[S-CG45] Ocean Floor Geoscience

[SCG45-P12] Geological structures in the southern part of the Okinawa Trough north off Ishigaki Island: Preliminary results of the KH-21-3 cruise

*Ayanori Misawa¹, Ryuta Arai², Masahiko Sato³, Saki Ishino¹, Misaki Horiuchi⁴, Keita Suzuki⁴, Chiori Tamura⁵, Katsura Kameo⁵, Satoshi Okumura⁶, Makoto Otsubo¹, KH-21-3 Shipboard Scientists (1.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology(AIST), 2.Research Institute for Marine Geodynamics, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 3.Department of Earth and Planetary Science, The University of Tokyo, 4.Nippon Marine Enterprises, Ltd., 5.Atmosphere and Ocean Research Institute, The University of Tokyo, 6.MOL Marine Co. Ltd.)

Keywords:Okinawa Trough, Backarc Basin, Yaeyama Graben, Normal fault

Backarc basins on continental margins are important constituents of the plate tectonic paradigm, and knowledge of their evolution is key to understanding the lithosphere’s development. Since the backarc basins in the rifting state at the continental margin are limited to the Okinawa Trough and the Bransfield Strait, they are not well understood due to the current rarity of such deformations. In the Okinawa Trough, intermittent rifting has been occurring since 2 Ma. 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) reported a magma intrusion directly beneath the narrow rift valley of the Yaeyama Rift. Arai et al. (2017) also identified a less reflective zone and a low-velocity zone in the trough-fill sediments and suggested an off-axis magma chamber in the Okinawa Trough. However, the spatial distribution of the normal faults around the Yaeyama Rift and the intrusive structure beneath the Yaeyama Rift has not yet been clarified.

To clarify the detailed geological structure of the southern part of the Okinawa Trough, an integrated marine geological and geophysical survey was conducted using R/V Hakuho-maru of JAMSTEC (KH-21-3 cruise) in January 2021. In this cruise, we conducted the following survey: swath bathymetric survey, multi-channel seismic (MCS) reflection survey, subbottom profiler survey, geomagnetic measurements (proton precession magnetometer and shipboard three-component magnetometers), and gravity measurement. The MCS survey used two 355 cubic inch GI guns (total volume of 710 cubic inches) and a 1,200 m 48ch streamer cable. In the MCS profiles, we could obtain clear images down to approximately 1.6 sec (Two-Way Travel Time: TWT) below the seafloor in the trough-fill sediments of the Okinawa Trough. In the Yaeyama Rift, the graben formed by innumerable normal faults were clarified. Moreover, total magnetic field data were acquired with a surface-towed proton magnetometer. The proton magnetometer sensor was towed approximately 290 m behind the ship to minimize magnetization effects from the ship. In this presentation, we will show the preliminary results of the KH-21-3 cruise in the southern part of the Okinawa Trough.