Pelagonian Margin Research Articles

Coherent underplating of HP–LT blueschist packages and basement during Hellenic subduction recorded by zircon U–Pb data, Pelion, Greece

Studies of HP–LT metamorphic complexes are crucial in advancing our understanding of subduction processes, such as underplating, metamorphism and exhumation. Detailed new U–Pb zircon data of blueschist facies metasedimentary units at Pelion indicate two distinct stratigraphically upright and coherent structural slices: (1) the South Pelion slice consisting of strata with Permian–Late Cretaceous zircon maximum depositional ages; and (2) the North Pelion slice consisting of strata with Triassic–Late Cretaceous zircon maximum depositional ages. Both slices have Late Cretaceous strata at the top of the section with cosmopolitan detrital zircon signatures. The ages from zircon U–Pb rim overgrowths correlate with existing constraints from subduction metamorphism during the Paleocene–Eocene, with a possible second stage recorded during basal underplating in the Late Eocene–Oligocene. Detrital zircon provenance data and multi-dimensional scaling comparisons demonstrate a tectono-metamorphic linkage between the Pelion rocks and the Cycladic Blueschist Unit, and between the southern Pelion basement and the Cycladic basement. Protolith deposition for Pelion blueschist facies rocks occurred during Permo-Carboniferous intra-arc extension and Adria–Pindos rifting. Our results show that the Pelion blueschist facies rocks, representing lateral equivalents of the Cycladic Blueschist Unit, consist of two separate, coherent upper crustal slivers that were underplated and metamorphosed during Hellenic subduction beneath the Pelagonian margin. Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists Supplementary material: Detailed methodology, further provenance and maximum depositional age methods and descriptions, sample location coordinates (Supplementary Table 1), and zircon U–Pb data (Supplementary Table 2). These data are available at https://doi.org/10.5281/zenodo.8300635 and https://doi.org/10.6084/m9.figshare.c.7008123

New stratigraphic and palaeontological data from carbonates related to the Vourinos–Pindos ophiolite emplacement: implications for the provenance of the ophiolites (Hellenides)

Microfacies analysis and conodont dating on carbonates associated with the Vourinos–Pindos ophiolite emplacement on the Pelagonian unit in northern Greece, combined with structural analysis, enhance our understanding of the palaeogeography and geotectonic evolution of the Hellenides. We focus on a dismembered Triassic Hallstatt Limestone succession incorporated as blocks in the Middle–Late Jurassic Avdella ophiolitic mélange, which is identical to a Triassic succession in the western Pindos (Hallstatt–Pindos succession). Initially, the Hallstatt–Pindos succession was deposited on the outer shelf along the eastern Pelagonian margin and later incorporated into the mélange during west-directed obduction processes. During the Middle to early Late Jurassic, ophiolites were obducted on the eastern Pelagonian margin, resulting in a nappe stack in front of the ophiolites in the lower plate position. Newly formed trench-like basins in front of the advancing nappe stack became filled with sedimentary mélanges, consisting of ophiolitic material mixed with blocks from the Hallstatt–Pindos succession, deposited in an outer shelf position. Subsequently, the Pindos ophiolites overthrust their foreland basin, forming a typical ophiolitic mélange (Avdella mélange). Consequently, the Triassic–Middle Jurassic Hallstatt–Pindos succession represents a far-travelled Middle–Late Jurassic nappe derived from east of the Pelagonian unit, which was bulldozed westward in front of the west-directed obducting Neotethys ophiolites. Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Reconstructing the southern Pelagonian domain in the Aegean Sea: Insights from U-Pb detrital zircon analysis, lithostratigraphic and structural study, and zircon (U-Th)/He thermochronology on Amorgos Island (SE Cyclades, Greece)

In the Attic-Cycladic Crystalline Complex (Aegean Sea, Greece), following early syn-convergence exhumation processes, the extensional tectonics resulted in the extreme denudation of the upper plate, preserved today in a few places. The Cycladic Blueschist Unit (CBU) and the underlying Cycladic Basement and the Basal Units have been exhumed in the footwall of detachment systems, juxtaposed against the Pelagonian-derived hanging wall fragments. Traditional views accept that Amorgos Unit occupies a structurally lower position, correlated to the Basal Unit. This study presents lithostratigraphic and structural data, integrated with detrital zircon MDA, provenance analyses, and (U-Th)/He ages, showing a new investigation of the paleogeographic and structural position of Amorgos in the Cycladic archipelago.The low-grade metamorphosed Amorgos Unit presents a tripartite lithostratigraphy: a basal metaconglomerate, a middle carbonate sequence with shallow-water to pelagic facies, and an Eocene (meta)flysch. An HP/LT metabasite huge block is incorporated within the metaconglomerate either by gravitational movements as an isolated olistolith or tectonic processes as a slice during the early deformation stage. Detrital zircon U-Pb analysis on the meta-siliciclastic deposits revealed that the basal metaconglomerate shows a dominant Ediacaran input with Mid-Permian and Precambrian MDAs for the matrix and the high-grade clasts, respectively. The metaflysch shows Paleozoic affinity (e.g., mainly Pelagonian-sourced) and Triassic-Jurassic MDAs. The deformation history of Amorgos includes an early-stage top-to-NW thrusting in the retro-wedge setting of the Late Eocene–Oligocene subduction zone and a late-stage top-to-SE extensional low- and high-angle normal faulting. Zircon (U-Th)/He ages reveal an Early–Mid Miocene (18–14 Ma) exhumation below ∼ 200 °C. Similarities on lithostratigraphy, structural inventory, exhumation history, and structural position of Amorgos Unit with the Santorini Detachment System Pelagonian hanging wall imply their close spatial relationship. From all the above, we propose that Amorgos belongs to the upper plate of the CBU and is paleogeographically located at the southern Pelagonian margin.

