Abstract The southern Baltic Sea is a peculiar area, where the Sorgenfrei‐Tornquist Zone (STZ), stretching from Bornholm into the North Sea, connects to the Teisseyre‐Tornquist Zone (TTZ) that continues SE up to the Black Sea. In this study, we show the structure and evolution of this controversially debated area, both on crustal and basin scale, by using three seismic reflection profiles combined with 2‐D potential field data. The results demonstrate that the southern Baltic Sea is underlain by a thick crust of the East European Craton with a Moho depth in the range of 38–42 km. The overall crustal architecture is shaped by three phases of localized stretching in the early Paleozoic, Devonian‐Carboniferous, and Permian‐Mesozoic. The most spectacular feature of the southern Baltic Sea is a zone of thick‐skinned compressional deformation produced by Late Cretaceous‐early Paleogene inversion, including a system of thrusts and back thrusts penetrating the entire crust in an 80–90 km wide inversion zone. ENE‐vergent thrusts are traced from the top of the Cretaceous down to the Moho and they are accompanied by back thrusts of opposite vergence, also reaching the Moho. Inversion tectonics resulted in the uplift of a block of cratonic crust as a pop‐up structure, bounded by thrusts and back thrusts, and the displacement of the Moho within the STZ and TTZ. The similar mechanism of intra‐cratonic inversion was recognized for the Donbas Foldbelt in eastern Ukraine, and it may be characteristic of rigid cratons, where deformation is localized in a few preexisting zones of weakness.
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.
DEFORMATION AND METAMORPHISM OF ROCK SERIES EAST OF THE SOWIE GORY BLOCK - NEW DATA AND INTERPRETATIONS
Summary
An important NNE-SSW - trending tectonic boundary, located between the Gory Sowie Gneiss Block to the west and the Strzelin Crystalline Unit to the east (Fig. 1), separates the structures of the West and East Sudetes in the area of the Fore-Sudetic Block. The West Sudetes have been traditionally included into the Saxothuringian and the East Sudetes into the Moravo-Silesian major facies-structural zones of the European Variscan belt. The boundary between the two zones continues further southwestward along the SE margin of the Bohemian Massif into the Moldanubian thrust [45] which separates Moldanubian and Moravian nappe piles. The ductile, NNE-directed, synmetamorphic displacements with a significant dextral, strike-slip component have been documented in this area throughout the entire length of the SE margin of the Bohemian Massif [26, 42, 21]. In contrast, the mylonites of the Niemcza Zone, located immediately to the west of the Western/ Eastern Sudetes boundary (Fig. 1), recorded effects of sinistral strike-slip displacements [28]. Metamorphic rocks cropping out to the east of the Gory Sowie Block are subdivided into three regional units. From west to east these are: the Niemcza Shear Zone, the Niemcza-Kamieniec Metamorphic Unit and the Doboszowice Metamorphic Unit (Fig. 1). The orientation of foliations and lineations in these units are shown in Figures 3, 4 and 7. On the basis of our field data, we established a tentative sequence of three tectonic events in the rock sequences of the study area (Fig. 8). The DJ event was related to E-directed tectonic transport under amphibolite facies conditions. The DJ fabric is preserved in paragneisses comprising the eastern part of the Doboszowice Metamorphic Unit (Fig. 6). The western part of this unit is composed of orthogneiss body (Fig. 6) representing a syntectonic granite intrusion emplaced during the D2 event. The D2 structures, well developed in this orthogneiss body and in coarse-grained mica schists exposed near to Kamieniec Ząbkowicki (Fig. 5) recorded a top-to-NE shearing under amphibolites fades conditions. The D3 event involved sinistral, strike-slip displacement in the Niemcza Shear Zone (Fig. 2) and a top-to-SW shearing in the Niemcza-Kamieniec Unit. The Niemcza Zone (Fig. 2), extending along the eastern edge of the Gory Sowie Block, consists of mylonites derived from the Gory Sowie gneisses during the D3. The mylonites occur as high- and low-temperature varieties produced under the amphibolite and greenschists facies conditions, respectively. The D2 event corresponds to synmetamorphic NNE-directed thrusting recognized along the entire SE margin of the Bohemian Massif. The displacements towards NNE were preceded by a separate stage of E-directed tectonic transport. The sinistral sense of shear in the Niemcza Zone is related to the subsequent D3. It seems to be comparable with late-orogenic, sinistral shearing localized in several NNE-SSW shear zones and ductile to brittle faults in the S and SE part of the Bohemian Massif [10].
