Evidence of Vertical and Horizontal Motions on Venus: Maxwell Montes

Home , Cleopatra (crater), Ishtar Terra, Lakshmi Planum, Maxwell (crater), Maxwell Montes, Tessera (Venus)

EVIDENCE OF VERTICAL AND HORIZONTAL MOTIONS ON VENUS: MAXWELL MONTES

V. ANSAN 1 and E VERGELY 2 l Centre National d'Etudes Spatiales, Division Instrumentation Radar, 18, avenue Edouard Be#n, 31055 Toulouse cedex, France; 2Laboratoire de Géologie Dynamique de la Terre et des planètes, Bät. 509, Université Paris-Sud, 91405 Orsay cedex, France

(Received 16 February 1995)

Abstract. Based on full-resolution Magellan radar images, the detailed structural analysis of central Ishtar Terra (Venus) provides new insight to the understanding of the Venusian tectonics. Ishtar Terra, centered on 65 ° N latitude and 0 ° E longitude includes a high plateau. Lakshmi Planum, surrounded by highlands, the most important being Maxwell Montes to the East. Structural analysis has been performed with classical remote-sensing methods. Folds and faults identified on radar images were reported on structural map. Their type and distfibution allowed to define the style of the crustal deformation and the context in which these structures formed. This analysis shows that Lakshmi Planum formed under a crustal stretching associated with a volcanic activity. This area then became a relatively steady platform, throughout the formation of Maxwell Montes mountain belt. Maxwell Montes is characterized by a series of NNW-SSE trending thrust faults dipping to the East, formed during a WSW-ESE horizontal shortening. In its NW quarter, the mountain belt shows a disturbed deformation controlled by pre-existing grabens and old vertical crustal fault zone. The deformation of this area is characterized by a shortening of cover above a fiat detachment zone, with a progressive accommodation to the southwest. All these tectonic structures show evidence of horizontal and vertical crustal movements on Venus, with subsidence, mountain belt raise, West regional overthrusting of this mountain belt, and regional shear zone.

1. Introduction

Venus, the second planet of solar system, has Earth-like parameters, like size, mass and mean bulk composition. Starting from similar conditions, the planet underwent an evolution drastically different from the Earth's. It therefore shows a great interest in geology. The study of its crustal deformation may provide new insight to the understanding of planet's evolution and dynamics. However, the observation of venusian surface has only been possible since the 1970s, because it was concealed under a thick and cloudy atmosphere. Space exploration and radar technology allowed to discover it. Radar images recorded from Earth-based radar, e.g., Goldstone and Arecibo (Plaut et al., 1990, Plaut and Arvidson, 1992, Senske et al., 1991, Campbell and Campbell, 1992) or by radar orbiters, e.g., Venera 15-16 and Magellan (Alexandrov et al., 1985, Basilevsky et al., 1986, Saunders et al., 1992) reveal that the venusian surface is characterized by a complex volcanic and tectonic history. In northern latitudes, a widespread area, Ishtar Terra, includes a high plateau Lakshmi Planum, standing at 4 km in elevation (Figure 1). This high-plateau is

Earth, Moon and P lanets 69: 285-310, 1995. @ 1995 Kluwer Academic Publishers. Printed in the Netherlands. 286 V. ANSAN AND R VERGELY

77.5*N

55°N

Fig. 1. In the globe, the grey box showsthe location on Venus of Magellan radar image C260N-333 centred at latitude 60 ° N and longitude 333 ° E and corresponding to Central Ishtar Terra. On the radar image, the radar beam is orientedto the East. The enclosedarea correspondsto the studiedarea, located by black box on the globe. surrounded by highlands, e.g. Freyja Montes to the North, Akna Montes to the Northwest, Danu Montes to the West, and Maxwell Montes to the East. The last one is the highest, standing more than 10 km. Owing to their high topographic relief and slope, and the intensity of their surface deformation, these highlands are interpreted as mountains belts resulting from horizontal shortening of the crust and lithosphere produced by large-scale compression (Campbell et al., 1983; Barsukov et al., 1986; Basilevsky, 1986; Crumpler et al., 1986; Pronin, 1986; Vorder Bruegge and Head, 1989, 1991; Bindschadler and Parmentier, 1990; Grimm and Phillips, 1990; Roberts and Head, 1990; Solomon et aI., 1991, 1992; Kaula et al., 1992, Ansan, 1993,Ansan et al., 1994). Based on Magellan full-resolution radar images, detailed tectonic interpretations of the central part of Ishtar Terra, especially Maxwell Montes (Figure 1, area in white box), are reported here. This study has led to define the tectonic deformation of Maxwell Montes and shows clearly that the venusian crust is locally deformed with both vertical and horizontal motions.

