JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 91, NO. B7, PAGES 7545-7554, JUNE 10, 1986
A Comparison of the Regional Slope Characteristicsof Venus and Earth'
Implications for Geologic Processeson Venus
VIRGIL L. SHARPTONI AND JAMESW. HEAD III
Departmentof Geolo•7icalSciences,Brown University,Providence,Rhode Island
The range of 3ø by 3ø regional slopesof the earth and Venus is similar (approximately0.0ø-2.4ø);
however, the surfacedistribution of these values differs significantly.On earth, cratonic and abyssal
plainsform extensiveregionsof 0.0ø slope.Within theseregionsa variety of features(mid-oceanridges,
volcanic island chains, subduction zones, and folded mountains) have regional slope characteristics
influencedby seafloorspreadingand plate recycling,as well as an activeweatheringregime.Continental
margins form laterally continuouszones of relatively high slope (passivemargins up to 1.9ø; active
margins up to 2.4ø).The plains provincesof Venus are much more rugged than earth's plains and are
marked by numerouscloselyspacedcircular and linear features(0.1ø-0.2ø regional slope)concentrated
into broad linear zones of global extent. Although Venus highlandsare bounded by narrow zones of
relatively steep slope, the margins of Aphrodite Terra and Beta Regio are not as steep as earth's
continentalmarginsand appear to be best developedparallel to the trends of major chasmatawithin
theseregions.Linear trendsformed by somehighlandmargin segmentsextendinto the plainsprovinces
as rugged belts denselypopulated with circular and linear features.Ishtar Terra's margins are significantly steeperand more continuousthan other highlandmarginsand are comparableto passivemargins
on earth. The Venus highlandsdo not contain appreciablesmooth, flat interior regions,implying that
highlandtopographyis not significantlymodifiedby erosionor deposition.Systematicvariationsin the
density of plains features,elongate planitia, highland margin trends, and aligned highland topography
form several major great-circle-like patterns oriented at generally less than 45ø to the equator and
differingin characterfrom both the mosaiclikepatternsof terrestriallithosphericplates,and the subdued
tectonic fracture grids of the smaller terrestrial one-plate planets (the moon and Mercury).
INTRODUCTION
Current efforts to understand the geology of Venus have
relied strongly upon the documentationand analysis of this
planet's topographic characteristics[Petten.qill et al., 1980;
Masursky et al., 1980; Arvidsonand Davies, 1981; Kaula and
Phillips, 1981; Solomonand Head, 1982; McGill et al., 1983;
Phillips and Malin, 1983: Sharpton and Head, 1985]. Global
coverageof Venus is limited to that derived from the Pioneer
Venus (PV) radar experimentwhich provided altimetry as well
as radar reflectivity and small-scaleroughnessmeasurements,
each at approximately 100 km resolution [Petten,till et al.,
1980; McGill et al., 1983; Head et al., 1985]. In addition,
higher-resolution images of the surface, from the Soviet
Venera 15-16 spacecraft (approximately 1-2 km resolution
[Barsukovet al., 1986; Basilevskyet al., 1986]) and Arecibo
radar measurements(1-3 km resolution [Campbell et al., 1983,
1984]) are beginning to provide insight into the nature of
geological processesoperating at the regional scale. These
images, however, cover only about a third of the surface of
Venus.Thus the PV data setscurrentlyprovide an unique and
important synoptic perspectivefrom which to evaluate the
global significanceof featuresand processesrevealed in the
high-resolutiondata.
(2) upland rolling plains which are extensiveregionswith elevations from 0.0 km to 2.0 km (this provinceincludesapproximately 65% of the surfaceof Venus),and (3) highlandswhich
include those surfaces above 2.0 km and make up approximately 8% of the total mapped surfaceof Venus.
Like elevation, regional slope is a scale-dependenttopo-
graphic relationshipwhich is fundamentalto the description
of a geologic surface [e.g., Selby, 1982; Scheidde•ter,1970].
Regionalslopedescribesthe planar surfacegradient measured
over a given area [Sharptonand Head, 1985]. As it is the first
scalar derivative of topography, it emphasizeshigh-frequency
variations in the data. Spatial variations in regional slope
values thus characterize the texture or topographic roughness
of a surface. To understand
better the nature
of the infor-
mation contained in the large-scaleVenus and earth topogra-
phy, we have assessedand compared the 3ø by 3ø regional
slopecharacteristicsof thesetwo planets.Elsewhere[Sharpton
and Head, 1985] we analyzethe global statisticsresultingfrom
this study and treat the effectsof removing the load of the
earth's oceans on regional slope measurements.Here we ana-
lyze the surfacedistributionof regionalslopevalues,examine
the large-scaletextural variationsof Venus and earth topography, and discusstheir implicationsregardingthe nature of the
geologicalprocesses
whichhaveshapedthe surfaceof Venus.
On the basis of elevation characteristics derived from PV
The global topography data for Venus [Petten,till et al.,
topography,Masurskyet al. [1980] have divided the surface
1980] and earth [Gates and Nelson, 1975a, b] are comparable
of Venus into three major provinces:(1) lowlands which are
in resolution:1ø by 1ø spatial resolution,100 m vertical accuregions below the Venus datum (0.0 km equals a planetary
racy [Sharptonand Head, 1985]. The slopeof the leastsquares
radius of 6051.0 km) and compriseabout 27% of the surface,
planar fit to the nine griddedelevationdata pointswithin each
3ø by 3ø region of the topographywas calculated,and maps
• Now at Earth PhysicsBranch, Energy, Mines and Resources were assembledusing a spatial filtering technique(e.g., Moik,
Canada, Ottawa, Ontario.
