JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 99, NO. El2, PAGES 26,015-26,028,DECEMBER 25, 1994
Structural history of Maxwell Montes, Venus:
Implications for Venusian mountain belt formation
Myra Keep and Vicki L. Hansen
Department
of Geological
Sciences,
Southern
MethodistUniversity,Dallas,Texas
Abstract. Modelsfor Venusianmountainbelt formationare importantfor understanding
planetarygeodynamicmechanisms.A rangeof datasetsat variousscalesmustbe consideredin
geodynamic
modelling.Longwavelength
data,suchasgravityandgeoidto topography
ratios,
needconstraints
from smaller-scale
observations
of thesurface.Pre-Magellanimagesof the
Venusiansurfacewere not of highenoughresolutionto observedetailsof surfacedeformation.
High-resolutionMagellanimagesof Maxwell Montesandthe otherdeformationbeltsallow us
to determinethenatureof surfacedeformation.With theseimageswe canbeginto understand
theconstraintsthatsurfacedeformationplaceson planetarydynamicmodels. Maxwell Montes
andthreeotherdeformationbelts(Akna,Freyja,andDanumontes)surroundthe highland
plateauLakshmiPlanumin Venus'northernhemisphere.Maxwell, thehighestof thesebelts,
stands11 km abovemeanplanetaryradius.We presenta detailedstructural
andkinematicstudy
of Maxwell Montes. Key observations
include(1) dominantstructuralfabricsare broadly
distributedandshowlittle changein spacingrelativeto elevationchangesof severalkilometers;
(2) thespacing,wavelengthandinferredamplitudeof mappedstructures
are small;(3)
interpretedextensional
structures
occuronly in areasof steepslope,with no extensionat the
highesttopographic
levels;and(4) deformationterminates
abruptlyat thebaseof steepslopes.
One implicationof theseobservations
is thattopography
is independent
of thin-skinned,
broadly
distributed,Maxwell deformation.Maxwell is apparentlystable,with no observedextensional
collapse. We proposea "deformation-from-below"
modelfor Maxwell, in which the crust
deformspassivelyover structurallyimbricatedandthickenedlower crust. This modelmay have
implicationsfor the otherdeformation
belts.
Introduction
beneath; lava from a small rille from the crater floods -64,000
km2 of easternMaxwellandwesternFortunaTessera.The
Maxwell Montes is one of four deformationbelts (Akna,
steepnorthern, western and southernslopesof the mountain
Freyja,Danu andMaxwell Montes)thatsurroundthehighland rangevary in slopefrom 2ø over-300 km (northandsouth),to
plateau Lakshmi Planum in Venus' northern hemisphere 30 ø over several tens of kilometers (west) [Smrekar and
(Figure1). LakshmiPlanumrises4 km abovemeanplanetary Solomon,1992]. Deformationterminatesabruptlyat the base
radius(MPR) with the fringingdeformationbeltsat elevations of thesesteep,deformedslopes.The easternslopeof Maxwell
of .-.5 km (Danu), 6 km (Akna), 7 km (Freyja), and 11 km gradesinto westernFortunaTessera.
(Maxwell) above MPR. Outboardof Lakshmi Planum, all the
Maxwell Montes is enigmaticbecauseof its vast elevation.
deformedbelts are adjoinedby areasof intenselywrinkled, Any mechanismfor the supportof Maxwell mustalsoapplyto
complexlydeformedcrustknown as tesserae,which lie at an IshtarTerra as a whole, and thushave significantimpacton
averageelevation of 5 km. Together, Lakshmi Planurn,the planetarygeodynamicmodels. Most workersfavor dynamic
deformation
belts and the tesserae define
a continent-sized
supportfor Ishtar Terra [e.g., Pronin, 1986; Basilevsky,1986;
Kiefer andHager, 1989;Bindschadleret al., 1990; Grimm and
Phillips, 1990, 1991; Bindschadlerand Parmentier, 1990].
Grimm and Phillips [1991] provide a discussionof the
geophysical
meritsof isostaticversusdynamicmodels.
Venusiandeformation
belts. It occupies
an area-850 km long
Current dynamic-supportmodels for Ishtar Terra suggest
by 700 km wide, and the adjacenttessera,westernFortuna thatit is thesiteof eitherlocalmantleupwelling[Pronin,1986;
Tessera,
covers
anarea
greater
than1.5million
km2. MaxwellBasilevsky,1986; Grimm and Phillips, 1990, 1991], or mantle
is characterized
bydominant,
northwest-trending,
radar-brightdownwelling[Bindschadleret al., 1990, 1992; Bindschadler
lineaments,andan impactcrater,Cleopatra(the highestcrater and Parmentier, 1990; Lenardic et al., 1991]. In order to
on Venus), that lies on the eastern slope. Ejecta from evaluatevariationsin crustaldensity,topographyand gravity,
regionreferredto asIshtarTerra.
Maxwell Montes lies at the eastern edge of Lakshmi
Planum. It is the highest and steepestfeature on Venus,
towering4 km aboveFreyja Montes,the next highestof the
Cleopatra
coversmostof centralMaxwell,obscuring
structures both models use scales greater than those observedfor the
individual deformation belts of Ishtar Terra. As a result, none
of these models are capable of predicting the nature of
Copyright1995bytheAmericanGeophysical
Union.
structures, orientations or kinematics
Papernumber94JE02636.
deformedbelts, althoughon a broad scalemantle-upwelling
and -downwelling mechanismsprovide a means of local
crust/mantlethickeningaroundIshtarTerra.
0148-0227/95/94JE-02636505.00
26,015
within
the Ishtar
26,016
KEEP AND HANSEN:
STRUCTURAL
HISTORY
OF MAXWELL
MONTES,
78øN
VENUS
30øE
Freyja
Akna
Lakshmi
Planum
Maxwell
56ON
Danu
300øE
Figure 1. (a) Portionof Magellan C2-MIDRP.60N333;2 showingIshtar Terra, includingAkna, Freyja, Danu, and Maxwell
Montes, and adjacenttesserae. Black areasare missingdata. (b) Topographiccontoursfor area representedin Figure la.
Contoursare in meters. Dashedline is easternsinuousboundarylineament.
The character, wavelength, and spatial distribution of
stamctures
on Maxwell providesconstraints
for the surfaceand
PhysiographicDivisions
deformation
of individual
Ishtar mountain
belts.
Maxwell Montes comprisesfive physiographicprovinces:
Geophysicaldata provide global constraintson lithosphere- the northwest arm, the eastern and western ridges, the area
and mantle-scale structures. The constraints from each of these
associated
with the impactcraterCleopatra,and the southern
in turnbelow.
data sets together provide insights into mechanismsof slope(Figure2). Theseprovincesarediscussed
mountain belt formation.
