A Revolution in Plate Tectonics?

These large shields all look much like shield volcanoes on Earth. They are mostly covered by long, radial lava flows. They all have very gentle slopes. And most also have some form of central vent or summit caldera. Thus, we think that these shields formed from basalts, much like the shield volcanoes in Hawaii. The venusian shields, however, show a map pattern which is quite different from that seen on Earth (see reference map). Namely, the shields on Venus are widely scattered, and they show no linear volcano chains like those on Earth. This suggests that Venus does not have active plate tectonics, and also that most volcanism on Venus is related to mantle hotspots.

Venusian geologic literature is difficult for outsiders to follow. About 700 of the larger 1000 or so of the old structures that I regard as of impact origin are given mostly unfamiliar designations (the others are ignored): corona, ghost or stealth corona, nova, astra, patera, arachnoid, mons, and volcano, and, because distinctions are arbitrary within the continuum that encloses all of these, diverse “hybrid” combinations. Most of these terms were first applied to express ambiguity as to origins but all are now used with exclusively endogenic connotations. Conjectural endogenic origins are debated in hundreds of published papers (e.g., Herrick et al., 2005, Krassilnikov and Head, 2003, and Stofan and Smrekar, 2005), but none of these speculations account for either circularity or impact-like morphology. Conjectures often are accompanied by rationalizations of gravity and topography in terms of isostasy plus dynamic upwelling or downwelling. As Herrick et al. (2005, p. 11) noted of their own modeling of such conjectures, the assumptions on which it is based are “certainly debatable and largely unconstrained.” Abundance and distribution of the structures are obscured in the literature because a third or so of the visible large old circular structures, and nearly all of the several thousand small old ones, are left, unmentioned, in wastebasket terrains.

Venus displays thousands of circular structures, typically rimmed depressions, with pristine to variably degraded or buried morphology of types expected for impact origins. The overwhelming consensus among Venusian specialists is that only the 1000 small pristine structures among these record impacts, all others being products of plumes and other hypothetical endogenic processes younger than 1 Ga. This consensus is based on a chain of questionable conjectures regarding planetary evolution, rather than on analysis of structures.

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A planet is covered in a wool blanket 50 miles thick moving at 300 miles an hour over it's surface, and you think features almost 4 billion years old would have survived that?

The age, however?

A continuum of increasing degradation, burial, and superposition connects the younger and truly pristine young impact structures with the most modified of the ancient structures. Younger craters of the ancient family are superimposed on older ones in impact-definitive cookie-cutter bites and are not deflected as required by endogenic conjectures. Four of the best-preserved of the pre-“pristine” circular structures are huge, with rimcrests 800–2000 km in diameter, and if indeed of impact origin, must have formed, by analogy with lunar dating, no later than 3.8 Ga. Much of the venusian plains is seen in topography to be saturated with overlapping 100–600 km circular structures, almost all of which are disregarded in conventional accounts. Several dozen larger ancient plains basins reach 2500 km in diameter, are themselves saturated with midsize impact structures, and may date back even to 4.4 Ga.

I don't know, Chiloe.... given all of the weirdnesses in the solar system:

Oceans of liquid methane
CO2 ice caps
Sulfuric acid volcanoes
Ice-plate tectonics
Walnut-shaped moons
Brown Dwarfs
Rings
Braided Rings!
Giant permanent storms
An ocean 62 miles deep
A volcano 28 miles tall

....the "Venusian mud volcano" probably takes the cake!

This chain of very large young structures is unique on Venus. Superpositions show that the structures become progressively younger southwestward. Nearly simultaneous impacts of fragments of a giant bolide are inferred.

Plume-induced crustal convection 3D thermomechanical model and implications for the origin of novae and coronae on Venus

There have been attempts recently to revive the impact theory. Vita-Finzi et al. (2005) suggest that many coronae are old, degraded impact craters, noting that the combined crater and corona population fit a log-normal frequency distribution, similar to the crater population of other terrestrial bodies. Hamilton (2005) questiontions the transfer of terrestrial plume theory to Venus, thus favouring an ancient impact origin. If impacts are indeed responsible for coronae, the implications are significant: the surface of Venus could be significantly older than thought and dominated by sediments, rather than volcanism.

However, this origin is still not in favour, because it cannot account for the clustering of coronae in rift zones, which are likely to be some of the youngest areas on Venus (Price et al. 1996), nor does it account for the complexities seen at individual coronae (e.g. Copp et al. 1998)

Looks like the Plume Invasion has started!

So how do you suppose, given the surface conditions mentioned by Fortmental, that these ancient impact sites are so well preserved, as compared to earth?

Limits on magnetic field strength from Magellan magnetometer data are 0.000015 times Earth's field. Current theories of the formation and evolution of the terrestrial planets do support an Earth scale magnetic dipole (magnetic field) on Venus for perhaps the first billion years or so after formation. During that time, remnant thermal energy from the heat of creation probably drove the Venusian dynamo. After this heat was fully dissipated, there appear to have been no other internal processes with which to generate the convective motion needed to support a global field. Today, the only magnetic field Venus may have comes from the interaction between Venus's upper atmosphere and the solar wind. The interaction causes electric currents to flow in the upper atmosphere which then create a weak magnetic field that streams behind the planet like a comet's tail.