Gravity study of the Pitcairn

Gravity study of the Pitcairn-Easter hotline
M Maia, G.. Dehghani, M Diament, J Francheteau, P Stoffers
To cite this version:
M Maia, G.. Dehghani, M Diament, J Francheteau, P Stoffers. Gravity study of the PitcairnEaster hotline. Geophysical Research Letters, American Geophysical Union, 1994, 21, pp.25272530. <insu-01354635>
HAL Id: insu-01354635
https://hal-insu.archives-ouvertes.fr/insu-01354635
Submitted on 19 Aug 2016
HAL is a multi-disciplinary open access
archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d’enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.
GEOPHYSICAL RESEARCH LETTERS, VOL. 2,1,NO..23, PAGES 2527-2530, NOVEMBER 15, 1994
Gravity study of the Pitcairn-Easterhotline
M. Maial,G.A.Dehghani
2,M. Diament
3,J.Francheteau
I andP. Stoffers
4
Abstract. Shipboardfree air gravity and bathymetricanoma-
several volcanic edifices exist. Some of these edifices, such as
theCroughSeamount(25øS-122øW)
werefoundto be partof a
systemof en-echelonridgesstrikingroughlyN65øE. These
associatedwith one of the positive geoid undulationsobserved ridgesare obliqueto the accretiondirectionsandto the lithosphericgrainandareformedby nat'row(20-30 lan large)linear
in the area from satellite data. Several smaller topographicfeatures, volcano-tectonicridgesorientedN65øE, are superimpo- featuresmore than 200 km long, with very abruptflanks (R.
Searle,unpublished
data).Volcanicedifices,somehigherthan
sed on the topographichigh. Admittancecomputationsand direct modelingshow that the swell topographyis compensated 2000 m above the sea floor, are emplacedon theseridges.
by a low densityzonewithin the lithosphere,4 to 8 km below The SO-80 cruise of the FS Sonne, in June/July 1992 aimed a
andpetrologicalstudyof the Pitcairn-Easter
Hotthe crust The volcano tectonicridges are locally compensated geophysical
in a classicalAiry sense.The swell and the associatedridges line. As part of the objectives,we carriedout a surveyof one
were probablycreatedby the actionof a thermalanomalyre- of theseridges(Fig. 2). Anotherobjectivewasto verify if the
with any partisultingfrom the interactionof the EasterIslandhotspotandof observedgeoidundulationcouldbe associated
culartopographic
signal.Three N-S profileswere thuscarried
the Easter Microplate accretioncenters.
lies with an extension of 400 km were identified
across the
Pitcairn-Easter hotline in the South Pacific. The anomalies are
out in order to investigatethe natureof the geoidhighs.
Introduction
Data and method
The existence of linear undulationsin the gravity field with
wavelengthsranging from 200 to 1000 Ion over the Central
Pacific Ocean is now well established[Haxby and Weissel,
1986; Baudry and Kroenke, 1991; Maia and Diament, 1991;
Cazenave et al., 1992]. The undulationsare generally associated with volcanicor magmaticfeatures.For example,volcanic
chains are located over linear geoid anomalies with wave-
lengthsof 500 to 800 km. Sublithosphericconvectiverolls
Eighteenascendingand descending
SEASAT trackswereused
in our analysis.Note that for the wavelengthsunderinvestigation more recent data such as GEOSAT would not have improved our results,since the spacingof satellite tracks is equiva-
lent. Also, improvementsin the resolutionof the altimetric
measurementsfor GEOSAT are more significantfor the short
than for the medium wavelengths,considered
here. Bathymetry
along the SEASAT profiles was computedfrom the DBDB5
data base.We shall keep in mind the poor quality of this data
basein the area when analysingthe results[Smith, 1993].
