3 research outputs found
Exhumation, crustal deformation, and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography
Two endâmember kinematic models of crustal shortening across the Himalaya are
currently debated: one assumes localized thrusting along a single major thrust fault, the
Main Himalayan Thrust (MHT) with nonuniform underplating due to duplexing, and the
other advocates for outâofâsequence (OOS) thrusting in addition to thrusting along the
MHT and underplating. We assess these two models based on the modeling of
thermochronological, thermometric, and thermobarometric data from the central Nepal
Himalaya. We complement a data set compiled from the literature with 114 ^(40)Ar/^(39)Ar,
10 apatite fission track, and 5 zircon (UâTh)/He thermochronological data. The data are
predicted using a thermokinematic model (PECUBE), and the model parameters are
constrained using an inverse approach based on the Neighborhood Algorithm. The model
parameters include geometric characteristics as well as overthrusting rates, radiogenic heat
production in the High Himalayan Crystalline (HHC) sequence, the age of initiation of
the duplex or of out-of-sequence thrusting. Both models can provide a satisfactory fit to the
inverted data. However, the model with out-of-sequence thrusting implies an unrealistic
convergence rate â„30 mm yr^(â1). The out-of-sequence thrust model can be adjusted to fit the
convergence rate and the thermochronological data if the Main Central Thrust zone is
assigned a constant geometry and a dip angle of about 30° and a slip rate of <1 mm yr^(â1). In
the duplex model, the 20 mm yr^(â1) convergence rate is partitioned between an overthrusting
rate of 5.8 ± 1.4 mm yr^(â1) and an underthrusting rate of 14.2 ± 1.8 mm yr^(â1). Modern rock
uplift rates are estimated to increase from about 0.9 ± 0.31 mm yr^(â1) in the Lesser Himalaya to
3.0 ± 0.9 mm yr^(â1) at the front of the high range, 86 ± 13 km from the Main Frontal Thrust.
The effective friction coefficient is estimated to be 0.07 or smaller, and the radiogenic
heat production of HHC units is estimated to be 2.2 ± 0.1 ”Wm^(â3). The midcrustal
duplex initiated at 9.8 ± 1.7 Ma, leading to an increase of uplift rate at front of the High
Himalaya from 0.9 ± 0.31 to 3.05 ± 0.9 mm yr^(â1). We also run 3-D models by coupling
PECUBE with a landscape evolution model (CASCADE). This modeling shows that the
effect of the evolving topography can explain a fraction of the scatter observed in the data but
not all of it, suggesting that lateral variations of the kinematics of crustal deformation and
exhumation are likely. It has been argued that the steep physiographic transition at the foot of
the Greater Himalayan Sequence indicates OOS thrusting, but our results demonstrate
that the best fit duplex model derived from the thermochronological and thermobarometric
data reproduces the present morphology of the Nepal Himalaya equally well