09:45 〜 10:00
[SCG55-15] An innovative approach to understanding tectonic evolution in the Eastern Nankai Trough, Central Japan.
キーワード：Eastern Nankai Trough, Piggyback basins, Tectonic activity, MCS profiles
The Eastern Nankai Trough, located at the offshore Tokai area in Central Japan, remains a focal point for ongoing structural geology geoscience due to the development of the accretionary complex formed when the Philippine Sea (PHS) plate subducts beneath the Japan Islands. Twenty-six migrated multi-channel seismic (MCS) profiles perpendicular to the trough slope, a combined bathymetric, and a recent topographic were used to investigate the area located approximately between the eastern rim of the Kumano Basin to the Kanasu-No-Se area off the coast of Omaezaki. The study area is divided into the Kanesunose – Yukie area (KYA: Northeast region) and the Tenryu Submarine Canyon Area (TSCA: Southwest region). These regions present slope basins that we have recognized as Piggyback basins. Twelve Piggyback basins were identified, including the Kanasunose Trough, Yukie Trough, and Shima-Oki Trough. Overall, these basins show three seismic units: the lower seismic unit (LSU), the middle seismic unit (MSU), and the upper seismic unit (USU), characterized by parallel to sub-parallel internal reflectors, continuous to discontinuous with variable amplitudes. We found that the seismic facies onlapping-fill are characteristics of our study area, suggesting an environment with relatively low-velocity turbidity currents, and the seismic units with parallel-subparallel internal reflectors may correspond to the Early to Late Pleistocene and Middle Miocene-Pliocene.
We identified the Frontal Thrust, Tokai Thrust, and Kodaiba faults, which exhibit landward vergence and delimit folds at their seaward edges revealing a structural style of a succession of faults-folds or faults-folds-Piggyback basins.
Between approximately 137°10’E and 137°30’E, Faults, folds, and Piggyback Basin are oriented from NE-SW to NNE-SSW or N-S directions. In contrast, between approximately 137°30’E and 138°10’E, faults, folds, and Piggyback Basins are oriented in NE-SW direction. East of 138°10’E, some faults, folds, and Piggyback Basin shift again from NE-SW to NNE-SSW or N-S directions, suggesting a morphological configuration resembling an "echelon" which is reflected by some Piggyback basins. This morphological shape is likely a result of (1) the indentation caused by the collision of the Paleo-Zenisu Ridge with the accretionary complex, which bends the accretionary prism with probably high friction at over 30% in the TCSA seeming local, and (2) the subduction of PHS Plate and the collision of Izu-Bonin in KYA which is related to the regional tectonic. This suggests that Piggyback basins should be considered as records of the tectonic evolution in the Eastern Nankai Trough.
We identified the Frontal Thrust, Tokai Thrust, and Kodaiba faults, which exhibit landward vergence and delimit folds at their seaward edges revealing a structural style of a succession of faults-folds or faults-folds-Piggyback basins.
Between approximately 137°10’E and 137°30’E, Faults, folds, and Piggyback Basin are oriented from NE-SW to NNE-SSW or N-S directions. In contrast, between approximately 137°30’E and 138°10’E, faults, folds, and Piggyback Basins are oriented in NE-SW direction. East of 138°10’E, some faults, folds, and Piggyback Basin shift again from NE-SW to NNE-SSW or N-S directions, suggesting a morphological configuration resembling an "echelon" which is reflected by some Piggyback basins. This morphological shape is likely a result of (1) the indentation caused by the collision of the Paleo-Zenisu Ridge with the accretionary complex, which bends the accretionary prism with probably high friction at over 30% in the TCSA seeming local, and (2) the subduction of PHS Plate and the collision of Izu-Bonin in KYA which is related to the regional tectonic. This suggests that Piggyback basins should be considered as records of the tectonic evolution in the Eastern Nankai Trough.