Current models for the structural
The northwestarm comprisesa triangular-shaped
regionthat
evolution of Maxwell [Vorder Bruegge and Head, 1989; juts westwardfrom the mainbodyof themountainbelt. It is
VorderBrueggeet al., 1990] did not have the benefitof high- separatedfrom therestof Maxwell by a sinuouslineamentthat
resolutionMagellan images,and somestructures
predictedby trends southwest from 69øN, 4øE to 66.5øN, 359øE, and then
these models are not seen in Magellan data. This paper trends south to 65øN, 0øE, where it marks the western
presentsa structuralanalysisof Maxwell Montes. We useour boundaryof Maxwell, extendingto 61.5øN, 2øE (Figure 2).
observationsto proposea mechanismfor the deformationand This lineament is herein referred to as the 0øE lineament. Most
support of Maxwell, in which the upper crust deforms of the northwestarm is at lower elevationthan the main body
passivelyover a structurallyimbricatedand thickenedlower of Maxwell. The highestareaof the arm (-10 km aboveMPR)
crust layer that supportsthe short-wavelength(500 km) is centered around 66.4øN, 357.5øE, and elevations decreasein
mountain-belt-scaletopography[e.g. Phillips and Hansen, all directionsaway from the 0øE lineament,to a low of-5.5
1994]. The favoredmodel explainsthe observeddeformation km above MPR around 67.3øN, 353.5øE. The northern flank
featuresof Maxwell Montes, and it may have implicationsfor of the arm slopes2 ø over -300 km [Smrekar and Solomon,
crustal
other Ishtar deformation
belts.
1992].
KEEP AND HANSEN:
STRUCTURAL HISTORY OF MAXWELL MONTES, VENUS
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.....--......•-.-.
'½:.:•:•
.•:
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-.:--.•
.....
..........
,;,..:..•?:,:.,
:...<•..:•-•:•.•••••,..•
.•••4:•::.,.•,•:..
.::•'•':.:4:.
'•..
,,
20øE
70.5øN
)
NW1&rm
Cleo
Eastern
Ridges
Western
Impact
Rid
area
s
Southern
Slope
60.5øN
350øE
Figure 2. (a) Compositeimageof Maxwell Montes. Black stripesare missingdata. Dark circle at centeris the impactcrater
Cleopatra. The 0øE lineamentis visible as offsetof radar-darkmaterialat left, just abovecenter. (b) Topographiccontoursfor
Figure2a showingthephysiographic
provincesdescribedin the text,locationsof Figures2a, 4a and5a, andthe locationof the0øE
lineament described in the text, shown as dashed line.
The easternand westernridges, crater-modifiedarea, and
the southernslopeare all part of the main body of Maxwell
Montes. Transitionsbetweenthesephysiographicprovinces
are gradational;they are distinguishedon elevation changes
and extentof impactejecta,both of which causevariationsin
radar-brightness.
The northernboundaryof the mainbodyof
Maxwell is the 0øE lineament. Steepelevationchangesmark
the western
and southern
boundaries
between
Maxwell
and
adjacent plains; the eastern boundary, a sinuous trough,
extendingfrom 62.5øN,11ø to 68øN 10øE,marksthebeginning
of an ~200
km
transition
between
Maxwell
and western
Fortuna Tessera.
The easternridges encompassthe radar-dark area east of
Cleopatra,extendingfrom 63ø to 68øN, and9ø to 13øE. They
occupythe topographicallylowest part of Maxwell, ranging
from 6.5 to 5.5 km above MPR over a distance of-200
km,
26,018
KEEPANDHANSEN:STRUCTURAL
HISTORYOFMAXWELLMONTES,
VENUS
haveseveralforms.In IshtarTerra,some
defining
theshallowest
slopeof Maxwell.Thewestern
ridges Singlelineaments
arelong,slightlyanastomotic,
radar-bright
areboundto thenorthby the0øElineament,
andto thewestby singlelineaments
a steepslopebetweenMaxwellMontesandtherelatively alongone side,andhave gradualtransitionsin brightness
undeformedLakshmi Planum. To the eastthe westernridges
acrossthem. These featuresare generallyinterpretedto be
ridges,
withbrightradar-facing
slopes
becoming
gradeintothecrater-modified
area.Theyoccupy
thehighest physiographic
moreradar-dark
wheretheydip awayfromthe
regionof Maxwell(average
elevation
>10km),although
the progressively
if
western
boundary
issteep,witha slopeof 30øoverseveral
tens radar[e.g.,Fordet al., 1989,1993]. Thiseffectis enhanced
to the radarbeam,and
of kilometers. A radar-brightband of Cleopatra ejecta the ridgesare orientedperpendicular
weakerwith smalleranglesbetween
extending
from63ø to 68øN,4ø to 8øEseparates
theeastern becomesprogressively
andradardirection.Ridgesarecommonly
andwesternridges. This band,hereafterreferredto asthe ridgeorientation
asfolds[e.g.,Campbell
et al., 1983;Crumpler
et
impactregion,spansanelevation
rangeof 6 to 9 km above interpreted
MPR and somewhat obscuresfeaturesbeneathit. To the east
andwest,relativelyabruptchanges
in radarbrightness
define
theboundaries
of thisregion.Theeastern
andwestern
ridges,
and the impact region, grade south into a narrow,
approximately
east-trending
bandof radar-bright
material
that
definesthesouthern
slope.Theslopein thisregionis 2ø over
al., 1986;Head, 1990;Solomonet al., 1991, 1992;Kaulaet al.
1992;Stofanet al., 1993], andthusof contractional
origin.
Radar-dark areas between bright ridge crests should be
topographicallylower than the crests(i.e., valleys), a
phenomenon
observed
locallyby theapparent
fillingof these
topographic
lows by radar-smooth
material,interpreted
as
the
-.-300km, and evidenceof crustaldeformationterminates flood-typelava flows. Stofanet al. [1993] interpreted
gradualtonaltransition
across
ridgesaspairedlightanddark
abruptlyat thistransition.
lineaments, whereas we define them as singular bright
Radar Image Interpretation
Magellansynthetic-aperture
radar(SAR) imagesderive
lineaments
withgradualtonalchanges;
eitherinterpretation
is
correct.
Othersinglelineaments
are thin,sharp,andstraight,
and
from echoesof electromagneticenergypulsestransmitted theyshoweitheruniformradarreflectivity
or havesharptonal
at theridgescarp.Thesesharplineaments
oftenend
perpendicular
to thelineof flightof theMagellan
spacecraft.contrasts
Analysis
of theintensity,
timedelay,andfrequency
shiftof the against
otherlineaments,
orotherlineaments
endagainst
them,
in juxtaposition
of contrasting
radar-bright
areas.On
echoes
produces
imageresolution
elements
thatareassigned
a resulting
brightness
level corresponding
to the strength
of the echo Earth, such featuresare commonlyinterpretedas fractures
[Ford et al., 1989]. The resultantimagesare surface [e.g.,Fordet al., 1989],andwe followthisinterpretation
for
representations
of echostrength(i.e., brightness).Radar Venus[Stofanet al., 1993].
brightness
is controlled
primarily
by surface
slopeandsurface Paired lineamentsare commonlystraight,parallel, and
byradar-smooth
material.Oneslopeof thepairis
roughness.
