Sedimentary basins analysis through Multi-Channel Seismic (MCS) profiles of the Eastern Nankai Trough landward slope, Japan.

抄録

Sedimentary basins analysis through MCS profiles suggests that basins present in the landward slope of the Eastern Nankai trough originate from the compressional stress produced by the subduction of the Philippine Sea plate beneath the Southwest Japan. 26 migrated MCS profiles-oriented northwest – southeast made during Geological Survey of Japan research cruises GC98 and GA99, as well as a bathymetric map generated by GMT5 software were used to identify the basins. Once a basin was confirmed both by the MCS profile and on the bathymetric map, from the Nankai trough to the landward slope, a number (from 1 to n) was assigned to it. Then, basins with the same number were grouped together, and the basin with the greatest thickness was chosen to analyse the area deposition, thickness deposits, and structural direction. A total of 14 basins were identified and interpreted as piggyback basins (basins created by anticlines or syndepositional growth structure (Ori and Friend (1984))), and among these basins Kanasunose Trough (basin 2), Yukie Trough (basins 5-7), Shima-Oki Trough (basins 13-14) were recognized. Five preferential directions were identified: North-South (basins 8, 11, 14), North northeast – South southwest (basins 7,12), Northeast-Southwest (basins 5, 6, 10, 13), East northeast – west southwest (basins 1, 2, 3, 9), and West northwest – East southeast (basin 4). Basin 4 has a peculiarity by the fact of its west northwest – east southeast direction corresponding to the direction of the compressional deformation meaning that this basin is probably syn-tectonic. These piggyback basins with a landward tilting are composed of two seismic sequences S2 and S1. S2 (sub-parallel to migrating wave internal reflectors, continuous to discontinuous with varying amplitude events) unconformable onlaps onto the basement, and S1 (parallel internal reflectors, continuous with low to moderate amplitude) unconformable onlaps onto S2. S2 and S1 respectively can probably correspond to the Unit II and Unit I defined by Hiroki et al. (2004) in the well BH-1 giving an age comprised between 0.5-1.2 Ma for Unit 2 and 0-0.5 Ma for Unit 1, using Quaternary standard nannofossil events (events 1-10) of Sato and Takayama (1992), and corresponding to the nannofossil zone CN15-CN13b of Okada and Bukry (1980).

References

Hiroki Y., Watanabe K., and Matsumoto R., 2004. Lithology, Biostratigraphy, and Magnetostratigraphy of Gas Hydrate-Bearing Sediments in the Eastern Nankai Trough. Resource Geology. Vol. 54, no 1, 25-34.

Okada H., and Bukry D., 1980. Supplementary modification and introduction of code numbers to the low-latitude coccolith biostratigraphic zonation (Bukry, 1973; 1975). Marine Micropaleont., 5, 321-325.

Ori G.G. and Friend P.F., 1984. Sedimentary basins formed and carried piggyback on active thrust sheets. Geology,12, 475±478.

Sato T., and Takayama T., 1992. A stratigraphically significant new species of the calcareous nannofossil Reticulofenestra asanoi. In Ishizaki K., and Saito T., (eds.) Centenary of Japanese Micropaleontology, Terra Scientific Publ., 457-460.