四国西端部秩父累帯の地体構造区分

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タイトル別名
  • Tectonic division of the Chichibu Composite-terrane in the westernmost part of Shikoku, southwest Japan
  • シコク セイタンブ チチブルイタイ ノ チタイ コウゾウ クブン

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説明

四国西端部の秩父累帯には，付加体である斗賀野層群，三島層（再定義）および三宝山層群と鳥巣層群相当層の田之浜層が分布し，地体構造区分上は南部秩父帯に帰属する．斗賀野層群と三宝山層群は明浜スラスト（新称）を介して構造的に重なり，構造的下位の三宝山層群が斗賀野層群の北側に地窓として露出する．南部秩父帯の付加体には南から北に向かって，変形・変成度が漸移的・累進的に上昇する変化が認められる．これは付加体の深度方向の層相変化を示している．斗賀野層群および三宝山層群が示す水平に近い構造は，付加体形成当時の構造そのものではなく，北側が相対的に上昇するという二次的改変を受けた結果である．二次的改変の開始時期は三宝山層群の形成終了時期（白亜紀古世）よりあとで，完成時期は三波川変成作用（100Ma前後）以降である．四国西端部で黒瀬川帯の分布が途切れる現象は，南部秩父帯の付加体深部が上昇する後生変形との関連で理解しうる． / The Chichibu Composite-terrane is tectonostratigraphically divided into three terranes (Northern Chichibu, Kurosegawa and Southern Chichibu), typically in Shikoku. Diversified interpretations in tectonic division have been proposed for the Chichibu Composite-terrane in the westernmost part of Shikoku. Extensive field work and radiolarian dating revealed that the study area is occupied solely by the Southern Chichibu Terrane composed of the Togano Group, Mishima Formation, Sambosan Group, and Tanohama Formation. The Togano Group is characterized by tectonic pile of chert-clastic sequences of Permian-Jurassic age. The Mishima Formation, which has been regarded as an equivalent to the Torinosu Group, is redefined as an accretionary complex characterized by melange facies. The Sambosan Group (Upper Jurassic-Lower Cretaceous) is an accretionary complex formed by seamount collision. The Tanohama Formation contains Torinosu-type limestones and is an equivalent to the Torinosu Group. The Southern Chichibu Terrane is characterized by low-angled structure of accretionary complexes. The Togano Group rests horizontally on the Sambosan Group. The boundary fault is newly introduced as the Akehama Thrust. The Sambosan Group crops out as tectonic windows in the northern part of the study area. Strata of the Togano and Sambosan groups show gradual facies change from weakly deformed and non-metamorphosed succession in the southern area to highly deformed and metamorphosed succession in the northern area. The polarity in deformation and metamorphic facies is interpreted to show lithological change in accordance to structural level of subduction-accretion complex. The present low-angled structure is not the original accretionary wedge structure but is the result of later (secondary) modification. The tectonic modification had clearly continued after the Late Cretaceous Sambagawa metamorphism. The missing of the Kurosegawa Terrane in the study area can be explained by the secondary modification.

収録刊行物

  • 地質学雑誌

    地質学雑誌 104 (9), 565-576_1, 1998

    一般社団法人 日本地質学会

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