2012

U.S. Department of the Interior

U.S. Geological Survey

Prepared for the National Aeronautics and Space Administration

Geologic Map of the Themis Regio Quadrangle (V–53), Venus

By Ellen R. Stofan and Antony W. Brian

Pamphlet to accompany

Scientific Investigations Map 3165

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Contents

The Magellan Mission ..................................................................................................................................1

Magellan Radar Data ..........................................................................................................................1

Introduction ....................................................................................................................................................1

Mapping Techniques ....................................................................................................................................2

General Geology ............................................................................................................................................2

Mappable Units ....................................................................................................................................2

Plains Material ............................................................................................................................2

Corona Flow Material .................................................................................................................3

Volcanic Edifice and Flow Material .........................................................................................4

Structures and Other Features .........................................................................................................4

Impact Craters .............................................................................................................................4

Surficial Material ........................................................................................................................4

Wrinkle Ridges ............................................................................................................................5

Broad Topographic Ridge Crests .............................................................................................5

Lineaments and Graben .............................................................................................................5

Coronae ........................................................................................................................................5

Rifts ................................................................................................................................................5

Volcanic Features .......................................................................................................................5

Geologic History ............................................................................................................................................6

References Cited ...........................................................................................................................................7

Tables

1. Coronae that produced deposits mapped in the Themis Regio quadrangle (V–53), Venus. .....10

2. Impact craters of Themis Regio quadrangle (V–53), Venus. ..........................................................11

3. Volcanoes of Themis Regio quadrangle (V–53), Venus. ..................................................................11

4. Tabular version of the map unit relations in the Themis Regio quadrangle (V–53), Venus. ......12

Figures

1. Sketch map of major features in Themis Regio quadrangle (V–53), Venus, shown on base 

image (map sheet) 

2. Magellan topographic data of Themis Regio quadrangle (V–53), Venus (map sheet)

3. Magellan rms slope map of Themis Regio quadrangle (V–53), Venus (map sheet)

4. Magellan Fresnel reflectivity data for Themis Regio quadrangle (V–53), Venus (map sheet)

5. Magellan SAR image of an unnamed channel on unit pm in Themis Regio quadrangle (V–53), 

Venus (map sheet)

6. Magellan SAR image of east-central Themis Regio quadrangle (V–53), Venus (map sheet)

7. Magellan SAR image of northwestern Themis Regio quadrangle (V–53), Venus (map sheet)
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1

The Magellan Mission

The Magellan spacecraft orbited Venus from August 10, 

1990, until it plunged into the Venusian atmosphere on October 

12, 1994. Magellan Mission objectives included (1) improving 

the knowledge of the geological processes, surface properties, 

and geologic history of Venus by analysis of surface radar characteristics, topography, and morphology and (2) improving the 

knowledge of the geophysics of Venus by analysis of Venusian 

gravity.

The Magellan spacecraft carried a 12.6-cm radar system to 

map the surface of Venus. The transmitter and receiver systems 

were used to collect three data sets: (1) synthetic aperture radar 

(SAR) images of the surface, (2) passive microwave thermal 

emission observations, and (3) measurements of the backscattered power at small angles of incidence, which were processed 

to yield altimetric data. Radar imaging and altimetric and radiometric mapping of the Venusian surface were accomplished in 

mission cycles 1, 2, and 3 from September 1990 until September 1992. Ninety-eight percent of the surface was mapped with 

radar resolution on the order of 120 m. The SAR observations 

were projected to a 75-m nominal horizontal resolution, and 

these full-resolution data compose the image base used in 

geologic mapping. The primary polarization mode was horizontal-transmit, horizontal-receive (HH), but additional data for 

selected areas were collected for the vertical polarization sense. 

Incidence angles varied between about 20° and 45°.

High-resolution Doppler tracking of the spacecraft took 

place from September 1992 through October 1994 (mission 

cycles 4, 5, 6). Approximately 950 orbits of high-resolution 

gravity observations were obtained between September 1992 

and May 1993 while Magellan was in an elliptical orbit with a 

periapsis near 175 km and an apoapsis near 8,000 km. An additional 1,500 orbits were obtained following orbit-circularization 

in mid-1993. These data exist as a 75° by 75° harmonic field.

MAGELLAN RADAR DATA

Radar backscatter power is determined by (1) the morphology of the surface at a broad range of scales and (2) the intrinsic 

reflectivity, or dielectric constant, of the material. Topography at 

scales of several meters and larger can produce quasi-specular 

echoes, and the strength of the return is greatest when the local 

surface is perpendicular to the incident beam. This type of 

scattering is most important at very small angles of incidence, 

because natural surfaces generally have few large tilted facets 

at high angles. The exception is in areas of steep slopes, such as 

ridges or rift zones, where favorably tilted terrain can produce 

very bright signatures in the radar image. For most other areas, 

diffuse echoes from roughness at scales comparable to the radar 

wavelength are responsible for variations in the SAR return. In 

either case, the echo strength is also modulated by the reflectivity of the surface material. The density of the upper few wavelengths of the surface can have a significant effect. Low-density 

layers, such as crater ejecta or volcanic ash, can absorb the incident energy and produce a lower observed echo. On Venus, a 

rapid increase in reflectivity exists at a certain critical elevation 

above which high-dielectric minerals or coatings are thought to 

be present. This leads to very bright SAR echoes from virtually 

all areas above that critical elevation.

The measurements of passive thermal emission from 

Venus, though of much lower spatial resolution than the SAR 

data, are more sensitive to changes in the dielectric constant 

of the surface than to roughness. They can be used to augment 

studies of the surface and to discriminate between roughness 

and reflectivity effects. Observations of the near-nadir backscatter power, collected using a separate smaller antenna on 

the spacecraft, were modeled using the Hagfors expression 

for echoes from gently undulating surfaces to yield estimates 

of planetary radius, Fresnel reflectivity, and root-mean-square 

(rms) slope. The topographic data produced by this technique 

have horizontal footprint sizes of about 10 km near periapsis 

and a vertical resolution on the order of 100 m. The Fresnel 

reflectivity data provide a comparison to the emissivity maps, 

and the rms slope parameter is an indicator of the surface tilts, 

which contribute to the quasi-specular scattering component.

