Int J Earth Sciences Geol Rundsch) 2001) 90 : 217±233 DOI 10.1007/s005310000104

ORIGINAL PAPER

A. I. Okay ´ M. Satõr ´ O. Tüysüz ´ S. Akyüz ´ F. Chen The tectonics of the Strandja Massif: late-Variscan and mid- deformation and metamorphism in the northern Aegean

Received: 3 June 1999 / Accepted: 29 March 2000 / Published online: 2 September 2000  Springer-Verlag 2000

Abstract The Strandja Massif is a mid-Mesozoic oro- The Srednogorie arc closed during the early Tertiary genic belt in the Balkans build on a late-Variscan through renewed northward thrusting of the Strandja basement of , and granites.New Massif. single-zircon evaporation ages from the gneisses and granites indicate that the high-grade metamorphism Keywords Strandja Massif ´ Variscan ´ Balkans ´ and plutonism is Early in age ~271 Ma). Metamorphism ´ Deformation ´ The late-Variscan basement was unconformably over- lain by a continental to shallow marine sequence of Early ±Mid- age.During the Late Introduction Jurassic±Early Oxfordian±) the lower Mesozoic cover and the basement were penetra- Metamorphic rocks occupy a large area of tively deformed and regionally metamorphosed in 400”200 km in the Balkans between the northern greenschist facies possibly due to a continental colli- Aegean and the western Black Sea Fig.1).This Bal- sion.An Rb±Sr biotite whole-rock age from a meta- kan metamorphic province, which includes the Serbo- granite dates the regional metamorphism as Late Macedonian, Rhodope and Strandja massifs, is Jurassic 155 Ma).Deformation involved north-ver- bounded in the south by the earliest Tertiary Vardar± gent thrust imbrication of the basement and the Intra-Pontide suture and in the north by the Eocene± emplacement of allochthonous deep marine Triassic Oligocene Balkanide thrust belt Fig.1).Views on the series over the Jurassic metasediments.The metamor- tectonic significance of the Balkan metamorphic prov- phic rocks of the Strandja Massif are unconformably ince during the Alpide orogeny range from a passive overlain by the Cenomanian shallow marine sand- role of a ±Variscan cratonic basement stones.During the Senonian, the northern half of the Foose and Manheim 1975; Dewey et al.1973; Burch- Strandja Massif formed a basement to an intra-arc fiel 1980; Boncev 1988) to having actually formed dur- basin and to a magmatic arc generated above the ing the Cretaceous to early Tertiary Alpide regional northward-subducting Tethyan oceanic lithosphere. metamorphism and compressive deformation Meyer 1968; Kronberg 1969; Dixon and Dimitriadis 1984; Burg et al.1990, 1996; Koukouvelas and Doutsos 1990; Ricou et al.1998).Probably the least inter- A.I.Okay  )) ´ O.Tüysüz nationally known part of this large metamorphic ter- Eurasian Institute of Earth Sciences, rain is the Strandja Massif in the northeast, which Istanbul Technical University, 80626 AyazagÆa, Istanbul, Turkey forms a northwest-trending metamorphic belt strad- E-mail: [email protected] dling the Turkey±Bulgaria border Fig.1). Here, we Phone: +90-212-2856208 Fax: +90-212-2856210 provide new stratigraphic, structural and geochrono- logical data from the Turkish part of the Strandja M.Satõr ´ F.Chen Massif and suggest a tectonic model for its post-Varis- Institut für Mineralogie, Petrologie und Geochemie, Universität Tübingen, Wilhelmstrasse 56, 72074 Tübingen, can evolution.The field data were obtained during Germany regional mapping of the Turkish sector of the Strandja Massif in 1991 and 1994, and during a field trip to the S.Akyüz Department of Geology, Faculty of Mines, Bulgarian part of the Strandja Massif in 1992. Istanbul Technical University, 80626 AyazagÆa, Istanbul, Turkey Although we have covered the whole of the Turkish 218

Fig. 1 Tectonic map of the northern Aegean and the Bal- kans showing the outcrops of metamorphic rocks dark areas) between the Vardar±In- tra-Pontide sutures and the Balkanide thrust belt

Strandja Massif, this paper concentrates on the central al.1969; Turgut et al.1991; Görür and Okay 1996). part of the Massif, where the stratigraphy and the Results from the oil wells in the Thrace Basin Alay- structural relations are best exposed.Our field and gut 1995) and a small outcrop of phyllites and slates geochronological results confirm that the Strandja north of Saros Bay Sümengen and Terlemez 1993; Massif is a late-Variscan unit that underwent regional Tüysüz et al.1998) indicate that metamorphic rocks metamorphism and compressive deformation during extend underneath the Tertiary sediments at least as the Late Jurassic±Early Cretaceous e.g. Chatalov far south as the Saros Bay Fig.1). In the east, the 1988; ÞagÆlayan et al.1988; Aydõn 1982).Our study Strandja Massif is bounded by the West Black Sea also shows that the mid-Mesozoic deformation was basin, which has a Cretaceous oceanic crust overlain thick skinned and involved thrusting of the basement by Upper Cretaceous±Cenozoic sediments over 13 km gneisses over the lower Mesozoic series.Furthermore, in thickness Letouzey et al.1977; Finetti et al.1988; zircon geochronological data indicate that the meta- Robinson et al.1996).The Istanbul Zone, which bor- morphism of the gneissic basement of the Strandja ders the Strandja Massif in the southeast, comprises a Massif, generally thought to be of Precambrian age, is continuous unmetamorphosed sedimentary sequence most probably Early Permian coeval with the granite of to age overlain uncon- intrusions. formably by Triassic rocks e.g. Görür et al. 1997). West of the city of Istanbul, the phyllites and slates of the Strandja Massif are separated by a strip of unde- Geological setting formed mid-Eocene sediments from the Carboniferous siltstones and , and Triassic limestones of the The Strandja Massif forms a northwest-trending Istanbul Zone Fig.2). The pre-mid-Eocene contact mountain belt, approximately 280 km long and 40 km between the Istanbul Zone and the Strandja Massif is wide, along the southwestern margin of the Black Sea believed to be a major N/S-trending dextral strike-slip Fig.1).It is bounded in the south by a major Tertiary fault, the West Black Sea fault Fig.1), which had a basin, the Thrace Basin, with up to 9-km-thick silici- significant role in the formation of the West Black clastic sediments of Eocene±Oligocene age Kopp et Sea basin.During the mid-Cretaceous opening of the 219

