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. ¿ 2001 American Geophysical Union