novae on venus: geology, classification, and evolution - brown

Novae on Venus: Geology, classification, and evolution

Anton S. KrassilnikovVernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, Moscow, Russia

Department of Geological Sciences, Brown University, Providence, Rhode Island, USA

James W. HeadDepartment of Geological Sciences, Brown University, Providence, Rhode Island, USA

Received 20 September 2002; revised 10 March 2003; accepted 9 June 2003; published 18 September 2003.

[1] Novae are ‘‘radially fractured centers’’ 100–300 km in diameter, and 64 have beenidentified on Venus. Dense radial fracturing and upraised topography are common, andmassive amounts of volcanism are seen in some. Novae are interpreted to be the result ofupdoming and fracturing of the surface due to interaction of mantle diapirs with thelithosphere and radial fracturing caused by dike emplacement. We analyzed thetopography and character of deformational and morphological structures of all 64 novae,leading to a better understanding of their evolution and relation to diapir dynamics, andtheir age. We subdivided novae into four topographic classes: (1) upraised, (2) annular,(3) flat and negative, and (4) plateau-like. Novae are located mostly in areas of regionalrises and rift zones, with a small number in the lowlands. About one half the novae studiedshow association with rifts; in some of these the rift troughs surround nova rises and formplateau-like novae. Plateau-like novae may predate, postdate, or form simultaneously withrifts. Our analysis generally confirms and updates previous models of novae evolution,with the following geological factors being important: (1) depth of the neutral buoyancylevel of the rising mantle diapir; (2) rheological characteristics of the overlyinglithosphere; (3) whether or not the visco-plastic material above the diapir undergoesspreading; and (4) the nature and influence of regional stress and rift structures. Thesecombined factors exert control on the direction of nova evolution and appear to determinethe primary characteristics of nova, such as topography, tectonics, and volcanism. Wefound clear evidence for (1) the important role of dike swarm emplacement in theformation of radial patterns of novae and (2) the formation of concentric tectonic featuresduring the relaxation stage of novae. Novae have multiple evolutionary stages and arelong-lived structures. Stratigraphic analysis of all 64 local areas showed a similarity in thesequence of regional geologic units. We found that 40.3% of the novae population startedto form before emplacement of regional plains with wrinkle ridges and that 11.3%completed their activity before this time; 88.7% of the population of novae was active afterregional plains formation. In contrast to novae, coronae activity was greatest beforeformation of regional plains, which may be due to thickening of the lithosphere with time.Detailed structural analysis shows that novae evolution does not always lead to theformation of corona-like features. INDEX TERMS: 6295 Planetology: Solar System Objects: Venus;

5430 Planetology: Solid Surface Planets: Interiors (8147); 5475 Planetology: Solid Surface Planets: Tectonics

(8149); 5480 Planetology: Solid Surface Planets: Volcanism (8450); 5470 Planetology: Solid Surface Planets:

Surface materials and properties; KEYWORDS: dike swarm, novae, Venus

Citation: Krassilnikov, A. S., and J. W. Head, Novae on Venus: Geology, classification, and evolution, J. Geophys. Res., 108(E9),

5108, doi:10.1029/2002JE001983, 2003.

1. Introduction

[2] Novae (single, nova) (Figure 1) were first identifiedand described on the basis of Magellan spacecraft radarimages [Schubert et al., 1991; Janes et al., 1992; Squyres etal., 1992; Stofan et al., 1992, 1997; Head et al., 1992]. The

initial use of the term reflected the radial, star-like nature ofthe structure. Aksnes et al. [2001] recommend that one usethe term ‘‘astra’’ (single astrum) for these structures becauseof its astrophysical meaning, but most planetary geologistsstill use the term ‘‘nova.’’ Presently, novae are variouslydescribed as ‘‘radially fractured centers’’ [Schubert et al.,1991], ‘‘radial corona-like features’’ [Stofan et al., 1992] or‘‘radially fractured domes’’ [Janes and Turtle, 1996]. Headet al. [1992] described novae in more detail: ‘‘these struc-

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. E9, 5108, doi:10.1029/2002JE001983, 2003

Copyright 2003 by the American Geophysical Union.0148-0227/03/2002JE001983$09.00

12 - 1

tures have prominent radial fracture patterns forming astarburst or stellate pattern in map plan which generallyconsist of a plexus of graben.’’[3] Novae are usually between 100 and 300 km in

diameter [Head et al., 1992; Crumpler and Aubele, 2000].In describing separate novae, many authors note that denseradial fracturing is common for these structures and thatthey have prominent upraised topography [Schubert et al.,1991; Janes et al., 1992; Squyres et al., 1992; Stofan et al.,1992, 1997; Head et al., 1992; Basilevsky and Raitala,1999, 2002; Aittola and Kostama, 2000, 2002]. In a studyof the stratigraphy and topography of several novae,Basilevsky and Raitala [1999, 2002] described age relation-ships of several novae and their environment. They showedthat some novae have two generations of radial fracturing.They also showed that the novae described started to formbefore the formation of regional plains with wrinkle ridgesand completed their evolution after emplacement of theseplains. Authors have also noted massive amounts of volca-nism connected with some novae [e.g., Head et al., 1992;Price, 1995; Basilevsky and Head, 2000b]. Aittola andKostama [2002] studied the geology of 20 novae andshowed that part of them have concentric structures, butdid not discuss the influence of regional tectonic structures.[4] The radial structures associated with novae are inter-

preted by several authors to be the result of updoming andfracturing due to the dynamic and thermal influence of hotmantle diapirs on the upper part of the lithosphere [Janes etal., 1992; Squyres et al., 1992; Stofan et al., 1992, 1997;Koch, 1994; Koch and Manga, 1996]. Novae are sometimesconsidered as the initial stages of corona development [e.g.,Janes et al., 1992; Squyres et al., 1992; Stofan et al., 1992,1997; Janes and Squyres, 1995], in which formation andevolution are connected with uplift and subsequent thermaland dynamic relaxation of hot mantle diapirs. Others havepointed out that the radial structure lengths commonlyexceed the distance anticipated for diapiric uplift and

fracturing, and have postulated that the radial fracturesand graben are linked to processes of dike emplacementand near-surface stress distribution and deformation [Headet al., 1992; Grosfils and Head, 1994a, 1994b, 1995, 1996;Parfitt and Head, 1993; Ernst and Buchan, 1998; Koenigand Pollard, 1998]. More recently, Parfitt and Head [1993],Ernst et al. [1995, 2001], and Ernst and Buchan [1998],have drawn attention to the similarities between novae andgiant radial dike swarms on Earth and Mars. The relativeimportance of uplift and radial dike emplacement in theformation and evolution of novae is still a matter of debateand will be assessed in this analysis.[5] There are a number of geological [Stofan et al., 1992,

1997], numerical geophysical [e.g., Koenig and Pollard,1998] and analogous tectonophysical [Krassilnikov et al.,1999; Krassilnikov, 2000, 2001] models for the process ofnova formation and evolution. Janes et al. [1992] andSquyres et al. [1992] analyzed the morphology of severalradially fractured centers. However, they did not assess indetail the tectonic structures of novae and the relationship ofnovae and surrounding stratigraphic units and tectonicstructures. Stofan et al. [1991] and Janes et al. [1992]completed numerical geophysical models describing thestress field, characteristics of the upper part of the litho-sphere, the shape of the rising dome above the upwellingmantle diapir and the possible duration of diapir activity.They suggested a model of corona evolution that includednova formation in the initial stages. All these models arebased on the description of selected individual structures ofthis class. Only a few papers describe the geology andevolution of some novae on the basis of a detailed strati-graphic and tectonic analysis [Squyres et al., 1992; Aittolaand Kostama, 2000, 2002; Basilevsky and Raitala, 1999,2002].[6] Crumpler and Aubele [2000] created a catalog of

volcanic structures of Venus, which includes 64 novae.They note that there are also a number of radially fractured

Figure 1. Typical example of ‘‘radially fractured center’’ or ‘‘nova’’ located at 14�S 164�E [Crumplerand Aubele, 2000]. (left) Fragment of SAR image (illumination from the left) C1-MIDR.15S163.101.(right) Sketch map showing nova radial structures represented mostly by graben and fractures and relatedlobate lava flows, which is evidence for volcanic/tectonic nature of these structures.

12 - 2 KRASSILNIKOV AND HEAD: NOVAE ON VENUS

centers associated with coronae, volcanoes, and arachnoidstructures, which are also included in the catalog. In thispaper we consider the 64 novae from the catalog that are‘‘radially fractured centers’’ [Crumpler and Aubele, 2000](Figure 2). We examine the geology, structure and evolutionof novae themselves through detailed geologic mapping andclassification. The results are used to assess the possibilityof an evolutionary sequence within novae, and betweennovae and coronae, and to address the issue of the formationof the radial structures.[7] The topographic shape of novae may reflect the

evolution of the shape of the diapir, which has beenproposed to produce this nova. Consequently, analysis ofnova topography and detailed geological analysis of thesestructures could lead to a better understanding of theevolution of novae and their dependence on diapir dynam-ics. Detailed study of the environment of novae could alsoreveal the relative age of these structures and their elements,and the relative importance of uplift and dike emplacementin formation of radial structure. This analysis also assessesthe possible connection between novae and regional geo-logical structures, such as rift zones and ridge and mountainbelts.[8] In summary the main goals of our work are: (1) To

study the topography of the 64 novae and to analyze theirgeological structures and adjacent environment. (2) Toclassify novae on the basis of their topography and geolog-ical structures. (3) To assess models of novae evolution. (4)To study the relationship of novae with large tectonic-volcanic structures, such as rifts and coronae. (5) To assessthe period of novae activity. (6) To assess the role of upliftand dike emplacement as mechanisms for the formation of

radial structure. (7) To compare the results of our studywith geological models of novae formation and with resultsof numerical geophysical and analogous tectonophysicalmodeling.

2. Methods of Study

[9] Our analysis includes the study of the topography andcharacter of deformational and morphological structures ofall novae and detailed geological mapping of typical exam-ples of novae. Gravity data are important in the analysis ofregional Venusian volcanic/tectonic structures and in deter-mination of ‘‘dead’’ and ‘‘live’’ structures [Bindschadler etal., 1992; Grimm and Phillips, 1992; Sjogren et al., 1997].However, the resolution of these data is not sufficient.[10] Methods of geological analysis of the surfaces of

other planets have been described in detail [e.g., Greeleyand Batson, 1990] and have been applied to the mapping ofthe surface of Venus [e.g., Tanaka, 1994; Ivanov and Head,2001]. The geological mapping and analysis were per-formed using C1-MIDR and F-MIDR Magellan radarimages [Saunders et al., 1992; Ford et al., 1993] in bothdigital and hard copy versions. Special attention was paid tothe relationship of nova structures, plains with wrinkleridges, and smooth and lobate plains, in order to assessthe relative ages of novae structures and their geologicalenvironment. Analysis of topography was undertaken usingMagellan GTDR altimetric data [Saunders et al., 1992;Ford et al., 1993] and artificial stereo images. We analyzedthe topography of novae paying special attention to theprominence of novae in relief and to the shape of the novarise. For analysis of topo data we used Transform PPC

Figure 2. Location of novae of different classes, overlain on the topographic map of Venus (simplecylindrical projection). Numbers of novae are given according to the ‘‘Catalog of volcanic structures ofVenus’’ [Crumpler and Aubele, 2000] (Table 1).

KRASSILNIKOV AND HEAD: NOVAE ON VENUS 12 - 3

software, and for correlation of geological and topo data weused perspective drawing by Bryce 3D (Figure 3). We alsogenerated topographic profiles for novae mapped (Figure 4),although analysis of 3-D images is more informative.

3. Geologic Map Units and Tectonic Structures

[11] We initially analyzed in a preliminary manner thegeology of several novae in order to develop a geologicalmapping strategy and to understand the scale of geologicalunits and the scale at which we would map individualtectonic structures. On the basis of initial analysis, we usedthe following stratigraphic units (Figure 5), which representthe generalized sequence from older units to younger on thebasis of the relationships observed and mapped in this study.We found agreement with the general stratigraphy ofBasilevsky and Head [1995a, 1995b, 1998b, 2000a,2002b]. Tessera terrain material (Tt); densely deformed bysets of ridges and fractures, therefore its surface is radarbright; fracture to fracture spacing is predominantlyhundreds of meters; tectonic structures mostly form an

orthogonal system; unit is usually rather elevated; in allcases observed is embayed by younger material. Denselyfractured plains material (Pdf); densely deformed and there-fore has radar bright surface; fractures are mostly straightand parallel, fracture to fracture spacing is predominantlyhundreds of meters; usually represented by kipukas, whichare embayed by younger material. Ridge belt material (Rb);occurs in the form of mostly broad elevated ridges, itssurface has intermediate radar brightness; the width andridge to ridge spacing of ridges average tens of kilometers,with lengths up to hundreds of kilometers; usually exposedas elongate islands embayed by younger material. Fracturebelt material (Fb); deformed by approximately parallelfractures with fracture to fracture spacing of hundreds ofmeters and more, therefore its surface is rather radar bright;sometimes looks similar to Pdf, but fractures of Pdf arepacked more densely and are straighter than in Fb; in ourmapping we describe Fb as a stratigraphic unit because inareas Fb is embayed by younger material. Corona denselyfractured material (Codf); composes concentric annulae ofcorona-like features; usually deformed by concentric frac-

Figure 3. Schematic topographic profiles and perspective views of different classes of novae;perspective views are made by superposing of SAR images on the topography models: (a) nova calledBecuma Mons (34�N 21.5�E), fragment of C1-MIDR.30N027.102, view is toward NNW, verticalexaggeration x20; (b) unnamed nova (63.5�N 090�E), fragment of C1-MIDR.60N097.201, view istoward NE, vertical exaggeration x20; (c) nova called Didilia Corona (18.5�N 37.5�E), fragment ofC1-MIDR.15N043.201, view is toward SSE, vertical exaggeration x25; (d) nova called H’Uraru Corona(9�N 68�E), fragment of C1-MIDR.15N060.101, view is toward NW, vertical exaggeration x20;(e) unnamed nova (8.5�S 106.5�E), fragment of C1-MIDR.15S112.201, view is toward SSW, verticalexaggeration x10; (f) nova called Maram Corona (7�S 221�E), fragments of C1-MIDR.00N215.101 andC1-MIDR.15S215.101, view is toward E, vertical exaggeration x15.


Figure 4. Topographic profiles of novae mapped (Figures 6–16). Tracks of profiles are shown inFigures 6–16.


tures of these features with fracture to fracture spacingaveraging hundreds of meters and more; has a rather highradar brightness due to intensive deformation; usually ratherelevated and in all cases is embayed by younger material.

Novae densely fractured material (Nodf); represented byradial fractured material of novae; has a rather high radarbrightness due to intensive deformation; fracture to fracturespacing is predominantly hundreds of meters; in all cases

Figure 4. (continued)


embayed by younger material; possible to subdivide theunits in some cases. Novae ridge belt material (Norb);represented by material of ridges which are concentric tothe novae centers; surface has an intermediate radar bright-ness; the width of ridges and the ridge to ridge spacingaverage tens of kilometers, while lengths are up to hundredsof kilometers; in all cases embayed from all sides byyounger material.[12] Material of plains with small shields (Psh); the

surface has an intermediate radar brightness; made up ofthe material of separate small shield volcanoes with diam-eters from 3–5 up to 20–30 km and with typically slightlysloping flanks; clusters of shield volcanoes are observedwith diameters of tens of kilometers. Material of plains withwrinkle ridges (Pwr); characterized by regional lava plains;the surface is rather radar dark; deformed by a regionalnetwork of wrinkle ridges, which are usually 1–2 km wide,up to hundreds of kilometers in length, and have a ridge toridge spacing from a few kilometers up to 30–40 km;possible to map subunits in some areas. Smooth plainsmaterial (Ps); the surface of this unit is radar dark; made upof smooth volcanic plains, deformed by local tectonicfeatures; usually embays all units described above; possibleto map subunits in some regions. Lobate plains material

(Pl); made up of radar bright lobate lava flows; usually notdeformed; possible to map subunits in some cases. Materialof aeolian patches and streaks (Sp + Ss); the surface isusually radar dark; within unit separate linear streaks areobserved, which are interpreted as wind streaks [Greeley etal., 1992]; the borders are often diffuse. Undivided materialof impact craters (Cu); is made up of material of impactcraters and their ejecta; the surface is usually radar bright,except for radar dark halos and dark parabolas around somecraters [Campbell et al., 1992]. Distinguishing between Pdf,Nodf and Codf is often difficult. We subdivided thesemorphologically similar units to bring attention to aspectson novae and coronae evolution.[13] On the basis of the scale of our mapping, we found

that we could map some individual structures of apparenttectonic origin, and could distinguish them from zones ofdense tectonic structures. Individual structures include fea-tures that we interpreted as fractures, normal faults, graben,and compressional ridges. Some authors consider densedeformed zones as geological units, depending on the scaleof the mapping [e.g., Wilhelms, 1990; Basilevsky and Head,2000a; Hansen, 2000]. In the case of fractures associatedwith rift zones, we mapped these as zones of intensivedeformation (like zones of melange on Earth) where they

Figure 5. Legend for geologic maps shown in Figures 6–16.


were too dense to map individually (densely deformed riftterrain; Rt). In some cases these zones were formed over along period and sometimes it is possible to separate locallyseveral stages in their evolution, relatively old (Rt1) andrelatively young (Rt2).

