Spatial distribution of the apatite fission‐track ages in the Toki granite,central Japan: Exhumation rate of a Cretaceous pluton emplaced in the East Asian continental margin |
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| Authors: | Takashi Yuguchi Shigeru Sueoka Hideki Iwano Tohru Danhara Masayuki Ishibashi Eiji Sasao Tadao Nishiyama |
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| Institution: | 1. Mizunami Underground Research Laboratory, Geoscientific Research Department, Tono Geoscience Center, Sector of Decommissioning and Radioactive Waste Management, Japan Atomic Energy Agency, 1‐64, Yamanouchi, Akiyo, Mizunami 509‐6132, JapanPresent address: Department of Earth and Environmental Sciences, Yamagata University, 1‐4‐12 Kojirakawa, Yamagata 990‐8560, Japan.;2. Natural Hazard Evaluation Group, Plant Technology Development Department, Monju Project Management and Engineering Center, Japan Atomic Energy Agency, 1, Shiraki, Tsuruga 919‐1279, Japan;3. Kyoto Fission‐Track Co., Ltd, 44‐4, Minamitajiri, Omiya, Kita‐ku 603‐8832, Japan;4. Mizunami Underground Research Laboratory, Geoscientific Research Department, Tono Geoscience Center, Sector of Decommissioning and Radioactive Waste Management, Japan Atomic Energy Agency, 1‐64, Yamanouchi, Akiyo, Mizunami 509‐6132, Japan;5. Department of Earth and Environmental Sciences, School of Science, Graduate School of Science and Technology, Kumamoto University, 2‐39‐1, Kurokami, Chuo‐ku, Kumamoto 860‐8555, Japan |
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| Abstract: | The Cretaceous Toki granitic pluton of the Tono district, central Japan was emplaced in the East Asian continental margin at about 70 Ma. The Toki granite has apatite fission‐track (AFT) ages ranging from 52.1 ±2.8 Ma to 37.1 ±3.6 Ma (number of measurements, n = 33); this indicates the three‐dimensional thermal evolution during the pluton's low‐temperature history (temperature in the AFT partial annealing zone: 60–120 °C). The majority of the Toki granite has a spatial distribution of older ages in the shallower parts and younger ages in the deeper parts, representing that the shallower regions arrived (were exhumed) at the AFT closure depth earlier than the deeper regions. Such a cooling pattern was predominantly constrained by the exhumation of the Toki granitic pluton and was related to the regional denudation of the Tono district. The age–elevation relationships (AERs) of the Toki granite indicate a fast exhumation rate of about 0.16 ±0.04 mm/year between 50 Ma and 40 Ma. The AFT inverse calculation using HeFTy program gives time‐temperature paths (t–T paths), suggesting that the pluton experienced continuous slow cooling without massive reheating since about 40 Ma until the present day. A combination of the AERs and AFT inverse calculations represents the following exhumation history of the Toki granite: (i) the fast exhumation at a rate of 0.16 ±0.04 mm/year between 50 Ma and 40 Ma; (ii) slow exhumation at less than 0.16 ±0.04 mm/year after 40 Ma; and (iii) exposure at the surface prior to 30–20 Ma. The Tono district, which contains the Toki granite, underwent slow denudation at a rate of less than 0.16 ±0.04 mm/year within the East Asian continental margin before the Japan Sea opening at 25–15 Ma and then within the Southwest Japan Arc after the Japan Sea opening, which is in good agreement with representative denudation rates obtained in low‐relief hill and plain fields. |
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| Keywords: | apatite fission‐track age chronological distribution in the pluton East Asian continental margin exhumation history HeFTy analysis Toki granite |
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