Abstract The timing of Svalbard's assembly in relation to the mid‐Paleozoic Caledonian collision between Baltica and Laurentia remains contentious. The Svalbard archipelago consists of three basement provinces bounded by N–S‐trending strike–slip faults whose displacement histories are poorly understood. Here, we report microstructural and mineral chemistry data integrated with 40 Ar/ 39 Ar muscovite geochronology from the sinistral Vimsodden‐Kosibapasset Shear Zone (VKSZ, southwest Svalbard) and explore its relationship to adjacent structures and regional deformation within the circum‐Arctic. Our results indicate that strike–slip displacement along the VKSZ occurred in late Silurian–Early Devonian and was contemporaneous with the beginning of the main phase of continental collision in Greenland and Scandinavia and the onset of syn‐orogenic sedimentation in Silurian–Devonian fault‐controlled basins in northern Svalbard. These new‐age constraints highlight possible links between escape tectonics in the Caledonian orogen and mid‐Paleozoic terrane transfer across the northern margin of Laurentia.
Comprehensive set of seismic and potential field data from the whole European Variscan belt is used to interpret the structure and evolution of the European Variscides as defined by Martínez Catalán et al. (2021). The gravity data show the presence of high amplitude, short-wavelength gravity anomalies correlated with the outcrops of eclogites, ultramafic rocks and ophiolites delineating the main body of the Mid-Variscan Allochthon (MVA) and the Devonian Mid-Variscan suture (MVS). The medium amplitude and elongated long-wavelength gravity highs, aligned parallel to the Variscan structural grain, correspond to the low-grade Proterozoic rocks of the MVA and Devonian arc – back-arc system. On the other hand, the short wavelength negative gravity anomalies developed in the central part of the belt coincide with Carboniferous (330–310 Ma) per- to meta-aluminous magmatic bodies. The magnetic data show two belts correlated with Carboniferous Rhenohercynian and Devonian Mid-Variscan magmatic arc granitoids. The Rhenohercynian and Mid-Variscan subduction systems are also well-imaged by moderately dipping primary reflectors in reflection seismic lines. Younger moderately dipping reflectors in the upper-middle crust coincide with outcrops of Carboniferous detachments, limiting granite plutons and core complexes along-strike the core of the Variscan orogeny. Deep crustal reflectors are considered as an expression of lower crustal flow resulting from extensional re-equilibration of the previously thickened Variscan crust. A P-wave velocity logs synthesis shows a high-velocity cratonic crust surrounding a thin Variscan orogenic crust defined by low-velocity lower and middle crusts. The latter crustal type coincides with regional outcrops of 330–310 Ma per- to meta- aluminous granitoids and associated gravity lows along-strike the belt. All these data are used to define the primary polarity of Devonian subduction systems defining the European Variscan belt (Schulmann et al., 2022) and discuss the Carboniferous extension forming specific structure of the Variscan crust. This geodynamic evolution is integrated into a paleomagnetically constrained model of the movements of continental plates and intervening oceans (Edel et al., 2018; Martínez Catalán et al., 2021).REFERENCES:Catalan, J.R.M., Schulmann, K. and Ghienne, J.F., 2021. The Mid-Variscan Allochthon: Keys from correlation, partial retrodeformation and plate-tectonic reconstruction to unlock the geometry of a non-cylindrical belt. Earth-Science Reviews, 220, 1–65.Edel, J.B., Schulmann, K., Lexa, O. and Lardeaux, J.M., 2018. Late Palaeozoic palaeomagnetic and tectonic constraints for amalgamation of Pangea supercontinent in the European Variscan belt. Earth-science reviews, 177, 589-612.Schulmann, K., Edel, J.B., Catalán, J.R.M., Mazur, S., Guy, A., Lardeaux, J.M., Ayarza, P. and Palomeras, I., 2022. Tectonic evolution and global crustal architecture of the European Variscan belt constrained by geophysical data. Earth-Science Reviews, 234,  p.104195.
