Werner Fielitz

VRANCEA99—the crustal structure beneath the southeastern Carpathians and the Moesian Platform from a seismic refraction profile in Romania

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.

Pre- and synorogenic burial metamorphism in the Ardenne and neighbouring areas (Rhenohercynian zone, central European Variscides)

Abstract The Rhenohercynian zone of the central European Variscides contains in the central Ardenne/Eifel/Brabant/Campine region of Belgium, France, Germany, and Luxemburg an omnipresent thermal overprint, which reaches in places uppermost epizonal conditions. Because of its distribution and structural relationships this metamorphism cannot be explained by a singular tectonometamorphic event. Field and microstructural observations, new illite crystallinity and other data relating to this metamorphism are presented. Together with a review of existing petrological, illite crystallinity, vitrinite reflectance, conodont alteration index, and fluid inclusion data, and their relationship to small- and regional-scale geological structures three distinct geotectonic situations can be described, which overlap and alternate in space and time. (1) The oldest metamorphism, which reached epizonal conditions, can be localized in the pre-Devonian basement rocks of the Brabant Massif and, with minor evidence, of the Stavelot Massif further south. It is clearly related to a pre-Variscan (Caledonian) deformation event and could be of burial origin. (2) The main regional metamorphism affected all Devonian and Carboniferous rocks and the lower Palaeozoic basement highs of the Ardenne/Eifel area. It does not exceed diagenetic conditions in some areas, but reaches maximal values of 500°C and 400 MPa in the Serpont and Rocroi Massif areas ( Theye and Fransolet, 1993 ; Robion et al., 1995 ). It is pre- to synkinematic to the main Variscan contractional penetrative deformation and correlates well with the age and thickness of the sedimentary cover rocks. The metamorphism is associated with the pre-Carboniferous preorogenic rifting stage of the Rhenohercynian basin and documents the peak of subsidence and sediment accumulation. We interpret it as diastathermal metamorphism. It reaches its maximum in the low-grade metamorphic zones of the Ardenne area, which are inverted synsedimentary structures. (3) The diastathermal metamorphism is overprinted in the central part of the Ardenne/Eifel area (Middle Devonian to Carboniferous rocks of the Dinant Synclinorium) by an anchizonal/beginning epizonal metamorphism. In the foreland basins north of the Midi Fault Zone it reaches diagenetic to lowermost anchizonal conditions. It is the expression of the burial by thick synorogenic mid- to Upper Carboniferous cover rocks, which were deposited in piggyback and foreland basins. This clastic wedge relates to the Carboniferous contraction of the Rhenohercynian basin, whose metamorphosed rocks were themselves affected by the deformation of the advancing orogenic front.

Variscan transpressive inversion in the northwestern central Rhenohercynian belt of western Germany

Abstract Important orogen-parallel strike-slip components during the convergence of the Variscan orogen of Central Europe have been increasingly recognized. These oblique-slip faults often develop from pre-existing tectonic boundaries and are influenced by pre-existing structural trends in the basement. In this paper evidence is presented for a 3–6 km wide distributed shear zone (here named the Monschau shear zone) in epizone metamorphic Cambro-Ordovician and Lower Devonian clastic rocks of the northwestern Rhenohercynian belt. Kinematic indicators including field observations of anomalous mullion and fold axes patterns, a set of two cleavages, stretching lineations, and shear and kink bands, microscopical observations of stretching-parallel asymmetric pressure shadows of quartz on prekinematic, epizone metamorphic pyrite porphyroblasts, and microfabrics of phyllosilicates by X-ray texture goniometry point to a sinistral tranpressive shear zone with emplacement of the southeast hangingwall under retrograde conditions. Coincidence of the anchizone-epizone metamorphic boundary with the change along strike from penetrative shearing to distributed thrusting suggests a ductile-brittle transition. The Monschau shear zone is situated on the backlimb of an allochthonous anticlinorium located above a basement ramp that corresponds roughly to a pre-orogenic synsedimentary normal fault. Its continuation shows up clearly in a DEKORP seismic reflection profile to a depth of 12–15 km.

Mafic Dykes from Heimefrontfjella and implications for the post- Grenvillian to pre-Pan-African geological evolution of western Dronning Maud Land, Antarctica

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.