Tectonic evolution of Ovda Regio: An example of highly deformed continental crust on Venus?

Planetary and Space Science 59 (2011) 1428–1445

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Tectonic evolution of Ovda Regio: An example of highly deformed continental crust on Venus? I. Romeo n, R. Capote ´mica, Universidad Complutense de Madrid, c/Jose´ Antonio Novais 12, 28040 Madrid, Spain Departamento de Geodina

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 December 2010 Received in revised form 30 March 2011 Accepted 17 May 2011 Available online 13 June 2011

A detailed structural analysis of several selected areas of Ovda Regio provides evidence of a complex tectonic evolution. We have reported thrusting in the marginal fold belts indicating together with the presence of short-wavelength folds a significant amount of shortening. Extensional tectonics postdate at least in some locations contraction, while the contrary was not observed. Both contraction and extension occur on a complex layered crust yielding contemporary structures of different wavelengths. The thrust and fold belts of the plateau margins are characterized by concentric contraction followed by concentric contraction with perpendicular extension and finally radial extension. Deformation in the thrust and fold belts of Ovda margins is gradually transmitted to the external plains. A complex tectonic history has been revealed in the internal area of Ovda, basically characterized by contraction in different directions generating basin and dome interference at different wavelengths. Small amounts of a non-coaxial component of deformation have been observed both in the margins and in the central area of the plateau. All the reported observations can be explained if Ovda Regio is a continent that survived a global subduction event. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Tectonics Volcanism Venus

1. Introduction The origin and evolution of crustal plateaus on Venus has been a controversial topic. Crustal plateaus are subcircular, with diameters in the range of 1500–2500 km, and elevations of 0.5–4 km above the surrounding plains. They show small gravity anomalies, low gravity to topography ratios and shallow apparent depths of compensation (ADC), all indicating a thickened crust (Smrekar and Phillips, 1991; Bindschadler et al., 1992a; Kucinskas and Turcotte, 1994; Grimm, 1994; Simons et al., 1997). They are made up of a intensely deformed terrain, known as tessera, characterized by different cross-cutting sets of structures indicating a complex tectonic history (Bindschadler and Head, 1991; Hansen and Willis, 1996; Hansen et al., 1999, 2000). The good spatial correlation of crustal plateau elevation and highly deformed tessera terrain indicates that deformation probably plays a major role during crustal thickening (Bindschadler and Head, 1991; Bindschadler et al., 1992a, 1992b). The process of gravitational relaxation of crustal plateaus has been studied in detail by Nunes et al. (2004, 2007). It has been proposed that tessera inliers cropping out in the regional volcanic plains are remnants of collapsed crustal plateaus. Hansen and Lo´pez (2010) have mapped large areas of tessera

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outcrops on the Venusian plains showing that they display planet-scale structural patterns indicative of a rich tectonic history predating the proposed global catastrophic resurfacing (Schaber et al., 1992; Strom et al., 1994). The composition of tessera terrains has been widely discussed because of the lack of direct data, but recent studies of thermal emission using data from two different missions, Galileo (Hashimoto et al., 2008) and Venus Express (Helbert et al., 2008), point out that tessera terrains are consistent with felsic rocks. The origin and evolution of crustal plateaus and tessera have been a focus of attention because they are locally the oldest terrains on Venus and therefore they are fundamental for understanding the geodynamic evolution of the planet. Initially two models were proposed considering crustal plateaus as either the surface expression of downwelling or upwelling flows in the mantle. The downwelling model involves a tectonic crustal thickening due to concentric compression caused by a subsolidus flow and horizontal accretion of a previous thin lithosphere on a cold mantle downwelling flow (e.g., Bindschadler and Head, 1991; Bindschadler et al., 1992a, 1992b; Bindschadler, 1995; Gilmore and Head, 2000). This hypothesis implies that the first recorded strain is contractional. The upwelling (plume) model accomplishes crustal thickening by magmatic underplating and volcanism due to interaction of a previous thin lithosphere with a large thermal mantle plume (Hansen and Willis, 1998; Phillips and Hansen, 1998; Ghent and Hansen, 1999; Hansen et al., 1999, 2000). The initial domical phase of the plume model requires the first recorded strain to be extensional.

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Crustal plateaus show both extensional and compressional structures with a wide range of wavelengths. Compressional structures are typically concentric and oriented parallel to the plateau margins, while extensional structures are mainly perpendicular to the margins and radially distributed (Ghent and Hansen, 1999). While there is agreement that the final tectonic stage is characterized by radial extension, the initial tectonic stages are debated. The supporters of the plume model distinguish between two families of extensional structures, long-narrow graben with small spacing, so called ‘‘ribbons’’, and wide short multiple-scarp graben largely spaced (Ghent and Hansen, 1999). These authors consider ribbons the first structure to be formed, later the concentric folds, and finally the wide graben. This interpretation is based on the assumption that the wavelength of each of these types of structures is indicative of the thickness of the brittle crust, being generated with a brittle– ductile transition deepened with time during the cooling of the plateau. However, Gilmore et al. (1998) and Romeo et al. (2005) studying the cross-cutting relationships between folds and extensional fabrics concluded that the deformation phases are first compressional and finally extensional. In this interpretation the extensional structures, including the long-narrow graben, postdate or are contemporary with the generation of the compressional structures. However both hypotheses, the downwelling and the plume model, have problems in explaining all the characteristics of crustal plateaus. On the one hand, the downwelling model requires too much time for the thickening by crustal flow (1–4 billion years) (Kidder and Phillips, 1996), using the flow laws of dry diabase (i.e. assuming a mafic composition of crustal plateaus). On the other hand, the plume hypothesis (1) has no explanation for the intense contractional tectonics observed indicated by folds of a wide range of wavelengths (Ghent et al., 2005; Hansen, 2006), and (2) the predicted timing of the extensional tectonics contradicts crosscutting relationships in different locations (Gilmore et al., 1998; Ivanov and Head, 1999; Romeo et al. (2005)). Although Gilmore et al. (1998) argued that the formation of ribbon-tessera terrain requires an excessive geothermal gradient, Ruiz (2007) indicated that the heat flow needed for generating ribbons is reasonable for a plume environment. The discovery of short-wavelength (1 km) folds (Ghent et al., 2005) caused Hansen (2006) to reject the hot spot model. Hansen (2006) proposed a new catastrophic model where crustal plateaus were formed by huge lava ponds generated by massive mantle melting due to large bolide impacts on a thin lithosphere. According to this model the mantle beneath the lava pond would be a depleted residuum compositionally buoyant with respect to the adjacent undepleted mantle, causing the plateau uplift by isostasy. The main challenges of this impact model are the following: (1) there is a wide discussion about whether large bolides are able to melt a significant portion of the mantle for generating such an amount of magma (Ivanov and Melosh, 2003); (2) the generation of the large wavelength folds requires stresses large enough to deform a brittle layer that is several km thick, but the underlying liquid magma in convection is not capable of transmitting such forces to the upper brittle layer. Recently, Romeo and Turcotte (2008) proposed a new explanation for the origin and tectonic evolution of crustal plateaus considering that tessera terrains, both tessera inliers in the volcanic plains and crustal plateaus, represent continental crust (or a crust of differentiated composition) that does not participate in the periodic recycling of the lithosphere through global subduction events (Turcotte, 1993, 1995, 1996; Turcotte et al., 1999). Romeo and Turcotte (2008) have studied the force balance on the boundary of a continental area that survives a global subduction event using an analytical model. The model predicts an initial

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Fig. 1. (a) Location of Eastern Ovda Regio in a radar map of Venus. (b) Location of the study zones of Figs. 2, 5, 7 and 10 on a left looking radar image of Eastern Ovda Regio.

strong contraction accommodated by concentric thrusts and folds caused by the gravitational force balance between the continentplateau and the surrounding mantle raised after a global subduction event. The subsequent stabilization of a new crust and lithosphere in the surrounding mantle changes the force balance, allowing a moderate gravitational collapse of the plateau-continent accommodated by radial graben. Here we show the results of a detailed structural analysis performed on several selected areas of Ovda Regio (Fig. 1), providing useful data to understand the structural evolution of the largest crustal plateau on Venus. Ovda straddles the planet’s equator from 501E to 1101E and from 51N to 151S, covering 1.5  107 km2. Four areas located both in the margins and in the central region have been selected for a structural analysis at the highest resolution available. The results are discussed considering the predictions of the existent models for crustal plateau origin and evolution.

