Coupled Lithosphere-Surface Processes in Collision Context

Abstract

From the mechanical point of view, a mountain range that exceeds a certain critical height (of about 3 km in altitude, depending on rheology and width) should flatten and collapse within few My as a result of gravitational spreading of its ductile crustal root. Even if the crustal root does not collapse, the mountain range would be levelled by gravity sliding and other surface processes that, in case of static topography, lead to its exponential decay with a characteristic time constant on the order of 2.5 My. However, in nature, mountains grow and stay as localized tectonic features over geologically important periods of time (> 10 My). To explain the paradox of long-term persistence and localized growth of the mountain belts, a number of workers have emphasized the importance of dynamic feedbacks between surface processes and tectonic evolution. Indeed, surface processes modify the topography and redistribute tectonically significant volumes of sedimentary material, which acts as vertical loading over large horizontal distances. This results in dynamic loading and unloading of the underlying crust and mantle lithosphere, whereas topographic contrasts are required to set up erosion and sedimentation processes. Tectonics therefore could be a forcing factor of surface processes and vice versa. One can suggest that the feedbacks between tectonic and surface processes are realized via two interdependent mechanisms: 1. Slope, curvature and height dependence of the erosion/ deposition rates 2. Surface load-dependent subsurface processes such as isostatic rebound and lateral ductile flow in the lower or intermediate crustal channel. Loading/unloading of the surface due to surface processes results in lateral pressure gradients, that, together with low viscosity of the ductile crust, may permit rapid relocation of the matter both in horizontal and vertical direction (upward/ downward flow in the ductile crust). In this paper, we overview a number of coupled models of surface and tectonic processes, with a particular focus on 3 representative cases: 1. Slow convergence and erosion rates (Western Alpes) 2. Intermediate rates (Tien Shan, Central Asia) 3. Fast convergence and erosion rates rates (Himalaya, Central Asia).