Introduction

Active convergent margins are recognized as the primary locations at which crust is both created and recycled back to the mantle (e.g. Clift et al. 2005; Fildani et al. 2008; Kochelek et al. 2011). Forearc basins, sited on the oceanward edge of actively deforming convergent margins, may contain an excellent record of the tectonic evolution of these dynamic plate boundaries (e.g. Dickinson and Seely 1979; Ingersoll 1983; Unruh et al. 2007; Trop 2008; Mitchell et al. 2010; Maravelis and Zelilidis 2013). Forearc basins constitute major sites of sediment accumulation located between the subduction complex and the magmatic arc, and they evolve in response to (1) tectonic forcing related to subduction processes, (2) eustatic changes in sea level and their effects on the depositional profile, (3) isostatic subsidence due to forearc sediment loading, which in turn is related to tectonic, climatic, and eustatic controls, and (4) flat-slab subduction of spreading ridges and thick oceanic crust (DeGraaff-Surpless et al. 2002; Clift et al. 2005; Fildani et al. 2008; Finzel et al. 2011; Paquet et al. 2011; Ridgway et al. 2012; Kortyna et al. 2013; Maravelis and Zelilidis 2013). Ancient forearc basins can be examined in outcrop in some instances (e.g. Clift et al. 2005; Maravelis et al. 2007; Fildani et al. 2008; Trop 2008; Wang et al. 2012), but typically are significantly altered by collisional tectonics and may be lost altogether because of underthrusting or uplift and erosion. Seismic data can add useful constrains into our understanding of the stratigraphic architecture of forearc basin fills (Laursen and Normark 2003; Berglar et al. 2008; Williams and Graham 2013) and can also provide information for basin-scale correlations, but their use in establishment of depositional environments is limited (Marcaillou and Collot 2008). Thus, the drivers of sediment accumulation and stratigraphic packaging in forearc basins are less well known than for basins in extensional or foreland settings (Mitchell et al. 2010). Sedimentary records of ancient forearc basins can provide a more complete history of arc magmatism and topography. Provenance analysis of forearc strata is also important because it can elucidate many arc–forearc region characteristics such as palaeotopography of the arc, regional drainage patterns between the arc and forearc basin, and the magmatic history of the arc (DeGraaff-Surpless et al. 2002).

Cenozoic rocks in the eastern Balkan Peninsula form one of the best-exposed active continental margin basin complexes in the world (Turgut et al. 1991; Turgut and Eseller 2000). The Thrace Basin extends across the Hellenic, Bulgarian, and Turkish domains (D’Atri et al. 2012; Maravelis and Zelilidis 2013) and contains a record of forearc–arc evolution fundamental to understanding the Paleogene convergent margin of African–Eurasian plate boundary (Fig. 1). The basin has been considered as a forearc basin of the “contracted” type (Görür and Okay 1996; Maravelis and Zelilidis 2010) and is the largest and thickest Tertiary sedimentary basin of the eastern Balkan region (Turgut et al. 1991; Turgut and Eseller 2000). A large accretionary prism in the central Aegean region (outcropping at Biga Peninsula of NW Turkey) bounds the basin on the seaward side, and the landward flank was bound by an active volcanic arc, the Rhodopian arc. Most of the basin strata are Lower Eocene to Upper Oligocene and form thick sedimentary successions (up to 9,000 m) consisting mostly of sand-rich deep-sea fan deposits (Turgut et al. 1991; Turgut and Eseller 2000). Previous studies have documented that the Hellenic part of the Thrace Basin was mainly influenced by two major sediment inputs (Maravelis and Zelilidis 2010, 2013). The southern (accretionary) part of the basin (Lemnos Island) was affected by the accretionary prism and associated ophiolitic units (Maravelis and Zelilidis 2010, 2013), while the northern (arcwards) part (Rhodope area) reflects a Rhodopian magmatic arc influence (Maravelis and Zelilidis 2010; Caracciolo et al. 2011). The present study provides additional insights into the provenance, palaeodispersal pattern, and sediment source nature of this complex two-sided forearc basin using new fieldwork and geochemical data.

Fig. 1
figure 1

Map depicting the plate tectonic configuration of the eastern Balkan Peninsula and the position of the study area in relation to the main Alpine orogenic elements of the region (from Dimitriadis et al. 1998; Maravelis and Zelilidis 2013; Tranos and Lacombe 2014)

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More particularly, this research employs: (1) sedimentological data to provide a comprehensive overview of the sedimentary facies, allowing the depositional processes and environments to be characterized, (2) palaeocurrent data to reveal the palaeoflow direction, (3) conglomerate clast composition data, and, (4) major- and trace-element provenance data to identify the source rock compositions. Based on the obtained results, it is suggested that Paleogene sediments in the Hellenic southern part of the Thrace Basin are affected by the active influence of the accretionary prism rather than the erosion of the Rhodopian arc. Consequently, the present study is also useful in documenting a basin example which can serve as a reference for future analysis of similar two-sided forearc basin successions characterized by significant sediment input from a trenchward prism.

Geologic setting

The Thrace Basin was formed as an elongate trough between the Rhodopian magmatic arc to the north and the accretionary prism (Pindic Cordillera in Greece and Biga Peninsula in Turkey) to the south (Görür and Okay 1996; Tranos 2009; Maravelis and Zelilidis 2010; D’Atri et al. 2012; Maravelis and Zelilidis 2013). This prism is considered to represent the westward continuation of the Intra-Pontide or the more southern İzmir-Ankara oceans which are related to the closure of the Neo-Tethyan oceanic basin (Okay et al. 2008). The sedimentary rocks of the Thrace forearc basin overlie the rocks of the Rhodope magmatic arc to the north and the southern basin fill is underlain by ophiolitic and metamorphic rocks, which form part of the accretionary prism (Elmas 2012). This sedimentary fill, and particularly the sediments located at the southern Hellenic part of the basin, can provide insights for the regional geodynamic evolution.

