A Global Survey and Regional Scale Study of Coronae on Venus

A thesis submitted for the Degree of Doctor of Philosophy of the University of London

Simon W. Tapper

Department of Geological Sciences University College London

1998 ProQuest Number: U643667

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ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 Abstract

Coronae are large-scale geological structures on Venus normally consisting of a planimetrically circular topographic rim which encircles a basin. They are considered to have formed by plume activity. The thesis describes and examines the characteristics of coronae using a new and comprehensive database which is used to further understanding of corona properties and the geological evolution of Venus. Topographic data were surveyed to identify coronae which are not easily detectable in synthetic aperture radar (SAR) images because they lack the annulus of brittle scale fractures that were previously considered to characterise all coronae. Data used to describe the distribution, morphology, geological setting and associated volcanic and tectonic structures were obtained from altimetry, high resolution Synthetic Aperture Radar (SAR) images returned by the 1990 Magellan mission and synthetic stereo images generated from Magellan data. Detailed geological mapping of the Scarpellini Quadrangle was then used to examine coronae on a regional scale and study their geological context and history. The 229 coronae identified by the survey were found in a variety of geological settings and materials. They occur on topographic rises and their margins and on ridge belts north of the Beta-Atla-Themis region. They have widths comparable to those described by earlier surveys but tend to be lower in height. A range of morphologies were identified, including a new class which have complexly deformed interiors. Most of th^ coronae have irregular fractures aligned with their rim topography, but fracture networks and radial structure are also seen. The large numbers of additional coronae strongly imply that they are a far more important global heat loss mechanism than previously thought. The results indicate that the plume’s ability to deform lithospheric materials varies not only spatially with lithospheric thickness but also with other factors such as regional stress regime. The morphological and stratigraphie evidence presented here indicates that coronae can develop in episodes, rather than by a continuous process of formation as suggested by some authorities. Contents

Abstract i

Contents ii

List of figures V

List of tables v ii

Acknowledgements v ii i

Statement of originality ix

Chapter 1: Introduction 1

Introduction 1 Mapping Venus 4 Previous investigations 5 Population and distribution 7 Corona evolution 8 Nomenclature 11 Terminology 11

Chapter 2: Global survey of coronae 14

The 1992 survey 14 Survey 16 Data Magellan altimetry 17 Radar interactions with geological surfaces 17 Incidence angle 18 Stereometry 22 Image processing 23 Procedure and survey design 25 Morphology 28 Stratigraphy 29 Volcanism 30 Tectonics 31

Chapter 3: Spatial and altitudinal distribution 32

Spatial distribution 32 Spatial analysis 41 Corona density distribution 1992 survey 41 1997 survey 43 All coronae 43 Nearest neighbour analysis 44 Clustering 46 Altitude 50 Distribution by morphology 53 Distribution by altitude 53 Chapter 4: Geological setting 55

Geological setting 55 Stratigraphy 58 Discussion 62

Chapter 5: Morphology and dimensions 67

Morphology 67 Dimensions 71 Height and width by morphology 75 Altitude and morphology 77 Altitude and relief 77 Discussion Morphology 79 Height 81 Width 81

Chapter 6: Volcanism associated with coronae 83

Volcanism and morphology 84 Volcanism and altitude 87 Discussion 90

Chapter 7: Tectonics associated with coronae 94

Stealth corona tectonics 94 Discussion 102

Chapter 8: Mapping the Scarpellini Quadrangle 105

Physiographic setting 105 Mapping techniques 111 Missing data and image processing artefacts 111 General geology 112 Material units and volcanic landforms 113 Tessera 113 Plains 116 Central volcanoes 118 Domes 120 Small volcanic edifices and edifice fields 120 Lava flow fields 121 Crater materials 121 Surface materials 123 Statistics 126 Rad iothermal properties 127 Structures Wrinkle ridges 128 Fractures 128 Graben 129 Geological history 130

Chapter 9: The coronae of Scarpellini 131

Introduction 131 M a 131 Juksakka 134 Thermuthis 136 Nabuzama 139 Mukylchin 141 Stealth coronae 143

111 Chapter 10; Stratigraphy and origin 151

Number of coronae 151 Type of coronae 152 Morphology and tectonics 160 Chronology 162 Stratigraphy 163 Global geology 167

Chapter 11: Summary and conclusions 169

Summary of results 169 Further work Further analysis of Magellan SAR 170 Laboratory experiments 171 New data 174 Numerical modelling 177 Coronae of the solar system 177

Appendixes 182

I 1997 Corona survey: data and notes 182 II 1992 Corona survey: data and notes 187

References cited 194

IV List of figures

1.1 SAR image of Aramaiti Corona 2 1.2 Quadrangles of the V-Map program 6 1.3 Squyres’s (1992) three stage model of corona evolution 9 1.4 Smrekar and Stofan (1997) model of corona evolution 10

2.1 The effect of surface roughness on radar backscatter 19 2.2 Geometric distortions in radar images 20 2.3 Radar backscatter variation with surface roughness 21 2.4 Directional filter matrices used to create shaded-relief images 25 2.5 Shaded relief image of the Scarpellini Quadrangle 26 2.6 Schematic illustrating terms used to describe corona morphology 27

3.1 Venus topography and names of major geographical features 33 3.2 Global distribution of coronae recorded by the 1992 and 1997 survey 34 3.3 Global distribution of different sized coronae 36 3.4-7 Distribution of coronae which have the same morphology 37-40 3.8 Density distribution of coronae of the 1992, 1997 and expanded corona database 42 3.9 Nearest neighbour distribution of coronae types 1-5 47 3.10 Nearest neighbour distribution of coronae types 6-8 48 3.11 Nearest neighbour distribution of coronae of the 1992, 1997 and expanded corona database 49 3.12 Base level of coronae identified by the 1992, 1997 surveys 51

4.1 Magellan image of Parga Chasma 56 4.2 Magellan altimetry of the Beta-Atla-Themis (BAT) region 56 4.3 SAR image of corona identified in tessera materials 59

5.1 Corona types. Topographic groups and percentage of the total population 68 5.2 Topographic profiles of coronae with different morphology 69 5.3 Graph showing the relative proportions of coronae identified by the 1992 and 1997 surveys 69 5.4 Three dimensional perspective view of Atete Corona 70 5.5 Three dimensional perspective view of Yavine corona 70 5.6 Corona height distribution 73 5.7 Corona width distribution 74 5.8 Mean width of coronae, ordered by morphology 76 5.9 Mean height of coronae, ordered by morphology 76 5.10 Mean altitude of coronae^ordered by type 78 5.11 Mean height of coronae plotted against altitude 78

6.1 SAR image of a corona dominated by volcanism 85 6.2 SAR image of a corona with a moderate amount of associated volcanism 85 6.3 Relative amount of volcanic activity associated with coronae ordered by morphology 86 6.4 Plot showing variation in the degree of volcanism with altitude 88 6.5 Interpretation of the relationship between volcanism and coronae at different altitudes 92

7.1 SAR image of Cerridwen and Neyterkob coronae 95 7.2 SAR image showing detail of the eastern rim of Neyterkob corona 95 7.3 Tectonic styles displayed by coronae identified in the new survey 96 7.4 SAR image of a stealth corona 97 7.5 SAR image of a stealth corona lacking tectonic structure 97 7.6 SAR image of a corona, previously classified as an arachnoid 100 7.7 SAR image of a corona with radial pattern of wrinkle ridges 101 7.8 Deformation patterns in materials surrounding coronae 101

8.1 SAR mosaic of the Scarpellini Quadrangle 106 8.2 Location map of named features within the Scarpellini Quadrangle 107 8.3 Geologic map of the Scarpellini Quadrangle 108-109 8.4 Magellan altimetry of the Scarpellini Quadrangle 110 8.5 Backscatter cross-section of geological units 114 8.6 Geological units and their chronological relationships 114 8.7 Geological cross section through the Scarpellini quadrangle 109 8.8 SAR image of tessera with a ridge and groove configuration 115 8.9 SAR image of tessera with convoluted configuration 115 8.10 SAR image of tessera with a radial configuration 115 8.11 SAR image of lineated tessera 117 8.12 SAR image of mottled and lineated plains material 117 8.13 SAR image of lineated plains material 119 8.14 SAR image of regional plains material 119 8.15 SAR image of craters Medhavi and Michelle 122 8.16 SAR image of wrinkle ridges in regional plains material 122 8.17 Map showing the distribution of surficial materials 124

9.1 SAR image of the Scarpellini Quadrangle showing the location of coronae 132 9.2 (a) SAR image and (b) topographic map of Ma Corona 133 9.3 (a) SAR image and (b) topographic map of Juksakka Corona 135 9.4 (a) SAR image and (b) topographic map of Thermuthis and Unnamed Corona 137 9.5 (a) SAR image and (b) topographic map of Nabuzama Corona 140 9.6 (a) SAR image and (b) topographic map of Mukylchin Corona 142 9.7 SAR image and topographic profile of Corona Cl 145 9.8 SAR image and topographic profile of Corona C2 146 9.9 SAR image and topographic profile of Corona C3 148 9.10 SAR image and topographic profile of Corona C4 149

10.1 Schematic diagram of lithospheric subduction at coronae 155 10.2 Schematic of the behaviour of materials in the plains surrounding Mukylchin corona 155

11.1 Cross section of a diapir model formed in a high speed centrifuge 173 11.2 Plan view of a diapir model formed in a centrifuge 173 11.3 Artist’s impression of a balloon aerobot in operation above the surface of Venus 175 11.4 Schematic diagram of balloon aerobot trajectory and operation 175 11.5 Sketch map of granite and migmatite domes of Zimbabwe 180 11.6 Sketch map of radial and arcuate fractures used to locate mantle plumes on Earth 180

VI List of tables

1.1 Categories for naming features on Venus 13 2.1 Data fields for corona survey 17 2.2 Varying incidence angle of cycle 1-3 images with latitude 22 3.1 Nearest neighbour analysis results for coronae of the 1992, 1997 and expanded corona database 45 3.2 Nearest neighbour analysis results for coronae ordered by morphology 46 4.1 Stealth coronae stratigraphy, types and proportions 60 4.2 Corona stratigraphy: percentage of materials in which coronae of different morphology have formed 61 6.1 Summary of volcanism observed in each population and interpretation 93 7.1 Stealth corona tectonics, types and proportions 98 8.1 Named features of the Scarpellini Quadrangle 105 8.2 Backscatter and radiothermal properties of Scarpellini material units 127 9.1 Attributes of named coronae of the Scarpellini Quadrangle 131 9.2 Attributes of stealth coronae of the Scarpellini Quadrangle 144 10.1 Model phase and corona morphology 160 10.2 Venus global stratigraphy 164

Vll Acknowledgements

I wish to extend thanks to the friends and associates who have helped to make my time at University College London an agreeable one. Firstly I wish to acknowledge the support and encouragement of Claudio Vita-Finzi, my supervisor. I would also like to thank John Guest and Ellen Stofan for their assistance and valuable feedback, and my friends Mark Biddiss, Anthony Brian, Cécile Chabot, Duncan Copp, Sarah Dunkin, Yasmin Durrant, Dave Heather, Mike Lancaster, Valerie Peerless and Maureen Evans, who as well as spurring me on have helped make the last three years fun. I am grateful to my parents for the support they have always given me.

The work was carried out with funding from PFARC (award 9075326).

Vlll Statement of originality

This thesis is based on the writer’s own work. Chapters 3-5 draw on material developed in a paper submitted to Geophysical Research Letters (1998), the contribution of co-authors Ellen Stofan and John Guest being largely editorial. Mapping of the Scarpellini Quadrangle was carried out by the writer and the interpretations described in Chapter 8 are his responsibility even though the version submitted to the US Geological Survey was checked by John Guest.

IX Chapter 1

Introduction

Coronae are large-scale geological features on the surface of Venus which generally consist of concentric and radial features arranged about a central depression (Stofan et al., 1992) (Fig. 1.1). They are considered to be the surface manifestation of mantle diapirs (Barsukov et al., 1986; Stofan and Head, 1990; Stofan et al., 1991, 1992;

Squyres et al., 1992) and hence are studied to improve our understanding of the geological evolution of Venus, its mantle dynamics and its heat budget. Coronae were first described by Barsukov et al. (1984, 1986) using Venera 15/16 data. SAR (synthetic aperture radar) data and altimetry returned by the Magellan spacecraft make possible detailed investigation of coronae by providing images of high resolution and enabling the generation of synthetic stereo images of the surface of Venus.

Coronae exhibit a wide range of topographic configurations. For example, the outer boundary may be a raised rim or a trough and the central feature may be a rise rather than a basin. Coronae have diameters ranging from 60 km to 2,000 km (Stofan et al.,

1997), although most of them have diameters of between 200 km and 400 km. The very large corona Artemis (2,600 km in diameter) is exceptional. Rim widths are in the order of 20 km to 40 km and rim heights range between 1,500 m and 5,000 m. In SAR images, coronae are identified by their annulus of concentric fractures, but many coronae lack an annulus of concentric fractures and are thus difficult to detect without altimetry data and synthetic stereo images.

A survey was carried out (Chapter 2) in order to create a feature gazetteer or database which encompasses the full range of features that may have formed through plume activity. m

Figure 1.1. Aramaiti Corona. Aramaiti corona, identified by survey of Stofan et al. (1992), is located in relative isolation at 26.0° S, 82.0° E, to the north of Aino Planitia. Classified as a 'concentric ring' type corona (Stofan et al., 1992), Aramaiti comprises a topographic rim which encircles an interior below the level of the surrounding plains and a central high. Brittle scale deformation is clearly visible. Concentric fractures can be seen on the southern rim and surrounding plains, and annular scarps on the interior flank of the northern rim. Fractured materials on the outer flank of the northern rim are embayed by radar-dark lavas which appear to have erupted from fractures located on the rim summit. Small volcanic edifices are discernible in the corona interior and southern rim interior. Lavas with a moderate backscatter and streaky appearance have erupted from the elevated central region. In this thesis the location, elevation, size, morphologic characteristics, geological setting, associated tectonic structures and associated volcanism were studied (Chapters

3-7) and their possible implications for the geological evolution of Venus evaluated.

Physiographic information was gathered from Magellan GTDR (global topography data record) framelets with the help of synthetic stereo images. The data were amalgamated with the observations by Stofan et al. (1992), and the expanded corona database examined in detail. Coronae which were overlooked, which were missed owing to a lack of data, or which did not fully meet the criteria of the survey of Stofan et al. (1992), were also incorporated into the new database.

The survey located 229 additional coronae, which, combined with the 362 identified by Stofan et al. (1992), brings the total count to 591. The features identified in the new survey which lack fracture annuli were provisionally termed stealth coronae, since they are hard to detect on SAR images (Tapper, 1997) (Fig. 7.5). Indeed many are only clearly visible in synthetic stereo images. They are roughly circular in plan, between

160 km and 1,700 km in diameter, and comprise a topographic rim 30 to 50 km across which encircles a basin. They are often topographically low, having elevations in the order of 2,000 m to 3,000 m above the surrounding surface, and lack the multiple trough and ridge configurations described by Stofan et al. (1992).

The additional coronae, which lack extensive brittle deformation in the form of a fracture annulus, yield clues as to how lithospheric thickness and other properties influence how plumes are able to modify the surface to form coronae. Coronae are also useful indicators of regional stress, as regional deformation modifies the tectonic pattern seen at coronae (Cyr and Melosh, 1991; McGill, 1993).

Examination of the distribution pattern of coronae and their geological setting shows that coronae are more common on ridge belts and on or near topographically elevated regions. Previous work (Squyres et al., 1993) had identified a single large cluster of coronae associated with chasmata in the Beta-Atla-Themis (BAT) region (Stofan, 1996).

The results of the new survey show that coronae are more extensive and more widespread than previously thought.

In this work coronae are generally interpreted as relatively youthful features, as very many of them deform materials considered to be the most youthful. In some instances, however, stratigraphie analysis of coronae, coupled with morphologic evidence including multiple ring/trough configurations, indicates that coronae have prolonged histories, a conclusion which is consistent with the findings of Copp et al. (1996).

Moreover, coronae are observed to have formed in materials which are recognised as the earliest interpretable units, indicating that corona formation has occurred throughout the interpretable geological history of Venus (Chapters 4 and 10).

The results presented in this work have a bearing on current models of corona evolution and major implications for our current understanding of the geological evolution of Venus (Chapter 10). The existence of a greater number of coronae is in itself significant. Among other things, the observation supports the argument of

Smrekar and Stofan (1997) that coronae, in the absence of plate recycling or catastrophic lithospheric turnover, are an important avenue of heat loss from Venus.

In summary, this thesis re-evaluates the intrinsic properties, distribution and setting of coronae and uses coronae to obtain further insights into the geological history of

Venus.

Mapping Venus

Venus is being mapped at global and regional scales. The program is administered by NASA and co-ordinated by the US Geological Survey’s branch of Astrogeology. SAR image and shaded relief maps at scales of 1:50 million and 1:10 million have been produced (Batson et ah, 1994). The Venus Mapping (V-Map) program will also produce a series of 62 geological maps of the planet’s entire surface using standard photogeological techniques (Wilhelms, 1972; Tanaka, 1994) whereby the stratigraphie relationships of structures and materials are studied to enable a chronology of geological evolution to be constructed. A series of full resolution image maps at 1:5 million scale which have been re-sampled to 75 meters per pixel have been generated directly from

SAR basic image data record (BIDR) strips are used for mapping. The series employs

Mercator, Lambert conformai conic and polar stereographic projections (Fig. 1.2).

The series will provide a useful tool for detailed geological and geophysical study of the planet. It should reveal much about the geological history of the planet as the stratigraphy of different regions can be compared within a standardised framework. The mapping process should help to characterise a number of very ambiguous features and geologically complex regions.

The Scarpellini Quadrangle contains several large coronae (Chapter 9) which are easily identifiable on Magellan SAR images, as well as stealth coronae. They are representative of the range of coronae found on Venus and include several types defined by Stofan et al. (1992) including concentric, concentric ring and basin coronae. They possess a variety of structural elements which are used in conjunction with stratigraphie evidence to interpret their geological history (Chapter 9).

Mapping enables this work to move from the study of coronae on a global basis using the new feature gazetteer, to focus on coronae in a regional setting. It provides structural and contextual detail vital for future modelling work involving coronae and in

Chapter 10 of this thesis, contributes to a proposed revision of current corona models. VI

SneguTOchka Planitia

V5 V6 V7 V2

Pandrosa Lakshmi Fortuna Meshkenet Atalanta Dorsa Planum Tessera Tessera Planitia

' VI4 V15 V16 V17 V18 V19 ' V8 V9 / VIO V ll \ / V12 V13 \ Nemesis Kawelu Beta Lachesis Sedna Bereghinys Bell / Tellus / Vellamo Tessera Planitia Regio Tessera Planitia Planitia Regio / Tessera / Planitia

V26 V27 V28 V29 V30 V31 V20 V21 V23 V24 V25 Atla Ulfrun Hecate Devana Guinevere Sif Eistla Niobe Russalka Mead Greenaway Regio Regio Chasma Chasma Planitia Mons Regio Planitia Planitia

V38 V39 V40 V41 V42 V43 V32 V33 V34 V35 V36 V37 Maat Taussig Galindo Phoebe Carson Alpha Scarpellini Ovda Thetis Diana Mons Regio Regio Regio Regio Chasma

\ Helen Themis / \ Barrymore Lavinia / \ Aino Juno / \ Artemis Mahuea / \ Planitia Regio / \ Nepthys Planitia / \ Planitia Dorsum / \ Chasma Tholus / \ V50 V51 / \ V52 V53 / \ V54 V55 / \ V46 V47 / \ V48 V49 /

Isabella Godiva Mylitia Fredegonde Fluctus V61

Figure 1.2. Mapping quadrangles used in the V-Map programme; the geological mapping of Venus at the 1:5,000,000 scale. Quadrangle names are designated after important geological structures or regiones in the area. Previous investigations

Associated with coronae are a range of tectonic structures including concentric fractures which are usually aligned with the topographic rim (Stofan et ah, 1990; 1992), compressional ridges, radial and/or concentric graben, and concentric wrinkle ridges which are found in the surrounding materials (Solomon et ah, 1992; Stofan et ah, 1992).

Arcuate concentric fractures in the surrounding plains are attributed by Cyr and Melosh

(1991) and McGill (1993) to the effect of coronae on the regional stress field.

Coronae often have volcanism associated with them (Barsukov et ah, 1986). In pre-

Magellan studies all coronae were considered to have volcanism associated with them

(Stofan et ah, 1991). Lavas which have erupted from the corona rim are common, as are central volcanoes which have flooded the interior with lava. Small volcanic edifices (20

- 50 km) and domes (Barsukov et ah, 1986), shields and cones and occasionally central volcanoes are observed on the topographic rim of coronae aligned with fractures that crosscut the fracture annulus (Stofan et ah, 1991).

Population and distribution

A count of 362 coronae was made by Stofan et ah (1992). In the course of the present study it soon became apparent that a population size was arbitrary: there are many arcuate features which could have formed as a result of plume processes but which have subdued, fragmentary or degraded topographies. The original survey also overlooked many substantial coronae which possessed rim-trough topography and others with easily discernible concentric annuli.

The global distribution of coronae is almost certainly not random as coronae are found in clusters and in linear chains associated with chasmata (Stofan and Head, 1990,

Stofan et ah, 1992; Stofan 1995). Visual inspection of the data is corroborated by statistical analysis. Although random populations tend to produce a certain degree of clustering, nearest neighbour analysis by Squyres et al. (1993) of thel corona population shows that the globally dominant concentration centred on the BAT region is not the product of a random distribution.

Corona evolution

Since coronae were first seen in Venera 15 and 16 images, a range of mechanisms have been proposed for their formation. Alternative ideas have included the rejuvenation of impact craters (Barsukov et al., 1986), ring dike intrusion (Masursky

1987), mantle sinkers (Stofan et al., 1987) and retrograde subduction of the lithosphere

(Sandwell and Schubert, 1992a; 1992b). The most widely accepted theoretical model is that of Squyres et al. (1992). The model involves three key stages (Fig. 1.3). Initially domical uplift occurs as a result of a plume rising to the surface. When it is near the surface the plume spreads laterally thinning the crust. When upwelling ceases, the plume cools; the domical topography is no longer supported and the surface subsides leaving a circular depression bounded by a topographic rim, trough, or both.

Tectonic evidence supports the three key stage hypothesis. There is evidence of early updoming such as radial fracturing (Solomon et al., 1992; Janes et al., 1992).

Compression of the surrounding surface during the spreading stage of formation then produces a series of concentric ridges or wrinkle ridges (Solomon et al., 1992). Annuli tend to be composed of extensional fractures which are ascribed to lithospheric flexure during the later stages of corona development (Solomon et al., 1992; Stofan et al.,

1992).

Although there is a general consensus that coronae are manifestations of mantle plume upwelling, there remain certain long-standing questions regarding the processes (a). Plume head rises to the surface, causing it to dome upwards.

(b). The plume head spreads laterally, thinning the lithosphere and creating a plateau.

(c). When the supply of plume material is shut off, relaxation of the plains surface occurs and a central depression is formed. The troughs are formed by topographic loading.

Figure 1.3. Schematic diagram illustrating the three stage model of Squyres et al. (1992). 800t 132 My SOOT 196 M y

400-- 4 0 0 "

-400-- -400"

225 My 246 My 800 T SOOT

400 -- 4 0 0 "

-400 - ■ -400- ■

-800-I-

800 T 275 My 800 T 304 My

400 " 400 -- -600 -400 200 400 600

-400 - - -400 ■ ■

-800-L

SOOT 800 T 324 My 392 M y 4 0 0 " 400 ■ ■

-4 0 0 " -4 0 0 "

Figure 1.4. Corona development (Smrekar and Stofan, 1 9 9 7 )Horizontal distance in kilometres. Vertical scale is height in metres. Model profiles generated by Smrekar and Stofan are reflected about the vertical axis to aid visualisation. Corona development age is indicated. Corona formation begins with dome formation. At 246 My viscous flows pull the delaminating layer to the centre, altering the position of the trough. At 275 My the trough merges with the central depression. The configuration observed at 324 My is caused by the downward movement of cool lithosphere. The subsequent rise in topography (392) My is the result of iso static rebound.

10 that lead to the development of the ridge/trough and more complex corona morphologies. Such questions were not fully addressed by the three stage model of

Squyres et al. (1992) or the numerical models of Janes & Squyres (1994; 1995a; 1995b),

Koch (1994) and Koch and Manga (1996). But recent work shows that mantle plume upwelling and subsequent lithospheric delamination can account for a range of coronae morphologies observed by the 1992 survey. In the model of Stofan and Smrekar (1997)

(Fig. 1.4) crustal thickening occurs from 75 to 195 km and is accompanied by a decrease of 20° C in mantle temperature. The plume operates for 140 My and the plume tail rises for about 165 My. Corona development is as follows: (a) the plume reaches the lithosphere at 100 My; (b) a dome forms at 132 My; at 225 My (c) the lithosphere thickens at the edge of the plume as it spreads outwards and downwards. Delamination

(d) pulls the surface downwards initially driven by the flow of the plume head but then sustained by the density difference between the lithosphere and the mantle. Viscous flows (e) pull the delaminating flow to the centre at 246 My, altering the position of the trough, which then merges into the central depression (278 My). The lowest corona topography is observed when the cold lithosphere pulls downward balancing the low density (mantle) layer pushing upward (304 My). The depleted layer thickens (f) and topography increases until about 324 My.

Nomenclature

Features on Venus are named according to certain guidelines outlined by the

International Astronomical Union (lAU). Table 1.1 outlines the convention for naming features on Venus.

Terminology

11 Detailed geological investigation of Venus has only just begun. There still exist ambiguities in the terminology used to describe geological structures revealed for the first time by Magellan. The definitions of geological terms used in the text are given below. There is also scope for confusion in some of the words used to describe certain geological phenomena: plume, mantle plume and mantle diapir for instance have been used interchangeably by some authors.

Corona and stealth corona were defined above. There exists a plethora of terms used to describe their morphologies and the structures that they contain. The term annulus refers to the small-scale fracture patterns (>1-5 km in width) that are often associated with coronae and which include lineaments and graben concentric to the corona. The rim topography and moat exhibited by some coronae are larger-scale features some tens of kilometres across.

Coronae have been classified according to structural criteria by several workers

(Stofan, 1992; Kreslavsky & Vdovichenko, 1996). The morphotectonic scheme outlined by Stofan (1992) includes concentric coronae, having a rim and trough with concentric fractures, double-ring concentric, whereby the basin is enclosed by two sets of concentric ridges, radial, which contain fractures radiating from the centre of the structure, asymmetric, which are non-circular coronae, in many cases- pear shaped, and multiple, which consist of two or more distinct coronae whose margins are in contact.

There are other circular structures which possess some of the characteristic features of coronae and which are sometimes referred to as corona-like features. Two forms that are mentioned by Head et al. (1992) are novae and arachnoids. A nova consists of a topographic rise highlighted by a dense pattern of radial graben which give the feature a stellate appearance (Head et al., 1992). The use of the term novae was discontinued as it suggested that these features were of recent origin. Arachnoids are composed of radial

12 and concentric ridges aligned with a circular topographic rim; the fracture system gives

the structure a cobweb-like appearance. Arachnoids possess the tectonic and

morphological features of coronae and in this work are treated as coronae rather than as

a distinct genetic entity.

The terms diapir and plume are both used in the literature in connection with corona

formation. The term diapir is avoided in this work because it is reminiscent of

terrestrial structures which can pierce through surface layers. On Venus mantle plumes

* originate at the mantle boundary and are considered to be responsible for the support of

regional scale topographic rises (Hansen et al., 1996). In this work mantle plume is used

to denote large-scale mantle upwelling. The upwellings which give rise to coronae are

unlikely to be so deep seated. In this thesis such upwellings are referred to simply as

plumes. Plumes which form coronae may occur on the upper surfaces of large scale

mantle plumes (Head and Wilson, 1992; Hansen et al., 1996).

Feature Definition Name Chasmata Canyons Goddesses of hunt; moon Colles Small hills, knobs Sea goddesses Coronae Rim and/or troughs encircling a Fertility goddesses depression Craters (large Impact structures Famous women Craters (small) Impact structures Female first names D orsa Ridges Sky goddesses Lineae Elongated markings Goddesses of war M ontes M ountains Goddesses, miscellaneous (and one male radar scientist) Paterae Irregularly shaped craters Famous women Planitiae Low plains Mythological heroines Planum (1 only) High plains Goddess of prosperity Regiones Large areas of moderate relief Giantesses and |t itanesses (also two Greek alphanumeric designations) Rupes Scarps Goddesses of hearth and home Tesserae Highly deformed terrain of ridges and Goddesses of fate or fortune grooves with second order perpendicular deformation Terrae Continents Goddesses of love Tholi Domical hills Goddesses, miscellaneous

Table 1.1. Categories for naming features on Venus. Source: USGS.

* Mantle/core boundary 13 Chapter 2

Global survey of coronae

The aim of the survey was to record the location and physical characteristics of coronae which lacked a concentric fracture annulus. This would make it possible to investigate their distribution, geological setting, morphology, associated volcanism and tectonic style. By amalgamating the survey data with those collected by Stofan et al.

(1992) a fully comprehensive corona database would be created, encompassing the full range of features ascribed to mantle upwellings.

The 1992 survey

The corona survey of Stofan et al. (1992) used Magellan Cl (compressed once) SAR photo-products to record the location, width measured from the outermost extent of the fracture annulus, and rim width. In the classification scheme used by Stofan et al.

(1992), coronae were assigned to categories according to a combination of morphological and tectonic criteria. A distinction was made between coronae which have a fractu re annulus, two sets of fracture annuli or were dominated by radiating fractures. Planimetrically asymmetrical or multiple structures were differentiated by their morphology. No distinction was made between coronae which had different rim and trough configurations. The classification was as follows;

Concentric: coronae which have well defined symmetric annuli.

Concentric double ring: coronae which are encircled by two sets of tectonic ridges, troughs, or both.

14 Radial/concentric: coronae which have interiors dominated by radial fractures and| an encircling annulus of concentric fractures.

Asymmetric:coronae which have asymmetry of form. Planimetrically they are pear or kidney shaped.

Multiple: two coronae, or small groups, which have a continuous bounding ridge and fracture annulus.

Coronae were also assigned a volcanic class. Volcanic flows vary widely in their detectability, however, and the amount of associated volcanism is not always clear from the examination of Cl images. Inspection of full resolution photo-products (F-maps) reveals that very few coronae lack associated volcanism. Flows sometimes have the same backscatter as the coronae or the plains which they have formed in, and are therefore difficult to see, so that the amount of associated volcanism risks being underestimated. Very often, however, repeated volcanic activity gives rise to flows which vary greatly in roughness. These radar-bright (interpreted as blocky) or radar- dark flows (interpreted as smooth) are easily distinguished from plains materials which have intermediate radar backscatter. The scale makes no distinction between styles of volcanism or the type of structures associated with coronae, but given that it is applied consistently, provides a useful indicator of the relative degree of volcanic activity linked to coronae:

1 Deficient in associated volcanic features 2 Moderate number of associated volcanic features 3 Dominated by volcanism

15 Several investigations have made use of the 1992 corona survey. They include a study of spatial variation of coronae and their altitudinal distribution by Squyres et al.

(1993), and of the morphology and regional setting of coronae by Stofan (1995). As they do not incorporate coronae which lack an intense fracture annulus, these studies are unrepresentative.

At the time of the survey by Stofan et al. (1992) the SAR dataset was not complete, and it was therefore expected that the new survey would find coronae which had lain within data gaps. It was also expected that the new survey would find coronae which had been overlooked by the first survey.

1997 Survey

Using Magellan digital altimetry images, synthetic stereo photo products and full resolution photo products, coronae which lack a concentric brittle deformation annulus were identified and their physical properties recorded.

The data and methods used are described in detail below. Table 2.1 summarises the variables recorded and the pertinent units. For the identification of these structures the primary dataset was the Magellan altimetry data. Location and morphological parameters-, relief, width and morphological class (corona profile) were recorded from the altimetry global topography data record (GTDR).

Synthetic stereo images were used to study the geological setting, stratigraphy, tectonic style and associated volcanism, and to provide additional checks that features observed in the altimetry were in fact coronae and not large craters, volcanic calderas or features ascribable to some other mechanism. Where necessary, full resolution photo products (F-maps) and digital data were examined.

16 Field Units Source Compatibility with 1992 Survey Latitude & longitude degrees GTDR absolute Diameter kilometres GTDR absolute Height metres GTDR collected for all coronae 1997 Mean altitude metres GTDR collected for all coronae 1997 Morphology class GTDR collected for all coronae 1997 Corona type (Stofan, 1992) class Stereo comparable Volcanic class (Stofan, 1992) class Stereo comparable Stratigraphy class Stereo N/A

Table 2.1. Data fields used in the corona survey.

