STABLE HOTSPOT VOLCANISM ON VENUS

Venus is dominated by hotspot type volcanism from deep mantle plumes. Yet there are no obvious hotspot tracks, indicating that plumes on Venus remain (relatively) stationary for hundreds of millions of years. This is surprising because in a vigorously convecting mantle plumes are expected to migrate unless anchored by topography on the core-mantle boundary or thermochemical 'piles’ such as LLSVPs (Large Low Shear Velocity Provinces) on Earth. I find that in stagnant-lid, spherical-shell convection calculations plumes, seeded by the structure of the initial condition, persist in a stable configuration for more than 1.5 Gyr. I show that in these calculations, topography on the base of the stagnant lid (i.e., the lithosphere-asthenosphere boundary) is responsible for the spatial stability of the plumes. If there is long-wavelength symmetry in the initial plume distribution, this long-wavelength symmetry can prevent the lithosphere from becoming unstable and overturning, leading to a significantly over-thickened lithosphere relative to predictions based on scaling laws. This is confirmed by considering an identical calculation where the long-wavelength symmetry in the plume distribution is broken. It has been previously shown that impactors with a radius greater than 300 km have a global effect on a coupled atmospheric/mantle system. I speculate that impactors greater than 300 km could disrupt the stagnant-lid mode of convection on Venus.