S Oceans 2. Midocean Ridge Crests

VOL. 84, NO. B9 JOURNAL OF GEOPHYSICAL RESEARCH AUGUST 10, 1979

An Analysis of Isostasyin the World's Oceans 2. MidoceanRidgeCrests

J. R. COCHRAN

Lamont-DohertyGeological Obseroatory of ColumbiaUniversity, Palisades, New York 10964

Cross-spectraltechniques are usedto analyzethe relationshipbetween gravity and bathymetryat the Mid-Atlantic Ridge and the East PacificRise crests.The resultingtransfer functionswere usedto study the nature of the isostaticmechanism operative at theseridge crests.The most satisfactoryresults were obtained for modelsin which the oceaniclithosphere is treated as a thin elasticplate overlyinga weak fluid. The bestfitting elasticthickness to explain gravity and bathymetryat the fast spreading(v > 5 cm/ yr) East Pacific Rise is in the range of 2-6 km and at the relatively slow spreading(v < 2 cm/yr) Mid- Atlantic Ridge is in the range7-13 km. Theseestimates are significantlysmaller than the elasticthickness of 20-30km obtained from surface loads formed on old (> 80m.y.) parts of theoceanic lithosphere. This differenceis consistentwith the fact that ridgetopography is formed near the ridgeaxis, where isotherms are shallowerand the lithosphereis thus weakerthan in older regions.The differencebetween the elastic thicknessof the East Pacific Rise and Mid-Atlantic Ridge is significant and may represent differing temperaturestructures at theseridges. Simple modelsin which it is assumedthat the elasticthickness representsthe depth to the 450øC isothermshow that thesevariations can be explainedby differencesin the spreadingrate at these ridges.Thus the lower effectivethickness at the East Pacific Rise can be attributedto higheraverage temperatures at shallowdepths in a regionsurrounding the ridgecrest. This is due to the faster spreadingrate which resultsin isothermshaving a shallowerdip away from the axisthan at the slowerspreading Mid-Atlantic Ridge. This modelcannot, however, explain gravity and bathymetry data over the RekyjanesRidge. The bestfitting elasticthickness for this slowspreading ridge is similarto the thicknessdetermined for the East PacificRise, suggestingan anomalousthermal regimeat this ridge crest.

INTRODUCTION Needhamand Francheteau,1974; A rcyana, 1975;Heirtzler and Midocean ridgesare recognizedin plate tectonictheory as Van Andel, 1977] and provided new information on the mor- the site of creationof oceaniclithosphere which subsequently phology and tectonics of the slow spreading Mid-Atlantic evolvesthrough cooling and the solidificationof hot mantle Ridge [e.g., Ballard and Van Andel, 1977;Ramberg and Van material. Simple modelshave beenconstructed for the cooling A ndel, 1977;Bryan and Moore, 1977;McDonald and Luyendyk, lithospherewhich generallyexplain the regional topography, 1977].The OBS studies[e.g., Orcutt et al., 1976;Rosendahl et heat flow, and free air gravity anomalies over ridge crests al., 1976] have revealed a low-velocity zone at very shallow [Sclater and Francheteau,1970; Parker and Oldenburg,1973; depthsbeneath the East PacificRise crestwhich may represent Lambeck, 1972]. These models have been mainly concerned a zone of partial melt or a magma reservoir for the basaltic with explainingthe evolutionof the lithosphereaway from the magmas erupted at the ridge axis [Rosendahl, 1976; Cann, ridge crest and therefore provide little information on sub- 1974]. surfaceprocesses occurring at the ridge axis. Although thesestudies have greatly increasedthe knowledge A number of models have now been constructed which of subsurfaceprocesses occurring at the ridge axis, there are a attempt to explainthe characterof seafloor topographyat the number of problemswhich need to be resolved.These include ridgeaxis. One classof modelconsiders the viscousinteraction the origin of ridgecrest topography, the causesof variationsin betweenthe ascendinghot mantle material and the adjoining the type of axial topographybetween ridge crests, and whether lithosphere [Sleep, 1969, 1975; Sleep and Rosendahl, 1979;' or not a low-velocity zone underliesall ridge crests. Sleep and Biehler, 1970; Osmaston,1971; Lachenbruch, 1973, A usefulapproach to someof theseproblems can be made 1976;Rea, 1975].The other considersthe imbalancebetween from studies of the relationship of gravity and bathymetry the width over which hot mantle material is intruded and the over ridge crests.The empiricalage againstdepth curve [Scla- distancewhich is requiredfor newly erupted material to reach ter et al., 1971] describesthe systematicincrease in sea floor the mean half spreadingrate [Defteyes, 1970; Andersonand depth away from the ridge crest. However, on a local scale Noltimier, 1973].Both classesof modelsappear able to explain there are significantdeviations from the empirical curve asso- the general observationthat slower spreadingridges (u is less ciatedwith topographicrelief. These have dimensions of a few than about 3 cm/yr) are characterizedby an axial rift valley, kilometersto a hundredor more kilometersand amplitudesof while faster spreadingridges (u is greater than about 3 cm/yr) a few hundred metersto a few kilometers.The gravity anoma- are characterizedby an axial 'block' or 'horst.' lies associatedwith short-wavelengthtopographic irregulari- Two studieswhich have provided useful observationalcon- ties are greatly reduced when a Bouguer correction is made straints on midocean ridge crest processesare the French- [Talwani et al., 1961, 1965], implying that they must be sup- American Mid-Ocean Undersea Study (Famous)project on ported by the strength of the lithosphere [Vening Meinesz, the Mid-Atlantic Ridge and a series of ocean bottom seis- 1948]. A number of recent studies suggestthat ridge flank mometer (OBS) studieson the East Pacific Rise crest. Project topography is created at or near the ridge axis and is sub- Famous involved extensivesurface ship and deep tow mea- sequently'frozen in' and transportedaway as the lithosphere surementsas well as bottom samplingfrom submersibles[e.g., thickens and cools [e.g., Needham and Francheteau, 1974; McDonald and Luyendyk, 1977]. The relationship between Copyright¸ 1979by the AmericanGeophysical Union. gravity and bathymetry over ridge flanks may thereforepro-

