GEOPHYSICAL
RESEARCH
LETTERS,
VOL.19,NO.16,PAGES
1707-1710,
AUGUST
21,1992
THERMAL BUOYANCY ON VENUS: UNDERTHRUSTING VS SUBDUCTION
JeffreyD. Bun andJamesW. Head
Department
of Geological
Sciences,
BrownUniversity
Abstract In aneffortto definetheconditions
distinguish- Modelsfor the formationof suchmountainbeltsandassoci-
ingunderthrusting
andsubduction
intheVenus
environment, atedfeatures
mustexplaincompressional
deformation,
crustal
wehavemodeledthethermalandbuoyancy
consequences
of
thickening,
andmeltproduction.
thesubduction
endmember.Predictionsof the thermalevolutionof a slabsubducting
at a fixed angleinto theVenusian
WhileVenuslacksclearevidence
of Earth-like
plate
tectonics,
features
resembling
subduction
zoneshavebeen
mantle
areusedto find slabdensities
basedon slabcompositionandchanges
of temperature,
pressure,
andphase.The
sustainability
of subduction
is assessed
by considering
the
effects
of slabbuoyancyandmantleflow on the subduction
identified.
Thetrough
belowUorsar
Rupesanditsassociated
risetothenorthresemble
theflexural
trough
andoutboard
rise
of terrestrial
subduct/on
zones[SolomonandHead,1990].
Sandwell
andSchubert
[1992]identifysimilartrough
andrise
angle.
Mantleflowinduced
by slabmotionapplies
torques
on
theslab,whichare comparedto buoyancytorques.Flow
torques
tendtodecrease
theangleof subduction.
Buoyancy
torques
alsoactto decrease
thesubduction
anglewhentheslab
ispositively
buoyant.
Thebasalt-eclogite
phasetransition
dominates
thetransformation
of positivelybuoyantslabsto
negative
buoyancy.
Slabsthathavedescended
to depths
less
thanabout275 km remainpositivelybuoyant.Beyond275 km
slabs
become
negatively
buoyant.
We predictthatthecombinationof flow andbuoyancytorqueswill tendto forceinitially
suMucted
slabsto assumean underthrusting
position,leading
tocrustal
thickening,deformation,melting,andvolcanism.
Thismayprovidea modelexplainingtheassociation
of compressional
mountainbeltsandtessera
blockswith apparent
flexural
rises,foredeeps,
andvoluminous
volcanicdeposits.
Onlyspecial
circumstances
appearcapableof promotingthe
conditions
for negativeslabbuoyancy.Thesemightinclude
onewhere an underthrust slab becomes attached to or included
in a densiftedcrustal root created in a zone of crustal thicken-
ing.Thesubsequent
delamination
andsinkingof therootmay
thenlead to the subduction of the slab.
features
formingtheouterboundaries
of somecoronae,and
propose
theirformation
thoughthedownwardflexureand
possible
subduct/on
of thesurrounding
lithosphere
underthe
radiallyspreading
centralcoronamass,with theflexuralload
supplied
byeithertheoverriding
plateor thesubducted
slab.
Furtherexamples
includearcuate
trenches
observed
in eastern
Aphrodite
TerraonVenusandinterpreted
by McKenzieet al.
[1992]tobesubduction
zonesonthebasisof theirsimilarity
to subduction zones on Earth.
Whiletheroleof platetectonic
processes
appears
minorin
theoverallevolution
of theVenusian
lithosphere,
underthrustingandsubduction
potentially
contribute
to thecreationof
manyobservedfeatures.To theextentthat the surfacetectonic
consequences
of thesetwo processes
woulddiffer,with un-
derthrusting
leading
primarily
tocrustal
thickening
andpossiblemeltproduction,
whiletheresultsof subduction
maynot
resemble
thosefoundonEarth,thefactorsgoverning
thedominanceof oneprocess
overtheotherarethereforeimportant.
In orderto evaluatetheconditions
distinguishing
underthrustingandsubducfion,
we havemodelledthe thermaland
buoyancyconsequences
of thesubductionendmember.This
studyconsiders
thefateof a slabfrom thetime it startsto
subduct,
butbypassing
thequestion
of subduction
initiation.
Introduction
Enhanced
surface
temperatures
anda thinnerlithosphere
onVenus
relativetoEarthhavebeencitedascontributing
to
increased
lithospheric
buoyancy.
