Middle Miocene volcanic flare up preceding and synchronous with the Langhian/Serravallian sea‐level decline in the North Pannonian Basin: Insights from 40Ar/39Ar dating, geo‐seismic analysis and 3D visualization of the subterranean Kráľová stratovolcano

The Pannonian Basin System originated from the collision of the African and European tectonic plates, followed by the Miocene extensional collapse that led to the development of a back‐arc basins. Accurate dating is essential to comprehend the tectono‐volcanic evolution of the region, particularly in the under‐studied Danube Basin. Single‐grain 40Ar/39Ar dating has revealed that volcanic activity in the Danube Basin commenced around 14.1 million years ago, aligning with previous biostratigraphic and radioisotope data from nearby volcanic fields. The initial Middle Miocene pyroclastic deposits were generated by intermediate high K calc‐alkaline magmas, contributing significantly to the deposition of thick layers of fine vitric tuffs. The timing and chemistry of the volcanism are consistent with the Badenian rift phase in the Middle Miocene within the Carpathian–Pannonian region, suggesting an intraplate back‐arc volcanic environment. Three‐dimensional imaging has exposed the buried Kráľová stratovolcano, revealing its impressive scale with a thickness between 2620 and 5000 m and a base diameter of 18–30 km. Such dimensions place it among the ranks of the world's largest stratovolcanoes, indicating its substantial impact on the evolution of the Carpathian–Pannonian area. The complex formation history of the stratovolcano points to multiple phases of growth. Furthermore, the basin controlling Mojmírovce‐Rába fault's intersection with the stratovolcano implies that fault activity was subsequent to the volcanic activity, being younger than 14.1 million years. Regional age data consistently indicates that volcanic activity in the Danube Basin reached its zenith just prior to and during the lower/upper Badenian sea‐level fall (Langhian/Serravallian). K‐metasomatism is unique to the stratovolcanic structures and is not observed in the wider regional setting. This study supports the notion of an intricate, interconnected subterranean intrusive system within the stratovolcano, underscoring the complex interplay between geological structures and volcanic processes.


| INTRODUCTION
The origin of the Pannonian Basins system is intrinsically tied to the complex geodynamic history of the Mediterranean region (Horváth et al., 2006(Horváth et al., , 2015)).The Mediterranean basins have undergone several phases of opening, closure, and collision since the Late Palaeozoic (Robertson, 2012).By the end of the Mesozoic era, the convergence of the African and European plates resulted in the closure of the Tethys Ocean, leading to the Alpine orogeny (Csontos & Vörös, 2004;Schmid et al., 2008).The subduction and collision of Africa with Eurasia, especially by Adria plate, during Cretaceous to Neogene was followed by complex post-collisional tectonic processes which formed extensional basins (Dilek, 2006).As a result of this convergence and subsequent extensional tectonics, several back-arc basins, including the Pannonian Basins system, began to form during the Miocene (Horváth et al., 2006(Horváth et al., , 2015;;Tari et al., 1992Tari et al., , 2020)).The Pannonian Basins system, in particular, was formed as a result of the extensional collapse of the earlier thickened crust of the Alpine-Carpathian Arc (Matenco & Radivojević, 2012).This extensional tectonic regime allowed for the basin's subsidence and the subsequent accumulation of thick sedimentary sequences including pronounced volcanic complexes (Balázs et al., 2016;Csontos & Vörös, 2004;Harangi & Lenkey, 2007;Konečný et al., 2002;Lexa et al., 2010;Pécskay et al., 2006;Schmid et al., 2008;Seghedi & Downes, 2011;Tari et al., 1992).Although these continental basins are referred to as back-arc type, some authors have suggested that magmatism was post-collisional, and the proper arc-type volcanism is absent, thus the term back-arc in the Carpathian-Pannonian region (CPR) is inappropriate (Seghedi & Downes, 2011).It is worth noting that the Pannonian Basin's evolution, which includes the studied Danube Basin, is characterized by its transition from a marine to a brackish/freshwater environment during the Late Miocene, a feature that sets it apart from other Mediterranean basins (Balázs et al., 2018;Harzhauser & Mandic, 2008;Magyar et al., 1999).
Although during the subduction event the significant volcanism was not present due to the overall compressive stress field and the presence of a thick crust and lithosphere, at that time the mantle lithosphere was supplied with various subduction components (Harangi & Lenkey, 2007;Lexa et al., 2010;Pécskay et al., 2006;Seghedi et al., 2004;Seghedi & Downes, 2011).Magmas reached the surface at the beginning of subductionretreat back-arc extension connected with translation and rotation of ALCAPA and Tisa-Dacia microplates (Brlek et al., 2023;Harangi & Lenkey, 2007;Konečný et al., 2002;Lexa et al., 2010;Lukács et al., 2018;Pécskay et al., 1995Pécskay et al., , 2006;;Schmid et al., 2008;Seghedi et al., 2004;Seghedi & Downes, 2011;Zelenka et al., 2004).The formation of extensional basins took place over several tectonic periods, which were accompanied by different volcanic activity (Fodor et al., 2021;Harangi & Lenkey, 2007;Horváth et al., 2015;Lexa & Konečný, 1998;Lukács et al., 2018;Pécskay et al., 1995Pécskay et al., , 2006;;Seghedi et al., 2004;Seghedi & Downes, 2011;Šujan et al., 2021).Competition between the different tectonic processes at both local and regional scales caused variations in the associated magmatism, mainly as a result of extension and differences in the rheological properties and composition of the lithosphere.The decompressional mantle melting and further underplating by asthenosphere uprise as well as assisting crustal melting correlated with increased amount of extension.Firstly, mantle-derived magmas caused extensive melting in lower crust.This assimilation together with fractional crystallization resulted in highly explosive felsic calc-alkaline volcanism along weak shear zones between microplates during early Miocene.Later, during the Middle Miocene, asymmetric extension accompanied by the upraising of the asthenosphere resulted in shift of depocentres in the Pannnonian Basins systems (e.g.Balázs et al., 2016;Fodor et al., 2021;Šujan et al., 2021).In this time, continuously thinning continental plate resulted in decreasing role of the crustal component in magmas and lead to high to medium K-calc-alkaline volcanism varying from basaltic andesite to rhyolite, shoshonite and also alkaline K-trachyte were present (Harangi & Lenkey, 2007;Lexa et al., 2010;Lukács et al., 2018;Pécskay et al., 1995Pécskay et al., , 2006;;Seghedi et al., 2004;Seghedi & Downes, 2011).The migration of volcanism from north to south was the result of break-off progress along the slab in the same direction.
The primary objective of this article is to provide a visual representation of the buried stratovolcano's structure and the basin surrounding it, while also quantifying its dimensions.Furthermore, this research aims to complement existing data on Danube Basin volcanism in the CPR, building on previously published findings related to the pyroclastic and volcaniclastic fill (refer to Rybár et al., in preparation;Šarinová, Hudáčková, et al., 2021, see Table 2).This study introduces new age data regarding the onset of volcanic activity in the Slovak section of the Danube Basin.Such data are crucial to either corroborate or challenge prevailing theories about the expansion of the Danube Basin's accommodation space during the Middle Miocene epoch.By refining our understanding of the timing of volcanic activity in the CPR, we can glean deeper insights into the sequence of the Pannonian basin system's individual basins openings and fillings, shedding light on the region's tectonic evolution.

