Résumé
In 2016 IODP/ICDP Expedition 364 drilled the peak ring of the Chicxulub impact structure, and recovered core from depths of 505 meters below sea floor (mbsf) to 1334 mbsf. The bottom 590 meters of core is largely composed of uplifted, heavily fractured and deformed granitic rocks. The core was scanned using x-ray computer tomography (CT), resulting in a three-dimensional model of the core imaged at 0.3mm resolution. Using orientation measurements of planar fractures in the granitic core and crosscutting relationships within CT-defined fracture facies, we seek to confirm the direction and relative timing of rock displacement and elucidate current models of crater formation. Using CT data, we grouped the fractures into four facies: open fractures; filled fractures; discrete fine fractures, and pervasive fine fractures. After orienting the core by matching the CT images to acoustic images of the borehole wall, we used CT images to measure the dip and dip direction of over 2,000 open, filled and discrete fine planar fractures. We verified the fracture facies before measurement by comparing the CT data with acoustic wellbore images and high-resolution linescan images of the split core. Pervasively fractured intervals proved too deformed to accurately measure individual fracture planes. We observe that cataclastically deformed zones appear as pervasive fractures in CT data and hypothesize that filled fractures on CT data are caused either by hydrothermal alteration or by cataclasis/ultracataclasis. Further, we hypothesize that uniform dip and dip directions of fractures reflect different stages of crater formation. We anticipate that fine fractures are formed within the rebound and early collapse stages and therefore will have different dips. We hypothesize that open and hydrothermally filled fractures are a result of late stage shear faulting and will have dips orthogonal to the peak ring crest, which is indicated in our preliminary results.