Geological and tectonic interpretation of the new Bouguer gravity anomaly map of Slovakia
Abstract: This paper analyzes the latest complete Bouguer gravity anomaly map of Slovakia in relation to geological architecture. The observed gravity field consists of regional and local gravity anomalies, as well as marked horizontal gravity gradients. The most remarkable regional feature on this map is the large field with low-density masses in the northern part of Central Slovakia (the so-called Western Carpathian gravity low), which is divided into two gravity sub-lows: the Outer and Inner Western Carpathian gravity low. The source of the first sub-low is the sediments of the Outer Western Carpathian flysch units, and the second one is a crust with prevailing granites and orthogneisses. It is suggested that this field is only the torso of the original one, which stretched along the entire length of Slovakia from the SW to the NE. However, in the youngest stages of tectonic development, the negative gravity anomalous field in the territory of West and East Slovakia changed to a positive one due to the thinning of the lithosphere and crust by the influence of asthenolithic masses from the mantle. The higher density masses in Central Slovakia south of the Carpathian gravity low are not caused only by asthenolithic action. The different tectonic segment with a predominance of metamorphic complexes and a higher average density, in comparison to the low-density granitized complexes in the north, also contributes to its manifestation. The boundary between these two segments in Central Slovakia is a linear and sharp tectonic zone and coincides with the extensive Pohorelá shear zone. Several local anomalies also occur on the complete Bouguer anomaly map, and they were also subjected to geological analysis. These include local areas with a predominance of heavier crust, such as the core mountains in western Slovakia, subvolcanic intrusions, metabasic complexes, and the Cadomian basements. Prominent horizontal gravity gradients reflect the tectonic interfaces (faults, shear zones) that originated mainly during the youngest period of the Western Carpathian tectonic development and were also interpreted. The faults shown in the complete Bouguer anomaly map were active mainly during the transpressional and extensional stage of the Neo-Alpine tectonic development.
Assessment of the deformation in the subparallel shear zone set: A case study from the Veporic Unit, Western Carpathians
Abstract: During the Alpine convergence of the Central Western Carpathians, a set of subparallel NW–SE shear zones formed in the eastern part of the Veporic Unit, near the tectonic contact with the Gemeric Unit. Deformation in low-grade metamorphic conditions was typical of the Alpine movements in the shear zones. Mylonitic rocks of the crystalline complex and their cover formations were studied on a macro and micro scale to characterise the evolution of deformation structures in the shear zones. The differential stress of dynamically-recrystallized quartz, which aggregates in mylonitic rocks, was determined using quartz paleopiezometry. Several types of foliation structures, but mainly penetrative subhorizontal stretching lineation in all observed mylonites, provide evidence of stretching and oriented ductile flow in NW–SE direction. Shear sense indicators show the subhorizontal movement of the shear zone hanging walls to the SE. Dynamically-recrystallized quartz aggregates in the mylonitic rocks show different values of grain sizes and differential stresses, depending on the structure in which they occur. The lowest differential stresses were found in the quartz aggregates parallel to the S planes of the mylonites. Higher values are related to the C shear bands. The character of the structural setting indicates the formation of the shear zones in the orogen-parallel extension conditions.
Remobilization of Ni–Co–As and platinum-group elements by carbonate metasomatic alteration (listvenitization) of metaultramafic rocks from Dobšiná, Slovakia
Abstract: Hydrothermal processes are mainly responsible for the release and accumulation of metals and metalloids in rocks. In this work, we investigated the mineralogy and geochemistry of altered metaultramafic rocks (listvenites) that are spatially associated with Ni–Co ores near Dobšiná aiming to identify the sources of the elements in the hydrothermal Ni–Co-mineralization. Optical microscopy, electron microprobe analysis, and laser-ablation inductively coupled mass spectrometry were used to correlate the mineralogy with the degree of rock alteration. The sulfides and silicates in the meta-ultramafic rocks host rare, tiny inclusions of cooperite (nominally PtS), sperrylite (PtAs2), Pty–Pd–Ir–Te phase, and Pt–Au–Cr-alloy. The results show that the metaultramafic rocks were the source of Ni and Co and that platinum-group elements (PGE) were also leached and mobilized from the metaultramafic rocks. LA-ICP-MS scans show that the sulfarsenides in the metaultramafic rocks host submicrometer inclusions of PGE minerals but the hydrothermal Ni–Co sulfarsenides contain much less PGE’s. These observations document the limited mobility of the PGE‘s. Changes in the sulfide mineralogy as a function of degree of alteration suggest that the fluids brought Fe and S, and probably a substantial amount of As whereas the metaultramafic rocks supplied Ni, Co, PGE, and perhaps also some As.
