Impact structure

Abstract: A major tool in the initial recognition and study of terrestrial impact craters, ∼20% of which are buried beneath postimpact sediments, is geophysics. The general geophysical character of terrestrial impact craters is compiled and outlined with emphasis on its relation to the impact process and as an aid to the recognition of additional impact craters. The most common and conspicuous geophysical signature is a circular gravity low. For simple bowl-shaped craters, gravity models indicate that the anomaly is largely due to the presence of an interior allochthonous breccia lens. In complex craters, modeling indicates that the main contribution to the gravity anomaly is from fractured parautochthonous target rocks in the floor of the crater. The gravity signature of both simple and complex crater forms can be modeled well, using known morphometric parameters of impact structures. The size of the gravity anomaly generally increases with increasing crater diameter reaching a maximum of ∼20–30 mGal at diameters D of ∼20–30 km. Further increases in D have a negligible effect on the magnitude of the gravity anomaly due to lithostatic effects on deep fractures. The general gravity signature of a simple low can be modified by target rock and erosional effects, and there is a tendency for larger complex structures ( D > 30 km) to exhibit a relative gravity high restricted to the crater center and extending out to 40 km) tend to exhibit central high-amplitude anomalies, with dimensions of <0.5D, due to remanently magnetized bodies in the target rocks. The sources of these bodies are wide ranging and include the effects of shock, heat, and chemical alteration. The few studies over craters involving electrical methods indicate resistivity lows coinciding with the extent of the potential field anomalies and related to fracturing. Seismic techniques, particularly reflection surveys, have provided details of the subsurface structure of craters. Incoherent reflections and reduced seismic velocities due to brecciation and fracturing are expected, the degree of coherency of reflections increasing away from and below the center of the structure. From the various geophysical techniques a set of general criteria can be established that correspond to the geophysical signature of impact craters. These criteria can be used to evaluate the hypothesis that any particular set of geophysical anomalies is due to impact. Confirmation of an impact origin, however, is based on geologic evidence.

Abstract: The origins of the Sudbury Structure and associated Igneous Complex have been controversial. Most models call for a major impact event followed by impact-induced igneous activity, although totally igneous models are still being proposed. Much of the controversy is due, in our opinion, to a misunderstanding of the size of the original Sudbury Structure. By analogy with other terrestrial impact structures, the spatial distribution of shock features and Huronian cover rocks at the Sudbury Structure suggest that the transient cavity was ∼100 km in diameter, which places the original final structural rim diameter in the range of 150–200 km. Theoretical calculations and empirical relationships indicate that the formation of an impact structure of this size will result in ∼104 km3 of impact melt, more than sufficient to produce a melt body the size of the Igneous Complex (present volume 4–8 × 103 km3). For the Igneous Complex to be an impact melt sheet it must have a composition similar to that of the target rocks. Evidence for this has been presented previously for Sr and Nd isotopic data, which suggest a crustal origin. Here, we also present new evidence from least squares mixing models that the average composition of the Igneous Complex corresponds to a mix of Archean granite-greenstone terrain, with possibly a small component of Huronian cover rocks. This is a geologically reasonable mix, based on the interpreted target rock geology and the geometry of melt formation in an impact event of this size. The Igneous Complex is differentiated, which is not a characteristic of previously studied terrestrial impact melt sheets. This can be ascribed, however, to its great thickness and slower cooling. That large impact melt sheets can differentiate has important implications for how the lunar samples and the early geologic history of the lunar highlands are interpreted. If this working hypothesis is accepted, namely, that both the Sudbury Structure and the Igneous Complex are impact in origin, then previous hybrid impact-igneous hypotheses can be discarded and the Sudbury Structure can be studied specifically for the constraints it provides to large-scale cratering and the formation of basin-sized (multiring?) impact structures.

Abstract: THE 200-km-diameter Chicxulub structure1–3 in northern Yucatan, Mexico has emerged as the prime candidate for the Cretaceous/Tertiary (K/T) boundary impact crater3–6. Concentric geophysical anomalies associated with enigmatic occurrences of Upper Cretaceous breccias and andesitic rocks led Penfield and Camargo1 to suspect that this structure was a buried impact basin. More recently, the discovery of shocked quartz grains in a Chicxulub breccia3, and chemical similarities between Chicxulub rocks and K/T tektite-like glasses3–6 have been advanced as evidence that the Chicxulub structure is a K/T impact site. Here we present evidence from core samples that Chicxulub is indeed a K/T source crater, and can apparently account for all the evidence of impact distributed globally at the K/T boundary without the need for simultaneous multiple impacts or comet showers. Shocked breccia clasts found in the cores are similar to shocked lithic fragments found worldwide in the K/T boundary ejecta layer7,8. The Chicxulub melt rocks that we studied contain anomalously high levels of iridium (up to 13.5 parts per 109), also consistent with the iridium-enriched K/T boundary layer9. Our best estimate of the crystallization age of these melt rocks, as determined by 40Ar/<39Ar analyses, is 65.2 ±0.4 (1σ) Myr, in good agreement with the mean plateau age of 64.98 ± 0.05 Myr recently reported10. Furthermore, these melt rocks acquired a remanent magnetization indicating that they cooled during an episode of reversed geomagnetic polarity. The only such episode consistent with40Ar/<39Ar constraints is chron 29R, which includes the K/T boundary.

Abstract: The Sudbury structure is interpreted as a 17-billion-year-old asteroid impact structure or "astrobleme" created by a $$3 X 10^{29}$$ erg event If traveling at 15 km/sec, the asteroid was about 4 km in diameter A crater was formed 30 miles across and 2 miles deep Along with melted country rock, the bolide, possibly a copper-rich nickel-iron meteorite, is still partially preserved, although converted to sulfides, as a marginal sheet along the crater wall and as injections into radial tension cracks Shock brecciation and rock were heaved up, forming the crater wall The collar is still easily recognized along the southern periphery of the Sudbury structure, but it can only be permissibly assumed in the massive granitic rocks around the northern periphery Because of its great magnitude, the Sudbury event triggered magmatism by offloading the lower crust and mantle and by adding shock heat Partial fusion of already critically warm rock resulted A saucer-shaped pool of magma, an extrusive lopolith, for