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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. E8, PAGES 13,533-13,544,AUGUST 25, 1992 Features on Venus Generated by Plate Boundary Processes DANMCKENZIE, 1 PETERG. FORD, 2 CATHERINE JOHNSON, 3 BARRYPARSONS, 4 SAUNDERS, 5 ANDSEANC . SOLOMON 6 DAVID SANDWELL,3 STEPHEN Various observationssuggestthat there are processeson Venus that produce features similar to those associatedwith plate boundarieson Earth. Synthetic aperture radar imagesof Venus, taken with a radar whose wavelengthis 12.6 cm, are compared with GLORIA images of active plate boundaries, obtained with a sound sourcewhose wavelength is 23 cm. Features similar to transform faults and to abyssal hills on slow and fast spreading ridges can be recognized within the Artemis regionof Venusbut are not clearly visible elsewhere.The compositionof the basalts measured by the Venera 13 and 14 and the Vega 2 spacecraftcorrespondsto that expected from adiabatic decompression,like that which occurs beneath spreading ridges on Earth. Structures that resemble trenches are widespread on Venus and show the same curvature and asymmetry as they do on Earth. These observationssuggestthat the same simple geophysicalmodels that have been so successfullyused to understand the tectonicsof Earth can also be applied to Venus. INTRODUCTION Perhaps the most important question about the evolution of Venusis to what extent its surfaceis being deformed in the same way as is that of Earth. If it is, then the understanding developedfor Earth can be directly used to understand the origin of featureson Venus. If it is not, progressis likely to be much slower, because synthetic aperture radar Our object in this paper is to showthat there are features on Venus that closely resemble oceanic transform faults, abyssal hills and trenches, and fold mountain belts and transform faults on continents. We have not yet found a convincingexample of a spreadingridge or of a basin produced by extension. Such features may be present but are not likely to be easyto map using altimetry and SAR images (SAR) images,altimetry with a resolutionof 10 km or less (seebelow). Of thesestructures,only analogsto terrestrial and a few low accuracy analyses of the surface composition are not likely to be sufficientto allow an understanding of new tectonic processesto be developed. Such observations would not have been sufficient for Earth, where our understandingof the processesinvolved dependsprincipally on seisinologyand palaeomagnetism. Since so few types of information are yet available for Venus, we must argue by analogy and comparefeatures on the two planets at the same scale,observedby similar types of sensors.If they look alike, we argue that they have been produced by similar processes. Though it is clear that such an argument may not be valid, there is at present no obvious alternative. Before Magellan imagesand altimetry becameavailable there was considerablecontroversyabout whether there were structures on Venus correspondingto plate boundaries on trenches, continental fold belts, and transform faults appear to be widespreadon Venus. Featuresresemblingabyssalhills and oceanicfracture zoneshave so far only been recognized in one region of the planet and extend over an area of about 500 x 1000 km 2. TRANSFORM FAULTS AND FRACTURE ZONES On Earth the most unambiguousmethod of mapping transform faults dependson offsetsin the magnetic anomaly patterns. Until this method came into general use there was some confusion between fracture zones and trenches. Though their topography can be similar, fracture zonestend to be narrower and more symmetric than trenches. The surface expression of the transform part of a plate boundary joining two spreadingridgesis a deep linear trough, whose width is about 20-30 km and whose depth is 1-3 km below Earth [Crumpier and Head, 1988; Grimm and Solomon, the ridge axis. The active strike-slip fault is generally near 1989]. It is now clear