The Bohemian Massif is the largest coherent surface exposure of basement rocks in Central Europe. It was consolidated during the Variscan orogeny. Irregular shapes of colliding blocks resulted in a very complex structure of the Palaeozoic convergence, lithospheric subductions and crustal shortening, which were followed by extensional processes and rifting. There seem to exist tectono-magmatic processes of interaction between the Earth’s crust und upper mantle in the crossing area of the Cenozoic Eger rift and the Marianske Lazne fault zone, resulting in periodically recurring swarm earthquakes at depths of 6 to 12 km, fluid emanations from the upper mantle and neotectonic crust movements. So, among important questions which remain unsolved, is the relation between near surface structures and the structure of the deep lithosphere. BOHEMA (BOhemian Massif Anisotropy and HEterogeneity) is a passive seismic experiment in the western part of the Bohemian Massif which is carried out in an international effort by various institutions in the Czech Republic, France, and Germany. Its scientific aim is to use all available techniques such as high-resolution tomography, receiver function analysis and 3D anisotropy studies to image the crust and upper mantle to depths of about 250 km. The investigation of crustal and upper mantle xenoliths will add petrological information. Results of the experiment will shed light on possible causes of the periodically recurring earthquake swarms as well as on a subcrustal source of the CO2 dominated gas emanations in mineral springs and mofettes at surface. We want to either confirm or deny the existence of a hypothetical active asthenospheric mantle finger beneath the western part of the Bohemian Massif. Therefore between the end of 2001 until 2003 84 temporary seismic stations were deployed by French, Czech and German institutions in addition to about 60 stations of already existing permanent Czech and German seismic networks in the area. First results of the receiver function investigation show a reduction of the crustal thickness in the area of CO2 emanations from 31 to 27 km. In the same area P-SV conversions at 6 s delay time are observed which might be caused by local seismic discontinuities in approx. 50 km depth or by small scale low velocity zones in the Earth’s crust just below seismogenic depths. Furthermore we found slightly delayed conversions from the 410 km and 660 km discontinuities compared to the model iasp91. This can be indicative for reduced seismic velocities in the upper mantle. The data also show an interesting northward dipping structure which might be a remainder of a paleosubduction zone.

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Heuer, B.; Geissler, W. H. (2004): The Earth's crust and upper mantle beneath the western Bohemian massif-structures derived from Receiver Functions.. 1st General Assembly European Geosciences Union (Nice, France 2004).