SE Carpathians Research Articles

Evaluating landslide response in a seismic and rainfall regime: a case study from the SE Carpathians, Romania

Abstract. There have been many studies exploring rainfall-induced slope failures in earthquake-affected terrain. However, studies evaluating the potential effects of both landslide-triggering factors – rainfall and earthquakes – have been infrequent despite rising global landslide mortality risk. The SE Carpathians, which have been subjected to many large historical earthquakes and changing climate thus resulting in frequent landslides, comprise one such region that has been little explored in this context. Therefore, a massive (∼9.1 Mm2) landslide, situated along the river Bâsca Rozilei, in the Vrancea seismic zone, SE Carpathians, is chosen as a case study area to achieve the aforesaid objective (evaluating the effects of both rainfall and earthquakes on landslides) using slope stability evaluation and runout simulation. The present state of the slope reveals a factor of safety in a range of 1.17–1.32 with a static condition displacement of 0.4–4 m that reaches up to 8–60 m under dynamic (earthquake) conditions. The groundwater (GW) effect further decreases the factor of safety and increases the displacement. Ground motion amplification enhances the possibility of slope surface deformation and displacements. The debris flow prediction, implying the excessive rainfall effect, reveals a flow having a 9.0–26.0 m height and 2.1–3.0 m s−1 velocity along the river channel. The predicted extent of potential debris flow is found to follow the trails possibly created by previous debris flow and/or slide events.

Earthquake Source Properties of a Lower Crust Sequence and Associated Seismicity Perturbation in the SE Carpathians, Romania, Collisional Setting

Romanian seismicity is mainly confined to the Eastern Carpathians Arc bend (ECAB), where strong subcrustal earthquakes (magnitude up to 7.9) are generated in a narrow lithospheric body descending into the mantle. The seismic activity in the overlying crust is spread over a larger area, located mostly toward the outer side of the ECAB. It is significantly smaller than subcrustal seismicity, raising controversies about possible upper mantle-crust coupling. A significant earthquake sequence took place in the foreland of the ECAB triggered on 22 November 2014 by a mainshock of magnitude 5.7 (the greatest instrumentally recorded earthquake in this region) located in the lower crust. The mainshock triggered a significant increase in the number of small-magnitude events spread over an unusually large area in the ECAB. The paper’s goal is to compute the source parameters of the earthquakes that occurred during the aforementioned sequence, by empirical application of Green’s function and spectral ratio techniques. Fault plane solutions are determined using multiple methods and seismicity evolution at regional scale is investigated. Our results highlight a still active deformation regime at the edge of the EE Craton, while the source parameters reveal a complex fracture of the mainshock and a very high-stress drop.

Tectonic regimes and stress patterns in the Vrancea Seismic Zone: Insights into intermediate-depth earthquake nests in locked collisional settings

Earthquake nests are anomalous clusters of seismicity located far from active collisional systems in intraplate, locked suture zones, or the deep part of relic subducted slabs, challenging classic earthquake generation mechanism theories. The Vrancea Seismic Zone in Romania is such an upper-mantle seismic nest located in the SE Carpathians, releasing the largest strain in continental Europe. To better understand earthquake generation and the relationship with lithospheric deformation, we estimate earthquake source parameters in Vrancea and surrounding regions between 2014 and 2020, and determine the stress field via focal mechanism inversion and unsupervised machine learning. In the crustal domain, maximum horizontal stress is in agreement with surface fault kinematics and GPS-derived S-SE trending horizontal plate velocities relative to Eurasia, implying that tectonic stress is vertically coherent on a crustal scale. The stress regime changes from transpression beneath the orogen to transtension towards the foreland where movement is accommodated along major crustal faults, and tension further away from the epicentre, in the Moesian Platform and the North Dobrogea Orogen. Inside the seismogenic body vertical tension and an overall compressive regime dominates, implying that vertical elongation may be the driving mechanism for brittle failure and that stress is transmitted along the sinking slab to the surface. However, the retrieved stress ratios are low: ~0.2 for mantle earthquakes Mw>4 and ~0.4 for Mw<4, challenging the brittle failure assumption. Increased pore fluid pressure has been shown to lower stress ratios, implying that dehydration embrittlement may contribute to generating intermediate-depth seismicity in the Vrancea slab. Comparisons with seismic tomography and anisotropy studies show excellent correlations between maximum horizontal stress directions, possible slab strike orientation, and seismic anisotropy, especially below ~130km depth, suggesting ambient mantle flow may also promote in-slab stress build-up and seismic potential.