that the resolutionof theseearly imthe center of the trough, whose depth below the seafloor ageswas not sufficient to allow this question to be decided changeslittle as the plate motions transport the seafloor one way or the other. But the new higher-resolutiondata away from the plate boundary. On ridges spreadingat less allow a much stronger and more detailed caseto be made. than about30 mm yr-1 half rate the crustalongthe axis of the fault is as thin as 3 km or considerably less than the oceanic average of 7 km. The thin crust probably re1Instituteof TheoreticalGeophysics, Departmentof Earth Scisults becauseless melt is producedat the ridge tip next to ences,Cambridge, England. 2Centerfor SpaceResearch,Massachusetts Institute of Tech- older, colder, lithosphere. The major cause of the trough is this difference nology, Cambridge, Massachusetts. in crustal thickness. On Venus the crustal thickness on transform faults is likely to be reduced by a 3Scripps Institutionof Oceanography, La Jolla,California. 4Departmentof Earth Sciences,Oxford University,Oxford, similar amount where they intersect ridges, but the depth England. of the trough will be reduced by a factor of about 0.7, the 5Jet PropulsionLaboratory,Pasadena,California. ratio of the density contrasts,becausethe trough is filled by 6Department of Earth, Atmosphericand Planetary Sciences, MassachusettsInstitute of Technology,Cambridge, Massachusetts. Copyright 1992 by the American GeophysicalUnion. Paper number 92JE01350. O148-0227/92/92JE-01350505.00 atmosphericgas, not water. The SAR on Magellan uses a radar whose wavelength is 12.6 cm and maps a swath width of about 20 km with a resolution of about75 m [Pettengillet al., 1991].The images are best compared with those produced by GLORIA, a side scansonar system that usesa wavelengthof 23.1 cm to map 13,533 13,534 MCKENZIE ET AL.: FEATURES ON VENUS GENERATED BY PLATE BOUNDARY PROCESSES a 20 km width swath with a similar resolution to the SAR the stress field associated with the ridge-transform fault [Somerset al., 1978;Searle,1986;Mitchell,1991].The most junction [Crane,1976;Searleand Laughton,1977;Lonsdale, important differencebetween the two systemsinvolves the 1978; Fox and Gallo, 1984] and is in the samedirectionas incidenceangle, measuredfrom the vertical. That of the the ridge offset. However, the location of the spreadingridge SAR variesslowlywith latitude from 17ø to 47ø [Pettengill et al., 1991] and is essentiallyconstantfor an image that itself cannot spansonly a few degreesof latitude. In contrast, that of GLORIA variesstronglywith range,from about 30ø at the edgeof the image nearestthe ship to more than 80ø at the greatestrange. GLORIA imageshave mostly been obtained SAR imagesof part of the Artemisregionof Venus(seeFigure 2 for a locationmap). Theseshowshortbrightreflectors for small areas of the seafloor. The only large areas that have been covered are around the United States, and most are dominatedby sedimentaryfeatures. Though the costof mapping all the ocean floor with GLORIA would only be about half of the cost of Magellan, it would take about 100 ship years to do so. Figure la showsa GLORIA image of part of the East Pacific Rise where it is offset by the Quebrada and Discov- be seen on the GLORIA record. Similar features to those in Figure 1a are visible on the that are linear over much of the image but sigmoidal where they cross what we believe are fracture zones. These traces are themselves curved. On Earth such curvature arises if the fracture zone is close to the pole of relative rotation between the plates involved or if the direction of spreadingchanges. It is not clear whether the faults in Figures 1c and 1d are associatedwith deep troughs, as they would be on E•rth. The altimeter has an along-track resolution of about 8 km and an across-track of about 20 km at -40øN [Pettengillet al., 1991] and may be sufficientto map any troughsassociery transformfaults [Searle,1983, 1986]. The half spreading