Introduction

Structures at three different scales have been attributed to plumes on Venus: large topographic rises, or regiones, extending 1000's of km (Solomon et al., 1991; Smrekar et al., 1997), fissure or dike systems radiating 100's of km (Grosfils & Head, 1994; Ernst et al., 2003), and the 300-500 circular constructs termed “coronae” with diameters generally 100-300 km (Stofan et al., 1992; DeLaughter & Jurdy, 1999). Approximately 1000 impact craters are located on Venus' surface, nearly randomly distributed. The modification of a crater or its ejecta by disruption or embayment by lava from outside documents local tectonic and/or volcanic activity (Phillips et al., 1991; Herrick et al., 1997). Parabolic deposits associated with some craters give their approximate age (Campbell et al., 1992; Basilevsky, 1993; Basilevsky & Head, 1998; 2002; Basilevsky et al., 2003), thus dating activity. Also, dip of the crater floor, which forms initially flat, indicates later reorientation (Connors, 1992). Thus, analysis of impact craters can provide clues about the timing, extent and nature of tectonic and volcanic processes to give further insights on the existence of diapirs or plumes on Venus.

The BAT Region

We focus on the Beta-Atla-Themis (BAT) region (± 30º N, 180º-300º E), the intersection of three rift zones defined in the global map of Venus (Price & Suppe, 1995) (Figure 1). BAT most probably has been active recently and may still be active currently, with planetary geoid highs, profuse volcanism, and numerous coronae. Because of the high level of volcanic and tectonic activity here, this is the most likely location on Venus for plume activity. Rifts and large volcanic edifices dominate both the Beta and Atla Regiones, with Atla containing some of the largest volcanoes in the planet (Crumpler et al., 1997). Themis Regio is dominated by coronae ranging in size from 200 to 500 km diameter (Stofan et al., 1992; Smrekar et al., 1997). Coronae have been defined as circular to irregular volcano-tectonic features (Price & Suppe, 1995) interpreted to result from rising diapirs (Stofan et al., 1992). These three regiones are part of the 9 broad topographic rises interpreted as surface manifestations of mantle upwelling (Smrekar et al., 1997). The BAT region as defined contains 153 impact craters: 102 pristine, 30 tectonized-only, 10 embayed-only, and 11 both tectonized and embayed. Although the BAT region covers just 1/6th of the planetary surface, fully 23% of Venus’ modified craters and most of Venus’ craters that have been both tectonized and embayed (61%) occur in this region (Matias & Jurdy, 2002).

We studied a total of 33 out of ~150 impact structures located within the BAT region. Impact craters smaller than ~15 km in diameter cannot be resolved from Magellan altimetry, but very few of these exist due to Venus’ thick atmosphere. Location limits the identification of craters in the altimetry data, with those craters in the rift zones generally indistinguishable from the surroundings. Figure 2 shows crater Melba, this crater on Atla has been classified as embayed. Lavas from the northeast flow over part of the ejecta blanket but not into the crater interior. This region currently dips moderately to the NE. It is possible that the radar-bright flows changed direction as the regional dips changed.

Some patterns emerge from the analysis of these craters in the BAT region. For instance, among the nine craters both tectonized and embayed, those located on Atla and Themis Regiones suffer predominantly tectonic alteration. However, craters Richards and Bashkirtseff on Atla do show evidence of multiple episodes of embayment. In contrast, on Beta the corresponding group of craters has experienced more embayment than tectonic disruption. Craters on Atla document its activity as recent and possibly even current. In general, Atla’s craters dip away from the rift, but Beta’s craters do not show the same dip pattern. Thus, it appears that Atla is more active than Beta, and possibly even currently active.

Figure 2: Melba (4.7 N, 193.4 E, 22.2 km). Stereo imaging topography (color) nicely overlays the F-MIDR radar image for Melba.

Impact Craters and the Giant Radiating Dike Swarms

We consider craters to further differentiate between models for radiating graben-fissure structures. Individual craters can provide a snapshot of the nature and degree of tectonic and volcanic activity as well as their timing at a specific location. The southern region of the Ernst et al. (2003) study area (24º-45º N) contains five of the impact craters in our data set. This area contains six of the giant radiating systems of which five were determined by Ernst et al. (2003) to be underlain by dike swarms based on extension beyond the uplift [Editor's comment: see also Giant Dikes and Giant Radiating Dyke Swarms pages].

A total of 23 craters are located between 24° and 45° N, with very few showing tectonic or volcanic modification. Based on the classification by Herrick et al. (1997), this region contains two tectonized-only craters and a single crater that has been both tectonized and embayed. Our approach differs from that of Grosfils & Head (1996) who only consider craters directly associated with lineaments and/or flows. We found little evidence of crater modification by embayment. The single crater showing encroachment by lava, Raisa (27.5º N, 280.3º E, 13 km) has been heavily embayed and also shows some tectonic disruption of its rim and ejecta. It lies ~400 km from the center of the R8 radiating system near a lineament. This system is considered the youngest of the radiating-graben systems in Beta Regio (Ernst et al., 2003).

Venus' radiating fissure swarms have caused surprisingly little modification of impact craters. Almost all craters near the extensional lineaments remain pristine. If shallow and recent magma sources had generated the radiating systems, then we would expect to find many more craters embayed by lava. The other scenario for the formation of radiating swarms requires an uplift caused by an ascending diapir. However, the craters within radiating systems do not dip away from the center. Instead, they dip more randomly, often toward the center of the radiating system, suggesting it has collapsed since the time of impact. Thus, the radiating systems may have formed before the impacts, as was concluded by Grosfils & Head (1996).

Impact Craters and Coronae

Coronae near Atla and Beta Regiones have very few impact craters adjacent to or inside their rims. Only four coronae contain craters: one tectonized and embayed, two tectonized-only, and one pristine. The single pristine crater (Aethelflaed) has recorded different regional dips that may document the evolution of a currently old-stage corona. The crater deficit within coronae and the adjacent region, along with the strong evidence for tectonic modification of associated craters argues for a tectonic rather than an impact origin for coronae. A diapir model for coronae could account for their topographic evolution as well as crater modification, orientation, and ultimate removal.

Conclusions

• Based on the higher geoid, crater dips and modification, we conclude Atla Regio is more active and possibly younger than Beta. This supports the results of Basilevsky and Head (2002) based on crater density and stratigraphy that Atla’s activity is more recent than Beta’s.
• Tilting results for the surroundings of 30 craters in the BAT region indicate that most of the impact craters around Atla’s geoid high dip away from the rift. On the other hand, craters on Beta are sparser and dip directions discordant.
• The radiating fissure swarms on northern Beta Regio have caused minimal modification of impact craters.
• Distribution and modification of craters associated with coronae in the BAT region strongly argues for a tectonic rather than an impact origin for coronae.

References

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