Denudation history of the Eastern Nepalese Himalaya constrained by
thermochronological methods
Abstract
The thermochronological method was applied to metamorphic rocks
distributed in 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 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 metamorphic rocks
are characterized by 1) gradual cooling (<30°C/Myr) followed
by rapid cooling (~150°C/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 which have been
proposed in the Central and Eastern Himalayas: 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). As a result, only the Flat-Ramp-Flat MHT model 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 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. The
result of the thermokinematic inverse analysis also indicates that the
denudation rate and its spatial distribution have been stable since ca.
9 Ma.