The mid-Langhian flooding in the eastern Central Paratethys: integrated stratigraphic data from the Transylvanian Basin and SE Carpathian Foredeep

The mid-Langhian (“Badenian”) flooding fully reconnected the semi-isolated Central Paratethys realm with the Mediterranean and, thereby, drastically changed the middle Miocene paleogeography of Central Europe. Due to the scattered stratigraphic record and scarcity of independent age constraints in some areas, the precise age and underlying mechanism are still debated. We present integrated chronostratigraphic data from five sections in the eastern part of the system to reconstruct the flooding event distal from the strait to the Mediterranean. By applying modern Mediterranean biochronology (planktonic foraminifera and calcareous nannofossils), supplemented by an 40Ar/39Ar age on a tuff, we demonstrate that the widespread open marine settings in the NW Transylvanian Basin were definitely installed after 14.9 Ma (MMi4d biozone), and in most areas before 14.4 Ma. In the marginal study area in the SE Carpathian Foredeep, fully marine conditions likely set in slightly later (14.6–14.4 Ma). There, short-lived marine incursions into the brackish environment occurred since the latest Burdigalian (“pre-flooding phase”). The new ages overlap with the flooding in the majority of the Central Paratethys (~ 14.9–14.4 Ma), and with marine overflow into the Black Sea (14.85 Ma). We suggest that the transgression was driven by subsidence of the Pannonian Basin, by creating accommodation space and diminishing barriers between sub-basins, but was likely enhanced by a global sea-level rise. Finally, we speculate that the scarcity of all calcareous material in the SE Carpathian Foredeep before the mid-Langhian flooding might be related to pulses of nutrient-rich brackish and low pH water from the neighboring Black Sea Basin.

Active tectonic deformation and associated earthquakes: a case study—South West Carpathians Bend zone

Active tectonic deformation is continuously shaping the landscape in the Alpine Orogenic system. Earthquakes are clear proves of neotectonic deformation process. In the Romanian Carpathians the most active deformation is recorded in the South Eastern Carpathian Bend Zone. Nevertheless, seismicity associated to latest orogenic stage of active deformation is not limited to this area. Other seismic regions, like the South West Carpathians Bend Zone, were identified. Recently, several earthquake sequences were recorded in the Caransebes-Mehadia (CMB) and Hateg basins (HB). These sedimentary basins developed over the South Carpathians, Getic/Supra Getic and Danubian basement napes, their evolution being related with the N-ward tectonic transport of the Carpathian Orogen, in the current position, during the Paleogene-Quaternary times. The aim of the present study is to perform a thorough analysis of the earthquakes recorded in these two basins in correlation with the observed geological structure to better understand and constrain the neotectonic processes affecting the SW Carpathians Bend Zone. The focal mechanisms determined for HB show a predominant strike-slip faulting component with the principal axes oriented approximately NW–SE (compression) and NE–SW (extension), a reorientation from the N-S one described for the Pliocene–Pleistocene phase. The focal mechanisms in CMB imply N–S extension with normal or strike-slip faulting. We argue that the present day tectonic processes are not restricted to the SE Carpathians Bend zone, as previously inferred. Instead they affect a much larger area of the Carpathian Orogen, like the SW Carpathians Bend zone.

Public Perception on Hydro-Climatic Extremes and Water Management Related to Environmental Exposure, SE Hungary

Increasing climate extremities, as consequences of climate change, highly affect the public and farmers in the SE Carpathian Basin. Our research aimed at the investigation of the perceptions and experiences of public, farmers and decision-makers on drought and inland excess water problems on the Hungarian part of this region, since their knowledge and cooperation are essential in the future planning of sustainable water management. Their opinions were explored by 481 questionnaires and 52 in-depth interviews addressing the perceived climate change impacts in everyday life and agriculture, causes of drought and inland excess water, possibilities of mitigation and adaptation, and sustainability of the present farming structure. The spatial distribution of the answers was compared with the spatial pattern of drought and inland excess water sensitivity based on environmental indicators. The results confirmed their high correspondence reflecting a realistic knowledge on severity, significance and the contributing factors. Individual responsibility, the lack of financial resources and an uncertain vision of public were considered as major weak points that could be improved to foster the implementation of an effective water management strategy. High efforts are necessary to outline the framework of inclusive planning processes with exact roles of all actors and find ways to motivate co-operation willingness and increase individual responsibility.

