Ovda Fluctus, the Festoon Lava Flow on Ovda Regio, Venus: Not Silica-Rich

摘要

Ovda Fluctus (6.1°S, 95.5°E) is a lava flow complex on the equatorial tessera highlands of Ovda Regio, Venus. We examined its morphology and geologic setting to determine if its apparent emplacement rheology is consistent with silica-rich (e.g., rhyolitic) lava; this has significant implications for the compositions of Venus's tesserae. Using Magellan radar data, we find that the radar properties (emissivity, roughness, and backscatter) of Ovda Fluctus are similar to those of the surrounding tessera, although the flow is smoother at multiple scales. Ovda Fluctus is a "festoon flow complex," whose surface is rumpled into arcuate folds. The flow includes multiple flow lobes and at least two distinct flow units. The flow complex includes a portion at high altitude (~5 km above mean planetary radius) that has low radar backscatter and high radar emissivity compared with lower-altitude portions of the complex. The high-altitude region of Ovda Fluctus is continuous with the lower-elevation portions: the change in radar properties does not represent different flows. Outlines of Ovda Fluctus flow lobes have fractal dimensions consistent with basaltic pahoehoe lavas. The margin of Ovda Fluctus is at significantly higher elevation than its center, a characteristic seen in basalt flows on Earth, but not on more silica-rich flows. Thus, the available evidence suggests that Ovda Fluctus had an emplacement rheology consistent with a basaltic composition; this result provides no support for hypotheses that Ovda Regio (a highlands tessera terrain) is composed of granitic rock.