The Mesozoic schists crop out in scattered areas resting on the crystalline basement as remnants of a former nappe of wide regional extent. In previous studies, they have been subdivided into two units: Mandrica and Maglenica groups in Bulgaria [8], which can be correlated with the Makri and Drimos-Melia units in Greece [23], respectively. The whole sequence both in Bulgaria and Greece comprises greenschists at the base, overlain by a sequence of basic extrusives, referred to as Evros ophiolite [19], and anchi-metamorphic sediments at the top. Fossiliferous formations yielded Triassic (greenschists), and Jurassic to Late Cretaceous ages (anchi- to non-metamorphic) [5,7,20,31]. The apatite fission-track data of basic rocks define an interval comprised between 140–161 Ma in Thrace [4], and K–Ar hornblende analysis gave an age of 155±7 Ma in Samothraki [33]. The allochthonous structure and inferred north-vergent thrusting is suggested through the general tectonic framework of the Balkanic orogen [8,14], or alternatively they are interpreted as southward-emplaced thrust sheet during the formation of Rhodope nappes [9]. Both crystalline basement and low-grade rocks are unconformably overlain by Upper Eocene to Miocene sediments [6] and intruded by Oligocene granitoids [10].

Field observations based on different lithologies, metamorphic grade and strain pattern show that the whole sequence can be divided into several units (Fig. 1B). The basement sequence includes a lower orthogneiss unit composed mainly of two-mica gneisses, and an overlying upper marble-schist unit that consists of marbles intercalated with gneiss and schists. Both units correspond to the migmatite–orthogneiss sequence and upper terrane [9], or represent equivalents of the Kechros and Kimi Complexes [17], respectively. The basement lithologies show mineral assemblage Qtz+Pl+Kfs+Bt+Ms+Grt+St and amphibolite facies fabric. Slight retrogression of biotite altered to chlorite is observed at the contact between the two units, indicative of temperature decrease to less severe metamorphic conditions. The gneiss foliation and schistosity, defined by preferred orientation of minerals is flat lying, dipping to the NNE (Figs. 1B and 2), and bear mineral stretching lineation with consistent NNE–SSW trend. Associated shear sense criteria unequivocally indicates a top-to-the-SSW ductile shear (Fig. 3A). Strong mylonitic fabric in gneisses underlying the marble-schist unit has led to the interpretation of this contact as an important tectonic boundary. We interpret this ductile–brittle shear zone as a detachment fault that carries the Mesozoic schists in its hanging wall. As it parallels both the mylonitic fabric and marble horizons at the base of the marble-schist unit, and does not cut through the section, it more likely represents a decollement surface.

The Mesozoic sequence starts with a greenschist unit at the base, overlying the basement along the tectonic contact of presumably extensional origin. It is composed of actinolite–chlorite and garnet–mica schists, and phyllites. Mineral assemblages Qtz+Ms+Act+Chl±Ep±Grt indicate greenschist facies metamorphism with temperatures below ca. 400°. The schistosity, associated to the metamorphic crystallizations, is affected by intense folding. Small-scale northeast-vergent tight folds that have axes oriented NW–SE appear as an earlier fold generation associated with syn-metamorphic actinolite laths. Asymmetric northwest-facing folds with discrete axial planar crenulation cleavage and NNE-SSW oriented axes are developed later, as deduced from crystallization–deformation relationships. Mineral elongation or stretching lineation is defined by aligned actinolite lath and streaky mica and chlorite flakes. It trends NW–SE, but swings to NE–SW at the vicinity of the tectonic contact that confines the greenschist unit from the marble-schist unit (Fig. 1B). Kinematic indicators demonstrate top-to-the-NW and NNE sense of shear (Fig. 3B) that occurred in greenschist facies. The reorientation of lineation at the vicinity of tectonically bounded kilometre-scale fold north of D. Lukovo is due to shortening that created this corrugated antiform of the basal detachment, whose axis parallels the kinematic direction.

The greenschist unit is overlain by tectonic or depositional contact of melange-like volcano-sedimentary unit. It consists of basalt lavas with Lower Jurassic radiolarian interlayers [31], Upper Permian siliciclastics [32] and Middle–Upper Triassic limestones [5] found as blocks in olistostromic packet, embedded in an Upper Jurassic–Lower Cretaceous shale-silty turbiditic matrix. The original sedimentary bedding of matrix lithologies is well preserved and only incipient seams-like cleavage is seen in thin sections, which suggests anchi-metamorphism.

