Finite-element modeling of large impact craters: Implications for the size of the Vredefort structure and the formation of multiple ring craters

Abstract

Impact cratering is a complex process which is not yet fully understood, especially in the cases of large planetary events. Most of the observations of impact craters created by such events are limited to remote sensing of their surface morphology; although there are large terrestrial craters whose sub-surface structures can be studied, most have been modified by subsequent geologic activity. Laboratory experiments are necessarily limited to very small impacts so their results need to be extrapolated over many orders of magnitude to compare to the largest terrestrial craters (>100 km in diameter). So, in order to study the formation of large craters it is useful to employ numerical simulations. Finite-element modeling is a numerical method that can accommodate complex structures and a variety of rheologies and can perform simulations at any scale. It is, therefore, useful for simulating impact crater collapse and I have used it to investigate different aspects of this process. In collaboration, E. Pierazzo and I used both hydrocode and finite-element modeling to recreate the formation of the Vredefort structure in order to predict where the pressure of an impact-generated shock wave would have been sufficient to form shatter cones and planar deformation features and to follow their subsequent displacement during crater excavation and collapse. By comparing the results of simulations of impacts by projectiles of various sizes to the observed locations of the shock features around Vredefort we constrained the projectile diameter to be 10-14 km. This corresponds to a final crater diameter of 120-200 km. I used finite-element models of crater collapse to investigate the ring-tectonic theory of multiple ring crater formation. The results of these models indicate that the ring-tectonic theory is consistent with the formation of circumferential faults around large terrestrial impact craters such as Chicxulub. The final project described in this dissertation uses the morphologies of impact craters on the icy Jovian satellite Europa to probe its lithospheric structure. Comparisons of simulated stress fields to the observed fracture patterns around Europan craters suggest that the elastic lithosphere in which the crater formed was at least 12 km thick.