258 SOLAR SYSTEM/Venus Figure 20 Relationships of shield plains (psh) with other units and structures (A) Some portion of a groove belt (gb) is covered by deposits of shield plains that are obviously younger A few shields, however, are cut by the fractures of the belt, suggesting that the formation of groove belt and shield plains partly overlapped in time Fragment of C1 MIDR.30N333; illumination is from the left (B) Occurrences of shield plains within regional plains are often characterized by a specific pattern of deformation confined within shield plains and a radar albedo that is different from the albedo of regional plains Small individual shields that are seen within regional plains have morphological characteristics similar to these, typical of the main occurrences of shield plains This suggests that the individual shields represent kipukas of more widespread shield plains covered by a mantle of wrinkle ridged plains (pwr) structures of coronae and arachnoids, and fractures and graben of novae commonly cut the plains Major Topographic Features The fact that the gravity and topography fields of Venus are highly correlated suggests that Venus may not have a low-viscosity layer, as in the asthenosphere of Earth, and that the Venusian mantle is strongly coupled with the lithosphere Thus, the mantle circulation on Venus could be directly responsible for the formation of large-scale tectonic and topographic features In the almost complete absence of erosion, on the other hand, the large-scale topographic features on Venus should much better reflect the balance between lithospheric buoyancy and mantle dynamics, compared to Earth Thus the distribution of the major topographic features on Venus combines the present pattern of mantle convection with contributions from extinct patterns The global altimetry data collected by Pioneer Venus, Venera-15/16, and Magellan show that three principal topographic provinces characterize the surface of the planet (Figure 1B) Lowlands (