Recognizing salt‐related structures and differentiating them from tectonic shortening‐related anticlines and synclines can be a very challenging task in salt‐bearing fold‐and‐thrust belts, especially ...in poor outcrop conditions. In this study, we explain several diagnostic structural and sedimentary features that may be used to distinguish pre‐orogenic halokinetic structures from shortening‐related structures on the example of the Silica Nappe (Inner Western Carpathians, Central Europe). Detailed structural mapping in this area resulted in the recognition of several pre‐orogenic salt‐related structures, including linear salt walls and minibasins. Initial evaporite movement started as early as the late Early Triassic, and widespread diapirism occurred during the Middle to Late Triassic, ultimately leading to facies differentiation, with carbonate platform growth in the subsiding minibasins and reduced basinal deposition on top of the diapirs. Later, the inherited salt structures localized the deformation during the Cretaceous Alpine orogeny, and exerted a strong control on the geometry and kinematics of the subsequent deformations. Our new interpretation explains previously unsolved structural problems in the Silica Nappe, like pre‐orogenic thickness variations during the post‐rift phase, frequent young‐on‐older type thrust contacts and multiple folding directions with variable vergencies. The results point out that the Silica Nappe is a fold‐and‐thrust belt, where pre‐orogenic salt tectonics and its effects on the fold‐and‐thrust belt evolution can be studied in detail.
Key Points
Structural mapping resulted in the first recognition of pre‐orogenic salt structures in the Western Carpathians
Triassic syn‐sedimentary evaporite deformation led to minibasin formation, variations in formation thickness and facies differentiation
The results provide guidelines on how to distinguish inherited salt structures from shortening‐related structures in fold‐and‐thrust belts
In the Transdanubian Range (Hungary), a wide spectrum of Triassic dolomites is known. Mechanism of dolomitization of the platform carbonate successions was subject of a number of studies but the ...study of dolomitization of basinal carbonates is very limited. Petrographical and isotope–geochemical characteristics of the Upper Triassic dolomitized carbonate deposits, formed in a fault-controlled intraplatform basin, and interpretation of the dolomite-forming processes are presented in the current paper. From the latest Carnian to Middle Norian under semi-arid climatic conditions density-driven flux of seawater derived mesohaline fluids was the dominant mechanism of the near-surface pervasive dolomitization of the thick platform carbonate succession. In the late Middle Norian incipient rifting of the Alpine Tethys led to establishment of an extensional structural regime and onset of the formation of the Kössen Basin. In the study area, above the dolomitized platform carbonate succession, platform-derived carbonate sediments were accumulated in a fault-bounded, semi-restricted sub-basin of the Kössen Basin, whereas talus breccias and debrites were deposited near a basin-bounding master fault. The basin deposits (lower and upper members of the Rezi Dolomite Formation) were subject of early diagenetic dolomitization in shallow burial setting. Enhanced salinity seawater was the dolomitizing fluid; the synsedimentary fracturing may have promoted the fluid flow and thereby the dolomitization. Thick-bedded carbonates (middle member of the Rezi Dolomite Formation) representing a progradational tongue of the ambient platform are intercalated between the basin deposits. The dolomitization of this unit can be explained by the reflux model.
The formation and deformation history of back-arc basins play a critical role in understanding the tectonics of plate interactions. Furthermore, opening of extensional back-arc basins during the ...overall convergence between Africa and Europe is a fundamental process in the overall tectonic evolution of the Mediterranean and adjacent areas. In this frame, Miocene tectonic evolution of the western Pannonian Basin of Central Europe and its connection to inherited Cretaceous structures of the Eastern Alpine nappes are presented.
Revision of published and addition of new structural and thermochronological data, as well as seismic profiles from the western Pannonian Basin is complemented by high-resolution thermo-mechanical numerical modeling in order to propose a new physically consistent tectono-sedimentary model for the basin evolution. The onset of extension is dated as ~25–23 Ma, and higher rates are inferred between 19 and 15 Ma at the south-western part of the area (Pohorje, Kozjak Domes, Murska Sobota Ridge, and Mura-Zala Basin). Rift initiation involved the exhumation of the middle part of the Austroalpine nappe pile along low-angle detachment faults and mylonite zones. The Miocene low-angle shear zones could reactivate major Cretaceous thrust boundaries, the exhumation channel of ultra-high-pressure rocks of the Pohorje Dome, or Late Cretaceous extensional structures. Miocene extension was associated with granodiorite and dacite intrusions between 18.64 and 15 Ma. The Pohorje pluton intruded at variable depth from ~4 to 16–18 km and experienced ductile stretching, westward tilting, and asymmetric exhumation of its eastern side. Terrestrial early Miocene (Ottnangian to Karpatian, 19–17.25 Ma) syn-rift depositional environment in supradetachment basins evolved to near-shore and bathyal one by the middle Miocene (Badenian, 15.97–12.8 Ma). Deformation subsequently migrated eastwards to the western part of the Transdanubian Range (Keszthely Hills) and to newly formed grabens. In this formerly emerged terrestrial area active faulting started at 15–14.5 Ma and continued through the late Miocene almost continuously up to ~8 Ma but basically terminated in the Mura-Zala Basin by ~15 Ma (early Badenian). These observations suggest a ~200 km shift of active faulting, basin formation, and related syn-tectonic sedimentation from the SW (Pohorje and Mura-Zala Basin) toward the Pannonian Basin center. Building on the above described observational and modeling data makes the Pannonian Basin an ideal natural laboratory for understanding the coupling between deep Earth and surface processes.
