STIIDIES OF THE GEM ] ALTIi'EIER DERIVÐ GEOID UIWUUTIOruS OVER SEAI.CII,I'ITS IN T}E. ImlAi,l 0cEA¡l(*)

BYA,ruw Cqzrrunvr, l(, brqlnu

Group de Reehez'ches de GeoC.esíe Spatiale,Centre Nationel d' Etu,Tes Spatiales,

lcy-l.ouse, Franee.

Kunr l¡reecrcResearch School of Earih Sciences,Aus tralian National Llniuensíty,

Car'berr.a, Aus tyalia.

( * ) - - . .' 'Þ18ÀÉres érú rõv t¡l¡clætg¡ roú yec,:er.ôoCrs noú êEñX'9rF,¡ ómó óÀtuue'rprxó orouxeicr

roü GEOS 3 rnv¡ cró ti,ç trnoÐoÀdocueç TrEpLoxéç roü 'Iv6uxoú '9xecrvoü.

+78

Cazenave, A., Dominh, K. and Lambeck, K., 1979. Studies of the GEOS 3 altimeter derived geoid undulations over seamounts in the Indian Ocean. In: The Use of Artificial Satellites for Geodesy and Geodynamics, (G. Veis and E. Livieratos, Eds), National Technical University Press, Greece, 478-499.

Int¡odr¡ction

Studies of t!.e defor¡nation of the l.itlnsphere sllbject to loacs prcnrides useful in-

forination on the næctnnicat pnoperties of the Earth, propo-rLies that are jÍPo:-ttil¡t

i¡¡ the wider ccntoct of nodels of litlrospheric evolution ar'¡d of rnantle convection.

In tìese studies, the upper mantle is usually nr¡delled by a lithosphere of fir-tité

stÌenght overlying a relative}y rnreak layer; t}re astherosphe¡e. TL¡e defor¡nation of

t1.e layer subjected to a surface load, corputed using conventional elastic plate

theoryr provides estimaLes of the elastic par¿meters.of tÌre litlnsphere. Problerrs

tl¡at have been ¡rnrch stud-iecl i¡r recent years include the ioadirg of the J.ithosphere

þz ice-sheets (eg Peltier'and Andre^s, 1976), by seåiment deposits (eg walcott,

1972) arÅ þ volcanoes (eg Watts and Cochra¡r, 1974). ftre last are parhicularly use-

ful i¡r tlnt t].ey are forned lln relatively short time pericds and they e)<ist over a

wide varieQr of geolcgical envj¡orrnents, i¡c1udi-ng the inte¡iors of oceanic. plates.

Studies of litilospherÍc J-oa€Iirg ìry rnarille volcanoes, or seãrvJunts were initiated

by F.A. Verrifig-Ì"lei¡esz in 1941 andR. GlffI in 1943. VerLi¡g-Ibùìesz fourd that gral

vitlz ancrnalies over t}re Har"¡aiia¡r islands could best be erçJ.airecl by a regional

ccnpensation (Heiska¡ren and Venùlg|-Meinesz, 1958). G]nn (1943 a,b) deveJ-cped a si-

rn-ilar a¡:proach, considerilg the defornation of both a loadcd u¡l¡roken litlosphere

ar¡d of a fractr:red littosphere. l{afcott (1 970 a) pursueo tÌ¡-is furthe¡ as t¡eve 9iatts

and Cochran (1974) . All of these studies invesi:igated the Ha"'aiia¡r ridge- lJatts et

aI, (1975) n:ade a sirn-ila¡ str:dy of the Gr'eat tleteor sea'rþunt i-n the Atl-antic Ocear¡.

ttte peái¡ent neclranical propert], of the litlnsptrere tlrat can be deduced frcrn the

Ioaùirg studies is tl.e flo<lrral- rigidity (Iove, 1921 , P. 464; Tj¡n¡shenko ä¡d l'¡oi-

ncn*sþ-Krieqæ, 1959) ,

EH3

t 2 Í - ø 2 )

(K + U . / 3 )

(K + 4 ' f r , /3)( r )

= 9 p"¡r3v1 (v?^ - v?l /vz5 L s ! ' , 5 Þ

where H is ttle thichneoc of t¡e elastj-c layer, E is Yourçrrs nodulus, o is Pois-

sonrs ratio, k is the Ìrr1k modulus, u the. rigi-dity arrd Vn, Vs are the cu-çressio-

nal ar¡d shear velocities of the litlosphere. fhe floc¿ral rigidity is a,neas:ie of

the resistance of tle J-itlnsphere to defornation. It represents ar effec-uive rigi-

dity irr that the liti¡csphereisconsidered t- be a layer wit¡ uniform n'ÊchanicaJ.

4 7 9

pr€perties t}roughout. lhe particulår LoadiJg probløn discussed here does not pro-

vide any j¡founation on tàe j¡relastic behas¡ior¡r of the lithosphere. Ttds is ade-

quate for-the present study sllce the ages of tlre seer¡rn:nts (: tO7-lO8 years) are

usua]-ly much greater than the tjme constants apprcpriate to the ijûelastic response

of the lithcsphere (:1Ol years). Litlrospheric tì-ickness H does, ho!ûever, var]t

r+itå age ard., since MaH-r scnE age dqendence of 1,1 rT¡ay be a¡ticipated. Seisn-ic

velocities are also age deperdent due to the petroì.ogical- evolution of t}te lithos-

phere (Botttuga a¡¡d Steirrnetz , 1978). Ilr-is introduces a further age dqerdence

of M . Alsc, due to the freguenq¿ depcrdence of elastic npduli of a dispersive IrE-

dia, the elaslic pâræneters, of the littosphere at seisnj.c freq'-rencies, n'ay be

guite di.fferent frcm those releuant to the rmrch longer, geological, tjme scale.

In previous cal-cr:fatior¡s, t1:e flocural rigidiÇ bes beerì estj¡r'ated þ coçarirg

the co -ncuted gravitlz with obs-s¡ed gravity. cæ]----------------d heights may li-Iiewise be used for

this, ard @OS 3 radar alt-imeter observations dreí paiticularly useful l-n tlnt they

have .:ennitted a rapid saçling of geoid undul-ations over tlre oceäns with a gcod

spatiâl- resolution and a precision of alput 50-100 cm. In tìis pa.per we rq¡ierv

briefly tl.e theory and apply t}lsn to seve¡al features j¡r the scuthern InC-i-an Ocean.

ïtteory

AtI of the above cited studies, except Ûtat of Watts et aJ-., are baseC on the as--

srrçtion tlrat tJ.e l-oad can be assi¡n-ilated try a trtÞ dirrensional feature. But for

sirgle seæTrf,unts, three djmensional nodels are regui-red. fhe theory is usually ba-

seC on t]re defor¡rillion of thi¡r pJ-ates (see. for e;<aple, Love, 1927.¡). fhe diffe- .

rentiai "qt;tion

ior the egui)-Í-Jcrir.m of a plate defor¡,ed þr a pollt load P is

(GÐr¡, 1943 a; Jeffrq¿s, 1970):

i . i 1l4z + (Éta - Pl) .Jz = P

z -- z(r) is the defornation of the plate at a distance r frcrn the poj¡t v¡Ìrere tlte

load is a5p1ier1 and is rreasured positive dc.¡,rwards. StJictly z is the defo:rnalion

of the median plane vritì-in tÌ¡e elastic plate hrt for tÌ¡-i¡r plates it is usr.lal to at-

sr.rne that alL surfaces defo::n by similar arûf,ulìts.pa ard PI are tbe de¡sÍties of

the astÌ¡erpspùrere ard lithosphere respecbively and g is gravity. The solution of

this equation v¡as fi-rst ùiscl¡ssed by H. Hertz in 1BB4 and is also given by Watts

et aI. (1975) for a ttrree dj¡ne¡¡sional poi¡¡t load. It is

( 2 )

d z ( x , y ) -r - ( Þ a - p î ) - 9

u 2 , t r t

480

r+here p is an integration ¡xraneter, ard o is defileJ þ

. ( p - p r ) sa " -

1:,i.

Ib obtajn t,he total deflection at x, y it is necessarlz to fu¡tegrate (3) or¡er Ûæ

voh¡rre of t¡e load. the approadr used by Watts et aI. (1975) (see also tÌ¡e cor -

rect-ion J- C,erph;l's. p6¡s., B0 p. 4116) has been oncloyed in this pafrer. Fj.gure 1

illustlates tlre rqrional ccrq>ensatj-on n¡cdel for a conic¿l seâmf,\rnt.

