Quaternary deformation and stress perturbations along the Digne thrust front, Southwestern Alps

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Quaternary deformation and stress perturbations along

the Digne thrust front, Southwestern Alps

Jean-Claude Hippolyte, Olivier Bellier, Nicolas Espurt

To cite this version:

Jean-Claude Hippolyte, Olivier Bellier, Nicolas Espurt. Quaternary deformation and stress

perturba-tions along the Digne thrust front, Southwestern Alps. Comptes Rendus Géoscience, Elsevier Masson,

2012, 344, pp.205 - 213. �10.1016/j.crte.2012.03.002�. �hal-03040680�

Tectonics,

tectonophysics

Quaternary

deformation

and

stress

perturbations

along

the

Digne

thrust

front,

Southwestern

Alps

De´formation

quaternaire

et

perturbations

de

contraintes

au

front

de

la

nappe

de

Digne,

Alpes

du

sud

Jean-Claude

Hippolyte

*

,

Olivier

Bellier,

Nicolas

Espurt

Aix-MarseilleUniv.,CEREGE,UMR7330,CNRS,CEREGE,UMR7330,IRD,CEREGE,UMR161,13545Aix-en-Provencecedex4,France

ARTICLE INFO Articlehistory:

Received20December2011 Acceptedafterrevision14March2012 Availableonline26April2012 Writtenoninvitationofthe EditorialBoard Keywords: Paleostresses Stressperturbations Quaternarytectonics Messinianevent Canyons Dignethrust SouthwesternAlps Motscle´s: Pale´ocontraintes Perturbationsdecontraintes Tectoniquequaternaire E´ve`nementmessinien Canyons NappedeDigne AlpesduSud ABSTRACT

InthesouthwesternAlps,theretrogradationofthecontinentalsedimentarywedgethat infilledtheValensoleforelandbasinaftertheMessinianevent,resultedinthedeposition ofrecent(3.4to2Ma)andthick(upto100m)conglomerateswithincanyonsintheDigne thrustsheet.Paleostressanalysis,intheseUpperPliocene-Quaternarysedimentsandina Quaternaryterrace,allowscharacterizingthelocalQuaternarystressfield.Thisstressfield displaystypicalstressperturbations(deviationofs1ofupto538)alongamaindextral fault(Be`sfault,0.7mm/yr)ofthestrike-slipedgeoftheDignesouth-vergingthrustsheet. Weproposeastructuralmodelthatexplainsthehighesttectonicupliftrecordedbythe paleo-drainagenetwork(345m;0.1mm/yr)atthenortherntipofthisdextralfault.We conclude that Messinian-Zanclean canyons allow one to characterize Quaternary geodynamicsfordeformationratesaslowas0.1mm/yr.

ß2012Acade´miedessciences.PublishedbyElsevierMassonSAS.Allrightsreserved.

RE´ SUME´

DanslesAlpesduSud,lare´trogradationduprismecontinentaldubassindeValensole, faisantsuite a` l’e´ve`nement messe´nien, a conduit aucolmatage de canyons par des conglome´ratse´pais(jusqu’a` 100m)etre´cents(3,4a` 2Ma)jusquedanslanappedeDigne. L’analysedesfaillesdecesse´dimentsduPlioce`nesupe´rieur-Quaternaireetd’uneterrasse quaternaire,permetd’imagerlechampdecontraintequaternairedelare´giondeDigne. Celui-ciestperturbe´ (de´viationsjusqu’a` 538)parlafailledextreduBe`squiaaccompagne´ le de´placementverslesuddelanappedeDignea` pre`sde0,7mm/an.Nousproposonsun mode`lestructuralquiinte`grecemouvementdextreetlesoule`vementmajeurenregistre´ parlepale´o-re´seauhydrographiquedeDignea` l’extre´mite´ septentrionaledecettefaille (345m;0,1mm/an).Lapre´sencedese´dimentsdecanyonsmessino-zancle´enssurlefront des Alpes permet donc de caracte´riser des structures Quaternaires a` faible taux de de´formation(0,1mm/an).

