Morphology of retrogressive failures in the Eastern Rhone interfluve during the last glacial maximum (Gulf of Lions, Western Mediterranean)

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Morphology of retrogressive failures in the Eastern

Rhone interfluve during the last glacial maximum (Gulf

of Lions, Western Mediterranean)

Shray Badhani, Antonio Cattaneo, Bernard Dennielou, Estelle Leroux, Florent

Colin, Yannick Thomas, Gwenael Jouet, Marina Rabineau, Laurence Droz

To cite this version:

Shray Badhani, Antonio Cattaneo, Bernard Dennielou, Estelle Leroux, Florent Colin, et al..

Mor-phology of retrogressive failures in the Eastern Rhone interfluve during the last glacial

maxi-mum (Gulf of Lions, Western Mediterranean).

Geomorphology, Elsevier, 2020, 351, pp.106894.

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Geomorphology351(2020)106894

ContentslistsavailableatScienceDirect

Geomorphology

j o ur na l h o me p a g e :w w w . e l s e v i e r . c o m / l o c a t e / g e o m o r p h

Morphology

of

retrogressive

failures

in

the

Eastern

Rhone

interﬂuve

during

the

last

glacial

maximum

(Gulf

of

Lions,

Western

Mediterranean)

Shray

Badhani

a,b,∗

_,

_Antonio

_Cattaneo

a

_,

_Bernard

_Dennielou

a

_,

_Estelle

_Leroux

a

_,

Florent

Colin

a,b

_,

_Yannick

_Thomas

a

_,

_Gwenael

_Jouet

a

_,

_Marina

_Rabineau

b

_,

_Laurence

_Droz

b a_IFREMER,_Unité_de_Recherche_Géosciences_Marines,_Centre_de_Bretagne,₁₆₂₅_Route_de_Sainte-Anne,_29280,_Plouzané,_France

b_Univ._Brest,_CNRS,_Univ._{Bretagne-Sud,}_Laboratoire_Géosciences_Océan,_UMR6538_-_IUEM,_rue_Dumont_d’Urville,_F-29280,_Plouzané,_France

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received28May2019

Receivedinrevisedform1October2019 Accepted2October2019

Availableonline15November2019

Keywords: Submarinelandslide Masstransportdeposits GulfofLions

WesternMediterranean Turbiditiclevees

a

b

s

t

r

a

c

t

TheGulfofLions(NWMediterraneanSea)isaSW-NEorientedpassivecontinentalmarginformedsince theOligocene.Itpresentssmalltolargescalemassmovementfeaturessuggestingalonghistoryof seafloorinstability.Ofparticularinterestarethetwosurficiallargemass-transportdepositsalongthe Rhoneturbiditiclevee,knownastheRhoneEasternandWesternMass-TransportDeposits(REMTDand RWMTD).Withthehelpoftherecentlyacquiredmulti-beambathymetric,sub-bottomprofiler, high-resolutionseismicandsedimentologicaldata,weinvestigatethemorphology,timing,kinematics,and possibletriggeringmechanismsofthesourceareaoftheREMTD,whichwerefertoastheEasternRhone InterfluveSlide(ERIS).ERIShasanestimatedrun-outdistanceofapproximately200km.Itcoversanarea ofabout700km2_and_the_volume_of_the_mobilized_material_is_{approximately}₁₁₀_km3_._Our_data_reveal

fourindividualglideplaneswithintheERIScomplexwhichweremostlikelygeneratedby retrogres-sivefailures.ThebasalsurfacesoftheERIScoincidewithhigh-amplitudeseismicreﬂectorssimilarto thosepreviouslyinterpretedastheexpressionofcondensedsectionsontheupperslope.Theturbiditic sequencessandwichedbetweenthecondensedsectionslikelycontrolthelocalisationofpotentialweak layersfavouringthefailures.AMSradiocarbondatingyieldsanageofapproximately21kacalBPforthe failures,whichfallswithinthepeakoftheLastGlacialMaximum.ThetoeareaoftheERISisincisedby severalactivelistricfaultsrootedintheMessinianstrata,whichcontrolthelocationoftheslidescarps. Thecombinationofseveralfactorssuchasslopesteepening,halokinesis,andexcessporepressure gen-erationduetorapidturbiditicsedimentationduringtheLastGlacialMaximumareconsideredasthe possiblecandidatesforthetriggeringofthefailuresintheinvestigatedslope.

1. Introduction

Submarinelandslidesareoneofthewidespreadphenomena knowntoshapethecontinentalmargins(PosamentierandKolla, 2003).Theyredistributelargevolumesofsedimentfromtheshelf or theupperslopes tothe lowerslope or deepbasin. Further-more, theycan befar largerthan any terrestrial landslide and threaten offshoreinfrastructure. Most importantly,theypose a risktoincreasinglypopulatedcoastlinesbytriggeringdevastating tsunamis(Brynetal.,2005,2003;Kvalstadetal.,2005;Tappinetal.,

∗ Correspondingauthorat:IFREMER,UnitédeRechercheGéosciencesMarines, CentredeBretagne,1625RoutedeSainte-Anne,29280,Plouzané,France.

E-mailaddress:shray.badhani@ifremer.fr(S.Badhani).

1999).Themostrecentexampleofasubmarinelandslidetriggered tsunamicametopublicattentionwhenanearthquakeof magni-tudeMw7.5mostlikelycausedasubmarinelandslideoffSulawesi, Indonesia.Thisresultedinatsunamiwithawaveheightofabout sevenmetres,killingmorethan4000peopleanddisplacingabout 200,000inhabitants(Carvajaletal.,2019;Heidarzadehetal.,2018;

SassaandTakagawa,2019).

Submarine landslidescan disintegrate and transport a large amountofslope-formingsediments,wheretheshearfailureoccurs along oneorseveralplanes (Eckel,1958;Hamptonetal.,1996;

SchusterandKrizek,1978).Duetotheirlocationsindeepwater set-tings,directobservationofsubmarinelandslidesandtheirdeposits isdifficult.Therefore,geophysicaltechniquesnamelymultibeam bathymetry,reflectionseismicareusedtoinvestigatesubmarine landslides.Inreflectionseismicdatasubmarinelandslidedeposits

https://doi.org/10.1016/j.geomorph.2019.106894

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2 S.Badhani,A.Cattaneo,B.Dennielouetal./Geomorphology351(2020)106894

aregenerallyreferredtoasmasstransportdeposits(MTDs)and masstransportcomplexes(Alves,2015;FreyMartinezetal.,2005). MTDsusuallycoverareasexceedingthousandsofsquare kilome-tresandhavevolumesinhundredsofcubickilometres,depositing slidematerialhundredsofkilometresfromtheirsourceareas,on thecontinentalshelforupperslope,ontothedeepoceans(Bull etal.,2009;FreyMartinezetal.,2005;Geeetal.,2006;Massonetal., 2006;McAdooetal.,2000;Shippetal.,2004;Solowetal.,2009). Inspiteofthelargesizesandvolumesofsedimentsinvolvedinthe failureevents,theirlinkwithlocalorglobalforcingisstillamatter ofdebate(Hamptonetal.,1996;HühnerbachandMasson,2004;

Lee,2009;Popeetal.,2015;Solowetal.,2009;Sultanetal.,2004;

tenBrinketal.,2009;UrgelesandCamerlenghi,2013;Urlaubetal., 2013).However,variousauthors(e.g.LocatandLee,2002and ref-erencestherein;Sultanetal.,2004;Vannesteetal.,2013)suggest thattheiroccurrenceiscommonlycontrolledbyoneora combina-tionoffavourablepreconditioningfactorsandtriggermechanisms suchasslopeover-steepening,earthquakes,storm-waveloading, rapidsedimentaccumulationandunder-consolidation,gas charg-ing,gashydratedissociation,lowtides,glacialloading,andvolcanic islandprocesses.

TheGulfofLions(GoL)presentstwolargesurﬁcialMTDswith volumesexceeding100km3_._These_MTDs_have_been_called_by

sev-eralnamesinpreviousstudies,suchas“EasternandWesternDebris Flows”(Bonneletal.,2005;Drozetal.,2001;Lastras,2007)or “East-ernandWesternMTDs”(Drozetal.,2006),ormorerecently“Rhone EasternMass-TransportDeposit(REMTD)”and“theRhone West-ernMass-transportDeposit(RWMTD)”(Dennielouetal.,2019).To allowaneasycorrelationandcomparisonbetweenthetwostudies, wefollowthenomenclatureofDennielouetal.(2019).