ated with thesetransformfaults [Ford and Pettengill,this rateof theridgeis about75 mm yr-• . The brightlineations issue]. are reflectionsfrom the edgesof abyssalhills and are zonesof SPREADING RIDGES small scaleroughnessproducedby faulting. As the seafloor movesaway from the spreading ridge axis, these zones beOn Earth, separating plate boundariesare marked by come lessreflective as they are coveredwith sediment. The topographicridgeswhoseelevatione is about 4 km abovethe transformfaults are marked by curvature of the marginsof oldestseafloor[Parsonsand Sclater,1977]. This elevation the abyssalhills. This curvature is believed to result from differenceis a direct consequenceof the production of plates (c) 129 130 --37 -37- (b) / 129 130 ,p Fig. 1. (a) Two-sidedGLORIA imageof parts of the Quebrada and Discovery fractue zones on the East Padtic Rise. The ship track is alongthe middle of the centraldark horizontalline. (b) An interpretation of Figure la. The fine lines are selected tectonic lineaments, the heavy line is the inferred plate boundary, half arrows show the slip directions, full arrows the spreading di- rections,and the ellipsesthe strain [from Searle,1983]. (c) and (d) SAR imagesof fracturezonesin the Artemisregionof Venus. 1ø of latitude corresponds to 111 km on Earth and to 105 km on Venus. Figure lc is from F-MIDR 35S130;1. Figure ld is from F-MIDR 40S131;1. o 85 50 KILOMETERS 75 loo MCKENZIE ET AL.: FEATURESON VENUSGENERATEDBY PLATE BOUNDARYPROCESSES 6051 13,535 6054 -2O -3O 1(c) , • -4O 120 140 l(d) 3(c) Fig. 2. The topographyof the Artemisregion,showingthe locationof Figureslc, ld and 3c. The circulardepressionof radius 1000 krn marks the outer boundary of Artemis Corona. from hot mantle. If T• is the mantle temperature, p• is the density of water, pm is the density of the mantle, • is the thermal diffusivity, c• is the volumetric thermal expansion coefficient,d is the thickness,and t is the age of the plate, then = where v is the thermal tion of ridge axes above old lithosphereis reducedto 1.2 km. The thermal time constant is also reduced, from 62.8 Ma for Earth to 28 Ma for Venus. Though a lithosphericthickness of 84 km is too small to satisfy the flexural observations,it is too large to allow the heat generatedby radioactive decay to escapeby conductionthrough the lithosphere. ((T1-Tø)d)F(t/v) (1) time constant d2 T-- 7f2 tg The other important causeof topographicvariationson Earth is variations in crustal thickness. On Venus, correspondingvariations in crustal thicknesswill produce topo- graphicvariationsreducedby a factor of (prn-- Pw)/prn• 0.70. In contrast, the topography associatedwith a spread- ForEarth,pw= 1.03Mg m-s, To= 0øCandT• • 1350øC, ing ridge is reduced by a factor of 0.31 if the lithospheric whereas for Venus,pw= 0.05Mg m-s, To• 470øC,andT• thicknessis 84 km. Hence ridges on Venus may be harder is likely to be similarto that of Earth (seebelow). These to recognizeby their topography compared with those on differencesalone reducethe value of e(0) from 3.9 km for Earth, becausethey are smaller both in height and in latEarth to 1.75 km for Venus[Kaula and Phillips,1981]. For eral extent relative to topographicfeaturesresultingfrom oceanic plates on Earth, d _ 125 km, but the value of d for crustal thicknessvariations. Furthermore,terrestrial ridge Venusis uncertain. If the heat flow throughold lithosphere axes cannot be distinguishedfrom adjacent abyssalhills on on Venus is required to be the same as that on Earth, or the GLORIA images(seeFigures1a and 3a) and are there36 mW m-2, its thickness canonlybe 84 km andtheeleva- forenot likely to be visibleon the SAR imageseither. These 13,536 MCKENZIE ET AL.: FEATURESON VENUS GENERATEDBY PLATE BOUNDARYPROCESSES (a) Fig. 3a. Mosaicof GLORIA imageson the axis of the Mid-Atlantic Ridge. (b) MEDIAN VALLEY Fig. 3b. An interpretationof Figure3a showingthe main tectoniclineations[Laughtonand Searle,1979]. argumentssuggestthat activespreadingridgeson Venuswill [Searleand Laughton,1977]. On fast spreadingridgesthe not be easy to map: only their linearity and symmetry may bright linear featuresare continuousover greater distances distinguishthem from other larger features supportedby variationsin crustal thickness. These argumentsprobably explain why no convincingexamplesof spreadingridgeson Venushaveyet beenidentified. Nonethelessspreadingridges have probably been responsiblefor producingthe structures in FiguresI and 3c. As Figure 2 shows,thesefeaturesoccur insideArtemis Corona. Its outer edgeis a flexuralbulgethat is cut by faults that are tangential to the coronaand closely (seeFigurel a). The existenceof featureson Venuswhose spacingand lengthis similar to thosein the North Atlantic (Figure3c) suggests that spreadingcentershaveexistedon resembles the outer rise of terrestrial trenches. Therefore the features that are like abyssal hills may be produced in Venus. But the processesthat produce the lineations on Earth are not yet well understood, and it is therefore not clear whether the Venusian ridges were also slowly spreading. There are no obviouslinear symmetricfeaturesvisible on the altimetric profiles associatedwith the lineations in Figureslc, l d, and 3b, so it is not clear whetherthe ridges that produced the lineations are still active. In the absence of erosion,the roughnessresponsiblefor the radar reflections a tectonic setting like that of back arc spreadingon Earth. The abyssalhills in Figure 3a are on either sideof a slowly will persist indefinitely. A striking feature of spreadingridges on Earth is that spreadingridge in the North Atlantic that is spreadingat 12 mm yr-• half rate [Laughtonand Searle,1979]. The they are the major volcanic features of the planet and pro- shortbrightfeaturesare thoughtto arisewhenshortindivid- ducebasalticmagmaat a rateof about20 km3 yr-•. The ual spreadingcentersare staggeredwithin a spreadingzone compositionof this basalt is distinctive in having uniform MCKENZIE ET AL.' FEATURES ON VENUS GENERATED BY PLATE BOUNDARY PROCESSES 132 (c) -40 133 I ß '"L•, •,. ,• '.... '""-.•'" -- , • >' ..•.,,• .•, .• -41- ,. • 134 I "•'- • " I -40 •'•L .>'•"•• ..•. •'•'• . ..r', • .• /•., ,.. •.'[' •..'.'•,-. '.• •,. ..• (/:...:...::• .... :-.• ...... :•;...... ...................... ............ . I 132 .• ,-._, 5 0 . .:........... ?:; ================= --41 I 133 _ 13,537 '1' 134 ," ._, 10 0 K I L 0 MET ERS • •.• •r• o.• •o•.) Fig. 3c. A SAR imagefrom F-MIDR 40S131;1of part of the Artemisregion(seeFigure2 for the location). An An (b) (a) ß 13 Ol el 13 Qz 01 Fig. 4. Projections onto the olivine-anorthite-quartz face from the chopsideapex of the tetrahedron of M cKenzie and Bickle [1988]. The location of the points in this tetrahedrondependsonly on the concentrationof SiOn_, Al•_O3,FeO, MgO and CaO in the rock, and is independentof that of Na•_Oand K•_O. The points corresponding to the compositions measuredby Venera13 and 14 and Vega 2 [$urkov et al., 1984, 1986] are shownwith open circlesat the center of ellipses,numbered 13, 14, and 2, respectively,producedby projecting the five-chmensional hyperellipsiod calculated from the standard errors given by Surkov et al. The heavy llne and the solid dot at the center of a circle show the point average and the point and depth average composition, respectively, generated by adiabatic decompressionto the surface when the potential temperature is 1330øC, calculated from the parameterisationof MCKenzie and Bickle [1988]and Watsonand MCKenzie [1991].The star showsthe average composition of cumulusgabbrofrom the Semailophiolite[Pallister,1984]. The Venuscompositions are compared with (a) mid-ocean ridgebasalt(MORB) measured on dredgedanddrilledsamples(R. S. White eta]., submitted manuscript,1992),(b) magmasfromoceanicislandsandseamounts (D. MCKenzieandR. K. O'Nions,manuscript in preparation,1992), (c) Mioceneand youngermagmasfrom westernTurkey [MCKenzie and Yilmaz, 1990],and (d) Jurassicmeltsfrom the North Sea[Latin et