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Petrostratigraphic evolution of the Thrace Basin(Bulgaria, Greece, Turkey) within the context ofEocene-Oligocene post-collisional evolution of theVardar-İzmir-Ankara suture zoneWilliam Cavazzaa, Luca Caracciolob, Salvatore Critellib, Azzurra d’Atric & Gian GaspareZuffaa

a Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Bologna,40126 Bologna, Italyb Dipartimento di Biologia, Ecologia e Scienze della Terra, Università della Calabria,87036 Rende (Cosenza), Italyc ENI S.p.a. Exploration and Production Division, 20097 San Donato Milanese, ItalyPublished online: 27 Nov 2013.

To cite this article: William Cavazza, Luca Caracciolo, Salvatore Critelli, Azzurra d’Atri & Gian Gaspare Zuffa ,Geodinamica Acta (2013): Petrostratigraphic evolution of the Thrace Basin (Bulgaria, Greece, Turkey) within thecontext of Eocene-Oligocene post-collisional evolution of the Vardar-İzmir-Ankara suture zone, Geodinamica Acta, DOI:10.1080/09853111.2013.858943

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Petrostratigraphic evolution of the Thrace Basin (Bulgaria, Greece, Turkey) within the contextof Eocene-Oligocene post-collisional evolution of the Vardar-İzmir-Ankara suture zone

William Cavazzaa*, Luca Caracciolob, Salvatore Critellib, Azzurra d’Atric and Gian Gaspare Zuffaa

aDipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Bologna, 40126 Bologna, Italy; bDipartimento diBiologia, Ecologia e Scienze della Terra, Università della Calabria, 87036 Rende (Cosenza), Italy; cENI S.p.a. Exploration and

Production Division, 20097 San Donato Milanese, Italy

(Received 31 May 2013; final version received 8 August 2013)

Eocene-Oligocene paleogeographic/paleotectonic reconstructions of the Rhodopian – northern Aegean – western BlackSea region largely ignore the Thrace Basin, a large sedimentary basin up to 9 km thick that has been long interpreted asa forearc basin developed in a context of northward subduction. Recent structural, stratigraphic, petrologic, and sedimen-tologic data challenge this notion and may instead be interpreted within a context of upper-plate extension during thecomplex transition between the collisional tectonic regime related to the closure of Vardar-İzmir-Ankara oceanic realmand the extensional regime characterizing the Oligocene-Neogene evolution of the Aegean and peri-Aegean regions. Thedetritus filling the Thrace Basin was derived from two main sediment source areas: (i) the mostly metamorphic terrainsof the Rhodopes to the west and (ii) the Vardar-İzmir-Ankara and Biga (intra-Pontide?) subduction-accretion prisms tothe southwest. During most of the Eocene-Oligocene, the entire basin was characterized by a complex physiography, asshown by commercial seismic lines in the subsurface and abrupt lateral facies change at the surface. Such physiographywas controlled by a series of basement highs trending from WNW-ESE (in the eastern and northern portions of thebasin) to WSW-ENE (in the western and southern portions of the basin) which influenced sediment dispersal and theareal distribution of paleoenvironments.

Keywords: post-orogenic collapse; upper plate extension; sandstone detrital modes

1. Introduction

The Thrace Basin is an important hydrocarbon provincecovering an area in excess of 15,000 km2 in Turkey,Greece, and Bulgaria. The complex historicalvicissitudes of the region have made collaborationamong the researchers of the three countries difficult.Consequently, unified geological analyses and interpreta-tions of the Thrace Basin are still missing despite thegreat wealth of outcrop and subsurface data availablefrom both academic and industrial sources. Integration ofsuch data with new stratigraphic, sedimentologic, petro-logic, and radiometric data provides a unique opportunityto delineate the sediment dispersal system of the entireThrace Basin and to generate new constraints on thegeneral evolutionary trends of the basin. This paper illus-trates how detailed definition of sandstone detrital modes(including heavy mineral analysis) and its integrationwith palaeocurrent analysis and sedimentological faciesrelationships can pinpoint significant within-basin prove-nance variations, thus providing important elements toconstrain in detail (i) the sediment dispersal pattern, (ii)the three-dimensional geometry of petrographic litho-somes, and (iii) the overall basin evolution. The sedi-ment paleodispersal system of the entire Thrace Basin ishere considered for the first time. Recent studies haveconstrained the palaeoenvironmental/palaeostructural set-ting and the chronostratigraphy of the southern portion

of the basin fill (d’Atri, Zuffa, Cavazza, Okay, & DiVincenzo, 2012; Okay, Özcan, Cavazza, Okay, & Less,2010; Özcan et al., 2010), thus integrating a large wealthof preexisting data, both from surface and subsurfacestudies (e.g. Doust & Arıkan, 1974; Görür & Okay,1996; Okay, Tüysüz, & Kaya, 2004; Önal, 1986; Sümen-gen & Terlemez, 1991; Turgut & Eseller, 2000; Yaltırak& Alpar, 2002). Similarly, sandstone petrology/geochem-istry and the overall paleogeographic evolution of thewestern margin of the basin in NE Greece and SWBulgaria have been studied recently by Caracciolo,Critelli, Innocenti, Kolios, and Manetti (2011, 2012) andCaracciolo et al. (2012).

2. Background geological information

The Thrace Basin is a complex system of depocenterslocated between the Rhodope-Strandja Massif to thenorth and west and the Biga Peninsula to the south(Figure 1). The southern margin of the basin is now cov-ered by the Marmara and northern Aegean seas anddeformed by the North Anatolian Fault system. TheThrace Basin is the largest and thickest Tertiary sedimen-tary basin of the eastern Balkan region and constitutesan important hydrocarbon province (Siyako & Huvaz,2007; Turgut & Eseller, 2000; Turgut, Türkaslan, &Perinçek, 1991). The older part of the basin fill crops out

*Corresponding author. Email: william.cavazza@unibo.it

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along the basin margins, but it is covered byPlio-Quaternary deposits in the basin center (Siyako,2006). In this area, subsurface data is abundant asTürkiye Petrolleri Anonim Ortaklığı has drilled morethan 350 wells and acquired in the 1980s and 1990s afairly dense network of seismic lines.

Most of the Thrace Basin fill ranges from the EarlyEocene (Ypresian) to the Late Oligocene. Maximum totalthickness, including the Neogene-Quaternary succession,commonly reaches 5,000 m and goes up to 9,000 m in anarrow depocenter bound by strike-slip faults (Siyako &Huvaz, 2007; Turgut & Eseller, 2000; Yıldız, Toker, &Şengüler, 1997). In terms of volume, most of theEocene-Oligocene sedimentary succession is made ofbasin-plain turbidites (Aksoy, 1987; Turgut et al., 1991).Sedimentation along the basin margins was characterizedby carbonate deposits during the Eocene and by deltaicbodies prograding towards the basin center in the Oligo-cene (Cavazza, unpublished data; Sümengen & Terlemez,1991; Sümengen, Terlemez, Şentürk, & Karaköse, 1987).The western margin of the basin, in Greek and Bulgarianterritory, was characterized already in the Eocene by aseries of coarse-grained fan-deltas prograding eastwardand feeding the depocentral basin-plain turbidites(Caracciolo et al., 2011, 2012).

