埼玉県立自然史博物館研究報告 第10号 (1992)
関東山地四万十帯の大滝層群
−関東山地四万十帯白亜系の変形過程とデュープレックス構造−
高橋 修・石井 醇
東京学芸大学教育学部地学科
ま と め
本論の結果を図6に模式的に表わした．
四万十帯形成当時，海洋プレートの沈み込みにより，まず構造的に最も浅い場所(wedge)ではぎ取りがおこなわれた．はぎ取られずに残ったものがさらに深部まで押し込められ，変形しながら底づけ付加され覆瓦状構造をなす地質体が形成された．より深部まで運びこまれ付加したものが大滝層群である．その時，増富層群・小河内層群がhinterland dipping duplexを形成したのに対し，大滝層群はforeland dipping duplexを形成した．
そしてその変形様式は，増富層群・小河内層群では脆性的な変形に留まっていたのに対し，大滝層群では流動変形へと変化していった．同時に，大滝層群の中でも構造的に下位のユニット，あるいは最も北方(深部)まで運ばれたところが片理の形成および再結晶作用などの一連の変成作用を被り，最終的には白泰断層の活動に伴って現在のような位置まで上昇してきたものと考えられる．
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図6 関東山地四万十帯の形成模式図．
変形様式および変成度から推定される関東山地四万十帯の形成深度．スケールはない．増富・小河内層群がhinterland dipping duplex (BOYER & ELLIOTT, 1982)を形成したのに対し，より深部で付加した大滝層群はforeland dipping duplex (BOYER & ELLIOTT, 1982)を形成した．また，付加された地質体は，浅い部分で脆性的な剪断変形を，深い部分では延性的な流動変形を，さらに深い部分では再結晶作用と片理の発達などの変成作用をうける．
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Tectonostratigraphic division and radiolarian biochronology of the Otaki
Group of the Northern Shimanto Belt, Kanto Mountains, central Japan
−The deforming processes and duplex structures of
the Northern Shimanto Belt in the Kanto Mountains−
Osamu TAKAHASHI and Atsushi ISHII
Department of Astronomy and Earth Sciences, Tokyo Gakugei University, Tokyo 184, Japan.
Abstract The Upper Cretaceous Otaki Group in the Kanto Mountains consists of the following four units, namely, the Wanakura Unit, the Koreisan Unit, the Futase Unit, and the Kawamata Unit. They are gently folded with WNW-trending anticlinal and synclinal axes. They are divided into two different type units.
The Wanakura and the Futase Units are melange-type units, including variable shapes of blocks of chert, limestone, and green rocks that are in a foliated shaly matrix.
The Koreisan and the Kawamata units are flysch-type units that are composed of sheared and deformed formations.
In general, these unites are rather common within accretionary complexes. A unit consists of a series of imbricate strata. Each stratum is bounded by thrust faults. At both the lower and the upper ends, each imbricate stratum and boundary thrust turn to be nearly parallel with the unit boundary thrusts. Therefore, it is difficult to reconstruct the original stratigraphic sequence. These features are of the duplex structure. A duplex can result in the thickening of accretionary prisms without isoclinal folding.
We attempt to explain the relationship among the Upper Cretaceous systems of the Shimanto Belt in the Kanto Mountains using a simple model.
The Otaki Group of the northern part of the area has ductile deformation and has undergone recrystallization in the deeper level, whereas the Ogouchi and the Masutomi Groups of the southern part of the area have brittle deformations and grade into the weakly deformed strata in the shallower level. These deformation and recrystallization in this region are explained as a result of accretionary thrusting from the south.