Western Ishtar Terra and Lakshmi Planum, Venus: Models of Formation and Evolution

Regional geologic mapping and gravity data reveal a variety of characteristics that must be accounted for in models for the formation and evolution of Western Ishtar Terra and Lakshmi Planurn, including: 1) high elevation, 2) plateau-shaped profile, 3) abnormally steep bounding slopes, 4) foredeeps, 5) polygonal outline, 6) adjacent orogenic belts, 7) volcanic plains, 8) plains emplaced synchronously with orogenic belts, 9) paterae, 10) variable topography of Lakshmi, 11) tessera-like material underlying Lakshmi, and 12) a large apparent depth of compensation. A tessera/peripheral deformation model, in which a preexisting block of tessera is the locus of convergence of adjacent thinner crust and lithosphere, underthrusting, mountain building, subsurface melting, and plateau uplift, is interpreted to account for most of the characteristics. The apparent depth of compensation is not simply explained by this model and appears to require a second, deeper mantle anomaly component, such as broad mantle upwelling or a hot spot.