Petrography of the Upper Miocene sandstones from the North Croatian Basin: Understanding the genesis of the largest reservoirs in the southwestern part of the Pannonian Basin System
Abstract: This paper presents a petrographic study of the Upper Miocene sandstones from exploration wells in the Sava and Drava Depressions in the North Croatian Basin (SW of the Pannonian Basin System), Central Europe. These sandstones represent the most important reservoir rocks for oil and gas in Croatia. A total of 130 core samples from depths of more than 3000 m were examined. The sandstones generally have a feldspatho–litho–quartzose (fLQ) composition. The modal composition of samples from the Sava Depression is Q39.2–61.0F8.9–26.0L26.1–42.3, and Q40.6–63.5F6.6–23.3L20.9–42.3 for those from the Drava Depression. Lithic fragments are dominated by extrabasinal carbonates with subordinate metamorphic grains (mostly schists) and less frequent magmatics (granitoids). Garnet, tourmaline, apatite, rutile, epidote, clinozoisite, zoisite, titanite, zircon, staurolite, and opaque minerals form the heavy mineral association and imply an area of provenance dominated by metamorphic rocks. The tectonic setting of the sandstones corresponds to recycled orogen, i.e., a subduction complex or fold-thrust belt. There are no significant compositional differences between the sandstone samples from the two depressions, thus indicating a common main source area and similar diagenetic processes in the subsurface. The sandstones originated from eroded parts of the uplifted Alpine–Carpathian fold belt and are distinct from the Lower and Middle Miocene sandstones in the North Croatian Basin. Observed variations in the sandstones’ composition with regards to geographic location and depth are associated with variations in the erosion of parent rocks, including subsequent modification of detrital sediment during transport, mixing, and deposition, as well as burial diagenesis. The Sava and Drava Depressions were part of Lake Pannon in the Late Miocene with no significant topographical obstacles between them for the inflow of the detritus. The study will therefore help with the correlation of the Upper Miocene reservoir rocks in various parts of the North Croatian Basin (as well as adjacent parts of the Pannonian Basin System) and thereby assist with future hydrocarbon exploration in this area.
Đurđevac Sands and the intraformational paleosoils (Podravina, N Croatia) are newly dated to Late Pleistocene/Holocene
Abstract: The Đurđevac Sands refer to an extensive sandy region south of the Drava River in northern Croatia, where it builds distinctive aeolian dunes. To date, their chronostratigraphical position has been based on stratigraphical inferences (superposition) without numerical and absolute age control. The recent discovery of a buried double paleosoil below and above aeolian dune sands in an abandoned sandpit (Draganci) have allowed the determination of the first absolute dates of the Đurđevac Sands. Field observations and laboratory analyses indicate that the degree of pedogenetic development of these paleosoils is very low. They appear to belong to the arenosol soil type, which is also the dominant recent soil type in the area. 14C analysis of charcoal from the paleosoils indicated their development during the Bølling–Allerød interstadial, approximately between 14.7 ka and 12.9 ka, as opposed to previous claims that they would be exclusively Holocene in age. Therefore, this shows the need for a detailed investigation of the Đurđevac Sands. The sands and paleosoils likely witnessed a series of alternating phases of landscape stability and instability during the Late Glacial and Holocene. Such episodes are known to have occurred in other sandy regions of the Carpathian basin as well.