plate convergence was accompanied by intense magmatic and volcanic activity in arc systems [START_REF] Jahn | The Central Asian Orogenic Belt and growth of the continental crust in the Phanerozoic[END_REF]. Final oceanic closure resulted in the formation of the Solonker suture zone. This foremost structure can be traced all along the Central Asian Orogenic Belt from Kyrgyzstan in the west, passing to the north of the Tarim (Xiao et al. 2008, this issue) and North China [START_REF] Windley | Tectonic models for accretion of the Central Asian Orogenic Belt[END_REF]) cratons, to northernmost North Korea in the east (Figs. [START_REF] Masago | Exhumation of the Sanbagawa high-pressure metamorphic belt, SW Japan-constraints from the upper and lower boundary faults[END_REF]2). Some terranes in northeastern China and Far East Russia belonged to active continental margins in Permian to early Triassic time [START_REF] Nokleberg | Phanerozoic tectonic evolution of the Circum-North Pacific[END_REF][START_REF] Nokleberg | Digital files for Northeast Asia geodynamics, mineral deposit location, and metallogenic belt maps, stratigraphic columns, descriptions of map units, and descriptions of metallogenic belts[END_REF][START_REF] Jia | Collision belt between the Khanka block and the North China block in the Yanbian Region, Northeast China[END_REF]. At the end of the Palaeozoic, magmatic arcs accreted to the Khanka superterrane [START_REF] Nokleberg | Digital files for Northeast Asia geodynamics, mineral deposit location, and metallogenic belt maps, stratigraphic columns, descriptions of map units, and descriptions of metallogenic belts[END_REF]) that collided with the North China craton, probably in the Early Triassic [START_REF] Zonenshain | Geology of the USSR: a Plate Tectonic Synthesis[END_REF][START_REF] Wu | Geochronology and petrogenesis of the post-orogenic Cu-Ni sulfide-bearing mafic-ultramafic complexes in Jilin Province, NE China[END_REF]. Arc plutons of Late Permian to Early Triassic age and a subduction-accretion complex occur in the Jilin area of northeastern China [START_REF] Li | Permian geodynamic setting of Northeast China and adjacent regions: closure of the Paleo-Asian Ocean and subduction of the Paleo-Pacific Plate[END_REF][START_REF] Lin | Permian-Triassic amalgamation of Asia: insights from Northeast China sutures and their place in the final collision of North China and Siberia[END_REF]. The Chongjin subduction-accretion complex in northernmost North Korea contains late Palaeozoic ophiolites, chert and limestone [START_REF] Nokleberg | Phanerozoic tectonic evolution of the Circum-North Pacific[END_REF]. These observations imply that the Solonker suture zone continued to the coastal area of the Sea of Japan (Figs. 1, 2;[START_REF] De Jong | Ordovician 40 Ar/ 39 Ar phengite ages from the blueschist-facies Ondor Sum subduction-accretion complex (Inner Mongolia) and implications for the early Paleozoic history of continental blocks in China and adjacent areas[END_REF]. The South Kitakami terrane of northeast Japan (Fig. 3) is classically regarded as part of the continental margin of the South China ctaton (Yangtze shelf; Isozaki 1997a). But South Kitakami's early Palaeozoic series have also been regarded as having been associated to the Khanka superterrane (Şengör and Natal'in 1996;[START_REF] Kojima | A correlation of accretionary complexes of southern Sikhote-Alin of Russia and the Inner Zone of Southwest Japan[END_REF][START_REF] Tazawa | Late Paleozoic brachiopod faunas of the South Kitakami Belt, northeast Japan, and their paleobiogeographic and tectonic implications[END_REF]. On the basis of such evidence, the Permian island arc system of the proto-Japan superterrane has been considered as the eastern extension of the Central Asian Orogenic Belt [START_REF] De Jong | Ordovician 40 Ar/ 39 Ar phengite ages from the blueschist-facies Ondor Sum subduction-accretion complex (Inner Mongolia) and implications for the early Paleozoic history of continental blocks in China and adjacent areas[END_REF]. A number of authors explored the possibility that the Central Asian Orogenic Belt continues southeastward from mainland Asia into the 250-235 Ma-old, mainly metasedimentary Hida-Oki terrane in Japan (Fig. 1; [START_REF] Arakawa | Crustal development of the Hida belt, Japan: evidence from Nd-Sr isotopic and chemical characteristics of igneous and metamorphic rocks[END_REF][START_REF] Jahn | Granitoids of the Central Asian Orogenic Belt and continental growth in the Phanerozoic[END_REF][START_REF] De Jong | Ordovician 40 Ar/ 39 Ar phengite ages from the blueschist-facies Ondor Sum subduction-accretion complex (Inner Mongolia) and implications for the early Paleozoic history of continental blocks in China and adjacent areas[END_REF]. Indeed, metamorphic rocks and synorogenic granites from the easternmost Central Asian Orogenic Belt in northeastern China yielded ca. 240 Ma U-Pb zircon dates [START_REF] Wu | Geochronology and petrogenesis of the post-orogenic Cu-Ni sulfide-bearing mafic-ultramafic complexes in Jilin Province, NE China[END_REF]) and ca. 225 Ma 40 Ar/ 39 Ar mica ages [START_REF] Xi | Isotopic chronology of the Hulan group and its geological significance in the central Jilin Province[END_REF].

In the models that advocate that the Qinling-Dabie-Sulu continued to Japan, it is far from clear what is actually colliding with what, as two essential elements are lacking in such models. Firstly, the Permo-Triassic subduction-accretion complexes in Japan stretch all the way to the southernmost islands of the Ryukyu arc (Yaeyama promontory; [START_REF] Ishiwatari | Paleozoic ophiolites and blueschists in Japan and Russian Primorye in the tectonic framework of East Asia: a synthesis[END_REF]. Secondly, the Japanese islands contain a number of terranes with fragments of a dispersed Permian magmatic arc system with a basement of Early Palaeozoic igneous and metasedimentary rocks, which have been grouped into the proto-Japan superterrane [START_REF] De Jong | Ordovician 40 Ar/ 39 Ar phengite ages from the blueschist-facies Ondor Sum subduction-accretion complex (Inner Mongolia) and implications for the early Paleozoic history of continental blocks in China and adjacent areas[END_REF].

Regional tectonic framework of the Japanese islands

The late Palaeozoic to early Miocene tectonic evolution of the Japanese archipelago has been explained in terms of a sequence of collisions of micro-continents with the East Asian margin and the closure of intervening relatively small oceanic basins [START_REF] Faure | The late Jurassic oblique collisional orogen of SW Japan. New structural data and synthesis[END_REF][START_REF] Faure | Late Permian/early Triassic orogeny in Japan: piling up of nappes, transverse lineation and continental subduction of the Honshu block[END_REF][START_REF] Charvet | Accretion and collision during East-Asiatic margin building-a new insight on the peri-Pacific orogenies[END_REF]. Usually, however, with the exception of the elements of the proto-Japan superterrane, most terranes oceanward of the Hida-Oki terrane, are interpreted as subduction-accretion complexes formed due to the subduction of lithosphere of different oceanic basins below the proto-Asian continent since the early Palaeozoic (Isozaki 1997a(Isozaki , 1997b;;[START_REF] Maruyama | Paleogeographic maps of the Japanese Islands: plate tectonic synthesis from 750 Ma to the present[END_REF][START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF][START_REF] Ernst | Permo-Triassic collision, subduction-zone metamorphism, and tectonic exhumation along the East Asian continental margin[END_REF]. Such complexes are composed of trench fill or fore-arc sediments and oceanic components, like chert, (pillow) basalt and limestone, that is, former reef caps of seamounts (Isozaki 1997a, b;[START_REF] Maruyama | Pacific-type orogeny revisited: Miyashiro-type orogeny proposed[END_REF][START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF][START_REF] Wakita | Ocean plate stratigraphy in East and Southeast Asia[END_REF]. Especially during the Jurassic to Early Cretaceous, extensive accretion took place including the Mino-Tamba-Ishio terrane, the Northern Chichibu terrane, the North Kitakami terrane, as well as the protoliths of the Mikabu and Sambagawa belts (Isozaki 1997a, b;[START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF][START_REF] Suzuki | Paleoceanographic affinities of radiolarian faunas in late Aalenian time (Middle Jurassic) recorded in the Jurassic accretionary complex of Japan[END_REF][START_REF] Wakita | Ocean plate stratigraphy in East and Southeast Asia[END_REF]. The Japanese Jurassic to Early Cretaceous subduction-accretion complexes form part of a huge system bordering the eastern Eurasian continental margin from Far East Russia to southwest Borneo [START_REF] Hamilton | Tectonics of the Indonesian region[END_REF][START_REF] Faure | The geodynamic evolution of the eastern Eurasian margin in Mesozoic times[END_REF]Zamoras andMatsuoka 2001, 2004;[START_REF] Wakita | Ocean plate stratigraphy in East and Southeast Asia[END_REF]. This was accompanied by the development of a vast, principally Cretaceous magmatic arc [START_REF] Maruyama | Paleogeographic maps of the Japanese Islands: plate tectonic synthesis from 750 Ma to the present[END_REF][START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF][START_REF] Takagi | Origin of magnetite-and ilmenite-series granitic rocks in the Japan arc[END_REF][START_REF] Nakajima | Mafic rocks from the Ryoke Belt, southwest Japan: implications for Cretaceous Ryoke/San-yo granitic magma genesis[END_REF]) that continues via east China [START_REF] Jahn | Rb-Sr ages of granitic rocks in southeastern China and their tectonic significance[END_REF] to Indochina and southwest Borneo [START_REF] Hamilton | Tectonics of the Indonesian region[END_REF]. A peculiar feature of the Japanese subduction-accretion complexes is the abundance of granitic and metamorphic detritus in sandstones and the rare occurrence of volcanic detritus (greywackes) that is so characteristic for other subduction-accretion complexes [START_REF] Suzuki | 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[END_REF][START_REF] Takeuchi | Changes in garnet chemistry show a progressive denudation of the source areas for Permian-Jurassic sandstones, Southern Kitakami Terrane, Japan[END_REF]). Jurassic turbidite sandstones of the Mino-Tamba-Ashio terrane contain detrital metamorphic minerals (like pyrope-almandine rich garnet, chloritoid and sillimanite), rare chloritoid-bearing phyllite and sillimanite gneiss fragments and intercalated polymict conglomerate lenses with pebbles of garnet-sillimanite-biotite gneiss and granitoids [START_REF] Adachi | Precambrian detrital monazites and zircons from Jurassic turbidite sandstones in the Nomugi area, Mino terrane[END_REF][START_REF] Tanaka | Occurrence of gneiss clasts with Mg-rich garnets from the Sawando conglomerate in the northeastern Mino terrane: source of detrital Mg-rich garnets in the Jurassic sandstone of the Mino terrane[END_REF][START_REF] Sano | Ion microprobe U-Pb zircon geochronology of the Hida gneiss. Finding of the oldest minerals in Japan[END_REF][START_REF] Nutman | 743 ± 17 Ma granite clast from Jurassic conglomerate, Kamiaso, Mino Terrane, Japan: the case for South China Craton provenance (Korean Gyeonggi Block?)[END_REF]. Whole-rock Rb-Sr dating [START_REF] Shibata | Rb-Sr whole-rock ages of Precambrian metamorphic rocks in the Kamiaso conglomerate from Central Japan[END_REF]) and mineral dating using the Chemical Th-U-total Pb Isochron Method (CHIME) on monazite [START_REF] Adachi | Precambrian detrital monazites and zircons from Jurassic turbidite sandstones in the Nomugi area, Mino terrane[END_REF]) and a sensitive high-resolution ion microprobe (SHRIMP) on zircon [START_REF] Sano | Ion microprobe U-Pb zircon geochronology of the Hida gneiss. Finding of the oldest minerals in Japan[END_REF][START_REF] Nutman | 743 ± 17 Ma granite clast from Jurassic conglomerate, Kamiaso, Mino Terrane, Japan: the case for South China Craton provenance (Korean Gyeonggi Block?)[END_REF] revealed that the majority of these pebbles are derived from Palaeo-and Mesoproterozoic gneisses with minor contributions of 250 and 180 Ma-old rocks. U-Pb isotope spot analysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) of igneous zircons from psammitic schist of the Jurassic complex that has undergone Cretaceous high-pressure metamorphism (Sambagawa belt, see below) yielded abundant Palaeoproterozoic ages in cores of crystals [START_REF] Okamoto | SHRIMP U-Pb zircon dating of quartz-bearing eclogite from the Sanbagawa Belt, south-west Japan: implications for metamorphic evolution of subducted protolith[END_REF]Aoki et al. 2007).

From a tectonic point of view, southwest Japan is generally subdivided into an Inner Zone (Asian Continent side) and an Outer Zone (Pacific Ocean side) (Fig. 3; Table 1) that are separated by the Median Tectonic Line (MTL). The MTL is regarded as a major left-lateral wrench fault zone that is associated with Late Cretaceous pull-apart basins with exceptionally thick turbiditic, shallow-marine, deltaic and continental sediments with abundant volcanic rocks [START_REF] Teraoka | Cretaceous sedimentary basins in the Ryoke and Sambagawa Belts[END_REF][START_REF] Whitaker | Cretaceous-Paleogene geology of Southwest Japan[END_REF][START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF]. However, deep seismic reflection data show that the fault zone is moderately dipping below the Inner zone [START_REF] Ito | Gently north-dipping Median Tectonic Line (MTL) revealed by recent seismic reflection studies, southwest Japan[END_REF][START_REF] Kawamura | Deep seismic reflection experiment using a dense receiver and sparse shot technique for imaging the deep structure of the Median Tectonic Line (MTL) in east Shikoku, Japan[END_REF]. The Outer and Inner zones have a similar general structure: a pile of flat-lying tectonic units that become progressively younger structurally downwards. In contrast to the Outer Zone, the Inner Zone contains Cretaceous to Palaeogene subductionrelated, mainly granitic volcanic-plutonic complexes [START_REF] Takagi | Origin of magnetite-and ilmenite-series granitic rocks in the Japan arc[END_REF][START_REF] Nakajima | Mafic rocks from the Ryoke Belt, southwest Japan: implications for Cretaceous Ryoke/San-yo granitic magma genesis[END_REF], locally associated with high-temperature low-pressure "Ryoke" metamorphism [START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF][START_REF] Brown | Unpairing metamorphic belts: P-T paths and a tectonic model for the Ryoke Belt, southwest Japan[END_REF].

The Outer Zone

The Outer Zone comprises (Fig. 3; Table 1) the Kurosegawa terrane and the Northern and Southern Chichibu terranes, which are grouped into the Chichibu composite terrane [START_REF] Kurimoto | K-Ar ages of the Sambagawa metamorphic rocks in the northern part of Wakayama Prefecture, Southwest Japan[END_REF][START_REF] Matsuoka | Unit division of the Chichibu Composite Belt from a viewpoint of accretionary tectonics and the geology of western Shikoku, Japan[END_REF]) that occurs as a klippe-like structure on the Mikabu and Sambagawa belts (north) and the Shimanto terrane (south), often straddling the contact between both [START_REF] Kurimoto | K-Ar ages of the Sambagawa metamorphic rocks in the northern part of Wakayama Prefecture, Southwest Japan[END_REF][START_REF] Matsuoka | Unit division of the Chichibu Composite Belt from a viewpoint of accretionary tectonics and the geology of western Shikoku, Japan[END_REF]Aoki et al. 2007). The Chichibu composite terrane truncates the imbricate structure of the Shimanto terrane [START_REF] Kurimoto | Chichibu" system in the area southwest of Koyasan, Wakayama Prefecture-Upper Cretaceous Hanazono Formation[END_REF][START_REF] Awan | Thermal structure and uplift of the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan: an illite crystallinity and b 0 lattice spacing study[END_REF]. The contact between the Northern Chichibu terrane and the underlying Sambagawa belt is a low-angle normal fault [START_REF] Masago | Exhumation of the Sanbagawa high-pressure metamorphic belt, SW Japan-constraints from the upper and lower boundary faults[END_REF]. Well-bedded, clastic series as old as early Late Jurassic overlie the Chichibu composite terrane and conglomerates contain pebbles of chert from the substratum, as well as granite and (sub) volcanic rocks [START_REF] Umeda | Paleozoic and Mesozoic radiolarians from pebbles of siliceous rocks of the Upper Jurassic Shiranezaki Formation in the Toba area, Mie Prefecture, Southwest Japan[END_REF][START_REF] Kashiwagi | Upper Jurassic to Lower Cretaceous Ikenoue Formation around the Kurosegawa Terrane in the western Kii Peninsula, Southwest Japan[END_REF].