Paper number 9B0063. 4713 0148-0227/79/009B-0063 $01.00 4714 COCHRAN: ISOSTASYAT MIDOCEAN RIDGE CRESTS vide information on subsurfaceprocesses occurring at the vatory of Columbia University for R/V Verna cruises17-24, ridge axis. 26, 28, 32 (1961-1974); R/V Robert D. Conrad cruises8, 10- Most previousstudies of the gravity field over midocean 13, 17, 20 (1963-1976); and USNS Eltanin cruise: 29 (1967), ridgeshave beenconcerned with the mannerin which the ridge (2) HydrographicOffice of the Royal NetherlandsNavy for flanks are supportedand thus have been concernedwith the HMNS $nellius cruises(1965-1966) (W. Langeraar, personal interpretationof long-wavelengthgravity anomalies.Talwani communication, 1966), (3) Woods Hole OceanographicIn- et al. [ 1961, 1965]analyzed the first continuousgravity profiles stitution for R/V Chain cruise100 (1971) (data obtainedfrom acrossthe Mid-Atlantic Ridge and East Pacific Rise. They National Geophysicaland Solar-TerrestrialData Center), and found that the ridge crestwas associatedwith relativelysmall (4) Pacific Marine Environmental Laboratory (formerly Pa- positive free air gravity anomalies and a Bouguer gravity cific OceanographicLaboratory, POL) for USS Oceanogra- anomaly low of about 140 mGal relative to the valuesin the pher cruises(1971-1972) (data obtained from National Geo- oceanbasins. This anomaly, which has a width of about 1600 physicaland Solar-Terrestrial Data Center). km in the North Atlantic [Talwani et al., 1965], was inter- The instrumentsand navigationused for the variousgravity pretedas beingcaused by low-densitymaterial underlyingthe surveysdiffered. The Lamont-Dohertyand Royal N etherlands ridge flanks and providing compensationfor the ridge. Re- Navy data wereobtained with Graf-Askania Gss2 seagravim- cently, Cochranand Talwani [ 1977] have shownthat the ridge eters [Graf and $chulze, 1961]. Different platforms were used crest is in general associatedwith a free air anomaly gravity with this meter. The Royal Netherlands Navy data and La- high of 20-30 mGal similar to that expectedfrom simple mont-Doherty data obtainedon USNS Eltanin and cruisesof thermal models of a cooling lithosphere[Sclater and Fran- R/V Vernaand R/V Robert D. Conrad prior to 1972(Verna, cheteau,1970; Parker and Oldenberg, 1973]. cruise29; Robert D. Conrad,cruise 15) utilized either an oil or Although many investigators[Talwani et al., 1961, 1965, electricallyerected Anschi•tz gyrotable. Since 1972, measure- 1971; Van Ancleland Bowin, 1968; Woodside,1972] have recog- ments on R/V Verna and R/V Robert D. Conrad have been nized that short-wavelengthfree air gravity anomalieswith obtained on an Aeroflex platform. This platform differsfrom amplitudesof up to ñ50 mGal exist over ridge crests,there the Anschi•tz platform mainly in the vertical referencewhich havebee n onlya fewattempts to interpretthem quantitatively. consistsof miniature integratinggyroscopes in the pitch and WoodSide[1972]argued from a breakin slopeof theBouguer roll axes to slave the platform and by the fact that torque gravity versusdepth relationshipthat the rift mountains are motors rather than gear trainsdrive the platform. The Woods supported by tectonic forces and/or a lower mantle density Hole data were obtained with the MIT seagravimeter which beneath the crest of the ridge. Talwani et al. [1972] examined utilizesa double-stringBosch-Arma type accelerometer[ Wing, the relationshipbetween gravity and bathymetryon a profile 1969] mounted on a Sperry gyrotable. The Pacific Marine acrossthe Mid-Atlantic Ridge at 320N and concludedthat the EnvironmentalLaboratory data were obtained with a gimbal- g.ravityanomalies could be well predictedfrom uncompen- mounted LaCoste-Romberg sea gravimeter [LaCoste, 1967]. satedtopographic relief, at leastfor wavelengthsof up to a few Most of the data usedin this studywere obtained with satellite hundred kilometers. navigation. Only Lamont-Doherty cruises prior to 1967 Recently, McKenzie and Bowin [1976] examined the rela- (Verna, cruise23, Robert D. Conrad, cruise 11) and the Royal tionshipbetween gravity and bathymetryalong two long pro- Netherlands Navy cruiseused celestial navigation. files,one of whichcrossed the Mid'-AtlanticRidge at about The overall accuracyof the gravity measurementsobtained 10øN. They usedWiener filter and cross-spectraltechniques to during thesesurveys is affectedby the accuracyof the gravime- quantitatively determine the relationship between gravity ter and the navigation.In general,those cruises which usedthe anomaliesand bathymetricrelief. They concludedthat their Gss2 seagravimeter or the MIT instrumentwith satellitenavi- results could best be explained by an elastic plate model of gation shouldbe accurateto about 2-5 mGal. However, some- compensation,in which the plate thicknessis about l0 km. what larger errors would be expectedfor cruiseswhich used They noted that this effectiveelastic thicknessis much less the gimbal-mountedLaCoste-Romberg gravimeter or celestial than previousdeterminations [ Walcott, 1970; Wattsand Coch- navigation. ran, 1974]from loadson relativelyold portionsof the oceanic The gravity anomalieswhich we usedwere reducedto the lithosphereand that this differencecould be explainedif the internationalreference ellipsoid (flattening equal to 1/297.0). topographywere creatednear the ridgecrest, where isotherms However, the choiceof a referenceellipsoid is not important, are higher in the mantle. since the mean and trend were removed before the data were The purpose of this paper is to examine the relationship used. Ship tracks along which the data were obtained are betweengravity and bathymetryover midoceanridge crests shownin Figures1 and 6. Profilesof the gravityanomalies and which differ in tectonicsetting and spreadingrate. We will use bathymetryprojected normal to the local trend of the ridgeare the cross-spectraltechniques of analyzing gravity and ba- shown in Figures 2, 7, 8, and 14. As mentionedabove, these thymetry data describedin paper 1 [Watts, 1978].The result- profiles have had the mean and trend removed, and also a ing transfer functionsor filters will be interpretedin termsof cosinetaper has been applied to the first and last 5% of each different modelsof isostasyat ridge crests.This analysiswill profile. They are not thereforeconventional gravity and ba- enableus to providenew constraintson the subsurfaceprocess thymetryprofiles. The meanis givenbeside each profile, sothe operating at ridge crestsand to examine the causesof differ- original valuescan be approximatelyreconstructed. encesbetween these processes at various ridge crests. METHOD OF ANALYSIS DATA COLLECTION,REDUCTION, ANDPRESENTATION It has been recognizedfor sometime that gravity anomalies In this study we have used surface-shipgravity and ba- at seamainly reflectthe effectof seafloor topographyand the thymetryprofiles over midoceanridge crests, obtained mainly manner in which it is compensated.Gravity and bathymetry on United Statesresearch vessels during the past 15 years.The generallycorrelate closely at shortwavelengths (less than a few sourcesof the data are (1) Lamont-DohertyGeological Obser- tensof kilometers)and poorly at longerwavelengths (greater COCHRAN:ISOSTASY AT MIDOCEANRIDGE CRESTS 4715

-125 ø -120 -I 15 -I I0 -105 -I00 -95 ø -90 ø -85 ø -80 ø -75 o 20 ø 20 ø

Clotion FroCtur

15ø 15ø

Guatemala

Basin Zo IO ø I0 ø FfeCtufe C•i•)•ertøn

o o

Bauer I0 ø I0 ø Basin o

Peru 15ø 15ø 7•o•e Basin

•e•dC •c

20" 20 ø

ZooB Fr BC'•Ur e-' QuirOS 25 ø 25 ø

• •0 ø 30 ø

EAST PACIFIC RISECREST 35ø •k5ø 7o•;•,,•Projectedand Bathyme•ry Freeairprofile. anomaly SCALE(approx.) 0 600 t I ' J KM 400 40 ø -125ø -120ø -115ø -I I0 ø -105ø -I00 ø -95 ø _90 o -85 ø -80 ø -75 ø Fig.1. Locationchart for 24 gravity and bathymetry profiles across the East Pacific Rise crest. Each profile is 300 km long and wasprojected perpendicular to the ridgecrest.

than a few hundredsof kilometers).The change is due to We requirea transferfunction or filterwhich, when applied isostaticcompensation. Thus in theabsence of noiseand other to an observedbathymetry profile, converts it to a serieswhich effectssuch as changesin the mean depth of the water, the resemblesthe observed gravity anomaly profile. We obtainthis relationshipbetween gravity and topography as a functionof filter usingthe cross-spectraltechniques described in paper1 wavelengthprovides information on isostaticcompensation. [ Watts,1978]. The method used in thesestudies has the advan- EAST PACIFIC RISE CREST

