三重県上野市東部の古琵琶湖層群にみられる喰代(ほうじろ)撓曲の形成過程  [in Japanese] Synsedimentary development of the Hojiro Flexure, on the Kobiwako Group, at the east of Ueno City, Mie Prefecture  [in Japanese]

三重県上野市東部の喰代に分布している古琵琶湖層群にみられる喰代撓曲は, 層序学的落差は最大4mと規模は小さいものの, 基盤の領家変成岩類中の逆断層から基盤を不整合におおう古琵琶湖層群の撓曲変形まで, 一連の変形が観察できる。撓曲変形を受けている地層は, 河川成・湖沼成および土石流成の合計23層の単層群からなる。このうち8層準の単層は撓曲を境に沈み込み側の層厚が厚い。これらの地層の下位では, これらの地層に削られるスランプ構造がみられる。したがって, 喰代撓曲は古琵琶湖層群の堆積中に, 基盤中の逆断層が断続的に活動することによってできたと結論できる。喰代撓曲の場合には, 撓曲を境に層厚の異なる地層の層厚から求められた撓曲変形による当時の地表面の変形の幅は, 基盤中の断層の垂直変位量に関係なく, 塑性変形をしている地層の層厚に相関関係が認められる。

The Hojiro Flexure (drape fold), which outcrops in Hojiro, south of Ueno City, Mie prefecture, is observed on the Pliocene to Pleistocene Kobiwako Group. The flexure is about 4m in the maximum vertical displacement and about 8m in the width of deformation. The flexure is observable as an upper extension of the reverse fault in the basement Ryoke Complex unconformably overlain by the group in the outcrop of Hojiro. The Kobiwako Group, of more than 20 min thickness from the unconformity, varies in thickness through the flexure, and thins to the relatively uplifting wall of the reverse fault. The flexured group is composed of 23 beds which are deposited influvial, lacustrine and debris environment. Eight beds of them vary in thickness extremely, and others have no variation in thickness or to the contrary, thicken slightly toward the relatively uplifting wall. Each bed underlying the bed varying in thickness has a slumping structure sliding from relatively uplifting wall to subsiding wall, and has an eroded surface. These facts show that the Hojiro Flexure is formed by the block movement of underlying basement rock during the deposition of the group. The width of the flexural deformed surface depends upon the thickness of plastie deformed underlying beds, without an effect of the vertical displacement of the uplifting block. Normal and reverse minor fault systems are observed in the flexuralde formed beds. Six stages are distinguished within them. According to their distribution and stages of development, the neutral surface existed and moved from lower to upper, to the surface as the flexural deformation developed. This might be ascribed to not only vertical displacement but also lateral compression caused by the reverse faulting in the underlying basement rock.