The Kurosegawa terrane forms a narrow, discontinuous belt of tectonic mélange containing a great variety of lithologies; partly set in a serpentinite matrix and that were formed in various geodynamic settings. Typical lithologies of this terrane are Early and Late Palaeozoic plutonic and high-grade to low-grade metamorphic rocks of different baric type, weakly to nonmetamorphic Pridolian to Eifelian tuffaceous clastic rocks, and Permo-Mesozoic continental shelf deposits [START_REF] Aitchison | Kurosegawa terrane: disrupted remnants of a low latitude Paleozoic terrane accreted to SW Japan[END_REF][START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF]Isozaki 1997a;[START_REF] Hada | U-Pb zircon ages for the Mitaki igneous rocks, Siluro-Devonian tuff, and granitic boulders in the Kurosegawa Terrane, Southwest Japan[END_REF][START_REF] Hada | Kurosegawa Terrane in Southwest Japan: disrupted remnants of a Gondwana-derived terrane[END_REF][START_REF] Kato | Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan[END_REF][START_REF] Takagi | Restoration of exotic terranes along the Median Tectonic Line, Japanese Islands: overview[END_REF]. Usually, a Permo-Triassic subduction-accretion complex is also regarded as part of this terrane. However, as pointed by [START_REF] Yamakita | What belongs to the Northern Chichibu Belt? Tectonic division between the Northern Chichibu Belt and the Kurosegawa Belt[END_REF] these rocks are not associated with the typical Kurosegawa lithologies and were therefore incorporated into the Northern Chichibu terrane, which we follow in this paper. The Northern Chichibu terrane is a subduction-accretion complex with virtually unmetamorphosed to pumpellyite-actinolite facies, Permian to late Jurassic pelites and basic phyllites with older tectonic blocks of sandstone, greenstone, chert and carbonate [START_REF] Kurimoto | Northern subbelt of the Chichibu Belt in the Shimizu-Misato area, Wakayama Prefecture, Southwest Japan[END_REF][START_REF] Kawato | Geotectonic boundary between the Sambagawa and Chichibu belts in central Shikoku, Southwest Japan[END_REF][START_REF] Matsuoka | Unit division of the Chichibu Composite Belt from a viewpoint of accretionary tectonics and the geology of western Shikoku, Japan[END_REF]. The Southern Chichibu terrane is an only locally metamorphosed Jurassic to Early Cretaceous subduction-accretion complex with blocks of Triassic cherts and Jurassic siliceous shales, sandstones, acidic tuffs and basaltic rocks [START_REF] Kurimoto | K-Ar ages of the rocks of the Sambagawa, Kurosegawa and Shimanto Terranes in the northwestern part of Wakayama Prefecture, Southwest Japan[END_REF][START_REF] Matsuoka | Tectonic division of the Chichibu Composite-terrane in the westernmost part of Shikoku, southwest Japan[END_REF].

The Mikabu belt is a greenstone-dominated subduction-accretion complex metamorphosed from pumpellyite-actinolite to clinopyroxene-lawsonite-actinolite facies [START_REF] Banno | Geology and metamorphic evolution of the Sanbagawa metamorphic belt, Japan[END_REF][START_REF] Banno | Pumpellyite-actinolite facies of the Sanbagawa metamorphism[END_REF][START_REF] Suzuki | Low-grade metamorphism of the Mikabu and northern Chichibu belts in central Shikoku, SW Japan: implications for the areal extent of the Sanbagawa lowgrade metamorphism[END_REF] in the middle Cretaceous [START_REF] Dallmeyer | Mesozoic tectonothermal development of the Sambagawa, Mikabu and Chichibu belts, south-west Japan: evidence from 40 Ar/ 39 Ar wholerock phyllite ages[END_REF][START_REF] De Jong | 40 Ar/ 39 Ar whole-rock ages and structural data from the western the Kii peninsula (western Japan): implications for extension and exhumation of the Mikabu and Sambagawa belts[END_REF][START_REF] De Jong | 40 Ar/ 39 Ar whole-rock dating of metapelites from the Mikabu and Sambagawa belts, western Kii peninsula, southwest Japan[END_REF]. [START_REF] Maruyama | Paleogeographic maps of the Japanese Islands: plate tectonic synthesis from 750 Ma to the present[END_REF] considered the belt as a remnant of a palaeo-oceanic plateau. The Sambagawa belt is a very low-grade to epidote-amphibolite facies metamorphic subduction-accretion complex [START_REF] Banno | Geology and metamorphic evolution of the Sanbagawa metamorphic belt, Japan[END_REF][START_REF] Takasu | 40 Ar/ 39 Ar age constraints for the tectono-thermal evolution of the Sambagawa metamorphic belt, central Shikoku, Japan: a Cretaceous accretionary prism[END_REF][START_REF] Masago | Exhumation of the Sanbagawa high-pressure metamorphic belt, SW Japan-constraints from the upper and lower boundary faults[END_REF]Aoki et al. 2007), in which eclogite-or granulite-facies metamorphic gabbro or peridotite complexes also occur [START_REF] Okamoto | SHRIMP U-Pb zircon dating of quartz-bearing eclogite from the Sanbagawa Belt, south-west Japan: implications for metamorphic evolution of subducted protolith[END_REF]Terabayashi et al. 2005;[START_REF] Miyamoto | Peak conditions of kyanite-bearing quartz eclogites in the Sanbagawa metamorphic belt, central Shikoku, Japan[END_REF]. Metapelites yielded Late Cretaceous whole-rock and white mica 40 Ar/ 39 Ar plateau ages [START_REF] Takasu | 40 Ar/ 39 Ar age constraints for the tectono-thermal evolution of the Sambagawa metamorphic belt, central Shikoku, Japan: a Cretaceous accretionary prism[END_REF][START_REF] Dallmeyer | Tectonometamorphic 40 Ar/ 39 Ar mineral ages from the Oki metamorphic complex, Oki-Dogo, southwest Japan: implications for regional correlations[END_REF][START_REF] Dallmeyer | Mesozoic tectonothermal development of the Sambagawa, Mikabu and Chichibu belts, south-west Japan: evidence from 40 Ar/ 39 Ar wholerock phyllite ages[END_REF][START_REF] De Jong | 40 Ar/ 39 Ar whole-rock dating of metapelites from the Mikabu and Sambagawa belts, western Kii peninsula, southwest Japan[END_REF], whereas zircons in eclogites have metamorphic rims with SHRIMP U-Pb ages of 132 -112 Ma [START_REF] Okamoto | SHRIMP U-Pb zircon dating of quartz-bearing eclogite from the Sanbagawa Belt, south-west Japan: implications for metamorphic evolution of subducted protolith[END_REF]. The Shimanto terrane is an Early Cretaceous to early Miocene zeolite to prehnite-pumpellyite, prehnite-actinolite and local greenschist facies subduction-accretion complex [START_REF] Toriumi | Tectono-metamorphism of the Shimanto Belt[END_REF][START_REF] Agar | Accretion and uplift in the Shimanto Belt, SW Japan[END_REF][START_REF] Taira | Photographic atlas of an accretionary prism: geological structures of the Shimanto Belt, Japan[END_REF][START_REF] Awan | Thermal structure and uplift of the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan: an illite crystallinity and b 0 lattice spacing study[END_REF][START_REF] Miyazaki | Thermal modelling in shallow subduction: an application to low P/T metamorphism of the Cretaceous Shimanto accretionary complex, Japan[END_REF]. Still younger accreted material extends offshore to the Nankai trough, the present-day eastern boundary of the Eurasian plate (Fig. 3; [START_REF] Taira | Photographic atlas of an accretionary prism: geological structures of the Shimanto Belt, Japan[END_REF].

The Inner Zone

The Inner Zone, to the south of the Hida-Oki terrane, comprises (Fig. 3;[START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF][START_REF] Nishimura | Geotectonic subdivision and areal extent of the Sangun belt, Inner Zone of Southwest Japan[END_REF][START_REF] Tsujimori | Eclogite-facies mineral inclusions in clinozoisite from Paleozoic blueschist, central Chugoku mountains, Southwest Japan: evidence of regional eclogite-facies metamorphism[END_REF]: a pre-Permian subduction-accretion complex [Oeyama ophiolites and underlying Carboniferous (330 -280 Ma) high-pressure metamorphic Renge belt], a Permian subduction-accretion complex (Akiyoshi, and associated Suo belt, Maizuru, and Ultra-Tamba terranes) and a Jurassic to earliest Cretaceous complex (Mino-Tamba-Ishio terrane). The Akiyoshi terrane contains limestone blocks that originally formed the coral reef caps on a palaeo-seamount chain and the Maizuru terrane is a remnant of a palaeo-oceanic plateau (Isozaki 1997a;[START_REF] Maruyama | Paleogeographic maps of the Japanese Islands: plate tectonic synthesis from 750 Ma to the present[END_REF]. The contact between the Mino-Tamba-Ishio terrane and the Hida-Oki terrane in central Japan is a narrow, complex tectonic zone that is affected by Jurassic right-lateral and Cretaceous left-lateral strike-slip deformation [START_REF] Otoh | Early Cretaceous strike-slip tectonics in the Hida and Hida Marginal belts, SW Japan: a preliminary study[END_REF][START_REF] Tsukada | Jurassic dextral and Cretaceous sinistral movements along the Hida Marginal belt[END_REF]. It consists of several long and narrow, faultbounded blocks that partly belong to the Renge, Suo, Akiyoshi and Maizuru subductionaccretion complexes, as well as rocks referred to as the redefined Hida Gaien belt by [START_REF] Tsukada | Redefinition of the Hida Gaien belt[END_REF]. The Palaeozoic subduction-accretion complexes are covered by a thick sequence of strongly deformed clastic sediments of the Early Jurassic Kuruma Group, which was deposited in a fore-arc setting [START_REF] Tsukada | Jurassic dextral and Cretaceous sinistral movements along the Hida Marginal belt[END_REF][START_REF] Tsukada | Redefinition of the Hida Gaien belt[END_REF]). The Hida Gaien terrane is only overlain by the Early Cretaceous series of the Tetori Group [START_REF] Tsukada | Jurassic dextral and Cretaceous sinistral movements along the Hida Marginal belt[END_REF].

The pre-Permian complex of the Inner Zone is correlated with the Kurosegawa terrane of the Outer Zone [START_REF] Isozaki | Pre-Jurassic klippe in northern Chichibu belt in west-central Shikoku, Southwest Japan-Kurosegawa terrane as a tectonic outlier of the pre-Jurassic rocks of the Inner zone[END_REF]Isozaki 1997a;[START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF]. This correlation was based on the similarity of virtually all pre-Jurassic lithologies and their structural position, as well as the analogous timing of tectonic and metamorphic events with the Inner Zone (Table 1). The Jurassic-earliest Cretaceous complex is considered to have the Northern Chichibu terrane as equivalent in the Outer Zone [START_REF] Isozaki | Pre-Jurassic klippe in northern Chichibu belt in west-central Shikoku, Southwest Japan-Kurosegawa terrane as a tectonic outlier of the pre-Jurassic rocks of the Inner zone[END_REF][START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF]. The Southern Chichibu terrane seems not to have an equivalent to the North of the MTL.

Hida-Oki terrane

Amphibolite-facies and locally granulite-facies, mainly paragneisses with discontinuous, concordant layers of subordinate amphibolite and marble, occur in the main Hida belt of central Japan and on the island of Oki-Dogo (Fig. 3; [START_REF] Suzuki | 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[END_REF]Isozaki 1997a; [START_REF] Dallmeyer | Tectonometamorphic 40 Ar/ 39 Ar mineral ages from the Oki metamorphic complex, Oki-Dogo, southwest Japan: implications for regional correlations[END_REF][START_REF] Arakawa | Crustal development of the Hida belt, Japan: evidence from Nd-Sr isotopic and chemical characteristics of igneous and metamorphic rocks[END_REF][START_REF] Arakawa | Geochemical characteristics of amphibolites in the Oki metamorphic rocks, Oki-Dogo Island, southwestern Japan: mixed occurrence of amphibolites with different geochemical affinity[END_REF][START_REF] Kawano | Whole rock geochemistry and Sr isotopic compositions of Phanerozoic sedimentary rocks in the Inner Zone of the Southwest Japan Arc[END_REF]. Some gneisses are migmatitic, and minor pelitic types may contain sillimanite or corundum. The Hida gneisses contain abundant granodiorite, diorite and tonalite intrusions that are not affected by the main Permo-Triassic metamorphism [START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF]. [START_REF] Arakawa | Crustal development of the Hida belt, Japan: evidence from Nd-Sr isotopic and chemical characteristics of igneous and metamorphic rocks[END_REF] noted that the Sr-Nd isotope systematics of the Palaeozoic to Mesozoic granitic rocks, and young Sm-Nd depleted mantle model ages of paragneisses imply that the Hida belt's protoliths were not older than early Palaeozoic. [START_REF] Suzuki | 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[END_REF] pointed out that the youngest detrital zircon and monazite grains in paragneisses, with CHIME ages of ca. 350 Ma (Oki-Dogo) and ca. 300 Ma (main Hida belt), show that their protoliths were deposited after the Early to Late Carboniferous. The chemical and isotopic signatures of the mafic igneous rocks indicate that the Hida belt was formed in a continental margin affected by a subduction zone or a continental island arc [START_REF] Arakawa | Crustal development of the Hida belt, Japan: evidence from Nd-Sr isotopic and chemical characteristics of igneous and metamorphic rocks[END_REF]. Amphibolites with tholeiitic to alkaline chemical affinity occurring in paragneisses on Oki-Dogo Island illustrate the complex and heterogeneous nature of the Hida-Oki terrane. Their geochemistry and Sr-Nd isotope systematics show that many amphibolites were derived from basalts of inferred Carboniferous to Permian age that extruded in a within-plate setting not affected by a subduction zone [START_REF] Arakawa | Geochemical characteristics of amphibolites in the Oki metamorphic rocks, Oki-Dogo Island, southwestern Japan: mixed occurrence of amphibolites with different geochemical affinity[END_REF]. However, some amphibolite that occur as thin concordant layers in calcareous psammitic gneisses, occasionally deformed into aggregates of lenticular blocks (chocolate tablet boudins), have island arc or mid ocean ridge basalt signatures [START_REF] Arakawa | Geochemical characteristics of amphibolites in the Oki metamorphic rocks, Oki-Dogo Island, southwestern Japan: mixed occurrence of amphibolites with different geochemical affinity[END_REF]. The latter two basalt types would imply the presence of fragments of oceanic crust.

