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Caracterização geofísica da estrutura de impacto de Araguainha, MT/GO; Gephysical Characterization of the Araguainha impact structure, MT/GO.

Vasconcelos, Marcos Alberto Rodrigues
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 17/04/2007 Português
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Araguainha é a maior e mais bem exposta cratera complexa da América do Sul, formada em sedimentos horizontais da Bacia do Paraná. Sua portentosa estrutura com aproximados 40 km de diâmetro e 25 km de cavidade transiente revela um contraste negativo de densidade e susceptibilidade magnética na região do soerguimento central. Esta feição permite inferir um limite de propriedade física entre o Núcleo Central Soerguido (NCS) e o Embasamento Granítico Soerguido (EGS), que por sua vez apresenta baixa razão de Koenigsberger. A caracterização geofísica de Araguainha é sustentada por modelamento 2,5D com base em informações gravimétricas, aeromagnéticas e magnéticas terrestres, o que permite classificá-la como uma estrutura concêntrica e simétrica, com profundidade média do embasamento a 1,0 km, exceto para a borda sul, que apresenta soerguimento mais acentuado. A interface embasamento/sedimento é seccionada por pares espelhados de falhas radiais que surgem no estágio de modificação da cratera. Estas estruturas rúpteis conferem as maiores profundidades à região da bacia anelar e promovem constricção dos sedimentos com tensão horizontal radial. A observação e interpretação destas deformações permitem caracterizar Araguainha como uma estrutura de impacto de domínio rúptil-dúctil.; Araguainha is the largest and the best-exposed complex crater of South America...

Condicionantes estruturais da drenagem e do relevo na cratera de Colônia e entorno, São Paulo - SP; Structural conditioning of drainage and relief in the Colônia crater and surrounding areas, São Paulo - SP

Santos, André Henrique Bezerra dos
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 21/06/2013 Português
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A pesquisa tem como objetivo compreender a evolução da drenagem e do relevo na cratera de Colônia e entorno, no setor meridional do Planalto Paulistano, em São Paulo-SP. Parte-se da hipótese de que o relevo da área resulta da degradação de uma zona de estruturas concêntricas, formada em conjunto com a cratera, por impacto de bólido extraterrestre. Essas estruturas seriam atribuídas a um ou mais mecanismos, entre eles: a) controle exercido por um sistema de falhas radiais e concêntricos, b) estratificação do alvo, c) pico anelar em cratera complexa, d) ejecta fluidificada e e) fluidificação de finos, que resultaram na geração de uma zona de formas concêntricas no entorno da cratera. Após sua formação, o relevo da cratera e seu entorno foram degradados pela drenagem, que promoveu trabalho erosivo mais intenso a leste que a oeste, produzindo níveis hipsométricos menores a leste. Com base na proposta metodológica de Oliveira (2003), elabora-se uma descrição do relevo e da drenagem para identificar elementos sugestivos de condicionamento geológico produzido pela estrutura de impacto, por meio de fotointerpretação, trabalhos de campo e morfometria. A partir dos dados de hipsometria, rupturas de declividade e lineamentos da drenagem...

Dynamic river channels suggest a long-lived Noachian crater lake on Mars

Bhattacharya, J.; Payenberg, T.; Lang, S.; Bourke, M.
Fonte: Amer Geophysical Union Publicador: Amer Geophysical Union
Tipo: Artigo de Revista Científica
Publicado em //2005 Português
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Recent images of the Noachian-age Holden Northeast Crater show spectacular meandering channels that form a complex 150 meter thick lacustrine river delta deposit confined to the northwest margin of the crater. We identify 11 major avulsion events in the topmost layer and suggest an avulsion frequency of a few hundred years. The meandering nature of the channels and numerous avulsions is incompatible with deposition on an alluvial fan and clearly reflects a complex history of multiple sustained flows. Assuming an average sedimentation rate of 1mm/year, we suggest that Holden NE Crater contained a lake that persisted for at least 150,000 years. Our analysis is incompatible with this feature being the result of a major bolide impact that would have released a catastrophic flood by groundwater sapping and supports the hypothesis that early Mars was both warmer and wetter during the Noachian.; Copyright 2005 by the American Geophysical Union.