| Geologic setting
The Danube Basin (Little Hungarian Plain), as part of Pannonian Basin system (Figure 1a,b), is around 200 km long and 100 km wide with an approximate NE-SW orientation.The Leitha and Malé Karpaty mts.separated this basin from neighbouring Vienna Basin on the west (Figure 1c).The Central Eastern Alpine and Central Western Carpathians units form their southern and northern boundary, respectively.Transdanubian Range separates the Danube Basin from the Great Hungarian Plain at the southeastern margin, while northeastern margin consist of Central Slovakia Volcanic Field (CSVF) and the Börzsöny-Visegrád-Pilis-Burda Field (BVPBF; Lexa et al., 2010).
The samples were loaded into two 1.9-cm-diameter and 0.3 cm-depth Al discs that contain multiple smaller sample wells; all sample wells containing the separated crystals were surrounded by sample wells that carried the Fish Canyon sanidine neutron fluence monitor (28.294 [±0.13%]Ma; Jourdan & Renne, 2007;Renne et al., 2011).The sample discs were Cd-shielded (to minimize undesirable nuclear interference reactions) and irradiated for 40 h in the TRIGA reactor (Oregon State University, USA), in a central position.The J-value and mass discrimination factor are given in Data S1.The correction factors for interfering isotopes were ( 39 Ar/ 37 Ar) Ca = 6.95 × 10 −4 (± 1.3%), ( 36 Ar/ 37 Ar) Ca = 2.65 × 10 −4 (± 0.83%) measured on CaF 2 and ( 40 Ar/ 39 Ar) K = 7.02 × 10 −4 (± 12%) determined on K-Fe glass (Renne et al., 2013).Ar isotopic data are corrected for blank, mass discrimination and radioactive decay.Individual uncertainties are reported in Data S1 at the 1σ level unless otherwise indicated.
For each sample, a series of single crystals were fused in a single step using a continuous 100 W PhotonMachine© CO2 (IR, 10.6 μm) laser fired on the aliquot material for 60 s.All standard crystals were fused in a single step.The gas was purified in an extra low-volume stainless steel extraction line of 240 cm 3 , set up to run with two SAES AP10 and one GP50 getter.Ar isotopes were measured in static mode using a low-volume (600 cm 3 ) ARGUS VI mass spectrometer from Thermo Fisher© set with a permanent resolution of ca.200.Measurements were carried out in multi-collection mode using three Faraday cups equipped with three 10 12 ohm (masses 40; 38; 37) and one 10 13 ohm (mass 39) resistor amplifiers and a low background compact discrete dynode (CDD) ion counter to measure mass 36.We measured the relative abundance of each mass simultaneously during 10 cycles of peak-hopping and 16 s of integration time for each mass.Detectors were calibrated to each other through air shot beam signals.Blanks were analysed for every three to four incremental heating steps and typical 40 Ar blanks range from 1 × 10 −16 to 2 × 10 −16 mol.Mass discrimination was monitored using an automatic air pipette, and values are provided in Data S1 in per Dalton (atomic mass unit).
Criteria for the determination of a convergent age are as follows: (1) an age must include at least three consecutive single crystal ages agreeing at 95% confidence level and satisfying a probability of fit (p) of at least .05.Convergent ages are given at the 2σ level and are calculated using the mean of all the plateau steps, each weighted by the inverse variance of their individual analytical error.The raw data (Data S1) were processed using the ArArCALC software (Koppers, 2002), and the ages have been calculated using the decay constants recommended by Renne et al. (2011).All analytical parameters and relative abundance values are provided in Data S1 and have been corrected for blanks, mass discrimination and radioactive decay.Individual errors in Data S1 are given at the 1σ level.Convergent ages include uncertainties on the decay constants and standard age and were calculated using the Monte Carlo approach of Renne et al. (2010).

| Petrography and geochemistry
The analysed rocks in this study were obtained from archived point-cored wells provided by Nafta Ltd.Petrographic analysis of volcanic formations drilled by the KRAL-1 well was conducted using thin sections prepared from four cored intervals (depths 2938-2942 m; 2635-2638 m; 2597-2602 m; 2504-2508 m).An additional sample was taken from the Modrany-2 well (Figure 1c; MO-2), where a fine vitric tuff layer was found at a depth of 1702-1700 m (Vlček, Hudáčková, et al., 2020;Vlček, Šarinová, et al., 2020).The representation of individual components was obtained by counting more than 450 points per sample in a set step (Data S2).In the case of KRAL-1 samples, it was not possible to clearly distinguish the boundaries of lithoclasts and groundmass due to the same composition of lithoclasts and groundmass additionally obscured by compaction and alterations.From this title, the composition is expressed only in terms of crystals/pseudomorphs and 'pseudogroundmass'.
The thin sections were examined in detail using a polarizing microscope and a Cameca SX 100 microprobe (at the State Geological Institute of Dionýz Štúr) employing WDS analysis with an accelerating voltage of 15 keV, a probe current of 20 nA, and a beam diameter of 10 μm for silicate minerals and 5 μm for carbonates.Raw analyses were recalculated to weight percent of oxide using the ZAF correction (Data S2).Feldspar composition was calculated based on 8 oxygen, while mica was calculated based on 22 positive charges (following Rieder et al., 1998).Amphiboles were recalculated using Locock's (2014) Excel spreadsheet, with classification following Hawthorne et al. (2012).
The chemical composition of whole-rock analyses was provided by Bureau Veritas Mineral Laboratories (Canada, Vancouver).Samples were pulverized and processed by lithium borate fusion.Major elements were analysed by ICP-ES, trace elements by ICP-MS, and carbon and sulphur content were measured using LECO analysis.In addition to the KRAL-1 pyroclastics and MO-2 tuff, other samples from the Badenian sedimentary record of the Danube Basin were included (Figure 1c; Data S2; data from Kováč et al., 2018, Rybár et al., in preparation).The carbonate content was recalculated from total carbon (C tot ; Data S2).

| Geophysics
Basic well logs, including spontaneous potential and resistivity, were assessed based on the methods outlined by Rider and Kennedy (2011).Seismic sequence and facies interpretations were conducted in accordance with the works of Brown and Fisher (1980), Coe (2003) and Fischer and Veeken (2015).In the analysis of geo-sesmic lines, a time-depth conversion was required to interpret the derived surfaces.This crucial step was indispensable for estimating the total thickness of Kráľová stratovalocano (Data S3 and S4).