On the basis of sketchy Sm-Nd and Rb-Sr isotope data, with huge errors of 10-50%, it has been suggested that the Hida-Oki terrane experienced early, locally granulite-facies, metamorphism as well as mafic magmatism and volcanism before the Permian (Isozaki 1997a;[START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF][START_REF] Arakawa | Crustal development of the Hida belt, Japan: evidence from Nd-Sr isotopic and chemical characteristics of igneous and metamorphic rocks[END_REF]. The timing of the main, medium-pressure type amphibolite-facies metamorphism and associated migmatite formation is well constrained. Unzoned monazites from paragneisses on Oki-Dogo as well as metamorphic overgrowths on older cores of zoned grains yielded PbO/ThO 2 ages of 250 ± 20 Ma [START_REF] Suzuki | 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[END_REF]. These dates are within error of a concordant U-Pb SHRIMP age of 236 ± 3 Ma, obtained by [START_REF] Tsutsumi | SHRIMP U-Pb dating of detrital zircons in paragneiss from Oki-Dogo Island, western Japan[END_REF] from the rim of a zircon grain in a paragneisses. A sillimanite-bearing paragneiss from the Hida belt in central Japan yielded a monazite with a CHIME age of 250 ± 10 Ma [START_REF] Suzuki | 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[END_REF], which is within error of the youngest concordant U-Pb SHRIMP age of 245 ± 15 Ma obtained by [START_REF] Sano | Ion microprobe U-Pb zircon geochronology of the Hida gneiss. Finding of the oldest minerals in Japan[END_REF] on zircon from a metavolcanic paragneiss. CHIME zircon ages in the 250 -230 Ma range for a granite [START_REF] Suzuki | The chemical Th-U total Pb isochron ages of zircon and monazite from the Gray Granite of the Hida terrane, Japan[END_REF], may indicate that metamorphism was accompanied by rare magmatism. On Oki-Dogo, the waning stages of metamorphism are constrained by earliest Jurassic 199 -192 Ma 40 Ar/ 39 Ar hornblende plateau and isochron ages [START_REF] Dallmeyer | Tectonometamorphic 40 Ar/ 39 Ar mineral ages from the Oki metamorphic complex, Oki-Dogo, southwest Japan: implications for regional correlations[END_REF]. Muscovite on Oki-Dogo yielded considerably younger 40 Ar/ 39 Ar plateau ages of 166.5 ± 0.6 and 167.8 ± 0.6 Ma [START_REF] Dallmeyer | Tectonometamorphic 40 Ar/ 39 Ar mineral ages from the Oki metamorphic complex, Oki-Dogo, southwest Japan: implications for regional correlations[END_REF].

The post-metamorphic Funatsu-type granitoid suite in central Japan yielded Rb-Sr wholerock isochron ages of 188.9 ± 4. 4 and 197.7 ± 15.4 Ma (Shibata and Nozawa 1984). Dykes and veins of leucocratic granite that intruded the gneisses on Oki-Dogo are correlated with Funatsu-type granitoids [START_REF] Dallmeyer | Tectonometamorphic 40 Ar/ 39 Ar mineral ages from the Oki metamorphic complex, Oki-Dogo, southwest Japan: implications for regional correlations[END_REF]. Widespread intrusion of 230 -180 Ma calc-alkaline plutons (late Triassic-early Jurassic) correspond to an important period of crust formation [START_REF] Arakawa | Crustal development of the Hida belt, Japan: evidence from Nd-Sr isotopic and chemical characteristics of igneous and metamorphic rocks[END_REF]. [START_REF] Kaneoka | Isotope geochemistry of igneous rocks in the Japan Sea[END_REF] noted that biotite (hornblende) granites that intruded the continental crust of the Yamato Bank area, have K-Ar and Rb-Sr whole-rock isochron ages in the range of 257 -196 Ma, and might thus be comparable to the Hida-Oki terrane.

The gneissic basement and the Funatsu-type granitoids are overlain by sediments of the Tetori Group [START_REF] Otoh | Early Cretaceous strike-slip tectonics in the Hida and Hida Marginal belts, SW Japan: a preliminary study[END_REF], the oldest rocks of which yielded Callovian and Oxfordian fossils (Kuzuryu subgroup, Tsukada 2003), implying a depositional age of 165 -156 Ma. Exhumation of the Hida-Oki terrane was thus completed in late Middle to early Late Jurassic time.

Often the Hida and Oki terranes are regarded as separate geological entities [START_REF] Sohma | Hida metamorphic belt[END_REF][START_REF] Banno | Metamorphic belts of Japanse Islands[END_REF]Isozaki 1997a;[START_REF] Maruyama | Paleogeographic maps of the Japanese Islands: plate tectonic synthesis from 750 Ma to the present[END_REF][START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF], a claim which is occasionally backed-up by geochemical and Sr-Nd isotope data [START_REF] Arakawa | Geochemical characteristics of amphibolites in the Oki metamorphic rocks, Oki-Dogo Island, southwestern Japan: mixed occurrence of amphibolites with different geochemical affinity[END_REF]. However, because both rock suites are petrologically and geochronologically essentially similar [START_REF] Suzuki | 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[END_REF][START_REF] Dallmeyer | Tectonometamorphic 40 Ar/ 39 Ar mineral ages from the Oki metamorphic complex, Oki-Dogo, southwest Japan: implications for regional correlations[END_REF], we do not want to focus on the possible differences between them. The small size of the outcrop (less than about 15 km 2 ) on the tiny island of Oki-Dogo with respect to the main occurrence in the Hida belt introduces a bias. Therefore and because of the complex tectono-metamorphic history of these rocks we prefer to focus on the similarities and group both as the Hida-Oki terrane.

Proto-Japanese superterrane

The principal elements that have been grouped into the proto-Japan superterrane are the South Kitakami terrane (northeastern Honshu) as well as parts of the Kurosegawa (western Honshu and Kyushu), Hida Gaien (or Marginal) (central Honshu) and Paleo-Ryoke (in restricted areas from Kyushu to the Kanto Mountains of central Honshu) terranes (Fig. 3). On the basis of similarities in litho-and bio-stratigraphy of Late Silurian to early Middle Devonian and Late Palaeozoic sedimentary series, as well as isotopic ages and petrochemistry of late Ordovician and Permian granitoids, parts of these terranes can be correlated [START_REF] Ehiro | Relationships in tectonic framework among the South Kitakami and Hayachine tectonic belts, Kurosegawa Belt, and "Paleo-Ryoke Belt[END_REF][START_REF] Hada | U-Pb zircon ages for the Mitaki igneous rocks, Siluro-Devonian tuff, and granitic boulders in the Kurosegawa Terrane, Southwest Japan[END_REF][START_REF] Takagi | Restoration of exotic terranes along the Median Tectonic Line, Japanese Islands: overview[END_REF][START_REF] Kurihara | Silurian and Devonian radiolarian biostratigraphy of the Hida Gaien belt, central Japan[END_REF][START_REF] Kawajiri | Petrology of gabbroic rocks in the Hida Gaien belt in the northern part of Takayama City, Gifu Prefecture, central Japan[END_REF]. Small and isolated outcrops of high-grade metamorphic rocks that yielded middle Cretaceous isotopic ages occurring in western Kyushu (Higo metamorphic complex, Fig. 3), western Shikoku (Oshima metamorphic rocks) and the Kanto Mountains (Yorii metamorphic rocks) are assigned to the Paleo-Ryoke terrane [START_REF] Ruffet | Large-scale displacement along the Median Tectonic Line, Japan: evidence from SHRIMP zircon U-Pb dating of granites and gneisses from the South Kitakami and paleo-Ryoke belts[END_REF][START_REF] Takagi | Restoration of exotic terranes along the Median Tectonic Line, Japanese Islands: overview[END_REF]. Petrologically similar high-grade metamorphic complexes crop out in northern Kyushu (Sefuri metamorphic rocks), which [START_REF] Osanai | The Higo metamorphic complex in Kyushu, Japan as the fragment of Permo-Triassic metamorphic complexes in East Asia[END_REF] correlated to the Hida Gaien terrane (Fig. 3), and furthermore in the western part of the Abukuma metamorphic terrane, NE Honshu (Fig. 3; Takanuki series, [START_REF] Takagi | Restoration of exotic terranes along the Median Tectonic Line, Japanese Islands: overview[END_REF][START_REF] Ruffet | Large-scale displacement along the Median Tectonic Line, Japan: evidence from SHRIMP zircon U-Pb dating of granites and gneisses from the South Kitakami and paleo-Ryoke belts[END_REF]. Like the Kurosegawa terrane, metamorphic rocks of the Paleo-Ryoke terrane are covered by early Late Cretaceous clastic sedimentary series (Takagi and Arai 2003, Fig. 2), and occur in the hanging wall of the Jurassic-Cretaceous subduction system.

The proto-Japan superterrane is fragmented and dispersed by strike-slip faulting and other tectonic movements since the early Cretaceous [START_REF] Hada | Northern Chichibu Terrane[END_REF][START_REF] Aitchison | Kurosegawa terrane: disrupted remnants of a low latitude Paleozoic terrane accreted to SW Japan[END_REF][START_REF] Hara | Tectonic evolution of the Sambagawa Schists and its implications in convergent margin processes[END_REF][START_REF] Maruyama | Paleogeographic maps of the Japanese Islands: plate tectonic synthesis from 750 Ma to the present[END_REF][START_REF] Hada | Kurosegawa Terrane in Southwest Japan: disrupted remnants of a Gondwana-derived terrane[END_REF][START_REF] Kato | Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan[END_REF][START_REF] Takagi | Restoration of exotic terranes along the Median Tectonic Line, Japanese Islands: overview[END_REF]. As a result of this, elements of the superterrane occur in different tectonic positions, both in the Inner and Outer Zones (Fig. 3). The Hida Gaien terrane fringes the southern boundary of the Hida-Oki terrane. As outlined above, the Kurosegawa terrane is part of the klippe of the Inner Zone on top of the Outer Zone. The Paleo-Ryoke terrane also occurs in the Outer Zone, always to the north of the Kurosegawa terrane. In the Kanto Mountains, the Paleo-Ryoke terrane occurs in a number of klippes on the Sambagawa belt (Fig. 4 of [START_REF] Takagi | Restoration of exotic terranes along the Median Tectonic Line, Japanese Islands: overview[END_REF], in part along a low-angle normal fault contact [START_REF] Wallis | Sense and direction of movement along the Atokura Fault at Shimonita, Kanto Mountains, central Japan[END_REF]). On Shikoku, the contact between the Paleo-Ryoke terrane and the Sambagawa belt, which only crops out in the easternmost part of the island (Fig. 3) due to a deeper erosion level, is not exposed because of widespread Quarternary volcanic deposits. The South Kitakami terrane is bounded by Neogene (strike-slip) faults (Fig. 3).

Southward extension of Japanese terranes

The southernmost outcrops of the Suo metamorphic belt in the Inner Zone of Japan occur on the islands of Ishigaki and Iriomote (Tomuru formation; Nishimura 1998), located near the southern tip of the Ryukyu Arc (Fig. 2). [START_REF] Faure | Doming in the southern foreland of the Dabieshan (Yangtze block, China)[END_REF] and Zamoras andMatsuoka (2001, 2004) reasoned that the Jurassic subduction-accretion complex continues southward into Taiwan, but [START_REF] Ishiwatari | Paleozoic ophiolites and blueschists in Japan and Russian Primorye in the tectonic framework of East Asia: a synthesis[END_REF] have argued against this. The eastern part of the Tananao metamorphic complex of eastern Taiwan (Fig. 2) comprises ocean floor rocks, such as metachert and Mn-rich rocks, greenstone, serpentinite and thick marble series [START_REF]Preliminary notes on the time relations of metamorphic rocks, Loshao-Tailuko area, eastern Taiwan[END_REF], which likely represent a subduction-accretion complex, and furthermore granitic gneiss and schists. Although geochronologic and microfossil data are lacking, it seems probable that the Tananao complex is the extension of the Inner Zone of Japan. Like the Inner Zone, these rocks were intruded by late Cretaceous dominantly granodiorite to quartz monzonite that yielded U-Pb zircon crystallization ages ranging from 80 to 90 Ma [START_REF] Jahn | Geochronology of the Tananao Schist Complex, Taiwan[END_REF], which are similarly related to westward subduction of oceanic lithosphere. The meaning of the granitic gneiss and schists is unclear, in Japan such rocks are principally found in the Hida-Oki terrane.

Farther to the south, the west central Philippines, that is the northern half of Palawan, the Calamian Islands, southwest Mindoro, northwest Panay and Tablas, comprises subductionaccretion complexes [START_REF] Hamilton | Tectonics of the Indonesian region[END_REF]Zamoras andMatsuoka 2001, 2004). These are regarded as having formed along the East Asian continental margin to southwest of Taiwan as continuation of Tananao complex, starting in the Middle Jurassic (Zamoras andMatsuoka 2001, 2004 andreferences therein). The easternmost of these subduction-accretion complexes has been correlated to the Southern Chichibu belt of the Outer Zone in Japan [START_REF] Zamoras | Malampaya Sound Group: a Jurassic-Early Cretaceous accretionary complex in Busuanga Island, North Palawan block (Philippines)[END_REF].

Sakaigawa unit

We concentrated our dating effort on the Sakaigawa unit [START_REF] Kurimoto | K-Ar ages of the rocks of the Sambagawa, Kurosegawa and Shimanto Terranes in the northwestern part of Wakayama Prefecture, Southwest Japan[END_REF], that forms a discontinuous <300 m wide zone of metamorphic psammitic, pelitic and siliceous schists that are associated with greenschists in the western Kii peninsula (Fig. 4). This rock association also occurs in a similar narrow band in the central and eastern part of the Kii peninsula [START_REF] Kato | Chichibu Terrane of Miyagawa-mura, Mie Prefecture and Tenkawa-mura, Nara Prefecture in the central Kii Peninsula, Southwest Japan[END_REF][START_REF] Kato | Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan[END_REF]. The unit overlies the Jurassic rocks of the Northern Chichibu subduction-accretion complex (Fig. 4). The Sakaigawa unit is overlain by conglomerates with coarse-grained, upward fining sandstones of the Cretaceous Futakawa formation, along a slightly undulating, primary sedimentary contact (Fig. 4). The Sakaigawa unit is an element of the Chichibu composite terrane. This complex klippe-like composite terrane is presently separated from the Shimanto terrane, in the south, by the Butsuzo tectonic line (BTL) and from the Sambagawa and Mikabu belts, in the north, by the Aridagawa tectonic line (ATL) (Fig. 4). The architecture of the eastern Kii peninsula [START_REF] Kato | Chichibu Terrane of Miyagawa-mura, Mie Prefecture and Tenkawa-mura, Nara Prefecture in the central Kii Peninsula, Southwest Japan[END_REF] and western Shikoku [START_REF] Matsuoka | Unit division of the Chichibu Composite Belt from a viewpoint of accretionary tectonics and the geology of western Shikoku, Japan[END_REF]) is similar. These generally steeply dipping fault zones, characterised by local fault gauges and cataclasites, truncate stacks of tectonic units and the thermal structure of the adjacent terranes [START_REF] Hada | Geology of the middle-Aritagawa district, Wakayama Prefecture, with special reference to the relationship between the Chichibu Belt and the Sambagawa Belt[END_REF][START_REF] Kurimoto | K-Ar ages of the Sambagawa metamorphic rocks in the northern part of Wakayama Prefecture, Southwest Japan[END_REF][START_REF] Awan | Thermal structure and uplift of the Cretaceous Shimanto Belt, Kii Peninsula, Southwest Japan: an illite crystallinity and b 0 lattice spacing study[END_REF]. Deformed, non-metamorphic, probably Cretaceous sandstones and mudstones occur discontinuously along the ATL, sandwiched between the Northern Chichibu and Sambagawa belts [START_REF] Hada | Geology of the middle-Aritagawa district, Wakayama Prefecture, with special reference to the relationship between the Chichibu Belt and the Sambagawa Belt[END_REF][START_REF] Kurimoto | Northern subbelt of the Chichibu Belt in the Shimizu-Misato area, Wakayama Prefecture, Southwest Japan[END_REF].