The uppermost sedimentary-volcanogenic unit is composed of sandstones at the base, and andesito-basalt lavas and gabbro-diorites interbedded with marly and tufaceous sediments that yield Late Cretaceous foraminifera [7]. It overlies depositionally the greenschist and melange-like unit, and is unmetamorphosed. This sequence is related to the Late Cretaceous arc/back-arc magmatic activity of the Sredna Gora zone to the north (Sd, in Fig. 5c), whose age is confirmed by the presence of Globotruncana taxa [7].

The greenschists display acicular texture and foliated fabrics of alternating quartz–mica–albite with chlorite–actinolite-rich domains. Their mineral assemblages and chemical composition and associated volcanic rocks, indicate that their protoliths were of sedimentary and volcanic origin. The basalts in melange-like unit are massive and variolitic lava flows with preserved primary aphyric to porphyric textures. The major phenocrysts are clinopyroxene ± orthopyroxene, plagioclase and Fe–Ti oxides, as well as olivine, when present, set in a microlitic matrix. The clinopyroxene crystallized earlier than plagioclase, as it is included in the latter; such a crystallization sequence is common in island arc basalt lavas [3]. The basalts are slightly overprinted by secondary minerals, such as chlorite, quartz and calcite, indicative of low-grade metamorphism. Prehnite–pumpellyite assemblages also occur resulting from ocean floor hydrothermal alteration, which have been reported also in Greece [19].

Selected greenschist and basalt samples were analysed by XRF for major and trace elements, and REE abundances were determined by ICP–MS at the University of Lausanne. They show the following characteristics: (a) low abundance of incompatible elements (e.g. Nb); (b) low Nb/Y and P₂O₅/Zr ratios characteristic of tholeiitic basalts; (c) low total REE concentrations. Using relatively immobile trace elements, the geochemical nature of basalts and greenschist has been defined on several discriminative diagrams. In a Zr/TiO₂ vs. Nb/Y plot [34] (Fig. 4a), samples fall in or near the sub-alkaline basalt field. Major and trace element abundances of lavas classify them as low-K and very low-Ti tholeiitic basalts, with some boninitic affinity on Ni vs. Ti/Cr plot [27]. Most samples plot in the island arc basalt field (Fig. 4b and c) except some of semi-pelitic schists, and all basalts fall below Zr/Y=3.0 ratio line on the Zr–Zr/Y diagram (Fig. 4b), typical for basalts erupted in intra-oceanic island arc setting [24]. The REE patterns (Fig. 4d) display LILE enrichment relative to the HFS elements, indicative of depleted mantle source. Basalt samples LREE depletion and slight HREE enrichment reflect subduction-related component [26]. The negative Nb and Ce anomaly consistently points to an island-arc environment, as shown by discrimination diagrams. Overall, trace element discrimination and REE patterns provide strong constraints for an origin of both rock types in a volcanic (intra-oceanic) island arc tectonic setting. Possible contamination by crustal material before their final emplacement is likely.

Petrologic and geochemical signature of greenschists and basalts strongly suggests a volcanic arc origin, which taken collectively with similar data and ages in Greece, indicates the existence of an intra-oceanic island arc system near the Rhodope continental margin in Middle–Late Jurassic time (Fig. 5a and b). Structural and kinematic data highlight the deformation history of the Mesozoic schists. The NW–NNE-directed shear in greenschist facies demonstrates a ductile-brittle tectonic emplacement. The northeast-directed shear is interpreted as related to detachment faulting, but with opposing sense of shear to that of the basement detachment, whose both surfaces have contributed to strong thinning, in between, of the marble-schist unit. However, northwest-directed shear possibly reflects an earlier thrusting event, which could represent deformation in a subduction–accretion complex. The low-grade rocks have escaped the Cretaceous UHP/HP conditions (Balkan orogenic events) recorded in the immediately underlying basement of the studied area [18], but high-pressure relics are preserved in Chalkidiki [21]. This suggests that they have been transferred to the upper plate in a supracrustal position. This is confirmed, in the Serbo-Macedonian domain, by ophiolites and CRB rocks found on top of the crystalline basement. They seem to have been involved in ENE-directed unroofing of Serbo-Macedonian basement rocks above the lower Rhodopian nappes, with lately reworked contact and omissions by Tertiary extensional tectonics [16]. These rocks remained in an upper plate position during the renewed Late Cretaceous subduction of the remnant Vardar ocean under the Rhodope, followed by the collision with the Pelagonian terrane and the Palaeocene closure of that ocean (Fig. 5c). The SSW-directed kinematics in the basement unit typically reflects the main syn-metamorphic deformation [9], as well as Eocene–Oligocene late-orogenic extension in this part of the eastern Rhodope [17]. We suggest on account of geochemical features and structural data, that the deformation history of Mesozoic schists corresponds to the Early Cretaceous tectonic emplacement related to ophiolite obduction onto the already accreted CRB domain and Rhodopian margin [22]. Later, the schists were strongly affected by Tertiary low-angle normal faulting.

