図10　西南日本の回転と関東山地周辺地域のテクトニクス(高橋, 1990). 関東山地は秩父盆地が堆積した直後の15Ma以降,およそ90°時計回りに回転した(HYODO & NllTSUMA, 1986).この90°の回転の半分は, 15Maの西南日本の回転によると考えられる.しかしながら,関東対曲構造の形成時期を示すと考えられる残りの40〜50°の回転のタイミングは,いまだ解明されていない.

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Geologic setting of the Miocene Chichibu Basin
in the Neogene Tectonics of central Japan

Masaki TAKAHASHI
Geological Survey of Japan, Ibaraki 305, Japan.

Abstract The Miocene Chichibu Basin, situated in the central part of the Kanto Mountains, is characterized by its very thick sediments inspite of the compact size of the basin. The Miocene strata in the Chichibu Basin are in contact with the basement rocks with high-angle normal faults along the southern margin of the basin and unconformably overlie along the northern and western margins. The planktonic foraminifera yielded from the basin indicate the age of zone N.8 (BLOW, 1969) , which represents a high sediment accumulation rate of the Miocene Chichibu Basin. The benthic microfossils suggest that the lowest part of the basin was deposited in the middle bathyal and the upper part in the inner sublittoral environments. The analyses by the vitrinite reflectance and by the minor faults revealed that the Miocene Chichibu Basin was formed by the basement tilting under the extensional stress field associated with the rifting of the Japan Sea.