Paradigm lost: Venus crater depths and the role of gravity in crater modification

Previous to Magellan, a convincing case had been assembled that predicted that complex impact craters on Venus were considerably shallower than their counterparts on Mars, Mercury, the Moon, and perhaps even Earth. This was fueled primarily by the morphometric observation that, for a given diameter (D), crater depth (d) seems to scale inversely with surface gravity for the other planets in the inner solar system. The unpredicted depth of fresh impact craters on Venus argues against a simple inverse relationship between surface gravity and crater depth. Factors that could contribute to deep craters on Venus include (1) more efficient excavation on Venus, possibly reflecting rheological effects of the hot venusian environment; (2) more melting and efficient removal of melt from the crater cavity; and (3) enhanced ejection of material out of the crater, possibly as a result of entrainment in an atmosphere set in motion by the passage of the projectile. The broader issue raised by the venusian crater depths is whether surface gravity is the predominant influence on crater depths on any planet. While inverse gravity scaling of crater depths has been a useful paradigm in planetary cratering, the venusian data do not support this model and the terrestrial data are equivocal at best. The hypothesis that planetary gravity is the primary influence over crater depths and the paradigm that terrestrial craters are shallow should be reevaluated

Petrology of Impact-Melt Rocks at the Chicxulub Multiring Basin, Yucatan, Mexico

Compositions and textures of melt rocks from the upper part of the Chicxulub structure are typical of melt rocks at other large terrestrial impact structures. Apart from variably elevated iridium concentrations (less than 1.5 to 13.5 +/- 0.9 ppb) indicating nonuniform dissemination of a meteoritic component, bulk rock and phenocryst compositions imply that these melt rocks were derived exclusively from continental crust and platform-sediment target lithologies. Modest differences in bulk chemistry among samples from wells located approximately 40 km apart suggest minor variations in relative contributions of these target lithologies to the melts. Subtle variations in the compositions of early-formed pyroxene and plagioclase also support minor primary differences in chemistry between the melts. Evidence for pervasive hydrothermal alteration of the porous mesostasis includes albite, K-feldspar, quartz, epidote, chlorite, and other phyllosilicates, as well as siderophile element-enriched sulfides, suggesting the possibility that Chicxulub, like Sudbury, may host important ore deposits

Morphology and evolution of coronae and ovoids on Venus

Coronae and ovoids on Venus were first identified in Venera 15/16 data. They are distinctive and apparently unique to the planet, and may be important indicators of processes operating in the Venusian mantle. Magellan images have provided the first high resolution views of coronae and ovoid morphology. Herein, the general geologic character is described of coronae and ovoids, and some inferences are drawn about their geologic evolution. Coronae are circular to elongate features surrounded by an annulus of deformational features, with a relatively raised or indistinct topographic signature and, commonly, a peripheral trough or moat. Ovoids are circular to elongate features other than coronae with either positive or negative topographic signatures, associated with tectonic deformation and volcanism. The relationship of these two geologic features to each other and to Venusian geology is briefly discussed

New Constraints on the Slate Islands Impact Structure, Ontario, Canada

The Slate Islands in northern Lake Superior represent the eroded remains of a complex impact crater, originally approximately 32 km in diameter. New field studies there reveal allogenic crater fill deposits along the eastern and northern portions of the islands indicating that this 500-800 Ma impact structure is not as heavily eroded as previously thought. Near the crater center, on the western side or Patterson Island, massive blocks of target rocks, enclosed within a matrix of fine-grained polymict breccia, record the extensive deformation associated with the central uplift. Shatter cones are a common structural feature on the islands and range from less than 3 cm to over 10 m in length. Although shatter cones are powerful tools for recognizing and analyzing eroded impact craters, their origin remains poorly constrained

A Model of the Chicxulub Impact Basin Based on Evaluation of Geophysical Data, Well Logs, and Drill Core Samples

