Lateral evolution of the rift-to-drift transition in the South China Sea: Evidence from multi-channel seismic data and IODP Expeditions 367&368 drilling results

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Lateral evolution of the rift-to-drift transition in the

South China Sea: Evidence from multi-channel seismic

data and IODP Expeditions 367&368 drilling results

Weiwei Ding, Zhen Sun, Geoffroy Mohn, Michael Nirrengarten, Julie Tugend,

Gianreto Manatschal, Jiabiao Li

To cite this version:

Weiwei Ding, Zhen Sun, Geoffroy Mohn, Michael Nirrengarten, Julie Tugend, et al.. Lateral evolution

of the rift-to-drift transition in the South China Sea: Evidence from multi-channel seismic data and

IODP Expeditions 367&368 drilling results. Earth and Planetary Science Letters, Elsevier, 2020, 531,

pp.115932. �10.1016/j.epsl.2019.115932�. �hal-02778542�

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Earth and Planetary Science Letters 531 (2020) 115932

Contents lists available atScienceDirect

Earth

and

Planetary

Science

Letters

www.elsevier.com/locate/epsl

Lateral

evolution

of

the

rift-to-drift

transition

in

the

South

China

Sea:

Evidence

from

multi-channel

seismic

data

and

IODP

Expeditions

367&368

drilling

results

Weiwei Ding

a

,

b

,

∗

,

Zhen Sun

c

,

∗∗

, Geoffroy Mohn

d

,

Michael Nirrengarten

d

,

Julie Tugend

e

,

f

,

Gianreto Manatschal

g

,

Jiabiao Li

a

a_Key_Laboratory_of_Submarine_Geosciences,_State_Oceanic_{Administration,}_Second_Institute_of_{Oceanography,}_Ministry_of_Natural_Resources,

Hangzhou310012, China

b_Shanghai_Jiao_Tong_University,_School_of_{Oceanography,}_Shanghai_20030,_China

c_CAS_Key_Laboratory_of_Ocean_and_Marginal_Sea_Geology,_South_China_Sea_Institute_of_Oceanology,_Chinese_Academy_of_Sciences_(CAS),_Guangzhou_{510301, China} d_Laboratoire_Géosciences_et_{Environnement}_Cergy_(GEC),_Maison_{Internationale}_de_la_Recherche,_Université_de_{Cergy-Pontoise,}_Paris_{95000, France}

e_Sorbonne_Université,_CNRS-INSU,_Institut_des_Sciences_de_la_Terre_Paris,_ISTeP_UMR_7193,_F-75005_Paris,_France f_Total_SA,_R&D_department_CSTJF,₆₄₀₀₀_Pau,_France

g_Institut_de_Physique_du_Globe_de_Strasbourg,_Université_de_Strasbourg,_{Strasbourg-CNRS,}_{F-67084, France}

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory: Received4April2019

Receivedinrevisedform18October2019 Accepted27October2019

Availableonline8November2019 Editor:A.Yin

Keywords:

rift-to-drifttransition tectono-magmaticprocesses breakup

SouthChinaSea IODPExpedition367&368

TheSouthChinaSeamarginsrepresentacriticalnaturallaboratorytostudytheprocessesandparameters controllingtherift-to-drifttransition.Highqualityseismicdatapreviouslysuggestedtheoccurrenceofa relativelysharptransitionbetweenthecontinentalandoceanicdomains,ahypothesisrecentlyvalidated bythe resultsoftheIODPExpedition367&368drilledinthenorthern SouthChina Seadistalmargin. DrillingresultsinthistransitioncoredMid-OceanRidgetypebasalticbasement(SiteU1502),whileits lateralequivalentshowedcontinentalaﬃnities(SiteU1499).Seismicdatadocumentthelateralevolution ofthisnarrowtransitionzone(∼15to25kmwide)andtectono-magmaticcontextrelatedtobreakup.A short-periodmagmaticeventoccurredduringthelateststageofcontinentalriftingandintrudedtheedge ofthethinnedcontinentalcrust,triggeringcrustalbreakupandonsetofsteady-stateseaﬂoorspreading. Thearchitectureofthetransitionaldomaindocumentedinthisworkisinmarkedcontrastwiththose interpreted as eithermagma-starvedor magma-richatbreakuptime, suggesting thatthe continental marginoftheSCSisintermediatebetweentheclassicalend-membermagmaticarchetypes.Wepropose thatthemagmaticeventtriggeringcontinentalbreakupisrelatedtodecompressionmeltinglinkedtothe ascendingasthenosphere.Itisassumedthatthiseventwasrapidatgeologicaltimescale(<10Ma)and wasfavoredbyahighmantletemperature.

1. Introduction

Newobservationsandtheacquisitionofnewdataarecrucialto describethearchitectureandnatureoftransitionaldomains lying betweencontinentalandoceanic crustsandunderstand the evo-lutionofthethermalstateandstrengthofthelithosphereduring breakup andsubsequent onset ofseaﬂoor spreading. Transitional

domainshavebeeninvestigatedinbothmagma-rich and

magma-*

Corresponding author at: Key Laboratory of Submarine Geosciences, State OceanicAdministration,SecondInstituteofOceanography,MinistryofNatural Re-sources,Hangzhou310012,China.

**

Correspondingauthor.

E-mailaddresses:wwding@sio.org.cn(W. Ding),zhensun@scsio.ac.cn(Z. Sun).

poorrifted marginsthanksto datasetsfromtheOceanic Drilling Program (ODP) (e.g. Larsen andSaunders, 1998), high-resolution multichannel seismic(MCS)data(e.g. Péron-Pinvidicetal., 2007) andﬁeld studiesinfossil analogues(e.g.Abdelmalaketal.,2015). These observations enabledthe development of conceptual ideas

inspired by analogue and numerical modeling (e.g. Lavier and

Manatschal,2006; Bruneetal.,2014).Magma-rich COTs,as inter-pretedintheSouthAtlanticandNorthwestAustralianmargins(e.g. Frankeetal.,2010; Sticaetal.,2014),arecharacterizedbySeaward Dipping Reflector sequences (SDRs). SDRs are built by the stack-ingoflandwardflowingbasalticmeltsinterbeddedwithsediments duringbreakup.Theirformationiseitherrelatedtolandward dip-pingfaultsand/orrelatedtooceanwardflexure(Patonetal.,2017). SDRs are typically floored by high-velocity bodies (Vp

∼

7.2-7.6

km/s) that are interpreted to result from magmatic

underplat-https://doi.org/10.1016/j.epsl.2019.115932

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ing/intrusions (e.g. White et al., 1992). Magma-poor transitional domainsshowevidenceforhyper-extendedcontinentalcrust, and locallyalso of exhumedsub-continental serpentinizedmantle, as exempliﬁed by the Iberia-Newfoundland margins (e.g., Boillot et al.,1980; Whitmarshetal.,2001).However,thisbipolarvisionof rifted margin is an over simpliﬁcation and case studies are ex-pectedtoexistin-between.