1980]. The regional slope maps of earth and Venus are preCopyright 1986 by the American GeophysicalUnion.
Paper number 5B5653.
0148-0227/86/005B-5653505.00
sented in Plate
1.
Regionalslopesrange from 0.0ø to approximately2.4ø for
both planetary surfaces,although the spatial distribution
7545
7546
SHARPTON AND HEAD: IMPLICATIONS OF REGIONAL SLOPE DISTRIBUTION--VENUS AND EARTH
Regional
.1
Slope Characteristics
.2
.:3
ß stable
.......
.4
.5
1.0
of Earth
1.5
2.0
2.5
degrees
cratons
Paleozoic
.........
mountain
Mesozoic
.............
belts
Recent
..........
,
and Venus
East
African
Rift
ß abyssal plains
ß.
' fast
.......
spreading ridges
slow
.........
seamounts
......
transforms
......
ee©eee
subduction
ß..
.... passive
......
zones
active
margins
lowlands
...... upland rolling plains
.................
margins
ß Lakshmi
Ishtar
Maxwell, Akna, Freyja
eeee
ß..
..................
interior
......
.....
margins
margins
Aphrodite
Beta
interior
........
chasmata
Fig. 1. Regionalslopecharacteristics
of selectedterrestrialand Venusiantopographicfeaturesdiscussed
in text. Values
representthe averagesurfacetilt of 3ø by 3ø regions.Solid lines give typical values' dots extend acrossthe full range of
slope valuesassociatedwith each feature.
(Plate 1) and the frequencyof occurrenceof theseslope values
differ significantly between earth and Venus [Sharpton and
Head, 1985]. Figure 1 summarizesthe regional slope values
characteristicof major featuresobservedon both planets.
REGIONAL SLOPE CHARACTERISTICS OF EARTH
(2-6 cm yr-1 [Heirtzler et al., 1968; Parsonsand Sclater,
1977]), is bounded by ill-defined flanks generally inclined less
than 0.2ø. Major fracture zones and transform faults associated with the East Pacific Rise can be recognizedas narrow
linear features trending at high angles to the ridge axis. As
these features exhibit slopes typically less than 0.2ø, the occurrence
Plains
and Related
Features
The stable interiors of the continents, as well as oceanic
abyssalplains, appear as extensiveregionsof 0.0ø slope in
Plate 1, top. At the scaleof this analysis,the continentalcratons appear flatter, broader, and more continuousthan the
ocean basins.
of transforms
and
fracture
zones
associated
with
slowerspreadingridgesis difficult to determinein Plate 1, top.
Less extensivelinear features of moderate slope define the
locations of seamountchainssuch as the Hawaii-Emperor and
the Tuamotu-Pitcairn Island chain, arranged in parallel systems in the central
Pacific.
These and other discrete linear
arrangementsof volcanic islands [Minster et al., 1974] are
The ocean basins of earth are marked by numerous linear produced as the lithospheremoves over relatively stationary
featuresindicative of a wide range of processes
related to sea- magma source regions in the mantle I-McDougall, 1971;
floor spreading.The flanks of mid-oceanridgesare expressed Wilson, 1963; Morgan, 1973; Burke and Wilson, 1976]. As the
as broad, low-slopinglinear regionsof marked continuityand volcanic topography migrates away from the hotspot, the
globalextent.The slopecharacteristics
of thesedivergentplate lithospherecools and subsidesunder the load, reducinglocal
boundariesare controlledby the spreadingrate of the ridge: relief toward the more distal (older) portions of thesechains.
Slow spreading ridges, such as the Mid-Indian and the Thus the flanks of these hotspot traces display a systematic
Mid-Atlantic(1-2.5 cm yr-1 and 1.25cm yr-1, respectivelypattern of regional slope magnitudesranging from lessthan
[Heirtzler et al., 1968; Parsonsand Sclater,1977]), are flanked 0.1 ø at the distal extents to 0.6 ø at the sites of the most recent
by relativelynarrow,well-definedregionsof 0.1ø-0.4ø regional volcanic activity (e.g., the eastermostislands in the aboveslope,whereasa fasterspreadingridge, the East Pacific Rise mentioned chains).
SHARPTON AND HEAD: IMPLICATIONS OF REGIONAL SLOPE DISTRIBUTION--VENUS AND EARTH
Finally, deep ocean trenchesstand out as narrow, distinct,
arcuate to linear bands of very high slope (0.3o-2.0ø) which
usuallyseparatebroad regionsof seafloorwith differingslope
expression.