The southern flank of the basin is deformed by the North Anatolian Fault (NAF). Estimated ages for the initiation of motion along the NAF range from the late Miocene to late Pliocene, and thus, it was not active during deposition (Dewey and Sengör 1979; Topuz et al. 2008). The NAF total displacement has been estimated using large river valleys and structural markers that indicate a Neogene slip rate of 6.5 mm/yr over 13 Myr (Hubert-Ferrari et al. 2002). A large part of the Thrace Basin is located in the North Aegean Trough (NAT) that constitutes the modern basin established over the Tertiary Thrace Basin (Tranos 2009). The NAT is a ~300-km-long narrow trough in the North Aegean Sea and is located along the Tethyan Ocean (Axios zone in Greece and Intra-Pontide suture zone in Turkey) (Mountrakis 2006) (Fig. 2). Structural analysis suggests that the formation and widening of the NAT were not related to the propagation of the NAF into the North Aegean Sea, but to the late collisional processes between Apulia and Eurasia plates during the late Oligocene–middle Miocene time (Tranos 2009).

Fig. 2
figure 2

Geologic sketch map of the Thrace Basin showing the pre-Eocene basement, the Eocene to Oligocene sediment, the post-Eocene deposits, and the volcanic domains (from Bonev and Beccaletto 2007)

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The Hellenic continental margin, which results from the oblique subduction of the oceanic African Plate beneath Eurasia, has played a key role in controlling magmatism in the Balkan Peninsula since late Cretaceous (Fytikas et al. 1984; Marchev et al. 2004; Bonev and Beccaletto 2007). A subduction mechanism has been proposed to explain late Cretaceous magmatism in southern Bulgaria, with the Rhodope being considered as the frontal part of Srednogorie arc (Marchev et al. 2004). A southward magmatic migration from 92 to 78 Ma is evidenced by U–Pb zircon dating (Peytcheva et al. 1998). The occurrence of Srednogorie zone into the southern Rhodope region is documented by the 70–42 Ma granitoid intrusions. Lower Miocene volcanic rocks are common in the North Aegean Sea, indicating further southward migration (Innocenti et al. 1994; Yilmaz et al. 2001; Maravelis et al. 2007). An extended subduction event has been invoked to account for the continuous regional magmatism (Marchev et al. 2004). The magmatic activity commenced in Rhodope in late Eocene and its migration suggests that the present day northward subduction in the Aegean region initiated during Eocene time (Fytikas et al. 1984; Spakman et al. 1988).

At the subduction zone, the accretion of oceanic crust and sediment to the upper plate caused the growth of an accretionary prism (Maravelis and Zelilidis 2010, 2013; D’Atri et al. 2012; Görür and Elbek 2013). The occurrence of eclogites and blueschists in the accretionary prism area (Biga Peninsula) suggests eclogite/blueschist-grade metamorphism (Topuz et al. 2008; Şengün et al. 2011). This metamorphic event along with the presence of an ophiolitic mélange has been ascribed to Tertiary subduction processes that led to the formation of the Hellenic orogen (Beccaletto et al. 2005; Okay et al. 2010; Maravelis and Zelilidis 2010, 2013). Between the Rhodopian magmatic arc and the accretionary prism, sediment filling the Thrace forearc basin comprises the Eocene to Oligocene clastic strata (Turgut et al. 1991; Turgut and Eseller 2000; Maravelis et al. 2007; Maravelis and Zelilidis 2011). Provenance analyses utilizing sandstone petrography, conglomerate clast composition and bulk-rock geochemistry indicate that the basin is two-sided (Maravelis et al. 2007; Maravelis and Zelilidis 2010; D’Atri et al. 2012).

Sedimentation along the arcward basin margin was dominated by deposition of carbonates during the Eocene and by deltaic bodies, prograding towards the basin centre, in the Oligocene (Turgut et al. 1991). The seaward part of the Thrace Basin is dominated by a sand-rich submarine-fan system that underlies slope and shallow-marine deposits; palaeoflow data indicate northeastward sediment transport (Perinçek 1991; Turgut et al. 1991; Maravelis et al. 2007; Maravelis 2009; Maravelis and Zelilidis 2011). The source area contributed a significant volume of ultramafic, gabbro, basalt, chert, and possibly some volcaniclastic detritus of variable grain size into the forearc basin (Görür and Okay 1996; Maravelis and Zelilidis 2010; D’Atri et al. 2012). The source area was probably rapidly eroded, causing the ophiolitic bedrock to be deeply incised and enabling a significant amount of coarse-grained material to be transported to the forearc region (Görür and Okay 1996; Maravelis and Zelilidis 2010; D’Atri et al. 2012). In the landward parts of the basin (Rhodope region), shelves fringed the Rhodopian arc and fed abundant coarse-grained sediment down the slope to the deep portions of the forearc basin. This sediment formed extensive submarine-fan deposits that crop out along the northern flank of the forearc basin (Maravelis and Zelilidis 2010, 2013).

During the Miocene, Thrace Basin was the site of volcanic activity resulting in accumulation of magmatic rocks, principally trachyandesites and dacites (Pe-Piper and Piper 2001). These rocks have been interpreted as belonging to the northern, “shoshonitic province” along the Aegean–Anatolian boundary that includes the islands of Samothrace, Lemnos, and Lesvos (Pe-Piper et al. 2009). These high-K rocks, mostly of intermediate composition, indicate ensuing calc-alkaline orogenic volcanism, emitted from large volcanic centres. Upwelling of asthenospheric mantle has been invoked to account for their genesis (Pe-Piper et al. 2009).

Basin stratigraphy, architecture, and evolution

The late Eocene to early Oligocene sand-rich submarine-fan systems in southern Thrace Basin (Lemnos Island and Biga Peninsula) have been extensively described and indicate an active tectonic setting (Perinçek 1991; Roussos 1994; Maravelis 2009; Maravelis and Zelilidis 2011). In Lemnos Island, this system has been subdivided into a basal outer and an overlying inner fan shape (Figs. 3, 4). The outer fan is equivalent to the low-stand fan and is composed of lobe, lobe-fringe, and fan-fringe deposits. The inner fan is equivalent to the low-stand wedge and is further subdivided into a channel-fill and levee system that evolves upwards into a conglomeratic channel-fill system. This submarine-fan system occurs as a laterally isolated body, with a single sediment source consisting of a proximal distributary channel. This source occupies a position approximately in the centre of the fan and contains the coarsest sediment in the study area. This stratigraphic architecture, in conjunction with the north-northeastward palaeoflow direction, indicates a curved fan shape.

