USING FOLD GEOMETRY TO INFER TESSERA FOLDING MECHANISMS IN OVDA REGIO, VENUS

Several tectonic models exist to explain the formation of the heavily deformed tessera plateaus on Venus. These models differ in the style, sequence, and magnitude of compression and extension acting to create the plateaus. Here we seek to use the geometry of tessera folds, measured from Magellan radargrammetry-derived topography, to constrain the processes by which the folds formed. We calibrate our approach by comparing the shapes of folds formed in analog models.

We have quantified a spectrum of fold shapes ranging from symmetrical buckle folds to asymmetrical thrust cored folds from published sandbox and clay analog models. We compare slopes on each side of folds to assess both the magnitude and consistency of their asymmetry. In analog models, thrust cored folds are found to have significantly (P < 0.001) greater slope asymmetry than buckle folds. This relationship persists over a range of shortening magnitudes. Slope analysis of wrinkle ridges on Mars reveals an asymmetry that is indistinguishable form thrust-cored analog models, consistent with previous interpretation of these structures as fault-related folds.

Similar analyses were performed on folds from two portions of Ovda Regio, Venus. Folds from this tessera terrain have been interpreted as thrust cored folds in one region towards the edge of the plateau and as buckle folds in the center of the plateau. Comparisons of slope data from both areas to the earth analog examples confirm previous interpretations in both regions. This suggests that the interior of the tessera terrain may be deforming by buckling and thus accommodating relatively low strains while the thrust cored folds at the periphery of the terrain accommodate greater strains. An alternative explanation is that the interior folds are also thrust cored, but that erosion or duplexing has led to symmetric topographic expression. Significant regolith-forming processes are not expected, however some authors have argued for multiple detachments and thus fold wavelengths.