In the northeast and northwest, the basin sedimentsoverlie nonconformably the metamorphic rocks of theStrandja and Rhodope massifs, respectively (Figure 1).The southern boundary of the Thrace Basin is less welldefined, with Eocene sedimentary and volcanic rocksextending southward into the Biga Peninsula, where theylie unconformably over the metamorphic rocks of theSakarya Zone (Siyako, Bürkan, & Okay, 1989). In thesouth, the North Anatolian Fault system cuts anddeforms the sedimentary rocks of the Thrace Basin.Small outcrops of ophiolitic rocks in this region havebeen interpreted as marking the Intra-Pontide suturebetween the Sakarya Zone and the Strandja-Rhodopemassifs (Beccaletto, Bartolini, Martini, Hochuli, &Kozur, 2005; Okay & Tüysüz, 1999; Şengör & Yılmaz,1981). The southwestern margin of the Thrace Basin iscovered by the northern Aegean Sea but Eocene-Oligo-cene sedimentary successions correlatable with those ofmainland Turkey and Greece crop out on several islandsin the region. Those of the islands of Lemnos andGökçeada were studied during this research and theresults are included in this paper.

The Thrace Basin can be subdivided into four parts(Figure 1): (1) Along the Strandja Massif to the north-east, there is a shelf region characterized by shallow-marine Eocene limestones passing southwestward intodeeper marine limestones, marls, and turbidites. (2) Inthe basin center, most of the Eocene–Oligocene succes-sion consists of siliciclastic rocks, up to 9000 m thick, asshown by a dense network of seismic sections and byhydrocarbon exploration wells (e.g. Siyako & Huvaz,2007; Turgut et al., 1991). The majority of the ThraceBasin sedimentary fill accumulated in this area. (3) The

southern portion of the basin in the Gelibolu peninsula ischaracterized by Eocene shallow-marine limestones over-lain by turbidites comprising large olistoliths of ophioliteand coeval limestone (Okay et al., 2010; Saner, 1985).(4) To the west (NE Greece and SE Bulgaria), marginalfan-deltas, shelf deposits, and laterally equivalent deepermarine turbidites crop out extensively (Maratos, Andro-nopoulos, & Koukouzas, 1977; Papadopoulos, 1980), butthe original basin margin geometries have been largelydisrupted by extensive exhumation of the RhodopianMassif during North Aegean extension (e.g. Krohe &Mposkos, 2002).

Contrasting hypotheses have been proposed toexplain the origin and the evolution of the Thrace Basin:(1) Keskİn (1984) and Perinçek (1991) considered thisbasin as intramontane in nature. (2) Turgut et al. (1991)and Tüysüz, Barka, and Yiğitbaş (1998) suggested atranstensional post-collisional origin following the clo-sure of the Intra-Pontide Ocean. (3) Görür and Okay(1996) proposed a forearc location between a subduc-tion-accretion complex to the south and a volcanic arc tothe north. (4) Şen (2002) championed a flexural origindue to the loading induced by backthrusts related to theİzmir-Ankara suture.

3. Plate-tectonic setting of the Thrace Basin

The Thrace Basin lies across a geodinamically complexarea characterized by three juxtaposed lithospheric blocks(terranes) distinguishable as to lithology, structural con-figuration, and geological evolution: the Rhodope-Stran-dja crystalline massif, the İstanbul Zone, and the SakaryaZone (Figure 1). (1) The Rhodope-Strandja massif hasLaurasian affinity and it is composed of Variscan conti-nental crust, Mesozoic metasedimentary rocks, and frag-ments of oceanic crust (Burg et al., 1996). Thisassemblage suffered repeated phases of crustal thickeningand exhumation during the Cretaceous and early Tertiary(e.g. Krohe & Mposkos, 2002). The main phase ofdeformation occurred in the Maastrichtian-early Paleo-gene following the closure of the Vardar Ocean (e.g.Stampfli & Borel, 2004). (2) Located at the southwesternmargin of the Black Sea, the İstanbul Zone is a continen-tal fragment about 400 km long and 70 km wide. It com-prises a Precambrian crystalline basement and a fairlycomplete Ordovician-Carboniferous sedimentary coverwhich was deformed during the Variscan orogeny (Görüret al., 1997; Okay, Zack, Okay, & Barth, 2011). Its strati-graphic, palaeobiogeographic, and palaeomagnetic char-acters show a marked Laurasian affinity. It was proposedthat this continental fragment rifted off the Odessa shelfand drifted southward during the opening of the westernBlack Sea backarc basin in the Cretaceous (Görür &Okay, 1996). (3) The Sakarya Zone, approximately1,500 km long and 120 km wide, is a continental blockseparated from the Rhodope-Strandja crystalline massifand the İstanbul Zone by the so-called Intra-Pontidesuture (Şengör & Yılmaz, 1981). The basement of this

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terrane is made of amphibolite-facies metamorphic rocksvisible in a few tectonic windows of limited areal extent(e.g. Cavazza, Okay, & Zattin, 2009; Okay, Satır, Zattin,Cavazza, & Topuz, 2008). In the Paleogene, the SakaryaZone collided with the Anatolide-Tauride terrane of Afri-can affinity to the south following the closure of theİzmir-Ankara ocean (Okay & Tüysüz, 1999; Stampfli &Borel, 2004).

Juxtaposition of the İstanbul and Sakarya Zonesalong the Intra-Pontide suture occurred in pre-Cenozoic

time (Cavazza, Federici, Okay, & Zattin, 2012), althoughthe exact timing has not been yet clearly defined. Thewestward continuation of the Intra-Pontide suture intothe Marmara Sea is controversial. Scattered outcrops ofthe ophiolitic Çetmi mélange in the Biga peninsula havebeen interpreted as marking the Intra-Pontide suturebetween the Sakarya Zone to the southeast and terrainsof Rhodopian affinity to the northwest (Beccaletto et al.,2005; Okay & Tüysüz, 1999; Şengör & Yılmaz, 1981;Siyako et al., 1989). Stampfli and Hochard (2009) dated

Figure 1. Schematic geological map of the Thrace and Marmara region (compiled from Andronopoulos, 1978; Konak, 2002;Maratos et al., 1977; Okay et al., 2010; Papadopoulos, 1980). The smaller box (lower left) shows the position of Thrace Basin withrespect to main tectonic domains. Legend refers only to geological map.

