EROSION AND SEDIMENTATION IN THE VICINITY OF A RAMP ANTICLINE - A CONTRIBUTION OF A NUMERICAL-MODEL

We developed a numerical model to study tectonic-erosion coupling. Alg orithms that describe the processes (thrusting, lithospheric tilting, superficial processes) are applied step by step on a regular grid, and allow us to predict a topographic evolution for an active thrusting w edge. With a simple tectonic system constituted with only one sheet, w e can test the influence of time parametres on the relief associated w ith the frontal ramp and the influence of the size of cells. We found that the high of the anticline and the volume of eroded sediments is a function of the ratio horizontal velocity / diffusion coefficient. Th erefore the influence of tectonic velocity is a fundamental factor, in sedimentary transfer studies. We also found that a modelled anticline structure with parametres values (Cl, V/Ks) is a similarity of a stru cture modelled with values (Cl/r, r.V/Ks), with Cl the size of a cell in the grid and r the ratio between the two structures. We performed a sedimentary analysis, by considering that grain coarseness deposit is a function of slope deposit. With this analysis, we found that syntec tonic sediments are removed and that the onlaps formed in the piggy-ba ck basin are eroded during the development of the structure. This kind of erosion could affect the Himalayan frontal thrust system, which ma kes the beginning of the frontal anticline formation became difficult to dare. For Mohand anticline in front of the Dehra Dun piggy-back bas in, we propose that its formation has begun about 1 Ma B.P., and has b een achieved with a shortening velocity near 5 mm/yr, and a transport coefficient between 50 and 150 m(2)/yr. We also recognize a cyclicity in sedimentary bodies geometry controlled by tectonic activity, which suggests that the topography of a growing ramp anticline does not reac h a steady-state geometry.