An overview of the Miocene tectonics in the Southwest Japan Arc from radiometric dating

2023 Fiscal Year Final Research Report

• Project/Area Number: 20K04136
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Review Section: Basic Section 17040:Solid earth sciences-related
• Research Institution: Tokyo Keizai University
• Principal Investigator: Shnjoe Hironao 東京経済大学, 全学共通教育センター, 教授 (60312013)
• Co-Investigator(Kenkyū-buntansha): 平田 岳史 東京大学, 大学院理学系研究科(理学部), 教授 (10251612) ; 折橋 裕二 弘前大学, 理工学研究科, 教授 (70313046)
• Project Period (FY): 2020-04-01 – 2024-03-31
• Keywords: 西南日本弧 / 日本海形成 / 中新世 / マグマ活動 / ウラン・鉛年代 / 瀬戸内火山岩類

Outline of Final Research Achievements

Southwest Japan underwent separation from the Asian continent through a clockwise rotation during the middle Miocene, yet numerous aspects concerning the timing and mechanisms of this event remain poorly understood. This study focused on examining the characteristics and dating the distinctive magmatic activities trench-proximal region of Southwest Japan, both preceding and following the rotation, to constrain the timing of this rotation and the contemporaneous plate configurations. Our findings reveal that the Setouchi Volcanic Rocks extend to the Amakusa area in western Kyushu. Furthermore, the formation age of the Murotomisaki Gabbro, which likely resulted from magma intrusion originating from the Shikoku Basin, is dated to approximately 15.6 million years ago. These results support the hypothesis that the rotation of Southwest Japan concluded 16 million years ago, with the Southwest Japan arc facing the Shikoku Basin on the Philippine Sea Plate at the cessation of this rotation.

Free Research Field

地質学

Academic Significance and Societal Importance of the Research Achievements

日本海の形成は日本列島が大陸から分離し，ほぼ現在の弧状列島の形となった出来事であった．したがって，日本海形成前後に起こった様々な地質学的な現象は，日本列島の地史を復元する上で重要な研究対象であると言える．本研究は特に西南日本が時計回り回転をして大陸から離れた時期とほぼ同時に起こったマグマ活動に着目して，マグマ活動の起こった時期やその成因について考察し，当時の西南日本の地質発展史の解明に寄与するものである．