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Planetary and Space Science 56 (2008) 1949–1966

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Formation and evolution of Lakshmi Planum, Venus: Assessment ofmodels using observations from geological mapping

M.A. Ivanova,b, J.W. Head IIIb,�

aVernadsky Institute, Russian Academy of Sciences, Moscow, RussiabDepartment of Geological Sciences, Brown University, Box 1846, Providence, RI 02912, USA

Received 29 April 2008; received in revised form 2 September 2008; accepted 3 September 2008

Available online 16 September 2008

Abstract

Detailed geological analysis of the Lakshmi Planum region of western Ishtar Terra results in the establishment of the sequence of

major events during the formation and evolution of western Ishtar Terra, an important and somewhat unique area on Venus

characterized by a raised volcanic plateau surrounded by distinctive folded mountain belts, such as Maxwell Montes. These mapping

results and the stratigraphic and structural relationships provide a basis for addressing the complicated problem of Lakshmi Planum

formation and for testing the suite of models previously proposed to explain this structure. We review and classify previous models of

formation for western Ishtar Terra into ‘‘downwelling’’ models (generally involving convergence and underthrusting) and ‘‘upwelling’’

models (generally involving plume-like upwelling and divergence). The interpreted nature of units and the sequence of events derived

from geological mapping are in contrast to the predictions of the divergent models. The major contradictions are as follows: (1) The very

likely presence of an ancient (craton-like) tessera massif in the core of Lakshmi, which is inconsistent with the model of formation of

Lakshmi due to rise and collapse of a mantle diapir; (2) The absence of rift zones in the interior of Lakshmi that are predicted by the

divergent models; (3) The apparent migration of volcanic activity toward the center of Lakshmi, whereas divergent models predict the

opposite trend; (4) The abrupt cessation of ridges of the mountain ranges at the edge of Lakshmi Planum and propagation of these ridges

over hundreds of kilometers outside Lakshmi; the divergent models predict the opposite progression in the development of major

contractional features. In contrast, convergent models of formation and evolution of Lakshmi Planum appear to be more consistent

with the observations and explain this structure by collision and underthrusting/subduction of lower-lying plains with the elevated and

rigid block of tessera. These models are capable of explaining formation of the major features of western Ishtar (for example, the

mountain belts), the sequences of events, and principal volcanic and tectonic trends during the evolution of Lakshmi. To explain the

pronounced north–south asymmetry of Lakshmi these models need to consider the likelihood that the major focal points of collision

are at the north and north-west margins of the plateau. We note that pure downwelling models, however, face three important

difficulties: (1) The possibly unrealistically long time span that appears to be required to produce the major features of Lakshmi; (2) The

strong north–south asymmetry of the Planum; the pure downwelling models predict the formation of a more symmetrical structure; and

(3) The absence of radial contractional structures (arches and ridges) in the interior of Lakshmi that would represent the predictions of

the downwelling models.

r 2008 Elsevier Ltd. All rights reserved.

Keywords: Venus; Lakshmi Planum; Modes of origin

1. Introduction

Lakshmi Planum, constituting the majority of the westernIshtar Terra, is a high-standing plateau (3.5–4.5 km above themean planetary radius, MPR), which is surrounded by the

e front matter r 2008 Elsevier Ltd. All rights reserved.

s.2008.09.003

ing author. Tel.: +1401 863 2526; fax: +1 401 863 3978.

ess: [email protected] (J.W. Head III).

highest (6–8, up to 12km above MPR, Fig. 1) mountainranges on Venus (Masursky et al., 1980; Pettengill et al., 1980;Campbell et al., 1983; Barsukov et al., 1986; Pronin et al.,1986; Stofan et al., 1987). Lakshmi and its surroundings forma roughly circular feature about 1800� 2000km (Fig. 1a) andrepresent a specific type of elevated region on Venus (Fig. 1b).In several important aspects, Lakshmi Planum is differentfrom the other first-order highlands of Venus such as the

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Fig. 2. Topographic profiles across Lakshmi Planum, Ovda, and Beta

Regiones (see Fig. 1 for the position of the profiles). Lakshmi and Ovda

are broad and elevated topographic plateaus, while Beta has a dome-

shaped topographic configuration. Elevation is above the mean planetary

radius, 6051km.

Fig. 3. The topographic distribution of the interior of Lakshmi Planum

coincides with the distribution of Ovda Regio and corresponds to the

higher peak of the topographic distribution of tessera terrain on Venus.

The mountain ranges surrounding Lakshmi Planum display a broad range

of elevations but mostly occur at higher elevations. The topographic

distributions of Beta and Atla Regiones are similar to each other and

distinctly shifted toward the lower elevations relative to Lakshmi and

Ovda. Elevation is above the mean planetary radius, 6051km; count is the

number of equal-area pixels for each feature.

M.A. Ivanov, J.W. Head III / Planetary and Space Science 56 (2008) 1949–1966 1951

dome-shaped and rifted rises (Fig. 1c and d) and the tessera-bearing crustal plateaus (Figs. 1e, f and 2). The mostspectacular and unique feature of Lakshmi is its mountainoussurroundings that do not occur either at the crustal plateausor the rises. Some large tessera plateaus (e.g. Alpha,Ovda) possess marginal zones of ridges that are parallel tothe plateau edges (Solomon et al., 1992; Bindschadler et al.,1992; Ivanov and Head, 1996). These zones, however, do notreach the elevations that characterize the mountain rangesaround Lakshmi Planum. Another important differenceof Lakshmi Planum from the tessera plateaus is that mildlydeformed plains units cover the surface of Lakshmi (e.g.,

Fig. 1. Major morphologic (a) and topographic (b) features of Lakshmi Planu

its periphery by the prominent mountain ranges of Akna, Freyja, Maxwell,

Tessera border the northern flank of Lakshmi. Low-lying volcanic plains, Sedn

and the north. Black and white lines indicate locations of topographic profiles.

the mean planetary radius, 6051 km; sinusoidal projection. Major morphologi

feature of Beta Regio is the system of broad zones of graben and fractures (rift

majority of the surface of Beta Regio. Topographically, Beta Regio represen

canyons of the rift zones. Major morphologic (e) and topographic (f) features o

Venus and is characterized by a heavily tectonized surface (bright areas) and sm

represents a broad elevated plateau. Volcanic plains occur within the local lows

Elevation is above the mean planetary radius, 6051 km; sinusoidal projection.

