Sediment Provenance Along the Middle Miocene-Pleistocene Nankai Subduction Zone From Sediment Transport to Accretion: Implications for Stratigraphy in the Accretionary Prism

Based on U-Pb dating of zircon crystals and petrographic analysis, this study provides new insights into the paleogeographic and accretion evolution along SW Japan. Our data are consistent with an older submarine fan identified from drilling in the Shikoku Basin (Kyushu Fan similar to 14.7-12.2 Ma), having a mixed sand provenance from the paleo-Yangtze/Yellow rivers and the Shimanto Belt, and the younger Zenisu Fan (similar to 9.2-7.6 Ma), which is mainly sourced from the Shimanto Belt and the Izu-Bonin/Honshu arc collision. Our results are in agreement with the hypothesis of very oblique subduction or strike-slip motion between the northern Shikoku Basin and mainland Honshu from similar to 12.2 to 9.2 Ma, after which essentially orthogonal subduction occurred after similar to 8 Ma. The two main sandbodies drilled at IODP Site C0002 within the inner Nankai Accretionary Prism have similar petrographic signatures to those of the Zenisu and Kyushu submarine fans in the Shikoku Basin. The incorporation of the Shikoku Basin deposits most likely resulted from the seaward propagation of in-sequence thrusts forming an outer accretionary wedge. The incorporation of the Kyushu Fan into the inner accretionary prism implies that the d & eacute;collement was located in the hemipelagic interval beneath the Kyushu Fan at least until similar to 2 Ma, whereas it is now located in the hemipelagic intervals below the Zenisu Fan. Such shifts in d & eacute;collement location are most likely related to changes in physical properties of the hemipelagic interval due to significant compaction and diagenesis during subduction. Sediment inputs exert a first-order influence over the tectonic behavior of subduction zones. Based on U/Pb dating of zircon and petrographic analyses, this study focuses on the provenance, transport and accretion of sediments along SW Japan where a large-magnitude earthquake occurred. U/Pb dating of zircon from sand sampled in the Shikoku Basin and in the frontal Nankai accretionary prism indicates a significant sediment input from SE China and from the Shimanto Belt from similar to 14.7 to similar to 8 Ma. The youngest submarine fan of the Shikoku Basin was mainly sourced from the Shimanto Belt and the Izu-Bonin/Honshu arc collision. This agrees with the hypothesis of a very oblique subduction or strike-slip motion of the Philippines Sea Plate between similar to 12.2 and 9.2 Ma, after which orthogonal subduction occurred after similar to 8 Ma. The deformed submarine fans observed in the accretionary prism show similar petrographic signatures to sandbodies of the Shikoku Basin located on the incoming plate. This indicates that the Shikoku Basin deposits were incorporated by seaward propagation of thrusts, likely after resurgence of orthogonal subduction similar to 8 Ma. To our knowledge, this is the first time that any study has correlated stratigraphy on a subducting oceanic plate to the internal stratigraphy of a linked accretionary prism. Contrasting sediment sources between the Shikoku Basin fans indicate strike-slip motion/oblique subduction of the Philippine Sea Plate Similar petrographic signature between accreted materials of the inner Nankai accretionary prism and submarine fans of the Shikoku Basin Changes in d & eacute;collement location between similar to 6 and similar to 2 Ma, likely related to changes in physical properties of hemipelagic intervals