JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 95, NO. B5, PAGES 6787-6798, MAY
10, 1990
Three-Dimensional Structure of the Upper Mantle Beneath the Atlantic Ocean
Inferred From Long-Period Rayleigh Waves
2. Inversion
A. MOCQUET1
Laboratoire de Gdophysiq•te Interne, Institut de Gdologie, Rennes, France
B. ROMANOWICZ
Laboratoire de Sismologie, Institut de Physique du Globe, Paris, France
The three-dimensional shear wave velocity structure of the upper mantle beneath the Atlantic
Ocean is investigated by inversion of fundamental mode of Rayleigh wave group and phase velocities. The results are reliable down to 300 km depth and improve upon the image of upper
mantle structure given by global tomographic models. The inversion of two-dimensional lateral
distributions of group and phase velocities, regionalized without a priori constraints, shows that
shear wave velocities are correlated with lithospheric age beneath the North Atlantic. Our data
set fails to quantify the correlation between shear velocities and lithospheric age, because of the
small lateral extent of young regions(<22 Ma) and of strong transversegradients. In contrast
with similar studies in the Indian and Pacific oceans,slow velocities associatedwith the location
of the Mid-Atlantic Ridge are restricted to the lithosphere. Therefore the Atlantic mid-oceanic
ridge behaves as a shallow structure, which is weakly related to deep upwelling asthenospheric
flows. At depths greater than 200 km, the central part of the Atlantic is characterized by the
highest velocity anomaly. This anomaly can be interpreted in different ways. Part of this anomaly
mightcorrespond
to a negative
(VsH-- Vsv)/Vsv ratioimplicitely
included
in theisotropic
inversion. We also tentatively interpret this anomaly as an important contributor to a shallow
degree two pattern of shear wave velocity distribution for the Atlantic area.
1. INTRODUCTION
In global tomographicmodels[e.g.,Woodho•seand Dziewonski, 1984; Natal et al., 1986; Taniraoto, 1986], the Atlantic upper mantle appearsrelatively homogeneous.Mocquet et al. [1989](hereafterreferredaspaper 1) haveshown
that a spatial resolution of 1500 km, at least, was neededin
order to detect fine lateral heterogeneitiesin the Atlantic.
In paper 1, 86 paths for group velocity and 114 paths for
locities and lithosphericageheld for group velocitiesbut not
for phase velocities. However, our data set did not allow us
to quantify precisely the correlations between surface wave
velocitiesand lithosphericage. In fact, similar studiesin the
Pacific [e.g., Forsyth,1977; Nishiraura and Forsyth,1988]
have shown that
the main
contrast
between
shear wave ve-
locitiesoccursfor regionsyoungerand older than 22 Ma. In
a slow spreading ocean such as the Atlantic, the former is
about 500 km wide, and lateral velocity gradients on each
phasevelocity,wereregionalized
usingMontagner's[1986] side of the Mid-Atlantic Ridge are high. Therefore paths
method of regionalizationwithout geographicala priori con- obliqueor perpendicularto the ridge are hardly sensitive,in
straints. Each path was corrected for surficial layer ef- the framework of the first-order optical approximation, to
fects before regionalization. Three subregionswith different this heterogeneity. Only paths parallel to the Mid-Atlantic
group and phasevelocitydistributionswere defined(Figure Ridge axis manage to determine preciselythe upper mantle
1). North of 35øN (regionA), we foundno correlationbe- structurebeneaththe ridge[e.g., Jacobyand Girardin,1980].
tween group or phasevelocitiesand lithosphericage. South
of 35øN (regionB), phasevelocitieswereslightlylowerbeneath young(< 22 Ma) oceanicregionsthan beneathold
ocean basins, but this velocity versus age relation was not
observedfor group velocities. In contrast, in the South Atlantic (region C), this correlationbetweensurfacewave ve-
1Temporarily
at Department
of EarthSciences,
Nagoya
University, Nagoya, Japan.
Therefore we will not attempt to construct shear wave velocity versusage models,and we will only mutually compare
the three main regionsA, B, and C.
After briefly presenting the inversion method, which
has already been extensively detailed by Montagner and
Joberr [1981] and Natal et al. [1986], we will discussthe
resolution at depth, and velocity versus depth profiles will
be constructedfor the different large subregionsdefined in
paper 1. Such a block regionalization will be useful to compare our results with previous studies, particularly with the
body wave analysisof Grand and Helmberger[1984b]in region B. Finally, we will interpret the lateral velocity varia-
Copyright 1990 by the American GeophysicalUnion.
tions of shear wave velocities
and discuss the differences
in
Paper number 89JB02839.
the group and phase velocity distributionsbetween regions
0148-0227]90! 89JB-02839$05.00
B and C.
6787
6788
MOCQUETAND ROMANOWICZ:
STRUCTURE
OF ATLANTICUPPERMANTLE,2
I
!
I
60'N --
30'N --
30'5
60'5 --
90øW
60'W
30'W
0
30'E
Fig. 1. The four subregionsin the Atlantic area for which mean velocity models
have been calculated[Mocqaetet al., 1989].
2. METHOD
matricesCd and Cp, respectively.The errorson the data
are assumedto be uncorrelated so that Cd is diagonal. In
In the isotropiccase,shearvelocitiesV$, compressional order to obtain a smoothly varying model with respectto
velocitiesVp, and densityp determinesurfacewave veloci- depth,eachmatrix elementCp0is definedusinga correla-
ties. The mostinfiuentparameteris V$, and the Rayleigh tion lengthLco, [Nata.[et al., 1986]:
wave fundamentalmode doesnot contain enoughinformation to invert simultaneously for those three parameters.
Nata.[et al. [1986]broughtforth a priori informationin or-
-(hi
- h2)
2]
=
der to retrieve theseparameters simultaneouslyin the transverselyisotropiccase,using the group and phasevelocities
of Rayleigh and Love waves. They found that the two best
2Lco,(hl)Lco,(h2)
(1)
resolved
parameters
areVs andtheratio(Vs, -Vsv )/Vsv,
which definesthe polarization anisotropyof shear wavesin
the transverselyisotropic case. Nevertheless,they found a
strong trade-off between these two parameters in the shallower upper mantle, down to 160 km. Furthermore, the
trade-offbetweendensityand Vsv is higherthan the density resolution,and Vp cannotbe resolvedeither [Nata.[et
al., 1986]. A more accuratedeterminationof the density
requireshigher modedata [Cara et al., 1984]. In our data
set, only the Rayleigh wavefundamentalmode is available.
Therefore
we restricted
our inversion
to the distribution
wherehi and h2 are depths,andao(h) is the a priori
error on V$ at depth h. In this paper, the subscriptzero
alwaysrefers to the starting model.
The problem is assumedto be linear, so that the relation
d=g(p)
(2)
which links the model p to the data d is written
of
shear wave velocitiesin the isotropiccase. Doing so, we
keep in mind that the match of our velocity maps to the
data could be improvedby taking into accountanisotropy
[Reganand Anderson,1984].
The inversionat depth of group and phasevelocity distributions is the secondstep of a two-step procedure im-
plementedby Montagnerand Joberr[1981], after the algorithm of least squaresinversiondevelopedby Tarantola and
with
G,j : (Ogi/Opj)
(4)
where the indice i refersto the velocitymeasurementat
Valeire[1982].The first step(paper1) wasthe regionaliza- period Ti and the indicej refersto the investigateddepth.
tion without geographicala priori constraintsof individual
path measurementsat constant period. The errors on the
data d (groupand phasevelocities)and the modelparame-
ters p (Vs in our case)are takeninto accountby meansof
Defining pe the best estimator of model p, the relation
ap
MOCQUET AND ROMANOWICZ: STRUCTURE OF ATLANTIC UPPER MANTLE,2
also holds in the linear case, and the inverse problem is
solved by
TABLE 1. Crustal Parameters of the Starting Model
Depth,
krn
where P0 refers to the starting model, and the resolution
matrix R is givenby [Tarantola and Valette,1982]
n- %oc*(c + c%oc*)-c
6789
(7)
In (6) and (7), the upperscriptT denotesmatrix transpose.
The final error on parameters is related to the resolution
by [e.g.,Montagnerand Joberr,1981;MontagnerandNatal,
1988]
p,
g crn
-3
gp,
gs,
krn/s
km/s
0.0
1.02
1.45
0.00
5.7
1.02
1.45
0.00
5.7
2.00
1.65
1.00
6.0
2.00
1.65
1.00
6.0
2.60
5.79
3.19
7.4
2.60
5.79
3.19
7.4
2.75
6.00
3.47
7.5
2.75
6.00
3.47
7.5
2.90
6.79
3.89
12.1
2.90
6.79
3.89
12.1
3.38
8.10
4.47
%( hl' h3)-- [•(hl, h2)- R(A1,h2)]' %0(h2,h3)
0
where•(hl, h2) ----1 if h1 ----h2and•(hl, h2) ----0 if not.
Equation(8) meansthat if a parameteris unresolved,the a
posterJorierror is equal to the a priori error.