Barsukov et al., 1986; Pronin et al., 1986), whereas the plainsare very restricted within large tessera regions (Solomon et al.,1992; Ivanov and Head, 1996; Banks and Hansen, 2000).Although Lakshmi is different from the crustal plateaus interms of the presence of high mountain ranges at its peripheryand the lava plains in its interior, the overall shape of thePlanum itself and the distribution of topography in itsinteriors resemble the topographic configuration of theplateau-like highlands (Figs. 2 and 3). Lakshmi Planum ismuch more dissimilar to the rises and differs from them dueto the presence of the peripheral mountain ranges, the generaltopographic shape, and the absence of rifts (Figs. 1a, c and 2).The surface of the rises, however, is covered by vast plains,which is similar to the situation in the interiors of LakshmiPlanum.The unique characteristics of Lakshmi Planum suggest

that it formed by an unusual combination of processes and

m. The Planum represents an elevated lava-covered plateau surrounded at

and Danu Montes. Heavily tectonized areas of Atropos and Itzpapalotl

a and Snegurochka Planitiae surround Lakshmi, respectively, to the south

Profile 1 is shown in Fig. 2, profile 2 is shown in Fig. 13. Elevation is above

c (c) and topographic (d) features of Beta Regio. The major morphologic

zones) that radiate away from the center of Beta. Volcanic plains cover the

ts a large dome-shaped rise, which is cut by relatively narrow and deep

f Ovda Regio. Ovda Regio is characterized by the largest tessera region on

all areas of volcanic plains (dark patches). Topographically, Ovda Regio

. In all images, black and white lines indicate traces of topographic profiles.


the large dimensions of this area indicate that it playedan important role in the geologic history of Venus. Thestructure of Lakshmi had been extensively studiedwith Venera-15/16 data (Solomon and Head, 1984, 1990;Sjogren et al., 1997; Pronin, 1986; Pronin et al., 1986;Head, 1990) and later with Magellan data (Kaula et al.,1992; Solomon et al., 1992; Basilevsky and Head, 1995).These studies resulted in two alternative classes of models,divergent and convergent, put forth to explain the unusualtopographic and morphologic characteristics of westernIshtar Terra (Table 1). The first, divergent, class ofmodels includes those that explain Lakshmi as a site ofmantle upwelling (Pronin, 1986, 1990, 1992; Grimm andPhillips, 1990). In these models, formation of LakshmiPlanum was initiated in its central area due to rising andsubsequent collapse of a mantle diapir (Pronin, 1986, 1990,1992). In a modified version of this model, a diapir wasconsidered to have impinged at the base of a thickenedblock of lithosphere (Grimm and Phillips, 1991). Thesemodels may explain emplacement of a lava plateau in theinteriors of Lakshmi Planum and, in some circumstances,formation of the mountain ranges. Models from the other,

Table 1

Models proposed to explain the structure of Lakshmi Planum (see references

Model Model features

Divergent, hotspot models Symmetric structure of Lakshmi as a who

Chaotically organized contractional struct

Central low instead of high-standing plate

Rift-like system or rift junctions (may be fl

Gaps between the mountain ranges

Compressional deformation becomes youn

Deformation progresses outward (‘‘strain-

Contemporaneous formation of the moun

Volcanism should proceed from the platea

Volcanism predates the mountain belts

Horst-and-graben structure of the mounta

topography

Convergent models

(1) Downwelling models Symmetric configuration of Lakshmi

Internal tectonics: earlier radial contractio

Internal tectonics: later concentric contrac

strike-slip faults

Tesserae (Atropos & Itzpapalotl) are colla

During mountain building: migrating of co

(2) Regional compression,

subduction, orogenesis

Contractional structures at the periphery o

Polyphase growth of the mountain belts

Migrating of contractional structures outw

Tessera and tessera-like outer welt is youn

(3) Convergence at ancient

tessera block

Contractional structures at the southern ed

Exposures of old tessera-like terrain in the

Outward migration of contractional struct

Volcanism in the interior of Lakshmi is at

(4) Deformation from below Domains of the Lakshmi structure are due

Lakshmi is underlain by a pond of mantle

convergent, class consider Lakshmi as a locus of mantledownwelling, convergence, underthrusting, and possiblesubduction (Head, 1986, 1990; Head et al., 1990; Robertsand Head, 1990a, b; Bindschadler et al., 1990; Lenardicet al., 1991; Hansen and Phillips, 1993, 1995; Keep andHansen, 1994; Ansan et al., 1996; Marinangeli andGilmore, 2000). The key features that are considered inthese models are the mountain ranges, high topography ofthe Lakshmi interiors, and the large volcanic centers in themiddle of the plateau. The divergent and convergentmodels proposed for explanation of the Lakshmi phenom-enon consider principally different mechanisms of itsformation and suggest different geodynamic regimes thatpossibly operated on Venus. Did Lakshmi form due to therise of an oversized mantle plume (divergent models)? Orcould elements of convergence and downwelling haveoperated in the geological past of the planet (convergentmodels)? Answers to these questions play an important rolein the understanding of types of large-scale tectonic stylesand mechanisms of heat loss on Venus.Almost all of the divergent and convergent models make

either explicit or implicit predictions about the type of

in text)

Observational support

Yes No

le X

ures in the center of Lakshmi X

au due to crustal thinning X

ooded) within Lakshmi X

X

ger inward (‘‘plume-traction’’ model) X

magnet’’ model) X

tain belts ? ?

u center outward X

X

in belts due to relaxation of X

X

nal features X

tional features, radial normal faults, X

psed mountain belts ? ?

ntractional structures outward X

f Lakshmi X

? ?

ard X

ger than the mountain ranges ? ?

ge of Lakshmi X

interior of Lakshmi X

ures in the mountain belts X

later stages of evolution X

to domains in the upper mantle/ Non-testable Non-testable

residuum Non-testable Non-testable

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Fig. 4. The generalized geologic map of Lakshmi Planum shows the areal distribution of major units and structures that make up the surface of the

western Ishtar Terra. Map scale is 1:5,000,000; Lambert conformal projection.


tectonic structures and volcanic landforms in the region ofLakshmi Planum and about the sequence of major eventsthat occurred during formation and evolution of Lakshmi(Table 1). These predictions can be tested with the resultsof detailed geological mapping of the western Ishtar region(Fig. 4) and, thus, there is a possibility to sort out themodels and estimate their applicability to explanation offormation and evolution of Lakshmi Planum. Recently, wecompiled a geological map of the Lakshmi Planum region(50–751N, 300–3601E) at scale1:5M as a part of the USGSprogram of geological mapping of Venus. The main goalsaccomplished during the mapping were: (1) define materialand structural units that compose Lakshmi Planum,(2) interpret these units in terms of volcanic and tectonicprocesses, and (3) establish the sequence of events during thegeologic history of Lakshmi. The mapping results putconstraints on the models proposed to explain the Lakshmiphenomenon and, thus, allow assessment of numerous modelsof formation and evolution of Lakshmi Planum.