Kráľová-1 samples
The drilled volcaniclastic strata of KRAL-1 well can be divided to three intervals.Lowermost strongly altered pyroclastics are represented by sample from the depth 2938-2942 m (c.40; Figures 2 and 3j-l).These pyroclastics can be macroscopically described as coarse tuff to lapilli tuff (clasts up to 0.5 cm in diameter).Microscopically, the strong alteration and compaction erased the internal texture.Only crystal pseudomorphs filed by secondary minerals are recognizable (Figures 4c,g,h and 5; Data S2).Other possible components were merged to pseudogroundmass which consists of secondary K-feldspar (orthoclase molecule Or 98.5 ) with minor quartz, magnetite, rutile, illite and dolomite (Figures 4g,h and 5; Data S2).The pseudomorphs of pyroxene shape are small compared to feldspar but numerous (Figure 4c,h).Feldspar pseudomorphs indicate initial albitization (Ab 99.6 ) followed by Kfeldspar (Or 98.6 ) formation in cracks and rims (Figures 4g  and 5; Data S2).Some pseudomorph shows original biotite cleavability and shape highlighted by opaque minerals (Figure 4c).Juvenile apatite is present inside pseudomorphs.The veins that cut the samples are filled with carbonate or quartz.
Following interval represented by core samples from the depth of 2635-2638 and 2597-2602 m (c.34-c.33; Figure 3d-i) contains pyroclastic rocks with preserved juvenile plagioclase and biotite crystals.Thus this interval was selected for 40 Ar/ 39 Ar dating.These lapilli tuffs (clast diameter up to 0.5-1 cm) contains crystals and lithoclasts with porphyritic texture and a microlithic groundmass.In both cases, phenocrysts and crystals are represented by relatively fresh plagioclase, biotite and pseudomorphs after other minerals (Figure 4a,b,e,f).Plagioclases (An 66-49 ) do not display significant chemical zonality within crystal.Similarly, composition of crystals and microliths in the groundmass is the same (Figure 5; Data S2).In addition, groundmass also contains the K-feldspar microliths.Biotites are large (0.5 cm in diameter) and phlogopite in composition (average Mg# 0.63; Figure 5c; Data S2).There, plagioclase and apatite inclusions are typical and abundant.The green pseudomorphs of amphibole shape are filled with chlorite in the central parts and illite in the marginal parts (Figures 4e and 5e; Data S2).The other pseudomorphs are fulfilled by secondary quartz and carbonates, which also fill veins.The groundmass is silicified, and especially in green-coloured parts also chlorite pore fillings are present (Figure 4a).
The uppermost interval of the Kráľová statovolcano consists of altered volcanic sandstones to conglomerates (reworked volcanic material) and coarse to lapilli tuff.This interval is represented by core samples from the depth of 2508-2504 m (c.31; Figure 3b,c) and 2454-2451 m (c.30; Figure 3a).Only volcanic sandy conglomerates from the depth 2508-2504 m were studied in detail.These subangular to angular volcanic grains consist of crystals and dominantly porphyritic lithoclasts, often with an oriented feldspar microcrystal in groundmass.In this case, plagioclase phenocrysts, crystals and microliths are fully replaced by albite or an albite-kaolinite mix (Figures 4d and 5; Data S2).Biotite phenocrysts and crystals are slightly altered (chloritization, sagenitization), and their composition correspond to the biotite from the depth of 2635-2597 m (Figure 5c-e).Additionally, pseudomorphs filled with cryptocrystalline quartz, kaolinite and/or carbonates are present.The lithoclasts' groundmass except the albitized microliths also contains quartz and K-feldspar (sanidine; Figures 4d and 5; Data S2).Apart from this, dark-coloured grains, strongly altered mineral grains and glauconite pellets occur.In the more reworked parts non-volcanic admixture is present: rounded carbonate clasts, mono-and polycrystalline quartz, muscovite and gneiss.The tuffitic matrix between clasts is darker in colour and contains diagenetic calcite crystals.Pore spaces are filled or rimmed by quartz, chlorite and chalcedony.
Based on whole-rock analysis, these samples are plotted in the latite/trachyandesite field (Figure 6a-c; Data S2), and exhibit enrichment in LREE; La N /Yb n is 10.1 for lowermost interval and 15.8-16.2for dated interval (Figure 6e; Data S2).Eu anomaly varies between 0.777 and 0.701.When the total carbon (C tot ) is recalculated to account for calcite, the carbonate content varies from 0.9 (dated sample) to 9.8 wt% (lowermost sample; Data S2).It reflects variable carbonatization during diagenesis.

Modrany-2 fine tuff layer
The MO-2 tuff (c.11, 1700-1702 m) is characterized by fine vitric tuff composed of glass shards, pumice fragments, crystals of plagioclase, amphibole and biotite (Figures 3m,n and 7a-d, Data S2).The association is complemented by rare K-feldspar, quartz, apatite and fossils.If only clasts are calculated, glass shards and pumice constitute 85%.The dominance of glass shards documents their juvenile origin, similarly as majority of plagioclase, biotite and amphibole crystals.Glass inclusions within plagioclase support their pyroclastic origin (Figure 6b).Glass shards and pumice fragments are altered to secondary minerals.Plagioclase crystals are predominantly labradorite to bytownite in composition (An 51-77 , Figure 5, Data S2), although less basic (An 30-39 ) plagioclases are also present.Biotites have an annite composition, and amphiboles correspond to magnesio-hastingsite to pargasite (Figure 5c,f).Based on chemical composition, rare Kfeldspar (Or 95.8 ) can be interpreted as an admixture from surrounding non-volcanic sediments, probably together with less basic plagioclases and rare quartz.The presence of fossils and pellets indicates subaquatic deposition (Figure 7d).
Based on whole-rock analysis, MO-2 tuff is plotted in the trachyte or dacite field, and tuff exhibit enrichment in LREE; La N /Yb n is 12.5 (Figure 6a-c,e; Data S2).More felsic composition and more pronounced Eu anomaly (0.419; Data S2) are result of glass shard proportion.The amount of carbonate calculated from total carbon (6.7%) and higher strontium content can be linked with presence of fossils (Figure 7d; Data S2).

