Relationship of coronae, regional plains and rift zones on Venus

Planetary and Space Science 68 (2012) 56–75

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Relationship of coronae, regional plains and rift zones on Venus A.S. Krassilnikov a,b, V.-P. Kostama b,n, M. Aittola b,c, E.N. Guseva d,e, O.S. Cherkashina d,e a

Department of Project Generation and Prospecting, 36/2, bld. 3, Arbat Str., Moscow, 119002, Russia University of Oulu, Department of Physics, Astronomy Division, P.O. Box. 3000, FIN-90014 Oulu, Finland University of Oulu, Oulu Southern Institute, Pajatie 5, FIN-85500 Nivala, Finland d Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Science, 119991 Moscow, Russia e Moscow State University, Geological Department, 119234 Moscow, Russia b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 December 2010 Received in revised form 4 March 2011 Accepted 29 November 2011 Available online 22 December 2011

Coronae and rifts are the most prominent volcano-tectonic features on the surface of Venus. Coronae are large radial–concentric structures with diameters of 100 to over 1000 km. They have varied topographical shapes, radial and concentric fracturing and compressional tectonic structures are common for their annuli. Massive volcanism is also connected with some of the structures. Coronae are interpreted to be the result of updoming and fracturing on the surface due to interaction of mantle diapirs with the lithosphere and its subsequent gravitational relaxation. According to Stofan et al. (2001), two types of coronae are observed: type 1—coronae that have annuli of concentric ridges and/or fractures (407 structures), and type 2 that have similar characteristics to type 1 but lack a complete annulus of ridges and fractures (107 structures). We analyzed 20% of this coronae population (we chose each fifth structure from the Stofan et al. (2001) catalog; 82 coronae of type 1 and 22 coronae of type 2, in total 104 coronae) for the (1) spatial distribution of rift structures and time relationship of rift zones activity with time of regional volcanic plains emplacement, and (2) tectonics, volcanism, age relative to regional plains and relationship with rifts. Two different age groups of rifts on Venus were mapped at the scale 1:50 000 000: old rifts that predate and young rifts that postdate regional plains. Most of young rifts inherit strikes of old rifts and old rifts are reworked by them. This may be evidence of rift-produced uplift zones that were probably mostly stable during both types of rifts formation. Evolution of distribution of rift systems with time (decreasing of distribution and localization of rift zones) imply thickening of the lithosphere with time. Coronaeproducing mantle diapirism and uplift of mantle material in rift zones are not well correlated at least in time in most cases, because majority of coronae (77%) of both types has no genetic association with rifts. Majority of coronae (72%) were mostly active before regional plains formation, and only 3% appear to have begun to form after the plains emplacement, which may be also due to thickening of the lithosphere. According to the relationship with regional plains type 2 coronae are in general older than type 1 coronae. Three types of corona-related volcanic activity were observed: shield volcanoes and their clusters, as well as extensive lobate lava flows and smooth volcanic plains. Shield volcanoes during coronae evolution were mostly active before regional plains emplacement. Most active phase of volcanism of corona may not coincide with the time of the major tectonic activity of corona, as majority of coronae (77%) were most active before regional plains formation, but almost half of all coronae have traces of post regional plains volcanism. Detailed mapping and stratigraphic analysis of seven regions with 34 examples of coronae showed a similarity in the sequence of regional geologic units. & 2011 Elsevier Ltd. All rights reserved.

Keywords: Venus Corona Regional plains Rift zones Volcanism Tectonics

1. Introduction Coronae (Fig. 1) are large radial–concentric volcano-tectonic structures on Venus (e.g., Stofan. et al., 1992; Janes et al., 1992; Head et al., 1992) with diameters of approximately 100 to over 1000 km (e.g., Head et al., 1992; Janes et al., 1992; Squyres et al.,

n

Corresponding author. Tel.: þ358 8 553 1946; fax: þ 358 8 553 1934. E-mail address: petri.kostama@oulu.fi (V.-P. Kostama).

0032-0633/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2011.11.017

1992; Stofan et al., 1992, 1997, 2001). They were first identified and described on the basis of the Venera 15/16 radar images (e.g., Barsukov et al., 1986; Nikishin et al., 1992). Coronae have varied topographical shapes; radial and concentric fracturing and compressional tectonic structures for their annuli are common, as well as massive volcanism that is characteristic with some of the coronae (e.g., Pronin and Stofan, 1990; Stofan and Head, 1990; Nikishin et al., 1992; Stofan et al., 1992, 1997; Janes et al., 1992; Squyres et al., 1992; Glaze et al., 2002). A comprehensive definition of coronae was given by Head et al. (1992): ‘‘these features

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Fig. 1. Typical examples of coronae. Left (a, b): Selu Corona (42.5S, 6E, diameter 300 km) of type 1 (Stofan et al., 2001) that have annuli of concentric ridges and/or fractures. a. Fragment of SAR image (illumination from the left) C1-MIDR.45S011.101. (b) Perspective view of SAR image shown in (a); view is toward NNW, vertical exaggeration  30. Right (c, d): unnamed corona (30.21N, 167.51E, diameter 182 km) of type 2 (Stofan et al., 2001), they have similar characteristics to type 1 but lack a complete annulus of ridges and fractures also, coronae of type 2 are less prominent in tectonics and relief. (c) Fragment of SAR image (illumination from the left) C1-MIDR.30N171.101; (d) Perspective view of SAR image shown in (c); view is toward NW, vertical exaggeration  30.

are defined by a dominantly concentric or circular structure consisting of an annulus of concentric ridges or fractures, an interior that is either topographically positive or negative, a peripheral moat or trough, and, frequently, numerous volcanic and tectonic landforms in the interior’’. Coronae are interpreted to be the result of updoming and fracturing on the surface due to interaction of hot mantle diapirs with the lithosphere and subsequent gravitational relaxation of the diapir (e.g., Janes et al., 1992; Squyres et al., 1992; Stofan et al., 1992, 1997; Head et al., 1992). The coronae have long intrigued researchers and as a result there are a number of basic geological (Squyres et al., 1992; Stofan et al., 1992, 1997; Krassilnikov and Head, 2003), numerical (Stofan et al., 1997; Janes et al., 1992; Koch, 1994; Koch and Manga, 1996) and analog (Krassilnikov et al., 1999; Krassilnikov, 2000; 2001) models for the processes of coronae formation and evolution. For now there are few catalogs of coronae (Crumpler and Aubele, 2000; Stofan et al., 2001; Kostama, 2006), and according to the detailed Stofan et al. (2001) list there are 514 coronae on Venus. Coronae have been classified into two types (Stofan et al., 2001; Glaze et al., 2002) (Fig. 1): type 1—coronae that have annuli of concentric ridges and/or fractures (Fig. 1a), and type 2 that have similar characteristics to type 1 but lack a complete annulus of ridges and fractures (Stofan et al., 2001) (Fig. 1b). Type 2 coronae are also less prominent in tectonics and relief (Stofan et al., 2001; Glaze et al., 2002). Few authors interpreted, that most of coronae predate formation of regional volcanic plains with wrinkle ridges (Basilevsky and Head, 1998a; Ivanov and Head, 2001), which have been argued to be usable as a general stratigraphic marker for Venus (Strom et al., 1994; Collins et al., 1999; Basilevsky and Head, 2000a; 2002). As was shown by several authors (e.g., Basilevsky and Head, 1998b; 2000a; 2002; Ivanov and Head, 1997a,b, 1998a,b, 2001)

