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Timing of Eocene–Miocene thrust activity in the
Western Axial Zone and Chaînons Béarnais
(west-central Pyrenees) revealed by multi-method
thermochronology
Gemma de Vicente I Bosch, Antonio Teixell, Marc Jolivet, Pierre Labaume,
Daniel Stockli, Mireia Domenech, Patrick Monie
To cite this version:
Gemma de Vicente I Bosch, Antonio Teixell, Marc Jolivet, Pierre Labaume, Daniel Stockli, et al..
Timing of Eocene–Miocene thrust activity in the Western Axial Zone and Chaînons Béarnais
(west-central Pyrenees) revealed by multi-method thermochronology. Comptes Rendus Géoscience,
Else-vier Masson, 2016, From rifting to mountain building: the Pyrenean Belt, 348 (3-4), pp.246-256.
�10.1016/j.crte.2016.01.001�. �insu-01301995�
Tectonics,
Tectonophysics
Timing
of
Eocene–Miocene
thrust
activity
in
the
Western
Axial
Zone
and
Chaıˆnons
Be´arnais
(west-central
Pyrenees)
revealed
by
multi-method
thermochronology
Gemma
V.
Bosch
a,b,*
,
Antonio
Teixell
c,
Marc
Jolivet
a,
Pierre
Labaume
d,
Daniel
Stockli
e,
Mireia
Dome`nech
c,
Patrick
Monie´
da
Ge´osciencesRennes,Universite´ Rennes-1,CNRS,35042Rennes,France
b
BRGM,45100Orle´ans,France
c
DepartmentdeGeologia,UniversitatAuto`nomadeBarcelona,08193Bellaterra,Spain
dGe´osciencesMontpellier,Universite´ deMontpellier,CNRS,34095Montpellier,France eJacksonSchoolofGeosciences,UniversityofTexasatAustin,Austin,TX78759,UnitedStates
1. Introduction
Unraveling the timing and dynamics of mountain building is a long-standing goal in collisional orogen
studies.Thisgoalistraditionallyaddressedby tectonics-sedimentationanalysisofsynorogenicdepositsand,more recently, by low-temperature thermochronology on the assumption that dated exhumation paths reflect the vertical component of the evolution of thrust belts. In thePyrenees,theforelandbasinrecordiswellknown,and thermochronology studies have focused in the past decades on the Paleozoic massifs of the Axial Zone (Fitzgerald et al., 1999; Gibson et al., 2007; Gunnell
ARTICLE INFO
Articlehistory:
Received23November2015
Acceptedafterrevision1stJanuary2016 Availableonline30March2016 HandledbyYvesLagabrielle Keywords:
Thermochronology Apatitefissiontracks Apatiteandzircon(U–Th)/He Thrusting
Pyrenees
ABSTRACT
Wepresentnewapatite(U–Th)/He(AHe),apatitefissiontrack(AFT)andzircon(U–Th)/He (ZHe)datatounravelthetimingofexhumationandthrustinginthewesternAxialZoneof thePyreneesandtheadjacentNorthPyreneanZone(ChaıˆnonsBe´arnais).Inthenorth,ZHe datayieldcoolingsignalsbetween26and50MaintheChaıˆnonsBe´arnais,whichare consistentwiththeonset ofthrust-relatedcoolingintheneighboring Maule´onBasin modeledbypreviousauthors.Non-resetTriassicagesarefoundinthefootwallofthe NorthPyreneanFrontalthrust(AquitaineBasin).Tothesouth,similarZHeagesinboththe hangingwallandfootwalloftheLakorathrustrecordLateEocenetoOligocenecoolingthat weattributetotheactivityoftheGavarniethrust.Thermalmodelingofsamplesfromthe LakorathrusthangingwallindicatescoolingfromEarlyEocenetimes,recordingactivityof theLakorathrust.PaleozoicdetritalsamplesfromthewesternmostAxialZoneandfrom theEaux-Chaudes andBalaitous–PanticosagraniticplutonsyieldAFTsignalsbetween 20and30MaandZHebetween20and25Ma.Modellingindicatesfastcoolingduringthis time,whichweattributetothemotionoftheGuargathrust.AHedatafromtheseAxial Zoneplutons,combinedwithmodelling,showapost-tectonicsignal(8–9Ma),which indicatesrenewederosionafteraperiodwithoutmajorcoolingandexhumationbetween 20to10Ma.
ß2016Acade´miedessciences.PublishedbyElsevierMassonSAS.Thisisanopenaccess articleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/ 4.0/).
* Correspondingauthorat:UMR6118Ge´osciencesRennes, B.15-Off.127,CampusdeBeaulieu,35042Rennescedex,France.
E-mailaddress:gemmav.bosch@gmail.com(G.V.Bosch).
