Seismic travel-time tomography under the peak ring of the Chicxulub impact crater

We present the results of a three-dimensional seismic tomography study of the upper crust beneath a quadrant of the peak ring structure of the Chicxulub meteorite impact crater, Mexico. Reflection and refraction travel-times from a grid of seismic profiles recorded by a 6 km streamer and 48 ocean bottom seismometer stations were inverted to give a well-resolved three-dimensional velocity model to a maximum depth of 6-8 km. The model comprised the thin water layer, a layer of low seismic velocity post-impact sedimentary infill, and the crater basement, which was separated from the fill by the interface representing the top of the crater, defined by normal incidence reflection picks. The crater basement shows a cylinder-shaped feature extending vertically downwards beneath the topographic peak ring to at least 8 km, the depth of resolution of this survey, characterised by slower seismic velocities than in the surrounding rocks at the same depth. This result supports and extends the observations of previous seismic refraction work in the peak ring, and also of scientific drilling, that the material in the peak ring has significantly reduced seismic velocity compared to typical granitic basement lithologies. We used the velocity model to perform a pre-stack depth migration of a key seismic reflection profile. In the best-fitting model presented here the prominent dipping reflector previously identified on seismic reflection profiles, which projects to the outer edge of the peak ring, dips inwards and crosses the low velocity cylinder without an apparent first order contrast in impedance. This result implies that the reflectivity of this dipping reflector is due to a thin, high-contrast layer such as entrapped impact melt or hydrothermal alteration within the overturned structures. We also identify well-imaged slump blocks from the crater rim/inner ring inwards, variations in the height and width of the peak ring, and associated variations in the velocity contrast that characterises the anomaly beneath the peak ring.