al., 1990]. ez 13,538 MCKENZIE ET AL.: FEATURES ON VENUS GENERATED BY PLATE BOUNDARY PROCESSES An An (d) (c) 14 13 13 Qz Ol ez 01 Fig. 4. (continued) (b) (a) 13 2 0 1 2 3 TiO2 4 o 5 14 1 2 % (c) ß ß ß ß ß ,ß ß ß ß ß ; ß ß 2 14• • , 1 i 2' I I 3 4 TiO2 % 2-,- 14•, 0 5 0 1 ' 2 •' 3 ' 4 Ti02 % Fig. 5. K20 as a functionof TiO2 for the samemagmasasin Figure4. The ellipsescorrespond to onestandard deviation. The magmasplotted in Figures5a-Sdare the sameas thosein Figures4a-4d. MCKENZIE ET AL.: FEATURES ON VENUS GENERATED BY PLATE BOUNDARY PROCESSES 13,539 (b) (a) lOO lOO 80 80 •o•6o o E '• 40 40 20 I I I I 5 10 15 20 0 25 0 I I I I 5 10 15 20 MgO % 25 MgO % (d) (c) lOO lOO 80 80 so • 60 o E 4O 40 20 i 5 I 113 I I 15 20 20 I I I I 5 10 15 20 0 0 25 MgO % MgO % Fig.6. ModalMg• asa function of MgOconcentration forthesamemagmas asin Figure4. Theellipses correspondto onestandard deviation. Themagmas plottedin Figures 6a-6darethesameasthosein Figures 4a-4d. and low concentrations of TiO2 and K20. Analyses of rocks The analysis from Venera 13 differs both from basalts from Earth and Venus[Surkovet al., 1984, 1986]are com- from spreadingridges and from basic melts from oceanicispared in Figures 4-6. Figure 4 showsthat the concentrations landsin havinga K20/TiO2 ratio of 2.5. Similarhigh values of SiOn, Al•O3, FeO+MgO, and CaO are similar to those from a variety of tectonic settings on Earth and are therefore little help in deciding how such rocks were produced are found in some continental volcanicson Earth. Figure 5c showsthat basic magmas from western Turkey have similar concentrationsof K20 and TiO2 to that of Venera 13, but on Venus. However,as Pearceet al. [1975]have pointed that out, the concentrations of TiO• and K•O are more diagnostic. Figures 4-6 showsthat the concentrationsof all elements except S analyzed by Venera 14 are similar to those of basaltsfrom spreadingridges. This agreementsuggeststhat the basalt covering the lower plains is formed by adiabatic decompressionand that the upwelling mantle comeswithin about 10 km of the surface. It also suggeststhat the source is like that of Earth and has the compositionof a garnet peridotite. The TiO2 and K•O concentrationsdepend strongly on the total amount of melting, which is in turn controlled by the upper mantle temperature. The agreement between the Venera 14 and MORB compositionssuggeststhat the upper mantle temperature on Venus is similar to that of in Figure 5d are richer in TiO2. On Earth, magmas associated with island arcs commonly have lower concentrations 1330 øC for Earth. those from of TiO• the Jurassic volcanics from the North Sea relative to that of K20 than do similar rocks from other environments, probably because TiO2 is retained by amphibole in the source regions. Amphibole can exist only if water is present in the source, and on Earth the water required is transported into the mantle beneath island arcs by subduction. Subduction of oceanic crust was occurring beneath western Turkey until about 5 Ma, but ceasedin the North Sea region at least 200 Ma beforethe volcanicsin Figure 5d were produced. It is somewhat surprisingthat water should be present in the mantle of Venus when there is so lit- tle in its atmosphere.However,in additionto the K20/TiO• ratio of Venera 13, the presenceof water within Venus is also 13,540 MCKENZIE ET AL.: FEATURES ON VENUS GENERATED BY PLATE BOUNDARY PROCESSES 1o ',8.o o •o 1o 6o -lO -20 -30 '• 140 160 Plate la. 180 The gravity field of the region between New Guinea and Fiji, calculated from the satellite altimetry [Ha•b!], 1987]. suggestedby the existenceof the high-viscositymelts that a differenceis not surprising if the Vega 2 sample contains are inferredto have producedthe pancakelikedomes[Head an appreciable fraction of cumulus gabbro, since cumulus et al., 1991;MCKenzieet al., thisissue].Thesedomesare rocks are usually strongly heterogeneous.The rock compoprobably made of rhyolite, a high-viscositymagma that con- tains about 