TheREMTD andRWMTD have previouslybeenmapped and describedindetail(Dennielouetal.,2019;Drozetal.,2006;Droz andBellaiche,1985);however,duetothelackofhigh-resolution dataontheuppercontinentalslopeoftheGoLmargin,little atten-tionhasbeenpaidtothesourceoftheREMTD.Thisstudytherefore focuses onanalyzing and characterizingthe sourceareaof the REMTD,whichwehereafterrefertoasthe“EasternRhone Inter-fluveSlide(ERIS)”.TheERISandREMTDtogetherformtheEastern RhoneInterfluveSlideComplex(ERISC).Wepresentforthefirst timeadetailedmappingoftheERISCwiththecombinationof high-resolutionmultibeambathymetry,high-resolutionmulti-channel seismic,sub-bottomprofiler,andnewlyacquiredsedimentcore datafromtheERISC.Weaimto1)identifythemorphologicaland sedimentologicalcharacteristicsofERIS,2)establisha chronolog-icalframeworkforthefailedsedimentaryunitsbasedondirect andindirectdatingmethods,3)identifythepresenceofpotential weaklayers,and4)identifypossibletriggermechanisms respon-siblefortheoccurrenceoftheERIS.Thedetailedmappingofthe ERISCiscomplementarytothesubmarinelandslidearchivethat currentlyexists for the Mediterranean region(e.g. Urgeles and Camerlenghi,2013).Evenifthecollectionoflandslideparameters isnotalwaysconducivetofullyunderstandthetriggering mecha-nismsofseafloorinstabilities(e.g.,HühnerbachandMasson,2004),

Clareetal.(2018)suggestthatthewideningoflandslidedatabase isakeytoimproveknowledgeandanimportantcontributionto tackleoutstandingscientiﬁcquestionsonhazardmitigation, espe-ciallyifthemeasuredparametersareconsistentlyacquired.

2. Geologicalsetting

TheGulfofLions(Fig.1)isaSW-NEoriented,youngpassive continental margin that formed after theOligocene time, with therotationofCorso-Sardinianblock,inacontextofconvergence betweenAfricaandEurope(LePichonetal.,1971).Duetoits lim-ited lateralextentand easy accessibility, theGoL is oneof the

best-studiedpassivemarginsintheworld(BernéandGorini,2005;

Séranne,1999;StecklerandWatts,1980).Themarginis charac-terisedbyawidecontinentalshelf(∼70km)thatwassub-aerially exposedduringtheLastGlacialMaximum(LGM)about21kaago andoffersacontinuoussedimentaryrecorddrilledontheupper slopebythePROMESSboreholes(Bassettietal.,2008,2006;Berné etal.,2007;Jouet,2007;Rabineauetal.,2005).Beingapassive mar-gin,earthquakesgreaterthanfourMwhaveneverbeenrecorded intheGoLsincerecordsexist(Caraetal.,2015).Thepresent-day shelfbreakoftheGoLoccursatwaterdepthsrangingfrom120to 170m.FifteensubmarinecanyonsfeedthedistalRhone Sedimen-tarySystemandincisetheGoLcontinentalslopedownto1000m waterdepth,themajoronebeingthePetitRhoneCanyon(PRC), whichisfedbyalpinesedimentsthroughthelargestriverRhone anditstributaries.

Thesedimentaryarchitectureof theGoLdrasticallychanged duringamajoreventknownastheMessinianSalinityCrisis,when amajorsea-leveldropintheMediterraneanSearesultedina km-scalethickevaporitedepositioninthedeepbasin(Hsu,1973).After refloodingofthebasinduringtheZanclean(5.33Ma;Lofietal., 2003;Roverietal.,2008),aprogradingandaggradingsedimentary prismofupto3kmthickdevelopedalongwithalargesubsidence (upto250m/Maontheshelf,Dennielouetal.,2019;Lerouxetal., 2014;Rabineauetal.,2006,2014andupto500m/Mainthedeep basin,Lerouxetal.,2015;Rabineauetal.,2014)throughoutthe Plio-Quaternarytimetoformthepresent-daymarginmorphology (Dennielouetal.,2019;Lerouxetal.,2014).Sincethen,the evo-lutionofthedeeperpartoftheGoLhasmainlybeencontrolled bysyn-sedimentarysalttectonicsandgravity-drivendeformation ofpost-Plio-Quaternarystratatunedbyrelativesea-level fluctua-tions.Thegravity-drivendeformationabovethesaltlayer,which formsa“décollement”surface,ledtothedevelopmentof exten-sionalareasdominatedbylistricfaultsandgrabensontheupper andlowerslope(Fig.1)andcompressionalareasinthedeepbasin drivenbysaltdiapirsanddomes(dosReisetal.,2005).

ThePleistocenesedimentationintheGoLwasmainlycontrolled byglacio-eustaticsea-levelﬂuctuationsrelatedtotheMilankovitch cyclicities(Bassettietal.,2008;Torresetal.,1995;Rabineauetal., 2005, 1998; Tesson and Gensous, 1998). During a major Qua-ternaryreorganizationcalledtheMid-PleistoceneTransition,the dominantclimatecyclicitychangedfrom40kato100kain fre-quency(DrozandBellaiche,1985;PenaandGoldstein,2014).These cycleswereaccompaniedbya changein sea-levelcycle ampli-tudesfrom50mto100m(Drozetal.,2006;PenaandGoldstein, 2014).AftertheMid-Pleistocene,sedimentationin theGoL was mainlydominatedbyterrigenousdepositionbroughtbytheAlpine rivers(mainly Rhone)intothebasinthroughturbiditicsystems. IntheLatePleistocene,theevolutionofthemarginandcanyons wasdrivenbysea-levelﬂuctuationswiththedevelopmentofthick forced-regressivesequencesduringsea-levelfallsboundedonthe slopebycondensedintervalsdepositedduringhighstands(Bassetti et al., 2006; Rabineau et al., 2006; Sierro et al., 2009).On the lowerslopeoftheGoL,whichismainlyfedbythePRC, sedimenta-tionratesfortheglacialanddeglacialperiodsweredocumented in theorder of 1−2m/ka,while during theinterglacial periods thesedimentationrates wereintherange of 0.05to0.10m/ka (Dennielouetal.,2019;LomboTomboetal.,2015;Sierroetal., 2009).

3. Materialandmethods

Multi-beambathymetricdatawerecollectedintheGoLsince 1997duringseveraloceanographiccampaignslistedinTable1.In thisstudy,weusedtwobathymetricdatasets:anEMODNET115m resolutiongrid(Fig.1;www.emodnet-bathymetry.eu,2018)and

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S.Badhani,A.Cattaneo,B.Dennielouetal./Geomorphology351(2020)106894 3

Fig.1.EMODNETbathymetricmapofGulfofLionsmarginhighlightingthedistributionofmainsedimentaryandmassmovementfeatures(blackdashedlines:sedimentary deposits;whitelines:MassTransportDeposits;greenlines:thalwegofmainsubmarinecanyons;blueline:shelfbreak;solidblackline:headwallofsubmarinelandslides; bluedashedline:upslopelimitofMessinianevaporites;reddashedroundedpolygon:raftdomaindominatedbylistricfaults;yellowdashedpolygon:compressionarea dominatedbysaltdiapirs).GulfofLionscanyons(Bernéetal.,2002):CC:CapdeCreus,LD:LacazeDuthier,P:Pruvost,Bc:Bourcart,He:Herault,S:Sète,CL:Catherine Laurence,M:Marti,PR:Petit-Rhone,GR:Grand-Rhone,Ms:Marseille,PL:Planier,C:Cassidaigne.REMTD:RhoneEasternMassTransportDeposit,RWMTD:RhoneWestern MassTransportDeposit.LimitsofsedimentarybodiesarefromDrozetal.(2006);MessinianevaporitelimitsarefromdosReisetal.(2005).

Table1

Oceanographiccampaignsanddatausedinthisstudy.

Cruise Year Vessel Data/attributes Reference

PRISME2 2013 L’Atalante EM122,710,HRseismic,3.5kHzSBP (Cattaneo,2013b)

PRISME3 2013 Pourquoipas? 3.5kHzSBP,Calypsopistoncores (Cattaneo,2013a) AM-MED-1 2013 LeSuroit EM300,HRseismic (Rabineauetal.,2013) SEEPGOL 2007 LeSuroit EM300,2–5.2kHzSBP (RabineauandAslanian,2007) PROGRESS 2003 LeSuroit EM300,HRseismic,2–5.2kHzSBP (Droz,2003)

CALMAR97 1997 L’Atalante EM12 (Loubrieu,1997)

GMO1 2001 LeSuroit EM300 (Cochonat,2001)

GMO2-CARNAC 2002 LeSuroit EM300 (SultanandVoisset,2002)

MARION 2000 LeSuroit EM300 (Berné,2000)

RHOSOS 2008 LeSuroit EM300 (BernéandDennielou,2008)

recent25–100mresolutionbathymetricgrids(Figs.2aand3a,b) thatwereacquiredusinghull-mountedmultibeamsystemsduring theoceanographiccampaignslistedinTable1.Thehigh-resolution bathymetricdataweremergedandgriddedataﬁnalgridsizeof 25musingArcMap10.3(www.arcgis.com).

High-resolution (HR)multi-channel seismic(MCS) reﬂection data usedin this studywere acquired usinggenerator-injector (GI)source,analogue24channelsstreamerand6.25mtrace spac-ingduringPROGREScruise(Droz,2003)andamini-GIsourceand digital72channelsstreamerwith6.25mtracespacingduring

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AM-4 S.Badhani,A.Cattaneo,B.Dennielouetal./Geomorphology351(2020)106894

Fig.2.a)High-resolutionmultibeambathymetricmapofthesourceareaoftheEasternRhoneInterfluveSlideComplex(ERISC)highlightingmultipleretrogressiveheadwall scarpsandassociatedsidewallscarpsmarkedwithdashedblacklines.RedcirclesrepresentthelocationsofsedimentcoresacquiredduringPRISME3cruise.Solidblacklines representlocationsoftheseismicprofilesshowninFigs.3–5a,band7.BlackrectanglesrepresentlocationsandextentofFigs.2band3a,b)Seafloorgradientmapofthe studyarea.c)BathymetricprofileA-A’illustratesthelocationsoffourindividualheadwallsandtheNorthernGrabenandd)BathymetricprofileB-B’showsmorphologic expressionsofthefaultscarpstowardstheeasternpartofthestudyarea.