POL4-51 ;'1 ^ .,, F 12 7øN -i 4 68o s]•½•*v•--• •--• • •[-,o ii 8os +12 2 FILTERED TOPOGRAPHY •v•••+l 9 TOPOGRAPHY -.-•...... --. ,.-.•••.•.•• 29908 ...... ,.....•;':.L•.-_.....,,...:..-,•.• 50980 ...... '...•...•ß- 30372

CI307•12 7øN - 0 7 POL4-471øS • +20 POL2-115os •.•.• ••• +149 •/.•••,••+ 2o +2• ••,...... ,•..."- -'• 29829 .,••-.ß ,' ;51164...... :• 30513 C2002l /1 /• A POL4-577øS • - 13 Vl90517'S • +149 •A•%•• _+26 ,..'.....'••-;"-•...,.... :50511, ,...'..••• ,'½ 31032 •• .,•,•• 29854

POL4-6 1 A ^ r ,,,.s POL2-2175•S • +95 +28 ½52 ...... •••'.•• 51454 '•* ß'•'••29985

71Oll ^ .. . A . [ POL4-7'I A r POL2-31 • A . r 92o Shv"V•'--•--'-• • +70 •---.._•-.-/• +2I ,'••lSI...... ß . • -...... ••.....,•, 3008 9 •-••...... 51129

V21041,.., ,, A /,.. F 55øSI •"--,./k/ •L+65 61714•20 I•S +74 -+23 .• 51258 ';•ß.,..•• • "•., . 29589 , -•'•• •067 I

POL4•-I'1 ,, A r POL4-899øS • +42 POL2-421•S • + I0 5 +22 '-.-• :51199

POL4-26 IøS • -0 6 114øS +57 28EL29 2•S •• -I 4 %•••• +28 rr I000

•: -I00 ' •• .... ::.'5107I .... 30342 '½•• 291•2

o ioo i K•M •

Fig.2. Observedgravity anomaly, filtered topography, and topography profiles across the East Pacific Rise. The mean andtrend have been removed from the observed gravity and bathymetry profiles. The mean is given to theright of each profile.The 'filtered topography' profiles are estimates ofthe gravity field produced byconvolving thefilter shown in Figure 4 withthe topography profiles. The rms difference between the observed gravity and the filtered topography profiles isgiven to theright of thefiltered topography profile. The location of theprofiles is shown in Figure1. COCHRAN: ISOSTASY AT MIDOCEAN RIDGE CRESTS 4717 tage that the filter and complexadmittance (Fourier transform The spacedomain representationof the complexadmittance of the filter) is obtained from many relatively short profiles is the filter shown in Figure 4, which was obtained by an centeredon the ridge crest rather than by subdividinga few inverse Fourier transform of the admittance. The filter can be long profiles.Thus we are assuredthat the resultingfilter is thought of as an impulse responsefunction (or Green's func- directlyapplicable to the ridge crestand is not an averagefilter tion) whichrepresents the gravity effectof a unit line load on influencedby data whichcross a numberof geologicfeatures. the earth's surface.The negativewings of the filter therefore The resultof the computationsis a filter which,from bathy- representthe effectsof isostaticcompensation. metric data, reproducesthe observedfree air gravity anomaly The degreeto which the filter shownin Figure 4 can repro- over the regionor feature which is beingexamined. This filter duce the observedfree air gravity anomaliescan be seen in is basedcompletely on the observedrelationship between grav- Figure 2. The filtered topography profiles in Figure 2 were ity and bathymetry and is not tied to any isostatic model. obtained by convolvingthe East Pacific Rise filter (Figure 4) However, as pointed out by McKenzie and Bowin [1976], the with the observedbathymetry. The predicted and observed observedfilter or complexadmittance can be easilycompared gravity anomaliesgenerally compare well. The rms difference with isostaticmodels based on different hypothesesfor the between observedand predicted anomalies does not exceed mannerin which seafloor topographyis supported. -t-3.7 mGal on any of the profilesand is lessthan +3.0 mGal for 66% of them. ISOSTASY AT MIDOCEAN RIDGE CRESTS The simplestmodel to explainthe observedadmittance is to In this sectionwe will use the cross-spectraltechniques to assumethat the gravityanomalies are due only to the seafloor investigatethe isostaticmechanism at midoceanridge crests. topography and that the topographyis uncompensated.The We will concentrateon two ridgesin this study.These are the admittance expressingthe relationshipof gravity to uncom- fast spreading East Pacific Rise, which is characterizedby pensatedtopography is given [Walcott, 1976; McKenzie and smoothtopography and an axial horst, and the slow spreading Bowin, 1976] by Mid-Atlantic Ridge, which is characterizedby rough topogra- Z(k,•) = 2a'GLo:- 1.03)e-•',a (1) phy and an axial 'rift valley.' We will thus be able to examine what featuresarefiharacteristic of ridgecrests in generaland in wherep: is the densityof the topographicrelief and d is the what mannerthe differingtectonic and morphologicnatures of mean water depth.This expressionis the first term of the series the two ridgesare expressed. usedby Parker [ 1972]to determinethe gravity effectof a two- East PacificRise. The data for the East PacificRise consist dimensionalbody and constitutesan approximationof the of 24 free air gravity anomaly and bathymetryprofiles across topographyas a densitylayer at a depthd beneaththe surface. the ridge crest.Profiles betweenlatitudes 13øN and 28øS were included,with a majority of them concentratedbetween 6øS 0.8 EAST PACIFIC and 21øS (Figure 1). Profiles near the Pacific/Nazca/Cocos RISE triple junction were excluded.Each profile was projectednor- N=24 mal to the local trend of the ridge and extends 150 km on z 0.4 either side of the ridge axis. The half spreadingrates for this portion of the East PacificRise vary from 5 to 8 cm/yr [Larson LU 0.2 0 and Chase, 1972; Herron, 1972; Handschumacher,1976], and r• 0 the data are thus from regionsless than about 3 m.y. old. The gravity and bathymetry profiles of the rise crest are presentedin Figure 2. The ridge crest in these profiles is characterizedby a positivehorstlike topographic feature about 20 km acrossand a few hundred metershigh. The character- istic gravity anomaly associatedwith the central high is a relativelysmall amplitude(15-25 mGal)high which is 20-30 km wide and is flankedby smaller-amplitudegravity lows.The bathymetryaway from the ridge crestis generallysmooth, and in many casesthe centralhigh is the largestfeature in both the bathymetry and gravity profiles. The gravity and bathymetric data from each profile were $ usedto obtain 24 independentestimates of the crossspectrum o -2.o - and of the power spectrumof the bathymetry. The resulting z averagedspectra were used to calculate the complex admit- I-- tanceZ(k,,), the coherence3,•'(k,,), and the phaseof the admit- • -3.o - tance ½(k,,). (See McKenzie and Bowin [1976] or Watts [1978] for the definitionof thesequantities.) These parametersare o plottedas a functionof wavenumber in Figure 3. The smooth- ._l -4.o - nessof the 1og•0admittance curve for wavelengthsof greater than about 15 km is evidencethat the samesignal was present I I I in the various profiles and that the averagingprocedure suc- o• 20 I0 5 K M cessfullyreduces noise. The phase is near zero for the same Fig. 3. The coherence,phase (in degrees),and 1og•0of the ampli- rangeof wavelengths,implying that the admittanceis real, as it tude of the admittance determined from the gravity and bathymetry should be for the earth; and the coherenceis high between profiles shown in Figure 2. The dashedline on the 1og•0admittance curve is a straightline fit to the curve for wavelengthsof 32 > 3, > 13 wavelengthsof 15 and 150 km. The coherenceis a measureof km and givesan estimateof the mean water depth and densityof the that portion of changesin the gravity field which can be topographicrelief of 3.219 km and 2.3 g/cm8, respectively.The actual directly attributed to changesin water depth. mean depth is 3.097 km. 4718 COCHRAN: ISOSTASY AT MIDOCEAN RIDGE CRESTS

0.010-

"- 0.008- EAST

z PACIFIC IJ.I O.006- RISE

laJ 0.004- N-24 0

n,' 0.002-

-0.002- I00 125 150 175 200 I I I I I KM Fig. 4. The filter determinedfrom the gravityanomaly and bathymetryprofiles over the East PacificRise. When con- volved with the bathymetricprofiles, this filter producesthe 'filtered topography'profiles in Figure 2. The filter is the inverseFourier transform of the observedadmittance values shown in Figures 3 and 5.

