Aphrodite Terra, the largest highland region of Venus, is a likely site of mantle upwelling, active volcanism, and extensional tectonics. We examine three alternative kinematic models for the interaction of mantle convection and the surface: one in which little horizontal surface displacement results from mantle flow (''vertical tectonics''), one in which shear strain from large horizontal displacements is accommodated only in narrow zones of deformation (''plate divergence''), and one in which strain from large horizontal motions is broadly accommodated (''distributed deformation''). In support of a divergent plate model for Aphrodite, Head and Crumpler have cited the presence of an organized system of lineaments held to be analogues to oceanic fracture zones; small-scale bilateral symmetry of topographic elements, hypothesized to be rifted and separated relief analogous to some terrestrial oceanic plateaus; and subsidence of topography proportional to the square root of distance, consistent with that expected for a divergent thermal boundary layer. We undertake quantitative tests of each of these assertions, and we compare the results with those from similar tests of the Mid-Atlantic Ridge.

We find that apart from the long-wavelength symmetry of broadly elevated regions, there is no convincing evidence for regional bilateral symmetry of features several hundred kilometers in size on either planet. The fit of the topography of Aphrodite to that of a thermal boundary layer is in general much poorer than for the Earth, and so other mechanisms such as dynamic uplift or crustal thickness variations must dominate the topography. The broad saddle-shaped region between Thetis and Atla Regiones shows the best fit to the root-distance relation, yielding apparent spreading half rates of a few centimeters per year, but the goodness of fit and apparent spreading rates are quite sensitive to the distance range analyzed. Calculation of a single pole of relative motion for the entire postulated system of transform faults shows that the inferred fracture zone traces are not consistent with a simple two-plate model, regardless of past pole motions. A multiple-plate geometry is therefore called for, and one or more of the lineaments must act as a plate boundary along nearly its full length, if the plate divergence hypothesis is correct. Such a boundary would be distinguishable from other lineaments by geological evidence for recent nontransform motion in forthcoming Magellan radar images. In the absence of such evidence, as long as the existence and distribution of the lineaments are verified, then broad disruption of a thin lithosphere is favored. In such a model, lineaments may be surface manifestations of mantle convective flow. A model dominated by primarily vertical tectonics is tenable only if the lineaments are not confirmed by Magellan. ¿ AmericanGeophysical Union 1989