付加体研究のツールとしての放散虫

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  • フカタイ ケンキュウ ノ ツール ト シテ ノ ホウサンチュウ
  • 付加体研究のツールとしての放散虫(付加体地質学の手法)(ジュラ紀付加体の起源と形成過程)
  • Radiolarians as tools for accretionary complex research

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付加体研究における放散虫の役割について，生層序，古生物地理，放散虫含有量と堆積岩の物性という観点から研究成果をレビューするとともに，最近進めつつある検討例を紹介した．生層序学研究に基礎を置く放散虫年代学の進展により，付加体の成長速度や成長様式について，定量的に議論を行うことが可能となった．古生物地理に関しては，今後この分野の研究が進展すると，付加体形成場の相対的位置関係を議論できるようになることを指摘した．また，“熱帯度”を表現するために，VEN（Vallupus/Eucyrtidiellum Number）という指標を提案した．放散虫の含有量と堆積岩の物性という点については，放散虫の絶滅事件により相対的に強度の弱い層準（下部トリアス系）が形成され，それが付加体形成時にデコルマ面として機能していることを例にあげた．また，付加体研究と古海洋学研究を有機的に結びつけていくことの重要性を強調した． / This paper reviews major roles of radiolarians in the study of accretionary complexes. The major roles include (1) age control, (2) spatial control and (3) mechanical control. The age control is related to radiolarian biostratigraphic investigations. The geochronologic calibration for zonal scheme makes it possible not only to give a basis for chronostratigraphic correlation among accretionary complexes but also to give data for the rate and mode of growth of accretionary complexes. The spatial control is connected to radiolarian paleobiogeographic research. Recognition of biogeographic realms offers a viewpoint for the relative position of the formation of accretionary complexes independent of other data such as paleomagnetic records. VEN (Vallupus(V)/Eucyrtidiellum(E) Number＝V/(V+E)×100) is proposed as one of paleoceanographic parameter which indicates a tropical environment. Mechanical control is related to the physical properties of sedimentary rocks. Radiolarian evolutionary events such as mass extinction and change in abundance affect lithology, particularly in pelagic sediments because radiolarian skeletons are the major components among rock-forming materials. The fluctuation of radiolarian content in pelagic sediments changes the physical properties along a lithologic sequence. This lithologic variation has an essential meaning in accretionary events because physically weak horizons act as a decollement above which the sedimentary package is off-scraped from a subducting oceanic plate. The Lower Triassic siliceous claystone unit, which was formed due to the end Permian mass extinction, is a good example for a decollement horizon.

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