New sedimentological, structural and paleo-thermicity data in the Boucheville Basin (eastern North Pyrenean Zone, France)

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Zone, France)

Roman Chelalou, Thierry Nalpas, Romain Bousquet, Maxime Prevost,

Abdeltif Lahfid, Marc Poujol, Jean-Claude Ringenbach, Jean-François Ballard

To cite this version:

Roman Chelalou, Thierry Nalpas, Romain Bousquet, Maxime Prevost, Abdeltif Lahfid, et al.. New

sedimentological, structural and paleo-thermicity data in the Boucheville Basin (eastern North

Pyre-nean Zone, France). Comptes Rendus Géoscience, Elsevier Masson, 2016, From rifting to mountain

building: the Pyrenean Belt, 348 (3-4), pp.312-321. �10.1016/j.crte.2015.11.008�. �insu-01300441�

Tectonics,

tectonophysics

New

sedimentological,

structural

and

paleo-thermicity

data

in

the

Boucheville

Basin

(eastern

North

Pyrenean

Zone,

France)

Roman

Chelalou

a,

*

,

Thierry

Nalpas

a

,

Romain

Bousquet

a

,

Maxime

Prevost

c

,

Abdeltif

Lahfid

b

,

Marc

Poujol

a

,

Jean-Claude

Ringenbach

c

,

Jean-Franc¸ois

Ballard

c a

Ge´osciencesRennes,UMR6118,Universite´ deRennes-1–CNRS,CampusdeBeaulieu,35042Rennescedex,France

b

BRGM,3,avenueClaude-Guillemin,45100Orle´ans,France

c

TOTAL,CSTJF,avenueLarribau,64018Paucedex,France

1. Introduction

TheN1108-trendingPyreneanbelt(Fig.1),separating theIberianmicroplatefromtheEuropeanplate,isanarrow

400-km-longcontinentalbeltthatdevelopedinresponse tothecollisionbetween theIberian microplateand the EuropeanplateduringtheLateCretaceousandtheTertiary (Beaumontetal.,2000;Chevrotetal.,2015;Choukroune, 1989; Deramond et al., 1993; Mun˜ oz, 1992; Roureand Choukroune, 1998; Teixell, 1998). Triassic, Jurassic and Cretaceousriftingeventsprecededthedevelopmentofthe Pyreneanbeltthatmainlyresultedintheinversionofthe

C.R.Geosciencexxx(2016)xxx–xxx

ARTICLE INFO

Articlehistory:

Received15September2015

Acceptedafterrevision16November2015 Availableonlinexxx

HandledbyMicheldeSaint Blanquat

Keywords: Boucheville

Structuralandsedimentologicalanalysis HT–LPmetamorphism

Albianextension Ramanspectroscopy

ABSTRACT

TheBouchevilleBasinisoneoftheeasternmostMesozoicbasinsoftheNorthPyrenean Zone (NPZ) that was opened during the Albian extension between the Iberian and Europeanplates.Duringtheextension,aHT/LPmetamorphismeventaffectedtheAlbian basinsneartheNorthPyreneanFault(NPF).Ouraimistobetterunderstandtheevolution oftheBouchevilleBasin duringthe Albian–Cenomanianlithosphericthinning,which occurred under high thermal conditions. Sedimentological and structural data were collectedinthebasinandareusedtoproducesyntheticstratigraphiccolumnsofdifferent portionsofthebasinandtorestoreselectedcross-sections.North–southcross-sections showthattheBouchevilleBasinisalargeand asymmetricaldeformedsynclinewith invertedborders.Syntheticstratigraphiccolumnsshowthatthesedimentationofthe BouchevilleBasin starts with carbonate platforms deposited underlow bathymetric conditionsshowingslopedepositsandevolvestodeepbathymetricconditionsofmarls depositedwithout evidenceof slopes.Ramanspectroscopyoncarbonaceous material (RSCM) was made on samples used to constructthe sedimentological stratigraphic columnsinordertoobtainatemperaturemapoftheAlbianmetamorphism.Theyreveal homogeneityinthetemperaturesbetween500and6008C.InsituLA–ICP–MSU–Pbdating oftitanitegrainsfoundinasyn-deformationlocatedintheAlbiancalcschistsprovidedan ageofca.97Mathatgivesatimeconstraintforboththedeformationandmetamorphism. Thesedata areusedcollectivelyto proposeamodelforthetectono-sedimentaryand metamorphicevolutionoftheBouchevilleBasinduringtheAlbianextension.

ß2016Acade´miedessciences.PublishedbyElsevierMassonSAS.Thisisanopenaccess articleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/ 4.0/).

* Correspondingauthor.

E-mailaddress:chelalou.roman@hotmail.fr(R.Chelalou).

ContentslistsavailableatScienceDirect

Comptes

Rendus

Geoscience

w ww . sc i e nce d i re ct . co m

http://dx.doi.org/10.1016/j.crte.2015.11.008

1631-0713/ß2016Acade´miedessciences.PublishedbyElsevierMassonSAS.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http:// creativecommons.org/licenses/by-nc-nd/4.0/).

