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Mitchell, Neil C. and Davies, John Huw 2018. Equatorial Pacific gravity lineaments: interpretations

with basement topography along seismic reflection lines. Marine Geophysical Research 39 (4) , pp.

551-565. 10.1007/s11001-018-9351-x file

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1

EquatorialPacificgravitylineaments:interpretationswithbasement

topographyalongseismicreflectionlines

NeilC.Mitchell1andHuwDavies2

1SchoolofEarthandEnvironmentalSciences,UniversityofManchester,

WilliamsonBuilding,OxfordRoad,ManchesterM139PL,UK.

2SchoolofEarthandOceanSciences,CardiffUniversity,MainBuilding,Park

Place,CardiffCF103AT,UK.

Keywords:oceanicthermalsubsidence,oceanicplateisostasy,freeairgravity

anomalies,Pacificplatedeformation,mantledynamics

Thisisthegreenopen-accessversionofthisarticleaccepted14March2018for

publicationinMarineGeophysicalResearch.

Abstract

ThecentralequatorialPacificisinterestingforstudyingcluestouppermantle

processes,astheregionlackscomplicatingeffectsofcontinentalremnantsor

majorvolcanicplateaus.Inparticular,themostrecentlyproducedmapsofthe

2

free-airgravityfieldfromsatellitealtimetryshowingreaterdetailthepreviously

reportedlineamentswestoftheEastPacificRise(EPR)thatarealignedwith

platemotionoverthemantleandoriginallysuggestedtohaveformedfrom

mantleconvectionrolls.Incontrast,thegravityfield600kmorfartherwestof

theEPRrevealslineamentswithvariedorientations.Somearealsoparallelwith

platemotionoverthemantlebutothersaresub-parallelwithfracturezonesor

haveotherorientations.Thisregioniscoveredbypelagicsedimentsreaching

~500-600mthicknesssobathymetryisnotsousefulforseekingevidencefor

platedeformationacrossthelineaments.Weinsteadusedepthtobasement

fromthreeseismicreflectioncruises.Insomesegmentsoftheseseismicdata

crossingthelineaments,wefindthattheco-variationbetweengravityand

basementdepthisroughlycompatiblewithtypicaldensitiesofbasementrocks

(basalt,gabbroormantle),asexpectedforsomeexplanationsforthelineaments

(e.g.,mantleconvectionrolls,viscousasthenosphericinter-fingeringor

extensionaldeformation).However,someotherlineamentsareassociatedwith

majorchangesinbasementdepthwithonlysubtlechangesinthegravityfield,

suggestingtopographythatislocallysupportedbyvariedcrustalthickness.

Overall,themultiplegravitylineamentorientationssuggestthattheyhave

multipleorigins.Inparticular,weproposethatafurtherasthenosphericinter-

fingeringinstabilitymechanismcouldoccurfrompressurevariationsinthe

asthenospherearisingfromregionaltopographyandsuchamechanismmay

explainsomeobliquelyorientedgravitylineamentsthathavenootherobvious

origin.

Introduction

3

HaxbyandWeissel(1986)discoveredaseriesoflineamentsinthemarine

gravityfieldrecordedusingsatellitealtimetry,formingelongatedtroughsand

swellsof5-20mGalamplitudespaced~200kminthePacificandIndianOceans.

AsthelineamentsinthePacificOceanwereparalleltothepresentplate

movementoverEarth’smantleandcrossfracturezones("cross-grain"),they

wereinitiallyspeculatedtobecausedbyelongatedsmall-scaleconvectionrolls

intheasthenosphericmantle,whichwereexpectedtodevelopwithinthefirst

fewmillionyears(BuckandParmentier1986;HaxbyandWeissel1986).That

convectionwouldprovideverticalstressesthatinitiallydistorttheoverlying

lithosphere(Figure1a),creatingtopographythatbecomes"frozenin"bythe

coolingplate.Subsequently,otherauthorshavecharacterisedandcommented

ontheseandlargersimilarfeaturesinsatellitealtimetrydata(Baudryand

Kroenke1991;Cazenaveetal.1992;FleitoutandMoriceau1992;Maiaand

Diament1991;McAdooandSandwell1989;Sandwelletal.1995;Wesseletal.

1996)andtheconvectionmodelhasreceivedrenewedinterestmorerecently

(Ballmeretal.2009;Harmonetal.2006).

Otherexplanationsforthegravitylineamentshavebeenputforward.

WintererandSandwell(1987)suggestedthatextensionarisingfrom

lithosphericcooling(TurcotteandOxburgh1973)couldexplainen-echelon

volcanicridgesfoundwithinthePacificplatethatareparalleltothesegravity

lineaments,withthelithospheredeforminginaboudinagemanner(Figure1b).

Alternatively,extensioncouldarisefromfar-fieldstressescausedbyslabpull

associatedwiththePacificplate'ssubductionzones(DunbarandSandwell

1988).GoodwillieandParsons(1992)usedseparationsoffracturezonesat

differentlocationstoassessplateextensionintheSouthPacificabouttheEast

4

PacificRise(EPR),butruledoutsignificantextensionasexplaininglineaments

there,aconclusionsimilarlyreachedbyGansetal.(2003).Gansetal.(2003)

suggestedinsteadthatflexingofthelithospherecausedbyvariedcoolingrates

withdepth(Figure1c)couldexplainthegravitylineamentspacingsand

amplitudes.Cormieretal.(2011)presentedmultibeamdatashowingfaultsthat

supportthepossibilityofsuchextensionoftheCocosplate.

Toexplainacorrespondencebetweenslowseismicvelocitiesbeneath

seamountchainsofanareaofthesouthernEastPacificRise,Weeraratneetal.

(2007)andHolmesetal.(2007)suggestedthataviscousinter-fingeringcould

occurinasthenosphereaffectedbyanoff-axismantlethermalanomaly.

Essentially,buoyancyinasthenosphereoff-axiscausesittorisetowardstheaxis

beneaththedippinglithosphericlid.Fingersofthemobileasthenospheremay

explainvolcanicseamountchains(Figure1a)thatdonotagesystematicallyas

expectedoftraditionalmantlehotspotorigins(Ballmeretal.2009).

Oneofthemostrecentlyreleasedgridsofthemarinegravityfield

(Sandwelletal.2014)showninFigure2brevealsthelineamentsoriginally

identifiedbyHaxbyandWeissel(1986).Thegreaterresolutionofthisnewgrid

revealsafinerstructureimmediatelywestoftheEastPacificRisethaninthe

earlieraltimeterdata,withtroughsandswellsspacedascloselyas~20km.

Figure3c(describedlater)showsahigh-passfilteredversionofthegravitydata.

Init,thelineamentswestoftheEastPacificRisecanbeseengivingway

westwardsintoapatternoflineamentsthatismoreconfusedthoughwiththree

ormoredominantorientations(redarrowsannotatedA-F),whichwefocuson

here.