ThreeN-S profileswerecarriedout duringthe cruise(Fig. l a),
chains of the area [Bonatti et al., 1976; Turner and Jarrard, one continuous,500 km-long (profile 1) and two about 300
km-long(profiles2A and 2B). The locationof the profiles(1,
1982; Diament and Baudry, 1987]. Some authorshowever in2A and 2B), 10 lan apart,and their orientationwere chosenin
ferred that mass heterogeneitieswithin the lithosphere were
causingthe gravity anomalies.Processes
at the origin of den- order to crossone of the en-echelonridges and to be perpendisity anomaliescan be boudinage,fractures and associated cular to the generaltrend of the geoid anomaly.Profiles2A
magmatismor convection(the densityanomaliescould have and 2B were used to composea single profile 500 km long
been frozen in the ageinglihosphere)[Winterer and Sandwell, (here called profile 2). In addition, the en-echelonridge was
surveyedby 50 km-long regularly spacedprofiles normal to
1987; MacAdoo and Sandwell, 1989]. Maia and Diament,
[1991] observedthat someof theseundulationscrossthe East the trend of the ridge (Fig. 2a). Bathymetry and free air anoPacificRise at particularpoints,whereEasterand JuanFernan- maly marineprofiles I and 2 were bandpassfiltered usinga
dez microplatesare located.A recentstudyshowedthat a rela- Butterworth recursive filter of the fourth order. The 200 km
tionshipbetweenthe patternof the anomaliesand the ridge cut-off wavelength was chosenin order to separatethe short
segmentation may indeed exist [Calgano and Cazenave, wavelength component of the signal from a longer wave1993]. Moreover the patternof geoid anomaliesmay display, lengththat could be visually identified.The long wavelength
residualwas obtainedby removingthe filtered signalfrom the
in the same area, both the directions of absolute and of relative
plate motions,superposed.
It is the casein the North Pacific originalnon-filteredprofile.SEASAT geoidprofiles,5300 km
in orderto removethe wave(J.L. Olivet, pers. comm.) and in the South Central Pacific long, were alsobandpass-filtered
[Buck and Parmentier, 1986] were invoked as a sourcefor the
anomalies [Baudry and IQ'oenke, 1991; Maia and Diament,
1991]. This hypothesiscould help to explain the deviations
from the classicalhot spot model shown by several volcanic
[Maia and Diament, 1991]. Such a pattern suggests that
different
sources for the anomalies
can exist.
One of the most striking geoid undulationsis associatedwith
the Pitcairn-Easter Hotline, which includes the Pitcairn and the
Easterhotspotsand the Sala y Gomez volcanicchain [Bonatti
et al, 1976] (Fig.l). The geoid anomalyhas an averageamplitude of 50 cm (in the 25-1500 km waveband) and crossesthe
ridge at the emplacementof the Easter Microplate. Between
the Pitcairn hot spot (23.9øS-130.8øW) and the microplate,
1. URA 1278 du CNRS and GDR 910, Brest, France
2. Institutf/ir Geophysik,UniversitfitHamburg,Germany
3. IPG Paris, France
4.Geologisch-Pal•ontologisches
Institut,Universitfit Kiel, Germany
Copyright1994by theAmericanGeophysical
Union.
Papernumber94GL01652
0094-853 4/94/94GL-01652503.00
lengthslongerthan 1500 lan and shorterthan 25 km. From
these,800 km segments,centeredover our area, were selected
avoidingcontributionof other structuresto the admittance.
Admittancewas computedfor the three data sets:the filtered
geoid and the syntheticbathymetricprofiles, the 500 km
shipboardprofiles and the ridge profiles. The experimental
pointswere thencomparedto theoreticalcurvescalculatedfor
different models. We consideredtwo models of compensation:
the classicalelastic plate model (the Airy model being an end
membercase when no rigidity is assumed)and a model where
the compensation
is achievedby a low densityzone in the upper mantlebeneatha crustof constantthickness(Airy II model) [Louden,1981]. For platemodels,four parameterswere allowed to vary simultaneously,
in a pre-determined
range:effective elastic thickness(E.E.T.), crustal thickness,average load
densityand averagedepthof the topography.For Airy II models, the E.E.T. is replaced by the depth to the low density
zone.Best fits were determinedby the minimumrms values.
