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  • Author: Martin Krajňák x
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Abstract

During the last years, many new results related to the thickness of the sedimentary fill and other geophysical and geological constrains on the structure of the Turčianska Kotlina Basin have been obtained. It allowed us to calculate the first, original stripped gravity map in this basin. To obtain this map the 3D gravity effect of the basin sedimentary fill had to be calculated. The gravity effect of the sediments was determined for two different density conditions. Firstly, the average density of the sediments was constant (2.45 g cm−3). Secondly, we supposed that the density of sediments varies exponentially from 2.00 g cm−3 on the surface up to 2.67 g cm−3 on the pre-Tertiary basement. On the maps of the calculated gravity effects it can be observed that the maximum amplitudes reach about -12 mGal. The gravity effect is larger in the case when the densities of the sedimentary fill vary exponentially. After subtraction of the gravity effects from the map of complete Bouguer gravity map, the resultant stripped gravity maps were defined. The detailed analysis of these maps indicates that the northern part of the pre-Tertiary basement of the basin could be built mostly by the Mesozoic rocks, which belong to the Hronic and Fatric units. The structure of the basement in the southern part of the basin seems to be more complicated. This feature probably reflects a presence of Neogene volcanites in the basin basement belonging to the Kremnické vrchy Mts. The picture of the gravity field in the stripped gravity map predicts that the Paleogene sediments probably do not build the sedimentary fill in the southern part of the basin. Finally, taking into account the differences in the size of the gravity gradients along the basin margins it could be suggested that the dipping of the Veľká Fatra Mts. beneath the Turčianska Kotlina basin is gentle in comparison with the Lučanská Malá Fatra.

Abstract

The paper deals with the revision and enrichment of the present gravimetric database of the Slovak Republic. The output of this process is a new version of the complete Bouguer anomaly (CBA) field on our territory. Thanks to the taking into account of more accurate terrain corrections, this field has significantly higher quality and higher resolution capabilities. The excellent features of this map will allow us to re-evaluate and improve the qualitative interpretation of the gravity field when researching the structural and tectonic geology of the Western Carpathian lithosphere. In the contribution we also analyse the field of the new CBA based on the properties of various transformed fields – in particular the horizontal gradient, which by its local maximums defines important density boundaries in the lateral direction. All original and new transformed maps make a significant contribution to improving the geological interpretation of the CBA field. Except for the horizontal gradient field, we are also interested in a new special transformation of TDXAS, which excellently separates various detected anomalies of gravity field and improves their lateral delimitation.

Abstract

New results related to the thickness and density of the sedimentary fill of the Turiec Basin allowed us to construct the first original stripped gravity map for this typical intramontane Neogene depression of the Western Carpathians. The stripped gravity map of the Turiec Basin represents the Bouguer gravity anomalies corrected for the gravity effect of the density contrast of its Quaternary-Tertiary sedimentary basin fill. It means that the map reflects the gravity effects of the density inhomogeneities which are located beneath the sedimentary basin fill. This map is therefore suitable for the interpretation of the structure and composition of the pre-Tertiary basement. Based on the new data analysis, two different density models of the sedimentary fill were constructed. The 3D density modelling was used to calculate the gravity effect of the density models. The stripped gravity maps were produced by subtracting the density model gravity effects from Bouguer anomalies. The regional trend was also removed from the stripped gravity maps. The residual stripped gravity maps were consequently used for geological interpretation of the pre-Tertiary basement of the Turiec Basin. The pre-Tertiary basement of the Turiec Basin can be divided into northern and southern parts due to its gravity characteristics. Furthermore the northern part can be split into two domains: western and eastern. The crystalline basement of the western domain is probably formed by the Hercynian crystalline basement of the Tatric Unit. In the eastern domain the basement could consist mostly of the Mesozoic complexes of the Fatric Unit. The southern part of the pre-Tertiary basement of the Turiec Basin is built predominantly by Mesozoic complexes of the Hronic Unit. It is suggested that the Hronic Unit also forms the bedrock of the volcano-sedimentary complex of the Kremnické vrchy Mts. The resultant stripped gravity maps and the map of total horizontal gravity gradients have also proven to be very useful for the interpretation of faults or fault systems in the study area. Various faults, particularly of NNE-SSW and NW-SE directions were discovered. The analysis of the faults indicates clearly that the contact of the Turiec Basin with the Malá Fatra Mts and the Veľká Fatra Mts is tectonic.