Local isostasy

The post-orogenic phase of most mountain belts is characterised by a gradual erosion of the topography created during the active tectonic phase. This erosion results in unloading of the underlying lithosphere and consequent isostatic adjustment. The principle of isostasy assumes that there is a region beneath the lithosphere (the compensation depth) where rocks are so weak that they cannot sustain any horizontal stress gradient over geological times, and hence the region adjusts to imposed loads by deformation. This implies that, at isostatic equilibrium, the weight of adjacent lithospheric columns must be equal. Erosion of surface topography results in a local reduction of the weight of the underlying lithospheric column and must therefore be compensated by vertical uplift.

As shown in Figure 11.1, surface erosion by an amount E0 (panel (b)) of a reference lithospheric column (panel (a)) of crustal thickness hc and total thickness hc + L results in an isostatic surface uplift by an amount u (panel (c)) such that the weights of the two columns ((a) and (c)) down to the compensation depth hc + L are identical.