In this paper we first outline the suite of proposedmodels and emphasize specific predictions and their logicalconsequences. Then we describe the regional geologyand material and structural units within western Ishtarand the sequence of major volcanic and tectonic events thatcan be documented in this region. Finally, we compare the

apparent sequences with the model predictions and sort themodels by their ability to explain the observations.

2. Proposed models of formation and evolution of

Lakshmi Planum

Two alternative classes of models were proposed toexplain the unique characteristics of Lakshmi Planum andeach model suggests a number of testable predictions aboutthe general shape, specific features, and major volcanic andtectonic episodes during formation and evolution ofLakshmi Planum (Table 1). The first class describesdivergent models that treat Lakshmi as a center of mantleupwelling (Pronin, 1986, 1990, 1992; Grimm and Phillips,1990, 1991). The upwelling models are consistent withsymmetric structure of Lakshmi due to spreading of anaxially symmetric diapir and may require contempora-neous formation of mountain belts. If the mountain rangeswere formed by a sublithospheric flow that moved materialof the lithosphere from the center of upwelling toward itsperiphery, it would cause formation of the central low dueto lithospheric thinning. The central uplift during theearlier stages of upwelling can lead to the formation of rift-like system or rift junctions within Lakshmi. The latercollapse of the central uplift could lead to formation of


younger and chaotically organized contractional structuresin the interior of Lakshmi. The upwelling models alsopredict an older age of volcanic plains in the interior ofLakshmi relative to the mountains and the generalprogression of volcanic events from the plateau centeroutward. In the models of interaction of a mantle plumeand a thickened block of lithosphere (Grimm and Phillips,1990), contractional deformation may progress eitheroutward (‘‘strain-magnet’’ model) or inward (‘‘plume-traction’’ model). These models would be also consistentwith a horst-and-graben structure of the mountain beltsdue to their growth from below and gravitational relaxa-tion from above. The divergent models also predictformation of gaps between mountain ranges due tooutward growth of Lakshmi as it evolves.

The general class of convergent models can be organizedinto four categories. The first category comprises modelsthat consider Lakshmi as a locus of downwelling (Kieferand Hager, 1989; Bindschadler and Parmentier, 1989;Bindschadler et al., 1990; Lenardic et al., 1991). The majortestable predictions of these models (Table 1) include:symmetric configuration of Lakshmi; earlier radial contrac-tional features in the central portion of Lakshmi; laterconcentric contractional features, radial normal faults, andstrike-slip faults; formation of tesserae outside Lakshmi bycollapse of mountain ranges; migration of contractionalstructures outward during the orogenic phase. The second

category of models explains formation and evolution ofLakshmi Planum by regional compression, possible subduc-tion, and orogenesis (Janle and Jannsen, 1984; Crumpleret al., 1986; Head, 1986, 1990). These models were put forthto explain contractional structures at the periphery ofLakshmi and they predict polyphase growth of themountain belts, migration of contractional structures out-ward, and a younger age of tessera massifs surroundingLakshmi relative to the mountain belts (Table 1). The mainfeature of the third category of models is the presence in thecenter of convergence of a stable lithospheric massif (Headand Burt, 1990; Roberts and Head, 1990a, b; Head et al.,1990). These models (Table 1) predict contractionaldeformation at the periphery of the massif, exposures oftessera-like terrain in the interior of Lakshmi, outwardmigration of contractional structures in the mountain belts,and volcanism in the interior of Lakshmi during later stagesof evolution. The fourth category consists of models propo-sing that the unique characteristics of Lakshmi Planum aredue to mantle downflow that causes formation andthickening of partial melt residuum just beneath Lakshmi(Hansen and Phillips, 1993, 1995). Differential flows withinthe residuum may lead to displacement of the lower crust,which, in turn, causes deformation of the upper crust. Thesemodels do not make specific testable predictions (Table 1).

3. Major units of the Lakshmi Planum area

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

such a pervasive part of the morphology of the terrain thatit becomes part of the definition of a unit. For example,several sets of densely spaced and intersecting ridges andgrooves deform the surface of tessera and these structuresalmost completely erased the initial morphologic charac-teristics of the tessera precursor material. Thus, the tesseramaterial unit is chiefly defined by its tectonic pattern(Barsukov et al., 1986; Basilevsky et al., 1986; Bindschadlerand Head, 1991; Sukhanov, 1992; Solomon et al., 1992;Ivanov and Head, 1996; Hansen and Willis, 1996). Themountain belt unit gives an example of a structural unitthat consists of densely packed ridges that are 5–15 kmwide and tens to a few hundreds of kilometers long.Theoretically, the ridges can deform different materialsand, thus, the mountain ranges may represent a compositematerial unit. The pattern of deformation and topographiccharacteristics of the mountains around Lakshmi, however,are so unique that they permit definition of a specificstructural assemblage of mountain belts (Pronin, 1986;Kaula et al., 1992), which is similar to some degree, forexample, to Ridge band material on Europa (Figueredoand Greeley, 2000). In many published Venus maps, thesame approach of unit definition has been successfullyapplied for mapping of several quadrangles that portraygeologically diverse provinces (Rosenberg and McGill,2001; Bridges and McGill, 2002; Campbell and Campbell,2002; Hansen and DeShon, 2002). In other cases, theapproach depends on scale and density of structures. Forexample, where the structures are more discrete, wemapped them separately and not as a specific unit. Inother cases, where structures are very dense, tend toobscure the underlying terrain, and are embayed byyounger material units (for example, tessera and denselylineated plains), we chose to map such occurrences ofpervasive tectonic structures as specific units. Here wesummarize the stratigraphic units and structures and theirrelations.The material and structural units that make up the

surface of Lakshmi Planum (Fig. 4) form three groups. Theoldest group consists of two material units and twostructural units (Figs. 4 and 5). Tessera (t, Fig. 5a) isdeformed by several sets of intersecting tectonic structures(Barsukov et al., 1986; Bindschadler and Head, 1991;Sukhanov, 1992). Tessera material is embayed by most ofthe other units (Fig. 5a) and, thus is among oldest units inthe region. Large and small fragments of tessera occur bothoutside Lakshmi Planum (e.g. Clotho and Moira Tesseraeat the southern slope of Lakshmi) and in the interiorof the plateau (Fig. 4). Based on the Venera-15/16 data(spatial resolution about 1–2 km/px), the areas of Atroposand Itzpapalotl Tesserae at the NE and N sides ofLakshmi Planum (Fig. 1a) were considered as tesseraterrain (Barsukov et al., 1986; Sukhanov, 1992; Pronin,1992). Images of this region taken by Magellan (spatialresolution 100–200m/px) show, however, that the surfaceof these tesserae displays morphology that is transitionalbetween highly deformed tessera and less tectonized plains