2. Data and Methods

Since 1990, Magellan synthetic aperture radar has scanned more than 98% venusian surface with a 120 to 300 m spatial resolution. During the first cycle of radar coverage, the radar beam was oriented to the East with an incidence angle that EVIDENCE OF VERTICAL AND HORIZONTAL MOTIONS ON VENUS: MAXWELL MONTES 287 decreased with latitude (Ford et al., 1989, 1993; Saunders et al., 1992). As the studied area is between 70 ° N and 60 ° N in latitude, it was constrained imaged with a 25 ° incidence angle. This links brightness variations on radar images to topography. Consequently, areas facing the radar beam appear bright, whereas areas facing away from the radar beam appear dark. This effect emphasizes the topographic relief. The studied area (Figure 1) is centered on 65 ° N latitude and 0 ° E longitude. It includes Eastern Lakshmi Planum, which appears dark on radar image, and the eastern highland of Ishtar Terra, Maxwell Montes, which comesponds to the eastern bright zone on image. This enclosed area is covered by two Magellan full- resolution radar images (F-MIDRPs 65N-354 and 65N-006). Each radar image covers an area of 500 × 500 km, with a sinusoidal projection (Figure 2 and Figure 4). The spatial resolution is 120 m. Despite the relatively low geometric distorsion of geologic features imaged by the spaceborne radar, detailed tectonic interpretations of each radar image are highly improved by the high resolution. Classical remote-sensing methods are used to establish a morphostructural map on which impact craters, volcanoes, folds and faults were recorded. Relative chronology between these different structural units can be inferred from superposition and/or cross-cutting relationships. Based on examination of the distribution and orientation of structural features, the style and type of crustal deforrnation am determined. The structural analysis is constrained by the Magellan altimetry data available in a regional topographic map (Ford and Pettengill, 1997). These data allow us to see the relationships between topography and morphologic features in the radar images. at large-scale. Therefore, all cross-sections shown in this paper are supported by Magellan altimetry data.

3. Structural Interpretations

3.1. LAKSHMI PLANUM

Lakshmi Planum corresponds to a dark, pear-shaped plateau 4 km high (Figure 1). Its dimensions are 7000 km E-W and 1500 km N-S. The eastem part of Lakshmi Planum presents a relatively flat floor with above several blocks standing 1 to 2 km (Figure 2a). Most are localized along the northern Lakshmi Planum boundary, marked by abrupt radar brighness change. These blocks are characterized by a mosaic of intersecting ridges and troughs (Figure 2b, box I and Figure 3). Their geometry ressembles that of a chocolate slab. Troughs are linear, limited by a flat floor and two parallel scarps hundreds of meters high. Scarps exhibit opposite facing front, with a relative vertical dip. The west-facing scarps are outlined by a relative bright line on the radar image. Because of their geometry, scarps seem to be fault-controlled. The formation of troughs would be due to normal faulting of 288 V. ANSAN AND R VERGELY these scarps. Consequently, troughs would correspond to grabens and rectangular ridges would be horsts. The distribution and orientation of these grabens are not randomly arranged (Figure 3). Three networks of grabens are observed: NW-SE, NE-SW and N- S trending. They keep their dominant orientation whatever observed block. As deduced from their geometry, their spacing and their cross-cutting relationships, the three networks of grabens were formed at different stages during the same crustal extensional history (Ansan el al., 1994). The network of NE-SW trending, narrow (< 2 km), and spaced at intervals of 5 km grabens seems to be formed first, because grabens are rectilinear. This network is crosscut by a set of NW- SE trending grabens that exhibit an irregular geometry controlled by the earlier fracturation. These grabens are discontinuous, more sinuous, wider (2 to 5 km) and spaced at intervals of 10 km. The last network of grabens formed, oriented N-S, is characterized by short and 5- to 10-km wide grabens that are spaced at intervals of 25 km. This structural analysis shows that these geologic structures formed during an intense extensional crustal deformation. The distribution of grabens seems comparable to that of terrestrial continental regions submitted to extension. These grabens exhibit a dark flat floor, as if they are filled by the same material that blankets Lakshmi Planum (Figure 2a). As this material covers a regional flat topography, its low radar backscatter would be due to surface textural characteristics. This material has then a fine-grained surface relatively to the 13.6 cm radar wavelength (Saunders et al., 1992). This material is interpreted to be volcanic, which is supported by characteristics of radar backscatter, reflectivity and emissivity, and as well as geologic correlation as the presence of a great volcano, Sacajawea Patera, located at 64 ° N-336 ° E. This volcanic material could also originate from other processes, because it covers a widespread area. As no other secondary vent is observed, this volcanic material could outcrop similarly to tholeiitic basalt ttows observed in several terrestrial continental areas such as Deccan traps in India. The lava flows are crosscut by a network of NNW-SSE linear troughs which extends over 100 km (Figure 2, lower part). These 5-km-wide troughs are bounded by two parallel lobate scarps, which suggests that they did not form by tectonic processes, but rather by cooling of lava flows or dike emplacement (McKenzie et al., 1992). However, the direction of troughs had to be controlled by the basement fracturation. In summary, Lakshmi Planum seems to have been the site of intense crustal extensional deformation from which faulted-blocks resulted. The extensional deformation of basement was followed by a volcanic activity and lava flows covered part of Lakshmi Planum. EVIDENCE OF VERTICAL AND HORIZONTAL MOTIONS ON VENUS: MAXWELL MONTES 289

A F-M~D~ 65N~54 C~~ 1 I o7~E

Fig. 2. (a) Magellan full-resolution radar image centred at latitude 65° N and longitude 354° displays Eastern Lakshmi Planum (F-MIDRP 65N-354, 1). The radar beam is oriented eastward. This image covers an area that is 450 km wide, E-W. The dark area corresponds to Lakshmi Planum, whereas the eastem bright area corresponds to Maxwell Montes. (b) The detailed structural map of Eastem Lakshmi Planum from F-MIDRP 65N-354. 1. Fault 2. Strike-slip fault 3. Normal fault 4. Thrust fault 5. Fold. Box 1 corresponds to horsts and grabens of Lakshmi Planum (cf. Figure 3). Box 2 corresponds to the foreland of Maxwell Montes (cf. Figure 5). Box 3 corresponds to the central part of the triangular zone of Maxwell Montes (cf. Figure 6). Box 4 corresponds to the northern slope of the triangular zone (cf. Figure 7). Box 5 corresponds to the western tip of the triangular zone (cf. Figure 8).