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the formation of the suture in the Biga peninsula at 200–180 Ma (Late Triassic–Early Jurassic) despite the factthat the blocks and the matrix composing the mélangereach up to the Early Cretaceous (Beccaletto, 2004).

Juxtaposition of the Sakarya and Anatolide-Taurideterranes occurred between the Late Cretaceous and thePaleogene following the closure of the Vardar Ocean andits continuation to the east, the İzmir-Ankara ocean(Okay & Tüysüz, 1999; Stampfli & Hochard, 2009). Thetransition between the collisional tectonic regime follow-ing the closure of these oceanic realms and the exten-sional regime characterizing the Neogene evolution ofthe Aegean and peri-Aegean regions is complex andrelatively poorly known (e.g. Bonev, 2006; Bonev &Beccaletto, 2007; Burchfiel, Nakov, Tzankov, & Royden,2000). The Thrace Basin developed during this transi-tional tectonic regime.

4. Stratigraphy and structure of the Thrace Basin

4.1. Western Thrace Basin

Traditionally, the term Thrace Basin has been usedwith reference to the depocentral area in Turkishterritory. Recently, Caracciolo et al. (2011, 2012) andd’Atri et al. (2012) demonstrated that theage-equivalent successions of Middle-Late Eocene andOligocene sedimentary and volcaniclastic rockscropping out extensively around the margins of theRhodopes in SE Bulgaria and NE Greece belong tothe same sediment paleodispersal system, and thus canbe considered the western portion of the basin. Faciesand paleocurrent analyses coupled with sandstonepetrology indicate that large volumes of the sedimentfilling the Thrace Basin were derived from the erosionand progressive unroofing of the Rhodopes. Present-day outcrops are separated by outliers of Rhodopianbasement rocks and are assigned for convenience toseparate sub-basins (e.g. Caracciolo et al., 2011, 2012),although it is difficult to assess the role of Oligocene-Miocene exhumation in this fragmentation.

In NE Greece, Lutetian coarse-grained fan-deltadeposits nonconformably overlie the basement rocks ofthe Rhodopes and grade upward into fluvial deposits andthen into laterally equivalent facies, from lagoonal to innershelf (Figure 2). This lower siliciclastic succession is over-lain – locally with a slight angular unconformity – by shal-low-water nummulitic limestone of Late Lutetian age(Maratos et al., 1977). The remaining Eocene section ischaracterized by a general paleoenvironmental trend fromshelf/slope deposits to the west to deeper-marine turbiditesto the east (Caracciolo et al., 2011). In the Early Oligo-cene, the western Thrace Basin system underwent anincreasingly complex tectonic and palaeoenvironmentalevolution (Innocenti et al., 1984), possibly the result of theprogressive exhumation/unroofing of the Rhodopes induc-ing higher local erosion rates and the progradation ofcoarse-grained clastic wedges from the basin margins.

Latest Eocene and Early Oligocene sedimentation ismarked by abundant volcaniclastic input ranging frompyroclastic flows to ash turbidites. Lava flows are alsopresent.

In SE Bulgaria, sedimentation began in the LateEocene (Figure 2) (Caracciolo et al., 2012). During thePriabonian, the Central-Eastern Rhodopes portion of theThrace Basin experienced a marine transgression markedby a thickening-upward sandy-pelitic turbidite succession.At the end of the Priabonian, the depositionalenvironment evolved into a continental–transitional–near-shore setting, represented by medium- to coarse-grained,carbonate-rich sandstones, alternating with microcon-glomerates. Similar to NE Greece, volcanic activitystarted in the latest Eocene. The structural signatures andthe coarse-grained nature of sediments associated withthe igneous activity suggest that extension occurred dur-ing and following magmatism. Volcanic activity andassociated sedimentation along the volcanic belt ceased atthe end of the Oligocene, although sedimentation contin-ued locally until the early Miocene (Burchfiel et al.,2000).

4.2. Northern Thrace Basin

The northern margin of the Thrace Basin has been tradi-tionally considered as the sediment source area for mostof the basin fill (e.g. Büyükutku, 2005; Görür & Okay,1996). This is at odds with the widespread presence ofshallow-marine limestone onlapping the basement rocksof the Strandja Massif (Figure 1). These limestones aretime-transgressive, drape the northern margin, and coverthe entire Late Eocene-earliest Oligocene time span(Less, Özcan, & Okay, 2011; Siyako, 2006): it is there-fore unlikely that any significant terrigenous input to theThrace Basin could have derived from its northern mar-gin, at least during this time frame. Significant detritalinput from the northern basin margin began only in theLate Oligocene (e.g. Turgut & Eseller, 2000) when animportant phase of delta progradation occurred alsoalong the southern margin.

4.3. Southern Thrace Basin

The sedimentary basin fill of the southern Thrace Basincrops out extensively in the Gelibolu peninsula and tothe north of the Gulf of Saros (Figure 1). Özcan et al.(2010) and Okay et al. (2010) have significantly refinedthe stratigraphy of this portion of the Thrace Basin. Inthe northern Aegean Sea, coeval sedimentary rocks cropout on the islands of Lemnos, Gökçeada, and Bozcaada,although the relationships between these and the age-equivalent deposits in mainland Turkey and Greece aredifficult to establish.

The stratigraphy of the southern Thrace Basin issomewhat different north and south of the Ganossegment of the North Anatolian Fault (e.g. Doust &Arıkan, 1974; Siyako & Huvaz, 2007; Sümengen & Ter-

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lemez, 1991; Turgut et al., 1991). North of the GanosFault low-grade metamorphic rocks belonging to the cir-cum-Rhodope belt are covered by a shallowing upwardsuccession of Bartonian-Priabonian distal-to-proximalturbidites, overlain by shelfal, deltaic, and continentalfacies of Oligocene age (Okay et al., 2010). Conversely,south of the Ganos Fault a rather complex Eocene suc-cession overlying the Çetmi mélange (Beccaletto et al.,2005; Okay et al., 2010; Okay, Siyako, & Bürkan, 1991)comprises thick olistostromes and large olistolithsderived from the mélange (Okay et al., 2010) and frompenecontemporaneous limestone (Figure 2).

