JournalofSouthAmericanEarthSciences26(2008)235–251 ContentslistsavailableatScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames Patagonia: A paleozoic continent adrift? Victor A. Ramos LaboratoriodeTectónicaAndina,CONICET,FCEyN,UniversidaddeBuenosAires,Argentina a r t i c l e i n f o a b s t r a c t Articlehistory: TheevolutionofPatagoniaasanindependentandexoticmicrocontinentfromtherestofSouthAmerica Received12January2008 wasarecurrenthypothesissincetheXIXcentury,reachingnotorietyduringthediscussiontimesofcon- Accepted5June2008 tinentaldrifttheory.Thearrivalofplatetectonicstriggereddifferenthypotheses,someofthemwithfixist interpretationsthatconsiderPatagoniaasanautochthonouspartofGondwana,andothersmoremobi- listic that postulate an allochthonous origin. After several decades, although some consensus exists Keywords: amongthosehypothesesthatpostulateitsallochthony,thereisnoagreementinitsboundaries,subduc- Terranes tion,accretion,andfinalamalgamationtimestotheGondwanasupercontinent.Inthisreviewthediffer- Paleozoic ent magmatic belts are analyzed, their deformation and metamorphism, the associated sedimentary Andes basins,aswellastheexistinggeochronologiccontrols.Awarethatimportantuncertaintiesstillremain, Exotic Allochthonous anewmodelisproposedwithtwomagmaticarcs:awesternbeltthatwasactivefromtheDevonian Para-autochthonous tothemidCarboniferous,andanorthernonepartiallycoevalthatledtothecollisionofPatagoniaagainst thesouthwesternmarginofGondwanaintheLowerPermian.Itishypothesizedthattheterminationof thewesternmagmaticarcactivitywaslinkedtothecollisionoftheAntarcticPeninsulaandassociated terranes.ThereconstructionoftheplatetectonichistoryofPatagoniaduringthePaleozoicshowsthe existenceofseveralepisodesoffragmentationandrifting,convergenceandaccretion,renewedperiods ofriftingandreaccretiontotheGondwanamargin.Thoseprocesseswereintrinsictotheformationof TerraAustralisorogen,controlledbytheabsolutemotionoftheGondwanasupercontinentandguided bysuccessiveglobalplatereorganizations. (cid:2)2008ElsevierLtd.Allrightsreserved. a r t i c l e i n f o r e s u m e n Articlehistory: LaevolucióndelaPatagoniacomouncontinenteindependienteyexóticoalrestodeAméricadelSurha Received12January2008 sidounahipótesisrecurrentedesdeelSigloXIX,alcanzandonotoriedaddurantelostiemposdeladiscus- Accepted5June2008 ióndelateoríadeladerivacontinental.Coneladvenimientodelatectónicadeplacascobrónuevoimpul- so,dividiéndoselasinterpretacionesenunaseriedehipótesisfijistasquelaconsideraroncomoparte autóctonadelGondwanayenotrasmásmovilistasquepostularonunorigenalóctono.Despuésdevarias Palabrasclaves: décadas, si bien ha ganado consenso las hipótesis que postulan su aloctonía, no hay acuerdo en sus Terrenos límites,tiemposdesubducción,acreciónyamalgamientofinalalsupercontinentedeGondwana.Enesta Paleozoico revisiónseanalizanlasdiferentesfajasmagmáticas,sumetamorfismoydeformación,lascuencassedi- Andes mentariasasociadas,asícomoloscontrolesgeocronológicosexistentes.Conscientequeaúnquedannota- Exótico Alóctono bles incertidumbres se propone un modelo con dos arcos magmáticos: uno occidental que fue activo Para-autóctono desdeelDevónicohastaelCarboníferomedio,yotroquesetraslapóparcialmenteeneltiempoyque llevaalacolisióndelaPatagoniacontraelmargensudoccidentaldelGondwanaenelPérmicoinferior. Sehipotetizaqueelcesedelaactividaddelarcomagmáticooccidentalestuvoligadoalacolisióndel basamentodelaPenínsulaAntárticayterrenosasociados.Lareconstruccióndelahistoriatectónicade placasduranteelPaleozoicoponeenevidencialaexistenciadevariosepisodiosderupturayrifting,con- vergenciayacreción,renovadosperíodosderiftingyreacreciónalmargendelGondwana.Estosprocesos sonintrínsecosalaformacióndelorógenodeTerraAustralis,controladosporelmovimientoabsolutodel supercontinentedeGondwanayguiadosporsucesivasreorganizacionesglobalesdelasplacas. (cid:2)2008ElsevierLtd.Allrightsreserved. E-mailaddress:[email protected] 0895-9811/$-seefrontmatter(cid:2)2008ElsevierLtd.Allrightsreserved. doi:10.1016/j.jsames.2008.06.002 236 V.A. Ramos/JournalofSouthAmericanEarthSciences26(2008)235–251 1.Introduction Martínez (1980) speculated that a displaced Patagonia could be thesourceofthesesediments.Thisproposalproducedanewwave Patagonia,oneofthelesspopulatedareasofSouthAmerica,and ofhypothesesthattriedtoevaluateviapaleomagneticstudiesthe avastsemidesertlandinacoldtotemperateregion,wastheinspi- apparentpost-DevoniandisplacementsofPatagonia(Valencioand rationformanyunusualhypothesesonitsgeologicorigin.There- Vilas,1985).However,thepresentknowledgeofprocessessuchas gionsouthoftheRíoColorado(seeFig.1),attractedthepremature tectonicerosionbysubduction,orverticalaxisrotationofthestud- attention of eminent naturalists such as the Perito Francisco P. iedblocks,couldeasilynegatealltheseproposals. Moreno as early as in the XIX century, who wondered about the Thefirstmodernideasthattriedtoexplainsomepeculiargeo- ‘‘exoticnature”ofitslandscapeandflora.Heemphasizeditsclosest logic and tectonic features of Patagonia were proposedby Frutos connections with the Antarctica Peninsula, Australia and and Tobar (1975). These authors were the first to envisage that New Zealand, and he wondered why some landscapes and rocks theDeseadoandSomúnCuramassifswereseparatedbyanearly of Patagonia were so different from the rest of South America Paleozoicsubduction zone (Fig. 2a). That inferencewas