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Fig.3.a)Three-dimensionalviewofthetoeareaoftheERISshowingmorphologyofthefaultgrabensandasmallslidelobelikelyoriginatingfromtheleveecollapse.Vertical exaggeration10×.b)high-resolutionbathymetricmapofseabedmorphologysurroundingtoeareaoftheERISshowingareasdominatedbyseaﬂoorspreading,collapseof PetitRhoneCanyonleveesandseveralscarpsrepresentingactivefaultsaswellasgrabens.

MED-1cruise(Rabineauetal.,2013).Theprocessingsequencefor the2DHRMCSdata includesqualitycontrol,filtering,binning, band-passfiltering,constantseawatervelocity(1500ms−1)normal moveout,stacking,andseawatervelocitypost-stacktime migra-tion. Seismicdata fromcruises PROGRES and AM-MED-1were processedusing Ifremerin-housesoftwaresSISPEED and Solid-QC,respectively.Sub-bottomprofiles(SBP)werecollectedduring

severalFrench oceanographiccampaignsand checked for qual-itycontrolwithIfremersoftwareQC SUBOP(Table1).Atypical problemwhenimaging submarinelandslidesisthepresenceof diffractionhyperbolaegeneratedattheedgesoflandslidescarps andsteepslopes(seeforexampleFig.4,in2DHRMCS,andFig.4

Insets1and2inSBP)and/orwithinMTDsduetothepresenceof scatteredblocks(Fig.5bin2DHRMCS;Fig.5aInsetinSBP).

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Seis-6 S.Badhani,A.Cattaneo,B.Dennielouetal./Geomorphology351(2020)106894

Fig.4. a)Uninterpretedandb)interpreteddipseismicprofileAM-MED018(NNW-SSE)crossingtheeasternpartoftheERIS.Itillustratesthesharptruncationoftheheadwall scarps,thesurficialMTD(REMTD)andtwoburiedMTDs-shallowMTDanddeep-seatedMTD(highlightedwithblueandgreencolour,respectively).Insets1and2show blow-upsofheadwallscarsHS2andHS3inthesub-bottomprofilesmarkedbysharptruncationsofwell-stratifiedsedimentsoverlainbytransparentreflectionsinterpreted asMTD.Solidblacklinesrepresentprominentfaultsimagedintheseismicprofile.Theseismicstratigraphyofthestudyareaishighlightedbysevenkeyreflectors(H1toH6) delineating7seismicunits(U1toU7).ThesekeyreflectorsandseismicunitsareshownwiththesamerespectivecoloursandnamesinFigs.5,6,7cand8.Notethereversed polarityofhorizonH4withrespecttotheseafloor.SeeFig.2forthelocationoftheprofile.

micprofilescollectedduringAM-MED-1cruiseshowlowfrequency andlowamplitudeartefacts40msbelowtheseafloor,particularly intheareasoftransparentreflections(Fig.6).Theseartefactsare commoninmarineseismicacquisitionandareassociatedto bub-blepulseartefactsthatareproducedduetooscillationsoftheair bubblegeneratedbyanairgun.Wehavepaidaspecialattentionto theseartefactstoavoidmisinterpretationofseismicdata.

For lithological characterization and age estimation of sedi-mentaryfacies, we used four sediment cores of 10–20m long (PSM3-CS041,42, 46 and47),collectedduringcruise PRISME3 (Cattaneo,2013a)inthetoeareaoftheERISusingaCalypso Pis-toncorer.Identiﬁcationoflithofaciesisbasedonvisualdescription performedonboard.Geotek®Multi-SensorCoreLogger(MSCL)was usedforgamma-densitymeasurementsat1-cminterval.Toobtain accurateages of theslide eventsbuildingupthe REMTD,each corewascarefullysubsampledinthepost-slidehemipelagicdrape

toavoid contaminationfrom reworked sediments,suchas tur-biditicdepositsorbioturbation.Hence,thesesamplesprovidea minimumageforfailure.Nineradiocarbondatesweredetermined by accelerator mass spectrometry (AMS). Dates were obtained fromhandpickedmonospeciﬁcplanktonicforaminifera (Globige-rinabulloides)extractedfromthehemipelagicsedimentsandwere analyzedattheBetaAnalytics,London,UK.Conventional14_C_years

BPwereconvertedintocalibratedcalendarageswithCALIBV7.1 (Stuiveretal.,2017)usingthemarinecalibrationcurveMarine13 curve(Reimeretal.,2013)withareservoircorrection(R)valueof 48±101yearsandreportedinthisstudywith2␴errors(Table2). Weusedthesamecalibrationprocedurepresentedin(Dennielou etal.,2019)toallowforan easycomparison.Alldatasetswere ﬁnallyintegratedandimportedtotheIHSKingdomSoftware®for furtherjointinterpretation.

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Fig.5.a)StrikeseismicprofileAM-MED17(orientedWSW-ENE)crossingthesourceareaoftheERISC(initsproximalpart).Theprofileimagesthesidewallsofheadwallscar HS2showingtruncationofseismicunitU2,theShallowMassTransportDeposit(SMTD,blueroundedpolygon,betweenH3andH4)andtheDeep-SeatedMassTransport Deposit(DSMTD,greenroundedpolygon,below2.2stwtt).Theleftpartoftheprofileshowsatransitionofwell-stratifiedsedimentstochaoticreflectionswithintactinternal reflectionsrepresentingseafloorspreadingaboveH1.Theinsetshowscoincidingsub-bottomprofileshowinghummockymorphologywithreflectionhyperbolae,b)NW-SE orientedstrikeperpendicularseismicprofileAM-MED04showsasmall-scalefailurewithinseismicunitU5.SeeFig.2forthelocationoftheline.

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Fig.6. SW-NEoriented,strikeseismicprofileAM-MED19crossingthetoeareaoftheERISCabout25kmdownslopeofAM-MED17.Theprofileimagessharptruncationof well-stratifiedseismicunitsatseverallevelsindicativeoffailuresurfaces.NotethattheSMTDisnotimagedintheprofilesuggestingashortrunoutofthislandslide.The DSMTDislimitedtotwopatches(markedbythedashedgreenline)of50−100msthickness.Theeasternandwesternpartsoftheprofilearedisturbedbynormalfaults.See

Fig.2forthelocationoftheproﬁles.

Table2

Radiocarbondatingcarriedoutonsedimentcores.

Core-section-depth(m) Labreference 14_C_age _Calibrated_age_(2␴)a _Sampled_material

PSM3-CS042-S01-0.15 508495 13390+/-50BP 15825-15140 G.bulloides PSM3-CS042-S03-2.4 508496 16990+/-60BP 20264-19612 G.bulloides PSM3-CS042-S04-3.085 508497 17450+/-50BP 20818-20171 G.bulloides PSM3-CS042-S04-3.55 508498 18200+/-60BP 21835-21103 G.bulloides PSM3-CS046-S06-5.04 508499 16240+/-60BP 19385-18,792 G.bulloides PSM3-CS046-S07-6.94 510578 17770+/-60BP 21262-20571 G.bulloides PSM3-CS046-S08-6.965 508501 18150+/-60BP 21785-21038 G.bulloides PSM3-CS047-S07-6.10 510579 17120+/-50BP 20446-19800 G.bulloides PSM3-CS047-S08-7.6 508503 17380+/-50BP 20710-20090 G.bulloides

a_Calibrated_calendar_ages_were_converted_from_conventional14_C_years_BP_with_CALIB_V7.1₍_Stuiver_et_al.,₂₀₁₇₎_using_the_marine_calibration_curve_Marine13_curve₍_Reimer etal.,2013)withareservoircorrection(R)valueof48±101yearsandreportedhereas2␴ages.

4. Results

4.1. Morphology

4.1.1. Overallmorphologyofthestudyarea

Theslopeofthecontinentalmargindipsprogressivelyfrom6◦ to1◦forwaterdepthsleadingfrom200mto1500m,respectively. Ourstudyarea,theinterﬂuveofthePRCandGrandRhoneCanyon (GRC),is characterisedbya gentleslope of1.8◦ and anareaof about1000km2_comparable_to_an_open_slope_environment._The

newly-acquiredmulti-beambathymetricdatashowtheseafloor morphologyoftheinterfluveareaingreaterdetailthanwhathas beenpublishedpreviously(Bernéetal.,2002;DrozandBellaiche, 1985; Torres etal.,1995).Our datashowmultipledistinct fea-turesrelatedtomass-wastingprocessessuchasslidescars,faulting activity,orseafloorspreading(Figs.2and3b).Themostprominent featuresinthestudyareaaretheNNW-SSEorientedlobateand trapezoidal-shapedscarpsinthebathymetry(blackdashedlinesin

Fig.2)thatweinterpretasasuiteofsubmarinelandslideswithin theEasternRhoneInterﬂuveSlideComplex.

