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On Venus, radar observations of the surface have highlighted two categories of craters: bright‐floored, interpreted as pristine, and dark‐floored, interpreted as being partially filled by lava. While volcanic resurfacing occurs within and outside craters in the plains, it seems mainly concentrated within the interior of dark‐floored craters in the crustal plateaus, suggesting that the magma is negatively buoyant there. Indeed, crater unloading may facilitate vertical ascent of a negatively buoyant magma by decompressing the underlying crust. However, the crater topography also generates a shear stress which would tend to horizontalize the vertical propagation of a dyke. We use numerical simulations of magma ascent in an axisymmetric crater stress field to demonstrate that, depending on the crust thickness and the magma‐crust density contrast, a negatively buoyant magma can indeed erupt only in the crater interior while remaining stored in the crust elsewhere. In particular, we identify four different behaviors depending on if and where a magma‐filled crack ascending below a crater reaches the surface. We draw a regime diagram as a function of two characteristic dimensionless numbers. For eruption to occur only in the crater interior requires a crust thinner than 45 km and a limited range of magma‐crust density contrasts, between 40 and 280 kg m−3 for crust thicknesses between 20 and 45 km, the permissible range decreasing for increasing crustal thicknesses. These results suggest that the crustal plateaus may not be particularly thick and could be slightly differentiated, but probably not very felsic. Plain Language Summary: The Magellan mission revealed two categories of impact craters at the surface of Venus: the pristine bright‐floored and the dark‐floored craters, which are interpreted as craters partially filled by smooth lava after their formation. In the crustal plateaus of Venus, the magma reaches the surface mainly within craters, suggesting that it is denser than the crust. Because of the crater negative topography, the underlying crust is decompressed relative to its surroundings, which, in turn, facilitates magma ascent below the crater despite its negative buoyancy. We first gather surface observations on a set of craters located in the crustal plateaus of Venus to construct a characteristic fresh crater topography. We then use a model of magma ascent below a crater in the crust of Venus to constrain the magma and crust densities as well as the initial magma storage depth that allow for magma eruption within the crater interior only. We show that magma reaches the surface only in the interior of the crater if the crust is slightly less dense than the magma and if it is not too thick (≤45 km in thickness). Key Points: We identify four different behaviors for magma‐filled crack propagation below cratersWe draw a behavior diagram as a function of two dimensionless numbers characterizing dyke propagation below a craterMagma infilling of dark‐floored craters in Venus' plateaus requires a crustal thickness ≤45 km and a small crust‐magma density contrast [ABSTRACT FROM AUTHOR]

Crustal plateaus are Venusian highlands characterized by tectonized terrains. It is commonly interpreted that their topography is isostatically supported and that they represent fossils of an extinct tectonic regime. Using gravity and topography we perform a comprehensive investigation of the lithospheric structure of six crustal plateaus. We computed the admittance (gravity to topography wavelength‐dependent ratio) for each region and compared them to modeled admittances. Three compensation scenarios were tested: Airy isostasy, a surface‐loading flexural model, and a flexural model with surface and subsurface loads. Our results show that the topography of most plateaus is supported by crustal thickening and that the addition of a mantle support component is not necessary at the investigated wavelengths. The elastic thickness was constrained to be less than 35 km with a best‐fitting average of 15 km, confirming that these regions are consistent with an isostatic regime. The average crustal thickness of the plateaus ranges from 15 to 34 km, and if they are in Airy isostasy, this implies that the global average crustal thickness of Venus is about 20 km. Phoebe Regio is the sole exception of our analysis in that crustal thicknesses that are compatible with the other plateaus are obtained only when a buoyant layer is included. Heat flow estimations computed from the elastic thickness indicate that the plateaus formed under higher heat flow conditions compared to the current global average and could have caused localized melting. Present‐day heat flow predictions suggest that eclogitization could occur where the crust is thickest. Plain Language Summary: Crustal plateaus are large and highly deformed highlands observed uniquely on Venus. It is generally assumed that these features are in an Airy isostasy regime, where the weight of the topography is balanced by a crustal root floating over a higher‐density mantle, and that the geologic processes responsible for their formation are no longer taking place. However, their origin, structure, and evolution are still topics of great debate. This study aims to investigate the lithosphere and crustal structure of six of these regions making use gravity and topography data. We use the observed topography and geophysical models of the lithosphere to predict the gravitational signature of each region which is then compared to the observed gravity. With this analysis we constrained the thickness of the elastic lithosphere, which is a proxy for the rigidity of the lithosphere, and estimated the crustal thickness of these regions. We found that the elastic thickness of the crustal plateaus is in general very small, which suggests that the common assumption of Airy isostasy is overall valid. Moreover, we found that the crustal thickness of these regions is about 25 km on average. Key Points: Crustal plateaus on Venus were investigated using a localized spectral analysis of gravity and topographyThe crustal plateaus are consistent with being in an Airy isostasy compensation regimeThe average crustal thickness of the plateaus ranges from 15 to 35 km [ABSTRACT FROM AUTHOR]

