Geologic evolution of Bulgaria in light of plate tectonics

Tecto~~~hys~cs, 40 (1977) 245-256 0 Elsevier Scientific Publishing Company,

GEOLOGIC EVOLUTION TECTONICS *

KENNETH

245 Amsterdam

OF BULGARIA

J. HSU I, IVAN K. NACHEV

-Printed

in The Netherlands

IN LIGHT OF PLATE

2 and VASSIL

T. VUCHEV

2

1 Geological Institute, Federal Znstitute of Technology, Zurich (Switzerland) 2 Geological Institute, Bulgarian Academy of Sciences, Sofia (Bulgaria) (S~brnjtt~d

June 2, 1976; accepted

for publication

September

28,1976)

ABSTRACT Hsii, K.J., Naehev, I.K. and Vuchev, V.T., 1977. Geologic plate tectonics. Tectonophysics, 40: 245-256.

evolution

of Bulgaria

in light of

The Balkanide is a mobile belt within a micro-continent, which included both the Moesian Platform and Rhodope Massif. This micro-continent was rotated counter-clockwise during the Jurassic mainly in response to the sea-floor spreading of the Vardar ophiolite trough. The rotation led to the consumption of Triassic Tethys along the DobrogeaCrimea-Caucasus Trend, producing the Cimmerian erogenic belt. Cretaceous rotation of the Italo-Rinaridian micro-continent led to the consumption of the Vardar ophiolites. An island arc (Macedonia-Rhodope-North Anatolia) was present at the consuming plate margin. Middle and Late Cretaceous marginal basins behind this arc included the Srednogorie and the Black Sea. Submarine volcanics, radiolarian cherts, and hemipelagit marls This sequence was folded during the were deposited in the Srednogorie “eugeosyncline”. early Tertiary Alpine orogeny, when the front of the Rhodope Massif was overthrust onto the Balkanides. The Black Sea meanwhile remained undeformed and can thus be considered a fossilized Cretaceous marginal basin.

INTRODUCTION The geology of Bulgaria was summarized by Foose and Manheim (1975) in a recent article. They have presented their synthesis within the framework of geosynclinal theory and with emphasis on deep faults. The question was raised during the visit by one of us (KJH) as guest of the Bulgarian Academy of Sciences, if different interpre~tions are possible when we adopt the postulates of plate tectonics as a working hypothesis. To explore this possibility we took six excursions during the months September and October 1974, visited the Fore-Balkan, the Stara Planina, the Srednogorie, the Kraistides, and the northern margin of the Rhodope Massif. We hesitate to claim

* Contribution zerland.

No. 71 of the Laboratory

of Experimental

Geology,

ETH, Zurich,

Swit-

246

that we are able to come up with definitive alternate interpretations of the Bulgarian geology. On the other hand, we would like to re-examine several of the old puzzles of Bulgarian geology with new premises. This paper is written with the aim of presenting a working hypothesis to stimulate discussion and to inspire further fact-finding towards a final solution. We might add that we have only referred to geological data particularly relevant to our thesis. For those English-speaking readers who are not familiar with the stratigraphy and structural geology of Bulgaria, they might consult the article by Foose and Manheim (1975) which includes an excellent resume of the basic geological information. TECTONIC

PROVINCES

OF BULGARIA

The morphotectonic units of Bulgaria are bounded by deep-seated faults (BonEev, 1971, see also Fig. 1). Those main units are from north to south (see Figs. 1 and 2): Moesian Platform Balkanides, subdivided into Fore-Balkan Stara-Planina, or Balkan (s.s.) Srednogorie Rhodope Massif Located in southwest Bulgaria are the Kraistides, and the northwest are the Southern Carpathians. The Moesian is an epihercynian platform with Mesozoic and Cenozoic cover. The Rhodope is a crystalline massif; the granitic and metamorphic rocks are commonly believed to be Precambrian in age. The Balkanide is a mobile zone between these two rigid elements. The Fore-Balkan consists of gently folded Mesozoic and Cenozoic sediments, south of the BrestnitsaPreslav flexure. The Stara-Planina is the main folded belt of Bulgaria. The range has a Paleozoic core, mantled by Mesozoic and early Cenozoic geosynclinal sediments. The Srednogorie is a structurally complicated province, composed mainly of sediments and volcanics of an Upper Cretaceous geosyncline. The folded Mesozoic sediments are cut by epizonal intrusions, which may represent the feeder-vents of former andesitic volcanism. The Srednogorie is separated from the Rhodope Massif by the Maritsa deep fault. The Kraistides are characterized by a set of NNW-trending lineaments. Like the Stara-Planina, the Kraistides are composed mainly of Mesozoic and Paleozoic sediments and have been considered a part of the Kraistide-Vardar zone. The South Carpathians run east-west across Romania fringing the Moesian Platform on the north. After a short incursion into northwestern Yugoslavia, the range turns sharply eastward and appears at the northwest comer of Bulgaria, where the meridionally folded sediments are mainly Cretaceous flysch formations. The morphotectonic provinces are Alpine and were created during the late Alpine erogenic movements.

