The NNE-trending Upper Rhine Graben (URG) of the European Cenozoic Rift System developed from c. 47 Ma onwards in response to changing lithospheric stresses in the northwestern foreland of the Alps. ...The composite graben structure consists of three segments, each c. 100 km long and 30–40 km wide, but flares to c. 60 km near its southern and to c. 80 km near its northern termination. Normal faulting induced a total extension of 5–8 km of the 1–2 km thick Mesozoic sedimentary Franconian platform and underlying Variscan basement rocks. However, distribution of an up to 3.5 km thick sedimentary graben fill and cumulative displacements near Eastern and Western Main Border fault systems suggest that subsidence of the graben floor and shoulder uplift created strong cross-sectional asymmetries. Cumulative W-down displacements >3 km along strongly segmented transfer faults in the east contrast with E-down displacements <3 km and major monoclinal “block fields” in the west. Both location and asymmetry of the URG appear to be related to lithospheric shear zones that originated within the central parts of the Variscan orogen between c. 330 and 315 Ma. Following pervasive deformation, HT/LP regional metamorphism and emplacement of granodioritic-granitic plutons a c. 50-km-thick orogenic crust were thinned to an about 30-km-thick two-layered crust above a reconsolidated and relatively planar crust-mantle boundary (Moho). In the URG area extensional thinning of the crust appears to have occurred mainly along a composite NNE-striking and mainly W-down “East Rhine Detachment”, which is partly exposed along the Wehratal, Omerskopf, Otzberg and other mylonitic-cataclastic shear zones in the basement of the eastern graben shoulder. These shear zones probably extend into lower crustal levels, where they are revealed as gently W-dipping seismic reflectors beneath and west of the URG. Major W-down displacements probably account for the mapped abundance of high-grade metamorphic basement rocks on the eastern graben shoulder in contrast to the predominantly low-grade metamorphic to unmetamorphosed sedimentary-volcanic rocks exposed on the western shoulder. Although between c. 310 and 270 Ma NE-trending Permocarboniferous volcanic-sedimentary basins of the URG area subsided along upper crustal faults that mimic the trend of Variscan faults, initial broad lithospheric cooling from c. 270–200 Ma led to subsidence of a distinctly NNE- to SSW-oriented embayment that was probably underlain by thinner Palaeozoic crust in the area of the NNE-trending East Rhine Detachment. After re-emergence of the platform above sea level in late Mesozoic times, the deep-reaching W-dipping “extensional defects” of the East Rhine Detachment exerted a primary lithospheric scale control on both location and cross-sectional asymmetry of the Cenozoic graben structure. NE- and NW-striking, strongly altered and more shallow rooted Permocarboniferous or Mesozoic faults exerted secondary upper crustal controls on transfer faults and the accommodation zones near the terminations and segment boundaries of the URG. Deep crustal to upper lithospheric asymmetries continue to influence the neotectonic setting of the URG, such as westward rising earthquake hypocentres. Seismic activity along the URG appears to be part of a >600 km long zone that delimits the trailing edge of a SW-moving lithospheric block. In the URG area, NE–SW-oriented seismic anisotropy at sublithospheric depths of c. 60–80 km suggest active mantle flow in this direction as a possible driving force for the reactivation of pre-graben lithospheric shear zones.
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 Braşov intramontane basin (western domain) to the external nappes and Focşani 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 Şuşiţa 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 (~240m) since the late Middle Pleistocene, whereas older strath- and younger dominant fill-terraces of the western domain designate a lower amount (~90m) 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.
•Terrace distribution in SE Carpathians indicate tectonics as their dominant control•Minimum terrace formation ages derived with infrared stimulated luminescence dating•Middle Pleistocene-Holocene strath-terraces show uplift-driven valley incision events•Loess deposition and palaeosol formation synchronous to terrace development•Fluvial incision document 240 m of tectonic uplift since Middle Pleistocene
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.
