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Suzuki, Kota*; Kawakami, Tetsuo*; Sueoka, Shigeru; Yamazaki, Ayu*; Kagami, Saya; Yokoyama, Tatsunori; Tagami, Takahiro*
Island Arc, 31(1), p.e12462_1 - e12462_15, 2022/09
Times Cited Count：1 Percentile：45.26(Geosciences, Multidisciplinary)Nakajima, Toru; Kawakami, Tetsuo*; Iwano, Hideki*; Danhara, Toru*; Sakai, Harutaka*
Journal of Geophysical Research; Solid Earth, 127(5), p.e2021JB023630_1 - e2021JB023630_33, 2022/05
Times Cited Count：1 Percentile：51.01(Geochemistry & Geophysics)The thermochronological methods were applied to the Higher Himalayan Crystalline (HHC) nappe and the underlying Lesser Himalayan sediments (LHS) to elucidate the denudation process of the middle- and upper-crust of eastern Nepal over the geological time scale. The thermochronological inverse analysis was undertaken for new results of fission-track (FT) age and FT length data of zircon and apatite in order to reconstruct the time-temperature (
) paths in the temperature range of 60-350 degree. Eight paths obtained along the across-strike section showed that the cooling process of the HHC nappe was characterized by following three aspects: (1) gradual cooling followed by rapid cooling and subsequent gradual cooling, (2) northward-younging of the timing of the rapid cooling, (3) gradual cooling followed by ca. 2 Myr rapid cooling in the frontmost part of the HHC nappe. The observed FT ages and paths were then compared with those predicted by forward thermokinematic modeling. The results of the thermokinematic modeling for the Flat-Ramp-Flat MHT model, in which the HHC and the underlying LHS are denudated accompanied with the movement of the Main Himalayan Thrust (MHT), well reproduced the observed paths and FT ages in eastern Nepal. This suggests that the observed FT ages and paths reflect a denudation process driven by the movement of the MHT showing the flat-ramp-flat geometry, and that the denudation rate and its spatial distribution have roughly been constant in eastern Nepal since ca. 9 Ma.
Kawakami, Tetsuo*; Sueoka, Shigeru; Yokoyama, Tatsunori; Kagami, Saya; King, G. E.*; Herman, F.*; Tsukamoto, Sumiko*; Tagami, Takahiro*
Island Arc, 30(1), p.e12414_1 - e12414_11, 2021/01
Times Cited Count：3 Percentile：42.47(Geosciences, Multidisciplinary)Kawakami, Tetsuo*; Sueoka, Shigeru; Tagami, Takahiro*
no journal, ,
no abstracts in English
Sueoka, Shigeru; Kawakami, Tetsuo*; Suzuki, Kota*; Yamazaki, Ayu*; Kagami, Saya; Nagata, Mitsuhiro; Yokoyama, Tatsunori; Tagami, Takahiro*
no journal, ,
no abstracts in English
Nakajima, Toru; Kawakami, Tetsuo*; Iwano, Hideki*; Danhara, Toru*; Sakai, Harutaka*
no journal, ,
The thermochronological method was applied to metamorphic rocks distributed to eastern Nepal to elucidate the denudation process of the upper-crust of the continental collision zone. New results of systematic fission-track (FT) age dating of zircon and apatite were utilized in the thermochronological inverse analysis to reconstruct the time-temperature (t-T) paths in the temperature range of 60-350 degree. Eight t-T paths obtained along the across-strike section showed that the cooling process of the upper-crust are characterized by (1) gradual cooling (30 deg./Myr) followed by rapid cooling (150 deg./Myr) and subsequent gradual cooling (gradual-rapid-gradual cooling: GRG cooling), (2) northward-younging of the timing of the rapid cooling since ca. 9 Ma. The observed FT ages and t-T paths were then compared with the FT ages and t-T paths obtained by forward calculations using 3-D thermokinematic models to test the following four tectono-thermal models: (1) The denudation of the upper-crust is associated with the movement of the plate boundary fault (Main Himalayan Thrust: MHT) showing flat geometry (the Flat MHT model) and (2) flat-ramp-flat geometry (the Flat-Ramp-Flat MHT model), (3) the denudation of the upper-crust is mainly controlled by the focused uplift associated with the growth of the Lesser Himalayan Duplex (the Duplex 01-03 model) or (4) slip of the splay fault of the MHT (the Splay Fault model). Only the Flat-Ramp-Flat MHT model could have reconstructed similar t-T paths and age distribution patterns obtained from eastern Nepal. This suggests that the observed FT ages and t-T paths reflect a denudation process driven by the movement of the MHT showing a flat-ramp-flat geometry. The result of the thermokinematic inverse analysis also indicates that the denudation rate and its spatial distribution have been stable since ca. 9 Ma.