InthenorthernSouthChinaSea(SCS)marginneitherSDRsnor exhumedmantle havebeen drilled,dredged or observedin seis-mic sections. However, evidence for a high-velocity layer (HVL) in the lower crust (Yan et al., 2001; Chiu, 2010), magmatic ad-ditionsinthecontinentalslope(Zhaoetal.,2016; Fanetal.,2017) andoccurrenceofdetachmentfaultsaffectingthehyper-extended crust have been described (Franke et al., 2014; Gao etal., 2015; Zhaoetal.,2018).TheSouthChina Seamargindoesnotresemble

the classical end-member magmatic archetypes and hasrecently

been recognized as an “intermediate” type (Larsen et al., 2018) based on the results of the International Ocean Discovery Pro-gram (IODP) Expeditions 367&368. Seven sites, spanning from a basement high in the distal margin to the initial oceanic crust, have been successfully cored (Sun et al., 2018). In complement to Larsen et al. (2018), herewe interpret four crustal-scale MCS proﬁlestoinvestigatethedeformationpattern,magmatic architec-tureandlateralvariabilityoftheCOTarchitectureofthenorthern SCS. We tentatively characterize the location, amount, and tim-ingofmagmaticadditionsthat occurredduring breakup,andaim to document further the mechanisms controlling the rift-to-drift transitioninthenorthernSCSbyfocusingonitstectono-magmatic evolution.

2. Geologicalsetting

SincetheTriassic,therealmfromwhichtheSCSdevelopedwas in a convergent setting caused by the subduction of the paleo-Paciﬁc(TaylorandHayes,1983).DuringthePaleogene,the

paleo-stress ﬁeld changed from compression to extension as a result

ofsubduction rollback(Taylor and Hayes,1983). This resultedin episodicrifting eventswith uplifting shouldersand local erosion thatcontinuedintoCenozoictime.Theextensionaltectonicsofthe

Cenozoicis evidencedby the development ofbasins boundedby

serial,parallel,toenechelonfaults,someofthesestructuresbeing interpreted aslong offset detachmentfaults (Franke etal., 2014; Zhaoetal., 2018).Twomain rifteventscan be distinguished:an earlier Paleocene to Eocene and a late Eocene to early Miocene (Cullenetal.,2010).Theformereventwaswidespreadandcanbe identiﬁedinnearlyallsedimentarybasinsinthecontinentalshelf. Conversely,thelattereventresultedinseaﬂoorspreadingandthe formationoftheSCS.

AmodelfortheopeningscenariooftheSCSwasfirstproposed by Taylor and Hayes (1983) and Briais et al. (1993) on the ba-sis of the magnetic anomalies, indicating that seafloor spreading occurred between32 and 16Ma. In contrast,Barckhausen et al. (2014) suggestedacessationageof20.5Mabasedonmorerecent ship-borne magnetic data.Althoughthere is 3–4Myr differences inthe cessationtime, mostauthorsagreethat seafloorspreading occurredintheEastSub-basinfirst,followedbyasouthwardridge jumpandare-orientationofthespreadinggeometryfroma west-ward to a southwestward ridge propagation (Briais et al., 1993; Dingetal.,2018).

Shipboardresultsofmicrofossilbiostratigraphyand palaeomag-netic results from the IODP Expedition349 (Li et al., 2015) and theIODPExpeditions 367&368(Sun etal., 2018),combined with analysisonrecently-acquireddeeptow magneticanomalies(Li et al.,2014),and40Ar/39Ardatingofbasaltsnearthefossilspreading ridge(Koppers,2014),haveshownthatseaﬂoorspreadingoccurred

between

∼

30Maand16Ma,supportingagemodelsproposedby

TaylorandHayes (1983) andBriaisetal. (1993).Thecessationtime coincideswiththeonsetofcollisionbetweenPalawanandBorneo andMindoro-CentralPhilippines(Cullenetal.,2010),suggestinga causalrelationshipbetweenthetwoevents.

Theseismicsectionspresentedheremainlyrunacrossthe dis-taldomainofthenorthernSCSmargindocumentingthestructure ofthe Continent-Ocean Transition(COT). TotheNW, thesections imagethemostdistal/southernpartofthePearlRiverMouthBasin

(PRMB) and adjacent oceanic crust. The PRMBconsists of

Ceno-zoic basins that formed during a syn-rift stage (

∼

53 Ma to 32 Ma), followed by a post-riftstage (32 Ma to 10.5Ma), anda ﬁ-nal rejuvenation stage (10.5 Ma to recent) (Clift and Lin, 2001; Wang et al., 2018). The crustal architecture of the southern part

of the PRMB has been well documented using wide-angle

seis-mic surveysandpresentsaseaward crustal thinningfrom24km to 12 km (e.g. Yan et al., 2001). The Cenozoic rift extension is

accommodated by detachment faults forming deep basins with

rampsynclineandroll-overanticlinestructures(Zhaoetal.,2018; Wangetal.,2018).

3. Reﬂectionseismic,drillholedataandinterpretational

approach

3.1. Reﬂectionseismicdataandprocessing

FourMCSsectionshavebeeninterpretedinthisstudy(for loca-tions ofsectionsseeFig.1).SeismicsectionsN2,N3andN4were obtainedduringthenationaloceanicprojectcruisebytheSecond Institute ofOceanography,MinistryofNaturalResources,in2004. The sectionswere obtainedby R/VTanbao through480channels, 6237.5 m-longstreamer, andwere recordedinSeg-Dformat at2 ms sampling intervals and 12 second (s) trace length. The shot interval is37.5 m. Seismic section SO49-17 was obtainedduring the“JointSino-GermanSouthChina SeaCruise”in1987.The

sec-tion was obtained by R/VSONNE through 48 channels. Pre-stack

processing ofthe seismic data includes amplitude compensation, staticcorrection,gainandmuteanalysis,predictivedeconvolution, multiple attenuation, velocity analysis, residual static corrections and frequency filtering. Post-stack deconvolution, band-pass and coherencyfilteringare appliedto thestacked data,followedby a finite-differencemigration.