Fig. 1. Tectonic provinces of Black Sea area. I = external forelands; 2 = present oceans; 3 = external molasse trough; 4 = Moesian Platform (MP); Colkhidian (Col. D) and Koura (KD) depression; 5 = internal depression: Panonian (PD) and Transilvanian (TD); 6 = North Dobrogea (ND), Central Dobrogea (CD), South Crimea (C), Great Caucasus (GCa) and south slope of Great Caucasus (SCa); 7 = Eastern Alps (EA), West Carpathians (WC), East Carpathians (EC), South Carpathians (SC) and Fore-Balkan and Balkan (B); 8 = Tatride zone; 9 = Slovakia (Sl) and Apusini zone (AP); 10 = Srednogorie (S), Pontides (PO), Anatolides (An), and Adzaro-Trialetian zone (ATz = Minor Caucasus); 11 = SerboMacedonian (SMM), Rhodope (R) and Kirsehir (KM) massifs; 12 = North ophiolite belt: Vardar zone (Vz) and Izmir-Ankara zone (IAz); 23 = Drina-Ivanitca (DI), Pelagonian (PM), and Mederes (MM) massifs; 14 = South ophiolite belt: Internal Dinarides (ID), SubPelagonian zone (SPz = Pindus zone) and Taurides (TA); 15 = External Dinarides (ED) and Hellenides (He = Apulian zone).

GEOLOGIC

HISTORY

Triassic The Triassic formations in Bulgaria consist of alluvial-lake (continental) sandstones in the lower part (Lower Triassic), shallow-water limestones and dolomites (Anisian-Camian), limestone breccias and conglomerates in the

“38

Fig. 2. Structural zones of Bulgaria. MP = Moesian Platform; Tz = Transitional zone; FB = Fore-Balkan; B = Stara Planina (Balkan s.s.); S = Srednogorie; R = Rhodope; K = Kraistides; SC = South Carpathians; SMM = Ser~ian~Macedonian (Dardanian) Massif (after Bon&‘ev, 1971).

upper part (Norian, Rhaetian). Notably one finds Norian flysch in the Preslav basin. Those sediments apparently constituted continents-m~gin deposits. If we accept the reconstructions according to the continental-drift theory, the Bulgarian Triassic should be located somewhere on the western margin of the great Tethys ocean. However, its relation to the Triassic of the Dinarides depends upon an interpretation of the significance of ophiolites of the Vardar zone. It was thought Bosnian ophiolites could be Triassic or even older, and this idea led to the postulate of a Triassic Tethyan arm, separating the Balkanides from the Dinarides (Hsii, 1971; Bosselini and Msii, 1973; see Fig. 3). However the recognition of the mblange nature of the ophiolites zone inv~idated the in~rpretation of pre-Jurassic ophiolites (Smith, 1973; Dimitrievich and Dimitrievich, 1973). The Vardar ophiolite trough apparently originated from sea-floor spreading during the Jurassic, more or less synchronous with the Penninic ophiolites of western Europe. We now believe that there was no Triassic Tethyan sea arm between Bulgaria and Yugoslavia. The Triassic carbonates of Bulgaria can be considered the eastern extension of the Dinaride and Carpathian carbonate shelf, as shown by Fig. 3. Furthermore, the Moesian Platform and the Rhodope Massif cannot have been separated by a wide stretch of Triassic Tethyan ocean, as postulated by Dewey et al. (1973, fig. 9), because there are no ophiolites nor oceanic sediments of Triassic age in the Balkanides. The Alpine chain, sensu lato, from Betic Cordilleras to Anatolia does not include fragments of Triassic ocean crust. The suture zone between the stable European continent and the microcontinents of southeastern Europe must, therefore, be sought north of the Moesian Platform. If we assume, according to the postulate of the plate-tectonic theory, that the Tethys and

