How to resist subduction: evidence for large-scale out-of-sequence thrusting during Eocene collision...

Journal of the Geological Society, London, Vol. 158, 2001, pp. 769–784. Printed in Great Britain.

How to resist subduction: evidence for large-scale out-of-sequence thrusting duringEocene collision in western Turkey

KLAUS GESSNER1,3, UWE RING1, CEES W. PASSCHIER1 &TALIP GU} NGO} R2

1Institut fur Geowissenschaften, Johannes Gutenberg-Universitat, 55099 Mainz, Germany.2Jeoloji Muhendisligi Bolumu, Dokuz Eylul U} niversitesi, 35100 Bornova, Turkey.

3Present address: CSIRO Exploration and Mining, PO Box 1130, Bentley WA 6102, Australia.(e-mail: [email protected])

Abstract: Significant along-strike variations have locked large parts of the Alpine subduction complex inthe Eastern Mediterranean in the Eocene, and defined the end of high-pressure accretion in westernTurkey. Structural analysis reveals that the Anatolide belt in western Turkey formed under greenschistfacies metamorphic conditions in the Eocene when a high-pressure metamorphic fragment of the Adriaticplate (the Cycladic blueschist unit) was thrust onto the imbricated mid-crustal units of the Anatolianmicrocontinent (the Menderes nappes). The contact between the Cycladic blueschist unit and theMenderes nappes, the Cyclades–Menderes thrust, represents an out-of-sequence ramp which cutsup-section towards the south. The lack of Alpine high-pressure fabrics below the Cyclades–Menderesthrust implies c. 35 km of exhumation of the Cycladic blueschist prior to its Eocene emplacement on topof the Menderes nappes. Structure and geodynamic evolution of the Anatolide belt are in striking contrastto the neighbouring Aegean and contradict the model of a laterally continuous orogenic zone, in which theAnatolide microcontinent is interpreted as an eastern extension of the Adriatic plate.

Keywords: Alpine orogeny, Turkey, Aegean Sea, metamorphism, thrust faults, exhumation.

The Hellenide–Anatolide orogen in the Eastern Mediterraneanformed by Cretaceous to recent accretion of continental frag-ments to the active southern margin of Eurasia (Sengor &Yilmaz 1981; Robertson & Dixon 1984). Tectonic units withinthe Hellenide–Anatolide orogen are aligned parallel to thepresent-day Hellenic subduction zone, and have traditionallybeen regarded as an arcuate arrangement of laterally continu-ous orogenic belts (e.g. Brunn 1956; Aubouin 1959; Durr et al.1978; Jacobshagen et al. 1978). A fundamental assumption ofthis hypothesis is that the Pelagonian zone (Aubouin 1959), theCycladic zone and the Menderes Massif (Parejas 1940) can begrouped together as a continuous ‘Median Crystalline Belt’(Durr et al. 1978). In this ‘classical’ interpretation the MedianCrystalline Belt represents Carboniferous basement andPermo-Mesozoic cover of the Adriatic plate (Fig. 1). It hasrecently been proposed that only the upper levels of theMenderes Massif (the ‘Cycladic blueschist unit’) were corre-lative with the Cycladic zone, while the lower units wereformed by the exotic Menderes nappes (Ring et al. 1999a). Thesubdivision of these authors will be adapted in this paper, andthe term ‘Menderes Massif’ will not be used in a tectoniccontext.

A striking characteristic of the Hellenides in the Aegean isthe southward propagation of nappe emplacement and high-pressure metamorphism. In this paper, it will be shown howthis propagation relates to the formation of the Anatolide belt,and that the crucial difference between the Hellenides and theAnatolides is due to along-strike variations in the Alpinesubduction complex. In detail, aspects of the tectonic develop-ment of the Cycladic zone and the Anatolide belt will bereviewed, followed by a structural study of the Cycladicblueschist unit in western Turkey and the Menderes nappes.It will be shown that the tectonometamorphic differences of

both units and the geometry of their thrust contact havesignificant implications on the timing of collision andexhumation processes, and on the palaeogeography of theeastern Mediterranean.

Overview

The nappe pile in the Aegean and western TurkeyThe Hellenides can be subdivided from top (internides) tobottom (externides) into (1) the internal zone, (2) the Vardar–Izmir–Ankara zone, (3) the Lycian Allochthon, (4) theCycladic zone, and (5) the external Hellenides (Fig. 2). A majordifference between the Aegean and western Turkey is thatin the latter the Menderes nappes instead of the externalHellenides form the lowermost tectonic unit (Fig. 2).

The internal zone consists of continental fragments of theEurasian plate, the south-easternmost being the Sakarya con-tinent (Fig. 1), underneath which oceanic crust of Neotethyswas subducted during Cretaceous convergence (S(engor &Yilmaz 1981). The related suture is the Vardar–Izmir–Ankarazone (S(engor et al. 1984), parts of which were metamorphosedunder blueschist-facies conditions in the Late Cretaceous(Sherlock et al. 1999) (Fig. 2c). The Lycian Allochthon(Collins & Robertson 1997) is a thin-skinned thrust belt, whichroots in the Vardar–Izmir–Ankara zone (Collins & Robertson1997, 1998, 1999). Within the Lycian Allochthon, top-to-the-south displacement occurred from the Late Cretaceous to theLate Miocene (Collins & Robertson 1998, 1999). Parts of theLycian Allochthon were metamorphosed under incipient high-pressure conditions (Bernoulli et al. 1974; Franz & Okrusch1992; Oberhansli et al. 2001).