Lower Cretaceous basins (Puigdefabregas and Souquet, 1986;Verge´s and Garcı´a-Senz,2001). The Lower Creta-ceousriftinginthePyreneanarea,coevalwiththeopening of the Bay of Biscay, was related to the anticlockwise rotationoftheIberianmicroplaterelativetotheEuropean plate(ChoukrouneandMattauer,1978;Gongetal.,2008; Jammesetal.,2009;LePichonetal.,1968;Olivet,1996; Rosenbaumetal.,2002;Sibuetetal.,2004).Theseeast– west-oriented basins, created during this rifting at the northernborderofthefuturePyrenees,arewellknownfor theirhigh-temperaturemetamorphism,whichis synchro-nous with the Aptian–Albian deposits (Albare`de and Michard-Vitrac, 1978; Bernus Maury, 1984; Casteras, 1933;Choukroune,1970;Mattauer,1968;Montignyetal., 1986;Ravier,1959;Verschureetal.,1969).

In the North Pyrenean Zone (NPZ), occurrences of subcontinentalmantlerocks(peridotites)associatedwith Mesozoicsedimentaryformations(Fig.1b)weredescribed earlybyLacroix(1895),andtheiroriginhasbeendebated formorethan100years.RecentstudiesintheNPZhave interpreted someof theselherzolites as subcontinental mantlematerialreworkedthroughsedimentaryprocesses withinCretaceousbasins(LagabrielleandBodinier,2008). Theyproposedthattheexhumationofthesubcontinental mantlewasassociatedwithcontinentalthinning during theMid-CretaceousPyreneanrifting(ClercandLagabrielle, 2014;Clercetal.,2015;Lagabrielleetal.,2010).Thatwas also proposed for the Maule´on Basin based on the interpretation of thegeophysical profilesand fieldwork (Jammesetal.,2009;Masinietal.,2014).

TheNPZiscurrentlyconsideredasaformerriftsystem (Tugendetal.,2014,2015)thathasbeenaffectedbyHT metamorphism during lithospheric thinning (Fig. 1c) (Clercand Lagabrielle, 2014; Clercet al.,2015; Golberg andLeyreloup,1990).SincetheNPZbasinswereexhumed during the Pyrenean compression, they are potential analogues fordepictingpassivemarginsthat recordeda HT metamorphism. These HT margins show original geometry, thermal evolution and subsidence (Clerc and Lagabrielle,2014;Clercetal.,2015;Vacheratetal.,2014). Thepresentstudyaimsatassistingintheunderstandingof thetectono-sedimentaryevolution ofextensionalbasins such as the Boucheville Basin under high thermal conditions.

2. Geologicalsetting

The Boucheville Basin is locatedin the southeastern partof theNPZ,just northof theNorth PyreneanFault (NPF).Itextendson30km,fromtheBesse`deMassif(tothe West)totheAglyMassif(totheEast),andis7kmbroadin itswidestpart(Fig.1).TheNPFseparatestheNPZfromthe AxialZone(AZ)(Fig.1).InthesouthernpartoftheNPZ, neartheNPF,thebiostratigraphicalrecordiserased;thisis probablyduetointensemetamorphism.Therefore,inthe Boucheville Basin,theagesofthesedimentsareusually determinedthrough facies analogywiththosefromthe less metamorphic Saint-Paul-de-Fenouillet Basin. This basinislocatedontheNorthoftheBouchevilleBasinon theothersideoftheAglyMassif(Fig.1.b).Thesediments

Fig.1.(a)StructuralmapofthePyrenees.(b)GeologicalmapoftheNortheasternPyreneesmodifiedfromClerc(2012).(c)Synthesisofknowntemperatures inthemetamorphicMesozoicbasinsoftheNPZ(modifiedfromClerc,2012).B.S.:Bouchevillesyncline.

of the basin of Saint-Paul-de-Fenouillet are well dated (Bergeretal.,1993),especiallytheNeocomiancarbonate platforms and Albian marls, which were dated by ammonites (Collignon et al., 1968). The Saint-Paul-de-FenouilletBasinisthereforeareferenceforindirectdating ofmetamorphosedsedimentaryseriesinthearea.

ThisstudyfocusesontheeasternpartoftheBoucheville Basin (Figs. 1 and 2a). Itwaspreviously described as a synclinorium where three phases of deformation were recognized(Choukroune,1970).AccordingtoChoukroune (1970),bycomparisonwithothertectoniceventsalready documented in the Pyrenees, the first one is a ductile compressional phase associated with high wavelength folds.Thesecondisaductile compressivedextralphase withshort-wavelengthfolds(bothLateCretaceous).The thirdisabrittlecompressivephase(Eocene).

ThesedimentsoftheBouchevilleBasinconsistofthree different lithologies (Bernus Maury, 1984). The first lithologyisinhomogeneousmarbleswhichformthebasis ofthebasin.Thesemarblesmaycontainthinphylliticbeds (rich in alumina),and someretrograde metamorphosed orthoclase and/or scapolite porphyroblasts have been reported(BernusMaury,1984).Thisauthoralsoobserved interbeddedlensesrichinmetamorphicminerals (clino-pyroxene,greenamphibole,feldspars,biotite,and scapo-lite). The second lithology is a calcschist previously described by Bernus Maury (1984) as sandstone com-posedofsmallroundedquartzgrains(50to100

mm).