| 40 Ar/ 39 Ar results
As was mentioned above, dating of Kráľová stratovolcano least altered core sample with large biotite crystals was performed (depth 2597-2602 m).All 15 single-grain biotite analyses yielded similar ages, but only the 10 youngest ages were used for calculation.The resulting biotite converging age is 14.09 ± 0.15 Ma (MSWD = 1.76; p = .07),and the inverse isochron age is 14.13 ± 0.20 Ma, with an MSWD of 1.91 and a p-value of .05(Figure 8a,b, Data S1).
The fine vitric tuff sample from MO-2 well (depth 1700-1702 m) contains small proportions of crystals, where majority of plagioclase, amphibole and biotite is considered to be juvenile.Plagioclase and amphibole were used for 40 Ar/ 39 Ar dating of the MO-2 tuff sample.The low content of K in the dated minerals, combined with the small crystal size, leads to greater measurement variance.Of the 15 single-grain plagioclase measurements, the five oldest ages were omitted.The resulting plagioclase converging age is 14.28 ± 0.49 Ma (MSWD = 1.46; p = .16),and the inverse isochron age is 14.87 ± 0.65 Ma, with a p-value of 0.46 (Figure 8c,d, Data S1).Only the eight youngest amphibole singlegrain ages were used for calculation, with a converging age of 14.55 ± 2.25 Ma (MSWD = 0.52; p = .82)and an inverse isochron age of 14.52 ± 2.56 Ma (p = .67;Figure 8e,f, Data S1).The low content of radiogenic 40 Ar in the amphiboles contributed to a larger deviation in the amphibole ages.
3.4 | Geo-seismic sequence and facies interpretation Seismic facies are critical in geological studies as they reveal valuable information about the subsurface geology (Figure 9).

| Observations
Seismic Sequence 0 (Sse0) predominantly aligns with the seismic facies Sf1.The defining features of Sse0 include an erosional truncation at its upper boundary, with its lower boundary remaining ambiguous.Within Sf1, the reflections are characterized by sub-parallel and wholly discontinuous patterns, coupled with low amplitudes and medium to broad frequencies.Moreover, within the Sf1 facies, conspicuous high-amplitude continuous reflections emerge.
Seismic Sequence 1 (Sse1).The upper boundary of Sse1 is delineated by an erosional truncation, while its lower boundary is characterized by downlaps.Sse1 encompasses two distinct seismic facies, Sf2 and Sf3.Within this sequence, Sf2 is marked by reflections that range from parallel/sub-parallel to sigmoid and oblique clinoform configurations.These reflections display variability in continuity, shifting between continuous and discontinuous patterns.Coupled with low-amplitude and mediumto-broad frequency characteristics.
In contrast, Sf3 is characterized by sub-parallel, discontinuous reflections, which are commonly of very high amplitude.This facies exhibits a medium-to-broad frequency spectrum (Figure 2).
Seismic Sequence 2 (Sse2) consists of two seismic facies, Sf4 and Sf5, which gradually transition into each other.Sse2 is defined by erosional truncation at the upper boundary and downlaps at the lower boundary.The reflection configuration of Sf4 facies varies from parallel to sub-parallel and features a sigmoid clinoform shape.Reflection continuity transitions from continuous (Sf4) to discontinuous (Sf5), and amplitude shifts from high to medium (Sf4) before becoming low (Sf5).The frequency spans from broad to medium.
Seismic Sequence 3 (Sse3) comprises two seismic facies, Sf6 and Sf7, stacked on top of one another.Sse3 is clearly defined by toplaps at the upper boundary and onlaps at the lower boundary.The reflection configuration of these facies transitions from parallel (Sf6) to sigmoid clinoform (Sf7) at the top.Reflections are continuous, with medium to low amplitude and medium to narrow frequency.
Seismic Sequence 4 (Sse4) consists of three seismic facies: Sf8, Sf9 and Sf10.The upper boundary of Sse4 is distinctly defined by the recent surface, while the lower boundary is marked by onlaps.Sf8 features a parallel configuration with continuous reflections, medium to very high amplitudes and medium frequency.The Sf9 is characterized by a sigmoid oblique clinoform with continuous reflections, medium to high amplitudes and medium frequency.Seismic facies Sf10 features continuous to discontinuous, parallel to sub-parallel reflections, with medium to low amplitude and medium to narrow frequency.This facies is occasionally disrupted by U-shaped features.

