Peak ring formation at Chicxulub: Unraveling its deformation path and rock mechanical behavior

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The 200-km diameter Chicxulub impact crater in Yucatán, Mexico, was drilled and cored within the framework of IODP-ICDP Expedition 364 Chicxulub: Drilling the K-Pg Impact Crater. This drilling has for the first time targeted a peak ring, which is a mountainous ring found in large complex impact craters that rises above the crater floor topography and is internal to the crater rim. This proposal is focused on addressing two major questions posed within the framework of Expedition 364: 1) What is the nature and formational mechanism of peak rings? 2) How are rocks weakened during large impacts to allow them to collapse and form relatively wide, flat craters? Regarding the first question, there are two competing peak ring formation models: i) A conceptual geological model based on geological and remote sensing observations of peak rings on the moon and other planetary bodies that emphasize the role of a large amount of impact melt for the peak ring formation, and ii) a numerical model which uses hydrocode simulations to compute the formation process. The two models predict distinctly different kinematic paths and structural deformational features in the peak rings, which a preliminary survey shows are inherent in the deformed Expedition 364 drill cores. We will analyze the cores using quantitative micro- and macro-structural methods to unravel the deformational history of the peak ring, and thus yield ground-truth data to prove or disprove the formation models.The second question addresses the long-standing issue of considerable temporal strength reduction of the target that is required for crater formation and remains an enduring problem in cratering mechanics. Three models have been proposed as weakening mechanisms: 1) acoustic fluidization, which assumes seismic vibrations to reduce friction, 2) thermal softening, which postulates shock heating and plastic deformation, and 3) strain rate weakening/frictional melting, where e.g. localized melts are proposed to locally reduce friction. The drill core allow us to assess the relevance of the three models, and we will investigate the cores to distinguish between the weakening mechanisms based on specific microstructural indicators. Moreover, the strength degradation by impact damaging will be measured by means of rock mechanical testing. In addition, we will also evaluate the significance of rate dependent brittle deformation leading to rock pulverization as a process that influences rock strength.Our macro-and microstructural analyses will be combined to a form kinematic model for Chicxulub's peak ring, and will thus contribute to a deeper understanding of the formation of peak rings in the Solar system. This will help to improve the interpretation of remote sensing studies on large impact craters on other planetary bodies, and can potentially help to understand details of the processes that occurred during the K-Pg mass extinction event caused by the Chicxulub impact.