足尾帯白岩地域における葛生コンプレックスのチャート-砕屑岩シーケンスの覆瓦構造  [in Japanese] Imbricate structure of chert-clastics sequence of the Kuzu Complex in the Shiraiwa area of the Ashio Belt, central Japan  [in Japanese]

白岩地域の足尾帯葛生コンプレックスを中心に, 放散虫化石層序を基に構造解析を行なった.調査地域では, チャート-砕屑岩シーケンス(CCS)の構成岩類からなるスラストシートが覆瓦構造を呈する.スラストはCCS中の中部ジュラ系珪質頁岩と下部三畳系珪質粘土岩に発達する.前者はCCSを珪質粘土岩-チャート-珪質頁岩で構成される上部と珪質頁岩-砕屑岩類からなる下部に2分し, 後者はCCSを古生界から分断する.これらのスラストの構造的上位と下位では変形様式が異なることから, これらのスラストがかつてのデコルマ面に相当すると考えられる.さらにシーケンス下部のみが累重する覆瓦構造は, これら2層準のデコルマ面に上下を境されるデュープレックス構造に相当する.したがって, 調査地域の構成岩類は付加体浅所のin-sequence thrustによる剥ぎ取り付加とともに底付け作用を受けたと考えられる.

Careful biostratigraphical and structural analyses of Units 1 and 3 of the Kuzu Complex in the Ashio Belt, central Japan, clarified accretionary processes of the chert-clastics sequence at shallow level of an accretionary prism. The units are characterized by tectonic repetition of the chert-clastics sequence consisting of siliceous claystone, bedded chert, siliceous shale, and coarse clastic rocks in ascending order. The chert-clastics sequence are structuraly repeated by the two decollements occurring within the Middle Jurassic siliceous shale and Lower Triassic siliceous claystone. The decollement in the former separates the sequence into types A and B sheets. Type A sheets are composed of the lower part of the sequence : siliceous claystone, chert and siliceous shale, and type B sheets consist of the upper part of the sequence : siliceous shale and clastic rocks. The latter decollement appears to have separated the chert-clastics sequence from the underlying Paleozoic strata. The imbricated type A sheets lie between the both decollemens. Therefore, the imbricate structure repeated by the repetition of the type A sheets may form a duplex structure. It suggests that accretion of the chert-clastics sequence of the Kuzu Complex occurred by the processes of in-sequence thrusting and, in part, by subsequent underplating at shallow level of the accretionary prism.