11:15 〜 11:45
[SCG47-02] Denudation process of high-grade metamorphic nappe in eastern Nepal constrained by thermochronological inverse analysis
★Invited Papers
キーワード：大陸衝突帯、ヒマラヤ、熱年代学、フィッション・トラック、削剥
The Himalayan ranges are the most typical example of orogenic belt built by continent-continent collision, and a thermochronological method has been widely applied to the high-grade metamorphic rocks distributed in the Himalayas to understand the denudation history of the high-grade metamorphic rocks in collision setting over the geologic time-scale (106–107 yr). However, the detailed cooling history of the high-grade metamorphic rocks is still unknown, and there is little consensus on a tectonic process which primarily drove the denudation. This is because the thermochronological studies have been focused on the distribution of the cooling age along an elevation profile or an across-strike section of the Himalayas, and another approach is required to reconstruct the cooling history with high temporal-resolution.
In this study, the thermochronological inverse analysis was applied to the Higher Himalayan Crystalline (HHC) nappe and the underlying Lesser Himalayan sediments (LHS) distributed in eastern Nepal to elucidate the denudation process of the high-grade metamorphic rocks in collision setting over the geologic time-scale. New results of systematic fission-track (FT) age dating and FT length measurements 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°C. Eight t-T paths obtained along the across-strike section in eastern Nepal showed that the cooling process of the HHC nappe is characterized by following three aspects: 1) gradual cooling (<30℃/Myr) followed by rapid cooling (~150℃/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, 3) gradual cooling followed by < 2 Myr rapid cooling in the frontmost part.
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. In this study, following four thermokinematic models were designed to test the tectono-thermal models which have been proposed in the Central and Eastern Himalayas and to constrain the primary factor that drove the denudation in eastern Nepal: 1) The Flat MHT model in which the HHC nappe and the underlying LHS are denudated accompanied with the movement of the plate boundary fault (Main Himalayan Thrust: MHT) showing flat geometry, 2) the Flat-Ramp-Flat MHT model in which the HHC nappe and the underlying LHS are denudated in the same process as the former one, whereas the MHT shows the flat-ramp-flat geometry, 3) the Duplex 01-03 model in which the denudation of the HHC nappe and the underlying LHS are mainly controlled by the focused uplift associated with the growth of the Lesser Himalayan Duplex (the Duplex 01-03 model), and 4) the Splay Fault model in which the movement of the splay fault of the MHT mainly controls the enhanced denudation in the high-mountain range. Only the Flat-Ramp-Flat model could have reconstructed similar t-T paths and age distribution patterns which were obtained from inverse analyses. This suggests that the observed FT ages and t-T paths from the HHC nappe reflect a denudation process driven by the movement of the MHT showing a flat-ramp-flat geometry. The GRG cooling and the northward-younging of the timing of rapid cooling indicate that the flat-ramp-flat geometry of the MHT was established by ca. 9 Ma and has been stable thereafter. It is assumed that the denudation rate and its spatial distribution pattern have roughly been constant in eastern Nepal since ca. 9 Ma.
In this study, the thermochronological inverse analysis was applied to the Higher Himalayan Crystalline (HHC) nappe and the underlying Lesser Himalayan sediments (LHS) distributed in eastern Nepal to elucidate the denudation process of the high-grade metamorphic rocks in collision setting over the geologic time-scale. New results of systematic fission-track (FT) age dating and FT length measurements 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°C. Eight t-T paths obtained along the across-strike section in eastern Nepal showed that the cooling process of the HHC nappe is characterized by following three aspects: 1) gradual cooling (<30℃/Myr) followed by rapid cooling (~150℃/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, 3) gradual cooling followed by < 2 Myr rapid cooling in the frontmost part.
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. In this study, following four thermokinematic models were designed to test the tectono-thermal models which have been proposed in the Central and Eastern Himalayas and to constrain the primary factor that drove the denudation in eastern Nepal: 1) The Flat MHT model in which the HHC nappe and the underlying LHS are denudated accompanied with the movement of the plate boundary fault (Main Himalayan Thrust: MHT) showing flat geometry, 2) the Flat-Ramp-Flat MHT model in which the HHC nappe and the underlying LHS are denudated in the same process as the former one, whereas the MHT shows the flat-ramp-flat geometry, 3) the Duplex 01-03 model in which the denudation of the HHC nappe and the underlying LHS are mainly controlled by the focused uplift associated with the growth of the Lesser Himalayan Duplex (the Duplex 01-03 model), and 4) the Splay Fault model in which the movement of the splay fault of the MHT mainly controls the enhanced denudation in the high-mountain range. Only the Flat-Ramp-Flat model could have reconstructed similar t-T paths and age distribution patterns which were obtained from inverse analyses. This suggests that the observed FT ages and t-T paths from the HHC nappe reflect a denudation process driven by the movement of the MHT showing a flat-ramp-flat geometry. The GRG cooling and the northward-younging of the timing of rapid cooling indicate that the flat-ramp-flat geometry of the MHT was established by ca. 9 Ma and has been stable thereafter. It is assumed that the denudation rate and its spatial distribution pattern have roughly been constant in eastern Nepal since ca. 9 Ma.