based on and strikingly similar, and with strong affinities, to the southern structural studiesof theobliqueNW-trendingpenetrativefabrics continents(Moreno,1882).Thishypothesiswasremindedduring ofsomemetamorphicrocksoftheDeseadomassif,incontrastwith theyearsofthecontinentaldriftdiscussionbyKeidel(1925)and thedominantnorth-trendingstructuresoftheAndeanbasement. Windhausen(1931),whoclaimedwithsimilarreasoningthatPat- Thishypothesis,butbasedondifferentcriteria,hasbeenrevisited agoniawasanisolatedcontinentduringpre-Cretaceoustimes,and by Gallagher (1990) who proposed that the Deseado Massif was hadbeenlaterweldedtotherestofGondwana.However,mostof accreted to Gondwana during Carboniferous times, in a similar these speculations were based on a poor and incomplete knowl- waytothatproposedmorerecentlybyPankhurstetal.(2006). edgeoftheprocessesrelatedtocontinentaldrift,anddonotresist Mostofthestudiesperformedatcontinentalscaleontheorigin amodernscreeningwiththepresentknowledgeoftheregion. oftheSouthAmericanbasementnotedthedifferencesbetweenthe PaleocurrentstudiesinsomePaleozoicbasinsalongthecoastin Brazilian(orSouthAmerican)platform(Fig.1),amalgamatedand southern Peru were consistent with an apparent provenance of consolidated as a craton by the end of the Proterozoic and Devonian sediments from the Pacific side (Martínez, 1980). This Patagonia consolidated during the Paleozoic (Almeida et al., last author to explain these observations resuscitated the old 1976).TwobasementmassifsweredifferentiatedinPatagoniaby hypothesisofaPacificacontinentofBurckhardt(1902).Although Harrington(1962)whodescribedthemasnesocratons,basement theoriginalproposalwasbasedonthegeosynclinaltheorythatre- areasremobilizedduringthePaleozoicorogenies. quired an inner supply of sediments in the orogenic stage, ThedevelopmentoftheterraneconceptbyMongeretal.(1982) fosteredsomenewideasabouttheoriginofPatagonia.Soonafter that proposal, complemented by the suspect terrane notion pro- posed by Coney et al. (1980), two different points of view were continuously debated. Some authors favored the autochthony of thePatagonianblock(eg.Forsythe,1982)whileothersinterpreted the Patagonia as an accreted terrane (Ramos, 1984, 1986). These conflictinghypothesesstimulatedmorethan20yearsofactivere- search, discussion and collection of new data on the Patagonian basement(seeFig.2bandc). 2.Generalfeaturesofpatagonia There are several ways to define the extension and limits of Patagoniaongeological,geophysicalandgeographicgrounds.The continent–ocean transition along the Atlantic sea is the natural eastern border, while the southern Andes are the western limit, althoughtheregionwestofthemaincordilleraisalsosometimes included as part of Patagonia. A focused study by the Geological Survey of Argentina determined that the characteristics of the basement,changes in crustalthickness,importanttruncationsby N70(cid:3)W-trending lineaments of the north–south fabrics of the northernbasement,andothergeologicalandgeophysicalfeatures, complementedbyhistoricalgrounds,definedthenorthernlimitof Patagonia as indicated in Fig. 3 (see discussion in Ramos et al., 2004). Based on these criteria, the Patagonia geological province encompassesaseriesofbasementoutcrops,mainlyexposedalong theeasternsideoftheAndes,whichcanbegroupedintwodistinct massifs following Leanza (1958) and Harrington (1962): the SomúnCuraandDeseadomassifs. The northern block known as the Somún Cura (or northern Patagonian) massif is bounded in the north by the Neuquén and Colorado basins (Fig. 3). However, the basement of the southern onethirdoftheNeuquénBasinhasWNW-trendingfabrics,which controltheorientationoftheHuinculridge,andhascommonbase- Fig.1. RegionallocationofthePatagoniaPlatformwithmostimportantbasement mentfabricswiththeSomúnCuraMassif(FranzeseandSpalletti, massifs:theDeseado(D)andSomúnCura(SC).Gu:Guyana,Bc:BrasilCentral,At: Atlántico(basedonAlmeidaetal.,1976). 2001;MosqueraandRamos,2006).Thesurfaceexpressionofthis V.A. Ramos/JournalofSouthAmericanEarthSciences26(2008)235–251 237 TheDeseadoMassifisexposedsouthoftheSanJorgeBasinand is bounded by the Austral (or Magallanes) Basin to the south (Fig. 3). Consequently, the Patagonia is composed by two large basement massifs, bounded by Mesozoic basins, which were mildlydeformedbytheAndeanorogeny(Ramos,2004b). 3.ThebasementofPatagonia The basement of northern Patagonia have been extensively studied by Caminos and Llambías (1984), Rapela and Llambías (1985), Rapela and Caminos (1987), Dalla Salda et al. (1992a,b, 1994), Von Gosen (2002, 2003), Varela et al. (2005, 2007), and Pankhurstetal.(2006),amongothers.Thesestudiesallowthedef- inition of two different metamorphic and magmatic belts: the northern and the western belts. The northern metamorphic and magmaticbeltispreservedparalleltothesouthernmarginofthe Neuquén and Colorado basins along the Río Limay valley from the city of Bariloche in the west up to the Sierra Grande region along the Atlantic coast (Varela et al., 1998a; Basei et al., 1999). The western metamorphic and magmaticbelt crosses the central Patagonia with a north–northwestern trend and continues into theDeseadoMassifandfurthersouth. 