Thematic Article Mesozoic to Tertiary tectonic history of ... · common mechanisms for ophiolite emplacement is obduction (i.e. the thrusting of internally unde- ... undeformed nappes

The Island Arc (2005) 14, 471–493

Blackwell Science, LtdOxford, UKIARThe Island Arc1038-48712005 Blackwell Publishing Asia Pty LtdDecember 2005144471493Thematic ArticleTectonics of Albania ophiolitesV. Bortolotti

et al.

*Correspondence.

Received 7 March 2005; accepted for publication 30 August 2005.© 2005 Blackwell Publishing Asia Pty Ltd

Thematic ArticleMesozoic to Tertiary tectonic history of the Mirdita ophiolites,

northern Albania

VALERIO BORTOLOTTI,1 MICHELE MARRONI,1,2,* LUCA PANDOLFI1,2 AND GIANFRANCO PRINCIPI1,3

2Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria, 53-56126 Pisa, Italy (email: [email protected]), 1Istituto di Geoscienze e Georisorse, CNR, Italy and 3Dipartimento di Scienze della Terra,

Università di Firenze, Italy

Abstract In this paper, a summary of the tectonic history of the Mirdita ophiolitic nappe,northern Albania, is proposed by geological and structural data. The Mirdita ophioliticnappe includes a subophiolite mélange, the Rubik complex, overlain by two ophiolite units,referred to as the Western and Eastern units. Its history started in the Early Triassic witha rifting stage followed by a Middle to Late Triassic oceanic opening between the Adriaand Eurasia continental margins. Subsequently, in Early Jurassic time, the oceanic basinwas affected by convergence with the development of a subduction zone. The existence ofthis subduction zone is provided by the occurrence of the supra-subduction-zone-relatedmagmatic sequences found in both the Western and Eastern units of the Mirdita ophioliticnappe. During the Middle Jurassic, continuous convergence resulted in the obduction ofthe oceanic lithosphere, in two different stages – the intraoceanic and marginal stages.The intraoceanic stage is characterized by the westward thrusting of a young and still hotsection of oceanic lithosphere leading to the development of a metamorphic sole. In theLate Jurassic, the marginal stage developed by the emplacement of the ophiolitic nappeonto the continental margin. During this second stage, the emplacement of the ophiolitesresulted in the development of the Rubik complex. In the Early Cretaceous, the finalemplacement of the ophiolites was followed by the unconformable sedimentation of theBarremian–Senonian platform carbonate. From the Late Cretaceous to the MiddleMiocene, the Mirdita ophiolitic nappe was translated westward during the progressivemigration of the deformation front toward the Adria Plate. In the Middle to Late Miocene,a thinning of the whole nappe pile was achieved by extensional tectonics, while the com-pression was still active in the westernmost areas of the Adria Plate. On the whole, theMiocene deformations resulted in the uplift and exposition of the Mirdita ophiolites asobserved today.

Key words: Mesozoic, Mirdita, northern Albania, obduction, ophiolites, tectonic, Tertiary.

INTRODUCTION

The interpretation of ophiolite sequences as on-land fragments of the oceanic lithosphere has orig-inated a great number of models for their emplace-ment within or onto the continental lithosphere.

Worldwide examples point out that one of the mostcommon mechanisms for ophiolite emplacement isobduction (i.e. the thrusting of internally unde-formed, huge slices of the oceanic lithosphere ontoa buoyant continental margin; e.g. Dewey & Bird1971; Gealy 1977; Coleman 1981; Moores 1982;Michard et al. 1991; Cawood & Suhr 1992; Searle& Cox 1999; Searle et al. 2004; and many others).Obduction is regarded as a multistage processinvolving intraoceanic detachments of ophiolite

472 V. Bortolotti et al.

slab followed by thrusting onto a continentalmargin (intraoceanic and marginal stages accord-ing to Michard et al. 1991). The main feature of theobducted ophiolites is their occurrence as giant,undeformed nappes displaced far from theiroriginal setting and floating on a continental crust.However, some features of this process are stillpoorly understood, such as, for instance, the rela-tionships among the subduction processes, theorigin of the metamorphic sole and the timing ofthe deformations related to the emplacement ontothe continental crust.

An exceptional opportunity to analyze the tec-tonic history of obducted ophiolites is provided bythe Mirdita ophiolitic nappe from northern Alba-nia. It is a good example of an obducted oceaniclithosphere derived from the eastern branch of theMesozoic Tethyan oceanic basin, located betweenthe Eurasia and Adria Plates. Different models forthe geodynamic history of the Albanian ophioliteshave been proposed recently (Collaku et al. 1991;Beccaluva et al. 1994; Shallo 1994; Kodra et al.2000; Robertson & Shallo 2000; Bortolotti et al.2002, 2004b; Hoeck et al. 2002; Saccani et al. 2004;Dilek et al. 2005), mainly based on petrologicaland/or stratigraphic data. Less attention has beenpaid to micro and mesostructural data derivedfrom the ophiolitic sequences, even if this informa-tion is able to provide fundamental constraints forthe reconstruction of their tectonic evolution.

In the present paper, a complete picture derivedfrom a full integration of the geological and petro-logical features with the structural data is pre-sented in order to outline the Mesozoic–Tertiarytectonic history of the Albanian ophiolites in theframework of the geodynamic evolution of theHellenic–Dinaric Belt. In addition, this recon-struction is discussed to provide further insightsfor general models concerning the obduction of theophiolites.

THE DINARIC–HELLENIC BELT

The Dinaric–Hellenic Belt (Fig. 1) is a north–southtrending collisional chain of alpine age runningfrom Slovenia and Serbia to southern Greece(Aubouin et al. 1970; Bernoulli & Laubscher 1972;Jacobshagen et al. 1978; Celet et al. 1980; Dimitr-ijevic 1982; Robertson & Dixon 1984; Smith 1993;Pamic et al. 1998; Robertson & Shallo 2000; Borto-lotti et al. 2004b; and many others). This belt hastraditionally been divided into four main tectono-stratigraphic zones, each corresponding approxi-

mately to the modern concept of terranes. Eachzone consists of an assemblage of variablydeformed and metamorphosed tectonic units ofoceanic and/or continental origin. These zones,from west to east, are (i) the deformed Adria zone;(ii) the external ophiolite belt; (iii) the Pelagonian–Korab–Drina–Ivanjica zone; and (iv) the Vardarzone. These zones are bounded to the west by theundeformed Adria zone and to the east by theSerbo–Macedonian–Rhodope Massif, generallyregarded as the stable margin of the Eurasia Plate(Fig. 1).

The deformed Adria zone consists of a west-verging imbricate stack of tectonic units derivedfrom the continental margin of the Adria Plate.These units are thrust onto the undeformed Adriamargin. From west to east, this deformed zone isrepresented by the Ionian, Gavrovo (Kruja in Alba-nia), Pindos (Krasta–Cukali in Albania) and Par-nassos units. All these units are characterized byunmetamorphosed sequences, each including Tri-assic–Paleocene neritic and pelagic carbonatesequences topped by widespread Upper Creta-ceous–Miocene siliciclastic turbidite deposits. Theage of inception of the flysch deposition, whichranges from Late Cretaceous in the Pindos unit toLate Oligocene in the Ionian unit, is related to thewestward migration of the deformation across thecontinental margin of the Adria Plate. In Montene-gro, Bosnia, Croatia and Serbia, the deformedAdria zone is represented, from west to east, bythe Budva, high Karst and pre-Karst units.Whereas the Budva unit can be regarded as thenorthward counterpart of the Pindos unit of Greeceand Krasta–Cukali of Albania, the other unitsprobably correlate to the Parnassos unit in Greece.

Eastward, the deformed Adria zone is thrust bythe external ophiolite belt, represented by a hugeoceanic nappe. This nappe is characterized by theoccurrence of ophiolites ranging in age from Tri-assic to Jurassic, which are regarded as represen-tative of the oceanic basin located east of the AdriaPlate. This nappe consists of a stack of ophioliteunits showing at their base a subophiolite mélange,consisting of an assemblage of continental- andoceanic-derived units. This mélange is known asthe Advella mélange in Greece or the Rubik com-plex in Albania, whereas in Bosnia and Serbia itprobably corresponds to the Bosnian–Durmitorunit. The external ophiolite belt is recognized as acontinuous nappe from Argolis, Othrys, Pindosand Vourinous in Greece to Mirdita in Albania,Bistrica and Zlatibor in Serbia, and up to Krivajain Croatia.

Tectonics of Albania ophiolites 473

In contrast, the Pelagonian–Korab–Drina–Ivanjica zone, hereafter simply referred to as the‘Pelagonian’ zone, is represented by an assemblageof tectonic units consisting of a prealpine basementintruded by Upper Paleozoic granitoids andcovered by Permian–Lower Triassic siliciclasticdeposits, followed by Middle Triassic–UpperJurassic carbonates. The units from the Pelago-nian zone are regarded as being derived from theeasternmost part of the Adria Plate (Bernoulli &Laubscher 1972; Zimmerman 1972; Vergely 1976;Jacobshagen et al. 1978; Collaku et al. 1992;Schermer 1993; Bortolotti et al. 1996, 2002, 2004b),or, alternatively, as belonging to a microcontinentbetween the Adria and Eurasia Plates (Jones &Robertson 1991; Shallo et al. 1992; Doutsos et al.1993; Beccaluva et al. 1994; Ross & Zimmermann1996; Kodra et al. 2000; Robertson & Shallo 2000;

Dilek et al. 2005). Westwards, the Pelagonian unitsare thrust by the units belonging to the Vardarzone.

