Metamorphic and cooling history of the Shimanto accretionary complex, Kyushu, Southwest Japan: Implications for the timing of out-of-sequence thrusting.

Illite crystallinity, K–Ar dating of illite, and fission-track dating of zircon are analyzed in the hanging wall (Sampodake unit) and footwall (Mikado unit) of a seismogenic out-of-sequence thrust (Nobeoka thrust) within the Shimanto accretionary complex of central Kyushu, southwest Japan. The obtained metamorphic temperatures, and timing of metamorphism and cooling, reveal the tectono-metamorphic evolution of the complex, and related development of the Nobeoka thrust. Illite crystallinity data indicate that the Late Cretaceous Sampodake unit was metamorphosed at temperatures of around 300 to 310°C, while the Middle Eocene Mikado unit was metamorphosed at 260 to 300°C. Illite K–Ar ages and zircon fission-track ages constrain the timing of metamorphism of the Sampodake unit to the early Middle Eocene (46 to 50 Ma, mean = 48 Ma). Metamorphism of the Mikado unit occurred no earlier than 40 Ma, which is the youngest depositional age of the unit. The Nobeoka thrust is inferred to have been active during about 40 to 48 Ma, as the Sampodake unit started its post metamorphic cooling after 48 Ma and was thrust over the Mikado unit at about 40 Ma along the Nobeoka thrust. These results indicate that the Nobeoka thrust was active for more than 10 million years. [ABSTRACT FROM AUTHOR]