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Fig. 5. Examples of material and structural units in the Lakshmi Planum region that form the group of oldest units (Group 1, see Fig. 4). In the order

from older to younger, the units are: (a) tessera, t, (b) densely lineated plains, pdl, (c) mountain belts, mt, and (d) groove belts, gb.


units. The surface of this unit (densely lineated plains, pdl,Fig. 5b) is cut by swarms of narrow and densely spacedfractures. Small fragments of the unit are seen in manyplaces outside the mountainous surrounding of LakshmiPlanum. The main occurrences of this unit are within theAtropos and Itzpapalotl Tesserae that adjacent to Lakshmifrom the NW and N (Fig. 4). There, broad archesadditionally deform the surface of densely lineated plainsand the surface of the unit resembles the tesseratransitional terrain described in the 301N geotraverse(Ivanov and Head, 2001). Mountain belts (mt, Fig. 5c)that surround the volcanic plateau of Lakshmi Planum arethe most important occurrences of contractional structuresin the region. Densely packed ridges that are 5–15 km wideand tens to a few hundreds of kilometers long characterizeall four belts, Danu, Akna, Freyja, and Maxwell. The beltscan extend for many hundreds of kilometers (e.g. Danu)and typically are a few hundreds of kilometers wide andseveral kilometers higher than the surface of LakshmiPlanum. The ridges of Danu, Akna, and Freyja abruptly

stop near the Lakshmi-facing edges of the belts (Fig. 5c).On the other side of Akna and Freyja Montes, themountain ridges are seen in massifs of densely lineatedplains of Atropos and Itzpapalotl Tesserae over distancesof hundreds of kilometers. Groove belts (gb, Fig. 5d) thatconsist of extensional structures such as fractures andgraben manifest the most important episodes of exten-sional tectonics. The belts can be many hundreds ofkilometers long and a few hundreds of kilometers wide.Groove belts are concentrated to the south of LakshmiPlanum and almost absent in the interior of the plateau.The second group consists of three units (Figs. 4 and 6).

Shield plains (psh, Fig. 6a) are characterized by abundantclusters of small shield-shaped features (1–5 km), many ofthem with summit pits. Geological mapping in VellamoPlanitia (Aubele, 1994, 1995) and in many other areas(Head and Ivanov, 1996; Ivanov and Head, 2004) showedthat clusters of the shields represent a distinct unit with aspecific stratigraphic position, which predates emplacementof the vast regional plains. Shield plains are widely

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Fig. 6. Examples of material units in the Lakshmi Planum region that constitute the group of units that occur at the middle stratigraphic level (Group 2,

see Fig. 4). In the order from older to younger, the units are: (a) shield plains, psh, (b) the lower unit of regional plains, rp1, and (c) the upper unit of

regional plains, rp2.

M.A. Ivanov, J.W. Head III / Planetary and Space Science 56 (2008) 1949–19661956

distributed outside of Lakshmi Planum but there are nooccurrences of them in the interior of the volcanic plateau(Fig. 4). The lower unit of regional plains (rp1, Fig. 6b) is adominant unit inside and outside Lakshmi Planum (Fig. 4).This unit is composed of morphologically smooth, homo-geneous plains material of intermediate–dark to interme-diate–bright radar albedo complicated by narrow wrinkleridges. Material of the plains embays all previous units butvolcanic edifices and sources of the plains material are notobvious. The upper unit of regional plains (rp2, Fig. 6c) hasnoticeably higher radar albedo than material of the lowerunit and usually is characterized by lobate boundaries.Wrinkle ridges cut the surface of the plains. The unit occursboth outside and inside Lakshmi Planum and clearlyassociate with large volcanic centers, such as Colette Paterain the interior of the Planum (Fig. 4).

The third group is comprised of the two youngestmaterial units (Figs. 4 and 7). The featureless and tectoni-cally undeformed surface of smooth plains (ps, Fig. 7a) has

uniform and preferentially low albedo. Small patches ofthis unit scattered throughout the map area but preferen-tially are concentrated within the volcanic plateau ofLakshmi Planum (Fig. 4). Lobate plains (pl, Fig. 7b) occuras large complex of lava flows to the south of LakshmiPlanum and in its interior in association with Colette andSacajawea Paterae (Fig. 4). A distinctive pattern of radarbright and sometimes radar dark flow-like features withlobate fronts characterizes this unit. The surface of theplains is tectonically undeformed and flow-like featuresembay most tectonic structures including wrinkle ridges.

4. Sequence of major events during the evolution of

Lakshmi Planum

The sequence of major events (emplacement of volcanicmaterials and their tectonic deformation) in the westernIshtar region is summarized in the correlation chart(Fig. 8). Various plains units heavily embay fragments of

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Fig. 8. The correlation chart of the major units and structures in the Lakshmi Planum region. See text for details.

Fig. 7. Examples of material units in the Lakshmi Planum region that form the group of youngest units (Group 3, see Fig. 4). In the order from older to

younger, the units are: (a) smooth plains, ps and (b) lobate plains, pl.


tessera in all localities inside and outside Lakshmi Planum(Fig. 9). Pieces of tessera terrain inside and outsideLakshmi resemble each other through their complexdeformational pattern and are distinctly different fromother tectonized units (Fig. 9). The consistent relationshipsof embayment and the complex and unique deformationalpattern of the surface suggest that tessera represents theoldest material unit in western Ishtar. This interpretationis in accord with the apparent stratigraphic positionof tessera material elsewhere on Venus (Ivanov andBasilevsky, 1993; Ivanov and Head, 1996; Hansen et al.,1997; Phillips and Hansen, 1998). Inside Lakshmi,outcrops of tessera material are seen in the eastern, north-western, and central parts of the plateau (Fig. 4). Such a

pattern of areal distribution suggests that tessera is moreextensive under the cover of younger plains units. Volcanicplains that have been densely fractured (densely lineatedplains) appear to be younger than tessera material wherethese units are in contact (Fig. 9c). The role of extension inthe formation of the structural pattern of this unit isevident. The largest massifs of densely lineated plains occurin Atropos and Itzpapalotl Tesserae to the NW and N ofLakshmi Planum where the plains are additionallydeformed by broad ridges and to some degree resemblethe tessera pattern of deformation (Fig. 10). The ridges aregenerally conformal to the strike of Akna and FreyjaMontes and occur within large areas of densely lineatedplains that are adjacent to the mountain ranges (Fig. 11).