The Vardar zone is represented by a compositeassemblage of continental and oceanic-derivedunits, including both Triassic and Jurassic ophio-lites. The latter now represent a more internalophiolite belt in the Dinaric–Hellenic chain.

The ophiolitic bodies constitute a semicontinu-ous ophiolite nappe from Greece to Macedonia,Serbia and Croatia. On the whole, the ophiolitesin the Dinaric–Hellenic Belt mainly occur alongtwo alignments (western, from Greece to Albania,Bosnia, Serbia and Croatia, and eastern fromGreece to Macedonia and Serbia), with minorklippes between them.

The relationships between the ophiolitic unitsand the neighboring continental units are sealed

Fig. 1 Tectonic sketch of the Dinaric–Hellenic Belt with location of the major ophi-olitic massifs (solid black), modified afterAubouin et al. (1970). Guev, Guevgueli; Oth,Othris; Parn, Parnassus zone; Vou, Vourinos.The location of Figure 2 is indicated in theboxed area.


by the deposits of the Meso-Hellenic trough,unconformably covering all of the nappe pile.These deposits, ranging in age from Eocene toMiocene, were sedimented in a northwest–south-east basin extending from southern Greece tonorthern Albania.

In all the proposed geodynamic models (e.g.Aubouin et al. 1970; Bernoulli & Laubscher 1972;Dimitrijevic 1982; Robertson & Dixon 1984; Smith1993; Bortolotti et al. 1996, 2004b; Kodra et al.2000; Robertson & Shallo 2000; Pamic et al. 2002;Dilek et al. 2005; and many others), the Dinaric–Hellenic Belt is regarded as the result of Mesozoic–Tertiary convergence and the subsequent conti-nental collision developed as a result of the closureof the eastern branch of the Tethyan oceanic basin.This oceanic area opened following rifting alongthe northern margin of Gondwanaland from LatePermian?–Early Triassic time onwards. Subse-quently, during Middle–Late Triassic time, thebreak-up led to the birth of an oceanic basin bor-dered by a pair of passive continental margins. Theoceanic basin underwent convergence in the EarlyJurassic as a result of motion between the Eurasiaand Africa Plates. This convergence led to an intra-oceanic subduction associated with the develop-ment of a wide oceanic basin above the subductionzone. In the Middle Jurassic, the continuous con-vergence between the Eurasia and Adria Platesresulted in the obduction of ophiolites onto theAdria continental margin before the continentalcollision. After the continental collision up to theNeogene, the continuous convergence affected thecontinental margin of the Adria Plate, which wasprogressively deformed in west-verging, thick-thinned fold and thrust sheets represented by theAdria-derived units. In the resulting orogenic belt,the ophiolites of the Dinaric–Hellenic Belt areincorporated as huge thrust sheets floating abovethe continental margin-derived units.

GEOLOGICAL OUTLINE OF NORTHERN ALBANIA

Northern Albania is considered to be the westernlinkage between the Dinaric Belt and the HellenicBelt (Aubouin et al. 1970). In this area (ISPGJ-IGJN 1983, 1985) three of the four tectonostrati-graphic zones of the Dinaric–Hellenic Belt cropout (Figs 2,3): the deformed Adria zone, the exter-nal ophiolite belt and the Pelagonian zone (Meco &Aliaj 2000). Northern Albania is bounded south-ward by a narrow east–west trending area, knownas the Vlora–Elbasan line (Nieuwland et al. 2001),

where erosion allows observation of the Krasta–Cukali unit cropping out below the Mirdita ophio-litic nappe. Northward, the boundary of northernAlbania is represented by the Shkoder–Péc line(Dercourt 1967). This line, probably a paleo-transform fault, is regarded as a still active,strike-slip fault.

UNITS OF THE DEFORMED ADRIA ZONE

In northern Albania, the units of the deformedAdria zone are represented by the Kruja andKrasta–Cukali units, which crop out in the westernareas (Figs 2,4). The succession of the Kruja unit(Robertson & Shallo 2000; and references therein)consists of Upper Cretaceous–Paleocene, shallow-water carbonates topped by Middle–UpperEocene pelagic carbonates showing a transition toUpper Eocene–Miocene siliciclastic turbidites. Atthe top of the shallow-water carbonates, a strati-graphic hiatus has been identified. The Krasta–Cukali unit is in turn characterized by a succes-sion consisting of Middle–Upper Triassic pelagiccherty limestones and radiolarites. At the top,Jurassic well-bedded, pelagic limestones andcherts and Lower–Upper Cretaceous deep-watercarbonates occur. This succession is topped by

Fig. 2 Tectonic sketch of northern Albania. (1) deposits of Peri-Adriaticand Meso-Hellenic Trough; (2) Mirdita ophiolitic nappe, western unit(WU); (3) Mirdita ophiolitic nappe, eastern unit (EU); (4) Rubik complex;(5) units of the Pelagonian zone; (6) deformed Adria zone, Kruja unit; (7)deformed Adria zone, Krasta–Çucali unit; (8) units cropping out in thecore of the Peshkopi and Sillatina tectonic windows; (9) deformed Adriazone, pre-Karts unit cropping out in the Albanian Alps. The line indicatesthe location of cross-section A–B represented in Figure 3; the Shkoder–Pec and Vlora–Elbasan lines are also indicated.


Maastrichtian–Upper Eocene siliciclastic turbid-ites. These units are deformed in a complexsequence of west-verging, north–south- to north-east–southwest-trending folds, developed undervery low-grade metamorphic conditions. Thesefolds are connected with a northeast–southwest- tonorth–south-trending, high-angle thrust charac-terized by brittle shear zones with a top-to-the-east shear sense (Fig. 3). This imbricate stack ofthrust units developed from the Early Oligocene–Middle Miocene during the progressive migrationof the deformation front toward the Adria Plate.

MIRDITA OPHIOLITIC NAPPE

The units from the deformed Adria zone are over-lain by the Mirdita ophiolitic nappe (Meco & Aliaj2000; and references therein). The boundarybetween these two units is represented by a west-

verging, high-angle thrust. This nappe consists oftwo end-members (Fig. 4): the ophiolite units andthe underlying Rubik complex (i.e. a subophiolitemélange; Bortolotti et al. 2004b; and referencestherein).

The Rubik complex (Bortolotti et al. 1996) con-sists of an assemblage of thrust slices derived fromboth continental and oceanic domains. In thegeological literature, the Rubik complex is alsoreported as a ‘carbonate periphery’ or ‘peripheralcomplex’ (Shallo 1991, 1992, 1994; Kodra et al.1993). The thrust slices are mainly represented bycoherent sequences of continental and oceanicorigin; however, slices made up of a sedimentarymélange, represented by blocks of carbonate,arenites and magmatic rocks set in a shaly or ser-pentinitic matrix, are common. The slices of conti-nental origin generally consist of Triassic–Jurassiccarbonate successions. According to Shallo (1991,1992) and Kodra et al. (1993), the commonest suc-cession consists of Middle Triassic cherty lime-stones grading upwards to Upper Triassic–LowerLiassic platform carbonates, topped by Middle–Upper Liassic, Ammonitico rosso-type nodularlimestones and Dogger–Malm, pelagic chertylimestones and radiolarites (Kodra et al. 1993;Marcucci et al. 1994; Bortolotti et al. 1996). How-ever, thick successions characterized by MiddleTriassic–Malm pelagic deposits, represented bycherty limestones alternating with radiolarites,are also common. In addition, slices consisting ofmagmatic rocks covered by cherts of Anisian age(Kodra et al. 1993; Beccaluva et al. 1994; Bortolottiet al. 1996) have also been identified. These slicesare characterized by pillow-lava picritic basalts,

Fig. 3 Cross-section of northern Albania. See Figure 2 for the location of the cross-section (line A–B). (1) Deposits of Peri-Adriatic and Meso-HellenicTrough; (2) units from deformed Adria zone (Krasta–Çucali and Kruja units; upper: the siliciclastic deposits; lower: the carbonate deposits); (3) Rubikcomplex and subophiolite mélange of the Peshkopi window; (4) western ophiolite unit (upper: crustal section; lower: mantle section); (5) eastern ophioliteunit (upper: crustal section; lower: mantle section); (6) metamorphic sole; (7) Simoni mélange and Firza flysch; (8) Barremian carbonate deposits; (9)units from Pelagonian zone (upper: sedimentary cover; lower: basement section); (10) Triassic evaporites of the Ionian unit cropping out at the core ofthe Peshkopi window.