5

Inthisstudy,wetakeadvantageofinterpretedseismicreflectiondata

fromthreeresearchcruises(Bloomeretal.1995;DuboisandMitchell2012;

Eittreimetal.1994;Gnibidenkoetal.1990)andarecentcompilationofseafloor

spreadinganomalies(Barckhausenetal.2013)toevaluatetheextenttowhich

basementreliefvarieswithgravityanomaliesacrosstheselineaments,

supportingsomeoftheexplanationsforthelineamentsthathavebeenput

forward.ThemodelprofilesshowninFigure1suggestthat,astheuppersurface

oftheplateisamajordensitycontrast,itsdeformationbythevarious

mechanismsshouldleadtogravityanomaliesvaryingsympatheticallyby

amountsexpectedofbasementwithahighdensitytypicalofmantlerocks.This

assumesthatthecrusthasuniformthicknesssoitcontributesanearlyuniform

amounttothegravityfield.Inpractice,however,thecrustmayvaryinthickness,

inparticular,wherevolcanismhasbeenpromoted(e.g.,atlocationsmarked“v?”

inFigure1),hencethecomparisonisnotstraightforward.Aspartofthework,

wealsore-evaluatethePacifictectonicplatesubsidence.Previoussubsidence

studies(CrosbyandMcKenzie2009;ParsonsandSclater1977;Trehu1975)

havetendedtousegriddeddatasets(e.g.,sedimentthicknessfromseismic

reflectiondata(Divins2003;LudwigandHoutz1979;Whittakeretal.2013)),

whereerrorsmayhavebeenpossibleduetoincompletesamplingandreflective

cherthorizons(Mitchell1998).Thenewresultsallowustoverifythoseearlier

subsidenceratesandlocatewheretheseabedliesaboveorbelowthosetrends.

Dataandmethods

Seismicreflectiondata

6

SeismicreflectiondatawereobtainedduringtheAMAT03cruiseofRV

Revellein2004,whichwasasite-surveycruiseforIODPExpedition320/321.

Recordingsweremadewitha4-channelstreamertowedat~10knotsbetween

sitesandwitha48-channelstreamertowedat~6knotsatthedrillsites.The

seismicsourcecomprisedtwo150cubicinchgenerator-injector(GI)guns.A

segmentofthedataisshowninFigure4,illustratingthetypicalvisibilityofthe

basementreflection.Thefulldatasetistoolargetopresenthere,thoughseveral

imagesofthedatacovering>1000kmwereshownbyTominagaetal.(2011)

andDuboisandMitchell(2012),andthefulldatasetcanbeobtainedfromthe

UTIGAcademicSeismicDataPortal(http://www.ig.utexas.edu/sdc/).

TherecordedseismicdatawereinterpretedasdescribedbyDuboisand

Mitchell(2012)(alsoseeMitchellandDubois(2014)),includingthebasement

reflectiontwo-waytime,whichisplottedagainstshiptracksinFigure5.To

computeaccuratelythedepthofthebasementbelowtheseabedfromthe

intervaltwo-waytimet,allowanceneedstobemadeforvariationsofseismic

velocitywithsedimentdepthbecauseofcompactionanddiagenesisinthese

sediments(carbonateooze).Mayeretal.(1985)estimatedinsituvelocityfrom

loggedcorevelocitydatafromthecentralequatorialPacific,correctingmainly

forlossofconfiningpressure.Thevelocityvariationwithdepth(~1500ms-1at

thesurfaceto>2000ms-1below400metresbelowseafloor(mbsf))was

approximatedwithalineartrend,V=V0+Bz,wherezisdepthbelowseabed,and

coefficientsV0andBfoundbyleast-squaresregression(V0=1479ms-1;B=

1.0286s-1).Thefollowingequationderivedbyintegrationwasusedhereto

estimatez:

7

(1)

Equation(1)wasusedtocalculatebasementdepthfromthetwo-waytimedata

derivedfromtheAMAT03seismicreflectiondataandtheVenture-1dataof

Bloomeretal.(1995).

AsdescribedbyEittreimetal.(1994),theRVAkademikSelskiydatawere

collectedwitha24-channel,2400-mstreamerwithairgunarraysof23or46L

andmostlyshotwith24-foldalongthelinemarkedinFigure2.Afterprocessing,

thedatarevealedMoho,crustalandbasementreflections.Theseabedand

basementreflectiontwo-waytimesinterpretedbyEittreimetal.(1994)were

digitizedandconvertedtodepthusingasedimentvelocityof1540ms-1typical

ofODPSite1219measurements(Lyleetal.2002)andawatervelocityof1500m

s-1.Eittreimetal.(1994)alsoidentifiedflat-lyingMohoreflectionsandmodestly

dippingreflectionsinthelowercrust.Two-waytimesfrombasementtothe

Mohoreflectionswereconvertedtodepthwithasimplifiedoceanicvelocity

structureofanuppercrustofconstant2.5kmthicknesswithvelocity(Vp)of5

kms-1overlyingagabbro-dominatedlowercrustofvariedthicknessandvelocity

of7kms-1(MutterandMutter1993).Asegmentofthereflectioninterpretations

areshowninFigure6.

Magneticanomalyidentifications

Magneticanomalieshavebeenre-evaluatedinthisregionbyBarckhausen

etal.(2013),exploitingthegreatercoverageprovidedbyrecentresearchvessel

transits.Onesetoftransitsinvolvedavectormagnetometer,whichaided

anomalyidentificationsinthislowfieldareanearthemagneticequator.The

z =V0

BeBt/2

−1( )

8

densityofidentifications(redcrossesinFigure2a)wassignificantlyimproved

comparedwithpreviousstudies.

Crustalagescorrespondingwiththeirmagneticanomalyidentifications

andsomeoftheirisochronsaroundfracturezonesweregriddedusingasurface-

fittingprogram(SmithandWessel1990).Figure2ashowsagecontoursderived

fromthatgrid.Thegridwasthensampledalongtheseismicsurveytracksto

obtaincrustalageateachseismicmeasurement.(CrustalagecontoursinFigure

2adonotperfectlyhonourtheisochronsofBarckhausenetal.(2013)overthe

wholearea,butthegridwasconstructedtomatchfaithfullyisochronswhere

theyunderlietheseismicshiptracks.)FortheAkademikSelskiydata,crustal

agesweresimilarlyassignedusingtheisochronsofBarckhausenetal.(2013)

and,wheretheywerenotavailable(<14Maand>76Ma),fromtheagegridof

Mülleretal.(2008).

Subsidencetrends

BasementdepthsderivedfromtheAMAT03,Venture-1andAkademik

Selskiycruiseswerecorrectedforisostaticsedimentloadingusingasediment

wetbulkdensityof1.4gcm-3(acolumn-averageofIODPcoremeasurements

(Lyleetal.2002;Pälikeetal.2010))andamantledensityof3.3gcm-3.Those

correcteddepthsareshowninFigures7and8versusthesquarerootofseafloor

age.Simpleleast-squaresregressionsofthedatawasusedtofindthe

subsidencerates,withthatinFigure8calculatedfromdatarestrictedtoless

than8Ma1/2wherebasementdepthsbreakfromthesubsidencetrend.

BasementelevationresidualsabouttheregressionlinesofFigures7and8are

9

shownalong-trackinFigures5and3(theseareresidualelevationsratherthan

residualdepthsaspositivevalueslieabovetheregressionline).

Gravityanomalies

Freeairgravityanomalieswerederivedfromsatellitealtimeter

measurements(SandwellandSmith1997).ThedatashowninFigure2bare

fromversion23ofthefree-airanomalygridavailablefrom

http://topex.ucsd.edu/.AccordingtoSandwelletal.(2014),thisversionhasan

averageaccuracyof~2mGalindeepwater,withmostoftheimprovementover

earlierversionsinthe12-40kmwavelengthbandandimprovedresolutionof

featurestoassmallas6km.

ForcrustthatislocallyAirysupported,variationsinbasement

topographyhavelittleeffectonthefree-airgravityfieldbecausetopographyis

supportedbythickenedcrustoflowerdensitythantheunderlyingmantle(i.e.,

theincreasedgravitationalattractionfromtheelevatedtopographyisalmost

balancedbythereducedattractionfromthegreatercrustalrootwhichdisplaces

densemantlerocks).However,short-wavelengthtopographycanbesupported

bytherigidityofthelithospheresoitcanaffectthegravityfield.Free-air

anomaliesthatarehigh-passfilteredthereforereflectcrustaldensityand

topographyvariations,whereaslow-passfilteredanomaliesshouldreflect

potentialdeepermantlesources(non-Airysupportedtopography).Relevantto

thefilterlength-scalerequired,accordingtoCochran(1979),theelastic

thicknessofthelithosphereis2-6kmabouttheEastPacificRise.Modellingby

Smith(1998)suggeststhatlittleeffectofcrustalvariationsappearinthegravity

fieldforspatialscaleslargerthan200kmfortheseelasticthicknesses.The

10

gravityfield(Figure2b)wasthereforeconvolvedwithacosine-taperedweight

of400kmfullwidth(WesselandSmith1991),whichcontainsmostweightover

200kmdistance.TheresultinFigure3awassubtractedfromthefield(Figure2)

toproducethehigh-passfilteredgravityfieldinFigure3b.Thus,Figure3ais

intendedtorepresentdeeper(mantle)sources,whereasFigure3bshould

representmainlylateralvariationsinbasementtopography,orcrustaldensities

andthickness.

LinesegmentsV1,A2,A4andA7locatedinFigure2arunalongcrustal

isochrons,soanyeffectoflithosphericthermalsubsidenceshouldbeuniform

alongthem.Fordiscussionofvariationsofgravitywithbasementstructure,

Figure9showscrosssectionsofbasementandseabedtopographyalongwith

theunfilteredfree-airanomalysampledfromthegridprovidedbySandwelletal.

(2014).

Thefree-airanomaliesshowninFigure9areaffectedbythesediment-

waterdensitycontrastattheseabedaswellasthebasementanddeeper

contraststhatweareprimarilyinterestedin.Toreducetheeffectoftheseabed

contrast,Bouguergravityanomalieswerecalculatedbyaddingthefollowing

quantitytothefree-airgravityanomalies:

Δg=2πGΔρh (2)

whereGisthegravitationalconstant(6.6726X10-11m3kg-1s-2),Δρisthe

densitycontrastbetweenseabedsedimentsandseawaterandhiswaterdepth.

Theformulaaccountsforthegravitationalattractionofalayerofmaterialof

infiniteextentandhenceweuseitassumingthatthetopographyoftheseabedis

slowlyvaryinglaterally.ThegridofBougueranomalieswasfilteredwitha400

kmfilterasdescribedearlierforthefree-airanomaliesandtheresultingresidual

11

anomalies(effectivelyhigh-passfiltered)weresampledalongthesurveylines,

producingtheresultsshownbytheredlinesinFigure9.Becausethedensity

contrastbetweensedimentsandseawaterissmall(only0.4gcm-3basedon

averagesedimentdensityof1.4gcm-3(Lyleetal.2002;Pälikeetal.2010)),the

correctionismodestbutitproducesanimprovement.E.g.,FAAdeclinesbyafew

mGalonaveragealongprofileA2inFigure9abutthisislargelycausedbythe

topographyoftheequatorialsedimentsanditseffectislargelyremovedinthe

Bougueranomalyshown.

ThebathymetrygridofRyanetal.(2009)wasusedforh.Thegridhas

variedresolutiondependingonthedistributionofthetracksofcruises

contributingbathymetry,althoughthemultibeamdatafromboththeAMAT03

andVenture-1cruiseshavebeenincorporatedinthegrid,soresolutionis

adequatefortheBouguercorrectionalongthoselines.Resolutionispoorer

alongtheRVAkademikSelskiyline.Bougueranomaliescomputedusingdepths

derivedfromtheseismicdatadifferedfromthosecomputedwiththeRyanetal.

(2009)gridwitharoot-mean-square(RMS)differenceof2.3mGal.Itwas

thereforedecidedtousetheformerBougueranomaliesforcomparisonwiththe

basementvariations.High-passfilteredanomaliesshownbytheredlinein

Figure6werecomputedbyfittinga5th-orderpolynomialofBougueranomaly

versuslongitudeandcalculatingresidualsrelativetothatpolynomial.

Tohelpassessmentofanyco-variationbetweenBougueranomaliesand

basementelevations,thesedataforeachlineareshowninFigure10(admittance

plots),usingtheresidualelevationsaboutthesubsidencetrendsforbasement

elevation(Figures7and8).Inthesegraphs,Bougueranomaliesareshownafter

removingthe400-km-filteredfield,buttheelevationshavenotbeensimilarly

12

high-passfiltered.InthecaseoftheAkademicSilskiyline,datawererestrictedto

113˚Wto145˚WtoavoidinfluenceofsomefeaturesneartheEPRvisiblein

Figure2bandtheLineIslandsSwelltothewest.Datafromthetwosegmentsof

apparentlyco-varyingbasementelevationandBougueranomaliesmarkedby

horizontalgreylinesinFigure6areshownwithredandbluecolourinthispanel

(seefigurecaptionfordetails).

Inadmittanceplotsconstructedasdescribedabove,weexpectchangesin

basementelevationwithassociatedcrustalthicknessvariations(localAiry

isostaticsupport)toformhorizontaltrends.Ontheotherhand,deflectionsof

basementcausedbymechanismssuchasshowninFigure1shouldleadtoalocal

correlationofBougueranomalywithbasementelevationandthegradientofthe

correlationtrendshouldcorrespondwithadensitycontrastofmantlerockswith

thesediments,ifthecrusthasuniformthicknessandthereforecontributeslittle

(alsonotethattheBouguercorrectionineffectreplacedwaterwithsedimentin

termsofitseffectonthegravityfield,hencethecontrastiswithsediment).If

topographyofthebasementissupportedbytherigidityofthelithosphere(e.g.,

becauseofoff-axisemplacementofseamountsorfaulting),othercorrelationsare

possibleandhenceweprovidemodeltrendsinFigure10forgabbroandbasalt

lithologies(3.0and2.7gcm-3)aswellasmantlelithologies(3.3gcm-3).

