Tectonic and paleogeographic interpretation of the paleomagnetism of Variscan and pre-Variscan formations of the Bohemian Massif, with special reference to the Barrandian terrane

Abstract

Paleomagnetic data from Variscan and pre-Variscan formations of the Bohemian Massif (BM) have been interpreted tectonically, and paleogeographically evaluated with respect to the European paleomagnetic results. The interpretation of data from the BM was preceded by the evaluation of the effect of the Trans-European Suture Zone (TESZ) on the dispersion of the European paleomagnetic pole positions during the Variscan orogeny. Stability of the European lithospheric plate is well documented by paleomagnetic results for the Early Permian to the Quaternary rocks of the regions between the Ural Mountains and Great Britain, north of the Alpine tectonic belt. For the Late Carboniferous and earlier times, the stable plate is defined in the regions NE of the TESZ in the East European Craton only. Late Carboniferous and older rocks from the regions SW of the TESZ, largely occupied by the West-European Variscides, show tectonic deformations controlled by prominent horizontal rotations. These rotations can be studied on a theoretical model simulating the distribution of pole positions controlled by rotation of rock formations about a vertical axis. In the BM, horizontal paleotectonic rotations are well documented for Variscan and pre-Variscan rocks: they reach several tens of degrees (up to 180° in extreme cases) of mostly clockwise sense. The study of the tectonics and paleogeography of Cambrian to Devonian formations of the Barrandian area, which is considered a peri-Gondwana terrane with affinities to Armorica, may serve as a case history of the study of a terrane incorporated into a stable lithospheric plate. Here, the Barrandian terrane became a part of the European Plate as a component of the emerging Pangea supercontinent in the final phase of the Variscan orogeny. High, almost peri-polar paleolatitudes for the Cambrian rocks of the Barrandian area are — within the limits of data scatter — in agreement with the presumed position of Gondwana and Armorica. Rocks of Ordovician age did not produce consistent results; they will require further paleomagnetic and paleoclimatologic studies in relation to the data for the Late Cambrian and Early Silurian rocks of the Barrandian area. The analysis of data presented here shows that paleomagnetic declinations of Variscan and pre-Variscan formations reflect paleotectonic rotations following the formation of the respective rocks including rotations induced by the Variscan orogeny and rotation of the European Plate to which the rocks became incorporated. Therefore, paleomagnetic declination of none of the pre-Variscan formations can be identified with paleotectonic rotation either of the Barrandian terrane or even of the whole BM. Magnitudes of horizontal paleotectonic rotations in the BM affected by Variscan tectonism are notably similar to those in the Western Carpathians affected by Alpine deformation. Senses of rotation are, however, different with clockwise rotation prevailing in the BM, and counterclockwise rotations prevailing in the Western Carpathians. Recognition of the effect of horizontal rotations of rock formations, if located in tectonically mobile belts and incorporated into a stable plate during their later history, should be respected while deriving the apparent polar wandering path (APWP).