The Late Jurassic oblique collisional orogen of SW Japan. New structural data and synthesis Michel Faure, Martial Caridroit, Jacques Charvet

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Michel Faure, Martial Caridroit, Jacques Charvet. The Late Jurassic oblique collisional orogen of SW Japan. New structural data and synthesis. Tectonics, American Geophysical Union (AGU), 1986, 5 (7), pp.1089-114. ￿10.1029/TC005i007p01089￿. ￿insu-00716159￿

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HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. TECTONICS, VOL. 5, NO. 7, PAGES 1089-1114, DECEMBER1986

THE LATE JURASSIC OBLIQUE COLLISIONAL OROGEN OF SW JAPAN. NEW STRUCTURAL DATA AND SYNTHESIS

Michel Faure, Martial Caridroit, and Jacques Charvet

DSpartement des Sciences de la Terre, Universit8 d'OrlSans, France

Abstract. The structural high pressure (HP) metamorphics (the configuration of SW Japan mainly reflects Sangun schists), the Permian Maizuru a late Jurassic-early Cretaceous orogeny. olistostrome, and the dismembered The region is divided into an inner belt Paleozoic Yakuno ophiolites. In the and an outer belt, on the Japan sea and northern part of SW Japan, the Tanba zone Pacific ocean sides respectively, by a is in faulted contact with the circum- strike-slip fault, the Median Tectonic Hida and the Hida zones. The former is Line (MTL). Both consist of a series of interpreted as the equivalent of the Oga stacked nappes. The inner belt is divided and Sangun-Maizuru nappes of the Chugoku into a Jurassic olistostrome known as the domain crushed by post Cretaceous Tanba zone and a hinterland area tectonics. The latter consists of comprising continental Triassic-Jurassic Paleozoic high temperature (HT) sediments. The Tanba zone is sliced into metamorphic rocks and late Triassic-early two units: a lower one with late Jurassic Jurassic granite, locally mylonitized and matrix and Triassic-early Jurassic covered by early Jurassic sandstone. The radiolarite olistoliths, tectonically outer belt is formed by a superficial overthrust by an upper unit comprising an nappe similar to the Tanba zone thrust olistostrome with middle Jurassic matrix upon a "deep domain" characterized by a and blocks which include late Paleozoic synmetamorphic ductile deformation. The limestone, basic lava, and radiolarite. "deep domain" is divided into a lower The Tanba zone is overthrust by a unit, the Oboke unit formed by Paleozoic nappe complex derived from the continental derived arenites and a Green hinterland. The basal sole of the nappe Schist nappe consisting of oceanic corresponds to a peculiar unit called the sediments and resedimented ophiolites.The ultra-Tanba zone. In the Chugoku area, Green Schist nappe overthrusts the Oboke the hinterland is divided into an upper unit under synmetamorphic conditions with nappe: the Oga nappe, formed by Permo- an eastward displacement. The two belts Carboniferous limestone and Permian are separated by the Ryoke zone which clastic rocks and a lower one: the corresponds to the southern part of the Sangun-Maizuru nappe, formed by Paleozoic Tanba zone affected by a Cretaceous HT metamorphism and sharply cut by the MTL. Copyright 1986 A geodynamic model is proposed for the by the American Geophysical Union. Jurassic orogeny of SW Japan assuming that the evolution of the inner and outer Paper number 6T0398. belts are linked. In the Late Triassic- 0278-7407 / 86 / 006T-0398510. O0 Early Jurassic SW Japan is an active 1090 Faure et al.: Late Jurassic Orogenof SWJapan

NEt JAPAN SW JAPAN CENTRAl, :akami

kuma

Hida

circumHida Z Kanto //Sangun f .Maizuru Z. Itoigawa - Oga nbaZ. Shizuoka F. Superficial Nappe reen Schist Nappe Oboke Unit

Kurosegawa. Sanbosan Z. 200 km Shimanto Z.

Fig. 1. Structural map of Japan, except Hokkaido,with emphasis on the late Jurassic structure.The distinctions between SW, Central, and NE Japan are Tertiary divisions due to the Tanakura and the Itoigawa-Shizuoka faults.

plate margin. The upper plate or South of Japan such as the opening of the Japan China block consisted of the hinterland Sea, the collision of the Izu peninsula, and the Tanba belt, a forearc basin; the and the present plate tectonic framework, lower plate consisted of an oceanic area it cannot be used to describe older and the South Japan continent. The basic deformations responsible for the mechanism of the orogeny is ascribed to progressive development of the present the oblique subduction and collision of Japanese Islands since Paleozoic times. the South Japan continent. It is now a widely accepted fact that the pre-Miocene structure of Japan is a INTRODUCTION result of three main orogenic cycles bounded by regional unconformities (e.g., The Japanese Islands can be divided Kobayashi, 1941; Kimura, 1973; Tanaka and into three domains by means of Cretaceous Nozawa, 1977). From youngest to oldest and younger faults (Figure 1), namely : they are: (1) NE Japan north of the Tanakura fault, 1. A Paleogene cycle or Shimanto (2) Central Japan between the Tanakura and orogeny responsible for the deformation Itoigawa-Shizuoka faults, (3) SW Japan of the outermost zone of SW Japan (the south of the Itoigawa-Shizuoka fault. Shimanto zone), for the reworking by This last domain is also divided by a brittle, superficial and local reverse large strike-slip fault, the Median faults of the older zones and for a Tectonic Line, into an inner belt on the calca!kaline magmatism related to the Japan Sea side and an outer belt on the subduction of an oceanic plate under the Pacific Ocean side. However, though this islands. geographic division can be used to 2. A Mesozoic cycle equivalent to the explain the Miocene and younger features early stage of the Sakawa orogeny of T. Faure et al.: Late Jurassic Orogen of SW Japan 1091

Kobayashi (1941). It begins in the late considered in this paper are: (1) the Triassic and ends in the middle Tanba zone and (2) the hinterland Cretaceous when the nappe structures are including all the other zones except the sealed in the outer belt by shallow-water Ryoke zone. This last one is considered Neocomian deposits and in the inner belt separately, since the post-Jurassic by middle to late Cretaceous acidic deformation and magmatism are by far volcanism or continental Cretaceous predominant. Inside the hinterland there deposits or are intruded by late is a distinction between the Chugoku Cretaceous-Paleogene granitoids. The domain on one hand and the Hida-circum- deformation related to this cycle reaches Hida domain on the other hand. This its climax in late Jurassic-early distinction is due merely to present Cretaceous times with the emplacement of geographic conditions, since their large nappes. It corresponds chrono- relationships are hidden under the Japan logically to the Daebo orogeny of Korea Sea. In the following the geology of the (e.g., Reedman and Um, 1975) and the Tanba zone, Chugoku domain, and Hida- early Yenshan deformations in China circum-Hida domain is described with (e.g., Klimetz, 1983). emphasis on Jurassic structure. 3. A late Paleozoic-early Triassic cycle or Akiyoshi orogeny (Kobayashi, The Tanba zone 1941) that very little is known about, since it is largely reworked by the early The Tanba zone is the largest one of Yenshan orogeny. SW Japan (Figures 1 and 2) well developed When considering the early Yenshan around Kyoto, i.e., the Tanba area sensu orogeny, the inner-outer division must stricto and North of Nagoya, i.e., the not be equated with the inner-outer Mino area. Recent sedimentological distinction of the alpine fold belt of studies and discoveries of radiolaria Europe where the inner belt is fossils in siliceous pelite, chert, and characterized by intensely deformed mudstone (e.g. Yao, 1972; Tanba Belt metamorphic rocks and the outer belt by Research Group, 1979; Mizutani et al., more superficial tectonics. The inner- 1981) show the importance of the outer division of SW Japan will be used olistostrome phenomenon. The Tanba zone here as it is a convenient division. should be now considered as an area of The early Yenshan orogeny is not Jurassic chaotic sedimentation. restricted to SW Japan. The Kanto area of Turbidites, diamictites, and Central Japan (Figure 1, Kimura, 1973; olistostromes are conspicuous facies in Tanaka and Nozawa, 1977; Guidi et al., the zone, and almost all of the 1984) is very similar in stratigraphy and radiolarian chert, limestone, and basic structure to the outer belt of SW Japan. volcanic are olistoliths. Local but In NE Japan also, the Kitakami and precise biostratigraphic studies suggest Abukuma massifs can be compared with the the existence of nappes, for instance, in Sanbosan zone, the Kurosegawa zone and the Kyoto area and up to the Japan sea the Green Schist nappe of SW Japan (Figures 2 and 3; Imoto et al., 1981; (Figure 1, Kimura et al., 1975; Faure, Ishiga, 1983; Caridroit et al., 1984, 1985a). 1985) or North of Nagoya(Kano, 1979) and This paper aims to present a new and perhaps south of the circum-Hida zone comprehensive structural map of the (Adachi and Kojima 1983). Jurassic orogen of Southwest Japan, based As the Kyoto area is the best known, on a detailed description of selected key it is given here as an example. There, areas and a synthesis of previous works. the lowermost unit observed in the core It then attempts to gather all the of the anticlines consists of an available data into a new geodynamic olistostrome whose siliceous siltstone model. and black shale parts of the matrix yield late Jurassic radiolaria. The olistoliths THE GEOLOGICAL STRUCTURE OF THE INNER are exclusively well bedded cherts BELT ranging from middle Triassic to early Jurassic in age (Isozaki and Matsuda, General outline , 1980). The lowermost member of the apparent sequence is often a large middle The inner belt is classically Triassic bedded chert mass but, it is subdivided into several zones roughly probably a huge olistolith. Thus the base trending east-west (Kimura, 1973; Tanaka of the sequence is unknown. and Nozawa, 1977). The main subdivisions The upper unit consists of late 1092 Faure et al.: Late Jurassic Orogen of SWJapan