3.1.Thenorthernmagmaticandmetamorphicbelt This belt of metamorphic and igneous rocks with a dominant NW to WNW trend, is well exposed along the Río Limay valley andinLaEsperanza,Yaminué,Valcheta,MinaGonzalito,andSierra Granderegions.ThemainfoliationoftheserocksnearbyBariloche hasatop-to-the-eastandnortheastvergence(Herediaetal.,2006). The eastern sector of the belt has Ordovician granitoids, well known since the early work of Weber (1983) and Ramos (1984). Recentstudies,withmorepreciseU–PbandSHRIMPdataindicate crystallizationagesof475±6Maand476±4Maforgraniticrocks of Arroyo Salado and Sierra Grande (Varela et al., 1998a, 2005, 2007;Pankhurstetal.,2006). The host rocks of these intrusives are metamorphic rocks in amphibolitefacies,wellrepresentedbytheMinaGonzalitoGneiss (Ramos,1975)(Fig.4)whichhasmetamorphiczirconsdatedbyU– Pb. The metamorphic peak of these rocks has 468.7±4.3Ma Fig.2. (a)EarlyproposalwhereanearlysubductionzonesplittheSomúnCuraand theDeseadoMassifs,implyingthattheDeseadoMassifwasallochthonous(based (Pankhurst etal.,2001), agesimilartosome previousRb–Srages on Frutos and Tobar (1975)); pars Pankhurst et al. (2006). (b) Autochthonous obtainedforthesemetamorphicrocks(Varelaetal.,1997,1998a, hypothesis where a wide magmatic arc crosses the entire Patagonia (Forsythe, 2005). Some associated low grade schists as the El Jagüelito 1982;CaminosandLlambías,1984;Rapelaetal.,1989;DallaSaldaetal.,1990, Ectiniteshaveanichnofaunathathelptoconstraintheageofdepo- amongothers).(c)AllochthonoushypothesisproposedbyRamos(1984,1986). sition between Cambrian and Early Ordovician (González et al., 2002). The geochemical study of these low grade metamorphic rocks of the eastern sector shows derivation from marine facies ridgeandthesubsurfaceevidencebasedon3Dseismicdata,gravi- associatedwithamagmaticarcinanattenuatedcontinentalcrust metricandmagnetometricsurveysdefinethenorthernmostlimits setting(Cagnonietal.,1993).Theinheritedzirconsoftheserocks ofPatagonia. (Fig.4)haveanimportantNeoproterozoiccontribution,indicating TheSomúnCuraMassifisboundedtothesouthbytheCañadón a Brasiliano–Panafrican source, which possibly implies a para- Asfalto Basin (Fígari, 2005). This basin was formed by NE–SW autochthonous origin within Gondwana for the Somún Cura extensionpriorto160MaduringtheopeningoftheWeddellSea Massif. (Ghidella et al., 2002; Ramos, 2004a). The Cañadón Asfalto Basin ThesemetamorphicrockshavebeenrecentlystudiedatYaminué extends to the north beneath the basalts of the central part of (Fig.5),whereLlambíasetal.(2002)describedLateCarboniferous SomúnCuraasnotedbyCortiñas(1996).Thisauthorproposedthat orthogneissesmetamorphosedinamphibolitefacies,emplacedby theSomúnCuraisformedbytwohighs,oneinthenorthwithan undeformedLatePermiangranitoids.Thehostrockoftheorthog- east–westtrend,andanotherwithaN30(cid:3)Wtrend,definedasthe neissesisamphibolite,marbleandphylliteofpossiblyearlyPaleo- Chubut high, consistent with the two igneous–metamorphic zoic age which are unconformably covered by the Sierra Grande beltsidentifiedinthepresentwork(seeFig.5).Thepresentsouth- Formation,aseriesoforthoquartzitesdepositedinapassivemar- ern border isenhanced by the subsidenceof the San Jorge Basin, gin setting. This last unit of Silurian to Early Devonian age has a which is interpreted as an aulacogen (De Wit, 1977; Fitzgerald ductile deformation with thrusts with top-to-the-south vergence etal.,1990;Ramos,1996)formedasaconsequenceoftheWeddell (VonGosen,2003).Therearealsoseveralmyloniticbeltswithduc- Sea opening, and reactivated during the opening of the South tile deformation that affected the Carboniferous–Early Permian Atlantic. rocks. U–Pb dating in the Yaminué region indicates 295±13Ma 238 V.A. Ramos/JournalofSouthAmericanEarthSciences26(2008)235–251 Fig.3. MaintopographicfeaturesofPatagoniaandsurroundingareas. in Puesto Peynecura, 307±23Ma in Treneta, and 300±6Ma in andSpikermann,1974;Lizuaín,1981)andextendedfurthersouth PuestoTardugno(Varelaetal.,1998b).Mostoftheseigneousrocks by Ramos (1983). These rocks were studied by Dalla Salda et al. are preserved as orthogneisses and recent paleomagnetic studies (1992a) who obtained K–Ar ages of 354±4Ma and 324±6Ma haveindicated,basedonthemagneticfabric,animportantductile fortonaliticgneisses,and376±9Maforabiotiticgranodioritein deformationconsistentwithaSW–NEcompression(Rapalinietal., theLagoLacararea,nearSanMartíndeLosAndes.TheseDevonian 2008).ThismagneticfabricisabsentintheLatePermiangranites. toCarboniferousageswereinterpretedasyoungertectonothermal Similar mylonitic rocks are seen in Cerro Los Viejos, at the eventsaffectingtheearlyPaleozoicmagmaticarc. northern margin of the Colorado Basin (see Fig. 9 for location) Theageofthemetamorphicrocksofthisregionwasassumedto whichalsorecordlatePaleozoicductilefabricsdescribedbyTickyj beeitherPrecambrianorearlyPaleozoicuntilthegeochronologic et al. (1997), as part of the same late Paleozoic deformation. studiesofBaseietal.(1999),thatfound345±4.3Maoldzircons Metamorphic rocks, mainly granitoids and gneisses preserved in inanamphiboliteintheCañadóndelaMoscaoftheBarilochere- amphibolite facies in Cerro Los Viejos, have northwest-trending gion,whichimpliesthattheserockshavebeenmetamorphosedin foliationwithS–Cstructuresthatindicatesatop-to-the-northeast latePaleozoictimes.Baseietal.