The steepestregionalslopes,however,do not correlate with the deepesttrenches:the Philippine, Japan, and
Kurile subduction zones display the highest regional slopes
(approximately2.0ø),whereasmaximumregionalslopevalues
associatedwith the deeper Mariana and Tonga-Kermadec
7547
regional slope value when calculated over regions averaging
about 300 km in width. For passivemargins this component
would be associated with the continental shelf and rise, both
of which have topographic gradients much lower than the
continental slope. For active margins, such as western South
America, the occurrenceof adjacenttrench topography and, in
some cases,coastal mountain rangesprovides additional components of relief which serve to increasethe calculated retrenches are between 1.0ø and 1.5 ø. Because narrow features
gional slope significantlyover that which is characteristicof
are undersampledat this scale, slope characteristicsare passive margins. The anomalously steep regional slopes asstronglyinfluencedby the width of the subductionzone as sociatedwith the (passive)easternmargin of Australia may be
influenced by the Great Dividing Range, located on the east
well as the depth of the trench.
coast and containing the highestelevationsfound on the AusContinental Interiors and Mountain Rankles
tralian continent JOllier, 1982, and referencestherein]. Large
The stable continental interiors, comprising Precambrian regional slope values for the southern margin, however,
shieldsand the associatedplatform sediments,display broad appear to reflect the unusually steep gradient (6o-8ø) [Wilexpansesof flat-lying surfaces;on the global scalethesere- liams and Corliss, 1982]) of the continental slope associated
gions appear smootherand lower in slopethan the oceanic with this passivemargin. The disparity in gradient between
abyssalplains. This differenceprobably reflectsthe signifi- the steeper continental slopes found here and those of the
canceof erosionas a planationprocessabovesealevel,as well North Atlantic passive margins (Plate 1, top) could be an
as the great age of continentsrelative to the ocean floor. expressionof different degreesof thermal and depositional
Folded mountain belts display a wide range of slope mag- evolution related to the relatively recent (Paleocene) separanitudes corresponding,primarily, to the age of the orogeny: tion of Australia from Antarctica [Williams and Corliss, 1982]
openingof the North AtlanRegionsof Paleozoicmountainbuilding,suchas the Appala- comparedto the Triassic-Jurassic
tic
[Heirtzler
et
al.,
1968;
Dietz
and
Holden, 1970].
chians along the eastern margin of North America, have
slopes generally less than about 0.2ø. Mesozoic to Recent
mountain belts, such as the Andes and the Rocky Mountains,
REGIONAL SLOPE CHARACTERISTICS OF VENUS
have characteristicregional slopesof 0.1ø to 0.8ø. In Plate 1,
Plains and Related Features
top, the Tibetan Plateau,just north of the Indian subconIn contrast to the vast expansesof uninterrupted plains
tinent, is enclosedby a bright ring of high regional slope
(0.5ø-1.3ø).Lower regionalslopes(0.2ø-0.3ø) typify the interior which typify the continental interiors and oceanic aybssal
of the plateau and a distinct asymmetryto the slopesof the plains on earth, the extensivelowlands and upland rolling
plateau margin is apparent.The southernmargin (the Hima- plains provinces of Venus are marked by numerous closely
layan Front) is the siteof major thrustingand suturingassoci- spacedfeatureswith regionalslopesgenerallyranging between
ated with the collision of India with Eurasia and yields re- 0.1c and 0.2ø, values which are significantly steeperthan those
gionalslopevaluesup to 1.3ø. In contrast,the marginto the characteristicof terrestrial plains. In terms of distribution, size,
north appearsto be related to the northward flow of weak and regional slope magnitude these featuresmost closely reTibetan crustand uppermantle in responseto continuedcon- semble the flanks of eroded mountain belts such as the Appavergence[Molnar and Tapponnier,1978] and regionalslope lachians or the Yablonovgy-Stanovoy Ranges of eastern Asia
values(0.8ø maximum)are significantlylower. Thus evenat 3ø (45ø-60øN; 100ø-150øE).Additionally, on earth, some (fasterby 3øresolution,regionalslopevariationscanbe distinguished spreading)mid-oceanridge crests(Figure 1) and a variety of
whichreflecttheologicaland structuralheterogeneitywithin a features within the volcanically and tectonically active portions of the ocean floor (e.g., the western Pacific and Indian
major physiographicprovince.
oceans)are similar in regional slope magnitude to the VenusContinental Mar.qins
ian features;however,the distributionand morphologyof feaThe continental margins are expressedas continuousre- tures within theseregionsare distinctlydifferentfrom thoseof
gionsof very high slopesurroundingthe major land masses the Venus plains. Within the plains of Venus the 0.1ø-0.2ø
on earth. In general,slopesassociatedwith activecontinental featuresform numerouscircular and linear systemson a varimargins appear steeperthan those associatedwith passive ety of scales.
Circular.features. Circular, elliptical, and a variety of irmargins(Figure 1). An importantexceptionto this generality
is observedin the southernand easterncontinental margins of regular enclosed regional slope features are distributed
Australiawhich exhibit slopesas greatas 1.4ø. Activemargins, throughoutthe plainsprovincesas shownin Figure 2b. These
particularly the Peru-Chile and the Aleutian subduction features display a range of regional slope characteristicsbut
zones,include regionsdisplayingthe steepestregional slopes must typicallyoccuras flat zones(0.0ø-0.1ø slope)boundedby
observedat this scale (2.4ø).More typically, however, active ringlike margins sloping at about 0.1ø-0.3ø. Although some
marginsare similarin slopemagnitudeto oceanicsubduction larger featuresmark the flanks of circularbasins(e.g.,Atalanta
zones but broader and more continuous.
