Rotational deformation in the Jurassic Mesohellenic ophiolites, …€¦ · tures that allow...

Lithos 108 (2009) 207–223

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Rotational deformation in the Jurassic Mesohellenic ophiolites, Greece, and itstectonic significance

Anne E. Rassios a, Yildirim Dilek b,⁎a Institute of Geology and Mineral Exploration, Lefkovrisi, 50100 Kozani, Greeceb Department of Geology, 116 Shideler Hall, Miami University, Oxford, Ohio 45056, USA

⁎ Corresponding author. Tel.: +1 513 529 2212; fax: +1E-mail addresses: [email protected] (A.E. Rassi

(Y. Dilek).

0024-4937/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.lithos.2008.09.005

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Article history:
The Jurassic Pindos and Vou Received 16 January 2008Accepted 4 September 2008Available online 24 September 2008
Keywords:Mesohellenic ophiolitesVourinos ophioliteSuprasubduction zone ophiolitesOphiolite emplacementHigh-T fabric in peridotitesRotational deformation

rinos ophiolites in the Western Hellenides of Greece are part of the Mesohellenicmafic-ultramafic slab underlying the Eocene–Miocene sedimentary basin (Mesohellenic Trough). Thetectonic incorporation of this oceanic slab into the western edge of the Pelagonian subcontinent occurred viatrench – 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 theinitial displacement of the Jurassic oceanic crust from its original igneous environment of formation, and itssubsequent tectonic emplacement eastward onto the Pelagonian margin. The heterogeneous deformation inthe mantle and crustal sequences of the Pindos–Vourinos ophiolites occurred in ductile, ductile-brittle, andbrittle fields synchronously, as the Jurassic oceanic lithosphere was translated eastward; it also resulted indifferential 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 movementdistances of at least kilometer scale. Pervasively mylonitic domains within the upper mantle peridotitessuggest 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 lowercrustal 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 geometricrelations imply that the Pindos–Vourinos ophiolites retain their relative orientations from the time ofcessation of ductile deformation within the ophiolitic slab. More than 90° anticlockwise horizontal rotationof the Vourinos ophiolite seems to have taken place during progressive simple shear deformation associatedwith the eastward emplacement of the Jurassic oceanic lithosphere. This kind of rotational deformationpostdating the igneous accretion of ancient oceanic crust may be common in many ophiolites and should becarefully distinguished from tectonic extension-related vertical rotational deformation developed duringtheir seafloor spreading evolution.

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1. Introduction

The strain during displacement and subsequent emplacement of anophiolitic slab onto a continental margin involves inhomogeneoussimple shear both within a N10-km-thick thrust nappe and in theunderlying tectonic basement. At the base of the emplacingoceanic slab,displacement is parallel to the shear plane marked by themetamorphicsole and an overlying zone of intensely mylonitic peridotite. Duringhorizontal compression associatedwith the displacement and emplace-mentof such anoceanic slab, anoriginally vertical section gets tilted androtated into anarc-shaped curve, concave towards the direction of shear.

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This rotational deformation may result in an overturn of the crustalsection in its upper portions (Twiss and Moores, 2007).

The recognition of such emplacement-related shear and deforma-tion patterns within ophiolites is difficult because different lithologi-cal units with varying rheology and physical properties accommodatedistributed shear quite differently in an internally heterogeneousophiolite pseudostratigraphy. It is also difficult to sort out emplace-ment-related deformation features from those associated with bothseafloor spreading-related extensional deformation and constrictionaldeformation in subduction-accretion settings. The paucity of struc-tures that allow precise measurement of primary horizontality (S0) inoceanic lithosphere is also problematic. Pillow lavas approximate butare rarely strictly parallel to horizontality. Sheeted dikes are assumedto intrude originally along vertical planes, although sheeted dikeintrusions with moderate dips have been observed in in-situ oceanic

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208 A.E. Rassios, Y. Dilek / Lithos 108 (2009) 207–223

crust (Karson, 1998). Inherent S0 mantle layering, depending onthe degree and nature of ductile lithospheric overprint, is presumedto dip away from rift axes (Nicolas, 1989a). For structures that do

Fig.1. Simplified geological map of northern Greece showing the distribution of theMesohellet al., 2006) depicts the crustal architecture of the Apulia-Pelagonia collision zone and the rowith their underlying mélange units). Key to lettering: Vourinos (V) and Pindos ophiolites (PAyios Stephanos locality (AS); Vrinena locality (VR); Koziakas Ophiolite (K).

approximate or can be used to reconstruct horizontality, rotationaldeformation resulting from imbrication during post-emplacementdeformation imposes further complications and often cannot be

enic ophiolite belt and other major tectonic zones. Cross-section (modified after Doutsosot zone of the Mesohellenic ophiolites (here Pindos to the west and Vourinos to the east); Koukourello–Pindos locality (KP); Mirdita ophiolite-Albania (M); Othris Ophiolite (O);

209A.E. Rassios, Y. Dilek / Lithos 108 (2009) 207–223

distinguished from rotations dating back to the main ophioliteemplacement period itself.

The Jurassic Vourinos ophiolite in the Mesohellenic ophiolite belt innorthwestern Greece (Fig. 1) provides an excellent opportunity to studyits internal architecture that is an artifact of deformation postdating itsigneous accretion. The Vourinos ophiolite is a typical Mediterranean-type suprasubduction zone ophiolite (Dilek, 2003), displaying seafloorspreading-generated extensional structures and crustal units, hetero-geneous mantle sequences, and underlying metamorphic sole andmélange units (Zimmerman 1968, 1972). It rests tectonically on thepassive margin and rift assemblages of the Pelagonian microcontinent(Moores 1969; Naylor and Harle 1976; Smith and Rassios 2003; Ghikaset al., 2007), and it is unconformably overlain by the Cenozoic clasticrocks of theMesohellenic Trough (Vamvaka et al., 2006). Earlier studiesby Rassios and Smith (2000) and Smith and Rassios (2003) haveevaluated variousmodels on the igneous environment of formation andtectonic emplacement of the Western Hellenide ophiolites. The recentpaper by Rassios and Moores (2006) has presented field-basedstructural observations pertaining to the crustal and upper mantleprocesses involved during the igneous accretion of the Pindos–Vourinosoceanic slab in an extensional environment and subsequently during itsobduction onto the continental margin.

