MODELING PLUME-RELATED UPLIFT, GRAVITY AND MELTING ON VENUS

Three roughly circular regions on Venus about 1000 km across are ident ified as potential Hawaii-scale plume sites on the basis of their grav ity (70-90 mgal) and topography (1.6-2.0 km) anomalies, and signs of m elt generation and rifting. Axisymmetric isoviscous convection models are used to reproduce gravity profiles across these plumes and the lin e-of-sight acceleration of the Magellan spacecraft as it passes over t hem. The best fitting models have a conductive lid thickness of less t han 150 km, mantle viscosity of 8.9 x 10(19) to 9.6 x 10(20) Pa s and a basal heat flux of 15-25 m W m(-2). The lid thickness is constrained by requiring a modest amount of melt generation and a potential tempe rature of about 1300 degrees C. The high mantle viscosity relative to that of Earth is probably a consequence of the absence of water in the mantle, and may help to explain the current absence of plate tectonic s on Venus. The low heat flux suggests that the thermal evolution of V enus has differed from that of the Earth.