Planitia), most are domical featuresin the PV altimetry. They
The high regionalslopevaluescharacteristicof earth'scon- range in diameter from approximately 200 km to over 2000
tinental marginsare primarily due to the steepgradientsex- km, and many are similar in sizeand regional slopecharacterpressedby continentalslopes(approximately4ø [Heezenet al., isticsto some terrestrial oceanicplateaussuch as the Ontong1959]; 2ø-6ø [National ResearchCouncil,1979]). However, as Java Plateau and the Bermuda Rise [Ben-Avraham et al.,
continentalslopesare usuallyonly 20-100 km wide, a variable 1981]. The smaller circular featuresmay be the topographic
componentof the surroundingterrain is averagedinto the expressionof structuresrevealed in the Venera 15-16 radar
7548
SHARPTON AND HEAD' IMPLICATIONS OF REGIONAL SLOPE DISTRIBUTION--VENUS
0
90
|shtar
[
180
AND EARTH
270
,9•talant•.,
I
I
•,..PI_..•.•'
0
FreyJ,
k•••xax
well
g•o
0
-
-60
-
'%IPho•)bc
R.
I
I
I
I
I
I
I
I
Mons
I
!
Fig. 2 Schematicmaps of regionalslopevariationsdiscussed
in the text and keyedto Plate 1, bottom. Bold solid lines
denotehighlandmarginsand interior mountain rangesas labelled.(a) Locationsof major features[Masursky et al., 1980]
discussedin the text. (b) Circular and elliptical features with elevated boundary slopes.Barbs on the larger features
indicate direction of regional slope. (c) The light stippled regions mark the location and the general trend of the more
prominent beltsof topographicallysmooth,flat terrain. The dark stippledregionsdefineprominentruggedbeltswithin the
plains provinces.
imagesas circularto ellipticalstructures
rangingin sizefrom
Severalof the larger circular featuresillustratedin Figure 2b,
200 km to 600 km [Barsukovet al., 1986]. These "coronae" or
"ovoids" [Barsukov et al., 1986] often occur with radar-bright
flowlike features [Barsukou et al, 1986] and are sometimes
grouped in linear arrays accompanied by linear features
[Stofan et al., 1985], suggestinga volcano-tectonicassociation.
such as Hathor Mons, also have radar characteristics which
imply a volcanic origin [Campbell and Burns, 1980]. Many
features with similar regional slope characteristicsare concentrated within linear belts of topographically rugged plains
and along the boundariesof elongatezonesof topographically
SHARPTON AND HEAD: IMPLICATIONS OF REGIONAL SLOPE DISTRIBUTION--VENUS AND EARTH
I
I
?70
180
o
7549
I
It
i,
ß
t%
ß ß
•.•.'
ß
ß
. ,,
dilli' M
J
'
I
I
t;;3
.5
Z.,
B
"
E S
Plate 1. (Top) Regionalslopemap of earth depictingthe maximumslopemeasuredover 3ø by 3ø regions.The rangeof
regional slope valuesfor earth, measuredat this scale,extendsfrom 0.0ø to 2.4ø. Standard error associatedwith slope
calculationsis 0.035ø [Sharptonand Head, 1985]. Each grid cell representsone degreeof latitude and one degree of
longitude (approximately 111.3 km at the equator). For regional slope less than 0.5ø, each map color representsa 0.1ø
slope increment; larger slope values are color-codedin 0.5ø increments(see color bar). (Bottom) Regional slope map of
Venus.The format, resolution,projection,and color scaleare equivalentto that of the top part. The range of regional
slopeson Venusextendsfrom 0.0ø to 2.4ø. At the equator,one degreeof latitude or longitudeequalsapproximately105.6
km.
7550
SHARPTON AND HEAD: IMPLICATIONS OF REGIONAL SLOPEDISTRIBUTION--VENUS AND EARTH
smooth plains, as discussedbelow. Thus the regional slope
data suggestthat the circular coronaerevealedin local, highresolution images have an obvious topographicexpression,a
global distribution,and an associationwith global scalelinear
trends.
Topographic fabric. Although variations in the distribution of regional slopefeatureson Venus are not as dramatic
as on earth, there are, nonetheless,broad, distinct variations in
the concentration
of the 0.1ø-0.2 ø features within
the Venus
topographywithin the highlandsis distinctlysteeperand more
rugged than that characteristicof the lowlands and upland
rolling plains. The interiors of western Aphrodite Terra and
Beta Regio are areas of 0.1ø-0.3ø regional slope, most similar
in regional slope characteristicsto the Rocky Mountains in
the westernUnited Statesor slow-spreadingoceanridgessuch
as the Mid-Atlantic Ridge. Central Aphrodite Terra (e.g.,
Thetis Regio)containszonesof anomalouslysteeptopography
(as great as 0.7ø) which trend northeast and generally define
plains: Several zones of considerableextent, characterizedby
relatively low abundancesof nonzero slope features are evident in Plate 1, bottom. Thesezonestypically occuras narrow
belts of topographically smooth terrain, as depicted schematically in Figure 2c, and are often bounded by distinct
linear slope features. These smooth belts are sometimes
flanked by other more rugged belts containing a relatively
high abundanceof 0.1ø-0.2ø features.The boundariesof these
belts are locally difficult to trace; small-scaleirregularitiesin
width and variations in orientation are apparent. Nevertheless,theselinear regional slope trends can be traced for thousands of kilometers with no significant offset. The northeast
trending smooth belt south of Aphrodite Terra betweenArtemis Chasma and Beta Regio extends over 20,000 km. This
zone, which parallels the axis of central Aphrodite Terra
lehmann and Head, 1982], is interrupted only in the vicinity of
Atla Regio by a northwest trending belt of 0.1ø-0.4ø slope
values which extends southeastward from Atla through
Themis Regio [Schaber,1982]. The truncation of this smooth
belt with the westernmargin of Beta Regio is sharp and linear
and appears to connect with a distinct linear regional slope
boundary extending southward to about -60 ø latitude, and
slightly east of Lise Meitner (- 56ø; 321ø).