Fig. 4 Sketch showing the relationships among the tectonic units innorthern Albania.


trachybasalts and trachytes showing within-plateto transitional geochemical affinity. However, themost widespread magmatic rocks are found as aslice at the top of the Rubik complex. These mag-matic rocks, reported as a ‘Volcano-sedimentaryFormation’ by Kodra et al. (1993) or the ‘PoravaUnit’ by Bortolotti et al. (2004a), are representedof a sequence up to 500 m thick of pillow-lavabasalts alternating with Middle–Late Triassicradiolarites and shales (Chiari et al. 1996).Geochemical data have revealed that the basaltsare characterized by flat high-field-strength-element patterns and by slightly light-rare-earth-element depleted patterns, typical of present-daybasalts generated at a mid-ocean ridge (Bortolottiet al. 2004a). In the Rubik complex, the otheroceanic-derived slices, not thicker than 100 m, arerepresented by lherzolites, generally highly ser-pentinized. At the base of the Rubik complex, a300-m-thick slice of ophiolite-bearing and carbon-ate turbidites of uppermost Tithonian–late Vala-nginian age occur. Slices of sedimentary mélangeare represented by clasts of serpentinite, basalt,gabbro and sedimentary rocks set in a shaly orserpentinitic matrix of undetermined age. Thedeformation history of the Rubik complex, mainlydeveloped in the volcano-sedimentary formationincludes two superimposed folding phases devel-oped under subgreenschist facies metamorphicconditions. The first phase is characterized by verytight to isoclinal folds associated with slaty cleav-age. The subsequent deformation is representedby open to tight folds and a spaced crenulationcleavage.

The Rubik complex is thrust by two ophioliteunits: the Western and Eastern units, according totheir geological and petrochemical features (Shallo1992; Shallo et al. 1992; Beccaluva et al. 1994; Bor-tolotti et al. 1996, 2002; Saccani et al. 2004). Theboundary between these units is represented bythe west-verging thrust developed during theCretaceous tectonic events (Fig. 4).

The Western unit is characterized by a north–south-trending assemblage of thrust slicesranging in thickness from 100 to 700 m. Thereconstructed stratigraphy (Fig. 5) includes, frombottom to top, the metamorphic sole, lherzoliticmantle tectonites, mafic–ultramafic cumulates, adiscontinuous sheeted dyke complex and a volcanicsequence (Shallo 1991, 1994; Beccaluva et al. 1994;Bortolotti et al. 1996, 2002, 2004b; Saccani et al.2004). However, the sheeted dyke complex as wellas the gabbroic complex are generally lacking, andthe crustal section can be only representative ofthe volcanic sequence (Cortesogno et al. 1998;Nicolas et al. 1999). The volcanic as well the intru-sive sequence show high-Ti (mid-oceanic ridgebasalt, MORB) affinity (Beccaluva et al. 1994; Bor-tolotti et al. 2002; Saccani et al. 2004). However, avolcanic sequence showing island arc tholeiites(IAT) and mid-ocean ridge (MOR)–IAT inter-mediate geochemical features directly overlyingthe more typical MORB sequences has been found(Bortolotti et al. 1996, 2002, 2004b; Saccani et al.2004). In addition, boninitic dykes cutting theophiolite sequence have also been discovered byBortolotti et al. (2002). The radiolarian cherts,referred as Kalur cherts by Bortolotti et al. (1996),

Fig. 5 Generalized stratigraphicsequences of the ophiolite sequences forthe western and eastern units, Mirditaophiolitic nappe (modified from Beccaluvaet al. 1994).


found at the top of the IAT basalts, displayradiolarian assemblages of late Bajocian/earlyBathonian to late Bathonian/early Callovian age(Marcucci et al. 1994; Marcucci & Prela 1996).

The Eastern unit shows a sequence up to 10 kmthick (Fig. 5), including, at the base, harzburgiticmantle tectonites with well-developed meta-morphic sole at the base, a thick intrusivesequence, a sheeted dyke complex and a volcanicsequence (Shallo 1992, 1994; Shallo et al. 1992;Beccaluva et al. 1994; Hoxha & Boullier 1995;Bortolotti et al. 1996, 2002, 2004b; Robertson &Shallo 2000; Saccani et al. 2004). According to thegeochemical data, the ophiolites from the Easternunit show low-Ti affinity (Beccaluva et al. 1994;Saccani et al. 2004). These petrological featuresindicate that the origin of these ophiolites was in asupra-subduction zone (SSZ) setting (Beccaluvaet al. 1994; Saccani et al. 2004). At the top of thepillow lava basalts, a sequence of radiolarites,referred to as Kalur cherts by Bortolotti et al.(1996), ranging in age from late Bathonian/earlyCallovian to middle Callovian/early Oxfordian hasbeen recognized by Marcucci et al. (1994), Prela(1994) and Marcucci and Prela (1996). In addition,decimeter-thick sequences of cherts recognized inthe uppermost part of the basalt flows show anupper Bajocian–lower Bathonian radiolarianassemblage (Chiari et al. 1994).

Both the ophiolite sequences from the Westernand Eastern units are unconformably covered bya thick sedimentary sequence that includes thelate Oxfordian to Tithonian Simoni mélange andthe upper Tithonian to upper Valanginian Firzaflysch (Bortolotti et al. 1996; Gardin et al. 1996).The Simoni mélange is a sedimentary mélangeapproximately 200–300 m thick, characterized byblocks ranging from several centimeters to severalhundreds of meters in size, set in a shaly matrix.The blocks consists of continental-derived litholo-gies such as Permian sandstones, Triassic volca-nics, Triassic cherts, Triassic–Liassic carbonatesand minor metamorphic rocks (Shallo 1991; Borto-lotti et al. 1996). The ocean-derived lithologies arerepresented by basalts, mantle ultramafics, gab-bros and cherts derived from both western andeastern ophiolite sequences. The occurrence oflayers of arenites in the uppermost levels of themélange marks the transition to the Firza flysch.This formation is represented by turbidite depos-its with ophiolite-bearing polimictic pebbly sand-stones and mudstones at different stratigraphiclevels. The ophiolite units and the Rubik complexare in turn unconformably covered by Barremian–

Senonian, shallow-water carbonate deposits, witha thickness of up to 1500 m (ISPGJ-IGJN 1983,1985).

UNITS OF THE PELAGONIAN ZONE

In the eastern area of northern Albania, the Mird-ita ophiolitic nappe is overlain by units from thePelagonian zone (Fig. 4). These units are charac-terized by a Paleozoic basement consisting of anOrdovician–Devonian sequence unconformablycovered by a Permo-Triassic clastic sequencegrading upward to Triassic and Jurassic neritic–pelagic, mainly carbonate, deposits (Robertson &Shallo 2000). In the Pelagonian zone, two mainunits, known as the Kollovoci and Muhur units,have been identified (Collaku et al. 1990). ThePelagonian units are deformed in a large antiformshowing at its core several tectonic windows(Fig. 6), mainly in the Peshkopi and Sillatina areas,where a pile of tectonic slices crops out (Collakuet al. 1990, 1992). In the Peshkopi window, this pileincludes, from top to bottom (Fig. 6): (i) a sliceconsisting of Upper Jurassic–Lower Cretaceousophiolite-bearing mélange associated with hugeserpentinites bodies; (ii) the Krasta–Cukali unitrepresented by a slice of Mesozoic carbonates; (iii)the Kruja unit represented by a slice consisting ofsiliciclastic turbidites of Oligo–Miocene age; and(iv) the Ionian unit represented by Triassic evapor-ites. The latter are dragged up in the footwall of amain thrust and remobilized as smeared-out sur-face extrusions at the core of the tectonic window(Velaj et al. 1999).

In the Peshkopi and Sillatina areas, a polyphasedeformation history has been identified by Kiliaset al. (2001). This deformation history includes twomain deformation phases. The first deformationphase is characterized by asymmetric, overturnedfolds with a northeast–southwest trend associatedwith thrusts with a top-to-the-southwest sense ofshear. Backthrusts with a top-to-the-northeastsense of shear and strike-slip faults have also beenidentified. This deformation phase is interpretedas having developed during the nappe stackingrelated to the progressive thrusting of the coupledMirdita ophiolitic nappe and Pelagonian units ontothe Krasta–Cukali, Kruja and Ionian units. Thisprogressive thrusting is well defined by the age ofthe siliciclastic turbidites found at the top of theunits derived from the Adria domain. Accordingto the available data, this progressive thrustingranges in age from Early Oligocene to MiddleMiocene. This deformation phase can be compared


with the deformations previously described in theunits from the deformed Adria zone. The seconddeformation phase is characterized by northeast–southwest to NNE–SSW-trending, low-angleextensional shear zones that crosscut all the pre-vious structures. These shear zones are brittle inthe Mirdita ophiolitic nappe and in the Pelagonianunits, whereas the underlying units show brittle–ductile features. In this frame, the older thrustsare reactivated as extensional faults with impres-sive thinning of the tectonic units developedduring the previous phase. These structures areassociated with northeast–southwest-trending,open folds with subhorizontal axial planes. Thisassociation of structures is coherent with exten-sional tectonics leading to the development of thePeshkopi and Sillatina tectonic windows, whereasthe compression was still active in the more east-

ern areas. The second deformation phase, whichaffects all the units of the Peshkopi and Sillatinatectonic windows, is probably Middle to LateMiocene in age.

