2010

U.S. Department of the Interior

U.S. Geological Survey

Prepared for the National Aeronautics and Space Administration

Geologic Map of the Lakshmi Planum Quadrangle (V–7), Venus

By Mikhail A. Ivanov and James W. Head, III

Pamphlet to accompany

Scientific Investigations Map 3116

75°

50°

25°

75°

25°

–25°

0°

50°

0°

270° 300° 330° 0° 30° 60° 90°

–25°

–50° –50°

–75° –75°

V–1

V–20 V–21 V–31

V–43

V–62

V–32

V–30

V–29

V–18

V–17

V–19

V–7

V–42 V–33

V–9

V–54 V–45

V–8

V–55 V–44

V–2

V–61 V–56

V–6 V–3

V–60 V–57

V–10

V–53 V–46

V–22

V–41 V–34

QUADRANGLE LOCATION

Photomosaic showing location of map area. An outline of 

1:5,000,000-scale quadrangles is provided for reference.

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Contents

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

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

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

Magellan Sar and Related Data ........................................................................................................2

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

Stratigraphy ...................................................................................................................................................4

Structures .......................................................................................................................................................6

Stratigraphic Relations of Units .................................................................................................................7

Geologic History ............................................................................................................................................9

Acknowledgements ....................................................................................................................................14

References Cited .........................................................................................................................................14

Tables

Table 1. List of volcanic and volcano-tectonic centers in the Lakshmi Planum quadrangle (V–7), 

Venus .............................................................................................................................................17

Table 2. List of impact craters within the Lakshmi Planum quadrangle (V–7), Venus ....................18

Table 3. Area of units mapped in the Laksmi Planum quadrangle (V–7), Venus .............................18

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 

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, altimetric, and radiometric mapping of the Venusian surface was done 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 meters. 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, 

there also exists a rapid increase in reflectivity 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. As such, 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 Lakshmi Planum quadrangle (V–7, fig. 1) is in the 

northern hemisphere of Venus and extends from lat 50° to 

75° N., and from long 300° to 360° E. The elevated volcanic 

plateau of Lakshmi Planum, which represents a very specific 

and unique class of highlands on Venus, dominates the northern 

half of the quadrangle (fig. 1). The surface of the planum stands 

3–4 km above mean planetary radius (fig. 2A) and the plateau is 

surrounded by the highest Venusian mountain ranges, 7–10 km 

high (figs. 2A and 3).

Before the Magellan mission, the geology of the Lakshmi 

Planum quadrangle was known on the basis of topographic 

data acquired by the Pioneer-Venus and Venera-15/16 altimeter (Masursky and others, 1980; Pettengill and others, 1980; 

Barsukov and others, 1986) and radar images received by the 

Arecibo telescope (Campbell and others, 1991) and Venera15/16 spacecraft (Basilevsky and others, 1986; Barsukov and 

others, 1986; Sukanov and others, 1989). These data showed 

unique topographic and morphologic structures of the mountain belts, which have no counterparts elsewhere on Venus, and 

the interior volcanic plateau with two large and low volcanic 

centers (Colette and Sacajawea Paterae) and large blocks of 

tessera-like terrain (Barsukov and others, 1986; Sukanov and 

others, 1986; Pronin, 1986). From the outside, Lakshmi Planum 

is outlined by a zone of complexly deformed terrains that occur 

on the regional outer slope of Lakshmi. Vast low-lying plains 

surround this zone. After acquisition of the Venera-15/16 data, 

two classes of hypotheses were formulated to explain the unique 

structure of Lakshmi Planum and its surrounding. The first 

proposed that the western portion of Ishtar Terra, dominated 

by Lakshmi Planum, was a site of large-scale upwelling (for 

example, Pronin, 1986, 1992; Grimm and Phillips, 1990) while 

the alternative hypothesis considered this region as a site of 

large-scale downwelling and underthrusting (for example, Head, 

1986; Bindschadler and others, 1990).
2

Early Magellan results (Kaula and others, 1992) showed 

important details of the general geology of this area displayed 

in the Venera-15/16 images. Swarms of extensional structures 

and massifs of tesserae populate the southern slope of Lakshmi. 

The zone of fractures and grabens form a giant arc thousands 

of kilometers long and hundreds of kilometers wide around the 

southern flank of Lakshmi Planum. From the north, the deformational zones consist mostly of contractional structures such 

as ridges. Corona and corona-like structures are not typical features of this zone but occur within separate branches of extensional structures oriented radial to the edge of Lakshmi. The 

southeastern edge of Lakshmi appears to be the source of large 

volcanic flows that extend to the south toward the lowland areas 

of Sedna Planitia. Colette and Sacajawea Paterae in the interior 

of Lakshmi are low volcanic centers with very deep central 

depressions (fig. 2A). Lava flows sourced by Colette and Sacajawea form distinctive radial patterns around these volcanoes. 

Magellan gravity data (Konopliv and Sjogren, 1994; Konopliv 

and others, 1999) show that the northern and northeastern portions of the quadrangle, which correspond to Lakshmi Planum, 

represent a significant geoid anomaly with the peak value of 

about 90 m over Maxwell Montes at the eastern edge of the 

map area. Maxwell is characterized also by very high vertical 

gravity acceleration values (as much as 268 mGal). The lowland 

of Sedna Planitia to the south of Lakshmi has mostly negative 

geoid values (down to -40 m).

The key geological structure of the quadrangle is Lakshmi 

Planum, the mode of formation of which is still a major unresolved problem. The topographic configuration, gravity signature, and pattern of deformation inside Lakshmi and along its 

boundaries make this feature unique on Venus. Thus, geological 

mapping of this region allows addressing several important 

questions that should help to put some constraints on the existing models of Lakshmi formation. What is the sequence of 

events in the formation and evolution of such a unique morphologic and topographic feature? What are the characteristics of 

the marginal areas of Lakshmi: the compact mountain belts and 

broad zones of deformation in the transition zone between Lakshmi and surrounding lowlands? How do the units in Lakshmi 

Planum quadrangle compare with the units mapped in neighboring and distant regions of Venus and what information do they 

provide concerning models for Venus global stratigraphy and 

tectonic history? These issues and questions are the basis for our 

geologic mapping analysis.

In our analysis we have focused on the geologic mapping 

of the Lakshmi Planum quadrangle using traditional methods 

of geologic unit definition and characterization for the Earth 

(for example, North American Commission on Stratigraphic 

Nomenclature, 1983) and planets (for example, Wilhelms, 

1990) appropriately modified for radar data (Tanaka, 1994). We 

defined units and mapped key relations using the full-resolution 

Magellan synthetic aperture radar (SAR) data (mosaicked fullresolution basic image data records, C1-MIDR’s , F-MIDR’s, 

and F-Maps) and transferred these results to the base map 

compiled at a scale of 1:5 million. In addition to the SAR image 

data, we incorporated into our analyses digital versions of 

Magellan altimetry, emissivity, Fresnel reflectivity, and roughness data (root mean square, rms, slope). The background for 

our unit definition and characterization is described in Tanaka 

(1994), Basilevsky and Head (1995a,b), Basilevsky and others 

(1997), and Ivanov and Head (1998).

Magellan Sar and Related Data

The synthetic aperture radar (SAR) instrument flown on 

the Magellan spacecraft (12.6 cm, S-band) provided the image 

data used in this mapping and interpretation. SAR images are a 

record of the echo (radar energy returned to the antenna), which 

is influenced by surface composition, slope, and wavelengthscale surface roughness. Viewing and illumination geometry 

also influence the appearance of surface features in SAR 

images. Guidelines for geologic mapping using Magellan SAR 

images and detailed background to aid in their interpretation 

can be found in Elachi (1987), Saunders and others (1992), Ford 

and Pettengill (1992), Tyler and others (1992), Tanaka (1994), 

and Campbell (1995). In the area of the Lakshmi Planum quadrangle, incidence angles (16.3–21.3 for Cycle 1 and 19.7–25.1 

for Cycle 2 [Ford and others, 1993]), are such that backscatter 

is dominated by variations in surface roughness at wavelength 

scales. Rough surfaces appear relatively bright, whereas smooth 

surfaces appear relatively dark. Variations also occur depending 

on the orientation of features relative to the incident radiation 

(illumination direction) with features normal to the illumination 

direction being more prominent than those oriented parallel to 

it. Full-resolution images have a pixel size of 75 m; C1-MIDRs 

contain the SAR data displayed at ~225 m/pixel. Altimetry data 

and stereo images were of extreme importance in establishing geologic and stratigraphic relations between units. Also 

essential in the analysis of the geology of the surface are data 

obtained by Magellan on the emissivity (passive thermal radiation), reflectivity (surface electrical properties), and rms slope 

(distribution of radar wavelength scale slopes). Aspects of these 

measurements were used in unit characterization and interpretation; background on the characteristics of these data and their 

interpretation can be found in Saunders and others (1992), Ford 

and Pettengill (1992), Tyler and others (1992), Tanaka (1994), 

and Campbell (1995).

General Geology

Several geologic processes have influenced the Lakshmi 

Planum quadrangle and have combined to form its geologic 

record. Volcanism is apparently the dominant process of crustal 

formation on Venus (Head and others, 1992) and produced most 

of the observed geologic units in the map area. These units typically have a plains-like appearance. There are two meanings of 

the term “plains” (Mescherikov, 1968). In a strict morphological sense, plains define morphologically uniform surfaces with 

relatively small differences in relief. In a broader sense, plains 

are counterparts of highlands and define vast flat terrains. In the 

strict sense, the term “plains” is simply a descriptor of a type 

of surface and does not bear an interpretative meaning. At the 

higher level of interpretation, the term “volcanic plains” implies 

knowledge or inference of the nature of material that makes up 
3

the plains. In the case of Venus, the materials that form plains 

are interpreted to be volcanic on the basis of a wide variety of 

data (for example, see Head and others, 1992). In the broader 

sense, the term “plains” usually describes large physiographic 

provinces (for example, North American Plain, Russian Plain). 

Thus, the same term can be applied to define two different classes of morphologic and (or) physiographic features. 