| Interpretations
Seismic Sequence 0 (Sse0) aligns with Sf1.Within Sf1, the reflections are characterized by sub-parallel and discontinuous patterns, coupled with low amplitudes and medium to broad frequencies.These characteristics mirror the lithological attributes observed in the region, substantiated by data from nearby wells (e.g.Biela, 1978;Fusán et al., 1987;Rybár & Kotulová, 2023;Šarinová et al., 2018;Vlček, Šarinová, et al., 2020).The lithological association for Sf1 points to granitoid intrusive rocks, notably within the Tatricum and Veporicum units, as delineated by Hók et al. (2014).These rocks trace their origins back to the Palaeozoic era (Kohút and Larionov, 2021).Moreover, Sf1 includes conspicuous high-amplitude continuous reflections which based on established literature (Magee et al., 2013;Morley, 2018;Planke et al., 2000;Smallwood & Maresh, 2002), are posited as indicators of sills, dikes, and potential laccolith complexes.The occasional saucershape of the sills reinforces this interpretation (Bischoff et al., 2019(Bischoff et al., , 2020)).These features either align with or conclude at major normal faults (Figures 10a,b and 11a,b).The salient reflectivity of the sills is attributed to their intrinsic high acoustic and elastic properties, as noted by Smallwood and Maresh (2002).Mapped sills exhibit lengths spanning from 1 to 5 km, it's deduced that a complex network of interconnected sills, complemented by short dikes and conduits, fueled the stratovolcano (Figures 10a,b and 11a,b).This interpretation aligns seamlessly with the perspective proffered by Morley (2018), which challenges the traditional notion of a solitary feeding conduit.Within the Pannonian Basin system similar sill features have been previously document by, for example Petrik et al. (2018).
Seismic Sequence 1 (Sse1) The upper boundary of Sse1 is delineated by an erosional truncation, while its lower boundary is characterized by downlaps.Sse1 encompasses Sf2 and Sf3.Sf2 is marked by reflections that range from parallel/sub-parallel to sigmoid and oblique clinoform configurations.These reflections display variability in continuity, shifting between continuous and discontinuous patterns.Coupled with low-amplitude and mediumto-broad frequency characteristics, Sf2 is interpreted as representing andesites, pyroclastics, deltaic, and possibly alluvial volcanic conglomerates originating from the stratovolcano.It should be noted that this interpretation is made in the absence of direct evidence from well records.Sf3 is characterized by sub-parallel, discontinuous reflections, which are commonly of very high amplitude.This facies exhibits a medium-to-broad frequency spectrum and is interpreted as being composed of andesites-as evidenced by the SP and RT well logs (refer to Figure 2)-and pyroclastics, the latter being verified by well core samples.These characteristics correlate with the andesitic and pyroclastic deposits associated with the Kráľová stratovolcano.Supporting these observations, Magee et al. (2013) posited that high-amplitude seismic reflections enveloping the basal flanks of volcanoes are indicative of alternating extrusive and clastic successions.This implies intermittent magma supply to the volcanic system.Furthermore, some of the pronounced amplitudes within Sf2 and Sf3 can also be interpreted as manifestations of intrusions, likely comprising sills, dikes and potential laccolith formations, consistent with findings from Planke et al. (2000), Smallwood andMaresh (2002), andMorley (2018).
Seismic Sequence 2 (Sse2) consists of two facies, Sf4 and Sf5.Sse2 is defined by erosional truncation at the upper boundary and downlaps at the lower boundary.The reflection configuration of Sf4 facies varies from parallel to sub-parallel and features a sigmoid clinoform shape.Reflection continuity transitions from continuous (Sf4) to discontinuous (Sf5), and amplitude shifts from high to medium (Sf4) before becoming low (Sf5).The frequency spans from broad to medium.Both facies (Sf4 and Sf5) are interpreted as volcanic sandstone-mudstone of the Pozba Formation.As they directly overlie the Kráľová structure and reflect its slope, Sf4 and Sf5 are considered to have been deposited in a coastal plain environment that extended from the inner to mid-volcanic shelf.The clinostratification of Sf4 may suggest a prodelta slope setting.gradually fill the basin and likely document the culmination of transgressive conditions (probably HST; Figures 9,  10a,b and 11a,b).These Sarmatian clinoforms have also been documented in other parts of the basin (Rybár & Kotulová, 2023;Šarinová et al., 2018).As a result, these facies are interpreted as sandstones-mudstones of the Vráble Formation, deposited in a mid-shelf to shelf-break slope environment.
Seismic Sequence 4 (Sse4) consists of three seismic facies: Sf8, Sf9 and Sf10.The upper boundary of Sse4 is defined by the recent surface, the lower boundary is marked by onlaps, indicating flooding related to early Lake Pannon.Sf8 features a parallel configuration with continuous reflections, medium to very high amplitudes and medium frequency.The continuous reflections and very high amplitudes suggest fine-grained sandstone alternating with mudstone of the Ivanka Formation, deposited in a deep basin environment accompanied by subaqueous sediment density flows.What is in line with the study of Sztanó et al. (2016).
Sf9 is characterized by a sigmoid oblique clinoform with continuous reflections, medium to high amplitudes and medium frequency.This facies is interpreted as finegrained sandstone and mudstone of the Ivanka Formation, deposited in a shelf-break slope environment, as supported by the sigmoid reflection arrangement.This progradation of the Lake Pannon clinoform has been described by, for example Magyar et al. (2013), Sztanó et al. (2016), and Šujan et al. (2021).Sf10 features continuous to discontinuous, parallel to sub-parallel reflections, with medium to low amplitude and medium to narrow frequency.This facies is occasionally disrupted by U-shaped features associated with channel-belt bodies.Generally, the facies is interpreted as mudstone with rare sandstones and conglomerates of the Volkovce and Beladice formations.These sediments were deposited in a delta plain to alluvial plain environment, as defined by Sztanó et al. (2016).

Danube Basin
The obtained biotite converging age of 14.09 ± 0.15 Ma from the KRAL-1 well and the plagioclase converging age of 14.28 ± 0.49 Ma from the MO-2 fine tuff represent the oldest acquired ages from the buried pyroclastics in the Danube Basin, except for the original K/Ar wholerock data (Table 2; Kantor, 1987).However, this K/Ar age does not align with biostratigraphic data, as the first volcanic and pyroclastic admixture in the Neogene fill of the Danube Basin appears alongside the fossil assemblage of the NN5 Zone (Kováč et al., 2018;Rybár et al., 2015Rybár et al., , 2016;;Vlček, Hudáčková, et al., 2020).Therefore, based on biostratigraphy, the volcanism is younger than ca.15 Ma (e.g.Agnini et al., 2017).Despite the significant error in the MO-2 fine tuff, the obtained 40 Ar/ 39 Ar age data are consistent with biostratigraphic ranking of volcaniclastic samples (Figure 12; Csibri et al., 2018;Holcová et al., 2019;Kováč et al., 2018;Rybár et al., 2015Rybár et al., , 2016;;Šarinová, Hudáčková, et al., 2021;Vlček, Hudáčková, et al., 2020;Zlinská, 2016).The reason that fine tuffs do not provide sufficiently accurate 40 Ar/ 39 Ar single-grain data are due to the absence of stable minerals with high K content (sanidine) and the small crystal size of the measured minerals with low K content (plagioclase and amphibole; Data S1).Moreover, some measured minerals in the MO-2 fine tuff do not lie on the inverse isochrones (Figure 8d,f), and they likely correspond to crystals assimilated from the basement or surrounding rocks.Considering the similar, fine vitric tuff within the NN5 Zone in the Blatné subbasin, the biotite from the Trakovice-4 fine tuff also did not provide accurate age data (Table 2, Figure 12; Rybár et al., 2016;Šarinová, Hudáčková, et al., 2021).Other tuff layers are strongly altered (zeolitization, albitization, replacement of mafic minerals) and therefore could not be dated.Additionally, the obtained time interval of Danube Basin volcanic activity agrees with neighbouring CSVF and BVPBF (Chernyshev et al., 2013;Karátson et al., 2000Karátson et al., , 2007)).
The younger and shortened age intervals are a result of improvements in dating methods and the equipment.These changes are typical for the entire CPR (e.g.Chernyshev et al., 2013;Lukács et al., 2018Lukács et al., , 2021Lukács et al., , 2022;;Šarinová, Hudáčková, et al., 2021;Šarinová, Rybár, et al., 2021).The new results conclusively confirm that calc-alkaline volcanic activity in the Danube Basin was younger than originally indicated by K/Ar data (Kantor, 1987).In light of this, it should be noted that existing review papers on the topic of volcanism in the Carpathian-Pannonian region (e.g.Lexa et al., 2010;Pécskay et al., 2006) and regional basin lithostratigraphy (Šurany Fm. after Vass, 2002) must be updated.

| Mineral and chemical composition of calc-alkaline pyroclastics
Although all analysed pyroclastic rocks from Kráľová stratovolcano are affected by compaction and alterations, the macroscopic and microscopic features indicated, that primary composition of studied samples was homogenous, without fossils and accidental non-volcanic clasts.The latter mentioned, as well as clasts from previous eruptions are present mainly in tuffs with glass shards/pumice component as dated MO-2/11 or NV-1/41 samples.In this case, accidental clasts admixture is result not only from explosions but also depositions in a shallow sea.Although samples with the highest abundance of pyroclastic material were selected for whole-rock analyses, the results may be affected by alteration as well as admixture of accidental clasts.The number of non-volcanic accidental clasts is estimated to be at the least 5% (Data S2), and therefore we consider the results to be applicable for general classification.To minimize misinterpretations only non-mobile elements were used for interpretations (Figure 6a-c; Hastie et al., 2007;Pearce, 1983Pearce, , 1996)).However, also trace element diagrams can be impacted by the mobility of some 'immobile' elements during the formation of bentonite or zeolite minerals (Christidis, 1998;Namayandeh et al., 2020).For example, the mobility of Y can cause a shift of vitric tuff samples towards a more alkaline composition in the diagram modified by Pearce (1996).Nevertheless, classification diagrams based on major oxides (Le Bas et al., 1986) and various trace elements display similar results (Figure 6d; Hastie et al., 2007;Pearce, 1983Pearce, , 1996)).Yet, the Na 2 O/K 2 O ratio is affected by alteration well expressed by K-metasomatized sample from KRAL-1 well, core 40.With respect to the character of existing samples, the first stage of magmatism in this region was build up by High K calc-alkaline latite to andesite, transitioning upward to basaltic andesite in the Želiezovce sub-basin (Figure 6a-d).The plotting of fine vitric tuff to a more felsic, trachyte field compared to coarse and lapilli tuff (Figure 6a-d) reflects the presence of a high amount of glass shards in the composition together with loss of crystals during transport.This is supported by pronounced Eu anomaly (0.412-0.499) and Ti depletion compared with lapilli and coarse tuff in similar stratigraphic positions (Eu* = 0.701-1.194;Figure 6e,f, Data S2).Decreasing role of the crustal component upward is also represented by the decreasing proportion of LILE, including LREE from beginning of volcanic activity upward (Figure 6e,f; Data S2).

| K-metasomatism and depositional environment of Kráľová stratovolcano
The Kráľová volcanic centre is built by pyroclastics as well as lava flows distinguished based on the RT log (Gaža, 1966), thus it is interpreted as stratovolcano, similar as Šurany stratovolcano (Konečný & Konečný, 2017;Mihaliková, 1962).In the Danube Basin, pyroclastic rocks as well as reworked volcanic material were deposited dominantly in marine environment (e.g.Csibri et al., 2018;Holcová et al., 2019;Kováč et al., 2018;Rybár et al., 2015Rybár et al., , 2016;;Šarinová et al., 2018;Vass, 2002; this study).However, based on the geo-seismic lines (Figures 10a,b and  11a,b), the Kráľová stratovolcano created positive structure, and erosion continued up to the onset of Pannonian shelf-break slope strata.Consequently, the stratovolcano acted as a barrier in the progradation of these clinoforms (Figures 10a,b and 11a,b;Hrušecký, 1999).If the sea-level drop at the lower/upper Badenian boundary (Langhian/ Serravallian, 13.82 Ma; e.g. de Leeuw et al., 2010;John et al., 2011;Mandic et al., 2019;Raffi et al., 2020) is considered, it suggests a high likelihood of terrestrial weathering of the stratovolcano.It should be noted that the dated part of the KRAL-1 well pyroclastics (depth 2638-2597 m) mainly exhibits silicification, and the presence of K in fluids is documented only by the formation of illite in pseudomorphs after dark minerals and the good preservation of biotite crystals.Furthermore, this part of the stratovolcano contains plagioclase phenocrysts, crystals and microliths without significant albitization.Thus it can be concluded, that this part of the stratovolcano was deposited in terrestrial setting.On the other hand, the Fechlorite which is present here as a secondary mineral is generally typical for marginal marine sediment diagenesis (Worden et al., 2020).It can be explained by the later flooding of the stratovolcano.The marine conditions during sedimentation in the uppermost part of the statovolcano (depth 2508-2504 m) are evidenced by the presence of glauconite grains in volcanic sandstones.Additionally, in this part significant albitization of plagioclases is repeatedly observed (Figure 5, Data S2).This can be explained by the upper Badenian (Serravallian) sea-level rise, along with the second rift phase, which opened the Rišňovce and Komjatice sub-basins (e.g.Hók et al., 2016;Šarinová et al., 2018;Šujan et al., 2021).
All analysed volcaniclastic samples from the Middle Miocene interval of the Danube Basin (Data S2; Kováč et al., 2018;Rybár et al., in preparation;Šarinová, Hudáčková, et al., 2021) display some degree of feldspar albitization, mafic mineral decay, chloritization, carbonatization and/or glass shard replacement by zeolite or smectite.However, only the lower part of the Kráľová volcanic centre (depth 2942-2938 m) is clearly impacted by K-metasomatism (K 2 O 9.39 wt%; Data S2).The Kmetasomatism is typical for regional crustal extension along normal faults (see Ennis et al., 2000, and references therein).These conditions are met by the opening of the Danube Basin as a back-arc basin type formed within the continental crust.Apart from high K 2 O content, K-metasomatism is also supported by higher values of Rb, As, Cu, Ni and depletion in Na 2 O and Sr (Data S2).
Although K-rich minerals should not be significantly affected by K-metasomatism (e.g.Ennis et al., 2000), only biotite pseudomorphs were observed in this part of the stratovolcano.This could be explained by mineral alteration/weathering before K-metasomatism.This can be confirmed by studying plagioclase crystals, which point to albitization preceding K-metasomatism (Figure 4g).During K-metasomatism, decay products of the original mafic minerals (probably pyroxene and biotite) and the groundmass were replaced by a mix of Fe-Ti oxides, illite, quartz and mostly K-feldspar.The final mineral phase is idiomorphic Fe-rich dolomite, which corresponds to burial diagenesis.
Generally, the source of K in fluids can be variable: magmatic, metamorphic or alkaline/saline water in hydrologically closed systems (see Ennis et al., 2000, and references therein).The higher K 2 O content is localized only in the lower part of the KRAL-1 well, which suggest local (hydrothermal) K-metasomatism, particularly due to a hydrologically closed system is not expected during this period (before 14.1 Ma; Kováč et al., 2018;Rybár et al., 2015Rybár et al., , 2016;;Vlček, Šarinová, et al., 2020).Additionally, surrounding sediments of the Danube Basin exhibit only weak K-addition during diagenesis.Hoch et al. (1999) documented that weathering and decay of K-rich volcanic glass in volcanic groundmass can produce fluids enriched in K and Si.This system can also function during hydrothermal circulation.

