THE SPATIAL-DISTRIBUTION OF CORONAE AND RELATED FEATURES ON VENUS

Coronae are large quasi-circular geologic features that are common on Venus. They appear to be the surface tectonic and volcanic expressions of mantle diapirs that have impinged on the underside of the venusian lithosphere. We have investigated the spatial distribution of 335 cor onae and related features identified in Magellan radar data. It is mor e clustered than a Poisson distribution, with a statistical certainty of more than 99%. It is dominated by a single large cluster centered n ear the equator at about 245-degrees longitude. The features axe prefe rentially found at elevation and geoid values close to the planetary m ean, with a paucity at both the highest and lowest levels of topograph y and geoid. Some coronae appear aligned in quasi-linear chains. We at tribute the clustering of coronae and related features to preferential formation of these features above regions of broad-scale mantle upwel ling, and suggest that a major mantle upwelling underlies the one larg e duster. We suggest that coronae axe rare at the lowest elevations be cause these may be regions of mantle downwelling. The shortage of coro nae at the highest elevations may result both from obscuration by othe r intense tectonism there and from suppression there of their formatio n by an unusually thick crust. Corona chains may be produced by enhanc ed passive mantle uplift below failed or incipient rifts.