The first (U-Th)/He thermochronology of pseudotachylyte from the Median Tectonic Line, southwest Japan

Kunimi Yamada, Takahiro Hanamuro, Takahiro Tagami, Koji Shimada, Hideo Takagi, Ryuji Yamada, Koji Umeda

    Research output: Contribution to journalArticle

    3 Citations (Scopus)

    Abstract

    To constrain the thermal and its spatial conditions of pseudotachylyte formation and cooling, we dated zircons from a pseudotachylyte in the Ryoke belt obtained about 100. m north from the Median Tectonic Line in the Taki area, southwest Japan, and its host rock, the Hatai Tonalite, by the (U-Th)/He method. The ages obtained are 60.3. ±. 2.7 and 55.5. ±. 4.5 (1SE) Ma, respectively, which are essentially the same within the error. This is the first (U-Th)/He age directly measured for a pseudotachylyte. The coincidence between these ages indicates that the pseudotachylyte formed at ca. 60. Ma under the ambient temperature of about 180. °C, the closure temperature of zircon (U-Th)/He method. The Fish Canyon Tuff zircon, one of the most well-known geological age standards, was also dated to confirm the validity of our analyses, which yielded the weighted mean age of 29.6. ±. 1.5 (1SE) Ma, consistent with other reported ages. The average cooling rates and denudation rates of the host rock were calculated based on these ages, reported zircon fission-track ages (ca. 70. Ma) and apatite fission-track ages (ca. 40. Ma) as about 12. °C/Myr and about 0.6. mm/yr from 70 to 60. Ma, and about 4. °C/Myr and about 0.2. mm/yr from 60 to 40. Ma, respectively, assuming that the closure temperatures of zircon and apatite FT methods are 300. °C and 110. °C, respectively, and a geothermal gradient of 20. °C/km. One possible reason of this slow down of denudation rates is the decrease of convergent rate and change in the movement direction of the Pacific Plate relative to the Eurasian Plate along the southwest Japan in the Paleogene.

    Original languageEnglish
    Pages (from-to)17-23
    Number of pages7
    JournalJournal of Asian Earth Sciences
    Volume45
    DOIs
    Publication statusPublished - 2012 Feb 2

    Fingerprint

    pseudotachylite
    thermochronology
    tectonics
    zircon
    closure temperature
    denudation
    host rock
    apatite
    cooling
    Eurasian plate
    tonalite
    geothermal gradient
    Pacific plate
    tuff
    Paleogene
    canyon

    Keywords

    • (U-Th)/He
    • Geochronology
    • Pseudotachylyte
    • Thermochronology
    • Zircon

    ASJC Scopus subject areas

    • Earth-Surface Processes
    • Geology

    Cite this

    The first (U-Th)/He thermochronology of pseudotachylyte from the Median Tectonic Line, southwest Japan. / Yamada, Kunimi; Hanamuro, Takahiro; Tagami, Takahiro; Shimada, Koji; Takagi, Hideo; Yamada, Ryuji; Umeda, Koji.

    In: Journal of Asian Earth Sciences, Vol. 45, 02.02.2012, p. 17-23.

    Research output: Contribution to journalArticle

    Yamada, Kunimi ; Hanamuro, Takahiro ; Tagami, Takahiro ; Shimada, Koji ; Takagi, Hideo ; Yamada, Ryuji ; Umeda, Koji. / The first (U-Th)/He thermochronology of pseudotachylyte from the Median Tectonic Line, southwest Japan. In: Journal of Asian Earth Sciences. 2012 ; Vol. 45. pp. 17-23.
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    AU - Tagami, Takahiro

    AU - Shimada, Koji

    AU - Takagi, Hideo

    AU - Yamada, Ryuji

    AU - Umeda, Koji

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    AB - To constrain the thermal and its spatial conditions of pseudotachylyte formation and cooling, we dated zircons from a pseudotachylyte in the Ryoke belt obtained about 100. m north from the Median Tectonic Line in the Taki area, southwest Japan, and its host rock, the Hatai Tonalite, by the (U-Th)/He method. The ages obtained are 60.3. ±. 2.7 and 55.5. ±. 4.5 (1SE) Ma, respectively, which are essentially the same within the error. This is the first (U-Th)/He age directly measured for a pseudotachylyte. The coincidence between these ages indicates that the pseudotachylyte formed at ca. 60. Ma under the ambient temperature of about 180. °C, the closure temperature of zircon (U-Th)/He method. The Fish Canyon Tuff zircon, one of the most well-known geological age standards, was also dated to confirm the validity of our analyses, which yielded the weighted mean age of 29.6. ±. 1.5 (1SE) Ma, consistent with other reported ages. The average cooling rates and denudation rates of the host rock were calculated based on these ages, reported zircon fission-track ages (ca. 70. Ma) and apatite fission-track ages (ca. 40. Ma) as about 12. °C/Myr and about 0.6. mm/yr from 70 to 60. Ma, and about 4. °C/Myr and about 0.2. mm/yr from 60 to 40. Ma, respectively, assuming that the closure temperatures of zircon and apatite FT methods are 300. °C and 110. °C, respectively, and a geothermal gradient of 20. °C/km. One possible reason of this slow down of denudation rates is the decrease of convergent rate and change in the movement direction of the Pacific Plate relative to the Eurasian Plate along the southwest Japan in the Paleogene.

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