P. M. Vermeesch

Structural uplift beneath the Chicxulub impact structure

Models of the central structure of large impact craters are poorly constrained, partly because of the lack of well-preserved terrestrial examples, and partly because of the extreme nature of impact events. Even large impact craters take only a few minutes to form, during which time rocks from the deep crust move upward many kilometers, interacting with impact melts and breccias before settling to their final position. We construct a new model of central uplift beneath the Chicxulub crater, based upon a well-constrained 3-D velocity model, obtained by jointly inverting seismic traveltime and gravity data. The input tomographic data set has good resolution, and many rays cross the central uplift in many directions. We use laboratory measurements to convert between velocity and density. Our velocity model possesses a high-velocity zone near the crater center, and velocity gradually decreases outside this zone. We use regional refraction data to interpret these velocities in terms of a broad 80-km-wide zone of structural uplift, in which the central rocks originate from the lower crust, and the surrounding rocks from the midcrust and upper crust. This is in contrast with previous models in which the zone of central uplift is either 40-50 km or 150 km wide. Our interpretation is consistent with scaling laws, Yucatan basement lithology, other velocity data, observations at similar-sized terrestrial craters, and dynamic modeling of peak ring formation. Our model of the uplift at Chicxulub can be used to help distinguish between competing models of effective target strength in numerical models of crater formation.

Three-dimensional joint inversion of traveltime and gravity data across the Chicxulub impact crater

In 2005 an extensive new seismic refraction data set was acquired over the central part of the Chicxulub impact crater, allowing us to image its structure with much better resolution than before. However, models derived from traveltime data are limited by the available ray coverage and the nonuniqueness that is inherent to all geophysical methods. Therefore, many different models can fit the data equally well. To address these issues, we have developed a new method to simultaneously invert traveltime and gravity data to obtain an integrated model. To convert velocity to density, we use a linear relationship derived from measurements on core from the Chicxulub impact basin, thus providing a reliable conversion equation that is typical for lithologies of the central part of this crater. Prior to utilizing the inversion on the observed data, we have run a suite of tests to establish the optimum weighting between traveltime and gravity constraints, using a synthetic model of central crater structure and the real experimental geometry. These synthetic tests indicate which inversion parameters lead to the best recovery of subsurface structure, as well as which parts of the model are well resolved. We applied the method to all existing gravity data and to seismic refraction data acquired in 1996 and the new, higher-resolution seismic refraction data acquired in 2005. We favor the traveltime model wherever we have sufficient ray coverage and the joint model where we have no ray coverage.