Planet. Space Sci., Vol. 42, No. 3, pp. 239-261, 1994 Copyright 6-1 1994 Elsevier Science Ltd Pergamon Printed in Great Britain. All rights reserved 0032-0633/94 $7.00 + 0.00 0032-0633(94)E0017-K
Tectonic interpretations of Central Ishtar Terra (Venus) from and Magellan full-resolution radar images
V. Ansan, P. Vergely and P. Masson Laboratoire de GCologie Dynamiquc de la Terre et des Plan&es (URA CNRS Dl369), b& 509, Univcrsitk Paris-Sud. 91405 Orsay cedcx, France
Received 20 July 1993; revised 6 January 1994; accepted 7 January 1994
Introduction Abstract. For more than a decade, the mapping of Venus has revealed a surface that has had a complex Since the 1960s space exploration and new technologies volcanic and tectonic history, especially in the northern have allowed scientists to observe the surface of Venus latitudes. Detailed morphostructural analysis and tec- concealed under its thick cloudy atmosphere. Soviet lan- tonic interpretations of Central Ishtar Terra, based ders provided data about physical and chemical properties both on Venera 15/16 and Magellan full-resolution of rocks and analysed the Venusian atmosphere at the radar images, have provided additional insight to the landing sites. Radar images recorded either by Earth- formation and evolution of Venusian terrains. Ishtar based radars (Goldstone or Arecibo) (Plaut et al., 1990; Terra, centred at 0”E longitude and 62”N latitude, con- Plaut and Arvidson, 1992; Senske rt al., 1991 : Campbell sists of a broad high plateau, Lakshmi Planum, partly and Campbell, 1992) or by radar orbiters ( Vmeru 15/16 surrounded by two highlands, Freyja and Maxwell and A4agellan) (Alexandrov et ul., 1985 ; Basilevsky et al.. Montes, which have been interpreted as erogenic belts 1986; Saunders and Pettengill, 1991 ; Saunders et al., based on Venera 15 and 16 data. Lakshmi Planum, 1992) have been used for geologic mapping that has the oldest part of Ishtar Terra, is an extensive and revealed a Venusian surface that has had a complex vol- complexly fractured plateau that can be compared to canic and tectonic history (Basilevsky et al., 1986). a terrestrial craton. The plateau is partially covered by Ishtar Terra, a high-standing block in the northern lati- fluid lava flows similar to the Deccan traps in India, tudes, displays evidence of intensive tectonism and vol- which underwent a late stage of extensional fracturing. canism. The results of detailed morphologic analysis and After the extensional deformation of Lakshmi Planum, tectonic interpretations of the central part of this area are Freyja and Maxwell Montes were created by regional reported here. As large as Australia, Ishtar Terra is cen- E-W horizontal shortening that produced a series of tred around an extensive high plateau, Lakshmi Planum, N-S folds and thrusts. However, this regional arrange- standing 4 km higher than the mean planetary radius. This ment of folds and thrusts is disturbed locally, e.g. the plateau is surrounded by several mountain belts which compressive deformation of Freyja Montes was closely include Freyja Montes and Maxwell Montes. Maxwell controlled by parallel WNW-ESE-trending left-lateral Montes, 10 km above the mean planetary surface, is the shear zones and the northwestern part of Maxwell highest point on Venus, and based on Veneru I5 and 16 Montes seems to be extruded laterally to the southwest, data it is interpreted as an erogenic belt (Campbell rt al., which implies a second oblique thrust front overlapping 1983 ; Crumpler et al., 1986). The current study focused Lakshmi Planum. These mountain belts also show evi- dence of a late volcanic stage and a subsequent period of relaxation that created grabens parallel to the high- land trends, especially in Maxwell Montes C’orrespondence to: V. Ansan 240 V. Ansan o/ al. : Tectonic interpretations of Central Ishtar Terra first on Maxwell Montes using Vwcru 15 and 16 data. I. 1. Venera 15 md 16 rudur ima,y:qPunulysis. then. using MN,~o/I~IMfull-resolution radar data. the study was extended to Freyja Montes and Lakshmi Planum. During the Vcwru I5 and I6 missions, the spacecraft was This approach permitted the comparison of tectonic oriented such that the radar beam was directed westward, interpretations and their relation to the changes and with a low incidence angle and with a low spatial resolu- improvements of resolution and incidence angle. More tion of about I km/pixel. The Northern Hemisphere of importantly, this study has led to a dynamic model of the Venus as far as 30 N latitude was scanned, revealing, for geologic history of Central lshtar Terra. the first time, a highly deformed surface, especially in lshtar Terra. This area includes a high plateau, Lakshmi Planum surrounded by highlands, e.g. Maxwell Montes, Freyja Mantes, Akna Montes, Danu Montes and Fortuna 1. Morphologic observations Tcssera. Maxwell Montes. the highest Venusian mountain belt Morphologic observations include identification of geo- (more than IO km above the mean planetary radius). is logic structures and the analysis of their distribution. bordered on the West by Lakshmi Planum and on the Based on the observation of surface contrasts on radar East by Fortuna Tessera (Fig. I a). This mountain belt has images, the morphostructural analysis correlates the geo- an asymmetric profile, with a steep western slope, a flat metric shape of illuminated features with geologic struc- summit. and a gently sloping eastern flank. Vorder tures. Ridges. troughs. volcanoes and impact craters are Brucgge and Head (1989) interpreted Maxwell Montes as recorded on a morphostructural map. A relative chron- a highland that consists of long linear ridges parallel ology between these different units can be inferred from to the general N-S trend of the mountain belt either superposition or cross-cutting relationships. or (Fig. la). both. This current detailed morphostructural analysis reveals Usually, the surface morphology of a planet depends that Maxwell Montes consists of a complex of parallel on three main factors : lithology. climate and geologic features (Fig. lb). Two parallel morphologic units are processes such as impact cratering. volcanism and tecton- observed on the western slope (Fig. I b, unit IO and I I). ics. With regard to lithology. the nature and chemical These units are characterized by NNW-SSE-trending par- composition of Venusian rocks are poorly known except allel symmetric ridges hundreds of kilometers long. These in some areas near the equator ( Vqa 2 and Vcwru 13,‘14 two units differ in their ridge spacing: the ridges at the landing sites). Petrologic analysis performed by the Soviet highest elevation exhibit the largest spacing of about 20 probes revealed that the Venusian surface seems to be km. A 100 km diameter circular depression, Cleopatra covered with basaltic lava, at least at the L’cewrrr landing Patera, occurs on the summit of Maxwell Montes. This 2 sites (Surkov. 1983; Saunders rt c/l., 1991). These results km deep depression shows a half-ring of domes on its however should not be extrapolated to the entire plan&try eastern side. Due to their morphology, these domes could surface. be interpreted as volcanic features. Furthermore, this Today, Venus is characterized by ;I dry climate (Kras- crater is surrounded by a radar-bright, hummocky, irregu- nopolsky and Parshev, 1983 ; Avduevskiy rt d., 1983). lar ring with divergent rectilinear features interpreted here Consequently, the lack of water implies a surface mor- as lava flows. Thus, Cleopatra Patera has some charac- phology devoid of geologic features that could have teristics of a volcano. However. its origin remains ambigu- resulted from fluvial erosion. Furthermore, the high sur- ous because of the hummocky terrain that surrounds the face temperature may facilitate chemical reactions crater, which could be ejecta deposits. This would then between the atmosphere and lithosphere (Lyons, 1991 ; imply that Cleopatra Patera is an impact crater (Nikishin Zolotov and Volkov, 1992). In the upper atmosphere, and Crumpler, I988 ; Basilevsky and Ivanov, 1990). high-speed winds were observed, which may facilitate Unfortunately, the Vcwcru I5 and I6 image resolution colian erosion and transport of fine particles on high- does not allow discrimination between these two hypoth- lands (Schubert. 