Introduction

The Themis Regio quadrangle (V–53), Venus, has been 

geologically mapped at 1:5,000,000 scale as part of the 

NASA Planetary Geologic Mapping Program. The quadrangle 

extends from lat 25° to 50° S. and from long 270° to 300° E. 

and encompasses the Themis Regio highland, the surrounding plains, and the southernmost extension of Parga Chasmata. 

Themis Regio is a broad regional topographic high with a diameter of about 2,000 km and a height of about 0.5 km that has 

been interpreted previously as a hotspot underlain by a mantle 

plume (Stofan and others, 1995). The Themis rise is dominated 

by coronae and lies at the terminus of the Parga Chasmata 

corona chain (Stofan and others, 1992, 1995) (fig. 1). Themis 

Regio is the only one of the three corona-dominated rises that 

contains significant extensional deformation (Stofan and others, 

1995). Fractures and grabens are much less common than along 

the rest of Parga Chasmata and are embayed by corona-related 

flows in places. Rift and corona formation has overlapped in 

time at Themis Regio (Martin and others, 2007).

The overall topography of the map area consists of lowlying plains, lying slightly below the mean planetary radius 

(MPR, 6051.84 km, Ford and Pettengill, 1992) and surrounding the Themis Regio highland (fig. 2). The highest point in the 

quadrangle, at 6,054.25 km, is found in the interior of Shiwanokia Corona (lat 42° S., long 277° E.); the lowest points are 

found along the Parga Chasmata rift and in the troughs around 

Tacoma Corona at about 1 km below MPR (for example, lat 

38° S., long 240° E.; lat 38° S., long 292.5° E.). The roughest 

regions in the quadrangle occur in the rim and trough regions 

of Semiramus and Ukemochi Coronae (~7.5° rms slope), but 

most of the map area is relatively smooth (comparable to the 

average Venus rms slope value of 2.84° of Ford and Pettengill, 

1992) (fig. 3). Fresnel reflectivity values in the map area do 

not vary greatly, except for small areas within the interiors of 

Shiwanokia and Shulamite Coronae, where values are as high 
2

as 0.23. The lowest reflectivity (0.05) occurs in the plains to 

the southwest of Shiwanokia Corona (fig. 4). The Visual and 

Infrared Spectrometer instrument on the Venus Express spacecraft (Helbert and others, 2008) measured high emissivity in the 

1 mm range at several coronae and large volcanoes in the map 

area, including at Mielikki Mons and Shulamite and Shiwanokia Coronae (Smrekar and others, 2010). The high emissivity 

values indicate that these flows are unweathered and, therefore, 

relatively young (<250,000 yr) (Smrekar and others, 2010).

The map area includes twenty-five coronae (table 1, fig. 1), 

as well as deposits from Bibi-Patma Corona, which is primarily located in the Nepthys Mons quadrangle (V–54). Thirteen 

coronae are on the Themis highland, with six in the surrounding 

plains or the rift leading into Themis Regio. The area covered 

by this quadrangle is part of the Beta, Atla, and Themis Regiones (BAT), which contains an unusually high concentration of 

volcanic features (Crumpler and others, 1997). Twelve impact 

craters are in the quadrangle (table 2). Other major features 

include the Parga Chasmata rift in the northwest corner of the 

quadrangle and six volcanoes greater than 100 km in diameter 

(table 3). Seven steep-sided domes are also located within the 

Themis Regio quadrangle.

Mapping Techniques

The geologic map of the Themis Regio quadrangle (V–53) 

was constructed using standard planetary geologic mapping 

techniques (Wilhelms, 1972, 1990). The base map is composed 

of Magellan synthetic aperture radar (SAR) data. Synthetic parallax stereo images with a 10x vertical exaggeration, produced 

by the U.S. Geological Survey (USGS) at Flagstaff, Ariz., were 

used extensively to determine stratigraphic relations. Additional 

information on map units was provided by Magellan altimetry, 

roughness, reflectivity, and emissivity data sets (Pettengill and 

others, 1992; Campbell, 1995).

Full-resolution Magellan image mosaics (FMaps) produced 

by the USGS were used extensively in mapping. Unit characteristics and boundary locations that were not resolvable at 

C1-MIDR scale (225 m/pixel) were clearly seen at FMap scale 

(75 m/pixel) (actual radar resolution is about 120 m). On the 

map, a solid line is used for a contact that is confidently located; 

a dashed contact is used where either the data resolution or 

surface expression of the units results in an approximately 

located unit boundary. Stratigraphic relations were best resolved 

utilizing the combination of large-format FMap prints, digital 

FMap data, and synthetic stereo data, although age relations 

were determined as a function of the scale and nature of the 

radar data utilized. Spatial resolution of data affects the ability 

to determine feature origin and age (Zimbelman, 2001); age 

relations between units and their mechanism(s) of formation in 

the map area are thus limited by the inherent resolution of the 

Magellan data.

The Correlation of Map Units does not show a broad range 

of ages, because unit relations do not clarify relative unit ages. 

For example, Rohina, Justitia, Angerona, and Kwannon Tholi 

and Tefnut and Mertseger Montes are shown as the same age, 

because they are in contact with a range of units but no units 

are younger. Therefore, they are shown as similar ages, but their 

actual ages may vary greatly. In addition, age relations of units 

that interfinger are difficult to represent on the Correlation of 

Map Units and are discussed in more detail in the text below.

Radar data presents unique challenges to constructing a 

geologic map (Ford and others, 1993; Hansen, 2000). As in 

any geologic map, contacts are mapped where complete data 

supports the interpretation. In radar data, however, distinguishing adjacent lava flow fields may not be possible, because flows 

are likely to have relatively similar backscatter characteristics 

and also because the surface of a unit may change laterally. For 

example, in this map area corona and large volcano flow aprons 

are in some cases seen to be divided into very few units, or not 

subdivided at all, due to the lack of clearly mappable contacts 

that can be followed well enough to delineate a mappable flow 

unit. This should not be taken to indicate that flow material 

units associated with features such as Mielikki Mons or Durga 

Corona that are >500 km across are actually made up of only 

one rock-stratigraphic unit, and caution should be used before 

interpreting these composite units as individual flow units 

(Romeo and Turcotte, 2009). In addition, structures tend to be 

more visible if they are oriented perpendicular to the radar look 

direction (Stofan and others, 1989). Therefore, in Magellan 

data, east-west-trending structures are likely to be less well represented in the data unless associated with rough talus slopes, 

while north-south-trending structures will be well defined.