regional plains with wrinkle ridges occupy most of the surface of Venus (  75%). The distribution of impact craters on the surface of the planet is not distinguishable from random (Phillips et al., 1992). Different authors (Strom et al., 1994; Collins et al., 1999; Basilevsky and Head, 2000a) used this argument and other evidence to infer that these plains were formed during a geologically relative short period. Therefore, it has been argued that these plains can be used as a general stratigraphic marker. According to the different methods of crater count interpretation, the age of regional plains is 288þ311/  98 Ma (Strom et al., 1994), 400–800 Ma (Phillips et al., 1992) and 750 Ma (from 300 Ma up to 1000 Ma) (McKinnon et al., 1997). Basilevsky and Head (1995a,b, 1998b, 2000a, 2002) proposed that the history of Venus was characterized by a series of periods, each of that are represented by a dominant set of volcanic and tectonic processes and their styles. Many of the styles of tectonic and volcanic processes were transitional between epochs, nevertheless, could be identified specific epochs where certain styles dominated, for example, material of regional plains emplacement or wrinkle ridge formation (Basilevsky and Head, 1998b, 2000a, 2002). Guest and Stofan (1999) criticized these observations and called this interpretation of general Venus evolution as a ‘‘directional history’’ model. Based on interpretation of Magellan data, Guest and Stofan (1999) concluded that the history of Venus represents events and processes that occur randomly throughout the history of Venus. They called it as the ‘‘nondirectional’’ model. Guest and Stofan (1999) also did not accept plains with wrinkle ridges as whole-planet-wide stratigraphic marker, they suggested that lavas of these plains ‘‘y erupted in a number of different styles, each occurring throughout the history represented by the exposed stratigraphy of the planet’’; the same geological approach these authors suggest for coronae and rifts formation on Venus.

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Based on our previous regional and detailed mappings of surface of Venus (Krassilnikov, 2002a,b; Krassilnikov and Head, 2003) and using results of other authors (Basilevsky and Head, 1995a,b, 1998b; 2000a, 2002; Ivanov and Head, 1997a,b, 1998a,b, 2010a, 2001, 2004) we conclude that in local areas studied the sequence of geological units is generally similar. Thus our early findings and interpretations (Krassilnikov, 2002a,b; Krassilnikov and Head, 2003) are consistent with the model of regular changes in style of predominantly regional to global geological processes in the observed history of Venus (Basilevsky and Head, 1995a,b, 1998b, 2000a, 2002). Consequently, in our interpretation as initial base, we used the model of Venus evolution by Basilevsky and Head (1995a,b, 1998b, 2000a, 2002). We did test this model also in this study using the new observations. Some authors have also described associations of coronae and rift structures (e.g., Solomon et al., 1992; Baer et al., 1994; Stefanick and Jurdy, 1996; Hamilton and Stofen, 1996; Stofan et al., 1997, 2001; Krassilnikov and Head, 2003; Martin et al., 2007) and association of coronae and novae (astra), or radially fractured centers (e.g., Squyres et al., 1992; Stofan et al., 1992; Janes et al., 1992; Aittola and Kostama, 2002; Krassilnikov and Head, 2003). Coronae and rift structures are features that are produced by uplift of mantle material, coronae by diapiric uplift (e.g., Squyres et al., 1992; Stofan et al., 1992; Janes et al., 1992), rifts by mostly linear uplift of hot material (e.g., Shelton and Tullis, 1981; Whitehead, 1982; Wilcock and Whitehead, 1991; Condie, 2001). Study of relationship between these two large-scale volcano-tectonic structures may lead to better understanding of internal dynamics of the planet and its evolution through time. Rift valleys on Venus were identified on Pioneer Venus topography data (e.g., Masursky et al., 1980; Schaber, 1982). After Magellan science mission it was shown that rift systems are widely spread (e.g., Solomon et al., 1992; Senske et al., 1992; Crumpler et al., 1993). Two types of rift structures were identified (Hansen et al., 1997; Tanaka et al., 1997): (1) rifts (chasmata) and (2) fracture belts. Both of them are hundreds to thousands of kilometers long and are deformed by linear systems of faults and graben. Rifts are represented by topographic troughs, fracture belts—by linear raises and/or depressions (e.g., Basilevsky and Head, 2000b). Both of them were mapped without subdivision as ‘‘fracture belts’’ or ‘‘rifts’’ by Price (1995). Synoptic mapping also showed the distribution in space and time of rifts and large volcanoes of Venus (Price, 1995; Basilevsky and Head, 2000b). All these maps are usable only for general analysis of correlation of mapped rift zones with hundred-kilometer-scale structures – like coronae – because of their scale. Because of the required detail our study needed a more comprehensive map of rift systems to check possible correlation of evolution of each given corona with formation of rift zones. To achieve this kind of analysis we undertook global mapping of rift structures at the scale 1:50 000 000 (Cherkashina et al., 2004) (Fig. 2). The main goals of our work were: (1) To study the geology of a sample of coronae of both types according to Stofan et al. (2001) and their local surroundings. We studied 20% of the whole population of coronae (Supplementary material, Tables 1S and 2S), each fifth structure from the list (Stofan et al., 2001). Altogether, 82 coronae of type 1 and 22 coronae of type 2 were studied, in total 104 coronae. (2) To study styles of tectonic and volcanic activity of these coronae and also ages of these activity relative to time of regional plains with wrinkle ridges emplacement. (3) To determine time and tectonic relationship of coronae activity with rift zones.

2. Rift structures Our analysis of rift structures includes the study of spatial distribution of rift structures and time relationship of rift zones

activity with time of regional volcanic plains with wrinkle ridges emplacement. We used C1 and C2-MIDRs SAR ‘‘Magellan’’ images (Saunders et al., 1992; Ford et al., 1993) for analysis and paid special attention on following objectives: (1) Global and regional photogeological mapping of rift zones at scale 1:50 000 000. Linear tectonic zones or fracture belts that may be interpreted as rift or rift-like zones (Solomon et al., 1992; Senske et al., 1992; Crumpler et al., 1993; Hansen et al., 1997; Tanaka et al., 1997; Price, 1995; Crumpler and Aubele, 2000; Basilevsky and Head, 2000b) were mapped. Not all of them are well prominent rift zones, some of these fracturing systems may be represented by linear dyke swarms (Condie, 2001; Ernst et al., 2001) or rifts on their initial stages of formation; Still, they have similar kinematic of formation (all of them of them are forming by linear extension (Condie, 2001; Ernst et al., 2001)) and may have similar characteristics (morphology of fracturing, relief, etc.). Because of that all of them were mapped without subdivision as an ‘‘axis’’ of belts of linear fracturing. For simplicity hereafter we will call these structures as rifts or rift-like feature or zone. (2) Study of relationship of time of rifts zones activity with time of regional plains with wrinkle ridges emplacement. Three possible types of this relationship were observed by Basilevsky and Head (2000b): All tectonic structures of riftlike feature are embayed by material of regional volcanic plains with wrinkle ridges, all tectonic structures of rift-like feature cut material of regional volcanic plains with wrinkle ridges, some of tectonic structures of rift-like feature are embayed by material of regional volcanic plains with wrinkle ridges and some cut this material. We also used gravity data (Bindschadler et al., 1992; Grimm and Phillips, 1992; Sjogren et al., 1997), that in general should show possible correlation between geoid high anomalies with recent rift structures location. The resolution of this data is not sufficient for analysis of each separate rift branch or rift zone, but recent rift structures should be located in regional areas of geoid high (e.g., Herrick, 1999). 2.1. Relationship of rift zones with regional plains with wrinkle ridges According to Basilevsky and Head (2000b) all rift zones were subdivided into two main age groups that were mapped separately (Fig. 2): (1) rift zones that predate the formation of regional plains with wrinkle ridges (old rift zones), and (2) rift zones that postdate emplacement of plains with wrinkle ridges (young rift zones). 2.1.1. Old rift zones that predate regional plains Main characteristic of an old rift zone is the embayment of all tectonic structures of these zones by material of regional plains with wrinkle ridges (Basilevsky and Head, 2000b). In most cases this is a sharp embayment contact, similar as ‘‘A’’ in Fig. 3. The sample area (Fig. 3) is located to north from Ovda Regio and south-west from Lemkechen Dorsa. In the southeastern part of the detail image dense deformed by linear fracturing old rift zone with north-northeast strike is located, it is embayed by material of regional plains deformed by wrinkle ridges. Ridges are marked as ‘‘B’’ in Fig. 3. Old rift zones are represented by linear deformation zones that are located around the planet with some concentration on highlands, they are also observed on flanks of regional volcanic rises and in lowlands (Fig. 2a). Some concentration of them is observed in equatorial regions, particularly south from