1983). eses. Volcanic domes (IO km in diameter) and short (5 km Lastly, geologic processes including impact cratering, in length) NNW-SSE-trending ridges occur southeast of volcanism and tectonics may be mainly responsible for Cleopatra Patera (Fig. I b, unit 8). The NNW-SSE-trend- the morphology of the present-day Venusian surface, and ing elongate belt between Maxwell Montes and Fortuna owing to very limited erosion. morphologic structures Tessera 7s characterized by 5 km long symmetric ridges once created are rarely modified. Mr!yr//rrn radar images separated by smooth floor valleys oriented N-S (Fig. 1b, revealed that the Venusian surface is poorly crateriLed unit 6). Fortuna Tessera, a widespread area extending with a uniform spatial distribution (Schaber ct u/., 1992). eastward. shows a typical tessera-like pattern of “plu- A part of ejecta should be transported by winds. mose” ridges (Fig. I b, unit 5). To the south, Maxwell In addition, the two major geologic processes that have Montes is limited by a steep scarp that exhibits a chaotic created the morphology of the present-day Venusian sur- network of short ridges and small domes (Fig. I b, unit 7). face are volcanism and tectonics. Volcanic features are The northern side extends in Semuni Dorsa (67 N broadly distributed, with a variety of morphologies and 0 E), where there is a N-S-trending ridge belt (Fig. lb, dimensions (Head r/ rrl., 1992). They arc associated with unit 4) and a wedge terrain formed by several diamond- dcformational features that result from tectonics. These shaped hills (Fig. lb, unit 9). include a wide variety of styles and spatial scales (Solomon The western boundary of Maxwell Montes seems to (11ol., 199 I, 1992). be coincident with Lakshmi Planum, a widespread high V. Ansan c’t ~1.: Tectonic interpretations of Central Ishtar Terra 241
N
60” N
0" E 1 1 15” E 200 km at latitude 65” N
Lambert conic conformal map
0” E 15” E &%i& at latitude 65“ N Lambert conic conformal map
InIl 6311
Fig. 1. (a) Mosaic of Vmmz 15 and 16 radar images of the region of Maxwell Montes. The Lambert conic conformal projection is used for this mosaic of radar images that covers an area of 1000 km in longitude x 2000 km in latitude. (b) The morphostructural map of Maxwell Montes from Venera 15/l 6 images. The Lambert conic conformal projection is used for this map which covers an area of 1000 km in longitude x 2000 km in latitude (I) Volcano. (2) Plain. (3) High plateau. (4) Ridge belt. (5) Tessera. (6) Intermediate terrain. (7) Chaotic zone. (8) Volcanoes and short ridges. (9) Magmatic terrain. (I 0) Short parallel ridges. (I I ) Long parallel ridges 242 V. Ansan et ccl. : Tectonic interpretations of Central Ishtur Terra
Fig. 2. (a) The box shows the location of Central Ishtar Terra on C2-hON-333. The image is 4500 km wide. (b) The regional morphostructural map of Central Ishtar Terra established from the observation of IO Mu~qrllrttzfull-resolution radar images [F-MIDRP 75N-332, 75N-35 I, 70N-339, 7ON-353, 70N- 007, 65N-342, 65N-354, 6SN-006, CON-355 and 60N-005, (cylc I)]. The Lambert conic conformal projection is used for this map which covers an area of 2000 km in longitude x 2400 km in latitude The white area was not studied. Impact crater: (1) impact ejecta deposit. Volcanic units : (2) volcano. (3) effusive flows. Intermediate terrain of 2-5 km elevation : (4) tessera, (5) fractured terrain, (6) ridge belt, (7) composite intermediate terrain. Highlands : (8) foreland, (9) Freyja Montes highland. Maxwell Montes: (IO) oblong blocks. (1 I) wide, parallel asymmetric ridges. (12) long parallel asymmetric ridges, (13) narrow ridges, (14) wide valleys, (I 5) plateau V. Ansan c’t 01. : Tectonic interpretations of Central lshtar Terra 243
Lambert Conic Conformal map 200 330” E at latitude 65” N
plateau along the southeast flank. on which long, linear, They include a widespread area extending from 57”N to NW-SE-trending grooves are particularly well developed 77’ N latitude and from l5’E to 320’ E longitude (Fig. 2a). (Fig. lb, unit 3). In contrast to the Vencru 15 and I6 mission, Muyllan In summary, Maxwell Montes is a mountain belt con- radar scanned the surface from the west (cycle I) wtth an sisting of parallel NNW-SSE-trending symmetric ridges. incidence angle that varied with latitude. Consequently, On the summit of Maxwell Montes. a large, circular in the northern high latitudes the incidence angle was 25 , depression, Cleopatra Patera, has tentatively been inter- whereas at latitude 60 N this angle increased to nearly preted both as caldera and as impact feature (Basilcvsky 30 Furthermore. the high spatial resolution (120-250 and Ivanov, 1990). m) significantly improved the morphostructural analysis. With regard to the cartographic projection, each full-res- olution image covers a 5 x 5 area in its own sinusoidal, 1.2. Magellan radar image anal_vsis equal-area projection. In order to provide a regional view of morphology, morphostructural units are plotted on a Based on Mugellun radar images, the study area was Lambert conic conformal prqjection map. extended West of Maxwell Montes into the Freyja Montes The following describes the characteristics of the major area. Ten Mugellun full-resolution radar images were ana- morphologic features observed on these images and classi- lysed (F-MIDRPs 75N332, 75N351, 70N339, 70N353, fies them into I5 morphostructural units (Fig. 2b). 70N007,65N342,65N354,65NOO6,6ON355 and 60N005). Impuct craters (Fig. 2b, Unit I). These are circular 244 V. Ansan et al. : Tectonic interpretations of Central Ishtar Terra depressions surrounded by a ring of ejecta. There is a Planum plateau and are similar to the extensive tholeiitic systematic change in morphology with increasing crater basalt flows existing in several terrestrial continental areas. diameter. Whereas the smallest (I 0 km in diameter) exhi- such as the Deccan traps in India (Fig. 2b, unit 3). bit no central peak. the largest display a central peak (4) The fourth group displays enigmatic features that or peak ring surrounded by complexly deformed strata. seem to have been formed by very low viscosity lava. Several impact craters are identified on Lakshmi Planum These features arc characterized by long sinuous channels (Phillips ct ul., 1991 ; Schaber et al., 1992). In the study extending more than 100 km and maintaining a constant area, they are mostly IO km or less in diameter and are width of I.5 km. This type of channel runs along narrow characterized by a central peak and a ring of ejecta. Except valleys in the northern part of Freyja Montes (76.5’ N for Maxwell Montes, impact craters are not observed on 335’ E) (see Fig. 6a) and led Head rt (I/. (1991) to suggest Freyja Montes. Mugellun images reveal that Cleopatra that low-viscosity lava erupting at high fluid rate was able Patera is. in reality, a 105 km diameter multi-ring impact to erode older terrain thermally and, because of great crater that consists of two concentric depressions. The turbulence, also physically. external depression exhibits a dark floor. Small dark Although volcanic units are extensive on the Venusian domes rising above the dark floor are observed. These surface, plutonic units also occur (Fig. 2b, unit 2). For domes are located around the inner depression. example, a 100 km diameter ovoid bulge, localized at Sometimes, dark lava flows originate from these domes. 72 N-342’ E, would be the superficial indication of in- Therefore, they are interpreted as volcanic domes. More- depth formation of a terrestrial-like pluton. This body dis- over, a lava flow passes through a breach in the eastern plays several sets of fractures, the origins of which are ap- crater rim and extends toward the north where it appears parently related to a separate, subsequent tectonic episode. to embay the eastern slope of Maxwell Montes. Owing to Tesserae (Fig. 2b, Unit 4). These units, characterized the presence of small volcanic domes and lava flows inside by at least two sets of intersecting ridges or grooves, or the crater, Cleopatra, although of impact origin, seems to both, generally stand I-2 km higher than the surrounding have undergone a late volcanic stage. plains. On Lakshmi Planum, tesserae are divided into Volcanic terrains (Fig. 2b, Unit 2 and 3). Unques- several blocks along the northern Lakshmi Planum tionably, volcanic terrains and associated features are the boundary. They exhibit three sets of linear fractures ori- most widespread of the morphostructural units and exhi- ented NW-SE, NE-SW and N-S. bit a large variety of morphologies, apparently related to The eastern slope of Maxwell Montes and all of Fortuna the viscosities of the different flows (Head ct ~1.. 