The Çetmi mélange crops out in situ in the Biga Pen-insula where it marks the suture between Rhodopianbasement to the northwest and the Sakarya terrane to thesoutheast (Figure 1). The mélange is intruded by aLower Eocene (ca. 53 Ma) granodiorite and is uncon-formably overlain by Eocene rocks (Figure 1, Beccaletto,Bonev, Bosch, & Bruguier, 2007; Okay et al., 1991).The limestone blocks in the mélange range from LateTriassic to Cretaceous in age, the youngest blocks beingTuronian–Coniacian (Okay et al., 1991). Following Okayet al. (2010) we consider the outcrops of mélange northof the Dardanelles as derived from tectono-sedimentaryreworking of the Çetmi mélange from the Biga penin-sula. More generally, the entire Upper Eocene succession(Çengelli Fm) south of the Ganos Fault is characterizedby an olistostromal, very coarse-grained turbidite series(Figure 2). The clasts in the coarser mass flows include

Eocene (Bartonian and Priabonian) neritic limestone, ser-pentinite, gabbro, basalt, metabasite, pelagic limestone,radiolarian chert, gabbro, greywacke-shale, and quartz-diorite. The source of the clasts in the mass flows wasan ophiolitic mélange unconformably overlain by neriticUpper Eocene limestones (d’Atri et al., 2012; Okayet al., 2010). Field observations and regional geologicalarguments indicate that the source was to the south, veryclose to the site of the deposition. The Gelibolu penin-sula thus represents the southernmost portion of theThrace Basin. In this area, Late Eocene sedimentationoccurred during a period of pronounced basin subsidence(Huvaz, Sarıkaya, & Nohut, 2005) in an extensional tec-tonic setting with clasts derived from north-facing nor-mal fault scarps (Okay et al., 2010; Turgut et al., 1991).

4.4. Central Thrace Basin

Accurate basin-wide chronostratigraphic correlation of theEocene-Oligocene succession of the central Thrace Basinin the subsurface is hampered by the poor fossil content ofthe siliciclastic turbiditic succession. Nevertheless, pro-longed hydrocarbon exploration has generated a largeamount of data which has made possible correlation withthe marginal areas. The top of the basement of the centralThrace Basin features a very irregular geometry (Perinçek,1991), with a series of troughs and intervening structuralhighs elongated in a WNW-ESE direction. Seismic reflec-tion profiles (e.g. Burke & Ugurtas, 1974) show the

Figure 2. Stratigraphic sections and correlations in the studied areas. For section location, see Figure 1. Palaeoflow measurementsare shown in each section. Geological timescale from Gradstein, Ogg, and Smith (2004).

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Middle-Late Eocene succession filling the troughs and on-lapping the surrounding highs. During Middle Eocene –Early Oligocene time – while carbonate sedimentationdominated along the northern margin and on some largehighs – the troughs were progressively filled by siliciclas-tic turbidites. Dacitic and andesitic tuffs are typically inter-bedded with the Late Eocene –Early Oligocene strata andindicate active volcanism during this time period, asshown also in other areas of the basin (see above).

5. Sandstone petrofacies

This paper summarizes the results of the petrographicstudy of 239 arenite samples collected from the studyarea mostly along the stratigraphic sections of Figure 2(Büyükutku, 2005; Caracciolo et al., 2011, 2012; d’Atriet al., 2012). All petrographic analyses were performedfollowing the Gazzi-Dickinson point-counting method(Dickinson, 1970; Gazzi, 1966; Gazzi, Zuffa, Gandolfi,& Paganelli, 1973; Ingersoll et al., 1984; Ingersoll,Cavazza, Graham, & Indiana Geologic Field SeminarParticipants, 1987). Particular care was taken forrecognizing spatial (intrabasinal vs. extrabasinal grains)and time relationships (grains coeval or non-coeval withrespect to the basin fill) of carbonate and volcanic grains(e.g. Critelli & Ingersoll, 1995; Ingersoll et al., 1987;Zuffa, 1980, 1987). Quantitative petrographic study alsoincluded the analyses of the heavy-mineral concentratesobtained from 40 representative arenite samples from theGökçeada, Tayfur, Şarköy, Alexandropoulis, Korudağ,and Ganos Mt. stratigraphic sections (Figure 2). Heavy

minerals were separated and analyzed following theprocedures described in Gazzi et al. (1973). Detaileddescriptions of the sandstone petrofacies of the westernand southern Thrace Basin can be found in Caraccioloet al. (2011, 2012) and d’Atri et al. (2012), respectively.Detrital modes from the western and southern portionsof basin have then been compared with those from thesubsurface in the depocentral area (Büyükutku, 2005).

Thrace Basin arenites are mostly lithic arkoses andarkosic litharenites of medium to low compositionalmaturity. All samples are prevalently made up of silici-lastic terrigenous grains with a significant component ofpenecontemporaneous volcaniclastic detritus startingfrom the Late Eocene and continuining into the EarlyOligocene. Aphanitic rock fragments comprise metamor-phic, volcanic, and sedimentary types. The dominantmetamorphic lithic grains are phyllites and slates. Ser-pentinite, chlorite-schist, and serpentine-schists, relatedto ophiolitic suites, can be present in considerableamounts, particularly in samples taken along the southernmargin of the basin. Volcanic rock fragments span thewhole compositional spectrum, including acidic, interme-diate, and basic types. Acidic lithics display phenocrystsof quartz and plagioclase in a microgranular felsiticgroundmass. Intermediate and basic lithics have micro-lithic and lathwork, commonly chloritized, texture. Dis-tinctive diabase rock fragments are present. Sedimentaryrocks fragments are represented by siltstone and chert.

Carbonate terrigenous grains encompass micritic andmicrosparitic limestone and minor dolostone, from

Figure 4. Ternary diagram showing compositional character-ization of Priabonian-Rupelian sandstone samples from differ-ent portions of the Thrace Basin. Qm, monocrystalline quartzgrains; F, total feldspar grains; Lt, total aphanitic lithic frag-ments. Stars and polygons indicate means and standard devia-tions from the means, respectively. Data from Büyükutku(2005), Caracciolo et al. (2011, 2012), and d’Atri et al. (2012).Fields within diagram are from Dickinson et al. (1983) andDickinson (1985).

Figure 3. Ternary diagram showing compositional character-ization of Ypresian-Bartonian sandstone samples from differentportions of the Thrace Basin. Qm, monocrystalline quartzgrains; F, total feldspar grains; Lt, total aphanitic lithic frag-ments. Stars and polygons indicate means and standard devia-tions from the means, respectively. Data from Büyükutku(2005), Caracciolo et al. (2011, 2012), and d’Atri et al. (2012).Fields within diagram are from Dickinson et al. (1983) andDickinson (1985).

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fine-to coarse-grained. They are relatively abundant (ca.5%) in the Karaağac Formation at the base of the Tayfursection (Figure 2). In a few cases, carbonate extrabasinalgrains could be tied to specific rock units in the sedimentsource area as; for example, in the Tayfur section wherecarbonate lithoclasts contain tintinnids tests.

As to heavy-mineral fraction, zircon, tourmaline,rutile, and garnet constitute >50% of the population andare present in all samples (d’Atri et al., 2012). Picotite ispresent in all samples (20% on average) except thosefrom the Alexandroupolis section, and clearly indicates aprovenance from ophiolitic rocks. Numerous other heavyminerals such as clinopyroxene, clinoamphiboles, mona-zite, epidotes, chloritoid, kyanite, staurolite, and glauco-phane are present in very low quantities or in traces. Inparticular, augite characterizes the upper part of the

Tayfur stratigraphic section and epidotes and glauco-phane characterize the Şarköy section.