Speleomorphology of M3-R2 – The highest cave in the SE Carpathians

High-alpine caves are currently in the spotlight of research as they are host to speleothems, key recorders of Quaternary environmental change. Based on modern survey and analytical techniques, we investigated the karst morphology of M3-R2 Cave, the highest alpine cave in the Romanian Carpathians. Spatial modeling in Compass has shown survey details and cave morphology. We recalculated the main cave parameters (development/extension/depth), obtaining 47 m/21 m/-18.15 m, respectively. Slope ranges between 25° and 45°, with short passage segments of 5° to 10°. The cave developed at the intersection of a SW-dipping schistosity plan with two fracture sets, occurred under compression conditions. It is a remnant of a once longer cave system, which was progressively eroded under glacial conditions. Speleothem diversity is uncommonly rich for a marble cave, with dominant moonmilk formations, stalagmites and draperies. Due to its location/elevation, evolution and speleothems, this cave has a great potential in providing important paleoclimatic information compared to other karst landforms in South Carpathians.

Combined magnetotelluric and petrologic constrains for the nature of the magma storage system beneath the Late Pleistocene Ciomadul volcano (SE Carpathians)

The Ciomadul is the youngest volcano of the Carpathian–Pannonian region, which erupted last time at 32ka. It produced high-K dacitic lava domes and pumiceous pyroclastic rocks. The dacite is crystal-rich and contains plagioclase, amphibole in addition to biotite, titanite, apatite, zircon and occasionally quartz, K-feldspar as well as olivine, clinopyroxene and orthopyroxene. There are two groups of amphiboles, characterized by low-Al and high-Al, respectively. They occur in the same samples and also as different zones of the same crystals. Thermobarometric calculations suggest that the low-Al amphiboles were formed from a low temperature (<800°C) silicic magma, whereas the high-Al amphiboles crystallized at about 950°C from a more mafic melt. A near-solidus silicic crystal mush body was stored at 7–14km depth, where an eruptible magma batch was produced by major reheating (about 200°C temperature increase) due to the intrusion of hot mafic magma into the silicic magma reservoir. A magnetotelluric survey was performed to reveal whether any melt-bearing magma body could presently reside beneath the volcano. Both the 2D and 3D inversion modeling calculations indicate low electric resistivity values in the depth interval of 5–25km, just beneath the volcanic centers. This can be interpreted as implying a partially melted zone, i.e. a crystal mush body containing about 5–15% melt fraction. In addition, the 2D modeling calculation indicates also a deeper low resistivity anomaly at 30–40km depth. The consistent petrologic and magnetotelluric constrains on the magma storage beneath Ciomadul are corroborated by the recent seismic tomography result, which pointed out a low-velocity anomaly at 8–20km depth zone. Thus, results of independent models suggest the presence of a melt-bearing crystal mush body beneath the seemingly inactive volcano. Since there are implications for long repose periods during the lifetime of the volcano as well as for effective and rapid remobilization of the low-temperature silicic crystal mush body prior to volcanic eruptions, the present existence of a low melt fraction silicic crystal mush beneath Ciomadul could mean that there is still a potential for a rejuvenation in the future. We suggest for long-dormant or seemingly inactive volcanoes, such as Ciomadul, having melt-bearing magmatic body at depths to term as ‘volcano with potentially active magma storage’ or PAMS volcano.

Reconstruction of the Late Holocene river channel shifts in the North-Eastern part of the Lower Danube Plain based on historical data