The best fit parametersdeducedfrom the admittanceanalysis
were usedto give initial constraintsto computedirect gravity
modelsusingthe Parker[1972] developmentup to the order3.
We computedthe gravity effects of the topography,of the
Moho interfaceand, in the Airy II model, of an interfacematerializing the density contrast.Models are one-dimensional,
2527
237'
238'
-22
....
239' 240' 241' 242' 243' 244' 24s' 24•'
247' 248' 249' 250' 2•1'
2•2' Geoidanomaly(m)
O.6
•,,•'*•i•/
' ' ' ' '' '/"•'•'" ' -.....
' '......
' ' -22'
o.5
B
O.4
0.2
I -3000
0.1 i•
-4000
0
..
100
200
300
400
500
600
700
o.o
.o.1
.o.2"•L
o.o
.0.3
-0.5
i
'
0
-0.5
I
'
100
!
200
'
I
'
I
300
400
Sou•
'
i
'
500
i
'
600
I
?00
Northwest
Figure1. A DBDB5bathymetry
(blackcontour
lines)superposed
to thefilteredgeoidanomalies
(25-1500
km waveband;gray shades).Thick lines show the emplacementof the SO-80 N-S profiles.The EasterMicroplate boundariesand the main topographictrendsare shownin white. The grid was obtainedusingthe cubic Bspline method of Inoue (1986) with a grid spacingof 0.25ø. Bathymetriccontourlines each 1000 m. White
dotted line showsthe emplacementof the SEASAT profile displayedin B. Inset on top fight showsthe location of all SEASAT profiles usedin our analysis.B Filtered geoidprofile (25-1500 km waveband)and correspondingDBDB5 bathymetrylocatedin A The surveyeden-echelonridgepositionis shownby the arrow.
but since we are mainly interestedin the long wavelengths,
this approximationis reasonable.For the short wavelengths,
this may underestimatethe anomaly and must be considered
while interpreting the results.
Results
Fig. lb showsone of the SEASAT geoid profiles, filtered in
the 25-1500 km waveband,and the correspondingDBDB5 derived bathymetry. The bathymetic signal of the swell seen on
the geoid and on the bathymetric shipboardprofiles is very
subdued,due to the poor quality of the DBDB5 bathymetry.
Fig. 3 shows the bathymetry and free air anomaly for shipboard profiles 1 and 2, unfiltered, filtered in the 1-200 lan wavebandand the residualobtainedby removingthe shortwavelengthsfrom the unfilteredprofiles.The shortwavelengthsignal correspondto topographicfeaturessuch as the volcanic
ridges. A broad anomaly (400 kan) can be identified on the residualsasa swell on which theridgesare superposed.
The location of this swell correspondsto the positive geoid anomaly
(Fig. 1). Fig. 2 displaysthe en-echelonridge bathymetry(2a)
and free air anomaly (2b). The morphologychangesfrom a
centralsmoothand narrow part to extremesoccupiedby two
large volcanoes(N. Binard unpublisheddata).
Fig 4 showsthe admittanceobtainedfor the filteredgeoidand
DBDB5 bathymetry(4a), the 500 km profiles 1-2 (4b) and the
profiles over the en-echelonridge (4c). The lines show the fits
for some of the different compensationmodels considered.