Although the specific meaning of the term “plains” is usually 

clear from the context, misunderstanding in the usage of it may 

occur. Fortunately, in planetary nomenclature the physiographic 

and physiographic-topographic meanings of the term “plains” 

are strictly defined. A vast plainlike and mainly homogeneous 

physiographic province is called planitia if it is at middle to low 

elevations (for example, Atalanta Planitia of Venus or Isidis 

Planitia of Mars) or called planum if it is elevated (for example, 

Lakshmi Planum of Venus or Hesperia Planum of Mars). Following this rule, we use the term “plains” in the nongenetic, 

morphological sense throughout the map text and the Description of Map Units. On the basis of their specific morphologic 

and physical property characteristics, we define various plains 

units, for example, lobate plains, shield plains, and so forth.

The principal structures of the western portion of Ishtar 

Terra and the map area are Lakshmi Planum and its mountain 

surroundings (figs. 1, 2). Lakshmi is a high standing volcanic 

plateau, the surface of which is covered by weakly to moderately deformed plains units. Two distinct volcano-tectonic 

features (Colette and Sacajawea Paterae) dominate the central 

portion of the plateau and appear to be sources of abundant lava 

flows. The most prominent parts of western Ishtar are mountain 

ranges that almost completely encircle the volcanic plateau of 

Lakshmi Planum. Topographically, the ranges are the highest 

tectonic constructs on Venus and morphologically they strongly 

resemble ridge belts elsewhere on the planet (Solomon and 

others, 1992; Squyres and others, 1992).

The distinct volcanic and volcano-tectonic features of 

smaller scale that occur within the quadrangle and are related 

to the larger and more distinct sources are listed in table 1. 

Tectonic activity has modified some of the crustal materials (for 

example, Pronin, 1992; Solomon and others, 1992; Kaula and 

others, 1992) in a variety of modes (extension and contraction). 

In places deformation is so extensive, as in the case of mountain 

belts and densely lineated plains, that the deformational features 

become part of the definition of the material unit (see also 

Tanaka, 1994; Scott and Tanaka, 1986). Impact cratering (table 

2) has also locally affected regions in the quadrangle but has not 

been an influential process over the map area as a whole.

In places, preferentially in the midlands that surround 

Lakshmi Planum to the south, clusters of small shield volcanoes 

less than about 15 km in diameter (Aubele and Slyuta, 1990; 

Guest and others, 1992; Aubele, 1994, 1995; Crumpler and 

Aubele, 2000; Addington, 2001; Ivanov and Head, 2004a,b) 

are common in spatial association with deformation belts. 

Individual small shields, however, are relatively rare within the 

vast extent of regional plains that cover the surface of lowlands 

to the south of Lakshmi Planum (Sedna Planitia). The small 

shields are low in elevation, commonly have a summit pit, do 

not appear to have distinct associated flows, and are commonly 

embayed by subsequent regional plains deposits (for example, 

Kreslavsky and Head, 1999; Ivanov and Head, 2001b; Ivanov 

and Head, 2004b). One steep-sided dome (Pavri and others, 

1992, Crumpler and Aubele, 2000, table 1) occurs inside the 

plateau of Lakshmi Planum near its mountain surroundings.

Five coronae and one arachnoid (fig.3, table 1) occur at the 

eastern and western edges of the map area. They are concentrated mostly within the heavily deformed midlands while the 

territory of Sedna Planitia lacks coronae (Stofan and others, 

1992, 2001; Crumpler and others, 1993; Crumpler and Aubele, 

2000). Extensive complexes of lava flows (fluctūs) occur to the 

south and west of Lakshmi Planum (table 1). The lava complex 

to the south of Lakshmi, Neago Fluctūs, appears to be associated with Muta Mons (fig. 1) and the fluctus to the west, Djata 

Fluctus, is at the northern edge of Omosi-Mama Corona and 

merges with another fluctus apparently sourced by Lunang 

Vallis (fig. 1). The flow-like features that compose fluctūs 

clearly indicate both the source regions and flow paths. The 

flows of the most distinct fluctūs follow the present-day topography, which indicates that the relief in the areas of the flows 

has not changed significantly since their emplacement.

The southern half of the map area and the central portion 

of Lakshmi Planum are dominated by plains of relatively homogeneous radar brightness that are interpreted to be of volcanic 

origin, and are mildly modified by sparse wrinkle ridges. The 

source vents of this plain-forming material are not known. The 

composition of the homogeneous and inhomogeneous volcanic plains is not known from data in this quadrangle, although 

Venera 9 (northeastern and southeastern slope of Beta Regio) 

and Vega 1 and 2 (Rusalka Planitia) lander geochemical analyses of sites in similar terrains suggest compositions similar to 

terrestrial basalts (Basilevsky and others, 1992; Abdrakhimov, 

2001a,b,c).

The area of the Lakshmi Planum quadrangle displays 

several occurrences of tesserae (fig. 3) where multiple tectonic 

styles have operated together or in sequence to produce terrain 

more heavily deformed than typical regional plains and where 

the deformation is so intense that it becomes a major part of the 

unit definition (for example, Bindschadler and others, 1992a). 

The largest (hundreds of kilometers across) tesserae, the surface 

of which consists of both contractional and extensional structures, are Clotho, Moira, and Itzpapalotl Tesserae (Itzpapalotl 

is within the Snegurocka Planitia, V–1 quadrangle). They occur 

within the heavily tectonized outer slopes of Lakshmi Planum 

to the south, southwest, and north of it. There are smaller 

and heavily flooded occurrences of tesserae in the interior of 

Lakshmi Planum. These occurrences display less evidence 

for contractional structures and are dominated by intersecting 

extensional structures.

Other tectonic features in the Lakshmi Planum quadrangle 

(fig. 3) are present in several deformation belts that consist of 

either contractional or extensional structures. Belts of contractional structures correspond to the mountain ranges of Danu, 

Akna, Freyja, and Maxwell Montes that are respectively to the 

south, west, north, and east of the upland volcanic plateau of 

Lakshmi Planum. These belts, which are about 200–300 km 

wide and thousands of kilometers long, are the most prominent 

topographic structures on Venus and represent dense swarms 

of ridges that are typically 5 to 15 km wide. Wrinkle ridges 
4

are widespread throughout plains units that cover most of the 

surface of the lowlands around Lakshmi Planum and make up 

the majority of the surface of the plateau itself. The typical 

length of wrinkle ridges is several tens of kilometers and width 

is a few kilometers. A large, 2,000-km long, easterly trending 

zone of extensional structures (fractures and grabens) extends 

from Omosi-Mama Corona through Ut Rupes to Clotho Tessera 

and outlines the southern edge of the Lakshmi Planum. The 

typical width of this zone is about 150–200 km. Another branch 

of grabens and fractures extends from Omosi-Mama Corona 

southward to Beiwe Corona at the southern margin of the map 

area. These belts of grooves outline rims of both coronae. To 

the northeast of Lakshmi Planum there is a corona-like feature; 

the rim of the feature is made up of swarms of arcuate grabens. 

Slightly elongated and heavily embayed outcrops of extensional 

structures also occur within the volcanic plateau of Lakshmi 

Planum and the rim areas of Colette and Sacajawea Patera are 

represented by arc-like zones of grabens and fractures. Extensional features in the quadrangle also include individual narrow 

(less than about 1–2 km) grabens hundreds of kilometers in 

length that are concentrated at the southern edge of Lakshmi 

Planum and apparently relate to some volcanic centers (Rangrid 

Fossae).

Twenty-three impact craters have been mapped in the 

quadrangle (table 2, fig. 3). The craters range in diameter from 

4.3 km for the unnamed crater at lat 55.1° N., long 360.6° E. to 

48.1 km for crater Cotton (Herrick and others, 1997; Schaber 

and others, 1998, table 2). Five splotches (surface markings and 

deposits interpreted to be formed from airblasts from projectiles 

traversing the atmosphere; for example, Schaber and others, 

1992; Ivanov and others, 1992) were detected in the quadrangle. 

The surfaces around craters Cotton, Stefania, Lind, and the 

unnamed crater (lat 55.1° N., long 360.6° E.) display radar dark 

haloes that likely were emplaced during the cratering event 

(Schaber and others, 1992; Phillips and others, 1992; Campbell 

and others, 1992; Schultz, 1992). There are no areas within the 

quadrangle apparently covered with fragmental surface materials that have been redistributed by aeolian processes.

Stratigraphy

On the basis of an analysis of the global size-frequency 

distribution and inferred flux of impact craters, a crater retention 

age of 800 Ma (McKinnon and others, 1997), 500 Ma (Schaber 

and others, 1992; Phillips and others, 1992) or 300 Ma (Strom 

and others, 1994), for the present surface of Venus has been 

proposed. The crater areal distribution cannot be distinguished 

from a spatially random population, which, together with the 

small total number of craters, means that crater size-frequency 

distributions cannot be used to date stratigraphic units for an 

area the size of the Lakshmi Planum quadrangle (Hauck and 

others, 1998; Campbell, 1999). Therefore, attention must be 

focused on the definition of geologic units and structures, and 

analysis of crosscutting, embayment, and superposition relations of structures and units to establish the regional geologic 

history.

Although we have mapped tectonic structures independently of geologic units, in a few cases tectonic features are 

such a pervasive part of the morphology of the terrain that it 

becomes part of the definition of a unit. For example, our unit 

of tessera material is analogous to aureole member 4 of the 

Olympus Mons Formation on Mars (“Forms broad...lobes; 

corrugated, cut by numerous faults that formed scarps and 

deep troughs and grabens”, Scott and Tanaka, 1986) and ridged 

plains material is similar to ridge band material on Europa (“...

linear to curvilinear features consisting of alternating ridges 

and troughs...”, Figueredo and Greeley, 2000). In recently 

published Venus maps, the same approach of unit definition 

was successfully applied for mapping several quadrangles that 

portray geologically diverse provinces (Rosenberg and McGill, 

2001; Bridges and McGill, 2002; Campbell and Campbell, 

2002; Hansen and DeShon, 2002). In other cases, the approach 

depends on scale and density of structures. For example, where 

the structures are more discrete and separated, we mapped them 

separately and not as a specific unit. In other cases, where structures are very dense, tend to obscure the underlying terrain, and 

are embayed by younger material units, we chose to map such 

occurrences of pervasive tectonic structures as specific units. 

