Transect III

    Transect III is one of the most complex transects of the project. It begins in the Massif Central, and crosses the Mediterranean region, approximately from W to E, for about 2200 km. for to end on the Danube near Oryahovo. On this distance, it runs first approximately North-South,and crosses through the Gulf of Lion, the Provencal basin and the western Sardinian margin. Then, it continues in a W-E direction through Sardinia and part of the Tyrrhenian basin. In the eastern part of the Tyrrhenian basin, the section shows a regional offset along the upper slope of the Campania continental margin. After the offset, the section, crosses the southern Tyrrhenian basin, the Southern Apennines and the Apulian foreland with a SW-NE trend, turning then W-E through the southern Adriatic sea,and the Albanides. The easternmost tracts of the section progressively turn to a SW-NE direction, crossing the Serbo-Macedonian massifs, the Balkanides and ending in the Moesian Platform, in northern Bulgaria.

    The section crosses 5 countries: France, Italy, Albania,Macedonia and Bulgaria. The team is composed of Eugenio Carminati, Carlo Doglioni, Andrea Argnani, Gabriela Carrara, Christo Dabovski, Nikola Dumurdzhanov, Maurizio Gaetani, Georgi Georgiev, Alain Mauffret, Shaquir Nazai, Renzo Sartori, Veronica Scionti, Davide Scrocca, Michel Seranne, Luigi Torelli and Ivan Zagorchev, under the coordination by Eugenio Carminati and Carlo Doglioni.

    The Bulgarian activities are sponsored by the Bulgarian Academy of Sciences and by the Ministry of Environment and Waters under the
project 204 "Geological Trans-border Sections and Itineraries".

Transect III - Bulgaria. Tectonic/geodynamic cross section
Authors: Ivan Zagorchev, Christo Dabovski, Georgi Georgiev with the participation of Aleksander Velev

   Information Sources: The main sources of information are mostly bearing on the surface geology (results of geological mapping on the scale 1:25000 shown on maps on the scale 1:100000 and 1:500000; Committee of Geology. 1989-1995), extensive shallow (down to 5 ˝ 8 km) seismic studies in the Moesian platform, and several boreholes reaching depths of 3 - 5 km.  The Moho discontinuity is deduced on the basis of complex gravimetric (gravity map on 1:200000) and seismic information as previously published by Petkov, Velchev and Dachev, and revised by Dachev (1988) and Boykova (2000). The deep structure is inferred also on the basis of the deep seismic profiling along the line Petrich - Nikopol (Dachev, 1988).  The deeper parts (Moho depth, lithospheric thickness) are inferred from seismological and gravimetric studies (Babuska et al., 1986; Volvovsky, 1987; Volvovsky and Starostenko, 1996; Boykova, 1999).
    Description of tectonostratigraphic and lithostratigraphic units: The following description is based mostly on Boyanov et al. (1989) and Dabovski et al. (2002). The transect crosses through the Morava-Rhodope, Srednogorie, Stara-planina and Fore-Balkan zones and the Moesian platform. It crosses through a number of Late Alpine (Palaeogene) and Neogene-Quaternary block structures (horsts and grabens).