ContentslistsavailableatScienceDirect
Comptes
Rendus
Geoscience
ww w . sci e nc e di r e ct . com
http://dx.doi.org/10.1016/j.crte.2016.01.001
1631-0713/ß2016Acade´miedessciences.PublishedbyElsevierMassonSAS.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http:// creativecommons.org/licenses/by-nc-nd/4.0/).
etal.,2009;Jolivetetal.,2007;Maureletal.,2008;Metcalf et al., 2009; Morris et al., 1998; Sinclair et al., 2005) (Fig. 1A). More recent thermochronology studies have included the Cenozoic sedimentary rocks of the South PyreneanforelandbasinandthePaleozoicandMesozoic rocksoftheNorthPyreneanZone,tobetterconstrainthe relationshipsbetween theexhumationintheAxialZone and the exhumation and burial in the adjacent basins (Beamudetal.,2011;FillonandvanderBeek,2012;Fillon et al., 2013; Meresse, 2010; Mouthereau et al., 2014; Rushlowet al.,2013;Vacheratet al.,2014; Whitchurch etal.,2011).
Whilemostofthethermochronologystudiesfocuson theeastern and east-centralPyrenees, oftenaroundthe ECORS-Pyrenees profile, the western Axial Zone and adjoiningareashavebeenlessinvestigated.Tounderstand the relationships between the exhumation in the Axial Zoneandthedynamicsofthefoldandthrustbeltinthe forelands,thewesternAxialZonegiveskeyinformation; thispartofthebasementinteractstothesouthwiththe
Tertiary Jaca basin, which contains the most complete forelandbasinsequencerecordingthestructural develop-ment (e.g., Barnolas and Teixell, 1994; Ca´mara and Klimowitz, 1985; Labaume et al., 1985; Milla´n et al., 2000;Teixell,1996;TeixellandGarcı´a-Sansegundo,1995). This work presents the first multi-method thermo-chronology database of the western Axial Zone of the Pyrenees(Fig.1A),includingapatitefissiontrack(AFT),(U– Th)/Heinzircon(ZHe)andapatite(AHe)data,withtheaim to investigate the Pyrenean Cenozoic evolution. A few samplesoftheadjacentNorthPyreneanZone(Chaıˆnons Be´arnais area) arealso included in thestudy. The area investigatedisparticularlyinterestingbecauseit compri-sesthewestern terminationoftheAxialZonePaleozoic units where it plunges under Upper Cretaceous and Paleogenerocks. Italsoconstitutes theonlyarea where can be observed the Cretaceous North Pyrenean basin overthrustingtheSouthernPyrenees(Teixell,1990,1998). Thethermochronologydataobtainedarecomparedwith thetectonostratigraphicrecordoftheforelandbasinsand
Fig.1.A.GeologicalsketchofthePyreneeswithpreviousthermochronologyresultsobtainedintheAxialZone(Fitzgeraldetal.,1999;Gibsonetal.,2007; Gunnelletal.,2009;Jolivetetal.,2007;Maureletal.,2008;Metcalfetal.,2009;Morrisetal.,1998;Sinclairetal.,2005).MB:Mauleonbasin;LT:Lakora Thrust;ChB:ChaıˆnonsBe´arnais;GT:GavarnieThrust.B.Simplifiedcross-sectionacrossthewest-centralPyreneesshowingthemaintectonicunits discussedinthisworkandprojectedstructurallocationsofthestudiedsamples(largestars:severalsamples;smallstars:singlesamples;Anso´-Arzaq transectofTeixell,1998).
integratedinthetectonicframeworkofthewest-central Pyrenees,providingamorecompletepictureofthehistory of thrust uplift and exhumation (both syntectonic and post-tectonic)ofthissegmentofthechain.
2. Geologicalsetting
The Pyrenees formed from Late Cretaceous to Early Miocenetimesdue toconvergencebetween theIberian and European continental margins (Choukroune et al., 1990;Mun˜ oz,1992).Asaresultofthecollision,the west-centralPyreneesroseasadoubly-vergingorogenicprism built by basement and cover-involved thrusts. This collisionbeltisunderlainbynorth-directedlowercrustal subduction(Lagabrielleetal.,2010;Teixell,1998;Teixell et al., 2016). The main upper crustal structures of this segment of thechain are shown on Fig. 1B. The North PyreneanZonewasarapidly-subsidingCretaceousbasin between the European and Iberian margins, floored by hyper-thinned continental crust and mantle exhumed during Albian–Cenomaniantimes (Jammes et al., 2009; Lagabrielle and Bodinier, 2008). This basin is now completely inverted, overthrusting to the north the AquitaineBasinalongtheNorthPyreneanFrontalthrust, andtothesouthalongtheLakorathrust(Fig.1).
The North Pyrenean Zone in the studied Chaıˆnons Be´arnais area contains thick and relatively complete Jurassic and Cretaceous successions and is internally deformedintoasystemoffoldsandthrustsdetachedin theTriassic Keuperfacies (Lagabrielle et al., 2010). The Lakorathrustcropsoutasagently-dippingfault,largely paralleltobeddingintheUpperCretaceouscoverofthe AxialZone(thefootwall),andcarryingathinthrustsheet ofPaleozoic,TriassicandMiddle–UpperCretaceousrocks initshangingwall(LakoraklippeandIguntze–Mendibelza massifs,Fig.1;Teixell,1990,1996).Eastward,theLakora thrustpasseslaterallytovariousthrustsalsocarryingthin Paleozoicbasementsliceslocatedatthesouthernedgeof theChaıˆnonsBe´arnais(e.g.,Eaux-ChaudesandCinq-Monts thrusts;Ternetetal.,2004).TheLakorathrustandthese easternextensionsderivefromtheinversionof extension-al structures in the upper Iberian continental margin (Teixelletal.,2016).