Cretaceous‐Paleogene Tectonics of the Pelagonian Zone: Inferences From Skopelos Island (Greece)

AbstractWe have investigated subduction‐exhumation processes in the Pelagonian zone, exposed on the Northern Sporades islands (Aegean Sea) related to successive episodes of ocean continent and continent‐continent convergence through integrating multiscale structural analysis, metamorphic petrology, and white mica 40Ar/39Ar dating. Two major progressive phases of ductile deformation are documented, which are related to distinct episodes of tectonic burial and exhumation of the Pelagonian continental margin, which was facing the Neotethys/Vardar ocean. Review of existing data sets from neighboring regions shows that both deformation phases can be correlated along strike up to the Dinarides. The first phase of tectonic burial and exhumation (D1) is characterized by NW‐SE tectonic transport, greenschist facies metamorphism, and Early Cretaceous (~105‐135Ma) 40Ar/39 Ar white mica single fusion ages. D1 is correlated with the initial closure of the Vardar ocean by top‐to‐the‐W to NW ophiolite obduction and the underthrusting of the Pelagonian margin below the oceanic upper plate. Underthrusting was followed by exhumation and the deposition of Late Cretaceous‐Paleogene sediments. The second phase of burial and exhumation (D2) is characterized by NE‐SW tectonic transport, greenschist to blueschist facies metamorphism, and latest Cretaceous‐Early Eocene 40Ar/39 Ar white mica ages of S2 fabrics. Top‐to‐the‐SW shearing is correlated with the tectonic burial of the Pelagonian zone below the Eurasian continent (Rhodopia), while top‐to‐the‐NE shearing is attributed to subsequent extensional exhumation. D2 fabrics record low‐grade P‐T conditions suggesting that the decoupled cover formations exposed on Skopelos were incorporated in an accretionary wedge that formed above the subducting Pelagonian basement during Paleogene times.

CORELLATION OF LATE TRIASSIC AND EARLY JURASSIC LOFER-TYPE CARBONATES FROM THE PELOPONNESUS PENINSULA, GREECE

Correlation of the Late Triassic carbonate successions, formed at the passive Pelagonian margin (SE of Dhidymi Mt.) with the Early-Middle Liassic successions of the Gavrovo-Tripolitza zone (SE of Leonidion), in eastern and central Peloponnesus respectively, is attempted. Detailed microfacies analysis revealed that the studied carbonate formations were deposited in analogous restricted inner-platform environments (lagoon-peritidal domain) and are composed of meter-scale, shallowing-upward, mostly incomplete peritidal cycles. The top of the supratidal and/ or shallow subtidal deposits are often affected by meteoric diagenesis tracing sea-level lowering and periodic emersions episodes. The well-developed pedogenic features observed in the Early Jurassic platform carbonates indicate long-lasting subaerial exposure intervals and semi-arid to arid climate. Instead, the Late Triassic strata preserve vadose diagenetic indices which point to shorter exposure events, weaker meteoric alteration and slightly wetter climatic conditions. The detected sedimentological features suggest the occurrence of wide lagoonal-peritidal depositional systems during Late Triassic and Early Jurassic, eastern and western of the Pindos basin, respectively. The basic facies pattern and the meter-scale cyclicity show many analogies with the Lofer cycles of the Alpine Triassic, supporting that in the Hellenides the Dachstein-type platform systems evolved till Early-Middle Liassic (Gavrovo-Tripolitza zone).