The Morava-Rhodope zone was formed in its actual shape during the Mid-Cretaceous thrusting and folding when the Morava unit had been thrust to the north-east over the Strouma unit. The Morava unit consists of several thrust sheets with slightly different lithologies. Only pre-Alpine rock units are present, at least on Bulgarian territory. The Eleshnitsa thrust sheet consists of Neoproterozoic - Cambrian greenschist-facies rocks gabbro-diorites to diorites (probably a volcanic-arc association) and Silurian - Devonian  phyllites, lydites and limestones (Ruzcha Formation,a product of a fore-arc basin). The Milevets thrust sheet consists of high-grade gneisses (pre-Cadomian granite gneiss with Cadomian overprint) and a sequence of metadiabases, different schists, metapsammites, marbles and quartzites (Cheshlyantsi Formation; Neoproterozoic - Cambrian volcanic arc association). The Poletintsi thrust sheet exhibits Silurian and Devonian basinal schists, lydites and limestones (Kosovo Formation) and Upper Devonian flysch-like sediments (Trunovdol Formation) issued from a Palaeozoic foreland basin.The Strouma unit consists of three subunits with different sections and geologic history. The transect traverses the Osogovo and the Trun-Louzhnitsa subunit. The basement of the Osogovo subunit consists of various gneisses, schists and amphibolites (orthoamphibolites included) of Precambrian age (Osogovo Formation). Cadomian metagranites as well as diorites and granites with a different degree of reworking are present. On Serbian territory (Bosilegrad district) they are unconformably covered with a depositional contact by Ordovician metasandstones. The Palaeozoic and Mesozoic cover has been deeply eroded, and only locally the basement is covered by Lower Triassic sandstones and Middle Triassic limestones and dolomites.They are tectonically covered by the thrust sheets of the Morava unit.
The basement of the Trun-Louzhnitsa subunit is locally represented by high-grade Precambrian metamorphics of a recycled continental crust. The Neoproterozoic - Cambrian Frolosh Formation (metadiabases, schists,metapsammites) contains rootless bodies of mafics and ultramafics and is intruded by the Strouma diorite formation. They are regarded as a volcanic arc association that contains some slices of oceanic crust or upper mantle. The basement is directly covered by Permian red beds (formed in intramountain depressions of the Variscan orogen).

The Triassic section begins over Permian or its basement with Lower Triassic mature conglomerate and sandstone followed by Middle-Upper Triassic carbonates (limestones, dolomites) and Carnian ˝ Norian marine red beds(red to purple shale, marl and conglomerate interbedded with oligomictic quartz sandstone and limestones).After a hiatus covering most of the Rhaetian,Hettangian and Sinemurian, the Jurassic begins locally with Lower Pliensbachian continental coal-bearing coarse terrigenous sediments followed by Upper Pliensbachian limestones. However, the transgression begins usually directly with Toarcian ˝ Aalenian quartz sandstone, and/or Aalenian ˝ Bathonian shelf limestones. In Aalenian to Kimmeridgian times, several environments have been formed, and namely:
(i) Svetlya zone of predominant carbonate sedimentation;
(ii) Treklyano zone of deeper basinal sedimentation with shales and radiolarites, some olistostromes, and transitions to (i). The Svetlya zone is regarded as a part of the carbonate platform that covered the Moesian platform and most of the Balkanides whereas the Treklyano zone is regarded as a local rift. A flysch basin (Nish-Troyan flysch trough; foreland basin) was superimposed over the previous environments in Kimmeridgian (locally ˝ beginning with the Callovian!) to Tithonian time, and was gradually closed and uplifted in Late Berriasian to Vallanginian times. After the Mid-Cretaceous folding and thrusting, the unit has been united with the Morava and Rhodope units into a single Morava-Rhodope superunit that represented an uplifted part ("plateau") of the complex Late Cretaceous volcanic island arc.

Srednogorie zone
The Srednogorie zone is regarded as a volcanic island arc of Late Cretaceous age, transformed by north-verging folding and thrusting in latest Cretaceous times in an orogen, and later thrusted northward over the Stara-planina zone. The following complexes are distinguished: (a) slivers and rootless bodies of ultramafic (ophiolitic?) character and mantle (or oceanic crust) origin (parts of the Precambrian Prerhodopian Supergroup); (b) principal mass of the Precambrian Prerhodopian Supergroup that represents typical continental crust of granitic composition (migmatites, gneisses, metagranites);(c) granitoid plutons of Carboniferous age (determined with Rb-Sr whole-rock isochrons at 340, 320-300 and 280-240 Ma) and mixed (I-type) and continental crustal signatures; (e) Triassic and Jurassic sedimentary complexes with Peri-Tethyan (Balkanide) signature; (f) Upper Cretaceous sedimentary formations represented by coal-bearing continental to shallow-marine Cenomanian and Turonian, terrigenous to carbonate marine Coniacian, pelagic carbonates and shales, locally with radiolarites (Santonian ľ Campanian) and Lower-Middle Maastrichthian flysch; (g) Cenomanian? to Early Maastrichthian volcanics(from pikrites and basanites to andesites and latites), in a volcano-plutonic association with gabbro to monzonites and granodiorites of mantle signature; (h) Danian conglomeratic formation of fluviolacustrine origin that contains well-rounded pebbles from all older rocks.