The southern part of the west-central Pyrenees is characterized bysouth-verging thrustsand includes (1) theAxialZone,abasementantiformalculminationcaused bytheGavarniethrust,and(2)thePaleogeneJacabasin,a large-scaleasymmetricsynformbetweentheAxialZone andtheSouthPyreneanFrontalthrust(Fig.1).
Paleozoicrocksof theAxialZone areunconformably coveredbyUpperCretaceousshelfcarbonates.Abranchof the Lakora thrust, the Larra thrust, propagated across UpperCretaceous–Eocene rocks oftheAxial Zonecover andthenorthernJaca basin.Eastof thestudyarea, the AxialZonecomprisesstackedbasementthrustsheetsthat caused a greater structural relief and a large basement exposure(e.g.,Mun˜ oz,1992;Roureetal.,1989).There,the northernboundary of the Axial Zone is marked by the North Pyrenean Fault, a steeply-dipping structure with complex kinematics that passes westward to a south-vergingde´collementatthesouthernedgeoftheChaıˆnons
Be´arnais.Thewestwardplunge oftheAxialZoneinthe studyareaprovidesconstraintonthestructuralreliefand shapeofthetopoftheAxialZoneandontherelationships betweenthemainstructuralunits.Non-exposedbasement thrustsunderlietheJacabasinandcausemajorvariations ofstructuralrelief(e.g.,theGuargathrust,Fig.1B;Ca´mara andKlimowitz,1985;Labaumeetal.,1985;Teixell,1996; TeixellandGarcı´a-Sansegundo,1995).
Pre-orogenicMesozoicsuccessionsintheJacabasinare relativelythinandincomplete;incontrast,thePaleogene infillisverythick(upto9km)andconformstoatypical flysch-to-molasse forelandbasinsequence (Mutti et al., 1988; Puigdefa`bregas, 1975). Tectonics–sedimentation relationshipsindicateapiggy-backsequenceofthrusting fromtheLakoratotheGuargathruststhatspanstheentire Pyreneanorogeny.TheLakorathrustprobablyinitiatedin theLateSantonian,asindicatedbyflexureinitsfootwall sediments,anditsmainactivitycontinueduntiltheMiddle Eocene(Bartonian).Thisincludesthefootwallsplaysofthe Larra thrust and the laterally equivalent Eaux-Chaudes thrust(Teixell,1996).TheGavarniethrustwasactivefrom theLateEocenetotheEarlyOligocene,whereastheGuarga thrusttookupfinalcompressivedeformationfromtheLate OligocenetotheEarliestMiocene(Teixell,1996).
Thechronology oftheNorthPyrenean thrustsis less known. TheinternalstructuresoftheChaıˆnonsBe´arnais wereinitiatedduringtheLateJurassic–EarlyCretaceousas diapiricsaltwallsinextensionalcontext(Cane´rot,1985; Teixellet al.,2016),but theirevolution inthePyrenean orogenyislessconstrainedintime. TheNorthPyrenean Frontal thrust appears as a long-lived structure partly contemporaneoustotheLakorathrustandextendinguntil morerecenttimes.Indeed,inthestudyarea,growthstrata in its footwall syncline indicate thrusting beginning in Campanian–Maastrichtian times and continuing during thePaleogene(Poitevinetal.,2014),and thermochrono-logically-constrained coolingintheMaule´onsegmentof theNorthPyreneanZone,tothewestofthestudyarea, begunsome50Maago(Vacheratetal.,2014).Inspiteof thisearlythrustingactivity,themolassedepositioninthe Aquitainebasinderived fromthePyreneanreliefs spans fromtheLate–MiddleEocenetotheMiocene(Biteauetal., 2006).
Theextenttowhichthedescribedsequenceofthrustsis reflected in the exhumation history is not known yet. NortheastoftheJacabasin,AFTinthegranitesoftheAxial Zone, yield Cenozoic cooling ages (e.g., Ne´ouvielle and Bielsamassifs;Jolivetetal.,2007),butthedegreeof post-Variscan reset and the amount of exhumation of the westernmost Axial Zone and the Lakora thrust are unknown.Inspiteoftherichtectonostratigraphicrecord oftheJacabasin,discrepanciesremainforthetimingof some major structures. Mun˜ oz et al. (2013) recently attributedtheemplacementoftheGavarniethrustsheetto theMiddleEocene,onthebasisofacorrelationbetween thebasementthrustandthegrowingandrotatingcover structuresintheAı´nsabasin.Thepreviousattributionof theGavarniethrusttomorerecenttimeswasbasedonthe refoldingitproducedintheoverlyingLarra–MontePerdido thrust,whichwaslinkedtotheBoltan˜ aanticline,inturn datedasLateLutetiantoBartonian(Teixell,1996).