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Fig. 9. Examples of tessera terrain: (a) tessera inside Lakshmi (at about

65.71N, 340.01E); (b) tessera to the northeast of Lakshmi (northern edge

of Itzpapalotl Tessera at about 71.11N, 357.01E); (c) tessera to the

southwest of Lakshmi (Moira Tessera, at about 58.01N, 309.51E). Both

inside and outside Lakshmi pieces of tessera are characterized by a similar

deformational pattern of intersecting ridges and grooves and are embayed

by the less-deformed plains units.

Fig. 10. Densely lineated plains in the area of Atropos Tessera to the

northwest of Lakshmi Planum have the background characteristics (fine

scale and uniformly oriented lineaments) similar to those in other outcrops

of densely lineated plains (see Fig. 5b). In the Atropos Tessera area,

however, the broad ridges (dotted lines) similar in width and orientation to

the ridges in Akna Montes, additionally deform the fine-scale fabric of the

plains.


The subparallel ridges within Atropos and ItzpapalotlTesserae are clearly related to the major episode ofcontractional tectonics in western Ishtar Terra thatcorresponds to the orogenic phase of formation of themountain belts (Pronin, 1986, 1992; Grimm and Phillips,

1990, 1991; Head, 1986, 1990; Head et al., 1990; Robertsand Head, 1990a, b; Bindschadler et al., 1990; Hansen andPhillips, 1993, 1995; Marinangeli and Gilmore, 2000). Theridges of the belts are densely packed and separated byV-shaped structural valleys (Fig. 5c). Similar but sparserridges deform the surface of densely lineated plains withinthe Atropos and Itzpapalotl Tesserae (Fig. 10). Regionalplains embay the mountainous ridges both outside andinside Lakshmi Planum. The relationships of mountainbelts with densely lineated plains and regional plains implythat the orogenic phase of the mountain belts formationwas stratigraphically bounded from below and above andshifted toward the earlier stages of observable geologicalhistory of western Ishtar (Fig. 8). The major extensionalstructures (groove belts) mostly predate emplacement ofshield and regional plains and, thus, also predominantlyformed near the apparent beginning of the geologic historyof the region (Fig. 8).Shield plains were emplaced in many areas of the

Lakshmi Planum region but exclusively outside of theplateau. Material of the plains clearly embays denselylineated plains and lacks prominent contractional struc-tures (except for small wrinkle ridges). This suggests thatemplacement of shield plains postdated the main phase offormation of the mountain belts (Fig. 8). The absence ofshield plains in the interior of Lakshmi Planum implies thatthis unit either never formed there or was completelycovered by younger plains units. The first interpretation is

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Fig. 11. The pattern of areal distribution of the broad ridges that

characterize the mountain belts around Lakshmi Planum (see also Fig.

1a). The ridges deform broad areas outside Lakshmi and abruptly stop at

the outer edges of the volcanic plateau.


more likely because embayed outcrops of the older unitsare exposed within the Planum in many places butfragments of shield plains are not associated with any ofthem. Subsequent to formation of shield plains, a broadunit of plains (the lower unit of regional plains, rp1) wasemplaced (Fig. 8). Occurrences of unit rp1 are concentratedto the south of Lakshmi Planum within the lowlands ofSedna Planitia and in the interior of Lakshmi. The wideextent of this unit and its uniform morphology suggest ahigh-effusion-rate mode of emplacement from a fewsources. The thickness of unit rp1 both inside and outsideLakshmi appears to be relatively small because outliers ofolder units commonly occur within the broad fields ofregional plains. The massive outpouring of lava duringemplacement of the lower unit of regional plains (rp1)changed to more localized eruptions that produced theupper unit of regional plains (rp2, Fig. 8). This unit formsdistinct lava flows that are cut by wrinkle ridges and occursinside Lakshmi Planum around Colette Patera and outsidethe plateau as well. The main tectonic structures in shieldplains, and both units of regional plains, are low and sparsewrinkle ridges that are significantly smaller than the ridgesof the mountain ranges and much less dense than structuresof either densely lineated plains or groove belts. This means

that the scale and intensity of tectonic deformation inwestern Ishtar waned significantly by the time of emplace-ment of the vast plains units (psh, rp1 and rp2).The youngest units in the western Ishtar, smooth plains

and lobate plains (Fig. 8), are undeformed by tectonicstructures and, thus, were emplaced after cessation oftectonic activity in western Ishtar. Smooth and lobateplains form extensive lava aprons around Colette andSacajawea Paterae, representing the latest volcanic activityinside Lakshmi Planum.

5. Discussion: testing the models of Lakshmi

Planum formation

Several features of Lakshmi Planum put importantconstraints on the proposed models of formation andevolution of western Ishtar Terra. First is the topographicconfiguration of Lakshmi Planum, which is a high-standingplateau-like feature (Figs. 1a and 2). The overall shape ofLakshmi resembles configuration of some steep-sided,tessera-bearing crustal plateaus (Fig. 2) that are thoughtto be isostatically compensated at relatively shallow depthand represent ancient blocks of thickened crust/lithosphere(e.g. Grimm, 1994). Elevation of Lakshmi Planum coin-cides with the topographic range of the highest tessera(e.g. Ovda Regio) regions (Fig. 3). The latitudinaltopographic profile of Lakshmi Planum and its elevationrange are very different from those of the rifted dome-shaped rises (Figs. 2 and 3) that likely represent sites ofrecent (e.g. Smrekar et al., 1997) or perhaps continuing(Basilevsky and Head, 2007) mantle upwelling. Thus, if oneassumes that Lakshmi Planum represents a specific stage ofevolution of a mantle upwelling site, this should be a latestage of development of a hot spot when significanttopography had been gained. In this case, a dome-shapedtopographic profile and a system of rift zones are expectedto form (McGill et al., 1981; Rosendahl, 1987; Chorowicz,2005) and neither are observed in Lakshmi Planum.Alternatively, important characteristics of the upwelling(e.g. dimensions of the upwelling site, duration of theupwelling, its rate, thickness of the lithosphere, etc.) weredifferent. In this case, however, the possible outcome ofsuch an upwelling is not readily predictable and, hence, nottestable. Second is the presence in the interior of LakshmiPlanum of blocks of heavily tectonized and embayedterrain that morphologically similar to tessera massifsoutside Lakshmi (Fig. 9). The tessera-like massifs are seenin many places of Lakshmi Planum, especially in thecentral and eastern parts of the plateau (Fig. 4). Thepattern of areal distribution and the evidence of embay-ment of the massifs by the plains imply that tessera-liketerrain is more extensive under the cover of younger unitsand suggest that this terrain constitutes the older basementof Lakshmi Planum (Roberts and Head, 1990a, b). Third isthe pattern of areal distribution of shield plains (Fig. 4) andclusters of shields catalogued by Crumpler and Aubele(2000) inside and outside Lakshmi Planum (Fig. 12). The


