On rates and styles of late volcanism and rifting on Venus

[1] We investigated the possibility of significant variations in the rates and styles of volcanism and rifting during the time postdating the formation of regional plains on Venus. We analyzed the age relations of all known impact craters 30 km in diameter (183 craters) with the neighboring geologic units. Of these we selected 164 craters which were superposed on regional plains and determined if post-regional-plains (PRP) volcanics and/or PRP rift structures were present in the crater vicinity and if these craters postdated the volcanics and rift structures or predated them. In 53 cases it was possible to determine these relations. On the basis of these relationships, it was found that the general rates of volcanism and rifting during PRP time were close to constant or at least had no drastic changes. This implies a significant change in the rates of volcanism and rifting in the vicinity of the boundary between PRP time and the preceding time, which is marked by the formation of the global wrinkle-ridge network. It was also found that (1) the role of rift-associated volcanism during PRP time was close to constant or slightly decreasing, (2) the role of corona-related volcanism was noticeably decreasing, and (3) the role of noncorona, hot spot volcanism was proportionally increasing. The latter changes may be due to thickening of the lithosphere during PRP time. The results imply also that at most if not all places where PRP rifting occurred, the rift-associated fracturing, when started, continued for a significant part of PRP time. Our conclusions are valid both (1) for the case of a globally synchronous transition from emplacement of regional plains to the PRP regime and (2) for the case of such a transition occurring at different times in different areas of Venus.