VRANCEA99 - the crustal structure beneath the southeastern Carpathians andthe 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 t he Eastern Carpathians in Romania. As part of the seismic experiment, a 300 -km-long refraction profile was recorded between the cities of Bacau and Bu charest, traversing the Vrancea epicentral region in NNE-SSW direction. The results deduced using forward and inverse ray trace modelling indicate a m ufti-layered crust. The sedimentary succession comprises two to four seismi c 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. V elocities in the upper crystalline crust are 5.9-6.2 km/s. An intra-crustal discontinuity at 18-31 Inn divides the crust into an upper and a lower lay er: Velocities within the lower crust are 6.7-7.0 lan/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. Constrai nts on upper mantle seismic velocities (7.9 km/s) are provided by Pn arriva l times from two shot points only. Within the upper mantle a low velocity z one is interpreted. Travel times of a PLP reflection define the bottom of t his low velocity layer at a depth of 55 km. The velocity beneath this inter face 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-s kinned shortening in the crystalline crust. On the autochthonous cover of t he Moesian platform several blocks can be recognised which are characterise d by different lithological compositions. This could indicate a pre-structu ring of the platform at Mesozoic and/or Palaeozoic times with a probable ac tive involvement of the Intramoesian and the Capidava-Ovidiu faults. Especi ally the Intramoesian fault is clearly recognisable on the refraction line. No clear indications of the important Trouts fault in the north of the pro file could be found. In the central part of the seismic line a thinned lowe r crust and the low velocity zone in the uppermost mantle point to the poss ibility of crystal delamination and partial melting in the upper mantle. (C ) 2001 Elsevier Science B.V. All rights reserved.