Dušan Plašienka

The Western Carpathians represent the northernmost segment of the Alpine–Carpathian orogenic system and are characterized by a complex polyphase Variscan–Alpine evolution. The Internal Western Carpathians preserve remnants of the Triassic–Jurassic Meliata Ocean and record a long history of subduction, nappe emplacement and subsequent tectonic overprinting. One of the best exposures of this evolution occurs in the Bretka village area, where the Meliata Unit is bounded to the south by a prominent ductile-to-brittle shear zone. The Bretka shear zone is characterized by the close association of serpentinites, metabasalts, mylonitized carbonates and pseudotachylite-like rocks. In addition, evaporites, mainly anhydrite, are known from borehole data, suggesting that deformation was probably localized within an evaporitic mélange. The serpentinite body is interpreted as a tectonic block incorporated into this mélange. The most significant feature of the shear zone is the relationship between carbonate mylonites and pseudotachylite-like rocks, documenting the transition from ductile to brittle deformation. Mylonites developed within Anisian Steinalm limestones and display a gradual transition from undeformed limestones to completely mylonitized marbles towards the shear zone. The marbles exhibit alternating calcite- and dolomite-rich layers. Calcite forms a fine-grained recrystallized aggregate, whereas dolomite occurs as elongated and rotated porphyroclasts, indicating deformation temperatures of approximately 250–350 °C. Shear bands and asymmetric folds consistently indicate a top-to-the-north sense of shearing. The mylonites are cross-cut by numerous thin calcite veins oriented nearly perpendicular to the mylonitic foliation. Pseudotachylite-like rocks occur mainly within marble layers adjacent to serpentinite bodies and consist of a fine-grained matrix composed of chlorite, epidote and actinolite containing fragmented clasts of albite, calcite, titanite and relict magmatic pyroxene. The observed structures suggest a three-stage evolution of the shear zone. A progressive deformation in ductile conditions caused mylonitization of limestones. A subsequent seismic event caused brittle fragmentation of marble and local frictional melting at the contact with ultramafic rocks. Subsequent fluid infiltration produced an extensive network of thin calcite veins and altered the pseudotachylite-like material to chlorite-, epidote- and actinolite-bearing assemblages. These observations indicate that the mylonitization developed at temperatures below approximately 350 °C, corresponding to depths of roughly 12–14 km with subsequent brittle deformation in the upper parts of the crust. Although kinematic indicators consistently record mainly north-directed transport, both the shear zone and the surrounding rock complexes currently dip to the north. We therefore suggest that the shear zone originally formed during northward nappe emplacement and was subsequently rotated into its present orientation during later tectonic disintegration of the Meliata Unit.Acknowledgments. The research was supported by the Slovak Research and Development Agency (APVV-21-0281 and SK-HU-24-0016) and the Slovak scientific grant agency VEGA (VEGA 1/0021/25).