Here we summarize the stratigraphic units and structures and 

their relations.

Tessera material (unit t, fig. 4) is interpreted to be among 

the stratigraphically oldest units in the quadrangle. The total 

area covered by tessera material within the quadrangle is about 

0.62 106 km2 or ~8.6 percent (table 3). The radar bright tessera 

material is deformed by at least two sets of intersecting ridges 

and grooves and is a result of tectonic deformation of some 

precursor terrain (Barsukov and others, 1986; Basilevsky and 

others, 1986; Bindschadler and Head, 1991; Sukhanov, 1992; 

Solomon and others, 1992). Tessera material is embayed by 

most of the other units within the map area. Arches, ridges, 

grooves, and grabens are tectonic features, so structure is an 

essential component of the tessera terrain and a key aspect of 

the unit definition. Globally, tessera occupies about 8% of the 

surface of Venus (Ivanov and Head, 1996) and occurs as large 

blocks and small islands standing above and embayed by adjacent plains. Several medium-sized occurrences of tessera are 

exposed in the map area: 1) Clotho Tessera (about 1000 by 400 

km, southern portion of the map area), 2) Moira Tessera (about 

300 by 300 km, southwestern portion of the quadrangle), and 3) 

southwestern portion of Itzpapalotl Tessera (about 500 by 300 

km, northeastern corner of the quadrangle). Outliers of tessera 

material also occur within the volcanic plateau of Lakshmi 

Planum. Based on the Venera-15/16 data (spatial resolution 

about 1–2 km/px), the area of Atropos Tessera at the northeastern corner of Lakshmi Planum was considered as tessera terrain 

(Barsukov and others, 1986; Sukhanov, 1992; Pronin, 1992). 

However, images of this region taken by Magellan spacecraft 

(spatial resolution 100–200 m/px) show that the surface of Atropos Tessera displays morphology that is transitional between 

tessera and less tectonically deformed units.

Densely lineated plains material (unit pdl, figs. 5, 6) forms 

apparently the oldest plains unit. This material is characterized 

by relatively flat surfaces on a regional scale and cut by swarms 

of parallel and subparallel lineaments (resolved as fractures 
5

if they are wide enough) having typical spacing of less than 1 

km. Densely lineated plains occur predominantly in the western 

portion of the quadrangle. In the southwestern corner of the 

map area, the plains are within the midland deformational zone 

surrounding Lakshmi Planum from the south, often in close 

association with belts of grooves. Occurrences of the unit are 

typically small (about 100–200 km across). In the northwestern 

portion of the map area densely lineated plains material forms 

the massif of Atropos Tessera area that is many hundreds of 

kilometers across. Broad arches additionally deform the surface 

of unit pdl in the region of Atropos Tessera (fig. 6). Along the 

boundary of Atropos Tessera and Akna Montes, the arches 

within the region of densely lineated plains material are roughly 

parallel to the trend of the Akna Montes mountain belt. Morphologically, densely lineated plains material in Atropos Tessera 

resemble the tessera transitional terrain described in the 30N 

geotraverse (Ivanov and Head, 2001b). The total area of the 

plains is about 0.43 x 106 km2 or about 5.9% of the area of the 

quadrangle (table 3). Although the unmodified precursor terrain 

for the densely lineated plains material is not observed, the flatness of the surface suggests that it was plains. The fractures that 

deform the densely lineated plains material are structural elements. They, however, are such a pervasive part of the morphology that it becomes a key aspect of the definition of the unit (see 

Campbell and Campbell, 2002; Hansen and DeShon, 2002, or 

several of the Mars examples cited above).

Ridged plains material (unit pr, fig. 7) is commonly 

deformed by relatively broad (2–5 km wide) ridges tens of 

kilometers long that dominate morphological appearance of 

the unit. In the Lakshmi Planum quadrangle, this unit is not 

abundant and occurs in the form of small (many tens of km long 

and a few tens of km wide) elongated fragments in the southern 

portion of the map area. The area occupied by ridged plains 

material is about 0.03 x 106 km2 or about 0.4% of the quadrangle (table 3). Although occurrences of this unit are small, 

they illustrate features that are analogous to the ridge belts of 

Kryuchkov (1990), Frank and Head (1990), Squyres and others 

(1992) and to units mapped in the other quadrangles on Venus 

(Rosenberg and McGill, 2001; Bridges and McGill, 2002). 

Although the ridges are structural elements, they are important 

features that help to define and map the material unit of ridged 

plains. For instance, there is not much doubt that wrinkle ridges 

deform materials of regional plains. Hypothetically, a wrinkle 

ridge or collection of them would represent kipukas of material 

of regional plains if younger lavas would flood the rest of the 

unit. If this hypothetical situation is mapped at a scale sufficient 

to outline individual wrinkle ridges, then the true structural 

elements, the ridges, should be mapped as a material unit. The 

scale of our mapping is mostly sufficient to map exposures of 

a material unit represented on the surface by ridges of ridged 

plains. This is unit pr and it is a material unit, although it is 

represented mostly by structures. Ridged plains material, thus, 

is probably volcanic plains material deformed by compressional 

stresses.

Shield plains material (unit psh, fig. 8) is characterized 

by abundant small shield-shaped features ranging from a few 

kilometers in diameter up to about ten to twenty kilometers, 

many of them with summit pits. Although small clusters of 

shields were recognized earlier globally (Head and others, 

1992), they were considered as localized occurrences possibly related to individual sources such as hot spots. Later 

work in the Vellamo Planitia, V–12 quadrangle (Aubele, 1994, 

1995) showed that many of these occurrences represented a 

stratigraphic unit in this region, and later this unit has been 

recognized in many areas on the planet (Basilevsky and Head, 

1995b; Basilevsky and others, 1997; Crumpler and Aubele, 

2000, Ivanov and Head, 1998, 2001a,b), including this quadrangle. Shields characterizing this unit occur in clusters, which 

give the unit a locally hilly texture, and as isolated outcrops in 

relatively smooth plains. The shields are probably volcanic in 

origin and are likely to be the sources of the adjacent smooth 

plains material. In the Lakshmi Planum quadrangle, however, 

there is no clear evidence for specific flow units associated with 

the small edifices of shield plains. The unit is widely distributed 

in the quadrangle but occurs predominantly in its southwestern 

part within the lowlands around the Lakshmi Planum upland. 

The area of the unit is about 0.55 x 106 km2 or about 7.6% of 

the map area (table 3). Quite often, occurrences of shield plains 

material are associated with swarms of grooves that outline 

the structures of Omosi-Mama and Beiwe Coronae. There are 

almost no concentrations of shield plains material within the 

volcanic plateau of Lakshmi Planum, where the plains were 

either completely buried by younger plains materials or never 

formed. Some isolated clusters of shields occur as kipukas in 

places where subsequent plains materials embay shield plains.

Pitted and grooved plains material (unit ppg, fig. 9) has 

intermediate radar albedo and is characterized by numerous 

narrow and wide and apparently shallow depressions with scalloped edges. Sometimes, chains of pits or slightly elongated 

rounded depressions are seen. In places, isolated pits and low 

shieldlike features complicate the surface of the pitted and 

grooved plains material. In the Lakshmi Planum quadrangle, 

unit ppg occurs in four small outcrops many tens of kilometers 

across at the periphery of the volcanic plateau of Lakshmi in 

spatial association with the mountain ranges. Three occurrences 

of the unit are at the southern edge of the plateau near Danu 

Montes and the fourth occurrence is in the northwestern corner 

of the plateau at the conjunction of Akna and Freyja Montes. 

The area of the unit is about 0.07 x 106 km2 or about 1.0% of 

the map area (table 3).

Regional plains material, lower unit, (unit rp1 fig. 10) 

forms the most widespread plains unit in the quadrangle. The 

total area of the plains is about 2.0 x 106 km2 or about 25.9% of 

the quadrangle (table 3). This unit is composed of morphologically smooth, homogeneous plains material of intermediate-dark 

to intermediate-bright radar albedo complicated by narrow, 

linear to anastomosing wrinkle ridges (a structural element) 

in subparallel to parallel lines or intersecting networks. This 

unit resembles the ridged unit of the plateau sequence on Mars 

(Scott and Tanaka, 1986), which is a plains unit defined by 

“long, linear to sinuous mare-type (wrinkle) ridges.” In the map 

area, the wrinkle ridges typically are less than 1 km wide and 

tens of kilometers long; in some areas they may be smaller, 

whereas in others they are larger. The trend of wrinkle ridges 

often varies locally even within one site (fig. 3). The unit is 

interpreted to be plains material of volcanic origin that was 
6

subsequently deformed by wrinkle ridges. Volcanic edifices and 

sources of the plains material are not obvious. The lower unit 

of regional plains material covers the northern portion of the 

plateau of Lakshmi Planum and isolated patches of it are seen 

in the southern portion of the plateau. This unit is also abundant 

to the south of the Planum where material of the lower unit 

of regional plains preferentially occurs within the lowlands 

of Sedna Planitia. The lower unit of regional plains material 

is noticeably less abundant on the regional slopes of Lakshmi 

Planum where older and highly tectonized units are concentrated.

Regional plains material, upper unit, (unit rp2, fig. 11) has 

noticeably higher radar albedo than material of the lower unit of 

regional plains and usually is characterized by lobate boundaries. Wrinkle ridges mark the surface of the unit (fig. 11). The 

largest area of this unit occurs in the southeastern, south-central, 

and southwestern portions of the quadrangle that correspond 

to the southern regional slope of the Lakshmi Planum upland. 

A smaller occurrence of this unit is on the northern flank of 

Colette Patera. This unit makes up about 0.92 x 106 km2 or 

12.8% of the area of the quadrangle (table 3).

Plains units that are younger than regional plains materials 

are characterized by morphologically smooth surfaces and often 

by distinctive lobate and digitate shapes and margins. Two types 

of the younger plains units are recognized.

