Report on DELP 1988 Cruises in the Okinawa Trough : Part 7. Geologic Investigation of the Central Rift in the Middle to Southern Okinawa Trough
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Department of Marine Sciences, University of the Ryukyus
Department of Earth Sciences, Kobe University
Department of Technology, Kyushu University
Department of Marine Mineral Resources, Tokai University
Ocean Research Institute, University of Tok
抄録
It has been suggested that the spreading of the sea-floor is expected only in a limited area along the central rift in the southern part of the Okinawa Trough if it occurs. Therefore, the active extensional features and the mode of occurrence of igneous rocks in the central rift were studied in detail by means of the analysis of 3.5 KHz sonic, profiling records and rock samples dredged up during the DELP 1988 Cruises along the middle to southern Okinawa Trough. In addition to these data, research findings from various other organizations were supplementally used. The result revealed that in trending E-W to ENE-WSE five segments (grabens) of the central rift are distributed in echelon along the trough axis, which has been active since early Pleistocene time. The length of the central grabens is 50-100km and the width is 10-20km. The present central graben formation is represented by the topographic, central valley first shaped in late Pleistocene time. Transform faults connecting segments of the central graben were not found. Calc-alkaline and tholeiitic rocks showing typical island arc nature were dredged from the Yaeyama Central Graben, which is the deepest in the Okinawa Trough. The rocks are younger than 1 Ma., and the oxygen fugasity of the rock rather indicates initial stage of a rifting environment shown in mid-oceanic ridges. A black smoker indicating an active rifting environment was discovered at the eastern end of the Aguni Central Graben in 1989. The geologic evidence of the central grabens strongly supports the finding that the middle to southern Okinawa Trough does not yet reach to sea-floor spreading but represents the back-arc rifting stage. The evidence is consistent with other geophysical one such as seismic refraction and magnetic measurements during the Cruises. Moreover, the whole Okinawa Trough is identified as being in a back-arc rifting stage in the continental crust, judging from previous data in addition to the present results.
沖縄トラフでもし海底拡大が行なわれているとすれば,それは南部の中央地溝付近に限定されるだろうとの指摘がされている.そこで,中-南部沖縄トラフ域で行なわれたDELP1988年度航海の際得られた, 3.5KHz音波探査資料およびドレッジ試料を中心に他の試・資料を補い,トラフ中央地溝の活動的伸張構造およびリフティングに関連する火成岩の産状等を詳しく調べた.その結果,同トラフの中軸に沿って,前期更新世以降活動している東-西から東北東-西南西方向の長さ50-100kmの5つのグラーベンが雁行状に発達し,それらがすべて地形的に中軸谷を形成していることが明らかになった.それらグラーベンは火山フロントより後背に形成され,現在沖縄トラフの伸張運動はこれらのグラーベンを中心として行われている.しかし,これらのグラーベンをつなぐトランスフォーム断層の存在は認められない.沖縄トラフ最深部の八重山中央地溝内より採取された岩石は, 100万年より若い島弧に典型的なcalc-alkali rockとtheleiitic rockであった.しかし,それらのoxygen fugacityは,大洋中央海嶺域で認められるように還元的な環境を示し.背弧のリフティングとの密接な関係が示唆された.活動的なリフトで認められるブラック・スモーカーが粟国中央地溝の東端で1989年に発見された.以上により,沖縄トラフ中-南部は背弧におけるリフティングが行なわれている段階であり,海底拡大に至っていないとみなされる.これは同時に行われた屈折法地震探査や地磁気探査等の地球物理学的な調査結果とも矛盾しない.また,従来の調査結果と合わせると,沖縄トラフ全域が大陸地殻内でのリフティングの段階とみなせる
雑誌名
東京大學地震研究所彙報 = Bulletin of the Earthquake Research Institute, University of Tokyo
巻
66
号
1
ページ
179 - 209
発行年
1991-06-28
ISSN
00408972
書誌レコードID
AN00162258
フォーマット
application/pdf
日本十進分類法
453
出版者
東京大学地震研究所
出版者別名
Earthquake Research Institute, University of Tokyo
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Report on DELP 1988 Cruises in the Okinawa Trough : Part 7. Geologic Investigation of the Central Rift in the Middle to Southern Okinawa Trough
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