Complicatingsomewhattheinterpretation,thelong-wavelengthvariationin

basementelevationcausedpotentiallybylarge-scalemantleconvection(Crosby

andMcKenzie2009;Crosbyetal.2006)willspreadoutthedatagenerally

paralleltothehorizontalaxis(anygravityeffecthavingbeenremovedbythe

high-passfiltering).However,thisspreadingoutofthedataisusefulasithelps

visibilityoftrendsduetobasementreliefinsmallersegmentsofthedata.

13

Inordertoconfirmtheseismicallyderivedstructure,whilealsoproviding

atestfortheapproachofprovidingcalculationsusingtheBouguerslabformula,

modelfree-airanomalies(greenlineinFigure6)werecalculatedforthe

AkademikSelskiylinewhereMohodepthsareavailablebyaddingthegravity

contributions(usingequation(2))ofthewatercolumn,sedimentandcrustusing

densitiesof1.0,1.4,3.0and3.3gcm-3forwater,sediment,crustandmantle,

respectively.Thesomewhatlarge3.0gcm-3wasusedforthecrustgiventhat

crustalseismiclayer3(gabbro)dominatesvariationsinseismicestimatesof

crustalthickness(MutterandMutter1993),whereastheseismiclayer2ismore

uniformandthereforevariationsinlayer2thicknessshouldhavelessaffecton

gravityfieldvariationsthanvariationsinthegabbroiclayerthickness.Minshull

etal.(1998)derivedsimilardensitiesof~3.0gcm-3forloweroceaniccrust

usingtheir~7kms-1velocitiesfromseismicrefractionexperimentsanda

relationofChristensenandShaw(1970),whichMinshulletal.(1998)suggested

wasaccuratetowithinafewpercent.Asthedeepermantledensitystructureis

unknownhere,theresultingmodelwassimplyde-trendedandthenre-trended

tothegradientoftheobservedfree-airanomalies.Theresultingmodelanddata

havearoot-meansquaredifferenceof2.3mGal,whichissimilartothe~2mGal

accuracyofthegravitygrid(Sandwelletal.2014).Thesmalldiscrepancycould

partlyarisefrominadequatemodellingofthecrustaldensities(e.g.,seamounts

mayhavealowerdensity(Mitchell2001))and/orfromtheBouguerslab

formulaapproach,whichdoesnotaccountwellfortopographicreliefandcrustal

structureawayfromeachpointofcalculationalongtheprofile.

14

Resultsandinterpretation

Inthefollowing,wefirstcharacterizethelarge-scalesystematicvariations

beforeexaminingthevariabilityduetothegravitylineamentsandother(e.g.,

volcanicstructures)superimposedonthosemajortrends.

Subsidencetrend

ThesubsidencerateobtainedfromtheAMAT03andVenture-1data

(Figure7)is313mm.y.-1/2,whichissimilartothatofMartyandCazenave

(1989)whofound329mm.y.-1/2fortheequatorialPacific(theircorridor26).It

isalsosimilartotheglobalaveragerateof323-336mm.y.-1/2ofKorenagaand

Korenaga(2008),tothe307mm.y.-1/2NorthPacificrateofHillierandWatts

(2005)andtothe320mm.y.-1/2averagerateforthePacificofCrosbyand

McKenzie(2009).Thelatterisperhapssurprising,asCrosbyandMcKenzie

(2009)selecteddepthswheretheycoincidedwithfreeairanomalymagnitudes

lessthan5mGal,toreduceeffectsofmantleupwellinganddownwellingontheir

subsidencerates.TheAMAT03seismiclinescrossabroadgravitylow(114˚to

123˚W)butencroachongravityhighsinthewestandeastendsoftheanalysed

region(Figure3a),soanyeffectofdownwellingmaybefortuitously

compensatedbyupwellingstoleaveasimilarsubsidencetrendasCrosbyand

McKenzie(2009)found.

ThesubsidenceratederivedfromtheAkademikSelskiydatasetwas343m

m.y.-1/2,whichissomewhatmorerapidthanthepreviousestimatesoutlined

above.Thisisperhapssurprisingifweexpectadynamiceffectassociatedwith

thelong-wavelengthfree-airanomaliesinFigure3,asthedatasetspans~30

mGalfroma-20mGallowintheeasttonearly+10mGalinthewest.Iftheeast

15

ofthelineweredynamicallydepressedbymantledownwelling,whilethewestof

thelinewereelevatedbyupwellingneartheLineIslandsswell,areduced

apparentsubsidenceratemightbeexpected.

Weinitiallycalculated2σuncertaintiesforourregressionsusing

standardmethods(Taylor1982)andobtainedvaluesof2and4mm.y.-1/2for

Figures7and8.Thesevaluesaremuchsmallerthanfortheothersubsidence

ratesquotedabove,whoseuncertaintieswerereportedtobe61m.y.-1/2(Marty

andCazenave1989),22-23mm.y.-1/2(KorenagaandKorenaga2008),21-29m

m.y.-1/2(HillierandWatts2005)and30mm.y.-1/2(ParsonsandSclater1977).

Oursmallnominaluncertaintiesarisefromthelargenumbersofdata

contributingtotheregressions.However,adjacentdepthvaluesalongsurvey

linesobtainedbydigitisingtheseismicdataarenotstrictlyspeaking

independentofeachotherbecauseofthefinitelength-scalesofabyssalhillsand

otherfeatures(Goff1992).Furthermore,theseismiclinesseverelyunder-

samplebasementacrosstheregion.Hence,trueuncertaintieswillbelarger.

However,theseissueshighlightproblemswiththeearliersubsidenceratesalso,

astheauthorshaveusedinterpolateddatasetswheretheunderlyingvariations

(e.g.,sedimentthickness)werenotmeasuredcontinuouslyandtherewasno

allowancemadeapparentlyforthestrongaveragingofsomedatathatoccurs

duringgridding.Wethereforesuggestthatuncertaintiesofsubsidencetrends

arenotsowellconstrainedinoursandintheotherstudies.Thesignificanceof

differencesinsubsidenceratesbetweenregionsmayinturnbedifficultto

assess.

Residualelevations

16

Theresidualelevationsofbasementcorrectedforsedimentloadingderivedfrom

theseismicdatahavesimilarmagnitudesandvarywithasimilarspatialpattern

tothoseofCrosbyetal.(2006),asshowninFigure11.Largerpositivereliefis

evidentalonglineV1andgenerallylargeramplitudesoccuralonglinesA2-A7

duetothenewdataresolvingseamountsandotherelevatedfeatures.

Relationsbetweenbasementelevationsandgravityanomalies

BasementelevationsvaryingsympatheticallywithBougueranomalies

Insomesegments,thebasementrisesandfallssympatheticallywiththe

Bougueranomalies,aswouldbeexpectedfromsomeoftheoriginsofthegravity

lineaments(Figure1).ThiscanbeseeninlineA2(Figure9a),wherelocal

gravityhighs"a"-"e"mimicbasementhighs.Thecorrespondingadmittanceplot

forthisline(seefiltereddatainredinFigure10)containstrendsthatgenerally

havelowergraph-gradientsthanthetrendsexpectedwiththevariousbasement

densities,althoughtheanomaliesinFigure9aaresmallinextentsothismaybe

duetoupwardcontinuationeffects.