3.2. Drillholedataandagecalibrationsofseismicunits

Drilling sites fromODP Leg184 (ODP Site 1148), IODP Expe-ditions 349,367&368 (U1435, U1499, U1500, U1501,U1502, and U1504)areusedtocalibratetheageofseismicunits.Seismic sec-tionN4liesveryclosetotheIODPExpedition367&368drillsites, andthereforethelithologiesandagesofbothsedimentsand base-ment rocks along this section are well constrained by the Sites

U1500, U1502 and U1504. Age of the unit boundaries is based

on biostratigraphic data (Li et al., 2015; Sun et al., 2018), and the locationofunit boundariesintheseismicsectionshavebeen estimated converting drilled depth to two-way travel time using P-wavevelocitiesderived fromtheloggingdataofrelativedrilling sites(Fig.2)(Sunetal.,2018).

The combination of seismic anddrill-hole data enabledus to deﬁnecharacteristicseismicunitsintheCenozoicinﬁllofthe dis-tal northern SCS margin that are interpreted to record distinct tectonic phases.The tectono/seismicunitsare describedhereafter frombottomtotop:

Thepre-riftunit: mainly identiﬁed below Cenozoic grabens in the distal margin and oftenits top is recognized asthe acoustic basement. A distinct angular unconformity (Tg) separates indis-tinctly thepre-rift or basement fromthe Cenozoicsediments on top.TheinternalreﬂectionpatternbelowTg ismainlychaoticand

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W. Ding et al. / Earth and Planetary Science Letters 531 (2020) 115932 3

Fig. 1. (a)MorphologicalfeaturesofthemiddlepartofthenorthcontinentalmarginandadjacentregionsoftheSouthChinaSea.Yellowlinesshowthelocationsofexample seismicsectionsdiscussedinthisstudy.Whitedashedlinesarethewide-angleseismicproﬁles.RedsquaresindicatethemagneticlineationfromBriaisetal. (1993).Drill sitesofODPLeg184,IODPExpeditions349,367&368areindicatedbyyellowcircles.(b)MajortectonicunitsandsedimentarybasinsintheSouthChinaSea.Reddashed squareshowsthestudyarea.(c)RTPmagneticanomalymap. (d)Free-airgravityanomalymap.(Forinterpretationofthecolorsintheﬁgure(s),thereaderisreferredtothe webversionofthisarticle.)

discontinuous withvarious frequency andintensities. Site U1501 located on a broad regional basementhigh, referred to as Outer MarginHigh(OMH)(Fig.1),drilledlithiﬁedpre-riftsandstonesand conglomeratesofuncertainage(Mesozoic?)belowanangular un-conformity (Sun et al., 2018; Larsen et al., 2018). Site U1504 is locatedintheeastwardprolongationoftheOMH,andpenetrated

a metamorphic basement composed of ﬁne - to coarse-grained

epidote-chloriteschiststhat lie belowtheTglevel. Theexact

na-tureoftheprotolithaswellasitsrelatedP-T-tevolutionremains unconstrainedforthemoment(Sunetal.,2018).

Unit1,orsyn-riftunit: deposited before the onset of seaﬂoor spreadingrangesfromEocenetoEarlyOligocene(before

∼

30Ma). Theacousticbasement(including thepre-riftunit) formsthebase

of this unit. This unit shows subparallel and low to -

moder-atelycontinuous reﬂectorswithlow frequencyandvarious inten-sities,sealedbyaregionalunconformityT70withanagebetween

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Fig. 2. CalibrationofseismichorizonsfromSiteU1502.Theinterpretedboundaries arecalibratedfromrecoveredlithostratigraphy.P-wavevelocityanddensityalong thesiteareshownindepth.SiteU1502haspenetratedintobasalticbasementand recoveredmostoftheNeogenedeposits.SeelocationinFig.1.

∼

32-30 Ma (Sun et al., 2018). Major thicknessvariations in the proﬁles imply that this unit has a syn-tectonic origin. This unit hasbeenpenetratedatSiteU1501,whereitrecoveredlateEocene siliciclasticsedimentspresentinga ﬁnningupward sequence(Sun etal., 2018). Thesesediments aresimilar in ageandtype tothe syn-rift sedimentsof Site U1435 (Li etal., 2015). A further sub-division ofthesyn-rift unit sequencewas not attemptedbecause ofalack ofdrillsitesinthe distalmargin. Furtheroceanward,at SiteU1499onabasementridge(referredtoasRidgeAFig.3),an undatedgravelunit withsiltyandsandy intervalshasbeen pene-trated(Sunetal.,2018).

Unit2:depositedsincetheinitialseaﬂoorspreading(

∼

30Ma) andlimitedbyaregionalunconformityT60,whichcorrespondsto ahiatusat

∼

23Ma,sampledatIODPSiteU1435 (Lietal.,2015). ThishiatusisalsopresentatSitesU1501andU1502corresponding toaremarkablelithologicalchangewithanageof

∼

24-

∼

26Ma (Sunetal.,2018).InseismicsectionsT60canbetracedbasin-wide byastrongnegativephasechange.Thisunitisinternally character-izedbysubparallelto- parallel reﬂectionsofhighto- intermediate continuityandmoderateintensity.Thearchitectureandlithological contentofthisunitchangesigniﬁcantlyacrossthedistalmarginas afunctionoftopographyandrelatedpaleowaterdepth.The repre-sentativesedimentary faciesof theOMHis found atSitesU1501 andU1505andiscomposedofnannofossilrichclay.AtSiteU1499, above the gravel Unit a condensed section of breccia (from

∼

30 to 26 Ma) was drilled. In addition, Site U1502 located on Ridge AeastwardofSite U1499(Figs.1&5),recovered veryhard,poorly sortedbrecciaandbiosiliceousclays(Sunetal.,2018).

Units3&4: depositedafterT60,spanningtheearlyMiocene syn-driftandmiddleMiocenetopresentpost-driftbasindevelopment.

These two units have very similar seismic reﬂectors separated

by T30 with an age of

∼

5Ma, characterized by parallel reﬂec-tionswithlow-to-moderateintensity,locallytransparentfaciesare observed as well. They are made of nannofossil-rich clays, silty clay,foraminifer-richclayeysiltstonetosandstone,andnannofossil oozes depositedindeepmarine environments,withminorcalcite andchalk(Sunetal.,2018).

