The shallow crustal structure of the Chicxulub impact crater from surface wave dispersion studies

A surface wave dispersion study has been conducted on high frequency (0.5-5 Hz) crustal Rayleigh waves propagating across the 65 Ma. Chicxulub impact structure in Mexico. These were recorded on a 20 station seismic array deployed along 4 radial arms across the region and originated from nearby quarries within the array. Events originating from the same quarry were stacked prior to the application of a multiple filter technique to produce group velocity dispersion curves. Using a genetic algorithm several one-dimensional shear wave velocity-depth models have then been obtained through the optimisation of the fundamental and higher mode dispersion curves. The models provide information on the velocity structure of the upper few kilometres of the crust and suggest an infilling of the crater from the crater rim inwards. An inverted velocity gradient is modelled over the upper few hundred metres across most of the region with the exception of a central radial area. This inverted velocity zone may be connected to dolomitization during a late Miocene regression. The base of the Tertiary sequence is modelled at c. 1-1.5 km depth and shows increased velocities compared to the overlying sediments. This velocity increase may imply some form of hydrothermal alteration of the sediments caused by a thermal blanket effect created by the underlying crater breccia and melt. Immediately below the Tertiary sediments a c. 200 m thick low velocity zone is interpreted as a layer of suevitic impact breccia. Models obtained at c. 35-45 km radius from the crater centre are consistent with the existence of a peak ring as a topographic high above the crater floor. The results from the velocity models provide fresh information on the sedimentation of the region and some constraints on the crater morphology.