Venus astra/novae: Estimates of the absolute time duration of their activity

In order to assess the age relations between astra/novae (features with extensive radial fracture-graben systems) and their surroundings, and to determine the duration of their activity, we undertook a photogeologic analysis of Magellan images of 78 astra, 49 dark-parabola craters and 114 clear-halo craters. For seven of these 78 astra it was found that the astrum-forming faulting started before or close to the time of emplacement of regional plains and extended into the second part of post-regional-plains time. Because the mean age of the regional plains is close to the mean surface age of Venus (which is estimated to be ∼750 m.y), this means that the duration of activity of these seven astra was several hundred millions of years. This is longer than the duration of activity of ongoing mantle plumes on Earth, but shorter than the duration of activity of the plume feeding the martian volcano Olympus Mons. The basic morphologic characteristics of these seven astra, as well as their age relations with other geologic units, are similar to those of the majority of other astra; therefore, such a long duration could also be typical of some other astra. We confirm the two-phase (pre- and post-regional-plains) evolution of astrum-forming faulting suggested in previous studies. For the first phase we show evidence for purely tectonic faulting caused by the diapiric rise of a mantle plume. For the second phase we find evidence supporting the interpretation of other studies that the observed faults resulted from subsurface dike intrusions produced by magmatism associated with the plume. We also found that faulting during the second phase was not instantaneous but distributed over a long period of time.