ß2012Acade´miedessciences.Publie´ parElsevierMassonSAS.Tousdroitsre´serve´s.

* Correspondingauthor.

E-mailaddress:hippolyte@cerege.fr(J.-C.Hippolyte).

ContentslistsavailableatSciVerseScienceDirect

Comptes

Rendus

Geoscience

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

1631-0713/$–seefrontmatterß2012Acade´miedessciences.PublishedbyElsevierMassonSAS.Allrightsreserved. doi:10.1016/j.crte.2012.03.002

1. Introduction

ThewesternAlpsarecharacterizedbyalow deforma-tionratewhichmakesdifficultcharacterizingtheiractive or Quaternary faults with classical geologicaland geo-physical approaches (e.g.,GPS measurements; Walpers-dorfetal.,2006).Seismotectonicanalyseshaveshownthat the inner Alps are deforming under radial extension whereastheouterAlpsarecharacterizedbycompression (Sueetal.,1999),butthecharacterizationofactivefaults through this approach remains exceptional (e.g. the BelledonneBorder fault, Thouvenotet al.,2003). Owing tohigherosionrates,ithasnotyetbeenpossibletoclearly characterize active deformation in the Subalpine units through geomorphologic studies. To bettercharacterize the recent geodynamics of the Subalpine front of the southwesternAlps,paleostressstudieshavebeenfocused on themost recent sedimentaryunits likethe Pliocene sedimentsoftheVarvalley,andtheMio-Plioceneseriesof theValensole Basin (Combes, 1984; Ritz, 1992). In the ValensoleBasin,Clauzon(1983)interpretedthedeposition ofEarlyPleistocenescreesastheconsequenceoftectonic deformationandupliftoftheDignethrustsheet.Northof thisarea,intheDignethrustsheet,Hippolyteetal.(2011) reconstructed a paleo-drainage network infilled with alluvialdepositsyoungerthan3.4Ma.Thispaleo-drainage networkhasbeendeformedduringtheQuaternary.

In this article, we present a reconstruction of the QuaternarystressfieldoftheDignethrustfrontbetween Digne and Barles (Fig. 1). Furthermore, we show how combined paleostress and structural analyses can help inferringthelocationandkinematicsofQuaternaryfaults inathrustbelt.

2. Geologicalsetting

TheDignethrustsheetistheoutermosttectonicunitof the southwestern Alps. It is mainly made of a thick Mesozoicrocksequence,thatinplaceswasthrustedover Miocenesedimentsof theValensoleBasin (e.g.Haccard etal.,1989;Fig.1).Itsde´collementlevelliesintheweak Triassicevaporitelayer(Goguel,1936;Fig.1).TheDigne thrustfrontextendsfromtheEcrinsexternalMassifinthe northtotheeast-westtrendingCastellaneArcinthesouth (Fig.1).WiththeCastellaneArc,itconstitutesthesouthern SubalpineChainsthatmovedtowardthesouth-southwest (Gidon and Pairis, 1986; Hippolyte et al., 2011). Their western front, along the Valensole Basin,is the dextral obliquerampofthisSubalpinethrustsystem,whereasthe ‘‘ArcdeCastellane’’representstheirleadingedge(Fig.1). Thewesternfront(Dignethrust)ischaracterizedbya complexgeometryprobablybecause oftheinfluenceof Liassicextensionalfaultsonthegeometryofsubsequent thrusts(Gidon,1982,1997;HaywardandGraham,1989) andalsobecauseofitsmainlystrike-slipdextralmotion. ImmediatelynorthofDigne,thisfrontshowsasalient(La RobineLobe),whichisalargesyncline,andare-entrant, theBarleshalf-window,correspondingtoalargeanticline (Fig. 1). In thesouthern partoftheBarles half-window appearsa complex syncline of Oligocene-Miocene sedi-ments,namedLe‘‘Ve´lodrome’’(Gigotetal.,1974).The‘‘La

Robine’’LobeisseparatedfromthemainbodyoftheDigne thrustsheetbytheBe`sfault(Fig.1).Thisfaultisarecent dextral faultthat offsets theleading edge of the Digne thrustsheetnearDigne(GidonandPairis,1988;Haccard etal.,1989;Hippolyteetal.,2011;Jorda,1982).

