Chicxulub central crater structure: Initial results from physical property measurements and combined velocity and gravity modeling

The Chicxulub crater in Mexico is a nearly pristine example of a large impact crater. Its slow burial has left the central impact basin intact, within which there is an apparently uneroded topographic peak ring. Its burial, however, means that we must rely on drill holes and geophysical data to interpret the crater form. Interpretations of crater structures using geophysical data are often guided by numerical modeling and observations at other large terrestrial craters. However, such endeavors are hindered by uncertainties in current numerical models and the lack of any obvious progressive change in structure with increasing crater size. For this reason, proposed structural models across Chicxulub remain divergent, particularly within the central crater region, where the deepest well is only similar to1.6 km deep. The shape and precise location of the stratigraphic uplift are disputed. The spatial extent and distribution of the allogenic impact breccias and melt rocks remain unknown, as do the lithological nature of the peak ring and the mechanism for its formation.The objective of our research is to provide a well-con strained 3D structural and lithological model across the central region of the Chicxulub crater that is consistent with combined geophysical data sets and drill core samples. With this in mind, we present initial physical property measurements made on 18 core samples from the Yaxcopoil-1 (Yax-1) drill hole between 400 and 1500 in deep and present a new density model that is in agreement with both the 3D velocity and gravity data. Future collation of petrophysical and geochemical data from Yax-1 core, as well as further seismic surveys and drilling, will allow us to calibrate our geophysical models-assigning a suite of physical properties to each lithology. An accurate 3D model of Chicxulub is critical to our understanding of large craters and to the constraining of the environmental effects of this impact

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

Importance of pre-impact crustal structure for the asymmetry of the Chicxulub impact crater

Impact craters are observed on the surfaces of all rocky planets and satellites in our Solar System(1); some impacts on Earth, such as the Cretaceous/Tertiary one that formed the Chicxulub impact crater(2,3), have been implicated in mass extinctions(4-12). The direction and angle of the impact - or its trajectory - is an important determinant of the severity of the consequent environmental damage, both in the downrange direction ( direction bolide travels) and in the amount of material that enters the plume of material vaporized on impact(2,13-15). The trajectory of the Chicxulub impact has previously been inferred largely from asymmetries in the gravity anomalies over the crater(2,3). Here, we use seismic data to image the Chicxulub crater in three dimensions and demonstrate that the strong asymmetry of its subsurface correlates with significant pre-existing undulations on the end-Cretaceous continental shelf that was the site of this impact. These results suggest that for rocky planets, geological and geomorphological heterogeneities at the target site may play an important role in determining impact crater structure, in addition to impact trajectories. In those cases where heterogeneous targets are inferred, deciphering impact trajectories from final crater geometries alone may be difficult and require further data such as the distribution of ejecta