Smooth plains material (unit ps, fig. 12) is characterized 

by uniform and low albedo and tectonically undeformed and 

featureless surfaces. Small patches of this unit are scattered 

throughout the map area, but preferentially are concentrated in 

its central (volcanic plateau of Lakshmi Planum), northwestern, 

and northeastern portions. The total area of smooth plains is 

about 0.31 x 106 km2 or ~4.3% of the map area (table 3).

Lobate plains material (unit pl, fig. 13) comprises a significant part of the quadrangle (about 0.79 x 106 km2 or 10.9%, 

table 3). A distinctive pattern of radar bright and sometimes 

radar dark flow-like features with lobate fronts characterizes 

this unit. The surface of the unit is almost tectonically undeformed and flow-like features embay most tectonic structures 

including those that deform the surface of the regional plains 

material, upper unit. There are three major areas where the 

lobate plains material occurs. The first is a large complex 

of lava flows, Neago Fluctūs, the source of which is in the 

complexly deformed zone at the southern regional slope of 

the Lakshmi Planum upland. The second is inside the volcanic 

plateau of Lakshmi Planum and represents lava flows from 

Colette and Sacajawea Paterae. Lava flows of Djata Fluctus to 

the west of Lakshmi Planum represent the third area of lobate 

plains material.

Impact craters and related deposits are observed in several 

places in the quadrangle (table 2, figs. 3, 14). Their locations 

are noted by symbols and they are mapped as undivided crater 

material (unit c) and crater flow material (unit cf). Four impact 

craters within the mapped area (Cotton, Stefania, Lind, and an 

unnamed crater at lat 55.1° N., long 350.6° E., table 2) are characterized by distinct surrounding radar dark material (Campbell 

and others, 1992; Ivanov and others, 1992) that partly to mostly 

obscures the underlying terrain. One crater (Gražina, table 

2) is deformed by extensional structures related to evolution 

of Freyja Montes. None of the craters in the quadrangle are 

embayed by volcanic materials.

Structures

A variety of extensional and contractional tectonic structures (some are in distinctive belts) is observed and mapped in 

the quadrangle (fig. 3).

Extensional structures 

Long linear fractures and some paired and inward-facing 

scarps mapped as sharp grooves interpreted to be grabens 

are seen in the eastern-central portion of the map area. These 

grabens form the complex of Rangrid Fossae at the southeastern 

edge of Lakshmi Planum. The grabens are several hundreds of 

kilometers long, oriented in the northwest direction, and produce a large set of parallel curvilinear structures that are slightly 

bent toward the west. Almost all structures of Rangrid Fossae 

are significantly broader at their southern ends and become 

progressively narrower northward. The southern ends of the 

grabens often have the shape of broad elongated depressions 

with scalloped edges or chains of pits. Morphologically, these 

depressions and pit chains resemble those that characterize the 

surface of pitted and grooved plains material (unit ppg).

In many places, the extensional structures such as fractures 

and grabens are so closely spaced that they tend to obscure 

underlying terrain and their presence takes on a defining character of the terrain. These concentrations are characterized by 

numerous short and long curvilinear subparallel lineaments that 

are usually wide enough to be resolved as fractures and grabens. 

These occurrences form linear belts (groove belts, unit gb; fig. 

15) that consist of individual swarms of extensional structures. 

The belts can be many hundreds of kilometers long and a few 

hundreds of kilometers wide. Individual swarms of grabens 

are typically tens of kilometers wide and are characterized by 

slightly higher topography relative to the surrounding plains. In 

detailed mapping at the F-Map scale, remnants of pre-existing 

plains can be seen between the neighboring tectonic features but 

cannot be mapped at the 1:5 M scale. Orientations of grooves in 

the belts are generally parallel to the trend of the groove belts as 

a whole.

Groove belts are concentrated in the southern half of the 

quadrangle; one of the most distinctive belts extends along the 

southern and western regional slopes of Lakshmi Planum from 

the area to the south of Clotho Tessera almost to Atropos Tessera 

(fig. 3). Groove belts (unit gb) are distinguished from densely 

lineated plains material (unit pdl) by their belt-like form, a lower 

density of tectonic features, and the character of the fractures 

(more distinctly recognizable grabens in groove belts). The belts 

also appear to be relatively distinctive stratigraphically. The 

structures of the belts are mostly embayed by shield plains material (unit psh) and by the lower unit of regional plains material 

(unit rp1). Thus, these plains units represent the upper stratigraphic limit for the groove belts. The lower limit of the belts 

is established by material and deformation of densely lineated 

plains material (unit pdl) and ridged plains material (unit pr) that 

are consistently cut by groove belts.
7

Apparently, there are two types of groove belts in the map 

area. The first, which outlines Lakshmi Planum from the south, 

does not show association with coronae. The second type of 

belt, which runs from the southern edge of the quadrangle in a 

northerly direction, both connects two large coronae (Beiwe and 

Omosi-Mama Coronae) and forms their rims. The association 

of coronae and zones of extensional structures (corona/groove 

belt chains) on Venus has been mapped and described in many 

regions (Baer and others, 1994; Ivanov and Head, 2004a, 2006).

Contractional structures 

Several types and scales of contractional features are 

observed within the quadrangle (fig. 3). Wrinkle ridges are 

widespread in the quadrangle and are so important in the 

broad regional plains that they in part define and characterize 

material of the plains (fig. 10). These features are mapped in a 

representative sense in terms of density and trend by individual 

wrinkle ridge symbols. There is no single predominant trend of 

wrinkle ridges in the quadrangle. Wrinkle ridges are common 

structures within the southwestern part of the map area where 

shield plains material and both units of regional plains material are the most important. In contrast, there are almost no 

wrinkle ridges in the southern portion of the volcanic plateau of 

Lakshmi Planum where relatively young plains units (ps and 

pl) are predominant. Thus these units appear to form an upper 

stratigraphic limit for the wrinkle ridges development within 

the map area.

Broader and less sinuous ridges dominate ridged plains 

material (unit pr, fig. 7). The general trends of these ridges are 

also represented in the units by symbols. Ridged plains material is largely equivalent to the ridge belts of Squyres and others 

(1992). Basilevsky and Head (1995a,b) described a structure 

(ridge belts, RB), which was a belt consisting of a cluster of 

densely spaced ridges 2–5 km wide and a few tens of kilometers 

long.

The most important occurrences of contractional structures 

in the map area are mountain belts (unit mb, fig. 16) surrounding the volcanic plateau of Lakshmi Planum. Densely packed 

ridges that are 5–15 km wide and tens to a few hundreds of 

kilometers long characterize all belts. There are four mountain 

belts: Danu, Akna, Freyja, and Maxwell Montes. Danu Montes, 

which are about 100 km wide and reach elevation about 1 km 

above the surface of the plateau of Lakshmi Planum, extend for 

about 1000-1200 km along the southern margin of Lakshmi (fig. 

3). Ridges at the inner side of this belt (with respect to Lakshmi 

Planum) are embayed by material of the lower unit of regional 

plains material of the plateau (fig. 17A).

Akna Montes represent a shorter (about 800–900 km), 

wider (about 200 km), and higher (about 3 km above the surface 

of the plateau) mountain range, which is slightly convex toward 

the plateau of Lakshmi (fig. 3). Some of the innermost ridges of 

Akna Montes with respect to the plateau appear to be embayed 

by materials of the lower unit of regional plains material 

from the Lakshmi interior (fig. 16). The surface of the plains, 

however, is tilted toward the mountain belt and deformed by 

wrinkle-type ridges that are parallel to the trend of the belt. 

At the outer side, the belt of Akna Montes is in contact with 

an extensive exposure of material of densely lineated plains in 

Atropos Tessera. Materials of this unit appear to be additionally 

deformed by long and broad ridges that continue the general 

trend of ridges in Akna Montes.

The mountain belt of Freyja Montes (figs. 1 and 3) forms 

the northern and northeastern boundary of Lakshmi Planum. 

This belt is wide (about 300 km) and high (about 3 km above 

the surface of Lakshmi) and characterized by complicated 

structures consisting of several domains with their own pattern 

of deformation. As in the case of Akna Montes, there is some 

evidence for embayment of structures of Freyja Montes by 

material of the lower unit of regional plains from the Lakshmi 

Planum side. The surface of the plains is tilted toward the belt 

and deformed by broad and low ridges that are parallel to the 

general orientation of Freyja Montes (fig. 17B).

The mountain range of Maxwell Montes is at the eastern 

edge of Lakshmi Planum and is the highest mountain construct 

on Venus (about 12 km above mean planetary radius). Within 

the map area, only the westernmost extension of Maxwell is 

seen. Relations of ridges of the range with the plains of the Lakshmi interior appear to be ambiguous and do not provide strong 

evidence suggesting relative ages of these units.

Stratigraphic Relations of Units

The material units and structures mapped within the Lakshmi Planum quadrangle commonly reveal relations of superposition (embayment) and crosscutting that either clearly display 

or strongly suggest relative ages among the units. Relations 

between the apparently oldest units in the map area, tessera 

material and densely lineated plains material, are shown in 

figure 18 in an area to the northeast of Lakshmi Planum where 

the large massif of Itzpapalotl Tessera is in contact with fragments of densely lineated plains material. Later plains materials 

broadly embay these highly deformed materials. This means 

that both tessera and densely lineated plains are at the bottom 

of the stratigraphic column. The surface of tessera material 

(unit t) is rough due to numerous chaotically oriented ridges 

and grooves. The most prominent of these are broad troughs 

oriented to the northwest, the floors of which are covered by 

dark plains material. The troughs cut both tessera and densely 

lineated plains materials and, thus, are relatively young structures. Densely lineated plains (unit pdl) occur as elongated 

massifs, the surface of which is deformed by densely packed, 

north-northwest-trending short and narrow lineaments. The 

surface of these materials appears to be smoother and structures that deform tessera material appear to be abutted at the 

northeastern edge of the massif of the unit pdl (fig. 18). The 

difference in the character, density, and orientation of structures 

suggests that densely lineated plains material was emplaced 

when tessera material was already in place and deformed. Some 

of the lineaments that cut the surface of densely lineated plains 

material cross the contact between the two units and are seen 

within tessera material. These relations suggest that material of 

the unit pdl was superposed on the surface of tessera material 

and covered some of the tessera structures and, thus, is younger. 

