Analysis of geomagnetic field along seismic profile P4 of the International Project POLONAISE'97

This paper presents relative secular variations of the total intensity of the geomagnetic field against a background of results of magnetic anomaly interpretation along seismic profile P4. Profile P4 crosses a Variscan folding zone in the Paleozoic Platform (PLZ), the Trans-European Suture Zone (TESZ), and the Polish part of the East European Craton (EEC). Secular geomagnetic field variations measured in 1966–2000 along a line adjacent to seismic profile P4 were analysed. The study of secular variations, reduced to the base recordings at the Belsk Magnetic Observatory, showed that the growth of geomagnetic field at the East European Craton was slower than in the Trans-European Suture Zone and the Paleozoic Platform.A 2D crustal magnetic model was interpreted as a result of magnetic modelling, in which seismic, geological and geothermal data were also used. The modelling showed that there were significant differences in the magnetic model for geotectonic units, which had been earlier determined based on deep seismic survey data. It should be noted that a fundamental change of trend of the relative secular variations was observed at the slope of the Precambrian Platform. After analysing the geomagnetic field observed along profile P4, the hypothesis that the contact between Phanerozoic and Precambrian Europe lies in Poland's territory can be proven.     

Interwedging and inversion structures around the trans-European suture zone in the Baltic Sea, a manifestation of compressive tectonic phases

During the evolution of continents, compressive tectonic phases can leave certain tectonic patterns in the lithosphere to be observed by reflection seismology. Also, in the area of the trans-European suture zone (TESZ) in the Baltic Sea, several relatively short, but occasionally strong, compressive phases have left their marks in the lithosphere in form of characteristic fault and thrust zones in the rigid parts of crust and mantle, especially clear and well investigated in some sediment troughs. At depth, interwedging processes seem to be generated by colliding tectonic units with different rheology, creating bi-vergent fault structures, possibly—but not necessarily—initiated by a previous subduction of intervening oceanic lithosphere. Near the surface, reactivation and inversion of previous faults are very selective. Transpressional processes and the reduced friction inside the faults are suggested to play a major role. It is assumed that the transfer of plate boundary stressed over long distances is performed mainly through the thick and rigid mantle lid, not through the thin, rigid, and heterogeneous upper crust. This assumption involves mechanisms of a vertical transfer of stresses from the mantle into the inversion area, and some signs of such a process are seen around the Tornquist Zone (TZ). Several examples of compressive transfer of stresses are shown.     

Crustal structure below the Polish Basin: Is it composed of proximal terranes derived from Baltica?

The large-scale seismic refraction and wide-angle reflection experiment POLONAISE'97 together with LT-7 and TTZ profiles carried out with the most modern techniques gave a high resolution of crustal structure of the Trans-European Suture Zone (TESZ) in NW and central Poland. The results of seismic investigations show the presence of relatively low velocity rocks (V_p < 6.1 km/s) down to a depth of 20 km beneath the Polish Basin (PB), and a high velocity lower crust (V_p = 6.8–7.3 km/s). The crustal thickness in the TESZ is intermediate between that of the East European Craton (EEC) to the northeast (40–45 km) and that of the Variscan crust (VB) to the southwest ( 30 km). Velocities in the uppermost mantle are relatively high (V_p = 8.25–8.45 km/s). The crust is three-layered with substantial differences in the velocities and thickness of individual layers. The area of the TESZ in NW and central Poland can be divided into at least two crustal blocks (terranes), called here Pomeranian Unit (PU, in the northwest) and Kuiavian Unit (KU, in the southeast). The postulated boundary between KU and PU is rather sharp at particular levels of the crust. Velocity distribution in the middle and lower crystalline crust in the TESZ area resemble values recognized in the EEC area, the fundamental difference being the much smaller thickness of both these layers. Our hypothesis/speculation is that the attenuated lower and middle crust of the TESZ belong to proximal terranes built of the EEC crust detached in the southeast and re-accreted to the EEC due to the process of anti-clockwise rotation of the Baltica paleocontinent during the Ordovician–Early Silurian.     

