CORONAE OF MNEMOSYNE REGIO - MORPHOLOGY AND ORIGIN

Coronae are circular to elliptical structures 150-600 km across, characterized in Venera 15/16 and Arecibo radar images by an annulus of concentric ridges. The interiors of the structures are geologically complex, displaying volcanic flows, domes, and radial, concentric, and/or obliquely trending ridges and are generally distinct from the exterior. The Mnemosyne Regio area, to the west of Ishtar Terra, contains seven coronae, the largest cluster of coronae identified on Venus. On the basis of the morphologic and topographic characteristics of the coronae in the Mnemosyne region, we assess several possible models of corona origin. Using cross-cutting and stratigraphic relationships, we have been able to identify a sequence of features and events for this group of coronae and have determined that a variety of processes contribute to the origin and evolution of coronae. We find that several processes associated with diapirism or hotspots and gravitational relaxation of high topography are most consistent with observed corona topography and morphology and are thus likely to have been significant in the formation and evolution of coronae. On the basis of these observations and stratigraphic relationships, the following sequence of events is outlined for the formation and evolution of coronae in this region: (1) regional volcanic and tectonic activity; (2) localized uplift and volcanism caused by diapirism or hotspots; (3) annulus and trough formation related to gravitational relaxation, accompanied by further volcanic activity; (4) continued local volcanic flooding of the corona, annulus, and trough. Differences in morphology between coronae reflect variations in amount and significance of these geologic processes. Coronae in other regions exhibit the same overall characteristics as the Mnemosyne coronae and support a similar origin for the majority of coronae. © 1990.