Deformation mode switches in the Penninic units of the Urtier Valley (Western Alps): Evidence for a dynamic orogen

Abstract

Multiple deformation mode switches from shortening to extension have been detected in the Penninic units of the Urtier Valley, to the north of the Gran Paradiso massif. An early shortening phase, whose traces are poorly preserved, led to the tectonic burial of rock units to high pressure conditions. Subsequent extensional shearing (SZ gr 1) led to the formation of mylonitic fabrics. During this event the eclogitic Piemonte unit was exhumed to greenschist facies conditions ( P = 0.2–0.3 GPa) in the footwall of the blueschist Piemonte unit. Exotic slices of continental basement and Mesozoic continental shelf carbonates are found along their contact, implying the existence of older thrust stacks dissected by the younger extensional shear zones. The extensional structures were later affected by upright folding (Fu), indicating yet another deformation mode switch, which led to re-burial to P = 0.6–0.8 GPa. Later multi-stage deformation culminated in an episode of extensional shearing with top-to-the-west kinematics (SZ gr 2). Therefore, at least two shortening–extension cycles affected the Piemonte units of the Urtier Valley. Existing geochronological data allow less than 20 Myr for the complex deformation history discussed here. Shortening–extension cycles can be inferred also from published studies from other sections of the Western Alps. However, while the first shortening–extension cycle was widespread in the Penninic units, the lateral extent and relative timing of the second cycle in different areas are still unclear.