249

Fig. 3. Reconstruction of Mediterranean micro-continents in Late Triassic (modified after Bosseline and Hsii, 1973). Future micro-continents are outlined by heavy lines. I = Iberia, 2a = Alboran (Rif-Betic), 2b = Alboran, 3 = Corso-Sardinia, 4 = Italo-Dinaridian, 5 = Bulgarian, 6 = Anatolia.

the Russian Platform were separated by a consuming plate margin, the best location of such a Triassic trench being subducted would be the belt extending from Dobrogea in Romania to southern Crimea to Great Caucasus (Dewey et al., 1973). Jurassic The continental and shallow marine sedimentation prevailed during the Early and Middle Jurassic in Bulgaria. Alluvial and lacustrine sands and clays were deposited before a marine transgression took place during the Hettangian stage. Within the interval of Hettangian-Bajocian shallow-marine sandstones, crinoidal limestones and marls were sedimented followed by the mainly terrigenous deposits of the early and middle Bajocian age. A minor regression took place during the late Bajocian and Bathonian, when shallowmarine bioclastic and sandy limestones were deposited locally. The fragmentation of the continental shelf and the “geosynclinal” subsidence started in Late Jurassic, resulting in the “Callovian transgression”. In large parts of Bulgaria shallow-marine Bathonian and older Jurassic deposits were overlain unconformably by the Callovian pelagic carbonates, apparently deposited in a deepening sea. We noted the occurrence of limestone breccia at Jabliano, in the Kraistides, which bear some resemblance to the Early Jurassic Limestone breccia of the Austro-Alpine regions of eastern Switzerland. Such breccias should be interpreted as mass-flow debris deposited at the floor of a

fault scarp, separating a subsiding block from the shallow carbonate sllelf. The history of the Alpine Mediterranean sysbem is closely related to the sea-floor spreading history of the Atlantic. The magnetic anomaly patterns of North Atlantic and thti results of tht> JOIDES drilling indic*atc~ that the history of the relat,ive motion between Africa and Europe (aan hf. subdivided into three stages (see Smith. 1971: Pitman and Talwani, 1972). During the first stage from 180 to 81 million years ago, namely from Early Jurassic to Late Cretaceous. Africa was separated from, while Eurasia was still attached to, North America. The net result was a left-lateral movement between Europe and Africa. During the second stage from 81 to 38 m.y., or from Late Cretaceous to Late Eocene, Eurasia was separated from North America and was moving at a greater rate eastward than Africa. During the last stage from Oligocene to Recent, there was very little lateral motion. When Africa was moving away from Europe during its left-lateral motion, extension prevailed between the two continents, resulting in the birth of Alpine Tethys. When Europe was catching up with Africa during its rightlateral motion, the region in between was subjected mainly to compression, leading to the formation of the Alpine mountain chain, sensu lato. Between the two plates are a number of micro-continents, and the relative motion between those blocks was responsible for the complicated geologic history of the Mediterranean Europe. We might consider the Alpine eugeosynclines a rifted ocean trough created during Jurassic, when micro-continents were split off from the stable Europe and from one another (Fig. 4). The Apennine geosyncline between the Iberian and Italo-Dinaric micro-continents, the Pennine geosyncline between the latter and stable Europe, and the Vardar geosyncline between the Italo-Dinaric and the Bulgarian microcontinents. All the micro-continents wcare rotated counter-clockwise under influence of the left-lateral motion. The Bulgarian micro-continents moved northeastward, toward the Dobrogea-Crimea front, and it was apparently stretching and fragmented during its rotating motion. The Moesian Platform was being pulled apart from the Rhodope Massif, and the Tithonian-Berriassian Nish-Trojan flysch trough was probably a down-faulted basin of extensional origin. However, the two blocks were never completely separated by an oceanic trough, as no ophiolites of Jurassic or any age are present in the Balkanides. While the Bulgarian block moved northeastward during the Jurassic, the ocean between it and the Russian Platform was being consumed along the DobrogeaXrimea-Caucasian front. Near this consuming plate margin of the southern edge of the Russian Platform a Triassic-Jurassic suture zone which includes pelagic carbonates and pillow lavas in the Northern Dobrogea and argillites (phyllites) in the Caucasus. The Nalbantian flysch trench existed during the Norian in the Northern Dobrogea. The Triassic-Lower Jurassic Tauric flysch trough appeared in Crimea. Tuff-flysch sediments were formed during the Middle Jurassic in the Caucasus. The flysch trough was deformed during the latest