(2005)basedonconventionalU– vergence. Pbzircondatingconstrainedtheageoftheplutonicrocksbetween Theageofthemetamorphicrocksofthenorthernbeltwastra- 420 and 380Ma. An age on U–Pb in titanite ca. 360Ma together ditionallyassignedtothePrecambrianalongtheRíoLimayvalley with K–Ar ages in the range of 375–310Ma were interpreted as and in the Bariloche region, until the studies of Basei et al. coolingagesofthemetamorphicpeak.Subsequentstudiesinthe (1999),whofoundaU–Pbageof345±4.3Maforanamphibolite northernpartofthewesternbeltwereabletorecognizetwodis- south of Bariloche, and Varela et al. (1999), that reported an age tinct episodes and precisely date the igneous emplacement and of292±9MaforatonaliticgneissofPasoFloresbothinterpreted thepeakmetamorphismoftheserocks(Pankhurstetal.,2006). asagesofcrystallizationoftheprotolith(Fig.5). Further south, these igneous–metamorphic complexes are ex- posedalongtheRíoChicovalley,andweredescribedbyDallaSalda 3.2.Thewesternmagmaticandmetamorphicbelt et al. (1994). In this area, metamorphic rocks in greenschist to amphibolitefaciesareassociatedwithfoliatedtonalitesandgran- Asecondandwesternbeltofmagmaticandmetamorphicrocks odiorites, mylonites and granitic cataclasites, formed in a colli- isexposedfromSanMartíndeLosAndes–BarilochetoPasodeIn- sional setting. The metamorphic grade increases to the east of dios,alongtheRíoChicovalley,withaNNWtoNWtrendidentified thevalley,andtheprotolithsofthemetamorphicrocksareshales astheChubutbasementridgebyCortiñas(1996)depictedinFig.5. and graywackes. The main metamorphic episode is syntectonic ThepresenceofthismagmaticarcalongtheeasternsideofthePat- withimportantanatexis,andhasaN49–34(cid:3)W-trendingmainfoli- agonian Cordillera was suggested by the K–Ar dating of ation. Dalla Salda et al. (1994) supported a magmatic arc setting 345±10MainLagoLacarand380±15MainLagoPuelo(Toubes followedbyacollisionalepisodebasedongeochemicalandisoto- V.A. Ramos/JournalofSouthAmericanEarthSciences26(2008)235–251 239 Fig.4. Relativeprobabilityplotforthebest-estimatedagesderivedfromtheMinaGonzalitoGneissandElJaguelitoEctinites.(a)Ageofmetamorphism;(b)and(c)dominant inheritedzirconsofbrasilianoevents(basedonPankhurstetal.,2001). picgrounds.Regionalconstraintsindicatethatcoevaltonalitesand throughtheSanJorgeBasinwiththeDeseadoMassifasdepicted granodioriteswereemplacedalongadecompressingpathslightly inFig.5.Mostoftheserocksareassumedtohavebeenemplaced postdatingtheclimaxofregionalmetamorphisminthebasement, inaPrecambrianbasement.U–PbSHRIMPdatafromzirconsfrom andthatmonzograniticintrusionoccurredatuppercrustallevels the Dos Hermanos phyllites corroborate a typical Brasiliano age, (López de Lucchi et al., 1992; Dalla Salda et al., 1994; Cerredo withsomestronginheritanceofGrenvilleMiddleProterozoiczir- and López de Luchi, 1998; López de Lucchi and Cerredo, 2008). cons between 1000 and 1060Ma (Fig. 6), similar to the age ob- The first reliable U–Pb ages were published by Varela et al. tained in Cabo Belgrano (Cape Meredith) in the Malvinas (2005). Recently a muscovite migmatite, west of Mamil Choique, (Falkland)IslesbyCingolaniandVarela(1976). hasbeendatedbyU–Pbin281±2Ma(Pankhurstetal.,2006). ItisinterestingtomentionthatthewesternhalfoftheLaModesta Theseigneous–metamorphiccomplexespermitidentificationof schists,alsoconsideredlateProterozoicbasedonsomeminimum a belt of exposures of (Devonian) Carboniferous to Permian de- K–Ar ages of 540Ma (Pezzuchi, 1978), have been recently dated formed rocks. In the north, the belt consists of two-mica granite byU–PbSHRIMPbyMoreiraetal.(2007).Theseauthorsfoundthat ofPiedradelAguilawithaU–Pbzirconageof290±3Ma(Varela the deposition of the muscovite–chlorite schists, metaquartzites, etal.,2005)andtheLaPotrancafurthersouthoftheRíoChubut, and tourmalinite strata-bound schists, was no older than with a deformed leucogranite associated with migmatite, dated (cid:2)473Ma, and that the source of the zircons was probably the byU–Pbina289±2Ma(Pankhurstetal.,2006). granitoids exposed to the east. They also found inherited zircons Thewesternigneous–metamorphicbeltwithitstypicalarcand ofBrasilianoandGrenvilleagesimilartotheDosHermanosphyl- collisionalsettingswithanorthwest-trendingstructure(seeFig.5) lite,togetherwitholderzircons(Moreiraetal.,2007).Thisclearly hasbeenpreciselydatedbyPankhurstetal.(2006).Theseauthors indicates that deposition and low grade metamorphism of La indicatethatthebasementhasexperiencedanimportantcollision ModestaschistsareyoungerthanMiddleOrdovician. in the mid Carboniferous prior to the emplacement of peralumi- ThicksequencesofCarboniferoustoEarlyPermiangraywackes, nous S-type garnet-bearing leucogranites of Paso del Sapo and shalesanddiamictitesoftheTepuelBasinareexposedwestofthe Sierra de Pichiñanes which yielded 314±2Ma and 318±2Ma igneous–metamorphicbeltbetweenEsquelandJosédeSanMartín crystallizationages. (Pageetal.,1984;Andreisetal.,1987).Theseforelandbasindepos- Thisbeltofexposuresisunconformablycoveredbytheforeland its reaching more than 5000m (López Gamundi and Breitkreuz, continental Cretaceous deposits of the San Jorge Basin (Fig. 5). 