Carpathian Orogen Research Articles

The bright spot in the West Carpathian upper mantle: a trace of the Tertiary plate collision-and acaveatfor a seismologist

The 2-D full waveform modelling of the mantle arrivals from the CELEBRATION 2000 profiles crossing the Carpathian orogen suggests two possible tectonic models for the collision of ALCAPA (Alpine-Carpathian-Pannonian) and the European Plate in the West Carpathians in southern Poland and Slovakia. Due to an oblique (NE-SW) convergence of plates, the character of the collision may change along the zone of contact of the plates: in the western part of the area an earlier collision might have caused substantial crustal shortening and formation of a crocodile-type structure, with the delaminated lower crust of ∼100 km length acting as a north-dipping reflecting discontinuity in the uppermost mantle. In the eastern part, a less advanced collision only involved the verticalization of the subducted slab remnant after a slab break-off. The lower crustal remnant of ∼10 km size in the uppermost mantle acts as a pseudo-diffractor generating observable mantle arrivals. Due to the similarity of synthetic data generated by both models, the question of the non-uniqueness of seismic data interpretation, that may lead to disparate tectonic inferences, is also discussed.

The Meliata suture in the Carpathians: regional significance and implications for the evolution of high-pressure wedges within collisional orogens

Abstract The Meliata nappe of the Western Carpathian orogen is comprised of Triassic deep-sea metasedimentary rocks and fragments of blueschist-bearing ophiolite. These were structurally emplaced onto Permian/Triassic shelf sequences and its Variscan basement. The nappe records a succession of deformational events which formed under decreasing pressure conditions. Subduction-related burial and resulting blueschist metamorphism is dated by 40 Ar/ 39 Ar plateau ages recorded by four phengitic muscovite concentrates (160–150 Ma). These crystallized during ductile deformation characterized by predominantly coaxial NW–SE stretching. The structures were overprinted by semiductile, nonpenetrative fabric elements which formed under greenschist facies conditions and contemporaneously with fabrics which developed in footwall tectonic units. Kinematic indicators record top north to NW shear during Middle Cretaceous loading of the Meliata unit onto the Inner Carpathian nappe complex recorded by a 40 Ar/ 39 Ar whole rock phyllite age of 105.8±1.5 Ma. A subsequent southeastern sense of shear is interpreted to have resulted from extension during Late Cretaceous uplift along hinterland margins of the tectonic wedge. The Meliata unit is part of a major Late Jurassic/Early Cretaceous suture which initially extended from the Alps to the Hellenides. It has been subsequently disrupted as a result of later strike-slip faulting following Tertiary collision of the Cretaceous orogen, and was transported onto extra-Alpine European units.

Long-term fluvial archives in Hungary: response of the Danube and Tisza rivers to tectonic movements and climatic changes during the Quaternary: a review and new synthesis

The paper discusses the Quaternary evolution of the Danube and Tisza rivers and their main tributaries in the context of evolution of the entire Pannonian Basin, which is Europe's largest intramontane basin, within the Alp–Carpathian orogen. The palaeo-drainage reconstruction of the Pannonian Basin for the pre-Quaternary period is outlined in connection with the gradual regression of Lake Pannon since the Late Miocene. Deltas of rivers that entered the basin from the northwest and northeast were gradually transformed into extended alluvial plains; thus, the earliest possible ancestor of the Danube coming southeastwards from the Alps could be as old as Late Miocene. By the Pliocene the whole Lake Pannon was infilled. The former extensional basin formation was replaced by a compresional stress field, which resulted in an uplift of the marginal flanks and late-stage subsidence anomalies. The increasing relief led to the formation of the Quaternary drainage pattern. The actively subsiding young basins were filled by fluvial sediments, transported by the Danube and Tisza river systems from the uplifting mountains. Between the subsiding regions of the Little and Great Plains, the Danube has formed an antecedent valley with terrace staircases between the uplifting sections of the Transdanubian Range and the North Midmountains. The formation of the terraces is attributed to periodic climate changes during the Pleistocene combined with differences in the uplift rate. The paper gives a complex overview of the classical chronology of the six terraces based on various data sources: mostly dating of loess/paleosol sequences, travertines, aeolian sand, and tephra strata overlying the fluvial sediments, complemented by scattered vertebrate faunal data and archaeological evidence directly from the terrace sediments.The Quaternary drainage pattern evolution of the Great Plain, with a strong tectonic control, is discussed in detail. Rivers originating from the uplifting marginal areas were drawn towards the subsiding depressions which served as local base level. Changes in subsidence rates in space and time throughout the Quaternary resulted in the evolution of a complex drainage pattern. A special emphasis is placed on the Late Pleistocene–Holocene development of the Middle–Tisza region and the Körös basin, where the Berettyó–Körös Rivers form an eastern tributary system of the Tisza River. A comparative evaluation of these two areas is especially relevant, as they provide insights into large-scale Late Pleistocene avulsion of the Tisza River. OSL dating, complemented with inferred transport directions determined from heavy mineral analysis of fluvial sediments in the Körös basin, has revealed an ancient large meandering river system that can be identified with the palaeo-Tisza, which was flowing along a tectonically controlled depression during the Late Pleniglacial. Successions in the Middle Tisza region have allowed differentiation between the older channels of the palaeo-Bodrog River and the Sajó–Hernád alluvial fan and the younger meander belts of the new course of the Tisza. In the Tisza system, changes in river style (braided to various scales of meandering) show correspondence to millennial-scale climate changes of the last 25ka, while in the Körös basin the effects of tectonics are overprinted onto the regional climatic signals.