Boninitic and tholeiitic basaltic lavas and dikes from dispersed Jurassic East Othris ophiolitic units, Greece: petrogenesis and geodynamic implications

ABSTRACTPillow lavas, massive lava flows, and sub-volcanic dikes of tholeiitic basaltic composition are found to be members of the Vrinena, Aerino, Eretria, and Velestino dispersed Middle–Upper Jurassic ophiolitic units in East Othris. The Vrinena and Eretria ophiolitic units appear to have been emplaced onto the Pelagonian continental margin during the Upper Jurassic–Lower Cretaceous, whereas the Aerino and Velestino units seem to have been finally emplaced during post-Palaeocene times. Geochemically these are divided into two groups: Group I includes subduction-related boninites and low-Ti basalts from the Vrinena and Aerino units, and Group II high-Ti basalts show spreading-type characteristics occurring in the Eretria and Velestino units. Primary magma of the Group I volcanics appears to have been formed after high partial melting degrees (~18%) of a highly depleted harzburgitic mantle source, under relatively high temperatures (mantle potential temperature ~1372°C). Petrogenetic modelling also suggests that the primary magma of the Group II volcanics were formed after lower partial melting degrees (~7%) of a moderately depleted mantle source. The petrological and geochemical data from the East Othris dispersed and diversely emplaced ophiolitic units provide evidence of a common intra-oceanic supra-subduction zone (SSZ) origin within the Pindos oceanic strand of the Western Tethys. Specifically, Group I lavas and dikes from Vrinena seem to represent the extrusive part of an almost complete fore- to island-arc ophiolitic sequence. Dikes of Aerino most likely correspond to fore-arc magmatic material that intruded within exhumed serpentinized ultramafic rocks through a subduction channel that developed close to the slab and towards the fore-arc and the accretionary prism. The Group II volcanics either corresponded to a fore-arc magmatic expression, which extruded earlier than Group I volcanics and prior to the establishment of a mature subduction zone, or represent back-arc to island-arc magmatism that was contemporaneous to the fore-arc magmatic activity during rollback subduction.

Balkan ophiolite-related geology

Springer-Verlag 2012 The present thematic set of papers resulted from the field symposium ‘‘Ophiolites 2008: Link between the Mesohellenic Ophiolites and Pelagonian Margin,’’ which was held September 14‐20 in West Macedonia and Thessaly, Greece. This was the second of a series of field meetings aiming to provide international researchers with an opportunity to examine the ophiolitic rocks and related geology of the Balkan region. The field localities studied during the field trip within mainland Greece included sites that are important to the development of plate tectonic

Kinematics and Ophiolite obduction in the Gerania and Helicon Mountains, central Greece

New structural, petrofabric and palaeostress data from the Beotia area (central Greece) were used to investigate the tectonic evolution of the suture zone between the External (Parnassus microplate) and Internal Hellenides (Pelagonian microplate). Petrofabric studies of ultramafic rocks were done using conventional U-stage analysis and the electron backscatter diffraction (EBSD) technique. Detailed structural analysis enabled us to distinguish three main deformation phases that took place from the Triassic to the Eocene. Triassic–Jurassic deformation is related to continental rifting and the progressive formation of an ocean basin. Ophiolites formed above a westward-dipping supra-subduction zone (SSZ) in the Early–Late Jurassic. Trench–margin collision resulted in the southeastward emplacement of the ophiolite nappe over the Pelagonian margin. There is also evidence for a north-westward thrusting of ophiolitic rocks over the Gerania and Helicon units during Berriasian time. This latter tectonic process is closely related to the deposition of “Beotian flysch” into a foreland basin. An extensional phase of deformation accompanied by shallow-water carbonate sedimentation is documented in the Upper Cretaceous. Later, during Paleocene the area was subjected to a compressional deformation phase characterised by SW-directed thrusting and folding, as well as NE-verging backthrusts and backfolds. Our proposed geotectonic model suggests the consumption of the ocean between the Parnassus and Pelagonian microplates. This model includes Late Jurassic eastward ophiolite obduction followed by Early Cretaceous west directed ophiolite thrusting.

Late Triassic, Early and Middle Jurassic Radiolaria from ferromanganese-chert ‘nodules’ (Angelokastron, Argolis, Greece): evidence for prolonged radiolarite sedimentation in the Maliac-Vardar Ocean

