COMPARISON OF THE BASEMENT-COVER RELATION SHIPS BETWEEN INTERNAL AND EXTERNAL ZONES IN THE CENTRAL ALPS

The comparison of the geometry of ductile deformations between basemen t and cover in the internal and external zones of the Alps highlights several differences. During the Tertiary collision, the original compe tence contrasts and the anisotropy between basement and cover produced heterogeneous deformation in the basements and a more homogeneous def ormation in the covers. In the external as in the internal zones, the measured patterns of shear zones in the basements are compatible with only one major Alpine deformation. The basement of the external crysta lline massifs is characterized by both sub-vertical schistosity and st retching lineations. These lineo-planar structures become progressivel y horizontal near the contact with the cover. In the Penninic zone, th e Alpine schistosity in the basement and in the cover is sub-horizonta l and associated with a SSE-NNW stretching lineation. At the scale of the Alps, the Penninic basement constitutes several thin nappes. The c overs are pinched between these crystalline nappes and art interpreted as large shear zones. The external zone is characterized by a lower i ntensity of strain but a higher deformation gradient at the basement-c over contact. In contrast in the Penninic zone, the strain intensity i s everywhere higher but the deformation gradient at the basement-cover contact is lower than in the external parts. The initial thickness of the continental crust, - thinned continental crust in the Penninic zo ne and normal thickness of the crust in the external massif - before t he Tertiary collision, could be partly responsible for these differenc es.