Three-dimensional density model of the sedimentary filling of the Carpathian-Pannonian region

Abstract

The work is devoted to the construction and calculations of a three-dimensional density model of the sedimentary filling of the Carpathian-Pannonian region in order to obtain a more detailed map of the residual gravity field (stripped gravity map). This research was facilitated by and in-depth analysis of a large amount of data highlighting the density properties of Neogene-Quaternary deposits (the Pannonian Basin, the Transylvanian Depression, the Transcarpathian Trough), molasse deposits of the Carpathian Foredeep and flysch deposits of the Outer Carpathians in the Czech Republic, Slovakia, Poland, and Ukraine. Basic data for the construction of a three-dimensional density model of sedimentary deposits were obtained from laboratory studies of rock samples from drill core logging and deep exploratory wells, as well as rock samples taken from numerous outcrops in the research region. The average value of the density for molasse and flysch deposits of the Romanian part of the Carpathians was estimated based on the results of comparing the lithologic-stratigraphic complexes of these deposits in the adjacent areas of the eastern part of the Ukrainian Carpathians with similar ones in the Romanian Eastern Carpathians and the analysis of available data on the density of the Carpathian Foredeep and the Outer Flysch Carpathians for the Ukrainian part. The research method, which is a modification of geological reduction, has been applied in the work. Its essence consists of the sequential calculation and extraction of the three-dimensional gravity effect of sedimentary layers, the parameterization of which is better defined than those layers that lie deeper, from the anomalous gravity field. As a result, a residual gravity field is formed due to deep inhomogeneities associated with the consolidated part of the crust and the upper mantle. Calculations of gravity effects were carried out on a scale of 1:4,000,000 on a 10—10 km grid using the modern GMT-Auto. The detailed map of the residual (cleared of the effects of sedimentary layers) gravity field of the Carpathian-Pannonian region (stripped gravity map) is an effective tool in understanding the sources of the dominant gravity features of the studied region. Thus, the Pannonian Basin manifests itself as a general maximum with a number of local positive anomalies (more than 50 mGal), which are observed over small depressions filled with low-density thick sedimentary deposits: the Danube, Solnok, Makó, Békés Basins, and the Transcarpathian Trough. The phenomenon of positive and not negative values of the residual gravity field for these structures can be explained by the intrusion of the sedimentary cover of volcanic rocks, or the presence of high-density bodies with a special petrophysical composition (metamorphic complexes?) in the consolidated part of the crust. Another reason may be the effect of the regional background, which is due to the rise of Moho boundary in the Pannonian Basin to 24—26 km. The gravity minimum of the Western Carpathians, which on the map of the anomalous gravity field, consists of two parts (northern and southern), is reflected by one intense minimum, the southern one (-60 mGal). The northern part of this gravity minimum is practically leveled after calculations of the gravity effect of sedimentary filling, so it can be assumed that its source is low-density flysch and molasse deposits. The southern part of the gravity minimum of the Western Carpathians can be explained by the mass deficit in the consolidated part of the crust. Since the intensity of the gravity lows of the Eastern (-80 mGal) and Southern (-100 mGal) Carpathians remained high even after being cleared of the effect of the layer of sedimentary deposits, it can be assumed that they are due not only to the low values of the density of the sediments of the Outer Carpathians and the Carpathian Foredeep, but additionally also due to the gravity effect of deep inhomogeneities of the consolidated part of the crust (crustal root).