70% SiO2. The productionof suchmagmason Earth involvesthe removalof AI2Os, FeO, MgO, CaO, and some SiO2 by crystal settling, to leave behind a melt that is rich in SiO2. This processis only possibleif the viscosity of the silica-rich magma is sufficientlylow to allow crystal settling. On Earth its viscosityis reduced by a factor of sition at the Vega 2 landing site is thereforeconsistentwith the exposure of cumulus rocks at the surface in the more elevated regionsof Venus. Another useful quantity that can be calculatedfrom the oxide concentrationsis the magnesium number Mg#, defined by 100[MgO] Mg#= [MgO] + [FeO] about10s by the presence of water. Whenthe magmais erupted, the water is exsolvedfrom the magma and the viscosity increases. The same behaviour can account for the pancakelikedomes on Venus, becausethe surface pressureis not sufficientlyhigh to prevent exsolutionof water. The compositionof the rock analysedby Vega 2 from where the brackets indicate molar concentrations. On ter- restrial ridges, partial crystallization reducesthe Mg# of the magma from its initial value of about 65 to valuesbe- tween60 and 40 (Figure 6). This reductionoccursbecause magnesiumis preferentially removedby the crystallization the northernslopeof AphroditeTerra [Surkovet al., 1986] of olivine and pyroxene. Figure 6 compares the Mg# calcontainsless TiO2 than any of the four basalt types shown culated for the basalts at the three landing sites with those in Figures4 and 5. As Surkovet al. [1986]remark,in this from a number of terrestrial environments. The error ellipses respect,and in its low K20 content, it resemblesa gabbro. for the Venusiansamplesare large, and Figure 6 showsthat The averagecompositionof the cumulusgabbro from the basalts from any of the four tectonic environments are simOman[Pallister,1984]is shownasa star in Figures4 and 5. ilar to those from Venus. Only the materiM sampled by Only the concentrationof CaO, of 7.5 +0.7% for Vega2 and Venera 14 suggeststhat some crystallization may have oc14.6%for the Oman gabbro,is significantlydifferent. Such curred before eruption. MCKENZIE ET AL.: FEATURES ON VENUS GENERATED BY PLATE BOUNDARY PROCESSES 13,$41 -io -30 14 o 16'0 I80 Plate 1b. The topographyof the regionbetweenNew Guinea and Fiji. TRENCHES AND FOLD BELTS On Earth the rate of plate productionon ridgesis the temperature of Venus is about 470øC, or closeto the brittleductile transition for Earth's mantle. If the mantle material same as the rate of consumption, either by trenches in of the two planetsis indeedthe same,as the basaltcomposoceanicregionsor by thrusting and folding in continental tionssuggest,then, sincethe surfacetemperatureis higher regions. Unlike trenches, where the relative movement be- on Venus,fold and thrust belts shouldbe morewidespread tweenadjacentplatesis largely taken up on a singlelarge than they are on Earth. thrust, shorteningin continentalregionsoccurswithin zones The first SAR imagesof Venusfrom Magellan[Solomon whosewidth can be as great as 2000 km. The processes that et al., 1991]showedthat fold belts werewidelydistributed occurwithin thesecontinentalzonesmust take up the rela- in someparts of Venusbut did not find any convincing evi- tive motion of the plateson either side but cannot be pre- denceof trenches.The greatercoveragenow available,and dictedin detail from a knowledgeof this motion. Though particularlythe high-resolution altimetry,showsclearlythat the reasonfor this differencein behavioris not properlyun- troughswhoseshapeis similar to that of terrestrial trenches derstood, it is believed to be connected to the difference in alsoexist on Venus. Plate 1 and Figure 7 showa comparhomologous temperaturet• = TITs, whereT is the temper- isonof gravitycalculatedfrom altimetry[Haxby,1987]and ature of the material and T$ is the temperature at which topographyin the East Indies regionwith that from eastern melting begins,both measuredin degreesKelvin, between Aphroditeon Venus,with sketchmapsof the major