A northeast oriented belt of smooth terrain occurs just
north of the highlands of eastern Aphrodite betweenAsteria
Regio and Thetis Regio and is interrupted by the northern
the intersectionof Aphrodite with the boundariesof the
northeasttrendingsmoothbelt discussedin the previoussection. Maximum regional slope values associatedwith this
regionare similarto thoseof someRecentmountainbeltson
extension
sectingsets of linear ridgesand grooves.Basilevskyet al.
[1986] cite similaritiesto conjugatefault systems,local offsets
of the aforementioned
Atla-Themis
trend. This belt
intersectsthe highlandsof Aphrodite at Thetis Regio (Figure
2a) in central Aphrodite. There is a distinct alignment of the
mountain rangeswithin Aphrodite at this point, and the trend
of the mountains, as well as the orientation of Diana Chasma,
parallel the trend of the smoothbelt. Distinct inflectionsin the
margins of central Aphrodite also occur at the zone of intersection.To the south of Aphrodite the belt of smooth terrain
continuessouthwestwardas a narrow zone defined by northeast trending 0.1ø-0.2ø slopefeatures.South of Ovda Regio in
western Aphrodite, this northeast trending belt intersectsanother linear belt of smoothterrain trending northwest.From a
point just south of Artemis Chasma the flanks of this smooth
belt can be tracedby alignednorthwesttrendingslopefeatures
to north of Rhea Mons in Beta Regio, a distanceof approximately 25,000 km. This feature follows the general trend of
Aino and Guinevere Planitia and is also distinctly recognizable within the upland rolling plains which divide these
two broad troughs.Numerousother trendsalignedwith these
belts of smooth terrain are evident throughout the plains
provincesof Venus thus producing a subtle but globally pervasiverectilineartopographicfabric distinctlyunlike the more
complex mosaic of plate-related features observed on earth.
The great-circle-likepatternsof the Venus fabric appear sinusoidal in Plate 1, bottom, and Figure 2c due to the map
projection.
Highland Interiors
Unlike the interiors of earth's continents,the Venus highlands do not contain appreciablesmooth, flat regions,and the
earth.
The steepestslopesfound within the Venus highlandsare
associatedwith the unique mountain systemsthat encircle
Lakshmi Planum in western Ishtar Terra. Maxwell, Akna, and
Freyja monteshave regionalslope valuesranging typically
between 0.6ø and 2.0ø; the western flank of Maxwell Montes
containsslopesas great as 2.4ø. The regionalslopeand elevation characteristicsof these mountain ranges are similar to
major compressionalfeatureson earth (active continental
margins and zones of continental convergence).Surfaces
within Lakshmi Planum range in slopegenerallybetween0.1ø
and 0.2ø and have a distinctsouthwardregionaltilt.
The interior of eastern Ishtar Terra (east of Maxwell
Montes) contains neither the systematicarrangementof
mountainbeltsnor the distinctplainsregionscharacteristicof
the westernIshtar. Instead,regionalslopesare highly variable
acrossshort distancesand range in magnitude from 0.1ø to
0.8ø. In terms of regionalslopeand elevationcharacteristics
this regionis similar to Ovda and Thetis regionesin Aphrodite Terra. High-resolutionradar images [Barsukovet al.,
1986] reveal that easternIshtar is dominated by "parquet"
terrain [Basilevskyet al., 1986] consistingprimarily of inter-
acrosslinear features, and associatedextensional features as
evidencefor sheardeformationthroughoutthis region.
Chasmata. Major chasmata,or steep-walled,linear, or arcuate valleys such as Artemis, Diana, Dali, and Devana
(Figure 2a) appear to be similar in regional slopeexpression
(0.1ø-0.4ø),althoughthoseoccurringwithin the highlandregions(e.g.,Dali and Diana) are not readilydistinguished
from
the relativelysteepand variable slopescharacteristicof the
highlandinteriors.Typically, chasmatadisplaymoderateregionalslopescomparableto thoseof the East African Rift on
earth (Figure 1), thus supportingthe interpretationof the
Venuschasmataas rift valleys[Masurskyet al., 1980; Kaula
and Phillips, 1981; McGill et al., 1981; Schaber,1982; Campbell et al., 1984].
Highland Margins
The major highland regions on Venus are bounded by
zones of relatively steep slope, although magnitudesare, in
general,significantlylessthan thoseassociatedwith terrestrial
continental margins. The margin slopesof Aphrodite Terra
rangein magnitudefrom about 0.1ø to approximately0.5ø and
are largestand most continuouson the north and southflanks
of westernAphrodite.The steepestslopesin centralAphrodite
occur along the axis of this narrow highlands regions and
appear to be associatedwith the extensivesystemof chasmata
found in this region [Schaber,1982]. Beta Regio is bounded
by slopesof 0.1ø-0.4ø which are most distinctly expressedon
SHARPTON AND HEAD: IMPLICATIONS OF REGIONAL SLOPE DISTRIBUTION--VENUS
the eastand west flanks.The westernflank is extremelylinear,
and this southeastlinear trend appearsto continue toward
Hathor Mons as a distinct systemof linear 0.1ø-0.2ø slope
featureswhich serveto divide a belt of relatively ruggedplains
to the east(containingHathor, Ushas,and Innini mons)from
the topographically
smoother,flatter regionto the west.The
marginssurrounding
IshtarTerra are significantly
steeperand
more continuousthan thoseof the other Venushighlands;the
regionalslope valuesof the Ishtar margins(as steepas 1.0ø)
are comparableto those of passivecontinentalmargins on
earth (Figure 1).