Fig. 3
figure 3

Detailed geologic map of the southern margin of the Hellenic Thrace Basin (Lemnos Island) where upper Eocene to lower Oligocene submarine fans evolve up-sequence into lower Oligocene slope and swallow-marine facies (Roussos 1994; Maravelis and Zelilidis 2012). Note the seven different recognized sub-environments. The black arrow marks the main palaeoflow direction and the black dots correspond to the sample locations (from Maravelis et al. 2007; Maravelis and Zelilidis 2011)

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Fig. 4
figure 4

Calcareous nannofossil biostratigraphic zonation and sequence stratigraphic framework for the Upper Eocene to Lower Oligocene depositional sequence of Lemnos Island. Note the NE palaeoflow direction of the underlying submarine fans (from Roussos 1994; Maravelis et al. 2007). Note the long-term (second order) progradational trend from the basin-floor fan to the shallow-marine deposits and the interpretation of the depositional facies in terms of third-order systems tracts (from Maravelis and Zelilidis 2012). FSST: falling-stage system tract; HST: highstand system tract; LST: lowstand system tract; BSFR: basal surface of forced regression; CC: correlative conformity; MRS: maximum regressive surface; MFS: maximum flooding surface; FAD: first appearance datum; FO: first occurrence; LAD: last appearance datum

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The sequence stratigraphic framework was constructed by observing changes in depositional setting, facies, and types of gravity-flow deposits with time (Catuneanu et al. 2011; De Gasperi and Catuneanu 2014). The base of the submarine-fan system in the southern part of the Thrace Basin is not exposed, and therefore, the basal surface of forced regression (BSFR) is not observed in outcrop in this case study. The correlative conformity (CC) that forms at the end of forced regression is one of the most prominent sequence stratigraphic surfaces in the deep-water setting, as it marks the end of high sediment supply to the deep-water environment, and consequently an abrupt shift to finer grained sediment. This surface is exposed within our study area, at the top of the conglomeratic fan system (Fig. 4). The maximum regressive surface (MRS), which forms during relative sea-level rise, is typically cryptic in the deep-water setting as there is no marked change in sediment supply from low-stand normal regression to transgression (Catuneanu et al. 2009). The MRS may be placed anywhere within the mudstone-dominated slope facies that overlie the conglomeratic fan system (Fig. 4). The maximum flooding surface (MFS), which also forms during relative sea-level rise at the time of maximum shoreline transgression on the shallow marine when sediment supply to the deep-water environment decreases to a minimum, may also be cryptic and placed within a condensed section. In our case study, the MFS is taken at the top of the mudstone-dominated slope facies (facies 4 in Fig. 4), which marks the base of the overlying coarsening-upward high-stand shallow-marine deposits (facies 5 in Fig. 4).

In summary, a sand-rich submarine-fan system along with overlying slope and shallow-marine deposits spans the Eocene/Oligocene boundary on Lemnos Island. During late Eocene (NP19/NP20), the outer fan system, the inner fan and the lower part of the conglomeratic fan accumulated. Subsequently, the remainder of the inner fan system was deposited during the early Oligocene (NP21). Eventually, during the late parts of the early Oligocene (NP21), the up-sequence slope and shallow-marine deposits accumulated (Maravelis and Zelilidis 2012).

Materials and methods

This investigation uses data derived from outcrop-based sedimentary facies analyses to reconstruct a palaeodepositional history of the basin; palaeodispersal data to reveal the palaeocurrent direction; conglomerate composition data and inorganic geochemical analyses of mudstone in order to unravel the provenance and tectonic setting. Depositional processes, environments, and sub-environments were interpreted by using primary sedimentological features. Calcareous nannofossils were utilized to constrain palaeobathymetry, palaeoenvironment, and timing of key stratigraphic levels in the basin (Maravelis and Zelilidis 2012). Palaeodirection analysis was performed for the first time on the lower Oligocene shallow-marine facies, expanding on previously published work conducted on the underlying submarine fans (Maravelis et al. 2007). Palaeoflow data were obtained by measuring sole marks and foreset laminae. For conglomerate compositional analysis, a one-square-metre frame was used and compositions noted at 100-mm grid intersections; simple average values are reported. X-ray fluorescence (XRF) spectroscopy was used to quantify the major and trace elements of the twenty (20) selected mudstone samples. Samples collected from natural outcrops in order to be representative of the regional basin history (Figs. 3, 5). A cut of each sample is thoroughly washed in cold water. Any large quantities of contaminants, such as lost circulation material, are removed. The washed contaminants or obvious cavings are removed. The sample is then air-dried at room temperature and subsequently crushed to a homogeneous powder and bagged in readiness for analysis. XRF analysis was performed using Niton XL3t Goldd + analyser. Thermo Scientific Niton XL3t analysers are equipped with excitation filters that optimize the analysers’ sensitivity for various elements. The “high-range” filter is used to optimize barium (Ba) through silver (Ag). The “main-range” filter provides optimum sensitivity for the elements manganese (Mn) through bismuth (Bi). The “low-range” filter is used to optimize the sensitivity for the elements from titanium (Ti) through chromium (Cr). The “light-range” filter is available only with He-purged and Goldd technology analysers and is used in light element analysis such as (Mg). Optimal results were obtained from powder samples with the use of a He purge system. Counting times were 30 s each on the low- and high-energy filters (for analysing elements K to U), and 60 s on the main and light filter setting (Mg to S) for a total analysis time of 180 s. All the analytical process was performed in the StratoChem Laboratories, Cairo, Egypt, with the use of a Thermo Scientific Niton XRF analyser.

