5:15 PM - 6:45 PM
[SCG41-P03] Applicability of low-temperature thermochronology to the Pliocene-Quaternary mountain building in the Japanese Islands
Keywords:mountain building, Japanese Islands, fission-track thermochronology, (U-Th)/He thermochronology
Low-temperature thermochronology, e.g., fission-track (FT) and (U-Th)/He thermochronometries, have been successfully used to reveal uplift-exhumation history of mountains over the world (e.g., Herman et al., 2013, Nature; Schildgen et al., 2018, Nature). On the other hand, most of the Japanese mountains are thought to be “young” mountains uplifted mainly in the Pliocene or Quaternary (Yonekura et al., 2001, “Regional Geomorphology of the Japanese Islands, vol. 1, Introduction to Japanese Geomorphology”). In such young mountains, low-temperature thermochronometries are difficult to detect cooling-exhumation histories related to the ongoing mountain building due to the small amount of exhumation after the onset of the uplift (Sueoka et al., 2016, Geosci. Frontiers).
This study aims at extracting conditions of mountains for which low-temperature thermochronology is able to detect the recent cooling-exhumation signals. We used the equation of Willet and Brandon (2013, G-Cubed), by which a temperature (T) at a desired time (t) and depth (z) can be obtained assuming a constant uplift rate (u) since a given time (t1). By using this equation, time-temperature histories of the rocks exposed at the present surface were simulated for a range of onset time of the uplift (t1), uplift rate (u), and onset time of the model (t0). The modeled time-temperature paths were converted into cooling ages of FT and (U-Th)/He systems in apatite and zircon based on the forward function of HeFTy ver. 1.9.3 (Ketcham, 2005, Rev. Min. Geochem.). The input parameters ranged at 0.5-5 Ma for t1 at intervals of 0.5 (10 cases), 0.01-10 mm/yr at intervals of 100.5 (7 cases), and 15 Ma, 60 Ma, and 120 Ma for t0 (3 cases). The surface elevation was assumed as in the following two cases: constant (elevation is constantly zero; uplift rate = exhumation rate), and increases exponentially by following the model of Ohmori (1978, Bull. Dept. Geogr. Univ. Tokyo).
The modeled FT and (U-Th)/He dates were consistent with the dates observed in some mountains for which uplift rates and onset times of the uplift are well known. In summary, uplift rates need to be greater than ~3 mm/yr to make FT and (U-Th)/He dates in zircon significantly younger than t0. On the other hand, apatite FT and (U-Th)/He dates can be significantly younger than t0 when the uplift is faster than ~0.3 mm/yr depending on t1 and presence/absence of the elevation change, and can be younger than t1 when the uplift is faster than ~1 mm/yr. Although t0 is equivalent to the formation ages of the rocks, the results were generally common for the three values of t0 attempted. Note, however, that t0 younger than 5 Ma can lead to different results; in such a case, cooling after the intrusion of the pluton and cooling by exhumation are promoted simultaneously.
The modeling results were applied to the low-temperature thermochronological database in Japan (Sueoka & Tagami, 2019, Island Arc) to illustrate the distribution of the uplift rate over the few million years. The results indicate that cooling ages younger than t1 are expected in the Japanese Alps, Kanto Mountains, Tanigawa-dake, and Iide Mountains. Ages younger than t0 are expected in the Hida Plateau, Mino Plateau, and Shikoku Mountains, whereas ages are less likely to be reset in the northern Kitakami Mountains, Abukuma Mountains, Rokko Mountains, and Awaji Island. The distribution of the uplift was generally consistent with that for 100 thousand years deduced from marine/fluvial terraces (Fujiwara et al., 2005, Jour. Nucl. Fuel Cycle Envir.), except for some areas, e.g., Muroto Cape, western Ise Bay, and Hokuriku districts. This disagreement might be brought by an accelerated uplift after 0.5 Ma and/or sampling bias related to conditions where terraces are developed and preserved (e.g., Malatesta et al., 2023, AGU abst.).
This study aims at extracting conditions of mountains for which low-temperature thermochronology is able to detect the recent cooling-exhumation signals. We used the equation of Willet and Brandon (2013, G-Cubed), by which a temperature (T) at a desired time (t) and depth (z) can be obtained assuming a constant uplift rate (u) since a given time (t1). By using this equation, time-temperature histories of the rocks exposed at the present surface were simulated for a range of onset time of the uplift (t1), uplift rate (u), and onset time of the model (t0). The modeled time-temperature paths were converted into cooling ages of FT and (U-Th)/He systems in apatite and zircon based on the forward function of HeFTy ver. 1.9.3 (Ketcham, 2005, Rev. Min. Geochem.). The input parameters ranged at 0.5-5 Ma for t1 at intervals of 0.5 (10 cases), 0.01-10 mm/yr at intervals of 100.5 (7 cases), and 15 Ma, 60 Ma, and 120 Ma for t0 (3 cases). The surface elevation was assumed as in the following two cases: constant (elevation is constantly zero; uplift rate = exhumation rate), and increases exponentially by following the model of Ohmori (1978, Bull. Dept. Geogr. Univ. Tokyo).
The modeled FT and (U-Th)/He dates were consistent with the dates observed in some mountains for which uplift rates and onset times of the uplift are well known. In summary, uplift rates need to be greater than ~3 mm/yr to make FT and (U-Th)/He dates in zircon significantly younger than t0. On the other hand, apatite FT and (U-Th)/He dates can be significantly younger than t0 when the uplift is faster than ~0.3 mm/yr depending on t1 and presence/absence of the elevation change, and can be younger than t1 when the uplift is faster than ~1 mm/yr. Although t0 is equivalent to the formation ages of the rocks, the results were generally common for the three values of t0 attempted. Note, however, that t0 younger than 5 Ma can lead to different results; in such a case, cooling after the intrusion of the pluton and cooling by exhumation are promoted simultaneously.
The modeling results were applied to the low-temperature thermochronological database in Japan (Sueoka & Tagami, 2019, Island Arc) to illustrate the distribution of the uplift rate over the few million years. The results indicate that cooling ages younger than t1 are expected in the Japanese Alps, Kanto Mountains, Tanigawa-dake, and Iide Mountains. Ages younger than t0 are expected in the Hida Plateau, Mino Plateau, and Shikoku Mountains, whereas ages are less likely to be reset in the northern Kitakami Mountains, Abukuma Mountains, Rokko Mountains, and Awaji Island. The distribution of the uplift was generally consistent with that for 100 thousand years deduced from marine/fluvial terraces (Fujiwara et al., 2005, Jour. Nucl. Fuel Cycle Envir.), except for some areas, e.g., Muroto Cape, western Ise Bay, and Hokuriku districts. This disagreement might be brought by an accelerated uplift after 0.5 Ma and/or sampling bias related to conditions where terraces are developed and preserved (e.g., Malatesta et al., 2023, AGU abst.).