Seismic crustal structure between the Transylvanian Basin and the Black Sea, Romania

In order to study the lithospheric structure in Romania a 450 km long WNW–ESE trending seismic refraction project was carried out in August/September 2001. It runs from the Transylvanian Basin across the East Carpathian Orogen and the Vrancea seismic region to the foreland areas with the very deep Neogene Focsani Basin and the North Dobrogea Orogen on the Black Sea. A total of ten shots with charge sizes 300–1500 kg were recorded by over 700 geophones. The data quality of the experiment was variable, depending primarily on charge size but also on local geological conditions. The data interpretation indicates a multi-layered structure with variable thicknesses and velocities. The sedimentary stack comprises up to 7 layers with seismic velocities of 2.0–5.9 km/s. It reaches a maximum thickness of about 22 km within the Focsani Basin area. The sedimentary succession is composed of (1) the Carpathian nappe pile, (2) the post-collisional Neogene Transylvanian Basin, which covers the local Late Cretaceous to Paleogene Tarnava Basin, (3) the Neogene Focsani Basin in the foredeep area, which covers autochthonous Mesozoic and Palaeozoic sedimentary rocks as well as a probably Permo-Triassic graben structure of the Moesian Platform, and (4) the Palaeozoic and Mesozoic rocks of the North Dobrogea Orogen. The underlying crystalline crust shows considerable thickness variations in total as well as in its individual subdivisions, which correlate well with the Tisza-Dacia, Moesian and North Dobrogea crustal blocks. The lateral velocity structure of these blocks along the seismic line remains constant with about 6.0 km/s along the basement top and 7.0 km/s above the Moho. The Tisza-Dacia block is about 33 to 37 km thick and shows low velocity zones in its uppermost 15 km, which are presumably due to basement thrusts imbricated with sedimentary successions related to the Carpathian Orogen. The crystalline crust of Moesia does not exceed 25 km and is covered by up to 22 km of sedimentary rocks. The North Dobrogea crust reaches a thickness of about 44 km and is probably composed of thick Eastern European crust overthrusted by a thin 1–2 km thick wedge of the North Dobrogea Orogen.

Crustal and upper mantle structure of the Western Carpathians from CELEBRATION 2000 profiles CEL01 and CEL04: seismic models and geological implications

SUMMARY CELEBRATION 2000 was a large international experiment that focused on the lithospheric structure in Central Europe. It consisted of several wide-angle reflection and refraction seismic profiles. Profiles CEL01 and CEL04 are located in the transition from the Precambrian and Palaeozoic platforms to the Carpathian orogen. The data were modelled using 2-D tomographic and forward modelling techniques. The profile CEL01 (about 900 km long) extends from the southwestern margin of the East European Craton to the Pannonian basin system. The depth of Moho boundary varies along the profile from 27‐33 km under the Pannonian basin system area, 30‐39 under the Carpathians and within the TESZ to 40‐45 km under the East European Craton. The CEL04 profile (630 km long) starts in the Polish trough (PT) and reaches the Pannonian basin system. Along the CEL04 profile, the Moho interface shallows from 40 km beneath the PT to about 35 km in the Maopolska unit. The crustal thickness is 43 km beneath the Carpathians and 25 km beneath the Pannonian basin system. The results obtained are consistent with a commonly accepted view of the Carpathians, thought to be formed largely due to the interplay between Palaeogene to Middle Miocene subduction of oceanic or suboceanic crust and subsequent collision. A feature that we interpret as the subduction-related orogenic root is at least partially preserved in both profiles and is manifested by a crustal thicknesses of 39

