GEOPHYSICAL MODELS FOR THE FORMATION AND EVOLUTION OF CORONAE ON VENUS

Coronae are large circular features on Venus characterized by an annulus of concentric tectonic features, interior fracturing, volcanism, and generally upraised topography. They are suggested to form over sites of mantle upwelling and modified by subsequent gravitational relaxation. We examine this proposition using two geophysical models to determine whether and under what conditions these mechanisms can produce the topography and tectonics exhibited by coronae in the Magellan altimetry data and radar images. Our results show that mantle diapirism can produce the domical topography of novae, which may be coronae in the earliest stage of formation. The model stresses induced at the surface by a mantle diapir imply the formation of radially oriented extensional fracturing as observed in novae. The dimensions of novae indicate that the diapirs responsible for them are smaller than about 100 km in radius and that the elastic lithosphere is less than 32 km thick. Diapirs that have reached the top of the mantle are expected to spread and flatten, producing plateaulike rather than domical topography. We model a flattened diapir at the top of the mantle and show that it will result in plateaulike uplift. The volume of the flattened model diapir is similar to that of the spherical diapirs derived for novae. We model gravitational relaxation of isostatically uncompensated plateaus and show that they relax to the topographic forms associated with coronae and that the model stresses are consistent with the development of the annulus of tectonic features around coronae.