Southeastern Carpathians Research Articles

Gravity and magnetic modelling in the Vrancea Zone, south-eastern Carpathians: Redefinition of the edge of the East European Craton beneath the south-eastern Carpathians

A 2D gravity and magnetic data model has been constructed along a 71km densely observed profile, called DACIA PLAN GRAV MAN's. The profile crosses part of the nappe pile of the south-eastern Carpathians and includes the seismically active Vrancea Zone and was acquired with the objective to illuminate the basement structure and affinity in this area. The modelling approach was to create an initial model from well constrained geological information, integrate it with previous seismic ray tracing and tomographic models and then alter it outside the a priori constraints in order to reach the best fit between observed and calculated potential field anomalies. The results support a realignment of the position of the TTZ (Tornquist-Teisseyre Zone), the profound tectonic boundary within Europe that separates Precambrian cratonic lithosphere of the East European Craton (EEC) from younger accreted lithosphere of Phanerozoic mobile belts to its west. The TTZ is shown to lie further to the south-west than was previously inferred within Romania, where it is largely obscured by the Carpathian nappes. The crust of the EEC beneath the south-eastern Carpathians is inferred to terminate along a major crustal structure lying just west of the Vrancea seismogenic zone. The intermediate depth seismicity of the Vrancea Zone therefore lies within the EEC lithosphere, generally supporting previously proposed models invoking delamination of cratonic lithosphere as the responsible mechanism.

Macroseismic field of the October 27, 2004 Vrancea (Romania) moderate subcrustal earthquake

On October 27, 2004, a moderate size earthquake occurred in the Vrancea seismogenic region (Romania). The Vrancea seismic zone is an area of concentrated seismicity at intermediate depths beneath the bending area of the southeastern Carpathians. The 2004 Mw = 6 Vrancea subcrustal earthquake is the largest seismic event recorded in Romania since the 1990 earthquakes. With a maximum macroseismic intensity of VII Medvedev–Sponheuer–Karnik (MSK-64) scale, the seismic event was felt to a distance of 600 km from the epicentre. This earthquake caused no serious damage and human injuries. The main purpose of this paper is to present the macroseismic map of the earthquake based on the MSK-64 intensity scale. After the evaluation of the macroseismic effects of this earthquake, an intensity dataset has been obtained for 475 sites in the Romanian territory. Also, the maximum horizontal accelerations recorded in the area by the K2 network are compared to the intensity values.

Shear-wave splitting within the Southeastern Carpathian Arc, Transylvanian Basin, Romania

We present new splitting measurements for SKS and SKKS phases recorded at four broadband seismic stations located within the Carpathian Arc, in the eastern part of the Transylvanian Basin. We made 75 measurements, of which 58 are nulls and 17 show clear splitting. We used two methods to estimate splitting parameters, ϕ (the fast polarization direction) and δt (the delay time between fast and slow split shear waves): minimization of energy on the horizontal component corresponding to the minimum eigenvalue of the polarization matrix, and simultaneous linearization of split waves from a suite of earthquakes recorded at a given station. We consistently observed no splitting at station IACB, in the northern part of the study area, for a range of well-recorded earthquakes at different backazimuths; we consider the station splitting to be null. Null splitting here is consistent with several possible tectonic scenarios, including downwelling of asthenospheric material as a result of the sinking Vrancea body, asthenospheric upwelling beneath Ciomadul volcano, or lack of anisotropic fabric development in the large Ciomadul magma chamber beneath the station. Fast polarized shear waves trend NW–SE at CHDM, in the Transylvanian Basin, and at PMAR, which lies at the boundary between the Transylvanian Basin and the E–W-striking Southern Carpathians; these fast directions are consistent with a regional trend in splitting ϕ azimuths over much of the Eastern Carpathians, the East European and Moesian Platforms, and the northwestern Transylvanian Basin away from the Vrancea Bend Zone. The regional NW–SE splitting trend parallels the NW-to-SE temporal migration of Neogene volcanism along the Eastern Carpathians, and is likely due to a combination of southeastward rollback of the Vrancea high velocity body and simultaneous southwestwards absolute plate motion of the thick leading edge of the East European Platform (Tornquist-Teisseyre Suture Zone), which together constrain regional mantle flow. Fast shear wave polarization, ϕ, at station COMD, at the juncture between the Vrancea Bend Zone and the Transylvanian Basin, trends NE–SW, consistent with splitting at stations VOIR and MLR in the Southern Carpathians. Splitting at these stations reflects perturbed mantle flow in the vicinity of the Vrancea body, which entails an abrupt change from NE–SW at VOIR to NNW–SSE at PMAR, consistent with redirection of mantle flow along the northwestern side of the Vrancea body. Delay times at the three stations with clear splitting vary from 0.96 to 2.73s, indicating either the presence of an anisotropic layer ranging between ∼100km thick beneath CHDM and 300km beneath PMAR, or an unusually strong anisotropic fabric beneath the latter.