Atawaterdepthof1750m,withinthetoeareaof theERIS, twoSW-NEorientedlineardepressionsincisetheseaﬂoor(Figs.2a, band3),previouslyinterpretedasnormalfaults(dosReisetal., 2005).However,weobservethatthesedepressionsaremarked bysigniﬁcantlylow-standinghangingblock(∼30−40m)bounded bynormalfaultsthatfaceoppositetoeachother(Figs.2aand3). Therefore,weinterpretthesedepressionsasfaultgrabensandrefer tothemhereafterastheNorthernGraben(NG)andtheSouthern Graben(SG).TheSGisaboutsixkmwideandextendsover approxi-mately100kmintheSW-NEdirection,crosscuttingtheentirePetit Rhonefan.(Fig.3).TheNGshowsastrikesimilartothatofSG,but itislimitedtothewestbytheEasternleveeofthePRC.The north-ernscarpofSGdisplaysastep-likemorphologywithanetslipof about20−30m,however,thesouthernwallshowsgentlerslopes of∼5◦_as_compared_to_the_10-25◦_of_that_of_the_northern_wall._The

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Table3

Headwallheight,length,areaandvolumesofthedifferentfailurestepsoftheEasternRhoneInterﬂuveSlide(ERIS).

Headwall Headwallheight(m) Evacuationlength(km) Averagearea(km2₎ _Average_volume_(km3₎

HS1 60 38 650 39 HS2 30 25 380 15.2 HS3 50 20 350 31.3 HS4 70 16 140 9.8 SS 50 8 35 1.75 Easternextension 150 10 165 9.75 Total 106.8

30◦andcreatingalmostanarcuate-shapedgeometrycomparable

toaslopefailureontheeasternlevee(Figs.2a,dand3).

Addition-ally,itslocationcoincideswiththefootoftheERIS.TheNGshows amaximumnetoffsetof150mwhereitcoincideswiththeeastern leveeofthePRC,however,theaverageoffsetofthefaultsisabout 50mandtheaveragewidthisapproximatelysevenkm.

4.1.2. TheEasternRhoneInterﬂuveSlide(ERIS)

TheERISconsistsoffourmainheadwallscarps(namedHS1to HS4)andseveralsecondaryscarps(Fig.2a-c).Theaverageheight oftheheadwallscarpsrangesfrom30mto70m,butitexceeds 100mintheareawheretheslidecomplexmergeswiththegrabens, towardstheeasternleveeofthePetitRhoneCanyon.Theheadwalls arenestedinastaircasestyle,withevidenceoffourmajorand sev-eralminorslidescarpswithexposedglidesplanes(Fig.2a,c).This geomorphologicalpatternisreminiscentofretrogressivefailures suchasreportedintheStoreggalandslidecomplex(Baetenetal., 2014;Brynetal.,2005).ThesourceareaoftheERISexceeds700 km2_,_covering_almost₇₀_%_area_of_the_interﬂuve.

Theheadwallscarpsoftheslideshaveanirregulargeometry,at timesmimickingtheshapeofthepreviousscarps.Theuppermost headwalloftheslide(HS1)islocatedatawaterdepthof700m,only sevenkmawayfromtheshelf-break.Aminorfailurescarpobserved attachedtotheheadwallscarpHS1isinterpretedasasecondary failurescaroftheHS1.TheupperpartoftheHS1isonly2.5km wide,butitwidenstoabout25kmat1700mwaterdepth.The aver-agewidthoftheslideis20kmandtheslidedeposits(i.e.REMTD) aremappedasfaras200kmawayfromthesourcearea(Drozand Bellaiche,1985;Torresetal.,1997).Overall,aslopegradientof1.8◦ characterisestheRhoneinterﬂuve;however,theslidescarpsshow asteepmorphologywithlocalslopesreachingupto25-30◦.The lowermostheadwallscarpHS4islocatedat1700mwaterdepth, alignedperpendicularlywiththeNG.Itextends30kmtowardsthe GrandRhonesedimentaryridge,coincidingwiththeNGand show-ingahorseshoe-shapedlandslideheadwallmorphology(Fig.2a). Theaveragearea,heightofheadwallscarp,widthoffailureand volumeofmobilizedmaterialofeachslidescarparesummarized inTable3.Thetotalvolumeofmobilizedmaterialfromthesource areabasedonthegeometryofeachheadwallscarpandoftheir respectiveheightaccountstoapproximately110km3₍_Table₃_).

The deposit area of the ERIS (i.e. REMTD) is characterised by a smooth morphology. The sediment ﬂow pathways of the mass-transportaremostlyunnoticeableinthebathymetricdata, mostlikelydue tothepost-slidesedimentarydrape(Dennielou etal.,2019)andlackofhigh-resolutiondatainthedepositarea (Figs.1and2a).TheoverallﬂowdirectionoftheREMTDistowards thesoutheast(Droz,1983).Inthenewly-acquiredhigh-resolution bathymetry,weobservefewslideblocksof10m–25mheightand abulgeinthebathymetricdataindicatingminorpathwayofthe REMTDlimitedtothedistalreachoftheNG(Figs.2aand3a).At awaterdepthof2000m,southoftheSG,weobservea250-km2

plateauofundisturbedsediments,previouslydescribedas“butte témoin”byDroz(1983).ThisplateauislocatedbetweenthePRC (towardsthewest)andasteepscarpof80mheight(towardsthe east).Abouttwokmeastoftheplateau,weobserveanN–S

ori-entedlinearsedimentridge,whichweinterpretastheremnant ofthefailure.Atthewesternendsofbothgrabens,weobservea breachoftheeasternleveeofthePRC(Fig.3b),coincidingwiththe headwallscarHS2,suggestingthatasigniﬁcantamountof canyon-sourcedturbiditeshavemostlikelyoverﬂowedandbeendeposited togetherwithintheREMTD.

4.2. Sub-seaﬂoorarchitecture

4.2.1. Seismicstratigraphyofthestudyarea

Ourstudyfocusesonthemass-wasting processeswithinthe upper part of Plio-Quaternary sediments. In this section, we describe theidentified units and key reflectors that will letus discuss specificmass-wasting features we observein a relative stratigraphicframeworkusedinthisstudy.Foradetailed strati-graphicframeworkoftheGoL,werefertoDrozetal.(2006)and

Torresetal.(1995).

SevenseismicUnits(U1toU7,youngesttooldest,Figs.4–7) areidentifiedinthestudyareabasedontheirreflectionpattern andtheerosionalordepositionalnatureofcontactwithunderlying sequences.Thehigh-amplitudereflectors(H1-H7,toptobottom) attheirbasesdefinetheboundariesoftheunits,whichhavebeen observedandpickedextensivelythroughoutthestudyarea. Paral-lel,moderatetohigh-amplitudereflectionpackagesinthewhole studyareacharacteriseU1toU7.Theseismicunits,ingeneral,show adecreasingupwardtrendininternalreflectionamplitudes.The contactsbetweenseismicunitsaresmoothandregular,exceptU7, whichliesunconformablyontheunderlyingsedimentsshowinga roughmorphologyatitsbase.TheaveragethicknessofU2,U5,U6 andU7isabout50ms,whileforU1,U3,andU4itisabout100ms. Fourtypesofseismicfaciesareobserved intheseismic sec-tions (Table 4): 1) Moderate to high-amplitude well-stratified faciesinterpretedasturbiditicleveefacies(Drozetal.,2006),2) high-amplitudereflectionpackages(HARPs),similartotheones observedintheAmazonFan(Pirmezetal.,1997)interpretedhere ascoarse-grainedmaterialdepositedwithinpaleo-Rhonechannel thalweg(Damuth,1980;Drozetal.,2006),3)Chaoticreflections interpretedasslidedeposits(Drozetal.,2006)and4)Acoustically transparentfaciestypicalofREMTDandRWMTD(Dennielouetal., 2019;Drozetal.,2006)

4.2.2. Slidesandfailureplanes

ThedipseismicprofileAM-MED18showstheinternalstructure oftheeasternsideoftheERIS(Fig.4).Thesourceareais charac-terisedbythreedistinctmorphologicalstepsof30,70and120ms height,whichcorrespondtotheheadwallscarpsHS2,HS3andHS4 respectively(Figs.2aand4).ThescarpsHS2andHS3truncatethe seismicunitsU1andU2respectively,whilescarpHS4truncates unitsU6toU1.ScarpHS4coincideswiththelistricfaultdelineating theNorthernGraben,whichtruncatestheseismicunitsU3andU4 (Fig.4).ThereflectorsH2andH3markthebaseunitsU2andU3that aretruncatedbytheheadwallscarpsHS2andHS3.Theheadwall scarpsHS2andHS3cutthroughwell-stratifiedsedimentswithout crosscuttingtheunderlyinghigh-amplitudereflections,suggesting

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Table4

Summaryofpropertiesandinterpretationoftheseismicfaciesencounteredinthestudyarea.

Faciesillustration Reﬂectionpattern Continuity Amplitude Boundaries

(upper/lower)

Depositional environment

Sedimentological interpretation

1 Paralleltosub-parallel Continuous,

semi-continuous, truncating

Moderatetolow Concordant Upper-middle

slope Hemipelagic nannofossilooze, turbidites 2 Sub-parallel,irregular orhummocky Variable, discontinuous

High Discordant Channelthalweg Coarse-grained

channeldeposits

3 Chaotic,hummocky Discontinuous Lowtomedium Erosive

(downlap/rough)

Upper-middle slope

Slumpdeposits

4 Transparent Discontinuous Low Concordantto

erosive

Middle-lowerslope Debrisﬂow deposits

thattheglideplanesofthefailureslieabovethehigh-amplitude

reﬂectors.