•Variable extensional structures characterize the western Pannonian Basin.•Exhumation of deeper crustal rocks along detachments occurred in the Miocene.•Depocenters and faulting shifted from basin margin toward basin center.•Thermomechanical modelling predicts depocenter migration within 12 Ma.•Miocene extensional structures reactivated and were bounded to inherited weakness zones.
Unraveling the age and kinematics of low temperature deformation events is crucial in understanding the late‐stage evolution of orogens. However, accurate age constraints can often be challenging to ...obtain due to unideal outcrop conditions, large sedimentary hiatuses or the lack of well‐defined thermal events. In this study, we show on the example of the Nekézseny Thrust, a poorly exposed late orogenic thrust in the southern Western Carpathians, that a combined approach of structural analysis and multi‐method thermochronology can provide the necessary temporal, kinematic and thermal constraints for a detailed reconstruction of the deformation history. While structural mapping revealed that the Late Cretaceous Uppony Gosau Basin in the footwall of the Nekézseny Thrust underwent a significant post‐Campanian and pre‐Miocene shortening, K/Ar dating of fault gouge samples from the main fault zone constrained the primary thrusting event to the Maastrichtian. Based on the acquired apatite fission‐track and (U‐Th)/He ages, subsequent heating of the Upper Cretaceous sediments due to tectonic burial was limited to 75–100°C, followed by deformation‐related and gradual cooling between the Eocene and Early Miocene. Considering the reconstructed deformation history, as well as the large‐scale tectonic affinity of the displaced units in its footwall and hanging wall, the Nekézseny Thrust is a far‐traveled (ca. 600 km) segment of the Late Cretaceous Alps‐Dinarides contact zone, whose development was linked to the switch from lower plate to upper plate position with respect to the Sava Zone and Alpine Tethys sutures, respectively.
Key Points
Detailed structural analysis is combined with three thermochronological methods to reconstruct low temperature deformation events
K/Ar age of fault gouges date the fault activity directly, while apatite fission‐track and (U‐Th)/He data constrain the exhumation history
The Maastrichtian age of the Nekézseny Thrust is interpreted in lights of the late‐stage Alpine‐Carpathian‐Dinaric tectonic evolution
Combined sedimentological and structural analysis was carried out in the field and on 2D seismic reflection profiles to recognize pre-orogenic structures in a Cretaceous fold-and-thrust belt. ...Detailed field observations were made in the Keszthely Hills, Western Hungary, while 2D seismic interpretation was carried out in the neighbouring Zala Basin. As a result, a fault-controlled intraplatform basin system was identified by a detailed analysis of bounding faults, and related outcrop-scale structures. The Norian–Rhaetian (227–201.3 Ma) synsedimentary faulting was associated with talus breccia formation, small-scale faulting, and dyke formation, in addition to slumping and other soft-sediment deformations. Based on the distribution of talus breccia, WNW–ESE-trending map-scale normal faults were identified in the Keszthely Hills, which is in agreement with the directly observed outcrop-scale synsedimentary faults. On seismic sections, similar WNW- or NW-trending Late Triassic normal faults were identified based on thickness variations of the syn-rift sediments and the presence of wedge-shaped bodies of talus breccia. Normal faulting occurred already in the Norian, and extensional tectonics was active through the Early and Middle Jurassic. The Late Triassic grabens of the western Transdanubian Range could be correlated with those in western part of the Southern Alps, and the Bajuvaric nappe system of the Northern Calcareous Alps. These grabens were situated on the proximal Adriatic margin, and they represent the first sign of the Alpine Tethys rifting. The locus of extension was laterally migrated westward, towards the distal Adriatic margin during Early and Middle Jurassic.