Fig. 5
figure 5

Lithological (metre scale) section of the studied shallow-marine sediments showing the stratigraphic position of the examined samples

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Facies associations

Each of the three sedimentary facies associations (FA1, FA2, and FA3; Fig. 5) described in this section is made up of a definitive set of lithofacies, which may consist of several smaller beds and bed sets with various sedimentary structures that are interpreted to have genetically related formative processes. These lithofacies sets were then grouped into a series of facies associations on the premise that several closely spaced lithofacies with similar sedimentary features and depositional processes are genetically related and have palaeoenvironmental significance (Dixon et al. 2012).

Facies association FA1: sandstone with minor mudstone

Description

This facies association consists of stacked and amalgamated sandstone (Fig. 6a). FA1 is composed of clean, normally graded or non-graded, well-sorted sandstone. Sandstone beds are characteristically sharp based and occur in thicknesses of up to 1.5–2 m forming sedimentary packages 7–10 m. They thin upward where they are interbedded with mudstone (Fig. 6b). Sandstone is light brown in colour, fine to medium grained, and thin to very thick bedded. Most of the sandstone is massive and structureless, although beds with parallel laminations and cross-lamination rarely occur (Fig. 6c). The base of the sandstone is commonly erosive into underlying mudstone (Fig. 6d). Locally derived mudstone clasts frequently line the base of the sandstone and internal erosion surfaces. Often, medium- to thick-bedded (0.5–1 m) conglomeratic units are present at the base of the amalgamated sandstone (Fig. 6e). These units are composed of moderately to well-sorted, polymictic, matrix-supported conglomerates. Most conglomerates are ungraded, low to medium sphericity, and moderately to well rounded. Some of the conglomerate beds show imbrication of clasts.

Fig. 6
figure 6

Outcrop photographs of FA1. a Thick-bedded, amalgamated sandstone. b Medium- to thin-bedded sandstone interbedded with mudstone. c Normally graded, medium-bedded sandstone bed with parallel lamination. d Medium-bedded sandstone bed. Note the sharp contact between the sandy fill and the underlying mudstone. e Well-sorted to moderately sorted, polymictic, matrix-supported conglomerate (Poliochni Bay in Fig. 3, pen is ~15 cm long)

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Interpretation

The erosional relations with the underlying muds support erosion by unidirectional currents, incision, and sand infill by high-density flows within a channelized environment (Lowe 1982; Mulder and Alexander 2001). The mudstone rip-up clasts record erosion and sediment bypass (Bowman and Johnson 2013). The absence of bioturbation is consistent with very rapid deposition (Pemberton et al. 2002). The weakly developed cross-lamination and parallel lamination suggest that the traction currents were transitional flows that were becoming stratified and beginning to entrain material into their upper layers (Bowman and Johnson 2013). Conglomerate was deposited from traction carpets beneath high-concentration currents. Its deposition implies a high-concentration base to the flow that would have inhibited entrainment of bed material (Piper and Normark 2009). We suggest that such straight hyperpycnal channels are characteristic of the largest high-concentration flows, which may have armoured the bed and minimized later erosion by smaller fully turbulent flows.

Facies association FA2: sandstone with interbedded mudstone

Description

This facies association consists of thin- to medium-bedded, medium- to fine-grained sandstone with interbedded mudstone (Fig. 7a). Sandstone beds are characteristically flat-based, normally graded or non-graded, and well sorted (Fig. 7b). In the sandstone, the main primary sedimentary structures are parallel and current ripple cross-lamination (Fig. 7c). The mudstone comprises both homogenous, 5–50-cm-thick structureless mudstone beds and thin- to medium-bedded laminated mudstone, which contains abundant coal debris in places. FA2 forms sedimentary cycles with fining upward trends and contains trace fossils. Bioturbation is rare and where it occurs, forms low abundance, but slightly diverse assemblage, with ophiomorpha, Thalassinoides, and Scolicia (Fig. 7d). Soft-sediment deformation occurs at a variety of scales: (1) convolute bedding (tens of centimetres), (2) dish structures (1–10 cm), and (3) load structures (millimetre to centimetre scale) (Fig. 7e).

Fig. 7
figure 7

Outcrop photographs of FA2. a Thin- to medium-bedded sandstone interbedded with mudstone. b Medium- to thin-bedded sandstone. Note the flat contact between the sandy fill and the underlying mudstone. c Medium-bedded sandstone with parallel and cross-lamination. d Branched trace fossils developed in the studied shallow-marine deposits (Thalassinoides paradoxicus). e Medium-bedded sandstone with convolute lamination (Poliochni Bay in Fig. 3, pen is ~15 cm long)

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Interpretation

Homogenous, structureless mudstone beds record “frozen” cohesive muddy debris flows (Mulder and Alexander 2001). Many sandstone beds record a spectrum of depletive, waning flow processes, which resemble low-density turbidities (Kneller 1995). The various styles of sediment deformation are indicative of deposition on a relatively steep and unstable slope (Porebski and Steel 2003). Sediment load structures and convolute bedding suggest sediment liquefaction, potentially triggered by pore-pressure changes associated with seismicity, growth fault activity, mud diapirism, and storm-related wave action (Lonergan et al. 2000). Dish structures are formed by the lateral and upward passage of water through sediment (Porebski and Steel 2003).

Facies association FA3: laminated mudstone

Description

Facies association FA3 is dominated by dark blue/grey coloured mudstone with rare millimetre to centimetre-thick interbeds of laterally extensive, sharp-based sandstone. This facies association consists of thick stratigraphic units, which contain parallel laminated mudstone (Fig. 8a). Laminations are 0.1 to <2 mm thick (Fig. 8b). Individual very fine sandstone beds are non-graded and occasionally contain asymmetrical current ripple cross-lamination.

