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.     

Exhumation and relief history of the Southern Carpathians (Romania) as evaluated from apatite fission track chronology in crystalline basement and intramontane sedimentary rocks

The combination of apatite fission track (FT) thermochronology from basement units and the FT age distributions of apatites in the Miocene intramontane sedimentary rocks allows describing the exhumation history of the central segment of the Southern Carpathians, Romania. Exhumation and cooling from the total track annealing temperature (>120°C) of the Cozia and Cibin massifs occurred in the Palaeocene–Early Eocene. Between the Eocene and Middle Miocene, there was a stagnation period concerning vertical displacement; the presently exposed part of the basement was buried in shallow depth. The present crests of the Cozia and Cibin Mountains were at temperatures around 80°C and 50°C, respectively. The second exhumation period occurred in Middle Miocene times. The magnitude of the Miocene vertical displacement is on the order of the present-day relief. The vertical apatite FT age distribution in the basement and the age clusters in the sedimentary rocks prove that the levels of the crests were already close to the surface in Palaeogene times. Therefore, the post-Palaeocene erosional removal from the crest zones is very limited.     

t*— an unsuitable parameter to characterize anelastic attenuation in the Eastern Carpathians

Investigation of teleseismic P -wave recordings at a temporary network in the Eastern Carpathians, equipped with predominantly short-period sensors, is compared with synthetic modelling of anelastic attenuation of teleseismic waves in the upper mantle. Using the t * approach, we examine variations of amplitude decrease over frequency for teleseismic recordings in the frequency band 0.5–1.5 Hz. The results reveal a consistent pattern of increased t * values in the centre of the network, in the Vrancea region at the bend of the Carpathian Arc, although the magnitude of the observed variation in t * is much higher than expected. Synthetic t * parameter computations for the same event-receiver configurations reproduce the observed pattern in terms of relative variations. However, the amplitude of the synthetic t * values explains only 10–20 per cent of the observed variation in t *. t * is not a direct measure for anelastic attenuation but rather for a combination of anelastic and other attenuating effects such as scattering and amplitude fluctuation related to velocity inhomogeneities. If regional amplitude variations are solely attributed to anelastic attenuation, all other effects are mapped into Q . We discuss the role of anelastic attenuation and other effects in the case of the Eastern Carpathians and conclude that t * is an unsuitable parameter to characterize anelastic attenuation in the Eastern Carpathians.     

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.     

A possible UHP-eclogite in the Leaota Mts. (South Carpathians) and its history from high-pressure melting to retrograde inclusion in a subduction melange

An unusual eclogite pod containing impure jadeite to omphacite pyroxene with anti-phase domain (APD) visible under the optical microscope is hosted in a medium-grade matrix in the Leaota Massif. Textural relationships support the hypothesis of garnet formation by peritectic reactions involving high-jadeite clinopyroxene, while inclusions entrapped in garnet, mainly a wide range of amphibole compositions, seemingly crystallised from a fluid phase in disequilibrium with the garnet host. Thermobarometric estimates indicate UHP, HT peak conditions (2.8–3.2 GPa, 780–840°C) inconsistent with those of the associated rocks, followed by a complicated exhumation path. The P–T-array of the retrograde path suggests syn-subduction exhumation and tectonic inclusion in a melange with a matrix derived from the accretionary wedge. Partial melting of the subducting slab, which induces tectonic weakening, may be an effective mechanism in detaching and enclosing small eclogite pods in metamorphic terrains.     

Petrology of two granite types from the Tribeč Mountains,Western Carpathians: An example of allanite (+ magnetite) versus monazite dichotomy

Two granitoid series have been distinguished in the Tribeč Mountains (Western Carpathians) on the basis of the contrasting petrological behaviour of their accessory minerals. The allanite–magnetite-bearing (AM) tonalite–granodiorite–granite series typically contains dark magmatic enclaves and is produced from oxidizing, more hydrous (about 5.2 wt.% water) melts that were emplaced at depths equivalent to 350 ± 100 MPa. The monazite-bearing (M) tonalite–granodiorite–granite series contains only metamorphic xenoliths and comes from reduced and drier (about 2.3 wt.% water) melts that were probably emplaced at a shallower depth. The inferred magma properties probably reflect source rock effects. The mineralogical discrimination scheme between the AM and M series corresponds broadly to I- and S-type subdivision and may be useful not only for the granitoid bodies in the Western Carpathians but also for the whole Variscan orogenic belt.     