plains and clusters are absent in the interior of Lakshmiand occur outside of it (Fig. 12a). This type of arealdistribution of small volcanoes is typical for the largetessera-bearing crustal plateaus (Fig. 12b) where thethickened crust/lithosphere possibly served as a rheologicalbarrier that prevented the distributed and shallow-sourcederuptions that likely characterized volcanic style of smallshields formation (Head et al., 1992). In contrast, withinthe dome-shaped rises (e.g. Beta or Atla Regios) clusters ofsmall shields occur commonly and these regions do notcorrespond to voids in the distribution of shield plains andshield fields (Fig. 12c and d). Fourth is the absence in theinterior of Lakshmi Planum of material units that formedoutside of the plateau after tessera and before emplacementof regional plains and tectonic events that occurred outsideLakshmi after tessera deformation and before emplace-ment of wrinkle ridges (Fig. 8). Such an incompletestratigraphic record inside Lakshmi Planum suggests thatthe plateau represented a geologically stable region wherethe geological activity was suppressed and that the majorvolcanic and tectonic events occurred outside of it duringthe beginning stages of the geological history of the region(Fig. 8).

The shape and elevation range of Lakshmi Planum, thepresence of fragments of heavily tectonized and embayedtesserae, the lack of shield plains and shield clusters in theinterior of Lakshmi, and an apparent large intermission inthe stratigraphic record within the plateau are collectivelyconsistent with and suggest that initially Lakshmi repre-sented a rigid, high-standing block of heavily tectonizedterrain similar to large tessera massifs elsewhere on Venus(Roberts and Head, 1990a, b; Grimm and Phillips, 1990).The presence of such a block (a ‘‘craton’’) stronglycontradicts the model of growth and subsequent collapseof a large dome-like structure at the initial stages ofevolution of Lakshmi Planum (Pronin, 1986, 1990, 1992)regardless of the unknown specific parameters of theproposed upwelling. Pronounced asymmetry characterizesthe longitudinal topographic profile of Lakshmi Planum(Fig. 13). The southern half of the plateau is tilted to thesouth and is bordered by the lower mountain range ofDanu Montes. The northern portion of Lakshmi iselevated, more horizontal, and is surrounded by thesignificantly higher ranges of Freyja Montes. This asym-metry of the large-scale topography of Lakshmi Planum isnot consistent with a model in which formation of themountain ranges is explained by the interaction of astationary and axial symmetric mantle plume with a blockof thickened lithosphere (Grimm and Phillips, 1990).

One of the important predictions of the divergent modelsis the probable presence of rift-like zones in the Lakshmiinterior, which are the characteristic structures of theupwelling sites, such as Beta Regio (Table 1, Fig. 1b).These zones are not seen within Lakshmi Planum and,thus, either never formed there, which argues against thedivergent models, or have been completely covered byvolcanic plains, which implies that the plains have

significant thickness and that the volcanism postdated therifting. There are two difficulties with the later hypothesis.First, the presence of small and large and heavily embayedmassifs of tessera-like terrain inside Lakshmi suggests thatthe layer of plains units is rather thin. Second, in the knownupwelling sites on Venus (Brian et al., 2005; Basilevsky andHead, 2007) and in zones of continental rifting on Earth(e.g. Chorowicz, 2005) phases of volcanism and tectonics(rifting) are often alternating instead of being strictlysequential. The other predictions of these models, such asthe older age of volcanism relative to the mountain belts,and progression of volcanism from the center of theplateau outward are not consistent with the observationsand the established sequences of events in the LakshmiPlanum region (Table 1, Figs. 4 and 8).The overall morphology of the mountain ranges

provides a possible additional argument against the modelof interaction of a plume with a thickened block of crust/lithosphere. If, as described in the model (Grimm andPhillips, 1990), the mountain ranges grew due to outwardsubsurface flow that thickened crust in the region below theranges, the morphologic manifestation of such a processwould likely resemble a horst-and-graben structure in thedirection of the mountain belts strike due to gravitationalrelaxation and spreading of the growing topography (e.g.Smrekar and Solomon, 1992). The same type of argumentapparently could be applied to the model of formation ofmountain ranges by deformation ‘‘from below’’ (Hansenand Phillips, 1993, 1995). The morphology of the mountainbelts that consists of ridges with morphologically smoothsurfaces separated by V-shaped structural valleys is similar,however, to the terrestrial fold-and-thrust belts (e.g.,Molnar and Tapponnier, 1975; Tapponnier et al., 1986,2001; Head, 1990; Suppe and Connors, 1992; Replumazand Tapponnier, 2003) that are formed in conditions ofregional compression. About the only prediction of thedivergent class of models that is supported by observations(Table 1) is the presence of gaps between neighboringmountain ranges. These gaps, however, are not a uniquefeature of the divergent models and could be explained bythe other models, for example, polyphase formation of themountain ranges (e.g., Crumpler et al., 1986).In the framework of the downwelling model, formation

of the mountain ranges is shifted toward the later stages ofthe process of continuous downwelling and is preceded bydevelopment of elevated topographic plateau over thelocus of the downwelling (Bindschadler and Parmentier,1989; Bindschadler et al., 1990). Specific parameters of thedownwelling (rheological properties of material, structureof lithosphere, temperature distribution, etc.) are highlyuncertain. Thus, the estimates of the duration of thedownwelling are poorly constrained. For example, themodel of downwelling (Kidder and Phillips, 1996) based onthe dry diabase rheology (Mackwell et al., 1996, 1998)suggests that growth of a plateau-like feature similar toLakshmi over a downwelling site may take billions ofyears. This result would imply an unreasonably long time

ARTICLE IN PRESS

Fig. 12. The areal distribution of clusters of small shields within the large physiographic provinces. (a and b) The interiors of the plateau-shaped

highlands, Lakshmi and Ovda, lack the shield clusters. (c and d) The clusters of shields typically occur throughout the area of the dome-shaped rises, Beta

and Atla Regiones.