•,1•• HidaZone Circum Hida Zone granit ,.•/•_(;••/• •==•u•- •r••, ,,•_ TsuvamaKamigori Maizuru' •• Maizulu Zo [• Na lasud

Akiyo• early Mesozoic molasse

nba Zone Ry oke Zone Uni' reen Schist Nappe Superficial Nappe Kurosegawa-SanbosanZone Shimanto Zone 50krn

Fig. 2. Schematic map of the Late Jurassic orogen in SW Japan. The Cretaceous and younger rocks have been omitted.

Carboniferous to early Permian red-brown olistoliths reworked into the early to chert, Permian hyaloclastite, and basic middle Jurassic matrix. Nevertheless, it lava sometimes having pillow structures, is clear that early to middle Jurassic dated by fusulina found in the black shale, sandy mudstone, tuffaceous interlayered limestone. These rocks are shale, and diamictite overlies similar overlain by an early to middle Jurassic but younger facies demonstrating the olistostrome. Permo-Carboniferous and existence of at least one late Jurassic Triassic cherts, Permian basic volcano- tectonic superposition. clastic rocks, and Permo-Carboniferous Olistostrome and nappe structure were limestones are included as olistoliths. already recognized in the eastern part of The chaotic formation is followed by a the Tanba area, north of Nagoya (Figure sandstone formation characterized by 2) by Kano (1979) on the grounds of slump structures and turbiditic sedimentological studies. The "nappes" alternations. In places, interstratified consist of kilometer-scale Triassic chert reworked granite, acidic lava and tuff slabs surrounded by olistostromes which conglomerates are known. It should also are interpreted as collapse structures in be noted that plagioclase and rock front of the advancing nappes. They are fragments derived from acidic and synsedimentary gravity-driven structures, calcalkaline rocks are conspicuous assumed to have slid from south to north. (Shimizu et al., 1978). As for the lower The origin of the gravity sliding is unit, the true stratigraphic succession thought to be the uplift of the Ryoke is unknown. According to Ishiga (1983), zone. This is in apparent contradiction Permian chert forms the base of the with the nappe motion in the Kyoto area, sequence followed by the greenrock of the which is from NW to $E, at least in the same age, it is conformably overlain by late stages. In fact, the two sets of the early-middle Jurassic olistostrome nappes differ in timing and emplacement followed by the turbiditic sandstone. mechanism. The nappes of the eastern However, Permo-Carboniferous greenrock Tanba zone are early (circa middle to and chert may also be kilometer-scale late Jurassic) synsedimentary structures, Faure et al.: Late Jurassic Orogen of SW Japan 1093

N INNERBEL' MTLOUTER BELT S PALEOZOICHINTERLAND TANBAZ. 'RYOKEZ.IGreenSchistN. Hida Sangun-Maizuru Oga • t Superficial N ß ...... neoge, nl•2 •1 / 9 I L• T,'ias2 4 • : 4 • l?•-[•'6' SouthI• Japan .T•anbøsan Block neoa

0 50

•-J•1 PaleozoicmetamorphicsHTitlOAZONE

HPand PermidnPerre,an olistostromeschistsSANGLJN HAIZURLI LateJurassic turbidireSANBOSAN ZONE

k•:t3 Permiannnmetamorphic rocks •GANAPPE K. [retaceo[Js,P Paleogene,I'1 Hiocene

:'::'•••HTJurassic schistsolisfostrome TANBARYOKE ZONE l_dtt.:E,'e.faLeOU, ruth,dre(,ZUI'tl GRUP) ••5 mefa-arenlfeOBOKE UNIT [r[:faceousand youngervoJcani['s :.:216Ju,• tipSchist 6REEN SEHISTNAPPE

Fig. 3. General N-S cross section in the upper crust of SW Japan. (A) Neogene and Quaternary deposits, (B) Cretaceous-Tertiary volcanism, (C) Intrusive granitoids ((K) Cretaceous, (P) Pa!eogene, (M) Miocene), (D) Neocomian molasse in the outer belt, (E) Maestrichtian turbiditic (Izumi group) on the northern side of the MTL, (F) Cretaceous HT metamorphic rocks and anatectic granitoids Ryoke zone, (G) Late Jurassic and Cretaceous-Paleogene turbiditc in the Sanbosan and the Shimanto zones, respectively, (H) Paleozoic HT Hida metamorphics, (I left Paleozoic HP Sangun metamorphics, right serpentinite, (J) Paleozoic ophiolite (Yakuno complex), (K) Permian Maizuru olistostrome, (L) Oga nappe, left, Late Carboniferous-Middle Permian reefal limestone facies, right, Late Permian noncalcareous facies, (M) Triassic sandstone, (N) Jurassic Tanba olistostrome and its extension in the superficial nappe of the outer belt, (0) Oboke sandstone unit, (P) Green Schist nappes undifferentiated, (Q) Basement of the outer belt (South Japan block) outcropping in the Kurosegawa zone. The deep crustal structure is from Hada et a1.1982, 6.1 and 7.8 are the layer velocities in kilometers per second. while the nappes of the Kyoto area are hinterland (Figures 1 and 2) but the two later ones formed after rock induration. units of the Kyoto area are not Moreover, the sense of displacement of definitely recognized owing to the small the eastern Tanba zone is not firmly surface of exposure. concluded from microtectonic Several problems remain with the Tanba observations. zone, among them, the source area of the West of the Kyoto area, Tanba type olistoliths, the environment and facies outcrops in several places, depositional conditions of the siliceous namely; the Masuda and Kuga (Figures 4, facies, and the nature of the underlying and 7C), Kuse (Figure 6), Kamigori and crust. The Tanba zone is generaly Wakasa areas, (Figures 5 and 7A; Toyohara considered by previous workers, to be an 1977; Tanaka,1980; Hayasaka and Hara, oceanic area whose crust has disappeared 1982; Hayasaka et a1.,1983; Faure and by subduction under the Hida zone; the Caridroit, 1983). They are interpreted sediments were accreted along the Hida- here as tectonic windows inside the circum-Hida margin (Chihara and Komatsu, 1094 Faureet al.: Late JurassicOrogen of SWJapan

Fig. 4. Structural mapof the Kuga-Masudaarea,modified from the 1/50000 geological mapof the Nishikigawa area. (A) Liassic molasse, (B) Noncalcareous Paleozoic rocks of the Oganappe, (C) HP Sangunmetamorphics, (D)ribbon facies, (E left) flysch facies, (E right) Jurassic olistostrome, (F)Southern part of the Tanbaolistostrome transformed into biotite-sillimanite schist by the HT Ryokemetamorphism, (G) CretaceousRyoke migmatite granite, (H) Late Cretaceousgranite, (I) Mineral and stretching linearion in the Ryoke metamorphics,arrows indicating the plunge, (J) Mineral and stretching lineation in the ribbon rock facies, arrowsindicating the senseof shear, (K) Location of the Jurassic radiolaria in the matrix of the olistostrome, from Hayasaka et al. (1983).