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The Cycladic zone consists of continental fragments of theAdriatic plate (Fig. 1) and can be subdivided into threetectonic units (Altherr & Seidel 1977; Avigad et al. 1997; Ringet al. 1999b), which are from structurally highest to lowest:(i) the heterogeneous Cycladic ophiolite nappe consisting ofunmetamorphosed to greenschist-facies ophiolitic and sedi-mentary rocks containing high-grade metamorphic blocks ofJurassic and Cretaceous age. (ii) The Cycladic blueschist unit,which is made up of a high-pressure nappe stack compris-ing from top to bottom an ophiolitic melange, a Permocarbon-iferous to Mesozoic shelf sequence, and a Carboniferousbasement. (iii) The basal unit, which is exposed in at least fourtectonic windows (Avigad et al. 1997; Ring et al. 2001a) (Fig.2) and is likely to be part of the Permian to PalaeogeneExternal Hellenides. Overall the temporal progradation ofnappe emplacement and high-pressure metamorphism towardsthe south mimics the southward retreat of the Hellenic sub-duction zone.

In contrast to parts of the External Hellenides, the Menderesnappes, which also occur tectonically below the Cycladic zone,do not show Alpine high-pressure metamorphism (Ring et al.1999a; Okay 2001). Another important difference is that thebasement of one of the Menderes nappes (Cq ine nappe, seebelow) is of Neoproterozoic to Cambrian age (Kroner &S(engor 1990; Hetzel & Reischmann 1996; Hetzel et al. 1998;Loos & Reischmann 1999). This basement preserves deforma-tion fabrics of latest Proterozoic age (the DPA event of Gessneret al. 2001a; the suffix ‘PA’ stands for pre-Alpine). The absenceof Alpine high-pressure metamorphism in the Menderesnappes and the lack of a well-defined subduction zone to thesouth of western Turkey suggest that subduction ceased after

the collision of the exotic Anatolide microcontinent in theEocene (Hetzel et al. 1995b; Ring et al. 1999a).

Lithology and tectonometamorphic evolution of theCycladic blueschist unit in the AegeanAspects of the tectonometamorphic evolution of the Cycladicblueschist unit in the Aegean are reviewed here to allowcomparisons with the development of the Cycladic blueschistunit in western Turkey. According to Ring et al. (1999b) theCycladic blueschist unit on Samos Island consists of threehigh-pressure nappes, which are from top to bottom: (1) theSelcuk nappe (ophiolitic melange), (2) the Ampelos nappe(Permocarboniferous to Mesozoic passive margin sequence),and (3) the Agios Nikolaos nappe (Carboniferous basement).

The Selcuk nappe contains blocks of metagabbro in a matrixof serpentinite and garnet–mica schist. Ring et al. (1999b)argued that the Selcuk nappe was correlative with theophiolitic melanges on the Cycladic islands of Syros and Tinos(Okrusch & Brocker 1990; Brocker & Enders 1999), where arock unit similar in lithology and tectonic position is exposed.High-pressure P–T conditions are in the range of 10–15 kbarand 400–500�C (Ring et al. 1999b).

The Ampelos nappe and correlative tectonic units across theentire Cyclades consist of quartzite, metapelite and metabasiteoverlain by marble containing metabauxite (e.g. Durr et al.1978). This succession has been interpreted as a former passivecontinental margin sequence (Altherr & Seidel 1977). Theunderlying Agios Nikolaos nappe contains marble and garnet-mica schist intruded by Carboniferous orthogneiss, and repre-sents part of the former basement of the shelf sequence. P–T

Fig. 1. Sketch maps of two commonpalaeogeographic models of the easternMediterranean illustrating the spatialarrangement of continents in the earlyCretaceous modified from (a) S(engor &Yilmaz (1981) and (b) Robertson et al.(1996). Notice that in both modelsPelagonia, Adria and the Anatolia(Menderes Tauride platform sensuS(engor & Yilmaz (1981);Anatolide–Tauride continent sensuRobertson et al. (1996)) represent thepre-Permian northern margin ofGondwana and that the lower LycianAllochthon is assumed to represent partof the sedimentary cover of AnatolianMicrocontinent.

Fig. 2. Map, cross-sections and table showing the major tectonometamorphic units of the Hellenide-Anatolide orogenic belt and the southwardprogression of high-pressure metamorphism. The Pelagonian and Cycladic zones are shown together. The Cycladic blueschist unit makes uplargest part of Cycladic zone, where it occurs below heterogeneous Cycladic ophiolite nappe and above post-middle Eocene metaflysch ofExternal Hellenides. Windows of External Hellenides below Pelagonian and Cycladic zones are marked with asterisk and abbreviated OW,Olympus window; APW, Almyropotamos window; PW, Panormos window; KN, Kerketas nappe. Cross sections A–A’ and B–B’ show differentunits in footwall of Cycladic blueschist unit: External Hellenides in Aegean and Menderes nappes in western Turkey. Box indicates location ofFigure 4. Modified after Schermer et al. (1990), Seidel et al. (1982), Avigad et al. (1997), Walcott (1998), Brocker & Enders (1999) and Ringet al. (1999b). Insert shows location of main map in Mediterranean and regional extent of the Hellenides and Anatolides.

770 K. GESSNER ET AL.

OUT OF SEQUENCE THRUSTING, WESTERN TURKEY 771

conditions of the shelf/basement unit across the entire Cycladiczone are of the order of 12–19 kbar and 450–550�C (e.g.,Okrusch & Brocker 1990; Will et al. 1998).

The basal unit below the Cycladic blueschist unit largelyconsists of marbles capped by post-middle Eocene metaflysch.The basal unit underwent Oligocene to Miocene high-pressure metamorphism (Ring et al. 2001), which reachedabout 8–10 kbar and 350–400�C (Avigad et al. 1997; Ringet al. 1999b). A subsequent Barrovian metamorphism over-printed all high-pressure units and reached greenschist- toamphibolite-facies conditions.

