Abstract
Porosity and its distribution in impact craters has an important effect on the petrophysical properties of impactites: seismic wave speeds and reflectivity, rock permeability, strength, and density. These properties are important for the identification of potential craters and the understanding of the process and consequences of cratering. The Chicxulub impact structure, recently drilled by the joint International Ocean Discovery Program and International Continental scientific Drilling Program Expedition 364, provides a unique opportunity to compare direct observations of impactites with geophysical observations and models. Here, we combine small-scale petrographic and petrophysical measurements with larger-scale geophysical measurements and numerical simulations of the Chicxulub impact structure. Our aim is to assess the cause of unusually high porosities within the Chicxulub peak ring and the capability of numerical impact simulations to predict the gravity signature and the distribution and texture of porosity within craters. We show that high porosities within the Chicxulub peak ring are primarily caused by shock-induced microfracturing. These fractures have preferred orientations, which can be predicted by considering the orientations of principal stresses during shock, and subsequent deformation during peak ring formation. Our results demonstrate that numerical impact simulations, implementing the Dynamic Collapse Model of peak ring formation, can accurately predict the distribution and orientation of impact-induced microfractures in large craters, which plays an important role in the geophysical signature of impact structures. Plain Language Summary The Chicxulub crater, Mexico, is widely known for its association with the extinction of the nonavian dinosaurs at the end of the Cretaceous period. The crater was first identified due to its gravitational and magnetic anomalies. Potential impact structures are often identified, in part, on the basis of geophysical anomalies, most commonly including a circular gravity low. Gravity is slightly weaker at craters because the impact cratering process removes mass from the impact site. In this study, we examine the cause of the Chicxulub gravity anomaly by combining observations from recent drilling of the crater, geophysical data measured across the crater, and numerical impact simulations. We demonstrate that porosity in rocks beneath the crater floor is primarily accommodated by fracturing during the impact cratering process, that the orientation of those fractures are consistent with predictions from numerical impact simulations, and that impact-induced porosity is one of the primary causes of gravity anomalies in large impact craters.