Icarus 203 (2009) 337–351

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Icarus

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Venus astra/novae: Estimates of the absolute time duration of their activity

A.T. Basilevsky a,c, M. Aittola b, J. Raitala b, J.W. Head c,*

a Vernadsky Institute of Geochemistry and Analytical Chemistry, RAS, Moscow, Russiab Department of Astronomy, Oulu University, Finlandc Department of Geological Sciences, Brown University, Providence, P.O. Box 1846, RI 02912, USA

a r t i c l e i n f o

Article history:Received 28 April 2008Revised 14 April 2009Accepted 17 April 2009Available online 30 May 2009

Keywords:VenusVenus, SurfaceTectonicsVolcanism

0019-1035/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.icarus.2009.04.035

* Corresponding author.E-mail addresses: atbas@geokhi.ru (A.T. Basilevs

(J.W. Head).

a b s t r a c t

In order to assess the age relations between astra/novae (features with extensive radial fracture-grabensystems) and their surroundings, and to determine the duration of their activity, we undertook a photo-geologic analysis of Magellan images of 78 astra, 49 dark-parabola craters and 114 clear-halo craters. Forseven of these 78 astra it was found that the astrum-forming faulting started before or close to the time ofemplacement of regional plains and extended into the second part of post-regional-plains time. Becausethe mean age of the regional plains is close to the mean surface age of Venus (which is estimated to be�750 m.y), this means that the duration of activity of these seven astra was several hundred millions ofyears. This is longer than the duration of activity of ongoing mantle plumes on Earth, but shorter than theduration of activity of the plume feeding the martian volcano Olympus Mons. The basic morphologiccharacteristics of these seven astra, as well as their age relations with other geologic units, are similarto those of the majority of other astra; therefore, such a long duration could also be typical of some otherastra. We confirm the two-phase (pre- and post-regional-plains) evolution of astrum-forming faultingsuggested in previous studies. For the first phase we show evidence for purely tectonic faulting causedby the diapiric rise of a mantle plume. For the second phase we find evidence supporting the interpreta-tion of other studies that the observed faults resulted from subsurface dike intrusions produced by mag-matism associated with the plume. We also found that faulting during the second phase was notinstantaneous but distributed over a long period of time.

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1. Introduction

Astra, sometimes called novae, are radial (stellate) fracture cen-ters typically 100–300 km in diameter identified (although by dif-ferent feature names) in early analysis of the Magellan images ofVenus (e.g., Head et al., 1992; Janes et al., 1992; Stofan et al.,1992; Squyres et al., 1992). They are often present as the internalpart of coronae and considered by some researchers as a possiblephase of corona formation and/or evolution (e.g., Hansen et al.,1997; Stofan et al., 1997; Crumpler and Aubele, 2000; Krassilnikovand Head, 2003; Aittola, 2003; Aittola and Kostama, 2002; Aittolaand Raitala, 2007). Astra often show evidence of associated volca-nism and are considered to form (like coronae) as a result of theemplacement of hot mantle diapirs (plumes) into the lithosphere(e.g., Janes et al., 1992; Stofan et al., 1992, 1997; Hansen et al.,1997). Many astra (again like coronae) show evidence that theystarted to form early in the morphologically observable part ofthe geologic history of Venus, before the emplacement of wrin-kle-ridged regional plains, and continued their evolution into

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ky), james_head@brown.edu

post-regional-plains time (Basilevsky and Raitala, 2002; Krassilni-kov and Head, 2003; Aittola and Raitala, 2007).

In this work we assess semi-quantitatively the absolute lifetimeof astra from the most ancient recognizable phases of their evolu-tion up to the most recent ones. For this analysis we use two kindsof absolute time markers. One of these is the estimate of the meanage of the wrinkle-ridged regional plains that has been found to beclose to the mean surface age of Venus (T) (Basilevsky and Head,1998, 2000a; Ivanov, 2008). The best estimate of T is 750 m.y. withany age between 300 m.y. and 1 b.y. considered to be acceptable(McKinnon et al., 1997). This is in agreement with earlier estimatesof Phillips et al. (1992) and Strom et al. (1994). Regional plainswere emplaced during a rather short time period, probably notlonger than 0.1–0.2T (see Discussion in Basilevsky and Head,2006, paragraphs 56–58 and Collins et al., 1999). If we see thatsome structures, in particular astra, show evidence of burial orembayment by regional plains, we may conclude that these struc-tures are older than time T ago.

Another time marker used is the degree of preservation of cra-ter-associated radar-dark halos. It has been shown in a number ofstudies that craters having radar-dark parabolas (DP) are the youn-gest on Venus and formed more recently than �0.1–0.15T ago. Cra-ters having a clear (but not parabolic) halo (CH) formed more

338 A.T. Basilevsky et al. / Icarus 203 (2009) 337–351

recently than�0.5T ago (but before 0.1–0.15T ago) and craters hav-ing a faint halo (FH) or no halo (NH) formed before �0.5T ago (e.g.,Arvidson et al., 1992; Campbell et al., 1992; Izenberg et al., 1994;Basilevsky and Head, 2002; Basilevsky et al., 2003). If astrum-re-lated faults cut craters with an associated clear-halo (CH) ordark-parabola (DP) we conclude that the astrum-related faultactivity continued through the time of �0.5T ago and �0.1–0.15Tago, respectively. In our study we undertook morphologic analysisusing Magellan F-Map images (maximum resolution is 75 m/px)downloaded from http://www.mapaplanet.org/ and for the DP-CH-FH-NH crater classification we used the database describedby Basilevsky and Head (2002) and Basilevsky et al. (2003). Tomake altitude profiles through the studied astra was used theGTDR data (Magellan CD ROM MG_3003 GTDR.3;2) with verticalresolution �100 m resampled to a 5 km spatial pixel size (Fordand Pettengill, 1992).