tectonic oceansand continents.To the first approximationthe creep features.On Earth the topography neartrenches is strongly propertiesof materials are controlledby the value of t•. Be- affected by variations in crustal thickness. If these were ab- causethe meltingpoint of mantlematerialis greaterthan is that of continentalcrust,the oceanicplatesarestrongerand sentthe topographywouldcloselyresemblethe gravityfield, with I km of topographycorresponding to 140 mGal when are more likely to deform by brittle failure than are continental plates whosecrust is thicker and weaker. The surface is submarine.The structuresshownin Plate lc and Figure the topography is above sea level and to !00 mGal when it 13,542 MCKENZIE ET AL.: FEATURES ON VENUS GENERATED BY PLATE BOUNDARY PROCESSES 6052 605 1 (kin) -io 1 .IO 1 Plate lc. Altimetry of the eastern part of the Aphrodite region. 7b resemble trenches on Earth. They are curved, with the high topographyon the concaveside. Both the high and low topography is strongly asymmetric. Trenchesin the East Indies have a complicated geometry, becausethere are several small plates in the area whose movement is not closely related to the relative motion of the two large plates involved, the Pacific and Australian plates. Similar complexity exists on Venus, where the SAR images from Magellan show that extensive normal faulting occurs, associatedwith the flexure of the plate that is being subducted. The images of these faults are similar to GLORIA images of comparable is not concentratedin the mantle beneath trenchesby subduction. If the large amplitude asymmetricfeaturesin Plate lc and Figure 7b are indeed trenches, then they must terminate on transform faults, as do similar features on Earth. A number of suchfaults are sketchedin Figures 7a and 7b. In Plates lb and lc they are marked by narrow troughsin the bathymetry and topography. DISCUSSION AND CONCLUSIONS We believe that all three types of plate boundary exist on Venus,but at presentthere is no way of knowingwhether structureson Earth [Massonet al., 1990]. One important any of these structures is active. In the absenceof erosion differenceis that there is no evidenceof magmatism being the features by which all three types of plate boundary have associated with subduction on Venus. Such a difference is been recognizedwill persist indefinitely. The only evidence to be expected, since melt generation beneath island arcs of their age dependson the crater density on the surfaceof on Earth is believed to be causedby water transported into Venus[Phillipset al., 1991],whichsuggests that the mean the mantle by the plate that is being subducted. When age of its surfaceis about 0.4 Ga. This estimate lies between the water is released,it lowersthe melting temperature of that of 0.06 Ga for Earth's oceansand a radiometric age of Plate tectonic motion is the mantle and generateswet magmas. The same process about 2.0 Ga for its continents. cannot operate on Venus because there is no water on its one of severalprocessesthat could causethe surfaceage of surface. Water may be present in the mantle of Venus but Venus to be so different from its planetary age of about 4.5 MCKENZIE ET AL.: FEATURES ON VENUS GENERATED BY PLATE BOUNDARY PROCESSES 13,543 that does so that can be recognized on the SAR images is dyke emplacement. Many areas of the planet show evidence of extensionby dyke intrusion that is not confinedto narrow linear belts, as it is on Earth's ridges. Similar distributed extensiondoes occur on Earth's continents but is generally taken up at the surfaceby normal faulting rather than dyke intrusion. It will not be straightforward to estimate the changein surface area produced by dykes on Venus, because their surface expressionis so easily removed by later lava flows. The examples of plate boundaries on Venus illustrated -2O in Figures1, 3, and 7 and Plate I showthat suchstructures are not confinedto Earth. Ridges and transform faults appear to be much less