Age data for high-pressure metamorphism show a consistentpattern across the Aegean. Zircons, which had overgrownhigh-pressure minerals in the ophiolitic melanges of Syros andTinos were dated by the U–Pb method at 78 Ma and 63–61 Ma(Brocker & Enders 1999). High-pressure metamorphism in theunderlying shelf/basement unit is usually placed into theEocene (53–40 Ma) and cooling during decompression belowc.350–400�C took place between 40 and 35 Ma (e.g. K/Ar,40Ar/39Ar, and Rb–Sr white mica ages of Altherr et al. 1979;Maluski et al. 1987 and Brocker et al. 1993). The Barrovian-type overprint occurred at about 25–15 Ma (e.g. Altherr et al.1979, 1982; Wijbrans & McDougall 1986, 1988; Wijbranset al. 1990; Brocker et al. 1993). High-pressure rocks of thebasal unit date from c. 40 to 35 Ma in the Olympus window(Schermer et al. 1990), from 24 to 21 Ma on Evia and Samos(Ring et al. 2001) and 24 to 20 Ma in Crete (Seidel et al. 1982;Thomson et al. 1999).

The tectonometamorphic evolution of the Cycladic blue-schist unit in the Aegean involved structures formed duringprograde high-pressure metamorphism. The structures youngstructurally downwards and were poikiloblastically overgrownby glaucophane, chloritoid and kyanite (Lister & Raouzaios1996 for Sifnos Island; Ring et al. 1999b for Samos Island).During subsequent decompression, thrusting of the Cycladicblueschist unit onto the basal unit was associated with moder-ate high-pressure metamorphism in the latter (Avigad et al.1997; Ring et al. 1999b; Ring et al. 2001). Succeeding mid-Oligocene to Recent crustal-scale extension occurred before,during and after the Barrovian metamorphic event. Extension

is manifest by greenschist-facies shear zones and brittle normalfaults, which are inferred to represent different generations ofdetachment systems (Forster & Lister 1999; Ring et al. 2001).

The exhumation of the Cycladic blueschist unit is generallyattributed to middle Oligocene normal faulting caused byrollback of the subducting Hellenic slab (e.g. Lister et al. 1984;Buick 1991; Raouzaios et al. 1996). Avigad et al. (1997) andRing et al. (1999b), however, argued that on the islands of Eviaand Samos up to 30–40 km of the exhumation of the Cycladicblueschist unit occurred before the middle Oligocene.

Lithology and tectonometamorphic evolution of theMenderes nappesThree Alpine deformation events, abbreviated DA3–DA5 (suf-fix ‘A’ indicates an Alpine age), have been recognized in theMenderes nappes (the Alpine deformation events DA1 andDA2 are only found in the Cycladic blueschist unit, see below).Some of the Menderes nappes were affected by a pre-Alpineevent (DPA; Fig. 3) (Gessner et al. 2001). The Menderes nappesconsist from top to bottom of (1) the Selimiye nappe, (2) theCq ine nappe, (3) the Bozdag nappe and (4) the Bayındırnappe (Ring et al. 1999a; Gessner et al. 2001a) (Figs 4 and 5).

The Selimiye nappe contains a metasedimentary sequence,the basal part of which is of Precambrian age (Hetzel &Reischmann 1996; Loos & Reischmann 1999). Preliminarystructural work suggests that an amphibolite-facies fabric of asyet unresolved kinematics and age was overprinted by theAlpine DA3 top-to-the-south Selimiye shear zone, which sepa-rates the Selimiye nappe from the underlying Cq ine nappe. TheSelimiye shear zone formed during prograde greenschist-faciesmetamorphism and related structures were overgrown bygarnet at temperatures >450�C (Hetzel & Reischmann 1996).Hetzel & Reischmann (1996) showed that 39Ar/40Ar muscoviteages of 43–37 Ma record slow cooling below 350–400�C(assumed closure temperature for Ar diffusion in muscovite)after DA3 and after garnet growth.

The Cq ine and Bozdag nappes are characterized by a distinctoverprinting sequence of ductile fabrics. The structurallyhigher Cq ine nappe consists of amphibolite to granulite-facies

Fig. 3. Diagram showing distribution ofpre-Alpine and Alpine events DPA andDA1 to DA5 in the tectonic units ofAnatolide belt. Temperature estimatesafter own observations, except for Cq ineand Bozdag nappes (Ring et al. in press)and Dilek nappe (Will et al. 1998).Diagram indicates that the Anatolidebelt was assembled during DA3. DA5 isoutlined for its brittle deformation style;suffix ‘PA’ denotes pre-Alpine and ‘A’Alpine deformation age.


ortho- and paragneiss with intercalated metabasite (Dora et al.1995; Lackmann 1997), while the Bozdag nappe is composedof amphibolite-facies garnet–mica schist and metabasite.Gessner et al. (2001) gave geochronological evidence thatthe DPA event in the Bozdag and Cq ine nappes occurredduring amphibolite-facies metamorphism at c. 550 Ma andcaused top-to-the-NE shear. DPA was overprinted by aDA3 greenschist-facies tectonometamorphic event. The corre-sponding SA3 foliation crosscuts SPA and produced a variablyspaced shear-band foliation and a well-defined stretchinglineation associated with top-to-the-south kinematic indi-cators in both nappes and also in Triassic metagranite (cf.Fig. 4) (Gessner et al. 2001a). Exact P–T conditions forAlpine greenschist-facies metamorphism during DA3 areunknown.

