Mantle plumes on Venus revisited

The Equatorial Highlands of Venus consist of a series of quasicircular regions of high topography, rising up to about 5 km above the mean planetary radius. These highlands are strongly correlated with positive geoid anomalies, with a peak amplitude of 120 m at Atla Regio. Shield volcanism is observed at Beta, Eistla, Bell, and Atla Regiones and in the Hathor Mons-Innini Mons-Ushas Mons region of the southern hemisphere. Volcanos have also been mapped in Phoebe Regio and flood volcanism is observed in Ovda and Thetis Regiones. Extensional tectonism is also observed in Ovda and Thetis Regiones. Extensional tectonism is also observed in many of these regions. It is now widely accepted that at least Beta, Atla, Eistla, and Bell Regiones are the surface expressions of hot, rising mantel plumes. Upwelling plumes are consistent with both the volcanism and the extensional tectonism observed in these regions. The geoid anomalies and topography of these four regions show considerable variation. Peak geoid anomalies exceed 90 m at Beta and Atla, but are only 40 m at Eistla and 24 m at Bell. Similarly, the peak topography is greater at Beta and Atla than at Eistla and Bell. Such a range of values is not surprising because terrestrial hotspot swells also have a side range of geoid anomalies and topographic uplifts. Kiefer and Hager used cylindrical axisymmetric, steady-state convection calculations to show that mantle plumes can quantitatively account for both the amplitude and the shape of the long-wavelength geoid and topography at Beta and Atla. In these models, most of the topography of these highlands is due to uplift by the vertical normal stress associated with the rising plume. Additional topography may also be present due to crustal thickening by volcanism and crustal thinning by rifting. Smrekar and Phillips have also considered the geoid and topography of plumes on Venus, but they restricted themselves to considering only the geoid-topography ratio and did not examine either the geoid and topography amplitudes separately or the shapes of anomalies.