Abstract: Recent construction of 1:10 M IMaps of Niobe Planitia and Aphrodite Terra paints a rich picture of Venus evolution. Using the Niobe‐Aphrodite map area (>25% of Venus) as an example, we illustrate the methodology and importance of detailed structural mapping at large regional scales in order to identify tectonic domains that lead to the discovery of global‐scale geodynamic evolution and operative processes. We highlight differences between lithodemic and lithostratigraphic units in geologic mapping. We step through a series of structural element maps that reveal the character of at least three different tectonic domains and evolving tectonic regimes that reflect changing geodynamic processes, here divided into three eras. The ancient era encompasses formation of tessera terrain and represents a time of unique global and environmental conditions marked by globally thin lithosphere. The second era resulted in formation of the Artemis superstructure. The ~13,000‐km‐diameter footprint of the Artemis superstructure is huge but not global, although associated mantle flow patterns could have had a global reach. As formation of the Artemis superstructure waned, tectonic activity became more focused leading to formation of the fracture zone complex. This complex extends beyond the map area connecting with Atla, Beta, and Themis regiones and associated fracture zones, marking the youngest era. Structural mapping of the Niobe‐Aphrodite map area indicates that Venus exhibits a complex, multistage history like other terrestrial planets, which probably extends to several billion years. Given that Venus never developed plate tectonics, Venus' preserved surface record is likely significantly richer than that of Earth. [ABSTRACT FROM AUTHOR]

Abstract: Ribbons are regularly spaced, between 2 and 6km, troughs that exist on venusian tesserae, which are mainly located in, and characterize to, venusian crustal plateaus. Independent of the geological or temporal relations with other features, regularly and similarly spaced ribbons on several tesserae strongly suggest a thermal control on the thickness of the deformed layer. This can be used to constraint the heat flow at the time of ribbon formation, which holds important implications for the viability of the hypotheses that address the origin and evolution of crustal plateaus. For a brittle–ductile transition ∼1–3km deep (as proposed from ribbon spacing), realistic strain rates, and a present-day surface temperature of 740K, the implied heat flow is very high, 130–780mWm−2. If Venus has experienced higher surface temperatures due to climate forcing by massive volcanism, then the heat flow could be greatly reduced. For surface temperatures of 850–900K the heat flow is 190–560, 60–230 and 20–130mWm−2 for brittle–ductile transition depths of 1, 2 and 3km, respectively. Heat flow values around 80–100mWm−2 are reasonable for venusian hotspots, based on terrestrial analogs, but hardly consistent with coldspot settings. High surface temperatures are also required to maintain the crustal solidus deeper than a few kilometers during the formation of ribbon terrains. For the obtained heat flows, a solidus deeper than ∼30km (the likely mean value for the crustal thickness) is difficult to achieve. This suggests that a substantial proportion of the crust beneath crustal plateaus was emplaced subsequently to the time when ribbon terrains were formed. Alternatively, at that time a magma reservoir inside the crust could have existed. [Copyright &y& Elsevier]

Detailed observations on Venus tessera terrain reveal the presence of tectonic structures, characterized by a subcircular trough, limiting an internal area with the average elevation of the surrounding terrain. We named this new type of tectonic structures Circular Troughs. Here we show the results of a morphometric study and a structural analysis based on data from 609 structures obtained through a global survey on Venus. Circular Troughs have an average outer radius of 12 ± 4.4 km, an average inner radius of 9 ± 3.9 km and an average trough width of 3 ± 1.2 km. A comparison with other tectonovolcanic structures on Venus shows that they are smaller and their size-distribution is more bounded. Although Circular Troughs show morphological similarities with some corona types, their size distribution is significantly smaller than coronae indicating a shallower geodynamic formation process. A shape preferred orientation analysis shows that Circular Troughs in Western Ovda are elongated parallel to long-wavelength tessera folds indicating that they are at least partially deformed by them, thus Circular Trough formation predates or it is contemporary with folding. A detailed structural analysis suggests that they were generated by small scale diapirism exclusively developed in tessera terrain. Several hypotheses could explain this small scale diapirism inside tessera terrain including volcanically driven compositional diapirism, small scale thermal diapirism or compositional cold diapirism, considering that the probable dessication of an ancient ocean would have generated evaporitic deposits in the past later buried by younger geological units. • A global survey of tessera terrain revealed a new type of circular tectonic structure. • These structures features a subcircular trough limiting an inner mesa. • Structural mapping shows compression at the inner margin and extension outside. • Observations support a diapiric origin probably generated by salt tectonics. [ABSTRACT FROM AUTHOR]