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Fig. 2. Global map of distribution of rift zones on Venus (initial scale 1:50 000 000). (a) Global map of distribution of rift zones with a background of global SAR mosaic of Venus with overlain location of coronae of different types (Stofan et al., 2001), simple cylindrical projection. Blue lines—rift zones that predate regional plains emplacement (old rift zones), red lines—rift zones that postdate regional plains formation (young rift zones). Type 1 coronae are shown as white circles with black borders; type 2 coronae are shown as black circles with white borders. Locations of coronae are given according to Stofan et al. (2001) (Tables 1 and 2). Yellow squares—location of typical examples of relationship with regional plains (A—Fig. 3, B—Fig. 4, C—Fig. 5). The areas of detailed mapping are shown by polygons (Figs. 7–13): transparent yellow polygons—areas where coronae are located in old rift zones (1-1, 1-2); transparent red—areas where coronae are located in young rift zones (2-1, 2-2, 2-3); transparent green—areas where coronae are located outside of any rift zones (3-1 and 3-2). In total 34 coronae (21 of type 1 coronae and 12 of type 2 coronae) were mapped. (b) Global map of distribution of rift zones overlain on the surface geoid map of Venus, simple cylindrical projection (data from NASA planetary data system) and also superposed on ‘‘Map of Rifts and Volcanoes of Venus’’ (modified after Price, 1995; Basilevsky and Head, 2000b). Black lines—old rift zones, white lines—young rift zones from our study. Legend is shown for ‘‘Map of Rifts and Volcanoes of Venus’’ by Basilevsky and Head (2000b). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Ovda Regio, close to Arthemis Corona and inside of it. Old rift zones show no visible correlation with geoid anomalies (Fig. 2b).

2.1.2. Young rift zones that postdate regional plains Young rift zones that postdate regional plains material clearly cut this material unit (Fig. 4). The sample area (Fig. 4) represents a young, well prominent rift zone which is located north of Atla Regio in Ganis Chasma. In detail image dense rift fracturing of Ganis Chasma rift zone (to west of image, Fig. 4) cuts material of

regional plains deformed by wrinkle ridges (‘‘B’’ in Fig. 4). Young rift zones are also represented by the linear deformation zones, they are less extended than old rifts and occupy region mainly from the 1801 to 3151 longitudes also in equatorial region (Fig. 2a). Clear concentration of these zones is observed in recent large volcanic rise and between them, for example in AtlaThemis-Beta Regio (Fig. 2a). Almost all young rift zones are located in areas with geoid high (Fig. 2b). Small number of young rift zones is located in lowlands without any connection with volcanic rises and/or geoid high.

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Fig. 3. SAR image (illumination from the left) of typical example of old rift zone that predates regional plains with wrinkle ridges (Basilevsky and Head, 2000b). Also, the shield plains which are typically older than regional plains are embaying the rift zone. The zone is to N from Ovda Regio and to SW from Lemkechen Dorsa (C1-MIDR.15N077.101), center of left image 9.51N, 811E. A—embayment contact; B—wrinkle ridges.

Fig. 4. SAR image (illumination from the left) of typical example of young rift zone that postdate regional plains with wrinkle ridges (Basilevsky and Head, 2000b) at N of Atla Regio in Ganis Chasma (C1-MIDR.15N197.101), center of right image 15.51N, 196.51E. A—fractures (normal faults) that cut regional plains with wrinkle ridges; B—wrinkle ridges.

2.1.3. Relationship of old and young rift zones In some cases young rift zones may cut previously formed old rift zones (Figs. 2 and 5) (Basilevsky and Head, 2000b). The sample area (Fig. 5) is located south of Parga Chasma and to northeast of Wawalag Planitia. In the detail image dense fracturing of old rift zone is sharply embayed (‘‘A’’ in Fig. 5) by regional plains that are deformed by a set of wrinkle ridges (ridges marked as ‘‘B’’ in Fig. 5). Both of them are deformed by well prominent fracturing (westnorthwest strike) of young rift zone of Chondi Chasma. Basilevsky and Head (2000b) also mapped this region and described similar relationship of rift activity. Often rift zones that postdate regional plains inherit strike of old rift systems (northern part of sample area (Figs. 2 and 5)) but sometimes young rift zones cut old rifts with another strike (southern part of the sample area (Fig. 5)). 2.2. Interpretation of key observations Two different age groups of rift zones on Venus were subdivided and mapped in this part of the study: old rift zones, which

predate emplacement of plains with wrinkle ridges, and young rift zones which postdate this event. In most cases young rifts inherit strikes of old rift zones (Figs. 2 and 5), and old rifts zones are reworked by young rifts (Figs. 2 and 5). This may imply that the axis of rift produced lithospheric extension was changed in some areas only, and dynamics of mantle upwelling during formation of both types of rifts were approximately the same. Rifts are long-lived structures that period of activity is comparable with age of regional plains. More widespread distribution of the old rift zones points to two possible interpretations: (1) there were more active mantle dynamics before formation of regional plains, and/or (2) the thickness of the lithosphere was reduced before the plains formation, and it became thicker with time and this reduced rift activity after the plains formation. This may be may be circumstantial evidence of the thickening of the Venusian lithosphere with time, which is predicted and discussed by different authors (e.g., Parmentier and Hess, 1992; Turcotte, 1995; Phillips and Hansen, 1994; Brown and Grimm, 1999). Correlation with geoid of young rift zones (Fig. 2b) may imply

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Fig. 5. SAR image (illumination from the left) of typical example of young rift zone that postdate regional plains with wrinkle ridges and cut regional plains and old rift zone that postdate regional plains (Basilevsky and Head, 2000b) to S from Parga Chasmata and to NE from Wawalag Planitia (C1-MIDR.15S232.101), center of right image 191S, 231.51E. A—embayment contact; B—wrinkle ridges; C—fractures (normal faults) that cut regional plains with wrinkle ridges and old rift zone.

Table 1 Time relationship of coronae of type 1 and type 2 activity (Stofan et al., 2001) with position of regional plains with wrinkle ridges (Pwr). Type 1 (82 coronae) Corona Corona Corona Corona

tectonics tectonics tectonics tectonics

Type 2 (22 coronae)

Total (104 coronae)

predates Pwr 32 Coronae (39%) 57 Coronae (69%) 14 Coronae (64%) 18 Coronae (82%) 46 Coronae (44%) mostly predates Pwr 25 Coronae (30%) 4 Coronae (18%) 29 coronae (28%) predates and postdates Pwr 22 Coronae (27%) 4 Coronae (18%) 26 Coronae (25%) postdates Pwr 3 Coronae (4%) 0 3 Coronae (3%)

75 Coronae (72%)

Table 2 Time relationship of coronae of type 1 and type 2 activity (Stofan et al., 2001) with rift zones formation. Coronae (in total 104 structures)

57 Coronae (55%) are located inside rift zones

35 Coronae (34%) in old rift zones

22 Structures (21%) in young rift zones

19 Coronae (18%) predate rift zones 16 Coronae (15%) were formed simultaneously with rift zones 14 Coronae (14%) predate rift zones 8 Coronae (8%) were formed simultaneously with rift zones

47 Coronae (45%) are located outside of rift zones

that these structures are not finished with their evolution, as opposite to the old rift zones.