TheVourinos ophiolite is rotatedN90° such that anE–Wcross-sectionalong its entire width presents an excellent geotraverse from its mantlesequence and the underlying metamorphic sole up to the upper crustalunits and the sedimentary cover. This rotation of the ophiolite, asoriginally documented by Moores (1969), poses a significant questionregarding its tectonic evolution and for ophiolite geology in general.

Fig. 2. Comparison of the pseudostratigraphy of a Penrose-type complete oceanic lithosphe2006). Reconstructed Vourinos pseudostratigraphy is analogous to the Penrose-type oceanicbeen highly deformed and tectonically imbricated. The Pindos Flysch underlies the ophiolit

Rassios (1981) and Beccaluva et al. (1984) described a monoclinaloverturnof theupper crustal section inVourinos thatwas relativelydatedback to the oceanic spreading period. However, our studies have shownthat the internal structure of Vourinos is, to a large extent, a result ofprogressive non-coaxial deformation following its magmatic evolution.

In this paper, we describe the internal architecture and the rotationaldeformation of Vourinos as an imprint of inhomogeneous shearassociatedwith the initial displacement of the ophiolite from its igneousenvironment of formation and subsequent emplacement onto thePelagonian continentalmargin.Wedocument the occurrence, geometryand kinematics of mesoscopic and macroscopic ductile to brittlestructures that developed during this progressive heterogeneousdeformation in the upper mantle and crustal sections of the Vourinosophiolite. This study has important implications for interpreting theinternal structural architecture of ophiolites in general. Our resultssuggest that differential rotational deformation within ophiolites is notalways associated with extensional tectonics during seafloor spreadingof ancient oceanic crust, and hence caution should be exercised whenstructural andpaleomagnetic studies are undertaken in those ophiolites,which appear to have experiencedwidespread heterogeneous deforma-tion and rotation.

2. Geology of the Mesohellenic ophiolites

The Vourinos and Pindos ophiolites are part of the Jurassic ophiolitebelt exposed along the margins of the Mesohellenic Trough crossingmuch of Greece and Albania (Fig.1; Smith,1993). The Vourinos ophioliteis analogous to the Penrose-type oceanic lithosphere (Fig. 2; Moores,

re with that of the Vourinos and Pindos ophiolites (modified after Rassios and Moores,lithosphere in terms of its inferred thickness and internal structure. Pindos ophiolite hase and its mélange (Avdella).


1969; Anonymous, 1972; Dilek, 2003) and includes, from bottom to thetop, upper mantle peridotites, 4 to 5 km thick ultramafic–maficcumulates, a sheeted dike complex, and a thin extrusive sequencecomposed mainly of pillow lavas (Jackson et al., 1975; Harkins et al.,1980; Vrahatis and Grivas, 1980; Rassios, 1981; Rassios et al., 1983a,b;Rassios and Moores, 2006). The approximate thickness of the Vourinossection is 12 km from its amphibolite sole to the overlying Calpionellidlimestones, making up its sedimentary cover (Mavrides et al., 1979). Thepresence in the sheeted dike complex and the dike/lava transition zoneof hydrothermal alteration zones, metalliferous jaspers, minor sulfidemineralization associated with narrow breccia pipes, and epidotizationof diabasic dikes (Rassios et al., 1983a,b) suggests that the Vourinosoceanic crust experienced seafloor spreading-related hydrothermalalteration during its igneous accretion, reminiscent of modern oceaniccrust (Humphris and Tivey, 2000; Kelley and Früh-Green, 2000).

By comparison, the Pindos ophiolite to thewest is distributed amongmulti-kilometer scale, petrologically overlapping nappe imbricates(Fig. 2; Rassios and Moores, 2006). The Pindos ophiolite is extremelythin, with its reconstructed thickness not more than 4 km from theamphibolite sole to the pillow lava–oceanic sediment interface (Jones,1990; Jones and Robertson,1991). Its cumulate section, from the level ofa transitional “Moho” to the base of the sheeted dike complex, is lessthana kilometer (Fig. 2; Kostopoulos,1989;Rassios,1991) and consists ofan upper plutonic suite including dunite, troctolite, and gabbrocumulates, and a lower plutonic suite composed of dunite, lherzolite,wehrlite, and pyroxenite (Kostopoulos, 1989). Sheeted dikes and pillowlavas, found in overlapping nappes, comprise a significant part of thisthickness; a reconstructed section is topped by at least a kilometer ofpillow lavas.

Despite their petrological and pseudostratigraphic differences, theVourinos and Pindos ophiolites have generally been grouped together asSSZ (supra-subduction zone) ophiolites (Dupuy et al., 1984; Robertson,2002; Pe-Piperet al., 2004; Saccani andPhotiades, 2004; Beccaluva et al.,2005; Garfunkel, 2006) as each includes dikes and lavas with island arctholeiitic (IAT) to boninitic affinities. The Vourinos dikes are, byabundance, overwhelmingly IAT in composition. Only about 20% of thediabasic dikes examined (Rassios, 1981) is clinopyroxene-bearing; mostof the boninitic rocks are distributed pervasively among aphyric IATdiabase dikes within the highest exposed levels of the sheeted dikecomplex. Several distinct exposures of boninitic sheeted dikes (each of ascale∼500m2) are locatedadjacent to theoutcrops offlaser gabbros andmagmatic mélange zones, which have been interpreted to haveexperienced oceanic transform faulting (Rassios, 1981), similar to theoccurrence of boninitic lavas and dikes in the Troodos ophiolite adjacentto the Arakapas fault zone (a fossil transform fault) in southern Cyprus(Cameron, 1985; Dilek et al., 1990).

Geochemical affinity of the Pindos displays a more complex pattern(Capedri et al.,1980;Kostopoulos,1989; Jones andRobertson,1991; Joneset al., 1991; Beccaluva et al., 2005). Themantle peridotites (mostly madeof harzburgite–lherzolite) and the olivine-rich upper plutonic suite withdunite, troctolite, and gabbro cumulate series have MORB affinity,whereas the lower plutonic suite composed of dunite, lherzolite,wehrlite, and pyroxenite series and most of the lavas of the Aspropo-tamos section (tectonically overridden by the mantle and cumulateimbricates, but including petrologically overlapping cumulate zones)have island arc tholeiite to boninitic affinities (Kostopoulos, 1989; Pe-Piper et al., 2004; Beccaluva et al., 2005). Isolated boninitic intrusions arealso encountered within the MORB-like cumulate section of Dramala(Rassios, 1991).