Fig. 2 Geological map of the Strandja Massif and the neigh- 1988; Okay et al.1994; S.Kasar and A.I.Okay, bouring regions.Compiled from Chatalov 1988; Chestitev and unpublished data).This relative autochthon is tectoni- Kancev 1989; Kasar and Okay, unpublished data; A.I. Okay et al., unpublished data) cally overlain by allochthonous Triassic units, pre- served mainly in the Bulgarian sector of the Strandja Massif e.g. Chatalov 1988; Dabovski et al. 1991; Gocev 1991).Cenomanian sandy limestones lie uncon- oceanic West Black Sea basin, the Istanbul Zone was formably over the metamorphic rocks both in the translated along the West Black Sea fault from its pre- Turkish and Bulgarian sectors of the Massif and pro- drift position south of the Odessa shelf to its present vide an upper age limit for the mid-Mesozoic regional position Okay et al.1994).North of the Strandja metamorphism. Massif is a belt of Upper Cretaceous volcano-sedimen- tary rocks and granodioritic plutons Fig.2). This Srednogorie Zone was formed in the Senonian as a Late-Variscan basement magmatic arc and intra-arc basin above the north- ward-subducting Tethys ocean Boccaletti et al.1974). Granites and felsic gneisses, which make up the bulk The contact between the metamorphic rocks of the of the basement of the Strandja Massif, form a discon- Strandja Massif and those of the Rhodope Massif in tinuous belt, 240 km long and approximately 13 km the west are covered by the Tertiary sediments. wide, along the southwestern rim of the Strandja Mas- sif from near Istanbul westward to Topolovgrad Fig.2).They also occur in the north along the Turk- Stratigraphy ish±Bulgarian border in the core of a faulted anticline. The metamorphic rocks are dominated by granitic The Strandja Massif in Turkey consists of a late-Varis- gneisses with minor amounts of , amphibo- can crystalline basement of granites and gneisses lite and micaschist, and are intruded by a large unconformably overlain by a Lower Mesozoic meta- number of granitic veins, dykes, small stocks and plu- sedimentary sequence Fig.3; Pamir and Baykal 1947; tons.The margins of large plutons are characterised Aydõn 1974, 1982; ÜsËümezsoy 1982; ÞagÆlayan et al. by numerous granitic veins and stocks, and there is no 220

The Kula metagranite, which straddles the Turkish± Bulgarian border Fig.2), is a little deformed, fine- grained, homogeneous, leucocratic granite that con- sists mainly of quartz, microcline and plagioclase with minor muscovite.Other plutons, such as the Düzor- man see Fig.10) and Üsküp metagranites, are strongly deformed and possess a penetrative foliation. Deformation and metamorphism in the intrusive metagranites are of mid-Mesozoic age and are described later.

Zircon geochronology

Up to now, the only available isotopic age data from the basement of the Strandja Massif was an Rb/Sr whole-rock isotope age of 24411 Ma based on three rock samples from the Kõrklareli metagranite Aydõn 1974).During the present study three samples from three different granitic bodies and two samples from the basement metamorphic rocks were dated using the single-zircon stepwise evaporation technique of Kober 1986, 1987).The sample locations are shown in the geological map in Fig.2.The petrographic features of the dated zircons and the analytical results are given in Table 1 and are summarised in the histograms in Figs.4 and 5. The single-zircon stepwise Pb evap- oration dating method has the advantage that only small zircon samples are needed, no chemical pro- cedures are involved and therefore there is no blank contamination, and one knows precisely which grain is being dated.It has given results that agree with the conventional dating techniques e.g. Cocherie et al. 1992).The analytical technique was the same as described by Okay et al.1996).Data collection Fig. 3 Stratigraphic section of the Strandja Massif in the started at a zircon evaporation temperature of approx- Kapaklõ syncline imately 1450 C.The temperature was increased by steps of 20±30 C to approximately 1520 C.Isotopic measurements were done after each temperature step. discernable contact metamorphism in the gneisses sur- rounding the plutons, which suggests that the granites represent deep-level intrusions.The granitic plutons Zircons from the metagranites show a range of petrographic and deformational fea- tures; however, there are several common traits Zircons from three samples from the Kõrklareli, Kula among the plutons such as the absence of hornblende, and Üsküp metagranites Fig.2) were dated.All three presence of magmatic muscovite and the modal samples consist essentially of quartz, K-feldspar and importance of microcline.Most common are porphy- plagioclase, and additionally magmatic biotite and ritic granites and monzogranites with pink K-feldspar muscovite occur in minor amounts in the Kõrklareli phenocrysts up to 3 cm across.The Kõrklareli and and Kula metagranite, respectively.The sample from Ömeroba metagranites are examples of such plutons the Üsküp metagranite shows a penetrative foliation Fig.2).These granites consist of quartz 28±46 modal and development of metamorphic muscovite ascribed %), plagioclase 20±34%), microcline 15±33%), mus- to the mid-Mesozoic orogeny. covite 7±13%) and minor biotite.Kõrklareli meta- Zircons from all three samples are dominantly granite, which is the only late-Variscan pluton from colourless, transparent and prismatic, with no or very which geochemical analyses are available, is a uniform few inclusions.They show generally fine growth zon- peraluminous granite s.s.with 71±74 wt.%SiO 2, ing typical for igneous zircons Fig.6). According to 11±14% Al2O3, 4±5% K2O, 3% Na2O and 1% CaO the morphological classification of Pupin 1980), the Aydõn 1974, 1982). zircons from all the three metagranites are dominantly 221

Table 1 Isotopic data from single grain zircon evaporation Lithology and sample Grain Zircon morphology Mass Mean 207Pb/206Pb 207Pb/206Pb age number scansa ratio and errorb Ma)