The Bayındır nappe at the base contains shelf sediments ofinferred Permo-Carboniferous age (Osman Candan pers.comm. 1998), which were metamorphosed under lowergreenschist-facies conditions at c.37 Ma (Lips et al. 2001). Theabsence of biotite in rocks of suitable bulk compositionsuggests temperatures �c.400�C (Yardley 1989). The Bayındırnappe was deformed by the synmetamorphic DA3 event, whichis the first deformation event in the Bayındır nappe. Thecorresponding SA3 foliation is penetrative and associated with

a fine-grained N-trending stretching lineation (LA3) (Fig. 6b).LA3 is expressed by stretched quartz, albite and chloriteaggregates and aligned tourmaline. In the structurally highestparts of the Bayındır nappe north of Aydın, DA3 ductile shearbands and sigma-type objects (Passchier & Simpson 1986)indicate a top-to-the-south shear sense. However, north ofBozdag Mountain, mylonite that formed during the intrusionof the middle Miocene Turgutlu and Salihli granodiorites(Fig. 4), shows consistent top-to-the-north shear senseindicators associated with ductile extensional deformation(Hetzel et al. 1995b). Because the structures north of BozdagMountain formed in the middle Miocene, they are about15–20 million years younger than the DA3 fabrics, whichformed during greenschist-facies metamorphism at c.37 Ma(Lips et al. 2001). Therefore, the structures north of BozdagMountain are likely to represent a separate DA4 ductileextensional event.

Late Alpine brittle extension (DA5) is expressed by normal-fault systems of Miocene to Recent age (Cohen et al. 1995;Hetzel et al. 1995a; Emre & Sozbilir 1997). Since the Pliocene,two opposite-facing normal-fault systems, the Kuzey detach-ment in the north and the Guney detachment in the south(Ring et al. 1999a), have evolved into rolling hinges and haveproduced the synclinal structure of the Central Menderes

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Fig. 4. Tectonic map of western Turkey and Samos Island after Hetzel et al. (1995b), Hetzel (1995a), Dora et al. (1995), Candan & Dora (1998),Gungor (1998), Ring et al. (1999a), Ring et al. (1999b) and own observations. Lines for cross sections A–A� to G–G� (Fig. 5) and location ofFigure 12 are indicated; CMT, Cyclades–Menderes thrust.


metamorphic core complex (Gessner et al. 2001b) (Figs 4and 5).

Overall, this summary reveals two important aspects: (1)there is no evidence for Alpine high-pressure metamorphism inthe Menderes nappes and (2) the available, albeit scarce datasuggest that grade and age of metamorphism associated withDA3 decrease structurally downward. Temperatures in theSelimiye nappe were >450�C and occurred before 43–37 Ma(Hetzel & Reischmann 1996), whereas in the Bayındır nappetemperatures barely reached 400�C and occurred later atc. 37 Ma (Lips et al. 2001).

The Cycladic blueschist unit in western Turkey

Lithology and metamorphismIn western Turkey the Cycladic blueschist unit is made up bythe Selcuk melange and the underlying Dilek nappe (Erdogan& Gungor 1992; Candan et al. 1997; Gungor 1998; Ring et al.1999a). The Selcuk melange consists of blocks of metagabbroand metabauxite-bearing marble, which are surrounded by amatrix of serpentinite and garnet-mica schist. The Dilek nappe

is a metamorphosed Permo-Mesozoic shelf sequence, whichincludes a quartzite conglomerate with interlayered kyanite-chloritoid schist, metabasite, phyllite and marble containingmetabauxite. Deposition age of the marble ranges from lateTriassic through middle Palaeocene (Durr et al. 1978; O} zeret al. 2001). The passive-margin sequence is overlain by lowerto middle Palaeocene flysch (O} zer et al. 2001). The Selcukmelange correlates with the Selcuk nappe; the Dilek nappe iscorrelative with the Ampelos nappe in Samos (Candan et al.1997; Ring et al. 1999a). No Carboniferous basement isexposed below the Dilek nappe in western Turkey.

Candan et al. (1997) reported P–T conditions of >10 kbarand <470�C for high-pressure metamorphism in the Dileknappe, which approximates the c.15 kbar and 500�C estimatedfor the Ampelos nappe on Samos (Will et al. 1998). 39Ar/40Arages for phengite were interpreted to date high-pressuremetamorphism at c. 40 Ma in the Dilek nappe (Oberhansliet al. 1998). 39Ar/40Ar work by Ring et al. (unpublisheddata) on Samos Island suggests that ages of c.37–40 Mareflect phengite recrystallization during a high-pressuredeformation event, which post-dated the peak of high-pressuremetamorphism.

Fig. 5. Cross sections A–A’ to G–G’ (position of sections given in Fig. 4). Section A–A’ shows that Cyclades–Menderes thrust cuts up-sectiontowards the south in direction of tectonic transport. For geometric viability section planes are oriented parallel to mean orientation of LA3.Trace of foliation is projected into section plane and used to infer geometry of sub-surface structures.


Deformation historyGiven the Late Cretaceous fossil evidence in the Dileknappe, deformation events in the Cycladic blueschist unit inwestern Turkey must be of Alpine age, and will accordinglybe abbreviated DA1, DA2 and DA3. DA1 fabric elementsoccur exclusively as microscopic relics in the Dilek nappe;mesoscopic DA2 and DA3 fabrics are present in both theSelcuk melange and the Dilek nappe.

DA1 is limited to the internal foliation SA1 in millimetre- tocentimetre-sized chloritoid and kyanite porphyroclasts in theDilek nappe (Fig. 7a–c). In some porphyroclasts, a diffuseopaque banding oriented at a small angle to SA1 may representa sedimentary or a pre-DA1 deformation fabric (Fig. 7b).

A second foliation SA2 developed heterogeneously in rocksof the Selcuk melange. Serpentinite and metapelite display apenetrative foliation, which is associated with a well-defined

Fig. 6. Orientations of stretchinglineations LA2 (a) and LA3 (b) in mapview and stereograms (equal-arealower-hemisphere projections). In (b), astereogram of LA3 in Bayındır nappe isshown for comparison.