1981; Hattort, 1982; Hara, 1982; Mizutani hinterland. The limestones are probably and Hattort, 1983; Hirooka et al., 1983). derived from the Oga nappe, since they However, those models proposed for the have the same age and facies. The basic eastern part of the Tanba zone cannot rocks can be provided by the Yakuno account for the Chugoku area. complex or the Carboniferous rocks at the The source of the Paleozoic base of the Oga nappe. This is in olistoliths can be easily found into the agreement with Adachi and Kojima's (1983) Faure et al.: Late Jurassic Orogen of SW Japan 1095

7 labbro ribbon ,peridotire Wakasa

Permian

Paleozoic ophioli Permian olistostro

Fig. 7

green sandstone flysch

J6rassic ,listostrome

;sic molasse

Kamigori 5km

Fig. 5. Structural map of the Kamigori-Wakasa area,adapted from the 1/50000 geological maps of Wakasa, Chizu, Sayo, and Kamigori, (A) Sangun metamorphics, (B left) serpentinized peridotite, (B right) metagabbro, (C) Paleozoic ophiolites, (D) Permian Maizuru olistostrome, (E)Triassic molasse, (F&G) Ultra Tanba zone, (F) Flysch (Triassic?), (G) Ribbon rock, (H&I) Tanba zone, (H) Greenish sandstone, (I) Jurassic olistostrome with Permo-Triassic olistoliths, (J) Antiform, (K) Synform, (L)Mineral and stretching lineation in the ribbon rock facies arrows indicating the sense of shear. suggestion that the limestone olistoliths Biotite-sillimanite gneiss provides in the eastern part of the Tanba zone Precambrian ages,(circa 1500 and 1700Ma, were derived from the circum-Hida zone, Shibata, 1979), unknown in the Hida zone which is the extension of the Chugoku but widespread in Korea and China area in our interpretation. Biotite- (Reedman and Um, 1975; Lee, 1980). sillimanite gneiss, garnet gneiss, Sandstone in the Tanba zone contains marble, orthoquartzite, granite, quartz seams of carbonaceous matter (Iijima et porphyry andesite, Permo-Carboniferous al., 1978) or plant fragments (Nishida et limestone are found as pebble in al., 1974). Most of this detritus was conglomerate. Isolated heavy minerals as provided from the Hida zone where such such as sillimanite, garnet, and rock and fossils are recognized. chloritoid are conspicuous in sandstone The cherts are often interpreted as (Adachi, 1971; Adachi and Kojima, 1983). deep pelagic ocean deposits in most of 1096 Faure et al.: Late Jurassic Orogen of SW Japan the geodynamic models. However, according The Triassic-Jurassic molasse facies: to sedimentological and geochemical The early Mesozoic rocks which form comparisons with present oceanic limited outcrops all along the Chugoku siliceous ooze, the Triassic-early domain, from Maizuru to Akiyoshi (Figures Jurassic Tanba cherts are considered to 2, 4, and 6) correspond to the post be deposited in an offshore or marginal orogenic molasse of the Paleozoic cycle. sea environment (Imoto and Fukutomi, They are shallow water sediments 1975; Shimizu and Masuda, 1977; Sugisaki containing brackish water, lacustrine and et al., 1982; Matsumoto and Iijima, 1983). deltaic facies; coal seams and plant With reference to the basement problem fossils are conspicuous (Nakazawa and of the Tanba zone, it is necessary to Shiki, 1954; Teraoka, 1959; Tanaka and distinguish between the present basement Nozawa, 1977). The Rhaetic-Liassic flora formed by late Cretaceous-Paleogene is similar to those known in China, Korea granite, the postorogenic basement and and Sikhore Alin (Kimura, 1980). The the preorogenic basement. As for the early Jurassic formations of the Chugoku former, it will be discussed with the area are correlated with the Kuruma group geodynamic model. As for the latter, of the circum-Hida zone upon the base of there is only one locality in the similar biofacies and lithofacies (Tanaka northern part of the Tanba zone, where and Nozawa, 1977; Yu, 1983). pillow lava and basic tuffs are closely All these formations are considered to associated with middle Triassic-early unconformably cover the Paleozoic rocks Jurassic pelite and chert (Hattori and (e.g., Kobayashi, 1941; Nakazawa and Yoshimura, 1983). Except this place, Shiki, 1954; Teraoka, 1959; Kimura, 1973). there is no evidence for Mesozoic However, the very unconformity is seldom ophiolite, even reworked in the whole observed, as it is often hidden by Tanba basin. Thus it is necessary to subvertical faults and sometimes has been suppose either that all the oceanic crust developed as a thrust plane since it has been subducted or that it was very corresponds to a mechanically weak zone. limited in extension and perhaps never The latter is the case in the Oga area, existed. (Figures 6 and 7B), where the basal conglomerate and sandstone overlying the The Hinterland Sangun schists suffered a brittle ,, deformation marked by tension gashes, This is a wide and composite domain, slickensides, Riedel planes, and crushed but in terms of the Jurassic orogeny, zones localized along high-angle reverse the characteristic feature is that all faults directed southwestward. the constituent zones of the hinterland The early Mesozoic rocks are never suffered to varying degrees the late metamorphosed nor schistosed, but very Paleozoic orogeny and are unconformably often they bear the marks of a weak overlain by shallow water facies of late superficial deformation in the form of Triassic to middle Jurassic age (Figures reverse faults and open folds. In the l, 2 and 8). Thus during the Jurassic Tsuyama area (Kawai, 1958; Mitsuno and orogeny the hinterland behaves as an Omori, 1964), the Triassic sandstone lies already structured "basement". Since the in the core of overtuned synclines. In relationships between the two parts of the Oga area (Figure 9), low angle the hinterland, the Chugoku domain and southward verging brittle shear zones the Hida-circum-Hida domain are hidden associated with drag folds are under the Japan sea, both areas are conspicuously observed in the Triassic presented separately. sandstone overthrusted by the Oga nappe. The Chugoku domain. Owing to Moreover, the age of the deformation is widespread extent of the Cretaceous and not clearly defined, since the early younger formations, the Jurassic and Cretaceous rocks also are sometimes older rocks outcrop only by patches. affected by reverse faults, for instance, Since this paper discusses the late in OEa or KamiEori areas (Faure and Jurassic orogeny, Triassic-Jurassic Caridroit, 1983). North of Tsuyama detrital rocks and their deformation, the (Figure 6) the Sangun metamorphic rocks stacking of nappes of the Chugoku zone thrust up the Miocene conglomerate with a with reference to the late Jurassic very steep (circa 70 N) reverse fault, structure and the relationships between (Kawai, 1958; Faure and Caridroit, 1983). the Chugoku domain and the Tanba zone are Synsedimentary normal faults are successively presented. observed in the Maizuru, Oga, and Faure et al.: Late Jurassic Orogenof SWJapan 1097

+ + + L•a9sic• + + + + + + + + +

• sic • ---- • )listostz•me -•_+•+•Kuze

g•t• SSCI•ijst•

,perieo•• + .+ . •te +++++++++++++ -I-_+ + + + + + + + + ++++ +++ + + + + + .4- + + + + + + + + +++ + + + + + + + + ++++++++++ + PERMIAN + + + + + Limestone ++++++++++++ Icareous OGA +++++++++++ + +++ rocks NAPPE + + + + + + + + + + + + + + + + + + + + + + P Permian a i+ + +• .k'i-+++ ++++ ++ ++ ++ ++ SC13iStS +++++ + + + + + + + +++++__ -+ + + + +++++ ++ +++++ + i+++++ ++ + + + + +++ ++ + + +--t---C retaceous

+++ ++ + +• + ++ + +' +++ ++ ++ ++ ++ +.+4• rocks + + + + +[ + + •-+• + + + + + +/';i ++ + + + + + +'•. '!"(+ + +1 ,["'l•J.t- + + +.+ ++ + + + + + +.i."1 + + + +• + J•++ +•++++++++ +•.-['7Z-Late Triassic + + + -:...... :./•.•:•_++++++++++..'.' molasse

+ ++++++! + + t' •',,•1•).'"'"':...::::...... '.."'•::.:i'.':T . :.o.: :,:' + ++ + •'1:I;•'••l•+ :•....'..•::•!.:_:i:-:-''"•...''..-:'--- '" -' ' ; ' ' ' .'

v:.'. ': t..' 5km, hJVvJ

FiE. 6, Structural mapof the Oga-Atetsuarea, (a) Cretaceousand younger granite, pyroclastites, and sediments,(b) Liassic molasse,(c) Late Triassic molasse, (d&e) Oganappe, (d) Late Permiannoncalcareous facies, (e) Late Carboniferous-Middle Permian reefal limestone facies, (f) Permian Sangun metamorphics,(g) serpentinizedperidotite,( unshadedg) metagabbro,(h) Basic schists into the $anKun metamorphics, (i) Jurassic olistostrome.

