Reconstruction of the Nankai Trough Margin during the Middle Miocene

2019 Fiscal Year Final Research Report

• Project/Area Number: 16K05583
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Research Field: Geology
• Research Institution: Tokyo Keizai University
• Principal Investigator: Shinjoe Hironao 東京経済大学, 全学共通教育センター, 教授 (60312013)
• Co-Investigator(Kenkyū-buntansha): 折橋 裕二 弘前大学, 理工学研究科, 教授 (70313046)
• Project Period (FY): 2016-04-01 – 2020-03-31
• Keywords: 西南日本弧 / 日本海形成 / 中新世 / 火成活動 / 苦鉄質火成岩 / U-Pb年代 / 40Ar/39Ar年代

Outline of Final Research Achievements

The radiometric age of the igneous rocks formed before and after the clockwise rotation of the Southwest Japan arc was analyzed. The zircon U-Pb method revealed that the Outer Zone felsic igneous activity started around 15.6 Ma and that there was no clear along arc age variation from Kyushu to the Kii Peninsula. On the other hand, the Neogene granitoids of the Koshikijima Islands off western Kyushu showed U-Pb ages of ca. 10 Ma, suggesting that the igneous activity is related to the commencing activities of the Okinawa Trough. The 40Ar/39Ar ages of the trench proximal mafic igneous rocks demonstrated that the igneous activity is basically post-rotation of the SW Japan arc. The whole rock composition of the mafic igneous rocks revealed the intense geochemical variations. Origin of the various mafic magmas was discussed based on their ages and whole rock compositions.

Free Research Field

地質学

Academic Significance and Societal Importance of the Research Achievements

日本海形成に伴って西南日本は時計回り回転してユーラシア大陸から離れたとされる。その回転時期の前後に起こったマグマ活動によって作られた火成岩試料を採取し、化学成分を分析するとともに年代測定を行った。これらの結果と過去の古地磁気や地質学的な研究結果を組み合わせることで、回転時期をより精密に評価し、マグマ発生の原因についての理解を深めた。これらを通じて当時の西南日本の地質発展史に寄与する成果を得た。