1,721,078 research outputs found
Shatter cones : nature and genesis
No full textShatter cones are a fracture phenomenon that is exclusively associated with shock metamorphism and has also been produced in the laboratory in several shock experiments. The occurrence of shatter cones is the only accepted meso-to macroscopic recognition criterion for impact structures. Shatter cones exhibit a number of geometric characteristics (orientation, apical angles, striation angles, sizes) that can be best described as varied, from case to case. Possible links between geometric properties with impact or crater parameters have remained controversial and the lack of understanding of the mechanism of formation of shatter cones does not offer a physical framework to discuss or understand them. A database of shatter cone occurrences has been produced for this introduction paper to the special issue of Meteoritics and Planetary Science on shatter cones. Distribution of shatter cones with respect to crater size and lithology suggests that shatter cones do not occur in impact craters less than a few kilometers in diameter, with a few, currently questionable exceptions. All pertinent hypotheses of formation are presented and discussed. Several may be discarded in light of the most recent observations. The branching fracture mechanism and the interference models proposed, respectively, by Sagy et al. (2002) and Baratoux and Melosh (2003) require further evaluation. New observations, experiments, or theoretical considerations presented in this special issue promise an important step forward, based on a renewed effort to resolve the enigmatic origin of these important features
Shatter cones : nature and genesis
No full textShatter cones are curved fractures decorated with divergent striations that are exclusively associated with impact metamorphism. The terminology "cone" was chosen from the observation of complete or near-complete, roundish, axisymmetric objects with a well-defined apex/apical area. It also imprecisely extends to the curved or subplanar fractures decorated with striations commonly observed at impact sites. The geometry of these objects is therefore highly variable but its significance was never addressed due to the lack of appropriate data. Here, we apply two methods to derive shape models of shatter cones. The first one is based on images acquired using a commercial camera and may be applied on large samples in the field. The second one uses an articulated arm equipped with a digital laser scanner and produces high-resolution and precision shape models of hand-sized samples. The analysis of 20 shape models of shatter cones from nine different impact sites indicates that the surface of shatter cones may be described by quadric surfaces and are generally consistent with hyperboloids, whereas occurrence of paraboloid objects cannot be ruled out. The surface characteristics are generally not consistent with the mathematical definition of a cone. The value of these shape models to discriminate between the different hypotheses of formation of shatter is still limited, as it remains to be resolved which type of surface pertains to which hypothesis. This requires theoretical developments, and experimental or numerical simulations of the propagation of tensile fractures associated with shock waves
Silicified cone-in-cone structures from Erfoud (Morocco): A comparison with impact-generated shatter cones
No full textShatter cones : nature and genesis
No full textAssociations between impact structures and meteorite occurrences are rare and restricted to very young structures. Meteorite fragments are often disrupted in the atmosphere, and in most cases, meteorite falls that have been decelerated by atmospheric drag do not form a crater. Furthermore, meteorites are rapidly weathered. In this context, the finding of shatter cones in Jurassic marly limestone in the same location as a recent (105 +/- 40 ka) iron meteorite fall near the village of Agoudal (High Atlas Mountains, Morocco) is enigmatic. The shatter cones are the only piece of evidence of a meteorite impact in the area. The overlap of a meteorite strewn field with the area of occurrence of shatter cones led previous researchers to consider that the meteorite fall was responsible for the formation of shatter cones in the context of formation of one or several small (<100 m) impact craters that had since been eroded. Shatter cones are generally not reported in association with subkilometer-diameter impact craters. Here, we present new field observations and an analysis of the distribution and characteristics of shatter cones, breccia, and meteorites in the Agoudal area. Evidence for local deformation not related to the structural High Atlas tectonics has been observed, such as a vertical to overturned stratum trending N150-N160. New outcrops with exposures of shatter cones are reported and extend the previously known area of occurrence. The area of in situ shatter cones (similar to 0.15 km(2)) and the strewn field of meteorites are distinct, although they show some overlap. The alleged impact breccia is revealed as calcrete formations. No evidence for a genetic relationship between the shatter cones and the meteorites can be inferred from field observations. The extent of the area where in situ shatter cones and macrodeformation not corresponding to Atlas tectonic deformation are observed suggest that the original diameter of an impact structure could have been between at least 1-3 km. For typical erosion rates in the Atlas region (similar to 0.08 cm yr(-1)), the period of time required for the erosion of such a structure (1.25-3.75 Ma) is much larger than the age of the meteorite fall. This line of reasoning excludes a genetic link between the shatter cones and the meteorite fall and indicates that the observed shatter cones belong to an ancient impact structure that has been almost entirely eroded
Nature of the Archean midcrust in the core of the Vredefort Dome, Central Kaapvaal Craton, South Africa