Interestingly,theglideplanesofeachfailurehaveasimilardipas

theseismicsequences,suggestingthattheslidesoccurredparallel

tothebeddingsurfacesanditisworthnotingthatthetruncated

faciesmainlyconsistoflow-amplitudereﬂections(Figs.4and5a).

Thisimpliesastrongstratigraphiccontrolonthebasalsurfacesof theslidescars.Additionally,thebasalreflectorsdisplayareverse polaritywithrespecttotheseafloor(Fig.4).Theamplitudeofthe reflectionschangeslaterallyattimesbutfollowsthenormaltrend ofreversedpolarity.

Thestrike profile AM-MED17(Fig.5a)crosses the sidewalls ofthefailureHS2.The sidewallofthefailureis markedbythe truncationoflow-amplitudereflectionsoverthehigh-amplitude basal surface H2. The profile clearly images 30-ms thick slide depositscharacterisedbychaotictotransparentacousticfacies typ-icalformass-transportdepositsthatweinterpretastheresultsof uppermostfailure(HS1)withintheERIScomplex.Thesedeposits distinctlycontrastfromtheundisturbed,well-stratifiedturbiditic leveesediments.

TowardsthewesternpartoftheERIS,theuppermostfailure appearslike a transitionof well-stratifiedsedimentsto chaotic reflections,withpartlyintactinternalreflectionsabovebasal sur-faceH1(Fig.5a,b).Ofparticularinterestwithintheuppermost unit(U1)isthehummockysurfaceexpressions,with downslope-dippinghigh-amplitude reflections showing“ridge and trough” morphology,whichisparticularlywellidentifiedshowing reflec-tionhyperbolaewithinthesub-bottomprofilerdata(Fig.5ainset). Basedontheirseismiccharacterandrepetitivepatternsofridges andtroughsorientedperpendiculartothedirectionofmovement inthebathymetricdata(Fig.3b),weinterpretthesereflectionsas seafloorspreading similartothat observedwithintheStoregga slidecomplex(Micallefet al.,2007).Similartotheglideplanes of failures originating from HS2 and HS3, the seafloor spread-ingobservedinourdataoccurslocallyabovethehigh-amplitude reflector(H1inthiscase).

WithintheERISheadwallarea,weobserveseveralerosional dis-ruptionsinthestratawithinthewell-stratifiedfacies,especially withinunitU5,betweenreflectorsH4andH5(Fig.5a,b).These disruptionsonlyaffectunitU5,whereathinningoftheentireunit ofaveragethickness100-mstolessthan50msisobserved.Inthe areasofreducedthicknesses,weobserveseismicfaciesrelatedto slidedepositsabovethehigh-amplitudereflections.These distur-bancesoccurlocallyintheareasofhighreliefapproximately200ms

belowtheseaﬂoor.Weattributethesedisturbancesasaresultof localslopefailurerelatedtoanoversteepenedslopethathasbeen inheritedfromthemorphologyoftheunderlyingsediments. 4.2.3. InternalseismicstructureoftheRhoneEastern

Mass-TransportDeposits(REMTD)

TheREMTDappearsasabodyoftransparentseismicreflections withlowpenetrationinthesub-bottomprofilerdata(Figs.6and7a, b).TheREMTDseismicfaciesarehomogeneousinmostpartsof thestudyareawithverylowtoabsentinternalreflectionsor struc-tures.ItinfillstheseafloordepressionspredatingREMTDimaged inthestudyarea(Fig.7a).TheaveragethicknessoftheREMTDis ∼100ms,withamaximumthicknessof150msinthedepressions oftheunderlyingstrata.Weobserveseveralcasesinthetoearea oftheERIS,wheretheREMTDerodesthebasalsurfacesleaving behindtheremnantsintheformofbasalramps(Fig.6).

At water depths between 2000–2200m, along the western boundaryoftheREMTD,werecognizetransformationsinthe struc-turalstyleofthestrataabovereflectorH4(Fig.7c).Theproximal partofthis areashows extensionalfeatures intheformof dis-placed and scatteredblocks.The middle partconsistsofhighly deformed and rotated blocks. The deformation becomes more apparenttowardsthefront,withanincreaseinthedegreeof rota-tion.Thefrontalpartoftheareashowsacompressionalcharacter withratherpreservedstratigraphyintheformofpressureridges withmorethansixindividualblockspushedupwardfromtheir originalstratigraphy:weinterprettheseblocksas“pop-upblocks”. In themostdistal area(inthedeep basin),where the MTD pinchesouttotheseafloor,weobserve∼5-mshighand50-mlong patchesoftransparentreflectionsinthesub-bottomprofiles fol-lowingtheREMTD(Fig.7b).Asthesepatchesappearindividually overthebasalsurfaceoftheMTDandaredetachedfromthemain MTDbody,weinterpretthesepatchesasthe“out-runnerblocks”, similartothatobservedintheNigeriancontinentalslope(Nissen etal.,1999).

4.2.4. BuriedMTDs

WithinthePleistocenesediments,weobservetwoburiedMTDs thatwerefertoas“shallowMTD(SMTD)”and“deep-seatedMTD (DSMTD)”.TheSMTDisobservedwithinunitU4between reﬂec-torsH3andH4(Figs.4and5a).Itdisplayswell-stratiﬁedsediments atplacesbutismainlycharacterisedbydisturbedtochaotic seis-micfacies (Figs.4 and5a).AsforERIS, itis interestingtonote that theglide plane of failure of theSMTD also occurs ontop

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Fig.7. a)SeismicprofilesProgres060and061(SW-NEoriented)showingatransectthroughtheREMTD.Thecolourscaleoftheseismicprofilehasbeenadaptedforbetter visualizationoftheseismicdata.b)NW-SEorientedsub-bottomprofilecrossingthesouthernlimitoftheREMTDshowingpatchesoftransparentfaciesinterpretedas outrunnerblocks.c)ExtractofseismicprofileAM-MED4e(NNW-SSE)throughthecompressionridge.d)Multibeambathymetricmapofthestudyareawithprofilelocations.

ofahigh-amplitudehorizonH4,withoutdisturbingthe underly-ingsediments(Figs.4and5a).TheaveragethicknessofSMTDis approximately40ms;however,atthemiddlepartoftheSMTD, itcrosses alocaltopographichighonwhichSMTDthinsoutto onlyﬁvems, healingtheprevioustopographyof thelocalhigh (Figs.4and5a).TheSMTDisobservedlocally,andthedepositsare notfoundoutoftheinterﬂuve.ThissuggestsanoriginoftheSMTD linkedtoalocalslidewithaveryshortrunoutdistance,compared tothatoftheERIS.

TheDSMTDisobserved300msbelowtheseaﬂoor(Figs.4–8) andisseparatedbyabout150msthicksedimentsfromSMTD.It coversa minimum areaofabout 3000km2_, _with_an_estimated

minimumvolumeofabout400km3 ₍_Droz,₁₉₈₃_)._The_headwall

scarpofDSMTDhasnotbeenmappedbecauseoflimitationsin datacoveragetowardsthenorth.However,basedonits

geome-try,weinferaheadwalllocationofDSMTDsomewherenearthe present-dayshelfbreak,alongtheeasternleveeofthePRC.DSMTD hasahighlyvariablethicknessrangingfrom200msinthe prox-imal areato 20−30ms in the distal area. DSMTD deposits are mappedatleast100kmfromthesourcearea.Theinternal archi-tecture ofthe DSMTDshows chaotic facies in theseismicdata (Figs.4and5a).Inplaces,notablyintheproximalarea,thetop boundaryoftheDSMTDisirregularshowingridgeandtroughs, appearinglikemoundsintheseismicprofiles,whichweinterpretas coherentslideblocksandraftedblocks(Figs.4and5).Thebasal sur-faceofDSMTDismarkedbyapackageofwell-stratifiedsediments lyingonthehigh-amplitudereflectionsofthepaleo-canyonofthe PetitRhonefan(Torresetal.,1995).ThecontactofDSMTDwiththe underlyingstrataisunevenatplaces,erodingtheunderlyingstrata (Figs.4and5a).ThewesternpartoftheDSMTDisdominatedby

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Fig.8. SeismicproﬁleAM-MED4e(NNW-SSE)crossingthestudyareashowingNGandSGgrabensformedbyfaultsrootedtothebaseoftheMessiniansalt.Zoomofthe faultscarpsininset1and2showrecentscarpsofheight100msand40msrespectively.SeeFig.2forthelocationoftheproﬁle.

kilometre-scalerotatedblocksidentiﬁedbysteeplydipping reﬂec-tions(Fig.5a).Nexttotherotatedblocks,weobservea50-mshigh and1-kmwideraftedblock.

4.2.5. Faults

Theentire GoLis dissected by severalsteeply dipping, low-amplitudeanomaliesinterrupting theseismicreflections ofthe well-layeredsediments andinterpreted asnormal faults.Faults in the GoL have been previously mapped and documented in detailusingHRseismicandmultibeambathymetricdata(dosReis etal., 2015, 2005).dos Reisetal. (2015)reportedthefaults in theGoL as shelf-parallel,basinward-dipping and listric, rooted intheMessinianevaporitesequence.Atleastfifteensuchfaults havebeendocumentedinourstudyarea,attimesfacingopposite toeachothertoformgrabensthatwenamed“NorthernGraben (NG)”and“SouthernGraben(SG)”(Fig.8).Thesegrabenshavea dominantSW-NEtrend and offsetsin therange of 10−100ms. In somecases, thefaults have impacted theseafloor morphol-ogyandcreatedbathymetricsteps,thereforesuggestinganactive syN–Sedimentarysalttectonicsuntiltoday.Inadditiontothe SW-NEtrendingfaults,we observealocalN–Strending faultalong the undisturbed plateau offsetting the shallow strata down to 300msbsf(Figs.6and7a).

