JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. E4, PAGES 20,933-20,946, NOVEMBER 25, 1991

CoronaStructures on ' Models of Origin

ELLENR. STOFAN, 1DUANE L. BINDSCHADLER,2 JAMES W. HEAD, AND E. MARC PARMENTIER

Departmentof GeologicalSciences, Brown University, Providence, Rhode Island

Coronaeon Venusare circularto elongatestructures with maximumwidths of 150-1000km characterizedby annuliof concentricridges surrounding complex interiors. The featureshave raisedtopography relative to thesurroundings, they are associated with volcanic activity, and mostare partiallysurrounded by a peripheraltrough. Variationsin morphologybetween individualcoronae are due to differencesin their stageof evolutionand/or differences in the relativesignificance of the geologic processes that occur in eachstage. We examinemodels for threeprocesses that may be involvedin coronaorigin and evolution: (1) a hotspotor rising mantlediapir model, (2) a sinkingmantle diapir model, and (3) gravitationalrelaxation of topography.Rising mantle diapirs are caused by heating at depth(e.g., hotspot), while sinking mantlediapirs may result from cooling or a phasechange causing increased density and negative buoyancyat the base of thelithosphere. The hotspot model is mostconsistent with the major characteristicsof coronae,with gravitationalrelaxation occurring as a modificationalprocess. The sinkingmantle diapir would produce dominant central compression that has not been observedat coronae;however, higher-resolution image and altimetry data from Magellancan be usedto distinguishmore fully between the two models. Coronae in variousstates of formation anddegradation can be identified in theVenera 15/16 data, suggesting that the process may be continuing today.

INTRODUCTION modes of origin for coronaehave been suggested including hotspots [Basilevskyet al., 1986], ring The 15/16 mission to Venus revealed a dikes [Masursky, 1987] and rising and sinking number of terrain types and classes of features of diapirism[Stofan et al., 1987]. All modelsof origin unknownorigin. Among these, coronaeare closedor must account for the raised topographyof coronae, partly closedcircular to elongatestructures surrounded which may have undergonemodification through by an annulusof concentricridges [Pronin and Stofan, processessuch as gravitationalrelaxation [Stofan et 1990] (Figure 1). The features, first describedby al., 1988]. Barsukov et al. [1986], have maximum widths of 150- We assess the mantle diapir models of corona 1000 km and interiors that are geologically complex. origin using the basic characteristicsof coronae' Tectonic features characteristic of the interior of relatively raised topography,annuli of compressional coronae include ridges and grooves in radial, ridges,volcanism, and peripheraltrough. Both rising concentric,oblique, and/or chaotic patterns. Volcanic and sinking diapirs are modeled quantitatively. features such as domes, central edifices, and flowlike Thermal instabilities in the mantle (hotspotsor rising features are also common in the interior of coronae. diapirs)may form resultingin uplift and volcanismat The annuli of ridges surrounding the structures are the surface. Sinking diapirs may result from cooling discontinuousand vary in width, generallycomposing or a phase change causing increased density and 15-60% of the maximum radius of a corona. The delamination at the base of the lithosphere. The general morphology,evidence of imbricationwithin effects of gravitational relaxation on the raised the annuli, and similarities to features of topography produced by these models are also compressionalorigin indicate that the majority of assessed.Predictions of the topographyand resulting annuli ridges are compressionalin origin [Stofan and stressesproduced by thesemodels are thencompared to Head, 1990; Pronin and Stofan, 1990]. Volcanic observed coronae characteristics. flowlike featuresfrequently overlap and/or surround the rim, which is also frequently cut by regional tectonic BACKGROUND lineaments [Stofan and Head, 1990]. Coronae are generallycharacterized by relatively raised topography The morphology, topography, and general (<1.5 km above the surrounding region), and are characteristics of coronae provide important sometimessurrounded by an exterior moat [Basilevsky information on their origin and evolution. Twenty- et al., 1986; Stofan and Head, 1986, 1990]. Several one coronae that fit the definition given above have been describedby Pronin and Stofan [1990], while a groupof coronaein the MnemosyneRegio regionwere 1Nowat Jet Propulsion Laboratory, Pasadena, California. characterizedby Stofanand Head [1990]. Three classes 2Nowat Department ofEarth and Space Sciences, of coronae have been identified: symmetrical, University of California, Los Angeles. asymmetrical,and subdued[Pronin and Stofan,1990]. Copyright1991 by the AmericanGeophysical Union. Symmetricalcoronae are characterizedby circular to oval form, with the annulussurrounding at least 75% Papernumber 91JE02218. of the structure. Asymmetricalcoronae are similar in 0148-0227/91/91JE-02218505.00 interior morphologyto the symmetricalclass but are

20,933 20,934 STOFANET AL.: MODELS OF ORIGIN OF CORONASTRUCTURES ON VENUS

Fig. 1. Venera15/16 radarimage of AnahitCorona, centered at 63øN, 264ø. The corona, approximately350 km across,has an interior characterizedby smoothplains, domes, and a volcanic edifice. The corona Pomona is seen to the northeast.

asymmetrical about a central axis or have an annulus Associated Tectonic Features on only one side of the structure.Subdued coronae are also similar in generalmorphology to the symmetrical The interiorsof coronaeare alsocharacterized by class, but their interiors are dominatedby smooth tectonicfeatures, some of regionalorigin and some plains, and the annuli have a flooded and embayed associated directly with corona formation and appearance. evolution(Figure 2). Both extensionalgrooves and compressionaltectonic ridges can be identified in the Corona Annulus interior of coronae, along with some tectonic lineamentsof unknown origin. Some extensional The symmetrical, asymmetrical, and subdued features,such as a group of lineamentsin Bachue coronae are all characterizedby annuli of concentric Corona,lie alongthe highesttopography within the ridges that compose15-60% of the maximum width of coronaand appearto be associatedwith uplift of the structure(Figure 2). Ridgeswithin the annulusare topography.Many coronae(e.g., Rananeida,Feronia) spaced 5-15 km apart [Stofan and Head, 1986; are characterizedby throughgoingcompressional Kryuchkov, 1988a], similar to ridge spacingin the ridgesthat appearto be relatedto regionaltectonic bandedterrain surroundingLakshmi Planurn [Solomon activity rather than coronaeformation and/or evolution and Head, 1984] whichis thoughtto be controlledby [Stofan and Head, 1990]. Other lineamentswithin the thicknessof a surfaceelastic layer in the crust. coronae appear to be related to small volcanic edifices Some evidence of imbrication can be seen within the (< 50 km in diameter),such as at Otau and Pomona annuli, and the annuli are similar in general coronae. Theselineaments surrounding the volcanic morphologyto featuresof compressionalorigin found edificesmay be tectonicin originor may be volcanic in Akna and Freyja Montes [Campbell et al., 1983; features. Nightingale and Fakahotucoronae are also Crumpler et al., 1986] and some ridge belts characterizedby chaotic terrain, consistingof [Kryuchkov, 1988b; Frank and Head, 1988]. The orthogonalto obliquelyintersecting ridges. Chaotic generalmorphology and topographyof the majorityof terrainoccurs in theinner annulus and may result from ridgeswithin the annulusstrongly suggest that they later modification(i.e., gravity sliding) within the are compressionalin origin [Stofanand Head, 1990; annulus[Pronin and Stofan, 1990]. In general,unless Pronin and Stofan, 1990]. tectoniclineaments are associatedwith regional STOFAN ET AL: MODELS OF ORIGIN OF CORONA STRUCTURESON VENUS 20,935