Kirklareli metagranite 1 Colourless, long and 147 0.051661211 2709 1346B 2 Prismatic 131 0.051718192 2739 3 126 0.051692151 2727 4 101 0.051701176 2728 5 126 0.051687185 2718 Mean 406 0.05167937 2712 Kula metagranite 1 Colourless, clear and prismatic 108 0.051715361 27316 1142B 2 146 0.051589428 26719 Mean 196 0.051674250 27111 Üsküp metagranite 1 Rose, prismatic and long 142 0.052541554 30924 837B 1 Rose, prismatic and turbid 170 0.051560317 26614 1323A 164 0.050968348 23916 2 Colourless, small and prismatic 136 0.051833433 27819 Migmatite 1 Colourless, prismatic and large 130 0.052314332 29915 1335A 144 0.051788271 27612 128 0.050959267 23912 148 0.050566316 22114 a Number of 207Pb/206Pb ratios evaluated for age assessment b Observed mean ratio corrected for non-radiogenic Pb where necessary

Fig. 4 Histograms show the distributions of radiogenic Pb-iso- schist facies regional metamorphism during the Late tope ratios derived from evaporation of individual zircon grains Jurassic±Early Cretaceous under metamorphic temper- from the metagranites of the Strandja basement atures of less than 500 C.Considering that zircon has an isotopic closure temperature of above 900 C for U and Pb e.g. Kober 1986), the zircon ages from the S-type, typical for zircons from the crustal melt gran- metagranites should represent magmatic crystallisation ites. ages of the plutons, and indicate that the granites Five zircon grains from the Kõrklareli metagranite have intruded the gneissic basement during the Early sample gave a precise age of 2712 Ma Fig.4a). A Permian.Plutonism during the Variscan orogeny in very similar but less precise age of 27111 Ma was central Europe spanned a period of 340 to 270 Ma obtained from two grains from the Kula metagranite e.g. Schaltegger 1997); therefore, from a temporal Fig.4b).A single-zircon grain from the Üsküp meta- and most probably also from a spatial aspect, the granite sample gave a slightly older age of 30924 Ma Early Permian metagranites in the Strandja Massif can Fig.4c). These metagranites have undergone green- be considered as part of the late-Variscan plutonism. 222

Fig. 5 Histograms show the distributions of radiogenic Pb- isotope ratios derived from evaporation of individual zir- con grains from the gneiss 1323A) and migmatite 1335A) of the Strandja base- ment

Fig. 6a±d Cathodolumines- cence photographs of zircons from the Strandja Massif. a Prismatic, colourless, transpar- ent magmatic zircon shows sector zoning, Kula metagran- ite, sample 1142B. b Prismatic, elongate, colourless, transpar- ent zircon with igneous sector zoning, Üsküp metagranite, sample 837B. c Prismatic, elongate, colourless, transpar- ent zircon with igneous sector zoning, leucosome from a mig- matite, sample 1335A. d Pris- matic zircon with a magmatic core and a homogeneous metamorphic ?) overgrowth, gneiss, sample 1323A

The isotopic ages obtained from the Strandja meta- ported by the oscillatory zoning and prismatic habit of granites confirm the Late ages assigned pre- the zircons.However, some zircon grains from this viously to the granitic intrusions on the basis of sample have homogeneous overgrowths possibly e.g. Chatalov 1988). formed during the late-Variscan metamorphism Fig.6). Two zircon grains from this sample were dated Fig.5; Table 1).The first grain gave two peak Zircons from the gneiss and migmatite ages of 26614 and 23916 Ma, corresponding to high and low evaporation temperature ages, respectively, Two samples from the basement metamorphic rocks while the second grain gave a single peak age of 278 were dated.Sample 1323A is a fine-grained, finely 19 Ma.The older ages are similar to those doc- banded gneiss of quartz, microcline, plagioclase, umented in the Kõrklareli and Kula metagranites, and biotite and muscovite.Rare, poorly preserved feldspar indicate that the orthogneiss was deformed and meta- porphyroclasts suggest a granitic origin, which is sup- morphosed during or very soon after its intrusion. 223