3.2. MAXWELL MONTES

Maxwell Montes, the highest venusian highland (10 km), lies along the eastem boundary of Lakshmi Planum and it is centered on 65 ° N of latitude and 5 ° E of longitude (Figure 1). This 1000-km-long x 700-km-wide highland appears relatively bright on the radar image. Its particular brightness may depend on three different surface variables: 1) surface roughness in relation to wavelength, 2) dielectric constant and 3) topographic variations. In northem latitudes where venusian surface was imaged with a 20-30 ° incidence angle, the roughness effects are less significant (Saunders et al., 1992; Ford et al., 1993). The bright radar backscatter of Maxwell Montes may be due to the high dielectric constant of metallic compounds 290 V. ANSAN AND P. VERGELY

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Fig. 2b. present at high elevations on Venus (Ford and Pettengill, 1983, and Kaula et al., 1992). However, at this resolution, local topographic variations may yield large changes in brightness, especially for low incidence angles. Owing to the illumination geometry, the slightest topographic relief leads to geometric distorsion on the radar image. Therefore, morphostructural interpretations of geologic features in mountains are to be made with precautions.

3.2.1. The central part of Maxwell Montes Maxwell Montes is characterized by a E-W trending asymmetric topographic relief. Along the AA ~topogaphic profile (Figure 4a,b), the highland is made up of ridges, tens to hundreds of kilometers long and a hundred of meters high. These ridges, typically 10 km apart, constitute a NNW-SSE parallel network. Because of the elevation-geometry relationships, ridges are interpreted as folds. Moreover, folds would correspond to original features due to the low rate of erosion (Avduevskiy et al., 1983; Krasnopolsky and Parshev, 1983; Schubert, 1983). The following is a description of these structural features along the WSW-ENE cross-section (noted AA' in Figure 4b).

3.2.2. The western slope of Maxwell Montes On the western side of Maxwell Montes, lava flows that covered flat Lakshmi Planum display a series of NNW-SSE, parallel ridges that have a high degree of EVIDENCE OF VERTICAL AND HORIZONTAL MOTIONS ON VENUS: MAXWELL MONTES 291

a T 100 7~

N T Fig. 3. Detail ofl Magellan full-resolution radar image F-M1DRP65N-354 corresponding to box 1 (Figure 2b) shows Lakshmi Planum tectonic structures. The radar beam is oriented eastward. This image covers an area thät is 275 km wide, E-W. This area consists of a three networks of rectilinear throughs bounded by scarps: NE-SW, NW-SE, and N-S. Troughs are interpreted as grabens and ridges as horsts. (a) Relative chronology of graben formation.

sinuosity and are parallel to their strike. They are 1- to 2-km wide, 50- to 100- km long, and spaced at intervals of 5 to 10 km. Because of their geometry and distribution, these ridges are interpreted as folds. As they deformed the lava flows that blanket Lakshmi Planum, they are younger than the flows. Ridges developed into three arcs of 50 km radius that crosscut each other at their tips. Each west- convex arc is located immediately west of the most deformed part of Maxwell Montes and in front of lobes of the long, sinuous boundary of Maxwell Montes. This geographic boundary, oriented NNW-SSE and extending on more than a thousand kilometers, is marked by a change in brightness. The latter is associated with the western 8 ° to 20 ° slope of Maxwell Montes facing the radar beam. On the western slope of Maxwell Montes, the deformation shows a regular change in shape and in geometry of ridges and valleys (Ansan et al.. 1994). This slope consists of linear, sinuous ridges oriented NNW-SSE. They are 10- to 100-km long and spaced at intervals of 10 km. Their geometry is characterized by an asymmetric profile with a narrow, western, bright limb and a wide, eastern, lowly tilted slope. Despite the distorsion effect on the radar image due to the illumination geometry, these ridges have a western sinuous to lobate outline. Consequently, these characteristics suggest that these ridges would correspond to overtumed folds and 292 V. ANSAN AND R VERGELY A 67,: 6%5~g

Fig. 4. (a) Magellan full-resolution radar image centred at latitude 650 N and longitude 60 E displays the highest relief of Venus, Maxwell Montes (F-MIDRP 65N-006,l). The radar beam is oriented eastward. This image covers an area that is 450 km wide, E-W. b) The detailed stmctural map of Maxwell Montes from F-MIDRP 65N-006. The circular feature centered at 660 N of latitude and 7 ° E of longitude corresponds to Cleopatra impact crater. 1. Fault 2. Strike-slip fault 3. Normal fault 4. Thrust fault 5. Ejecta. Schematic SW-NE trending cross-section of Maxwell Montes. It corresponds to a fold and thrust belt lately deformed by gravitational relaxation.

imbricated structures bounded by low eastward-dipping thrust faults (Figure 4b). These folded and imbricated structures are associated with NW-SE, narrow, long, linear troughs that are intrepreted as left-lateral strike-slip faults. Their formation and kinematics are consistant with the same period of formation of compressive structures. In summary, all the tectonic structures result from a WSW-ENE trending crustal shortening. The geometry of the western sinuous boundary of Maxwell Montes is characteristic of a thrust margin that overlaps Lakshmi Planum westward. The foreland associated with this thrust front correspond to the series of folds arranged in three arcs parallel to the mountain belt. EVIDENCE OF VERTICALAND HORIZONTALMOTIONS ON VENUS: MAXWELLMONTES 293