Thiswouldlimit[Phillips
andMalin,1982],orprevent[Anderson,
1981]subduction
on
Venus,
andfavortheconsa
uction
ofthickened
crustthrough
underthrusting.
Underthrusting
maycontribute
totheformation
ofanumber
offeatures
onVenus.
Forexample,
Freyja
Montes,
aneast-west
trending
linearmountain
bekin northern
ishtar
Terra,
liessouth
ofahighplateau
ofcomplexly
deform-
Thermalchanges
in slabssubducting
intoa mantlehavinga
rangeof initialgeotherms
areusedtopredictdensitychanges
and,thus,slabbuoyancy.Thisthenformspartof anargument,utilizinga modelfor subduct/on-induced
manfieflow,
wherebytheangleof slabdiphelpsdifferentiatebetween
underthrusting
andsubduction.
Mantleflow appliestorquesto
theslabwhich,in combination
with torquesdueto slab
buoyancy,
actto changetheangleof slabdip.
edtern/n,Itzpapalotl
Tessera,
thatcontains
evidence
of exten-
sive
volcanic
activity
[BunandHead,1990].Itzpapalofi
is
bounded
tothenorth
byUorsar
Rupes,
a steep
scarp
marking
adement
of2 to3 kmelevation
toavolcanic
plains-filled
foredeep.
Further
north
areanoutboard
riseandthenorth
polar
plains
ofVenus.
Freyja
Montes
isinterpreted
asanorogenic
belt
[(haanpier
etal.,1986]andtheregion
a zoneofconvergence
andunderthrusting
of thenorthpolarplainsbeneath
ishtar
Terra,
withconsequent
crustal
thickening
[Head,
1990].
Copyright
1992
bytheAmerican
Geophysical
Union.
Subduction Model
In themodel(Figure1), slabshavinga thickness
setby
90%of thebasaltsolidussubduct
at a fixedangleintothe
manfie.Geotherms
proposed
for Venusrangefrom 5øC/lma
[SandwellandSchubert,1992]to 25øC/!an[Solomonand
Head,1990].Theinitialmodelgeotherms,
matchingsurface
thermalgradients
of 10øC/km,15øC/•n, and25øC/lma
[Hess
andHead, 1989],werecalculatedusingtheinstantaneous
coolingof a semi-infinite
halfspace,andcovermostof the
proposed
range.Themodelpenttitscoolingof themantlewith
time,allowingtheevolutionof initial highgeotherms
toward
Paper
number
92GL01702
thelowerendof therange.A basalticmantleandslabis
0094-8534/92/92GL_01702503.00
assumed
for thepurpose
of thethermalevolutioncalculations
1707
1708
BurtandHead:ThermalBuoyancy
onVenus
depth
0cm)
¾crust
temp
Parmentier,1977; Parmentierand Hess, 1992]. We do not
(Oc)
meanto implythatcrustformsin a ridge-typespreading
center.Densitychangesincludethosedueto phasetransitions.
The sampleshownin Figure3 is for a 25 km crustanda
65O
ßY- -
•lithosphereJ/A
depleted
mantle
t
transmon
1250
10øC/kminitial surfacegeotherm.The lithosphereincludes
layersof depleted
andundepleted
mantlematerial.
undepleted
mantieI
Results
300•,
I enstatite/forsterite
plus
1450
400
transition
stishovite
_
Fig. 1. Model sumcture
for a 10øC/kmgeothermanda 25 km
thick basaltic crust.
(basaltandperidotitematerialproperties
differinsignificantly
in termsof heat conduction).Slabsheatvia conduction,radio-
activity,phasechanges,andadiabaticcompression.
Phase
changes
involvingtheconversion
of basaltto eclogiteat depths
of 60 to 160km andthenenstatiteto forsterire
plusstishovite
between
260and360km generate
0.13x10
'sergs/cm3-s
and
0.36x10'sergs/cmLs
respectively
[MinearandToksoz,1970].
Theassumed
slabradiogenic
heatproduction
is2.63x10"
ergs/gm-s
[TurcotteandSchubert,1982]whileradioactivity
in
themantleis considered
negligibleandignored.Adiabatic
compression
adds0.5øC/km.A dynamicmantlecould
significantly
affectslabtemperatures.