No full textExtreme uplift associated with the formation of the 2.02 Ga Vredefort dome has exposed a substantial cross section through the crystalline early Archean basement complex rocks of the Kaapvaal craton. The rocks comprise polydeformed high-grade tonalite-trondhjemite-granodiorite (TTG) gneisses, migmatites and late-tectonic intrusive granitoids that straddle the upper amphibolite-to granulite-facies transition. Field, petrographic and geochemical data indicate that compositional heterogeneity occurs on a local scale and reflects the migmatitic character of the rocks rather than crustal-scale layering as has been previously proposed. No evidence has been found to support exposure of either a melt-depleted, refractory, lower crust or an upper crustal batholithic granite layer; however, the immense volume of granitic leucosome in the rocks suggests that the exposed section represents an intermediate level between these two zones. Granitic leucosomes in the upper amphibolite-facies migmatites appear to be intrusive into the predominantly trondhjemitic host rocks, rather than of in situ derivation. Leucosome compositions in the granulite-facies migmatites are more variable, ranging from granitic and charnockitic to enderbitic, probably reflecting at least some local derivation. Leucosomes and small granitoid bodies show local-scale geochemical variation that can be explained in terms of variable amounts of melt segregation and migration, and fractionation of minerals such as K-feldspar within the melts. Copyright (C) 2004 Elsevier Ltd
Evidence for shock-induced anhydrite recrystallization and decomposition at the UNAM-7 drill core from the Chicxulub impact structure
No full textDrill core UNAM‐7, obtained 126 km from the center of the Chicxulub impact structure, outside the crater rim, contains a sequence of 126.2 m suevitic, silicate melt‐rich breccia on top of a silicate melt‐poor breccia with anhydrite megablocks. Total reflection X‐ray fluorescence analysis of altered silicate melt particles of the suevitic breccia shows high concentrations of Br, Sr, Cl, and Cu, which may indicate hydrothermal reaction with sea water. Scanning electron microscopy and energy‐dispersive spectrometry reveal recrystallization of silicate components during annealing by superheated impact melt. At anhydrite clasts, recrystallization is represented by a sequence of comparatively large columnar, euhedral to subhedral anhydrite grains and smaller, polygonal to interlobate grains that progressively annealed deformation features. The presence of voids in anhydrite grains indicates SOx gas release during anhydrite decomposition. The silicate melt‐poor breccia contains carbonate and sulfate particles cemented in a microcrystalline matrix. The matrix is dominated by anhydrite, dolomite, and calcite, with minor celestine and feldspars. Calcite‐dominated inclusions in silicate melt with flow textures between recrystallized anhydrite and silicate melt suggest a former liquid state of these components. Vesicular and spherulitic calcite particles may indicate quenching of carbonate melts in the atmosphere at high cooling rates, and partial decomposition during decompression at postshock conditions. Dolomite particles with a recrystallization sequence of interlobate, polygonal, subhedral to euhedral microstructures may have been formed at a low cooling rate. We conclude that UNAM‐7 provides evidence for solid‐state recrystallization or melting and dissociation of sulfates during the Chicxulub impact event. The lack of anhydrite in the K‐Pg ejecta deposits and rare presence of anhydrite in crater suevites may indicate that sulfates were completely dissociated at high temperature (T > 1465 °C)—whereas ejecta deposited near the outer crater rim experienced postshock conditions that were less effective at dissociation
Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136)
No full textIn recent years, several major international drilling and field projects on terrestrial impact structures, as well as new spacecraft missions to the Moon, neighboring planets, asteroids, and comets, have delivered important new insights into impact cratering processes within the solar system. LMI VI will provide a forum for discussion of recent results and advances, based on natural observations, spacecraft data, experimental results, and numerical simulation studies.Institutional support Barringer Crater Company, Institute of Geosciences, Brasília, IRD - Institut de Recherche pour le Développement (France), FINATEC Foundation at University of Brasília, Lunar and Planetary Institute, The Meteoritical Society, Universities Space Research Association, University of BrasíliaConference convener Wolf Uwe Reimold, University of BrasíliaPARTIAL CONTENTS: The Tin Bider Impact Structure (Saharan Platform, Algeria): New Inputs on Structural Aspects / F. Kassab and D. Belhai--Update of the Terrestrial Impact Crater Record: Crater Discovery Statistics, Size, and Age Frequency Distributions. / T. Kenkmann--Shock Deformation in the Cleanskin Impact Structure, Northern Territory, Australia / T. Kenkmann, P. W. Haines, I. Sweet, and K. Mitchell--India´s Third Impact Crater: Ramgarh, Rajasthan / T. Kenkmann, G. Wulf, and A. Agarwal--High-Precision Geochronology and Thermal Modeling of the Morokweng Impact Melt Sheet, South Africa / G. G. Kenny, J. L. Crowley, M. D. Schmitz, M. A. G. Andreoli, and R. L. Gibson--Cretaceous-Paleogene Boundary Stratigraphy of Belize / D. T. King and L. W. Petruny--What Do We Know About the Formation of Libyan Desert Glass? / C. Koeberl--Timing of Emplacement of Vredefort Granophyre Dykes / E. Kovaleva, M. S. Huber, M. Clark, and F. Fourie--Twisted Kink Bands: New Shock Deformation Microstructure in Zircon from the Vredefort Impact Structure / E. Kovaleva, D. Zamyatin, and H. Leroux--An Eroded Peak Ring Impact Recording a Tsunami on Earth: Rochechouart / P. Lambert--Current Stage of the CIRIR Research and Outreach at Rochechouart / P. Lambert--Global Impact Heritage Association: A Project Serving the Public and Impact Research / P. Lambert--The Effects of Impactor and Target Properties on the Formation of Basin Structures on the Moon / T. Lompa and K. Wünnemann--Melting Induced by Giant Collisons in the Earth-Moon System / L. Manske, N. Güldemeister, and K. Wünnemann--Orientation of Fractures in the Chicxulub Peak Ring / N. McCall, S. Gulick, A. Rae, M. Poelchau, U. Riller, J. Morgan, and J. Lofi
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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