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3.2.3. The summit of Maxwell Montes Eastward, the 50-km wide western slope of Maxwell Montes tends to flatten. The summit of Maxwell Montes corresponds to a 200- to 300-km wide high plateau where folds and imbricated structures are covered by a bright material. This would be the vesult of a chemical transition at high elevation that would lead to a higher dielectric constant in the rock (Pettengill et al., 1988; Klose et al., 1992). The mountain belt summit also exhibits a 105-km-in-diameter circular feature, called Cleopatra, that consists of two concentric depressions. These depressions have a flat, dark floor, and the depth of the inner one is 2 km. The circular feature is surrounded by an irregular ring of bright, rough surface material that would correspond to ejecta. Ejecta and inner depression provide evidence that this feature is a double-ring impact crater (Basileysky and Ivanov, 1990; Ivanov et al., 1992; 294 V. ANSAN AND R VERGELY

Schaber et al., 1992). Because it kept its original circular shape, it is assumed the impact crater postdated the formation of Maxwell Montes. To the north and south of Cleopatra impact crater, some rectilinear, 30 km long, 10-km wide troughs developed (Figure 4a). They are bounded by two parallel straight scarps corresponding probably to grabens controlled by normal faulting (Figure 4b). These grabens are oriented NNW-SSE, parallel to old folded and imbricated structures. Because of their orientation and their location on the mountain belt summit, these grabens would result from gravitational relaxation (Froidevaux and Ricard, 1987; England and Houseman, 1989; Solomon et al., 1992; Ansän et al., 1993, 1994). Furthermore, small shield volcanoes, marked by 10 km diameter dark circular features are observed along and within the grabens, indicating that gravitational relaxation would be associated with volcanic activity (Head et al., 1992). Although no direct crosscutting relationships is observed between grabens and Cleopatra impact crater, their formation would be simultaneous or immediately after the impact crater formation. The sequence of events is inferred from lava flows observed inside the two features types. As we saw previously, the impact crater floor appears dark and relatively smooth, implying that the material covering the impact crater basement is a fine-grained surface material, with its dark radar characteristics, the material would correspond to volcanism resulting from impact melt and/or volcanism triggerred by thermal-chemical changes in rocks after the impact. Consequently, the lava flows observed both within the graben and the impact crater are likely to originate from the same place.

3.2.4. The eastern slope of Maxwell Montes Some lava flows pass trough a rim breach located in the NE quarter of Cleopatra impact crater and partially filled the valleys of the eastern side of Maxwell Montes (Figure 4a). The hinterland of Maxwell Montes is thus characterized by a 2 ° Eastem facing slope on which the style of deformation is similar to that of western Maxwell Montes, i.e., NNW-SSE parallel overturned folds and imbricated structures facing to the West (Figure 4b, cross-section). However, fold spacing is wider. The valleys display a dark flat ftoor, interpreted to be volcanic material that embays the valleys. Some lava flows emanate from Cleopatra impact crater. But, owing to their great geographic extent, this suggests that lava flows are also produced by another process as melt impact. In summary, the previous structural description shows that Maxwell Montes corresponds to a fold and thrust belt which resulted from a WSW-ENE trending crustal shortening inducing the formation of a 10 km high mountain belt. The west facing folds and imbrications, the long, sinuous, western convex, NNW-SSE thrust front and the foreland located in eastern Lakshmi Planum provide evidence of the western crustal overthrusting of Maxwell Montes on Lakshmi Planum (Suppe and Connors, 1992; Ansan et al, 1993, 1994). Our structural analysis confirms and furthers these studies by demonstrating that after the regional crustal shortening, Maxwell Montes shows evidence of a late gravitational relaxation associated with EVIDENCE OF VERTICAL AND HORIZONTAL MOTIONS ON VENUS: MAXWELL MONTES 295 volcanic activity. Therefore, the deformation of Maxwell Montes was also controlled by vertical crustal deformation.

3.3. THE NORTHWESTERN PART OF MAXWELL MONTES

The regular pattern of folds and imbricated structures within Maxwell Montes is disturbed in its northern area (Figure 2a, b, upper right corner). This area corresponds to a 100-km-side triangular zone bounded by the main thrust front of Maxwell Montes to the East, the end of the northern polar plain to the North and Lakshmi Planum to the Southwest. Its topography is characterized by a radial divergent slope from a summit reaching 6 km in elevation and located at 66.5 ° N of latitude and 0 ° E of longitude.