Depositional environments and fluvial system changes in the dinosaur-bearing Sânpetru Formation (Late Cretaceous, Romania): Post-orogenic sedimentation in an active extensional basin

The Sânpetru Formation (Haţeg Basin, Romania) was deposited in an active extensional basin in response to the post-orogenic collapse of the Southern Carpathians. Despite being one of the best exposed latest Cretaceous continental successions of Eastern Europe and having great potential to improve knowledge about the Mesozoic evolution of the Carpathian orogen and the latest Cretaceous terrestrial paleoenvironments of Europe, this formation has been the subject of limited sedimentologic research. Fourteen stratigraphic sections were measured in the Sânpetru Formation throughout an 860-m-thick interval exposed along the Sibişel Valley in order to conduct the most detailed stratigraphic and sedimentologic study of this formation to date. The Sânpetru Formation is a repetitive succession of fining-upward units composed of stratified and structureless sandstones, conglomerates, and mudstones. Facies analysis reveals that the Sânpetru Formation was deposited by braided streams that flowed through a symmetrical, extensional basin. The low variability of paleocurrent direction, the sheet-like architecture of the deposits, and the paucity of channel-shaped scours indicate poorly channelized flow across a shallow, broad braidplain. Small-scale fluctuations in sandstone/mudstone ratio and maximum grain size of channel deposits in the lower Sânpetru Formation reflect autocyclic shifts in paleochannel position. In the upper Sânpetru Formation, sandstone/mudstone ratios and maximum grain size of channel deposits increase, paleocurrent direction changes significantly, and hydromorphic paleosols become the sole type of paleosols present. These changes reflect an episode of rapid uplift of the source area and the basinward creation of accommodation space below the local water table, which resulted in the creation of extensive wetlands. The wetlands of the upper Sânpetru Formation were not an environment favorable for the formation of dinosaur bonebeds, which are found in the lower part of the formation. The apparent absence of fossils in the upper part of the formation led previous researchers to incorrectly interpret the Cretaceous–Tertiary boundary at the transition between the lower and upper Sânpetru Formation.

The beginning, development and termination of the Middle Miocene Badenian salinity crisis in Central Paratethys

Middle Miocene Badenian evaporites of the Carpathian region are underlain and overlain by deep-water deposits, the onset of evaporite deposition was sudden but not synchronous in all facies zones and the deposition of evaporites was controlled by the evolution of Carpathian orogen. In the Carpathian Foredeep (and most probably in other basins) the Badenian evaporites represent the lower part of the NN6 zone. Halite and associated deposits in the central part of the Badenian evaporite basin show the same facies successions and marker beds can be traced across and between individual basins. Characteristic marker beds made it possible to correlate various facies zones of the marginal Ca-sulfate platform. These marker beds seem to reflect events that may be related to sudden and widespread changes in water chemistry, which in turn imply major changes in basin hydrology. The onset of the evaporitic deposition in the Carpathian Foredeep was clearly diachronous and the evaporites deposited in the basin centre preceded the beginning of evaporite sedimentation in the marginal basin, however, depositional history in the marginal basin and the basin centre was the same. A general transgressive sequence of evaporites found in the Carpathian Foredeep resulted from the migration of facies zones induced by the nappe movement. Isotopic studies of Badenian foraminifers occurring below evaporites suggest that the interrupted communication of the Paratethys with the ocean was a consequence of eustatic sea-level fall, possibly related to climatic cooling, and it was coupled with a tectonic closure of connection with the Tethys. Thus both tectonics and eustacy have contributed to the origin of salinity crisis. Sedimentological and geochemical data indicate recycling of evaporites throughout most of the evaporite deposition. The recycling at the end of gypsum deposition in the marginal sulfate platform was accompanied by a change in the hydrology of the Central Paratethys that was tectonically-driven, and possibly related to the block tectonic phase manifested in the marginal part of the Carpathian Foredeep Basin. The change in hydrology implied the dilution of brines by inflowing marine water and this terminated the Middle Miocene Badenian salinity crisis. The onset of the Badenian salinity crisis shows great similarities to the onset of the Messinian salinity crisis and the terminations of both crises were different.

