THE CORONA CONUNDRUM, AGNESI QUADRANGLE (V-45; 25-50S/30-60E), VENUS

Venus preserves ~500 coronae, quasi-circular tectonomagmatic features (60-1100 km diameter; 200-km median) that occur in chains (68%) and clusters (21%) spatially associated with mesoland chasmata and highland volcanic rises, respectively, or isolated within the lowlands (11%). Coronae are widely accepted as the surface expression of endogenic diapirs, yet they record a wide range of characters with regard to topography, geomorphology, tectonic features, and flow features. We examined lowland coronae and mons within the Agnesi VMap quadrangle (~7x10⁶ km²). Agnesi (V-45) is situated within Venus' lowland, and hosts large to small arcuate exposures of ribbon-tessera terrain, nine coronae, two mons, 11 impact craters, and extensive lobate flows sourced to the west from Sephari Mons and Ubastet Fluctus. Geologic mapping is conducted as part of the NASA-USGS Planetary Mapping program, with the goal of producing a 1:5 M scale geologic map. Data include correlated digital NASA Magellan data sets: full resolution (~100m/pixel) synthetic aperture radar (SAR) images, altimetry, synthetic stereo images, RMS slope data, and emissivity data. The features are below the resolution of available gravity data. Geologic mapping allows us to divide the coronae into four groups: 1) features likely representing the intersection of seemingly unrelated lineament trends [and probably not genetically single features]; 2) domical features with characteristic radial fractures and/or flows; 3) domical to plateau-shaped features marked by tectonic fabric with perpendicular contractional and extensional structures, perhaps akin to ribbon-tessera terrain that characterizes Venus' crustal plateaus; and 4) circular basins, marked by concentric structures and generally lacking radial features, which we call circular lows. The mons are similar in character to the radial coronae. The results of mapping suggest that these features may record different genetic processes, including: 1) intersection of unrelated tectonic events; 2) endogenic diapirs (radial coronae and mons); 3) large lava ponds, or the intersection of different tectonomagmatic events; and 4) bolide impact on rheologically relatively weak crust (circular lows). Thus, perhaps the term ‘coronae' should be used in a descriptive sense, free of genetic implications.