ARTICLE IN PRESS

Fig. 13. A longitudinal topographic profile across Lakshmi Planum demonstrates the significant asymmetry of the structure (see Fig. 1). The southern

portion of Lakshmi is tilted to the south and bordered by the relatively low mountain ranges of Danu Montes. The northern half of Lakshmi is higher,

more horizontal, surrounded by the higher mountain range of Freyja Montes, and shows a steep regional scarp at the transition to Snegurochka Planitia.

A prominent topographic moat occurs between the high-standing Itzpapalotl Tessera and the low-lying Snegurochka Planitia. Elevation is above the mean

planetary radius, 6051km.


of formation for both the high-standing plateau ofLakshmi Planum and its mountainous surroundings.Although the downwelling model is able to explainformation of the unique mountain ranges at the laterstages of the evolution, two important model predictionsappear to be weakly consistent with the observablecharacteristics of Lakshmi Planum. First is the pronouncednorth–south asymmetry of the Planum (Fig. 13), whereasthe model suggests a more symmetrical structure(Bindschadler and Parmentier, 1989). Second is the modelrequirement of formation of radial contractional structures(ridges) in the interior of Lakshmi during the growth of theplateau (Kiefer and Hager, 1989; Bindschadler et al., 1990).According to the model, these ridges must predate themountain ranges (Bindschadler et al., 1990). Within theinterior of Lakshmi, however, these structures are notobserved. If they formed, they must have been completelyflooded by later plains units such as the lower unit ofregional plains, and this layer appears to be relatively thin.Elsewhere on Venus, contractional structures usually havehigh topography (several hundred meters) (Ivanov andHead, 2001; Young and Hansen, 2005). The thickness ofthe later lava plains would need to be significantly largerthan the height of the ridges to completely hide them.Otherwise, the surface of the cooling plains would drapeover the ridges and produce a specific pattern of structuresthat, for example, mark impact craters buried by lavaplains on the Moon and Mars (e.g., Colton et al., 1972;Raitala, 1988). These ‘‘ghost’’ structures are absent inLakshmi Planum.

The convergence models that treat formation ofLakshmi Planum due to regional compression, under-thrusting/subduction, and orogenesis due to collision oflow-lying plains with the edges of an ancient crustal(tessera) plateau (Crumpler et al., 1986; Head, 1986, 1990;Roberts and Head, 1990a, b; Head et al., 1990) appear tobe more consistent with the observable characteristics andsequence of major events in western Ishtar Terra. Thesemodels are partly based on the presence of embayed andheavily deformed terrain in the interior of Lakshmi Planum

(e.g., Roberts and Head, 1990a, b) and explain theconcentration of contractional structures at the peripheryof Lakshmi. They also can account for the prominentasymmetry of Lakshmi if the major axes of the convergencewere at the northern and northwestern edges of thePlanum.Fragments of tessera-like terrain in the interior of

Lakshmi Planum are heavily tectonized and embayed bythe lower unit of regional plains that also embays theinnermost ridges of the mountain belts. Neither the blocksof tesserae inside Lakshmi and near the belts are broadlyridged, nor do the ridges in the central parts of the beltsdisplay evidence for involvement of the tessera-like terrainin the deformation. The additional deformation of alreadytectonized terrain by the mountain-type ridges is evident,however, outside Lakshmi within Atropos and ItzpapalotlTesserae where the surface of densely lineated plains isdeformed by broad ridges over distances of hundreds ofkilometers (Fig. 11). The abrupt cessation of the broadridges at the edge of Lakshmi suggests that two types oftectonized terrains, tessera inside Lakshmi and mountainbelts outside of it, had an independent tectonic history.This is expected if initially the interiors of Lakshmirepresented a rigid craton-like massif the existence ofwhich is supported by a number of lines of evidence. Thesmooth surface of the ridges that form the ranges suggeststhat (1) the central tessera massif had not been significantlyinvolved in formation of the ranges and (2) it was mostlythe material of plains units that was deformed to producethe ridges of the mountain belts.The sequence of volcanic activity and progression of

tectonic deformation are among the key predictions of theconvergent models (Table 1). Volcanism is predicted tolargely postdate formation of the mountain belts (Table 1)because the collision of plains with the rigid and possiblystationary block of tessera and tectonic deformation of theplains must predate volcanic activity within the rigid block.This prediction appears to be in complete agreement withthe observation that the first recognizable plains unit in theinterior of Lakshmi (material of the lower unit of regional

ARTICLE IN PRESSM.A. Ivanov, J.W. Head III / Planetary and Space Science 56 (2008) 1949–1966 1963

plains, rp1) embays structures that correspond to theorogenic stage of formation of the mountain belts (Figs. 5cand 8). The convergent models further suggest thatvolcanic activity should migrate inward as the slabs ofsubducted/underthrusted plains move toward the center ofoverlying tessera block. This prediction is supported by theobservation that the younger plains units inside the plateau(the upper unit of regional plains, rp2, smooth plains, ps,and lobate plains, pl, Fig. 8) are concentrated around thecenter of Lakshmi and sourced there by Colette andSacajawea Paterae (Fig. 4).

