MOUNTAINS IN MINUTES AND MELT SEAS IN SECONDS ON THE MOON: THE ROLE OF IMPACT BASINS IN LANDSCAPE EVOLUTION

Very large hypervelocity impact events are rare in the last half of Solar System history, but were common throughout the Solar System in its earlier history. Individual basin-scale impact events have a profound effect on huge areas, excavating crust and perhaps mantle, redistributing it laterally over significant portions of a planetary body, and creating multiple major mountain ranges during the excavation and collapse phase. Huge amounts of melted target rock settle into the collapsed basin interior as a sea of impact melt that undergoes subsequent cooling and differentiation. New data from lunar orbiting spacecraft provide insight into the formation and evolution of these basins. The 930 km diameter Orientale basin is the youngest and most well-preserved large multi-ringed impact basin on the Moon; it has not been significantly filled with mare basalts and thus the nature of the basin interior deposits and ring structures are very well-exposed and provide major insight into the formation and evolution of planetary multi-ringed impact basins. Pre-basin structure had a major effect; dozens of impact craters and basins, ranging up to the 630 km diameter Mendel-Rydberg basin, underlie both the Orientale ejecta (Hevelius Formation-HF) and the unit between the basin rim (Cordillera ring-CR) and the Outer Rook ring (OR) (known as the Montes Rook Formation-MRF). The distribution of these features supports the interpretation that the 620 km diameter OR ring is the closest approximation to the basin excavation cavity. Total basin interior topography ranges ~6-7 km from mountain peaks to basin floor. The inner basin depression is about 2-4 km deep below the Inner Rook (IR) plateau. The Maunder Formation (MF) consists of smooth plains (on the inner basin depression walls and floor) and corrugated deposits (on the IR plateau); observed in the melt sheet are depressions interpreted to be due to local drainage, and cracks related to cooling and solidification. This configuration supports the interpretation that the MF consists of different facies of impact melt, and that the central plains unit was an impact melt sea several kilometers deep. These new data reveal the dynamics of such basins and provide insight into the formative years of planetary crusts.