NE- to east-trending stretching lineation. Some metabasic andultrabasic lithologies occur as massive, largely unfoliatedblocks. In the Dilek nappe, DA2 fabrics are best preserved inmarble, quartzite and kyanite-chloritoid schist, whereas inphyllite only relics of DA2 occur. In quartzite and marble, thearrangement of quartz, calcite, white mica and chlorite definesa pervasive SA2 foliation parallel to lithological layering.In kyanite-chloritoid schist, SA2 is the dominant foliation,which is expressed by the preferred orientation of white micaand flattened quartz. On SA2, a stretching lineation LA2 isexpressed by elongated quartz pebbles and quartz-fibreaggregates in quartzite, kyanite–chloritoid schist and phylliteand by elongated aggregates of white mica in marble.Stretched LA2 quartz-fibre aggregates commonly occur asfolded rods or in microlithons between later SA3 shear bands.Locally, kyanite laths, epidote and blue amphibole are alignedsubparallel to LA2. In zones where a DA3 fabric is weak orabsent, LA2 generally trends NE (Fig. 6a), whereas in zonesof high DA3 strain, the orientation of LA2 scatters around thenorth direction.

In zones of high DA3 strain, LA2 was apparently alignedsubparallel to LA3. The present difference of c. 45� between theazimuths of the LA2 and LA3 stretching lineations in rockswhich are not or only weakly deformed by DA3 is probablyclose to the original angular separation between LA2 and LA3

Fig. 7. Relation between foliations SA1 and SA2 and the inferredsense of shear in kyanite-chloritoid schist intercalations in quartziteconglomerate of the Dilek nappe. Mineral abbreviations:ky, kyanite; cld, chloritoid). Location of outcrop: 37�54.24N,027�21.92E. (a) Chloritoid �-type porphyroclast (sensu Passchier &Simpson 1986) with the internal foliation SA1 oriented at a highangle to SA2 in the matrix. Notice that crenulation of opaquebanding is indicated by arrow. Sense of shear is top-to-the-NE.Plane polarized light, field of view is 13�18 mm. (b) Chloritoidporphyroclast with curved inclusion trails illustrating angularrelationship between pre-SA1 opaque banding, internal foliation SA1,and foliation SA2 in matrix. Sense of shear is top-to-the-NE. Planepolarized light, field of view is 9�13.5 mm. (c) Kyaniteporphyroclast with �-type geometry indicating top-to-the-NE senseof shear. Field of view is 1.8�2.7 mm.

Fig. 8. Cartoon illustrating different regional stretching directionsduring (a) DA2 and (b) DA3 defined by mean orientations of LA2 andLA3; notice the DA1 relics in kyanite-chloritoid schist layers withinlenses of quartz conglomerate.


(Fig. 8a, b). The original orientation of LA2 can therefore beassumed to have been about NE–SW.

Shear sense associated with DA2 is not obvious in theoutcrops, but a consistent top-to-the-NE sense of shear canbe detected at the microscopic scale in kyanite–chloritoidschists. These contain �-shaped objects made up of chloritoidand kyanite porphyroclasts with strain shadows containingrecrystallized quartz (Fig. 7a, c). The asymmetries displayed bythe external and internal foliations of kyanite and chloritoidporphyroclasts can be explained by two kinematic modelswhich yield opposite shear sense (Passchier & Trouw 1996, fig.7.34a) (Fig. 9a and b). In the first model (Fig. 9a), theasymmetries are caused by dextral (i.e. top-to-the-SW in ourspecific case) rotation of the clast with respect to the floweigenvectors and the external foliation. In the second model(Fig. 9b), the external foliation rotates sinistrally (i.e. top-to-the-NE) with respect to the flow eigenvectors, while the clast isrotating little or not at all because it is coupled with the strainshadows. The latter model applies to the kyanite and chloritoidporphyroclasts described here, because of the asymmetric,stair-stepping shape of the strain shadows. Moreover, thetop-to-the-NE displacement along shear bands in the matrixindependently point to the same conclusion.

The contact between the Selcuk melange and the Dileknappe is exposed west of Tire. There, garnet–mica schist of theSelcuk melange overlies kyanite-bearing calcschist of the Dileknappe. At the contact, DA2 structures are pervasive. SA2

layering is parallel to the contact and is associated withNE-trending LA2, defined by stretched calcite–mica aggregatesand up to 30 mm long kyanite laths.

DA3 structures deformed the DA2 fabrics in the Selcukmelange and the Dilek nappe. These overprinting relations anddifferences in structural style allow for a distinction betweenDA2 and DA3 fabrics in many places. DA3 is well-documentedin phyllite and greenschist of the Dilek nappe whereas, in theSelcuk melange, DA3 structures are rare. SA3 occurs as apenetrative cleavage in phyllosilicate-rich lithologies, in whichSA2 is strongly transposed and SA3 usually forms the dominantfoliation. In marble and quartzite, SA3 is mostly absent. Incomposite lithologies like phyllite-quartzite alternations andcalcschist, SA3 represents a heterogeneous shear-band folia-tion, in which DA2 fabrics are offset by SA3 shear bands.In microlithons between the SA3 shear bands, crenulation ofSA2 is frequently observed. A north-trending LA3 stretchinglineation (Fig. 6b) is expressed by fine-grained whitemica-chlorite-quartz aggregates. In zones of high DA3 strain,folding of SA2 and LA2 is common with FA3 axes orientedsubparallel to LA3 (Fig. 10a) and locally LA2 quartz rodsare folded into FA3 sheath folds (Fig. 10b). Macroscopickinematic indicators associated with DA3 fabrics in theDilek nappe display a uniform top-to-the-south shear sense(Fig. 11a, b).

�� ;

�� ;

�

�

Fig. 9. Schematic model for the development of porphyroclasts.(a) The porphyroclast rotates freely with respect to floweigenvectors; internal and external foliation in this case belong tosame generation (i.e. SA2) and the inferred sense of shear would bedextral, i.e. top-to-the-SW. (b) Case, in which the foliation rotatessinistrally (i.e. top-to-the-NE) with respect to the porphyroclastwhich largely remains stationary because it is coupled with strainshadows. Because foliation rotates relative to porphyroclast newcleavage domains (SA2) form between strain shadows. Case (b) ispreferred, because it agrees with sense of shear indicated by shearbands in matrix. Notice that steep dip of internal foliation inporphyroclast is likely to be close to original attitude of SA1.