2. Methodology We explore the geometries and structural relationships between different sets of the tectonic structures that make up the structural framework of Ovda Regio, with the explicit goals of estimating the tectonic origin of the structures, their time relationships when possible and, finally, a coherent strain history. The spacing of some sets of structures are very small (o1 km) compared to the size of Ovda Regio (5000  2000 km). For this reason, working at the highest resolution requires the selection of small areas for detailed structural analysis. Four small areas have been selected for detailed structural analysis in Eastern and Central Ovda Regio.

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The data sets used for structural mapping were the S-band synthetic aperture radar (SAR) and the altimetry from the NASA Magellan mission. The SAR imagery used includes full-resolution ‘‘F’’ (75–100 m/pixel), right and left illuminated images and stereodata set. Magellan altimetry has a spatial resolution of  8 km along-track, 20 km across-track and  30 m average vertical accuracy, which improves to  10 m in smooth areas. Data were downloaded from the USGS Map-a-Planet website (http://www. mapaplanet.org/explorer/venus.html). The nature of observed tectonic structures was interpreted considering the effects of radar artifacts (Connors, 1995). Lineaments with a gradual variation of radar brightness across their trace were interpreted as folds. Sharp lineaments, white when facing to the radar and black when facing the opposite, were interpreted as fault scarps. The spacing of the different mapped structures have been determined using an interception line method. The measured distance of a line perpendicularly transecting a small group of individual structures is divided by the number of spaces between structures in order to obtain a direct measurement of the average spacing. This procedure is repeated along the map using a regular grid of 0.251 for latitude and 0.251 for longitude for each set of tectonic structures. Inside each quadrangle of the grid, the best location for applying the line interception method was selected. The spacing measurement was made for the most regular spatial occurrence of structures inside each quadrangle. Usually the location of tectonic structures is not homogeneous for the complete mapped area. Some quadrangles have not enough structures or no structures at all, consequently these quadrangles were omitted. The procedure was different for the largest mapped structures (long-wavelenght folds). Due to the small number of these structures, the spacing data were obtained directly measuring the distance between adjacent folds. In this case the statistics is less robust than using the interception line method, but is the only way to obtain all the available spacing data from each map. The interpretation of the tectonic origin of distinct structures was made analyzing their radar characteristics combined with the information provided by stereo-pairs when available. Magellan altimetry was used to map the largest topographic features.

3. Structural analysis 3.1. N margin of Central Ovda Regio The detailed structural analysis of the northern margin of Central Ovda has been performed on an area from 01 to 21N and from 79.251 to 81.251E. The resultant structural map is shown in Fig. 2. Two different structural domains can be easily distinguished: a northern area out of the crustal plateau and a southern area corresponding to Ovda Regio itself. The northern area out of the crustal plateau is characterized by a smooth radar-dark regional volcanic plain (Pb) that appears to have flooded a radar-bright lower unit (Pa). The radar-bright lower unit is probably volcanic in origin because it features a set of lava ridges with a chaotic orientation pattern very similar to pahoehoe ‘‘ropy’’ lavas on Earth, these structures are labeled as ‘‘lava ridges’’ in Fig. 2. The Ovda Regio domain features two types of units: highly deformed (folds and graben) radar-bright tessera terrains (T) and onlapping intratessera volcanic plains. Three types of intratessera volcanic plains have been differentiated using radar brightness: (1) the oldest located in the core of some folds (P1) is radar-bright and it is affected by both folds and graben, (2) on lapping with this unit there is a radar-medium-bright unit (P2) filling the valleys between tessera folds and sometimes affected by graben

and finally (3) the youngest unit appears to fill the largest basins with a radar-dark lava (P3) featuring wrinkle ridges but unaffected by graben. The Magellan altimetry of the area mapped in Fig. 2 indicates a general north-dipping slope in the crustal plateau descending towards the plains. It is also observed a large wavelength alternation of topographic ridges and valleys (large folds in Table 1, Fig. 3) striking parallel to the plateau margin previously observed by Hansen (2006). The tectonic pattern observed in the northern margin of Central Ovda (Fig. 2) is complex, featuring both extensional and compressional structures. The main tectonic feature is the presence of medium sized anticlines (medium folds in Table 1, Fig. 3) developed both inside and outside the plateau trending parallel to the northern margin of Ovda. The northern limit of the crustal plateau seems to control the orientation of this main fold set with a strike rotating from WNW–ESE in the western part of the map to WSW–ENE in the eastern part (Fig. 2). This fold set is closely associated to a parallel set of north-facing scarps that we interpret as thrust fronts on the basis of their lateral terminations and geometries of relief. A selection of detailed areas where the northfacing scarps can be interpreted as thrust fronts is shown in Fig. 4. The examples of Fig. 4a and b are particularly clear. A relief between two anticlines associated with two thrusts is shown in Fig. 4a. Note the cross-cutting relationship between the two anticlines. The eastern anticline cut the western one through a WSW–ENE striking scarp facing NNW (radar bright in the leftlooking image) that we interpret as the thrust front of the eastern nappe. Therefore the eastern nappe is thrusting on the western one through an oblique ramp. Previous structural interpretations in the area considered only the presence of gentle folds (Ghent and Hansen, 1999; Hansen, 2006). These authors considered the north facing scarps simply as the north-dipping limbs of the folds. But looking in detail to the lateral terminations of the folds shown in Fig. 4a and b the interpretation of simple folding has to be ruled out; the observed relief structures cannot be explained without lateral thrust faults. The thrust fault nature of these lineaments is specially evident in Fig. 4a, because in this area the oblique valley between the nappes featuring a WSW–ENE striking scarp is partially flooded by the external radar-dark volcanic unit Pb. The acute termination of unit Pb in this valley evidence a ‘‘v’’ shaped topography for the valley between both nappes, this again strongly suggests the existence of thrust faults between anticlines rather than synclines. The evidence of oblique thrust ramps located in the relief structures between anticlines strongly suggests that the north-dipping scarps separating anticlines are probably caused by thrust faults breaking the surface. Similar thrust relief structures are shown in the center of Fig. 4b and c. A more complex structure is found in Fig. 4d, which can also be interpreted as the evidence of several nappes with lateral terminations indicating thrusting. Other independent evidence of the presence of thrusts between anticlines is found when analyzing the topography with stereo-pairs. The valleys between anticlines that are not flooded by intratessera volcanic plains are very narrow with respect to the anticline widths. A gradual variation of radar brightness indicating the presence of a syncline between anticlines is not observed; therefore the best explanation is the presence of thrust faults between anticlines. Moreover, the lateral terminations of intratessera plains have almost always an acute ‘‘v’’ shape (Fig. 2), which indicates that these units are embaying ‘‘v’’ shaped valleys and not ‘‘u’’ shaped valleys. A ‘‘u’’ shaped valley is expected if it is made up by a syncline, while the observed ‘‘v’’ shaped valleys strongly suggest the presence of thrust faults between anticlines. This criterion has been used for mapping the thrust fault set. The structural style characterized by anticlines separated by thrust faults has been extrapolated also to the

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Fig. 2. FMAP left-looking radar image and structural map of the northern margin of Central Ovda Regio. The location of the images a–d of Fig. 4 is indicated. For the interpretation of non-coaxial deformation indicated by arrows on the radar image see the discussion section.

valleys partially flooded by intratessera plains. Some of these flooded valleys show differences between the northern and southern contacts. The southern limit of a specific volcanic plain

filling a valley between anticlines is usually more straight than the northern one, suggesting that the northern limit is a real volcanic limit of the volcanic plain generated by embayment on

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Table 1 Spacing of structures mapped in Fig. 2.