The Pindos and Vourinos ophiolites are likely connected as part of alarge mafic-ultramafic oceanic slab beneath the Mesohellenic Trough(Brunn, 1956; Rassios and Moores, 2006; Rassios and Smith, 2000;Rassios,1991). This is evident in anaeromagneticmap (Fig. 3) that showsthe presence of a massive positive magnetic anomaly corresponding toan ultramafic “keel” deep beneath the sedimentary strata of theMesohellenic Trough. Both the Pindos and Vourinos ophiolitic units, as

mappedat the surface, have their origin in this peridotite keel, andhencethey likely represent bidivergent ophiolitic thrust sheets derived fromthe sameophiolitic root zoneunderneath theMesohellenic Trough.BothPindos and Vourinos have similar crystallization ages (168.5–171 Ma;Liati et al., 2004), and near-contemporaneous metamorphic sole ages(∼162–167 Ma; Roddick et al., 1979; Spray and Roddick, 1980; Thuziatet al., 1981; Spray et al., 1984), supporting their genetic relationship.

3. Internal deformation in the Vourinos ophiolite

Detailed structural analyses within the Mesohellenic ophioliteswere produced during reconnaissance studies for chromite ores(Economou et al., 1986) and delineation of tectonic criteria forexploration drilling strategies of chrome ore districts over a 12-yearperiod (1984–1996). The results of these reconnaissance studies havebeen reported mostly in internal reports of the Institute of GeologyandMineral Exploration of Greece (IGME), and in some peer reviewedpapers (i.e. Rassios et al., 1986; Roberts et al., 1988; Rassios andKostopoulos, 1990; Rassios, 1991; Rassios et al., 1991; Rassios andKonstantopoulou, 1992; Konstantopoulou, 1992; Grivas et al., 1993;Rassios, 1994; Rassios and Grivas, 1999, 2001). Compilation of thismassive data resource, in synthesis with new research observationsand paradigms, continues to provide new insights for the structuralarchitecture of the Mesohellenic ophiolites, particularly for thosestructures associated with their displacement and emplacementkinematics. In this section, we document and discuss the nature ofheterogeneous deformation within the lower, ductile portions and inthe higher, crustal sections of the Vourinos and Pindos ophiolites.

3.1. Rheology and deformation of ophiolitic mantle rocks

An ophiolitic–oceanic slab should record strain in a heterogeneousfashion, as eachoceanic layer (Fig. 2a) differs in its degree of competencyand rheological properties. Competency varies within oceanic layers, aswell; a pillow lava outcrop with abundant hyaloclastic sedimentinterlayers is less competent than pillow lavas lacking significantsediments within pillow interstices. In many well-preserved ophiolitecomplexes around the world, the majority (∼80%) of a completepseudostratigraphy consists of variously serpentinized upper mantleperidotites (Coleman, 1977). Original mantle lithologies, essentially amixture of harzburgite or lherzolitewithminor dunite, provide a furthersource of heterogeneous rheology as demonstrated in rock deformationexperiments (Avé Lallemant and Carter, 1970; Nicolas et al., 1973; AvéLallemantet al.,1980;Chopra andPaterson,1981). Peridotite deformsviaplastic intracrystalline mechanisms (dislocation creep) that initiate attemperatures of about 1200 °C (referred to as “asthenospheric” or“diapiric” fabric) and continue in cooling conditions through about900 °C (Mercier andNicolas,1975; Nicolas and Poirier,1976;Watts et al.,1980; Calmant, 1987; Kohlstedt et al., 1995); it then deforms as a ductilematerial via intercrystalline mechanisms (diffusion creep) down toabout 700 °C, at which point it enters the brittle field (Calmant, 1987).Peridotites beneath the rift axis at a typical seafloor-spreading centerexperience high-enough temperatures to record ductile deformation.Mantle layering (Nicolas,1989a) is considered as themost primitive sub-ridgecrest ductile structure recorded and preserved in ophiolitic mantlesections, and is generally utilized as an “S0” fabric for tectonic analysis ofmantle peridotites in ophiolite complexes. As the newly formed oceaniclithosphere is translated away from the spreading center, it coolsdownward and the ductile-brittle transition is depressed to successivelygreater depths (Harper, 1985). The first and quickest parts of the slab tocool are the shallowest sections; thus, the highest temperature (nearmagmatic condition) fabricswithin a preserved ophiolite are commonlylocated near the petrological Moho. In many cases, these “frozen-in”fabrics record shallow mantle processes occurring synchronously withridgecrest activities, such as deformation of magmatically growingchromite ores (Christensen and Roberts, 1986), and are affected to a


lesser degree by continuing ductile deformation still occurring at deeperlevels in the slab.

Rassios and Smith (2000)modeled the time span and distance overwhich an off-axis and passively spreading oceanic crust is capable ofundergoing ductile deformation. With greater depth, the slab remainsin ductile conditions for a longer time span, and higher-temperaturedeformation is gradationally overprinted by cooler but still ductiledeformation in the off-axis environment. Thus, “S0” structures such asmantle layering and lithological contacts are deformed in a grada-tionally evolving tectonic environment. For an ophiolite of thethickness of Vourinos, ductile deformation would be recorded for a

Fig. 3. Aeromagnetic anomalymap showing the distribution of a high-magnetic slab beneathSkianis, 1991). The surface outline of the Pindos, Vourinos and Koziakas ophiolites is shown ithe continuity of the mafic-ultramafic rocks at depth beneath the Mesohellenic sedimentar

period of about four million years in succeedingly deeper levels(Rassios and Smith, 2000), at which point it crosses entirely into thebrittle field. The near-contemporaneous crystallization and meta-morphic sole dates of the Vourinos and Pindos ophiolites, that is,within ∼4 million years, essentially forces us to consider that theductile imprint on the spreading slab coincides with ductile-fieldemplacement motions. Thus, plastic mantle fabrics preserve aspectrum of deformation dating from the asthenospheric diapiricflow beneath the ridgecrest into homogeneous, spreading-relatedmantle flow away from the ridgecrest and displacement-emplace-ment processes.

the Cenozoic sedimentary units of the Mesohellenic Trough (modified fromMemou andn green. High-magnetic anomaly between the Pindos and Vourinos ophiolites indicatesy overburden.