The extraordinary Western Carpathian tectonic structure, the Pieniny Klippen Belt (PKB), is characterized by its complex internal structure, particular tectonic units and a range of specific lithostratigraphic formations. Among them, the terminating coarsening-and-thickening upward deep marine clastic formations record the sequential thrust propagation of the principal Oravic tectonic units of the PKB. The Oravic palaeogeographic domain included the southern Kysuca–Pieniny basin (later Pieniny nappe) as a marginal part of the South Penninic oceanic realm (Piemont–Ybbsitz­­–Váh ocean) that separated the Oravic continental ribbon from the Central Carpathian (Austroalpine) realm. The central Czorsztyn ridge and its rifted slopes formed the Subpieniny unit, while the northern ridge foots (Šariš unit) were facing the North Penninic ocean (present Magura units). This pattern resulted from complex rifting and breakup episodes related to opening of the Alpine Atlantic during the Jurassic – Early Cretaceous.The syn-orogenic clastic sedimentation commenced in the inner Pieniny basin during the Coniacian–Santonian. Material of siliciclastic turbidites and mass-flow conglomerates was derived from advancing fronts of the Central Carpathian Fatric cover nappe system (e.g. Klape nappe). The clastic material was largely recycled from mid-Cretaceous “exotic” conglomerates related to intra-continental subduction that generated the Fatric nappes. Distal northern turbidite lobes reached slopes of the Czorsztyn ridge and bypassed it up to the Šariš depositional area, so forming a large-scale, northward thinning classic wedge.After elimination of the Váh ocean, shortening progressed by detachment of the Pieniny unit and its thrusting over the Czorsztyn ridge. This event is recorded by deposition of the Maastrichtian–Danian calcareous wildflysch complex that was terminated by olistostromal breccias composed of erosional debris released from the Pieniny nappe fronts. After the basement of the Czorsztyn ridge was thrust under the Central Carpathian orogenic wedge, its variable cover successions were attached to the developing Oravic accretionary complex. Depended on the variable sedimentary settings and resulting heterogeneous composition, sedimentary successions of the Subpieniny unit were partly fragmented and gravitationally transported by mass-wasting mechanisms to the northern Šariš basinal area. Consequently, the Maastrichtian to Lower Eocene calcareous wildflysch of the Šariš unit is inserted by huge bodies of olistostromes and (mega)olistoliths derived predominantly from the Subpieniny successions. As a matter of fact, numerous klippen that gave the name to the entire PKB are olistoliths, in addition to the tectonic klippen that originated by deformational disintegration of competent sedimentary layers and their embedding in incompetent matrix formations.

The Internal Western Carpathians comprise a complex Alpine thin-skinned nappe system derived from the Meliata domain. In its lowermost structural position, the Meliata Unit (Meliaticum) represents a remnant of the Neo-Tethyan oceanic suture and includes the blueschist-facies Bôrka Nappe, low-grade syn-orogenic accretionary complexes of the Meliata Unit s.s., and chaotic ophiolite-bearing mélanges. In this study, we focus on deep-sea sediments from the Meliata Unit and on comparable units of similar age and structural position within the Internal Western Carpathians, particularly the Turnaicum and Silicicum. Our dataset consists mainly of radiolarites in association with shales and claystone. Although the samples can be broadly divided into Triassic and Jurassic age groups, their geochemical composition reveals information that goes beyond biostratigraphic age constraints. As pelagic sediments deposited far from major continental input, radiolarites preserve signatures of seawater chemistry, hydrothermal activity, and detrital supply. These records allow us to reconstruct depositional conditions, sediment provenance, and the tectonic evolution of the Meliata oceanic domain. All 31 samples processed by whole-rock analysis were recalculated to eliminate the carbonate component and loss on ignition. The results show pronounced variability in the processes affecting the Meliata oceanic domain through time and space. In the Al₂O₃/(Al₂O₃ + Fe₂O₃) versus Fe₂O₃/TiO₂ discrimination diagram after Murray (1994), most samples define a trend from continental margin to ocean ridge affinity. Provenance discrimination for shales after Roser and Korsch (1988) indicates mature continental sedimentary sources. PAAS-normalized REE patterns further distinguish lithological groups. Shales display higher total REE concentrations and flatter profiles, whereas radiolarites are characterized by lower REE abundances and significantly more variable patterns. Several samples show a clear negative Ce anomaly, suggesting oxidizing marine conditions or seawater influence during sedimentation. A weak positive Eu anomaly in some samples may reflect a minor terrigenous contribution. These geochemical signatures demonstrate that radiolarites and associated shales from the Internal Western Carpathians record both local depositional conditions and broader tectonic evolution of the Meliata oceanic domain. Together, they provide a geochemical framework for tracing the transition from continental-margin sedimentation to more oceanic settings. This approach highlights the value of pelagic sediments as archives of ancient basin evolution and plate-boundary processes. Additional analyses of radiolarite and siliciclastic pelagic sediment samples from other tectonic units and paleoprovenances, together with comparisons to well-characterized geological settings, are necessary to fully and reliably interpret the obtained geochemical data. Acknowledgements: The research was supported by projects of the Slovak Research and Development Agency (APVV-21-0281; SK-SRB-25-0031), Grant Agency for Science, Slovakia (VEGA 1/0021/25) and National Science Centre, Poland (2021/43/B/ST10/02312). Murray R.W., 1994. Chemical criteria to identify the depositional environment of chert: general principles and applications. Sedimentary Geology, 90: 213-232.Roser B.P. and Korsch R.J., 1988. Provenance signatures of sandstone-mudstone suites determined using discrimination function analysis of major-element data. Chem Geol, 67: 119-139.