Fig. 8
figure 8

Outcrop photographs of FA3. a Laminated mudstone with rare thin-bedded sandstone. b Mudstone with fine lamination (Poliochni Bay in Fig. 3)

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Interpretation

The mudstone is deposited from suspension under low-energy conditions. The lamination is possibly due to cyclic breaksup of flocs and turbulence damping at bed, or via bedload transport of flocs (Talling et al. 2012).

Provenance

Palaeodispersal patterns

To estimate the palaeoflow direction, palaeocurrent data from the shallow-marine sediments were measured from flute and groove marks and current ripples. Data were collected in both FA1 and FA2, although in the amalgamated sandstone strata (FA1), the only available palaeocurrent indicators were cross-lamination foresets. Several measurements were taken from each facies association (25–30), and these data are plotted in rose diagrams that document a northeastern palaeocurrent direction (Fig. 9). Palaeocurrent data are not adjusted for post-depositional rotation, but the regional 25° clockwise rotation since 17 Ma (Kondopoulou 2000) is elaborated in the discussion.

Fig. 9
figure 9

Rose diagrams from the studied section showing an NE trend of the palaeocurrents’ drift and conglomerate clast count results from conglomeratic units of southern Thrace Basin (Lemnos Island)

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Sandstone petrography and conglomerate clast composition

Previous petrographic investigations of the submarine-fan sandstone and the overlying shallow-marine sandstone in both Hellenic and Turkish domains of the southern Thrace Basin suggest diversity of mineralogical compositions (Maravelis and Zelilidis 2010; Caracciolo et al. 2011; D’Atri et al. 2012). These studies have reached similar conclusions, namely derivation from diverse mineralogical provinces, producing immature sandstone that is rich in lithic rock fragments. The sandstone from the southern Thrace is litharenite and wacke with abundant low-grade metamorphic rock fragments (Maravelis and Zelilidis 2010; D’Atri et al. 2012). The framework grains are mainly lithic clasts, feldspar (both K-feldspar and plagioclase), and less quartz. This sandstone is typified by the prevalence of polycrystalline quartz grains at the expense of monocrystalline quartz (Maravelis and Zelilidis 2010; D’Atri et al. 2012). Metamorphic, sedimentary, and igneous lithic fragments are abundant (Fig. 10). Metamorphic clasts consist of schist, while sedimentary lithic fragments include those of limestone, microcrystalline chert, and sandstone. The igneous lithic fragments mainly consist of mafic basalt. Other common framework components are gabbro and peridotite rock fragments as well as glaucophane and picotite grains.

Fig. 10
figure 10

Photomicrographs of lithic fragments identified within the selected sandstone samples of the southern Thrace Basin (Lemnos Island). (A) Basalt rock fragment (BAS). (B) Basalt (BAS) and Chert (CH)

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Conglomerate clasts can provide detailed provenance information, including rock type, age, bulk geochemistry, and isotopic constraints (Trop and Ridgway 2007; Fildani et al. 2008; Trop 2008; Bradshaw et al. 2012). In the study region, the conglomerate units are rare and consist of mature, moderately to well-rounded pebbles of highly resistant rock types. For compositional analysis, a one-square-metre frame was used and compositions noted at 100-mm grid intersections; simple average values are reported here (Fig. 9). Three principal rock types were distinguished within the conglomerates: (1) mafic and ultramafic igneous rocks consisting of basalt, gabbro, and peridotite, (2) low-grade metamorphic rock fragments consisting of schist, and (3) sedimentary rocks comprising sandstone, limestone, and chert. The most abundant clasts are chert (47 %) and basalt (26 %). The remaining 25 % were mainly sandstone (8 %) and limestone (9 %) as well as peridotite, gabbro, and schist (10 %). Felsic volcanic clasts were not observed.

Geochemistry

Several studies have employed major, trace, and rare earth element (REE) abundances in order to define sedimentary provenance and basin tectonic setting (e.g. Garcia et al. 1991; Nesbitt and Wilson 1992; Roser and Korsch 1986; Van de Kamp and Leake 1995; Zhang 2004; Fildani et al. 2008; Konstantopoulos and Zelilidis 2013; Tao et al. 2014). REE abundance, especially the study of the Eu anomaly in clastic sediments, is a good fingerprint for source rock characterization (Cullers 2000). Sedimentary rocks of felsic provenance exhibit negative Eu anomalies, whereas sediments with mafic influence exhibit slightly positive or no Eu anomalies (Gao and Wedepohl 1995; Cullers 2000). The study region has been influenced by a mafic source during the late Eocene to early Oligocene, as demonstrated by the geochemical and petrographic signatures of the submarine fans that underlie the lower Oligocene shallow-water sediments (Maravelis and Zelilidis 2010). Their chondrite-normalized REE patterns exhibit minor positive Eu anomalies, suggesting influx of juvenile mafic material (Maravelis and Zelilidis 2010). Here, we present the geochemistry of the overlying Oligocene shallow-marine deposits (Table 1) in order to unravel their provenance and document the presence or absence of continued mafic influence on the Oligocene section.

Table 1 Whole-rock major and trace element compositions of mudstone from the southern Thrace Basin (Lemnos Island). Major elements in %, trace elements in ppm
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The contents of Cr and Ni abundance, the Cr/Ni ratio in mudstone, and the plot of Cr/V versus Y/Ni ratios are sensitive indicators of mafic–ultramafic provenance and can provide insights into determining the timing and extent of ultramafic-derived material (Hiscott 1984; McLennan et al. 1993; Garver et al. 1996; Hessler and Lowe 2006; Amorosi et al. 2007; Pantopoulos and Zelilidis 2012). The studied samples are rich in Cr and Ni, with low Y/Ni, increased Cr/V ratios, and a moderate Cr/Ni ratio (Fig. 11; Table 1). They also exhibit low Al2O3/TiO2 and SiO2/Al2O3 ratios as well as high Ti/Al, 100*Fe2O3/Al2O3, and 100*TiO2/Fe2O3 ratios (Fig. 12; Table 1). The examined samples exhibit high Ti/Zr ratios and low Zr/Ni and Zr/Cr ratios (Fig. 13; Table 1). The multi-element diagrams, of Fig. 13c, display the element concentrations of the examined mudstone, normalized to the upper continental crust (UCC) composition (Taylor and McLennan 1985), on an element-by-element basis. In addition, the normalized element concentrations of sources of different compositions have been plotted for comparison. The diagram exhibits elevated abundances of V, Cr, Ni and Ti, (>1), K normalized values below unity, a marked positive CaO anomaly and also a slight positive Sr anomaly (Fig. 13c).