2. Observations and analysis

In this study we undertook a search for astra showing (1) clearage relations between their most ancient phases and the surround-ing regional plains, and (2) having in their vicinities craters thathave been cross-cut by astrum-related faults. For each of thefault-crossed craters, we determined to which class of the DP–CH–FH–NH crater classification system it belonged. If we observedevidence that an astrum started to form before, or close to, the timeof emplacement of regional plains, but its youngest faults cut CH orDP craters, we concluded that its activity lasted from �T agothrough 0.5T or 0.1–0.15T ago, respectively. In most cases we didnot study the age relations of astrum-related faults with FH andNH craters because these relations do not put significant con-straints on the duration of astrum evolution.

In our descriptions and analyses we use terms for material andstructural units as they were identified and described by Basilev-sky and Head (1998, 2000a) and Basilevsky and McGill (2007).From the oldest to the youngest the material units are: (1) tesseraterrain material (tt), (2) material of densely fractured plains (pdf),(3) material of fractured and ridged plains (pfr) typically formingso-called ridge belts, (4) material of shield plains (psh), (5) materialof plains with wrinkle ridges (pwr), (6) materials of lobate plains(pl), and (7) and material of smooth plains (ps). We sometimes de-scribe the suite of psh and pwr plains as ‘‘regional plains”; whilethe pwr plains typically postdate psh, in some localities psh is

Table 1Summary of characteristics of the seven astra studied.

Wohpe Astrun CoronaJunkgowa

Corona Gertjon

Number on the Fig. 8 1 2 3Coordinates of the

astrum center41.5�N, 288�E 37�N, 257�E 29�S, 277.5�E

Distance reached byfaults from astrumcenter (km)

Most 400–600,some up to1000–1500

Most 200–400,some up to1000–1200

Most 150–300 km

Base to summit height(km)

2 1 1.5

Old astrum faulting Predatesemplacement ofpwr plains

Predatesemplacement ofpwr plains

Predates or close toemplacement of pwplains

Young astrum faulting Postdates CHcrater Zvereva

Postdates CHcrater Gentileshi

Probably postdatesCH crater Kitna

Observed associatedvolcanism

pwr abundant pwr2 abundant,pl minor

pl moderate

Time durationestimate

From >T agothrough 0.5T ago

From >T agothrough 0.5T ago

From �T ago through0.5T ago

the younger unit. In addition to these material units, two structuralunits are used in our descriptions and analysis: (1) fracture belts,and (2) rifted terrain. Fracture belts, usually considered as old rifts(Hansen et al., 1997; Basilevsky and Head, 2000b), postdate thematerial unit pfr and are partly overlapped in time with unitspsh and pwr. Rifted terrain, sometimes simply called rifts, post-dates units psh and pwr and is broadly contemporaneous withunits pl and ps.

In this analysis we studied images of all 78 astra listed by Crum-pler and Aubele (2000), Aittola (2003), Aittola and Kostama (2002)and studied images of all 49 DP craters and images of all 114 CHcraters with diameters P15 km. When a DP or CH crater was cutby fault(s) we traced the source of the fault(s). This procedure re-sulted in finding 2 DP and 5 CH craters cut or apparently cut byfaults radiating from 7 astra (Table 1). We were able to determinethat the early faults of six of these astra predated emplacement ofwrinkle-ridged regional plains. The faults of the seventh astra post-dated their emplacement but predated emplacement of wrinkleridges deforming these plains. In the first six cases we concludedthat the astra started to form before time T. In the seventh casewe concluded that this astrum started to form close to time T.The latter conclusion is based on results of McGill (2004a) and Bas-ilevsky and Head (2006) who showed that emplacement of wrinkleridges happened within the first 10–15% of the time following theemplacement of the plains. Below we describe these seven astraand their relations with regional plains and with dark-parabolaor clear-halo craters.

2.1. Wohpe Tholus Astrum

There is a prominent structure consisting of radiating faults ofat least two generations centered at Wohpe Tholus, a small(�40 km across) and low (a few hundred meters) hill (Fig. 1). Weinformally call this structure Wohpe Astrum. Located in GuineverePlanitia adjacent to Beta Regio it is centered at 41.5�N, 288�E(Fig. 1). Its position was mapped (among 163 others) by Grosfilsand Head (1994a), described under the name R3 among other‘‘radiation centers” of Venus by Ernst et al. (2003) and later studiedin the process of 1:5,000,000 geologic mapping of the V17 Beta Re-gio quadrangle (Basilevsky and Head, 2007; Basilevsky, 2008).

The older generation of astrum faults is confined by the�600 km diameter discontinuous annulus composed of the geo-logic unit of densely fractured plains (pdf). This unit is character-

Corona Minona Jokwa Linea CoronaAudhumla

Becuma Mons

4 5 6 723.5�N, 218.5�E 14.1�S, 205.5�E 46.2�N 11.8�E 34�N, 22�E

Most 50–100,Some up to 200

Most 100–200,Some up to 300–400

Most 200–300 Most 50–100

2 2 0.5 1.5

rPredatesemplacement ofpwr plains

Predatesemplacement ofpwr plains

Predatesemplacement ofpwr plains

Close toemplacement ofpwr plains

Postdates DPcrater Boleyn

Postdates CHcrater Fouquet

Probablypostdates CHcrater Kemble

Possiblypostdates DPcrater Noreen

pl moderate pl moderate pl minor None

From >T agothrough 0.1–0.15T ago

From >T agothrough 0.5T ago

From >T agothrough 0.5T ago

From �T agothrough 0.1–0.15T ago

Fig. 1. Wohpe Astrum. (a) Context image showing Wohpe Astrum and its surroundings, including the craters Deken and Zvereva, which are used for estimation of theduration of this astrum tectonic activity; double-headed arrows show lines of topographic profiles; (b) Faint-halo crater Deken; 1—a wrinkle ridge deforming the crater floor,2—a fault radiating from Wohpe Astrum and partly buried by a Deken crater ejecta outflow, 3—a fault radiating from Wohpe Astrum and cutting Deken crater ejecta; (c)Central part of Wohpe Astrum; 1—young post-pwr faults, pdf—the older astrum-related fracturing, inlet shows structural map of the astrum, hereafter thinner black linesdesignate the younger astrum faults, thicker black lines designate the older faults; (d) clear-halo crater Zvereva; 1—faults radiating from Wohpe Astrum; (e) topographicprofiles, black arrows show location of astrum center, where the two profiles intersect.

