A north-south section across the Queyras Schistes lustrés (Piedmont zone,Western Alps): Syn-collision refolding of a subduction wedge
Résumé
The 10 km thick pile of Queyras Schistes lustrés has a complex structure, resulting from successive deformation phases (D1 to D5). The tectonic history starts with the building of a syn-subduction wedge through accretion of nappes displaying oceanic and distal European margin series. We relate this early wedging to primitive isoclinal folding (D1) associated with blueschist metamorphism in Paleocene-Eocene times. Variations in the P-T conditions allow for distinguishing four early-formed regional-scale tectonic units. During subsequent collision, the wedge continues growing through new shortening phases (D2 to D4) while metamorphism evolves toward greenschist conditions, mainly during Oligocene times. Orogen-perpendicular F2 and F3 folds have opposite facing directions. They still control most of the present-day multiscale structure, which is best visible in an orogen-parallel section. Orogen-parallel F4 folds produce most of the present-day westward dip of the tectonic pile. Their western equivalents in the nearby Briançonnais zone correspond to the classical back-movements. Extension (D5) prevails from the Miocene onwards, resulting in west-dipping detachment faults. The major structures associated with D4 and D5 are also better visible in an orogen-perpendicular section. Special attention is paid to dolomitic and ophiolitic bodies presently scattered amidst the Schistes lustrés structure. They may be of sedimentary or tectonic origin and are again best visible along the proposed orogen-parallel section.