In the Argolis, the Basal Sequence, constituting the eastern Pelagonian margin which bordered the Maliac-Vardar oceanic domain, includes shallow-water carbonates of Late Triassic-Early Jurassic, condensed pelagic limestones of Early-Middle Jurassic, radiolarian cherts of late Middle-Late Jurassic age and siliceous mudstones and sandstones rich in ophiolite fragments. Up-section, coarse breccias, including clasts of boninites derived from the ophiolite obducted onto the Pelagonian margin in Late Jurassic times crop out. Near Angelokastron a small quarry exposes pervasively sheared dark reddish-brown, radiolarian-bearing cherty shales with disrupted fragments of chert and chert nodules impregnated by ferro-manganese oxides. These shales occur in the footwall of a thrust bringing them into contact with the Pantokrator Limestone of the Basal Sequence. We collected more than 30 samples of the chert fragments and the shaly matrix. Thirteen nodules and one matrix sample yielded determinable radiolarians. Low to non-detectable concentrations of trace metals such as Co, Cr, Cu, Ni, Zn, and Pb indicate a hydrothermal origin of the ferro-manganese mineralization. The radiolarian taxa found indicate four age groups for the nodules that are embedded in the siliceous shale matrix that yielded a Middle Jurassic age (middle Bathonian). The first group includes a nodule of Late Triassic age (late Norian to Rhaetian); the second group nodules of Early Jurassic age (late early to late Pliensbachian and probably middle-late Toarcian); the third group nodules of early Middle Jurassic age (Aalenian–Bajocian); the last group finally includes nodules of late Middle Jurassic age (Bajocian–Bathonian). The presence of Upper Triassic to Middle Jurassic Mn-impregnated chert nodules in a Middle Jurassic matrix indicates a deep oceanic environment of deposition outside the Pelagonian realm (easternmost Adria Plate), which at that time was a shallow-water carbonate platform with a thin pelagic limestone cover. The chert nodules are with all certainty derived from the oceanic Maliac-Vardar domain and were, together with their host formation, tectonically emplaced onto the Pelagonian margin. We speculate that these nodules, more lithified than their matrix, were exhumed on the slope of an intra-oceanic accretionary wedge and were redeposited in the Middle Jurassic siliceous mudstones on the floor of the subducting Maliac-Vardar Ocean.

The Hellenic ophiolites: eastward or westward obduction of the Maliac Ocean, a discussion

Ophiolitic bodies in the Dinaro-Hellenic mountain belt are among the most important ones in the Peri-Mediterranean Alpine chains. The characteristic feature of this ophiolitic belt is its Middle to Late Jurassic age of obduction. The ophiolitic bodies form two major belts on each side of the Pelagonian zone: an east Pelagonian belt in the Vardarian domain and a Supra-Pelagonian ophiolitic belt (SPO) to the west. The different hypotheses relative to the origin of the SPO present geodynamic evolution models accounting for most of the available data: a mid-Triassic episode of rifting; a Ladinian–Jurassic episode of sea-floor spreading forming notably the Maliac Ocean; a Middle to Late Jurassic convergent period with subduction and obduction episodes, and finally, a late episode of Tertiary compressional deformation responsible for the westward thrusting of the Jurassic ophiolitic nappes over the external zones. Despite many studies dating from the early 1970s, the eastern or western Pelagonian origin of these ophiolites, especially the SPO, is still under dispute. Whatever the adopted hypothesis, we consider that the main SPO bodies (N-Pindos, Vourinos, Othris, Evia, Argolis) have the same origin because of their geographic continuity and of the similarities in their geological characteristics. We propose that this ocean corresponds everywhere to the Maliac Ocean, defined in Othris from the well-preserved sedimentary (oceanic margin) and ophiolitic nappes thrust during the Late Jurassic obduction onto the Pelagonian platform. There is strong evidence for the existence of two deep basins on both sides of the Pelagonian continental ridge during Triassic–Jurassic times. They correspond, respectively, to the Vardar area to the east and the Pindos domain to the west, one of these domains being at the origin of the SPO. The hypothesis of an eastward emplacement of the SPO from the Pindos domain is based mainly on sedimentological data from the margin series and on structural analyses of ophiolitic bodies. However, we conclude the westward obduction of the Maliac Ocean, originating from the Vardar area, to be the best fitting model. This westward model is supported by paleogeographic and structural constraints on regional scale. Notably, the absence of obducted ophiolites in the Jurassic series of the Koziakas units (units attributed to the western Pelagonian margin) and of the Parnassus domain (on the eastern side of the Pindos basin) is difficult to reconcile with an eastward obduction from the Pindos domain. Other observations, such as the distribution of ophiolitic detritus in the internal and external zones, also promote the westward Late Jurassic obduction of the Maliac Ocean. Our preferred model offers a consistent explanation for the mechanism and timing of the emplacement of the SPO, as well as providing insight on the origin and emplacement of the Vardarian ophiolites. Following this hypothesis, there is no need for a clear boundary between the SPO and the west Vardarian ophiolitic bodies as they were obducted from the same oceanic basin and during the same Jurassic tectonic event. In this paper, we develop evidence in favor of the eastern Pelagonian origin for the SPO (our adopted model) and provide discussion on the data supporting the main alternative hypothesis (western origin for the SPO).