Climatic implications of cirque distribution in the Romanian Carpathians: palaeowind directions during glacial periods

The many glacial cirques in the mountains of Romania indicate the distribution of former glacier sources, related to former climates as well as to topography. In the Transylvanian Alps (Southern Carpathians) cirque floors rise eastward at 0.714 m km⁻¹, and cirque aspects tend ENE, confirming the importance of winds from some westerly direction. There is a contrast between two neighbouring ranges: the Făgăraş, where the favoured aspect of cirques is ENE, and the Iezer, where the tendency is stronger and to NNE. This can be explained by the Iezer Mountains being sheltered by the Făgăraş, which implies precipitation‐bearing winds from north of west at times of mountain glaciation. Palaeoglaciation levels also suggest winds from north of west, which is consistent with aeolian evidence from Pleistocene dunes, yardangs and loess features in the plains of Hungary and southwestern Romania. In northern Romania (including Ukrainian Maramureş) the influence of west winds was important, but sufficient only to give a northeastward tendency in cirque aspects. This gave stronger asymmetry than in the Transylvanian Alps, as the northward (solar radiation incidence) tendency in these marginally glaciated mountains was less diluted by wind effects. Cirque floors in northern Romania are lower also in northeast‐facing cirques. In general, cirque aspects result from several factors and the mean tendency is not downwind, but is displaced from poleward by wind and by minor effects. Copyright © 2010 John Wiley & Sons, Ltd.     

Stoneflies (Plecoptera) in some volcanic mountain ranges of the West Carpathians (Slovakia) and the impact of human activities

44 stonefly species were recorded in the Pol'ana mountains (Central Slovakia). A typical representative of the west-east fauna, Rhabdiopteryx navicula, was found. The Hučava stream basin represents the southern border of the montane and submontane distribution of stonefly species, which are bound to the river basins in high and central mountain regions of the West Carpathians. Six characteristic associations of stoneflies were recorded. Shredders predominated in the upper, and scrapers and collectors in the lower section of the Hučava stream. Detritivores were more abundant in the gravel-to-sandy mesohabitat, whereas predators and scrapers prevailed in rocky mesohabitat. Total annual production was low — 340.4 g · m⁻² (dry weight), though being three-times higher in the rocky mesohabitat than in the gravel-to-sandy mesohabitat. The annual P/B ratio of stoneflies reached 3.3. Clear-cutting and operation of small hydropower stations were found to have a significant impact on the taxocoenoses of stoneflies.     

Historical and contemporary aspects of pluriactivity in the Curvature Sub–Carpathians of Romania

This paper focuses on the Curvature Sub–Carpathians, especially with respect to Buzau County. The region as a whole has always been relatively well-settled and agriculture has usually been combined with a range of local industrial and handicraft employments. A study of traditional domestic activities brings out a close connection between specific natural resources and characteristic features of traditional peasant household work. The Prahova Valley has been particularly favoured on account of the oil industry and a major transport axis connecting Bucharest with Brasov via Ploiesti, but elsewhere industry did not spread extensively beyond the county centres (Buzau, Focsani, Pitesti and Targoviste) and their immediate environs such as Colibasi and Doicesti. During the present transition phase the rural population is struggling to survive by combining elements of the modern and traditional economies. The main rural centres are relatively well off while the outlying villages are experiencing severe poverty problems. Apart from brandy distilling, traditional rural industries are no longer viable, although alternative systems of pluriactivity are under discussion. This revised version was published online in July 2006 with corrections to the Cover Date.     

Cretaceous evolution of a metamorphic core complex,the Veporic unit,Western Carpathians (Slovakia): P–T conditions and in situ40Ar/39Ar UV laser probe dating of metapelites

Alpine metamorphism, related to the development of a metamorphic core complex during Cretaceous orogenic events, has been recognized in the Veporic unit, Western Carpathians (Slovakia). Three metamorphic zones have been distinguished in the metapelites: 1, chloritoid + chlorite + garnet; 2, garnet + staurolite + chlorite; 3, staurolite + biotite + kyanite. The isograds separating the metamorphic zones have been modelled by discontinuous reactions in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (KFMASH). The isograds are roughly parallel to the north‐east‐dipping foliation related to extensional updoming along low‐angle normal faults. Thermobarometric data document increasing P–T conditions from c. 500 °C and 7–8 kbar to c. 620 °C and 9–10 kbar, reflecting a coherent metamorphic field gradient from greenschist to middle amphibolite facies. ⁴⁰Ar/³⁹Ar data obtained by high spatial resolution in situ ultraviolet (UV) laser ablation of white micas from the rock slabs constrain the timing of cooling and exhumation in the Late Cretaceous. Mean dates are between 77 and 72 Ma; however, individual white mica grains record a range of apparent ⁴⁰Ar/³⁹Ar ages indicating that cooling below the blocking temperature for argon diffusion was not instantaneous. The reconstructed metamorphic P–T–t path is ‘clockwise’, reflecting post‐burial decompression and cooling during a single Alpine orogenic cycle. The presented data suggest that the Veporic unit evolved as a metamorphic core complex during the Cretaceous growth of the Western Carpathian orogenic wedge. Metamorphism was related to collisional crustal shortening and stacking, following closure of the Meliata Ocean. Exhumation was accomplished by synorogenic (orogen‐parallel) extension and unroofing in an overall compressive regime.