JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 95, NO. B5, PAGES 7119-7132, MAY 10, 1990
Venus Trough-and-RidgeTessera:
Analogto Earth OceanicCrustFormedat SpreadingCenters?
JAMES W. HEAD
Departmentof GeologicalSciences,
BrownUniversity,Providence,RhodeIsland
One type of tesseraterrainon Venus,the trough-and-ridge
tessera,exhibitsa distinctivemorphology
composedof throughgoing
generallyparallellinearvalleysand shorterorthogonalvalleysand ridges.The
similarity of this patternto oceaniccrustaltopographyand morphologyis examined.Oceaniccrust on
Earth is characterizedby a distinctive orthogonalpattern, with transformsand fracture zones in one
directionand linear abyssalhills developedparallel to the rise crestin the other. Similaritiesbetweenthe
components
of the tesseraterrainandterrestrialseafloorfabricand structureare foundin termsof linearity,
parallelism,length, width, spacing,orthogonality,and generalmorphology.A differenceis that trough
andridgetesseratendto occuras plateau-likeregionsat intermediate
elevations(average-•2 kin) abovethe
surrounding
plains, while terrestrialseafloorrangesin elevationfrom that observedat rise crestsand
oceanicplateaussuchas Iceland,down to depthsof severalkilometerson the abyssalfloors, wherethe
orthogonalpatternis oftenmaskedby sedimentation.
On the basisof the similaritiesit is proposedthat
trough-and-ridge
tesseramay have originatedthroughprocesses
analogousto thoseresponsiblefor the
oceanfloor fabric on Earth, formingat a rise crestsimilarto divergentboundarieson the Earth'sseafloor,
and evolvingto its presentmorphologyand configurationthroughprocesses
of crustalspreading.The
plateau-likenatureof the trough-and-ridge
tesseradistributioncould be the resultof localizedcrustal
thickeningin the spreadingprocess,producingelevatedIceland-likeplateauswhosetextureis preserved
from subsequent
lowlandvolcanicflooding.Some regionsof more complextesserapatternsmay result
from deformationof the basic trough-and-ridgepattern of tessera.This hypothesiscan be tested with
globalhigh-resolution
imagingand altimetrydataandgravitydatafrom the Magellanmission.
1. INTRODUCTION
Kozak and Schaber, 1986; Markov et al., 1989; Smrekar and
Tessera("tile" in Greek; also informally called "parquet")is
Phillips, 1988] or gravitationalrelaxation[Bindschadlerand
Parmentier,
1987]; (4 seafloor spreading where structural
anupland
terrain
typeoccurring
in areas
known
to be patterns
arerelated
torise-crest
processes
[Bindschadler
and
distinctive
onthebasis
ofPioneer
Venus
altimetry,
roughness,
Head,1988c].
Recent
workbyHeadandCrumpler
[1987]
andreflectivity data [Head et al., 1985; Bindschadlerand Head, suggeststhat divergenceand crustalspreadingis occurringon
1989a,b].
Thisterrain
wasshown
byVenera
15/16
radarVenus
inWestern
Aphrodite
Terra,
tothesouth
oftheregion
images
tohave
adistinctive
morphology
andtexture
dominated
covered
byVenera
15/16
data,
although
theglobal
extent
and
byclosely
spaced
intersecting
grooves
andridges
thatproduce
significance
ofthisprocess
isuncertain
[Kaula
andPhillips,
a variety
of patterns
(hence
thetermtessera)
including
1981].
Analysis
ofterrestrial
spreading
centers
extrapolated
to
orthogonal,
diagonal,
chevron,
andchaotic
[Barsukov
etal., Venus
conditions
[Sotin
etal.,1988;1989a,b]
shows
thatthe
1986; Basilevskyet al., 1986; Sukhanov, 1986; Bindschadler gravity and topography data for western Aphrodite are
andHead,1988b,c].
Thespacing
between
ridges
in tessera
consistent
witha spreading
hypothesis.
There
isalso
evidence
terrain
varies
butgenerally
isintherange
of5-20km.Ridgethatsome
oftheequatorial
regions
thought
torepresent
crustal
heights
aregenerally
notmore
thanseveral
hundred
meters
spreading
have
thesame
radar
characteristics
asthetessera
in
[Basilevskyet al., 1986]. Tesseraoccur in both small isolated the north [Bindschadler and Head, 1988a,d; Senske and Head,
patches
andin largeregional
configurations
thatcomprise
1989].
Thepurpose
ofthispaper
istoinvestigate
further
the
about
15%
ofthearea
north
of30øN.
Themorphology
of general
similarity
ofmorphology
between
the
trough-and-ridg
tesserahas been interpretedto be related to deformationwhich tesseraand the fabric of the ocean floor and to investigateone
has acted with primarily horizontal componentsover broad of the hypothesesfor tesseraorigin, that is, that the tessera
areas(the areal deformationof Basilevskyet al. [1986], but the texturemight be relatedto a crustalfabricproducedat spreading
origin of the deformationis still controversial.
centers.
A range of modelsfor tesseraorigin has been outlined and
summarizedby Bindscharierand Head [1988c] andincludes(1
2. CHARACEERISTICS OF TESSERA ON VENUS AND
horizontal compression and crustal thickening due to
Ttm T•OUGH-aND RnX]E TE•
iN LmM• Tv.ss•.RA
asthenosphericcurrents [Basilevsky, 1986; Pronin, 1986]; (2
Pioneer Venus data show that tesseraterrain lies at higher
vertical uplift due to shallowmantleprocesses[Phillips, 1986;
Bindschadlerand Parmentier, 1987]; (3 gravity-driven(generally
2 +1 kin) elevations
thanthesurrounding
plainsand
deformation
manifested
as gravitysliding[Sukhanov,1986; thatit is characterized
by relativelyhighervaluesof surface
roughnessat the centimeter-scaleand the meter to decmeter
scale [Head et al., 1985; Bindscharier and Head, 1989b] than
most other surface units. Gravity data for one of the major
Copyright
1990
bytheAmerican
Geophysical
Union.
regions
oftessera
(Tellus)
ledSjogren
etal.[1983]
tosuggest
Paper
number
89JB03068.
thatTellusis compensated
at relatively
shallow
depths.