The rocks are flattened, with a well-defined schistosity outlinedbysomebiotitelamellasandopaquematerials. The lastlithologyis a rockcalled ‘‘hornfelsof Vira’’by

Ravier(1959)whodidnotnoticethepreferredorientation shownbythemetamorphicassemblages.Eventhoughthe ‘‘hornfels of Vira’’ donot properly result from contact metamorphism (see below), we will use that classical terminologyhereafter.Theserocksarerichinroundor almond-shapedplagioclase(80%oftherock)borderedby blackmica(BernusMaury,1984;Ravier,1959).Thelast

two lithologies are stratigraphically located over the marbles,butnoparticularorganizationwasdocumented accordingtotheseauthors.

In theBoucheville Basin, themaximum temperature recordedis 570108Cinthewestanddecreasesto 400-4508Cintheeast(GolbergandLeyreloup,1990)(Fig.1c).In thenearbyPaysdeSault(betweentheBesse`deMassifandthe Saint-Barthe´le´myMassif,Fig.1),thereisadirectrelationship betweenexhumedmantle,thethicknessofthecrustandthe degreeofmetamorphism.Closetothelherzolites,thedegree ofmetamorphismisthehighestandthemaximum tempe-ratures estimated are higher than 6008C (Golberg and Leyreloup,1990).

3. Results 3.1. Stratigraphy

WepresentamapoftheeasternpartoftheBoucheville Basin and the cross-sections associated with the strati-graphic columns (Figs. 2 and 3). Fig. 3a represents four stratigraphiccolumnsbuiltfromsectionA–A’thatillustrate, fromsouthtonorth,the foursides ofthe two synclines (Fig. 2). Based on mapping, field observations and thin sectionsdata,ageneralsyntheticstratigraphiccolumnof thebasinallowsustoreconstructageneralevolutionofthe depositionalenvironments(Fig.3b).Threemainlithologies canbedefined:marbles,hornfelsesandblackcalcschists.

Marbles formthe baseof the Mesozoicsedimentary series (Fig. 3b, Sms, e and f). Because of the lack of biomarkers,themarblesfromtheBouchevilleBasinwere notdirectlydated.Wecannotconcludewhetherthislack wasprimaryorduetometamorphicerasing.However,by correlation with similar limestones/marbles from the nearbySaint-Paul-de-Fenouillet Basin, Jurassicand Ber-riasian(equivalenttotheUrgonienfacies)ageshavebeen proposed(Bergeretal.,1993;Collignonetal.,1968).These Fig.2.(a)StructuralandgeologicalmapoftheeasternpartoftheBouchevilleBasinwithlocationofthecross-sectionspresentedinFig.6.Modifyafter Fonteillesetal.(1993).(b)Stereogramofstratificationplanspoles.(c)Stereogramofisoclinalfoldsplansandhinges.

marblesarecomposedoffullyrecrystallizedlimestonesand ofthefollowingmetamorphicminerals:scapolite,diopside, K-feldspar,plagioclase,biotite,phlogopite, andpyrite.To theeastofthebasin,saccharoidwhiteoryellowmarblesare outcropping, occasionally associated with cargneules, probably Triassic in age (Bergeret al., 1993). These are discontinuousandcanreachamaximumthicknessof10m (Fig.3b).Onthenorthernandsouthernbordersofthebasin, awhitemarblewithgreylevelsisobservedandisassociated withtheJurassicandLowerCretaceous(Berriasian)(Berger etal.,1993). Thisseriesof marblesreachesa maximum thicknessof400mandpresentsboudinagedmorphologyin themap(Fig.2).Furthermore,weobservedboudinagedsills atthecentimetricscale(seebelow),andboudinageatthe decimetrictoplurimetricscalehasbeendocumentedinthe BouchevilleBasinmarbles(ClercandLagabrielle,2014).The baseofthesemarblespresentsstratificationswith alterna-tionsofslumpsandmonogenictopolygenicsedimentary breccias(Sm.1e),someofwhichcontaindecimetricclasts fromtheProterozoic–Paleozoicbasement.Supplementary material1fpresentsacross-sectionofaslumpinmarbles showing feldspar clasts. These slumps and breccias are related to slopes located at the basin border. These sedimentary structures are good polarity criteria for structuralreconstruction.Thetopofthesemarblespresents carbonate platform sedimentation without slumps or breccias.

The contact between the marbles and the black calcschistsisgradualover10m,withoutdeformation.