MESO-HELLENIC DEPOSITS

Finally, transgressive, marine–continental depos-its of the Burrel Basin, belonging to the Meso-Hellenic trough, unconformably covered thestructures of the Mirdita ophiolitic nappe (Fig. 4),including the Rubik complex as well as the West-ern and Eastern ophiolite units (ISPGJ-IGJN1990). These deposits, ranging in age from Eoceneto Miocene, are found as northwest–southeast-striking belts extending from southern to northernAlbania (Fig. 2). In the easternmost areas, thenappe pile is unconformably covered by the Neo-

Fig. 6 Schematic geological map ofthe Peshkopi area and related cross-section.


gene ‘molasse’ deposits of the Peri-Adriatic trough(ISPGJ-IGJN 1990).

GEOMETRY AND KINEMATICS OF DEFORMATION IN THE OPHIOLITE SEQUENCE

As are all the other worldwide examples ofobducted slices of oceanic lithosphere, the Mirditaophiolites are characterized by deformations local-ized at the base of the mantle section – the meta-morphic sole. The metamorphic sole developedduring the intraoceanic stage of obduction, when athick and hot section of oceanic lithosphere wasdetached and emplaced over the neighboring oce-anic domain. The metamorphic sole originated atthe base of the oceanic slice as a strongly deformedand metamorphosed shear zone. However, furtherdeformations were acquired by the ophiolitesequence in the subsequent marginal stage, duringits emplacement over the continental margin.These deformations are easily identified in thesedimentary cover of the ophiolites, mainly in theradiolarites that show pervasive deformation asfolds and thrusts developed under very low-grademetamorphic conditions (Carosi et al. 1996b).Therefore both metamorphic sole and radiolaritesrepresent suitable lithologies where the deforma-tions, including folds, shear zones and foliations,can be analyzed in order to outline the tectonicevolution of the Mirdita ophiolitic nappe.

DEFORMATION HISTORY OF THE METAMORPHIC SOLE: INTRAOCEANIC STAGE-RELATED STRUCTURES

According to Collaku et al. (1991), Carosi et al.(1996a) and Dimo-Lahitte et al. (2001), the meta-morphic sole of the northern Albanian ophiolites ischaracterized by an assemblage, up to 700 m thick,of garnet-bearing amphibolites, coarse- to fine-grained amphibolites, quarzites, garnet-bearingmicaschists, calcschists, garnet-bearing para-gneisses and minor mafic granulites. All theserocks are strongly deformed under high pressure/low temperature metamorphism. Based on theirgeochemical signature (Carosi et al. 1996a), theinferred protoliths for the amphibolites are basaltsand gabbros, showing oceanic island basalt orMOR geochemical affinity, whereas quarzites,paragneisses and micaschists probably representsiliciclastic, deep-sea oceanic sediments. The rarelayer of calcschists–impure marble can be inter-preted as being derived from deep-sea carbonateturbidite. The metamorphic sole occurs below the

peridotites, whose basal levels show obduction-related, low temperature mylonitic deformation(Hoxha & Boullier 1995; Rassios & Smith 2000).The metamorphic sole is in turn thrust ontothe Rubik complex, mainly onto the volcano-sedimentary formation (Fig. 7a), where only sub-greenschist facies metamorphism has beendetected (Carosi et al. 1996a). Where the relation-ships are well exposed, the boundary is repre-sented by a shear zone up to 30–40 m thickcharacterized by cataclastic and ultracataclasticfault rocks derived from both the metamorphicsole and the volcano-sedimentary formation. Insome places, the amphibolite facies metamorphicrocks can be found as boudinaged bodies inside thevolcano-sedimentary formation. The amphibolites,the paragneisses and the micaschists display evi-dence of a polyphase deformation history thatincludes almost three phases developed under dif-ferent pressure and temperature conditions. Thestructures of the D1 phase are recognized only inthe thin sections of the coarse-grained amphibo-lites and paragneisses where relics of the S1 schis-tosity are preserved in the microlithons betweenthe S2 schistosity. In thin sections, the S1 foliationis defined by coarse grains, oriented plagioclaseand amphibole grains. The main deformationstructures recognized at the mesoscale can bereferred to the D2 phase. In all the outcrops, themain foliation is represented by the S2 schistosity(Fig. 7b), generally bearing well-defined L2 miner-alogical lineations consisting of elongated fibers ofamphibole or plagioclase, whereas in the gneissesand micaschists, the L2 lineations are representedby elongated fibers of white mica or orientedpressure shadows around garnets. Mineralogicallineations (Fig. 8) show WNW–ESE to northwest–southeast strikes in the western areas (Carosiet al. 1996a), whereas in the Eastern units, theselineations display northwest–southeast strikes(Collaku et al. 1992; Carosi et al. 1996a). In thinsections, the amphibolites are characterized bylayers of plagioclase showing oriented granoblas-tic structure alternating to bands enriched inprismatic or acicular nematoblastic amphibole(tschermakitic hornblende to magnesium-hornblende) and clinopyroxene (diopside–salite)showing well-preferred orientation (Fig. 7c). Inthe granulites, the association of clinopyroxeneand orthopyroxene with garnet and plagioclase isdetected. The S2 schistosity in the paragneissesand micaschists is defined by oriented minerals ofwhite mica (muscovite), biotite, plagioclase andgarnet (almandine) (Fig. 7d,e). Decimeter-thick


mylonite shear zones with widespread kinematicindicators as intrafoliar folds (Fig. 7f) or σ struc-tures are found parallel to the S2 schistosity inboth amphibolites and paragneisses. The S2 schis-tosity is deformed by D3-phase structures. The D3phase is mainly represented by F3 tight to isoclinalfolds characterized by a northwest–southeasttrend and steep axial plane. The associated folia-tion range from a well-developed crenulationcleavage in micaschists to a disjunctive cleavage inamphibolites and gneisses. Both in western andeastern areas, the F3 folds show an asymmetrycoherent with a top-to-the-west or -southwestsense of shear. During the D3 phase, shear zonescharacterized by cataclastic structures arerecognized.

According to Carosi et al. (1996a), differentpressure–temperature (P–T) conditions of themetamorphism are recorded in slices of metamor-phic sole. No differences for the P–T conditions ofthe peak metamorphism for the D1 and D2 phaseshave been reported. The related metamorphicoverprint ranges from low (T = 680° ± 20°C;P = 0.2–0.4 GPa) to intermediate (T = 740° ± 25°C;P = 0.4–0.5 GPa) and high (T = 850° ± 20°C;

Fig. 7 The metamorphic sole of the Mirditaophiolitic nappe. (a) View of contact amongthe lherzolites (lh), the metamorphic sole (ms)and the volcano-sedimentary formation (vs),western ophiolites, Gomsique Massif; (b) fieldoccurrence of amphibolites with well-developed S2 foliation, Fushe Lura; (c) amp-hibolites characterized by bands enriched inprismatic or acicular nematoblastic amphibole(amp) showing oriented granoblastic structurealternating with layers of plagioclase (pl); (d,e)S2 schistosity in the paragneisses and mica-schists defined by white mica (muscovite),quartz, plagioclase and garnet (grt); shearsense is indicated by S–C and s structuresaround garnet porphyroblasts; (f) myloniticshear zones with intrafoliar folds indicating theshear sense.

Fig. 8 Equal area, lower hemisphere stereographic representation ofstructural data from the metamorphic sole in western and eastern areasof the Mirdita ophiolitic nappe. (a,b) L2 mineral lineation; (c,d) S2foliation.


P = 0.4–0.5 GPa) temperature amphibolite facies(Carosi et al. 1996a). In addition, a granulite faciesmetamorphism (T = 800–860°C; P = 0.9–1.1 GPa)is reported by Dimo-Lahitte et al. (2001). How-ever, data about high pressure values are stillscarce and contrast with the majority of the anal-yses performed on the amphibolites, which indi-cate P < 0.5 GPa. In addition, the granulites fromthe Mirdita metamorphic sole are characterized bythe occurrence of orthopyroxene (Dimo-Lahitteet al. 2001), the lack of which is a diagnostic featurefor high pressure granulites (O’Brien & Rotzler2003). On the whole, the granulite as well theamphibolites seem to have originated at a pressureof <0.5 GPa. A retrograde, low-grade metamor-phism associated with chevron folds is alsorecorded. It is very important to underline that allthe samples clearly show evidence of amphibolite–granulite facies metamorphism. So, a major jumpof metamorphic grade can be identified betweenthe metamorphic sole and the underlying volcano-sedimentary formation, where only a subgreen-schist metamorphic mineral assemblage has beendetected (Carosi et al. 1996a). In addition, theresults of structural and petrological analysesshow that the tectonic setting of the metamorphicsole from the Albanian ophiolites consists of anassemblage of multiple slices, each with a differentP–T metamorphic climax, without evidence of aclear inverse zonation of the metamorphism. Theoccurrence of slices with different metamorphismsis a common feature in the metamorphic sole, asrecognized, for instance, by Hacker and Mosen-felder (1996) in the Oman ophiolite.

