HR: 17:20h
AN: U34A-05    [Abstracts]
TI: 3-D Variation in peak ring structure of the Chicxulub impact crater
AU: * Surendra, A T
EM: ats36@cam.ac.uk
AF: Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge, CB4 OEZ, United Kingdom
AU: Barton, P J
EM: barton@esc.cam.ac.uk
AF: Bullard Laboratories, University of Cambridge, Madingley Road, Cambridge, CB4 OEZ, United Kingdom
AU: Morgan, J
EM: j.v.morgan@imperial.ac.uk
AF: Earth Science and Engineering, Imperial College London, Royal School of Mines, Prince Consort Road, London, SW7 2BP, United Kingdom
AU: Gulick, S P
EM: sean@utig.ig.utexas.edu
AF: Institute of Geophysics, Jackson School of Geosciences, University of Texas at Austin, 4412 Spicewood Springs Rd., Austin, TX 78759, United States
AU: Christeson, G L
EM: gail@utig.ig.utexas.edu
AF: Institute of Geophysics, Jackson School of Geosciences, University of Texas at Austin, 4412 Spicewood Springs Rd., Austin, TX 78759, United States
AB: The lithology and structure of the Chicxulub impact crater's peak ring is of key importance when trying to understand the kinematics of large crater collapse, and can be investigated by examining models of the velocity structure within the crater. A tomographic model of the top 10 km of the peak ring was created using data from a 3-D grid of 50 ocean bottom seismometers over the north-western quadrant of the crater. Results from this 3-D tomographic inversion show that the velocities within the peak ring topographic high, which has now been imaged on numerous reflection profiles offshore and forms a prominent feature 375 -- 675 m high above the crater surface, are the same as that of the Tertiary sediments on either side of it. However, at 2 -- 8 km depth beneath the topographic peak ring there is a radial low velocity zone (LVZ) 15 -- 25 km wide which has an outer radius of ~ 45 km and is contiguous with the horseshoe shaped gravitational low over the crater. This LVZ may be caused by low velocity impact breccia that interacts with the collapsing transient crater rim during emplacement. The width and velocity anomaly of the LVZ at different azimuths from the crater centre are shown to be variable within the segment of the peak ring imaged in this study. This segment can be divided into three sections, each of which has distinct characteristics.

1. In the NW of the crater, over an azimuthal range of 290° - 320°, the peak ring is clearly defined on reflection profiles in this region and forms a pronounced topographic high. The LVZ beneath it has a width of 15 km and at a depth of 2 km has a velocity anomaly of -0.6 km/s compared to the material either side of it. The magnitude of this anomaly decreases with depth so that at 6 km depth it has reduced to -0.3 km/s.
2. Moving clockwise around the peak ring, underneath the break in the gravitational low, the LVZ is much less pronounced and more poorly defined than the NW crater section. The velocity anomaly has a magnitude of -0.3 km/s at depths of ~ 2 km but disappears almost completely at depths of 7 -- 8 km.
3. To the NNE of the crater between 350° - 045° the peak ring forms a wider, lower relief structure on reflection profiles. The LVZ beneath it is much broader (~ 25 km width) and spreads further in towards the centre of the crater. Its magnitude of -0.6 km/s is similar to the NW crater section but spread out over a wider region. However at depth the anomaly only continues in its full magnitude in the outer section of the LVZ; the inner part fades out at depths of 5 -- 6 km.