Anatomy of the Cycladic Blueschist Unit on Sifnos Island (Cyclades, Greece)

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Vincent Roche, Valentin Laurent, Giovanni Luca Cardello, Laurent Jolivet, Stéphane Scaillet

To cite this version:

Vincent Roche, Valentin Laurent, Giovanni Luca Cardello, Laurent Jolivet, Stéphane Scaillet.

Anatomy of the Cycladic Blueschist Unit on Sifnos Island (Cyclades, Greece). Journal of Geody-

namics, Elsevier, 2016, 97, pp.62-87. �10.1016/j.jog.2016.03.008�. �insu-01293235v2�

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Contents lists available atScienceDirect

Journal of Geodynamics

j o u r n a l h o m e p a g e :h t t p : / / w w w . e l s e v i e r . c o m / l o c a t e / j o g

Anatomy of the Cycladic Blueschist Unit on Sifnos Island (Cyclades, Greece)

Vincent Roche

^a,b,c,∗

, Valentin Laurent

^a,b,c

, Giovanni Luca Cardello

^a,b,c

, Laurent Jolivet

^a,b,c

, Stéphane Scaillet

^a,b,c

aUniversitéd’Orléans,ISTO,UMR7327,45071Orléans,France

bCNRS/INSU,ISTO,UMR7327,45071Orléans,France

cBRGM,ISTO,UMR7327,BP36009,45060Orléans,France

a r t i c l e i n f o

Articlehistory:

Received11December2015

Receivedinrevisedform11March2016 Accepted17March2016

Availableonlinexxx

Keywords:

High-pressureandlow-temperature metamorphism

Syn-orogenicexhumation Post-orogenicextension Strainlocalization Sifnos

Aegeandomain

a b s t r a c t

Since35Ma,thekinematicsoftheAegeandomainhasbeenmainlycontrolledbythesouthwardretreat oftheAfricanslab,inducingback-arcextension.Themainstructuresandassociatedkinematicsarewell constrained,butthekinematicsofdeformationbefore35Ma,coevalwiththeexhumationofblueschists andeclogitesoftheCycladicBlueschistUnit,isstillpoorlyunderstood.TheearlierEocenesyn-orogenic evolutionisstronglydebatedandverydifferentgeometricalinterpretationsandkinematichistorieshave beenproposedintheliterature.Thisstudyfocusesonthehigh-pressureandlow-temperature(HP-LT) paragenesesspectacularlyexposedandwellpreservedonSifnosIsland.Thenewﬁeldworkprovidesnew structuralconstraintsonthetectonichistoryofHP-LTunitsgeneratedinthesubductionzoneduringthe Eocene.Itfurthershowshowlithologicalheterogeneitieslocalizestrainwithinanaccretionarywedge andhowthelocalisationofstrainevolvesthroughtimeduringexhumation.Weshow,throughnew geologicalandmetamorphicmaps,cross-sectionsandanalysesofkinematicindicators,thatSifnosis characterizedbyshallow-dippingshearzonesreactivatingweakzonesduetocompetencecontrastsor earliertectoniccontacts.Structuresandkinematicsassociatedwiththeseshearzones,showatop-to- the-Nto−NEductiledeformation.Thelowerpartofthetectonicpileshowsadownwardgradientof shearingdeformationandisactuallyathicktop-to-the-NEshearzone,whichwenametheApollonia ShearZone.Throughtimeshearingdeformationtendstolocalizedownward,leavingtheupperpartof thesubductioncomplexpreservedfromlatedeformation.Thepresent-dayshapeandtopographyofthe islandislargelycontrolledbylatebrittlefaultsreworkingtheearlierductileshearzones.Comparingwith thenearbyislandofSyros,weproposeanewtectono-metamorphicevolutionoftheCycladicBlueschist Unit,whichpartlyexplainsthedifferentdegreesofretrogressionobservedontheCycladicIslands.

1. Introduction

Theexposureofhigh-pressureand low-temperature(HP-LT) metamorphicrocksresultsfromtheexhumationofmaterialburied alongtheplateinterfaceinsubductionzones.Partoftheexhuma- tioncanbeachievedduringsubductionindifferentways:1)by decouplingofcrustalunitsofvariablethicknessfromthesubduct- inglithospherewithintheplateinterface(Chemendaetal.,1995;

Jolivetetal.,2005;BrunandFaccenna,2008;Agardetal.,2009;

JolivetandBrun,2010;Ringetal.,2010;Tireletal.,2013);2)by

∗Correspondingauthorat:Universitéd’Orléans,ISTO,UMR7327,45071Orléans, France.

E-mailaddress:v.roche@brgm.fr(V.Roche).

circulationofweakmaterialwithinathickaccretionarycomplex (cornerﬂowmodel);3)inthesubductionchannel(Englandand Holland,1979;ShreveandCloos,1986;Platt,1993;Burovetal., 2001Geryaetal.,2002).Themaindifferencebetweenthedifferent typesofmodelsisthedegreeofstrainlocalization.Thesubduction channelconceptishighlyvariableintheliterature,fromtheini- tialsimpledoublecirculationmodelinthespaceleftbetweenthe twoplates(EnglandandHolland,1979;ShreveandCloos,1986)to moresophisticatedmodelswithseverallevelsofcirculationlikein Burovetal.(2001).Thistypeofmodelsdoesnotshowthedetails ofhowtectonicunitsareﬁnallydecoupledfromthesubducting plateand itonly suggestsexhumation fromlargedepthin low viscositymaterial,whichimpliesdistributeddeformation inthe channel.Othermodels,basedondecouplingofcrustalunitsofvari- ablethicknessfromthesubductinglithosphere(Chemendaetal.,

http://dx.doi.org/10.1016/j.jog.2016.03.008

0/).

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Pleasecitethisarticleinpressas:Roche,V.,etal.,AnatomyoftheCycladicBlueschistUnitonSifnosIsland(Cyclades,Greece).J.Geodyn.

Fig.1.TectonicmapoftheAegeandomain.

TectonicmapoftheAegeandomainandschematicgeologicalmapoftheCycladesshowingthemainstructuresrelatedtobothsyn-orogenicandpost-orogenicepisodes, modiﬁedafterJolivetandBrun(2010).Theoriginalmaphasbeenmodiﬁedincorporatingrecentworks(Kumericsetal.,2005;Rosenbaumetal.,2007;Huetetal.,2009;

Grasemannetal.,2012;Augieretal.,2015;Laurentetal.,2015;Beaudoinetal.,2015).Stretching-parallelcross-sectionthroughAegeandomain(Fig.1a)andcentralAegean (Fig.1b)areindicated.Redbox:LocationofSifnos.(Forinterpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

1995;Jolivetetal.,2005;BrunandFaccenna,2008;Agardetal., 2009;JolivetandBrun,2010;Ringetal.,2010;Tireletal.,2013), alsopermittheexhumationofhigh-pressurerocks.However,the pieceofcrustisextrudedupwardbetweenafrontalthrustanda normalfaultabove,whichimpliesastronglocalizationclosetothis interfaceonly.Thesemodelsonlydifferfromtheprecedentbythe degreeoflocalizationofdeformation.Dependingonthesizeofthe exhumedunit,themodelismoreorlesssimilartothebasicsub- ductionchannelmodel.Theextremelocalizationcanbefoundin thepropositionsofChemendaetal.(1995),orBrunandFaccenna (2008)wheretheexhumedunitisonlyone,totallyrigid,between thefrontalthrustandthenormalfault.Whenexhumedunitsare morenumerousandsmallerthegeneralpictureisquitesimilarto thesubductionchannelmodel.Someofthemainproblemsarethus thedegreeandthecontrollingfactorsofstrainlocalizationinsub- ductioncomplexes.Inthispaperweshowhowstrainwaslocalized duringexhumationthroughanewﬁeldstudy.

IntheCycladicArchipelago,theexhumationofHP-LTmetamor- phicrockswasachievedintwostages:theﬁrststagecorrespondsto theexhumationwithinthesubductionchannelduringEoceneand thesecondstagetotheexhumationbelowshallow-dippingdetach- mentsintheback-arcregionintheOligoceneandMiocene.Inthe following,wenametheﬁrststage‘syn-orogenicexhumation’as opposedtothesecondstage‘post-orogenicexhumation’fordistin- guishingexhumationacquiredduringlithosphericshorteningfrom exhumationacquiredduringlaterlithosphericextension(Jolivetet al.,2003;JolivetandPatriat,1999;Jolivetetal.,2003,2013;Bonev andBeccaletto,2007;ForsterandLister,2009).Thesyn-orogenic partofexhumation, whichisofconcernhere,is alsocontrolled bykinematicboundaryconditionsofthesubductionchannel,with slabretreatprobablyfavouringfastexhumation(Jolivetetal.,1994;

Beaumontetal.,1996;BrunandFaccenna,2008;Tireletal.,2013).

Metamorphicunits maythus have differentP-T-time histories, startingwithinthesubductionchannel inHP-LTconditions and

endingwithintheback-arcregioninhigh-temperaturemetamor- phiccorecomplexes.

SyrosandSifnosislandsarelocatedinthecentralpartofthe CycladesArchipelago(Fig.1a)andshowspectacularpreservation of blueschistsand eclogites parageneses (Fig.1b;Altherr etal., 1979;Blakeetal.,1981;Avigadetal.,1992;Trotetetal.,2001a,b;

Rosenbaumet al.,2002; Philipponet al.,2011)that areworld- wideknownforthestudyofthegeodynamicprocessesoperating attheplateinterface.Severalpetrologicalstudies,P-Testimates andgeochronologicalconstraintsareavailableonthesetwoislands (Altherretal.,1979;Wijbransetal.,1990;Avigad,1993;Listerand Raouzaios,1996;Trotetetal.,2001a;SchmädickeandWill,2003;

Schumacheretal.,2008;Groppoetal.,2009;Dragovicetal.,2012;

Bröckeretal.,2013).However,structuralstudiesdevotedtothe relationsbetweendeformationandmetamorphism(Trotetetal., 2001b;Rosenbaumetal.,2002;Ringetal.,2003;Keiteretal.,2004, 2011;Bondetal.,2007;Philipponetal.,2011;Ringetal.,2011),are stilldebatedanddifferentgeometricalinterpretationsandkine- matichistorieshavebeenproposedrenderingdifﬁcultthechoice ofamodelfortheexhumationmechanism.

