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Color: gravity over the Chicxulub impact structure (courtesy of Alan Hildebrand and Mark Pilkington, Geological Survey of Canada). In September-October 1996, 34 Ocean Bottom Seismograph (OBS) deployments were made along two of the three multi-channel seismic lines shot by the M/V GECO Sigma. Deployments were made by investigators from UTIG aboard the R/V Longhorn, operated by the University of Texas Marine Science Institute in Port Aransas, Texas. The data recorded by these instruments, and a set of onshore seismic receivers, were used to model the crustal structure of the Chicxulub impact crater along the Chicx-A/A1 and Chicx-B/F profiles.  Details can be found in 2 publications:

Christeson, G.L., R.T. Buffler, and Y. Nakamura, Upper crustal structure of the Chicxulub impact crater from wide-angle ocean bottom seismograph data, in Dressler, B.O., and V.L. Sharpton, eds., Large Meteorite Impacts and Planetary Evolution II: Boulder, Colorado, Geological Society of America Special Paper 339, 291-298, 1999.

Christeson, G.L., Y. Nakamura, R.T. Buffler, J. Morgan, and M. Warner, Deep Crustal Structure of the Chicxulub Impact Crater, J. Geophys. Res., 106, 21751-21769, 2001.

UTIG scientists involved with the Chicxulub OBS program: Yosio Nakamura, Ben Yates, Gail Christeson, Richard Buffler, John Brittan (from Imperial College).

Velocity model of the upper crustal structure along Chicx-A/A1 - velocities are only shown where constrained by the OBS data. The lowered velocities from 0-1 km depth between model offsets 85-225 km are interpreted as Tertiary sediments infilling the crater.  The 4.5-5.0 km velocities beneath the Tertiary Basin at 1-2 km depth are interpreted as suevitic breccias.  A suevite is a typical impact breccia containing melt clasts and a mixture of fragments from the target rock (melting occurs because of the high pressures at impact).  

Velocity model of the upper crustal structure along Chicx-B/F - velocities are only shown where constrained by the OBS data. The high velocities at 4-8 km depth at the center of the crater are interpreted as central uplift - uplift of deep material during the crater process.  The high velocities to the northwest, labeled northwest uplift, are probably not related to cratering mechanics but instead indicate a pre-existing basement high in that region.

The figure above shows Moho (crust-mantle boundary) structure overlain on line-drawing of crustal reflectivity from the coincident seismic reflection profile along Chicx-A/A1.  The primary feature at the base of the crust is a regional deepening in Moho depth (red dashed line) from ~33 km at the western end of the profile to ~35.5 km at the eastern end of the profile.  Superimposed on the regional trend is Moho uplift of ~1 km near the center of the profile, with adjacent Moho deepening of ~1.25-1.5 km.  This Moho topography may be related to deformation processes associated with the formation of the outer ring.

Density models, based on velocity structure (1 km/s velocity contours are plotted with yellow dashed lines), for Chicx-A/A1 and Chicx-B/F.  The large gravity gradient at 210-240 km on Chicx-A/A1 corresponds to the eastern edge of the low-density, low-velocity Tertiary impact basin.  The low-density, low-velocity regions beneath the Tertiary impact basin are interpreted as suevitic breccias (see previous text).  For Chicx-B/F the central uplift and northwest uplift have similar magnitudes, but gravity signal is much less over central uplift.  We model this with a low-density suevitic breccia layer similar to that observed on Chicx-A/A1.  The suevitic breccia layer extends across the center of the crater, and has a diameter of ~50-70 km.