Balkan (Stara-planina) zone
The western parts of the Stara-planina zone are characterized with a north-verging thrusting and folding in pre-Late Eocene times. The zone includes pieces of units of Mid-Cretaceous and Late Cretaceous folding. The following complexes are distinguished:
(a) pre-Ordovician volcano-sedimentary complexes of island-arc and (partially) oceanic signature;
(b) Palaeozoic(Ordovician to Carboniferous) sedimentary formations intruded by Carboniferous granitoids;
(c) Permian red beds;
(d) a Triassic Peri-Tethyan sequence ending with folding and covered with unconformity by
(e) a Lower Jurassic to Kimmeridgian Peri-Tethyan sequence covered by
(f) a Jurassic - Upper Lower Cretaceous carbonate sequence (carbonate platform);(g) Upper Cretaceous shallow-water carbonate sequence;(h) Danian to Middle Eocene continental and shallow-marine sediments; (i) unconformable Upper Eocene molasse.

Svoge unit
The Svoge unit consists of a Neoproterozoic - Cambrian greenschist-facies volcano-sedimentary complex (Berkovitsa and Dalgi-Del groups) covered by an almost full section of the Palaeozoic. Ordovician (psephytic, psammitic to pelitic sediments), Silurian (sandstones, lydites, graptolite shales),Devonian (shales passing into Upper Devonian - Lower Carboniferous flysch),Carboniferous (Upper Carboniferous coal-bearing complex) and Permian (red beds) sedimentary formations are proven. Pre-Permian igneous rocks (diorites to granites) build up several intrusive bodies. The Mesozoic and Cenozoic section corresponds to that described above for the whole Stara-planina (Balkan) zone.

Berkovitsa unit
The typical pre-Mesozoic section of the Berkovitsa unit is represented by greenschist-facies pre-Ordovician volcano-sedimentary rocks with Neoproterozoic - Cambrian age, and by Ordovician metasediments. The Berkovitsa Group consists of metapelites and marbles (in the lower parts) and metasilstones and metasandstones(in the upper parts) all interlayered with spilites, keratophyres and their tuffs. Relics from an ophiolitic association are preserved as fragments. The Dalgi-Del Group is built up of metaterrigenous rocks (metaconglomerate,metasandstone, metasiltstone, phyllite) that contain olistolites of older igneous rocks. These rocks are intruded by Variscan (mostly, Carboniferous) granites. The Mesozoic section corresponds to that already described for the Stara-planina zone.

The Fore-Balkan is the transitional zone between the Balkan fold belt and the Moesian platform. The transitional character is indicated both by the lower degree of deformation, and by the transitional Alpine facies, e.g., lack of Jurassic - Lower Cretaceous flysch. In the eastern parts of the zone, the Triassic and Jurassic rocks are deeply buried beneath the Cretaceous and Palaeogene (partially) cover.

Moesian platform
The Moesian platform exhibits on Bulgarian territory only Cretaceous and younger complexes usually buried beneath Neogene cover and Quaternary loess. The oldest sediments crossed by the boreholes belong to the Permian. Several unconformities reflect partial uplift and erosion throughout the Mesozoic and Cenozoic history. The cover contains:
(a) continental Lower Triassic followed by Middle Triassic to Norian carbonates, and Carnian to Norian marine red beds;
(b) terrigenous and carbonate Jurassic, the marine transgression over the deeply eroded Triassic beginning often directly with Middle Jurassic;
(c) Upper Jurassic - Lower Cretaceous carbonate sequences;
(d) (parallel unconformity) Upper Cretaceous shallow-marine carbonate and terrigenous deposits, and continental or marine Palaeogene;
(e) locally Badenian to Pontian Neogene marine sediments belonging to a transitional zone between the Central and Eastern Paratethys.