Inthiswork,wethusrevisitthegeologyofSifnos,basedonnew geologicalmapsatthe1:20,000scale.Ouraimistostudythedis- tributionofdeformationthroughevolvingP-Tconditionstobetter describethedistributionofstrainwithinthesubductioncomplex andconstrainthemechanismsofexhumationofHP-LTunitsfrom thesubductionzone.

2. Geologicalsetting

TheAegeandomain,locatedintheeasternMediterraneanSea (Fig. 1a),has undergonea tectonic and metamorphic evolution deﬁned bytwo main steps.Firstly,fromthelate Cretaceousto the Eocene,the Africa-Eurasia convergence hasled to the for- mationoftheHellenides-Taurideschain(seelocationonFig.1a)

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Fig.2. SimpliﬁedgeologicalmapofSifnosmodiﬁedafterAvigad(1993)andTrotetetal.(2001a,b).

that is composedof a stack of oceanic and continental nappes belonging to Apulia (Aubouin, 1957; Jacobshagen et al., 1978;

Jacobshagen,1986;Stampﬂi,2000).Thestratigraphicsequencesof themainHellenicnappesandtheirpaleogeographicorigincanstill berecognizedandcorrelationscanbemadefromthemainlandto theislands,despitetheintensityoflaterpost-orogenicextension (Bonneau,1984;Jolivetetal.,2004;VanHinsbergenetal.,2005).

Secondly,since35Ma,thekinematicsintheupperplatesofthe Mediterraneansubductionzoneshasmainlybeencontrolledbythe southwardretreatoftheAfricanslab(MalinvernoandRyan,1986;

JolivetandFaccenna,2000;Jolivetetal.,2008;JolivetandBrun, 2010),responsibleforback-arcextension.TheCycladesarchipelago resultsfromthecollapseoftheHellenides-Tauridesbeltcausedby thisretreatingsubductionwhichchangedthekinematicbound-

aryconditionsfromcompressionaltoextensionalintheback-arc region(Fig.1b;Listeretal.,1984;Jolivetetal.,2013).

ExtensionwasrecordedintheRhodopeMassifearlierthanin theAegeanSea.TheRhodopeMassifislocatedinthenorthernpart ofthestudieddomain,southoftheBalkans.TheRhodopeMas- sifunderwentaHPmetamorphism(i.e.12–17kbar,750–811^◦C) withlocallyultra-HPconditions,nolaterthanthelateCretaceous (Liatietal., 2002;Bonev etal., 2006;Bauer etal.,2007).Then, MT-MPmetamorphism(i.e.8–10kbar,560–650^◦C)wasrecorded in thePaleocene and exhumation of these rocksstarted in the centralRhodopemassif∼55Maagobelowtop-to-theNEductile- brittledetachmentsinback-arcdomain(Burgetal.,1996;Bonev etal.,2006).ExtensionstartedintheRhodopeearlierthaninthe Cyclades,sometimesintheEocene(BrunandSokoutis,2007).Dur- ing thesame period, syn-orogenicexhumation occurred in the

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Fig.3.P–TconditionsanddeformationageswithinCheronissosandFarosunits.

(a)GeologicalpileonSifnosshowingdifferentagesofmetamorphicpeakconditions,andblueschist-facies(BS)togreenschist-faciestransition(GS)withtheassociated depths.Superscriptnumbersindicatethecitations,1)Altherretal.,1979;2)SchliestedtandMatthews,1987;3)Wijbransetal.,1990;4)Avigadetal.,1992;5)Trotetetal., 2001a;6)SchmadickeandWill,2003;7)ForsterandLister,2005;8)Spearetal.,2006;9)Groppoetal.,2009;10)Ringetal.,2011;11)Dragovicetal.,2012;12)Bröckeretal., 2013;13)Ashleyetal.,2014.(b)CompilationofresultsofK-Ar,⁴⁰Ar/³⁹ArandRb-Sragesonphengite,paragoniteandfissiontrackonzircon.(c)DetailedP-Tpathmodified afterAshleyetal.(2014),subscriptnumbersindicatetheReferences.(Forinterpretationofthereferencestocolourinthisfigurelegend,thereaderisreferredtotheweb versionofthisarticle.)

CycladesregionwithintheHellenicsubductionzoneandtheCBU underwentHP metamorphismat70–90km depthwhereas par- tialexposureatthesurfaceofthecentralRhodopecorecomplex

occurredintheback-arcdomain(JolivetandBrun,2010).Extension thenmigratedtotheCycladessome35MyrsagoandtheAegean Seastartedtoform.Thisnewepisodeofextensionassociatedwitha

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fastmigrationofthemagmaticarc,duringslabretreat,whileduring theﬁrstextensionalepisode,thearcwasmoresteady,suggesting thatslabretreatwasquiteslow(JolivetandBrun,2010;seealso Menantetal.,2016).

2.1. Cycladicarchipelago

ThegeologyoftheCycladicarchipelagoisdividedintothree units,which arefrom toptobottom: theUpper Unit(UU),the CycladicBlueschistUnit(CBU),andtheCycladicBasementUnit.

In this framework,we focus ontheCBUthat mainlycrops out inthecentralpartofAegeandomain(Fig.1b).Thisunitiscorre- latedwiththePindosUnitincontinentalGreece(Bonneau,1982, 1984)andconsistsofalternatingmarbles,metapelites,metabasites andmeta-ophiolite.CBUrockshaveundergoneadominantHP-LT metamorphismintheEocene(K-Ar,⁴⁰Ar-³⁹ArandRb-Sronphen- giteafterAltherretal.,1979;BröckerandFranz,1998;Huet,2010), andareparticularlywellexposedinthemappedarea.Different paragenesesdisplaycontrastingP-T-timeevolutionsbutwiththe samepeakP-Tconditionsintheeclogite-faciesaround21kbarand 550^◦C(Trotetetal.,2001b;Groppoetal.,2009;Dragovicetal., 2012;Ashleyetal.,2014).ThisHP-LTmetamorphismispartially overprintedbyOligo-MioceneHT-LPmetamorphism(Altherretal., 1979,1982;Keayetal.,2001;Vanderhaeghe,2004;Ducheneetal., 2006;Bröckeretal.,2013),whichtookplacecontemporaneously withback-arcextension.Theoverprintisgenerallycharacterized by greenschist-facies parageneses and locally by amphibolite- faciesones,reachingpartialmelting conditionsinNaxos, Ikaria andMykonos(Keayetal.,2001;Denèleetal.,2011;Laurentetal., 2015;Beaudoinetal.,2015).TheCBUrecordedtheirretrograde peaktemperaturein thecentreof theback-arc areaduringthe Miocene,atabout20MainNaxos(K-Ar,⁴⁰Ar/³⁹ArandRb-Sron phengite,Altherretal.,1982;Wijbransetal.,1990;Bröckeretal., 2013)or16 MainIkaria(Beaudoinetal.,2015).Previousstud- ies(Listeretal.,1984;Uraietal.,1990;GautierandBrun,1994;

Jolivetetal.,1994;Tireletal.,2009;JolivetandBrun,2010;Ring etal.,2010)haveshownthatMiocenelow-anglenormalfaultshave accommodatedthelate-stageexhumationofthemetamorphiccore complexesduringextension.Asaresult,severaldetachmentswere formedandarenowexposedinthenorthernandwesternCyclades, theyaretheNCDS(NorthCycladicDetachmentSystem)andWCDS (WestCycladicDetachmentSystem)(Jolivetetal.,2010;Lecomte etal.,2010;Iglsederetal.,2011;Grasemannetal.,2012).Those structuresareassociated withtop-to-the-NEandtop-to-the-SW kinematicindicators,respectively(Fig.1b).Duringthisperiodthe ﬁrstformedMCC(MetamorphicCoreComplex)wereexhumedin theOligoceneandEarlyMiocene,andtheypartlyescapedfromthe later(MiddleMiocene)completeretrogressionandpartialmelting observedinNaxos,Paros,MykonosorIkaria.

2.2. SifnosIslandgeology

Sifnosliesintheback-arcregionoftheHellenicsubductionand issituatedinthesouth-westernpartoftheAegeanSeabetween theSerifosmetamorphiccorecomplex(Iglsederetal.,2009)and theMilosQuaternaryvolcano(Fig.1b).AccordingtoRingetal.

(2011),themorphologyofSifnosispartlycontrolledbyNE-and SW-dippingnormalfaults,andE-orW-dippingyoungerfaults.This islandischaracterizedbyanapparentlyinversemetamorphicgra- dientwithintheCBU(SchliestedtandMatthews,1987;Wijbrans etal.,1993;ListerandRaouzaios,1996).Indeed,blueschistsfound atthetopofthestructuralpilearewellpreservedwhilethebase isstronglyretrogradedundergreenschist-faciesconditions.Nev- ertheless,high-pressuremarkersarelocallypreservedinallunits onSifnos(Avigadetal.,1992).Twomaintectonicsunitsseparated byashallow-dippingtectonicdiscontinuityhavebeenrecognized

(Trotetetal.,2001a):theCheronissosandtheFarosunits(Fig.2).

ThecontactbetweenthetwounitsisnamedCOF(CheronissosOver Faros)inthefollowing.

Cheronissos Unit (CU) layson top and it is furtherdivided intothreelithologicsubunits,whicharefromtoptobottom,1) theUpperMarbleComplex,exposedinthenorth-westernpartof theisland(mainlycalciticand dolomiticmarbleswithlensesof quartzitesand wellpreservedblueschists and eclogites),2) the EclogiteandBlueschistUnit(interbeddedquartzites,metabasites, metasedimentsandacidicgneisses),3)theMainMarbleUnit(a successionofthickmarblelevelsalternatingwithmetasediments andmetabasitesmetamorphosedintheblueschist-togreenschist- facies) (Schliestedt and Matthews,1987; Wijbrans et al.,1990;

Schmädickeand Will,2003;Ringet al.,2011;Fig.2).Faros unit (FU)cropsoutunderneathCheronissosUnitmainlyinthesouth- easternpartoftheisland(Fig.2).It iscomposedofmetabasites andmetasedimentsstronglyretrogradedinthegreenschist-facies.

Locally,lensesof albite-andepidote-bearingblueschists(AEBS- faciesofEvans,1990)arepreservedatthebaseofCheronissosUnit andthetopofFarosUnit(Trotetetal.,2001a).

