3D gravity modelling of the Chicxulub impact structure

The Chicxulub impact structure is the youngest and best preserved of the three largest impact craters on Earth (Chicxulub, Sudbury, Vredefort). The crater gained high interest because of its possible association with the large mass extinction at the K/T boundary. Based on seismic, gravity and borehole data a 3D structural model of the Chicxulub crater has been evaluated. In a first step the 3D thickness of the Tertiary infill was reconstructed from reflection seismic profiles and borehole data. Removing this infill results in a basin of 140-160 km diameter and 1-1.5 km depth with a central plateau of 90 km diameter and similar to 200 m height surrounded by a depression. The basin palaeotopography has a rather smooth appearance compared to intact craters of similar size on other planetary bodies e.g. Keeler crater on the Moon. This smoothness may be an indication of erosional processes by high energy water wave action immediately after the impact on the shelf area. Modelling the gravity signature of the 3D Tertiary infill results in too small amplitudes compared to the pronounced Bouguer gravity anomalies of the Chicxulub structure. In the next step the 3D sub-Tertiary structure was modelled on the basis of seismic, borehole and gravity data. The resulting model consists of a central uplift of 20 km radius surrounded by a ring of megabreccia out to a radius of 35 km. A region of slumped blocks follows up to a radius of 70 km. Borehole data provide support for a central melt body. The central zone of intermediate and short wavelengths of relatively high amplitude magnetic anomalies with a diameter of 90 km may indicate the outline of this body. It coincides with the 90 km central plateau of the palaeobasin. Thus, a central melt body of 90 km diameter covers the deeper structures. Immediately below the Tertiary infill a layer of fallback and ejected breccia is included. The final model shows several deviations from radial symmetry thus it is more suitable for discussions of future borehole Locations than 2D models. The diameter of the palaeotopograghic basin of about 150 km supports estimates of a total crater diameter including a crater wall of less than 20() km. The volume of the central melt body corresponds very well with a volume estimate of I;Kring (JGR 100 ( 1995) 16,979 -16,986) who assumed a crater diameter of 180 km and an impact velocity of 11 km/s based on impact mechanics. (C) 2001 Elsevier Science Ltd. All rights reserved.