IDEALIZED CORONA SKETCHMAP characterizedby volcanicflows, found in the interior and overlappingand surroundingthe rim (Figure 2). Flows frequently pond in the peripheral trough and inside the coronaein topographiclows borderingthe annulus. Aside from some small volcanic edifices, no clear sources for the flows can be identified. Some coronae, such as Otau, Pomona, and Feronia, are also associated with small volcanic edifices, 20-50 km in diameter (Figure 2). The edifices always lie along lineament trends that cut the annulusand appear to be related to regional tectonic activity. The volcanic edifices are surroundedby flows and apparenttectonic FEATU RE CHARACTER I STICS ridges or dikes. Coronae differ significantly from ANNULUS Compose 15-60% of the maximumradius of a most volcanic complexes in that they are not corona. Discontinuous, varies in width, often characterizedby a dominantcentral edifice, indicating • asymmetrical.imbricationseen. Ridges Interpretedspacedas 5-15 compressionalkm apart, that coronae have had a more complex volcanic in origin. evolution. INTERIOR Less than 100 km long,radar-bright and dark. RIDGES Of compressional,extensional and unknown Topography origin.In radial,concentric and oblique • orientations. Both Pioneer Venus and /16 topographic data indicatethat coronaeare generallyraised <1.5 km INTERIOR Randomlyoriented, less than 100 km long. GROOVES Graben 5-20 km wide. Interpretedas • extensional in origin. PIONEER VENUS TOPOGRAPHIC PROFILES

CHAOTIC Ridgesand groovesintersecting in orthogonal 1. NIGHTINGALE CORONA TERRAIN to obliqueorientations. Occur in 2 coronae, [ TROUGH • interpretedascomplex deformation ofannulus. 1 _ DOME 5-15 km diameter,some with summitpits. Located in annulus, interior and exterior. 0 ß Interpreted as volcanic in origin. 123 ø 136 ø FLOW Up to 100's of kms long, radar-brightand dark. 2. BACHUE CORONA Few sources identified,interpreted as volcanic :• inorigin.

EDIFICE 30-50 km in diameter, summitpits or calderas. 1 which generally lie along tectonictrends. :•• FlowsInterpretedandradialas volcanicridges insurround origin. the edifices, 249 271 Fig. 2. Idealized sketchmap of a corona showingpossible 3. FERONIA CORONA associated tectonic and volcanic features. A I I I A I

tectonic activity or local volcanic activity within a corona, the lineaments do not cut the annulus and 274 286 appear to be produced by extensional and/or 4. COATLICUE CORONA compressionalforces related to coronaevolution.

Associated Volcanism

All coronae are associatedwith volcanic activity, occurring at various times throughouttheir evolution. Small domes (10-15 km in diameter) are found in the 271 277 interior, in the annulus, and in the region surrounding VE: 50X A: LOCATION OF ANNULUS coronae(Figure 2). Barsukovet al. [1986], noting the presence of summit craters on many of the small Fig. 3. Topographicprofiles acrossseveral coronac based on domesin the Venusianplains, interpretedthe domesto Pioneer Venus altimetry measurements. Profile 1, be volcanic in origin, possibly produced by Nightingale Corona; profile 2, Bachue Corona; profile 3, Feronia Corona; profile 4, Coatlicue Corona. The profiles strombolian-typeeruptions [Head and Wilson, 1986]. were created with the 1985 NSSDC Pioneer Venus data set Analysis of the dome populationfor the sevencoronae (footprint size in the region is approximately100 x 100 km, in the Mnemosyne Regio area shows that the vertical accuracyabout 200 m [Pettengillet al., 1980]). The populationof domes inside the coronaeis higher than profiles show that some coronae are characterized by relativelydomelike topography, while othersare characterized in the surroundingregion, indicating that formation of by a centraltopographic low. Approximatelyhalf of coronae the domes is related to coronae origin and evolution, mapped in the northern hemisphereare at least partially as well as regional volcanic activity. Coronae are also surroundedby a peripheraltrough. 20,936 STOFANET AL.:MODELS OF ORIGIN OF CORONA STRUCTURES ON VENUS

above the surroundingregion. Profiles acrossseveral Corona such as Bachue exhibit high topographyand coronae are seen in Figure 3. The central region of central extensionthat are consistentwith domal uplift. many coronaeis at a lower elevationthan the annulus Coronalike volcanic structures such as Sekmet Mons (profiles 1, 4, Figure 3). The annuluscorresponds to and 58øN, 255ø provideevidence that uplift and raised or sloping topography,with ridges within the volcanic construction precede annulus and trough annulusgenerally paralleling topographic trends. Over formation. These volcanic structures have been half of coronae are also characterizedby a peripheral interpreted as protocoronae[Stofan and Head, 1990]. trough(Figure 4), approximately500 m deepand 50- In the secondstage (2, Figure 4), volcanismcontinues 75 km wide in the case of the Mnemosyne coronae combinedwith formation of the annulusand peripheral group. The troughgenerally lies outsidethe annulus. trough. Last (3, Figure 4), topographic degradation and volcanism are the dominant processes, with EvolutionarySequence covering of the annulusand infilling of the trough by local volcanic activity. Late stage regional tectonic Stofan and Head [1990] examinedthe morphology activity postdates corona formation, with bands of of seven coronae in the Mnemosyne Regio area and lineaments frequently cutting coronae annuli. derived a general evolutionary sequence for these Volcanism, in the form of small volcanic edifices features(Figure 4). Comparisonsto the morphology discussed above, tends to concentrate along these of a large number of coronae [Pronin and Stofan, lineament bands. Despite the general similarities 1990] provide further support for this sequence. between coronae that allow them to be classified as a Regionaltectonic and volcanicactivity were generally group, significant differences in morphology exist found to precedecorona formation. The initial stage of corona formation (1, Figure 4) involves uplift of between individual coronae. We interpret these differences to reflect the differing amounts and the surface, volcanic activity, and construction. significanceof the processesdiscussed above in the formationof each corona. For example, some coronae have been significantly overprinted by regional CORONA EVOLUTION tectonic activity (e.g., Rananeida), while others postdatemuch of the regionaldeformation (e.g., Otau). All coronaeappear to follow the evolutionarysequence , describedabove, but the stage that different coronae are at may vary as well as the relative significanceof the processesthat occur in each stage.

