SIMULATING THE FORMATION OF MERCURY’S CALORIS BASIN TO DETERMINE IF IT LIKELY FORMED AS A MULTIRING BASIN

The 1550-km diameter Caloris basin, Mercury’s largest, best-preserved impact structure, is often regarded as the planet’s only multiring class basin, a hypothesis that has persisted since the Mariner 10 flybys. Previous researchers have proposed a range of ring diameters primarily through inferred connections between their presumed burial beneath superposed volcanic material and the observed transitions between faulting styles at the surface. Previous hydrocode modeling applied to lunar multiring basins attributed ring formation to normal faulting during transient crater collapse with the pre-impact thermal structure being a key factor in their location. Here, we apply a similar approach to simulate the formation of Caloris basin using the observed basin diameter and crustal thickness distribution to constrain impactor and planetary characteristics at the time of impact, such as Mercury’s pre-impact thermal structure. We then use our best-fitting models to explore whether Caloris formed as a multiring basin.

Our simulations imply that Caloris likely developed shear zones at locations broadly consistent with previously inferred ring locations, with the exact locations dependent on parameter choices within the best-fitting model set. Our best-fit models assume thermal gradients that correspond to surface heat fluxes of 60 mW/m², a higher yet still plausible value than previously suggested. This thermal structure implies Caloris is either older than ~3.8 Ga when Mercury was much hotter, or that a warm interior structure was maintained for a protracted timeframe. In contrast to the lunar studies, shear localization in Caloris models is not solely resultant from extensional failure of the lithosphere. Instead, alternating periods of basin-wide extension and compression during the cratering process cause portions of the inner-most crust to detach from itself, forming discrete blocks bounded by shear zones. This is a consequence of warmer thermal gradients which produce weaker interior structures. While not ring faults in the sense of normal faults with significant offsets, the shear zones produced in our best-fit models suggest that Caloris may have indeed formed as a multiring basin. Further modeling is required to determine how these shear zones respond to subsequent cooling and isostatic adjustment.