Acloserelationshipseemstoexistbetweenthefaultscarpsand thefailureplanesoftheERIS.EspeciallyatthetoeoftheERIS,the scarpoftheNorthernGrabensuperimposewiththeslidescarHS3 andHS4whereitformsa50−100mshighscarp(Figs.4,6,8).To thewest,thecontactofthefaultgrabenswiththeEasternleveeof theRhonecanyonischaracterisedbytruncationsofthefanlevees indicativeoffailureandcollapseofthelevee(Figs.3a,band8).We observethatthescarpsareonlyvisibleatthewesternand eastern-mostpartsoftheERIS,likelyduetothealterationoftheseaﬂoor morphologyafterthedepositionoftheREMTD.Additionally,the eastwardextentoftheNGtowardstheGRCformsanelongated ruptureinthebathymetrysimilartothatobservedinslope

fail-ures.Wethusinterpretthesescarpsastheeastwardextentofthe ERIScomplex(Figs.2aand3).

4.3. Lithologyandageofsedimentcores

FoursedimentcoreswererecoveredeastoftheRhone subma-rinevalleyintheturbiditiclevees;two inareaswitha smooth bathymetry(PSM-CS041&42),twoinareaswithseaﬂooraffected bymass-wasting(PSM-CS046&47).Lithologicalobservationsmay besummarizedintothreelithofacies(L1toL3;Fig.9):

-Lithofacies L1 is composed of foraminifera and calcareous nannoplankton oozes that correspond to pelagic-hemipelagic sedimentationduringtheHoloceneperiod(Fig.9).

-LithofaciesL2is composedoflaminated claywithsilttovery ﬁnesandlaminaeinterpretedasturbiditesdepositedbyspillover fromtheRhonevalley(Dennielouetal.,2019).

-LithofaciesL3iscomposedofstiffmudofgenerallyhigherbulk density than the background values (1.85–1.90g.cm−3). This lithofaciesattimesshowcolourbandingcorrespondingto lami-naeandfewsiltlayerscharacteristicsoflithofaciesintheRhone fan (Dennielou et al., 2019; Lombo Tombo et al., 2015). The laminaeare highlydeformed andfolded and show,at places, signaturesofcracks.Basedonthesimilaritiesobservedinother studiesinthestudyarea(e.g.Bonneletal.,2005;Dennielouetal., 2019,2006;LomboTomboetal.,2015),weassociatethe lithofa-ciesL3toREMTD.

CorePSM3-CS41wascollectedontheundisturbedplateauat 1996mwaterdepthand penetrated19.33mdeep(Fig.9).This corewasacquiredonthetopographichighandsamplesonlythe backgroundsediments,henceservesasareferencecoreforthe studyarea.Atthetop,itpresents6cmlithofaciesL1.From6cm tothebottomofthecoreitpresentslithofaciesL2.Thelithological uniformityofthecoreiswellrepresentedbythegammadensity

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Fig.9. a)Gammadensityandlithofaciesofsedimentcores(fromlefttoright,PSM3-CS047,46,42and41)collectedatthetoeoftheRhoneEasternMassTransportDeposit (REMTD).b)Sub-bottomproﬁlecrossingtheSouthernGrabenindicatingthepositionandpenetrationofsedimentcores(solidredrectangles).c)Locationmapofthesediment coresandthesub-bottomproﬁle.WR:wholeroundsamplescollectedformechanicalpropertiesanalysis(nolithostratigraphicdescription).

curvewithbulk densityfallingbetweenthe backgroundvalues 1.85–1.95g.cm−3.

CorePSM3-CS42is9.29mlongandwascollectedat2015m waterdepth,onekmeastfromcorePSM3-CS41.Theuppermost 20cmofthecoreiscomposedoflithofaciesL1.From20cmto3.7m, thecoreiscomposedoflithofaciesL2.From3.7mtothebottomof thecoreitpresentslithofaciesL3.Sedimentsarehighlydeformed withsteeplyinclinedandsometimesfoldedcolouredbands.The inclinationofthecolourbandsﬂuctuates throughthecore.The upper3.7mofthecoreischaracterisedbyuniformbulkdensity lowerthan1.9g.cm−3,whilebelow3.7mthebulkdensityvalues increaseto1.95–2.0g.cm−3andﬂuctuatethroughouttothebottom ofthecore.

CorePSM3-CS46wascollectedat1985mwaterdepthandis 9.07mlong.Theuppermostsevencmofthecoreiscomposedof lithofaciesL1.Fromsevencmto7.02m,thecoreiscomposedof lithofaciesL2withsiltylaminaeoccurringeveryfewcms.The fre-quencyofthelaminaedecreasestowardsthetop.From7.02mto thebottomofthecore,belowa sharp,erosivecontact,thecore presentslithofaciesL3.Thebulkdensityvaluesforthetop7.02m varyfrom1.75to1.9g.cm−3andgraduallyincreaseto2.05g.cm−3 from7.02mtowardsthebottomofthecore.

CorePSM3-CS47is10.87mlongandislocatedattheeastern flankofthePRCatawaterdepthof1810m.Fromthetoptofive cm,thecorepresentslithofaciesL1.Fromfivecmto7.75m,the corepresentslithofaciesL2.SimilartothecorePSM3-CS46,this

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Fig.10.ComparisonofradiocarbondatesatthebaseofthepostMTDdrapeontop oftheREMTD(thisstudy),RWMTD(Dennielouetal.,2019)andtheBalearicAbyssal PlainMegabed(Rothwelletal.,1998).

partofthecoreshowsfrequentappearanceofsiltytosandy tur-bidites.However,theturbiditesinthisunitshowerosivebasesand areﬁningupwards.Below7.75mtothebottomofthecore,the sed-imentsshowlithofaciesL3.Thebulkdensityvaluesarewellbelow thebackgroundvaluesof1.8–1.9g.cm−3throughoutthecore.The upperpartofthecore(downto7.75m)ischaracterisedbydensity ﬂuctuationswhereasthelowerpartshowsmorestablevaluesthat generallyincreasetowardsthebottomofthecore.However,the bulkdensityvaluesfortheentirecoreremainlowerthanthemean valuesobservedintheothersedimentcores.

RadiocarbondatingatthebaseoflithofaciesL2,i.e.post-slide sedimentswithintheundisturbedplateau,easternPRCﬂankand thefaultgrabenyielda2␴agebetween20and 21.8kacalBP, withanaveragemedianageof20.85kacal.BP(Fig.9).The post-slidesedimentsdatedinthecoresatthebaseoftheERISwithinthe SouthernGraben(corePSM3-CS46)andontheundisturbedplateau (corePSM3-CS42)yielda2␴agebetween21and21.8kacalBP, withanaveragemedianageof21.4kacalBP.However,thisdoes notdiscardthepossibilityofmultiplestagesofslidingwithinthis time.Wesuggestthattheyoungeragesattheeasternﬂankofthe PRC(atthebaseoflithofaciesL2withinthecorePSM3-CS047,Fig.9) relatetoayoungerfailureca1kalaterthanthemainslideevent. OurresultscoincidewiththeemplacementofRWMTDat21kacal BP(Dennielouetal.,2019),suggestingasynchronousageforboth MTDs(Fig.10).

Ourresultsareingeneralagreementwiththepreviousstudies thatthesedimentationratesfortheHolocenefallintherangeof 0.05to0.10m.ka−1whereasthesedimentationratesfortheonset oftheinterglacialperiod,whichconcernslaminationsofmuddyto siltyclaymaterialthatisinterpretedasﬁne-grainedturbidites,the sedimentationratesexceed1m.ka−1.

5. Discussion

5.1. Emplacement,kinematicsandtimingoffailuresinthe EasternRhoneInterﬂuve

Multipleheadwallscarps HS1-HS4intheERIS complex sug-gest multiple failures in the source area. The presence of the REMTD facies within the uppermost headwalls in the seismic data(Figs.4and5a)andthepresenceofseveralﬂat-lyingglide planessuggestaretrogressivetranslationalpropagationofthe fail-ure,meaningthatthefailurestarted atthetoeof theERIS and propagatedstepwisetowardstheshelfalongthehigh-amplitude reﬂectors.Ourresultsareconsistentwithpreviouslypublished

evi-dencethattheoverallflowdirectionofREMTDwastowardsthe southeastduetotheinheritedtopographyoftheRhoneFan(Droz etal.,2006);however,thebetterresolutionofthenewly-available data allows usto reconstructmore finely theslide flow based onsmall-scale morphologicalfeatures resultingfromthefailure (Fig.11).

