EGU Stephan Mueller Special Publication Series, 3, 105–120, 2002 c European Geosciences Union 2002 Late Cenozoic tectonic evolution of the Transylvanian basin and northeastern part of the Pannonian basin (Romania): Constraints from seismic profiling and numerical modelling D. Ciulavu1 *, C. Dinu1, and S. A. P.L. Cloetingh2 1Bucharest University, Faculty of Geology and Geophysics, 6 Traian Vuia st, 70 139 Bucharest, Romania *Present address: Shell Canada Ltd, 400-4th Ave S.W., P.O. Box 100, Station M, Calgary, Alberta T2P 2H5, Canada 2Vrije Universiteit, Department of Sedimentary Geology, Institute of Earth Sciences, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands Received: 22 December 2000 – Accepted: 16 July 2001 Abstract. The Transylvanian and the Pannonian basins are 50 mW/sqm (e.g. Demetrescu and Veliciu, 1991), (2) the av- presented as basins with different tectonic evolution. In this erage elevation of the Transylvanian Basin above sea level paper, seismic lines are used to highlight the late Ceno- is relatively high, around 600 m; the difference in eleva- zoic structural pattern of the Transylvanian Basin and of the tion of the Upper Miocene deposits from the Transylvanian northeastern sector of the Pannonian Basin. Late Miocene and the Pannonian basins is more than 1000 m (Ciupagea transpressional strike-slip faults have been interpreted in both et al., 1970). Moreover, a Miocene extensional origin of areas. North- to northeast-oriented regional stress field has the Pannonian Basin is documented, while a Miocene retro- been inferred from the structures interpreted in both areas. foreland setting (e.g. Sanders, 1998) or a Miocene compres- The similar structural pattern and stress field indicate a com- sional/transpressional setting have been suggested for the mon late Cenozoic tectonic evolution of these two areas. A Transylvanian Basin (e.g. Ciulavu et al., 2000). finite element model has also been carried out in order to in- Since the Pliocene, an overall compressional/- vestigate which orientation of late Cenozoic far-field stress transpressional stress regime is interpreted in the Pannonian are most likely to be responsible for the complex structural Basin (Horvath and Cloetingh, 1996), and in the Tran- pattern of these two areas. The finite element modeling also sylvanian Basin (Ciulavu et al., 2000). Both basins are indicates that the complex late Cenozoic structural pattern characterised by low to medium level of crustal seismic observed within the studied areas, but also around them, is activity (Horvath and Cloetingh, 1996; Ciulavu et al., 2000). most likely the result of a coherent northeast-oriented far- The average magnitude (M) of the earthquakes is about 4, field compressional stress. with a maximum magnitude up to 6 recorded only in the Our results indicate a Pliocene intraplate stress field simi- central part of the Pannonian Basin (Horvath and Cloetingh, lar to the one documented by Horvath and Cloetingh, (1996) 1996). in the Pannonian Basin. In the studied area, the boundary between the Transyl- vanian and Pannonian basins is the narrowest, being repre- sented only by few isolated metamorphic basement blocks 1 Introduction (Fig. 1). This closest position of the two basins makes the The Transylvanian and the Pannonian basins are located in comparison between them easier and more reliable. Un- the eastern sector of the European Alpine chain (Fig. 1). til now, structural and seismic data from the Transylvanian The Apuseni Mountains and few isolated mountains related Basin have been published (De Broucker, 1998; Ciulavu, to them represents the boundary between the Transylvanian 1999; Ciulavu et al., 2000) but not from the northeastern and the Pannonian basins. These two basins display differ- sector of the Pannonian Basin. Two seismic lines from the ent geological and geophysical characteristics: (1) the heat- Transylvanian Basin, acquired by former Shell Romania ex- flow of the Pannonian Basin is about of 100–120 mW/sqm, ploration BV, and four seismic lines from the northeastern while the heat-flow of the Transylvanian Basin is about 40– part of the Pannonian Basin, acquired by Prospectiuni SA are presented and compared in this paper. Geological bound- Correspondence to: D. Ciulavu ([email protected]) aries have been interpreted using borehole data. Published 106 D. Ciulavu et al.: Late Cenozoic tectonic evolution of the Transylvanian basin and the Pannonian basin (Romania) 28ºE 51ºN 12ºE 14ºE 16ºE 26ºE 18ºE 20ºE 22ºE 24ºE 60 km 50ºN UKRAINE EUROPEAN C N A R E T R PLATFORM S P E BVF A W T H I R. A D an ube N S E 48ºN A Satu Mare S Wien T E R Baia Mare EASTERN Budape st N C Y E P ALPS Transylvanian er A Fig.3 i-Adr basin R iati APUSENI c R Fa . R + . ult N D MOUNTAINS A a P n 46ºN a u A b G e NTF T N s PANNONIAN H U CARPATHIANS I t RN A BASIN E H N H T GETIC D