3.3. Interpretationofmagmaticadditions:methodologyand terminology

While thecombinationofdrilling resultsandseismicdata en-ables the determination of sedimentary units that can be corre-lated across the distal margin, the identification of the amount of magmatic additions related to breakup is challenging and re-latedtolargeuncertainties(Tugendetal.,2018).Inthisstudy,we used seismic criteria, locally supported by drilling results of the IODP expeditions 367&368,to interpret magmatic additions that occur as magmatic sills characterized by spatially limited, high-amplitude reflectors, which can be easily distinguished from the relativelow-amplitude,chaoticinternalsurroundingseismic reflec-tions.(e.g.Trudeetal.,2003; Zhaoetal.,2016).

4. Stratigraphicandtectono-magmaticcharacteristicsoftheSCS

structuraldomains

We use four seismic sectionsimaging the along strike north-ernSCSmargintodocumentthetectonicandmagmaticstructures

related to breakup. Based on these seismic data combined with

geophysical dataanddrillingresults,wetentativelyidentiﬁed dif-ferentbasementtypesalongthemargin:theunambiguousthinned continentalandoceanicdomains.Atransitionaldomain,wherethe rift-to-drifttransitionisrecorded,occursinbetween.

Therecognitionoftheseriftdomainsonreflectionseismicdata ismainlybasedoncrustalthicknessvarications,changesinMoho reflectivity, stratigraphic architecture, andtop-basement architec-ture (Figs. 3–6). Magnetic data and the free-air anomaly signals wereinvolvedtoprovidecomplementaryconstraintstodelimitthe spatial extent ofthesedomains onthe profiles(see map view in Fig.1c&d,andalong-profileviewinFigs.3–6).Thesedatawere col-lectedbythe1:500000scalingGravity,Magnetic,andBathymetric Survey in2003to 2007by theSecond Institute ofOceanography, MNR.VariableinclinationRTP(reduction-to-the-pole)isemployed forthemagneticdata.

4.1. Thinnedcontinentaldomain

Based onseismicobservationsthethinnedcontinentaldomain ischaracterized by:(1)well developedfaultboundedbasinswith thick syn-tectonic sediments; (2) a major reﬂector around 9-10

s TWT that shallows oceanwards identiﬁed as the Moho; and

(3) generally negative free-air anomaly, increasing oceanwards to the SE endof thisdomain.Exception is SO49-17,where the FAA signalschange alot. The crustalthicknessranges from5.5to 3s TWT(16-

∼

7km),graduallythinningoceanward.Thecrustinthis domainissigniﬁcantlythinnedcomparedwiththe

∼

30km

thick-ness of the continental crust documented onshore South China

(Braitenbergetal.,2006).

OnbothN2andN3seismicsections,twomainbasementhighs

delimit a

∼

30 kmwide sedimentary basin. The most

continent-wardbasementhigh(between0- 15kminN2,Fig.3;between0 -13kminN3,Fig.4)isdelimitedbyseveral normalfaultsaffecting

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Fig. 3. MCSsectionN2inasoutheasterndirectionandthealongprofilefree-airgravity(blue)andRTPmagneticanomaly(red).Middleistheoriginalseismicsection.Black arrowsindicatestronginternalreflectorsandMoho;Blowisageologicalinterpretationwithlinedrawing.Coloreddashedlinesarethesequenceboundariesseparating differentsedimentaryunit.Ahugebasementhighwithpeakclosetotheseafloorcanbeobservedinthedistalpartofthemargin,boundedbyseverallistricnormalfaults. Boththegravityandmagneticanomaliesofthisbasementhighincrease.Verticalexaggerationis3.

thecontinental crust andthe overlying syn-rift sediments.These normalfaults dipgenerallytoward theocean. Locallystrong hor-izontalintra-basement reflectorsare identified in seismic section N2(Fig. 3) at8.0 sTWT. The observed normalfaults are consis-tentlysolingoutintheseintra-basementreflectors,showinga typ-icallistricfaultshape.Thereforethesereflectorsat

∼

8.0sTWTare interpreted asa major decollement corresponding to the brittle-ductile transition. The syn-riftUnit1 shows locally growth struc-turesconﬁrmingtheirdepositionduringactivefaulting(Fig.8).

Abasement high(between45- 75 kminN2,Fig. 3;40– 50 kminN3,Fig.4)markstheoceanwardterminationofthethinned continentaldomain.InseismicsectionsN2andN3,thebasement highis onlappedon both sidesby post-driftsediments while no clearevidenceforsyn-riftsedimentsareobserved.

Towardsthewest,thisthinnedcontinentaldomainshowsa dif-ferentarchitecture.InseismicsectionN4,abasementhigh, identi-ﬁedastheOMH(between20-40km,Fig.5),showsaremarkable moundshapewithitstopbeingexposedattheseaﬂoor.TheOMH

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Fig. 4. MCSN3inasoutheasterndirectionandthealongproﬁlefree-airgravity(blue)andmagneticanomaly(red).Middle:Originalseismicsection;Blow:ageological interpretationwithlinedrawing.Coloreddashedlinesarethesequenceboundariesseparatingdifferentsedimentaryunit.Abasementhighseparatesthecontinentaldomain fromthetransitionaldomain.Twooceanwarddippinglistricfaults(solidblackline)boundstheSEsideofthisbasementhighandsolesoutintheMoho.Verticalexaggeration is3.

was drilled at three locations (Sites U1504, U1501 and U1505). TwoSiteswereabletopenetratethepre-riftunit/acousticbasement. SiteU1504liesveryclose(

∼

10km)totheeastofthesectionN4, recoveringﬁne- tocoarse-grainedepidote-chloriteschistwith un-knownage(Sunetal.,2018).InSite U1501,locatedfurthertothe SW,thepre-riftUnit/acousticbasement iscomposedofwelllithiﬁed sandstones interbedded with siltstones and conglomerates, con-tainingmagmaticclastspossiblyofMesozoicage(Sunetal.,2018; Larsenetal.,2018).

On the NW side of the OMH, a thick sedimentary package

(1.2

∼

1.7sTWT)isobservedthatconnectswiththesouthern part

ofthe PRMBtowards theNW. Inthisarea,the topofthe pre-rift unit/acousticbasement ismarked by a continuous strongreﬂector without evidenceforsigniﬁcant normalfaulting.ThisOMHis de-limitedbytwooceanward-dippingnormalfaultsintheSEside.

