57 BULGARIAN GEOLOGICAL SOCIETY, 80-th Anniversary MECHANISMS OF LATE ALPINE EXTENSION IN THE EASTERN PART OF THE BALKAN PENINSULA Ivan Zagorchev Geological Institute, Bulgarian Academy of Sciences; e-mail: zagor@geology.bas.bg Key words: Balkan Peninsula; Alpine extension Two principal types of Alpine orogens can be outlined in the Balkan Peninsula, namely: orogens (Dinarides- Albanides-Hellenides; Vardarides) issued from former Tethyan oceanic and marginal basins, and orogens (Srednogorie, Balkanides) issued from peri-Tethyan basins and epicontinental island arcs, flysch and molasse troughs, and rifts. For most of the Alpine evolution these two types of tectonic environments have been separated by a domain of thickened continental crust – the Thracian plateau. This domain constituted in post- Palaeozoic times an area of shallow basins (Peri-Tethys; Paratethys) and island arcs, with almost continuous uplift and exhumation of metamorphic and igneous rocks of Cadomian, Hercynian and Alpine age. The extension processes within the Late Alpine orogens on the Balkan Peninsula followed different mechanisms and patterns. They depended both on the configuration of the basins and the structure of the crust, and on more general interplate and intraplate processes. The orogens usually had a complex and composite character inherited from the composite structure and pattern of the sedimentation systems. Thus, the complex Srednogorie volcanic island arc system consisted of separate basement-built islands, and of individual diachronously set on and evolving troughs. It evolved in Late Maastrichtian times through folding and complex thrusting into the Srednogorie orogen. The latter and its frontal arc (Morava-Rhodope zone) were subject to intense Palaeocene to Middle Eocene extensional collapse. Crustal areas built of light masses (migmatites, Palaeozoic and Mesozoic granites) exhibit a steady uplift tendency due to isostatic forces, and local rheological contrasts between basement and mantle have favoured their transformation in domal or horst structures. Palaeogene sedimentation occurred mostly in grabens situated between the uplift structures, and the sediment composition reflected a provenance from adjacent horsts. In the same time, linear marine troughs continued to evolve to the north (Balkan and Forebalkan). The Mid- Eocene retrocharriage thrusting in the Stara planina belt, and possible coeval thrusting events in the Rhodope region were followed by extensional collapse in Late Eocene and Early Oligocene times. Extension proceeded again in different manner in function of scale, configuration of near-surface structures, crustal thickness, and influence of existing or newly-formed fault belts and large faults. Collapse of the thickened crust and related decompression triggered in Late Eocene – Early Oligocene times melting at different depths, with mixing and mingling of magmas of mantle and crustal origin. The marine regression in Late Oligocene times was closely followed by transtension along some of the principal fault belts (Maritsa, Strouma, Sava), and formation of huge lakes. In earliest Miocene times a compression phase formed the Balkan orogen, and changed from transtension to transpression the stress state along the fault belts. Early to Middle Miocene planation followed, thus forming the principal peneplain of the Balkan Peninsula. Neogene tectonic movements followed several principal mechanisms that led to a new disintegration into a block mosaic: (1) Final shaping of the two orogen “branches” and orogenic collapse of the thickened continental crust (the Central-Balkan neotectonic region); (2) Continuing subduction under the Aegean arc, and back-arc extension; (3) Relative motions of the microplates; (4) Right-lateral strike-slip along main fault belts; (5) Continuing role of subducted lithospheric slabs; (6) Isostatic phenomena; (7) Irregular melting processes in depth; (8) Updoming and rifting; (9) Crustal extension and necking. The principal fluvio-lacustrine systems set on in Middle Miocene times, and continued their evolution throughout Neogene and Quaternary times. A few episodes of drastic change in the rate of vertical and horizontal movements along the principal fault belts are recorded, the most important being in Late Pontian and in Late Pliocene times. Two composite collapse centers were activated diachronously in SE Europe (Fig. 1). They coincided with the loops of the Alpine orogenic edifices, and possessed different collapse behaviour and rotation characteristics.