Basin inversion in a strike-slip regime: the Tornquist Zone, Southern Baltic Sea

Abstract

Abstract The southern section of the deep seismic reflection profile BABEL crosses the Tornquist Zone between the mainland of Southern Sweden (Scania) and the island of Bornholm. This marks the boundary between the Sorgenfrei Tornquist Zone to the northwest and the Teisseyre Tornquist Zone to the southeast. The region represents a large-scale releasing bend in the dextral strike-slip system of Tornquist and has led to the development of pull-apart basins such as the Rønne and Arnager grabens, from the Carboniferous to the Late Jurassic. The area around BABEL is covered by proprietary surveys (courtesy of JEBCO Seismic Ltd, BGR Germany and the Swedish Geological survey) which allow the three-dimensional interpretation of sediment/structure interactions within these basins. Within the area various classical (positive) inversion structures are seen on seismic sections, including harpoon structures and inversion monoclines. These features are found in association with both basin-bounding and intra-basinal faults. There is also evidence for inversion of basement blocks within the upper crust. Inversion took place between the late Cretaceous and early Tertiary, probably as a response to Carpathian/Alpine movements to the south. Strike-slip structures indicate that movements were dominantly dextral with rigid blocks such as Bornholm channelling deformation around them. At the same time, progradational sequences of sediment are seen to build off these blocks. The BABEL section images a number of features believed to have resulted from the post-Carboniferous strike-slip movements of the Tornquist fault system. Structures imaged within the upper part of the section are consistent with those interpreted in three dimensions from the Jebco survey, as a brittle response within the upper crust. Below these, between 7 and 12 seconds TWT, a zone of high reflectivity is observed. This is similar in character to regions of lower crust imaged on either side to the northeast and southwest, but it is shallower below the Tornquist Zone. This is interpreted as a region of lower crust in which deformation is transferred in a more ductile fashion. Below this again there are coherent reflectors that may represent a more brittle transfer of the deformation into the upper mantle. The extent of upper crustal deformation imaged on this profile is around 40 km in width whilst in the lower crust it is at least 90 km.