The Siwaliks of western Nepal II. Mechanics of the thrust wedge J.L. Mugnier a, *, P. Leturmy a , P. Huyghe a , E. Chalaron b a Laboratoire de Geodynamique des Chaines Alpines et UPRESA CNRS 5025, rue Maurice Gignoux, 38031, Grenoble, France b Institute of Geological and Nuclear Sciences, New Zealand 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 diusion 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 deÂcollement 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 aects 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-o-sequence thrusting in the frontal thrust belt of western Nepal, whereas numerical results suggests that episodic out-o sequence reactivation could be a general characteristic of the Himalayan wedge evolution often hidden by erosion. # 1999 Elsevier Science Ltd. All rights reserved. 1. Introduction Recent progress in the study of orogenic thrust belts has arisen in: (a) mechanics, from the development of the critical taper model (Davis et al., 1983; Dahlen and Suppe, 1984); (b) kinematics, from the use of balancing procedures (Dahlstrom, 1969) and forward kinematic models (Endignoux and Mugnier, 1990); and (c) syn- tectonic basin development, from erosion and sedimen- tation modelling (Flemings and Jordan, 1989). A com- puter based approach, which incorporates these three aspects is presented and applied to the Neogene Siwalik Group of western Nepal. Comparison of its predictions with structural ®eld data presented in a companion paper (Mugnier et al., 1999) helps to improve the understanding of the kinematic history of thrusting and the relationships between development of the intramountain basins (duns) and the thrust sys- tem geometry. Lesser Himalayan tectonics is also dis- cussed in the light of the implications of the wedge model. 2. Mechanics of the thrust wedge growth 2.1. Coulomb-wedge theory The mechanics of fold and thrust belts has been Journal of Asian Earth Sciences 17 (1999) 643±657 1367-9120/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S1367-9120(99)00039-5 * Corresponding author. E-mail address: mugnier@ujf-grenoble.fr (J.L. Mugnier).