Magnetotectonics in the Piemont Tertiary Basin

Abstract

Characteristic remanent magnetization (ChRM) directions from late Oligocene sediments of the Piemont Tertiary Basin show a 45° anticlockwise deviation from the present-day field direction. Up to 30° of this deviation can be ascribed to large-scale (plate) movements, as deduced from earlier studies. The inclinations show a considerable scatter and are too low for the presumed latitude of the area during the time of deposition.

The samples show a strong anisotropy of magnetic susceptibility (AMS) and in one section there is evidence that the magnetic foliation is produced by the inhomogeneous distribution of titanomagnetite in the specimens. The general pattern of the AMS strongly suggests that the rocks have been deformed with a NW-SE stretching axis. This suggestion is supported by scanning electron microscope observations, which reveal a fabric in accordance with the above-mentioned strain orientations and indicate that the shortening axis makes a small angle with the bedding. It is tentatively assumed that this deformation has resulted in the extra anticlockwise deviation of 15° of the ChRM directions. Since there were no signs of deformation visible in the field, the AMS proved to be the main indicator for deformation.