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Chicxulub — Subsurface structure of impact crater infered from gravity and magnetic data
Abstract
Why did the dinosaurs become extinct? The theory proposed in the late 1970s by L.W. Alvarez et al. related the massive biota extinction at the transition from the Cretaceous to the Tertiary (K/T) and the global climatic changes to the impact of a major meteorite.
... Com um diâmetro de 180 km, é 33 considerada a mais jovem e bem preservada das três maiores crateras da Terra (Pilkington e Hildebrand, 2000). Campos Enriquez et al. (1996) definiram assinaturas magnéticas e gravimétricas para Chicxulub. A anomalia magnética associada com a estrutura de impacto é um exemplar único produzindo cinco anéis circulares representando uma bacia multianelar. ...
This thesis presents 3D subsurface models built for two Brazilian impact structures
located in the Paraná Basin: Vargeão and Vista Alegre Structures. Both structures
are located over basaltic flows of Serra Geral Formation and are rare examples on
Earth.
Through high resolution terrestrial gravimetric surveys, during the studies,
petrophysical data with previous geological mapping, a 3D subsurface model has
been built for both impact structures. The gravity data consists of a total of 763
stations (419 in Vargeão and 344 in Vista Alegre) newly collected in the region and
allows the improvement of the subsurface knowledge of both impact structures.
The model proposed to Vargeão structure shows a large sedimentary block with the
shape of a vertical truncated cone and a center ~650 meters thick. This block
corresponds to the Pirambóia/Botucatu formations and it windows the upper basalt
layers. The model is consistent with sandstones outcrops located in the central uplift
area of Vargeão, suggesting that the cratering process was responsible for
positioning these rocks in a higher level than the surrounding basalts. The model also
displays that rocks of the impact region were modified until about 2 km of depth.
Vista Alegre model shows a medium-sized complex impact structure (9.5 km
diameter), with a central elevation of 650 meters and a radial extension of 3.5 km.
The main region contains the fractured polymictic breccias and target rocks is about
100 meters thick. The rocks of the impact region are disturbed at depths of ~1 km,
which represents the contact between Pre-Triassic layers and the sandstones of the
Pirambóia/Botucatu Formations.
Both models are consistent with a meteoritic impact formation and confirm the
theoretical values of structural uplift, central core dimension and depth of excavation.
Lastly, the integration of the studies developed in Vargeão and Vista Alegre with
previous studies of the region provides new insights about formation of impact
structures on basaltic targets on Earth, and may contribute to analogous studies
craters of other planetary bodies such as the Moon, Mars, Venus, and others.
... This feature is associated to a central uplifted basement suggested by gravity and magnetic field data which has been modeled previously by several authors (e.g. Hildebrand et al., 1994;Espíndola et al., 1995;Campos et al., 1996). ...
A magnetotelluric (MT) survey was carried out in the Yucatan Península to assess the capabilities of the method to detect structural changes along two radial profiles crossing the sinkholes ring, a surface feature thought to be directly associated to the Chicxulub impact crater. A total of 22 long period MT soundings were measured. Eleven of them were located along a N-S profile (orAA while the other ten were measured along a profile of azimuth NW-SE (orBB. A two-dimensional model using the Smith and Booker (1991) algorithm was obtained for each profile from the simultaneous inversion of the TE and TM modes. The interpreted structural sections along the two measured profiles yielded significant differences. The NS profile shows a smooth change of resistivity structure and a deeper undisturbed basement away from the crater center. However, the NW-SE profile renders a sharp electrical contact related to the major outer crater ring and a shallower undisturbed basement. In addition, a structural high is observed in this profile towards the postulated crater center. Based on the two resistivity cross sections the crater diameter of the main cavity is set to 195 ± 4 km while the base of the crater central structural high to about 80 ± 4 km diameter.
... The magnetic method has thus expanded from its initial use solely as a tool for finding iron ore to a common tool used in exploration for minerals, hydrocarbons, ground water, and geothermal resources. The method is also widely used in applications other than exploration, such as studies focused on water-resource assessment (Smith and Pratt, 2003;Blakely et al., 2000a), environmental contamination issues , seismic hazards (Blakely et al., 2000b;Saltus et al., 2001;Langenheim et al., 2004), park stewardship (Finn and Morgan, 2002), geothermal resources , volcano-related landslide hazards , regional and local geologic mapping (Finn, 2002), mapping unexploded ordnances (Butler, 2001;Hansen et al., 2005), locating buried pipelines (McConnell et al., 1999), archeological mapping (Tsokas and Papazachos, 1992), and delineating impact structures (Campos-Enriquez, et al., 1996;Goussev et al., 2003), which can sometimes be of economic importance (Mazur et al., 2000). ...
The magnetic method, perhaps the oldest of geophysi- cal exploration techniques, blossomed after the advent of airborne surveys in World War II. With improvements in instrumentation, navigation, and platform compensation, it is now possible to map the entire crustal section at a variety of scales, from strongly magnetic basement at re- gional scale to weakly magnetic sedimentary contacts at lo- cal scale. Methods of data filtering, display, and interpreta- tion have also advanced, especially with the availability of low-cost, high-performance personal computers and color raster graphics. The magnetic method is the primary explo- ration tool in the search for minerals. In other arenas, the magnetic method has evolved from its sole use for map- ping basement structure to include a wide range of new applications, such as locating intrasedimentary faults, defining subtle lithologic contacts, mapping salt domes in weakly magnetic sediments, and better defining tar- gets through 3D inversion. These new applications have increased the method's utility in all realms of explo- ration — in the search for minerals, oil and gas, geother- mal resources, and groundwater, and for a variety of other purposes such as natural hazards assessment, map- ping impact structures, and engineering and environmental studies.
Crustal magnetic fields were first measured systematically by airborne surveys in the 1930s. Ground-based surveying had been developed much earlier as a tool for mineral exploration. Today, crustal magnetic fields are also measured in boreholes, from ships and balloons, and from terrestrial and planetary spacecraft. Metamorphism, petrology, and redox state all have important effects on the magnetism of crustal materials. Mapping of the crustal magnetic field provides a third dimension to surface observations of the Earth's composition and geologic structure, and a suite of mathematical tools has been developed to assist with this interpretation. Studies of crustal magnetism have contributed to geodynamic models of the lithosphere, geologic mapping, and petroleum and mineral exploration. Inferences from crustal magnetic fields, interpreted in conjunction with other information, can locate kimberlite pipes, impact structures, and other geologic entities which have a magnetic contrast with their surroundings. Present-day efforts are focused on (1) the development of a global, near-surface map of the crustal magnetic field that integrates airborne and satellite coverage, (2) extending the techniques to the other planets and moons of our solar system, and (3) extending regional and detailed mapping of the crustal magnetic field as an exploration and reconnaissance tool.
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