ORIGIN OF PEAK-RING BASINS ON THE MOON: IMPLICATIONS FOR CRUSTAL STRUCTURE

Impact basins provide windows into the Moon's crustal structure and stratigraphy. Interpreting the origin of basin materials requires constraints on the processes controlling basin formation and morphology. Peak-ring basins provide important insight into the basin-formation process, as they are transitional between complex craters with central peaks and multi-ring basins. New image and altimetry data from the Lunar Reconnaissance Orbiter have permitted a reassessment of the origin of lunar peak-ring basins. We synthesize new morphometric observations of lunar peak-ring basins to construct a model for their formation via modification of an interior melt cavity. At the onset diameter of peak-ring basins, the volume and depth of melting are great enough to modify the interior morphology of the basin. The depth of this nested melt cavity is near the transient crater depth, creating a strengthless interior cavity that facilitates gravitational collapse of the transient crater. On the Moon, maximum melting depths are near or exceed the crust-mantle boundary; impact melt in peak-ring basins is thus composed primarily of lower crustal material and some mantle material. Peak rings are formed by the interaction of the inward and upward collapse of the rim of the transient cavity, which uplifts the unmelted base of the excavation cavity. The diameter of the melt cavity is about a factor of five to ten smaller than the current peak-ring diameter, suggesting that peak rings are formed from material uplifted between the edge of the melt cavity and rim of the transient crater. This predicts peak-ring sampling depths much shallower than the maximum depth of melting. The final configuration of the peak-ring basin has a several kilometers-thick slab of cooled impact melt on an uplifted mantle plug with little solid crustal material. Highly faulted and fractured and possibly thickened crust should occur below and outward from the peak ring due to inward and upward translation of collapsed transient crater rim material. This formation scenario has important implications for the interpretation of GRAIL gravity data over basins. The gravity structure should reflect a high density, uplifted impact melt plus mantle zone confined within the peak ring, which is surrounded by a highly fractured, low density zone of possibly thickened crust.