Within even relatively simple mantle peridotite suites such as inVourinos, the difference in competency is such that chromite-richlithologies (chromitite or olivine chromitite) are the most competentat all temperatures, and have been suggested to provide the “breakingpoint” for initial brittle behavior (Grivas et al., 1993). Massiveharzburgite is more competent than pure dunite (Nicolas et al.,1980; Moat, 1986; Roberts et al., 1988). Once ductile deformation hasinitiated, producing inter-crystalline and essentially mylonitic formsof strain, early fine-grained neoblastic-crushed zones preferentiallyaccommodate later strain via dynamic recrystallization (Nicolas,1989a). When subsequent serpentinization of peridotite occurs inbrittle field conditions, these differences in competency are further

Fig. 4. Outcrop photos of representative shear fabric elements in various upper mantle pertectonic foliation in dunite near Skoumtsa, Vourinos ophiolite. Foliation defined by the eserpentinized wehrlite, Kissavos (Pilori), Vourinos. Width of the image is about 1 cm. c. Mylwithin the mylonite. Temperature of formation of this mylonitic zone is estimated as ∼900Shape of the fold is accentuated by dashed line. Vergence is to the NE. e. Strong high-temperaoceanic period ductile-brittle sinistral fault cutting pyroxenite dike swarms and offsettingphase ramp shears imprinting “diapiric fabric” in harzburgite at Doumaraki, Vourinos Ophi

accentuated, as, for example, between easily deforming totallyserpentinized dunite and partially serpentinized harzburgite. Byviewing deformation within mantle peridotites as an example of theaccommodation of continuous strain associated with coaxial defor-mation in a heterogeneous medium, kinematics of deformation anddirection of tectonic translation can be deciphered (Ceuleneer andNicolas, 1985; Holtzman, 2000).

During this ductile interval, shallow-level peridotite above theductile-brittle transition zone deforms in a brittle mode synchro-nously with ductile deformation at depth. Thus, peridotites at bothshallow and deeper structural levels record the same orientation ofshear strain but by different mechanisms. As the oceanic lithosphere

idotites in the Vourinos ophiolite (see text for further explanations). a. Chrome-spinellongated spinel (black) grains is parallel to the scale bar. b. Mylonitic shear fabric inonite zone in harzburgite, near Voidolakkos, with red arrows indicating the shear sense°C (Ross et al., 1980). d. “Z-Fold” imprinting chrome ore layers at Xerolivado, Vourinos.ture chromite lineation on chrome ore layer surface at Frourio, Vourinos Ophiolite. f. Latea thin gabbro vein, Aliakmon River below Mikroklisoura, Vourinos ophiolite. g. Brittle-olite.


becomes older and cools further, the ductile-brittle boundary getspushed down and brittle structures overprint older ductile structures(Harper, 1985); however, both ductile and brittle structures record asimilar orientation of shear strain and kinematic direction.

The constrictional fabric within an ophiolitic mantle sequenceprovides an abundance of structures that show displacement directionswithin the slab. Intra-slab imbrication, ramps and brittle shearingphenomena are pervasive to all structural levels in the slab anddemonstrate “over-riding” emplacement directions. The deformationof earlier formed high-temperature peridotite structures by later stagelower-temperature but still ductile shearing creates folds, which occuron a range of scale from the size of an entire ophiolite massif (atVourinos, by the bowing of structures parallel to the sole zone describedin the following sections), to the kilometer-scale synformal shape of theXerilivado dunite body delineated by its petrologic contact, and tosheath folds tens of meters in scale defined by form lines within thissynform. Fold vergence of these and evenmore minor parasitic folds, aswell as ductile shear features such as “δ” and “σ” structures, shearboudins, and tectonic “fish” structures are widespread to demonstrateinternal vorticity (Passchier and Trouw,1996; Twiss and Moores, 2007).Lineations showing preferred alignment (as dragged, or over-ridingtrains of chrome spinel, for example) can be observed directly, ormeasured as “stretching” lineations coinciding geometrically with foldaxes of verging fold systems, used for interpreting the sense of shearingand the direction of tectonic transport.

Fig. 5. Composite structural cross section of theVourinos ophiolite. TheKrapaMonoclinewith andepict various intra-oceanic structural elements in the ophiolite (structural data are taken fromR(upper right) shows the generalmap of Vourinos withmajor ophiolitic subunits, and localitiesmV = Voidolakkos; K = Kissavos: P = Pilori Section; D = Doumaraki; F = Frourio; AK = Aga Kouri;

Some examples of these structures have been described previously(Rassios and Smith, 2000; Rassios and Moores, 2006) and severalmore are represented in Fig. 4. Fig. 4a shows a typical spinel foliation(vertical) in dunite: most spinel grains are aligned in trains and someindividual grains display asymmetrical “heads” and “tails.” The largestgrain (upper right corner) shows minor shortening, overriding alongthe foliation trace toward the top of the photo. Fig. 4b and c showmylonitic fabrics that demonstrate E–NE-directed sense of shearing.Fig. 4b is from a mylonitic band several cm thick and traced over tenmeters within lower cumulates at Vourinos, and Fig. 4c is a mylonitezone from Voidolakkos (Ross et al., 1980; Grivas et al., 1993) includingover-riding ductile imbricates.