1997)continueinthesubsurfaceoftheSanJorgeBasin,wherethey However,inthenorthernandsouthernflanksofthisbasin,several have been described in the Pastos Blancos well by Cortiñas and wells recovered cores of the late Paleozoic basement (Sylwan, Arbe (1982). Sedimentological analyses indicate submarine fans 2001). Drilling cores of some of these wells have been dated by prograding to deltaic systemsthat up-sequence culminate in flu- K–Ar yielding ages between Middle Carboniferous and Early vial continental deposits (López Gamundi and Limarino, 1985). Permian (Linares and González, 1990). The range of the drilling The outcrops have dominant paleocurrents from northeast to coreagesissimilartotheagesobtainedintheexposuresbetween southwest, which are also observed in the subsurface data Barilocheand Paso de Indios(see Fig. 5), and are alongthesame (CortiñasandArbe,1982).TheEarly-MiddleCarboniferoustoEarly structural trend as the northern sector of this belt. Some wells Permiandepositionageisbasedonthebrachiopodfauna(Andreis north of Comodoro Rivadavia, to the east of this granitoid belt, etal.,1987).Therocksaremildlydeformedandingeneralpostdate reachedthemetamorphicbasementcomposedofamphibolitesof an early Paleozoic deformation. These deposits are intruded by unknownage(Lestaetal.,1980). tholeiiticgabbroswithK–Aragesof211–243Ma,whichareinter- Further south, the basement is again exposed in the Deseado pretedasminimumagesduetotheverylow-Kcontentsofthegab- Massif,wherescatteredexposuresofgranitoidsandmetamorphic bros (Page, 1984; Poma, 1986). These authors interpreted the rockswiththesameNWtrendoccur(GiacosaandMárquez,2002). magmatic suites as to have been emplaced in an extensional AlthoughavailableU–Pbzirconages ofthe granitoidsare mainly setting.Dominantvergenceistowardthesouthwest.Thetectonic early Paleozoic, ranging in age between 472 and 454Ma (Loske setting of these sequences was interpreted as forearc (Forsythe, et al., 1999), some new SHRIMP data on these zircons yielded 1982;UlianaandBiddle,1987);marginalbasinonattenuatedcrust DevonianandMiddleCarboniferousages(Pankhurstetal.,2003). (Ramos, 1983; Page, 1984); forearc to foreland (López Gamundi Based on the continuity of the magmatic belt with dominant and Breitkreuz, 1997), and collisional foreland (Pankhurst et al., northwest structures, and the range of U–Pb ages, it is assumed 2006). that the igneous and metamorphic western belt is connected 240 V.A. Ramos/JournalofSouthAmericanEarthSciences26(2008)235–251 Fig.5. ExposuresofmagmaticrocksofthenorthernandwesternbeltsoftheSomúnCuraMassifbasedonCortiñas(1996),andextensionofthewesternbeltintheDeseado Massif.U–PbagesinthenorthernbeltaremainlybasedonBaseietal.(1999,2005)andVarelaetal.(1998a,b,1999,2005,2007).TheU–Pbagesinthewesternbeltaremainly fromVarelaetal.(2005)andPankhurstetal.(2003,2006).ThesubsurfaceK–AragesfromdrillingcoresarefromLestaetal.(1980),LinaresandGonzález(1990),andlocation ofthewellsfromSylwan(2001). AkeybasintounderstandthegeologicsettingforthePermian Triassic sag sequence and reactivated extension during the Early intheDeseadoMassifistheLaGolondrinaBasin,bestexposedin Jurassic. the proximity of Dos Hermanos, Bajo La Leona and La Dulce (see for location Fig. 5). Continental conglomerates, sandstones, and siltstonesupto2500mthick,weredepositedinhalfgrabensys- 4.ThepassivemarginofGondwana tems,withabundantflorathatindicatesanEarlytoLatePermian age(BellosiandJalfin,1989).Thisbasinhasbeeninterpretedasa Astableplatformalsequencecorrespondingtotheoldpassive riftbyRamosandPalma(1996),aninterpretationconsistentwith margin of Gondwana is preserved in the Sierras de la Ventana the seismic expression depicted by Homovc and Constantini (Fig. 7). Mostof the present reconstructions of Gondwana accept (2001). These authors illustrate an important Permian rift, a thatitssouthwestmarginconsistedofacontinuousclasticpassive V.A. Ramos/JournalofSouthAmericanEarthSciences26(2008)235–251 241 marginthatextendedfromSierradelaVentanatotheCapeSystem event associated with the unconformity between the Devonian (Milani,2007,andreferencesherein),andcontinuesfurthernorth orthoquartzites and the Late Carboniferous sequences (Massabie alongthePacificsidethroughnorthernArgentina,Bolivia,andPeru andRossello,1984). (Ramos,2008). The Ventania fold-and-thrust belt of Sierra de la Ventana is The continental margin of Gondwana facing Patagonia is par- characterized by isoclinal folds associated with a high strain in tially located beneath the present Colorado Basin. A gravimetric the orthoquartzites, with vergence typically to the northeast transectacrossthebasinshowsanasymmetrybetweenthethick- (Dimieri et al., 2005, and references therein). The southwestern nesses of the two crusts, northern Gondwana being thicker than part of the belt, where the basement is exposed, has evidence of Patagonia(Ramos,1996).Thisdifferenceisinterpretedasevidence thrusts(seeFig.7)associatedwithlowgrademetamorphism,da- ofajuxtapositionofanoldPrecambriancontinentalmarginbear- tedbyK–Arbetween282and260Ma(Varelaetal.,1986;Buggi- ing the Río de la Plata craton, with Transamazonian ages of ca. sch, 1987) constraining the deformation between Lower and 2.0Ga (Tohver et al., 2007), against a younger Patagonian litho- MiddlePermian.Althoughthereisevidenceofstrike-slipdisplace- sphere according to Stern et al. (1990). A series of stages can be ments,themaindeformationinthislatePaleozoicfold-and-thrust recognizedintheGondwanamarginintheSierrasdelaVentana, beltischaracterizedbySW–NEshorteningandtransport(Tomezz- alsoknownastheVentaniaSystem. oli and Cristallini, 1998; Dimieri et al., 2005). As a result of the AnearlystageofriftingaffectingtheProterozoicbasementwas thruststacking,theClaromecóforelandbasin(Fig.8)wasformed postulatedbyRapelaetal.