ALateEocenetoOligoceneagefortheGavarniethrust hasalsobeenfavoredbyJolivetetal.(2007)onthebasisof AFTagesofca.35Maofthethrusthangingwallathigh elevationintheNe´ouviellegranite.Ontheotherhand,AFT agesaround20MadominatethesouthernmostAxialZone in thecentral Pyrenees,in thefootwall of theGavarnie thrust(Fitzgeraldetal.,1999;Jolivetetal.,2007;Sinclair et al., 2005). In post-tectonic times, an acceleration of exhumationratesat9MawasdetectedbyAHemodeling intheeasternpartoftheSouthPyreneanforelandbasin (FillonandvanderBeek,2012).Todate,thiseventhasnot beenreportedintheAxialZoneexceptfora10.91.0Ma sampleobtainedbyJolivetetal.(2007)intheBielsamassif. 3. Samplingandmethods
Atotalof18sampleswerecollectedforZHe,AFTand AHestudiesandfivemoresamplesfromMeresse(2010) wereusedtocompletethedataset(seelocationonFig.2). FivesamplesweretakeninthePaleozoicgranitesofthe Balaitous–PanticosaandEaux-Chaudesplutons,andthree inPaleozoicdetritalrockstothewesttounravelthetiming of exhumation of the western Axial Zone. These were complemented with the five previous AFT resultsfrom Meresse(2010)andnewHedatamadeonthesesamples. TogaininsightintotheactivityoftheLakorathrust,two samplesweretakeninUpperCretaceousandLowerEocene turbiditicsedimentsfromthefootwall(AxialZonecover) andthreeinAlbianconglomeratesfromthehangingwall. To complete thestudyto thenorth,foursampleswere
taken in the Chaıˆnons Be´arnais (NPZ) and two in the Aquitaine basin, in the footwall of the North Pyrenean FrontalThrust.
Theninesamplesthatcontainedasufficientnumberof high-qualityapatiteswereanalyzedforfissiontracksand/ or (U–Th)/He (Tables 1 and 2 in Annex 1, online supplementary data). Zircon grains could be retrieved frommostsamples,andZHeanalyseswereperformedon 18 samples (Table3 in Annex1, online supplementary data). Sample JA2 did not provide zircon suitable for analysis.Thedataobtainedallowedthermalmodelingof fourprofilesinthegraniticmassifsofBalaitous–Panticosa andEaux-ChaudesandintheLakorathrustsheet.
The AFT analyses were performed following the proceduredescribedbyJolivetetal.(2007).Themounted samplesweresenttoOregonStateUniversityfor irradia-tion.Ageswerecalculatedusinganoverallzetavalueof 3442acm2(GVB)obtainedonbothDurango(McDowell
etal., 2005) andMountDromedary(Green,1985;Tagami, 1987)apatitestandards.
Apatite(U–Th)/Hedatingmeasurementswere perfor-medatGeosciencesMontpellier,followingtheprocedure describedinRomagnyetal.(2014).Prismaticapatiteswere selected,withtwoornopyramidsandsizesrangingfrom 50 to 200
m
m. For the samples from the Balaitous– Panticosapluton,twotothreeapatitegrainsofthesame size were used for each aliquot with the exception of sampleGPY15forwhichsinglecrystalsweredated.Inthe Eaux-Chaudes pluton, only two aliquots from sample GPY09containedtwograins.Fig.2.MapofthewesternPyreneanAxialZoneandadjacentChaıˆnonsBe´arnais,showinglocationofsamplesandthermochronologicalresults;zircon(U– Th)/He(ZHe)resultsingreen,apatitefissiontrack(AFT)inredandapatite(U–Th)/He(AHe)inblue.ForHedata,weindicatetheageranges(forfurther information,seeTables1,2and3inAnnex1,onlinesupplementarydata).Thebluedashedlineindicatesthedrainagedivide.
Zircon (U–Th)/He dating was carried out at the University of Texas-Austin using laboratory procedures describedinWolfeandStockli(2010).Individualageswere calculated using standard
a
-ejection corrections (e.g., Farley et al., 1996; Farley, 2002) and reported age uncertaintiesofabout8%(2s
)arebasedonthe reproduc-ibilityofreplicateanalysisoflaboratorystandards(Farley etal.,2001;Reinersetal.,2004).Bothuncorrectedanda
-ejectioncorrectedagesarereported(Table3inAnnex1, onlinesupplementarydata).4. Results
Inwhat follows,dataare organizedaccordingtothe structuralpositionofthesamplesinthedifferenttectonic orlithologicunits(Fig.2).ForZHeages,wepresent age-elevationplotsandagesversusUeplotsinAnnex1(online supplementarydata).
4.1. TheChaıˆnonsBe´arnaisandtheAquitaineBasin The sample set of the Chaıˆnons Be´arnais and the AquitaineBasinconsistsofterrigenousrocksof Carbonif-eroustoCretaceousagethatprovidedonlyzirconcrystals suitableforanalysis.IntheAquitaineBasin,samplesASS1 andNAY2arepoorlycementedsandstonesofCampanian andMaastrichtian,respectively,andyieldadispersionof ZHeagesolderthanthedepositionalage,rangingbetween 150and 270 Ma (for error marginssee Table 3 in Annex1,onlinesupplementarydata),indicatingnoreset afterdeposition.IntheChaıˆnonsBe´arnais,ZHeagesrange between 26 and 50 Ma, which is younger than the depositional ages, and attest for exhumation in the hangingwalloftheNorthPyreneanFrontalthrust.Sample CTH1yieldstheoldestagerange(41–50Ma)inaccordance withitshigheststructuralpositioninan Albian–Cenoma-niansyncline.ThesouthernmostsampleGPY17fromthe Carboniferoussandstoneof theChaıˆnonsBe´arnais base-mentyieldstheyoungeragerange,between26and29Ma, inspiteofahigherelevation.SamplesfromthePermian– Triassicredbeds(LBT2andMCT7)yieldintermediateages between33and42Ma.