We computedthe partial derivativesof group and phase
velocitiesfor the starting model using Wiggins'[1972] algorithm and usedsimultaneouslythe distributionsof group
and phasevelocitiesobtainedin paper I for a periodrange
from 50 to 175 s. The inversionwas performedat each 5ø
by 5ø cellof the regionalized
maps.The a posteriorierrors
on group and phase velocitiesafter regionalizationat con-
200
•00
stant period were used as errors on the data. The choice
of the a priori error on parameters is important because
it definesthe domain of variation allowed for the final V$
model. The amplitude of the velocity anomaliesis greater
at shallowdepth. Forinstance,in the Vsv modelof Natal et
al. [1986],maximum valuesof 0.6 km/s are reachedlocally
at a depth of 50 km. Nevertheless,the maximum values
foundin this latter model are more generallyequalor lower
than 0.3 km/s. Thus the a priori error on parametershas
beenfixedto 0.3 km/s downto the low-velocityzone(LVZ),
and to 0.25 km/s below this depth. These values correspond to a maximum relative variation of 6% with respect
to M1066A [Gilbertand Dziewonski,1975]Vsv model.
500km
0
The correlationlengthLcor (equation(1)) wasfixed to
100 km down to 88 km depth. Below this depth, Lcor increasedlinearly to reach a value of 200 km at a depth of
400 km. For the period range used in this study, surface
waves are weakly sensitive to the upper mantle discontinuities of a spherically symmetric Earth. Therefore we chose
km
M1066Awith a modifiedoceaniccrust(Table1) asstarting
model. This choiceis supportedby the result of Grand and
km
Helmberger[1984b],who did not observediscontinuitiesin
the upper 400 km of the mantle beneaththe Atlantic using Fig. 2. Resolutioncurvesat selecteddepths. The target depths
SH body wave data.
areindicatedby an arrow;O'p,corresponding
a posteriori
errorin
krn/s (equation(8)). A priorierrorsare 0.3 and0.25 km/s above
3. MEAN
SHEAR
VELOCITIES
UPPER
IN THE
ATLANTIC
and below 190 km depth, respectively.
MANTLE
Before describingthe results and discussingthem, it is
necessary
to investigatethe resolutionat depth. In Figure 2,
we plotted for different depths typical resolutioncurvesobtained using equation(7). These curvesdo not vary significantly from one geographicalpoint to another. Down
to 300 km, the maximum of the curves coincides with the
target depth, except at 190 km depth. Below300 km depth,
the peaks widen and the resolution is poor. Therefore we
will only be interested in the results above 300 km.
6790
MOCQUET AND ROMANOWICZ:STRUCTUREOF ATLANTIC UPPER MANTLE,2
The resolution kernels are 100 km wide at midheight, at
a depth of 150 km. This width enlargesto 200 km at a
I
I
I
I
I
i
[
I
]
!
i
I
i
i
i
depth of 300 kin. These values are close to those obtained
by Weldnet[1974]in the Atlanticand by Nata! et al. [1986]
on a global scale. They are better by an order of magnitude than thoseobtainedby Tanimoto[1986].Furthermore,
in this latter study, the target depth and the peak of the
resolution kernel coincidedonly at a depth of 200 kin; in
lOO
the work of Nata! et al. [198½;],
the coincidence
wasreached
at depthsgreaterthan 150 kin. Thereforeour study will
help to detail the shallow structure of the upper mantle.
At 190 km (Figure 2), the target depth and the peak of
the resolution kernel are separated by an amount of 30 km.
This discrepancymight be associatedwith the presenceof
the LVZ in this depth range, and with the high-velocitygradient it involvesvertically. If such gradients are too sharp
to be modeled by the inversion method used, our continous
approachwill tend to smooththem. The differenceof 30 km
betweenthe target depth and the resolutionpeak can hence
be consideredas the uncertainty upon the location at depth
of the LVZ
200
eA
OB
OC
ß CAR
in our model.
We will now present mean velocity models for a tectonic province: the Caribbean, and three oceanic regions
300
definedin paper 1 (Figure 1). Continentaland tectonicareas bordering the Atlantic ocean were unavoidable in our
i
study(paper 1). Nevertheless,apart from the Caribbean
where the path coveragewas good, they were not sufficiently resolved to attempt a modelization of their mean
shear velocity structure. Specific continental studies for
shear waves are, among others, those of Hadio•tche and
Joberr[1988]for Africa, Panza et ai. [1980]for Europe, Osagie [1986] for South America, and Cara [1979], Grand and
Helmberger[1984a],and Grand [1987]for North America.