Several assøçtions are jrùerent in the formal-isn (2). Thû¡ plate thecry is used

but the stucry ty Walcott (1 977) suggesls t-trat '.h-is is adequate if the wav-elerghts'of thc 'defo::nation are long curpared with the J-ittospheric th-icknels- A'seiòird as-

surption is that the qohericiþr of the EartÌ¡ is igncred, êr-i asstûnption that is

aìso acleguate if the vravelelqths of t¡e deforrnation are short ccrrçxred v;ith ttie

radius of the EarLh. lhe oceanic li+-Ìosphere has an average thick¡ess of scnre 70

kn whereas the lravelerçttrs. cf the defornatior¡s stu<liel her-e ere of the orde¡ of

sevel'.i1 hudred ki1cr.'.eters a¡d the above assuq>ü-on-s ap¡:eer justifieo. Thi-rd,r'iscc-

el¡si-ic prcperties cf -.he litÌ:osphere are i.gnored. If tids is jradeguate, it will

leail to a fley-ural rigidity that is time deperxient. It v¡il-I not be jnportant so

nnrch j¡¡ tÌ¡e ¡rode1 as iJr the subsequent interFretation of tl]e elastic parðrEters.

Once the deflection due to a k-noqrn l-oad is deteuni¡ed, the grravitlz or geoid arrc-

naly. can be evaluated directly þ iltegrati-rg c¡¡er the load and the deforned sur-

face, usirg net-l:cds outU¡ed þr Talwani (1975). f+- is usual, Ì¡¡^revc-r, to use a

sj.nptified approach in which the attrâction at a given poilt is asswnd to equal

that due to an j¡fj.nite layer ui.th densJ-ty an tlickness equal {o ttre density ano-

nuly imnediately beloa that point. This a;proxirnation is nostly adequate if the

slopes of the tcpcgraphy are snall and tJ.e lithosphere Ís not evcessively tlúck-.

Scrne trial calculations suggest that tl¡-is appro><imation, vCrich corrsidecably s1m-

plifies ttre calculation of t̡e gravity field, is satisfactory for the problars

åiscussd here. Tt¡e câlculation also ass.¡nes that tjre observed. topogaphy c¿¡ be

seParated i¡rto the pert cqnÈDrisiry the load and the part ccnprisj..ng tlre defo:rr'a-

Lion. The nnoel graviÇ is ccrqârted by tãki¡g j¡tc acorJnt i*re seãrÞunt, the de-

formed litloqpheric la1er, the '*'âter displacd by tt¡e seanor:nt, the der:siþr con-

trast at tlre littrosphere, asthencspheric layer ard a sediment layer wtrich has

filled i¡ the litlrospheric dqrression ai:out tÌ¡e seanìount. Ttr-is sedj¡ne¡rt layer

ftrtÌrcr loads tl¡e litJrosphere and leads to a¡r additional Ceformation. In atl .,he

4 8 1

above disc,r¡ssed npdels tt¡is has nct been taken into account. fhe sa¡re convention

is adopted i¡ this paper. Ba¡ks et aI. (1977) outlj¡red a nnrctr ¡ûf,re rigorous apÊ

p::oach to solvilg t}Ie loadirg problsn a.rd r¡)llidr merits an application to tfìe GEûS

3 altimeter- radar data.

In npst geological studies tl1e quantiþr corputed is t}le gravity disbEbânce 69 not

tìe graviQr ancrnaly Ag. The relation betr¡reen 6g and åg Ís:

 s = ø - S N ( 4 )

wlrere N is tlE geoia ircigl-rt, "

*" Eravitational co¡lst¡nt and R the rad.ius of the

Earth. The second term, descriJcirg tJle defornration of the geoid on vitr-icir gravÍty

is neasr:red, is often igrored ì¡ con¡xrisong of thg model gravity v¡ith observations.