Fig. 10. Outcrop photographs of DA2 fabrics. (a) Folding of SA2

and LA2 about N-trending FA3 fold axes in calcschist-phylliteintercalation. Location of outcrop 37�54.24N, 027�21.92E. (b) Sheathfolding of LA2 quartz rod in albite–epidote greenschist of Dileknappe. LA3 is expressed by north-trending fine-grainedwhite-mica-aggregate lineation; plan view, outcrop is located at38�00.76N 027�06.23E.


The Cyclades–Menderes thrustThe Cyclades–Menderes thrust cuts through several nappes ofthe underlying Menderes nappe pile (Figs 4 and 5). In theSelcuk–Tire region, the Cyclades–Menderes thrust separatesthe Dilek nappe from the Bozdag nappe. East of O} demis, theDilek nappe overlies para- and orthogneiss of the Cq ine nappe.South of Selimiye, the Cyclades–Menderes thrust is obscuredby a series of imbrications, but most likely separates the Dileknappe from the Selimiye nappe.

The Cyclades–Menderes thrust has been studied in detail inthe Selcuk–Tire region, where it is well exposed along a ridgecrest NW of Yemisler (Fig. 12). There, the base of the Dileknappe consists of quartzite and marble, which make up thetwin peaks Ballikkayası Tepe and Bozkaya Tepesi. The south-ern slope of the ridge consists of garnet-mica schist andamphibolite of the Bozdag nappe in the footwall of theCyclades–Menderes thrust. South of Yemisler, amphibolite-facies DPA structures dominate the fabric in the Bozdag nappe.The SPA foliation is expressed by flattened potassium feldsparand quartz and by preferred orientation of white mica andbiotite. SPA is associated with a NE-trending LPA stretchinglineation made up by elongated biotite-white mica and quartz-potassium feldspar aggregates. Shear bands indicate a top-to-the-NE sense of shear. In a several hundred metres thick

section north of the village of Yemisler, the amphibolite-facies DPA structures in the Bozdag nappe are progressivelydestroyed within a greenschist-facies shear zone (Fig. 13a, b).Overprinting criteria, style and metamorphic grade of thefoliation and fold axes, as well as the orientation of thestretching lineation (cf. Fig. 11) and associated kinematicindicators allow the correlation of these greenschist-faciesstructures with DA3 structures in the overlying Dilek nappe.

Interpretation of deformation–metamorphism–timingrelationships

DA1 and DA2

Chloritoid and kyanite form a peak high-pressure assemblagein the Ampelos nappe in Samos (Will et al. 1998) and thecorrelative Dilek nappe in western Turkey. The temporalrelation between the formation of SA1 and SA2 and the growthof chloritoid and kyanite reveals aspects of the tectonometa-morphic history of the Dilek nappe during high-pressuremetamorphism. An interpretative sequence of the relationshipbetween the development of structures and mineral growth isshown in Figure 14a–d. Growth of chloritoid and kyaniteporphyroblasts occurred after the formation of SA1 and over-lapped with the onset of DA2 structures. Early during DA2,chloritoid and kyanite ceased to grow (Fig. 14c), which makesa relation of DA2 to decompression likely. 40Ar/39Ar phengiteage data obtained by Ring et al. (unpublished) for SamosIsland suggest an age of c. 40 Ma for DA2.

DA3

Deformation–metamorphism relations during DA3 are com-plex and heterogeneous. In the Selimiye nappe, Hetzel &Reischmann (1996) reported growth of garnet (i.e. tempera-tures exceeding 450�C) after the formation of the DA3 Selimiyeshear zone. Accordingly, 40Ar/39Ar white-mica ages from theSelimiye shear zone were interpreted to date cooling aftershearing (Hetzel & Reischmann 1996). In the Bayındır nappeat the base of the Menderes nappes, DA3 fabrics formed atpeak-metamorphic temperatures of �400�C (Fig. 3). Hence,the 40Ar/39Ar white-mica ages of Lips (2001) closely date DA3

in the Bayındır nappe. As suggested above, downward propa-gation of DA3 thrusting was associated with decreasing tem-peratures.

Deformation–metamorphism relations across the Cyclades–Menderes thrust indicate that the breakdown of garnet andbiotite to chlorite in the Bozdag nappe at temperatures�c.400�C (Yardley 1989) occurred during DA3 mylonitiz-ation. As temperatures during deformation along theCyclades–Menderes thrust appear to have been at least 50�Clower than in the Selimiye shear zone and the inverted meta-morphic gradient in the Menderes nappe pile may be olderthan the Cyclades–Menderes thrust. It therefore appearsreasonable to regard the Cyclades–Menderes thrust as a lateDA3 structure. In concert with geometric constraints (Figs 5and 15), the age data suggest that the emplacement of theCycladic blueschist unit was by out-of-sequence thrusting.

Discussion

Tectonic implicationsDetailed structural work across the contact of the Dilek nappewith the underlying Bozdag nappe reveals that this contact is a

Fig. 11. Macroscopic DA3 shear-sense indicators. (a) DA3 shear zonein calcschist of Dilek nappe with �-type object of marble indicatingtop-to-the-S sense of shear; location is 37�55.01�N, 027�20.12�E.(b) Asymmetric DA3 fold in quartzite of Bozdag nappe with closelyspaced axial-planar SA3 cleavage. Outcrop is located at 38�01.75�N,27�41.11�E.


late DA3 greenschist-facies shear zone, the Cyclades–Menderesthrust, along which the Cycladic blueschist unit was emplacedon top of the Menderes nappes by out-of-sequence thrusting.Significant differences in tectonometamorphic history ofthe hanging wall and footwall prior to movement along theCyclades–Menderes thrust imply large displacements along thelatter. In the Cycladic blueschist, the Cyclades–Menderesthrust overprinted a two-phase Alpine high-pressure history; inthe footwall, the Cyclades–Menderes thrust crosscuts pre-Alpine structures. During mylonitization along the Cyclades–Menderes thrust the temperature was relatively low at least inthe upper portion of the Menderes nappe pile, if compared tothe temperature at which DA3 fabrics formed in the Selimiyenappe. This suggests that the Menderes nappes had beenassembled early during DA3, before the Cycladic blueschistunit was emplaced.