3.3.1. The southern side of the triangular zone In the southem side of the triangular zone located at 64 ° N of latitude and 0 ° E of longitude, a series of ridges oriented NW-SE to WNW-ESE developed obliquely to the main thrust of Maxwell Montes (Figure 2b, box 2). Their orientation is not consistent with shortening trend previously inferred from folded and imbricated structures within Maxwell Montes. Furthermore, their detailed geometry is difficult to determine because ridges are nearly parallel to the radar illumination. A close- up view allows to define their geometry and distribution more accurately (Figure 5a, b). In the dark area, WNW-ESE ridges appear sinuous and flat-topped and are interpreted as folds. They intersected and deformed the NNW-SSE trending folds parallel to the main thrust front of Maxwell Montes located in eastern part of image (a, Figure 5a and B2, Figure 5b). Therefore, WNW-ESE folds are younger than NNW-SSE ortes. In addition, to the north, ridges appear asymmetric with a northeastern lowly tilted limb and lobate southwestern outline, indicating that they would correspond to southwestem facing overturned folds. These folds deformed the older NNW-SSE folds with a visible leit lateral offset. Their visible offset can be explained by the southwestward propagation of overturned folds oriented WNW-ESE that postdated the formation of NNW-SSE folds. Consequently, the triangular zone seems to have overthrusted Lakshmi Planum southwestward. As NNW-SSE folds are deformed by WNW-ESE folds, the main thrust front of Maxwell Montes formed first (Ansan et al., 1993, 1994). Moreover, the WNW- ESE thrust zone displays a progressive change of deformation type to the West. Overtumed folds and imbricated structures vanished progressively and two groups of orthogonal, flat-topped, step-sided ridges developed. Ridges are arranged at an acute angle with the thrust trend. They have the same orientation as the horsts that formed Lakshmi Planum basement (Figure 2, box 1). Consequently, these ridges would be the result of reactivation in compression of old horsts.

3.3.2. The triangular zone The defonnation within the triangular zone (Figure 2a, upper right corner) is characterized by a series of parallel, asymmetric, NW-SE trending ridges bounded 296 V. ANSAN AND R VERGELY A

a b

Fig. 5. (a) Detail of Magellan full-resolution radar image F-MIDRP65N-006 corresponding to (Figure 2b) shows the foreland of Maxwell Montes. The radar beam is oriented eastward. The dark area corresponds to Lakshmi Planum, whereas the bright one corresponds to Maxwell Montes. Box a and box b display detail and relative chronology of tectonic structures (cf. Figure 5 b). EVIDENCE OF VERTICAL AND HORIZONTAL MOTIONS ON VENUS: MAXWELL MONTES 297

B a

N I -- "" ~7--3 "1 2 3 ,4 b

BI~j Iß2 1 Fig. 5. (b) Structural map of this area. a. Detailed structural map. b. Schematic structural map and relative chronology. To the North (B 1 ), NNW-SSE folds and thrusts were deformed by WNW-ESE folds and thrusts with a visible left-lateral offset, indicating that the latter are the younger. To the South (B2), tectonic structures are folds and thrusts that formed in the same sequence as previous]y. So, there is a NE deformation gradient with the transition of ductil to brittle deformation. 1. Fold 2. Thrust fault 3. Triangular zone 4. Maxwell Montes.

by a network of parallel, narrow, discontinuous, curvilinear troughs. Figures 6a and b display the two types of features in detail. Ridges generally exhibit a southwestem lobate outline, with a round-topped profile, characteristic of overturned folds and imbricated structures. They stopped perpendicularly to obliquely to discontinuous, curvilinear narrow troughs locally oriented ENE-WSW. In some place, folds show a visible right-lateral offset along the troughs. However, no local deformation such as tip flexion is observed. This suggests that the troughs would correspond to surface discontinuities. Throughout the regional crustal shortening of Maxwell Montes oriented WSW-ENE, the surface geologic layer would be deformed above 298 V. ANSAN AND R VERGELY A

Fig. 6. (a) Detail of Magellan full-resolution radar image F-MIDRP65N-006 corresponding to box 3 (Figure 2b) displays the central part of the triangular zone of Maxwell Montes. The radar beam is oriented eastward. (b) Structural map. This area consists in NW-SE thrusts that were controlled by right-lateral shear zone. 1. Fault 2. Strike-slip fault 3. Formal fault 4. Thrust fault 5. Shear zone.

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a relatively flat shear zone, that might correspond to surface between Lakshmi Planum basement and lava flows. However, the detachment sheet had to move at unequal rates over the flat shear zone accommodated by right-lateral slip along the ENE-WSW discontinuities. Therefore, it appears that southeast compartments move more fastly to the southwest than NW ones. EVIDENCE OF VERTICAL AND HORIZONTAL MOTIONS ON VENUS: MAXWELL MONTES 299

3.3.3. The northern side of the triangular zone The northem slope of the triangular area (Figure 2, box 4) consists in a pattem of orthogonally intersecting straight troughs that bound rectangular ridges. Figures 7 a and b show the central part of the northern slope. In the lower left comer, the slope is characterized by two orthogonal networks of troughs bounding rectangular, flat-topped, step-sided ridges oriented NE-SW, perpendicularly to the slope. The orientation of ridges and troughs is similar to that of Lakshmi Planum basement (Figure 2, box 1). As their arrangement and geometry are the same, these ridges and troughs would correspond to pattem of horsts and grabens and they might have fonned currently with the Lakshmi Planum basement. But, in opposite to their arrangement on the flat Lakshmi Planum, they are disturbed by movement along the slope. At the top of the slope, grabens are wider than the ones located at its bottom. Furthermore, horsts seem to be tilted to the Northwest. At the bottom of the slope, horsts show an asymmetric profile with a NW lobate outline, indicating they were overturned to the Northwest. This association of structures along a slope is characteristic of a deformation by gravity sliding (Figure 7b). Several authors (Smrekar and Phillips, 1988; Smrekar and Solomon, 1992) demonstrated that gravity spreading could occur on Venus, when the topographic slope reaches 30 °. In the western part of Figure 7, the morphology is complex because sliding occured along a concave topographic slope and affected an old pattern of horsts and grabens. This progressively vanish to the East and Northeast because it was deformed by oblique, sinuous ridges oriented N-S to NE-SW trending. Ridges show a round-topped profile, a W to NW sinuous outline characteristic of overtumed folds and imbricated structures oblique to the slope. Therefore, they do not correspond to gravity folds. Furthermore, their geometry was closely controlled by earlier fracturation, especially by NW-SE trending normal faults. In conclusion, the northwestern edge of Maxwell Montes resulted from the WNW-ESE trending crustal shortening of Maxwell Montes. The deformation of the triangular zone is characterized by SW-facing folds and imbricated structures. Their formation was controlled by old basement shear zone and by an old pattem of horsts and grabens present on the proto-Lakshmi Planum. The deformation was accommodated in relation with these factors and the topography.