3. Coronae structures Our analysis of coronae structures includes the study of characteristics of deformational and morphological structures of all selected coronae (Tables 1 and 2), which is 20% of population of identified coronae (Stofan et al., 2001). Detailed geological mapping of typical examples of coronae was completed within seven selected regions including 34 examples of coronae (21 of type 1 coronae and 12 of type 2 coronae) (Figs. 2a and 6–13). The selected areas have three different types of spatial association of coronae with rift zones: (1) areas where coronae are located in old rift zones that predate regional plains emplacement (1-1 and 1-2 in Fig. 2a); (2) areas where coronae are located in young rift zones that postdate regional plains formation (2-1, 2-2 and 2-3 in Fig. 2a); (3) areas where coronae are located outside of any rift zones (3-1 and 3-2 in Fig. 2a). Gravity data is important in the analysis of regional volcanic/ tectonic structures and in determination of ‘‘dead’’ and ‘‘live’’ structures (Bindschadler et al., 1992; Grimm and Phillips, 1992;

Sjogren et al., 1997; Johnson and Richards, 2003). However, the resolution of this data is not sufficient especially for detailed study of small coronae. Methods of geological analysis of the surfaces of other planets have been described in detail (e.g., Greeley and Batson, 1990, Wilhelms, 1990; Hansen, 2000) and have been applied to the mapping of the surface of Venus (e.g., Tanaka, 1994; Ivanov and Head, 2001; Krassilnikov and Head, 2003). The geological mapping and analysis were performed using C1-MIDR and F-MIDR Magellan radar images (Saunders et al., 1992; Ford et al., 1993) in both digital and hard copy versions. Special attention has been paid to the relationship of corona structures with plains with wrinkle ridges as well as smooth and lobate plains, in order to assess the relative ages of coronae structures and their geological environment. Analysis of topography was undertaken using Magellan GTDR altimetric data (Saunders et al., 1992; Ford et al., 1993) and artificial stereo images. For analysis of relationship of coronae with rift structures we used following criteria the same way as previously for novae analysis (cf. Krassilnikov and Head, 2003). To have a genetic association with a rift, corona should be located inside or close to the rift zone. We analyzed this on the basis of our map of rifts (Fig. 2). We also noted the age of the rift (is the rift old or young)

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Fig. 6. Legend for geologic maps and correlation charts shown in Figs. 7–13.

for all coronae under study (104 structures), but in the case of spatial association with rift, corona may predate or postdate the rift zone. Because of that, we also analyzed the structural association of corona with rift. If corona predates or postdates rift there should be no structural association between them and rift does not have influence on the shape, morphology and distribution of the deformational structures of corona. Thus diapiric uplift that produce corona does not have relation with the regional uplift of mantle material and regional extension that produced the rift zone (e.g., Shelton and Tullis, 1981; Whitehead, 1982; Wilcock and Whitehead, 1991; Condie, 2001). If coronaproducing event took place on the background of regional uplift and extension, corona should have structural traces of that the same with novae (Krassilnikov and Head, 2003). Distribution of deformational structures by formation of uplifted domes on the background of regional extension has been modeled (e.g., Withjack and Scheiner, 1982) and described for novae (Koenig and Pollard, 1998; Krassilnikov and Head, 2003), and related to coronae structures (Janes et al., 1992; Squyres et al., 1992; Stofan et al., 1992; 1997; Koch, 1994; Koch and Manga, 1996). Thus only in case when the tectonic structures of corona have traces of rift influence (that shows in their distribution and kinematics), corona may have a genetic association with the rift. In this case, corona-related uplift may have been formed due to the inhomogeneous uplift of hot material along the rift zone (e.g., Shelton and Tullis, 1981; Whitehead, 1982; Wilcock and Whitehead, 1991). In our study we considered corona as rift-connected only if the structural association is observed.

authors consider densely deformed zones as geological units, depending on the scale of the mapping (e.g., Wilhelms, 1990; Basilevsky and Head, 2000a,b; Hansen, 2000). In the case of fractures associated with rift zones, we mapped these as zones of intensive deformation (like zones of melange on Earth) where they were too dense to be mapped individually (densely deformed rift terrain).

5. Geological structure of coronae and their relationship with regional plains and rift zones The stratigraphy and tectonics of each mapped region are shown in geological maps and corresponding charts (Figs. 7–13). Attention was given to the key observations of corona activity, time relationship of corona activity with time of emplacement of regional plains and also rift zones formation. Seven areas were selected regarding three types of possible association of coronae with rift zones mapped by Cherkashina et al. (2004) (Fig. 2): (1) areas where coronae are located in old rift zones that predate regional plains emplacement (1-1 and 1-2 in Fig. 2); (2) areas where coronae are located in young rift zones that postdate regional plains formation (2-1, 2-2 and 2-3 in Fig. 2); (3) areas where coronae are located outside of any rift zones (3-1 and 3-2 in Fig. 2). Hereafter, geographic coordinates and diameters in the beginning of coronae description are given according to the catalog by Stofan et al. (2001), and the names of the coronae and other geographic features are taken from the USGS planetary names web site: http://planetarynames.wr.usgs.gov/vgrid.html.

4. Geologic map units and tectonic structures 5.1. Coronae in old rift zones We used stratigraphic units (Fig. 6) which represent a generalized sequence from older units to younger on the basis of the relationships observed and mapped in this study. The used stratigraphical approach and the units are based on the studies of Basilevsky and Head (1995a,b, 1998a,b, 2000a, 2002). In our mapping we describe Fb as a stratigraphic unit because in the studied areas Fb unit is embayed by younger material. In some cases this unit composes concentric annulus of corona-like features and is usually deformed by concentric fractures of these features with fracture to fracture spacing averaging hundreds of meters and more. Fb unit has rather high radar brightness due to intensive deformation and in some areas it is possible to divide it to subunits. The unit is usually rather elevated and in all cases embayed by younger materials. On the basis of the scale of our mapping, we found that some individual structures of apparent tectonic origin can be mapped, and be distinguished from the zones of dense tectonic structures. Individual structures include features that we interpreted as fractures, normal faults, graben, and compressional ridges. Some

Two sites of relationship of coronae with rifts, regional plains and associated volcanism in old (pre-Pwr) rift zones were selected for the detailed studies (1-1 and 1-2 in Fig. 2). The areas include six coronae, four coronae of type 1 and two coronae of type 2. 5.1.1. Coronae in the southeastern part of Aino Planitia (V-46 quadrangle) Geologic map of the region is shown in Fig. 7b and the stratigraphic sequence is described in Fig. 7f. Two coronae are located in this area, both are type 1 coronae—(1) Iang-Mdiye Corona (in the paper by Stofan et al. (2001) this is part of Makh corona) (471S, 861E), diameter 275 km, (2) Khotun Corona (46.61S, 81.51E). Old (pre-regional plains) rift zone is located in the area (1-1 in Fig. 2) with strike to east–northeast. Formation of this rift zone predates formation of regional plains (Pwr). Iang-Mdiye Corona (type-1) tectonic evolution started and finished before formation of regional plains on the background of regional extension produced by pre-plains rift. Thus corona

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Fig. 7. Coronae in old rift zone (1-1 in Fig. 2a), SE part of Aino Planitia (V-46 quadrangle). (a) SAR image (illumination from the right) fragment of C1-MIDR.45S074.201, center of image 47.51S, 82.71E; (b) geologic map of area shown in Fig. 4a; (c) topographic map of area shown in Fig. 4a; (d, e) perspective views of SAR image shown in Fig. 4a (d) and geologic map shown in Fig. 4b (e); view is toward SW, vertical exaggeration  30; (f) correlation chart. Hereafter in correlation charts in Figs. 4–10 R means rift activity; reg?—possible influence of regional stress (main role in formation of these tectonic structures belong to regional stress); type of corona-related volcanism—(a) certain corona-related volcanism (dark gray contoured by solid line), (b) volcanic edifices that are located inside or close to corona but apparently have no connection with corona evolution (light gray contoured by dashed line).