Two main stages have been inferred in the metamorphic evolution of Sifnos during the Eocene and Miocene. The ﬁrst wascharacterizedbyHP-LTmetamorphismunderblueschist-to eclogite-faciesconditionspossiblyduetothesubductionofApulia andthePindosoceanicbasinbelowEurasia(Avigad,1993)inthe Eocenewithpeakconditionsaround21kbarand550^◦C(Fig.3a) (Trotetetal.,2001b;Groppoetal.,2009;Dragovicetal.,2012,2015;

Ashleyetal.,2014).Inarecentpaper,Aravadinouetal.(2015)have relatedthistectono-metamorphicevent(theirD1andD2stages) toa top-to-the-SE shearing event.Thisepisode hasbeendated around45MawithdifferentmethodssuchasK-ArandRb-Sron whitemicas(seecompilationonFig.3aandb;Altherretal.,1979;

Wijbrans etal.,1990), ⁴⁰Ar/³⁹Ar apparentagespectraonwhite micas(Fig.3a;ForsterandLister,2005)andSm-Ndisochronson garnets(Fig.3a;Dragovicetal.,2012,2015).Thesecondstage(D3 inAravadinouetal.,2015)overprintedtheblueschist-toeclogite- facies assemblages in the greenschist- to amphibolite-facies; it rangesinagefrom30to19MabasedonK-Ar,Rb-Srand⁴⁰Ar/³⁹Ar datingonwhitemicas(Fig.3aandb;Altherretal.,1979;Wijbrans etal.,1990;ForsterandLister,2005;Ringetal.,2011;Bröckeretal., 2013).

2.3. MaingeologicalcontroversiesaboutSifnos

Oneofthemaindifferencesofinterpretationofthegeological evolutionofSifnosrelatestothesigniﬁcanceofthesecondstageof deformationandtheretrogressionoftheeclogite-andblueschist- faciesparagenesesintogreenschist-faciesassemblages.Ithasbeen forinstanceeitherinterpretedasanextensionalepisode(Trotet etal.,2001b)orasacontractionalepisode(Aravadinouetal.,2015).

Inmoredetails,thenatureandoriginofthecontactbetweenChero- nissosandFarosunits,whichcanbebestobservedalongtheroad fromApolloniatoVathy(Fig.2),hasbeendebatedintheliterature (Avigad,1990,1993;Wijbransetal.,1990;ListerandRaouzaios, 1996;Trotetetal.,2001b;Bröckeretal.,2013).Thistectoniccon- tacthasbeendiverselyinterpretedas:abrittledetachmentwith top-to-the-northeastkinematics(Avigad1993);atop-to-the-south thrust(ListerandRaouzaios,1996);atop-to-the-northeastductile shearzone(Trotetetal.,2001a)and,morerecently,asadetachment markedbytop-to-the-southbrittle-ductiledeformation(Ringetal., 2011).

Conventional thermobarometry and multiphase equilibria (Matthewsand Schliestedt,1984; Schliestedt,1986; Schliestedt and Matthews, 1987; Avigad, 1993; Trotet et al., 2001b;

Schmadicke and Will, 2003; Groppo et al., 2009; Dragovic etal.,2012,2015),oxygenisotopethermometry(Matthewsand

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Schliestedt,1984),Zr-in-rutilethermometry(Spearetal.,2006)and quartzinclusioningarnets(Ashleyetal.,2014)havebeenusedto estimatetheP-Tpathsandtheconditionsofpeakmetamorphism fortheEclogiteandBlueschistsubunitinCheronissosUnit(Fig.3a andc).The shapeoftheprogradepathissimilarforallstudies andfollowsaHP-LTgradientinferredfromthepresenceofpseu- domorphsoflawsonite(Trotetetal.,2001a).However,themain controversyrelatestotheretrogradeP-TpathsinCheronissosand Farosunits.SchmädickeandWill(2003)gaveasimilarP-Tgradi- entof10–12^◦C/kmforboth unitswhereas Trotetetal.(2001b) proposedtwo differentpathswithacooler oneforCheronissos Unitstartingfroma commonpeakataround600^◦Cand20kbar (Fig.3c).Consequently,differentexplanationswereproposedfor theapparentinversemetamorphic gradient in Sifnos(eclogites andblueschistsrocksoverlyinggreenschists;Altherretal.,1979;

MatthewsandSchliestedt,1984;Schliestedt,1986;Schliestedtand Matthews,1987;Trotetetal.,2001b;Wijbransetal.,1990;Avigad 1993; Groppoet al., 2009; Ringet al., 2011).It was suggested thatthegreenschistoverprintinFarosUnitresultedfromperva- sivefluidinfiltration(MatthewsandSchliestedt,1984;Schliestedt and Matthews,1987), theMain MarbleUnit being a relatively impermeablelayer,protecting theEclogite-BlueschistUnitfrom it.Wijbransetal.(1990)assumed insteadthatoverprintingwas duetoatemperatureincreaseatthebaseofthemetamorphicpile, fittingtheyounger⁴⁰Ar/³⁹Aragesfoundthere(Fig.3b).Another interpretationsuggeststhatbothCheronissosandFarosunitswere derivedfromdifferentstructurallevelsandwerejuxtaposednextto eachotherbyathrustduringthepost-orogenichistory(Listerand Raouzaios,1996),involvingasignificantdisplacementbetweenthe twounitsandimplyingdifferentpeaksofmetamorphisminthetwo units.Alternatively,Avigad(1993)andTrotetetal.(2001b),have proposeddifferentcoolinghistoriesbetweenthetwounitswitha similarpeakofmetamorphism,leadingtodifferentdegreesofret- rogression(Fig.3c).Inordertoclarifythesekindsofdiscrepancies wefocusedourfieldstudyontherelationsbetweendeformation andmetamorphismatdifferentscales.

3. AnewgeologicalmapofSifnos

Ournewgeologicaland structuralmapatthe1:20,000 scale (Fig.4)isentirelyredrawnfollowingﬁeldobservationsandsatel- liteimagesanalyses.Hence,amoredetaileddescriptionofthe3D geometryoflithologicalassemblagesandstructure,is provided.

Thenewdatashedlightontheshapeoflithologicalboundariesand onthenatureofthecontactbetweenCheronissosandFarosunits.

Themainoccurrencesofserpentiniteandeclogitearereportedon themap(Fig.4).Thegeometryandkinematicsoflaternormalfaults havealsobeenredrawn(Fig.4).Thismapisconsistentwiththe moredetailedmaprecentlyproposedforthenorthernpartofthe islandbyAravadinouetal.(2015).

3.1. Brittledeformation

The general structure of the island and the topography are controlledbya setof majornormal tooblique faultsthat con- troltheﬁrst-orderdistributionoflithologiesonSifnos(Ringetal., 2011).Wecarefullyremappedthesefaultsthatshouldnotbecon- fusedwiththemaincontactbetweenCheronissosandFarosunits (Fig.5a).ThelargestoftheselatefaultsaretheKastroFaultandthe TosoNeroFaultrespectivelyalongtheeasternandwesterncoastof theisland,whereasthesouthernandcentralpartsshowaseriesof SW-dippingnormaltoobliquefaultswithdifferentslicken-ﬁbres generationssuchastheKamaresFaultortheVathyFault(seeFig.4 forlocation;Fig.5aandb).Inmostoutcrops,theyoungestbrittle movementistop-to-the-SWandbrittle-ductilecriteriaalsoindi- catetop-to-the-SWmotion(Fig.5c).Oneofthesefaults,theVathy Fault,locallymarkstheCOF(Fig.5d).Thishighanglefaultcross- cutsthecontactthatisparalleltotheregionalfoliation.Thefault showsacorewithathicktalc-richfaultgougeandthefaultplane showsslickensides(Fig.5f)andotherbrittle-ductilecriteriaindi- catingtop-to-the-SWand −Smovements(Fig.5eandg). Afew metersawayfromthedamagezone,theolderductiledeformation, characterizedbytop-to-the-northeastshearisvisiblelikeinmost oftheisland.Inthefarsouthof Sifnos,S-toSSE-dippingfaults occur(Fig.4).Theirexactkinematicsisnotwellestablishedbut theireffectonthedistributionofmetamorphicrocksandtheasso- ciatedscarpshowanormalcomponent.Thesefaultscutprobably theSW-dippingfaults.

3.2. Regionalfoliation

The intense ductile deformation is characterized by well- developed foliationsand stretching lineations shown on Fig.4.

Withinthesameunit,foliationplanesaregenerallyparalleltoeach otherindifferentlithologies.Atthescaleoftheisland,inthenorth, thefoliationdipstowardtheNW,whereasintherestoftheislandit

Fig.4.GeologicalmapofSifnos.

NewstretchinglineationmapofSifnoswithfoliationtrajectories.Lithologiccontourshavebeendeﬁnedonthebasisofourﬁeldobservationsandsatelliteimages.Wehave alsoincludedsomeinformationofAravadinouetal.,2015.Themajorstructuresareindicated(TNF:TosoNeroFault;KF:KatroFault;VF:VathyFault;SSEF:S-SEFaults).

Lineationsassociatedwithblueschist-andgreenschist-faciesassemblagesarerespectivelyindicatedinblueandgreen;arrowsindicatethesenseofshear(top-to-the-Nto

−NEindifferentcoloursandtop-to-the-SWinblack).North–southcross-sectionshowingthegeneralarchitectureofSifnosandrelativesequenceofhigh-andlow-angle extensionalfaults.(Forinterpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

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Fig.4. (Continued)

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Fig.5.BrittledeformationonSifnos.

(a)Locationoffault-slipdatafromhigh-anglenormalfaults.Thediagramsshowgreatcirclesoffaultplanesandtheprojectedtraceofslickensidelineationsinalower hemisphereequal-areaprojection.Arrowsindicateextensiondirectionsfromthemeasuredfaultsets(paleostressorientationpatternswerecalculatedwiththewin-tensor computer-aidedinversionsoftwareofDelvauxandSperner,2003).Readersunfamiliarwiththemethod,ormoregenerallywithfault-slipdatainversionarereferredtothe workofAngelier(1990).(b)KamaresFaultshowingtwoslicken-ﬁbregenerations.(c)Top-to-the-SWbrittle-ductilekinematicindicatorsassociatedwithKamaresFault.

(d)VathySWdippingnormalfaultwith(e)stereographicprojectionsofstriations.(f)Faultplanecharacterizedbyafewmetersthickreddishfaultgougeshowing(g) top-to-the-SWbrittle-ductilekinematicindicators.NotetheconsistencyoftheNNE-SSWtoE-Wdirectionofbrittleextensionallovertheisland.

