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A complete structural model and kinematic history for distributed deformation in the Wharton Basin
Duncan Stevens, Lisa Mcneill, Timothy Henstock, Matthias Delescluse, Nicolas Chamot-Rooke, Jonathan Bull
To cite this version:
Duncan Stevens, Lisa Mcneill, Timothy Henstock, Matthias Delescluse, Nicolas Chamot-Rooke, et al..
A complete structural model and kinematic history for distributed deformation in the Wharton Basin.
Earth and Planetary Science Letters, Elsevier, 2020, 538, pp.116218. �10.1016/j.epsl.2020.116218�.
�hal-02900789�
Contents lists available atScienceDirect
Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
A complete structural model and kinematic history for distributed deformation in the Wharton Basin
DuncanE. Stevensa,∗,Lisa C. McNeilla, TimothyJ. Henstocka, Matthias Delescluseb, Nicolas Chamot-Rookeb, Jonathan M. Bulla
aOceanandEarthScience,NationalOceanographyCentreSouthampton,UniversityofSouthampton,Southampton,S0143ZH,UnitedKingdom bENSLaboratoiredeGéologie,CNRSUMR8538,PSLResearchUniversity,Paris,France
a r t i c l e i n f o a b s t ra c t
Articlehistory:
Received28June2019
Receivedinrevisedform3February2020 Accepted9March2020
Availableonline23March2020 Editor:J.-P.Avouac
Keywords:
intraplatedeformation IndianOcean diffuseplateboundary IODP
The equatorial eastern Indian Ocean hosts a diffuse plate boundary, where widespread deformation accommodatestherelativemotionbetweentheIndian,AustralianandCapricornsub-plates.Weintegrate IODP Expedition 362 borehole data, which for the first time provides an accurate, ground-truthed chronostratigraphy ofthesedimentary sequenceeastoftheNinety EastRidge(NER),with 2Dseismic reflectionprofilesandmultibeam bathymetrytoassessthestylesoffaultingbetweentheNERandthe Sundasubductionzone,timingofactivityandcomparisonwithphysicalandrheologicalproperties.We identifyfourdistinctfaultsetseastoftheNERinthenorthernWhartonBasin.N-S(350-010◦)orientated faults,associatedwiththeN-SfracturezonesformedatthenowextinctWhartonspreadingcentre,are stillactiveandhavebeencontinuouslyactivesinceatleast10Ma.NNE- andWNW-trendingfaultfabrics developbetweenthefracturezones.Theorientationsandlikelysenseofdisplacementonthesethreesets offaultsdefinesaRiedelshearsystemrespondingto∼NNE-SSWleft-lateralstrike-slipactivityatdepth, demonstratedbytherecent2012greatintraplateearthquakes.Wealsofindevidenceof∼NE-SWreverse faults,similarinstyletoE-W reversefaultsobservedwestoftheNER,where reversefaultingismore dominant.Weshow thattheactivityofthisstrike-slipsystemincreased ca.7-9Ma,contemporaneous withreversefaultingandintraplatedeformationwestoftheNER.
©2020TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense (http://creativecommons.org/licenses/by/4.0/).
1. Introduction
TheoceaniclithosphereoftheequatorialIndianOceanhostsa 3000km wide (West to East)zone ofcomplex deformation rep- resentingadiffuseplateboundary betweentheIndian,Capricorn andAustralianplates(DeMets et al.,1990; Stein andOkal,1978;
Sykes,1970;Wiensetal.,1986).Therelativemotionbetweenthe threeplates,andthetransferofstressfromplateboundariessur- roundingtheIndianOcean,resultsinavariationinsenseandstyle ofdeformation acrossthediffuseplateboundary(Fig.1) (DeMets etal., 1990; Gordon et al., 1998; Wiens et al., 1986). The Nine- tyeast Ridge (NER), a north-south aseismic ridge, that stretches from∼34◦Sto ∼10◦N(∼90◦E), seemstoplay arole inthewest toeastvariation indeformation.Westofthe NER,deformationis characterisedby crustalshortening(BergmanandSolomon,1985;
Bull,1990; SteinandOkal, 1978; Wiensetal., 1986),whereas in theequatorialIndian Ocean,atthe northernendof theNERand
* Correspondingauthor.
E-mailaddress:des1g12@soton.ac.uk(D.E. Stevens).
east of the NER (the northern Wharton Basin, Fig. 1), primarily left-lateral strike-slipmotiondominates (Deplus, 2001; Deplus et al., 1998;Sager etal., 2013). Thechange indeformation is likely duetotheincreasedproximityandinfluenceoftheSundasubduc- tionzone.