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Fig. 8. Coronae in old rift zone (1-2 in Fig. 2a), SE part of Themis Regio (V-53 quadrangle). (a) SAR image (illumination from the left) fragment of C1-MIDR.45S286.101, center of image 47.71S, 286.51E; (b) geologic map of area shown in Fig. 5a; (c) topographic map of area shown in Fig. 5a; (d, e) perspective views of SAR image shown in Fig. 5a (d) and geologic map shown in Fig. 5b (e); view is toward NNW, vertical exaggeration  30; (f) correlation chart.

may have genetic association with the rift. Associated volcanism includes few generations of shield volcano clusters that mostly predate regional plains (two of them predate regional plains formation (Psh1,2) and one postdate (Psh3)), and also smooth plains and lobate lava flows. Almost all Khotun Corona (type-1) structures predate emplacement of regional plains, while just few structures were formed after the formation of these plains. Rift structures do not have apparent influence on the corona, indicating that they have no genetic connection. Cluster of pre-plains shield volcanoes at southern flank of the corona (Psh2 unit) may present the corona-related volcanism. Some small field of smooth and lobate plains is located inside the corona depression, which may also have connection to corona activity.

5.1.2. Coronae in the southeastern part of Themis Regio (V-53 quadrangle) Four corona structures are located and mapped in this area (Fig. 8). Two are type 1 coronae—(1) Turesmat Corona (51.51S, 289.51E), diameter 150 km, (2) unnamed corona (49.51S, 289.51E), diameter 225 km (#1 in Figs. 8a and f) and two are type 2 coronae—(3) Nzambi Corona (451S, 286.51E), diameter 181 km, (4) unnamed corona (47.51S, 2861E), diameter 655 km (#2 in Fig. 8a and f). A zone of parallel fracturing mapped as a rift zone, also locates in this area (1-2 in Fig. 2) with a strike to north– northwest. Formation of this rift zone predates the late stages of the regional plains, Pwr formation, because there are traces of formation of fracturing of the zone after Pwr1, but Pwr2 completely embay the fracturing of the zone.

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Fig. 9. Coronae in young rift zone (2-1 in Fig. 2a), N part of Hinemoa Planitia, Hecate Chasma (V-28 quadrangle). (a) SAR image (illumination from the left) fragment of C1-MIDR.15N249.101, center of image 121N, 253.51E; (b) geologic map of area shown in Fig. 6a; (c) topographic map of area shown in Fig. 6a; (d, e) perspective views of SAR image shown in Fig. 6a (d) and geologic map shown in Fig. 6b (e); view is toward NNE, vertical exaggeration  30; (f) correlation chart.

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Fig. 10. Coronae in young rift zone (2-2 in Fig. 2a), W of Hinemoa Planitia (V-40 quadrangle). (a) SAR image (illumination from the left) fragment of C1-MIDR.0N249.101, center of image 2.71S, 252.51E; (b) geologic map of area shown in Fig. 7a; (c) topographic map of area shown in Fig. 7a; (d–g) perspective views of SAR image shown in Fig. 7a (d, f) and geologic map shown in Fig. 7b (e, g); (d, e) view is toward WNW; (f, g) view is toward ENE, vertical exaggeration  30; (h) correlation chart.

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Fig. 11. Coronae in young rift zone (2-3 in Fig. 2a), E part of Themis Regio (V-53 quadrangle). (a) SAR image (illumination from the left) fragment of C1-MIDR.45S286.101, center of image 39.51S, 2881E; (b) geologic map of area shown in Fig. 8a; (c) topographic map of area shown in Fig. 8a; (d, e) perspective views of SAR image shown in Fig. 8a (d) and geologic map shown in Fig. 8b (e); view is toward NW, vertical exaggeration  30; (f) correlation chart.

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Fig. 12. Coronae without spatial association with rift zone (3-1 in Fig. 2a), W part of Eistla Regio (V-20 quadrangle). (a) SAR image (illumination from the left) fragment of C1-MIDR. 15N009.101, center of image 11.51N, 4.81E; (b) geologic map of area shown in Fig. 9a; (c) topographic map of area shown in Fig. 9a; (d–g) perspective views of SAR image shown in Fig. 9a (d, f) and geologic map shown in Fig. 9b (e, g); (d, e) view is toward NW; (f, g) view is toward SE, vertical exaggeration  30; (h) correlation chart.

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Fig. 13. Coronae without spatial association with rift zone (3-2 in Fig. 2a), N part of Ganiki Planitia, NW part of Ulfrun Regio (V-15 quadrangle). (a) SAR image (illumination from the left) fragment of C1-MIDR.45N223.101, center of image 421N, 217.21E; (b) geologic map of area shown in Fig. 10a; (c) topographic map of area shown in Fig. 10a; (d, e) perspective views of SAR image shown in Fig. 10a (d) and geologic map shown in Fig. 10b (e); view is toward NW, vertical exaggeration  30; (f) correlation chart.

Turesmat corona (type-1) started to form after the formation of the material of the Fb unit. Tectonic features are represented by dense concentric extensional fracturing, but the strike of fracturing has no connection with the strike of the regional Fb fracturing

mapped as an old rift zone. Judging by the different strike of fracturing in the regional linear fracturing that may represent rift structures and concentric fracturing of corona, evolution of the corona should not have connection with formation of this rift zone.

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Unnamed corona #1 (type-1, Fig. 8a and f) formed dense concentric extensional fracturing before the plains that, in turn, deformed the Fb1 unit. The last traces of observed activity – the radial bilateral fracturing and concentric extensional fracturing on the western flank of the corona – took places after Pwr1 formation. The asymmetry, strike and time of this fracturing may have connection with the regional stress caused by the rift-like zone formation. These late stages of the corona formation may have traces of influence of regional stress. Two associated generations of volcanism are observed—both of them are clusters of shield volcanoes. The connection with the rift formation is possible. The dense radial fracturing of Nzambi Corona (type-2) mostly predates formation of the regional plains. There is no evidence for the formation of the corona on the background of rift formation. Formation of the concentric ridges after Pwr formation apparently took place on the background of regional compression by wrinkle ridges formation. This corona has a large amount of associated volcanism that is displayed in formation of long lobate lava flows (Pl unit) that are radiating from the nova rise. Corona #2 (Fig. 8a and f) is not well prominent either structurally or in the regional topography. More or less clear traces of the corona formation can be seen only in the formation of concentric compressional ridges that are morphologically similar with the regional wrinkle ridges. Formation of this corona apparently postdates the regional plains. 5.2. Coronae in young rift zones Three regions were chosen for the detailed mapping of coronae in young post-Pwr rift zones (2-1, 2-2 and 2-3 in Fig. 2). These areas included 17 coronae (12 coronae of type 1 and five coronae of type 2). 5.2.1. Coronae in the northern part of Hinemoa Planitia, Hecate Chasma (V-28 quadrangle) The geologic map of the region is shown in Fig. 9b and the stratigraphic sequence is described in Fig. 9f. Six coronae from the list (Stofan et al., 2001) are located in this area: five type 1 coronae—(1) Ak-Ene Corona (9.51N, 254.51E), diameter 150 km, (2) unnamed corona (111N, 248.51E), diameter 300 km (#1 in Fig. 9a and f); (3) unnamed corona (13.51N, 2531E), diameter 200 km (#2 in Fig. 9a and f); (4) unnamed corona (141N, 154.51E), diameter 125 km (#3 in Fig. 9a and f); (5) unnamed corona (141N, 256.51E), diameter 180  125 km2 (#4 in Fig. 9a and f) and one type 2 coronae—(6) unnamed corona (12.51N, 254.61E), diameter 245 km (#5 in Fig. 9a and f). Young (post-regional plains) rift zone, that was mapped by Cherkashina et al. (2004), is located in NW part this area (2-1 in Fig. 2) with strike to NE. Formation of this rift zone postdates the Pwr formation. Ak-Ene Corona (type-1) formation started after the emplacement of the material within Fb unit, before the formation of Pwr. After that corona was embayed by the regional plains and deformed by a wrinkle ridges network. Sources of lobate lava flows are partly inside the corona, but most of them are located in Nipa Tholus to the southeast from the corona. There are five unnamed coronae within this mapped region. Of these, four predate the formation of the regional plains. Corona #1 (Fig. 9a and f; type-1) predates completely the formation of the regional plains. Concentric extensional fracturing deforms Fb unit, which is in turn embayed by the Pwr. There is part of the Fb unit also approximately radial fracturing to the SE from corona, but it may have no association to corona and could be regional. Also, a young rift cuts the corona at NW flank. Volcanic features have no distinguishable association with the corona. Corona #2 (Fig. 9a and f, type-1) was formed in conditions of regional