A.T. Basilevsky et al. / Icarus 203 (2009) 337–351 339

ized by closely-spaced (0.5–2 km) faults arranged radially in rela-tion to Wohpe Tholus. The pdf unit and its component radial frac-tures on it are embayed by wrinkle-ridged shield plains and byplains with wrinkle ridges (Fig. 1c). This observation suggests thattectonic activity at Wohpe Astrum started before emplacement ofthese plains.

The younger radiating faults cut materials of psh and pwr plainsboth inside and outside of the annular pdf zone. The pdf material is

also cut by these younger faults. The younger faults are narrow(typically less than 1 km wide) and several tens to several hun-dreds of kilometers long. Some of these faults can be traced for dis-tances up to 1000–1500 km from the astrum center. Spacing of theyounger faults varies from a few km just beyond the pdf annulus toa few tens of kilometers and larger further outside (Fig. 1c).

As mentioned above, this astrum is centered on Wohpe Tholus,a relatively low and gently-sloping hill. It is circular in planimetric

340 A.T. Basilevsky et al. / Icarus 203 (2009) 337–351

view and its surface, which gradually merges into the surroundingpwr plains, is modified by concentrically arranged wrinkle ridges.It is interpreted to represent an accumulation of lavas above the lo-cal volcanic source vent, which may also have fed pwr plains of thisarea. Its position in the astrum center, and its timing (in betweenthe older and the younger episodes of the astrum faulting) suggeststhat it is genetically linked to astrum-forming activity.

There is no obvious evidence of young (younger than psh andpwr regional plains) volcanism associated with Wohpe Astrum.At the northeastern base of Wohpe Tholus are a few radar-darkflow-like features from 1 � 5 to 5 � 13 km across. They appear verysimilar to small fields of the amoeboid-type smooth plains de-scribed by Basilevsky and Head (2007) in the area west and south-west of the astrum. These smooth plains are certainly a product ofyoung volcanism, but they are probably associated with the north-ern termination of the Devana Chasma rift zone rather than withWohpe Astrum. Far to the north–east and south–west of the centerof Wohpe Astrum (1000–1500 km), in the areas which are reachedby the most distant young astrum faults, there are a few fields ofpl-type lava flows, but they are probably genetically linked to theadjacent fracture belts composing Dodola Dorsa and BlathnetCorona.

Fig. 1e shows that the highest place within the astrum is the topof the Wohpe Tholus volcanic hill (6052.2 km above the planetcenter of mass). The wrinkle-ridged plains around the hill are sev-eral hundred meters lower. The pdf annulus stands a few hundredsmeters km above these plains (see for example the NW-SE profilein Fig. 1e). The surface within it is typically inclined outwards witha gradient of 3–5 m/km. Beyond the pdf annulus, the psh + pwrplains lie at the 6050.5–6051 km level, and are slightly inclinedoutside the astrum center with a mean gradient of about 1–2 m/km. The observation that the pdf annulus stands above the pwr-plains located inside it probably indicates that at the time of pdfformation the central part of Wohpe Astrum was higher in altitudethan now observed, and that it then subsided and was flooded bypwr plains.

A few of the younger faults approach the 48-km crater Deken,which has a faint radar-dark halo (Fig. 1b). Deken crater is super-posed on pwr plains and its floor is deformed by a wrinkle ridge,whose orientation is consistent with the orientation of wrinkleridges in the crater vicinity (arrow 1 in Fig. 1b). Therfore, crater for-mation postdated the emplacement of pwr plains but predated thedeformation of these plains by the wrinkle ridge network. As re-ported by McGill (2004a) and Basilevsky and Head (2006), wrinkleridges were emplaced within the first 10–15% of post-pwr-plainstime; therefore, Deken crater probably formed early in this timeperiod. This is consistent with observation that radar-dark haloassociated with Deken crater is faint. Deken crater has hummockyejecta and ejecta outflows. Southwest of the crater is seen an ejectaoutflow that fills shallow criss-crossing graben and thus acquires across-like planimetric shape. One of the faults propagated fromWohpe Astrum is buried by this ejecta outflow (arrow 2 inFig. 1b) while another fault cuts the outflow (arrow 3 in Fig. 1b).This means that some astrum-related faults here predate forma-tion of Deken crater, while other postdate it.

A few of the younger faults approach the 23-km crater Zvereva,which has a clear radar-dark halo. One of the faults cuts the crater,which is most obviously seen on the crater floor (Fig. 1d). Thisobservation suggests that the Wohpe Astrum-related faultinglasted through the time of formation of craters that still preservea radar-dark halo. In combination with the observations relatedto Deken crater described above, and to the pdf zone of the astrum,this implies that the Wohpe Astrum faulting was already occurringin pdf time, prior to the emplacement of the psh and pwr plains,and continued through the first half and then into the second halfof post-pwr-plains time.

2.2. Corona Junkgowa Astrum

A second astrum, centered at 37�N and 257�E, is also locatedclose to Beta Regio (Fig. 2). It is represented by the radial fracturingcomponent of the Junkgowa Corona, which is about 300 km indiameter and outlined by concentrically arranged wrinkle ridgesdeforming the psh and pwr plains (Fig. 2). Inside the corona locallyare seen small subareas of fracturing of the pdf type generally ar-ranged radially in relation to the corona structure. Spacing of thisold fracturing, where it is not embayed by later materials, is only0.5–2 km. The densely fractured areas are embayed by psh andpwr plains and thus represent the older stage of fracturing of thisastrum (Fig. 2e). Inside and outside the corona annulus are seennumerous faults of a few hundred meters to 1 km wide, whichcut psh and pwr plains and radiate approximately from the coronacenter. They are typically a few hundreds of kilometers long, butsome are even longer. These faults represent the younger stageof fracturing of the Junkgowa Corona astrum.