The kinematic indicators from the metamorphicsole of the Western ophiolite unit clearly reveala top-to-the-west sense of shear (Carosi et al.1996a). The same structural elements in theeastern metamorphic sole provide contrastingkinematics. According to Collaku et al. (1991) andCarosi et al. (1996a), the sense of shear is top-to-the-west, whereas Dimo-Lahitte et al. (2001)indicate a coexistence of top-to-the-west and top-to-the-east indicators, with the latter prevailing.This contrasting evidence can be explained with adeformation where simple and pure shear coex-isted. In this frame, the kinematic indicators iden-tified within the mylonitic shear zones developedduring the amphibolite metamorphism can providemore valuable data. In the eastern metamorphicsole, these shear zones provide evidence for a top-to-the west sense of shear (Fig. 7f). Ar–Ar datingranging from 159 ± 2.6 to 171.7 ± 1.7 Ma by step-heating of mineral concentrates, laser-probe step-

heating and spot fusions on single grains (amphi-boles and micas) from the metamorphic sole is pro-vided by Dimo-Lahitte et al. (2001), without sharpdifferences between the Western and Easternunits. These datings can be interpreted as the agethe metamorphism developed during the intraoce-anic stage of obduction.

DEFORMATION HISTORY OF THE RADIOLARITES: MARGINAL STAGE-RELATED STRUCTURES

The tectonic setting of the Western unit is repre-sented by an imbricate stack of thrust sheets,showing a thickness ranging from 1 to 2 km. Thesesheets are mainly composed of mantle ultramaficswith remnants of the metamorphic sole at theirbase. However, sheets represented either bybasalts with the associated sedimentary sequence,mainly consisting of Kalur cherts, the Simonimélange and the Firza flysch, or by layered and/orisotropic gabbros are also common. Each thrustsheet displays an internal structure characterizedby minor folds. The related deformation historyhas been reconstructed in the Kalur cherts byCarosi et al. (1996b). Structural analysis revealeda polyphase deformation history, with two phasesof folding followed by faulting events. The firstfolding D1 phase, identified only in the Kalurcherts, consists of recumbent, tight to very tightF1 folds with geometry ranging from similar toparallel (Fig. 9a). The A1 axes are moderatelyscattered with a cluster between N120E/N150E(Fig. 10). The associated S1 axial plane foliationhas been identified as disjunctive cleavage, mainlydeveloped by a pressure solution mechanism(Fig. 9b). The D2 phase, which also affects theSimoni mélange and the Firza flysch, is marked byoverturned, asymmetric F2 folds with concentricgeometry. The F2 folds, characterized by axesranging from N100E to N130E (Fig. 10), show ashort overturned limb associated with steep togently sloping axial planes. The F2 folds areassociated with southwest- to south-verging andnorthwest- to north-trending thrusts. Associatedwith the thrust systems, northwest–southeast- tonortheast–southwest-trending strike-slip faultsare recognizable at the map scale. The vergence ofthe F1 and F2 folds as well as the kinematicsalong the thrust planes is from southwest to westin both the Western and Eastern units. On thebasis of structural evidence, the D1 phase predatesthe deposition of the Simoni mélange. Thetimespan for this phase is presently estimated tobe between the age of the top of the Kalur cherts


(early Callovian) and the base of the Simonimélange (Tithonian). According to Carosi et al.(1996b), the D1 phase recognized in the Kalurcherts can be related to the inception of the ophio-lites emplacement onto the continental margin.The last deformation event is represented by thesecond D2 phase recognized in the Kalur cherts,which also affected the Simoni mélange and Firzaflysch. This phase, which probably marked thefinal stage of emplacement of the Albanian ophio-lites onto the continental margin, developed in theHauterivian. The sedimentation gap recognized

between the Firza flysch and the shallow-waterBarremian–Senonian carbonate sequence, whichare unaffected by the folding phases, can berelated to the D2 phase.

RECONSTRUCTION OF THE TECTONIC HISTORY OF THE ALBANIAN OPHIOLITES

Some remarks about the geological history follow,in order to provide valuable tools for the recon-struction of the tectonic evolution of the Mirditaophiolitic nappe.

AGE OF THE OCEANIC BASIN

In the Rubik complex, the occurrence of Middle–Upper Triassic MOR basalts, associated every-where with slices of lherzolite, suggests that thephases of oceanization had already been reachedin the Albania area during the Middle Triassic,probably after a rifting phase developed in theEarly Triassic (Bortolotti et al. 2004a). This findingis confirmed by the occurrence of Middle–LateTriassic, Early Jurassic and Middle Jurassic MORbasalts reported by Bortolotti et al. (2002) in theDhimaina ophiolite sequence, Argolis Peninsula,which represent the southward extension of thePindos, Vourinos and Koziakas ophiolites (Borto-lotti et al. 2004b). In contrast, the SSZ ophiolitesfrom the Mirdita nappe are Middle Jurassic in age,according to the age of the radiolarites found in(late Bajocian–early Bathonian) or at the top of(late Bajocian–early Bathonian to middle Callovian–early Oxfordian) the IAT basalts (Bortolotti et al.1996; and references therein). On the whole, theMOR oceanic lithosphere is Middle–Late Triassicin age according to evidence from the Rubik com-plex, whereas the SSZ oceanic lithosphere isMiddle Jurassic in age as detected in both theWestern and Eastern units of the Mirdita ophio-litic nappe.

Fig. 9 Photographs of structures recognized in the Kalur Cherts. (a)tight parallel F1 folds; (b) S1 axial plane foliation highlighted by pressuresolution surfaces and by recrystallization of quartz fibers around radiolar-ian (arrow).

Fig. 10 Equal area, lower hemispherestereographic representation of the struc-tural data from the Kalur cherts. (a) A1axes; (b) S0 bedding; (c) A2 axes.


GEODYNAMIC SIGNIFICANCE OF THE MOR AND SSZ OPHIOLITES FROM THE MIRDITA NAPPEThe Middle–Upper Triassic basalts found in theRubik complex represent the remnants of a MORoceanic lithosphere without the influence of asubduction-related magmatism (Bortolotti et al.2004b). In addition, the western ophiolites arerepresentative of a MOR oceanic lithosphere, butthe occurrence of intermediate MOR–IAT, IATbasalts and boninites indicates that the oceanicbasin, from which these ophiolites were derived,has experienced two different stages of crustalgrowth (Bortolotti et al. 1996, 2002; Bébien et al.1998, 2000). In the first stage, a MOR-type oceaniclithosphere was generated at a mid-ocean ridgespreading center. Subsequently, during the secondstage, a portion of this lithosphere was trapped inthe supra-subduction setting (most probably in aproto-forearc region) with consequent generationof intermediate MOR–IAT and IAT basalts as wellas boninitic dykes. In order to explain the coexist-ence of these geochemically different magmagroups, Bortolotti et al. (2002) have proposed amodel based on the complexity of the magmaticprocesses that may take place during the initiationof a subduction in the proximity of an active mid-ocean ridge. This model implies that the initiationof subduction processes close to an active mid-ocean ridge leads to contemporaneous eruptions ina forearc setting of MORBs generated from theextinguishing mid-ocean ridge, and of intermedi-ate MOR–IAT basalts generated in the SSZ man-tle wedge from a moderately depleted mantlesource. The development of the subduction in ayoung, hot lithosphere caused the generation ofIAT basalts and boninites from strongly depletedmantle peridotites in the early stages of subduc-tion, soon after the generation of intermediateMOR–IAT basaltic rocks. On the whole, the west-ern ophiolites can be interpreted as a MOR oceaniclithosphere trapped over a subduction zone andsubsequently affected by subduction-related mag-matism (Bortolotti et al. 2002, 2004b). In contrast,the eastern ophiolites represent an oceanic litho-sphere entirely developed in an SSZ (Beccaluvaet al. 1994; Saccani et al. 2004). The same age ofthe IAT basalts in the Eastern and Western units,as demonstrated by the radiolarian assemblagefound in the radiolarites (Marcucci & Prela 1996),indicates these sequences originated in the sameoceanic basin in Middle Jurassic time. This oceanicbasin, located over a subduction zone, was there-fore characterized by a trapped MOR lithospherewhere a younger oceanic lithosphere, originated

entirely in a supra-subduction setting, wasemplaced.

AGE OF THE SUBDUCTION INCEPTION

If the Albanian ophiolites are representative of abasin located in a supra-subduction setting inMiddle Jurassic time, not older than Bajocian–Bathonian (Bortolotti et al. 1996), subduction of anolder oceanic crust is required to produce IATmagmatism. Assuming that a timespan of 10–15 Ma from the inception of subduction is requiredto develop the SSZ magmatism, the convergenceshould have started during the Early Jurassic(Bortolotti et al. 2004b). No clear evidence for thedipping of this subduction is available. However,the occurrence of calcoalkaline magmatic rocks ofLate Jurassic age in the Vardar zone (Bebien et al.1986), eastward of the present-day location of theMirdita ophiolites, suggests an east-dipping of thesubduction. Therefore, in the proposed evolution(Fig. 11), a subduction characterized by the under-thrusting of the oceanic lithosphere below theEurasia Plate is assumed. However, the high angleof rotation proposed for the Dinaric–Hellenic Beltin the Jurassic–Neogene timespan (Kondopolou2000) implies that the direction of the Jurassicsubduction cannot be regarded as coinciding withthe present-day trend of the ophiolites from theDinaric–Hellenic Belt.