Equation (1) givesa straight line on a log admittanceplot, A similarprocedure is usedto determinean expressionfor and thus an estimateof o: and of d can be made by fitting a the admittancewhen a plate model of compensationis as- straightline to valuesoflog•oZ [McKenzie andBowin, 1976]. A sumed, although the computationsare more involved, since straight line fit to the observedadmittance for a wavelength the deflectiondue to the load must be calculatedas part of the range of 32 > 3, > 13 km gaveestimates ofd = 3.219 km andp: computation of the gravity effect of the compensation. = 2.33 g/cm8. The estimated mean water depth, although McKenzieand Bowin [ 1976]outline a procedurefor the deriva- slightlygreater than the observedmean depth, is within the tion. The expressionwhich we use for the admittance,Z(kn), rangeof observedvalues. The mean densityestimate, however, differsslightly from that given by McKenzie andBowin [ 1976]. is somewhatlower than densitiesusually assumed for oceanic The expressionwhich they use(their equationA 16) assumes layer 2. One explanationis that the densityappropriate to the that the entire crust has the samedensity as the topographic topographicrelief is that of layer 2A rather than of layer 2. relief. For the low densities which we have deduced for the Houtz and Ewing [ 1976]have documenteda l- to 1.5-km-thick topographyat the East Pacific Rise, this would result in a layer in crestalregions of midoceanridges with P waveveloci- density contrast of greater than I g/cm• at the base of the ties of 3.1-3.3 km?s.These velocities correspond to densitiesof crust, which is unrealistically high. We have therefore in- approximately2.3 g/cm8 usingthe Nafe-Drake curves. troducedanother density contrast within the crust.The result- Although the short-wavelengthportion of the logaoZcurve ing admittanceis then can be fit by a straightline, the admittancedecreases rapidly at Z(k,,) = 2•rG(p:- 1.03)e-•-a{1- [e-•,,t,(•oa- •o:) longer wavelengthsbecause of the effectsof isostasy.As a compensatedtopographic feature becomeswider, the gravity + e-n"t4pm- Ps)]/[(P,n - p:) + 4(p,,,- 1.03)Mkt:AB-']} anomaliesover its centerdecrease, and the patternsof gravity and bathymetry become lesssimilar. The nature of the rela- (3) tionshipbetween gravity and bathymetryfor varioustheoreti- wheret: is the thicknessof layer2; tc is the meanthickness of cal isostatic models can be determined [Walcott, 1976; the crust;pa is the densityof layer3, takento be2.9 g/craB; McKenzieand Bowin, 1976]and comparedto observedvalues. is the densityof the uppermantle, taken to be 3.4 g/cma;M = The admittancefor the Airy model of compensationis given E/3gh(p,,- 1.03),where E is Young'smodulus and the plate by the sumof two densitylayers, one at the meanwater depth, thicknessis 2h; k' = kh; A = [(sinh2k')/2k']: - 1; andB = representingthe topography,and one at the depth of com- [(sinh 4k')/4k'] + 1. (We note that there shouldbe a 2 in the pensation, representing the compensation [McKenzie and denominatorof the c__on_stantsat the beginningof the ex- Bowin, 1976].The resultingexpression is pressionfor Z givenby McKenzieand Bowin [1976, equation Z(k,, ) = 2•rG(p:- 1.03)e-•',,a(l- e-•',,t) (2) A16].) Calculatedadmittance curves for the Airy model and the where t is the distance below the sea floor at which com- plate model of compensationare superimposedon the ob- pensationis assumedto occur. For simple Airy isostasy,t servedadmittance values from the EastPacific Rise in Figure would normally be the averagecrustal thickness.The other 5. The verticallines through the observedpoints are the stan- parametersare as in (1). dard errorsof the admittanceestimates calculated following COCHRAN:ISOSTASY AT MIDOCEANRIDGE CRESTS 4719

Munk and Cartwright[1966]. At short wavelengths,both sets 0 to 0.45 is 6 km. This value is considerablyless than the of calculatedcurves asymptotically approach the straightline effective elastic thickness of 20-25 km deduced from loads representinguncompensated topography. The differencebe- formedon old oceaniclithosphere [ Walcott, 1970; Watts et al., tween the two schemesof compensationappears mainly at 1975;Watts, 1978] and is alsoless than the estimateof 10km long wavelengths. deducedby McKenzieand Bowin [1976] from topographyin The calculatedadmittance curves for the Airy model show the Atlantic Ocean.It can easilybe seenfrom Figure 5 that the that an averagecrustal thickness of 10-20 km is requiredto elasticplate thickness in the Pacificcannot be as greatas 10 explain the obervedadmittance. When the admittancevalues km. for wave numbersof 0-0.45 (3, > 14 km) are considered,the We have also determinedthe admittancefor setsof profiles bestfitting crustalthickness in a leastsquares sense is 13 km. 150 km and 960 km long acrossthe East Pacific Rise to This valueis significantlygreater than the crustalthickness of determineif thereis any changein the elasticplate thickness 5-6 km determined from seismic refraction measurements in with crustalage. The bestfitting plate thicknesses are 5 km for the region [Shor et al., 1970; Rosendahlet al., 1976]. The the 150-km-longprofiles and 6 km for 960-km-longprofiles. crustalparameters deduced for an Airy modelof localisostatic Analysesof setsof profilesof differentlengths therefore, compensationthus do not agreewell with directmeasurements includingthose confined to theimmediate vicinity of theridge in the vicinity. crest and those extendinglarge distancesacross the ridge The plate modelof regionalcompensation results in calcu- flank, give similar valuesfor the admittanceover the East lated admittancevalues which adequately explain the observed PacificRise crest. This supportsthe hypothesis[e.g., Le Pichon admittance and show that an elastic plate thicknessin the et al., 1973;Needham and Francheteau,1974] that ridgeflank range of 2-6 km is required.The bestfitting effectiveelastic topographyis createdat or nearthe ridge axis, frozen in, and plate thicknessin a leastsquares sense for wavenumbers from subsequentlytransported from the ridge atop the rapidly

EAST PACIFIC RISE 300-Krn Profiles 0.05, AIRY MODEL

a• O.04- E

r'• 0.01 Dmeon= 3.097Km

0.0 0 .62 0•4 d6 0•8 I• I• .1'4 .1• .1• .•0 K I I I I00 Km 50 Km 30 Km t•

300-Km Profiles 0.05 PLATE MODEL

0.04-

•0=2.3

D mean =3.097 Km

o .& .68 .,b x i i IOOKm 50Km 30Km )• Fig.5. Observedadmittance values (dots) for the East Pacific Rise crest compared with theoretical admittance curves for two isostaticmodels. The verticalbars extend 1 standarddeviation on eitherside of the valueand arecalculated by the methodof Munkand Cartwright [ 1966]. An oceaniccrustal thickness of 10-20km is required to fit theobserved admittance valuesusing the Airy model.The bestfitting elastic thickness for the platemodel is 6 km. 4720 COCHRAN: ISOSTASY AT MIDOCEAN RIDGE CRESTS