The
0148-0227/90/89JB-03068505.00
range of surface morphology observed in the tessera led
7119
7120
HEAD: VE2qUSTESSERA-EARTH OCEANIC CRUST ANALOG?
280
320
0
40
120
160
200
60
-30
-30
-60
-60
24O
280 r
320
•)
'
'
40
80
120
160
200
240
Fig. la. Locationand settingmaps.Globalmap of Venusshowingthe locationof LaimaTesseraand its relationto Ishtar
Terrato the northandto AphroditeTerrato the south.Areasindicatedin blacklie belowthe 0-km datum.Contourintervalis
1 km. Box showslocationof Figure lb.
Bindschadler
andHead[1988b;alsoModelsfor theoriginand morphologic
variabilityalongstrike.The individualfeatures
evolution
of tessera
terrain,Venus,submitted
to Journalof themselves
(Figures
2b-2c)havethreemodes
of expression:
(1)
Geophysical Research,1989] to proposea three-parttrough-shaped
in cross
section,
withinwarddipping
wallsanda
classification
scheme,outlinedin order of increasingflatfloorwhichusually
appears
tobecovered
bysmooth
plains
complexity
of structural
patterns:
(1) subparallel
ridgedterrainunitsof probable
volcanicorigin[Basilevsky
et al., 1986;
(subparallel
linearridgeswith narrowzonesof disruptionSukhanov
et al., 1987](Figures
2b-2c,marked
T); (2) groovedoriented
at a variety
of angles
totheridges;
typeareaWesternshaped
in crosssection,
wherethe inwarddippingwalls
Fortuna
Tessera);
(2) trough
andridgeterrain
(largetroughs
in converge
without
thedevelopment
of a flatfloor(Figures
2bonedirection,
smallerridgesandvalleysin theother,generally2c, markedG); and,(3) lineaments,
wherea linearfeatureis
orthogonal
direction;
typeareaEastern
LaimaTessera);
and(3) observed,
buttopography
isnotasdistinct
asin thecaseof the
disrupted
terrain
(complex
andchaotic
orientation
of shortto troughs
andgrooves
(Figures
2b-2c,marked
L).Troughs
range
intermediate
lengthridgesandtroughs;typeareacentralTellus
Regio).In thisanalysis,
the generalcharacteristics
of thetype
exampleof the trough-and-ridge
terrain(EasternLaimaTessera)
(Figure1) are assessed.
30 ø
eøø,
/
/
50 ø
I
I
70 ø
•
\
\
60ø
LaimaTessera
covers
over2 x 106 km2 andliessouthof
FortunaTesseraand EasternIshtar Terra (Figure 1). It is
bounded
on the eastby a generallynorth-south
trending
ridge
belt (Kamari Dorsa),and alongthe rest of its bordersit has a
transitionalboundarywherethe topographydescends
toward
'
the surrounding
plainsandappears
to be embayed
by them, s0o.
50 ø
particularlyto the southand west (Figure lb). The major
morphologic pattern associated with Laima Tessera is a
structuralfabric composedof two elements(Figuresl b-lc).
Oneof theseelements
consists
of WNW to NW trendinglong
and throughgoinglineamentsand faults [Basilevskyet al.,
1986;Sukhanov
et al., 1987]andlargetroughsup to 30 km in
0o
width.
Thesecond
isa NNEtoNEtrending
setofmuch
shorter40ø30o•
linearelements
of alternating
ridgesandvalleysspaced6 to 12
km apartandorientedlocallyorthogonally
to thelargerWNW
trending
structures.
4•o
Iø
50
•.../
6•3
ø
[400
Fig.lb.Sketch
map
showing
thelocation
ofLaima
Tessera
and
The troughs/lineaments
(Figures1 and2a-2c) are nomenclature
ofadjacent
features
and
structures.
Dorsa
arelinear
belts
characterized
by their linearity,significantlength,and ofdeformation.
Planitia
areplains
interpreted
tobeofvolcanic
origin.
HEAD: VENUS TESSERA-EARTH •C
ß
..
_
CRUST ANALOG?
i
ß
7121
I
I
I
t
Fig.lc. Structure
ofthetessera
incentral
andnorthem
Laima
Tessera
(centered
on48ø,53øN)[from
Basilevsky
etal. 1986].
1, lavas;2, ridgesof linearbelts;3, lineaments
in tessera;
4, circularblocks;5, volcanoes;
6, impactcraters;7, mainfaults.
Verticalwhitestripesaredatagapswhichwerefdled in laterin themission.AusraDorsa,upperleft; KamariDorsa,right.
in width from about8-30 km with most in the rangeof 10-12 Detailed examinationof the areasbetweenmajor troughsoften
km. In some casesthe trough walls become irregular and reveals the presenceof much more subtle linear features
convexoutward,and a bead-like appearanceresults(Figures1 orientedparallel and subparallelto the troughsand producing
and 2c, markedB). Thesebead-likeor oval structuresrangein minor disruptionsin the orthogonalfabric (Figures 1 and 2b,
width from 15 to 45 km and in length from 30 to 50 km and markedD).
appearto be more commonin the southernpart of Laima
The secondstructuralelementoccursin the terrainbetween
Tessera than in the northern. Often, the flat-floored trough the troughs/lineaments
and is characterizedby parallel ridges
narrows and the walls merge into a single linear valley, and valleys oriented generally perpendicular to the
generallyless than 8 km in width. A crosssectionacrossthe troughs/lineaments,giving the impressionof a corrugated
strikeof the troughs(Figure2d) showsthatthe troughsaverage appearance.