The other dated sample 1335A) comes from the metasiltstones and phyllites.Occasionally graded bed- granitic leucosome of a migmatite.It consists of ding and current bedding are preserved in the meta- quartz, microcline, plagioclase and minor muscovite. sandstones, and show that the sequence is right way Zircons from the sample are dominantly prismatic, up.In the northern limb of the Kapaklõ syncline there colourless and transparent with strong oscillatory is a gradual upward transition from these medium- igneous zoning Fig.6), although there are also turbid, grained metaclastic rocks to a carbonate-rich sequence milky grey to pale-brown zircons.Two grains were of calc-schist, phyllite, slate and thin marble horizons. dated.Like the previous orthogneiss sample, the Some of the black marble horizons around the village results show two age peaks, one at 28513 Ma and the of TasËtepe Fig.7) contain abundant undetermined other at 22811 Ma.The 285-Ma age is similar to that lamellibranch and gastropod fossils.This mixed car- obtained from the orthogneiss and the metagranites bonate±siliciclastic sequence is overlain by a calcitic and indicates high-grade metamorphism and partial and dolomitic marble series of over 600 m in thickness melting during the earliest Permian.The Triassic iso- preserved in the core of the Kapaklõ syncline Fig.7). topic ages from both samples are more difficult to The marble is white to grey, medium to thickly bed- interpret, especially since the sedimentary sequence, ded and represents recrystallised neritic limestone. which lies stratigraphically on the exhumed and Carbonates are stratigraphically the youngest unit eroded basement of the Strandja Massif, have been preserved in the core of the Kapaklõ syncline.Howev- dated palaeontologically as earliest Triassic to mid-Ju- er, northeast of a complex fault zone, east of the vil- rassic in age Chatalov 1988); therefore, these Triassic lage of Kula, there is a sequence of black, dark-grey ages from the gneiss and migmatite are regarded as slates with minor intercalated metasiltstones, over being due to Pb loss during the Late Jurassic±Early 400 m in thickness, which lie stratigraphically over Cretaceous metamorphism. dark recrystallised limestones Fig.7). These fine- An important result of the zircon geochronology is grained metaclastics extend to Bulgaria, where they that the age of the basement metamorphic rocks of are palaeontologically dated as Mid-Jurassic Chatalov the Strandja Massif is similar to that of the intrusive 1988). granites.This result is consistent with the absence of The abundance of metaconglomerates, erosional contact metamorphism at the granite gneiss contacts channels in the metasandstones, and large-scale cross- and indicates high-grade metamorphism, migmatisa- bedding in the lower part of the metaclastic sequence tion and formation of crustal melts during the Early indicate a fluviatile environment, whereas the shelly Permian in the Strandja Massif. faunas in the carbonates suggest shallow marine con- ditions for the upper part of the sequence.The cover series exhibits some lithological variation between the Mesozoic metasedimentary cover two limbs of the Kapaklõ syncline.In the southern limb of the syncline the metaclastics are dominated by The late-Variscan metamorphic rocks and metagran- metaconglomerates, which pass up abruptly to the ites are unconformably overlain by a transgressive marbles, whereas in the northern limb arkosic to continental to shallow marine metasedimentary quartzitic metasandstones are more abundant than the sequence, over 2.5 km in thickness, which is best metaconglomerates, and there is a thick siliciclastic± preserved in the central Strandja Massif in the core carbonate transitional interval between the metasand- of the northwest-trending Kapaklõ syncline Figs.2, stones and the marbles Figs.3, 7).These lithological 3, 7).The metasedimentary sequence consists of variations suggest a southerly source for the clasts in metaconglomerates and metasandstones at the base, the conglomerates and a northward deepening sed- which are overlain by carbonates in the core of the imentary environment. syncline. Palaeontological ages of the cover series in the The unconformity between the basement gneisses Turkish Strandja are not known.In contrast, the con- and the overlying metaconglomerates can be clearly odont and bivalve ages of the cover units in the Bul- observed 1 km northwest of the village Erikler garian Strandja are relatively well determined Chata- Fig.7). Metaconglomerates dominate the lower part lov 1985a, 1988, 1990).Geographic and lithological of the metaclastic sequence.The well-rounded and correlations with the Bulgarian sector of the Strandja poorly sorted clasts in the metaconglomerate are Massif indicate an Early Triassic age for the basal usually 3±10 cm across but may reach up to 50 cm and metaclastic unit and for the overlying phyllite±calc- consist mainly of granite, gneiss, aplite and quartz in a schist±slate sequence, and a mid-Triassic Anisian) age sandy-silty matrix.The metaconglomerate beds, 1±5 m for the overlying marbles.The slates, which outcrop thick, alternate with thinner arkosic to quartzitic meta- southeast of the Kula metagranite, are probable equiv- sandstone and metasiltstone beds.In some cases meta- alents of the Jurassic phyllites which occur immedi- conglomerate occurs in the erosional channel fills in ately across the Bulgarian border Chatalov 1988). the metasandstones.Upwards in the metaclastic The Triassic sequence in the Strandja autochthon can sequence, the metaconglomerate beds become scarce, be compared with the Germanic Triassic series of cen- and the sequence is dominated by metasandstones, tral and eastern Europe e.g. Schröder 1982) with the 224

Fig. 7 Geological map and cross section of the central Strandja basal metaclastic series corresponding to Buntsand- Massif around the Kapaklõ syncline.For location see Fig.2 stein, and the overlying marbles to the Muschelkalk. In the Bulgarian part of the Strandja Massif, the age of the authochthonous cover series of the Strandja Massif extends up to the Mid-Jurassic Bathonian) with an unconformity between the Carnian and Het- tangian series Chatalov 1988). 225

Triassic allochthons of the Strandja Massif

The allochthonous Triassic metasedimentary and metavolcanic rocks, which lie tectonically over the Jurassic metasediments of the Strandja autochthon, are known as the Strandja-type Triassic as opposed to the stratigraphically, lithologically and structurally dif- ferent Balkanide-type Triassic of the Strandja autoch- thon Chatalov 1980, 1988; Dabovski and Savov 1988). The allochthonous Triassic is found mainly in a large klippe in Bulgaria Fig.2), and consists dominantly of phyllite, slate, marble, metabasite, metakeratophyre, metasandstone and metasiltstone, including thick packages of turbidite sequences.The Strandja-type Triassic shows intense deformation, which has largely disrupted the stratigraphic continuity.Conodont ages from the Strandja-type Triassic range from the earliest Triassic Griesbachian) to Chatalov 1985b, 1988).With its lithological features, chaotic internal structure and allochthonous tectonic position, the Strandja-type Triassic may represent a subduction±ac- cretion complex SË engör et al.1984); however, ultra- mafic rocks or are conspicuously absent from the whole of the Strandja Massif. Fig. 8 Small-scale geological map of the area immediately east of the town of IÇgÆneada illustrates the Cenomanian unconformity over the metamorphic rocks of the Strandja Massif.For location see Fig.2 Cretaceous±Tertiary volcano-sedimentary cover of the Strandja Massif pluton in the Turkish Strandja is the Demirköy gran- Along the Black Sea coast the metamorphic rocks of odiorite Fig.2, Aykol and Tokel 1991) with biotite the Strandja Massif are unconformably overlain by and hornblende K/Ar ages of 782 and 792 Ma, widespread Upper Cretaceous sedimentary and vol- respectively Moore et al.1980).A small microdiorite canic rocks, which constitute the Srednogorie Zone.In stock northeast of Dereköy has also yielded similar the Turkish sector the base of the Cretaceous is well K/Ar biotite ages of 833 Ma Aydõn 1982).Similar exposed west of IÇgÆneada Fig.8), where shallow isotopic ages are obtained from plutons in the Sredno- marine carbonate-rich sandstones, 40 m thick, lie over gorie Zone in Bulgaria e.g. Zagorcev and Moorbath the phyllites of the Strandja Massif with an angular 1987).These Cretaceous calc-alkaline intrusive rocks unconformity.The sandstones contain abundant Orbi- are abruptly terminated southwest of Dereköy, which tolina sp.and Orbitolina concava indicating an age marks the magmatic arc front, and there are no Creta- range of Aptian to Cenomanian Pamir and Baykal ceous intrusions in the southern basement strip of the 1947; O.Sungurlu, unpublished data).They are fol- Strandja Massif. lowed up by hemipelagic siltstone and marl, and by Along the southwestern margin of the Strandja andesitic tuffs and agglomerates, over 250 m in thick- Massif the metamorphic rocks are unconformably ness Fig.8).The more widespread Cretaceous series overlain by Eocene shallow marine limestones Fig.2; in Bulgaria also start with Cenomanian conglomerate, Kopp et al.1969).The Thrace basin south of the marl, and sandstone with coal lenses and pass up into Strandja Massif started to form in the mid-Eocene Coniacian to Campanian andesitic tuffs, agglomerates, and was filled by Eocene±Oligocene siliciclastic sed- lava flows intercalated with marl and siliciclastic turbi- iments over 9 km in thickness Turgut et al.1991; dites, more than 3 km in thickness Aiello et al.1977; Görür and Okay 1996). Boccaletti et al.1978; Nachev 1993; Nachev and Dimi- trova 1995).Maastrichtian is unconformable over the volcanoclastic rocks and is represented by shallow Structure marine limestones.The Upper Cretaceous volcano- sedimentary sequence was deposited in an intra-arc The Strandja Massif comprises penetrative structures basin above the northward-subducting Vardar-Intra- of Early Permian, and Late Jurassic±Early Cretaceous Pontide ocean Boccaletti et al.1974).South of the age, and largely brittle post-Cenomanian structures. intra-arc basin there is a belt of Upper Cretaceous The late-Variscan structures, which are found in the sills, dykes, stocks and plutons related to the Sredno- gneissic basement, must have been overprinted by the gorie magmatic arc Fig.2). The largest Cretaceous mid-Mesozoic deformation; however, it is difficult to 226

