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

We present a paleomagnetic study performed in the Brianconnais sedimentary cover of the western Alpine Arc (France). Sampling was focused on Upper Jur assic rocks of the Brianconnais cover in the Briancon-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 a lternating field demagnetizations revealed three components of magnetizatio n carried dominantly by magnetite: (1) a low-temperature component with pre sent-day magnetic field direction of viscous origin; (2) a well defined rev erse-polarity component with a maximum unblocking temperature of 440 degree s C (A component); and (3) a weak high-temperature component (B component) that experienced a Viscous overprint during heating and was difficult to is olate. The A component exhibits a negative fold test and shows a better clu stering when corrected for tilting associated with late Alpine extension. T his 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 degrees, I = -57 degrees, k = 44, alpha(95) = 8 degrees. This di rection significantly differs by 47 degrees +/- 13 degrees from the expecte d direction for Eurasia. We interpret this difference to be related to a po st-Eocene counterclockwise rotation about a vertical axis of the Penninic z one relative to stable Europe. (C) 1999 Elsevier Science B.V. All rights re served.