Three Dimensional Gravity Field Modelling of the Chicxulub Impact Crater. A

Introduction: The structure of the Chicxulub crater has been actively investigated by potential field modeling, seismic reflection and refraction surveys, and drilling during the decade since its recognition as the crater responsible for mass extinction which terminated the Cretaceous Period. Given the data sets that are now available to delineate crater structure a surprisingly diverse number of structural models remain proposed. For example, crater diameters ranging from the originally proposed 180 km to ~300 km are still current in the literature, and collapsed disruption cavity sizes (the observable parameter which leads to assessment of the impact's environmental perturbation) range from ~85 km to >120 km. We feel that the available data provide enough constraints to remove most of the variability in published sizes for the structural elements of Chicxulub. We have undertaken 3D modeling of the gravity field over the crater to refine our working structural model [e.g. 2, 3], and to compare our results with those of another 3D modeling effort [4]. The 3D gravity model also establishes an interesting target for scientific drilling. 3D Gravity Modelling: The 3D gravity modeling method employed is that of [1]. In this forward model-ing procedure body geometry is specified using horizontally oriented polygons at arbitrary depths. The calculated field is integrated exactly in x and y, and then integrated across the third dimension, depth (z). The grid size was limited to 50 by 50 stations for computational practicality. Gravity data: The gravity data compilation used (Figure 1) is based on land data collected five decades ago by Pemex (we have digitized unpublished Pemex maps filling in data gaps), results of an airbourne gravity survey conducted for Pemex which fills a data gap west of the crater, several recent land surveys (which have more than doubled available station coverage over the crater), and a shipbourne survey (which has filled in data gaps over the submarine portion of the crater). We have found that recently acquired density constraints have led to substantial revision of our 2D gravity models, and have been guided by those of [2], but anticipate additional progress in improving the validity of gravity models with additional constraints. The Bouguer gravity anomaly expression of Chicxulub has a magnitude of up to ~30 mGals, but still suffers substantial interference from regional anomalies unre