Impact‐Induced Porosity and Microfracturing at the Chicxulub Impact Structure

Auriol S P Rae,Ulrich Riller,Sean P S Gulick,Jody Leung,Gordon R Osinski,Michael Poelchau,Gareth S Collins,Johanna Lofi,Richard A F Grieve,Tobias Salge,Gail L Christeson,Catalina Gebhardt,Joanna V Morgan,Iodp‐Icdp Expedition 364 Scientists

doi:10.1029/2019je005929

Abstract

AbstractPorosity 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.

Highlights

Impact cratering is the dominant surface process on most solid bodies of the solar system
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
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

Summary

Introduction

Impact cratering is the dominant surface process on most solid bodies of the solar system. The process of hypervelocity impact causes irreversible changes to the nature and physical properties of rocks (Melosh, 1989). These changes produce anomalies in geophysical data that make craters conspicuous in comparison to the surrounding rocks. Negative residual gravity anomalies at craters are caused by the replacement of high-density intact rocks in the preimpact target with fractured para-autochthonous rocks, porous impact breccias, and/or the relative loss of mass due to excavation and basin infill (whether it is filled by atmosphere, water, post-impact sedimentary rocks, or vacuum). Within complex craters, which are shallow compared to their diameters, the main contributor to the gravity signature derives from fractured para-autochthonous target rocks beneath the floor of the crater (Pilkington & Grieve, 1992). Understanding how small-scale properties of impactites link to the large-scale geophysical characteristics of craters is of critical importance

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