Fig. 11
figure 11

Histograms, ratios, and chemical classification diagrams from the shallow-marine facies of the southern Thrace Basin (Lemnos Island). a Cr content. b Ni content. c Cr/Ni ratio (a, b, and c are from Garver et al. 1996). d Cr versus Ni (after Taylor and McLennan 1985). e Cr/V versus Y/Ni (after Hiscott 1984). The Cr/V versus Y/Ni diagram illustrates the importance of ophiolitic provenance. Ultramafic sources have very low Y/Ni and high Cr/V ratios. Mafic sources are with low Y/Ni ratio and high to moderate Cr/V ratios. All plots indicate mostly a mafic influence

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Fig. 12
figure 12

Provenance discrimination diagrams from the studied sediments (Lemnos Island). a TiO2 versus Al2O3 (after McLennan et al. 1980). The “granite” and “3 granite + 1 basalt” lines from Schieber (1992). b TiO2 versus Zr (after Hayashi et al. 1997). c Fe2O3/Al2O3 versus TiO2/Al2O3. d TiO2/Fe2O3 versus TiO2/Al2O3 (c and d are from Nesbitt and Wilson 1992). e Cr versus MgO. f Ni versus MgO (e and f are from Van de Kamp and Leake 1995 and Zhang 2004)

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Fig. 13
figure 13

Chemical classification diagram, ternary plot, and spider diagram from the southern Thrace Basin (Lemnos Island). a Zr/Cr versus Zr/Ni (from Hiscott 1984). b 15*Al2O3–300*TiO2–Zr (from Garcia et al. 1991). c UCC-normalized spider diagram. The normalized element concentrations of granite, felsic volcanics, andesite, and basalt are also plotted for comparison (Condie 1993). UCC average is from Taylor and McLennan (1985). Elements are arranged from left to right in order of increasing normalized abundance in Mesozoic–Cenozoic (Condie 1993)

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The geochemical results indicate mafic (and intermediate) sources, with a lack of any felsic component. This conclusion is in agreement with results from previous provenance studies of Eocene–Oligocene deep-marine sediments in the area (Maravelis and Zelilidis 2010, 2013) and is evidenced by: (a) elevated values of Cr and Ni in the studied sediments and mudstone Cr/Ni ratios of 1.3–1.5 (Garver et al. 1996; Cingolani et al. 2003), (b) the plot of Cr/V versus Y/Ni ratios, (c) relatively low Al2O3/TiO2 ratios ranging below 21 (Schieber 1992; Hayashi et al. 1997), (d) high Ti/Zr ratios, much higher than those commonly recorded in felsic igneous rocks (<20) and close to the threshold of mafic igneous rocks (c. 50) according to Taylor and McLennan (1985), (e) clustering of Fe, Al, and Ti oxide discrimination diagrams around the average compositions of andesites and basalts, (f) Cr/Zr ratios well above the values seen in felsic igneous rocks (>0.5) and within the field of the mafic igneous rocks (>1), (g) SiO2/Al2O3 ratios ranging close to c. 3 (Hayashi et al. 1997), (h) low Zr/Ni and Zr/Cr ratios Al, Ti, and Zr combined abundances (Garcia et al. 1991), (i) the presence of marked positive CaO and Sr anomalies in the multi-element (“spider”) diagrams and similarity of UCC-normalized profiles with geochemical patterns of basaltic composition.

Discussion

Sedimentology, lithostratigraphy, sequence stratigraphy, palaeocurrent analysis, petrography, and geochemistry results suggest a forearc origin for the Thrace Basin between the Rhodopian magmatic arc in the north and the accretionary prism in the south (Pindic Cordillera and/or Biga Peninsula). The basin was influenced by the uplift of the accretionary prism, which served as major contributor of sediment into the southern basin margin. Consistent with previous interpretations (e.g. Görür and Okay 1996; Elmas 2003), this forearc interpretation is also supported by numerous recent studies conducted on different portions of the basin, within both Hellenic and Turkish domains (e.g. Zattin et al. 2005; Tranos 2009; Sen and Yillar 2009; Islamoglu et al. 2010; Maravelis and Zelilidis 2010; Black et al. 2013; Görür and Elbek 2013; Maravelis and Zelilidis 2013; Tranos and Lacombe 2014).

An alternative interpretation suggests that the sedimentary assemblages on the southern Thrace (Lemnos Island) were derived from terranes of the north (Caracciolo et al. 2011), but this model provides no field evidence to support the Rhodopian influence (Maravelis and Zelilidis 2013). Although most forearc basin fills are derived chiefly from an attendant volcanic arc, in Thrace Basin, such influence is of minor importance in the southern basin margin. Instead, an accretionary prism source is indicated by: (1) the arcward palaeoflow pattern, (2) landward migration of the basin depocentre towards the magmatic arc, and (3) the abundance of mafic source rocks. The Thrace Basin is therefore one of the best examples of a mature, two-sided forearc basin succession with significant sediment input from a trenchward prism.

Provenance

The Cenozoic sedimentary successions in the Thrace Basin lie unconformably over different basement types, which are separated by the NAF (Siyako and Huvaz 2007). In the north, the Eocene–Oligocene clastic deposits of the Thrace Basin overlie the metamorphic rocks of the Rhodope, whereas in the south they are underlain by ophiolitic and metamorphic rocks (Elmas 2011). The geochemical signature of the mudstone samples from the southern Thrace Basin reflects a mafic source terrane, typical of active tectonic settings. Mudstone-normalized REE values exhibit a pronounced LREE depletion indicating a fore-arc origin, and the minor positive Eu anomalies suggest accumulation in an active setting typified by the influx of juvenile mafic material (Maravelis and Zelilidis 2010). The sandstone lithic fragments suggest that mafic/ultramafic rocks and associated metamorphic rocks and sedimentary carapace served as the main sediment source for the studied deposits. Glaucophane suggests a provenance consisted of blueschist facies metamorphic source rock (an exhumed accretionary prism). Rare presence of neovolcanic lithic grains (Caracciolo et al. 2011) may represent a minor arc-derived component from the northern sources (Maravelis and Zelilidis 2010).