Magellan altimetry

Using knowledge of the spacecraft's orbital position, in nadir-pointing altimetry mode, the Magellan radar system was used by NASA to produce a topographic map of the surface of Venus by measuring the travel time of the radar signal. The Doppler- frequency shift of the returned signal was used to separate altitude measurements of specific surface areas (Plaut, 1993). There are two considerations of particular relevance to the data collection undertaken in this thesis. Examination of the global topography data record (GTDR) reveals spikes in the data, especially in regions of steep terrain, which result from strong radar return signals and lead to ambiguity in height determination. In altimetry mode, as in SAR mode, the resolution of the data varies with spacecraft altitude, and therefore latitude. Altimeter footprint dimensions vary between 12 km cross-track, 8 km along-track at 10° latitude and 27 km cross-track, 15 km along-track at latitudes 80° and 60° S. The global altimetry dataset is provided in 32

Mercator and 8 polar stereographic ffamelets.

Radar interaction with geological surfaces

Some understanding of the properties of the Magellan SAR images and knowledge of the way in which microwaves interact with geology is necessary in order to make a

17 sound interpretation of the data. The radar return or backscatter of the surface varies with its orientation with respect to the radar beam, with surface texture (Fig. 2.1) and with the dielectric properties of the surface (Elachi, 1983; Drury, 1993). There are a number of peculiarities common to radar images which can sometimes present obstacles to interpretation. These difficulties are shared by Earth-orbiting imaging radar systems such as SEAS AT, ERS-1 (Earth resources satellite) and the SIR (shuttle imaging radar) experiments, and are well documented. Often the low incidence angle of the radar beam means that some regions are in shadow because they are masked by features occupying the foreground (Fig. 2.2c\ Foreshortening may occur, whereby upstanding structures can appear to have a steep or shortened foreslope closer to the radar and a shallow back slope further away from the imaging system (Fig. 2.2 a). Layover is common in very rugged or mountainous terrain, and occurs when the radar beam impinges on elevated terrain before a simultaneous burst reaches ground level, so that the elevated region appears closer to the sensor than it is (Fig. 2.2 b). In some instances, areas of radar rough terrain may appear to occupy a lower elevation that is actually the case, because material is so oriented that it reflects the beam not back to the radar receiver but towards either the surface or another object. The action of corner reflectors increases the distance that the beam has to travel, thus giving the impression that the region is topographically depressed.

SAR incidence angle

Incidence angle has a strong influence on backscatter cross-section. It will also determine the degree to which SAR images are affected by foreshortening, overlay and radar shadow. Incidence angle varies with latitude owing to the configuration of the orbit and the imaging system in relation to the surface (Fig. 2.3). The incidence angle of

18 Isolated flows « 1 Smooth: no return T

Smooth plainsplains

Slightly rough: slightly diffuse T

Mottled plains

Moderately rough: moderately diffuse

Tessera

» Pi

Very rough: very diffuse T

Figure 2.1. The effect of surface roughness on radar backscatter. Surfaces which are less rough than the radar wavelength scatter the beam in a specular direction. Rough surfaces scatter energy in all directions, and back to the sensor; they therefore appear brighter in radar images. Terrain types which exhibit different radar backscatter properties are indicated. After Farr (1993).

19 SAR instrument

Look angle

Radar-image plane Radar beam

Radar-image (a) A1 . format

Near range Far range

SAR instrument

Radar-image plane

Radar beam

(b) Radar-image A 1 fomiat

F ar rangeNear range Far rangeNear

A C SAR instrument

Radar-image plane Radar beam

Near range Far range

A C D

Figure 2.2. Geometric distortions in radar images: (a) foreshortening, whereby the slope AB is compressed in the image plane; (b) layover, whereby the top of the topographic obstacle is imaged before the base; (c) shadow, where the back slope of the obstacle BC and surface CD are not illuminated by the radar and data are not acquired. After Farr (1993).

20 Smooth

Surface

T3 Moderately rough

-10

Rough

-20

-30 0 20 40 60 80

Incidence angle 0, deg

Figure 2.3. Radar backscatter as a function of incidence angle for surfaces with different backscatter properties. After Farr (1993).

21 cycle 1 imagery varies widely, but is quite high. Cycles 2 and 3 have substantially lower incidence angles. The bulk of the data for the study area was obtained during cycle 2; both radar shadow and swamped pixels (terrain having very a radar bright signature) result from the low incidence angle relative to the terrain. Incidence angle can be used to derive height data from the SAR images, providing both left and right looks are available for the area of interest.

Latitude Cycle 1 Cycle 2 Cycle 3 Cycle 3 (Stereo)

0° 44.9 24.9 - 24.5

-5° 43.8 24.9 - 2T6

-10° 42.3 24.9 - 2 2 6

-15° 40.4 25.0 - 21.4

-20° 38.1 25.1 - 20.1

-25° 35.5 25.1 - 18.7

Table 2.2. Varying incidence angle of cycles 1-3 with latitude.

Stereometry

Although it is possible to use left and right looking imagery from different mapping cycles, it is difficult to make the images fuse satisfactorily. However, superb synthetic stereo image pairs have been created by the United States Geological Survey at

Flagstaff. Synthetic stereo images are generated using radar images of a single look direction. Image co-ordinates are fitted to the altimetry data; then to create the stereo pair a second image is made by projecting the original image from an imaginary viewpoint to generate parallax. The synthetic stereo imagery was derived from Cl

(compressed once) images, which have a pixel resolution of 225 m.

A number of differences exist between synthetic and conventional stereo imagery.

These present potential pitfalls that need to be kept in mind when using the stereo pairs.

The synthetic stereo prints have been generated with vertical exaggerations of lOx and

22 50x. For most terrain, the lOx series is adequate. The distortion of the surface that occurs with greater exaggeration causes some difficulty in the interpretation of highly deformed unit types, such as the ' tesserae. Images generated with a high vertical exaggeration factor are useful for detecting subtle variations in plains topography, and low-lying and relatively small features. The low plainimetric resolution of the radar altimeter is a restricting factor; it means that smaller features, even though they may be topographically quite high, cannot be differentiated from adjacent areas. The stereo imagery, despite these potential difficulties, forms an invaluable mapping tool. It makes it far easier to differentiate between material units, and stratigraphie and unit relations are made clear. Differences in tectonic style and structure also become more apparent.

The surface expression of regions which initially appear as level plains due to very low and uniform backscatter is also made visible.

SAR and altimetry enhancement and processing

The main product for mapping are full resolution images or F-maps. These were also consulted during the corona survey. Where particularly difficult interpretation conditions exist, digital F-maps can be examined. Areas of interest can be enlarged, as well as subject to suitable enhancement techniques. As the region covered by the F-map photo-product may contain surfaces which are very radar rough, radar smooth or both, the range of tones or stretch may not be the most appropriate. Basic image processing techniques such as logarithmic or exponential enhancement can be used to provide a better range of tone in images which contain large areas which are either radar-bright or radar-dark. The use of contrast-stretching techniques, which increase the range of tones in an image, can assist the visualisation of materials with similar backscatter properties.

Convolution matrices can be applied to images to reduce or amplify either low or high

23 frequency information. Median filtration which replaces a high pixel (DN) value with a median value can be used to enhance images containing pixels with spuriously high values. Image processing techniques such as these are not limited to SAR images.

Altimetry data are prone to errors in topographically complex regions (Jankowski,

1995), which results in data spikes of high DN values, so median filtration can be usefully applied to these data to remove outliers.

Frequency filtration reduces the amount of noise, effectively eliminating the surface roughness and dielectric components of the radar backscatter, allowing the interpreter to analyse slope more effectively. A particularly useful techniques involves applying a 5 x

5 mean or low-pass filter matrix to the contrast enhanced images. The original unfiltered image is added back at a ratio of 25 % to the filtered version (Tapper, 1994).

This preserves a useful amount of high frequency information, but the speckled appearance and variable brightness of the unfiltered image does not now detract from the low frequency information which describes broad-scale structural geometry.

First derivative directional filters (Fig. 2.4) can be applied to altimetry data to create shaded relief images (Fig. 2.5). The technique converts raw topography data into an image of topography in which shadow and light are used as visual cues to depth.

Shaded relief images were created to complement raw altimetry images which are sometimes difficult to interpret when extremes of altitude are encountered or if the region of interest is fairly extensive.

Shaded relief maps have some advantages over raw data, though for analytical work and measurements the raw data must be used. Viewing shaded relief is a more natural way of looking at topography data and is easier to interpret. Conventional maps as well as satellite image maps use the technique to assist the visualisation of topography.

24 When applied to Venus altimetry data it facilitates the viewing of extreme topography as well as local topographic trends. The shaded relief map allows very subtle variation in topography to be picked out. Shaded relief images are not dependent on the contrast

stretch of the original image, so that areas of low, intermediate and high relief can be

viewed simultaneously. The technique is very versatile, as different illumination effects

can be generated. The matrix used here (Fig. 2.4) does not create extreme shadow and retains some of the original data, allowing relative topography to be seen, as well as

gradient.

1 1 1 1 1 -1 1 1 1 1 1 1 1 -1 -1 1 1 1 -1 1 1 -1 -1 -1 1 1 -1 -1 1 -1 -1 -1 -1 1

Figure 2.4. Directional filter matrices (3 x 3) and (5 X 5) used to simulate illumination from the northwest.

Another important aid to visualisation of altimetry data is the use of 3D rendering techniques. A variety of PC based software packages can be used to plot the altimetry

data as a three dimensional landscape, the most versatile of which are Surfer and

Landscape Explorer. Using Landscape Explorer it is possible to drape a SAR image

over the digital elevation model. Three dimensional images (Figs. 5.4 and 5.5) of the

venusian landscape form a useful analytical tool as they reveal how structure relates to morphology. They are particularly useful in geologically complex regions or where

SAR data^redifficult to interpret.

25 -25°-,

NJ Figure 2.5. Shaded relief image generated from Magellan altimetry using first derivative directional filtering to simuate illumination from the 0\ northwest. Such images were used to assist the interpretation topography in the survey for stealth coronae. This example is the Scarpellini Quadrangle. a.

D' Fracture annulus Topographic rim

Corona diameter measured from the outermost extent of the fracture annulus (D ), as in the survey of Stofan et al. (1992) and the outermost extent of the toppographic rim (lÿ ), as in the present survey.

M aximum altitude

Base level

Height (H) is measured from the maximum altitude of the corona to the level of the surrounding plains or base level.

Figure 2.6. Diagrams (a) and (b) illustrate terms used in the description of corona morphology.

27 Procedure and survey design

Before commencement of the survey, coronae recorded by the 1992 survey of Stofan et al. (1992) were identified on the altimetry ffamelets to ensure that coronae were not recorded more than once. A search was then made for additional planimetrically circular or elliptical features. The criterion adopted for the selection of these features was that (1) any arcuate rim/trough had to be more than ~70 % complete; or that (2) multiple rim/trough systems could be identified. Final verification of each new feature was made whilst using the synthetic stereo images to record stratigraphie, volcanic and tectonic information.

When new structures were identified their location and physical attributes were recorded. Cross sections were taken and the maximum altitude and the level of the surrounding plains recorded for all coronae (Fig. 2.6), including those of the previous survey. Diameters of the 1992 survey (Stofan et ai., 1992) were measured from the outermost extent of their fracture annulus. The newly identified features, however, lack a fracture annulus. Therefore diameter was measured from the outermost discernible topographic extent of the corona.

Morphology

The morphological classification scheme designed for the survey recognises 11 distinct forms of coronae morphology (Fig. 5.1), ranging from simple basins and domes to complex structures that have multiple ridges, troughs and chaotic interiors. The classes are:

1 Dome 2 Plateau 3 Rim encircling an elevated interior

28 4 Rim encircling a low-lying interior 4b Trough surrounding a rim and basin 5 Rim surrounding a central rise 6 Rim encircling a basin with inner rim and central depression 6b Two rims encircling a central rise 7 Rim only 8 Basin 9 Unclassified 10 Rim or trough encircling chaotic interior of arcuate troughs, ridges, or both

The classification scheme is similar to that used by Smrekar and Stofan (1997) to describe the morphology of coronae recorded by the 1992 survey. Types 4b and 6b are created to ensure continuity with their scheme.

It is important to realise that coronae are hard to classify. In some cases, there exists a series of encircling rims and troughs, and the configuration is not clear, for example if rim/trough morphology varies for part of the corona circumference. Another difficulty encountered is that sometimes it is hard to determine whether topographically high structures located in the interior constitute part of the corona's morphology or whether they are simply a central volcano, albeit related to corona formation.

Stratigraphy

The stratigraphie classification scheme is used to give some indication of the materials in which coronae have formed:

Rp Regional plains Mp Mottled plains

Ip Intermediate plains Rp/Lp Regional plains and lineated plains Rp/Lt Regional plains and lineated terrain

29 Lp Lineated plains Lt Lineated terrain T Tessera

The scheme includes all the major material types found on Venus from regional

plains, interpreted as the most youthful unit, through to tessera materials, which are

interpreted as being amongst the earliest units to have formed. Although it is not

possible to correlate individual units globally, it is possible to construct regional

stratigraphies and examine the age relations of coronae. This information cannot form

the basis of globally applicable corona dating, but provides much useful information.

Volcanism

The volcanic classification scheme of Stofan et al. (1992) was applied to the coronae

identified by the new survey to enable a comparison of the relative amount of volcanism associated with the two corona populations to be made. The degree of associated volcanism

determined using C1 images and the survey therefore can be considered consistent with

that made by Stofan. Consistency in the recording of the amount of volcanism is

assured by the simplicity of the scheme, whereby coronae are simply deficient, have a

moderate amount, or are dominated by volcanism:

Deficient in associated volcanic features. Little or no associated volcanism. Only minor flows discernible. Sparsely distributed volcanic edifices may be present.

Moderate number of associated volcanic features. Volcanic flows are present which have erupted from the corona rim; these may form an annular flow apron, occupy the corona interior or the surrounding plains. Volcanic edifices are more numerous and are clearly associated with the corona.

Dominated by volcanism. Numerous and extensive lava flows mantle the corona. Individual flows which have erupted from vents located on the flanks of the

30 corona rim may be discernible. Clusters of small edifices are present. Larger central volcanoes associated with the corona may be located in the interior or on the rim of the corona.

Tectonic style

Though many coronae lack an intense concentric fracture annulus, virtually all coronae show signs of some tectonic deformation. The categories used to describe the tectonic features associated with the coronae are as follows:

AN Annulus ACF Aligned concentric fractures PACF Partially aligned concentric fractures ABN Aligned braided network RF Radial fractures ARF Arcuate radial fractures CW R/G Concentric wrinkle ridges/graben

Summary

The survey for stealth coronae gathers tectonic and stratigraphie information, but morphological attributes were collected for all coronae, to be compared in this thesis.

The data are supplied in Appendix I and II. To enable comparison between the datasets and enable modifications to be made easily, the data are listed separately.

Together, however, they form the most complete corona catalogue yet compiled. Many errors and inconsistencies which afflicted the 1992 database have been remedied and additional nomenclature proposed by the USGS and confirmed by the lAU assigned to the coronae.

31 Chapter 3

Spatial and altitudinal distribution

This chapter describes the spatial and altitudinal distribution of the coronae that were identified in the survey. The distribution of coronae which lack an intense fracture annulus is compared with the distribution pattern of those identified by the 1992 survey.

Density maps are used to illustrate spatial trends occurring in the data, and nearest neighbour analysis is used to assess the significance of any tendency towards spatial clustering; it is considered prerequisite for any discussion of mechanisms.

Spatial distribution

Indicated on a global topography map are the major geographical features of Venus

(Fig. 3.1). The global distribution of coronae is given in Fig. 3.2. The new survey identifies significant additional concentrations of coronae, although coronae were found in chains and near clusters already documented by Stofan et al. (1992). A large group of newly identified coronae is centred on 25.0° N, 170.0° E, situated to the north of

Rusalka Planitia and northwest of Nokomis Montes. South of this cluster is a dog-leg chain of coronae centred on 10.0° N, 165.0° E. Other clusters are centred on 40.0° N,

15.0° E, in northwestern Bereghinia Planitia, in southeastern Aino Planitia at 50.0° S,

115.0° E and to the west of Ovda Regio at 20.0° S, 35.0° E. Additional coronae were recorded at 20.0° S, 215.0° E and at 50.0° S, 290.0° E, southeast of Themis Regio, which augment clusters identified by the 1992 survey. Stofan et al. (1992) found that plotting coronae in equal area latitude bins revealed a greater concentration at mid-latitudes.

Although the new survey finds major clusters in the northern hemisphere, the tendency towards mid-latitudes is seen in the new data as well. Relatively few additional coronae

32 : iSH

H R m iT

300 330

Planetary Radius (km) 6048 6050 6052 6054 6056 6058 6060 6062

Figure 3.1 Contour map derived from Magellan altimetry. Large scale geological structures and regiones are labelled. CO CO a

330

Planetary Radius (km) 6048 6050 6052 6054 6056 6058 6060 6062

Figure 3.2. Black dots represent coronae recorded by the 1992 survey. White dots represent coronae recorded by the 1997 survey. CO 4k were identified by the survey in the Beta-Atla-Themis (BAT) region, where the coronae identified by the 1992 survey are concentrated (Stofan et al., 1992; Squyres et al., 1993).

The distribution of different sized coronae (Fig. 3.3) shows that coronae of similar diameter often occur in clusters. Small coronae appear to occur in clusters more frequently than large coronae, possibly because they are more common. Chains tend to consist of coronae of intermediate size ranging from about 300 km to 450 km in diameter. Very large coronae with diameters of over 700 km have a more widespread distribution and tend to occur in isolation in the plains.

Coronae which have similar morphology were plotted (Figs. 3.4-7) to assess their distribution and to identify any pattern. Although there appears to be a tendency for coronae with late stage morphologies to be more spatially dispersed than early stage coronae, measures of central tendency suggest that this is a merely a function of the different population sizes.

The distribution of coronae of different types was examined for possible global trends which might imply variation in the age of coronae spatially but no pattern was identified. Although no obvious trends emerged, the distribution plots (Figs. 3.4-7) give an indication of how coronae of different morphology vary in their distribution. In certain regions some morphologies are predominant whereas other types are either sparsely distributed or absent. The relative abundance of coronae of different morphology is discussed in Chapter 5.

It was found that coronae that lie along chasmata or ridge belts in chains exhibit a variety of morphologic styles but no pattern or trend along corona chains was revealed.

35 6 0 ' i Corona diamter

30° - , km

# > 150 # 151-300 e 301-450 • 451-600 -30° - e 601-750 • 750 + -60° J

40= 80= 120° 160° 200° 240= 280° 320°

Figure 3.3. Distribution of coronae of different sizes. All coronae were classified by diameter using the size bins given in the key and plotted on a graph to reveal the distribution pattern of different sized coronae. o\OJ (a) Type 1 : Dome

(b) Type 2; Plateau

90° 150° 210° 270° 330°

80°

60 °

40°

2 0 °

0 ° • • •

- 20 °

-40°

-60°

-80° 30° 90° 150° 210 ° 270° 330°

Figure 3.4. Distribution of corona types 1-3, plots (a), (b) and (c) respectively. Filled circles represent coronae identified by the 1992 survey (Stofan et al., 1992). Hollow circles represent coronae identified by the new survey.

37 Type 4: Rim encircling a depressed interior (a)

• • •

• • O o

- 20 '

-40'

-60'

-80' 30' 90' 150° 210° 270° 330°

(b) Type 4b: Trough surrounding a rim and basin —

8 0 " • • • 60° ■

40° " •

20° ■ :

0° ■ 0

. • • O ° • -20° ■ • ^ -40° ■ • Oo ° o • -60° ■ •

-80° 30° 90° 150° 210° 270° 330°

60°

40° • • • O • •

20 ° « • 0 °

- 2 0 °

-40°

-60°

-80° 30° 90° 150° 210° 270° 330°

Figure 3.5. Distribution of corona types 4-5, plots (a), (b) and (c) respectively. Filled circles represent coronae identified by the 1992 survey (Stofan et al. 1992). Flollow circles represent coronae identified by the new survey.

38 (a) Type 6: Rim encircling inner rise and central basin

90° 150° 210° 270°

Type 7: Rim only —------A A_

Type 8; Basin

Figure 3.6. Distribution of corona types 6-8, plots (a), (b) and (c) respectively. Filled circles represent coronae identified by the 1992 survey (Stofan et al. 1992). Hollow circles represent coronae identified by the new survey.

39 (a) Type 9: Unclassified

150° 210° 270° 330°

(b) Type 10: Rim or trough encircling chaotic interior

60° --

4 0 °--

Figure 3.7. Distribution of corona types 9 and 10, plots (a) and (b) respectively. Filled circles represent coronae identified by the 1992 survey (Stofan et al. 1992). Flollow circles represent coronae identified by the new survey.

40 spatial analysis

Nearest neighbour distances were derived from great circle distances between all coronae and used to construct maps of corona density over the surface of Venus. Mean minimum distances were used to determine the degree of clustering in each distribution.

Density maps (Fig. 3.8) were constructed by counting the number of coronae that lie within a radius of 2,100 km of each other. This distance, equivalent to a search radius of 20°, was used to ensure that the resulting density maps could be used to identify variation in corona density over large areas. The number of neighbours of each corona were used to generate a surface map using a technique known as kriging (Matheron,

1963) to interpolate between the values. The plots need to be evaluated with caution because no interpolation technique can give an equally accurate representation of density over the entire surface, although kriging minimises the effect of spatial .location by weighting values dependent on their position relative to one another, assigning low weights to distant samples and the converse. Comparison with the original point distribution shows that the density maps do not distort the distribution and that the potential drawbacks of the technique do not present an obstacle to interpretation.

Corona density distribution

1992 survey

The density map (Fig. 3.8a) of coronae recorded by the 1992 survey clearly shows the concentration of coronae in the BAT region, where the density of coronae ranges from 1.5 to 2.5 coronae per 10^ kml The global average for the 1992 survey population is 0.73 coronae per 10® km^ (Squyres et al., 1993). Squyres et al. generated a contour map indicating the BAT cluster where density exceeds 1.67 coronae per 10® km^ and where clustering is significant at the 99 % level. Other smaller (less significant)

41 m 0 0.5 1.0 1.5 2.0 2.5 (b)

0 0.5 1.0 1.5 2.0 (c)

m ''"

H 0 0.5 1.0 1.5 2.0 2.5 3.0 Coronae per 10 >6 km, 2

Figure 3.8. Density of coronae of (a) the 1992 survey, (b) the 1997 survey and (c) the expanded database. Warm tones indicate high corona density, cool tones indicate low corona density. Solid line indicates significant clustering at the 95 % level. Broken line indicates density exceeding 1.67 coronae per 10^ km ^ .

42 concentrations are observed south of Aino Planitia and south of Sedna Planitia. The corona distribution appears, at least in part, to be topographically constrained. Low densities correspond to very high and very low altitudes, with coronae concentrated in the most extensive regions of intermediate altitude. The altitudinal distribution of coronae is discussed below.

1997 survey

The 1997 survey (Fig. 3.8b) has a more dispersed distribution than that observed of the 1992 survey where coronae (in general) are concentrated in the BAT region. A cluster of coronae significant at the 95 % level is located north of Rusalka Planitia.

Clusters are also located north of Sedna Planitia and east of Phoebe and Asteria Regio, and coincide with regions of intermediate altitude. The most extensive of these clusters is the cluster north of Rusalka, which has a distribution density ranging from 1.4 to 2.3 coronae per 10^ km^. The density map reveals additional clusters in Ulfrun Regio, south of Atalanta Planitia, southwest of Fortuna Tessera, and north of Atla Regio. At equatorial latitudes, including the BAT region, corona density is only 0.5 per 10^ kml

All coronae

The trend surface generated for the expanded corona database (Fig. 3.8c) identifies groups of coronae encircling the BAT region which correspond to chains of coronae aligned with chasmata and fracture belts. Along these chains, corona density may exceed 3 coronae per 10® kml The largest of these are: (1) east of Aphrodite Terra at

15.0° S, 220.0° E, (2) south west of Asteria Regio at 30.0° S, 280.0° E and (3) west of

Themis Regio at 5.0° N, 245.0° E. Slightly lower corona densities are recorded in the interior of the BAT region, where there are 2.5 to 3 coronae per 10® km^. Density of

43 coronae at mid-latitudes, north and south of Aphrodite Terra, ranges from 1.5 to 2.5 coronae per 10^ km \ The cluster north of Sedna Planitia seen in the 1997 survey surface plot augments a region which yielded a slightly higher than average density in the 1992 survey. Density values of 2.5 to 3 coronae per 10® km^ identify the group as a significant cluster. Indicated on Fig. 3.8c are clusters significant at the 95 % level, clearly illustrating the main results: that significant clusters are (1) more numerous, and

(2) more extensive than previous investigations have shown.

Nearest neighbour analysis

Nearest neighbour analysis was used to determine whether the degree of clustering observed in the new and expanded data sets is significant. For each corona population it was first necessary to calculate the expected density:

re = 0.5(VA/N) [1]

Where: N = number of coronae and A = surface area of Venus = 4.17 x 10^ km^.

The coefficient r, or nearest neighbour function, is the statistic used to determine the degree of clustering and is calculated thus: r = ro / re [2]

Where: ro (observed) = mean minimum distance.

The value of r gives an indication of the degree of clustering, where a value of 1 indicates a random distribution, a value of 0 indicates perfect clustering, and a result of

2.1491 indicates that points are regularly dispersed (forming a triangular lattice). To test the significance of the result the z score was calculated:

44 z = (ro - re) / S.D [3]

Where: S.D (standard deviation) = 0.26136 / V (N (N/A)).

Inspection of the distribution of the expanded corona population displayed as a trend surface (Fig. 3.8c) revealed a number of clusters (above). Nearest neighbour results

(Table 3.1) show that the distribution of coronae identified by the new survey, at a significance level of 99 %, is not spatially random. A comparison of the value of r obtained for the 1997 survey indicates that the coronae identified are more clustered than those of the 1992 survey.

1992 1997 ALL # 335 229 563 re 585 709 451 ro 505 470 377 r 0.86 0.66 0.84 SD 15.93 23.31 9.48 z -5.03 -10.27 -7.81

Table 3.1 Nearest neighbour analysis results. # number of coronae, re expected density, ro observed density, r statistic, SD standard deviation, z scores.

The nearest neighbour function r was plotted against the number of coronae with nearest neighbour distances > r. This enabled a direct visual comparison between corona distribution from survey results and spatially random populations. The cumulative frequency plot (Fig. 3.11) shows that, at separation distances of between 200 km and 500 km, the coronae of the 1997 survey are clustered at a significance level > 99

%. The plot shows that the coronae of the 1992 survey have a greater degree of clustering at separation distances of 300 km to 700 km. Plotting the distribution of observed and randomly generated coronae for the expanded corona database illustrates the non-randomness of surveyed coronae. The values of r and the z-scores confirm that the expanded database distribution is non random at a significance level of > 99 % and

45 that the combined survey results indicate not a more random, but a more clustered distribution.

Are coronae with the same morphology clustered?

Further nearest neighbour tests were carried out to make an assessment of the degree of clustering of coronae which have the same morphology. Cumulative separation distances are shown in Figs. 3.9-11 which shows the percentage of the corona population with separation distances < r. Table 5.1 lists the types of coronae and Figs.

3.4-7 show the spatial distribution of the different types. For types 2 (plateaux), 3 (rim surrounding an elevated interior), and 6b (two rims surrounding a central depression, there are too few coronae to detect clustering. Inspection of their distribution, however, reveals that pairs of coronae occur more often than might be expected of a random distribution. For coronae that are more numerous (about 30 upwards) some clustering is apparent. Nearest neighbour analysis was carried out to determine whether these observations were significant (Table 3.2).

1 2 3 4 4b 5 6 7 8 9 # 20 17 79 61 41 118 35 37 30 23 re 2084 2407 1238 781 1595 851 1702 1614 1805 2177 ro 1620 2271 879 637 1406 783 1140 940 1575 2128 r 0.78 0.94 0.71 0.82 0.88 0.92 0.67 0.58 0.87 0.97 SD 222 296 75 31 130 37 148 133 167 243 z -2.09 -0.45 -4.79 -4.6 -1.45 -1.83 -3.79 -5.05 -1.38 -0.20

Table 3.2. Nearest neighbour analysis results ordered by corona type. # number of coronae, re expected density, ro observed density, r statistic, SD standard deviation, z scores. Type 6b coronae were not included because they were too few in number.

The r values and z scores obtained for types 2 (plateaux), 3 (rim surrounding an elevated interior, and 9 (unclassified), indicate that they are randomly distributed. Types

1 (dome), 4 (rim surrounding a low-lying interior), 6 (two rims encircling basin with

4 6 (a) (b )

100 100 T

% %

10 ..

0 2000 41X10 6000 8000

Separation distance (km) Separation distance (km)

100 -r 90 ■ ■

70 - • 70 60 % % 60 50

40 -•

30 20 ■■

1000 2000 3000 4000

Separation distance (km) Separation distance (km)

(e) (0

100

90 90

60 60 % % 50

40 40

0 lOlX) 21XX) 3000 40 (X) 5000 6000 0 500 1000 151X) 2000 2500 3000 3500

Separation distance (km) Separation distance (km)

Figure 3.9. Nearest neighbour distribution of corona types 1,2,3,4,4b and 5. Shaded area represents the distribution of 100 random sample populations of equivalent size. Clustering is indicated where the actual population (line) falls below the shaded region.

47 100 100

% %

0 2000 4000 6000 2000 4000 6000 8000

Separation distance (km) Separation distance (km)

too too 90 80 70 60 50 % % 40 30 20 10 0 1000 2000 3000 4000 5000 6000 0 2000 4000 6000 8000

Separation distance (km) Separation distance (km)

Figure 3.10. Nearest neighbour distribution of corona types 6, 6b, 7 and 8. Shaded area represents the distribution of 100 random sample populations of equivalent size. Clustering is indicated where the actual population (line) falls below the shaded region.

4 8 (a) (b )

100 1997 Survey 100 1992 Survey

60 % % 50

0 0 400 800 1200 1600 2000 500 1000 15002000 2500

Separation distance (km) Separation distance (km)

0 200 600 1000 1400 1800

Separation distance (km)

Figure 3.11. Nearest neighbour distribution of corona of (a) the 1997 survey, (b) the 1992 survey (after Squyres et al. 1993) and (c) the expanded corona database. Shaded area represents the distribution of 100 sample populations of equivalent size. Clustering is indicated where the actual population (line) falls below the shaded region.

49 central depression),and 7 (rim only), are clustered at the 99 % significance level. Type 5 coronae, comprising a rim surrounding a central rise, are clustered at the 96 % significance level. Types 8 (basin), and 4b (trough encircling a rim and central plateau), are clustered but at a significance level of 67 %.

When the r coefficient of these coronae is plotted against the number of coronae with separation distances greater than r, some types display a clustered distribution at certain distances. Type 8 have a clustered distribution at separation distances at 500 km to 1,500 km. Type 1 have a clustered distribution at separation distances of 1,000 km to

2,000 km.

Altitude

Corona altitude or base-level was measured from GTDR (global topography data record) framelets. Fig. 3.12 shows the coronae of the 1992 survey of Stofan et al.

(1992) and the new survey plotted as percentage of coronae against altitude. Although the distributions are reasonably well matched, the coronae of the 1997 survey are concentrated at lower altitudes reducing the mean height of the expanded corona database. About 45 % of the coronae recorded by the 1992 survey occur below MPR

(mean planetary radius = 6051.4 (Pettengill et al., 1992)), whereas 65 % of coronae recorded by the new survey occur below MPR.

Corona altitude was compared with hypsography, that is the percentage of the surface of Venus above a particular elevation. Although a greater number of coronae are found below mean altitude, 70 % of coronae coincide with the 6051 - 6052 km level which constitutes 50 % of the planet’s surface. At very high altitudes a paucity of coronae is observed. At altitudes in excess of 6053 km (10 % of Venus’s surface area) only 2 % of coronae are found. Of coronae identified by the new survey only 10 %

5 0 Corona base level

1 0 0 90 80 70 ♦ 1992 Survey 60 ♦ 1997 Survey 50 Planetary radius Cl 40 30 20

6050 6051 6052 6053 6054 6055

Altitude of corona base level (planetary radius in km)

Figure 3.12. Base level (altitude of the terrain in which coronae form) of coronae identified by Stofan et al. (1992) and coronae identified by the new survey. The dotted line shows percentage surface area of Venus. The percentage of coronae above or below a certain altitude can be determined from the graph by selecting the altitude required. Percentages either side of the cumulative frequency/altitude intersection can be read from the y axis. A higher percentage of coronae which lack an intense fracture annulus occur at lower altitudes.

51 occur at altitudes in excess of 6052 km. At low altitudes more coronae are identified by the new survey. Only 5 % of the planet’s surface has a radius of below 6051 km. At this altitude are found 8 % of coronae identified by the 1992 survey and 15 % of coronae identified by the new survey.

Interpretation of spatial analysis

The density plot of the expanded (Fig. 3.8c) database reveals more spatially dispersed clusters than the original survey. This indicates that the conditions which permit coronae to form are more globally widespread and more numerous than formerly thought.