4. Geological Structure and Topography of Novae

[14] Magellan topography is available for all but two ofthe 64 novae (Figure 2; Table 1). We did not assess in detailthe two novae that are located in gaps in Magellan radaraltimeter data because of the importance of topography inthe study of these features. The average diameter of the 62novae (Tables 2–5; Crumpler and Aubele [2000]) is 180 ±58 km (for asymmetric novae, the larger diameter was usedin this calculation). According to Crumpler and Aubele[2000] the diameter of novae ‘‘is the diameter defined byusing the median fracture length; fractures that are contin-uations of longer rift or fracture belt were not considered inarriving at the value for the median length.’’[15] We subdivided novae into four broad topographic

classes (Figures 2 and 3): (1) upraised novae (Table 2),which have a well-defined and prominent cone- or dome-like rise in the central part of the novae. (2) Annular novae(Table 3), have a central rise and a rather elevated annulus,which is concentric to the central nova rise. (3) Flat andnegative novae (Table 4), these novae usually have a flat,irregular shape, without a well-defined or prominent centralrise. We also included in this class two novae with negativerelief. (4) Plateau-like novae (Table 5), which have a well-defined and prominent plateau-like shape.[16] Some of novae show topographic characteristics

transitional from one class to another. We put each of thesein the class that was defined by their most prominentfeature. We studied the radial and concentric tectonicstructures of each nova and paid special attention tomorphology, kinematics and evolutionary sequence of novadeformation structures, and also the embayment relation-ships of the different material units and structures. We alsoassessed the period and style of novae activity relative to theemplacement of regional plains with wrinkle ridges (Pwr) ineach local studied area. We examined the association ofnovae with such regional tectonic-volcanic structures as riftzones. We used the base map of the ‘‘Tectonic and VolcanicMap of Venus’’ by Price [1995], as modified by Basilevskyand Head [2000b] in their ‘‘Map of Rifts and Volcanoes ofVenus.’’ If we found that the tectonic structures of novaeconnected with rift-associated fracturing, we say that thisnova is associated with a rift zone. In both of these maps[Price, 1995; Basilevsky and Head, 2000b] some novaewere described as volcanoes. In some cases the novae withmassive amounts of volcanism are located in rift zones. Inthis case we say that these novae show an association withrift zones. If a nova has no clear connection with a rift zoneand is shown in the maps of Price [1995] and Basilevskyand Head [2000b] as a volcano, we consider this nova as anova with a large amount of volcanism, and which has noconnection with rift systems.

4.1. Upraised Novae

[17] There are 16 structures in this class (25.8% of novae)(Table 2, Table 6). Their average diameter is 161 ± 64 km,

Table 1. Location and Diameters of ‘‘Radial Fractured Centers’’

or Novae Listed by Crumpler and Aubele [2000] and Used in This

Analysisa

Number of Nova Latitude, deg Longitude, deg Diameter, km

1 69 88 1002 68 282.5 200 � 1003 66.5 278 1504 63.5 90 2005 54 18 250–3006 53 151 1007 34 21.5 1508 32 312 709 29 348 15010 27 342 25011 24 346.5 6012 22.5 256.5 15013 20 24 22514 18.5 37.5 25015 17.5 252 22016 17 48 20017 17 234 22018 14.5 39 20019 12.5 49 300 � 20020 11.5 37 300 � 20021 11 14.5 20022 9 68 22523 �5 250.5 15024 �6.5 251 15025 �7.5 221 15026 �8 243 20027 �8.5 47 25028 �8.5 106.5 13029 �9 214 20030 �9 109 15031 �10.5 47 10032 �11 211 20033 �11 172.5 30034 �12 251 20035 �14 164 35036 �15 215 20037 �16 57.5 20038 �16 205.5 10039 �16.5 162 20040 �17 203.5 150 � 10041 �20 335 20042 �20 102 100–15043 �20.5 193.5 15044 �22 271 15045 �26 238 15046 �27.5 273 25047 �28 232 25048 �30 95 5049 �32 99.5 20050 �32 95 10051 �32 276 10052 �34.5 287.5 15053 �36 150 20054 �37 3 20055 �37 136.5 15056 �38 245.5 20057 �39.5 296.5 15058 �42 75 20059 �42 278 20060* �45.5 303.5 15061 �47.5 58 20062 �49 85 15063 �49 85 10064* �59.5 56 150

aThe diameter of radial fractured centers or stellate patterns is defined byusing the median fracture length; fractures that are continuations of longerrifts or fracture belt trends were not considered in arriving at the value forthe median length. Novae 60 and 64 are located in gaps of Magellan radaraltimeter data and are indicated by an asterisk.


and they have a cone-like or dome-like topographic profile(Figure 3a) elevated above the background from severalhundreds of meters up to 2–3 km, with slopes up to 2–3�.We first describe two typical examples of these in detail andthen describe the general characteristics of this population.

[18] The typical examples of upraised novae are BecumaMons (34�N, 21.5�E) and Mbocomu Mons (15�S, 215�E).Hereafter geographic coordinates and diameters in thebeginning of each nova description are given according tothe catalog by Crumpler and Aubele [2000], and the names

Table 2. Characteristics of Upraised Novae

Novae Characteristicsa Radial Extentional Structures Concentric Structures

Numberof nova

Coordinate,Latitude/Longitude

LargestDiameter,

km

Numbersof VisibleGenerations

StratigraphicUnits

EmbayingEach

Generationb Compressional Extentional

StratigraphicUnits

EmbayingStructuresb Commentsc

3 66.5/278 150 3 1 - Pwr, Psh, Pl 2, 3 - Pl massive volcanism5 54/18 300 2 1, 2 - Pwr, Psh, Ps ridges Pwr, Psh, Ps6 53/151 100 1 Pwr, Psh7 34/21.5 150 1 Psh, Ps, Cu8 32/312 70 1 Pwr, Psh21 11/14.5 200 1 Ps, Psh massive volcanism23 �5/250.5 150 2 1 - Psh, Ps, Pl 2 -Ps, Pl ridges graben, normal faults Pl is located in rift24 �6.5/251 150 2 1 - Psh, Ps, Pl 2 - Ps, Pl ridges graben, normal faults Pl is located in rift36 �15/215 200 3 1 - Ps, Pl 2 - Ps, Pl 3 - Pl graben massive volcanism42 �20/102 150 2 1 - Pwr, Psh 2nd - Ps43 �20.5/193.5 150 1 Pwr, Psh, Ps46 �27.5/273 250 2 1 - Ps, Pl is located in rift48 �30/95 50 2 1 - Ps 2 - not is located in rift51 �32/276 100 2 1 - Psh, Ps 2 - Pl two generations of graben 1 - Ps is located in rift56 �38/245.5 200 1 Psh, Ps, Pl61 �47.5/58 200 2 1 - Pwr 2 - Psh, Ps is located in riftaFrom Crumpler and Aubele [2000].bRelative ages of generation are given by 1, 2, 3, where 1 is oldest. Indexes of stratigraphic units are in agreement with model described in text.cAssociation of novae with rifts and massive volcanism determined on the basis of the ‘‘Map of Rifts and Volcanoes of Venus’’ [Basilevsky and Head,

2000b].

Table 3. Characteristics of Annular Novae


Numberof Nova


LargestDiameter,

km

Numbers ofVisible

Generations

StratigraphicUnits

EmbayingEach


StratigraphicUnits


Transitional Novae9 29/348 150 2 1 - Pwr1 – 2, Psh 2 - Pwr2, Psh ridges graben62 �49/85 150 2 1 -Pwr, Psh 2 - not is located in rift63 �49/85 100 2 1 -Pwr, Psh 2 - not is located in rift

Annular Novae1 69/88 100 2 1 - Pwr, Psh, Ps, Pl 2 - Ps, Pl ridges Ps, Pl4 63.5/90 200 2 1 - Pwr, Psh, Ps, Pl 2 - Ps ridges Pwr, Psh, Ps massive volcanism10 27/342 250 2 1 - Pwr 2 - not ridges11 24/346.5 60 2 1 - Pwr, Psh 2 - Cu massive volcanism14 18.5/37.5 250 1 Pwr, Ps, Pl graben Pwr, Ps, Pl16 17/48 200 3 1 - Ps, Cu 2 - Ps, Cu 3 - Cu ridges Pwr, Ps18 14.5/39 200 2 1 - Psh, Ps, Pl 2 - Ps, Pl, Psh19 12.5/49 300 2 1 - Psh, Ps, Pl 2 - Psh, Ps, Pl ridges Pwr35 �14/164 350 2 1 - Ps, Pl 2 - Pl graben,

normalfaults

is locatedin rift

41 �20/335 200 1 Psh45 �26/238 150 1 Pwr, Psh, Ps is located

in rift47 �28/232 250 2 1 - Pwr 2 - Psh is located

in rift53 �36/150 200 1 notaFrom Crumpler and Aubele [2000].bRelative ages of generation are given by 1, 2, 3, where 1 is oldest. Indexes of stratigraphic units are in agreement with model described in text.cAssociation of novae with rifts and massive volcanism determined on the basis of the ‘‘Map of Rifts and Volcanoes of Venus’’ [Basilevsky and Head,

2000b].


of the novae and other geographic features were taken fromthe USGS Web site: http://planetarynames.wr.usgs.gov/vgrid.html. The elevations of novae structures are givenabove the datum, 6051 km above the center of mass ofVenus. When we discuss radar brightness of the surface ofsome unit, we mean the radar brightness of material of theunit itself, and if radar brightness increases or decreases forsome reason, it is specifically noted. In the description weuse terms ‘‘radar bright’’ or ‘‘radar dark’’ in a relative sensewithin the local areas mapped. Note that the surface of someunits has high radar cross section in a global sense, e.g.,tesserae or material of impact craters. In the descriptions weuse the letter symbols for stratigraphic units describedabove.4.1.1. Becuma Mons[19] Becuma Mons (Figure 6) is located in the NE part of

Bereghiniya Planitia (D � 150 km). It has an approximatelycone-like shape with a small depression on the top with adiameter 24–28 km and depth 300–500 m. The nova islocated on the SW part of a regional ridge, which is �450–500 km long and up to 100–130 km wide. The ridge strikesNW, and its elevation above the lowlands is �500–700 m.The nova is elevated �300–400 m above this ridge, andabout 900–1100 m above the lowlands. Slopes associatedwith this nova are up to 1–1.5�.[20] Fb is located in SW part of the area studied and lies

at �450–700 m altitude. Fb is slightly radar dark and is

deformed by almost parallel fractures, striking mostlyNNW, with fracture to fracture spacing of �2–3 km. Codfunit has an intermediate radar brightness and is located inthe NW part of the area on the regional ridge. Codf makesup a concentric annulus around a corona-like feature (D �120 km), center is located �150 km NW of the nova center.The elevation of Codf is �580–730 m. Codf is denselydeformed by concentric fracturing, with fracture to fracturespacing �1.5–2 km. The central depression is �1–1.2 kmdeep. Fb and Codf are embayed by Pwr. The contactbetween Codf and Fb is not clear, and we cannot examinethe age relationships between them.[21] Pwr is located almost everywhere in the area. The

elevation of the plains ranges from �500 to 1300 m on theregional ridge. Pwr is deformed by a network of wrinkleridges, which are up to 200–300 km long and up to �2 kmwide. The ridge to ridge spacing is from several kilometersup to 20–30 km and the strike is mostly to the NW. In theNE part of the area, ridges make up an orthogonal systemwith NWand NE strikes, and in the depressed interior of thecorona-like feature they have chaotic strikes. Ridges alsodeform material of Codf and Fb.[22] Psh has intermediate radar brightness and can be

subdivided into two subunits, a lower one (Psh1) isdeformed by wrinkle ridges and embayed by Pwr. Psh1 islocated at the NW flank of the nova at �1 km altitude. Psh2has a different stratigraphic position and is discussed below.

Table 4. Characteristics of Flat and Negative Novae


CommentscNumberof Nova


LargestDiameter,

km

Numbers ofVisible

Generations

StratigraphicUnits

EmbayingEach


StratigraphicUnits

EmbayingStructuresb

Flat Novae2 68/282.5 200 2 Ps graben Ps massive

volcanism12 22.5/256.5 150 2 1 - Psh, Ps 2 - Psh graben,

normal faultsis locatedin rift

13 20/24 225 2 1 - Pwr, Psh 2 - Psh20 11.5/37 300 2 1 - Ps, Pl, Cu 2 - Cu22 9/68 225 2 1 - Pwr, Psh 2 - not26 �8/243 200 2 1 - Ps, Pl 2 - Pl graben,

normal faults

massivevolcanism

27 �8.5/47 250 2 1 - Pwr, Psh Ps, Pl 2 - Ps, Pl ridges graben graben - Pwr,ridges - Psh, Pl

31 �10.5/47 100 2 1 - Pwr, Psh 2 - Ps, Pl graben37 �16/57.5 200 2 1, 2 - Pwr, Psh39 �16.5/162 200 2 1, 2 - Ps, Pl graben Ps, Pl is located

in rift54 �37/3 200 2 1 - Psh, Ps, Pl 2 - Psh, Ps, Pl graben,

normal faultsPs is located in rift

55 �37/136.5 150 2 1 - Psh, Ps, Pl 2 - Psh, Pl graben Psh, Ps58 �42/75 200 2 1 - Pwr, Psh, Ps, Pl, 2 - Pl ridges graben,

normal faultsridges - Pwr, Pl is located

in rift

Negative Novae28 �8.5/106.5 130 2 1 - Ps 2 - Ps graben Ps is located in rift30 �9/109 150 2 1 - Psh, Ps 2 - not is located in riftaFrom Crumpler and Aubele [2000].bRelative ages of generation are given by 1, 2, 3, where 1 is oldest. Indexes of stratigraphic units are in agreement with model described in text.cAssociation of novae with rifts and massive volcanism determined on the basis of the ‘‘Map of Rifts and Volcanoes of Venus’’ [Basilevsky and Head,

2000b].


[23] The fracturing associated with the nova deformsCodf, Fb, Pwr and Psh1. The fractures radiate from thesmall depression on the top of the cone-like nova rise. Thesetectonic features are represented by long narrow graben (upto 70–80 km long, up to 2 km wide) as well as straightfractures. The spacing of fracturing is less than 250 m in thecentral part of nova and greater than 2–4 km on the flanks.The width of the graben decrease close to the margins of thenova, fracturing gradually disappears. Some of the radialfractures extend away from the topographic rise of nova formore than 50–70 km. In the SE part of the nova the strikeof the radial fractures changes from radial to the nova toparallel to the regional linear rise.[24] The radial fracturing of the nova is embayed by Psh2

and Ps and overlapped by Cu. Psh2 is located in the SE partof the area and is characterized by a cluster of shieldvolcanoes with slightly sloping flanks and diameters from�1.5 to 4–8 km. Psh2 embays Pwr and radial fracturing ofthe novae at its SE flank. There are two small occurrencesof smooth plains (Ps). One of these is located close to thecentral part of the nova, and another at the W flank and hasan intermediate radar brightness. Ps embays Fb, Pwr andPsh1. The impact crater Noreen (D 18.6 km) [Schaber et al.,1998] is located in the SE part of the area and its materialand its ejecta (Cu) are extremely radar bright. To the WSWradar bright and dark streaks with diffuse borders areobserved. They strike to the SW and are 150–180 km longand 20–30 km wide. Streaks overlie Pwr and Fb and areinterpreted as ejecta of Noreen impact crater reworked bywind.