For plotting purposes, arenite samples were groupedobjectively by merely discriminating by age (Ypresian-Bartonian vs. Priabonian-Rupelian) and location (wes-tern, southern, and central sampling areas) (Figures 4and 5, Table 1). This approach clearly represents a firstobjective step providing a general picture of the ThraceBasin arenitic detrital modes; a more sophisticated statis-tical treatment of such a large analytical dataset can beenvisioned.

5.1. Ypresian-Bartonian detrital modes

Ypresian-Bartonian sandstone samples from the westernbasin margin have quartzolithic composition(Qm53F19Lt28) (Figure 3) and reflect major contributions

Figure 5. Large vitric tuff olistolith (thickness ca. 15 m) and associated slump horizon within slope turbidites of the upper TayfurFormation (Early Oligocene) at Tayfur Reservoir (Gelibolu Peninsula) (Figure 2). 40Ar–39Ar analyses of feldpars from the tuff olisto-lith yielded an eruption age of 30.22 ± .20 Ma (d’Atri et al., 2012). The abundance of penecontemporaneous volcaniclastic detritusthroughout the upper Tayfur Fm. points to a virtual coincidence between tuff age and depositional age of the upper Tayfur Fm. Orien-tation of slump fold axes indicate a north-facing paleoslope, in agreement with paleoflow indicators from the turbidite beds (Figure 2).

Table 1. Means and standard deviations of sandstone petrofacies.a

Age Provenance Composition QmFLt%Qm QmFLt%F QmFLt%Lt

Priabonian-Rupelian W margin Quartzo-feldspathic 46.0 ± 9.2 46.0 ± 10.2 8.0 ± 5.9W margin Volcaniclastic 7.8 ± 8.2 36.7 ± 14.5 55.5 ± 16.2S margin Volcaniclastic 13.2 ± 10.3 47.7 ± 24.1 39.1 ± 20.0Depocenter Mixed 26.8 ± 7.5 40.8 ± 9.1 32.5 ± 10.9

Ypresian-Bartonian W margin Quartzo-lithic 53.1 ± 19.8 18.5 ± 12.3 28.3 ± 18.2S margin Feldspathic litharenite 21.0 ± 10.9 29.8 ± 15.8 49.2 ± 20.6Depocenter Mixed 46.6 ± 20.4 30.3 ± 20.3 23.1 ± 15.0

aSee Figure 4 for explanation of petrographic parameters.

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from metamorphic terrains and subordinately from car-bonate rocks. Samples from this area and time framewere taken from continental deposits (Figure 2), andhence display a high degree of compositional variance.Lithic fragments are low-grade metamorphic and ophio-lite-related grains. Phyllites, fine-grained schists, andminor amphibolite grains are characteristic in SW Thraceand less represented in the Bulgarian sector, where ser-pentinite and serpentinite-schist fragments occur. Compo-sition of Middle Eocene deposits points to a provenancefrom the Circum-Rhodope Belt and – subordinately –from the Variegated Complex (Caracciolo et al., 2011,2012), as shown by low- to medium-grade metamorphicrock fragments and heavy-minerals associations of epi-dote + actinolite (e.g. Bonev & Stampfli, 2003). Increas-ing amounts upsection of amphibolite and gneissicmetamorphic grains reflect the progressive unroofing ofthe Variegated Complex.

Ypresian-Bartonian sandstone samples from thesouthern basin margin are feldspathic litharenites(Qm21F30Lt49) (Figure 3). Abundant epimetamorphicrock fragments are associated with a significant amountof ophiolitic and terrigenous carbonate detritus (d’Atriet al., 2012). Picotite is the only distinctive heavy min-eral, in agreement with the presence of ophiolitic rockfragments. Within this succession, the Middle Eocenesection is richer in low-grade metamorphic rock frag-ments that can be attributed to the erosion of the Rhodo-pian terrane of the Biga peninsula. Ophiolitic rocks andpicotite occur as in the Early Eocene section, but terrige-nous carbonate grains are virtually absent. Traces of epi-dote and titanite are sporadically present.

Ypresian-Bartonian sandstone samples from a numberof oil wells in the basin central portion were studied byBüyükutku (2005) (Figure 1). Samples from the turbi-dites of the Hamitabat Group (Middle Eocene) have acomposition (Qm47F30Lt23) (Figure 3), largely similar tothe one of coeval sandstone samples from the westernmargin. Phyllite and serpentinite lithic grains are present.

5.2. Priabonian-Rupelian detrital modes

Priabonian-Rupelian sandstone samples from the westernbasin margin in NE Greece and SE Bulgaria show twodistinct, interbedded petrofacies: quartzofeldspathic andvolcaniclastic (Figure 4, Table 1). The quartzofeldspathicpetrofacies is characterized by similar amounts of quartzand feldspar. Phaneritic rock fragments are representedby medium- to high-grade metamorphic rocks whereasplutonic rock fragments are minor. Aphanitic lithic grainsmainly consist of phyllite, chlorite schist, epidote-bearinggreenschist, and micaschist. Although the detrital supplyfrom the Circum-Rhodope Belt is appreciable, most ofthe sediment was derived from the Rhodopes sensustricto (Caracciolo et al., 2012).

Neovolcanic grains, both as single crystals and asrock fragments, characterize the volcaniclastic petrofaciesof the western margin (Caracciolo et al., 2011, 2012)

(Figure 4). Phaneritic rock fragments consist of volcanicrock fragments showing association of plagioclase andamphibole phenocrysts. Aphanitic lithic grains consist offine-grained to vitric volcanic fragments and subordinatemetamorphic (phyllite and mica schists) and sedimentary(extrabasinal carbonates) lithic fragments. Heavy-mineralcontent is generally high (up to 2% of total grains) andconsists of pyroxenes and amphiboles, with subordinatesphene. Noncarbonate extrabasinal grains are representedsolely by plagioclase, quartz, and K-feldspar.

The Priabonian-Rupelian succession south of theGanos Fault (“southern margin” petrofacies in Figure 4)is characterized by arkosic litharenites. This interval fea-tures abundant neovolcanic and intrabasinal carbonategrains (some samples are hybrid arenites, sensu Zuffa,1980). Other common framework components are (i)epimetamorphic and ophiolitic rock fragments and (ii)glaucophane and picotite grains, but their amounts aresignificantly lower compared to those of theEarly-Middle Eocene section in the same region becausethe abundant carbonate intrabasinal and neo-volcanicgrains dilute all other detrital components. Within theheavy-mineral association, titanite and augite characterizefurther this petrostratigraphic interval. In the upper partof the Tayfur section (Figure 2), the volcanic componentis very abundant and slumped turbidite slope deposits,including a large olistolith of penecontemporaneousvitric tuff, indicating a north-facing palaeoslope(Figure 5). Despite the fact that significant neovolcanicdetritus is present only in the upper parts of the Tayfurand Şarköy sections, it should be noted that volcanicdetritus – mostly penecontemporaneous – is somewhatpresent in the uppermost Eocene–Oligocene sectionthroughout the study area. Overall, detrital modes andthe presence in several samples of glaucophane andepidote suggest a provenance from an exhumedsubduction-accretion prism affected by volcanism.