AGE OF OBDUCTION

The age of the inception of the obduction processis well constrained by the age of the amphibolitesfrom the metamorphic sole. The Ar–Ar radiometricdatings of the amphiboles from the northernAlbanian metamorphic sole range from 159 ± 2.6 to171.7 ± 1.7 Ma (Dimo-Lahitte et al. 2001). There-fore, the intraoceanic stage of the obduction startedin the Middle Jurassic and developed up to earlierLate Jurassic time. Despite problems regardingthe correlations between the paleontological andradiometric ages, these data point to an inceptionof convergence in the oceanic basin slightly olderthan the magmatic events, as detected in otherexamples of obducted ophiolites (e.g. the Omanophiolites; Michard et al. 1991).

EVIDENCE FROM THE INTRAOCEANIC STAGE OF OBDUCTION

The stage of intraoceanic thrusting is recorded bythe occurrence of slices of metamorphic sole at


Fig. 11 2-D sketch illustrating the different steps (from A to J) of the tectonic evolution proposed for the Mirdita ophiolitic nappe in the Mesozoic–Tertiary timespan. MOR, mid-ocean ridge.


the base of the mantle ultramafics in both theWestern and Eastern ophiolites (e.g. Collakuet al. 1992; Shallo 1992; Carosi et al. 1996a;Dimo-Lahitte et al. 2001). The metamorphic soleconsists of an assemblage (up to 700 m thick)of ocean-derived rocks metamorphosed underclimax P–T conditions related to granulite andamphibolite facies (Carosi et al. 1996a; Dimo-Lahitte et al. 2001). No rocks showing greenschistfacies climax metamorphism have been found inthe metamorphic sole. Tectonic models indicatethat the high temperature required for the devel-opment of the metamorphic sole could have beenproduced in the shear zone formed at the baseof an overridden section of oceanic lithosphere(McCaig 1981, 1983; Michard et al. 1991; Cawood& Suhr 1992; Suhr & Cawood 1993; Searle & Cox1999; Searle et al. 2004). The occurrence of differ-ent P–T conditions of peak metamorphism in thedifferent slices can be interpreted as beingrelated to the origin of the metamorphic sole by atectonic assemblage of slices developed at differ-ent stages and assembled in a single body duringthe thrusting of the ophiolites (Hacker & Mosen-felder 1996). In order to explain the very highgeothermal gradient suggested by the occurrenceof the amphibolite and granulite metamorphism, asource of heat is required. Residual heat from ayoung and still hot oceanic lithosphere could havesupported the temperatures required for themetamorphism that developed during the obduc-tion process. Recent models have regarded theobduction process located in the lower plate dur-ing its underthrusting in a flattening subductionzone (Dimo-Lahitte et al. 2001). However, thevery high geothermal gradient is inconsistentwith a subduction process where the metamor-phisms are characterized by high pressure condi-tions. Therefore, the geochemical features of theMirdita ophiolites, that indicate their location is inan SSZ, as well as the tectono-metamorphic fea-tures of the metamorphic sole, are coherent withthe location of the obduction process in the upperplate over a subduction zone, where young and astill hot oceanic lithosphere was affected by com-pression immediately after their origin. In addi-tion, the metamorphic sole can provide evidencefor the kinematics of the ophiolite emplacement.The L2 mineral lineations show WNW–ESE tonorthwest–southeast strikes in western areas(Carosi et al. 1996a), whereas in eastern areasthese lineations display strikes ranging fromeast–west to northwest–southeast (Collaku et al.1992; Carosi et al. 1996a). On the whole, the struc-

tural data suggest that the displacement duringthe intraoceanic stage ranged from east–west tonorthwest–southeast. In turn, the shear sensedetected in the amphibolites determined on themicrostructures related to amphibolite faciesmetamorphism is recognized in both the Westernand Eastern ophiolite units as top-to-the-west orto-the-northwest (Collaku et al. 1992; Carosi et al.1996a).

ORIGIN OF THE RUBIK COMPLEX

The Rubik complex originated during the marginalstage as the result of the emplacement of theobducted oceanic lithosphere onto the continentalmargin (Bortolotti et al. 1996, 2004b). During thisemplacement a wedge consisting of slices detachedfrom their continental basement developed at thebase of the obducted oceanic lithosphere. Theresult of this process is a tectonic wedge – the Rubikcomplex – sandwiched between the obductedophiolites and the underlying continental margin.This interpretation is suggested by the tectonicsetting of the Rubik complex and its features,as, for instance, the occurrence of slices consist-ing of carbonate successions representative ofthe continental margin where the ophiolites wereemplaced. However, oceanic slices have also beenidentified. These slices can probably be inter-preted as remnants of an older, pre-existing accre-tionary wedge, developed during the LowerJurassic subduction. In this hypothesis, the accre-tionary wedge was enclosed in the Rubik complexduring the displacement of the obducted oceaniclithosphere towards the continental margin, assuggested by Bortolotti et al. (2004b). Many of theoceanic and/or continental sequences were con-temporaneously deformed and eroded during themain tectonic stage – the shedding of blocks, par-ticularly from their basal levels, lead to the forma-tion of a true sedimentary mélange, which wassubsequently incorporated as slices into the Rubikcomplex. The origin of this complex can thereforebe regarded as the result of a mixing betweentectonic and sedimentary processes that tookplace during the ophiolite emplacement onto thecontinental margin in Late Jurassic time. Duringthese events, the lithologies from the Rubik com-plex were deformed under a subgreenschist faciesmetamorphism. Further thrusts occurred duringthe subsequent Lower Cretaceous intracontinen-tal deformation; during these stages, slices ofupper the Tithonian–Upper Valanginian ophiolite-bearing mélange were accreted to the slice of


carbonate sequences, leading to the present-daysetting of the Rubik complex.

INTERPRETATION OF THE SIMONI MÉLANGE AND FIRZA FLYSCH

The first deformation event recognized in theKalur cherts is followed by the sedimentation ofthe Simoni mélange and Firza flysch (Shallo 1991;Bortolotti et al. 1996; Gardin et al. 1996). Thelarge-scale unconformity at the base of the Simonimélange, above the cherts, as well as the pillowlava and massive basalts, are recognized through-out the Mirdita region (Bortolotti et al. 1996). Thissuggests that the deposition of the mélange wasassociated with an important submarine erosionphase of the underlying deposits. In addition, thesedimentation of the Simoni mélange, as well asthat of the Firza flysch, corresponds to a sharpinception of turbiditic debris flows and slidedeposits derived from both oceanic and continen-tal source areas. These features suggest that theSimoni mélange, as well as the Firza flysch, areprobably the sedimentary response to the maintectonic stage that affected the Albanian ophio-lites. The involvement of the neighboring conti-nental margins in this stage is suggested by thediffuse occurrence of continental-derived blocksin the Simoni mélange, as well as by the debriscomposition of the Firza flysch. This tectonic-related sedimentation began during the Tithonianand continued until late Valanginian. The origin ofthe Simoni mélange and Firza flysch is still amatter of debate. Robertson and Shallo (2000)have proposed for these deposits an origin con-nected with mud-diapir(s) along large-scale faultsin the ophiolitic nappe, where fragments of theRubik complex were dragged up to the top ofthe ophiolitic nappe. An alternative explanation(Bortolotti et al. 2004b) is represented by a thrustlocated in the inner part of the ophiolitic nappeand able to expose the Rubik complex and theunderlying continental margin (i.e. the sourceareas of the Simoni mélange and Firza flysch).Recently, Dilek et al. (2005) have proposed asource area of the Simoni mélange and Firza fly-sch in correspondence with a tectonic wedgerelated to eastward emplacement of the Mirditaophiolitic nappe. Despite these suggestions, theorigin of these deposits still represents an openproblem. However, all the interpretations aboutthe origin are able to explain the stratigraphicfeatures detected in these deposits, such as, forinstance, the Triassic volcanics and Triassic–

Liassic carbonates, analogous to that recognizedin the underlying Rubik complex.

THE CARBONATE SEQUENCES AT THE TOP OF THE MIRDITA OPHIOLITES

The final emplacement of the Mirdita ophioliticnappe is followed by the unconformable sedimen-tation of the Barremian–Senonian, mainly shallow-water carbonate deposits at the top of the ophio-litic nappe. The age of these deposits confirms thatthe emplacement of the ophiolitic nappe was com-pleted in Early Cretaceous time and, from theLate Cretaceous onwards, the convergence mainlyaffected the Adria continental margins. Thesedeposits, as well as the overlying Meso-Hellenicdeposits, are affected by brittle deformations asinverse and normal faults (Kilias et al. 2001).These deformations can be regarded as the resultof the Lower Oligocene–Middle Miocene tectonicsconnected with the thrusting of the coupled Mird-ita ophiolitic nappe and Pelagonian units over theKrasta–Cukali, Kruja and Ionian units and laterextensional tectonics.

TECTONIC SIGNIFICANCE OF THE PESHKOPI AND SILLATINA WINDOWS

The occurrence of the Peshkopi and Sillatina tec-tonic windows, and the same structures along theShengerij corridor clearly demonstrate that thepresent-day tectonic setting of northern Albania isthe result of a large-scale, westward thrusting ofthe coupled Mirdita ophiolitic nappe and Pelago-nian units onto the units of the deformed Adriazone (i.e. the Ionian, Kruja and Krasta–Cukaliunits; Collaku et al. 1990, 1992; Kilias et al. 2001).This thrusting developed from the Early Oligo-cene to the Middle Miocene, according to the ageof the turbidites at the top of the Krasta–Cukali,Kruja and Ionian units. In addition, the structuresidentified in the Peshkopi and Sillatina tectonicwindows provide evidence for extensional tecton-ics that affected the eastern areas of northernAlbania in the Middle–Late Miocene timespan.The result of these extensional tectonics, charac-terized by folds and shear zones, is a strong mod-ification the previous structural setting.