4.2. TheLakorathrustsheet
ThreesamplesfromtheAlbianMendibelza conglomer-ate(BoirieandSouquet,1982)providedzirconssuitable foranalysis,but no apatites. Thethree samplesprovide ZHe ages in the Late Eocene–Early Oligocene interval, rangingfrom27and29Maattheloweraltitude(312m, GPY03)to30and35Maathigheraltitude(1800minthe Lakoraklippe,GPY04).
4.3. TheAxialZonecover
Inthepost-VariscancoveroftheAxialZone(hanging wall of the Gavarnie thrust), a Maastrichtian turbiditic sandstone(GPY07,1579m)provides ZHeages of31–36 Ma, younger than the depositional age and strikingly similartotheageintheLakora thrustsheetjust above (Fig.2).AsampleofLowerEoceneflyschlocated5kmto
thesouth(GPY08,1810m)yieldsawidedispersionofZHe ages(37–80Ma),someofthemolderthanthedepositional age,indicativeofpartialreset.
4.4. ThewesternAxialZone:detritalPaleozoicrocks SamplesfromthewesternmostPaleozoicexposuresof the Axial Zone also belong tothe hanging wall of the GavarniethrustandcompriseCarboniferousand Perm-iansandstonesthatprovidedapatiteandzirconcrystals with large age dispersion. SamplesJA2and JA3of the southern Axial Zone near the Somport pass yield comparable AFT(Meresse, 2010)andZHe centralages thatmayindicaterapidcoolingat25–30Ma,asdosome zirconcrystalsfromsamplesGPY05andGPY06fromthe UpperArago´nSuborda´nvalley.Thelattersamplesshow however a greater dispersion, as does Carboniferous sandstonefromtheLescunareafurthernorth (GPY14), whose age dispersion ranges between 14 and 52 Ma (Fig. 2 andTable 3 in Annex 1, online supplementary data).
4.5. ThewesternAxialZone:theEaux-Chaudesand Balaitous–Panticosaplutons
These granitic bodies provided apatite and zircon crystalsthatweresuitableforAFT,AHeandZHeanalyses. MostofthesamplesshowZHeagesindependentoftheeU concentration,suggestingcompletereset(seeAnnex1).In theEaux-Chaudespluton,samplesGPY11andGPY12yield similarZHeandAFTagesthatmaybeindicativeofrapid coolingbetween20and25Ma,furtherattestedtobythe similarity in age between the samples in spite of a difference in elevation of 531m. Sample GPY09 at an intermediatealtitudeyieldsaZHeageof24–28Ma,while theAFTageissignificantlyyounger,around12Ma(Figs. 2and3,Tables1and3inAnnex1,onlinesupplementary data). In theBalaitous–Panticosa granites, ZHeages are also markedly clustered at 20–25 Ma for different elevations. However, samples BA1 and BA5 from the BalaitousmountaingiveAFTagesof28–29Ma(Meresse, 2010),slightlyolderthantheZHeages obtainedforthe same samplesinthis study (Table 3in Annex1,online supplementary data). Sample GPY15 at lower elevation yieldsanAFTageof18Ma,whereasZHeagesaremore dispersed(18–36Ma).TheAHeagesobtainedforthissetof samples range between 21 and 6 Ma, with a cluster between 6and10Ma,againyoungerthantheAFT ages (Fig.3andTable2inAnnex1,onlinesupplementarydata), butwithsimilarageifwetakeintoaccountthe2
s
errorsof bothAFTandAHedata.5. Thermalmodeling
ThermalhistorymodelingwasperformedusingQTQt software(Gallagher,2012;Gallagheretal.,2009).ForAFT modelingoftheonlysamplewithconfinedtracklengths (GPY11), we used Ketcham et al.’s (2007) multikinetic annealing model, with the Dpar parameter as kinetic
constraint.(U–Th)/Heagesweremodeledusingaspherical diffusion domain (based on the crystal’s equivalent
sphericalradius),and takingintoaccounteU-dependent radiationdamagemodulateddiffusivityforHediffusion, following the models of Flowers et al. (2009) and Guenthner et al. (2013).In cases whereAHe ageswere obtainedbymultigrainaliquots,wedidnotuseaspecific diffusion model. Models were run for the sub-vertical profiles at the Balaitous–Panticosa and Eaux-Chaudes granitesandattheLakorathrustsheet.Allmodelswere forcedtobeatsurfacetemperatureatpresenttimeandwe allowedthetemperatureoffsetbetweensamplestovary throughtimeinarangeequivalenttogeothermalgradients of15to358C/km.
