Bohemian Massif

Palaeomagnetic studies and palaeogeographic interpretation of the Barrandian Lower Palaeozoic with respect to the Bohemian Massif and peri-Gondwanan Europe (P. Pruner, D. Venhodová, O. Man, P. Štorch & V. Suchý)

The project was focused on unresolved problems in palaeomagnetism and palaoegeographic interpretation of the Lower Palaeozoic of the Barrandian terrane, with the aim to contribute to palaeogeographic and plate-tectonic reconstruction of the peri-Gondwanan Europe. Paleomagnetic data from the Variscan and pre-Variscan formations of the Bohemian Massif were tectonically interpreted and palaeogeographically evaluated with respect to the European palaeomagnetic results. The study provided the calculation of palaeomagnetic and virtual pole positions to the Lower and Middle Silurian of Armorica and for the Ordovician to Devonian of the Barrandian. Palaeogeographic affinity of newly inferred virtual pole positions to pole positions for the Ordovician and Silurian was tested on a theoretical model simulating the rotation-controlled distribution of pole positions. The values of palaeolatitudes correspond with the regions northeast of the Trans-European Suture Zone (the stable East European Craton). The peri-polar latitudes derived by some authors are in good agreement with the cold climate indicators for the Ordovician (Late Ordovician glaciation of N Africa) but, in case of the paleogeographic affinity of the Barrandian to Armorica, they imply an anomalously rapid drift of Armorica and Gondwana as well. In the Bohemian Massif, horizontal paleotectonic rotations were well documented for the individual bloks of Silurian and Ordovician rocks. They reach several tens of degrees, but the total palaeorotation relative to the Permian reaches up to 200° in extreme cases for the Devonian rocks, of mostly clockwise sense. We also documented very similar values of palaeomagnetic inclinations for both investigated areas, which show palaeolatitude values of -27^o for the Barrandien and -24^o for the Armorican Massif. The study of the tectonics and palaeogeography of Ordovician to Devonian formations of the Barrandian area, which is considered a peri-Gondwanan terrane with affinities to Armorica, may serve a case history for 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.

Barrandian Area – Middle Ordovician to Middle Silurian virtual pole positions. Left side: 36V - Karlštejn, Kosov Quarry, Early to Middle Silurian, black shale; 37V - Hlásná Třebáň, Levín, Early Silurian, limestone and claystone; 48V – Žebrák - Točník, Middle Ordovician, silicite; 49V - Praha Libeň, Černá skála, Middle Ordovician, black shale. Armorician Massif. Right side: RO – France, Crozon, Rosan form., Asghilian, limestones.Thin lines indicate distribution of pole positions due to paleotectonic rotation. Full small circles denote pole positions due to paleorotation at the paleomagnetic declination step of ΔD = 20°. APWP for a stable Europe during the period of Middle Triassic (T2) to the Early Devonian (D1) is presented by a thick line (according to M. Krs and P. Pruner, 1995).

Results:

Krs M., Krsová M., Pruner P., Chvojka R., Havlíček V. (1987): Palaeomagnetism, palaeogeography and the multi-component analysis of Middle and Upper Cambrian rocks of the Barrandian in the Bohemian Massif. Tectonophysics, 139: 1-20, Elsevier, Amsterdam.
Pruner P. (1987): Palaeomagnetism and palaeogeography of Mongolia in the Cretaceous, Permian and Carboniferous - preliminary data. Tectonophysics, 139: 155-167, Elsevier, Amsterdam.
Pruner P. (1992): Palaeomagnetism and palaeogeography of Mongolia from the Carboniferous to the Cretaceous - final report. Phys. Earth Planet. Inter., 70, 169-177, Amsterdam.
Krs M., Pruner P. (1995): Palaeomagnetism and palaeogeography of the Variscan formations of the Bohemian Massif, comparison with other European regions. - Journal of the Czech Geological Society, 40/1-2, 3-47, Prague.
Krs M., Krsová M., Pruner P. (1996): Palaeomagnetism and palaeogeography of the Western Carpathians from the Permian to the Neogene. In A. Morris and D.H.Tarling (eds), Palaeomagnetism and Tectonics of the Mediterranean Region. Geological Society Spec. Publ. No. 105, pp. 175-184, London.
M. Krs, M. Krsová and P. Pruner (1997): Palaeomagnetism and Palaeogeography of the Variscan and pre-Variscan Formations of the Bohemian Massif.In: Vrána S. and Štědrá V. (eds.): Geological model of Western Bohemia related to the KTB borehole in Germany - Journal of Geological Sciences, Geology 47, Czech Geological Survey ,162 -171, Prague.
M. Krs, J. Pešek, P. Pruner, V. Skoček and J. Slepičková (1997): The origin of magnetic components of Westphalian C to Stephanian C sediments, West Bohemia: a record of waning Variscan tectonism. In Gayer, R. and Pešek, J. (ed.): European Coal Geology and Technology, Special Publication No. 125, The Geological Society of London,p. 29 - 47, London.