2.5D seismic velocity modelling in the south-eastern Romanian Carpathians Orogen and its foreland Andrei Bocin a , Randell Stephenson b, * , Ari Tryggvason c , Ionelia Panea a , Victor Mocanu a , Franz Hauser d , Liviu Matenco a,b a Faculty of Geology and Geophysics, University of Bucharest, 6 Traian Vuia St., RO-70139, Bucharest, Romania b Faculty of Life and Earth Sciences, Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands c Department of Earth Sciences, Uppsala University, Uppsala, Sweden d Geophysical Institute, University of Karlsruhe, Hertzstrasse 16, D-76187 Karlsruhe, Germany Received 4 May 2004; received in revised form 25 November 2004; accepted 14 May 2005 Available online 4 November 2005 Abstract The DACIA-PLAN (Danube and Carpathian Integrated Action on Processes in the Lithosphere and Neotectonics) deep seismic reflection survey was performed in August–September 2001, with the objective of obtaining new information on the deep structure of the external Carpathians nappe system and the architecture of the Tertiary/Quaternary basins developed within and adjacent to the Vrancea zone, including the rapidly subsiding Focsani Basin. The DACIA-PLAN profile is about 140 km long, having a roughly WNW–ESE direction, from near the southeast Transylvanian Basin, across the mountainous south- eastern Carpathians and their foreland to near the Danube River. A high resolution 2.5D velocity model of the upper crust along the seismic profile has been determined from a tomographic inversion of the DACIA-PLAN first arrival data. The results show that the data fairly accurately resolve the transition from sediment to crystalline basement beneath the Focsani Basin, where industry seismic data are available for correlation, at depths up to about 10 km. Beneath the external Carpathians nappes, apparent basement (material with velocities above 5.8 km/s) lies at depths as shallow as 3–4 km, which is less than previously surmised on the basis of geological observations. The first arrival travel-time data suggest that there is significant lateral structural heterogeneity on the apparent basement surface in this area, suggesting that the high velocity material may be involved in Carpathian thrusting. D 2005 Elsevier B.V. All rights reserved. Keywords: Vrancea Zone; Tomographic inversion; Velocity model 1. Introduction A controlled source seismic experiment (DACIA- PLAN 1 ) was carried-out in August–September 2001, in Romania, at the same time as a deep regional 0040-1951/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2005.05.045 * Corresponding author. Tel.: +31 20 598 7347; fax+31 20 598 9943. E-mail address: randell.stephenson@falw.vu.nl (R. Stephenson). Tectonophysics 410 (2005) 273– 291 www.elsevier.com/locate/tecto