For theBalaitous–Panticosa andEaux-Chaudes grani-tes, independent constraints derived fromfield geology (unconformity of the Cenomanian carbonates above Paleozoic rocks of the Axial Zone) were used as input parameters in the thermal models to force thecooling curvestopassnearthesurfaceinCenomaniantimes.The profile in the hanging wall of the Lakora thrust was constrained to pass near the surface in Albian times (stratigraphic age of the Mendibelza conglomerate), whereasamodelofthewesternmostAxialZone(Paleozoic rocks andtheir cover)wasconstrained tobeatshallow levelsfromtheCenomaniantotheEarlyEocene.
ModelsoftheEaux-Chaudesand Balaitous–Panticosa sub-vertical profiles show similar results, with a fast exhumationfrom30Mato20Ma(Fig.4).Priortothat ageinterval,thethermalhistoryisnotwellconstrainedby thedata,asreflectedbythehighdegreeofuncertaintyin the thermal path, taking into account the 95% credible intervals(Fig.4A–B).After20Ma,theEaux-Chaudesprofile showsa slowercoolingtowardsthesurface.Incontrast, theBalaitous–Panticosaprofileshowsaperiodofstability, althoughnotwellconstrainedbythedata,withalastrapid coolingeventat8–9Ma.Themeangeothermalgradient inferredfromthetemperatureoffsetbetweensamplesisof 258C/km,withnomajorvariationovertime(Fig.4C–D). ZHeandAHeagespredictedfrommodelingarecoherent withinerrormarginswiththeobservedages. AFTshow worse predicted ages; therefore,the predicted ages are
withintheerrormarginsoftheobservedAFTagesonlyin thecaseofGPY09andGPY15(Fig.4E–F).
ModelingresultsoftheLakorathrustsheetshowthat thesamplescrossedthelowerlimitofthezircon partial-retention zone (ZPRZ) at 50–42 Ma (bottom and top samples), with a moderate rate of cooling, and passed throughtheupperlimitoftheZPRZat25–30Ma(Fig.5). Thepre-Eocene thermal historyis not wellconstrained, duetolargeuncertaintiesinthemodels,reflectedbythe 95%credibleintervals(Fig.5A).Therefore,theonsetageof exhumation cannot be determined from the models, althoughmodelingsuggeststheywerealreadyexhuming by 50–42Ma. Abovethe ZPRZ, thethermal path is not constrained, as no AFT and AHe ages wereobtained in thosesamples.AsshownonFig.5E,themodel-predicted ages are consistent, within error margins, with the observedages.
The modeled thermal path of the Lakora thrust’s footwallindicatesafastcoolingat25Ma,followedby a slower rate finalcooling from 25 Ma tothe Present. However,thisfinalcoolingpatternisnotwellconstrained, since AHe ages were only obtained from one sample (Fig. 5B). Data modeling shows a constant geothermal gradient of 258C/km, inferred from the offset between samples. In this case, ZHe ages are badly predicted, as shownonFig.5F.Samplesfromthisverticalprofileshow largeintra-sampleagedispersionthatcannotbepredicted bytheQTQtsoftware,indicatingthatthedispersioncannot be explained by crystals eU content or size. The age dispersion couldbeproduced, for example,by complex internalzonationinzircongrains,whicharenot incorpo-ratedinthemodelingduetolackofinformation.Thepoor predictions showed by the model imply that this last coolinghistoryshouldbeconsideredwithcaution.
6. Interpretationanddiscussion
Single-graindatingperformedinthisstudyprovidethe followingfivemainresults:
Fig.3.Apatitefissiontrack(AFT)age-elevationplotoftheEaux-ChaudesandBalaitous–Panticosagranodioritesdataset(inred),andoftheGPY14sample fromPaleozoicsedimentsinthewesternmostAxialZone(black).
in the Aquitaine Basin, ZHe data indicate no post-depositionalresetofages;
theChaıˆnonsBe´arnaisoftheNorthPyreneanZonerecord aprotractedexhumationbetween50and26Ma(Eocene toOligocene);
theLakorathrustsheetwasexhumedthroughtheZHe closure temperaturetogether withitsfootwall (upper levelsoftheGavarniethrustsheet)at30–36Ma(Late EocenetoOligocene);
the granitic massifs of Eaux-Chaudes and Balaitous– Panticosa,andtheUpperPaleozoicrocksoftheSomport area,alllocatedintheGavarniethrustsheet,recordarapid exhumationat26–20Ma(LateOligocenetoAquitanian); the granitic massifs record a final acceleration of exhumation at 8–9 Ma (Late Miocene), which is constrainedbytheAHedata.
Thermalmodelsfurtherconstrainthecoolinghistoryof theLakorathrustsheet,indicatingthatitwasexhumingat leastbetween50–42Maand30–25Ma(EarlyEoceneto Oligocene),andbetterdefinetherapidexhumationhistory oftheEaux-ChaudesandBalaitous–Panticosagranites.