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dipstowardtheNNEwithalargediprange,between0^◦and90^◦and dipincreasingnorthward(Fig.6a).Differentfamiliesoffoliationcan bedistinguisheddependingontheassociatedparagenesesandthe apparentintensityofshearingdeformation(e.g.densityofshear bands,intensityofstretching...).Inthenorthernpartoftheisland, themainfoliation(S₂)generallystrikesENE-WSWanddipstoward theNNW(Fig.6a).AnearlyS₁foliationisseenasrelicswithingar- netsporphyroblasts(Trotetetal.,2001a;Aravadinouetal.,2015).

IntheMainMarbleUnit,foliationisgenerallycharacterizedbyNW- SEstrikeanddipstotheNE(Fig.6a).AtthebaseofCheronissosUnit, lensesofmetabasicrockswrappedwithinmarblesarepreserved atthemesoscopic-scale.Locally,anewAEBS-faciesfoliation(S3)is developed,overprintingtheearlierfoliations(Trotetetal.,2001a;

Aravadinouetal.,2015).InFarosUnit,themainfoliation(S₃)iswell developedwithaNNW-SSEstrike,dippingtowardtheNNE.Finally, alatergreenschist-faciesfoliation(S₄)ischaracterizedbyalow northwarddipatthebaseofFarosUnit(Trotetetal.,2001a).Hence, fromCheronissostoFarosUnit,asuccessionofdeformationfrom blueschist-togreenschist-faciesconditionsisrecognized.More- over,goodexposuresofthetectoniccontactbetweenCheronissos andFarosUnitshowthatthefoliationplanesretainthesamestrike anddipacrossthecontactandacrossmajorlithologicalcontactsas well.

3.3. Stretchinglineation

Stretchinglineationsarewidespreadthroughalllithologiesand havebeendistinguishedaccordingtotheassociatedmineralogy, dominantmetamorphicfacies,and/orintensityofdeformation.In calciticanddolomiticmarbles,whitemicasandcalcitedefinethe lineation; inbasic rocks, theelongationand boudinage ofpris- maticminerals(glaucophane,epidote,albiteandchlorite)reflects acomponentofstretching,whichisalsolocallyexpressedwithin metaconglomeratesbyelongatedpebblescarryingpressureshad- ows.TheselineationsareprojectedonthemapofFig.4.Atthescale oftheislandandonmostoutcrops,stretchinglineationstrendcon- sistentlyNE-SW(Fig.6b),exceptforsomelocalNNW-SSE-trending lineations observedin thenorthernpartof theisland (seealso Aravadinouetal.,2015).Theplungeoflineationisgenerallyshallow anditrarelyexceeds40^◦.Wehavedistinguishedthreesuccessive generationsoflineationsbasedonthemetamorphicgradecoeval withdeformation:1)underblueschist-faciesconditions,thetrend oflineations,definedbyglaucophaneinmostcases,showsadisper- sionbetweenN330^◦EandN30^◦EwithanaveragearoundN350^◦E (Fig.6b);2)inFarosUnit,thelineationcharacterizedbyalbiteand chlorite,islessdispersedwithanaveragearoundofN52^◦E(Fig.6b);

3)closetothemaincontact,inAEBS-faciesconditions,lineations deﬁnedbyepidote,albiteandrareglaucophane,trendinaverage aroundN30^◦E(whichcorrespondtotheaverageofthetrendof blueschistandgreenschistlineation,seestretchinglineationonthe mapinFig.4).

3.4. Folddirections

Threefolding eventshave beenidentiﬁed intheﬁeld.Inthe structurallyhighestandnorthernpartoftheisland,earlyisocli- naltoverytightmacroscopicfoldsF1 canbeseentoaffectthick massivedolomitesalternatingwithcalciticmarbles(Figs7band7;

seealsoAravadinouetal.,2015).Thesefoldsareasymmetricand generallysouth-vergent,withanaxialplaneparalleltothefolia- tion.Thegeometryandkinematicinterpretationofthesefoldsis detailedinSection6.AsecondgenerationoffoldsF₂isparticularly welldevelopedatsmallscaleinbothunitsandgenerallyassociated withanintenseshearingdeformation.F₂foldingisdominantlyNE- vergentbutcharacterizedbyvariableaxialtrendwithrespecttothe lineation.Theaxialplaneoftheﬁrstfamilyislyingparalleltothe

foliationwiththefoldaxissub-paralleltothelocalstretchinglin- eations.ThetighttoisoclinalfoldsareoverturnedtowardtheNor NE.Thesefoldsaremainlyobservedinareaswherethedeformation isintensesuchasnearVroulidia(northernpartoftheisland,Fig.7d) orChryssopigi(southeastpartoftheisland,Fig.7e).Theyarebest expressedinmetabasitesandmetapelites.Thesecondsethasits axialplanesdippingtowardthenorthandtheaxesareroughlyper- pendiculartothelineation(Fig.7f).Thesefoldshavebeenobserved atsmallscaleinallunits.Finally,latestageF₃foldsrefoldthemain foliationandtectoniccontactswithanorientationperpendicularto thesecondgeneration.TherelationbetweenF1,F2,F3foldsandthe top-to-the-northor−northeastshearingdeformationisdiscussed inSection6.

4. Kinematicindicatorsandmetamorphicconditions withinCheronissosUnit(CU)

4.1. Eclogite-Blueschistsubunit

Thisunit,whichoutcropsinthenorthernpart(Fig.4),iscom- posedbymaficandsiliceousrocks.Mostkinematicindicatorsare top-to-the-Nto−NE.Alllithologiesareaffectedby asymmetric structuresatvariousscales:1)Decametricasymmetricfoldsaffect themainfoliationeitherinmafic(Fig.8a)orinsiliceouslayers(e.g., quartzandphengiteaggregatesinFig.8b);2)Meter-scaleasym- metricboudinageofmafic(eclogite)layersembeddedinaweaker matrixindicatingatop-to-the-northeastshearsense(Fig.8c);3) Shearbandsinallunits(Fig.8dande).Spacingbetweenshearbands rangesfromafewmillimetrestoafewcentimetresintheweakest levels.Theanglebetweenshearbandsandfoliationvariesfromone outcroptoanother,dependingonlithology.Shearbandsarecom- monlymarkedbythinglaucophanecoatingsinmaficrocks(Fig.8f) andquartz/micainacidicrocks.Shearbandsgiverisetosigmoidal foliationatthecentimetricormetricscalecompatiblewithtop-to- the-northeastsenseofshear;4)Pressureshadowswithingarnet porphyroblastsconsistingofquartz,mica,rareglaucophaneand chloritealsoindicateatop-to-the-NEsenseofshear(Fig.8gandh).

4.2. UpperMarbleComplexsubunitandMainMarblesubunit

The marblesand schist lenses composingthe Upper Marble ComplexandtheMainMarbleunitsdisplaykinematicindicators showing a top-to-the-northeast sense of shear (Fig. 4) consis- tentwiththestructuresdescribedintheEclogite-BlueschistUnit, butwitha morelocalizeddistribution.Inmarbles,stretchingis expressedby boudinage at variousscales. In thenorthern part of theislandfor instance,competencecontrastbetweencalcite and dolomite resultsin domino and asymmetrical sigma/delta clastsindicatingatop-to-the-northeastshearsense(Fig.9bandc).

Stretchingisalsomarkedbytensiongashesandcalciterecrystal- lizationwithinmarbles(Fig.9d).Isoclinalfoldsoverturnedtoward the north,withan axisgenerally perpendicular to thestretching lineation,arecommonly observed inthe Main MarbleUnit (Fig.9e).Fewkinematicsindicatorsareobservedinotherlithologies inthesetwosubunitsofCheronissosUnit.Theyallindicateatop-to- the-northeastsenseofshear(e.g.,asymmetricshapeofstretched carbonatepebblesinmetaconglomeratelayersinFig.9f).

4.3. Metamorphicconditionsofshearingdeformationin CheronissosUnit

In the northern part of the island, Upper Marble Complex andEclogite-BlueschistUnitarecharacterizedbyanoverallgood preservationofHP-LTparagenesessuchasfreshandveryabun- dantglaucophaneandomphacitewithinmetabasites.Shearbands andsometimesstrainshadowsaroundgarnetsareoftenﬁlledwith

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Fig.6.Mainplanarfabricsandstretchinglineation.

(a)StatisticsofthemainfoliationofdifferentunitsshowingdistributionfrequencyofdipschistosityandSchmidt’slowerhemisphereequal-areaprojectionofschistosity poles.(b)StatisticsofthepreferredorientationofstretchinglineationpresentedinSchmidt’slowerhemisphereequal-areaprojection.Weusedthe1%areacontouringto deﬁnethecontourlines.Inourcase,weusedacontourintervalvalueof2.

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Fig.7. Differentfoldingphasesatvariousscales.

(a)Locationofﬁeldphotographs.(b)and(c)showﬁeldevidencesoffoldingofdolomiticmarblesandcalciticmarblesinthenorthernpartoftheisland.(d)Syn-blueschist foldingaffectedearlyfoldsinametabasite.NotethatF2foldaxesaremostlyperpendiculartothestretchingdirection.Twosetsofaxialplanesshowing,(e)axesparallelto thestretchingand(f)perpendiculartothestretchinglineationinthesouth-easternpartoftheisland.

glaucophaneandphengites,indicating syn-HP-LTconditionsfor thetop-to-the-Nto−NEshearing deformation(bluearrows on Fig.4).IntheMainMarbleUnit,HP-LTparagenesesaregenerally moreretrogradedwhenapproachingthecontactbetweenChero- nissos and Faros units. In this latter unit, top-to-the-northeast kinematic indicators in metabasite lenses are associated with glaucophane, epidote, phengite and sometimes chlorite, albite characteristicofAEBS-faciesP-Tconditions(green/bluearrowson Fig.4).

5. Kinematicindicatorsandmetamorphicconditions withinFarosunit

5.1. Top-to-the-NEductilesheardeformation

Top-to-the-northeast kinematic indicators are widely dis- tributedthroughFarosUnitbutmorecommoninzonesofstrain localization.Severaltypesofshear-derivedobjectsareobserved:1) metricshearbandsindicatingtop-to-the-northeastsenseofshear

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Fig.8.MicrostructuresandmacrostructuresobservedinSifnosEclogite-BlueschistUnit.

