5:15 PM - 6:30 PM
[SCG47-P02] Uplift and denudation histories in Kitakami Mountains since the Cretaceous based on thermochronology
Keywords:Kitakami Mountains, Thermochronology, Uplift/denudation
Tectonics on the fore-arc side in an arc-trench system is strongly affected by mechanical interaction between plates along a trench. Thus, revealing crustal movements, such as uplift/denudation histories on the fore-arc side, is important for comprehensive understanding of tectonics of a subduction zone.
Northeast Japan arc is a typical island arc because the geologic components are clearly identified due to their arc-parallel arrangements. Hence, many studies on tectonics of island arc have been carried out in terms of various scientific fields (e.g., Amano and Sato, 1989; Sato, 1992; Hashimoto et al., 2008; Ikeda et al., 2012; Nakajima, 2013; Yoshida et al., 2013). Nakajima (2013) suggested that the back bone range and back-arc side have been uplifted by east-west compression stress initiated at 6.5 Ma and enhanced since 3 Ma. However, it is unclear whether these stresses affected the uplift on the fore-arc side. On the fore-arc side, uplift history of Kitakami Mountains has been estimated by marine terraces or alluvia in <105 years scale (Niwa, 2019). On the other hand, only Fukuda et al. (2020) discussed the uplift/denudation quantitatively in >106 years scale. However, Fukuda et al. (2020) focused only on tendency over the whole island arc. Therefore, this study aims to reveal the detailed uplift/denudation history in Kitakami Mountains in >106 years scale by applying thermochronology.
Apatite fission-track (AFT) and apatite uranium-thorium-samarium helium (AHe) thermochronometers were adopted. Samples were collected from granitoids in Kitakami Mountains. As a result, AFT ages of 156.8-70.3 Ma and AHe ages of 76.3-33.6 Ma were obtained. Note that AFT ages older than 100 Ma have large errors, ranging over 20-30 Mry (2σ).
In order to clarify the east-west trends of the cooling ages, age vs. longitude data were plotted including the previously reported ages. AFT ages get older to the east side, whereas AHe ages are spatially uniform (~40-30 Ma) except for the westernmost localities indicating older ages. Given that these spatial trends arise from the differential denudation histories between the east side and west side, the AFT age trend suggests that the west side have been denudated more after ~120 Ma. This observation agrees with the fact that Cretaceous volcanic rocks are distributed mainly on the east side. On the other hand, the AHe age trend indicate no spatial variation in denudation rates over the Kitakami mountains since ~40-30 Ma, except for the westernmost localities.
The future prospects are (a) to increase localities to apply thermochrometries, (b) to conduct thermal inverse modeling based on AFT ages and AFT lengths, and (c) to apply ultralow temperature thermochronology (e.g., electron spin resonance dating) etc.
Acknowledgements: 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 2018 to 2020)”. This study was also supported by the Grant-in-Aid for Scientific Research on Innovative Areas (KAKENHI No. 26109003) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT).
Northeast Japan arc is a typical island arc because the geologic components are clearly identified due to their arc-parallel arrangements. Hence, many studies on tectonics of island arc have been carried out in terms of various scientific fields (e.g., Amano and Sato, 1989; Sato, 1992; Hashimoto et al., 2008; Ikeda et al., 2012; Nakajima, 2013; Yoshida et al., 2013). Nakajima (2013) suggested that the back bone range and back-arc side have been uplifted by east-west compression stress initiated at 6.5 Ma and enhanced since 3 Ma. However, it is unclear whether these stresses affected the uplift on the fore-arc side. On the fore-arc side, uplift history of Kitakami Mountains has been estimated by marine terraces or alluvia in <105 years scale (Niwa, 2019). On the other hand, only Fukuda et al. (2020) discussed the uplift/denudation quantitatively in >106 years scale. However, Fukuda et al. (2020) focused only on tendency over the whole island arc. Therefore, this study aims to reveal the detailed uplift/denudation history in Kitakami Mountains in >106 years scale by applying thermochronology.
Apatite fission-track (AFT) and apatite uranium-thorium-samarium helium (AHe) thermochronometers were adopted. Samples were collected from granitoids in Kitakami Mountains. As a result, AFT ages of 156.8-70.3 Ma and AHe ages of 76.3-33.6 Ma were obtained. Note that AFT ages older than 100 Ma have large errors, ranging over 20-30 Mry (2σ).
In order to clarify the east-west trends of the cooling ages, age vs. longitude data were plotted including the previously reported ages. AFT ages get older to the east side, whereas AHe ages are spatially uniform (~40-30 Ma) except for the westernmost localities indicating older ages. Given that these spatial trends arise from the differential denudation histories between the east side and west side, the AFT age trend suggests that the west side have been denudated more after ~120 Ma. This observation agrees with the fact that Cretaceous volcanic rocks are distributed mainly on the east side. On the other hand, the AHe age trend indicate no spatial variation in denudation rates over the Kitakami mountains since ~40-30 Ma, except for the westernmost localities.
The future prospects are (a) to increase localities to apply thermochrometries, (b) to conduct thermal inverse modeling based on AFT ages and AFT lengths, and (c) to apply ultralow temperature thermochronology (e.g., electron spin resonance dating) etc.
Acknowledgements: 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 2018 to 2020)”. This study was also supported by the Grant-in-Aid for Scientific Research on Innovative Areas (KAKENHI No. 26109003) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT).