Denudation history of the Eastern Nepalese Himalaya constrained by thermochronological methods

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

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.