Rms for differentmodelsare shownin Fig. 5. For the geoid,
the poor bathymetrypreventsa good analysis,but the best fit
(dots and long dashesin Fig. 4a, rms 0.0244/0.0247 m/km)
correspondsto a plate model with very low effective elastic
thickness(2 km), thick crust(8 lan) andrelativelylow density
6 km) andlow densityloads(2.4x103kg/m3).Higherdensities
would require unreasonablythin crust(Fig. 5c)
Fig. 6 displays the total and the filtered (wavelengths < 200
km removed) residuals(observedfree air anomaly - calculated
anomaly) obtained with the direct models. Fig 6a shows the
residualsfor one of the 500 km profiles. Curve a• showsthe
residualobtainedwith a flexure model and curve a2 the residual
for an Airy II type of compensation.For a low rigidity plate
model (2 km), the residuals (curve a•) are very close to zero
(total residualrms 6.1 mgal; filtered residualrms 2.4 mgal) and
most of the short wavelengths are well modeled. Some very
high frequency signal is probably related to either the one-dimensionality of the model or to lateral density variations. A
positive residual, comparable to the long wavelength of the
observed signal, still exists. This is an indication that either
the crustal thickness required by the flexure models to compensatethe topographicload is too large or that the densities
assumedare too high. The residuals(curve a2) obtainedfor a
model assuming an Airy II compensation with the density
anomaly 4 to 8 lan below the crust, as indicated from the admittance results, are stronger for the short wavelengths, but
the long wavelength residual associatedwith the swell is well
accounted for (total residual rms 10.9 mgal; filtered residual
rms 1.5 mgal). A slight negative residual is probably associated with some flexural compensationof the larger topographic
featureswhich was not consideredin the model. Fig. 6b shows
the model for the en-echelon ridge, with a local compensation
(no rigidity, classical Airy model). The residual close to zero
(rms 3.8 mgal) confirms that these features were created on a
weak lithosphere. Fig. 6c presentsthe residuals (total residual
rms 6.8 mgal; filtered residual rms 0.8 mgal) for a composite
model for the 500 km profiles, consideringa local compensation for the short wavelength topography(1-200 km wave-
loads(2.6x103kg/m3).Bestfits alwayscorrespond
to a flexu- band)andanAiry II compensation
for theswell()• > 200km),
ral model with low plate thicknesses.A spectralanalysisof
the DBDB5 bathymetrywas performedto check whether this
resultmight be a consequence
of artefactsinta'oduced
by the interpolation methods used to create the data set [Smith, 1993].
The amplitudespectrumdisplayedslopesof -1 for this waveband.Only for a small fractionbetween35 and50 km slopes
were closeto -2, showinga reductionof energyin the signal.
Experimentalpointsin this wavebandshowedvery low coherence values and were not consideredin the analysis.The better constrainedfits for the shipboardprofiles 1 and 2 (dots, solid and dashesin Fig. 4b; rms in Fig. 5b) are alwaysobtained
for Airy II modelswith relatively low compensation
depths(4
to 8 km below the crust) a rather thick crust (8-10 lan) and a
with constantcrustal thicknessand a low density zone within
the lithosphere.This is a more realisticmodel, and the long
wavelength
residualis verycloseto zero(rms0.8 mgal).
241' 0(Y
241' 30'
242* Off
241'00'
241'30'
i
-25'00' .
i
242'00'
i
i
,,
•?'
highloaddensity(2.8x103kg/m3).Platefits requireeitheran
unreasonablecrustal thickness(14-16 km) or else display a
systematicmisfit for the long wavelengths.For the en-echeIon ridge (Fig. 4c), the fits are of low quality (rms in Fig. 5c),
probablydue to the small size of the feature.Best fits (dashes
in Fig. 4c) are for Airy modelswith low crustalthicknesses
(4-
/"9
241'0•
,
241'30'
-25* 30 ',
242'0•
241'0•
,
,
241'3ff
242'0ff
Figure 2. En-echelonridge bathymetry(2a) and corresponding free air anomaly(2b). Surveylines are shownin inset.
sity layer below the lithosphere[Robinsonet al., 1987]. Ac-
Profile1
c I•
•o
-100
-100
• orth
•lerth 100 200 300 400
South
500
0
c
0
• -1ooo
-]
25.•
'S
.•000
b, s
0
100
200
300
tive convection in this layer could help to dynamically sup-
port the topographyand create the gravity signal. The high
viscositycontrastin the layercouldmaskthe prese.
nceof density variationsat greater depths.If active convectionis pre100 200 300 4O0
500 sent and contributingto the signal, the thicknessof the low
South
viscosityzone shouldbe roughlyof the sameorderas the wavelengthof the gravity anomaly[Buck and Parmentier,1986].