The Sakaigawa unit is dominated by light grey coloured, mica-and sometimes chloritebearing quartzites that have a well-developed platy tectonic foliation and a grain shape fabric.

These banded quartzites contain centimetre-decimetre thick intercalations of dark grey to black coloured quartz-rich phyllites, with numerous quartz veins parallel to the main tectonic foliation. Thicker phyllite series contain sometimes rounded pluri-metre scale lenses and boudins of quartzite. Up to 10-m thick layers of foliated and well lineated chlorite schists also occur, which may contain darker and coarser grained greenstones, locally with metapelite intercalations. The rocks contain lawsonite, chlorite, K-white mica, albite and stilpnomelane [START_REF] Hada | Geology of the middle-Aritagawa district, Wakayama Prefecture, with special reference to the relationship between the Chichibu Belt and the Sambagawa Belt[END_REF] pointing to very-low-grade, high-pressure metamorphism in a subductionaccretion complex, which agrees with the encountered lithologies.

The main tectonic foliation is characterised by a strong quartz-mica differentiation and wraps around isolated quartz lenses that often are fold hinges with pinched limbs. This foliation is sub-vertical to steeply southward dipping and contains a weakly to moderately W or E plunging mineral and stretching lineation (Fig. 4). This tectonic fabric is deformed by tight centimetre-scale folds with curvilinear axes and also by mesoscopic D 3 folds that are associated with steeply dipping, anastomosing faults. These faults form conjugate systems of steeply N and S dipping sets with a normal movement sense and an important strike-slip component indicated by moderately plunging slickenside striations.

The basal contact of the Cretaceous Futakawa formation is slightly oblique to the main foliation in the substratum and also cuts later cleavages. Currently this contact and Futakawa formation are steeply dipping or northward overturned. Locally, the contact is formed by steep faults with weakly plunging linear structures. The conglomerate contains unoriented, well-rounded to sub-rounded pebbles and cobbles. Most of these are grey cherts, as well as quartzites and phyllites that are probably derived from the Sakaigawa substratum. Subordinate components are substratum-derived greenschists, and in addition, epidote veined platy greenstones, dark-bluish green and reddish-purple quartzites and red jasper/chert, as well as rare thin bedded fine-grained carbonates, described from the substratum in the central and eastern Kii peninsula [START_REF] Kato | Chichibu Terrane of Miyagawa-mura, Mie Prefecture and Tenkawa-mura, Nara Prefecture in the central Kii Peninsula, Southwest Japan[END_REF][START_REF] Kato | Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan[END_REF]. Other conspicuous components are autoclasts of coarse, pebbly sandstone. The matrix-supported, unsorted rocks contain fluidization channels and may therefore be deposited by mass-flows (e.g. [START_REF] Postma | Classification for sediment gravity-flow deposits based on flow conditions during sedimentation[END_REF]). These observations imply that the metamorphic recrystallisation and penetrative ductile deformation in the Sakaigawa unit are of pre-Cretaceous age and that a subduction-accretion complex was being eroded in Cretaceous time. Deposition of these brackish water or shallow marine clastic sediments occurred in a forearc basin [START_REF] Kato | Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan[END_REF]. The steep dip of the unconformity and the local fault contact with the substratum point to syn to post-Cretaceous deformation that seems in part strike-slip related. This deformation, which may be related to major left-lateral Campanian-Maastrichtian strike-slip along the MTL, has profoundly modified the earlier klippe-like juxtaposition of the different subduction-accretion terranes in the Chichibu composite terrane.

We correlate the Sakaigawa unit with the Agekura and Ino units composed of similar rocks occurring in a comparable structural position in the classical Kurosegawa terrane on central Shikoku. [START_REF] Adachi | Discovery of the Late Triassic radiolarians from the Ino Formation, central Shikoku[END_REF] reported Triassic radiolarians from the Ino unit; whereas [START_REF] Matsuda | Re-examination based on the conodont fossil from the crystalline schists of the Chichibu belt, central part of Shikoku[END_REF] found Early Carboniferous to Late Permian conodonts in limestone. The Agekura unit is overlain by the Shirakidani unit (Table 1), comprising weakly to unmetamorphosed greenstone, sandstone, mudstone and limestone, which [START_REF] Isozaki | Pre-Jurassic klippe in northern Chichibu belt in west-central Shikoku, Southwest Japan-Kurosegawa terrane as a tectonic outlier of the pre-Jurassic rocks of the Inner zone[END_REF] considered as equivalent to the weakly metamorphic Permian Akiyoshi subductionaccretion complex of the Inner Zone. The latter complex is closely related to the high-P/T metamorphic Suo belt [START_REF] Nishimura | Geotectonic subdivision and areal extent of the Sangun belt, Inner Zone of Southwest Japan[END_REF]. The Shirakidani unit, at its turn, is covered by serpentinites that enclose blocks of metamorphic rocks, including garnet-clinopyroxene granulites, glaucophane-lawsonite-pumpellyite and jadeite-quartz rocks with isotopic ages that are comparable to those of the Inner Zone [START_REF] Isozaki | Pre-Jurassic klippe in northern Chichibu belt in west-central Shikoku, Southwest Japan-Kurosegawa terrane as a tectonic outlier of the pre-Jurassic rocks of the Inner zone[END_REF]; Table 1).

A metapelite from the Sakaigawa unit yielded a 210 ± 5 Ma K-Ar whole-rock age [START_REF] Kurimoto | K-Ar ages of the rocks of the Sambagawa, Kurosegawa and Shimanto Terranes in the northwestern part of Wakayama Prefecture, Southwest Japan[END_REF]. The Agekura rocks contain muscovite with 175 -230 Ma K-Ar ages [START_REF] Isozaki | Pre-Jurassic klippe in northern Chichibu belt in west-central Shikoku, Southwest Japan-Kurosegawa terrane as a tectonic outlier of the pre-Jurassic rocks of the Inner zone[END_REF][START_REF] Hara | Tectonic evolution of the Sambagawa Schists and its implications in convergent margin processes[END_REF]. [START_REF] Dallmeyer | Mesozoic tectonothermal development of the Sambagawa, Mikabu and Chichibu belts, south-west Japan: evidence from 40 Ar/ 39 Ar wholerock phyllite ages[END_REF] obtained strongly disturbed 40 Ar/ 39 Ar age spectra on two phyllites with progressively decreasing apparent ages from about 255 to 235 Ma for the most mica-rich main component in these whole-rock samples.

As indicated above, classically the very-low-grade metamorphic pelites of the Sakaigawa and Agekura units are regarded as northern limit of the Kurosegawa terrane [START_REF] Kurimoto | K-Ar ages of the rocks of the Sambagawa, Kurosegawa and Shimanto Terranes in the northwestern part of Wakayama Prefecture, Southwest Japan[END_REF][START_REF] Dallmeyer | Mesozoic tectonothermal development of the Sambagawa, Mikabu and Chichibu belts, south-west Japan: evidence from 40 Ar/ 39 Ar wholerock phyllite ages[END_REF]. [START_REF] Kato | Chichibu Terrane of Miyagawa-mura, Mie Prefecture and Tenkawa-mura, Nara Prefecture in the central Kii Peninsula, Southwest Japan[END_REF] and [START_REF] Kato | Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan[END_REF] assign comparable rocks on the central and eastern Kii peninsula to the Kurosegawa terrane too. However, we correlate the Sakaigawa unit with the Northern Chichibu terrane. In this we follow [START_REF] Yamakita | What belongs to the Northern Chichibu Belt? Tectonic division between the Northern Chichibu Belt and the Kurosegawa Belt[END_REF], who pointed out that the Permian subduction-accretion complex on Shikoku is not associated with typical Kurosegawa lithologies but with clastic rocks of Cretaceous age, like the Sakaigawa and Agekura units.

Samples and experimental procedures

We sampled five dark grey graphite-rich, quartzitic, very low-grade metamorphic pelites from the Sakaigawa unit. These have K-white mica, chlorite and quartz as main metamorphic minerals with minor albite. The samples possess a well-defined tectonic quartz-mica differentiation foliation that is not overprinted by younger penetrative ductile deformation or late stage brecciation related to strike-slip faulting. JK49 is taken from about 10 m below the Cretaceous unconformity and contains conspicuous millimetre-thick foliation parallel quartz laminae; JK57 is taken 4 m away from a breccia zone. The grain-size of the samples is too small for a successful mineral separation, hence, we isotopically dated whole-rocks instead following a procedure outlined by [START_REF] Ruffet | Comparison of 40 Ar-39 Ar conventional and laser dating of biotites from the North Trégor Batholith[END_REF][START_REF] Ruffet | Large-scale displacement along the Median Tectonic Line, Japan: evidence from SHRIMP zircon U-Pb dating of granites and gneisses from the South Kitakami and paleo-Ryoke belts[END_REF] and that is summarised below.

We obtained thin, 0.7 -1.2 mm diameter fragments by handpicking the sieve fraction of five crushed phyllites under a binocular zoom microscope. Handpicking enabled selection of pristine whole-rock fragments without alteration or with too many quartz veins. Each fragment was thoroughly ultrasonically rinsed in distilled water and subsequently put in a 10 × 10 × 0.5 mm Al foil envelope. Envelopes were stacked in an irradiation canister together with aliquots of flux monitor hornblende HB3gr (K-Ar age: 1071.7 ± 5.4 Ma, [START_REF] Turner | 40 Ar-39 Ar ages and cosmic ray exposure ages of Apollo 14 samples[END_REF], inserted after every eight to ten samples, which allowed to determine the flux gradient with a precision of ±0.2%. The total neutron flux density during irradiation in position 5C of the McMaster University research reactor (Hamilton, Canada), which lasted 149.92 h, was 9 × 10 18 n × cm -2 . Fragments were step-heated at Géosciences Azur (CNRS-University of Nice) with a continuous, defocused, laser beam of a Coherent Innova 70-4 argon ion laser probe; fusion during the final step is achieved by beam focusing. The homogeneity of the energy distribution over a step-heated fragment was monitored by checking its hue with a joined video-binocular microscope system. Gas cleaning was achieved with a SAES GP10 getter pump and a cold trap. Argon isotopes were measured on a VG3600 noble-gas mass spectrometer equipped with a Daly photomultiplier. System blank values were determined at the beginning of an experiment and repeated typically after each third step and were subtracted from subsequent steps. Measured mass spectrometer Ar peak intensities were corrected for mass spectrometer discrimination (1.02797; measured from analysis of air Ar), radioactive decay of 39 Ar and 37 Ar and interference of Ca-and K-derived Ar isotopes. The ( 36 Ar/ 37 Ar) Ca , ( 39 Ar/ 37 Ar) Ca and ( 40 Ar/ 39 Ar) K correction factors used were 0.000279, 0.000706 and 0.0295, respectively. Errors are quoted at the 2σ level; step errors include analytical uncertainties only; the 2.15% uncertainty in the 40 Ar*/ 39 Ar K ratio of the monitor is propagated into the errors on integrated and pseudo-plateau ages. Decay constant and isotopic abundance ratios used: 40 K tot = 5.543 × 10 -10 a -1 ; 40 K/K = 0.01167 atom % [START_REF] Steiger | Subcommission on geochronology: convention on the use of decay constants in geo-and cosmology[END_REF]. We use the time scale of [START_REF] Gradstein | A new geologic time scale, with special reference to Precambrian and Neogene[END_REF] to compare isotopic and time stratigraphic ages.

Results

We performed 40 Ar/ 39 Ar laser step-heating analyses of small, single whole-rock fragments to ensure a thorough degassing over an extended energy range, aiming to separate gas fractions released by different constituents of these polymineralic assemblages. The 40 Ar/ 39 Ar analytical data are presented in Table 2, and depicted as age spectra with corresponding 37 Ar Ca / 39 Ar K ratio spectra (Fig. 5; lower and upper panels, respectively), with 37 Ar Ca / 39 Ar K being 0.459 × (CaO/K 2 O).

The main degassing of 39 Ar K and 40 Ar* is strongly correlated; additional 36 Ar AIR and 37 Ar Ca release occurred in the high temperature steps (Table 2). 37 Ar Ca / 39 Ar K ratios are slightly elevated during early degassing and sharply increase for the final 39 Ar release (Table 2; Fig. 5 upper panels). All samples yielded 40 Ar/ 39 Ar spectra with much younger apparent ages for the first laser increments (Fig. 5 lower panels). None of the samples met the strict plateau criteria, viz. 70% or more of the 39 Ar K released in three or more contiguous steps, the apparent ages of which agree to within 2σ of the integrated age of the plateau segment. Yet, using the above criteria, pseudo-plateau ages could be calculated for flat sections of age spectra of three samples that corresponded to 49-63% of the released 39 Ar. The pseudo-plateau ages (2σ) obtained were 218.4 ± 0.4 Ma (JK09) in the eastern part of the unit, and almost concordant dates of 228.8 ± 0.9 Ma (JK57) and 231.9 ± 0.7 Ma (JK61) for the westernmost part (Figs. 4,5). Two other samples from the eastern part of the unit (JK40 and JK47; Fig. 4) lack flat sections, but their apparent ages are between ca. 205 and 225 Ma, over 80-90% of the 39 Ar release (Fig. 5; Table 2).

Interpretation

Obviously, isotopic ages of polymineralic whole-rocks are only meaningful if radiogenic argon ( 40 Ar*) of the original detrital minerals is completely outgassed during tectonometamorphic recrystallisation [START_REF] Dodson | Potassium-argon ages of slates and phyllites from south-west England[END_REF], as the influence of a very small, substantially older detrital component can be quite significant [START_REF] Reuter | K-Ar and 40 Ar/ 39 Ar dating of cleavage formed during very low-grade metamorphism: a review[END_REF]. Resetting of detrital mica components is regarded to have completed during low-grade (epizonal) metamorphic conditions [START_REF] Reuter | K-Ar and 40 Ar/ 39 Ar dating of cleavage formed during very low-grade metamorphism: a review[END_REF], that is above 300-350°C [START_REF] Leitch | The age of orogenesis in the Nambucca Slate belt: K-Ar study of low-grade regional metamorphic rocks[END_REF][START_REF] Hunziker | The evolution of illite to muscovite: mineralogical and isotopic data from the Glarus Alps, Switzerland[END_REF]. In line with interpretations by [START_REF] Muecke | Hercynian/Alleghenian overprinting of an Acadian terrane: 40 Ar/ 39 Ar studies in the Meguma zone[END_REF], [START_REF] Dallmeyer | 40 Ar/ 39 Ar whole-rock phyllite ages from late Precambrian rocks of the Avalon composite terrane, New Brunswick: evidence of Silurian-Devonian thermal rejuvenation[END_REF] or de [START_REF] De Jong | 40 Ar/ 39 Ar whole-rock dating of metapelites from the Mikabu and Sambagawa belts, western Kii peninsula, southwest Japan[END_REF], we interpret the early 36 Ar AIR and 37 Ar Ca release, that is unrelated to that of 39 Ar K and 40 Ar*, (Table 2) by the degassing of slightly weathered, carbonaceous and chlorite-rich material. The important 37 Ar Ca release and elevated 37 Ar Ca / 39 Ar K ratios for the final 5-15% of the degassing (Fig. 5, upper panels) may be due to the presence of detrital plagioclase. Apparently, detrital feldspars were strongly or completely outgassed during the main tectono-metamorphic recrystallisation as this Ca-rich component is not associated with much older ages. The main and correlated release of 39 Ar K and 40 Ar* is probably related to degassing of K-white mica as main component of the wholerocks.

