Deformation of a thrust complex in the Col de la Vanoise, Vanoise Massif, French Alps

A structural analysis of the area surrounding the Col de la Vanoise in the Vanoise Massif of the French Alps demonstrates the importance of two major thrusting events in this part of the Alpine orogenic belt. This study emphasizes the three-dimensional complexity of structure on the kilometre scale which characterizes the Vanoise Massif. Four generations of structures can be distinguished on the basis of systematic overprinting criteria on the mesoscale. Tracing of these structures across the well-developed lithostratigraphy allows correlation with kilometre-scale structures, produced during each of these folding events. The youngest structures can be explained by horizontal shortening (D₄) which produced upright folds and kinks. On the limb of a major D₄ dome in the northwestern part of the area, originally shallow-dipping structures are thrown into steep attitudes. D₄ may be related to either large-scale imbrication associated with the second thrust event or to an event in which considerable differential uplift took place. The second thrusting (D₃) took place under greenschistfacies conditions and culminated in movement eastward along two major gently dipping shear zones. The zone of relatively low D₃ strain between both horizons allows distinction of a pre-D₃ deformation associated with tight upright folds (D₂). This generation of structures overprinted recumbent folds and shear zones which represent the earliest recognizable deformation (D₁). The D₁ event culminated in formation of fold nappes and thrust slices and can be inferred to have involved a major component of northward translation.