北部フォッサマグナにおける上部新生界の変形機構 DEFORMATIONAL MECHANISM OF UPPER CENOZOIC SYSTEM IN NORTHERN FOSSA MAGNA, CENTRAL JAPAN

北部フォッサマグナのグリーンタフ堆積盆地は, NNE-SSW方向に伸長し, 南東側の複背斜(筑摩帯)と北西側の複向斜(水内帯)に区分される。筑摩帯に軸をもつ南東フェルゲンツの広域的な曲隆運動によって盆地の伸長方向に連なるいくつかの大規模なドームが形成され, 下部中新統のグリーンタフやそれに貫入した石英閃緑岩が広く露出した。グリーンタフと中部中新統の細粒砕屑岩との境界部を剥離層として, より上位の地層群は北西へ向かって重力滑動し, 短波長の非調和褶曲群を形成した。水内帯では, 曲隆にともなうアンチセチックな基盤断層運動と被覆層の北西への重力滑動との相互作用の結果, 基盤断層群に沿うchevron型の背斜-断層とそれらの間の幅広いcurvilinearな向斜群とが交互に並走する複雑な複向斜構造が形成された。

Interacting with the gravity, a regional asymmetric upwarping brought a complicated deformation to the upper Cenozoic system in the northern Fossa Magna by means of antithetic faulting and gravity gliding. Such a rather simple mechanical model is reduced in this article through structural analysis. Most prominent structure in the area is a large scale undulation of the NNE-SSW axial trend, which forms a pair of anticlinorium and synclinorium. The anticlinorium of sortheastward vergence is composed of culminations and depressions linked in the longitudinal direction. The Early Miocene volcanics intruded by quartz diorite stocks crops out extensively as cores of the culminations, and the Middle Miocene to Pliocene clastics of upward-coarsening tendency covers the volcanics in the depressions. Due to decollement between these two groups of sediment the covering strata is shrunk rather independently of the underlying one. The northwestern synclinorium is an association of the three subparallel synclines of curvilinear shape and the bounding anticlines of chevron shape which accompany with axial plane thrusts dipping to the northwest. The Middle Miocene to Early Pleistocene thick strata is concerned with such a complicated deformation. Taking into consideration the modification by the assembling intrusion of quartz diorite stocks into some foci, the above structural features of the anticlinorium with some culminations appear to have been formed by an upwarping of southeastward vergence. The upwarping may have promoted also gravity gliding of the well-stratified covering strata, especially in its lowest horizon of monotonous black shale and mudstone. The convex direction of the rounded axial traces of detached folds on the northwestern slope of the anticlinorium confirms that the covering strata glided in the northwestern direction toward the synclionrium. In the synclionrium, the axial plane thrusts accompanied by the chevron-shaped anticlines seem to have turned from the antithetic faults in the pre-Neogene basement which were generated in an extensional stress field by the regional upwarping. Namely, gravity spreading of the hanging wall and rotation of the northwestern limb of the upwarp due to the advancement of upwarping can bend the upper parts of the surfaces of antithetic fault, resulting in transformation from normal faults in the basement into thrusts in the cover. Thereby the boundary faults which separate the three synclines in the synclinorium got cylindrical forms convex to the northwest. Further more, the northwestward gravity gliding of the covering strata on the northwestward slope extending from the anticlinorium to the synclinorium surely promoted the conspicuous folding in the synclinorium, as well as the shrinkage by axial plane thrusts of the chevron-shaped anticlines. Thus, it is concluded that the complex structure of the late Cenozoic sedimentary basin in the target area was formed by the regional upwarping of southeastward vergence through the interaction with the gravity. The origin of the asymmetric upwarping can be traced back upto the begining of the Middle Miocene when the zone occupied by the anticlinorium at present turned from subsidence to embryonic uplift. Advancement of the upwarping during the Middle Miocene to the Pleistocene brought about the complicated tectonic deformation at last.