First paleomagnetic data from the sedimentary cover of the French Penninic Alps: evidence for Tertiary counterclockwise rotations in the Western Alps

We present a paleomagnetic study performed in the Briançonnais sedimentary cover of the western Alpine Arc (France). Sampling was focused on Upper Jurassic rocks of the Briançonnais cover in the Briançon–Guillestre area. More than 100 samples from 11 sites were collected essentially from Ammonitico rosso limestones and sedimentary dykes of the Upper Jurassic. Thermal and alternating field demagnetizations revealed three components of magnetization carried dominantly by magnetite: (1) a low-temperature component with present-day magnetic field direction of viscous origin; (2) a well defined reverse-polarity component with a maximum unblocking temperature of 440°C (A component); and (3) a weak high-temperature component (B component) that experienced a viscous overprint during heating and was difficult to isolate. The A component exhibits a negative fold test and shows a better clustering when corrected for tilting associated with late Alpine extension. This component is interpreted as a secondary remagnetization acquired during the cooling path related to Late Eocene–Early Oligocene metamorphism. The mean A component direction, corrected for late Alpine extensional tilting, is D=142°, I=−57°, k=44, α95=8°. This direction significantly differs by 47° ± 13° from the expected direction for Eurasia. We interpret this difference to be related to a post-Eocene counterclockwise rotation about a vertical axis of the Penninic zone relative to stable Europe.