Beograd S I U N O DEPRESSION Fig.8 A S R Bucuresti 44ºN I Adriatic D MOESIAN PLATFORM Cluj E R. Danube S Sea Black + Apuseni Sea B A L K A N I D E S Tirgu Mures Sofija 0 100 200 km Mountains 42ºN C Transylvanian a Mures River East European Basin r TF MOLDAVIA p Platform Sibiu Olt River a t h i E Brasov AIN a UKR South Carpathians n s Getic Ploiesti Depression S E R B CF Bucuresti IA Craiova IMF J i Moesian u Black Sea R i v e a r Platform e ãr un D BULGARIA Legend East Carpathians Apuseni Mountains Southern Carpathians Northern Apuseni Getic and Metamorphics Mountains Supragetic nappes Post-tectonic cover Southern Apuseni Flysch Nappes Mountains Severin Nappe and folded Danubian realm sedimentary strata Political Neogene volcanics Paleogene deposits Normal fault boundary Fig. 1. Geologic sketch of Romania. Location of study area is highlighted by the black rectangle. Symbols represent CF – Cerna Fault, IMF – Intra-Moesian Fault, TF – Trotus Fault, NTF – North Transylvanian Fault and BVF – Bogdan Voda Fault. The inset represents the geologic sketch of the Carpathian-Pannonian area. paleostressFig. data are1 Ciulavu also incorporated. et al.... The late Cenozoic responsible for the structural pattern in both areas. structural pattern of both areas is characterised by transpres- sional strike-slip faults indicating a common tectonic evolu- tion for this time span. A finite element model has also been 2 Regional geological setting carried out in order to investigate which orientation of late The arcuate Carpathian thrust-and-fold belt represents the Cenozoic far-field stress are most likely to be responsible for result of the Cretaceous to Miocene subduction and colli- the complex structural pattern of these two areas. A coherent sion between European plate and several smaller continental northeast-oriented far-field compressional field is most likely blocks to the south (e.g. Sandulescu, 1984). Southward, the D. Ciulavu et al.: Late Cenozoic tectonic evolution of the Transylvanian basin and the Pannonian basin (Romania) 107 Carpathians connect to the north-directed Bulgarian Balkan vath and Cloetingh, 1996). At the crustal scale, this stress chain along a north-trending dextral wrench system (Fig. 1), is reflected in transpressive structures in both areas (Tari, to which Cerna Fault belongs. To the west, the Carpathians 1994; Ciulavu et al., 2000). The importance of these faults is are linked to the eastern Alps. The eastern Alps are separated pointed out by the fact that movement along them are respon- from the Southern-Dinaric Alps by the peri-Adriatic line. sible for the uplift of the isolated mountains which cropout The main area of paleo-oceanic units in Romanian in the Pannonian Basin (Tari, 1994; Fig. 1). Carpathian is the Vardar-Mures unit, which derives from a Strike-slip deformation is responsible for a succession of former oceanic lithosphere that formed during Mid Trias- subsidence and uplifted areas from the Pannonian Basin, sic and Mid-Jurassic rifting phases (e.g. Sandulescu, 1984). from the southeastern sector of the Transylvanian Basin and The rocks of the Vardar-Mures unit cropout in the South- from a zone of the Carpathians and their foreland, bordered ern Apuseni Mountains and are interpreted to extend beneath by the seismically active intra-Moesian and Trotus faults the eastern part of the Transylvanian Basin (e.g. Sandulescu, (Horvath, 1993; Ciulavu et al., 2000). In the foreland area, 1984; Fig. 1). between the Trotus and the Intra Moesian faults, intermedi- West and east of the Mures unit, in the Northern Apuseni ate foci occur in a rather small, quasi-vertical area down to Mountains, East and South Carpathians, a system of base- about 200 km. These earthquakes are interpreted as a result ment and cover nappes was developed during the Cretaceous of stress-concentration at the bent of the Carpathians (Ro- (Bleahu, 1976; Sandulescu, 1984; Fig. 1). man, 1970). After the Paleogene, thin-skinned deformation is docu- mented in the external part of the Carpathians, where about 180 km of shortening took place (e.g. Roure et al., 1993; Ma- 3 Geological setting of the studied areas tenco, 1997). Late Burdigalian thin-skinned deformation is also documented in the internal part of the Carpathians, in the To highlight the geological setting of the studied areas, Pienides (Sandulescu, 1988). The Miocene thin-skinned de- chrono-stratigraphic charts of the Transylvanian Basin, af- formation from the external part progressively migrated east- ter Ciulavu et al. (2000), and of the northeastern part of the ward, along the Carpathian chain (e.g. Sandulescu, 1988). Pannonian Basin, after Bleahu et al. (1967) are presented The Miocene extension documented in the Pannonian Basin (Fig. 2). Ages are given in million of years after Rogl¨ is coeval with an important stage of nappe emplacement in (1996).