Fission-track constraints on the thermal and tectonic evolution of the Apuseni Mountains (Romania)

New zircon and apatite fission-track (FT) data, including apatite thermal modelling, are combined with an extensive literature survey and reconnaissance-type structural fieldwork in the Eastern Apuseni Mountains. This leads to a better understanding of the complex structural and thermal history of a key area at the boundary between two megatectonic units in the Balkan peninsula, namely the Tisza and Dacia Mega-Units. Following Late Jurassic obduction of the Transylvanian ophiolites onto a part of the Dacia Mega-Unit, that is, the Biharia nappe system, both units were buried to a minimum of 8km during late Early Cretaceous times when these units were underthrust below the Tisza Mega-Unit consisting of the present-day Codru and Bihor nappe systems. Tisza formed the upper plate during Early Cretaceous (‘Austrian') east-facing orogeny. Turonian to Campanian zircon FT cooling ages (95-71Ma) from the Bihor and Codru nappe systems and the Biharia and Baia de Arieş nappes (at present the structurally lowest part of the Dacia Mega-Unit) record exhumation that immediately followed a second Cretaceous-age (i.e. Turonian) orogenic event. Thrusting during this overprinting event was NW-facing and led to the overall geometry of the present-day nappe stack in the Apuseni Mountains. Zircon FT ages, combined with thermal modelling of the apatite FT data, show relatively rapid post-tectonic cooling induced by a third shortening pulse during the latest Cretaceous (‘Laramian' phase), followed by slower cooling across the 120°-60°C temperature interval during latest Cretaceous to earliest Paleogene times (75-60Ma). Cenozoic-age slow cooling (60-40Ma) was probably related to erosional denudation postdating ‘Laramian' large-scale updomin

Cenozoic granitoids in the Dinarides of southern Serbia: age of intrusion, isotope geochemistry, exhumation history and significance for the geodynamic evolution of the Balkan Peninsula

Two age groups were determined for the Cenozoic granitoids in the Dinarides of southern Serbia by high-precision single grain U-Pb dating of thermally annealed and chemically abraded zircons: (1) Oligocene ages (Kopaonik, Drenje, Željin) ranging from 31.7 to 30.6Ma (2) Miocene ages (Golija and Polumir) at 20.58-20.17 and 18.06-17.74Ma, respectively. Apatite fission-track central ages, modelling combined with zircon central ages and additionally, local structural observations constrain the subsequent exhumation history of the magmatic rocks. They indicate rapid cooling from above 300°C to ca. 80°C between 16 and 10Ma for both age groups, induced by extensional exhumation of the plutons located in the footwall of core complexes. Hence, Miocene magmatism and core-complex formation not only affected the Pannonian basin but also a part of the mountainous areas of the internal Dinarides. Based on an extensive set of existing age data combined with our own analyses, we propose a geodynamical model for the Balkan Peninsula: The Late Eocene to Oligocene magmatism, which affects the Adria-derived lower plate units of the internal Dinarides, was caused by delamination of the Adriatic mantle from the overlying crust, associated with post-collisional convergence that propagated outward into the external Dinarides. Miocene magmatism, on the other hand, is associated with core-complex formation along the southern margin of the Pannonian basin, probably associated with the W-directed subduction of the European lithosphere beneath the Carpathians and interfering with ongoing Dinaridic-Hellenic back-arc extensio

The problems of the post-Cenomanian tectonic evolution of the central parts of the Sredna Gora Zone. The wrench tectonics - how real is real?

The Sredna Gora Zone holds a unique place in the tectonic subdivisions of the Balkanide orogen and its evolution is still a subject of debate. In the last twenty years, the idea of strike-slip-related evolution of the zone has been invoked. However, for the moment, the number of thorough studies where such a scenario is envisaged is limited, and substantial evidence based on detailed fieldwork is still missing. In this article, we discuss some of the major problems of the suggested wrench tectonic concept in the evolution of the central part of the Sredna Gora Zone. These are the character of some major shear zones in the area, to which strike-slip movements are attributed, and the transtension-transpression evolution scenario for the Chelopech and Panagyurishte basins. Despite refuting completely their wrench tectonic-related evolution, we confirm the presence of strike-slip and oblique slip structures cutting the sediments, whereas the time of their activity and role in the deformation of the basin fill are yet to be revealed. Finally, we present a model based on natural examples and analogue modeling, in which the long-lived dextral Maritsa shear zone represents a zone of localized strain partitioning, separating the opposite vergent thrust belts of the Rhodope to the south and the Sredna Gora and Balkan fold-thrust belt to the north, during oblique or possibly orthogonal convergence

Main fault zones controlling the late Alpine structure in the area east of Sofia (Srednogorie Zone, Western Bulgaria)

The northwestern narrow tip of the Upper Cretaceous Panagyurishte sedimentary strip is cropping out along the eastern-north-eastern margin of the Neogene Sofia graben. In this area the sedimentary strip is split in two parts separated by an uplifted pre-Upper Cretaceous basement block. Both parts show syncline geometries with cores built of Upper Cretaceous rocks. The synclines are striking NW-SE and are extremely asymmetrical with vertical to overturned southwestern limbs. Within the overturned limbs meso-scale structural features such as imbrications, folds and small-displacement faults indicate strong flattening of the rock volume under generally top-to-the-N shear. These features were observed along the Kamenitsa-Rakovishki and Negushevo fault zones. Due to the relatively poor outcrop conditions the precise dips of these fault zones are still poorly known and there are no data about the fault cores. Thus, the general north-vergent character of the both fault zones is assumed on the basis of the semi-penetrative small-scale structures within their immediate footwalls and field relations. Our new data suggest that the northward translations along the Negushevo fault zone resulted in formation of approx. 30 long and 2 km wide basement uplift

