11:00 〜 13:00
[SCG42-P06] Fission-track data from the Mount Everest massif: Sigmoidal cooling profiles since 15 Ma
キーワード：冷却プロファイル、フィッショントラック、エベレスト山塊、熱年代学、チョモランマデタッチメント
This paper presents low-temperature cooling profile of the Higher Himalayan Crystalline (HHC) in the Mount Everest massif since its southward extrusion in the Middle Miocene time. We investigated a NNE-SSW trending area across the Mount Everest, with 80 km distance between the Main Central Thrust (MCT) and the Qomolangma Detachment (QD). Previous thermochronologic data gave an initial condition that is the entire HHC within the Everest massif was hot enough to recrystallize zircon until immediately prior to rapid cooling of the top of the HHC at 15–14 Ma (Iwano et al., 2021). In this study, the downward cooling direction at that time was supposed to be perpendicular to the QD, not along elevation. We used zircon and apatite fission track (FT) age data as 250°C and 120°C isotherm, respectively.
Both newly obtained and previously reported data provided the following two cooling profiles: (1) a 1-D relationship between age and structural distance from the QD as a downward cooling component and (2) a 1-D relationship between age and horizontal distance from the MCT as a lateral cooling one in the NNE-SSW cross section. As a result, both components showed sigmoidal profiles with rapid, slow and then relative fast cooling zones from top to bottom of the HHC, all of which were reasonably supported by fission-track length distribution patterns.
In the downward cooling profile, we can see that zircon FT with higher closure temperatures showed a wider range of the rapid cooling (15-Ma isochron) zone, over 4 km from the QD, compared with apatite FT of the QD surface only. In the slow cooling zone, the downward moving rate for zircon FT (250°C isotherm) is constant during 15-6 Ma with a smaller value of ca. ~0.25 km myr-1 in the middle part of HHC (5-8 km from the QD). As for apatite FT (120°C isotherm), the downward moving rate is also constant being ~0.23km myr-1 during 15-3 Ma in the uppermost part of the HHC (3 km from the QD). The slow and constant downward moving rates of FT isotherms suggests the upper to middle part of HHC had been static without uplift during 15-3 Ma. In the relative fast cooling zones, the downward moving rate for zircon and apatite FT isotherms were changed to be 1.6 and 3.4 km myr-1, respectively. Between the slow and relatively fast cooling zones, there are break-in-slope points of moving isotherms at 6 Ma for zircon FT and 3 Ma for apatite FT. The amounts of increase for downward moving rates after the break-in-slope are calculated to be 1.4 km myr-1 and 3.2 km myr-1 for zircon and apatite FT, respectively. These are additional components of downward moving rates after the break-in-slope point for each isotherm. According to the method described above, we obtained the lateral moving rates in the second zones to be very small or negligible for both zircon and apatite FT isotherms. Therefore, additional components of lateral moving rates after the break-in-slope points are given to be 8.3 km myr-1 and 20 km myr-1 for zircon and apatite FT, respectively. Using the additional components from downward and lateral moving rates after the break-in-slope points, the resultant 2-D moving rates of isotherms in the cross section were estimated to be 8.4 km myr-1 (zircon) and 20 km myr-1 (apatite), indicating a strong and southward-lateral cooling in this study area.
Our scenario is that the high-temperature massif (HHC) rose against the geothermal temperature structure and gradually cooled from the top perpendicular to the QD. This implies a situation where the HHC had thrusted rapidly along the MCT and exposed on the Earth's surface just before 15 Ma. The cooling from the highlands to the south may have been caused by topographic change, e.g., folding of the HHC and subsequent sliding of the Tethys deposits overlying the HHC.
Reference
Iwano et al. (2021) Chemical Geology, https://doi.org/10.1016/j.chemgeo.2020.119903
Both newly obtained and previously reported data provided the following two cooling profiles: (1) a 1-D relationship between age and structural distance from the QD as a downward cooling component and (2) a 1-D relationship between age and horizontal distance from the MCT as a lateral cooling one in the NNE-SSW cross section. As a result, both components showed sigmoidal profiles with rapid, slow and then relative fast cooling zones from top to bottom of the HHC, all of which were reasonably supported by fission-track length distribution patterns.
In the downward cooling profile, we can see that zircon FT with higher closure temperatures showed a wider range of the rapid cooling (15-Ma isochron) zone, over 4 km from the QD, compared with apatite FT of the QD surface only. In the slow cooling zone, the downward moving rate for zircon FT (250°C isotherm) is constant during 15-6 Ma with a smaller value of ca. ~0.25 km myr-1 in the middle part of HHC (5-8 km from the QD). As for apatite FT (120°C isotherm), the downward moving rate is also constant being ~0.23km myr-1 during 15-3 Ma in the uppermost part of the HHC (3 km from the QD). The slow and constant downward moving rates of FT isotherms suggests the upper to middle part of HHC had been static without uplift during 15-3 Ma. In the relative fast cooling zones, the downward moving rate for zircon and apatite FT isotherms were changed to be 1.6 and 3.4 km myr-1, respectively. Between the slow and relatively fast cooling zones, there are break-in-slope points of moving isotherms at 6 Ma for zircon FT and 3 Ma for apatite FT. The amounts of increase for downward moving rates after the break-in-slope are calculated to be 1.4 km myr-1 and 3.2 km myr-1 for zircon and apatite FT, respectively. These are additional components of downward moving rates after the break-in-slope point for each isotherm. According to the method described above, we obtained the lateral moving rates in the second zones to be very small or negligible for both zircon and apatite FT isotherms. Therefore, additional components of lateral moving rates after the break-in-slope points are given to be 8.3 km myr-1 and 20 km myr-1 for zircon and apatite FT, respectively. Using the additional components from downward and lateral moving rates after the break-in-slope points, the resultant 2-D moving rates of isotherms in the cross section were estimated to be 8.4 km myr-1 (zircon) and 20 km myr-1 (apatite), indicating a strong and southward-lateral cooling in this study area.
Our scenario is that the high-temperature massif (HHC) rose against the geothermal temperature structure and gradually cooled from the top perpendicular to the QD. This implies a situation where the HHC had thrusted rapidly along the MCT and exposed on the Earth's surface just before 15 Ma. The cooling from the highlands to the south may have been caused by topographic change, e.g., folding of the HHC and subsequent sliding of the Tethys deposits overlying the HHC.
Reference
Iwano et al. (2021) Chemical Geology, https://doi.org/10.1016/j.chemgeo.2020.119903