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.