JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 96, NO. E4, PAGES 20,933-20,946, NOVEMBER 25, 1991 CoronaStructures on Venus' 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 Venera 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 Venera 15/16 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