The reason for proposing that DA3 and the associatedCyclades–Menderes thrust resulted from crustal shortening isthat the Cycladic blueschist unit ramped upwards relative tothe Earth’s surface in the direction of DA3 transport. As

illustrated in Figure 5, the Cyclades–Menderes thrust cutsup-section through the Menderes nappe pile, which was as-sembled during an earlier stage of DA3. Within the Menderesnappes, the inverted metamorphic gradient also suggests crus-tal shortening. Even if the original dip of the Menderes nappecontacts had been subhorizontal, the Cyclades-Menderesthrust still would have had to be somewhat steeper in order tocut up-section in the direction of transport. This geometry isdisplayed in Figure 15, which illustrates the proposed thrustsequence and the thermochronological data. The myloniticrocks in the Selimiye shear zone cooled below 350–400�Cbetween 43 and 37 Ma (Hetzel & Reischmann 1996) (thrust 5in Fig. 15). DA3 thrusting in the Menderes nappes progressedstructurally downwards and affected the Bayındır nappeat c.37 Ma (Lips 2001) (thrusts 6 and 7 in Fig. 15). Ifthis interpretation is correct, phengite 40Ar/39Ar ages of 40 Mareported by Oberhansli et al. (1998) from the Dilek nappewould overlap with the cooling ages below the Cyclades–Menderes thrust. This implies that greenschist-faciesdisplacement of the Cycladic blueschist unit along the

Fig. 12. Structural map and cross sectionfrom area southwest of Tire, where theCyclades-Menderes Thrust is exposed(box in Fig. 4 shows location of map).The dashed line indicates the extentof DA3 fabrics across theCyclades–Menderes thrust.


Cyclades–Menderes thrust occurred shortly after the end ofblueschist-facies metamorphism; DA2 and DA3 in part mighteven have overlapped in time. It remains unclear whetherdeformation of the Bayındır nappe occurred before, duringor after motion at the Cyclades–Menderes thrust. Furtherthermochronological research is needed to better constrain theage of the Cyclades–Menderes thrust.

Overall, the tectonic evolution in western Turkey as outlinedin Figure 15 is in striking contrast to the orogenic developmentin the Aegean, where the Cycladic blueschist unit rests onthe External Hellenides. The latter show early Oligocenehigh-pressure metamorphism in tectonic windows and earlyMiocene high-pressure metamorphism in Crete and thePeleponnesos (Fig. 2). The downward propagating high-pressure metamorphism in the Aegean was probably con-trolled by progressive southward retreat of the Hellenicsubduction zone, which most likely commenced in the Eocene(Thomson et al. 1998). In western Turkey, subduction-zoneretreat was probably halted when the exotic Anatolide micro-continent (Fig. 16) entered the subduction zone in the Eocene

(Hetzel et al. 1995a; Ring et al. 1999a). We speculate that theoverall continental architecture of Anatolia was different fromthat of easternmost Adria. While the crust of the latter wasthinned and maybe even oceanic (Pindos Basin), and hadprobably been easier to subduct than the crust of Anatoliathere is no indication that the crust of Anatolia was thinnedat all before entering the subduction zone; it therefore suc-cessfully resisted deep subduction (Fig. 16). Wijbrans &McDougall (1988) made a similar proposition by suggestingthat the Cycladic zone had formerly been a collage of smallfragments of easily subductable continental crust. We specu-late that accretion of Anatolia caused early DA3 thrusting inthe upper Menderes nappes (thrusts 5 and 6 in Fig. 15).Accretion of the Bayındır nappe may have caused a change inthe geometry of the orogenic wedge, hence triggering ad-ditional shortening, back-stepping of thrusting towards thehinterland and out-of-sequence emplacement of the Cycladicblueschist unit onto the Menderes nappes. This model allowssimultaneous in-sequence motion of the Bozdag nappe ontothe Bayındır nappe and out-of-sequence movement at theCyclades–Menderes thrust. Given the scarce thermochrono-logical data, the proposed thrust sequence remains speculativebut provides a testable working hypothesis.

The observation that the Cycladic blueschist unit sits on topof the Menderes nappes has also implications for the LycianAllochthon above the Cycladic blueschist unit. Because oro-genic development progressed structurally downward, high-pressure metamorphism in the Lycian Allochthon should beolder than that in the Cycladic blueschist unit (i.e. pre 40 Ma).Such an inference fits well into the recently proposed tectonicmodel for the Lycian Allochthon by Collins & Robertson(1998, 1999). However, the tectonic position of the LycianAllochthon above the ophiolitic Selcuk melange implies thatthe lower units of the Lycian Allochthon were either depositedon a promontory of Adria or Sakarya (‘Lycia’ in Fig. 16), oron a separate continental fragment rather than being part ofAnatolia, as is generally assumed (cf. Fig. 1) (Sengor & Yilmaz1981; Sengor et al. 1984; Collins & Robertson 1998).