Surfaces
orientedapproximately
perpendicular
to separated
towardthecenter,
whereas
theotherisradar-dark,
the incidentbeamreflect the greatestamountof radarenergy, radar-bright
oppositely
dippingslopes
towardthecenterof the
producing
a strongly
radar-bright
area.Surface
slopes
oriented indicating
pair. Thesefeatures
aretypicallyinterpreted
to be
awayfromtheradarproduce
weaker,thusdarker,areasonthe lineament
representing
bounding
SAR images.This effectemphasizes
topography
andvaries graben,with the pairedlineaments
with the changein look angleof the radarbeamat different
latitudes[Ford et al., 1989;Farr, 1993]. Surfaceroughness
affectsimagebrightness
in thefollowingway:givenuniform
slope,smooth
surfaces
(e.g.,flood-type
lavaflows)reflectmost
of theradarenergyawayfrom thereceiverandare therefore
darkon radarimages,whereasroughsurfaces
(e.g.,deformed
areas)cause scatteringof radar echoesin all directions,
resulting
in radar-bright
areas[Fordet al., 1989;Farr, 1993].
More detaileddiscussions
on Magellan SAR imagingand the
effectsof surfaceslopeandroughness
canbe foundin Fordet
al. [1989, 1993],SaundersandPettengill[1991],Pettengillet
al. [1991],Saunders
et al. [1992],Ford andPettengill[1992],
Tyleret al. [1992],andreferences
therein.
Surfaceradar-brightnessis therefore a useful tool for
structural
mapping
onVenus.Changes
in radar-brightness,
and
thetextureof radar-bright
areas,allowinterpretation
of relative
topographyand/or surfaceroughness(i.e., deformation).
normal faults and the radar smooth material representing
grabenfloors[Smrekarand Solomon,
1992;Stofanet al.,
1993]. Radar smoothgrabenfloors may preserveradar
brightness
contrasts
or may be uniformlyradarbright.
Generally,
thetransition
across
onelineament
to theinterpreted
graben
flooris sharp,in contrast
to thegradualtransition
in
radarbrightness
across
ridges.Thesegrabenareinterpreted
as
evidenceof local crustalextension[e.g., Basilevskyet al.,
1986;Bindschadler
and Head, 1991; Solomonet al., 1991;
Smrekar and Solomon, 1992].
Althoughlimited to two-dimensional
imagesof the
VenusJan
surface,we may determinethe relativetimingand
depthof extension
fractures
fromtheirspacing
andorientation
withrespect
totopography.
Terrestrial
fracture
studies
indicate
thatterminated
fractures
areyoungerthanolder,through-going
fracturesets,becausethe older fracturesact as a free surface
blocking
propagation
of younger,
terminated
fractures
[e.g.,
Engelderand Geiser,1980;Pollard and Aydin,1988].
physiographic
andgeologicfeatures,
generallymanifested
as Fracturefill is important,and if an older, through-going
Within the overall radar-bright or -dark framework,
lineaments, may be discerned on the basis of shape
(straightness
or sinuosity),spacing,length,natureof changes
betweenradar-bright
and-darkareas(eithergradualor abrupt),
andwhetherstructures
arepairedor single[Stofanet al., 1993].
We emphasize
herethatphysiographic
features
discernible
on
SAR imagery must be distinguishedfrom a geologic
interpretation
of them.
fractureis filled, it no longeractsasa freesurface,soyounger
fractures
maypropagate
across
it. Also,if a younger
fracture
initiatesat greaterdepththan an older fracture,it may
propagate
beneath
andaroundtheolderfracture.Fracture
spacing
is relatedto thedepthof a fracture
set;closely
spaced
or penetrative
fracturesindicateshallowdepth,andmore
broadly
spaced
fractures
indicate
greater
depths
[e.g.,Engelder
26,020
KEEP AND HANSEN: STRUCTURAL HISTORY OF MAXWELL MONTES, VENUS
Figure3a. Magellan
F-MIDRP.65N006;1
showing
detail
of themainpartofMaxwell
Montes.
Darkcircular
areaisCleopatra.
Blackstripesrepresent
areasof missingdata. Arrowis screen
cursor.
0 km100
I
•
• •k'•
I
\\
•Arcuate normal
faults
Dominant
NW-
trending
ridges
65ON
\
Penetrative
N-trending
i lineaments
ß
1OE
,
1
63ON
Figure
3b.Geologic
intepretation
ofFigure
3ashowing
structural
features,
image
latitudes
andlongitudes,
andthelocation
of
transects
in Figure4.
KEEPAND HANSEN: STRUCTURALHISTORY OF MAXWELL MONTES,VENUS
a.
25
Transect
A Modified
Solomon,
1992]. Grabenfloorsaregenerally
smooth,
but,like
the northwest-trending
grabens,they preservevariationsin
radar brightness,indicatingthat they are not lava filled.
i--FloodedE
area
,•eIe
20-
area
W
Graben orientation is consistent with northwest-southeast
E
o• 15
LAKSHMI
.-:
Q-
sw
•
,•
ß _
el I
FORTUNA
I/'.
I /e
ß p
ß
lO
5
•::•::•::•::•::•,
NE
ß .10m
)
•
'
•: structures
thatdisplayvariationin strike,fromridge-parallel
in
................
•::::•:•:•::•
i
i
i
i
i
i
i
i
1O0
200
300
400
500
600
700
8t')0
- fl
elsewhere.The resultis that the penetrativelineamentsare
' •
modifiedto a sigmoidal
formdue to interaction
with
'0• northwest-trending
ridges
and
grabens.
The
sigmoidal
nature
900
Modified
area ,---
B
E 17- E
ßß
'
W
ß
•
ß
planesform synchronously
and require specific angular
relationships
(0-45ø). Althoughthe penetrativeand spaced
lineaments
arecommonly<45ø apart(e.g.,centralMaxwell),in
ß ,-0
6 "•"-•--'"''•'"'"'"'"'•':""•"••••••iiii
!i•ii!ii•ili!iiii!ii?•i•:i:iiiiii?•i?•11•
':"'""•::.....-"•i•
6'
southernMaxwell they are perpendicularto one another
(Figure3b). Theseangularrelationsarenotcompatible
withSC geometry,
butratherwithcrenulation
cleavage
development
[Ramsayand Huber, 1983], in which one planar fabric
overprints
another.The penetrative
lineaments
andthespaced
ridgesare thereforenot synchronous;
the spacedridgesand
grabens
postdate
formationof thepenetrative
lineaments.
The spacedridgesandnorthwest-trending
grabens
parallel
oneanother,
withthegrabens
generallylocatedin thevalleys
betweenthe spacedridges. The grabensareprominentto the
southeast
and are spatiallylimitedto the southernslope;the
'•:•:•:•:•:•:•:•
.....
•:•.....•....:•:•:::::::::::::::::::::::::
----.-.,•........•.•..••.:•-:•••••••••
•iii•iiiiii::i;•
:::::::::::::::::::::::::::::::::::::::::::::::::::
0
,
,
100
200
,
300
_3
........
'i
400
500
600
700
"0
:33
800
21
Transect C
E
C.