Severalpaleostressstudieswerecarriedoutalongthe westernfrontoftheDignethrustsheetfordeterminingits geodynamics. Combes (1984) characterized ENE-WSW trendsofcompressionintheMioceneconglomeratesofthe ValensoleBasin.Faucheretal.(1988)measuredfaultsand schistosityintheDignethrustsheetandconcludedfortwo successivedirectionsofshortening:N30andN80, respec-tively. Ritz (1992) also identified two directions of compression, but with a NE-SW compression followed byanorth-southcompression.IntheBarleshalf-window (Fig. 1), Fournier et al. (2008) found a single trend of compressionorientedNNE-SSW.Inthisarticlewepresent the results of a paleostress analysis in the same area (Fig. 1), but in younger (Upper Pliocene-Quaternary) alluvial deposits.Therecentageof thealluvialdeposits used for fault slip analysis, and of the paleo-drainage network,ascertainthatthetectonicdeformationandthe reconstructedstressfieldareofQuaternaryage.

3. TheupperPliocene-Quaternaryalluvialdeposits OnthewesternfrontoftheDignethrust,Jorda(1970, 1982) and Jorda et al. (1988,1992) described tectonic deformation in alluvial deposits supposed of Early and Middle Pleistoceneage.Theseoutcropsof conglomerate areactuallytheremnantsofthethicksedimentaryinfillof paleo-canyons that were mainly incised during the Messinianevent(Fig.1;Clauzon,1999;Hippolyteetal., 2011).DuringtheMessiniansalinitycrisis,from5.96Mato 5.332 Ma (Gautier et al., 1994; Krijgsman et al., 1999; Lourensetal.,2004),theMediterraneansea-leveldropped byca.1500m,whichresultedindeepincisionofthe peri-MediterraneanriversincludingtheRhoˆneandtheDurance Rivers(Clauzon,1979,1982;Clauzonetal.,1996).Atthe end of this eustaticcycle, theMediterraneanBasinwas refilled,andthelowercourseofthesedeeplyincisedrivers (canyons)wasflooded.Theseriaswereprogressivelyfilled upwithmarinesediments,whereastheuppercourseof thecanyonswasprogressivelyinfilledbyalluvialdeposits. Thisunusualinfillofdeeplyincisedcanyonsresultedin particular sedimentaryprocessesincludingthe aggrada-tion and the retrogradation (upstream) of continental deposits(Hippolyteetal.,2011).Inthestudiedarea,this aggradation resulted in a second infillof the Valensole Basin. A Pliocene-Quaternary conglomerate, named the Valensole-IIconglomerate, coveredmostof theMiocene andpre-MessinianValensole-Iconglomerate,anditstop surfacenowformstheValensolePlateau(Clauzon,1996; Dubar,1983).Theretrogradationofthecontinentalwedge explainstheoccurrenceofthickalluvialdepositsinfilling canyons upstream of the Valensole Basin, in the Digne thrustsheet(Fig.1).Segmentsofpaleo-canyonsofallthe main modern rivers of Digne were identified: paleo-Ble´one, paleo-Be`s, paleo-Eaux-Chaudes, paleo-Mardaric (Fig. 1; Hippolyte et al., 2011). At Digne, the base of their retrograding alluvial infill is dated using pollen

J.-C.Hippolyteetal./C.R.Geoscience344(2012)205–213 206

Fig. 1. Location of the study area on a shaded relief view of the Southwestern Alps and structural sketch of the front of the Digne thrust sheet with the Messinian-Gelasian drainage network (yellow dashed lines). Schmidt’s diagrams present the striated surfaces measured in the alluvial deposits, with the computed paleostress axes resulting from the fault slip inversions (Angelier, 1990, 1994). Five-branch star =s1

(maximum principal stress axis); four-branch star =s2(intermediate principal stress axis); three-branch star =s3(minimum principal stress axis).