Structure and tectonics of subophiolitic mélanges in the western Hellenides (Greece): implications for ophiolite emplacement tectonics

The Jurassic Vourinos ophiolite is part of the Western Hellenide ophiolite belt in Greece and rests tectonically on the Pelagonian microcontinent. The Vourinos and coeval Pindos ophiolite to the west display suprasubduction-zone geochemical affinities, and represent remnants of oceanic lithosphere formed in a rifted incipient arc–forearc setting within the Pindos Basin. In structurally descending order, and from west to east, the subophiolitic mélange beneath the Vourinos ophiolite contains the Agios Nikolaos Formation (ANF) and a rift assemblage, both of which display ENE-vergent thrust faults, shear zones, and folds. The ANF comprises schistose mudstone with pebbles, cobbles, and boulders of arenite and wacke derived from the crystalline basement of Pelagonia. Imbricated along ENE-directed thrust faults and metamorphosed up to lower amphibolite facies, the ANF represents continental rise deposits of the rifted Pelagonian margin. The rift assemblage includes blocks of basaltic lavas, ribbon chert, micritic cherty limestone, metagabbro, dolerite dikes, and serpentinite breccia that are commonly in thrust contact with each other and are tectonically imbricated with the Pelagonian carbonates; however, primary intrusive and depositional contacts are locally well preserved. Gabbro and dolerite dikes are locally intrusive into the recrystallized carbonates and metapelitic rocks of the Pelagonian microcontinent. Lavas display mid-ocean ridge basalt within plate basalt affinities and represent Upper Triassic rift units that erupted during the separation of Pelagonia from Apulia. Gabbro, dolerite, and serpentinite breccia are the products of a magmatic rifting episode prior to the onset of seafloor spreading in the Pindos Basin. The Vourinos subophiolitic mélange thus consists of passive margin and rift assemblages that were tectonically overridden by the Vourinos ophiolite in the Middle Jurassic. The Avdella mélange beneath the Pindos ophiolite to the west includes a chaotic matrix including ophiolitic clasts and blocks, reefal to pelagic limestones, and olistostromal turbidites that range in age from Triassic to Jurassic–Cretaceous. The Avdella mélange rests tectonically on a Cretaceous–Eocene flysch unit and the Cretaceous–Eocene shelf and slope deposits of the Apulian margin along west-directed thrust faults. The evolutionary history of the Vourinos and Avdella subophiolitic mélanges indicates diachronous tectonic emplacement of the Mesohellenic oceanic slab, first in the east as a result of a trench-continent (Pelagonia) collision in the Middle Jurassic, then in the west due to the Eocene collision of Apulia with Eurasia.

Ophiolites 2008 Guidebook: Link between the Mesohellenic Ophiolites and the Pelagonian Margin

Ophiolites 2008 Guidebook: Link between the Mesohellenic Ophiolites and the Pelagonian Margin

Rotational deformation in the Jurassic Mesohellenic ophiolites, Greece, and its tectonic significance

The Jurassic Pindos and Vourinos ophiolites in the Western Hellenides of Greece are part of the Mesohellenic mafic-ultramafic slab underlying the Eocene–Miocene sedimentary basin (Mesohellenic Trough). The tectonic incorporation of this oceanic slab into the western edge of the Pelagonian subcontinent occurred via trench – passive margin collision in the late Jurassic. Much of the tectonic architecture of these ophiolites, particularly Vourinos, was acquired during progressive inhomogeneous deformation associated with the initial displacement of the Jurassic oceanic crust from its original igneous environment of formation, and its subsequent tectonic emplacement eastward onto the Pelagonian margin. The heterogeneous deformation in the mantle and crustal sequences of the Pindos–Vourinos ophiolites occurred in ductile, ductile-brittle, and brittle fields synchronously, as the Jurassic oceanic lithosphere was translated eastward; it also resulted in differential horizontal rotations within the displaced oceanic slab. Areas retaining high-temperature (diapiric) mantle fabric appear to have been “passively” translated by lower temperature ductile shearing, commonly along mylonite zones. Individual dunite bodies in the harzburgite tectonites indicate movement distances of at least kilometer scale. Pervasively mylonitic domains within the upper mantle peridotites suggest elongation on the order of five to ten times in the direction of ophiolite emplacement. Seafloor-spreading related, high-temperature mantle structures within the Pindos and Vourinos ophiolites are sub-parallel. Imprinted ductile kinematic indicators and ductile shear zones pervasive to the mantle and lower crustal sections in both ophiolites are also sub-parallel, consistent with the direction of tectonic vergence, and appear traceable across the sedimentary overburden of the Mesohellenic Trough. These geometric relations imply that the Pindos–Vourinos ophiolites retain their relative orientations from the time of cessation of ductile deformation within the ophiolitic slab. More than 90° anticlockwise horizontal rotation of the Vourinos ophiolite seems to have taken place during progressive simple shear deformation associated with the eastward emplacement of the Jurassic oceanic lithosphere. This kind of rotational deformation postdating the igneous accretion of ancient oceanic crust may be common in many ophiolites and should be carefully distinguished from tectonic extension-related vertical rotational deformation developed during their seafloor spreading evolution.