Subunits (members) in the map area associated with a 

specific source are numbered, if stratigraphic relations between 

them could be determined (for example, units fSe1 and fSe2), 

or lettered, if not all members are in contact and, therefore, age 

relations are ambiguous (for example, units fTya, fTyb, fTyc, 

fTyd). Small volcanic edifices and structures in the map area are 

too numerous to be mapped individually at map scale (Tanaka, 

1994); approximately one in ten features are shown, although 

fewer are mapped in the chasmata troughs where they are particularly dense.

General Geology

Five types of materials have been mapped in the Themis 

Regio quadrangle (V–53): plains, volcanic edifice and flow, 

corona, crater, and surficial materials. Materials are mapped as 

units, except for surficial material, which is shown by a stippled 

pattern. All types of material units occur throughout the quadrangle.

Mappable Units

Plains Material

We have identified seven plains-material units within the 

map area: densely fractured plains material (unit pdf), lineated 

plains material (unit pl), Themis Regio fractured plains material (unit pfT), mottled plains material (unit pm), Helen Planitia 

composite plains material (unit pcH), Themis Regio lobate 
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plains material (unit plT), and smooth plains material (unit ps) 

(table 4). Most of these plains units occur in relatively small 

patches (<1,000 km) and are composed of multiple smaller units 

that are grouped together to form a mappable unit. None of the 

plains units within this quadrangle are analogous to regionalscale plains units mapped in other quadrangles, such as the Aino 

composite plains material in Aino Planitia quadrangle (V–46) 

(Stofan and Guest, 2003). This is largely due to extensive resurfacing of much of the region by flows associated with coronae 

and volcanoes (Hansen and DeShon, 2002; Stofan and others, 

2005). Plains units on Venus are generally interpreted to be volcanic in origin based on (1) the similarity to volcanic plains on 

other planets (Masursky and others, 1980; Barsukov and others, 

1986), (2) the Venera lander surface chemical compositions and 

images (Surkov and others, 1984), and (3) the lack of water on 

the planet for at least 2–3 billion years (Kasting and Pollack, 

1983), which make a sedimentary origin implausible.

The oldest plains units in the map area are the densely fractured plains material (unit pdf) and the lineated plains material 

(unit pl), both of which tend to be high standing and embayed 

by younger units. The two units are not in contact, so an age 

relation between them is not determinable, but they are always 

both the locally oldest units. In addition, these units may be a 

single time-stratigraphic unit or a mixture of different ages of 

kipukas of fractured plains. Unit pdf is very densely fractured 

and somewhat resembles tessera terrain mapped elsewhere on 

the planet, but it lacks the two or more orientations of structures 

typically characteristic of tessera terrain. Relative age relations 

between these and other units is shown in table 4.

The Themis Regio fractured plains material (unit pfT) is 

composed of multiple occurrences of heavily fractured plains on 

the Themis Regio volcanic rise. They may represent plains units 

off the rise that were elevated and deformed (such as units pm

or ps), or they may represent earlier corona or volcano flows 

that have been subsequently deformed.

The mottled plains material (unit pm) is exposed in variably sized patches off the Themis Regio rise; one exposure 

is located on the northern margin of the rise north of Erigone 

Corona. It has a patchy or mottled appearance and is cut in 

places by fractures and wrinkle ridges. Not all occurrences of 

the unit are cut by wrinkle ridges, and the wrinkle-ridge sets do 

not have a consistent trend in all areas of the map (for example, 

they trend north in the southwest corner of the map and eastnortheast near the west-central edge of the map). Based on the 

unit’s lobate boundaries and volcanic edifices, we interpret it 

to be composed of multiple materials of volcanic origin. The 

unit is also cut by two channels, Sinaan Vallis and a nearby 

unnamed channel at about lat 48°–50° S., long 272.5° E., of 

probable volcanic origin (Baker and others, 1992; Komatsu and 

others, 1993). The channels are cut in places by wrinkle ridges. 

However, at one location, material from the channel appears to 

superpose wrinkle ridges, which suggests that multiple episodes 

of deformation have occurred (fig. 5). A few occurrences of 

unit pm are queried (lat 37.5° S., long 275° E.; lat 38° S., long 

276.5° E.), because it is not certain whether the mapped unit is 

unit pm or unit plT.

The Helen Planitia composite-flow plains material is made 

up of numerous lobate subunits of variable radar backscatter. It 

is interpreted to be volcanic in origin, from unidentified sources, 

based on the identification of diffuse flow fronts within the 

unit and its lobate boundaries. The Themis Regio lobate plains 

material (unit plT) is mapped on the Themis Regio volcanic 

rise. Unit plT has variable radar backscatter and lobate boundaries and is interpreted to be volcanic in origin. Within the unit, 

multiple flows can be discerned but are generally not mappable, 

and no sources can be identified. Different outcrops of this unit 

are not likely to be the same age; some may be volcanic flows 

that have originated from rift fractures, while others may have 

come from unknown edifices or coronae.

The youngest plains unit is smooth plains material (unit 

ps). The smooth plains unit (unit ps) forms extensive, generally featureless level plains. It is interpreted to be volcanic in 

origin based on its lobate boundaries, although no sources can 

be identified.

Corona Flow Material

Coronae are volcano-tectonic features that are interpreted 

to form over mantle upwellings owing to their associated volcanism and large size (>100 km across) and the raised topographic 

relief of many coronae (Basilevsky and others, 1986; Stofan 

and others, 1991; Koch and Manga, 1996). They have been 

classified according to morphology as concentric (circular), 

asymmetric (elongate or irregularly shaped), multiple, concentric/double ring (circular with a double annulus), or radial/

concentric (circular with dominant radial fractures) (table 1; 

Stofan and others, 1992). Twenty-three coronae have associated 

flow material units in the map area (tables 1, 4). Flows originate 

from the interior of coronae, as well as from the flanks and rim. 

Flows associated with coronae range in morphology from sheetlike to digitate, and both embay and are cut by corona-related 

deformation. Some of the coronae have multiple associated flow 

units, including Ukemochi Corona, with five associated units, 

and Tamiyo Corona, with four associated flow-material units. 