Several periods of increased fluvial activity and accelerated runoff / deposition have been identified during the Holocene in different regions of Europe (Macklin et al, 2006). According to Starkel (2002) the older phases were characterized by an incision of deeper and wider tracks and, since Roman period, especially during the past millennium, tendency had changed to aggradation, braiding and river avulsions. The turn to a more stable fluvial regime caused further incisions and the development of narrow and smaller meanders. All these fluvial responses can be mainly correlated with the climatic fluctuations (cooler and wetter phases alternating with drier and warmer ones), socio-economic changes (increased anthropic pressure) or both. The last major period of concentration of extreme meteorological and hydrological events is the Little Ice Age (LIA), which conventionally took place between the 16th and mid 19th century (Lamb, 1984). However in Romania the study of LIA and the associate fluvial effects is rather poor. The data regarding hydro-climatological events (weather conditions, floods) in the medieval period (mostly collected by Topor, 1960) is scarce and based on the few local chronicles and diaries of the foreign travelers. However these confirm the increased frequency and magnitude of the rainfalls in the summer (with several important peaks in the 1590 – 1690 interval) as well as the hard winters well documented for the rest of the continent. Changes in the configuration of the stream network, due to avulsions, can be evaluated based on historical maps only with the first half of the 17th century and the dating of the fluvial records is still missing. The North-Eastern part of the Lower Danube Plain (Fig. 1) is a large alluvial floodplain where Siret River collects several major tributary streams draining SE Carpathians (Putna, Râmnicu Sărat, Buzău) or the Moldavian Tableland (Bârlad), before joining Danube, near Galaţi. Surprisingly Siret’s floodplain (until the confluence with Buzău) it is larger by a couple of kilometers than Danube’s, despite the anastomosed character of the last one. This can indicate a progressive Holocene shift of the Siret’s river course from the Carpathians to the eastern edge of the floodplain or a transformation from a multi-channel to a single channel pattern. The instability was also specific to main tributaries, as is confirmed by the presence of multiple abandoned sinuous courses and relict meander belts in the area. An important shift of the Siret River’s lower course to a more eastern position during medieval times was briefly discussed, for the first time, by Antonovici (1929) based on couple of historical documents. The few questions we will try to answer to in this study are: When was formed the actual configuration of the river network in the study region? What was the impact of the LIA? How relevant can be the analysis of the historical data in the assessment of the river channel shifts?

Late Holocene soil and vegetational changes in the foothills of the SE Carpathians (Ukraine), based on the study of pedochronocatenas

A study of pedochronocatenas which include soils of archaeological sites is a reliable approach to reconstruct environmental changes since the time of the settlements existence. The studied pedochronocatenas include: 1) soils buried under ancient earth ramparts, 2) soils formed on the surface of these ramparts, and 3) natural undisturbed soils located close to the ramparts. The studied sites are located on the plateau between the Prut and Siret rivers (in the Glyboka district of the Chernivtsi region, South-Eastern part of the Ukrainian Carpathians). At present, this area is covered mainly by Haplic Greyzems and Dystric Cambisols under broad-leaved forests. Pedochronocatenas at two sites have been studied: 1) the Glyboka defensive rampart built during the IX-Xth centuries AD (1100-1000 yr BP, 14 C 1040±190, the Early Slavic time) and 2) the Grushivka rampart built as a defence from the Scythians during the Vth century BC (2500-2400 BP), but also with the new material deposited on the old rampart during the Early Slavic times. According to archaeologists [3–6] the ramparts were built in order to defend those sides of the settlements which were unprotected by natural barriers. Morphological and morphometrical soil analysis, as well as pollen study have been carried out in order to trace pedogenic, vegetational and climatic changes since the time of the rampart’s construction.

Assessment of recent tectonic evolution and geomorphic response in SE Carpathians (Romania) using hypsometric analysis

The study investigates correlations between the recent tectonic evolution of the external South-Eastern Carpathians and the characteristics of the hypsometric curves, derived for various Horton-Strahler basin orders. The analysis focuses on an area of about 8000 km 2 , delineated by the Trotus Fault in the north, the southern limit of the Buzau River catchment, Brasov and Focsani Basins in the west, respectively, east, known to conserve the youngest topography in the Romanian Carpathians. Considering the effects of the Quaternary uplift, these limits include the less elevated region of Subcarpathians. Spatial distribution of the hypsometrical integrals (HI) values was evaluated taking into account basin order, main lithological features and distance from major structural lines as well as some local indices of spatial autocorrelation. The results show that HI reflects both the specificity of regional tectonic evolution and denudation. However this is better sustained for the higher order basins (>3).

