Geologic History of the Northern Portion of the South Pole-Aitken Basin on the Moon

Plain Language Summary We conducted a detailed photogeological analysis of the northern portion of the South Pole-Aitken (SPA) basin (10-60 degrees S, 125-175 degrees W) and compiled a geological map (1:500,000 scale) of this region. Our new absolute model age for the Apollo basin, 3.98+0.04/-0.06Ga, provides a lower age limit for the formation of the SPA basin. Some of the plains units in the study area were formed by distal ejecta from remote craters and basins. The characteristic concentrations of FeO and TiO2 of other plains are indicative of their volcanic origin. The oldest volcanic materials occur near the center of the SPA basin and have an Early Imbrian age of 3.80+0.02/-0.02Ga. Late Imbrian volcanic activity occurred in and around the Apollo basin. In total, the volcanic plains cover 8% of the map area and cannot account for the extensive SPA iron signature. The sources of the signature are the oldest materials on the SPA floor (FeO 11-14.5wt%). In contrast, the ejecta composing the SPA rim are significantly poorer in FeO (<7.5wt%). The signature could be related to the differentiation of the SPA impact melt. However, the spatial segregation of the ancient iron-rich and iron-poor materials suggests that the SPA iron signature predated the basin. Thus, the signature might be explained by a pre-SPA lunar crust that was stratified with respect to the iron concentrations, so that the SPA impact excavated the upper, iron-poorer portion of the crust to form the SPA rim and exposed the deeper, iron-richer portion on the floor of the basin. We conducted a geological analysis of the northern portion of the South Pole-Aitken basin, which is the largest recognized and likely the oldest impact structure on the Moon. Results of our mapping efforts permitted the unraveling of the major sequence of impact and volcanic events that have shaped the basin throughout its evolution and resulted in the discovery of the oldest materials related to the basin formation. Analysis of the distribution and concentrations of iron and titanium in the materials of different age within the South Pole-Aitken basin allows the characterization of the structure of the ancient lunar crust and mantle. These results introduce important constraints on the current models of the early evolution of the Moon.