This tern has beerr retaj¡red i¡r tÌús study.. The ccrpute<l geoid height N follclr¡s fr<¡n

(4) and the Stokes i-ntegral {eg Heiskanen and t'4oritz, 1967).

s . ( ' 9 ) d s ,

vrhere S (rþ) is tle Stokes function a¡rd rl¡ is .the argular separatior¡ between Ág and

tàe poht at vrtlich N is cu¡¡cuted. Àn ii:erative ¡:rocedure has been aCcpted to al-lo¡¡

for the cleperdence of the free ai-r ancrnaly on the geoid height. In reEional studies

it is necessar¡r to r€rrove frcrn the global gïavity field or frcrn the gecid the lorg

vravelength ancrnalies rvb-ich are rÐt àssociated v¡ità the l-oad arrd lithrospheric det-or-

r¡ation and which origjnate deeper in the.¡nantl-e. In ttús stud¡r, we have adopted a

different approach jrr that, i-nstead of r€rnoviJq long vravelenth undulatior¡s frcrn the.

observed gercid, vre liave added tJ:rase long vravel-erqth u¡dulations to tÌ¡e corputed

geoid nnCel.llris apprctech j-s guite .equivalent for coqrarisons betrveen observed and.

cor-in'r:ted geoid.

the sout̡-r+rest Indian Ocean qeoid

Duriry a l-jrnited cãrpa-ign in 196, 153 altirneûy profiJ-es of GOS 3 were obtainedj-n t}re South Indian Ocean arou¡rd the Kerguelen and Crozet islands, as a oc@erative

progrãn between France anC the tlnited States (figure 2). À detailed geoiô for the

re-gion Ìns Le-¡r ccnp:ted by Balnfuo et al. (1978) (figure 3). the spatial- r'esolu-

tion is of tïe order 0.5o ancl the height precision is of the o¡der 50-100cn.Ocean:

cgraphic ds¡iations of sea-leve} fron the rrcan geoid have been igncred.

fhe geoid i¡ tì-is area is dc¡nj¡rated þr a ¡nsitive arunaly centered over tlre Kergue-

1en plateau rnñ.ich incl-udes trre Kerguelerr and Heard isla¡rds. Thi-s plateau is betie-

ved to be of oceanic origin (Houtz et a1-. 1977¡ watkins et al. 1974; Luyerdl'k ard

Rennick, 19?7) anC-probaH.y resulted frcrn an upliftirg of the ocean basil þ a

" l l

4 c I I -',

J J

482

mantle hotsFot. The bro.rd grraviLy high over this plateau is n¡rre readily seen in

the satellite solutions (eg. Ga;nsctrkin, 1977; Balmù¡o et al., 1977) and probably

originates j¡¡ the uo¡:er mantle, due to thermal processes associated with tte hot-

spot. To t¡e north and nortf¡-east, tle geoid heights beccr¡Enegative a¡¡d a;e asso-

ciated with tÌ¡e t€si-ns of the eastern India¡ Ocean. To¿ards the r.æst, a secord

broad high is ce¡¡tered over the CYozet pJ-ateau, a seanount cLrain to the scuth ard

the intervenirg CtozeL Basin. Ðespite scme rcre¡t !,Ðrk (Schliclt, 1975t Goslj¡r ard

Schlich, 1978) tle soutj:-vestern part of tåc Ir¡dian Ocea¡r is still poorly under-

s+.oocl. CYozet isla¡d is on tle eâqtelin lxrt of a fai-rly extensive subnnrine p1a-

teau e':terdirg to Pri¡ce lÌd.¡a¡d Island arrl the vJestern branchr of the Irdian Ocean

Ridge (Fig,u::e 4). PetxolcAical and geoctrønical stud.ies by O:nr et al. (1970) on

f1e de lrEs"- and earliern¡f,rk on the fìe de Ìa Possession irdicate that the islards

ccrçr.i-sjrg tle group are rsÌnants of shield volcanoes daLLrg fron the Pl-iocer.e or

ÍÞre recent. The a]-kaJ-ic chãracteristics of the irasalts also suqqests ttrat tierej.s1a¡*ls fo::ned avay frcrn the ridges.