compression as a part of ridge belt and completely predates formation of Pwr. No volcanic edifices are observed in the corona area, and it has no genetic association with young rift zone. Corona #4 (Fig. 9a and f) also completely predates Pwr unit. It is embayed by the regional plains and is deformed by wrinkle ridges. No corona-related volcanic edifices are observed. In addition there is no influence to the rift zone that cut the corona with W–E strike. This corona is embayed by the regional plains and also predates the formation of young rift zone, and has no associated volcanism. Corona #5 (Fig. 9a and f) is poorly prominent in tectonics and in its relief. Just small piece of Fb unit is observed in the southwestern part of the corona. The unit is deformed by dense fracturing that may be hardly considered as concentric to corona. This corona-like feature predates the regional plains, and is deformed by the rift zone that has no influence on its shape. Clusters of shield volcanoes are observed inside the corona-like feature, but they have no distinguishable connection with its formation because same clusters are located also rather far from the corona. One of the unnamed coronae within the region (#3 in Fig. 9a and f) postdates Pwr formation. Visible activity of the corona postdates the emplacement regional plains, but the formation of the corona may have concurrent with the formation of wrinkle ridges that form radial features of corona. Thus, corona may have been formed on the background of the regional compression. If the formation of the corona took place simultaneously with the wrinkle ridges formation (that means it is close to time of regional plains emplacement (Collins et al., 1999)) and has no influence of young rift zone, it predates the post-plains rift zone.

5.2.2. Coronae to the west of Hinemoa Planitia (V-40 quadrangle) Six coronae from the list (Stofan et al., 2001) are located in this area (Fig. 10): three type 1 coronae—(1) Javine Corona (5.51S, 251.21E), diameter 450 km, (2) Viardot Patera corona (7.51S, 2541E), diameter 60 km; (3) Tenisheva Patera corona (1.51S, 2551E), diameter 109 km and three type 2 coronae—(4) unnamed corona (2.21S, 248.31E), diameter 209 km (#1 in Fig. 10a and h); (5) unnamed corona (1.41S, 251.41E), diameter 118 km (#2 in Fig. 10a and h); (6) unnamed corona (3.51S, 254.61E), diameter 189 km (#3 in Fig. 10a and h). Young (post regional plains) rift zone can be seen as a broad zone that cross the mapped area (2-2 in Fig. 2) from W to E, with the strike of rift fracturing from SE to NE. Formation of this rift zone postdates Pwr formation, because there are no clear evidence for its activity before Pwr unit formation. Javine Corona (type-1) is the most complex corona in the region. There are two centers of radiation of fracturing inside the corona core—in central part and at southeastern part of the core. Second generation of radial fracturing took place after the formation of Fb2 and Psh1 materials. It is possible, that this secondary fracturing is evidence of renewal or increase in activity (Aittola and Kostama, 2002) in these locations of the corona. After formation of the regional plains this corona was still active. Massive volcanism is associated with the corona: there are a cluster of shield volcanoes in the middle part and extensive lobate lava flows mostly in the central parts and at the southwestern, southern and southeastern flanks of the corona. Formation of the lobate lava flows, the last generation of radial fracturing and thrusts were formed simultaneously. Viardot Patera corona (type-1) completely predates emplacement of regional plains and has no connection with the young rift zone. Associated volcanism is questionable. The status of Tenisheva Patera corona (type-1) is similar. Fractures that postdate Pwr and cut the young interior of the corona are all rift related and the rift has no influence on the morphology and

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tectonics of the corona. The associated volcanism is presented by clusters of shield volcanoes. Corona #1 (type-2, Fig. 10a and h) is located on the edge of a young rift zone and is not very prominent in tectonics or in its relief. The tectonic structures of this corona-like feature were formed after the formation of regional plains by wrinkle ridges. The local rift has no influence on the corona and the corona has no association to the rift. The associated volcanism of this corona looks problematic, but it is probable that the mapped shield plains as well as smooth and lobate plains are corona-related. Corona #2 (type-2, Fig. 10a and h) is not well distinguishable in its tectonics; there are just few concentric extensional fractures that postdate the formation of the Pwr. The rift branch located to the south from the corona, has no influence and no association with the structure. The volcanism of the corona is also problematic, but the shield plains as well as smooth and lobate lava plains may be associated with the corona. Unnamed corona #3 (type-2, Fig. 10a and h) was tectonically active mostly before the formation of plains, partly on background of regional extension. Just a few traces of tectonic activity after emplacement of regional plains is observed (concentric ridges), which may have a regional character. The corona associated volcanism includes clusters of shield volcanoes that postdate and predate the Pwr, and smooth-/lobate lava plains postdating the Pwr.

5.2.3. Coronae in the eastern part of Themis Regio (V-53 quadrangle) Five coronae from the list (Stofan et al., 2001) are located in this area (Fig. 11): Four type 1 coronae—(1) Shulamite Corona (38.81S, 284.51E), diameter 275 km, (2) Latta Corona (38.71S, 2871E), diameter 225 km; (3) Zywie Corona (38.61S, 291.21E), diameter 200 km; (4) Ukemochi Corona (391S, 296.11E), diameter 325 km and one type 2 corona—(5) unnamed corona (40.61S, 288.21E), diameter 155 km (#1 in Fig. 11a and f). Young (post regional plains) rift zone is located in belt-like zone that cross mapped area (2-3 in Fig. 2) from W to E, strike of rift fracturing is various from SE to NE. Formation of this rift zone postdates the formation of Pwr unit. Geologic map of the region in shown in Fig. 11b and the stratigraphic sequence is presented in Fig. 11f. Shulamite Corona (type-1) tectonics mostly predates, but the less dense radial fracturing postdate regional plains. Most of tectonic activity predates the rift formation. The rift influences the corona by producing concentric extensional fractures, but there is no genetic association between the corona and the rift, because most of the corona-related tectonics predates the Pwr. Corona volcanism is dominated by the voluminous, extensive post regional plains lobate lava flows. Latta Corona (type-1) is composed of rift tectonics and topographic features, rift-related dense extensional fracturing and topographical troughs that surround the corona rise. This indicates that the corona postdates regional plains formation. There are two observed generations of smooth lava plains (Ps1,3 units) and a possible Pl unit, which may, however, have the main source in Shulamite Corona. Zywie Corona (type-1) postdates regional plains emplacement, because there is no evidence for activity of this corona before the plains formation. The corona is composed of rift fractures and troughs that outline the corona rise. The corona has a genetic association with the young rift. There are also signs of associated volcanism: two generations of smooth lava plains (Ps2,3 units) and extensive lobate lava flows (Pl unit) are observed. Tectonic activity of Ukemochi Corona (type-1) started before the regional plains. All post-Pwr corona-related tectonic activity belongs to the rift structures, which outline corona at the southern flank. The young rift fracturing influence the corona tectonic

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and topography, but the formation of the corona has no clear genetic connection with the rift because corona predates the rift formation. There are voluminous eruptions of extensive lobate lava flows that radiate from the corona rise, clearly postdating the emplacement of the regional plains. The unnamed corona #1 (type-2, Fig. 11a and f) is located to the south from a young rift zone. Formation of this corona predates as well as postdates the formation of regional plains, but the post-plains stage of corona evolution was less prominent and it occurred on the background of the regional compression. The rift does not have a genetic association with the corona. PrePwr shield volcanoes and post-plains clusters of shield volcanoes and smooth lava plains present the volcanism associated with the corona. 5.3. Coronae without spatial association with rift zones Two places of detail mapping of coronae without any rift zones were selected (3-1 and 3-2 in Fig. 2), they include 11 coronae (five of type 1 and six of type 2). 5.3.1. Coronae in the western part of Eistla Regio (V-20 quadrangle) The general view and the geologic map (Fig. 12a and b) shows five coronae from the list (Stofan et al., 2001) that are located in this area: one type 1 coronae—(1) Changko Corona (10.91N, 6.21E), diameter 200 km and four type 2 coronae—(2) unnamed corona (12.71N, 1.31E), diameter 109 km (#1 in Fig. 12a and h); (3) unnamed corona (11.71N, 2.11E), diameter 100 km (#2 in Fig. 12a and h); (4) unnamed corona (11.61N, 3.31E), diameter 109 km (#3 in Fig. 12a and h); (5) unnamed corona (8.31N, 71E), diameter 491 km (#4 in Fig. 12a and h). There are no rift zones in studied area (3-1 in Fig. 2). Changko Corona (type-1) was formed after the formation of regional plains. There is no evidence of any activity of the corona before the formation of Pwr unit. The corona has no association with any rift zone. Volcanism of the corona is represented by lobate lava flows that also postdate Pwr. The coronae #1, #2, #3 and #4 (type-2, Fig. 12a and h) all more or less completely predate formation of the regional plains. They have no observed association with rift structures. Corona-associated volcanism is absent in all cases, except for some shield volcanoes (#1 and #4). 5.3.2. Coronae in the northern part of Ganiki Planitia, NW part of Ulfrun Regio (V-15 quadrangle) Geologic map of the region in shown in Fig. 13b and the stratigraphic sequence is described in Fig. 13f. Six coronae from the list (Stofan et al., 2001) are located in this area (Fig. 13): four type 1 coronae—(1) Tituba Corona (42.51N, 214.71E), diameter 250  150 km2; (2) unnamed corona (41.41N, 217.51E), diameter 180 km (#1 in Fig. 13a and f); (3) unnamed corona (431N, 2191E), diameter 270 km (#2 in Fig. 13aand f); (4) unnamed corona (41.71N, 2221E), diameter 160 km (#3 in Fig. 13a and f) and two type 2 coronae— (5) unnamed corona (40.41N, 212.31E), diameter 136 km (#4 in Fig. 13a and f); (6) unnamed corona (38.71N, 2181E), diameter 173 km (#5 in Fig. 13a and f). There are no rift zones in studied area (3-2 in Fig. 2). Tituba Corona (type-1) is deformed by a network of wrinkle ridges that compose some concentric features to east and west from the corona, which, however, have regional characteristics and do not have direct connection to the corona formation. Associated volcanism is represented by the cluster of shield volcanoes in the central part of the corona. The corona almost completely predates regional plains emplacement and has no association with the rift. There is some associated volcanism in

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the form of shield volcanoes. Some tectonic activity may be displayed on the background of wrinkle ridges formation. Corona #1 (type-1, Fig. 13a and f) almost completely predates regional plains, just small part of tectonic features may be evidence of possible traces of activity which was simultaneous with the wrinkle ridges formation. The corona has no genetic association with the rift, and no volcanic activity is observed. The unnamed corona #2 (type-1, Fig. 13a and f) started to form before the formation of regional plains by the formation of dense concentric fracturing that deformed Fb unit. A network of ridges (W, SE, NW flanks) makes up the radial and concentric set of features relative to the corona. This may be interpreted as evidence that the part of tectonic activity of corona may have been displayed in ridges formation on background of regional compression. Corona has no association with rift, but several indications of associated volcanism: Pre-Pwr shield volcanism as well as post-Pwr smooth plains and lobate lava flows. Majority of the tectonic activity of the unnamed coronae #3 and #4 (type-1 and 2, Fig. 13a and f) predate the regional plains formation, but they were still active after the plains formation. Both coronae have no association with the rift, and their associated volcanism is represented by smooth and lobate lava plains. Corona #5 (type-2, Fig. 13a and f) both pre- and postdates the regional plains formation. Some stages of the corona evolution took place on the background of regional compression that formed regional wrinkle ridges. Corona associated volcanism includes pre-Pwr shield fields as well as post-Pwr smooth and some lobate lava plains.

Fig. 14. Age relationship of coronae formation studied and regional plains with wrinkle ridges (Pwr) emplacement. Type 1 coronae (82 structures): 96% started to form before Pwr, 31% were tectonically active after Pwr, only 4% started to form after Pwr. Type 2 coronae (22): all of them started to form before Pwr, 18% were tectonically active after Pwr formation. In total (104): 97% of coronae started to form before Pwr, 28% were tectonically active after Pwr, only 3% started after Pwr.

most active before regional plains emplacement, only 4% started to form after regional plains formation. Type 2 coronae (22): all of them started to form before regional plains, 82% were most active before regional plains formation. 6.2. Time relationship of coronae with rift zones activity The relationship of coronae with old and young rift zones was also analyzed on the base of described in the methodical chapter criteria (Fig. 15). We subdivided all the studied coronae into two groups (Tables 2–4):

6. Summary of key observations 6.1. Period of coronae tectonic activity We estimated the age of all the studied coronae (20% of total population of 514 coronae (Stofan et al., 2001), 104 coronae in total) similarly as we did for the 34 coronae in the seven mapped areas (Fig. 14, Table 1) and subdivided all coronae into four categories (Table 1) based on the relationship of any corona-related tectonic activity with the position of regional plains. We used the period of formation of these structures as the primary characteristic for the definition of age of corona. Within the stage of intensive tectonic processes that form deformational features during corona evolution, corona should be most dynamically active. The studied 104 coronae were subdivided into four classes (Table 1): (1) Coronae that tectonically predate the regional plains: all tectonic structures are embayed by regional plains (46 coronae, 44%). (2) Coronae which have tectonic activity that mostly predates the regional plains; small amount of tectonic features were formed after plains formation (29 coronae, 28%). (3) Coronae which show tectonics that both predates and postdates regional plains, approximately in equal proportion (26 coronae, 28%). (4) Coronae where tectonics postdates the regional plains: there are no traces of corona tectonic activity before regional plains formation (3 coronae, 3%). However, there is of course the possibility that pre-regional-plains activity may have occurred, but it is effectively covered by the regional plains cover. Because a single corona may have had a long period of formation, the most important phase for us is the most active period of them. Thus we have merged the two first types in our analysis and summarized their amount as coronae that were more active before regional plains formation. In total 97% of coronae started to form before regional plains formation, and 72% of the coronae were most active before plains emplacement, 3% of coronae have no traces of activity before Pwr. Type 1 coronae (82 structures): 96% started to form before regional plains, 69% were