251

Fig. 4. Alpine Tethys in Early Cretaceous. The Alpine geosyncline reached extent (see Colom and Escandell, 1962). Whether the Anatolia block started to away from Africa during the Cretaceous or in an earlier episode (Triassic? depends upon the eventual determination of the ophiolites of the Taurides. A = Betic, L = Ligurian, P = Pennine, V = Vardar.

its widest be ripped Jurassic?) Atlas, B =

Triassic or earliest Jurassic in the Northern Dobrogea; during the Middle Jurassic in Crimea; after the Bajocian and before the Callovian in the Caucasus. These erogenic movements have been designated as the Cimmerian Orogeny with old Cimmerian, Donetsian, Istrian and Adigian phases. Extensive Jurassic andesitic volcanism prevailed behind the consuming margin, as the plate-tectonic model predicts, constituting the Cimmeridian magmatic cycle (Bleahu et al., 1962; Khain, 1975). The counter-clockwise rotation of the Bulgarian block must have been faster than that of the Italo-Dinaric block. The rifting of these sialic blocks gave rise to the Vardar ophiolite trough, which may be compared to the Neogene Red Sea. The sea-floor spreading of the Vardar “geosyncline” started in Jurassic and continued into Early Cretaceous as indicated by the age of pelagic sediments there (Smith, 1973; Dimitrievich and Dimitrievich, 1973). 0-e taceous Deep-sea sedimentation continued in the Nish-Trojan trough from Jurassic to Early Cretaceous. There was a gradual northward shift in the shoreline. The olistostrome facies of the Tithonian flysch, bearing clasts of the Rhodope lithology, is common in the Kraiste region, but the Berriasian olistostromes are best developed in the Fore-Balkan. A northward shift in shoreline is also suggested by the upward increase of terrigenous elastics in