16 -
E
11
of the interferenceis similar to S-C sheargeometriesin
terrestrialshearzones[Berthdet al., 1979]. However, in S-C
structures,
the S (penetrative)and C (non-penetrative)
shear
Flooded
area
v
o
spatiallocation. The penetrativelineamentsare the only
to a spaced
ridge,to generally
north-trending
-5 '•= proximity
transect
•,
above
areinferred
onthebasis
of cross-cutting
relations
and
'
b. 22
0
(slope-parallel)extension.
Timing relationsbetweenthe four structuralsuitesdescribed
.
0
•
26,021
Modified
area W
ß
ßo
ß
ß
ß
ß
ß
-
Flooded
area
ß
ß
ß
ß
ß
ß
ß
ß
-10
m
::j:.,?....::•
ßß :'""•....
-:::::-o•:::.:,.•
• ß ß'•ßßßßßßß
::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::iiii::i
::•:::::."•iii•::•::•i•
::'...::.:...•
ß
o
'o
........................................................................ "•iiiiii:••.•:•
E: 5
o
0
i
i
i
i
................
i"
100
200
300
400
500
............
.::..::.•
.....
:5
'
600
700
'o
-o
800
Distance (km)
Figure 4. Topographicprofilesof transectsA, B, and C,
showing
ridgespacing
across
MaxwellMontes.Dotsrepresent
ridgespacings
measured
betweenridgecrests.Plotsaredivided
into four sections:easternunmodifiedsection(E), western
unmodified section(W), and flooded and modified sections.
Error bars of standarderror are given for the easternand
westernareas.Topography
has35 X verticalexaggeration.
ridgesare prominentelsewhereto the northweston Maxwell.
Bothsetsof structures
crosscut
thepenetrative
lineaments
but
are themselvescut by northeast-trendinggrabenson the
southernslope. We thereforeinfer the northwest-trending
grabens
andthespaced
ridgesto beessentially
coeval,forming
in differentplacesat thesametime. Geologically
it is easierto
envision
preservation
of early-formed
ridges,withlatergraben
development,
than the preservation
of early-formedgrabens
duringlatercontraction.We thereforeinterpretthegrabensas
beingrelativelyyoungerthanthenorthwest-trending
ridges,
althoughabsolutetimingrelationscannotbe discerned.
The northeast-trending
grabenson the southernslope
crosscutboth the spacedridgesand the northwest-trending
grabens.In places,thenortheast-trending
grabens
alsocrosscut
the penetrative lineaments,causing irregularly-shaped
depressions
thathavethetipsof thepenetrative
lineaments
and
partsof theridgespreserved
as "islands"
(e.g.,63øN,6øE). We
thereforeinterpretthe northeast-trending
grabensto be the
Solomon,1992] (Figure5). The grabensaverage60 km in
length,7 km in width,andat theirclosesttheyarespaced
-10
km apart.Grabenfloors,although
generally
smooth,
preserve
variations
in radar-brighmess,
indicating
thattheyarenotfilled
with flood-typelava. We interpretsteeply-dipping
graben-
youngestof the four structuralsuites.
1992].
The fourth set of structures trends northeast and occurs
perpendicular
to slope.
Relative timing between structuralsuites is thus (1)
penetrative
lineaments,(2)
spaced
ridges,(3)
northwest-trending
grabens,and(4)northeast-trending
grabens,althoughall four
suitesmaybe manifestations
of progressive
deformation.Both
graben
sets
occur
only
in
areas
of
steep
topographic
slope,
boundingfaultsbasedon the straightness
of the lineaments
suggesting
that
extension
is
related
to
the
abrupt
decrease
in
relativeto topography.
Grabenorientations
suggest
northeastsouthwest(slope-normal)extension[Smrekarand Solomon, topography,with extensiondirectionsboth parallel and
Northwest
Arm
predominantly
on the southern
slope,in areasof 2" regional
slope (Figure 5). This set comprisesa seriesof narrow,
The northwest arm, in addition to the four structural suites
straight,paired,parallel-sided
lineaments,
with an average described above, contains three other sets of structures.
lengthof 20 km, interpretedto be grabens[Smrekar and Vestigesof the first-formed,penetrativelineamentfabricare
26,022
KEEP AND HANSEN:
STRL•CT.URAL HISTORY OF MAXWELL MONTES, VENUS
%..:
.
11øE•
': !!.,.-'-'•,: ........".,-X- -,"¾', .-"
IiJ ',\, ,,'.,',,,,.
"'":'
":'•'
'""?'
"•'""'-"
'" "
c•,\x••-;,,
,-•%...
o
I
km
lOO\"',
I
,
- ,•, •
62.5ON
• "',,"b'.' •,
, ,,' .,,".", ,,'•;,,,-,•' '.,,'.' ,.
NW-trending'•,. NW-trending
graben.::::::::::.
NE-trending
graben
spaced ridges
61ON
Figure5. (a) Portionof F-MIDRP.60NO05-1
showing
soud•em
slopeof Maxwell. Blackstripes
represent
missingdata.
Co)Generalized
structure
mapof areain Figure5a, showinginterpreted
structures
andimageboundaries.
restrictedto a small areaaround67øN, 0øE. The spacedridges
dominatethe northwestarm,but are spatiallyseparatefrom the
penetrativelineaments,so the crenulationcleavage-typefabric
is absent. Northwest-tendinggrabensbetween the spaced
ridges are prominent in areas of steep topographyon the
northern slope, but they do not fan out as they do on the
southernslope(Figure6). They extendto thenorthernterminus
of the mountain range, recording local northeast-southwest
extension(i.e., normal to slope). Northeast-trendinggrabens
alsooccurin areasof steepslope,but they are closely spaced,
forming a penetrativefabric that extendsfrom ~67øN, 355øE,
to 68øN, 357øE. These grabensrecordnorthwest-southeast
(slope-parallel)extension(Figure6). Relativedepthsanddips
of these
structures
are inferred
to mimic
those
in central
Maxwell. However, cross cutting relations between the
northwest-trending
grabens,andthe herepenetrativenortheast3OE
Spaced NE-
'• trending
ridges
Penetrative NE-
0
',"
trending
graben
I
•
lineaments
,,
,'.
,,
,,
Arcuate
km
1O0
I
,, NW-trendinggraben
•
Dominant NW-trending
ridges
Penetrative
N-
trendinglineaments
/
% %'•%%II/
t
I
I
I II
-:
....
:.•:.:•:.::?..•:•:
::5
ff'f •
Figure 6. (a) Compositeimageof northwestarmandslump
structureof Maxwell Montes. Black stripesare missingdata.
(b) Generalizedstructuremapof areain Figure6a, indicating
imageboundaries
andthelocationof theslumpstructure.Note
that the strike of penetrativenortheast-trending
grabenis
modifiedin the vicinityof theslump.