Geochemical and Sr–Nd–Pb isotopic compositions of Mts Pieniny dykes and sills (West Carpathians): Evidence for melting in the lithospheric mantle

Mineralogical, geochemical and Sr–Nd–Pb isotopic data are presented for the Middle Miocene (13.5–10.8 Ma) Mts Pieniny magmatic rocks which occur in a portion of the outermost post-collisional volcanic arc in the Carpathian orogen. The analysed rocks cover a compositional range from basaltic andesites to andesites of the medium-K calc-alkaline series. Despite their arc-like trace element signature, Pieniny rocks show unusual isotopic compositions different from the Neogene–Quaternary calc-alkaline volcanic rocks erupted in the nearby Carpathian–Pannonian region. Their 87Sr / 86Sr (0.7053–0.7070) and 143Nd / 144Nd (0.5121–0.5124) ratios suggest that these magmatic rocks have originated from lithospheric sources enriched in Rb with low Sm / Nd ratios. Pb isotopic compositions ( 206Pb / 204Pb = 18.62–18.86; 207Pb / 204Pb = 15.66–15.75; 208Pb / 204Pb = 38.74–39.84) are also consistent with a lithospheric origin of Pieniny rocks. Sr and Nd depleted mantle model ages suggest that parental magmas of Pieniny rocks derived from a metasomatized subcontinental lithospheric mantle recording ancient (> 1.1 Ga) subduction-related enrichments. Geochemical features of Pieniny basic rocks allow an estimate of the mineralogy of their lithospheric mantle source region. The small range (from 11 up to 15) of La / Yb and the lack of significant Eu anomalies preclude the presence of residual garnet and plagioclase, respectively; the low Rb / Sr and Rb / Ba ratios as well as the range observed in Ba / Th, K / Ba and La / Yb ratios require involvement of an heterogeneous source in terms of amphibole abundance. The delamination process of the European plate that occurred in this sector of the Carpathian orogen during Middle Miocene should have favoured partial melting of metasomatized amphibole-bearing veins within the lithospheric mantle of the overriding Alcapa micro-plate, giving rise to magma genesis in the Mts Pieniny post-collisional arc segment.

Crustal constraints on the origin of mantle seismicity in the Vrancea Zone, Romania: The case for active continental lithospheric delamination

The Vrancea zone of Romania constitutes one of the most active seismic zones in Europe, where intermediate-depth (70–200 km) earthquakes of magnitude in excess of M w = 7.0 occur with relative frequency in a geographically restricted area within the 110° bend region of the southeastern Carpathian orogen. Geologically, the Vrancea zone is characterized by (a) a laterally restricted, steeply NW-dipping seismogenic volume (30 × 70 × 200 km), situated beneath (b) thickened continental crust within the highly arcuate bend region of the Carpathian orocline, and (c) miscorrelation of hypocenters with the position of known or inferred suture zones in the Carpathian orogenic system. Geologic data from petroleum exploration in the Eastern Carpathians, published palinspastic reconstructions, and reprocessing of industry seismic data from the Carpathian foreland indicate that (1) crust of continental affinity extends significantly westward beneath the external thrust nappes (Sub-Carpathian, Marginal Folds, and Tarcau) of the Eastern Carpathians, (2) Cretaceous to Miocene strata of continental affinity can be reconstructed westward to a position now occupied by the Transylvanian basin, and (3) geologic structure in the Carpathian foreland (including the Moho) is sub-horizontal directly to the east and above the Vrancea seismogenic zone. Taken together, these geologic relationships imply that the Vrancea zone occupies a region overlain by continental crust and upper mantle, and does not appear to originate from a subducted oceanic slab along the length of the Carpathian orogen. Accordingly, the Vrancea zone appears to potentially be an important place to establish evidence for active lithospheric delamination.

Tectonics of Miocene–Pliocene fresh-water molasses in the Carpathian Foredeep (Witów Series, South Poland)

Normal faults of different orientations appear to be the youngest manifestations of faulting in the Polish Outer Carpathians, composed of the Lower Cretaceous trough Lower Miocene strata, and the related Carpathian Foredeep, which is filled with the Lower to Middle Miocene sediments. In the Outer Carpathians, the folds and thrusts produced by accretion-related shortening were formed between the Paleocene and early Late Miocene. The origin of normal faults is still debatable, since it is not known whether these faults were a result of multidirectional extension produced in a single collapse event, or differently oriented extension proceeding in a series of successive events. Structural studies of the Late Miocene–Pliocene(?) fresh-water molasses of the Witów Series provide a possibility to reconstruct the Late Neogene stress field in the central part of the Polish Carpathian Foredeep and, indirectly, in the central part of the Polish Outer Carpathians. The strata of such an age are unique features in the Polish Carpathian Foredeep, proving thereby a key record of structural deformation during the latest stages of orogenic evolution of the Carpathian orogen. The molasses are cut by joints, and normal and strike-slip faults which were formed in two successive events: (1) a syn-depositional one, proceeding under NNW-SSE to N-S oriented horizontal compression, possibly coeval with reactivation of a NE-striking sinistral fault of the Kurdwanów-Zawichost Fault Zone in the basement; and (2) a post-depositional one, during N-S to NE-SW-oriented extension. In the first event, reactivation of the NE-striking sinistral fault led to formation of N-S-oriented joints, as well as NW-striking dextral, and NNW-trending normal faults. This event was probably contemporaneous with sinistral reactivation of some thrusts in the Western Outer Carpathians, induced by eastward-directed extrusion of crustal blocks in the Carpathian internides. In the second event, both W-E and NW-SE-oriented joints and WNW-striking normal faults were formed. The latter most probably originated due to reactivation of the Early Paleocene WNW- and NW-striking normal faults in the basement. Therefore, normal faults detected in the Outer Carpathians and Carpathian Foredeep appear to be a result of not a single collapse event but of different successive events. This extensional episode lasted at least to the late Pleistocene. No evidence for the recent NNE-directed tectonic compressive stress, typical for that segment of the Carpathian arc, has been found yet.