(a)DecametricasymmetricfoldsinmaficrocksclosetoKormos,showingaxesmostlyperpendiculartothestretchingdirection.(b)Asymmetricfoldsinsiliceouslevels withS1preservedingarnetsintheupperlevelofEclogite-BlueschistUnit.(c)Boudinageofeclogitesshowingatop-to-the-NEsenseofshear.(d)Top-to-the-NEshearbands affectingcompetentmaficrocksand(e)acidicrocksinthenorthoftheisland.(f)Detailsofshearbandsmarkedbyglaucophaneinmoreacidicrocks.(g)Pressureshadow ongarnetporphyroblastsinafieldphotographand(h)thinsection.Theyshowtop-to-the-NEsenseofshearclosetothecontactbetweenEclogite-BlueschistUnitandMain MarbleUnit.

nearKamaresFault(Fig.10b);2)asymmetricboudinsofcompe- tentlithologies,suchasthosepreservingmaﬁceclogiteboudins (Fig.10c);3)shearedquartz-carbonateveinsperpendicularorpar- alleltothemainfoliationshowingpressureshadowsﬁlledwith chloriteandalbitesuchasnearVathy,withNE-vergingshearingin greenschist-faciesconditions(Fig.10dande);4)syn-greenschist asymmetricfoldingaffectinglayersrichinglaucophaneandepi- doteclosetotheCOF(Fig.10f).Crenulationofanearlierfoliation compatible with top-to-the-northeastshearing is also frequent withinFarosUnitandespeciallynearzonesofstrainlocalization suchasChryssopigi(Fig.10g).

Greenschist-facies ductile deformation is more localized in FarosUnitthaninotherunitsalongshearzones.Onespectacu- larexampleofsuchshearzonescanbeobservedatthesouthern

coast of the island (Fig. 11a). This area, located near thebase ofFarosUnit,showsadistinctgradientofshearingdeformation.

Inaverage,thefoliationstrikesalmostE-Wanddipsmoderately toward the southwest.The shear zone develops within micas- chistsandminormarbles, displaysthesameoverall distributed top-to-the-NEkinematicindicatorsoccurringatvariousscalesover thewholeFarosUnit.Suchkinematicindicatorsarealsoexposed neartheport of Faros.The N50^◦Etrending stretchinglineation ismarkedby stretchedmicasand albitepods. Thedeformation stronglyincreasestowardthenorthwestandthegradientisclearly visibleonChryssopigipeninsula(Fig.11a).Approachingthepenin- sula,anincreasingnumberofveinscomposedofquartz,chlorite, andcarbonatesareshearedandtransposedintothemainfoliation (Fig.11c).Thefoliationandtheveinsarestronglyandasymmetri-

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Fig.9. MicrostructuresandmacrostructuresobservedinUpperMarbleComplexandMainMarbleUnit.

(a)Locationofﬁeldphotographs.(b)Differenceofcompetencebetweencalciteanddolomiteshowingthedevelopmentofdominosand(c)delta/sigmaclastsindicating top-to-the-NEsenseofshear.(d)North-southstretchingshownbycalciterecrystallizationwithintensiongashes.(e)Centimetricfoldsshowingaxesmostlyperpendicular tothestretchingdirection.(f)Top-to-the-NEshearsensedeﬁnedbytheasymmetricshapeofstretchedcarbonatepebblesinmetaconglomeratelayers.

callyfoldedconsistentlywiththemeanshearingdirection.Curved foldaxes aredispersed aroundadirectionperpendicular tothe stretchinglineation(Fig.11a,number1).Here,anaxialplanecrenu- lation cleavage develops in thehinge of these folds (Fig. 11d).

Shearedlensesofmetabasite,orquartzandcalciteveinsindicate atop-to-the-northeastsenseofshear(Fig.11e).Towardthetopof thepeninsula,movingupwardtowardthecoreoftheshearzone, thenon-coaxialdeformationincreasessharply.Foldaxestendto progressivelyrotatetowardtheshearingdirectionandareﬁnally deﬂectedparallel to thestretching lineation,although noclear closedeye-structurehasbeenobservedintheYZplane(Fig.11g).

Thesefoldsaretypically a-foldsthatshowa largesimple shear componentinthedirectionofthestretchinglineation(Fig.11a, number3).In themostdeformedareas,observations oftheXZ planeoftheﬁnitestrainellipsoidshowadensenetworkofsmall- scaleshearbands,allcompatiblewithtop-to-the-northeastsense ofshear(Fig.11f).Theirspacingrangesfromafewmillimetresto afewcentimetresand theyareassociatedwithwhitemicaand albite.Chlorite,calciteandquartzveinsarestretchedandfolded andtotallytransposedintothefoliation.

5.2. Metamorphicconditionsofthetop-to-the-northeast deformation

In the Faros unit, metabasitesthat are deformed bytop-to- the-NE deformation show strongly retrograded blueschist- or greenschist-facies rocks. From top to bottom 1) eclogite and blueschistlensesarelocallypreservedclosetotheCOFinthenorth- ern part of theisland (Fig.10c). At macro-structural scale, the dominantparagenesisischaracterizedbyglaucophane,epidote, phengiteandalbite.Inthisstructurallevel,glaucophanefillingof top-to-the-NEshearbandsindicatesthatdeformationhaspartly actedinHP-LTconditions(seeFig.12);2)inthemiddlepartof FarosUnit,deformationislocallymoreintenseandtheretrogres- sionintogreenschist-faciesmoreadvanced.Chlorite-filledpressure shadowsdeveloparoundquartz-calcitelenses,attestingthattop- to-the-northeastdeformationhasalsoactedingreenschist-facies conditions(Fig.10e).Wehavefurthermoreobservedtop-to-the- northeastshearbandsfilledbylow-grademineralssuchaschlorite andalbite, leadingtothesameconclusion;3)thebaseofFaros Unitisthemostretrogradedzone(e.g.,Chryssopigi).Here,veins filledbycalcite,chloriteandalbitearemostly(ortotally)trans-

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Fig.10.MacrostructuresobservedinFarosUnit(FU).

(a)Locationoffieldphotographs.(b)S-CStructureswithtop-to-the-NEsenseofshearcharacterizedbymetricshearbandsinametapelite.(c)Asymmetricmaficeclogite boudinscrosscutbyquartz-carbonateveinsshowinganorth-southstretching.(d)Shearedquartz-carbonateveinscrosscutthemainfoliationand(e)arecommonly transposedinthefoliationwithpressureshadowsfilledinchloriteandalbiteshowingtop-to-the-NEsenseofshear.(f)Centimetricfoldsshowingaxesmostlyperpendicular tothestretchingdirection.(g)Axialplanecrenulationcleavageassociatedwiththehingeoffoldsatthebaseofthisunit.

posed in the main foliation. At the micro-structural scale, in thinsectionscollectedfromChryssopigi,shearingdeformationis localizedaroundsyn-kinematicporphyroblastsofalbitethatare surroundedbyquartz-micastrainshadows.Thesecriteria attest thattop-to-northeastdeformationhasactedduringacontinuum fromblueschist-togreenschist-faciesconditionsduringexhuma- tion.

6. Kinematicsofdeformationinthemainshearzones 6.1. ContactbetweenCheronissosandFarosunits(COF)

TheCOFcontact,asshownonthecross-section(Fig.4),isfolded byopenfoldsandthenlaterfaultedbyhigh-anglefaults.Thiscon- tactappearssub-parallel (oronly locally slightly discordant)to

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Fig.11.Chryssopigishearzonedeﬁnedbyadistinctgradientofshearingdeformation.

(a)LargescalerepresentativeviewoftheChryssopigishearzoneinthesouth-easternpartoftheisland.AlbitepodsmarkedthestretchinglineationtrendingN50^◦E.All informationissummarizedwithsynthetic3-Ddiagrams.Thisshearzoneisdividedinthreeareas;zone1correspondsto(c)anintensefoldingand(a)associatedSchmidt’s lowerhemisphereequal-areaprojectionsshowingfoldaxesdispersion;zone2isdeﬁnedby(d)axialplanecrenulationcleavageassociatedwiththehingeoffolds.(a)Fold axestendtoprogressivelyrotatetowardtheshearingdirection.(e)Shearedlensesofquartzandcalciteshowingatop-to-the-NEsenseofshear;zone3ismarkedbyintense non-coaxialshearingconsistentwithanoveralltop-to-the-NEkinematics.(f)Shearbandsspacingrangesfromfewmillimetresand(g)a-foldsarecommonlyobserved.Note thatfoldsaxesarenowparalleltothestretchingdirection.

themainfoliation.Itisparticularlywellexposedinthesouthern partoftheislandalongthecountryroadthatbringsfromVathy toFikiadasbeach(Fig.4).Thisareashowsadistinctgradientof

shearingdeformationclosetothemaincontactwiththeChero- nissosUnitandashallownortheast-dippingfoliation,sometimes stronglyfolded(Fig.12d).There,shearedcarbonatepebblesand

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Fig.12.COFcontact.

TheCOFcontactiswellexposed(a)Intenseductiledeformationshownbyfoldinginalllithologies.Top-to-the-NEshearbandsassociatedwithglaucophaneaffectthefoliation.

(b)Asymmetricboudinageofepidotiteboudinsshowingatop-to-the-NEshearsense.(c)Closely-spacedshearbandsassociatedwithwhitemicaindicatetop-to-the-NE senseofshearclosetoVathy.(d)SimilarfeaturesobservedclosetoKastro,justbelowthemaincontact.

asymmetricboudinageofepidotiteboudinsindicateatop-to-the- northeastshearsense(Fig.12bandc)togetherwithcloselyspaced shearbandsassociatedwithglaucophaneandwhitemicas.TheNE- trendingstretchinglineationisdeﬁnedbytheelongationofalbite spotsandglaucophaneneedles.

ThesamefeaturesareobservedclosetoKastro,justbelowthe maincontactwiththeupperunit (Fig.4).In thisareathefolia- tionstrikesalmostN-Sanddipsshallowlytowardtheeast.The lineationismarkedbyepidote,glaucophaneandalbite.Numerous quartzlensesaretransposedinthefoliationandinshearbands withanasymmetrycompatiblewithatop-to-the-northeastsense

ofshear(Fig.12eandf).Asymmetricboudinageofepidotitesis alsowell developed and indicatesa top-to-the-northeast shear sense(Fig.12g).Lateveins,perpendiculartotheregionalstretching, ﬁlledwithchlorite,quartzandhematitecuttingthemainfoliation, suggestacontinuumofstretchingduringmorebrittleconditions ingreenschist-faciesconditions.Tosummarize,theCOFcontact ischaracterizedbyamoreintenseshearingdeformationandthe senseofshearisclearlytop-to-the-northeast,asobservedwithin CheronissosandFarosunits.Thisshearinghasbeenactiveduring theretrogressionfromblueschist-togreenschist-faciesconditions.