Akiyoshi areas at the outcrop and Hase (1964), field surveys in the Kuga- microscopic scales (Figure 10). These Masuda area (Figure 4), Kamigori-Wakasa data fit well with the Triassic area (Figure 5) and Oga-Atetsu area distension inferred in our model. (Figure 6) allows the subdivision of the The stack of Paleozoic nappes: The Chugoku domain into the Oga nappe, the Chugoku domain has been interpreted by Sangun metamorphics, the Yakuno early workers as being formed by nappes ophiolite, and the Maizuru zone. The last and recumbent folding. Kobayashi (1941) three are not differentiated in Figure 2. defined the late Paleozoic Akiyoshi The Oga nappe is the uppermost unit. It orogeny in the westernmost part and the comprises non metamorphic limestones and late Jurassic-early Cretaceous Oga nappe noncalcareous sediments ranging from late in the central part. In agreement with Carboniferous to late Permian. The age of 1098 Faure et al.: Late Jurassic Orogen of SWJapan

• 1T:Triassic molasseJ:Jurassic Permianolistostrome- MAIZURU GROUP flysch ULTRA-TANBA ZONE •[•.2 limestone PERMIAN• 8 ribbon rock OGA greensandstone JURASSIC TANBA ZONE [:•i!7[!i3non-calcareous faciesNAPPE [L•9 olistostrome EARLY CRETACEOUS '•4 peridøtiteHP schist PERMIAN i'!•10HT schist METAMORPHICS RYOKE ZONE •-15olistolithbasic Schist (SANGUN) i-•-]11migmatite 6Paleozoic ophiolite (YAKUNO) granite- LATE CRETACEOUS-PALEOGENE

Fig. 7. Submeridiancross sections, (A) Kamigori-Wakasaarea, (B) Oga-Atetsu area, (C) Kuga-Masudaarea. SeeFigures 4, 5, and& for location.

the Oga nappe is a debated question. In is preferred (Figure 7C) given that in the Oga area, the Triassic rocks the Akiyoshi area the Triassic formations unconformably overlie both the Oga nappe are covering basal contact of the Oga and its substratum of Sangun metamorphic nappe (Sakamoto,19&9; Tanakaand Nozawa, rocks (Teraoka, 1959; Oto, 1984; Figure Z977). &). However, a deformation surely As already stated, the Sangun occurred between the late Triassic and metamorphics are relevant to the middle Cretaceous, since in the Oga and Paleozoic orogeny; however, they suffered Atetsu areas the Paleozoic rocks thrust a reworking during the late Jurassic as the Triassic sandstone which is folded shownby the K-At ages clustering about and overturned (Figure 9). A two stage 260 Ma and 170 Ma (Shibata and Igi, 1969; deformation can account for the Nishimura, 1981; Nishimura et al.,1983). contradictory observations. At first, North of Kamigori (Figures 5 and 7A) the between late Permian and late Triassic Sangun metamorphics thrust up the Yakuno times the Oga nappe thrusts the Sangun ophiolite developing decameter scale drag metamorphic rocks, then the contact is folds. The Yakuno ophiolite in turn covered by the Triassic deposits and thrusts a Permian olistostome called the during the late Jurassic orogeny, the Oga Maizuru group (Faure and Caridroit, 1983; nappe moves again overthrusting the Caridroit et al., 1984, 1985). The Triassic deposits. In discussing the Triassic molasse, when it is present, is Jurassic orogeny, there is a need to always trapped between late subvertical estimate the amplitude of the Jurassic faults, so that a Jurassic age for the thrust. It could be a multikilometer thrust cannot be ascertained. The Maizuru scale overthrust of a minimum of lO0 km group is the lowermost part of the assuming a displacement perpendicular to hinterland stack of nappes, it thrusts up the regional NE-SW trend, since there is the Tanba zone by means of a complex no room for a root zone before the basal sole which is described in the circum-Hida zone, or only a limited high following section. angle thrust. This second interpretation The relationships between the Paleozoic Faure et al.: Late Jurassic Orogen of SWJapan 1099 [ c[ I •9 1 Permian

++++ 20 km •4 / MeSC

•ml7 8 •Juras•icmolasse•.Makido

•g. •. S•uc•u•a• mapo• •he H•da-c•cum-H•da doma•, (• C•e•aceous mo•asse, (2• Jurassic mo•asse, (3• •a•y Neso•o•c •u•a•su g•a•e, a•ows •d•ca•g •he sense o• shea• •o •he my•o•c •ac•es, (• •a•eo•o•c H•da H• me•amo•ph•cs, (5 • No•me•amo•ph•c•e•m•a• •oc•s, (5L• •mes•o•e •ac•es o• 0m• a•d ;se, (6• No•me•amo•ph•c S•u•o-Oevo•a• •oc•s, (7• Ne•amo•ph•c •oc•s o• •he c•cum-H•da •o•e, (•e• Se•pe••e; (•gh• early •a•eo•o•c me•abas•es sometimes w•h H• assemblages, (• Jurassic o•s•os•ome (•a•ba •o•e•, (9• •ea•o• • •he g•a•e my•o•e w•h •he sense o• shea•. rocks and the Jurassic Tanba zone: It is Kyoto area are recognized, but the upper in the Maizuru zone that the relation- unit is overlain by another one called ships between the Jurassic Tanba zone and the ultra-Tanba zone. Upwards, the cross the Paleozoic rocks are the best known section is as follows: The early to (Caridroit et al., 1984, 1985). The two middle Jurassic olistostrome is covered units of the Tanba zone defined in the by greenish sandstone; it is thrusted by

NW SE

Fig. 9. Deformation of the late Triassic sandstone, below the Oga nappe near Oga. 1100 Faure et al.: Late Jurassic Orogen of SW Japan

N Sandstoneandconglomerate S black siltire Finegrained sands[one

50 cm 1 cm • ,,

Fig. 10. Example of synsedimentary faulting in the Triassic molasse. a late Permian cherty formation called boudinaged and pull-apart clasts. the "ribbon facies" because chert and Crystallization of quartz ribbons and pelite form millimeter-scale pressure shadows is conspicuous along the alternations. The ribbon facies is lineation. Sometimes asymmetric pressure followed by a flyschoid formation assumed shadows (Figure 11) and discrete shear to be early Triassic. It is itself bands directed eastwards are also covered by an olistostrome assumed to be observed. The linearion is interpreted as early to middle Jurassic since it an "a" lineation showing the sense of resembles the olistostrome of the Tanba transport of the hinterland nappe. Such upper unit. The ultra-Tanba zone is in an eastward sense is in agreement with turn thrusted along late Permian black the eastward nappe motion inferred from pelite belonging to the Maizuru group. microstructures in the outer belt (Faure, The ribbon facies and the flysch are !985b) and with the deformations related characteristic formations. They have been to the Jurassic orogeny in the Hida zone; recorded in the Kuga and Masuda areas see below. (Figure 4), in the Kamigori and Wakasa At the scale of the whole inner belt areas (Figure 5), in the Kuze area (Figure an eastward nappe displacement is rather 6), and other small places (Figure 2). difficult to explain unless by assuming a The Chugoku area forms in its entirity a two stage deformation with an early vast nappe overlying the Tanba zone. The ductile eastward motion reworked by a ultra-Tanba zone appears as the sole of southeastward brittle thrust. As a matter this nappe where almost all strain is of fact, the present thrust contact is a concentrated. The bedding surfaces of the reworked one, since the whole succession flysch are coated by neoformed white is seldom found except in the Maizuru micas and chlorite, and the ribbon facies area. But the age of reworking is is extensively recrystallized. The unknown. This interpretation is in initial bedding of the ribbon facies is agreement with the deformation sequence enhanced by a spaced cleavage formed by observed in the Maizuru area. There alternation of polycrystalline quartz southward verging subhorizontal brittle bands and micaceous layers. The same shear zones cut obliquely the foliation anastomosing cleavage is also observed in of the ribbon facies and the underlying the sandy facies around clasts. Pressure Tanba pelites. Though the Tanba rock are solution is obviously the dominant generally undeformed, in places, deformation mechanism as shown by buckled especially in the northern windows, a veins normal to the cleavage with rough cleavage surrounding the clasts in dissolved hinges or pressure shadows pebbly mudstone underlined by chlorite around detrital clasts in sandy facies and sericite is observed. An E-W (Figure !1) and pyrite in siliceous preferred orientation of pebble and a facies. The ribbon rocks bear a faint true stretching marked by pull-apart mesoscopic mineral amd crenulation pebbles is also conspicuous. linearion trending N60E in the Maizuru The Hida and circum-Hida domains. The area and N80E-NllOE in the Kamigori- Hida zone is the inner most zone of SW Wakasa and Kuga areas (Figures 4 and 5). Japan, (Figure 1). It consists of In thin section, stretching is marked by Paleozoic metamorphics and granites Faure et al.: Late Jurassic Orogen of SW Japan 1101