Shear velocity versus depth profiles for each of the four
selectedregionsare shownin Figure 3, and the corresponding values and standard deviationsof the mean are listed
in Table 2. Above the LVZ (down to 150 km) shear velocitiesare identical in the North (regionsA and B) and
i
4.2
i
4.4
4.6
4.8
Vs , KM.S-I
Fig. 3. Mean V$ profilesbeneaththe fourlargesubregions
defined in Figure 1. Error bars are standard deviations of the
mean. All values are listed in Table 2. Solid curve, Weldher's
[1974]
model
fortheAtlantic
normal
basins;
dashed
curve,
V$H
Atlantic modelof Grand and Helrnberõer[1984b].
depth, Grand's model for the Caribbean becomes simi-
lar to model ATL [Grand and Helmberger,1984b]. Similarly, in the 200-300 km depth range (Figure 3), shear veSouthAtlantic (regionC), closeto Weldher's[19741model, locities for regions CAR. are in good agreementwith this
whereasthe Caribbean (CAR) shear velocitiesare signif- model,correctedfor attenuationeffects[Kanamoriand Anicantly slower, of the order of 0.2 km/s. Below 250 km, derson,1977].
our V$ valuesare higherthan Weldher's[1974]model. We
In contrast, though model ATL correspondsto a sampling
are more confident in our models than in Weidner's below
from regionB, the two modelsdiffer significantly,exceptat a
that depth, becauselonger periods have been used in our depthof 200 km (Figure 3). Grandand Helmberger
[1984b]
study.
Below the LVZ, Caribbean shear velocities become similar to those of regions A and C. The similarity of the
Caribbean seismicstructure with those of regionsA and C
below 150 km depth indicates that the tectonic character of
the Caribbean disappearsbelow that depth. Grand [1987]
constructeda V$H tomographicmodelof the upper and
lower mantle beneath North America, the northwestern At-
constrained the upper 200 km of the mantle by adjusting
the thicknessand the velocity of an homogeneous
lid to the
travel times and waveformsof the SH motion at regional
distances(10ø-16ø). A 100-km-thickhigh-velocitylid gave
the best fit to their SH body wave data. Our approachis
different becausewe use Rayleigh waves and we assumea
smoothly varying velocity with depth. The uncertainty on
model ATL is estimated at 0.04 km]s [Grand and Helmberger,1984b]. Using the a priori errors previouslymen-
lantic and the Caribbean, using S and SS body waves. In
his model [Grand, 1987, Figure 17], the Caribbeandisplays tioned, the a posteriorierror on our model(equations(7)
lowvelocities
(-3%) downto the 140-235km depthrange. and (8)) increases
regularlyfrom 0.06 km/s abovethe LVZ
In fact, studiesof the upper mantle using body wavesand
surface waves are complementary. The resolvingpower of
surfacewavesis best above200 km for the period range used
in our study, whereasSH body waveshavemore resolution
at depths greater than 200 kin. Taking into account the
respectivevertical resolvingpowersof body wavesand surface waves down to 300 kin, we consider that Grand's model
and ours are mutually consistent. Between 235 and 320 km
to 0.2 km/s at 318 km depth (Figure 2). Thereforethe difference between
model
ATL
and our model
above the LVZ
is too large to be entirely explained by the different techniques used, or by the respectiveerrors of the models. At
greater depth, model ATL lies at the edge of the domain
of variation allowed for region B. Polarization anisotropy
is implicitely included in the isotropic models, and we can
tentatively explain their differencesin terms of anisotropy.
MOCQUET AND ROMANOWICZ:STRUCTUREOF ATLANTIC UPPER MANTLE,2
6791
TABLE 2. Mean Shear Wave Velocities in the Four RegionsDefined in Figure 1
A
B
C
CAR
45
4.59
0.11
4.56
0.19
4.62
0.07
4.45
6:2
4.56
0.09
4.53
0.16
4.58
0.07
4.38
0.03
0.03
88
4.49
0.10
4.48
0.]4
4.5:2
0.07
4.:29
0.0:2
113
4.41
0.11
4.40
0.10
4.43
0.07
4.27
0.01
139
4.34
0.09
4.34
0.08
4.34
0.08
4.27
0.01
165
4.32
0.07
4.36
0.09
4.31
0.07
4.31
0.01
190
4.35
0.04
4.43
0.07
4.34
0.04
4.37
0.01
216
4.39
0.03
4.52
0.08
4.38
0.04
4.41
0.01
242
4.45
0.02
4.63
0.10
4.44
0.06
4.46
0.01
267
4.52
0.04
4.72
0.12
4.50
0.08
4.50
0.01
293
4.60
0.07
4.79
0.14
4.57
0.13
4.55
0.01
318
4.66
0.07
4.83
0.11
4.65
0.08
4.61
0.01
Velocitiesin km/s.
