日本海南部およびその沿岸域における中新世の構造運動  [in Japanese] Miocene tectonics of the southern Japan Sea and its adjacent area  [in Japanese]

西南日本の北陸-山陰地域の下部中新統堆積盆地には, マグマ性隆起部での陥没で生じた小規模で多角形の盆地と, より大規模で長方形の盆地がある。他方, 同地域沖海域の下部中新統は, 大規模な地溝を埋積している。小規模な盆地は地溝の南北再縁部に分布し, 両者は対をなして形成された。海進は, 海域の地溝と陸域の地溝の一部では前期中新世初期に始まるが, 小規模な盆地においては中期中新世からである。この地溝の南縁は前期〜中期中新世の間は同じ位置にあったが, その北縁は中期中新世における正断層の再活動によって北方に移動している。中期中新世中頃に西南口本弧の主な部分が隆起したのと同時に, 地溝内の堆積の中心は北〜北東に移動し, それによって褶曲の形成が始まった。北陸-山陰地域沖の海域の広範囲が中新世末から鮮新世初頭に隆起し, それとほぼ同時期の正断層の再活動によって隠岐海嶺が形成された。

Formative processes and growth patterns of the Miocene sedimentary basins distributed in the Hokuriku and San'n districts, southwest Japan and also in the southern Japan Sea on the south of the Oki Ridge, are discussed in this paper. On shore, the Lower Miocene series consists mainly of thick volcanic products deposited onland and/or inland-water environments. It fills two kinds of basins. One is a small-scale polygonal type of basins considered to be formed by collapse resulting from igneous activity. The other is along and narrow type of basin, larger than the former. Areas offshore from the Hokuriku and San'in districts can be divided into two subdivisions based on the litho-facies of the Miocene sediments; the offshore Hokuriku-eastern San'in and offshore middle San'in areas. The Lower Miocene strata in the former consist mainly of volcanic rocks, whereas in the later they consist almost entirely of thick muddy sediments. However, they were both deposited under marine conditions with a large-scale graben. The long and narrow type of onshore basin is thought to be continuation of the larger offshore graben. Small polygonal basins are situated along both the northern and southern margins of the graben. Consequently, it can be concluded that the graben and small basins make one genetic set. Transgression is thought to have started in the early stages of the Early Miocene in the offshore graben and parts of the onshore graben, but not until the Middle Miocene in the small onshore basins. The lower Middle Miocene series of this region, both onshore and offshore, are composed mainly of muddy sediments deposited in bathyal environments. Onland, this series unconformably covers the Lower Miocene beds, and also fills small basins and a part of the large graben. But generally, offshore this series conformably covers the Lower Miocene beds and fills the large graben. The southern border of the graben remained at the same position during the Early and Middle Miocene, but its northern border was shifted northward by reactivation of normal faults during the Middle Miocene. An unconformity between the lower and middle parts of the Middle Miocene can be traced widely on land and the offshore middle San'in area. It may be concluded that the depocenter of the graben has shifted north-ward or northeastward. This resulted in the strata having large-scale fold geometry. These movements were concurrent with uplift of the major part of the southwest Japan arc in the middle of the Middle Miocene epoch. The wide area of the offshore Hokuriku-San'in districts was uplifted at the end of the Miocene. The Oki Trough was formed by rapid subsidence caused by reactivation of normal faults at the beginning of the Pliocene epoch.