The Hanö Bay part of the Fennoscandian Border Zone has been investigated by means of seismic reflection profiling. These data have been interpreted on the basis of correlation with two offshore boreholes. Hanö Bay is found to be a tectonically formed sedimentary basin. Mesozoic sedimentary rocks are deposited on an eroded surface consisting partly of remnants of presumed Lower Cambrian sandstone and Precambrian basement. The Hanö Bay Basin is divided into a shallow and a deep part by the Nävlinge Flexure in the southeastern extension of the northern scarp of the Nävlingeåsen Horst. The basin is bounded to the SW by the Linderödsåsen and Christiansö Horst blocks. South of the dividing flexure, Upper Triassic-lower Upper Cretaceous deposits are present in the deeper part of the basin, whereas Santonian and Campanian strata cover all of the Hanö Bay Mesozoic basin. Post-Campanian movements have occurred in the Linderödsåsen-Christiansö Horst blocks and in the flexure where, in addition, indications of volcanic or intrusive activity have been observed in the seismic sections.