Formation of Ishtar Terra, Venus: Surface and gravity constraints

We examine the geology of Ishtar Terra, Venus, as viewed in high-resolution Magellan synthetic-aperture radar imagery and altimetry, in order to interpret regional surface strain patterns. We combine constraints imposed by the surface strain with gravity modeling ofMagellan degree-75 high-resolution gravity data. Long-wavelength (>2500 km) and short-wavelength (500–2500 km) features show different apparent depths of isostatic compensation. Ishtar straddles a long-wavelength topographic bulge that is compensated at ;130 6 40 km depth, whereas the short-wavelength features (individual mountain belts and tesserae) are compensated at varying depths in a range of ;25–70 km. These two ranges of compensation, taken together with surface strain patterns, lead us to postulate a new model for the evolution of Ishtar Terra. In the model, mantle downflow results in ponding and thickening of partial melt residuum that compensates the long-wavelength bulge of Ishtar Terra. Thickened lower crust, resulting from shear forces associated with structurally deeper residuum, isostatically compensates short-wavelength topography. Relative displacement of the lower crust beneath the upper crust, which deforms in ruglike fashion, results in the surface strain patterns.