THE HISTORY OF DEFORMATION DURING PEAK-RING EMPLACEMENT: LINKING OBSERVATIONS WITH MODELS

The dynamic mechanisms that cause impact crater collapse are poorly known. In order to form peak rings, rocks must be significantly and temporarily weakened during transient cavity collapse. The mechanism(s) remain unresolved, in part, due to a lack of micro- and macro-scale structural data from pristine peak rings necessary to kinematically describe the emplacement of peak rings. IODP-ICDP Expedition 364 recovered nearly 600 m of shocked, deformed, and uplifted, crystalline basement rocks within the peak ring of the Chicxulub Crater. Investigation of these rocks will allow for the deformation history of the peak ring to be resolved.

Here, we show that the recovered target rocks experienced a sequence of discrete deformation events during peak-ring formation; that there was little to no relative rotation between the recovered target rocks; and that faulting occurred on thousands of individual fractures with displacements ranging from mm’s to dm’s and potentially up to the km scale. Furthermore, whilst the recovered basement rocks have experienced no relative rotation, the orientation of shock micro-structures (Feather Features), combined with numerical simulations, indicate that the entire sequence was rotated by ~90 degrees after the passage of the shock wave. Finally, we show that the deformation of the crystalline rocks by the shock wave and large-scale movement during peak-ring formation, has resulted in extraordinary porosities of 8-10%.

These geological observations are invaluable for calibrating numerical models of peak-ring formation. We have determined the incremental strain history of peak ring material within iSALE simulations of the Chicxulub impact. The modelled deformation history is consistent with the observations, allowing us to place quantitative constraints on the magnitudes, directionality, and rates of rotations, pure and simple shears, and volumetric strain, during the stages of peak-ring emplacement.

Our results provide further support for the dynamic collapse model of peak ring formation. Additionally, these results quantitatively describe the kinematics of peak-ring formation, linked to observed deformation styles within Chicxulub peak-ring material. This information provides crucial kinematic constraints on the dynamics of complex crater collapse.