Basin Crustal Structure at the Multiring Basin Transition

Abstract

AbstractTwo 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.