There are at least two phases of volcanism associated withJunkgowa Corona. The older phase is represented by the outskirtsof the radar-brighter wrinkle ridged plains (pwr2), which extend150–200 km outward from the wrinkle-ridged annulus of the cor-ona mostly to the north and northwest. The younger phase is rep-resented by the smaller radar-bright flow-like features (unit pl)seen in the southwestern sector of the corona (Fig. 2a).

Fig. 2f shows topographic profiles through the Junkgowa Coronaastrum structure. It is seen from these that altitudes within thisstructure are on the level of 6051–6052 km above the planet centerof mass and their areal distribution of the altitudes is rather cha-otic except its eastern flank, where there is a general inclinationto the east with a mean trend of a few meters per kilometer.

One of the younger faults of the Junkgowa astrum radiatingabout 1000 km NNE from the corona cuts the ejecta of the 20-kmcrater Gentileshi, which is superposed on regional plains withwrinkle ridges and has a clear radar-dark halo (Fig. 2b). Anotherfault, also radiating from the astrum, is covered by the crater ejecta(Fig. 2b, black arrow). This situation resembles that described pre-viously for Deken crater and faults radiating from Wohpe Astrum:the Deken crater ejecta was cut by one of the astrum faults andcovers another one. So the observations described here suggestthat early faulting of the Corona Junkgowa astrum was alreadyhappening in pdf time, that is before or close to the emplacementof psh and pwr plains, while the younger faulting extended into thesecond half of post-pwr time. The observation that one astrum-associated fault approaching Gentileshi crater predated the craterwhile another one postdated it suggests that the younger faultingdid not happen instantaneously, but rather was more widely dis-tributed during post-pwr time.

2.3. Gertjon Corona Astrum

Gertjon Corona is one structure in the chain of coronae in The-mis Regio (Fig. 3). It is a double-ring corona centered at 30�S,276�E, with an outer annulus diameter of about 250 km. The coro-na annulae are rings of deformation representing the fracture belt(fb) structural unit and the majority of these fractures are embayedby wrinkle ridged psh and pwr plains. These plains materials sur-round the corona and fill the area in between the two corona ann-ulae and inside the internal annulus. In the NE sector of the outerannulus, faults strike to the north, northeast and east, these forman astrum-like structure in conjunction with the faults of the cor-ona’s outer annulus, centered at 29�S, 277.5�E (Fig. 3a). The olderfaults, where they are not partly covered with psh and pwr mate-rials, have a 0.5–2 km spacing. The faults radiating to the north,northeast and east mostly cut the psh and pwr plains and thus rep-resent the younger component of the astrum. Their spacing is typ-

Fig. 2. Astrum that forms a component of Junkgova Corona. (a) Synoptic image of the area; white dashed oval outlines the corona; double-headed arrows show locations ofthe altimetry profiles, inlet shows structural map of the astrum; (b) Gentileshi crater, with a clear radar-dark halo; white arrows point to the fault which radiates fromJunkgova Corona and cuts the crater ejecta; black-and-white arrow points to the fault also radiating from the corona, but covered by crater ejecta; (c) Area SW of Gentileshicrater; arrows point to the same faults; (d) Area further SW; arrows point to the same faults; (e) NE part of the corona; 1—points to the older faults; (f) Topographic profiles,black arrows show location of astrum center, where the two profiles intersect.

A.T. Basilevsky et al. / Icarus 203 (2009) 337–351 341

ically a few kilometers. Close to the astrum center the youngerfaults are often buried by the lobate lava flows (unit pl) (see lower

right of Fig. 3a). These lava flows surround the astrum center andrepresent young volcanic activity associated with this astrum.

Fig. 3. Astrum that is a component of Gertjon Corona. (a) Synoptic image of the area; white dashed outline shows the outer annulus of the corona; double-headed arrowsshow locations of altimetry profiles; inlet shows structural map of the astrum; (b) area in between the two corona annulae showing old fracturing (1), wrinkle ridgesdeforming plains (2) and young fractures (3); (c) Kitna Crater, with a clear radar-dark halo cut by young faults extending from the center of radiating fractures (black arrows);white arrow designates an astrum-associated fault covered by the crater ejecta; (d) topographic profiles, black arrows show location of astrum center, where the two profilesintersect.

342 A.T. Basilevsky et al. / Icarus 203 (2009) 337–351

Fig. 3d shows topographic profiles through this structure. It isseen that altitudes within this astrum vary from 6051 and almost

6054 km above the planet center of mass. The highest point corre-sponds to the astrum center and 20–40 km outward from it the

A.T. Basilevsky et al. / Icarus 203 (2009) 337–351 343

altitudes gradually decrease by 1–3 km. However further outwardthe surface altitudes remain at approximately the same level oreven increase (to the west).

About 70 km north of the astrum center there is the 15 km cra-ter Kitna which has a clear radar-dark halo. Its ejecta may be cut byyoung faults radiating from the astrum (Fig. 3c, dark arrows), butthe cross-cutting relations here are less obvious than in the caseof the Zvereva and Gentileshi craters described above. One of thefaults north of Kitna crater, which judging by its orientation is anastrum-associated fault, is overlapped by crater ejecta (white ar-row in Fig. 3c).

Therefore, these observations suggest that early faulting of theCorona Gertjon astrum was already happening in fb time, that isbefore or close to the emplacement of the psh and pwr plains,while younger faulting extended into the second half of the post-pwr-plains time. The observation that one astrum-associated faultapproaching crater Kitna predated the crater while others post-dated it suggests that the younger faulting was not instantaneousbut rather was distributed within post-pwr time, as in the cases ofWohpe Astrum and Corona Junkgowa asrum was distributed with-in post-pwr time.