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6.2. ShearzonesawayfromtheCOFcontact

TheEclogite-BlueschistUnitoverlaystheMainMarbleUnitand thecontactisexposedalongtheeastcoastclosetoTroulaki(see location onFig. 4).The average dipof foliation is about 50^◦W (Fig. 13a). The stretching direction is provided by the mineral lineation(glaucophane,chlorite,albiteandcalcite)andtheelonga- tionofpebbleswithinmetaconglomeratelayersinterleavedwith metabasitesandmarbles.Dependingonthemineralparagenesis, thetrendoflineationschangesfromN70−80^◦Eforthemoreretro- gradedlevelstoN45^◦Eforthebetter-preservedblueschist-facies.

In particularcloseto theMainMarbleUnit, ametric metacon- glomerate level, which mainly contains calcitic and dolomitic pebbles, shows near its base eclogite pebbles when approach- ingtheEclogite-BlueschistUnit(Fig.13c).Immediatelyabove,a pileoffullyretrogradedmetabasitescropsout(Fig.13d).Associ- atedwithgreenschist-faciesparageneses,quartzveinsparallelor perpendiculartothemainfoliationareobserved(Fig.13e).Well preservedeclogite-andblueschist-faciesparageneses withpris- tinegarnetsoccurjustabove(Fig.13g).Kinematicindicatorssuch asdominostructures(Fig.13b),stretchedpebblesinmetaconglom- erates(Fig.13c),shearbands(Fig.13f)andasymmetricaleclogite boudins(Fig.13g), arecommonin allunitsatall metamorphic gradesandarecharacterizedbyatop-to-the-NEto−Esenseof shear.

In thenorth-eastern partof theisland, closeto Cheronissos (Fig.4),thecontactbetweenUpperMarbleComplexandEclogite- BlueschistUnitiswellexposedalongthecoast.Thiscontactappears similartothepreviousoneexceptfortheabsenceofmetaconglom- eratelevel.Kinematicindicatorsaretop-to-the-northeastinboth unitsandthedegreeofretrogressionseemstobethesameasin thecontactdescribedabove.

7. Discussion

7.1. Timingofdeformationandmetamorphism

Basedonkinematicindicatorsandtheirrelationswithmeta- morphicparagenesesdescribedaboveandthosereportedinthe literature,weproposeanewmetamorphicmap(Fig.14).Thismap isintendedtodisplaytheﬁrst-orderdistributionofthedominant paragenesesasseenintheﬁeld,keeping inmindthatthepeak ofmetamorphismwaseverywhere insimilarP-T conditions,in theeclogite-facies.Inthissection,were-evaluatethechronolog- icalevidenceusedtosupporttherelativetimingoftectonicand metamorphiceventsrecordedonSifnos.

ThroughoutmostofCheronissosUnit,HP-LTparagenesesare particularlywell preserved. In the northern part of the island, syn-kinematicmineralsinshearbandandpressureshadowsare developedduringsyn-orogenicdeformation.Locally,anearlyS1

foliationhasbeenobserved asrelicsintogarnet porphyroblasts butkinematicindicatorsrelatedtothiseventareunknown.Most kinematicindicatorsassociated withtheretrogradepath(under HP-LTconditions)aredistributedheterogeneouslyinalllithologies andindicateapenetrativetop-to-the-Nshearsenseintheupper partandtop-to-the-NEto−Eintherestofthisunit.Aravadinou etal.(2015)haverecognizedtop-to-the-SEkinematicindicators in thin-sectionscut in zonespreserved fromthebulk of retro- gradedeformationinnorthernSifnos.Ifoneadmitsthattheoriginal stretchingdirectionhasbeenpreservedinthesezones,theshear senseduringthebeginningofS₂formationwastop-to-the-SE.This isdebatablebecausethefoldsrefoldingS2areacuteanditisnot surewhethertheoriginaldirectionofstretchingdirectionwaspre- servedbutthisistheonlyavailableinformationinfavourofatop-SE shearingevent,whichcouldrepresenttheearlyshearingrelatedto

thrustingnearthepeakofpressure,asproposedbyAravadinou etal.(2015),beforethetop-to-the-NEshearing.Thissecond-stage shearingisinsteadobservedallovertheislandanditstartedbefore therockshadlefttheblueschist-faciesduringtheirexhumation.

Itispossiblethatthischangeofshearsensecorrespondstothe transitionfromburialtoexhumationathighpressure.

Ductiledeformationislocallymoreintenseincertainareassuch asinthenorthernpartoftheislandatVroulidia.Thispervasive deformationis linkedwiththeexhumationof CheronissosUnit fromitsmaximumburialdepthandisdatedaround45Ma(Altherr etal.,1979;Wijbransetal.,1990;ForsterandLister,2005;Dragovic etal.,2015).AtthebaseofCheronissosUnitandtopofFarosUnit, closetothemaincontact,astrongdeformationgradientinAEBS- faciesP-TconditionsisobservednearKastroandclosetoRizomata Mountain(Fig.12).Greenschist-faciesconditionsarerecordedin thewholeFaros Unitandlocally alsoin CheronissosUnitclose totectoniccontacts(Fig.14),andarealwaysassociatedwithtop- to-the-NEor−Ekinematicindicators.StrainlocalizationinFaros Unitissystematicallyassociatedwithgreenschist-faciesconditions suchasnearChryssopigi(Fig.11).Availableradiochronologicaldata (Altherretal.,1979;Wijbransetal.,1990;Ringetal.,2011;Bröcker etal., 2013), obtainedonrockssampledinFaros Unit,datethe greenschist-faciesretrogressionfromtheOligoceneandMiocene likeonmostotherislandsoftheCyclades(e.g.,SyrosIsland,Maluski etal.,1987;Baldwin,1996;Wijbransetal.,1990;TinosandAndros Islands,Bröckeretal.,1993;Huet,2010).

AlllithologiesareaffectedbyearlyisoclinalfoldsF₁,overturned totheSWFigs.4and7b)andparticularlywellexpressedbyalter- natingfoldedcalciticanddolomiticmarblesinthenorthernpartof theisland,whereHProcksarebestpreserved.Therelationbetween F1foldsandthetop-to-the-northto−northeastsheardeformation isdebated.Avigad(1990)suggestedthattheywereformedbefore thepeakofmetamorphismintheorogenicwedge(orsubduction channel)duringthesubductionoftheAfricanslabinvolvingduc- tilekinematicindicatorstop-to-the-SW,hencetheiroverturning totheSW.Alternatively,theymayhaveformedcontinuouslydur- ingexhumationfromblueschist-togreenschist-faciesconditions duringthedominanttop-to-the-northeastshearingandtheirover- turningtotheSWis onlyapparent.Basedonﬁeldobservations (e.g.,relationshipsbetweendolomiticmarblesandmarbles),we suggest that themetamorphic pile ofSifnos underwentimpor- tant ﬂattening andshearing beforethepeakof metamorphism.

Thisfoldingcouldbeassociatedwithtop-to-the-southdeformation thatwasactivewithinthesubductionzone,insyn-orogeniccon- ditions(Huetetal.,2009;Philipponetal.,2011;Aravadinouetal., 2015).Theexcellentpreservationofeclogite-andblueschist-facies paragenesesinthenorthofCheronissosUnitsuggeststhatthese foldsstartedtoformwithinHP-LTconditionsandevolvedcontin- uouslyduringexhumationfromblueschist-togreenschist-facies conditionsduringthedominanttop-to-the-northeastshearing.A laterfoldgenerationF3deformstheCOFcontactwherewehave observedacertainlocalizationofgreenschist-faciesdeformation.

7.2. P-T-timepathsofSifnosandlinkstodeformation

Ourstudyshowsthattheintensegreenschistretrogressionof Faros Unitisclosely associated withthedeformationalong the maintectoniccontactbetweenCheronissosandFaros Unitdur- ingexhumation.ConspicuouspreservationofHP-LTparageneses in Cheronissos Unit largely reﬂects the fact that greenschist- facies deformation localized in deeper parts of CBU and only locallyaffectedthisunit.Thus,CheronissosUnitwasalreadypartly exhumedwhileFarosUnitwasexhumingbelowit.Theobserva- tionthatthelowerpartofFarosUnitisdominatedbylow-pressure parageneses suggests that deformation progressively migrated downwardduringexhumation.TheP-TestimatesofTrotetetal.

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Fig.13. ContactbetweentheEclogite-BlueschistUnitandMainMarbleUnit(a)Thiscontactischaracterizedbyanaveragedipofthefoliationisabout50^◦westwardand showingalocalgreenschist-faciesretrogression.Allkinematicindicatorsindicatetop-to-the-NEsenseofshear.Bottomtotop,(b)dominosstructuresinmarblesand(c) asymmetricmetabasitepebbleswithinametaconglomeratelevelshowingatop-to-the-NEsenseofshear.(d)Metabasitefullyretrogradedinthegreenschist-faciesconditions isassociatedwith(e)quartz-calciteveinswhichareparallelorperpendiculartothefoliation.Atmicroscale,(f)millimetricshearbandsﬁlledinchloriteshowingaconsistent top-to-the-NEto−Esenseofshear.(g)Metabasitesformasymmetricpinchboudinageindicatingtop-to-the-NEkinematicsinblueschist-faciescondition.

(2001a,b)plotatslightlyhighertemperature(around600^◦C)than thoseofGroppoetal.(2009),whosepeakisaround560^◦C(Fig.3c).

Whetherthisdiscrepancyisduetodifferentsamplingstrategies ortodifferentpetrologicalapproachesisbeyondthescopeofthis paper.However,thehighertemperatureproposedbyTrotetetal.

(2001b)forthegreenschist-faciesconditionsﬁtsquitewelltheHT pulseproposedaround19MabyWijbransetal.(1990)basedon

40Ar/³⁹ArdataonphengiteinFarosUnit(seeagesinFig.3aandb).

Furtherpetrologicalandchronologicalstudiesareneededtoclarify theblueschist-togreenschist-faciestransitiononSifnos.