Summary - UPLIFT Coronae appear to form by uplift and volcanic -VOLCANISM construction,followed by formationof the annulusand - CENTRAL EXTENSION peripheral trough. Both extensional and compressionalfeatures can be found in the interior of the structures, with volcanism occurring throughout . their evolution. The interior of some coronae are also topographicallylower than the annulus,which does not alwayscompletely surround the corona. The basic characteristics of coronae that must be accounted for in any model of origin include relatively raised topography,annuli of compressionalridges, interior volcanic and tectonic activity, presenceof a peripheral - ANNULUS FORMATION trough, and the evolutionarysequence described above. - TROUGH FORMATON These general characteristicssuggest some basic processesrelated to thermal perturbations,diapirism, . and gravitationalrelaxation. We have pursuedmodels of these processes in order to test whether they producesurface features and sequencessimilar to that observedfor coronae. In this paper, we discussthree analytical models that may be significant in corona origin and evolution: mantle diapir models (both - REDUCTION OF TOPOGRAPHY rising and sinking anomalies) and gravitational - LOCAL VOLCANISM relaxationof a plateauand dome. -INFILLING OF TROUGH Fig. 4. Generalizedsequence of eventsof coronaevolution. MANTI• DIAPIR MODELS In the first stage, coronaeare characterizedby uplift, volcanism, and central extension. Annulus and trough formation occur in the second stage, with reduction of Diapirism involves the rising or sinking of topography,local volcanism,and infilling of the trough material. Density contrastsmay result from heating occurringin the third stage. and partial melting, causing upward movement of STOFAN ET AL: MODELS OF ORIGIN OF CORONA STRUCTURESON VENUS 20,937

A. RISING ANOMALY conditions on the half-space and matching conditions between individual layers. Velocities are required to vanish at great depths (z--, oo), while at the surface , > shear stresses vanish and vertical normal stresses are those due to topography. Horizontal and vertical componentsof velocity and shear and vertical normal stressesare required to be continuousacross viscosity interfaces within the halfspace, except at the crust- mantle boundary and at the location of the density B. SINKING ANOMALY anomaly. Here, vertical normal stresses are discontinuous by amounts dependent upon the deflection of the crust-mantle boundary and the magnitude of the density anomaly, respectively. In practice, the density anomaly representingthe rising diapir is deconvolvedinto individual harmonicsusing two-dimensional fast Fourier transforms (FFT's). Boundary and matching conditionsform a set of linear Fig. 5. (a) Idealized rising anomaly model of corona origin. equations for each harmonic which is solved using A thermal anomaly or hotspotresults in partial melting, with matrix inversion methods and whose solution yields lighter buoyant material rising causinguplift and volcanism at the surface. (b) Idealized sinking anomaly model of corona constantsof the equationsof flow. We describeresults origin. A sinking diapir may form due to a phase change of the various models in terms of surface topography producedby thickening of the lithospherebelow a critical and style of deformation. Deformationis examinedby depthor coolinginstabilities at the baseof the lithosphere calculating principal stressesat the surface at different times during the rise of the diapiric body. Stressesare material (Figure 5a). Conversely,cooling or a phase calculated directly from the equations of flow. changemay cause the density of material to increase, Following Anderson's theory [1951], fault types are resulting in downward motion (Figure 5b). We have determined by the orientations of greatest and least formulated a model to study the effects on the surface compressivestresses. Strictly speaking, the above model only allows us of both upward and downwardmotion from the interior to consider the effects of a steady (fixed in space) of a planet. source of mantle flow. In order to considerdynamic effects, continuousmovement of the diapir is simulated Hotspot or Rising Mantle Diapir Model by placing the diapir at a depth d for a time interval At First, we examine the topography and deformation that is short compared to the characteristicresponse that are predicted to result from the upward movement time of the crust [Bindschadler and Parmentier, 1990] of a diapiric body in the Venus mantle. The crust and and calculatingaccumulated topography at the surface mantle are treatedas a linear viscoushalf-space that is layered in both density and viscosity, following the approach of Bindschadler and Parmentier [1990] z = •5(t) (Figure 6). In all models,we assumea 10% density contrast between the crust and mantle, and a crustal densityof 3000km/m 3. Effectivelinear viscosities for individual layers are obtained from flow laws for diabase[Shelton and Tullis, 1981], representingcrustal rocks, and olivine [Goetze, 1978], representingmande rocks. Temperatures are assumedto follow an error function distribution with depth, appropriate for a thermal boundary layer. Near the surface, such a distribution is approximately linear but at depth approaches an asymptotic value appropriate to a convectivesystem (1500 K). Flow is driven by an axisymmetric density anomaly within the halfspace, representingthe rising diapiric body. We take the rising body to have a stress magnitude of about 30 MPa (the stress necessary to support I km of Fig. 6. Schematicof the model used for the rising and topography). A number of shapeswere examined for sinkingdiapir modelsand the gravitationalrelaxation model, [after Bindscharier, 1990]. In the simplifiedcase shown, the the shape of the diapiric body, including spherical, half-space is stratified in both density and viscosity, with multilayered shapes. Results were found to be topography at the surface and at z = h (h = upper layer insensitive to choice of diapir shape, and a Gaussian thickness)represented by harmonicfunctions (d(x, t)= F(t) function was used in the results described below, cos kx, e(x,t) = G(t) cos kx, where k is the wavenumberand F largely for computationalconvenience. and G are time-dependentamplitudes of the topographyat the surface(F) and along the interface(G)). The casedescribed in In this multiple-layered linear model, solutions to the text utilized multiple layers with viscosities varying the equationsof flow are fully determinedby boundary accordingto flow laws for diabaseand olivine. 20,938 STOFAN ET AL.: MODELS OF ORIGIN OF CORONA STRUCrURES ON VENUS

a. 0.2 and the crust-mantle boundary. The diapir is then I I I I moved a distance Az = w At, where w is determined

from Stoke's law and the resulting Az is much less 0.15 - x d = 300 km - than the crustal thickness. At each new location of the diapir, additional topography is added to that 171 km 0.1 - 139 km _ which has already accumulated. Results were tested for 106 km convergence by halving the time interval and comparing topography and surface stressesand were 0.05 found to be identical within 1%. In this model, the responseof the surface to a rising diapiric body is most strongly affected by the strength (viscosity) structure of the crust and I I I I uppermostmantle, or the theological lithosphere. The -0.050 1O0 200 300 400 500 strength of these layers is most strongly affected by R (km) the crustal thicknessand thermal gradient and, to some extent, is also a function of the characteristic value of deviatoric stress used to obtain linear viscosities from b. 250 I I I I flow laws [Grimm and Solomon, 1988]. We have 2OO examined model results for crustal thickness values of • RADIAL STRESS 150 10 and 30 km; surfacethermal gradientsof 10, 20, and ,-o- HOOP STRESS

