新生代地質構造の研究  [in Japanese] Analytical study of geological structure of the Cenozoic  [in Japanese]

It has recently become possible to discuss the mechanism of formation of geological structure on the basis of kinematical theory or analytical study in the field. These researches have made progress in connection with the discussion on regional tectonics as reviewed below. 1) Analytical study of geological structure: To establish the method for analysing the tectonic stress states was the important problem. For this purpose, analytical studies of conjugate joints or faults were introduced (HOSHINO, 1965; HIRAYAMA & KAKIMI, 1965). Many stress systems were described in the same area, and historical process of their formation was investigated (Fig. 1). Magnitude or sense of fault displacement was also another important problem. KODAMA (1974) and KAKIMI (1980) discussed it as related to crustal deformation. Regional stress fields which prevailed over island arc during the Neogene were analyzed with "dyke method" (NAKAMURA, 1969). They are considered to be N-S trend tensional type in the early stage, but later changed to E-W trend compressional type (TAKEUCHI, 1977). Since NAGUMO (1966) and others introduced kinematical theories of buckling fold (BIOT, 1961; RAMBERG, 1964), the relations between wavelength and thickness of competent layers were investigated in the field (KOMATSU, 1969; KATO, 1970). Mechanism of buckling fold was also discussed in relation to the existence of minor reverse faults (KINUGASA & KAKIMI, 1972) or neutral surface (Fig. 2). IKEDA & SHIMAMOTO (1974) and SHIMAMOTO (1982) examined their mechanism with model experiments (Fig. 3). SUZUKI et al. (1971) showed that the anticlines of the first class in Niigata Tertiary basin were formed by vertical movement of the basement blocks. Presice survey by NAKAMURA (1982) revealed that folds were initiated to the unit of the sedimentary basin, and developed by tilting of the basement blocks (Fig. 4). Recently another type of folding mechanism has been proposed. KUNIYASU (1981) showed that an anticline was formed step by step by alternative flexures of each wing, SHIBUYA & SHINADA (1985) examined the process by precise analysis of Sakuna anticline in Boso Peninsula. II) Study of the formation of the late Cenozoic sedimentary basins: During the recent twenty years, the studies on the formation of sedimentary basins have advanced particularly in the field of the Green Tuff and Quaternary basins. Recent offshore surveys also contributed to these researches. Mechanism of formation of sedimentary basins can be classified into the following six types. Warped basins were formed at the syncline of foundation fold (MAKIYAMA,1965) under the horizontal compression of the crust. This type of deformation has been discussed in Osaka Bay (Fig. 5). Tectonic history of the Kinki area was summarized by HUZITA (1973). Collapse basins form a relative depression surrounded by faults and its process and mechanism have been summarized by FUJITA (1960, 1971). He considered that their origin is ascribed to the dome like upheavals of the crust. Tilted basins appear near boundaries of tilted blocks of the crust (Fig. 8). TAKEUCHI (1978) explained that these basins were formed by the movement along cylindrical faults (Fig. 9). YANO (1983) considered that these basins were developed along antithetic faults which were formed by regional warping of the island arc (Fig. 10). Volcano-tectonic depressions are simillar in shape to the collapse basins, but are correlated with igneous activities. Green Tuff basins are the representatives and their mechanism are confirmed in many areas. "Shogi-daoshi-structure" is summarized as a type of sedimentary structure in which the center of maximum depression uniquely shifts toward a certain direction (FUJITA, 1951; YAN0, 1982). Two types of mechanisms have been proposed. One is the wavelike shifting of crustal depression (FUJITA, 1973a; KUWABARA, 1979; KAWABE; 1983), and the other is the unidirectional expanding of strained area on the hinge of crustal