The southern Thrace Basin, in Hellenic and Turkish domains, contains granule to boulder conglomerates that form beds up to 5 m thick (Maravelis et al. 2007; Okay et al. 2010). Conglomerate types, seen in the deep-sea fans, range from unstratified poorly sorted pebble to cobble conglomerate with dispersed larger boulders, through well-organized clast-supported pebble to cobble conglomerate with sandstone, to beds of sandstone with granules and rare pebbles (Maravelis et al. 2007). Most conglomerates are ungraded, grading is seen near the base or near the top in some instances whereas, reverse grading occur in a number of beds (Maravelis 2009). The main rock types observed within the conglomerates are: (1) igneous rocks clasts of mafic and ultramafic composition, (2) low-grade metamorphic rock fragments, and (3) sedimentary rocks (Maravelis et al. 2007; Maravelis and Zelilidis 2010). In particular, these deposits contain chert, peridotite, schist, and igneous fragments, such as basalts and gabbros in addition to sandstone and limestone clasts (Maravelis et al. 2007; Okay et al. 2010). The results presented in this investigation suggest that the conglomerates in the overlying shallow-marine facies are composed of clasts of similar composition. The dominance of mafic and ultramafic igneous rocks, cherts and metamorphic rock fragments in association with the absence of felsic volcanics, makes the Rhodopian volcanic arc an unlikely source of the sediments. Potential source-area candidates are the Axios or sub-Pelagonian zones and the chert rock fragments in the conglomerates suggest that the latter is the most likely candidate.

All data suggest that metamorphic, sedimentary, and igneous rocks in a recycled orogenic environment were the most important source rocks. This is related to an accretionary prism provenance and indicates that the Thrace Basin is an example of a forearc basin that developed under the active influence of a nearby accretionary prism (Pindic Cordillera and/or Biga Peninsula).

Basin tectonics

The northward subduction of the African oceanic lithosphere beneath the Eurasian continental plate was the driving mechanism for the development of the active continental margin on the southeastern Balkan Peninsula (Fytikas et al. 1984; Le Pichon et al. 1995; Marchev et al. 2004; Maravelis and Zelilidis 2010; D’Atri et al. 2012). The subduction during the closure of a branch of the Neo-Tethyan Ocean formed the Rhodopian volcanic arc, the Thrace forearc basin, and the accretionary prism, which is exposed in northeast Greece (Pindic Cordillera) and northwest Turkey (Biga Peninsula) (Tranos 2009; Maravelis and Zelilidis 2010; D’Atri et al. 2012; Görür and Elbek 2013). Data from blueschist facies suggest active subduction and accretionary crustal growth associated with trenchward decrease in the degree of metamorphism from ultrahigh-pressure conditions in the Rhodope to blueschist–eclogite facies on Biga Peninsula (Topuz et al. 2008). During late Eocene time, this blueschist facies basement was partly exhumed and was enveloped by upper Eocene radiolarian chert, while its post-Miocene exhumation took place along a transpressional segment of the NAF (Topuz et al. 2008).

Late Cenozoic plutonic rocks in the North Aegean Sea result from subduction migration along the Hellenic Arc (Fytikas et al. 1984; Yilmaz et al. 2001). Their late Eocene–early Oligocene ages correspond to the late stages of subduction, whereas their late Oligocene–late Miocene ages indicate regional extension and exhumation (Black et al. 2013; Görür and Elbek 2013; Maravelis and Zelilidis 2013). Zircon ages indicate that the plutonic rocks on Biga Peninsula derived from different magma injections, with their late Eocene–early Oligocene ages recording the initial crystallization of magmas generated above the subducting slab (Black et al. 2013). Late Oligocene to Miocene ages document continued crystallization during exhumation related to the subduction slab roll-back along the Hellenic trench (McKenzie 1978; Bonev and Beccaletto 2007). Geochronologic studies indicate that exhumation was active during the early Miocene time (Cavazza et al. 2009).

The late orogenic processes in the broader area of Thrace Basin were related either to extension (Caracciolo et al. 2011; Cavazza et al. 2009) or a more complicated pattern of transpression to transtension regimes (Tranos 2009, Tranos and Lacombe 2014). U–Pb geochronology of plutonic rocks suggests regional subduction and suturing during late Eocene time, followed by late Oligocene to Miocene extension and exhumation of the plutons (Black et al. 2013). Additional evidence for this Late Paleogene exhumation event comes from metamorphic core complexes exhumed along the regional detachment faults from late Oligocene to the Miocene (Cavazza et al. 2009; Okay and Satır 2000). Tranos (2009) documented that during late Oligocene time, a NNW-SSE to N–S compression resulted in E–W to ENE–WSW open buckle folds and intense reverse faulting on the late Eocene to early Oligocene submarine fans. Structural analysis reveals a folding event characterized by open to gentle, slightly asymmetric buckle folds with interlimb angles of about 140°. The average fold axis plunges towards the WSW at subhorizontal to shallow (~11°) angles (Tranos 2009). This folding event indicates a regional NNW-SSE contraction that occurred during the Oligocene and before the early Miocene as it does not affect the lower Miocene volcanics (Tranos 2009; Maravelis and Zelilidis 2010). Age-equivalent compression has been reported within the eastern parts of the basin, in the Turkish domain (Görür and Okay 1996; Elmas 2012), and also in the Hellenic eastern Rhodope area (Koukouvelas and Doutsos 1990; Karfakis and Doutsos 1995; Tranos et al. 2009). Seismic stratigraphic data suggest that the bulk of the shortening was due to thin-skinned thrusting and folding that occurred from the base Paleocene to Eocene, with subsequent doming of overthrust tips in Oligocene time (Sinclair et al. 1997).

