Cooling and inferred exhumation history on the Ryoke metamorphic belt in the Yanai district, South-west Japan: Constraints from Rb-Sr and fission-track ages of gneissose granitoid and numerical modeling

The Ryoke metamorphic belt in south-west Japan consists mainly of I-type granitoids and associated low-pressure/high-temperature metamorphic rocks. In the Yanai district, it has been divided into three structural units: northern, central and southern units. In this study, we measured the Rb-Sr whole-rock-mineral isochron ages and fission-track ages of the gneissose granodiorite in the central structural unit. Four Rb-Sr ages fall in a range of ca 89-87 Ma. The fission-track ages of zircon and apatite are 68.9±2.6 Ma and 57.4 ±2.5 Ma (1σ error), respectively. Combining the newly obtained ages with previously reported (Th-)U-Pb ages from the same unit, thermochronologic study revealed two distinctive cooling stages; 1) a rapid cooling (>40°C/Myr) for a period (∼7 Myr) soon after the peak metamorphism (∼95 Ma) and 2) the subsequent slow cooling stage (∼5°C/Myr) after ca88 Ma. The first rapid cooling stage corresponds to thermal relaxation of the intruded granodiorite magma and its associated metamorphic rocks, and to the uplift by a displacement along low-angle faults which initiated soon after the intrusion of the magma. Uplift by the later stage deformation having formed large-scale upright folds resulted in progress of the exhumation during the first stage. The average exhumation velocity of the stage is ≥ 2 mm/yr. During the second stage, the rocks were not accompanied by ductile deformation and were exhumed with the rate of 0.1-0.2mm/yr. The difference in the exhumation velocity between the first and second cooling stages resulted from the difference in the thickness of the crust and in the activity of ductile deformation between the early and later stages of the orogenesis.