[S-CG46] Uncovering stress accumulation and fault strengthening of megathrust earthquakes

[SCG46-P03] Preliminary results of radiolarian biostratigraphy from IODP Expedition 405 JTRACK

*Minori Hagino¹, Masao Iwai², Marianne Conin³, Patrick Fulton⁴, Jamie Kirkpatrick⁵, Shuichi Kodaira⁶, Christine Regalla⁷, Kohtaro Ujiie⁸, Nobu Eguchi⁶, Lena Maeda⁶, Natsumi Okutsu⁶, Sean Toczko⁶, IODP Expedition 405 Science Party (1.Graduate School of Science and Engineering, Yamagata University, 2.KCC/MaCRI, Kochi University, 3.University of Lorraine, 4.Cornell University, 5.University of Nevada, Reno, 6.JAMSTEC, 7.Northern Arizona University, 8.University of Tsukuba)

Keywords:IODP, JTRACK, Radiolaria, biostratigraphy, Japan Trench

The International Ocean Discovery Program (IODP) Expedition 405, Tracking Tsunamigenic Slip Across the Japan Trench (JTRACK), was planned to drill the rupture zone that caused 2011 Mw 9.0 Tohoku-oki Earthquake and the pelagic sequence of the incoming oceanic plate around Japan Trench off Miyagi Prefecture, and was conducted using the D/V Chikyu, from September to December 2024. In this expedition, we drilled at two sites across the Japan Trench, C0019 (land side) and C0026 (ocean side), to obtain cores and examined almost all core catcher samples for radiolarian biostratigraphy to date cored material.

Radiolarian biostratigraphy at Site C0019 documented the frontal prism structure. Overall, the estimated ages of the sediments in the prism range from the Miocene to the Pleistocene. Especially, the ages of the samples from Holes C0019L and C0019M (0 to 107.485 mbsf) were almost Quaternary. Also, several age gaps were found in Cores from Hole C0019J (~210 mbsf, ~610 mbsf, at least), which were consistent with the diatom stratigraphic results. In addition, the chert sample of the deepest Core from Hole C0019J yielded Cretaceous radiolarians.

In Site C0026, radiolarian abundance were common to abundant. From radiolarian biostratigraphy, the hemipelagic muds are continuously deposited and dated from the Early Miocene to the Quaternary. The estimated sedimentation rate is ten times or more higher during the Quaternary than during the Pliocene. An Early Miocene age was suggested by radiolarians for the Core C0026B-19K, below which there is a sequence of approximately 30 meters which did not yield any age marker species. Below the Core C0026B-26K, radiolarians from brownish–black clay, and color-banded clay immediately above the chert suggest a Cretaceous or earlier age.