ThebasalrampobservedatthetoeoftheERIS(Fig.6)isacrucial kinematicindicatoroftheflowdirectionoftheREMTD.Our obser-vationisinagreementwiththegeneralobservationthattheramps trendperpendiculartotheflowdirectionofthefailedmass(Alves, 2015;Bulletal.,2009;GawthorpeandClemmey,1985;Trincardi andArgnani,1990).WesuggestthatthepresenceoftheN–S ori-entedfaultnexttotheundisturbedplateau(Figs.2a,a)havemost likelyfunnelledandchannelisedtheflowofthefailedsediments, eventuallyenhancingtheerosionoftheseafloor.Alvesetal.(2014)

suggestedthatthepresenceofbasalrampsisakeyindicator relat-ingtothecombinedeffectoftheerosionalpoweroftheMTDand seaﬂoormorphology.SincetheERISCoccurredinagentleslope of1.8◦,wesuggestthatbasalrampsatthetoeoftheERISshow anerosionaltendencyofthemassmovement. Theminor topo-graphichighsalongthegrabens(Fig.3a,11)suggestthedeposition offailedsedimentsoriginatingfromtheleveecollapseduringthe majorslideeventoratalaterstage.

SimilartootherlargeMTDsaroundtheworld(e.g.RWMTD,

Dennielou etal., 2019; BIG’95 debrisflow, Lastras etal., 2005; Saharalandslide,Lietal.,2017),theREMTDhasalongrunout dis-tanceofabout200kmandanaveragewidthof50km.Itfilledthe topographicdepressionsformingasmoothseafloorandshowsvery littletonoreflectionsintheseismicdataalongwithits transpar-entfaciesinthesub-bottomprofilerdata(Figs.1,2a,7a,b).This evidencesuggeststhattheemplacementoftheREMTDwashighly fluidandmobile,suchasinthecaseofadebrisflow(Lastrasetal., 2005).Theseargumentsinadditiontothepresenceof transpar-entseismicfacieswithintheREMTDarein agreementwiththe previousstudiesonsubmarinedebrisflow(e.g.BIG’95debrisflow,

Lastrasetal.,2005).Additionally,thelargewidthoftheREMTD sug-geststhatitmusthavetransformeddownslopeintoamoreﬂuidic ﬂowthatallowedittospreadlaterally.

Weobserveafrontallyconfinedpressureridgeatthewestern boundaryoftheREMTD(Fig.7c).Pressureridgesaretypicalwithin thelobesofdebrisflows(MoscardelliandWood,2008)andprovide informationabouttheflowdirectionoftheMTDs.However, con-sideringthatthepressureridgeoccurredonastratigraphiclevel muchlower thantheREMTDandit isnotlocatedin the direc-tionofthemainpathwayoftheREMTD,wediscardageneticlink ofthepressureridgerelating totheREMTD.GamboaandAlves (2016)suggestedthatpressureridgesarecommonwithin mini-basinssurroundedbyburiedsalt-relatedstructures.Similarly,the pressureridgeinthestudyareaoccurswithinaminibasinbounded bya buried diapirinthesouth and aburied roll-overanticline inthenorth(Fig.8).Therefore,theemplacementofthispressure ridgewaslikelycontrolledbythegrowthoftheunderlyingsalt structures,byanalogytowhatsuggestedfortheBrazilianMargin (Gamboaetal.,2011;GamboaandAlves,2016).

Therecurrenceof theMTDs (REMTD,SMTD,andDSMTD)at differentstratigraphicintervalssuggestsalonghistoryof mass-wastingintheGoL.TheDSMTDhasbeenpreviouslydescribedas theresultofa westwardshiftof thepaleo-Rhonevalleyduring theMid-Pleistocenethatcoincidewiththeoccurrenceofamajor unconformity(∼450ka,Bellaicheetal.,1986; Droz,1983;Droz etal.,2006;Torresetal.,1995).AsfortheSMTD,ithasneverbeen datedwithdirectorindirectmethods.Basedontheaverage sedi-mentationratesanditsstratigraphicplacement,weestimatethat theSMTDwasmostlikelyemplacedaround350ka.TheREMTD, ontheotherhandhasbeendirectlydated,fortheﬁrsttime,using AMSradiocarbondating.Radiocarbondatingofthepost-REMTD

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Fig.11.Geomorphologicalinterpretationmapofthestudyarea.REMTD:RhoneEasternMassTransportDeposit,NG:NorthernGraben,SG:SouthernGraben.Thickblack arrowsindicateprobablemainpathwaysoftheREMTD.ActiveandburiedfaultsaremodiﬁedafterdosReisetal.(2005).

sedimentsconﬁrmsthatitwasemplacedbetween21and21.8ka calBP(Figs.9and10).

TheERISCtookplacebefore21.4kacalBP,duringthepeakof theLGM,whilethesea-levelwas120mlowerthantoday.Within theresolutionlimitsofgeophysicaldata,wedonotﬁndevidence ofmultiplefailureevents.However,MTDlithofaciesfoundonthe Rhonelevee(corePSM-CS047;Fig.9)weredepositedca.1kaafter theREMTD.Thissuggeststhatmass-wastingoccurredoveralong periodoftime,althoughnotalwaysinbigvolumes.Overall,theage oftheemplacementoftheREMTDfromourstudyiscoevalwith theemplacementoftheRWMTD(21.5kacalBP;Dennielouetal., 2019)andwithindatinguncertaintieswiththemega-turbiditein theBalearicAbyssalPlain(21.95kacalBP;Rothwelletal.,1998), suggestingacommontrigger(Fig.10).

5.2. Thesigniﬁcanceofthehigh-amplitudehorizonsintheERIS GlideplanesoftheERIS,aswellastheSMTDandpressureridge, coincide withhigh-amplitudereversed polarity beddingplanes

showingastrongstratigraphiccontrolonthefailures(Figs.4–6,7c). IntheWesternMediterranean,repetitionofthesehigh-amplitude reﬂectorsin thelast sixsedimentarysequencesis attributedto 100kyr-glacio-eustaticcyclesintheupperslopesuccessionsinthe GoL(Jouetetal.,2006;Rabineauetal.,2005;Sierroetal.,2009), in theAdriaticSea(Ridenteet al.,2009)andin theTyrrhenian Sea(Toucanne et al.,2015).Especially in theGoL, thecyclicity expressed by the superposition of high-amplitude reﬂectors is widelyreportedasa resultofdepositionofcondensed sections (Torresetal.,1995;Jouetetal.,2006;Rabineauetal.,2006,2005;

Riboulotetal.,2014;Sierroetal.,2009).Condensedsectionsare described ashard groundsrepresentinglongperiodsof geolog-ical time (Baraboshkin, 2009; Cattaneo and Steel, 2003; Crews et al., 2000; Loutit et al., 1988). In the GoL, these successions of condensedsectionsformedduring periodsof sea-level high-stand, when sources of terrigenous sediments from theAlpine rivers shifted landwards. During highstands, deltaic sediments were trapped in theﬂooded inner shelf and hemipelagic sedi-mentationledtothepreferentialaccumulationofpelagicskeletal

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fractionsin thelowerslope (Frigolaet al.,2012).We therefore suggestthat thehigher fractionsof pelagicskeletalmaterial as observed in the condensed sections in the PROMESS borehole (Jouet,2007;Riboulotetal.,2014;Sierroetal.,2009),mostlikely createhighbulkporositiesinthesedimentsandeventuallyexplain thereversedpolaritiesidentifiedinourseismicdata(Figs.4–6,7c). Submarinelandslidescantakeplaceatslopesasgentleas1◦ alongpotentialweaklayers,whentheshearstressbecomes signif-icantlyhigherthantheshearstrength(Fieldsetal.,1982;Urlaub etal., 2015, 2018).Weaklayersconsist of sedimentsthat have strengthslowerthantheadjacentlayerstoprovideapotentialfocus forthedevelopmentofthesurfaceofrupture(sensuLocatetal., 2014).Inhigh-latitudesettings,asinthecaseoftheStoreggaslide complex,lithologicalcontrasthasbeeninvokedasthemainreason fordevelopingweaklayers,wherecontouriticandhemipelagic sed-imentsareinterbeddedwithglacigenicdebrisflowdeposits(Laberg andCamerlenghi,2008).Alsoinmid-latitudedeepwater environ-mentssuchcontrastinlithologyexists.Danetal.(2009)invoked thatlithologicalcontrastofmarinemudandthin-beddedturbidites couldbecomeanimportantpredisposingfactorinordertoemplace majorfailuresontheslopes.Condensedsectionsarecomposedof clayeysedimentsthatareabundantofmicrofossilsandcould rep-resentasignificantcontrastinlithology.Theyshowhighershear strengththantheoverlyingsequencesand arewellacceptedas glideplaneofthefailureratherthantohaveactedasaweaklayer, asproposedintheUrsaBasin(Sawyeretal.,2009; Sawyerand Hodelka,2016)andintheAdriaticSea(Sultanetal.,2008).The turbiditicsequencessandwichedbetweenthecondensedsections withintheERIS complex mostlikely controlthe localisationof potentialweaklayers,especiallybecausecoarser-grainedmaterials (suchasturbidites)arepronetoliquefactionevenincaseofamild seismicloading(Danetal.,2009).Thiscouldexplainwhya vari-etyofmassmovementsoccurextensivelyoverthehigh-amplitude horizonsatseveralstratigraphiclevelsinthestudyarea(Figs.4–6, 7c).