The Romanian East Carpathians display large-scale heterogeneities along the mountain belt, unusual foredeep geometries, significant post-collisional and neotectonic activity, and major variations in ...topography, mostly developed in the aftermath of late Miocene (Sarmatian; ∼11 Ma) subduction/underthrusting and continental collision between the East European/Scythian/Moesian foreland and the inner Carpathians Tisza-Dacia unit. In particular, the SE corner of the arcuate orogenic belt represents the place of still active large-scale differential vertical movements between the uplifting mountain chain and the subsiding Focşani foredeep basin. In this key area, we have analysed the configuration of the present day landforms and the drainage patterns in order to quantify the amplitude, timing and kinematics of these post-collisional late Pliocene–Quaternary vertical movements. A river network is incising in the upstream a high topography consisting of the external Carpathians nappes and the Pliocene–Lower Pleistocene sediments of the foreland. Further eastwards in the downstream, this network is cross-cutting a low topography consisting of the Middle Pleistocene–Holocene sediments of the foreland. Geological observations and well-preserved geomorphic features demonstrate a complex succession of geological structures. The late Pliocene–Holocene tectonic evolution is generally characterised by coeval uplift in the mountain chain and subsidence in the foreland. At a more detailed scale, these vertical movements took place in pulses of accelerated motion, with laterally variable amplitude both in space and in time. After a first late Pliocene uplifting period, subsidence took place during the Earliest Pleistocene resulting in a basal Quaternary unconformity. This was followed by two, quantifiable periods of increased uplift, which affected the studied area at the transition between the Carpathians orogen and the Focşani foreland basin in the late Early Pleistocene and the late Middle to late Pleistocene. Both large-scale deformation events affected the western Focşani basin flank, tilting the entire structure with ∼9° during the late Early Pleistocene and uplifted it as a block during the early Late Pleistocene. The late Early Pleistocene tilting resulted in ∼750 m uplift near the frontal monocline and by extrapolation in a presumed 3000 m uplift near the central parts of the Carpathians. The late Middle to late Pleistocene cumulative uplift reaches ∼250 m and correlates with a contemporaneous progradation of the uplifted areas towards the Focşani Basin. The uplifting events are separated by a second Quaternary unconformity. On the whole, the late Pliocene–Quaternary evolution of the Carpathians orogen/Focşani basin structure indicate large-scale differential uplift during the latest stages of a continuous post-collisional orogenic evolution.
VRANCEA'99 is a seismic refraction line that was carried out in 1999 to investigate the deep structure and physical properties of the upper lithosphere of the southeastern Carpathians and its ...foreland. It runs from the city of Bacau to the Danube River, traversing the Vrancea epicentral area of strong intermediate-depth seismicity and the city of Bucharest.
Interpretation of P-wave arrivals led to a velocity model that displays a multi-layered crust with velocities increasing with depth. The range of P wave velocities in the sedimentary cover increases from N to S and a structuring of the autochthonous basement of the Moesian Platform is observed. The crystalline crust displays thickness variations, but at the same time the lateral velocity structure along the seismic line remains almost constant. An intra-crustal boundary separates an upper crust from the lower crust. Within the upper mantle a low velocity zone is detected at a depth of about 55-km.
The interpretation of observable S-waves resulted in a velocity model that shows the same multi-layered crust, with S-velocities increasing similarly with depth as the P-waves. The corresponding Poisson's ratio is highly variable throughout the crust and ranges from 0.20–0.35 for the sedimentary cover to 0.22–0.25 for the crystalline crust. The interpretation of the
V
p,
V
s and Poisson's ratio in petrological terms suggests a large variety of rocks from sand and clay to sandstone, limestone and dolomite within the sedimentary cover. Within the crystalline crust the most probably rock types are granite, granodiorite, granite–gneiss and/or felsic amphibolite–gneiss in the upper part and gneiss and /or amphibolite in the lower part.
Based on the 2-D seismic velocity model, a density model is developed. Density values are assigned to each layer in agreement with the P-wave velocity model and with values accepted for the geological units in the area. After several iterations a good fit between the computed and observed Bouguer anomalies was obtained along the seismic line.