In severalplacesthe zonesof steepslopemarking the highland marginsappearto extenddowninto the plainsregionsas
linear belts of relatively rugged terrain, with regional slopes
typically between0.1ø and 0.2ø (Figure 2c). One such belt of
ruggedterrain appearsto extendfrom the southernmargin of
Ishtar Terra (Ut Rupes) southeastwardto merge with the
westernmargin of Aphrodite Terra. Another ruggedbelt connectsthe linear northeasttrendingspine of central Aphrodite
Terra with Beta Regio, as discussedby Schaber [1982]. A
short, but distinct, northwest trending belt extends from the
steeplyslopingnorthern margin of Aphrodite Terra as shown
in Figure 2c. This linear trend is approximatelyparallel to the
major northwest trends discussedin previous sections(and
others in Plate 1, bottom) and appears to cut or modify the
interior of central Aphrodite. There may also be a relationship
AND EARTH
7551
regional
slopecharacteristics
alone.However,
thisstudysupports the observationsconcerningsubductionzones:With the
possible exception of the mountains within western Ishtar
Terra, there are no regional slope values as consistentlyhigh
as those associatedwith terrestrial convergentboundary features. Furthermore, terrestrial hotspot traces possessdistinct
regional slope characteristicsindicative of seafloor spreading,
i.e.,narrow linear zonesof relativelyhigh slope,the magnitude
of which decreases with distance from the zone of active vol-
canism. Although many narrow linear zones of moderate
slopeare evidentin the Venusregionalslopemap (Plate1,
bottom), none show the systematic along-axis variation in
slopedisplayedby terrestrial hotspot traces.Thus evidenceto
support lateral motionsof the sort associatedwith the present
terrestrial style and rate of plate recycling is not found on
Venus.
While this regionalslopeanalysissupportsthe findingsof
Arvidsonand Davies [1981] that major tectonic and volcanic
landforms are recognizableat PV resolution,the absenceof
topographicallyanalogousfeatureson Venus does not necessarily rule out a terrestrial style of plate recycling[Solomon
and Head, 1982; Brassand Harrison, 1982; Phillips and Malin,
1983, 1984; McGill et al., 1983]. Important seafloorlandforms
would becomemore subduedand lessdistinguishablewithout
the loading and cooling effectsof the terrestrial hydrosphere
[-Headet al., 1981], and giventhe extremelyhigh temperatures
between the orientation
of the chasmata associated with the
(approximately 750 K) typical of the Venus surface,viscous
highlands and the developmentof highland margin slopes: degradation of topography would be enhanced [-Weertman,
The well-developednorth and south margins of Aphrodite 1979; Solomonet al., 1982], thereby smoothing the relief and
Terra parallel the trend of Dali Chasmaand the steeplyslop- regional slope magnitudesof Venusian features.In addition,
ing east and west flanks of Beta Regio are aligned with differencesin thermal gradient, volatile content, and distriDevana
bution
Chasma.
ANALYSIS AND DISCUSSION
On earth the low regional slopescharacteristicof the continental interiors and oceanicabyssalplains are the productsof
vigorous, hydrosphere-drivenplanation processesacting on
regions which are relatively free of relief-generatingvolcanic
and tectonic activity. The conspicuousabsenceof major regionsof smooth,flat-lying surfacesassociatedwith the highland interiors or the plains provincesof Venus suggeststhat
the large-scalemorphologyof these regionsis not severely
affectedby erosion or depositionof sediments.Instead, it appears that the redistributionof surfacematerials is not nearly
as efficient at completely removing topographic relief on
of heat sources as well as the surface conditions
of
Venuscould havedramaticeffectson the thicknessand buoyancy of the lithosphere,the rate of spreading,and the topographic expressionof major volcanic and tectonic landforms
[Weertman, 1979; Anderson,1980, 1981; Phillips et al., 1981;
Solomonand Head, 1982; Phillips and Malin, 1983].
rl'he distribution and orientation of the large-scale topo-
graphic featureson earth reflect the boundariesof large, irregular lithosphericplates,along which volcanic and tectonic
activity are concentrated.In the case of intraplate features
suchas hotspottracesand fracturezones,the distribution and
orientation reflectsthe relatively independentlateral motions
of these plates. Even passivecontinental margins mark the
sites of previous divergent plate boundaries or transform
Venusasit is onearth.Thisissupported
by a recentanalysisfaults [-e.g.,Cogley, 1984]. This resultsin a relatively plateof PV radar reflectivityand small-scaleroughness[Head et al., specificarrangementto the large-scalelandforms which are
1985] which indicatesthat the surfaceof Venus is covered recognizableas distinct featuresin the regional slope map of
predominantly by rocky material with very little soil. It is not earth (Plate 1 top). Typically, linear trends truncate or are
clear from these observations alone, however, whether the ab- offsetat plate boundaries,and there is no recognizableglobal
senceof broad-scaleplanation on Venus is indicative of low consistencyto the orientation of linear features becauseof
weathering rates, or, of a more ubiquitous, perhaps more plate-to-plate variations in direction of motion.
active, style of volcanic and tectonic modification. In either
In contrast to the plate-specificmosaiclikearrangementof
case,it appearsunlikelythat the major topographicfeaturesof features on earth, the orientation and distribution of Venusian
Venus are primarily depositionalor erosionalin origin.
topographic featuresdisplay a noticeableglobal consistency.