(2003),basedongeochemicalcharacter- byflexuralloadingoftheGondwanamarginwithaforedeepmore isticsandtheageofsome531–524Magranitesandrhyolitesinter- than 10km thick (Ramos, 1984; López Gamundi and Rossello, pretedasaCambrianriftandcorrelatedwithsimilarrocksinthe 1992).ThechangesinthesedimentationintheLateCarboniferous conjugatemarginofSouthAfrica.Depocentersboundedbynorth- predate the low-grade dynamic metamorphism associated with west-trending normal faults have been observed in the seismic importantshorteninginthefoldandthrustbeltofthesouthwest- linesoftheClaromecóBasin,perpendiculartothemarginandcor- ernsectorofVentaniaasdenotedbythestudiesofVonGosenand relatedwiththisrifting(RamosandKostadinoff,2005). Buggisch(1989). Sequencesofplatformalorthoquartzitesuptoseveralthousand Thesefacts,togetherwiththesyntectonicsedimentationinthe meters thick of the Curamalal and Ventana Groups were uncon- uppermost part of the Lower Permian sequence (López Gamundi formablydepositedonmetamorphicbasement.Paleocurrentanal- et al., 1995), indicate a major episode of deformation along the yses of these mature sequences of orthoquartzites indicate a Gondwana margin at these latitudes. There is no doubt that the provenance from the northeast. The biostratigraphic control is main episode is Early Permian in age, but the unconformity be- scarce but is bracketed between Middle-Late Cambrian and tweentheDevonianandLateCarboniferousdepositsindicatesthat Devonian times, based on a well dated basement (Rapela et al., upliftmayhavestartedpriortotheLateCarboniferous.Deforma- 2003),andanoverlyingunconformity(Andreisetal.,1989). tionlastedatleastuntilMiddlePermian,asthecompletesequence Amolassesequenceexposedeastofthethrustfront(seeloca- ofEarlyPermiandepositsisfolded. tioninFig.8),andcomposedofarkosesandwackesofthePillahu- The evolutionof the Gondwanapassive margin and Patagonia incóGroup,unconformablyoverlyingtheDevonianquartzitesand should be tied to the southern Africa counterpart, the Cape fold associatedwithglacialdepositsinthelowersectionhasaLateCar- and thrust belt and the Karoo foreland basin. Since the work of boniferoustoEarlyPermianage.TheageisbasedontheEurydesma Du Toit (1927) there is a growing consensus that the Ventania Faunaofthemarinedeposits(Harrington,1955)andU–Pbageof fold-and-thrust belt is the continuation of the Cape fold belt and 274±10Ma from a tuff layer in the upper part of the sequence thattheClaromecóforelandbasinisthewesternendoftheKaroo (Tohver et al., 2007). These immature sandstones with volcanic Basin,bothofthemwithathicknessexceeding10km(Milaniand clastshaveasouthwesternprovenance.Thechangesbetweenthe De Wit, 2008). Crustal thicknesses have a similar pattern: below stable clastic platform and these immature deposits indicate an the Karoo Basin the crust is about 38km thick, increasing to importantmodificationin thetransportdirectionfrom NE toSW 43km below the Cape fold belt, and abruptly decreasing to the in the base, toSW tothe NE in the uppersection;an increaseof south to 30km at the southern coast of South Africa, and from instability in the basin, and the existence of a positive relief to theretolessthan20kmthickacross(cid:2)250kmofcontinentalshelf the south (Andreis and Cladera, 1992; López Gamundi and uptotheAgulhasFractureZone(DeWitetal.,2007).Asimilarpat- Rossello,1992).Thischangeindicatestheexistenceofafirstuplift ternisobservedintheClaromecóBasin,withthethickestpartbe- lowtheVentaniabelt,andwithanewdecreasebelowtheColorado Basin(Introcaso,2003). The basal sequence is represented by the Kango Group, the lowermost rift sequence of the Cape Supergroup, which on the basis of U–Pb data on detrital zircons is, in part, Early Cambrian in age (Armstrong et al., 1998). An angular unconformity sepa- rates these deposits from underlying Neoproterozoic metasedi- ments intruded by Cambrian A-type granites as young as 520– 540Ma. U–Pb dating and geochemistry of these granites and associated rhyolites below the Cape unconformity, which sepa- rates them from the overlying siliciclastics of the Cambrian–Or- dovicianTableMountainGroup,showthattheyareequivalentin age and composition to the granites and rhyolites of Sierra de la Ventana (Rapela et al., 2003). The siliciclastic Cape Supergroup ranges in age from mid Cambrian (ca. 500Ma) to Upper Devo- nian (ca. 360Ma) and comprises a number of well-defined mar- inetransgression–regressionsequencesthatmatchtheCuramalal and Ventana Groups. The Pillahuincó Group correlates with the Karoo Supergroup. Both start with an extensive sequence of gla- Fig.6. Relativeprobabilityplotforthebest-estimatedagesderivedfromtheDos Hermanosphyllites(basedonPankhurstetal.,2003). cial sediments, but the southernAfrica counterpart has up to se- 242 V.A. Ramos/JournalofSouthAmericanEarthSciences26(2008)235–251 Fig.7. BasementexposuresofSierradelaVentanaanditsearlyPaleozoicsequencesthrustwithadominantnortheastvergence(basedonCingolaniandVarela,1973;Rapela andKostadinoff,2005).SeelocationinFig.8. Fig.8. ThedifferentunitsoftheVentaniafoldandthrustbeltassociatedwiththeClaromecóforedeep,formedbycrustalloadinginEarlytoMiddlePermian.Notethelocation ofthethrustdeformationfrontandtheaxisofthe>10kmforedeep(basedonRamosandKostadinoff,2005). V.A. Ramos/JournalofSouthAmericanEarthSciences26(2008)235–251 243 ven major ice advance-retreat episodes representing the Carbon- Thegravimetricandmagnetometricsurveysconductedon-land iferous–Early Permian Dwyka glaciations (De Wit et al., 2007). byKostadinoffetal.