Denudation process of crystalline nappes in a continental collision zone constrained by inversion of fission-track data and thermokinematic forward modeling; An Example from Eastern Nepalese Himalaya

フィッション・トラックデータの逆解析とサーモ・キネマティックモデルの順計算による大陸衝突帯における結晶質岩ナップの削剥過程への制約; 東ネパールにおける例

中嶋 徹   ; 河上 哲生*; 岩野 英樹*; 檀原 徹*; 酒井 治孝*

Nakajima, Toru; Kawakami, Tetsuo*; Iwano, Hideki*; Danhara, Toru*; Sakai, Harutaka*

大陸衝突帯における結晶質岩ナップの削剥過程を明らかにするために、東ネパールヒマラヤに分布する高ヒマラヤ変成岩ナップとレッサーヒマラヤナップに熱年代学的手法を適用した。

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.