The VRANCEA99 seismic refraction experiment is part of an international and multidisciplinary project to study the intermediate depth earthquakes of the Eastern Carpathians in Romania. As part of the ...seismic experiment, a 300-km-long refraction profile was recorded between the cities of Bacau and Bucharest, traversing the Vrancea epicentral region in NNE–SSW direction.
The results deduced using forward and inverse ray trace modelling indicate a multi-layered crust. The sedimentary succession comprises two to four seismic layers of variable thickness and with velocities ranging from 2.0 to 5.8 km/s. The seismic basement coincides with a velocity step up to 5.9 km/s.
Velocities in the upper crystalline crust are 5.9–
6.2 km/s. An intra-crustal discontinuity at 18–31 km divides the crust into an upper and a lower layer. Velocities within the lower crust are 6.7–7.0 km/s. Strong wide-angle PmP reflections indicate the existence of a first-order Moho at a depth of 30 km near the southern end of the line and 41 km near the centre. Constraints on upper mantle seismic velocities (7.9 km/s) are provided by Pn arrival times from two shot points only. Within the upper mantle a low velocity zone is interpreted. Travel times of a PLP reflection define the bottom of this low velocity layer at a depth of 55 km. The velocity beneath this interface must be at least 8.5 km/s.
Geologic interpretation of the seismic data suggests that the Neogene tectonic convergence of the Eastern Carpathians resulted in thin-skinned shortening of the sedimentary cover and in thick-skinned shortening in the crystalline crust. On the autochthonous cover of the Moesian platform several blocks can be recognised which are characterised by different lithological compositions.
This could indicate a pre-structuring of the platform at Mesozoic and/or Palaeozoic times with a probable active involvement of the Intramoesian and the Capidava–
Ovidiu faults. Especially the Intramoesian fault is clearly recognisable on the refraction line. No clear indications of the important Trotus fault in the north of the profile could be found. In the central part of the seismic line a thinned lower crust and the low velocity zone in the uppermost mantle point to the possibility of crustal delamination and partial melting in the upper mantle.
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.
Middle Miocene (Sarmatian) convergence created the fold and thrust belt of the Eastern Carpathians of Romania, which subsequently experienced post-collisional crustal deformation combined with ...calc-alkaline and alkalic-basaltic volcanism in late Miocene–Quaternary time. This deformation led to the rise of the Cǎlimani–Gurghiu–Harghita volcanic mountains and to the subsidence of the N–S-oriented intramontane Borsec/Bilbor–Gheorgheni–Ciuc and Braşov pull-apart basins, and the E-oriented monocline-related Fǎgǎraş basin. The regional drainage network is the composite of:
(1)
Older E-, SE- and S-flowing rivers, which cross the Carpathians, radiate towards the foreland and were probably established during the Middle Miocene (Sarmatian) collision event.
(2)
A more recent drainage system related to the contemporaneous development of the volcanoes and intramontaneous basins, which generally drains westward into the Transylvanian Basin since late Miocene time and has been capturing the older river system.
The older river drainage system has also been modified by Late Pliocene–Quaternary folding, thrusting and monoclinal tilting along the Pericarpathian orogenic front and by reactivated transverse high angle basement faults, which cross the Eastern Carpathian foreland.
Two groups of geochemical different dykes have been identified in the Grenville-aged basement of Heimefrontfjella. The first group comprises dykes of continental tholeiite composition which probably ...intruded during the final stage of indentation of the Kaapvaal—Grunehogna Craton into Laurentia. One dyke of this group yielded an U—Pb zircon SHRIMP age of 1033 ± 7 Ma. The second group has an E-type MORB composition and may be related to ocean floor basalts of the Mozambique Ocean between East and West Gondwana. A preliminary U—Pb SHRIMP age of 586 ± 7 Ma for a single zircon crystal was obtained from a dyke of the second group. During the Pan-African orogeny both dyke groups underwent metamorphism and tectonism at different grades: up to amphibolite-facies in the eastern and southern Heimefrontfjella, and at greenschist-facies in the western and northern Heimefrontfjella. The older dykes may be correlated with the Equeefa suite of southern Natal whereas the younger dyke group is not correlatable with any known mafic intrusions or lava flows in adjacent regions.