ThegraphforprofileA3inFigure10containsvariedtrendsthatcouldbe

compatiblewithbasementdensitiessmallerthan2.7tolargerthan3.3gcm-3

(i.e.,largegravityvariationsoccurinFigure9bwithmodestvariationsin

basementelevation).InFigure9a,profileA4mostlydoesnotshowsympathetic

behaviour,althoughanomaly"a"andthefurtheranomalyimmediatelysouthof

italongthatlinedoshowsympatheticvariations.Intheadmittanceplot(Figure

10)sometrendsforA4arenearlyparalleltotheexpectedtrends.

17

ThegraphforprofilesA5andA6inFigure10mostlyshowstrendsthat

areequaltoorshallowerthanthe2.7gcm-3trend.ThatforA7ismoreconfused,

thoughtheprofileinFigure9asuggestssomesympatheticvariation.

TheAkademikSelskiydataalsocontaintwosegmentswherethereare

basement-Bouguercorrelations(marked"c"and"d"inFigure6andhighlighted

bygreybars).ThecorrespondinggraphinFigure10containsdatasegments

withtheexpectedgradientsforbasementdensities.

BasementtopographywithlittleornoBougueranomalyvariation

Inafewsegmentsofthedata,wefindbasementelevatedbyafew

hundredmetresormorecoincidingwithsubduedgravityanomalies.For

example,at"c"inprofileA5-A6(Figure9b),thereisabasementhighofafew

100mwithtwoextremelocalhighs(seamounts)thatisassociatedwithaweak

Bouguerhighofonly~4mGal.At1˚NinprofileA3,asimilarlyextremereliefis

associatedwithagravitylow.At5˚NinprofileA7,abasementhighof>500mis

accompaniedbynegligibleBougueranomaly.

Thesefeaturesaremostlikelycausedbythickenedcrustunderlying

seamountsorgroupsofseamounts.Figure12ashowsseamountsoccurringin

bathymetrycollectedwithmultibeamsonarsinthenortherlypartsofprofiles

A4-A7.IntheAkademikSelskiydata,localbasementhighsEH1andEH2arealso

associatedwithsubduedBougueranomalies(Figure6).TheMohoimaged

seismicallybeneathEH1appearstobe500-1000mdeeperthanareasafew

degreesoflongitudeeithersideofthehigh.Thereislessclearlylocaldeepening

oftheMohobeneathEH2,thoughtheMohoisnoticeablyshallowerunderthe

adjacentlowEL2.(Althoughanexplanationisunclear,EH2interestingly

18

coincideswiththechangeinPacificplatemotionindicatedbythebendofthe

Hawaiian-Emperorseamountchain(ClagueandDalrymple1987).)

InlineA4(Figure9a),thebasementdeclinesby~300moncrossingthe

GalapagosFZoveradistanceof~200km,buttheBougueranomaliesarelittle

changed.ThisformsatransitiontoanimportantlowL2inresidualelevation

thatis~300-400kmacross(Figure3a).Wemodelledthechangeinfree-air

anomalygoingoverthisfracturezone(arrowinFigure9aforA4)usingthe

gravityslabformula(equation(2))withtwodifferentassumptions.Themean

basementdepth,bathymetryandfree-airgravityanomalywerefirstcalculated

foreachof-1˚to0˚andfor1.5˚to2.5˚N.

First,weassumedthebasementvariationislocallycompensatedby

crustalthicknessvariations.Thedifferenceinaveragedbasementelevation

betweenthetwoareasis311m.IfthatwereAirycompensated,thecrustis

predictedtothinby2383mfrom0.5˚Sto2˚NasillustratedinFigure13.This

valuewasderivedusingasimpleisostaticcalculationusingmantle,crustaland

waterdensities(ρm,ρc,ρw)of3.3,3.0and1.0gcm-3.Thegabbroiclayerwas

assumedtodominatecrustalthicknessvariationssoanearlyconstant2.5km

thicklayer2(MutterandMutter1993)doesnotcontributetotheisostatic

calculationandweuseagabbroic3.0gcm-3densityforthecrustasexplained

earlier.Sedimentisostaticeffectswereignored.Withtheseassumptions,the

changeinMohodepthΔMispredictedfromthechangeinbasementdepthΔBto

be

ΔM=ΔB(ρc-ρw)/(ρm-ρc) (3)

ThegreenbarinFigure9ashowstheexpectedfreeairanomaly(free-air

anomalyat0.5˚Spluschange)computedusingthegravityslabformulafromthe

19

changesinseabed,basementandMohodensityinterfaces(a1.6gcm-3density

wasassumedforthesediments(MayerandTheyer1985)).Thepredicted

anomalyisnotmuchdifferentfromthefree-airanomalyintheSandwelletal.

(2014)grid(bluecircleat2˚NinFigure9a).

Second,ifthebasementdepthchangeinsteadariseswithoutchangein

crustalthickness(topographynotlocallycompensated),theeffectongravityis

shownbytheredbarinFigure9a(free-airanomalyat0.5˚Spluschange).This

calculationinvolvedapplyingthegravityslabformulatothechangeinseabed

andbasementdensityinterfaces,applyingamantle3.3gcm-3densitytothe

basementchangeasthegravityeffectofthecrustwouldbeuniforminthiscase.

Thismodelclearlydoesaworsejobofpredictingthefree-airgravityvariations.

Thereappearsthereforetobeasignificantthinningofthecrustby~2km

crossingtheGalapagosFZnorthwards.

Discussion

Large-scalevariations

Atthelargerscale(Figure3a),theresidualelevationscorrelatepoorly

withtheregionalfree-airgravityanomalies.Forexample,aresidualelevation

highEH1liesoverthelowestfree-airanomalies(<20mGal),whereasthelowest

residualelevationEL2isfarfromthegravitylow.Crustalthicknessvariations

alsoaffectbasementelevations,soanalysisoflong-wavelengthresidual

elevationsalsorequiresacorrectionforcrustalthickness(Hoggardetal.2016;

Hoggardetal.2017).

WethereforederivedaresidualelevationfortheAkademikSilskiydata

insteadusingthesameglobalsubsidencerateof324mm.y.-0.5ofHoggardetal.

20

(2017)andwiththeirprocedure.Basementdepthswerecorrectedforthe

effectsofcrustalthicknessvariationsusingcrustalandasthenosphericmantle

densitiesof3.0and3.3gcm-3.Theresultsoffsettoazeromeanareshownby

thepinklineinFigure6.Althoughclosertothelarge-scalevariationobservedin

thefree-airanomalies,thisprofilestilldoesnotreproducethelowinthose

anomalies.Whatevercausestheregionalgravitylow,itisapparentlynotso

straightforwardlylinkedheretoasurfaceelevationchange.

