11:00 〜 13:00
[SCG42-P05] フィリピンルソン島中央コルディエラ山地の削剥史の地質; 熱年代学的予備研究
キーワード：地質年代、熱年代、U-Pb年代測定、フィッション・トラック年代測定、U-Th/He年代測定、削剥
Two end-member models, doming and pop-up uplift models, have been proposed as uplift mechanisms for mountain ranges in island arcs (Sueoka et al., 2015; Fukuda et al., 2021). In this study, we apply geo- and thermochronological methods to the Cordillera Central, Luzon, Philippines, to test these models and discuss the uplift mechanisms.
The Cordillera Central, a 300-km-long north-south trending mountain range in northern Luzon, corresponds to a magmatic arc which resulted from early Miocene-Recent subduction along the Manila Trench (Bellon and Yumul, 2000; Queaño et al., 2007). We collected five diorite samples from the east and west blocks of the Abra River Fault, a strike-slip fault along the western margin of the Cordillera Central. The zircon U–Pb Concordia ages range from 32.5 ± 0.4 to 6.1 ± 0.08 (1σ) Ma, indicating that the intermediate magma intermittently intruded the middle and upper crust during the Cenozoic volcanism. The zircon fission-track (ZFT) pooled ages range from 36.7 ± 3.9 to 6.9 ± 0.7 (1σ) Ma, which are generally comparable with the U–Pb ages. These results suggest that these diorites were rapidly cooled below ~200–300 ℃, the partial anneal zone of the ZFT, immediately after the intrusions. On the other hand, the apatite (U–Th–Sm)/He (AHe) weighted mean ages ranging from 9.3 ± 0.3 to 1.8 ± 0.4 (1σ) Ma indicate a decrease in cooling rates around ~80–200 ℃. This observation indicates that the ZFT ages reflect the initial cooling of the intermediate magma, whereas the AHe ages reflect the cooling process associated with the regional exhumation after the initial cooling.
The AHe ages do not show a younging pattern toward the axial part of the mountain range and do not support the doming-uplift of the Cordillera Central (Sueoka et al., 2015). Inverse analysis using a thermokinematic model (Braun et al., 2012) proposes a pop-up uplift mechanism with almost no exhumation during the Neogene and rapid exhumation (>3.0 mm/yr) during the Quaternary. A significant difference in exhumation rates was also not observed between the eastern and western blocks of the Abra River Fault, suggesting that faulting had little contribution to the regional exhumation. This interpretation is also supported by the good correlation of the age-elevation relationship regardless of the fault. Our initial geo- and thermochronological data infer that exhumation of the Cordillera Central during the Quaternary resulted from the pop-up uplift of the entire mountain range.
ACKNOWLEDGMENTS
This study was supported by the Grant-in-Aid for Scientific Research (C) (No. 21K03730).
REFERENCES
Bellon, H., Yumul, G.J.P. (2000). C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences, 331, 295–302.
Braun, J., van der Beek, P., Valla, P., Robert, X., et al. (2012). Tectonophysics, 524–525, 1–28.
Fukuda, S., Sueoka, S., Tagami T. (2021). Radioisotopes, 70, 173–187.
Queano, K.L., Ali, J.R., Milsom, J., Aitchison, J.C., Pubellier, M. (2007). Journal of Geophysical Research, 112, B05101, doi:10.1029/2006JB004506.
Sueoka, S., Tsutsumi, H., Tagami, T. (2015). Earth Science (Chikyu Kagaku), 69, 47–70.
The Cordillera Central, a 300-km-long north-south trending mountain range in northern Luzon, corresponds to a magmatic arc which resulted from early Miocene-Recent subduction along the Manila Trench (Bellon and Yumul, 2000; Queaño et al., 2007). We collected five diorite samples from the east and west blocks of the Abra River Fault, a strike-slip fault along the western margin of the Cordillera Central. The zircon U–Pb Concordia ages range from 32.5 ± 0.4 to 6.1 ± 0.08 (1σ) Ma, indicating that the intermediate magma intermittently intruded the middle and upper crust during the Cenozoic volcanism. The zircon fission-track (ZFT) pooled ages range from 36.7 ± 3.9 to 6.9 ± 0.7 (1σ) Ma, which are generally comparable with the U–Pb ages. These results suggest that these diorites were rapidly cooled below ~200–300 ℃, the partial anneal zone of the ZFT, immediately after the intrusions. On the other hand, the apatite (U–Th–Sm)/He (AHe) weighted mean ages ranging from 9.3 ± 0.3 to 1.8 ± 0.4 (1σ) Ma indicate a decrease in cooling rates around ~80–200 ℃. This observation indicates that the ZFT ages reflect the initial cooling of the intermediate magma, whereas the AHe ages reflect the cooling process associated with the regional exhumation after the initial cooling.
The AHe ages do not show a younging pattern toward the axial part of the mountain range and do not support the doming-uplift of the Cordillera Central (Sueoka et al., 2015). Inverse analysis using a thermokinematic model (Braun et al., 2012) proposes a pop-up uplift mechanism with almost no exhumation during the Neogene and rapid exhumation (>3.0 mm/yr) during the Quaternary. A significant difference in exhumation rates was also not observed between the eastern and western blocks of the Abra River Fault, suggesting that faulting had little contribution to the regional exhumation. This interpretation is also supported by the good correlation of the age-elevation relationship regardless of the fault. Our initial geo- and thermochronological data infer that exhumation of the Cordillera Central during the Quaternary resulted from the pop-up uplift of the entire mountain range.
ACKNOWLEDGMENTS
This study was supported by the Grant-in-Aid for Scientific Research (C) (No. 21K03730).
REFERENCES
Bellon, H., Yumul, G.J.P. (2000). C. R. Acad. Sci. Paris, Sciences de la Terre et des planètes / Earth and Planetary Sciences, 331, 295–302.
Braun, J., van der Beek, P., Valla, P., Robert, X., et al. (2012). Tectonophysics, 524–525, 1–28.
Fukuda, S., Sueoka, S., Tagami T. (2021). Radioisotopes, 70, 173–187.
Queano, K.L., Ali, J.R., Milsom, J., Aitchison, J.C., Pubellier, M. (2007). Journal of Geophysical Research, 112, B05101, doi:10.1029/2006JB004506.
Sueoka, S., Tsutsumi, H., Tagami, T. (2015). Earth Science (Chikyu Kagaku), 69, 47–70.