fhe Ob, Lena and l{ârion Duifresnes searount-s, tó the soutl'r of Crozet, nln ro:ghly

i¡r a v¿est-east òi-rection with a fourt}r 3€âr:'ount betrveenler¡a end I"¿rion )ufresnes(fiigure 5) . fhis trend Ís rougl:-I1, parallel to the maginetic ar¡crnal-ies and the sea-

nÐulìts aFpeiìr tc be situated on lithospher-e of appro><imately consiznt age. Tlús is

unl-ile ;rost seãÍ,ount cha-i-ns r.ìrere +-he ages charçe q¿stenatically írcrn one e¡:d to

ttre otho:. This is suggestive of a different oligix tìan, for exa:qpIe, the r,þre

extel'Eive Êçeror seã."orni:s in the Pacific. Ob and LeJta aDproach to -rlthiJl ab,ctt

250 n of sea-Icvel while t].e tv¡c others also have their surnr-its near about 1 000 m

depth, the ccnparatively shallo"¡ s¡rmits of Ob and L€rìa suggest tlnt they coutd

have forned ar*ay frcn ttre ocean rídge otherwise thei¡ subsidence, <ìue to the cæLirrg

a¡¡d contraclion of the lit¡osphere hþul-d probabJ-y have been grreate+, unless the i:ri-

tial fornations rose at least 1000 reters above sea-1eve1. The age of tl',e li-.hosphe-

re appears to be Uper Cretacests (Schlich, 'l 975) but the age of the seærrcunts is

u*nce¡n. Sc¡re shal-Ic¡.r seisnúc structure arourd the base of rrhrion Dufresne is inter-

preted by Gosiin a¡d SchLich (1978) as inpiying a¡ old structure. ltrese authors also

fi¡d that prcvisÍonal graviQr profiles cver this seanlcrlnt do not ed--i}it a broad re-gional- variatj.on a:rC tley conclude thet it do:s not represent a loaC sr:perposed on

tlte lithosptrer-e krut rathe¡ that it for¡ed æntsnporaneolsly v,'ith tJ:e littos-oher.e a¡d

i.s locally ccrçersa+-ed.

To tlte ncrtt¡ of tåe searncunt- chain, the magnetic ancrnal-ies are offset right-J-ate-rally þ ssveral frasture zon-s r¿itÌ¡ a tre¡rd li26o E (SchliC:l. 19?5). Neitlrer tllea¡¡crnalies rËr the fractuÍe zones hav'e been iclentified i¡ the nortÌ¡ern part cifErCerþ Basj¡r j¡rmeCiately belcw the sea¡u¡unt chai¡r (Gos1i¡r a¡d Schlich, 19?B). :ft}te fracttlrc zotìes can be exterdel sortìrward, ft ðrd lena çoul-d have tcen sE)ara-ted frcrn the otirer tvo, vrith possì-bly a furtler separation of l4arion D¿Íresnesfron the fc¡rrtÌ¡ seârnoxlE.

4 8 3

Às ocq¡rs.i¡,tÌ¡e otìer oceans (eg l.¡atts et al., 1975; Cochran ard Ta-IuanÉ, 1977)

strorg correLations are for:nd betlJeen batlryrnetqr ard gavity or, j¡ this case,

geoid heights. The 4ergn:ele¡rHea¡d Plateau, t}re Crozet, Plateau, tlle Ob-Lena-¡4arion

Dtfresnes searþunt ctnin a¡¡d various other sear¡¡unts, are all associated witl re

l-ative maxirrìa in qeoid treight of varyirq nragrn-itude and rnraveJ-erg"h. Th-is is sugge-

st-lve that much of t¡e gravity field over ttr-is part of tìe ocea¡ is donintated by

near surface featr:res or that bcth gnawity ard topography are conseçßtences ofmantle Proc€sses. Prcbably a cûnbùration of the LaÐ ocqrrs. Thre geoid a¡rcr¡aIies

over tl¡e seamountsare a conseque-nce of neâr surface structrre dhile the a¡ronal-ies

over the Kergr:elea Plateau are a consequence, jr part, of an ancrnalo:s u¡per mantle

structue. Figure 6 illustrates battÐ/rnetric and géoid profiles al.org the searr¡Jurrtdr.ain, fron LeJìa ó ùozet artd aeoss tte Kerguelen pl.ateau. To the North-East ofCrozet the volcanic islands of St. Paul and A¡nste¡dan do not result 5n geoid aro-

nalies. The petrotqy of these islards, cl-ose tci.,tireìoctLiri ridge, ÌËs been discus-

sed by G-ìrn et a1. (1975). ProbabJ-y these sea--nounts on a.thin oceân lithosphere,

and fo:med at about the same tjme as tfris lilùciþhere, are 1oca1ly, rat¡er tlnnregi.onally, ccnpen-sated.