Also, we could expect somerecent magmaticactivity associa-
• -1ooo
-]
4o0
0
500
100
Distanc• 0cm)
25.9'5
200
300
400
ted with the swell or with the superposeden-echelonridges, as
is the case for other mid-oceanic swells. A rock composition
close to OIB basaltscould also be expected.The petrological
500
analysisof the rock samplesdredgedfrom the ridgesshows
that the compositionof the basaltsis close to volcanicsfrom
Distanc• (kin)
the southwest rift of the microplate and unrelated to other
known South Pacific intraplate magmatism. The material is
old and no trace of presentday activity was found (R. H6kithe shortwavelengths
andlowercurves(a andb) the unfiltered nian, unpublisheddata). Our resultsfor the en-echelonridge
on a weak andfractured
signaland the long wavelengthsignal.Arrow showsthe lati- indicatea very near-ridgeemplacement
lithosphere,confirmingthe hypothesisrelating thesefeatures
tude at which the swell height is maxhnal.
to the activity close to the southernborder of the Eastermicroplate.The absenceof recentmagmatis
m and the petrology
Figure 3. 500 km N-S profiles: a) unfiltered, b) filtered
(wavelengthsshorterthan 200 km removed) and c) residual (1200 km waveband).On each graph, upper curves (c) display
resultsstrengthen
the hypothesis
relatingthe swellto a ther-
Discussion
mal/chemical anomaly located close to the Easter microplate
and the Easterhotspot.Even if the presenceof a low viscosity
zone below the Pacific lithosphereis highly probable,the ac-
The existenceof gravity and thereforegeoid undulationsof
medium wavelengthassociatedwith a topographicswell bet-
tual existence of active small-scale convection is still contro-
ween the Easter microplate and the Pitcairn hot spot has been
confirmed. The resul.tsof the geoid modeling accordwith the
geodynamicsetting of the area, a young lithosphere,deeply
fracturedby the proximityof the Eastermicroplate,and affected by long-lived thermal anomalies.The analysisof geoid
verseand our results'do not supportthis hypothesis.
Conclusions
The existenceof geoid anomaliesassociatedwith the Easter-
profiles,coveringa broaderareaintegratingdifferenttectonic
provinces,tendsto a resultwith a thin platewith an equivalent
elasticthicknesscorresponding
to the averageage of the area.
Resultsfrom the shipboardprofilesindicatean Airy II type of
compensationfor the swell, with the topographicload being
supportedby a low density body lying at relatively shallow
depthsbeneatha 8 km thick crust(4 to 8 km below the crust),
rather than by a simple crustal thickening as in flexure models. Lithosphericagesrange from 5 to 7 Ma [Naar and Hey,
1991], correspondingto a thermal lithosphericthicknessof
30 km. The densityanomalieswould then be roughly halfway
throughthe lithosphere,suggestingthat the swell is probably
Pitcairn
related to a thermal/chemicalanomaly close to the microplate
and the Easter hotspot,with resulting density anomaliesnow
embeddedin the lithosphere.It has been arguedthat the apparent shallow depths of compensationgenerally observedfor
mid-plateswellscouldresultfrom the presenceof a low viscoA
1.0 I,,,, , , . ß
t,
•
]•
seikip = 2.5 s/cn•,d m2500m,Tc m8 kin,Tom0 km
dotsp = 2.6f/era2,d ,• 3500m,Tc,• 8 kin,To= 2 km
0.9
1• d•h• p = 2.6•',
is confirmed.
90
lingdash. Tc= 81nn.d•*2km
90
ß •iid Tc=10km, de,=4km
• 80
dots
Tc*=
8Irm,
dc= 6km
•
0.2
"
0
........
! ........•
! ........