1991 ; Tessera consist of tesserae with short, parallel. asymmetric Head and Wilson, 1992). Moreover, Mupyrllun radar data ridges. Although the asymmetry could be related to the reveal evidence of two principal types of igneous activity : observational effect of radar geometry, the asymmetric volcanic and plutonic. morphology of the ridges is inferred from their western Volcanic activity, the widespread igneous process, pro- sinuous limb. These ridges, slightly tilted to the east, trend duced several types of morphologies related to the lava NW-SE and have an oblique west-facing cliff several viscosities. Volcanic features are classified into four hundred meters high. The wide flat-floor valleys between groups based on the apparently relative decreasing vis- these ridges may correspond to old troughs filled by lava cosity of the lava from which they were formed. floods. (1) The first group consists of typical volcanoes with or In ltzpapalotl Tessera, which borders the northern part without radial lava flows (Fig. 2b. unit 2). The largest one. of Freyja Montes. the complex geometry of the ridges has Sacajawea Patera, is located at 64 ‘N-336’ E on Lakshmi resulted from intense and chaotic deformation. However, Planum. This volcano is an elliptical depression. 200 x 300 they seem to be asymmetric, and near the boundary of km wide and 2 km deep, surrounded by a network of tessera blocks, some of the ridges have a sigmoid pattern. concentric fractures and by radial lava Ilows extending Fractured terrains (Fig. 2b, Unit 5). These are located over hundreds of kilometers. This morphology suggests near the highlands. They have a flat floor that is cut by that the magma reservoir emptied and subsidence occured linear to curvilinear troughs. either simultaneously or immediately after eruption. Sev- Ri&e belts (Fig. 2b, Unit 6). These lie several hundred eral similar features are identified in the northern part of meters above the adjacent terrains and consist of curvi- Lakshmi Planum. The 200 km diameter circular structure linear and subparallel ridges. the margins of which are located at 68 ‘N-346’ E and the half-circular structure approximately parallel to the margins of the belt. Semuni located at 70.5’ N-340 ‘E were interpreted as coronae by Dorsa, an extension of Maxwell Montes to the north, is a Kaula and co-workers (1992). Because of their large num- typical ridge belt, but the geometry of ridges is unusual. ber of concentric and radial fractures, these structures are The ridges display a sinuous outline and an asymmetric interpreted as old subsident volcanoes. profile that suggest that they are overturned westward. The most widespread volcanoes of this class are small, An oblique scarp, hundreds of meters high faces west, e.g. I km in diameter, lack visible caldera and generally whereas the eastern side of the ridges slopes gently have a dome-like shape. NNW-SSE-trending chains of eastward. these small volcanoes are widespread near Semuni Dorsa Composite intermediute terrains (Fig. 2b, Unit 7). These (67 N-0’ E). have developed on flat surfaces 5 km above the mean (2) The second group includes lava Ilows without visible planetary radius and are located 66”N-0”E. They consist vents (Fig. 2b, unit 2). These flows occur mainly north of small ovoid-shaped blocks separated by narrow, short and south of Maxwell Montes. valleys that in places have evolved to rhomboid-shaped (3) The third group. volcanic floods, cover the Lakshmi basins. V. Ansan rt ~11.: Tectonic interpretations of Central Ishtar Terra 245
Forelund (Fig. 2b, Unit 8). Located in front of the partially embayed by lava flows. The second is the Freyja highlands, this unit is characterized by sets of ridges that and Maxwell highlands characterized by complex struc- are parallel to the main trend of the highland. On the ture and morphology, the major features of which are western side of Maxwell Montes, the foreland is 50 km ridges with western sinuous limb. Furthermore, Mugellan wide and covered by deformed lava flows. Two types of data reveal that the Venusian surface shows some unusual deformation are observed. In the south, N-S-trending geologic characteristics. For instance, a thin bright layer long curvilinear ridges developed into three arcs of 50 km partially covers the summit of highlands, and although radius that crosscut each other at their tips. Each arc is the origin of this material remains controversial, two located immediately west of the most deformed part of explanations have been proposed. First, this thin layer Maxwell Montes. The northern part of the foreland is could be composed of small particles carried by high- characterized by an E-W-trending zone consisting of two speed wind at high elevation where it could blanket the sets of short ridges oriented either NW-SE or WSW- summits of the highlands. However, a deflation zone is ENE. This pattern also occurs in the southern part of not observed, although dunes lie immediately southwest Freyja Montes. of some of the tessera ridges on Lakshmi Planum. A Highlunds. Located in the vicinity of Lakshmi Planum, second explanation is that this bright material could result they have high elevations. a complex morphology and from chemical reactions between rocks at high altitudes have been compared to terrestrial mountain belts and the hot Venusian atmosphere essentially composed of (Campbell et al., 1983 ; Crumpler et al., 1986 ; Head et CO?; the temperature is almost 370°C at the summit of ul., 1990; Mueller et al., 1991 ; Kaula et al., 1992). Maxwell Montes. This hypothesis was suggested by Strom (1) Freyja Montes, bordering Lakshmi Planum to the et ul. (1992) and Fegley et al. (1992). Moreover, Ford and North, is an arcuate mountain belt that can be divided Pettengill(l983) and Kaula et al. (1992) suggested that the into two morphologic areas (Fig. 2b, unit 9). Eastern source of the bright blanketing material is an emissivity- Freyja Montes consists of a series of long, parallel, N-S- related compositional effect combined with altitude, trending asymmetric ridges with a western sinuous outline. rather than a roughness effect. On each ridge, several WNW-ESE-trending linear Another unusual characteristic is the small number of troughs, 10 km wide, are discontinuous and show appar- impact craters on Ishtar Terra. Based on crater size fre- ent right-lateral offset. The western part of Freyja Montes quency distribution on 89% of the Venusian surface, Sch- is characterized by narrow valleys, oriented either NW- aber et al. (1992), Strom et al. (1992) and Phillips et al. SE or NE-SW. which bound diamond-shaped blocks. The (1992) estimated that the Venusian crust is 500 Myears southern part of this area is limited by short sigmoid ridges old. Impact craters on Ishtar Terra are mostly 10 km in that overlap Lakshmi Planum southward. The two parts diameter, with the exception of Cleopatra, 105 km in of the mountain belt are separated by a 100 km wide diameter. This crater experienced a late volcanic episode transition zone along longitude 335”E, that may cor- during which a lava flow breached the crater rim. Basi- respond to an interference zone between the two structural levsky and Ivanov (1990) pointed out that this breach belts. might have been a conduit for post-impact drainage of (2) The eastern boundary of Lakshmi Planum is limited shock melt from the crater floor. by a similar arcuate mountain belt, Maxwell Montes, which displays a regular change in shape and in geometry of ridges and valleys (Fig. 2b, units 10-15). The border of 1.3. Discussion the western part of Maxwell Montes is oriented NNW- SSE generally along longitude O’E. The western mor- The morphologic observations described in the previous phostructural unit 10 consists of small oblong, flat-topped sections show that the regional morphostructural features