ALBERT

Description: The large-scale POLONAISE'97 seismic experiment investigated the velocity structure of the crust and upper mantle in the Trans-European suture zone (TESZ) region between the Precambrian east European craton (EEC) and Paleozoic platform that comprises terranes added during the Caledonian and Variscan orogenies (530–370 and 370–225 Ma, respectively). This experiment included 64 shots recorded by 613 seismic stations during two deployments. Very good quality data were recorded along five profiles, and the longest and most important one (P4) is the focus of this paper. Clear first arrivals and later phases of waves reflected/refracted in the crust and Moho were interpreted using two-dimensional (2-D) tomographic inversion and ray-tracing techniques. The crustal thickness along the profile varies from 30–35 km in the Paleozoic platform area to ∼40 km below and due northeast of the TESZ, to ∼43 km in the Polish part of the EEC, and to ∼50 km in Lithuania. The Paleozoic platform and EEC are divided by the Polish basin, so the upper crustal structure varies considerably. In the area of the Polish basin, the P wave velocity is very low (V P 〈 6.1 km/s) down to depths of 15–20 km, indicating that a very thick sedimentary sequence is present. We suggest two possible tectonic interpretations of the velocity models: (1) Baltica indented Avalonia, obducting its upper crust and underthrusting its lower crust in a tectonic flake structure and (2) a rifted margin of Baltica underlies the Polish basin. This model is similar to other interpretations of seismic profiles recorded in the Baltic Sea. The second model implies that the Paleozoic platform solely consists of Avalonian lithosphere and the EEC of Baltica lithosphere. It offers a simple explanation of the difference in crustal thickness of the two platforms. It also implies that the Caledonian and Variscan orogenies in this area were relatively “soft” collisions that left this continental margin largely intact.

Description: The TOR experiment (Teleseismic TOmography TORnquist) carried out in winter 1996/97 across the Trans-European Suture Zone (TESZ) in Germany, Denmark and Sweden has collected new data to investigate the transition zone between Precambrian and Palaeozoic Europe. In this study, seismograms of teleseismic earthquakes recorded by the broad-band TOR stations have been used to calculate the receiver functions. The time-domain inversion method has been applied to the receiver functions to compute S-wave velocities in the crust and uppermost mantle beneath each station. The results of inversion down to 60 km depth provide new, independent information about the distribution of S-wave velocity in this area. Beneath the Swedish stations on Baltica, the thickness of the crust varies from about 45 to 50 km with mostly gradually increasing S-wave velocity and no sharp discontinuities while for Danish and German stations the crust is thinner (29-38 km) with a sharp Moho discontinuity. A very distinct S-wave low velocity layer was found at depth of 8-16 km in the upper crust of Baltica, supported by the results of refraction/deep seismic sounding experiments using both P and S waves. The map of V p /V s ratio beneath the c. 1000 km long TOR 'profile' was obtained using the V p velocity model from previous investigations. The values of V p /V s = 1.73 were found in the uppermost crust of Baltica and upper Avalonian crust. The low velocity layer in the upper crust of Baltica is characterized by high value of V p /V s = 1.85. Relatively low S-wave velocities are observed in the lower crust of Variscides (V p /V s = 1.79), Baltica (V p /V s = 1.83) and Avalonia (V p /V s = 1.91); in the uppermost mantle V p /V s values are 1.77, 1.79 and 1.82, respectively.