Thermo-Mechanical Models for Coupled Lithosphere-Surface Processes: Applications to Continental Convergence and Mountain Building Processes

Abstract

Simple mechanical considerations show that many tectonic-scale surface constructions, such as mountain ranges that exceed certain critical height (about 3 km in altitude, depending on rheology and width) should flatten and collapse within few My as a result of gravitational spreading that may be enhanced by flow in the ductile part of the crust. The elevated topography is also attacked by surface erosion that, in case of static topography, would lead to its exponential decay on a time scale of less than 2.5 My. However, in nature, mountains or rift flanks grow and stay as localized tectonic features over geologically important periods of time (>10 My). To explain the long-term persistence and localized growth of, in particular, mountain belts, a number of workers have emphasized the importance of dynamic feedbacks between surface processes and tectonic evolution. Surface processes modify 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 realised via 2 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), and (3) fast convergence and erosion rates (Himalaya, Central Asia).