Analysesof PV altimetric data have establishedthat Venus Phillipset al. [1981] noticedthat a great circle arc inclined 30ø
exhibitsnothing topographicallysimilar to present-dayterres- to the equator could be fit to the linear "equatorial" highlands
trial mid-ocean ridges [•Arvidson,1981; Arvidson and Davies, formed by Aphrodite Terra and Beta Regio. Schaber[1982]
1981; Masursky et al., 1981; Phillips et al., 1981; Kaula and recognized three major linear topographic trends in this
Phillips, 1981; Head et al., 1981] or subduction zones [Mas- region: (1) the "Aphrodite-Beta"zone, trending northeastward
ursky et al., 1980; McGill et al., 1981; Schaber,1982]. The for approximately 21,000 km from the south slope of Ovda
regional slope valuesassociatedwith terrestrial divergentplate and Thetis regiones in Aphrodite Terra to the west side of
boundariesare highly variable and sometimesindistinct; thus Beta Regio,(2) the "Themis-Atla"zone,trendingnorthwestfor
the occurrenceof suchfeatureson Venus cannot be testedby about 14,000km from a point just east of Themis Regio to the
7552
SHARPTON AND HEAD' IMPLICATIONS OF REGIONAL SLOPE DISTRIBUTION--VENUS AND EARTH
northwest border of Atla Regio, intersectingthe AphroditeBeta zone, and (3) the "Beta-Phoebe" linear, a short discontinuous zone extending southward from Beta Regio to
Phoebe Regio. Becausethese linear zones contain elevated,
highly variable topography and are associatedwith various
chasmata, Schaber proposed that they are zones of lithosphericweaknessalong which limited extensionand volcanic
activity have occurred. The variations in regional slope
propertiesevident in Plate 1 bottom, reveal that similar northeast and northwest linear trends are also expressedby the
orientationand distributionof topographicfeaturesthroughout the lowlands and upland rolling plains: Extensivebeltlike
zonescontainingeither topographicallysmoothor ruggedter-
Western Ishtar Terra is unique among the Venusian highlands in many ways; topographicallyit containsthe highest
elevations[Masursky e• al., 1980; Pe•engill e• al., 1980] and
highland features and therefore is linked to the formation and
with these features.
thesteepest
regionalslop'es
observed
on Venus.It contains
a
broad, elevatedplains region, Lakshmi Planum, which is sur-
roundedby continuous,
steep-Sided
mountainrangesrivaling
those on earth associated with the Tibetan Plateau. The mar-
gins of Ishtar are only slightlylesssteepthan thoseof terres-
trial passive
margins.
In addition,earth-based
radar!mages
of
Ishtar Terra• show evidenceof extensiveregional lineations
similarin appearanceto foldedmountainterrain [Campbellet
al., 1983]. Such characteristicshave led to suggestionsthat
Ishtar might be the only true continenton Venus [Phillips et
rain, and numerousotherlinearfeatureswithinthe plains,are al. 1981] and may have been producedby subductionand
aligned with either the Aphrodite-Betahighlandstrend or the plate collisionprocesses
duringan earlierstageof VenushisThemis-Atla trend, thus forming a globally pervasivetopo- tory when lower temperaturesand the availability of free
graphic fabric.
water permittedplate recycling[Phillips and Malin, 1983]. If
The topography within central Aphrodite appearsto be af- Ishtar Terra is ancientcomparedto the other Venus highfectedby the intersectionwith theseterrain belts,as supported lands, then the anomalouslysteepregional slopesassociated
by the alignmentof steeptopographywithin Ovda Regio,the with its marginsand interior mountainrangesare difficultto
northeastward trend of Diana Chasma, the inflections in the explain.While its basic crustal propertiesmight have orighighland boundariesnear theseintersectionsand the continu- inated during an earlier stage in the tectonic evolution of
ation of highland margin trends as linear zones of rugged Venus,the mountainousterrain encirclingLakshmiPlanum
terrain which extendinto the plains.Thus the processwhich and the highland margins of Ishtar Terra would seemto reappears to have controlled the distribution and orientation of quire more recent processesto generateor maintain the unthe plains topographyalso has influencedthe morphologyof characteristically
steepslopesand high elevationsassociated
modificationof highlandsand plainsalike.