Fig. 1. Situation de la zone d’e´tude sur un relief ombre´ des Alpes du Sud et sche´ma structural du front de la nappe de Digne avec le re´seau hydrographique Messinian-Ge´lasien (tirets jaunes). Les diagrammes de Schmidt pre´sentent les plans strie´s mesure´s dans les alluvions, ainsi que les axes de contraintes calcule´s (Angelier, 1990, 1994). E´toile a` cinq branches =s1; E´toile a` quatre branches =s2; E´toile a` trois branches =s3.

Hippolyte et al. / C. R. Geoscience 344 (2012) 205–213 207

assemblagesbetween 3.4 and 2.6 Ma (Hippolyteet al., 2011).Theageofitstopisabout2Maaccordingtoplant leaves(Jordaetal.,1988),whichisincloseagreementwith the estimated age of the top of the Valensole Basin (Clauzonetal.,1990;Dubaretal.,1998;Hippolyteetal., 2011).Consequently,theincisionoftheriverswasactivein theAlpsduringtheMessinianandtheZancleanwhereas downstream,intherias,itwasstoppedbytheZanclean flooding.Thetectonicdeformationofthispaleo-drainage network,infilledbyUpperPliocene-Gelasiansediments,is thereforeofQuaternaryage.

4. Paleostresses 4.1. Dataandmethod

The fault kinematics and paleostress analyses pre-sentedinthis studyarebased onmeasurement offault slips, in particular in the Upper-Pliocene-Quaternary alluvialdeposits.The slickenlinesanalysed weremainly measured on relatively flat surfaces of boulders and cobbles (Fig. 2). However, we also found faults cutting both the bedrock (Liassic limestone) and the alluvial deposits(Fig.2).Asthegeometryofthestriatedsurfacesof clastsissimilartothoseofthelargestfaultplanescutting

throughtheconglomerates,thesetwotypesoffaultsare mixed in the dataset.Note that, with the exception of radial striation in sand rich alluvial deposits, previous studies already showed the similarity of the stress inversionresultswhenusingstriatedclastsorconsolidated rocks(e.g.Hippolyte,2001).

In the recent sediments of the Digne thrust sheet, striatedsurfacesofclastswerefirstobservedintheGrand Colle Mountain (immediately north of Digne) by Jorda, 1970, 1982; Fig. 1). We measured fault slips in the sedimentsinfillingtheMessinian-Zanclean canyons,but also at the baseof the Champourcin alluvial terrace of probableRissage(Fig.1;Haccardetal.,1989).Incontrast withthevery thinslickenlines foundintheQuaternary terraces,thestriationsobservedintheinfillof Messinian-Zanclean canyons sometimes showed calcite steps and styloliths. The presence of these typical dissolution-crystallisationstructuresisprobablyduetotherelatively largeburialdepthandlithostaticcharge(forriverdeposits) that prevailedwithinthesepaleo-canyonsduring defor-mation.Thethicknessofalluvialdepositsstill preserved fromerosionisinplaceupto100m,whereasthethickness ofthesedimentsbeneaththeUpperQuaternaryterracesis usuallylessthan20metersinthisarea(Hippolyteetal., 2011).

Fig.2.Examplesofstriatedsurfacesusedforpaleostressanalysis.Upperphoto:areversefaultcutsthebaseofapaleo-canyoninfillatsiteMeunie`re(Fig.1). Thestriatedsurfacesaremeasuredinthealluvialdeposits.Lowerphoto:detailofparallelslickenlinesonthesurfaceofaclastinthealluvialdepositsofsite Gd.Colle.

Fig.2.Exemplesdesurfacesstrie´esutilise´espourlareconstructiondepale´ocontraintes.Photographiesupe´rieure:unefailleinversecoupelabased’un pale´o-canyonausiteMeunie`re(Fig.1).Lessurfacesstrie´esonte´te´ mesure´essurdesgalets.Photographieinfe´rieure:de´taildelasurfacestrie´ed’ungaletdu siteGd.Colle.

J.-C.Hippolyteetal./C.R.Geoscience344(2012)205–213 208

We used the Angelier’s sofware to compute the orientation of the three principal stress axes (

s

1,

s

2 and

s

3)andashaperatio(

s

)ofthestresstensor(Angelier, 1990,1994).TheinversionresultsarepresentedinTable1, andthefaultdiagramsareshowninFig.1.