Middle to late Miocene sequence stratigraphy of the Transylvanian Basin (Romania)

The history of Middle to Late Miocene evolution of the Transylvanian Basin was determined by the bordering Carpathian orogen evolution, the tectonic events being well recorded by the sedimentary history. The basin evolved in a back-arc setting, under a regional, compressional stress field. The major tectonic events produced during the Late Sarmatian and Post-Pannonian were related to the reactivation of the pre-Badenian fault systems. The Transylvanian Basin got uplifted after the Late Pannonian (? during the Pliocene), and at least 500 m of sedimentary cover was eroded. Based on seismic and well-log interpretation, core and outcrop sedimentology, and microfauna, eight sequences were defined. The early Middle Miocene sequences are roughly synchronous to five 3rd order global sea-level cycles. Most of the recognized sequence boundaries are enhanced by regional tectonic events. The sedimentary evolution was also strongly influenced by salt-tectonics, active starting with the Late Sarmatian. Two sequences were identified in the Lower Badenian deposits. The third sequence (late Early Badenian to early Mid Badenian) preserves information about deeper shelf settings. The lowstand of the following sequence was responsible for the deposition of the salt formation (late Mid Badenian), an important lithostratigraphic marker in the sedimentary record of the basin. In general, the Upper Badenian deposits (parts of the 4th and 5th sequences) belong to deep marine submarine fan systems. The Sarmatian (partially 5th, 6th and partially 7th sequences) was characterized by diverse salinity conditions, stretching from brackish to hypersaline, and by high tectonic instability, which induced several significant relative sea-level falls. During that time, deltaic (north) and fandeltaic (east) systems fed submarine fans, stacked between salt-related submarine heights (“channeled” deep-marine depocenters). Most of the Pannonian deposits (partially 7th and 8th sequences) belong to submarine fan systems, but shallower facies were also found in the western and eastern part of the basin.

Ongoing tectonic reactivation of the Outer Carpathians and its impact on the foreland: Results of borehole breakout measurements in Poland

The direction of present-day maximum horizontal compression (= S Hmax = tectonic stress) was interpreted for 62 wells in Poland, using the method of borehole breakout analysis of 4-arm and 6-arm dipmeter logs. The study area covers complex tectonic junction of the Carpathian orogen and its foreland, which comprises the East European Craton (EEC) divided by the Teisseyre–Tornquist zone (TTZ) from the Palaeozoic platform of western Poland. For this area, frequent deviation of the S Hmax direction from NW–SE characteristic for the Atlantic ridge push has been interpreted in terms of the ALCAPA tectonic push. In the Upper Silesian segment of the Polish Outer Carpathians (POC), NNE–SSW-oriented S Hmax in the accretionary wedge differs significantly from NNW–SSE S Hmax in the autochthonous basement. The above discrepancy points to uncoupled type of the suture in this segment of the POC. In this scenario the ALCAPA push involves the nappes and is compensated in the top of the basement, which is expressed by systematic S Hmax rotations. In the accretionary wedge of the eastern Małopolska segment, S Hmax follows the trend perpendicular to the strike of nappes. It is in general agreement with NNE–SSW-oriented S Hmax in the autochthonous basement that also parallels the ALCAPA push direction. Similarity in stress orientation between these structural levels implies coupled suture zone in this segment of the POC. Further to the north, ALCAPA push is transmitted into the foreland plate where it causes N–S orientation of S Hmax, as determined for sedimentary cover of the EEC. Within the Baltic portion of the EEC, further S Hmax rotation towards the intermediate NNW–SSE position suggests balance between the ALCAPA push and the ridge push components. Within the TTZ, common S Hmax rotations from N–S to NW–SE indicate structurally controlled accommodation of the ALCAPA push. In the Palaeozoic platform of western Poland, Mesozoic complex of the Fore-Sudetic Monocline reveals NNE–SSW-directed S Hmax that differs from NW–SE-oriented S Hmax in the Variscan accretionary wedge. Here, mechanical decoupling along Zechstein evaporates is likely. Presented set of breakout data from Poland shows that recent compressive reactivation of the Carpathians exerts strong impact on the stress field of the foreland plate at a distance of 700 km from the suture. Due to this effect, the Fore-Carpathian stress domain within the North European stress province can be discriminated.