Wehaveearlierhighlightedthe~400kmextentofdepressedresidual

elevationL2alonglineA4andthatitappearstocorrespondwithcrustthatis~2

kmthinnerthantothesouthoftheGalapagosFZ.Suchachangeissimilartothe

fullvariabilityofcrustalthicknessescompiledbyChen(1992)frompost-1970

seismicrefractiondata.Thisisanunusuallylargeareaofthinnedcrustcreated

atafast-spreadingridge.Ifthecrustsouthofthefracturezonehasathickness

moretypicalofaveragecrust(~7km(MutterandMutter1993)),thatnorthof

thefracturezonehas5kmthickcrust.Ifthisweretohavebeencreatedbyan

areaofmantlethatisunusuallycold,modellingsuggeststhetemperature

anomalycouldbe~25-30˚(BrownandWhite1994;Suetal.1994).Sucha

localisedmantletemperatureanomalywithanabruptsouthernboundaryseems

unlikely,however.Wesuggestthiswascreatedbyabodyofmantlethathad

beendepletedbyapriormeltingevent,e.g.,lithosphericmantleearlierrecycled

intothemantleatasubductionzone.

Finer-scalevariations

Thegraphsdescribedabove(Figures6,9,10)suggestthatthelineaments

observedinthegravityfield(Figure3c)couldpotentiallyhavearisenfroma

21

localdeformationoftheplate,suchasbythemechanismsillustratedinFigure1.

Althoughtheapparentdensitiesarenotresolvedsufficientlywelltoaddress

theirformationmechanisms,someaspectsofthespatialpatternmadebythe

lineamentsinFigure3csupportsomemechanismsoverothers.

Eastof"F"inFigure3c,lineamentsarefinelyspacedandorientedWNW,

parallelwiththerecentmotionofthePacificplateoverthemantle(Figure3b).

Mantlesmall-scaleconvectionlikelytakesafewmillionyearstoinitiate(Buck

1985),sotheirinitiationinFigure3cveryclosetotheEPR(within20km?)

suggestsanotherorigin.Theseareprobablycrustalstructures(e.g.,seamount

chains)createdabovemeltinganomaliesthatarefixedinthemantle(Fleitout

andMoriceau1992).Thefinest20kmscaleoftheselineamentsequalsthe

spacingofP-wavevelocityanomaliessuggestiveofmantleupwellingsinalarge-

scaleseismicrefractionexperimentontheEPRbetweentheClippertonand

Siqueirosfracturezones(Toomeyetal.2007).

Immediatelywestofthere,lineamentsareorientedN060˚E(parallelwith

arrowmarkedby"F"inFigure3c)andhaveawiderspacing(>100km).These

lineamentsareshorter(upto~500km)thanothersinFigure3c.Astheylie

obliquetoflowlinedirections(Figure3b),theirorientationsareincompatible

withsmall-scaleconvection(BuckandParmentier1986;HaxbyandWeissel

1986)orwithplatecontraction,whichisexpectedtooccurparalleltotheridge

(Cormieretal.2011;DunbarandSandwell1988;Gansetal.2003;Wintererand

Sandwell1987).Giventheproximityoftheareaofelevatedtopographyand

seamountsoverthewestwardextensionoftheSiqueirosFZ(Figure2a),we

speculatethatthestaticcomponentofpressureintheasthenosphereherecould

beaffectedbythistopographyandtheasthenospheremobilisedinamanner

22

somewhatlikelowercrustalchannelsproposedoncontinents(Beaumontetal.

2004;ClarkandRoyden2000;Roydenetal.1997).Theasthenospheremaythen

havebeenaffectedbyviscousinter-fingeringassuggestedbyWeeraratneetal.

(2007),thoughherearisingfromtopographicpotentialenergyratherthanthe

buoyancyfromoff-axishottermantlethattheyhadsuggested.

Theelevationdifferencedrivingsuchaninter-fingeringmovement

betweentheelevatedregionandthat~500kmtothesouthisroughly200-500

m(Figure2a)sotheexcesslithostaticpressureintheasthenospherewouldbe4-

10MPa(200-500mtimes2000kgm-3times10ms-2).Thisissmallcompared

withthelithostaticpressuredrivinglowercrustalflowinmountainregionswith

elevationdifferencesofmanykilometresandwithcrustalrockscontrastingin

densitywithairratherthanwater(e.g.,100MPaforthe4kmaltitudeofthe

TibetanPlateau).However,thedepth-extentofweakrheologyformingany

potentialasthenosphericchannelcouldbe~50-100kmorlargerbasedon

seismicvelocityprofiles(Harmonetal.2011;Maggietal.2006;Weeraratneetal.

2007),comparedwith<30kmforlowercrustalchannels(Beaumontetal.2004).

Buck(1985)assumedanasthenosphereviscosityof1018Pas,comparedwith

lowerviscositiesof1018-1019Passuggestedtooccurinthecrustalchannels

(Beaumontetal.2004;ClarkandRoyden2000).Furthermore,thevolumeof

asthenospheremovedtodeformtheplatebyuptoafew100matmost(Figure

9)wouldbelessthanthatelevatingorogensbykilometres.

Somelineamentswithtrend"C"inFigure3c,whichparallelfracture

zones,mayhavearisenfromspreadingcentreprocesses.Fromtheabove

interpretationoftheresidualelevationsinlineA4andtheapparentcontinuation

ofelevationlowstolineA3totheeastandpossiblypartsofA6tothewest

23

(Figure3b),thesemayaccompaniedlong-livedprocessesthatledtothinnerthan

normalcrustforseveralmillionyears,perhapsaslongas10m.y.(Figure2a).

Furtherlineamentswiththisorientationcanbeobservedinthisspreading

corridortothewest(144˚-135˚W)andsouthoftheClippertonFZ.

ThoselineamentsinFigure3cappeartobecross-cutbylineamentswith

orientation"A",whichlieparalleltothemotionofthePacificplateforthepast

20m.y.orlonger(seeflowlineinFigure3b).Thelineamentmarked"A"appears

tocontinuetotheEastPacificRise,whereithasafinerstructure,presumablyin

partduetotheshallowwaterdepth(smallerupwardcontinuationeffect).

FurtherlineamentsbeneaththeAkademikSilskiyline("B"inFigure3c)are

orientedclockwiseofthoseat"A",butareparallelwiththedirectionofmotionof

thePacificplatepriorto30Ma(Figure3b).Giventhiscorrespondence,small-

scaleconvectionmayexplainbothsetsoftheselineaments(Buckand

Parmentier1986;HaxbyandWeissel1986).Alternativeexplanationsinvolving

crustalconstructionattheEastPacificRisemodifiedbymeltinganomaliesthat

arefixedinthehotspotreferenceframe(FleitoutandMoriceau1992)seemtobe

ruledoutinthisareabytheircontinuationacrossfracturezonesandother

structureswherecrustalthicknessisreduced.

Returningtothepossibilityofextensioncausingthesefeatures,

unfortunatelywehavefoundtheseismicreflectionrecordsarenotadequateto

resolvethis,despitesedimentupto600mthickinplaces(MitchellandLyle

2005;Mitchelletal.2003).Asthesedimentispelagic,itcommonlydrapesthe

underlyingbasement(DuboisandMitchell2012;MitchellandHuthnance2013;

Tominagaetal.2011),makingfaultorfolddisplacementsdifficulttoseparate

fromdepositionalgeometry.Faultscanalsoarisebecauseofdifferential

24

compaction.Nevertheless,pelagicdrapingmeansthattheseabedrelief

commonlyappearsasanattenuatedversionoftheunderlyingbasementrelief

(Figure4).AsshowninFigure12,multibeamsonardatafromthisareaclearly

showtheabyssalhillsoriginallycreatedattheEastPacificRise.Atfracture

zones(suchastheGalapagosFZinFigure12b),therecanbetransversefeatures,

butotherwisewehaveobservednoclearevidenceforfaultscrossingtheabyssal

hillsherethatwouldariseifthegravitylineamentswerecausedbyplate

extension(Cormieretal.2011;DunbarandSandwell1988;Gansetal.2003;

WintererandSandwell1987).