Fig. 11. Thin section of the ribbon rock facies, in section parallel to the lineation and perpendicular to the foliation showing asymmetric quartz pressure shadows around detrital quartz and feldspar, Kuga area. intruded by Triassic-early Jurassic present. To the naked eye, the lineation granites, the Funatsu granites. It is is not always well observed because the unconformably overlain by Jurassic to mylonitic texture is overprinted and Cretaceous shallow water detrital rocks partially removed by K-feldspar and intruded by late Cretaceous granites. porphyroblasts without preferred The Jurassic orogeny in this domain will orientation. Moreover, the flattening be described in the context of the component of the deformation is probably deformation in the Funatsu granites, the important, as shown for instance by the early Jurassic sediments, and looking for symmetric pressure shadows, conjugate reactivaton marks inside the Paleozoic shear zones, and cracks in feldspathic basement. porphyroclasts. However, the study of The early Mesozoic granite: These are deformation from the outcrop scale to the granodiorite, diorite, and tonalitc microscopic scale in mylonites and mainly distributed along the southern and orthogneisses (Figure 12) shows eastern margin of the Hida zone (Tanaka rotational criteria in section normal to and Nozawa, 1977; Nozawa, 1979; Figure 8). the foliation and parallel to the Their radiometric ages range from 215 Ma lineation, such as asymmetric pressure to 170 Ma (e.g., Ishizaka and Yamaguchi, shadows, oblique pull-apart of the 1969; Shibata et al., 1970; Shibata, porphyroclasts, and sigmoidal and 1979; Shibata and Nozawa, 1978,1984). The retort-shaped amphiboles. Such a most recent data provide a Rb-Sr whole deformation feature is interpreted as the rock isochron age of 189 and 198 Ma for result of a ductile shear along the the two main facies. Another facies gives strike of the linearion and directed from a Rb-Sr whole rock age of 297 Ma and a W-SW to E-NE, in the southern part of the mineral isochron age of 211Ma. Thus a Hida zone (Figure 8). Late Triassic-Early Jurassic age is In the eastern part of the Hida zone, likely. The Funatsu granites are covered the gneissic Funatsu granite trends N-S by a late Jurassic conglomerate with a steep westward dip. It is deformed consisting of reworked pebbles of into augen gneiss, mylonite and ultra- andesitc, ryolite, and acidic tuffs mylonite. The mylonitic foliation bears a suggesting volcanic activity cogenetic subhorizontal linearion. In the augen with the granitic plutonism (Nozawa, gneisses asymmetric pressure shadows and 1979). The •eodynamic significance of the shear bands make it possible to infer a Funatsu magmatism will be discussed with dextral shear sense. The deformation the geodynamic model of the chain. history of this area appears rather Along the southern margin of the Hida complex. At first, during the late zone, the granites are transformed into Permian or early Triassic, the Hida augen gneisses and mylonites (e.g., Kano, gneisses thrust eastward kyanite- 1975, 1983; Isomi and Nozawa, 1957; Nozawa sillimanite schists (Hiroi, 1978), et al., 1975). In the Kamioka area developing mylonitic facies with E-W (Figure 8) the foliation trends N60E to trending lineation (Ohta, 1961). Then N90E and dips northward at high angle. during late Jurassic the rocks suffered a Sometimes a subhorizontal stretching right lateral shear, which is clearly lineation trending N40E to N90E is also observed in the early Mesozoic granite. 1102 Faure et al.: Late Jurassic Orogen of SWJapan

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•g. 12. De•o•ma•on o• •he my•on•e •ae•e8 o• •he •una•au g•an•e •n hand 8pee•men, (•e•) and m•e•oaeop•e 8eaSe (•gh•). 8ee•on8 a•e pe•pend•eu•a• •o •he •o•a•on •d pa•a•e• •o •he •nea•on. (a) Aa•mme•o p•e88u•e shadow8 a•ound •he K •e•dapa•8, (b) po•ye•y•a•ne qua• •bbona, (e)•e•o• shape o• •he 8mph•bo•e, Kam•oka a•ea.

Finally in Cretaceous times or later, the sedimentation started in the early eastern Hida area suffered brittle Jurassic. The lithofacies are quite thrusting similar to the deformation similar to those of same aged deposits in observed in the circum-Hida zone. the Chugoku domain. The early Jurassic As shown in Figure 8, there is a Kuruma group is located on the Eastern progressive rotation of the structural margin of the circum-Hida zone while, the trend from E-W in the south to N-$ in the middle Jurassic to early Cretaceous east. This curvature is a late feature Tetori group is widespread in the Hida related to the left-lateral motion of the area (Figure 8). The early Jurassic rocks Itoigawa-$hizuoka fault during Miocene are lacustrine, brackish water and tectonics such as during the opening of shallow marine sandstone and ill-sorted the Japan sea or the collision of the Izu conglomerate bearing Rhaeto-Liassic flora peninsula with the main part of Honshu. and late Pliensbachian ammonites. In the Thus, the mylonitization of the Funatsu southern margin of the Hida zone, middle granite can be attributed to the late Jurassic clastics unconformably overlie Jurassic orogeny. In the whole Hida area the Hida gneisses and the Funatsu a coherent eastward shear is inferred. granite. There is an unconformity between Given the present subvertical dip of the the upper Jurassic and the early foliation, it is rather difficult to say Cretaceous interpreted as a sedimentary if the shear corresponds to a strike-slip echo of the tectonic movements. The movement or if it is related to an structure is very simple; the Jurassic initial flat lying thrust steepened by molasse is folded in upright synclines later tectonics. overturned southward in the southern part The Jurassic orogeny in the Hida and eastward in the eastern part gneisses: It is very difficult to associated with high-angle reverse faults separate with certainty the deformation of the same vergence (Nozawa et al., 1975; structures due to the Jurassic orogeny Uemura and Yokota, 1981). from older ones into the Hida gneisses or The circum-Hida zone: The circum-Hida even to prove the existence of a zone marks the boundary between the Hida retrograde metamorphism related to the zone and the Tanba zone. It is largely Jurassic deformation. As a clear result hidden by Cretaceous granite, lava, and of this deformation, there is an isotopic sediments (Figure 8). The Omi area, the rejuvenation as shown by the Mesozoic widest, is the best known (Chihara et radiometric ages around 170-190 Ma al., 1979; Chihara and Komatsu, 1981). It obtained from the Paleozoic gneisses is characterized by Paleozoic sedimentary (Shibata et al., 1970; Yamaguchi and and metamorphic rocks unconformably Yanagi, 1970; Shibata and Nozawa, 1978, covered by the early Jurassic Kuruma Shibata, 1979). group. The main rock facies are (1) early The Jurassic molasse facies: Contrary Carboniferous to early late Permian to the Chugoku domain, Triassic deposits massive limestone. (2) noncalcareous are lacking in the Hida-circum-Hida sedimentary rocks. Recently, middle domain. In this latter domain detrital Permian radiolaria have been obtained Faure et al.: Late Jurassic Orogen of SW Japan 1103