[25] This nova has a cone-like shape, was formed duringa broad stage of deformation, and formed after emplacementof Pwr and before formation of Psh2 and Ps. Perturbationsto the wrinkle ridge patterns however, suggest that it mayhave started to form just before the emplacement of regionalplains and formation of wrinkle ridges, but we have clearevidence of this nova activity only after Pwr.4.1.2. Mbokomu Mons[26] Mbokomu Mons (D � 200 km) (Figure 7) is located

to the N of Wauwalag Planitia. The shape of this nova isclose to cone-like. On the flanks of the nova a topographicstep is observed. This ledge is located approximately in themiddle part of the slope. This nova is elevated above thesurrounding plains by �900–1300 m, and the step onthe nova slope is located at �500–800 m above thesurrounding plains. Topographic slopes on the nova areup to 1–1.5�. This nova has some traces of an annulus, butthe dome-like shape is dominant.[27] Pdf is located in the NE, NW and SE parts of the

area, is radar bright due to intensive deformation, and isdeformed by dense fractures with fracture to fracturespacing averaging 200–300 m and less. Fractures aremostly parallel, their strikes are varied, with most fracturesoriented NE. Pdf is embayed by lavas of Ps + Pl.[28] Fb is located in the SW corner of the area, has a

rather high radar brightness due to intense deformation, andis deformed by dense fracturing with a NE strike. Fb can besubdivided into two subunits: a lower (Fb1) and an upper(Fb2). Fracture to fracture spacing for Fb1 is 300–500 m, forFb2 �1–1.5 km. In some areas, embayment of Fb1 by Fb2 is

Table 5. Characteristics of Plateau-Like Novae


Numberof Nova


LargestDiameter,

km

Numbersof VisibleGenerations

StratigraphicUnits

EmbayingEach


StratigraphicUnits


Transitional29 �9/214 200 2 1 - Ps, Pl 2 - Pl graben is located in rift38 �16/205.5 100 1 Pl is located in rift40 �17/203.5 150 1 Ps, Pl is located in rift49 �32/99.5 200 2 1 - Ps, Pl 2 - Pl graben is located in rift50 �32/95 100 2 1, 2 - Ps, Pl is located in rift

Plateau-Like15 17.5/252 220 2 1 - Ps 2 - Ps, Pl ridges graben is located in rift17 17/234 220 2 1 - Ps 2 - not graben Pl is located in rift25 �7.5/221 150 2 1 - Ps, Pl 2 - not graben,

normal faultsis located in rift

32 �11/211 200 2 1 -Ps, Pl 2 - Ps graben is located in rift33 �11/172.5 300 2 1 - Ps, Pl 2 - Pl graben,


34 �12/251 200 2 1 - Ps, Pl, Psh 2 - Pl graben,normal faults

Psh is located in rift

44 �22/271 150 2 1 - Ps, Pl 2 - Ps, Pl ridges graben is located in rift52 �34.5/287.5 150 2 1 - Ps, Pl, Psh 2 - Ps, Pl, Psh graben is located in rift57 �39.5/296.5 150 2 1 - Ps, Pl 2 - Ps, Pl graben,


59 �42/278 200 2 1, 2 - Pl, Psh ridges graben Is located in riftaFrom Crumpler and Aubele [2000].bRelative ages of generation are given by 1, 2, 3, where 1 is oldest. Indexes of stratigraphic units are in agreement with model described in text.cAssociation of novae with rifts and massive volcanism determined on the basis of the ‘‘Map of Rifts and Volcanoes of Venus’’ [Basilevsky and Head,

2000b].


observed, fractures of Fb2 crosscut subunit Fb1. ThereforeFb1 is older than Fb2. Fb is embayed by Ps + Pl. In the areathe age relationship between Fb and Pdf is not clear, but inthe neighboring areas (to the NE and to SE) Pdf is embayedby Fb and is deformed by fractures, which are typical of Fb;we interpret the Pdf unit to be older than Fb.[29] Located very close to Fb in stratigraphic position and

similar in morphology of tectonic structures is Nodf, whichis located in the central part of the nova. It can be dividedalso into two subunits (Nodf1 and Nodf2). Nodf1 is radarbright due to dense fracturing and located on the NE andSW slopes of the nova, mostly on the flat part of the slope.Dense radial fracturing, with fracture to fracture spacingaveraging 200–300 m and less, deforms Nodf1. Nodf1 isembayed by Nodf2 and Ps + Pl. Nodf2 is located on theslopes of the nova and on the central rise and is rather radarbright due to dense radial fracturing. Fracture to fracturespacing is from �200–300 m up to �2–3 km. Nodf2 isembayed by Ps + Pl.[30] At the SW flank of the nova, material and tectonic

structures of unit Nodf2 gradually merge with Fb2 without adistinctive and sharp geologic contact. Nodf2 and Fb2 heregenerally have the same material, and the same spacing andmorphology of fractures. The difference between them isprimarily in the orientation of fractures. In Fb2 they aremostly parallel and have a NE trend, while in Nodf2 thefractures are radial to the nova center. Fb2 and Nodf2 mayhave formed simultaneously. The same relationship is ob-served between Nodf1 and Fb1. These units have the samespacing, morphology of fractures, and stratigraphic position.We separate Fb and Nodf to emphasize the central structureduring nova evolution and to bring attention to the possiblelink between nova radial fractures and regional stress.[31] Owing to the scale of the mapping here, we find that

it is difficult to distinguish between Ps (generally equidi-mensional shape) and Pl (generally linear and lobe-like) andtherefore we combine these plains into one unit, Ps + Pl,which occupies most of the area and is located at 500–2200 m altitude. Ps + Pl is observed mostly at the peripheryof the nova, but some occurrences of Ps + Pl are alsolocated in the central part. Separate radar bright lobate flowsare superposed on the background of radar dark Ps. Lobateflows are located mostly on the periphery, and the majorityof them begin to descend down the nova slopes fromapproximately the same elevation, somewhat lower thanthe horizontal step on the nova slope. Ps + Pl is deformed byradial fractures of the nova. Fracture to fracture spacing isfrom 4–5 km up to 10–12 km, and some extend away fromthe nova rise for 60–80 km. The majority of fractures cutlobate flows, but in some cases these fractures are embayedby lobate flows. In some cases lobate flows appear toemerge from these fractures, evidence that this fracturingand the emplacement of the lobate flows were simultaneous,and that the radial fractures are a possible source for lobatelava outflow. The youngest tectonic activity is observed onthe W and E slopes of the nova, displayed in the grabensystem formation which is concentric to the nova center.These graben are more concentric on the E than on the Wflank. On the E flank of the nova some of the graben forman en echelon system. Most of them are located on thetopographic bench on the nova slope. These graben are notembayed, and cut Ps + Pl.T

able

6.Characteristicsof62Novae

Studied

Class

ofNovae

Numbers

and

Percents

of

Total

Population

of62Novae

Average

Diameters,a

km

NumbersandPercentsofNovae

inEachClass

Novae

WithOne

Generation

ofRadial

Fracturing

Novae

With

Twoor

More

Generations

ofRadial

Fracturing

Novae

With

Concentric

Extentional

Structures

Novae

With

Concentric

Compressional

Structures

Novae

With

Both

Concentric

Extentional

and

Compressional

Structures

Novae

Started

Before

Pwr

Emplacement

Novae

Finished

Before

Pwr

Emplacement

Novae,

Which

Were

Active

After

Pwr

Emplacement

Novae

Show

Association

With

Riftsb

Upraised

novae

16novae

25.8%

161±64

6novae

37.5%

10novae

62.5%

4novae

25%

3novae

18.7%

2novae

12.4%

7novae

43.7%

3novae

18.7%

13novae

81%.3

6novae

37.5%

Annularnovae

16novae

25.8%

188±66

4novae

25%

12novae

75%

3novae

18.7%

6novae

37.5%

1nova

6.3%

12novae

75%

3novae

18.7%

13novae

81.3%

5novae

31.2%

Flatandnegativenovae

15novae

24.2%

192±50

–15novae

100%

10novae

66.7%

2novae

13.3%

2novae

13.3%

6novae

40%

1nova

6.7%

14novae

93.3%

6novae

40%

Plateau-likenovae

15novae

24.2%

179±51

2novae

13.3%

13novae

86.7%

12novae

80%

3novae

20%

3novae

20%

––

15novae

100%

15novae

100%

aAfter

CrumplerandAubele[2000].

bFrom

Basilevsky

andHead[2000b].


Figure 6. Becuma Mons, nova at 34�N 21.5�E and geological interpretation of region surrounding(Figure 2, nova 7); (a) SAR image (illumination from the left) fragment of C1-MIDR.30N027.102;(b) geologic map of area shown in Figure 6a; (c and d) perspective views of SAR image shown in Figure6a (Figure 6c) and geologic map shown in Figure 6b (Figure 6d); view is toward NNW, verticalexaggeration x20; (e) correlation chart.


[32] This nova was formed during three distinguishablestages of radial fracture formation. The first two generationsof radial fracturing were formed simultaneously with theformation of Fb, possibly in connection with backgroundregional extension. The youngest radial fracturing occursafter and/or simultaneously with Ps + Pl formation. Thelatest activity of this nova involves the formation ofconcentric graben, which are located on the plateau on thenova slope. There is no Pwr in this area, but in neighboringareas we observe embayment of Pwr by Ps + Pl, thereforethis nova was active after Pwr formation. There is no clearevidence of activity of the nova before Pwr.4.1.3. Summary of Upraised Novae[33] In these novae extensional straight fractures and long

narrow graben define novae radial structures. They radiatefrom the center of the nova rise and mostly are located in the

central elevated part and on the flanks of the nova. How-ever, some parts of the radial structures extend away fromthe nova rise for hundreds of kilometers. In 6 novae (37.5%,hereafter percents are shown for upraised novae) one stageof radial fracturing is observed. Another 10 novae (62.5%)show two or three visible stages of radial fracture formation.Fracture to fracture spacing of the older fracturing issystematically narrower than in younger ones. Our obser-vations lead us to suggest that this fracture system is asource of lobate lava flows. These flows are located mostlyin the middle of the novae rise and extend down theirslopes. In some cases lobate flows begin to emerge fromapproximately a single elevation on the nova rise. Radialfracturing of novae is usually covered by younger materialof different types of volcanic plains, and sometimes bymaterial of aeolian deposits and by ejecta or outflow of

Figure 7. Mbokomu Mons, nova at 15�S 215�E and geological interpretation of region surrounding(Figure 2, nova 36); (a) SAR image (illumination from the left) fragment of C1-MIDR.15S215.101; (b)geologic map of area shown in Figure 7a; (c and d) perspective views of SAR image shown in Figure 7a(Figure 7c) and geologic map shown in Figure 7b (Figure 7d); view is toward WSW, verticalexaggeration x20; (e) correlation chart.


impact craters. In 15 structures (93.8%) some of these coverthe last radial fracturing. However, in one case, we found noevidence for embayment of radial fracturing.[34] Concentric extensional and compressional structures

are observed in some upraised novae. In 4 novae (25%)concentric graben are identified, in 3 novae (18.7%) con-centric ridges are observed, and 2 novae have both grabenand ridges. Concentric tectonic structures may or may notbe embayed by the materials described above.[35] Seven novae (43.7%) started to form before Pwr

emplacement (Tables 2 and 6). Three novae (18.7%)showed no evidence of activity after Pwr formation. Thir-teen novae (81.3%) of this class were tectonically andvolcanically active after Pwr emplacement. Six novae ofthis class (37.5%) show an association with rift structures,but rift structures apparently have no visible influence onthe shape of the novae in this class.

4.2. Annular Novae

[36] There are 16 structures in this class (25.8% of novaestudied) (Table 3, Table 6). The average diameter of thesenovae is 188 ± 66 km, the central rise and annulus areusually elevated above the surroundings for �1–2 km, andslopes are up to 2–3�. We can identify two subtypes ofannular novae (Figures 3b and 3c). Subtype 1: Radialextensional fracturing in elevated central parts of novaeand concentric compressional structures at annulae andperiphery. Graben and normal faults characterize radialfracturing and ridges characterize concentric compressionalstructures. Some concentric extensional structures (graben)are present amidst dominantly concentric compressionalstructures. Subtype 2: Radial extensional fracturing inelevated central parts and in annulae of novae and concen-tric extensional fracturing in annulae. Graben and normalfaults characterize radial and concentric fracturing.4.2.1. Annular Novae Subtype 1[37] Typical examples of these novae include an unnamed

nova (63.5�N, 90�E), a nova called Ops Corona (69�N,88�E) and a nova called Tutelina Corona (29�N, 348�E).4.2.1.1. Unnamed Nova[38] This unnamed nova (Figure 8) is located to the SW

of Meskhenet Tessera (D � 200 km). The central part of thenova being rather high and dome-like, elevated above thesurrounding plains by �900–1100 m, and with slopes of upto 1–1.5�. On the top of the nova, a small depression isobserved with a diameter of �20–25 km, and a depth up to1 km. To the NW a piece of elevated annulus is observed,which is concentric to the nova center and is elevated abovethe surrounding plains by �700–800 m. A depressionconcentric to the nova center is located between the annulusand central rise, its depth relative to the surrounding plainsis �1 km.[39] Tt is located in the NE and NW parts of the mapped

area, and lies at �1.2–1.7 km altitude. Tt has extremelyhigh radar brightness because of intense deformation byseveral fracture systems. In the piece of Tt to the NW, thedensest fracturing strikes WNW, with a fracture to fracturespacing of �400–500 m and less. In the piece of Tt to theNW, the densest fracturing strikes N, with fracture tofracture spacing of �300–400 m and less. Both of thesepieces of Tt are deformed by fracturing which is normal tothe dense one. This fracturing has fracture to fracture

spacing from 3 to 15 km. Tt in all cases is embayed byyounger material.[40] Pdf is also located in the NW and NE corners of the

area, is densely deformed by mostly parallel straight frac-tures, and therefore its surface is radar bright. In the NWpiece of Pdf, fracturing strikes to the NE, while in the NEpiece of the Pdf unit fracturing strikes to the NNW, fractureto fracture spacing in this system is �400–600 m. Pdf isdeformed less extensively than Tt and by one fracturesystem only. The contact between Pdf and Tt is highlytectonized, and we can say little about their embaymentrelationships. However, Tt is deformed by fractures, whichare typical for Pdf, and Pdf is not deformed by fractureswhich are typical for Tt. We conclude that Tt is older thanPdf. Pdf in all cases is embayed by younger material.[41] Rb is located in the NE part of the study area and to

the SE of the nova. Rb is relatively radar bright. Ridges,which are up to 150–170 km long and 3–8 km wide,deform it; ridge to ridge spacing of 8–10 km. Rb isembayed by younger Pwr. The contact between Rb andPdf is highly tectonized, but close to the contact ridges,which are common for Rb deform the Pdf, and Rb is notdeformed by fracturing of Pdf. We conclude that Rb isyounger than Pdf.[42] Nodf is located at the central part of the nova and to

the N and S from nova for a distance of more than 200 km.Nodf has a high radar brightness due to intensive deforma-tion and is defined by a fracture system which is radial tothe nova center, with a fracture to fracture spacing in thecentral part of the nova of up to 300–400 m, and in theperiphery up to 2–5 km. This fracturing also cuts Rb. Nodfis embayed by Psh, Pwr and Ps. Norb is located to the NWfrom the nova center, partly on the slopes of the concentricannulus. Norb is relatively radar bright and is deformed byridges that are concentric to the nova rise, and up to 100 kmlong, 2–5 km wide and with ridge to ridge spacing of �5–10 km. Norb is embayed by Psh, Pwr, and Ps, is locatedvery close to Nodf, is not deformed by dense radial fracturesof the nova, and Nodf material is deformed by ridges ofNorb. We thus suggest that Norb is younger than Nodf.[43] Psh is located everywhere in the area and is repre-

sented by separate shield fields with diameter up to 60–70 km, and sometimes by small kipukas, which are sur-rounded by Pwr. In Psh abundant separate individualvolcanic constructs with diameters �3–10 km are visible.In most cases in the area Psh is embayed by Pwr and isdeformed by a network of wrinkle ridges. In some cases wedo not observe clear contact between Pwr and Psh. Weconclude that Psh mostly predates Pwr, in some cases maybe simultaneous, but predates wrinkle ridge formation.[44] Pwr occupies most of the area, is located in the

central part of the nova and in the surrounding lowlands, ismostly radar dark, and is deformed by a regional networkof wrinkle ridges that are up to 150–170 km long and 3–5 km wide. Ridge to ridge spacing is �20–40 km, theirstrike is variable, and in some cases ridges form anorthogonal system. Pwr embays Pdf, Nodf, Norb and Psh,and wrinkle ridges deform most of them as well. Theyoungest nova fracturing cuts Pwr. This fracturing radiatesfrom the depression in the nova center. Most fracturesextend to the N and to the S from the nova, with fractureto fracture spacing of up to 3–5 km in the center of nova,


and up to 6–10 km at the periphery. Numerous fracturesextend away from the nova center for more than 200 kmand cut all units described and are embayed only by Ps.[45] Pl is represented by several radar bright lava flows in

the NW part of the area, which are located at 600–850 maltitude and embay Pdf, Pwr, and Psh. The Pl unit isdeformed only by several local fractures. Separate lavafields make up Ps to the W of the nova at 500–600 melevation. Ps is radar dark and embays local fractures, whichin turn cut Pl. The Ps unit is not deformed and embaysNodf, Norb, Pwr, Psh and Pl.[46] There were two specific stages of radial fracture

formation during the history of this nova. It appears as ifthe nova began to form between formation of Rb andNorb, and before emplacement of Pwr. Norb unit, whichmakes up the prominent annulus, was formed between theformation of Nodf, Psh, and Pwr. The latest activity of thenova is observed after formation of Pwr and before Psemplacement.4.2.1.2. Ops Corona[47] Ops Corona (D� 100 km) (Figure 9) is located to the

SE of Ishtar Terra. A central rise is elevated �900–1100 mabove the surrounding plains and a piece of concentricannulus on the S flank of the nova is elevated �1.5–1.6 kmabove the surroundings. Two depressions are observed, in thecentral part and on the NW flank of the nova, which are�1.5 km deep relative to the surroundings. Slopes of thecentral rise of the nova are up to 0.9–1.1�, and external slopesof the annulus are up to 2.5–3�.[48] Tt is located in the NW, SW and SE corners of the

mapped area at 1.5–2 km altitude. Tt is radar bright due tointense deformation by structures with two main strikes,NW and NE. Fracturing with a NW strike is more densewith a fracture to fracture spacing of �0.5–1 km. The NEoriented fracturing is mostly represented by graben, whichare up to 150–200 km long and 2–7 km wide. Tt isembayed by younger plains, and is covered by Cu of impactcrater Gloria and by Sp + Ss. Pdf is located in the NE part ofthe area and has an intermediate radar brightness. Approx-imately parallel fractures with an E-W strike deform Pdf,with fracture to fracture spacing of �0.5–0.7 km. Pdf isembayed by Psh1, Pwr, and Ps. Nodf is located on the S andN flanks of the central part of the nova at 1.1–1.3 kmaltitude. Nodf is rather bright because of intense deforma-tion by fracturing, which is radial to the nova center and hasfracture to fracture spacing of �2–2.5 km. Nodf and radialfracturing are embayed by Pwr and by Ps2.[49] Pwr occupies most of the area around the nova and

the rather elevated annulus to the S from the nova. Pwr haslow radar brightness and is deformed by a regional networkof wrinkle ridges. To the W and to the NE from the nova,ridges are up to 150–170 km long and 1–1.5 km wide, withvariable strike. At the S and E flanks of the nova, ridges areconcentric to the nova center and more prominent (up to180–200 km long and up to 4–4.5 km wide). Ridge toridge spacing is from several kilometers up to �20–30 km.

