著者

磯崎 行雄 丸山 茂徳

出版者

学術雑誌目次速報データベース由来

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.100, no.5, pp.697-761, 1991

被引用文献数

77 207

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The Japanese Islands consist fundamentally of late Paleozoic to Cenozoic accretionary complexes that formed in situ in a subduction zone along the East Asian continental margin, i.e. 2.0 Ga Yangtze craton (South China) and 450 Ma fore-arc ophiolite. Recent research utilizing microfossil and radiometric dating has distinguished several major accretionary complexes, including high-P/T metamorphosed parts, and subordinate ophiolites. In particular, recognition of oceanic plate stratigraphy and age of subduction-related metamorphism for individual accretionary complex allows the geotectonic subdivision of the Japanese Islands be emended with a new definition of geotectonic units and their mutual boundaries. Removing the effect of arcrelated magmatism and secondary tectonic modification by microplate activities such as backarc basin opening, fore-arc sliver movement, and arc-collision, a remarkable oceanward younging polarity is recognized among the accretionary complexes. This polarity in growth is well observed in Southwest Japan where seven distinct units occur, i.e. from the Japan Sea side to the Pacific side: 400-300 Ma high-P/T schists, Permian (250 Ma) accretionary complex, 230-180 Ma high-P/T schists, Jurassic (180-140 Ma) accretionary complex, 100 Ma high-P/T schists, Late Cretaceous (80 Ma) accretionary complex, and Tertiary (50-20 Ma) accretionary complex. The sinuous surface trajectories of these geotectonic boundaries and occurrence of several tectonic outliers and windows indicate that all these complexes, including high-P/T schists, occur as subhorizontal (or gently northward dipping) thin tectonic unit, i. e. nappe. Thus the Japanese Islands form a huge pile of nappes that become younger structurally downward to the modern Nankai accretionary complex. What is remarkable in this subhorizontal orogen is that high-P/T units are tectonically intercalated between low-P units, e. g. the thin nappe of 100 Ma Sanbagawa blueschists between Jurassic and Late Cretaceous accretionary complexes of the prehnite-pumpellyite facies. Uplift of the Sanbagawa high-P/T unit appears to correlate with the arrival of the Kula/Pacific spreading ridge at the trench, suggesting that this high-P/T accretionary complex may have been extruded and uplifted into low-P domain in fore-arc by buoyant subduction of the spreading ridge at the trench. Evidence of ridge subduction at that time is supported by reconstructed paleoplate motion and the coeval climax of arc-related Ry-oke magmatism associated with low-P/T regional metamorphism. Formation of older high-P/T blueschist nappes sandwiched between low-P units can be explained likewise. Subduction of major spreading ridges seems most critical for the episodic oceanward development not only of subhorizontal high-P/T nappes but also of continent side granitic belts.

著者

丸山 茂徳 生駒 大洋 玄田 英典

出版者

日本地質学会

雑誌

日本地質学会学術大会講演要旨

巻号頁・発行日

vol.114, 2007-09-01

著者

鈴木 和恵 丸山 茂徳 山本 伸次 大森 聡一

出版者

Tokyo Geographical Society

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.6, pp.1173-1196, 2010-12-25

被引用文献数

11 35

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The Japanese Islands have long been considered to be the most evolved of all the island arcs in the oceans. A simple scenario has been implicitly accepted for the growth of the Japanese Islands: since subduction started sometime around 520 Ma, the TTG crust has increased over time in association with the steady-state growth of the accretionary prism in front. Here, we show very different dynamic growths of TTG crusts over time than previously thought, <i>i.e.</i>, four times more TTG crusts than at present must have gone into the deep mantle due to tectonic erosion, which occurred six times since subduction was initiated at 520 Ma. Tectonic erosion is a major process that has controlled the development history of the Japanese islands. It can be traced as a serpentinite mélange belt, which indicates the upper boundary of past extensive tectonic erosion.

著者

磯崎 行雄 丸山 茂徳 柳井 修一 下司 信夫 佐野 修 笠原 順三

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.2, pp.187-189, 2010-04-25 (Released:2010-07-06)

参考文献数

15

著者

丸山 茂徳 磯崎 行雄 柳井 修一 下司 信夫 佐野 修 笠原 順三

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.6, pp.959-962, 2010-12-25 (Released:2011-03-17)

参考文献数

28

著者

青木 一勝 大藤 茂 柳井 修一 丸山 茂徳

出版者

Tokyo Geographical Society

雑誌

地學雜誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.2, pp.313-332, 2010-04-25