Facies analysis of Lofer cycles (Upper Triassic), in the Argolis Peninsula (Greece)

The Upper Triassic carbonate sediments of Argolis Peninsula are part of the Upper Triassic–Lower Jurassic extensive and thick neritic carbonate formations (Pantokrator facies) that formed at the passive Pelagonian margin and are considered as Dachstein-type platform carbonates. Facies analysis of the Upper Triassic “Lofer-type” lagoonal–peritidal cycles in the Dhidimi area, proved that cycles, although mostly incomplete, were regressive shallowing-upward. The ideal elementary cyclothems are meter-scale in thickness and begin with a subtidal bed (Member C), represented by a peloidal dolostone with megalodonts (wackestone or packstone), being followed by a stromatolitic intertidal dolomitic mudstone and/or fenestral intertidal dolomitic mudstone (Member B) that is overlain by dolocrete (terrestrial stromatolites or pisoidic dolomite) or a supratidal “soil conglomerate” in red micritic matrix (Member A). Lofer-cycle boundaries are defined at the erosional surfaces and accordingly the Lofer cyclothems are unconformity-bounded units. Due to common post-depositional truncation of the subtidal and intertidal facies, the supratidal members prevail, being developed, in places, directly upon subaerial exposure surfaces (erosionally reduced cyclothems). Peritidal layers are characterized by a well-expressed lamination, sheet cracks, tepee structures, fenestral pores and karst dissolution cavities. The studied lagoonal–peritidal cycles are considered to have been deposited in a tidal-flat setting (inner platform), repeatedly exposed under subaerial conditions, in the context of a broader tropical rimmed platform. Although the studied area was tectonically active due to rift-activity and the autocyclic processes should also be taken in consideration, the great lateral correlatability of cycles, the facies shifting and the widespread erosion that resulted in superposition of supratidal-pedogenic facies directly upon subtidal members (subaerial erosional unconformity), indicating a sea-level drop, reflect allocyclic control via high-frequency eustatic sea-level oscillation (orbital forcing). Sediment deposition occurred during low-stand system tract (LST), that probably continued also in the transgressive system tract (TST) and reflects an overall sea-level fall. Under these conditions dissolution and cement precipitation episodes, as well development of paleosols and karsts, were triggered, during a relatively less arid interval.

PETROLOGY, GEOCHEMISTRY AND GEOTECTONIC SETTING OF THE TITAROS OPHIOLITE AND ASSOCIATED PELAGONIAN BASEMENT ROCKS IN THE AREA NW OF MT. OLYMPOS, GREECE

The Titaros ophiolite in Greece is a coherent thrust sheet of oceanic rocL· that lies atop Permo-Carboniferous granitic orthogneisses and amphibolites of unknown age of the eastern Pelagonian margin in the area NW of Mt. Olympos. It comprises a harzburgite tectonite mantle sequence with chromite mineralisation and a welldeveloped magma chamber with cyclic units ofdunite-lherzolite-wehrlite-pyroxenite that pass upwards into massive gabbros cut by diabase dykes and locally containing plagiogranite ponds. Ophiolite mineral and whole-rock chemistry are strongly in favour of a supra-subduction zone origin in a purely oceanic setting with the occasional brawny signature of melts from subducted sediments. By contrast, the basement amphibolites display a clear within-plate tholeiitic affinity with a slight imprint of subduction-zone fluids. Mineral stretching lineations and kinematics indicators of the orthogneisses and amphibolites suggest a consistent transport direction to the WSW. It is proposed that the basaltic protoliths of the amphibolites were emplaced during Permo-Triassic rifting of the eastern Pelagonian margin that led to the subsequent formation of the Vardar Ocean. The Titaros ophiolite was formed during closure of the Vardar Ocean via northeast-directed intra-oceanic subduction and subsequent obduction towards the southwest onto the eastern Pelagonian margin, probably in the Lower Cretaceous.

GEOCHEMISTRY AND TECTONO-MAGMATIC SIGNIFICANCE OF HP/LT METAOPHIOLITES OF THE ATTIC-CYCLADIC ZONE IN THE LAVRION AREA (ATTICA, GREECE)