The flanksof theridgesandvalleysareoftensharp
less than 500 m deep along this profile but that the most and linear and are interpretedto be fault bounded,in contrastto
prominenttrough(Baba-jagaChasma)approaches1 km depth the hills and swales typical of the mountain belts of Ishtar
where it crosses this profile. The range and frequency Terra which are interpretedto be anticlines and synclines
distributionof trough/lineamentlengthsare related in part to [Crumpier et al., 1986]. Thereforethe hills and valleys of the
their preservation,since many can be seen to be embayedby corrugated terrain appear to be more similar to horsts and
plains units surroundingLaima Tessera (Figure 1). Several grabenthan to anticlinesand synclines(Figure 2).
troughscompletelycrossthe width of Laima Tessera,and one
In many cases,particularlywherethe edgeof the corrugated
(Baba-jagaChasma)extendsfor a distanceof about 1400 km. terrainis definedby a trough,individuallinear elementsdo not
Most of thesefeaturesare lessthan750 km in length,andthere appearto crossfrom one domainacrosstrough/lineaments
into
is a distinctdifferencein the length and continuitynorth and the adjacentcorrugatedsegment.In other cases,particularly
southof Baba-jagaChasma(Figure 1).
where the edgeof a portionof the corrugatedterrainis defined
The majortroughsgenerallyparalleleachother(Figure1) by a lineament,manylinearsegments
canbe tracedacrossthe
with the distancebetweenany two being similar alongtheir structure,whereasothersterminateagainstit. Few corrugated
strike. The spacing between successive major elementscarry furtherthan acrossone trough/lineament
zone
trough/lineaments
in a traversenormal to the strike of the (see map patterns in Figure 1). Many elements terminate
features,however,is variable.ANNE orientedtransectacross against the more subtle lineaments between major
easternLaima Tesserashowsthat the spacingrangesfrom 20 to troughs/lineaments(see particularly central Laima Tessera,
100 km with an averagespacingfor 13 troughsof about50 kin. Figure 1).
7122
HEAD: VENUS TESSERA-EARTH OCEANIC CRUST ANAIXX;?
....
'•-•::"i,•
...........
•.......
":;"•'
::?...
•?•"•'.•
•-:•::---:::•'•
• '•:• %[:
"'
::.:•:•:..•:
• •'..-.-.•i:::
.....•
.•:• % - 60•
......
'•":-•'t
....
•,
.......
.:.
..•.
55 ø
45 ø
50 ø
55 o
60 o
Fig. ld. Venera 15/16 image of easternLaima Tesserashowingthe textureof the trough-and-ridge
tesseraand the locationof
the enlargements,sketchmaps, and topographicprofile shownin Figure 2. Kamari Dorsa, right side of image.
The lengths of the valleys and ridges in the corrugated
terrain are thus generally comparable to the separation
distancesbetween the WNW oriented troughs/fractures.
These
distancesare commonly in the range 20 to 100 km and are
much shorter than the WNW oriented troughs/fractures.
The
separationdistancesof the hills range fxom about 6 to 12 km,
averaging about 8-10 kin, in general agreement with the
average8 km crest-to-crest
distancefoundfor LaimaTesseraby
Ivanov [1988].
interpretedto be of volcanic origin are noted adjacentto the
tesseraand and are sometimesassociatedwith smoothplainsin
the tesserain general [Slyuta et al., 1988; Aubele, 1989]. The
abundanceof domeswithin the tesseraitself, however,cannot
be easily determined because of the extremely rough
topographycomprisingthe corrugatedterrain (Figure 2).
In summary,the trough and ridge type of tesserain Laima
Tessera is characterized by a distinctive pattern of
throughgoing troughs/lineaments, and orthogonally oriented
AlthoughLaimaTesseraitself is surrounded
by plainsridgesandvalleyscomprising
thecorrugated
terrain(Figure2).
interpreted
to be of volcanicorigin[Basilevsky
et al., 1986], We now proceed
to examinethe basiccharacteristics
of the
thetessera
terrainitselfdoesnotappear
to displayevidence
of terrestrialseafloorin order to compareand contrastthe
abundant
volcaniccenters
exceptfor thesmooth
flat floorsof characteristics.
the trough/fracturezones, and the beaded areasor ovals within
these
zones
(Figure
1).A fewelongate
dome-like
features,
having
dimensions
of40kmx 70kin,areoriented
parallel
to
3.NATURE
OF
THESE•RFORMED
ATSPREADING
CENTERS
thefabricof thecorrugated
terrainandareof possible
volcanic
The terrestrialseafloorpossesses
threemajorlandformsin
origin (Figure2a). Smallerdome-likehills (<20 km diameter) areasof oceanicridges:(1) the linearrise crestat the spreading
HEAD: VENUS TESSERA-EARTH OCEANIC CR•T
)dqALOG?
7123
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......:. -:. ":-.- .'t' '%.....---::
'..:?'.-:
•..:.-:
..'•:-::•.'?....•:.:%)•:•
'..:•i::.
::::•."•"'-::;'-•i:'i'.;-'•.
:-:-•.-..•
'•
;'..•:":::'•..;.:'!•:..--'
....... ".!:':.:.7:
':--.-.::'•:
'::
i:i'::'i:•:
-•:.::•:..:--t:'•:•;..:..?'.½..,
:?.•i:},
."•:•c::::?ii:iii,
i:::
...........
'-:::..;.:.•..
:.•.?:::'-')
'"•:•.-:..!:'
•":--•
.............
=======================
:i:':',
½•/***
';'-:':. ..::.
.........
::':•-..i?,---..:•:::•
......
'.......... :" ":•
'....
:':;:'?':""
':•:'½;•"";'•;'"
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Fig. 2a. Venera 15/16 imagesand sketchmapsof the troughs/fractures
and theocorrugated
terrainin Laima Tessera.T,
o
troughs;G, grooves;L, lineaments.
Southeastern
LaimaTessera,centeredon 51 , 48 N; widthof imageis 500 km. The
generallyorthogonal
natureof the fabricis clearlyvisible,with troughs(paireddarklinesparallelto the long axisof photo
with hatchuresindicatingdepressions)and parallel fractures(dark lines) orthogonalto the shorter,more closely spaced
linear hills of the corrugatedterrain.Note alsothe circulardepression
of probableimpactorigin and the two linearhills.
7124
tLEAD: VENUS TESSERA-EARTH OCEANIC CRUST ANAIX)G?
