An integrated palaeomagnetic and AMS study of the Tertiary flysch from the Outer Western Carpathians

Abstract

SUMMARY The Carpathians belong to the European Alpine system, which was formed during the convergence and collision of the European and African plates. The study area, which comprises the Magura and Silesian nappes of the Outer Western Carpathians, were the locations of intensive folding and nappe transport during the Tertiary. There are a number of models depicting the changes in shapes and positions of the original sedimentary basins as a consequence of the above processes. However, it is this study that provides the first palaeomagnetic constraints for reconstructing the displacement history of the Silesian and Magura nappes during Tertiary. For the palaeomagnetic and anisotropy of the low-field magnetic susceptibility (AMS) studies 554 independently oriented cores were drilled, mostly from claystones, at 57 geographically distributed localities of Oligocene (and partly of Late Eocene in the Magura nappe) age. In the laboratory, the magnetic mineral identified was always magnetite, invariably accompanied by paramagnetic pyrite. The AMS measurements showed that the magnetic fabric was dominantly foliated. The foliation and bedding poles were very close at most of the localities, suggesting that the foliation was of sedimentary/compaction origin. Magnetic lineations, even if clustered at the locality level, did not exhibit a regional pattern for the Magura nappe. On the other hand, lineations related to folding were NE–SW oriented in the western and E–W oriented in the central and eastern segments of the Silesian nappe. Following the AMS measurements the samples were subjected to detailed stepwise demagnetization and analysis of the demagnetization curves that provided the input data for regional tilt tests. Both the tilt and inclination only tests suggested that the remanence is of post-folding age for the Magura nappe, showing a general 50° counterclockwise (CCW) rotation. Similar palaeomagnetic directions were observed for the central and eastern segments of the Silesian nappe, whereas a significantly larger CCW rotation for the western segment. The orientations of the AMS lineation directions averaged for the central, the eastern and the western segments of the Silesian nappe correspond to a ‘rotated’ palaeomagnetic pattern, as they are practically E–W oriented for the first two and NE–SW oriented for the third. The palaeomagnetic results of this study are interpreted in terms of a 50°‘en bloc’ rotation in the CCW sense of the Magura and Silesian nappes during their Miocene emplacement. This result has two important implications. The first is that the CCW rotation of the nappes that accompanied the displacement of the flysch basins from SW to NE is not of Palaeogene age, as earlier suggested, but younger. The second is that the change in the stress field orientation during the Miocene, observed for the Silesian nappe is not a CW far field stress rotation, but the consequence of CCW block rotations. The larger CCW rotation observed in both the palaeomagnetic and AMS data sets for the western segment of the Silesian nappe relative to the central and eastern segments is connected to local block rotations in a left-lateral wrench corridor.