Phylogeography of a subalpine tall-herbRanunculus platanifolius(Ranunculaceae) reveals two main genetic lineages in the European mountains

A phylogeographical analysis of Ranunculus platanifolius, a typical European subalpine tall-herb species, indicates the existence of two main genetic lineages based on amplified fragment length polymorphism (AFLP) markers. One group comprises populations from the Balkan Peninsula and the south-eastern Carpathians and the other includes the remaining part of the range of the species, encompassing the western Carpathians, Sudetes, Alps, Pyrenees and Scandinavia. The main phylogeographical break observed in this species runs across the Carpathians and separates the main parts of this range (western and south-eastern Carpathians), supporting a distinct glacial history of populations in these areas. The high genetic similarity of the Balkan Peninsula and south-eastern Carpathian populations could indicate a common glacial refugium for these contemporarily isolated areas of species distribution. The western and northern part of the species range displays an additional weak differentiation into regional phylogeographical groups, which could have been shaped by isolation in glacial refugia or even by a postglacial isolation. The observed weak phylogeographical structure could also be linked with ecological requirements, allowing survival along streams in relatively low, forested mountain ranges. © 2013 The Linnean Society of London

Distribution and Phytocoenotic Context of Kobresia simpliciuscula (Wahlenb.) Mack. in South-Eastern Carpathians

This study proposes a critical analysis of the distribution and habitat requirements of the rare arctic-alpine plant species Kobresia simpliciuscula (Wahlenb.) Mack. in the South-Eastern Carpathians. The species was recorded in this part of Carpathians only from Romania, in Bucegi Mountains. The mention of K. simpliciuscula in Rodna Mountains (Eastern Carpathians) is considered to be erroneous. K. simpliciuscula was found in the Southern Carpathians in a different habitat type compared to the one characteristic for populations in the Arctic and the Alps. The species does not grow in the pioneer phytocoenoses of the Caricion bicoloris-atrofuscae alliance but, on the contrary, in dry calciphilous alpine vegetation included in Oxytropido-Elynion. The plant communities where K. simpliciuscula was found in Bucegi Mountains belong to Achilleo schurii-Dryadetum (Beldie 1967) Coldea 1984. These phytocoenoses are very similar to those described for the species in Belianske Tatra Mountains (Western Carpathians, Slovakia).

Is Evolution of Blind Mole Rats Determined by Climate Oscillations?