| Geophysical analysis and comparative study of the Kráľová stratovolcano: dimensions, seismic data interpretation and global comparisons
The Kráľová stratovolcano, as elucidated by the geophysical data (Figure 13a-c), is deduced to have a thickness of approximately 5000 m, and its diameter at the base spans between 18 and 30 km (time-depth conversion based on Figure 13b, Data S3 and S4).In comparison, the nearby Štiavnica stratovolcano during its 1st stage of development is believed to have attained an elevation of 3000-4000 m with a diameter of 35-40 km at its base (Chernyshev et al., 2013).Meanwhile, the submerged Pásztori volcano, situated southeast of the primary study area in the same basin, is postulated to reach a thickness of 2000 m (Pánisová et al., 2018).To provide a more comprehensive perspective, the Kráľová volcano was compared to some renowned stratovolcanoes around the globe and their measurements: Mount St. Helens, Washington, USA: Known for its monumental eruption in 1980, this volcano has an elevation of approximately 2550 m.It measures around 8 km in width at its base and has a crater diameter of about 2 km formed after the 1980 eruption (Clynne et al., 2005).Mount Rainier, Washington, USA: Dominating the horizon with an elevation of 4392 m, its base diameter extends to about 20 km.Its rich eruptive history includes frequent eruptions over the past 2600 years (Sisson & Vallance, 2009).Mount Fuji, Japan: An emblematic symbol of Japan, Mount Fuji stands at an elevation of 3776 m and has a broad base diameter of roughly 40 km.Eruptive records indicate activity going back about 11,000 years (Miyaji & Koyama, 2003).Mount Cotopaxi, Ecuador: Among the highest active volcanoes in the world, Cotopaxi towers at an elevation of 5897 m.Its base diameter measures approximately 23 km (Mothes et al., 2004).Mount Shasta, California, USA: soaring to an elevation of 4322 m, its base diameter is about 17 km (Sisson & Grove, 1993).
Consequently, the dimensions of the Kráľová stratovolcano remain within a comparable spectrum.Nonetheless the process of deriving precise dimensions for the Kráľová stratovolcano from the geophysical data has been challenging, partly due to variations in seismic velocities.In the examined seismic lines (MXS3_93, MXS7_93), the seismic velocities within the volcanic interval (comprising alternations of andesites and tuffs) are not explicitly determined.However, in an adjacent line (557a_81), these velocities average around 3500 m/s.This estimation seems reasonable when aligned with Press's (1966) findings, which indicate P-wave velocities in andesites typically reach 5230 m/s, while tuffs have velocities around 2160 m/s.It is crucial to acknowledge that these velocities can be influenced by various factors, including the rock's composition, porosity and degree of alteration (Toksöz et al., 1976).
Consequently, the uncertainty in seismic velocity measurements, estimated at 47.56%, translates to a plus-minus error of ±2378 m in determining the thickness of the stratovolcano.Therefore, the actual thickness of the volcano could deviate from the estimated range of 5000 ± 2378 m.Given that thicknesses between 5000 and 7378 m are deemed unrealistic, a more plausible thickness for the stratovolcano lies somewhere between 2622 and 5000 m.
Additionally, the possibility that a fault blow the stratovolcano was active and experiencing slip during the volcanic period cannot be disregarded.This suggests that the 'active' height of the volcano, measured from the top to the base in the fault's hanging wall, might have been less than the estimated 5000 ± 2378 m.
Further insight into the volcano's dimensions can be gained through a simple model, examining parameters like the base diameter (ranging from min.18, average 24, to max. 30 km) and the volcano's thickness (considering the minimum estimated value of 2622 m, the average estimated value of 5000 m, and an intermediate value of 3811 m).The maximum thickness of 7378 m is excluded from this model, as it represents an unrealistic value.By evaluating these parameters, we can estimate the slope angles and volume of the volcano.The boundary condition data are derived from the measured stratovolcano volume (measured at 762.045 km 3 from visualizations in Figure 13a,c) and the slope angle (measured directly from the examined geo-seismic lines as ca.12° under 1:1 elevation conditions).The parameters that best match the data suggest a base diameter of 30 km and a thickness of 2622 m, yielding a slope angle of 9.92° and a volume of 617.79 km 3 , both roughly aligning with the measured values.
In conclusion, the overall height of the volcano was likely closer to 2622 m rather than 5000 m.It is also noteworthy that while stratovolcanoes typically reach slope angles of 29 to 27° (Karátson et al., 2010), the calculated slope of 9.92° indicates pronounced effects of compaction and erosion.
Despite all the given limitation the provided dimension is the best that can be derivate using the available data and methods.Thus the Kráľová stratovolcano formed major volcanic structure which must have hade pronounced effect on the evolution within the CPR and vicinity.
Additionally, the Kráľová stratovolcano is intersected by a normal fault, identified as the Mojmírovce-Rába fault system (Mfs;Figures 1c, 10a,b and 11a,b;Hrušecký, 1999).For instance, the Komjatice sub-basin opened along this fault system (Hók et al., 2016;Šarinová et al., 2018).Given that the Kráľová stratovolcano was in place before any activity on the Mfs fault affected its structure, the presented date of 14.09 ± 0.15 Ma must have preceded the opening of the Komjatice sub-basin.The Sarmatian syn-tectonic activity of Mfs is supported through indirect dating of rhyolite clastics (with an age around 12 Ma), found near the base of the Komjatice sub-basin fill (in the Vráble-1 well; Chernyshev et al., 2013;Lexa & Pécskay, 2010;Šarinová et al., 2018)

