Deeply rooted inversion tectonics in the southern Baltic Sea

We performed reinterpretation of the DEKORP-BASIN’96 offshore deep reflection seismic profiles PQ-002 and PQ-004-005 running ENE-WSW in the South Baltic area through the transition zone between the East European Craton (EEC) in the NE and the Palaeozoic Platform in the SW. These profiles intersect the Teisseyre-Tornquist Zone (TTZ) and the Sorgenfrei-Tornquist Zone (STZ) to the south and north of the Bornholm Island, respectively. While the STZ is considered to be an intra-cratonic structure within the EEC, the TTZ is often believed to represent the actual edge of the Precambrian craton. Regardless of their origin and tectonic position, both zones are characterized by intense compressional deformations associated with the Alpine inversion of the Permian-Mesozoic basins at the transition from the Cretaceous to Paleogene.

Our research aimed to explain the structure of the transition zone between the EEC and the Palaeozoic Platform and check whether its structure differs north and south of Bornholm. We also aimed at documenting the nature of the Late Cretaceous deformations and their relationship to the STZ and TTZ, as well as the marginal zone of the EEC.

Both PQ profiles show a continuation of the EEC crust toward the WSW beyond the STZ and TTZ. The cratonic crust has a considerable thickness and is characterized by a deep Moho position along the entire length of the profiles. The depth of Moho is in our interpretation much greater than that postulated in previous interpretations. Consequently, numerous reflections once interpreted as upper mantle reflections occur within the lower crust in our opinion.

The most spectacular feature of both PQ profiles is related to the zones of thick-skinned compressional deformation associated with the Alpine inversion along the STZ and TTZ. Crustal-scale, ENE-vergent thrusts have been traced from the top of the Cretaceous down to the Moho in terms of the detachment faults through the entire crust. They are accompanied by back thrusts with vergence toward the WSW, which also reach the Moho. The Late Cretaceous deformation resulted in the uplift of a block of cratonic crust as a pop-up structure, bounded by thrusts and back thrusts, and displacement of the Moho within the STZ and TTZ. It also led to the formation of the Late Cretaceous syn-inversion troughs on both sides of the uplifted wedge providing evidence for the age of deformation.

The STZ and TTZ, imaged by the PQ profiles, appear as zones of the localised Late Cretaceous thick-skinned deformation that is superimposed on the EEC crust and its sedimentary cover. Within these zones, the Moho is faulted in several places and a large block of the basement is uplifted as a crustal-scale pop-up structure. A similar crustal architecture characterises the Dnieper-Dontes Paleorift, which was also inverted in the Late Cretaceous. A special position is occupied by the island of Bornholm, located in the middle of the pop-up structure, which owes its formation to the Late Cretaceous inversion of the sedimentary basin in this place.

This study was funded by the Polish National Science Centre grant no UMO-2017/27/B/ST10/02316.