Another specific feature of the regional compression andunderthrusting/subduction models is the prediction ofoutward migration of contractional deformation as newportions of plains have been accreted to the outer edge ofthe rigid tessera block. The areas of Atropos andItzpapalotl Tesserae present evidence for this progressionof deformation (Fig. 11). There, the broad ridges, whichare conforming to the general strike of Akna and FreyjaMontes, deform the older material of densely lineatedplains. The ridges extend over distances of hundreds ofkilometers and occur within the majority of Atropos and

Fig. 14. A sketch illustrating the major features of the model that considers the

with a rigid tessera-like craton and underthrusting of the plains. Numbers corr

Itzpapalotl Tesserae (Fig. 11). Inside Lakshmi, the ridgesof Akna and Freyja Montes abruptly stop near the base ofthe mountain range (Figs. 5c and 11). Such an asymmetryof the distribution of the mountain-type ridges on bothsides of the mountain ranges suggests that the ridgespreferentially formed outside Lakshmi and, thus, indicatesoutward progression of deformation.The major features of the convergence models with the

central rigid tessera massif are summarized in Fig. 14. Thepossible initial (pre-deformational) configuration of wes-tern Ishtar corresponds to stage 1 (Fig. 14) when a tesseracraton was surrounded by a layered suite of low-lying lavaplains. Compression from the north led to deformation ofthe plains against the foreland of the tessera massif andformation there of the higher mountain ranges (Fig. 14,stage 2). The craton may have been displaced by the forcesfrom the north to cause formation of the lower mountainrange of Danu Montes along the southern edge of thecraton (Fig. 14, stage 2). Continued underthrusting couldfinally cause a limited uplift of the northern mountainranges (Freyja Montes) and the northern portion ofLakshmi Planum (Fig. 14, stage 3), which created the

formation and evolution of Lakshmi Planum due to collision of lava plains

espond to sequential stages of the process. See text for detail; not to scale.


asymmetry of Lakshmi (Fig. 13). As underthrustingcontinued, there may have been two different sequencesof events, one with and one without delamination (Hessand Head, 1990).

In the beginning of the delamination process, fertilemantle could flow toward the base of the tessera massif,melt there due to decompression, and lead to widespreademplacement of the lower unit of regional plains in theinterior of Lakshmi (Fig. 14, stage 4a). On the more maturestages of delamination, the deepest portion of the slabcould start to melt and cause emplacement of the youngestlava plains, particularly at Colette and Sacajawea Patera(Fig. 14, stage 5a). If no delamination occurs (for example,due to a higher thermal gradient), then formation of thelower unit of regional plains could have formed due tobroad melting of the underthrust slab as it crosses themelting isotherm (Fig. 14, stage 4b). As the underthrustingproceeded, the relatively colder slab deflected the isothermdownward and new (the deepest) portions of the slabmelted, producing the younger lavas near the center ofLakshmi Planum (Fig. 14, stage 5b). When eitherdelamination or continued underthrusting waned, thethicker lithosphere of the northern mountain ranges couldstart to rise epirogenically, which could lead to theadditional elevation of the ranges and the northern portionof Lakshmi. The late phases of the epeirogenic rise alsocould have lead to a broad warping of the lower unit ofregional plains and its distinct tilt away from the mountainranges.

6. Conclusions

The detailed geological analysis of Lakshmi Planum inwestern Ishtar Terra allows both definition of materialunits and tectonic structures that make up the surface ofthis region (Fig. 4) and establishing of the sequence ofmajor events during formation and evolution of westernIshtar (Fig. 8). These results permit addressing one of themost complicated problems in the geology of Venus,formation of Lakshmi Planum, and testing the suite ofmodels proposed to explain the mechanisms of formationof this structure.

The interpreted nature of units and the sequence ofevents strongly contradict the predictions of divergentmodels. The major contradictions are as follows. (1) Thevery likely presence of an ancient (craton-like) tesseramassif in the core of Lakshmi. Such a core is inconsistentwith the model of formation of Lakshmi due to rise andcollapse of a mantle diapir (Pronin, 1986, 1990, 1992).(2) The absence of a rift zone (or system of such zones) inthe interior of Lakshmi. These zones appear to be a naturalconsequence of growth of the surface topography due todiapiric rise (e.g. Condie, 2001). (3) The apparent migra-tion of volcanic activity toward the center of Lakshmi. Thedivergent models are consistent with the opposite trend ofvolcanism (Table 1). (4) The abrupt cessation of ridges ofthe mountain ranges at the edge of Lakshmi Planum and

propagation of these ridges over hundreds of kilometersoutside Lakshmi within Atropos and Itzpapalotl Tesserae.The divergent models predict the opposite progression inthe development of major contractional features (Table 1).The convergent models of formation and evolution

of Lakshmi Planum appear to be more consistent withthe observations. The pure downwelling models (e.g.Bindschadler et al., 1990), however, faces three importantdifficulties. (1) The possibly unrealistically long time spanthat can be required to produce the major features ofLakshmi (Kidder and Phillips, 1996). (2) The stronglyasymmetrical north–south topographic profile of Lakshmiand striking difference in the height and thickness of themountain belts to the north-west and north (Akna andFreyja Montes) and to the south of Lakshmi (DanuMontes). The pure downwelling models would requireformation of more symmetrical structure. (3) The absenceof radial contractional structures (arches and ridges) inthe interior of Lakshmi. These structures represent thenecessary result of the downwelling models.Convergence models of formation of Lakshmi are most

consistent with the observations and explain this structureby collision and underthrusting/subduction of lower-lyingplains with the elevated and rigid block of tessera (Headet al., 1990; Roberts and Head, 1990a, b). These models arecapable of explaining formation of the major features ofwestern Ishtar (for example, the mountain belts) and thesequences of events and principal trends in evolution ofvolcanism and tectonics. To explain the pronouncedlongitudinal asymmetry of Lakshmi, however, thesemodels have to consider the major axes of the collision tobe at the north and north-west of the plateau in the areas ofAtropos and Itzpapalotl Tesserae. There is a little evidencefor the features indicative of the collision and under-thrusting/subduction at the southern edge of LakshmiPlanum and these processes may have not been operatedthere.The results of our analysis of the geology of western

Ishtar Terra suggest that the processes related to significantlateral movement of lithosphere (elements of plate tec-tonics) may have been active on Venus in the beginning ofits observable geologic history. These processes may causeformation of Lakshmi Planum. The unique characteristicsof Lakshmi strongly suggest, however, that some mechan-isms of plate tectonics, if they indeed acted on Venus, werereally restricted and did not form a global system similar tothat which currently that characterizes the Earth.

Acknowledgements

We gratefully acknowledge helpful discussion withAlexander Basilevsky, Slava Solomatov. Senthil Kumarand Debra Hurwitz, and our many colleagues in the VenusMapping program. This work was supported by grantsfrom the NASA Planetary Geology and GeophysicsProgram, and the NASA Applied Information SystemsResearch Program, to JWH. Thanks are extended to James

ARTICLE IN PRESSM.A. Ivanov, J.W. Head III / Planetary and Space Science 56 (2008) 1949–1966 1965

Dickson and Anne Cote for help in data presentation andmanuscript preparation and submission.

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