Deep seismic reflection/refraction interpretation of crustal structure along BABEL profiles A and B In the southern Baltic Sea

In 1989 the BABEL Working Group collected 2268 km of near‐vertical reflection data in the Baltic and Bothnian Seas. As an integrated part of the field survey, the marine airgun shots were recorded by 64 multicomponent land stations. In this paper results are presented from interpretation of profiles B and A in the Baltic Sea, extending from the Åland Archipelago (Finland) into the Bay of Lübeck (Germany). In the shield part of the profiles northeast of the Sorgenfrei‐Tornquist Zone, crustal reflectivity is observed at all levels and its termination in depth coincides largely with the crust‐mantle boundary. the wide‐angle data indicate a three‐layer crust with velocities of 6.1‐6.4, circa 6.6, and 6.9‐7.2 km s^‐1. the Moho is found between 40‐48 km depth, corresponding to 12‐15 s TWT. In the northeastern part of profile A and the southern part of profile B, steeply northeast‐dipping reflections are found at all crustal levels. the tectonic inversion of the Sorgenfrei‐Tornquist Zone is clearly imaged above a thickened, high‐velocity lowermost crust (7.1‐7.4 km s^‐1). At depth the Sorgenfrei‐Tornquist Zone widens and displays some asymmetry that is believed to be indicative of crustal shortening across the zone. an undulating Moho is observed along profile A where the lateral variability in structure and velocity field primarily is in the lower crust. Beneath the Skurup Basin south of the Sorgenfrei‐Tornquist Zone, no intracrustal discontinuities are seen in the wide‐angle data and a highly reflecting lowermost crust between 8 and 10 s TWT corresponds to a zone between 25 and 31 km depth with high velocity gradient (6.7‐7.1 km s^‐1). A bright upper mantle reflection at 12s TWT below the Skurup Basin can be explained by a velocity increase from 7.8 to 8.2 km s^‐1. Southwest‐dipping reflections in the basement of the Møn High, an eastward continuation of the Ringkøbing‐Fyn basement High, indicate that the Caledonian Deformation Front is located at least 50km further north than previously believed. the crust below the Møn High is 38 km thick with high velocities (7.1‐7.4 km s^‐1) in the lower crust. In the North German Lowlands, the crystalline crust below the 10 km thick post‐Caledonian sedimentary sequence is only 20km thick and has velocities between 6.0 and 6.9 km s^‐1. It is hypothesized that during the Caledonian evolution, Baltica's Pre‐Cambrian crust protruded into the docking Avalonian terrain as a major crustal flake structure.