20
-
20
i ........
seikiTc,•4km, To=0km
•o 60 Ira8
daahe.
sTc=6km,
To=01ma
• so
10
,
to a
• 70 short
dashes
Tc=4km,
Tos0km
30
• •o
0.1
0.0
are related
dotsTc,•6'km, To=0km
80
Tc=Skm,
d•--Skm
maditnndashes
Tc•10km,
de,=Skm
_•0.7dashes
p=2.4gJcm•,d=•m,
Tc=101nn,
To=2km -- 70dashes
• o.4
0.3
..
The anomalies
100
d = 3O00
m
0.8Tc=8
kin,Tea2
km
hotline
broad (400 lan) topographicswell. Both results from adznittance calculationsand direct modeling indicate that the anomalies supportingthe swell topographyare locatedwithin the lithosphere,4 to 8 lan below the crust.The znechaniszn
of coznpensationis compatiblewith the presenceof a low density
body in the upper mantle. The en-echelonridges located on
top of the topographichigh show a local compensation
in an
Airy type mechanism,with thin crust.They are probablyrelated to the southwesternborder of the Easter microplate as well
as to the Easter Island thermal anoznalyand formed on a weak
lithosphere.We cannotcompletelyexcludea convectiveorigin for the swell. It is possiblethat densityanomaliescreated
by convectionnear the microplateare frozen into the lithosphere. Also, the observedsignal could be directly related to
deeperconvectiverolls if a thick low viscosityzone exists,
!
/"/ ..... ' '
1o
o
........
! ........
Wavelength
(kin)
Wavelength(kin)
t ........
Wavelength
(kin)
Figure 4. Admittancecomputedfor the filtered geoid and syntheticbathymetricprofiles(4a) the 500 km
marineprofiles(4b) andtheridge(4c).Curves
showthe fits for differentcompensation
models.Te=effective
elasticthickness;
Tc=crustalthickness;
de=depthof compensation
(for Airy II models);d=averagedepthof the
topography;
p=average
loaddensity.
Forthe500km.profiles,onlythefitsfor Airy II models
areshown.
0.18
I
I
I
I
I
I
,
0.16
0.14
o.
12
•
0.06
034
p=2.6
d.=3500
Tc•10•_
P=2-6
d•000T•10I•
i
[
!
2
4
6
8
,
i
,
I
,
I
,
I
•
op--2.4
d=2500
To=8
•.
8
Elasticthickness(kin)
i
i
10
12
•
o tic=30kin
• tic=•24km
•
•
,,
o
• dc•18
l•n
<•O o
ode=2km
'
I
2
'
•
4
'
v
•
6
'
,,
!
8
rrns(mgal/km)
o
'
10
,, p=2.91T•2 km
oTc=8 Lm
OTc-• km
•Tc=4 km
•g> 60
e
o p=2.4,d=1500,Tc=-2km
'
I
12
I
, Tc=8 km
n Tc-• iun
•
odC•8 k3n
•rd•--2inn
,l,l,l,l,l,l,l,l,l,l,l,l,l,l,l,l,!,
I
p=2.8,
d=::•}00
p--2.4
d*2500
T•10
[_ 10
• •
!
I
12
p=2.5
d=2500Tc--8
I:'
032
0.00
p=2.6
d-=-3500
Te--.8
•
O.lO
0.08
O
3
i
4
'
i
5
'
i
6
'
I
7
'
i
8
'
i
9
'
i
i
'
i
'
I
'
i
i
i
i
10 11 12 13 14 15 16 17 18 19 20 21
rms(m•al/lcm)
Figure 5. rms for the admittance
fits for someof the considered
models.For all figures,crustalthicknesses
(Tc) andeffectiveelasticthicknesses
are in km, averageloaddensities(p) arein g/cm• andaveragebathymetry
depths(d) are in meters.5a rms for the geoidadmittances;
5b rms for the 500 km marineprofiles(de is the
depthof thedensityanomalybelowthe baseof thecrustin km) and5c rmsfor the en-echelon
ridgeprofiles.