Furtherlineament"E"(Figure3c)isneitherparalleltoadjacentfracture

zones,nortoflowlinedirections.Thisfeatureis~1000kmlong.Somesimilarly

orientedfeatureslienorthoftheClippertonFZalso.Iflithospherewasweak

alongthefracturezone,itisunclearhowfeature"F"couldhaveformedby

extensionwiththisobliqueorientation.Byelimination,aviscousinter-fingering

mechanism(Weeraratneetal.2007)seemstheonlyviableexplanationforthese

lineamentdirectionsatpresent.

Insummary,thediversityoflineamentorientationsisdifficulttoexplain

byanysinglemechanism(Figurer1).Itseemslikelythatmultipleprocesses

havecreatedthegravitylineamentshere,ofwhichwefavourspreadingcentre

processes,small-scaleconvection,viscousinter-fingeringandmeltinganomalies

thatarefixedinthemantlereferenceframe.ThePacificplatechangedabsolute

velocityfrom~0.5˚/m.y.priorto30Mato~0.9˚/m.y.after30Ma(Wesseland

Kroenke2008),thoughneitherthisnorthechangesindirectionofmotion

markedinFigure3bappeartocorrelatewithanyofthesechangesinlineament

25

style.Furtherprogressontheseissueswouldbenefitfromfurtherdeep-seismic

datacapableofmappingtheMoho.

Conclusions

Pelagicsedimentsupto600mthickobscurebasementintheequatorial

Pacific,preventingasimplecomparisonofthegravityfieldwithbathymetry

data.Depthsofbasementwerethereforederivedfromthesethreeseismic

reflectiondatasetsthatcrossthisregionandcombinedwithre-evaluated

seafloorspreadingmagneticanomalies(Barckhausenetal.2013).After

correctionforsedimentisostaticloading,thesedepthsrevealasimpleplate-

coolingtrendwithasubsidencerateof313mm.y.-1/2southoftheClippertonFZ

(AMAT03andVenture-1datases)and343mm.y.-1/2immediatelynorthofit

(AkademikSelskiydataset).Theseratesarebroadlyconsistentwithresultsof

previousstudiesofthisareathathadbeenbasedoninterpolateddatasets.

Bougueranomalieswerecomputedfromfree-airgravityanomaliesusing

theslabformulatocorrectfortheseabeddensitycontrast.Theresulting

anomaliesinsomeplacesdonotvarywithbasementtopography,suggesting

areasofcrustalthicknessvariation,suchasaroundseamounts.Inotherareas,

however,basementtopographyvariesinasimilarwaytothatnotedfrom

bathymetrydatasetselsewhereovergravitylineaments,forminglowrelief(100s

ofm)swellsover~100kmorsmallerlength-scales.Bouguergravityanomalies

varyoverthesefeatureswithanapparentbasementdensitythatvaries,but

includes2.7-3.3gcm-3densitiesexpectedofbasalttomantlelithologies.

ThegravitylineamentsintheequatorialPacifichaveremarkablyvaried

orientations.SomeareorientedparalleltoPacificplateflow-lines,changing

26

directionnorthoftheClippertonFZ,asexpectedfromthedifferentplate

directionpriorto30Maifamechanismsuchassmall-scaleconvectionformed

them.However,otherlineamentsareorientedwithdirectionsthatarenot

explainablebyplatemotion,sotheyrequireothermechanismstobeconsidered,

suchasviscousinter-fingeringintheasthenosphere.Despitethethicksediment,

thefine-scaleabyssalhillmorphologyiscommonlyobservedbecausethe

sedimentispelagicandtendstodrapetheunderlyingbasement.Wenotethat

thereareveryfewindicationsoffaultscrossingtheabyssalhillsthatmightbe

expectedifthegravitylineamentswereproducedbyextension.

Ourdetailedinvestigationhashighlightedanarea~400kmacrosswhere

residualbasementelevationsareunusuallydepressed.FromasimpleAiry

isostaticargument,oceaniccrustinthisareais~2kmthinnerthanoutsidethe

area.WesuggestthatthisthincrustwascreatedattheEPRforaperiodwhen

theridgeoverlaydepletedmantle.

Acknowledgements

NathalieDuboisinterpretedtheseismicreflectiondatafromRVRevellecruise

AMAT03andkindlymadeherinterpretationsavailabletous.UdoBarckhausen

providedthemagneticanomalyidentificationsandadviceontheisochron

mapping.SteveEittreimprovidedapapercopyoftheseismicdatasummarized

inEittreimetal.(1994).ThefreeairanomalygridwasprovidedbyDavid

SandwellandothersoftheScrippsSatelliteGeodesygroup.TheLamontgroup

providedthebathymetricgrids(Ryanetal.2009).Figureswerecreatedwiththe

GMTsoftwaresystem(WesselandSmith1991).TheRevelledataacquisitionat

seaandseismicinterpretationwassupportedinpartbyNaturalEnvironment

27

ResearchCouncilgrantNE/C508985/2.WethanktheeditorRogerUrgelesand

threeanonymousreviewersforinsightfulandusefulcommentsthatsignificantly

improvedthisarticle.

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Figures:

35

Figure1.Someexplanationsforthegravitylineaments.(a)Verticalstresses

(arrows)causedbysmall-scaleconvection(BuckandParmentier1986;Haxby

andWeissel1986)orviscousinter-fingeringintheasthenosphere(Holmesetal.

2007;Weeraratneetal.2007)deformthelithosphere.(b)Extensionleadsto

boudinageofthelithosphere(WintererandSandwell1987).(c)Thermal

contractionincreaseswithdepthintheplate,creatingatorquethatbucklesthe

lithosphere(Cormieretal.2011;Gansetal.2003).Volcanicedifices("V?")can

beemplacedatlocationsofextensionlyinginbasementvalleysin(b)and(c)but

athighsoverlyinghotasthenospherein(a).

36

a:

b:

Figure2.(a)BathymetryofthecentralequatorialPacific(Ryanetal.2009)

alongwith(whitelines)shiptracksforcruisesAMAT03(RVRevelle)and

Venture1(RVWashington)and(greyline)RVAkademikSelskiy.Tracksare

showninboldwhereseismicdatawereinterpreted.BoldannotationV1andA1

toA7locatesegmentsofthetracksreferredtointhetext.Redcrossesare

magneticanomalylocationsofBarckhausenetal.(2013)andsolidblack

contoursarecrustalagesevery1m.y.derivedfromthem(boldcontoursevery

37

10m.y.).SmallsolidcirclesalongtheEastPacificRiselocatesampledaxial

depthsshowninFigure7.Opencirclesrepresentscientificdrillingsites."FZ"

represents"fracturezone".(b)Free-airgravityanomaliesofthecentral

equatorialPacificderivedfromsatellitealtimetrymeasurements(Sandwelletal.