Part of the irregularities of the age spectra is probably due to the use of fine-grained material that is made up of minerals with a strong K-contrast between K-white mica on the one hand and K-poor minerals like chlorite and plagioclase on the other [START_REF] Reuter | K-Ar and 40 Ar/ 39 Ar dating of cleavage formed during very low-grade metamorphism: a review[END_REF]. The potential energy of fast neutrons used during irradiation of such material may cause 39 Ar K to recoil from a K-rich mineral into an adjacent K-poor mineral at the sub-microscopic scale (e.g. 39 Ar K recoil distance = 0.08-0.16 μm; [START_REF] Turner | Possible effects of 39 Ar recoil in 40 Ar-39 Ar dating (Suppl 5[END_REF][START_REF] Onstott | Recoil refinement: Implications for the 40 Ar/ 39 Ar dating technique[END_REF]. As chlorite and plagioclase degas before and after K-white mica, respectively [START_REF] Reuter | K-Ar and 40 Ar/ 39 Ar dating of cleavage formed during very low-grade metamorphism: a review[END_REF], young apparent ages during first steps (all samples) and final gas release (JK09 and JK57), may point to 39 Ar K recoil into these K-poor components. The hump-shape displayed by older age steps just following the young apparent ages of the first 10% of 39 Ar release of samples JK09, JK57 and JK61, may represent degassing of the material that has lost 39 Ar K by recoil. Recoil of 37 Ar Ca (recoil distance about four times that of 39 Ar K ; [START_REF] Onstott | Recoil refinement: Implications for the 40 Ar/ 39 Ar dating technique[END_REF] from a Ca-rich mineral to the first degassing components provides an alternative interpretation for the presence of carbonate amongst the earliest degassing, weathered material that is rich in 36 Ar AIR .

The strongly disturbed 40 Ar/ 39 Ar age spectra obtained [START_REF] Dallmeyer | Mesozoic tectonothermal development of the Sambagawa, Mikabu and Chichibu belts, south-west Japan: evidence from 40 Ar/ 39 Ar wholerock phyllite ages[END_REF] on whole-rock phyllites of the Agekura unit (central Shikoku) have unrealistically young apparent ages (<150 Ma) for the first 20-25% of 39 Ar release. 39 Ar K recoil from the most mica-rich main component to earlier degassing, less K-rich material would explain these young ages, and would raise the apparent ages for the main degassing to the observed 255-235 Ma values. The youngest apparent ages of about 235 Ma at the end of the trajectory of progressively decreasing apparent ages would thus be least affected by 39 Ar K recoil and thus be the best age estimate, which is comparable to our results.

Despite the complexities of our age spectra, we interpret the 218, 229 and 232 Ma pseudoplateau ages as geologically meaningful and dating the very-low-grade, high-P/T tectonometamorphic recrystallisation in the Sakaigawa unit. These Ladinian-Carnian (late Middle to early Late Triassic) isotopic ages are comparable to those of the Agekura unit of the Outer Zone [START_REF] Dallmeyer | Mesozoic tectonothermal development of the Sambagawa, Mikabu and Chichibu belts, south-west Japan: evidence from 40 Ar/ 39 Ar wholerock phyllite ages[END_REF] and the Suo belt of the Inner Zone (Table 1), including a 225 ± 4.8 Ma 40 Ar/ 39 Ar plateau age on phengite from the Tomuru formation on the southern Ryukyu Islands [START_REF] Faure | Microtectonics and 39 Ar-40 Ar dating of high pressure metamorphic rocks of the south Ryukyu Arc and their bearings on the pre-Eocene geodynamic evolution of Eastern Asia[END_REF]. This confirms the correlation of the Sakaigawa unit with the Agekura unit on the one hand and with the Suo belt on the other.

Regional implications: Japanese Islands

The 218-232 Ma 40 Ar/ 39 Ar pseudo-plateau ages that we obtained in the Sakaigawa unit are not only comparable to isotopic ages in the Permian subduction-accretion complex in the Outer and Inner Zones of Japan, but also to the 250-235 Ma range of isotopic ages of the Hida-Oki terrane, as well as to the 245-225 Ma age assigned to the ultrahigh-pressure metamorphism in the Qinling-Dabie-Sulu suture [START_REF] Hacker | Subduction, collision, and exhumation in the Qinling-Dabie Orogen[END_REF]) between the North and South China cratons. Extension of the latter suture zone to the Japanese islands through Korea [START_REF] Maruyama | Orogeny and relative plate motions: Example of the Japanese Islands[END_REF][START_REF] Sohma | Hida metamorphic belt[END_REF]Isozaki 1997a;[START_REF] Maruyama | Pacific-type orogeny revisited: Miyashiro-type orogeny proposed[END_REF][START_REF] Maruyama | Pacific-type orogeny revisited: Miyashiro-type orogeny proposed[END_REF][START_REF] Oh | A new concept on tectonic correlation between Korea, China and Japan: Histories from the late Proterozoic to Cretaceous[END_REF][START_REF] Osanai | The Higo metamorphic complex in Kyushu, Japan as the fragment of Permo-Triassic metamorphic complexes in East Asia[END_REF][START_REF] Tsujimori | Triassic paragonite-and garnet-bearing epidote-amphibolite from the Hida Mountains, Japan[END_REF][START_REF] Ernst | Permo-Triassic collision, subduction-zone metamorphism, and tectonic exhumation along the East Asian continental margin[END_REF][START_REF] Oh | The Late Permian to Triassic Hongseong-Odesan collision belt in South Korea, and its tectonic correlation with China and Japan[END_REF], where major Permian to Triassic tectono-metamorphic events are also recorded, is virtually the only model used to explain the medium pressure metamorphism in the Hida-Oki and Hida Gaien terranes. Recent reconstructions basically follow the scheme of [START_REF] Sohma | Hida metamorphic belt[END_REF] with only few, relatively minor modifications [START_REF] Oh | A new concept on tectonic correlation between Korea, China and Japan: Histories from the late Proterozoic to Cretaceous[END_REF][START_REF] Osanai | The Higo metamorphic complex in Kyushu, Japan as the fragment of Permo-Triassic metamorphic complexes in East Asia[END_REF][START_REF] Oh | The Late Permian to Triassic Hongseong-Odesan collision belt in South Korea, and its tectonic correlation with China and Japan[END_REF].

Below we will elaborate on a new model in which the proto-Japan superterrane is regarded as a Permian magmatic arc that collided with the East Asian margin around Permian-Triassic boundary time, giving rise to the main metamorphism in the Hida-Oki terrane. Deep erosion of the collision zone shed detritus into trench fill sediments of the Jurassic subductionaccretion complexes.

Higo complex correlative of the Hida-Oki terrane and eastward extension of the Qinling-Dabie-Sulu suture [START_REF] Osanai | The Higo metamorphic complex in Kyushu, Japan as the fragment of Permo-Triassic metamorphic complexes in East Asia[END_REF] and [START_REF] Oh | The Late Permian to Triassic Hongseong-Odesan collision belt in South Korea, and its tectonic correlation with China and Japan[END_REF] proposed to correlate the Qinling-Dabie-Sulu suture zone, through the southwestern Gyeonggi terrane in South Korea, with the Hida belt of central Japan passing via the Higo metamorphic complex of west-central Kyushu (Fig. 3). Isozaki (1997a) correlated the Takanuki series (Fig. 3) of the Abukuma metamorphic terrane of NE Honshu to the Hida belt. Isotopic data imply that such correlations may be incorrect.

The Higo metamorphic complex predominantly comprises a metasedimentary sequence that increases in grade structurally downwards from amphibolite-facies to granulite-facies, including ultrahigh-temperature sapphirine-or spinel-bearing types as blocks in peridotites [START_REF] Ruffet | Large-scale displacement along the Median Tectonic Line, Japan: evidence from SHRIMP zircon U-Pb dating of granites and gneisses from the South Kitakami and paleo-Ryoke belts[END_REF][START_REF] Miyazaki | Low-P-high-T metamorphism and the role of heat transport by melt migration in the Higo Metamorphic Complex, Kyushu, Japan[END_REF][START_REF] Osanai | The Higo metamorphic complex in Kyushu, Japan as the fragment of Permo-Triassic metamorphic complexes in East Asia[END_REF]. Migmatites and diatexites point to abundant partial melting in the highest-grade areas, accompanied by plutonic rocks [START_REF] Miyazaki | Low-P-high-T metamorphism and the role of heat transport by melt migration in the Higo Metamorphic Complex, Kyushu, Japan[END_REF]. A garnet-biotite-cordierite paragneiss yielded essentially concordant 238 U/ 206 Pb SHRIMP zircon ages in the 330-184 Ma range (33 out of 36 grains), whereas zircon rims defined a mean age of 116.5 ± 18.7 Ma [START_REF] Ruffet | Large-scale displacement along the Median Tectonic Line, Japan: evidence from SHRIMP zircon U-Pb dating of granites and gneisses from the South Kitakami and paleo-Ryoke belts[END_REF]. On the basis of the latter age, the authors argued that the metamorphic recrystallisation of this sample was a Late Cretaceous event. They pointed out that the depositional age of the protolith should thus be younger than the youngest detrital zircon with an age of 184.4 ± 8.5 Ma (Early Jurassic). In addition, Sakashima et al. ( 2003) obtained 238 U/ 206 Pb SHRIMP ages of 110.4 ± 4.1 and 111.4 ± 2.7 Ma on zircon from the foliated Miyanohara tonalite, which had yielded a ca. of 211 Ma Sm-Nd hornblende, whole-rock isochron age (references in [START_REF] Osanai | The Higo metamorphic complex in Kyushu, Japan as the fragment of Permo-Triassic metamorphic complexes in East Asia[END_REF]. The failure to obtain correct mineral-whole-rock isochron ages on the Higo metamorphic complex that generally yielded Permo-Triassic Sm-Nd and Rb-Sr dates (see Table 1 in [START_REF] Osanai | The Higo metamorphic complex in Kyushu, Japan as the fragment of Permo-Triassic metamorphic complexes in East Asia[END_REF] points to isotope disequilibrium between the coexisting minerals. Also the major and trace element sector zoning of garnets (Figs. 4, 5 of Osanai et al. 2006) indicates nonequilibrated growth during prograde metamorphism in the complex.

The upper amphibolite-facies Takanuki series (Fig. 3) mainly comprise quartz-feldspar and pelitic paragneisses intruded by 90-110 Ma granitoids [START_REF] Banno | Metamorphic belts of Japanse Islands[END_REF][START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF][START_REF] Hiroi | Cretaceous high-temperature rapid loading and unloading in the Abukuma metamorphic terrane, Japan[END_REF]. Zircons from metapelites yielded 280-200 Ma SHRIMP U-Pb ages [START_REF] Hiroi | Cretaceous high-temperature rapid loading and unloading in the Abukuma metamorphic terrane, Japan[END_REF]) that were interpreted as detrital ages. The authors regarded that these data indicate that the Takanuki series originated from continental shelf sediments deposited from earliest Jurassic time, and were affected by low-pressure high-temperature metamorphism that resulted in andalusite-sillimanite-cordierite assemblages at about 110 Ma.

It is striking that the youngest detrital zircons are younger than the main metamorphism in the Hida-Oki terrane. A probability distribution diagram (Fig. 7b of [START_REF] Ruffet | Large-scale displacement along the Median Tectonic Line, Japan: evidence from SHRIMP zircon U-Pb dating of granites and gneisses from the South Kitakami and paleo-Ryoke belts[END_REF] shows that the overwhelming majority of the 238 U/ 206 Pb ages from cores of zircon grains from the garnet-biotite-cordierite paragneiss they dated span the 250 -175 Ma range. This age range corresponds to the timing of the main metamorphism in the Hida-Oki terrane and the intrusion of Funatsu-type granitoids. This may imply that the Higo complex and Takanuki series are metamorphic equivalents of the Early Jurassic molasse-type deposits like the Kuruma or Tetori groups, that is the erosional products of the Hida-Oki terrane, but not the terrane itself. Consequently, the Qinling-Dabie-Sulu suture zone would not extend to the Higo metamorphic complex as proposed by [START_REF] Osanai | The Higo metamorphic complex in Kyushu, Japan as the fragment of Permo-Triassic metamorphic complexes in East Asia[END_REF] and [START_REF] Oh | The Late Permian to Triassic Hongseong-Odesan collision belt in South Korea, and its tectonic correlation with China and Japan[END_REF]. Such a correlation is furthermore highly unlikely for two tectonic reasons. This loop-like belt along the eastern margin of the North China craton marks the collisional suture with the Yangtze block of the South China craton, without specifying which Japanese rocks would belong to the latter craton. It is important to underline that the Higo metamorphic rocks and correlatives occur in a thin klippe on top of the Sambagawa belt, which would be difficult for a major structure that separates two lithospheric plates. Therefore, we follow [START_REF] Ruffet | Large-scale displacement along the Median Tectonic Line, Japan: evidence from SHRIMP zircon U-Pb dating of granites and gneisses from the South Kitakami and paleo-Ryoke belts[END_REF] and [START_REF] Takagi | Restoration of exotic terranes along the Median Tectonic Line, Japanese Islands: overview[END_REF] who correlated the Takanuki series to the Higo metamorphic complex, the latter being associated with the Paleo-Ryoke terrane, an element of the proto-Japan superterrane.

Tectonic position of the Hida-Oki terrane

In Isozaki and Itaya's 1991 model, progressive accretion of younger oceanic rocks took place below crystalline rocks of the Hida-Oki terrane, regarded as reworked Precambrian crust of the East Asian continental margin, formed by the South China craton (Isozaki 1997a;[START_REF] Maruyama | Paleogeographic maps of the Japanese Islands: plate tectonic synthesis from 750 Ma to the present[END_REF], or the North China craton [START_REF] Oh | A new concept on tectonic correlation between Korea, China and Japan: Histories from the late Proterozoic to Cretaceous[END_REF]. However, Sm-Nd isotope systematics and SHRIMP and CHIME zircon and monazite ages showed that the Hida-Oki terrane is not a reworked Precambrian crustal segment and the protoliths of the Hida-Oki terrane's paragneisses were deposited in Carboniferous time in a sedimentary basin that did not have an important Precambrian crystalline basement. This would suggest that the sedimentary rocks of the Hida-Oki terrane were deposited on a strongly thinned continental or oceanic crust. The multiple age, Th and U zonation of some zircon and monazite grains point to a complex history before the early Triassic metamorphism [START_REF] Suzuki | 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[END_REF]. The basin received detritus from Archaean (very minor), Palaeo-to Mesoproterozoic (dominant), as well as from Early and Late Palaeozoic sources (minor) [START_REF] Shibata | Rb-Sr whole-rock ages of Precambrian metamorphic rocks in the Kamiaso conglomerate from Central Japan[END_REF][START_REF] Suzuki | 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[END_REF][START_REF] Yamashita | U-Pb and Rb-Sr dating of the Oki metamorphic rocks, the Oki Island, Southwest Japan[END_REF][START_REF] Sano | Ion microprobe U-Pb zircon geochronology of the Hida gneiss. Finding of the oldest minerals in Japan[END_REF][START_REF] Tsutsumi | SHRIMP U-Pb dating of detrital zircons in paragneiss from Oki-Dogo Island, western Japan[END_REF]. It has been argued that the presence of both Neoarchaean and Palaeo-to Mesoproterozoic detrital material in the metasedimentary gneisses of the Hida-Oki terrane points to a mixed contribution from both the North and the South China cratons [START_REF] Sano | Ion microprobe U-Pb zircon geochronology of the Hida gneiss. Finding of the oldest minerals in Japan[END_REF][START_REF] Tsutsumi | SHRIMP U-Pb dating of detrital zircons in paragneiss from Oki-Dogo Island, western Japan[END_REF]. Whole-rock geochemistry and Sr-Nd isotope systematics of metasediments of the Hida-Oki terrane corroborate this notion as these imply that their protoliths may have been derived from several terranes of varying age and geochemical composition [START_REF] Kawano | Whole rock geochemistry and Sr isotopic compositions of Phanerozoic sedimentary rocks in the Inner Zone of the Southwest Japan Arc[END_REF]. Although the presence of a Mesoproterozoic basement below the Hida belt cannot be excluded, [START_REF] Arakawa | Crustal development of the Hida belt, Japan: evidence from Nd-Sr isotopic and chemical characteristics of igneous and metamorphic rocks[END_REF] interpreted 1.4-2.2 Ga Sm-Nd depleted mantle model ages of some paragneisses as inherited from their source rocks, such as the Yangtze block of the South China craton. However, the 750 -650 Ma zircon signature of Neoproterozoic rifting that is widespread in the Yangtze block and constitutes its most useful fingerprint [START_REF] Hacker | Subduction, collision, and exhumation in the Qinling-Dabie Orogen[END_REF]) has so far not been recognised in Hida-Oki rocks. This would suggest that the importance of the South China craton as source was limited. Sm-Nd isotope systematics of the Hida gneisses and Precambrian rocks of the North China craton are different [START_REF] Arakawa | Crustal development of the Hida belt, Japan: evidence from Nd-Sr isotopic and chemical characteristics of igneous and metamorphic rocks[END_REF]. This may suggest that the clastic sediments were not principally derived from the East Asian continents. This makes it also unlikely that the Qinling-Dabie-Sulu suture zone would continue to the Hida-Oki terrane. It is a distinct possibility that the clastic sediments of the Hida-Oki terrane were deposited in a deep marine continental margin of the proto-Japan superterrane and that some of the basaltic lavas represent accreted or imbricated parts of an oceanic basin bordering it. Pre-Devonian fossil flora and fauna of one of the elements of the proto-Japan superterrane, and the Kurosegawa terrane, have affinities with Australia, suggesting that it may have been Gondwana-derived [START_REF] Yoshikura | Kurosegawa terrane[END_REF][START_REF] Aitchison | Kurosegawa terrane: disrupted remnants of a low latitude Paleozoic terrane accreted to SW Japan[END_REF][START_REF] Aitchison | Allochthonous Silurian volcaniclastic rocks of the Kurosegawa terrane: longdispersed early Gondwana fragments[END_REF]. Consequently, the possibility that Precambrian detritus in the Hida-Oki metasediments was Gondwana-derived has to be considered. It is striking that in spectra of mainly SHRIMP U-Pb ages in zircons of bedrock and detritus in sediments in Australia, New Zealand and other parts of East Gondwana [START_REF] Veevers | Gondwanaland from 650-500 Ma assembly through 320 Ma merger in Pangea to 185-100 Ma breakup: supercontinental tectonics via stratigraphy and radiometric dating[END_REF], data in the range of 750-650 Ma are virtually lacking.

