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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：22.72(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.
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.
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.
Iwano, Hideki*; Sakai, Harutaka*; Danhara, Toru*; Nakajima, Toru; Hirata, Takafumi*
no journal, ,
This paper presents low-temperature cooling profile of the Higher Himalayan Crystalline (HHC) in the Mount Everest region since its southward extrusion in the Middle Miocene time. We investigated an NNE-SSW cross section, which runs near the Mount Everest, with 80 km distance between the Main Central Thrust (MCT) and the Qomolangma Detachment (QD). Zircon and apatite fission track (FT) data provided the following two cooling profiles: a 1-D relationship between age and structural distance from the QD and a 1-D relationship between age and horizontal distance from the MCT along the QD. The resulting sigmoidal cooling curves showed (1) rapid, (2) slow and then (3) relative fast cooling zones from top to bottom of the HHC, all of which were reasonably supported by fission-track length distribution patterns.
Nakajima, Toru; Kawakami, Tetsuo*; Iwano, Hideki*; Danhara, Toru*; Sakai, Harutaka*
no journal, ,
Understanding the processes of continental crustal denudation in a collision zone has advanced significantly with the development of thermochronological methods. Currently, the most common technique for reconstructing denudation history is to investigate the distribution pattern of a cooling age on elevations or transects. However, this technique is considered to be insufficient mainly in terms of temporal resolution in a collision zone with spatiotemporal changes in the denudation rate. In this study, we attempted to improve the temporal resolution of the reconstructed thermal history of Himalaya by performing the thermochronological inversion, and to quantitatively reconstruct the spatiotemporal variation of the denudation rate by thermokinematic modeling. Thermochronological inversion 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 (t-T) paths in eastern Nepal. Eight t-T paths calculated along the across-strike section show that the cooling process of the upper crust in this study area is characterized by: 1) gradual cooling followed by rapid cooling and subsequent gradual cooling, 2) northward-younging of the timing of the rapid cooling. The observed FT ages and t-T paths were then compared with those predicted by forwarding thermokinematic modeling. The result of the thermokinematic modeling indicates that:1) the cooling pattern observed in the field mainly occurred when the upper crust overthrusted accompanied with the activity of the plate boundary fault showing the flat-ramp-flat geometry, 2) and its geometry has been stable since the underplating of the Indian continental plate before 9 Ma.
Nakajima, Toru; Niki, Sota*; Kudo, Shumpei*; Kawakami, Tetsuo*; Higashino, Fumiko*; Hirata, Takafumi*; Sakai, Harutaka*
no journal, ,
no abstracts in English