0
,
km
100
II
I
,
\
Fig.2b.Eastcentral
LaimaTessera,
centered
on55ø, 52.5øN;
height
ofimage
is 180kin.Notethetrough-like
nature
ofthe
majortrough/fracture
in the southeastern
part of the image(Mots Chasma),how it widensin the centralpart and then
changesto a narrowfracturein the northwest.Most elementsof the orthogonal
corrugated
terrainterminateagainstthe
trough/fracture,while some extend acrossit.
o
I
lOO
i
•
km
Fig.2c.CentralLaimaTessera,
centered
on48ø, 53.5øN;heightof imageis 110kin.Notethevariation
in widthof the
trough(Baba-jagaChasma),the development
of smalloval structures
filled with volcanicplains(center),andits evolution
intoa fractureon the northwest
sideof the image.Corrugated
terrainis orientedgenerallynormalto the trough/fracture.
A
mountain40 x 50 km, of possiblevolcanicorigin,is locatedin the southeast.
LAIMA
I^
Venera16
I
I
2/16
TESSERA
aaba-JagaC.
Mots
C. IMezas-Mate
C.
Kozhla-Ava C. /
/
/KaygusC.
I
I
mountainsor abyssalhills). These abyssalhills producea
closelyspacedpatternof linear ridgesand valleysextending
away from the rise crest,and orientedgenerallynormal to the
transforms/fracturezones [Macdonald, 1986].
Following the terminology of Fox and Gallo [1986],
Sandwell and Schubert[1982] and Kastens [1987], transforms
are defined as fault zones(along which strike slip motion is
Fig.2d.Venera
15/16altimetry
profde
across
Laima
Tessera
showingoccurring)
connecting
offsetrise-crest
segments
andoriented
the
topography
associated
withnamed
troughs
andchasma
(labeled
generally
orthogonal
to therisecrest(Figure
3). Fracture
aboveprofile) and unnamedtroughsor troughextensions(labeledTR
below
profde).
Location
ofprofile
shown
inFigure
1.
zones
aretheaseismic
along-strike
extension
of transform
faults (Figure 3). For this analysis,the term fracturezone (FZ)
is used to refer to both fracture zones and transform faults,
center, (2) orthogonallyorientedpatternsof transformsand unlessindicatedotherwise.
fracture
zones[Lonsdale,
1977;Macdonald,
1986;Fox and
Fracture
zones
arepervasive
in theocean
basins
(Figure
3)
Gallo,1986;Kastens,
1987](Figure3), and(3) linearridgesandmayrangefrom20 to 60kmin width,consisting
of ridges
and troughsthat are formedparallelto the rise crest(rifted andtroughs
parallelto thegeneralstrikeof thezone.A typical
HEAD: VENUS TESSERA-EARTHOCEANIC CRUST ANALOG?
7125
Morphology•
Structure
_
/
//
•
Tr,..__•
.,.,.F
1t
--
•/ /
,/
'--- ......
,--*-'*-•
)>:_//:.
f, /
'- -O? ",.,,....,•/•,,/
1•
Transform
...,,,
Fracture
Zone
Domain
(20-eO
Transv©rs©
•
',,.•.•
FZ Valley
Fig. 3a. Earth'soceanfloor.Morphology,
structure,
andnomenclature
of fracturezone/transforms,
andabyssalhills
(modified
fromFoxandGallo[1986]).At thetop,therift valleysareseparated
by about100km alongthetransform
fault
andthegeneraltopographic
characteristics
of the abyssal
hills,thetransform,
anda smallpartof its aseismic
extensions
are
shown.The structure
andnomenclature
are shownbelow. NVZ, neovolcanic
zone;TFZ, transform
fault zone;TTZ, transform
tectonizedzone;PTDZ, principaltransformdisplacement
zone.SeeFox and Gallo [1986]for details.
fracturezonecontainsa centraltrough5-20 km in widththat andvolcanism
is oftenlocalizedwithinfracturezones[Lowrie
hasregionalinwarddippingconvex-downward
slopesdownto et al., 1986]. Fracturezonesare observedto changetheir
the troughfloor. Subsidiarybasins,troughs,and ridgesare morphology
alongstrike.A sonograph
mosaicof the Charliepresentalongthe valley floor. Floor depthis a functionof Gibbstransform
andfxacture
zonein theNorthAtlantic(Figure
transform
offsetandmaybe asmuchasseveralthousand
meters 3b) showsits distinctlylinearappearance
overshortdistances
[FoxandGallo,1986]dueto differences
in ageandsubsidence
andits complexityalongstrikeandalongits flanks[Searle,
level acrossthe fracture zone. Ocean floor topography 1979].
typically changesacrossthe fracture zone due to the
In map view, fracturezonescan be linear,arcuate,or
juxtaposition
of crustand lithosphere
of differentages. slightlysinuous,
depending
on a number
of factors
including
Transverse
ridgesoftenparalleltheedgeof thefracture
zone changes
in spreading
rate,location
andvariability
of polesof
valley(Figure3a). The actualmovement
at anygiventime rotation,
andinterplate
andintraplate
deformation
subsequent
occurs
primarilyalonga narrowtransform
faultzone(TFZ),but to crustalformation[Candeet al., 1988].On Earth,fracture
themovement
canmigrate
andsuchmigration
tends
to producezones(FZ) reachthousands
of kilometers
in lengthandare
a broader
transform
tectonized
zoneCITZ)(Figure3a).Talusand apparently
limitedonlyby thewidthof theoceanbasin.
sedimentation
oftenobscure
thedetailsof thefloorstructure, The spacingbetweenmajorFZs is relativelyconstant
7126
•:
t
VENUS TESSERA-•TH
•C
CRUST ANALOCr?
-' .... ......
-..
..
--.
