西南日本における海溝‐島弧‐縁海系の地殻構造―南海トラフから大和海盆北縁まで―

Abstract

The first attempt to make a crustal-scale cross-section of a trench-arc-backarc sea system has been successfully accomplished from the Nankai trough to the northern margin of the Yamato basin across Southwest Japan, using three seismic profiles: Nankai trough to Japan Sea coast (Ito et al., 2009), Oki trough (Tanaka and Ogusa, 1981), and Yamato basin (Sato et al., 2006). The section contains much new information that will be useful for research on the structural development of the Japanese island arc. The highlight of the information is the substantial difference in the structure between the Outer and the Inner zones. The Outer zone is constructed mainly by N-dipping accretionary complexes in its upper crust with a poorly developed lower crust. On the contrary, the Inner zone exhibits predominantly horizontal structures in its upper crust, beneath which thick horizontal lower crustal laminations occur as in the continental lower crust. Furthermore a 10-km-thick lens-shaped olivine-pyroxene cumulate body underlies the lower crust. The N-dipping Median Tectonic Line juxtaposes two substantially different zones from the upper to lower crusts The section, however, has the following serious defects caused by the poor specifications of the seismic profilings used here: (1) Shallow structures (<about several km) are poorly imaged, which makes it difficult to interpret geological structures. (2) Seismic images beneath the on-and-offshore zones of the Japan Sea coast are missing, which also makes it difficult to understand the rifting process associated with the opening of the Japan Sea. Thus, a new crustal-scale cross-section that is free from these defects is necessary. Fortunately, both the theory and the techniques of seismic profiling have advanced tremendously recently. This situation enables us to conduct seismic profiling along a long seismic line simultaneously on both land and sea (so-called “integrated land-and-sea seismic profiling”), and to provide a new section. The new section is expected to replace the former one soon.