Testing the viability of topographic relaxation as a mechanism for the formation of lunar floor-fractured craters

Historically, two modes of origin for lunar floor-fractured craters have been proposed: subcrater laccolith emplacement and topographic relaxation. To test the viability of the topographic relaxation model, we employ an elastoviscoplastic finite element model and ductile rheology measurements of a devolatilized terrestrial diabase, an analog to lunar crustal materials. Trials were run under two situations: (1) under extreme conditions (e.g., high heat flow) to determine the maximum degree of relaxation that the model can accommodate, and (2) under more nominal conditions in order to resolve how much relaxation can be expected to occur under more plausible lunar conditions. We find that in the extreme case, topographic relaxation is a viable means only for explaining the observed shallowing of the largest floor-fractured craters (>100 km in diameter). Results from the less extreme case demonstrate that lunar materials are simply too rigid to experience degrees of relaxation as large as those observed in the floor-fractured crater population. These results indicate that topographic relaxation is not the primary mechanism for the formation of floor-fractured craters. Because the topographic relaxation model does not apply for the vast majority of floor-fractured craters (i.e., those with diameters less than ~60 km), we conclude that the laccolith intrusion model is a more viable alternative.