The discrepancybetweenour modelfor regionB and model
At depths greater than 100 km, regionsB and C display
ATL wouldcorrespond
to a (Vsn-Vsv )/Vsv ratioofabout different lateral shear wave velocity variations. In region B,
+5% abovethe LVZ and of-5% below,valueswhich are
consistentwith the result of Nard! et al. [1986,Figure 30] in
the North Atlantic. Nevertheless, the comparison between
two isotropicmodelsonly providesan upper bound on polar-
the best visualcorrelationbetweenlithosphericage and Vs
is observedat 139 km and disappearsdeeper. In region C,
such a correlation is not observed but, in contrast, the ve-
locity anomaliesstrike along an east-westdirection. In paization anisotropy[Reganand Anderson,1984; Montagner per 1, we presentedthe azimuthal path coverageof our data
and Anderson,1989]. Hencethe inclusionof Love wavesin set and showed that the east-west direction is not overs amthe inversionis neededto test theseproposedhigh valuesof pled in region C. Such large east-westtrending anomalies
polarization anisotropy. This will increase the difficulty of are in good agreementwith the observationsof Honda and
the inversion because, as discussedin paper 1, continental Tanimoto[1987]in this depth range.
Below 200 km depth, the most striking feature is a strong
regionsare unavoidablein such a study. Continental marginsare characterizedby strongtransversevelocitygradients high-velocity anomaly beneath the central part of the Atwhich induce surfacewave scatteringor, equivalently,mode lantic, which mainly contributesto the high mean veloccoupling[e.g., $nieder, 1987]. Thereforethe joint inversion ity of regionB (Figure 3). In the Pacific [Montagnerand
of Love and Rayleigh waves will require the use of newly Joberr,1981; Wielandtand Knopoff,1982]andIndian oceans
developedformalisms [$nieder and Romanowicz,1988; Ro- [Montagner and Joberr, 1988], slow shear wave velocities
rnanowiczand $nieder, 1988] which describe,for instance, associatedwith mid-oceanic ridges can be followed down
to depths greater than 350 km. These deep slow velochigher-modecontaminationby short-periodLove waves.
4. LATERAL
VARIATIONS
OF SHEAR
VELOCITIES
The regionalizedgroup and phase velocitiesobtained in
paper I were inverted at depth in each 5ø by 5ø cell, using the method describedin section2. The resultinglateral
variations
of shear wave velocities
in the Atlantic
area are
presentedat selecteddepths in Figure 4. Montagner and
Joberr[1988] showedthat one of the advantagesof Montagher's[1986]method is that the a posterioritrade-offbetween
parameters vanishesif a suitable correlation length is used
in the regionalization. For instance,we found no trade-off
between the North and South Atlantic, using a correlation
length of 1500 km (paper 1). The use of this correlation
length is equivalentto a spatialfiltering and determinesthe
smoothnessof the final model. On the other hand, assuming
smooth lateral velocity variations, together with the firstorder optical approximation,doesnot permit to modelsharp
lateral velocity gradients. These are the reasonswhy, at a
depth of 88 km (Figure 4), the Mid-Atlantic Ridge only displays two local slow velocity anomalies,in regionsB and C.
The absenceof slow velocities in other regions along the
ridge (in regionA, for instance)doesnot mean that they
do not exist but only that our data set and the method of
regionalizationusedare not adequateto model them.
ity anomaliesare usually interpreted in terms of large-scale
upwelling asthenosphericflows beneath mid-oceanicridges.
In contrast, in our model, the Mid-Atlantic Ridge displays
slow velocities only in the lithospheric depth range. Below the LVZ, the distribution of negative anomaliescannot
straightforwardlybe interpreted in terms of asthenospheric
upwelling flows beneath the Mid-Atlantic Ridge. If such
flows exist, their lateral extent is too small to be detected
by our data set and our method of investigation, so that
their
influence
on the distribution
of shear wave velocities
is maskedby the global asthenosphericstructure. In M84C
[Woodho•seand Dziewonski,1984], the Atlantic area is a
major contributorto the high-velocityanomalyof the global
degree two pattern of shear wave velocity distribution, for
depthsgreater than 450 km. Romanowiczet ai. [1987] suggested several depths for the global degree two pattern of
shear wave velocity distribution, one of them concentrated
in the depth range 300-500 km. In our study, the highestvelocity anomaly, in the depth range 200-300 km, might
correspondto the top of this latter heterogeneity.
Test o! the Model
The final Vsv modelwastestedby calculatingthe correspondinggroup and phasevelocitydistributions.The misfit
6792
MOCQUET
ANDP•OMANOWICZ:
STRUCTURE
OFATLANTIC
UPPER
MANTI,E,2
60 ø W
30øE
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242Km
139"K rn.........................
.
'VSv
Fig.4. Lateralvariations
ofshearwavevelocities
at selected
depths.