Seismic section SO49-17 lies in the westernmost part of the studyarea(Figs.1,6).TotheNW(between0- 40km,Fig.6),the acoustic basement documents contrasted topographies related to bothlandwardandoceanward-dippingnormalfaults.These exten-sionalstructurescontrolled theformationofriftbasinsassociated withwedge-shapedgeometriesbelongingtothesyn-riftunit1. Lo-cal intra-basement reﬂectors are identiﬁed at about 6.5 s TWT,

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Fig. 5. MCSsectionN4inasoutheasterndirectionandthealongprofilefree-airgravity(blue)andmagneticanomaly(red).Middle:Originalseismicsection;Below:a geolog-icalinterpretationwithlinedrawing.LocationsofSitesU1502andU1500areprojectedalongtheline.Coloreddashedlinesarethesequenceboundariesseparatingdifferent sedimentaryunit.Abasementhigh(referredasOuterMarginHigh,OMH)withamoundshapecanbeobserveddrilledatSiteU1504P,withthetoppiercingtheseafloor. Severalstronginternalreflectorsareinterpretedasoceanwarddippinglistricnormalfaults(dashedblackline)developedwithintheRidgeAinthedistalmargin.RidgeAis penetratedatU1502P.FurtheroceanwardistheRidgeBofbasalticbasement,drilledbyU1500P.Verticalexaggerationis3.

possible corresponding to the depth of the décollement level of listricfaultscharacterizingthethinnedcontinentaldomain.Similar to theother proﬁles, the endof thethinned continentaldomain ismarkedbyacontinentalhigh(between30- 50km,Fig.6)that showsquitehighgravityandmagneticsignals(Fig.1a,c&d). Two oceanward-dippingnormalfaults can be identiﬁedin theSE side ofthisbasementhigh, whichare associatedwithseveralstepsof thetopofthe acousticbasement.Thecrustal thicknessdecreases graduallywiththeMohoshallowingupfrom

∼

9sto

∼

8sTWT. Thebasementisruggedandoverlainbywavysyn-riftUnit1with varyingthickness. Thisunit extendsoceanwardtoabouttheedge ofabasementhigh.

Onlylittle evidenceformagmaticactivityisinterpretedinthe thinned continental domain. Discontinuous sub-horizontal strong reﬂectors within the sedimentary packages present a saucer- or bowl-shaped geometry characteristic of magmatic sills (seismic sectionN4between15- 20km,Fig.5).Localdiscontinuous

reflec-tivepackagesabovetheMohocould correspondtointrusivedikes inthelowercrust,notably towardstheoceanwardendofthe do-main (Fig.3–6).Twowide-angle seismicprofiles (OBS2008(Chiu, 2010) andOBS1993(Yanetal.,2001)),whichliebetweenthe seis-mic sections N2-N3 andN3-N4 respectively(Fig. 1) indicatethat thelowercrustbelowthecontinentalslopecorrespondstoahigh velocity layer(HVL)interpretedasrelatedto magmaticmafic un-derplating.

However,thedeterminationofthetimingofmagmaticaddition

emplacement is challenging notably within basement and some

are likely to predate continentalbreakup. In addition, the lateral repartition ofthesepotential maﬁcunderplating islikely variable assuggestedbythelackofevidencefora HVLinthewide-angle seismic proﬁle OBS2006-1 lying in the west (Fig. 1, Ding et al., 2011).Thelowresolutionofboththegravityandmagneticsignals provides limitedconstraintsgiventhesmallamountof magmatic additions that can be interpreted. Still, they do not support the

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Fig. 6. MCSsectionSO49-17inasoutheasterndirectionandthealongprofilefree-airgravity(blue)andmagneticanomaly(red).Middle:Originalseismicsection;Below:a geologicalinterpretationwithlinedrawing.Coloreddashedlinesarethesequenceboundariesseparatingdifferentsedimentaryunit.Abasementhighseparatestheabyssal basinandthecontinentalslope.Fragmentary,butoccasionallystronginternalreflectorscanbeidentifiedbelowthisbasementhigh.Verticalexaggerationis3.

occurrenceoflargemagmaticintrusionsalongtheproﬁles investi-gated.

4.2. Transitionaldomain

Thetransitional domainalong thefourseismicsections repre-sentsa

∼

15

∼

25kmwideareabetweenthecontinentalandthe oceanic domains. This domain is characterized by (1) an irregu-larcrustalthicknessrangingfrom1.2to1.8s TWT(

∼

7kmto 11 km);and(2)complexandchaoticintra-basementhigh-amplitude reﬂectors(Fig.8).Evidenceofmagmaticadditionsismorefrequent inthisdomainthaninthethinnedcontinentaloneandtheyseem tobecontrolledbytectonicactivity.

The transitional domain in seismic section N2 starts at the southeastward edge of the basementhigh (Fig. 7a). This edge is characterizedby anoceanwarddippinglistricfaultrootingdeeper than in the thinned continental domain, possibly nearthe

inter-preted Moho. The top basement of the transitional domain lies

at

∼

7 s TWT and appears faulted. Several strong amplitude

re-ﬂections are observed within the basement above the Moho at

9 s TWT (between68 – 84km,Figs. 3&7a). Some ofthem have

bowl shape withsharp edges and could correspond todykes

in-truded in the lower crust. We propose that the footwall of the majorlistricnormalfaultiscomposedofthinnedcontinentalcrust withmagmaticadditions,whereasthehanging wallisinterpreted asvolcanoclasticmaterial(includingsedimentsandbasalticﬂows) possibly restingon remnants ofcontinentalcrust. Thelower part

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Fig. 7. Enlargedoriginal(left)andinterpreted(right)sectionsdisplayexpandedviewofthetransitionaldomain.Fig.7afromN2,Fig.7bfromN3,Fig.7cfromN4,andFig.7d fromSO49-17,respectively.

oftheUnit2 (Tg-T60)presentsgrowthstructuresonlappingonthe majorfaultandsealedbyundeformedsediments.