-I
351OE
66ON
KEEP AND HANSEN:
STRUCTURAL
HISTORY
OF MAXWELL
MONTES, VENUS
26,023
penetrative northeast-trendinggrabens. However, several
northwest-trending
grabensare prominent,with inferreddeep
depths; these grabens crosscut the penetrative northeastcomprisesarcuate lineaments that trend northeast in the trendinggrabens. Cross-cuttingrelations are dependent,in
northernpart of the arm andwest-southwest
in the westpartof part, on the depth of the structuresinvolved: older structures,
the arm. The arcuate lineaments are long (>200 km), thin, where deep and/oruntilled, may apparentlytruncateyounger,
conversely,youngstructures
may propagate
relativelycontinuous,
sporadically
spaced(30 to 55 km), sharp, shallowstructures;
andgenerallyradar-bright.Severalof thelineaments
bifurcate under or across older structures where the latter are shallow
(e.g., 66.4øN, 356øE), and many northwest-trending
ridges and/orfilled. We thereforeproposethatthenorthwest-trending
terminateabruptly at the lineaments. We suggestthat the grabensare generally older and deeper than the northeastlineamentsmark sharp,concentricfaults (Figure 6). Their trending grabens (as on central Maxwell), but that at the
continuitywith respectto topography
indicatesrelativelysteep northernslope break, the youngernortheast-trendinggrabens
relations.
dips,andtheir broadspacingsare indicativeof relativelydeep are locally deeper,resultingin mutualcross-cutting
We interpret the slump structure to be younger than
penetration.The shapeanddistributionof thesestructures
are
reminiscent of rotational normal faults in landslip or slump penetrativenortheast-trending
ridge-grabenpairs that boundit,
areas[Varnes, 1958], and would, in this case, indicate down- as the slump structureapparentlydeflects the latter. Similar
apparentdeflectionof structuresmay occurif the slumpwas a
dropto thenorthwest,consistent
with topography.
An irregularly shapedstructure(the sixth set) lies at the rigid object aroundwhich the folds formed. However, by
northeastern-most extension of the arm, around 68.1øN, 0øE
definition,a slumpis not a rigid body, and we proposethat it
(Figure6). This structureis boundedby northeast-trendingpostdatesthepenetrativenortheast-trending
ridge-grabenpairs.
ridge-grabenpairs,the orientationsof which are deflectedto Similarly,we suggestthatthe spacednortheast-trending
ridges
enclosea balloon-shaped
areaon thesteepnorthernslope.The that essentiallyboundMaxwell deformationare the youngest
center of the structure is filled with deflected penetrative structureson the arm, as they physicallyboundthe slumpand
grabensthat becomerandomlyorienteddownslope. The locallymodify thetrendof thepenetrativegrabens.
irregular shapeof the structure,the deflectionof grabens
The arcuatefaultscut the northwest-trending
grabensat the
around the structure and the random orientation of lineaments
head of the slope, inferring that they are younger than the
at its center, lead us to suggestthat the structureformed as northwest-trending
grabens,but are otherwisenot temporally
resultof slumping,possiblyrelated to the steeptopographic constrained. We interpret them to postdatethe northwestgradient and local extensional deformation. Similar trendinggrabens,andto be relatedto downslopefailure. They
interpretations
for thisstructurehavebeensuggested
by Kaula may haveformedat any time beforeor duringthedevelopment
et al. [ 1992].
of the slumpstructure.
A seventhsetof structuresboundsthe slumpstructureto the
northwest(Figure 6). This set comprisesspaced,northeastSummary of Structural Styles
trending,continuouslineaments,that extendfrom ---67.5øN,
Central Maxwell
and the northwest
arm share similar
354øE, to 69.4øN, IøE. The lineamentsare radar-bright on
theirnorthwestsides,andthey are separatedby parallel,paired, deformational histories up to the time of formation of the
grabens(i.e., penetrativelineaments,spaced
continuous,radar-darkbands. We interpretthesefeaturesto be northeast-trending
ridges,separatedby narrowgrabens,respectively.The ridge ridges, northwest-trending grabens, northeast-trending
spacing(-6 km) is greater than that of the penetrative, grabens). We briefly discussformationof structurescommon
northeast-trending
grabens. At 67.5øN, 355øE and 68.2øN, to both central Maxwell and the northwest arm, and then those
0øE,thepenetrative
northeast-trending
grabensaredeflectedby uniqueto the northwestarm.
The penetrative lineaments and the northwest-trending
thesespacedridges,forming sigmoidalshapesthat trendinto
ridges are contractional structures, folds, possibly thrustparallelism with the spaced ridges as they intersect. The
spaced northeast-trendingridges on the northern slope of related, and thus are interpretedto have formed duringoverall
Maxwell essentiallyboundthe deformation,althoughtracesof crustal shortening. The two structural suites may be
manifestations of a single, progressive, non-coaxial
northwest-trendinglineamentsparallel to those on Maxwell
deformationepisode,with the penetrativelineamentsreflecting
gradeintothe plainsaround69øN,355øE.
Vestigesof the penetrativefabric, the spacedridgesand the either an early phaseof deformation,or an earlier, unrelated
northwest-trendinggrabenshave similar spatial relations to episodeof crustal contraction. It is also possiblethat the
thoseobservedfor analogousstructureson central Maxwell.
penetrative lineaments represent original, early-formed,
We thereforeinfer similar timing relationsbetweenthem: (1) parallellinearionsof Banerdtand Sammis[1992], formedas a
penetrative lineaments, (2) spaced ridges, (3) northwest- result of linearion-normal extension, that were later contracted
trendinggraben,and(4) northeast-trending
grabens.Temporal during crustalshortening,as they are found throughoutIshtar
relations between the two graben sets, and among other deformed belts [Phillips and Hansen, 1994; Hansen and
structuralsuiteson thenorthwestarm, aremore complex.
Phillips, 1993]. Northeast-trendinggrabenson the southern
As discussed
abovefor centralMaxwell, northwest-trending slopeparallel the directionof maximumshorteninginterpreted
grabensare inferred to be older and deeper than northeast- from the spacedridges, and are thuscompatiblewith a single
trendinggrabensthatcrosscutthem,with bothsetsof structures strain regime. Northwest-trendinggrabenson the southern
confined to areas of steep topography. However, on the slope are not compatiblewith a single strainregime, as they
northwestarm, mutual cross-cuttingrelationsexist betweenthe parallel the spacedridges. However, the spatialrestrictionof
grabensets. Most of thenorthwest-trending
grabensdisappear the northwest-trendingridges to the steepsouthernslopemay
indicate that their distribution is somewhat controlled by
toward the northern terminus of Maxwell (inferring
progressivelyshallower depths), and are crosscutby the topographicgradient,and is thusrelatedto the mechanismof
trendinggrabensare more complex(seediscussion
of timing
relationshipsbelow).
A fifth set of structures, unique to the northwest arm,
26,024
KEEP AND HANSEN: STRUCTURAL HISTORY OF MAXWELL MONTES, VENUS
mountainbelt support. We thereforeconcludethat the main
structures
of both central
Maxwell
and the northwest
arm
(penetrative ridges, northwest-trendingridges, northwesttrendinggrabens,northeast-trending
grabens)are compatible
with deformationduringregionalcrustalcontraction,with late,
local,northeastextensionalongthe southernslope.