ThenorthernWhartonbasin(northof10◦S)includessediments oftheNicobarFan,partoftheBengal-NicobarFansystem(McNeill etal., 2017a).The completesedimentary sectionwas sampledby International Ocean Discovery Program (IODP) Expedition 362 to basementat1415 mbelowseafloor(mbsf)(McNeilletal.,2017b).
Fansedimentsaredominatedbysiliciclastic sedimentgravity-flow deposits(e.g.,turbiditycurrentsanddebrisflows),thatrangefrom clay to silty clay to fine-grained sand. These are underlain by pelagic andtuffaceous sedimentsoverlying ocean basement(Mc- Neilletal.,2017b).Sedimentsaremostlyunlithified,withlithified sedimentsonlyencounteredinthedeepestintervals.
CompressionaldeformationwestoftheNER,between6◦Nand
∼8◦S,iswelldocumented(e.g.BergmanandSolomon,1985;Bull etal., 2010;BullandScrutton,1990,1992;Chamot-Rooke, 1993;
Krishna et al., 2001, 2009; Stein et al., 1989; Stein and Weissel, 1990; Stein andOkal, 1978) by theintegration ofseismic reflec- https://doi.org/10.1016/j.epsl.2020.116218
0012-821X/©2020TheAuthor(s).PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/).
Fig. 1.A)Mapshowingregionaltectonicconfiguration.Reddashedlineindicatesex- tentofdiffusedeformation(Gordonetal.,1998).Redtoothedlinesshowschemati- callytheareaaffectedbyprominentreversefaults(BullandScrutton,1992;Deplus, 2001).Earthquakefocalmechanismswithin>2degreesofnarrowplateboundaries, scaledbasedonmagnitude:brown=strike-slip;orange=thrust;blue-grey=nor- mal(GordonandHouseman,2015,events5.3<Mw<8.6,1976-2014).Lightblue boxshowsenlargedinpartB(andapproximateareashowninFig.10).NER–Ninety EastRidge;CR–CarlsbergRidge;CIR–CentralIndianRidge;SWIR–SouthWest IndianRidge;RTJ–RodriguezTripleJunction;SEIR–SouthEastIndianRidge;CAP –Capricornplate.Bathymetry contouredat2000mintervals.B).Yellowdotsin- dicateIODPdrillsites.BlacklinesaremultichannelseismicprofilesA-C,sections highlightedinyellowindicatethelocationsofseismicdatadepictedinFigs.5,6 and8.Lightblueshadedareashowsmultibeambathymetrycoverageusedinthis study(presentedinFig.4)Red,dashedlinesarefracturezones(Jacobetal.,2014) (nomenclaturefromSinghetal.,2011).FocalmechanismsforApril2012events inred,datafromGlobalcentroidmomenttensorproject[www.globalcmt.org].Or- angeshadedareashowsruptureplanemodelfor2012earthquakesfromWeietal.
(2013).SSZ–SumatraSubductionZone.
tiondatawithinformationfromoceandrillingsites(ODPLeg116, DSDP 215,218).Deformation takesplacealong E-W trendingre- versefaults,i.e.∼N-Scompression,interpretedasreactivatingthe originalridge-parallel spreadingfabric duetostresstransfer from theN-ScontinentalcollisionbetweenIndiaandEurasia(e.g.,Bull andScrutton,1992,1990;Chamot-Rooke,1993).Faultreactivation herestartedat ∼14-15.5 Ma (Krishnaetal., 2009), withsugges- tions of increased activity at ∼7-8, 4-5 and 0.8Ma (e.g. Bull et al., 2010; Krishnaetal., 2001).Bulletal.(2010) showedthat in- creasedcompressionaldeformationat∼7-8Ma(e.g.Krishnaetal., 2009) coincideswiththeaccelerationofIndian-Capricornrotation andrelativeconvergencefromplatemotionmodels(DeMetsetal., 2010,2005).Manyhavesuggestedthatthetimingofpulsesindis- talfaultactivityarerelatedtoupliftoftheHimalayasandTibetan Plateau (Bulletal.,2010;Gordon, 2009; MerkourievandDeMets, 2006;MolnarandStock,2009),bucklingtheIndo-Australianlitho- sphere(Copleyetal.,2010;Weisseletal.,1980).However,studies oftheHimalayasandTibethavegeneratedarangeoftimingsfor activityandaccelerateddeformationoverthelast20 Ma(e.g.Clift etal., 2008; Molnaretal., 1993;Molnar andStock,2009),there- forecorrelating discrete collisional/uplift episodeswithphases of distalcompressionaldeformation ischallenging.Alternatively,Iaf- faldano etal., 2018have suggested that an acceleration in com- pressionaldeformationat7-8MawithintheIndo-Australianplate
maybelinkedtoincreasedasthenosphericflowresultingfromthe re-emergenceofvolcanismalongtheRodriguesRidgeat11Ma.