Abundant evidence now shows that the buried Chicxulub structure in northern Yucatan, Mexico, is indeed the intensely sought-after source of the ejecta found world-wide at the Cretaceous-Tertiary (K/T) boundary. In addition to large-scale concentric patterns in gravity and magnetic data over the structure, recent analyses of drill-core samples reveal a lithological assemblage similar to that observed at other terrestrial craters. This assemblage comprises suevite breccias, ejecta deposit breccias (Bunte Breccia equivalents), fine-grained impact melt rocks, and melt-matrix breccias. All these impact-produced lithologies contain diagnostic evidence of shock metamorphism, including planar deformation features in quartz, feldspar, and zircons; diaplectic glasses of quartz and feldspar; and fused mineral melts and whole-rock melts. In addition, elevated concentrations of Ir, Re, and Os, in meteoritic relative proportions, have been detected in some melt-rock samples from the center of the structure. Isotopic analyses, magnetization of melt-rock samples, and local stratigraphic constraints identify this crater as the source of K/T boundary deposits

Project Unit 94-46. Clastic Breccias at the Slate Islands Complex Impact Structure, Northern Lake Superior

Provides a summary of the impact process. Also some of the results of laboratory investigations conducted in 1995 and 1996. Describes the various clastic breccias encountered on the islands during 1994 and 1995 field work and relates them to the various phases of the impact process.B. O. Dressier, V. L. Sharpton, B. Schnieders and J. Scot

The Chicxulub impact crater and its influence on the regional hydrology in Northwestern Yucatan, Mexico

In 1980 Luis Alvarez et al. proposed a revolutionary theory: the large mass extinction that occurred at the Cretaceous–Tertiary (K–T) limit (65 million years ago) was a consequence of the impact of a large bolide with Earth (Alvarez et al. 1980). This theory was considered highly controversial, particularly because it took over a decade to “discover” the crater. However, research about these phenomena on Earth became relevant only a couple of decades ago. The investigations on some important craters, such as Sudbury crater (e.g., Dressler and Sharpton 1999), Manson crater (e.g., Grieve 1989), Popigai crater (e.g., Masaitis et al. 1999), and Ries crater (e.g., Stöffler 1977; Hörz et al. 1983; Newsom et al. 1990; Von Engelhardt 1990), along with about 120 other craters discovered so far on the surface of Earth, proved that these events are not as rare as once thought. With the discovery of Alvarez et al. (1980) it was also proposed that these kinds of phenomena might have had a relevant, if not main, role in the evolution of life on Earth. In this chapter we present a brief summary of the importance of the Chicxulub impact crater, its discovery, and how it influences regional hydrogeology

The Chicxulub impact crater and its influence on the regional hydrology in Northwestern Yucatan, Mexico

In 1980 Luis Alvarez et al. proposed a revolutionary theory: the large mass extinction that occurred at the Cretaceous–Tertiary (K–T) limit (65 million years ago) was a consequence of the impact of a large bolide with Earth (Alvarez et al. 1980). This theory was considered highly controversial, particularly because it took over a decade to “discover” the crater. However, research about these phenomena on Earth became relevant only a couple of decades ago. The investigations on some important craters, such as Sudbury crater (e.g., Dressler and Sharpton 1999), Manson crater (e.g., Grieve 1989), Popigai crater (e.g., Masaitis et al. 1999), and Ries crater (e.g., Stöffler 1977; Hörz et al. 1983; Newsom et al. 1990; Von Engelhardt 1990), along with about 120 other craters discovered so far on the surface of Earth, proved that these events are not as rare as once thought. With the discovery of Alvarez et al. (1980) it was also proposed that these kinds of phenomena might have had a relevant, if not main, role in the evolution of life on Earth. In this chapter we present a brief summary of the importance of the Chicxulub impact crater, its discovery, and how it influences regional hydrogeology

Evidence for a meteoritic component in impact melt rock from the chicxulub structure

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155788/1/Koeberl_et_al_1994_Evidence_for_a_meteoritic.pd

Isotopic comparison of K/T boundary impact glass with melt rock from the Chicxulub and Manson impact structures

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/155769/1/Blum_et_al_1993_Isotopic_comparison.pd