A review of palaeomagnetic and magnetic anisotropy results from the Alps

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Abstract
Summary: Numerous palaeomagnetic studies in the Alpine region have documented the effects of tectonic displacement and rotation. Regions in the southern Alps are characterized by palaeomagnetic directions which are more similar to directions predicted by the African path of apparent polar wander than by the Eurasian path. Attempts to define the extent of the region with this African affinity are found in the internal zone of the western Alps, and also in the Lepontine of southern Switzerland, yet directions with European affinity have been reported from S of the Insubric Line. Although the northern calcareous Alps are of southern Tethyan origin, palaeomagnetic directions reveal that these allochthonous units have undergone substantial rotations, in some places clockwise and in others anticlockwise, during emplacement. Several palaeomagnetic studies in the eastern and central Alps show rotated directions due to strong local tectonic effects. In the western Alps, European style directions are found in the undeformed rocks of the Dôme de Barrot and the external Dauphinois zone, whereas directions from the internal zone tend to have African affinity. Deformation produces not only a deflection of the stable palaeomagnetic vector in a rock. If the deformation is penetrative, grain rotations falsify the palaeomagnetic vector. Preferential grain alignment caused by deformation produces a magnetic anisotropy which can be measured easily and quickly in many rocks. Analogously to the strain fabric, the magnetic fabric can be described by an anisotropy ellipsoid. Several studies in the Alps have confirmed that the principal axes of the magnetic anisotropy are aligned with the corresponding axes of the strain ellipsoid. In deformed Permian red beds in the Alpes Maritimes the anisotropy ellipsoid changes shape and orientation like the strain ellipsoid during progressive deformation. A good qualitative agreement between magnetic fabric and petrofabric is also found in Jurassic shales and slates from the Dauphinois zone of the western Alps, where magnetic lineations often correspond to the direction of nappe transport. In strongly deformed Permo—Triassic red beds from the Lower Glarus nappe complex in central Switzerland the quantitative relationship between strain and magnetic fabric was used to derive equivalent strain ellipsoids in rocks that had no conventional strain markers. In the eastern Alps, the sense of motion on the Insubric Line has been postulated from magnetic fabric observations. Magnetic anisotropy results have been used in attempts to correct palaeomagnetic vectors for the effect of deformation. This technique may be viable in weakly deformed rocks, but it fails in strongly deformed samples; the limits of its applicability have not yet been determined satisfactorily.