3.3.4. The western end of the triangular zone The western end of the triangular zone is characterized by a low northward slope on which just two curvilinear troughs vanished to 353 ° E of longitude (Figure 2, box 5). Along these discontinuities locally oriented E-W, some NW-SE to W-E trending symmetric ridges interpreted as folds are observed. To the West, a network of rectilinear troughs developed with a radial and concentric distribution (Figure 8b, A). Troughs are bounded by two parallel scarps, indicating they would correspond to grabens. Grabens that are arranged radially are often narrow, discontinuous although their direction remains constant. These are crosscut by two wide, longer and half-circular troughs (Figure 8b, B). Their distribution is reported on 300 V. ANSAN AND E VERGELY A N T

Fig. 7. (a) Detail of Magellan full-resolution radar image F-MIDRP65N-006 corresponding to box 4 (Figure 2b) shows the northern slope of the triangular zone of Maxwell Montes. The radar beam is oriented eastward. (b) Structural map: To the SW, the defomaation is characterized by extensional structures, whereas it is characterized by compressive structures to the NE. The topographic slope is radially oriented to NW. 1. Fault 2. Strike-slip fault 3. Normal fault 4. Thrust fault 5. Fold 6. Volcano 7. Shear zone 8. extensional deformation zone 9. gradient of compressive deformation. Schematic cross-sections: AA') Thrusts resulting from W-E crustal shortening of Maxwell Montes. BB ~) Gravitational sliding above a detachment zone, tilting old grabens and forming gravity thmsts at the bottom. The black circle displays the geographic boundary between the thrust and fold area and the gravitational structures.

a rose-diagram (Figure 8b, C2). Although their distribution covers a large range of directions, the three trends of grabens present in Lakshmi Planum basement are predominant (Figure 8b, C 1). Based on cross-cutting relationships, the relative chronology of graben formation was established. As in Lakshmi Planum basement, NE-SW grabens formed first. They were crosscut by NW-SE grabens that are deformed by N-S ortes. All were crosscut by E-W curvilinear grabens. In the eastern area, folds deformed the extensional structures. These folds indicate that EVIDENCE OF VERTICAL AND HORIZONTAL MOTIONS ON VENUS: MAXWELL MONTES 301 B • 22krn 1 2 3 4 5 6 7 8 9

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22 km i

Fig. 7b. this area was deformed lately by compressive strength. However, grabens present in this area do not belong to Lakshmi Planum basement. They deformed lava flows that covered Lakshmi Planum. How can be explained the extensional deformation 302 ~ ANSAN AND R VERGELY A N T

Fig. 8. (a) Detail of Magellan full-resolution radar image F-MIDRP65N-006 corresponding to box 5 (Figure 2b) displays the western tip of the triangular part of Maxwell Montes. The radar beam is oriented eastward. (b) Structural Interpretations. A. Detailed structural map. 1. Normal fault 2, Fold 3. Fault. B. Relative chronology of structural units. C1 Rose-diagram of grabens locate Lakshmi Planum (box 1 in Figure 2b); C2. Rose diagram of grabens within the area. in the previous regional chronology? It is difficult to include another extensional deformation episode between the volcanic activity occured on Lakshmi Planum and the regional compressive deformation that created the mountain belt. Conse- quently, grabens that crosscut lava flows in this area might be indicative of local accommodation of regional shortening by vertical motion.

4. Discussion

The previous structural study of the central part of Ishtar Terra has established that this area is characterized by two distinct structural and temporal tectonic units: Lakshmi Planum and Maxwell Montes. Their crustal detormation has geometric characteristics similar to those observed on Earth. In this pärticular venusian highland, Ishtar Terra, it seems that venusian deformation results from horizontal and vertical crustal motions. What are the dynamical implications for the formation of Lakshmi Planum and Maxwell Montes? Which kinematic model can explain the deformation of the two areas?

4.1. DYNAMICAL IMPLICATIONS FOR LAKSHMI PLANUM

As we saw previously, Lakshmi Planum, the oldest structural area, was formed under a regional extensional stress regime that led to the formation of a network EVIDENCE OF VERTICAL AND HORIZONTAL MOTIONS ON VENUS: MAXWELL MONTES 303

T 38 km ~.',% . l, %.r ,4 az "%'J'-"~ '~'" | t ~

,~ g,5"NIJ?'Nq~~ - .-. ..,, ~., tN., ,, ~

~g,~.,4£_ .... -,

B ® ®

w~'E w E

S S

Fig. 8b.