(2005)betweenthecityofNeuquénandthe Rhyolitic–andesitic volcanic tuffs present in the Dwyka Group coastline recognized a strong contrast of behavior between the have U–Pb dates on zircons from 297±1.8Ma (Bangert et al., GondwanamarginandthePatagonianplatformdefiningadiscon- 1999), whereas zircons in the overlying tuffs have U–Pb ages tinuityjustwestoftheColoradooff-shorefeature.Thisdiscontinu- of 288±3 and 289±3.8Ma. These tuffs are older than the ity coincides with the Huincul fault, a regional transcontinental 274±10Ma tuff found above the glacial deposits in the Pillahu- strike-slip fault defined in this segment by Ploszkiewicz et al. incó Group. Late Early Permian–through Middle Permian (280– (1984),andseparatesbasementwithdifferentcharacteristics. 260Ma) tuffs are also detected in the Paraná Basin of Brazil, Theairbornemagnetometricsurveyscombinedwiththegravi- Paraguay, and Uruguay (López Gamundi, 2006). New SHRIMP metric data led Chernicoff and Zapettini (2004) to recognize a data on the Paraná Basin of Brazil constrain the age of the main sharptruncationofthemagneticfaciesintheGondwanamargin. tufflayerto278.4±2.2MaincloseagreementwiththeVentania The ophiolitic belts and the north–south fabrics among Chilenia tuffs (Santos et al., 2006). and Cuyania, as well as between Cuyania and Pampia, are trun- AscanbeseeninFig.9a,itisevidentthatinthepre-breakup cated along the Huincul fault, confirming the observations of paleogeography,thereisnotasimplelinearcontinuationbetween Kostadinoffetal.(2005)furthertotheeast.Thedifferentcharac- theVentaniafoldandthrustbeltandtheCapefoldbelt.Thenorth- teristics between the Gondwana, more specifically the Río de La ernigneous–metamorphicbeltdescribedinFig.5hasnoobvious Plata craton, and Patagonia led Dalla Salda and Francese (1989) counterpart in on-land southern Africa, and it has been only de- to interpret this discontinuity as a suture produced during early tected in the M. Ewing bank (see Fig. 2c) by the ODP dredging Proterozoictimes. anddrilling(seeRamos,1986). The Neuquén Basin is segmented in two parts by the Huincul In order to precisely locate the different features betweenthe high, a series of half grabens inverted during the Andean orog- Agulhas and the Malvinas plateaux, the southern plateau should eny as a strike-slip fault zone (see for structural details Silvestro becontractedatleast20%,inwhatistheassumedW–Estretching and Zubiri, 2008). The half graben system was controlled by the betweentheM.EwingBankandtheSouthAmericancoastbased suture of Patagonia with Gondwana according to Franzese and on the continental crustal attenuation (Sandwell and Smith, Spalletti (2001). The basement beneath the sedimentary cover 1997). Even so, there is a north–south truncation and displace- has been screened using the available 3D seismic data in differ- ment among the late Paleozoic features of South America and ent blocks surveyed by the industry along the Huincul fault by South Africa, larger than 600km. The structural trend of the Mosquera (2008). These data show more clearly than the other Argentine continental platform changes from N70(cid:3)W to almost potential geophysical methods (Chernicoff and Zapettini, 2004) north–south in the Valdés and Rawson basins (Fig. 9a), adopting that the N–S trend of the basement structural grain is again a similar WNW trend in the San Jorge Basin. The region of truncated by an east–west trending fabrics (Mosquera and Ra- north–southtrendfitswiththeamountofdisplacementofthelate mos, 2006). The Huincul fault continues to the west in a series Paleozoicfeatures. of conspicuous east–west lineaments first described in Chile by Chotin and Giret (1979). Basedonthesedata,itispossibletoproposethatalthoughthere 5.Discussion is no direct evidence of an ophiolitic belt separating Gondwana fromthePatagoniaplatform,thepresenceofacrustaldiscontinu- Inordertoexplainthedifferentgeologicalfeaturespreviously ityexpressedbyatranscontinentalfaultzone,afirstorderstruc- described and summarized in Fig. 5, it is necessary to integrate tural feature related to the Huincul fault zone, the truncation of thefactswithsomeothergeologicalevidences.Fig.10integrates thebasementfabricsandtheoffshoremagneticanomaly,together thewesternandnorthernigneous–metamorphicbeltswithsome withthegeologicalevidence,providearobustindicationofapo- otherstructuralandgeophysicalevidence,tothenorthandsouth tentialsuturebetweenthetwocontinentalblocks.AlthoughCho- of the previously described study area. The different problems tin and Giret (1979) identified those transversal lineaments and uncertainties will be discussed from north to south to cover between Temuco and Valdivia in Chile, north of Temuco there is the diverse kind of evidence and topics that are pertinent to the awell-knowntruncationofthelatePaleozoicarcfrontthatjumps proposedevolutionofPatagonia. fromthePacificcoasttothewaterdivideoftheAndes(Fig.10). Independent evidence for the suture is the location of the 5.1.Theproposedsuture Cañadón Asfalto rift basin. This basin is developed in the hang- ing-wallofthepotentialsutureofthewesternmetamorphicbelt The ophiolitic belts among different early Paleozoic terranes that coincides with the Chubut high of Cortiñas (1996). The