blocks blanketed by a radar-bright thin material (Solo- appear similarly on both Venera 15/16 and Mugellun mon et al., 1992; Kaula et al., 1992). Wide, parallel, images, despite the different radar illuminating angles. sinuous, asymmetric flat-topped ridges (Fig. 2b, unit 11) However, the high-resolution Magellun images provide are separated by long narrow valleys whose floors are additional morphological details that lead to a better covered by the same bright material. These long asym- understanding of the formation of Central Ishtar Terra. metric ridges oriented NNW-SSE become progressively For instance, Maxwell Montes was previously described narrower and more sinuous eastward (Fig. 2b, unit 12). as a highland characterized by long sinuous ridges parallel Westward, they may be covered by thin radar-bright to the trend of the mountain belt, but Magellan data have material, and short, sinuous, asymmetric ridges (Fig. 2b, resulted in improved morphostructural interpretation of unit 13) are superposed at an acute angle on the long the ridges. They are now seen to be asymmetric because NNE-SSE trending ones. Arcuate 10 km wide and 100 of their asymmetric morphology associated with a western km long valleys (Fig. 2b, unit 14) oriented N-S define the sinuous outline. high plateau of Maxwell Montes. The flat summit (Fig. Another issue clarified by Mugellan images is the impact 2b, unit 15) consists of several parallel short ridges covered origin of Cleopatra, which was revealed to have a ring of by a thin layer of fine material. To the north, several ejecta deposits surrounding it. parallel valleys 20 km wide are bounded by small vol- Based on Magellan data, a relative chronology between canoes and straight scraps. the different morphostructural units can be established In summary. the studied area involves two major mor- according to either their superposition or cross-cutting phostructural units (Fig. 2b). The first is Lakshmi Planum, relationships, or both. Lakshmi Planum seems to be the a widespread plateau on which several tessera troughs are oldest area based on the presence of tessera blocks char- V. Ansan of trl. : Tectonic interpretations of Central Ishtar Terra
zones or structural discontinuities are shown. Because of r’oungest their geometry, the long symmetric ridges parallel to the highland trend that were described earlier, are interpreted low volcanic activity within plains and highlands as terrestrial-like folded features. They are not presented 4 on this structural map. As previously noted, the folds that Cleopana Patera impact crater developed at high clcvation confirm that Maxwell Montcs is a mountain belt comparable in size and complexity to a terrestrial fold-mountain belt. The western boundary of Maxwell and Freyja Montes t Maxwell Montes is a long and sinuous boundary inter- preted to be a thrust front that overlaps Lakshmi Planum Lakshmi PlamJm volcanic flows westward. This thrust boundary is associated with a nar- Lakshmi Planum tessera row foreland composed of folds parallel to the mountain belt. Inside the highland, folds are deformed by two sets t Oldest of long narrow furrows oriented NW SE (Fig. 4a) and NNE SSW (Fig. 4b). Due to their geometry, these long
Fig. 3. Relative age of Mq~/lut~ morphostructur~~l units based furrows appear to mark the location of conjugate strike- on superposition or cross-cutting relationships, or both slip faults. The NW~ SE trending discontinuities cor- respond to left-lateral strike-slip faults, as indicated by the systematic curve of the folds, whereas the NNE SSW- acterized by a pattern of intersecting ridges and troughs. trending fault set seems to correspond to a right-lateral Since the tessera troughs were embayed by lava flows, the strikeeslip fault set. Both fold and fault kinematics are tessera blocks were formed prior to volcanic embayment. consistent with E-W oriented horizontal shortening. The lava flows that blanketed Lakshmi Planum indicate However, it is difficult to estimate the amount of dis- that this plateau has experienced intensive volcanic placement along the strike-slip faults and the rate of hori- activity. Based on the deformation of volcanic material zontal shortening. North of Maxwell Montes in Semuni near the highland boundaries, the development of Freyja DorSa, several parallel curvilinear furrows bound and Maxwell montes occurred after the volcanic episode. Because of their similar morphology, these mountain belts are believed to have formed during the same period of time, although no clear superposition or cross-cutting relationships have been observed. Because it still possesses its original circular shape, the impact event that produced Cleopatra occurred after the deformation that created Maxwell Montes. At a late stage, volcanic flows were emplaced both in the highlands and in the plains. A rela- tive chronologic summary of events is presented in Fig. 3. Currently, it is diflicult to date these different mor- phostructural units, despite the presence of a few impact craters on Ishtar Terra. Are these units contcmporeneous. and arc they relatively older or younger than the mean age of the Venusian surface that is estimated to be about 500 Myears (Schaber ef ~1.. 1992)‘~
2. Tectonic interpretations
Tectonic interpretations are based on the mor- phostructural analysis of Vmcw 15/l 6 and Mqc//ul~ data. The morphologic structures observed on Venus appar- ently have not been modified significantly by weathering \/ or by erosional processes, hence. radar images can be 6O"N OoE 15”E 60”N interpreted directly in terms of tectonic and volcanic struc- 200 at latitude 65” N tures. Although the rock composition of the studied arca is unknown. the main purpose of this study is to define the geometry of tectonic structures and their relative chronology in order to reconstruct the tectonic history of Tshtar Terra. Lambert conic conformal map Fig. 4. The structural map of Maxwell Mantes from b’~zr~tr 15: 16 data. The Lambcrt conic conformal projection is used for 2.1. Venera 15/l 6 fec’tonic irztt~r-~~retutions this map which covers an area of 1000 km in longitude x 2000 km in latitude. (I) Fault, (2) strike-slip fault, (3) normal fault. Figure 4 is a structural map of Maxwell Montcs prepared (4) thrust fault. (5) crater. (6) volcano. Fold axes arc not plotted from Vmrrrr I5/16 images. On this map, only the fracture on this structural map 241 diamond-shaped domes that are arranged et?cdwlor~. The an irregular geometry controlled by earlier fractures. long curvilinear furrows arc interpreted as right-lateral These youngest grabens are spnccd at intervals of 25 km. strike ~slip faults whose movement postdated the volcanic At the close of the extensive tectonic activity, a radar- activity. In the southern part of Maxwell Montes. an E dark material intcrprctcd to bc lava flows filled the grabcns W-trending, 50 km wide chaotic 7one is characteri/cd by and covered Lakshmi Ptanum. small ridges and by valleys orthogonal to the slope. The In southeast Lakshmi Planum (62.5 No 357 E) (Fig. Sa. inner part of this zone is cut by small C/I c~~/lc,lor~faults b), a set of NNW -SSE-trending lincar troughs developed oriented NE-~SW. but no long rectilinear discontinuity in the law taycr. These linear troughs extend more than exists. This ;lone may reprcscnt 21zone of distributed sheal IO0 km and arc bounded by two parallel lobate scarps. deformation, and the linear depressions orthogonal to The troughs mostly consist of discontinuous chains of the slope could bc interpreted as grabens in which IIXISS oval-to-cil-culat- depressions 2. 