Due to the complexity inherent to the systems, the interpretation of the structure of the Northern Calcareous Alps and Inner Western Carpathians has traditionally relied on assuming a simplified pre-contractional configuration. The assumptions made include, for instance, that stratigraphy was either layer-cake or that it varied linearly from proximal to distal facies, that there is no or only limited decoupling between cover and basement during extension, and that rift systems have a one-directional distribution from proximal to distal domains. In the case of the Eastern Alps and Western Carpathians, these starting assumptions require complex interpretations of the orogenic structure, many of which have internal contradictions such as conflicting orogenic vergence or the unresolved debate on presence and/or the location of Mesozoic oceanic domains.We argue that by incorporating modern concepts of salt tectonics and of rift system development, some long-standing paradoxes in these orogens can be successfully addressed. In the Eastern Alps, the development of a salt-rich rift system can explain the origin of the pelagic Hallstatt facies without requiring the presence of an oceanic domain. In the Western Carpathians, the presence of salt in a hyper-extended rift system can explain the confusing relationships of the Silica and its neighboring tectonic units as well as the apparent absence of its original basement. Incorporating the non-linear development of rift systems in turn can explain the nature of Mesozoic oceanic domains, from the Eastern Alps to the Western Carpathians.In this contribution we review the key aspects of salt tectonics and rift systems that we believe could contribute to the understanding of the Eastern Alps and Western Carpathians and how they may be explored for in other segments of the Dinaric-Caraptho-Balkan system.

The ALCAPA (Alps-Carpathians-Pannonia) megaunit, encompassing most of the Eastern Alps (Austroalpine units) and the Western Carpathians (Inner and Central part), has traditionally been interpreted to represent the Neo-Tethys-facing Permo-Triassic passive margin of Pangaea. Multiple tectonic units within the ALCAPA and wider Dinaridic-Balkanic domain have been regarded as continental sutures formed during the closure of the Neo-Tethyan Ocean. A long-standing debate concerns whether the Neo-Tethys comprised a single or multiple oceanic basins. In the ALCAPA region, the Neo-Tethys is commonly called the Meliata-Hallstatt Ocean and considered to represent an oceanic branch located south (in present-day coordinates) of the salt-rich Austroalpine-Carpathian passive margins. This interpretation assumes a simple and linear arrangement of Permo-Triassic facies, from shallow water platforms in the north to pelagic and bathyal deposits in the south. This facies scheme has been generally used as the basis to explain the distribution of Permo-Triassic units in the present-day fold-and-thrust belts.Recent advances in the understanding of salt tectonics and passive margin geodynamics, however, calls this linear paleogeographic model into question. In this contribution we present new observations from the Inner Western Carpathians and a reevaluation of the Austroalpine domain and argue that the Permo-Triassic palegeography of ALCAPA was far more complex than previously assumed. We propose that the Permo-Triassic rift system formed an anastomosing network of rift branches, some of which remained aborted while others progressed to continental breakup and mantle exhumation. This evolution produced a complex mosaic of microcontinental blocks separated by rift basins and domains of exhumed subcontinental mantle, in stark contrast to the conventional linear continent-to-ocean model. This revised paleogeographic framework has significant implications for deciphering the distribution of Permo-Triassic facies in the pre-orogenic setting. Understanding the role of continental hyperextension in the ALCAPA also satisfactorily explains the frequent (and apparently incongruous) contacts of shallow crustal units on sub-continental mantle (e.g., evaporites or platform carbonates on serpentinized mantle).We argue that Jurassic subduction nucleated preferentially along domains of Triassic exhumed mantle, and that recognizing the complex paleogeographic architecture substantially simplifies the tectonic interpretation of the subsequent Mesozoic evolution of the region. These observations further call for a redefinition of what is considered the Neo-Tethys Ocean within the ALCAPA domain.