Inseismic section N3,the architecture of the transitional do-mainisverysimilarwiththatofN2.Twooceanwarddipping nor-malfaults boardtheNEpartwithstepped-downbasement possi-blyrootingneartheMohoat

∼

9sTWT(Fig.7b).Thesefaultsseem toseparatedifferenttypesofacousticbasement,showingdifferent seismicfacies. The footwallis characterized by low - to medium amplitude, discontinuous reflectors which are similar with those ofthethinnedcontinentalcrust inthenorthwest. Stronginternal reflectorsexistwithinthebasementabovetheMoho(between45 - 55 km,Figs. 4&7b). We interpretthese light reflectorsas

mag-maticdykesinthelowercrust.Thetop basementonthehanging wallsideshowsmediumto- strong amplitude,sub-continuous re-flectors, overlain by very strong amplitudereflectors on the top (basalticflows?).ThesedimentsofUnit2 areundeformedand un-affectedbymagmatism whichimpliesa shortinterval of magma-tismatbreakup.

ThetransitionaldomaininseismicsectionN4isseparatedfrom the continental domain by the Ridge A (Fig. 7c). Disconnected strong oceanward dipping reﬂectors can be identiﬁed within the basement.Weinterpretedthemasmarkersoflistricnormalfaults

possibly rootingnearMoho between8- 9 s TWT.The basement

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am-Fig. 8. Gatheredgeologicalinterpretationsofallfourseismicsectionsfromeasttowest.Threebasementdomainsareidentiﬁed,includingthethinnedcontinentalcrustwith magmaticadditions;thetransitionaldomain,andtheoceanicdomainwithslightnormalfaults.Thethinnedcontinentalcrustthinstowardtheoceanicbasinwiththickness between7and16km,andisdominatednormalfaultingwithsyn-tectonicsediments.Thetransitionaldomainisnarrow(<20km)anddominatedbyhyper-extendedcrust complicatedbymagmaticintrusion,indicatingarapidrift-to-drifttransitionandinteractionbetweentectonicsandmagmaticprocesses.Theoceanicdomainissteady-state oceaniccrustwithslightnormalfaulting.

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plitude,sub-continuous,parallelto sub-parallelreﬂectors.Drilling resultsat Site U1502 locating a few km east of the lineon this ridgepresentalteredbasalticbreccia,brecciatedbasaltsandaltered pillowbasalts(Sunetal.,2018),conﬁrmingmagmaticactivity. Dif-ferent typesof veins, including composite silica-rich veins, com-posite epidote-richveins andcomposite carbonateveins crosscut thebasaltsandindicate intense hydrothermalcirculation.Clay to claystonewascoredimmediatelyabovebasalticbasementwithan assemblage ofagglutinated benthicforaminifera,indicating a late Eoceneage(Sunetal., 2018). SiteU1499,which is

∼

40kmwest ofSite1502alongRidgeA,endedbelowTginundatedgravel.The

last age datum comes from matrix-supported breccia deposited

between 30 to 26 Ma (Sun et al., 2018). All these drilling re-sultsprovethattheRidgeAwasformedbeforetheinitialseafloor spreading.Thesyn-riftunit1is depositedrightabovethebasement betweenthe OMH andRidge A, and isoverlain by Units 3, 4&5 withhorizontal,parallelreflections.Transparentandcoherent pat-terns are observed in the footwall of the normal faults and are interpreted to consist of rotated continental blocks. Some strong internalreflectorsexistwithinthebasementabovetheMoho (be-tween55 –70km,Figs. 5&7c),possibly indicatingmagma intru-sions. As emphasized by Larsen et al. (2018), the Moho can be followedfrom the thinned continentalcrust until the oceanward

edge of Ridge A sampled by the Site U1502 on the section N4.

This suggests that ﬁrst magmatic additions were emplaced over

andwithinthinnedcontinentalcrust.

Thetransitional domainin seismicsection SO49-17 isdeﬁned

asa

∼

15km-widesegmentbetweenthethinnedcontinentaland

the oceanic domains (Figs. 6, 7d), characterized by a highly re-ﬂectivetop andshallowest Mohodepth (

∼

8 s TWT).No obvious normalfaulthasbeenidentiﬁedwithinthebasement.Thedeposits aboveareun-deformedfeaturedwithparalleltosub-parallel, con-tinuousreﬂectors.

4.3.Oceanicdomain

Alongthe fourseismic sections(Fig. 8) the following features characterizethe architecture ofthe oceanicdomain: (1)thebase ofthe crust isclearly markedby a set ofmoderate tohigh con-tinuity reﬂections, interpreted as oceanic Moho located between 8-9s TWT. (2) the top of igneous basement is characterized by high-amplitude,subhorizontalandcontinuousreﬂections; (3)The crustalthicknessvariesfrom1.8to2.8s TWT,butranges mostly

between1.8 and 2.2 s TWT; and (4) The appearance of oceanic

magneticanomaliesontheproﬁles(MagneticanomalyC10onN3, Fig.4;C11onN4,Fig.5;andC11,C10onSO49-17,Fig.6).In ad-dition,usingwideanglevelocities(Chiu,2010),theoceaniccrustal thicknesswas inferredto rangebetween5.4and8.4km,andthe

top basement morphology is not entirely ﬂat and smooth, but

complicatedbyasetoffaults andtiltedblocks.Inseismicsection N4,the acoustic basementis clearly offsetby severaloceanward normalfaults, forming severalisolated basins (Fig. 5). The lower partoftheUnit2 appearstobesyn-tectonic,forminggrowth struc-tureswiththicknessvariation.TheupperpartoftheUnit2,andof Units3&4 aregenerallynotperturbedbydeformationandsealsthe fault-boundedtiltedblocks.

ThisdomainwasdrilledduringIODPexpeditions367&368.The Site U1500 is located close to the seismic section N4 and sam-pledtheedgeofRidgeB.IODPSite U1500cored

∼

115mbasaltic

basement with a recovery of 77%. Shipboard petrological

stud-ies showed that the basalts have a classical MORB composition

similar to the basalts sampled on the central part of the SCS

oceanicdomain during the IODPExpedition 349 (Liet al., 2015; Sunetal.,2018).Thesebasaltsareoverlainbypost-driftsediments madeofdeepmarineclay,claystone,sandstonewithsiltandsandy siltintervals.

