- 地球科学 (ISSN:03666611)
- vol.31, no.2, pp.70-82, 1977-03-25 (Released:2017-07-26)
The Niitsu anticline, situated in the Niitsu hill, oil-field of Niigata, Japan, consists mostly of the middle Miocene to the Pliocene sediments with some volcanics, and is an asymmetrical fold with northwesterly dipping axial surface in the main part, and displays some irregular shape in the northern one. The trend of the anticlinal axis changes from NE-SW in the main part to N-S or NNE-SSW in the northern one drawing a curve, and it plunges at 10°-20° toward the north (Fig. 1). Most of the minor faults developed in the Neogene formations belong to the longitudinal fault system, where constituent faults have their strikes parallel to the fold axis. Orientations of principal stress axis can be determined by the conjugate fault sets of this system as follows: σ1 (maximum compressive principal stress) is nearly normal to the bedding, σ2 (intermediate principal stress) is nearly parallel to the anticlinal axis, and σ3 (minimum compressive principal stress) is parallel to the bedding and normal to the anticlinal axis (Fig. 6). These represent the stress field in the past, that is at the faulting of this system. It is remarkable fact that these directional relationship between the principal stress axes and the fold axis is consistently maintained even if the trend of fold axis changes from NE-SW to N-S as in the northern part. Such a close relation strongly suggests that these faults and the Niitsu anticline might be formed under the same tectonic field of stress. In the Pleistocene formation unconformably orerlying the Neogene strata, some conjugate sets of minor fault with strikes normal to the fold axis are developed to compose the transversal fault saystem. Based on them, orientations of principal stress axes at the faulting of this system are restored as follows: σ1 is vertical, σ2 and σ3 are both flat, and nearly normal and parallel to the elongation of the Niitsu hill respectively (Fig. 5). It is noteworthy that these orientations are fixed through the whole region surveyed independent on the turning of the fold axis. This reveals that the stress field at the faulting of this system is different from that at folding, and it might correspond with that of the later stage, namely at the general upheaval of the Niitsu hill. With respect to the genetical type of fold, the Niitsu anticline is intensely suggested to be of transversal bending through discusions about the stress field expected by the fold of buckling or longitudinal bending and of the transversal bending as well as the effect of the movement of basement block on the overlying strata. From the above mentioned morphological characteristics and dynamic structures clarified in the Niitsu anticline, it is reasonably concluded at present that an uplifting with some tilting of the basement block is most adequate as a model for the mechanism of folding. Judging from the arrangement and orientations of fold axes around the Niitsu anticline, especially those in its northern and southern extension, differently oriented two types of basement blocks are supposed to exist beneath the Neogene strata; one is arranged in nearly NE-SW direction and the other is in N-S. Problems about the causes of such a blocking and its later uplifting with tilting are remained for future research.