2014)(theirversion23).

38

Figure3.(a)Free-airgravityanomaliesofFigure2bfilteredwitha400-kmwide

cosine-weightedfilter(WesselandSmith1991)highlightinganomalieswitha

likelydeep(mantle)source.Alsoshownalongtheshiptrackshereandin(b)

withthecolourscalelowerrightareresidualelevationsrepresentingthe

39

deviationsofthebasementelevations(correctedforsedimentisostaticloading)

fromtheregressionsinFigures7and8.(b)High-passfree-airanomalies

obtainedbysubtractingthefilteredanomaliesin(a)fromtheanomaliesin

Figure2b,effectivelyrepresentinganomaliesmorelikelyarisingfrombasement

andseabedtopographyandcrustaldensityvariations(anomalyscaleasin(a)).

Dashedanddottedlinesareflow-linereconstructionsfromapointat110˚W,3˚S

usingtheWK08-GandWK08-AmodelsofWesselandKroenke(2008),with

every10m.y.markedwithclosedcircles.(c)As(b)thoughshownwithhigh

contrasttohighlighttrends.FAA<-5mGalisshownblack,FAA>5mGalinwhite

and-5mGal<FAA<5mGalinmid-grey.Greenlinesshowseismiccruisetracks.

Redarrowsannotated"A"-"F"indicatedifferentlineamenttrendsinthismap

thatarediscussedinthetext.

Figure4.ExampleofAMAT03seismicreflectiondatawithbasementmarked

(whitearrows).SurveylineismarkedinFigure2a.

40

Figure5.Interpretedseabedandbasementreflectiontwo-waytimesfrom

RevellecruiseAMAT03(DuboisandMitchell2012).Foreachline,reflection

timesareplottedwithfixeddirections,asindicatedbytheopenarrows,andwith

thescaleshownbythe1000msscalebar(aftersubtractinga5000msoffset).

Basementisinbold.Alsoshownalongtheshiptrackswiththecolourscale

shownupper-rightareresidualelevationrepresentingthedeviationofthe

basementelevation(correctedforsedimentisostaticloading)fromthe

regressioninFigure7.

41

Figure6.Inlowerpanel,depthsofseabed(lightline),basement(boldline)and

Moho(crosssymbols)reflectionsalongtheAkademikSelskiyline(Figure2a)are

shown,alongwithcrustalthickness(greyline,scaletoright).Crustalagesgiven

alongthebaseofthegrapharefromMülleretal.(2008).Middlepanelshows

(blueline)free-airgravityanomalies(FAA)derivedfromsatellitealtimetry

measurements(Sandwelletal.2014)and(greenline)modelanomaliesderived

usingthegravityslabformula.Inupperpanel,redlineshowsBouguer

anomalieswithlongwavelengthvariationsremoved.Blacklineshowsresidual

elevationsrelativetothesubsidencetrendinFigure8,whilepinklineshows

42

residualelevationscalculatedallowingforcrustalthicknessvariationsas

Hoggardetal.(2017).EH1,EH2,EL1andEL2arehighsandlowsinresidual

elevationsmentionedinthemaintext.Greybarshighlighttwosegmentswith

somewhatstraightforwardcorrelationsbetweenBouguerandresidualelevation.

Figure7.Depthofbasementversussquarerootofcrustalageforthedatain

Figure5,withagesbasedonidentificationsofBarckhausenetal.(2013).Points

representindividualseismichorizonpicksandredcirclesrepresenttheir

averagescomputedevery0.5m.y.1/2.Dashedlineisasimpleleast-squares

regressionthroughthefullseismicdataset(notaverages).AnnotationL1,L2

andH1-H4representareasofresidualelevationlowsandhighsreferredtointhe

text.BluedotsatzeroageareEPRdepthssampledfromthebathymetrydataas

43

showninFigure2aandgreencircleistheiraverage.TheseEPRdepthswerenot

usedintheregressionbutareshowntohighlighttheoffsetatzeroage.

Figure8.SubsidencetrendofbasementdepthintheRVAkademikSelskiy

seismicreflectiondata(Eittreimetal.1994)locatedinFigure2a.Crustalage

wasderivedfromtheisochronsofBarckhausen(2013)andglobalagegridof

Mülleretal.(2008)(theirversion3.6).Dashedredlineisleastsquares

regressionfittedtothedataovertherangeshown.

44

a:

b:

45

Figure9.Profilesofseabedandbasementelevation(blacklines)andfree-air

gravityanomaly(FAA,blue)andhigh-passfilteredBouguergravityanomaly

(red)locatedinFigure2a:(a)paralleltocrustalisochronsand(b)crossing

isochrons.FinedarkredlineshowsthoseBougueranomaliessmoothedwitha

50-kmwidecosine-taperedfilter.Annotation"a",etc.,refertoFAApeaks

discussedinthetext.IngraphforA4,bluesolidcirclesrepresentaveragedFAA

for-1˚to0˚and1.5˚to2.5˚N.Inthatgraph,horizontalbarsareFAApredictedat

1.5˚to2.5˚Nfromthechangeinbathymetryandbasementfrom-1˚to0˚forAiry

compensatedtopography(green)andtopographynotlocallysupported(red).

46

Figure10.Variationinhigh-passfilteredBougueranomalywithbasement

residualelevationforthevariousseismiclines(markedinupper-leftofeach

47

panel)."A.Silskiy"representstheRVAkademikSilskiyline.Overlaininredfor

V1-A7arethedatashownaftera50-kmcosine-taperedfilterwasappliedto

removeshortwavelengthfluctuations.AlldatafortheAkedemikSilskiydata

havethisfilterapplied.Redandblueinthatpanelshowdatacorrespondingto

theextentsofthetwogreybarsinFigure6.Threelinesinupper-leftpaneland

reproducedinotherpanelsshowthetrendstobeexpectedifgravity-elevation

changeswereduetoasingleinterfacebetweensediment(1.5gcm-3)andthe

densityshown.

Figure11.Residualelevationsderivedherefromtheseismicreflectiondatasets

comparedwiththoseofCrosbyetal.(2006).

48

Figure12.Examplesofbathymetrydatafrommultibeamsonarsincorporatedin

theRyanetal.(2009)dataset(mapscreatedusingtheGeoMapAppdata

browsingtool(www.geomapapp.org)).(a)AreaadjacenttoClippertonFZ

49

includingnorthernpartsoflinesA4-A7showingregionofsmallseamounts.(b)

SouthernareaencompassingtheGalapagosFZwithfewseamounts.Inboth

panelscoloursfromdarkbluetowhitecorrespondto4800to3800mdepths

andilluminationisfromN80˚W.Multibeamdatalieabouttracksmarkedwith

solidblacklines,whereasotherlower-resolutionbathymetryineachmapwas

derivedfromsatellitealtimetryorsparsesingle-beamecho-sounderdata(Ryan

etal.2009).

Figure13.IllustrationoftheestimatedchangeinMohodepthresultingfrom

localcompensationofthechangeinbasementdepthoncrossingtheGalapagos

FractureZonealonglineA4inFigure9a.