INSIGHTS INTO THE GEOLOGIC PROCESS OF PEAK RING FORMATION FROM THE CHICXULUB IMPACT CRATER

The Chicxulub impact crater in Mexico is a unique natural laboratory for investigating geological processes associated with large diameter impacts. Chicxulub is the only impact directly associated with a mass extinction event and the only large impact structure on Earth with an intact peak ring. Peak rings are diagnostic of large impacts on silicate planetary bodies and peak ring diameter may be directly related to impact energy. Despite likely originating from middle to upper crustal depths, the peak ring at Chicxulub has a distinct low velocity and low-density signature, possibly originating from extensive brecciation, and exhibits high-amplitude seismic reflections consistent with some melt lithologies. A dipping reflector imaged by seismic reflection data marks the outer boundary of the material that underlies and forms the topographic peak ring; this reflector may be a pseudotachylitic fault, a zone of mineralization from a post-impact hydrothermal system, or the result of an impedance contrast with the less brecciated lithologies external to the peak ring. At all imaged azimuths, the inwardmost downthrown blocks of the terrace zone underlie the outer edge of the peak ring and comes in contact with the dipping reflector. These observations support a emplacement process wherein acoustically fluidized crust rebounding upward subsequent to the initial impact come into contact with collapsing large slump blocks of the terrace zone; this impingement of the uplifting material by the gravitational slumping process may then result in the outward collapse of the central uplift to form the peak ring. Vital to this process may be the rheologic contrast between intact large blocks of the terrace zone deforming along discrete faults up to the kilometer scale with the central uplift material that is undergoing brittle deformation at all scales (acoustic fluidization). As the central uplift recedes into the subsurface outward emplaced peak ring may then form a natural barrier trapping the majority of the melt sheet within the crater center.