Percentages
areanomalous
velocities
withrespect
to M1066A[GilbertandDziewonski,
1975].
between
synthetized
andobserved
velocities
should
attempt wavevelocityanomalyat $$ km depth beneaththe Brazilto matchthe observational
error. In Figures5 and 6, the ian shield. As pointedout in paper 1, the lack of data consynthetized
velocitydistributions
arecompared
with the re- cerning the thicknessof the crust made accurate surficial
gionalized
mapsobtainedin paper1. Figures7 and8 show layercorrections
impossible
in this region,anda thickconthe relation between data residuals and observational er- tinentalcrustmightaccount
for thislow-velocity
anomaly.
rorsfor phaseand groupvelocities,respectively.In what Therefore,evenin the ideal caseof a perfectfit between
follows,we will only discussthe resultsbeneath oceanicar- synthetized
and observed
groupand phasevelocities,
this
eas, because of the lack of resolution in continental areas. anomalyhas no meaningconcerning
the structureof the
For instance,our modeldisplaysa strongnegativeshear- upper mantle.
73S
102S
lllllllllll!11111111111111111
!I •
/
•
Ji i i • iIIIII1111111-•
i • • • • • -.'
i :: • • •1111•
171S
•
•
-
-
::111111111111111!
! :•
SYNTHETIZED
REGIoNALIZED
I [ J J•l I ! :: •-•.11111111111HIIl[:-":'...'.:.::•
Fig. 5. Comparison
between
synthetized
andobserved
phase
velocities
at threeselected
periods.
Anomalous
velocities
arein percentwith respectto M1066A[GilbertandDziewonski,
1975].
70S
100S
175S
SYNTHETIZED
REGIoNALIZED
Fig. 6. Same as Figure 5 for group velocities.
73S
i
171S
_
lli !•"g
_
....
crD
Fig. 7. Comparison
between
a posterJori
phase
velodtyresiduals
andobservational
erroreYD.Thescale
for eY
D is in percent
withrespect
to M10OOA
[G//Mr/auclDzieumuski,
1975].
70S
lOOS
175s
13
RES
Fig. 8. Same as Figure 7 for group velocities.
MOCQUET AND ROMANOWICZ: STRUCTURE OF ATLANTIC UPPER MANTLE,2
Our
model
tends
to overestimate
the
relation
between
group and phasevelocitiesand lithosphericage (Figures 5
and 6). In the North Atlantic, the increaseof short-period
(T=73 s) phasevelocitiestoward old oceanbasinsis synthetizedwith an accuracyof 2% (Figure5). The synthetized
groupvelocitiesalsodisplaysucha relation with lithospheric
age, though it was not observedin the regionalized maps
(Figure 6). This remark alsoholdsat intermediateperiod
(T=100 s). In paper 1, we observedin the North Atlantic
a velocity-age correlation for phase velocities but not for
group velocities. In the South Atlantic, this correlationwas
observedfor group velocitiesbut not for phasevelocities. In
this latter region, the lack of denselydistributed refraction
6797
errors and data residuals, together with the associatedun-
certainty on the location at depth of the LVZ (Figure 2),
indicate
that
the structure
of the LVZ
must be more com-
plicated. Furthermore, all these studies,including ours, deal
with isotropic models, and it has been recognizedthat the
inclusion of anisotropy in the inversion at depth of surface
wave data substantially reduces the importance of the LVZ
[e.g., Dziewonski and Anderson, 1981; Regan and Anderson, 1984].
5. CONCLUSION
Considering the previously discussedlimitations of our
model, the following conclusionscan be drawn: (1) the
profiles had led us to fix crustal parameters to standard
oceanic values. Therefore the correlation between group
velocities and lithospheric age might still be explained by
surficial layer effects, such as lateral variations in crustal
slower velocities beneath the Mid-Atlantic Ridge than beneath old ocean basins. The largest velocity contrast is 6%
velocity data appear to be too highly sensitive,in a nonlin-
deep slow-velocity anomalies below the location of the mid-
North Atlantic lithosphere displays classical structures:
with an uncertaintyof 2%. (2) In contrastwith otheroceans,
thickness.More generally,short-period(T < 100 s) group the Atlantic asthenosphericstructure is not characterizedby
ear way (paper 1), to surficiallayer effectsto provideuseful oceanicridge, but the central regionof the Atlantic displays
information concerning the structure of the upper mantle. a high-velocity(+6% 4- 3%) anomaly in the depth range
In contrast, the better fit between synthetized and observed
phase velocities in the North Atlantic permits interpretation of the velocity-age correlation in this region in terms
of lithosphericcooling and plate thickeningwith age [e.g.,
Nishim•tra and Forsyth,1988]. The amplitude of the velocity anomaliesand the associatedtransversegradients will
have to be better constrained by incorporating amplitude
data
in the inversion.