30 K km-1;and characteristic stress values of 10 and lOO 25 MPa. These values representlikely limits on the model. In all of these runs, the diapir was placed 50 initially at a depth of 300 km and was allowed to rise o to a depth of approximately 100 km. Calculations were halted at that point becauseof the likelihood that -5o the interactionbetween the diapir and the overlying -lOO - lithospherewould result in significantflattening of the diapir and subsequentalteration of the flow field. -150 i i i i The nominal case of the model was run with a 10- 250 I I I I km-thickcrust, a 20 K km-1 surfacethermal gradient, 200 and a characteristicstress value of 10 MPa (Figure 7). The rising mantle diapir or hotspotresults in uplift of 15o the surface,with the magnitudeof the uplift increasing and the width decreasingas the diapir approachesthe surface (Figure 7a). An increase in the thermal gradientresults in decreasingtopographic uplift due to weakening of the lithosphere permitting crustal thinning to begin before the diapir reachesnear-surface levels. An increasein the crustal thicknesssimilarly reducesthe amount of uplift. Over the center of the uplift, both hoop and radial stressesremain extensionalthroughout the rise of the 250 diapir (Figure 7b). Hoop stressesare larger in I I I I magnitude, predicting formation of radial extensional 200• featuresduring uplift. As the diapir nearsthe surface,a 150 zone of radial compressionalstresses develops along the base of the growing region of uplift (Figure 7b, 100 middle). Sincehoop and radial stressesare oppositein sign and comparablein magnitude,strike-slip faulting 5O is expected. As the diapir continues to near the surface (Figure 7b, bottom), compressional radial stressesat the base of the uplifted region have become -50 large comparedto hoop stresses,predicting formation -1 oo of concentriccompressional features. -150 I I I I 0 100 200 300 400 500 R (km) Fig. 7. Rising mantle diapir model results. (a) Surface example,for a risers•ress of about30 MPa anda crustal topographyshown at five differentdepths of the diapir. As densityof 3000 kg m'ø, a dimensionlessheight of 1 is equal the diapir approachesthe surface, uplift increasesand to I km. (b) Stress at the surfacedue to a rising manfie narrows. The topographyresults are sealedby the ratio of diapir. Radial, hoop, and vertical stressesare shownat three the densityanomaly which represents the diapir(expressed as depthsof the diapir;(top) depth= 220 km, (middle)depth = a surfacemass density) to the densityof the crust. For 171 km, and (bottom)depth = 106 km. STOFAN ET AL: MODELS OF ORIGIN OF CORONA STRUCTURESON VENUS 20,939

Variations of model parameters within the limits A) STAGE 1. FORMATIONAND DETACHMENTOF described above do not alter the prediction of radial SINKING BODY extension in the interior of the uplifted region and strike-slip faulting along the periphery. Relative magnitudesof radial compressionand azimuthal (hoop) extension along the periphery of the uplifted region depend on the strength (viscosity) structure of the lithosphere,which is a function of thermal gradient, crustal thickness and flow laws chosen for crust and mantle material. The magnitudeof radial compression is increasedfor strong, thin near-surfacelayers, which B) STAGE 2. SINKING, ISOSTATIC UPLIFT is likelyto occurfor moderate tohigh (< 20 K km-1) thermal gradients or increased (> 10 km) crustal thickness. In summary, the hotspot or rising mantle diapir model predicts uplift of the surface, with the uplifted region becoming narrower and higher as the diapir approaches the surface. Radial normal faults are predicted over the center of the uplift with azimuthal compression or strike-slip faulting at the margins. Increasingthe thermal gradientor the crustal thickness C) STAGE 3. CONTINUED SINKING, SUBSIDENCE results in crustal thinning, decreased uplift, and increasedazimuthal compression.

SinkingMantle Diapir Model The process by which a portion of the lower lithospheredetaches and sinks is extremely complex; thus its effects on the surface cannot be addressed in a simplemodel as was done for the rising mantlediapir. The processof a sinkingmantle diapir can be broken into three stages(Figure 8): Stage 1. An instabilitydevelops at the baseof the Fig. 8. Stagesin the evolutionof a sinkingmantle diapir. thermallithosphere due to coolingor at depthdue to a (a) Stage 1. Formationand detachmentof a portionof the lower lithosphere. A topographic low with interior phasechange, leading to the detachmentof material compressionis predicted. (b) Stage2. Sinkingof the body forming a sinking diapir (Figure 8a). Understanding with isostaticuplift causedby replacementof the sinking the early time variations in surface topographyand portionof the lithospherewith warmermaterial. A troughis deformationas the instability developsand begins to predictedaround the uplift. This stagewould most likely be detach involves a greater complexity of calculation accompaniedby volcanism. (c) Stage 3. Continuedsinking of the detachedportion of the lithosphere,accompanied by than warranted here. Qualitatively, this stage is subsidenceof the topographyand accompanyingdeformation. expectedto lead to the formationof a topographiclow At this stage,crustal thickening would also occur. and compressionaltectonic features due to downward vertical normal stresses associated with the formation and initial detachmentof the sinkingbody. relaxation time, which determines the rate at which Stage 2. The detachmentand sinkingof a portion thermal supportfor topographydecreases. In order to of the lithosphereproduces three distinct sourcesof assess the relative importance of these effects, we topography:upward vertical normal stressesassociated derive the characteristic time scales associated with with isostatic adjustment to hot material that has each sourceof topography. replacedthe sinkingbody, downwardvertical normal Consider a portion of the lithosphere that has stressesassociated with the sinking of the detached detached and begun to sink due to its negative part of the lithosphere,and downwardvertical normal buoyancy. The mean temperatureof the lithosphereis stresses associated with loading of the surface by TL and that of the convectingmantle below is TL + volcanism(Figure 8b). The shapeof topographyand AT. The temperaturedifference AT leadsto a density the stress field depend on the interactions of these differenceAp = p•xAT,where •x = 3 x 10-5 K-1 is the sourcesof stress and the rate at which each changes coefficientof thermal expansionand p is the density with time. Effects due to the sinking body are of mantle material at temperatureT. To first orderwe stronglyattenuated as it sinks and thus dependupon ignorethe detailsof the shapeof the sinkingbody, its downwardvelocity. The rate at which the surface and considerit as a spherethat obeysStokes' law as it respondsisostatically to hot material intrudedat the sinks. The diameter of the body divided by the baseof the lithosphereor to a topographicload caused sinkingvelocity yields a characteristicsinking time by volcanismdepends primarily upon the viscosityof the mantle and the characteristic wavelength of the 9•trn xD = • (1) load. Also relevant is the characteristic thermal Apga 20,940 STOFAN ET AL.: MODELS OF ORIGIN OF CORONA STRUCTURES ON VENUS

