Characteristics of lunar mare deposits in Smythii and Marginis basins: Implications for magma transport mechanisms

An analysis of 34 lunar lava flows and ponds in the eastern limb Smythii and Marginis basins was undertaken to examine and model the first stages of secondary crustal formation and assess processes involved in magma transport and eruption. In order to isolate the characteristics of single eruptive pisodes, we focused on discrete mare ponds adjacent to the major mafia. Mean values for areas and volumes of deposits estimated to be good candidates for single eruptive phases are large, approximately 950-1000 km 2 and -200 km 3 respectively. These ruptive volumes are commensurate with the largest known terrestrial eruptions (flood basalts). The lack of geomorphological structures indicative of shallow magma reservoirs indicates that deep, probably subcrustal source regions are prevalent. With respect to crustal thickness relationships, the magnitude and frequency of eruptive events are observed to be greatest in areas of thinnest crust. Specifically, regions of major maria (Mare Smythii, Mare Marginis) occur in areas of thinnest crust, while isolated ponds occur where the crust is relatively thicker. This is consistent with the correlation observed globally between crustal thickness and the magnitude and frequency of eruptive events. Ages of volcanic flows and pyroclastic events range from Early Imbrian to Imbfian-Eratosthenian but are concentrated most heavily between 3.80-3.60 Ga, suggesting a period of peak volcanism beginning around 3.85 Ga and lasting approximately 200 Ma. However, the existence of dark-halo crater clusters in non-mare units within the region suggests he presence of cryptomaha, which would indicate an earlier onset of volcanism and a volume of mare matehal potentially greater than that currently exposed on the surface. Typical nearest-neighbor distances suggest deposits derive from reservoirs <-100 km in diameter. The observations made here are consistent with a magma transport model in which plumes rising diapirically stall at a density boundary under the lunar crust and propagate dikes to the surface through overpressufization.