The supposed Meliata suture divides the Austroalpine-related Central Western Carpathian (CWC) units from the Internal Western Carpathian (IWC) units of the Adria and/or Dinaridic affinity (Transdanubian and Bükk, respectively). The tightly imbricated and dismembered Triassic–Jurassic complexes of the Meliata Unit (Meliaticum) record the subduction/accretionary processes connected with the opening, expansion, subduction and closure of the Neotethyan Meliata Ocean in the southern Western Carpathian zones. The Meliaticum exhibits a lithologically variable composition. In general, three main partial units or complexes can be distinguished: i) the blueschist-facies Bôrka Nappe representing the distal continental margin exhumed from the subduction channel during the Late Jurassic and subsequently thrust over the foreland lower CWC plate; ii) variable chaotic, in part ophiolite-bearing mélange complexes; iii) relatively coherent Jurassic successions of deep-marine hemipelagic and distal flysch deposits with olistostrome bodies which include Triassic carbonate blocks revealing the late Mid-Anisian (Pelsonian) breakup of the Meliata Ocean. This study focuses on the examination of the chaotic complexes of the Meliata Unit at the crucial Čoltovo locality. Due to the poor outcrop conditions caused by soil and debris cover and tectonic reworking typical for the Meliaticum, heavy equipment was used to expose the bedrock and establish the relationships between various components. Based on lithological study and biostratigraphy of radiolarians, the Čoltovo locality represents a chaotic mélange structure of Lower Jurassic clastic pelagic deposits and Middle Jurassic olistostromes with cherty intercalations and blocks of Middle Triassic limestones, terrigenous siltstones and variegated radiolarites, Upper Triassic red radiolarites, siltstones and basic volcanics.Geochemical analyses were undertaken on various types of sedimentary rocks. Majority of the samples fall within the fields of the pelagic environment affected by terrigenous input from the marginal continental area and a continental slope/rise environment, where they intercalate the radiolaritic shales. In the provenance discrimination diagram, most of the sample's plot fall in the fields of mature quartzose, i.e. continental terrigenous provenance. Basic volcanics plot in the border between the within-plate tholeiites with volcanic arc basalts and MORB. According to primitive mantle normalized REE patterns and other immobile trace elements, samples indicate a typical oceanic crust origin.Čoltovo locality records gradual spreading and deepening of the Meliata Ocean which was the NW branch of the Neotethys. First phase of deep-water sedimentation is represented by distal terrigenous clastics and radiolarite sedimentation with significant volcanic and tectonic activity during the Middle–Upper Triassic. The second, Jurassic phase, points to the formation of olistostromes in connection to the subduction process. In its current form the Meliatic complexes are fragmented into allochthonous blocks/sheets, comprising the Meliatic Basin and the northern continental margin, together incorporated into subduction–accretion mélanges. Acknowledgements: The research was supported by following projects: APVV-17-0170, APVV-21-0281 and VEGA 1/0435/21.

