Mean age of rifting and volcanism on Venus deduced from impact crater densities

UNLIKE the extensively cratered highlands of the Moon and Mars, the surface of Venus does not preserve a record of heavy bombard-ment from the early history of the Solar System 1-3 . Those craters that are found on Venus appear to be statistically indistinguishable from a random spatial population and rarely show modification by folds, faults and lava flows 1-3 . Although the volcanic and tec-tonic history of Venus is still much debated 2-5 , there is mounting evidence for near-global resurfacing ~300-500 Myr ago 1,2,6 . Moreover, it has recently been noted that the density of impact craters on large volcanic structures is less than the average crater density of the planet, suggestive of significant activity after the resurfacing event 7 . It is not clear, however, whether these features represent late remnants of the global event or continuing volcanism and tectonism of a still active planet. To address this question, we have used the regional variations in crater density to date volcan-oes, rifts and coronae which, based on stratigraphic evidence, clearly post-date the main resurfacing event 8-11 . The calculated mean ages of 70-125 Myr exclude the possibility that the majority of these features represent the final stages of the global event.