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Shallow crustal structure of Chicxulub impact crater imaged with seismic, gravity and magnetotelluric data: inferences about the central uplift

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SUMMARY

The structure of the onshore portion of the Chicxulub impact crater (Yucatan, Mexico) has been studied with seismic, gravity and magnetotelluric (MT) exploration methods. A dispersion analysis of Rayleigh waves along a 150-km long, east–west profile permitted the shallow Tertiary cover to be imaged to a depth of 400 m. Three layers were mapped. The thickness of the first two layers increase as we approach the sinkhole ring from the exterior of the crater (consistent with the existence of a central basin). Those thicknesses change from 100–150 m outside the crater rim to more than 200 m in the terrace zone. At the crater centre, the first layer is again approximately 100 m thick and has a low shear wave velocity (approximately 1 km s−1). Its velocity is slightly larger in the rest of the profile. Outside the central basin, we observed a third layer with a shear wave velocity larger than 1.8 km s−1, but its thickness could not be determined. The velocity distribution along our profile increases monotonously with depth, without low-velocity layers. The inferred inward slope of the two shallow layers immediately outside the central basin correlates well with a smooth gravity gradient. In addition, we present results of detailed gravity measurements along a profile that coincides (in part) with the line covered by the seismic experiment. These gravity measurements were inverted to obtain a density model. In this model (which is of course non-unique), the steep gravity gradients are explained by the faults along which the slumping of the terrace blocks took place. Two of these may correspond to the faults observed at radial distances of 85 and 96 km in offshore seismic data. Finally, we show results obtained using MT soundings. They provide additional, independent constraints on the structure of the Chicxulub crater. MT soundings at the centre of the crater show a sharper increase in resistivity than elsewhere in the impact structure. This sharper increase in resistivity may be related to the uplifted basement of the structural high at a depth of 4 km. Only the MT soundings located over the central gravity high display this feature. Our three different data sets are independently consistent with the existence of a central structural high in the crater and support a twin-peak shape for this high.
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Keywords: Bouguer gravity; Chicxulub impact crater; MT; Rayleigh wave dispersion; central structural high; sinkhole ring

Document Type: Research Article

Affiliations: 1: Instituto de Geofísica, UNAM, 04510 Coyoacán, México, D.F., Mexico 2: Instituto de Ingeniería, UNAM, Apdo. Postal 70-472, Coyoacán 04510 México, D.F., Mexico 3: Departamento de Sismología, CICESE, Ensenada, B.C., Mexico 4: Departament of Earth and Planetary Physics, University of Tokyo, Japan 5: Centro de Geociencias, UNAM, Juriquilla, Qro., Mexico 6: Institute for Geophysical Research, University of Alberta, Edmonton T6G 2J1, Alberta, Canada 7: Facultad de Ingeniería, UNAM, Mexico City 04510, México, D.F., Mexico

Publication date: 01 May 2004

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