The Lavrion area corresponds to the northwestern end of the Attic-Cycladic Complex and mainly consists of metamorphic rocks formed during the Eocene high-pressure/low-temperature (HP/LT) event and the Upper Oligocene-Lower Miocene medium-pressure metamorphic event. These metamorphic rocks are found in two superimposed tectonic units: the Kamariza Unit, which includes metavolcanic rocks and the overlying Lavrion blueschist Unit, which is largely represented by metaophiolites. Protoliths of metavolcanic rocks in the Kamariza Unit are calc-alkaline basalts displaying a marked enrichment in Th, U, and LREE and depletion in Ta, Nb, Hf and Ti, which point to a genesis from a depleted mantle source further enriched by subduction components. The Lavrion blueschist Unit mainly includes metavolcanic rocks with tholeiitic composition, as well as subordinate metagranites and metavolcanic rocks with calc-alkaline affinity. The tholeiitic metavolcanic rocks are mainly represented by enriched-type (E-) mid-ocean ridge basalts (MORB) and subordinately by normal-type (N-) MORB. E-MORB chemistry implies a genesis from a depleted asthenospheric source modified by an OIB component, whereas the N-MORB has chemical features typical for rocks generated in a mid-ocean ridge setting from a primitive asthenospheric source. Previous works suggested that the magmatic protoliths of similar HP/LT metamorphic rocks from elsewhere in the Cyclades reflect an arc-back-arc tectonic setting, which developed during the Cretaceous closure of the Pindos oceanic basin. However, recent geological studies have shown that the Lavrion metamorphic Units, unlike similar units from the Cycladic zone, represent Triassic Pelagonian sequences metamorphosed under HP/LT conditions typical of the Cycladic zone. The geochemical and petrological characteristics of the Lavrion metamorphic rocks support this conclusion. In particular, calc-alkaline protoliths display many similarities with the Triassic calc-alkaline rocks associated with the rift of Gondwana, whereas MORB-type protoliths are similar to the Triassic MORB found in the Subpelagonian ophiolitic melanges. The magmatic protoliths of the Lavrion HP/LT metamorphic rocks are thus compatible with a paleotectonic evolution which encompasses the Triassic continental rift, followed by the early oceanization stage of the Pindos ocean, and emplaced on the border between the Pelagonian continental margin and the Pindos basin. These rocks were probably included into melanges during the Jurassic closure of the Pindos basin, and finally they were involved in the Eocene and Upper Oligocene-Lower Miocene metamorphic events that affected the Cycladic zone.

Heterogeneous mantle complex, crustal processes, and obduction kinematics in a unified Pindos-Vourinos ophiolitic slab (northern Greece)

Abstract The Eocene-Miocene Mesohellenic Trough is an elongate sediment-filled tectonic basin trending NW across central Greece and into Albania. Neotethyan oceanic rocks, including Triassic-Jurassic rift-related volcanic rocks and deep-sea sediments, accretionary mélange and ophiolitic complexes, crop out along its margins. These units were tectonically emplaced onto the Pelagonian microcontinent to the east and the Apulian-African continental margin to the west. In northern Greece, the mid-Jurassic Vourinos ophiolite on the eastern margin of the trough is geographically separated from the synchronous Pindos ophiolite along the western margin by a minimum c. 20 km distance. The sedimentary fill of the trough obscures their presumed subsurface continuation, although magnetic surveys identify thick ‘ophiolitic’ rocks beneath the basin. We interpret these ophiolites as parts of the same oceanic slab, two parts of a single larger oceanic complex we now term the Mesohellenic ophiolite. Comparable ophiolitic complexes to the south (the Koziakas and Othris) and the ophiolites of the Mirdita complex to the north in Albania are considered as members of this same complex. Geological and petrological data from the Vourinos and Pindos ophiolites define intra-slab heterogeneity. Vourinos essentially is a ‘Penrose-style’ ophiolite with ‘supra-subduction’ compositions; the less continuous Pindos ophiolite shows coexisting mid-ocean ridge basalt and island arc characteristics. Ophiolitic rocks that seem to represent geographical overlap between these characteristic associations crop out along their northern (Dotsikos strip) and southern (Mesovouni) margins. Variations in mantle strain conditions across the ophiolitic slab have been mapped, and demonstrate a single orientation of deformation; this is explained by variable strain kinematics that persisted across the ductile-brittle boundary. A continuity from ductile to brittle emplacement structures spans the Mesohellenic Trough, independent of petrological association, and indicates the original relative positions of these ophiolites within the oceanic slab. These structures illustrate tectonic ‘steps’ of obduction from the ridge crest onto the Pelagonian margin to the east, and can be relatively timed by the overlap of magmatism with ductile deformation in different parts of the slab. Hence, rotations of original horizontality are dated to the period preceding cessation of ductile field deformation, while still in the oceanic environment. The morphology defined by these structures and the horizontal rotation of stratigraphy are analogous to a spoon- or scoop-shaped nappe originating in the ductile field at its base, and crossing into the brittle field rapidly at its leading edge (Vourinos), whereas the mylonitic deformation characterizes the ‘trailing’ end (Pindos). Age relations require that geochemical variation between the two complexes must be explained within a model of synchronous generation, possibly with apparent ‘supra-subduction zone’ rifting of originally heterogeneous mantle, or an overlapping series of diverse processes of magma generation in an initially homogeneous mantle. Indications of the original ridge crest directions suggest the operation of several simultaneous spreading centres, separated by transform faults or ‘pseudofaults’. A palinspastic reconstruction of the slab constrains applicable oceanic models and provides the basis of future research.

