Structure of the Brunovistulian Unit (E part of the Czech Republic) on the basis of seismic and magnetic data


Authors: Gnojek I, Hubatka F

Published in: Bulletin of Geosciences, volume 76, issue 3; pages: 157 - 168;

Keywords: Magnetic anomalies, Magnetic modelling, Seismic survey, Basement paleorelief, Basement structure, Eastern Moravia, Czech part of Silesia,

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The extensive South Moravian-Beskydy (SMB) regional magnetic anomaly is developed in the southern and eastern Moravia, i. e., in the area built by the Carpathian Foredeep and by flysch nappes of the Outer Carpathians. Magnetic field of similar features was also found in northern Moravia and in the Czech part of Silesia forming the Silesian (SIL) regional magnetic anomaly here. Magnetic susceptibility studies of all rock complexes occurring in the area of interest revealed that the Neogene, Paleogene, Cretaceous, Jurassic, Carboniferous and Devonian sedimentary sequences cannot act as sources of the mentioned regional anomalies. Some volcanic rocks incorporated in the Palaeogene, Cretaceous and Devonian complexes can produce magnetic anomalies but their role is minor. On the contrary, the crystalline basement complex formed by the Cadomian orogeny and pertaining to the Brunovistulian unit was proved to be the particular source of these anomalies. The two regional SIL and SMB anomalies correspond to two partial Brunovistulian blocks. Large volume of seismic survey was carried out in the Carpathian part of the CR in the last decades. This method determined reliably the boundary between the sedimentary sequences and the crystalline basement in the geological cross-section. A rather detailed scheme of buried basement paleorelief could be drawn on the basis of the results of seismic survey in the region of South Moravia-Beskydy Mts. Exceptionally applied seismic profiles with a prolonged registration time of reflected waves permitted to reveal the structure of the basement down to a depth of 12 km indicating the presence of bodies of plutonic and/or of metamorphic rocks. Magnetic modelling of the sources of magnetic anomalies contributed to the definition of mutual relationships between the SIL and the SMB basement blocks. The accepted solution suggests that Brunovistulian plate, originally probably not fragmented, was broken along a west-east-striking fault zone within which only weakly magnetic fragments rested. The existence of this displacement indicates an old (pre-Variscan) plate collision in the region south of the Baltic Shield. The consequent diverse position of the SIL and SMB blocks necessitated their different role during the Alpine orogeny. Complex interpretation of seismic and magnetic data points to the presence of several partial blocks within the Cadomian basement. These are separated mostly by NW-SE- or W-E-striking fault systems. The interpretation also indicated elevation structures that may imply the presence of plutonic rock cupolas.