To the N from the nova, Pwr is deformed by fractures thatare radial relative to the nova center. They are straight, long(up to 140–160 km) and have fracture to fracture spacing of�3–5 km. Ridges and fractures also deform material of Tt,Pdf, Nodf, Psh1.[50] Psh1 is located at �1 km altitude and has an

intermediate radar brightness. Within it are observed sepa-rate volcanic constructs with diameters of 2–7 km. We donot observe a clear contact between Pwr and Psh1, but Psh1is deformed by the wrinkle ridge network and by fracturesystems, which are usual for Pwr. Psh1 embays just oneolder unit, Pdf. We conclude that Psh1 is close in astratigraphic sense to Pwr. Both Psh1 and Pwr are embayedby Ps1–2 and Pl, and are covered by Sp + Ss. Pwr is alsoembayed by Psh2 and is covered by Cu.[51] Psh2 is located to the NW from the nova center and

has intermediate radar brightness. Psh2 is characterized byseveral volcanic constructions with diameters 3–7 km andby long lava oriented in a NW direction; this flow unit is�100 km long and is running apparently through a fracturewhich deforms Pwr. Psh2 embays Pwr and is embayed byPs2 and Pl.[52] Ps1 is located in the NE part of the area, has a rather

low radar brightness, and is deformed by almost parallelstraight fractures with a NW strike and fracture to fracturespacing of �2–10 km. This fracturing also cuts Pdf andPsh1. Ps1 is embayed by Ps2. Ps2 is located in the centralparts of the nova, to the N of the nova, and in the NE part ofthe area. Fractures are oriented approximately radial to thenova and deform Ps2 in the central parts of the nova, withfracture to fracture spacing of �3–20 km. These fracturesalso deform Nodf and Psh. Unit Ps2 is embayed by Pl. Pl islocated to the SW of the nova center at 0.5–1.1 km altitudeand has low radar brightness. Numerous lobate lava flowsare observed within this unit, and these extend to the Wfrom the nova for a distance of more than 100 km. Pl is notdeformed and not embayed.[53] To the SE of the nova center, a huge area of aeolian

deposits is observed (Sp + Ss). Streaks located within thisarea are interpreted as wind streaks and also located thereare features which have been interpreted as a dune field[Greeley et al., 1992; Ford et al., 1993]. Sp + Ss covers Ttand Pwr. The lower age limit of aeolian activity is notconstrained from the observed relationships.[54] To the N of the field of Sp + Ss lies the crater Jadwiga

(D� 12.7 km) at 870 m altitude [Schaber et al., 1998]. It hasa prominent radar bright ‘‘parabola’’ [Campbell et al., 1992].The bright material of this parabola overlies Pwr, Ps2, andSp + Ss. Within the area of Sp + Ss, material of thisparabola has traces of the influence of wind activity; thematerial of the parabola is concentrated into streaks with aWNW strike. Thus crater Jadwiga happened simultaneouslywith the time of aeolian activity. Crater Gloria (68.5�N,94.2�E) is located to the E of the previous crater [Schaber etal., 1998]. Material of the outflow of this crater (Cu)extends away for �80–100 km and embays Tt, Pwr and

Figure 8. (opposite) Unnamed nova at 63.5�N 090�E and geological interpretation of region surrounding (Figure 2, nova4); (a) SAR image (illumination from the left) fragment of C1-MIDR.60N097.201; (b) geologic map of area shown inFigures 8a; (c and d) perspective views of SAR image shown in Figure 8a (Figure 8c) and geologic map shown in Figure 8b(Figure 8d); view is toward NE, vertical exaggeration x20; (e) correlation chart.


Figure 9. Ops Corona, nova at 69�N 88�E and geological interpretation of region surrounding (Figure 2,nova 1); (a) SAR image (illumination from the left) fragment of C1-MIDR.75N074.101; (b) geologic mapof area shown in Figure 9a; (c and d) perspective views of SAR image shown in Figure 9a (Figure 9c)and geologic map shown in Figure 9b (Figure 9d); view is toward NE, vertical exaggeration x20;(e) correlation chart.


probably Sp + Ss. To the N of this crater, Cu flows intothe fractures that deform Pwr and produces polygon-likefeatures. The size of the blocks or ‘‘islands,’’ which areembayed by the crater outflow, is up to 10–12 km indiameter. To the N of the crater, part of the outflow ofGloria crater overlaps the material of the radar brightparabola of Jadwiga crater and wrinkle ridges, which havetraces of brightening due to the Jadwiga crater parabola.Gloria does not have any parabola, but is presumablyyounger than crater Jadwiga, which has a radar brightparabola, as further documented by Basilevsky and Head[2002a].[55] Three visible stages of radial fracturing occurred

during the formation of this nova. The nova started to formbefore emplacement of Pwr; formation of concentric com-pressional structures occurred after or simultaneously withwrinkle ridge formation. The latest activity of this nova isobserved after formation of Pwr, and before Pl emplacement.4.2.1.3. Tutelina Corona[56] Transitional between this subtype of annular novae

and upraised novae is a structure called Tutelina Corona(D � 150 km) (Figure 10) which is located to the S of SednaPlanitia. We placed it in a transitional category due to thesubdued prominence of the relief of the concentric annulus.The central rise of this nova stands 500–800 m above thesurrounding plains, and slopes are up to 1–1.6�. To the SWof the nova, pieces of low concentric annulus and a depres-sion are observed. The depression is up to 900–1000 mdeep relative to the surrounding plains.[57] Rb is located in the NW corner of the mapped area at

700–800 m altitude, and is rather radar bright. Rb ischaracterized by material of ridges that are embayed fromall sides by younger material, ridges being up to 60–80 kmlong, 2–4 km wide and with ridge to ridge spacing of�5–15 km. Rb is embayed by Pwr1–2. Nodf is located onthe nova slopes at �400–900 m altitude and is radar brightbecause of intense deformation by fracturing which is radialto the nova center. Fracture to fracture spacing is 200–300 mand less. Graben are observed, which are usually 15–20 kmlong and 2–3 km wide. Nodf is embayed by Pwr1 and byPsh2. Psh1 is located mostly on the surrounding plains to theNE and to the SE of the nova at �400–800 m altitude.Some edifices of Psh1 are observed near the nova. It isrepresented by steep-sided kipukas with diameters up to10–15 km, which are embayed from all sides by material ofPwr1–2 and which are exposed through it. Psh1 is embayedby Pwr1–2 and by Psh2, which is discussed below. Wecannot to constrain the lower stratigraphic age of Psh1; wecan document that it predates Pwr.[58] Pwr occupies most of the mapped area at 0–850 m

altitude and can be subdivided into two subunits, Pwr1 andPwr2. The surface of Pwr1 has rather low radar brightness.Pwr1 embays Rb, Nodf and Psh1, and is deformed by youngnova fracturing. This fracturing radiates from the nova for adistance of more than 130–150 km. Both fractures andgraben make up this deformation; graben are up to 50–60 km long and up to 2–3 km wide. Fracture to fracturespacing of the system is about 5–20 km. This fracturing cutsPwr1 and is embayed by Pwr2 and Psh1. The surface of Pwr2has intermediate radar brightness. Pwr2 embays Rb, Nodf,Psh1, and Pwr1. Pwr2 is located at the same elevation as Pwr1.Psh2 is located in the central part of the nova at 800–900 m

altitude and is rather radar dark. Psh2 embays Nodf, Psh1,Pwr1 and young fracturing of the nova. Psh2 has a verydifferent contact with Pwr1 and different radar brightness thanPsh1. We cannot clearly establish the relationship betweenPsh2 and Pwr2; they may have formed simultaneously.[59] Pwr1–2 and Psh2 are deformed by a network of

wrinkle ridges that mostly strike NE and are up to 90–120 km long, 1–2 km wide, and have ridge to ridge spacingof �10–15 km. To the SW of the nova, they are moreprominent (with widths of up to 4–5 km) and are approx-imately concentric to the nova. These ridges are located inthe concentric depression, in pieces of the annulus, and alsoon the surrounding plains. A graben system is located on therise slope concentric to the nova. Graben are 40–50 kmlong and 4–5 km wide and cut Nodf, Psh2 and Pwr1.Unclear are the age relationships between graben, wrinkleridges on plains, ridges to the SW of the nova, and youngnova radial fracturing.[60] Two visible stages of radial fracturing occurred

during the evolution of the nova. The nova started to formwith formation of Nodf prior to emplacement of Pwr. Youngradial fracturing of nova occurred between formation ofPwr1 and Pwr2. Wrinkle ridge deformation occurred afterformation of young nova fracturing. Ridges, which areconcentric to the nova, were formed simultaneously or afterregional wrinkle ridges. The latest activity associated withthis nova (formation of concentric graben) is observed afterformation of Psh2.4.2.2. Annular Novae Subtype 2[61] A typical example of the second subtype of annular

novae is Didilia Corona (18.5�N, 37.5�E) (D � 250 km)(Figure 11), which is located in Eistla Regio. This nova hasa central upraised part and a rather elevated annulusconcentric to the center of the nova. The central rise iselevated above the interior part of the nova by about 1.2–1.4 km, while the concentric annulus is elevated above thesurrounding areas by about 1.4–1.6 km, with the centralpart being �300–400 m higher than the annulus. The slopeof the central rise is up to 2.5–3�, whereas the externalslopes of the annulus are usually 0.5–1�, but in some casesup to 3�.[62] Nodf is located in the central rise and in the elevated

annulus of the nova at 2–2.8 km altitude; it is rather radarbright due to intense deformation. Nodf is deformed bydense fracturing, which is radial to the nova center; fracturesare mostly straight and parallel, with fracture to fracturespacing up to 300–500 m. Sometimes graben are observed,and these are usually 3–5 km wide and up to 100–120 kmlong. In the E part of the annulus, Nodf is deformed bytectonic structures which are represented by graben andnormal faults and are concentric to the nova center. Grabenare 4–6 km in width and 50–70 km long. In the SE cornerof the studied area a piece of the same unit is observedwhich belongs to the annulus of Pavlova Corona; themorphology of fracturing is the same as the Nodf describedabove. Although materials of both Didilia and PavlovaCoronae are embayed by Ps2-Pl, the relative age of thetwo structures cannot be determined. Nodf is embayed byPwr, Ps1, and Ps2-Pl, and is covered by Sp + Ss.[63] Rb is located in the SW corner of the area studied at

1–1.5 km altitude and is rather radar bright. Rb is deformedby long (up to 100–150 km) and broad (up to 5–7 km)


Figure 10. Tutelina Corona, nova at 29�N 348�E and geological interpretation of region surrounding(Figure 2, nova 9); (a) SAR image (illumination from the left) around fragment ofC1-MIDR.30N351.101; (b) geologic map of area shown in Figure 10a; (c and d) perspective views ofSAR image shown in Figure 10a (Figure 10c) and geologic map shown in Figure 10b (Figure 10d);view is toward NNE, vertical exaggeration x20; (e) correlation chart.


Figure 11. Didilia Corona, nova at 18.5�N 37.5�E and geological interpretation of region surrounding(Figure 2, nova 14); (a) SAR image (illumination from the left) fragment of C1-MIDR.15N043.201;(b) geologic map of area shown in Figure 11a; (c and d) perspective views of SAR image shown inFigure 11a (Figure 11c) and geologic map shown in Figure 11b (Figure 11d); view is toward SSE, verticalexaggeration x25; (e) correlation chart.


ridges with ridge to ridge spacing up to 10–20 km, and astrike of the ridges to the NW. Rb is embayed by Pwr. Rband Nodf are not in contact, so we cannot determine therelative ages of these units.[64] Both Rb and Nodf are embayed by Pwr which is

located in the NE, NW and W parts of the area at 0.7–1.5 km elevation and is rather radar dark. Pwr is deformedby a network of regional wrinkle ridges, which are usually150–200 km long and 2–4 km wide, with ridge to ridgespacing of up to 20–30 km. The strike of the ridges ismostly concentric to the nova. To the NE of the structure,long narrow graben are observed which extend from thenova center for more than 200 km, they are close to radial tothe nova and are apparently nova-related; graben are 2–3 km wide, cut Nodf and Pwr materials and are radial to thenova. Pwr is also deformed by regional fracturing, whichdoes not have connection with nova activity. The strike ofthis fracturing is variable. These fractures are up to severalhundreds of kilometers long and have fracture to fracturespacing from 4–5 km up to 20–30 km. Pwr is embayed byPs1 and Ps2-Pl.[65] Ps1 is located to the N of the nova at 0.8–1.1 km

altitude and is rather radar dark. Ps1 is deformed by aregional fracture system striking NE, with a fracture tofracture spacing of �6–10 km. Ps1 is embayed by Ps2-Pl.Ps2-Pl is located inside the nova annulus and forms lobateoutflows on the slopes outside the nova. We are unable tosubdivide consistently these two materials at this scale andthus we combine them into one unit. Ps2-Pl is situated �1–2 km altitude and has variable radar brightness, fromintermediate in the central part of the nova to rather radarbright on the periphery. Some lobate lava flows extend fordistances of up to 170–200 km from the nova rise. The unitis deformed by only a few local fractures that are mostlyradial to the nova center and are located to the N from thenova center and to the S from the nova annulus. Ps2-Pl iscovered by Sp + Ss. In the SW part of the nova annulusradar bright areas are observed, whose material overlapsNodf and Ps2-Pl. In some cases within these areas radarbright and dark streaks are observed which can be inter-preted as wind streaks. We map it as a separate unit andinterpret this material to be of aeolian origin (Sp + Ss).[66] Several stages of evolution can be distinguished in

this nova. The radial fracturing of nova located in the centralrise of the nova, as well as in the elevated annulus, occurredmostly before the emplacement and deformation of Pwr.The concentric fractures in the annulus were also formedbefore Pwr emplacement, but after radial fracture formation.After Pwr emplacement, nova activity was focused in theformation of separate fractures radial to the construct and inthe emplacement of Ps2-Pl.4.2.3. Summary of Annular Novae Population[67] The radial fracturing of novae of this class is the

same as that of upraised novae in terms of both morphologyand behavior. Four novae have one generation of radialfracturing (25%; hereafter percents are shown for annularnovae) and 12 novae (75%) have two or more visiblegenerations of radial fracturing. The radial fractures maybe embayed by material of different types of volcanicplains, and covered by material of eolian deposits, or byejecta or outflow of impact craters. In 4 novae (25%) the lastradial fracturing is not embayed by any material.

[68] Concentric deformation structures are observed insome of these novae, and are represented by ridges andgraben. These structures are located mostly in the annulusstructure. Six novae show the existence of concentric ridges(37.5%) and in 3 novae concentric graben are observed(18.7%). One nova (6.3%) has both concentric extensionaland compressional structures. Concentric tectonic structuresand the elevated annulus of this type of novae may or maynot be embayed by some material. Radial fracturing of novamay deform the annulus. Concentric compressional struc-tures are usually younger than most of the radial novafracturing. In some cases the last nova radial fracturingdeforms concentric compressional structures.[69] Twelve novae (75%) started to form before Pwr

emplacement and 3 novae (18.7%) completed their visibleactivity before Pwr formation. Thirteen novae (81.3%) ofthis class were active after Pwr emplacement. Five novae(31.2%) show association with rift zones, but rift structureshave no visible influence on the novae shape.

4.3. Flat and Negative Novae

[70] There are 15 structures in this class (24.2% of novaestudied) and their average diameter is 192 ± 50 km (Table 4,Table 6). The central parts of these novae are usually flat(Figure 3e). One of the two having prominent negativerelief is up to 2.5 km deep relative to the periphery.4.3.1. H’Uraru Corona[71] A typical example of flat novae is a structure called

H’Uraru corona (D � 225 km) (9�N, 68�E) (Figure 12),which is located to the N of Manatum Tessera. The shape ofthis nova is approximately flat. Some variations in topog-raphy are observed within the nova, but are only �500 m inelevation.[72] Pdf is located on the N flank of the nova at 700–800m

altitude and is radar bright because of intense deformationby straight fractures, which strike to the NW and havefracture to fracture spacing of �300–500 m. Pdf isembayed by Nodf, Pwr and Ps. Rb has the same strati-graphic position, is located in the SW and W parts of thearea studied at 0.7–2 km altitude and is exposed aselongated kipukas, which are embayed by younger material.Rb has an intermediate radar brightness and is deformed bylong (up to 100–130 km) and broad (up to 10–15 km)ridges, which mostly have W and WNW strike. Rb isembayed by Nodf and Pwr. Nodf is located in the centralpart of the nova and to the S, SE and N from the center ofthe feature. Nodf is rather radar bright due to intensedeformation by straight radial fractures, with fracture tofracture spacing of �1–1.5 km. These fractures also deformPdf and Rb. Nodf is embayed by Psh, Pwr, and Ps.[73] Pwr occupies most of the area and is located at 0.6–

2.1 km altitude. Pwr has rather low radar brightness and isdeformed by a regional network of wrinkle ridges which areup to 130–150 km long and up to 2–5 km wide. The strikeof the ridges is variable; they also cut Nodf and Psh.[74] Psh is located at 1.1–1.3 km altitude at the S flank of

the nova. Psh is rather radar dark, and within this unit areobserved separate volcanoes with diameters 1.5–6.5 km.The relationship of this unit with Pwr is unclear, but wrinkleridges deform Psh. We can conclude that Psh was formedbefore wrinkle ridge formation, and Psh and Pwr wereformed rather close to each other in time. They may have


Figure 12. H’Uraru Corona, nova at 9�N 68�E and geological interpretation of region surrounding(Figure 2, nova 22); (a) SAR image (illumination from the left) fragment of C1-MIDR.15N060.101;(b) geologic map of area shown in Figure 12a; (c and d) perspective views of SAR image shown inFigure 12a (Figure 12c) and geologic map shown in Figure 12b (Figure 12d); view is toward NW, verticalexaggeration x20; (e) correlation chart.


