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repository: http://orca.cf.ac.uk/110665/
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Citation for final published version:
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.