lhc2 Lower Cretaceous sediments of the Balkanides. Meanwhile, relief on land and under the sea becamct more moderate. Dominantly flysch sedimentation during the ‘~it~l~nian was replaced by d~rnirla~lt‘ly h~rni-I~ela~(, marly sedimentation during the Berriasian and Valanginian (Nachuv and Vuchev, 1972). The basin becamtl so shallow that molasscl-like facies and shtllf-carbonatr of lJrgonian typts w(*re being deposited in the Pirot-‘I‘urnovo hasin during tht: late Early Cretacrous (Nachev, 1969a). I.ocal uplifts and changes on l~~ld-~tl~l.~~~~t distril)L~tic~n occurred during the l%rIy Crttaceou?; as indic.ated by detailed facies analyses (Nachcv, 1969a). The transitional natural of’ the sedimentation and the gradual moderation of the topographical relief and bathymctry indicate> that thtl Bulgarian block ~.~)tltil~u~d to mov(’ ~~ort~~~.~astw~rdtoward Dc~br~g~~ and Crimtla without_ suffering intense deformation. The active consuming margin was lxohably still locxted north of the Bulgarian block. while the contac*t hcbtween the Rhodope Massif and the Vardar Sea was a passive margin. ‘1 major tectonic crisis took place sometime during t,htAmiddle Cretaccous f ~~u~triat~ phase) in so~lt~~e~st~rrl Europt:. Evidence of a major oro~el~ic.~ clcformation could be traced from Austria (prc-Gossau phase) through the Carpathians, to Bulgaria and thence to C;recc.c. The I Jpper Cr&aceous formations of Bulgaria are c*haracterized try a fat:& and assume a completely different c4~;trac%er from that of the Lower Cretaceous. The c~~al-b~~iIi~ Cc~~~~nl~~liaI~and Turonian sediments of tht> BaIkanides area ~ornparablc~ to the lmst-orogcnic, molasse sednnents. However, in the Srednugorie zone’ pelagic sediments (limestones, radiolarites, and Couches-Rougcx), t,urbidites, and submarine volcanics and pyrot:lastic s~dirn~~nt~ were deposited in a bathyal environ~le~~t. The rapid facies variations (Nachev, 1969b,c*) indicate that, there may have been local basins (Srednogorian, Eminian and North Bulgarian) separated by shallow sills. The mainly andesitic volcanism. particularly well represented in the Srednogorie )trovince, is the typic-al igneous activit,y behind an island arc. The Srednogorie facies consists of int~rt~edd~cl volcanics and pelagic marls (Nachev. 1969b,c*). Thch scquenc~t~ is very similar to the section at thcl JOIDES Philippine Sea Sites 53 and 54, which are located behind the Mariana Arc (see Fischer et al., 1971). The nat.ure of sediments and of the volcanics indicatth to us that. t.hcbd~posit,ic)t~~l ~I~vironIn~~~l~ might be ~~on~)~~~dto that of the Aegean Sea today. which is a hack-art* sea with a dimension similar to that of the Upper C’rc~taceous Srednogorir SW. The tectonic’ crisis in middle Cretaceous apparently coincided with a reorganization of the splint”ering plates. The northeast motion of the Bulgarian block was slowed to a halt. after its collision with the Russian Platform, leaving the Cimmerian mountains of Dobrogea and Crimea as the scar of the suture zone. The consuming plate margin between the southern European Platform and the ‘I’ethyan ocean (locally the Vardar geosyncline) was probably located south of the Rhodope Massif during &he Crttaceous. While the Vardar ophiolites were being subducted under a trench wall south of the

253

Pm

Pm

Pm

Pm

Pm

Vz

Rm

Vz

MP

Rm

Vz

Vz

Vz

Dz

Rm

Rm

Rm

Sz

KOTr

MP

EP

BbDz

EP

52

EP

Fig. 5. Interpretation of the evolution of Dinarides and Balkanides. 1 = continental crust; 2 = oceanic or pseudo-oceanic crust; 3 = mantle; 4 = ophiolites; 5 = andesites; 6 = flysch; 7 = molasse; 8 = pelagic rocks; 9 = carbonate and shallow-water rocks; I0 = shallow-water elastic rocks; 11 = volcanitcs; 12 = intrusions; 13 = ptastic folds; Pm = Pelagonian massif; Rm = Rhodope massif; MP = Moesian platform; Vz = Vardar zone; EP = Eurasian plate; CD = Central Dobrogea; D = Dobrogea; FDTr = Pre-Dobrogea trough; NTr = Nalbant flysch trough; Bd = Babadag basin; Dz = Dobrogea zone; NTTr = Nish-Trojan flysch trough; PTTr = Pirot-Tarnovo molasse trough; STr = Srednogorie volcanic-sedimentary trough; ETr = Emine flysch trough; KOTr = Kula-Obsor motasse trough; Sz = Srednogorie; B = Balkan; FB = Fore-Balkan.

Rhodope Massif, which probably constituted a part of an island arc. Calcalkaline andesitic volcanism was active in the Upper Cretaceous Srednogorian basin behind the arc (Fig. 5, see also Boccaletti et al., 1974).