In addition, three additional coronae that are located within the 

quadrangle have no mapped units (table 1); they are strictly 

tectonic features that deform the materials of units in which 

they are located. These coronae are located on units pl (Partula 

Corona), pfT (Parvati Corona), and fO1 (Hutash Corona). 

Some corona flows do overlap (table 4). Ikas Coronae 

flow material (unit fIk) is superposed by Nzambi flow material, member 2 (unit fN2), Tacoma Corona flow material (unit 

fTa), Zwyie Corona flow material (unit fZ), Shulamite Corona 

flow material (unit fSh), Latta Corona flow material (unit 

fLt), Semiramus Corona flow material, member 2 (unit fSe2), 

and Shiwanokia Corona flow material, member 2 (unit fS2). 

Shiwanokia Corona flow material, member 2 (unit fS2), flows 

also superpose Nzambi Corona flow material (unit fN2), Ama 

Corona flow material (unit fAm), and Shulamite Corona flow 

material (unit fSh). Tacoma Corona in turn is overlain by flow 

material from four other coronae. Several coronae have interfingered flows; Ukemochi Corona flow material, member a (unit 

fUa), is superposed by Tamiyo Corona flow material, member 

a (unit fTya), but Ukemochi Corona flow material, member 

c (unit fUc), is superposed on Tamiyo Corona flow material, 

member a (unit fTya). Similarly, Semiramus Corona flow mate-
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rial, member 1 (unit fSe1), is superposed by Tamiyo Corona 

flow material, member a (unit fTya), and Semiramus Corona 

flow material, member 2 (unit fSe2), is superposed on Tamiyo 

Corona flow material, member a (unit fTya).

Four coronae have superposed impact craters (table 4). 

Jocelyn and Elza are on Obiemi Corona flow material, member 

1 (unit fO1), Maret is superposed on Rigatona Corona flow 

material (unit fR), Bernadette is on Nzambi Corona flow material, member 2 (unit fN2), and Shiwanokia Corona flow material, member 2 (unit fS2), and Sabin is on Shiwanokia Corona 

flow material, member 1 (unit fS1).

A few coronae are in contact with volcanic and edifice 

flow materials. Chloris Mons flow material, members 1 and 2 

(units fC1 and fC2), superpose Navolga Corona flow material 

(unit fNa), Nzambi Corona flow material, member 2 (unit fN2), 

Bibi-Patma Corona flow material, members 1–3 (units fB1, fB2, 

fB3), Orbona Corona flow material (unit fOr), and Ukemochi 

Corona flow material, member e (unit fUe). Tefnut Mons flow 

material (unit fTe) superposes Ukemochi Corona flow material, member c (unit fUc), and Tamiyo Corona flow material, 

member a (unit fTya). Angerona Tholus flow material (unit 

fAn) superposes Durga Corona flow material (unit fD), Rohina 

Tholus flow material (unit fRh) superposes Zywie Corona flow 

material (unit fZ), and Mertseger Mons flow material (unit fMe) 

superposes Obiemi Corona flow material, member 1 (unit fO1). 

Abeona Mons flow material (unit fA) superposes Ama Corona 

flow material (unit fAm) but, in turn, is superposed by Shiwanokia Corona flow material, member 2 (unit fS2). Mielikki Mons 

flow material, member 2 (unit fM2), superposes Durga Corona 

flow material (unit fD), Anjea Corona flow material (unit fAj), 

and Rigatona Corona flow material (fR) but, in turn, is superposed by Gertjon Corona flow material, member 1 (fG1).

Coronae that have abundant associated flows are located 

both on and off the rift and on and off the volcanic rise. Recent 

work by Martin and others (2007) indicates that coronae on the 

Themis volcanic rise have more associated volcanism than other 

coronae in the Parga region. Along Parga Chasmata, coronae are 

located along the rift, as well as to the north and the south of the 

rift (Martin and others, 2007), similar to coronae along Hecate 

Chasma (Hamilton and Stofan, 1996). Most of the coronae in 

the map area formed synchronously with rifting, although some 

clearly predate the rifts and others postdate extensional deformation (Martin and others, 2007).

Volcanic Edifice and Flow Material

Thirteen units associated with eleven named volcanic 

edifices have been mapped, including those associated with 

the large volcanoes, Chloris Mons, Mielikki Mons, and Tefnut 

Mons (tables 3, 4). Some of the named edifices are tholi, defined 

as small domical hills or mountains (such as, Justitia Tholus).

Occurrences of volcanic center material (unit vc) and edifice field material (unit ef) are located throughout the map area. 

Although most volcanic center material or edifice field material outcrops have similar morphology, different occurrences of 

each have differing relations with surrounding units, indicating 

that they did not form at the same time. For example, Chloris 

Mons flow material, member 1 (unit fC1), is superposed by flow 

deposits from an edifice field but superposes deposits from a 

different edifice field. Volcanic center material consists of both 

large individual volcanic features, such as steep-sided domes 

and small unnamed volcanoes or small flow fields. The seven 

steep-sided domes in the map area are located in the plains, in 

the interior or on the rims of coronae, or at large volcanoes. 

They may represent more evolved compositions (McKenzie 

and others, 1992). However, their relatively smooth, fractured 

upper surfaces are not consistent with more silicic compositions, which would tend to form thick crusts that break into 

blocks (Stofan and others, 2000). Instead, they may be basaltic 

in composition and have early-stage crusts that were either 

annealed or entrained to form the final, relatively smooth upper 

dome surfaces (Stofan and others, 2000). Edifice field material 

consists of clusters of small edifices that have associated lava 

flows and outcrops ranging in size from ~100 to 400 km across 

(Head and others, 1992; Guest and others, 1992; Addington, 

2001). The unit is mapped on the basis that the edifices share a 

common source and (or) are stratigraphically related.

Most of the volcanic flow units are not in contact with each 

other or with consistent plains or corona units, which makes 

the regional volcanic stratigraphy difficult to determine. The 

stratigraphic relations between coronae and volcanoes were 

discussed above. The only deposits associated with named volcanic edifices that are in contact are Siduri Mons flow materials (unit fSi) that are overlain by Chloris Mons flow material, 

member 1 (unit fC1).