Amphibole perspective to unravel pre-eruptive processes and conditions in volcanic plumbing systems beneath intermediate arc volcanoes: a case study from Ciomadul volcano (SE Carpathians)

Ciomadul is the youngest volcano in the Carpathian–Pannonian region produced crystal-rich high-K dacites that contain abundant amphibole phenocrysts. The amphiboles in the studied dacites are characterized by large variety of zoning patterns, textures, and a wide range of compositions (e.g., 6.4–15 wt% Al2O3, 79–821 ppm Sr) often in thin-section scale and even in single crystals. Two amphibole populations were observed in the dacite: low-Al hornblendes represent a cold (<800 °C) silicic crystal mush, whereas the high-Al pargasites crystallized in a hot (>900 °C) mafic magma. Amphibole thermobarometry suggests that the silicic crystal mush was stored in an upper crustal storage (~8–12 km). This was also the place where the erupted dacitic magma was formed during the remobilization of upper crustal silicic crystal mush body by hot mafic magma indicated by simple-zoned and composite amphiboles. This includes reheating (by ~200 °C) and partial remelting of different parts of the crystal mush followed by intensive crystallization of the second mineral population (including pargasites). Breakdown textures of amphiboles imply that they were formed by reheating in case of hornblendes, suggesting that pre-eruptive heating and mixing could take place within days or weeks before the eruption. The decompression rim of pargasites suggests around 12 days of magma ascent in the conduit. Several arc volcanoes produce mixed intermediate magmas with similar bimodal amphibole cargo as the Ciomadul, but in our dacite the two amphibole population can be found even in a single crystal (composite amphiboles). Our study indicates that high-Al pargasites form as a second generation in these magmas after the mafic replenishment into a silicic capture zone; thus, they cannot unambiguously indicate a deeper mafic storage zone beneath these volcanoes. The simple-zoned and composite amphiboles provide direct evidence that significant compositional variations of amphiboles do not necessarily mean variation in the pressure of crystallization even if the Al-tschermak substitution can be recognized, suggesting that amphibole barometers that consider only amphibole composition may often yield unrealistic pressure variation.

Middle Pleistocene to Holocene fluvial terrace development and uplift-driven valley incision in the SE Carpathians, Romania

Abstract This study reveals that in the SE Carpathians terrace development and fluvial incision during the Middle Pleistocene–Holocene are predominantly controlled by tectonic uplift as shown by terrace distributions and uplift amounts and rates. The work focuses on a transect from the internal nappes and Brasov intramontane basin (western domain) to the external nappes and Focsani foredeep basin (eastern domain). New infrared stimulated luminescence ages were obtained and minimum terrace formation ages were determined to derive fluvial incision rates, and thereby, to constrain tectonic uplift. In the eastern domain, non-uniform terrace distributions in adjacent sub-parallel more active Punta and less active Susiţa rivers and an eastward migrated fluvial incision from the orogen to the foredeep basin indicate tectonic uplift as dominant control on terrace development. Strath-terraces in the western and eastern domains indicate repeated events of vertical fluvial incision and lateral erosion during the early Middle Pleistocene and late Middle Pleistocene–Holocene, respectively. These events imply successive recurrent disturbances of equilibrium conditions due to pulses of increased tectonic uplift. Fill-terraces in the western domain show that initial aggradation periods were followed by uplift-driven vertical incision during the late Middle–Late Pleistocene. As fill-terraces show a wide-spread development, climatic change and complex response cannot be excluded as contributing factors. Synchronous to terrace development, loess deposition periods during the late Middle–Late Pleistocene and Latest Pleistocene and intercalated episodes of palaeosol formation during the Late Pleistocene imply comparable climatic conditions across the SE Carpathians. Dominant strath-terraces of the eastern domain indicate stronger fluvial incision (~ 240 m) since the late Middle Pleistocene, whereas older strath- and younger dominant fill-terraces of the western domain designate a lower amount (~ 90 m) since the early Middle Pleistocene. Middle Pleistocene–Holocene fluvial incision rates document higher tectonic uplift in the external nappes and lower towards the western intramontane and eastern foredeep basins.