However,sincesubductingterrestrialslabsaregenerallyin compression,
mantle
motionpresumably
doesnotkeeppacewiththatof theslab.If
a similarsituationoccurson Venus,thethermalevolutionmay
morecloselyresemblethecasefor an immobilemantlethanfor
one in which the mantle moves with the slab. Therefore, for
thepurpose
of thisinitialtreatment,
we neglecttheeffectsof a
dynamicmantlein thethermalevolutioncalculations.
Results
of thethermalevolutiontaketheformof tempera-
taredistributions
in theslabandmantle.The principaleffect
produced
is slab-induced
mantlecooling.Figure2 shows
typicalresultsof thedensitymodelingcomputations,
corresponding
to thecasepresented
in Figure1 for a 10øC/km
geotherm,
25 km crust,and5 km/Masubduction
rate.Density
contoursdelineatethe slaband its crustallayer. The crustis
distinctlylowerin densitythanits surroundings
abovetheI I0
km depthassumed
for thebasalt-eclogite
phasechange.
Below
thephasechangethecrustalmaterialis of greaterdensity
than
themantle.The depletedmantleportionof the slabis less
clearlydelineated
andis boundedby thethinundepleted
mantle
layerincludedin thelithosphere.
The undepleted
mantleportionof the slabis denserthanthe surrounding
mantlebecause
of its lower temperature.
The net slabdensitiesin theregionabovethe basalt-
eclogitephasetransition
arelowerthantheirmantlesurroundingsfor everygeotherm,
subduction
rate,andcrustalthickness.Abovethe 110 km depth,crustaldensitiesarelowerthan
thoseof themantleoutsidethe slab.The mantleportionsofthe
slabdiffer slightlyin densitywith mantleoutsidetheslab.
Above110km depththecrustalportionis muchlessdense
thanthemantleslablayers.Belowthatdepththereverse
is
true.Belowthebasalt-eclogite
phasechangenet slabdensities
alwaysexceedthosein theneighboring
mantle.
We follow the slabthermalevolutionusinga finite
differencetechnique[MinearandToksoz,1970].The model
regionmeasures
approximately
800km horizontally
by400
km deep.Convergence
ratesrangefrom5 to 100mm/yrand
processing
endsat thepointwhereslabtipsreacha 300km
depth,implyingtimeintervalsof 10 to 100Ma.
Slabdensitychanges
derivefromthethermalresults
through
calculation
of thethermal
expansion
(av=3x104
/øK
[Stacey,
1977])andtheeffects
of pressure
(b=lx10'•/kb
[TurcotteandSchubert,1982]) on an initialdensitydistributionsetfor zeropressureandtemperature.
While thecoefficientof thermalexpansionchangeswith temperature
and
pressure
( thevalueusedappliesto shallowdepths,butav
decreases
to 2x10's/ øKby thebaseof themodelregion
[Stacey,1977]),theerrorintroduced
by theuseof a constam
valueis on the orderof 1%. The assumed
initial density
structure[OxburghandParmentier,1977]includesa 10 or 25
km basaltic
crust(p=3.0gm/cma);
a corresponding
25 or65
km thickdepleted
mantlezone(p=3.295gnUcm');
andan
underlying
undepleted
mantle(p=3.36grrffcm•).
Thecrustal
thicknesses
wereselected
to illustratetheeffectsof changing
the crustalthickness,andrepresenta rangeproposedfor
Venus[Zuber, 1987; Grimm andSolomon,1988], although
the exactaveragethickness
is unknown.The initialdensity
structure
is basedupontheassumption
thatbasalticcrustis
derivedby thepartialmeltingof themantle.Thiscreates
a lowdensitydepletedmantlelayerbelowthecrust[Oxburghand
Discussion
Qualitatively,subduction
is likely to be enhanced
bynegativeandhinderedbypositiveslabbuoyancy.
For all geotherms
positivenetbuoyancy
persists
abovethebasalt-eclogite
phase
change.
Full lengthslabswerenegatively
buoyantin allcases.
We used a model for slab motion-induced mantle flow to
evaluate
theeffectsof buoyancy
onsubduction
[Turcotte
and
Schubert,
1982].Theflowapplies
a torqueto thebodyofthe
slab(Figure3). Mantledynamics
couldcontribute
to the
depth
Ocm)
density
(gm/cm')
x4•$•:•il
3.5
100
--':.,'&;•
3.4
3.3
•:i•, '••"'••••;•-
"•••--•--••••-••••••.