Kapaklõ syncline show a rough spaced fracture cleav- age subparallel to the bedding.The original sedimen- tary features, such as graded and current bedding, are commonly preserved in the metasandstones.The shapes of the clasts in the metaconglomerates suggest weak strain intesity.In contrast, phyllites and calc- schists in the Kapaklõ syncline exhibit a penetrative foliation, and very rarely a weak subhorizontal north- trending mineral lineation.They are deformed into mesoscale north-vergent folds with east-trending sub- horizontal axis.Crenulation cleavage subparalel to the east-west fold axial planes has developed widely in the fine-grained metaclastic rocks. The Kõrklareli and Ömeroba metagranites on the southern limb of the Kapaklõ syncline and the Kula metagranite on the northern limb show clear effects of recrystallisation, although penetrative foliation, apart from that in the rare shear zones, has not been observed in these plutons. The Kapaklõ syncline is an open, symmetrical, non- cylindrical fold with a half wavelength of ~10 km Figs.7, 9b).It is modified by faulting along its south- western and northeastern limbs.On the southwestern limb the Kõrklareli metagranite and the gneisses have been thrust along a steeply south-dipping reverse fault Fig. 9a±d Structural data from the Strandja Massif plotted on over the Mesozoic metasediments.Along the north- equal area streonets.In the contoured streonets the contours are at 1, 3, 5, 7, 9 and 11%. a Foliation planes from the late-Varis- eastern limb of the Kapaklõ syncline there is a major can gneisses. b Foliation planes from the Triassic metasediments dextral transtensional fault between the Triassic cover overlie the late-Variscan basement. c Foliation planes from the rocks and the underlying basement granites and Üsküp metagranite. d Foliation planes from the Triassic meta- gneisses.This Malkoçlar fault Fig.7) extends for over sediments tectonically underlie the Üsküp metagranite 60 km in the Bulgarian and Turkish sectors of the Strandja Massif and has a curved pattern Fig.2).In the east between the village of Kula and the Bulgarian differentiate these two structure generations in the border, the Malkoçlar fault is subvertical.The steep gneissic basement, where the dominant structure is a attitude of the fault and the mapped offset in Bulgaria south-dipping compositional banding and foliation Fig.2) suggest dextral strike-slip movement along Figs.2, 9a). This gneissic banding is discordant with this section of the fault.The western part of the Mal- the generally northeast-dipping foliation in the over- koçlar fault dips at 45±60 to south and is a normal lying cover series Figs.7) and is also cut by the gran- fault. itic veins and stocks of the Early Permian Kõrklareli The large-scale structure in the eastern central and Ömeroba metagranites.These two observations Strandja Massif is a strongly deformed, probably root- indicate that the south-dipping foliation and composi- less granitic thrust sheet, the Üsküp metagranite, tional banding in the basement metamorphic rocks are which lies over strongly strained Mesozoic metased- pre-Triassic. iments Fig.10). The Üsküp metagranite shows a The deformation in the Triassic±Jurassic metased- strong mylonitic foliation and compositional banding, iments in the central Strandja Massif shows a marked and a very weak subhorizontal north-trending line- increase in intensity, defined by the development of ation.The foliation in the Üsküp metagranite dips con- foliation, from the relatively little deformed Kapaklõ sistently south at 35 and is parallel to the basal thrust syncline in the northwest to a highly strained thrust plane, and to the foliation in the underlying Mesozoic belt in the southeast.Although foliation of varying cover series Figs.9c,d). The deformation intensity in intensity is present throughout the Strandja Massif, the metagranite, as defined by the intensity of folia- mineral lineation is only very rarely observed.The tion, increases downwards towards the basal thrust. general absence of lineation is a structural feature that Near the basal thrust the metagranite is completely distinguishes the Strandja Massif from the strongly lin- transformed into banded gneisses which are difficult to eated Rhodope Massif e.g. Burg et al. 1996). distinguish from the underlying paragneisses.The par- The large-scale post-Variscan structure of the west± agneisses under the Üsküp metagranite occur in a central Strandja Massif is a broad, symmetrical syn- shear zone between the Kõrklareli-Düzorman meta- cline, the Kapaklõ syncline Figs.2, 7).The metacon- granites at the base and the Üsküp metagranite at the glomerates and metasandstones in the limbs of the top Fig.10).They consist of fine-grained, white quart- 227