from the siliceous facies (Tazawa et al., Therefore the main difference is 1984). (3) Metamorphic rocks such as structural. The circum Hida zone is biotite pelitic schists, garnet squeezed along faults giving rise to amphibolite, glaucophane-jadeitic schist, schuppen structure. This brittle and metagabbro. K-At and Rb-Sr deformation is unrelated to the Jurassic measurements on biotite and muscovite orogeny, since Cretaceous rocks are provided Paleozoic ages of 310-370, 415, involved. However, the present outcrop 442, and 676Ma (Shibata and al., 1970; conditions do not allow any conclusion to Shibata and Ito, 1975). (4) Serpentinites be drawn on the Jurassic related (5) Ordovician, Silurian, and Devonian deformation. The circum-Hida zone is undeformed limestone, shale, and tuff. considered here to be the eastern According to Chihara and Komatsu extension of the Chugoku area dismembered (1981), the structure of the circum-Hida by post-Cretaceous tectonics. zone in the Omi area consists of schuppen Nevertheless, pre-Cretaceous relation- directed eastward. As a whole, the ships between the Hida zone and the circum-Hida zone is considered by Circum Hida zone are unresolved. Given previous workers as a suture zone between that the whole Chugoku domain is a wide the Hida and the Tanba zones. All the nappe overlying the Tanba zone, a large elements of the circum Hida and Tanba overthrust of the Hida zone upon the zone have been progressively accreted by circum-Hida-Tanba zone is proposed here a continuous subduction from the late s a working hypothesis. Paleozoic to Cretaceous (e.g., Chihara and The inner belt of SW Japan, is a stack Komatsu, 1982; Hattori, 1982; Hara, 1982; of nappes formed during the late Jurassic Mizutani and Hattori, 1983; Hirooka et orogeny. Paleozoic structures such as the al., 1983). However, the deformation is Sangun metamorphics, Oga nappe, and Hida obviously polycyclic, since the HP zone are seen as "basement nappeg". The metamorphism is early Paleozoic, the Tanba zone was deformed for the first Paleozoic rocks are reworked into the time during this orogeny. A two stage Mesozoic Tetori group, and the Cretaceous motion is inferred for the nappeg formations are folded and cut by thrusts. emplacement. Microtectonic observations It should be noted that there is no suggest that an early pervasive eastward record of a Mesozoic HP metamorphism shear was followed by a more superficial related to any subduction. Moreover, south to southeastward thrust. Such a Mesozoic ophiolites are lacking, so that deformation history is in agreement with the suture hypothesis is not factually that which is observed in the outer belt supported. as described below. The structural relation between the Tanba zone and the circum-Hida zone is AN OUTLINE OF THE GEOLOGY that of a subvertical fault zone. There OF THE OUTER BELT are striking similitudes in lithology, sedimentology, metamorphism, and South of the Median Tectonic Line is stratigraphy between the circum-Hida the outer belt of SW Japan (Figures 1, 2, rocks and the Paleozoic rocks of the and 3) composed of a southern part Chugoku area. The limestones have the deformed during the Paleogene Shimanto same reefal character; their age ranges orogeny and a northern part where the from early Carboniferous to middle main structure was formed during a late Permian, as that of the Oga limestones. Jurassic orogeny and is covered by The metamorphic rocks are comparable to Neocomian shallow water deposits. As its the Sangun metamorphics and the Yakuno litho-stratigraphy, deformation, ophiolite. One important misfit is the megastructure, and geodynamic evolution radiometric age, since up to now the have already been described in some Sangun metamorphic rocks are younger detail (Faure, 1985a• b) only an outline (around 240-280 Ma) than the circum-Hida is presented here. ones. However, early Paleozoic rocks are The Jurassic orogeny is responsible also recognized among the Sangun for a stack of nappes from top to bottom metamorphic rocks (Nishimura, 1979). Thus (Figure 3): the Superficial nappe and the the early Paleozoic ages, though of Green Schist nappe. The Superficial nappe unclear meaning, also have equivalents in consists of a Middle Jurassic the Sangun-Maizuru zone. Moreover, the olistostrome whose matrix is quite resemblance between the Kuruma-Tetori similar to the Tanba olistostrome in age groups and the Jurassic molasse of the and facies. The olistoliths are Permo- Chugoku zone has already been underlined. Carboniferous limestone, late 1104 Faure et al.: Late Jurassic Orogen of SW Japan

Carboniferous basic volcanoclastics, red sandstone Oboke unit of unknown but shale and chert (resemblinE the facies of likely Mesozoic aEe is interpreted as the OEa nappe) and Permo-Triassic havinE been deposited at the transition radiolarites (similar to the olistoliths between an oceanic area where the found into the Tanba olistostrome). Given elements of the Green Schist nappe were these analoEies and microtectonic deposited and a continental mass called evidence that the superficial nappe the South Japan continent. Paleozoic emplaced from N-NW to S-SE, it is sedimentary, metamorphic, and Eranitic correlated with the upper unit of the rocks, unconformably covered by late Tanba zone. The olistostrome of the Triassic-early Jurassic shallow water ultra-Tanba zone is also a possible root sediments, outcrop as multikilometer- zone but too little is known about it as scale lenses in the KuroseEawa zone. This it is well identified only in the zone is interpreted as the presently Maizuru area. outcroppinE part of the South Japan This unmetamorphosed, weakly deformed continent. The southern marEin of this nappe tectonically overlies a "deep continent, called the Sanbosan zone, is domain" characterized by an HP/MT made up of a late Jurassic flyschoid metamorphism, the famous SanbaEawa sequence reworkin E Paleozoic rocks metamorphism (e.E., Miyashiro, 1972), and derived from the South Japan continent a penetrative ductile deformation. Upon and also late Triassic and late Jurassic litholoEical aspects, the deep domain is limestone, chert, and basic volcano- subdivided into an overlyinE Green Schist clastite of unkown oriEin. The Sanbosan nappe and an underlyinE sandstone unit, zone is followed southward by Cretaceous reworked continental detritus, called the and PaleoEene turbidites, belonEinE to Oboke unit. The Green Schist nappe the Shimanto zone. The main deformation consists of pelitic schists, meta- of the Sanbosan-Shimanto basin occurred radiolarites and basic rocks both of in Eocene and Miocene times durinE the iEneous and sedimentary oriEin and a few Shimanto oroEeny, nevertheless, a pre- limestones. The major part of the meta- Albian deformation is likely by radiolarites and basic rocks comprises comparison with equivalent units in the olistoliths included in the pelitic North Kitakami massif (FiEure l) but has matrix. Since the uppermost part of the uot yet been recoEnized. nappe is less metamorphosed, weakly The mechanism of Eenesis of the schistosed red shale forminE the matrix Jurassic oroEeny in the outer belt of SW of a Eabbroic olistostrome provided late Japan has been explained by the oblique Jurassic radiolaria (M. Iwasaki et al. subduction of an oceanic domain followed unpublished manuscript, 1988). Micro- by the continental subduction of the tectonic analysis of the deep domain, South Japan continent and the related shows that the ductile deformation is thrustinE of the oceanic-derived Green characterized by a conspicuous E-W Schist nappe (Faure and Charvet, 1•84; trendinE stretchinE and mineral Faure, 1•85a, b). However, it is lineation. The deformation reEime is necessary to replace this with a wider rotational. It was produced by a shear model accountinE for both the outer and mechanism directed from west to east inner belts. Before that, the EeoloEical alone the lineation which is thus the relationships between the two belts have transport direction (Faure, 1•85b). The to be examined. schistosity and lineation are refolded by southward overturned folds trendinE N50E THE LINK BETWEEN THE TWO BELTS: to NlOOE. This second phase occurred in THE RYOKE ZONE more superficial levels. It is General Presentation responsible for the slicinE of the Green , Schist nappe into two units, so that the hiEher-Erade metamorphic rocks overthrust The Ryoke zone is important as it the lower-Erade ones, EivinE rise to an forms the junction between the inner and apparent inverse metamorphism, and also the outer belts. However, its structure for the thrustinE of the superficial has been formed by Cretaceous and younEet nappe upon the "deep domain". The whole tectonics. Overlain by Maestrichtian pile of nappes and the Neocomian turbidites, called the Izumi Eroup, the unconformity are folded into upriEht Ryoke zone forms a EranitoEneissic ridEe. folds equivalent to the middle Cretaceous In aEreement with previous works (e.E., upriEht folds of the inner belt. The Nureki, 1980; Okamura, 1980; Suwa, 1973; Faure et al.' Late Jurassic Orogen of SW Japan 1105

