THE COMPLEX MORPHOLOGIES OF MAHUEA THOLUS, VENUS

Mahuea Tholus (164.7°E, 37.5°S), is a large volcano (250 km across) on Venus with an extensive and morphologically complex lava flow field. Prior studies suggest that it has an evolved (i.e., silica-rich) composition based on the interpretation that the parallel arcuate ridges on the surface of Mahuea Tholus are folds, and the scale of those folds is similar to scales seen on evolved lava flows on Earth: wavelengths of 10s to 100s of meters. Fold wavelengths are thought to result from a viscosity contrast between the crust and flow interior, and reflect the lava composition and emplacement conditions. However, it remains unclear whether the Venusian mantle can generate such large volumes of evolved lavas (e.g., rhyolite, dacite), or if the large fold wavelengths may be explained by mafic lavas emplaced under Venusian conditions.The diversity of basaltic lava flow morphologies observed on Earth includes flows with a range of fold wavelengths. For example, the mafic South Sugarpine lava flow, in the central Oregon High Cascades, has folds with wavelengths of 10s of meters. Observations of basaltic flows at Craters of the Moon National Monument, in Idaho, USA, show fold wavelengths that span a few centimeters to a few meters.

Although these fold wavelengths and amplitudes are smaller than those observed on Mahuea Tholus, the Venusian environment (atmospheric pressure almost 100 times greater than Earth’s and temperatures of around 400°C) will result in different crustal cooling rates, which may help to explain the large fold wavelengths on Mahuea Tholus. The Venusian environment allows for crusts on lava flows to form rapidly, creating insulation for the lava underneath, and potentially different viscosity contrast between flow crust and interior. We will present our preliminary mapping results, along with fold wavelength measurements, of Mahuea Tholus, to explain its unique morphology, along with comparisons to diverse folded terrestrial basaltic lava flows.