different ages as we have observed in other areas, but wecannot establish that here. Young radial fracturing of thenova deforms Psh and Pwr. This fracturing is less promi-nent than the older one and is located on the S flank of thenova only. It is represented by long straight fractures, whichcut also Rb. Fracture to fracture spacing of this system is3.5–5 km.[75] To the S of the nova are observed several regional

fractures that are mostly represented by graben. The strikeof these fractures is NW; they are up to 50–80 km long.These fractures cut Nodf, Psh, and Pwr. Pwr on the N flankof the nova is embayed by the Ps unit, which is located at500–800 m altitude. Ps is radar dark and is not deformed.This material embays Nodf, Pdf and Pwr. The latestgeological activity documented in the area is seen in theformation of aeolian deposits. Within these deposits, sepa-rate diffuse streaks up to 180–200 km long and 5–8 kmwide are observed, and these are interpreted as wind streaks.These deposits mostly cover material of Pwr and Ps to theW and N from the nova. The structure of underlying unitscan be seen through these deposits, which indicates thethinness of this material, and therefore we do not map it as aseparate stratigraphic unit at this scale. This nova recordstwo visible stages of radial fracture formation, mostly beforePwr emplacement. The last stage of nova activity, whichoccurred after formation of Pwr, was much less importantthan the first phase.4.3.2. Unnamed Nova[76] One of the negative structures is an unnamed nova

(8.5�S, 106.5�E) (D � 130 km) (Figure 13) located in OvdaRegio in the N part of Aphrodite Terra. This nova hasprominent funnel-like negative relief. The center of the novais depressed relative to the periphery by 2–3 km, withslopes of the central nova depression of up to 2–3�. To theSW of the nova are located several smaller depressions withdepths up to 2–2.5 km, relative to the edges.[77] The oldest stratigraphic unit in this area is Tt, which

is located in the NW corner of the area studied at 3–5 kmaltitude and occupies the most elevated areas. Tt is veryradar bright because of the extremely intense deformationby fracturing mostly with a NE strike, with fracture tofracture spacing of 500–800 m. Another fracture system hasa NW strike, with fracture to fracture spacing of 1–10 km.Tt is embayed by Pdf and Ps1.[78] Pdf is located mostly on the change of slope of the

nova depression at 1.8–3 km altitude. There are somepieces of Pdf in the SE part of the area. Here Pdf is locatedat 3–5 km altitude. Pdf has a high radar brightness becauseof intense deformation. Around the depression Pdf isdeformed mostly by dense radial fracturing of nova, withfracture to fracture spacing of 1–2 km. Part of this fractur-ing is composed of graben which usually are up to 60–70 km long and up to 3–5 km wide. These fractures alsodeform Tt and extend away from the nova center for morethan 200 km. Another fracture system is concentric to thenova, deforms Pdf, and is located on the flexure of the novadepression. It is represented mostly by graben which areusually up to 70–80 km long and 3–5 km wide. In the SEarea, most fracturing with a NE strike deforms Pdf; fractureto fracture spacing is 1–2 km. Pdf is embayed by Ps1.[79] Ps1 is located in the central part of the nova depres-

sion and in rather low areas in the S part of the area. Ps1 is

located mostly at lower altitude relative to Tt and Pdf. Ps1 israther radar dark. Young radial fracturing of the novadeforms Ps1, and this fracture system has fracture to fracturespacing of 1–3 km and also deforms Pdf and Tt. Part of it isrepresented by graben which are usually up to 50–80 kmlong and 2–3 km wide. In depressions to the SW of thenova, extensional fracturing is also observed. This fracturesystem is represented mostly by narrow graben whichconform to the edges of depressions. Ps1 is embayed byPs2. Ps2 is radar dark and is located at 300–900 m altitudeon the floors of depressions to the S of the nova. Ps2 isdeformed by small ridges, which are generally radial to thedepression edges.[80] This nova had two visible stages of formation. In the

first stage, dense radial and concentric extensional fractureswere formed, and in the second stage, only radial exten-sional fracturing occurred. The lower limit of this novaactivity is not clear in the area studied. In neighboring areasPs1 embays Pwr, and in the mapped area Ps1 is deformed bynova radial fracturing; thus this nova was active after Pwremplacement.4.3.3. Summary of Flat and Negative Novae[81] Extensional fracturing and graben comprise the

radial structures of these novae. The morphology andbehavior of this radial fracturing are the same as radialfracturing of the novae classes previously described. Allnovae of this class have two or more generations of radialextensional fracturing. Radial fracturing may be embayedby different volcanic plains or covered by material ofaeolian deposits, or by ejecta of impact craters. In 2 novae(13.3%; hereafter percents are shown for flat and negativenovae) the last stage of radial fracturing is not covered.[82] Concentric features are observed in part of these

novae and are made up of both extensional and compres-sional tectonic structures. Ten novae (66.7%) have concen-tric graben and normal faults, and in 2 novae (13.3%)concentric ridges are observed. Two novae (13.3%) haveboth concentric extensional and concentric compressionaltectonic structures. Commonly some material embays them.[83] Six novae (40%) started to form before Pwr

emplacement and 1 nova (6.7%) completed its activitybefore that time. Apparently, 93.3% of flat and negativenovae were active after emplacement of Pwr. Six novae ofthis class (40%) show association with rift zones, but riftstructures have no visible influence on the nova shape.

4.4. Plateau-Like Novae

[84] There are 15 structures in this class (24.2% of novaestudied) (Table 5, Table 6), and their average diameter is 179± 51 km. The plateau-like rise of these novae is elevatedabove the surrounding areas by up to 1–2 km (Figure 3f ). Inthis class of structures we consider five novae as transitionalbetween upraised novae and plateau-like.4.4.1. Maram Corona[85] One of the typical plateau-like structures is a nova

called Maram Corona (7.5�S, 221�E), (D � 130 km)(Figures 14a and 14b), which is located on the S flank ofKhicheda Chasma, part of Parga Chasmata. This nova has aclear plateau-like shape. The N flank of the plateau is higherthan the central part and the S flank. The nova is located onthe edge of a rift valley which strikes E-W. The width of therift valley is 250–300 km and it is up to 4.5–4.8 km deep


Figure 13. Unnamed nova at 8.5�S 106.5�E and geological interpretation of region surrounding(Figure 2, nova 28); (a) SAR image (illumination from the right) fragment of C1-MIDR.15S112.201;(b) geologic map of area shown in Figure 13a; (c and d) perspective views of SAR image shown inFigure 13a (Figure 13c) and geologic map shown in Figure 13b (Figure 13d); view is toward SSW,vertical exaggeration x10; (e) correlation chart.


relative to the surrounding plains. The elevation of the Npart of the nova plateau above the floor of the rift valley is5.1–5.3 km, while the elevation of central and S parts of theplateau above the surrounding plain is 1–1.5 km. The slopeof the nova plateau to the rift valley is up to 4–8�, to thesurrounding plains up to 1–2�. To the S of the novaplateau, a shallow trough is observed, with a depth up to700–900 m. This trough surrounds the S part of the novaplateau.[86] Pdf is located mostly in the S part of the area

studied, although some pieces of Pdf are observed in the Nand W parts. Pdf is located at 0–1.8 km altitude and israther radar bright because of intense deformation. Pdf isdeformed by densely spaced, mostly parallel fractures withfracture to fracture spacing of hundreds of meters (100–300 m and less). The strike of this fracturing is variable,but mostly N-NE. Pdf is embayed by Pwr and Ps-Pl1. Pshis located in the N part of the area studied, lying at 1.4–1.8 km altitude. Psh has intermediate radar brightness andforms separate volcanic fields in the NE part of the area,which are embayed by Pwr. Separate volcanic constructswith slightly sloping flanks and diameters from 2–4 km to11–15 km are observed. In the NW part of the area Psh isobserved as a less deformed tectonic remnant in thedensely deformed rift zone.[87] Pwr is located in the SW and NE corners of the area,

is rather radar dark, and is located at 0–1.9 km altitude. Pwris deformed by a regional network of wrinkle ridges whichmostly strike NW, are up to 150–200 km long and 3–4 kmwide, with ridge to ridge spacing of up to 10–30 km. Pwr isalso deformed by a fracture system. This fracturing strikesNNE in the S part of the area studied and NNW in the N part.Fracturing has fracture to fracture spacing of up to 2–3 kmand also cuts Pdf and Psh. Pwr embays Pdf and Psh and isembayed by Ps-Pl1. The age relationship of Pdf and Pshis unclear because these units have no contact, but Psh islocated very close to Pdf and is not deformed by the densefracturing of Pdf, and thus Psh is younger than Pdf.[88] Nodf is located in the central and N parts of the nova

plateau rise at 2.5–4.8 km altitude and is radar bright becauseof dense radial fracturing; fracture to fracture spacing is 200–300 m and less. Nodf is embayed by Ps-Pl1. The agerelationship between Nodf and Psh, Pwr, Pdf are not clear;we can note that Nodf is older than the Ps-Pl1.[89] Ps-Pl1 is located in the central part of the area at 1.5–

2.8 km altitude. At this scale of mapping we cannot separatelobate and smooth plains. Ps-Pl1 is generally radar dark, andonly several radar bright lobate flows are visible on thebackground of radar dark smooth plains. Ps-Pl1 is located inthe nova plateau and also composes the surrounding plains.It is deformed by two fracture systems. The first radiatesfrom the center of the nova and has a fracture to fracturespacing of 1–2 km in the central part of the structure and upto 6–10 km on the flanks. Fractures and narrow grabenmostly represent the radial structures of the nova. Thisfracturing extends away from the nova center for 250–300 km and is denser to the N of the nova but runs forlonger distances to the S-SE. Another fracture system isconnected with the rift. Denser fracturing is observed in thecentral parts of the rift valley, with fracture to fracturespacing in the central parts of the rift up to several hundredsof meters (200–300 m and less) and even larger on the

flanks. Some of the fractures extend away from the riftvalley for 300–400 km. This fracturing cuts units describedabove, as well as young radial fracturing. We do not see thenature of the material that is deformed by the rift, and wetherefore mapped this area as a tectonic feature.[90] Part of the rift fractures surrounds the nova plateau

rise. These fractures are represented mostly by normal faultsand usually are located in the rift valley and on the marginsof the nova plateau. This fracturing is denser in the N part ofthe nova, which neighbors the rift, and has a lower densityin the S part of the nova, where it is also concentrated on thenova plateau margin, and in the shallow trough to the S ofthe nova. On the N flank of the nova, rift fracturing cuts offboth pieces of Nodf and Ps-Pl1. These blocks are lessdeformed by rift fracturing, and in the backside of theseblocks numerous graben and normal faults are observed;apparently these blocks sloped down to the rift valley. In therift valley itself some blocks or tectonic remnants ofsomewhat less deformed material are observed, such asblocks of Psh and Ps-Pl1. Part of the rift fractures cut thenova through from W-NW to E-SE without any changes instrike. This fracturing is represented mostly by long narrowgraben, which are up to 300–400 km long and about 3–5 km wide.[91] In the SE part of the area, the rift fractures are

embayed by radar bright material of Pl2 and Pl3 located at0.8–2 km altitude. Some of the rift fractures are embayedby Pl2 and some cut it. Pl3 embays all visible rift fractures.Rift activity started after formation of Ps-Pl1, the Pl2 unitformed simultaneously with rift activity, and Pl3 embays riftfractures.[92] Two visible stages of radial fracture formation

occurred during the evolution of this nova. Before forma-tion of the Ps-Pl1 unit, dense radial fracturing of the novawas occurring. After Ps-Pl1 emplacement, a second gener-ation of radial fracturing occurs. The rift zone began to formafter nova formation. We see clear evidence for activity ofthis nova only after Pwr emplacement.4.4.2. Oduduva Corona[93] Another example of a plateau-like structure is Odu-

duva Corona (D � 200 km) (11�S, 211�E) (Figure 15),which is located to the NE of Wawalag Planitia and is a partof Veleda Linea. The nova is located in the rift that strikesWSW-ENE. In some places the rift valley is up to 2.3–2.5 km deep relative to the surrounding plains. The plateau-like part of the nova is elevated 1.5–2 km above thesurrounding plains and rift structure. Shallow troughs areobserved around the nova, mostly to the N and S and are0.5–1.5 km deep relative to the surrounding plains. Slopesof the nova are up to 10–13�.[94] Pdf is located mostly in the SE corner of the area

studied, at 1.5–1.7 km altitude; some pieces of Pdf areobserved close to the rift zone and in the N part of the areaat 2–2.2 km altitude. Pdf is rather radar bright because ofintense deformation by features that mostly strike NW, withfracture to fracture spacing of 0.5–2 km. Another fracturesystem has larger fracture to fracture spacing (4–8 km) andstrikes approximately WSW, with some variations. Pdf isembayed by Ps1-Pl and is overlapped by Cu of the impactcrater Izakay.[95] Ps1-Pl occupies most of the area; it is located at 1.2–

1.6 km altitude and is mostly radar dark. Radar bright lobate


Figure 14. Maram Corona, nova at 7�S 221�E and geological interpretation of region surrounding(Figure 2, nova 25); (a) SAR image (illumination from the left) fragments of C1-MIDR.00N215.101 andC1-MIDR.15S215.101; (b) geologic map of area shown in Figure 14a; (c) correlation chart. (Figures 14dand 14f) Perspective views of SAR image shown in Figure 14a and (Figures 14e and 14g) geologic mapshown in Figure 14b; (d and e) view is toward E; (f and g) view is toward SW, vertical exaggeration x15.


flows are concentrated in some places, especially around thenova. We can not clearly separate Ps and Pl, at this scale,therefore we combine these materials into one unit. Ps1-Pl isdeformed by several fracture systems, including rift fractur-ing and late nova-associated fracturing.[96] The rift strikes from WSW to ENE; the zone of dense

deformation is up to 230 km wide. Fracture to fracturespacing within the rift is 200–300 m and less. We cannotdetermine the nature of material that is being deformedinside the rift, and therefore we mapped this structure as atectonic feature. More and less deformed parts of the rift areobserved, and in some parts of the rift a possible volcanicembayment contact is seen with one unit more deformedthan the other. We can subdivide two types of rift terrain ortectonic zones. The first zone (Rt1) is deformed moredensely than the second one (Rt2), and Rt2 in some placescuts material which composes and is densely deformedwithin Rt1. Ps1-Pl mostly embays the Rt1 zone and isdeformed by structures of Rt2. This suggests that Rt1 isrelatively older than Rt2.[97] Nodf is observed in the central part of the rift. It is

located at 2.5–2.7 km altitude, mostly on the flat top of theplateau-like nova rise. Nodf is rather radar bright because ofintense deformation by radial fracturing of nova, withfracture to fracture spacing of 200 m and less. Nodf isdeformed by young rift fracturing at the nova plateau

margins. The latest nova radial fracturing also deformsNodf and extends away from the nova for 120–150 km,mostly to the N and S. Young rift fracturing cuts Pdf, themajority of Ps1-Pl, the latest fracturing of nova, and isembayed only by several lobate flows of Ps1-Pl, by themajority of Ps2 and is covered by Cu. Ps2 also embay Nodfand the young radial fracturing of nova. Cu of Izakayimpact crater (D � 10.2 km) is radar bright and overliesPdf, Ps1-Pl, and young rift fracturing. The crater is locatedat �1 km altitude [Schaber et al., 1998].[98] This nova was formed during several stages. In early

stages, the first generation of radial fracturing of nova isforming on the background of old rift zone evolution (Rt1).After formation of the majority of the Ps1-Pl unit, on thebackground of young rift zone evolution (Rt2) the secondgeneration of radial fracturing of nova forms. Part of theyoung nova radial fracturing composes the young riftfracturing. We cannot define the relation with Pwr withinthe area studied, but in neighboring areas the Ps1-Pl unitembays Pwr. Therefore we see only evidence for activity ofthe nova after Pwr emplacement.4.4.3. Unnamed Novae[99] As examples of transitional novae between upraised

and plateau-like we consider two unnamed novae (17�S,203.5�E and 16�S, 205.5�E) (Figure 16), which are locatedin the N part of Wawalag Planitia in Jokwa Linea. Hereafter

Figure 14. (continued)

Figure 15. (opposite) Oduduwa Corona, nova at 11�S 211�E and geological interpretation of region surrounding(Figure 2, nova 32); (a) SAR image (illumination from the left) fragment of C1-MIDR.15S215.101; (b) geologic map ofarea shown in Figure 15a; (c and d) perspective views of SAR image shown in Figure 15a (Figure 15c) and geologic mapshown in Figure 15b (Figure 15d); view is toward WSW, vertical exaggeration x10; (e) correlation chart.