Results

Tttë¡rcretical- geoid heigh+.s have bee¡r ccnputed ov-er the Crozet Plateau and the Ob,Lena artc l\4arion Drfresnes seðncunts, on t:l-e assr,lqption tLrat they ca¡r be modelled

as surface loads tlnt a:re regiornlly, rathgr than locally, crcrpensated. Ihe baü!'-

netr!¡, frqn Schlich (1975) a¡nl Goslj¡r ard Schlich ('1978) is illustrated i¡ Figu-

res 4 a¡d 5. The calculatior¡s sketched out in Section 2 have been ørolcryed for dif-

ferent values of tl¡e fleaxural rigidity M. Scrne of tl¡e'refl¡e¡rents àiscussed abc¡¡e'

have'r¡ct been rnade, nor has an j¡verse solution been attanpted. Figure 7 i1lu.stra-

tes the nodelgeoids over tlre Crozet Plateau as r"¡ell as the observed geoid ard the

bathymeûy. The anplitude and vraveJ-erçth of the rndet geoid changes signì-ficantJ-y

with the diffe¡.¡t assuned values of ¡f ard the best fit is obtained for lrf-0.75 x? n

10"" dyne-cn. fn so far as lcraer r,'al-ues for Iq inq>ly a lncre local coq>ensation,

these calcul-ations suggest -J¡at a npdel of regiornl ornperr-sation is adeguate. Si-

núlar ccnçrarisons have been rnade for the Ob and Le¡a seãrÞunts w'ith si¡n-itar resuLts(rigure B). Agai-n, the regional ccnpensation nrcdel gives results i¡ agreelent vrith

the observed geoid. fhe best fit is obtai¡led with ìF0.6 x 1030 dyne-on. For the tv,rr

rs¡ajjLirlg se¿nþunts, tlE one b,et¡neen l''larion Dr:fresres a¡d Ler¡a can be satisfacto-

rily nrodelJ.ed lrrt }4arion Dufrlsnes is unsatisfactory (Figure 9); the broail geoid

ancrnaJ.y ærprted with 1tr0.?5 x 1030 dyne-cm is not seeri.i¡ the observed geoid.Ítris

nay þ a conseguence of a lithos$rere of 1o"¡er-f1exura1 rigidity in this area hrt

tåis is unlÍJ<ely i¡¡ vier,¡ of +åe sacisfactory results obtai¡ed for tåe other sea-

milnts unless a fracture zone separates t4erion Dr:Fresnes frcrn tle ot¡ers. Alter-

4-84

natively, I'larion Du.fresnes nay L,e locally corænsaLed, as tentatively suggested þGosli¡r ard, Schtich (1g7ïl , br¡È ttús assulrÞs an origin for tñis particular searor¡nt

that, is different fron the otehrs. ltobably tåe differenc€ is a conseguence of ¿¡n

Sladequate geoid in tltis area, since tJ.e nr¡nber of altjmeter passes in tle inmedia-te vicinity of this seãrÞunt is relatively snatl coro.qreA v¡ith the other areas, arda number of tests sho¡¡ that t¡re solution is ur,stable for this area.

The'¡alues for M fc¡:nd here a-re ccrparable to tÏ¡ose fourd eLse+,trere for oceanic Ii-thosphere of a simil-ar age (Table 1) . Tirere is r¡c evidence that the Irdia'l Ocean ìi-t-ttosph,ere is very significantly djJferent frcrn the Àtlantic arrl pacj.fic ocean litÌ¡os-pheres. Conti¡ental- J,itlcspheres, on the other hand, have higher .¡alues (Table 1),a difference tlnt can be attril¡uted to an average contj¡entaì- lithosphere tLnt isabout twice as thick as ocean fithosohere, irr agrcsrellt w-ith 'ç.t¡at has been fcr.:ndfrcrn seisnic.a¡d other evidence (see, for e><arple, Le pichon et al.,1973). Thus,v¡lûLe tire geoid obserJations hâve not provided ne..r insj-ght ùrto the nature of the

.seëlþunts a,'ld sulnal:ùte pÌateaus i¡ tþe sounth-ue:tern Fårt of tlre Indian ocean,r¡orcontrihìtsi to a cjetec+ion of i:egional variations in ocean lithosphere par-n"t".",tl¡e resurts are sufficiently encor:ragi¡g to u;rcertake a sysb.snatic stÌ:oy of oceangeoid anonal-ies else"¡ìrere ar¡l to develop a rrþre refj¡reC theory for riadelli-ng thecrustal and lithospheric structures.