2530
A
MAIA ET AL.: GRAVITY
HOqLINE
a2)Atry1Imodel
pc2.8
g/cm
• p,3.0g/½m
• T½8 kmdc8km
al) Flexure
model
p•2.8g/em
{ Tc8kmTe2 km
a
•
STUDY OF THE PITCAIRN-EASTER
•-
C
AiryH parameters
pc2.8
g/cm
• p,3.0g/cm
• Tc8 kmdc8 km
50
Flexure
parameters
p•2.4
g/era
• T½
6kmTe0km
••0•"
•/'•••••
''
.•0•0
o
•o
-50 •
40
-30
',,",......
0
• .........
•00
• .........
200
• .........
300
• ..........
400
.•
g•0_I
•L--•%•g
••
• 40
70
-30
•
..
0
'•
, .... • .........
• .........
•0
D•stance
(kin)
20
• .......
•
',, • ,'...... ',
40
-30 I,, • ' ' ", •" I ' "• ......
50 •
0
100
Distance
(kin)
! ......
200
' .........
300
400
500
Distance
(kin)
Figure 6. Residualsfrom the direct models(observedfree air anomay- model anomaly).Curvescorrespond
to the total residual and to the filtered (wavelengths < 200 km removed) residual (smooth lines). 6a. Models
for the 500 km long profiles. Only one profile is shown. (a•) Airy II model, assuminga constantcrustal thi-
ckness,with the Moho interfacefollowingthe swell topography
and a low density(p=3000 kg/m3) bodyin
the upper mantle. The strong high frequency signal presentin the total residual resultsfrom not considering
any compensationmodel for the short wavelength topography.A sligth negative residual for the long wavelengths is still present and may correspondto a certain deflection of the lithosphere beneath the long wave-
length topography.(a2) Flexuremodel,with a thin elasticplate (E.E.T.=2 km). The shortwavelengthtopography is well accountedfor, but the strong positive residual related to the topographicswell shows that
flexure models alone are inadequateto explain the observedanomaly. 6b. Flexure (Airy model) total residual
for the obliqueridge (averageprofile). The model predictsmost of the observedsignal, except for some small
variationsprobablyrelated to crustaldensity heterogeneitiesand the presenceof sediments.6c. Composite
model for the 500 km profiles.The shortwavelength(1-200 km) topographyis modeledby a local compensa-
tion in an Airy senseand the long wavelengthby an Airy II model,with constantthickness
crustandlow density in the uppermantle.The model accountsfor the differentprocesses
that act in both wavebands.
but values of viscosity required to bring apparent compensation depths up to 12 km below seafloor would be very low.
Such
values
could
however
result
from
the interaction
of a
Haxby, W.F. and J.K. Weissel, Evidencefor small-scalemantle convection from Seasat altimeter data, J. Geophys.Res., 91, 35073520, 1986.
complex accretion pattern and the Easter Island hotspot. The
identification of superposedrelative and absoluteplate directions in the geoid anom'Mypattern of the North Pacific (J.L.
Olivet, pets. comm.) shows that at least in some parts of the
Pacific, geoid undulationsmay be related to convection.
Based on our multi-scale analysis and on the petrological cha-
Inoue,
H., A least-squares
'smooth
fittingforirregularly
spaced
data:
racteristics
of the en-echelon
ridgeSlwe preferthe alternative
Maia, M. and M. Diament, An analysisof the alfimetric geoid in va-
hypothesis.The density heterogeneitieswere created by a regional thermal anomaly, resulting from the interaction of the
Microplate accretion processesand the Easter Island hotspot.
finite-element approach using the cubic B-spline basis,
Geophysics, 51, 2051-2061, 1986.
Louden, K., A comparisonof the isostaticresponseof bathymetric
featuresin the north Pacific and PhilippineSea. Geophys.J. R.
astr. Soc., 64, 393-424, 1981.
rious wavebands
in the central
Pacific
Ocean:
constraints
on the
origin of intraplatefeatures.Tectonophysics,190, 133-153,
,
1991.