Inwhatfollows,weinterprettheobtainedcoolingages andpathsasindicatingexhumationprimarilylinkedtothe activity of tectonic units, although we understand that climatic events mayhave playeda role toan unknown degree.ThesamplesfromtheUppermostCretaceousrocks oftheAquitaineBasinpreserveZHedetritalagesolderthan thestratigraphicage,indicatingthatburialunderCenozoic forelandbasindepositswasnotenoughtoresettheZHe system. This is consistent with the limited degree of diagenesisandcementationobservedintheserocks.This pattern changes across the NPFT, where rocks from
Fig.4. A,B.ModeledthermalhistoryoftheEaux-ChaudesandBalaitous–Panticosasub-verticalprofiles.Theredlinecorrespondstothepathofthehottest (lowestelevation)sample(with95%credibleintervalrangeinmagenta)andthebluelinecorrespondstothecoolest(highestelevation)sample(with95% credibleintervalrangeincyan).Intermediatesamplesareshowningrey.Blackboxescorrespondtotheconstraintsimposedonthemodeling(seetextfor furtherinformation).C,D.Modeledgeothermalgradientsinredwith95%credibleintervalrangeingrey.E,F.Observedandmodel-predictedapatite(U–Th)/ He(AHe)andzircon(U–Th)/He(ZHe)uncorrectedagesversuselevation.ThepredictedtracklengthdistributiononsampleGY11isplottedinredwith95% credibleintervalsinorangeincomparisonwithobservedtracklengthdata(histograms).
Paleozoic to Albian age have been buried and heated enoughtoresettheZHesystem.IntheChaıˆnonsBe´arnais area,theeffectofburialwasreinforcedbyhighheatflow duringtheMiddleandLateCretaceoustimes,detectedby Ramanspectroscopyofcarbonaceousmaterialthat provid-edpaleo-temperaturesof250–3008C(Clercetal.,2015). TherangeofZHecoolingagesobtainedattestforlong-lived exhumation in the North Pyrenean Zone initiating at 50 Ma (including the Lakora thrust hanging wall), consistent with thermal modeling by Vacherat et al. (2014)intheMaule´onbasin.Totheeastofthestudyarea, AFTagesbyMeresse(2010)fromtheBagne`res-de-Bigorre NorthPyreneanmassifwerecenteredatca.41Ma.Farther east,alongtheECORS-Pyre´ne´estransect,mostoftheAFT data from the North Pyrenean basement rocks indicate exhumation during the Eocene (Fitzgerald et al., 1999; Morrisetal.,1998).Thesedatatogethersupportanearly exhumationoftheNorthPyreneanZoneduringtheEarlyto Middle Eocene. In the study area, we associate the
exhumation of the Chaıˆnons Be´arnais with the pop-up extrusionoftheformerNorthPyreneanbasinbytheNPFT to the north and the Lakora thrust and its eastern extensions such as the Eaux-Chaudes thrust (Teixell etal., 2016). Younger coolingages obtainedin the area, especiallyatdeepstratigraphiclevels,indicatethat thrust-related exhumation proceeded during younger times, causedbycontinuedupliftontheNPFTandprobablyalso by the thick-skinned basement thrusts of the southern Pyrenees,suchastheGavarnieandGuargathrusts(Fig.1B). ZHeagescenteredon34–40MainPermian–Triassicrocks indicate exhumationduring Middle and Late Eocene to Oligocene,whichisconsistentwiththeonsetofmolasse sedimentationofthis ageintheAquitainebasin (Biteau etal.,2006).Ontheotherhand,thesouthwardextentofthe Lakorathrustsheetisconstrainedbythenon-completely resetEocenesamplefromPicoMatz(GPY08),inagreement withthehangingwallrampoftheLakorathrustobserved intheLakoraklippe,6kmtothenorthofPicoMatz.
Fig.5.A,B.ModeledthermalhistoryoftheverticalprofilelocatedinthefootwallandhangingwalloftheLakorathrust.Theredlinecorrespondstothepath ofthehottest(lowestelevation)sample(with95%credibleintervalrangeinmagenta).Thebluelinecorrespondstothecoolest(highestelevation)sample (with95%credibleintervalrangeincyan).Intermediatesamplesareshowningrey.Blackboxescorrespondtotheconstraintsimposedintothemodeling (seethetextforfurtherinformation).C,D.Modeledgeothermalgradientsinblackwith95%credibleintervalrangeingrey.E,F.Observedandmodeled predictedapatite(U–Th)/He(AHe)andzircon(U–Th)/He(ZHe)uncorrectedagesversuselevation.