2.4. Minona Corona Astrum

Minona Corona is in the Ganiki Planitia plains, close to the wes-tern flank of the Fea Fossa-Zevana Chasma rift zone. Its90 � 130 km oval annulus, consisting of several generations of con-centric and radial faults, is centered at 23.5�N, 218.5�E (Fig. 4). Theannulus is surrounded by an extended apron of relatively small(only a few kilometers wide) coalescing lobate flows (unit pl) sug-gesting the presence of volcanic activity associated with this struc-ture. Numerous faults radiating from the corona center areobserved inside and outside the annulus; thus, one can considerthis structure an astrum.

Inside the annulus and west of it are observed plains with rarewrinkle ridges, which in the eastern part of the corona interiorintersect with radially-oriented faults (Fig. 4b). Wrinkle ridges atthese intersections show no lateral displacement, suggesting thatthese faults postdated wrinkle ridges and thus represent the youn-ger component of astrum faulting. Wrinkle ridges intersecting ra-dial faults with no lateral displacement are also locally observedin small windows of pwr plains within the apron of lobate flows(Fig. 4c–f). In general the younger component of radial faulting ofthe Minona Corona astrum is prominent. The faults are 0.5–1 kmwide, up to 100–150 km long and have a spacing from a few kilo-meters near the central part of the structure to more than 10–20 km at greater distances.

A remnant of the older pdf-like component of radial faulting ofthis astrum that is particularly well-displayed forms a�100 � 100 km cluster of radiating faults about 100 km east ofthe annulus. These faults are slightly to significantly embayed byplains with wrinkle ridges (Fig. 4a). Individual faults are 0.5–1 km wide and, because of embayment by pwr plains, only a fewtens of kilometers long. In the least-embayed places their spacingis comparable with the width of the faults.

Fig. 4h are topographic profiles through this structure, showingthat its altitudes vary from 6052 to almost 6054 km above the pla-net center of mass. The highest point corresponds to the astrumcenter and outward the altitudes gradually decrease by 1–3 km,with the gradient of a few meters per kilometer, to locally 30–50 m/km.

About 140 km northeast of the astrum center is the 70 km cra-ter Boleyn, which has a radar-dark parabola. The ejecta and floor ofthis crater are cut by several faults radiating from the astrum(Fig. 4g). Therefore, our observations suggest that early faultingof Corona Minona astrum predated emplacement of the pwr plains,

while the younger faulting extended into the latest 10–15% of post-pwr-plains time.

2.5. Astra of Jokwa Linea chain

Jokwa Linea is an approximately 200 � 1200 km area of denselyfractured plains (unit pdf) arranged as a combination of severalstellate structures, and so can be considered a chain of astra(Fig. 5a). Unit pdf and most of the astra-related faults are embayedby shield plains (psh) and plains with wrinkle ridges (pwr)(Fig. 5b), and locally by lobate flows (unit pl) (Fig. 5a). These faults,which are 0.5–1 km wide and locally subparallel, represent the old-er component of the astra. From some astra of this chain the 0.5–1 km wide faults radiate. They cut pwr and psh plains, but are cov-ered by lobate flows (unit pl). These faults represent the youngercomponent of these astra.

North of Jokwa Linea there is a 48 km crater, Fouquet, centeredat 15.1�S, and 203.5�E. It has a clear radar-dark halo and its ejecta iscut by a few faults. One of them (black arrow in Fig. 5c) radiatesfrom the astrum centered at 14.1�S, and 205.5�E, judging by its ori-entation. Another fault also probably radiates from this astrum,and appears to be covered by the crater ejecta outflows (white ar-row in Fig. 5c). The previously mentioned lobate flows are associ-ated with the northern boundary of this astrum.

Fig. 5d shows topographic profiles through this structure. Theseprofiles show that altitudes within this structure vary from 6051 to6053 km above the planet center of mass. The highest point corre-sponds to the center of the astrum; outward from the center alti-tude gradually decreases, with a gradient of 5–10 m/km withinthe pdf part of the astrum and only a few meters per kilometerwithin the adjacent pl and pwr plains.

In summary, our observations suggest that early faulting associ-ated with the Jokwa Linea astra was already occurring at the end ofpdf time, before the emplacement of the psh and pwr plains. Mean-while, younger faulting, which postdated the clear-halo crater Fou-quet, extended into the second half of post-pwr-plains time. Theobservation that one post-pwr astrum-associated fault approach-ing crater Fouquet predates the crater while another postdates it,suggests that the younger faulting did not happen instantaneouslybut was more widely distributed within post-pwr time, as in thecases of Wohpe, Junkgowa and Gertjon.

2.6. Corona Audhumla Astrum

Audhumla Corona is a 225-km feature centered at 45.5�N and12�E (Fig. 6a). The northern segment of its annulus consists of abroad arc of densely fractured plains (unit pdf). The pdf faults inthe southern part of this segment are arranged concentrically, buttoward the north they converge in a way that they are radiatingfrom a point at 46.2�N, 11.8�E. The southern segment of the annulusis represented by arcs of wrinkle ridges, which deform the surfaceof the pwr and local psh plains that are embaying the pdf unit.North of the corona there is another area of pdf plains whose faultsalso radiate from 46.2�N, 11.8�E. Together with the northern seg-ment of Audhumla Corona, these faults form an astrum structure.

The pdf faults, 0.5–1 km wide, densely spaced and embayed,represent the older component of the astrum faulting. Also radiat-ing around this center are 0.5–1 km wide but less closely spacedfaults which cut the pwr and psh plains. These faults representthe younger component of astrum faulting. The radial faultingstructure described is well seen in the 1:5,000,000 geologic mapof this area (McGill, 2004b).