Some additional coronae identified by the new survey were found in chains, but most of them were found in clusters or in isolation. Conditions either precluding the formation of a fracture annulus, or factors which have meant that coronae formed more recently and have yet to develop annuli, therefore predominate in regions outside the

BAT region. The coronae that were found by the new survey in alignment with chasmata in the BAT region, usually possessed intense fracture annuli.

The distribution pattern of the expanded database conforms with intermediate topography, as fewer coronae were observed in the low-lying plains or at very high altitudes. The corona chains within the BAT region are associated with chasmata

(Stofan et al., 1996), but there may be additional factors which have led to the observed distribution, because not all coronae within the BAT region are associated with chasmata.

Squyres et al. (1993) suggest that: (1) clustering (the BAT cluster) results from mantle processes concentrating the development of plumes in specific areas to the exclusion of others but also that it is possible that (2) variation in crustal properties

5 2 allows corona formation in some areas and prevents it in others. Such conditions are mutually compatible and can exist (Tackley et al., 1992) where pressure-release- melting

(PRM) occurs in a large convected upwelling, promoting Raleigh-Taylor instabilities of dimensions consistent with those thought to form coronae. Many coronae within the

BAT region, however, are oriented in chains along chasmata and fracture belts; a single, large plume is therefore not a viable clustering mechanism. The linear alignment of coronae in terms of mantle processes is hard to explain, but lithospheric conditions

(thinning) along chasmata may permit coronae to form preferentially.

Distribution of coronae with the same morphology

Coronae having the same morphology display no ordered spatial trends. Although certain coronae of the same type tend to occur in clusters (Figs. 3.9-11), there is no identifiable correlation between the morphology (and therefore possibly age) of coronae and location (Figs. 3.4-7). Clustering of coronae of the same morphology, however, strongly implies that they are coeval.

Altitudinal distribution

The coronae identified by the new survey, like the coronae of the 1992 survey, are less common at very low altitudes and at very high altitudes. The main difference between the two survey populations is that coronae identified by the new survey tend to occur at lower altitudes than those previously recorded. The concentration of coronae which lack an intense fracture annulus at lower altitudes suggests either (1) that coronae forming plumes are less able to cause brittle fractures to form in the lithosphere, or (2) that a greater proportion of the population found in low lying areas are incipient and have- yet to form annuli. A third scenario whereby crustal thickening has prevented

53 coronae from achieving end member morphology and to form concentric fracture annuli is possible but less likely. Geological evidence exists, including wrinkle ridges which postdate these coronae, that is consistent with a thin and mobile lithosphere. As corona morphology has to be taken into account, these alternatives are discussed more fully in

Chapter 10.

In discussing the paucity of coronae at high altitudes, Squyres et al. (1993) invoke tectonic processes as an agency of corona removal. But a thick lithosphere in upland areas may explain the lack of coronae in upland areas equally well. A crust which has thickened over time may account for the lack of new coronae in these regions. The new survey finds coronae in upland areas which have complex forms but have not been modified by tectonic processes. This is not consistent with the suggestion that tectonic processes destroy coronae, but is consistent with change, either in lithospheric conditions (thickening) or in the sense that corona-forming plumes have ceased to be active.

5 4 Chapter 4

Geological setting

In Chapter 3 it was observed that clusters of coronae are widespread and extensive.

These findings need to be looked at in conjunction with their geological setting. In this chapter the geological setting of coronae identified by the new survey, including stealth coronae, is compared with that of the coronae of the 1992 survey. Associations with large scale geological structures, broad scale topography and regiones are described.

Geological setting

Coronae are found in three geological settings: on volcanic rises, along chasmata/fracture belts, and occasionally in isolation in the plains (Baer et al., 1994;

Stofan, 1995; Stofan et al. 1997). Coronae identified by the new survey are found in each of these settings. Additional coronae are found on corona chains including Parga

(Fig. 4.1) and Hecate Chasmata in eastern Aphrodite Terra (Fig. 4.2), alongside coronae which exhibit intense concentric fractures. Coronae were found associated with topographic rises, frequently on the rise itself but also occurring on the margins of the rise. Additional coronae were identified on the margins of regional scale geological structures including Ishtar Terra, Alpha Regio and Artemis Corona, and smaller rises including Hyndla and Imdr Regio (Fig. 4.2). Some coronae identified by the survey are found in clusters or in isolation in relatively low lying plains but do not appear to be associated with either topographic rises or chasmata/fracture belts.

Because coronae identified by both surveys occur in a variety of geological settings it is very difficult to discern the differences between the populations. The density distribution map in Chapter 3 shows that for the 1992 database (coronae which have an

5 5 k

Figure 4.1. Magellan SAR image of Parga Chasma. The image shows the chasmata setting of coronae identified by the 1992 survey and which possess heavily fractured concentric annuli. The image is centered on 30.0° S, 272° E. Scale is correct at latitude 30°.

60.0

40.0

20.0

Rusalka ^ Deyana Planitia

-20 .0°.

-40.0

1000 km -60.0 160.0 180.0 200.0 220.0 240.0° 260.0° 280.0° 300.0°

Figure 4.2. Magellan altimetry of the BAT (Beta Atla Themis) region. The BAT region is characterised by chasmata, a high volcanic flux and elevated topography (I km-3km) above mean planetary radius (6051.0) km. Coronae identified by the 1992 survey are concentrated on Dali, Hecate and Parga Chasmata. Coronae identified by the new survey are concentrated on the ridge belts north of Rusalka Planitia (a) and south west of Themis Regio (b). Some coronae identified by the new survey were found near to topographic rises such as Imdr (c) and Hyndla (d). Scale is correct at latitude 57°.

56 intense fracture annulus), the main corona cluster is centred on the BAT region.

Coronae identified by the new survey which lack fracture annuli are identified on the periphery of the BAT region on ridge belts which extend northwest and southeast of the chasmata. The clearest example is the ridge belt system north of Rusalka Planitia, north west of the BAT region (Fig. 4.2). In the southeast of the BAT region additional coronae were found associated with ridge belts near Themis Regio.

The altitude of coronae in different geological settings varies v^dely. Overall, coronae identified by the new survey avoid low plains but are found at slightly lower altitudes than those which have concentric annuli (Chapter 3). Coronae found on the ridge belts north west and south east of the BAT region and the Sedna Planitia cluster are at altitudes 0 km - 1 km above mean planetary radius. Much of the BAT region, however, is 2 km - 3 km above planetary radius. Coronae identified in relative isolation tend to be found at or slightly below mean planetary radius but are absent from the low- lying plains.

It was possible using the distribution (Chapter 3) to observe the relationship between coronae and other large scale geological structures. Stofan and Smrekar (1998) describe the geology of three corona dominated rises (CDRs): Themis, Eastern Eistla and Central

Eistla regiones, where coronae are observed to occur in clusters. Additional coronae were identified on Tellus Tessera, a topographically high region of highly deformed terrain.

Coronae are frequently observed to occur on the margins of topographically high, regional scale features. Small groups of coronae in the new survey are found to be associated with the volcanic rises of Asteria and Imdr Regio, and in close proximity to large volcanoes such as Sif Mons. Coronae occurring specifically on the margins of

5 7 rises, of which Beta Regio is the only cited example (Squyres et ah, 1993), are not

special cases and are more prevalent than previously indicated.

Stratigraphy

The stratigraphy of coronae and the materials they deform and by which they are

embayed were examined with the aid of synthetic stereo images. Most coronae

identified occur in and deform the extensive regional plains. The regional plains are

extensive radar dark materials which are interpreted as being relatively youthful because

they overlie and embay* earlier units. Cratering statistics (Phillips et al., 1992) indicate

that the regional plains are chronologically youthful and were emplaced about 500 Ma

±300 Ma ago. Some coronae were observed to deform mottled plains, which is an earlier

plains unit considered to have formed by continued shield forming volcanism. Older,

highly deformed and tectonised units such as lineated plains and lineated tessera, which

are locally overlain by regional plains materials, contain fewer coronae.

Table 4.1 shows the proportions of different materials in which stealth coronae have

formed. Stealth coronae are most frequently found in regional plains units (69 %),

which are generally interpreted as the most youthful materials. Tessera materials, which

are normally distinguished as the oldest materials, only contain 2 % of the coronae

identified (Fig. 4.3). Lineated plains and lineated terrain are each modified by 8 % of

coronae. Corona formation is slightly less common in mottled plains materials; just 5 %

of coronae deform these materials. The intermediate stratigraphy class contains a

handful of coronae for which it was not possible to determine the type of material

deformed or where various units had been deformed by coronae and it was not clear at

E m b a y m e n toccurs when a geological material floods or e m b a y s existing units which are low-lying, leaving elevated portions of the original surface exposed.

5 8 \

Figure 4.3. Example of a corona identified using Magellan altimetry in Tessera materials (Alpha Regio). Arrows are used to mark the rim of the corona which forms an escarpment encicling an interior below the level of the surrounding materials. The materials to the north of the corona (between the black lines) have deformed obliterating the structure, thus indicating that the corona is coeval with the tessea. Some structures are observed which deform tessera, but cannot be shown to have formed at the same time because they postdate all other deformation structures. Similar structures are identified on Tellus Tessera.

59 which stage coronae development had taken place. Coronae which have deformed more than one unit constitute 7 % of the total.

Unit/s Description % of Coronae Regional plains Interpreted as the most youthful materials. 62 Characterised by low radar backscatter. Regional plains / lineated Deforms both units. 12 terrain Lineated terrain Materials/terrain often associated with 11 chasmata; highly deformed (high backscatter). Difficult to determine stratigraphie context. Mottled plains Plains with mottled appearance, 4 interpreted to have formed shield forming volcanism. Intermediate backscatter. Lineated plains Extensive tectonised (compressed) unit, 3 locally overlain by regional plains materials. Tessera Widely interpreted as the earliest materials 3 unit. Characterised by intense deformation, having undergone episodes of extension and compression. Often displays a ridge and trough pattern. Very high radar backscatter. Intermediate plains Plains characterised by moderate amounts 2 of deformation. Intermediate backscatter. Regional plains / lineated Deforms both units. 1 plains

Table 4.1. Units deformed by coronae identified in the new survey.

The proportions of each materials unit deformed by coronae of different morphology

(Fig. 5.1) vary (Table 4.2). Differences with respect to detectability and other factors, such as the distribution and relative cover of the units themselves, may obscure any underlying trend in the stratigraphie position of coronae. A trend is hard to discern but some multi-staged coronae have arisen in a range of units and the evolution of a significant proportion of individual coronae has been ongoing, disrupting at least two geological horizons. This is consistent with the results of detailed mapping of the

Scarpellini Quadrangle (Chapters 8 and 9) and the mapping of selected coronae by Copp

6 0 et al. (1996), which demonstrates that coronae have lengthy evolutionary histories, sometimes spanning several geological episodes.

Some common multi-stage coronae (types 5 and 6) appear to deform a diversity of units because they are found more frequently, in contrast to some complex/chaotic coronae (type 10) which appear to deform only a few or even a single stratigraphie horizon. Coronae of type 7 (rim only) and type 4 (rim encircling a low-lying interior) are common and are found in a variety of stratigraphie settings.

Unit Corona t] 1 2 3 4 4b 5 6 7 8 9 10 Regional plains 100 25 31 68 50 56 56 72 75 17 43 Tessera 25 23 1 7 56 3 17 Mottled plains 25 8 3 3 17 Lineated plains 8 1 5 25 14 Regional pi. / Lineated pi. 8 6 Intermediate plains 2 5 56 3 17 Lineated PI. / tessera 1 Regional PI. / Lineated terrain 15 8 21 19 11 16 0 29 Lineated terrain 1 14 11 3 Unknown 25 8 8 7 5 11 33 29

Table 4.2. Corona stratigraphy. Percentage of materials in which coronae of different morphologies (see Fig. 5.1) have formed.

Complex/chaotic coronae tend to be found in materials that are interpreted as the oldest (tessera and lineated terrain) or have evolved during several geological episodes.

The majority of type 8 (basin) and 4 (rim encircling a low-lying interior), are found in regional plains. Type 1 coronae (domes), which are interpreted to be the earliest manifestation of plume development and have been consistently shown in numerical models (Squyres et al., 1992, Janes et al., 1992; Koch, 1994; Musser and Squyres, 1997;

Stofan and Smrekar, 1996; Smrekar and Stofan, 1998) to represent the first stage of corona formation, are found only in regional and lineated plains.

61 Coronae which comprise a topographie rim surrounding an interior below the level of the surrounding plains and an additional central depression only occur in regional plains

Summary

Few additional coronae were found in chains in the chasmata setting described by

Stofan et al. (1992). Stealth coronae were observed on the margins or near large topographic rises in small groups and, more commonly, associated with ridge belts to the northwest and southeast of the BAT region. Stealth coronae were also frequently found in isolation in relatively low-lying plains regions, where it was impossible to correlate them with any particular geological setting.

The majority of stealth coronae appear to have developed in a single stratigraphie horizon, most frequently regional plains materials. Fewer coronae were identified in plains materials that are older and which are locally overlain by regional plains. It was also noted that simple, possibly incipient forms, deform one unit, whereas more complex forms deform several. In cases where multiple units are deformed, there is often a lack of evidence, such as deformation annuli in the older material, to show whether any part of the corona predates more recent materials; and it is only possible to be certain that these coronae formed (or continued to form) after the emplacement of the regional plains. There is no distinct correlation between the morphologic types identified by the new survey and stratigraphie setting.

Interpretation of geological setting

Coronae occur in a variety of geological settings (Stofan et al., 1995). Coronae were found by the new survey in similar settings, in relative isolation in the plains, in clusters

6 2 and in groups on or near topographic rises. Coronae were identified by the new survey on the periphery of the BAT region. Most were seen on the ridge belts to the northeast

(northeast of Rusalka Planitia) and to the southwest of the BAT area.

location of stealth coronae on the ridges marginal to the BAT region indicates that

lithospheric thickness is sufficient for coronae to form but not for them to develop

intense fracture annuli and is consistent with the idea (Chapter 3) that lithospheric

thinning along chasmata may permit coronae to form preferentially. Thus stealth

coronae on these ridges may suggest that they are incipient chasmata or alternatively

that chasmata-forming processes have not led to sufficient lithospheric thinning for the

development of brittle fractures in the form of concentric annuli or of steep topography.

The morphology and lack of fracture annuli which is discussed in detail in Chapters

5 and 7, indicates that plume sources are unable to deform the lithosphere (hence the

low topography) or to form brittle deformation fractures. The prevalence of coronae,

however, suggests that the thinned lithosphere necessary for corona formation extends

beyond the core of the BAT region.

Coronae identified by the new survey in two distinct geological settings indicate that

coronae have been active on Venus since the early part of the interpretable geological

history of Venus until recent times. The earliest coronae previously identified are those

found on corona dominated rises (CDRs) including eastern Eistla Regio, which have

end member configurations comprising a rim surrounding a raised interior and model

ages of 300 Ma (Smrekar and Stofan 1997; 1998). The coronae found by the new survey

on Tellus Tessera may be much older. Tellus is an elevated region of intense crustal

deformation, embayed by regional plains. It may have formed in the same way as Alpha

Regio (Herrick and Phillips, 1990; Phillips et al., 1991) via mantle upwelling, as they

are similar in topography, morphology, terrain and gravity signature. Tellus Tessera

6 3 may, like Alpha Regio, be considered senescent, supported by a thickened crust. The existence of coronae on Tellus Tessera demonstrates that they can be preserved in upland regions and not necessarily or easily destroyed by tectonic processes. The geology of upland regions such as Tellus Tessera further suggests, besides the preservation state of coronae and lack of new structures, that corona forming plumes are no longer active beneath the region. The coronae of CDRs Eistla Regio and Themis are predominantly end stage coronae indicating that that in some other upland areas coronae are no longer forming.

Indicating that coronae have formed during more recent venusian history is their proximity to volcanic rises, as it suggests that corona formation is closely related to rise development. Coronae identified by the new survey, which lack intense brittle deformation were found near volcanic rises including Imdr and Sif; these rises are considered relatively youthful because lava from the edifices located on these rises overlies existing materials, including regional plains and mottled plains formed by repeated shield forming volcanism (Grimm and Phillips, 1992).

Summary

Coronae are found in clusters on or near topographic rises which are sites of mantle upwelling. Coronae which lack fracture annuli are more common on ridge belts (e.g. near Rusalka) and near small rises (e.g. Sif and Imdr Mons) and therefore may be interpreted as more youthful than end stage coronae which possess intense fracture annuli and which occur on chasmata or rises such as Themis and Eistla. Structures which are tentatively identified as highly modified coronae and which may represent early plume activity can be identified on Tellus and Alpha Regio.

6 4 Interpretation of corona stratigraphy

Although it is impossible to correlate stratigraphie types across the whole surface of

Venus, the units in which coronae form provide a broad indication of the relative age and regional history of corona development. No attempt is made, however, to extrapolate these units to any global stratigraphie system.

Detailed stratigraphie investigation of the coronae recorded by the 1992 survey was not carried out, but for purposes of comparison it is possible to draw on the studies of

Copp et al. (1997) and on the mapping the Scarpellini Quadrangle (Chapters 8 and 9).

Copp et al. (1997) studied a selection of coronae and found that they had protracted developmental histories. These results are corroborated by detailed stratigraphie analysis of the coronae of the Scarpellini Quadrangle. The abundance of coronae which appear to have undergone complex developmental histories contrasts with the simple stratigraphie configuration of stealth coronae, which generally deform the youngest locally interpretable unit and thus would appear to be more youthful than those comprising the original (1992) database.

When the relative proportions of materials deformed by coronae are considered, it is seen that the number of coronae is inversely proportional to the age of the units in which they occur. This may reflect the degradation of coronae forming in units which have become deformed and tectonised, but as the older units are limited in area, a trend of this kind is difficult to establish.

Complex coronae identified by the new survey might have been expected to deform multiple units, but many deform only regional plains materials, consistent with the modelling work of Smrekar and Stofan (1997), which demonstrates that even complex coronae may form over relatively short timescales 150 - 300 Ma. Some coronae therefore would appear to have begun to evolve soon after the emplacement of the

6 5 regional plains and continued to evolve. The observation of complex coronae deforming regional plains is therefore inconsistent with a thickening lithosphere as proposed by Phillips (1997). A thickening lithosphere ought to have halted corona development because coronae deform thin lithosphere (Smrekar and Stofan, 1997). The observation further suggests that stealth coronae which deform regional plains materials have yet to reach end stage morphologies.

In summary, the stratigraphie results are consistent with a thin lithosphere and the interpretation of stealth coronae as relatively youthful features which have yet to reach more complex end member morphologies, to develop an annulus of concentric fractures, or both.

66 Chapter 5

Morphology and dimensions

In this chapter, the morphologic characteristics of stealth coronae are described in detail. Height, width and form are considered and compared with those of coronae identified by the 1992 survey (Stofan et ah, 1992).

Morphology

The coronae of the expanded database were grouped into 12 classes according to morphology (Figs. 5.1 and 5.2). The most common are those v^th a rim encircling a low-lying interior (-28 %) and those with a topographic rim which encircles a central rise (-21 %). Complex and chaotic coronae are found infrequently but two important variants were found, which combined constitute -4 % of all coronae. The variants of group 10 coronae comprise a rim or trough which encircles terrain composed of fragmented concentric ridges or troughs. Unclassified (-4 %) coronae fall into none of the groups shown in Fig. 5.1 because their topographic signature was low, their morphology varied for part of their circumference, or it was hard to discern the effect on corona topography of adjacent features or terrain.

The coronae identified by the new survey typically comprise a topographic rim encircling a low-lying interior, though there exists a range of rim and trough configurations similar to that recorded by Stofan et al. (1992) and Stofan and Smrekar

(1997) representing various stages in the evolution of coronae.

The 1992 survey encompasses a greater range of coronae morphologies than the new survey. There are a number of corona morphologies that were not discovered during the survey of stealth coronae. A comparison of the different morphologies recorded by the

6 7 Profile Description % Fig.

1 Dome 3.48 2 —/------^ Plateau 2.96 3 y------^ Rim encircling a raised interior 13.74 6.1

4 Rim encircling a low-lying interior 28.00 7.5,7.6,7.8

4b Trough surrounding rim and basin 7.13 7.4,9.2

5 Rim surrounding central rise 20.70 5.5,5.7 Rim encircling basin with inner 6 6.09 9.4 rim and central depression 6b Two rims encircling a central rise 0.87

7 _ _ A A _ Rim only 6.43 5.5,6.2 8 Basin 5.22

9 ------Unclassified 4.17

10 Rim or trough encircling chaotic interior 3.13

Figure 5.1. Corona types. Topographie groups and percentage of the total population. Figure based on Stofan et al. (1997) with the addition of type 10. Types 4b and 6b are created to ensure continuity with the previous classification scheme. Type 3 have interiors above the level of the surrounding plains and type 4 have interiors below the level of the surrounding plains.

68 Vertical

Figure 5.2. Topographic profiles of coronae, shown to scale. Profiles were generated by calculating the mean width and mean height of each corona type.

30 □ 1997

25 m 1992

0 (N 00 o\ o

Corona type

Figure 5.3. Graph showing the relative proportions of coronae of different morphology identified by the survey of Stofan et al. (1992) and the 1997 survey.

69 Figure 5.4. Three dimensional perspective view of Atete Corona. The corona has a relatively complex morphology, comprising a trough which surrounds a rim and a central plateau (Type 4b). Atete is measures 600 km north to south and 450 km west to east and is located at latitude 16.0° S, longitude 244.0° E. Image scale is distorted by perspective but the 20 km wide data gaps provide a way of estimating the size of features.

Figure 5.5. Three-dimensional perspective view of Yavine Corona. Yavine was originally classified as an asymmetric corona. It comprises an irregular topographic rim which surrounds a central rise (Type 5). Yavine is approximately 500 km in diameter and located at latitude 5.0° S, longitude 248.5° E. The simulated viewing direction is northeast. Beyond Yavine to the northeast is a stealth corona (a) comprising a topographic rim (Type 7), and a central volcano. Image scale is distorted by perspective but the 20 km wide data gaps provide a way of estimating the size of features.

70 two surveys (Fig. 5.3) reveals that there are three morphologies that were found more frequently in the stealth survey. These are type 4 (rim encircling a low lying interior), 6

(rim encircling basin with inner rim and central depression), and 7 (rim only). Features containing complex terrain of discontinuous arcuate ridges and troughs were predicted

(Stofan, personal communication) but not identified in the previous survey. The new survey found these structures to be present when the coronae were re-surveyed using both altimetry and synthetic stereo images.

A significant number of arcuate ridge and trough fragments were found which did not meet the survey criteria, and were therefore not recorded. Although a proportion of these might be attributed to topographic alignments or volcanic activity unrelated to coronae formation, many of them may have been coronae, and the chronological population may accordingly be far higher than the modem record suggests.

The survey encountered many coronae which possessed a mixture of topographic forms. For example, some coronae, if found on the flank of a topographic rise might consist of a basin cut into the elevated topography which grades into a topographic ridge with distance from the elevated region.

Dimensions

Corona height and diameter recorded by the new and the 1992 survey were combined and plotted as histograms to evaluate the properties of the total corona population. Corona frequencies of the two populations were plotted as percentages of the total to allow a direct comparison to be made between the results of the two surveys.

Corona heights were taken from altimetry framelets and measured from the level of the surrounding plains or base level to the highest point on the corona, which in the majority of cases is the corona rim. The height distribution of all coronae is shovm in

71 Fig. 5.6a. Fig. 5.6b shows corona heights for the 1992 and 1997 as percentages of each

survey population. There are two main differences between the two populations. The

coronae of the 1992 survey have a mean height of just over 1,000 m, the coronae of the

1997 survey, 750 m. Whilst the height distribution of the 1992 survey exhibits a fairly normal distribution, the population of the 1997 survey is heavily skewed towards lower

mean corona height; the majority of 1997 survey coronae (70 %) have heights of

between 250 m and 1,000 m.

Those coronae recorded by the new survey with heights greater than 2,000 m are

large coronae that were missed by the 1992 survey, that lay within data gaps, or that

simply did not meet the original survey criteria because they lacked a distinct tectonic

annulus.

Width was measured between opposing points on the outer margins, where the

topography of the trough, rim or slope is first discernible. This should give widths

comparable to, but slightly greater than, the 1992 survey, which used the extent of the

fracture annulus to define the width. The mean diameters of asymmetrical or elliptical

coronae are used in this analysis. Fig. 5.7a shows the population distribution of the

diameter of all the coronae listed by the 1992 survey and the new surveys. Fig. 5.7b

shows corona diameter as percentages of each survey population. The width

distribution of the two populations is very similar. They exhibit comparable numbers of

coronae at all widths and a similar degree of skewness toward smaller corona diameters.

Large numbers of coronae in both populations have diameters of 150 - 200 km. Both

population distributions are normal but have a greater than expected number of coronae

in the 200 - 250 km size range.

7 2 (a) Corona height (all coronae)

140

120 --

100 -- §

80 --

60 --

40 --

20 --

Ü L _CL o o o o o o o o in in in in m in in in (N r- (N r- (N r- «N r~- (N fN (N m m "4-

Corona Height

(b) Corona height (1992 and 1997 surveys)

25 0c 1 3 20 -- CL 0 1997 Survey o Cu

15 -- 1 I 1992 Survey

10 Ü o,

0 j£L h ■Ilf-liH I—n _| CH C=L- 0 500 1000 1500 2000 2500 3000 3500 4000 4500

Corona height

Figure 5.6. Corona height distribution of (a) total population, and (b) the survey of Stofan et al. (1992) compared with that recorded by the new survey.

73 (a) Corona width (all coronae)

Corona width

(b) Corona width ( 1992 and 1997 surveys)

I 1997 Survey

I I 1992 Survey

S 10 --

» o o o o o o o o o o o o o o O o in o in o in o m o in o in ^ (N (N m m -rt Tf in in l O

Corona width

Figure 5.7. Corona width distribution of (a) total population, and (b) the survey of Stofan et al. ( 1992) compared with that recorded by the new survey.

74 Very large coronae, over 750 km in diameter, are not shown on these histograms.

The 1992 survey has 4 coronae over 750 km in diameter and the 1997 survey 11 coronae greater than 750 km in diameter.

Height and width by morphology

For each corona type in each survey, average width (Fig. 5.8) and relief (Fig. 5.9) were plotted to compare and assess morphological variation between coronae which do and those which do not have an annulus of closely spaced fractures.

The widths of coronae of the two surveys are broadly comparable but some differences between the two survey populations were noted. Types 2 (plateau) and 3

(rim encircling an elevated interior) in the 1997 survey have smaller widths than those of the previous survey. Coronae identified by the 1997 survey which have greater widths include the types: 4b (trough surrounding a ridge and plateau), 6 (rim surrounding a basin with central rim and depression), 7 (rim only), 9 (unclassified), and

10 (rim or trough that surrounds a chaotic interior).

It was expected that the relief of coronae of different morphology of the 1997 survey would be consistently lower than that recorded by the 1992 survey. This was however, found not to be the case. Some coronae identified by the 1997 have greater heights than those recorded by the earlier survey, such as those which comprise a trough surrounding a ridge and central plateau. Others have similar heights including types 4 and 6. The remainder have heights several hundred metres lower than coronae identified by the 1992 survey.

In summary, coronae widths for the two surveys are largely similar, although the new survey finds a number of coronae types that have significantly greater widths than those identified by the original survey. In terms of height, most coronae identified by

7 5 □ 1997 m 1992

- a

Figure 5.8. Mean width of coronae of different morphology for the 1992 survey and the new survey.

1600 □ 1997 1400 n 1992 200

1000 OD 'S 800 I 600

400

200

0 (N X) oo Os o

Figure 5.9. Mean height of coronae of different morphology for the 1992 survey and the new survey.

76 the new survey are lower in height (type 4b being the main exception), but several

coronae types were found to be of comparable height. This similarity indicates that

corona relief is less clearly related to the presence or absence of brittle scale deformation

than previously thought.

Altitude and form

There was no distinct correlation observed between corona altitude and form (Fig.

5.10). There is no indication that the population can be differentiated morphologically

(and therefore chronologically) on the basis of altitude. Broad morphological types,

however, seem to exhibit slight variation in mean altitude. Domes and basins occur at

high mean altitudes but coronae with morphologies of intermediate complexity,

including coronae which comprise a rim encircling a plains and rim only coronae, have

low mean altitudes. There is a slight preponderance of more complex forms at higher

altitudes. A possible reason is that they are older coronae which have survived

inundation by lavas at lower altitudes.

Altitude and relief

The relationship between corona altitude and corona height was investigated. Fig.

5.11 shows these variables plotted. The trend observed is towards increasing corona

height, with altitude, for which a high degree of correlation is observed (r = 0.81). The trend is more clearly seen in the 1992 survey, where coronae height shows a more rapid

increase with altitude than the 1997. At very high altitudes, however, lower mean

corona heights are observed, possibly because there are too few coronae for an adequate

sample.

7 7 6051.95 -- □ 1997 J 6051.85 -- m 1992 6051.75 -- a 6051.65 -- 6051.55 --

6051.45 --

6051.35 --

6051.25

Figure 5.10. Mean altitude of each corona type.

7000

6000

5000 O both

4000 j'5C

3000 ...0 .O

2000

1000

0 6048 6050 6052 6054 6056 6058

Altitude

Figure 5.11. Mean height of coronae at altitude for each survey and expanded corona database.

78 Summary

In summary, coronae which lack an annulus of concentric fractures exhibit a similar range of morphologies to those recorded by the 1992 survey and those which do have a well defined fracture annulus. The stealth population, however, tends to consist of morphologically less complex coronae.

The survey identifies features v^th unusual characteristics, notably, coronae which share rim topography. Also identified were coronae with complex interior topography, which were predicted by the modelling work of Smrekar and Stofan (1997) but had not been previously described

Morphometric analysis of stealth coronae shows that they are significantly lower in height than those recorded by the 1992 survey, though some morphologic types are of comparable height. In terms of width the population distributions are similar. The coronae of the new survey, however, appear to have slightly greater diameters.

Discussion

Morphology

The range of morphologies found is similar to that outlined by Smrekar and Stofan

(1997). A slightly more diverse range was recorded by the 1992 survey since it is a larger sample and includes most of the coronae that are easily detectable in SAR images.

The population identified by the new survey encompasses a range of coronae, but fewer which have complex rim/trough configurations, the most commonly found comprising a single encircling rim. Nearly twice as many type 4 (rim encircling basin) coronae were found by the 1997 survey as by the 1992 survey as a proportion of the respective totals, and nearly five times as many type 7 (rim only) coronae. This observation shows that

7 9 the common morphologic types which lack a fracture annulus are not fully accounted for by the previous survey.

Type 8 coronae (rim encircling a basin containing an interior rim) represent only a slightly higher percentage of the 1997 population than the 1992. The percentage of all other types appears to be greater in the 1997 population. This in part may be due to a difference in the detection level of certain types of corona. Domes, plateaux and basins, for example, which lack the degree of fracturing observed in the 1992 survey are harder to detect. Furthermore, domes which are characterised by radial fractures may resemble novae, central volcanoes which are sometimes dissected by fractures (Head et al., 1992).

Some domes therefore may be erroneously ascribed to volcanic activity.

There is evidence from modelling work and observation (Stofan et al., 1992;

Squyres et al., 1992; Smrekar and Stofan, 1997) which suggests many coronae go through a series of developmental stages exhibiting very different topographic signatures. Using additional stratigraphie and tectonic evidence, it is possible to arrange these coronae into an evolutionary sequence. Such a sequence categorises simple forms as representative of the initial stages of corona formation and more complex forms as the later stages. It may be, however, that not all coronae go through all theoretical stages of evolution and that not every coronae will eventually develop a complex end- stage morphology.

The new survey identified a new class of structure: complex and chaotic coronae, which comprise a rim or trough surrounding an interior of fragmentary concentric rims and troughs. They appear to be end-stage coronae that have undergone several episodes of uplift and subsequent relaxation. The fragmentary nature of the interior rim and trough series could be attributed to (1) the deformation of an inhomogeneous overburden or (2) multiple phases of corona development in a deforming crust.

8 0 Consistent with the modelling results of Smrekar and Stofan (1997) is the finding within the expanded database of a clear correlation between size and complexity. This

suggests that larger coronae have longer histories and smaller coronae are more likely to be incipient.

Height

All corona types recorded by the 1997 survey have relief of between 50 % and 66 %

that displayed by corona morphologies of the 1992 survey. This result shows that

coronae with lower topography lack an intense fracture annulus irrespective of type,

implying that the presence of a fracture annulus is not dependent on evolutionary stage

and that other factors such as lithospheric properties determine whether coronae develop

an annulus of fractures.

