The Deep Structure of Lunar Basins' Implications for Basin Formation and Modification

We present models for the structure of the crust and upper mantle beneath lunar impact basins from an inversion of gravity and topographic data from the nearside of the moon. All basin models display a thinner crust and an elevated Moho beneath the central basin region compared to surrounding areas, a signature of the processes of basin excavation and mantle uplift during collapse of the transient cavity. There is a general decrease in the magnitude of apparent uplift of mantle material with increasing basin age; we attribute this relation primarily to enhanced rates of ductile flow of crustal material early in lunar history when crustal temperatures were relatively high and the effective elastic lithosphere was thin. The more relaxed topographic and Moho relief associated with older basins on the central nearside may, in particular, be at least partly a consequence of the extensive subsurface heating associated with the formation of the large Procellarum basin. The deep structure of the youngest basins constrains the geometry of the cavity of excavation and the amount of crustal material ejected beyond the basin rim. From the volumes of the topographic basin, of mare basalt fill, and of uplifted mantle material, the volume of crustal material ejected beyond the basin rim for an Orientale-sized event was of the order of 107 km 3. A near-constant thickness of nonmare crustal material beneath the central regions of young basins of various diameters and preimpact crustal thicknesses uggests that the transient cavity excavated to at least the base of the crust for the largest basins; significant excavation into the mantle may have been impeded by an abrupt increase in strength at the lunar Moho.