Southeast of the center of radiation are seen relatively small (3–5 � 10–15 km) lobate flows with an intermediate-bright surface(unit pl) (inset in Fig. 6a). A similar-appearing surface is observedwithin the �50 km across area around the center of radiation

Fig. 4. Astrum of Minona Corona centered at 23.5�N, 218.5�E. (a) Synoptic image of the area; white dashed oval outlines the corona; double-headed arrows show locations ofthe altimetry profiles; inlet shows structural map of the astrum; (b–e) Subareas showing plains, with wrinkle ridges (white arrows) that cut the younger faulting of theastrun; (f) Subarea showing pl plains, with wrinkle ridges (white arrows) protruding through the plains material; (g) Crater Boleyn, with a parabolic radar-dark halo cut byyoung faults extending from the radiating astrum center (white arrows); (h) Topographic profiles; black arrow show location of astrum center, where the two profilesintersect.

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(Fig. 6b). Probably the result of a coalescence of lobate flows, thissurface suggest astrum-associated young volcanism. The surface

in the center of radiation is cut by faults, some of which are ori-ented radially and others approximately concentrically.

Fig. 5. A chain of astra of Jokwa Linea centered at 17�S, 210�E. (a) Synoptic image of the area, double-headed arrows show locations of the altimetry profiles; inlet showsstructural map of the astrum; (b) sub-area showing the older component of the astra embayed by plains with wrinkle ridges (arrow); (c) ejecta outflows of the clear dark halocrater Fouquet cut by young faults extending from the radiating astrum center (black arrow) and covering another astrum-related fault (white arrow); (d) topographicprofiles; the maximum and minimum altitudes are 6053.1 and 6051.5 km for the N–S profile and 6053.1 and 6051.2 km for the NW–SE profile; black arrows show location ofastrum center, where the two profiles intersect.

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Fig. 6d shows topographic profiles through this structure. Theseshow that altitudes within this structure vary from 6051.6 to

6052.2 km above the planet center of mass. The highest point cor-responds to the astrum center; outward from this, the altitudes

Fig. 6. Astrum of Audhumla Corona centered at 45.5�N and 12�E. (a) Synoptic image of the area; white dashed outline shows the outer annulus of the corona; double-headedarrows show locations of the altimetry profiles; inlet shows structural map of the astrum; (b) sub-area showing older (1) and younger (2) components of astra; (c) the cleardark halo crater Kemble cut by young faults extending from the astrum center of radiation (white arrows); the crater rim is outlined by black arrows; 1—designates post-pshastrum-associated fault filled with crater ejecta outflows; (d) topographic profiles; the maximum and minimum altitudes for both the N–S and W–E profiles are 6052.2 and6051.6 km; black arrows show location of astrum center, where the two profiles intersect.

346 A.T. Basilevsky et al. / Icarus 203 (2009) 337–351

Fig. 7. Becuma Mons astrum of Audhumla Corona 34�N, 22�E. (a) Synoptic image of the area; double-headed arrows show locations of the altimetry profiles; (b) sub-areashowing offsets (white arrows) of wrinkle ridges along the older faults of the astrum and absence of offset along the younger faults (black arrow); (c) the dark-parabola craterNoreen probably cut by young faults extending from the astrum center of radiation (see inset); white arrow shows the place where the young fault is covered by crater ejecta;(d) topographic profiles, the maximum and minimum altitudes are 6052.1 and 6051.1 km for the N–S profile and 6052.6 and 6051.3 km for the W–E profile; black arrowshows intersection location of the profiles.

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348 A.T. Basilevsky et al. / Icarus 203 (2009) 337–351

gradually decrease with a gradient of 5–6 m/km for approximately100 km around the radiation center and with a smaller gradientfurther outward.

About 200 km northeast of the astrum the 24 km diameterKemble crater is superposed on wrinkle-ridged psh plains and cen-tered at 47.7�N, and 14.9�E (Fig. 6c). Kemble crater has a clear ra-dar-dark halo suggesting that it was formed in the second part ofpost-pwr time. The crater is approached from the southwest byboth older and younger faults radiating from the astrum. Some ofthe faults appear to cut the ejecta and inner wall of the crater.The northern and eastern parts of crater ejecta are representedby the ejecta outflow facies. In the northern terminations of this fa-cies the ejecta outflow partly fills a post-psh fault (1 in Fig. 6c).Judging from its orientation, the partially-filled fault is astrum-associated; its infilling with ejecta outflow deposits suggests thatits formation predated the formation of Kemble crater.

In summary, our observations suggest that early faulting of theAudhulma Corona astrum had already begun by the end of pdftime before the emplacement of psh and pwr plains, while theyounger faulting, which postdated the clear-halo Kemble crater,extended into the second half of post-pwr-plains time. The obser-vation that one post-psh astrum-associated fault predated craterKemble while others seem to postdate it suggests that as in thefour cases described above, the younger faulting happened notinstantaneously, but was more widely distributed during post-psh-pwr time.

2.7. Becuma Mons Astrum

Becuma Mons (Fig. 7) is a gently-sloped mountain about 1 kmhigh and 200 km across in the central part of Bereghinya Planitia.Plains with wrinkle ridges (unit pwr) are observed in this region.The mountain top (34�N, 22�E) is a center a radiation of numerousfaults (Fig. 7) forming an astrum. This feature is also well seen inthe 1:5,000,000 scale geologic map of this area (McGill, 2004b).

Astrum faults are typically 0.5–1 km wide. Near the center ofradiation they are combined into 2–3 km wide parallel abuttingclusters. Spacing for the individual faults and the clusters variesfrom 3 to 10 km and more. The faults cut plains with wrinkle ridges(pwr) and in many cases the wrinkle ridges show lateral off-set along the faults where they intersect (white arrows inFig. 7b). This is considered as evidence that these faults formedprior to emplacement of wrinkle ridges: The latter are featuresformed due by compressional deformation (e.g., McGill, 1993).When compressional stress forming a wrinkle ridge is across theearlier formed fault (in this case an astrum fault) the latter beinga discontinuity in the deforming material controls propagation ofthe compressional deformation in the way that the wrinkle ridgeis formed with lateral displacement along the fault. If the astrumfault postdates wrinkle ridge and is extentional without strike-slipcomponent (Squyres et al., 1992; Krassilnikov and Head, 2003) itcrosses wrinkle ridge without lateral displacement of the latter.

