Abstract:
The key geological observations and structural interpretations associated with the current plume hypothesis for Venusian crustal plateau formation are that (1) short-wavelength, spatially periodic tectonic structures originated as mechanical instabilities in a regionally extensive surface brittle layer whose basal boundary was thermally generated and controlled; (2) characteristic wavelengths of spatially periodic structures record the brittle layer thickness at the time of deformation; and (3) structural wavelengths increased with time, driven by brittle layer thickening in response to cooling. The plume hypothesis accommodates these constraints by proposing that crustal plateaus formed above mantle plumes impinging on thin lithosphere; it further suggests that surface temperatures temporarily elevated to ?1000 K may have been required to maintain a sufficiently thin brittle layer for formation of the shortest-wavelength structures. We report here on finite element simulations designed to test the feasibility of the proposed thermal conditions. Specifically, we model formation of short-wavelength folds thought to have initiated as contractional layer instabilities early in the plateau formation process. Under high surface temperatures, the finite element meshes develop semibrittle zones in which short-wavelength folds can form, but development of even modest structural relief requires unrealistically high total mesh shortening. Thus elevated surface temperatures inhibit development of short-wavelength folds because the models' effective integrated mechanical strength under such hot conditions is excessively low. Decreasing the surface temperature increases structural relief but produces tectonic wavelengths that are larger than those observed. We conclude that a model with solely thermal control of mechanical properties cannot explain the observed structures.