The concept of climate variability facilitating adaptive radiation supported by the “Court Jester” hypothesis is disputed by the “Red Queen” one, but the prevalence of one or the other might be scale-dependent. We report on a detailed, comprehensive phylo-geographic study on the ∼4 kb mtDNA sequence in underground blind mole rats of the family Spalacidae (or subfamily Spalacinae) from the East Mediterranean steppes. Our study aimed at testing the presence of periodicities in branching patterns on a constructed phylogenetic tree and at searching for congruence between branching events, tectonic history and paleoclimates. In contrast to the strong support for the majority of the branching events on the tree, the absence of support in a few instances indicates that network-like evolution could exist in spalacids. In our tree, robust support was given, in concordance with paleontological data, for the separation of spalacids from muroid rodents during the first half of the Miocene when open, grass-dominated habitats were established. Marine barriers formed between Anatolia and the Balkans could have facilitated the separation of the lineage “Spalax” from the lineage “Nannospalax” and of the clade “leucodon” from the clade “xanthodon”. The separation of the clade “ehrenbergi” occurred during the late stages of the tectonically induced uplift of the Anatolian high plateaus and mountains, whereas the separation of the clade “vasvarii” took place when the rapidly uplifting Taurus mountain range prevented the Mediterranean rainfalls from reaching the Central Anatolian Plateau. The separation of Spalax antiquus and S. graecus occurred when the southeastern Carpathians were uplifted. Despite the role played by tectonic events, branching events that show periodicity corresponding to 400-kyr and 100-kyr eccentricity bands illuminate the important role of orbital fluctuations on adaptive radiation in spalacids. At the given scale, our results supports the “Court Jester” hypothesis over the “Red Queen” one.

New Seismic and Tomography Data in the Southern Part of the Harghita Mountains (Romania, Southeastern Carpathians): Connection with Recent Volcanic Activity

The southern part of the southeastern Carpathians represents the site of the most recent volcanic eruptions of the entire Carpathian-Pannonian region. The products of these eruptions range from 42 to 10 Ka radiocarbon ages in the South Harghita Mountains (high K calc-alkaline rocks with adakite-like features), and at 1.2–0.6 Ma K–Ar ages in the Persani Mountains (alkali basalts). They were emplaced in a post-collisional regime. Ciomadul volcano is located at the southernmost part of the NW–SE oriented Călimani-Gurghiu-Harghita range crossing the inner part of the southeastern Carpathians and in the rough proximity of the Vrancea seismic zone (at ca. 60 km toward NW). Its magma generation is attributed to geodynamic events closely related to the seismogenic area. A number of particular geophysical and geochemical features located in the study region, including (1) the abrupt attenuation of the seismic waves originating from the Vrancea intermediate-depth foci, (2) the most intense heat-flow anomaly in Romania, (3) the most prominent 3He/4He anomaly measured in natural “postvolcanic” gas emanations, are all in favor of the hypothesis of a still existing hot local magma chamber. Data acquired during recent seismic monitoring of the Vrancea zone and its neighborhoods suggest an enhancement of the local seismicity beneath the southern edge of the South Harghita Mts., both at crustal and subcrustal levels. At the same time, recent tomography images obtained using local earthquake data correlate well with the presence of a vertically extended low-velocity zone coming from the upper mantle to the assumed magmatic chambers located in the crust. The present data, supporting the presence of an active crustal magma chamber beneath Ciomadul, allow us to consider that future volcanic activity at this volcano cannot be discarded.

The effect of dynamic recrystallization on olivine crystal preferred orientations in mantle xenoliths deformed under varied stress conditions

Spinel peridotite xenoliths from Persani Mountains, Southeastern Carpathians, Romania, were submitted to deformation by dislocation creep accompanied by dynamic recrystallization under variable stress and temperature conditions. Predominance of low-angle boundaries parallel to (100) with well-defined [0vw] rotation axes in olivine indicates that subgrain rotation is the main recrystallization mechanism and that dislocation glide occurs mainly in [100]{0kl} systems. Analysis of olivine crystal preferred orientations highlights that recrystallization results in dispersion of the orientations of recrystallized grains relatively to the parent grains. This dispersion may be quantified by a dimensionless dispersion factor defined as the ratio of the random component of the [100] axis distributions between the recrystallized grains and porphyroclasts. This factor is largely independent from the overall fabric strength, the number of grains analyzed, and the recrystallized grain size. The olivine CPO strength and anisotropy at the rock scale, quantified by J-index, are largely controlled by the volume of recrystallized grains, which depends on finite strain. Comparison of olivine CPO in coarse- and fine-grained porphyroclastic peridotites shows that deviatoric stress and temperature conditions play nevertheless a role on the CPO evolution during recrystallization; selective grain growth, which is favored at high temperature and low stresses, does counteract the dispersion produced by the nucleation processes.