we will call the nova at 17�S, 203.5�E nova 1 (D� 150 km),and the nova at 16�S, 205.5�E nova 2 (D � 100 km). Bothof these two novae are located in the rift zone. Nova 1 islocated in a rather deep rift valley in the center of the areastudied. The central part of the nova is located at the samealtitude as the shoulders of the rift, at 1–1.1 km above thesurrounding plains. Between the elevated central part of thenova and the elevated periphery of the nova, which islocated on the rift shoulders, rift troughs are observed whichsurround the elevated central part of the nova and are up to2 km deep relative to the nova rise and the rift shoulders.The slopes of this nova rise are up to 5–8�. Nova 2 islocated on the rift zone also in the NE corner of the area.The central part of the nova is elevated 1.3–1.4 km abovethe surrounding plains. The shallow troughs thatsurround the central part of the nova are 700–800 m deep.The slopes of the nova rise are up to 2–4�.[100] Tt is located to the SW from nova 1 and in the SE

corner of the area at 1100–1200 m altitude. Tt is radarbright due to intense deformation. Tt is represented bykipukas of densely deformed material with sizes up to60 � 60 km; these are embayed from all sides by youngermaterial. Densest fracturing strikes NE and fracture tofracture spacing is �1–1.5 km; lower density fracturingstrikes NW and fracture to fracture spacing is �6–10 km.Psh is located to the N and to the S from the rift at 1–1.2 kmaltitude. Separate shield volcanic fields are embayed byyounger material (Pwr, Ps1). Psh has intermediate radarbrightness and is sometimes deformed by a fracture system,as in the shield field to the SW of nova 1. The strike of thisfracture system is NE and fracture to fracture spacing is�2–3 km.[101] Pwr is located to the S and N of the rift. Pwr is radar

dark and is located at 700–1500 m altitude. Pwr embays Ttand Psh. Pwr is deformed by a regional network of wrinkleridges which are usually 30–60 km long, 0.5–1.5 km wideand have a ridge to ridge spacing of �7–20 km. Most of thewrinkle ridges strike NNW and also deform Psh. Fracturingof both the rift and nova 1 deforms Pwr. The rift strikesWSW-NNE and is 60–70 km wide. Nova fracturing com-poses part of the rift structures. Nova fractures, radial to thenova center in its central part, change strike to parallel therift fracturing and gradually proceed to the rift fracturing atthe periphery. Some fractures of the nova to the N and to theS cross rift structures and extend away from the rift zone for120–150 km. Fracture to fracture spacing in the central partof nova is 200–300 m and less. The same fracture tofracture spacing of rift fracturing is observed inside the riftzone without nova structure. In the central parts of the riftwe cannot identify the nature of the precursor unit to theextremely highly deformed material, and we thereforemapped these structure as tectonic. To the sides of the rift,fracture spacing becomes greater. Most of the rift fracturescut Pwr and are embayed by Ps1–2 and Pl1–2, but some ofthe fractures cut Pl1 and Ps1. Some fractures, which are

parallel to the rift, are observed to the S and to the N of thevalley at distances of more than 200–250 km.[102] Units of Ps and Pl alternate. Ps1 is located in the

NW corner of the area at 1–1.2 km altitude. Ps1 is radardark and embays Psh, Pwr and most rift and nova 1fracturing. However, part of the rift and nova 1 fracturingcut Ps1. The unit is embayed by material of lobate plainsPl1–2. Ps2 is discussed below. Pl1 is located on the slopesof the rift shoulders to the N and to the S of nova 1 at1.2–1.5 km altitude, and to the N of nova 2. Pl1 is madeup of long (up to 70–90 km) radar-bright lava flows,which embay most of the rift fractures but are deformedby some of them. On the S flank of the rift, severalgenerations of lava flows are observed. Ps2 is located inthe rift zone at 600–800 m altitude. Ps2 is radar dark andembays rift fracturing and Pwr. Ps2 is not deformed by riftfracturing and nova 1 fracturing, but is deformed byradial fracturing of nova 2. Radial fracturing of nova 2has fracture to fracture spacing of 2–4 km in the centralpart of the nova and 6–10 km at the periphery. Thisfracturing also cuts rift structures. Pl2 has a radar brightsurface and is located on the N slope of the rift shoulderand to the N of nova 2. Pl2 is located at 1.2–1.7 kmaltitude and embays Pwr, Ps1 and Pl1, rift fracturing, andnova 2 radial fracturing. Pl2 is not deformed.[103] Both to the S and to the N of the rift zone, dark areas

with diffuse borders are located. Within these areas ratherradar dark and bright streaks 30–50 km long and 1–3 kmwide are observed. The strike of these streaks is NW to theS of the rift and NE to the N of the rift; these are interpretedas wind streaks of aeolian deposits. The structure ofunderlying units appears through these deposits, whichindicates the thinness of this material; we therefore do notseparate it into a specific stratigraphic unit.[104] Impact crater Fouquet (D � 47.8 km) is located in

the N part of the area at 1 km altitude. To the S of the rift,crater Loretta (D � 13.5 km) is located at 900 m altitude[Schaber et al., 1998]. Cu is extremely radar bright andoverlies Psh, Pwr and Ps1 and is not deformed.[105] Nova 1 started to form after Pwr emplacement, in

close association with rift formation. The latest activity ofthis nova is observed after emplacement of Ps1 and beforePl1. Nova 2 began and finished its formation after Ps2emplacement and before formation of Pl2.4.4.4. Summary of Plateau-Like Novae Population[106] The radial fracturing of novae of this class is the

same as that of previously described novae classes in termsof both morphology and behavior. In just 2 transitionalnovae (13.3%, hereafter percents are shown for plateau-likenovae), one generation of radial fracturing is observed, andin the other 13 novae (86.7%), two or more stages of radialfracturing formation were identified.[107] Most plateau-like novae (80%) have concentric

extensional structures which are represented by grabenand normal faults and are located on the nova plateau

Figure 16. (opposite) Unnamed novae at 17�S 203.5�E (nova 1) and at 16�S 205.5�E (nova 2) and geologicalinterpretation of region surrounding (Figure 2, novae 38, 40); (a) SAR image (illumination from the right) fragment ofC1-MIDR.15S197.201; (b) geologic map of area shown in Figure 16a; (c and d) perspective views of SAR image shownin Figure 16a (Figure 16c) and geologic map shown in Figure 16b (Figure 16d); view is toward WSW, verticalexaggeration x20, on the foreground is nova 2, on the background is nova 1; (e) correlation chart.


margins. Three novae (20%) without this fracturing belongto the novae that we consider as transitional. Three novae(20%) show the existence of both concentric extensionaland compressional structures.[108] All these novae are located in rift zones. Rift

structures influence the shape of the novae; rift troughsusually surround plateau-like rises of novae. The riftcontrols the shape of the novae. Three types of agerelationship of these novae with rift zones are observed:(1) nova predating rift zone activity; rift structures cutnova structures; orientations of radial fracturing do notdepend on orientation of rift structures; (2) nova formingsimultaneously with rift zone evolution; orientation ofnova radial fracturing depend on the orientation of riftstructures; nova fracturing partly composes rift fracturing;part of nova fractures may extend away in a directionnormal to the rift zone for hundreds of kilometers; (3)nova postdating rift activity; orientation of nova radialfracturing does not depend on orientation of rift structures.[109] We considered 5 novae as transitional between

upraised and plateau-like on the basis of the followingreasoning: these novae are located in the rift zones andare surrounded by rift troughs, but the rise of these novae, incontrast to other novae of this class, has no clear plateau-like shape. We did not find any structures in plateau-likenovae, which are embayed by Pwr. This suggests that thesenovae started to form after Pwr formation or that traces ofpre-Pwr nova activity were erased by subsequent nova orrift activity.

5. Summary of Key Observations

5.1. Radial Fracturing of Novae

[110] The radial fracturing that define novae as a class offeatures has the following characteristics: (1) Dense radialfracturing is usual for all novae; most of them have severalvisible generations of radial fracturing (usually two, up tothree); fracture to fracture spacing systematically increasesfrom older generations to younger. The effect of ‘‘multipleradial fracturing’’ may be created by partial embayment ofone stage of fracturing by volcanic plains, but we do not seeextensive evidence for that. (2) These fracture systems arecharacterized by straight, long extensional fractures andnarrow graben, which radiate from the centers of the novarises and are mostly located in the central elevated parts andon the flanks of novae; part of radial novae fracturingextends away from the nova rises for hundreds of kilo-meters. (3) In many cases young radial fracturing of novaeis the source of lobate lava flows, which in some casesemerge at the same elevation on nova slopes; this is trueonly for the youngest generation of radial fracturing,because older generations are usually observed as kipukasembayed by younger material. (4) The distribution of radialfractures of numerous novae is asymmetric, and mostlybilateral.

5.2. Four Topographic Classes of Novae are Observed

[111] (1) Upraised novae (Figure 3): an upraised topo-graphic profile, transitional between cone-like and dome-like; some of the novae have concentric compressional and/or extensional structures. (2) Annular novae: have anelevated central part and rather elevated annulus concentric

to the nova center. Two subtypes of annular novae areobserved: the first subtype has radial extensional fracturingin the elevated central parts of the novae and mostlyconcentric compressional structures at their annulae andperiphery, the youngest radial fracturing is usually youngerthan the concentric compressional structures. The secondsubtype has radial extensional fracturing in the elevatedcentral parts and in the annulae of novae and concentricextensional fracturing in the annulae; concentric fracturingis usually younger than the radial fracturing. (3) Flat andnegative novae: flat, irregular shapes. Some of these haveprominent negative relief. A small number of concentricstructures are seen in flat novae and negative novae haveconcentric extensional structures. (4) Plateau-like novae:prominent plateau-like topography or shapes transitionalto plateau-like. These are located in rift zones, and mostof them have concentric graben and normal fault systemsapparently formed under the influence of the rift zone.Three types of relationship between these novae and riftstructures are observed: nova predates rift zone formation,nova forms simultaneously with rift evolution, nova post-dates rift zone activity.

5.3. Location on Planet

[112] Novae are located (Figure 2) mostly in areas ofregional volcanic rises and rift zones (Figures 2 and 17)[Price, 1995; Basilevsky and Head, 2000b], and someconcentration is observed in the Beta-Atla-Themis triangleregion [Crumpler et al., 1993; Crumpler and Aubele, 2000].A small number of novae are located on the lowlandswithout any connection with other regional volcanic-tectonicstructures like rifts, volcanic rises or coronae. Most low-lands are sites of accumulations of regional plains withwrinkle ridges [Ivanov and Head, 2001] and thus somenovae might be buried by these deposits and not currentlyvisible.

5.4. Relationship of Novae With Rifts andAssociated Volcanism

[113] Most novae of all classes show an association withrift zones (Figure 17) and display the following relation-ships: (1) rift does not have a visible influence on topog-raphy of upraised, annular, flat and negative novae; (2) rifthas a visible influence on the topography of plateau-likenovae; rift troughs surround novae constructions and formtheir shape. Most of the volcanic activity of novae is seen inlobate lava outflows, which is usual both for novae whichare located inside and outside of rift zones. With somenovae small shield volcanoes and their clusters areconnected. Large amounts of volcanism are common forupraised, annular and plateau-like novae.

5.5. Main Distinctions From Other Radial-ConcentricVolcanic-Tectonic Structures

[114] 1. Coronae: Radial fracturing is observed in onlypart of the coronae population. Coronae have more variedshapes and concentric annulae are common [e.g., Proninand Stofan, 1990; Stofan and Head, 1990; Nikishin et al.,1992; Stofan et al., 1992, 1997; Janes et al., 1992; Squyreset al., 1992; Glaze et al., 2002]. The evolution of part of thenovae may lead to formation of corona-like features orannular novae, but numerous novae completed their evolu-


tion without formation of a morphologic or structuralconcentric annulus.[115] 2. Arachnoids and calderas: The majority of arach-

noids have radial ridge patterns, and the majority of calderashave concentric fracture patterns; a very small portion of thearachnoid and caldera population has radial extensionalfracturing [Head et al., 1992; Aittola and Kostama, 2000;Tormanen and Kauhanen, 1994; Krassilnikov and Head,2003; Pace and Krassilnikov, 2003]. Numerous novae haveconcentric compressional structures, which is unusual forarachnoids and calderas. Almost all arachnoids and allcalderas have negative relief, while most novae havepositive relief.[116] 3. Volcanoes: Judging by the large amount of

associated volcanism, some novae could be described asvolcanic constructions, but in contrast to volcanoes, muchmore numerous tectonic features commonly characterizenovae. Volcanoes show a dominance of volcanic activity,while novae show a dominance of tectonic activity.

5.6. Period of Novae Activity

[117] Part of the population (40.3%) began to form beforePwr emplacement (Figure 18), 11.3% completed theiractivity before Pwr formation, and 88.7% were active afterPwr emplacement. Some of the post-Pwr novae may startbefore Pwr emplacement, but evidence of that may havebeen erased by later nova or rift activity.

5.7. Stratigraphic Sequence

[118] In all local areas studied the sequence of geologicalunits is generally similar. Our findings are consistent withthe model of regular changes in style of predominantlyregional to global geological processes in the observed

history of Venus [Basilevsky and Head, 1995a, 1995b,1998b, 2000a].

6. Mechanisms and Scenarios of Novae Evolution

[119] The formation of novae is interpreted by differentauthors to be the result of updoming and fracturing of thesurface of Venus due to the dynamic and thermal influenceof hot mantle diapirs on the upper part of the lithosphere

Figure 17. Distribution of novae superposed on ‘‘Map of Rifts and Volcanoes of Venus’’ (modifiedfrom Basilevsky and Head [2000b] and Price [1995]).

Figure 18. Period of novae activity. (a) Novae finishedany activity before Pwr emplacement. (b) Novae startedbefore Pwr formation and active after this moment.(c) Novae without traces of activity before Pwr emplace-ment. Forty percent of novae show activity before Pwrformation. Eighty-nine percent of novae show activity afterPwr formation.


[e.g., Janes et al., 1992; Squyres et al., 1992; Stofan et al.,1992, 1997]. The term diapir has been used to describesome mantle plumes in which a mass of material in themantle rises because of its buoyancy [e.g., Condie, 2001].Mantle diapir is used to describe a small mantle plume(<300 km across) that never developed a tail or lost its tailand thus ceases to grow [Herrik, 1999; Condie, 2001]. Wewill use the term diapir because the diameter of novaestructures is usually between 100 and 300 km [Head et al.,1992; Crumpler and Aubele, 2000], and the prominence ofthe structure on the surface may depend on the size of theplume head [Griffiths and Campbell, 1991]. We do not ruleout that part of novae activity may be related to a normalplume (head + tail). In our interpretation we will also callon models of plume evolution [e.g., Whitehead, 1982;McKenzie, 1984; White and Mckenzie, 1989; Griffiths andCampbell, 1991; Mege and Ernst, 2001; Condie, 2001;Dombard et al., 2002] and radial dike emplacement fromcentral magma sources associated with plumes [e.g., Ernstet al., 1995].[120] According to the interpretation of many authors

[e.g., Janes et al., 1992; Squyres et al., 1992; Stofan etal., 1992, 1997; Cyr and Melosh, 1993; Koch, 1994; Kochand Manga, 1996], the mechanical uplift of the surface andthe consequent increase of the area of the nova rise by theinfluence of the rising mantle diapir, cause extension andthe formation of radial fracturing seen in novae. The diapirthen flattens along a neutral buoyancy level (n0), spreadslaterally, and forms the corona structure. There are severalgeological [Squyres et al., 1992; Stofan et al., 1992, 1997],geophysical [Stofan et al., 1991; Janes et al., 1992; Cyr andMelosh, 1993; Koch, 1994; Koch and Manga, 1996] andanalogous tectonophysical [Krassilnikov et al., 1999, 2001;Krassilnikov, 2000, 2001, 2002] models of this scenario ofnovae and coronae evolution. Processes of nova formationabove the rising hot mantle diapir, in the mechanical sense,are similar to the formation of deformation structures aboverising and uplifting salt diapirs or to the formation of radialfractured domes on Earth. Therefore we also used the resultsof modeling of salt diapirism processes [e.g., Parker andMcDowell, 1955; Ramberg, 1981; Davison et al., 1993] andradially fractured dome formation [e.g., Withjack andScheiner, 1982]. In an attempt to understand the processesof novae gravitational relaxation, we used the results ofgeological observations and analogous tectonophysicalmodeling of gravitational spreading of volcanic construc-tions [Merle and Vendeville, 1995; Merle and Borgia, 1996;Borgia et al., 2000]. In developing our model of novaeevolution, we built on the model of corona evolutiondescribed by Squyres et al. [1992] and Stofan et al.[1992, 1997].

6.1. Mechanisms and Scenarios of Novae EvolutionOutside of Rift Zones

[121] Upraised, annular, flat and negative novae arelocated outside the prominent influence of rift zones. Asfollows from models noted above a major factor of novaeevolution is the position of the neutral buoyancy zone of theascending mantle diapir (n0). The mantle diapir is rising andtrying to reach its neutral buoyancy level (n0). Hereafter wewill use term n0 for the neutral buoyancy zone of the diapir.The position of each diapir depends on thermal, dynamical

and compositional buoyancy of the diapir. We do notaddress the causes of diapir buoyancy [Kreslavsky, 1994;Hansen, 2001], but only the position of n0 relative to thesurface and the surface manifestation of the influence of thediapir on the lithosphere.6.1.1. Upraised Novae[122] As suggested by the modeling, a rising diapir of a

given volume produces a more or less prominent topog-raphic rise on the surface with the magnitude depending onposition of n0 (Figure 19). The shallower n0 is, the moreprominent and steep-sloped is the relief of the nova andmore dome-like. The deeper n0 is, the lower the relief andless prominent is the topography and the more cone-like isthe nova shape.[123] Most authors consider nova radial fracturing as

forming due to uplift on the surface and consequentincreasing of the area of the nova rise. The central partsof the novae are elevated above the periphery for heightsfrom several hundreds of meters up to 1–2 kilometers, anddiameters of novae are 100–300 km [Head et al., 1992;Crumpler and Aubele, 2000]. The formation of such denseradial fracture systems described is hard to explain solelydue to mechanical uplift of the surface by such small scalesof topographic uplift. Dense radial fracture formation alsowas not observed in the analogous modeling of novae andupraised dome formation [Withjack and Scheiner, 1982;Krassilnikov et al., 1999, 2001; Krassilnikov, 2001, 2002],nor in the influence of rising salt diapir on the overlyinglayers and observation of natural salt dome structures [e.g.,Parker and McDowell, 1955; Ramberg, 1981; Davison etal., 1993]. According to these observations in the centralparts of the dome, which is produced by uprising diapir, a‘‘broken plate’’ structure or polygonal extensional fracturesystem forms.[124] This discrepancy could be explained by an addi-

tional mechanism of formation of nova radial fracturingfrom the influence of the emplacement of radial dikeswarms in association with diapiric magmatism. Radial dikeswarms on the surface of Venus, Earth, and Mars aredescribed in detail by different authors and this mechanismhas previously been suggested for formation of radialfractured centers on Venus [Head et al., 1992; Parfitt andHead, 1993; Grosfils and Head, 1994a, 1994b, 1995, 1996;Ernst and Buchan, 1998; Koenig and Pollard, 1998; Ernstet al., 1995, 2001, 2003]. Studies of dike emplacementindicate that most dikes are injected laterally and occur asvertical slabs centered at levels of neutral buoyancy in thecrust [Halls, 1982; Ernst et al., 1995; Grosfils and Head,1994a, 1994b, 1995, 1996; Parfitt and Head, 1993; Wilsonand Head, 2002]. Formation of these dike swarms occurs byincreased pressure inside magmatic reservoirs centeredalong their n0 for hundreds and thousands of kilometersaround the chamber [Grosfils and Head, 1994a, 1994b,1995, 1996; Parfitt and Head, 1993; Rubin and Pollard,1988; Rubin, 1992; Wilson and Head, 2002; Scott et al.,2002]. Such reservoirs could readily be formed due topartial melting in the rising mantle diapir head, whichresults from adiabatic decompression during their rise nearthe base of the lithosphere. The numerical plume meltingmodel by Ribe and Christiansen [1999] suggests that majormelting occurs in a primary melting zone in the plume heador at a shallow level in the plume tail [Condie, 2001].


