Insights Into Venus' Crustal Plateaus From Dyke Trajectories Below Craters.

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]