Width

The widths of coronae of the two surveys are comparable, with only slight variation

between the morphologic groups recorded by each survey (Fig. 5.5). Domes and

plateaux recorded by the 1997 survey have smaller apparent widths. This could be due

to the way in which the coronae were measured. Stofan et al. (1992) measured corona

width from the outermost extent of fracturing, whereas in the 1997 survey width was

measured from the outermost topographic expression of the corona. In a domical

feature, plateau or basin, the radial fractures could propagate into the surrounding plains

beyond the region where topographic deformation is discernible, and thus have greater

measured widths. In coronae of type 3 (rim surrounding an elevated interior, 4 (rim

surrounding a low-lying interior), and 5 (rim surrounding a central rise), the concentric

fractures normally occur on the rim topography, and topographic expression extends

81 beyond the zone where fractures are concentrated; thus the coronae of the 1992 survey

appear to have smaller widths. The slight differences between broadly comparable populations are entirely accounted for by the difference in the data collection technique.

The comparable corona widths suggest that the mantle upwellings responsible for

forming these structures are also of comparable size. The coronae of the 1997 survey

J5 (i.e. those which lack an annulus) emerge not^a class of small coronae but rather as

coronae which have low height to width ratios.

From comparison of height data, coronae of the 1992 population and all coronae of the 1997 survey population (Fig. 5.6), it appears that coronae of the 1997 population are

lower in height. But when coronae are distinguished on the basis of their morphology

(Fig. 5.9) not all coronae types identified by the new survey are lower in height.

Although some coronae had markedly lower heights, some coronae, including

frequently observed coronae (Type 4) had heights which were comparable with coronae identified by the 1992 survey. The relationship between the presence of brittle deformation in the form of a fracture annulus (the principal characteristic of the 1997 population) and height, whereby coronae exhibiting greater broad scale deformation also exhibit greater brittle scale deformation, does not hold true. Coronae with comparable heights are seen to exhibit a wider range of associated tectonic styles than first thought.

The relationship between topography and tectonic pattern is examined in more detail in

Chapter 7.

8 2 Chapter 6

Volcanism associated with coronae

In this chapter, volcanism associated with stealth coronae is described and the degree of volcanism compared with that of the coronae recorded by Stofan et al. (1992).

The relationships observed between the degree of volcanism observed in stealth coronae and morphology and between volcanism and altitude are then discussed.

Although stealth coronae in general have a lesser degree of volcanism associated with them than the 1992 survey coronae, a complete range of volcanic forms and styles is observed comparable with those recorded by previous surveys (Stofan et al., 1992) and analyses (Stofan and Head, 1990).

The most commonly observed volcanic phenomenon associated with stealth coronae are flows which have erupted from the topographic rims. Superimposed radar-bright and radar-dark flows (Fig. 6.1) are frequently observed and indicate that lavas have been erupted in distinct episodes. Some eruptions can be traced to a single vent. In general, however, flows which flank coronae have no traceable source and are assumed to originate at fractures located on the summit of the corona rim. Central volcanoes are more commonly associated with larger coronae and their flows sometimes flood the corona interior. In addition, edifice fields which comprise small volcanic cones and shield volcanoes (about 5 km in diameter) are sometimes associated with coronae in dense clusters on corona rims and interiors (Fig. 6.1). Lava flows associated with small volcanoes may coalesce, as seen clearly on Ma Corona (Chapter 9), and form an extensive layer. Larger volcanic domes and scalloped margined domes (SMDs) are less often found in proximity to coronae.

83 The scheme used to assess the degree of associated volcanism was described fully in

Chapter 2. To siunmarise: stealth coronae which are dominated by volcanism will display extensive flows which have erupted from their rims or a central volcano, and may also be associated with numerous small edifices. Fig. 6.1 shows a stealth corona dominated by volcanism. Fig. 6.2 shows a coronae with a moderate amount of volcanism. Although this corona may initially appear to have little or even no volcanism associated with it, closer examination reveals lavas which have erupted from the west rim onto the surrounding plains and from the northwest rim into the corona interior. Located within the interior of the coronae are numerous small ( < 5 km) volcanic shields and cones.

Figs. 6.3a and 6.3b show the relative amount of volcanism associated with each type of corona for both the 1992 and 1997 surveys. Clearly the main difference between the two survey populations is the amount of volcanism irrespective of type. The coronae of the 1992 survey in general display a moderate amount of volcanic activity whereas the coronae of the 1997 survey, on the whole, exhibit little or no volcanism.

Volcanism and morphology

No clear trend exists between the degree of volcanism and corona type (Figs. 6.3a and 6.3b), and complex forms or end stage coronae appear on average to have as much volcanism as incipient forms such as domes and plateaux. Examination of the database shows that most complex/chaotic coronae are dominated by volcanism. There are, however, complex/chaotic forms which have a far lesser degree of volcanic activity associated with them, while there are simple forms, such as domes, basins and plateaux, which have a moderate to high amount of volcanism. Type 7 coronae (rim only) are associated with relatively little or no volcanism in both the 1992 survey and the 1997

8 4 - 9.5

- 10.0 °

- 10.5

307 . 0° 307 .5° 308 .0° 308 . 5° 309.0 ° 309. 5°

Figure 6.1. Corona identified by the new survey dominated by volcanism. Radar-bright flows have erupted from the corona rim. Dense clusters of small volcanic shields and cones overlie the corona rim and interior and surrounding plains.

H o

27 .0 °

Figure 6.2. Corona identified by the new survey with a moderate amount of associated volcanism. Radar-dark lavas which have erupted from the west rim have flowed onto the surrounding plains. Lavas erupted from the northwest rim have flowed into the corona interior. Small ( > 5km ) volcanic edifices can be seen in the corona interior.

85 (a) Volcanism: coronae of the 1992 survey

E "c _o

3c O E <

2 3 4 4b 5 6 6b 7

Morphologic type

Volcanism: coronae of the 1997 survey

Ic s 0 > "O 1 o

c 3 o E <

4b 5 6 6b 7

Morphologic type

Figure 6.3. Relative amount of volcanic activity assiciated with coronae of each morphologic type of: (a) the 1992 survey of Stofan ct al. (1992) and (b) the 1997 survey. 1 - little or no volcanism. 2 = moderate amount of associated volcanism. 3 = dominated by volcanism.

86 survey. Type 2 coronae (rim about a low-lying interior) are also relatively poor in volcanism.

Volcanism and altitude

Analysis of the relationship between degree of associated volcanism and corona altitude was carried out for each survey. Corona altitudes were extracted from topography data, and for each contour band the average degree of volcanism was calculated (Fig. 6.4). The first important difference to note between the two populations is that the coronae of the 1997 survey at altitudes between 6048 m and 6052 m have less volcanism than the 1992 survey. More striking, however, is the change exhibited by both populations in the degree of volcanism with altitude. It can be seen that the coronae of the 1992 survey have progressively lower amounts of volcanism with increasing altitude. The amount of volcanism falls slowly at intermediate altitudes

(6051 km - 6053 km) but at higher altitudes (exceeding 6053 km) there is markedly less volcanism. The coronae recorded by the 1997 survey, however, have increasing amounts of volcanism at higher altitudes, the amount of observed volcanism increasing rapidly from moderate amounts of volcanism at 6052 km to a tendency for coronae to be dominated by volcanism at altitudes over 6053 km.

Summary

The data collected by both surveys are comparable. Compressed images (Cl photo products) were used in the new survey to ensure consistency in the expanded corona database. But because compressed images (with a pixel resolution of 225) were used, the survey results are likely to underestimate volcanism associated with coronae. In addition to lower resolution, the contrast between flow units may not be the optimum

8 7 Degree of associated volcanism v. altitude

3.00

2.75

2.50 Both

2.25

2.00

1.75

.50

OX) 1.25

1.00

6048 6049 6050 605 6052 6053 6054 Altitude (planetary radius)

Figure 6.4. Average degree of associated volcanism at altitude for each data set. 1 = Little or no associated volcanism. 2 = Moderate amount of associated volcanism. 3 = Dominated by volcanism. for distinguishing between flows which may have very similar backscatter properties

(Fig. 6.2). Assessing the amount of volcanism is difficult because some of the styles are more readily apparent and it is sometimes hard to be sure whether volcanic landforms are in fact related to a corona.

Further studies of volcanism attributable to coronae should employ full resolution digital images, because contrast enhancement techniques can then be used to discern between units and identify flow fronts. Nevertheless, despite the limitations of the Cl products, an indication of the relative amounts of volcanism was obtained which gave some useful insights into the nature of volcanism associated with coronae.

Coronae have been previously described as structures with abundant volcanism

(Stofan and Head, 1990; Stofan et al., 1991). The coronae of the expanded database exhibit a greater range in the amount of detectable volcanism, and there are many more coronae which have little or no volcanism associated with them than previously thought.

On the whole, stealth coronae have less volcanism associated with them than the coronae identified by the 1992 survey and sometimes none. A few coronae were found to be dominated by volcanism. Furthermore, those coronae which had large amounts of volcanic activity displayed a range of volcanic structures comparable to that previously observed of the coronae identified by the 1992 survey.

Two important trends were identified by plotting altitude against average associated volcanism: (1) the 1997 coronae have greater amounts of associated volcanism with increasing altitude and (2) for the 1992 survey population the converse is true.

Coronae identified by the new survey which lack an intense fracture annulus, as stated in Chapter 5, are lower in height than coronae recorded by previous surveys. The conditions under which these coronae form appears also to mean less volcanism. The

8 9 lower height of coronae has been ascribed to a thicker lithosphere, where rising coronae forming plumes are less able to deform the surface. The implications for volcanism are twofold: (1) less surface deformation will mean that there are fewer and smaller pathways for magmas to exploit, and (2) pressure-release melting at plume boundaries is less effective because the greater overburden means that the plume is (relatively) deep seated. A third possible implication from the modelling results of Smrekar and Stofan

(1997) is that the chemically (Fe) depleted mantle which results in coronae having a low topographic signature is more viscous and less easily penetrated by partial melts.

Altitude and volcanism

Coronae of the 1992 survey have a greater degree of associated volcanism at low altitudes. This is consistent with the suggestion that atmospheric pressure on Venus plays an important role in determining the amount of material discharged from volcanoes. Head and Wilson (1992) predict greater volcanic outpourings at lower altitudes and large deep magma reservoirs and less volcanism at high elevations, because high atmospheric pressures will reduce volatile exsolution and tend to prevent the formation of neutral buoyancy zones (NBZs) which stall magma movement.

Curiously, the 1997 survey population shows an increase in the amount of associated volcanism with altitude. Bearing in mind that the coronae identified by the 1992 survey are lower in height, they therefore may have NBZs which are less deep seated (relative to corona rim); so that at comparatively high altitudes they have more associated volcanism than structures which have more pronounced topography and thus a relatively deep NBZ. Bearing in mind that they have less brittle deformation associated with them, they therefore have fewer alternative pathways for magmatic intrusion and dyke propagation, and volcanic activity is thus more concentrated. Several factors may be

9 0 producing the trend in the stealth population towards lower amounts of volcanism at altitude, the converse of the 1992 population. To begin with the chemically depleted mantle in low lying areas where stealth coronae are observed could mean that magmas are more viscous.

If magma chambers were associated with coronae at low altitudes with low topography (i.e. the 1997 population), we would be able to detect them. Atmospheric pressure is even greater at the low altitudes where these coronae are found. The greater atmospheric pressures imply greater confining pressures and reduced exsolution of volatiles, possibly sufficient to inhibit NBZ formation; therefore magmas do not stall because they do not encounter a neutral buoyancy zone. A number of plausible reasons exist for the paucity of volcanism at such altitudes: (1) in these regions, very low topography is possibly due to a thickened crust, which inhibits volcanism; alternately (2) lower topographies may be caused by less vigorous upwelling, in which case they are accompanied by lesser amounts of pressure-release melting.

Table 6.1 and Fig. 6.5 summarise the relationship between volcanism and altitude and possible interpretations for (a) the 1992 corona population (Stofan et al., 1992) and

(b) the new (1997) survey.

Summary

The relationship between volcanism and altitude is a complex one. Many factors will determine the style and amount of volcanism observed. Exogenetic factors or environmental factors such as lithospheric properties, geological setting and atmospheric pressure may play a part as may endogenetic factors relating to the plume and coronae formation process such as the degree of resulting brittle deformation, plume intensity and plume modification.

91 FRACTURE ANNULUS LACKING FRACTURE ANNULUS

1992 Survey population 1997 Survey population

High altitude (less volcanism) High altitude (more volcanism)

Atmospheric Vent Atmospheric pressure Fracture pressure annulus More associated volcanic activity

HIGH TOPOGRAPHY Dike NBZ system Magma chamber

Low altitude (more volcanism) Low altitude (less volcanism)

Atmospheric Atmospheric Low topography pressure More associated pressure volcanic activity Little or no volcanism

Magma chamber at shallow LOW depth relative to altitude TOPOGRAPHY

Figure 6.5. Large amounts of volcanism are associated with coronae at low altitudes (1992 population) and at high altitudes (1997 population). The 1997 population lack annular fractures, which reduces magma dispersal; they probably have relatively shallow magma chambers. At low altitudes volcanism is inhibited by the lack of pressure-release melting in topographically low coronae. Lithospheric thickness probably plays the principal role in determining volcanic

activity by limiting crustal deformation. However, the corona rim height achieved may

also be an important factor in determining the degree of volcanism because it affects the

relative depth of the magma chamber. The 1997 survey coronae at high altitudes

(exhibiting volcanism) have significantly lower topographies than their fractured

counterparts.

Extensive volcanism caused by the further inhibition of the exsolution of volatiles at

very low altitudes is not observed perhaps because the coronae with low height/width

ratios at these altitudes lack magma chambers altogether.

(a)

1992 Population: characterised by elevated topography and intense fracture annuli

Observation Interpretation At high altitudes low amounts of Higher rim topography Deeper magma chamber volcanism detected More deformed More opportunities for magma dispersal by dykes

At low altitudes high amounts of Lower rim topography Shallower magma chamber volcanism detected Less deformed Fewer opportunities for magma dispersal

(b)

1997 Population: characterised by low topography and lack of fracture annuli

Observation Interpretation At high altitudes high Rim height (relative to Shallow magma chamber (relatively) amount of volcanism 1992) low detected

Less deformed Fewer opportunities for magma dispersal At low altitudes low and zero Lack rim deformation May lack a magma chamber volcanism detected

Table 6.1. Summary of volcanism observed in each population and interpretation.

9 3 Chapter 7

Tectonics associated with coronae

In this chapter the results of analysis of stereo pairs and regional studies, including the mapping of Scarpellini (Chapters 8 and 9), are used to build a picture of the tectonics associated with stealth coronae and, in conjunction with morphological information (Chapter 5), to study the relationship between morphology and structure.

It will be recalled that coronae were originally described as morphotectonic structures characterised by an annulus of concentric fractures (Stofan and Head, 1990;

Stofan et al., 1992; Squyres, et al., 1992). Stofan et al. (1992) found a few coronae which lacked fracture annuli, including the large double corona comprising Cerridwen and Neyterkob (Figs. 7.1 and 7.2), but in general coronae lacking a fracture annulus (e.g.

Figs. 7.4 and 7.5) were not incorporated into the early database.

The original classification scheme was based partly on morphology and partly on tectonic configuration. The different tectonic styles of corona that the scheme encompassed were: (1) radially fractured; (2) possessing a concentric fracture annulus;

(3) possessing a concentric fracture annulus and with radial fractures; (4) with a double annulus (two sets of concentric fractures). The survey presented in this work distinguishes between coronae on the basis of their morphology (Fig. 5.1), partly because first-order deformation (morphology) provides a better indication of stage in their evolution and partly because the variety and intensity of the tectonics associated with coronae vary so greatly.

Most stealth coronae have second order deformation structures associated with them

(Table 7.1 and Fig. 7.3). Although 97 % of coronae surveyed lacked a well defined tectonic annulus, only 14 % lacked a tectonic structure that could be attributed to corona

9 4 Figure 7.1. Cerridwen (left) and Neyterkob (right) coronae. The image is centered on 50.0° N, 202.0° E. This double corona, with easily discernible morphology, was identified in the 1992 survey (Stofan et al., 1992). The coronae have fractures aligned with their rim topography, but lack the intense concentric fractures which were previously thought to characterise coronae.

Figure 7.2. Detail of the eastern rim of Neyterkob Corona. The browse image above (7.1) shows radar-bright rims, but the full resolution framelet shows that these are caused by wrinkle ridges and scarps, not an intense fracture annulus. Wrinkle ridges are seen in the surrounding plains units too and are embayed by lavas which have erupted from the corona rim.

95 Radiating arcuate fractures Reticulate network aligned with topography

Braided network aligned Concentric wrinkle ridges with topography

Radial fractures Well defined tectonic annulus

Partially aligned concentric Aligned concentric fractures fractures and radial fractures and radial fractures

Partially aligned Aligned concentric fractures concentric fractures

Figure 7.3. Different types of tectonic structure associated with coronae identified by the new survey. The sketch maps do not show the relationship of the tectonic pattern to morphology to preserve clarity; concentric fractures tend to be aligned with coronae rim or trough. The shaded circle represents the corona interior.

96 50.7°

50 .2 '

49. 7 '

4 9.2° -

194.4 194.9°

Figure 7.4. Stealth corona. The rim is easily discernible, highlighted by co-aligned fractures and the dark radar return of rim backslopes.

- 2.0

- 3 .0°

-4 0

- 5.0

Figure 7.5. Example of a stealth corona lacking tectonic structures clearly ascribable to corona formation. Stealth coronae like these are hard to detect using SAR images alone. Arrows indicate the outer margin at the base of the topographic rim. To the southeast is a radar-bright region inferred to have resulted from a failed impact.

97 development. Stealth coronae exhibit a diversity of tectonic patterns, but they have deformed the surface to a far lesser extent than the coronae described by previous investigators.

Tectonic style Percentage Aligned concentric fractures 29.5 Partially aligned concentric fractures 29.5 Aligned concentric fractures and radial fractures 19.5 Lacking in associated tectonic structures 14.5 Partially aligned concentric fractures and radial fractures 4.5 Well defined tectonic an n u lu s 3.5 Radial fractures 2.5 Braided network aligned with topography 2.5 Concentric wrinkle ridges 1.5 Reticulate network aligned with topography 1.0 Radiating arcuate fractures 1.0

Table 7.1. Types of tectonic pattern associated with coronae and the percentage of stealth coronae which exhibit them.

Concentric fractures and fractures that are partially aligned with the corona, that is concentric to the coronae for the greater part of their length, are found in -75 % of stealth coronae. Concentric fractures occur at each break of slope, and radial fractures extend from the centre of the feature, terminating at the outer edge of the topographic rim. In some cases the concentric fracturing takes the form of a series of arcuate fractures that are aligned with the encircling topographic rim. Such a pattern is also common in many coronae recorded by the 1992 survey (McGill, 1993). In many stealth coronae, however, all that can be seen is a single, sometimes discontinuous fracture, 120 m to 240 m (± 60 m) across, which is located either on the break of slope at the outer edge of the topographic rim or on the corona rim itself. They are hard to see in compressed photo-products, partly ovsdng to the low resolution (225 m / pixel), partly owing to the lack of contrast in some images. Digital images and full resolution photo products helped to identify faint irregular fractures associated with stealth coronae.

9 8 A further 27 % of stealth coronae were associated with tectonic structures other than concentric fractures. Nearly a quarter displayed radial fractures, which were often pronounced and extended far into the adjacent plains. A number of different styles of radial fracturing were encountered (Figs. 7.6 and 7.7). In one case, it was possible to discern radial graben. Associated with some coronae were arcuate fractures which extended radially from the coronae rim into the plains. Coronae displaying this form of tectonic deformation possessed two bunches of fractures, their foci located at opposite sides of the corona (Fig. 7.7).

Braided fractures and reticulate networks which are aligned with corona topography were also found. Braided patterns of tectonism were found in 2.5 % of coronae. They consist of two or three irregular fractures which intertwine. Reticulate structures were found in 1 % of coronae and consist of interconnecting fractures which form a network greater in extent than braided fractures. In most cases these networks were found on the topographic rim of coronae, but a number of coronae had reticulate fracturing concentrated in the interior.

The coronae which exhibit greater tectonic complexity (2 %) do not fall into any one of the categories represented in Fig. 7.3. They have a variety of tectonic deformation styles associated with them which may include concentric and radial fracturing but also feature radial or arcuate graben and/or wrinkle ridges in the surrounding materials.

Some of the more unusual tectonic styles observed in the search for stealth coronae include coronae with networks of fractures aligned with the corona topography (e.g. C2,

Chapter 9) and coronae with a fracture pattern consisting of radial and concentric wrinkle ridges contiguous with the compressional ridges in the surrounding plains (e.g.

C3, Chapter 9).

9 9 1 ' ^ ^ ' .1". î;:;^

W.. ,

Figure 7.6. SAR image of a feature previously termed 'arachnoid' and classified separately from coronae. Such structures are here generally considered to have formed in the same way as coronae and are thus incorporated into the expanded database as type 4 coronae (rim encircling a low-lying interior). This example is centered on 40.0° N, 18.0° E. Indicated aie; (a) partially aligned concentric fractures; (b) a well defined annulus; (c) radial fractures (in this case, cross-cut by concentric fractures); and (d) a reticulate network associated with rim topography.

100 mmm

Figure 7.7. Corona identified by the new survey. The corona, located at 45.0° S, 195.0° E, has a well defined concentric fracture annulus. Associated with the corona are wrinkle ridges located in the plains but concentrated at the north and south margins of the corona, and attributable to the effect of the corona on the regional stress field.

Figure 7.8. Image is centered on the corona located at 9.5° S, 69.0° E. The fracture pattern (a) associated with the corona is considered to have formed by the effect of the corona on the regional stress regime (McGill, 1993). Concentric fractures associated with corona development are located at (b) at the interior rim and (c) in the surrounding plains. Lava flows with moderate backscatter overlie concentric fractures associated with the rim, mostly to the north and south of the corona. Two other coronae, (d) and (e), are identified in the image.

101 In summary, although stealth coronae lack an annulus of intense fractures, most of them are associated with some tectonic structures. The most common tectonic element is a single irregular fracture aligned with the inner or outer rim of the corona.

Tectonics of stealth coronae in the Scarpellini Quadrangle

Mapping results from the Scarpellini Quadrangle are consistent with the findings of the global survey. Extensive brittle deformation appears to be absent and they lack a well defined fracture annulus. It may be that fractures associated with these features are simply difficult to detect because they are too narrow to be visible on the SAR data or because they have a muted surface expression that makes them indistinguishable from the surrounding material. Fractures are commonly overlain by volcanic materials, or by surficial debris associated with impact craters. Nonetheless most stealth coronae have some detectable brittle deformation associated with them, usually in the form of small scale fractures which partially encircle the corona at the break of slope between the rim and the surrounding plains.

Interpretation of stealth corona tectonics

Most stealth coronae are associated with some tectonic structure even though they lack an annulus of closely spaced fractures. The most commonly observed fracture configuration consists of a few irregular fractures aligned with the rim on the break of slope. They may be related to rim development but could also be attributed to topographic loading and flexure of the surface when the rim is in place (Sandwell and

Schubert, 1992). The large number of fractures of this kind which propagate into the plains rather than being concentrated on the corona rim point to topographic loading as the key process. Wrinkle ridges partially aligned with coronae (Fig. 7.8), resembling

1 0 2 those occurring in proximity to large coronae (McGill, 1993; Cyr and Melosh, 1991) occur probably where coronae have interfered with the regional stress field causing otherwise linear ridges within the plains to bend round the corona. The bunches of arcuate fractures at opposing margins and extending into the plains resemble those generated by the compression occurring around a hole (Savin, 1961). They are concentrated at the points of maximum compression and are aligned parallel to the minor axis.

Some coronae display a network of fractures (Chapter 9) consisting of a series of wrinkle ridges radiating from the centre which intersect with single irregular wrinkle ridges aligned with the topography. This pattern, which has not been described previously, may indicate the following mode of formation: (1) uplift of the plains surface gives rise to radial extensional fractures, (2) development of rim topography causes fracturing at the rim apex, and (3) topographic loading causes fractures to develop at slope margins. Lateral compression (4) in the region causes wrinkle ridges to form in the surrounding areas and where the radial and concentric extensional fractures had formed during corona development.

Many tectonic structures occur on coronae which are unrelated to corona development. These structures often help to identify coronae which lack intense annular fractures. For example, wrinkle ridges appear to curve when they cross cut topographic rims, creating a pattern of ripples which help to delineate topographic rims. Again, it is not always possible to determine whether wrinkle ridges post-date or pre-date the corona itself, but as deformation is likely to follow lines of weakness the wrinkle ridges in linear swarms probably pre-date corona development.

Factors which may vary regionally such as lithospheric properties or lithospheric thickness can prove more important in the formation of brittle scale annuli than the

103 establishment of fractures during the growth of corona topography (first order deformation) and flexure. Compression of the lithosphere is capable of producing concentric fractures (McGill, 1993; Cyr and Melosh, 1991). In some cases regional stresses of the lithosphere have led to the only visible tectonism associated with coronae. This suggests that regional lateral movements of the lithosphere are more important to annuli formation than previously thought and may be the key to understanding why some coronae of similar morphology and dimensions are associated with very different patterns of tectonism.

Summary

Brittle deformation in stealth coronae is less intense than in other coronae, owing in part to the lower degree of first order deformation, but is still common. Less extreme first order deformation in a slightly thicker lithosphere almost certainly implies less brittle scale fracturing and is consistent with the models of Cyr and Melosh (1991) which required a very thin lithosphere to recreate the fracture configurations observed in venusian coronae. Coronae which lack intense fracture annuli are more common in regions where the ridge belts suggest the lithosphere to be thicker.

There is some evidence that the regional stress regime accounts for at least some variation in tectonic style and that brittle scale deformation under plume development and flexure of the surface is not necessarily the most important factor in determining the amount of concentric deformation. Lateral compression may lead to the formation of partially concentric annuli, arcuate radial fractures and the development of wrinkle ridge networks.

104 Chapter 8

Geological mapping of the Scarpellini Quadrangle

This chapter describes the geology of the Scarpellini region (Fig. 8.1) before proceeding to examine in detail its coronae. The names of the key features and their location in the quadrangle are given in Fig. 8.2; their size and location are given in

Table 8.1; and a sketch map of the geology is presented as Fig. 8.3.

Name Feature Latitude Longitude Diameter lAU Status r s ) n (km) Rae Crater 9.0 58.5 5 Proposed Evika Crater 5.0 31.4 16 Approved Vashti Crater 6.8 43.7 15 Approved Valerie Crater 6.4 31.0 10 Provisional Munter Crater 15.3 39.3 36 Approved Fatima Crater 17.8 31.9 15 Approved Michelle Crater 19.5 40.4 14 Approved Medhavi Crater 19.5 40.6 30 Approved Magdelena Crater 12.0 57.5 10 Provisional Gillian Crater 15.2 49.9 16 Approved Georgina Crater 20.3 58.6 5 Provisional Recamier Crater 12.0 57.5 24 Approved Laurencin Crater 15.4 46.4 30 Approved Bathsheba Crater 15.1 49.3 36 Approved Scarpellini Crater 23.4 34.4 26 Approved Mukylchin Corona 12.5 46.0 375 Proposed Juksakka Corona 19.5 44.5 300 Proposed Thermuthis Corona 8.0 33.0 330 Approved Nabuzama Corona 8.5 47.0 525 Approved Ma Corona 22.5 57.0 420 Provisional Nekhebet Fluctus 0.0 35.0 N/A Provisional Salus Tessera 4.0 48.0 N/A Proposed Manatum Tessera 7.0 60.0 N/A Proposed

Table 8.1. Named features in V33.

Physiographic setting

The Scarpellini quadrangle extends from latitude 0° to 25° S and longitude 30° to 60

° E. It is bounded by Aphrodite Terra and the Ovda Regio plateau to the east, Eistla

Regio to the north and Tinatin Planitia to the northwest. The southeast of the quadrangle contains the northwestern fringes of Aino Planitia (Fig 3.1). The average

105 25.0° o On 30.0= 35.0= 40.0= 45.0= 50.0= 55.0= 60.0=

Figure 8.1. SAR mosaic of the Scarpellini Quadrangle. -5.0

- 10.0

-15.0° Laurencin

Juksakka Corona Gcoraina

- 20.0

Scarpel& ^_ W'»&.'w , . 4 Corona

250 km

-25.0 30.0° 35.0° 40.0° 45.0° 50.0° 55.0' 60.0' o Figure 8.2. SAR mosaic of the Scarpellini Quadrangle showing the location of named features. 108 -0.0 s -0.0 S 30.0 E 60.0 E

7 /

+ ++ + A X

■++

.++

/ / + + 4-+ + +

+ + -25.0 S -25.0 S 30.0 E 60.0 E Units Map Symbols

Tessera assemblage Edifice assemblage Ridge O Volcano; diameter 20 > km 1 1 Tessera 1 1 Nekhebet flow 1 I Nabuzama flow (2) —^— Wrinkle ridge ® Volcano: diameter > 20 km 1 1 Lineated tessera 1 1 Thermuthis flow 1 1 Nabuzama flow (1) —J — Trough + Volcano: diameter > 5 km Plains assemblage 1 1 Ma flow ------Lineament 0 Impact crater ♦ Graben 1 1 Scarpellini lineated plains 1 II 1 Other volcanic materials — Flow direction 1 1 Scarpellini mottled and lineated plains

1 1 Scarpellini regional plains

Figure 8.3. Geological map of the Scarpellini Quadrangle.

Volcanic

Units n£2 Plains nfl Nabuzama Manatum Corona Tessera spr

Tectonic sip spml t

250 km O CD Figure 8.7. Stratigraphie cross section. Transect of Scarpellini between (A) 11.0 S, 30.OE and (B) 8.2 S, 60.0 E. 0.0'

-5.0'

»' %''

- 10.0'

-15.0'

A # ' % . • »

- 20.0'

' *SL iL""? 3R?i

-25.0' 30.0' 35.0' 40.0' 45.0' 50.0' 55.0' 60.0'

Figure 8.4. Magellan altimetry of the Scarpellini quadrangle. Light tones represent topographically elevated regions. Dark tones represent areas which are topographically low. elevation of the Scarpellini quadrangle is 6051.98 km, which is close to the mean planetary radius of 6051.84 km (Ford and Pettengill, 1992). The elevation range in the quadrangle is from 1 km below to just over 2 km above the mean planetary radius. The plains are low-lying and relatively level and form a broad swath which runs through the centre of the quadrangle. In the southern central part of the quadrangle is the northern extent of a broad topographic rise extending to Lada Terra.

Mapping techniques

Magellan SAR images cover 98 % of the quadrangle. For geological mapping F- maps were used. They have a scale of 1:1.5 M, a re-sampled pixel size of 75 m and a resolution of about 120 m (Ford, 1993). Considerable use was also made of the synthetic stereo Cl images. The altimetry data of the GxDR series simplified the broad scale interpretation of topography. The SAR images were interpreted using standard photogeological techniques (Wilhelms, 1972) modified for radar.

Missing data and image processing artefacts

Only 2 % of the Scarpellini quadrangle is affected by data gaps (Fig. 8.1). They are not concentrated in any single region. In the full resolution images, low signal to noise values adjacent to the black data gaps are readily apparent, especially in the south of the quadrangle. There is also a small region of whitewash, a data error associated with cycle 3 images which gives the data an uncharacteristically bright signature. Whitewash affects an area of around 150 - 200 km^ in the southern part of Manatum Tessera.

Probably the most significant data processing artefact is the Venetian blind effect

(Broome, 1995). The stripes occur where basic image data record (BIDR) strips have been concatenated. The radiometric properties of the near and far range vary owing to

111 factors that include small variations in the radar pointing direction, spacecraft orientation and topographic modelling, and thereby cause banding. Bands are more noticeable in areas which have high backscatter cross-sections, such as regions of tessera. The effect can be clearly seen in the eastern part of Manatum Tessera, where the stripes extend south into the plains materials near crater Georgina. The effect is also prominent in the deformed terrain 200-300 km north of crater Vashti, and the tessera north east of the Nabuzama Corona. Another artefact of the data arises from a technique known as seam correction, which is carried out in order to remove the dark banding described above. The procedure results in a series of light bands along the margins of concatenated BIDR strips.

General geology

The Scarpellini quadrangle is dominated by plains which are interpreted as volcanic in origin (Tapper and Guest, 1997). The regional plains contain linear features which vary widely in style and distribution. They include wrinkle ridges, reticulate and braided networks, and linear fractures often associated with topographic features. There are six coronae present in the quadrangle. The southern flanks of the volcano Nekhebet

Mons extend into the northwest of the region. Small volcanic features are distributed throughout Scarpellini, including domes, scalloped margined domes, shields and conical edifices. Clusters of edifices are located on the plains materials and are often associated with fracture swaths. A large region of tessera, known as Manatum Tessera, dominates the northeast of the quadrangle.

