Search

In this paper, we present results of the 3-D tomographic modelling of the crustal structure and Moho topography applied to data recorded in SE Poland during the CELEBRATION 2000 seismic experiment. The target area covers ca. 500 km × 500 km and represents a complex geological setting from old Precambrian platform (East European Craton, EEC), through the crustal blocks (terranes) that form the Trans-European Suture Zone, to the young Alpine orogen—the Carpathians. We test two different inversion strategies using two different algorithms: (i) coupled inversion of and arrivals to constrain both the crustal velocities and the Moho depths; (ii) decoupled inversion of arrivals only using previously obtained smooth 3-D crustal velocity model. The coupled inversion of and arrivals results in a much smoother crustal velocity field than the one previously obtained by inversion of first arrivals only. Also, the obtained Moho structure is much smoother than the Moho map compiled from the existing 2-D models. Decoupled inversion of the reflections provides Moho structure comparable in resolution to the compiled map. Synthetic tests indicate that with our data set we are able to resolve larger than 100-km-size Moho structures. The modelled Moho is shallowest in the area of the Upper Silesian Block ( ca. 32 km), then it is deepening by ca. 10 km over 100-km-wide zone along the margin of the EEC and finally it reaches up to 48 km depth in the area of the EEC. Our favoured strategy for modelling large refraction/wide-angle reflection data set consists of derivation of a smooth crustal velocity model by a first-arrival tomography and then complementing this model by a decoupled inversion of reflections in order to constrain minimum-structure Moho topography. [ABSTRACT FROM AUTHOR]

In our paper, we compile a detailed 3-D velocity model of the crust for the territory of Poland and investigate the effect of a 3-D density distribution in the crust on geoid undulations. The major tectonic feature in the study area is the Trans European Suture Zone (TESZ) separating the Precambrian East European Craton from younger tectonic units of Palaeozoic Europe. High-resolution seismic models of the crust and upper mantle from recent controlled-source experiments were interpolated into a 3-D P-wave velocity model for the whole Poland. The model was converted to a density model assuming different equations connecting seismic velocities to densities for crustal rocks, sediments and the uppermost mantle. The crustal density model was used to calculate synthetic geoid undulations for the territory of Poland and to estimate influence of separate layers (topography, sediments, crust and uppermost mantle) on the geoid. To calculate the gravity effect of a large-scale 3-D density model, we developed a code based on a point mass algorithm as well as prisms algorithm that uses some easy to implement optimizations. The synthetic geoid undulations were compared to an existing gravimetric quasi-geoid solution. The modelling showed that the difference in crustal structure between the East European Platform and Palaeozoic Platform explains significant part of the observed geoid undulations in the study area. The remained residual could be related to density inhomogeneities in the upper mantle. [ABSTRACT FROM AUTHOR]

We present analysis of lateral variations of density in the upper mantle in the area of contact of the precambrian East European Craton (EEC) and the Palaeozoic Platform (PP) in Poland, obtained by analysis of the gravimetric geoid undulations. A precise 3-D density model of the crust in the study area down to a depth of 50 km, discussed in the first part of this paper ( Majdański et al. , in press) did not explain all features of the observed geoid. This suggests that these features can be due to density inhomogeneities in the upper mantle. To estimate them, we performed inversion of a residual between the observed geoid and undulations caused by the 3-D density distribution in the crust. Basing on the assumption of local isostatic compensation and Pratt–Hayford isostasy model, the density distribution in the upper mantle was parametrized as a 40-km-thick layer located above the assumed compensation depth of 140 km and subdivided into irregular blocks. The boundaries of the blocks were defined according to boundaries of major tectonic units in the study area and position and shape of the most pronounced anomalies in the residual geoid. A series of sensitivity tests calculated for such density heterogeneities in the upper mantle showed that they can produce geoid undulations of the order of several metres. The density values in each unit were taken as model parameters for the inversion procedure, and inverse problem was solved using global optimization with constraints. The density variations in the upper mantle in the final model correlate well with the surface heat flow. This suggests that these variations can be due to diversity in mantle temperature. The Trans-European Suture Zone (TESZ), which is a major suture separating the EEC from the PP, is not observed as a distinct unit in the mantle. Instead, our study suggests continuation of the lithosphere of the EEC beneath the PP and confirms subdivision of the TESZ into terranes with distinctly different evolution. A simple calculation shows that condition of local isostatic equilibrium is almost achieved at the assumed compensation depth, with the exception of several small-scale units. However, different loads and mass deficiencies in the lithosphere are compensated at different depth levels. This proves that the lithosphere is alternation of mechanically strong and weak layers, both in the crust and upper mantle, and that different loads can be compensated at different depth. However, the total compensation of all lithospheric loads occurs in the upper mantle. [ABSTRACT FROM AUTHOR]