Sueoka, Shigeru; Kawakami, Tetsuo*; Suzuki, Kota*; Kagami, Saya; Yokoyama, Tatsunori; Nagata, Mitsuhiro; Yamazaki, Ayu*; Higashino, Fumiko*; King, G. E.*; Tsukamoto, Sumiko*; et al.
no journal, ,
Young plutons of 10-0.8 Ma, including the world's youngest Kurobegawa pluton, are exposed in the Kurobe area of the Hida Range, central Japan, indicating rapid rock uplift and exhumation in the Quaternary. However, reconstructing the exhumation history has been challenging because interpretations of the thermochronologic data are difficult due to the complicated, recent and active thermal history/structure. Here we are determining the exhumation history by estimating formation depths and ages of the young plutons based on Al-in-Hb geobarometry and zircon U-Pb geochronometry, respectively. At JpGU2021, we reported the preliminary results based on three datapoints from the ~5 Ma plutons and one datum from the ~0.8 Ma pluton, suggesting the three following interpretations: (1) formation depths of ~5 Ma and ~0.8 Ma plutons were estimated to be ~6-9 km, which indicates an increase of the exhumation rate after ~0.8 Ma, (2) mean exhumation rate since ~0.8 Ma was computed at 8-10 mm/yr in the Baba-dani area, (3) the formation depths are uniform in the E-W direction, disagreeing with the eastward tilting model. As of January 2022, we have obtained 14 datapoints in total: five from the ~5 Ma plutons, two from the ~2-1 Ma plutons, five from the ~0.8 Ma plutons, and two from the 
65 Ma plutons. These additional data are basically consistent with the previous data, reinforcing the three interpretations above. Recently, new uplift models of the Kurobe area were proposed, in addition to the eastward tilting model. Ito et al. (2021) suggested that the Kurobegawa pluton is a resurgent pluton of the Jiigatake caldera and was uplifted by the resurgence. Kawasaki (2021) proposed that displacements related to the E-W compression are localized along the Kurobe area due to the high geothermal gradient caused by presence of thermal fluid interstratified layers. We are planning to discuss consistency between our data and these two models.
Suzuki, Kota*; Kawakami, Tetsuo*; Sueoka, Shigeru; Yamazaki, Ayu*; Kagami, Saya; Yokoyama, Tatsunori; Tagami, Takahiro*
no journal, ,
Kudo, Shumpei*; Kawakami, Tetsuo*; Nakajima, Toru; Sakai, Harutaka*
no journal, ,
In Himalaya, how high-grade metamorphic rocks, or High Himalayan Crystallines (HHC), exhumed up to the surface has long been studied and various models for the exhumation mechanism have been proposed (e.g., Jamieson et al., 2004). Estimating pressure-temperature-time (P-T-t) paths of the metamorphic rocks is indispensable to discuss the exhumation mechanism. In this study, we performed metamorphic zone mapping using metapelites collected from the north of Dhankuta, eastern Nepal and estimated a P-T path of a garnet-biotite-kyanite gneiss of HHC. Based on the field survey and microscopic observation, we newly defined the kyanite-in isograd in the study area. We also confirmed sillimanite-in and muscovite-out isograds consistent with those reported in Groppo et al. (2009). The studied sample was collected from the north of the muscovite-out isograd where Ms+Qtz is unstable. The garnet is about 3 mm in diameter and consists of the core with abundant inclusions and the rim with a few inclusions. The core encloses kyanite, plagioclase, quartz, rutile, ilmenite and zircon, whereas the rim encloses plagioclase, rutile, ilmenite, zircon and nanogranitoid inclusions. Geothemobarometry and micro-texture indicate that this sample have experienced the P-T evolution starting from the kyanite stability field (ca. 8 kbar, ca. 700 degree: garnet core stage) to the sillimanite stability field (ca. 4-6 kbar, ca. 600 degree: garnet rim stage), and replacement structure of Bt+Ky+Qtz+Pl was formed in between these P-T conditions. The studied path is similar to that of lower HHC reported in Imayama et al. (2012). Therefore, we confirmed that lower HHC rocks in the HHC nappe share similar P-T path with those from the root zone.
Nakajima, Toru; Fukuda, Shoma; Niki, Sota*; Sueoka, Shigeru; Kawakami, Tetsuo*; Danhara, Toru*; Tagami, Takahiro*
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We conducted monazite fission-track (MFT) etching experiments for Quaternary monazite dating. Quite low-temperature annealing of the MFT system (45-25 degree: Jones et al., 2021) has been proposed, and application as an ultra-low-temperature thermochronometer is expected. The practical application of MFT dating gives us access to a geological event at ultra-low temperatures, such as shallow crustal denudation and faulting. Jones et al. (2019) examined the etching conditions and suggested that the etching rate can vary between grains depending mainly on the accumulated radiation damage. Since the monazites used in previous studies were pre-Mesozoic (Weise et al., 2009; Jones et al., 2021), younger monazites with less radiation damage would be expected to have higher etching resistance. In this study, we attempted to etch FT using the Quaternary monazites, which are expected to have less radiation damage, to investigate appropriate etching conditions and discuss a relation between etching rate and radiation damage. Monazites from the Toya Ignimbrite (ca. 0.1 Ma: Niki et al., 2022) and the Kurobegawa Granite (ca. 0.8 Ma: Ito et al., 2013) were used in this study. Euhedral monazite-(Ce) crystals have weak magnetism and can be separated by the conventional magnetic separation method. The Cretaceous monazite from the Kibe Granite (ca. 98 Ma: Skrzypek et al., 2016) was also etched for comparison.
