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 fr om lateral density variations. These lateral density variations are du e to the lateral variations in the thicknesses of the crust and of the lithosphere. Thermal processes tend to restore the mechanically thick ened or thinned lithosphere to a 'normal' thickness. The lithosphere m antle and the asthenosphere are assumed to have different densities. I n the particular example studied using this model, buoyancy forces ari sing from deformation-induced lateral density variations augment, rath er than oppose, the shortening and thickening of the shortened and thi ckened lithosphere, which means that the growth of an orogenic belt is not necessarily limited by its high topography, although it is not cl ear whether this is the exception or the rule. The results also suppor t the prevailing notion that the collapse of an orogenic belt is proba bly caused by mechanical instability of the lower part of the lithosph ere mantle (e.g. delamination of part of the thickened lithosphere man tle, or convective thinning), rather than the slow thermal conduction relaxation process.