The palaeotectonic reconstructions in the western Tethys [28,29,35] have shown sequential back-arc basin openings along the Eurasian margin from the Late Permian–Triassic to Early–Middle Jurassic time, due to the Palaeotethys closure and Neotethys widening. The Meliata–Maliac Ocean had been separated from the Neotethys, and was affected by intra-oceanic subduction zone during Jurassic times (Fig. 5a). This subduction and slab roll-back of Meliata gave birth to a new back arc basin – the Vardar ocean, which by Late Jurassic time had replaced the Meliata–Maliac ocean (Fig. 5b). Our results from the Mesozoic units of eastern Rhodope and western Turkey [2] fit this palaeogeographic framework. We consider that the Meliata–Maliac ocean northern passive margin (Rhodopian lower plate) could be the only source for the olistostromic Upper Permian siliciclastics and carbonates and Middle–Upper Triassic limestone blocks found within the melange-like unit, whereas turbidites and basic lavas originated in an island arc/accretionary complex (upper plate) or possibly from the already accreted CRB domain. The latter corresponds to a segment of the Middle–Late Jurassic intra-oceanic Vardarian island arc system, related to the northwestward slab retreat (roll-back) of the Meliata–Maliac Ocean. The collision of the CRB with the Rhodopian margin followed by the obduction of the intra-oceanic arc (Mandrica–Melia units) resulted in the Early Cretaceous Balkan orogen, which can be followed up to the Carpathians and Eastern Alps [12]. The same Vardarian island arc system collided also with the Pelagonian margin, and provoked a large-scale Late Jurassic–Early Cretaceous ophiolitic obduction in the Hellenides–Dinarides (Fig. 5b). Subduction reversal (northeast-directed subduction of the remnant Vardar) in Late Cretaceous times gave birth to the Srenogorie arc [14] and to the back-arc extension of the Black Sea region (Fig. 5c).

These new structural and petrologic data shed a new light on the tectonic setting of the Mesozoic units and their geodynamic context in the frame of the Early Jurassic to Late Cretaceous evolution of the Vardar Ocean. The subduction–accretion history along the limits of the Rhodope metamorphic province has been recently lively debated [1,11,21,22]. The subduction setting of Mesozoic schists situated near the Rhodope margin in pre-Cretaceous time was generally adopted as a possibility [25]. Early Cretaceous cooling of the accretionary wedge by underplating of weakly metamorphosed material may have resulted in emplacement of Mesozoic schists onto the margin and clearly reinforce the maximum allochtonous hypothesis in view of regional correlations and interpretation [11,16,21]. Obviously, the Mesozoic schists were strongly reworked by late extensional and strike-slip tectonics in the entire CRB belt [16,25]. More studies are necessary to unravel the significance of the Circum-Rhodope belt, and test the model of a ‘single’ Vardar Ocean, but that developed several arc systems.

From our field and laboratory studies of the Rhodopian upper nappe system, we can draw the following conclusions. (1) The Mesozoic schists form allochthonous sheets found above the crystalline basement, they present complex internal deformation that could preserve relics of Late Jurassic–Early Cretaceous subduction–accretion tectonic processes, and mainly reflect strong imprint of Tertiary late-orogenic extension. (2) Petrologic and geochemical characteristics of the greenschists and basalts define their island-arc signature, and their protoliths as being derived from depleted mantle source. (3) These results indicate their origin in a Vardarian intra-oceanic island-arc setting during the Middle–Late Jurassic, whose collision with the Rhodope promontory was responsible for the Early Cretaceous north-directed Balkan orogenic system.