50 ø 40 30 ø 20 ø I0 ø 0 ø Talwani, 1972;Ladd et al., 1975], the profiles extend out to 50 ø 50 ø crust of 10-17 m.y. in age. The gravity and bathymetryprofiles in the North Atlantic are presentedin Figure 7, and thosein the South Atlantic in Figure 8. The ridge crestis associatedwith a rift valley in every profile acrossthe Mid-Atlantic Ridge. The rift valley variesin 40 ø depth from up to 1500m in profilessuch as V2805 and SN05 in the North Atlantic and V2604 and C0801 in the South Atlan- tic, where it is quite distinct,to no more than a few hundred meters in profiles suchas V1913 in the North Atlantic. It is, however, always associatedwith a very prominent free air 30 ø gravity anomaly low and, wherethey are well developed,with large gravity highsover the rift mountains.The gravity anomalies associatedwith the rift valley and rift mountains are .,.•SN05 generallythe dominantfeature on the free air gravity profiles. 20 ø 20 ø The complexadmittance, coherence, and phaseof the admittance were calculatedfrom the averagedspectra of the profilesas wasdone for the EastPacific Rise and are plottedas a function of wave numberin Figure 9. As was the casewith IO o IO o the East Pacific Rise, the 1og•0admittance curve is smoothfor wavelengthsof greaterthan about 15 km. Also, in this rangeof wavelengthsthe phaseis near zero, and the coherenceis high, • zor•e indicating that a definite signal is present in the group of 0 o 0 o profiles. The degreeto which the Mid-Atlantic Ridge filter, shown in Figure 10, is capable of reproducingthe gravity anomaliescan be judged from the 'filtered topography'and I0 ø I0 o 'differencegravity' profiles in Figures 7 and 8. The observed admittance values with their standard errors are plotted in Figure 11 along with calculated admittance •"'•000 curvesfor the Airy and plate modelsof compensation.It can 20ø SOUTHATLA NTI 20 ø easilybe seenthat an averagecrustal thickness of greaterthan 20 km is requiredto explain the relationshipbetween gravity ..• OCEAN and bathymetry according to the Airy hypothesis.This is several times the observed oceanic crustal thickness. The effec-

30o / .R,(9-•r•de(i. :::' :' 30 ø tive plate thicknessand densitywhich best fit the observed admittancein a least squaressense are 9 km and 2.6 g/cm3. .•.:: • _.-R•se,, ooo •:::.:•r• .::::::':::::::::::..... :1 This value for the elasticplate thicknessis nearly the sameas the value of 10 km determinedby McKenzie and Bowin [1976] for the Atlantic. The Atlantic value is somewhatgreater than • M ID-A TLA NTIC "• ,,'.Z':::' 4Oøh RIDGECREST 40 ø that determined for the East Pacific Rise, and the difference / •,o• Projected appearsto be real and significant.The differencecan be easily /I e" Free Air Anomaly and '. ' q,;coø 2000 seenby comparingthe filters from the two ridges.Although H Bathymetryprofile the mean depth is nearly the samefor the two setsof profiles II SCALE(opprox) (3159 m for the Atlantic and 3097 m for the Pacific), the Mid- 50øtt • • • • 50 ø Atlantic Ridge filter (Figure 11) has an amplitude nearly 50% 50 ø 40 ø 30 ø 20 I0 greater than the East Pacific Rise filter (Figure 4). It is also Fig. 6. Location chart for 16 gravity and bathymetricprofiles somewhat wider than the East Pacific Rise filter. acrossthe Mid-Atlantic Ridge. Each profile is 400 km long and was projectedperpendicular to the ridge crest. TECTONIC IMPLICATIONS A result of particular interest is that the effective elastic thickeninglithosphere. Even thoughthe lithospherethickens thicknessdetermined from small-scaletopography appears to with age, material is added in sucha manner that it doesnot differ betweenridge systems.Low values of the elasticthick- alter the original deformedconfiguration of the crust giving ness(2-6 km) were determinedfor the East Pacific Rise crest, rise to the gravity anomalies. while higher values(7-13 km) were determinedfor the Mid- Mid-Atlantic Ridge. The data for the Mid-Atlantic Ridge Atlantic Ridge. Thesedifferences appear to be real and signifi- consistof 16 free air gravity anomalyand bathymetryprofiles cant. acrossthe ridge crest between45øN and 47øS. Each profile The differencein the relationshipbetween gravity and bath- was projectedperpendicular to the ridgeand extends200 km ymetry over the two ridge crestsis illustratedin Figure 12, in on either sideof the ridgeaxis. The locationof the profilesis which the isostaticresponse function ½ is plotted againstwave- shown in Figure 6. Profile V1713 was analyzedin detail by length.The responsefunction is obtainedby writing the admit- Talwani et al. [1965] and is also discussedby Talwani et al. tance in the form [ 1972].The SN05 profileat 22øN is closeto the areastudied by Z(k,.,) = 2•rG(p, - 1.03)e-k.d½(k,) (4) Van A ndel and Bowin [1968]. Since the half spreadingrate in the Atlantic varies from about 1.2 to 2 cm/yr [Pitman and It thus serves to normalize the admittance for variations in COCHRAN:ISOSTASY AT MIDOCEANRIDGE CRESTS 4721

MID-ATLANTIC RIDGE NORTH ATLANTIC

V2805]'•1.•.• - v _^•'" \ 'h-,,A. "'• •L:o"L-"" • -20zøo t DIFFERENCE GRAVITY ,.... +430 • -zo OBSERVED GRAVITY o FILTERED TOPOGRAPHY -20 2690 • o TOPOGRAPHY • -IOOO

SN09•,• _,,.. +-88 28ONSN07 ly-..-- •..•---•",.-%.•/"--•,...•v-•[ •e 0 +59

_..]F,,,A_• 2795 3317

Vl713 SN05 .... ,--, _,.?, ,.-. r+67 52øN 22øN ,•••+ 39 .•.,,1• d:...... _•.• -•'•• 3688

SN08 V2306 1 •-.•.•__Av/• • _ r+_93 31øN 85195 17øN• 245 :•"• ::' 3258 •'• ':'• 3558

Vl913 1/-• • .. •,.••___ r+74 Cl311 1 .-_._ .._• A.X• • ^ 30ON Y v -"'•'•N.•?-•--'" - '-L- 2ON +06

3056

KM Fig.7. Observedgravity anomaly, topography, and difference gravity anomaly profiles across the Mid-Atlantic Ridge crestin the North Atlantic.The meanand trendare removedfrom the observedgravity and bathymetryprofiles, and the meanis givento theright of eachprofile. The filteredtopography profiles are estimates of thegravity field produced by convolvingthe filter shown in Figure10 with the topography profiles. The differencegravity is thedifference between the observedgravity and filtered topography. The rms difference isgiven by each difference gravity profile. Location of profiles is shown in Figure 6. water depthand topographicdensity. If 4• = 1, thenZ(kr,) = both convolvedwith profilesacross the Mid-Atlantic Ridge. 2•rGLo:- 1.03)e-k,a, which representsthe conditionthat to- The differencebetween the resultinggravity anomalies is easily pographyis uncompensated.If 4• = 0, thenZ = 0, implying seen,as is the fact that the East Pacificfilter doesa relatively zero free air gravityanomalies and, for sufficientlylarge wave- poorjob of reproducingthe observedanomalies. lengths,complete compensation. It can be easilyseen that Differencesin the elasticplate thicknessat ridge crestscan while the East Pacific Rise and Mid-Atlantic Ridge valuesfit be interpretedin termsof differencesin the thermalregime the patternexpected for a plate model,the valuesdefinitely beneaththe ridgecrests. Our datawould thus imply relatively representseparate curves with differentplate thicknesses. highertemperatures under the East Pacific Rise and relatively The differencein the relationshipbetween gravity and bath- lower averagetemperatures under the Mid-Atlantic Ridge ymetryover the two ridgecrests is alsoillustrated in Figure13, crest.Sleep [1975] and Lachenbruch[1973, 1976]have sug- wherethe Mid-Atlantic Ridge and East PacificRise filtersare gestedthat temperaturedifferences may reflectdifferences in 4722 COCHRAN:ISOSTASY AT MIDOCEANRIDGE CRESTS

