THE INTERACTION OF MANTLE PLUMES WITH SURFACE THERMAL AND CHEMICAL-BOUNDARY LAYERS - APPLICATIONS TO HOTSPOTS ON VENUS

Large volcanic swells on Venus are believed to be a manifestation of m antle upwelling, or hotspots. The study of these regions provides impo rtant information on the interior of the planet. Numerical experiments are carried out to examine the interaction of mantle plumes with the thermal lithosphere and a layer of depleted mantle, a product of press ure-release melting, using an axisymmetric finite element and finite d ifference code that incorporates temperature-dependent viscosity and p ressure-release melting. The lithosphere is defined as a high-viscosit y lid; plumes are initiated and maintained by a prescribed temperature at the base of the computational domain The topographic uplift, the g eoid-to-topography ratio, and the volume of pressure-release melt are compared to estimated values for possible hotspots on Venus to constra in the properties of plumes and the lithosphere. The effects of lithos pheric thickness, depleted layer viscosity and thickness, mantle tempe rature, and plume temperature and duration on the surface observables are predicted. Models with a thermal lithospheric thickness of approxi mately 100-150 km are consistent with observations, assuming a mantle temperature of 1300 degrees C, a maximum plume temperature of approxim ately 1500 degrees C, and mantle plume durations of 150-250 m.y. To be consistent with estimated volumes of volcanics at Venusian hotspots, a significantly thinner thermal lithosphere requires a much cooler man tle; a thicker lithosphere requires a hotter mantle. Models with a dep leted layer thickness of 100-250 km, in addition to a 100-km thick the rmal lithosphere, predict the range of parameters found on Venus. Inte rpretation of large volcanic swells on Venus based on the evolutionary sequence predicted here implies that Beta, Atla, Western Eistla, and Imdr Regiones overlie either active or recently active mantle plumes. The geoid-to-topography ratio at Beta Regio is much larger than values found at other hotspots and may indicate that it is in an early stage of evolution or that the chemical or thermal boundary layers are sign ificantly thicker. Bell, Dione, and Themis Regiones appear to represen t very late stage, possibly now extinct, hotspots. An estimate of the total plume buoyancy flux for Venus is much lower than the value obtai ned for Earth. The small areal distribution of volcanism is consistent with a low level of ongoing resurfacing at a few active hotspots.