西南日本弧のアクティブテクトニクス : 前弧スリバーの西進運動にともなう変形像  [in Japanese] Active tectonics of Southwest Japan arc controlled by the westward translation of the forearc sliver  [in Japanese]

西南日本弧を大地形および要素的活構造の特徴をもとに, 南より 1) 陸側海溝斜面帯, 2) 前弧海盆帯, 3) 四国帯(前弧隆起帯), 4) 瀬戸内(剪断)帯, 5) 中国帯, 6) 山陰帯, の島弧にほぼ平行に配列する6つの構造帯に分割した。四国帯には剣山背斜, 紀伊向斜など, 帯に平行な方向の圧縮の結果と考えられる南北軸の構造が卓越する。MTLの北側の幅80〜100 kmの瀬戸内帯は顕著な右ずれ剪断運動を示す構造が発達し, 中部九州の雁行地溝群分布域まで連続する。西南日本弧の活構造を形成する基本的メカニズムは, フィリピン海プレートの斜め沈み込みを原因とする前弧海盆帯および四国帯からなる前弧スリバーの西進運動である。前弧と背弧の境界であるMTLの右ずれ運動および瀬戸内帯の右ずれ剪断運動もそれで説明できる。南九州南方ではトラフ軸・島弧の屈曲にともない, プレート間収束方向がプレート境界に対してほぼ直交するために, 前弧スリバーを西進させる力がなくなる。その結果, 前弧の"追突現象"が起き, 四国帯の南北軸の圧縮構造が形成されたと考えられる。

The most significant active structure of Southwest Japan arc is the right-lateral strike-slip faulting of the Median Tectonic Line (MTL), The slip- rate of MTLis estimated as much as 5-10 m/1000 years (Okada, 1980). MTL is the main boundary fault between forearc and backarc of Southwest Japan arc. South of MTL, the forearc sliver is not a rigid plate and has characteristic deformed structures. It can be divided into two zones, namely the zone of forearc rise (Shikoku zone) and that of forearc basins. Both of them are characterized by folds and reverse faults oriented at high angle to MTL, although the making processes for those structures are quite different from each other. The forearc basin zone is composed of five major basins divided by reverse faults and folds. These surface structures are explained to have been made by oblique thrust faulting, as are bound to the subducting plate (Awata & Sugiyama, 1989). In the Shikoku zone, NNW-SSE trending structures are predominant like synclines of Kii-suido and Bungo-suido, Tsurugi-san anticline and Hijikawa monocline, which indicate an arc-parallel shortening. This evidence, together with the right-lateral strike-slip motion of MTL, supports the hypothesis of westward movement of forearc sliver. The Seto'uchi longitudinal basin (Setonaikai or Seto inland sea) on the north of MTL has characteristic dextral shear structures. The Seto'uchi basin is divided into four major en echelon basins, namely Osaka bay, Harima-nada, Hruchi-nada, Iyo-nada. The long axis of these basins (syncline axis) is oriented to NE-SW, at the angle of 30 to 40 degrees to MTL. The northwestern margin of basin is bounded by reverse faults dipping to the northwest. These synclines and reverse faults can be explainedas structures in a right-lateral shear zone. This shear zone about 80-100 km wide is named as "Seto'uchi shear zone" (SSZ:Tsukuda, 1988). SSZ extends to central Kyushu of volcanic region where E-W trending en echelon grabens and normal faults are intensively developed. The Unzen graben shows a typical distribution of normal faults around volcano in E-W compressional stress field. The right-lateral shear of SSZ can be understood as a drag feature made by westward translation of forearc sliver. In the east of Southwest Japan arc, the oblique subduction of the Philippine Sea plate is quite obvious (Seno, 1977), which gives the driving force of westward translation of forearc sliver. In the west, it becomes not clear because the trench axis rotates counterclockwise. This means that there is no driving force for the westward translation. The gradient of translation rate along the forearc sliver to west can make the arc-parallel compressional structures of Shikoku zone. In conclusion, the active tectonic features of Southwest Japan is mostly originated by the westward translation of the forearc sliver caused by the oblique subduction of the Philippine Sea plate.