Geological Evolution of Venusian Deformation Belts : Examples from Poludnista Dorsa and Oya Dorsa and Implications

Deformation belts are complex linear features in Venus' lowlands, analogous in scale to terrestrial mountain ranges. While the precise mode of origin remains unclear, they are clearly shaped by a complex interplay of folding and faulting at various scales. Detailed geological mapping of Magellan SAR images combined with radar altimetry profiles allow an examination of the sequence of deformation belt evolution.

Poludnista Dorsa, a complex 2000-kilometer long deformation belt in Rusalka Planitia, is segmented by orientation and tectonic style. Much of this belt is made up of topographic arches up to a kilometer high and a couple of hundred kilometers wide, overprinting a structural core of local ridges and lineaments approximately an order of magnitude smaller. The flacks of the arch component typically extending beyond the structural core into surrounding smooth materials, and control some of the regional short-wavelength deformation (wrinkle ridges). As the smooth materials locally embay the structural core, and if they represent a local stratigraphic marker, archs appears to be a late feature of the topography of Poludnista Dorsa. Simultaneous regional development of short and long wavelengths of deformation is not supported by the mapping; thus a mechanically stratified lithosphere of regional extent is not immediately required by the local geological history. Local lithospheric thinning within the belt (spatial localization) or progressive regional thickening of the mechanical layer (temporal evolution) better explain the observed sequence of events. While the broad arch topography may reflect in-plane contraction, the location of the arch may simply represent strain focused on an existing belt dominated by shorter-wavelength structures of more ambiguous origin. Evidence for coeval local volcanism, superposed contraction and extensional faulting, abrupt segmentation may indicate a role for out-of-plane forces (for example, delamination, detachment or crustal overthickening and collapse) in the structural core of the belt. Similar results are found for the simpler belt Oya Dorsa in Llorono Planitia.

Evolution in the scale of deformation and the existence of intertwined volcanism demand an improvement in our models of these enigmatic features.