MID- ATLANTIC RIDGE SOUTH ATLANTIC

V2604 ].-.----- • • .... r,48 o DIFFERENCE GRAVITY [ -202o] = -ao OBSERVEDGRAVITY 105øS•'"'"'-""'"'••Y 92 [ 20 F'LT"O •- '0%0t TOPOGRAPHY '?::•:• 2935• -I000 J

C0801 V2412 '1• .... • L 25øS 34os J ""J "'/hv---"-,-'--/ -'1_-68 65 155

3459 3289

V2011 ] A •-•._/% •, __,-, _ r+6 6 CI214 28 3øs y • vv _ L/ .... '-I.- 42øS 120 •'-•••• 3118

V2204 91 47os 254

2907

_ 0 50 I00 KM Fig.8. Observedgravity anomaly, topography, and difference gravity anomaly profiles across the Mid-Atlantic Ridge inthe South Atlantic. The mean and trend are removed from the observed gravity and bathymetry profiles, and the mean is givento the right of each profile. The filtered topography profiles are estimates ofthe gravity field produced byconvolving thefilter shown in Figure10 with the topography profiles. The difference gravity is the difference between the observed gravityand filtered topography. The rms difference isgiven by each difference gravity profile. Location of profiles isshown in Figure. 6. the geometryof the magmareservoir or conduit for the as- crestcan be investigated by consideringthe Reykjanes Ridge, cendinghot mantle material.High averagetemperatures whichis the exceptionto the generalobservation [Anderson would representrelatively wide conduits,and low average andNoltimier, 1973] that fastspreading ridges are associated temperaturesrelatively narrow conduits.A number of model with axialblocks and slow spreading ridges with rift valleys. studieshave been carried out [Sleep,1969, 1975; Sleep and Accordingto the modelswhich we havejust discussed,it Rosendahl,1979; Lachenbruch, 1973, 1976; Cann, 1974] which would be expectedthat the elasticplate thicknessunder the relateridge crest topography to thegeometry of the magma ReykjanesRidge crest would be quite small. conduit,viscosity of themagma, and the static pressure differ- Figure 14 showsthe resultsof convolvingthe EastPacific enceat the baseof the conduit.In thesemodels the styleof Rise and Mid-Atlantic Ridge filterscorrected for the differ- axialtopography isrelated to theconduit width by considering encein meanwater depth with five profiles across the R eyk- the effectof viscousdissipation on the sidesof theconduit. For janes Ridge crest.It can be seenthat the East PacificRise filter wideconduits, viscous forces are small,allowing magma to consistentlygives predicted anomalies which are closerto the ascendto an elevationcontrolled by hydrostaticpressure, thus observedanomalies than does the Mid-A tlantic Ridge filter. In favoring the formation of an axial horst. For narrow conduits particular,the Mid-AtlanticRidge filter resultsin gravity the viscousstresses are larger,retarding the riseof materialin anomaliesover the centralblock which are largerthan are the conduitand favoringthe creationof an axialvalley. In observedand doesnot reproducethe gravitylows on either addition, the viscous stresseson the walls of the conduit will sideof thecentral block. In fact,on mostof theprofiles the act to help support the rift mountains, the maintenanceof EastPacific filter does not completely reproduce the amplitude whichimplies the presenceof quite largestresses. of thelows either, suggesting that the elastic thickness may be The relationshipof plate thicknessand thustemperatures slightly lessthan that at the East Pacific Rise. beneaththe ridge crest to thestyle of topographyat a ridge The thermalmodels for theridge axis topography also have COCHRAN: ISOSTASY AT MIDOCEAN RIDGE CRESTS 4723

I0 MID-ATLANTIC implicationsfor the state of isostasyat the ridge axis. These RIDGE 0.8 N= 16 models can be tested using the relationshipof gravity and bathymetryover ridge crests.Since for wide conduits(or ab- 0.6 normally low viscosities)the ridge crestaxial elevationis determinedby hydrostaticequilibrium, these models predict that 0.4 axial blocks should be in a state of isostatic equilibrium. 0.2 Conversely,at ridgeswhere an axial rift valley is present,the viscousstresses are relatively large, and the ridge axis is de- pressedto a level below that of hydrostaticequilibrium. Thus the ridge axis shouldbe associatedwith a massdeficiency and not be in isostaticequilibrium when a rift valley is present. The state of isostatic equilibrium at ridge crests can be consideredby examinationof simpleregional isostatic anomalies which assumethe plate model of compensation.Bouguer and regional isostaticanomaly profilesacross the R eykjanes Ridge and the Mid-Atlantic Ridge are shown in Figure 15. Although both profilesshow a Bougueranomaly low centered at the ridge crest,there is a differencein the characterof the lows. The Bougueranomaly low over the R eykjanesRidge is smooth, gentle, and at least 100 km wide. This low is the

-I.O central portion of the broad Bouguer anomaly low which expressescompensation of the entireridge and is eliminatedin the regionalisostatic anomaly. In contrast,the Bougueranom- aly low, over the Mid-Atlantic Ridge is bounded by steep gradients,is confinedto the inner portion of the rift valley,and is not removedin the regionalisostatic anomaly. The presence of a gravity low in both Bouguerand isostaticanomaly pro- m -3.o files impliesthat not only is the rift valley uncompensatedbut that in addition, a subsurfacemass deficiencyis associated with it. o -J 40 Several investigators[e.g., Andersonand Noltirnier, 1973] •o 50 20 I0 KM have suggestedthat the type of axial topographyis spreading rate dependent,with axial horstscharacterizing fast spreading Fig. 9. The coherence,phase (in degrees),•nd log•0of theampli- ridgesand rift valleysslow spreadingridges. We prefer, how- tude of the admittance determinedfrom the gravity and bathymetry ever, to consider the axial topography to be thermally con- profiles shown in Figures 7 and 8. The dashed line on the 1og•0 admittancegives the admittancefor uncompensatedtopography with trolled and thus to be primarily a function of the plate thick- a densityof 2.6 g/cm8 and a water depth of 3.159 km. ness.This is demonstratedat the Reykjanes Ridge, where a

0.016 -

0.014

MID - o.o•?1 RIDGEATLANTIC o.olo1 N=16 0'0081 0.004 o.oo -o.oo21 I • 0 125I 150I 175I 200I 225i 250I 275i 300i KM

Fig. 10. The filterdetermined from thegravity anomaly and bathymetryprofiles over the Mid-AtlanticRidge shown in Figures7 and 8. When convolvedwith the bathymetricprofiles, this filter producesthe 'filteredtopography' profiles in Figures7 and8. The filteris theinverse Fourier transform of the observedadmittance values shown in Figures9 and 11. 4724 COCHRAN: ISOSTASY AT MIDOCEAN RIDGE CRESTS

MID- ATLANTIC RIDGE 400-kmProf,les AIRY oõ MODEL $ 04

, •

• .01 = '