Structures and Other Features

Impact Craters

There are twelve impact craters in the Themis Regio 

quadrangle (V–53) (tables 2, 4). They range from 11 to 52 km 

in diameter. Six have bright floors, three have completely dark 

floors, and three have small patches of dark material on their 

radar bright floors. Two craters, Peck and Bernadette, have a 

complex relation with the surrounding units. Peck appears to 

be superposed on unit pdf, but its ejecta has been fractured 

and embayed, and its interior has been embayed by later flows 

that occurred within unit pdf. Therefore, we interpret it to have 

formed during the apparently protracted evolution of unit pdf. 

The crater Bernadette is superposed on Nzambi Corona flow 

material, member 2 (unit fN2), but it is also embayed by small, 

difficult-to-map, later Nzambi flows. It is unclear if the flow that 

extends to the west of the crater and overlaps unit fS2, which 

we have mapped as part of Bernadette, is actually related to 

Bernadette or is a Nzambi-related flow.

Surficial Material

Radar-dark, diffuse material is mapped as surficial material (lat 41.5° S., long 283° E.). The material is superposed on 

Shulamite Corona flow material (unit fSh) and does not have 

an associated impact crater. We interpret this deposit to be the 

product of the breakup of an impactor in the atmosphere (Phillips and others, 1991). The resulting shock wave pulverizes the 
5

upper layers of the surface, producing a roughly circular region 

of impact debris (Phillips and others, 1991; Zahnle, 1992).

Wrinkle Ridges

Wrinkle ridges in the map area are tens of kilometers to a 

few hundred kilometers long, sinuous, and less than a few kilometers wide. Very few plains units in the map area are deformed 

by wrinkle ridges, which is similar to plains units in the Hecate 

Chasma quadrangle (V–28) (Stofan and others, 2012) to the 

northwest. In the southern region of the map area, some mapped 

north-northwest-trending wrinkle ridges deform units pcH, 

pm, plT, and pfT. Near the west-central edge of the map area, 

east-northeast-trending ridges deform unit pm. Wrinkle ridges 

are common on plains units in other regions on Venus (Squyres 

and others, 1992; McGill, 1993) and are interpreted as forming 

from limited amounts of horizontal contraction (Plescia and 

Golombek, 1986).

Broad Topographic Ridge Crests

In several locations in the map area, we have indicated the 

location of broad topographic ridge crests, 10–20 km wide and 

several hundred kilometers long, that correspond to the raised 

topographic rims of Shiwanokia and Shulamite Coronae and a 

topographic ridge near the southern boundary of the map in unit 

pcH.

Lineaments and Graben

Radar-bright, sharply defined straight lineaments that are 

distributed throughout the map area have lengths to several 

hundred kilometers and widths of a few kilometers. Graben 

are mapped where two parallel lineaments border a depression. 

We interpret most lineaments to be extensional in origin, given 

their overall morphology. Other nonrift-associated fractures 

in the map area are associated with coronae and volcanoes. In 

particular, several coronae and volcanoes have central radial 

sets of fractures that are interpreted to be the surface expression 

of dikes (Grosfils and Head, 1994; Grindrod and others, 2005). 

Below, we describe two specific sets of lineaments: faults and 

fractures at coronae and those associated with Parga Chasmata.

Coronae

Fractures and faults, along with some ridges of possible 

compressional origin, make up the annuli surrounding coronae 

and are generally spaced tens of kilometers to a few kilometers 

apart. These are mapped representatively in red for emphasis. 

Some annuli, such as those at Anjea and Orbona Coronae, 

appear to have formed relatively early in the history of the 

corona, because they are embayed by flow material. Deformation at Ukemochi Corona is complex—interior deformation, 

followed by annulus formation, followed by another period of 

interior deformation—as indicated by crosscutting relations. 

Detailed mapping of individual coronae (Grindrod and Guest, 

2006) is necessary to unravel the complex stages of annulus 

formation and volcanism.

The topography of the coronae has been classified following the scheme presented in Smrekar and Stofan (1997) 

(table 1). Seventeen of the coronae are plateaus or topographic 

highs, six are rimmed depressions, and three consist only of a 

topographic rim surrounding a flat interior. Four of the plateaushaped coronae have outer rises surrounding deep troughs that 

are proposed subduction trenches (Schubert and Sandwell, 

1995).

Rifts

Radar-bright lineaments interpreted to be extensional faults 

and fractures associated with Parga Chasmata dominate the central portion of the map area, though most units in the map area 

exhibit some amount of fracturing. Parga Chasmata fractures 

are hundreds of kilometers long and vary in spacing from tens 

of kilometers to <5 km. Most are too narrow to be identified 

as graben. The rift fractures trend more or less east-southeast, 

parallel to the rift trough. Some of the fractures appear to be 

deflected around coronae, while others terminate at them.

Parga Chasmata extends for over 10,000 km from Atla 

Regio through Themis Regio. It is a discontinuous trough 

and fracture system with en echelon offset along strike that is 

characterized by numerous coronae along its length. Regions 

where the topographic trough is more distinct tend to be characterized by more closely spaced fractures (Martin and others, 

2006), similar to Hecate Chasma (Hamilton and Stofan, 1996). 

The origin of chasmata systems is controversial, including 

theories ranging from extension and diapiric upwelling (Stofan 

and others, 1992; Hansen and Phillips, 1993; Bleamaster and 

Hansen, 2004) to subduction (Schubert and others, 1994; 

Schubert and Sandwell, 1995).

The Parga Chasmata rift consists of numerous branches; 

only the main branch of the rift is located in the map area. The 

rift trough varies in depth along the rift and, in most places, 

the zone of fracturing extends beyond the trough (Martin and 

others, 2007). In some places, the fractures or faults are very 

closely spaced, and in other places, the fractures are more 

widely spaced and tend to meander slightly.

The Parga Chasmata rift does extend across the Themis 

Regio rise but dies out in the plains to the east of Themis Regio. 

Most of the coronae in this region of the Parga Chasmata rift 

seem to have formed concurrently with rifting.

Volcanic Features

Six symbols are used in the map area for volcanic features: 

depression, volcanic edifice >5 km, steep-sided dome, volcanic 

edifice with diameter <5 km, flow direction, and channel. Channels are described in the Plains Material section above; both 

channels in the map area are in unit pm. Steep-sided domes 

are located throughout the map area and are discussed in more 

detail in the Volcanic Edifice and Flow Material section above. 