Until now the timing of intraplate deformation in the Indo- Australian plate has not been well-constrained east of the NER.
There are fewer studies of deformation (e.g. Deplus et al., 1998;
Geersenetal.,2015;Singhetal.,2017;Hanantoetal.,2018),and until recently there hasbeen no direct agecontrol. Seismicity is generally characterised by left-lateral strike-slip motion on reac- tivated N-S trending fracture zones that offset the E-W trending WhartonRidge, afossilspreadingcentre(Deplus,2001;Depluset al., 1998). Spreadingatthe Wharton Ridgewas active atleastas earlyas84Ma,separatingIndiafromAustralia,butceasedaround 42 Ma (alongwithrelatedtransform fault/fracturezone activity), withtheIndianandAustralianplatesbecomingasingleplate(Liu etal.,1983).Thesametectonicdrivingmechanismresponsiblefor compressionaldeformationwestoftheNERmayalsohavereacti- vatedtheWhartonRidgefracturezonesbutthereactivationtiming ofthelatterisunclear. Thedifferenceindeformation styleonei- ther side of the NER maybe dueto variation in the stress field within the Indo-AustralianplatewiththeSundasubduction zone causing the principal stress in the Wharton basin to be NNW- SSE, compared to N-S west of the NER (Gordon and Houseman, 2015).
In April 2012, thelargest andmostcomplex strike-slipearth- quakes ever recordedruptured aset ofWNW-ESE and NNE-SSW fault planes oblique to the N-S fracture zones. The initial main- shockrupturedalongaWNWtrendingfaultwithacentroiddepth of ∼30 kmwitha Mw of8.6, that initiated multipleruptures to thenorthandsouthonNNEtrendingfaultsatsimilardepths,this was then followed by another Mw 8.2 earthquake that ruptured a WNWtrending fault ∼180kmto the south(Fig. 1; e.g. Dupu- tel etal.,2012;Hilletal., 2015; Weietal.,2013).Recent studies have identified faults/lineationsfrombathymetry data withsimi- lar orientation to faults modelled to haveruptured (e.g. Geersen etal., 2015; Hanantoetal., 2018;Singhetal.,2017).The various orientationsobservedhavebeeninterpretedasaRiedelshearsys- tem relatedto thereactivated N-Sfracture zones(Geersen etal., 2015;Hanantoetal.,2018).Geersenetal.(2015) suggestedthese Riedel shearfabricsdevelopedaround20Ma,withfracturezones reactivated since 40 Ma. However, these timings were based on pre-drillingchronostratigraphicestimates.
Data from IODP Expedition 362 boreholes provides the first ever core-stratigraphic ages for the sedimentary sequence east of the NER (McNeill et al., 2017b). These data indicate that the chronostratigraphy of the sedimentary sequence is significantly different to that previously assumed. Based on this new infor- mation, we update herethe activity timingsof all types of fault in the northern Wharton Basin, east of theNER. In addition, we have conducted a thorough integration betweenthe seismic and bathymetrydatainthestudyarea,toderivefaultorientationsand fault types.Together, thesenewintegrateddataenable usto per- form a more complete fault analysis. We also directly compare our results, east of the NER, with previous studies west of the NER,forthefirsttimewithchronostratigraphyinbothlocations,to correlateandexamineexistingdiscrepanciesbetweendeformation events and fault activity timing across the eastern Indian Ocean and discuss potential deformation forcing mechanisms. We also usetheIODPresultstotestrelationshipsbetweenfaultingandthe lithologicalandphysicalpropertiesoftheoceanicplatesediments.
2. Methods
2.1. MCSdataandinterpretation
We havere-interpreted multichannel seismic reflection(MCS) data fromthe northern Wharton Basin(e.g. Fig. 2) (building on
Fig. 2.RegionalseismicprofileBandpositionofIODPSiteU1481(verticalpinkline).(A)Uninterpretedand(B)interpreted,withfaults(black),horizons/reflectors(coloured) R1toR14andchannels(red).Dashedblueboxesindicatethelocationsofexpandedsections(Fig.6E(seebelow)andexamplesX1andX2(seesupplementarymaterial).
NC=NicobarChannel;NER=NinetyEastRidge.Fig.9showsalongprofileanalysisforthisprofile.
Deanetal.,2010;Geersenetal.,2013,2015;McNeilletal.,2017a).