Interaction of mantle dynamics, crustal tectonics, and surface processes in the topography of the Romanian Carpathians: A geomorphological approach

Tectonic processes and dynamic mantle flow impart a unique imprint on topography and geomorphic responses over time scales of 104 to 106yr. First-order topographic features in a tectonically active landscape represent ways to quantitatively characterise the interaction between crustal tectonics, mantle dynamics, and geomorphology, providing a basis for modelling landscape evolution. We analysed the topographic features of the Romanian Carpathians, a mountain range characterised by two straight segments connected by a narrow curvature zone. The deformation started in the Late Jurassic and includes two collisional phases during the Cretaceous and Miocene. We examined the tectonic geomorphology of the Romanian Carpathians focusing on regional and local topographic setting, drainage pattern, and river long profiles. Our main database is composed of DEM-based topographic analysis, supplemented with field investigations in the Slănic River basin, located in the Carpathian curvature zone. The longitudinal profiles of rivers draining the southern Carpathians are close to the equilibrium shape, in agreement with the older emersion of the chain. The longitudinal profiles of the rivers draining the eastern and southeastern Carpathians are in a transient state of disequilibrium as a consequence of a more recent emersion of the chain and of the Pliocene–Pleistocene tectonic activity in the Bend Zone. Filtering the topography at different wavelengths, we observe a relative depression in correspondence with the Carpathian Bend, where mantle seismicity and a high-velocity zone in tomography data are located and commonly interpreted as related to an almost inactive and dying subduction zone. Contrastingly, the filtered topography presents a high in the Transylvanian basin, where tomography data show a low-velocity area, interpreted as upwelling of hot asthenospheric materials. We hypothesise that local mantle convection generates positive and negative dynamic topographies. In the Slănic River basin, the foredeep deposits (Middle Miocene–Upper Pliocene) are folded and tilted, suggesting active compression beginning in the Lower Pleistocene. During the Middle–Late Pleistocene and Holocene, four terrace orders developed by the interaction between regional uplift and climate changes. The Carpathian topography results from a diachronous uplift superimposed on crustal tectonics. This diachronous uplift influenced the chain topography, the shape of river longitudinal profiles, and the formation of strath terraces. Crustal tectonics dominated the hydrographic net organisation. In correspondence with the Carpathian Bend and the Transylvanian basin, mantle flow driven by slab pull produced negative and positive dynamic topographies.

Geometrical constraints for the configuration of the Vrancea (Romania) intermediate-depth seismicity nest

For a seismogenic area like Vrancea (Romania) with well-defined geometrical features of the seismicity production in space and time, the numerical simulation of the earthquake generation process (e.g. cellular automaton) looks highly attractive. The delimitation, as accurately as possible, of the geometrical features of the seismically active system in the Vrancea subcrustal zone is essential to constrain the simulation modeling. As a first approximation, the seismicity pattern is close to a fault plane NE–SW oriented, extended roughly vertically between 60 and 170 km depth. A characteristic median plane is defined by minimizing the distance of hypocenters. The average distance of the hypocenters to the median plane is around 5 km. However, a more detailed investigation of the geometrical configuration of seismicity indicates a fragmentation of the active body located in the upper mantle in two segments. The seismicity pattern is well approximated by a planar distribution in each segment. In the transition zone, between the upper and lower segment, the hypocenter distribution is more dispersed and shows a disruption among the two planar segments, measured by about 9 km apart laterally one relative to the other. The two segments hosted the major Vrancea events recorded in the last two centuries (for which we have available location of acceptable accuracy). The narrow transition zone at about 100 km depth is interpreted as a weaker segment, possibly caused by a dehydration process or by an infiltration of asthenosphere material from the back side of the South-Eastern Carpathian arc system. It is still debatable if fragmentation in two seismically active segments reflects the existence of two neighbouring separate blocks (upper, continental and lower, oceanic block) or a consequence of a breaking process separating a continental block into two parts. The segmentation of the descending lithosphere and the edge effects are apparently stationary, at least for the time interval since 1985 to the present, for which the earthquake catalogue is reliable (homogeneous).