5 km wide: trough width wasting has occurcd. Moreover. this shear zone seems to decreases to the north. Based on this observation. it is extend westward and to affect ;I set of NW--SE-trending deduced that thcsc troughs postdate the formation of the troughs in Lakshmi Planum. Thcsc linear troughs, inter- tesscra and the volcanic activity. although their origin is preted to be grabens. show systematic Icft-lateral dis- not yet fully understood. The fact that these troughs arc placement. providing additional cvidencc of the /one bounded by festooned scurps implies that. rather than being one of distributed shcnr. having resulted from tectonic acti\ ity. they co~ild bc In summary, Maxwell Montes seems to bc a typical fold related tither to the cooling of the lava llow controlled by belt. The N S-trending parallel folds are cross-cut by a the old fractured bascmcnt (tcsscra units) or to magma network of conjugate strike slip faults, and fault kin- withdrawal. McKcnzic c’[ (I/. (1992) suggested that they ematics arc mechanically consistent with the E W-ori- could be the surface expression of dike emplacement cnted horizontal shortening related to the folding ot where dikes failed to reach the surface. Maxwell Montcs. The regional thrust front of Maxwell In summary. Lakshmi Planurn can be compared to a Mantes. ovcrlaping Lakshmi Planum Lvestward. is accon- craton-like platform that was submitted to intense crustat modated both by right-lateral strike -slip faults to the extensional stress. This stress gcncrated three sets of gra- North and a left-lateral shear zone to the South (Vordei bcns in the tcsscra units, follo\hing which volcanic activity Bruegge and Head. 1989). Lakshmi Planum is a stable led to the fooding of the valleys. platform on which ;I number of NW SE-trending grubens 3-.-.-. 3 3 Fwl,jrr A4mtc.v. This arcuate highland. rising 6 km developed at ;I later time. above the mean planetary radius and bounding northern Lakshmi Plan~~m. is made ~tp of ridges that occur in two distinct areas (Fig. &I. b). The eastern part of IJrqja 2.2. Magellan fcc’torlic i~1~clp~c~trrtiotl.s Montes is a series of long, sinuous. N S-trending asytn- metric ridges with 30 km spacing. Their sinuous western With Mqqc~/ltr/l full-resolution radar images. major struc- limbs and their asymmetric topographic profiles indic;tte tures such as folds. strike-slip faults. thrust faults and that these ridges correspond to overturned folds or wcst- normal faults can bc identified and mapped in greater facing imbricated structures. They are thus assumed to detail and with more certainty than with ~CJ/XY~/15:‘16 have been generated by E&W-oriented crustal shortening. images. The results of the tectonic interpretation of Lak- On the gently tilted backlimbs of these structures, ;I nct- shtni Plan~~m. Frey.ja Montes and Maxwell Montes are work of WNW ESE-trending narrow discontinuous reported hcrc. troughs 5 km wide occurs. Thcsc troughs, bounded by 2.2. I. Luk.s/mi Planun~. The surface of Lakshmi Planum two parallel straight scarps, arc inlcrpreted as grabens. presents two major morphostructural units of distinct They are arranged C/I CC/~C~/O~~with apparent right-lateral ages. The tcssera units. characterized by intersecting ridges of&t. Their visible otTset can be explained by the late and troughs. arc older than the smooth lava flows that formation and westward propagation of overturned folds blanket the high plateau. The northern boundary of Lak- oriented No S, which with the itnbricatcd structures prob- shtni Planum shows several areas of tessera whose geo- ably postdate the grabcn formation. These grabens also metric pattern resembles that of a chocolate slab (Fig. 5). occur on a volcanic dome (72 N--349 E) (Fig. 21. b) that The tesscra consists of three networks of parallel rec- is contemporaneous with the fracturing of the tcsscra 01 tilincar troughs that outline rectangular blocks. The the Lakshmi Planum basctnent. ge:)mctry of troughs seems to be controlled by normal The western part of Freyja Montes is the most intcn- faults. as indicated by the presence of two parallel scarps sivoty deformed part of this highland. This area, which hundreds of meters high. Hence, these troughs are intcr- extends perpendicularly to eastern Frcyja Mantes. is one preted as grabens that bound rectangular horsts. in which ridge geometry is closely related to topography. Based on their cross-cutting relationships and spacing. Thcsc ridges are disrupted by a network ofconjugate shear ;I relative chronology of these grabens was proposed ~oncs that are oriented WNW--ESE and ENE WSW. (Ansan (71rrl.. 1992). Along the set of WNW ESE-trending shear zones, the The set of NE-SW-trending parallel grabens seems to relative displacetnent is left-lateral. whereas it is right- have formed first. These narrow grabens bounded by two lateral along the ENE WSW-trending shear yoncs. Exten- parallel straight scarps are regularly spaced at intervals of sional basins. arranged (JIIc~c/~c~/o~I along these shear zones. 5 km and are cross-cut by a set of NW- SE-oriented. Z 5 are common. This arrangement fits Anderson’s model km wide, sinuous grabens spaced of intervals of IO km. A ( I95 I ) where extensional fractures bisect the acute angle set of N-S-trending grabens formed later, and they exhibit between two conjugate sliding plants. Hence. by dcf- 248 V. Ansun ct (11.: Tectonic interpretations of Central Ishtar Term nition, the kinematics of these structural features is mech- westward. The foreland associated with this thrust front anically consistent with compressive stresses oriented Em is 75 km wide and consists of long arcuate overturned W. Along longitude 335 ‘E. a zone occurs in which these folds parallel to the mountain belt. These deformed the two styles of deformation interfere. lava fows that cover Lakshmi Planum indicating that In the northern latitudes, the deformation of ltrpapalotl Maxwell Montes is younger than the lava that covered the Tessera is closely controlled by a network of narrow val- high plateau. leys oriented WNW-ESE, that bound parallel-banded ter- The regular pattern of thrust faults within Maxwell rains 2 km high. The geographic boundary of ltzpapalotl Montes is disturbed at the northwestern end of the moun- Tessera with Freyja Montes coincides with the southern tain belt. This fan-like area contains thrusts that are now valley hundreds of kilometers long. Their regional sig- oriented NW-SE. together with small Icns-shaped basins. moidal “Z” shape (Fig. 6a, b) and the deformation of On its western side, the area is limited by long, narrow. overturned folds on parallel-banded terrains along these parallel curvilinear furrows that seem to correspond to valleys indicate that these discontinuitics are left-lateral right-lateral strike slip faults. Along its castcrn tnargin. strike-slip faults. The banded terrains arc dominated by this area is bounded by the main No S trending thrust fault NW-SE-trending folds or imbricated structures, or both, of Maxwell Montes. In this fan-like area. all the thrusts or arranged CIZc~&lon. In places. NE SW trending extcn- overfolds seem to propagate southwestward to Lakshtni sionat basins occur. Hence, the distribution of the struc- Planum. The deformation of this block seems to result tures is in good agreement with Anderson’s model ( I95 I) from lateral extrusion to the southwest that was controlled and is consistent with E-W-oriented crustal shortening. both by the westward propagation of thrusts of Maxwell The pre-existing faults have been reactivated during the Montcs and by the right-lateral strike-slip faults observed last stage of the formation of Freyja Montcs. especially to to the North. The dcfortnation was accommodated within the east near Semuni Dorsa. These narrow valleys with flat Maxwell Montes by a second network of imbricated struc- floors enlarge eastward and are bounded by two parallel tures oriented N S, which at-e superposed on the NNW- scarps. These extensional structures are filled with lava SSE-trending thrusts. flows that erupted from small volcanoes along the fracture Southwest of this laterally extruded block, a foreland zones. developed in which occur two groups of (‘17c~I~clo/~ folds. Evidence for extension and volcanism within this moun- These folds, oriented NE SW and NW SE, tend to fortn tain belt is provided by small randomly distributed vo- an acute angle with the thrust front and are interprctcd canoes and lava flows filling all the depressions. As Freyja to result frotn the transpression of horsts that form the Montes was thrust westward, folding occurred along the Lakshmi Planum basement (tesscra unit). The defor- border of Lakshmi Planum. The foreland of eastern mation of the foreland provides evidcncc of the relative Freyja Montes consists of typical N-S-trending folds par- chronology between the two thrust zones (64 N 0 E) allel to the tnountain bctt. The style of deformation of the (Fig. 5a, b), i.e. the NW-SE-trending folds were super- western foretand is characterized by a system of trap- posed onto the N S-trending folds (Ansan ct (I/.. 