Fig. 3b. Section of sonographmosaic and tectonic bte•retation from •e sou•em tr•sfo•
fault (str•bg essentia•y
east-westM •e •nff• pa• of •e •age] of •e Charlie-GibbsFracture•ne • •e No• Afl•tic Oc•n. •s•fficafion is
fr• no• to sou• [from Searle, 1979].
betweenany pair of adjacentFZs, but the spacingbetweenFZs al., 1959]whichcoverupto 80%of theseafloor[Menardand
is variable along the length of an oceanic rise. Fracture zone Mammerickx, 1967]. Abyssal hills are elongate, oriented
spacing increaseswith spreadingrate [Fox and Gallo, 1984; generally parallel to regional magnetic anomalies,and normal
Sandwell, 1986; Abbott, 1986]. Average spacingin the North to fracturezonesand possessrelief of 40-1000 m and widthsof
Atlantic is about 55 km [Macdonald, 1986; Vogt, 1986]. In 2-35 km [Krause and Menard, 1965]. Sonographsof the Midregions of complex plate interactions,there are fracture zones Atlantic Ridge show the linear nature and relatively regular
which convergetoward each other, such as the Mendocinoand spacingof abyssalhills (Figure 3c [Laughton, 1981]). R. A.
Pockalny et al. (A morphological comparisonof abyssalhill
BlancoFZs boundingthe Gordaplate [Stoddard,1987].
A distinctivelinear fabric of abyssalhills occursparallel to topography using high-resolution, multibeam bathymetry
the rise crestand normal to the fracturezones(Figures3a and data, submitted to Journal of Geophysical Research, 1989)
3b). Along the Mid-Atlantic Ridge (MAR), normal faults that reviewed three models for the formation of abyssal hills: (1)
dip toward the rift valley develop within 1-5 km of the the horst and grabenmodel [Needhamand Francheteau, 1974],
spreadingaxis. This faulting createsthe inner walls of the rift whereabyssalhills were formedby inwardand outwarddipping
valley. Adjacent to this in some areas,in a zone 10-20 km normal faults;(2) the listric fault model [Harrison and Stieltjes,
wide, occur the relatively flat, faulted terraces, outwardly 1977], where listtic faulting occursto offset the effects of the
boundedby the outer wall, the rift mountains,and the abyssal inward facing faults of the rift valley walls, rotating crustal
hills [Macdonald, 1986]. The major scarps are generally blocks and producing hills shaped triangularly in cross
continuousalong the 40-60 km length betweenfracture zones section, which result from the rotated half grabens; (3) the
[Macdonald, 1986]. The nature of the transformation of the rift episodicmagmatismmodel [Kappel and Ryan, 1986; Pockalny
valley outer wall into the abyssal hills is a matter of et al., 1987; Barone and Ryan, 1988], where abyssal hill
controversy but may involve tilting of the rift valley walls topography is generated by the interplay between episodic
back to a horizontalposition[Verosub and Moores, 1981], or magmatismand extensionaltectonism.
overprintingby outwarddippingnormal faults [Macdonaldand
Recentwork (R. A. Pockalnyet al., submittedmanuscript,
Atwater, 1978], or a combinationof both. The presenceand 1989; P. J. Fox, personal communication, 1989) has shown
wide occurrenceof abyssalhills distally from oceanicrises was that the spacing of abyssal hills is inversely related to
noted early in the explorationof the ocean floor, and they were spreading rate. Slow spreading ridges (<30 mm/yr) have
used in the definition of morphotectonicprovinces[Heezen et relatively high abyssal hills (about 200 m) and an average
HEAD:VENUSTESSERA-EARTH
OCEANICCRUSTANALO6'?
7127
29øW
29øW
39øN
30øW
..
....
';';•::;;:
;5;
39øN
30øW
o
I
5o
,,
I
lOO
I
km
Fig. 3c. Two paralleltwo-sidedsonographs
on the crestof the Mid-AtlanticRidgenearthe Azoressin,wingthe linear
tectonicfabricof the abyssalhills.The sonographs
havebeendigitized,corrected
for slantrangeerrors,andmergedby the
JetPropulsion
Laboratory.
Widthof imageis approximately
100km [fromLaughton,1981].
spacing
of 10 km. Abyssal
hills derivedfromslowto 4.2 km).Overall,
thevariation
in thelength
of abyssal-hill
intermediate
spreadingridges(-40 mm/yr) have an average ridgesalongstrikeis 10 km to 40-50km.
spacing
of 8.2 km.Abyssal
hillsassociated
withintermediate- In somecases,
thebasicorthogonal
pattern
canbemodified
to-fastridges(-100 mm/yr)showan average
spacing
of 4.4 by several
processes.
Actonet al. [1988]described
a complex
km, andthosefromfastspreading
ridges(-160 mm[yr)areless curved seafloorfabric in the Galapagosrise which they
elevated(50-60m) andlesswidelyspaced(averagespacingof interpretedto be causedby rift propagation
and changesin
7128
HEAD: VENUS TESSERA-EARTH •C
CRUST ANALO•
but not the more
spreadingrate. Menard [1984] showedthat serrateridges and can see surficial expressionsof troughs/ridges
transforms could develop during asymmetrical spreading. subtle effects of the fracture zone thermal structure. Additional
Stoddard [1987] describedsinuousand complex patternsof high-resolutionaltimetry data are required to determine the
magnetic lineations and abyssal hills on the Gorda plate actual width of the zones or domains in which the troughs
locatedbetweenthe convergentBlanco and Mendocinofracture occur,but preliminaryanalysisof Venera 15/16 altimetrydata
zones, and he analyzed models related to mechanismsof (Figure 2d) suggeststhat they may be wider than the trough
rotation of rigid subplatesand nonrigid deformationto explain observed in the images. The two features (FZs and
are comparablein their generalparallelism
the origin of these patterns on the plate moving toward the troughs/fractures)
and spacing,althoughdetailed analysesof tesseraterrain are
apexof the two fracturezones.
In summary,oceaniccrust is commonlycharacterizedby a required to establish the full range and variability of
distinctive orthogonal pattern (fracture zones and abyssal trough/fracturespacingon Venus. The two featuresare also
hills) whose orthogonalelementsdiffer in their characteristics comparablein their general morphology(Figures 1-3), with
in the two directions(Figure 3). We now proceedto compare both having similar shapes and along-strike variations
the generalfabric of the oceanfloor to the fabric of the trough- (changesin strike, basins).
and-ridgetesseraon Venus.
