Basin Crustal Structure at the Multiring Basin Transition

Two impact basins on the Moon-Freundlich-Sharonov and Hertzsprung-are nearly the same size but exhibit different surface morphologies and subsurface structures. Gravity data reveal a bench-like transitional structure in the crust-mantle interface between the outer ring and the inner basin cavity in Hertzsprung, unlike that beneath both Freundlich-Sharonov and larger multi-ring basins. We use iSALE-2D to model the formation of impact basins into a 40-km thick pre-impact crust with a range of thermal conditions to understand the divergent development of these basins and gain insight into the factors affecting whether a basin forms with a peak-ring or multiring basin structure. We find that thermal gradients of at least 30 K/km result in Freundlich-Sharonov-type basins, in agreement with previous work. Cooler thermal gradients of approximately 15-20 K/km are needed to develop Hertzsprung-like multiring basins with observed bench-like structures in the crust-mantle topography. We find that for cooler models, the bench structure develops early in the cratering process as a rotated inner normal fault cutting the crust-mantle interface, whereas models with higher thermal gradients instead develop diffuse deformation zones instead of a discrete inner fault. The peak rings of both basins develop later in the cratering process as the collapsed central uplift. These results highlight the complex interplay between a strong lithosphere needed to develop ring faults and accessible ductile rocks that facilitate multiring basin formation. The varying thermal conditions giving rise to Hertzsprung and Freundlich-Sharonov impact basins may be a possible constraint on the lunar cooling rate and chronology.