[S-CG52] Ocean Floor Geoscience

[SCG52-12] Origin of Philippine Sea basins
-Tectonic setting leading to subduction initiation of Izu-Bonin-Mariana arc-

*Osamu Ishizuka^1,9, Kenichiro Tani², Rex N Taylor³, Susumu Umino⁴, Izumi Sakamoto⁵, Yuka Yokoyama⁵, Gen Shimoda¹, Yumiko Harigane¹, Yasuhiko Ohara^6,9,10, Chris E Conway¹, Americus Perez⁷, Shun Sekimoto⁸ (1.Geological Survey of Japan, AIST, 2.National Museum of Nature and Science, 3.University of Southampton, 4.Kanazawa University, 5.Tokai University, 6.Hydrographic and Oceanographic Department of Japan, 7.Philippine Nuclear Research Institute,, 8.Kyoto University, 9.JAMSTEC, 10.Nagoya University)

Robust tectonic reconstruction of the evolving Philippine Sea Plate for the period immediately before and after subduction initiation at ~52 Ma to form the Izu-Bonin-Mariana arc is prerequisite to understand cause of subduction initiation. Understanding of nature and origin of overriding and subducting plates is especially important because plate density is a key parameter controlling subduction initiation based on numerical modelling. There is increasing evidence that multiple geological events related to changing stress fields took place in and around Philippine Sea plate about the time of subduction initiation at ~52 Ma (Ishizuka et al., 2011). To understand tectonics during the period of subduction initiation, it is important to understand the pattern and tempo of these geological events, particularly the duration and extent of seafloor spreading in the Mesozoic arc terrane (Daito Ridge Group), and its temporal relationship with spreading in the West Philippine Basin.
The recent cruises in the Philippine Sea using submersible Shinkai 6500 and its mother ship Yokosuka aimed to investigate origin and age of formation of ocean basins in and around the Daito Ridge Group. Recovery and characterization of igneous crust of these basins will provide crucial information about the possible basement of the Izu-Bonin-Mariana arc.
Eocene andesitic magmatism (Northern Philippine Sea volcanics) has been discovered in and around the Kita-Daito Basin (Ishizuka et al., 2022). Ar/Ar and zircon U-Pb dating results of these igneous rocks indicate that this magmatism occurred mainly between 45 and 42 Ma. These andesites show contribution of slab-derived material, however, lack of any systematic spatial variation of the slab component in the Northern Philippine Sea volcanics as well as great distance (over 400 km) from the arc front make unlikely that the Northern Philippine Sea volcanics is associated with contemporaneous subduction either along the Izu-Bonin-Mariana arc or Philippine arc. We argue that this magmatism was not associated with on-going subduction, but related to the rifting/spreading event forming the Kita-Daito Basin. As onset of activity of the Northern Philippine Sea volcanics is essentially synchronous with the magmatism associated with the Oki-Daito mantle plume, it is possible that both magmatism and rifting of the Kita-Daito Basin were triggered by the arrival of the Oki-Daito mantle plume in this region. These results demonstrate that the Kita-Daito Basin postdates subduction initiation of the Pacific Plate along the Izu-Bonin-Mariana arc.
In this contribution, we will also present geochronological and geochemical data of igneous rocks from the oldest part of the West Philippine Basin as well as the Palau Basin, and discuss their implications to the tectonics in the very early stage of the Philippine Sea Plate development.