0 o .&' .& .& .,k .do 200km IOOkm 50km 30km •

400-kmProfiles PLATE MODEL

.0 5

• .04 • o3 ø• oa

• 01 Dmeon=3159km

0 , , I I I I 200km IOOkm 50km 30krn Fig. 11. Observedadmittance values (dots) for the Mid-AtlanticRidge crest compared with theoreticaladmittance curves for two isostatic models. The vertical bars extend 1 standard deviation on either side of the value and are calculated by the methodof Munk andCartwright [ 1966].An oceaniccrustal thickness of greaterthan 20 km is requiredto fit the observedadmittance values using the Airy model.The bestfitting effective elastic thickness for the plate modelis 9 km.

slow spreadingridge is characterizedby an axial horst. This elastic thicknessis probably both temperature and pressure ridgeprobably has the smallestelastic plate thickness of any of dependent,we will assume, following the results of Watts the ridgesconsidered in this study. [1978], that the 450øC isotherm definesthe elasticthickness. The averagetemperature in the vicinityof the ridge crestis, With Ts = 0, T,• = 1325øC, T• = 450øC, Z = Te = 9 km (value however, dependenton the spreadingrate. The depth to an obtainedfor the Mid-Atlantic Ridge), u = 1.5 cm/yr, and k = isotherm is a function of age [Sclater and Francheteau,1970], 0.007 cm•-/swe obtain x0 = 132 km. Substitutingx0 = 132 km so as the spreadingrate increases,isotherms migrate outward, in (5) resultsin a curve relating Te and u. This curveis plotted resultingin higher averagetemperatures beneath the immedi- in Figure 16. ate vicinity of the ridge crest. The East Pacific Ridge crest falls close to the theoretical Turcotte[ 1974]gives an expressionfor the temperatureas a curve,which is constrainedto passthrough the Mid-Atlantic functionof depth and of distancefrom the ridge crestin a form Ridge value.Thus the observeddifferences in the elasticthick- which is suitablefor usenear the ridge axis. His expressionfor nessbetween the Mid-Atlantic Ridge and EastPacific Rise can the temperatureTz at a depth Z near or at the ridge axis is be explainedby a simple spreadingrate dependentthermal model for the accretingplate boundary. It is apparent,however, that differencesin spreadingrate T,.-Z--ffTT:- Ts ' =erf{•[x_•l •/:} (5) cannot explain the values of elasticthickness determined for where Ts is the surfacetemperature, T,. is the temperatureof the ReykjanesRidge crest.The ReykjanesRidge hasa spread- the solidus,u is the spreadingrate, k is the thermal diffusivity, ing rate of about 1 cm/yr, which correspondsto a predicted and Xois a parameterwhich describes the thermal structureof value for the elasticthickness of about 11 km (Figure 16). We the ridge crest. The expressionis calibratedthrough the pa- havepreviously concluded that it is not possibleto explainthe rameter Xo, which has dimensionsof distance.Although the observedgravity anomaliesover the R eykjanesRidge for an COCHRAN: ISOSTASY AT MIDOCEAN RIDGE CRESTS 4725

.L I0- T [ T ,gUNCOMPENSATED

RiseEast CrestPac,f,c 08-

• M,d-AtlanhcR,dge Crest

•02-

0 COMPENSATED •o'oo ,•oo •$o 4;o •$o •bo ,$o •b WAVELENGTH (KM) Fig. 12. Plot of the isostaticresponse function ½ as a functionof wavelengthon a log plot for the EastPacific Rise crest and Mid-Atlantic Ridge. The variouscurves represent expected response functions for variousthickness elastic plates. The isostaticresponse function normalizes the admittancefor differencesin water depth and density. elastic thicknessgreater than about 6 km. The differences order to explain thesedifferences, temperatures would be re- between the ReykjanesRidge crest value and the predicted quiredto be anomalouslyhigh beneath the R eykjanesRidge. curve which fits the East Pacific Rise and Mid-Atlantic Ridge The presenceof unusuallyhigh temperaturesbeneath the can be explainedif the thermal structureof the R eykjanes ReykjanesRidge has been suggestedby severalinvestigators. Ridge crestdiffers significantly from the other ridge crests.In Schilling [1973] explainedanomalously shallow topography

NORTH ATLANTIC

SN07 OBSERVED GRAVITY 28øN M.AR FILTER EPRFILTER "•/•%'•• TOPO GRA P H Y

SOUTH ATLANTIC

V2412 54øS SCALE MGAL GRAVITY[2_02 ¸o IOOO TOPOGRAPHY 0 METERS -I000

0 50 KM

Fig. 13. Representativetopography profiles from the North Atlantic and South Atlantic wereconvolved with the filters determinedfrom the East PacificRise and the Mid-Atlantic Ridge to producethe curveslabelled E.P.R. filter and M.A.R. filter. The observedgravity is shown at the top of each set of profiles.The differencein the relationshipof gravity to bathymetryis shownby the fact that while the M.A.R. filter fits the data well, the E.P.R. filter producesanomalies which are much too small. 4726 COCHRAN:ISOSTASY AT MIDOCEANRIDGE CRESTS

REYKJANES RIDGE

MID-ATLANTIC RIDGE FILTER EAST PACIFIC RISE FILTER V2303-2 _"'81 GRAVITY 619ON • V"",J"V"•N[ I -22•] DIFFERENCE 619ONV2303- 2].,'-,J%,-•..•,•L/._,-•L -.,- 56 +48 8 •: -20JoOlGRAVITY •,•2••+ 488 -20J TOPOGRAPHY • I000tTOPOGRAPHY i•,••• 1577

V2303-4].,.,,...,•__ •[_i' 49 V2303-4]'"""•,•/.•-'""•t 88 616øN .1' "" - '"' L 616øN•••/•+ 467 +467

• 1427 ß. "},-,•J'••1427

V2303- 5 ].._.-,• _ ..-...-.[ 614øNv2303-5 ]"-'-'"•-,....._...•/"•_+ 82 614oN _[ v • L_+ 49 •.••/•• +479

• 1448 • 1448

605øN _'1'6 8 60V2303-8] 5øN J'.__,.•_._.•.,•x___^[ +- 4 2 ••••,•+ 522 •%/'/•*• +52 2

• 1535 • 1535

V2303-9604øN •[i75 604øNV2303- 9_F] ...... ,•,,___.,._../-..,.._ _.,..,•+4 5 ,•••½,,• +45 0 ],-,•/••,•'"' 450