The tectonic deformation, however, continued and structures 

that deform the surface of the unit pdl material have added com-
8

plexity to the tessera surface. This additional complication of 

the tessera surface, the presence of the long broad troughs, and 

the example of Atropos Tessera (fig. 6) indicate that the tesseraforming deformation continued subsequent to the emplacement 

of younger materials.

Vast plains units, such as shield plains material and the 

lower unit of regional plains material, embay ridged plains 

material (unit pr, fig. 19) and, thus, appear to provide the upper 

stratigraphic level for the ridged plains material. The lower 

level of this unit is poorly determined. Direct relations between 

ridged plains material and either tessera material or densely 

lineated plains material cannot be established because the units 

are not in contact with each other within the map area. The simpler pattern of deformation may suggest, however, that ridged 

plains material was deformed later than the units t and pdl and 

did not accumulate as much deformation as tessera and densely 

lineated plains materials. Elsewhere on Venus, units that have 

morphology similar to that of ridged plains material within V–7 

quadrangle appear to be about at the same stratigraphic level as 

densely lineated plains material or slightly higher (Ivanov and 

Head, 2001a, 2005, 2006). This may be circumstantial evidence 

that in the Lakshmi Planum quadrangle, unit pr is also near 

the bottom of the regional stratigraphic column. This apparent 

stratigraphic position of ridged plains material is supported by 

the observation that structures of groove belts appear to cut the 

surface of the ridge plains material in places (fig. 19) and appear 

to be embayed by units psh and rp1 materials (figs. 19, 20).

Relations between groove belts and shield plains material are much more clear and consistent throughout the map 

area (fig. 20). In the area shown in figure 20, a large fragment 

of a groove belt is in contact with a broad field of shield plains 

material. The density of structures that form the belt varies 

across the scene and wide gaps between swarms of grooves are 

covered by shield plains material, which appears to be tectonically undeformed. In places, shield plains material abuts the 

grooves. These relations clearly indicate that emplacement of 

shield plains material occurred after formation of the majority 

of grooves that form the belt. Some of the grooves, however, 

cut through the surface of unit psh and appear to deform shields 

(fig. 20). This suggests that development of grooves continued 

after emplacement of shield plains material, but at much lower 

rate.

Throughout the map area, consistent relations of embayment occur between shield plains material (unit psh) and the 

lower unit of regional plains material (unit rp1, fig. 21). In 

many areas within the quadrangle, the shield plains material 

occurs as tight clusters of small shieldlike structures, which is a 

characteristic feature of this unit elsewhere on Venus (Crumpler 

and Aubele, 2000; Addington, 2001). Clusters of shields form 

equidimensional fields tens of kilometers across surrounded 

by wrinkle ridged plains material as shown in figure 21. The 

clusters may have slightly different radar albedo and internal pattern of deformation that is absent within surrounding 

regional plains. There is no evidence for flows emanating from 

any of the shields and superposed on the surface of surrounding plains. The density of shields is sharply diminished across 

the cluster boundaries and only a few small isolated edifices are 

seen between neighboring clusters.

These relations between the shield plains material unit 

and the lower unit of regional plains material strongly suggest that the shields (clustered or individual) represent kipukas 

of a unit (shield plains material, unit psh) heavily flooded by 

younger regional plains material (Ivanov and Head, 2004b). 

Kreslavsky and Head (1999) have shown detailed quantitative 

examples of such relations that illustrate how isolated shields 

become smaller and less dense away from the contact, suggesting embayment by younger plains units. Although these relative 

ages are typical within the map area, one cannot rule out the 

possibility of partly contemporaneous emplacement of shield 

plains material and the lower unit of regional plains material. 

To account for this uncertainty, these two units are shown partly 

overlapping in the Sequence of Map Units.

Regional plains material units rp1 and rp2 are deformed 

by a pervasive network of wrinkle ridges and, thus, predate 

the episode(s) of ridge formation (fig. 22). The upper unit of 

regional plains material is typically brighter in SAR images 

and, in places, this unit is characterized by lobate boundaries 

resembling lava flows and internal flowlike features. Outside 

Lakshmi Planum, occurrences of the upper unit of regional 

plains material apparently extend along local depressions and 

tend to outline local highs on the surface of the lower unit of the 

regional plains material. Inside Lakshmi, northern flows from 

Colette Patera are superposed on vast featureless regional plains 

material and also are cut by wrinkle ridges, thus representing 

the upper unit of regional plains material (unit rp2). This apparent topographic position and more distinctive flows within the 

upper unit of regional plains material suggests that unit rp2 generally postdates, but may be locally correlative with, the lower 

unit of regional plains material (unit rp1).

Consistent relations exist between the upper unit of 

regional plains material (unit rp2) and lobate plains materials 

(unit pl, fig. 23). In each locality where these units are in contact, lobate plains material is almost tectonically undeformed 

and clearly embays structures of wrinkle ridges. In the example 

shown in figure 23, the surface of the upper unit of regional 

plains material is cut by a system of subparellel wrinkle ridges 

oriented northwest. The ridges deform the surface of unit rp2

that displays faint internal flowlike and channellike features. 

More bright flows of lobate plains material (unit pl) spread over 

the surface of unit rp2 and mostly lack wrinkle ridges. This 

means that volcanic activity responsible for the emplacement 

of the lobate plains material began near terminus of wrinkle 

ridge formation. However, the distal portion of the larger flow 

of the unit pl appears to be deformed by a wrinkle ridge (fig. 

23). This relation implies that, as in the case of groove belts and 

shield plains material, the majority of wrinkle ridges formed 

before emplacement of the lobate plains material but some of 

the ridges still continued to form after its deposition. Relations between regional plains material that cover the northern 

portion of Lakshmi Planum with lobate plains material and 

smooth plains material are similar to those observed outside the 

Planum. In each locality inside Lakshmi where these units are in 

contact the lobate and(or) smooth plains materials show almost 

no tectonic structures and embay wrinkle ridges.

The deformational episodes that have formed the mountain 

ranges around Lakshmi have stratigraphic constraints from both 
9

stratigraphically below and above. The lower stratigraphic limit 

for Akna Montes appears to be provided by the additionally 

deformed densely lineated plains material in Atropos Tessera 

(fig. 6) where the surface of the plains is deformed by broad 

arches parallel to the general strike of ridges in Akna. The lower 

unit of regional plains material within Lakshmi Planum embays 

ridges of Danu and Akna Montes (figs. 16, 17A) but the surface 

of the plains is tilted away from the Akna Montes mountain 

range. This suggests that the long wavelength warping of the 

plains continued until the shorter wavelength deformation that 

produced ridges of the range had ceased. The warping of the 

plains may indicate the late-stage gravitational readjustment 

of the bulk of the mountain range. Emplacement of the upper 

unit of regional plains material and lobate plains material inside 

Lakshmi Planum appears to be controlled by the tilted surface 

of the lower unit of regional plains material. Thus, the episodes 

of young volcanism may indicate the upper stratigraphic limit 

of the warping. The massif of Freyja Montes displays more 

ambiguous relations with the lower unit of regional plains material (fig. 17B). Although some structures of the range appear to 

be embayed, suggesting that the short wavelength deformation 

of Freyja is mostly older than emplacement of the lower unit 

of regional plains material, the surface of the plains sometimes 

displays broad and low ridges resembling those that form the 

Freyja massif (fig. 17B). Some of these broad ridges apparently 

control the distribution of wrinkle ridges and, thus, predate 

them. These relations suggest that the short wavelength deformation responsible for the formation of the ridges of the Freyja 

massif mostly took place before emplacement of the lower unit 

of regional plains material but some deformation occurred after 

formation of the unit and before formation of wrinkle ridges. 

The surface of the lower unit of regional plains material adjacent to Freyja Montes is tilted away from the mountain range 

suggesting, as in the case of Akna Montes, that broad warping 

of this material occurred after emplacement and deformation by 

wrinkle ridges.

Geologic History

The area of Lakshmi Planum quadrangle (western Ishtar 

Terra, figs. 1, 2) is a unique area on Venus where the highest 

mountain ranges surround a high-standing volcanic plateau (for 

example, Barsukov and others, 1986). Extensive studies of this 

region using Venera-16/16 and Magellan data have resulted in 

two groups of models put forth to explain the unusual topographic and morphologic characteristics of western Ishtar Terra. 

The first group includes models that explain Lakshmi as a site 

of mantle upwelling (Pronin, 1986, 1990, 1992; Grimm and 

Phillips, 1990). In this model, formation of Lakshmi Planum 

initiated in its central area due to rising and subsequent collapse 

of a mantle diapir (Pronin, 1986, 1990, 1992). In the modified 

model (Grimm and Phillips, 1990), a diapir was considered to 

have impinged at the base of a thickened block of lithosphere. 

Models from the other group consider the planum as a locus of 

mantle downwelling, convergence, underthrusting, and possible 

subduction (Head, 1986, 1990; Head and others, 1990; Roberts and Head, 1990; Bindschadler and others, 1990; Lenardic 

and others, 1991; Hansen and Phillips, 1993, 1995; Keep and 

Hansen, 1994; Ansan and others, 1996; Marinangeli and Gilmore, 2000). For both groups of models, the understanding of 

the progression of large-scale deformation is important. Thus, 

the main goal of the geological mapping, establishing of the 

sequence of events, should help constrain the models of formation and evolution of the western Ishtar Terra. Here we discuss 

stratigraphic positions of units, their temporal correlation, and 

the implied geologic history of the region. We also examine the 

sequence of tectonic deformation and its interpretation as well 

as the evolution of volcanic and tectonic styles.

