HR: 11:05h
AN: U22A-04    [Abstracts]
TI: Broad zone of structural uplift beneath the Chicxulub impact structure
AU: Morgan, J V
EM: j.v.morgan@imperial.ac.uk
AF: Imperial College London, Department of Earth Science and Engineering, South Kensington Campus, London, SW7 2AZ, United Kingdom
AU: * Vermeesch, P M
EM: peggy.vermeesch@mail.utexas.edu
AF: Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, J.J. Pickle Research Campus Bldg. 196, 10100 Burnet Road (R2200), Austin, TX 78758, United States
AB: Although meteorite impacts are a ubiquitous and fundamental geologic process affecting the terrestrial planets, they are relatively poorly understood. The Earth has comparatively few pristine craters, and only three large (>150 km diameter) impact basins: Chicxulub, Vredefort and Sudbury, of which Chicxulub is the best preserved. Seismic reflection data acquired across the offshore half of the Chicxulub crater in 1996 and 2005 reveal clear images of the target rocks and impact basin. There is no reflection data across the crater center, and therefore central crater structure at Chicxulub, and large impact craters in general, is a matter of some debate. Although we know that large craters possess particular features (structural uplift, impact melt rocks, impact breccias, a peak ring), the precise geometric relationship between these features remains uncertain. Models of Chicxulub constructed from geophysical data are diverse in part due to the lack of terrestrial examples and the inherent ambiguity of geophysical modeling, and also because drill holes within the impact basin have penetrated the uppermost crater deposits only. We have constructed a new model of central crater structure across Chicxulub, based upon inversions of geophysical data. Previous interpretations of the width of structural uplift beneath Chicxulub vary from 50 to 150 km. In 1996 and 2005 we acquired tomographic seismic and gravity data, and have performed both 3D travel-time and joint gravity and travel-time inversions to produce a well-constrained velocity model across the central crater. This model possesses a 15-25 km wide high-velocity-zone near the crater center, where rock velocity is >6.3 km s^-1 below 5 km depth and, outside this zone, velocity gradually decreases. We interpret these velocities in terms of a broad 80-km wide zone of structural uplift, in which the central rocks originate from the lower crust, and the surrounding rocks from the mid and upper crust. The new velocity model across the central crater, which incorporates gravity constraints, is a major advance. The resolution of the new 3D tomographic velocity model significantly surpasses that of the 1996 model. Our interpretation of the velocities from the joint travel-time and gravity inversion in terms of lower, mid and upper crustal rocks is supported by regional refraction data, general crustal models, the lithology of basement clasts in Chicxulub impact breccias, impact scaling laws, observations at the similar-sized crater Vredefort, and dynamic models of crater formation.
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
SC: Union [U]
MN: 2007 Fall Meeting