Acknoderìgenent

The @OS 3 altfneter res'Jlts v¡ere obtai¡ed ttrro:gh a coc1erative effont Leth¡ee¡-l-the United States and France ar'¡il ue tln¡k all agencies j-rn¡olved. J. Coslfur provided.us witjtr the bathyÍr3try results. fhe program used to cû-ncute the lit¡opheric defle-cLions is tlnt developed by À.8. Watts, J.R. Cochran ar.d G. Selzer.

References

Ba]mirn, G., DrossierÆ. ,CazenaverÀ., Dcnj¡h,K.,

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4E7

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Nougrer, J. and Baksi, A,

197 4.

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J . R . , 1 9 7 4 .

ConiÍ¿\ft f ioii (i:: ;,: :: ;' i i"l?L!;.:(; l 'I.i ' i:::l ':À!, íiic].Ti

Olr'.i ' i..1ìru:r t'o:r :'i:ta CiÍ)t:¡i't i:;:-. C.r, i. i.t'ì:À :_ì:i;,i.irj '.:i ': '::

ltl'If I DÈIiiF.: : I Ì:;tîI O:::-{ ¡'¡i:):.i rJ'¡': ii:!.: S'¡ üCl SS

:r.'-!ìl,f: I

) . J t Ì ios ; :hcrc

!¡r¿ri.l: ..*v ¿.:i.'. ì : .' 't:

!dyrrc - r : r : )

À t c

o f t l : ( )

l cad* r r , 6 , ,

! ì e i ¡ : : r ' : : c c : ;

cci i i ' i NIiìt! ' .:: ( : ; Ì : i : - i : ì : ; : r : r : :

t a i ¡ r s ., . . . i ' - l

J F;J .ì c c. i:

\ .V . r r r ¡ \ r : : \ r - j \ : r

COiII'I Ì¡Ði{l'.r:l Àr,cr,cIr cc! i ( l : . í c " ' d ^ . . L r - ^ t f . : ' :

Çcì iÀ: : Icl i l i f ' - 1 ' ; / ¡ . . . \

i !¡H¿ i :r-r ' i ì:¡ ' í.t 'cr

: : : r . . . ' ¡ i Í .a : l : i LS 5 ' . : C ' - "

( 8C t : r y ii . : . : i cor- î" i : : ,ùr; l ' j 6 . t O ? ' ) l.i<i:"s .3. iì I

i<. : ' ¡ ! '

t o u

( ' ! : i 7 . ( :a ' ( l ' i : f .

: , c f ì J c ( . ' . : : . , ; : ' : : ) t : : ì . i i l : 1O! : r ¿ r - ' : -c : : : : ; : . - :

4BB

r

clc

seo woterP = l .O3

sedimentsP - 2 . 8

Figure 1 . Schernát ic mode l o f. to the load ing by a

seo surfoce

t h e r l e f l - t : c t i o n o f t l r e c l a s t i c p a n t o f t h e o c e a n i c l i t h o s p h e r t : d u es ; c a n ì o u r ì t a n d s e r l i m e r ¡ t s .

geo¡d

elost ic pf oteP = 2 . 8

mont leP - 3 . 4

r

ii\; ;r,r'i.:s\>tì''lE;>

":i*r;$Dfll

----FSt'-----r.--

-----t-

ft, : À l t l r r l r : t f J r ( l ( r l . ö covc rðq€

- tn ^-r'

l . r ¡ l : l re Sor r t l r I ¡x I lan Ocean

F

66

[ ' i r ¡u re 3 , ' l ' l r c a l t in r , : ' t , - ' r ' t l , ¡ ¡ ' i v t r t l ¡ i ' :o i rJ l ' , r r

LOI . J I TUDE

t l r e : ; o r ¡ t l r t ¡ r ' t r i n r . ì i . ¿ t t t 0ce¿ r r t a ro t t t t d l . l l e C ¡ ' o2 . , : t

ssanc l K , . : r8uc le t t p la t t :au> i .