Proto-Japan superterrane: Late Carboniferous-Permian magmatism and Triassic metamorphism

Compelling evidence has been produced that demonstrates that the South Kitakami terrane, as well as parts of the Kurosegawa, Hida Gaien and Paleo-Ryoke terranes that we have grouped as the proto-Japan superterrane were situated in an active continental margin, an immature volcanic arc and/or a back-arc basin in late Carboniferous and Permian time. Coarse-grained sandstones and conglomerates of Kurosegawa and South Kitakami occur as clastic wedges deposited by gravity flows in Permian off-shore muddy facies, overlying a Carboniferous series with a regional unconformity [START_REF] Takeuchi | Changes in garnet chemistry show a progressive denudation of the source areas for Permian-Jurassic sandstones, Southern Kitakami Terrane, Japan[END_REF][START_REF] Hada | U-Pb zircon ages for the Mitaki igneous rocks, Siluro-Devonian tuff, and granitic boulders in the Kurosegawa Terrane, Southwest Japan[END_REF][START_REF] Hada | Kurosegawa Terrane in Southwest Japan: disrupted remnants of a Gondwana-derived terrane[END_REF][START_REF] Yoshida | Provenance of Permian sandstones, South Kitakami Terrane, Northeast Japan: implications for Permian arc evolution[END_REF]. These clastic rocks contain abundant detritus of andesites, granites and their contact metamorphic aureoles and skarns, as well as silicic volcanic and hypabyssal rocks [START_REF] Takeuchi | Changes in garnet chemistry show a progressive denudation of the source areas for Permian-Jurassic sandstones, Southern Kitakami Terrane, Japan[END_REF][START_REF] Hada | U-Pb zircon ages for the Mitaki igneous rocks, Siluro-Devonian tuff, and granitic boulders in the Kurosegawa Terrane, Southwest Japan[END_REF][START_REF] Hada | Kurosegawa Terrane in Southwest Japan: disrupted remnants of a Gondwana-derived terrane[END_REF][START_REF] Yoshida | Provenance of Permian sandstones, South Kitakami Terrane, Northeast Japan: implications for Permian arc evolution[END_REF]. U-Pb zircon ages of granitic boulders derived from the magmatic arc fall within the range of stratigraphic ages for the conglomerates [START_REF] Hada | U-Pb zircon ages for the Mitaki igneous rocks, Siluro-Devonian tuff, and granitic boulders in the Kurosegawa Terrane, Southwest Japan[END_REF]references in de Jong et al. 2006). Sr-Nd isotope systematics of a garnet-biotite granodiorite pebble from Kurosegawa's middle Permian Kozaki formation (western Kyushu), which is comparable to the Usuginu conglomerate of the South Kitakami terrane, suggest a source rock in an intra-oceanic arc [START_REF] Shimizu | Rb-Sr isotopic and REE geochemistry on garnet-biotite granodiorite clasts from the Permian Kozaki Formation, Kumamoto, Southwest Japan: a slightly enriched initial Sr isotope ratio and Mtype REE tetrad pattern[END_REF]. The Permian intrusive suites of the Paleo-Ryoke terrane are considered as the source of such clastic rocks in the South Kitakami and Kurosegawa terranes [START_REF] Takagi | Restoration of exotic terranes along the Median Tectonic Line, Japanese Islands: overview[END_REF]. The Hida Gaien terrane experienced widespread, mainly felsic pyroclastic volcanism in the late Carboniferous and Permian, possibly in a back-arc basin [START_REF] Tsukada | Redefinition of the Hida Gaien belt[END_REF][START_REF] Kawajiri | Petrology of gabbroic rocks in the Hida Gaien belt in the northern part of Takayama City, Gifu Prefecture, central Japan[END_REF].

At least, part of the terranes of the proto-Japan superterrane was affected by metamorphism in the Triassic. Metamorphic titanite from a tuff in the Siluro-Devonian volcano-sedimentary sequence of the Kurosegawa terrane in Shikoku yielded a U-Pb Concordia age of 210.5 ± 3.6 Ma [START_REF] Hada | U-Pb zircon ages for the Mitaki igneous rocks, Siluro-Devonian tuff, and granitic boulders in the Kurosegawa Terrane, Southwest Japan[END_REF]. Muscovite and biotite from the Unazuki schists of the Hida Gaien terrane yielded Rb-Sr ages in the 250 -210 Ma range (references in: [START_REF] Banno | Metamorphic belts of Japanse Islands[END_REF][START_REF] Dallmeyer | Tectonometamorphic 40 Ar/ 39 Ar mineral ages from the Oki metamorphic complex, Oki-Dogo, southwest Japan: implications for regional correlations[END_REF]. The Unazuki schists principally comprise ferroan peraluminous metasedimentary rocks associated with well-bedded metamorphosed carbonates, which yielded Late Carboniferous bryozoa and foraminifera [START_REF] Banno | Metamorphic belts of Japanse Islands[END_REF]Isozaki 1997a, and references therein). Metamorphism has given rise to kyanite-sillimanite assemblages and some micaschists are chloritoid-and staurolite-bearing [START_REF] Banno | Metamorphic belts of Japanse Islands[END_REF]. Other chloritoid-bearing metamorphic assemblages have until now only been encountered in the Ryuhozan metamorphic rocks of the Paleo-Ryoke terrane on Kyushu (Sakashima et al. 2003). The Ryuhozan series furthermore contain bedded limestone including Early to Middle Permian fusulinid fossils (Sakashima et al. 2003, and references therein). Both the Unazuki and the Ryuhozan series did not form part of subduction-accretion complexes but are regarded as deposited in a continental shelf environment that was deformed and metamorphosed in the Triassic [START_REF] Sohma | Hida metamorphic belt[END_REF]Isozaki 1997a;[START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF][START_REF] Takagi | Restoration of exotic terranes along the Median Tectonic Line, Japanese Islands: overview[END_REF].

Triassic collision of the Proto-Japan superterrane Since the work of [START_REF] Isozaki | Pre-Jurassic klippe in northern Chichibu belt in west-central Shikoku, Southwest Japan-Kurosegawa terrane as a tectonic outlier of the pre-Jurassic rocks of the Inner zone[END_REF], subduction-accretion is envisaged to have occurred ocean ward of the Hida-Oki terrane. Most models do not take into account that the Suo belt, which yielded white mica with a 225 Ma 40 Ar/ 39 Ar plateau age, continues to the southernmost islands of the Ryukyu arc (Fig. 2; [START_REF] Faure | Microtectonics and 39 Ar-40 Ar dating of high pressure metamorphic rocks of the south Ryukyu Arc and their bearings on the pre-Eocene geodynamic evolution of Eastern Asia[END_REF][START_REF] Nishimura | Geotectonic subdivision and areal extent of the Sangun belt, Inner Zone of Southwest Japan[END_REF][START_REF] Ishiwatari | Paleozoic ophiolites and blueschists in Japan and Russian Primorye in the tectonic framework of East Asia: a synthesis[END_REF]. The rocks of the Suo belt must have accreted against some kind of margin, which could not have been the Hida-Oki terrane as this was undergoing deformation and metamorphism at about the same time. This is an indication that the Permian subductionaccretion complex continued far southward and formed against a margin that is independent of the North and South China cratons. We envisage that this tectonic entity was the proto-Japan superterrane (Figs. 6,7). In pre-Cretaceous time, we position the proto-Japan superterrane between Hida-Oki terrane and the subduction-accretion complexes (Figs. 6,7). Active subduction occurred along the southeastern margin of proto-Japan leading to formation of Permian subduction-accretion complexes (Akiyoshi, Maizuru and Ultra-Tamba terranes). Late Middle to early Late Triassic metamorphism not only affected the associated Suo belt, but also the Sakaigawa (Kii peninsula) and Agekura (Shikoku) units (Fig. 7a). Post Early Jurassic tectonic movements related to strike-slip displacements in the Hida Gaien belt, formation of the Jurassic to early Cretaceous subduction-accretion complexes and exhumation of the Cretaceous Sambagawa belt, and finally the strike-slip movement along the MTL, resulted in disruption of the palaeo-Pacific margin of proto-Japan. De [START_REF] De Jong | Ordovician 40 Ar/ 39 Ar phengite ages from the blueschist-facies Ondor Sum subduction-accretion complex (Inner Mongolia) and implications for the early Paleozoic history of continental blocks in China and adjacent areas[END_REF] argued that older subduction-accretion complexes like the Hayachine belt in the northern margin of South Kitakami terrane, and the Renge belt, which may contain fragments of the palaeo-Pacific ocean floor of Late Devonian age, and possibly as old as Cambrian (Oeyama ophiolite), had already accreted to the southeastern margin of the proto-Japan superterrane.

The late Permian to middle Triassic metamorphism in the Hida-Oki terrane may be related to the collision of the proto-Japan superterrane with East Asia's active margin along the Central Asian Orogenic Belt's extension (Fig. 6). The metamorphism that affected part of the proto-Japan superterrane in the Triassic, notably the Unazuki and the Ryuhozan series, may similarly be due to this collision. Proto-Japan was fringed by pre-Permian subductionaccretion complexes and high-pressure metamorphic belts and carried a Permian magmatic arc, all formed during subduction of palaeo-Pacific oceanic lithosphere below the microcontinent (Figs. 6,7a). [START_REF] Kamada | Coarse clastic sedimentation in the Triassic offshore sequence of the southeastern Kitakami Mountains, Japan[END_REF] and [START_REF] Ehiro | A time-gap at the Permian-Triassic boundary in the South Kitakami Belt, Northeast Japan: An examination based on the ammonoid fossils[END_REF] have shown that South Kitakami's Early Triassic series rests unconformably on Late Permian rocks with a basal conglomerate. The Triassic clastic wedges too were deposited by gravity flows with provenance from the west in a fan delta/submarine-fan sedimentary system that was developed in a margin parallel strike-slip fault zone [START_REF] Kamada | Coarse clastic sedimentation in the Triassic offshore sequence of the southeastern Kitakami Mountains, Japan[END_REF]. Such coarse-grained clastic series may have been deposited in a foreland basin to the southeast of the collison zone. Large amounts of acidic to intermediate tuffs and volcaniclastic rocks in Late Triassic series of the South Kitakami terrane suggest continuing volcanism and/or exhumation of such rocks in the hinterland [START_REF] Takeuchi | Changes in garnet chemistry show a progressive denudation of the source areas for Permian-Jurassic sandstones, Southern Kitakami Terrane, Japan[END_REF]. Also zircons from granitic pebbles in series of early Middle Jurassic age of the Kurosegawa terrane that yielded U-Pb ages of 204 ± 15 Ma [START_REF] Hada | U-Pb zircon ages for the Mitaki igneous rocks, Siluro-Devonian tuff, and granitic boulders in the Kurosegawa Terrane, Southwest Japan[END_REF], suggest that magmatic activity related to subduction continued into the Late Triassic. Interestingly, also the Late Triassic Mine group, which overlies the Permian Akiyoshi subduction-accretion complex of the Inner Zone, records a change in detritus from sedimentary rocks derived from the immediate substratum to granulite-to amphibolite-facies metamorphic rocks, granitoid and skarn [START_REF] Kametaka | Provenance of the Upper Triassic mine group, Southwest Japan[END_REF]. This may point to erosion of a magmatic arc [START_REF] Kametaka | Provenance of the Upper Triassic mine group, Southwest Japan[END_REF] and/or of exhumed regional metamorphic rocks of a collision zone situated to the north of the Palaeozoic subduction-accretion complex of the Inner Zone.

The position of the proto-Japan superterrane in the late Permian to middle Triassic, close to the South China craton at a palaeo-equatorial latitude in the palaeo-Pacific (Fig. 6a) is in agreement with reconstructions based on Middle Permian fusulinacean fossils (Kurosegawa terrane, Colania-Lepidolina territory; [START_REF] Hada | Kurosegawa Terrane in Southwest Japan: disrupted remnants of a Gondwana-derived terrane[END_REF], Early to Middle Permian ammonoids (South Kitakami terrane; [START_REF] Ehiro | Permian ammonoids Prostacheocers and Perrinites from the Southern Kitakami Massif, Northeast Japan[END_REF] and Early to Middle Permian rugose coral faunas that are comparable to those in South China but absent in North China (South Kitakami terrane; [START_REF] Wang | Permian rugose coral faunas of Inner Mongolia-Northeast China and Japan: paleobiogeographical implications[END_REF].

