The age of lunar mare basalts south of the Aristarchus Plateau and effects of secondary craters formed by the Aristarchus event

Using crater size-frequency distributions (CSFDs) and careful discrimination of primary and secondary impact craters in the mare region south of Aristarchus Crater, we reexamined the age of unit 'P60', determined previously by Hiesinger et al. (2003) to be one of the youngest basaltic surfaces on the Moon. Owing to the apparently young age of these basalts, this region is a potential site for future sample return missions. We investigated this 55,000 km2 region with Lunar Reconnaissance Orbiter Camera (LROC) images to assess potential variations in CSFDs across the unit, particularly in light of heavy contamination by secondary craters from Aristarchus Crater, and to determine the age(s) of P60 over its full areal extent. We are able to match, within one standard error, the previously determined age using approximately the same counting area. In addition, we defined twelve regions between the rays of secondaries to determine if and how the ages of P60 might vary across the unit. For these inter-ray regions, we find a systematic progression of ages from west to east, ranging from youngest (∼1 Ga) in the west, to 1.8 Ga southwest of Aristarchus, to 2.2 Ga south of Aristarchus, and finally to 2.7 Ga southeast of Aristarchus. This variation in ages is not solely attributable to secondary cratering, indicating it must be at least partially due to volcanic resurfacing. The northwestern-most extension of P60 may belong to a different unit owing to topographic and crater distribution differences. Analysis of the summed CSFD for P60 provides evidence for emplacement of the younger basalts (<2.5 Ga) on top of an older surface of ∼3.6 Ga. We observe that ∼2-3 km diameter craters within an older mare unit are embayed by the younger lava flows, some of which are 'ghost' craters, with barely visible rims. In addition, we identified six volcanic vents that are possible sources for the younger P60 flow(s). This work indicates that volcanism spanned a significant range, from ∼3.6 Ga to 1.0 Ga ago across this region.