Report on DELP 1987 Cruises in the Ogasawara : Part II : Seismic Reflection Studies in the Ogasawara Trough

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概要

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• Multichannel seismic profiles along two track lines in the Ogasawara Trough were obtained through DELP 1987 cruises. The trough is located between the old volcanic (Ogasawara) Ridge and the present volcanic front (Shichito-Iwoto Ridge). Stratigraphic units are acoustically distinguishable between eastern and western parts of the trough. The sedimentary unit of the trough in the eastern part is acoustically transparent and that in the western part, on the contrary, is opaque. The sedimentary layer becomes thicker toward the southern margin of the trough accompanied by scattering of reflected waves as well as decrease in free air gravity anomaly values. Although the acoustic basement was not clearly identified by the present study, it seems likely that it is dipping eastward in a stepwise manner. This feature can be reasonably interpreted as caused by normal faults which were formed in a tensile stress field during rifting of the trough in the Tertiary period (ca. 26-28 Ma). This tectonic deformation is possibly caused by differential opening of the Parece Vela Basin and Shikoku Basin in a narrow time interval.1987年11月小笠原トラフ周辺海域においてDELP研究航海が実施された.小笠原トラフは,第三紀の火山弧である小笠原海嶺と現在の火山弧である七島・硫黄島海嶺の間に位置する.トラフ内の堆積構造は,トラフ内の東側と西側で,音響学的に大きく異なっている.東側では音響学的に透過性の良い堆積物が,西側では音響学的に散乱の激しい堆積物が堆積している.トラフ南部に向かうほど堆積物は厚くなり,その散乱の度合も激しくなる傾向がある.音響基盤については,東西測線のトラフ底中央部においてのみ明瞭に確認することができた.それによると,七島・硫黄島海嶺から続いてくると思われる音響基盤が,東落ちの正断層を伴いながら,ブロック状にステップダウンしている.このことは,かつてこの地域がテンションの場にあったことを示唆し,小笠原トラフがリフティング起源であることを示す.その時期については,四国海盆拡大開始の前後で,しかも非常に短期間のうちに形成されたと推定する.このような構造的変形は四国海盆とパレスベラ海盆の拡大過程の違い,特に拡大開始時期のわずかな時間的ずれによって引き起こされたものと考えられる.

• 東京大学地震研究所,Earthquake Research Institute, University of Tokyo,千葉大学理学部地学科,東京大学海洋研究所,東北大学理学部地震予知・火山噴火予知センターの論文
• 1989-09-30

著者

• SUYEHIRO Kiyoshi

  IFREE, Japan Agency for Marine-Earth Science and Technology

• Tokuyama Hidetoshi

  Faculty Of Pharmaceutical Sciences The University Of Tokyo

• TOKUYAMA HIDEKAZU

  Ocean Research Institute, University of Tokyo

• Suyehiro Kiyoshi

  Ifree Japan Agency For Marine-earth Science And Technology

• Nishizawa A

  Maritime Safety Agency Tokyo Jpn

• Shimamura Kiyoshi

  Ifree Japan Agency For Marine-earth Science And Technology

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