of grabens and horsts (Figure 3). Normal faults corresponding to graben-bounding scarps are indicative of a crustal thinning that can be produced by two dynamical processes. First, the extensional deformation of Lakshmi Planum could result from the emplacement of a mantle diapir at shallow depth. This would imply a regional domal uplift, volcanism, and a radial-concentric graben formation as observed on venusian equatorial highlands (Blondel, 1992; Ansah, 1993; Ansan et al., submitted). However, the distribution of grabens on Lakshmi Planum is not expected one. It results rather from the second dynamics process: the extension of lithosphere (Figure 3). Moreover, the three systems of regularly distributed grabens, extending over more than 2,000 km 2 suggest that the stretching could have occured at a low strain rate, e.g., 10 -~6 s -~ (Kusznic and Park, 1986). At the low strain rate, the deformation can be transfered laterally to an undeformed area along a flat detachment horizon. This would coincide with the transition from the brittle to ductile regime in the venusian crust estimated to be at a depth of 15-20 km (Phillips, 1990 and Arkani-Hamed, 1993). The lithospheric extension has led to three implications: firstly, the lithosphere was thinning, secondly, it was associated with regional subsidence, and thirdly, the lithosphere became hotter because the asthenosphere was nearer the surface. This third consequence might have triggered the partial melting of the mantle and produced magma covering Lakshmi Planum (Figure 2 and Figure 3, dark flat zone). The style of diffuse volcanism observed on Lakshmi Planum seems to be consistent with the terrestrial volcanism present during the last stage of continental rifting 304 V. ANSAN AND R VERGELY prior to oceanic spreading, e.g., traps in Deccan, India. Since the extensional and volcanic stage, Lakshmi Planum has been a stable plateform. However, an unan- swered question remains. When did Lakshmi Planum reach its current elevation? Was it before, during or after the lithospheric extension? And in the last case, what is the dynamical process that supports the high elevation of this plateau?

4.2. DYNAMICAL IMPLICATIONS FOR MAXWELL MONTES

The previous structural analysis showed that Maxwell Montes has characteristics of a terresrrial fold and thrust belt. In the main part, the mountain belt displays a series of regularly arranged NNW-SSE folds and imbricated structures bounded by thrust faults dipping Eastward a 30 ° to 40 ° (Figure 4). This type of mountain belt typically involves a relatively low crustal thickening. The WSW-ENE crustal shortening estimated to reach 80% (Ansan, 1993) was associated with the westward movement of the massif induced by detachments along a weak layer and propagation of ramp (Figure 4b, cross-section). In western Maxwell Montes, area compared to a critical taper wedge (Suppe and Connors, 1992), the depth of detachment is calculated to be about 1.5 km with a 5 ° to 10 ° Eastward dip. Although the series of thrust accommodated at shallow depth (Zuber, 1987) and connected to a main detachment zone, where did the detachment zone go Eastward? Did the shallow detachment zone connect to the lithospheric ramp? And what happened to the manfle portion of the lithosphere above which the rocks of the thin sheet resided? Crustal shortening resulted in the formation of a 300-km-wide, high plateau on which normal faults developed (Figure 4). On terrestrial mountain belts, the formation and evolution of a high plateau are the late tectonic events that are often combined with gravitational relaxation (Froidevaux and Ricard, 1987; England and Houseman, 1989; Molnar et al., 1993). However, the high plateau formation involved two processes that are physically and probably temporally distinct. The horizontal shortening of the lithosphere might be sufficient to have doubled the crustal thickness, e.g. Himalaya, but the lithospheric thickness had to remain constant for the formation of a high plateau. Consequently, the high elevation could trigger the extension in the area. However, for extension to have occured, the bal- ance between the compressional forces applied by the underthrusting lithosphere and buoyancy within the plateau is associated with thermal evolution of the lithosphere. Indeed, the removal of cold lithosphere beneath the high plateau and its replacement by warmer asthenosphere (England and Houseman, 1989) leads to thermal change, implying a horizontal thermal gradient and a horizontal density gradient within lithosphere. Consequently, driving forces are maintained by convection. Furthermore, the thermal desequilibrium of lithosphere could trigger the lithospheric melting and volcanism. It seems that volcanism observed on Maxwell Montes is characterized by basaltic flows, owing to radar characteristics (Figure 4). In this case, basaltic fiows could originate from the lithosphere if they are enriched in incompatible element or from asthenosphere as for the Himalaya (McKenzie EVIDENCE OF VERTICAL AND HORIZONTAL MOTIONS ON VENUS: MAXWELL MONTES 305 and Bickle, 1988; Amaud et al., 1992; Turner et al., 1993; Molnar et al., 1993). The chemical composition of Maxwell Montes lava flows remains is however still unknown, and cannot decide this particular point.

4.3. DYNAMICAL IMPLICATIONS FOR THE FORMATION OF MAXWELL MONTES TRIANGULAR ZONE

The deformation of the triangular zone located to the northwestem edge of Maxwell Montes is the best example of deformation resulting from both vertical and horizontal motion of the crust. The compressive deformation characterized by fold and imbricated structures was closely controlled by the pre-existing pattern of grabens affecting the basement (Figure 5, 6, 7). As soon as the WSW-ENE crustal shortening occured, folds and imbricated structures overthrusted Lakshmi Planum to the SW. The compressive deformation intensity decreases radially to the SW periphery of the triangular zone. This type of deformation is characteristic of the shortening of cover above a detachment zone that would correspond to the interface of Lakshmi Planum lava flows and its basement. This interface is estimated to be 1- to 2-km at depth (Ansah, 1993). Furthemlore, in the northeastem part of the triangular zone, the folded and imbricated structures stopped obliquely to NE-SW discontinuous, visible right-lateral strike-slip faults. This suggests that the deformation rate of cover to the SW was unequal. It appears that southeast compartments moved laster than the northwestern ones. Westward, the compressive deformation vanishes with curvilinear discontinuities locally oriented E-W: folds are parallel to the faults with a WNW-ESE direction. The deformation is then characterized by relative vertical motions (Figure 8), as shown by the reactivation of old grabens belonging to Lakshmi Planum basement.