Two closed circular depressions (diameters ~100 and 50 km) 

are seen at Angerona Tholus and in unit pm to the northeast of 
6

Angerona. Both of these features are interpreted to be calderas. 

Mielikki Mons also appears to have a summit caldera, but the 

lack of topographic expression suggests that it is filled with 

lavas. Hundreds of volcanic edifices <5 km diameter (Head and 

others, 1992; Guest and others, 1992) are mapped in this quadrangle; where there is a distinct mappable boundary surrounding a cluster, they are mapped as edifice fields, as described 

above. In the map area, we used a circle with exterior hachures 

to designate volcanic edifices >5 km diameter (not including 

steep-sided domes) that are too small to map at map scale. Flow 

direction symbols are used to illustrate distinct digitate flow 

paths.

Geologic History

The earliest geologic activity in the Themis Regio quadrangle (V–53) is recorded by plains units pdf and pl, which 

suggests that the region has been subjected to multiple episodes 

of plains-forming volcanism and deformation that culminated 

in the current (ongoing?) tectonic and volcanic activity at 

Parga Chasmata and Themis Regio. Unlike many other areas 

on Venus, the map area lacks regional-scale plains units. The 

scale of plains units tends to be on the order of hundreds rather 

than thousands of kilometers, and much of the region has been 

resurfaced by flows from volcanoes and coronae (Stofan and 

others, 2005). We have chosen to lump plains units of relatively 

similar morphology into units (for example, unit plT, unit pm) 

that are unlikely to have originated from the same source or 

have formed at the same time. It is also apparent that most of 

the volcanic units, including plains units such as units pcH and 

pm, corona units such as fD and fSh, and volcanic units such 

as fTe, fM1, and fM2, are made up of multiple episodes of flow 

emplacement covering an undetermined amount of time. Their 

scale should not be interpreted to indicate a single resurfacing 

event (Romeo and Turcotte, 2009).

Because many units in the map area are not in contact, 

we are unable to confidently define the actual stratigraphic 

sequence, except for the events discussed below. This results in 

a Correlation of Map Units that shows a limited range in relative unit ages and uncertainty in the timing and extent of unit 

formation. For example, the observation that flows from Rigatona Corona overlie flows associated with both Erigone Corona 

and Anjea Corona has led us to place the latter two coronae at 

a relatively similar position in the Correlation of Map Units. 

However, we have no real insight into the relative unit ages 

between Anjea and Erigone Coronae, because their associated 

flow materials are not in contact. Therefore, a number of possible geologic histories could be developed for this quadrangle, 

most of which cannot be tested without age-dating surface rocks 

(Stofan and others, 2005).

Mapping of Themis Regio has permitted the delineation of 

some stratigraphic relations. Early-stage flows from Rigatona, 

Santa, and Semiramus Coronae overlie the mottled plains material (unit pm) to the north of Themis Regio, indicating that formation of these coronae generally postdated formation of these 

plains. Plains units originating from unknown sources within 

Themis Regio (units plT, pfT) and generally overlain by corona 

deposits also overlie the unit pm, suggesting that rise formation 

in general has postdated formation of this unit. One patch of 

unit pm does lie on the northern margin of the rise, suggesting 

that this unit may have been partially uplifted during the formation of the rise. Corona flows also overlie plains units (units pm, 

pcH) to the south, west, and east of Themis Regio, testifying to 

the potentially young age of the Themis rise.

Some stratigraphic relations among coronae can also 

be determined. Flows from Shiwanokia, the largest corona, 

overlie flows from Shulamite, Nzambi, Ama, and Ikas Coronae, 

while flows from Shulamite overlie flows from Latta, Erigone, 

and Rigatona Coronae and are overlain by flows from Santa 

Corona. Deposits from Ukemochi and Tamiyo interfinger (fig. 

6), illustrating that formation of the two coronae overlapped in 

time. Older flows from Santa Corona (unit fSa1) are overlain by 

Durga Corona flow material (unit fD), but later Santa flows (unit 

fSa2) overlie Durga flows. The great complexity of the coronae 

in this region and their abundant volcanic deposits suggest long 

histories. For example, if flows from two coronae establish a 

relative age relation (such as the superposition of Shiwanokia 

flows over Nzambi flows), we cannot determine if the apparently older corona (Nzambi in this case) ceased all activity prior 

to emplacement of the younger flow from the other corona. In 

fact, the apparently older corona may have begun formation 

after the apparently younger feature but ceased activity sooner.

Age relations between coronae and volcanoes indicate that 

the two types of features formed throughout the history of the 

quadrangle. In some cases, volcanic flows are superposed on 

corona flows. For example, Chloris Mons flows overlie flows 

from Nzambi Corona, and flows from Abeona Mons overlie 

Ama Corona materials. Mielikki Mons flows overlie flows from 

Anjea, Durga, and Rigatona Coronae (fig. 7). At other locations, 

corona flows are younger. Gertjon Corona flows superpose 

Mielikki Mons flows, and Shiwanokia Corona materials overlie 

Abeona Mons materials. However, as with corona-to-corona 

age relations, we can only determine the relation between sets 

of flows; we cannot determine when each feature originated or 

the duration of activity. We see no clear evidence that coronae 

predate volcanoes, as suggested by McGovern and Solomon 

(1998).

A strong association between volcanism and coronae 

along rifts has been noted elsewhere on Venus (Magee and 

Head, 1995). In V–28 (Stofan and others, 2012) and V–53 map 

areas, some coronae along rifts do not have much associated 

volcanism; coronae with the most associated volcanism in these 

quadrangles are located at least 500 km off the rifts or on the 

Themis Regio highland. Whereas extension clearly plays a role 

in volcanism at coronae, it is not the only contributing factor 

(Martin and others, 2007). Coronae at Themis Regio may have 

greater than average associated volcanism owing to the possible 

mantle plume beneath the rise.

