An analysis of tectonic structures in Venusian crustal plateaus: Structure, kinematics, and numerical modeling

Venusian crustal plateaus are ancient features with complex and unique structural histories. Crustal plateaus exhibit steep sides, subdued topography, and are isostatically supported via crustal roots. Tectonic hypotheses for crustal plateau formation consist of a downwelling model, in which crustal thickening and surface deformation occur by horizontal flow above cylindrical mantle “drips;” and an upwelling model, in which crustal thickening occurs by magmatic processes above a hot plume. The downwelling model predicts early and pervasive contraction expressed at the surface as folds and thrust faults. The upwelling model predicts early and pervasive extension of a thin layer in a hot environment, with possible minor contraction perpendicular to extension producing folds or other shortening features normal to extensional features. Subsequent cooling leads to an increasingly thick strong surface layer accompanied by a secular increase in the characteristic wavelength of deformation. The first paper of this dissertation consists of a detailed structural and kinematic analysis of the tectonic features preserved at Ovda Regio, Venus' largest crustal plateau. The results indicate that Ovda preserves a surface strain history consistent with the plume hypothesis for crustal plateau origin, and inconsistent with the downwelling hypothesis.; Following on this work, Paper 2 presents results of a global Fourier and Walsh spectral analysis of early extensional “ribbon” structures in crustal plateaus and other occurrences of “tessera,” the characteristic terrain of crustal plateaus. The purpose of this work is to constrain local thermal conditions during ribbon formation. The results indicate that wherever they occur, ribbon structures record a strong surface layer less than 3 km thick—and thus, hot conditions. This analysis further supports the plume hypothesis, because it indicates an early, uniformly thin strong surface layer underlain by a weak substrate over each crustal plateau.; Finally, Paper 3 reports on finite element simulations of formation of short-wavelength (100–500 m) contractional crustal plateau features in crust with elasto-visco-plastic (EVP) rheology, and simultaneous cooling. The purpose of this work was to elucidate the role of these features in the broader context of crustal plateau evolution. The results indicate that (1) like ribbons, the very short-wavelength tectonic features preserved in crustal plateaus require a thermal state sufficiently hot to place the local brittle-ductile transition very close to the surface; (2) at least some of the short-wavelength topography is fault-related; and (3) it is possible to achieve simultaneous development of very short-wavelength fault topography and longer-wavelength viscous folding.; The unifying theme of this dissertation, then, is that the tectonic structures preserved in crustal plateaus record hot conditions during the early stages of plateau evolution, as predicted by the plume hypothesis.