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 we dge theory, a 2.5-D forward kinematic system, progressive tilting of t he underthrust basement, and an erosion/sedimentation model in order t o characterise the sequence of fault activation in a thrust system and the syn-orogenic sedimentation in such a system. The Siwalik belt, th e frontal thrust system of the Himalayas, represents a structure on wh ich the model can be applied. It presents mainly simple structures mad e of one fault system branched on an underlying decollement. Conglomer ate deposits of the Plio-Pleistocene (Upper Siwalik) are overthrust by Miocene formations (Lower Siwalik) at the front of the wedge and alon g inner faults. Structural maps show that thrust sheets are laterally relayed and simultaneously emplaced. The comparison between the deform ation pattern in the Himalayan foothills and the modelling results sup ports the concept that the Himalayan thrust wedge has a steady-state-t ype evolution controlled by horizontal convergence and superficial mas s transport, and characterised by an irregular spatial and temporal di stribution of shortening within the whole wedge. During this evolution , piggy-back basins or strongly displaced internal sheets hide some th rust sheets. The exhumation path of these sheets is then discontinuous , though the shortening of the wedge is regular.