A geodynamic reconstruction introducing the proto-Japan superterrane relieves the somewhat forced efforts to correlate terranes in Korea and Japan. It specifically implies that the Qinling-Dabie-Sulu suture zone, the position of which is not resolved even in Korea, cf. [START_REF] Ernst | Permo-Triassic collision, subduction-zone metamorphism, and tectonic exhumation along the East Asian continental margin[END_REF] and [START_REF] Oh | The Late Permian to Triassic Hongseong-Odesan collision belt in South Korea, and its tectonic correlation with China and Japan[END_REF], does not continue to Japan, but joins the southern extension of the Central Asian Orogenic Belt (Fig. 6). The currently available time constraints do not permit to distinguish between the age of the collision of the Yangtze block with the North China craton, the main metamorphism in the Hida-Oki terrane, and the age of metamorphic recrystallisation in the Permian subduction-accretion complexes in Japan. Consequently, this reconstruction envisages that these events were essentially coeval, and in a way independent. The Hida-Oki terrane is thus not a backstop to the Japanese subduction-accretion complexes, but is a part of a complex collision zone between proto-Japan and East Asian margin. The complex and heterogeneous nature of the Hida-Oki terrane is particularly well illustrated by the geochemistry of amphibolite layers in paragneisses that points to formation in a withinplate setting not affected by a subduction zone, a continental margin affected by a subduction zone or a continental island arc, an island arc or a mid ocean ridge [START_REF] Arakawa | Crustal development of the Hida belt, Japan: evidence from Nd-Sr isotopic and chemical characteristics of igneous and metamorphic rocks[END_REF][START_REF] Arakawa | Geochemical characteristics of amphibolites in the Oki metamorphic rocks, Oki-Dogo Island, southwestern Japan: mixed occurrence of amphibolites with different geochemical affinity[END_REF]. Stacking of thrust slices with different provenance or even subduction-accretion could easily account for the diversity of these basaltic rocks of inferred Carboniferous to Permian age. [START_REF] Takeuchi | Changes in garnet chemistry show a progressive denudation of the source areas for Permian-Jurassic sandstones, Southern Kitakami Terrane, Japan[END_REF] documented a fundamental change in the sources of sedimentary rocks around Triassic-Jurassic boundary time, not only in the South Kitakami terrane, but also in most other Japanese terranes. He interpreted the abundance of detrital chromian spinels and pyrope-almandine-rich garnets in Early Jurassic sandstones, in contrast to the pre-Jurassic deposits, as derived from eroding upper mantle peridotite and cumulate rocks of an island arc ophiolitic complex and a medium-to high-grade metamorphic belt. The Hida-Oki terrane is the only possible source of such rocks in Japan. In this respect, the occurrence of chloritoid in South Kitakami's Early Jurassic deposits is paramount, as this mineral was not formed during metamorphism of the pelites of the subduction-accretion complexes because they were too poor in Al. As already mentioned, chloritoid has only been encountered in ferroan peraluminous rocks of the proto-Japan superterrane, namely the Unazuki schists (Hida Gaien terrane) and the Ryuhozan schists (Paleo-Ryoke terrane). Middle to Late Jurassic clastic series of the South Kitakami terrane contain many pebbles of acidic volcanic and granitic rocks that are petrographically different from those found in Permo-Triassic conglomerates [START_REF] Takeuchi | Changes in garnet chemistry show a progressive denudation of the source areas for Permian-Jurassic sandstones, Southern Kitakami Terrane, Japan[END_REF], suggesting erosion of a Jurassic magmatic arc, of which Funatsu-type intrusions were part (Fig. 7b).

Jurassic exhumation and erosion of the collision zone

As indicated earlier, sandstones and conglomerate in trench fill deposits of the Japanese Jurassic subduction-accretion complexes demonstrate a substantial influx of granitic and metamorphic sources [START_REF] Adachi | Precambrian detrital monazites and zircons from Jurassic turbidite sandstones in the Nomugi area, Mino terrane[END_REF][START_REF] Tanaka | Occurrence of gneiss clasts with Mg-rich garnets from the Sawando conglomerate in the northeastern Mino terrane: source of detrital Mg-rich garnets in the Jurassic sandstone of the Mino terrane[END_REF][START_REF] Sano | Ion microprobe U-Pb zircon geochronology of the Hida gneiss. Finding of the oldest minerals in Japan[END_REF][START_REF] Okamoto | SHRIMP U-Pb zircon dating of quartz-bearing eclogite from the Sanbagawa Belt, south-west Japan: implications for metamorphic evolution of subducted protolith[END_REF][START_REF] Nutman | 743 ± 17 Ma granite clast from Jurassic conglomerate, Kamiaso, Mino Terrane, Japan: the case for South China Craton provenance (Korean Gyeonggi Block?)[END_REF]Aoki et al. 2007). [START_REF] Sano | Ion microprobe U-Pb zircon geochronology of the Hida gneiss. Finding of the oldest minerals in Japan[END_REF] favoured a mixed contribution from both the North and the South China cratons to explain the presence of both Neoarchaean and Palaeo-to Mesoproterozoic detrital material. On the basis of euhedral zircons without obvious inheritance that yielded a SHRIMP U-Pb date of 743 ± 17 Ma, [START_REF] Nutman | 743 ± 17 Ma granite clast from Jurassic conglomerate, Kamiaso, Mino Terrane, Japan: the case for South China Craton provenance (Korean Gyeonggi Block?)[END_REF] underlined the importance the South China craton as source area. However, the 250 and 220 Ma-old zircons [START_REF] Sano | Ion microprobe U-Pb zircon geochronology of the Hida gneiss. Finding of the oldest minerals in Japan[END_REF] and the youngest ca. 180 Ma-old monazite [START_REF] Adachi | Precambrian detrital monazites and zircons from Jurassic turbidite sandstones in the Nomugi area, Mino terrane[END_REF] and179.3 ± 12.1 Ma-old igneous zircons [START_REF] Nutman | 743 ± 17 Ma granite clast from Jurassic conglomerate, Kamiaso, Mino Terrane, Japan: the case for South China Craton provenance (Korean Gyeonggi Block?)[END_REF]) could equally well point to erosion of the metamorphic rocks of the proto-Japan superterrane, and/or of the Hida-Oki terrane with Funatsu-type granitoids. [START_REF] Dallmeyer | Tectonometamorphic 40 Ar/ 39 Ar mineral ages from the Oki metamorphic complex, Oki-Dogo, southwest Japan: implications for regional correlations[END_REF] suggested that pebbles of (sillimanite) gneisses, chloritoidbearing phyllite and bedded limestone that occur in Jurassic trench fill sandstones of the Mino-Tamba-Ashio subduction-accretion terrane, may have been derived from the Hida-Oki terrane and the Unazuki schists.

We interpret this evolution as indicative of deep erosion of the collision zone between East Asia and the docked proto-Japan superterrane, of which the Hida-Oki terrane was an element (Figs. 6b,7b). The occurrence of late Early Jurassic igneous material shows that this event took place during continued westward subduction of palaeo-Pacific oceanic lithosphere (Figs. 6b,7b). During this process, the huge Jurassic subduction-accretion complex and the Funatsu-type granitoids, which show calc-alkaline chemical affinity indicating emplacement in a continental margin or continental arc environment [START_REF] Arakawa | Crustal development of the Hida belt, Japan: evidence from Nd-Sr isotopic and chemical characteristics of igneous and metamorphic rocks[END_REF], were formed (Fig. 7b).

Permian to Triassic active margin in East Asia

To the south of the Ryukyu Arc of Japan, Jurassic subduction-accretion complexes occur as isolated belts, probably in Taiwan, the Philippines and Indonesia (e.g. [START_REF] Hamilton | Tectonics of the Indonesian region[END_REF]Zamoras andMatsuoka 2001, 2004;Wakita and Metcalfe 2005, and references therein). The part of Taiwan located to the west of the subduction-accretion complexes (Tananao complex), belongs to the eastern margin of the South China craton (Figs. [START_REF] Masago | Exhumation of the Sanbagawa high-pressure metamorphic belt, SW Japan-constraints from the upper and lower boundary faults[END_REF]2). A peculiar feature of the craton is the ca. 1300 km-wide South China Indosinian Orogen that occupies its southeastern half [START_REF] Xiao | Early Mesozoic thrust tectonics of the NW Zhejiang region (Southeast China)[END_REF]Li and Li 2007, and references therein). The southeastern part of the craton, the Cathaysia block, contains an extensive belt of granitoids that has been interpreted as a continental margin magmatic arc within which a granitic belt of Mesozoic and possibly Palaeozoic age was generated due to northwestward subduction of oceanic lithosphere [START_REF] Jahn | Rb-Sr ages of granitic rocks in southeastern China and their tectonic significance[END_REF][START_REF] Li | Initiation of the Indosinian Orogeny in South China: evidence for a Permian magmatic arc on Hainan Island[END_REF]. Syntectonic calc-alkaline I-type granites that yielded SHRIMP U-Pb zircon ages of 267 ± 3 and 262 ± 3 Ma [START_REF] Li | Initiation of the Indosinian Orogeny in South China: evidence for a Permian magmatic arc on Hainan Island[END_REF] indicate that this arc was present at least as far south as Hainan Island (Figs. 2, 6a) in late Early Permian time. Also Vietnam experienced important Indosinian magmatism, high-temperature metamorphism and ductile deformation in the Late Permian to Early Triassic period (Lepvrier et al. 2004 and references therein).

South China

The South China craton is divided by the 2,000-km long northeast-southwest trending Jiangshan-Shaoxing fault zone into the northwestern Yangtze block and the southeastern Cathaysia block. The fault zone contains an ophiolitic mélange with blocks of ultramafic and mafic rocks, including basalt and blueschist [START_REF] Zhao | Tectonothermal evolution of the Mayuan assemblage in the Cathaysia Block: Implications for Neoproterozoic collision-related assembly of the South China craton[END_REF][START_REF] Xiao | Early Mesozoic thrust tectonics of the NW Zhejiang region (Southeast China)[END_REF]. On the basis of ca. 870-820 Ma isotopic ages for such rocks, the fault zone has been interpreted as a suture zone along which both blocks collided in the Neoproterozoic, forming the South China craton that remained a single entity thereafter [START_REF] Zhao | Tectonothermal evolution of the Mayuan assemblage in the Cathaysia Block: Implications for Neoproterozoic collision-related assembly of the South China craton[END_REF]. [START_REF] Li | Formation of the 1300-km-wide intracontinental orogen and postorogenic magmatic province in Mesozoic South China: a flat-slab subduction model[END_REF] regarded the South China Indosinian Orogen, which was accompanied by granitoid intrusions of mainly Triassic age, as intracontinental. They presented a flat-slab model to explain the very broad nature of the belt, and the northwest migration of magmatism in time and space, from the coastal region (mid-Permian) to the interior (Late Triassic or Early Jurassic). They suggested that the transformation of a passive continental margin of the South China craton into an active one was triggered by the onset of its collision with the North China craton in the late Early Permian, that is around ca. 270 Ma (Fig. 6a). [START_REF] Faure | Doming in the southern foreland of the Dabieshan (Yangtze block, China)[END_REF] and [START_REF] Li | Tectonic history of the major East Asian lithospheric blocks since the mid-Proterozoic-a synthesis[END_REF] explained Triassic deformation developed in the northern part of the South China craton up to 400 km to the south of the Dabieshan, by a far-field stress regime related to the collision with the North China craton.

The late Palaeozoic to early Mesozoic tectonic evolution of South China has also been discussed within the framework of accretion and collision of terranes in an archipelago palaeogeography similar to the present-day Southwest Pacific [START_REF] Hsü | Tectonic facies in an archipelago model of intra-plate orogenesis[END_REF][START_REF] Xiao | Early Mesozoic thrust tectonics of the NW Zhejiang region (Southeast China)[END_REF]. The latter authors pointed out that a number of isotopical, micropalaeontological, sedimentological and structural data imply that the Cathaysian block on the one hand and the Yangtze block and the Lower Yangtze sub block on the other, were independent microplates in the late Palaeozoic to Early Triassic. In the first place, Neoproterozoic and younger sedimentary series of the Cathaysia block are disrupted along the Jiangshan-Shaoxing fault [START_REF] Zhao | Tectonothermal evolution of the Mayuan assemblage in the Cathaysia Block: Implications for Neoproterozoic collision-related assembly of the South China craton[END_REF], and isotopic ages as young as Triassic have been reported (references in: [START_REF] Xiao | Early Mesozoic thrust tectonics of the NW Zhejiang region (Southeast China)[END_REF]. The latter authors underscored that Carboniferous to Late Permian fossils, including radiolaria in chert blocks in ophiolite mélange, occur in rocks of inferred Neoproterozoic age along this fault zone and associated faults. A gneissic granodiorite with island arc geochemical signature that occurs as fragment in a mélange, yielded zircons with a 242 ± 2 Ma U-Pb age (Kong et al. 1995, in: Xiao and[START_REF] Xiao | Early Mesozoic thrust tectonics of the NW Zhejiang region (Southeast China)[END_REF]. The southeastern continental margin of the Yangtze block and Lower Yangtze sub block (Fig. 6a) formed a SE-facing continental margin that evolved from shallow marine condition, during most of the Permian, to a deep water environment through the Early Triassic [START_REF] Xiao | Early Mesozoic thrust tectonics of the NW Zhejiang region (Southeast China)[END_REF]. Subsequently, a foreland fold-and-thrust belt developed in these continental margin series that is unconformably overlain by weakly deformed, Late Triassic continental, molassetype deposits. [START_REF] Xiao | Early Mesozoic thrust tectonics of the NW Zhejiang region (Southeast China)[END_REF] emphasised that these observation can only be explained by regarding the Cathaysian block as a magmatic arc related to northwestward subduction of the Pacific plate in late Palaeozoic and Early Triassic times. The ophiolite mélange zones along the Jiangshan-Shaoxing fault and associated faults represent, in their view, the remnants of a back-arc basin, which was closed in mid-Triassic time. As a result of this, the Cathaysian arc collided with the Yangtze block and the Lower Yangtze sub block, leading to deformation of the sedimentary prisms of the continental margins of these blocks, before the Late Triassic. Xiao and He's tectonic model of the Cathaysian arc colliding with the Yangtze block is similar to our proposed model for the proto-Japan superterrane.

Indochina and Southeast Asia

The Jurassic subduction-accretion complexes of the west central parts of the Philippines (Zamoras andMatsuoka 2001, 2004) were grouped into the North Palawan block by [START_REF] Hamilton | Tectonics of the Indonesian region[END_REF]. The nature of the Southeast Asian margin off Indochina between Palawan and the small oceanic basin of the South China Sea (Fig. 2) in the area of the Spratly Islands and Dangerous Grounds is not well known. This 400 km wide and 900 km long terrane ("Nansha block"), mostly shallower than 2 km, consist of thinned continental crust with a tilted fault block and horst block topography covered by thin Neogene sediments [START_REF] Hamilton | Tectonics of the Indonesian region[END_REF][START_REF] Morley | A tectonic model for the Tertiary evolution of strike-slip faults and rift basins in SE Asia[END_REF][START_REF] Yan | Tectonic-stratigraphic division and blind fold structures in Nansha Waters, South China Sea[END_REF]. Locally occurring Mesozoic marine sediments, including Late Triassic/Early Jurassic siltstone, shale and volcanic rocks, overlie an acoustic basement [START_REF] Yan | Tectonic-stratigraphic division and blind fold structures in Nansha Waters, South China Sea[END_REF].

U-Pb zircon and 40 Ar/ 39 Ar mineral ages in Indochina span the ca. 260 -240 Ma range (see [START_REF] Lepvrier | The Early Triassic Indosinian orogeny in Vietnam (Truong Son Belt and Kontum Massif); implications for the geodynamic evolution of Indochina[END_REF] for a review). Relics of ca. 260 Ma-old ultrahigh-temperature metamorphism occur in the Kontum massif of central Vietnam [START_REF] Owada | Timing of metamorphism and formation of garnet granite in the Kontum Massif, central Vietnam: Evidence from monazite EMP dating[END_REF]. The main Indosinian magmatism, high-temperature metamorphism and ductile deformation of Late Permian to Early Triassic age is usually explained as resulting from collision between Indochinan terranes and South China [START_REF] Lepvrier | The Early Triassic Indosinian orogeny in Vietnam (Truong Son Belt and Kontum Massif); implications for the geodynamic evolution of Indochina[END_REF] and references therein). In the light of the occurrence of late Palaeozoic to early Mesozoic magmatic arcs on microcontinental fragments that collided with the East Asian margin farther north, as discussed above, the docking of such an arc, like the Nansha continental block, with Southeast Asia's active margin during westward subduction of Pacific oceanic lithosphere below the amalgamated Indochinan terranes (Fig. 6), might have contributed equally well.