Ono, 1969; Kutsukake, 1980) the However, arguments are scarce, since the constitutive elements of the Ryoke zone othoderived character of gneisses is not are, from youngest to oldest, certain. All the radiometric ages are 1. Late Cretaceous granites belonging reset by the Cretaceous granites, and all to the Late Cretaceous-Paleogene the structural arguments reviewed by magmatism widespread into the inner belt. Yoshida (1981) are diputable. In the same They are undeformed except sometimes way, granitic conglomerate in the Ryoke along their margins, and are intrusive metamorphic rocks does not imply an into element 2. unconformity, since there are also 2. Acidic to intermediate volcano- interstratified conglomerate bearing clastic rocks. They petrographically metamorphic rocks in the Tanba resemble the undeformed pyroclastic olistostrome. Evidence for a sialic deposits overlying the Tanba zone. Thus basement underlying both the Ryoke and their age is assumed to be the same, Tanba zones is brought by the xenoliths around 110 Ma (Seki, 1978). Moreover, found in the Tertiary volcanoes. they are deformed by a subvertical According to petrologic studies (Nureki cleavage. and Murakami, 1979; Asami and Asami, 3. The metamorphic complex: the Ryoke 1982), they are of granite, granulite, zone sensu stricto is divided into the and amphibolite whose minerals are not in Ryoke metamorphic rocks and the older equilibrium with the Ryoke metamorphism. granites. It is established that the It is evident that they were formed initial facies of the Ryoke metamorphic deeper in a continental crust. rocks have the same facies as the Tanba Nevertheless, it is not certain that they rocks ('Nureki, 1960; Kano, 1978; are pre-Jurassic. According to Ishizaka Kutsukake, 1980). Metaradiolarite, marble et al. (1984), their radiometric age and sandstone olistoliths are included in ranges between lO0 and 200 Ha, but a late a pelitic or sandy mudstone matrix. resetting also cannot be discarded. Therefore a late Jurassic age for the Finally, an underlying sialic basement is initial sediments is likely. The necessary to provide the enormous volume sedimentary features have been erased out of acidic magma produced in late by an HT metamorphism (Miyashiro, 1972; Cretaceous and Paleogene. However, it Suwa, 1973). The metamorphic grade could be either a the pre-orogenic increases from north to south from basement of the Tanba-Ryoke zone or a biotite schist through andalusite- continental block tectonically emplaced cordierite and to sillimanite schist. during the Jurassic orogeny as proposed Migmatite and granite are the end member in our model. of this metamorphic sequence. Thus the so called older granites are formed by The Median Tectonic Line and anatexis of the Tanba sediments. Owing to the paleo•R•oke.. concept a late Cretaceous resetting, the age of the HT metamorphism is not precisely The present boundary between the inner known. It occurred before the Aptian- and the outer belts is the Median Albian acidic volcanism and after the Tectonic Line (MTL). Generally, the fault late Jurassic sedimentation in the Tanba zone is marked by gouge and cataclastic zone. An early Cretaceous age is rocks lying between the Green Schist therefore likely. The Ryoke metamorphic nappe to the south and the Maestrichtian rocks and the migmatites are folded by Izumi sandstone or the Cretaceous upright E-W trending folds. The foliation granites to the north. The oldest known bears an E-W mineral lineation (Figure 4, motion along the fault corresponds to a Nureki, 1960; Okamura, 1960) but it is left lateral strike slip leading to the •ery difficult to show the evidence for a mylonitization of the Ryoke metamorphic Jurassic deformation. rocks. Nevertheless, the deformation is posterior to the Jurassic orogeny (Hara .The pre-R•oke Concept et al., 1980; Ichikawa, 1980). It is worth to note that the HT Ryoke Some exposures of the older granites metamorphism, though younger is are described as orthogneisses (Kojima undetected into the Green Schist nappe. and Okamura, 1968; Nureki, 1979) There is a deep structural and interpreted as a Paleozoic or Precambrian metamorphic gap between the Ryoke substratum of the Ryoke metamorphic metamorphic rocks and the Green Schist rocks, called the pre-Ryoke basement. nappe. The Ryoke metamorphic rocks form 1106 Faure et al.' Late Jurassic Orogen of SW Japan an antiform whose southern limb is cut by explained by subduction on the south side the MTL. The metamorphic isograds both in of the South Japan continent, responsible the Green Schist nappe and the Ryoke for the Shimanto orogeny. As the rocks are oblikely cut by the MTL. structural features of the outer belt can Therefore it is suggested that during the be followed from Okinawa to eastern motion of the MTL some intermediate Hokkaido, along more than 2500 km, a fragments called the "missing Ryoke" considerable displacement is necessary in (Miyashiro, 1972; Kojima, 1973; Nureki such a hypothesis. Such a model is a kind and Okamura, 1977) have disappeared. of "collage," as defined by Helvig (1974) Moreover, in the whole SW Japan there and is largely used to explain the is no detrital material from the Green structure of the western North America Schist nappe, though granite, Paleozoic Cordillera (e.g. Coney et al, 1981; Jones sediments, and Ryoke metamorphic rocks et al., 1983; Monger, 1984). are abundantly reworked into the However, in the case of SW Japan, several Maestrichtian molasse. Thus it is assumed similarities between the two belts that the Green Schist nappe was overlain should be emphasized. First, the main by a part of the Ryoke zone, called the deformation occurred at the same time in paleo-Ryoke (Yabe, 1963; Miyashiro, 1972, the late Jurassic, second, the same Kojima, 1973; Nureki and Okamura, 1977; deformation style and history occurred at Ichikawa, 1980). It was located south of the same time. The nappe motion is the MTL and has been eroded off between characterized everywhere by an early late Cretaceous and Paleogene times to eastward displacement followed by a produce the Izumi molasse. The concept of south-southeastward one. Third, matrix a paleo-Ryoke is independent of that for and olistoliths of the Superficial nappe a pre-Ryoke. The former could be formed of the outer belt are identical to the either by a pr-eRyoke basement and its Tanba olistostrome and the Paleozoic sedimentary cover or only by this last rocks of the Chugoku area, respectively. part, i.e., the Tanba rocks and the We assume that parts of the Paleozoic Superficial nappe. rocks at first redeposited into the Tanba Thus in conclusion, the Ryoke zone basin in middle to early late Jurassic represents the southern part of the Tanba times, then these were thrusted together zone, deeply perturbed by a thermal with the Tanba sediments above the Green metamorphism. The Ryoke metamorphism and Schist nappe in late Jurassic-early the deformation related to the MTL Cretaceous times. Hence a model linking correspond to a late feature overprinted the evolution of the two belts is upon the Jurassic chain. However, it is developed in the following. likely that some segments between the Ryoke zone and the Green Schist nappe are The Late Triassic-Earl • Jurassic presently missing. It is now well established that the THE MESOZOIC PALEOGEOGRAPHY AND Japan sea opened in the Miocene (e.g., GEODYNAMICS Otofuji and Matsuda, 1984; Lallemand and Jolivet, 1986) detaching the Japanese Collase versus linkase islands from Asia. The Triassic-Jurassic molasse facies of the hinterland is Since the MTL presently represents an similar to the continental terrigenous important gap, two kinds of relationships deposits of Manchuria (Wang and Sun, between the inner and outer belts of SW 1983), Shikhote Alin (Kimura, 1973; Japan can be imagined. One assumes that Gnibidenko et al., 1977), Korea (Reedman paleogeographic and tectonic relation- and Um, 1975; Y• 1983) and bears the same ships never existed between them before Rhaetic-Liassic flora (Kimura 1980). In middle Cretaceous. Each belt South Korea the late Jurassic Daebo independantly developed its late Jurassic orogeny (Reedman and Um 1975, Lee, 1980) structure and metamorphism in remote is recognized. As in the hinterland of SW areas. Afterwards, during Middle to Late Japan the deformation is weak, only Cretaceous times they were placed in marked by brittle high angle faults and close contact with each other by left open folds. The early Mesozoic Funatsu lateral motion of the MTL. Since that granite has an equivalent in North Korea time, inner and outer belts have behaved (Lee, 1980) but it cannot be compared as a single unit whose evolution is with the South Korean Daebo granite, Faure et al.: Late Jurassic Orogen of SW Japan 1107

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•:'.'" . uplift Middle basic / / • .early Late Jurassic olcanism _•. /••• + + +'+ ..... +• ++ +. .. , +++ +++ ++ +11++ ++++ ++ ++ ++ ++ + +•V + + + + + + + • [ I i •/•,.• ,•.-•;;'.:-"• • i i i • ' •'••/'••gB;•aizuruTanba• eenSch•t Sanbosan flysch

- • . • • ..'":"'' + + + + + • + • .• , ß

HTmetamorphism•e•e• •• Missing RYøk• •molasse flysch Neocomian

Fig.13. Geodynamic model for the Jurassic orogeny of SW Japan, emphasizing the nappe structures and the continental subduction of the South Japan continent. The detailed structure of the Green Schist nappe and the Tanba zone is not represented. since they differ in age, petroloEy, and agreement with sedimentology but does not geochemistry, (e.g. Iiyama and Fonteilles, fit with paleomagnetic data (Hirooka et 1981; Ishihara et al., 1981). Thus the al., 1983, Mizutani and Hattori, 1983; hinterland of SW Japan represents a part Sasajima, 1984) which indicate that of a pre-Triassic northern continent. The during the Jurassic the hinterland of SW South China block, which includes the Japan was at the same latitude as at southern part of Korea is the most present and that the Tanba zone was at likely. subequatorial latitudes. However, the The Tanba zone remains the most data for the Hida zone are derived from problematic element of the inner belt. the Funatsu granite, and for the Tanba Late Triassic-Early Jurassic normal and zone, from Paleozoic olistoliths and possibly strike-slip faulting gave rise shaly matrix of the olistostrome, so that to a kind of horst and graben pattern. In comparison is difficult, not to mention the proximal parts of the Tanba basin the correction difficulties for the flyschoid deposition took place, while in deformed Jurassic granite. the distal parts siliceous sedimentation South of the Tanba basin there was an occurred (Figure 13). Since the amount of oceanic domain subducting under the Tanba Mesozoic neoformed oceanic crust is small basin in which radiolarite, limestone, the distension did not produce a large and pelite were deposited together with oceanization. In our model the Tanba zone some reworked ophiolitic rocks. They form is seen as a forearc basin floored by the the material of the Green Schist nappe. thinned continental crust of the South The cause of the ophiolitic reworking is China block with some limited oceanic disputed; spreading ridge, trough, sea parts and filled by continental derived mount, transform fault, or intraoceanic sediments. A remote source for the tectonic wedge related to the subduction Permian radiolarian cherts is not are suitable environments for providing a necessary; it may represent in situ sufficient topographic gradient necessary reworked rocks from the substratum of the for this kind of reworking. Tanba basin. This interpretation is in The Funatsu and North Korean granites 1108 Faure et al.' Late Jurassic Orogen of SW Japan