Permian rhyodacites, melaphyres and tuffs from the Cracow area (South Poland) were sampled for the palaeomagnetic and isotope studies. Single-grain U-Pb dating of most zircon grains separated from the rhyodacites gave mean age of magma emplacement of 294.2 ± 2.1 Ma. Some zircons, however, displayed younger ages (268.7 ± 3.4 Ma), probably related to the metasomatic alterations of these rocks. Two Permian components of magnetizations related to these processes were isolated and together with previously defined Late Carboniferous–Permian palaeomagnetic poles from South Poland were used for construction of the regional apparent polar wander path (APWP). The Early Permian segment of this APWP shows a certain departure from the coeval part of the Fennoscandian APWP due to anticlockwise rotations of studied rocks most probably caused by mid-Permian sinistral tectonic movements along reactivated prominent Variscan faults of Central Europe. This sense of tectonic mobility does not support the hypothesis about transformation from Pangea ‘B’ to Pangea ‘A’ along an intra-Pangea dextral megashear during the Permian. Older than previously assumed ages of the post-Variscan igneous rocks of Central Europe reduce overlap of Gondwana's and Laurussia's parts of the Early Permian Pangea ‘A’. [ABSTRACT FROM AUTHOR]

Organic matter preservation and associated conditions during deposition, important in the context of fossil fuel exploration, are commonly determined by advanced geochemical analyses. However, the relation between organic matter preservation and magnetic mineral composition remains poorly constrained. The aim of the studies was to check the potential of magnetic mineral differentiation between facies containing various amounts of organic matter as a factor to better understand the processes which influence water chemistry at the bottom of sedimentary basins, and thus to better understand factors controlling the preservation of organic matter. To determine the composition and the properties of magnetic minerals, detailed low-temperature measurements of Saturation Isothermal Remanent Magnetization and hysteresis loops were performed on two types of rocks, Silurian shales from the Baltic Basin (northern Poland). The analysed shale facies are characterized by similar thermal evolution, but different amounts of organic matter: the Pelplin Formation, containing a modest content of organic matter, in which we also examined early diagenetic carbon concretions; and the Jantar Formation, which represents an organic-rich 'sweet spot' layer. In both facies, the results indicate the presence of multi- or pseudo-single domain magnetite, which is interpreted as detrital in origin. However, the main observation gained from this study is the relation between magnetic mineral assemblage in the studied shales and the amount of organic matter: in the rocks with modest amounts of organic matter we observed hematite, while in organic-rich layers hematite was absent. Hematite (mostly single-domain grains) preserved in the Pelplin Formation suggests that stable oxygen-rich conditions were present at the bottom of the sedimentary basin continuously during deposition, concretion cementation and compaction. In turn, its absence in the Jantar Formation suggests that during sedimentation and early diagenesis more anoxic conditions appeared. Generally, findings show that the presence of hematite is related to the significantly lower amount of organic matter in sedimentary rocks. Thus, presence of this mineral may be a useful indicator of organic matter preservation. [ABSTRACT FROM AUTHOR]

Seismic interferometry and beam-forming techniques were applied to the ambient noise recorded during January 2014 at the '13 BB star' array composed of thirteen seismic stations located in northern Poland. The circular and symmetric geometry of the array allowed the evaluation of the azimuths of noise sources and the velocities of recovered surface waves with a good reliability. After having pre-processed the raw records of the ambient noise in time- and frequency-domain, we studied the associated power spectral density to identify the frequency bands suitable for the recovery of the surface waves. Then the cross-correlation was performed between all the station pairs of the array to retrieve the Green's function, from which the velocity range of the surface waves can be determined. Making use of that analysis, the direction of the noise wavefield was linked to the maximum amplitude of the beam-power, estimated by the mixing in the frequency-domain of all the corresponding noise records. The results were related day by day to the mean wind velocity around Europe at 10 m above ground level obtained from global surveys carried out during the same month. Significant correlation between the direction of maximum beam-power associated to the ambient noise recorded at '13 BB star' and the average wind velocity was found. [ABSTRACT FROM AUTHOR]