Geological history of the Southwest Japan from the triple junction of the Shimanto, Chichibu, and Sanbagawa belts

2023 Fiscal Year Final Research Report

• Project/Area Number: 19K04013
• 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: Shinshu University
• Principal Investigator: Tokiwa Tetsuya 信州大学, 学術研究院理学系, 准教授 (50649241)
• Co-Investigator(Kenkyū-buntansha): 森 宏 信州大学, 学術研究院理学系, 助教 (80788183)
• Project Period (FY): 2019-04-01 – 2024-03-31
• Keywords: 西南日本 / 地質構造発達史 / ジルコンU-Pb年代 / ラマン分光分析

Outline of Final Research Achievements

Geological survey, structure analysis, zircon U-Pb dating and Raman spectra of carbonaceous-material thermometry were carried out on the Shimanto, Chichibu and Sambagawa belts. The results confirm that these belts consist of several thrust sheets and that there is a geological unit with peak temperature of about 300°C, which is the boundary region between accretionary and metamorphic rocks. Detrital zircon U-Pb ages for Cretaceous trench-fill deposits show no significant age gap between their youngest zircon ages and true depositional age inferred from fossils. In addition, Cretaceous zircons dramatically increased during the mid-Cretaceous, and this large amount of Cretaceous zircons were most likely supplied by Ryoke Granitoids and Abukuma Granitoids.

Free Research Field

地質学

Academic Significance and Societal Importance of the Research Achievements

紀伊半島には付加体と変成岩の境界領域である約300℃の被熱温度を受けた地質体が存在することが明らかになり，今後の付加体・変成岩の境界領域の研究場を提供することになった．また，ジュラ紀から白亜紀の海溝充填堆積物の砕屑性ジルコンU-Pb年代の最若年代が示準化石から求めた堆積年代と大きな差がないことが分かり，ジルコン年代による堆積年代の制約能力の可能性を見出した．加えて，海溝充填堆積物のジルコン年代構成は，東西で違いが認められるものの，その原因は近接する内帯の火成岩の分布状況で説明できる．このことは，大規模な左ずれを必要とせず，西南日本の地質構造発達史の解釈において重要な情報を提供する．