Fig. 10 Geological map and cross section of the region around the allochthonous Üsküp metagranite.For loca- tion see Fig.2

zo-feldspathic gneisses and rare biotite micaschists orogeny in the Strandja Massif was north-vergent.The which show fine mylonitic banding but no mineral line- mineral and streching lineation in the cover series, ation.Only the rare clasts in the paragneisses reveal although weak and rarely preserved, is consistently their sedimentary origin.The thrust contact between north±south.The paragneisses directly under the the Üsküp metagranite and the underlying Mesozoic Üsküp metagranite show intense micro- and mesoscale metasediments can, therefore, be regarded as a deep- folds with a consistent northward vergence.Similar seated greenschist facies shear zone. overturned folds are described from the cover meta- Several lines of evidence indicate that the defor- sediments in the Bulgarian sector of the Strandja Mas- mation and thrusting during the mid-Mesozoic sif e.g. Dabovski and Savov 1988). North-vergent 228 character of the mid-Mesozoic orogen is also sup- tional banding defined by millimetre-thick alternations ported by the Triassic palaeogeography of Bulgaria. of quartz+feldspar, and muscovite+biotite+epidote lay- The Triassic sequences north of the Strandja Massif ers.The only magmatic relics are the larger feldspar are continental to shallow marine with no evidence of and very rarely muscovite porphyroclasts, the latter an intervening deep marine Triassic basin, from which showing extensive recrystallisation to smaller grain the Triassic allochthons could be derived. aggregates at their margins.The quartz and feldspar in the matrix of these metagranites form equant and equal-sized grain aggregates largely free from the Metamorphism effects of deformation features such as undulose extinction, bending or fracturing of the grains. All the pre-Cenomanian rocks of the Strandja Massif The mineral assemblages in the metapsammites have undergone one or two periods of regional meta- and in the metagranites indicate that the Strandja morphism.The late-Variscan metamorphism has Massif was metamorphosed in low greenschist facies affected only the basement, whereas the mid-Mesozoic at temperatures of less than 500 C and pressures of regional metamorphism affected both the basement <8 kbar e.g. Yardley 1989), as also deduced by Cha- and the cover series. talov 1988) for the neighbouring areas in Bulgaria. The widespread Triassic metapsammites and con- There is no marked areal change in the grade of meta- glomerates in the cover series typically contain morphism in the Turkish Strandja Massif.The age of quartz+albite+K-feldspar+muscovite assemblage.The the regional metamorphism is stratigraphically brack- original sandstone fabric with poorly sorted quartz eted between Bathonian, the youngest palaeontologi- and feldspar grains in a sparse matrix is petrograph- cal age from metamorphic rocks in the Bulgarian ically recognisable in the rocks of the Kapaklõ syn- Strandja Chatalov 1988), and Cenomanian, the age of cline.The only metamorphic effects in these meta- the oldest sedimentary rock that lies unconformably sandstones is the recrystallisation of quartz into on the metamorphic rocks Fig.8).K/Ar biotite ages smaller polygonal grain aggregates, and the formation from three samples of the Kõrklareli metagranite of sericitic white mica along grain boundaries.In con- yielded isotopic ages ranging from 155 to 149 Ma Ay- trast, in the southwest in the thrust belt under the dõn 1974, 1982).During the present study Rb±Sr iso- Üsküp metagranite, the metapsammites are strongly topic data were obtained from the biotite and whole recrystallised and show compositional banding.The rock of the Kõrklareli metagranite sample 1346B), mineral assemblage in these rocks is quartz+albite+ which was dated by zircons as Early Permian K-feldspar+muscovitebiotiteepidote.The composi- 271 Ma).The analytical techniques were the same as tional bands in the metapsammites consist of approx- described in Okay et al.1993).The biotite whole- imately 1-mm-thick alternating muscovite-rich and rock age of the Kõrklareli metagranite is 1552 Ma quartzo-feldspathic layers.Rare coarse feldspar grains with an initial 87Sr/86Sr ratio of 0.7167318 Table 2). with a mantle structure are the only relics from the Considering that the biotite from the Kõrklareli meta- original sandstone. granite is texturally magmatic, this Late Jurassic Rb/Sr Deformation and metamorphism in the late-Varis- age gives a maximum age for the mid-Mesozoic green- can granites are likely of mid-Mesozoic age.This is schist facies metamorphism in the Strandja Massif. obvious in the case of the allochthonous Üsküp meta- The common mineral assemblage in the micaschists granite, where the marked compositional banding is in the late-Variscan basement is quartz+plagioclase+ parallel to the basal thrust plane and to the foliation biotite+muscovite+garnetstaurolite.Staurolite occurs in the underlying Mesozoic series Fig.9c,d). The only to the west, north of LalapasËa Fig.2). Rare Kõrklareli and Ömeroba metagranites on the southern amphibolites consist essentially of plagioclase and limb of the Kapaklõ syncline and the Kula metagranite green hornblende.The voluminous gneisses in the on the northern limb show clear effects of recrystalli- basement are made up of quartz, plagioclase and sation.The magmatic quartz crystals have recrystal- K-feldspar with minor muscovite, biotite and epidote. lised into aggregates of polygonal grains with straight Several gneiss samples comprise rounded feldspar por- grain boundaries.Feldspar porphyroclasts show recrys- phyroclasts, which may be relics from a granitic proto- tallisation into smaller grains at their margins.Newly lith.The mineral assemblage in the micaschists indi- formed metamorphic minerals in the metagranites cates upper greenschist to lower amphibolite facies include muscovite, epidote and chlorite.The more metamorphism.Migmatites indicate a high amphibo- strongly deformed metagranites, such as the Düzor- lite facies metamorphism during the Early Permian, man and the Üsküp metagranites, show a composi- although mineral assemblages characteristic for this