Thrusts Medium folds Large folds NNW–SSE graben N–S external graben ENE–WSW graben Wrinkle ridges Lava ridges

Average spacing (km)

Standard deviation (km)

4.97 3.98 28.42 1.34 0.80 2.23 4.51 2.23

2.21 1.34 8.77 0.42 0.27 0.78 1.49 0.89

Fig. 3. Frequency of the spacing of structures mapped in Fig. 2.

tessera, while the southern limit has a structural origin caused by the partial thrusting of the southern nappe on the volcanic plain. Taking into account the effect of radar artifacts for the analysis of structures observed in Fig. 4 do not significantly change the interpretation. The fold crests are dragged towards the radar beam due to the effects of foreshortening of the radar-facing limb and elongation of the opposite limb (Connors, 1995), but the observed structures of thrust relief and lateral termination of nappes through oblique ramps are not essentially modified by radar artifacts. The age of the thrust and fold belt is constrained by its crosscutting relationship with the mapped units. Thrust and folds are essentially developed on the tessera terrain of Ovda (T) and on the external bright lava ridged terrain (Pa). The bright intratessera volcanic plain (P1) is affected by thrusting and folding. The medium brightness intratessera volcanic plains (P2) are probably simultaneous with the generation of the thrust and fold belt, because they appear to have flooded the valleys between anticlines (postdate the first stages of compression) and in some places seem to be affected by thrusts. Both the external (Pb) and intratessera (P3) radar-dark volcanic plains clearly postdate the thrust and fold belt formation, although some late movements of the thrusts nappes over these units cannot be ruled out. The extensional tectonics developed inside Ovda in Fig. 2 is accommodated by a set of narrow graben (NNW–SSE graben in Table 1, Fig. 3). This set of extensional structures has a very constant NNW–SSE strike in all the tessera terrains of Ovda in the mapped area compared to the thrusts and anticlines that rotate parallel to the plateau margin. Consequently, in the eastern

Fig. 4. Detailed FMAP left-looking radar images and structural maps from the area mapped in Fig. 2 showing evidences of thrusting. The north-facing scarps are interpreted as thrust fronts on the basis of their lateral terminations and geometries of relief through oblique ramps.

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portion of Ovda in Fig. 2 the thrusts are approximately perpendicular to the graben, while in the western part the compressional and extensional structures cross-cut at  451. Except for a small area in the NW corner of the map, the set of NNW–SSE striking graben is restricted to the plateau area. The radar-bright external volcanic unit (Pa) features a set of very fine graben striking N–S (Fig. 3) (N–S external graben in Table 1). This set of extensional structures can also be observed in a  30 km wide band located in the northern portion of Ovda in Fig. 2. Both extensional sets, the NNW–SSE striking graben of Ovda and the external N–S striking fine graben, do not spatially overlap and the change of strike is abrupt following a line parallel to the plateau boundary. A relative chronology for the formation of these extensional structures can be deduced from the crosscutting relationships with different mapped units. Inside Ovda, the NNW–SSE striking graben clearly postdate the tessera terrain (T) and the radar-bright intratessera unit (P1). Nevertheless, the

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medium brightness intratessera unit (P2) usually postdate graben although in some places the graben appear affecting these units, indicating that the graben began to form before the medium brightness intratessera units (P2) were emplaced but continued developing after that. The radar-dark intratessera unit (P3) clearly postdate everywhere the NNW–SSE striking graben; equally in the external area the radar-dark volcanic plain (Pb) always postdate the N–S striking graben. Two final tectonic stages took place after the radar-dark plains emplacement, whose relative chronology is uncertain. On the one hand, the radar-dark external plains (Pb) and the radar-dark intratessera plains (P3) show N–S striking wrinkle ridges (Table 1, Fig. 3), indicating an E–W contraction of these lava units. On the other hand, there is a set of ENE–WSW striking graben (Fig. 3) generated on the external units (both the bright lava ridged terrain, Pa, and the radar-dark plains, Pb). This extensional fabric is parallel to the lava filled collapse pits probably formed under the same

Fig. 5. FMAP left-looking radar image and structural map of the NW margin of Eastern Ovda Regio. For the interpretation of non-coaxial deformation indicated by arrows on the radar image see the discussion section.

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stress tensor that appears inside Ovda crossing all the units in Fig. 2. The origin of this WSW–ENE striking graben discontinuously developed in the northern area of the map (Fig. 2) may be linked to an extensional belt, with the same WSW–ENE strike, present northwards out of the mapped area. 3.2. NW margin of Eastern Ovda Regio The detailed structural analysis of the northwestern margin of Eastern Ovda has been performed on an area from 3.31 to 5.31N and from 87.71 to 88.91E. The resultant structural map is shown in Fig. 5. Similar to the detailed map described in the previous section, in this map (Fig. 5), two different structural domains can be separated: a northern area out of the crustal plateau and a southern area corresponding to Ovda Regio. In the northern area out of the crustal plateau, four units can be distinguished. The older and lower unit (Pa) is a radar-bright terrain deformed by thrusts and associated anticlines striking parallel to the plateau margin and by several N–S graben. Overlapping this unit there are three volcanic units: the lower of these units is a radar-bright (Pb) unit formed by a group of small coalesced volcanic edifices, on those a radar-medium volcanic plain was emplaced (Pc) and finally a radar-dark volcanic plain (Pd) flooded from the north overlapping units Pa and Pc. The Ovda Regio domain features two types of units: highly deformed (folds and graben) radar-bright tessera terrains (T1 and T2) and on-lapping intratessera volcanic plains. The distinction between the two tessera units is based on the presence of a set of penetrative fractures striking N–S in the older unit (T1). This set of fractures is not observed in T2, suggesting that this is a younger superimposed unit postdating the tectonic event of fracture generation. Also two types of intratessera volcanic plains have been differentiated through radar brightness: (1) the oldest emplaced on some valleys (P1) is radar-medium and it is affected by graben, but not by folds, (2) on lapping with this unit there is a radar-dark unit (P2) postdating

Table 2 Spacing of structures mapped in Fig. 5.

Thrusts Medium folds Large folds NW–SE graben N–S graben En echelon graben

Average spacing (km)

Standard deviation (km)

8.95 6.46 48.77 2.21 2.90 2.05

3.79 1.99 20.97 0.62 0.78 0.92

Fig. 6. Frequency of the spacing of structures mapped in Fig. 5.

folds and only affected by a normal fault in one location, so generally postdating graben formation. The Magellan altimetry of the area mapped in Fig. 5 also indicates a general north-dipping slope of the crustal plateau descending towards the plains with a superposed large wavelength alternation of topographic ridges and valleys (large folds in Table 2, Fig. 6) striking parallel to the plateau margin previously reported by Hansen (2006). The tectonic pattern observed in Fig. 5 is complex, featuring both extensional and compressional structures. The main tectonic structures found are medium sized anticlines (Table 2, Fig. 6) developed both inside (on T2) and outside (on Pa) trending parallel to the Ovda boundary. The extensional tectonics developed inside Ovda in Fig. 5 is accommodated by normal faults developing narrow graben (Table 2, Fig. 6); these narrow grabens sometimes coalesce forming wider graben. Two different sets of extensional structures can be distinguished according to their strike: the inner part of Ovda in Fig. 5 features a set of NW–SE striking graben that is gradually substituted by N–S striking graben in the outer portion of the deformational belt of Ovda. To establish cross-cutting relationships in the area where both sets overlap is a difficult task, probably indicating that they are contemporary. The N–S set of graben is also found on Pa outside of the plateau, therefore was formed before the emplacement of Pc. The formation of graben at least partially postdate folding, considering that, in different locations, the unit P1 can be observed emplaced in the valleys between folds (postdating folding) and affected by several graben. The general structural framework is completed by a set of grabens that is only observed outside the plateau. It postdates the formation of the N–S striking graben set, because it affects units Pa and also Pc. It is characterized by an en echelon right-stepped pattern indicating a small degree of non-coaxial sinistral deformation along the plateau boundary, marked by arrows in Fig. 5. 3.3. NE margin of Eastern Ovda Regio The detailed structural analysis of the NE margin of Eastern Ovda has been performed on an area from 41 to 61N and from 981 to 1001E. The resultant structural map is shown in Fig. 7. Similar to the previous study areas of the margins of Ovda, two different geologic domains can be distinguished: a northeastern volcanic area out of the crustal plateau and a southwestern area corresponding Ovda Regio. Two volcanic units have been differentiated in the external area. The older unit (Pa) has a medium radar brightness and is located closer to the plateau. It is deformed by thrusts and folds and is partially covered from the NE by a radar-dark unit (Pb) showing inlier textures indicative of embayment on Pa. The Ovda Regio domain features only two units: highly deformed (folds and graben) radar-bright tessera terrains (T) and on-lapping intratessera volcanic plains (P1) with a medium radar brightness. The presence of intratessera volcanic plains in this area, first remarked by Ivanov and Head (1999), has been controversial, since dark areas between anticlines can be simply synclines and no lava filled valleys (Ghent and Hansen, 1999). We are confident of the existence of at least the mapped intratessera plains (P1) in Fig. 7. The presence of these smooth undeformed plains is clearly visible when left-looking and rightlooking images are compared and the area between two anticlines shows a similar homogeneous radar brightness in both images indicating the presence of a smooth horizontal surface. The Magellan altimetry of the area mapped in Fig. 7 indicates a general NE-dipping slope in the crustal plateau descending towards the plains, and superimposed there is a large wavelength fold set (Table 3, Fig. 8) indicated by the alternation of topographic ridges and valleys striking parallel to the plateau margin.

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Fig. 7. FMAP right-looking radar image and structural map of the NE margin of Eastern Ovda Regio. The location of the images a–d of Fig. 9 is indicated.

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Table 3 Spacing of structures mapped in Fig. 7.

Small folds Thrusts Medium folds Large folds NW–SE graben NE–SW graben

Average spacing (km)

Standard deviation (km)

0.82 8.18 5.76 52.48 1.21 1.08

0.30 2.87 1.89 6.59 0.33 0.26

Fig. 8. Frequency of the spacing of structures mapped in Fig. 7.

The structural analysis of the northeastern margin of Eastern Ovda (Fig. 7) indicates a complex tectonic framework, featuring both extensional and compressional structures. The main tectonic feature is, like in the other studied margins, the presence of medium size (Table 3, Fig. 8) anticlines, developed both inside and outside the plateau. These anticlines are parallel to the NE boundary of Ovda (Fig. 7). This fold set is closely associated to a parallel set of northeast-facing scarps that we interpret as thrust fronts in the basis of their lateral terminations and geometries of relief. Detailed radar images and their geological interpretation for selected areas indicating the presence of thrust faults are shown in Fig. 9. In the three images Fig. 9a–c, several lateral relief structures between thrust nappes are shown. Similarly to the relief structures shown in Fig. 4 for the northern margin of Ovda, the interpretation of simple folding has to be ruled out, the observed relief structures cannot be explained without lateral thrust faults. The evidence of lateral thrusting associated to the structures of lateral relief of nappes is also indicative of the nature of the northeast-facing scarps that are probably caused by thrust faults breaking the surface. Like in the maps of Figs. 2 and 5, an evidence of the presence of thrusts between anticlines is found when analyzing the topography in detail with stereo-pairs in the mapped area of Fig. 7. The valleys between anticlines are very narrow with respect to the anticline widths probably indicating the presence of thrust faults between anticlines.

Fig. 9. Detailed FMAP right-looking radar images and structural maps from the area mapped in Fig. 7 showing evidences of thrusting. The northeast-facing scarps are interpreted as thrust fronts on the basis of their lateral terminations and geometries of relief through oblique ramps.

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Fig. 10. FMAP left-looking radar image and structural map of the central basin and dome area of Ovda Regio. For the interpretation of non-coaxial deformation indicated by arrows on the map see the discussion section.

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Taking into account the effect of radar artifacts for the analysis of structures observed in Fig. 9 do not significantly change the structural interpretation. The observed structures of thrust relief and lateral termination of nappes through oblique ramps are not essentially modified by radar artifacts. Nevertheless, the sinuosity of the thrust fronts is significantly increased in Figs. 9 and 7 due to the effects of foreshortening and elongation. The timing of thrust formation is indicated by their crosscutting relationship with the mapped units. These structures are essentially developed on the tessera terrain of Ovda (T) and on the external medium brightness lava unit (Pa). The dark external plains unit (Pb) appears flooding the thrust and fold belt developed on Pa, but some low-amplitude folds parallel to the main thrust and fold belt of the northeastern margin of Eastern Ovda are present on Pb. The intratessera volcanic plains (P1) appear flooding the elongated valleys between anticlines, and their undeformed surfaces seem to indicate that they essentially postdate the formation of the thrust and fold belt, although in some locations late movements of the thrusts nappes over these units cannot be ruled out. Parallel to the anticlines and associated thrusts, there is a very penetrative fabric of fine lineaments (Table 3, Fig. 8). When looked at the highest radar resolution these lineaments do not look like sharp fault scarps, they rather seem to show gradual radar brightness variation in a perpendicular direction to their strike. Although their scale is very close to the resolution of the images, we can interpret these fine lineaments as short-wavelength folds according to the structural analysis of Hansen (2006). These short-wavelength folds are restricted to the area of the fold and thrust belt. The main extensional tectonics developed in the NE portion of eastern Ovda mapped in Fig. 7 is radially distributed from the interior of the crustal plateau towards the margin covering all the fold and thrust belt. The extension is accommodated by a set of NE–SW normal faults (Table 3, Fig. 8). The normal faults are usually conjugated forming long narrow graben (shear fracture ribbons using the nomenclature of Ghent and Hansen, 1998) although these normal faults in some places coalesce and form wide, short and deep graben. Both graben morphologies (narrow and wide) cannot easily be separated in two different sets, they usually are formed by the same faults. Considering that both narrow and wide graben have the same cross-cutting relationships with the mapped geologic units we cannot separate their origin in time and therefore we have mapped all of them as NE–SW graben. They are formed on the tessera terrain (T) and the medium bright external lava unit (Pa) and also discontinuously formed over the external radar-dark volcanic plain (Pb). The intratessera plains (P1) cover and postdate this graben set although in some places the normal faults cut P1 indicating that the generation of these faults continued after the intratessera plains emplacement. Located in the SW portion of the map of Fig. 7, corresponding to the internal area of Ovda that do not show short-wavelength folds and medium-wavelength folds and thrusts, there is a penetrative extensional fabric parallel to the plateau margin. These extensional structures are NW–SE normal faults (Table 3, Fig. 8). These normal faults are conjugated forming long narrow graben. They are formed on the tessera unit (T), and are cross-cut by the previously described NE–SW graben set. This is specially evident when looking to the wide NE–SW graben (centered in 98.51E 4.51N) where the NW–SE normal faults are cut and cannot be observed inside this wide graben.

Willis, 1996); this is the case of the detail study zone mapped in Fig. 10 from 0.251N to 1.751S and from 96.251 to 97.751E. The high abundance of tectonic structures with cross-cutting directions and different wavelengths makes difficult the task of distinguishing between different geologic units in this study area. After a detailed analysis of the full-resolution radar images, two different units have been separated. There is a highly deformed tessera terrain (T1) characterized by the presence of abundant NW–SE short-wavelength folds. Overlapping T1 with inlier textures indicatives of embayment there is a medium brightness intratessera volcanic plain (T2) also deformed but with scarce NW–SE short-wavelength folds. The Magellan altimetry of the area mapped in Fig. 10 reflects the basin and dome interference pattern caused by a large wavelength alternation of ridges and valleys with two main cross-cutting directions, NW–SE and NE–SW. The interference structural pattern shown in Fig. 10 is very complex. It is dominated by contractional structures although a small amount of scattered graben have been observed. The contractional structures are characterized by the alternation of ridges and troughs with a gradual variation of the radar brightness across their traces, which indicates a folded surface. Clear evidence of the presence of thrusts between anticlines has not been found, but considering the thrust and fold nature of the plateau margins, the existence of thrusts in the highly deformed basin and dome internal area cannot be ruled out. These fold sets show different wavelengths and two general cross-cutting directions: NW–SE and NE–SW. The NE–SW contractional folds occur at three wavelengths (Table 4, Fig. 11) that show small variations in their average strike: (1) large-wavelength striking on average N551E, (2) medium-wavelength striking on average N401E and (3) short-wavelength striking on average N501E. The average strike of each set of structures separated according to their wavelengths indicates that the largewavelength set and the short-wavelength set are approximately parallel, while the medium-wavelength set is slightly oblique showing a right-stepped en echelon pattern with respect to the

Table 4 Spacing of structures mapped in Fig. 10.

NW–SE NE–SW NW–SE NE–SW NW–SE NE–SW NW–SE

small folds small folds medium folds medium folds large folds large folds graben

Average spacing (km)

Standard deviation (km)

1.21 0.59 7.91 2.57 29.12 30.58 0.83

0.45 0.16 2.57 1.77 8.46 6.75 0.32

3.4. Central area of Eastern Ovda Regio The central area of crustal plateaus on Venus usually features basin and dome interference structural patterns (Hansen and

Fig. 11. Frequency of the spacing of structures mapped in Fig. 9.

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large-wavelength folds (Fig. 10). It is very remarkable that the extremely penetrative character of the NE–SW short-wavelength folds keep a very constant strike for 100–200 km. The characteristics of the NW–SE contractional folds are a bit different, showing also three wavelengths (Table 4, Fig. 11): (1) large-wavelength striking on average N1251E, (2) mediumwavelength striking on average N1451E and (3) short-wavelength striking on average N1401E. The strike of the NW–SE largewavelength folds is less constant than their NE–SW equivalent. It locally rotates from NW–SE to WNW–ESE. In the areas where this rotation take place, the medium-wavelength folds show a left-stepped pattern with respect to the strike of the largewavelength folds. The NW–SE short-wavelength folds are less penetrative than their NE–SW equivalent, and their wavelength is a bit larger (1.21 km for the NW–SE set and 0.59 km for the NE– SW set, Table 4, Fig. 11). The difference in the penetrativeness of both fold sets can be easily explained considering their temporal relationships with respect to the emplacement of T2. The development of the NW–SE set mainly predates T2 emplacement while on the contrary the NE–SW set clearly postdates T2. The structural pattern is completed by narrow non-penetrative NW–SE graben with a spacing between faults of 0.83 km (Table 4, Fig. 11) striking on average N1401E. These structures are mainly found on T2.

4. Discussion 4.1. Tectonic evolution of Ovda margins We have performed a detailed structural analysis using fullresolution radar data on three selected areas of the North and Northeast margins of Ovda Regio. The obtained results contrast with previous structural works carried out on the same areas (Ghent and Hansen, 1999; Hansen, 2006; Chety et al., 2010). From our detailed structural analysis we can deduce a general tectonic evolution for Ovda margins, which can be summarized as follows: (1) a thrust and fold belt began to be developed parallel to the plateau margin, initially affecting tessera terrain (T in Figs. 2 and 7 and T2 in Fig. 5) and later intratessera volcanic plains (P1 and P2 in Fig. 2 and P1 in Figs. 5 and 7); (2) a perpendicular extension took place on the thrust and fold belt contemporary with contraction and partially postdating it; (3) the contractional tectonics with subsequent perpendicular extension was gradually transmitted out of the plateau affecting younger volcanic units (Pa in Figs. 2,5 and 7). This general tectonic history is discussed below, considering also the relative time relationship between tectonic structures and volcanic units. Initially the emplacement of the material forming tessera terrain (T in Figs. 2 and 7 and T2 in Fig. 5; in Fig. 5a there are two outcrops of an older fractured unit, T1) took place. Later, the thrust and fold belt trending parallel to the plateau margin was formed. This highly deformed thrust and fold belt is characterized by contractional structures generated at different wavelengths indicating the deformation of a complex layered crust with decollements developed at different depths. The long-wavelength (30–50 km, Figs. 3,6,8 and Tables 1–3) alternation of ridges and valleys is probably indicative of the generation of nappes of crustal or lithospheric scale indicating high contraction and crustal thickening. Other possible explanation could be that the longwavelength ridges are caused by the generation of anticlinal stacks of nappes with a moderate thickness. The medium-wavelength thrusts (5–9 km, Figs. 3,6,8 and Tables 1–3) and anticlines are indicative of imbrication of thrusted nappes associated to a shallower decollement. The short-wavelength (0.8 km, Fig. 8 and Table 3) folds have been found only on certain areas of the maps

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shown in Fig. 7. They probably correspond to simple folding or thrusting with associated folding of a thin external layer not always present, which explains the lack of these short-wavelength folds, for example, in Fig. 2. The distinction between simple folding or thrusting with associated folding in these fine tectonic structures cannot be solved with full-resolution Magellan radar images. Although these fine lineaments were first reported by Ghent et al. (2005) and Hansen (2006) interpreting them as simple folds, the reported presence of thrusts between the medium-wavelength anticlines (Figs. 4 and 9) seems to indicate that thrusting cannot be ruled out for the short-wavelength folds. We have not found any evidence indicating that the observed contractional structures at different scales were formed at different times. All of them are parallel and can be interpreted as formed under the same stress field. The directions of maximum shortening (perpendicular to the contractional structures) are mainly controlled by the plateau geometry being perpendicular to the plateau boundary in all the studied areas. The topography associated to the thrust and fold belt is a general slope lowering from the inner area of the plateau to the external plains. The vergence on the thrusts is directed radially outside the plateau, which is well established from the structural detailed analysis of Figs. 4 and 9. Thus, from the general topography and the vergence direction we can deduce that the deformed margins of Ovda consist of a thrust and fold wedge that thickens towards the plateau. From the study of thrust and fold belts on Earth (Moores and Twiss, 1995) we know how they were formed and evolved. The active front of the Ovda thrust and fold belt is located in the plateau external boundary and the generation of new normal prograde thrusts take place out of the plateau, adding newly generated nappes to the thrust and fold wedge by tectonic underplating. Applying to Ovda margins, the well known process of growing of terrestrial thrust and fold belts by underthrusting, we deduce that the last formed thrusts are probably those located outside of the plateau. This is also supported by the cross-cutting relationships between these contractional structures and the volcanic units mapped out of the plateau. The deformation of the concentric thrust and fold belt affecting the plateau margins is transmitted to the surrounding external volcanic plains, which show a lower density of compressional structures but also has developed thrusts and folds also parallel to the plateau margin. The older external volcanic units (Pa in Fig. 2, Pa in Fig. 5 and Pa in Fig. 7) were probably emplaced after the generation of the thrust and fold belt but they were later affected by the propagation of the contractional deformation outside the plateau generating thrusts and anticlines parallel to the plateau boundary. The youngest radar-dark volcanic plains (Pb in Fig. 2, Pc and Pd in Fig. 5 and Pb in Fig. 7) flooded the external thrust and fold belt generating inlier textures; therefore they clearly postdate the generation of the thrust and fold belt, but they also show some evidence of small compressional deformation. Pb in Fig. 2 could be in some areas overthrusted by Pa nappes, although the distinction between mechanical or flooding boundaries between both units is a difficult task (a similar structural relationship is found between Pa and Pd in Fig. 5). Pb in Fig. 7 is clearly affected by contraction developing some soft folds parallel to the thrust and fold belt. Therefore, the formation of the thrust and fold belt in Ovda margins starts before the emplacement of any intratessera plains, propagating outwards, and their deformation ends after the emplacement of the external units and the emplacement of different intratessera plains (P1 and P2 in Fig. 2 and P1 in Figs. 5 and 7). The syntectonic emplacement of the intratessera plains P1 and P2 in Fig. 2 and P1 in Figs. 5 and 7 resembles the filling of piggy-back basins on terrestrial thrust and fold belts. In Venus, these basins between anticlines are filled by lava, probably raised form the interior along thrust planes. P3 in Fig. 2 and P2 in Fig. 5 postdate the thrust and belt contraction.

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Fig. 12. Schematic structural evolution of contraction in Ovda margins. Note that the different wavelengths of structures are caused by contraction of a layered crust. (a) Lower thick rigid unit. (b) Soft unit acting as a decollement. (c–e) Upper rigid units with different thicknesses. Note that the position of the set of very small folds developed on unit d with respect to the large wavelength folds can change with time. In the final stages the contractional deformation is transmitted out of the plateau affecting the external volcanic units. The intratessera volcanic plains that were emplaced first are also affected by contraction while the last one not. The generation of short or medium wavelength folds depends on the local stratigraphy. The large wavelength folding is caused by the deformation of a thick rigid layer, crustal or lithospheric (unit a); different mechanisms can build up a large antiform: an imbricated thrust fan (I), an anticlinal stack with duplex (II) and a simple thrust in the very thick rigid layer (III). Extensional structures are omitted from this figure.

A model for the general evolution of the contraction associated to the thrust and fold belt of Ovda margins developed on a multilayer crust is outlined and summarized in Fig. 12. The initial diagram shows different areas occupied by different units with distinct thicknesses. Three upper rigid units (c–e) with different thicknesses lay on a soft unit (b) that will act as a decollement. Under the decollement there is a thick rigid unit (a). Contraction starts with the generation of nappes on the thick lower rigid unit (a) and detachment folds in the upper thin rigid units with wavelengths proportional to their thicknesses. Contraction continues with thrusting of the thick unit under the decollement and folding and thrusting of the upper rigid units. Note that the position of the set of very small folds developed on unit d with respect to the large wavelength folds can change with time, for instance, the set of short-wavelength folds of the unit d is initially located on a large syncline and finally on a large anticline. In the final stages the contractional deformation is transmitted out of the plateau affecting the external volcanic units. The intratessera volcanic plains that were emplaced first are also affected by contraction while the last one does not. The generation of short and medium wavelength folds depends on the local stratigraphy.

The large wavelength folding is caused by the deformation of a thick rigid layer (crustal or lithospheric); different mechanisms can build up a large antiform: an imbricated thrust fan (large antiform I of Fig. 12), an anticlinal stack with duplex (large antiform II of Fig. 12) and a simple thrust in the very thick rigid layer (large antiform III of Fig. 12). The hypothesis of the contraction of a layered crust is rejected by Hansen (2006) considering that the contraction of a layered crust will generate small folds only in the synclines of the large ones, while in Ovda small folds can be observed on the top of large anticlines. We find this argument very weak for several reasons: (1) the extension associated to the outer arc of a large anticline does not exceed the amount of general contraction, thus although there could be less contraction over a large anticline the upper layer is still contracted; (2) the contraction of a layered crust implies the presence of decollements, which prevents the transmission of the extension of the outer arc to the upper rigid layer; (3) a scenario with thrusts is more complex producing the change of the location of a set of small folds with time from a large syncline to the crest of an anticline (central upper thin unit in Fig. 12). The presence of small secondary folds developed on

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large anticline crests are usual on Earth (Hayes and Hanks, 2008; Hanks et al., 2006) and has also been proposed for explaining the general structure of Maxwell Montes on Ishtar Terra (Keep and Hansen, 1994; Hansen and Phillips, 1995). When the thrust and fold belt generation had started and some intratessera plains were emplaced (P1 in Fig. 2) extension perpendicular to the thrust and fold belt took place generating radial graben and fractures probably at the same time when contraction continued. There is not any tectonic evidence of extensional structures formed before the contractional ones. On the contrary, two independent evidences of long narrow graben postdating folds have been observed in all the studied plateau margins: (1) graben wider at the fold crest than at their flanks, and (2) graben cutting intratessera volcanic plains emplaced between anticlines. Both observations were previously reported. The existence of long narrow graben wider at the fold crest than at their flanks was first reported by Gilmore et al. (1998) in Fortuna tessera and by Romeo et al. (2005) in the southern margin of central Ovda Regio. Ivanov and Head (1999) indicated the presence of graben cutting intratessera volcanic plains emplaced between anticlines for the NE margin of Eastern Ovda, and a similar observation was reported by Romeo et al. (2005) in the southern margin of central Ovda Regio. In the present study, the units emplaced in the valley between anticlines and cut by long narrow graben are P2 in Fig. 2 and P1 in Figs. 5 and 7. These units postdate the thrust and fold belt generation and also postdate the formation of the majority of the radial graben fabric that appear cut by these intratessera plains, but predate the last formed narrow graben. We do not find support in our observations for the distinction of two different extensional events associated to the two types of graben: wide short and deep graben and long narrow graben (ribbons), as was reported by Ghent and Hansen (1999). Both types show the same orientation and time relationship with other structures and units; moreover the same faults usually form both types of graben. We consider that the presence of these two types of graben is again indicative of a complex layered crust. The long narrow graben corresponds to the extension of the most external thin rock layer, while the wide, short and deep graben (observed in Fig. 7) corresponds to the extension of a thicker rock layer, that could correspond to the brittle domain of the crust at the time when extension took place. Both types of structures were generated during the same extensional process. The brittle extension affected the external rock layer generating the very penetrative long narrow radial graben set; this extension was concentrated in some weaker corridors where the normal faults are more abundant, producing the deep wide graben accommodating the extension of a thicker structural layer. The general radial strike of the extensional fabrics in the Ovda margins extends also to the external plains, but with a small angle rotation, from NNW–SSE inside the plateau to N–S outside in Fig. 2, from NW–SE inside to N–S outside in Fig. 5 and from NE–SW inside the plateau to N–S outside in Fig. 7. Interpreting the orientation of the long narrow graben as a good indicator of the maximum horizontal stress, it can be deduced that the general N–S direction of the horizontal maximum stress in the area north to Ovda is refracted (the change of orientation is sharp) in the plateau boundary (or close to the plateau boundary for Fig. 5) striking with a radial pattern inside Ovda. The changes in the characteristic spacing (Fig. 3) of normal faults inside and outside the plateau observed in Fig. 2 are probably due to the different rheological behavior of the units affected by deformation. The strike of the extensional structures inside Ovda is usually perpendicular to the contractional ones, with the exception of the western half of the area of Ovda shown in Fig. 2. While the orientation of the contractional structures is mainly controlled by the orientation of the plateau boundary (from WNW–ESE in the western part of the map to WSW–ENE in the eastern part, Fig. 2),

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the extensional structures show much more constant strike (NNW–SSE). Therefore, in the eastern portion of Ovda in Fig. 2, both structures are perpendicular, while in the western part they cross-cut at  451. From the point of view that the formation of both structure sets was in part contemporary, we interpret this structural relationship as an evidence indicating that the orientation of the compressional structures is mainly determined by the plateau boundary, while the strike of the extensional structures is mainly controlled by the orientation of the stress tensor. This interpretation implies that the contractional structures of the western part of Ovda in Fig. 2 were generated at  451 of the shortening direction indicated by the orientation of graben. This seems to indicate the prevalence of a non-coaxial contraction with a transpressional dextral component of deformation for the mentioned area. This interpretation is also supported by the ‘‘eye’’ shaped structures found in the area, each of these structures is characterized by a sigmoidal area inside the main thrust and fold belt where there is an oblique right-stepped set of en echelon folds (Fig. 2). The dextral deformation accommodated by these en echelon folds is indicated by arrows in Fig. 2. The last recorded deformation observed in Fig. 5 was a small amount of transtension that generated a set of right-stepped en echelon graben on the external plains (Pa and Pc). This is indicative of a moderate component of sinistral shear deformation of the units outside the plateau in a direction parallel to the plateau boundary. This deformation is coherent with a N–S shortening direction. Large conjugated shear zones are defined by Chetty et al. (2010) to be the main characteristic of the last tectonic deformations on Ovda Regio. Our detailed structural map of the NE margin of Eastern Ovda (Fig. 7) is located inside of one of the shear zones defined by Chetty et al. (2010). We have not found any evidence of dextral deformation along the fold traces in Fig. 7. Although some amount of transpresion cannot be ruled out, definitively a significant shear deformation is not supported by our detailed structural analysis contradicting the interpretation of Chetty et al. (2010). 4.2. Tectonic evolution of the central basin and dome area The detailed structural study of the map shown in Fig. 10 was performed to unravel the complex tectonic evolution of the central area of Ovda Regio. Ghent and Hansen (1999) indicated the presence of a basin and dome interference pattern in the central area of Ovda, caused by two sets of cross cutting folds. Our structural analysis of Fig. 10 corroborates the presence of a basin and dome interference pattern caused by two general crosscutting directions, NW–SE and NE–SW, but the interference has been observed at different scales according to different fold wavelengths (Table 4 and Fig. 11). We interpret the presence of folds with different wavelengths as the evidence of the contraction from different directions of a complex layered crust. The basin and dome interference of folds could be generated by two different tectonic histories: (1) both sets (NW–SE and NE–SW) were simultaneously formed undergoing a constriction regime in all directions (Fig. 13a), or (2) first a contraction took place in a unique direction generating a fold set, and later a change in the stress tensor provides a perpendicular contraction generating the second structure set, providing the interference with the first one (Fig. 13b). These two possible interpretations are simple endmember tectonic evolution models. More complex and possibly more realistic tectonic evolutions can be considered between both models. For example, first the contraction in a direction generated a fold set, later the tectonic regime changed to a constriction in all directions and finally the contraction in a perpendicular direction dominated at the end (Fig. 13c).

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Fig. 13. Hypotheses of formation of the basin and dome fold interference for the central region of Ovda. (a) Both sets (NW–SE and NE–SW) were simultaneously formed undergoing a constriction regime in all directions. (b) First a contraction took place in a unique direction generating a fold set, and later a change in the stress tensor provides a perpendicular contraction generating the second structure set, providing the interference with the first one. (c) First the contraction in a direction generated a fold set later the tectonic regime changed to a constriction in all directions and finally the contraction in a perpendicular direction dominated at the end.

Although identifying geological units in this highly deformed area is a difficult task (Fig. 10), we are confident of the presence of two tessera units: T1 is the older unit affected by all the observed tectonic structures, and specially by the NW–SE short-wavelength folds and T2 is probably a younger medium-radar brightness volcanic plain affected by all the tectonic structures except for the NW–SE short-wavelength folds that are very scarcely developed in this unit. These two units were initially identified in this area by Bleamaster and Hansen (2005) in the map of the USGS 1:5000000 v-35 Ovda Regio quadrangle (T1 named tO and T2 named mma). From the cross-cutting relationships it can be deduced that first the generation of the NW–SE short-wavelength folds on T1 took place, and near the end of this tectonic stage, the emplacement of T2 took place. The rest of the tectonic structures postdate both units T1 and T2. The current regional topography controlled by the basin and dome interference pattern of the large wavelength alternation of ridges and valleys directed NW–SE and NE–SW was not present when T2 was emplaced, because T2 is not restricted to the valleys and T1 is not restricted to the ridges, as would be expected if the topography at the time of the emplacement of T2 was the same as the topography observed today. While the wavelengths of the medium and large folds are equivalent for the NW–SE and NE–SW sets, this is not the case for the short-wavelength folds. The NW–SE short-wavelength folds show an average wavelength of 1.21 km while the NE–SW set shows an average wavelength of 0.59 km (Table 4, Fig. 11). The formation of both sets is diachronic, as indicated by the crosscutting relationship with T2. The NW–SE set was generated first. The emplacement of T2 took place near the end of this tectonic stage. Later, the NE–SW set was formed. Therefore, the crosscutting relationships with T2 indicate that the wavelengths do not increase with time, contradicting the plume (Ghent and Hansen, 1999) and lava pond hypotheses (Hansen 2006). The differences in wavelength of both sets of fine folds can be due to a difference in the thickness of the deformed layer but also due to different amounts of contraction. If that is the case, the initial contraction directed NE–SW that generated the NW–SE set

was less intense than the following NW–SE contraction that generated the NE–SW set. The development of the NE–SW short-wavelength folds is characterized also by the contemporary development of N–S sinistral strike-slip shear zones indicated by arrows in Fig. 10. The shear zones are 20–70 km long and 5–10 km wide. They are characterized by a rotation and in some cases disruption of the fabric defined by the fine folds lineaments. We consider the development of these non-penetrative shear zones contemporary with the generation of the NE–SW short-wavelength folds, considering that it is also compatible with a NW–SE shortening direction. The development of these shear zones with a small displacement would indicate a small sinistral component of deformation contemporary with the NW–SE contraction. Unfortunately, we cannot unravel the timing relationships between the different sets of medium and long wavelength folds. Their generation can follow any of the models proposed in Fig. 13 or more complex evolutions. Considering the formation of the medium and long wavelength folds to be contemporary, their small differences in strike provide evidence for non-coaxial deformations. For the NW–SE sets, the medium-wavelength folds show a left-stepped en echelon pattern with respect to the longwavelength folds indicating a sinistral component of deformation contemporary with the NE–SW contraction. Similarly, for the NE–SW sets, the medium-wavelength folds show a right-stepped en echelon pattern with respect to the long-wavelength folds indicating a dextral component of deformation contemporary with the NW–SE contraction. Both regimes are compatible with a contraction in all the directions but with the maximum shortening direction oriented E–W. By correlation with the timing deduced for the other maps we consider the development of the NW–SE graben probably the last tectonic stage. Summarizing, the simplest tectonic evolution of the area mapped in Fig. 10 that provides a satisfactory explanation for all the observations is (1) initially a NE–SW contraction generated the short-wavelength folds on T1, the emplacement of T2 took place near the end of this tectonic regime, (2) a strong NW–SE contraction with a small sinistral component of deformation took place on all the area generating the NE–SW short-wavelength folds and the associated sinistral shear zones, (3) a contraction in all the directions with a maximum E–W shortening could account for the contemporary formation of all the medium and long wavelength folds observed and (4) finally a soft NE–SW extension generated a small number of graben. 4.3. Implications for the models of crustal plateau origin and evolution Ghent and Hansen (1999) interpreted the structural evolution of the plateau margins of Ovda according to the plume hypothesis of crustal plateau formation: (1) initially radial long narrow graben (so called ‘‘ribbons’’) were formed, (2) later concentric soft (large wavelength and low amplitude) folds were developed and (3) finally radial short wide graben were superimposed to all previously formed structures. Hansen (2006) performed detailed structural analyses on selected areas of the northern margin of Eastern Ovda, mapping new structure sets: long-wavelength alternation of topographic ridges and valleys and short-wavelength folds, together with the structures described by Ghent and Hansen (1999). The observations carried out by Hansen (2006) were used to support the large impact and huge lava pond hypothesis. Both works (Ghent and Hansen, 1999; Hansen, 2006) assigned a relative time of formation for each structure set according to their wavelength or spacing: the short-wavelength structures are the first to be formed, the medium-wavelength were formed next, and the long-wavelength are the last formed structures. This adjudication of different

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ages for different wavelengths is based on the assumption that all the observed structure sets affect the whole brittle domain of the crust at the time when they were formed. Both models propose a cooling environment from a hot beginning (plume or huge lava lake) to the present cold stage. The wavelength or spacing of the structures is proportional to the thickness of the layer deformed, and assuming that this layer is the brittle domain of the crust, a cooling environment where the brittle domain of the crust thickens with time provides that the short-wavelength structures are expected to be formed in the initial hot stage while the longwavelength structures would correspond to the final cold stage. Ghent and Hansen (1999) suggested that if the short-wavelength structures (high thermal regime) are formed in the final stages then the older long-wavelength would relax and disappear by a process of mechanical annealing; therefore the only possible tectonic evolution implies a cooling environment with a thickening brittle domain of the crust. This conjecture is only valid if all the structures, including the smallest, were generated by the deformation of the whole brittle domain of the crust, which is a strong assumption not based on any independent evidence. Nevertheless, several observations contradict the hypothesis of a cooling environment and thickening brittle domain: fine graben postdating medium-wavelength folds (Gilmore et al., 1998; Ivanov and Head, 1999; Romeo et al., 2005; and this contribution) and folds with 0.59 km of average wavelength postdating folds with 1.21 km of average wavelength (Fig. 10). For structural geologists establishing relative time relationships through wavelength differences between different structure sets seems to be an extremely unusual method. Deformation belts on Earth, probably with no exception, show a wide range of structure sets characterized by different wavelengths from the microscopic to the cartographic scale. Usually, different wavelengths are interpreted to be formed simultaneously, according to the common layered character of terrestrial rocks. There is no evidence indicating that Venusian rocks are more homogeneous preventing different wavelengths to be formed simultaneously. On the contrary, Venusian surface rocks are usually interpreted to be volcanic in origin, and therefore they will surely be layered at different scales. Thus, we consider the assumption that all the structures have been formed due to the deformation of the whole brittle layer of the crust unlikely. The existence of a very penetrative set of short-wavelength folds with a very constant strike for more than 200 km in the basin and dome central area of Eastern Ovda (NE–SW shortwavelength folds of Fig. 10), reported for the first time in this contribution, is very difficult to explain by the lava pond model. Following this model, the short-wavelength folds were formed in the initial stages of the cooling of the lava pond, due to the deformation of the thin solid external layer by underlaying convective forces. The traces of the folds formed on a huge convecting lava pond would be very irregular according to the unstable geometries of convective cells. We conclude that the extremely constant strike (for more than 200 km) in the central area of Ovda of the observed short-wavelength folds can only be caused by a regional tectonic stress field applied on solid rocks, and they cannot be originated on a convecting lava lake. The discovery of short-wavelength folds on crustal plateaus (Ghent et al., 2005) implied higher amounts of shortening with respect to previous estimates (Ghent and Hansen, 1999) that considered only the shortening accommodated by the mediumwavelength folds. Moreover, the existence of thrusts between anticlines reported in this contribution (Figs. 4 and 9) significantly increases the estimation of the amount of shortening of the thrust and fold belts of Ovda margins. Evidences indicating that Ovda has suffered a high amount of concentric shortening in their margins and a complex tectonic