Petrographic analyses of Late Eocene–EarlyOligocene sandstone samples from the subsurface in thecentral area of the Thrace Basin are unavailable. Thesamples closest to the central portion of the Thrace Basinare those from the middle and upper Korudağstratigraphic section (Figures 1 and 2). This group ofsamples has a composition intermediate between thewestern quartzofeldpathic petrofacies and the volcaniclasticpetrofacies of both the western and southern margins(Figure 4).

5.3. Reconstruction of sediment dispersal pathwaysand paleoenvironments

The study area covers a significant portion of theThrace Basin and the results of this study provide newcompelling constraints on its sediment dispersal patternand overall stratigraphic architecture. Figure 6 is a ten-tative reconstruction of the overall sediment dispersalpattern of the Thrace Basin. In the figure, consideringthat the study area comprises the most active strand of

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the North Anatolian Fault system (Figure 1), a conser-vative Plio-Quaternary dextral strike-slip offset of ca.70 km (Armijo, Meyer, Hubert, & Barka, 1999) wasrestored.

Paleocurrent trends, sedimentological/stratigraphicanalysis, and sandstone detrital modes indicate that animportant sediment source area was located to the southand southwest, along the Vardar-Izmir-Ankara suture(d’Atri et al., 2012; Okay et al., 2010). This is evident inthe Gelibolu peninsula south of the Ganos Fault, whichmarked the site of an escarpment active throughout theEocene and separating a shallow marine area to the southfrom a deep basin to the north (Doust & Arıkan, 1974).Sediment derived from the erosion of the Vardar-Izmir-Ankara orogenic prism is characterized by ophioliticdetritus, including a deep-sea sedimentary cover.Epimetamorphic and granitoid detritus is also present.Starting from Late Eocene time, a significant penecon-temporaneous volcanic component is characteristic.Within this later phase, the coexistence of pure neovolca-nic layers (crystal tuffs) and hybrid carbonate-rich are-nites with detritus derived from a continental basementindicates the presence of episutural basin(s) where shal-low-water carbonates were deposited on top of theexhuming subduction-accretion prism (d’Atri et al.,2012). These carbonates were mixed with penecontem-

poraneous neovolcanic components and redeposited indeeper marine environments. The entire southern ThraceBasin was fed from the south and southwest. The precisedelineation of the sediment dispersal pattern toward thesouthwest (i.e. in the northern Aegean) is hampered bythe fact that most of this region is now underwater, butobservations on the islands of Lemnos (Caracciolo et al.,2011; Maravelis, Konstantopoulos, Pantopoulos, &Zelilidis, 2007), Gökçeada (d’Atri et al., 2012), andBozcaada (Temel & Çiftçi, 2002) substantiate thisreconstruction.

Other elements pointing to a southern provenance are(i) large olistoliths and olistostromes enclosed in theEocene turbidites in the Şarköy region along the south-ern margin of the basin (Okay et al., 2010), and (ii) largeOligocene deltaic bodies generically prograding north-eastward in the same region (Osmancık Formation, Ata-lik, 1992). Both elements point to a prolonged history ofnorthward sediment dispersal, in agreement with oursandstone petrographic data and sedimentological obser-vations. During most of the Eocene, the entire basin wascharacterized by a complex physiography featuring a ser-ies of structural highs trending generically east-west, asshown by both commercial seismic lines (e.g. Turgutet al., 1991) and dramatic lateral facies changes at thesurface (e.g. Siyako & Huvaz, 2007). Such configuration

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Figure 6. Schematic reconstruction of Middle Eocene–Early Oligocene provenance and overall sediment dispersal pathways in theThrace Basin. Small arrows show mean azimuth paleoflow directions. Plio-Quaternary dextral offset of about 70 km along the NorthAnatolian Fault was restored. Red lines are normal (ticks on downthrown side) and strike-slip faults. Depth-to-basement contour lines(in meters) from Siyako (2006).

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influenced greatly the sediment dispersal pattern and thelocal rates of sedimentation.

A second, important sediment source area was theplutono-metamorphic Rhodope Massif west of the basin(Caracciolo et al., 2011, 2012). The coarse-grained fan-deltas characterizing the Eocene section along the wes-tern margin of the basin in Greece and Bulgaria were theentry points associated with this second sediment sourcearea. The detritus generated in this area was then dis-persed eastward within a complex system of depocenters(Figure 6). The Rhodopian provenance underwent signif-icant changes through time, the progressively increasingcontent of amphibolite, and gneissic metamorphic grainsreflecting progressive unroofing of the Variegated Com-plex (Caracciolo et al., 2012). Since the late Eocene, thesource-basin system underwent an abrupt change instructural style with the onset of general extensionleading to the exhumation of the Gneiss–Migmatite corecomplex and concurrent widespread volcanism. As aconsequence, sandstone composition evolved from quar-tzolithic to quartzofeldspathic/volcaniclastic.

Middle Eocene–Early Oligocene sedimentary depositscropping out in the Alexandropoulos-Komotini region andin southern Thrace show consistent eastwardpaleocurrents. Therefore, supply could be thought as thesame for both the Greek and Turkish sectors of the ThraceBasin. However, sandstone gross compositions for the tworegions are substantially different (Figures 4 and 5). In theAlexandropoulos-Komotini region sandstone detritalmodes can be linked directly to the Rhodopian Massif(Caracciolo et al., 2011; Caracciolo, Critelli, Innocenti,Kolios, & Manetti, 2013; Caracciolo et al., 2012; Marchevet al., 2004), whereas in the southernmost Thrace Basinthe provenance was from an exhumed accretionary wedge(d’Atri et al., 2012). In the latter region, sedimentpalaeodispersal was then deflected toward the east alongthe axes of a number of elongated depocenters (Doust &Arıkan, 1974; Perinçek, 1991; Turgut et al., 1991).

The Korudağ and Ganos Mountain stratigraphic sec-tions cover together the Middle Eocene–Late Oligocenetime span and are both made of deep-sea turbidites grad-ually evolving upsection into slope, shelfal, and deltaicdeposits (Figure 2). Palaeocurrent measurements north ofthe Ganos Fault indicate palaeoflows consistently direc-ted toward the ESE (d’Atri et al., 2012; Görür & Okay,1996; Siyako & Huvaz, 2007; Sümengen & Terlemez,1991; Turgut & Eseller, 2000). Such palaeocurrents maybe compatible with turbidite flows derived from either (i)the Rhodopes or (ii) the eastward deflection of turbiditeflows originally derived from the southern basin margin.The fact that this petrofacies does not match neither aRhodopian (cf. Caracciolo et al., 2011) nor a southern(cf. d’Atri et al., 2012) provenance points either to amixture of the two detrital inputs or to an unspecifiedthird input from the region now covered by the northernAegean Sea.