Moho topography and lower crustal wide-angle reflectivity around the TESZ in southern Scandinavia and northeastern Europe

The Moho topography is strongly undulating in southern Scandinavia and northeastern Europe. A map of the depth to Moho shows similarities between the areas of the Teisseyre–Tornquist Zone (TTZ) in Poland and the Fennoscandian Border Zone (FBZ), which is partly coinciding with the Sorgenfrei–Tornquist Zone (STZ) in Denmark. The Moho is steeply dipping at these zones from a crustal thickness of approximately 32 km in the young Palaeozoic Platform and basin areas to approximately 45 km in the old Precambrian Platform and Baltic Shield. The Moho reflectivity (P_MP waveform) in the POLONAISE'97 refraction/wide-angle seismic data from Poland and Lithuania is variable, ranging from ‘sharp’ to strongly reverberating signals of up to 2 s duration. There is little or no lower crustal wide-angle reflectivity in the thick Precambrian Platform, whereas lower crustal reflectivity in the thin Palaeozoic Platform is strongly reverberating, suggesting that the reflective lower crust and upper mantle is a young phenomena. From stochastic reflectivity modelling, we conclude that alternating high- and low-velocity layers with average thicknesses of 50–300 m and P-wave velocity variations of ±3–4% of the background velocity can explain the lower crustal reflectivity. Sedimentary layering affects the reflectivity of deeper layers significantly and must be considered in reflectivity studies, although the reverberations from the deeper crust cannot be explained by the sedimentary layering only. The reflective lower crust and upper mantle may correspond to a zone that has been intruded by mafic melts from the mantle during crustal extension and volcanism.     

Potential field imaging of Palaeozoic orogenic structure in northern and central Europe

The Trans-European Suture Zone (TESZ) is the most fundamental lithospheric boundary in Europe, separating the ancient crust of the Fennoscandian Shield–East European Craton from the younger crust of central Europe, and extending deep into the mantle. Geophysical potential field images provide an overview of the entire Palaeozoic orogenic system of northern and central Europe for the first time. The TESZ is largely concealed by sedimentary basins of Permian–Cenozoic age; geological observations are largely restricted to local basement highs and deep boreholes, and the coverage of deep seismic surveys is widely spaced, despite experiments recently acquired within the EUROPROBE programme. By contrast, the potential field data offer a relatively detailed coverage of standardised observations throughout the TESZ. While some features of the images may be sourced in the near surface, particularly in the gravity image, much of their content reflects the structure of the underlying Palaeozoic basement. At the scale presented, the images highlight the most fundamental features of the crustal structure of the TESZ. These include the strong contrast between the highly magnetic crust of the East European Craton and the less magnetic Palaeozoic-accreted terranes of central Europe; the lateral continuity of terranes and their internal structure, particularly where arc-magmatic complexes are involved; and the location and geometry of the terrane boundaries (oceanic sutures and strike-slip zones) that separate them.     

Seismic and geological structure of the crust in the transition from Baltica to Palaeozoic Europe in SE Poland—CELEBRATION 2000 experiment, profile CEL02

The CELEBRATION 2000 together with the earlier POLONAISE'97 deep seismic sounding experiments was aimed at the recognition of crustal structure in the border zone between the Precambrian East European Craton (Baltica) and Palaeozoic Europe. The CEL02 profile of the CELEBRATION family is a 400-km long SW–NE transect, running in Poland from the Upper Silesia Block (USB), across the Małopolska Block (MB) and the Trans-European Suture Zone (TESZ) to the East European Craton (EEC). The structure along CEL02 was interpreted using both 2D tomography and forward ray-tracing techniques as well as 2D gravity modelling.The crustal thickness along CEL02 varies from 32–35 km in the USB to 45–47 km beneath the TESZ and the EEC. The USB is a clearly distinctive crustal block with the characteristic high velocity lower crust (7.1–7.2 km/s), interpreted as a fragment of Gondwana. The Kraków–Lubliniec Fault is a terrane boundary produced by soft docking of the USB with the MB. The Małopolska crust fundamentally differs from the USB and has a strong connection with Baltica. It is a transitional, 150- to 200-km wide unit composed of the extended Baltican lower crust and the overlying low velocity (5.15–5.9 km/s) Neoproterozoic metasediments in the up to 18-km thick upper crust. The Łysogóry Unit has its crustal structure identical with that of Małopolska, thus it is connected with Baltica and cannot be interpreted as a Gondwana-derived terrane. Higher velocity and density bodies found below the Mazovia–Lublin Graben at a depth of 12 km and at the base of the lower crust, might be a result of mantle-derived mafic intrusions accompanying the extension of Baltica. By the preliminary 2D gravity modelling, we have reconfirmed the need for considering the increased TESZ mantle density in comparison to the EEC and USB mantle.