Introduction. The TTZ-South experiment is the latest in a series of three wide-angle reflection/refraction (WARR) seismic studies situated along a NW-SE oriented line in Central Europe. This paper presents the results of a WARR study on the ~550 km long УДК 550.341
<p>The southern Baltic Sea area is in the transition zone between the Fennoscandian Shield as part of the East European Craton (EEC) and the West European Platform. This area is characterised by a mosaic of various geological blocks separated by several fault zones formed throughout the Phanerozoic. The most prominent tectonic feature is the NW&#8211;SE trending Sorgenfrei-Tornquist Zone, crossing the southern Baltic Sea area between Scania in Sweden and Pomerania in Poland. Recently, this area was covered with the new multi-channel seismic data (MCS), acquired during the &#8220;BalTec&#8221; cruise of the German R/V Maria S. Merian. In addition to MCS data, hydroacoustic and gravity data were collected along the same profiles.</p> <p>The new data, acquired during the &#8220;BalTec&#8221; cruise in 2016, include 3500 km of MCS data and 7000 km of gravity data. This is the first such a regional survey in the southern Baltic Sea, which provides a gapless image of sedimentary layer with a high resolution from seafloor to the base of Permian salt (North German-Polish Basin) or Palaeozoic strata (EEC). In addition, a 230-km long refraction/wide-angle reflection (WARR) profile was acquired across the transition zone to image its deeper structure. This profile is nearly parallel to the western Polish coast in half a distance to Bornholm.</p> <p>The main topic of our study is the structure of Phanerozoic sedimentary cover in the southern Baltic Sea and its relationship to the geological evolution of the area situated at the junction of two major tectonic units of NW Europe. In the methodological part of our research, we are going to develop the process of integration of potential field modelling into seismic interpretation workflow. Another important point is testing the capability of marine versus satellite gravity data to reflect the geometry of shallow tectonic structures.</p> <p>The first step in analysis of potential field data was integration of marine gravity with a regional gravity dataset. The result was a coherent gravity grid, which was used for further advanced processing, involving calculation of transformations and derivatives. We also included a regional magnetic grid in the advanced processing. Calculated derivatives and filters of gravity and magnetic data were applied for qualitative interpretation, i.e., compilation of a structural map based on the location and nature of gravity and magnetic anomalies. In addition, a preliminary 2D forward model was produced for the WARR profile to provide an image of the broad crustal structure. The next 2D models will be built upon seismic reflection profiles acquired during the &#8220;BalTec&#8221; cruise. The results will be eventually used to calibrate the three-dimensional model for the top of crystalline basement derived from gravity inversion.</p> <p>This study was funded by the Polish National Science Centre grant no UMO-2017/25/B/ST10/01348.</p>
<p>Crustal and uppermost mantle structure along the Teisseyre-Tornquist Zone (TTZ)&#160; was explored along the ~550 km long, NW-SE-trending TTZ-South profile, using seismic wide-angle reflection/refraction (WARR) method. The profile line was intended to follow the border between the East European Craton (EEC) and the so called Palaeozoic Platform (PP) of north-central Europe, believed to contain a number of crustal blocks that were accreted to the craton during pre-late Carboniferous times, defining the Trans-European Suture Zone (TESZ).</p><p>The seismic velocity model of the TTZ-South profile shows lateral variations in crustal structure. Its Ukrainian segment crosses the interior of the Sarmatian segment of the EEC, where the crystalline basement gradually dips from ~2 km depth in the SE to ~12 km at the Ukrainian-Polish border. This part of the model shows a four-layered crustal structure, with an up to 15 km-thick sedimentary cover, an underlying crystalline upper crust, a 10-15 km-thick middle crust and a ~15 km thick lower crust. In Poland, the profile passes along the TESZ/EEC transition zone of complex crustal structure. The crystalline basement, whose top occurs at depths of 10-17 km, separates the sedimentary cover from the ~10 km thick mid-crustal layer (Vp=6.5-6.6 km/s), which, in turn, overlies a block of 10-15 km thickness with upper crustal velocities (Vp~6.2 km/s). The latter is underlain by a ~10-15 km-thick lower crust. Along most of the model one can see conspicuous velocity inversion zones occuring at various depths. At intersections of the TTZ-South profile with some previous deep seismic profiles (e.g. CEL02, CEL05, CEL14, PANCAKE) such inversions document complex wedging relationships between the EEC and PP crustal units. These may have resulted from tectonic compression and thick-skinned thrusting due to either Neoproterozoic EEC collision with accreting terranes or intense Variscan orogenic events. Five high velocity bodies (HVB; V<sub>p</sub> = 6.85-7.2 km/s) were detected in the middle and lower crust at 15-37 km depth. The Moho depth varies substantially along the profile. It is at ~42 km depth in the NW and deepens SE-ward to ~50 km at ~685 km. Subsequently, it rises abruptly to ~43 km at the border of the Sarmatian segment of the EEC and sinks again to ~50 km beneath the Lviv Paleozoic trough at ~785 km. From this point until the SE end of the profile, the Moho gently shallows, up to a depth of ~37 km, including a step-like jump of 2 km at ~875 km. Such abrupt Moho steps may be related to crust-scale strike-slip faults. Along the whole profile, sub-Moho velocities are ~8.05-8.1 km/s, and at depths of 57-63 km Vp values reach 8.2-8.25 km/s. Four reflectors/refractors were modelled in the upper mantle at ~57-65 km and ~80 km depths.</p>
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