[1] 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 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.

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S U M M A R YThe 430 km long DSS Baltic profile in SE Finland runs through the Rapakivi intrusion, the Svecokarelian geosynclinal complex, including the Ladoga-Bothnian Bay zone, and the Archean basement complex. A brief description of field operations in 1982 is presented. Record sections of P-and S-waves were used for the interpretations. A starting model was constructed by inverse methods and improved by dynamic ray tracing modelling. Three blocks are indentified on the cross-section from SW to NE. These are the southern, associated with the Rapakivi Massif, depth to Moho (M) about 40 km; the central, Ladoga-Bothnian Bay zone, with M of approximately 55-60 km, and the northern, Archean crystalline basement, with M about 40-45 km. The average crustal velocity is 6.6-6.7 kms-'. The crustal structure along the Baltic profile is compared with the surrounding DSS profiles, Finnish Sveka and Soviet Kern-Tulos and Ladoga profiles. The eastern border of the Ladoga-Bothnian Bay zone was located and specific features of the crust and uppermost mantle along the profile area are discussed.

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