Mapping Chicxulub crater structure with gravity and seismic reflection data

Abstract

Aside from its significance in establishing the impact-mass extinction paradigm, the Chicxulub crater will probably come to exemplify the structure of large complex craters. Much of Chicxulub’s structure may be ‘mapped’ by tying its gravity expression to seismic-reflection profiles revealing an ∼180 km diameter for the now-buried crater. The distribution of karst topography aids in outlining the peripheral crater structure as also revealed by the horizontal gradient of the gravity anomaly. The fracturing inferred to control groundwater flow is apparently related to subsidence of the crater fill. Modelling the crater’s gravity expression based on a schematic structural model reveals that the crater fill is also responsible for the majority of the negative anomaly. The crater’s melt sheet and central structural uplift are the other significant contributors to its gravity expression. The Chicxulub impact released ∼1.2 × 10 31 ergs based on the observed collapsed disruption cavity of ∼86 km diameter reconstructed to an apparent disruption cavity ( D ad ) of ∼94 km diameter (equivalent to the excavation cavity) and an apparent transient cavity ( D at ) of ∼80 km diameter. This impact energy, together with the observed ∼2 × 10 11 g global Ir fluence in the Cretaceous-Tertiary (K-T) fireball layer indicates that the impactor was a comet estimated as massing ∼1.8 × 10 18 g of ∼16.5 km diameter assuming a 0.6 gcm −3 density. Dust-induced darkness and cold, wind, giant waves, thermal pulses from the impact fireball and re-entering ejecta, acid rain, ozone-layer depletion, cooling from stratospheric aerosols, H 2 O greenhouse, CO 2 greenhouse, poisons and mutagens, and oscillatory climate have been proposed as deleterious environmental effects of the Chicxulub impact with durations ranging from a few minutes to a million years. This succession of effects defines a temperature curve that is characteristic of large impacts. Although some patterns may be recognized in the K-T extinctions, and the survivorship rules changed across the boundary, relating specific environmental effects to species’ extinctions is not yet possible. Geochemical records across the boundary support the occurrence a prompt thermal pulse, acid rain and a ∼5000 year-long greenhouse. The period of extinctions seems to extend into the earliest Tertiary.

Structural uplift beneath the Chicxulub impact structure

“…Our final velocity models are consistent with previous geophysical models, which all contain a 40-50 km-wide dense and/or highly magnetized body [Espindola et al, 1995;Sharpton et al, 1996;Hildebrand et al, 1998;Campos-Enriquez et al, 1998;Pilkington and Hildebrand, 2000;Ebbing et al, 2001] close to the location of the zone of high velocity in Fig. 3 and 4.…”

“…3 and 4) and we conclude that the top of 11 the intact uplifted lower crust has a complex 3D shape in contrast to the flat top of uplift proposed by Hildebrand et al [1998]. The width of structural uplift is 15-25 km, 40-60 km and ~80 km for the lower-, mid-and upper-crustal rocks respectively (Fig.…”

“…Models of Chicxulub redrawn from a) Sharpton et al [1996], and b) Hildebrand et al [1998] re-scaled with a Vertical Exaggeration (V.E.) of 2.…”

“…2a). In the model by Hildebrand et al [1998] the central uplift is relatively narrow (~50 km in diameter), surrounded by megabreccia (defined as allogenic polymict breccia containing clasts of target rock from several stratigraphic levels in Visnevsky and Montanari [1999]), and separated from the peak ring by megabreccia and/or melt sheet (Fig. 2b).…”

Three-dimensional gravity modeling of Chicxulub Crater structure, constrained with marine seismic data and land boreholes

“…Low gravity anomaly values form a ring with an extension towards the NW. When applying horizontal derivatives to this anomaly, several rings become apparent in the anomaly pattern, which enhance the high horizontal gravity gradient (Sharpton et al, 1993;Pilkington et al, 1994;Hildebrand et al, 1998).…”