[S-CG55] Ocean Floor Geoscience

[SCG55-15] An innovative approach to understanding tectonic evolution in the Eastern Nankai Trough, Central Japan.

*Philomene Vanessa EYANG ONDO EP ELLA NDONG¹, Asahiko TAIRA^1,2, Ayanori MISAWA³, Kohsaku ARAI³, Yuka YOKOYAMA¹, Izumi SAKAMOTO¹ (1.TOKAI Univ., 2.Japan Agency for Marine-Earth Science and Technology (JAMSTEC) , 3.Geological Survey of Japan (GSJ), AIST)

Keywords: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.