1 1 2 Material units and volcanic landforms

Each material unit within the Scarpellini quadrangle is described below. Backscatter properties are plotted in Fig 8.5 and the chronological relationships between different

geological units, subdivided by type, are shown in Fig. 8.6; they are tessera, volcanic,

crater and surficial* materials. To further clarify the interpretation of geology of

Scarpellini a topographic profile was drawn across the quadrangle and the geologic map

used to construct a stratigraphie cross section across the region between 11.0° S, 30.0°

E and 8.2° S, 60.0° E (Fig. 8.7).

Tessera

Tessera materials (t) occupy a fifth of the Scarpellini region, or some 15,000,000

km2. They have a high radar backscatter coefficient, suggesting that their surfaces are

rough. Manatum Tessera is the most extensive tessera region, dominating the northeast

of the quadrangle. Another significant region of tessera is that of Salus Tessera, north of

Nabuzama Corona. There are other smaller blocks of isolated tessera, embayed by

plains units. These blocks, like those west of Nabuzama Corona, have the same

alignment and ridge trough wavelength as Manatum Tessera.

Three different styles of tessera deformation are identified in the Scarpellini

Quadrangle, but there are no sharp contacts between them. Ridge-and-trough tessera

consists of parallel ridges and grooves (Fig. 8.8). For example, Manatum Tessera possesses this characteristic configuration with an approximately SE-NW alignment. A

second form has a more chaotic surface expression and contains ridges that are highly

deformed and convoluted (Fig. 8.9). The southern part of Manatum Tessera is of this type. A third form was identified within Manatum Tessera and consists of a radial

The term surficial is used in USGS mapping literature and is synonymous with superficial.

113 ■5

"O -25 k f2

M n f2 'q -10 n f 1 0o spml 1 -15 I S -20 30 40 45 50 Incidence angle (0)

Figure 8.5. Backscatter cross-section of geological materials within the Scarpellini Quadrangle.

V olcan ic C rater Surficial U nits M aterials M aterials

CO si Plains Units

cm spr s2

Tessera Units spml

spl

spr Scapellini regional plains t Tessesa spml Mottled and Lineated Plains It Lineated tessera nf 1,2 Nabuzama flow material si Surficial material (radar dark) k fl,2 Nekhebet flow material s2 Radar bright splotches f Flow materials cm Crater materials CO Crater outflow materials

Figure 8.6. Geological units of the Scarpellini Quadrangle and their chronological relationships.

114 Figure 8.8. Tessera with characteristic ridge and groove configuration in Manatum Tessera. Ridges and troughs are oriented NW-SE. The grooves of the tessera are embayed by regional plains materials.

Figure 8.9. Tessera with a convoluted deformation pattern. Embaying materials are regional plains.

- 2 2 . 3 “

Figure 8.10. Tessera (Alpha Regio) with a - 2 2 . 8' radial system of ridges and grooves. The central region has a lower elevation and is embayed by regional plains materials.

- 2 3 . 3 '

-23.8'

15 pattern of narrow ridges extending from a central depression which may be flooded by

smooth plains materials (Fig. 8.10).

Lineated tessera materials (It) (Fig. 8.11) are distinguishable from tessera by their

fractured but less rugged appearance. They are concentrated in the southwest of the

quadrangle. The unit is embayed by plains materials in the region of Scarpellini Crater,

and patches occur as inliers for about 1,200 km to the north. The material is dissected

by north-south trending lineaments which include tension fractures and graben. The

largest area of this unit lies north of Scarpellini Crater. It has a western margin which is

difficult to see because the linear features of the adjacent plains materials are of similar

morphology and magnitude and have the same orientation.

Tessera have been interpreted as the oldest materials unit on the planet (Sukhanov,

1992; Basilevsky & Head, 1996). In the Scarpellini Quadrangle, however, it is embayed

only by regional plains materials and therefore does not necessarily constitute the

earliest materials unit. The age relations between the two tessera units, tessera and

lineated tessera, cannot yet be determined. Lineated tessera is embayed by mottled and

lineated plains, whereas tessera is embayed only by regional plains. The lineated tessera

at 17.5° S, 23.5° E is dominated by north-south trending fractures which extend into the

adjacent regional plains unit, suggesting that some deformation continued after plains

emplacement.

Plains

There are three plains units which can be distinguished by their stratigraphie

relationships and backscatter cross-sections: Scarpellini regional plains material (Spr),

Scarpellini mottled and lineated plains material (Spml), and lineated plains material

(Spl).

116 - 8.6 ° -

- 9 . 1° -

-9.6°-s'

-10 r

- 10.6 °

3 2 .8 ° 33 , 3 ° 33 . 8 ° 3 4 .3 ° 3 4 .8 °

Figure 8.11. Lineated tessera (a) embayed by Scarpellini regional plains (b).

-9.9°-

-10.4°

-10.9°

30%ii -11.4°

30.6° 31.r 31.6° 32 1' 32.6°

Figure 8.12. Mottled and lineated plains formed by continued eruption of small volcanic domes and shields.

117 The Scarpellini mottled and lineated plains (Fig. 8.12) are characterised by deformed and heavily fractured terrain of variable backscatter. They occupy a higher elevation than the surrounding Scarpellini regional plains materials which embay them.

Numerous edifice fields make up the unit, which contains a wide range of volcanic and tectonic structures. The mottled and lineated plains were apparently built up by shield- forming volcanism and lava flows.

Lineated plains materials (Fig. 8.13) have a moderately high backscatter. The unit contains anastomosing wrinkle ridges but is dominated by arcuate fractures and graben which are concentric with Nabuzama Corona. The unit is embayed by Scarpellini regional plains materials and appears to comprise volcanic materials which have been subject to several episodes of tectonic modification.

The Scarpellini regional plains unit (Fig. 8.14) is the most extensive plains unit and covers about 70 % of the quadrangle. The materials are moderately radar-dark but considerable variation exists owing to the presence of dark surficial material. The unit embays mottled and lineated plains materials, lineated plains materials and tessera materials. The unit contains a plethora of deformation structures which catalogue the prolonged tectonic evolution of the Scarpellini region. All the coronae, except Ma and

Juksakka, formed after emplacement of the regional plains materials.

Central volcanoes

There are several central volcanoes, approximately 50 - 60 km in diameter, in the

Scarpellini Quadrangle (Fig. 8.9). They have a conical form and are cut by radial fractures. A good example can be seen at the centre of Nabuzama Corona (Fig. 8.2) flanked by flows (Nabuzama flow materials (nfl) and unnamed flows (f)) which have a higher backscatter than the surrounding plains units. The volcanic centre at 17.0° S,

118 5 0 . 8 ° 51 .3 ° 5 2 .3 °

Figure 8.13. Lineated plains materials which embay Tessera (a) are in turn embayed (b) by Scarpellini regional plains.

-11.4°

-11.9°

-12.4°-l

31.8° 33 8°

Figure 8.14. A 20 km diameter, intermediate volcano at 11.9° S, 32.7° E. To its north and east are pancake-domes. The summit pits of smaller volcanic edifices can be seen at the top of the image. The moderate to radar-dark materials about the central volcano are regional plains.

119 38.0° E has a large scalloped margined dome (Guest et al., 1992), 25 km in diameter, on its summit and an annulus of circular fractures which may have been caused by lithospheric bending induced by the overburden of volcanic materials. Central volcanoes are found on the topographic ridge of Thermuthis Corona and some of them at least appear to predate formation of the coronae ridge as they are cut by fractures that have the same alignment as the annular lineaments of the corona. A heavily fractured volcano is located southeast of Mukylchin Corona at 15.0°S 47.0°, and flows from the northern flank have entered the corona.

Domes

Steep-sided domes and scalloped margined domes (Guest et al., 1992) as well as pancake-domes (Mackenzie et al., 1992) are found in the Scarpellini Quadrangle (Fig.

8.14). An example of a 25 km steep sided dome can be found on a topographic high,

located at 18.0°S, 38.5° E ; it has fluted scarps and a pit at the centre of the flat

summit. Part of the collapsed volcano appears to be embayed by regional plains material. A 40 km scalloped margined dome is located north west of Crater Scarpellini

(22.0°S, 33.0°E); it has scalloped flanks and a concave summit, at the centre of which is

a pit. A faint annulus of fractures can be detected which could be due to stress caused by the volcanic load.

Small volcanic edifices & edifice fields

The Scarpellini region contains numerous small (1-20 km diameter) volcanic

edifices (Fig. 8.12), most of which are <2 km in diameter. The majority have shallow

slopes and are shield-like, but some are steep-sided cones (Guest et al., 1992). They are

distributed throughout the region, often in clusters which are sometimes associated with

1 2 0 linear features. Clusters of small edifices or edifice fields are common on mottled and

lineated plains and regional plains units, and are more numerous in the southwest of the

quadrangle. To the northwest of Ma Corona there are several large clusters which lie on

sets of SW-NE trending fissures. Other concentrations associated with deformation of the plains materials are found at 27.0°S, 43.0° E, in the region south of Juksakka

Corona.

Lava flow fields

Extensive lava flows are common within the Scarpellini quadrangle. They are

associated with coronae, small volcanic edifice fields and volcanic centres. Flows are

identified by their lobate margins and generally homogenous backscatter. The flow margins tend to have a higher backscatter cross-section than the main body of the flow

and may be less homogenous. This is indicative of rougher material at the margins.

Nekhebet Fluctus (kfl and kf2) is the most extensive lava field in the Scarpellini

quadrangle. Lavas from Nekhebet have a less homogeneous backscatter than other

flows in the quadrangle, indicating that the surface is composed of lavas which have more than one type of surface texture.

Crater materials

The Scarpellini quadrangle contains 16 impact craters which range in diameter from

2.5 km to 39 km. They appear pristine, having well defined, sharp morphologies (Fig.

8.15). The materials which form the crater rim (cm) and continuous ejecta (co) have a high backscatter coefficient, indicating the surface is rough at the radar wavelength

scale. It also appears hummocky. The ejecta margins are usually irregular to lobate. A

few craters have extensional fractures which dissect their floor and sometimes the ridge

1 2 1 -19.1'

-19.3°

-19.5°

-19.7° 40 .r 40.3° 40.5° 40.7° 40.9°

Figure 8.15. Craters Medhavi and Michelle. Crater rim material (cm). Crater outflow material (co). The radar dark region to the east of the craters are interpreted as fine debris deposited after the impact. The radar bright region to the west is possibly caused by the scouring effect of the impact, winnowing fine material from surface interstices.

-5.9°

-6.4°

-6.9°

-7.4°

51.4° 51.9° 52.4° 52.9°

Figure 8.16. Wrinkle ridges in regional plains material. The radar bright unit embayed by regional plains is tessera. The wrinkle ridge configuration changes when tessera outlier are encountered.

122 crest and which evidently indicate some tectonic modification. Many of the larger craters (15.3° S, 39.3° E) are associated with flows of high backscatter cross-section.

They extend from the outer margins of the crater ejecta and are interpreted by Asimow

(1992) as impact melt.

Surficial materials

About 17 % of the Scarpellini region is mantled by surficial deposits (si) which have a low backscatter coefficient (Fig. 8.17). They are associated with impact craters and have a circular or paraboloid distribution pattern (Campbell et al., 1992). Schaber et al. (1992) suggest that shock waves generated in the atmosphere by the impact entrain fine-grained ejecta material which is later deposited. The global radar mosaic shows many of these large parabolas. A significant number have the apex of the parabola situated eastward of the main body of fallout material, suggesting that the material is carried by winds. The distribution pattern may depend partly on impact direction.

Schultz (1992) suggests that objects which have a steep entry angle tend to produce circular deposits, whereas impacts which enter the atmosphere obliquely produce large parabolas.

A large parabola of dark surficial debris is associated with Bathsheba Crater. Within the parabola are two large craters, Medhavi and Munter, which have generated additional surficial deposits. Regions of surficial material which are more circular and compact are associated with craters Vashti and Evika. Radar-dark materials in the southeast of the quadrangle are associated with crater Bassi located west of Ma Corona in the adjacent quadrangle.

In several localities, aeolian activity appears to have redistributed surficial materials.

Southwest of crater Medhavi can be found a series of bright and dark diffuse linear

123 0.0°-i

-5.0 iSI Ü

- 10.0

M unter % -15.0 filfc

i Georgina

- 20.0 m

’*2 30.0= 35.0= 40.0= 45.0= 50.0= 55.0= 60.0= Figure 8.17. SAR mosaic of the Scarpellini Quadrangle showing the distribution of radar-dark surficial materials (outlined) and to their relation to impact craters. Arrows are used to indicate the inferred direction of deposition where a non-circular debris apron has been formed. The materials to the west of crater Georgina are the extreme parts of a large parabola associated with Crater Bassi in the adjacent quadrangle. streaks which are aligned south of several upstanding blocks of embayed materials. The

configuration suggests that obstacles have produced a wind-shadow effect (Greeley et

al., 1992). In the north of the quadrangle, at the margins of tessera, are materials with a

low backscatter coefficient. The low backscatter is interpreted here as particulate debris

which has accumulated on the flanks of the upstanding tessera units and which has

perhaps been driven there by wind.

Some circular radar-bright regions (s2) can be seen. They have probably been

formed by the scouring effect of failed impacts. Sometimes at the centre of the radar-

bright splotch can be seen irregular structures. Shock waves firom the impact may

winnow small particulate debris from the surface, which thus appears rougher than the

surrounding materials.

The surficial materials map is a useful supplement to the geological map of the

Scarpellini Quadrangle. It serves to indicate areas where boundaries may be obscured

completely by fine grained materials or where boundaries have been inferred very tentatively. It indicates why units have been identified where there is no obvious

geological boundary. Certain features are more easily detected in regions which are mantled by surficial materials. Small volcanoes highlighted by the presence of

surficial material may have been preferentially mapped. The map also indicates the direction in which the materials were entrained before deposition. In instances where the parabola is pronounced this is fairly straightforward. Where impacts have subsequently occurred, the distribution pattern is not so clear and other evidence is used to determine direction, such as the presence of wind streaks produced by topographic obstacles.

125 Statistics

The physical properties of each materials unit mapped are listed in Table 8.2. Type

localities were identified and | analysed with software written \^y Campbell (1995).

Backscatter, emissivity, fresnel reflectivity, root mean square (RMS) slope and elevation were calculated for each unit. The algorithm used to calculate observed backscatter

compensates for variation in incidence angle of the SAR sensor with respect to the planet’s surface, removes the Muhleman backscatter calibration formula, and calculates

backscatter from DN converting it to a decibel value. The backscatter properties of the

different units are plotted in Fig. 8.5.

For the derivation of these statistics a box of appropriate size was used to allow

sufficient pixels for a suitably representative region to be evaluated. Care was taken to

avoid contamination of the selected area by other units or unrepresentative structures.

For example, the high backscatter cross section of lineaments oriented perpendicular to the radar would severely bias the character of many of the units, particularly those that

are predominantly radar dark.

A single image box was used for each of the units, except in the case of the mottled plains. Distinct regions of mottled plains were interrogated to determine whether these units consisted of materials with comparable backscatter properties or simply possessed

similar variance co-efficients. The stratigraphie context, and the statistics derived,

indicate that the units are similar. Other units vary considerably in their backscatter

over small areas. Lineated terrain, for example, is hard to characterise because its backscatter properties are affected by the density of fractures.

Care was taken to exclude from the data boxes any regions subject to data

acquisition or processing errors. They include data gaps, regions of low signal-to-noise

(SN) ratio, regions of high DN value caused by seam correction, and areas where the

126 dark banding effect of the adjoining orbital swaths can give DN value readings of 26 below the mean, equivalent to a reduction in backscatter of 5.2 dB over the affected areas (Broome, 1994).

U nit Longitude (°E), latitude (°S) N 0° Planetary radius ®rms ° Spr 50.03°-50.91° 18.52°-20.39° 199225 34.4 6051.51 (6051.27, 6051.83) 2.64(1.2, 4.1) Spml 34.18°-34.86° 11.70°-12.56° 107869 41.5 6051.49 (6051.35, 6051.75) 2.02(1.2, 4.3) Spm 48.37°-49.46° 20.48°-21.3r 238425 37.7 6052.03 (6051.72, 6052.42) 3.36 (1.6, 6.6) Ip 37.37°-37.84° 12.25°-12.64° 28565 41.4 6051.48 (6051.35, 6051.52) 1.85(1.5, 2.8) n f l 48.90°-49.42° 9.90°-10.81° 94809 42.2 6051.74 (6051.52, 6052.06) 1.58(0.8, 2.4) nf2 45.93°-46.74° 9.59°-10.03° 66045 42.4 6051.45 (6051.18, 6051.75) 2.12(1.2,3.1) k f l 35.21°-35.63° 0.75°-1.08° 26055 44.8 6052.28 (6052.26, 6052.34) 2.16(1.6, 2.8) kf2 35.21°-35.70° 0 .ir-0 .6 2 ° 32929 44.8 6052.24 (6052.20, 6052.28) 3.33 (2.3, 3.7) m f 58.20°-58.57° 21.86°-22.57° 56265 37.2 6051.44 (6051.27, 6051.52) 3.79 (2.5, 5.6) f 37.51°-37.69° 17.41°-18.00° 15521 39.2 6051.56 (6051.47, 6051.72) 2.44(1.8,3.4) t 56.59°-57.58° 10.93°-11.61° 212629 41.9 6052.18(6051.61,6052.73) 3.50 (0.1, 8.5) It 38.93°-39.32° 23.27°-24.48° 92105 36.1 6052.16(6051.92, 6052.48) 3.10(0.7,5.1) si 47.78°-48.19° 17.43°-17.85° 34645 39.2 6051.44 (6051.33, 6051.52) 1.81 (1.0, 2.5) s2 48.90°-49.05° 24.52°-24.90° 11505 35.7 6051.62 (6051.61,6051.66) 3.28 (2.5, 4.2)

N = number of image pixels in data box, 0° = incidence angle, ®rms° = Root mean square slope.

Unit Backscatter a (dB) S.D (S.D/x)100 Reflectivity p Emissivity 8 Spr -14.03 (-19.24, -7.97) 1.294 -9.22 0.100 (0.075, 0.120) 0.840 (0.828, 0.845) Spml -16.96 (-23.98, -6.58) 1.445 -8.52 0.106 (0.080, 0.125) 0.840 (0.832, 0.860) Spm -11.71 (-21.05, 0.91) 1.692 -14.44 0.085 (0.060, 0.120) 0.860 (0.826, 0.888) Ip -12.01 (-22.44, 3.04) 1.665 -15.82 0.119(0.105, 0.140) 0.826 (0.822, 0.832) nfl -14.98 (-22.91,-6.13) 1.498 -10.00 0.110(0.080, 0.165) 0.831 (0.825, 0.838) nf2 -14.89 (-23.17, -6.20) 1.456 -9.77 0.090 (0.070, 0.110) 0.840 (0.829, 0.850) kfl -19.31 (-29.60,-13.41) 1.306 -6.76 0.113 (0.090, 0.135) 0.819(0.814, 0.825) kf2 -11.05 (-37.73, -4.23) 1.436 -12.99 0.108 (0.95, 0.130) 0.812(0.794, 0.826) mf -18.65 (-24.92, -10.92) 1.372 -7.35 0.127 (0.085, 0.160) 0.845 (0.842, 0.849) f -18.08 (-23.52,-11.51) 1.432 -7.92 0.112(0.095, 0.140) 0.846 (0.843, 0.850) t -10.06 (-21.22, 5.36) 2.170 -21.50 0.068 (0.020, 0.016) 0.897 (0.886, 0.907) It -11.69 (-21.03,0.55) 1.883 -16.10 0.103 (0.075, 0.145) 0.872 (0.859, 0.883) si -21.71 (-26.94, -8.12) 1.477 -6.80 0.100 (0.075, 0.120) 0.840 (0.828, 0.845) s2 -10.99 (-16.47, -6.12) 1.293 -11.76 0.150 (0.135, 0.155) 0.824 (0.821, 0.829)

Table 8.2 Backscatter and radiothermal properties of Scarpellini units. Extreme values are given in brackets. Unit names are: Spr (Scarpellini regional plains), Spml (Scarpellini plains mottled and lineated), Spm (Scarpellini plains mottled). Ip (intermediate plains), nf (Nabuzama flows), kf (Nekhebet flows), Mf (Ma flows), f (other volcanic flows), t (tessera), It (lineated tessera), si (radar- dark surficial deposits), and s2 (radar-hright splotches).

Radiothermal properties

Emissivity is used as guide to the dielectric properties of the surface materials. It is used to determine the effect of these properties on SAR imagery. Materials of high dielectric constant are good reflectors and poor emitters. For example, low emissivity

127 is associated with the impact parabola of Crater Bathsheba (0.80). Emissivity varies widely, though the mean surface emissivity is 0.845, indicating a smooth surface dielectric constant of around 4.0 (Pettengill et al., 1992).

Rough surfaces yield higher emissivity values. Tessera is such an example where a rough surface gives a high emissivity reading. Plains have a moderate to low emissivity.

Some lavas, for example those emanating from Nekhebet or those located on the NW flank of Nabuzama, have a very low emissivity, whereas those associated with the rim of Thermuthis have a moderately high emissivity. Rough surfaces, tessera, some lava flows and crater ejecta have emissivities 0.05-0.10 higher than the surrounding plains

(Plant, 1993).

Structures

Wrinkle ridges

Wrinkle ridges (Fig. 8.16) are identified as upstanding features by their leading edge of high backscatter and shadowed back-slope with a low radar return (Squyres et al.,

1992). They appear to have been produced by compression and are often found in the plains materials associated with coronae. Such an association can be seen in Scarpellini, where wrinkle ridges occur concentric with Nabuzama Corona at 7.0° S, 52.0° E.

Anastomosing wrinkle ridges can be seen in the regional plains materials at 13.0°S,

41.0° E. It is not clear how they formed, but they indicate some kind of regional-scale tectonisation of the plains surface.

Fractures

Intricate polygonal fracture networks commonly occur on venusian lava flows

(Johnson, 1992) and are considered to be extensional features. In the Scarpellini region

128 polygonal fractures can be found at 6.0° S, 56.0° E in the volcanic plains embaying a topographically low region within Manatum Tessera.

Reticulate fractures form a trellis-like network which is considered by Banerdt and

Sammis (1992) to have been formed by stretching of the plains surface. Reticulate networks are clearly visible in the regional plains north of Thermuthis Corona (5.0° S,

44.0° E) and at 17.5 ° S, 38.0° E.

Graben

Most of the graben found in the quadrangle are associated with the lineated tessera

(21.0° S, 35.0° E). They are generally 1 - 2 km in width, 10-20 km long and tend to intersect each other at right angles. Some graben are found in Manatum Tessera; they are wider (> 5 km) and longer (> 200 km) than the majority. Graben found within the tessera intersect and cross cut the ridge-and-trough terrain of the tessera.

Parallel lineaments at 3.0° S, 33.5° E trend southward from the flanks of Nekhebet

Mons. They are sufficiently wide to be identified as graben. They may have been generated by uplift and stretching of the regional plains while Nekhebet Mons was forming.

Arcuate graben form part of the tectonic annulus east of Nabuzama Corona. They stretch for several hundred kilometres and, though shallow, are up to 20 km in width.

Some of the arcuate graben have scalloped edges, but there is no visible debris apron which could signify that collapse of the escarpment has occurred. A few lineaments transect the graben, but in general the graben floor is characterised by numerous smaller lineaments which terminate at the scarp.

129 Geological history

The interpretable history of the Scarpellini Quadrangle begins with severe tectonic deformation and disruption of primitive crustal materials, Manatum and Salus Tessera.

Plains formation followed, and continuing tectonic activity fractured the surface. New plains were constructed by the widespread eruption of lavas and shield-forming volcanism. Numerous small volcanic edifices erupted more material onto the surface.

Mantle plumes started to deform these volcanic materials, initiating the development of the coronae Ma and Juksakka. The next major geological event was the emplacement of large quantities of volcanic materials in the low-lying areas to form extensive plains which embay the tessera and the fractured margins of the older materials. These plains continued to deform, with the development of additional coronae in the new surface.

Eruption of lava from the coronae and various volcanic centres followed, Nabuzama

Corona and Nekhebet Mons both experiencing episodes of volcanic activity. Eruptions from isolated volcanic structures and clusters of small volcanic edifices occurred on the plains surface while it continued to deform. Surficial materials associated with late bolide impacts were deposited over much of the regional plains, and in recent times wind has deposited fine-grained material against topographic obstacles.

130 Chapter 9

Coronae of the Scarpellini Quadrangle

The coronae of the Scarpellini quadrangle (Fig. 9.1 and Table 9.1) range widely in geologic setting, morphological configuration, stratigraphy and associated tectonic

structures.

Name Latitude Longitude Diameter Height Altitude MCV (°S) n (km) (m) (m) Ma 23.5 57.0 450 3411 6051.72 3 1 2 Thermuthis 9.0 33.0 525 2034 6051.22 6 1 2 Mukylchin 12.0 44.5 350 2531 6051.51 5 2 2 Nabuzama 8.5 47.0 400 3081 6051.81 5 3 3 Juksakka 19.5 44.5 330 990 6052.34 4 1 1

Table 9.1. Attributes of named coronae in the Scarpellini Quadrangle. Altitude is expressed as a planetary radius. Symbols used are: M (morphological type), C (corona class, after Stofan et al. (1992)) and V (degree of associated volcanism).

Ma Corona (Fig. 9.2a) is located in the plains south of Manatum Tessera, centred on

22.5° S, 57.0° E, on the flank of an extensive topographic rise. It has a mean diameter of 400 km, and consists of a concentric steep sided trough which encircles the remnants of a plateau (Fig. 9.2b).

Materials with a low backscatter, interpreted as lavas, occupy the southern part of the corona interior. Lavas which appear to have erupted from the corona rim can be found on the northern and eastern slopes of the corona. Three units can be recognised within Ma Corona. Mottled lineated plains (containing concentric fractures) make up the oldest detectable unit, and are embayed by regional plains materials. The regional plains materials are overlain by volcanic materials which have erupted from shield

131 f

" V

- 10.0

-15.0

- 20.0

-25.0° 30.0° 35.0 40.0 4 5 .0 ' 50.0' 55.0' 60.0' U) Figure 9.1. SAR mosaic of the Scarpellini Quadrangle showing the location of named coronae and coronae (C1-C4) K) identified by the new survey. (a)

53.0° E 60.0° E 19.0° S 19.0° S

6054.0

6053.6

6053.2

6052.8

6052.4

6052.0

6051.6

6051.2

6050.8

60.0° E 26.0° s k------26.0° S

(b)

-20.5

- 21.0

-21.5

- 22.0

-22.5

-23.0

-23.5

-24.0°

-24.5

54.5° 55.0° 55.5° 56.0° 56.5° 57.0° 57.5° 58.0 58.5'

Figure 9.2. (a) Contour map and (b) Magellan SAR image of Ma Corona.

133 volcanoes located in the interior. They are presumably the most youthful unit associated with Ma Corona.

The structure of the interior is characterised by swaths of irregularly spaced fractures with a southwest trend. The topographic rim and trough are heavily fractured; most of the lineaments are concentric, but there are radial fractures that are cross-cut by the concentric swaths and extend into the plains for several hundred kilometres.

History of Ma

• Emplacement of mottled and lineated plains • Deformation of mottled and lineated plains Radial fracturing associated with uplift Concentric fracturing associated with basin formation • Emplacement of Scarpellini regional plains • Deformation of Scarpellini regional plains Formation of plateau Relaxation of plateau and disruption of surface • Eruption of lavas associated with corona rim

Juksakka

Juksakka (Fig. 9.3a), located at 19.50° S, 44.50° E, is situated on the northern flank of an extensive topographic rise west of Ma Corona. Juksakka is a concentric ring corona with a diameter of 400 km. The relief of the corona is not great, but a wide topographic rim can be discerned (Fig. 9.3b). The northern part of the corona lies within the parabola of surficial material associated with crater Bathsheba.

The northern part of the corona has been inundated by lavas which form part of the unit that now makes up the regional plains. More recently, lavas have erupted from the corona rim and flowed toward the central basin. Debris associated with crater

Bathsheba overlies these materials and may obscure structural detail.

134 (a)

4 1 .5 °E 47.0° E 22.5° S 22.5° S

6053.6

6053.4

6053.2

6053.0

6052.8

6052.6

6052.4

6052.2

6052.0

605 1.8

6051.6 km 41.5° E Î 47.0° E 27.5° S 27.5° S

(b)

-17.5'

-18.0°

-18.5°

-19.0°

-19.5°

- 2 0 . 0 °

-20 5°-

- 21 . 0°

-21.5° 42.5° 43.0° 43.5° 44.0° 44.5° 45.0° 45.5° 46.0°

Figure 9.3. (a) Contour map and (b) Magellan SAR image of Juksakka Corona.

135 A few lineaments are observed in the central basin, but they have no preferred

alignment. The southern margins of the corona are heavily fractured. Annular fractures

are found on the south western rim and are cross-cut by groups of linear fractures which trend northeast.

The highly deformed and fractured southern rim comprises materials interpreted as

mottled plains which were emplaced through the eruption of numerous small volcanoes.

These plains are embayed by more youthful plains which have a lower backscatter co

efficient. Juksakka, like Ma Corona, appears to have formed before the emplacement of the regional plains which embay the corona's broad-scale deformation annulus. Unlike

Ma corona, however, there is no evidence for Juksakka’s having experienced further

episodes of deformation.

History of Juksakka

• Emplacement of mottled and lineated plains • Deformation of mottled and lineated plains Radial fracturing associated with uplift Concentric fracturing associated with dome formation Basin formation • Emplacement of Scarpellini regional plains • Eruption of lavas associated with corona rim

Thermuthis and Unnamed

Thermuthis (Fig. 9.4a) is a 550 km diameter corona located at 8.0° S, 33.0° E, south of Nekhebet Mons. It consists of a raised rim, interrupted in places, which encloses a basin (Fig. 9.4b). Within this basin, located in the northern hemisphere, is a smaller circular structure of about 150 km in diameter, which consists of a raised rim and a topographically low interior. On the northern rim of Thermuthis is a small unnamed

100 km diameter corona which consists of a ridge enclosing a basin with a central upstanding region. The resulting trough has been flooded by lavas.

136 29.5° E 36.5° E 4.0° S 4.0° S

6 0 5 3 .8

6 0 5 3 .4

6 0 5 3 .0

6 0 5 2 .6

6 0 5 2 .2

6 0 5 1 .8

6 0 5 1 .4

6 0 5 1 .0

29.5° E 36.5° E 11.0° S 11.0° S

(b)

-5.0

- 6.0

-7.0

- 8.0

-9.0

- 10.0 II

- 11.0

- 12. 0° 0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00

Figure 9.4. (a) Contour map and (b) Magellan SAR image of Thermuthis Corona.

137 Small lava fields with a low backscatter can be discerned on the eastern, interior slopes of the basin. Flows of moderate backscatter which include streaks of material with high backscatter erupted from a heavily fractured structure on the southeastern margin of the corona. The small corona on the northern rim of Thermuthis contains lavas which have a moderate to high backscatter. They occupy the circular trough and mantle the upstanding western rim, from where they appear to have erupted. Part of the flow descends northwards towards Nekhebet Mons and encounters flows which have erupted from this large central volcano.

The dominant fracture swaths of the basin are formed by east-west trending

lineaments. There are also fractures associated with the circular structure at the centre of the basin and a distinct centre of fracturing associated with the volcano located on the

southeastern rim of the corona. Annular fractures and scarps encircling the corona are particularly prevalent on the eastern rim.

The northern half of the corona has developed within regional plains material. The

southern half of the corona, however, deforms a region of mottled plains. Because contiguous structures occur in the mottled plains and regional plains we can infer that the corona postdates the formation of the mottled plains. Lava flows and small volcanoes may mask recent brittle scale deformation in the region of mottled plains.

History of Thermuthis and Unnamed

• Emplacement of mottled and lineated plains • Emplacement of Scarpellini regional plains • Deformation of mottled and lineated plains and Scarpellini regional plains Radial fracturing associated with uplift Concentric fracturing associated with basin formation • Concentric fracture annulus associated with the formation of central ridge/basin structure during secondary phase of corona development • Development of corona (unnamed) on the northern rim • Eruption of lavas associated with corona rim and unnamed corona

138 Nabuzama

The 450 km diameter circular basin of Nabuzama (Fig. 9.5a) is centred on 8.50° S,

47.0° E. It consists of a circular depression which lies below the level of the

surrounding plains surface (Fig. 9.5b). Occupying the centre of the depression is a

volcanic structure, the flow apron of which occupies most of the floor of the corona.

Material of variable backscatter, with a streaked appearance and lobate margins, can be

interpreted as lavas which have flowed radially from the edifice. Lavas also appear to

have erupted from the fractured rim of the corona, some of the material flowing onto the

adjacent plains but most of it into the basin. Volcanic materials of low radar backscatter

from the corona rim are discernible to the west of the corona, overlying the regional

plains.

Nabuzama has a distinct tectonic annulus which is aligned with the steep interior

wall of the depression. Concentric lineaments, clearly associated with the corona, can

be found in the surrounding plains materials. Arcuate graben, concentric to the corona,

are found to the south east in lineated plains materials. These vary in width, but can be

up to 20 km across and 200 km long. Concentric wrinkle ridges exist in the

surrounding plains material up to a distance of 200 km from the corona rim. The

wrinkle ridges suggest that the corona underwent a phase of radial expansion. The

structure at the centre of the basin, which is interpreted as a central volcano, is criss

crossed by numerous fractures that give it a stellate appearance.

