[S-CG48] Ocean Floor Geoscience

[SCG48-P27] Submarine geological structures in the southern part of the Okinawa Trough: Preliminary results of the KH-23-11 cruise

*Ayanori Misawa¹, Ryuta Arai², Akane Yamamoto³, Katsura Kameo⁴, Ryoji Toda⁴, Hikaru Iwamaru⁵, Makoto Otsubo¹, KH-23-11 Shipboard Scientists (1.Geological Survey of Japan, AIST, 2.Japan Agency for Marine-Earth Science and Technology , 3.Nihon University, 4.Atmosphere and Ocean Research Institute, The University of Tokyo, 5.Nippon Marine Enterprises, Ltd.)

Keywords:Okinawa Trough, Backarc Basin, Normal Fault, Submarine Geological Strcture, Seismic Reflection Survey

Backarc basins on continental margins are important constituents of the plate tectonic paradigm, and their evolution is critical to understanding the lithosphere’s development. The Okinawa Trough is one of the few backarc basins in the rifting state at the continental margin, and intermittent rifting has occurred since 1.5 Ma (Sibuet et al., 1998). Based on seismic profiles from the Yaeyama Rift, Arai et al. (2017) and Nishizawa et al. (2019) reported normal faults bounding the Yaeyama Rift and the stratigraphy of well-stratified trough-fill sediments. Arai et al. (2017) also reported a magma intrusion directly beneath the narrow rift valley of the Yaeyama Rift. During the KH-21-3 cruise in 2021, we conducted a seismic reflection survey by setting grid-shaped survey lines around the existing survey lines offshore the northern coast of Ishigaki Island. This exploration revealed the subseafloor geological structure of the Okinawa Trough and the Yaeyama Rift, particularly the development of normal faults and intrusion structures around the Yaeyama Rift. However, only a tiny portion of the Yaeyama Rift could be investigated in the 2021 exploration. To clarify the overall detailed geological structure of the southern part of the Okinawa Trough, an integrated marine geological and geophysical survey was conducted on a wide area from the Yaeyama Rift to the Yonaguni Rift using R/V Hakuho-maru of JAMSTEC (KH-23-11 cruise) from December 2023 to January 2024. In this cruise, we conducted the following survey: Multi-Channel Seismic (MCS) reflection survey, swath bathymetric survey, subbottom profiler survey, geomagnetic measurements (proton precession magnetometer and shipboard three-component magnetometers), and gravity measurement. Especially, the MCS survey used two 355 cubic inch GI guns (total 710 cubic inches) and a 1,200 m 48ch streamer cable.
In the MCS profiles, we could obtain clear images down to approximately 2.5 seconds (Two-Way Travel Time: TWT) below the seafloor in the trough-fill sediments of the Okinawa Trough. In the Yaeyama Rift area, continuous, high-amplitude parallel reflectors characterize the internal structure of the trough-fill sediments. The thickness of the trough-fill sediments in this area is approximately 1.3 seconds (TWT) and generally increases toward the axis of the Yaeyama Rift. Similar to the 2021 exploration, the development of normal faults is observed in the area around the Yaeyama Rift, and a structure presumed to be an intrusive structure from deep is observed. The trough-fill sediments have a maximum thickness of 2.5 seconds (TWT) and are characterized by stratified internal reflectors. Numerous normal faults are recognized to have developed in the trough-fill sediments. In the Yonaguni Rift area, the maximum thickness of the trough-fill sediments is 1.2 seconds (TWT), which tends to be thinner than in other areas. Normal faults are recognized inside and around the Yonaguni Rift, and the intrusion structures are identified from the deeper part. Some intrusion structures are thought to have reached the seafloor, forming submarine volcanoes. This presentation will show the latest results about the submarine geological structure in the southern part of the Okinawa Trough, which was revealed from seismic sections obtained during the KH-23-11 cruise.