4.3. Abyssalhills and corrugatedterrain. Oceanic abyssal
hills are similar to the ridges and valleys of the corrugated
4. COMPareSONS
terrain in their distinct parallellism and their consistent
occurrence and relatively constant widths over large areas
The basic dimensions, spacing, and geometric
(Figures2, 3a, and 3c). They are also similar in terms of the
characteristicsand relationshipsbetweenthe terrainsexamined
fault-like
boundariesof the valleys and ridges,in their spacing
on Venus and Earth are summarized in Table 1, and this
(Table 1), and in their relation to the troughs/fracturezones
providesa basisfor comparisonof similaritiesand differences
(predominantly terminating against them). More detailed
between the two terrains. There are numerous
similarities
analysesof the spacing of the elementsof the corrugated
betweenthe componentsof the trough-and-ridgetesseraterrain
terrain are needed to establishvariability within and between
type and basic terrestrialseafloorfabric and structure.
domainsand between different areasof tesseraand to compare
these to terrestrial data which show relations of spacing to
spreadingrates (R. A. Pockalny et al., submittedmanuscript,
1989). Additional data (radarclinometry) are also needed to
TABLE 1. Comparisonof Dimensionsof VenusTroughsand Ridges
establish the heights and slopes of the elements of the
and EarthFractureZonesand AbyssalHills
corrugatedterrainon Venus.
Significant differencesexist between the environmentand
Dimensions. km
characteristics of the Earth's seafloor and the general
environmentof Venus and the trough-and-ridgetesseraas seen
Venus troughs
Earth fracture zones
Len•
Widtl•
Sp•ing
up to 1400
up to 4000
10-12
5-20
-50
30-70
Dimensions.
Length_
Venusridges
Earth abyssalhills
20-100
10-50
km
Heidi
Sp•$igg
?
0.05-0.2
6- ! 2
4-10
in Laima Tessera.
4.4. Differences in environmentbetweenVenus and Earth.
Venus is characterizedby much higher surface temperatures
than the Earth (450øC) and the lack of oceans.These factors
have potentially important influences on seafloor spreading
processesin terms of crustal formation processes,crustal
modification processes, and hydrothermal cooling.
Assessmentof terrestrial spreading centers under Venus
environmental conditions [Sotin et al., 1988, 1989a,b]
indicates that the major differences between Venus and the
Earth
4.1. Orthogonal orientation of different components.
Oceanic abyssalhills and fracture zones bear relationshipsto
eachother similar to thoseof elementsin the corrugatedterrain
and troughs in the tessera, i.e., a much greater length of
troughs and fracture zones, and orientation of the shorter
corrugated terrain elements and abyssal hills orthogonal to
troughsand fracturezones.
4.2. Fracture zonesand troughs/fractures.
Oceanicfracture
zonesand tesseratroughs/fractures
are similar in their general
linearity, in their often arcuate tendency, in their length
(measuredin hundredsof kilometerswith the largestextending
over 1000 km across the width of Laima Tessera), and in the
width of the centraltrough(Figures1-3 andTable 1). On Earth,
the "transformdomain"is deftnedas the width of the portionof
the seafloororientedperpendicularto the strike of the FZ that
is affectedby the presenceof the FZ itself (i.e., thinnedoceanic
crust) (see Figure 3a). With existing Venus imaging data, we
would
be
the
influence
of
the
enhanced
surface
temperatureon upper mantle temperatureson Venus and the
resulting increase in crustal thickness and elevation of
isostatically compensated topography. Crust on Venus
producedat averagespreadingcenterswould be about 15 km
thick, in contrastto the average crustal thicknesson Earth of
about 5 km. Along-strike variations in upper mantle
temperaturesat Venus spreadingcenters (e.g., Icelandic-like
hot spots) could produce enhanced crustal thickness and
increased isostatically compensated topography (e.g.,
Icelandic-like plateaus). Application of these concepts to
Western Aphrodite Terra (Figure la), proposedas a site of
crustalspreadingon Venus [Head and Crwnpler, 1987], showed
that the topographyand gravity datafor Ovda Regio in Western
Aphrodite were consistent with the hypothesis of crustal
spreading[Sotin et al., 1988, 1989a,b]. Specifically,much of
the crust on the flanks of Western Aphroditemay be about 15
km average thickness and could be produced by crustal
spreading.Ovda Regio, a central oval-like plateau along the
Aphrodite rise, appears to be an area of enhanced crustal
}IF.M): VENUS TESSERA-EARTH •C
CRUST ANALOG?
7129
thickness(about 30 km) which was initiated by an increasein explanationis requiredfor 4. The apparentlack of trough-and-
uppermantletemperature
of about100øCandwhichhasbeen ridge patternin the lowlandscouldbe due to volcanicflooding
spreadingat about 1 cm/yr for the last 200 m.y.
In general, the environmentalvariations between the two
planets modifies but does not seem to preclude crustal
spreadingprocesses.Left undeterminedby existing studiesis
the influence
that the different
environmental
conditions
will
have on the nature, morphology,and spacingof abyssalhills
and fracture
zones
formed
in the Venus
environment.
These
factors are of the utmost importancebecauseit is known that
the thermal structureand theology of the oceaniccrest and the
interplay of volcanism and tectonismhave an important but
not yet fully understoodinfluence on the characteristicsof
fracture zones and abyssal hills on Earth. Some of the
variationsin otherwisegenerallysimilarfeatures(Table 1) may
be due to these factors, although it cannot be ruled out that
differencesin the environmentprecludethe formationof these
featuresin a Venus crustal spreadingregime. Thus particular
attentionmust be paid to the continuallyimprovingmodels for
the formationand evolutionof fracturezonesand abyssalhills
on Earth and the application of these models to the Venus
environment. Continued investigation of the nature of
spreadingprocessesunder Venus conditionswill also be of
significancein terms of understandingthe Earth's Archean, a
time when the thermal structureof the Earth is thoughtto be
similar to that of present-dayVenus.
