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

[SCG46-03] IODP Exp 405 reveals how to form a chaotic accretionary prism in the ‘non-accretionary’ Tohoku margin

★Invited Papers

*Christine Regalla¹、James D Kirkpatrick²、Marianne Conin³、Kohtaro Ujiie⁴、Patrick Fulton⁵、Shuichi Kodaira⁶、Lena Maeda⁶、Natusmi Okutsu⁶、IODP Expedition 405 Scientists (1.Northern Arizona University、2.University of Nevada Reno、3.University of Lorainne、4.University of Tsukuba、5.Cornell University、6.Japan Agency for Marine Earth Science and Technology )

キーワード：IODP, JTRACK, Japan Trench, Frontal Prism, Tohoku-oki earthquake

International Ocean Discovery Program (IODP) Expedition 405 (JTRACK) completed the first transect of boreholes across the trench from the undeformed reference section to the frontal prism of the Tohoku margin. These data, combined with prior IODP, seismic reflection, and bathymetric data, reveal several key findings regarding the composition and evolution of the frontal prism in the portion of the margin that hosted large magnitude shallow slip during the 2011 Tohoku-oki earthquake and tsunami. First, stratigraphic correlations suggest that the décollement preferentially forms within pelagic clays near the base of the input section, and that the upper ~240m of muds are tectonically offscraped. Second, core and logging data provide evidence of at least three lithologic repetitions within the prism sequence, and two 100-300 m scale, open to tight, seaward-vergent fault-related folds. A major fault identified at ~610 mbsf at Site C0019J, which places Late Miocene muds over Pleistocene muds, may be a major active splay fault in the prism and may provide a path for fluid migration. Stratigraphic repetition and folding suggest that the long-term strain within the prism is contractional and associated with plate convergence. Third, the prism notably contains numerous Plio-Quaternary slope, shelf, and prism-derived mass transport, debrite, turbidite, and slump deposits that accumulated as trench fill, and to a lesser extent, prism slope basin deposits, which were subsequently offscraped or overthrust during prism formation. The generation of mass transport deposits is likely enhanced at the Japan trench due to the subduction of seafloor relief generated by subducted outer rise normal faults, and may lead to multiple cycles of offscraping, slope failure, and re-accretion. The abundance of Pleistocene-aged sediments in the prism muds may be linked to increased sediment supply to the trench in response to Plio-Quaternay uplift and erosion of the Tohoku arc, and could be influenced by climatically modulated increases in sediment flux during Quaternary glaciations and sealevel lowstands. Collectively these data paint a picture of a dynamic and structurally complex frontal prism, formed by a combination incoming plate sediment accretion, downslope transport, and tectonic contraction, that accommodates a portion of the convergent strain budget in the shallow Tohoku plate boundary.