The Vrancea seismic zone and its analogue in the Banda Arc, eastern Indonesia

It is now widely, although not universally, accepted that the Carpathian orogen marks the site of an arc–continent collision that followed the subduction of a now vanished small ocean basin. Seismic tomography has defined a high-velocity anomaly in the upper mantle similar to those associated with subduction zones worldwide. There is, however, no recognisable Wadati–Benioff Zone (WBZ), and intermediate-depth seismicity is confined to a relatively small, roughly cylindrical and vertically elongated region beneath the extreme southeastern corner of the mountain chain. There is no consensus in the published studies as to the origin of this ‘Vrancea Zone’. The Banda Sea region of eastern Indonesia has sometimes been cited as an analogue for the Pannonian/Transylvanian basin and the enclosing Carpathian orocline, but at first sight the patterns of seismicity appear very different. Intermediate depth seismic activity defines a subducted slab that dips north, south and west beneath the Banda Sea, a configuration explained as a consequence of the rapid expansion of the sea during roll-back subduction. If the similar scenario proposed for the Carpathians is correct, then it is the absence of a Carpathian WBZ that is actually anomalous. Closer examination of Banda Arc seismicity shows that it can be divided into two parts, these being a scoop-shaped WBZ and an adjacent ‘Damar Zone’ of much more intense intermediate-depth activity. At its eastern end the Damar Zone merges with the WBZ, but in the west there is evidence for separation from it. A plausible explanation of this pattern is that a lower layer of the downgoing slab is peeling away from the remainder. The Banda/Australia collision is now almost complete and the activity in the WBZ proper can be expected to decrease. Damar Zone activity, on the other hand, may persist for a much longer period, migrating towards the foreland as the detaching layer separates from the remainder of the subducted lithosphere. In a few million years the seismicity of the Banda region could well resemble the present day seismicity of the Carpathian orogen.

Three-dimensional numerical modeling of contemporary mantle flow and tectonic stress beneath the earthquake-prone southeastern Carpathians based on integrated analysis of seismic, heat flow, and gravity data

The principal purpose of the study is to understand the interplay between intermediate-depth large earthquakes in the southeastern Carpathians (Vrancea) and tectonic stress induced by a high-velocity body (lithospheric slab) descending into the mantle beneath the region. To analyze processes of stress generation and localization in and around the descending slab, we develop a three-dimensional (3D) numerical model of contemporary mantle flow and stress beneath the Vrancea region. The input data of the model consist of: (i) temperatures derived from seismic P-wave velocity anomalies and surface heat flow, (ii) crustal and uppermost mantle densities converted from P-wave velocities obtained from seismic refraction studies, (iii) geometry of the Vrancea crust and slab from tomography and refraction seismic data, and (iv) the estimated strain rate in the slab (as a result of earthquakes) to constrain the model viscosity. We find that major crustal uplifts predicted by the model coincide with the East Carpathian orogen and surround the Transylvanian basin and that predicted areas of subsidence are associated with the Moesian and East European platforms. We show a correlation between the location of intermediate-depth earthquakes and the predicted localization of maximum shear stress. Modeled tectonic stresses predict large horizontal compression at depths of about 70–220 km beneath the Vrancea region, which coincides with the stress regime defined from fault-plane solutions for the intermediate-depth earthquakes. This implies that buoyancy-driven descent of the lithospheric slab beneath the Vrancea region is directly linked to intermediate-depth seismicity.