Our petrographic analyses indicate that the Ypresian-Bartonian sediment source areas for the Thrace Basin

sediments were characterized by ophiolitic source rocksand their pelagic sedimentary cover. The Vardar-İzmir-Ankara subduction/accretion wedge associated withthe closure of a branch of the Neotethys and the morenorthern Biga subduction/accretion wedge (Çetmimélange) –whether or not associated with the westwardcontinuation of the Intra-Pontide suture (Okay, Satır,Tüysüz, Akyüz, & Chen, 2001; Okay et al., 2008; Okay& Tüysüz, 1999) – are the obvious source-areacandidates for the detritus forming the Ypresian-Barto-nian succession. An important southern provenance isalso shown by palaeocurrent trends and palaeoslope ori-entation as shown by slump features and olistoliths.

Pure neovolcanic fallout beds and turbidite strata ofhybrid arenites compositionally characterize the LateEocene–Early Oligocene succession of the Thrace Basin.The composition of the arenite framework, made of car-bonate intrabasinal grains (bioclasts and peloids) and bypristine neovolcanic grains locally coated by carbonaterims, points to the existence of a shallow-water intrabasi-nal source area where carbonate grains were generatedand mixed with pyroclastic detritus from penecontempo-raneous volcanism. Such shallow-water sediment accu-mulations were then periodically mobilized as gravityflows and redeposited into slope/basinal environments,resulting in hybrid arenitic turbidites (d’Atri et al.,2012). As for the lower part of the southern petrofacies,granitic/gneissic, epimetamorphic and ophiolitic lithicgrains, and picotite are present, but their amount is sig-nificantly lower because of the abundant carbonate intra-basinal grain and neo-volcanic grains that dilute all othercomponents.

The turbiditic succession of the Çengelli Formationalong the Şarköy stratigraphic section (Figure 2) featuresthick proximal olistostromes and giant olistoliths (Okayet al., 2010). Clast composition is scale-invariant, fromsand grains to olistoliths up to 1 km across: serpentinite,gabbro, basalt, greenschist, greywacke, Cretaceous-Paleo-cene pelagic limestone, and the underlying Upper Barto-nian-Lower Priabonian Soğucak Limestone (Özcan et al.,2007). Composite olistoliths consisting of pelagic lime-stone or basalt overlain by the Soğucak Limestone arecommon, providing further evidence for synsedimentarytectonics. The Upper Eocene mass flows of the ÇengelliFm were formed in an extensional setting and werederived from the south from the flanks of large normalor transtensional faults related to the opening of thesouthern Thrace Basin (Okay et al., 2010). The entireŞarköy section indicates a provenance from the ophioliticsuite of an exhumed subduction-accretion prism locatedto the south. The occurrence of serpentinite lithic frag-ments and glaucophane grains is in agreement with thisinterpretation.

6. Discussion

A thorough paleogeographic/paleoenvironmental recon-struction of the Thrace Basin is hampered by the high

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degree of tectonic overprint – including large-scaleextension in the Aegean and peri-Aegean regions as wellas strike-slip deformation related to the westward propa-gation of the North Anatolian Fault – which has largelyobscured the original facies pattern in most of the south-ern and western basin margins. Despite these complexi-ties, the integration of preexisting and new datapresented in this paper places significant constraints onthe overall evolution of the basin and its tectonic inter-pretation.

The Thrace Basin has been interpreted as a forearcbasin which developed in a context of northward subduc-tion (Görür & Okay, 1996). This interpretation is chal-lenged by the lack of a coeval magmatic arc along itsnorthern (Strandja) margin. Besides, penecontemporane-

ous volcanism kicked off late in the basin evolution(Caracciolo et al., 2011, 2012; d’Atri et al., 2012;Innocenti et al., 1984; Yanev, 1998; Yanev, Innocenti,Manetti, & Serri, 1998) in contrast with typical forearcbasins (for a review, see Dickinson, 1995). The interpre-tation of the Thrace Basin as a forearc basin was alsobased on the occurrence, along its southern margin inthe Gelibolu peninsula, of a belt of chaotic deposits longinterpreted as a tectonic mélange formed in situ in anaccretionary prism (Beccaletto, 2004; Beccaletto et al.,2005). Recently, Okay et al. (2010) demonstrated insteadthat such belt was derived from the erosion and sedimen-tary reworking (olistoliths and large submarine slumps)of an older mélange unit (Çetmi mélange) located to thesouth. Despite several examples of sedimentary rework-

Figure 7. Lutetian-Bartonian (Middle Eocene) paleogeographic reconstructions (from Stampfli and Hochard (2009), modified). Fol-lowing the closure of the Vardar-Izmir-Ankara ocean, the subduction of the Pindos and Troodos oceanic slabs triggered upper-plateextension and the opening of numerous rifts and associated core-complexes in Turkey, in the Cyclades, and in the Balkans. TheThrace Basin developed within this framework of post-collisional extension in the area facing the subducting remnant Pindos Ocean.Abbreviations: Adr, Adria; Ana, Anatolides; Ant, Antalya; Apu, Apulia; Bdg, Beydaglari; BS, Black Sea; Hat, Hatay; Lyc, Lycian;Men, Menderes; Moe, Moesia; Pel, Pelagonian; Rho, Rhodope; Sak, Sakarya; Tau, Taurus; TB, Thrace Basin; Tro, Troodos.

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ing of tectonic mélange from accretionary prisms intoadjacent forearc basins (see Cavazza and Barone (2010),for a review), the long time span (>30 Ma) between theyoungest age of the Çetmi mélange (Turonian) and thebase of the Thrace Basin fill indicates that the accretion-ary prism was inactive when the Thrace Basin wasformed. We propose that the Thrace Basin is instead theresult of one of the following processes: (i) post-orogeniccollapse after the continental collision related to the clo-sure of the Vardar ocean, (ii) upper-plate extensionrelated to slab retreat in front of the Pindos remnantocean (d’Atri et al., 2012), or (iii) a combination of thesetwo processes.

