LARGE TOPOGRAPHIC RISES ON VENUS - IMPLICATIONS FOR MANTLE UPWELLING

Topographic rises on Venus have been identified that are interpreted t o be the surface manifestation of mantle upwellings. These features ar e classified into groups based on their dominant morphology. Atla and Beta Regiones are classified as rift-dominated, Dione, western Eistla, Bell, and Imdr Regiones as volcano-dominated, and Themis, eastern Eis tla, and central Eistla Regiones as corona-dominated. At several topog raphic rises, geologic indicators were identified that may provide evi dence of uplifted topography (e.g., volcanic flow features trending up slope). We assessed the minimum contribution of volcanic construction to the topography of each rise, which in general represents less than 5% of the volume of the rise, similar to the volumes of edifices at te rrestrial hotspot swells. The total melt volume at each rise is approx imated to be 10(4)-10(6) km(3). The variations in morphology, topograp hy, and gravity signatures at topographic rises are not interpreted to indicate variations in stage of evolution of a mantle upwelling. Inst ead, the morphologic variations between the three classes of topograph ic rises are interpreted to indicate the varying influences of lithosp heric structure, plume characteristics, and regional tectonic environm ent. Within each class, variations in topography, gravity, and amount of volcanism may be indicative of differing stages of evolution. The s imilarity between swell and volcanic volumes for terrestrial and Venus ian hotspots implies comparable time-integrated plume strengths for in dividual upwellings on the two planets.