The corona appears to have formed in regional plains materials, although it deforms

a region of lineated plains material which pre-dates the regional plains. Several

episodes of volcanism have occurred. The earliest detectable volcanic eruptions

occurred on the corona margins and produced flows which occupy basin floor. They

were superseded by flows which have erupted from the central volcano.

139 (a)

44.0° E 51.0° E 5.0° S 5.0° S

-H 6053.8

6053.4

6053.0

6052.6

6052.2

6051.8

6051.4

6051.0 44.0° E 51.0° E 12.0° S 12.0° S

(b)

-5.5

45.0° 45.5° 46.0° 46.5° 47.0° 47.5° 48.0° 48.5° 49.0° 49.5° 50.0° 50.5

Figure 9.5. (a) Contour map and (b) Magellan SAR image of Nabuzama Corona.

140 History of Nabuzama

• Initial uplift Tectonisation of Scarpellini lineated plains • Emplacement of Scarpellini regional plains Deformation of the Scarpellini regional plains • Concentric wrinkle ridges associated with outward expansion of plume head. • Concentric fracturing associated with basin formation Eruption of lavas associated with corona rim Updoming of central edifice Eruption of lavas associated with central edifice

Mukylchin

Mukylchin is located south of Thermuthis Corona at 12.5° S, 46.0° E (Fig. 9.6a). It

is a circular basin (Fig. 9.6b) 400 km across which has deformed regional plains

material. The low radar backscatter over much of the area occupied by the corona can

be attributed to the presence of the surficial materials associated with Crater Bathsheba.

Morphologically, Mukylchin is similar to the adjoining Nabuzama Corona. It

consists of a deep depression encircled by a steep wall. In the western half of the corona

basin is a 130 km diameter depression which appears to be a volcanic caldera.

Radar dark materials, which are probably lavas, are located on the southwestern rim

of the corona. Lavas which have erupted from the volcanic centre located on the plains just beyond the southeastern rim of the corona have penetrated the corona.

The tectonics of the interior are dominated by faint radial fractures which extend

radially from the margins of the caldera cross-cutting the materials that occupy the

corona interior. A series of concentric lineaments associated with the corona’s rim topography are concentrated on the crest of the eastern ridge. Faint radial fractures and wrinkle ridges can be found in the plains to the south-west. They are narrow and radar

bright and extend for several corona radii from the topographic rim into the surrounding

materials. Flows from the central volcano on the rim of Mukylchin and lavas which

141 (a)

43.25° E 48.25° E 9.50° S 9.50° S

6057.0 6056.5 6056.0 6055.5 6055.0 6054.5 6054.0 6053.5 6053.0 6052.5 6052.0 6051.5

43.25° E 48.25° E 14.50° S 14.50° S

(b)

- 10,0

-10.5

-15.0 43.5° 44.0° 44.5° 45.0° 45.5° 46.0° 46.5° 47.0° 47.5° 48.0°

Figure 9.6. (a) Contour map and (b) Magellan SAR image of Mukylchin Corona.

142 have erupted from the rim itself may obscure further radial fractures, which are only

discernible in the regional plains initially deformed by the mantle upwelling.

History of Mukylchin

• Emplacement of Scarpellini regional plains • Deformation of the Scarpellini regional plains Radial fractures associated with uplift of plains surface Concentric wrinkle ridges associated with outward expansion of plume head. Concentric fracturing associated with basin formation • Eruption of lavas associated with corona rim volcanic centres • Caldera/pit formation

Stealth coronae

The stealth coronae of the Scarpellini region (Fig. 9.1) range in diameter between

235 km and 445 km and tend to be smaller than the average reported by Stofan’s 1992

global survey. They have relief of between 394 m and 787 m, which is less than the

global average. Location and physical data for the stealth coronae of Scarpellini are

given in Table 9.2. Stealth coronae are unnamed features; in this thesis they are

assigned a number and referred to as Cl, C2,...C229.

Four morphological types are found in the Scarpellini Quadrangle: type 4 (rim

encircling a low-lying interior, 5 (rim surrounding a central rise), 6 (rim encircling a rise

and central depression), and 7 (rim only). The encircling rims of coronae C1-C3 are

shared. Because the coronae lack an annulus of intense concentric fractures or marked

radial fracturing it is not possible to categorise them according to the morphotectonic

scheme of Stofan et al. (1992). The stealth coronae of V33 are deficient in associated

volcanism. Coronae Cl and C3 deform mottled plains, C2 and C4 deform regional

plains materials.

143 Name Latitude Longitude Diameter Height Altitude M V s (°S) n (km) (m) (m) 1 17.58 42.44 265 647 6051439 7 1 Spml 2 14.75 41.65 355 731 6051249 6 1 Spr 3 17.16 39.50 275 563 6051246 7 1 Spml 4 6.500 44.00 360 530 6051330 6 1 Spr

Table 9.2. Attributes of the stealth coronae of the Scarpellini Quadrangle. Symbols used are: M (morphologic type), V (degree of associated volcanism) and S (stratigraphie unit).

Corona 1

Corona 1 (Fig. 9.7) consists of a circular topographic rim, the northern part of which forms the southern encircling rim of C2. The surficial debris associated with crater

Bathsheba has highlighted discontinuous concentric fractures on the crest of the topographic rim; these fractures occur in mottled and lineated plains which the corona deforms. The surface may have been subsequently embayed by Scarpellini regional plains materials and further deformed, although any unit boundary that may have existed is now obscured by surficial materials. A few concentric lineaments can be distinguished on the inner and outer breaks of slope on the northern topographic rim.

Linear fractures cross-cut the interior of the corona but they do not appear to be associated with the development of Cl. No volcanism appears to be associated with the corona.

Corona 2

Corona 2 (Fig. 9.8) is one of the most extensive stealth coronae of the quadrangle. It is also the most complex. The topographic rim encircles a basin which contains an upstanding circular structure which itself contains a depression. To the northeast, the corona is bounded by an arcuate escarpment. The tectonics of the feature are quite unusual. A web-like network of wrinkle ridges occupies the floor of the basin and the

144 100 km

Figure 9.7. Magellan SAR image of corona Cl centered on latitude 17.5 degrees south, longitude 42.5 degrees. Indicated are (a) craters Medhavi and Michelle, (b) the shared rim of coronae C1-C3. The arrows are used to demark the rim of the corona and (c) the centre. The topographic profile between points A and B was created using the Magellan altimetry. Although the corona deforms regional plains materials, earlier materials are identifiable on the corona rim at (b). The corona lacks an intense fracture annulus but a number of faint fractures can be identified on the rim, partly obliterated by materials which are associated with the crater Medhavi. Corona Cl lies within the radar dark parabola of crater Bathsheba, the fallout from which may also serve to disguise associated tectonic structures.

145 m

100 km

Figure 9.8. Magellan SAR image of corona C2 centered on 14.75° S, 41.0° E. Arrows are used to indicate the rim of the corona. The topographic profile between points A and B was created using the Magellan altimetry. The corona deforms regional plains materials which contain wrinkle ridges (a). The wrinkle ridges are aligned with corona topography. Fractures (b) are seen at the centre of the corona which has a topographically depressed interior. Radial fractures associated with the corona, indicated by unbroken lines, have their origin at the centre of the corona and extend to the corona rim. Lineated plains materials (c) can be seen at the shared rim of corona C 1. Structures (d) to the north east of the coronae are shield volcanoes with summit caldera.

146 central edifice. The network is regular and there is a clear pattern of brittle deformation clearly related to the topographic expression of the corona. Several linear fractures radiate from the centre of the inner basin, cross-cut the inner rim and extend toward the outer rim. Fractures appear to be more concentrated at the centre of the structure where they cannot be discerned from wrinkle ridges, and are more pronounced where there is change in slope gradient, for example at the outer edge of the central edifice. Only the pattern of ridges and fractures identifies the coronae in SAR images, and it is quite difficult to see in non-stereo images because the faults that cross-cut the feature grade into a complex braided pattern that extends into the surrounding plains for several hundred kilometres to the north and west. The corona appears to have formed in

Scarpellini regional plains materials.

Corona 3

Corona 3 (Fig. 9.9) consists of a rim which encircles a low topographic rise. It shares its western rim with Cl and C2 and is located within the parabola of surficial material associated with crater Bathsheba; the debris serves to highlight discontinuous fractures which occur on the rim crest. The corona appears to have deformed mottled and lineated plains, which were later embayed by Scarpellini regional plains materials.

Isolated fragments of embayed lineated tessera are found within the corona basin. The central topographic rise is cross-cut by faint irregular fractures. The flanks of the rise are radar-bright, whereas the plateau-like summit and the low-lying corona interior are radar dark. Surficial materials which mantle the corona make it hard to distinguish flows associated with the topographic rise, but the backscatter and distribution of materials suggests that lavas have erupted from the flanks of the rise. To the west of the central rise is a second, smaller rise and SMD encircled by fractures. Lavas have

147 *

100 km

Figure 9.9. Magellan SAR image of C3 centred on 17.5° S, 39.5° E. Indicated on the image are; (a) Medhavi crater, (b) Munter Crater and (c) lineated terrain. Located within the corona interior are several volcanic edifices. The structure (d) comprises a small rise with a volcanic dome and a larger scalloped margined dome. Flows from the fractured summit mantle the flanks of the rise. The white arrows indicate the outer margins of the corona rim. The centre of the corona (e) is dominated by an elevated region (radar bright) which is cross cut by a reticulate fracture network. The eastern rim of the corona is shared with corona C 1 and appears to contain materials which are embayed by regional plains materials. The low backscatter over much of the region is attributable to the parabola of surficial material associated with crater Bathsheba. The topographic profile (above) was created using Magellan altimetry data.

148 \ 4^1 .

'-,"1

100 km

A B

Figure 9.10. Magellan SAR image of corona C4 centered on 6.75° S, 42.5° E. The crater at the centre of the corona (a) is Crater Vashti. Arrows are used to indicate the rim of the corona. Wrinkle ridges in the Scarpellini regional plains (b) can be seen to the northwest of the corona and embayed tessera (c) to the southwest. The topography of the corona is very subdued and it appears to lack assocated tectonic structures making it difficult to see without synthetic stereo images.

149 erupted from the flanks of the second smaller rise. Small volcanic cones and domes are found in the interior to the northeast between the corona rim and the central rise.

Corona 4

Corona 4 (Fig. 9.10) comprises an extensive basin at the centre of which is a low topographic rise. Its topography is disrupted by a 40 km wide linear ridge which

extends 1,500 km to the southwest. Crater Vashti is located at the centre of the corona,

surficial debris from which, mantles a 125 km diameter circular part of the interior. The

corona is bounded to the west by large blocks of embayed tessera. The embaying

materials are Scarpellini regional plains deformed by the corona. No brittle deformation

can clearly be attributed to corona growth. Fracture swaths to the north of the corona

are not aligned with topography, are not annular, and appear to be part of the regional tectonic system. Linear fractures unrelated to basin morphology dissect the basin floor.

Lavas have erupted from the southern rim of the corona; some of the flows have

entered the basin whilst others have flowed out onto the surrounding plains.

150 Chapter 10

Stratigraphy and origin

Early heat loss calculations which suggested that coronae could make a contribution to heat loss (Stofan et ah, 1992) were based on a small population of coronae. This work shows there to be more coronae, adding weight to the argument that in the absence of episodic plate recycling (Turcotte, 1993; 1995) or catastrophic lithospheric turnover

(Strom et ah, 1994) they provide a significant heat loss mechanism. Stratigraphie evidence suggests that coronae have formed a heat loss mechanism for the entire interpretable history of Venus because they incorporate geological materials of all ages.

Furthermore, the number of arcuate structures, degraded or partially embayed, which did not fully meet the database criteria suggests that even more coronae than have been described in previous chapters have been present at one time or another on the surface of

Venus.

Rate of heat loss

Assuming similar proportions of heat generating materials are present in the mantle as on Earth, a heat loss rate of 6 x 10-^^ W kg-i is required. Stofan et al. (1992) calculated that the rate of heat loss (Qd) from mantle diapirsi is 2.46 x 10'^^ kg i, accounting for justi 4.1 x 10'^ or 0.41 % of the estimated flux

The results of the present survey suggest that a new value of Qd might be obtained and that coronae account for a more significant fraction of the heat loss from Venus.

Equation 10.1 of Stofan et al. (1992) was used where p is density (3250 kg m-^i),c is conductivity (1 kJ kg-^ K’^), AT is the temperature difference (275 °K), Tg is the time

151 scale (2001 Ma), and M is the mass of the mantle 4 x 10^4 kg. Old values of R^j and rj were substituted; R^, (diapir diameter) becomes 200 km and p (number of coronae) is now 700.

Q d = ^ R 3 £ £ ^ [1 0 .1 ] 3

These values give an estimate of the rate of beat loss of 4.598 x 10-'^, or ~8 % of the total flux. In the corona development model of Smrekar & Stofan (1997), which invokes plume upwelling and subsequent delamination of the lower lithosphere at the plume head, coronae are estimated to account for 25 % of heat loss from the planet.

Assuming that similar proportions (about half) of coronae recorded by the new survey are active, coronae could account for a third of the total heat loss from Venus. The likelihood that coronae form by more than one mechanism makes it difficult to provide accurate heat loss estimates until such uncertainties are resolved. These new estimates, however, strengthen the case for steady state heat loss on Venus through the activity of plumes and mantle plumes (Bindschadler et al., 1992; Phillips and Hansen, 1994). The findings corroborate objections to other heat-loss mechanisms such as catastrophic lithospheric overturn or plate re-cycling.

Morphology

Existing models for the evolution of coronae on Venus are found wanting by the novel data presented above. The evidence discussed in the preceding chapters shows that coronae evolve over longer time-scales than predicted or assumed by existing

152 models and that coronae may follow a variety of evolutionary pathways rather than the single route the models all favour.

A three stage model of corona evolution was proposed by Squyres et al. (1992) in which corona development | takes place as upwelling, spreading and relaxation of the surface when the plume is shut off (Chapter 1). Most early numerical models (Janes et al., 1992; Janes and Squyres, 1994; Kreslavsky, 1994; Koch, 1994; Koch and Manga,

1996) echo this pattern of development. For example Koch and Manga (1992) model a rising spherical diapir which produces a dome (type 1 in this thesis). The diapir spreads laterally at the depth of neutral buoyancy forming a plateau (type 2). When the diapir begins to subside a rim which surrounds an elevated interior (type 3) is formed. As the centre of the corona subsides, a rim-only corona (type 4) exists for a time until further relaxation of the surface leaves a rim which encircles a low-lying interior (type 7).

Unfortunately their model fails to account for the range of corona morphologies, including coronae which contain central rises and those which are encircled by a trough.

Sandwell and Schubert (1992a; 1992b) and Schubert et al. (1994) explain trough formation at coronaeby retrograde subduction. In their model (Fig. 10.1), the lithosphere beneath a rising plume is weakened and breached; the lithosphere at the margins of the corona is rolled back as the plume materials spread outwards. Despite some similarities between the model profiles they generate and large coronae on Venus and regions of back-arc spreading on Earth, geological evidence from SAR images goes against this mode of evolution. Smrekar and Stofan (1997 point out that the large coronae which are suggested to have formed by lithospheric subduction lack transform faults at their margins. Furthermore, it appears that many! coronae interiors are comprised of the same material as the surrounding plains material.

153 Although the three stage model was widely accepted and favoured over alternatives,

Copp et al. (1996) demonstrated that corona formation is far more complex. Copp presented evidence, from the detailed geologic mapping of selected coronae, that they evolve over prolonged periods, highlighting the shortcomings of the early models. [ They showed that materials which form the deformation annulus of coronae are sometimes embayed by more recent materials which have also been deformed or fractured by plume activity.

The model of Smrekar and Stofan (1997) accounts for most of the morphologies reviewed in this survey. The model (outlined in Chapter 1) involves delamination at the plume head, generating the trough observed at more evolved coronae, and isostatic rebound which in the later stages leads to the formation of a topographic rim which encircles an interior above the level of the surrounding plains. The model dispels the assumption that positive topography is an indication of the youthfiilness of coronae and low (or negative) topography is an indicator of senescence. Domes, incipient coronae at

130 M a, have positive topography and end stage coronae, with model ages of 200 -

300 Ma , have positive topography.

One important feature of the Smrekar and Stofan model is that subsequent stages may be superimposed upon one another resulting in complex rim structures (Stofan, personal communication). The complex forms found during the survey for stealth coronae (Chapter 5) usually comprise a well defined topographic rim which encircles an interior containing a series of discontinuous arcuate ridges. Though such structures may be consistent with the model proposed by Stofan and Smrekar (1997), the evolution of multiple arcuate rims could be attributed to multiple phases of plume growth, an idea first mentioned by Phillips and Hansen (1994).

154 Outer Corona Outer Trench Trench

(a)

(b)

Figure 10.1. (a) Schematic cross-section of a mature corona formed by lithospheric subduction, (b) Stages in the evolution of coronae where: (a) plume upwelling occurs thinning the lithosphere and causing volcanism, (b) the volcanic load and further weakening cause a breach in the surface layer, and (c) the edges of the lithosphere sink, forming a trench that expands outwards as retrograde subduction progresses. After Sandwell and Schubert (1992b).

Tessera embayed a. Plains surface emplaced ywvyvF Tessera /wvvv Uplift of plains surface Relaxation of surface c.

f Plains surface drops Hinge (rim) format/ion \ Drag

Delamination

Figure 10.2. Regional plains materials in the Scarpellini Quadrangle northwest of Mukylchin corona embay tessera. The low elevation of the same plains materials near Mukylchin indicate that they have been warped downward, possibly by delamination of lithospheric materials beneath the corona.

155 Geological observations made during the mapping of Scarpellini (Chapter 8) are not inconsistent with delamination occurring at corona margins as proposed by Smrekar and

Stofan (1997). The configuration of geological materials (Fig. 10.2) northwest of

Mukylchin Corona indicates that unless the tessera or lineated plains have undergone uplift, for which there is no evidence, the plains have been warped downward. The plains materials may have been bent downward by the topographic load of the corona rim and volcanic materials but the downwarped region is very extensive (greater than one radius) whereas the effect of topographic loading and flexure might be expected to be more localised.

The range of morphologies observed by the new survey is consistent with the delamination model of Smrekar and Stofan (1997). Stratigraphie evidence is harder to interpret, and although there exists evidence for the Smrekar and Stofan model, there are some indications that corona evolution may actually differ from the proposed scheme.

The observation of a range of morphologies within regional plains units which have an estimated average age of 500 Ma (Phillips et al., 1992) supports the time scales of evolution yielded by the model.

There is stratigraphie evidence (consistent with the findings of Copp et al., 1997) that coronae develop over longer periods than suggested by the Smrekar and Stofan model. Many coronae have flooded interiors and it is difficult to determine the nature of the materials in which coronae development was initiated, although materials from earlier corona development episodes are sometimes preserved on or near the topographic rim. On Ma Corona, for example, annular deformation was taking place after regional plains emplacement as indicated by the fractured and down-warped materials on the eastern rim. Annular deformation at Ma Corona was also occurring for

156 a considerable time before the plains emplacement as shown by the fractured materials embayed by regional plains.

Examination of the trough which encircles Ma Corona reveals a third rim. Such a

configuration is difficult to reconcile with superimposition of morphologies in a single

continuous phase of corona formation proposed by any of the schemes previously

proposed. Detailed study of the morphology, tectonics and stratigraphy of Ma Corona

shows that its evolution is consistent with the comment by Phillips and Hansen (1994)

that corona development may occur in pulses. The episodic development of coronae

would help to explain how multiple rims form and why a single model cannot easily

account for the range of morphologies observed. The existence of a secondary pulse

would help to explain the support of the plateau-like interior associated with some

coronae.

The earliest interpretable units associated with Ma Corona are the embayed

materials on the western rim. Fractures aligned with these in later materials indicate

that the corona rim was forming before and after rim deformation. Because the

materials comprising the interior consist of emplaced flood lavas, they can be inferred to

have deformed after the rim. Such a configuration can be explained by a resurgence of

the plume, or by the evolution of a new plume within the confines of the first. The steep

interior rim occurring in the later stages of corona development is not accounted for by

iso static rebound.

Copp et al. (1996) observed that corona annuli do not always coincide with the ridge

and trough topography. This, while not inconsistent with the Smrekar and Stofan

model, tends to support the idea that some coronae develop in phases whereby plume

activity is shut off and relaxation of the deformed and fractured surface occurs, followed

157 by the resurgence or birth of a fresh plume in a different place, which may fail to

generate the stresses required to lead to extensive brittle deformation.

The Smrekar and Stofan (1997) model reproduces many of the observed

morphologies, including late stage morphologies. It rules out ‘blob’ detachment

(separation of the plume head from source), and favours a continuous viscosity-driven

cycle. Such a hypothesis is incompatible with the idea of pulses of plume activity,

which, unlike the Smrekar and Stofan model, accounts for the formation of a more

complete range of coronae with complex topographies.

Study of SAR images of the Scarpellini region (Chapters 8 and 9) reveals evidence

for the three stage corona formation model of Squyres et al. (1992). The three stage

model can account for the range of morphologies observed (Chapter 5), if, as Phillips

and Hansen (1994) suggest, corona forming activity can be renewed in pulses.

The initial stages of corona formation are assumed by Kiefer (1994), Phillips and

Hansen (1994) and Parmentier and Hess (1992) to consist of the development of Rayleigh-

Taylor instabilities at the interface of a density boundary. Then, fed from below, a hot

low density mantle plume rises though the cooler, denser upper layer (Stofan and Head,

1991; Squyres et al., 1992).

Plains surface and overburden updomed by rising plume. As the plume rises the

upper layers are flexed upward creating a bulge in the surface. Most coronae which

have a domical topography display a system of fractures which radiate from an origin on

the summit (Janes, 1992). The radial fractures on the outer flanks of Mukylchin

Coronae are extensional and suggest that updoming of the plains surface has already

occurred. Updoming is accompanied by downwarping of the surrounding plains

surface, whereby the neck of the plume is compressed by the denser surrounding

158 material. Horizontal compensation may result in a localised drop in the level of the plains surface surrounding the dome. Downwarping is consistent with both the Smrekar and Stofan model and the three stage model of Squyres (1992). The surface surrounding coronae sometimes contains regions which are topographically low compared to the average elevation of the regional plains that embay outliers of tessera.

Lateral spreading of plume head. The plume head is more dense than the surficial materials and cannot breach this layer. It is however more buoyant than the layer through which it has travelled. The neutrally buoyant plume head, still being fed from below, spreads radially just beneath the crust. The plains to the northeast of Nabuzama contain a series of wrinkle ridges that are concentric to the coronae. These linear features are interpreted as compressional and corroborate the idea that outward expansion of a plateau takes place at some point during the evolution of the corona.

Cooling of plume and development of basin. As the plume cools, it becomes more dense. This cooling, perhaps combined with a lower rate of supply, causes the plume to contract; as it does, the surface relaxes; at the margins of the plateau a ridge is formed as the basin develops (Squyres et al., 1992). Nabuzama and Mukylchin coronae exhibit this pattern of topography. They comprise a basin, lower than the surrounding plains encircled by an outer rim. Pressure release melting may occur at this stage. With a decrease in pressure occurring at the plume head, lava reservoirs form. Lava is able to penetrate the fault system established at the boundaries of the basin, where stresses are greatest and erupt from the coronae rim. Both Nabuzama and Mukylchin are associated with abundant volcanism, including extensive lava flows on their rims. Nabuzama also contains a central volcano.

159 Secondary plume development and relaxation. Resurgence of the plume occurs as a new central dome is formed, later followed by relaxation of the interior. This leads to the formation of a second ridge and disruption of the interior. The complex morphologies of many coronae such as Ma are accounted for by rejuvenation of the plume.

Morphology Model phase (1) Dome Incipient coronae. Initial uplift and (2) Plateau spreading. (3) Rim encircling elevated interior Relaxation of plains surface. (4) Rim encircling low-lying interior Formation of basin with or without (7) Rim only rim. (8) Basin (5) Rim encircling central rise Initiation of secondary phase of coronae development. (4b) Trough encircling a rim and basin Formation of secondary structures. (6) Rim encircling basin and central rise with Repeated uplift and relaxation. depressed interior (6b)Two rims encircling a central rise (10) Chaotic terrain of arcuate ridges (higher or lower Further activity. Disruption of than the surrounding plains) encircled by a rim or existing morphologies. trough.

Table 10.1. Corona types and proposed sequence of development.

Morphology and tectonics

Altimetry and synthetic stereo images reveal coronae which are hard to detect in

SAR images because they lack a fracture annulus. The observation that some coronae do not have an annulus suggests that either not all coronae form annuli or that these coronae are yet to develop an annulus.

A range of conditions may exist which prevent coronae from developing annuli.

The plume itself may not be able to deform the lithosphere sufficiently to form intense brittle fracturing. This scenario is unlikely, however, as the widths of coronae of the

160 new survey indicate that the plumes are of similar dimensions to those forming the coronae identified in the 1992 survey. Lithospheric conditions may vary spatially, such that in areas where the crust is thinner, topographic expression is likely to be pronounced and brittle fracturing evident. In areas where the crust is thicker, plumes are less able to deform the surface or cause brittle deformation or both.

Alternatively coronae are yet to form or have been prevented from forming annuli either because: (a) the coronae identified by the 1997 survey are incipient and have not yet formed a brittle scale annulus, or (b) the development of coronae has been halted by lithospheric thickening, preserving coronae that have low heights and which lack intense fracture annuli.

The major difference between the coronae of the 1992 survey and the 1997 survey is the low height of the coronae of the new survey. The amount of flexure associated with a corona is the main factor influencing the amount of associated tectonic (brittle scale) deformation. However, some types of coronae exhibit fracture patterns of varying intensity but have very similar mean heights. This discovery suggests that not all coronae form fracture annuli (or at least not at the same stage), even if many coronae which lack fracture annuli are interpreted as relatively youthful features.

Chronology

A means of dating coronae and thus estimating the rate at which they have formed would provide useful insights into variations in lithospheric conditions and mantle behaviour over time. A geological means of estimating corona age would clearly improve our understanding of the geological evolution of Venus and provide a means of checking the validity of numerical and theoretical models.

161 Three kinds of technique have been employed by investigators in attempting to establish the age of coronae: (1) stratigraphie techniques (discussed at length in the next section), (2) crater statistics (Phillips et ah, 1992; Namiki, 1994), and (3) corona morphology (Kreslavsky, 1995; 1996).

The use of crater statistics for this purpose involves counting the number of craters that fall 'within coronae. Owing to the lack of very large coronae, and the paucity of craters, it is not possible to obtain a good estimate of the average corona age. Of 319 coronae examined by Namiki and Solomon (1994), 89 % were found not to contain impact craters.

Two approaches rely on the existence of a correlation between coronae age and morphology. To establish rates of corona formation in the plains, Kreslavsky (1995) attempted to distinguish between young and old coronae on the basis of whether their topography was elevated (as in a dome or plateau) or depressed (as in a basin). Besides deficiencies in the dataset and the crudity of the resulting estimate, many coronae with positive topographic profiles have been subject to secondary uplift.

The second approach is based on the assumption that older coronae are more complex. Although it is likely that a more complex corona will be older than a less complex structure, evidence from the Scarpellini Quadrangle shows that this relationship does not always hold true, as complex or multi-phase coronae (e.g.

Nabuzama) sometimes deform a single recent unit.

Stratigraphy

The stratigraphy of coronae identified by the new survey was outlined in Chapter 4, the detailed mapping of the Scarpellini Quadrangle in Chapter 8. Most of them have formed in a single unit (regional plains). Fewer occur in plains which are older and

162 which are locally overlain by regional plains. This suggests that the majority of stealth coronae are relatively youthful features. Some complex morphologies have evolved within regional plains materials, indicating that they have formed over time-scales of

>500 My. Some coronae, particularly coronae identified by the 1992 survey (e.g. Ma

Corona), deform several units and have either developed over a long time scale (thus spanning several units) or formed a complex morphology episodically.

The global stratigraphy and stratigraphie analysis of coronae undertaken by

Basilevsky and Head (e.g. 1995; 1996; 1998), is fundamentally flawed: it assumes global synchroneity of emplacement and geological processes, and claims that coronae comprise materials which can be distinguished from the materials in which they form.

The findings presented here are corroborated (at least in part) by mapping results from other Quadrangles (Copp et al., 1996; 1997) and more recent work by Ivanov (1998) and

Ivanov and Head (1998) which demonstrate that coronae formed in more than one stratigraphie unit.

Global stratigraphie model

Basilevsky and Head (1994; 1995) selected 36 sample sites centred on impact craters

(considered to be randomly distributed) and studied the age relations of materials. The global stratigraphy which they derive is given in Table 10.2. Although such a broad stratigraphie framework may be useful, the scheme makes a number of unrealistic assumptions.

Their stratigraphy is used to correlate units globally which have similar surface

(backscatter) properties. Although the scheme has been applied to a number of quadrangles, it does not follow (for example) that regional plains emplaced in one

163 region were emplaced at the same time as materials with similar surfaces properties

elsewhere on the planet.

Geologic Time Time-Stratigraphic Rock-Stratigraphic Units Units Units

Aurelian Aurelian Aurelia Cdp Period System Group

Atla Ps, PI Group Guineverian Guineverian Guineverian Rusalka Pwr Group Period System Supergroup Lavinia Pfr,RB Group Sigrun Pdf, Group COdf Fortunian Fortunian Fortuna Tessera Period System Group Pre-Fortunian Pre-Fortunian ?? Period System

Table 10.2. Global stratigraphy sequence proposed by Basilevsky and Head (1995). Units include: Cdp (crater deposits), Ps, (smooth plains), PI (lineated plains), Pwr (wrinkle ridge plains), Pfr, (fractured plains), RB (ridge belts). Pdf (deformed plains).

Basilevsky and Head name units on the basis of the structures they contain and

assign major geological events to distinct epochs of venusian history. For example there

is a wrinkle ridge forming epoch represented by the unit Pwr (wrinkle ridge plains).

Mapping of the Scarpellini Quadrangle reveals two units {Ip and Rp), intermediate

plains and regional plains, which contain wrinkle ridges. Basilevsky and Head (1995)

also identify an episode of plains formation dominated by shields {Psh). Least

consistent with the geological evidence presented in this work is that coronae are

consigned to a discrete corona forming episode of venusian history. Assigning

structures to stratigraphie units does not work. Study of the stratigraphy of coronae

shows that coronae post and predate regional plains and post and predate wrinkle ridge

formation.

164 In summary, basing a global stratigraphy on a series of sample sites is problematical.

It is not yet possible to correlate regional geological units over the whole planet. A global stratigraphy will result from the detailed mapping of contiguous areas with the aid of synthetic stereo images. Attributing geomorphological processes to discrete geological epochs is demonstrated by the mapping results presented in this work to be unsound.

The new survey identified coronae in a variety of units. Within the mapping quadrangles V30, V31 and VI9 (Guinevere Planitia, Sif Mons and Sedna Planitia respectively) coronae have also been seen to have formed in a variety of geological units

(Copp et al., 1996; 1997). The new survey, however, identifies coronae which have formed in materials interpreted as the most youthful (regional plains) and materials interpreted to be the oldest (tessera), thus expanding the period over which coronae are considered to form and completely refuting the idea of a corona-forming epoch.

Recent work (Chapman and Zimbelman, 1998) distinguishes between coronae occurring in the plains and those associated with rifting. Using standard geologic mapping techniques they show that the coronae in these different settings formed at different times. This contradicts the interpretation of Basilevsky and Head (1995) who treat the deformed annuli of coronae as discrete geological units (COdf) which formed simultaneously and globally.

The geology of Ma Corona clearly indicates that coronae deform ambient units (in addition to forming over several geologic episodes). The material comprising the rim of

Ma Corona can be seen to contain volcanic materials which have been deformed by corona development and not emplaced, but simply physically altered as a result of corona development. At Ma corona, a 10 km diameter volcanic cone that has been tectonically

165 modified is located within a chain of concentric outcrops of mottled and lineated plains.

These have been cross cut by arcuate lineaments which are interpreted as resulting from annular deformation by a rising plume. Outliers of this material become more extensive and contiguous to the south, where a large region of mottled and plains is encountered.

These mottled plains contain the intensely fractured annulus of Ma corona. Cross cutting the Ma Coronae fracture annulus, within material which is clearly discernible as mottled and lineated plains, is another annular belt of fractures which is aligned with the topographic rim of C6. The material that embays the mottled and lineated plains outliers consists of Scarpellini regional plains. This material can be seen to have been deformed; it contains the annular fractures that encircle Ma corona to the north and east.

Synthetic stereo reveals a trough within this material into which lavas erupting jfrom the corona margin have flowed. The central plateau contains both mottled and lineated and regional plains materials, suggesting that the plateau formed more recently.

