A COUPLED THERMAL-MECHANICAL MODEL FOR CORONA FORMATION ON VENUS

We develop a model of the combined thermal and mechanical evolution of diapirs in the mantle of Venus. The diapir is treated as an oblate sp heroid rising through a viscous fluid and ultimately impinging on a ri gid overlaying ''lid''. Drag forces imposed by the lid cause the diapi r to spread and flatten as it rises. We parameterize the heat loss fro m the rising diapir using a Nusselt number formulation and treat the r esulting loss in buoyancy in our flow calculation. The model predicts the evolution timescale and degree of flattening of a diapir as it ris es, as well as the stresses exerted on the underside of the lithospher e. In order to explore diapir behavior further, we check our analytica l model against the predictions of the finite element code MANTLE. Fro m the combined results of these models and observations of Venusian co ronae, we are able to make a number of inferences about Venusian diapi rism and mantle properties. We find that the diapirs responsible for f ormation of Venusian coronae have an initial size distribution extendi ng from about 30 to 100 km in radius. Typical evolution timescales for these diapirs are 30-50 Myr. If Venus' global average resurfacing age is 300-500 Myr, then our results are consistent with a rate of diapir ism that has been roughly constant over this period, and with an effec tive mantle viscosity of 10(21) Pa s.