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In contrast to multiring basins, megabasins lack clear ring structures and display crustal profiles characterized by a thin layer of crust at their center that gradually increases in thickness to the surrounding region. The 1,550‐km‐diameter Caloris basin on Mercury is often speculated to be a multiring basin; however, its crustal structure is indicative of a megabasin, perhaps explaining the lack of discernible rings. Here, we model the formation of Caloris basin using iSALE‐2D under a range of preimpact thermal conditions to determine whether the processes responsible for its megabasin crustal structure could lead to the formation of basin rings. We find that thermal gradients between 22 and 30 K/km—hotter than previously inferred—facilitate the reproduction of Caloris basin's crustal structure through crustal flowback, though such preimpact thermal structures preclude its formation as a multiring basin. Instead, repeated thrusting and necking events during transient crater collapse induce instances of fault reactivation, which diminish initial fault offsets and produce a series of discrete crustal blocks. Models assuming cooler thermal gradients lead to overly thin or non‐existent crust at the basin center, in contrast to observations. Even if crustal deficits were made up via a differentiating melt pool, the crust elsewhere would be overly thick, supporting the idea that the crustal structure of megabasins is primarily associated with transient crater collapse. We conclude that the transition to a megabasin morphology on terrestrial planetary surfaces, like the multiring transition, is dependent upon the size of the basin compared to the lithospheric thickness. Plain Language Summary: Impact craters are the most pervasive geologic features in our Solar System. At the largest scales, aptly termed "impact basins" can present with varying surface and crustal morphologies. For example, some impact basins contain multiple concentric topographic "rings," while others do not. Additionally, basins with and without rings exhibit contrasting crustal structures. Such differences are owed to the properties inherent to the impacted body at the time the basin‐forming impact occurred. On Mercury, the Caloris basin has frequently been suggested to have formed multiple rings, though its crustal structure suggests otherwise. Using numerical techniques, we model the formation of Caloris basin with an emphasis on reproducing its inferred crustal structure to determine whether these processes can also produce basin‐concentric rings. We find that a warm and weak interior structure allows the impact‐fractured crust to flow back toward Caloris' center during the impact event. The fracture patterns in the crust, and its fragmentation into several blocks, however, do not suggest it formed a series of rings. These results suggest that Mercury may have been warmer than previously thought, and that a planetary body's internal temperature compared to the size of the basin is what dictates whether rings will form. Key Points: The formation of the Caloris megabasin is marked by fault reactivation and the segmentation of its crust into several rafted blocksCrust at Caloris' center can be explained by the inward flow of crustal material weakened during transient crater collapseNear‐surface temperature gradients determine whether impact basins form with multiring or megabasin morphologies [ABSTRACT FROM AUTHOR]

Multiring basins dominate the crustal structure, tectonics, and stratigraphy of the Moon. Understanding how these basins form is crucial for understanding the evolution of ancient planetary crusts. To understand how preimpact thermal structure and crustal thickness affect the formation of multiring basins, we simulate the formation of lunar basins and their rings under a range of target and impactor conditions. We find that ring locations, spacing, and offsets are sensitive to lunar thermal gradient (strength of the lithosphere), temperature of the deep lunar mantle (strength of the asthenosphere), and preimpact crustal thickness. We also explore the effect of impactor size on the formation of basin rings and reproduce the observed transition from peak‐ring basins to multiring basins and reproduced many observed aspects of ring spacing and location. Our results are in broad agreement with the ring tectonic theory for the formation of basin rings and also suggest that ring tectonic theory applies to the rim scarp of smaller peak‐ring basins. Plain Language Summary: The largest impact craters on the Moon are multiring basins that exhibit three or more topographic rings. Great volumes of material were ejected and redistributed during the formation of these 1,000‐km‐scale basins. Formation of these basins is the predominant process driving change of the lunar crust, the outermost layer of the Moon. Why large basins have multiple topographic rings and what they might tell us about the Moon remain poorly understood. Here we simulate the formation of these basins and their rings during an asteroid impact. We explore how thickness of the lunar crust, size of the impacting body, and interior temperature of the Moon affect the formation of basins and their rings. With well‐persevered multiring basins and an abundance of high‐quality gravity and topography data, the Moon is an ideal location to explore the formation of multiring basins. A better understanding of the formation of these basins will help us understand how similar basins may have affected the crusts of the Earth, Mars, Mercury, and Venus. Key Points: We simulate the formation of multiring basins exploring the effect of impactor size, target thermal structure, and crustal thicknessOur simulations reproduce observed trends in ring spacing including the transition from peak‐ring to multiring structuresOur results show that basin ring formation is quite sensitive to target thermal structure in general agreement with ring tectonic theory [ABSTRACT FROM AUTHOR]