17:15 〜 19:15
[SCG44-P04] Uplift and denudation history of the Chugoku Mountains using (U-Th)/He thermochronology: preliminary results
キーワード：中国山地、低温熱年代学、隆起/削剥、(U-Th)/He法
The Chugoku Mountains are distributed roughly along the volcanic front of the Southwest Japan Arc, being the water divide. The uplift of the Chugoku Mountains has been attributed to geotectonic factors such as the subduction of the Philippine Sea Plate [1], however, the detailed process remains unclear. Tanaka and Suzuki (2021) [2] suggested that the Chugoku Mountains has not experienced significant uplift during most of the Cenozoic because the Kibi Plateau, depositional surfaces of the Paleogene sediments, are widely distributed at elevations of 900-1200m [2]. The major uplift of the Chugoku Mountains is thought to have occurred after the mid-Miocene considering the distribution of the middle Miocene marine deposits on the Kibi Plateau. The mountain building process of the Chugoku Mountains after the middle Miocene is poorly understood. Therefore, we have applied thermochronometric approaches to estimate the long-term uplift and denudation history of the Chugoku Mountains.
Thermochronology is a field of geoscience that reconstructs the time-temperature history of rocks and minerals using cooling ages of radiometric dating methods with different ranges of temperature sensitivity. In this study, we applied apatite and zircon (U-Th)/He (hereafter, AHe and ZHe, respectively) methods across the mountains. Their typical closure temperatures are 55–80°C for the AHe system [2] and 160–200°C for the ZHe system [3]. Rock samples were collected along the traverse of the eastern Chugoku Mountains (Hiroshima–Okayama Prefecture).
As a result, we obtained AHe ages ranging from 41.3 to 12.7 Ma (n=6) and ZHe ages ranging from 66.9 to 15.1 Ma (n=9), indicating younging toward north from south. The youngest AHe age ~13 Ma was obtained at the northernmost locality in the traverse, around the northern part of Shimane Prefecture. Two possible interpretations can be considered for this youngest age. First, since this locality is within the cluster of the late Cenozoic volcanic rocks [4], the age might be reset by a local reheating event due to the late Cenozoic magmatic activity. Alternatively, the young age might be derived from the uplift of the Chugoku Mountains after the back-arc spreading and the clockwise rotation of the Southwest Japan Arc at 18–16 Ma [5]. However, if a large-scale tectonic event like the uplift of the Chugoku Mountains were responsible, it is unlikely that the young age would be obtained at only a single location along the Sea of Japan coast. Therefore, based on the present dataset the younger age can more likely be attributed to reheating due to magmatic activity although the current number of sampling points is insufficient to provide a conclusive interpretation.
Future research will apply AHe/ZHe thermochronometers along a further westward traverse (Shimane-Hiroshima Prefectures), avoiding areas where volcanic rock clusters are distributed. This strategy will allow the reconstruction of thermal histories resulting from uplift and denudation of the Chugoku Mountains, with little magmatic/volcanic heating. We also plan to further constrain the thermal history using the apatite fission-track method, which can resolve detailed cooling patterns at ~60–120°C.
Acknowledgments: This study is part of the results supported by JSPS KAKENHI (Project Number: 21K14021).
References:
[1] Nakajima (2018) The Journal of the Geological Society of Japan, 124.9: 693-722.
[2] Tanaka and Suzuki (2021) Okayama University Earth Science Report, 27.1: 19-27.
[3] Farley (2002) Reviews in Mineralogy and Geochemistry, 47.1: 819-844.
[4] Reiners (2004) Geochimica et Cosmochimica Acta, 68.8: 1857-1887.
[5] Nguyen et al. (2020) Journal of Geophysical Research: Solid Earth, 125.10:
Thermochronology is a field of geoscience that reconstructs the time-temperature history of rocks and minerals using cooling ages of radiometric dating methods with different ranges of temperature sensitivity. In this study, we applied apatite and zircon (U-Th)/He (hereafter, AHe and ZHe, respectively) methods across the mountains. Their typical closure temperatures are 55–80°C for the AHe system [2] and 160–200°C for the ZHe system [3]. Rock samples were collected along the traverse of the eastern Chugoku Mountains (Hiroshima–Okayama Prefecture).
As a result, we obtained AHe ages ranging from 41.3 to 12.7 Ma (n=6) and ZHe ages ranging from 66.9 to 15.1 Ma (n=9), indicating younging toward north from south. The youngest AHe age ~13 Ma was obtained at the northernmost locality in the traverse, around the northern part of Shimane Prefecture. Two possible interpretations can be considered for this youngest age. First, since this locality is within the cluster of the late Cenozoic volcanic rocks [4], the age might be reset by a local reheating event due to the late Cenozoic magmatic activity. Alternatively, the young age might be derived from the uplift of the Chugoku Mountains after the back-arc spreading and the clockwise rotation of the Southwest Japan Arc at 18–16 Ma [5]. However, if a large-scale tectonic event like the uplift of the Chugoku Mountains were responsible, it is unlikely that the young age would be obtained at only a single location along the Sea of Japan coast. Therefore, based on the present dataset the younger age can more likely be attributed to reheating due to magmatic activity although the current number of sampling points is insufficient to provide a conclusive interpretation.
Future research will apply AHe/ZHe thermochronometers along a further westward traverse (Shimane-Hiroshima Prefectures), avoiding areas where volcanic rock clusters are distributed. This strategy will allow the reconstruction of thermal histories resulting from uplift and denudation of the Chugoku Mountains, with little magmatic/volcanic heating. We also plan to further constrain the thermal history using the apatite fission-track method, which can resolve detailed cooling patterns at ~60–120°C.
Acknowledgments: This study is part of the results supported by JSPS KAKENHI (Project Number: 21K14021).
References:
[1] Nakajima (2018) The Journal of the Geological Society of Japan, 124.9: 693-722.
[2] Tanaka and Suzuki (2021) Okayama University Earth Science Report, 27.1: 19-27.
[3] Farley (2002) Reviews in Mineralogy and Geochemistry, 47.1: 819-844.
[4] Reiners (2004) Geochimica et Cosmochimica Acta, 68.8: 1857-1887.
[5] Nguyen et al. (2020) Journal of Geophysical Research: Solid Earth, 125.10: