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Tectonic Interpretations of Central Ishtar Terra (Venus) from and Magellan Full-Resolution Radar Images

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Tectonic Interpretations of Central Ishtar Terra (Venus) from and Magellan Full-Resolution Radar Images 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:qP unulysis. 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).
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