are interpreted as one of the effects of owing to the buoyancy of the continental the subduction. The granite, acidic and crust. The collision gave rise to large calcalkaline lava reworked into the Tanba nappes pilled up upon the South Japan sediments, may also have been derived continent. From bottom to top are the from rocks formed by this subduction. Green Schist nappes, the Tanba and This plate also incorporates the South Superficial nappes, the ultra-Tanba zone, Japan continent, upon which shallow water the hinterland nappe where the Paleozoic detrital rocks were deposited. On the thrusts were reused,and the uppermost outer side, chert, limestone and basic Hida zone. At depth the crust of the volcanoclastic rocks accumulated in the hinterland is probably sliced in the same Sanbosan zone. way that the Austro-Alpine nappes have There is the possibility that since been in the Alps (e.g., Aubouin 1981). The this time, deformation and metamorphism Tanba nappes are assumed to lie occurred in the oceanic domain but the tectonically upon the extension of the main deformation was induced by the Green Schist nappes, themselves obducted subduction of the South Japan continent. upon the South Japan continent. In the Sanbosan zone the flysch deposit is seen T.he Midd!e..Jprassi.cTEa?l • Late Jurassic as the sedimentary echo of the collision. As stated above, microtectonics show that As the molasse sedimentation continued nappe displacement occurred, in the early in the hinterland, the sedimentation stages, with a dominant eastward became more and more chaotic in the Tanba component parallel to the trend of the basin, prograding from the hinterland. chain. The early E-W motion is best Olistostromes were formed in the upper recognized in the Green Schist nappes, unit and in the ultra-Tanba zone, while but eastward directed deformation is also in the lower unit, i.e., the most distal clear into the Funatsu granite and the one, sedimentation of cherts was still ultra-Tanba zone. As in other orogenic possible. The origin of the olistostrome zones, nappe displacement shown by the is tentatively suggested to have been lineation and asymmetric criteria is related to a progressive uplift of the assumed to be close to the direction of hinterland due to subduction. Southward convergence (e.g., Escher and Watterson, the South Japan continent entered the 1974; Mattauer ,1975; Mattauer et al., subduction zone. As a consequence, the 1981; Malavielle et a1.,1984). Therefore edge of the Tanba basin was uplifted; the convergence between the South China this may be the origin of the southward and South Japan continents is assumed to gravity sliding deposits of the Mino have been one of oblique subduction. area. At depth the HP/MT metamorphism and early tectonics may have begun in this The Neocomian time. The future Green Schist nappe suffered its early HP metamorphism The orogeny was completed in the recorded into the metagabbro and middle Cretaceous when the MTL divided eclogites. the chain into two belts. The nappe contacts were covered by the continental The Late Jurassic-Earl • Cretaceous formations and by the acidic volcanism in the inner belt, and in the outer belt, by At that time the main nappe tectonics shallow water deposits. The left lateral occurred. The basic mechanism for orogeny displacement along the MTL induces en is the convergence between the South echelon upright folds (Hara et al., China and the South Japan continents. The 1980). At that time a HT Ryoke convergence at first accommodated by the metamorphism developed whose geodynamic oceanic subduction of the Green Schist causes are not well understood. One could sediments was followed by the subduction argue that the crustal thickening due to of the continent itself. The subduction the collision was responsible for the HT stopped, whether owing to global reasons metamorphism. However, this explanation inducing a change in relative plate alone is not sufficient, since the Ryoke motion, for instance, from pure metamorphism forms a well-delimited shortening to strike-slip motion, or only linear zone. It is likely that it was for regional mechanical reasons; for related to the ductile deformation along instance, when enough continental crust the MTL. A kind of large-scale shear was subducted,the subduction was choked heating, enhanced by the still hot, Faure et al.' Late Jurassic Orogen of SW Japan 1109

underlying South Japan continental crust, several peculiar features of SW Japan is proposed here as a working hypothesis. must be emphasized here: A new subduction south of the Sanbosan 1. There are no ophiolites in the zone began from middle Cretaceous times. sense of large masses of basic-ultrabasic Its evolution led to the Shimanto associations. Even inside the Green orogeny, and the huge late Cretaceous- Schist nappe which can be regarded as a Paleogene magmatism widespread in Japan, kind of ophiolitic nappe, the magmatic as on the eastern margin of Asia from part is underrepresented and always Southeast China to Eastern Siberia. In reworked into sediments before the our model the origin of the calcalkaline deformation. magma is located in the postorogenic 2. The famous HP/MT Sanbagawa basement of the Tanba zone, that is to metamorphism is in fact an atypical one, say, a part of the South Japan continent. since true high pressure assemblages are absent. CONCLUSION 3. Moreover, tectonic analysis reveals the importance of early Two sets of models aim to account for longitudinal displacements, related to the geology of Japan. The "Pacific type nappe tectonics as soon as in the syn- orogeny" proposed for the Cenozoic metamorphic deformation stage. For that tectonics is extended to older orogens reason, pure convergence has to be (e.g., Miyashiro, 1972; Kimura, 1973; coupled with strike slip into an oblique Uyeda and Miyashiro, 1974; Chihara and collision model. This feature is probably Komatsu, 1981; Hara, 1982; Hada et the most original character of the al., 1982; Hirooka et al., 1983; Mizutani Jurassic orogen of SW Japan. et al., 1983). It assumes that the constitutive units of Japan were progressively accreted onto Asia by the Acknowledgements. J. Aubouin, J.P. continuous subduction of an oceanic plate Cadet, J.T.Iiyama, M. Iwasaki, K. since Paleozoic. The second group Ichikawa, and Lee Byong Joo are consists of collisional models(Ogawa, acknowledged for enlighting discussions 1978; Ono, 1980; Ichikawa, 1981, Charvet and help during the field and laboratory et al., 1983; Sasajima, 1984; Faure and works. An anonymous reviewer helped Charvet, 1984; Faure, 1985a). Depending greatly to improve the English. The on the authors, two or three of the Hida, authors wish to thank the University of pre-Ryoke, and Kurosegawa blocks are Tokyo, the University of Tokushima, and involved. However, the precise geological the Osaka City University for providing data are seldom integrated into the material facilities for M.F. and M.C. The scheme. work expenses were supplied by grants The nappe structure and HP/MT from the Ministry of Education of Japan metamorphism of SW Japan related to the for M.F., from the Minist•re des late Jurassic orogeny fits better to a Relations Ext•rieures for M.C., and from model where orogeny is driven by the the KAIKO project for J.C. continental subduction of the South Japan mass than to with an oceanic subduction REFERENCES driven model. Basically, the late Jurassic orogeny in SW Japan is Adachi, M., Permian intraformational responsible for a stack of nappes with conglomerate at Kamiaso, Gifu prefecture, Central Japan, J. Geol. hundred-kilometer displacements. From top , to bottom there is (1) a "basement SOC. J•n• 77, 471-482, 1971. nappes" complex formed by the hinterland: Adachi, M., and S., Kojima, Geology of Hida and Chugoku areas, (2) a forearc- the Hikagedaira area, East of Takayama, derived nappe complex with the ultra- Gifu prefecture, Central Japan, J. Tanba zone, and the Tanba and Superficial Earth Sci. Nagoya Univ.• 31, 37-67, nappes, and (3) an oceanic derived nappe 19•3. '' complex with the Green Schist nappes. The Asami, M., and S. Asami, Granulite whole pile lies upon the South Japan xenoliths in andesites from continent. Amagiriyama, Kagawa Prefecture, Such a model is similar to the Mem..Geol. Soc. J•n• 21, 151-161,1982. collisional models exemplified by the Aubouin,J., Aboutmountain building, Western Alps and the Himalayas. However, Geol. Soc. China• Mem..4, 33-53, 1981. 1110 Faure et al.: Late Jurassic Orogen of SW Japan

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