Geological study of the Southern Vanoise Massif units from Pralognan to Aussois (Savoie, France) Structural and sedimentary evolution
A sedimentary and structural study of the Mesozoïc cover around the Paleozoïc culmination of Chasseforêt massif (Southern Vanoise basement) is performed with detailed mapping (1 : 10 000) and analysis of the superimposed deformalions. Stratigraphy - The stratigraphic composition of the sequences demonstrates the existence of three distinct paleogeographic domains, each with different pre-Bathonian erosion level. Viz. from West to East: - the Briançonnais series of Western Vanoise type, deeply eroded as far as the lower Triassic quartzites. - the Briançonnais series of Arc Valley, marked by the strong pre-Bathonian uncomformity and by preservation of the upper Triassic units under this erosion levaI. - the Dent Parrachée series (Grande Motte type), characterized by thick Liassic Iimestone inserted in the typically Briançonnais sequence. Age and distribution of the transgressive Jurassic terms prove that these three paleogeographic domains were adjacent and allow recognition of a tiIted block paleotopography created during the major extensional phase of the middle Lias. Fitted into the regional context of the Vanoise Briançonnais Zone, these facts provide new evidences of an intra-Briançonnais origin for the Grande Motte Unit. Tectonics - Four major tectonic events have been recognised. These are: D1 : thrusting and local replacement of the Briançonnais cover by the Dent Parrachée unit on the Southern Vanoise basement, D2 : a general folding of the basement and of its cover with E-W trending axes and N-vergence, D3 : backthrusting and folding with ESE-vergence, D4 : uplift and NW-vergent thrusting of the Paleozoïc basement with a N-S to NE-SW trending large-scale antiformal fold. After geometric analysis and study of the features of the D1 strain, we suggest a tectonogenetic model for the thrusting and displacement of the Dent Parrachée unit, based on inheritance of Liassic extensional paleostructures. This model explains the "cover substitution" during a compressive and synmetamorphic tectonic event.