NEW EVIDENCE FOR GRABEN ORIGIN OF OCEANUS-PROCELLARUM FROM LUNAR SOUNDER OPTICAL IMAGERY

The Procellarum Basin is thought by some to be the result of a gargant uan impact that occurred very early in lunar history. It is suggested that later impacts, such as Imbrium, would have partially obliterated the outline of that ancient ''Gargantuan'' basin in most areas, leavin g only its western margin with a circular shape. An alternative idea i s that the Procellarum Basin is the result of faulting associated with the Imbrium event. We have detected a subsurface horizon in Oceanus P rocellarum using the Apollo lunar sounder experiment optical-format da ta. The horizon is deepest at the western edge of the basin, and shall ows eastward. We did not observe peak ring structure in the areas wher e peak rings were predicted to exist according to the ''Gargantuan imp act'' hypothesis. This observation implies that the alternative (grabe n) formation hypothesis for Oceanus Procellarum may be more likely. hi this paper, we also review other geophysical and geologic evidence th at supports the graben hypothesis. Finally, we report on some new rada r evidence of a 500-km-diameter pre-Imbrian impact basin beneath Mare Insularum, in the Copernicus region.