4.2. Reconstructionofthepaleostressfield

The distribution of the outcrops of Upper Pliocene-Quaternaryalluvialdepositsallowedfaultanalysisalong theBe`sand Saint Benoıˆtfaults (Fig.1). TheQuaternary deformation resultingfromtheslip on thesefaultswas alreadycharacterizedusingtheelevationofthethalwegof thepaleo-canyons(Hippolyteetal.,2011).Thesedextral faultscutanddisplacedlaterallythepaleo-canyonofthe Ble´oneRiverbyabout2.3kmata sliprateof0.7mm/yr (Fig.1,Hippolyteetal.,2011).NorthofDigne,theBe`sfault trendsN15Eanditsslipismainlyalong-strikeasshownby thelowverticaldisplacement(+45m)oftheBle`one paleo-canyonatGrandColle(thalwegat790m;160mabovethe modern river) relative to its elevation in the Valensole Basin(thalwegat696m;115mabovethemodernriver) (Fig.3).AroundDigne,thisfaultturnstoaNW-SEstrike and branches on the Saint Benoıˆt fault and on thrusts rampsthatupliftedthethalwegofthepaleo-EauxChaudes Riverby245m(Pliocene-Quaternarythalwegat1010m elevationatsiteFonse;Fig.3).

We measured striations at six sites in the alluvial depositsofthesepaleo-canyonsalong theBe`sandSaint Benoıˆt faults. In contrast, we have not observed any tectonic striation inthe sedimentsof theBle`one paleo-canyonwestofsiteBonnette(Fig.1).Allfaultsitesalong theBe`sandSaintBenoıˆtfaultsshowstrike-sliptoreverse fault slips in agreement withthe strike-slip to reverse motiononthesemainfaults(Fig.1).

OntheeasternsideoftheBe`sfault,inthethreesitesof the Grand Colle Mountain, the computed trends of compressionaresimilarand rangedbetweenN628E and N708E (Figs. 1 and 3). To the south the trend of compression rotates counter-clockwise to N458E at La Fonse.NorthofGrandColle, atsiteChauvet(Fig.3),the trendofcompressionrotatesclockwisetoN988E.Allthe sitesshowedmonophasedeformationandtheformations

are tooyoungtointerpret theselarge variationsof the trend of compression (538) as the result of rotation of faultedblocksduringthestrike-slipdeformation. There-fore,we interpretthesevariationsofthetrendof

s

1 as ‘‘stressdeviations’’(Rebaı¨ etal.,1992;Rispoli,1981).

Itis wellknown that variations in maximum stress directioncanoccuralongstrike-slipfaults(e.g.Homberg etal.,2004;Lacombe,2012;Rispoli,1981).AtDigne,our paleostressanalysissuggestsaperturbationofthestress fieldinagreementwiththemotionoftheBe`sfaultthatis mainly dextral on its N15-trending portion (Fig. 3). A similarstressdeviationmayoccuralongtheSaintBenoıˆt fault,wherethetrendofcompressionrotatesfromN79Eat Champourcin,toN50EatBonnette(Fig.1).

4.3. Originofthestressdeviations

Stressperturbationsoccurgenerallyattheextremities offaults(e.g.Hombergetal.,2004;Lacombe,2012;Rispoli, 1981).Nearthesouthernextremityof theBe`sfault,the counter-clockwisestressrotationatLaFonseoccurswhere thefaultchangesofstrikeanddiptobecomeanoblique thrust(Fig.3).Onitsnortherntip,theBe`sfaultdoesnotcut LeVe´lodromesyncline(Fig.1)andwecaninferthat,ina similarway,itsmotionistransferredtothrusting.Thatthe strike-slipmotion oftheBe`sfaultendsinLeVe´lodrome synclineisinagreementwiththeclockwiserotationof

s

1 at Chauvet (Fig. 3). Finally the recent stress field reconstructedisinagreementwiththedextralstrike-slip motionoftheBe`sfaultbetweenDigneandLeVe´lodrome syncline. The way this strike-slip motion ends in Le Ve´lodromesynclineisdiscussedbelow.