,z•',•--••
•
400
0
3.1
2.7
200
400
600
800
Fig.2.Finaldensity
distribution
for10øC• initialgeotherm,
25kmbasaltic
crust,and5 km/Masubuction
rate.Dia?
shows
contours
of density
in grn/cm
3,0.1gm/cm
• spacing.
ButtandHead:
Thermal
Buoyancy
onVenus
1709
I
Overriding
plate
$,x• •
•odu-•
IOip •,/_,.<,•" [,• ...........
1.8e+21
U
1.5e+21
mantle
)
• y .,,;•
•
o
'
•
.•
• •
•
•
••
•
•
Induced
mantler
flow
1.2e+2I
q
u 9.0e+20
6.0e+20
3.0e+20
body force
O.
Fig.3.Diagramof mantleflowinduce,
clby slabmodon(from
I
5.
l.O.
1.5. 20.
T•rcotteand Schubert,1982).
25.
:
30.
I
35.
I
40.
I
45.
!
5;0.
!
55.
- I
60.
•
65.
'70
subclucfion
angle
torques
bearingontheslab,however,thenatureof mantle
dynamics
onVenusis unknown.
Mantleflow couldbepartof
Fig.4. Torques
imposed
ontheslabbyinduced
mantleflow
fora rangeof subducfion
angles.
Torques
arein unitsof Nm.
alargeconvective
systemindependent
of slabmotion.Because
oftheuncertainty
involvedin modellingmantleflow, we assmethesimplestcaseof flow dueto slabmotion.Induced
flowdepends
on assuming
a rigidslab.Slabheatingwill diminish
thatrigidity,anddecrease
thecalculated
flowtorques.
Buoyancy
bodyforcesalsoresolveintoa torqueonthe
slab,
andbothtorquesare takento acton the slababoutits
junction
withthesurface.
Table1 listsbuoyancy
torques
for
longslabbuoyancy
torques
arenegative
andlarge,exceeding
flowtorques
at angles
over10degrees.
If slabssomehow
subductbeyond
theneutral
buoyancy
depth,thenegative
net
foursubducfion
rates,both crustalthicknesses,and shallow
andfulllengthslabs.Figure4 plotsflow torquesversussubducfion
anglefor a 100km/Masubducfion
rate.Flowtorques
arepositive
andsmall(relativeto buoyancy
torques)
for subducfion
angles
overabout15degrees,
becoming
largefor
small
angles.
Positivetorquestendto decrease
thesubducfion
angle.
Fortheshortslabsdip anglestendto decrease
dueto
both
thebuoyancy
andflow torques.
No matterwhatinitial
angle
a slabwouldtakeinto a mantleundertheseconditions,
buoyancy
couldhelpdrivesubducfion.
Buoyancy
torques
for
allbuttheshallowest
dipangles
arenegative
andgreater
than
themantle
flowtorques,
resulting
inincreased
slabdip.
Forcrustal
thicknesses
outside
of themodelled
range,
somebuoyancy
effectsmaydiffer.For crustthinnerthan10
km,thepositivebuoyancy
resulting
fromthelow crustaland
depletedmantledensitieswouldbe decreased.The increaseof
density
duetotheconversion
of basalt
toeclogite
wouldalso
besmaller.
Thus,slabshavingthincrustallayerswouldfollow thesamebuoyancytrendasthosemodelledhere,butwith
a smallernetbuoyancy
difference.
If thecrustwereabout100
kmthickthelithospheric
slabmaybecomprised
entirely
of
crustanddepleted
mantle,or evenentirelycrust,depending
on
thegeotherm.
Thenetpositive
or negative
buoyancy
of such
slabs,
aboveorbelowthebasalt-eclogite
transition,
respectively,wouldbemoreextreme
thanforthinnercrustal
layers.
thebuoyancy
andmantleflowwouldcombine
toprevent
subducfion
andunderthrusting
wouldresult.Further,sincethe
These resultsindicate that for all casesconsideredhere of
crustal
portion
of theslabwouldbepositively
buoyant,
though
themantle
partof theslabisnegatively
buoyant,
perhaps
deassumed
Venusgeotherm
a lithospheric
slabwhosesubduction has been initiated will be forced to underthrustthe overridlamination
couldbeexpected.
In thiscasecrustal
slices
may
underthrust
and imbricatewhile the rest of the slabcould sink.
Forallcases
of subducfion
rateandgeotherm
the400km
ing lithosphere.