Table 2 Rb±Sr data from the 87 86 87 86 sample 1346B from the Kirk- Rb ppm) Sr ppm) Rb/ Sr Sr/ Sr Apparent age Ma) lareli metagranite, Strandja Rock 1205 104,00 1115,7152 0.7293512 155.41.5 massif Biotite 1024 112,04 2111 5.3786 229 high-grade metamorphism have not been found, pos- Triassic±Jurassic epicontinental sedimentation sibly because of the scarcity of the metapelitic litho- and the mid-Mesozoic orogeny logies. By the end of the Permian the late-Variscan metamor- phic and granitic rocks of the Strandja Massif were Geological evolution exhumed and continental sedimentation started by the Early Triassic.The detritus for the 2-km-thick Lower Late-Variscan orogeny Triassic coarse clastic rocks came from the late-Varis- can gneisses and granites.By the mid-Triassic the clas- The close spatial and temporal association of granites tic sedimentation in the Strandja Massif was suc- and gneisses in the Strandja basement, the high alkali ceeded by neritic carbonate deposition Fig.11a).The feldspar content high K/Na ratio) of the granites, the Triassic sedimentation is similar in facies to that of scarcity of hornblende and the zircon typology suggest central and northern Bulgaria and central Europe, that the granites formed by partial melting of crustal where an initial continental clastic series of Early Tri- material during the Early Permian.An older crystal- assic age is succeeded by a shallow marine carbonate line basement or Paleozoic metasediments has not platform.This suggests that the Strandja Massif con- been identified.These Early Permian granites and tinued to be part of the Variscan Europe during the metamorphic rocks are regarded as the eastward Triassic.In the Bulgarian Strandja the shallow marine extension of the central European Variscan belt, espe- sedimentary sequence is known to extend up to the cially since the overlying post-orogenic Triassic Bathonian with a possible break in sedimentation dur- sequences bear close stratigraphic resemblance to ing the Norian and Rhaetian Chatalov 1988).North- those found in central Europe.Terrigeneous Lower vergent, Late Triassic folds are recorded in the seismic Triassic sequences, similar to the Central-European sections in the Moesian Platform Tari et al.1997); Triassic, occur also widely in the rest of Bulgaria e.g. however, this latest Triassic compression appears to Mader and Chatalov 1988), where they lie unconform- have been not important in the Strandja Massif, as the ably over terrigeneous Permian sediments or older sedimentation continued during the Early and mid-Ju- formations.However, in the Balkanides and in the rassic with the deposition of shallow marine sequences Moesian Platform the Variscan deformation was of Chatalov 1988). Late Carboniferous and earliest Permian in age Ten- In the Late Jurassic±Early Cretaceous interval the chov 1989; Haydoutov and Yanev 1997), somewhat authochthonous Strandja sequence was overthrust by earlier than that in the Strandja Massif.The vergence allochthonous sedimentary and volcanic rocks, proba- of the late-Variscan orogen in the Strandja Massif is bly representing a subduction±accretion complex, and not known directly.However, the presence of the whole package of Mesozoic sediments and the deformed but unmetamorphosed Paleozoic sequences underlying basement were penetratively deformed and in the Moesian platform north of the Strandja Massif regionally metamorphosed Fig.11b). The north-ver- e.g. Foose and Manheim 1975), possibly forming a gent deformation involved folding, generation of folia- foreland belt, implies a northward vergence. tion and thrusting of the basement granites over the Late-Variscan metamorphism and plutonism can be Mesozoic metasediments. traced as far north as the Balkanides, where Lower The age of the mid-Mesozoic orogeny in the Triassic continental sandstones and conglomerates lie Strandja Massif is stratigraphically bracketed between unconformably over the Upper Paleozoic granites and Bathonian and Cenomanian.Its age can be further metamorphic rocks e.g. Jubitz 1960). Metamorphism narrowed using the age of the foreland basin in the and plutonism are not present in the Moesian Plat- north SË engör and Natalin 1996, p 579).During the form in the north and in the Istanbul Zone, to the Oxfordian a narrow east/west-trending siliciclastic southeast of the Strandja Massif Fig.1), which show basin developed between the Strandja-Rhodope mas- uninterrupted Paleozoic sedimentation from Ordovi- sifs and the Moesian platform Fig.11b).This foreland cian to the Carboniferous followed by minor Carbon- basin, called the Nish-Trojan trough, received silici- iferous deformation.This suggests a major tectonic clastic and carbonate turbidites from the south break, a Variscan suture or a major fault, between the Tchoumatchenko et al.1989, 1990; Tchoumatchenko Strandja Massif in the south and the Moesia-Istanbul and Sapunov 1994; Harbury and Cohen 1997).The block in the north cf.Burchfiel 1980).On the other Nish-Trojan trough migrated northwards and persisted hand, the basement of the Strandja Massif appears to until the Barremian.The thick turbidite sequences in be similar to that of the Pelagonian Zone in Greece, the Nish-Trojan trough contrast with the neritic car- which has a metamorphic basement of quartzo-feld- bonates deposited on the Moesian platform in the spathic micaschist and amphibolites intruded by Late north during the Oxfordian-Barremian Tchoumat- Carboniferous granites and unconformably overlain by chenko et al.1989, 1990; Harbury and Cohen 1997). Triassic to Jurassic metasedimentary rocks e.g. Yar- Data from the Nish-Trojan foreland basin indicate wood and Aftalion 1976; Nance 1981; Schermer et al. that the compression in the Strandja Massif started in 1990). Oxfordian and continued until the Barremian.This 230

Fig. 11a±d Cross sections illustrate the geological evolu- tion of the Strandja Massif during the Mesozoic. a Middle Triassic, shallow marine lime- stone deposition over a pene- plained late-Variscan base- ment. b Latest Jurassic, deformation and metamor- phism due to continental colli- sion; development of the Nish- Trojan fore-deep basin in front of north-vergent thrust slices. c Cenomanian, shallow marine transgression over a peneplained surface. d Devel- opment of a magmatic arc and an intra-arc basin over the northward subducting Neo-Te- thyan ocean with the Strandja Massif occupying a compres- sional fore-arc setting

