Notion of dynamics
	Gravitational potential energy and gravitational force
	The case of crustal thinning: • Gravitational force (Fg) due to instantaneous homogeneous stretching (at time to) and following thermal relaxation (at time to+200Ma). The yellow shaded areas represent the gravitational force acting on the lithosphere. Fg is given by the integratation with depth of the difference in lithostatic pressure (σzz) between the thinned and the undeformed lithosphere. • Following homogeneous thinning by a factor of 2 (50% reduction of the thickness of the crust and that the whole lithosphere), Fg is rather small (<1012 Nm-1). • As thermal relaxation and cooling proceeds, the thickness of the lithospheric mantle increases. This leads to a decrease in the extensional gravitational force acting on the lower part of the lithosphere and an increase in the compressional gravitational force acting in its upper part. The integration of Fg with depth gives a value > 1012 Nm-1. For contraction structures to develop Fg must be larger that the integrated strength of the thinned lithosphere.