Observing folds directly within mantle peridotites is difficult sincethe host petrology is uniform in appearance. However, as shown inFig. 4d, moderate-scale folds can be seen within chrome-ore bearingareas, and can be useful in indicating local kinematics (here verging tothe northeast in the direction of ophiolite emplacement). Elsewhere,folding is delineated by careful studies of mineral foliation (for example,Ayrton, 1968; Rassios et al., 1994). Mineral lineations, where observedwithin massive lithologies (spinel lineations in dunite or chrome ore,spinel and pyroxene lineations in harzburgite) correspond to thegeometrically determined position of fold axes defined by foliations.Some brittle structures, such as those depicted in Fig. 4e, can becircumscribed to specific tectonic environments based on whether ornot they continue into younger Cretaceous to recent formations, and

overturned crustal section is shownat theWSWextremeof the section. Contour diagramsassios,1981; Rassios et al.,1991;Mavrides andKelepertzis,1994, and this study). Insetmapentioned in text. Cross section line (a–b–c) is also shown. Key to lettering: X = Xerolivado;

MD = Mikroklisoura pyroxenite dike swarm; KRAPA = Krapa Hills area.


whether or not they reconcile with the regional direction of emplace-ment. Brittle ramp shears are commonly oriented in the generaldirection of emplacement override. In Fig. 4g, these brittle shears areimprinted on peridotites with a high-temperature “diapiric” fabric thatare lacking those structures originated from intervening temperatureconditions. This observation demonstrates the need for envisioning amechanism for displacing mantle rocks from high-temperature ductileconditions into brittlefield conditionswithout undergoing intermediatedeformation.

3.2. Rotational deformation in the crustal sequence

The crustal structure and the upper contact of Vourinos are wellexposed within the Krapa Hill locality in the northwestern part of theophiolite (Fig. 5; some lithological characteristics of the uppermostsection are shown in Fig. 6). Layering in the ultramafic to gabbronoriticanddioritic cumulates is nearly verticalwith an “up-section”direction tothe west. Cumulate layers are rotated into slightly overturned (to theeast) orientations near the contact with the overlying sheeted dikecomplex. Diabasic dikes (Fig. 6a) first occur in a transition zonemade ofmixed dike-cumulate screen zones in the east, and then grade intosheeted dikes toward the west over an interval of about 0.5 km. Thesesubhorizontal sheeted dikes run parallel to the strike of cumulatelayering, but dip perpendicular to it. The uppermost volcanicmember ofthe ophiolite includes a minor screen (b5 m in outcrop) of small pillowlavas within mixed dikes and flows and hydrothermal jaspers

Fig. 6. Various lithologic units within the uppermost part of and overlying the Vourinos opc. Conglomerate at the base of the Cretaceous limestone including clasts of diabase, radiolarophiolite. e. Pre-Cretaceous folds in a Calpionellid limestone.

(exhalative sediments) in the uppermost 200 m of the section.Metamorphic zoning, typical of crustal units in other ophiolites(Coleman, 1977) is consistent with the observed up-section trend suchthat amphibolite-facies mineral assemblages occur at deeper structurallevels mainly within the mafic cumulates. Greenschist-facies assem-blages are encountered at the level of dike-screen units and gradeupward into a zeolite-facieswithin theuppermost sheeteddike complexand in the lowermost extrusive sequence.

Part of the upper contact of the ophiolite is overlain by a JurassicCalpionellid-bearing limestone (Fig. 6b; Brunn, 1956; Moores, 1969;Mavrides et al., 1979; Mavrides, 1980), as also found within parts of thePindos ophiolite (Jones, 1990; Jones and Robertson, 1991). Red “ribbon”chert-limestone units also appear within this unit at Vourinos. Thesesedimentary rocks are interpreted as the “Layer 1” member of theVourinos oceanic crust. At their thickest (northward) outcrop, theselimestones appear to have been deposited directly onto an erodedsurface of greenschist-facies dikes and lava flows. The thickness of thissedimentary unit decreases to the south beneath the Cretaceousunconformity (Fig. 6c, d), and the southernmost outcrops are seen torest on higher igneous stratigraphic levels, containing zeolite-faciesdikes and lava flows. Using the horizon of the zeolite-facies meta-morphic zone as a rough indication of the original stratigraphy, weestimate that at least 250 m of volcanic rocks were eroded prior toCalpionellid limestone deposition.

Beddingwithin the Calpionellid limestone is oriented∼N–NWwithdips (∼85°) to the east. The best estimation of original horizontality

hiolite. a. Sheeted dikes. b. Belemnites in Calpionellid-bearing red oceanic limestone.ian chert, and Jurassic Calpionellid limestone. d. Reefal Cretaceous limestone above the


within the upper parts of the volcanic unit of the ophiolite (lava flows,metalliferous sediment horizons) shows that the uppermost volcanicrocks were rotated ∼10°–12° in a counter-clockwise sense about ahorizontal axis before the deposition of the overlying Calpionellid unit.In addition, the Calpionellid unit exposes sub-meter scale folds vergingto the east (Fig. 6e); this deformation fabric is not seen in the overlyingCretaceous carbonates. Timing of this folding is thus synchronouswiththe ophiolite emplacement in the late Jurassic.

Layering in the Cretaceous limestone defines a 40°–60° counter-clockwise rotation about a N–NW horizontal axis, apparently acontinuation of the rotation observed in the Jurassic sediments. It isnot possible to date the timing of formation of the unconformity at thebase of the Cretaceous more accurately than to say that it wouldappear to be coeval with the late emplacement period.

A section through the entire Vourinos ophiolite and its sedimen-tary cover, from the amphibolite sole at the bottom to the east into theCretaceous limestone on top to the west (Figs. 5 and 7) demonstratethe structural continuity and allows documentation of a gradualsteepening of “S0” fabrics. Above the basal sole, these fabrics dipmoderately to the west (Fig. 5), rotate towards verticality around theposition of the petrological Moho, and then overturn within thecumulate section to dip about 60° to the east. To attain this flexingstructural pattern, it is not necessary to invoke any other mode ofdeformation other than inhomogeneous shear (Fig. 7) consistent witha deforming nappe sheet. The “topping” direction would imply thisshear to be generated by an eastward motion along the base of theophiolite, with a westward lag at the top surface and creating an

Fig. 7. Cartoon demonstrating the flexure of an idealized oceanic lithosphere via simple inhomseen in the KrapaMonocline. The modern oceanic lithosphere is presumed to include a heterridgecrest. The attitude of mantle layering in the modern lithosphere implies that the ridgeupper levels of the “flexed” ocean lithosphere are similar in orientation to the structures obsewithin the composite structural section of Vourinos (Fig. 5) delineates a pattern parallel to

apparent monoclinal fold of the Krapa Hills. At the time of thedeposition of the Cretaceous limestone, the ophiolitewas oriented as amafic-ultramafic slab with a vertical pole plunging about 30–40°eastward, and “original horizontality” dipping to the west. The furtherrotation of the obducted ophiolite was probably due to post-Cretaceous subsidence and shortening of the Mesohellenic Trough.