| CONCLUSIONS
Despite its size, the Danube Basin remains underexplored, presenting challenges in data acquisition and sampling.However, the importance of the presented findings cannot be overstated as they provide precise determinations of the tectono-volcanic context of the basin.
The 40 Ar/ 39 Ar single-grain dating and 3D visualization of the buried Kráľová stratovolcano have established that volcanic activity in the Danube Basin and its vicinity initiated around 14.1 million years ago.This timeline is consistent with existing biostratigraphic records of basin sedimentation and radioisotopic data from surrounding volcanic fields.The data indicate that peak volcanic activity in the northwest Pannonian Basin occurred just prior to and during the sea-level decline in the lower/upper Badenian (Langhian/Serravallian) age.While volcanic activity was diminishing by 13.8 million years ago in the central basin, it persisted into later periods in the northeastern sub-basins (Komjatice and Želiezovce), in line with the continued growth of the neighbouring Štiavnica stratovolcano.The nature of the early Middle Miocene volcanism in the Danube Basin is typified by intermediate High K calc-alkaline magmas, which likely gave rise to several meters thick fine vitric tuff layers, indicating significant eruptive events.Despite the tectonic setting, we observe that only hydrothermal K-metasomatism is present, which is in harmony with the broader volcanic trends of the Carpathian-Pannonian region.
The Kráľová stratovolcano is bisected by the basin controlling Mojmírovce-Rába fault, which contributed to subsequent rift phases in the Danube Basin.Presented research confirms that tectonic activity along this fault occurred after 14.1 million years ago.
The Kráľová stratovolcano's scale, with a thickness of between 2622 and 5000 m, supplements the nearby formations like the Pásztori volcano.Its base, spanning 18-30 km in diameter, rivals the Štiavnica stratovolcano's first developmental stage and stands alongside global giants such as Mount St. Helens, Mount Rainier, and Mount Fuji.While its size is at the comparative spectrum, discrepancies in dimensions could be attributed to variance in seismic velocity measurements.
Within Seismic facies we identify distinct highamplitude reflections that can be associated with geological features such as sills, dikes and potential laccoliths.These features support the notion of a sophisticated and extensive subsurface network that fuelled the stratovolcano's activity.

ACKNO WLE DGE MENTS
This study was supported by The Ministry of Education, Science, Research and Sport of the Slovak Republic: VEGA-1/0526/21, and by the Slovak Research and development agency under the contracts No. APVV-20-0120, APVV-16-0121.Our gratitude also goes to Nafta ltd.management, for allowing sampling campaigns in their repository.Our thanks also go to Schlumberger ltd.for the Petrel software donation.We express our heartfelt thanks to the editor, Dr. Craig Magee, and to our reviewers, including Dr Dougal Jerram and those who have chosen to remain anonymous, for their perceptive feedback and constructive recommendations that have substantially improved this manuscript.Contributions of the Authors: Samuel Rybár and Ľubomír Sliva were responsible for the interpretation and visualization of the geo-seismic data.Katarína Šarinová conducted the petrography and geochemistry research.Fred Jourdan and Celia Mayers took charge of the mineral separation and dating using the 40 Ar/ 39 Ar method.
Additional data related to this manuscript, including detailed data sets and analyses, will be made available upon reasonable request to the corresponding author.The request process is intended to ensure the responsible sharing of sensitive data in accordance with ethical and privacy guidelines.Please note that some restrictions may apply due to the nature of the data or agreements with third-party data providers.

Highlights•
Danube Basin's volcanism start dated to 14.1 million years ago.• Peak activity aligned with Badenian sea-level change.• Kráľová stratovolcano, a giant at 2660-5000 m thick.• Tectonic activity post-dates 14.1 million years.• Evidence of extensive underground volcanic structures.

F
I G U R E 1 (a) Location of the Pannonian Basin System in Europe; (b) Position of the Danube Basin within the Pannonian Basin System.(c) Danube Basin with well locations and volcanic fields adapted from Well table of the Kráľová-1.

F
Classification of studied pyroclastics based on whole-rock chemistry: (a) Zr/Ti versus Nb/Y classification of volcanic rocks for altered rocks(Pearce, 1996); (b) Co versus Th classification of altered volcanic rocks(Hastie et al., 2007); (c) The Th/Yb versus Ta/ Yb Pearce (1983) diagram supporting observed trend = decreasing alkalinity upward.(d) Comparison results with TAS diagram (Le Bas et al., 1986).Plotting of analysed samples in this diagram should be regarded with caution due to the high content of volatile elements (see LOI and CO 2 in Data S2), and due to observed alteration (albitization; K-feldspatitization; silicification).It is visible in KRAL-1 Kmetasomatized sample (core 40).(e) Chondrite-normalized REE patterns (normalization values after Sun & McDonough, 1989).(f) Upper continental crust-normalized trace elements (McLennan, 2001); for primitive mantle-normalized (Sun & McDonough, 1989) trace elements pattern see Data S2.F I G U R E 7 Thin sections from Modrany-2 well, core 11: (a) plane polarized light; (b-d) BSE; Abbreviations: see Figure 4.

F
Uninterpreted (a) and interpreted (b) reflection seismic profiles through the Gabčíkovo-Győr sub-basin, based on line MXS3-93, with the projected Kráľová-1 well (red line represents the erosional surface).Refer to Figure 1 for the location.

Seismic
Sequence 3 (Sse3) comprises of stacked, Sf6 and Sf7.Sse3 is defined by toplaps at the upper boundary and onlaps at the lower boundary.The reflection configuration of these facies transitions from parallel (Sf6) to sigmoid clinoform (Sf7) at the top.Reflections are continuous, with medium to low amplitude, and medium to narrow frequency.The onlap of Sf6 onto the underlying facies suggests transgressive conditions (most likely TST), transitioning into prograding clinoforms Sf7, which F I G U R E 1 1 Uninterpreted (a) and interpreted (b) reflection seismic profiles through the Gabčíkovo-Győr sub-basin, based on line MXS7-93, with the projected Kráľová-1 well.Refer to Figure 1 for the location.

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I G U R E 1 2 Compilation of all age data obtained from volcanic materials in the Danube Basin (Data from Rybár et al., in preparation; Šarinová, Hudáčková, et al., 2021; this study).

F
Map of the Danube Basin, showcasing the top pre-Cenozoic basement, with an overlay of the top Kráľová stratovolcano, which is marked in red.The map is divided into three sections: (a) presenting data in the Two-Way Time (TWT) realm in milliseconds, (b) displaying a time-depth conversion model derived from check shots from the Diakovce-1 well, and (c) illustrating data in the True Vertical Depth (TVD) realm in meters.
. The timing of the sub-basin opening is also evidenced by the thickening of the Sarmatian interval towards the Mfs (Figures 10a,b and 11a,b).The cut through the volcanic centre is visible on the presented pre-Cenozoic basement 3D visualization of the Danube basin, overlain by the top of the Kráľová stratovolcano (Figure 13a,c).

40 Ar/ 39 Ar dating
Radioisotopic age data from buried pyroclastic rocks and volcanic sandstones of Danube Basin.