,,ry.*Olr^r,_----tu,/'

litiu

5 1 ' a 2

Þ å c : Í ¡ ¡ ^ i i " . ì o

l i3l .Fe 4. Barh'/net:- '1: around the C:'ozet pl_areau. Isobaths are eiven at 5O0m interval-( F ron Sch i ich , 1 -o7 5 ) .

492

r(o(t

Figurc 5 . Bat : l ry rnc t ry . r t 'oun tJ t l re 0 l ¡ , [ , tna and Mar ion du [ ' resnes seanount r ì . I so t :a t l rs a rc 6 l i ver r a t 500r r r i l ¡ te r .va ] .( F rom Scf r j c l r , 1c7 t ) .

ooF

oc o'3 'iG ==

-rt

Tt 4Of

ã ? |cfr ; I

ï i ssl-t . P Iq r o I3 - s Io 3 o L

and ¡ ' ,eo id (upper ' ) p ro f i les a lo t rg a l ine a longdu [ ' r t : snes

2000

E

L.+,oE+¿(Ìtm

t ' i ¡ ' ,u re 6a . Ba t hynr r r t r i c ( tower ' )0b . Lena ar rd l4ar ion

4000

+ror

t imeter geoid

^tr-: 301P?

.9. o

pooL' 20

o

G'fn

| / reqional oeoid/- trõ* sate-ll i te orbit

?G'Noo

Gtro

J

10o

2000

4000

i i ø rn - â Ì r . Â ¡+ r ^ r 'ma t r i c and gec id : ¡ o f i l e - " a f cnc - a l i ne f r on Lena t o C roze : '

4 9 5

r

ct)

E

.9o

:cto(uo

30

25

20

ço0,ãtt)o

Y

rttúo:E

15

10

Fi .gune t ìc . Bathymetn ic and gco id p ro f i le rs a long a L i r re ex tend in ¡1 f rom Kergue len in a southeas ten ly < I i rec t ion ac t 'os$ t t re Kengue len- l l cand p la teau.

o

2 O O O ' -E

V

4000

6000

4 8 ,

. CROZET':: -.5 J \

PLATEAU

484 5 r

-E as t I onq i t ude

Obsenved geo iC

45

,r/

f-9,o

É

o

Bath yme try

1. rc29 dyn-cm 0 , 7 5 . 1 0 3 0 d y n - c m

? nr . . I U O y n - c m - 48L- 1 . 1031

48

I4ODTLS

Figure ?. Observed 5ath1'¡¡s¡¡- and geoi<ì fcr the Crozet clateau (top) and computedgeoid heights for the f l-exu¡'e rnode] with four <i i f ferent vaLues for thef.Iexural r igid!. ty. Geoi.d contours ate at 1m lnterval .

4.97

Longi tude- 5 1

OB & LTNA SEAMOUNÏS

;East Lonq i tude

C5servei ( f o:. ;e: ') ani :cnPutedfor the 0b anC Lene seanountso f O. 5x1033 dyne-cn . Ccntcurs

l t r ¡ ¡ o ¡ l r r , r ì , ì h a ì e þ ! g

{ n - ¡ f ì r 5 ' , ^ r ¡ - r ì n i ø i r ì i r r '- ¿ + - r q ! ! - t

arE at 1: : r lnte: 'vei .

C onrpu ted

0bservedo

4 9 8

I" lARION DUFRESNES SEAI"IOUNT

Fig.ure 9. Observed ( J-orver ) a;rd co¡:lputccj ( u¡:pe:- ) ;.eoidhe igh t s f . r t nÊ i l : r - : cn du i i . c s , .¿s s3ãno i : : ì t : c ra f . Lex ì ¡ ¡¿ .1 r - ! r i d i : . ¡ o f 0 . 2> :LC3 i . j , , ' ne - cn .Co:ìtouÌ.s ere ai I :ìl ii.Ìte:"vel.

4 9 9

o€l

o)

C or:lpu ted

0bse rved

E a s t L o n g ì t u d e

D