Structure, texture, and physical properties of accretionary prism sediments and fluid flow near the splay fault zone in the Nankai Trough, off Kii Peninsula

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  • Anma Ryo
    Graduate School of Life and Environmental Sciences, University of Tsukuba
  • Ogawa Yujiro
    Graduate School of Life and Environmental Sciences, University of Tsukuba Present Address: Tokyo Electric Power Services Co., Ltd.
  • Kawamura Kiichiro
    Fukada Geological Institute
  • Moore Gregory
    SOEST, University of Hawaii at Manoa
  • Sasaki Tomoyuki
    School of Engineering, The University of Tokyo Present Address: Atmosphere and Ocean Research Institute, The University of Tokyo
  • Kawakami Shunsuke
    Graduate School of Life and Environmental Sciences, University of Tsukuba Present Address: Earth-Appraisal Co., Ltd.
  • Hirano Satoshi
    IFREE4, JAMSTEC Present Address: Dept. OD Sci. Tech. Support, Marine Works Japan Ltd.
  • Ota Teppei
    Graduate School of Life and Environmental Sciences, University of Tsukuba
  • Endo Ryota
    Graduate School of Life and Environmental Sciences, University of Tsukuba Present Address: INPEX CORPORATION
  • Michiguchi Yoko
    Graduate School of Life and Environmental Sciences, University of Tsukuba Present Address: Japan Nuclear Energy Safety Organization
  • YK05-08 Shipboard Science Party

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Other Title
  • 潮岬海底谷に露出する南海トラフ分岐断層付近の付加体堆積物の構造・組織・物性と流体移動
  • シオノミサキ カイテイコク ニ ロシュツ スル ナンカイ トラフ ブンキ ダンソウ フキン ノ フカタイ タイセキブツ ノ コウゾウ ソシキ ブッセイ ト リュウタイ イドウ

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Shionomisaki Canyon, off Kii Peninsula, SW Japan, cuts through five E–W-trending ridges developed in the Nankai accretionary prism. Using the submersible SHINKAI 6500, we investigated outcrops along the eastern canyon wall in the landward-most ridge. Four dives yielded information on lateral variations in the structure, texture, and physical properties of sediments along the splay fault zone in the accretionary prism. The ridge itself is disrupted by E–W-trending gullies that continue to bifurcations in the splay fault. Southward (seaward) dipping strata are predominant in gently folded, commonly steeply inclined sediments of gravity flow origin. The south-younging sequence is consistent with the radiolarian biostratigraphy. The sediments in this region are Pliocene to Recent in age (<4.3 Ma). Detailed observations reveal soft-sediment deformation structures such as web structure, vein structure, and deformation bands. The distribution of structures and chemosynthetic biocommunities (vesicomyid bivalves and tube worms) marks the position of faults that accompany active cold seepages. Porosity decreases southward toward the splay fault, showing a negative correlation with the age of the sedimentation (i.e., older sediments have higher porosity). This finding indicates progressive tectonic compaction toward the splay fault. The uniaxial compressive strength of sandstone, as calculated from a needle penetration test, shows a marked increase above the deduced fault zones. We attribute this finding to the precipitation of carbonate cement from CaCO3-saturated fluids that migrated along the splay fault and its bifurcations. Sandstones with high pore-connectivity and permeability, located above the faults, acted as a channel through which CaCO3-saturated fluids migrated. The related cementation acted to strengthen the frontal part of the ridge, which may act as an indenter for the inner wedge and as a backstop for the outer wedge. The proposed model accounts for the southward decrease in porosity behind the ridge front.

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