Dynamics of isolatedSaponaria bellidifoliaSm. populations at northern range periphery

AbstractFour populations ofSaponaria bellidifoliasituated at the species’ northern range periphery (Apuseni Mountains, southeastern Carpathians) were monitored over a period of 5 years. They were chosen to represent different habitat types (rocky, fixed screes, open screes and grassy), disturbance regime (fire), and population sizes (categorized as large and small). The reproductive effort was quantified, and matrix models were used to describe the population dynamics and to assess population viability.Saponaria bellidifoliahad very stable population dynamics in the harsh and stable abiotic conditions of the outcrops where populations occur. Habitat conditions exerted a notable influence on the species’ population reproductive performance, growth rate, and vital rates, whereas population size and climate did not have a clear‐cut effect on the dynamics of the species.Saponaria bellidifoliamaintains viable populations in the southeastern Carpathians, at its northern range periphery.

Divergence and speciation in the Carpathians area: patterns of morphological and genetic diversity of the crane flyPedicia occulta(Diptera:Pediciidae)

Morphological and genetic data are increasingly used in combination to test taxonomic hypotheses of species status. We compared the morphological and genetic diversity of the spring-dwelling crane fly, Pedicia occulta (Diptera), a widely distributed western Palearctic species considered to be monotypic throughout its range. We studied 11 morphological characters from 360 male individuals from the entire recorded range of P. occulta, and analyzed a subset of 77 individuals from the southeastern Carpathians, Rila, Pirin, and Rhodope for genetic variation at the mitochondrial cytochrome c oxidase I subunit (COI) gene. In contrast to expectations, our data on morphology and genetic structure of populations showed a structured pattern in the Carpathian region. Two well-separated groups were identified with nonmetric multidimensional scaling based on morphology. COI data revealed 2 divergent lineages (13.4% sequence distance) that corresponded to the 2 morphological groups and to differences in phenology and behavior. Sequence data showed additional genetic subdivision within one of these morphological groups, perhaps indicating the existence of additional cryptic entities. Our results also point out the importance of the Carpathian spring and mountain aquatic ecosystems as Pleistocene refugia. Our study emphasizes the power of combining morphological, genetic, and behavioral approaches in taxonomic hypothesis testing.

Disentangling phylogeography, polyploid evolution and taxonomy of a woodland herb (Veronica chamaedrys group, Plantaginaceae s.l.) in southeastern Europe

Southeastern Europe is a centre of European biodiversity, but very little is known about factors causing the observed richness. Here, we contribute to fill this gap by reconstructing the spatio-temporal diversification of the cytologically variable and taxonomically intricate complex of Veronica chamaedrys (Plantaginaceae s.l.), growing in open forests, forest edges and grasslands, with flow cytometry, molecular markers (AFLPs, plastid DNA sequences) and morphometry. Our results show that both diploid and tetraploid cytotypes are widespread, but diploids predominate on the southern Balkan Peninsula. Plastid sequences suggest a first split into three main lineages in the mid-Pleistocene and a continuous diversification during the last 0.4my. Two of the identified plastid lineages coincide with geographically distinct AFLP clusters. Altogether, the genetic data suggest forest refugia on the southern-most Balkan Peninsula (Greece), in Bulgaria, Istria (Croatia and Slovenia) and maybe the southeastern Carpathians (Romania). Morphometric and genetic data show little congruence with current taxonomy.

Delamination or slab detachment beneath Vrancea? New arguments from local earthquake tomography