1992). ezoidal ridges arranged et7 ddon along the belt bound- The hinterland of Maxwell Montcs and the adjacent ary. The fold geometry suggests that these ridges initially area to the East, Fortuna Tessera (Fig. I b) are highly corresponded to old horsts of the Lakshmi Planum base- deformed regions of loiver elevation that extend over sev- ment but were later modified under compressive stress. eral hundred thousand square kilomctcrs. Their style 01 These observations suggest that the creation of Freyja deformation is similar to that of Western Maxwell Montes postdated the crustal extension of Lakshmi Montes. i.e. parallel oicrturned folds or imbricated struc- Planum. tures, but their 20 km spacing is greater. The valleys arc In summary, Freyja Montes is a fold and thrust belt filled with dark smooth material that is interpreted to be that resulted from E-W-oriented crustal shortening. In volcanic flows. Itzpapalotl Tessera. the crustal shortening was controlled The southern side of the fold and thrust belt exhibits by old WNW -ESE-trending tectonic structures that were a chaotic zone where imbricated structures vanish and reactivated into left-lateral shear zones. Following this grabens develop. Thcsc grabcns are not parallel to the crustal shortening. the mountain belt apparently experi- slope but arc oriented WNW ESE. This implies that they enced late extension and volcanism. do not result simply from gravity sliding but rather from 2.2.3. MU.YUYJ/~Montr~s. Maxwell Montes, the highest a tectonic process. The associated normal faults are con- Vcnusian highland (IO km), lies along the eastern bound- trolled by older faults. ary of Lakshmi Planum (Fig. 7a. b) and is tnade up of The summit of Maxwell Montes exhibits the Cleopatra ridges, tens to hundreds of kilometers long. These arcuatc impact crater. Because it retains its original circular shape. ridges, typically IO km apart. constitute a parallel network it is assumed that the crater postdated the formation of oriented NNW SSE. Because of their asymmetric topo- Maxwell Montes. graphic profile and their sinuous pattern. these ridges are After this regional E W oriented crustat shortening. interpreted as overturned folds and imbricated structures Maxwell Montes underwent extension and volcanism. bounded by eastward-dipping thrust faults. The tnost Some grabens, paratlcl to the mountain belt. indicate that intense deformation was located on the western slope of the formation of Maxwell Montes included a period of a this thrust belt, the sinuous western boundary of which is relaxation. These grabens. IO km wide, arc bounded by oriented NNW-SSE and extends more than a thousand small volcanoes that indicate that the relaxation process kilometers along Semuni Dorsa. This geometry is charac- was associated with volcanic activity, especially on the teristic of a thrust margin that overlaps Lakshmi Planum eastern side of Maxwell Montcs. Valleys between the V. Ansan et rrl. : Tectonic interpretations of Central Ishtar Terra 249
a
346.5 E 350 355 0 1.7 E
67.5 N - 67.5 N
67
65
64
63 . 63
62.4 N _ 62.4 N
347.8 E 350 355 0 0.38 E , 1 b F-MIDRP 65N-354,l 100 km
-ti~l-r-r+--+ 0
1 2 3 4 5 6 Fig. 5. 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. (b) The detailed structural map of Eastern Lakshmi Planurn from F-MIDRP 6SN-354. (I) Fault, (2) thrust fault. (3) strike-slip bull. (4) normal fault, (5) fold. (6) volcano 250 V. Ansan et al. : Tectonic interpretations of Central Ishtar Terra
a
t A 100 km
329.0 B 330 335 340 345.2 E
77.5N 77.5N
76 76
75
74 74
7
?: 8. 13 - c 73
72SN ,: .I* 72.5N I . I
329. 5 E 330 - 335 340 341.4 E b - 100 km F-MIDRP 75N-332,l
-u/“-S -t-t-l--a
1 2 3 4 .5 6
Fig. 6. (a) Central part of Freyja Montes (F-MIDRP 75N-332,l). The radar beam is oriented eastward. This image covers an area that is 450 km wide, E-W. (b) The detailed structural map of Central Freyja Montes from F-MIDRP 75N-332. (1) Fault, (2) thrust fault, (3) strikeeshp fault, (4) normal fault, (5) fold, (6) volcano 251 V. Ansan rt al. : Tectonic interpretations of Central Ishtar Terra
a
5 100 km 358.5 E 0 10 13.7 E
67.5~ i7.5N
67 61
66
65 65
63 63
62.5N J[ \\\ J 52SN I I 359.8 E 11 5 10 12.8 E 100 km F-MIDRP 65N-006,l b -S -G+n-fT+-+ *
1 2 3 4 5 6
Fig. 7. (a) Maxwell Montes (F-MIDRP 65N-006.1). The radar beam is oriented eastward. This image covers an area that is 350 km wide, E-W. (b) The detailed structural map of Maxwell Montes from F-MIDRP 65N-006.1. (I) Fault, (2) thrust fault. (3) strike-slip fault, (4) normal fault, (5) fold, (6) volcano V. Ansan C/ trl. : Tectonic interpretations of Central Ishtar Terra 253 Lambert Conic Conformal map 200 at latitude 65” N
75” N
LAKSHMII PLA
d Thrust fauIt Impact crater + Strike slip fault - Fault 0c -r-r-r- Normal fault - Fold @ Tessera Fig. 8. Regional structural map on which the main tectonic structures are plotted according to a Lambcrt conjc-~~l~f~rrn~~l projection from If) ,~,l~~~~~ilfif?full-rcs~luti~~l radar images. This structural map covers an arca of 2000 km in longitudcx7400 km in latitude. (I) Fault. (2) thrust fault. (3) strike -slip fault, (4) normal fault. (5) fold. (6) impact crater, (7) volcano, (8) tcssera
imbricated structures are filled with lava flows, some of which seem to have originated in the crater Cleopatra. The principal tectonic features of Central Ishtar Terra are shown on a Lambert conic-conformal map (Fig. 8). On the basis of the tectonic study, including the relative Lakshmi Planum, an extensive faulted platform, resulted chronology of structural units, a model is proposed for from crustai stretching and was covered by lava flows. the regional tectonic history of Central lshtar Terra (Fig. The mountain belts surrounding Lakshmi Planum consist 10). Central Ishtar Terra was formed under two different of a series of N-S-trending to NNW-SSE trending folds tectonic conditions : (I) Lakshmi Planum formed under and thrusts formed by E--W-oriented crustal shortening. an extensional stress regime (phases I and II); and (2) Figure 9 summarizes tectonic features of each area and Freyja Montes and Maxwell Montes formed under a later their relative age. compressional stress regime (phases III, IV and V). V. Ansan 1’1(I/. : Tectonic intcrprctations ol‘<‘cntral Ishtar Tcrra
;l series of West-fitting parallel IhrusIs. This fold and thrusI belt is bounded on its western side by ;I long sinuous thrust border which extends more than ;L thous:ind kilo- meters along Semuni Dorsa. As the horizontal shortening continued, ;I second thrust appeared in l’ront of the first one :ind conncctcd \vilh the thrust front of Maxwell Monlcs. The long NE-SW trending curvilinear faults that limit the northwestcrn end of Mauwcll Montcs prcvontcd Lateral block the westward propag;ltion ofthcsc thrusIs. C’onsqucn~ly, extrusion the block limited by the main thrust of M;~x\vcll Mantes southwestward on its castcrn side vnd right-lateral strike-slip f:lults on leading to NW-SE oblique thrust its wes1crii side, w;ls latelally cxtrudcd lo the southwest. Soulhwes(w:trd of this extruded block. ;I forelnnd
Left-lateral dc\elopcd. The superpositional rclutionships bctu.een strike-slip of these Iwo thrusts indicate that the NW SE Ircnding thrust Itzpapalotl I’ormed lirst. The inner part of Lakshmi PI;inum remained Tessera faults ;I stable platform without compressive dcform:~tion. P/rr/.sc~ IV. The imp;tcl event th:lt croated C’leopatra N-S to NNW-SSE folds and thrusts crater effectively m:lrkcd the end of the Icctonism that created Maxwell Mantes. because the original circular Volcanic lava 110~ sh;~pc of Cleopatra has not been altered. Tessera : ‘ILlton P/Jc/.Q, V. Volcnnic ncti\ity associated with tectonic ‘J-S grabens N-S grabens rcl:tx:ition occured \h ithin the highland. especially in W-SE grabens I NW-SE grabens M:~xwell Montcs. The summil ofthis highland has narrow\ \INE-SSW grabens NNE-SSW graber gr;tbens pnrullel to the trend of the mountain belt. These W-SE Itzpapaiotl ressera faults frabens ;lrc‘ bounded by smnll volcilnocs. Furthermore, I the cltatcrn side of Maxwell Montcs displays imbricated slructurcs bounded by valleys that ;Irc filled with I:IXI Oldest m Stretching m Shortening flows.
