ON THE GROWTH AND COLLAPSE OF WIDE OROGENIC BELTS

A numerical model is designed to investigate the build-up and collapse of wide orogenic belts. The lithosphere is treated as a thin viscous layer subjected to boundary forces and also buoyancy forces arising from lateral density variations. These lateral density variations are due to the lateral variations in the thicknesses of the crust and of the lithosphere. Thermal processes tend to restore the mechanically thickened or thinned lithosphere to a 'normal' thickness. The lithosphere mantle and the asthenosphere are assumed to have different densities. In the particular example studied using this model, buoyancy forces arising from deformation-induced lateral density variations augment, rather than oppose, the shortening and thickening of the shortened and thickened lithosphere, which means that the growth of an orogenic belt is not necessarily limited by its high topography, although it is not clear whether this is the exception or the rule. The results also support the prevailing notion that the collapse of an orogenic belt is probably caused by mechanical instability of the lower part of the lithosphere mantle (e.g. delamination of part of the thickened lithosphere mantle, or convective thinning), rather than the slow thermal conduction relaxation process.