New advances in the understanding of Bulgarian geology through fission-track analysis - a review

The paper is focused on the development and application of fission-track analysis in Bulgaria from the first experiments in the 1980s to the present. The results from two different approaches (the conventional method and fission-track analysis using LA-ICP-MS), obtained for deciphering the thermal and tectonic evolution of several zones of the Balkanides, are summarized. They demonstrate the significance of the method and its considerable contribution to the understanding of Bulgarian geology

High-grade metamorphic complexes in Bulgaria

Various in age, metamorphic overprint and rock compositions high-grade metamorphic complexes are exposed in the southern parts of the Balkan Peninsula. Until recently all the complexes were presumed to be Precambrian in age. Despite of this groundless assumption, the current geochronological data have shown that the Precambrian age clusters are quite rare and concern mainly the protoliths. To avoid the controversial views about the evolution of the high-grade complexes in Bulgaria we propound a new scheme of their subdivision based on both field and geochronological data. The main difference from the other available schemes is that we use only the ages of the metamorphic overprint and not of the protolithic age. Isolated Cadomian crustal fragments are distinguished only in low-grade metamorphic complex of Krayshte and Stara Planina Mountain. Carboniferous high-grade metamorphics are exposed in Southwestern Bulgaria and Srеdna Gora. Rocks building up the core of the Alpine Orogen of the Balkans suffered Jurassic, Cretaceous and Tertiary high-grade metamorphic overprints. The metamorphics in Sakar and Osogovo are Early Alpine in age. The Rhodope metamorphic complex is a composite unit with polymetamorphic evolution comprising both pre-Mesozoic and Mesozoic protoliths affected by Mesozoic and Tertiary orogenic events. The tectonic structre of the Rhodopes has resulted from the processes of syn-metamorphic thrusting as well as syn- to post-metamorphic extension. Fragments of high-grade complexes with well constrained pre-Mesozoic ages are distinguished only in the uppermost parts of the Rhodopian metamorphic section. В южните части на Балканския полуостров се разкриват разнообразни по състав, степен на наложени изменения и възраст високостепенни метаморфни комплекси. До скоро всички те по презумпция се считаха за докамбрийски, но публикуваните в последните години геохроноложки данни показаха, че доказаните прекамбрийски възрасти са твърде малко и се отнасят главно за протолити. Основавайки се на собствени теренни наблюдения, в съчетание с геохроноложките данни, предлагаме нова схема за подялба на високометаморфния фундамент на територията на България. За разлика от предишни схеми, тази подялба е базирана на възрастта на високостепенните метаморфни изменения, запечатани от скалите, а не на данни за протолитните възрасти. Малки по площ, кадомски корови фрагменти са установени единствено в херцинските нискостепенни метаморфни комплекси, разкриващи се в Краището и Старопланинската област. В югозападна България и Средногорието широко разпространени са мигматизирани гнайси, запечатали метаморфизъм с карбонска възраст. Ядрените части на Алпийския ороген на Балканите са засегнати от юрски, креден и терциерен метаморфизъм. С ранноалпийска възраст са метаморфните комплекси в Сакар и Осогово. Родопският метаморфен комплекс е сложна полиметаморфна единица включваща до-мезозойски и мезозойски протолити, засегнати от мезозойски и терциерни динамо-термални събития. Тектонското разслояване в Родопите е резултат от синметаморфни навличания и син- до постметаморфна екстензия. Фрагменти от домезозойски високометаморфни комплекси в Родопите уверено се установяват само в най-високо разположените в разреза единици

Timok fault and Tertiary strike-slip tectonics in part of western Bulgaria

The existence of NW-SE and NNW-SSE trending, regional-scale faults is well-known feature in the area of Tran and Breznik (West Bulgaria). Despite the numerous studies in the area, there is a lack of direct data about the kinematics of the main fault zones. Our investigations allow to define three groups of faults and also demonstrate the dominant dextral strike-slip kinematics of the faults from Pernik fault zone, as well as of several segments of Tran-Kosharevo fault. The field data, together with analysis of the existing maps, suggest the existence of another main strike-slip fault zone with almost N-S strike – the Timok fault. This fault is well-documented in Eastern Serbia, as its continuation in the area of Tran (Kraishte zone) was already suggested by Karaguleva et al. (1980) and Krautner and Krstic (2003). In the westernmost parts of Bulgaria, the Timok fault is traced along the fault segments, previously interpreted as parts of Tran-Kosharevo fault. To the Southward the zone is following the Serkirna fault. Unlike the northeast Serbia the translations along the Timok fault in western Bulgaria are much smaller – probably of not more than few kilometers. Additionally, our new data do not support the idea that these fault zones are part of Maritsa fault zone, well-defined southeast from Sofia

The Balkan Fold-Thrust Belt: an overview of the main features

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