DETAILED GEOLOGIC MAPPING OF VENUS' CRUSTAL PLATEAUS AND IMPLICATIONS FOR PLATEAU FORMATION

Formation of Venus' crustal plateaus (CPs) is one of the most hotly debated topics to emerge from NASA's Magellan mission. CPs form large (1500-2500 km diameter) quasi-circular plateaus (0.5-4 km high) marked by extensive tectonic deformation. Gravity topography ratios indicate shallow apparent depths of compensation, generally taken as thickened crust. The mode of deformation and crustal thickening form the center of debate. Three CP hypotheses have been proposed. Mantle downwelling calls for surface contraction and crustal thickening as thin lithosphere is dragged downward by a sinking diapir. The plume hypothesis calls for thickening and surface deformation related to magmatic accretion resulting from thin lithosphere-mantle plume interactions. The lava pond hypothesis calls for surface deformation due to concurrent deformation and solidification of ‘scum' on a huge lava pond, with pond formation the result of massive partial melting of the mantle due large bolide impact on ancient thin lithosphere. In this case plateau topography results from low-density (depleted) mantle rather than thickened crust.

Each of these hypotheses makes different predictions for CP surface history. We present detailed geologic maps of three CPs, Alpha, Ovda, and Tellus Regiones in order to evaluate current hypotheses. Mapping employs Magellan data including high-resolution (100m/pixel) synthetic aperture radar (SAR) data (left-, right- and stereo-look), altimetry, and synthetic stereo images. Mapping to date reveals widespread layer extension and contraction, and intricate surface flooding across each CP. Contractional and extensional structures occur in multiple λs. Short-λ structures dominantly predate long-λ structures. Flooding is prominent throughout structurally defined lows, including many local lows preserved along the crests of long-λ folds. These observations indicate that low viscosity fluid (likely magma) was able to leak to the surface across huge expanses of each CP during surface deformation, implying that CP deformation likely occurred under extremely high temperature conditions. High temperatures would also be consistent with the nature of tectonic structures. These observations challenge the downwelling and plume hypotheses, but might be accommodated within the lava pond hypothesis.