where ix is the viscosity of the convectingmantle, g is cooling of the intrusion of warm mantle material. The the acceleration due to gravity, and a is the uplift is surroundedby a peripheral trough due to characteristic radius of the sinker. sinkingof the detachedportion of the lithosphere.As Uplift of the surfacedue to the intrusionof warmer the diapir sinks and the heated mantle cools, it seems mantle material into the void left by the sinking body likely that the rise of warm mantle material to occurs at a rate governed by the time constant for relatively shallowdepths will result in partial melting, isostatic rebound, familiar from studies of glacial intrusion, and extrusion of melts. Because of the rebound [e.g., Turcotte and Schubert, 1982]. In the relatively long-lived nature of the thermal anomaly limit that load wavelengths are greater than associatedwith delamination,volcanism and plutonism lithospheric thickness (most coronae are greater than axe expectedto continue into the third stage of the 250 km across): sinking process. Stage 3. At this point in the sinking process,a all Xr = -- (2) topographic high would exist on the surface due to pga thermaluplift, the sinkingbody wouldbe near enough where 4a is taken to be the characteristicwavelength to the surface to have an effect on topographyand stressfields, and volcanismwould be likely to occur of the load. Since the load of any volcanic deposit (Figure 8c). We therefore consider a model for this associatedwith a corona is likely to be the same size stage in which a sinking body and a volcanic load at (within a factor of a few), the same characteristictime the surfacecombine to producetopography and surface scale appliesto relaxationof sucha load. Note that xD deformation. We use a versionof the rising mantle and xr are proportionalto ix/a, and thus their ratio is constant even if mantle viscosity or sinking body diapirmodel modified to includean initial topographic load on the surface. Similar cases were run for the radius are varied. model: crustal thicknessesof 10 and 30 km; thermal Cooling of the thermal intrusion beneath the gradientsof 10,20, and30 K km-1;and characteristic surface (Figure 8) is governed by transient one- stresses of 10 and 25 MPa. We assumed that the dimensional cooling and is characterized by a time scale sinking body was 200 K cooler than the mantle through which it sank, and for all models the initial Xc = -- (3) topographic load due to volcanism was taken to be Gaussianin shape with an amplitudeof 500 m. The initial depth of the sinkingbody was taken to be 100 whereAL is the verticaldimension of the regionwhere km; runs were continueduntil a depth of 300 km was lithosphere has been replaced by warmer mantle reached at which point the only significantchange material and •c is thermal conductivity. We first with increasing depth was an overall decrease in consider the case where AL = 2a, adopting the surface horizontal stress levels. parametersshown in Table 1. For these parameters, The combinedvertical loads of the sinking body we find that xc > xD as long as a > 23 km. The effect and the volcanic load result in the initial formation of of our approximationsis likely to overestimatexc and a topographic trough surrounding the assumed underestimatex D, but while a lengthscale of the order topographichigh (Figure 9). Figure 9 illustrates a of 20 km could representthe thermallithosphere on nominal case which assumeda 30 km thick crust, 20 K Venus (particularlypostdating a delaminationevent, as k m-1 surfacethermal gradient, and 10 MPa supposedhere), it is a factor of-5 smaller than the characteristicstress. Relaxation of the topographic smallestcorona radius. More indicativeis an example in which we take L = 50 km and a = 100 km. We then high occursquite rapidly. This is consistentwith the findx c = 83x 106 yearsand xD = 17x 106years. For relationshipbetween '1;r and '1;D as derived in equations (1) and (2). The bottom of the trough moves inward thesesame parameters, xr = 35,000 years. and the trough shallows as the diapir sinks and the We thereforeexpect that the secondstage of the topographic high relaxes. Although details of the sinking processwill be accompaniedby uplift of the topographicshape are different for different values of surfaceand by deformationsimilar to that predictedby crustal thickness, thermal gradient, and characteristic the rising diapir model. This uplift occursrapidly stress,all casesexhibited a trough surroundinga high, compared to the sinking of the diapiric body and which moved inward and shallowed while the topographichigh relaxed. In all cases the maximum

Table 1. Model Parameters depth of the trough was approximately100 m or less. This model implies that troughsare transientfeatures; Symbol Parameter Value troughs around coronae also might be producedby a coefficientof thermalexpansion 3 x 10-5 K-1 flexure of the lithosphere. a radius of sinker variable The deformation due to the interaction of stresses AT temperaturedifference between 200 K due to a sinking body and a topographic load is lithosphereand convectingmantle p densityoflithospheric material at 3300kg rn -3 complex, showing a great deal of variation both with time and with different values of model parameters. temperatureT /• thermalconductivity 10-3 m s-2 We summarizepreliminary resultshere, with the caveat Pm viscosityof mantle at temperature 1021Pa s that more extensiveexamination would be requiredto fully characterize the processes that produce the T+AT deformation. The general pattern of stressesin the STOFANET AL.: MODELS OF ORIGIN OF CORONA STRUCTURES ON VENUS 20,941

SINKER MODEL •

0 25OO5000 =1• I I 0.25m• •• • ------250 OOPSTRES -0.75 -5000I (a)-1 2000 I I I I 0.1 d = 144 km 15OO 1000

-0.2 T 500 -_ _ -- ..... _%•-•__2 •_- !_- .... o -0.3 Et -500 -1000 -0.4 (b) -1500 I I I I _ -0.5 I GRABEN 75O I I I I 0.1 d = 144 km 5OO o • THRUSTFAULTS -o.1 m• 250 Fig. 10. DefOrmationalfeatures predicted by the sinking mantle diapir model. Interior radial compressionalfeatures -0.2• are surrounded by concentric graben associated with the o -0.3'• peripheral trough. Concentric compressionalfeatures are predictedto form outsidethe trough. -250 -0.4

I I I I -5OO -o.5 0 100 200 300 400 500 surfacesurrounded by a trough, with radial extensional R (km) featuresand possiblyperipheral compression. Uplift Fig. 9. Surfacetopography and stressespredicted by the occursrapidly comparedto the sinkingof the diapiric sinkingmantle diapir model for three depthsof the sinking body and the cooling of intruded warm material. As mantlediapir. As the diapir sinks,topography decreases and the detachedportion of the lithospherecontinues to the troughshallows and narrows. The topographyresults are sink, the topographichigh produced in the second scaledby the ratio of the densityanomaly which represents stage relaxes. The trough around it shallows and the diapir(expressed as a surfacemass density) to the density of the crust. For example,for a sinker stressof about 30 narrowsas sinkingproceeds. Deformation produced by MPaand a crustaldensity of 3000kgm -3 a dimensionless this final stage is characterized by radial heightof 1 is equalto 1 km. Shorteningis predictedwithin compressional features associated with the interior the relaxing topographichigh, with marginal radial extension high topography,concentric graben associatedwith that follows the trough as it moves inward as the diapir sinks. Radial compressionoccurs at later stages. the troughand compressionalfeatures surrounding the trough.

GRAVITATIONALRELAXATION MODEL sinkingmantle diapir model implies an inner zone of radial compressionalfeatures associated with the high Coronaerepresent a topographicload on the surface topographyof the volcanic load, a zone of concentric of Venus. The dominance of volcanic features extensionalfeatures associated with the trough itself, indicates that some portion of corona topography and a zone of concentriccompression surrounding the consistsof volcanic material depositedon the surface trough (Figures 9 and 10). The innermostzone is due of the planet. In addition, thermal support for the combinedeffects of the sinking body and the topography will decay as lithosphere heated by topographicload; both tend to cause compressional upwelling or delaminationcools. Dynamic support hoop and radial stressesat the center of the load. The will ceaseas a rising diapir encountersthe theological zone of extensionrelated to the troughis due to the lid of the lithosphere. Gravitational relaxation of surface load, which causes stress similar to that of an topographyis a processthat has been suggestedas elastic layer. The outer zone of radial compression likely to occur on Venus on geologic time scales appears to be due to flow inward toward the center, [Weertman, 1979; Solomon et al., 1982a]. Materials related to the sinking body. at relatively shallow depths are expected to exhibit In summary, the sinking mantle diapir model is ductile flow due to high Venus surfacetemperatures. characterizedby an early time depressionwith interior Solomonet al. [1982a] suggestedthat an impactbasin compression.This stageis followedby uplift of the severalhundred kilometers across and 3 b.y. old or 20,942 STOFAN ET AL.: MODELS OF ORIGIN OF CORONA STRUCTURESON VENUS

greater shouldhave little topographicrelief at present. vertical normal stress. Analytic solutions for Stephens et al. [1983] predicted that the high topographyand stressesare obtained as a function of topographyof (over 8 km above the time. mean planetary radius (6051.9 km)) would relax in a Two topographic forms were chosen as likely few hundredmillion years if not supportedby dynamic starting conditions for a corona: (1) a steep-sided processes. At these time scales,and in the absenceof topographichigh or plateau and (2) a gently sloping significant erosion rates [Iranov et al., 1986], topographic high or dome. Two starting conditions gravitationalrelaxation may be of great importancein were used for each topographic form: (1) initially modifying and reducingsurface relief on Venus. isostatically (Airy) compensatedtopography and (2) We apply a gravitationalrelaxation model to two initially uncompensatedtopography topographicforms that might representearly forms of For the cases of an initially isostatically coronae on Venus, a dome and a plateau. The compensatedplateau and dome, the topographicfeature topographicforms are relaxed over time in an attempt relaxes uniformly to zero (Figures 11a and 11b). to reproduce coronalike topography. In addition, Heights for the dome and plateau are equal to one at stresses produced by the relaxing topography are t=O. The topographic high does not undergo any analyzed to see if they are consistent with corona significant change in form as it relaxes; the elevation tectonic features. decreases at approximately the same rate for all The relaxation model uses the same layered wavelengths. The model fails to reproducethe trough theology as the rising mantle diapir model characteristic of most coronae, as well as the central [Bindschadlerand Parmentier,1990] (Figure 6). low typical of many coronae. Solutionsare thoseobtained by Bindschadler [1990], Relaxation of initially isostaticallyuncompensated extended to axisymmetricgeometries using two- topographyresults in the more rapid loweringof the dimensional FFTs. In all cases examined, shear feature and developmentof a peripheraltrough (Figure stressesvanish at the surface. Relaxationis drivenby 12). Relaxation of a plateau results in the formation topographic loads at the surface and/or crust-mantle of a central low, while relaxation of a dome does not. boundary,which are approximatedas discontinuitiesin The topographyof the relaxed structureis similar to

GRAVITATIONAL RELAXATION MODEL: GRAVITATIONAL RELAXATION MODEL: COMPENSATED PLATEAU COMPENSATED DOME

a.

TIME 1 TIME 1

TIME 3 TIME 5

TIME 5 TIME 3

Fig. 11. Topographyresulting from gravitationalrelaxation of an initially isostatically compensated(a) plateauand (b) dome. The initialform of the topographyis preservedas the topographicrelief decreases,shown for threearbitrary time steps. STOFAN ET AL.' MODELS OF ORIGIN OF CORONA STRUCTURES ON VENUS 20,943

RELAXATION MODEL (and the thickness of the theological lithosphere 1.5104 1 increased),initial compressionin the center of the featurebecomes greater compared to later extension,as 0.5 1ß0104 more of the earlier deformation is taken up in sagging o T of the stronglayer [Bindschadler,1990]. rn 5ß0103- -o.5 • The topographicform and stressesresulting from -1 T gravitationalrelaxation differ based on the initial 0ß0100- ___- -15 •' degree of isostaticcompensation of the topographic ß 3 v feature. Initially uncompensateddomes and plateaus -5.01031- .J• • - RADIAL_ST_R_E_SSq-2 undergo a changein shape as they relax, forming I ..•_• -o- HOOP STRESS -•-2.5 peripheral troughs, and, in the case of the plateau, -1.0104 I I I I (a)1-3 central sags. Stressesare initially compressionalin the center and extensional at the periphery, then 0.5 changein sign and decreasein magnitudeas relaxation 1'ø1ø4•/ I Itl o proceeds.Initially compensatedfeatures maintain their 5.OLO3 shape as they relax. -o.5 • -1 --I DISCUSSION -1.5 O.Ol o0 ..... :_-__:, -2 Model Predictions -2.5 We have formulated and tested three models that -5.0103T..... I I I I / -3 may be involved in corona origin and evolution' 4OOO 0.5 I I I I mantle diapirism (rising and sinking anomalies) and 3OOO gravitationalrelaxation of topography. Hotspot model. A hotspotor rising mantle diapir 2000 producesuplift at the surface, with extensional radial lOOO featurespredicted in the central region surroundedby azimuthal compressionalfeatures. Higher, narrower o uplift is producedas the rising diapir is placednearer -lOOO to the surface. Sinking diapir model. For a sinking mantle diapir, -2000 I I -2 0 1 O0 200 300 400 500 an initial depressionis predictedfollowed by uplift and R (km) volcanic loading. A peripheral trough is predicted, along with an inner zone of radial thrusting, Fig.12. Topographyand stresses predicted by therelaxation concentric graben associated with the trough, and of an uncompensatedplateau. Resultsfor the dome are concentric features of compressional origin similar. Topography,vertical, hoop and radial stressare shown,at (a) 260 years, (b) 5700 years,and (c) 25,000 surroundingthe trough. years. As the topographichigh lowers in relief, a centralsag Gravitational relaxation. The gravitational and peripheralti'ough develop. Relaxationoccurs on more relaxation model was tested for several cases of initial rapid time scales than in the compensated case. isostaticcompensation for a plateauand dome. In the Uncompensatedrelaxation of a domeresembles Figure 1 lb; initially compensatedcase, both the plateau and dome no centralsag is formed,although a troughdoes form. Stress levelsare quitehigh due to the high viscosityof near-surface relax without a change in shape. The initially layers. As time proceeds,the shapeof the stresscurves uncompensatedcase does predict a change in shape, remainsapproximately the same,until very late stageswhen with a peripheral trough and central sag produced as the stresslevels are extremelylow and the relaxationmore the dome or plateau relaxes. The central region is resemblesthat of compensatedtopography. initially characterized by both radial and azimuthal compression. As the plateau changes shape, radial compression is focused along the rim and radial that of a relaxing impact crater [Solomon et al., extensionin the trough surroundingthe plateau. 1982a, b; Hall et al., 1981]. Relaxation proceeds at an initially faster rate than the compensatedcases, as Comparisonsto Corona Characteristics much of the deformation is taken up in the relatively inviscid half-space. Early time stresses for the In the hotspot model, early uplift and central uncompensatedcases are much higher than in the extension are predicted, consistentwith the swell-like compensatedcases. Plots of surface stressfor the topography, tectonic features, and central volcanism plateau (Figure 12) indicate that at early times, the characteristicof coronae. The peripheraltrough and center of the plateau undergoes both radial and central low region typical of many coronae are azimuthal compression,while radial extensionoccurs consistent with uncompensated relaxation of the along the base of the plateau. As relaxation uplifted region. The central low characteristicof many progresses, compressional stresses in the center coronae is only produced by the uncompensated decrease,hoop stressesdecrease in magnitude,radial relaxation model. The hotspot or rising mantle diapir compressionalstresses are focusedalong the rim, and model does predict that the topographynarrows as the radial extensional stresses are focused in the anomaly approachesthe surface. Late stage behavior developingtrough. If the thermalgradient is decreased of the rising mantle diapir, when flattening occurs as 20,944 STOFANET AL.:MODELS OF ORIGIN OF CORONASTRUCTURES ON VENUS

