Physics – Geophysics
Scientific paper
May 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agusm.u34a..04b&link_type=abstract
American Geophysical Union, Spring Meeting 2007, abstract #U34A-04
Physics
Geophysics
0905 Continental Structures (8109, 8110), 3025 Marine Seismics (0935, 7294), 3036 Ocean Drilling, 3045 Seafloor Morphology, Geology, And Geophysics, 8136 Impact Phenomena (5420, 6022)
Scientific paper
The Chicxulub impact crater includes the only known peak ring structure preserved on the Earth's surface, and as such represents an important natural laboratory for the study of peak ring craters, seen commonly on other planets. Seismic reflection data collected in the offshore part of the crater in 1996 and 2005 includes refracted arrivals recorded on a 6 km multichannel seismic streamer, and the reflection and refraction information together give powerful constraints on the structure of the upper ~2 km of the subsurface, which includes the Cenozoic infill, the crater surface, and about 1 km of underlying material. The refracted travel-time data from the streamer are transformed via tau-p into a detailed velocity map which may be superimposed onto the reflection image in two- way-time or depth. Inside the inner rim of the crater we observe a very consistent pattern, with basin infill of 2.0-3.4 km/s and top crater velocities of 3.4-5.4 km/s. About 1500 m beneath the present day sea level, and 750-1000 m below the crater surface, we detect a high velocity layer, which is seen inside and outside the topographic peak ring interpreted from the reflection profile, but does not appear to run continuously either over the peak ring or beneath it, at least within the depth range of our data. This high velocity feature maps onto a low frequency reflector mapped intermittently on the reflection profiles, and we interpret it as the top of the melt sheet expected in the interior of the crater. The layer is at least three hundred metres thick, and has a seismic velocity of about 5.5- 6.2+ km/s, the maximum resolvable using the given acquisition geometry. We have mapped this surface using the 5.5 km/s velocity contour as a proxy, and will compare this in detail with a map of the low frequency event identified on the reflection profiles. The high velocity layer forms a relatively smooth, though not strongly reflective, surface, possibly modified post-emplacement by hydrothermal alteration. In some cases the feature appears to shallow and possibly finger at the inside edge of the peak ring, and seems to be deeper immediately outside, where it may truncate against the inward-dipping reflectivity observed under the outer edge of the peak ring. Outside the peak ring, this feature remains at a constant depth of about 750 m beneath the crater surface, shallowing with this surface towards the inner rim. Penetration of the melt sheet is one of the key objectives of the proposed ICDP drillhole planned onshore in the Chicxulub crater. We believe that the information from the adjacent offshore surveys provide excellent constraints on the depth, geometry and seismic characteristics of this target.
Barton Penny
Blackburn M.
Christeson Gail
Gulick S. S.
Morgan Jaqueline
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