AtChauvet,thealluvialdepositsareabout100mthick andthepaleo-thalwegisat1020melevation,280mabove themodernBle´oneRiver(Fig.4).Thiselevationispartly relatedtothrustdeformationassuggestedbythetiltof thesealluvialdepositstothesoutheast,andtheirtectonic deformation(Figs.4and5).Comparedwiththethalwegof thepaleo-Ble´oneriverintheValensoleBasinimmediately westofDigne,115metersabovethemodernBle´oneRiver, thethalwegatChauvethasbeenupliftedof165m(Fig.3). Note that we quantify anuplift that occurred afterthe incision of the paleo-canyon at 3.4–2.6 Ma (Hippolyte

Table1

ResultsofthestressinversionspresentedinFig.1.s1,s2,s3:maximum,intermediaryandminimumprincipalstressaxesrespectively.Trend(northto east)andplungeindegrees.s=(s2–s3)/(s1–s3).Qualityestimators:ANG(averageanglebetweencomputedshearstressandobservedslickenside lineation,indegrees)andRUP(0RUP200)(Angelier,1990).

Tableau1

Re´sultatsdesinversionsdecontraintes;diagrammesdefaillesdelaFig.1.s1,s2,s3:directionetplongementdesaxesdecontraintesprincipales respectivementmaximales,interme´diairesetminimales.s =(s2–s3)/(s1–s3).Crite`resdequalite´ :ANG(anglemoyenendegre´sentrelacontrainte cisaillantecalcule´eetlastrieobserve´e)etRUP(0RUP200)(Angelier,1990).

Site Numberoffaults s1 s2 s3 PHI ANG RUP Trend Plunge Trend Plunge Trend Plunge

Champourcin 8 79 10 201 71 347 16 0.16 21 42 Bonnette 21 50 0 140 0 249 90 0.29 10 32 Chauvet 18 98 1 189 5 357 85 0.60 8 20 Meuniere 25 242 2 332 6 134 83 0.34 10 38 Rochassas 23 66 2 335 16 162 74 0.32 15 47 GD.Colle 20 250 3 150 71 341 19 0.08 11 32 LaFonse 21 45 4 315 4 177 85 0.01 11 34

et al., 2011). Another segment of this paleo-drainage network is present to the northwest of Chauvet at the Escuichie`repass(Fig.3).Therethethalwegofthe paleo-Be`sriverisat1200melevationandwasupliftedby345m (Figs. 3 and 6). This area of the Digne thrust sheet is thereforecharacterizedbythehighestupliftraterecorded bythepaleo-drainagenetworkofDigne(between0.10and 0.13mm/yr).

Theupliftofthepaleo-thalwegattheEscuichie`repass confirmsthatthestrike-slipmotionontheBe`sfaultisnot accommodated north of this site by the Digne thrust. Effectively,beneaththeEscuichie`repass,theDignethrust dips tothesouth,and asouthwarddisplacementofthe Digne thrust sheeton this south-dippingthrust contact would have loweredtheelevationof thepaleo-thalweg (Fig.6).

Fig.3.Quaternarystressfielddeducedfromthecalculatedorientationofthes1axis.Doublearrow=calculatedtrendofs1.Ingreenthefaultsprobably

activeduringtheQuaternary(Be`sfault,EsclangonandTanaronblindthrusts).Thetectonicupliftofthesegmentsofpaleo-thalwegintheDignethrustsheet iscomputedtakingintoaccounttheelevationoftheBle´onepaleo-thalwegintheValensolebasinandthemodernriverprofiles.Thestressdeviationsare compatiblewiththedextralstrike-slipmovementoftheBe`sfault.Thediagramshowsthebestfittingstressmodelfromfocalplanemechanismsofthe DigneareausingtheexactmethodofGephartandForsyth(1984)andtheaveragemisfitvaluesassociatedwitheachconfidencelimit(Hippolyte,2001). Fig.3.ChampdecontraintesQuaternairede´duitdel’orientationdes1calcule´ea` partirdeplansstrie´s.Doublefle`che=directiondes1.Envert,lesstructures