Individual coronae in Themis Regio have a range in 

complexity of morphology and in the amount of associated 

volcanism. In comparison to the other two corona-dominated 

rises (eastern Eistla Regio and central Eistla Regio), coronae 

at Themis Regio tend to have more associated flows, and no 

large volcanoes are located on the rise. Most of the coronae on 
7

the rise (Shiwanokia, Semiramus, Ukemochi) have undergone 

multiple stages of volcanism and deformation. At Ukemochi, 

interior deformation was followed by annulus formation, which 

in turn was followed by another episode of interior deformation. Initial Ukemochi flows that were more sheetlike have been 

followed by more digitate flows and the formation of an edifice 

on the annulus. This edifice has been subsequently deformed by 

continuing annular deformation. The complex, long-duration 

histories of these coronae are consistent with complex histories 

of coronae described by Copp and others (1998).

In the map area, we find no evidence to support a directional geologic history of Venus (Basilevsky and Head, 1995; 

Guest and Stofan, 1999). The directional model predicts that 

specific types of geologic units (coronae, small edifices) formed 

at distinct time periods (Basilevsky and Head, 1995, 1998). The 

Correlation of Map Units illustrates that all types of geologic 

activity, from large volcano formation to plains formation to 

corona formation, has occurred throughout the visible geologic 

history of this quadrangle. Overlapping episodes of corona, 

volcano, and plains formation have overlapped with episodes 

of dominantly extensional deformation. Evidence for several 

episodes of extensional deformation is provided by the rift, the 

lineated plains unit, and the deformed plains unit.

Smrekar and Stofan (1999) studied the morphology and 

gravity for the three corona-dominated rises on Venus, central 

and eastern Eistla Regiones and Themis Regio. Gravity data for 

Themis was best fit by a bottom-loading model with an elastic thickness estimate of 22 km, a crustal thickness of 10 km, 

and an apparent depth of compensation of 80–110 km. While 

the broad topographic rise and associated gravity anomaly at 

Themis Regio indicate the possible presence of an underlying 

plume, the complex sequence of corona formation favors formation by a series of small-scale upwellings over time rather than 

nearly simultaneously from the break-up of a single large-scale 

mantle plume (Smrekar and Stofan, 1999). This would indicate 

that buoyancy flux and plume duration are the primary factors 

controlling the difference between corona-dominated rises, 

such as Themis Regio, and volcano-dominated rises, such as 

Atla or western Eistla Regio. In addition, elastic and thermal 

lithospheric thicknesses that are estimated for Themis Regio 

are consistent with Earth-like values, and the range of stages 

of evolution of coronae, the delamination signal in the gravity data, and the broad topographic swell indicate that Themis 

Regio is likely to be underlain by an active plume with ongoing surface deformation due to delamination. The evidence for 

delamination at the coronae at Themis Regio is consistent with 

coronae contributing to heat loss on Venus through both upwellings and delamination and to the conclusion that lithospheric 

thickness is not a limiting factor on corona formation (Smrekar 

and Stofan 1997, 1999).

The geologic history of Themis Regio quadrangle (V–53) 

is dominated by volcanism and corona formation at Themis 

Regio and extensional deformation associated with Parga Chasmata, with interspersed episodes of plains formation. As shown 

in the Sequence of Map Units, all of this activity overlapped in 

time; however, the duration of the activity is unknown. Stofan 

and others (1995) suggested that Themis Regio is a hotspot 

underlain by a mantle plume. Terrestrial hotspots are thought to 

have lifetimes of hundreds of millions of years (Courtillot and 

others, 2003); we would expect Venusian hotspots to have relatively similar lifetimes. Given that the average crater-retention 

age of Venus is ~750 m.y. (McKinnon and others, 1997) and 

that this region has previously been suggested to be relatively 

recently formed (Price and others, 1996; Smrekar and Stofan, 

1999), it is possible that this region is still geologically active. 

Recent observations of the Themis Regio region by the Visual 

and Infrared Thermal Imaging Spectrometer instrument on 

the European Space Agency (ESA) Venus Express spacecraft 

(Mueller and others, 2008; Helbert and others, 2008) show a 

strong correlation between volcanic flows and high emissivity 

signatures that indicate possible unweathered surfaces and support possible ongoing formation of Themis Regio (Stofan and 

others, 2009; Smrekar and others, 2010).

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10

Table 1. Coronae that produced deposits mapped in the Themis Regio quadrangle (V–53), Venus.

Coronae or

corona name

Latitude

(°S)

Longitude

(°E)

Maximum

diameter (km)

Feature type* Topography*

Ama –45.7 278.2 300 concentric rimmed plateau

Anjea –29 282.5 145 concentric double ring rim-inner high

Bibi Patma (V–54) –47 302 450 concentric plateau

Durga –31 286 227 concentric rim-inner high

Erigone –34.5 284 325 multiple partial rim

Gertjon –30 276 236 concentric double ring outer rise-trough-plateau

Hutash –34.5 275.2 91 concentric rimmed depression

Ikas –40.5 288.5 127 multiple rim only

Latta –38.7 287 200 concentric double ring rimmed depression

Lilwani –30.5 271.5 491 asymmetric outer rise-trough-plateau

Navolga –48.6 296.5 170 concentric rimmed depression

Nzambi –45 287.5 225 asymmetric rimmed plateau

Obiemi –31.9 276.6 300 asymmetric rimmed plateau

Orbona –47.5 293.5 182 concentric depression, partial rim

Partula –49.5 290 145 concentric depression, partial rim

Parvati –36 276.5 173 concentric rimmed depression

Rigatona –33.5 278.5 300 concentric double ring outer rise-trough-rimmed plateau

Santa –34.5 288 200 radial/concentric plateau

Semiramus –37 293 375 asymmetric plateau partial rim

Shiwanokia –42 279.8 409 concentric plateau

Shulamite –38.8 284.3 275 concentric plateau

Tacoma –37 288 500 concentric plateau

Tamiyo –36 298.5 400 asymmetric rim only

Ukemochi –39 296.1 300 asymmetric outer rise-trough-plateau

Xmukane –28.2 269.5 200 radial/concentric rim-trough-inner high

Zywie –38.6 291.2 200 concentric dome

*From Stofan and others, 1992.
11

Table 2. Impact craters of Themis Regio quadrangle (V–53), Venus.