Thethicknessofthecalcschistsis200minthenorth and 1000m in thesouth (Fig. 3), and is related to the

asymmetryofthebasin.Thechangeinthedepositional environmentfromtheBerriasiantotheAlbianimpliesan increaseinsubsidenceandinthedeepeningofthebasin duringtheAptian–Albiandeposition.Asforthemarbles, theageofthecalcschistsissuggestedbyfacies compari-sonwithlessmetamorphicbasinsfromthenorthernpart of theNPZ (Bergeret al.,1993; Collignonetal., 1968). These calcschists are extremely homogeneous, with a quartzgrainsizevaryingfromverythin(80–70

mm)

tosilt (Fig. 3, Sms. 1b and d), with angular grains (Sm. 1d). Severallayersaredevoidofquartz(Sms.1candd),and becauseoftheexcessofcalcite,ofthepresenceofonlya few silicate minerals and the lack of quartz sills, we believethatthislackofquartzgrainsisprimaryandnot duetometamorphism.

IntheSouthofthebasin,betweenSournia(south)and Vira(north),thebaseofthecalcschistseriesismadeofa formation rich in plagioclase and K-feldspar porphyro-blasts (100 to 500

mm

in size) embedded in a calcite cement(Sm.1a).Thefeldspar grainsaresurrounded by biotite and opaque filaments,but the rockis devoid of quartz. This formation corresponds to the ‘‘hornfels of Vira’’ from the literature (Bernus Maury, 1984; Ravier, 1959).Themacroscopicaspectofthisformationissimilar tothatoftheothercalcschists,andthusitisimpossibleto preciselydetermineitsthickness.Nevertheless,usingthin sections reported on thestratigraphic column to deter-minethevariationsinthelithology,thethicknessofthe so-called‘‘hornfels’’canbeestimatedat200matitsmost.The darkcolorof thewholeseriesindicates thepresenceof largequantitiesoforganicmatter(Sm.1a).Thethickness Fig.3. (a)FourstratigraphiccolumnsmadealongthesectionA–A’,fromsouthtonorth,reflectingthethicknessvariationsinthebasinandthepreferential localizationofthedeformationalongthenorthernedge.(b)Syntheticstratigraphiccolumnofthebasin,thechangeinthedepositionalenvironments illustratesthedeepeningofthebasin.1,Aptian–Albian;2,JurassictoBerriasian;3,Triassic;4,basement;5,carbonateinterbeddedlayers;6,slump.

andhomogeneityoftheseriespreventusfromconsidering thesehornfelsastheresultofa contactmetamorphism. Furthermore, we can estimate that the protolith is substantiallydifferent,andisprobablycomposedofmarls containingsufficientclaytosupplythealuminumrequired fortheformationofplagioclase.

Onthenorthernedgeofthebasin,wherethe‘‘hornfels’’ donotoutcrop,thebaseofthecalcschistsseriespresents interbedded carbonate layers with parallel or wavy laminations (Sms. 1c and d), suggesting tidal or storm deposits.However,thetransitionfromplatform sedimen-tation (Lower Mesozoic limestone/marbles) to a lower offshore sedimentation Cretaceous calcschists, Fig. 3) impliesdeepening,favoringstormdeposits.The homoge-neity of this series indicates a constant depositional environment located in the lower offshore. The lack of turbiditessuggeststheabsenceofsignificantslopesinthe BouchevilleBasinduringAlbiantime.Thetransitionfrom anisopachcarbonateplatform(marbles)toan asymmet-ricaldepositofloweroffshoresedimentation(calcschists) impliesadecreaseinthewavelengthofthedeformation, withthecreationofasub-basin(namelytheBoucheville Basin). The lack of slope during the Albian is thus consistentwithaccommodationrelatedtoflexurationat localscale.InalltheseMesozoicseries,weobservethatS1 isparalleltoS0,andthereforeS1isusefultofindS0inthe homogeneous calcschists when the stratigraphy is not clear.

3.2. Structureanddeformation

The map and the cross-sections present two ENE– WSW-trending isoclinal synclines parallel to the basin borders(Figs.2aand2b).Thesouthernoneis1kmwide andsometimepartiallydeformedbyreversefaultstothe southofthebasin,whilethenorthernoneisabout2km wide.

ThesouthernborderofthebasinistheNPF,wherethe graniteofMillasthrustsovertheBouchevilleBasintothe north.Thenorthernedgeofthebasinisaverticalor north-dipping reverse fault, which brought in contact the ProterozoicseriesoftheAglymassifandtheBoucheville Basin.Reverseborders,reversefaultstothesouthandthe basingeneralfoldedstructureillustratesthemajorpartof thecompressionaldeformation.

In this section, we will focus on the extensional deformationsandrelationshipswithmetamorphism.

In alltheseMesozoicseries,weobserve foliation S1, whichinmostcasesissubparalleltostratificationS0.This foliation istheaxial planeofisoclinal folds(seebelow) whoseaxisdirection,variable,isparalleltothelineationof intersectionS1–S0(L1)andisveryoftenemphasizedby metamorphicminerals(Choukroune,1970)suchasbiotite, plagioclasediopside,andscapolite.

