EPS Planetary Science Talk<br>Wednesday, January 11<br>11:00 - 12:00<br>Hoffman Laboratory, Faculty Lounge (4th floor)<br><b><br>Broad zone of
structural uplift beneath the Chicxulub impact structure</b><font face="Arial" size="2"><b>
<div><br></div></b><span>Peggy</span> M. Vermeesch and Joanna V. Morgan<br><br>1) University
of Texas Institute for Geophysics, Austin, Texas 78758, USA<br>2) Department
Earth Science and Engineering, Imperial College London, SW7 2AZ,
UK<br><br> <br>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. <br> <br>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. <br> <br>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 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. <br> <br>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.</font><br clear="all"><br><br><br>-- <br>Sarah T. Stewart-Mukhopadhyay<br>Asst. Professor of Planetary Science<br>Dept. of Earth & Planetary Sciences, Harvard University
<br>Office 617.496.6462 Lab 617.496.5782 Fax 617.384.8249<br><a href="mailto:sstewart@eps.harvard.edu" target="_blank">sstewart@eps.harvard.edu</a><br><a href="http://www.fas.harvard.edu/%7Eplanets/sstewart/" target="_blank">
http://www.fas.harvard.edu/~planets/sstewart/
</a><br><br>Assistant: Ben Tobin, <a href="mailto:tobin@eps.harvard.edu" target="_blank">tobin@eps.harvard.edu</a>, 617-495-2350