It was recently shown that emplacement of wrinkle ridges hap-pened within the first 10–15% of the time following the emplace-ment of the plains (McGill, 2004b; Basilevsky and Head, 2006).Thus emplacement of these astrum-forming faults was even closerin time to emplacement of the pwr plains. Some faults do not showthis offset (black arrow in Fig. 7b) and could be formed afteremplacement of wrinkle ridges. So this astrum, like the ones de-scribed above, may have both older and the younger phases offaulting.

We did not find evidence for volcanism associated with this as-trum. The pwr plains material predates the observed astrum-form-ing faulting and might be derived from sources not associated withthe astrum and younger lavas are not seen in its close vicinity.About 200 km east and southeast of the astrum center are ob-

served radar-bright, flow-like features belonging to the young unitpl. However they are in obvious association with two other struc-tures: Ilmatar Corona and Modron Corona.

Fig. 7d shows topographic profiles through the astrum. Theseprofiles show that altitudes within this structure vary from6051.1 to 6052.6 km above the planet center of mass. The highestpoint corresponds to the astrum center and outward from this, thealtitudes gradually decrease with a gradient of 10–20 m/km.

About 200 km southeast of the astrum center the 19 km diam-eter crater Noreen (centered at 33.6�N, 22.7�E) is superposed onpwr plains and on astrum faults (Fig. 7c). Noreen crater has a darkparabolic halo suggesting that it was formed in the latest 10–15%of post-pwr time. Obvious cross-cutting relationships of this craterby the astrum-associated faults are not observed, but the Noreencentral peak shows linear features with orientations generally par-allel to orientation of the astrum faults (inset in Fig. 7c). These maybe young astrum-associated faults postdating the crater but be-cause the central peak of an impact crater is an uplifted block oftarget material we cannot exclude the possibility that these arepre-crater faults enhanced in visibility by the peak-forming uplift.The northwestern extension of the crater ejecta clearly covers theyoung astrum-associated fault (white arrow in Fig. 7c) so if the cra-ter central peak is cut by the young faults this means that the Be-cuma Mons young faulting was also (as in the cases of fivedescribed above astra) somewhat distributed in time.

In summary, these observations suggest that early faulting ofthe Becuma Mons astrum had already begun soon after theemplacement of pwr plains, while the younger faulting could post-date the dark-parabola crater Noreen, formed close to the end ofthe post-pwr-plains time. But we cannot exclude the possibilitythat Noreen crater postdated the astrum-associated faults and inthis case the time of the end of this astrum activity is notconstrained.

3. Discussion

Results of our observations and analyses are summarized in Ta-ble 1 and Figs. 8 and 9. The seven astra for which we have foundevidence of an extended duration of radial fracture activity (Table1) have generally typical characteristics compared with other astraon Venus (e.g., Head et al., 1992; Janes et al., 1992; Stofan et al.,1992; Squyres et al., 1992; Krassilnikov and Head, 2003). They con-sist of faults arranged in a stellate manner and often they are partsof coronae. The component faults typically extend 50–600 km fromthe center of radiation, and sometimes reach as far as 1000–1500 km. The astra base-to-summit heights are from 0.5 to 2 km.We found evidence of associated volcanism, mostly young volca-nism of the pl type, for six of the seven astra. All seven astra haveboth an older and younger component of astrum-forming faultingin six of seven cases separated by lava plains units, a characteristicshared with many other astra (Basilevsky and Raitala, 2002; Kra-ssilnikov and Head, 2003; Aittola and Raitala, 2007). Therefore,the astra described here are very typical and so one may suggestthat their long duration activity may also be typical, for at leastsome of the other astra on Venus.

The earlier faulting (pre-psh-pwr) of the seven astra studied, aswell as the majority of other astra which show phases in their tec-tonic evolution (Basilevsky and Raitala, 2002; Krassilnikov andHead, 2003; Aittola and Raitala, 2007), has very dense spacing.Where not partially flooded by later lavas, the early phase 0.5–1 km wide faults are practically touching each other. The smallspacing may suggest that the layer in which these faults formedwas relatively thin and brittle (Zuber, 1987; Banerd and Golombek,1988).

Fig. 8. Geographic positions of the seven astra (white stars) showing evidence of relatively recent tectonic activity, on the background of global Magellan SAR mosaic ofVenus. Sinusoidal projection. Central longitude is 180�E.

Fig. 9. Diagram showing minimum estimated durations of activity of the seven astra.

A.T. Basilevsky et al. / Icarus 203 (2009) 337–351 349

The later (post-psh-pwr) faults have a noticeably larger spacing,from several kilometers close to the center of radiation to morethan tens of kilometers at distances of hundreds kilometers. Thismay suggest that these younger faults formed in a relatively thick-er brittle layer (Zuber, 1987; Banerd and Golombek, 1988). The lat-ter agrees with the model of geological evolution of Venus byPhillips and Hansen (1998) and probably is not the issue of smalleror greater strain. For five or perhaps six of the seven astra it wasfound that their post-psh-pwr faulting was not instantaneous,but rather was more distributed in time. The case of Wohpe As-trum shows that this distribution in time may be rather long; somepost-pwr faults of this astrum were formed within the first 10–15%of post-pwr time, while others formed within the second half ofthis time period.

Two of the seven astra are within Beta Regio (#1 and #2 inFig. 8), one (#3) is in Themis Regio, two (#4 and #5) are within AtlaRegio. Since early analysis of the Magellan data the Beta-Atla-The-mis region of Venus is known as an area where volcanic featuresare concentrated (e.g., Head et al., 1992; Crumpler et al., 1997).From our analysis it follows that it is also an area where recent vol-

canic/tectonic activity was concentrated, a finding that agrees withthe earlier analyses of this region involving the dark-parabola andclear-halo craters (Basilevsky, 1993; Strom, 1993; Basilevsky andHead, 2002, 2007; Basilevsky et al., 2003; Vezolainen et al., 2003,2004). The last two of the described astra are in the boundary areabetween Bereghinya Planitia and Ishtar Terra. Material units sug-gesting post-pwr volcanic activity are present here, although theyare not abundant (McGill, 2004b; Ivanov, 2008).

