ALBERT

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.