The Western Carpathians—interaction of Hercynian and Alpine processes

The complicated structural and rheologic properties of Western Carpathian lithosphere reflect the complex geodynamic history of the Carpathian orogen. Based on critical analysis of earlier models, new interpolation of existing geophysical data and results of integrated modelling, a new map of the lithosphere thickness for the Carpathian–Pannonian region has been constructed. The map allows for the distinction of a frontal orogen collision zone in the NE (from increased lithosphere thickness) as well as a zone of oblique collision with the Bohemian Massif in the West, where lithosphere is not significantly thickened. The MOHO discontinuity beneath the Western Carpathian hinterland (Danube and East Slovak Basins), as defined by deep reflection seismic profiling, is relatively shallow. This probably reflects recent crustal extension related to oblique collision between the European plate and the ALCAPA block and an increase of the asthenospheric updoming from the Middle Miocene onward. Crustal thickness reflects the combined effects of deep-seated orogenic processes and mantle thermal evolution beneath the Pannonian Basin system. In this study, we focus particularly the structures of: (1) the Late Alpine collision and Neogene back arc basin development, including deep-seated contacts between colliding plates, a zone of slab detachment, the compressional accretionary wedge of the Outer Western Carpathian Flysch Belt, and extensional structures produced by subduction rollback and asthenosphere upwelling; (2) Early Alpine structures related to Cretaceous thrust-stacking, including subhorizontal reflection packages (interpreted as multi-generational extensional structures), the underplated intra-Penninic (Oravic) continental ribbon, and ophiolite traces of the Meliatic oceanic suture; and (3) north-dipping reflectors interpreted as remnant Hercynian lithotectonic fragments with opposed vergency to the subducted Alpine units.

Lithosphere in the Western Carpathians and its surrounding tectonic units - Geophysical study

Geophysical methods are important tools for the investigation of the structure and geodynamic development of the lithosphere. The central and eastern parts of the Western Carpathians are bordered in the north by a thicker and stronger lithosphere of the European platform (100-150 km), which is underthrust (about of 50 km) beneath the margin of the overriding Carpathian orogen. This thickening is interpreted as remnants of subducted slabs. In contrast, the “thin” lithosphere at the western margin of the Western Carpathians can be considered as a result of oblique collision along a deep-seated transform zone between the platform and orogenic lithosphere. Neo-Alpine “soft” collision and retreating subduction of this orogen can also be discovered by means of quantitative interpretation of observed gravity field. The crustal thickness in the Western Carpathians ranges among 27-35 km. The central Western Carpathians are characterized by thicker crust (30-55 km) in comparison with thinner crust (25-30 km) in the Pannonian Basin System. This feature is probably the result of the youngest lithosphere processes from the Middle Miocene. Rheological properties of the Western Carpathian lithosphere show that the mechanical strengths decrease within the whole lithosphere from the area of the European platform via the Western Carpathians to the Pannonian Basin. The most remarkable and important first-order tectonic structures (seismo-tectonic zones) in the Western Carpathians are the zones of the Pieniny Klippen Belt, the Mur-Mürz-Leitha fault zone, the Čertovica fault zone and the Hurbanovo line. Map of neo-Alpine fault systems and neotectonic regions (blocks) of Slovakia was defined.

3-D upper crustal tomographic structure across the Vrancea seismic zone, Romania

The VRANCEA99 and VRANCEA2001 seismic refraction experiments are part of a multidisciplinary project to study the Eastern Carpathians in Romania. The objectives of these studies are intended to disclose a more detailed picture of the crustal and upper mantle structures above the seismically active Vrancea region. In this paper we provide additional constraints for the upper crustal structures of the area. The 1999 campaign consisted of a 320-km-long N–S profile and a 70-km-long E–W profile. The intersecting 2001 profile extended in E–W direction from the Hungarian border to the Black Sea. In order to enhance the model resolution, first arrival data from local crustal earthquakes were also included. This configuration allowed for the first time to derive a 3-D velocity model for the upper crust of the Romanian Carpathian Orogen, within a 115×235 km wide region, centred over the Vrancea seismic zone. The 3-D model reveals lateral velocity variations, which were not visible on the in-line interpretations. It allows us to distinguish between foreland platform areas, foreland basins and the Carpathian Orogen. Clear velocity differences between the foreland basins south and southeast of the Eastern Carpathians and the Focsani Basin further north indicate different pre-Miocene sedimentary compositions and geological evolutions of these foreland platforms. The involved Moesian and Scythian platforms are separated by the Trotus Fault system, which is observed as a velocity discontinuity. An upper crustal high-velocity zone, above the northern Vrancea seismic zone, could also be identified. This high-velocity zone is explained by a Middle Pliocene to Pleistocene E–W oriented out-of-sequence thrust of the crystalline basement, below the decollement of the flysch nappes.