Crater Name Latitude (°S) Longitude (°E) Diameter (km) Unit

Peck –28.9 294.3 30.4 on/under pdf

Kitna –28.9 277.3 15.3 on pfT

Koinyt –30.9 293.2 11.7 on pm

Jocelyn –33.2 276.4 14 on fO1

Maret –33.3 280.2 11.7 on fR

Elza –34.4 275.9 18 on fO1

Sabin –38.5 274.7 33.1 on pm, fS1

Aksentyeva –42 271.9 42.5 on plT

Kenny –44.4 271.1 52.7 under fA

Bernadette –46.4 285.6 12.8 on/under fN2, fS2

Abington –47.8 277.7 21.7 on pm

Nastya –49 275.8 12.5 on pm

Table 3. Volcanoes of Themis Regio quadrangle (V–53), Venus.

Edifice Name Latitude (°S) Longitude (°E) Diameter (km) Unit

Kwannon Tholus –26.3 296.8 135 unit fK

Ts'an Nu Mons –27.2 272.9 310 unit fTs

Mielikki Mons –27.8 280.5 450 units fM1, fM2

Justitia Tholus –28.7 296.5 60 unit fJ

Angerona Tholus –29.8 287.2 200 unit fAn

Mertseger Mons –38.1 270.3 450 unit fMe

Tefnut Mons (V–54) –38.6 304 182 unit fTe

Rohina Tholus –40.6 295.4 30 unit fRh

Siduri Mons –42.3 297.3 105 unit fSi

Abeona Mons –44.8 273.1 375 unit fA

Chloris Mons –45.4 294.6 180 units fC1, fC2
12

Table 4. Tabular version of the map unit relations in the Themis Regio quadrangle (V–53), Venus, shown in graphic 

format in the Correlation of Map Units. Unit age relations were determined by the identification of superposition 

relations and the use of cross-cutting structures.

Unit Relations Unit Name Older Units Younger Units

Volcanic edifice and flow 

material

fTs pfT, pm fX, plT

fSi pdf fUe, vc, fC1

fM1 pl, fD fM2

fM2 fM1, pm, pl, pfT, fAj, fR fG1

fA pfT, plT, fAm fS2

fC1 pdf, pl, pm, plT, fSi, fUe, fNa, fB1,

fB2, fB3, fOr, ef

fC2, ef

fC2 fC1, fB2, fOr, ef, pl, fN2

fAn pfT, ef, fD

fTe pfT, fUc, fTya

fMe pfT, pm, plT, fO1

fJ pdf, pm, ps, ef

fK pdf, ps

fRh plT, fZ, vc, fUa, fUe

Corona flow material fIk pm, fN2, fTa, fZ, fSh, fS2, fLt, fSe2

fNa pl fC1, ef

fTa fIk fSh, fSa2, fSe2, fLt, plT

fE fR, ef, fSh, fSa1, fD

fSa1 pfT, pm, plT, fE fD, fSa2

fSa2 fSa1, pfT, pm, plT, fD, fTa, fE fSh

fAj pm fM2, fR

fSe1 fSe2, fTya, fTyc

fSe2 pm, plT, fSe1, fTa, fIk, fTya, fUa fZ, ef

fLt fTa, fIk fSh

fN1 pl fN2, ef, fOr

fN2 pl, pm, pcH, ef, fIk c, fC2, fS1, fS2, ef

fR pfT, pm, fE, fAj c, ef, fO1, fO3, fM2, fG1, fSh

fD pl, pm, pfT, fSa1, fE fAn, ef, fM1, fM2, fSa2

fUa fUb, fUc, fUd, fUe, fRh, fTya, fSe2, fZ

fUb fUa, fZ

fUc fUa, pfT, fTya fTe, fUd, fUe

fUd fUa, fUc

fUe fUa, fUc, pfT, plT, vc, fSi fC1, fRh

fB1 pfT, plT fB2, fC1, ef

fB2 fB1, ef fB3, fC1, fC2

fB3 fB2, ef fC1

fTya fSe1, fSe2 fTe, fTyb, fTyc, fTyd, fUc

fTyb fTya, pm, plT fTyc

fTyc fTya, fTyb, fSe1, pl, plT fTyd

fTyd fTya, fTyc
13

Table 4. Tabular version of the map unit relations in the Themis Regio quadrangle (V–53), Venus, shown in graphic 

format in the Correlation of Map Units. Unit age relations were determined by the identification of superposition 

relations and the use of cross-cutting structures—continued.

Unit Relations Unit Name Older Units Younger Units

fOr pl, fN1 fC1, fC2, ef

fAm pm, plT fS2, fA

fZ fSe2, fIk, pm, plT, fUa fRh, vc, fUb

fO1 fR, plT, pm, pfT c, vc, fO2, fO3, fMe

fO2 pfT, plT, fO1 vc, fO3

fO3 fO1, fO2, pfT, fR

fS1 pfT, pm, pcH, fN2 c, fS2

fS2 fS1, pcH, pm, pfT, plT, fAm, fN2,

fA, vc, fIk, fSh

c

fG1 pfT, fM2, fR fG2

fG2 fG1, pfT

fSh pfT, fIk, fLt, fTa, fSa2, fE, fR fS2, vc

fX plT, pm, fTs fL

fL pm, plT, pfT, fX, fIk

Plains material pdf c, ps, pm, fK, fSi, fJ, vc, ef

pl fC1, fC2, fTyc, fNa, fOr, fN1, fN2, pm,

pcH, ps, plT, fO, fM1, ef, fM2, fSe2

pfT c, pm, plT, fL, fG1, fG2, fR, fD, fSh,

fS1, fS2, fSa1, fSa2, fA, fUc, fUe, fO1,

fO2, fO3, vc, fTe, fMe, ef, fTs, fM2,

fAn, fB1

pm pdf, pl, pfT, fIk c, fM2, fTs, fMe, fC1, fR, fSa1, fSa2,

fSe2, fD, fL, fX, fO1, fAm, fTyb, fS1,

fS2, fZ, fAj, fN2, ef, vc, pcH, ps, plT

pcH pm, pl fS1, fS2, fN2

plT pfT, pm, pl, fX, fAm, fO2, fTyb, fTyc, fZ, fL, fSe2,

fSa1, fSa2, fTa, fUe, fB1, fS2, vc, ef,

fMe, fC1, fRh, fA

ps pm, pdf fK, fJ