common on Venus than they are on Earth 140 160 180 Fig. 7a. A sketchmap of the major tectonicfeatures[Abetsand McCaffrey, 1988; Cooper and Taylor, 1989; Louat and Pelletlet, 1989; Maillet et at., 1989; Wells, 1989]of Plates la and lb. and for this reason were not discovered until most of the planet had been imaged. Trenches, however, are widespread, and detailed comparison of their topography with those on Earth should provide constraints on the theology of the Venustanlithosphere. Acknowledgments. We wish to thank K. Burke, R. Grimm, C. Small, and R. Livermore for their help and the Natural Environment Research Council and the Royal Society for support. The paper was written while D. MCKenzie was at Scripps Institution of Oceanography, supported by the Cecil and Ida Green Foundation and the Scripps Institution of Oceanography. Earth Sciences contribution 2241. REFERENCES -2O 140 160 180 Fig. 7b. A sketchmap of the tectonic features of Plate I c. Abers, A., and R. McCaffrey, Active deformation in the New Guinea fold-and-thrust belt: Seismologicalevidence for strikeslip faulting and basement-involved thrusting, J. Geophys. Res., 93, 13,332-13,354, 1988. Cooper, P., and B. Taylor, Seismicity and focal mechanismsat the New Britain Trench related to deformation of the lithosphere, Tectonophysics, 16•, 25-40, 1989. Crane, K., The intersection of the Siqueiros transform fault and the East Pacific Rise, Mar. Geol., 21, 25-46, 1976. Crumpier, L. S., and J. W. Head, Bilateral topographicsymmetry patterns acrossAphrodite Terra, Venus, J. Geophys. Res., 93, 301-312, 1988. Ford, P. G. , and G. H. Pettengill, Venus: Topography and Ga. However,its globalimportancein resurfacingthe planet is likelyto be minor,becausePhillipset al. [1991]foundno systematicvariations in crater density with position. Such variations would be obviousif the Venustanlithosphere was formed on spreadingridges and aged as it moved towards the subduction zones, in the same way as it does on Earth. Transform faults and abyssal hills on Venus have only been clearly recognizedwithin the Artemis region, though features that somewhat resemble those illustrated in Figures I and 3 do exist elsewhere. The bright lineations in these images result from small rough escarpments. Since these can easily be buried by lavas, and perhaps even by tuff falls [Surkovet al., 1984], abyssalhills and transform faults could underlie the smooth plains that cover so much of the surface,though there is no evidencethat they in fact do so. In contrast there are many large scalefeatures that resembletrenches:thoseillustrated in Figure 7b and Plate lc were chosenbecausetheir topography most closelyresembles that of similar structures on Earth. Since there are so many trenches on Venus, there must be an equally widespreadprocessthat generatesnew surface. Other than plate generationby spreadingridges, the only other process kilometer-scale slopes, J. Geophys. Res., this issue. Fox, P. J., and D. G. 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MCKenzie, Melt generation by plumes: A study of Hawaiian volcanism, J. Petrol., 32, 501-537, 1991. Wells, R. E., Origin of the oceanic basalt basement of the Solomon Islands arc and its relationship to the Ontong Java PlateauInsights from Cenozoic plate motion models, Tectonophysics, 165, 219-235, 1989. P. G. Ford, Center for Space Research, Room 3%641, MassachusettsInstitute of Technology,Cambridge, MA 02139. C. Johnson and D. Sandwell, Geological Research Division, ScrippsInstitution of Oceanography,9500 Gilman Drive, La Jolla, CA 92093. D. MCKenzie, Bullard Laboratories, Madingley Road, Cambridge CB3 0EZ, England. B. Parsons,Department of Earth Sciences,Parks Road, Oxford OXl 3PR, England. S. Saunders, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109. S.C. Solomon, Department of Earth, Atmospheric and Planetary Sciences,MassachusettsInstitute of Technology,Cambridge, MA O2139 (ReceivedSeptember30, 1991; revised June 11, 1992; acceptedJune 11, 1992.)