• 1543 o 50 ioo • 1543 KM

Fig. 14. Comparisonof observedfrcc air gravity anomalies with the anomalies produced by convolving the East Pacific Riscand Mid-Atlantic Ridgc filtcrs with fivc topographic profiles across thc Reykjancs Ridgc. Thc obscrvcd gravity and topographyprofilcs arc discusscd by Talwaniet al. [1971],and thc numbcring system for thcprofilcs corrcsponds to that uscdin thcir papcr. Thc mcan watcr depth and frcc air anomaly for cach profilc arc givcn ncxt to thcobscrved profilcs, and therms diffcrcncc bctwccn thc obscrvcd and calculated gravity anomalics isgivcn by cach differcnce gravity profilc. andgeochemical anomalies on theReykjanes Ridge in termsof R eykjanesRidge, processesother than variations in thermal a narrowmantle plume situated beneath Iceland. l/ogt [1974] structuredue to differentspreading rates appear to operate. has constructed a model for the evolution of the northern There is good evidencethat these are related to anomalous North Atlantic basedon the hypothesisof a narrow mantle thermalprocesses immediately beneath the ridgecrest. plume beneathIceland throughwhich hot mantlematerial is broughtto the surfaceand then flows away from Iceland SUMMARY AND CONCLUSIONS beneaththe ridge crests. Recently, Talwani and Eldholm [ 1977] We have examinedthe relationshipbetween gravity and and Cochranand Talwani[ 1977]have argued in favorof a hot bathymetryover midocean ridge crests by usingcross-spectral spotin the uppermantle which is muchlarger than the island filteringtechniques. The followingconclusions may be drawn of Iceland.In eithercase there is compellinggeophysical and from this study. geochemicalevidence for anomalouslyhigh temperatures be- 1. The most satisfactorymodel for the compensationof neaththe ReykjanesRidge crest. seafloor topographynear the ridgecrest is the regionaliso- Theseresults therefore suggest that differences in spreading , static or elasticplate model. The effective elastic plate thick- ratemay explain the differences in elastic thickness (and thus nessesare 2-6 km overthe East Pacific Rise and Reykjanes of thestyle of axialtopography) between most 'normal' ridge Ridgeto 7-13km overthe Mid-Atlantic Ridge. crests.However, in regionsof anomalousdepth such as the 2. Theseestimates for the elastic thickness are significantly COCHRAN:ISOSTASY AT MIDOCEANRIDGE CRESTS 4727

REYKJANES RIDGE CREST (61.4øN) BOUGUERANOMALY ( •c= 2.6g/cm 3) V2305-5 ,601 ...j1401 :•1201 I00 • REGIONAL ISOSTATIC ANOMALY(Te =2 km) • ,o] • _•,.,,o.:o.•_ •

TOPOGRAPHY o V2505-5

MID-ATLANTIC RIDGE CREST(37.øN,FAMOUS)

BOUGUERANOMALY (pc = 2.6g/cm •) v32o$

to Gap 1 130 J REGIONAL ISOSTATIC u 'øot • :•-201 TOPOGRAPHY ,.-RIFTVALLEY 2.• o V$205 OUTER OUTER WALLINNER WALL •12 IWALL I• ,:,..:.../• 0 I0 20 30 I I I I KM Fig. 15. Bouguergravity anomaly and regionalisostatic (plate model) gravity anomaly profiles over the Reykjanes Ridgeat 61øNand the Mid-Atlantic Ridge at 37øN.Note that although both profiles show a pronouncedBouguer anomaly lowover the ridge crest, the regional isostatic anomaly profiles vary greatly. The ReykjanesRidge crest is characterized by smallregional isostatic anomalies, while the Mid-Atlantic ridge crest is characterized by a regionalisostatic anomaly low of about-30 mGal overthe rift valley.A regionalisostatic gravity low characterizesthe axial regionof the Mid-Atlantic Ridgein boththe North andSouth Atlantic. The Mid-AtlanticRidge profile was chosen because it passes through the well- studiedFamous area. It wasnot usedin generatingthe Mid-Atlantic Ridge filter (Figure 10) because it is not sufficiently long. lessthan •he valuesof 20-30km determinedfrom loads nessrepresents the depthto a particularisotherm (taken to be formed on olderparts of the oceaniclithosphere [Watts and 450øC), the difference between the East Pacific Rise and the Cochran,1974; Watts, 1978].We attributethis differenceto the Mid-Atlantic Ridge canbe explainedas the resultof differing origin of ridgetopography in the axial portionsof the ridge spreadingrates. crests,where the lithosphereis hot and weak. 4. The ReykjanesRidge results cannot, however, be easily 3. The differencein elasticplate thicknessesbetween the explainedin terms of spreadingrate differences.The elastic East PacificRise and the Mid-Atlantic Ridge appearsto be platethickness for thisslow spreading (about 1 cm/yr) ridgeis real and significantand is interpretedas representingdiffer- similar to that of the fast spreading(> 5 cm/yr) East Pacific encesin the thermalstructure of the region underthe ridge Rise and appearsto indicate anomalousthermal conditions crest.With the assumptionthat theeffective elastic plate thick- beneaththe R eykjanesRidge. 4728 COCHRAN: ISOSTASY AT MIDOCEAN RIDGE CRESTS

Cann, J. R., A model for oceaniccrustal structure developed, Geophys. J. Roy. Astron. Soc., 39, 169-187, 1974. Cochran, J. R., and M. Talwani, Free-air gravity anomaliesin the world's oceansand their relationshipto residualelevation, Geophys. J. Roy. Astron. Soc., 50, 495-552, 1977. Defteyes,K. S., The Axial Valley: A steady-statefeature of the terrain, NTIC in The Megatectonicsof Continentsand Oceans,edited by H. John- son and B. L. Smith, pp. 194-222, RutgersUniversity Press,New u.i io Brunswick, N.J., 1970. ,,,,, . Graf,A., andR. Schulze,Improvemerits onthe sea gravimeter Gss2, J. Geophys.Res., 66, 1813-1821, 1961. Handschumacher,D. W., Post-Eoceneplate tectonicsof the eastern EAST PACIFIC RISE Pacific, in The Geophysicsof the Pacific Basin and Its Margin, Geophys.Monogr. Ser., vol. 19, edited by G. Sutton, M. Man- ghnani, and R. Moberly, pp. 177-202, AGU, Washington,D.C., 1976. Heirtzler, J. R., and T. H. Van Andel, Project Famous: Its origin, programsand setting,Geol. Soc. Amer. Bull., 88, 48•-487, 1977. Herron, E. M., Sea floor spreadingand the Cenozoic history of the east central Pacific, Geol. Soc. Amer. Bull., 83, 1671-1692, 1972. Houtz, R. E., and J. I. Ewing, Upper crustal structureas a function of plate age,J. Geophys.Res., 81, 2490-2498, 1976. o Lachenbruch,A. H., A simplemechanical model for oceanicspread- ing centers,J. Geophys.Res., 78, 3395-3417, 1973. SPREADING RATE (cm/yr) Lachenbruch, A. H., Dynamics of a passive spreadingcenter, J. Fig. 16. Plot of the bestfitting elasticthickness for the various Geophys.Res., 81, 1883-1902, 1976. ridgecrests against the half spreadingrate. The soliddots are the best LaCoste, L. J. B., Measurementof gravity at seaand in the air., Rev. fitting values,and the hatchedboxes are subjectiveestimates of the Geophys.Space Phys., 5, 477-526, 1967. possiblerange of thevalues based largely on thestandard deviations of Ladd, J. W., G. O. Dickson, and W. C. Pitman, III, The age of the the admittancevalues (Figures 5 and 11). We alsoshow a theoretical South Atlantic, in The OceanBasins and Margins, vol. 1, The South curve[Turcotte, 1974] calculated with the assumptionthat the baseof Atlantic, edited by A. E. M. Nairn and F. G. Stehli, pp. 555-573, the elasticlithosphere is the 450øCisotherm [Watts, 1978]and which Plenum, New York, 1975. is constrainedto passthrough the Mid-Atlantic Ridge value. Lambeck, K., Gravity anomaliesover oceanridges, Geophys. J. Roy. Astron. Soc., 30, 37-53, 1972. Larson, R. L., and C. G. Chase, Late Mesozoic evolution of the western Pacific Ocean, Geol. Soc. Amer. Bull., 83, 3627-3644, 1972. 5. The presenceof a positive(horst) or negative(rift val- Le Pichon, X., J. Bonin, and J. Francheteau,Plate Tectonics,300 pp., ley) topographic feature at the ridge crest appears to be a Elsevier, New York, 1973. functionof the averagetemperature in the vicinityof the ridge McDonald, K. C., and B. P. 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