5. Discussion

5.1. ArchitectureoftheCOTandlateralvariability

Some along-strikesimilaritiescan befoundinthearchitecture ofthe transitional domainshownin thiswork.The thinned con-tinentaldomain isgenerallyterminated by a basementhigh sep-arated from the transitional domain by one or two major listric normal fault(s)rooting nearthe interpreted Moho.Oceanward of themajorlistricfault(s),theupperpartofthebasementis reflec-tive, locallyshowinggrowthsequences towardsthe fault(seismic sections N2&N3),possibly corresponding to volcano-clastic mate-rial.Thesmootherandhighly reflectivetopbasementtowardsthe oceanicdomainmightindicatebasalticflow,whichhasbeen pen-etrated by Site U1502. Below thissequence, we suggest the oc-currenceofpieces ofcontinentalremnants.Wethusinterpretthis

domain as an igneous crust with possible variable remnants of

continentalcrust. Thehybrid typeof crustof thetransitional do-mainisinterpretedtorecordtherift-to-drifttransition.Referredto astransitionaldomaininthiswork(usingthesameterminologyas Welfordetal.,2010)(Fig.8),itcomparestothe‘embryonicoceanic crust’ (Jagoutz et al., 2007), ‘proto-oceanic’ (Gillard et al., 2015; Tugendetal.,2018) or‘outerdomain’(Péron-Pinvidicetal.,2007) describedatotherriftedmargins.

A lateral variability of the COT architecture nevertheless oc-curs along strike. In the Epart asshown inseismic sections N2 and N3 (Fig. 8), the transitional domain shows very similar ar-chitecturesfeaturedwithabruptchangesfromthinnedcontinental domainwithlistricnormalfault(s),extremelythinnedcontinental blocksinthelowerpartcoveredbypossiblevolcanoclastic materi-als.Thelowerpartofthesedimentarysuccessioniscontrolledby normal faults.While further to the W,theequivalent ofRidge A isthickerthanintheseismicsectionsN2andN3,possiblerelated withthickercontinentalcrustremnantandintensivemagmatic ac-tivities.

The transitional domain ofthe SO49-17 lying inthe western-mostpartshowssomedifferences. Noobviousnormalfaultswere observed(thiscouldbeduetothelowerresolution)andthewidth isnarrower(

∼

15kmcomparedwith

∼

15-

∼

25kmoftheabove three sections in the east) (Fig. 8). Magmatic additions appear more limited, andno HVL is evidenced beneaththe lower crust. Cameselleetal. (2017) suggestedthatthenarrowcharacterofthis transitionaldomainwasduetoanabruptthinningofthe continen-tal lithosphere in the Northwest Sub-basin(NWSB). A transform faulttermedasZhongnanFracturezonehasbeenreported,which separatestheNWSBfromtheESB,delimitingachangeinthedepth ofacoustic basement(e.g.,Lietal., 2015; Cameselleetal., 2017; Larsenetal.,2018)andmayalsomarkthechangeinthe architec-tureofthetransitionaldomain.

5.2. Tectono-magmaticevolutionoftherift-to-drifttransitionatthe SCS

Rifting at the SCS margin appears at ﬁrst order as a

sym-metric process, strongly controlled by the occurrence of a weak lower crust (Franke et al., 2014; Brune et al., 2017). Polyphase extension was mainly controlled by a major décollement located between the brittle upper crust and ductile lower crust (brittle-ductiletransition),interpretedtoformintra-basementshearzones actingasthemaincrustalthinningmechanism(Frankeetal.,2014; Gao et al., 2015; Zhao etal., 2018). In additionto thismain dé-collement, additional shallower décollement levels can be inter-preted on seismic sections (Fig. 8), controlling the variability in the structural style observed in the thinned continental domain. The occurrence of severaldécollements atvarious depths is also

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Groundarea(e.g.Dingetal.,2013; Liangetal.,2019),highlighting alongstrikevariationsofthearchitectureoftheSCSmargins. How-ever,theyallimagelistricnormalfaultssolingoutinlowercrustal levelsandminorhigh-angle normalfaults showingonlylittle off-set(Fig.8).Thishigh-angle typefaults cannotaccommodatea lot of extension suggestingthat normalfaulting in thebrittle upper crustcannot bethe onlymechanismthat thinnedthecontinental crust.Thepresenceofaweaklower crustattheonsetoftheSCS rifting might havetriggered a ductilenecking of the lower crust withinintra-basementshearzonesinterpretedasthemaincrustal thinningmechanism(e.g.Clift andLin,2001; Franke etal., 2014; Zhaoetal., 2018). Crustalthinning attheSCSwas therefore con-trolledbya non-uniformextensionalmoderesultingfroman im-portantpartitioningofextensionaldeformationbetweenthebrittle uppercrustandaweakductilelowercrust.

Studieson major andtrace elements ofEocenevolcanic rocks frompetroleumdrillholesamplesinthePRMBindicatethelocal occurrence of basaltic rocks emplaced during rifting (Zou et al., 1995).Theyappearlimitedintimeandspace(Gaoetal.,2015).

Thedistal marginrecordsthe latestrifting stageofthe north-ern SCSduring late Eocene(Larsen etal., 2018).Extension likely

remainedsymmetric,accommodatedby pureshearextension

un-tilprogressive coupling betweencrustal andmantle deformation triggeredbreakup(Brune etal.,2014).Thiscouplingbetween up-per crustalandmantledeformation maybeexplained bythe ex-treme thinning of ductile lower crustal levels enabling a crustal embrittlementenhancedbycrustalcoolingduringthinning.Faults that cut the entireresidual crust androoted near the Moho are interpretedasstructuringtheedgeoftheextremelythinned con-tinental crust (locally lessthan 10 km thick). Oceanward, inthe transitionaldomain,magmaticadditionsbecomedominant(Fig.8). The transitional domain seems characterized by the emplace-mentofextrusivebasaltsontopofthehyper-extendedcontinental

crust (MORB-typemagmatism drilledatSitesU1500 andU1502),

intrusivedykesinthelowercontinentalcrust.

Based on theseinterpretations, we suggestedthat continental breakupalongthecentralnorthernSCSoccurredrapidlyduringthe lateststageofrifting,andmagmapotentiallyusedfaultsystemsof thedistalmargintopercolateandinjectthroughthehyper-thinned continentalcrustasexempliﬁedatSiteU1502wereavolcanic ed-iﬁce built over a fault block. The transitional domain of the SCS margin seems characterizedby a progressiveincrease of magma-tism in time andspace fromlatest rifting to breakup, overprint-ing the hyper-thinned continental crust. Breakup likely occurred rapidly in continuum with the crustal and lithospheric thinning thattriggereddecompressionmelting.Thismagmaticeventmight also contribute to the formation of the HVL beneaththe middle andeastpartofthecontinentalslope.

