房総半島清澄背斜の形成機構の実験的研究  [in Japanese] Experimental study of formation mechanism of the Kiyosumi Anticline Boso Peninsula, central Japan  [in Japanese]

厚い海成層からなる鮮新-中新統三浦層群は, 房総半島中部において大規模な背斜と向斜構造を形成している。それらはそれぞれ清澄背斜と清澄向斜と呼ばれ, 約30 kmの長さをもち, ほぼ東西に走る軸をもつ。同時間面を示すと考えられる火砕岩層を鍵層に用いて, 清澄背斜と清澄向斜の西部における本層群の層厚変化を調べた。鍵層を基準にして本層群を4つの単元に区分し, 各単元が最も厚くなる位置は, 単元毎に異なる。すなわち, Stage 1の最大層厚部は清澄向斜南翼にあり, Stage 2では清澄背斜南翼, Stage 3では清澄背斜北翼へと最大層厚部は移動した結果, 清澄背斜と清澄向斜の原型が形成されたと推定される。さらに, この層厚変化のデータを用いた仮想基盤変位法によるシミュレーション実験の結果によれば, 各Stageに基盤中に形成される断裂は特定の部分に集中し, 3系統の断層が生じることが明らかになった。また, 最大層厚部の移動は, 基盤ブロックの垂直差別運動によるものと推定された。この実験結果と野外における断層分布とを比較すると, 調査地域内に発達する断層系統の中の古期断層系に属する断層の特徴は, 仮想基盤変位法によって推定される断裂急系の特徴に良く一致する。これらの古期断層系を, 小玉ほか(1974)の実験結果と比較すると, 基盤の垂直運動によって形成される断層系に良く一致する。以上の理由により, 調査地域の褶曲構造の原型は, 三浦層群の堆積期間に基盤ブロックの垂直差別運動によって生じた堆積の中心の移動の結果, 形成されたものといえる。

Two large folds which have E-W trending axes about 30 km long are developed in the Mio-Pliocene Miura Group, called Kiyosumi Anticline and Kiyosumi Syncline respectively. Many pyroclastic beds which are throught to be useful achronological criteria are used as marker-bed to estimate variation of thickness of sediments in western part of both folds. The period of the deposition of the Miura Groupls subdivided into 4 stage by well continued marker-bed. The thickest parts of the sediments of each the stage, which may be correspond to the depocenter during the stage, is defferent from each other; namely, one of stage 1 is located in the south wing of the Kiyosumi Syncline, stage 2 in the south wing of the Kiyosumi Anticline, and stage 3 in the north wing of the Kiyosumi Anticline. It is assumed from drift of depocenter that fundamental forms of two folds have begun to be made during the sedimentation of the group. The simulation, called Vertical Basement Displacement (KODAMA et al., 1986) shows that three fracture systems were formed in basement rock and the Miura Group and concentrated in certain zones. Moerover, the simulation shows that migration of dipocenters were caused by vertical movement of basement blocks. Older three systems of faults developed in the study area shows the same features as ones assumed by the simulation, and their destribution are good agree with ones that is caused by vertical movement of basement blocks in clay model experiments by KODAMA et al. (1974). The above results show that both folds were formed by migrating depocenter which was caused by vertical movement of basement blocks during the sedimentation of the Miura Group.