Themostimportantandpenetrativedeformationsare observedinazonethatisapproximately500mwidealong thenorthernborderof thebasin.Thiszoneis stratigra-phically located at the base of the Aptian–Albian calc-schists series. Boudinaged calc-quartz veins (Sm. 2a) resulting from Mesozoic metamorphism (Boulvais, 2016),isoclinalfolds(Fig.2c,Sms.eandf)andboudinage

in interbedded carbonate layers are observed. The boudinagedveinsareusuallysubparalleltothe stratifica-tion, but someare deformed by theisoclinal foldsand crosscutting them (Sms. 2e and f). Therefore, we may assumethattheisoclinalfoldsarecontemporaneouswith thegrowthoftheseveins.

Deformationismuchmorelocalizedandless penetra-tiveonthesouthernedge ofthebasin.In thevillageof Sournia (Fig. 2), the low-dipping S0/S1is deformed by high-dipping shearing (C, with N20 and N40–trending stretchinglineation)producingsigmoidalstructures(Sms. 2candd).Sigmoidaldeformationlocalizedalongthecontact suggestsarelativelybrittledeformationofthe carbonate layers. However, we failedto determine whether these deformationsweredueonlytothePyreneancompressive phase or to the Albian extensional phase with later reactivationduringcompression.

Magmaticsills(eastofSourniaontheroadtoPratde Sourniatothesouth(Fig.4star4)andclosetotheVivier villagetothenorth,Fig.4,star3)canbedistinguishedfrom the calcite- and quartz-rich veins studied by Boulvais (2016).Thesesillsoccurinthe‘‘hornfels’’seriesoratthe baseoftheblackcalcschists.Theyareplurimetricinlength andmillimetrictocentimetricinthicknessandpresenta strongboudinage(Sm.2g).Themineralogicalcomposition ofthesesillsisdescribedbelow.

IntheEastofthebasinatthebaseofthecalcschists series,ontheroadtoMontalba,wenoticeoccurrencesof metamorphicdiopside.Thesediopsidesrecordedatleast twostrainregimes:asyn-metamorphicone(S1),showing rotationsand pressure shadowpatterns(Sm. 2b)and a secondone,withfoldS1andthestratification(Sm.2b). However, thewhole structure has beenaffectedby the laterPyreneancompression,asindicatedbymanyreverse faultsintheSouthofthebasin.

3.3. Metamorphism

TheMesozoicextensioncausedanextremelithospheric thinningwithmantleexhumation(Clerc,2012;Clercand Lagabrielle,2014;Clercetal.,2015;Jammesetal.,2009; Lagabrielle and Bodinier, 2008; Lagabrielle et al., 2010; Tugend,2013;Tugendetal.,2015),andtheriseofthemantle

Fig.4.MapofthemaximumtemperaturesrecordedduringtheMesozoic metamorphismmeasuredbyRamanspectroscopyandthecharacteristic mineralogicalassemblages.Dataaredetailedinsupplementarymaterial #3.

producedaHT/LPmetamorphismthatischaracteristicof thearea(Clerc,2012;ClercandLagabrielle,2014;Clercetal., 2015; Golberg and Leyreloup, 1990; Ravier, 1959). We sampledtheBouchevilleBasintogetmoredataaboutthe temperatureevolutionatthescaleofthebasinbyproducing atemperaturemapbyRamanspectroscopy.

Themetamorphicoverprintinthebasinisindicatedby thefollowing mineral assemblages: (1) In the marbles: tremolite,chloriteandphlogopite-richbiotite(Fig.4,star 1)orpurephlogopite,diopside,plagioclaseandK-feldspar (Fig.4,stars2,Sms.2.band4.a1);and(2)inthemagmatic sillstotheeastofSournia(Fig.4,star4):quartz,feldspar, chlorite,tremolite,muscovite,biotite,andphlogopite.To thesouthofVivier(Fig.4,star3andSm.2.g):microcline, titanite,biotite,plagioclaseandapatite(see Supplementa-rymaterial);thetitanitegrainswereusedtodatethesill (see below).Generally, anorthite-rich plagioclase grains formtypicalsyn-metamorphicporphyroblastes,growing ontosomelargeK-feldspar crystals,probablyofdetrital origin(disaggregationofthenearbyAnsignancharnockite or granite?). Some K-feldspar grains occur as small (millimeter)grainsinthinsectionsandhavea metamor-phicorigin.Fromthediopsideandscapoliteoccurrences,

GolbergandLeyreloup(1990)estimatedametamorphic peaktemperaturebetween4008Cinthewesternpartto 5008Cintheeasternpartofthebasin.Thesetemperature conditionsareslightlylowerthanourestimateusingthe RSCMgeothermometer.