The broadening of the resolution curves below 200 km
depth (Figure 2) implies a greater uncertaintyon the location at depth of the V$ v anomalieswith increasingdepth.
This uncertainty results in a shift in period between synthetized and observed phase velocity distributions for peri-
200-300 kin. Part of this anomaly might be explained by a
negative polarization anisotropy implicitely included in the
isotropic model. We also tentatively interpret this anomaly
as a contributor to a shallow global degree two pattern of
shear wave velocity distribution for the Atlantic area.
Acknowledgements. This work was conductedunder grant
GEOSCOPE
ASP 1987, sponsored by INSU of CNRS. We are
thankful to J.P. Montagner, who provided us with his programs.
His help at the early stage of this work was greatly appreciated.
Constructive reviews from the editors, M. Cara, and an anonymous reviewer have been helpful to improve this paper. This is
IPG contribution nø 1081.
odsgreater than 100 s (Figure 5). This is particularly clear
for the Central Atlantic high-velocityanomaly,whoselargest
amplitude(6%) was observedover a large area at a period
of 102 s, and which is better synthetized at a period of 171 s.
Therefore,as can alsobe seenin Figure 2 (h > 190 km),
our model only providesan averagedestimate of the
anomaliesfor depths greater than 200 kin.
As statedpreviously,the misfit a! betweensynthetized
REFERENCES
Cara, M., Lateral variations of S velocity in the upper mantle
kom higher Rayleigh modes, Geophys. J. R. Astron. Soc., 57,
649-670,
1979.
Cara, M., J.J. L•v•que, and V. Maupin, Density-versus-depth
models from multimode surface waves, Geophys. Res. Left.,
11, 633-636, 1984.
and observedgroup and phasevelocity distributions should
attempt to match the observational error a,t. This is Dziewonski, A.M., and D.L. Anderson, Preliminary reference
Earth model, Phys. Earth Planet. Inter., •5, 297-356, 1981.
achieved for all periods in the western part of the North
Forsyth,
D.W., The evolution of the upper mantle beneath midAtlantic, becausethis region displayedthe best path cover-
age (seeFigure 1 in paper 1). The misfit increasebeneath
young regionsof the North Atlantic reflectsour difficulty to
model velocity versuslithospheric age relations. The small
erroron phasevelocitydistributions
(ad < 1%, Figure7)
appears to be underestimated. An error of 2% is more realistic in view of the group velocity distributions, for which
a! canbe smallerthanad (Figure8).
The largestdifferencebetweena! and ad is obtainedfor
intermediateperiodgroupand phasevelocities(T - 100 s)
which are mostly sensitiveto the V$ structure of the LVZ
ocean ridges, Tectonophysics,38 (1-2), 89-118, 1977.
Gilbert, F., and A.M. Dziewonski, An application of normal mode
theory to the retrievM of structural parameters and source
mechanisms from seismic spectra, Philos. Trans. R. Soc. London Set. A, 278, 187-269, 1975.
Grand, S.P., Tomographic inversion for shear velocity beneath the
North American plate, J. Geophys. Res., 92, 14,065-14,090,
1987.
Grand, S.P., and D.V. Helmberger, Upper mantle shear structure
of North America, Geophys. J. R. A stroh. Soc., 76, 399-438,
1984a.
(see Figure 13 in paper 1). In this depth range (Figure 4), shearwave velocitiesare homogeneously
distributed Grand, S.P., and D.V. Helmberger, Upper mantle shear structure
beneath the northwest Atlantic Ocean, J. Geophys. Res., 89,
throughout the Atlantic Ocean. Similarly, we noted previ-
ouslythat the Vsv modelsfor regionsA, B, C, and CAR
11,465-11,475,
1984b.
coincidedwithin a pronouncedLVZ (Figure 3). Though Hadiouche, O., and N. Joberr, Geographical distribution of
this result is consistent with previous works in this region
[e.g., Weidner, 1974], the large misfit betweenobservational
surface-wave velocities and 3-D upper mantle structure in
Africa, Oeophys. J., 95, 87-110, 1988.
6798
MOCQUET
ANDROMANOWICZ:
STRUCTURE
OFATLANTICUPPERMANTLE,2
Honda, S., and T. Tanimoto, Regional three-D heterogeneities Romanowicz, B., and R. Snieder, A new formalism for the effect
by the wave form inversion-Application to the Atlantic area,
of lateral heterogeneity on normal modes and surface waves,
Geophys. J. R. Astron. $oc., 91, 737-753, 1987.
2, General anisotropic perturbation, Geophys. J., 93, 91-99,
1988.
