ALBERT

Description: The Trans-European Suture Zone (TESZ) comprises a series of sutures that developed by Palaeozoic amalgamation of the Phanerozoic Central Europe onto the Proterozoic Baltica and East European Plate during the formation of Pangea. Within TESZ, the Permian Basin in Poland froms the easternmost part of the Permian Central European Basin, which is bordered on the east by the East European Craton (EEC) and on the southwest by the Bohemian Massif. The axis of the basin, the Mid-Polish Trough, parallels the edge of the EEC. The Teisseyre-Tornquist Zone (TTZ) is a geological inversion zone in the Polish Trough. A large seismic experiment, the POLONAISE´97 project, was conducted in Poland during May of 1997 and targeted the deep structure of the TESZ and the complex series of upper crustal features associated with it. It included contributions from the geophysical communities in Poland, Denmark, the USA, Lithuania, Germany, Finland, Sweden and Canada. This large lithospheric seismic experiment deployed 613 instruments to record 64 shots along five profiles with a total length of about 2000 km. Moreover, five multichannel seismic reflection stations (90 and 120 channels) recorded all singnals from shots. Two dimensional velocity models were derived by P-wave tomographic inversion. These models provide an initial interpretation of this massive data set and allow us to present a few significant seismic and tectonic observations. One of the most important is a very distinct asymmetry between the maximum thickness of the sedimentary cover in the Polish Trough (16-20 km) and the crustal root (~50 km) associated with TESZ/TTZ.

Description: The structure of the mantle transition zone between two seismic discon-tinuities at depths near 410 and 660 km is one of the most important issues to understand the Earth’s dynamics. Most of what we know about this zone is based on seismic interpretation. Seismic waves observed in the distance range of about 15-28° turn in the transition zone and give us information about structural details of penetrated region. We present new results on the upper mantle velocity struc-ture beneath Eastern Europe obtained from individual earthquakes for which seismic data has been collected during two passive experiments, TOR and SVEKALAPKO. Comparison with the earlier models describing the upper mantle velocity structure allowed to study lateral changes in the topography of the 410 and 660 discontinuities. Variations in the average thickness of the transition re-gion are thought to reflect variations in the mantle temperature. We cannot ex-clude non-thermal effects occurring in the Earth’s mantle but the thickness of the transition zone may be a useful thermometer for indication of lateral temperature variations. The size of velocity jumps could be translated into chemical composi-tion, mainly for olivine content at discontinuity.

Description: During EUROBRIDGE"96 seismic data were acquired along a 544-km NW-SE profile, from the East Lithuanian Belt (EL) to the Ukrainian Shield. Explosive sources from 16 shotpoints at 30-km intervals were recorded by 114 three-component seismographs deployed at 3-4-km intervals along the profile. Tomographic inversion and raytrace modelling, integrated with results from the EUROBRIDGE"95 experiment, established a two-dimensional P-wave velocity lithospheric model and the spatial variation of Vp/Vs. Sedimentary cover in Belarus consists of two principal layers with P-wave velocities of about 2.3 and 4.0 km/s. Upper, middle and lower crystalline crust exhibit velocities of 6.1-6.3, 6.4-6.8 and 6.9-7.2 km/s, and are characterised by low velocity gradients and small contrasts at boundaries. The crust below Belarus is about 50 km thick with Moho elevations of a few kilometres. Mantle P-wave velocities immediately beneath the Moho are generally 8.2-8.4 km/s. A lower lithosphere reflector occurs at 65-70 km depth. S-wave velocities are high in the upper crust and low in the lower crust. Our crustal model shows similarities to results from Scandinavia. High lower-crustal velocities and a crustal thickness of about 50 km (common features of Proterozoic crust) are observed throughout the EUROBRIDGE"96 profile. The boundary between the EL and West Lithuanian Granulite Domain (WLG) is associated with pronounced crustal velocity changes, and a thinning of crust towards the northwest. The WLG may be part of a larger southern Baltic Sea tectonic unit. Correlation of our seismic structure with near-surface geology tentatively suggests that contact zones between the EL, Belarus-Baltic Granulite Domain, Central Belarussian Belt (CB), and the Osnitsk-Mikashevichi Igneous Belt all dip slightly to the southeast, consistent with successive docking of the terranes during craton growth. A spectacular feature of our model is high velocities throughout the CB crust, which marks the Fennoscandia-Sarmatia suture. Here we observe a change from typical shield/platform crust in the northwest to highly heterogeneous crust with pronounced lower crustal reflectivity in the southeast. Our results are consistent with CB uplift during continental collision. Our model implies significant tectonic involvement of middle crust in the formation of the Pripyat Trough.

Description: The EUROBRIDGE deep seismic sounding (DSS) profile is a key component of a EUROPROBE project to examine Palaeoproterozoic processes of continental collision and crustal accretion. Its purpose is to establish the deep lithospheric structure of the East European Craton between the exposed Proterozoic and Archaean complexes of the Baltic and Ukrainian Shields. In 1994 a DSS experiment was recorded across the Baltic Sea from Västervik (Sweden) to Shventoji (Lithuania). We report on EUROBRIDGE'95, the first onshore stage of the seismic profile. It is 280km long, recorded from NW to SE on the Lithuanian part of the East European Platform, traversing the Proterozoic West Lithuanian Granulite Domain (WLG) and East Lithuanian Belt (EL) terranes. Explosive shots of up to 1000kg TNT were detonated at 10 shotpoints (SP01-SP10) at intervals of about 30km. Arrivals were recorded at 76 3-component seismograph stations with an average station spacing of 3.5km, providing high quality records. A 11th shot of 3000kg (SP00) was fired in the Baltic Sea close to Gotland. Raytracing analysis of refracted and reflected P-waves has been used to determine a 2-dimensional seismic velocity model for the crust and uppermost mantle below EUROBRIDGE'95. The thickness of the Phanerozoic sedimentary cover decreases from 2.2km in the north-west near the Baltic Sea coast to 0.4km at the south-east end of the profile near the Lithuania/Belarus border. Crust in the north-west and central part of profile consists of two major layers with a thickness of about 44km, increasing to 50km and three layers in the south-east. Crystalline upper crust is about 20km thick, thinning in the south-east, with P-wave velocities of 6.0-6.3km/s. A very weak low velocity zone, with a velocity contrast of 0.1-0.2km/s, occurs at 8-13km depth below the north-west and central part of the profile only. Lower crust exhibits velocities of commonly 6.5-6.9km/s, and thickens to the south-east with P-wave velocities up to 7.0km/s in the deepest parts. Crystalline crust is characterised by low velocity gradients and small velocity contrasts at most seismic boundaries. Major lateral changes in crustal velocity structure at all depths can be spatially correlated with the WLG-EL boundary determined from near surface geological information. Very strong reflections from the Moho boundary are observed. The mantle P-wave velocity immediately below the crust is 8.2-8.35km/s. A reflector in the lower lithosphere at a depth of almost 70km was found below Lithuania. Reflectivity modelling of the Gotland shot data suggests that this interface is absent offshore, where mantle velocities are lower. The DSS model supports the interpretation of the WLG and EL as terranes of Proterozoic age forming part of Fennoscandia. Later modification of crustal structure may have occurred, possibly by a mantle-heating episode centred on the Baltic Sea area.

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.