[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 YNew seismic refraction data were collected across the western Svalbard continental margin off Kongsfjorden (NyÅlesund) during the cruise leg ARK15/2 of RV Polarstern. The use of onshore and offshore seismic receivers and a dense air-gun shot pattern provide a detailed view of the velocity structure of Svalbard's continental interior, the continent-ocean transition, and oceanic crust related to the northern Knipovich Ridge and the Molloy Ridge.The proposed Caledonian central and western terranes of Svalbard are not distinguishable on the basis of seismic velocity structure. Below a 7 to 8 km thick Palaeozoic sedimentary cover the crystalline crust reveals a three-layer structure with seismic velocities ranging between 6.1 and 6.9 km s −1 . The geological suture between the terranes is imperceptible. The middle and upper crust below the Tertiary Forlandsundet graben shows low velocities. This can be related to faulting during the Early Palaeozoic movements between Svalbard and northern Greenland, followed by the continental break-up. Moreover, a sedimentary Palaeozoic core is may be buried below the Forlandsundet graben.The continent-ocean transition can be classified as an obliquely sheared (transform) continental margin. The Moho dips with an angle of 45 • eastwards at the continent-ocean transition that exhibits higher seismic velocities of more than 7.2 km s −1 on the continental side. The sheared margin evolution is linked to the Spitsbergen Transform Fault, today located north of the Molloy Ridge spreading segment. During a later evolutionary stage the Molloy Ridge passed the continental margin. The separating boundary between continental and oceanic crust off northwestern Svalbard is today part of the inactive Spitsbergen Fracture Zone. The high seismic velocities at the continent-ocean boundary can be interpreted as minor mantle-derived intrusions, probably induced by interaction of the passing spreading ridge during the sheared margin evolution.The oceanic crust generated at the Knipovich Ridge and the Molloy Ridge is thin (2 to 4 km), compared to the global mean, and is thinner as previously observed. The oceanic crust is characterized by the absence of oceanic layer 3. These observations can be ascribed to conductive cooling of the ascending mantle as a result of the extremely low divergence rate.The underlying mantle is slightly serpentinized below the Knipovich Ridge segment, reflected by low seismic velocities of ∼7.7 km s −1 . A thicker sequence of syn-and post-rift sediments and sedimentary rocks are observed on the Molloy Ridge oceanic segment, which most likely results from greater subsidence relative to the Knipovich Ridge segment.