Evidence for Neotethys rooted within the Vardar suture zone from the Voras Massif, northernmost Greece

Three conflicting models are currently proposed for the location and tectonic setting of the Eurasian continental margin and adjacent Tethys ocean in the Balkan region during Mesozoic–Early Tertiary time. Model 1 places the Eurasian margin within the Rhodope zone relatively close to the Moesian platform. A Tethyan oceanic basin was located to the south bordering a large “Serbo-Pelagonian” microcontinent. Model 2 correlates an integral “Serbo-Pelagonian” continental unit with the Eurasian margin and locates the Tethys further southwest. Model 3 envisages the Pelagonian zone and the Serbo-Macedonian zone as conjugate continental units separated by a Tethyan ocean that was sutured in Early Tertiary time to create the Vardar zone of northern Greece and former Yugoslavia. These published alternatives are tested in this paper based on a study of the tectono-stratigraphy of a completely exposed transect located in the Voras Mountains of northernmost Greece. The outcrop extends across the Vardar zone, from the Pelagonian zone in the west to the Serbo-Macedonian zone in the east. Within the Voras Massif, six east-dipping imbricate thrust sheets are recognised. Of these, Units 1–4 correlate with the regional Pelagonian zone in the west (and related Almopias sub-zone). By contrast, Units 5–6 show a contrasting tectono-stratigraphy and correlate with the Paikon Massif and the Serbo-Macedonian zone to the east. These units form a stack of thrust sheets, with Unit 1 at the base and Unit 6 at the top. Unstacking these thrust sheets places ophiolitic units between the Pelagonian zone and the Serbo-Macedonian zone, as in Model 3. Additional implications are, first, that the Paikon Massif cannot be seen as a window of Pelagonian basement, as in Model 1, and, secondly, Jurassic andesitic volcanics of the Paikon Massif locally preserve a gneissose continental basement, ruling out a recently suggested origin as an intra-oceanic arc. We envisage that the Almopias (Vardar) ocean rifted in Triassic time, followed by seafloor spreading. The Almopias ocean was consumed beneath the Serbo-Macedonian margin in Jurassic time, generating subduction-related arc volcanism in the Paikon Massif and related units. Ophiolites were emplaced onto the Pelagonian margin in the west and covered by Late Jurassic (pre-Kimmeridgian) conglomerates. Other ophiolitic rocks formed within the Vardar zone (Ano Garefi ophiolite, Unit 4) in latest Jurassic–Early Cretaceous time and were not deformed until Early Tertiary time. The Vardar zone finally sutured in the Early Tertiary creating the present imbricate thrust structure of the Voras Mountains.

Pre-tertiary blueschist facies metamorphism in the Hellenides evidenced by detrital blue amphiboles in the Paleocene flysch of the Othrys mountain

Analyses of detrital blue amphiboles from Paleocene flysch deposits of the western Othrys Mountain show a narrow compositional variability and are comparable with published analyses from the Cycladic Islands. In contrast, analyses from Pelagonian blueschists (Olympos, Ossa, Ambelakia) show a wide compositional variation that is rather due to the lithological variability of their parental rocks and incomplete overprinting by blueschist facies metamorphism. Combined with published evidence, the above features exclude the Pelagonian rocks from being the potential source of the detrital blue amphiboles. The latter were derived from a source with "Cycladic" affinities that (1) presently is not exposed, but may have been covered tectonically during the Tertiary and (2) has been already exposed to erosion in response to subduction at the Pindos oceanic realm at the active external Pelagonian margin during Maastrichtian/Paleocene times. The data demonstrate that blueschist facies metamorphism in the Hellenides started at Pre-Tertiary times.

Tectono-stratigraphy and evolution of the Mesozoic Pindos ophiolite and related units, northwestern Greece

The northwest Pindos Mountains expose Mesozoic and Tertiary thrust sheets, which include the Jurassic Pindos ophiolite, composed of ultramafic and mafic oceanic crustal rocks. The tectono-stratigraphy of these units is: (i) mainly ultramafic ophiolitic thrust sheet and basal metamorphic sole (Dramala Complex and Loumnitsa Unit); (ii) Late Cretaceous platformal limestones (Orliakas Group); (iii) dismembered intrusive and extrusive ophiolitic rocks (Aspropotamos Complex): (iv) tectonic melange and olistostromes, dominated by Triassic-Jurassic volcanic and sedimtntary rocks (Avdella Melange); (v) coherent thrust sheets of Late Jurassic–Late Cretaceous deep-water sediments (Dio Dendra Group); (vi) Early Tertiary flysch (Pindos Flysch). Immobile trace-element studies indicate that Triassic and Jurassic extrusives of the volcanic-sedimentary melange formed mainly at within plate and/or mid-ocean ridge settings. By contrast, structurally overlying ophiolitic extrusives include boninitic volcanics and island arc tholeiites, of supra-subduction zone origin. Initial ophiolite displacement ( c . 165 Ma) is recorded in metamorphic sole formation, including basal peridotite mylonite, amphibolites and greenschists. The sole rocks have MORB and WPB chemical affinities. In our favoured model, the Pindos ophiolite formed above a Mid-Jurassic westerly-dipping intra-oceanic subduction zone, accompanied by accretion to form melange, followed by collision and emplacement with the Pelagonian margin. Final suturing of the Pindos ocean, however did not take place until Early Tertiary times.