Also,sincethecrustalmaterialin a shallow
slabis positively
buoyant,
whiletherestof theslabmaybe
TABLE 1' Torquesdueto slabbuoyancy.
10kmcrust
'
25 kmCrUst
buoyancytorque(Nm)
buoyancytorque(Nm)
25
25
25
25
subd. rate
(km/Ma)
100
50
25
5
400 km slab
-1.0x10 21
-7.1x102ø
-5.6x10 2ø
-3.6x102ø
110 lcmslab
1.5x102ø
1.9x102ø
2.1x10 2ø
2.5x102ø
400 !cmslab
-2.5x10 21
-2.2x102z
-2.0x10 2z
-1.8x102•
1!0 km slab
2.8x10 2ø
3.2x102ø
3.4x10 2ø
3.8x10 •9
15
15
15
15
100
50
25
5
-2.6x10 21
-2.0xlO 21
-1.7xlO 21
-9.8x10 2ø
-1.5xlO 19
3.9x10 •9
7.1xlO •9
1.6xlO2ø
-3.8x10 2•
-3.1xlO 21
-2.8x10 21
-2.1xlO 21
4.9x10 •9
1.OxlO2ø
1.3xlO2ø
2.2x10 2ø
lO
lO
lO
lO
lOO
50
25
5
-3.8x10 2•
-3.1x10 21
-2.8x10 21
-2.4x10 21
1.2x102ø
1.4x102ø
1.5x102ø
1.6x102ø
-3.7x10 2•
-3.1x10 21
-2.5x10 2•
-2.3x10 21
9.1x10 •9
1.1x102ø
2.8x10 2ø
1.3x10TM
geotherm
_(øC/krn)
1710
BurtandHead:ThermalBuoyancy
onVenus
References
STEEP
THERMAL
GRADIENT
Anderson,
D.L., Platetectonics
on Venus,Geoph¾s.
Re•.
Gravitationa
L•tt.. 8, 309-311, 1981.
Burt, J.D., andJ.W. Head, Venus:Tectonicandvolcanic
Collapse
consequences
of subduction
andunderthrusting,
•
andPlanetaryScienceXXI, 149-150, 1990.
COLD
Conduction
WARMER
Crumpier,
L.S., J.W.Head,andD.B. Campbell,Orogenic
belts on Venus, Geology. 14. 1031- !034, 1986.
SHALLOW
•
Grimm, R.E., and S.C. Solomon,Viscousrelaxationof
impactcraterreliefonVenus:Constraints
oncrustal
THERMAL
1• Delamination(?) thicknessandthermalgradient,J. Ge0phy$.Res.,93,
GRADIENT
[
Fig.5. Possible
resultsof crustalthickening
including
melting
of thecrustal
rootand,alternatively,
delamination
afterroot
hasdeepened
through
thebasalt-eclogite
phasetransition
(from
VorderBrueggeandHead, 1991).
negativelybuoyantor neutral,somesortof crustaldetachment
may bepossible.Thesebothcouldthenleadto crustalthicken-
ing,meltingandvolcanism,
andpossibly
provideonemodel
to explaintheassociation
of compressional
mountainbeltsand
blocksof highstanding
tessera,
with apparent
flexuralrises,
foredeeps,
andlargevolumesof volcanicdeposits.
Furtherwork is requiredto constraintherateat whicha
slabmayriseto assume
anunderthrusting
positionafter
initiallysubducting.
Nevertheless,
we expectthatwheresubduction
isinitiatedit will soonevolveintounderthrusting.
The
flexuralsignature
shownby thelithosphere
atFreyjaMontes
[SolomonandHead, 1990] and aroundcoronaesuchas
ArtemisandLatona[Sandwelland Schubert,1992] should
thusbedueto theloadingprovidedby theoverriding
lithosphereandnotto a negativelybuoyantslab.
Exceptin thecaseof anunderthrusting/subduction
eventof
shortduration,perhapsto be expectedarounda corona,the
tendencyto convertsubduction
into underthrusting
shouldlead
to the creationof zonesof thickened,deformedcrust.As
thickening
continues,
andthecrustalrootis isostatically
forced
to deeperlevelsin theuppermanfie,heatingof crustalmaterial
couldtriggertheformationof magmasderivedby crustal
remelting(Figure5). In the anhydrousmantleof Venussuch
meltsmightbe SiO2-poortrachytesandphonolites
or ferrobasaks[Hessand Head, 1990]. These could extrudeasflood
11,911-11,929, 1988.
