CONTROL ON THRUST TECTONICS IN THE HIMALAYAN FOOTHILLS - A VIEW FROM A NUMERICAL-MODEL

The evolution of an intracontinental accretionary wedge is controlled by: (a) the displacement of several thrust sheets along faults and an underlying decollement; (b) the vertical motion of the rigid basement beneath the accretionary wedge; and (c) superficial processes (erosion/sedimentation). A numerical model is proposed based on the Coulomb wedge theory, a 2.5-D forward kinematic system, progressive tilting of the underthrust basement, and an erosion/sedimentation model in order to characterise the sequence of fault activation in a thrust system and the syn-orogenic sedimentation in such a system. The Siwalik belt, the frontal thrust system of the Himalayas, represents a structure on which the model can be applied. It presents mainly simple structures made of one fault system branched on an underlying decollement. Conglomerate deposits of the Plio-Pleistocene (Upper Siwalik) are overthrust by Miocene formations (Lower Siwalik) at the front of the wedge and along inner faults. Structural maps show that thrust sheets are laterally relayed and simultaneously emplaced. The comparison between the deformation pattern in the Himalayan foothills and the modelling results supports the concept that the Himalayan thrust wedge has a steady-state-type evolution controlled by horizontal convergence and superficial mass transport, and characterised by an irregular spatial and temporal distribution of shortening within the whole wedge. During this evolution, piggy-back basins or strongly displaced internal sheets hide some thrust sheets. The exhumation path of these sheets is then discontinuous, though the shortening of the wedge is regular.