TERRANES OF SERBIA Eds. Knežvić, V. & Krstić, B. pp. 31-36, Belgrade 1996 PRE-PALAEOGENE ALPINE TECTONIC UNITS AND TERRANES IN THE BORDER AREA OF SW BULGARIA AND YUGOSLAVIA Ivan Zagorchev Geological Institute, Bulgarian Academy of Sciences, Acad.G.Bonchev-Street build. 24, 1113 Sofia Abstract: Southwest Bulgaria belongs to the Late Cretaceous Srednogorie and Morava-Rhodope regions (superunits). The Mid-Cretaceous Morava and Struma superunits are distinguished in the Morava-Rhodope region. Although the boundaries between the superunits are thrusts of medium to considerable amplitude, existing geological evidence usually shows close relationships that point at parautochthonous position in the southern margin of Eurasia. Thus, the superunits cannot be considered as terranes (in the strict sense) that have accreted during Alpine orogenies, and rather as Alpine native, or indigenous terranes. KEY WORDS: Alpine tectonic units; SW Bulgaria 1. INTRODUCTION The complex geological structure of Southwest Bulgaria and the adjacent territories of Yugoslavia has been subject of considerable controversies in Bulgarian and Serbian publications. The modern tectonic approach to the Alpine thrust tectonics of the area is much indebted to S. Radovanovic, S. Boncev, V. Petkovic and E. Boncev. Later on, the pre- Palaeogene (mostly, of Mid-Cretaceous age) fold and thrust tectonics was revealed as the most important for the formation of the present pattern, and the pre- Palaeogene Alpine tectonic units were correlated across the border. Plate tectonics and terrane analysis approaches have been recently also performed, the Alpine orogen being regarded as a superposition of several orogenic edifices. The present paper aims to demonstrate the relations between the Alpine tectonic units in the border area, and to discuss whether some of them could represent real tectonic terranes accreted during the Alpine plate motions and orogenies. It is also a tribute to the role of Prof. S. Karamata in the application of modern approaches to the tectonics and geodynamics of the Balkan Peninsula. 2. TECTONIC SETTING Considering that the effects of the late Alpine (Palaeogene and early Neogene) tectonics were of minor importance, the general tectonic picture in Southwest Bulgaria and the adjacent territory of Yugoslavia is dominated (Figure 1) by the Late Cretaceous tectonics. The Late Cretaceous complex arc of the Srednogorie zone was bordered to the South by the Morava-Rhodope (Macedonian- Rhodope) region (superunit) of thickened continental crust that consisted mostly of two Mid-Cretaceous superunits: the autochthonous to parautochthonous Struma Superunit and the allochthonous Morava Superunit. The Morava Superunit consists of several thrust sheets (units) each of them containing a different set of pre-Alpine formations. Pre-Alpine type sections. The comparison of the pre-Alpine (and usually, pre-Permian) basement in the Alpine tectonic units reveals (Figure 2) the presence of several section types. Their succinct characteristics (ZAGORCHEV, 1990a) are as follows: 1) Rhodope type: superpositional position of the Precambrian Rhodopian Supergroup above the Ograzhdenian (Prerhodopian) Supergroup; both are intruded by Precambrian and Hercynian granitoid plutons; 2) Vlahina type: transgressive covering of the Ograzhdenian Supergroup by the Vendian - Early