The Siwaliks of western Nepal: II. Mechanics of the thrust wedge

Abstract

Comparison between numerical models and structural data is used for a better understanding of the evolution of the Siwalik thrust belt of western Nepal. The numerical model involves discontinuities within a critical wedge model, a kinematic forward model of serial cross sections, and a linear diffusion algorithm to simulate erosion and sedimentation. In western Nepal, large Piggy-back basins (Duns) are located above thick thrust sheets that involve more than 5500 m of the Neogene Siwalik Group, whereas Piggy-back basin sedimentation is less developed above thinner thrust sheets (4300 m thick). Numerical model results suggest that thrust sheet thickness and extension of wedge-top basins are both related to an increase of the basal décollement dip beneath the duns. The West Dang Transfer zone (WDTZ) is a N–NE trending tectonic lineament that limits the westward extent of the large Piggy-back basins of mid-western Nepal and is linked to a thickening of the Himalayan wedge eastward. The WDTZ also affects the seismotectonics pattern, the geometry of the thrust front, the lateral extent of Lesser Himalayan thrust sheets, and the subsidence of the foreland basin during middle Siwalik sedimentation. Numerical models suggest that the individualisation of the Piggy-back basins at the transition between the middle Siwalik and upper Siwaliks followed the deposition of the middle Siwaliks that induced a geometry of the foreland basin close to the critical taper. As WDTZ induces an E–W thickning of the Himalayan wedge, it could also induce a northward shift of the leading edge of the ductile deformation above the basal detachment in Greater Himalayas of far-western Nepal. Field data locally suggest episodic out-off-sequence thrusting in the frontal thrust belt of western Nepal, whereas numerical results suggests that episodic out-off sequence reactivation could be a general characteristic of the Himalayan wedge evolution often hidden by erosion.