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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