Tectono-Volcanic Structures

Fotla Corona

This image shows a small, fairly simple corona. This corona has two parts. There is a central plains region roughly 150 km across, and an outer fracture ring about 45 km wide. The central plains stand slightly higher than the outside plains, while the fracture ring seems to lie in a slight trough. There are also many signs of volcanism in the area. For instance, three pancake domes lie close to the fault ring (1,2,3). There is also a string of small shields along the bottom of the image (5). And the fault ring itself is buried by plains lavas in several places (arrows). All the pancake domes are cut by some ring faults, as are the younger plains. Thus, the corona was still forming when these lavas were erupted. This mix of major faulting and volcanism is why coronas are called tectono-volcanic features.

Note: there is a fourth round feature in this corona that is not a pancake dome. Rather, its edge is a set of concentric faults that seem to have formed around a central uplift. Possibly, this marks deformation over a magma intrusion. Or maybe it is somehow related to the faulting in the outer ring.

Idem-Kuva Corona

Idem-Kuva is about 230 km across, and is a more complex corona. It lies just north of Gula Mons, and it shows a multi-ringed plan. At the center is a large plateau, about 100 km wide and 600 m high. This looks somewhat like a pancake dome, but it is much bigger. It probably formed by heating and uplift rather than by eruption. On the edge of the central high are two arcuate troughs, and these are bounded in turn by a partial ring of faults. These faults may have once ringed the entire feature, but they seem to be buried on the north by younger plains lavas. Two long, bright lava flows also run north out of the troughs and over the fault ring. These flows are about 150 and 350 km long, and they mark the clearest signs of volcanism in Idem-Kuva.

This image also shows two other features. First, there is a major line of faults running northwards into Idem-Kuva (arrows). These faults are part of a rift zone on Gula Mons, and they may have fed the two lava flows seen in Idem-Kuva. Second, the pale circle in the upper left marks an older volcanic center (Nissaba). This volcano is fairly large but very flat. It is 300 km long by 200 km wide, but it is only about 400 m high. Note how the leftmost flow out of Idem-Kuva curves around its base. This suggests that Nissaba formed before the flow. It may even have formed before Idem-Kuva.
(Image is part of Magellan C1 MIDR 30N351.)

Corona Chain

Many coronas lie in groups or chains. Here, we see one such grouping of 4 or 5 coronas about 200 to 250 km across. It is part of a much longer chain over 4000 km long. The coronas overlap in places, but they are also linked by a number of bright faultlines. These faults are part of a major rift zone, Parga Chasma, that seems to have helped the coronas form. Indeed, the Parga rift has the highest density of coronas to be found on Venus. It also runs into Atla Regio and may have helped to feed the large shields found in that region. Thus, while rifts can bring lavas to the surface, this rift may also lie over a line of deeper mantle melting and uplift.
(Image is part of Magellan C2 MIDR 30S284, centered near 34S, 275E.)

A Simple Arachnoid

This arachnoid lies in the plains northwest of Atla Regio. Like a Corona, it has a clear fault ring and a central (mostly) smooth region. Unlike a Corona, it also has a set of radial ridges which arc away from the fault ring. It is also much smaller than most coronas. In this case, the fault ring is about 95 km across, and most arachnoids are about the same size. In contrast, most coronas are over 200 km across. Still, outside the fault ring, the ridges seem to run 100 to 200 km before merging with other ridge systems.

Most arachnoids show few signs of volcanism. Here, there are a few small shields in the center, but the plains lavas show the clearest signs of volcanism. Note the bright lava flows on the northwest and southeast that seem to come out of the fault ring. Also note how the dark plains arc above the arachnoid, and how they ring the crater in the bottom center (arrows). These all suggest that many large, thin lava flows were erupted as the arachnoid was forming.
(Image is part of Magellan C1 MIDR 45N223, centered near 41N, 214E.)

Complex Arachnoids

Arachnoids often lie in faulted plains, and their outer ridges tend to turn into regional patterns. Thus, they often look like spiders on a bright web. Indeed, the name Arachnoid means Rspider-likeS. Here, we see two arachnoids in a faulted plains region (A and B). These features are each about 120 km across, with an outer ring of ridges maybe 150 km wide. Again, there is little sign of volcanism. Only a few bright lava flows are seen in the upper left. A third feature (C) may also be an arachnoid. However, it has only a few radial ridges, and it also shows a caldera-like pattern of nested cliffs. Thus, it is harder to classify. The edge of a larger corona (arrows) also can be seen at the top of the image.
(Image part of Magellan C1 MIDR 45N011, centered near 40N, 19E.)

Simple Nova

Unlike Coronas and Arachnoids, Novas do not have major ring features. Rather, they show a striking starburst of faults and graben. Also, there is often a clear rise at the center of these faults. This example is some 250 km across, and it lies in Themis Regio. There is no clear sign of volcanism, but a few small lava flows may run out from faults in the upper left and upper right. Still, there is a lot of volcanism nearby. Namely, this nova lies within a very long chain of coronas. Indeed, it lies just above the corona chain image in Parga Chasma. Novas mostly occur next to large shields or major corona chains. Thus, they seem to be volcanic. They probably mark an early stage of uplift and faulting that could later become a corona or a large shield volcano.
(Image is part of Magellan C1 MIDR 30S279, centered near 27S, 272E.)

Transitional Form

This image may show the future of other novas on Venus. It shows a complex volcano with several clear parts. Its raised center has a clear starburst of nova-like faults. Around this nova is a partial ring of corona-like faults (arrows). Then, outside the corona, is a broad apron of thin bright lava flows. These flows lie on darker lavas which, in turn, clearly bury a much older area of heavily faulted lowlands. In all, the outer lava flows mark the start of a shield volcano some 450 km long by 350 km wide. The central nova itself is about 150 km across. Maybe, in 10 or 50 million years, more flows will finally bury the central nova to make a shield looking like
Ushas Mons.
(Image part of Magellan C1 MIDR 15S215, centered at 15S, 215E.)

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