There exist many other examples of multiple unit deformation within the Scarpellini

Quadrangle. The rim shared by coronae C1-C3, (Chapter 8) comprises materials interpreted as mottled plains materials which were embayed by regional plains before further deformation took place.

Because of the scantiness of the crater record, the lack of contiguous mapping, and uncertainties in the rates of corona formation (supply rates) and senescence caused by external factors (e.g. lithospheric thickening), it is not possible to provide either specific corona ages or a reliable chronology. It is however possible to assert on a regional basis that coronae are not necessarily coeval. Coronae which form in tessera or early plains units (exhibiting annular deformation from this episode) and are emabyed by more youthful units are probably older than structures which deform a single unit. There remains the possibility that in some cases, very early episodes of corona deformation are

166 completely obscured by the emplacement of plains materials, which would give the impression that the corona population is more youthful than it is.

Global geology

The findings resulting from the analysis of the expanded corona database have implications for the global geology of Venus. Coronae are more abundant and more widely distributed, from which it may be inferred that corona-forming sources are more numerous. Coronae are also observed in a wider range of geological settings, indicating that the conditions for corona formation are more prevalent than previously thought. In this section, the implications for global geology, including heat loss and lithospheric thickness, are examined.

Brown and Grimm (1996) and Phillips (1997) suggest that Venus has undergone crustal thickening owing to a decline in heat-flux to the base of the lithosphere, and that the transition from a mobile “thin” lid phase to a “thick” lid phase involved lithospheric thickening to 100-150 km. Geological evidence shows that crustal thickening may support some highland regions, although gravity data indicate that dynamic support is responsible for the maintenance of some topographic highs. Estimates of elastic lithospheric thickness from analysis of lithospheric flexure and spectral admittances

(Johnson et al., 1997) are indicative of regional cooling and thickening. The geological evidence, such as the paucity of coronae at high altitudes, is consistent with regional thickening of the lithosphere, accounting for the support of some crustal plateaux. The evidence is not, however, consistent with a global transition from a thin to thick lid, as globally widespread plumes have been able to deform the surface within the last 500

My. Physical evidence, such as tectonic pattern and morphology (Chapters 5 and 7) also indicate that Venus has a mechanically thin crust. Brittle deformation patterns observed

167 at coronae are consistent with a lithospheric thickness in the order of 2-5 km (Cyr and

Melosh, 1991). The complex physiography of the coronae suggests that the lithosphere is thin. The lithosphere can flex and deform as plumes rise to the surface and then relax as the plume cools and recedes. If the lithosphere of Venus was thick it would not respond in the complex manner observed.

168 Chapter 11

Summary and conclusions

The preliminary survey (Stofan et ah, 1992) identified a large number of coronae but the limiting definition of a concentric fracture annulus led to the description and analysis of an unrepresentative sample. The present survey, with the aid of synthetic stereo images, expanded the corona population by 229, bringing the population to 563. This expanded corona database provides an improved picture of the dimensions, morphology, stratigraphy and geological setting of coronae.

Coronae which lack a tectonic annulus tend to be lower in height. However, the degree of first order deformation associated with coronae (formation of a topographic rim) does not imply a correlative level of brittle scale deformation. It appears therefore that the ability of the plume to deform lithospheric materials varies, probably spatially with lithospheric thickness, but with other factors, such as deformation rates and the regional stress regime, playing a role in the amount of tectonism observed at coronae.

The existence of a greater number of coronae on Venus suggests that they are a far more important heat loss mechanism than previously thought. Moreover, stratigraphie results indicate that coronae have been active throughout the interpretable geological history of Venus. Results from the analysis of the distribution of coronae show that the conditions necessary for corona development are more widespread than previously indicated and that coronae occur in a wider variety of geological environments, including topographic rises and their margins, and ridge belts.

Although the Smrekar and Stofan (1997) model reproduces a range of morphologies, the work presented here, on the basis of stratigraphie evidence, favours the three stage model of Squyres et al. (1992) but with the rider that plume activity can occur in phases.

169 Plume rejuvenation may explain the complex morphologies unaccounted for by previous models.

Given the existence of a complex range of morphologies, occurring in a range of geological settings, throughout the interpretable geological history of Venus, it seems probable that corona formation follows or has followed different pathways. This may underlie the difficulty in ascribing a single model of corona evolution to the range of morphologies observed.

The work highlights the need for an improved stratigraphy, for the detailed analysis of coronae occurring in different geological settings, and for further investigation of the nature of corona formation. These areas of potential research and the possibilities of new data are discussed below.

Coronae have played a crucial role in the evolution of Venus and have been operative on a global scale for the interpretable history of the planet. They tell us about the properties of the lithosphere and its configuration, and they are markers of regional stress. As the visible manifestation of heatloss on Venus they are a window on interior processes. They can be considered the key to understanding the geological history of

Venus.

Further work

Analysis of the Magellan dataset

There remains much useful potential in the existing data. Further insights into the geological evolution of Venus could be obtained through the stratigraphie analysis of all coronae identified by the 1992 survey. At present only a small number of coronae of the original survey have been analysed in detail (e.g. Copp, 1998). A reliable corona stratigraphy database, compiled with the aid of synthetic stereo images and completed

170 V-Maps, could be used to correlate more accurately the units in which coronae form and provide a chronology of corona forming events. The data could reveal changes in the rates and style of corona formation globally. The identification of spatial trends in the age distribution of coronae may^help us to understand the evolution of the lithosphere and the resurfacing of Venus.

Detailed comparison of coronae which occur in different geological environments, and have therefore evolved under different conditions could help to explain why coronae exhibit such a diversity of morphological styles. Although lithospheric thickness probably plays an important role, it is still not clear how or the extent to which the geological setting in which coronae form or the plume source itself determines corona morphology. This work would have to be carried out in conjunction with very detailed stratigraphie analysis as, for reasons that are not clear, many coronae which share the same geological setting have very different morphologies and structural characteristics (Fig. 5.5). For example: if two very different sized coronae which are in proximity are coeval, and it can be shown that neither is incipient, this may indicate formation by plumes which originate from density boundaries co-existing at different depths.

Laboratory experiments

Centrifuge experiments carried out by Ramberg (1967) generated models which possessed a circular deformation ring resembling those of coronae on Venus.

Ramberg’s dynamic models comprised layers of different coloured clays. Plasticine, or composites which were spun at high speed in a centrifuge to simulate the behaviour of geological materials over long time scales. A plume could be formed by using a low density source material and the inclusion of an initiation spike.

171 While it is possible to build a dynamically similar model, the effect of non- mecbanical processes including chemical reactions, nuclear processes, beat flow and diffusion must be studied by other means. Another drawback is that our knowledge of the rbeological properties, strength, viscosity and yield point of venusian rocks is sparse: models would require many assumptions regarding the precise nature of venusian materials to be made, and there exist difficulties in finding and developing materials with rbeological properties suitable for scale modelling. Despite the drawbacks, the centrifuge technique has proved effective in the study of buoyancy-driven processes

(Ramberg 1973) and has many significant benefits. It allows experimentation with materials which have far greater viscosities than those which are required for non centrifuge laboratory work, thus permitting far more versatility in experiment design.

Alternately coloured materials can be used to indicate the location and magnitude of stress fields. The method gives greater freedom of control over initial conditions: for example, variation in crustal structure and composition can be fine tuned.

One such model (Fig. 11.1) shows a body of mid-density material which approaches the surface as a density driven plume. The model consisted of three density layers including a low density surface layer which prevented intrusion of the plume, causing it to spread laterally near the surface. As the dome spread deflation at the centre and buckling of the surface layer at its extremities took place. Cross sections of these models reveal structures that are analogous to the to the hypothetical three stage model (Fig.

1.2) of Squyres et al. (1992).

Our understanding of coronae evolution and tectonic development could be further enhanced by the development of laboratory models which simulate the process of corona development. Physical models of corona evolution might be used to support

172 1 cm

Figure 11.1. Cross-section through a centrifuge model (Ramberg, 1967). The model is constructed from coloured clays and plasticines to reveal the pattern of deformation. This example shows a well developed plume, the head (a) of which has spread laterally on reaching the surface. Deep rim synclines (b) can be seen in the material about the plume. Buckling of the surface (c) occurred as the plume head evolved.

(b)

Figure 11.2. Plan view of a centrifuge model in which two diapirs were formed. The margins are buckled by the spreading domes (a). Tensile cross joints (b) can be seen on the buckled rim. After Ramberg (1967).

173 numerical models. They could also be used in an attempt to explore the effect of local and regional geological conditions upon coronae formation. Dynamic models could be developed to study multiple and asymmetric coronae. Modem high speed centrifuges can be used to simulate tectonic processes. Dixon (1975) and Dixon and Summers

(1983) used a centrifuge capable of generating accelerations of 20,000 g to study complex fold and fracture stmctures. The ability to create such high accelerations permits constmction of models from materials of lower viscosity which can form brittle fractures. High speed centrifuge techniques could therefore be used to study the tectonics of coronae; centrifuge experiments could tell us why some coronae do not form fracture annuli.

New data

Whilst the Magellan mission made possible the detailed mapping of surface stmctures, the use of SAR images for geological mapping has limitations and there remain many uncertainties regarding the nature of venusian geological materials. Future missions using airborne probes known as aerobots may provide the key to resolving current issues in Venus geology.

Fig. 11.4 shows the trajectory of a hypothetical Venus mission which employs a balloon filled with reversible fluids to control and maintain altitude. A number of practical and theoretical studies demonstrate the feasibility of such a mission. In 1985 two Teflon-coated helium filled balloons each carrying an instrument ‘gondola’ were deployed into the atmosphere of Venus and monitored for two days as they drifted at an altitude of 54 km (Kremnev, 1986). In 1994 the fourth balloon experiment of the ALICE series (Altitude Control Experiment on Earth) was carried out by JPL. Reversible fluid control was used to perform 5 altitude adjustment cycles at 5 - 10 km. More recently,

174 Figure 11.3. Artist's impression of a balloon aerobot drifting over the a venusian landscape. Suspended from the liquid filled balloon is the instrument gondola and landing snake. (NASA picture).

C lo u d s 60 km -10° C 0.2 atm

Close 48 km Water starts 1.3 atm to condense Inflate balloon Reascend - 42 km . by natural 143° C heating clouds 2.7 atm Water starts O ptical Landing to vaporise “ 30 km G ondola Water and navigation snake 222° C ammonia data 9.5 atm

Descent to surface C lea r Reservoir and heat exchanger Open valve to vaporise Surface trapped water operations 0 km 460° C 92 atm

Figure 11.4. Schematic showing the stages in deployment and flight trajectory of a Venus Balloon aerobot which may one day provide optical wavelength images of the surface of Venus. Altitude is controlled by using reversible fluid. The balloon contains a fluid which is vapor low in the atmosphere and becomes a liquid at high altitudes. The 'landing snake' is used to warn of proximity to the surface and to help sustain the weight of the aerobot when it comes into contact with the ground. Source: JPL.

175 the Balloon Experiment: Venus (BEY) spacecraft system and mission summary of

DiCicco et al. (1995) and the 1997 Venus Geoscience Aerobot Study (VEGAS) show that an aerobot can be constructed to endure venusian conditions for about a year and perform a range of scientific objectives.

NASA’s Jet Propulsion Laboratory (JPL) lists among the objectives for a Venus aerobot: ‘make high resolution, visible wavelength transects across [the] surface of

Venus to obtain “ground truth” for interpreting Magellan data globally’ and ‘obtain detailed surface morphological data of selected targets by means of visible imagery’.

It is unlikely that new high resolution images will provide an improved means of visualisation of coronae in their entirety because their topography is very low over large distances; the Magellan SAR system enabled the identification of coronae and stealth coronae because radar is so acutely sensitive to variation in slope angle. There are, however, several ways in which new data could assist our attempts to understand coronae. High resolution images will enable very small structures to be resolved and will provide further information relating to surface processes, as the study of small scale features such as wrinkle ridges and fracture patterns will tell us more about regional scale geologic processes. There may exist structures beyond the resolution of Magellan that will prove diagnostic in many areas of Venus research. High resolution images could detect a wider range of tectonic structure associated with coronae, perhaps associated with coronae which in Magellan images appear to lack brittle scale fractures.

Multispectral images will advance the study of coronae and thus the geological evolution of Venus by enabling true geological mapping as opposed to geomorphological mapping which forms the basis of the current V-Map program.

Ground-truth of even a few sample sites would enable verification of the mapping of materials carried out on a geomorpholgical basis. Multi-spectral images could be used

176 to distinguish between materials on basis of their composition rather than their roughness and help to understand the origin and properties of geological materials associated with coronae.

Numerical modelling

Future numerical modelling might attempt to describe the types of coronae which seem to form in different geologic settings, for instance in association with volcanic rises and in chasmata or ridge-belts. Coronae forming in a thin lithosphere might be expected to have different morphologic characteristics to those which form in a thickening lithosphere or a chemically (Fe) depleted layer. Characteristics observed in this work including (1) the abundance of coronae in the plains, (2) their low rim height, and (3) their lack of deformation annuli, are consistent with the model of Smrekar and

Stofan (1997), which invokes a depleted mantle. Alternative models might be developed to account for coronae with morphologic characteristics which are not entirely consistent with this model.

Coronae of the solar system

Plume activity which has led to the formation of venusian coronae may have played a role in the evolution of other planets in our solar system. Many authors, however, state that coronae are unique to Venus (e.g. Janes et al., 1992), and although this appears to be the case, the existence of coronae on other terrestrial bodies cannot be ruled out. It is possible that elsewhere in the solar system plume structures which lack intense fracture annuli have remained undetected, either because they are not so obviously manifest as on Venus or because they have not been preserved. The existence of coronae or analogous structures on other solar system bodies is discussed below, though

177 stmctures which match the criteria employed by the team which undertook the 1992 survey (Stofan et al., 1992) are not likely to be uncovered.

Mars

Watters and Janes (1995), Tanaka et al. (1996) and IllesAlmar (1997) think that coronae are not unique to Venus. A study of the Tharsis region of Mars (Tanaka et al.,

1996) revealed 15 coronae, previously interpreted as deep-seated intmsives (Scott and

Dohm, 1990). Although the stmctures are partially buried or degraded, lack the outer troughs common to many venusian coronae, and have less volcanism associated with them, there are geological and contextual similarities between coronae on Venus and

Mars. Martian coronae have (1) similar diameter ranges, (2) annular topographic rims cut by concentric graben and (3) associated volcanism, and (4) many of them are identified on major fracture belts (Tanaka et al., 1996). But they are very hard to see.

Although Viking lOrbiter images have resolutions varying between 40 -100 m and provide good regional coverage, images obtained by Mars Global Surveyor (MGS) currently orbiting Mars and the scheduled MGS 98 are of extremely high quality and offer an alternative look angle to those obtained by Viking. High resolution images (10 m per pixel) from MGS might be used to examine the detail of rim stmctures identified by Tanaka et al. (1996), and may yield clues as to the origin of the stmctures. Altimetry returned by MGS, which is of a far greater resolution than that obtained by the Viking orbiter, could be used to determine whether there is any correlation between regional scale topography and the stmctures identified by Tanaka et al. (1996).

178 Earth

Although no directly comparable structures are known to exist on Earth, it is possible to draw analogies with a range of terrestrial features that have a density-driven evolution. The concentric and radial fissures which may form on the surfaces of salt diapirs resemble those observed in Venusian coronae. The granite and migmatite domes of the Shamvaiian-Bulawayan-Sebakwian orogenic belt of Zimbabwe (Macgregor,

1951) form bollard-like plumes and exhibit concentric fractures on their surfaces (Fig.

11.5). Such domes are more physically analogous to venusian coronae than salt diapirs:

(1) the size range is closer to that of venusian coronae (the Mtoko, Charter and Manyika domes have diameters of 100 - 120 km) and (2) they consist of silicic rocks. They appear to be dynamically related to venusian coronae, in that (3) they evolve over long time-scales and (4) they are interpreted to have formed by the upward flow of low density materials (pegmatites) accompanied by the downward flow of denser materials

(metamorphosed lavas).

Ernst and Buchan (1998) describe a number of localities (e.g. Fig. 11.6) where they identify giant radiating dyke swarms, which have been used to locate mantle plume centres on Earth (Ernst and Buchan, 1997), and arcuate dyke swarms, which may have once constituted a concentric annulus. The annuli that the remnant fractures potentially represent would have diameters in the order of 600 km. Ernst and Buchan (1998) suggest that these structures represent eroded coronae, but acknowledge that evidence indicating that a corona morphology once existed is required to strengthen the hypothesis.

Detecting coronae topography on Earth, if it had existed, would be very difficult. It is possible, however, that coronae are preserved in some marine environments and, with the help of sidescan-sonar images such as those returned by the GLORIA system, could

179 K W W ' YOUNGER ROCKS MAD2IWA I i g r a n i t e s

o ^ ^ U ^ O A M O R O MTOKO 9- (fzWIMBA) ( li

^(^>^GOROMONZI

m^irP —

SHANSANK^CHillMANZI \ V t'{I) 8IKITA # CHIBI

too krn

Figure 11.5. Granite and migmatite domes of the Shamvaiian-Bulawayan-Sebakwian orogenic belt of Zimbabwe. They form bollard-like plumes and have concentric fractures on their surfaces. They are analogous to venusian coronae in that they are interpreted to have formed by the upward flow of low density materials. After Macgregor (1951).

Figure 11.6. The 930 Ma Blekinge- Dalama (B) and ~ 900 Ma North Sea Hunnedalan (H) dyke swarms. R marks the 931 ±2 Ma Roagland m assif. The plum e centre is indicated by a star. After Ernst and Buchan (1998). Caledonian

250 km Phanerozoic 10° E 20° E

180 be surveyed for morphological or structural features which might be diagnostic of terrestrial coronae.

Corona-like structures observed on other planets may merely resemble venusian coronae in part, having formed via a very different series of processes; alternatively, they may represent a widespread and crucial planetary phenomenon. If this is found to be the case, their detection on other planets would be highly ironic. Early interpretation of Magellan and pre-Magellan studies attempted to discern and attribute to Venus a terrestrial-style system of plate-tectonics, yet one of the most intriguing structures observed on Venus and one vital to the history of the planet, may have played a role in the evolution of other terrestrial planets, including Earth.

181 Appendix I

1997 Corona survey: data and notes

M = Morphological class, (modified after Stofan et al., 1996): (1) dome, (2) plateau, (3) rim encircling a raised interior, (4) rim surrounding an interior below the level of the surrounding plains, (4b) trough surrounding a rim and basin, (5) rim surrounding a central rise, (6) rim encircling a basin with an inner rim and central depression, (6b) two rims encircling a central rise, (7) rim only, (8) basin, (9) unclassified, (10) complex or chaotic interior.

H Height: relief of coronae measured fi'om maximum altitude to base level (surrounding plains level).

W = W idth: measured froin ôutérrriost extent of topography .

C ^ M orphotectonic class, (after Stofan et al., 1992): (1) concentric, (2) concentric double ring, (3) radial/concentric, (4) asymmetric, (5) multiple.

V = Associated volcanism: (1) deficient in associated volcanism, (2) moderate number of associated volcanic features, (3) dominated by volcanism.

Name Longitude Latitude M Maximum Base level H WCV Tectonics Stratigraphy altitude degrees degrees class m m m km class class class class cI13 0.959 -21 10 6 0 5 4 1 8 8 6051064 3124 690 0 1 - Rp/. c69 2 .5 4 2 43.3 5 6051747 6051215 532 100 3 2 ACFRF - cl55 5.706 -2.7 9 6051972 6050991 981 1338 0 1 ACF R p/M p c70 6.233 4 1 .9 4 6051629 6050860 769 180 0 2 PACF Rp/Lp c74 7.947 28.5 4 6 0 5 2 1 0 8 6 0 5 1 3 1 4 794 185 0 2 ARN(B) Rp c71 12.17 42.2 4 6051451 6050978 473 195 0 1 ARN Rp c73 12.39 45 .6 4 6 0 5 1 6 8 8 6 051333 355 135 3 1 ACFRF Rp/Lp c77 12.79 1.36 4 6050944 6050653 291 160 0 3 AF ACF Rp c66 13.79 50.4 4 6052339 6051511 828 335 0 3 PACF - c75 13.835 1.69 6 6051129 6050685 444 385 0 3 PACF (R. B) Rp c50 15.86 4 1 .7 4 6 0 5 2 0 0 0 6 0 5 1 3 0 0 700 180 3 1 - Rp c51 16.82 4 3 .6 4 6 0 5 2 2 2 0 6 0 5 1 3 0 0 920 300 3 1 - Rp c20I 19.812 10 7 6 0 5 2193 6 0 5 1 3 7 8 815 515 0 2 - Rp c213 20.559 27.9 7 6051514 6051174 340 215 0 1 AF(R) Rp cl07 20.82 -13 9 6051842 6051374 468 420 4 1 ACF(R) R p/M p c54 21.04 44.1 4 6 0 5 2 1 2 0 6 0 5 1 3 4 0 7 8 0 230 0 1 PACF Rp c49 21.17 41.6 4 6 0 5 2 0 0 0 6 0 5 1 2 3 0 770 253 3 1 PACF Rp c47 21.31 50.4 4 6053300 6051870 1430 335 3 2 ACFRF Rp c8 2 1 .6 1 4 23.3 3 6052057 6051276 781 160 1 2 ACF(R)RF(0) Rp (Lp interior) c225 21.746 26.1 7 6051480 6051208 272 245 0 1 AF(R) Rp cl3 24.25 3 5 6051982 6051774 208 0 0 - Rp c53 24.25 4 7.6 4 6053192 6051821 1371 155 3 2 - Rp cll9 2 4 .4 6 -4.1 6 6051594 6051044 550 660 0 1 ARN Rp cl9 24.73 40.4 7 6051820 6051004 816 288 0 3 ABN(I) Rp c43 25.745 31.9 6 6051752 6 0 5 1 2 0 8 544 2 0 0 3 2 ACF(B) RF (O R) Rp c52 2 8 .3 8 4 8 .9 4 6053863 6052493 1370 145 3 2 - Rp c60 29.832 -34 4b 6052750 6051897 853 420 1 1 ACF (R) Rp c41 33.611 -51.1 7 6052808 6051972 836 755 1 2 ACF(R) Rp/Lt c72 36.6 -31 9 6052282 6051897 385 1480 2 1 - various? *H c44 39.32 37 7 60 5 2 2 9 5 6 0 5 1 5 1 4 781 130 1 2 ACF(R) Rp c l4 2 39.5 -17 7 6051622 6051264 358 275 0 1 AF(R) Rp

182 cl43 41.65 -15 6 6051704 6051347 357 355 0 1 ARN Rp cl44 42.44 -18 7 6051842 6051264 578 265 0 2 AF(R) Rp c45 42.84 39.1 4 6052159 6051582 577 155 0 1 ACF(R) Rp cl25 43.63 -7.1 9 6052612 6051842 770 450 0 - c31 44.597 12.2 10 6053042 6051684 1358 450 1 3 ACF(R) Lp/? c42 47.585 7.86 7 6052737 6051174 1563 310 0 2 PACF (?) Rp cl 53.079 -37.6 10 6052254 6051484 770 965 0 1 ACF Rp/Lt cl45 54.31 -24 5 6052227 6051704 523 275 0 1 ACF(R) Rp/Lt c95 58.57 -26 4 6052007 6051539 468 275 0 1 stereo data gap Data Gap c48 59.8 61.3 4b 6052493 6052389 104 1828 0 1 PACF Rp c56 61.6 41.7 4 6051406 6050850 556 195 0 1 - - clO l 62.791 22.1 5 6052240 6050933 1307 135 0 1 ARN Rp c90 63.186 21.6 ■ 5 ■ 6052053 6051166 887 125 1 ACF(R) Rp/Lt c63 65.47 44.1 4 6051050 6050759 291 205 1 2 ACF Ip c55 66 43.4 5 6052045 6050759 1286 255 1 1 ACFRF Rp/T c59 66.75 49.4 5 6051548 6050776 772 420 1 2 ACF (B) Rp cl37 71.271 71.2 5 6056041 6052429 3612 295 0 2 RF (I) PACF (I) R p /L t cl38 75.825 70.4 4 6054028 6052429 1599 255 0 2 PACF Rp Clio 76.589 -47.5 4 6051564 6051352 212 265 0 2 AF Mp cl24 78.127 5.77 6 6052847 6051960 887 175 0 2 RF(0) Rp/Lt cl09 79.358 -50.6 5 6052476 6051967 509 305 0 2 - Mp cl39 79.91 73 4 6052666 6051295 1371 160 0 2 PACF Rp c58 81.12 42.6 2 6050776 6051540 -764 380 3 1 ACFRF - cl08 85.466 -59.8 4b 6052709 6051903 806 1000 1 3 PACF(B) Rp/Lt c7 86.389 -37 3 6051648 6051648 0 150 0 2 ACF (R) Rp (Lt interior) c68 89.861 12.1 5 6052520 6051776 744 110 1 1 ACF(R) Rp c57 92.19 8.91 6 6052847 6051680 1167 140 1 2 AN Rp c62 92.63 54.3 9 6052365 6051434 931 1500 0 2 PACF - c79 92.893 13.8 5 6051820 6051447 373 145 3 2 RF (O.I.R) AN (R) Rp c65 97.2 50.8 4 6052009 6051359 650 205 0 2 PACF RF(R) Rp c61 97.51 48.2 5 6051690 6051359 331 175 3 3 ACFRF Rp c46 97.639 24.1 3 6051773 6051306 467 265 0 1 ACFCR.B) RF (A) - c64 101.7 50.8 4 6051938 6051441 497 190 1 2 ACF Rp c97 106.3 -47.4 5 6051349 6050949 400 160 3 1 ACF(I) RF(0) Rp c94 107.7 -42.1 5 6052015 6051349 666 225 0 1 - Rp/T c227 109.8 44.4 4 6052201 6051530 671 270 1 1 ACF (B) Rp c96 109.86 -42.2 4 6051749 6051535 214 195 0 1 - Rp/Lt cl48 113.11 24.9 4 6051629 6050913 716 135 0 1 ACF(R) Rp cl4 0 113.19 72.9 6 6052488 6052015 473 645 0 CF PACF Lt/Rp c98 114.16 -47.8 4b 6051775 6051269 506 125 0 1 PACF(R) Lt cl5 0 115.17 24.5 4 6051252 6050913 339 160 0 1 ACF (B) Rp c99 115.48 -49.1 4b 6051482 6050949 533 100 0 1 PACF(R) Lt cl49 115.75 25.7 4 6051591 6050763 828 135 0 ACF (B) Rp clOO 118.07 -48.8 5 6051802 6051215 587 185 0 1 PACF (R) Rp/Lt cl05 119.52 -64.1 4b 6051752 6051440 312 360 1 1 ACF(I/R) Rp cl23 124.4 -0.3 6 6053915 6053050 865 340 3 ACG RF AN(I) Rp c226 124.5 51.2 4 6052261 6051649 612 330 3 ACF RF (R. 0) Rp cl41 127.1 66.8 6 6052787 6051997 790 550 0 1 PACF Lt cl26 127.9 -17 4b 6056331 6053460 2871 325 1 1 ACF RF(0) Lp/Lt cl03 128.97 -44.9 7 6052015 6051695 320 395 0 1 AF(R) Rp cl0 6 129.15 -47.3 4 6051935 6051535 400 283 0 1 AF(R) Rp cl0 4 131.74 -56.1 4b 6051509 6050842 667 400 1 PACF(R) Rp cl02 133.32 -43 4b 6052921 6052388 533 300 1 1 ACF(R) Rp/Lt cl4 6 141.5 28.1 5 6051666 6051177 489 355 0 1 ACF(R)RF (0) Rp/Lt

183 cl35 142.2 31.2 7 6051327 6051023 304 453 0 1 ACF(R)RF (0) Rp/Lt cl47 142.38 24.8 5 6051967 6051177 790 475 0 1 - Rp/Lt cl27 143.1 -18 4 6055009 6053323 1686 215 0 1 ACF (R) Lt cl28 144.3 -11 7 6056331 6052913 3418 300 0 2 ACF RF(R) Lt cl29 144.8 -7.7 7 6054007 6052822 1185 180 0 2 PACF Rp cl21 151 -20 4 6053541 6052937 604 175 0 2 PACF Rp clS l 151.17 11.4 3 6052088 6051029 1059 320 0 2 ACF (R) T clSO 151.74 16.2 6 6051636 6051129 507 385 0 2 WR PACF(R) T (Rp interior) cI20 152.7 -19 4 6053390 6052585 805 135 0 1 PACRF Rp/Lp cll5 153.49 -47.7 7 6051717 6051223 494 390 1 2 - Rp c llS 153.5 -13 4b 6053440 6052233 1207 300 0 2 PACF Rp/Lt cl82 156.17 17.1 7 6051987 6051180 807 378 0 1 ACF (B) ARCRDS T (Rp interior) cl 17 158:85 -45.2 5 6051618 6051190 428 435 0 1 AF(R) Rp c212 160.9 35.2 4 6051483 6051230 253 235 0 1 ARN Rp c210 161.27 29.7 7 6051886 6050978 908 320 0 1 ACF (R) Rp/Lt c211 161.28 23 4 6051533 6051079 454 400 0 1 - Rp c209 161.54 32.1 4 6051483 6051129 354 240 0 1 - - cl83 161.62 17.4 7 6052088 6051281 807 298 0 2 ACF(R) PACF (0) Rp cll4 161.84 -52 3 6052377 6051470 907 1200 0 3 AF(R) Mp c224 162.9 48.7 4 6051160 6050608 552 180 1 1 ACF (B) RF (R) Rp cl84 163.65 16.3 4 6052239 6051432 807 120 0 2 - Rp cl85 164.88 13.3 4 6052290 6051432 858 120 1 2 WR (0) ACF(R) Rp c207 165.14 29.7 7 6052189 6050978 1211 530 0 2 ACF (R) Rp c206 166.19 23.5 7 6052037 6051180 857 130 0 1 ACF (B) Rp c215 167.47 27 7 6051533 6050978 555 115 0 1 ABN (B) Rp clll 167.69 -55.5 4 6051915 6051322 593 185 3 2 ACF(R) RF(0) Rp/Lt c218 167.85 2.66 5 6052239 6051483 756 230 1 2 ACF (R) Rp cl87 168.65 7.64 4 6052037 6051382 655 113 0 2 ACF (B) ARCRDS Rp cl22 168.83 -39.2 7 6051780 6051277 503 250 0 1 WR Rp c217 169.09 5.42 4 6052037 6051281 756 300 3 1 RF (0) ACF (R) Rp c216 169.89 6.73 4 6052088 6051483 605 268 0 2 ACFRF Rp c220 170.37 0.55 5 6052744 6051785 959 190 1 2 ACF (R) PACF Rp c204 172.87 24 8 6052189 6051548 641 330 0 2 ACF (R) ABN(I) Rp cl95 173.97 31.7 4 6051684 6051129 555 305 0 1 PACF (0) Rp c205 173.97 21.9 9 6051936 6051548 388 80 2 0 - Ip cl 96 175.2 27 4 6052088 6051382 706 213 0 1 (?) Rp c200 175.91 25.6 4 6052189 6051684 505 175 0 2 ACF (R) Rp cl92 176.1 34.5 3 6051987 6051382 605 225 0 1 ACF (0) T c203 177.27 24.3 6 6052643 6051634 1009 245 0 2 - Ip cl98 177.53 27.5 4 6052693 6051785 908 345 0 3 - Rp c22I 178.06 2.83 5 6051836 6051533 303 160 0 1 - Rp cl93 178.4 34.6 2 6051936 6051382 554 200 0 1 - Rp/T def sim. c223 178.5 43.1 4 6051989 6051364 625 235 1 2 ACF (B) RF (R.0) Rp cl94 178.67 33.9 7 6052996 6051483 1513 65 1 1 AN Ip c202 178.67 25.3 4 6052643 6051785 858 185 0 1 PACF (R) Rp/Lt cl99 179.69 26.8 4 6052693 6051987 706 240 0 2 ACF (R) Rp c lI6 184.48 -44.1 6 6051503 6051289 214 460 0 1 - Rp cl88 184.96 30.8 6 6052693 6051281 1412 640 0 2 - Rp c214 185.4 35.5 4 6052390 6051281 1109 105 0 1 ACF (B) - cl90 191.16 29.8 4 6052441 6051483 958 190 4 2 WR(RA) ACF (R) Rp/Lt cl36 192.6 73.3 4 6052226 6051582 644 175 0 3 CF Rp/Lt cl91 193.09 28.9 4 6052542 6051533 1009 163 4 2 ABN Rp/Lt c222 193.4 50 3 6051594 6051002 592 190 1 3 ACF (B) RF (R.O) Rp cl59 195.29 -41.1 7 6051767 6051322 445 220 1 2 AN ARCRDS Rp/Lt