Fig. 9. Rclati\c ape of tectonic l’caturcs in each arca ztudicd
Phtrsc~ I. Lukshmi Planum. the oldest area. \V;IS a highly 3. Discussion fuul~cd one that formed under qional extension. creating cxtensi\.e tesscr;l. Three networks of srabens developed. Since the beginning of Venusian radar mapping, the for- oriented NE SW. NW~ SE and N S. mation and evolution of Ishtar Terra has been con- The northern part of Lakshmi Planum seems to be troversial. To date. it is commonly accepted Ihal its crustal bounded by long WNW ~ESE trending faul& localized in delormution is related to mantleconvectivestrength. Until ltzpapalotl Tcssera. During this first phase. the kinem:ttics 1990, the model of mantlc upwclling IocaliLed under Lak- ;trc unknown because of the lack of 21 geologic record. shmi Pl;inum appeared to bc in good agrcemcnt with At the sitme time. a large volcanic dome formed (72 N volcanic features observed on the high plateau. e.g. SX:I- 142 E) (Fig. ?a) which displays fractures Lh;it were rc;ic- jnwea and Colette paterae (Pronin. 1986; Grimm and tivaled during the extensional phase of Lakshmi Planum. Phillips. 1990). but the formation of peripheral mountain Ph.w II. The previous tectonic deformaGon wits fol- belts was not well explained. To explain these. several auth- lowed by \olcanicnctivity which covered Lakshmi Planum ors proposed 21 model in which Ishtar Terra ~;IS formed with low-viscosity lava How. This volcanic material W:IS by compression :md lithosphcric thickening :tbo\,c ;I cylin- ilself dcformcd by long n;trrow NNW SSE-trending driciil mantlc downwclling (Bindsch:tdler C/ ul.. 1990. I992 : troughs called Rangrid Fossae in the south of Lakshmi HCX~ (21ol.. 1990 ; Phillips. 1990 ; Kicfcr :tnd Hager, 1991 ). Planum (62.5 N 357 E) (Fig. 5~1, b). The location ;~nd The block-diagram in Fig. 1I displays the principal orientation 01‘ these troughs were controlled by old l’rac- tectonic fc:itures c!‘CcntraI I&tar Terra th;it ;irc observed turcs in the tesseril of the Lakshmi Pl:tnum b;rsement. on Mr@/rr/l imngcs. The dilfercnt cross-sections also P/ltr.rc~ III. During this phase. Maxwell Montes and included in Fig. I I show ;I schematic nrrangement of Frey.j:l Montcs formed by E W oriented crustni short- tectonic structures down to IO km depth, uith ;I x 20 ening. The mountain belts ilrc the youngest tectonic units exagerulcd vertical scale. of Central Ishtar Terra and arc composed of ;I series of Lakshmi Planum is 21 broad stable platform that may
N-S-trending to NW SE-trending overturned folds and be compared to a terrestrial cralon. In Ihe past. Lakshmi thrusts that overlap to the West. The deformation of Planum experienced a period of crustal extension that Freyja Mantes was closely controlled by the old, long, created the tessern. These tesserae ilre characterized by narrow WNW-ESE-trending zones of Itzpapalotl three systems of regularly distributed horsts and grubens. Tcssero, which are left-laleral shear y/ones ;ts indicated by extending over 2000 km’. Normal Faults corresponding to their regional sigmoidal “Z” inflection and by defol-= graben-bounding scarps. maintain ;I uniform dip over ;I mation Ltlong this zone. Maxwell Mantes itself consists of hrond ;lrc;l. Thcsc normal f;~ults ;tre interpreted to fatten V. Ansan et 01.: Tectonic intcrprct~~tions of Central lshtar Terra 255
PHASE V PHASE II
PHASEIV ” PHASE I
600 km at latitude 65” N
6
PHASE 111 ”
Fig. IO. The regional tectonic history of Central Ishtar Terra. The Lambcrt conic conformal: projection is used for the maps. Each map covers an area of 2000 km in longitude x 2400 km in latitude. The oldest tectonic event is shown as phase I. whereas the youngest one is phase V. (I) Fault, (2) thrust fault, (3) strike--slip fault, (4) normal fault, (5) volcano, (6) impact cjecta deposit, (7) tessera, (8) Lakshmi Planum lava flows, (9) mountain belts, (IO) late volcanic Rows 256 V. Ansan et nl. Tectonic interpretations of
DORSA MAXWELL ITZPAPALOTL TESSERA
Fig. 11. diagram of Ishtar Terra tectonic interpretations text). (1) Impact eiecta deposit, (2) Lakshmi Planum lava flows, (3) volcano belt at northeast Semuni Dorsa, (4) volcanic embayment in eastern Maxwell Montes
with depth and to terminate in an incompetent layer, i.e. of mantle upwelling may have triggered the partial melting they are listric normal faults (Fig. 12a, b). This incom- of the mantle. Owing to chemical and thermal processes petent layer or detachment zone should coincide with the and the thinned and fractured crust, magma could have transition from the brittle to ductile regime in the crust. reached the surface where it would have spread over Lak- Referring to the strength envelope in relation to depth in shmi Planum (Fig. 12~). However, on Mr~geyellunimages, an extensional regime, this transition is estimated to occur a localized narrow volcanic zone such as terrestrial oceanic at a depth of 15520 km (Phillips, 1990 ; Arkani-Hamed, rifting zone is not observed, but rather Ishtar Terra is 1993). Listric normal faulting is a common brittle response characterized by diffuse volcanism, such as produced the to crustal thinning, hence, the crustal extensional defor- basaltic traps on Earth. This observation seems to be in mation of Lakshmi Planum could have been produced by good agreement with the model of continental rifting in two kinds of dynamical process. First, this platform could its volcanic paroxysmal stage (e.g. traps in Deccan, India). have been initiated as a result of the emplacement of a and prior to oceanic spreading. Perhaps, there were several mantle plume or diapir, which would produce both a zones of fissure volcanic emission of tholeiitic traps in broad domal uplift and volcanism. The results of such addition to the vent eruptions of the great volcanoes, processes are observed on the equatorial highlands in Sacajawea Patera and Colette Patera. This volcanic western Aphrodite Terra (Ansan et al. 1994). The expected activity progressively decreased and after a long period of crustal deformation, such as concentric and radial time, a thermal readjustment and thickening of the crust grabens, is not observed on Lakshmi Planum, so it is occurred. Eventually, Lakshmi Planum behaved as a rigid suggested that Lakshmi Planum tessera was produced block in response to the compressive stresses that formed entirely as a result of extension of the lithosphere. The the peripheral mountain belts. three sets of grabens would have been created in the same The formation of mountain belts were the product of extensional stress field, but the maximum extensional horizontal shortening related to crustal thickening stresses would have changed in orientation, throughout (Vorder Bruegge and Head, 1989). The northern moun- the existence of the extensional regime. Because of the tain belt, Freyja Montes, was interpreted as a site of large- great extent of the horst-graben system, it is assumed that scale convergence and imbrication involving the under- this stretching could have occurred at a low strain rate thrusting of the north-polar plain beneath Lakshmi (e.g. 10 ” ss’) (Kusznic and Park, 1986). These authors Planum (Head, 1990). The deformation beginning with a demonstrated that at low strain rate, local strain hard- N-S oriented compressional deformation caused flexure, ening might be expected to transfer deformation laterally buckling and underthrusting of the crust and lithosphere to an undeformed area along a flat detachment horizon. of the northern plain. This model, based on the obser- Thus, the zone of active deformation would be pro- vation of Venrru data, was called into question by Muel- gressively widening. The process of lithosphere extension ler’s flexural model ( 199 1), based on A4u~c~llurr images and probably resulted in two opposite effects. First, the thin- altimetry. ning acted to strengthen the lithosphere and second, a On the basis of the current study, it is concluded that steepening of the geotherm, brought about by bringing Freyja Montes is a fold and thrust belt whose imbrication the hotter asthenosphere nearer the surface, weakened the and fold kinematics are consistent with a regional hori- lithosphere. The rising of isotherms in the crust because zontal shortening oriented E-W. The foreland of this V. Ansan et (11.: Tectonic interpretations of Central Ishtar Terra 257
Lakshmi Planum Maxwell Montes _ _