Lakshmi Planum quadrangle is made up of the western 

side of Ishtar Terra, the eastern portion of which represents one 

of the largest tessera regions on Venus, Fortuna Tessera (Ivanov 

and Head, 1996). Smaller massifs of tessera populate western 

Ishtar and occur to the south (Moira and Clotho Tesserae) and 

to the north (Itzpapalotl Tessera) of Lakshmi Planum. Outcrops 

of tesseralike material, which is deformed by several sets of tectonic structures, occur also inside Lakshmi Planum where it is 

heavily embayed by the material of lower unit of regional plains 

material. The largest of these outcrops are seen in the eastern 

part of the plateau and smaller fragments of tessera material 

occur in its northwestern part. Such a pattern of areal distribution suggests that tessera material forms the older basement of 

the plateau. The complex and unique deformational pattern of 

the surface, which is not repeated within the other tectonized 

units, suggests that tessera material outside and inside Lakshmi 

Planum are among the oldest units within the map area. This 

interpretation is in accord with the stratigraphic position of tessera material elsewhere on Venus (Ivanov and Basilevsky, 1993; 

Ivanov and Head, 1996; Hansen and others, 1997; Phillips and 

Hansen, 1998). Although exposures of tessera material make 

up only about 9 percent of the map area (table 3), the likely 

extent of tessera in the subsurface may significantly increase the 

total amount of tessera material within the V–7 quadrangle. It 

is important to emphasize that the distinctive pattern of deformation seen in tessera material is not visible on the surface 

of ridges that compose the mountain ranges. Their surfaces 

appear to be morphologically smooth as if these structures 

were produced by the deformation of morphologically smooth 

plains-forming material. Thus, tessera material was probably 

not significantly involved in the later deformation that produced 

the mountain ranges and could compose an ancient craton-like 

massif as the basement of the plateau of Lakshmi. Such a massif 

is the key feature of almost all downwelling models of Lakshmi 

formation (for example, Head, 1990; Roberts and Head, 1990; 

Hansen and Phillips, 1995) and some upwelling models (Grimm 

and Phillips, 1990).

Volcanic plains material that has been densely fractured 

(unit pdl) appears to be younger than tessera material (fig. 18) 

where these units are in contact. The deformation patterns in 

densely lineated plains material is simpler than in tessera material and consists of very dense and unidirectional lineaments. 

The role of extension in the formation of the structural pattern 

of unit pdl is evident. Fragments of densely lineated plains 

material are locally elevated and irregularly shaped and predominantly populating the western part of the quadrangle outside of 
10

the mountain belts that surround Lakshmi Planum. The largest massif of the unit occurs in the Atropos Tessera area where 

the surface of the unit pdl is additionally deformed by broad 

ridges and to some degree resembles pattern of deformation 

of tessera material (fig. 6). The pattern of deformation of this 

unit indicates areas of limited extension and possible shear that 

occurred in the map area roughly contemporaneous with continuing tessera deformation. There are no outcrops of densely 

lineated plains material inside Lakshmi Planum but they may be 

completely covered by a composite layer of later plains units. 

The alternative is, of course, that the specific tectonic regime 

responsible for formation of the pattern of deformation of unit 

pdl never operated inside Lakshmi.

A less deformed plains unit (ridged plains material, unit 

pr) was emplaced either contemporaneously or, in part, following the formation of the tessera and densely lineated plains 

materials. Although no evidence exists for sources, the smooth 

surface of the deformed material of the unit strongly suggests 

that it is of volcanic origin. This unit makes up a very small 

fraction of the map area (about 0.4%, table 3) and, thus, does 

not appear to be areally important. Small fragments of the 

unit occur within the deformational zone along the southern 

regional slope of Lakshmi Planum southeast of Clotho Tessera. 

The most important features of ridged plains are ridges that 

are several kilometers wide. Elsewhere on Venus, these ridges 

are usually compose prominent ridge belts (Kryuchkov, 1990; 

Frank and Head, 1990; Squyres and others, 1992; Ivanov and 

Head, 2001a,b, 2006). The belts have significant relief, reaching 

several hundred meters (Ivanov and Head, 2001b), and thus represent significant zones of compression. The lack of prominent 

ridge belts within the V–7 quadrangle suggests that compressional environments outside Lakshmi occurred locally and in 

rather small areas.

The major contractional structures within the map area are 

concentrated in the mountain ranges surrounding the plateau of 

Lakshmi Planum (for example, Barsukov and others, 1986). The 

ranges consist of densely packed parallel ridges that are 5–10 

km. They form compact and high-standing zones mapped as 

mountain belts (unit mb). There is a general consensus that the 

belts were formed under large compressional stresses (Pronin, 

1986, 1992; Grimm and Phillips, 1990, 1991; Head, 1986, 1990; 

Head and others, 1990; Roberts and Head, 1990; Bindschadler 

and others, 1990; Hansen and Phillips, 1993, 1995; Marinangeli and Gilmore, 2000). Relations of mountain belts with the 

other material units and structures suggest that formation of 

the morphologically distinctive, short-wavelength structures of 

the belts (an orogenic phase) took place after emplacement and 

deformation of tessera and densely lineated plains materials but 

mostly before emplacement of the lower unit of regional plains 

material. The long-wavelength component of the deformation is probably related to the later evolution of the belts and 

is manifested by broad tilting of plains at the contact with the 

mountain ranges. This tilting continued after the emplacement 

of the lower unit of regional plains material. This broad warping 

is similar to the situation at large tessera regions where materials of plains units are tilted away from the tesserae (Head and 

Ivanov, 1996; Ivanov and Head, 1996), interpreted to be the 

consequence of late epeirogeny of regions of thickened crust. 

The warping of the plains occurs at Akna and Freyja Montes but 

not at the base of Maxwell Montes (Suppe and Conors, 1992) 

and Danu Montes.

The most extensive zone of groove belts that outlines the 

southern regional slope of Lakshmi Planum apparently formed 

approximately contemporaneously with the deformation of 

the orogenic phase of the mountain belts. The structures of the 

southern branch of groove belts that outline some coronae in the 

southwestern corner of the map area do not display clear relations with the other occurrences of the belts. These structures, 

however, mostly predate both shield plains and the lower unit 

of regional plains materials and, thus, appear to have about the 

same stratigraphic position as the other occurrences of groove 

belts.

Another distinctly different plains unit (shield plains material, unit psh) was emplaced in many parts of the map area 

after emplacement and deformation of ridged plains material 

and the first orogenic phase of formation of the mountain belts. 

The lack of extensive development of contractional features 

such as arches and ridges within the areas of unit psh implies 

that regional compressional stresses significantly waned before 

emplacement of this material. Presently exposed occurrences of 

shield plains material are seen preferentially in the southwestern 

and northeastern parts of the map area in close spatial association with groove belts.

Clusters of small shields characterizing the unit have 

slightly different radar albedo than surrounding regional plains 

and often are outlined by clear curvilinear to sinuous contacts 

that outline individual shields at the boundaries of the clusters 

(fig. 21, see also Aubele, 1994). There is no evidence for flows 

either emanating from the shields or superposed on the surface 

of surrounding plains. The density of shields is sharply diminished across the cluster boundaries and only a few small isolated 

edifices are seen between neighboring clusters. All these criteria 

collectively suggest that the clusters of shields and shield plains 

material largely predate emplacement of the vast lower unit of 

regional plains material (Ivanov and Head, 2004b).

The abundant shield volcanoes and inter-shield plains that 

are characteristic of this unit are noticeably different from the 

volcanic style of subsequent plains units and indicate widespread local and shallow magma sources during the emplacement of shield plains. Shield plains material embays densely 

lineated plains material (fig. 21) and, due to a lack of the broad 

ridges, is likely younger than the deformation characteristic of 

the ridged plains material.

Two reasons may explain the close association of shield 

plains material with groove belts. First, groove belts form local 

highs and the topographic position of shield plains material on 

flanks of the belts could be an important factor in the present 

areal distribution of unit psh where it survived flooding by the 

lower unit of regional plains material (Ivanov and Head, 2001b, 

2004b). This is consistent with broad embayment of shield 

plains material by later regional plains materials and the areal 

distribution of outcrops of unit psh suggests that the shield 

plains material probably underlies a significant part of regional 

plains, at least within the lowlands surrounding Lakshmi 

Planum from the south. Second, the association of unit psh with 

groove belts may have genetic links. The most extensive areas 
11

of unit psh in the western part of the map area are associated 

with groove belts that form rims of two coronae (Beiwe and 

Omosi-Mama) that may be the surface manifestations of mantle 

diapirs (for example, Stofan and others, 1992) and serve as volcanic centers. The belts may represent the initial tectonic phase 

of interaction of the diapirs with lithosphere and shield plains 

material may correspond to the later volcanic stage of evolution 

of the diapirs. This interpretation is partly supported by the fact 

that the association of groove belts and shield plains material 

is less typical within the deformational zone of groove belts to 

the south of Lakshmi where corona and coronalike features are 

absent (fig. 3).

Shield plains material is absent in the interior of Lakshmi Planum suggesting that it either never formed there or 

was completely covered by younger plains materials. The first 

interpretation appears to be more favorable because outcrops 

of the tesseralike units embayed by the lower unit of regional 

plains material are exposed within Lakshmi Planum (fig. 3) but 

fields of shield plains material are not associated with any of 

them. Small fragments of pitted and grooved plains material 

(unit ppg), which vaguely resemble outcrops of unit psh and 

probably have been formed due to distributed volcanic eruption (fig. 9), occur along the inner edge of the mountain ranges. 

This unit is superposed on ridges of the ranges and is embayed 

by unit rp1. Thus, it formed approximately contemporaneously 

with shield plains material outside Lakshmi. The unit ppg indicates small pulses of volcanic activity within Lakshmi after the 

waning of the orogenic phase of the mountain ranges and before 

and (or) concurrent with the emplacement of the lower unit of 

regional plains material on the surface of the Planum.

Subsequent to the emplacement of shield plains material, 

the style of volcanism changed again (Head and others, 1996). 