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history of high shortening in different directions in the internal area are strong and multiple: folds developed at wavelengths ranging from 60 to 0.5 km (Tables 1–4, Figs. 3, 6, 9 and 11), and thrusts found at medium wavelengths but probably present at short and large wavelengths. With the present data, it is very difficult to perform a rigorous estimation of the shortening accommodated by thrust displacements, but using an analogy with terrestrial thrust and fold belts, the presence of thrusts probably indicates that the total shortening is significantly higher than the estimations of shortening accommodated only by folding. A high crustal shortening produced by contraction necessarily implies an important amount of crustal thickening. Crustal plateaus are considered to be formed by a thickened crust (Smrekar and Phillips, 1991; Bindschadler et al., 1992a; Kucinskas and Turcotte, 1994; Grimm, 1994; Simons et al., 1997). Thus, probably tectonic contraction was the main process causing crustal thickening, which is in contradiction with the plume and lava pond hypotheses. On the contrary, crustal thickening by tectonic contraction is the process advocated by the pulsating continent model (Romeo and Turcotte, 2008) for crustal plateau formation. The tectonic evolution of Ovda Regio deduced from the present study can be summarized as follows: (1) a strong shortening in Ovda margins produces concentric contractional structures (thrusts and folds) with different wavelengths probably contemporary with high amount of contraction in different directions in the central areas producing basin and dome interference patterns of contractional structures at different wavelengths, during this tectonic stage the crust is significantly thickened causing the plateau to raise by isostatic compensation; (2) the contraction of Ovda margins is transmitted to the external plains producing the accretion of newly formed nappes to the thrust and fold wedge, and contemporary with contraction, a radial family of graben is generated and also gradually transmitted to the external area; (3) the final stage is characterized by extension accommodated by radial graben present in the central area and the margins and it is also transmitted to the external plains. The interpretation of the tectonic evolution of Ovda deduced from our detailed structural analysis is in good agreement with the strain history deduced from the pulsating continents model (Romeo and Turcotte, 2008), and is also compatible with the downwelling model (Bindschadler et al., 1992a, 1992b). The pulsating continents model considers that crustal plateaus have a differentiated composition (continental crust) whose buoyancy prevents their recycling through global subduction events proposed by Turcotte (1993, 1995, 1996) and Turcotte et al. (1999). The amount of lithospheric mantle that remains attached to a continent after a global subduction event will determine the evolution that continent (Romeo and Turcotte, 2008). If the crustal thickness is less than 2/5 of the lithospheric mantle thickness, then the continental area will be compressed, but if the crustal thickness is greater than  2/5 of the lithospheric mantle thickness, it will spread out and collapse. The lithospheric mantle under areas of thick continental crust (crustal plateaus) is expected to be significantly delaminated during a global subduction event causing the extensional collapse of the plateau. The lithospheric mantle under areas of thin continental crust (collapsed crustal plateaus) is expected to remain attached to the crust during a global subduction event causing the compression of the collapsed plateau that will generate an elevated crustal plateau again. Following this model, Venusian continents would have a pulsating behavior due to the alternation of contraction and gravitational collapse. A continental area that survives a global subduction event is expected to suffer first strong concentric contraction caused by the gravitational force balance between the continent-plateau and the surrounding hot mantle raised after the subduction event. Later, the stabilization of a new crust and lithosphere on the

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surrounding mantle changes the force balance allowing a moderate gravitational collapse of the plateau-continent accommodated by radial graben; during the first stages of this gravitational collapse the compression and perpendicular extension of the plateau margins is transmitted to the external plains (to the newly formed lithosphere that surrounds the plateau). The extensional collapse ends when the strength of the newly formed lithosphere surrounding the plateau-continent exceeds the gravitational collapse force. The interpretation of the tectonic evolution is more complex when the non-coaxial character of deformation is taken into account. Conjugated dextral (western half of Ovda margin in Fig. 2) and sinistral (the northern limit of Ovda margin Fig. 5) strike-slip deformations according to a N–S dominated contraction are recorded in the northern margin. These small components of non-coaxial deformation are probably due to the changes in orientation of the plateau boundary with respect to a regional stress field. The irregular boundary of the plateau, considering the pulsating continents model, is determined by the original shape of the continental area. A similar explanation can be provided for the origin of the complex dextral shear belt of southern Ovda margin studied in detail by Romeo et al. (2005). A perfect concentric contraction can be expected only for a circular continent, which is not the case of Ovda. The irregular shape of continents produces differences in the orientation of the continent boundary with respect to the stress field yielding non-coaxial deformations. The general tectonic evolution described in the pulsating continents model (Romeo and Turcotte, 2008) is simple and does not take into account the complexities derived from the process of subduction. The general model as described by Romeo and Turcotte (2008) considers the global subduction event to be instantaneous, thus the new generated tectonic stresses are oriented perpendicular to the plateau margins. But this is an oversimplification since global subduction takes a time span and subduction trenches have specific locations and orientations. These complexities would generate differences in the tectonic evolution of distinct margins of the same plateau and complex strain histories for the internal areas. The high contraction recorded by the northern margin of Ovda could be indicative of an ancient subduction zone located in this margin. The pulsating continent model is in good agreement with the structural observations of Ovda Regio exposed in this contribution but it also received support from new analyzed data of two different missions: Galileo (Hashimoto et al., 2008) and Venus Express (Helbert et al., 2008). Both works detected, using different data, that the thermal emission of tessera terrains on Venus is coherent with a felsic composition. Hashimoto et al. (2008), using the Near-Infrared Mapping Spectrometer of the Galileo spacecraft, revealed that the emissivity of the lowlands is generally higher than that of the highlands (including crustal plateaus). Helbert et al. (2008) estimated the thermal emissivity of Lada Terra using the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on Venus Express, and detected a significant anomaly associated to Cocomama tessera, also consistent with a felsic composition.

5. Conclusions The exposed detailed structural analyses of several selected areas of Ovda Regio have provided new clues to unravel the complex tectonic evolution of crustal plateaus on Venus. The observations have been discussed in the framework of already proposed hypotheses for crustal plateau origin and evolution. The main contributions of our structural analysis are the following: (1) we have reported the presence of thrusts in the marginal fold belts indicating that contraction was underestimated in previous

works; (2) we do not find any evidence of extension in the initial tectonic stages, nevertheless we have reported several evidences of extensional tectonics postdating contraction; (3) both contraction and extension occur on a complex layered crust yielding contemporary structures at different wavelengths, no evidence has been found for an increasing wavelength with time; (4) the thrust and fold belts of the plateau margins are characterized by concentric contraction followed by concentric contraction with perpendicular extension and finally radial extension; (5) deformation (both concentric contraction and radial extension) in the thrust and fold belts is gradually transmitted to the external plains; (6) locally small amounts of a non-coaxial component of deformation have been observed both in the plateau margins and in the internal area; (7) the extremely constant strike of shortwavelength folds in the internal area of Ovda for several hundreds of kilometers is incompatible with a magmatic origin associated to the cooling of a huge lava pond, and it only can be generated by a regional tectonic contraction. All the reported observations can be explained considering Ovda Regio to be a continent that survived a global subduction event, following the model of pulsating continents of Romeo and Turcotte (2008). Detailed structural analyses of other tessera terrains and crustal plateaus on Venus are needed in order to improve the comprehension of the tectonic evolution of the locally oldest mapped terrains of the planet.

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