5.3. Preconditioningandtriggermechanisms

Thepotentialrelationshipbetweensea-levelandlandslide fre-quencyhaslongbeeninthedebate(Lee,2007.;Leynaudetal.,2009;

Owenetal.,2007;Urlaubetal.,2013).UrgelesandCamerlenghi (2013)highlightedanincreaseinlandslideactivityinthe Mediter-raneanregionduringandaftertheLGM. Besides,Brothersetal. (2013)proposedadirectlinkbetweensea-levelinduced seismic-ityandslopefailures.Evenifsomeuncertaintiesremainondating accuracies(Urlaubetal.,2013),andwithlimitedanalysis,itis difﬁ-culttodirectlydemonstrateacausallinkbetweenclimaticforcing andseaﬂoorinstabilities.Consideringthesynchronicityintheages ofmultipleMTDs,wesuggestthatitshouldnotbediscardedthat thesea-levelfallduringtheLGMdirectlyorindirectlyplayed a majorroleintheemplacementoftheselargeMTDsintheWestern Mediterranean.Therefore,wediscussthelikelihoodoflarge-scale triggersthatmayhaveinducedslopeinstabilitiesandeventually favouredemplacementoftheselargeMTDs(especiallyREMTDand RWMTD)intheGoL.

Rapidsedimentationandexcesspore-pressuregeneration:Rapid sedimentationcangovernthestressandstrengthconditionsona slopeovertime,leadingtofailure,onceacriticalamountof precon-ditioningisreached(Stoecklinetal.,2017).TheRhonefanduring theend ofthelast glacialperiod receivedsediment loadupto 5timesthepresent-dayduringearlyphasesof glaciersmelting (KettnerandSyvitski,2009;LomboTomboetal.,2015;Sierroetal., 2009).Forinstance,upto25mofsedimentsweredepositedwithin 12kabetween30-18kaontheupperslope(Sierroetal.,2009)and fourmofsedimentweredepositedintheRhoneturbiditicvalley within1.4kaattheendoftheLGM(LomboTomboetal.,2015).

Additionally,below-averagebulkdensity(<1.9g.cm−3)may sug-gestanunder-consolidatedstateoftheturbiditicunits(Lithofacies L2;Fig.9)thatweredepositeduntiltheonsetofinterglacialperiod. SlopefailureanalysisattheBourcartCanyonheadshowthat sed-imentloadingwithonlytwomofsedimentrapidlyaccumulated cangeneratesigniﬁcantslopefailure(Sultanetal.,2007).Evenif themorphologyandsedimentaryprocessesonthefanleveesand atthebaseoftheslopearedifferentfromthoseofcanyonheads. Onecanassumethatthemuchthickeraccumulationontheopen slopeandfanleveesmusthavefavouredthefailure.Thisforcing isfurthersuggestedbytheageoftheREMTD,between20–21.7 ka cal BP(Fig. 10), exactly duringthemaximum lowstand and beforetheonsetofsea-levelrise(Clarketal.,2004;Lambecketal., 2014),whenriversandcanyonswereconnectedtotheshelfedge, maximumsedimentloadingoccurredontheupperslopeand max-imumsedimentwastransferredtotheriseand ontobasin fans. Thethresholdbetweenshearstrengthandstressunderhigh accu-mulationratescouldlikelybereached,particularlybecauseitcan generateexcessporepressureanddevelopweaklayers(Leynaud etal.,2007;Locatetal.,2014).Inourcasesedimentationrates,inthe orderof1₋₂m.ky-1₍_Lombo_Tombo_et_al.,₂₀₁₅_;_Sierro_et_al.,₂₀₀₉₎

arenotextremelikeonglaciatedmargins(Hjelstuenetal.,2004), prodeltas(Fangetetal.,2013;Zhouetal.,2016)orevenondeep-sea turbiditicchannelmouthlobes(Dennielouetal.,2017;Rabouille etal.,2017).However,sub-seaﬂoorarchitectureatplaces (espe-ciallywithintheERISheadwallarea)revealssteepslopesinheriting topographiesofpreviousfailuresaccompaniedbylocalslope fail-ures(e.g.Figs.5a,b).Theconcurrenceofover-steepeningandlow shearstrengthandunder-consolidationbyhighsediment accumu-lationcouldconstituteanaggravatingfactorforsliding.Inaddition tosedimentloadingandover-steepening,theoccurrenceof poten-tialweaklayerslikesiltlayersmaybedeterminantinthetrigger ofsliding(Danetal.,2009;Locatetal.,2014).Thereforetheporous siltlayersoccurringintheleveedturbiditicfacies(Fig.9)mayhave promotedanincreaseinexcessporepressureandhavefavoured sliding,particularlybecausetheyaresandwichedbetween clay-rich,lesspermeablelayers.

Gravitationalgliding ofpost-Messinian stratarelatedto haloki-nesis:In theGulf of Lions listric faultsrelated tosalt-sediment interactionsextendfromthebaseofthePlio-Quaternary(ca.two kmdeep).Themajorityofthefaultsreachtheseaﬂoor correspond-ingtoescarpmentswithverticaloffsetsexceeding20m(dosReis etal.,2005),suggestinganongoingslopesteepeningdueto salt-sedimentinteraction.GrowthstrataatthefootwalloftheNGand SGclearlyshowthatthefaultsweremostlikelyactiveatthetimeof emplacementofERIS(Figs.4,6,8).Therefore,anaseismicslipofthe maingrowthfaultmaylikelyhaveresultedinaregional steepen-ingoftheseaﬂoorandmostimportantlyexposingthestrataatthe toeoftheERIS.Ineithercase,thegrowthfaultcontrolsthelocation oftheheadwallscarpsandwithaslipofabout100manexternal triggermightnothavebeenrequired,ahigh-sedimentationrate coupledwithhighslopeanglesmighthavebeenenoughtoinitiate thefailure(Stoecklinetal.,2017).

ConsideringthecoincidingagesofRWMTD(Dennielouetal., 2019)andREMTD(thisstudy)atthepeakoftheLGM,andthe pres-enceofthefaultscrosscuttingtheentireGoL,wesuggestthatthe growthfaultslocatedatthetoeoftheERISinpresenceof pref-erentialfailureplanes andhigh-sedimentation ratesmostlikely favoured theemplacement of the slide.Vertical movementsof listricfaultsduetoisostaticreboundmaylikelyoccur preferen-tiallyduringthelowstandandbeapositivefeedbackofsediment loadingontheRhonefan.Althoughtherateofverticalmovements isnotquantiﬁed,wesuggestthatitcanbedeﬁnitelyconsideredasa furtherfactorforsedimentinstabilitythataddsuptohighsediment accumulation.

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6. Conclusions

TheGulfofLionsisapassivemarginofabout200kmlength and70kmwidthwithmainsedimentsupplyfromtheRhoneRiver depositingalpinesedimentsontheRhonedelta.Theslopeofthe GoLisdissectedbyfifteensubmarinecanyons,thelargestonebeing thePetitRhoneCanyon.SeafloorinstabilityintheGoLoccursin canyonheadsandflanks,butalsointheinterfluveslopeareasand theleveesofthemainsubmarinevalleys.Previousstudies high-lightedthepresenceofseverallargesurficialMTDswithvolumes exceeding100km3_._We_presented_here_details_on_the_REMTD_that

wasemplaced21.4kaduringthepeakoftheLastGlacialMaximum. Thenewly availablehigh-resolutiongeophysicaldataalloweda detailedinvestigationofthesourceoftheREMTDthatwereferred toastheEasternRhoneInterﬂuveSlide.Thecombinationofa diver-siﬁedgeophysicaldatasetprovidesinsightsintotheslopefailure processesthatcanbesummarisedinthefollowingpoints. • Theoverallmorphologyof theERISis dominatedbystaircase

geometryindicativeofaretrogressivefailureonmultipleglide planes,extendingfromawaterdepthof500m–2000m. • PreconditioningoftheERISbylocalizationofweaklayersmay

havebeenfavouredbyrapidlyaccumulatedturbiditicsequences sandwichedbetweenreversed polaritystrongreﬂectors inter-pretedascondensedsections,whichweremostlikelydeposited duringthesea-levelhighstands.

• Thelocationoflandslidescarsseemstobeassociatedwiththe presenceoflistricfaultsrootedintheMessinianevaporitelayer actingasadécollementsurface,drivenbythegravitational glid-ingofpost-Messinianstrata.

• Thetriggeringmechanismresponsiblefortheemplacementof theslideisnotyetclear,butacombinationofprocessessuchas localandregionalslopesteepeningduetohalokinesisandexcess porepressuregenerationduetorapidsedimentaccumulation likelyaffecttheslopestability.

Dataavailability

Sediment cores are curated at IFREMER core repository in Plouzané(France).Coredatarelatedtothisarticlecanberequested at:

CorePSM3-CS041:IGSNBFBGX-87805( http://igsn.org/BFBGX-87805)

Acknowledgement

Wethankthecaptains,crewandsciencepartiesofthePRISME-2 andPRISME-3campaignsonboardR/VL’AtalanteandR/VPourquoi pas?,respectivelyfortheirsupportduringthecampaigns.ThePhD ofShrayBadhaniisfundedbytheEuropeanUnion’sHorizon2020 researchandinnovationprogrammeunderthe Marie-Skłodowska-CuriegrantviaprojectITN-SLATE(grantagreementNo721403). Sylvain Bermel, Mathilde Pitel Roudot and Arnaud Gaillot are thankedfortheirhelponGISdataprocessing.Thisstudyhasbeen greatly beneﬁtted from several fruitful discussions with Shane Murphy,JacquesDéverchèreandElodieLebas.WealsothankTiago M.Alvesandananonymousreviewerfortheircommentsthat sig-niﬁcantlyimprovedourmanuscript.

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