Acknowledgements. We are indebtedto Captain M. Kull and crew
MacAdoo, D. and D. Sandwell,On the sourceof cross-grainlineafions
in the Central Pacific gravity field. J. Geophys. Res., 94, 9341-
of theFS SONNE aswell asto our colleagues
of theScientificPartyfor
their help duringthe SO-80 cruise.Discussions
wifl• J.L. Olivet, R.
Naar, D. and R. Hey, Tectonicevolutionof the EasterMicroplate.J.
H6kinian and C. Devey were very helpful. Commentsof D. Gibert and
two anonymousreviewers much improvedthis work. Figureswere
drawnusingthe GMT softwareof Wesseland Snfith (1991). This work
Parker,R.L., The rapid calculationof potentialanomalies.Geophys.
J. R. astr. Soc.,31, 447-455, f972.
9352, 1989.
Geophys.Res., 93, 8735-8745,1991.
wasfundedby ATPG6osciences
Marines(INSU)andbyEECcontract Robinson,
E., B. Parson•and S. Daly, The effectof a shallowlow
n ø CEE SC1 CT92 0782.
References
Baudry, N. and L. Kroenke, Intermediate wavelength (400-600 kin),
South Pacific geoidal undulations:their relationshipto linear volcanic chains. Earth Plan. Sci. Lett., 102, 430-443, 1991.
Buck, W.R. and M. Parmentier,Convectionbeneathyoung ocanic lithosphere: implications for thermal structure and gravity. J.
Geophys. Res., 91: 1961-1974., 1986.
Bonatti, E., C.G.A. Harrison, D.E. Fischer, J. Honnorez, J.-G.
Schilling, J.J. Stipp and M. Zentilli, Easter volcanic chain
(SoutheastPacific): a mantlehot line. J. Geophys.Res., 82, 24572475, 1977.
Cazenave,A., S. Houry, B. Lago and K. Donfinh,Geosat-derived
geoid
anomaliesat medium wavelengths.J. Geophys.Res..97, 70817096, 1992.
Calgano,P. and A. Cazenave,Presentand pastregionalridge segmentation: evidence in geoid data. Geophys. Res. Lett., 20, 18951998, 1993.
Diament, M. and N. Baudry,Structuraltrendsin the SouthernCook and
Austral archipelagoes(South Central Pacific) basedon an analysis
of SEASAT data: geodynamicimplications.Earth. Plan. Sci. Lett.,
85, 427-438, 1987.
viscosity zone on the apparent compensationof mid-plate swells,
Earth. Plan. Sci. Lett., 82, 335-348, 1987.
Smith, W., On the accuracyof digital bathymetricdata. J. Geophys.
Res., 98, 9591-9603,
1993.
Turner, D.L. and R.D. Jarrard,K-Ar dating of the Cook-Australisland
chain'a testof tlie hot-spot
hypothesis.
J. Volcanol.Geotherm.
Res., 12, 18%220, 1982.
Winterer, E.L. and D. Sandwell,Evidencefrom en-echeloncross-grain
ridgesfor tensionalcracksin the Pacificplate.Nature, 329, 534537, 1987.
M. Maia and J. Francheteau,Universit6de BretagneOccidentale,
Laboratoirede G6ophysique,
Av. Le Gorgeu,BP 452, 29275, Brestcedex, France
G.A. Dehghani,Instimt ffir Geophysik,Universit/itHamburg,Bundestrasse
55, 20146 Hamburg13, Germany
M. Diament, Institut de Physiquedu Globe, 4 Place Jussieu,75 252
Paris cedex 5, France
P. Stoffers, Geologisch-Palaontologisches
Institut, Christian-Albrechts-Universitat,Olshausenstrasse
40, 24118, Kiel, Germany
(Received' March 16. 1994; Revised' June 6. 1994;
Accepted:June23. 1994)