Exhumation of the Cycladic blueschist unitThe overgrowth of SA1 by a blueschist-facies mineral assem-blage suggests that DA1 occurred during prograde high-pressure metamorphism and is probably related to nappestacking in the Cycladic blueschist unit (see Lister &Raouzaios (1996) for Sifnos Island and Ring et al. (1999b) forSamos Island). The subsequent DA2 event occurred duringinitial decompression, i.e., exhumation, and may have reacti-vated the nappe contact of the Dilek nappe and the Selcukmelange. If the age of 40 Ma produced by Oberhansli et al.(1998) represents phengite crystallization, the Cycladic blues-chist unit in Turkey must have been rapidly exhumed byc. 35 km before its greenschist-facies emplacement onto theMenderes nappes during late DA3 at about 37 Ma (Lips et al.2001). How this pronounced exhumation was accomplished islargely unknown. Avigad et al. (1997, fig. 13) inferred anOligocene extensional event for Tinos; Ring et al. (1999b)estimated 30–40 km of Eocene to early Oligocene exhumationof the Cycladic blueschist unit by vertical ductile thinning anderosion for Samos.

There are two possible tectonic interpretations of DA2

top-to-the-NE shear. (1) DA2 is related to back thrusting ofthe Cycladic blueschist unit onto its hinterland due to the

Fig. 13. Microstructures across the Cyclades–Menderes thrust.(a) Photomicrograph showing retrograde growth of chlorite (chl) atthe expense of garnet (gt) in garnet-biotite schist of the Bozdagnappe. Asymmetric tails of chlorite indicate top-to-south directedshearing during retrogression; plane polarized light, field of view3.5�5.3 mm, location of outcrop 38�01.94N, 27�41.22E.(b) Photomicrograph of quartzite mylonite of the Dilek nappe in thehanging wall of the Cyclades–Menderes thrust with shear bandsshowing top-to-the-south displacement; plane polarized light, field ofview 11�16 mm, location of outcrop is 38�02.59N, 27�40.58E.


development of topographic gradients in the accretionarywedge (e.g., Willett et al. 1993). Accordingly, SA2 should haveoriginally dipped towards the SW, but pervasive DA3 shearingmay have rotated SA2 into subparallelism with the north-dipping penetrative SA3 foliation. The c.35 km of exhumation

would then be due to vertical ductile thinning and erosion. (2)DA2 is related to normal faulting, which aided exhumation ofthe Cycladic blueschist unit during DA2 in the Eocene. Aninitially high angle between SA1 and the instantaneous stretch-ing axis of DA2 can be inferred from SA1 and SA2 in those

Fig. 14. Interpretative sequence ofdeformation and mineral stages inkyanite–chloritoid schist of the Dileknappe. (a) Transposition of pre-DA1

fabric by progressive shearing orpressure solution. (b) DA1`DA2

intertectonic mineral growth stageindicated by poikiloblastic growth ofkyanite and chloritoid. (c) Continuedgrowth of kyanite and chloritoidsynchronous with onset of top-to-the-NEshearing documented in syn-DA2

chloritoid rims. Flow is partitioned incleavage domains that wrap aroundclasts. (d) Formation of SC fabric and�-shaped clast-matrix system.

Fig. 15. Interpretative thrust sequence during formation of Anatolide belt. Late Cretaceous to early Eocene imbrication ofVardar–Izmir–Ankara suture zone and Cycladic blueschist unit during high-pressure metamorphism (thrusts 1–4), followed by collision ofAnatolia causing greenschist-facies imbrication within Menderes nappes (thrusts 5–7).


porphyroclasts which retained their initial position betweenthe strain shadows during shearing (Fig. 9). The high anglebetween SA1 and SA2 may reflect a pronounced change of theflow field, i.e., a strain reversal, and would lend strong supportinto an extensional interpretation of DA2. If DA2 reworked thecontact between the Dilek nappe and the Selcuk melange, theDilek nappe should have decompressed faster in the Eocenethan the Selcuk melange. Further work is needed to resolvethis issue, which may have significance for exhumationprocesses in the eastern Mediterranean.

Conclusions

The Anatolide belt of western Turkey was assembled bytop-to-the-south thrusting of the Cycladic blueschist unit ontothe Menderes nappes during greenschist-facies metamorphismin the Eocene. The Menderes nappes as part of the Anatolianmicrocontinent successfully resisted subduction, whereas theeasternmost part of the Adriatic plate had been subject tosustained high-pressure metamorphism. The different tec-tonometamorphic history of these units contradicts the modelthat the Menderes nappes are the eastern continuation of theCycladic blueschist unit. The Cycladic blueschist unit displaysa prograde Alpine DA1 fabric, which was overgrown by ahigh-pressure mineral assemblage. The onset of DA2 occurredduring initial decompression after this high-pressure event andis associated with top-to-the-NE shearing. A subsequent top-to-the-south greenschist-facies DA3 event affected the Cycladicblueschist unit and the Menderes nappes together. Late duringDA3 the contact between the Cycladic blueschist unit and theMenderes nappes, the Cyclades–Menderes thrust, formed. TheCyclades–Menderes thrust defines an out-of sequence rampstructure, which cuts up-section through Menderes nappestowards the south. In the Cycladic blueschist unit, late DA3

fabrics associated with the Cyclades-Menderes thrust crosscuthigh-pressure DA2 structures. In the footwall, DPA structuresin the Bozdag nappe were deformed by the Cyclades–Menderes thrust. The Cycladic blueschist unit was exhumed byc. 35 km before the Cyclades–Menderes thrust formed in the

Eocene. DA2 structures aided this early exhumation, either byvertical ductile thinning, normal faulting, or a combination ofboth.

Funded by the Deutsche Forschungsgemeinschaft (grants Ri 538/4-1,538/4-2 and 538/4-3 and Graduiertenkolleg 392, ‘Stoffbestand undEntwicklung von Kruste und Mantel’). We thank J. M. Bartley andA. C. Collins for constructive reviews.

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Received 13 October 2000; revised typescript accepted 13 June 2001.Scientific editing by Mike Fowler.


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