Selected references
Babushka, V., Plomerova, J., Spasov, E. 1986. Lithosphere thickness beneath the territory of Bulgaria - a model derived from teleseismic P-residuals. - Geologica Balcanica, 16, 5; 51-54.
Boykova, A. 1999. Moho discontinuity in central Balkan Peninsula in the light of the geostatistical structural analysis. - Physics of the Earth and Planetary Interiors, 114; 49-58.
Committee of Geology. 1989-1995. Geological map of Bulgaria on the scale 1:100000. Sheets Kozloduj (eds. G. Cheshitev, L. Filipov), Bjala Slatina (L. Filipov), Vratsa (Tz. Tzankov), Berkovitsa (I. Haydutov, R. Dimitrova), Sofia (S. Yanev), Bosilegrad & Radomir (I. Zagorchev),Kriva Palanka & Kyustendil (I. Zagorchev).
Dabovski, C., Boyanov, I., Khrischev, Kh., Nikolov, T., Sapounov, I.,Yanev, Y., Zagorchev, I., Structure and Alpine evolution of Bulgaria, Geologica Balcanica, 32, 2-4, 2002.
Dachev, Ch. 1988. Structure of the EarthÝs crust in Bulgaria. Tehnika,Sofia; 334 pp. (in Bulgarian).
Haydutov, I., Peri-Gondwanan terranes in the pre-Palaeozoic basement of Bulgaria, Geologica Balcanica, 32, 2-4, 2002.
Haydutov, I., Yanev, S., The Protomoesian microcontinent of Balkan peninsula - a peri-Gondwanaland piece, Tectonophysics, 272, 2-4, 303˝313,1997.
Haydoutov, I., Gochev, P., Kozhoukharov, D., Yanev, S., Terranes in the Balkan area. In: Papanikolaou, D. (ed.), IGCP Project 276. Terrane map and terrane descriptions. Annales Geol. Pays Helleniques, Athens; 479-494, 1997.
Lilov, P., Zagorchev, I., Peeva, I., Rubidium-strontium isochrone data on the age of the metamorphism in the Ograzhdenian complex, Maleshevska Mountain, Geologica Balcanica, 13, 2, 31-40 (in Russian, with English abstract), 1983.
Nikolov, T., The Mediterranean Lower Cretaceous. Publ. House Bulg. Acad. Sci., Sofia; 269 pp., 1987.
Volvovsky, I.S., Dachev, Ch., Popova, O.G., Babinets, V.A. 1987. The Earth's crust of Bulgaria from the data of the Petrich-Nikopol wide-angle reflexion ˝ deep seismic sounding profile. ˝ Mezhduvedomstvenniy geofizicheskiy komitet AN SSSR, Moscow; 112 pp. (in Russian)
Volvovsky, I. S., Starostenko, V. I. (eds.), Geophysical parameters of the lithosphere in the southern sector of the Alpine orogen. Kiev, Naukova dumka; 216 pp. (in Russian), 1996.
Yanev, S., Gondwana Palaeozoic Terranes in the Alpine Collage System of the Balkans, Journal of Himalayan Geology, 4, 2, 257-270, 1993..
Zagorcev, I., Neotectonics of the central parts of Balkan Peninsula: basic features and concepts, Geologische Rundschau, 81, 3, 635-654, 1992.
Zagorchev, I., Multiphase crustal thickening in the central parts of the Balkan Peninsula, Bull. Geol. Soc. Greece, 28, 1, 87-97, 1993.
Zagorchev, I., Alpine evolution of the pre-Alpine amphibolite-facies basement in South Bulgaria, Mitt. der Osterreich. Geol. Geselschaft, 86, 9-21, 1994.
Zagorchev, I., Rhodope controversies, Episodes, 21, 3, 159-166, 1998.
Zagorchev, I., Pre-Triassic sections and units in West Bulgaria, IGCP Project No 276, Newsletter No 6, Spec. Publ. Geol. Soc. Greece, 3, 42-53, 1998.
Zagorchev, I., Geology of SW Bulgaria: an overview, Geologica Balcanica, 21, 1-2; 3-52, 2001.
ę Geological Institute of the Bulgarian Academy of Sciences
Author: Ivan Zagorchev
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