4.5. Plateau-like nature of the trough-and-ridgetessera.
Tessera terrain in general lies at elevations of 2 km (+1 km
standarddeviation [Bindschadlerand Head, 1989b]) above the
mean planetaryradius, while most plains units (which do not
show the distinctive patternsof the trough-and-ridgetessera)
lie at or within 1 km of mean planetaryradius.Two types of
topographicboundariesare observedat the edge of the troughand-ridgetesseraterrain: (1) abrupt,where the edge is marked
by a distinctive topographic change over relatively short
distancesand by a zoneof deformation,as in Kamari Dotsum at
the eastern edge of Laima Tessera (Figure 1), and (2)
transitional,where the trough-and-ridgetesseraterrain slowly
decreasesin elevation and is embayedby the volcanic deposits
forming the lowland plains, as in southernand westernLaima
Tessera(Figure 1). There are severalpossibleexplanationsfor
theseobservations,many of which can be testedwith existing
data and data from the upcoming Magellan mission: (1) the
trough-and-ridgetessera are high because they represent the
thermally most youthful part of a crustal spreading system.
This would require that spreadingcenters be located within
zones of trough-and-ridgetesseraand that they be currently
active. (2) The trough-and-ridgetesseraare high becauseof
enhanced
crustal
thickness
due to increased
crustal
thickness
of the pattern, as suggestedby the embaymentrelationships
along the southernand westernmarginsof Laima Tessera(and
preferentialpreservationof tesseraterrain in areasof younger
and/orthicker crest). In this case,lava thicknessesin the range
of severalhundredmeterswouldbe requiredto coverthe texture,
and one would expectto see transitionalareasand patchyareas
of exposedtrough-and-ridge
tessera.Sukhanov[1986] cited the
patternsof irregularpolygonsin volcanicplains units that are
adjacent to some tessera as evidence for regions of buried
tessera.On the basis of the distributionof thesepatterns,he
proposedthat the actualabundanceof tesseramight be a factor
of two more than what is presently exposed, perhaps
representing some global process of "tesserization."
Comprehensiveanalysisof existingand Magellan data will be
required to evaluate fully this possibility. Alternatively, if the
trough-and-ridgepattern is producedpreferentially in regions
of thicker crest, perhapsthere is somefactor in the production
of normal thicknesscrust at Venus spreadingcenters(different
thermal regime or different ratio of extrusionto intrusion)that
might result in the lack of productionor preservationof the
orthogonal texture (i.e., thicker crust is better preserved than
thinner crust, or only thick crest exhibits the orthogonal
texture).Additional analysesand data are clearly requiredto test
theseideas and to establishthe relationshipof the trough-andridge tesserato the surroundingplains.
Severaladditionalpropertiesof tesseramay help to resolve
the origin of the trough-and-ridgetessera.Tesseraein general
are characterizedby distinctive and anomalouslyhigh surface
roughnessat a range of scalesfrom centimetersto decameters
comparedto plains units in general [Bindschadlerand Head,
1989b]. Localized volcanismand pervasivefaulting producing
rift mountains/abyssalhills are responsiblefor the abnormally
rough topographyat the centimeterto decameterscale seenon
the seafloor [Gallo et al., 1984, Figures 8-9; Searle, 1984],
which is very similar to the type of pervasiveroughnessseen
in the tesseraterrain [Bindschadler and Head, 1988a, 1989b].
High-resolution data obtained by the Magellan mission will
permit the correlationof areasof enhancedsurfaceroughness
with specificgeologicfeaturesand will allow furtherevaluation
of the similarities
and differences
between
the features
on
Venus and Earth.
Line-of-sight (LOS) gravity data exist for all or part of
severaltesseraregions(Tellus Regio, Laima Tessera,and Alpha
Regio) [Sjogrenet al., 1983]. Maps of LOS gravity anomalies
show that these three regions are characterizedby very small
anomaliesdespite their topographicelevation, in contrastto
the very large anomalies associatedwith other areas of high
topography, such as Beta Regio. The small LOS gravity
anomalies,togetherwith the high topographycharacteristicof
the tessera,may indicate compensationdue to crustal thickness
variations or shallow mantle processes.More widespread
coverageand higher resolutiongravity data will be of extreme
importancein distinguishingbetweenmodelsfor the formation
and evolution of tessera in general, and trough-and-ridge
tesserain particular.
productionalong strike at the spreadingcenter(the Iceland hot
spot effect). This would require evidencethat this processis
operatingat spreadingcentersand would predictthat the terrain
is in isostaticequilibrium.(3) The trough-and-ridgetesseraare
high because of enhanced crustal thickness related to
deformationalprocessesoperatingsubsequentto its formation
in the spreading center environment. This would seem to
require significant surface deformationreflecting the deeper
processesof crustal thickening. (4) The trough-and-ridge
tesseraare completely unrelated to processesassociatedwith
5. DISCUSSION AND CONCLUSIONS
crustal spreading and the morphological and geometrical
On the basis of the similaritiesin morphology,geometry,
similaritiesare unrelatedin tern of processes
of formation.
Explanations1-3 must also accountfor the apparentlack and spacing,it is concludedthat processesanalogousto those
of the trough-and-ridgepattern in the lowlands,while no such responsible for the ocean floor fabric on Earth are good
7130
HEAD: VENUS TESSERA-EARTH•C
CRUSTANALOG?
candidatesfor origin and productionof the trough-and-ridgevery similarto thoseof the tessera(suchas LaimaandTellus)
tesseraterraintypeon Venus.In thismodel,elongatetroughs in the regionscoveredby Venera 15/16. High-resolution
are analogous
to fracturezones,and linear elementsof the Magellandatawill permita comparison
betweenLaimaTessera
corrugated
terrainto abyssalhills; the trough-and-ridge
tessera and the terrain and structurein Aphrodite Terra and the nature
originatedat a rise crestanalogous
to divergentboundaries
and and relationshipof the intervening region.
The more complex tessera patterns observed by Venera
spreadingcenterson the Earth'sseafloorand evolvedto its
present morphology and configurationthrough similar 15/16 (subparallel ridged terrain and disrupted terrain
[Bindschadlerand Head, 1988b] are not discussedin this paper
processes
of crustalspreading.