[125] The following observations suggest that dikeswarms emplacement is the dominant process in nova radialfracturing formation: (1) Part of the radial novae fracturesextend away from the nova rise for hundreds of kilometers,well beyond the topographic uplift [Grosfils and Head,1994a, 1994b, 1995, 1996]. (2) The central parts of manynovae are elevated above the flanks for several hundreds ofmeters. However, since the diameters of these radial struc-tures are hundreds of kilometers this upraised region is nothigh enough to produce dense radial fracturing by simplemechanical uplift of the surface. (3) Radial fractures andgraben are straight and long, which is hard to achieve due topassive extension of upper parts of the lithosphere. (4) Inmany cases young radial fracturing associated with novae isthe source of lobate lava flows. In some cases lobate flowsemerge on the nova slopes from the same elevation,indicating a possible connection with the position of themagma chamber which is predicted to produce nova radialdike swarms. (5) We do not observe simultaneously devel-oped radial and concentric extensional structures in theactual nova rises as predicted by models. Concentric exten-

sional fractures of novae are always younger than radialfractures and appear to have a connection with the relaxa-tion phase of nova evolution.[126] As was described by Basilevsky and Raitala [1999,

2002] radial fracturing of novae can occur in multiplephases. Most novae (81%) have several generations of radialfracturing and fracture to fracture spacing systematicallyincreases from older generations to younger (Figure 7). Wesuggest the following explanation for these observations:(1) One of the more important conditions of radial dikeemplacement is increased pressure inside the magmaticreservoir [Grosfils and Head, 1994a, 1994b, 1995, 1996;Parfitt and Head, 1993]. Pressure should be higher in theinitial stages of nova formation because of the initialenhanced pressure-release melting associated with theimpinging diapir. Large amounts of volcanism are associ-ated with numerous novae, and decreasing generation andassociated decreasing pressure in the magma chamber couldlead to decrease in density and extent of each subsequentradial dike swarm. (2) The influence of the emplacement ofradial dike swarms on the surface depends on the position of

Figure 19. Sequence of events in evolution of upraised novae with prominent relief and annular novae(subtype-1), where n0

d, neutral buoyancy level of the diapir; n0r, neutral buoyancy level of the magmatic

reservoir; n0l , level of lithospheric isostatic equilibrium; h, thickness of the upper brittle part of the

lithosphere (includes crust), h1 < h2 < h3.


the n0 of the magma chamber relative to the surface. Thedeeper n0 is, the less is the manifestation of the dike emplace-ment on the surface. Cooling of the chamber and subsequentassociated increase of the depth of n0 should cause thesame sequence to be observed, an increase of the fracture tofracture spacing of radial nova fracturing with time.[127] It is possible, that both of these reasons lead to an

increase in the fracture to fracture spacing of radial fractur-ing with time. We can conclude that the mechanisms ofradial dike swarm emplacement are clearly operating in theradial fracturing of novae, because it explains the discrep-ancy between real and modeled structures. Of course, radialdike emplacement occurs in the presence of a stress fieldand if this is extensional due to uplift, graben formation willbe favored [e.g., Wilson and Head, 2002]. Thus thesefractures can be formed in part due to mechanical uplift,but the leading role appears to be played by dike swarms.Studies by Grosfils and Head [1994a] show that 72% ofradial fractured structures in their analysis must haveinvolved subsurface dike swarm emplacement and only7% appear to have formed chiefly from uplift [see alsoCondie, 2001].[128] More developed novae should have more genera-

tions of radial fracturing. In the history of a single nova thenumbers of generations of fracturing increase with theevolution of the nova. The highest number of single-stagenovae are observed in upraised novae (Table 6), which maybe evidence that these novae are in the early stages of theirevolution. The upraised topographic shape of these novaesuggests the existence of internal support of these novae.[129] The influence of the regional stress field on the

distribution of the radial fracturing of some novae is clearlyobserved. Many authors [e.g., Withjack and Scheiner, 1982;McKenzie et al., 1992; Koenig and Pollard, 1998] describedthe distribution of radial fractures in conditions of a regionalstress field during dome formation and by radial dikeemplacement around a magma chamber. The distributionof radial fractures of novae is often asymmetric (Figure 6).Most of radial fracturing of this nova is localized in the NEand SW parts of the nova rise. We suggest that radialfracturing of this nova was formed in conditions of NE-SW compression and NW-SE extension (s3 � NE-SW,s1 � NW-SE). This is in agreement with the main strikeof wrinkle ridges (NW-SE). Formation of wrinkle ridgesand radial fracturing may have different ages, but they alsomay be formed in one stress field and have the same ages.Asymmetry of radial-fractured centers on Venus formed bydike swarm emplacement was described and used for thereconstruction of global and regional stress state [Grosfilsand Head, 1994a, 1994b; Ernst et al., 2003].[130] Large amounts of volcanism are associated with

some novae. Magmatism is likely to have a connection tomelting of the mantle diapir head. According to models ofmantle plume head evolution, the peak of partial melting ofthe plume and therefore plume-related magmatism andvolcanism on the surface takes place after maximum uplift.Models suggest that topographic uplift varies during plumeascent, but at the time of peak magmatism, the outer bound-ary of uplift approximately coincides with the edge of theplume head [Griffiths and Campbell, 1991]. The beginningof gravitational relaxation of some novae may be the result ofsubsidence of the surface due to several reasons [Condie,

2001; Campbell, 2001]: (1) As the plume head thins andspreads laterally, it loses buoyancy and begins to sink overthe plume axis; (2) removal of magma from the top of theplume results in deflation; (3) gradual heat losses from thesurface over hundreds of millions of years also result in somethermal contraction and subsidence. Traces of the beginningof gravitational relaxation and subsidence of the rise andalso massive volcanic activity are observed in some novae(Figure 7). The shape of this nova and the appearance ofthe concentric extensional fracturing may be revealing thebeginning of gravitational relaxation processes.[131] We suggest the following general sequence of

events of upraised novae evolution (Figures 19 and 20):Dome-like or cone-like rise forms on the surface from theinitial stages of influence of the diapir on the lithosphere.The position of n0 of the diapir controls the prominence ofthe nova relief and its shape. Upraised novae with promi-nent relief form with a rather shallow n0, and upraised novawith low relief form if n0 is rather deep. Partial melting inthe mantle diapir head occurs due to adiabatic decompres-sion. The magma reservoir forms and produces radial dikeswarms, which create the dense radial fracturing of novae.Massive volcanism connected with the reservoir occurs onthe surface, which is most prominent after maximum upliftof the nova rise. The beginning of gravitational relaxation ofthe novae topography takes place due to cooling andflattening of the diapir, which leads to the beginning ofsubsidence of the surface and the decreasing of pressureinside the magma chamber, and, as a result, an increase infracture to fracture spacing of nova radial fracturing pro-duced by radial dikes. Part of upraised novae completedtheir activity before Pwr emplacement. This mostly likelymeans that relaxation has been arrested. These novae aresmall; perhaps relaxation of nova was arrested due to theirsmall size and lithospheric strength.6.1.2. Annular Novae[132] Coronae have elevated concentric annulae, some-

times surrounded by a depression, with numerous exten-sional and compressional structures [e.g., Barsukov et al.,1986; Pronin and Stofan, 1990; Stofan and Head, 1990;Nikishin et al., 1992; Squyres et al., 1992; Stofan et al.,1992, 1997; Janes et al., 1992]. In these novae pieces ofannulus are observed that make them morphologicallyintermediate structures between ‘‘classical’’ novae andcoronae. The interior parts of coronae are usually depressed;central rises are observed in annular novae (Figures 3b, 3c,and 8–11).[133] The morphology and distribution of dense radial

fracturing of these novae (Figures 8–11) are the same as forupraised novae; we therefore consider the mechanism offormation to be the same. Following many previous work-ers, we believe that radial dike swarms form the radialextensional structures of these novae, and we interpret theformation of morphologic and tectonic structures into novaeannulae as related to relaxation of topography.[134] We have separated annular novae into two subtypes

(Figure 3b, 3c, and 8–11). The first subtype has radialextensional fracturing in the elevated central parts of novaeand concentric compressional structures dominating at theannulae and its periphery (Figures 8–10). Tectonophysicalmodeling [Krassilnikov, 2000, 2001, 2002] shows thatformation of compressional structures by gravitational


relaxation following nova construction is possible under theconditions that there is a rather small thickness of the layerabove the diapir or a rather shallow n0. The lower thethickness of the layer above the relaxing diapir, the moreprominent are the compressional structures. The presence ofradial compressional stress on the flanks of the nova rise maybe due to several reasons, which have previously beensuggested: (1) owing to the lithostatic pressure of novaconstruction and its gravitational spreading; (2) owing tothe lateral pressure of the flattening diapir; (3) owing togravitational sliding on nova slopes. These mechanisms maytogether control radial compressional stress and lead toformation of concentric compressional structures. Theydepend on gravity and should be more prominent duringrelaxation of upraised nova with prominent relief.[135] Formation of concentric compressional structures in

novae is apparently analogous to the formation of wrinkleridges above mantle plumes, which formed in response tocontractional forces associated with plume emplacementand collapse [Mege and Ernst, 2001; Condie, 2001]. The

characteristics and geological position of concentric com-pressional structures of novae are in general agreement withsimilarities of ridges, described by Mege and Ernst [2001]and reviewed by Condie [2001] for the Yakima fold belt inthe Columbia River on Earth and the Tharsis region onMars: (1) ridge size; (2) concentric distribution about aplume center; (3) development in stratified basalts; (4)association with flood basalts erupted on thin, brittle crustunderlain by a ductile layer (in our case the analog of flood,stratified basalts are volcanic plains, which are believed tobe basaltic in composition [Barsukov et al., 1982; Surkov etal., 1984]); (5) ridge growth rate parallels lava flow eruptionrate; (6) periodic ridge spacing; (7) ridge formation by acombination of folding and thrust faulting. Most of thesecharacteristics are in agreement with our observation ofnovae concentric compressional structures (Figures 8–10).[136] Analogous tectonophysical modeling [Krassilnikov,

2001, 2002] also provides evidence that the formation ofconcentric compressional structures and an elevated annulustakes place by gravitational relaxation of the nova rise and

Figure 20. Sequence of events in evolution of upraised novae with prominent relief and annular novae(subtype-2), where n0





the diapir body, as the diapir influences the lower visco-plastic part of the lithosphere. Formation of these structuresis possible under conditions in which there is a smallthickness of the upper brittle layer of the lithosphere andlateral spreading of visco-plastic material of the lower partof the lithosphere above the diapir, and possible flatteningof the diapir body itself.[137] Part of the radial fracturing could also be formed by

relaxation of the nova rise. However, if radial fractures areformed mostly due to radial dike emplacement during theuplift stage of nova formation, the relaxation stage mayinvolve conditions for simple mechanical formation of thisfracturing. The thinner the layer above the diapir, the moreprominent might this fracturing be, but it will not be asdense as that produced by dike emplacement [Krassilnikov,2000, 2001, 2002].[138] There are some upraised novae with concentric

compressional structures (Tables 2 and 6). This may beexplained by processes, also described above, operatingduring the beginning of the gravitational relaxation of thispart of upraised novae. We identify a transitional formbetween upraised and annular novae characterized by alow prominent concentric annulus (Figure 10). In this casecompressional ridges, which apparently were formed due todownslope movement, represent the structural annulus; inthe proximal parts of this movement a concentric grabensystem is observed, which is usual for this type of move-ment. In two other novae (Figures 8 and 9) of this type,mapped annulae are more prominent in relief. We canthus suggest an interpreted sequence from well-upraisednovae (Figure 6), through novae with a low prominentannulus (Figure 10), to novae with prominent annulae(Figures 8 and 9).[139] We suggest the following general sequence of

events in the evolution of annular novae of subtype 1(Figure 19): Early stages of evolution are the same as withupraised novae. The diapir influences the lower visco-plastic part of the lithosphere, and the thickness of theupper brittle layer is rather small. Following this, gravita-tional relaxation of the novae rise takes place due to coolingand flattening of the diapir, which leads to subsidence of thesurface. Lateral spreading of the diapir and material ofthe lower visco-plastic part of the lithosphere takes place.The concentric annulus and compressional structures formafter the start of gravitational relaxation due to the lithostaticpressure of nova construction and its gravitational spread-ing, related to the lateral pressure of the flattening diapir anddue to gravitational forces on the nova slopes.[140] The second subtype has radial extensional fracturing

in the elevated central parts and in the annulae of thenovae and concentric extensional fracturing in the annulae(Figure 11). Tectonophysical modeling [Krassilnikov, 2001,2002] shows that formation of concentric extensional struc-tures by gravitational relaxation of nova construction ispossible due to a rather shallow n0, but in contrast to theprevious subtype of annular novae, this has a rather thickupper brittle deforming layer, or upper brittle part of thelithosphere.[141] We consider this type of novae as one more possible

direction of evolution of upraised novae with prominentrelief. We consider the upraised nova, Mbocomu Mons(Figure 7), as transitional to this type of annular novae

because of the same set of tectonic structures and traces ofannulus formation. In this case [Krassilnikov, 2001, 2002]the diapir influences the lower visco-plastic part of thelithosphere. We thus suggest a sequence from well-upraisednovae (Figure 6), through novae (Figure 7), with tracesof the beginning of gravitational relaxation and annulusformation, to novae with a prominent annulus (Figure 11).[142] We suggest the following general sequence of

evolution of annular novae of subtype 2 (Figure 20): Earlystages of evolution are the same as upraised novae. Thediapir influences the lower visco-elastic part of the litho-sphere; the thickness of the upper brittle layer is rather large.Gravitational relaxation of the nova rise takes place due tocooling and flattening of the diapir, which leads to subsi-dence of the surface. Lateral spreading of the diapir andmaterial of the lower visco-plastic part of the lithospheretakes place. Plastic bending at the periphery of the novaforms a concentric annulus due to the lithostatic pressure ofnova construction, which leads to extensional structureformation in the annulus after the start of gravitationalrelaxation.[143] The peak of plume related magmatism and volca-

nism takes place on the surface by flattening of the plumeand subsidence of the surface after maximum uplift[Whitehead, 1982; Griffiths and Campbell, 1991], whichdepend on partial melting of the plume head. Most of theannular novae of both subtypes have prominent traces ofvolcanic activity, which is seen in the formation of ratheryoung volcanic plains, which often embay the centralparts and concentric annulae of these novae (Figures 8, 9,and 11). This volcanic activity is apparently the trace ofdiapir-related magmatism associated with relaxation of thenova rise.6.1.3. Flat and Negative Novae[144] These novae also have dense radial fracturing, in all

cases multiple, with its morphology and distribution thesame as that described above for upraised and annular novae(Figures 12 and 13); we thus infer that mechanisms offormation should be the same. Following previous workers,we believe that radial dike swarms play a very prominentrole in forming the radial extensional structures of thesenovae. In some flat novae (Figure 12) no concentric struc-tures are observed which could be connected with gravita-tional relaxation. It is possible that these novae were notupraised and in this case their relaxation could have takenplace without formation of any deformation structures. It ispossible that stress by relaxation of such constructions wasconcentrated into a zone of elastic deformation and may notcross its limits. On the basis of this, formation of these novaecould take place by relaxation of upraised novae with lowrelief. All these novae have dense radial fracturing, but werenever prominently upraised, which we believe is one morepiece of evidence for formation of dense radial fracturingdue to dike swarm emplacement. Part of the novae of thisclass has separate concentric extensional and/or compres-sional structures, which is evidence for some traces ofrelaxation, but they are not as prominent as concentricstructures in annular novae. We cannot determine whichpart of the lithosphere influences the diapir, but we canconclude that the n0 of these novae-producing diapirs shouldhave a deeper position than in previous cases, because theydo not form a prominent rise on the surface.