Vrancea, located at the southeastern Carpathians Arc bend, is one of the areas in the Alpine‐Himalayan belt that features strong earthquakes occurring at intermediate depths (60–200 km). In this study we investigated the crustal and lithospheric structure beneath the Vrancea seismic area using a local earthquake tomography approach. We used an updated and revised catalog, spanning from 1982 to 2006 that uses data from both permanent and temporary networks in the target area. Simultaneous tomographic inversion for the Vp and Vs anomalies and the Vp/Vs ratio and source locations was done using the LOTOS code. The reliability and robustness of the results were rigorously checked using various tests (e.g., by studying the role of different parameters on the results of the inversion, performing the inversion using random data subsets, and synthetic modeling). The tomography results clearly indicate the presence of a high‐velocity material beneath Vrancea at a depth interval of about 60–200 km that coincides with the distribution of intermediate‐depth seismicity. This result agrees generally with previous tomographic studies. We compare two scenarios leading to this structure: (1) subduction and slab detachment and (2) “drop forming” or delamination. The latter mechanism presumes that the thickening of the crust due to continent‐continent collision causes transformation of the mafic lower crust into denser eclogite. This material accumulates until it reaches a critical mass, at which point it forms a large drop that begins to fall down. We propose that the high‐velocity anomaly we observe in our tomogram might represent the descending eclogitic lower crust material enveloped by the entrained lithosphere. It is possible that a similar delamination process can be observed in other parts of the Alpine‐Himalayan belt, such as in the Pamir Hindu‐Kush area.

Architecture of the south-eastern Carpathians nappes and Focsani Basin (Romania) from 2D ray tracing of densely-spaced refraction data

A velocity model of the upper crust (maximum depth 10–15 km) of the earthquake-prone Vrancea area (Romania) from 2D forward ray tracing of densely-spaced refraction data is presented. The model is derived from more than 11,000 travel times recorded at stations 100 m apart picked from 42 shot gathers along a 140 km line crossing the south-eastern Carpathian bending zone and the adjacent deep (foreland) Focsani Basin. The model refines basement structure beneath the south-eastern Carpathian nappe stack and Focsani Basin and documents reverse faults on which crystalline rocks or highly metamorphosed Mesozoic sedimentary cover of the crystalline basement have been elevated to depths as little as 3.5–4 km (with a vertical displacement of at least 2–2.5 km) beneath the external Carpathian nappes in the Vrancea Zone. Fault systems (flower structures) and normal faults associated with the overall tectonic subsidence of the foreland basin are also inferred. Some of the basement fault systems appear to be recently active, since they involve overlying young (Tertiary–Quaternary) sedimentary layers, and may be associated with intracrustal earthquakes recorded in the area.

Heavy rains and extreme rainfall-runoff events in Central Europe from 1951 to 2002

Abstract. Runoff data were used to better select historically significant precipitation events. The suggested criterion Qx expresses the increase of a stream runoff over up to four days in a row. Tests confirmed that Qx maxima correspond to maxima of areal precipitation in the respective catchment. Ten significant precipitation events in summer half-years from 1951 to 2002 were selected in 25 catchments each, and further studied in respect to spatial extent, simultaneous occurrence in various river basins, seasonal distribution, and temporal variability. Four regions were recognised within Central Europe that show related seasonality and simultaneous occurrence of events. The main coincidence of significant precipitation events was confirmed between the Austrian Alps and Bohemia and Saxony on one hand, and Moravia, Silesia, and Western Slovakia on the other hand. Significant events typically emerge here during peak summer, in the south-eastern area of the Alps during autumn months, in the South-Eastern Carpathians from May to July, and in Western Germany in spring or autumn. Episodes with less significant precipitation events (around 1960 and 1990) alternate with inverse episodes (1970's, second half of the 1990's). A reasonable selection of reference events opens the door to a quantitative evaluation of dynamic and thermodynamic conditions typical for heavy rains in various parts of Central Europe.

Genetic structure of peripheral, island-like populations: a case study of Saponaria bellidifolia Sm. (Caryophyllaceae) from the Southeastern Carpathians

Geographically peripheral populations often experience a reduction of genetic diversity and divergence from the core populations. Habitat geometry and quality can induce a local genetic diversity pattern, which overlies the regional variability issued from the range-wide phylogeo- graphy. We evaluated the genetic variation and genetic divergence of Saponaria bellidifolia Sm. on limestone outcrops within peripheral island-like populations from the Southeastern Carpathians, using RAPD markers. We also determined the degree of isolation related to other European populations, using AFLP. The Romanian populations had a decreased overall genetic diversity shared among popula- tions, with lower level in small populations. Potential habitat size had a positive effect on genetic diversity estimates. Fisher's exact tests of genetic differentiation revealed sig- nificant divergences only between the geographically most distant populations. Romanian populations were genetically pauperised as compared to Bulgarian and Italian populations and our results suggest that they might have originated from a recent range expansion from southern glacial refugia.

