The Balkan Fold-Thrust Belt is a part of the northern branch of the Alpine-Himalayan orogen in the Balkan Peninsula and represents a Tertiary structure developed along the southern margin of the Moesian Platform. The thrust belt displays of two clearly distinct parts: an eastern one dominated exclusively by thin-skinned thrusting and a western part showing ubiquitous basement involvement. A wide transitional zone is locked between both parts where the structural style is dominantly thin-skinned, but with significant pre-Mesozoic basement involvement in the more internal parts. For the western thick-skinned part the poorly developed syn-orogenic flysch is a characteristic feature that along with the very restricted development of foreland basin suggests a rather limited orogenic shortening compared to the eastern part of the belt. The Tertiary Balkan Fold-Thrust Belt originated mainly through a basement-driven shortening and this is explained by the occurrence of compatibly oriented reactivated basement weak zones of pre-Carboniferous, Jurassic and Early Cretaceous ages. The proposed re-definition of the Balkan thrusts system and internal structure of the allochthons also call for significant re-assessment of the existing schemes of tectonic subdivision.
The easternmost coherent tract of the pre-Mesozoic basement of the Carpatho-Balkanide belt is exposed in Tvarditsa Stara Planina Mountain. Two metamorphic units – the Lazovo and Shivachevo complexes as well as the Tvarditsa pluton were distinguished, as their ages are still not well constrained. Here, we present U-Pb (ID-TIMS) data for the Shivachevo complex and the Tvarditsa pluton, putting this information in the regional context of the late Variscan geodynamic evolution of the region.
Gneisses from the deep structural levels of the European Variscan Belt are well exposed in the Central Sredna Gora in Bulgaria. In general, migmatites predominate, but unmigmatized domains (or domains with incipient migmatization) are also documented in this area. This paper presents new structural, petrographic and U-Pb isotope geochronological data from such an unmigmatized part of the Variscan high-grade metamorphic basement (the Koprivshtitsa Unit). A predominant part of this unit represents an alternation of metagranitoids and metabasites. The protolith crystallization age of the metagranitoids is constrained at 491.5 ± 7.6 Ma by U-Pb laser ablation method on zircons. This age coincides with the previously available Late Cambrian protolith ages of metabasic rocks that crop out within the adjacent migmatitic unit. The Korpivshtitsa Unit comprises also lesser orthogneisses with Late Neoproterozoic protoliths. Based on the available local and regional paleogeographic reconstruction schemes, we suggest that the Upper Cambrian magmatic rocks intruded Upper Neoproterozoic crust during the initial opening stages of the Rheic Ocean or a related basin. Subsequently, both were involved in the Variscan high-grade deformation. The contact of the Koprivshitsa Unit with the migmatitic part of the metamorphic complex coincides with a north-vergent greenschist facies thrust zone – the Chuminska Shear Zone. The exact time of the shearing is still not well constrained but it clearly postdates the Variscan high-temperature metamorphism of the gneisses.
Тhe late- to post-Variscan magmatic activity in the South Black See region took plaсe during a considerably long period of ca. 80 from Late Carboniferous up to Early–Mid Triassic as two episodes are distinguished. The obtained age of 253.6±1.6 Ma for the Klisura/Rozino granite gives further evidence of the contemporaneous intrusive and extrusive magmatism during late Permian and Early Triassic in the Sredna Gora and Balkan zones.
The rhyolites from the Central Balkan and Sinite Kamani areas are formed in a close time span from 254.6 to 250.7 Ma, while those next to the village of Vlado Trichkov are a little bit earlier (260.3±1.0 Ma). Both age groups follow the voluminous late Carboniferous to early Permian igneous activity that ceased around 290 Ma. The geochemical characteristics of the Vlado Trichkov dacite are comparable to those from the earlier magmatic event. On the other hand, the rocks from the Central Balkan and Sinite Kamani areas show trends of A-type granitoids. The origin of this magmatic products could be attributed to a crustal melting by mantle-derived melts, penetrating or underplating the crust in a post-orogenic setting during the initial extension and rifting, preceding the opening of the Vardar-Meliata Ocean.
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