Geometric Evolution of a Plate Interface-Branch Fault System: Its Effect on Tectonics in Himalaya

The collision between India and Eurasia is still going on in Himalaya. The present convergence rate between the Indian and the Eurasian plates has been estimated as 50 mm/yr. About 40 % of the total convergence rate is consumed at the collision boundary along the Himalayas by the subduction of the Indian plate beneath the Eurasian plate. The plate boundary fault system in Himalaya is characterized by a large-scale ramp beneath the high Himalayas and a series of thrust-type branch faults. The long-term duration of steady slip along the fault system has caused crustal deformation in Himalaya. The essential point is that the geometry of the fault system changes with time because of the internal deformation caused by the slip of the fault system itself. Therefore, it is necessary to reveal this feedback mechanism to understand the topographic evolution process of the Himalayas. In this study, the interaction between the Indian and the Eurasian plates is represented by steady slip along an interface that divides a stratified elastic-viscoelastic half-space into two blocks. The geometric evolution of the fault system is simulated by considering the changes in fault geometry with time caused by internal deformation. Through numerical simulations we revealed the fundamental properties of geometric evolution of faults. When the plate interface has a ramp, the ramp moves horizontally toward the hanging wall side at a half of the plate convergence rate. The offset of the ramp decreases with time. When the plate interface has an thrust-type branch fault, we can find the accelerative increase in dip-angle of the branch fault. Since the branch fault with a steeper dip-angle is harder to consume the horizontal convergence, we may conclude that the increase in dip-angle results in the cessation of slip along the branch fault at last. The shallower the depth of the branching point is, the larger the increase rate of the dip-angle is. It means that the branch fault with a shallow branching point can not produce the large-scale mountain range, because large amount of slip can not be accommodated by the branch fault.Incorporating these simulation results into geological observations, we propose a scenario on the tectonic evolution of the Himalayas in the last 30 Myr.