Two-scale modelling of Saponaria bellidifolia Sm. (Caryophyllaceae) abundance on limestone outcrops from its northern range periphery (Southeastern Carpathians)

We modelled the effect of habitat heter- ogeneity on the abundance of the submediterranean Saponaria bellidifolia, a red list species in Romania. The study was designed at two scales: 100 and 0.5 m 2 . At larger scale, generalized additive models and canonical correspondence analysis were used to model the density of ramets, whereas at microscale, binomial logistic regression was employed to model the species' occurrence. S. bellidifolia abundance responded sensitively to habitat type (classified as ''grassy'', ''rocky'' and ''scree''), rather than to microclimatic variables. At both scales, habitat type was the best predictor of ramet abundance, followed by slope and vegetation cover. At 0.5 m 2 , soil depth was also a good predictor of species occurrence. The data revealed that screes are the most suitable habitats for hosting relatively large populations of this rare species, because of occasional natural disturbances and presumably lower interspecific competition.

Lithosphere–asthenosphere interaction at the Southeastern Carpathian Arc bend: Implications for anisotropy

The Vrancea region, in the Southeastern Carpathians (Romania), represents a unique case among the seismic areas in the world taking into account the extreme concentration and persistence of seismicity and the tectonic stress field. Subduction in a post-collisional phase is still active in a narrow area located at the sharp bend of the mountain belt. Our goal is to show that the particular shape of the shear-wave splitting can be interpreted in the light of the decoupling and slab-retreat processes, which hypothetically induce a specific configuration of the upper-mantle flow. Shear-wave splitting of SKS phases shows a relatively coherent pattern outside the epicentral area, suggesting a prominent NE-SW anisotropy, in agreement with previous estimations performed in Central and Eastern Europe and following the trends of the deformation field as outlined by the GPS measurements. A clear change is pointed out inside the Vrancea area, where strike-parallel polarization is emphasized. Toward the NW (wedge side), the polarization turns to a strike-perpendicular direction in agreement to an upwelling asthenospheric flow in the back-arc region (i.e., polarization aligned to the local strike of the slab). These shear-wave splitting attributes are not consistent with conventional models of 2-D mantle flow near subduction zones, nor with a sub-vertical down-dipping flow driven by the sinking of the slab. They correlate well with lateral inhomogeneities outlined by the tomography image, heat flow, seismic-wave attenuation and thermal field. We suggest that the eastward slab retreat, and decoupling between the underlying asthenosphere and the slab itself, have induced strike-parallel mantle flow, likely favoring detachment of the slab along the arcuate mountain belt. These processes are directly related to the strong anisotropy observed in the SE Carpathians. The anisotropy and GPS data suggest a strong coupling of the surface and mantle processes.

Evidence of strong lateral inhomogeneous structure beneath SE Carpathians and specific mantle flow patterns

The Vrancea region, in the South-Eastern Carpathians (Romania), represents a unique case among the seismic areas in the world taking into account the extreme concentration and persistence of seismicity and tectonic stress field. Our goal is to show that the particular shape of attenuation and shear wave splitting properties can be interpreted in the light of the decoupling and slab retreat processes, which hypothetically induce a specific configuration of the upper mantle flow. Delamination and break-off processes combined with retrograde motion of the slab imply lateral asymmetry in flow geometry and geotectonic properties. Particularly relevant is the strike-parallel flow localized in front of the downgoing slab, in contrast with the steeply dip flow along the slab in the back side. The local upper mantle flows around the descending seismic active body explains the contrast of the seismic wave attenuation in the back-arc region against fore-arc region and the strong anisotropy anomaly observed in the South-East Carpathians (time delays of 1.5–2 s). Outside the epicentral area, the coherent pattern of the shear wave splitting follows the trends of the deformation field as outlined by the GPS measurements, in favour of a strong coupling between the surface and mantle processes.