Instead of abundant small shield volcanoes, the lower unit of 

regional plains material (unit rp1) broadly occurring and regionally deformed by wrinkle ridges, was emplaced from sources 

that are poorly known. Occurrences of unit rp1 are concentrated 

mostly in the southern portion of the map within the lowlands 

of Sedna Planitia surrounding Lakshmi Planum from the south 

and in the interior of Lakshmi. The wide extent of this unit and 

its uniform morphology suggest a high-effusion-rate mode of 

emplacement from a few sources. Such a volcanic style during 

the formation of the lower unit of regional plains material is in 

distinct contrast to the widespread and abundant small shield 

volcanoes just preceding this phase. The widespread lower unit 

of regional plains material appears to be relatively thin (Ivanov 

and Head, 2001b), because small and large outliers of older 

units commonly occur within the broad fields of regional plains 

materials.

The massive outpouring of lava during emplacement of 

the lower unit of regional plains material (unit rp1) changed 

to more localized eruptions that produced the upper unit of 

regional plains material (unit rp2). Wrinkle ridges, which 

belong to the same family of structures that deform lower unit 

of regional plains material, as well as materials of other older 

units, deform flow-like features characterizing unit rp2. This 

means that the new radical change of volcanic style (from unit 

rp1 to rp2) occurred before the main phase of wrinkle ridge 

formation. On the basis of the outcrop distribution of the upper 

unit of regional plains material it appears that the sources of 

this material may have been distributed in different parts of the 

map area. The most abundant and the largest fields of unit rp2

are seen in the southern portion of the map area at the base of 

the regional slope of Lakshmi Planum. Distinct flows of the 

unit rp2 flow to the south toward the lowlands of Sedna Planitia indicating that the main topographic configuration of the 

region was established before emplacement of the upper unit of 

regional plains material. Another field of unit rp2 is at the northern flank of Colette Patera inside Lakshmi. This occurrence of 

the unit probably indicates some early stages of volcanism in 

the interior of Lakshmi. Sacajawea Patera and its surroundings 

does not display lava flows that are deformed by wrinkle ridges. 

All visible volcanic activity of Sacajawea Patera postdates both 

the emplacement of the upper unit of regional plains material 

and its deformation by wrinkle ridges.

The youngest units in the Lakshmi Planum quadrangle, 

smooth plains material and lobate plains material, are undeformed by tectonic structures and obviously were emplaced 

after formation of the pervasive networks of wrinkle ridges. 

Smooth plains material (unit ps) is concentrated preferentially 

at the distal edges of volcanic fields to the south of Colette and 

Sacajawea Paterae. Such an association suggests that smooth 

plains material in these localities represents volcanic material. 

Smooth plains material is not deformed by wrinkle ridges and, 

thus, postdate emplacement of the upper unit of regional plains 

material that corresponds to the earlier phases of volcanism at 

Colette Patera. Lobate plains material is clearly superposed on 

smooth plains material and, thus, whereas unit ps may represent 

the middle stages of volcanism at the Paterae in the interior of 

Lakshmi, unit pl represents late-stage volcanism.

Lobate plains material apparently represents the youngest material in the map area. The unit is concentrated in three 

areas: outside the deformational surrounding of Lakshmi 

Planum to the west and to the southeast of it and in the interior 

of the planum, around Colette and Sacajawea Paterae. Outside 

Lakshmi, lobate plains material occurs as extensive complexes 

of large and interfingering lava flows (fluctūs) that suggest 

multiple episodes of voluminous eruptions. The larger of the 

fluctūs is Neago Fluctūs (fig. 1) to the southeast of Lakshmi. 

By its dimensions (hundreds of kilometers), and complex 

superposition of lava flows, Neago Fluctūs resembles Mylitta 

Fluctus and other complexes of lava flows in the transition 

zone from the elevated Lada Terra to the lowlands of Lavinia 

Planitia within the Mylitta Fluctus, V–61 quadrangle (Roberts 

and others, 1992; Bridges and McGill, 2002; Ivanov and Head, 

2006). Djata Fluctus to the west of Lakshmi (fig. 1) is significantly smaller than Neago Fluctūs and is in spatial association 

with Omosi-Mama Corona. In contrast to large lava complexes 

elsewhere on Venus, for example, Mylitta Fluctus, the source 

of which is Jord Corona (Roberts and others, 1992), and 

Juturna and Cavillaca Fluctūs sourced by Boala Corona, Neago 

Fluctūs displays no distinctive source features. This suggests 

that the episodes of the latest volcanic activity at the southeastern base of Lakshmi Planum may not be related directly 

to mantle diapirs manifested by coronae. These episodes also 

appear to be not persistent enough to build significant volcanic 

constructs. Lobate and digitate flows that make up unit pl flow 
12

along the regional slopes in both sites where the unit occurs 

outside Lakshmi, providing evidence that the long-wavelength 

topographic configuration of the exterior of Lakshmi Planum 

already existed before emplacement of lobate plains material. 

Inside Lakshmi, the unit pl occurs on the floor of Colette and 

Sacajawea Paterae and forms a broad apron around them. The 

apron extends mostly to the south of both Paterae following 

the regional slope of the Lakshmi surface to the south. Such a 

distribution of young lava material indicates that the slope was 

established prior to the late stage volcanism at the paterae. In 

places where flows from Colette Patera (unit pl) come close to 

Akna Montes, the flows appear to be stopped at the tilted surface of the lower unit of regional plains material. This implies 

that the later phases of evolution of the mountain belts (broad 

warping of the surrounding plains) were completed before the 

later stages of volcanism of Colette Patera and perhaps Sacajawea Patera as well.

Of the twenty-three impact craters in the map area (table 

2), only one, Gražina (lat 72.5° N., long 337.5° E., 16.5 km), 

is cut by structures associated with Freyja Montes (fig. 17B). 

The other impact craters display no evidence of either tectonic 

deformation or volcanic embayment. This means that the shortwavelength tectonic deformation, which could be morphologically detected, and volcanic activity in the quadrangle mostly 

ceased before formation of the craters.

In summary, there are clear trends in the evolution of 

both tectonic regimes and volcanic styles within the Lakshmi 

Planum quadrangle. Contractional deformation first appeared 

after emplacement of ridged plains material but according to the 

areal distribution of the unit pr it was not very abundant in the 

map area. The peak of contractional deformation was reached 

during formation of the compact mountain ranges (unit mb) 

before emplacement of the vast plains units. Then it significantly dropped to the level of formation of pervasive but small 

structures of wrinkle ridges that deform the lower and the upper 

units of regional plains material (units rp1 and rp2). Contractional structures are absent within younger plains units such as 

smooth and lobate plains materials (units ps, and pl). This lack 

of contractional structures suggests cessation of compressional 

stresses by the time these materials were emplaced. Thus, the 

most important episode of contractional deformation within the 

map area was shifted toward the earlier stages of the observable 

geological history of the Lakshmi Planum quadrangle.

Extensional deformation that overprints densely lineated 

plains material (unit pdl) and played a major role in the formation of groove belts (unit gb) appears to have an opposite trend. 

The older fractures that have shaped the surface of unit pdl

material are very narrow, pervasive, and do not form belt-like 

concentrations. The later occurrences of extensional structures 

are organized into belts of broader and longer structures. The 

majority of extensional structures predate emplacement of the 

shield plains material (unit psh). After emplacement of the 

materials of shield plains and the lower unit of regional plains, 

only individual grabens appear to cut the surface of the plains. 

Some of the grabens form a loose system of Rangrid Fossae at 

the southeastern edge of Lakshmi Planum. Only minor extensional deformation is observed in either smooth plains material 

(unit ps) or lobate plains material (unit pl), and as such, these 

units appear to mark a significant waning of the extensional 

tectonics within the map area.

The evolution of volcanic styles is clearly seen in the stratigraphic sequence of the moderately deformed plains units (psh, 

rp1, rp2, ps, and pl) where either sources, or nature of material, 

or both are recognizable. The older of these units is the shield 

plains material (unit psh), which is characterized by small 

shields suggesting eruptions from distributed shallow sources 

(for example, Head and others, 1992) with limited supply of 

magma. The lack of shield plains material in the interior of Lakshmi Planum suggests that this area probably was not a favorable site for unit psh-style volcanism, which is consistent with 

the possible presence of a massif of thick crust and lithosphere 

in this area. The unit rp1 material formed both outside and 

inside Lakshmi and in both localities has similar morphology 

and surface properties. Inside Lakshmi, possible candidates for 

the sources of the unit rp1 are Colette and Sacajawea Paterae 

simply because these structures are the only obvious centers of 

volcanism within Lakshmi. If the paterae are indeed the sources 

of the lower unit of regional plains material in the interior of 

the Planum, then they began to be active when formation of the 

main bodies of the mountain belts (unit mb) was completed. 

Smooth plains material (unit ps) and both units of lobate plains 

material (units rp2 and pl) manifest another distinctive change 

of volcanic style from the vast lava flooding to voluminous 

and multiple eruptions from specific sources. Such a change of 

volcanic activity indicates localization and possibly deepening 

of the sources.

The tectonic and volcanic trends and the sequence of major 

events established during the geological mapping of the V–7 

quadrangle outline the evolution of the large-scale topography 

of Lakshmi Planum and put constraints on the models of its possible formation.

