Multi-OSL-thermochronometry using deep borehole core for thermal history
over 0.1 Myr in Rokko Mountains
Abstract
Optically stimulated luminescence (OSL) thermochronometry is a tool for
constraining cooling histories in low-temperature domains (several tens
of degree Celsius) during the past
104–105 years
[1][2][3]. This method is currently applied only to rapidly
denuded regions (~5mm/yr when a general geothermal
gradient in is assumed to be ~0.03℃/m) 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. In addition, using deep borehole core enable to
compare the results between samples at multiple depths, which is useful
to isolate the denudation history from other events, such as faulting or
hydrothermal activity. We applied multi-OSL-thermochronometry [2] 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
[4][5][6]. We used the Kabutoyama core collected by National
Research Institute for Earth Science and Disaster Resilience
[5][7]. 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. Our results showed that useful thermal
information can be extracted from the infrared stimulated luminescence
signals of samples collected at depths ≥408 m. 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. Further
research is required to quantify the denudation rate. This study was
funded by the Ministry of Economy, Trade and Industry (METI), Japan as
part of its R&D supporting program titled “Establishment of Advanced
Technology for Evaluating the Long-term Geosphere Stability on
Geological Disposal Project of Radioactive Waste (Fiscal Years
2019-2021)”. References: [1] Herman et al. (2010). Earth and
Planetary Science Letters, 297, 183-189; [2] King et al. (2016).
Quaternary Geochronology, 33, 76-87; [3] Herman and King (2018).
Elements, 14, 33-38; [4] Huzita (1968). The Quaternary Research, 7,
248-260; [5] Sueoka et al. (2010). Journal of Geography, 119,
84-101; [6] Matsuhi et al. (2014). Transactions, Japanese
Geomorphological Union, 35, 165-185; [7] Yamada et al. (2012).
Technical Note of the National Research Institute for Earth Science and
Disaster Prevention, 371, 27p.