Almost all structuresunique to the arm are interpretedas
extensionalfeatures(orientedperpendicularto slope), and are
associated
with steeptopographicslope. We suggestthat the
penetrativenortheast-trending
grabens,the arcuatelineaments,
the slump structureand the spacednortheast-trending
ridges
boundingthe slumparemanifestations
of downslopeextension,
perhapsrelated to gravity sliding [Smrekar and Solomon,
1992]. In supportof this hypothesis,the spacednortheasttrendingridges(the sixth set of structuresdescribedfrom the
arm) occurnearthe breakin slopewith theplainsto thenorth.
Similarly, the slumpextendsdownslope,and is terminatedby
spacednortheast-trendingridges at the slope break. The
arcuate lineaments (interpreted normal faults) occur further
upslope.The densedistributionof structuresat the slopebase,
with more widely spacedstructuresupslope,is consistentwith
formationdue to slumping[e.g., Varnes,1958,Shreve,1968].
CentralMaxwell and the northwestarm appearto havebeen
deformed simultaneously for most of their respective
deformation histories, as shown by several structural
similarities: (1) the dominant structural fabrics parallel
topographiccontours,are broadly distributed,and show no
changein spacingrelative to elevationchanges;(2) structures
on Maxwell Montes have relatively short wavelengthsand
closespacingwith respectto elevation;(3) grabensoccuronly
in areasof steepslope(>2ø); (4) no grabensoccurin areasof
highesttopography;and (5) deformationterminatesabruptlyin
areasof steepslope (with the exceptionof the more gentle
Tectonic
Models
for Ishtar
Terra
Models for the formation of Ishtar Terra propose that
Lakshmi Planum is the site of either local mantle downwelling
[Bindschadler et al., 1990; Bindschadler and Parmentier,
1990], or local mantleupwelling[Grimm and Phillips, 1990,
1991], producing crustal contraction. A third model,
[Crumpieret al., 1986;Head, 1990]proposes
thatIshtarTerra
is the locusof regionalcompression,
with crustconvergingon,
andsubducting
beneath,LakshmiPlanum.
Bindschadlerand Parmentier [1990] and Bindschadleret al.
[1990, 1992] proposethat deformationaroundIshtar Terra
results from crustal thickening and contraction above a
cylindricalmantledownwelling. Accordingto theseworkers,
vertical normal stresses from downward flow, and shear
couplingof horizontalmantleflow with the baseof the crust,
cause flow and thickening in the lower crust and resultant
surface uplift and contractional deformation. Using
Andersoniancriteria, Bindschadleret al. [1990] predicteda
relativedeformationsequencewith increasingcrustalthickness
as follows: (1) radial thrustsabove the downflow and strike-
slip faultstowardtheperiphery;(2) radialandconcentricthrust
faults, with strike-slipfaultstowardthe periphery;(3) outward
migrationof radial thrustingandstrike-slipmotionat thecenter
of the uplift; and (4) at steady-state,
radialnormalfaultingand
strike-slip faults. Contractionis inferred to young outward
from theperipheryof the belt.
In contrast,Grimm and Phillips [1990, 1991] proposethat
LakshmiPlanurnrecordscrustalthickeningabovean upwelling
mantle plume, and that crustalthickeningand mountainbelt
deformationresult from downwardreturn flow of the plume.
Structures
predictedusingAndersonian
criteriaare similarto
thosepostulated
by Bindschadler
et al. [1990], includinginitial
radial thrustfaults with peripheralstrike-slipfaults,changing
transition to western Fortuna Tessera. Extensional structures
to concentricthrustfaults with peripheralradial thrustfaults,
related to gravity sliding occurredlate in the deformation andultimatelyto radial normalfaultsandperipheralstrike-slip
historyof thenorthwestarm.
faultswith time and increasingcrustalthickness.Contraction
is inferredto younginwardfrom theperipheryof thebelt.
Both modelspredictcontractionalstructuresorientedboth
Discussion
radially and concentricallywith respectto LakshmiPlanum,
Our observationsof structuralstyleson Maxwell Montes and abundantperipheralstrike-slipfaults at variousstagesof
on Maxwell areradiallyoriented
have severalimplications.Broadlydistributeddeformation, uplift. Noneof the structures
over severalhundredkilometersperpendicularto strike and around Lakshmi; rather, orientations of each of the structural
elevation changesof several kilometers, suggeststhat suitesareuniform acrossLakshmiPlanurnandcannoteasilybe
deformation is independentof topography. Similarly, interpretedas concentric. Predictionsas to the younging
topography
is independent
of deformation
(i.e.,notstructurally direction of structures on Maxwell cannot be tested because
supported);
assuming
foldamplitudes
of <2 km [11ansen
and temporalrelations parallel to strike cannot be determined.
Phillips,1993], deformation
is modest,andnotableto support Both of thesemodels thereforepredict structuresthat are not
the 12 km elevation of Maxwell.
If the elevation of Maxwell
derivesfrom crustalthickeningalone,thenfold amplitudesof
tensof kilometersare implied,which is difficult to envision
giventhe regularridgespacingof 6-12 km, andno apparent
structuralbreakparallelto ridge traces. Lack of extensional
structures
at thehighestelevations
of thebeltsuggest
a lackof
extensionalcollapse;therefore,Maxwell crustis apparently
observed,and fail to predictobservedstructuralrelations[see
alsoPhillips and 11ansen,1994; 11ansenand Phillips, 1993],
and neither model discusses how stresses within the mantle are
transferredto the crust. Both models suggestthat mountain
beltsresult from thickenedcrust. If this were true, the highest
elevations,underlainby the thickestcrust,wouldbe theloci of
gravitationalcollapse;yet this is not observed. Also, recent
stable at elevations of 11 km. Tectonic models for the
gravitydata [Sjogren,1993] suggestthat westernIshtarTerra
formation of Maxwell Montes must address these observations may have a larger componentof crustalcompensation
than
previouslythought,and thereforegeodynamicmodelsmay not
andimplications.
Models for the tectonic evolution of venusian mountain belts
needto infer deep-mantleflow mechanisms.It is possiblethat
fall intotwo groups,thoseaddressing
IshtarTerraasa whole, the gravitydatausedfor bothof thesemodelsmay resultfrom
compensation
mechanisms.
andthosespecifically
addressing
MaxwellMontes.We givea bothlong- andshort-wavelength
Althoughbothstudiesare modeledat scalesfar greaterthan
brief overviewof eachof thesegroups,andthenpresenta new
model for the tectonic evolution of Maxwell Montes.
that of individual Ishtar deformedbelts, and make few unique
KEEP AND HANSEN: STRUCTURAL HISTORY OF MAXWELL MONTES, VENUS
26,025
Maxwell
E Lakshmi
•1.
MAXWELL
'"::--.':•
.... upper
crust
J•
lowercrust
FORTUNA
onsouthern
slope
.....
:':m'"'""
•'•.•-:.••••i•:
•
MAXWELl.._
.........
.•
w
"•'••••
.........
ß
.:-:•::
-..:::•
......
' ',•••••4•'"'•:•
'•-•••
:....
-"':':':':
:'':
:• •'::: :•:•:•:•:
-?•.
LAKSHMi
w
••"
......
