[Baps] Chixculub talk Wed Jan 11, 11 AM, Peggy Vermeesch

[Sarah Stewart-Mukhopadhyay]

EPS Planetary Science Talk
Wednesday, January 11
11:00 - 12:00
Hoffman Laboratory, Faculty Lounge (4th floor)
*
Broad zone of structural uplift beneath the Chicxulub impact structure**
*Peggy M. Vermeesch and Joanna V. Morgan

1) University of Texas Institute for Geophysics, Austin, Texas 78758, USA
2) Department Earth Science and Engineering, Imperial College London, SW7
2AZ, UK


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 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.



-- 
Sarah T. Stewart-Mukhopadhyay
Asst. Professor of Planetary Science
Dept. of Earth & Planetary Sciences, Harvard University
Office 617.496.6462 Lab 617.496.5782 Fax 617.384.8249
sstewart at eps.harvard.edu
http://www.fas.harvard.edu/~planets/sstewart/
<http://www.fas.harvard.edu/%7Eplanets/sstewart/>

Assistant: Ben Tobin, tobin at eps.harvard.edu, 617-495-2350
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