The processing of the gravity data of the Paris Basin has enlightened the existence of crustal ridges. These ridges correspond to the zones of different crustal thicknesses. In plan view, the ridges are concentric with the Alpine orogen and were probably controlled by this Tertiary Alpine mountain building. The thinned zones related with the Mesozoic and pre-Mesozoic basins, were already existing before the Alpine deformations. These are of limited size and clearly associated with the basins.On a large scale, two different domains have been recognized. The first one corresponds with a zone where the crust is 37 km thick. It is also the place where the Tertiary depocentres developed. The second one relates with a zone where the crust is 29–30 km thick. It has been uplifted during the Alpine orogeny. These two observations suggest the existence of a Tertiary lithospheric buckling beneath the Paris Basin.