Structures
on
•<'•••'•••Xnorthwest
arm
-:::¾•
•'-••
'-':'•:•:•
L:
."•i• •:
thickened
Figure7. (a) Schematic
cross
section
fromLakshmi
Planum
easttoward
Fortuna
Tessera,
showing
proposed
changes
inmantle
residuum
thickness
thatsupport
Maxwell
Montes.(b) Schematic
blockdiagram
indicating
therelationship
between
mantle
residuum
thickness
andthelocation
ofextensional
structures.
Extension
occurs
onlyinareas
ofsteep
topography
thatmarkthe
transition
fromcrustoverlying
thickened
residuum
tocrust
overlying
normal
residuum.
Figure
hassame
spatial
orientation
as
Figure
7a. (c) Schematic
blockdiagram
looking
south
fromtheNWarmtothemainbodyof Maxwell,
illustrating
thetearfault
originproposed
fortheOøElineament.
A section
of crust/mantle
isshown
removed
forillustrative
purposes.
Notethatthetear
faultdoesnoteverywhere
intersect
thesurface;
in factit maynotintersect
thesurface
at all.
predictions
asto thenatureof crustaldeformation,
theymay, compression
[e.g.,Crumpleret al., 1986;Head, 1990;Roberts
with modifications,
provideinsightsinto possibleprocesses and Head, 1990]. In this model, based on deformation
drivingcrustaldeformation.The presenceof a varietyof analysesof the surfaceusing Venera images, the Ishtar
volcanic
constructs
onVenushaveledmanyauthors
topropose deformedbeltsresultfrom crustalthickeningas a resultof
that Venusloosesmuchof its heat throughmantleplumes underthrusting
of lithosphereunderLakshmiPlanum. Sucha
risingto thesurface[seereviewin PhillipsandHansen,1994]. plateconvergence
modelwouldpredict(1) variationsin strain
It is possiblethat plume activity at some scalemay be intensity,with high strainat the plateboundary,and lower
responsiblefor some of the featuresof Ishtar Terra, and we strainaway from it; (2) major structures
parallel to plate
explorethis possibilitymore fully below (see Proposed margins,with nonorthogonal
convergence
accommodated
as
TectonicModel for Maxwell Montes).
margin-parallel
displacements;
(3) graduallateraltermination
A third model for the tectonicevolution of Ishtar Terra, of intensedeformation,with possibletransitionto different
similarto platetectonic
modelsfor Earth,is regional structures(e.g., terminationof terrestrialorogenicbeltsat
26,026
KEEP AND HANSEN:
STRUCTIYRAL HISTORY
boundingstrike-slipfaults);and (4) extensionalcollapseof the
deformationbelt during later stagesof convergence. The
kinematicpatternspredictedby thismodelarenot observedin
Maxwell Montes, or regionally throughout Ishtar Terra
[PhillipsandHansen,1994]. We rejecta regionalcompression
model becausepredictedsurfacestructuresare not observed,
and crustalthickeningis inferred to be solely due to crustal
deformation,yet the observedsmall-wavelengthand small
amplitudestructures,interpretedas folds and thrusts,cannot
accountfor the topographyof Maxwell . In fact, crustal
thickeningon the orderof only severalkilometersis predicted
[e.g.,VorderBruegge,1994].
Tectonic
Models
for Maxwell
Montes
Pre-Magellanmodelsfor the formationof Maxwell Montes,
based on a thorough analysis of Venera and Arecibo data
[Vorder Bruegge and Head, 1989; Vorder Bruegge et al.,
1990], propoundthat Maxwell Montes comprisesten distinct
crustal domains,forming an original, linear, terrestrial-style
mountainbelt, telescopedalong nine right-lateralstrike-slip
faults(with up to 125 km displacement/fault).The telescoped
packageis proposedto lie betweentwo boundingshearzones,
a northeast-striking,
right-lateralzoneat the northernboundary,
andan east-striking,left-lateralzoneat thesouthernboundary,
along which Maxwell Montes was rotatedand translatedinto
its present configuration. This model hinges on the
identificationof nine north-northwest-trending
strike-slipfaults
and two crustal-scale
shear zones that are not documented with
Magellan data [Kaula et al., 1992; R. W. Vorder Bruegge,
personal communication,1993; this study]. However,even
with relatively low-resolutiondata, certain structuralfeatures
were identified [Vorder Bruegge et al., 1990]: (1) the
sigmoidalnature of the penetrativelineaments;(2) grabens
spatiallyrestrictedto northernand southernslopes;and (3) a
lack of extensional features on central Maxwell, indicating
stabilityof the belt.
UsingMagellandata,Suppeand Connors[1992] proposed
thatdeformationof thesteepwesternslopeof Maxwell Montes
resultedfrom east-directedunderthrustingof Lakshmi Planum
beneathMaxwell Montes, analogousto the toe of a criticaltaper wedge as proposed for terrestrial fold and thrust
deformation. This model addressesonly deformationon the
western slope of Maxwell and does not explain other
deformation
fabrics
or structural
features.
Because
the
structures
presentalongthewesternslopearenot limitedto this
region,but extendto the restof Maxwell, we believethat any
model must address formation of the entire range and its
structures,
not simplya smallpart thereof.
ProposedTectonicModel for Maxwell Montes
Our proposed
modelfor thetectonicevolutionof Maxwell
Montes seeks to explain the following observationsand
OF MAXWELL
MONTES,
VENUS
The apparentshort wavelengthsand small amplitudesof
structures on Maxwell Montes, coupled with their broad
distribution,indicatethattheyrepresentlittle morethansurface
deformationof a thin layer, analogousto contractionaland
shearstructures
in flowing lava thatdevelopasthecoolsurface
of the flow is alecoupledfrom and draggedalong with the
flowing lava underneath[Borgia et al., 1983]. If this analogy
is correct, it implies that processesoccurring in the region
below the thin surfacelayer are responsiblefor the surface
deformation of Maxwell Montes. Also, as we have noted, the
small amplitudesof Maxwell deformationstructures
areunable
to accountfor the high topography,yet Maxwell is apparently
stable, with no evidence of extensionalcollapse even at the
highest elevations. This also strongly implies that the
mountain belt is being supportedfrom below, by processes
operatingat deeperlevels than the thin upper crust. Finally,
the Maxwell structurespreserveno apparentstrain gradient
acrossthedeformedbelt; structuralspacingremainsunchanged
despitelargetopographic
changes.
We follow the model of Phillips and Hansen [1994; Hansen
and Phillips, 1993], that deformationin the mountainbelts
surroundingLakshmi Planum are surface manifestationsof
deformationprocessesoccurring at depth in the crust and
mantle (deformation-from-belowmodel). In this model, the
strong,uppercrustis separatedfrom a stronglower crustby a
weak zone (a "jelly sandwich"analogy) [e.g., Zuber, 1987;
Banerdtand Golombek,1988] (Figure7). The uppercrustmay
be broadly alecoupledfrom the lower crust [Basilevskyet al.,
1986;Phillips, 1986, 1990] by a weak layer that actsasboth a
decollement surface, allowing the upper crust to deform
independently,and as a stressconduit, transmittinglower
crustalandmantlestressesto the uppercrust. The uppercrust
therefore deforms independentlyof the lower crust, but in
response
to stresses
imposedby mantlestrainepisodes.Largescale deformation
in the lower crust and mantle
will thus be
manifestedas "cover"deformationin the upper crust (Figure
7).