The seismic data are two composite SW-NE profiles A (BGR06 101-102)and B(BGR06103-104-105)fromGaedickeetal. (2006;
Cruise SO186 on FS Sonne) which extend from the NER to the SundasubductionzoneatNorthSumatra(Fig.1),andNW-SEpro- file C (MD116-ANDAMAN84) from Chamot-Rooke (2000; Cruise MD116 on the R/V Marion Dufresne). The seismic data have a verticalresolutionof∼10-15 m.Weinterpretallfaultswithmax- imumverticaloffsets>10 msoftwo-way-travel-time (TWT)(ap- prox.10 m),includingblindfaults(someobviousblindfaultswere interpreted that have less than <10 ms vertical separation). We interpretanduseroughly evenlyspacedseismichorizons R1-R14 that can be continuously correlated.Some of thesereflectors are equivalenttothoseofGeersenetal.(2015),whileR11corresponds tothe‘High-Amplitude-Negative-Polarity’ (HANP)pre-décollement reflectorof Deanetal.(2010). Welocally interpretadditionalre- flectorstoallowdetailedanalysisofselectedfaults.
2.2.IntegrationwithIODPExpedition362boreholedata
Allthree seismic profiles intersect IODPExpedition 362 bore- holesitesU1480andU1481wheresoniclogandcore-logseismic integration(McNeilletal.,2017b)allowustodeterminethedepth intervalsofreflectors R3 to R12,and R14 andcorrelateborehole datatotheseismicstratigraphy.Agecontrolisbasedprimarilyon calcareousnannofossils,andwenotethattheagemarkersareap- proximate (see Fig. 3; McNeillet al., 2017a,2017b). The section isdominatedbyNicobarFansedimentationsince9-10Maincon- trasttotheolderpre-drillingpredictedagesusedbyGeersenetal.
(2015).Wealsocomparefaultdisplacementwithotherparameters intheIODPboreholes,includinglithology,physicalproperties(e.g., porosity,velocity),andfractureintensity.
2.3.Multibeamprocessingandinterpretation
We reprocessed a subset of the multibeam bathymetry data fromcruiseSO186collectedusingtheSIMRADEM120Multibeam System.Weappliedanupdated andimprovedsoundvelocitypro- file correction, a ship roll correction and removed spikes from
individual pings, to improvethe signal to noise ratioof the true seafloor topography. We then re-interpreted this data and re- interpretedmultibeamdatafromcruiseMD116.
2.4. Verticaloffsetmeasurements
Wemeasuredtheverticaloffset/throws(hereafterreferredtoas displacements)forreflectorsR2toR12acrossallinterpretedfaults on the NE-SW seismic profiles. We developed a semi-automated methodwhichreliesonthepreciseandconsistentinterpretationof thereflectorsacrosseachfaulttocalculateoffset.Seismichorizons werepickedincommon-depth-point(CDP)–two-way-time(TWT) (ms) space. CDPspacingis 6.25m.Seismichorizonswere picked so horizonsterminateatthe fault-reflector intersection,leavinga horizontalgapacrosseach normal-offset fault.Thevertical gapis the fault throw in TWT. Where fault drag is apparent, horizons were picked so that the maximumvertical offsetof thereflector acrossa faultis maintained(i.e.extrapolationfromeitherside of a fault unaffected by drag). We are therefore confident that our interpretationrepresentstrueverticalseparation.Theseismichori- zons provide adatabase ofTWTpicks at every CDPwherepicks weremade.
WeconvertthefaultdisplacementTWTmeasurementstome- tresusingseismicintervalvelocitiespickedat∼1000CDPspacing from the original processingof the MCSreflection data by BGR.
ThisisanadditionalimprovementonGeersenetal.(2015),which used a single set of estimated interval velocities throughoutthe seismiclines.
Aroundfaultsofparticularinterest,weincreasethenumberof interpreted horizonsandgrid all ofthe observedvertical separa- tionsasaheatmap(seeFig.6Cand G).