184 c29 195.6 50.2 4 6051430 6051211 219 115 1 1 ACF (B) RF (O R) Rp cl58 195.9 -43.1 7 6052317 6051678 639 240 1 2 PACF Rp/Lt cl30 197.4 -6.2 4b 6054056 6052840 1216 220 1 2 ACF (R) Rp c28 197.7 46.1 10 6051802 6051177 625 320 3 2 ACF (B) Rp cl5 7 200.34 -47.9 7 6051806 6051449 357 193 0 2 - Rp cll2 203.8 -4.3 4 6053529 6052476 1053 230 0 1 ACF (B) Rp cl56 204.25 -47.1 7 6052317 6051525 792 290 0 1 PACF Rp c229 205.22 29 4 6052304 6051865 439 245 3 3 ACF (R) RF (R.0) Rp cl54 206.49 -48.8 4 6051729 6051449 280 295 3 1 ACF (B) RF (O R) Rp c27 207.2 50.7 4 6052139 6051453 686 290 3 2 ACF (R) RF (0. R) Lp/Rp cl52 207.55 -47.5 7 6052087 6051525 562 190 0 1 AN (R) RF (0. R) Rp c3 208.25 19.5 3 6052646 6051914 732 200 0 1 PACF Rp cl51 209.39 -66.2 5 6052996 6052240 756 315 1 2 ACF WR Rp c2 209.6 -30 5 6051989 6051787 202 315 0 2 - - c4 210.3 40.9 2 6052158 6051377 781 100 0 2 AN (O.I) R Rp c208 210.8 -27 5 6052111 6051706 405 175 4 2 ACF Rp/Lt c219 211.1 -29 8 6052030 6051544 486 90 2 1 ANRF(O) Rp cl97 214.6 -18 5 6052759 6051787 972 185 0 2 ACF (R) RF(R) Lp cl33 217.6 75.3 8 6050438 6051582 -1144 185 0 2 CFWR Rp cl 86 218.4 -14 6 6052840 6051989 851 250 0 3 RF(R) Rp c35 218.7 -32 5 6052273 6051463 810 225 3 1 ACF RF (0) Rp cl53 219.1 -1.8 4 6052111 6052111 0 75 1 1 ACF (R) Lp cl75 219.1 -19 5 6052151 6051827 324 235 1 2 - Mp cl64 219.3 -5.1 5 6052962 6052111 851 315 0 2 ACF (R) RF (A) Lp cl34 220.54 79.1 4 6051523 6051553 -30 115 0 2 RF Rp c25 221.8 45.1 4 6052240 6051312 928 150 1 2 ACF (R) Rp c26 225.3 44.7 4 6052358 6051582 776 160 3 2 ACF (B) RF (R) Rp cl60 226.18 -46.3 4b 6051908 6051091 817 245 0 1 PACF (B) Rp c5 226.18 31.9 5 6052402 6051670 732 390 0 2 ACF(R) Rp cl61 226.75 -44.3 7 6051781 6051117 664 275 1 1 PACF (B) Rp c6 228.46 30.3 4 6052256 6051621 635 85 0 1 AF(R) Rp cl62 231.76 -44 7 6051678 6051091 587 185 0 1 PACF (R) Rp c24 236.46 -34.4 5 6051142 6050938 204 300 0 2 PACF Lt/T (T interior) c228 236.5 5 4 6052011 6051474 537 255 0 2 RF Rp cl 69 244.07 -52.5 4 6051540 6050982 558 295 0 1 - T (Rp interior) c20 249.11 5.73 5 6051516 6050978 538 245 1 1 ACF (B) Rp cl66 249.21 -58.3 6 6051726 6051205 521 715 0 2 - Rp cl32 259.94 83.2 6 6051202 6050821 381 285 0 2 PACF Lt cl73 263.09 -35 3 6051824 6051555 269 140 0 2 ACF (R) Rp c21 263.18 33.6 8 6052508 6052053 455 100 3 2 ACF (R) Lp c22 265.3 34.9 3 6053500 6051970 1530 140 3 2 ACF (R) RF (0) Lp cl8 266.7 6.95 4 6051722 6051309 413 115 1 1 AN(R) Rp c23 267.36 33.7 3 6052838 6052053 785 190 3 2 PACF (R) T cI74 267.59 -33.6 3 6051891 6051555 336 190 0 2 ACF (R) Rp/Lt cl63 273.42 -48.2 7 6052432 6051707 725 320 0 1 - Rp cl68 274.26 -55.2 7 6051837 6051205 632 685 0 3 ACF(R) Mp cl65 275.97 -50.4 4 6052079 6051949 130 260 0 1 AF(B) RF (R.I) Rp cl79 276.3 -20 3 6052863 6052058 805 335 0 2 RFPACF Rp cl78 276.41 -35.7 4 6052829 6051924 905 215 1 2 PACF Rp/Lt cl31 278.82 64.2 5 6052287 6051551 736 120 0 3 RF Rp cl76 282.2 -29 4 6052326 6052025 301 245 1 1 ACF (R) RF (R) Rp c93 285.5 62.6 5 6051361 6051116 245 175 1 2 PACF (R) - cl5 286.5 -11 5 6052686 6052115 571 520 0 3 ACF (R) Rp/Lt c89 286.6 62.4 4 6051967 6050836 1131 185 1 2 PACF (R) Rp/Lp

185 c l4 286.7 -7.5 2 6052263 6051989 274 160 0 3 PACF (B. I) Mp c l2 286.8 -8.9 4 6052337 6052115 222 155 0 2 PACF (R. B) Mp c33 287.62 13 4 6052318 6051144 1174 210 0 2 PACF Rp c91 291 61.9 4 6050836 6050556 280 50 0 2 ACF(R) RF (R.0) Lp/T cl72 291.75 -51 4 6052283 6051860 423 190 1 1 AF(R) Rp c34 292.8 11 4 6052502 6051408 1094 130 3 2 ACF (B) Rp cl6 292.8 -11 7 6053257 6052115 1142 465 1 2 ACF (R. B) Rp c92 293.4 62.4 4 6052027 6050556 1471 200 1 2 PACF Ip c88 294.3 48.7 4 6050871 6050484 387 515 0 2 PACF - cl7 0 294.56 -45.2 1 6051752 6052004 252 295 3 1 ARCRDS (G) ACF Rp cl71 296.01 -48.6 4 6052101 6052423 322 200 1 1 ACFRF Rp c36 296.71 9.64 5 6051995 6051378 617 290 0 3 - Rp c38 297.06 12.7 5 6051643 6051320 323 105 3 3 ACF (B)RF(O) Rp c37 299.66 10.5 10 6051936 6051467 469 223 0 3 ACF (B) Rp c40 299.79 16.7 5 6052171 6051613 558 290 3 3 ACF (R) RF (R) Rp c39 300.4 14.2 4 6051995 6051672 323 173 0 2 AF Rp cl7 302.4 -0.8 5 6052559 6051925 634 420 0 3 ACF (R) Rp clO 305.4 -8.1 4 6052813 6051989 824 380 0 2 ACF (R) RF Rp c87 306.2 55.8 6 6051256 6050836 420 395 0 2 PACF (R) - c83 306.7 64.3 10 6052727 6051466 1261 425 0 3 PACF - c82 307.1 64.7 5 6052272 6051361 911 410 0 2 PACF - c32 308.01 9.08 4 6053081 6051701 1380 375 0 3 - Mp c9 308.2 -9.5 4 6052749 6051925 824 165 0 3 ACF (R) Rp c84 309.3 58.8 4b 6053532 6051081 2451 680 0 2 PACF (R) - cll 309.5 -13 4 6052432 6051957 475 100 1 3 AN (R) RF (B) Rp c30 314.07 8.47 4 6051995 6051496 499 130 0 1 PACF (B) Rp c85 320.4 50.8 4 6051256 6050661 595 395 0 2 Data Gap Data Gap c80 322.6 56.6 5 6052307 6051081 1226 335 0 2 PACF Rp c86 323.4 46.6 6 6050976 6050381 595 525 0 1 Data Gap Data Gap c81 325.6 57 5 6052062 6050941 1121 433 0 2 PACF (R) - cl77 339.6 -11 5 6051583 6051355 228 508 0 2 ACF Mp cl67 343.6 -28 10 6052263 6051464 799 1515 0 1 PACF Rp/Lt cl89 347.6 -28 5 6052045 6051355 690 200 3 3 ACFRF Rp c78 348.48 9.38 4 6052029 6050944 1085 125 0 2 ACF (B) Rp c76 355.69 12.7 4b 6052294 6051023 1271 395 1 2 ACF (B) Rp c67 359.3 43.5 4 6055417 6050860 4557 240 0 2 PACF Rp

1 8 6 Appendix II

1992 Corona survey (modified after Stofan et al., 1992): data and notes

H = Height: relief of coronae measured from maximum altitude to base level (surrounding plains level).

W = Width: measured from outermost extent of brittle scale annulus.

C = Morphotectonic class, (after Stofan et al., 1992): (1) concentric, (2) concentric double ring, (3) radial/concentric, (4) asymmetric, (5) multiple.

V = Associated volcanism: (1) deficient in associated volcanism, (2) moderate number of associated volcanic features, (3) dominated by volcanism.

Name Longtude Latitude M Maximum Base level H W C V altitude degrees degrees class m m m km class cl

Schumann-Heink 214.5 74.5 1 6052431 6051582 849 150 1 - Bachue 261.5 73.5 1 6053806 6051319 2487 522 4 - 288 41.5 1 6051221 6050556665 900 3 2 271 35 1 6052714 6051309 1405 160 1 1 264 24.5 1 6055980 6052714 3266 280 6 3 265.5 19.5 1 6052094 6052384 -290 150 1 2 6 15.5 1 6052558 6052029 529 120 1 2 205 14.5 1 6051718 6052353 -635 200 1 3 Anala 14 11 1 6053591 60516851906 240 6 3 311 5 1 6051936 6051760 176 150 1 2

57.5 3 1 6053586 6052397 1189 200 4 X Dhorani 215 -2 1 6055028 60525972431 180 1 1 243 -8 1 6052494 6051354 1140 305 3 1 214 -9 1 6053894 60524351459 275 3 1 Tumas 255.5 -16.5 1 6053668 6051824 1844 180 1 2 270 -28 1 6052997 6052326 671 200 3 2 Lilwani 271.5 -29.5 1 6054036 60523261710 500 4 2 Kunapipi 86 -34 1 6053932 6051508 2424 220 6 3 Santa 288 -34.5 1 6053288 6052242 1046 200 3 1 Onatah 3.5 49 2 6052280 6051629 651 280 4 2 33 26.5 2 6052227 6051174 1053 110 3 1 Sappho 15.4 14 2 6053382 6051582 1800 300 1 3 Libera 24 12.5 2 6052805 6051752 1053 350 1 2 248.5 11 2 6051846 6051102 744 300 1 1 246 10 2 6051846 6051102 744 200 2 2 273.7 5 2 6052218 6051268 950 287.5 5 3 236 1.5 2 6052988 6051523 1465 350 5 2 243 -2 2 6051924 6051019905 212.5 5 1 Ceres 151.5 -16 2 6056760 6052736 4024 675 1 1 Shulamite 284.5 -38.5 2 6053869 6052461 1408 275 1 -

1 8 7 84 -66 2 6052158 6051882 276 277 5 -

Anahit 277.5 77 3 6052287 6051600 687 385 1 -

Maslenitsa 204 77 3 6052402 6051348 1054 223 1 -

Earhart 136 71 3 6053714 6051624 2090 370 1 -

Tusholi 101 69.5 3 6053951 6052033 1918 236 4 - Upunusa 252 66 3 6051949 6050864 1085 300 5 1 Nightingale 129.5 63.5 3 6054076 6051563 2513 520 1 2 Rananeida 263.5 62.5 3 6052266 6051067 1199 469 1 1 Mokosha 255 57.5 3 6053329 6051564 1765 200 1 3 Holde 156 53.5 3 6051949 6051101848 200 1 2 Ban 259 53 3 6052718 60509901728 497.5 1 2 Beiwe 306.5 53 3 6051571 6050766 805 600 5 2 Ki 227 43.5 3 6052898 60516161282 300 1 3 219 43 3 6052578 60515321046 270 5 2 207 35.5 3 6051914 6051621 293 150 1 3 12 35 3 6051526 6050997529 320 1 3 312 31 3 6051672 6050997675 225 2 2 Metra 348 29 3 6052135 6051341 794 180 3 1 Beyla 16 27 3 6052023 6051004 1019 290 4 1 Boann 136.5 27 3 6051629 6051101 528 300 1 2 Anqet 98 26.5 3 6051960 6051120840 225 1 2 Nissaba 355.5 25.5 3 6052638 6051950 688 250 1 3 Maya 98 23 3 6052100 6051306 794 225 1 2 Ereshkigal 84.5 21 3 6053174 60513061868 320 6 3 Kunhild 80.1 19.3 3 6053221 6051400 1821 200 6 3 37.5 18.5 3 6053178 6051854 1324 320 1 2 Perchta 234.5 17 3 6054504 6051670 2834 500 3 1 260 17 3 6052094 6051350 744 370 6 2 48 17 3 6053518 6051548 1970 550 1 2 Omeciuatl 118.5 16.5 3 6052269 6051327 942 175 1 2 Benten 340 16 3 6052003 6050785 1218 310 4 3 Belet-ili 20 6 3 6052125 60509371188 300 1 2 316.5 5.5 3 6052494 6051613 881 250 5 2 223 2.5 3 6053134 6052011 1123 412.5 4 3 Poloznitsa 302 0.5 3 6052288 6052083 205 675 1 3 264.5 0 3 6051764 6051350 414 200 1 2 Rosmerta 124.5 0 3 6053662 6053172490 300 4 2 Kuan-Yin 10 -4.3 3 6051500 6050919581 250 1 2 Cybele 20.7 -7.5 3 6052034 6051127 907 480 1 2 Maram 221.5 -8 3 6055717 60527592958 450 4 2 Atargatis 8.6 -8 3 6052263 6051173 1090 410 4 2 Nabuzama 47 -8.5 3 6053410 60517871623 525 3 3 Sith 176.5 -10.5 3 6054648 6052585 2063 405 4 1 Oduduwa 211.5 -11 3 6054299 6052354 1945 175 3 1 Zemina 186 -11.5 3 6054949 6052635 2314 530 1 2 250.5 -12.5 3 6052729 6051254 1475 300 1 3 Miralaidji 163.8 -14 3 6054245 60520822163 300 3 2 Atete 243.5 -16 3 6053567 60518241743 500 4 3

Aeracura 238.5 -19 3 6052557 6051868689 225 1 - Latona 171 -20 3 6057213 6052635 4578 810 2 1 291 -22 3 6053859 6052052 1807 400 1 3 265.3 -23.5 3 6052963 6052125 838 347.5 4 2 103 -25.5 3 6053373 6052160 1213 225 2 2 Mama-Allpa 31 -27 3 6052337 6051677660 300 1 2

188 Mayaeul 50.5 -27.5 3 6052334 6051732 602 375 2 2 Eve 359 -32 3 6053062 6051718 1344 330 1 2 Nott 202 -32.3 3 6052070 6051584 486 150 1 3 303.3 -33.3 3 6052940 6052211 729 200 1 2 Tamiyo 298.5 -36 3 6052908 6052722 186 252.5 4 2 Inanna 35.9 -37 3 6052165 6051952 213 287.5 2 2 Shiwanokia 279 -42 3 6053846 6052020 1826 675 1 3 Nzambi 287.5 -45 3 6053212 6052248 964 187.5 5 3 Enekeler 264 -46 3 6052042 6051484 558 337.5 4 2 Dunne-Musun 85 -60.5 3 6052858 6051946912 675 4 2 Quetzalpetlatl 0 -67 3 6053348 6052806 542 800 4 - Semele 208 66 4 6051515 6050980535 180 5 2 283 61.5 4 6051746 6050909 837 225 2 2 245 61.1 4 6052221 6051135 1086 150 1 2 326.5 54.8 4 6051676 6050976 700 180 4 2 Ashnan 357 50.2 4 6052398 6051629769 300 4 1 B a’het 0.3 48.3 4 6052221 6051629592 245 1 2 Audhumla 12 45.5 4 6052043 6051274 769 225 4 1 Tituba 214.3 42.5 4 6052308 6051402 906 200 5 2 306 42 4 6051326 6050486 840 120 2 2 222 41.7 4 6051966 6051899 67 160 1 2 Trotula 19 41.2 4 6051850 6051300 550 175 1 2 Rauni 271 41 4 6051587 6050909 678 268.5 2 2 Olwen 67.5 37.5 4 6051727 6050980 747 175 1 2 Emegen 290.5 37.5 4 6052259 6051525 734 150 1 2 277.5 36 4 6053541 6052549 992 125 5 2 Ved-Ava 143.5 33 4 6052193 6050988 1205 300 1 3 Zamin 258.5 31.5 4 6052260 6051433 827 320 1 1 246.5 31 4 6052094 6051144 950 162 5 1 250.5 31 4 6051764 6051102 662 200 1 2 241.5 28 4 6051929 6051144785 100 6 3 Qetesh 243.5 24 4 6051350 6050978372 362.5 5 1 240 22.5 4 6052590 6051350 1240 150 1 2 256.7 22.5 4 6051640 6051309 331 150 4 2 Chlun 340.5 18.3 4 6051394 6050679715 150 1 2 240 18 4 6052714 6051474 1240 125 1 2 240 17.8 4 6052301 6051474 827 325 1 1 Taranga 251.5 16 4 6053789 6051268 2521 525 4 1 256.5 14 4 6051598 6051102 496 152.5 1 2 253 13.5 4 6052053 6051392 661 200 1 1 311.8 12.3 4 6051936 6051525 411 100 1 2 251.5 10.5 4 6051681 6051392 289 300 1 2 94.7 10 4 6051867 6051633 234 120 1 2 254.5 9.5 4 6051764 6051350 414 150 2 2 247.5 8 4 6052632 6051020 1612 150 1 2 313.5 7.5 4 6052142 6051613 529 90 1 2 264.7 6.2 4 6051474 6051061 413 75 1 2 226 5.5 4 6052500 6051718 782 375 5 2 Gala 21.5 3.5 4 6051684 6050903781 400 5 2 214 3.5 4 6052549 6052109 440 137.5 1 2 233.7 3.5 4 6053134 6051914 1220 225 1 2 Habonde 81.8 3 4 6053454 6052147 1307 125 1 2 280 2.5 4 6052508 6051764744 187.5 5 3 219 2 4 6052451 6051764687 85 1 2

1 8 9 240.5 0 4 6052094 6051433 661 125 1 2 255 -1 4 6051723 6051287436 100 1 2 211.5 -1.5 4 6053529 6052516 1013 240 1 2 Seia 153 -3 4 6052485 6051931 554 225 5 2 220.5 -3 4 6052557 6051625 932 150 1 2 154.8 -4 4 6052434 6051831603 150 1 2 Krumine 261.5 -5 4 6051757 6051522 235 225 5 2 232.5 -5 4 6052313 6051706 607 125 1 2 Javine 251 -5 4 6052259 6051354 905 500 4 1 Verdandi 65.2 -5.5 4 6054165 6052580 1585 180 1 2 235.5 -5.5 4 6051949 6051503 446 150 1 2 Thouris 12.9 -6.5 4 6051536 6051282 254 290 1 2 234.5 -11:3 4 6052232 6051746 486 195 1 2 215.5 -15 4 6053083 6052192 891 220 2 3 Erkir 234 -16.5 4 6052881 6051949 932 195 5 2 201 -18 4 6052759 6051989 770 150 2 2 221.5 -19 4 6052192 6051908 284 230 1 2

Juksakka 44.5 -19.5 4 6052337 6051347 990 400 1 - 212.2 -20.5 4 6052192 6051989203 125 1 3 220.3 -21 4 6052151 6051746 405 260 5 2 250 -24 4 6053500 6051421 2079 420 4 2 Ohogetsu 85.7 -27 4 6052021 6051461 560 175 1 2 261.2 -27.5 4 6052729 6051891 838 225 1 3 245.5 -29 4 6051924 6051421 503 322.5 5 2 Colijnsplaat 151 -31.5 4 6053591 6052485 1106 350 4 2 266.3 -34.7 4 6051757 6051790 -33 180 1 2 Tamfana 247 -36.5 4 6051824 6051220 604 225 5 2 43 -37 4 6052172 6051649523 150 1 2 289.5 -50 4 6052095 6051742353 225 1 2 Tureshmat 289.5 -51.5 4 6052083 6051401682 110 1 2 Melia 119.5 62.9 5 6053326 6052004 1322 175 4 3 Aspasia 188 56 5 6051732 6050568 1164 200 4 2 Xilonen 329 51 5 6052027 6050696 1331 323.5 1 2 Neyterkob 203 49 5 6052679 6051453 1226 217.5 5 2 36.5 38 5 6052872 60516161256 180 2 2 206 37.5 5 6052402 6051425977 180 1 2 Junkgowa 257 37 5 6051958 6051020938 400 1 2 217 36.5 5 6052402 6051767 635 300 4 2 Blathnat 293.5 35 5 6052670 6051674 996 262.5 5 2 Kayanu-Hime 57 33.5 5 6052329 6051242 1087 150 1 2 Mawu 241 31.5 5 6051846 6051102 744 357.5 1 3 255 31.5 5 6051805 6051102 703 300 1 1 Eurynome 94.5 26.5 5 6052193 6051306887 200 2 2 234.5 26.5 5 6052207 6051718 489 125 1 2 314.5 26 5 6052024 6051202 822 290 1 3 207 25.8 5 6052646 6051767 879 205 4 3 Idem-Kuva 358 25 5 6053538 6052400 1138 230 1 3 218.5 23.5 5 6053769 6051865 1904 145 4 2 Ituana 153.5 20 5 6052189 6050978 1211 187.5 1 3 227.5 19.5 5 6054990 6052011 2979 350 3 1 63.5 17 5 6052287 6051353 934 100 1 2 Allatu 114 15.5 5 6051629 6051365 264 150 1 2 Kubebe 132.5 15.5 5 6051892 6051403 489 125 4 2 Dhisana 111.7 14.5 5 6051741 6051365376 100 1 2

19 0 Nehalennia 10 14 5 6052876 6051658 1218 345 4 3 254.1 14 5 6051970 6051232 738 125 1 1 258.8 14 5 6051746 6051144 602 125 1 2 228.5 12 5 6053183 6052060 1123 250 4 2 Hulda 308 12 5 6052347 6051467 880 300 1 1 Arum 262 9 5 6052632 6051350 1282 400 4 3 Atse Estsan 92 8.5 5 6052894 6051680 1214 150 1 2 Calakomana 43.5 6.5 5 6102906 605171851188 575 2 2 258 1.5 5 6051846 6051185 661 100 1 2 231.3 -0.5 5 6052273 6051408865 175 1 2 254.5 -3.5 5 6051857 6051287570 325 1 2 210.5 -4 5 6052881 6052476 405 230 1 2 Elgin 175 -5 ■ ■ 5 6052334 6051780 554 200 1 2 Unnamed 34 -5.5 5 6051787 6051319468 190 2 2 214 -6.5 5 6052719 6052151 568 175 1 2 259.3 -8.8 5 6051857 6051287 570 100 1 2 173 -11 5 6055050 60522332817 300 3 2 Mukylchin 44.5 -12 5 6052117 6051319 798 150 2 2 261.5 -12.5 5 6053835 6051924 1911 170 6 3 213 -13.2 5 6052719 6052070649 210 1 2 188 -15.5 5 6054899 6052585 2314 125 1 2 Ekajata 347.5 -16.4 5 6051863 6051355508 180 5 2 292 -16.5 5 6052591 6052211 380 100 1 2 227.8 -17.7 5 6052273 6051787 486 100 1 2 Nagavonyi 259 -18.5 5 6052595 6051857 738 225 5 3 250.5 -18.5 5 6051991 6051522 469 200 2 2 Bemth 233.5 -19 5 6052313 6051868 445 520 5 2 196 -19.5 5 6052759 6052151608 230 1 2 273 -20.5 5 6052963 6052092 871 150 1 2 97.3 -21.5 5 6052860 60516481212 160 1 2 210.2 -21.7 5 6052313 6051787 526 100 1 2 Bona 157.5 -24 5 6054094 6052233 1861 275 3 1 Nishtigri 72 -24.5 5 6052021 6051135 886 275 4 2 Fildias 177.3 -24.5 5 6053994 6052384 1610 150 3 1 Aramaiti 82 -26.3 5 6052067 6051275 792 350 1 2 Mokos 272.8 -27.2 5 6053802 60520251777 300 3 1 Agraulos 165 -27.5 5 6052988 6052283 705 150 2 1 216 -27.5 5 6051949 6051625 324 172.5 2 2 Epona 209.5 -28 5 6052111 6051584 527 380 5 2 259.5 -31.5 5 6051891 6051589 302 400 1 2 Oanuava 255.5 -32.5 5 6051757 6051354 403 290 5 2 Tai Shan 95 -32.5 5 6052673 6052021 652 290 5 2 6 -36.3 5 6052445 6051718727 400 1 2 Tacoma 288 -37 5 6052908 6052115 793 500 4 2 270 -38 5 6053533 6051757 1776 162.5 3 2 Nungui 245.2 -42.5 5 6051465 6051186 279 125 2 2 Khotun 80 -46.5 5 6051924 6051373 551 200 1 2 85 -48.7 5 6051988 6051375 613 175 3 2 Obasi-Nsi 291 -53 5 6052201 6051307 894 275 1 2 95.5 -53.5 5 6052009 6051415 594 312.5 4 2

68 -56 5 6053749 6052073 1676 275 4 - Otygen 30 -57 5 6053039 6051857 1182 455 2 2 Vasudhara 2.7 43.2 6 6052280 6051215 1065 160 1 2 Umm Attar 65 28 6 6051773 6050886887 262.5 4 3

191 205 27.5 6 6052256 6051572 684 230 1 3 Abundia 125 18.5 6 6052005 6051327 678 250 1 2 Bhumiya 118 15 6 6052080 6051327 753 125 1 2 H'uraru 68 9 6 6052100 6051540 560 150 3 1 211 6 6 6052988 6052207 781 60 1 2 Thermuthis 33 -9 6 6052309 6051347 962 330 1 2 154 -27.5 6 6052937 6052585 352 200 2 1 280 -27.7 6 6053466 6051958 1508 150 6 3 232.1 -28 6 6052232 6051746 486 200 3 2 Rigatona 278.5 -33 6 6052796 6052259 537 300 1 1 Annapurna 152 -35.5 6 6052635 6051931 704 300 4 2 Indrani 70.5 -37.5 6 6051881 6051228 653 200 2 2 277 -46 6 6052172 6052098 74 300 1 2 Makh 85 -47 6 6052158 6051225 933 275 2 2 Banba 209.2 -47.2 6 6051908 6051525 383 225 1 2 Lilinau 22 34 7 6053040 6052268 772 200 3 1 Pasu-Ava 318 29.5 7 6051496 6051056 440 300 1 2

323 28 7 6051320 6051114 206 - 1 - 244 11.5 7 6052838 6051516 1322 290 2 2 50 -33.5 7 6052667 6051732 935 130 1 2 Colette 323 66.5 8 6055843 6054859984 200 1 3 Sacajawea 336 64.5 8 6055238 6053417 1821 300 1 2 Nalwanga 247 49 8 6051671 6050924747 380 1 2 243 29 8 6052218 6051226 992 200 1 2 Mesca 342.6 27 8 6052426 6050579 1847 190 3 2 Nintu 123.5 19.2 8 6051967 6051290677 75 1 2 228.5 10 8 6052695 6051816 879 150 2 2 Madderakka 315.5 9 8 6052377 6051467910 200 1 2 Sunrta 11.7 8.3 8 6052055 6051526 529 170 1 2 215 -3 8 6053619 6052111 1508 300 2 2 254.2 -7 8 6052092 6051421 671 100 1 2 133.5 -12.5 8 6056376 6054462 1914 450 3 1 224 -14 8 6052273 6051949 324 300 1 2 223.7 -16.7 8 6052557 6051908 649 125 1 2 230.3 -20.3 8 6052557 6051868 689 225 2 2 266.4 -21.3 8 6052528 6052159 369 90 1 3 213.5 -21.5 8 6052111 6051787 324 162.5 5 2 229 -24 8 6052313 6051706 607 225 5 2 210.5 -25 8 6051989 6051706 283 100 1 2 196 -25.5 8 6052678 6051868 810 100 1 2 258 -28.5 8 6052058 6051790 268 240 1 1 Teteoinnan 149.5 -38.5 8 6052275 6051865 410 180 1 2 256.5 -42 8 6051837 6051409 428 100 1 2 Sarpanitum 14.6 -52.3 8 6053131 6051912 1219 170 1 2 Fotla 163.5 -58.5 8 6051750 6051008 742 150 1 2

97 -73 8 6051900 60519000 200 1 -

220 75.2 9 6051904 6051406 498 100 1 - Maa-Ema 102.5 40.8 9 6052193 6051400 793 300 4 2 Purandhi 343.5 26.1 9 6052347 6051473 874 170 1 2 Kamadhenu 136.5 21 9 6051666 6051290376 400 1 2 311.3 16 9 6052054 6051349705 80 1 3 313 5.5 9 6051995 6051672 323 100 1 2 Heng-o 355 2 9 6051897 6050864 1033 1060 1 2 281 1 9 6052218 6051846 372 170 1 2

1 9 2 259.5 -3.5 9 6051991 6051421 570 212.5 4 2 237.7 -13.2 9 6052151 6051746 405 180 1 2 271 -21.5 9 6053466 6052293 1173 225 4 2 240.5 -22 9 6051857 6051589 268 220 2 2 258 -28.7 9 6051958 6051857 101 200 1 2 Semiramus 293 -37 9 6054873 60526542219 375 4 1 Copia 75.5 -42.5 9 6052434 6051352 1082 550 4 1 55.5 -45.5 9 6052117 6051482 635 200 3 2 Mou-nyamy 59.5 -49 9 6053183 6051886 1297 175 1 3 Jord 349.5 -58.5 9 6053023 6051695 1328 130 3 3 Coatlicue 273 63 10 6052628 6051746 882 315.5 4 2 Fakohotu 108 59 10 6053030 6052111919 310 4 1 307 39 10 6051173 6050850 323 120 2 2 Nefertiti 49 36 10 6053790 6051582 2208 362.5 4 3 224 22 10 6052939 6051963976 350 3 1 Pavlova 39.5 14.5 10 6053756 6051955 1801 500 2 2 Isong 49.5 12 10 6054095 60516842411 540 1 3 102.5 -20.2 10 6052953 6052254699 150 3 1 Gertjon 276.5 -31.7 10 6053198 6051991 1207 262.5 4 1 Erigone 284 -34.5 10 6052561 6052025 536 200 3 I Artemis 135 -35 10 6056422 6052548 3874 2600 1 3

Pomona 300 79 4b 6052876 60518451031 360 1 -

Otau 298 68 4b 6052582 6051452 1130 206 1 -

Feronia 280 67 4b 6052287 6051551736 393 4 - Ninkarraka 221 65.5 4b 6051936 6050898 1038 125 1 3 Vacuna 95 61 4b 6052436 6051974 462 560 4 2 Haumea 21.8 54 4b 6053600 6053150450 317.5 3 1 Mari 151 54 4b 6051989 6051121 868 200 4 2 217.5 41.4 4b 6052088 6052088 0 180 2 2 Pani 231.5 20 4b 6053769 6052256 1513 325 1 3 226.5 13 4b 6052890 6052060830 300 5 2 226.5 9 4b 6052304 6051865 439 100 1 2 Blai 134.5 -0.4 4b 6054508 6053551 957 125 3 1 Hepat 145.5 -2 4b 6053050 6052275775 150 2 2 Fatua 17.2 -16.5 4b 6052309 6051319990 310 2 2

Bhumidevi 343 -17 4b 6052190 6051391 799 200 1 - Inari 120.3 -18 4b 6054518 6053050 1468 250 3 1 Ma 57 -23.5 4b 6052612 6051512 1100 450 1 2 Hervor 269 -25.5 4b 6052394 6052125 269 250 2 2 Chikisanti 276 -30 4b 6052628 6052192436 300 2 2 Ge^un 98.5 -33.5 4b 6053606 6051788 1818 300 4 2 Carpo 3 -37.5 4b 6052880 60512821598 215 3 1 Derceto 20.2 -46.8 4b 6052780 6051742 1038 155 1 2 Tangba 258.2 -46.8 4b 6051248 605118662 175 1 2 Cailleach 88.3 -48 4b 6051373 6051182 191 125 1 2 Eithinoha 8.2 -57.3 4b 6053483 6051912 1571 510 1 2 Seiusi 241 -62 4b 6052246 6051037 1209 150 1 1 Demeter 295 55 6b 6051957 6050486 1471 500 1 3 Reneti 326.5 32.7 6b 6051467 6051202 265 200 2 3 Ninhursag 23.5 -38 6b 6052062 6051869 193 125 1 2 Ukemochi 296 -39 6b 6054398 6052813 1585 325 1 2 Selu 6 -42.5 6b 6052589 6051234 1355 300 3 1

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