Magnetic fabric of Araguainha complex impact structure (Central Brazil): implications for deformation mechanisms and central uplift formation

Yokoyama, Elder; Trindade, Ricardo Ivan Ferreira da; Lana, Cristiano de Carvalho; Souza Filho, Carlos Roberto de; Baratoux, D.; Marangoni, Y?ra Regina; Tohver, Eric
Fonte: Universidade Federal de Ouro Preto Publicador: Universidade Federal de Ouro Preto
Tipo: Artigo publicado em periodico
Português
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The weakening mechanisms involved in the collapse of compleximpact craters are controversial. The Araguainhaimpact crater, in Brazil, exposes a complexstructure of 40 km in diameter, and is an excellent object to address this issue. Its core is dominated by granite. In addition to microstructural observations, magnetic studies reveal its internal fabric acquired during the collapse phase. All granite samples exhibit impact-related planar deformation features (PDFs) and planar fractures (PFs), which were overprinted by cataclasis. Cataclastic deformation has evolved from incipient brittle fracturing to the development of discrete shear bands in the center of the structure. Fracture planes are systematically decorated by tiny grains (< 10 ?m) of magnetite and hematite, and the orientation of magnetic lineation and magnetic foliation obtained by the anisotropies of magnetic susceptibility (AMS) and anhysteretic remanence (AAR) are perfectly coaxial in all studied sites. Therefore, we could track the orientation of deformation features which are decorated by iron oxides using the AMS and AAR. The magneticfabrics show a regular pattern at the borders of the central peak, with orientations consistent with the fabric of sediments at the crater's inner collar and complex in the center of the structure. Both the cataclastic flow revealed from microstructural observations and the structural pattern of the magnetic anisotropy match the predictions from numerical models of compleximpactstructures. The widespread occurrence of cataclasis in the central peak...

Magnetic fabric of Araguainha complex impact structure (Central Brazil): Implications for deformation mechanisms and central uplift formation

Yokoyama, E.; Trindade, R. I. F.; Lana, C.; Souza Filho, C. R.; Baratoux, D.; Marangoni, Y. R.; Tohver, E.
Fonte: ELSEVIER SCIENCE BV; AMSTERDAM Publicador: ELSEVIER SCIENCE BV; AMSTERDAM
Tipo: Artigo de Revista Científica
Português
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The weakening mechanisms involved in the collapse of complex impact craters are controversial. The Araguainha impact crater, in Brazil, exposes a complex structure of 40 km in diameter, and is an excellent object to address this issue. Its core is dominated by granite. In addition to microstructural observations, magnetic studies reveal its internal fabric acquired during the collapse phase. All granite samples exhibit impact-related planar deformation features (PDFs) and planar fractures (PFs), which were overprinted by cataclasis. Cataclastic deformation has evolved from incipient brittle fracturing to the development of discrete shear bands in the center of the structure. Fracture planes are systematically decorated by tiny grains (<10 mu m) of magnetite and hematite, and the orientation of magnetic lineation and magnetic foliation obtained by the anisotropies of magnetic susceptibility (AMS) and anhysteretic remanence (AAR) are perfectly coaxial in all studied sites. Therefore, we could track the orientation of deformation features which are decorated by iron oxides using the AMS and AAR. The magnetic fabrics show a regular pattern at the borders of the central peak, with orientations consistent with the fabric of sediments at the crater's inner collar and complex in the center of the structure. Both the cataclastic flow revealed from microstructural observations and the structural pattern of the magnetic anisotropy match the predictions from numerical models of complex impact structures. The widespread occurrence of cataclasis in the central peak...

Caracterização geológica da estrutura de impacto de Riachão, MA; Geologic characterization of Riachão impact structure, MA

Mariana Velcic Maziviero
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 01/03/2012 Português
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Atualmente são conhecidas em toda superfície terrestre cerca de 180 estruturas formadas por impacto de meteoritos. No Brasil, foram diagnosticadas seis estruturas originadas por este tipo de evento: Araguainha (MT-GO), Vargeão (SC), Vista Alegre (PR), Cerro do Jarau (RS), Serra da Cangalha (TO) e Riachão (MA). A área de estudos é a estrutura de impacto de Riachão (7º43'S/46º39'W), uma estrutura de impacto do tipo complexa, ligeiramente elíptica, em avançado estágio erosivo e com diâmetro de aproximadamente 4 km. Esta estrutura situa-se na porção centro-oeste da Bacia do Parnaíba, onde predominam arenitos e, subordinadamente, rochas carbonáticas intercaladas com siltitos pertencentes à parte superior do Grupo Balsas depositados entre o Pensilvaniano e Permiano. Estudos geológicos dessa estrutura foram realizados nas décadas de 1960 e 1980, resultando na caracterização geológica preliminar da mesma. Neste trabalho, a partir da integração de dados geológicos e de sensoriamento remoto, foi possível caracterizar geológica e geneticamente a estrutura de Riachão. As unidades litoestratigráficas identificadas correspondem a diferentes níveis da Formação Pedra de Fogo, e, possivelmente, da Formação Piauí...

