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Two sandstone samples from the upper and lower part of the Ordovician succession of Svoge Unit were analyzed in order to determine their detrital zircon U–Pb age spectra using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). One sample was collected from the Hirnantian siliciclastic glaciomarine deposits of the Sirman Formation. The other sample is amiddle Darriwilian sandstone from the lower half of the Grohoten Formation. Additionally, micropetrographic studies were performed. They are composed predominantly of detrital monocrystalline quartz grains with uniform or rarely undulose extinction and only sporadically presented feldspar grains, muscovite flakes and single rock fragments. The heavy mineral assemblages are also characterized mostly by detrital zircon. As suggested from previous studies of the Hirnantian glaciomarine deposits of Sirman Formation, the provenance of siliciclastic material was most probably associated with sedimentary recycling of maturesands deposited across the North Gondwana Platform. Multi-dimensional scaling statistical technique allowed a reliable objective identification of the potential source areas in Northern Africa and palaeogeographic reconstructions were made. Both samples are close to the Trans-Saharan Belt provenance, which is the most probable source for the detrital component. Our data supportthe idea proposed by previous researchers that the present-day position of some of these terranes implies significant dextral strike-slip displacement, probably due to the movement on the Pangea megashear during the Carboniferous and Permian.

Two sandstone samples from the upper and lower parts of the Ordovician succession of the Svoge Unit were analyzed in order to determine their detrital zircon U-Pb age spectra using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Multi-dimensional scaling statistical technique allowed a reliable objective identification of the potential source areas in North Africa and paleogeographic reconstructions were made. Both samples indicate a close proximity to the Trans-Saharan Belt provenance, which is the most probable source of the detrital component.

Two contrasting Variscan units are juxtaposed in the area of Central Stara Planina Mountain. They are represented by two metamorphic complexes: a high-grade (Central Srednogorie high-grade metamorphic complex -CSHGMC) and low-grade (Stara Planina low-grade metamorphic complex -SPLGMC). Unlike the area to the west (Zlatitsa Stara Planina Mountain), the contact between them is severely overprinted by Palaeocene-Eocene thickskinned thrusting and Miocene-Quaternary extension. The suturing/docking of the two units is well constrained as Visean (∼ 330 Ma) and after a gap of about 15-20 Ma voluminous batches of granite magma were emplaced into the SPLGMC. These granites (Karlovo-Ribaritsa suite) are almost everywhere intensively foliated and they delineate a Late Variscan strike-slip dominated tectonic belt. As a contrast, another widespread granitoid suite (Kalofer granitoids) does not display gneissic fabric and can be regarded as a part of the batholithic-scale, composite Hisarya-Pastrovo pluton (303 ± 3 Ma). Our study documented contacts of the Kalofer granitoid suite, the type of the internal fabric and the presence of inclusions. The perfectly outcropped Palaeogene Botev Vrah allochthon is built by a sill-like body of Kalofer granitoids, emplaced conformably into SPLGMC. These granitoids are foliated along their margins as well as in shear zones. Despite this solid-state overprint, numerous meso-scale features indicate intrusive relations. On the other hand, it is well known that Kalofer granitoids are intrusive into CSHGMC gneisses and they also containThe work on this paper was initiated during the realization of PhD project (Eleonora Balkanska) funded by grant VU-13/06 from the Bulgarian National Science Fund (NSF). Additional support was provided by another NSF grant (DMU-03/41). 709km-scale stopped blocks/rafts from them. These relations undoubtedly indicate that the composite Hisarya-Pastrovo pluton is stitching one of the most important Variscan contacts in the central part of the Balkan peninsula.

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<p>Sredna Gora Zone in Bulgaria is confined between the Balkan fold-thrust belt to the north and the Rhodope metamorphic complex to the south. The pre-Mesozoic basement of the central parts of the zone consists of Variscan high-grade metamorphic rocks intruded by Late Carboniferous granitoid plutons. They are transgressively overlaid by Triassic epicontinental, arc-related Upper Cretaceous volcaniclastic and Paleocene continental deposits. The Paleogene-Neogene sediments of the Thrace basin cover unconformably the older rock sequences. The zone experienced several compressional and extensional events during the Alpine time followed by post-orogenic extension in the Cenozoic.</p><p>We performed apatite fission-track analysis on rocks from the central, topographically highest parts of the Sredna Gora Zone in order to constrain the cooling history of the Variscan basement. With this aim four granitic samples were collected at different altitude (between 565 and 1604 m) from the tectonically uninterrupted section along the slope of Sredna Gora Mountains. The samples were processed and analyzed in the newly established Low-Temperature Thermochronology Laboratory in Bulgaria using LA-ICP-MS technique.</p><p>The samples yield apatite FT ages between 41.6 ± 2.6 (the highermost sample) and 39.4 ± 3.1 (the lowermost sample). The obtained confined mean tracks lengths are between 12.81 and 14.06 µm with standard deviation between 0.99 and 2.11 µm. The Dpar values vary from 1.75 µm to 1.46 µm (with standard deviation of approx. 0.20 µm).</p><p>The obtained positive age-altitude correlation suggests indeed that the studied part of the basement has cooled as one single block. The apparent exhumation rate is estimated to 0.46 mm/year. However, the positive Dpar-age correlation implies that the age dispersion could be influenced by apatite kinetic variability and hence relatively different closure temperature for the analysed samples may be suggested. Therefore, we consider the estimated apparent exhumation rate as only the minimum possible rate. The thermal models of the analysed samples (using HeFTy software) also show moderate cooling rates in the period between 45 and 35 Ma. This cooling could be related to the period of post-orogenic denudation and extension during the Eocene, associated with formation of the Thrace basin to the south-southeast. This extensional event, known from the whole Balkan Peninsula, is well documented in the neighbouring Balkan fold-thrust belt and the Rhodope metamorphic complex from where much faster exhumation rates were reported.</p><p> </p><p><strong>Acknowledgements</strong>. The study is supported by the grant 04/9 funded by the National Science Fund, Ministry of Education and Science, Bulgaria.</p>

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