The Salvan-Dorénaz intramontane basin formed between ca. 308-293Ma as an asymmetric graben along crustal-scale transtensional fracture zones within the Aiguilles-Rouges crystalline massif (Western Alps) and represents a feature of the post-collisional evolution of the Variscan orogens. It contains 1.5-1.7km of continental clastic deposits which were eroded from granitic, volcanic, and metamorphic rocks. Textural and compositional immaturity of the sandstones, and the numerous lithic fragments with low chemical and physical stability suggest only short-range transport. 40Ar/39Ar analyses of detrital muscovite are interpreted to represent cooling of the crystalline basement below the respective closure temperatures. Ages from detrital muscovite range between ca. 280-330Ma. 40Ar/39Ar white mica plateau ages from granitic boulders range between 301-312Ma and suggest rapid cooling. The very short time interval recorded between the 40Ar/39Ar cooling ages and the stratigraphic age of the host sediment suggests that considerable portions of the upper crust were removed prior to the formation of the basin. Late Variscan granitic boulders document surface exposure and erosion of Late Carboniferous granites during early stages of the infilling of the basin. Therefore, unroofing of basement units, magmatic activity, and formation of the fault bounded Salvan-Dorénaz basin were acting concomitantly, and are highly suggestive of extensional tectonics. When compared with other orogens, this situation seems specific to the Variscan, especially the exclusively young ages of detrital material, however, modern analogous may exis
