Counterclockwise rotation of the western Alps since the Oligocene: New insights from paleomagnetic data

[1] A paleomagnetic study of Penninic units in the southern part of the Alpine arc has been carried out. More than 200 samples (23 sites) were collected in Brianconnais Ammonitico rosso limestones of the high Ubaye valley and the Ligurian Alps. A characteristic component of magnetization of reverse polarity was isolated on most of the sites. This component does not pass the fold test and is interpreted as a Tertiary overprint related to Alpine metamorphism. Mean directions in geographic coordinates are D = 121degrees, I = -52degrees, alpha(95) = 11degrees, and D = 72degrees, I = -48degrees, alpha(95) = 15degrees for the Ubaye and the Liguria localities, respectively, indicating large counter-clockwise rotations about vertical axis of 68 and 117 relative to stable Europe. These rotations, in agreement with a previous study conducted in the Briancon area, together with other paleomagnetic data from the western Alps, show that the internal Alps suffered a large but nonhomogeneous counter-clockwise rotation since the Oligocene. The rotations are in agreement with the combination of earlier separately proposed processes: the rotation of the Adriatic plate accounts for about 25degrees of rotation, remaining rotation and southward gradient would be related to left-lateral shear accommodating the displacement between Adria and Europe at the southern border of the western Alps. Furthermore, the southward extrusion of the western Alps south of the Simplon fault zone may account for up to 10degrees of rotation. Rotations appear therefore as a major process accommodating deformation in the western Alps since the Oligocene.