Miocene rifting concentrated in Permo-Triassic collision zones

The late Oligocene-Middle Miocene and younger back-arc basins of the Sea of Japan and the Okinawa trough [START_REF] Kaneoka | Isotope geochemistry of igneous rocks in the Japan Sea[END_REF][START_REF] Maruyama | Paleogeographic maps of the Japanese Islands: plate tectonic synthesis from 750 Ma to the present[END_REF][START_REF] Van Der Werff | Backarc deformation along the eastern Japan Sea margin, offshore northern Honshu[END_REF][START_REF] Hall | Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations[END_REF] coincide with the trace of the Hida-Oki terrane (Fig. 2). The continental crust of the Yamato Bank in the centre of the Sea of Japan rift may be a part of the Hida-Oki terrane [START_REF] Kaneoka | Isotope geochemistry of igneous rocks in the Japan Sea[END_REF]. The South China Sea rift basin may similarly have formed in lithosphere affected by thickening during the Triassic "Indosinian" event. The most important extension in the area between the small oceanic basin of the South China Sea and Palawan including the Spratly Islands and Dangerous Grounds (Fig. 2) occurred during the Eocene-Oligocene [START_REF] Morley | A tectonic model for the Tertiary evolution of strike-slip faults and rift basins in SE Asia[END_REF]. Due to rifting and subsequent seafloor spreading during the mid-Oligocene to early Miocene, which created the South China Sea [START_REF] Hall | Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations[END_REF]Morley 2002, and references therein), the North Palawan block broke away from the East Asian continent, migrated southward and collided with the Philippine island arc system in the middle Miocene (Zamoras and Matsuoka 2004, and references therein).

In a review, [START_REF] Cloetingh | Lithospheric memory, state of stress and rheology: neotectonic controls on Europe's intraplate continental topography[END_REF] pointed out that during rifting the weakest parts of the lithosphere will yield first once stress levels equate their strength. These authors underlined that lithological discontinuities that enhance the mechanical anisotropy of the lithosphere can weaken the strong upper part of the mantle-lithosphere of former orogenic belts. Compared to the crust of old stable cratonic areas, younger orogenic belts have a higher crustal thickness and an elevated heat flow; their strong upper parts of the crust are thus thinned and weakened. Moreover, former orogens contain deep-reaching crustal discontinuities, such as thrust-and strike-slip faults, which can further weaken the upper parts of the crust and can be tensionally reactivated. Former orogens, thus, constitute rheologically weak zones that can be preferentially reactivated during rifting. Hence, the stress exerted on the East Asian continental margin since middle Eocene time by slab pull forces generated by the subducting Pacific oceanic lithosphere [START_REF] Hall | Cenozoic geological and plate tectonic evolution of SE Asia and the SW Pacific: computer-based reconstructions, model and animations[END_REF] seems to have preferentially reactivated Permo-Triassic collision zones of the margin. This very mechanism is thus an argument in favour of the presence of collision zones along the East Asian continental margin formed around Permo-Triassic boundary times, and may have been the extension of the Central Asian Orogenic Belt into the circum-Pacific region.

Conclusions

The 218, 229 and 232 Ma 40 Ar/ 39 Ar pseudo-plateau ages date the very-low-grade highpressure metamorphism in the Sakaigawa unit, which is part of the Permian subductionaccretion complex that continues to the southernmost part of the Ryukyu arc. This late Middle to early Late Triassic tectono-metamorphic event is virtually coeval with the main metamorphism in the Hida-Oki terrane and the docking of the North China and Yangtze cratons. The Hida-Oki terrane is not a backstop to the Japanese subduction-accretion complexes, but is part of a complex collision zone along the East Asian margin, which is considered as the southward extension of the Central Asian Orogenic Belt. The Japanese islands contain dispersed terranes that include fragments of a Permian island arc system with a basement of Early Palaeozoic igneous and metasedimentary rocks. These terrane fragments have been grouped into the proto-Japan superterrane, also situated in the extension of the Central Asian Orogenic Belt. The main metamorphism in the Hida-Oki terrane occurred during the collision of the proto-Japan superterrane with the East Asian active margin. Subduction-accretion and metamorphism of the Sakaigawa unit took place on the Pacific side of the proto-Japan superterrane, during and following this collision.

The concept of the proto-Japan superterrane implies that the Qinling-Dabie-Sulu suture zone did not continue to Japan, but joined the Central Asian Orogenic Belt to the east of the North China craton. It is possible that this orogenic system continued farther to the south via Taiwan and the Cathaysia block in China to Indochina. The Late Permian to Middle Triassic Indosinian thermo-tectonic event might, consequently, also be due to docking of Pacificderived terranes with Southeast Asia's continental margin.

Major implications of this model are that (1) early Palaeozoic subduction-accretion complexes formed alongside the proto-Japan superterrane, before it collided with East Asia, and (2) only Jurassic and younger rocks derived from subducted Pacific plates directly accreted to the East Eurasian margin and received detritus from the earlier docked proto-Japan superterrane, the exhumed and deeply eroding Hida-Oki collision belt and Eurasia's eastern margin. 3 Tectonic sketch map of Japan, compiled using the 1:1,000,000 map sheets of the 1995 on-line version of the geological atlas of the Geological Survey of Japan (http://www.aist.go.jp/GSJ/PSV/Map/mapIndex.html), with modifications after Nishimura (1998), [START_REF] Kato | Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan[END_REF], [START_REF] Takagi | Restoration of exotic terranes along the Median Tectonic Line, Japanese Islands: overview[END_REF], [START_REF] Ishiwatari | Paleozoic ophiolites and blueschists in Japan and Russian Primorye in the tectonic framework of East Asia: a synthesis[END_REF], [START_REF] Tsukada | Redefinition of the Hida Gaien belt[END_REF], [START_REF] Tsujimori | Eclogite-facies mineral inclusions in clinozoisite from Paleozoic blueschist, central Chugoku mountains, Southwest Japan: evidence of regional eclogite-facies metamorphism[END_REF] The pre-Permian subduction-accretion complexes and high-pressure metamorphic belts had already accreted to the southeastern margin of the proto-Japan superterrane during earlier events. b Deep erosion of the collisional zone (Hida-Oki terrane) between the proto-Japan superterrane and the East Eurasian margin in the Jurassic. Clastic sediments (Early Jurassic Kuruma Group, Hida-Oki terrane) were deposited in a fore-arc and comparable rocks as trench fill deposits in the Jurassic complex. Westward subduction of palaeo-Pacific oceanic lithosphere continued, forming the Jurassic subduction-accretion complex and a continental margin arc (Funatsu-type suite) Tables Table 1 Correlation of tectonic units in the Outer and Inner Zones of Japan on central Shikoku, which are separated by the Median Tectonic Line Modified after Isozaki (1997a, b), [START_REF] Nakajima | Regional metamorphic belts of the Japanese islands[END_REF], [START_REF] Nishimura | Geotectonic subdivision and areal extent of the Sangun belt, Inner Zone of Southwest Japan[END_REF], [START_REF] Takagi | Origin of magnetite-and ilmenite-series granitic rocks in the Japan arc[END_REF][START_REF] Takagi | Restoration of exotic terranes along the Median Tectonic Line, Japanese Islands: overview[END_REF][START_REF] De Jong | Ordovician 40 Ar/ 39 Ar phengite ages from the blueschist-facies Ondor Sum subduction-accretion complex (Inner Mongolia) and implications for the early Paleozoic history of continental blocks in China and adjacent areas[END_REF]

Figures

Fig. 1

 1 Fig. 1 Tectonic sketch map of Central and East Asia (modified after de Jong et al. 2006). The Central Asian Orogenic Belt has a dark shading; cratons have a light hatching; Kazakhstan, a composite continent or a terrane assemblage formed by amalgamated microcontinental fragments with Proterozoic basement and volcanic arcs, separated by Palaeozoic subduction-accretion complexes (Windley et al. 2007; Cocks and Torsvik 2007), is not distinguished from the Central Asian Orogenic Belt. HO, Hida-Oki terrane that may be the prolongation of the Central Asian Orogenic Belt into the Pacific region. Thick lines, main subduction zones fringing the eastern margin of the Eurasian continent. Strings of dots, Solonker zone

  Fig. 1 Tectonic sketch map of Central and East Asia (modified after de Jong et al. 2006). The Central Asian Orogenic Belt has a dark shading; cratons have a light hatching; Kazakhstan, a composite continent or a terrane assemblage formed by amalgamated microcontinental fragments with Proterozoic basement and volcanic arcs, separated by Palaeozoic subduction-accretion complexes (Windley et al. 2007; Cocks and Torsvik 2007), is not distinguished from the Central Asian Orogenic Belt. HO, Hida-Oki terrane that may be the prolongation of the Central Asian Orogenic Belt into the Pacific region. Thick lines, main subduction zones fringing the eastern margin of the Eurasian continent. Strings of dots, Solonker zone

  Fig.3Tectonic sketch map of Japan, compiled using the 1:1,000,000 map sheets of the 1995 on-line version of the geological atlas of the Geological Survey of Japan (http://www.aist.go.jp/GSJ/PSV/Map/mapIndex.html), with modifications after Nishimura (1998),[START_REF] Kato | Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan[END_REF],[START_REF] Takagi | Restoration of exotic terranes along the Median Tectonic Line, Japanese Islands: overview[END_REF],[START_REF] Ishiwatari | Paleozoic ophiolites and blueschists in Japan and Russian Primorye in the tectonic framework of East Asia: a synthesis[END_REF],[START_REF] Tsukada | Redefinition of the Hida Gaien belt[END_REF],[START_REF] Tsujimori | Eclogite-facies mineral inclusions in clinozoisite from Paleozoic blueschist, central Chugoku mountains, Southwest Japan: evidence of regional eclogite-facies metamorphism[END_REF]. The high-grade to ultrahigh-grade metamorphic Sefuri (S) and Higo (H) rocks and the peraluminous Unazuki (U) and Ryuhozan (R) metasediments occur associated with the Hida Gaien and Paleo-Ryoke terranes, respectively; (T) Takanuki paragneisses. The Paleo-Ryoke terrane occurs as scatted outcrops (P and R), generally in a zone running just north of the Kurosegawa terrane. HEMF Hayachine Eastern Marginal Fault, HTL Hatagawa Tectonic Line, ISTL Itoigawa-Shizuoka Tectonic Line, MTL Median Tectonic Line, TTL Tankura Tectonic Line, NK North Kitakami, O Oki-Dogo Island. Ryukyu Islands (see Fig. 2) with local outcrops of Palaeozoic rocks, northernmost Honshu and Hokkaido are omitted for simplicity. Dashed line northern margin of the continental shelf of Japan (500 m depth). E-PS boundary of the Eurasian and Philippine Sea plates, NA-P boundary of the North American and Pacific plates, NA-PS boundary of the North American and Philippine Sea plates. The location of Fig. 4, the geological map of the western Kii peninsula where the samples were taken, is outlined

Fig. 5 40

 40 Fig.540 Ar/ 39 Ar age spectra (lower panels) and 37 Ar Ca / 39 Ar K ratio spectra (upper panels) of whole-rock metapelites from: a the main outcrop of the Sakaigawa unit; b the westernmost isolated outcrop. The pseudoplateau ages have errors quoted at 2σ and correspond to 49.0% of 39 Ar release in five steps for JK09; 62.7% in four steps for JK57; 57.6% in seven steps for JK61

Table 2 40

 2 Ar/ 39 Ar analytical data of quartz-rich whole-rock metapelites from the Sakaigawa unit, northern Wakayama prefecture, western Kii Peninsula JK

09 J factor: 0.03694458 Step # Atm. contam. (%) 39 Ar K (%) 37 Ar Ca / 39 Ar K 40 Ar*/ 39 Ar K Apparent age (Ma)

	1	50.927	0.02	0.000	1.318	85.6 ± 172.0
	2	47.609	0.26	0.276	1.046	68.3 ± 8.9
	3	18.074	0.35	0.229	1.706	110.1 ± 6.6
	4	11.081	1.54	0.144	1.239	80.7 ± 1.4
	5	5.875	2.14	0.148	1.840	118.5 ± 1.8
	6	3.155	2.46	0.123	2.812	178.1 ± 2.2
	7	1.447	2.79	0.068	3.211	201.9 ± 1.0
	6	1.095	3.55	0.060	3.415	214.1 ± 1.1
	9	1.214	3.28	0.044	3.499	219.0 ± 1.3
	10	1.354	3.90	0.054	3.452	216.3 ± 1.2
	11	0.866	7.09	0.048	3.343	209.8 ± 1.0
	12	0.567	9.21	0.053	3.427	214.8 ± 0.9
	13	0.579	15.94	0.045	3.488	218.4 ± 0.7
	14	0.688	10.56	0.061	3.496	218.8 ± 0.8
	15	0.507	11.70	0.047	3.498	218.9 ± 0.8
	16	0.908	5.06	0.066	3.480	217.9 ± 1.0
	17	0.737	5.77	0.069	3.468	217.2 ± 0.9
	18	0.729	7.07	0.182	3.391	212.7 ± 0.7
	19	1.084	5.49	0.526	3.403	213.4 ± 0.9
	20	1.750	1.57	1.466	3.505	219.4 ± 2.0
	21	8.277	0.25	6.461	3.214	202.1 ± 17.9
					Integrated age 210.2 ± 0.3
	JK 40 J factor: 0.03689923
	1	30.615 1.71 0.097 1.184	77.1 ± 2.2
	2	5.834 13.27 0.031 2.794	177.0 ± 0.8
	3	1.238 8.92 0.031 3.352	210.3 ± 0.8
	4	1.342 13.67 0.021 3.349	210.2 ± 0.8
	5	1.791 10.01 0.049 3.406	213.5 ± 0.9
	6	1.339 9.53 0.034 3.459	216.6 ± 0.8
	7	1.480 6.55 0.037 3.472	217.4 ± 1.0
	6	1.415 11.69 0.038 3.484	218.1 ± 0.8
	9	1.176 9.40 0.042 3.549	221.9 ± 1.0
	10	0.851 4.06 0.092 3.549	221.9 ± 1.0
	11	1.778 5.87 0.153 3.527	220.6 ± 1.0

Acknowledgments

This study was initiated while the senior author (KdJ) was a visiting scientist at the Geological Survey of Japan (Tsukuba) supported by grants of the European Union and the Agency of Industrial Science and Technology of Japan. The analytical work in France (CNRS, UMR 6526, Géoscience Azur, Nice) was financed by the Geological Survey of Japan. The paper was written while KdJ was associated to the Institute of Advanced Studies Studium ® (CNRS and Région Centre) on the invitation of Prof. Michel Faure, while being based at the University of Orléans. The senior author's interest in subduction-accretion processes in Southeast Asia was raised by Warren Hamilton's captivating lectures on the tectonics of the Indonesian region delivered as Guest Professor at the University of Amsterdam. Many discussions with my friends Katsumi Kimura and Takashi Nakajima at GSJ greatly improved his knowledge of the intriguing geology of the Japanese islands. KdJ and CK gratefully acknowledge the invitation by Prof. Hideo Takagi (Waseda University, Tokyo) to join an excursion to the Kanto Mountains, enabling to fathom the intricacies of the Paleo-Ryoke terrane. The invitation by Prof. Ching-Hua Lo to visit the Taiwan National University financially supported by the National Science Council of Taiwan by means of grant NCS92-2811-M-0022-016-enabled KdJ to get acquainted with Formosan geology. The paper benefited from insightful comments from topic editor Wenjiao Xiao, and reviews by Brian Windley and Shoufa Lin. Comments on an earlier draft by Richards Spikings are acknowledged.