On the importance of melt to localize large-scale deformation on Venus

Based on Earth’s dynamics, water is often considered as a key-ingredient to allow subduction, Plate Tectonics and the continuous resurfacing of a planet, because it softens rocks rheology and lubricates faults and the subduction interface. However, there is another way to strongly localize deformation: two-phase flow 

Subduction and Plate Tectonics require the strong localization of deformation. Based on Earth’s dynamics, water is often considered as a key-ingredient since it softens rocks rheology and lubricates faults and the subduction interface. Bands of small-size grains such as mylonites have also been proposed. But melt is also ubiquitous around Earth’s plate boundaries. So we studied if and how deformation was localizing in a diphasic fluid using laboratory experiments and DEM (Discrete Element Methods) modeling. 

As a model system, we used convection-evaporation in Ludox colloidal dispersions. These fluids are constituted of water, ions and nanoparticles of silica. As the solute (water) is removed, a skeleton of nanoparticles aggregates develops (Gerardi et al, 2026). Therefore as water content decreases, the fluid density increases and its rheology changes from newtonian to visco-elasto-plastic to brittle (Di Giuseppe et al, 2012). Under evaporation, such a system develops solutal convection under a denser and more viscous skin, analogous to the denser and more viscous lithosphere on a planet undergoing thermal convection. The experimental lithosphere is heterogeneous at the meso-scale, with the presence of particle aggregates and free water channels. This results in a highly non-linear rheology at the large scale, capable of producing sufficient shear localization to generate self-consistently asymmetric subduction and rifting. 

This suggests that a liquid-solid mixture such as melt-rock could allow subduction on a rocky planet.
That could explain why Venus offers evidences of subduction around some large coronae despite the absence of a water ocean on its surface: pervasive melt could do the trick. Indeed, the surface of Venus is littered by volcanoes, suggesting the presence of melt almost everywhere, and lava flows are observed in the trenches of the subduction candidates. Not only water, but melt could also serve as a lubricant of plates interfaces.