Crustal Block and Muted Ring Development During the Formation of Mercury's Caloris Megabasin

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.