AGE OF RIFTING AND ASSOCIATED VOLCANISM IN ATLA-REGIO, VENUS

An estimation of the age of rifting and associated volcanism in Atla R egio is carried out based on photogeologic analyses of the stratigraph ic relationships between rift-associated features and impact craters w hich have associated radar-dark paraboloids. The paraboloids are belie ved to be thin mantles of debris derived from the fallout of crater ej ecta, and their ''parent'' craters are considered to be among the youn gest 10% of Venus' crater population [Campbell et al., 1992]. In the a rea under study, there are three impact craters with associated dark p araboloids: 1) Von Schuurman, which is clearly superimposed on the Dal i Chasma Rift; 2) Sitwell, which is superimposed on Ganis Chasma and m ay have been followed by subsequent rift-associated fracturing; and 3) Luxemburg, which is superimposed on lavas from Ozza Mons and dissecte d by Dali Chasma fractures which, in turn, are covered by lavas from M aat Mons. This means that at least the last stages of tectonic and vol canic rift-associated activity in Atla Regio are contemporaneous with the formation of impact craters which are young enough to preserve the associated dark paraboloids. If these craters represent the youngest 10% of the Venus crater population, and the average age of the venusia n surface is about 500 m.y. [Schaber et al., 1992], this means that ri fting and rift-associated volcanism in Atla Regio were active as recen tly as during the last approximately 50 m.y. In the current debate ove r models of Venus resurfacing the results of our study favor the equil ibrium resurfacing model [Phillips et al., 1992].