Structural expression of large rotations: the Alpine example

Paleomagnetic methods are used to measure the magnetic north-direction in rocks at the time of formation of the rocks. As continents move across the earth surface due to continental drift, they also rotate about vertical axes. Therefore the north direction changes through geologic time relative to the rocks observed. These changes are characteristic for continents and are plotted in curves. Measurement of the north direction in sedimentary successions covering the last 200 Million years in the Eastern and Southern Alps between the Northern and Southern foreland confirmed the affinity of the sampled rocks to the Adriatic or European plate in some places. However, the results in most sampling sites could not be related to any of the known curves. A younger group of magnetic directions from rocks aged 5 Million years and older point to the northwest, and an older group from rocks up to 30 Million years in age point to the northeast. Most of the magnetic information was imprinted on the rocks after their formation, in most cases also after folding of the rocks (secondary directions). The younger group of secondary directions disappears to the north across the northernmost thrust of the Alps. Only a few kilometres of offset can be deduced from the geometry of this thrust in seismic images of the subsurface. Interpretation of the magnetic data in terms of 25 relative counterclockwise vertical axis rotation would require very large offset and must be excluded. Similar problems arise with the interpretation of the older group of secondary magnetizations. We have to question the underlying assumption that every paleomagnetic direction represents an old north-direction, or that the characteristic curves the of Adriatic and European plate are wrong. Whatever the secondary magnetizations truly mean, we have now the possibility to date folds relative to the secondary magnetizations. The investigation of thrust- and fold geometries of the Subalpine Molasse, which is the northernmost part of the Alps, showed that thrust activity propagated to the north until approximately 9 Million years. Younger thrusts formed within the Alps and truncate the previously formed structures. This is consistent with the observation that folds in the inner part of the Alps (Helvetic unit) tilt the younger secondary magnetization, indicating very young folding.