5. Conclusion

Based on the previous structural interpretation, a kinematics model is proposed (Figure 9). Central Ishtar Terra shows evidence of both vertical and horizontal crustal motions as indicated by regional extensional and compressive deformation Vertical crustal motions am predominant in Lakshmi Planum and the north polar plain located between Maxwell Montes and Freyja Montes, another mountain belt characterized by NNW-SSE folds and thrusts (Kaula et al., 1992; Ansah, 1993, Ansan et al., 1994). The relative vertical motion of these areas would correspond to their subsidence, marked by the formation of grabens. In addition, the mountain belt also displays evidence of local extensional deformation: Maxwell Montes is lately deformed by gravitational relaxation, marked by grabens and volcanism oriented parallel to mountain trend. But, the major vertical motion of Maxwell Montes is the consequence of crustal shortening that leads to the formation of a high topography reaching 11 km in elevation. 306 V. ANSAN AND R VERGELY

i ;~llOkm , 340"E at latituda GS°N

iO=N

3l]U-l:. O"E

A ~ l~r~_vi~ Mc~nt~_~ Maxwell Montes A'

Fig. 9. Kinematic model ofMaxwell Montes. Schematic structural map and cross-sections. 1. Fault 2. Strike-slip fault 3. Normal fault 4. Thrust fault 5. Transfer zone 6. Mountain belt 7. Lakshmi Planum 8. North polar plain 9. Direction and qualitative amount of horizontal motion in mountain belt 10. Relative left-lateral motion 11. Negative vertical motion 12. Positive vertical motion. In Maxwell Montes mountain belt, a high westward displacement is observed to the south, in opposition to the one observed to the north. The displacement difference was transferred to Freyja Montes by left-lateral movement of cover in the transfer-zone.

However, the main characteristics of Ishtar Terra's deformation is the evidence of deformation resulting from horizontal crustal motions. Ishtar Terra is the only venusian area that is characterized by such a compressive deformation associated with horizontal crustal motions. These are inferred from NNE-SSW folded and imbricated structures present in mountain belt, the main, long, sinuous, NNE-SSW thrust front of Maxwell Montes that overthrusts Lakshmi Planum Westward, and folded foreland deforming the lava ftows covering Lakshmi Planum (Figure 9, cross-section BBr). EVIDENCE OF VERTICAL AND HORIZONTAL MOTIONS ON VENUS: MAXWELL MONTES 307

On the regional structural map (Figure 9), the Maxwell Montes thrust front presents a curvature located at 66 o N oflatitude and 1 ° E oflongitude. To the North, the thrust front is more Eastward and the intensity of compressive deformation seems to be lower than that observed to the South of the mountain belt. Freyja Montes, a mountain belt located 650 km westward developed with a relative low deformation gradient (Figure 9, cross-section AAr). Between the two mountain belts, the plain is characterized by grabens and volcanism that developed before the mountain belt formation (Ansah et al., 1994). In the southem part of Maxwell Montes, the intensity of deformation is higher and the thrust front propagated 200 km westward in comparison to the north Maxwell Montes' thrust front (Figure 9, cross-section BBr). The geographic transition between the two areas corresponds to a wide zone parallel to 66.5 o N of latitude. It is characterized by a compressive deformation to the East (Triangular zone: Figure 2, box 2, 3, 4) and extensional deformation to the West (Figure 2, box 5). To the East, the deformation is characteristic of cover compressive deformation above a shallow, flat shear zone. The folded and imbricated structures are distributed obliquely to the Maxwell Montes ones, and accommodated locally along right-lateral tear faults (Figure 3 and Figure 9). The shortening of cover decreased progressively to the west and southwest. This wide zone links Maxwell Montes to Freyja Montes. Although the deformation of this zone appears complex, with gradual change of deformation style, this zone could correspond to transfer zone between Maxwell Montes and Freyja Montes. In terms of kinematics, Maxwell Montes and Freyja Montes move to WSW, with different, spatial and temporal rates, leading to overlap Lakshmi Planum. The transition zone would correspond to a pre-existing cmstal "vertical fault zone" along which the compressive deformation is transferred from Maxwell Montes to Freyja Montes with a left-lateral movment, during the compressive stage of Ishtar Terra. In conclusion, Central lshtar Terra shows evidence of crustal deformation resulting from both vertical and horizontal motions. However, the observation of a "convergence zone" on Venus similar to those observed on Earth is not associated with the observation of a spreading zone, e.g., rift zone. This seems to suggest that the consequences of Venus' dynamics at the surface would differ from that of the Earth (plate tectonics).

Acknowledgments

The Magellan radar images used in this study were extracted from the series of "Magellan" CD-ROM provided by NASA-JPL, California, U.S.A, and aväilable through the Photothèque Planétaire d'Orsay, Nasa Regional Planetary Image Cen- ter. We thank Ph. Blondel for their critical review of this paper. 308 V. ANSAN AND R VERGELY

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