THE MORPHOLOGY AND EVOLUTION OF CORONAE ON VENUS

Coronae on Venus are large, circular to ovoidal surface features that have distinctive tectonic, volcanic, and topographic expressions. They range in diameter from less than 200 km to at least 1000 km. New data from the Magellan spacecraft have shown coronae to be among the dominant tectonic forms on the planet and have revealed their morphology in unprecedented detail. Typical coronae are dominated by concentric tectonic features and have a raised rim, a central region higher than the surounding plains but in many instances lower than the rim, and, commonly, a peripheral depression or "moat". Some coronae also show significant amounts of radial tectonic structure, and in most cases this predates the concentric features. In addition, there are other features on Venus, recognized for the first time in Magellan data, that consist of domical rises with intense radial tectonic patterns and little or no concentric structure. All of these features commonly are associated with moderate to large quantities of volcanism. In fact, some radially fractured domes have undergone so much volcanism that volcanic construction appears to have played a significant role in establishing their topography. We explore a model of corona formation that links these forms into a genetic sequence. The model begins with the ascent of a mantle diapir. Upward mantle flow driven by its ascent forces the lithosphere above the diapir upward, producing a gentle dome with a radiating pattern of extensional fractures. As the diapir impinges on the underside of the lithsphere it flattens and spreads, transforming the uplift to a more flat-topped shape. In this flattened, near-surface configuration the diapir can cool rapidly. With the resultant loss of buoyancy the raised plateau can relax to form a central sag, a raised rim, and a depressed moat. Concentric tectonic features develop primarily during the latter stages of corona formation and hence are best preserved on mature coronae. Volcanism takes place during all phases of the uplift and may diminish as the relaxation occurs. Our analyses to date suggest that this scenario is broadly consistent with many of the coronae on Venus. However, there is enormous diversity in corona morphology, and features are present that require substantial deviations from this simple model. In particular, some circular depressions appear corona like in synthetic aperature radar images but may in fact be large calderas. Some of the variations observed in corona morphology may ultimately be interpretable in terms of variations in the behavior of individual diapirs and in the local properties of the Venusian lithosphere.