Occurring in the center of the Krapa Hills monocline is an exposureof pegmatitic pyroxenite dike swarms between the upper mantleperidotites and the highest cumulate gabbros (Fig. 8; Rassios, 1981;Rassios et al., 1983a). This dike swarm crops out along the AliakmonRiver section of the Krapa Hills as a ∼250 m thick exposure (Rassioset al., 1983b). Pyroxenite dikes of 20 cm to 2.5 m thickness (Fig. 8a–c)constitute as much as 75% of the exposed outcrop made ofserpentinized dunite, which is the host rock of the dike intrusions.The dikes are composed of websterite and clinopyroxenite that showatypical pegmatitic fabric where the largest pyroxenes, up to meterscale, grew perpendicular to the dike walls; smaller pyroxene crystals,1 to 20 cm long, occur with more random orientation in the centralparts of the dikes (Fig. 8d–e). Pyroxenite dikes with this pegmatiticfabric run N–NW with steep dips to the E. In general, these dikes aretabular in appearance and do not display any evidence of ductiledeformation as in the underlying mantle peridotites. They crosscutolder ductile structures in the peridotites and are deformed by latebrittle structures such as minor pre-Tertiary strike-slip faults andneotectonic features. The exception to this structural “rule” seems tobe the deformation of some pyroxenite dikes within ductile mylonitezones such as at Voidolakkos (see inset map in Fig. 5 for location; Ross

ogeneous shear and its resemblance to the composite structural section of Vourinos asogeneous competency due to its diverse lithology and temperature gradient beneath thecrest lies to the left relative to this representation. Deformation style and geometry inrved in the KrapaMonocline. A continuity in the observed direction toward “up section”that of the flexured idealized section.

Fig. 8. Pegmatitic pyroxenite dike swarms in Vourinos along the Aliakmon River. a. Intersecting pyroxenite pegmatite dikes surrounding a “Λ” shape dunite inclusion in the dikeswarm. b. Single pyroxenite dike, approximately 2 mwide and 200 m in length (surface exposure) continuing across the Aliakmon River. c. Single, relatively narrow pyroxenite dikewithin serpentinized dunite. Even in this smaller dike, pyroxene crystals are blocky and several cm in size. The dunite in contact with dikes shows a blackened serpentine envelope.d. Single orthopyroxene crystal above hammer. Optically continuous pyroxene crystals range in size from about a cm to nearly a meter. While both ortho- and clinopyroxenes havebeen observed (Rassios, 1981), their size range precludes statistical analyses of modal proportions and chemical composition. e. Typical pegmatitic fabric in pyroxenite dikes. Hand isset on dark serpentinized dunite margin. Alongmargin, large pyroxene crystals grow perpendicular to the dike contact, while smaller andmore random orientations occur within thecentral portions of the dike.


et al., 1980; Grivas et al., 1993) and within mylonitic harzburgiteimmediately above the basal sole. The relative timing of the intrusionof these dikes can hence be constrained as sometime between on-axiscrustal construction of the Jurassic oceanic crust and late stage ductiledeformation associated with the initial displacement of the ophiolite

Fig. 9. Examples of structures within the Mesohellenic ophiolites depicting intra-slab rotatioalong the “bowed” basal thrust zone of Vourinos (by Wright, 1986). Green lines outline mtemperature form lines of orthopyroxene and spinel mineral foliations in red and traces of masedimentary cover. Nearly E–W-trending strike-slip faults and shear zones (locally dashed) aras the ophiolitic units. b. Synformal-antiformal composite structure of the Dramala imbricapositionwith bowed antiforms delineated inmineral form lines in the Dramala peridotite, anthe peridotite. Elongate arrows in the peridotite in the diagram at left indicate differential m7 km. c. Bowing of structures at Ayios Stephanos (Othris) synchronous to “hot” nappe emplfabric at the Aga Kouri ore district. Mylonitic and ductile-shear zones transport a “diapiric”sense kinematics around this core zone is shown by folding along ductile margins, the bowKoukourelo “Ductile Highway” of the Pindos ophiolite nappe. A zone of mylonitic-cataclassedimentary units. The fabric strikes NE, and a parallel magnetic lineament possibly is cont

from its igneous tectonic setting. Therefore, these pegmatitic pyroxenitedikes may have formed as a result of precipitation of late-stage fluidswithin shear fractures that opened up during inhomogeneous shearingassociated with the eastward displacement of the Vourinos lithospherein a suprasubduction zone environment in intraoceanic conditions.

n. a. Upper map (a1) shows the emplacement movement directions (arrows) observedajor imbricate structures associated with the ophiolite. Lower map (a2) shows high-gnetic lineaments in black. Magnetite-rich serpentinized zones show upwell under theemid-late Tertiary brittle structures, crosscutting the younger sedimentary rocks as wellte group of the Pindos nappe. Synforms in the overridden Avdella Mélange coincide ind antiforms in themélange occur below “tear” systems delineated by bowed synforms inovement within the nappe. The diagram at right is a cross section; scale on the order ofacement (see text for explanation). d. “Ductile Rafting” of high-temperature peridotitefabric terrane towards the east and into a deeper part of the mantle section. The sheared shape of the eastern major ore body, and smaller scale kinematic indicators. e. Thetic fabric peridotite extends to the northeast where it is covered by the Mesohellenicinuous with the southern emplacement margin of Vourinos.

Original_text: -


3.3. Rotational deformation as evidenced by the mantle suite

Structural observations presented here apply to all mantle rocksstudied within the Mesohellenic ophiolites, with an emphasis onVourinos due to its relatively intact section. A common method ofstudy ofmantle structures is to draw “form lines,” continuations of high-temperature foliation measurements, across the peridotite suite (Rosset al., 1980; Frison, 1987; Rassios and Moores, 2006). On a broad scale,major ophiolitic imbricates display “bowed” structural distributions ofmantle form lines and outcrop patterns. Vourinos itself provides thestrongest example of this phenomenon: the basal sole, over 40 km alongoutcrop, is arcuate in shape (Fig. 9a, uppermap), and the general trend ofhigh-temperature form lines in theoverlyingmantle sectionparallels thesole (Fig. 9a, lower map), as pointed out by Wright (1986), Grivas et al.(1993), Rassios (1994), and Ghikas et al. (2007). The chrome-ore richmetalliferous zone (Grivas et al., 1993) within the mantle suite, about5 km structurally higher than thebasal sole, spans the length of Vourinosand also parallels this arcuate synformal geometry of the ophiolite.