EVIDENCE FOR AN EVENT OF OUT-OF-SEQUENCE THRUSTING IN THE PESHKOPI AND SILLATINA TECTONIC WINDOWS

The occurrence of a slice consisting ofUpper Jurassic–Lower Cretaceous ophiolite-bear-


ing mélange associated with serpentinite bodiesbetween the Pelagonian and the Krasta–Cukaliunits is a puzzling feature detected in the Peshkopiand Sillatina tectonic windows. This feature canbe interpreted as result of an event of out-of-sequence thrusting in Miocene time. The proposedreconstruction includes (i) the thrusting of the cou-pled Mirdita ophiolitic nappe and Pelagonian unitsonto the Krasta–Cukali unit in Early Oligocenetime; and (ii) the subsequent out-of-sequencethrust that affected the advancing front of theMirdita ophiolitic nappe (i.e. the Upper Jurassic–Lower Cretaceous ophiolite-bearing mélange andthe associated serpentinite bodies), today found inthe Peshkopi and Sillatina tectonic windows. Thisevent could have produced a tectonic structurewhere the Krasta–Cukali unit and the oceanic-derived slices are thrust by the Pelagonian unitswith the Mirdita ophiolitic nappe at the top.

A MODEL FOR THE TECTONIC HISTORY OF THE ALBANIAN OPHIOLITES

Different models have been proposed for thetectonic evolution of the Albanian ophiolites (seeShallo & Dilek 2003 for a complete review). Thesemodels (Collaku et al. 1991; Beccaluva et al. 1994;Shallo 1994; Kodra et al. 2000; Robertson & Shallo2000; Bortolotti et al. 2002, 2004b; Saccani et al.2004; Dilek et al. 2005) show relevant differences,mainly concerning the features of the Triassic–Early Cretaceous geodynamic history.

For instance, the rifting processes related tothe opening of the oceanic basin, from which theMirdita ophiolitic nappe was derived, are generallyreported as Early Jurassic in age by mostresearchers. However, Bortolotti et al. (2004b)have proposed an Early Triassic age for the riftingprocess, based on new geochemical and paleonto-logical evidence. In addition, different inter-pretations have been proposed about thepaleogeographic location of the oceanic basin inJurassic time. Most authors (Beccaluva et al. 1994;Shallo 1994; Kodra et al. 2000; Robertson & Shallo2000; Saccani et al. 2004; Dilek et al. 2005) suggesta location between the Adria and PelagonianPlates (i.e. in the same position as the present-dayMirdita nappe). However, other authors (Collakuet al. 1991; Bortolotti et al. 2004b) suggest an orig-inal location east of the Pelagonian zone (i.e. in theVardar domain), mainly through structural evi-dence. In these interpretations, the emplacementof the Mirdita ophiolitic nappe is the result of a

large-scale displacement toward the Adria Plate.Another matter of debate is represented by thedipping of the subduction during the LateJurassic–Early Cretaceous timespan. Generally,this subduction zone has been envisaged as beingwestward dipping below the Pelagonian or Eur-asian Plate (Collaku et al. 1991; Beccaluva et al.1994; Shallo 1994; Kodra et al. 2000; Bortolottiet al. 2002, 2004b). However, recent models haveproposed a dip of the subduction below the AdriaPlate (Robertson & Shallo 2000; Saccani et al.2004; Dilek et al. 2005). In contrast, there is a gen-eral agreement on the dipping of the subductionzone in Tertiary time about a westward dippingsubduction zone. Finally, the sense of shear duringthe obduction of the ophiolites is also under discus-sion. Most authors suggest a bi-divergent model(Beccaluva et al. 1994; Shallo 1994; Kodra et al.2000; Robertson & Shallo 2000; Saccani et al. 2004)with a coeval opposite sense of shear on both thewestern and eastern sides of the Mirdita ophioliticnappe. Alternatively, other authors have proposeda westward (Collaku et al. 1991; Bortolotti et al.2002, 2004b) or eastward sense of shear (Dileket al. 2005).

Taking into account all the models discussedpreviously, a tentative reconstruction of the tec-tonic history of the Albania ophiolites in the Meso-zoic–Tertiary timespan is provided here, mainlybased on geological and structural data aboutthe Mirdita ophiolitic nappe, as well as evidencederived from the underlying units of the Pelago-nian and deformed Adria zones. A sketch of thisevolution is shown in Figure 11.• The history started in Triassic time, as sug-

gested by the occurrence of Triassic MORbasalts in the Rubik complex. This occurrenceprovides the evidence for an oceanic basinalready having opened between the Adria andEurasia Plates in Middle Triassic time. Thisoceanic basin developed through a Lower Trias-sic rifting stage, the evidence of which is pre-served in the carbonate slices of the Rubikcomplex, where pelagic succession of Triassicage is recognized. The phase of spreading con-tinued from Middle to Late Triassic (stage A inFig. 11) according to the age of radiolaritesassociated with MOR basalts in the Rubikcomplex.

• Subsequently, in Early Jurassic time (stage B inFig. 11), the oceanic basin was affected byconvergence, when a subduction zone developedas result of the sharp change in the motionbetween the Adria and Eurasia Plates. The


existence of this subduction zone is provided bythe occurrence of the SSZ-related magmaticsequences found in the Western and Easternunits of the Mirdita ophiolitic nappe, whereupper Bajocian–lower Bathonian cherts inter-calated in IAT basalts have been found. In theSSZ, coexistence of a MOR oceanic lithospherewith SSZ magmatism has been found in theWestern unit of the Mirdita nappe (Bortolottiet al. 1996, 2002; Hoeck et al. 2002; Saccani et al.2004). This MOR lithosphere is regarded asbeing trapped in the SSZ basin (most probablyin a proto-forearc region) with consequentemplacement of intermediate MOR–IAT andIAT basalts, as well as boninitic dykes.

• In the same basin, the SSZ lithosphere of theEastern unit was generated at a subsequentstage of the subduction process (stage C inFig. 11). On the whole, all the Jurassic ophio-lites from northern Albania represent a com-posite oceanic crust belonging to the sameoceanic basin (i.e. a supra-subduction basin),which experienced two different accretionevents, in a mid-ocean ridge spreading centerand, subsequently, in a supra-subductionsetting. According to Dewey (1980), the processof the opening of an oceanic basin in a supra-subduction setting can be regarded as the resultof the roll-back of the hinge zone belonging tothe down-going plate. When the rate of the roll-back is higher than that of plate convergence,the extension in the supra-subduction takesplace with the opening of an oceanic basin (e.g.Beccaluva et al. 2004; Dilek et al. 2005). There-fore, the resulting picture for the earlier MiddleJurassic timespan includes an oceanic basinlocated eastwards of the Adria Plate and char-acterized by a subduction zone separating thelower plate with a MOR oceanic lithosphere,today preserved only in the Rubik complex,from an upper plate where a trapped MORoceanic lithosphere coexisted with the SSZlithosphere.

• During the Middle Jurassic, the continuous con-vergence between the Adria and Eurasia Platesresulted in the obduction of the SSZ oceaniclithosphere; this event was probably connectedwith the involvement of the continental crust inthe subduction zone. This event produced asharp decrease of the rate for the roll-back ofthe down-going plate that became lower thanthe rate of the plate convergence. Subsequently,the transfer of the compression in the SSZ lead-ing to the inception of the obduction process

occurred (stage D in Fig. 11). According toMichard et al. (1991), the obduction processconsists of two different stages – the intraoce-anic and marginal stages. The intraoceanicstage is characterized by the thrusting of a sec-tion of oceanic lithosphere over the neighboringone. The development of high-grade meta-morphic rocks (i.e. the amphibolites and thegranulites) occurs in correspondence with thehigh-temperature shear zone between the twosections of the young and still hot oceaniclithosphere. According to radiometric dating,this stage occurred from 159.0 ± 2.6 to 171.7 ±1.7 Ma (i.e. from the Middle Jurassic to the ear-lier Late Jurassic; Dimo-Lahitte et al. 2001).The sense of shear indicators collected in themetamorphic sole from both the Western andEastern units provided evidence for a displace-ment from east to west or from southeast tonorthwest during the intraoceanic thrusting.According to Collaku et al. (1992), this evidencesuggests that the paleogeographic location ofthe Jurassic oceanic basin was eastward of thePelagonian zone, which can be interpreted asthe easternmost portion of the Adria Plate. Inthis frame, the Mirdita ophiolites were derivedfrom an oceanic domain eastward of the AdriaPlate. Probably, the roots of this oceanic basinare located in the Vardar zone or at its boundarywith the Serbo–Macedonian–Rhodope Massif.For instance, Brown and Robertson (2004) sug-gested a configuration of the Vardar zone withtwo oceanic basins of different ages. In theirhypothesis, the easternmost oceanic basin ofJurassic age can be regarded as the area fromwhich the Albanian ophiolites were derived.This oceanic basin, probably totally destroyedin the Early Cretaceous, was separated fromthe westernmost oceanic basin by a continentalmicroplate where a magmatic arc wasemplaced. This basin developed in Late Juras-sic time and definitively closed only in EarlyTertiary time, as suggested, for example, byPamic et al. (2002).