the body approachesthe surface, may cause additional causedby a mantle diapir at depth acting on layers of deformation not included in this model. No correlation prescribed viscosity. In the hotspot or rising diapir between age or complexity of structure and size of model, uplift is accompaniedby central extensional coronae has been identified at this time. In general, features surrounded by azimuthal compressional the hotspot model combined with gravitational features. As the force distribution representingthe relaxation is consistent with most of the major diapir is placed closer to the surface,narrower higher characteristics of coronae. This combination of uplift results along with radial normal faulting and models predicts relatively late stage formation of the some amount of concentriccompression. The sinking central low and trough. mantle diapir model predicts uplift of the surface A sinking mantle diapir model would produce an surrounded by a peripheral trough. The high initial depression followed by uplift and trough topography increases and the trough narrows and formation. The predicted topographyis consistent shallows as the diapir is placed at deeper levels. with coronae, with the exception of the lack of Radial thrust faults, concentric graben, and outer identification of basins that may be precursors to concentric compressional features are predicted. coronae. As the diapir sinks, the peripheral trough Gravitational relaxation models were also investigated narrows and shallows. Troughs surroundingcoronae and were found to produce a central topographiclow are generally 50-75 krn acrossand approximately500 and peripheraltrough. m deep, with no obvious change with increasing corona degradation. Coronae interpreted to be the Origin of Coronae oldest (those 'm the subdued class) are not surrounded Coronae on Venus have several defining by troughs, which could, however, be below the characteristicsincluding relatively raised topography, resolution of available altimetry data [Pronin and annuli of concentric ridges, peripheral troughs, and Stofan, 1990]. Or the troughsaround these coronae association with volcanism. Studies of individual may be infilled with volcanic material. Radial coronamorphology have defineda sequenceof events. compressionalfeatures, concentric graben, and outer Uplift and volcanism occur in the early stages, concentric compression are predicted at corona. followedby annulusand troughformation, topographic Concentric graben are not typical of coronae in the degradation,and continuedvolcanism. This sequenceof Venera 15/16 data, and the concentriccompressional events is most consistentwith formation by a hotspot featureslie outsidethe trough unlike at coronae. Some accompaniedand followed by gravitationalrelaxation. interior linear featuresare of unknownorigin and thus However, the sinking mantle diapir model cannotbe may fit the model. The raised topographyproduced in completelyruled out, as it predicts many features this model would undergo gravitational relaxation as characteristic of coronae. None of the models describedabove. In general,the sinkingmantle diapir adequatelyexplains formation of the annulus. We are can account for many of the observed corona currentlydeveloping more detailedrelaxation models characteristics,but the prediction of dominant central as well as examining other possiblemechanisms of thrusting, concentric graben, outer concentric annulus formation. compressionalfeatures, and an early time topographic Relaxation time scales suggest that some coronae low makes it a less favored model. Neither model are relatively young features. The processof corona adequatelyexplains why deformationis concentratedin formation is an ongoing one; some structures appear the annulae of coronae. highly degraded,and othersstill may be in the process The average age of the surfaceof the northern of formation. Variations in morphology between hemisphere of Venus has been estimated at 0.5-1.0 coronae are interpretedto result from differencesin b.y. [Iranov et al., 1986] or younger [Schaber et al., their individual stage of evolution as well as 1987]. Since gravitational relaxation is a time- differencesin the significanceof various processesin dependentprocess, upper limits can be placed on the their evolution. Coronae occur as both isolated ageof coronae.For a mantleviscosity of 1021Pa s, features in the low- latitude plains and in clustersat relaxationtime for a 400-km-diameteruncompensated higherlatitudes. Regardless of whethercoronae formed plateau is approximately 100 m.y. or less by rising or sinking mantle diapirs, their formation [Bindschadler, 1990]. Relaxation times for involveda significanttransfer of heat from the interior compensatedfeatures will be longer, and are sensitive to the surface. We are currently assessing the to crustal thicknessand temperaturegradient. These significanceof coronaeto heat lossprocesses, as well relativelyfast relaxationtime scalessuggest that many as their relationshipto global tectonicpatterns. coronae are relatively young in age, with most probably similar in age to the overall age of the Testsfor northernhemisphere. The processof coronaformation appearsto have been occurring over a period of time, High resolutionimage and altimetry data from the with some coronae apparently almost completely Magellan mission to Venus which arrived in mid-1990, relaxed (e.g., Demeter), others in the process of can be used to differentiate between models of corona relaxation (e.g., Feronia), and others still may be origin. The sequenceof events,initial topographyand forming (e.g., protocoronae). evolutionof the trough, and nature of interior tectonic featuresare critical in understandingcorona formation. CONCLUSIONS Sequenceof events. The risingmanfie diapir model We have developed models that can be used to predictsearly uplift and volcanismfollowed by later predict topographyand stressesresulting from forces trough formation. The sinking model predicts STOFAN ET AL.: MODELS OF ORIGIN OF CORONA STRUC'U.NtF• ON VENUS 20,945 formation of a topographic low, followed by a deformationon Venus: Analysis of Veneras 15 and 16 topographichigh surroundedby a trough. Formation data, J. Geophys.Res., 91, 399-411, 1986. Bindschadler,D.L., Modelsfor the originof tesseraterrain: A of a compressionalannulus could occurat early stages studyof the tectonicsof Venus,Ph.D. thesis,241 pp., or at later stagesoutside the trough. At present,the Brown University,Providence, R.I.,May, 1990. geologicalrelations that have beenestablished [Stofan Bindschadler, D.L., and E.M. Parmantier, Mantle flow and Head, 1990; Pronin and Stofan, 1990] are not tectonicsand a ductilelower crust:Implications for the sufficiently detailed to permit