北部フォッサマグナ後期新生代層の層序と古地理の変遷  [in Japanese] Late Cenozoic stratigraphy and paleogeographic changes in the Northern Fossa Magna  [in Japanese]

北部フォッサマグナ新第三系の層序について検討をくわえ, それに基づいて古地理の変遷を論じた。関東山地北縁部や諏訪湖北方には中新世初期の海成層が小規模に分布しており, 当時太平洋に大きく開いた海域がフォッサマグナ中央部にまで入り込んでいた可能性がある。中新世初期〜中期初頭には海底火山の活動を伴いながら海域がフォッサマグナ中央部に広がり, 一次的にはこの地域が太平洋側と日本海側の海をつなぐ連絡通路となった時期もあった。中新世中期は水内帯地域で大規模な堆積盆地が形成されたが, 後期になると次第に埋積が進み, 海は一般に浅海化していった。鮮新世〜更新世前期には, 戸隠・荒倉山・聖山・塩嶺などの各地で安山岩質の火山活動が発生した。また大規模な断層に沿う落ち込みの結果, 大峰帯, 小諸陥没盆地などの新たな盆地が形成された。

The Northern Fossa Magna region is a well defined geologic unit that occupies the central part of the Honshu Arc. Late Cenozoic Sedimentary strata and volcanic rocks in this region are divided into seven formations in ascending order; Moriya, Uchimura, Bessho, Aoki, Ogawa, Shigarami and Sarumaru formations. In this paper the authors discuss the stratigraphy of the central part of the Northern Fossa Magna region with some new data on microfossils and radiometric datings. The paleogeographic history of this region is summarized as follows. 1 Uchiyama stage (18 Ma): Thick marine pelitic deposits were accumulated in the outer belt of the present Japanese Islands. 2 Uchimura stage (16〜15 Ma): Large extent of the Northern Fossa Magna region subsided with a thick accumulation of marine deposits. Active submarine volcanic activities occurred mainly in the Takai-Utsukushigahara area. 3 Bessho stage (15〜14 Ma): The Minochi sedimentary basin is newly formed be side the northwestern area of the Takai-Utsukushigahara belt. In this area, pelitic sedimentary rocks were accumulated. 4 Aoki stage (14〜7 Ma): The Minochi sedimentary basin became much more extensive than those of the preceding age, and a large amount of turbidite sediments were accumulated under a lower middle bathyal and reducing environment. 5 Ogawa stage (7〜5 Ma): The eastern part of the Minochi and Takai-Utsukushigahara areas turned to an upheaval and the sea resulted in retreating gradually to the Japan Sea side. 6 Lower-Middle Shigarami stage (5〜3 Ma): In the Minochi sedimentary basin the marine regression continued, and fan-delta deposits were accumulated under a shelf environment in the Takafu basin area. Andesitic volcanic activities occurred in Togakushi, Arakurayama and Hijiriyama areas, The Omine Belt of a depressed zone was formed along the Itoigawa-Shizuoka Tectonic Line which defines the western border of the Fossa Magna region. 7 Upper Shigarami〜Sarumaru stage (3 Ma〜): The Minochi basin was buried rapidly by coarse clastic sediments, and fluvial environment appeared at the end of this stage. Acidic volcanic rocks erupted and alluvial fan deposits were accumulated in the Omine Belt. The Komoro collapse basin was formed between the Takai-Utsukushigahara belt and the Kanto Mountainland.