Possible deep lithospheric roots beneath South-Eastern Carpathians back-arc region

The Vrancea seismic region represents a unique case of well-defined and intense intermediate-depth earthquake activity as a consequence of specific geodynamic processes at the continental contact between East-European, Moesian and Intra-Alpine plates. Apart the subcrustal earthquakes generated in the Vrancea slab, the analysis of seismicity puts into evidence two other clusters of subcrustal earthquakes (h > 50 km) toward the back-arc side of the SE Carpathians bend, one to the west (Sinaia), the other to the north-west (Brasov Depression) relative to the Vrancea seismic source. The hypocenters lay down between 50 to 105 km in Sinaia and between 50 and 136 km in Brasov Depression. The rate of seismic energy release is much lower (Mw ≤ 3.7 in Sinaia and Mw ≤ 3.1 in Brasov Depression) than for Vrancea activity. The seismicity pattern in the SE Carpathians back-arc region upper mantle correlates well with the high-velocity structures depicted by seismic tomography investigations suggesting possible remnant deep lithospheric roots apart from the narrow well-defined slab generating Vrancea major earthquakes. The results outline significant lateral heterogeneities in the mantle and provide new data for incorporating seismological, geotectonic and volcanological data in a unified modeling of the complex processes taking place in the study region.

From nappe stacking to out‐of‐sequence postcollisional deformations: Cretaceous to Quaternary exhumation history of the SE Carpathians assessed by low‐temperature thermochronology

Apatite fission track (AFT) and (U‐Th)/He (AHe) thermochronology have been combined to constrain the exhumation history of the SE Carpathians. Cooling ages generally decrease from Cretaceous for the internal basement nappes (AFT ages), to Miocene–Quaternary (AFT and AHe, respectively) for the external sedimentary wedge. The AFT and AHe data show a Paleogene age cluster, which confirms a suspected but never demonstrated tectonic event. The new data furthermore suggest that the SE Carpathians have been affected by a middle Miocene exhumation phase related to continental collision, which occurred at rates of ∼0.8 mm/yr, similar to the one previously inferred for the East Carpathians. The SE Carpathian tectonic evolution, however, is overprinted by two younger exhumation events in the Pliocene–Pleistocene. The first exhumation phase (latest Miocene–early Pliocene) occurred at high exhumation rates (∼1.7 mm/yr) and is interpreted as a tectonic event and/or associated with a sea level drop in the Paratethys basins during the Messinian low stand. The youngest recorded tectonic phase suggests rapid Pleistocene exhumation (∼1.6 mm/yr) and is interpreted to represent crustal‐scale shortening different in mechanics from collisional processes. The data suggest that the SE Carpathians did not develop as a typical double‐vergent orogenic wedge; instead, exhumation was related to a foreland‐vergent sequence of nappe stacking during collision and was subsequently followed by a large out‐of‐sequence shortening event truncating the already locked collisional boundary.

Surface kinematics in the Alpine–Carpathian–Dinaric and Balkan region inferred from a new multi-network GPS combination solution

The understanding of the intraplate tectonics of Central Europe requires a detailed picture of how stress is transferred from the interaction of the Eurasian, Nubian and Anatolian plates to the Alpine, Carpathian, Pannonian and Dinaric regions. Recent strain distribution is controlled by the Adria horizontal push, by the Vrancea vertical slab pull and associated horizontal displacements, and by the Aegean/Anatolia extension and slab-roll back. We present a horizontal velocity field for the Alpine-Carpathian-Pannonic-Dinaric and Balkan regions resulting from a new combination of seven different GPS networks formed from permanent and campaign stations. Dedicated velocity profiles in two specific regions are studied in detail. One is the Alpine Pannonian region, with a detailed picture of the NS indentation of the Adria microplate into the Southern Alps, in NE Italy, the deformation in the Tauern Window and the eastwards kinematics of a Pannonian plate fragment. The second study region includes Transylvania, the Southern Carpathians up to the Aegean sea and Albania, where a major right lateral shear deformation exists as a consequence of the NE convergence of the Apulia platform towards the Dinarids, and the SSW motion of Macedonia, Western Bulgaria and Rumania, related to the Hellenic arc dynamics in the Eastern Mediterranean. The profiles in the Alpine–Pannonian area indicate that a velocity drop of 2.5 +/− 0.4 mm/yr associated with the Adria indentation concentrates on a segment of some 50 km south of the Periadriatic fault. The deformation becomes extensional by a similar amount just north of the Periadriatic fault, in the Tauern Window, where the updoming of the Tauern Window implies vertical motion which could well be associated with surface extension. In the EW profile, we observe a sudden velocity change of 1.5 +/− 0.2 mm/yr in 20 km, corresponding to the right lateral Lavant fault, which seems to mark the border between dominant indentation kinematics to the West and dominant extrusion kinematics to the East. Three profiles are considered in Southern and Eastern Europe: one across the lower Adriatic sea from Apulia in Italy to the southern Dinarides, which enables it to constrain the velocity drop associated with the subduction of the Adria microplate into the Dinarides to 3.2 +/− 0.5 mm/yr in 140 km. The second profile is longitudinal and constrains the velocity inversion of 7.4 +/− 1.0 mm/yr in 350 km associated with right lateral shear faults in Albania. The third profile crosses the Transylvania region with a shortening of 2.3 +/− 1.0 mm/yr in 220 km, and the Wallachian–Moesian region up to the Chalcidic peninsula in N Greece. This lower part of the profile implies an extensional stretch of the upper crust of 3.2 +/− 0.9 mm/yr in 440 km, culminating in the Hellenic arc. Strain rate maps are presented in this regional scale, showing the excellent agreement between fault plane solutions of crustal earthquakes and the eigenvectors of the GPS derived two dimensional strain rate tensor. Three profiles are considered in the Balkan and SE Carpathians: one across the lower Adriatic sea from Apulia in Italy to the southern Dinarides, which enables to constrain the velocity drop associated to the subduction of the Adria microplate into the Dinarides to 3.2 +/− 0.5 mm/yr in 140 km. The second profile is longitudinal and constrains the velocity inversion of 7.4 +/− 1.0 mm/yr in 350 km associated to right lateral shear faults in Macedonia, a highly seismic region. The third profile crosses the Transylvania with a shortening2.3 +/− 1.0 mm/yr in 220 km, and the Wallachian–Moesian region up to the Chalcidic peninsula in N Greece. This lower part of the profile implies an extensional stretch of the upper crust of 3.2 +/− 0.9 mm/yr in 440 km, culminating in the Hellenic arc.