Variscan to Alpine heterogeneous palaeo-stress field above a major Palaeozoic suture in the Carpathian foreland (southeastern Poland)

The tectonic evolution of the region adjacent to the SW border of the East European craton is crucial for understanding the Palaeozoic amalgamation of Europe and its later evolution. We present a structural analysis of the brittle deformation (fault-slip data inversion) and folding carried out in the Cambrian to Cenozoic rocks of the European platform in SE Poland to characterize the Late Palaeozoic, Maastrichtian–Palaeocene and Miocene tectonics of this region. The Variscan (Late Carboniferous) palaeo-stress pattern was characterized by N–S to NNE–SSW-oriented compression, as inferred from the folding of the Palaeozoic (mainly Devonian) rocks. The compression was associated with the northwesterly (present-day coordinates) motion of Gondwana towards the northerly situated Laurussia. Subsequently, the Mid-Polish Trough (MPT) developed in an extensional regime. The stress regime changed to compressional at the Maastrichtian–Palaeocene transition in response to the Africa–Europe plate kinematics. The compressional regime induced the tectonic inversion of the Mid-Polish Trough. The stress pattern was characterized by NE–SW-oriented compression in the Holy Cross Mts. (HCM) area and in the Radomsko High. Folds and faults in the Mesozoic cover revealed a sinistral reactivation of the Holy Cross Fault (HCF) and of NW–SE-trending faults. The Alpine stress patterns were related to the subduction of the European platform beneath the Carpathian orogen. The flexural bending of the subducted lithosphere induced a homogeneous Middle Miocene N–S extension in the Carpathian foreland. Subsequently, the northward propagation of the Carpathian front resulted in late Middle Miocene N–S compression, restricted to the proximal area of the frontal thrusts, implied by a strain partitioning at a low friction contact between the Carpathian nappes and the European platform. Since the Palaeozoic, the Teisseyre–Tornquist Zone (TTZ) behaved as lithospheric weak zone, which has been reactivated during the Maastrichtian–Palaeocene change in stress regime. It resulted in the tectonic inversion of the Polish Trough. Inherited faults conditioned superficial stress perturbations and focused the deformation.

Determination of the continental lithospheric thermal structure in the Western Carpathians: integrated modelling of surface heat flow, gravity anomalies and topography

Abstract Various interpretations of seismic, magnetotelluric and geothermal data have resulted in distinctly different models of the structure and thickness of the lithosphere in the Western Carpathian–Pannonian Basin region. The model inconsistencies, which are at least partly due to the use of non-unique single-method data inversion, led us to the application of integrated lithospheric multi-method modelling. Our approach combines the interpretation of surface heat flow, gravity and topography (local isostasy) data in order to constrain the continental lithospheric thermal structure along five transects crossing the Western Carpathians from the Polish platform and the Bohemian Massif to the Pannonian Basin. The results indicate clear variations of the lithosphere thickness along the Western Carpathian orogen not only in NS direction but also in WE direction. The lithospheric thickness is found to be 90–120 km underneath the Bohemian Massif, 115–140 km beneath the Polish platform and 75–100 km beneath the Pannonian Basin. A thick lithosphere (up to 130–150 km) is indicated underneath the central and eastern segments of the Western Carpathians with a tendency of thickening eastwards. This structure is interpreted as a remnant of a subducted slab of the European plate. In contrast, no lithospheric root shows up in our models of the westernmost segment of the Western Carpathians. We suggest that this difference indicates that in the transition zone from the Eastern Alps to the Carpathians, strike-slip movements predominated during the entire convergence period and no clear slab was produced in this area.

Transylvanian heat flow in the presence of topography, paleoclimate and groundwater flow

The Transylvanian Depression (TD) is located in the inner part of the Carpathian orogen and shows a marked heat flux low of 30 mW m −2 in the centre, increasing to 60 mW m −2 towards the margins. It is the purpose of the paper to asses the possibility that the observed heat flux in the TD is a consequence of disturbances of the shallow geothermal gradient caused by topography, topographically driven ground water flow and climatic changes. The thermal effects of topography, topographically driven groundwater flow and paleoclimate have been investigated using two-dimensional finite element models. The results show that topography can induce a decrease in heat flux at hill crests and an increase at the bottom of valleys. Topographically driven ground water flow accentuates the effect above ∼1000 m and decreases it below. For the characteristic amplitude and wavelength of topography in Transylvania the heat flux corrections for the cumulated effect of topography and water flow are maximum 1 mW m −2, which is insignificant. Paleoclimate potentially has a large thermal effect. Temperature variations associated with the last glaciation can produce a significantly increasing gradient with depth, which is present in the majority of measured wells. Inversion of six wells confirmed that the curvature in the temperature profiles is consistent with the climatic cooling of the last glaciation. The paleoclimatic effect may induce heat flow corrections in the range 3–7 mW m −2.