CONCLUSIONS
The topographicfabric of Venus is suggestiveof the welldocumentedgridlike arrangementof linear topographicfeaThe surfacedistributionof 3øby 3øregionalslopevalueson
tures on the moon (i.e., the lunar grid [Spurt, 1944; Fielder, Venus and earth are fundamentallydifferent. On earth, re1963; Strom, 1964]) and Mercury [Dzurisin, 1976; Masson and gional slope characteristicsreflect spatial variations in the
Thomas,1977]. Involvementof impact basin-relatedtopogra- nature and extent of various geologic processesrelated to
phy implies that these grid systemsmust have formed at an plate tectonicsand an activeweatheringregimeand providea
early stageof planetaryevolutionand thereforeprobablyrep- basis for understandingthe regional slope characteristicsof
resenta basicthermal, tidal, or rotationally producedfabric of Venus.From this analysiswe derivethe followingconclusions
the ancient lithosphere.The moon and Mercury both have relevantto geologicalprocesses
on Venus:
globally continuouslithospheres;planetary heat loss has oc1. There are no major regionsof 0.0ø slopeassociatedwith
curred principally by conductionwith only limited levels of the highland interiors or the plains provincesof Venus indivolcanic and tectonic activity relative to the earth [Head and cating that the major topographicfeaturesof Venus have not
Solomon,1981]. The topographicfabric of Venusdiffers,how- beensignificantlyaffectedby erosion,transportation,or depoever, in significant ways from the gridlike patterns of the sition of surface materials.
moon and Mercury. On the moon and Mercury the patterns
2. The Venus highlandsare boundedby distinctivezones
are most often manifestedin subtlealignmentsof linear crater of relativelyhigh slope,indicatingthat they are differentfrom
wall segments,rilles, and scarpsbut are not part of the major the plains provincesin termsof thermal structure,tectonics,or
topographicsignature of the planet. On Venus the fabric is crustalcompositionand configuration.Major differencesbeformed of severalgreat-circle-liketrends that are oriented at tween the magnitudeof boundingslopessuggestpossibledifanglesgenerallylessthan 45ø to the equator and which form a ferencesin age or mode of formation of the highlands.
basicaspectof the topographiccharacterof the planet.
3. Circular featuresare abundantthroughoutthe plains of
The Venus topographic trends appear globally extensive, Venus. On the basis of high-resolutionradar images,these
and no major offsetsare detectable.Becauseof the regional features are probably mostly coronae with some volcanoes.
nature of the slope data [•Sharptonand Head, 1985], detection Thus the coronae structuresobservedin radar imagesof the
of lateral offsetsof the type associatedwith present-daysea- northernplains are probablyvery widespreadon the planet
floorspreading
wouldbe unlikely.As previously
discussed,and may represent a fundamental differencefrom earth in
high-resolutionimages of eastern Ishtar Terra have revealed termsof heattransfermechanisms
[SolomonandHead, 1982].
evidenceof local shearing in conjunction with the parquet
4. WesternIshtar Terra is uniquein regionalslopecharacterrain [Basilevskyet al., 1986]. The regionalslopeand eleva- teristicsand contains the highest regional slope values on
tion characteristics
of this region closelyresemblethose of Venus. Radar images of Ishtar reveal a distinctive banded
centralAphroditeTerra (Ovda and Thetis regiones).Although appearanceto the terrain of the steeply sloping mountain
no high-resolutioncoverage of Aphrodite currently exists, ranges[Campbellet al., 1983] and a compressional
origin has
these topographicsimilaritieswith easternIshtar suggestthe been suggested[Solomonand Head, 1984]. As the largestrepossibleoccurrence
of extensivedeformationin this regionas gional slopeson earth are associatedexclusivelywith major
well.
active zones of horizontal compression(i.e., subduction or
SHARPTON AND HEAD: IMPLICATIONS OF REGIONAL SLOPE DISTRIBUTION--VENUS
convergence),the high slopes within Ishtar lend additional
support to the compressionalorigin of the banded terrain and
further serve to distinguishIshtar Terra from the extensiondominatedequatorial highlands[Schaber,1982].
5. Eastern Ishtar Terra is fundamentally different from
Ishtar west of Maxwell Montes. Its regional slope and elevation characteristics
closelyresemblethoseof centralAphrodite
Terra. This suggeststhat the intensedeformation observedin
radar imagesof easternIshtar [Basilevskyet al., 1986] may
also occurin the centralregionsof Aphrodite.
6. There is a global tectonicfabric on Venus, consistingof
broad terrain belts and alignedregionalslopefeatureswithin
the plains, trends of elongateplanitia, highland margin segments,chasmata,and linear tropographywithin the highlands.
The fabric is characterizedby severalmajor great-circle-like
patterns oriented at lessthan about 45ø to the equator. This
fabricdiffersfrom both the complexterrestrialmosaicof lithosphericplate boundariesand the more subtle and pervasive
ancient lunar and Mercurian grids.This fundamentallydifferent pattern suggeststhat the major mechanismof heat transfer
and associatedtectonicson Venus is neither simpleconduction, as on the moon, nor a well-integratedpattern of plate
recycling,as on earth. Venus may have a distinctivetectonic
style related to less complex,more fundamentalpatterns of
heat loss.
Acknowledgments.This researchwas supportedby NASA grants
NGR-40-002-088
and NAGW-713,
and the William F. Marlar Me-
morial Foundation for J.W.H. Additional support was provided by
the Earth Physics Branch, EMR Canada. We are also grateful to
Roger Phillips, an anonymousreviewer, and the associateeditor for
their helpful suggestions.Critical readingsby Richard Grieve, Marc
Parmentier,Carle Pieters,Paul Hess,and Stan Zisk are also appreciated, as is the careful work of photographerCathy Carver of Brown
University.
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