5.3. ImplicationsfortheinterpretationoftheSCSmargintype

Consideringtheapparentreasonableamountofmagmatic sup-plyduringrifting,theabsenceofexhumedmantleorSDRsinthe transitionaldomain,theSCSdoesnotseemtoﬁtintoclassical

end-member magmatic archetypes. Instead, the SCS margin appears

‘intermediate’ in term of magmatic volumes at breakup. Instead ofconsideringmagmaticvolumes,riftedmarginsmayalsobe con-sideredbasedontherelativetimingofdecompressionmeltingand meltextractionrelativetotheamountofcrustalthinning(Tugend etal.,2018).Interestingly,magma-poorsystems,suchasthe Iberia-Newfoundlandcontinentalmargins,arecharacterizedbyadelayin decompression meltingandmeltextraction thatpost-dates litho-sphericbreakup(e.g.Péron-Pinvidicetal.,2007; Bruneetal.,2017; Tugend et al., 2018). This enables the formation of several tens

to hundreds of kilometerswide zones of exhumed

subcontinen-tal mantle potentially experiencing long-duration tectonism and

multi-stage stuttering magmatic activity (Sutra et al., 2013). In

contrast, magma-rich systems show an early onset of magmatic

production and emplacement relative to crustal separation. Ex-treme crustal andlithospheric thinning of the SCS margin likely triggered a rapid onset decompression melting of the ascending asthenosphere.

5.4. Originofsyn-breakupmagmatism

The parameterscontrollingtheonset ofmagmaticactivityand

magmatic supply at breakup are numerous. These include the

mantletemperature(e.g.NielsenandHopper,2004),theupwelling rateofasthenosphere(e.g.Forsyth,1992), theinheritedextension history before seaﬂoorspreading (e.g. Minshullet al., 2001), and possiblethesedimentaryinsulation(e.g.Lizaraldeetal.,2007).An anomalouslylowmantletemperaturewillsuppressmeltingatthe time of continental breakup. These “cool” mantle spots are rare andexistedundersome typicalmagma-poormargins,suchasthe Australian-Antarcticmargin(Cochranetal.,1997) andWestIberia margin(Minshulletal.,2001).Regionsofanomalouslyhighmantle

temperatureappearcommonlyassociatedto magma-richmargins

(e.g. Skogseid,2001). CliftandLin (2001) suggestedthe presence of a signiﬁcant positivemantle thermalanomaly in the northern continentalmarginoftheSCSatthetimeofbreakupbasedonthe calculation of the total amountof subsidence. Other studies also

conﬁrmed anomalously hot thermal condition of the continental

lithosphereatthetimeofextension,implyingthatthecontinental crustwasthermallyweakened(Clift,2015).

We suggest that magmatic additions in the transitional

do-main mainly originated from decompression melting of the

as-cending asthenosphere resulting in typical MORB signatures, as

conﬁrmed by preliminary geochemical data on Sites U1502 and

U1500(Larsenetal.,2018).FastextensionintheSCSmargin even-tually leadingtocontinentalbreakup,seems likeakeyfactorthat enhanced the magmatic production (Larsen et al., 2018). Exten-sion appears to have started in the Eocene (Briais et al., 1993; Bruneetal.,2017), asalsoevidencedfromtheSiteU1501.Inthis site,thelastagedatumisat34Ma,recordedat200m abovethe baseofthesyn-riftsequence. Basedonthisdataset,Larsenetal., 2018inferredalateEoceneagefortheoldestsedimentslyingjust above acousticbasement.Thus,themainrift eventthat preceded breakuplastedlessthan10Myrs.Thisrapidextensionwouldhave favored theupwellingofhotmaterial,withaminimalcooling ef-fect. This mayexplain the generation ofenhanced and localized

magmaduringbreakup.

Future geochemical studies on the cored basaltic rocks from IODPSitesU1500andU1502couldprovidemoredetailed informa-tionaboutthetype ofmantlethat isatthe originofthemagma, themantletemperatureanddegreeofdepletion.

6. Conclusions

Thanks to high quality seismic data and beneﬁting from the IODPExpeditions367&368thatdrilled acrosstheCOT,the north-ernSouthChinaSeamarginrepresentsacriticalnaturallaboratory to studytheprocessesandparameterscontrolling therift-to-drift transition. Threetypesofbasementdomains havebeenidentiﬁed basedonthebasementarchitectureanditsinterpretednature,i.e. thinnedcontinentaldomain withtypicalextensionalstructures, tran-sitionaldomain whererift-to-drift processesarerecorded,andthe oceanicdomain.

Preliminary drilling results in the rift-to-drift transition zone

coredMOR-type basalticbasementwithno evidenceofexhumed

mantle, implying a remarkable difference with the end-member

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suggestthatthistransitionisnarrowandcharacterizedbya com-plexbasementstructure,includingremnantsofhyper-thinned con-tinentalcrustoverprintedbyextrusiveandintrusivemagmatic ma-terial.We suggest that the formation of thishybrid basement is bestexplainedby a magmaticeventthatoccurredrapidly during thelateststage ofrifting totrigger continentalbreakup,followed bysteady-stateseaﬂoorspreading.

Consideringthelimitedmagmaadditionsduringriftingandthe

absence ofexhumed mantle or SDRs, the SCS does not seem ﬁt

into the classical end-member magmatic archetypes. Instead, re-gardingthemagmaticvolumesatbreakup,theSCSmarginappears ‘intermediate’.Breakup occurredafterthecrustal andlithospheric thinningwouldhavetriggeredthedecompressionmeltingonsetof theascendingasthenosphere,andthefollowingseaﬂoorspreading.

Declarationofcompetinginterest

Theauthorsdeclarethattheyhavenoknowncompeting ﬁnan-cialinterestsorpersonalrelationshipsthatcouldhaveappearedto inﬂuencetheworkreportedinthispaper.

Acknowledgement

We are grateful to Dieter Franke, AnneBriais, and Brian Tay-lor for the constructive suggestions and fruitful comments for themanuscript. Funding forthisresearch is provided by the Na-tionalNaturalScience Foundation ofChina (91858214,41890811, 41676027),andtheGlobalchangeandAir–SeaInteractionspecial project(GASI-GEOGE-01,GASI-02-SHB-15).IODPChina isthanked forofferingthepositionsin“JODIESRESOLUTION”.

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