3. Results
3.1. Biostratigraphy(fromIODP)
Biostratigraphictiepoints fromtheIODP(Fig.3)boreholedata show two distinct periods of sediment deposition in our study
Fig. 3.Interpolationofreflectoragesfromcore-seismicintegration.Colouredcrossesareseismicreflectors(Fig.2).Theageofreflectorsisestimatedbytime-depthconversion andlinearsedimentationaccumulationratesbetweenbiostratigraphictiepoints(McNeilletal.,2017a,2017b).Errorsofreflectoragesare±5%duetovelocityuncertainty andtheagerangesoftiepoints.
areaoccurringatdifferentrates(McNeilletal.,2017a,2017b).The age of the oceanicbasement is ∼68 Ma. Basal materials, domi- nantlypre-fanpelagicsediments,are∼150mthickatSiteU1480 (Exp.362UnitsIII-V)andaccumulatedatanaveragerateof∼2.5 mMyr−1overthe∼60Myrperiodfrom68to9Ma(rateexcluding hiatuses).TheoverlyingNicobarfansediments(Exp.362UnitsI-II) are∼1250mthickatboreholeU1480andaccumulatedover9Myr atanaveragerateof∼139mMyr−1.Sedimentthicknessoverlaying theoceanicbasementincreasesto∼4 kmtowardsthesubduction zone dueto theinfluence ofplateflexure and fillingofthe sub- ductionzonetrench(SU1,Fig.2).Mostofourisochrones(laterally continuousseismicreflectors)arewithinthethickfansediments.
3.2. Faultgeometry
Seismicprofiles (Figs.1 and2) show pervasivefault deforma- tionacrossanapproximately300kmwide areabetweentheNER andtheSundasubductionzoneinthenorthernpartoftheWhar- tonbasin.Thebathymetricdataimagethesamestructuresinplan view andindicate a range of orientations,which vary acrossthe oceanicplate.ThelargenumberoffaultsimagedonprofilesAand Bareroughlyevenlydistributedfromwesttoeast.Thefaultdipis between60◦ and75◦ withbothlandwardandseawarddipdirec- tions(along-profile).ProfileCimagessub-verticalfaults,whichare moreheterogeneouslydistributedfromnorthtosouth.
Weobservenumerouslineamentsinthemultibeambathymetry data(Fig.4). Weareconfident thattheselineamentsare thesur- face expression of faults, since they can be tiedto faults in the seismic data that displace the seafloor. Lineaments show three distinct orientationsbetween theNER andthe Sunda subduction zone that are geographically distinct from each other. We group theselineamentsandassociatedfaultsintothreeClasses.Between the NER and ∼91◦45E Class A faults are the dominant fabric, trendingNNE-SSW (020-030◦);between∼91◦45Eand∼92◦20E Class B WNW-ESE (100-120◦) lineaments dominate; and ∼be- tween 92◦20E and 93◦ Class C N-S (350-010◦) dominate. It is unclear whether the Class A and B fabricsoverprint each other, butin thearea of ClassB faulting, very few NNE-SSW faults are observed in the bathymetry suggesting that there is a genuine change fromone orientation to theother. In the area of ClassC
faulting,ClassAandBfaultorientationsareeffectivelyabsent.The N-Strendingfaults(ClassC)areingoodagreementwithboththe positionandorientationofpreviouslymappedWhartonRidgefos- silfracturezones(e.g.Singhetal.,2011).
3.3. ClassAfaults
Class A faults are only observed on seismic profile C in the west of our study area (Fig. 1). They are sub-vertical, propagate through the entire sedimentary section and deform the oceanic basement.The senseofdisplacement isunclear,sedimentary lay- ers frequently show a v-shaped patternthat is expressed at the seafloor (Fig. 5A, B). We also note differences in sedimentary layerthicknessacrossthesefaults andcomplexdisplacementpat- ternsthatindicatebothapparentnormalandreversedisplacement (Fig. 5C).These featuresare verysimilar tointerpretedstrike-slip faults attheNER(Sager etal., 2013) andstrike-slipfaultsingen- eral.Thecomplexdisplacementprofilesforthesefaultslimitsour ability toconstrain thetiming oftheir formation andsubsequent activity.
3.4. ClassBfaults
ClassBfaultsareimagedbyseismicprofilesAandB,makingup the majorityoffaults ontheseprofiles.ClassB faultsformstruc- turesofconjugatenormalfaultsthatconvergeatdepthwithinthe sedimentary section, showing little to no offset at or below re- flector R12 (e.g.Fig.6,EandF).Typically,a centralpairoffaults that reach theseabed meetat around 1.5 s TWTbelow seafloor (∼1800-2000 mbsf)inthevicinity ofreflector R10(8.6 Ma).One orboth ofthesecentral faultsmaypenetratetheentiresediment section, butdie out beforeor very shortlyafterreaching the top oftheoceanicbasement.Thecentralpairoffaultsaresurrounded by multiple, typically 10-20, seismically-resolvable faults, with a roughlyevendistributionbetweenseaward- andlandward-dipping geometries. These minor faults include multiple blind faults that branchfromthecentralpair(hard-linked)andfaults thatarejust spatiallyrelated(soft-linked).Thenumberofblindfaultsincreases below reflector R5 (2.6 Ma), but then decreases below reflector