Upper-mantlemelt residuumpondsat depthbeneathIshtar,
drawndownwardandinward as a resultof mantledownwelling
[Phillips and Hansen , 1994; Hansen and Phillips, 1993].
Maxwell Montes is underlain by imbricated lower crust,
stackedup asa resultof thepondingresiduum;thickenedlower
crustsupportsthe mountain-belt-scale
topography(Figure7).
Uppercrustal"cover"deformspassivelyin response
to lowercrustal translation,imbrication and thickening. Structurally
imbricatedlower crustwould have steepboundariesat the front
and sides(westernslope and northernand southernslopesof
Maxwell, respectively), and shallow boundariesbehind the
imbricated package (eastern slope of Maxwell) [Dahlstrom,
1970; Butler, 1982; Boyer and Elliot, 1982;Mitra, 1986]. At
thecenterof theupliftedarea,structures
aresupported
andthus
stable (Figure 7). At the periphery, sharp boundariesexist
between upper crust that overlies normal lower crust and
mantle and upper crust that overlies thickenedlower crust.
interpretations:
(1) broadlydistributed
surface
strain,withlittle Upper crustoverlyingthickenedlower crustis stretchedto
changein intensityoverhundreds
of kilometers
perpendicular accommodate the increased surface area created by the
to strike or elevation changesof several kilometers; (2)
inability of short-wavelength/small
amplitudestructuresto
supporttopography;
(3) apparentstabilityof crustwithin the
mountainbelt,particularlyat thehighestelevations;
(4) abrupt
terminationof deformation,corresponding
to steeptopographic
gradient;and(5) thatevidencefor extension
is limitedto areas
of steeptopographic
slope.
thickening(much like a table cloth is stretchedout over a
table). At the boundariesbetweenthickenedand unthickened
areas, contractional structures will end abruptly, and
extensional structures accommodate the increased surface area
(Figure7). In this model then, the lower crustthickensand
supports
themountjan-beltscaletopography,
whereasa pond,
or kneelof residuumsupportsthe long-wavelength
(2,000 km)
KEEP AND HANSEN: STRUCTURAL HISTORY OF MAXWELL MONTES, VENUS
welt of Ishtar Terra.
This keel of mantle residuum
would
26,027
be
strainover hundredsof kilometersperpendicularto strike and
acrosselevationchangesof severalkilometers.Suchpassive
1975, 1981]. The thickenedupper-mantleresiduum(and deformationresults in short-wavelengthstructuresthat are
possibly mantle downwelling) is responsiblefor the large incompatiblewith high topography,implying supportof the
apparentdepth of compensationat the wavelengthof Ishtar uppercrustby underlyingthickenedlower crustand thickened
Terra. The highelevationof Maxwell is supportedby variably mantleresiduum. Supportfrom below alsoexplainsthelack of
thickenedlower crust, and detailed topographyis associated extensional collapse of the surface features,the abrupt
with folded, modestlythickenedcrust [Phillips and tlansen, termination of deformation at slope boundaries, and the
distribution
of extensional
structures
limited
to these
1994;Hansen and Phillips, 1993].
The 0øE lineament
between central Maxwell
and the
boundaries.Tear faultingduringlowercrustalimbricationmay
northwestarm markschangesin slopefacingdirection,and the havetriggeredgravityslidingon thenorthwest
arm,explaining
spatial limit of localized gravity sliding. It may be the discontinuity between the northwest arm and central
similar to mantle keels beneath continentson Earth [Jordan,
accommodated
within the above described "deformation-from-
Maxwell.
below" model as a surfacemanifestationof a lateral ramp or
tear fault, bounding the imbricated lower crustal package. Conclusions
Terrestrialfold and thrustbeltsarenot continuousalongstrike;
The followingconclusions
canbe drawnfrom thisstudy:(1)
differential displacementon different parts of the systemare
accommodated
by tear faults or transformfaults that segment central Maxwell and the northwest arm record similar
the belt (Figure 7). Thrustsystemsmay terminatelaterally at deformationhistories,with thenorthwestann recordinga late
these structures, much like the Lewis thrust of the northern
episodeof gravitysliding;(2) the aspectratio, abruptalongRocky Mountains, or the Pine Mountain thrust of the strike terminationof deformationat the base of the steep
AppalachianMountains[Dahlstrom, 1970; Boyer and Elliot, northernandsouthernslopes,andthebroadlydistributedstrain
1982; Butler, 1982; Mitra, 1986]. We proposethat during on and around Maxwell Montes make it unlike terrestrial
lower crustal imbrication differential displacementon the deformationbelts; (3) the modestshorteningindicatedby
overthrust sheet was accommodatedby a tear fault or lateral short-wavelengthstructureson Maxwell is unable to account
ramp beneathwhat is now the discontinuitybetweencentral for thegreatelevationof thebelt;(4) Maxwell Montesmaybe
Maxwell and the northwestarm (Figure 7c). This fault was underlainby a "block"of anomalouslythickenedlower crust
mountain-belt-scale
topographyandcontrolsthe
activatedlate in the deformationhistory,after similar structural that supports
suiteshad developedubiquitouslyacrossMaxwell. Activation spatialdistributionof deformation;
and (5) failure alonga
of the tearfault at depthmay havemechanicallyseparatedthe possiblelower crustaltear fault boundingthe anomalously
residuum beneath the northwest
arm from that beneath central
Maxwell, with failure triggeringgravity sliding on the arm.
Movement on the fault may also have causedthe arm to be
translatedwestward,resultingin the apparentoffsetof the arm
from central Maxwell.
Recentlyacquiredgravitydata,andnewly derivedflow laws
for Venusmay haveimplicationsfor theproposed
model. New
deformationexperimentsfor materialsscaledto the crust of
Venus[Mackwellet al., 1994] indicatethatthe depth-averaged
strengthof the crustmay be more similar to that of the mantle
thanpreviouslythought. Thus the strong-weak-strong
profile
may be less distinct than indicated by previous experiments
[Zuber, 1994]. Preliminary evaluationof the relationship
between tectonic length scalesand a stronglayer thickness
[Neumann and Zuber, 1994] indicates that multiple
deformationmay still occur when brittle-ductile transitions
occurin both the crustand the uppermantle. Also, even with a
muchstrongercrust,contractionalmorphologies
on Venusmay
be explained by horizontal shortening of a laterally
thickenedlower crustmay have resultedin gravity sliding
structures on the northwest
arm.
Acknowledgments.
Thisresearch
wassupported
by NASA operating
grantNAGW-2915awardedto V.L.H. We thankJohnGoodgefor his
suggestionsand comments,and Bruce Banerdt and Bob Grimm for
their reviews.
Warm thanks also to Dianne Hess Goldbach for her
patience,supportandmarveloushumour.
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(ReceivedOctober4, 1993;revisedJune4, 1994;
acceptedSeptember30, 1994.)