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

墨西哥尤卡坦州直径为200公里的希克苏鲁布撞击坑是在国际大洋钻探项目-国际综合钻探项目第364次考察希克苏鲁布：钻探K-Pg撞击坑的框架内钻探和取芯的。这一钻探首次以一个峰环为目标，这是在大型复杂撞击坑中发现的一个多山的环，它高出撞击坑底部地形，位于撞击坑边缘内部。这个建议的重点是解决在364远征的框架内提出的两个主要问题：1）峰环的性质和形成机制是什么？2)在大撞击中，岩石是如何被削弱的，从而使它们坍塌并形成相对较宽、较平的陨石坑？关于第一个问题，有两个相互竞争的峰环形成模型：一个是基于对月球和其他行星上峰环的地质和遥感观测的概念地质模型，强调大量撞击融化对峰环形成的作用，另一个是使用水文代码模拟来计算形成过程的数值模型。这两个模型预测明显不同的运动学路径和结构变形特征的峰环，初步调查显示，这是固有的变形远征364岩芯。我们将使用定量的微观和宏观结构方法来分析岩芯，以揭示峰环的变形历史，从而产生地面实况数据来证明或反驳形成模型。第二个问题解决了长期存在的问题，即陨石坑形成所需的目标的时间强度大幅降低，这仍然是陨石坑力学中的一个持久问题。已经提出了三种模型作为弱化机制：1）声流化，其假设地震振动以减少摩擦，2）热软化，其假设冲击加热和塑性变形，以及3）应变率弱化/摩擦熔化，其中例如提出局部熔化以局部减少摩擦。钻芯使我们能够评估这三种模型的相关性，我们将研究芯，以区分基于特定的微观结构指标的弱化机制。此外，将通过岩石力学试验测量冲击损伤引起的强度退化。此外，我们还将评估速率相关的脆性变形导致岩石粉碎作为一个过程，影响岩石强度的意义。我们的宏观和微观结构分析将结合到一个形式的运动学模型希克苏鲁布的峰环，从而有助于更深入地了解在太阳系中的峰环的形成。这将有助于改进对其他行星上大型撞击坑的遥感研究的解释，并可能有助于了解希克苏鲁布撞击造成的K-Pg大灭绝事件期间发生的过程的细节。

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.