Puleogene The Balkanides were app~ently subjected to compression at t,he start of Tertiary. The Eminian flysch was folded after the Paleocene (Late Laramide phase). The clayey-sandy sediments of the Ypressian and Lutetian age within the Kula-Obsor basin were folded before the IJpper Eocene (Illirian phase). During the Upper Eocene and Oligocene coarse elastics were deposited as molasse-like sediments in the Balkanides and Rhodopes. We were particularly impressed by the fact that boulder-sized clasts of granite and gneisses, apparently derived from the Rhodope or from the Srednogorie are common in an Ypressian Lutetian conglomerate of the DjadoDjanko district of Gabrovo in the Fore-Balkan, whereas none of the clasts could be identified to have come locally from the Balkanides belt. We suggest, therefore, the possibility that the Rhodope Massif had marched far to the north and may have covered part of the Srednogorie as a large thrust plate. Erosion has removed much of the material from this thrust plate, so that rocks from the Rhodope Massif should be found as boulders in the molasse-like conglomerates of the Fore-Balkan. The only remnant of this great thrust plate is the Klippe of South Bulgarian Granite north Karlovo. The ~~ossibility of large-scale ove~h~sting is also suggested by an interpretation that the Kotel zone of chaotic blocks (see Nachev et al., 1967) may represent a tectonic melange, and this zone of olistostrome-like rocks marks the site where rocks of the Srednogorie facies were thrust upon the Balkan. If this interpretation is correct, some of the blocks of unusual facies, such as the Triassic and Liassic blocks, may have been detached by the thrust plate from a location far to the south of their present location and transported tectonically by the thrusting like erratic boulders in a ground moraine. Neogenc

The Neogene deposits are ~h~acterized by post-erogenic sediments, changing from marine in the Oligocene and Early Miocene to largely brackish in Late Miocene and Plio-Quaternary. Major faults had dominantly vertical movements. The faults served to mark the boundaries separating the various geological provinces and the vertical movement was responsible for the physiographic relief evident today. CONCLUSION

This paper on the geology of Bulgaria is developed on the premise that geosynclinal sedimentation and erogenic deformation of the Alpine belt of Europe are related to interactions among Europe, Africa and rotating microcontinents in response to sea-floor spreading of the Atlantic. Bulgaria cannot be separated into two micro-continents (Rhodope and Moesia) as postulated by Dewey et al. (1973) because Bulgaria has no major suture zone that could

‘255

mark the site of subduction of oceanic crust. The mobile zone between the Rhodope and Moesia has undergone extension during the early Mesozoic and compression during the late Mesozoic and Cenozoic. The extensional movement was apparently related to a phase (Jurassic and early Cretaceous) of sea-floor spreading which gave birth to the Vardar Trough. The compression followed the Late Cretaceous consumption of the ophiolite trough which led to a collision of the Italo-Dinaridian and the Rhodope part of the Bulgarian micro-continents. Bulgaria is not separated from the Romanian Carpathians by a major suture zone. One might envision the former existence of a micro-continent south of the Apusine and east of the East Carpathian ophiolite troughs; this micro-continent should include practically all of Bulgaria, and much of south Romania (W~lachia). We might further postulate that the northern Anatolia is also a segment of this Bulgarian micro-continent. Several features of Bulgarian geology can be satisfactorily answered by the adoption of a plate-tectonic model. One such puzzling example is the sedimentation” in the Srednogorie during the very local “eugeosynclinal Late Cretaceous, resulting in a sequence in submarine volcanics, radiolarian cherts, and pelagic marls. Plate-tectonic model suggests that this Cretaceous basin was a small back-arc basin when the Vardar ophiolite trough was being subducted under the Rhodope island arc (Fig. 5, Kz). Further compressional movement converted this marginal basin into a folded belt. This interpretation of the genesis of Srednogorie led us to speculate that the Black Sea might also have been a Cretaceous back-arc basin, which has not yet been eliminated by mountain building. We cannot accept the postulate by Dewey et al., that the Black Sea is a relic of the Triassic Tethys. The northern Black Sea margin was a zone of active subduction and suturing during the early Mesozoic. The partial submergence of this suture zone probably took place in the Cretaceous, when the Black Sea originated as a back-arc basin behind the North Anatolian Arc. The early Mesozoic active margin was converted into a late Mesozoic and Cenozoic passive margin. Such a model could explain the existence of an oceanic crust under the Black Sea. The great thickness of the sediments in the Black Sea is comparable to that of the eastern Mediterranean. The more than lo-km pile could have been deposited during the 100 million years at a reasonable hemipelagic sedimentation rate of 10 cm per thousand years, which seems to be the rate of pre-~uatern~ rate of sedimentation in the Black Sea as one of us (KJH) deduced from a study of deep-sea drilling cores. ACKNOWLEDGEMENTS

We are indebted to the Bulgarian and U.S. Academies of Sciences who sponsored a Scientific Exchange Program that enabled the visit of one of us to Bulgaria. We are also grateful to our colleagues in the Geological Institute, B.A.S., for their helpful discussions.

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