Proto-Lakshmi Planum C + 4- -m a- -0 - -.a--& a’ 0 I- 200 km 2 3 Fig. 12. Schematic dynamic model of Central Ishtar Terra. Materials shown are: (1) upper mantle, (2) crust and (3) volcanic flows. Arrows indicate stress direction. The larger the arrow, the larger is the stress. The sequence of events is : (a) the beginning of extensional deformation of proto-Lakshmi Planum. (b) Horizontal stretching with crustal thinning. (c) Topographic uplift and volcanism. Possible tectonic transitions : (d) convergent zone such as “subduction zone” westward of mountain belt, or (e) convergent zone such as “foreland thrust belt” where the active block moved westward. (f) Relaxation of mountain belt related to normal faulting and crustal stretching/volcanism 258 V. Ansan C/ r/l. : Tectonic interpretations ofCentral lshtar Tel-r-a mountain belt does not have a trench to the South, imply- ing that FreyJa Montcs overthrust Lakshmi Planum in that direction. The formation and westward propagation of thrusts involved the presence of a regional detachment zone to which the thrusts are connected. This zone could have been the brittle-~ductilc transition zone in the crust. estimated to be at depth of IO km in a compressional regime (Phillips. 1990 ; Arkani-Hamed. 1993). The E W oriented crustal shortening of Freyja Montes was closely controlled by the WNW-ESE trending shear zones of Itzpapalotl Tessera. During compressional deformation. these long shear zones played an important part in the formation of the mountain belt. The shear /ones moved with a left-lateral displacement, which was consistent with the E W oriented compressional stresses. The series of N-- S-trending to NW SE-trending thrusts took root along these WNW ESE trending shear zones. and half a flower structure formed along their southern margin (Fig. I I ). Hence. the deformation of Frey_ja Montes would have been partially absorbed by displacement along these shear ~oncs, which could explain the relative low elevation of Frey.&1 Montcs in comparison to that of Maxwell Mantes. Thus, contrary to Head’s model (1990). the present study suggests that Freyja Montes was formed by the oblique underthrusting of Lakshmi Planurn beneath ItLpapalotl Tcssera. Maxwell Montes is a typical fold and thrust belt. formed during Em~W-oriented crustal shortening. Geographic con- 6048.5 ElevaGon [km] 6061.2 tinuity of the convergent zone between Freyja and Maxwell montcs does not exist. Figure 13 displays the M~7,4d/ur7 altimetric map centered on Maxwell Montes together with three cross-sections orthogonal to the thrust and to the imbrication trend. A good correlation exists between altimetry and structures along these topographic profiles and four /ones can be distinguished. (1) The western side of Maxwell Montes is bounded by a I.5 km deep. and X0 km wide foredeep that can be interpreted as the response of the gentle flexure of Lakshmi 50 '"3 150 200 250 300 350 400 450 500 550 600 650 700 Planurn as it underthrust Maxwell Montes. (2) This forcdeep is followed by a narrow 20 km wide zone characterized by a westward slope of X 20 This topographic slope correlates well with prominent dcfor- mation such as the first fold and thrust units of Maxwell Montcs, which deformed the thin volcanic layer blank- cting Lakshmi Planurn. Thcsc thrusts are regularly arranged and the graben-horst features of Lakshmi Planum did not influence their formation. The thrusts thus may mark the eastern tectonic boundary of Lakshmi Planum. The imbricated structures of Maxwell Montes seem to dip castward at 30 40 . and they could bc con- nectcd to a shallow detachment zone, as is shown in Fig. 1 I. This narrow zone. 5 km higher than the adjacent plateau. has been compared by Suppe and Connors (1992) to a critical taper wedge. Using Coulomb’s law for a dry diabase crust, they calculated the thickness of the cohesive boundary layer. and thus the depth to the d&ollemcnt. to bc I .5 km. Furthermore. the estimated basal dip is no 0 50 ':I 150 200 250 300 350 400 450 500 550 600 650 730 more than 5 -10 in relation to the surface slope (Suppe distance (km) and Connors. 1992). in good agreement with their model Fig. 13. Mup~//rm north-polar stereographic altimetry image and of a taper wedge. three cross-sections orthogonal to Maxwell Mantes. The inserts (3) Eastward, this topographic slope bends down and show forcland topographic profiles without vertical exag- becomes almost horizontal. According to Suppe and Con- gcration V. Kansan et rd. : Tectonic interpretations of Central Ishtar Terra 259 nors ( I992), this change of slope can be explained by the the results of eventual relaxation related to horizontal efrect of the brittle-ductile transition zone, approximately stretching that was marked by volcanism. IO IS km deep. This high plateau consists of a series of But, what was the dynamical process that formed monoclinal imbricated structures covered by a bright layer Maxwell Mantes? What were the mechanical and thermal more than 300 km wide. What is the arrangement of processes that led to thechange of tectonic regime between these thrusts at depth‘? Do they connect to a widespread the extensional one that formed Lakshmi Planum and the horizontal d&ollemcnt as suggested in Fig. I I? This flat compressional one that created the mountain belts‘? Two horizontal d&collement should be unstable. and thus there hypotheses have been proposed; that the compressive were probably several deep crustal imbrications that ter- stresses arose either within Lakshmi Planum, or that they minated in the brittleductile transition zone. Never- originated elsewhere. theless, the high topography cannot be explained by the In the first hypothesis. the convergent zone would be relatively slight horizontal shortening that produced the associated with a mantle downwelling, which involved a thin (200~300 m thick) monoclinal imbrications. mantle upwelling at the centre of the high plateau (Fig. The presence of the high plateau on Maxwell Montes l2d). This implies that the convective cells would be lim- can bc explained by relaxation. and some grabens parallel ited in the Venusian mantlc. an explanation that is not to the mountain trend could be the result of mechanical consistent with the observation that no trench exists along and thermal processes during the relaxation. Froidevaux Freyja Montcs and a low foredcep exists along Maxwell and Ricard (1987) showed that the deformation and cvol- Montes. South of Maxwell Montes. the deformation is ution of high plateau in mountain belts was the last tec- marked by a wide zone interpreted aa shear zone, implying tonicevent. an event that combines relaxation with normal that deformation was absorbed at depth. faulting. Furthermore, this high elevation can be the result The second hypothesis is that the comprcssivc stresses of thermal evolution of a thickened lithosphere owing to originated by the underthrusting of Lakshmi Planum thrusts of lithosphcric blocks (England and Houseman, beneath Maxwell Montes, i.e. in a foreland thrust belt 1989). The hot asthenosphere should support this regime. This type of mountain belt typically involves rela- elevation. Furthermore. this hypothesis seems to be con- tively little crustal thickening (Fig. 12e). This second tirmed by the observation and interpretation of the 2 model seems to agree with observations, i.e. a broad eastern slope of Maxwell Montes. mountain belt with a series of thrusts accommodated at The dip of thrusts seems to bc greater than that on shallow depth (Zuber. 1987). Currently, no evidence exists the western side of the highland. Owing to the volcanic for underthrusting of eastern terrain (Fortuna Tessera) embayment of valleys, it is concluded that the entire cast- under Maxwell Montes. Evidence of such a rone would ern slope of Maxwell Montes underwent a late stretching be the presence of a trench. a foreland, and westward episode. leading to a vertical clockwise rotation of imbri- dipping thrusts in the eastern region of Maxwell Montes. cations. In this instance, thrusts became normal faults Other unanswered questions remain. What is the cxpla- related to volcanism. This process would correspond to nation for the strong correlation between the high top- the final stage of the relaxation of a mountain belt (Fig. ography of Maxwell Montes and the high positive geoid IX). Therefore, the fold-and-thrust belt experienced three (60 m) and (55 mgal) gravity anomalies centred southeast episodes of fcjrmation : of Maxwell Mantes? The inversions of Piorwrr Venus (a) fold-and-thrust wedge propagation to the west ; gravity and altimctric data show that Western Ishtar Terra (b) high-plateau with normal faulting ; and posscsscs an apparent compensation of 130 ~180 km (c) relaxation in the hinterland (Fig. IX). (Grimm and Phillips, 1991). Did the high plateau form The origin of Cleopatra’s volcanism is still debated. before, during, or after the mountain belt? Was it the product of crustal partial melting after the impact shock wave? Or, did it come from the mantle. rising to the surface along crustal faults? Considering the foreland of Maxwell Montes, the Conclusion northwestern edge of the highland was extruded laterally to the southwest. This deformation is controlled both by Detailed morphostructural and tectonic analysis using the western propagation of the main mountain belt and l/i~nrra 15;‘16 and Mqrllm data has lead to a better by right-lateral strike slip faults on the northwestern understanding of the tectonic evolution of Central Ishtar side of this wedge. This tectonic event was recorded Terra. A relative chronology of different structural units within the highland by the change of trend of short has been established on the basis of their superposition imbricated structures superposed on the NW-SE- and cross-cutting relationships. Lakshmi Planum, the old- trending thrusts. The change in trend is indicative of est area, was formed under a regional extensional stress a readjustment of crustal strain opposite an opposing regime that led to the development of tessera, char- rigid crust. acterized by a network of grabens and horsts. After this In summary, the formation of Maxwell Montes intense normal faulting, lava erupted through the frac- occurred in three successive events. On the western side tured crust and covered Lakshmi Planum. Since then, of the highland, we observe the youngest area, charac- Lakshmi Planum has been a stable platform as mountain terized by a fold and thrust wedge. The thrust wedge is belts developed to the East and North. These mountain adjacent to the high plateau, which should be supported belts are fold-and-thrust belts whose tectonic structures by the strength of the mantle. as indicated by grabens were created by regional E-W oriented horizontal short- parallel to the mountain belt. The eastern part shows ening. The compressional deformation of Freyja Montes 260 V. Ansan (‘I trl. : Tectonic interpretations of Ccnlral Ishtar Terra
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