The Vardar-İzmir-Ankara-Erzincan suture representsthe major neo-Tethyan suture between Laurasian andGondwanian terranes in southeastern Europe and AsiaMinor (Figure 1). It can be traced for at least 2,000 kmfrom the Balkans to the Lesser Caucasus, where it con-tinues southeastward as the Sevan-Akera suture (Adamiaet al., 2011; Khain, 1994; Okay & Tüysüz, 1999). TheVardar segment of this important suture system separatesthe Rhodope and Serbo-Macedonian massifs in the northfrom the Hellenides in the south. Subduction and closureof the Vardar Ocean occurred in the Late Cretaceous–Early Paleogene. The nappes were stacked by southwardthrusting in the hanging wall of the north-dipping sub-duction of the Vardar slab, rooted in the present-day Var-dar suture zone (Bonev & Beccaletto, 2007; Ricou,Burg, Godfriaux, & Ivanov, 1998). Despite contrastingmodels for the details of the following evolution of theVardar orogen (e.g. Burg et al., 1996; Krohe &Mposkos, 2002), stratigraphic evidence clearly points towidespread overall extension from the Late Eocene (e.g.Burchfiel, Nakov, & Tzankov, 2003). The İzmir-Ankara-Erzincan suture can be traced south of the Biga penin-sula; in this region, the structural evolution is similar tothe one outlined above for the Vardar segment.

Despite being ideally located for recording the com-plex transition from compression to extension along theVardar-İzmir-Ankara-Erzincan orogenic wedge, theThrace Basin has been largely ignored in most paleotec-tonic-paleogeographic reconstructions. Notable excep-tions are Stampfli and Borel (2004) and Stampfli andHochard (2009), showing the Thrace Basin straddlingthe inactive Vardar-İzmir-Ankara-Erzincan suture andfacing the northward subducting Pindos remnant ocean(Figure 7). This reconstruction implies that upper-plateextension was largely driven by slab rollback along theretreating subduction zone along the northern margin ofthe Pindos Ocean. We argue that the complex physiogra-phy of the Thrace Basin described earlier was inheritedby a previous structural regime, most likely during late-collisional strike-slip tectonism. This is demonstrated bythe fact that the basin-fill succession onlaps passivelysuch physiography (Turgut et al., 1991). Further accom-modation was generated over a wider area during the restof the Eocene and in the Oligocene during extension, inagreement with the timing and areal distribution of

crustal stretching phenomena common during this timespan over the entire northern Aegean region. This latterphase is characterized by a significant penecontempora-neous volcanic detrital input, as shown by this study.The coexistence of pure neovolcanic layers (crystal tuffsand cinerites) and hybrid arenites rich in penecontempo-raneous carbonate grains with sands derived from acontinental basement and ophiolitic suites indicates thepresence of episutural basins where shallow-water car-bonates were deposited on top of the exhuming subduc-tion-accretion prism. These carbonates were mixed withpenecontemporaneous neovolcanic and terrigeneous com-ponents and redeposited in deeper marine environments.

Zattin, Okay, and Cavazza (2005) and Zattin et al.(2010) showed that the Ganos segment of the NorthAnatolian Fault had a late Oligocene precursor with asignificant dip-slip component. Abrupt palaeoenviron-mental variations between time-equivalent stratigraphichorizons across the Ganos Fault (e.g. Doust & Arıkan,1974) support the notion that during the Eocene a struc-tural discontinuity was already active along the trace ofthe present-day Ganos Fault, in line with the documentedexistence of a number of elongated structural highs andlows influencing the sediment dispersal pattern and theareal distribution of palaeoenvironments across theThrace Basin (Perinçek, 1991; Turgut et al., 1991). Inthe Palaeocene, a paleo-Ganos Fault may have beenactive taking up the lateral component of oblique sub-duction, similar to the strike-slip faults north of theSumatra-Java trenches in southeast Asia (e.g. Hamilton,1979), as first proposed by Okay et al. (2010). In the Pli-ocene (Armijo et al., 1999), the present-day North Ana-tolian Fault reactivated this older tectonic structure.

Such strike-slip-dominated tectonic scenario duringthe late- and post-collisional stages related to the closureof the Vardar-İzmir-Ankara Ocean is further corroboratedby the presence of an important strike-slip shear zone ofcrustal relevance in the region just southeast of the Mar-mara Sea (Okay et al., 2008). Such shear zone is at least225 km long, has an horizontal offset of about 100 km,and has a trend similar to the present-day North Anato-lian Fault.

7. Conclusions

The integration of (i) paleoflow measurements, (ii)qualitative sedimentological observations, and (iii)detailed petrographic observations for discriminating pal-aeo- vs. neovolcanic and penecontemporaneous vs. non-coeval terrigenous sands leads to a substantial revisionof the geodynamic interpretation of the Thrace Basin,formerly considered a forearc basin. The Thrace Basinsedimentary fill was mostly derived from (i) theRhodopian basement complex to the west and (ii) theVardar-İzmir-Ankara and Biga (?Intra-Pontide) subduc-tion/accretion complexes to the south and southwest.Significant detrital input from the northern margin startedonly in the Oligocene. Proximal facies along the

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southern basin margin consistently show northwardpalaeocurrents whereas most palaeocurrent indicatorsmeasured downcurrent point to an eastward palaeoflow,likely the result of the deflection of primary gravityflows originated along the southern basin margin. Rho-dopes-derived, Eocene proximal facies in northeasternGreece and southeastern Bulgaria feature a series ofcoarse-grained fan-deltas and alluvial fans progradinggenerically eastward. The latter were the sediment entrypoints likely feeding the basin-plain turbidites of the cen-tral portion of the Thrace Basin, now concealed in thesubsurface.

The Thrace Basin developed during the transitionbetween the collisional tectonic regime following the clo-sure of Vardar-İzmir-Ankara oceanic realm and theextensional regime characterizing the Oligocene-Neogeneevolution of the Aegean and periAegean regions. It waslong interpreted as a forearc basin which developed in acontext of northward subduction but the lack of a coevalmagmatic arc in the Strandja Massif indicates otherwise.This element – along with the correspondence betweensubsidence pulses in the basin and lithospheric stretchingin the metamorphic core complexes of southern Bulgariaand the northern Aegean region – may indicate insteadthat the Thrace Basin was the result of upper-plate exten-sion and/or post-orogenic collapse following the conti-nental collision related to the closure of the VardarOcean. Preliminary data indicate that initial subsidence(Ypresian-Bartonian) was localized in small depocentersdelimited by a system of strike-slip faults, probably dur-ing the late stages of collision. Further, extension-drivensubsidence over a wider area occurred during Priabo-nian-Oligocene, in agreement with the timing and arealdistribution of crustal stretching phenomena evident dur-ing this length of time over the entire northern Aegeanregion. This hypothetical two-stage evolutionary trendmight represent a predictive tool in the tectonostrati-graphic interpretation of other post-orogenic sedimentarybasins.

AcknowledgementsMany thanks to Aral I. Okay, who patiently introduced WilliamCavazza to the geology of Turkey, sharing graciously his exper-tise through the years. Thanks are also due to Rocco Dominici,Niko Kolios, Piero Manetti, Francesco Muto, and the late Fab-rizio Innocenti for their participation during early stages of thisstudy. Thorough reviews by Ercan Özcan and Muzaffer Siyakogreatly improved the manuscript. This research was sponsoredby MIUR (Italian Ministry of Public Education, University andResearch).

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