7.3. NatureofthecontactbetweenCheronissosandFarosunits

In this study,we interpret theCOF contactas a top-to-the- northeastductileshearzoneasproposedbyTrotetetal.(2001a) (Figs.4 and14).Atsmall-scale,localbrittledeformationmaybe observed,butnottothepointofsuggestingalarge-scalebrittle

detachment.Locally,brittleorbrittle-ductilekinematicindicators withatop-to-the-SWsenseofshearareobserved,whichledRing etal.(2011)tosuggestthatthiscontactbelongstoatop-to-the- southdetachmentsystem.Ourobservationsindicateinsteadthat thesekinematicindicatorsarerelatedtothesouth-dippinglate- stagenormalfaultsthatcutthroughtheregionalfoliation.

This shear zone along the COF contact is coeval with the greenschist-faciesdeformationobservedinthewholeFarosUnit, andmorelocallyinCheronissosUnit,aswellasalongafewlocal- izedshearzonessuchastheChryssopigiShearZone.TheCOFShear Zonethusbelongstoathickstrainlocalizationzonethatencom- passesthewholeFarosUnitwithadownwardgradientofstrain andprogressivedownwardlocalizationthroughtime,withlocal smallerscaleshearzoneswhereshearinghasbeenpreferentially partitioned.WethenrefertothisthickshearzoneastheApollonia ShearZone.

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Fig.14.AnewmetamorphicmapofSifnos.

Colourscorrespondtothedominantmetamorphicfaciesobservedintheﬁeldandcolourgradientsshowthegradualtransitionsduetoprogressivetop-to-the-NEshearing deformationandcoevalretrogression(redshowsdominanteclogite-facies,bluedominantblueschist-faciesandgreendominantgreenschist-facies).Thewhitearrows indicatethesenseofshearinallunits.Themainshearzonesareindicatedinthemap.(Forinterpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferred tothewebversionofthisarticle.)

7.4. Evolutionofshearzonesandstrainlocalization

AlthoughsomeuncertaintiesremainonthedetailedP-Tevolu- tion,weshowthat,fromtheEocene(45Ma)totheMiocene(19Ma), acontinuumofsyn-syn-blueschisttosyn-greenschisttop-to-the- N/NEshearingalongtheApolloniaShearZoneaccommodatedthe exhumation.Inthenorthernpartoftheisland,peaktoretrograde HP-LTdeformationislocallywellpreservednearVroulidia,without signiﬁcantgreenschist-faciesretrogressionnordeformation.This partofthetop-to-the-northshearzonewasthusdeactivatedearly

intheexhumationprocess.Thedeformationisprogressivelylocal- izedwithinthemajorApolloniaShearZone,towardthebaseofthe CBUofSifnosduringexhumation.Thislarge-scalestructureappears tobemoreretrograded(fromAEBS-togreenschist-facies)toward itslowerpart.Moreover,progressivelocalizationofdeformationis linkedwitharejuvenationofapparentages,fromnorthern(around 47Ma)tosouthern(30–19Ma)partoftheisland,especiallyshown by⁴⁰Ar/³⁹ArandRb-Srdataonphengite(seecompilationonFig.3a andb;Altherretal.,1982;Wijbransetal.,1990;Ringetal.,2011;

Bröckeretal.,2013).Hence,fromageodynamicpointofview,we

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proposethatthisprogressivedownwardlocalizationofstrainfirst occurredatthestageofexhumationwithinthesubductionchan- nel(Eocene,syn-orogenicexhumationsensuJolivetetal.,2003).In alaterstage,strainlocalizationoccurredwithintheback-arcregion (OligocenetoMiocene,post-orogenicexhumation).Theobserved shearzones,intheAEBS-orgreenschist-facies,tendedtolocalize duetorheologicalcontrastalonglithologicalboundariessuchas thebaseoftheMainMarbleUnit.Someshearzones(e.g.,Chrys- sopigi)arelocalizedwithinasinglelithologicalunit. Hence,the downwardmigrationofdeformationcouldbelinkedto1)theini- tiallithologicalheterogeneityoftheCBU.Shearzoneswerethus activeduringsyn-orogenicandpost-orogenicstagesofexhuma- tionshowingthatthekinematicoftheAegeanextensioninthe OligocenetoMiocenewasinfluencedbyheterogeneitiescreated duringburialandsyn-orogenicstage;2)anintensefluidcircula- tionwhichisreflectedbymanysyn-kinematicveins;3)increased thermalinfluxatthebaseofthemetamorphicpileassuggested byvariousauthors(MatthewsandSchliestedt,1984;Schliestedt andMatthews,1987; Avigad,1993;Trotetetal.,2001b).What- evertheactualcauseforthis downwardstrainlocalization one importantconsequenceisthatitleavestheupperpartoftheaccre- tionarycomplexpreservedfromlatedeformationandallowsitto reachthesurfacealmostintact.Thisprocessofprogressivelocal- isationisimportanttoexplainhowpristineblueschists-faciesor eclogite-faciesunitscannowbeobservedatthesurface.

7.5. Tectono-metamorphicevolutionofSifnos

Ourobservationsofthetectono-metamorphicevolutionofthe cycladic blueschists of Sifnos show that, after a phase of progradetop-to-the-SEshearing(Aravadinouetal.,2015),exhumation involvedatop-to-the-NEshearingcoevalwithdecompressionand progressivelocalizationofstraininthelowerpartofthetectonic pile,fromtheEocenetotheEarlyMiocene.Thisevolution thus encompassesboththesyn-orogenicstagewithinthesubduction zoneandthepost-orogenicstageinaback-arcposition.

Inmoredetailsthetectono-metamorphicevolutionofSifnoscan besummarizedinfourstages(Fig.15):

•Before 47 Ma, during the convergence between African and EurasianplatesandthesubductionofthePindosoceanicbasin belowthesouthernmarginofEurasia,theCBUofSifnosrecorded afirstdeformationphaseinthesubductionchannel,creatingan earlyfoliationS1,onlypreservedasinclusionsingarnets,inpar- ticularinthenorthernpartofSifnoswhereHP-LTparageneses arewellpreserved.ThesporadicpreservationofS₁ structures doesnotallowconstrainingthekinematicsassociatedwiththis firstdeformationstage.However,weinterpretthelarge-scale foldsobservedinthenorthernpartoftheislandastheconse- quenceofthisfirstductiledeformationevent(Figs.4,7band c).Thistop-to-the-SSWkinematicsmayberelatedtothetop-to- the-SEshearingdescribedonSifnosbyAravadinouetal.(2015) orthetop-to-the-SW senseofshear described intheCBUon SyrosIslandbyPhilipponetal.(2011)andinterpretedbythese authorsassynchronouswiththeprogradehigh-pressuredefor- mationphase.OnSifnos,duringthisfirststageofdeformation inthemid-Eocene,theMarbleComplex,Eclogite-BlueschistUnit andMainMarbleUnitwererespectivelythrustsouthwardonthe topofFarosUnit,buttheamountofdisplacementcannotbefur- therconstrained.AllrocksofSifnoshaveprobablyrecordedthis progradetop-to-the-SWkinematicsthatwasalmostcompletely overprintedbythelaterdeformationphases.

•Between47and30Ma,themetamorphicrocksofSifnosreached peakP-Tconditionsandstartedtheirexhumation.During this syn-orogenicexhumationperiod,asyn-blueschistdeformation phasehasresultedintheformationofthefoliationinEclogite-

BlueschistUnitwithtop-to-the-Nto−NEsenseofshear(Fig.14).

Manyfoldshavedevelopedduringthisphase,somewiththeir axesperpendiculartothetransportdirection;locally,a-typefolds wereformedindicating astrongnon-coaxialshearingcompo- nent.

•From30Ma,whichcorrespondstothebeginningofpost-orogenic extension and formation of the Aegean Sea, P-T conditions evolvedfromAEBS-togreenschist-faciesconditions.Duringthis phase,thedeformationwasmainlydistributedinthewholeFaros Unit,and morelocally inCheronissosUnit.Kinematic criteria indicateatop-to-the-NEsenseofshear.TheentireFarosUnitcan beinterpretedasathickshearzone(ApolloniaShearZone)with strainlocalizationalongtheCOFcontactand,later,nearthebase ofFarosUnitalongdiscreteshearzonessuchastheChryssopigi Shear Zone (Fig. 14). Extensive downward greenschist-facies overprint during this event may be related to enhance ﬂuid circulationandincreasedthermalinﬂuxatthebaseofthemeta- morphicpile (Matthewsand Schiestedt,1984;Schiestedt and Matthews,1987;Avigad,1990;Trotetetal.,2001b).

•Between19MaandPresent,thetimingofCBUexhumationis constrainedbyzirconﬁssiontrackdata(Ringetal.,2011).ZFT agesof13–10MasuggestthatFarosUnitwasexhumedacross the250–300^◦Cisothermduringthisperiod.Theyoungestages associatedwithductiledeformationarethoseobtainedonmicas withthe⁴⁰Ar/³⁹Armethod(Wijbransetal.,1990)around19Ma.

ThelatefoldsF₃ developedduringthebeginningofthisperiod andtheyrefoldbothFarosandCheronissosUnit.Thesouthwest- dippingnormalfaultsthatoffsetthemainCOFcontactarelocally low-anglestructuresdisplaying brittle-ductilemicrostructures Fig.5candg).However,theirthrowisnotlargeanditisdifficultto saywhetherthesestructureshadasignificanteffectonthefinal exhumationofFarosUnit.Therefore,thetimingofthetransition fromtop-to-the-NEductileshearing tolate brittletop-to-the- SW normalfaults observedonSifnosis not wellconstrained.

SimilarSW-dippinglistric normalfaultsandﬂat-ramp system havebeendescribedrespectivelyonFolegandrosIsland(Augier etal.,2015)andonSyrosIsland(Philipponetal.,2011).Likeon Sifnos,faultscrosscuttherethemetamorphicpileoftheCBUand thegreenschist-faciesfoliation,whicharebothassociatedwith ductiletop-to-the-northshearsense.Thesigniﬁcanceofthese SW-dippingfaultsisdebated.Ringetal.(2011)interpretedthe contactbetweenCheronissosandFarosUnitasatop-to-the-SW brittledetachment,whichtheycoinedasthe“Sifnosdetachment”

belongingtothe“SouthCycladicDetachmentSystem”outcrop- pingontheislandsofSerifos,IosandSifnos.Thetop-to-the-south senseofshearpreviouslydescribedonIos(Listeretal.,1984)has beenreinterpretedbyHuetetal.(2009)asathrust-relatedshear- ingevent,thuspredatingthetop-to-the-northAegeanextension.

