The Fennolora Experiment and Crustal Structure in the Bohemian Massif

Abstract Seismic measuring along the southernmost part of the Fennolora profile (Fennoscandian Long Range Project 1979) between shot-points Bischofswerda and Stankov were mostly performed by the Geofyzika n.p., Brno, and the Geophysical Institute of the Czechoslovak Academy of Sciences, Prague. The profile between the above mentioned shot-points could not be covered continuously with the recording stations, because of the concentration of industry and agricultural areas along the profile. The total length of the profile Bischofswerda — Stankov is 192.5 km. In the seismic records individual wave groups were identified : the first onsets correspond to a refracted wave propagating in the upper crustal layers with gradually increasing velocity, in epicentral distance of 100 km a head wave related to the Conrad discontinuity was recorded and, finally, a particular attention was paid to the waves associated with the Mohorovicic discontinuity — reflected P M and head P n waves. The onsets of further waves, arriving at about 20 s, are obviously reflected mantle waves and seem to confirm the transitional character of the interfaces between the crust and the mantle. The travel-time curves were compiled in unreduced time-scale for the above mentioned groups of waves. For both shot-points the velocity-depth curves were compiled. In general it can be stated that in the block of Saxo-Thuringicum (comprising the Bischofswerda shot-point in the north) we can find lower crustal velocities than in the Tepla-Barrandian block (comprising the Staňkov shot-point in the south). On the basis of the obtained travel-time curves & schematic cross-section of the crust was worked out (Fig. 1.), from which one may infer a deeper immersion of the crust below the northern part of the Plzeň basin; the Mohorovicic discontinuity may be located in the depths of about 32 to 36 km. In the region of the Kladno-Rakovnik basin and under the North Bohemian Neogenic basins the thickness of the crust is reduced. In the region of the Bohemian Cretaceous Table and the Krusne hory Mts the thickness of the crust again gradually increases. In general, the relief of the Mohorovocic discontinuity follows the shape of shallower horizons of the crustal structure within the northern part of the Plzeň basin, contrary to the anticlinal structure, which is characteristic of the distribution of seismically ascertained interfaces below the Neogene of the North Bohemian basins. The contribution presents only the preliminary information obtained from measurements and the results will be steadily completed with other geophysical data.