Rifts and large volcanoes of Venus: Global assessment of their age relations with regional plains

We report on the results of synoptic global mapping of rifts and large volcanoes on Venus and detailed mapping in six areas illustrating the relationships among these features. Rifts and large volcanoes were subdivided into old, transitional (for volcanoes only), and young features. The geologic event separating old features from young ones is the formation of wrinkle ridges on the regional plains. Global mapping showed that the postplains rifts dominate in the population of rifts both in terms of number of mapped segments and in area. The postplains volcanoes are more abundant in terms of number and area occupied than those that are transitional in age, and these, in turn, dominate over the old ones. These data show that in post-regional-plains time rifting and formation of large volcanoes are significantly more important processes than in the immediately preceding period. The time period during which regional plains were emplaced, and old (and partly transitional) large volcanoes formed, was much shorter than the postplains time. Therefore the mean rate of formation of large volcanoes during this first period was significantly higher than in the subsequent time. Similarly, the relatively short time duration of the plains-emplacement time indicates that the mean rate of old rifting was higher than in more recent time. This study shows prominent changes with time in the absolute rates and roles of rifting and large volcano formation. This supports the ``directional'' model of the geologic evolution of Venus during the last several hundred million years, as opposed to the ``nondirectional'' model, which predicts a more random occurrence of geologic processes throughout history.