A bowed shape is also shown by form lines in the ∼20 km2

Dramala imbricate of the Pindos ophiolite above an amphibolite sole(Rassios, 1991) where the form lines delineate an antiformal structureverging to the NE in the peridotite massif. Based on the morphology offolds in the sedimentary units of the Avdella Mélange underlying theophiolite, Jones (1990) deduced a synformal structure for the identicalarea. Fig. 9b is a cartoon showing the coexistence of these structures,antiforms in the Dramala peridotite above the sole and a synform inthe Avdella Mélange immediately below the ophiolitic nappe.

Smaller scale imbricateswithin the Othris ophiolite to the south alsoshow bowing structures of form lines such at those in the area of theAyios Stephanos Chrome Mine (Fig. 9c). This area displays boweddeformation of orthopyroxene form lines within a plagioclase lherzolitenappe overriding a harzburgite nappe. Plastic fabric elements (mainlymylonitic) are observed around andparallel to the contact zonebetweenthe nappes, suggesting that still “hot”, plastic conditions existed withinthe peridotite at the time of peridotite nappe imbrication. The thrustitself apparently was accommodated along a chrome-bearing dunitebody that was extensively attenuated during this deformation (verylittle remnant dunite remaining in outcrop), concentrating chrome oresinto massive boudins and facilitating a weak zone for intrusion ofgabbroic dikes. In this case, the bowing phenomenon and nappeformation can be relatively dated as an off-axis deformational event inintraoceanic conditions.

Such “bowing,” including the coexistence of synformal-antiformalcomposites, is characteristic of many thrust nappes, and is by nomeansunique to emplaced oceanic lithosphere. Rock masses within suchnappes are rotated in semi-circular fashion around apparent verticalaxes. “Bowing” of primary structures within oceanic slabs, such as inVourinos and in some areas of the Pindos and Othris described above, isa mode of vertical rotation affecting the internal parts of ophiolitic slabsundergoing thrusting.

3.3.1. Rafted diapiric fabric domainsSeveral research groups identified diapiric fabrics in the mantle

section of the Mesohellenic ophiolites (Ross et al., 1980; Frison, 1987;Nicolas, 1989b; Rassios and Moores, 2006). Small areas dominated byblocky-shaped pyroxene fabrics, interpreted (Nicolas, 1989a) as havinginitiated beneath a seafloor spreading rift axis, lack overprinting byyounger ductile structures (i.e. elongated pyroxene fabric or significantintergranular deformation fabrics). These domains are locally cut bybrittle emplacement features such as thrust ramps and shear zonesshown in Fig. 4. They range up to several km2 in sizewithin the Vourinosmantle section and down to about 0.5 km2 in the Pindos. How couldthese rocks be translated from conditions such as those described byRoss et al. (1980) at ∼120 km depth and temperatures of ∼1200 °C toshallow crustal conditions without having acquired deformationstructures expected to develop at intermediate depths?

Diapiric fabrics and planar mylonite zones were first documented inophiolites within the Voidolakkos Chrome Mining District (Ross et al.,1980). Grivas et al. (1993) demonstrated that thesemylonite zones, eachonmeter-scale thickness and traceable for distances of several hundredsof meters, serve as ductile ramp structures that cause dislocation (over-riding to the NE) of dunite–harzburgite lithological contacts. Dunitebodies (including chrome ore deposits) are deformed within thesemylonitic ramps at temperatures of ∼900 °C (Ross et al., 1980).Harzburgites hosting these mylonite zones show no evidence of thislower temperature ductile imprint. Mineral foliations show them to bepassively folded. Rassios et al. (1994) describe “tongue-shaped” folds atVoidolakkos analogous in geometry to ductile condition imbricationphenomena.

In the nearby Aga Kouri district (see inset map in Fig. 5 for location),we observe a kilometer-scale area showing a “high temperature” core,that is, a diapiric fabric zone, retaining undeformed chrome spinels. Thehigh-temperature core zone has been translated to the NE by lateralductile ramps with rotation of its internal fabric (Fig. 9d): these rampsare accompanied by folding of lithological contacts between the duniteand harzburgite, and by the formation of mineral stretching due tolower-temperature ductile deformation. In the case of Aga Kouri, weestimate aminimumtranslationdistanceof this hot core zoneofonekm,that is, from the nearest diapiric terrain following the lateral rampingstructures towards the west. The translation of early high-T diapiricfabric areas by lower-temperature ductile ramp structures can explainthe lack of intermediate deformation within the core zone area. Thetranslation of these core zones shows additional rotation due todifferential lateral shear that gives the appearance of vertical rotationwithin small areas inside an ophiolitic slab.

3.3.2. Mylonitic shear zones as ‘ductile highways’Rassios (1991) described mylonitic domains cropping out over

areas 0.2–1 km-wide and up to 10 km-long within the Pindos mantleunit (Fig. 8d); similar mylonitic rocks occur even in larger areas withinwestern Othris. These domains consist of peridotites that exhibitmineral fabrics that range from mylonitic in appearance (that is,formed at ∼900 °C–700 °C), to deformation fabrics inwhich individualmineral grains of pyroxene are elongated to strain ratios of 5–10:1.These rocks appear in microscopic examination as long ribbons ofhighly strained orthopyroxene haloed by pyroxene neoblasts in amatrix of primarily olivine neoblasts. The distinctive appearance ofthese fabrics allows us to map them as described in Rassios and Smith(2000). The conditions of formation of these highly elongated mineralfabric domains may reach temperatures nearing the ductile-brittleboundary around 700 °C. We have traced individual mylonitic fabricdomains in the Pindos for distances up to 3 km (Rassios, 1991; RassiosandMoores, 2006) and for 10 km in the Kedron area of Othris (Rassios,1994) before they are occluded by younger sedimentary rocks ordisplaced by younger imbrication structures.

