Multi-OSL-thermochronometry using deep borehole core for thermal history over 0.1 Myr in Rokko Mountains

Ogata, Manabu ; King, G. E.*; Herman, F.*; Yamada, Ryuji*; Omura, Kentaro*; Sueoka, Shigeru   

Optically stimulated luminescence (OSL) thermochronometry is a tool for constraining cooling histories in low-temperature domains (several tens of degree Celsius) during the past 10-100 kyr. This method is currently applied only to rapidly denuded regions (about 5 mm/yr), because luminescence signals in slowly denuded regions saturate before the rocks are exhumated to the surface. However, cooling histories in slowly denuded regions may be constrained if unsaturated samples are obtained from deep boreholes. We applied multi-OSL-thermochronometry to the deep borehole core drilled at the Rokko Mountains, Japan, where slow denudation rates (0.1-1.0 mm/yr) are expected from previous studies. We used the Kabutoyama core collected by National Research Institute for Earth Science and Disaster Resilience. The total length of Kabutoyama core is 1,313 m and we collected the samples at 408, 642, 818 and 1048 m for OSL-thermochronometry. We found that the sample temperatures remained around the present ambient temperature at each depth for the last 0.1 Myr, indicating that the Rokko Mountains is topographically stable, which was consistent with previous findings. Thus, the thermal denudation history of slowly denuded regions may be constrained by multi-OSL-thermochronometry using samples from deep borehole cores. However, the denudation rates in the Rokko Mountains were too low and could not be determined by this method.