The Chicxulub crater is an ∼180‐km‐diameter peak‐ring crater based on drill hole logs and samples, potential fields, seismic reflection profiles, and surface fracture patterns. A structural cross section produced based on these constraints has the features expected for a large complex impact crater. The Bouguer‐gravity anomaly consists of a broad ∼90‐km radius, ∼30‐mGal low with a central ∼20‐km radius, ∼20‐mGal high and two <5‐mGal concentric lows at ∼35‐ and ∼60‐km radius. The gravity anomaly is disrupted by large‐scale basement anomalies and possibly by large‐scale slumping and backwash erosion effects. The magnetic field anomaly over the crater consists of three zones, an outer zone from ∼45‐ to ∼90‐km radius of low‐amplitude, short‐wavelength anomalies with an irregular perimeter, a middle zone from ∼20‐ to ∼45‐km radius of high‐amplitude, short‐wavelength anomalies slightly elongated NNW‐SSE, and an inner ∼20‐km‐radius single large‐amplitude anomaly. Magnetic field modeling indicates that the melt pool averages ∼90 km in diameter and the melt volume in the crater is estimated at ∼20,000 km3. The melt pool size constrains the collapsed transient cavity diameter to ∼90 km. Gravity and magnetic field modeling indicate that the structural uplift is irregular in shape but ∼40 km in diameter and underlies or protrudes into the melt pool. The preliminary structural cross section indicates that the inferred peak‐ring is decoupled from the structural uplift. The geometry and Bouguer gravity signature of the crater indicate that no significant uplift of the Moho or relaxation of the crater has occurred.

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.

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The buried Chicxulub impact structure is marked by a dramatic ring of sinkholes (called cenotes if containing water), and adjacent less prominent partial rings, which have been shown to coincide with maxima in horizontal gravity gradients and a topographic depression. These observations, along with the discreteness and spacing of the features, suggest a formation mechanism involving faulting in the outer slump zone of the crater, which would thus have a diameter of approximately 180 km.An opposing view, based primarily on the interpretation of gravity data, is that the crater is much larger than the cenote ring implies. Given the association of the known cenote ring with faults, we here examine northern Yucatan for similar rings in gravity, surface features and elevation, which we might expect to be associated with outer concentric faults in the case of a larger, possibly multiring, structure.No such outer rings have been found, although definite patterns are seen in the distribution of karst features outside the crater rim. We explain these patterns as resulting mainly from deformation related to the block fault zone that parallels the shelf edge of eastern Yucatan.

In this paper we introduce an image reconstruction technique for imaging permittivity distributions using electrical capacitance tomography, based on global optimization by very fast simulated annealing. Electrical capacitance measurement data are obtained between electrodes placed around the outer wall of an electrically insulating pipe. Such data are used to infer material distributions inside the pipe. The data are processed in order to reconstruct an image of the spatial distribution of the relative electrical permittivity (also known as dielectric constant) inside the pipe, which reflects a material distribution. In the very fast simulated annealing method, the permittivity image is reconstructed by minimizing iteratively a cost function related to the difference between the measured data and those calculated for an estimated permittivity distribution that is repeatedly updated, in a semi-random search process that mimics the thermodynamic phenomena of annealing (as metals slowly cool down) or crystallization (as liquids freeze). The images are refined until their calculated capacitance data match the measured data, in which case it is considered that such images properly resemble the permittivity distribution that produced the measured capacitance data.