The syn- and post-collisional evolution of the Romanian Carpathian foredeep: New constraints from anisotropy of magnetic susceptibility and paleostress analyses

The first results of anisotropy of the magnetic susceptibility (AMS) data from the Romanian Carpathian foredeep reveal compression directions generally perpendicular to the highly bended Carpathian orogenic arc. The distribution of the present-day stress field in the Pannonian–Carpathian region reflects pre-imposed plate boundaries that were established during late Miocene collision. Analysis of the AMS along the eastern and southern Carpathian foredeep, at the contacts of the orogenic nappe pile, the lower plate, and the overlying sedimentary rocks, indicates that two factors are critical for the distribution of the stress field during and after collision, i.e. 1) the inherited highly bended plate geometry and 2) the Quaternary deformation that lead to differential vertical and horizontal movements in the SE Carpathians. The AMS analysis was successfully used in the (weakly) deformed Mio–Pliocene sedimentary rocks from the Carpathian foredeep, revealing information about the deformation history of the soft, non-competent rocks where the poor preservation of traditional kinematic indicators (e.g., faults and folds) is difficult.

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.

Deformation and seismic anisotropy of the lithospheric mantle in the southeastern Carpathians inferred from the study of mantle xenoliths

Peridotite xenoliths with a broad range of textures provides evidence for consistent microstructural evolution in a vertical transect of the shallow lithospheric mantle (35–55 km depth) beneath the Persani Mountains, SE Carpathians, Romania, due to ongoing plate convergence in the Carpathian Arc nearby. The recrystallized grain size, crystal preferred orientations strength, and resulting seismic anisotropy vary continuously and display a strong correlation to equilibrium temperatures, suggesting a continuous change in deformation conditions with depth. The shallowmost xenoliths have microstructures typical of high stress deformation, marked by strong recrystallization to fine grain sizes, which results in weak crystal preferred orientations and anisotropy. The deepest xenoliths have coarse-grained porphyroclastic microstructures and strong crystal preferred orientations. Replacive orthopyroxene structures, consuming olivine, and high H 2O concentrations in the pyroxenes are observed in some xenoliths indicating limited percolation of fluids or volatile-rich melts. Despite the high stress deformation and high H 2O contents in some of the studied xenoliths, analysis of olivine crystallographic orientations indicates that [100] slip systems, rather than “wet” [001] accommodate most of the deformation in all samples. Seismic anisotropy estimated from the measured olivine and pyroxene crystal preferred orientations suggests that the strike-parallel fast SKS polarization directions and ~ 1 s delay times measured in the SE Carpathians are likely the consequence of convergence-driven belt-parallel flow in the lithospheric mantle.