Reflection seismic evidence for Caledonia deformed sediments above Sveconorwegian basement in the southwestern Baltic Sea

Two complexes of orogenic structures are interpreted to the south of the Baltic Shield from seismic reflection sections in the southern Baltic Sea. Dipping reflections from the pre-Mesozoic basement are interpreted as (1) an upper, Caledonian (450-440 Ma) complex of SSW dipping low-angle thrusts that formed in relation to Avalonia's accretion to Baltica and (2) a lower, Sveconorwegian (1150-900 Ma) complex of west to SW dipping ductile thrusts and shear zones in the crystalline crust of Baltica. The two complexes are separated by a regional seismic marker, the O horizon, from the basal part of Baltica's lower Palaeozoic cover and, in places, also by reflections from an underlying Neoproterozoic clastic rift sequence. The complex tectonic structures were resolved from a network of commercial seismic reflection profiles to 4 s two-way travel time. The results imply that (1) the late Proterozoic crust of southwestern Baltica extends southward, with little lateral offset, across the Sorgenfrei-Tornquist Zone, below the southwestern Baltic Sea and the North German Basin: (2) in the study area a distinction should be made between the northernmost occurrence of Caledonian deformation structures and the Caledonian suture proper: (3) Caledonian deformation did not involve the crystalline basement in the study area, but was accommodated by the main decollement, near the top of the O horizon, and by thrusts in the overlying lower Palaeozoic sequence; and (4) with such data it is possible to add more detail and partly revise previous interpretations of the Neoproterozoic and Caledonian tectonic evolution of the southwestern part of Baltica.