• In the Late Jurassic (stage E of Fig. 11), themarginal stage developed by the emplacementof the ophiolitic nappe onto the continental mar-gin (Shallo 1991, 1992, 1994; Kodra et al. 1993;Bortolotti et al. 1996; Robertson & Shallo 2000).As for the worldwide examples of obductedophiolites (e.g. Dewey & Bird 1971; Gealy 1977;Coleman 1981; Moores 1982; Michard et al.1991; Cawood & Suhr 1992; Searle & Cox 1999;Searle et al. 2004; and many others), the Mird-


ita ophiolitic nappe is characterized by well-preserved sequences unaffected by the ductileand pervasive deformations and related meta-morphism. All the obduction-related deforma-tions are accumulated in the metamorphic soleand the original features of the Western andEastern ophiolitic units remain slightly modi-fied. The result of this continental marginemplacement is a tectonic wedge (i.e. the Rubikcomplex), consisting of continental- andoceanic-derived slices detached during theemplacement of the ophiolite nappe and sand-wiched between the obducted ophiolites and thecontinental margin. The origin of this complexis probably a multistage process, with interfer-ence of sedimentary and tectonic events. Theoccurrence of ophiolite slices involved in thecomplex is a puzzling feature. They can be inter-preted as remnants of an older, pre-existingaccretionary wedge developed during theLower to Middle Jurassic subduction. In thishypothesis, the accretionary wedge wasenclosed in the Rubik complex during the pro-gressive displacement of the ophiolitic nappetowards the continental margin. If the ophioliticnappe was derived from an SSZ basin, as testi-fied by the geochemical affinity of the intrusiveand magmatic sequences, part of the accretion-ary wedge, developed in correspondence withthe subduction zone, can be deformed and par-tially enclosed at the base of the ophiolitic nappeduring its displacement. During this secondstage, a basin filled by ophiolite-bearing depos-its (i.e. the Simoni mélange and the Firza flysch;Gardin et al. 1996) developed at the top of theophiolitic nappe. In the Early Cretaceous, theemplacement of the ophiolites onto the eastern-most area of the Adria continental margin,today represented by the units from the Pelag-onian zone, was completed.

• The final emplacement of the ophiolites ismarked by the unconformable sedimentationof the carbonate deposits at the top of theophiolitic nappe. The Barremian age of thesedeposits confirms that the emplacement of theophiolitic nappe was ultimated in the EarlyCretaceous (stage F in Fig. 11).

• Subsequently, from the Late Cretaceousonward (stage G in Fig. 11), the compression-related deformation was transferred to theAdria continental margin. This progressivedeformation, which affects the units derivedfrom the deformed Adria zone, is well repre-sented by the shifting in the age of the turbid-

ites at the top of the successions as well as thetime of the inception of the deformation in eachunit.

• The Maastrichtian–Upper Eocene turbiditesfrom the Krasta–Cukali unit represent the firstforedeep deposits, subsequently deformed inthe Early Oligocene (stage H in Fig. 11).

• The second foredeep is represented by theUpper Eocene–Lower Miocene turbidite depos-its from the Kruja unit, in turn deformed in thelatest Early Miocene (stage I in Fig. 11). Prob-ably, the Lower Miocene deformation wasassociated by an out-of-sequence thrust, theevidence for which is provided by the tectonicsetting of the Peshkopi windows, where slices ofophiolite-bearing deposits are sandwichedbetween the Krasta–Cukali and Pelagonianunits. In this frame, the thrusts recognized inthe Meso-Hellenic deposits at the top of theMirdita ophiolitic nappe can be related to thisout-of-sequence thrust event. In the sametimespan, the Pelagonian units are thrust by thecontinental and oceanic units from the Vardarzone, which represents a suture where the Alba-nian ophiolites were derived (Collaku et al.1992). On the whole, the progressive thrustingfrom the Late Cretaceous to the Early Mioceneproduced an imbricate stack of tectonic units atthe front (i.e. at the western side) of the Mirditaophiolitic nappe (Fig. 3). The progressive east-ward thrusting into the Adria domain ischaracterized by the steepening of the thrustdeveloped in the first stage of the intracontinen-tal deformation. This process can explain theoccurrence of very steep, up to vertical attitude,thrusts among the ophiolitic units, the Rubikcomplex and the Krasta–Cukali units, asdepicted in Figure 3.

• In the Middle to Late Miocene (stage L inFig. 11), a thinning of the whole nappe pile wasachieved by extensional tectonics, while thecompression was still active in the westernmostarea of the Adria Plate. The extensional tecton-ics mainly resulted in the exhumation of theIonian, Kruja and Krasta–Cukali units asobserved today at the core of the Peshkopi andSillatina windows, regarded here as first-orderextension-related deformation structures. Thenormal and reverse faults related to theMiocene deformations are also recognized inthe Mirdita ophiolites (Kilias et al. 2001). On thewhole, the Miocene deformations resulted in theuplift and exposition of the Mirdita ophiolites asobserved today.


CONCLUSIONS

The Mirdita ophiolitic nappe preserves the recordsof a long-lived, Triassic–Miocene history of a frag-ment of the eastern branch from the Tethyan oce-anic basin, located between the Adria and Eurasiacontinental margins. This basin was originated bya Lower Triassic rifting phase, followed by aMiddle Triassic spreading phase, leading to a widearea characterized by a MOR oceanic lithosphere.This oceanic basin was affected in Early Jurassictime by convergence, with the development of asubduction zone, probably dipping below the Eur-asia Plate. During the Middle Jurassic, the sub-duction processes resulted in the development ofa wide SSZ characterized by an SSZ oceanic litho-sphere. The continuous convergence resulted, inthe latest Middle Jurassic, in the obduction of theophiolites, represented by the Mirdita ophioliticnappe. The collected data confirm that theobducted ophiolites are representative of a sectionof SSZ oceanic lithosphere deformed when it wasyoung and still hot, immediately after the end ofthe magmatic processes. The obduction occurredin the upper plate of the subduction zone, probablycharacterized by a dipping below the EurasiaPlate. This geodynamic frame is analogous to thatof the SSZ ophiolites from Oman, Cyprus,Newfoundland, etc. Different from these exam-ples, the Albanian ophiolites provide evidence forthe involvement in the obduction process of MORophiolites, as detected in the Western units. How-ever, the Western ophiolites can be interpreted asrepresentative of a trapped oceanic crust in anSSZ geodynamic setting. This confirms that all theobducted ophiolites were derived from an SSZgeodynamic setting. According to Michard et al.(1991), the obduction history developed throughtwo steps, referred to as the intraoceanic and mar-ginal stages. During the intraoceanic stage, themetamorphic sole developed at the base of theAlbanian ophiolites, represented by a level of oce-anic-derived rocks up to 700 m thick, highlydeformed under amphibolite and granulite faciesmetamorphism. This metamorphism is associatedwith a polyphase ductile history developed at highgeothermal gradient, as expected for an intraoce-anic thrusting of hot oceanic lithosphere. Duringthe intraoceanic stage, the deformation is localizedonly at the base of the overriden section of oceaniclithosphere. In contrast, in the marginal stage, thedeformation is brittle and affects the whole ophio-lite sequence, from mantle to sedimentary cover.In the Albanian ophiolites, the marginal stage-

related deformations are represented by apolyphase history developed by thrusts and foldsunder very low-grade metamorphic conditions, asdetected for the Kalur cherts. The marginal stageis interpreted as the result of the thrusting ontothe continental margin of a cold oceanic litho-sphere. On the whole, the deformations detectedin the Albanian ophiolites are mainly referred tothe obduction history, from the intraoceanic to themarginal stage. However, the Albanian ophiolitesprovide evidence for a large-scale displacementwithout strong internal deformations during thepost-obduction history. This displacement wasacquired during the intracontinental convergence,by Upper Cretaceous–Miocene compressive andextensional events. After these events, the Mirditaophiolites acquired their present-day tectonic set-ting – that is, a giant oceanic nappe floating overthe units derived from the Adria Plate.

ACKNOWLEDGEMENTS

This research was supported by M.I.U.R (ProjectPRIN), by C.N.R (Istituto di Geoscienze eGeorisorse) and by funds from ATENEO grantfrom Firenze and Pisa Universities. We thankAlaudin Kodra and Farouk Mustafa for their field-work assistance and stimulating discussions. Pro-fessor Minella Shallo and Dr Dede Kolndreu arealso acknowledged for their careful review. Thispaper greatly benefited from discussions with L.Beccaluva, M. Chiari, L. Cortesogno, M. Fazzuoli,L. Gaggero, M. Marcucci and E. Saccani.

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Thematic Article Mesozoic to Tertiary tectonic history of ... · common mechanisms for ophiolite emplacement is obduction (i.e. the thrusting of internally unde- ... undeformed nappes