5:15 PM - 7:15 PM
[SCG46-P02] Prism structure around the shallow rupture zone of 2011 Tohoku earthquake: Initial results of IODP JTRACK Exp.405 diatom biostratigraphy
Keywords:IODP Exp.405, Japan Trench, 2011 Tohoku earthquake, shallow rupture zone, diatom biostratigraphy, JTRACK
International Ocean Discovery Program (IODP) Expedition 405 operated from September through December 2024 by using DV Chikyu for Tracking Tsunamigenic Slip Across the Japan Trench (JTRACK) off Sendai, where large fault slips exceeding 50 meters were observed near the trench axis and caused a devastating tsunami. During the expedition, we drilled at two Sites C0019 and C0026 to obtain cores from shallow fault slip zone and input material of the Pacific Plate respectively. We examined almost all core catcher material and some additional inter section samples for diatom biostratigraphy to date cored material.
Diatom biostratigraphy at Holes C0019J and C0019K clearly documented the frontal prism structure and constrained the locations of major faults. Overall, the ages of the sediments in the prism range from Pleistocene to Miocene, though in general there are few intervals that show coherent age-depth trends. Major reversals defined faults at ~210 mbsf, ~610 mbsf, and the plate boundary fault zone at ~826 mbsf. Most prism units contain diatoms with Pleistocene, Pliocene, and upper Miocene ages that are interpreted as evidence for deposition by slope deformation processes, soft sediment deformation, and mass transport deposits (MTDs) or thickening of prism sediments by compressional deformation. Biostratigraphic constraints suggest thickness of sedimentary units in the prism is up to ~300 m.
Diatoms in samples of cores recovered Site C0026 were common to abundant, providing tight constraints of the ages of hemipelagic muds as Quaternary to Middle Miocene. Over this time interval, the diatom biostratigraphy revealed the average sedimentation rate was up to 33 m/m.y. in the Pleistocene and Pliocene, whereas average rates were lower in the upper Miocene interval (~5-15 m/m.y.), and very slow during the lower Miocene (< 2 m/m.y.). Sedimentation rate change is concidered to be affected by the distance from trench. The brownish-black clay below Core C0026B-20K was barren of diatoms. It will be suggested that before the early Miocene, drilled site was located within the subtropical gyre, southward of the Kuroshio extensional current.
Diatom biostratigraphy at Holes C0019J and C0019K clearly documented the frontal prism structure and constrained the locations of major faults. Overall, the ages of the sediments in the prism range from Pleistocene to Miocene, though in general there are few intervals that show coherent age-depth trends. Major reversals defined faults at ~210 mbsf, ~610 mbsf, and the plate boundary fault zone at ~826 mbsf. Most prism units contain diatoms with Pleistocene, Pliocene, and upper Miocene ages that are interpreted as evidence for deposition by slope deformation processes, soft sediment deformation, and mass transport deposits (MTDs) or thickening of prism sediments by compressional deformation. Biostratigraphic constraints suggest thickness of sedimentary units in the prism is up to ~300 m.
Diatoms in samples of cores recovered Site C0026 were common to abundant, providing tight constraints of the ages of hemipelagic muds as Quaternary to Middle Miocene. Over this time interval, the diatom biostratigraphy revealed the average sedimentation rate was up to 33 m/m.y. in the Pleistocene and Pliocene, whereas average rates were lower in the upper Miocene interval (~5-15 m/m.y.), and very slow during the lower Miocene (< 2 m/m.y.). Sedimentation rate change is concidered to be affected by the distance from trench. The brownish-black clay below Core C0026B-20K was barren of diatoms. It will be suggested that before the early Miocene, drilled site was located within the subtropical gyre, southward of the Kuroshio extensional current.