Neogene remagnetization of normal polarity in the Late Jurassic black shales from the southern Subalpine Chains (French Alps). Evidence for late anticlockwise rotations

This study presents paleomagnetic investigations of 23 sites from the Late Jurassic black shales, the so-called Terres Noires, that outcrop in the southern Subalpine Chains (SSC). Six sites are located in the Diois, six sites in the Trieves and Devoluy, and eleven sites in the Digne nappe. The magnetizations are weak and are carried by titanomagnetites. We found generally one component per site within the medium range of temperature (200 degrees to 500 degrees C). All magnetizations are characterized by the lack of reverse polarity. The directions of magnetization are generally better grouped in geographic coordinates in the Trieves, the Devoluy and the Digne nappe. Fold testing at one site from the Trieves demonstrated a late synfolding acquisition of magnetization. In contrast, pre-tilting magnetizations are suggested in the Diois and locally, in the Digne nappe. Given an average inclination close to late Tertiary expected inclinations in geographic coordinates, we propose that these magnetizations are Neogene overprints. Our results point to two major results. (1) The lack of reverse polarity which is puzzling during the Neogene where 50% of the magnetic field is reverse. (2) The discovery of quite systematic anticlockwise rotations. These anticlockwise rotations do not occur in the Diois and are larger in the southern part of the Digne nappe. We presume that these rotations were active during the late stage of Alpine folds emplacement and are still active today. (C) 1999 Elsevier Science B.V. All rights reserved.