withnorth–southtrendsthatseparateChilenia,Cuyania,andPam- SomuncuraBasinofthisauthorisalsolocatedinthehanging-wall piafromtheRíodelaPlatacraton(Ramos,1988)aretruncatedby ofthenorthernbeltsuture. an east–west structural fabric in the basement. This fact can be seenindiversedatasets.Forexample,basementfabricsinthedif- 5.2.Ageofductiledeformationinthenorthernbelt ferent off-shore basins constrained by seismic reflection profiles and aeromagnetic surveys shows WNW Paleoproterozoic trends, Ductile thrusting affecting the supracrustal rocks of Yaminué andcontrolthenucleationandorientationofhalf-grabensystems wasaccompaniedbygreenschisttoloweramphibolitefaciesmeta- developedintheriftbasins(Ramos,1996).SouthoftheColorado morphism during peak deformation, forming extensive mylonite Basin, a N–S trend dominates the Valdés and Rawson basins and ultramylonite ductile shear zones, including deformation of (Fig. 9a). Just along this boundary Ghidella et al. (1995) define a (cid:2)300Ma old orthogneisses (Llambías et al., 2002; Von Gosen, magnetic lineament that crosses the entire platform from the 2003;Baseietal.,2005).Similartrendsandductiledeformationoc- mouthoftheColoradoriverwithaNWtrend.ThisColoradomag- curinamphibolitegrademetamorphicrocksoflatePaleozoicage neticdiscontinuityhasbeeninterpretedbyMaxetal.(1999)asthe inCerroLosViejos(Fig.10)describedontheoppositeGondwana southernboundaryofthePrecambrianRíodelaLaPlatacraton,a marginbyTickyjetal.(1997).AlongthismarginintheSierrade potential suture with the continental crust of the Patagonia laVentanaareathereisclearevidenceoflowgrademetamorphism platform. as well as penetrative deformation between 282 and 260Ma 244 V.A. Ramos/JournalofSouthAmericanEarthSciences26(2008)235–251 Fig.9. (a)StructureofthecontinentalplatformsofsouthernSouthAmericaandsouthernAfricabasedontheearlyfitofMartinetal.(1981)andcomplementedwithnew structuralfeaturesbyRamos(1996);(b)DetailofthelatePaleozoicprovincesintheSouthAmericasidesshowingthatthecontactbetweenthenorthernigneousand metamorphicbeltofSomúnCuraandtheClaromecóforedeepiscoveredbytheLateJurassic–CretaceousaulacogenicColoradobasin(DeWit,1977). affecting the basement, as well as independent evidence derived (1988) or Pankhurst et al. (1993) were found to be geologically from Early Permian syntectonic sedimentation of the Pillahuincó meaningless. The first regional U–Pb dates performed by Varela GroupofEarlyPermianagedisturbinga274±10Maoldtufflayer etal.(2005)inthenorthernPatagonianCordilleraandinthewes- (Tohveretal.,2007). ternsectoroftheSomúnCuraMassifsuggestthatthelatePaleo- All together these data constrain the peak of deformation be- zoic granitoids extend to the east at least up to the Yaminué tween Early and Middle Permian, as previously established by area.TheonlypreciseU–PbSHRIMPageobtainedintheoldphases manyauthors(Ramos,1984;Andreisetal.,1989;LópezGamundi is 273±2Ma (Pankhurst et al., 2006) should be interpreted as a etal.,1995;Rapalinietal.,2008). syn-tolate-tectonicphase,duetotheageofthedeformationre- corded at that time in the Sierra de la Ventana region. These 5.3.AmagmaticarcalongthenorthernsectoroftheSomúnCura authorsrecognizeda320Mainheritedzirconsinrocksofthisarea, Massif but no systematic study was performed. More precise ages are needed in this area as well as in the Yaminué region, to confirm A collision model is supported by magmatic evidence for the theU–Pbagesfrom295to307Mafortheorthogneissesand de- postulated subduction and collision phase. Petrological studies formed rocks described by Varela et al. (1998b) and Von Gosen performedinthewesternandcentralsectorsofthenorthernbelt (2003). recognizedmetaluminousgranitoidsthatevolvedtoperaluminous granites(Llambíasetal.,1984;LlambíasandRapela,1985;Rapela 5.4.Thewesternbeltanditssouthernextension and Llambías, 1985). Geochemical studies recognized an initial tonalitic phase followed by widespread granodioritic facies that ThislatePaleozoicmagmaticbeltwasfirstrecognizedongeo- evolved from typical calcalkaline subduction related metalumi- chronological grounds by the pioneer work of Halpern (1968), nous granitoids to syncollisional peraluminousgranites, followed and strongly suggested the existence of continental drift along by postcollisional peralkaline granites and rhyolites (Llambías theSamfrauorogenicbeltfromSouthAmericatoAustralia.Further etal.,1984;RapelaandCaminos,1987).Theseauthorsemphasized studies(e.g.Forsythe,1982;Ramos,1983)interpretedthisbeltas that these petrological characteristics can be traced from the La to have formed along the Pacific side. Some authors recognized EsperanzaareainthewesternsectortotheYaminuécentralsector, two magmatic cycles by U–Pb dating, one in the Devonian (ca. andevenfurthereast. 390Ma)andayoungerinthelatePaleozoic(ca.280Ma)withcool- Themainuncertaintyoftheserocksisthepreciseageoftheold- ingK–Aragesof260–250Ma(Varelaetal.,2005).Amorecompre- er phases with typical subduction related character. For many hensive study based also on U–Pb dating, mainly with SHRIMP years these complex systems were dated by Rb–Sr, commonly ages, constrain the magmatic arc from the Devonian to mid Car- resultinginconflictsbetweenrelativeagesandfieldrelationships. boniferoustime(ca.320Ma)basedontheageofsomeperalumi- EvenwelldefinedRb–SrisochronesaspresentedbyCaminosetal. nous granites from Paso del Sapo and Sierra de Pichiñanes