Large quantities of Cretaceous Fe-oxidising bacteria (FeOB) are documented here for the first time in carbonate injection dykes. These injection dykes occur in fissures of a Palaeozoic metamorphic ...complex in the Geresd Hills, Hungary, and are made up of microlaminated haematitic carbonate that contain very well preserved helical FeOB stalk fossils similar to recent iron-oxidising Gallionella and Mariprofundus. The presence of filament clusters with semi-parallel textures suggests aerotactic bacterial cell movements. Fractal analysis of the filament clusters showed higher fractal dimension values (Df ∼ 1.9) compared to the typical fractal dimension values of the texturally similar, but inorganic Diffusion-Limited Aggregation (DLA) structures (Df ∼ 1.7). This finding is interpreted to suggest that the observed filamentous texture is of microbial origin. Cretaceous basaltic intrusions that penetrated the gold-bearing pyritic metamorphic basement launched a low-temperature flow of reductive iron-rich fluids. Well‑oxygenated bottom waters percolated through thin pelagic sediments and picked up ferrous iron from the pyritic metamorphic bedrock and made it available for the Fe-oxidising microbes. Reactivation of a major fault zone (Mecsekalja Zone) resulted in the development of extensional faults in the metamorphic basement beneath the seafloor. Pelagic carbonate sediments with FeOB mats were drawn into these fissures, forming a network of injection dykes. This study shows that fractal dimension analysis and directional analysis may be a helpful method to differentiate FeOB textures from inorganic textures in ancient rocks. The presented results are expected to provide useful analogues to aid the reconstruction of similar palaeoenvironments worldwide.
•First globally: Cretaceous iron bacteria in injection dykes.•FeOB's stalks suggest aerotactic motion.•Fractal dimensions distinguish FeOB from inorganic structures.•Submarine pyritic rocks served as electron donors for Fe-bacteria.•Pelagic sediment drawn into seismic shock induced fractures to form injection dykes.
Fossil plants were discovered by geologists in the dolomite quarry on Strázsa Hill, west of Zsámbék, central Hungary, during December, 2020. The analysis of the plant remains and subsequent field ...studies suggest that the fossil plants are preserved in sediments of the Mány Member of the Törökbálint Formation and the sandy-clayey layers overlying Mesozoic dolomites were formed during the Oligocene. The fossil plant assemblage includes a small number of taxa with a relatively high frequency of gymnosperms. Among the gymnosperms taxodiaceous twigs, assigned to ?Taxodium, are dominant. Angiosperms are represented by the family Lauraceae (Daphnogene and Laurophyllum), Betulaceae (cf. Alnus), Ulmaceae (cf. Ulmaceae gen. et sp.), and “Rhamnus” warthae, a taxon with unknown systematic relations and a putative endemic element of the Intra-Carpathian area of the Central Paratethyan region. The flora is dominated by gymnosperms and “Rhamnus” warthae, other taxa are present but at lower frequency. Although the plant fossils of Zsámbék represent wetland vegetation types, less suitable for climate reconstruction, the relatively high ratio of lauraceous elements suggests a frostfree, warm climate. The floristic composition of the Zsámbék flora is clearly comparable to other late Oligocene floras of Hungary, therefore earlier climate estimates based on other similar-aged floras may also be considered for Zsámbék.
In this study, we investigated the structural evolution of the Vértessomló (VT) Thrust and the Környe-Zsámbék (KZ) Fault, which are located in the Transdanubian Range in the center of the Miocene ...Pannonian back-arc basin. Our study is based on surface and well data. The Transdanubian Range was located on the Adriatic passive margin during the Late Triassic, where a thick succession of platform carbonates was deposited. Intercalations of intraplatform basin deposits occur in the eastern part of the study area. South-directed thrusting and the formation of the VT Thrust took place during the Cretaceous, related to the Austroalpine orogeny. Asymmetric half-grabens were formed during the Eocene in the hanging wall of the segmented dextral normal KZ Fault. The geometry and kinematics of the KZ Fault were influenced by the pre-existing VT Thrust located in the Mesozoic basement of the Paleogene sub-basins. These Eocene half-grabens suffered mild inversion due to the dextral reverse reactivation of the VT Thrust and the KZ Fault during the Oligocene–Early Miocene. The geometry of Miocene normal faults indicates that the VT-KZ Fault system was an active transfer fault during the Miocene extension of the Pannonian Basin, as well. We found a positive correlation between the rheology of the Triassic basement and the mode of Paleogene fault reactivation. Our results show that reactivation of the pre-existing thrust took place along that segment, where the Triassic basement is made up of homogeneous platform carbonates. In contrast, a diffuse fault zone developed, where the Triassic basement is represented by the weak layers of intraplatform basins.
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