Jacoby, W.R., and N. Girardin, The evolution of the lithosphere
at the southeast flank of Reykjanes ridge from surface wave Romanowicz,B., G. Roult, and T. Kohl, The upper mantle dedata, J. Geophys., 47, 271-277, 1980.
Kanamori, H., and D.L. Anderson, Importance of physical dispersionin surfacewave and free oscillationproblems: Review,
Rev. Geophys., 15, 105-112, 1977.
Mocquet, A., B. Romanowicz, and J.P. Montagner, Threedimensional structure of the upper mantle beneath the Atlantic
Oceaninferred from long-periodRayleigh waves,1, Group and
phase velocity distributions, J. Geophys.Res., 94, 7449-7468,
1989.
Montagner,J.P., Regionalthree dimensionalstructuresusinglong
period surface waves, Ann. Geophys., ,l, 283-294, 1986.
Montagner, J.P., and D.L. Anderson,Petrologicalconstraintson
seimic anisotropy, Phys. Earth Planet. Inter., 54, 82-105,
1989.
Montagner, J.P., and N. Joberr, Investigation of upper mantle
structure under young regions of the southeastPacific using
long-period Rayleigh waves, Phys. Earth Planet. Inter., 27,
206-222,
1981.
gree two pattern:
Constraints from GEOSCOPE
fundamen-
tal spheroidal mode eigenfrequencyand attenuation measurements, Geophys. Res. Left., 14, 1219-1222, 1987.
Snieder,R., Surfacewave scatteringtheory: With applicationto
forward and inverseproblemsin seismology,Ph.D. thesis,Univ.
of Utrecht, Netherlands, 1987.
Snieder, R., and B. Romanowicz, A new formalism for the effect
of lateral heterogeneityon normal modesand surfacewaves,1,
Isotropicperturbations,perturbationsof interfacesand gravitational perturbations, Geophys. J., 92, 207-222, 1988.
Tanimoto, T., The backus-Gilbert approach to the three-D structure in the upper mantle, II, SH and SV velocity, Geophys. J.
R. Astron. Soc., 84, 49-69, 1986.
Tarantola, A., and B. Valette, Generalized non linear inverse
problems solved using the least squares criterion, Rev. Geophys., 20, 219-232, 1982.
Weidner, D.J., Rayleigh wave phase velocities in the Atlantic
Ocean, Geophys. J. R. Astron. Soc., 36, 105-119, 1974.
Montagner, J.P., and N. Joberr, Vectorial tomography,II, Appli- Wielandt, E., and L. Knopoff, Dispersionof very-longperiod
P•yleigh waves along the East Pacific Rise: Evidence for S
cation to the Indian Ocean, Geophys. J., 9,1,309-344, 1988.
wave velocity anomaliesto 450 km depth, J. Geophys. Res.,
Montagner, J.P., and H.C. Nataf, Vectorial tomography,I, The87, 8631-8641, 1982.
ory, Geophys. J., 94, 295-308, 1988.
Nataf, H.C., I. Nakanishi, and D.L. Anderson, Measurementsof Wiggins, R.A.,The general linear inverse problem: Implication
mantle wavevelocitiesand inversionfor lateral heterogeneities, of surfacewavesand free oscillationsfor Earth structure, Rev.
Geophys., 10, 251-285, 1972.
3, Inversion, J. Geophys.Res., 91, 7261-7307, 1986.
Woodhouse,
J.H., and A.M. Dziewonski,Mapping the upper manNishimura, C.E., and D.W. Forsyth, Rayleigh wave phaseveloctle: Three-dimensional modeling of Earth structure by inverities in the Pacific with implicationsfor azimuthal anisotropy
sion of seismicwaveforms,J. Geophys. Res., 89, 5953-5986,
and lateral heterogeneities, Geophys. J., 94, 479-501, 1988.
1984.
Osagie, E.O., Anelasticity of the crust and upper mantle of South
America
from the inversion
of observed surface wave attenua-
tion, Geophys. J. R. Astron. Soc., 86, 1-17, 1986.
Panza, G.F., St. Mueller, and G. Calcagnile,The grossfeatures
of the lithosphere-asthenosphere
system in Europe from seismic surfacewavesand body waves,P•re Appl. Geophys.,118,
1209-1213,
1980.
Regan, J., and D.L. Anderson, Anisotropicmodelsof the upper
mantle, Phys. Earth Planet. Inter., 35, 227-263, 1984.
A. Mocquet, Department of Earth Sciences,NagoyaUniversity,
Chikusa, Nagoya 464, Japan.
B. Romanowicz,Laboratoire de Sismologie,Institut de Physique du Globe, 4 place Jussieu, 75252 Paris, Cedex 05, France.
(Received December 15, 1988;
revised August 22, 1989;
accepted September 6, 1989.)