In a more recent paper, Mizera and Behrmann (2015) have addressedthisdebateagainandarguedinfavouroftheexten- sionalmodelfortheCycladicBlueschist/BasementUnitcontact onIos.Theyhavestudiedthedistributionofkinematicindica- torsand quantiﬁedtheamountofstretchinginthebasement orthogneissandconcludedthatthebasementhasbeenstretched andﬂattenedbyrespectively70%and40%.Theyusetheupward straingradientinthebasementtoconcludethatthecontactwas mainlyextensionalandtheyestimatethetotaldisplacementdur- ingextensiontosome13km.Theyalsorecognizedthat,before extension,themaincontacthadtobeathrustasshownbyHuet etal.(2009).Themainpointofdisagreementstemsfromdifferent interpretationsofthedeformationinthebasement.Huetetal.

(2009)didnotconsiderthattheupwardstraingradientinthe basementwasindicativeofextensionandwesticktotheirinter- pretation.Itshowstheexistenceofamajorshearzonethere,butit doesnotsaywhetheritwasextensionorcompressional.Besides, the late extension is clearly shown in the overlying cycladic

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Fig.15.Seriesofstep-by-stepnorth-southcross-sectionsoftheAegeandomainfrom65Mato10MamodiﬁedafterJolivetandBrun(2010).SeelocationinFig.1a.

Reconstructionsindicatethetectono-metamorphicevolutionofSifnos,SyrosandTinos,respectivelyshownbynumbers1,2and3.Thelastcross-section,seelocationin Fig.1b,showingthePresentgeometryoftheseislands.AbbreviationsWCDS,VD,NCDSindicaterespectivelytheWestCycladicDetachmentSystem,theVariDetachment andtheNorthCycladicDetachmentSystem.(Forinterpretationofthereferencestocolourinthisﬁgurelegend,thereaderisreferredtothewebversionofthisarticle.)

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blueschistsbyretrogradetop-to-the-Nshearbandsatallscales, whiletop-to-the-Ssyn-blueschistsshearinghasbeenpreserved onlyinthevicinityofthemaincontact.Suchintensetop-to-the-N shearing,thatHuetetal.(2009)attributetoextensionisobserved allovertheisland,alsointhemaincontactanditisdifﬁcultto conﬁdentlysaywhetherthetop-to-the-Nandtop-to-the-Sshear zonesinthebasementwerecoevalornot.

Grasemann et al. (2012) have described the West Cycladic DetachmentSystem(WCDS)exposedonKea,KythnosandSeri- fosIslands.TheWCDSconsistsintwomaindetachments,aductile oneintrudedbyagranodioriteplutonandabrittleonecuttingthe roofofthesamepluton(seealsoRabillardetal.,2015).Thesenseof shearontheWCDSiseverywheretop-to-the-south,fromSerifosall thewaytoLavrion(Fig.1b).Basedonournewfieldobservationsof thedeformationrecordedonSifnosandespeciallyalongthecontact betweenCheronissosandFarosunits,weproposeaninterpretation significantlydifferentfromthatofRingetal.(2011)whoassociated IosandSifnoswithintheSouthCycladic Detachment.AsAugier etal.(2015)onFolegandrosIslandandJolivetetal.(2015),wesug- gesttoassociatethetop-to-the-SWfaultsthatoffsettheCOFShear ZonetoabrittleexpressionoftheWCDSthatmaybeextendedfrom SerifostooffshoreSifnos.Inthisscenario,theN-S-trendingnormal faultsdippingeitherwestoreastoneithersidesoftheislandare takentoreflectthelatestbrittlestagesaccommodatingtheendof exhumationonSifnos.

7.6. ImplicationsfortheexhumationoftheCycladicBlueschist Unit(CBU)

Currentassumptionsonthetectonicandmetamorphichistory oftheCBUneedtoberevisedin thelightof ournewobserva- tions.ForsterandLister(2005)proposedareconstructioninvolving thesuccessionofseveralsubduction/exhumationepisodeswith repeatedinversionofthesenseofmotionalongthemaincontacts.

However,theexhumationoftheCBUismainlycharacterizedbya top-to-the-Nto−EductiledeformationsincetheEocenewith,most notably,thesamemetamorphicpeakconditionsacrossthewhole pile.Thesecanhardlybereconciledwiththeinversionshypothe- sizedfortheevolutionoftheCBUonSifnos.Basedontheextrusion wedgemodel ofChemenda etal. (1995),Ringand Reischmann (2002)suggestedadecouplingofcrustalunitsaccommodatedby adetachmentatthetopandathrustatthebasetoexplainapart ofCBUexhumation.Brunand Faccenna(2008)proposedasim- ilarmodel withanextremelocalizationofstrainatthetopand baseoftheCBU,theirmodelalsoemphasizingtheimportanceof slabretreattofavourexhumation.However,ourﬁeldobservations showtheexistenceofseveralshearzonesactivesincetheEocene duringexhumation,showingthattheCBUdidnotbehaveasasingle coherentunit.

Thisstudyhasclariﬁedsomeoftheuncertaintiesaboutthegen- eralstructureoftheCBUofSifnos,whichshowssomesimilarities withSyrosIsland.Indeed,onSyrosthemetamorphicpeakisesti- matedaround20kbarand500–550^◦C(Trotetetal.,2001b)and datedaround52Ma(Lagosetal.,2007).TheHP-LTparagenesesare alsowellpreservedandthesyn-exhumationkinematicsarechar- acterizedbytop-to-the-Esenseofshearactiveinblueschist-facies andgreenschist-faciesconditions(Trotetetal.,2001a,b;Laurent etal.,2016).Hence,theoverallstructureofSyrosisverysimilar tothatofSifnoswiththenotableadditionoftheVariDetachment croppingoutonSyros.Bothislandsshowthesamearchitecture withtheuppermostunitsbelowthemaindetachmentdisplaying thebest-preservedHP-LTparageneseswithaprogressivelocaliza- tionofdeformationtowardthebaseofthetectonicstackduring exhumation(seeTrotetetal.,2001aandLaurentetal.,2016for SyrosIsland).However,thisEocenesyn-orogenicexhumationgra-

dientisassociatedwithdifferentshearingdirections(E-vergingon Syros,N-NE-vergingonSifnos;Trotetetal.,2001a,b).Everywhere else,theCBUshows mostlytop-to-the-N orNEkinematic indi- catorsrelated tosyn-blueschistexhumation.Thesearefoundto varyaccordingtotheirpositionwithinthearcsubduction,north- directedinthefront,(e.g.,Ios),moreeast-directedonthenorthern sideofthearc(e.g.,Andros,Tinos,Fig.1b).

ThehistoryofexhumationoftheCBUthusinvolvedathrustat thebase(seenonIosorSikinos),amaindetachmentatthetop (theVariDetachmentseenonSyros;Trotetetal.,2001a;Soukis andStöckli,2013;Laurentetal.,2016)andadistributionofstrain withintheexhumedunitalongaseriesofshearzonessyntheticof themainupperdetachment(top-to-the-Nor−E).Sotheexhuma- tionscenarioshouldbeintermediatebetweentheextrusionmode where allthe deformation is concentrated atthe topand base and a subductionchannel inthe senseofEngland and Holland (1979)orShreveandCloos(1986)wherethedeformationisfully distributed.WesuggestthatHPunitsareprogressivelydetached fromthesubductinglithospherewhentheyreachthepeakofpres- surebytop-to-the-Ssyn-HPthrustsandarethenexhumedwithin thechannel,progressivelyshearedtop-to-the-Nor−NEwhenthe approachtheroofofthechannel,likeinJolivetetal.(2005).Pre- ciselydatingthepeakofmetamorphisminthedifferentunitsas wellasstrainlocalizationalongthemainshearzonewouldhelp reachingaconsensusonthisquestion.

AftertheEocenesyn-orogenicepisode,exhumationwascom- pletedfrom35to30MaduringtheformationoftheAegeanSea bycrustalthinningandaccommodatedbytheNCDS(Jolivetetal., 2010),theNaxos-Parosdetachment(Gautieretal.,1993)orthe WCDS(Grasemannetal.(2012).Thispost-orogenicexhumationis characterizedbyaprogressivelocalizationofdeformationalong extensionalshearzonesandaconsistentsenseofshearduringthe Oligo-Miocene.ItiswellexpressedonTinos,markedbyaNE-SW gradientofnon-coaxialstrainacrossamajorextensionalshearzone (Parraetal.,2002).Agreenschist-faciesdeformationgradient,asso- ciatedtotheNCDS,separatestheCBUfromtheUpperUnit.The top-to-the-northretrogradeshearingdeformationobservedonIos isprobablyrelatedtothisepisode(Huetetal.,2009).Radiometric agesshowthatthisepisodeisalsorecordedonSifnosduringthe latelocalizationofshearingtowardthedeeppartsoftheediﬁce.

Wethensuggestthatsouth-dipping brittlenormalfaultsare thebrittleexpressionoftheWCDS.TheCBUofSifnosarelocalized withinthefootwalloftheWCDS,justasKea,KythnosandSerifos, butprobablydeeperandtheydidnotseetheintenseductileshear- ingassociatedwiththeWCDS,theyonlyrecordedthisextension assomelatebrittlenormal faults.Similarsouth-dippingnormal faultsareobservedonFolegandrosIsland, furthertothesouth- eastandthesearetheonlyexpressionoftheWCDSthere.Atthis period,theCBUofSifnoshasundergonepenetrativetop-to-the-NE deformationwhereastheCBUofSerifoshasrecordedpost-orogenic deformationwithtop-to-the-Wsenseofshear.Apossibilityisthat themaindetachment(WCDS)associatedwithductiledeformation diesouttowardthesoutheastorreachesthesurfaceoffthewest coastofSifnosandFolegandrosIslands.

Combining our new structural observations with published petrological and geochronological data, we propose a tectonic modelfortheexhumationoftheCBUintheAegeandomainwith particularemphasisplacedonevolutionoftheCycladicislandsof Sifnos,SyrosandTinos.Theinitiallithosphericscalegeometryof thesubductionandthedispositionofthedifferentunitsaretaken fromJolivetandBrun(2010).Ourmodelispresentedasaseries ofstep-by-stepnorth-southcross-sectionsoftheAegeandomain from50Matothepresentdaywithdifferentnumberstrackingthe evolutionofSifnos(1),Syros(2)andTinos(3)(seeFig.15).

Early Eocene(50Ma)to Late Eocene-EarlyOligocene (30–35 Ma):