Mylonitic peridotite zones on the order of fifty to a few-hundredmeters in thickness are common features of ophiolites immediatelyabove theirmetamorphic soles (Nicolas,1989a), as in thebasalmylonitesdocumented above the Dramala (Pindos) and the Vourinos soles(Rassios, 1991; Rassios et al., 1994). However, within the Dramalanappe pervasive mylonitic fabric zones extend from the sole upwardinto higher parts of the mantle section. Their outcrop pattern does notalways agree with the orientation of mineral foliation form lines asshown in Rassios and Moores (2006), who hypothesized that volatile-rich zones within the mantle rocks could locally affect mechanisms ofperidotite deformation. These volatiles would promote a more pro-nounced elongation fabric (as if at cooler temperatures), while regionaltemperature-pressure conditions are uniform. In general, pervasivemylonitic zones inDramala trendN70E, although several appear to crosstowards the NW, perhaps outlining incipient ductile thrust patterns.

South of the Dramala imbricate, several contiguous areas of thePindos mantle section contain similar pervasive mylonitic fabric


domains as described above. In the Koukourello area (Fig. 9e), thesemylonitic fabrics grade into complete cataclastites resulting in a fine-grained rock comprised entirely of neoblasts. This zone trends about050° with awidth of 1 km and is exposed over a distance of 8 km. Bothextremities are occluded by imbrication and younger sedimentarycover, and hence the entire length of this feature cannot bedetermined. Using elongation of pyroxene grains (averaging about8:1) as way to make a rough estimate, we determine the original pre-deformed length of the preserved section on the order of a kilometer orless. Zones of this nature clearly translate material within an oceanicslab undergoing ductile to brittle deformation for significant distances.

We interpret these zones of pervasive late-ductilemylonitic fabric asbroad lateral ramp phenomena, informally referred to as ‘ductilehighways’. These shear zones likely played a major role in lateraltransport of the mantle section during the initial displacement of theJurassic oceanic crust. As such, they have contributed to the rotation ofthe mantle section while still in ductile conditions.

Fig.10. Cartoonmodel suggesting the effect of ductile deformation at depth in the oceanic lithan idealized complete oceanic lithospherewith frozen igneous (pillows, dikes, cumulates) andductile field, with higher temperatures toward the base. a. “Bowing” of structures in displacedshear zone in displaced oceanic lithosphere. See text for further discussion.

4. Tectonic implications

Accretion of ophiolites into continental margins is a first-ordertectonic phenomenon causing significant internal deformation withinthe ancient oceanic crust. The mode and nature of this internaldeformation have been rarely documented in the literature, andgenerally are only considered as secondary in emphasis to primaryseafloor spreading processes responsible for the igneous accretion ofoceanic crust.

We constructed a composite cartoon (Fig. 10) to show the potentialeffect of displacement-emplacement related shear deformationwithin the mantle peridotites in higher levels of an oceanic slab, andin the distribution of magnetic anomalies on the seafloor as recordedby the striped patterns. Detached over a basal thrust, internal nappedeformation via formation of a “bowing” structure should result indeformation of themagnetic-stripes in a similar fashion and geometry(Fig.10a). Ductile rafting as recognized at Aga Kouri requires that high-

osphere-ophiolitic slab on surface pattern of magnetic stripes. At left, a block diagram ofhigh temperaturemantle structures. Lower in themantle section, peridotite remains inoceanic lithosphere. b. Ductile rafting as recognized at Aga Kouri. c. A “Ductile Highway”


temperature asthenospheric fabric is translated down-section, thusresulting in this “fossil diapiric” fabric surrounded by lower tempera-ture ductile deformation structures (Fig. 10b). In this cartoon, the slabhas been rotated to a position to allow such movement via mylonitezones as in Aga Kouri. This inferred rotation agrees in a broad sensewith the pre-Cretaceous orientation of the Vourinos slab. A “DuctileHighway”, a pervasively mylonitic-cataclastic shear zone, may deflectmagnetic stripes on the seafloor to a transform fault-looking pattern(Fig. 10c). Distinguishing between an ancient oceanic transform faultand a displacement-emplacement related ductile shear zonewould bedifficult if the two systems are parallel in orientation.

Our study of the internal structural fabric of the Mesohellenicophiolites, particularly Vourinos, suggests that differential rotationaldeformation as recorded in their crustal and mantle rocks is not alwaysrelated to extensional or transform fault tectonics during seafloorspreading of ancient oceanic crust. The time gap between the igneousconstruction of oceanic crust and its emplacement into a continentalmargin appears to be less than 10 million years in the case of manyTethyan ophiolites (Dilek and Moores, 1990; Robertson et al., 1991;Hacker and Gnos, 1997; Dilek et al., 1999; Robertson 2004; Dilek et al.,2005;Warrenet al., 2005; Çelik et al., 2006;Dileket al., 2008), andhenceage constraints on the timing of formation of some of the brittle andductile structures in these ophiolites are not reliable unless they arewelldocumented by the occurrence of carefully dated crosscutting dikes andplutons. Some of the well documented rotations of normal fault blocksaround ridge-parallel, subhorizontal axes or vertical axis rotations in thevicinity of fossil transform faults in the Troodos ophiolite (Morris, 1996,and references therein) are likely to hold true; however, in some othercases the cause of horizontal and vertical rotations in some ophioliteshave been potentially misinterpreted within the context of paleomag-netic studies, resulting in incorrect regional scale tectonic reconstruc-tions (Morris et al., 2002). Therefore, caution should be exercised whenstructural andpaleomagnetic studies are undertaken in thoseophiolites,which appear to have experiencedwidespread heterogeneous deforma-tion and rotation during their original displacement and subsequenttectonic incorporation into continental margins.

Acknowledgments

Rassios gratefully acknowledges the fruitful discussions with andhelp during fieldwork by Litsa Konstantopoulou, Sifis Vacondios, EliasGrivas, and Dina Ghikas. We have benefited from our discussions onthe regional geology of the Western Hellenide ophiolites with A.G.Smith over the years. Dilek's fieldwork in the Vourinos ophiolite hasbeen supported by research grants from IGME (Greece) and MiamiUniversity (USA) that are gratefully acknowledged. Constructive andthorough reviews by Dimitrios Kostopoulos and an anonymousreferee were helpful to improve the paper. We thank Guest EditorA.H.F. Robertson for his encouragement to prepare this paper for theLithos Special Issue and for his editorial handling.

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