Head, J.W., Formationof mountainbeltson Venus:Evidence
for large-scale
convergence,
underthrusting,
andcrustal
imbrication
in FreyjaMontes,IshtarTerra,Geology,1•
99-102, 1990.
Hess,P.C. andHead,J.W., (1990),Derivationof primary
magnaas
andmeltingof crustalmaterialson Venus:some
preliminarypetrogenetic
observations,
Earth.Moon,and•
Planets.50/51. p. 57-80.
Hess,P.C., andJ.W. Head,Derivationof primarymagmas
andmeltingof crustalmaterialson Venus:some
preliminaryconsiderations,
Abstractsof the28th
InternationalGeological Congress.2-55, 1989.
McKenzie,D., P.G. Fred,C, Johnson,
B. Parsons,
G.H.
Pettengi!l,D. Sandwell,S. Saunders,andS.C. Solomon,
Features
on Venusgenerated
by plateboundaryprocesses,
J. Geoohvs.Res. in press,1992.
Minear,I.W.• andM.N. Toksoz,
Thermal
regimeof a
downgoingslabandthenew globaltectonics,J.
Geoph¾•.
Res.. 75. no.8, 1397-1419, 1970.
Oxburgh,E.R., andE.M. Parmentier,Compositional
and
densitystratification
in oceaniclithosphere
- causes
and
consequences,
J. •eol. Soc.Lond.. 133, 343-355, 1977.
Parmentier,
E.M., ancfP.C.Hess,Chemicaldifferentiation
of
a convecting
planetary
interior:Consequences
for one-plate
planetssuchasVenus,LunarandPlanetaryScience
XXIII.
1037-1038, 1992.
Phillips,R.J.,andM.C. Malin, The interiorof Venusand
tectonicimplications,
in ..Venus.
Hunten,D.M., et al.eds.,
U. AZ Press, Tucson, 159-214, 1983.
Sandwe!l,D.T., andG. Schubert,Is the Venusianlithosphere
subducting?,
LungrandPlanetary_
Science
XXIII. 1209,
1992.
Solomon,
S.C., andJ.W.Head,Lithospheric
flexurebeneath
theFreyjaMontesforedeep,Venus:constraints
on
lithospheric
thermalgradientandheatflow,•
Res. L•tm, 17, no.9, 1393-1396, 1990.
volcanics.The volcanicsobservedatopItzpapalofiTessera
maybe oneexampleof suchmaterials.
If meltingdoesnot occur,becauseof a low geotherm,the
rootmaydeepento thelevelof thebasalteclogitephasetransition (Figure5). This densification
of thedeepest
portionof the
thickenedcrustmay leadto its detachmentanddescentintothe
mantle.Sucha delamination
wouldclearlycreatestrongtectoniceffectsastheremainingcrustreequilibrates
isostatically.
If an underthrust
lithosphericslabsuturesto thecrustalrootit
couldbe carriedalongwhenthatrootdensitiesanddetaches.
This scenariomay thenleadto self-sustaining
subduction
as
slab-pullprovidesthedrivingforce.
Stacey,F.D., A thermalmodelof theEarth,Phys,Earth
p!.anet.Interiors,15, 341-348, 1977.
Turcotte,D.L., andG. Schubert,
Geodynamics:
.Applications
uAckn.0wledgrnents.
The authorsgratefullyacknowledge
thefinancialsupportof theNASA undergrantNAGW-1873
to JWH. The paperbenefitedfrom discussions
with Marc
Parmentierandthe commentsof two anonymous
reviewers.
02912.
of Continuum
Physics
,q)Geological
Problems.
J.Wiley&
Sons,New York, 450 pp., 1982.
VorderBruegge,R.W., andJ.W. Head, FortunaTessera,
Venus:Evidence
of horizontal
convergence
andcrustal
thickening,
Geophys.
Res.Lett.. 16. no.7, 699-702,
1989.
Zuber,M.T., Constraints
onthelithospheric
structure
of
Venus from mechanicalmodels and tectonic surfacefea-
tures,Pr0½,LunarPlanet.Sci.Conf.17th,Part2., 1987.
J.D.BurrandJ.W.Head,Department
of Geological
Sciences,
Box 1846,BrownUniversity,
Providence,
RI
(ReceivedMay 12, 1992;
accepted
June25, 1992.)