3.3.1. Ramanspectroscopy

TheRamanspectroscopyhasbeenusedsuccessfullyon 43 samplesintheBoucheville Basintocharacterize the structural evolution of carbonaceous materials (CM), reflecting a transformation from disordered to well-orderedCMduringmetamorphism(WopenkaandPasteris, 1993).Theirreversiblepolymerizationandreorganization ofthesematerialsisreflectedintheirRamanspectrumby the decreasing width of the graphite G band and the gradualdisappearanceofthedefectbands,firstD3andD4, thenD1andD2.TheRamanspectrumofwell-orderedCM (perfectgraphite)containsonlytheGband.Thisspectral evolutionwithincreasinggraphitizationwasrelatedtothe temperatureandquantified,providingawaytodetermine thepeaktemperaturesreachedbythemetamorphicrocks (Beyssac et al., 2002). This is the basis of the Raman spectroscopy CM geothermometer (RSCM), which was calibratedintherange330–6508CbyBeyssacetal.(2002), extendedtotherange200–3208C(Lahfidetal.,2010)and 100–2008C(Lahfidetal.,2014).Forourconcern(Sm.3), RSCMhasbeencalibratedfortherange200–6408Cwithan absoluteaccuracy of 508C and a much better internal reproducibilityofabout10–158C(Lahfidetal.,2014).This approach, associated with petrological observations, may allowustodistinguishthedifferentthermalandstructural events(Wiederkehretal.,2009;Wiederkehretal.,2011).

Asafirst-orderresult,high-temperaturehomogeneity bracketingbetween530and5808Cisobserved,exceptfor afewareasseparatedbytectonicstructures(Fig.4).This temperatureuniformityisconsistentwiththeexistenceof anoverlayingsedimentarycover,thethicknessofwhich willbediscussedlater.

3.3.2. P–Testimations

In addition, to better constrain the metamorphic evolution of the sediment outcropping in the basin, we performed P–T estimations using a thermodynamic approach for one sample (sample B01363, Fig. 4, star 2 and Sm. 2.b). All calculations were performed by minimizing Gibbs-free energy (De Capitani and Brown, 1987)usingthesoftwaresuiteTheriak-Domino(DeCapitani andPetrakakis,2010)andtheJUN92d.bsdatabase.

The studiedsampleis a marble containingdiopside about 1mm wide and 5mm long. The metamorphic mineralogicalassemblagedocumentingthetemperature peakreachedbythesample(Fig.6,star2andSm.2.b)is madeofdiopside-rich clinopyroxene–anorthite–phlogo-pite-rich biotite–orthoclase (K-feldspar)–calcite–pyrite (Sm.4.a1).Raresmallquartzgrainshavebeenidentified usingthemicroprobe.Theyarealwaysoccurringbetween diopsideandanorthitecrystals(Sm.4.a2).Inouropinion, this texture indicated a clear growing of quartz after thediopside.Thesequartzgrainsareclearlypostdating the temperature peak.We notealso the occurrence of some large K-feldsparclasts of detrital origin.For the modelling,toaccountfortheoccurrenceofpyriteinthe rock,weuseamagnetite(Mt)asanO2buffer.Thestability

fieldofthestudiedmineralogicalassemblageislarge,with aT-rangeofatleast2008C,anddoesnotprovideanygood constraints for the pressure. This assemblage can be stable up to 0.8GPa(Sm. 4.b). InT XCO2H2O diagrams,

the stabilityfield ofthe T-peak assemblageis strongly P-dependent(Sm.4.c)anditisconstrainedbythe quartz-appearance reaction. By decreasing the pressure by 0.15GPa, thepeaktemperaturemaydecreaseby608C. During aprogressive isobaricheatingofcarbonate-rich rocks assiliceous dolomites,thefluidcomposition will change:theoccurrenceofdiopsideinsuchrocksisonly possibleifCO2activityincreasesuntilreachinga value

around0.8(BrownandSkinner,1974;BucherandFrey, 1994).Highervalues ofXCO2 willdecreasethestability

fieldofH2O-bearingmineral,asbiotite.Knowingthepeak

temperaturereachedbythissample(5808C)byRaman thermometry, we compute the P XCO2H2O diagram to

knowthemaximumsedimentarythicknessofthebasin. ConsideringaXCO₂around0.8,thepressurecannotexceed

0.35GPa. At higher pressures, assemblages containing quartz willbe stable(Sm. 4.b).However, theobserved mineralogicalassemblageinsampleBO1363willbestill stableatpressureslowerthan0.35GPa.Asasecond-order result,weobservethattheisothermsareroughlyparallel to the stratigraphic bedding and appear to follow the basin’s double syncline structure. On the map of

Fig.4,thehighesttemperaturesareobservedinthecore of the folded structure and are associated with the deepestlevels ofthebasin.Basedonthese stratigraph-ic-thermal correlations, we estimate the minimum temperaturegradientinthebasintobebetween708C/ kmand808C/km.

3.3.3. U–Pbgeochronology

Inordertoobtainageochronologicalconstraintonthe metamorphismandthedeformation,weperformedinsitu LA–ICP–MSU–Pbdatingofthetitanitegrainsfoundinthe

magmaticsilldescribedpreviously(Sm.2.g).Datingwas performeddirectlyinthinsectionsatGe´osciencesRennes using anESINWR193UCExcimerlaser coupledwithan Agilent7700quadripoleICP–MS.Theanalytical proce-dureisdetailedinBoutinetal.(2015).Fivetitanitegrains (Sm.6.a)wereanalyzed(24analyses)andthedatacanbe foundintheSupplementary material.Plottedina Tera– Wasserburgdiagram(Sm.6.b),thedatadefineadiscordia withalowerinterceptdateof972.3Ma(MSWD=3.9).If thediscordiaisanchoredtothecommonPbvaluecalculated followingthemodelofStaceyandKramers(1975),weendup withalowerinterceptdateof991.5Maequivalentwithin themarginoferror.Therefore,weinterpretthisdateofca 97Maasthecrystallizationageofthesetitanitegrains. 4. Interpretationanddiscussion