probablementactivesauQuaternaire(failleduBe`s,chevauchementsaveuglesd’EsclangonetdeTanaron).Lesoule`vementtectoniquedessegmentsde pale´o-thalwegdanslanappedeDigneestcalcule´ parrapportaupale´o-thalwegdelaBle´onedanslebassindeValensoleeta` lapentedesrivie`resactuelles. Lesde´viationsdedirectionsdecontraintessontcompatiblesaveclemouvementdextredelafailleduBe`s.Lediagrammepre´senteunre´sultatd’inversion desme´canismesaufoyerdese´ismesdelare´giondeDigne(Hippolyte,2001)enutilisantlame´thodeGephartetForsyth(1984).

J.-C.Hippolyteetal./C.R.Geoscience344(2012)205–213 210

Inthisarea,theDignethrustsheethasbeenanticlinally foldedandtheerosionofthisanticlinehasproducedthe Barleshalf-window(Fig.3).Theupliftofthepaleo-canyons ismuchprobablyrelatedtothisfolding(Fig.6).However, thisanticlineandatectonicwindowpartlyexistedinthe Upper Pliocene-Gelasian because the alluvial deposits infilling the canyons contain clasts of Oligocene rocks eroded from the autochtonous of the Barles area, in particularattheEscuichie`resitethatisasegmentofthe paleo-Be`s canyon (Hippolyte et al., 2011; Jorda, 1970, 1982;Fig.1).Moreover,thesouthernlimboftheBarleshalf windowisjuxtaposedacrosstheBe`sfault,withasyncline of theDignethrust sheet(Figs. 1and 6). Therefore,the Barles anticline is a complex structure that cannot be explainedbytheactivityofasinglesubsequentthrust.We proposeastructuralmodeltoexplainhowtheQuaternary strike-slipmotionontheBe`sfaultistransferredtoupliftin theEscuichie`rearea.

AtChauvet,areversefaultbroughttheLiassiclimestone above the Upper Pliocene-Quaternary alluvial deposits (Fig.5).ThisfaulttrendsNW-SEanddipstothenortheast (Fig. 5). Along its strike, Le Ve´lodrome syncline is a complexstructurewithtwofoldaxeseast-westand NNW-SSE(Fig.1)probablyresultingfromtwophasesoffolding (GidonandPairis,1992).ItsNNW-SSEtrendcorresponds totheeasternlimboftheEsclangonsyncline(Gidonand Pairis,1992)butalsotothewesternlimboftheEsclangon anticline(Fig.6).ThelocationoftheEsclangonanticline, beneaththesoutheasternpartoftheBarlesanticlineinthe Dignethrustsheet(Fig.6),togetherwiththeNW-SEtrend ofthereversefaultatChauvet(Fig.5), suggeststhatits formationoritsreactivationhasparticipatedtotheuplift ofthepaleo-canyons(Fig.6).

To takeinto accountthestress perturbationandthe terminationofthedextralmotion ontheBe`sfaultinLe Ve´lodromearea,thetrendandstructureoftheEsclangon anticline,therecentNW-SEreversefaultatChauvet,and thehighestQuaternaryupliftoftheDignethrustsheetat

theEscuichie`repass,weproposethat,tothenorth,apartof thedextralslip oftheBe`sfaultistransferredtoablind thrust below the NNW-trending Esclangon anticline (Figs.3and6). Wenametheunitthattiltedtheeastern edgeofLeVe´lodromesyncline(Fig.5)theEsclangonhorse. Itsemplacementwaspartlyaccommodatedbyapassive roof thrust as suggested by NW-trending disharmonic foldsintheEocene-Oligocenesandstoneandmarls(Fig.6). Our model does not exclude the activity of another blindthrustthatmayparticipatetotheaccommodationof the Late Pliocene-Quaternary 2.3km right lateral dis-placementoftheBe`sfault(at>0.7mm/yr,Hippolyteetal., 2011).WestoftheBe`sfault,amajorrecentthrusthastilted the Miocene series of the Ve´lodrome syncline and the overlyingDignethrustsheet,andhascreatedtheLaRobine syncline (Fig. 6). We name this deep-seated NW-SE structuretheTanaronhorse(Fig.6).Itwasemplacedafter the deposition of the Valensole-I conglomerate and Tanaron Formation (after the Messinian, Fig. 6), and probably partly existed in the Upper Pliocene because thepaleo-Be`sriverincisedtheOligocenesandstoneofthe Barleshalf-window(Fig.1).ItmaybranchtotheWeston the rightlateral Monges fault (Gidon and Pairis, 1992; Fig.3).