被引用文献数

10 24

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The Sanbagawa metamorphic belt in SW Japan was previously considered to extend in the E-W direction from the Kanto Mountains to Kyushu Island, a distance > 800 km. However, Aoki <i>et al.</i> (2007) recently demonstrated that protoliths of metamorphic rocks in the Oboke area of the belt in central Shikoku accumulated at the trench after <i>ca.</i> 90-80 Ma. Furthermore, Aoki <i>et al.</i> (2008) showed that these rocks suffered blueschist metamorphism at 66-61 Ma, which differs from the timing of the Sanbagawa metamorphism. Thus, these results show that the Sanbagawa belt in Shikoku is a composite metamorphic belt. We, therefore, redefine the traditional Sanbagawa belt; the structurally upper part is the Sanbagawa metamorphic belt (<i>sensu stricto</i>). It formed as an accretionary complex at <i>ca.</i> 140-130 Ma and subsequently experienced BS-EC facies metamorphism at <i>ca.</i> 120-110 Ma (Okamoto <i>et al.</i>, 2004). By contrast, the structurally lower segment termed the Shimanto BS facies metamorphic belt, formed as an accretionary complex after <i>ca.</i> 90-80 Ma and experienced peak metamorphism at <i>ca.</i> 60 Ma. Our observations have important implications for the lateral extension of these two metamorphic belts in SW Japan. The accretionary ages of the traditional Sanbagawa belt in the Kanto Mountains are younger than the Sanbagawa peak metamorphic age (Tsutsumi <i>et al.</i>, 2009), clearly indicating that the entire region of Kanto Mountains Sanbagawa must belong to the Shimanto metamorphic belt. The same timing relationships were also found for the Sanbagawa belt on Kii Peninsula (Otoh <i>et al.</i>, 2010). These results, therefore, indicate that the Shimanto metamorphic belt is exposed in Shikoku, Kii, and Kanto, thus the spatial distribution of Sanbagawa belt (<i>ss</i>) is less than half of its previous extent. The metamorphic grade of the Kanto Mountains in the Shimanto metamorphic belt ranges from pumpellyite-actinolite facies to epidote-amphibolite facies. Therefore, the higher-grade rocks of the Shimanto metamorphic rocks are exposed in the Kanto Mountains in comparison with Shikoku and Kii Peninsula. Hence, these two distinct BS-EA-EC (?) metamorphic belts are virtually equivalent in terms of spatial distribution, metamorphic range of grade, and facies series. Pacific-type orogenic belts typically comprise accretionary complex, high-P/T metamorphic belt, fore-arc sediments, and batholith belt landward from the trench (Maruyama <i>et al.</i>, 1996). In SW Japan, the Sanbagawa belt (<i>ss</i>) is paired with the Ryoke low-P/T metamorphic belt and with the <i>ca.</i> 120-70 Ma Sanyo TTG batholith belt. Furthermore the related fore-arc basin may have developed penecontemporaneously with the Shimanto BS-EA orogeny, which is paired with the late Cretaceous to early Tertiary San-in TTG belt, which extending along the Japan Sea coast. In-between the intervening Izumi Group, a fore-arc basin deposit formed during the Campanian to Maastrichtian. Thus, these two groups of orogenic units, which formed during independent orogenies were both extensively modified during the opening of the Japan Sea <i>ca.</i> 20 Ma. The southward thrusting of the Ryoke and Cretaceous TTG belts over the Sanbagawa extended beyond the southern limit of the Sanbagawa, leading the up-down relationship of the Sanbagawa (<i>ss</i>) and the Ryoke belts.

著者

青木 一勝 飯塚 毅 平田 岳史 丸山 茂徳 寺林 優

出版者

日本地質学会

雑誌

地質學雜誌 (ISSN:00167630)

巻号頁・発行日

vol.113, no.5, pp.171-183, 2007-05-15

被引用文献数

10 65

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四国中央部大歩危地域に分布する三波川結晶片岩類は,原岩層序に基づき構造的上位から下位に向かい三縄層,小歩危層,川口層に分類されてきた.今回,それぞれの層から火成ジルコンを分離し,LA-ICP-MSを用いてU-Pb年代測定を行った結果,三縄層中のジルコンの多くが1900-1800Maの火成年代を示した.一方,小歩危層・川口層中のジルコンから得られた火成年代は三縄層よりも若く,その中で最も若い年代は,それぞれ92±4Ma,82±11Maであった.両層の堆積年代は更に若く,その年代は,四万十帯北帯の付加年代と一致する.このことから,従来,三波川帯と見なされてきた小歩危層,川口層は,四万十帯北帯であることが明らかになった.したがって,三縄層と小歩危層の層境界は,三波川帯/四万十帯境界にあたる.また,大歩危地域における構造的累重関係は,三波川変成帯が四万十帯の構造的上位に位置することを示している.