Thickness variations and lateral facies variations between theedgesandthecenterofthebasinallowus tosuggestthattheBouchevilleBasinwasanindependent basin or a subbasin separated from the Saint-Paul-de-FenouilletBasinbytheshoal,whichwastheAglyMassif during the Cretaceous extension. Based on geological maps, isotherms map and inferred cross-sections, we suggest that the current architecture of the basin correspondstoalargesynclinehavingtworeversedflanks withoppositeverging.Choukroune(1970)describedthe ‘‘synclinorium’’ structureoftheBoucheville Basinasthe resultofthestudyoffoliations, schistosities,and micro-structures,withtheideathatfoldscouldbeformedonlyin acompressivecontext.However,ourstudyshowsthatthe isoclinalfoldswereformedduringtheCretaceous exten-sion.

A remarkable feature is the rapid variation in the structure oftheMesozoicmarblesinterpretedas boudi-nage.Itcanberelatedtoaductilestretchingandfrequent pinchedstructure.Anasymmetryofthethicknessofthe calcschists is observed, witha greater thickness of the wholesedimentarypiletothenorth(Fig.5).

Penetrativeandductilesyn-metamorphicdeformation iswelldevelopedina500-m-wideareaonthenorthern edgeofthebasin(Fig.5).Inthisarea,theisoclinalfolding andtheformationofcalciteandquartz-richveins,which arethedirectresultofmetamorphism(Boulvais,2016),are coeval.Inaddition,wedatedaboudinagedsillat97Ma, which intrudesthecalcschists. Thissillemplacement is posteriortothedepositionofthesedimentsbutanterior and/or contemporary with both metamorphism and deformation(isoclinalfoldingandboudinage).

Based on the relation between deformation and metamorphism, we suggest that the thermal overprint ofthewholebasiniscontemporaneouswiththeductile deformation,andwasprobablyproducedbyanextreme crustal thinning during the Albian–Cenomanian rifting (ClercandLagabrielle,2014;Clercetal.,2015;Lagabrielle andBodinier,2008;Lagabrielleetal.,2010;Tugendetal., 2014; Vacherat et al., 2014; Vielzeuf and Kornprobst, 1984).

On thesouthern edge of thebasin, thedeformation shows brittle features and is more localized along the contactthantheoneobservedonthenorthernedgeofthe

basin.Thisdeformationisdocumentedby a well-devel-oped N20/N40 stretching lineation. If we consider the structuresobservedinthemarbles(Sm.2.c),we cannot excludethatthePyreneanshorteningwasresponsiblefor theobservedstructuralfeatureswithacompleteerasingof previous extensional effects. In addition, the observed shearingindicatesacompressionaldeformationwiththe beddingnowinareverseposition,butmayindicatealsoan extensionaldeformationifthebeddingisrestoredasbeing inanormalposition.

Deformationsandsedimentthicknessaregreaterinthe north, suggesting more subsidence there. A strong asymmetryfortheBouchevilleBasinisclearlyindicated bytheasymmetry ofthedifferentstructures (Fig.5)as sedimentthicknessorintensityandtypeofdeformations. Altogether,wemayassumethatthemainlimit,i.e.thearea that recorded the highest extension in the Boucheville Basin,islocatedalongitsnorthernborderzone.

The minimum temperaturegradient (708C/km) esti-matedfortheBouchevilleBasinissimilartothatproposed in other metamorphic Mesozoic basins along the NPZ (Clercetal.,2015;Vacheratetal.,2014).Accordingtoour calculation, pressure was lower than 0.35GPa. If we assumeapressurevaluebetween0.3and0.2GPaforthe temperaturepeak and a sediment densityvarying from 2500 to 3000kg/m3_{, we estimate the thickness of the} Fig.5.Cross-sectionA–A’,B–B’,andC–C’intheBouchevilleBasinwith isothermsplacedaccordingtothetemperaturemapshowninFig.6.The blackcrossessymbolizethechurchesinthevillages.

sedimentarycoverduringthemaximumpickof tempera-ture(Albian–Cenomanian)tobebetween6and10km

However,regionalgeologicaldata(geological mapof RivesaltesandSaint-Paul-de-Fenouillet,inprep.)suggest that thesedimentary pile could not have reached such thickness;tothenorthofSalvezine(Fig.1),Cenomanian depositssealtheAlbiansedimentarypile,which arenot exceedingthreekilometersinthickness(geologicalmapof Saint-Paul-de-Fenouillet,inprep.).Consideringallthis,we estimatethethicknessofthesedimentarypileduringthe Albian–Cenomanianmetamorphismtobebetween6and 7km.Therefore,astrongergradientisneededtoexplain ourmeasuredtemperaturesandarelativethinsediment cover. Despite slight mappable temperature variations, correlated withthe stratigraphy of the basin, we point relative temperature homogeneity between 530 and 5808Cinvolvingalowthermalgradient(seeabove).This makesthestudiedcasecomparabletotheSaltonSeabasin system,whereaveryhighthermalgradient(3008C/km) isfoundinthefirstkilometerofshalesediment,andan almost adiabatic thermal gradient prevailing in the underlyingsandstone(Helgeson,1968).