The presence of heavily tectonized and embayed tessera 

material and the lack of shield plains material in the interior of 

Lakshmi are consistent with and collectively suggest that initially it was a high-standing block of heavily tectonized terrain 

similar to large tessera massifs elsewhere on Venus (Roberts 

and Head, 1990, 1990; Grimm and Phillips, 1990). The presence of such a block (a craton) contradicts the model of growth 

and subsequent collapse of a large dome-like structure at the 

initial stages of evolution of Lakshmi Planum (Pronin, 1986, 

1990, 1992). Pronounced asymmetry characterizes the general 

topographic configuration of Lakshmi Planum. The southern 

half of the plateau is tilted to the south and is bordered by the 

lowest mountain range of Danu Montes. The northern portion 

of Lakshmi is more horizontal and is surrounded by the significantly higher ranges of Akna and Freyja Montes. This asymmetry of the large-scale topography of Lakshmi Planum appears 

to be inconsistent with a model in which formation of the 

mountain ranges is explained by the interaction of a stationary 

and axial symmetric mantle plume with a block of thickened 

lithosphere (Grimm and Phillips, 1990). The other predictions 

and consequences of the divergent class of models such as the 

presence of rift-like zones in the Lakshmi interior, older age 

of volcanism relative to the mountain belts, and progression of 

volcanism from the center of the plateau outward, are also not 

supported by the observations and the established sequences 
13

of units. The morphology of the ranges provides an additional 

argument against the model of interaction of a plume with a 

thickened block of crust/lithosphere. If, as described in the 

model (Grimm and Phillips, 1990), the ranges grew due to outward subsurface flow that thickened crust in the region below 

the ranges, the surface manifestation of such a process would 

likely resemble a horst-and-graben structure of the mountain 

belts due to gravitational relaxation of the growing topography. The same type of argument apparently could be applied 

to the model of formation of mountain ranges by deformation 

“from below” (Hansen and Phillips, 1995). The morphology of 

the ranges, consisting of ridges with morphologically smooth 

surfaces separated by V-shaped structural valleys, is similar, 

however, to the terrestrial fold-and-thrust belts (for example, 

Head, 1990). About the only prediction of the divergent class 

of models that is supported by observations is the presence of 

gaps between neighboring mountain ranges. These gaps, however, are not a unique feature of the divergent models and could 

be explained in the framework of other models, for example, 

by polyphase formation of the mountain ranges (for example, 

Crumpler and others, 1986).

Another class of models proposed to explain the unique 

topographic characteristics of Lakshmi Planum considers processes of convergence. There are two groups of these models. In 

the first, Lakshmi Planum and its mountainous surroundings are 

considered as the sequential result of a continuous downwelling 

under Lakshmi (for example, Bindschadler and others, 1990). In 

the second group of models, the morphologic and topographic 

characteristics of Lakshmi Planum are explained by regional 

compression and underthrusting (for example, Head, 1986, 

1990). In the framework of the downwelling model, formation 

of the mountain ranges is shifted toward the later stages of the 

process of continuous downwelling and was preceded by development of elevated topographic plateau over the locus of the 

downwelling (Bindschadler and Parmentier, 1989; Bindschadler 

and others, 1990). Although such a model is able to explain formation of the unique mountain ranges, three important characteristics of Lakshmi Planum appear to be weakly consistent with 

this model. First is the asymmetric topographic configuration of 

Planum, whereas the model might predict a more symmetrical 

structure (Bindschadler and Parmentier, 1989). The second is 

the requirement of formation of radial contractional structures 

in the interior of Lakshmi that predate the mountain ranges 

(Kiefer and Hager, 1989; Bindschadler and others, 1990). The 

evidence for such structures is absent. If they formed, they must 

have been completely flooded by later plains materials such as 

material of the lower unit of regional plains (unit rp1). The layer 

of this material, however, appears to be relatively thin because 

outliers of older tectonized terrain are scattered throughout the 

interior of Lakshmi. Contractional structures (especially those 

that could be formed during persistent downwelling flow) usually have high topography of several hundred meters (Ivanov 

and Head, 2001b, Young and Hansen, 2005) and the thickness 

of the later lava plains would need to be significantly larger than 

the height of the ridges to completely hide them. Otherwise, the 

surface of the cooling plains would drape over the ridges and 

produce specific pattern of structures that, for example, mark 

impact craters buried by lava plains on the Moon and Mars 

(for example, Colton and others, 1972; Raitala, 1988). These 

“ghost” structures are also absent in Lakshmi Planum. The third 

is the prediction of radial normal and strike-slip faults during 

the latest stages of evolution of the downwelling system. These 

tectonic features have not been found within Lakshmi Planum.

The convergence models that explain formation of Lakshmi Planum due to regional compression, underthrusting/

subduction, and orogenesis due to collision with the edges of 

an ancient crustal (tessera-like) plateau (Crumpler and others, 

1986; Head, 1986, 1990; Roberts and Head, 1990, Head and 

others, 1990) appear to be more consistent with the basic 

characteristics of western Ishtar Terra. These models explain 

the concentration of contractional structures at the periphery 

of Lakshmi and can account for the prominent asymmetry of 

this structure if the major axes of the convergence were at the 

northern and northwestern edges of the planum. These models 

are partly based on the obvious presence of heavily deformed 

terrain flooded by the later plains materials within the interior of 

Lakshmi Planum (for example, Roberts and Head, 1990). This 

terrain (the tessera material unit) clearly predates formation of 

the mountain ranges. The smooth surface of the ridges that form 

the ranges suggests that (1) the central tessera massif had not 

been significantly involved in formation of the ranges, and (2) 

it was mostly the material of plains units that was deformed to 

produce the ridges of the mountain belts.

The sequence of volcanic activity and progression of 

tectonic deformation play a key role in these models. Volcanism 

is predicted to largely postdate formation of the mountain belts 

with its younger phases shifted toward the center of Lakshmi 

Planum. This progression of volcanic events is consistent with 

the model in which the slabs subducting under the central craton 

of Lakshmi progressively melt at different depth [Ivanov and 

Head, 2008]. This is in complete agreement with the observations that material of the lower unit of regional plains (unit rp1), 

the oldest recognizable plains unit in the interior of Lakshmi, 

covers the majority of the interiors and followed the orogenic 

stage of the evolution of the mountain ranges. The younger 

plains units (materials of smooth plains, unit ps, and lobate 

plains, unit pl) occur at the center of Lakshmi around Colette 

and Sacajawea Paterae.

Another requirement of the regional compression and 

underthrusting/subduction models is the outward sequence of 

deformation. The Atropos Tessera area appears to illustrate such 

a progression of contractional deformation at the northwestern flank of Lakshmi Planum. There, the broad ridges, which 

conform to the general strike of Akna Montes, deform older, 

densely lineated plains material and make the surface of this 

material resemble the complexly deformed surface of tessera. 

The ridges extend over distances of hundreds of kilometers 

and occur within the majority of Atropos Tessera. On the other 

side of Akna Montes, inside Lakshmi, the ridges abruptly stop 

near the base of the mountain range. Such an asymmetry of the 

distribution of the ridges on both sides of Akna suggests that 

the ridges preferentially formed outside Lakshmi and, thus, may 

indicate outward progression of deformation.

Thus, the results of the geological mapping of the V–7 

quadrangle and established sequence of events in this region 

mostly contradict the divergent models of formation of Lakshmi 
14

Planum and apparently are in poor agreement with the important predictions of the downwelling models. The results of the 

mapping are consistent with the predictions, major features, 

and consequences of the models explaining formation of the 

unique topographic and morphologic characteristics of Lakshmi 

Planum due to large-scale compression and underthrusting/subduction of plains units from the north against an ancient crustal 

block in the core of Lakshmi.

Acknowledgements 

Thanks are extended to Alexander Basilevsky and Jayne 

Aubele for detailed discussion about regional and global units 

and relations. We particularly appreciate the helpful discussions 

that occurred during the annual Planetary Geologic Mappers 

meetings and during an informal mapping workshop held at 

Brown University in the summer of 1995 and attended by 

George McGill, Steve Saunders, Jim Zimbelman, Alexander 

Basilevsky, Elizabeth Rosenberg, and Nathan Bridges. Their 

comments and discussions were most helpful in this effort. Constructive reviews of the map by Jim Dohm and Robert Herrick 

are deeply appreciated. Financial support from the NASA Planetary Geology and Geophysics Program (Grant NNX06AB29G), 

and the NASA Applied Information Systems Program (Grant 

NNG05GA61G) is gratefully acknowledged.

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Table 1. List of volcanic and volcano-tectonic centers in the Lakshmi Planum quadrangle (V–7), Venus (modified from Crumpler and 

Aubele, 2000).

[—, no official name has been assigned]

Feature name Latitude

(deg N.)

Longitude

(deg E.)

Diameter

(km)

Lava fields (fluctūs)

Djata Fluctus 66.5 307 230x100

Neago Fluctūs 50 347 >600

Coronae

— 68 346 230–250

Omosi-Mama Corona 64.5 306.0 480

Beiwe Corona 53.5 307 180x400

Xilonen Corona 51 321 250x200

Ashnan Corona 50.2 357.0 300

Paterae

— 74.5 315 100

Colette Patera 66 323 600

Sacajawea Patera 64.5 335 225x150

Siddons Patera 61.5 340 70

Steep-sided dome

— 60 324.5 25

Arachnoid

— 54.5 326.5 280
18

Table 3. Area of units mapped in the

Lakshmi Planum quadrangle (V–7), Venus.

Unit

label

Area

(km2)

Area

(%)

cf 4,000 0.0

c 23,000 0.3

pl 787,000 10.3

ps 313,000 4.1

rp2 923,000 12.0

rp1 1,986,000 25.9

ppg 71,000 0.9

psh 549,000 7.2

gb 1,020,000 13.3

mb 459,000 6.0

pr 28,000 0.4

pdl 425,000 5.5

t 620,000 8.1

data gaps 455,000 5.9

TOTAL 7,663,000 100

Table 2. List of impact craters within the Lakshmi Planum quadrangle (V–7), Venus (modified from Schaber and others, 1998).

[—, no official name has been assigned to this feature]

Crater

name

Latitude

(deg N.)

Longitude

(deg E.)

Diameter

(km)

Gražina 72.448 337.474 16.5

Wanda 71.244 323.145 21.7

Osipenko 71 321 30

Rita 70.97 334.8 8.3

Cotton 70.758 300.197 48.1

Ivka 68.206 303.831 14.9

Magda 67.041 329.675 10.1

Zlata 64.63 333.918 7

Sigrid 63.555 314.394 16.1

Lyudmila 62.128 329.672 14.1

Berta 62 322 20

Tamara 61.63 317.315 11.9

Akhmatova 61.349 307.885 41.4

Volyana 60.636 359.938 5.3

Magnani 58.616 337.208 26.4

Kartini 57.793 333.039 23.4

— 55.1 350.6 4.3

Sévigné 52.625 326.474 29.6

Stefania 51.29 333.313 11.7

Lotta 51.06 335.909 11.8

Bahriyat 50.3 357.5 5

Lind 50.22 355.027 25.8

Aftenia 50 323.9 7