Lunar multiring basins and the cratering process

Numerous studies of the lunar gravity field have concluded that the lunar Moho is substantially uplifted beneath the young multiring basins. This uplift is presumably due to the excavation of large quantities of crustal material during the cratering process and subsequent rebound of the impact basin floor. Using a new dual-layered crustal thickness model of the Moon, the excavation cavities of some nearside multiring basins (Grimaldi and larger, and younger than Tranquillitatis) were reconstructed by restoring the uplifted Moho to its preimpact location. The farside South Pole-Aitken (SPA) basin was also considered due to its importance in deciphering lunar evolution. Restoring the Moho to its preimpact position beneath these basins resulted in a roughly parabolic depression from which the depth and diameter of the excavation cavity could be determined. Using these reconstructed excavation cavities, the basin-forming process was investigated. Excavation cavity diameters were generally found to be on the small side of most previous estimates (for Orientale the modeled excavation cavity lies within the Inner Rook Ring). Additionally, with the exception of the three largest basins (Serenitatis, Imbrium, and South Pole-Aitken) the depth/diameter ratios of the excavation cavities were found to be 0.115 @ 0.005, a value consistent with theoretical and experimental results for impact craters orders of magnitude smaller in size. The three largest basins, however, appear to have significantly shallower depths of excavation compared to this trend. It is possible that this may reflect a different physical process of crater formation (e.g., nonproportional scaling), special impact conditions, or postimpact modification processes. The crustal thickness model also shows that each basin is surrounded by an annulus of thickened crust. We interpret this thickened crust as representing thick basin ejecta deposits, and we show that the radial variation in the thickness of these deposits is consistent with scaling laws obtained from small-scale experimental studies. If multiring basins ever possessed a terraced main crater rim, this terraced zone may be presently unrecognizable at the surface due to the emplacement of ejecta deposits that exceed a few kilometers in thickness exterior to the excavation cavity rim. We also show that the interiors of many basins were superisostatic before mare volcanism commenced. Those basins that were closest to approaching a premare isostatic state lie close to or within an anomalous geochemical province rich in heat-producing elements. (C) 1999 Academic Press.