[145] Radial and concentric extensional structures areobserved in negative novae (Figure 13). Concentric exten-sional fractures of novae may be analogous to caldera-likefeatures, and on the basis of results of tectonophysicalmodeling [e.g., Roche et al., 2000; Troll et al., 2002],formation of calderas is possible by relaxation of a magmachamber (in our case it is the diapiric body) through itsinfluence on the upper brittle part of the lithosphere. By therelaxation of this condition, concentric extensional fractur-ing forms mechanically the same as caldera-concentricfracture formation. From this interpretation it follows thatnegative novae can be formed from distinctly upraisednovae as well as from low upraised novae. For the forma-tion of negative novae, the diapir should penetrate the lowervisco-elastic part of the lithosphere to reach the upper brittlepart. Both of the negative novae are located inside theAphrodite Terra tessera (Figure 13). Many authors hypoth-esize that a larger thickness of the crust exists inside tesseraterrain [e.g., Philips and Hansen, 1994; Hansen et al., 1997]that leads to a larger thickness of the deforming layer abovethe diapir. We did not observe any other novae in thetessera. Although there are some novae that are locatedrather close to the tessera ‘‘islands’’ (Figure 9), they are notsurrounded by it (Figure 13), as are the negative novae. Thelocation of negative novae may also be evidence for athicker deforming layer (includes crust) above the diapirin tessera terrain, perhaps a relatively thicker brittle litho-sphere. In this case mantle diapirs possibly reach the base ofthe brittle part of the lithosphere, which in tessera terrainshould have a lower position and is more accessible fordiapir head influence, which apparently did not happenduring formation of other nova classes.[146] We suggest the following general sequence of

evolution of flat novae (Figure 21): Early stages of evolu-tion are the same as upraised novae. The upraised novaewith low relief may evolve to flat novae due to gravitationalrelaxation of the rise by a deep n0. The diapir cools andflattens, and the surface subsides. The amount of volcanismis small due to the relatively deep n0 and magmaticreservoir. Rare concentric structures form and completeflattening of the surface happens because of the low reliefof the uplift stage. Complete relaxation of the construct andlithosphere occurs. In this case we cannot determine theconditions of the diapir influence on the lithosphere butpresumably it influenced the lower visco-plastic part.[147] We suggest the following general sequence for

evolution of negative novae (Figure 22): The early stagesof evolution are the same as upraised novae, but in this casethe mantle diapir penetrates the lower visco-plastic part ofthe lithosphere and influences the brittle upper part essen-tially thinning the lithosphere as it tries to reach n0. Thedepression forms due to cooling and flattening of the diapirand complete relaxation of the construct, with extensionalstructures on its bend because of lithosphere thinning byupdoming; late stages are mechanically the same as those ofcalderas. Negative novae may be formed by relaxation ofupraised novae with prominent relief as well as low relief.Parfitt and Head [1993] argued that these features couldoriginate from shallow reservoirs fed by vertically ascend-ing dikes above very deep-seated thermal anomalies. Thisway topographic uplift is not necessary for formation of aradially fractured center. We observe very similar radial

fracturing in all nova classes. Therefore we believe that allnovae have been generally formed by the processes wedescribe.

6.2. Relationship of Novae and Rift Zones;Mechanisms and Scenarios of Novae EvolutionWithin the Rift

[148] We found 6 upraised novae, 5 annular novae, 6 flatand negative novae and 15 plateau-like novae within andassociated with rift zones [Price, 1995; Basilevsky andHead, 2000b], for a total of 32 novae (51.6%) showingassociation with rift zones. This is made even more clear ifwe overlay the distribution of 64 novae from the ‘‘Catalogof Volcanic Structures of Venus’’ [Crumpler and Aubele,2000] on the ‘‘Map of Rifts and Volcanoes of Venus’’[Basilevsky and Head, 2000b] (Figure 17). We observetwo types of relationship between novae and rift zones:(1) The rift does not have a visible influence on topographyof upraised, annular, flat and negative novae; the nova-forming processes apparently have no connection with riftsor dominate over rift-forming processes. (2) The rift has avisible influence on plateau-like novae topography, which isdisplayed in rift troughs that surround novae and form thetopographic shape of the structures; rift-forming processespredominate or are equivalent to nova-forming processes. Itfollows that formation of plateau-like novae should beconsidered in connection with rift-zone evolution, while inother cases we can dismiss a role for rift activity. Thereforewe described two main scenarios of novae formation,outside of rift influence (above) and within the influenceof the rift zone (below).[149] All plateau-like novae have dense radial fracturing,

mostly multiple sets, whose morphology and distributionare the same as the classes of novae described above;therefore the mechanism of formation should be compara-ble, that is, primarily due to radial dike swarm emplace-ment. Most of these novae (80%) have concentric grabenand normal fault systems, but these extensional structureswere apparently formed not by nova activity, but by riftinfluence. Three types of relationships between these novaeand rift structures are observed and are discussed below.6.2.1. Nova Predates Rift Zone Formation[150] Part of the rift fracturing outlines the plateau-like

constructions of this novae and forms rather deep troughs,which outline the nova construction (Figure 14a). This partof the rift fracturing is represented mostly by normal faultsand makes this nova appear like a more competent inho-mogenity (Figure 14a); rift fracturing outlines the moreridged nova in the sense of mechanics and distribution ofdeformation. This is one more piece of evidence that thesenovae were formed before rift formation. Another part ofthe rift fracturing is represented by long narrow graben thatcut through the nova plateaus, and sometimes have lavaflows associated with them. We interpret these graben asmanifestations of dike emplacement which are connectedwith the rift system [Fahrig, 1987; Ernst et al., 2001]. Thefractures, which surround the nova plateau apparently wereformed due to passive regional extension and outline themore competent nova construction (Figure 23). Graben,which were formed due to dike emplacement, are moreactive in the deformation sense and cut through the novaplateaus without changes in strike. As the formation of these


Figure 21. Sequence of events in evolution of upraised novae with low relief and flat novae, where n0d,

neutral buoyancy level of the diapir; n0r, neutral buoyancy level of the magmatic reservoir; n0

l , level oflithospheric isostatic equilibrium; h, thickness of the upper brittle part of the lithosphere (includes crust),h1 < h2 < h3.

Figure 22. Sequence of events in evolution of upraised and negative novae, where n0d, neutral buoyancy

level of the diapir; n0r, neutral buoyancy level of the magmatic reservoir; n0

l , level of lithospheric isostaticequilibrium; h, thickness of the upper brittle part of the lithosphere (includes crust), h1 < h2 < h3.


novae predates rift activity, rift recycling may expose novaof any class.6.2.2. Nova Forms Simultaneously With Rift Evolution[151] In these novae part of their radial fracturing forms

rift fracturing (nova 1 in Figure 16) and fractures becomeconformal to the strike of rift fracturing at the periphery ofthe nova rise. This shows that this fracturing of novae wasformed simultaneously with rift fracturing during conditionsof regional extension, which was controlled by rift forma-tion. Rift troughs surround the central rises of these novae.The mechanism of formation of these novae (Figure 24)should be the same as the upraised novae described above(Figures 19–22), but in this case the rift influences theorientation of nova radial fracturing. In rift zones thelithosphere is tectonically and/or thermally thinned, whichpromotes conditions of partial melting of the diapir head[e.g., McKenzie, 1984; White and McKenzie, 1989] relativeto the influence of a diapir on non-thinned lithosphere. Itappears to account for the large amount of volcanismobserved in these novae (Figures 15 and 16). Novae whichform simultaneously with rift formation may be formed dueto inhomogeneous uplift of hot material along the rift zone[e.g., Shelton and Tullis, 1981; Wilcock and Whithead,1991].6.2.3. Nova Postdates Rift Formation[152] These novae apparently are younger than the rift,

but the influence of the nova-producing diapir on thesurface was not sufficient to produce a prominent rise andto erase the rift trough that was inherited by nova riseformation (nova 2 in Figures 16 and 25). The mechanism offormation of these novae should be the same as upraisednovae described above (Figures 19–22), but the shape ofthis novae is controlled by rift structures predating forma-tion of nova structures.[153] In some of the plateau-like novae, a small number of

concentric compressional structures are observed (Tables 5and 6). Formation of these structures may be evidence forgravitational relaxation of the novae with a rather thin brittlepart of the lithosphere, the same with annular novae ofsubtype 1.

7. Interpretation of Results

7.1. Relationship Between Novae and Coronae

[154] The evolution of part of novae can form coronaestructures, as was suggested by many authors. A mostprominent part of this process is shown in annular novae.However, numerous novae completed their evolution with-out formation of the concentric morphologic or structuralannulus. Some coronae have radial fracturing which may beinterpreted as traces of nova formation [Janes et al., 1992;Squyres et al., 1992; Stofan et al., 1992]. However, manycoronae do not have radial tectonic structures [Janes et al.,1992; Squyres et al., 1992; Stofan et al., 1992]. Therefore apart of the corona population was formed due to gravita-tional relaxation of novae-like constructions, and anotherpart was formed without early formation of a nova. There-fore formation of novae does not in all cases lead toformation of corona-like structures, and not all coronaewere formed by gravitational relaxation of novae-like fea-tures. Our models of novae evolution are generally consis-tent with other geological, geophysical and analogous

tectonophysical models of novae-coronae formation andplume evolution noted above. Our results support theinterpretation of Aittola and Kostama [2002], who arguedthat the morphology, topography and age relations of thenova and corona depend on phase of corona maturity duringthe nova-forming period. These authors also suggested thatformation of novae and coronae might be represented byseparate and multiple diapiric events that lead to formationof radial fracturing following the formation of concentricstructures. We do not see evidence for multiple diapiricuplift in the evolution of individual novae. Instead we haveshown that in all novae studied, radial fracturing largelypredates concentric fracturing. In some cases concentricfractures are forming due to regional stress and do notdepend on nova-forming processes (e.g., in plateau-likenovae).

7.2. Period of Novae Activity

[155] As was shown by several authors [Basilevsky andHead, 1998b, 2000a; Ivanov and Head, 1997a, 1997b,1998a, 1998b, 2001] regional plains with wrinkle ridgesoccupy most of the surface of Venus (�70%). The distri-bution of impact craters on the surface of the planet is notdistinguishable from random [Phillips et al., 1992]. Differ-ent authors [Strom et al., 1994; Collins et al., 1999;Basilevsky and Head, 2002b] used this argument and otherevidence to infer that these plains were formed during ageologically short period. Therefore it has been argued thatthese plains can be used as a general stratigraphic marker.According to different methods of crater count interpreta-tion, the age of regional plains is 288 + 311/�98 myr[Strom et al., 1994], 400–800 myr [Phillips et al., 1992]and �750 myr (from 300 myr up to 1000 myr) [McKinnonet al., 1997].[156] Most coronae started to form and finished most of

their activity before formation of regional plains [Basilevskyand Head, 1998a; Ivanov and Head, 2001]. Most novaewere active after formation of these plains (Figure 18).Formation of ‘‘classical’’ coronae may occur due to a ratherthin upper deforming layer above the relaxing diapir [Janeset al., 1992; Squyres et al., 1992; Stofan et al., 1992, 1997;Krassilnikov, 2001, 2002] which is necessary for formationof the concentric compressional and extensional structureswhich are so characteristic of coronae. As follows from themodeling described, the formation of the rise on the surfacemay take place due to the relatively larger thickness of theupper deforming layer above the diapir. The differentperiods of coronae and novae activity may be circumstantialevidence of the thickening of the Venusian lithosphere withtime, which is predicted and discussed by different authors[e.g., Parmentier and Hess, 1992; Turcotte, 1995; Phillipsand Hansen, 1998; Grimm, 1994; Brown and Grimm,1999].

8. Conclusions

8.1. Novae Classification

[157] The topographic shape of novae was used as thebasis of classification because novae have a very widevariety of sets of tectonic structures and a smaller numberof characteristic topographic shapes. We subdivided thenovae population into four classes (Figures 3 and 26): (1)


upraised novae; (2) annular novae; (3) flat and negativenovae; (4) plateau-like novae.

8.2. Scenarios of Novae Formation

[158] Interpretation of the results of our analysis generallyconfirm and update models of novae formation and evolu-

tion due to the influence of rising mantle diapirs on theupper part of the lithosphere. We suggest scenarios of theevolution of different nova classes depending on the fol-lowing geological factors (Figures 20–26): (1) the depth ofthe neutral buoyancy level of the rising mantle diapir (n0);(2) the rheological characteristics of the part of the litho-

Figure 23. Sequence of events in formation of plateau-like novae which predate rift activity, where n0d,




Figure 24. Sequence of events in formation of plateau-like novae which form simultaneously with riftactivity, where n0





Figure 25. Sequence of events in formation of plateau-like novae which postdate rift activity, where n0d,



Figure 26. (opposite) Formation of tectonic and morphologic structures in novae by uprising and gravitational relaxationof the mantle diapir. Astersik means that annular novae and part of plateau-like novae satisfy coronae definition and weredocumented by some authors as coronae. The location of plateau-like novae in diagram does not mean that they had norelaxation stage. Numbers represent examples shown by figures in the text. For details, see text.


sphere which the evolving diapir influences, on the upperbrittle part (including the crust) or the visco-plastic lowerpart of the lithosphere; an important factor we are also usingis the thickness of upper brittle part of the lithosphere; (3)whether or not the visco-plastic material of the lowerlithosphere above the diapir spreads; (4) the character ofthe influence of regional stress and rift structures. In somecases we suggested new directions of novae evolution(Figure 26) which are not described in previous investiga-tions. We distinguished two geodynamic conditions ofnovae formation, outside and within rifts (Figure 26).8.2.1. Novae Evolution Outside of Rifts[159] Upraised novae with prominent relief (Figures 19,

20, and 26) form by a rather shallow level of neutralbuoyancy of the diapir (n0). A dome-like or cone-like riseon the surface forms during the initial stages of diapirinfluence on the lithosphere. The position of n0 of thediapir controls the prominence of nova in relief and itsshape. Partial melting in the mantle diapir head occurs dueto adiabatic decompression. The magmatic reservoir formsand produces radial dike swarms, which create dense radialfracturing. Massive volcanism connected with this reservoiroccurs on the surface (Figures 7, 19, 20, and 26), which ismost prominent after the maximum uplift of the nova rise.The beginning of gravitational relaxation of the novae risetakes place due to cooling and flattening of the diapir,which leads to the beginning of subsidence of the surfaceand pressure loss inside the magmatic chamber. This inturn leads to an increase of fracture to fracture spacing ofnova radial fracturing produced by radial dikes. Severalpossible directions of evolution by gravitational relaxationof these novae are thus suggested (Figures 19, 20, and 26):1. Diapir influences the upper brittle part of the lithosphereby relaxation (including the crust). In this case, negativenovae form, and late stages of evolution are mechanicallythe same to calderas (Figures 22 and 26). 2. Diapirinfluences the lower visco-plastic part of the lithospherewith its lateral spreading above the diapir and possibleflattening of diapir body. Following this, gravitationalrelaxation of the novae rise takes place due to coolingand flattening of the diapir, which leads to subsidence ofthe surface. In this case two possible scenarios are possibledue to relaxation of these novae depending, on the thick-ness of the upper brittle part of the lithosphere (Figures 19,20, and 26): (1) Thickness of the upper brittle part of thelithosphere (including the crust) is rather small. In thiscase annular novae with mostly concentric compressionalstructures in their annulae form (Figure 26). The formationof the annulus and compressional structures occur dueto the lithostatic pressure of nova construction and itsgravitational spreading, due to lateral pressure of theflattening diapir and due to gravitational forces on the novaslopes. (2) Thickness of the upper brittle part of the litho-sphere (including the crust) is rather large. In this case theannular novae with mostly concentric extensional structuresin the annulae form (Figures 20 and 26). Formation ofthe annulus and extensional structures occurs due to thelithostatic pressure of nova construction and plastic bendingat the periphery of the nova.[160] Upraised novae with low relief form due to a rather

large depth of neutral buoyancy level (n0) of the upliftingmantle diapir (Figures 21 and 26). Partial melting in the

mantle diapir head occurs due to adiabatic decompression.The magmatic reservoir forms and produces radial dikeswarms, which create the dense radial fracturing of novae.Negative novae (Figures 22 and 26) form by the relaxationof upraised novae with low relief if the diapir influences theupper brittle part of the lithosphere (including the crust). Flatnovae (Figures 21 and 26) form by relaxation of upraisednovae with low relief with or without small amounts ofconcentric tectonic structures.8.2.2. Novae Evolution Within the Rift[161] Plateau-like novae form in conditions of prominent

rift zones, and three scenarios are possible (Figures 23–26):(1) Nova predates rift zone formation (Figures 23 and 26).These novae form according to the described scenario andafter that are made more prominent by rift recycling.(2) Nova forms simultaneously with rift evolution (Figures24 and 26). These novae form in the same way as upraisednovae, but on the background of uplift of hot material alongthe rift zone. (3) Nova postdates rift formation (Figures 25and 26). These novae form in the same manner as upraisednovae, but inherit rift structures during their formation.

8.3. Period of Novae Activity and its Interpretation

[162] In our study it was shown that novae have multiplestages and are long-lived structures. Most novae (88.7%)were active after formation of regional plains with wrinkleridges. In contrast to novae activity, coronae activity wasgreatest before formation of these plains [Basilevsky andHead, 1998a; Ivanov and Head, 2001]. This may beevidence for thickening of the lithosphere with time, whathas been predicted on the basis of other observations[Parmentier and Hess, 1992; Turcotte, 1995; Phillips andHansen, 1998; Brown and Grimm, 1999]. We also showedthat formation of novae does not in all cases lead toformation of corona-like structures, and not all coronaewere formed by gravitational relaxation of novae.

[163] Acknowledgments. We thank Alexander Basilevsky, MikhialIvanov, and Lionel Wilson for productive discussions and comments on themapping and interpretations. Special thanks to reviewers Richard Ernst andEric Grosfils for extremely useful comments, which helped us improve thepaper significantly. This work was supported by a grant from the NASAPlanetary Geology and Geophysics Program to JWH and from the RussianFoundation for Basic Research (02-05-65068) to Anton Krassilnikov.Thanks to Peter Neivert for advice and help on figure preparation andAnne Cote for help in manuscript preparation and submission.

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��J. W. Head, Department of Geological Sciences, Brown University,

Providence, RI 02912, USA. ( [email protected])A. S. Krassilnikov, Vernadsky Institute of Geochemistry and Analytical

Chemistry, Russian Academy of Sciences, 119991 Moscow, Russia.([email protected])


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