(1) Coronae and rifts that were formed simultaneously and have a genetic association. (2) Coronae that predate or postdate rift zones formation and do not have a genetic association with rift formation. Type 1 coronae (82 structures): 10% are associated with the young rift zones and 16% are associated with the old rift zones. In total 26% of type 1 coronae have association with rift zones and 74% are located outside of a rift or predate/postdate rift zones formation. Type 2 coronae (22 structures): There are no coronae associated with the young rift zones while 14% are connected to the old rift zones. Altogether 14% of type 2 coronae have association with rift zones while 86% are located outside of rift or predate/postdate rift zones formation. Therefore, from all the coronae studied, 8% have genetic association with young rift zones, 15% have genetic association with old rift zones, indicating that 23% have genetic association with rift zones and 77% are located outside of rift or predate/postdate rift zones formation. 6.3. Type of corona-related volcanism Based on our observations of all 104 studied coronae (Table 5), shield volcanoes and/or their clusters (Psh unit) associate with 77% of coronae of both types, 72% of coronae have shield volcanoes that predate formation of regional plains and 10%—shield volcanoes that postdate these plains. Lobate lava flows (Pl unit) formation is associated with 47% of all coronae and smooth volcanic plains (Ps unit) with 40% of the coronae. Type 1 coronae: shield volcanoes are associated with 74% of coronae, 68% of all structures have shield volcanoes that predate formation of regional plains and 11% have shield volcanoes that postdate these plains. Lobate lava flows formation (Pl unit) is associated with 50% of all coronae, 44% of coronae have smooth volcanic plains (Ps). Type 2 coronae: shield volcanoes are associated with 86% of coronae, 86% of coronae have shield volcanoes that predate

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Fig. 15. Relationship of coronae and rift zones activity. From all the coronae studied, 8% have genetic association with post regional plains (young) rift zones, 15% have association with rift zones predating regional plains (old), indicating that 23% have genetic association with rift zones and 77% are located outside of rift or predate/ postdate rift zones formation. Table 3 Time relationship of type 1 coronae activity (Stofan et al., 2001) with rift zones formation. Coronae, type 1 (total 82 structures)

49 Coronae (60%) are located inside rift 30 Coronae (37%) in old rift zones 17 Coronae (21%) predate rift zones zones 13 Coronae (16%) were formed simultaneously with rift zones 19 Structures (23%) in young rift 11 Coronae (13%) predate rift zones zones 8 Coronae (10%) were formed simultaneously with rift zones 33 Coronae (40%) are located outside of rift zone

Table 4 Time relationship of type 2 coronae activity (Stofan et al., 2001) with rift zones formation. Coronae, type 2 (total 22 structures)

8 Coronae (36%) are located inside rift zone

5 Coronae (23%) in old rift zones

3 Structures (14%) in young rift zones

2 Coronae (9%) predate rift zones 3 Coronae (14%) were formed simultaneously with rift zones 3 Coronae (14%) predate rift zones 0 Coronae (0%) were formed simultaneously with rift zones

14 Coronae (64%) are located outside of rift zone

Table 5 Style of corona-related volcanism and relationship of position of shield volcanoes and their clusters with position of regional plains.

Coronae, type 1 (82 structures) Coronae, type 2 (22 structures) Coronae, type 1 1 and 2 (104 structures)

Shield volcanoes activity

Shield volcanoes activity that predate Pwr

Shield volcanoes activity that postdate Pwr

Smooth plains activity

Lobate lava flows activity

61 Coronae (74%)

56 Coronae (68%)

9 Coronae (11%)

36 Coronae (44%)

41 Coronae (50%)

19 Coronae (86%)

19 Coronae (86%)

1 Corona (5%)

6 Coronae (27%)

8 Coronae (36%)

80 Coronae (77%)

75 Coronae (72%)

10 Coronae (10%)

42 Coronae (40%)

49 Coronae (47%)

formation of regional plains and 5% have shield volcanoes that postdate these plains. Lobate lava flows (Pl unit) are associated with 36% of all coronae and smooth volcanic plains (Ps unit) with 27% of coronae.

7. Conclusions 7.1. Relationship between coronae and rifts Previous studies have proposed that the formation of coronae may have a relationship with the formation of rift systems (e.g., Solomon et al., 1992; Baer et al., 1994; Stefanick and Jurdy, 1996; Hamilton and Stofen, 1996; Krassilnikov and Head, 2003; Martin et al., 2007). However, our new observations actually show that the majority (77% of all studied coronae) of the coronae of both types (Table 2 and Fig. 15) do not have genetic associations with the rifts. This result implies that coronae-producing mantle

diapirism (e.g., Squyres et al., 1992; Stofan et al., 1992; Janes et al., 1992) and uplift of mantle material in rift zones (e.g., Shelton and Tullis, 1981; Wilcock and Whitehead, 1991; Condie, 2001; Whitehead, 1982) are not well correlated in most cases at least in time. Some of the coronae (23%) have genetic association with the rift formation, which shows, that this part of the structures may be formed due to the prominence of both types of mantle material uplift—isometric corona-related on background of rift-related linear uplift. The same geodynamic model has been developed previously for novae (Krassilnikov and Head, 2003). This study also confirms that coronae and rifts are indeed longlived structures (i.e., Basilevsky and Head, 2000b; Stofan et al., 2005; Ivanov and Head, 2010b). Majority of the coronae (72%) were most active before the formation of the regional plains with wrinkle ridges, and only 3% begun to form after the plains emplacement or at least show only signs of post-plains activity (Fig. 14). That may have two reasons—decreasing of corona-producing diapirism

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activity with time and/or the thickening of the Venusian lithosphere with time, which is predicted and discussed by different authors (e.g., Parmentier and Hess, 1992; Turcotte, 1995; Phillips and Hansen, 1994; Brown and Grimm, 1999). The evolution of distribution of rift systems with time (decreasing of distribution and localization of rift zones) is also evidence for this model of Venus evolution caused by lithosphere thickening. Our observations also showed that rift-produced uplift zones were probably mostly stable at least from the period of formation of regional plains. According to our observations type 2 coronae are in general older than type 1 coronae (Fig. 14) and less prominent in tectonics and relief, supporting the earlier observations (i.e., Stofan et al., 2001; Glaze et al., 2002). That may indicate that the type 2 coronae may be more embayed by the later volcanic events, e.g., regional plains formation and/or more gravitationally relaxed than the type 1 coronae. 7.2. Coronae and coronae-related volcanism The most active volcanism of coronae may not be strongly correlated with time of most of the tectonic activity of coronae. Majority of coronae were most active before the regional plains formation, but almost half of all coronae have traces of post regional plains volcanism. It again shows that coronae are longlived structures with complex evolution and some of evolution processes (e.g., tectonics and volcanism) may be distributed in time. Three types of volcanic activity connected to coronae were observed; shield volcanoes (separate volcanoes and their clusters) as well as extensive lobate lava flows that are often merged with smooth volcanic plains. Shield volcanoes during coronae evolution were mostly active before regional plains emplacement. The position of the lobate and/or smooth plains may not be an evidence for some volcanic evolution, but decreasing of shield volcanoes activity with time supports the model of ‘‘directional’’ model of evolution of Venus by Basilevsky and Head (1995a,b, 1998a,b, 2000a, 2002), this is in correspondence with the global study of shield volcanoes on Venus (Ivanov and Head, 2004).

Acknowledgments The authors wish to thank Alexander Basilevsky and one anonymous reviewer for their work in reviewing this paper. We also acknowledge the RFBR Grant #02-05-65068, Russian Science ¨ al ¨ a¨ Support Foundation (AK), Academy of Finland (VPK) and Vais Foundation (MA) for the support in this study.

Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.pss.2011.11.017.

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