157- to 125-Ma period is compatible with the scarce and Sakarya zones.In the Sakarya Zone sedimenta- Rb/Sr and K/Ar age data on the mid-Mesozoic tion was continuous throughout Late Jurassic±Early regional metamorphism 155 to 149 Ma; Aydõn 1974, Cretaceous and the Alpide deformation occurred dur- 1982, this study). ing the Paleocene Okay and Tüysüz 1999).In the The mid-Mesozoic orogeny in the Strandja Massif Istanbul Zone, the weak Alpide deformation also took is characterised by prolonged thick-skinned defor- place during the Paleocene±Eocene.Similarity in this mation associated with regional metamorphism.The point is again with the Pelagonian Zone in Greece northward vergence is perpendicular to the main e.g. Mountrakis 1984). structural trend of the Strandja Massif cf.Fig.2). These features are hallmarks of an important orthogo- nal continental collision.The main tectonic problem Late Cretaceous±Tertiary history with the mid-Mesozoic collisional orogeny in the Strandja Massif is the apparent absence of the collid- The northern margin of the Strandja Massif was ing block. exhumed by the mid-Cretaceous after the enigmatic Before collision, the Strandja Massif was part of Late Jurassic±Early Cretaceous orogeny, and Cenoma- the passive margin of Laurasia, and it formed the foot- nian shallow marine sandy carbonates were deposited wall of the orogen during deformation.It must be sep- with angular unconformity over the Mesozoic meta- arated through a pre-Cenomanian oceanic suture from morphic rocks Figs.8, 11c).By early Senonian a mag- a continental block that had an opposing active mar- matic arc and intra-arc basin, characterised by gran- gin during the Late Triassic to odioritic plutons, stocks, dykes and voluminous Fig.11b).This continental block may be buried under andesitic lavas and tuffs, were established over the the Thrace Basin. northeastern half of the Strandja Massif Boccaletti et Late Jurassic±Early Cretaceous deformation or al.1974).This Srednogorie magmatic arc, which can regional metamorphism are not present in the Istanbul be traced along the whole length of Bulgaria, was gen- 231 erated during the northward subduction of the ceased by the Maastrichtian, and the intra-arc basin Tethyan ocean along the Vardar and Intra-Pontide closed during the Early Tertiary by north-vergent sutures Fig.11d). The southern part of the Strandja thrusting leading to the formation of the Eocene±Oli- Massif may have been in a fore-arc position during gocene Balkanide mountain chain. the Late Cretaceous.The absence of Cretaceous A major unsolved problem is the geological rela- deposits in the southern Strandja or in the Rhodope tion between the Strandja Massif and the Rhodope massifs indicates either that the magmatic arc was Massif s.l., which includes the Rhodope s.s., Serbo- compressional, or that such deposits were eroded dur- Macedonian, and Circum-Rhodope massifs.There are ing the subsequent continental collision Fig.11d). many similarities but also major differences between Continental collisions leading to the generation of the these two large metamorphic terranes.The presence Vardar, Intra-Pontide sutures occurred during the of a late-Variscan gneissic and metamorphic basement Paleocene to Eocene.The ensuing compression in the Rhodope Massif s.l., strongly overprinted by the resulted in north-vergent thrusting of the Strandja Late Tertiary heating, is indicated by some isotopic Massif over the Cretaceous deposits leading to the clo- data Borsi et al.1965; Wawrzenitz and Krohe 1998) sure of the Srednogorie intra-arc basin Fig.2). The and there is stratigraphic evidence for a second period compressional structures propagated northward pro- of metamorphism and deformation during the Late ducing the Balkanide fold and thrust belt, which is Jurassic and Early Cretaceous especially in the Cir- mainly of Eocene and Oligocene age Fig.1; e.g.Sin- cum-Rhodope Zone.However, the Rhodope Massif clair et al.1997). includes eclogites and metamorphosed ophiolites pos- sibly of Early Cretaceous age Liati 1988; Kolceva and Eskenazy 1988; Wawrzenitz and Mposkos 1997) Conclusion absent in the Strandja Massif.Furthermore, the Meso- zoic compressive deformation in the Rhodope Massif The Strandja Massif is a composite orogenic belt s.l. is generally south- to southwest-vergent in stark deformed and regionally metamorphosed during the contrast to the northward vergence found in the late-Variscan and Late Jurassic±Early Cretaceous Strandja Massif. orogenies.The Early Permian orogeny in the Strandja Massif involved amphibolite facies regional metamor- Acknowledgements We thank the Turkish Petroleum Company phism, crustal anatexis, and generation and emplace- TPAO) for supporting our fieldwork in the Strandja Mountains during 1991 and 1994, and the British Petroleum Company for ment of crustal melt granites.Early Permian zircon organising a field trip to the Bulgarian part of Strandja Massif. ages are obtained from both the deep-level granites G.The late G.Chatalov is thanked for leading the field trip in and the surrounding gneisses and migmatites. the Bulgarian Strandja.The late S.Kasar, M.Siyako, M.I ÇsËbilir During the earliest Triassic the basement was over- and H.S.Serdar, all from Turkish Petroleum Company, are thanked for help during the fieldwork, and J.Loeschke for a lain by fluviatile coarse-grained sediments with abun- critical reading of the manuscript. J.-P. Burg and L.-E. Ricou dant detritus derived from the basement granites and provided helpful reviews, which improved the paper. gneisses.The transgressive cover reach up to the mid- Jurassic and is characterised by continental to shallow marine sedimentation similar to that in central References Europe.During the Late Jurassic±Early Cretaceous, both the cover series and the basement of the Strandja Aiello E, Bartolini C, Boccaletti M, Gocev P, Karagjuleva J, Massif were penetratively deformed and regionally Kostadinov V, Manetti P 1977) Sedimentary features of the metamorphosed in greenschist facies.Rb/Sr biotite Srednogorie Zone Bulgaria), an Upper Cretaceous intra-arc whole-rock ages of the metamorphism is Late Jurassic basin.Sediment Geol 19:39±68 ~155 Ma).Compression involved north-vergent Alaygut D 1995) The petrographic investigation of the Turkish Strandja under the Thrace sedimentary basin; NW Turkey. thrusting of the basement granites over the mid-Meso- Int Earth Sci Colloq on the Aegean Region, IESCA-IÇzmir, zoic cover series, and the emplacement of the Triassic Güllük, Turkey:1 allochthons including basic volcanic rocks and deep Aydõn Y 1974) Etude petrographique et gØochimique de la par- marine turbiditic sequences, over the epicontinental tie centrale du Massif d©Istranca Turquie).PhD thesis, Univ Nancy, France, 131 pp Jurassic cover series of the Strandja Massif.An upper Aydõn Y 1982) Geology of the Yõldõz Istranca) Mountains. age limit for the mid-Mesozoic orogeny in the Thesis, Istanbul Technical University, 106 pp in Turkish) Strandja Massif is given by the Cenomanian shallow Aykol A, Tokel S 1991) The geochemistry and tectonic setting marine sandstones, which lie unconformably over the of the Demirköy pluton of the Srednogorie-Istranca granitoid chain, NW Turkey.Mineral Mag 55:249±256 metamorphic rocks. 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