If trough-and-ridgetesserafabric is producedby crustal but could be due to deformationof the basic trough-and-ridge
spreading,where is the spreadingcenter located in Laima tesserapattern (analogous to more complex patterns seen on
Tessera?On the basis of the asymmetryof the fabric, candidate the terrestrialseafloor [Acton et al., 1988; Stoddard, 1987]), or
spreadingcenterswould be expectedto be foundparallelto the to complextectonicpatternsfrom other sourcesof deformation
corrugatedterrain and normal to the trough/fractures,
oriented (Bindschadlerand Head, Models for the origin and evolutionof
in an approximatelyNNE-SSW direction (Figure lc). If the tesseraterrain, Venus, submitted to Journal of Geophysical
crust and lithosphereis relatively old, topographicvariations Research, 1989). Recognitionof the key basic patternsof the
due to thermalevolutionmay no longerbe detectable.At active trough-and-ridge tessera (long troughs/fractures and short
spreading centers on Venus topographic rises should be orthogonal corrugated terrain) could provide a "marker"
preserved and detectable for relatively slow spreadingrates structuraltype and help in decipheringthe origin of the more
[Kaula and Phillips, 1981; Phillips and Malin, 1983]. complex tesseraoccurrences.
In conclusion, the similarities in morphology, geometry,
Alternatively, regional slopesmight provide information on
and
spacingof elementsof the trough-and-ridgetesserato the
the direction to higher, younger crust and lithosphere.
Preliminary analysis of topograpic contour maps (500-m terrestrial ocean floor fabric suggest that trough-and-ridge
of divergenceand
contour interval) has revealed no distinctive local rises, but tesseramay haveformedby similarprocesses
crustal
spreading
on
Venus.
This
hypothesis
can be further
more detailed analysisis underwayusing individual altimetry
tested
by
detailed
analyses
of
the
nature
of
Laima
Tesseraand
profiles. Although there does not appear to be a systematic
trend in regionaltopographyin an E-W direction,the extentof other tessera in the northern hemisphere, application of
volcanic flooding of Laima Tesseraappearsto be greaterin the models for the formation of fracture zones and abyssalhills on
west (Figure lc), suggestingthat the easternportion of Laima Earth to the conditions of the Venus environment, and the
may be slightly higher, and thus a candidatefor youngercrust analysis of global high-resolution imaging, altimetry, and
and lithosphere. The eastern boundary of Laima Tessera is gravity data from the Magellan mission.
characterizedby a complex NNW striking ridge belt and a
Acknowledgments.The financial supportof National Aeronautics
topographic drop of several kilometers, suggesting the
presenceof a tectonicboundary(Figure lc). Detailedmapping and Space Administrationgrants NGR-40-002-116 and NAGW-713,
from the Planetary Geology and Geophysics Program of the Solar
from Venera 15/16 altimetry and imagesof Laima Tesserais System Exploration Division, Office of Space Science and
presenfiy underway to documenttopographictrends and to Applications,are gratefully acknowledged.Mary Ellen Murphy, Angel
Hilliard, and Peter Neivert assistedgreatly in the preparationof the
locate possiblespreadingcenters.
The trough-and-ridgetesseraterrain in Laima Tesseralies manuscript.Particularthanksare extendedto Duane Bindschadler,Ellen
between Ishtar Terra in the north (a region of compressional Stofan, and Marc Parmentier for helpful discussionsrelating to
conceptsin this paper and to John Mustard, Jack Hermance, Scott
deformation and crustal thickening [Crumpier et al., 1986; Murchie, David Senske,Larry Crumpier,RichardVorder Bruegge,Kari
Vorder Bruegge and Head, 1989]), and equatorialregionsof Roberts, Sharon Frank, John Grant, Susan Keddie, Jack Kliever, Phil
extensionaldeformation [Schaber, 1982] in AphroditeTerra to Horzempa, and Annette deCharonfor reviews of earlier drafts of this
the south (Figure la). Patterns of topography, morphology, paper. Helpful comments and reviews of the final manuscriptwere
provided by Paul Morgan, Robert Pockalny, Roger Seafie, and Paul
and structurecomparableto terrestrial spreadingcentershave Stoddard.Thanksare extendedto the USSR Academyof Sciencesfor
been observedin the Western Aphrodite Terra region several permissionto use the data and to colleaguesat the VernadskyInstitute
thousands
of
kilometers
to
the
south
of
Laima
Tessera
for helpful scientific discussions.
[Crumpieret al., 1987;Head and Crumpier,1987;Crur•ler and
Head, 1988] (Figure la). Although high-resolutionimages of
the Venera
15/16 mission are not available at these low
Abbott, D., A statisticalcorrelation between ridge crest offsets and
latitudes, evidence exists from Pioneer Venus and Arecibo data
spreadingrate, Geophys.Res. Lett., 13, 157-160, 1986.
for the presence of a spreading center. Observationsfrom Acton, G., S. Stein, and J. F. Engeln, Formation of curved seafloor
fabric by changesin rift propagationvelocity and spreadingrate:
Western Aphrodite Terra show (1) a segmentedrise crestoffset
Application to the 95.5øW Galapagos propagator,J. Geophys.
right-laterally
along fracture zone-like cross-strike
discontinuities(similar to the troughs in Laima Tessera); (2)
bilaterally symmetrical topography normal to the rise crest;
and (3) a thermal boundary layer topography suggesting
spreadingrates of about a centimeter a year and Iceland-like
plateaus.Also revealedin the moderateresolutionimagesand
topographydata are patternsof orthogonalelementsparallel to
the rise crest and in the general direction of the cross-strike
discontinuities (Crumpier and Head, Crustal spreading on
Venus: Evidencefrom topography,morphology,symmetryand
map patterns, submitted to Tectonophysics, 1989). In
addition,the radarpropertiesof the WesternAphroditeIcelandlike plateausalong the rise crest(Ovda andTheftsregiones)are
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dome-like hills on Venus, Lunar Planet. Sci. XX, 28-29, 1989.
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plate boundaryzone of the southernsegmentof the Endeavour
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D. B., andJ. W. Head,Diffusescattering
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D. B., andJ. W. Head,Definition
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(ReceivedJune 14, 1989;
revisedAugust14, 1989;
acceptedAugust14, 1989)