GRAVITY EVIDENCE FOR AN ALPINE BUCKLING OF THE CRUST BENEATH THE PARIS BASIN

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 concent ric with the Alpine orogen and were probably controlled by this Tertia ry Alpine mountain building. The thinned zones related with the Mesozo ic and pre/Mesozoic basins, were already existing before the Alpine de formations. 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 sec ond one relates with a zone where the crust is 29-30 km thick. It has been uplifted during the Alpine orogeny. These two observations sugges t the existence of a Tertiary lithospheric buckling beneath the Paris Basin.