MostlyfocusedonthenorthernedgeoftheBoucheville Basin,thehistoryofitsdepositionalenvironmentsmaybe outlinedasfollows.

Afteranevaporiticeventduringthebeginningofthe Triassic, carbonate platforms were deposited from the Liassic to the Berriasian (Fig. 6). The occurrence of slope breccia and slumps in the Mesozoic carbonates announcea changein thebasin’s topography,probably relatedtothelocalexhumationofthebasement,whichis probablyrelatedtodeformationandfaultscreationduring rifting initiation (Fig. 6). From Albian times, the basin deepensandsedimentationchangesfromcarbonate plat-formstostormdepositsfollowedbyloweroffshoremarls (Fig.6).Thedepositsareasymmetrical,butslopedeposits arenolongerobserved.Duringtheincreaseintemperature, thedeformationtendstobelocalizedinthenorthernedgeof thebasin,suggestingtheformationofashearzone(roughly 500mthick,Fig.6).Thisshearingseemstobesynchronous withthe boudinage ofthe wholebasal carbonateseries. Altogether,thisindicatesthatthetectonicsettingevolves fromcoldandbrittletohotandductileduringtheonsetof BouchevilleBasinopening.

Itisworthnotingthattheobservedthicknessvariations and the characteristics of the thermal regime during deformationsuggest thata boudinage ata crustalscale controlledtheshapeofthebasin.Byunfoldingitspresent geometry,weinferthatthewidthoftheBouchevilleBasin probablydidnotexceedtenkilometersattheendofthe Albian.This model is coherentwiththe current model, which proposes that theMesozoic extensioncaused an extreme lithospheric thinning with mantle exhumation (Clerc,2012;ClercandLagabrielle,2014;Clercetal.,2015; Jammes et al., 2009; Lagabrielle and Bodinier, 2008; Lagabrielle et al., 2010; Tugend, 2013; Tugend et al., 2015)andariseofthemantlewhichproducedthe well-knownHT/LPmetamorphism.Westressthefactthatthe mostsignificantlimitationtothedevelopmentofamore sophisticatedmodelforthebasinevolutionisprecludedby the lack of biomarkers or of any other way of dating

deposits. Metamorphism deeply transformed the series and it isthereforerisky tocomparethis basinwiththe neighboringones.Thepresenceofafoliationparalleltothe beddingcomplicates thereadingofthestratigraphyina veryhomogeneoussedimentarysequencesuchasthatof thecalcschistsoftheBouchevilleBasin.

5. Conclusion

Our study allowed us toproposea thermal-tectono-sedimentaryevolutionmodel,whichshowsthat: the Boucheville Basin is a large asymmetric syncline

upliftedinitsmiddlepartwithreversebordersrelatedto the Pyrenean compression. The basin was already asymmetricalwhenthesedimentsdeposited;

the Boucheville Basin evolved in two phases: (i) initializationoftheriftingintheJurassicwithcreation Fig.6.TectonicandsedimentologicalevolutionmodeloftheBoucheville Basin,focusedonthenorthernedgeofthebasin.

of the topography, exhumation of the basement and settingupoftheslopedeposit;thebasinsubsidenceand asymmetry increased during the Cretaceous, and we noticed that more significant boudinage and ductile deformationappearinthenorthernborder;

thesedimentaryevolution,geometryanddeformationin the Boucheville Basin allow us to propose that the northernborderactedasamainextensionalAlbianshear zone;

Ramanmeasurementsonorganicmatterweretakenin cohesionwiththestratigraphiccolumnsandshowthat the isotherms are organized subparallel to the basin bedding. These measurements show homogeneous temperatures between 530 and 5808C, which are coherent with the metamorphic mineral assemblages observedindifferentpartsofthebasin;

theoccurrenceofmagmatic sillsat 97Maimpliesthe existenceofpartialmeltingofthecrustalrocksbelow, relatedtothecrustalthinningandmantleexhumation currentlyusedtoexplaintheHT/LPPyrenean metamor-phism.

Acknowledgments

WeacknowledgeTotalforfinancialsupport(contract number:AD13-521).WethankS.Mullinforhereditingof theEnglishstyle,Y.LagabrielleandS.Fourcadefortheir suggestions and comments on an early version of the manuscript.Finally,wearegratefultoG.MohnandtoA. Vacherat for their detailed review and constructive comments which helped us improve the manuscript. SpecialthanksareduetoM.deStBlanquatforhiseditorial work.

AppendixA. Supplementarydata

Supplementarydataassociatedwiththisarticlecanbe found,intheonlineversion,athttp://dx.doi.org/10.1016/j. crte.2015.11.008.

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