• The figures on the right show the gravitational force (Fg) due to instantaneous homogeneous thickening (at time to), and following convective thinning (to+30 Myr). Convective thinning is the gravitational process upon which the cold and dense lithospheric keel is dragged into the convective mantle. Following homogeneous thickening, the excess in gravitational potential energy in the upper part of the thickened lithosphere is more or less balanced by a deficit in gravitational potential energy in the deeper part of the lithosphere. Indeed the integration of σ_zz with depth gives a very small graviational force (Fg = 0.77 10¹² N.m^-1).

• Following convective thinning (the cold and heavy lithospheric keel has been removed), the deformed lithosphere has a larger excess in gravitational potential energy. This excess in GPE gives a gravitational force Fg = 6.7 10¹² N.m^-1. Such an Fg could easily balance or overcome tectonic forces, and in some circumstances could be sufficient to drive extensional collapse of the mountain belt.