Lunar multiring basins and the cratering process

Numerous studies of the lunar gravity field have concluded that the lunar M oho is substantially uplifted beneath the young multiring basins. This upli ft is presumably due to the excavation of large quantities of crustal mater ial during the cratering process and subsequent rebound of the impact basin floor. Using a new dual-layered crustal thickness model of the Moon, the e xcavation cavities of some nearside multiring basins (Grimaldi and larger, and younger than Tranquillitatis) were reconstructed by restoring the uplif ted Moho to its preimpact location. The farside South Pole-Aitken (SPA) bas in was also considered due to its importance in deciphering lunar evolution . Restoring the Moho to its preimpact position beneath these basins resulte d in a roughly parabolic depression from which the depth and diameter of th e excavation cavity could be determined. Using these reconstructed excavati on cavities, the basin-forming process was investigated. Excavation cavity diameters were generally found to be on the small side of most previous est imates (for Orientale the modeled excavation cavity lies within the Inner R ook Ring). Additionally, with the exception of the three largest basins (Se renitatis, Imbrium, and South Pole-Aitken) the depth/diameter ratios of the excavation cavities were found to be 0.115 @ 0.005, a value consistent wit h theoretical and experimental results for impact craters orders of magnitu de smaller in size. The three largest basins, however, appear to have signi ficantly shallower depths of excavation compared to this trend. It is possi ble that this may reflect a different physical process of crater formation (e.g., nonproportional scaling), special impact conditions, or postimpact m odification processes. The crustal thickness model also shows that each bas in is surrounded by an annulus of thickened crust. We interpret this thicke ned crust as representing thick basin ejecta deposits, and we show that the radial variation in the thickness of these deposits is consistent with sca ling laws obtained from small-scale experimental studies. If multiring basi ns ever possessed a terraced main crater rim, this terraced zone may be pre sently unrecognizable at the surface due to the emplacement of ejecta depos its that exceed a few kilometers in thickness exterior to the excavation ca vity rim. We also show that the interiors of many basins were superisostati c before mare volcanism commenced. Those basins that were closest to approa ching a premare isostatic state lie close to or within an anomalous geochem ical province rich in heat-producing elements. (C) 1999 Academic Press.