distinguishingbetween formationof large-scalefeatures on Venus,J. Geophys. Res., 95, 21,329-21,344, 1990. the two models. Volcanic complexes interpreted as Campbell,D.B., J.W. Head, J.K. Harmon, and A.A. Hine, proto-coronaeand coronaesuch as Bachuesuggest that Venus' Identification of banded terrain in the mountains uplift and volcanismpredate major annulusand trough of , Science,221, 644-647, 1983. formation, favoring the hotspotmodel. Crumpier,L.C., .J.W.Head, and D.B. Campbell,Orogenic Topography. Detectionof basinswhich may be beltson Venus,Geolo3y, 14 , 1031-1034, 1986. Frank,S. L., andJ.W. Head,Classification of ClassH ridge precursorsto coronae in the Magellan data would belts on Venus (abstract),Lunar Planet. Sci., XIX, 351- provide supportfor the sinking diapir model. In 352, 1988. addition,the sinkingmantle diapir model predictsthat Goetze, C., The mechanismsof creep in olivine, Philos. the troughwill becomemore narrowand shallowwith Trans. R. Soc. London, Set. A, 288, 99-119, 1978. time. Neither of these effects has been identified in Grimm, R.E., and S.C. Solomon, Viscous relaxation of crater relief on Venus: Constraints on crustal thickness and available (50-100 km resolution) altimetry data. thermal gradient, J. Geophys.Res., 93, 11,91l- 11,929, Current data suggestthat the troughs around coronae 1988. are all about the same width and depth, which supports Hall, J.L., S.C. Solomon, and J.W. Head, Lunar floor-fractured the predictionsof the gravitationalrelaxation model. craters: Evidence for viscous relaxation of crater topography,J. Geophys.Res., 86, 9537-9552, 1981. Interior tectonic features. Many ridges in the Head,J.W., and L. Wilson,Volcanic processes and landforms interior of coronae are of unknown origin. An on Venus: Theory, predictions and observations, J. extensionalorigin for the features would supportthe Geophys. Res., 91, 9407-9446, 1986. hotspot model, while a compressionalorigin would Ivanov, B.A., A.T. Basilevsky, V.P. Kryuchkov, and I.M. Chemaya,Impact craters of Venus:Analysis of Veneras15 support the sinking diapir model. In addition, the and 16 data,J. Geophys.Res., 91, 413-430, 1986. hotspot model predicts that the topography will Kryuchkov,V.P., The featurelength and spacingcomparison narrow and become higher as the anomaly approaches among ridge and groove belts, coronae, and mountain the surface. No correlation between complexity of surroundingof Lakshmi, H (abstract),Lunar Planet. Sci., X/X, 651-652, 1988a. structure and size has been determined, but higher Kryuchkov,V.P., Ridge belts on the plains of Venus, I resolution images of the deformed interior of many (abstract),Lunar Planet. Sci., XIX, 649-650, 1988b. coronaemay permit detectionof this effect. The study Masursky,H., Geologicalevolution of coronae(complex of the interior of coronae and their evolution and circular features)on Venus (abstract),Lunar Planet. Sci., topographic characteristics with high-resolution XVIII, 598-599, 1987. Pettengill, G.H., E. Eliason, P.G. Ford, G.B. Loriot, H. imagesand altimetry from Magellan shouldallow us to Masursky, and G.E. McGill, Pioneer Venus radar results: distinguish between models of origin. In addition, Altimetry and surfaceproperties, J. Geophys.Res., 85, more complex models analyzing effects of flattening 8261-8270, 1980. of the diapir as it approachedthe surfaceare needed. Pronin, A.A., and E.R. Stofan, Coronae on Venus: Gravity signature. Hotspots on the Earth have a Morphologyand distribution,Icarus, 87, 452-474, 1990. Schaber,G.G., E.M. Shoemaker, and R.C. Kozak, Is the characteristic gravity signature (positive geoid Venusiansurface really old? (abstract),Lunar Planet. Sci., anomalies). Magellan gravity data will be used to XVIII, 874-875, 1987. determine if coronae have a characteristic gravity Shelton,G., and J. Tullis, Experimentalflow laws for crustal signature and how it compares to the gravity rocks (abstract),Eos Trans. AGU, 62, 396, 1981. Solomon, S.C., and J.W. Head, Venus banded terrain: signaturesof terrestrial hotspots. Tectonicmodels for bandformation and their relationship to lithosphericthermal structure,J. Geophys.Res., 89, Acknowledgments. This work was supportedby NASA 6885-6897, 1984. grantsNAGW-713, NGR40002088, and JPL contract957088, Solomon,S.C., S.K. Stephens,and J.W. Head, On Venus as well as the William F. Marlar Foundation. One of the impactbasins: Viscous relaxation of topographicrelief, J. authors(E.R.S.) was supportedby a NASA GoddardSpace Geophys.Res., 87, 7763-7771, 1982a. Flight Center GraduateStudent Fellowship. We gratefully Solomon,S.C., R.P. Comer, and J.W. Head, The evolutionof acknowledgehelpful discussionswith Alex Pronin, A.T. impactbasins: Viscous relaxation of topographicrelief, J. Bas/levsky,Ray Fletcher,Carle Pieters,and Pete Schultz. We Geophys. Res., 87, 3974-3992, 1982b. would like to thank helpful reviews by Bob Grimm and an Stephens,S.K., S.C. Solomon,and J.W. Head,On the age of anonymousreviewer. Venus highlandtopography: Constraints on the viscous relaxation of relief (abstract), Lunar Planet. Sci., XIV, R•FERENCES 747-748, 1983. Stofan, E.R., and J.W. Head, Pioneer Venus characteristicsof Anderson, E.M., The dynamics of faulting and dyke ovoids on Venus: Preliminary results (abstract),Lunar formation, with applicationsto Britain, 2d ed., Oxford Planet. Sci., XVII, suppl.,1033-1034,1986. Press,Edinburgh, 1951. Stofan,E.R., andJ.W. Head,Coronae of MnemosyneRegio, Barsukov,V.L., et al., The geologyand geomorphologyof Venus: Morphology and origin, Icarus., 83, 216-243, the Venussurface as revealedby the radarimages obtained 1990. by Veneras15 and 16, J. Geophys.Res., 91, 378-398, Stofan, E.R., J.W. Head, and E.M. Parmentier, Corona 1986. structureson Venus: Models of origin (abstract),Lunar Basilevsky,A.T., A.A. Pronin, L.B. Ronca, V.P. Kryuchkov, Planet. Sci., XVIII, 954-955, 1987. A.L. Sukhanov, and M.S. Markov, Styles of tectonic Stofan, E.R., J'.W. Head, and E.M. Parmentier, Corona 20,946 STOFAN ET AL.: MODELS OF ORIGIN OF CORONA STRUCTURES ON VENUS

structureson Venus: Evidence for a diapiric origin I.W. Headand E.M. Parmentier,Department of Geological (abstrac0,Lugtar Planet. Sci., X/X, 1129-1130, 1988. Sciences,Box 1846, BrownUniversity, Providence, RI Turcotte,D. L., and G. Schubert,Geodynamics: Application 02912. of continuumphysics to geologicalproblems, 450 pp., E.R. Stofan, Jet PropulsionLaboratory, MS 230-225, JohnWiley, New York, 1982. 4800 Oak Grove Drive, Pasadena,CA 91109. Weenman,J., Heightof mountainson Venusand the creep propertiesof rock, Phys. Earth Planet. Inter., 19, 197- 207, 1979. (Received January17, 1990; D.L. Bindschadler,Department of Earthand Space revised August27, 1991; Sciences,UCLA, Los Angeles,CA 90024. acceptedAugust 27, 1991 )