ZHeagesindicatethattheLakorathrustsheetandits immediate footwall of theAxial Zone cover underwent jointexhumationat30–36Ma(LateEocenetoOligocene), whichwemustattributetothrustfaultingundertheAxial Zone.ThiscoolingageiscorrelativewithLateEoceneto Early Oligocene conglomerate pulses in the Jaca basin, which are dominated by clasts derived from Lower to Middle Eocene turbidites (Puigdefa`bregas, 1975; Roige´ etal.,2016).Weattributethiseventtothemotionalong theGavarniethrust(Fig.1B),becauseitisthefirstmajor south-directedthrustthatunderliestheAxialZoneinthe studyarea,producingasignificantduplicationofPaleozoic rocks and creating a marked structural relief. This interpretation is in agreement with the timing of the GavarniethrustproposedbyTeixell(1996),Jolivetetal. (2007), and Labaume et al. (in review)on the basisof structuralrelationshipsandtectonics-sedimentation rela-tionshipsintheJacabasin,anddiffersfromMun˜ ozetal. (2013)attributionofthethrusttotheMiddle Eocene.It couldbe arguedthat the entireprofile of theGavarnie thrustsheetwasemplacedbelowtheZPRZ,andthatallthe exhumationwasdrivenbytheunderlyingGuargathrust (Fig. 1B), from the Late Eocene to the Miocene (e.g., samplesfromBalaitous,PanticosaandEaux-Chaudes).We considerthis unlikely forthestudyareaon thebasisof faultslip magnitudesand cross-sectionbalancingin the Jacabasin(e.g.,Teixell,1996), eveniftheGuargathrust causesacomponentofupliftontheAxialZone.Nodistinct thermochronology signal canbe attributedto theLarra thrust,probablybecausethisthin-skinnedbranchofthe Lakora thrustdid not createsignificant structural relief during its propagation in the Jaca basinfill during the Lutetian–Bartonian.
Followingtheseconsiderations,weattributethesecond ZHeageclusterat20–26MaobservedinthewesternAxial Zoneandthefastcoolingbetween30and20Mamodeled inthegraniticmassifstotheeasttocontinuedupliftofthe AxialZone along theGuargathrust. Thisactivityof the Guargathrustwascorrelatedtothemainemergenceofthe South Pyrenean thrust front of the External Sierras, recordedbytheLateOligocenetoAquitanian conglome-ratesand fluvialsandstonesoftheUncastilloFormation (Milla´netal.,2000;Puigdefa`bregas,1975;Teixell,1996). AHe data show exhumation signals between 6 and 10 Ma in the Eaux-Chaudes and Balaitous–Panticosa plutons,withsampleslocatedonbothsidesofthepresent drainagedivide(Fig.2).IntheBalaitous–Panticosaprofile, samplesonbothsidesofthedividecanbefitinasingle coherentmodel,indicatingthattheyexperiencedasimilar coolinghistory. Therefore, theincisionduring this time couldnotbecausedbythecaptureofthesouth-flowing EbroRiver,asdefendedbyFillonandvanderBeek(2012).
7. Conclusions
A low-temperature thermochronology study of the westernAxial ZoneofthePyrenees andof theadjacent Chaıˆnons Be´arnais (North Pyrenean Zone) provides the followingconstraintsonthetectonicanderosionalhistory ofthissegmentofthechain.
TheuppermostCretaceousforelandbasinsedimentsof theAquitainebasinhavenotbeenburiedenoughfor post-depositionalreset of theZHethermochronologysystem andpreservePermiantoJurassicdetritalsignals.
Within the Chaıˆnons Be´arnais, the ZHe system was resetandrecordcontinuedpop-up-likeexhumationofthe NorthPyreneanZonebetweentheNorthPyreneanFrontal Thrustandthemajorsouth-directedthrustsofLakoraand others further south (Gavarnieand Guarga) from50 to 26Ma(EocenetoOligocene).
TheleadingedgeoftheLakorathrustsheet,whichwas reportedtobringtheNorthPyreneanZoneontopofthe AxialZoneduringLateCretaceoustoMiddleEocenetimes, shows a cooling path at least from Early Eocene to Oligocene times.The Lakora thrust sheettogether with itsimmediatefootwallformingthepost-Variscancoverof the AxialZone wereexhumed throughtheZHe closure temperatureat36–30Ma(LateEocenetoOligocene)along theunderlyingGavarniethrust.Hence,thecoolingpathof theLakorathrustsheetistheresultoftheactivityofthe Lakorathrustitselfand oftheunderlyingGavarnie(and possiblyGuarga)thrusts.
Paleozoic sediments of the westernmost Axial Zone often yield scattered ZHe thermochronology results indicating partial reset. The granite samples from the Eaux-ChaudesandBalaitous–Panticosaplutonsprovidea good ZHe and AFTL Late Paleogene to Miocene signal, clearlyreflectedinmodelsasfastcoolingbetween30and 20 Ma(Late OligocenetoAquitanian).Weattribute this cooling to thrusting on the Guarga thrust, ultimately upliftingtheolderGavarniethrustsheetlyingabove.
AHe results from the Eaux-Chaudes and Balaitous– Panticosa plutons cluster at 9–8 Ma (Late Miocene) attesting for post-orogeniccoolingthat wasdetected in previousstudiesofthesouthernforelandbasin.
Acknowledgements
ThisworkwassupportedbyprojectsCGL2010-15416 and CGL2014-54180-P (MINECO, Spain) and PYRAMID (ANR, France), and by Ge´osciences Montpellier. We acknowledge the constructive reviews by Luis Barbero, Camille Clerc and theco-editor Yves Lagabrielle, which helped to improve the manuscript. Andreu Badia is thankedforfieldassistanceduringsampling.
AppendixA. Supplementarydata
Supplementarydataassociated withthisarticlecanbe found,intheonlineversion, athttp://dx.doi.org/10.1016/j. crte.2016.01.001.
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