Typical forearc basins are characterized by the deposition of thick successions of submarine fans at the expense of shallower marine sediments (Trop and Ridgway 2007; Fildani et al. 2008; Mitchell et al. 2010; Maravelis and Zelilidis 2013). The southern edge of the Thrace Basin, within both the Hellenic and Turkish domains, is mainly dominated by deep-marine strata with a shoaling upward trend into shallow-marine and non-marine strata observed at the top of the succession. Because global eustatic sea-level fluctuations were negligible during this time (Miller et al. 2011), the regional shoaling upward trend requires tectonically driven uplift (Maravelis and Zelilidis 2010). The driving force behind this uplift is most likely related to uplift of the accretionary prism (Fig. 14). The late Eocene–early Oligocene basin fill consists of two intervals: (1) a lower, 500–1000 m sand-rich submarine-fan system, and (2) an upper, 100–150-m-thick shallow-marine to deltaic sedimentary unit (Lalechos 1986; Perinçek 1991; Turgut et al. 1991; Roussos 1994; Turgut and Eseller 2000; Maravelis et al. 2007; Maravelis and Zelilidis 2011; D’Atri et al. 2012). Subduction tectonics affected the palaeogeography and gravity-flow deposits migrated northeastward against the continental slope. These flows, largely turbidity currents, accumulated widespread, thick, sand-rich sediments.

Fig. 14
figure 14

Paleotectonic cross section that depicts the regional plate tectonic configuration (from Maravelis and Zelilidis 2010). Palaeogeographic map illustrating the depositional setting of Thrace Basin. The basin fill records the exhumation of a coeval accretionary prism (Pindic Cordillera and Biga Peninsula). This uplift leads to a two-sided forearc basin succession with significant sediment input from the trenchward prism. Both arcward- and trenchward-derived sediment forms submarine-fan systems that fill the basin depocentres

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Uplift of the accretionary prism shifted the locus of coarse-grained sedimentation landwards, towards the magmatic arc. Once subaerially exposed, this prism contributed sediment northeastwards towards the magmatic arc (Maravelis and Zelilidis 2013). The submarine-fan system prograded towards the NE-NNE, as evident by the overlap of the inner fan above the outer fan (Turgut et al. 1991; Maravelis et al. 2007; Maravelis 2009; Maravelis and Zelilidis 2010; 2011; D’Atri et al. 2012). On southern Thrace Basin, extensive palaeocurrent analysis performed on submarine fans on both Hellenic and Turkish domains revealed a NE-NNE-ward flow, making the northern Rhodopian magmatic belt an unlikely source of sediment (Perinçek 1991; Turgut et al. 1991; Maravelis et al. 2007; Maravelis 2009). Palaeoflow results presented in this study also indicate a dominant NE-NNE-ward palaeoflow direction. Palaeomagnetic studies show no rotation of the eastern Rhodope–Thrace region from the Oligocene onwards and imply that the NE-NNE palaeocurrent pattern was the primary direction (Kissel and Laj 1988). Kondopoulou (2000) suggested a c. 25° clockwise rotation since 17 Ma for the Lemnos Island. If it is correct, an S–N palaeoflow direction is indicated, documenting the influence of the southern accretionary prism.

The tectonically driven uplift of the accretionary prism and its influence on forearc basins has been documented in similar ancient settings such as, in the Maastrichtian Great Valley Basin, California (Mitchell et al. 2010). In Great Valley Basin, the accretion of oceanic crust and sediment to the upper plate caused the growth of an accretionary prism. Structural uplift and emergence is documented from fanning of seismic reflector dips that is consistent with syntectonic growth strata. Following this emergence, prograding submarine fans and shelf deposits filled the basin from north to south. This progradational package marked the onset of a shoaling upward trend within the basin. The importance of accretionary prism growth into forearc settings can also be observed in the modern forearc basins of Peru and Chile (Laursen and Normark 2003), the Philippines (Hayes and Lewis 1984), Indonesia (Matson and Moore 1992), and Alaska (Dobson et al. 1991). All of these profiles show the importance of progressive arcward onlap as well as angularities within the basin fill created by progressive arcward folding and tilting of the basin due to accretionary prism growth. In particular along the Sunda arc–trench system of Indonesia, several islands represent an emergent outer arc (e.g. Nias). Erosion of these islands feeds an arcward prograding system. In particular, the Nias delta systems have constructed a coastal plain extending 15 km from the centre of the island. From the coastline, the shelf and slope have prograded another 15 km into the forearc trough.

Conclusions

The lower Oligocene shallow-marine facies in the southern margin of the Hellenic Thrace Basin (Lemnos Island) offers insights into active convergent margins. Integration of sedimentological, stratigraphic, palaeocurrent, and provenance investigations indicate a direct relationship between sedimentary packaging and regional tectonics. During late Eocene to early Oligocene, the basin served as a forearc basin with significant sediment input from a nearby, southern positioned, accretionary prism (e.g. Pindic cordillera and/or Biga Peninsula).

The shallow-marine sediments correspond to the upslope evolution of a regional well-studied upper Eocene to lower Oligocene sand-rich submarine-fan system. This system is exposed on both Turkish and Hellenic domains and forms part of the larger number of fan systems that filled the southern part of the basin. With respect to the underlying deep-sea sediments, the studied rocks are typified by the same: (1) NE-NNE palaeodispersal direction, towards the toe of the Rhodopian magmatic arc, (2) landward migration of the basin depocentre, (3) no evidence of structural deformation involving normal faulting, (4) sedimentary petrography data that suggest an oceanic origin, (5) geochemical values that reflect a mafic source terrane, and (5) conglomerate clast composition that is most likely derived from the accretionary prism.

These data indicate consistent influence of the accretionary prism into the basin fill, as the accretionary prism underwent tectonically driven uplift and compression.