SUB-SURFACE ASYMMETRIES OF THE CHICXULUB IMPACT CRATER

Formation and subsequent collapse of the 65 Ma Chicxulub impact crater are of key interest due to the impact's role in the K-T mass extinctions and Chicxulub being the only preserved large impact crater currently accessible in the solar system. Seismic reflection profiles imaging the offshore portion of the crater suggest that it includes multiple, largely circular rings at distances of up 140 km from the center of the crater. However, in cross section the crater is surprisingly asymmetric. At depth the rings in the western part of the crater sole into one or two mid-crustal detachment surfaces overlying reflective lower crust while in the eastern part the rings are all deep rooted and nest inside each other like bowls. The inner ring of the crater, which largely traps the Tertiary basin sediment inside of it, is subdued to the northeast due to the presence of a pre-existing basin modifying final crater relief. Within the inner ring, the terrace zone slump blocks reach the greatest depth in the northwest part of the crater and exhibit rougher topography than other azimuths; on the eastern side the terraces are smooth topographically yet faulted pervasively. Lying above the terrace zone closer near the center of the crater is the peak ring, which rises higher above the crater floor in the west and northwest relative to the east and northeast. A dipping reflector lies beneath the topographic peak ring along all imaged azimuths defining the thickness of the peak ring material. This material, which is likely some combination of breccia and melt, is significantly thicker in the west and northwest and thinner in the east and northeast.

Some of these asymmetries are very likely the result of an oblique impact. However, features such as the subdued topography of the northeast inner ring likely result from structural heterogeneities in the carbonate platform at the time of impact. Impact characteristics such as the direction and angle of impact must interact with target lithologies, basins, and crustal features to result in both the final asymmetric crater and the biogeochemical effects that lead to mass extinction. However, suggested impact directions and angles based on differing aspects of the subsurface asymmetries can be contradictory requiring comparison with physical experiments, modeling, and remote sensing of other solar system craters.