Astra are part of the broader domain of systems of radiatingfaults on Venus. Grosfils and Head (1994a) identified 163 suchstructures with maximum radii from 40 to P2000 km. Their stud-ies and comparisons with terrestrial dike swarms (e.g., Halls andFahrig, 1990; Parker et al., 1990; Ernst and Buchan, 2003) led tothe conclusion that the graben-fissure systems on Venus formedthrough subsurface dike swarm emplacement (e.g., Grosfils andHead, 1994b; Ernst et al., 1995, 2003). Centers of radiation of faultsare interpreted to indicate the position of upwelling mantleplumes which produced melts and shallow magma reservoirs.

Overpressurization of these magma reservoirs caused radialdike emplacement, often out to great radial distances, depending

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on whether the reservoir was buffered or unbuffered (Parfitt andHead, 1993; Head and Wilson, 1993). Emplacement of shallow ra-dial dikes created near-surface extensional stress fields thatformed fractures and graben above the dikes. Our observation thatsix of the seven astra studied show evidence of volcanism associ-ated with the graben adds support to these interpretations, as wellas the results of other studies (Head et al., 1992; Janes et al., 1992;Stofan et al., 1992; Squyres et al., 1992).

This dike-emplacement interpretation relates most clearly tothe later (post-psh-pwr) faults. The older, very densely spacedfaults typically are part of the pdf unit and show no associationwith any recognizable volcanic events. These older tectonic fea-tures were perhaps formed by plume topographic uplift; such up-lift was likely to have been significantly larger than the 0.5–2 kmastra heights now observed. The evidence for the pre-pwr subsi-dence of the central part of Wohpe Astrum described in Section2.1 supports this suggestion.

As shown in Table 1 and Fig. 9, the duration of activity of mostof these seven astra started forming >T or �T ago, and then stoppedforming after �0.5T ago or after 0.1–0.15T ago. If T is �750 m.y.(the most probable estimate of McKinnon et al. (1997)) then theduration of astra activity is P375 m.y. If T is the upper possibletime estimate of 1 b.y. then the duration is >500 m.y, but if T isthe lower possible limit of 300 m.y. then the duration isP150 m.y. Keeping in mind the rather low accuracy of these esti-mates it is reasonable to say that the duration of activity of theseastra was of the order of hundreds of millions of years.

The duration of activity of ongoing mantle plumes on Earth are:the Tristan plume (�140 m.y.), the Kerguelen plume (P116 m.y.),the Hawaiian plume (P75 m.y.), the Reunion plume (65 m.y.)and so on (Condie, 2001 and references therein). These durationsare shorter than our estimates of the astra activity duration, whenwe translate these ages to absolute ages using the most probableand upper limit of estimates of the mean surface age of Venus(�750 m.y. and 1 b.y. respectively, McKinnon et al., 1997). Butthe estimates of the duration of astra activity and estimations ofthe durations of activities of terrestrial mantle plumes becomecomparable when we use the lower limit of the mean surfaceage of Venus estimate (�300 m.y.). This may mean that the lowerlimit estimate better corresponds to reality but at the same timewe should keep in mind that the terrestrial plumes discussed areongoing and may last into future for a significantly longer time.

An extremely long duration of activity was found for the plumefeeding the martian volcano Olympus Mons. Crater counts on thehuge landslide deposits on the flanks of Olympus Mons volcanosuggest that they were formed �3.9 m.y ago, while the latest lavaflows of this volcano have been emplaced only a few tens millionsto a few millions years ago (Hartmann and Neukum, 2001; Neu-kum et al., 2004). Analysis and modeling of magmatic activity ofthe large Tharsis volcanoes (including Olympus volcano) led Wil-son et al. (2001) to the conclusion that the very long durationwas episodic, with active phases lasting less than 1 m.y. alternatingwith �100 m.y. quiet phases. So Olympus Mons gives an exampleof very long activity of the plume, even longer than activities ofthe plumes responsible for formation of venusian astra.

4. Conclusions

From photogeologic and stratigraphic analysis of images of 78astra, 49 dark-parabola craters and 114 clear-halo craters we havefound seven astra whose astrum-forming faulting started before orclose to the time of emplacement of psh-pwr regional plains andclear examples of several of these that extended into the secondpart of post-pwr time. Because the mean age of psh-pwr regionalplains is close to the mean surface age of Venus, which is estimated

to be �750 m.y with any age between 300 m.y. and 1 b.y. consid-ered to be acceptable, this means that the duration of activity ofthese astra was likely to have been several hundred millions ofyears. This is longer than the duration of activity of ongoing mantleplumes on Earth, but shorter than the duration of activity of theplume feeding the martian volcano Olympus Mons. The basic mor-phologic characteristics of these seven astra, as well as their agerelations with other geologic units, are similar to those of themajority of other astra on Venus; thus, a long duration of formationmay be typical, at least for some other astra. We confirm the two-phase (pre-pwr and post-pwr) evolution of astrum-forming fault-ing suggested in previous studies. For the first phase we suspectthat faulting may have been predominantly tectonic, caused bymantle diapiric rise. For the second phase we find evidence sup-porting the conclusions of other studies that the observed faults re-sulted from subsurface dike intrusions related to a plume-relatedmagma reservoir. We also found that in many cases, the secondphase of faulting was not instantaneous, but was more distributedin time.

Acknowledgments

Thanks are extended to Terhi Tormanen, Mikhail Ivanov, Mik-hail Kreslavsky, James Dickson and Anne Côté for the help in prep-aration of this paper.

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