著者

下條 将徳 大藤 茂 柳井 修一 平田 岳史 丸山 茂徳

出版者

公益社団法人 東京地学協会

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.119, no.2, pp.257-269, 2010-04-25 (Released:2010-07-06)

参考文献数

32

被引用文献数

32 43

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The Japanese Islands have grown through the formation of igneous rocks and accretionary prism caused by subduction of oceanic plates. However, the timing of the initiation of the subduction is not well defined. The South Kitakami Belt (SKB) in NE Japan is the best field to solve this problem. Here, basement igneous rocks are covered by successions of Ordovician to Early Cretaceous beds (450-100 Ma). We obtained LA-ICP-MS U-Pb zircon ages from the following localities. Along the Yakushigawa-Valley section in the northeastern part of the SKB, we examined the ages of (1) trondhjemite of the Kagura Complex of basement igneous rocks, (2) felsic tuff of the Koguro Formation conformably covering the Kagura Complex, and (3) Yakushigawa Formation covering the Koguro Formation and lying under the Silurian Odagoe Formation. In the Ohasama area in the northwestern part of the SKB, we examined the age of (4) Nameirizawa Formation, which probably lies below the Silurian Orikabetoge Formation. Moreover, we examined the ages of four samples from the Hikami Granite body in the central part of the SKB. The ages of trondhjemite (08331-5: 466±6 Ma) of the Kagura Complex and felsic tuff (08331-4b: 457±10 Ma) of the Koguro Formation indicate that the subduction of an oceanic plate had already started at 466 Ma, and that the Koguro Formation is the oldest age-known formation of the SKB. The tuffaceous sandstone of the Yakushigawa Formation (08331-3) has detrital zircons with the youngest age cluster of around 425 Ma, and is probably a Silurian formation. The tuffaceous sandstone of the Nameirizawa Formation (08331-9) has detrital zircons with the youngest age cluster of around 430 Ma. The formation is probably a Silurian formation and is correlated with the Yakushigawa Formation. Precise ages of around 412 Ma were obtained from the Hikamiyama body of the Hikami Granitic Rocks (08330-1,-3,-4), clearly suggesting that at least some parts of the body are not pre-Silurian basement.

著者

河村 雄行 廣瀬 敬 圦本 尚義 丸山 茂徳 神崎 正美 加藤 工

出版者

東京工業大学

雑誌

特定領域研究

巻号頁・発行日

2000

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本研究では世界最古の岩体であるカナダのアカスタかこう岩体の地質調査を行い、ジルコンを用いた年代測定と地質構造上の関係から、岩体の形成史をあきらかにした。またオーストラリアのピルバラ地塊の調査により、太古代中期30-35億年前の中央海嶺や沈み込み帯における火成活動や熱水活動をあきらかにした。その結果当時の海水中には多量の二酸化炭素が含まれており、熱水活動により炭酸塩が多量に地殻に生成され、さらに沈み込みにより地球深部へ多くの炭素が運ばれていたことを明らかにした。また太古代に特徴的なコマチアイトマグマ中のメルト包有物の測定を行った結果、太古代のプルームマントル中には多くの水が含まれていたことがわかった。またマルチアンビルプレスを用いた高圧高温実験により、下部マントル上位におけるマントルかんらん岩と玄武岩の融解実験を行い、融解相関係や微量元素分配を決定した。さらにこの結果から現在進行中のコア・マントル境界における融解現象において、微量元素分配はCaSiO3ペロブスカイトに支配されることを示した。実験結果から期待される特徴的な微量元素の濃度パターンは現在のプルームマントル中の捕獲岩に残されていることから、ハワイや南太平洋のプルームはコア・マントル境界由来である可能性を指摘した。本研究ではダイヤモンドセルを用いたマントル最下部までの条件の超高圧高温実験をも行った。その結果、SiO2相は下部マントル中で2回の相転移を起こし、マントル最下部ではα-PbO2相が安定であることなどがあきらかになった。

著者

丸山 茂徳

出版者

富山大学

雑誌

一般研究(C)

巻号頁・発行日

1986

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最終年度である本年度はこの2年間の研究成果をまとめ公表することを目的とした。高圧型変成相系列の標準となるフランシスカン層群の研究はJournal of Petrologyに印刷された。フランシスカン層群のテクトニクスはTectonophysics誌に、研究のまとめは月刊地球にそれぞれ印刷された。この2年間続けてきたカナダバンクーバー島の低圧型変成相系列の研究と中圧型変成相系列をまとめて、低温の変成作用一般を論じ、"Low-Temperature Metamorphism and Related Tectonics"と題してケンブリッジ大学出版会からB5判約450ページの専門書を出版する契約を結び、アメリカ人研究者2名と共同して第1稿の約2/3を仕上げたところである。

著者

清水 健二 丸山 茂徳 小宮 剛

出版者

日本地質学会

雑誌

日本地質学会学術大会講演要旨

巻号頁・発行日

vol.104, 1997-09-30