The Central Western Carpathians represent a tectonic system that extends eastward from the Alps and may be well correlated with the Austroalpine units of the Alps. The pre-Tertiary complexes of the Central Western Carpathians originated during the Cretaceous collisional events following the closure of the Meliata ocean by Late Jurassic times. Alpine metamorphism, related to the development of a metamorphic core complex during Cretaceous orogenic events, has been recognised in the Veporic unit (Janák et al. 2001). Increasing P-T conditions from greenschist to middle amphibolite facies reflect a coherent metamorphic field gradient. The isograds are roughly parallel to the north-east dipping foliation related to extensional updoming along low-angle normal faults, 40Ar/39Ar data constrain the timing of cooling and exhumation in the Late Cretacous. To constrain the timing of prograde metamorphism we dated monazite using electron microprobe. One reason for that is a shortage of geochronological data on Alpine metamorphism from the polymetamorphosed rocks which include pre-Alpine, mostly Variscan relicts. The dated monazite occurs in semi-pelitic schists of the highest grade Alpine metamorphic zone, exposed from the deepest levels of the Veporic dome. The peak metamorphic assemblage consists of garnet + biotite + muscovite + paragonite + rutile + quartz. Garnet occurs as clusters involving the fragments of fractured (pre -Alpine) garnet, and newly-formed, idioblastic Alpine garnet. Alpine garnet is zoned with decreasing Ca (XGrs = 0.23-0.16) and increasing Mg (XPrp = 0.05-0.12) from the core to the rim. Thermodynamic modelling indicates that Alpine garnet was growing during the burial (up to 1.2-1.4 GPa; 580-600°C). The metamorphic P-T-t path is “clockwise”, reflecting post-burial decompression (down to 0.8-0.1; 600-610°C) and cooling during Alpine orogenic cycle. Monazite (30-50 µm in size) occurs in the matrix occasionally associated with allanite (REE-epidote) and xenotime, suggesting monazite formation via allanite breakdown during the prograde P-T path. Chemical Th-U-Pb dating yielded Cretaceous age of 95.8 ± 2.7 Ma (MSWD = 1.13) with point ages ranging from 75 to 124 Ma. Similar age of 97 Ma was reported from the ICP-MS dating of monazite in a lower grade chloritoid-kyanite schists. This timing is in excellent agreement with metamorphism between c. 100 and 90 Ma in the Eo-Alpine HP/UHP belt in the Austroalpine Nappes of the Eastern Alps (Miladinova et al. 2022). We argue that the Veporic Unit represents direct continuation of the Eo-Alpine high-pressure belt into the West Carpathians. Janák, M., Plašienka, D., Frey, M., Cosca, M., Schmidt, S. T., Lupták, B. & Méres, Š. 2001. Cretaceous evolution of a metamorphic core complex, the Veporic unit, Western Carpathians (Slovakia): P-T conditions and in situ 40Ar/39Ar UV laser probe dating of metapelites. Journal of Metamorphic Geology, 19, 197-216. Miladinova, I., Froitzheim, N., Nagel, Th., Janák, M., Fonseca, R.O.C., Sprung, P. & Münker, C. 2022. Constraining the process of intracontinental subduction in the Austroalpine Nappes: Implications from petrology and Lu-Hf geochronology of eclogites. Journal of Metamorphic Geology 40, 423-456.

Volcanic rocks in the Pieniny Klippen Belt (PKB) of the Western Carpathians have been the focus of geologists for over a century (e.g. Uhlig, 1890; Małkowski, 1921). Miocene volcanism is most common in the PKB. However, there are infrequent occurrences of Cretaceous volcanic rocks. Several magmatic bodies of Cretaceous age have already been described in the PKB, including basalts at Hanigovce and Biała Woda, as well as peperites at Vršatec, and Velykyi Kamenets. The magmatic body in Vršatec occurs within the Upper Cretaceous marlstones of the Lalinok Formation, the age of which was previously determined to be younger than 100 Ma (Spišiak et al., 2011). Our new U-Pb zircon dating indicates the magmatic age to be ca. 80 Ma. This new age can be used as a benchmark for the forthcoming provenance studies of the surrounding clastic rocks in the PKB and the Outer Carpathians flysch.