Fig. 5.Tectonic superposition of Liassic limestones on the Upper-Pliocene-Quaternary alluvialdeposits of theBle´one paleo-canyon at siteChauvet(northernborderofthepaleo-canyon,Fig.3).Striationswere notpreservedatthisplaceandthereversefaultslipsshowninthefault diagramaremeasuredontheothersideofthisoutcrop(Fig.4). Fig.5. FailleinverseportantlescalcairesduLiasdelanappedeDignesur lesalluvionsduPlioce`nesupe´rieur-Quaternairedupale´ocanyondela Ble´onea` Chauvet(bordurenorddupaleo-canyon,Fig.3).Lesstriesn’e´tant pasconserve´esa` cetendroit,lesplansdefailleinversepre´sente´ssurle diagrammeonte´te´ mesure´sdel’autrecoˆte´ del’affleurement(Fig.4). Fig.4.Viewlookingtothesouthwestofthe100mthickalluvialdeposits

atChauvetontheBarlesanticline(Fig.3).Thealluvialdepositsaretilted about108towardthesoutheast.Theyareabout100metersthickandthe elevationofthepaleo-thalwegis1020meters.

Fig.4.Vueverslesud-ouestdel’affleurementdeconglome´ratsdusite Chauvetsurl’anticlinaldeBarles(Fig.3).Lesalluvionssontbascule´es d’environde108verslesud-est,ellessonte´paissesdepre`sde100me`tres etlepale´o-thalwegesta` 1020me`tresd’altitude.

The Tanaron horse, with its passive roof thrust, transferredtherightlateraldisplacementoftheBe`sfault tothe west (Fig.3). The two horsesparticipated tothe upliftedbymorethan345meterstheDignethrustsheetand thepaleo-canyonsabovetheBarlesAnticline(Figs.3and6). 5. Conclusion

Paleostressanalysisfromstriationin Upper Pliocene-Quaternaryalluvialdepositsallowedustocharacterizethe Quaternary stress field of an oblique strike-slip thrust front.Thedeformationoftherecentdepositsismonophase butischaracterizedbystressperturbations.Weinferthat thevarious trends of compression determined in some previousfaultstudiesinolderrockformationsalongthis dextral-reverse thrust front, might result from stress perturbationsandnotnecessarilyfromsuccessivephases of the Digne thrust sheet emplacement. The NNE-SSW trendofcompressionfoundbyFournieretal.(2008)inthe Valensole-IconglomerateoftheVe´lodromesynclinecould berelatedtotheformationoftheBarlesanticlineandthe emplacementoftheTanaronhorse.Despitethelarge(538) deviation of the trend of compression near Digne,

paleostress analysisis founduseful torevealtherecent activityoftheBe`sFaultandofdeepintercutaneousthrust wedges, the Esclangon and the Tanaron horses. The proposedstructuralmodelallowstakingintoaccountall the characteristicsof theQuaternarydeformation along theDignethrust:locationofthedeformation,variationin thetrendsofcompression,highestupliftrateofthe paleo-canyons.Inconclusion,thisstudyunderlinestheinterest of combining fault kinematics studies in recent rock formationswithregionalstructuralanalysesto character-izetherecentgeodynamicsoflowdeformationrateareas. Acknowledgements

ThanksgotoGeorgesClauzonforconstructive discus-sions.WearethankfultoDr.DamienDelvauxandPr.Marc Fournierfortheirconstructivereviews.

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