Inversion of pre-orogenic extensional basins in the external Western Alps: Structure, microstructures and restoration

During collision, continental margins are shortened along with the inversion inherited syn-rift basins. In this contribution, we explore the internal deformation of pre-orogenic basins during their inversion in the external Western Alps. New structural and microstructural data allow us to provide a new kinematic scenario for the shortening of two inherited basins in the Oisans External Crystalline Massif. Two cleavages and three vein sets are documented by field observations in the sedimentary cover metamorphosed in the greenschist facies. Their spatial and temporal development is strongly dependent on the structural style that is controlled by the initial basin geometry. In the basement, shear zones accommodate the shortening while the cover is disharmonically folded. We present a new approach for the restoration of cross-sections where ductile deformation prevailed, thus for which classical methods do not apply. Both length conservation for basement top and area conservation for the sedimentary cover, as well as fixed hinge kinematics for the cover folds, are assumed. Such restoration is discussed in the light of cover strain ellipsoid quantification based on field data analysis. We apply this approach to the Morcles nappe and propose a new restored geometry, which is further compared to the Oisans section.