Magnetic fabric of Araguainha complex impact structure (Central Brazil): Implications for deformation mechanisms and central uplift formation

Yokoyama, E.; Trindade, R. I. F.; Lana, C.; Souza Filho, C. R.; Baratoux, D.; Marangoni, Y. R.; Tohver, E.
Fonte: Elsevier; Amsterdam Publicador: Elsevier; Amsterdam
Tipo: Artigo de Revista Científica
Português
Relevância na Pesquisa
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The weakening mechanisms involved in the collapse of complex impact craters are controversial. The Araguainha impact crater, in Brazil, exposes a complex structure of 40 km in diameter, and is an excellent object to address this issue. Its core is dominated by granite. In addition to microstructural observations, magnetic studies reveal its internal fabric acquired during the collapse phase. All granite samples exhibit impact-related planar deformation features (PDFs) and planar fractures (PFs), which were overprinted by cataclasis. Cataclastic deformation has evolved from incipient brittle fracturing to the development of discrete shear bands in the center of the structure. Fracture planes are systematically decorated by tiny grains (<10 mu m) of magnetite and hematite, and the orientation of magnetic lineation and magnetic foliation obtained by the anisotropies of magnetic susceptibility (AMS) and anhysteretic remanence (AAR) are perfectly coaxial in all studied sites. Therefore, we could track the orientation of deformation features which are decorated by iron oxides using the AMS and AAR. The magnetic fabrics show a regular pattern at the borders of the central peak, with orientations consistent with the fabric of sediments at the crater's inner collar and complex in the center of the structure. Both the cataclastic flow revealed from microstructural observations and the structural pattern of the magnetic anisotropy match the predictions from numerical models of complex impact structures. The widespread occurrence of cataclasis in the central peak...

Caracterização geológica da estrutura de impacto meteorítico de Santa Marta (PI); Geological characteristics of Santa Marta meteorite impact structure (PI)

Grace Juliana Gonçalves de Oliveira
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Dissertação de Mestrado Formato: application/pdf
Publicado em 10/08/2015 Português
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A estrutura de Santa Marta, localizada no Estado do Piauí, Nordeste do Brasil, torna-se a sexta estrutura de impacto meteorítico a ser reconhecida no país. Trata-se de uma cratera do tipo complexa de tamanho moderado, centrada em 10°10'S / 45°14'W, com ~10 km de diâmetro, borda elevada e núcleo central de 3,2 km. Foi formada em estratos sedimentares (conglomerados, arenitos, siltitos) que pertencem a duas bacias distintas: a Bacia Paleozoica do Parnaíba e a Bacia Mesozoica Sanfranciscana, que se sobrepõem na região da cratera. Evidências macroscópicas e petrográficas da natureza da estrutura incluem shatter cones e feições de deformação planar em grãos de quartzo (planar deformation features - PDFs). Orientações cristalográficas dos PDFs que ocorrem em vários conjuntos em grãos de quartzo são indicativos de pico de pressão de choque de 20-25 GPa nas rochas próximas ao centro da cratera. As feições ocorrem dominantemente em arenitos e brechas areníticas. Estudos estratigráficos apontam idade na faixa de 66-93 Ma para o evento do impacto meteorítico.; The Santa Marta structure, located in Piauí State, northeastern Brazil, became the country's sixth impact structure to be recognized. Santa Marta is a complex crater structure of moderate size...

Dropping the Ball: The effect of anisotropic granular materials on ejecta and impact crater shape

Drexler, Philip; Keim, Nathan; Arratia, Paulo
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 11/10/2013 Português
Relevância na Pesquisa
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In this fluid dynamics video, we present an experimental investigation of the shape of impact craters in granular materials. Complex crater shapes, including polygons, have been observed in many terrestrial planets as well as moons and asteroids. We release spherical projectiles from different heights above a granular bed (sand). The experiments demonstrate two different techniques to create non-circular impact craters, which we measure by digitizing the final crater topography. In the first method, we create trenches in the sand to mimic fault lines or valleys on a planetary target. During impact, ejecta move faster in the direction of the trenches, creating nearly elliptical craters with the major axis running parallel to the trench. Larger trenches lead to more oblong craters. In the second method, a hose beneath the surface of the sand injects nitrogen gas. The pressure of the gas counters the hydrostatic pressure of the sand, greatly reducing static friction between grains above the injection point, without disturbing the surface. The affected sand has lower resistance to impact, creating a knob in the otherwise circular crater rim.; Comment: Two videos for submission in the 2013 Gallery of Fluid Motion at the 66th Annual Meeting of the APS Division of Fluid Dynamics

Role of the granular nature of meteoritic projectiles in impact crater morphogenesis

Bartali, Roberto; Rodríguez-Liñán, Gustavo M.; Nahmad-Molinari, Yuri; Sarocchi, Damiano; Ruiz-Suárez, J. C.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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By means of novel volume-diameter aspect ratio diagrams, we ponder on the current conception of crater morphogenesis analyzing crater data from beam explosions, hypervelocity collisions and drop experiments and comparing them with crater data from three moons (the Moon, Callisto, and Ganymede) and from our own experimental results. The distinctive volume-diameter scaling laws we discovered make us to conclude that simple and complex craters in satellites and planets could have been formed by granular vs. granular collisions and that central peaks and domes in complex craters were formed by a dynamic confinement of part of the impacting projectile, rather than by the uplift of the target terrain. A granulometric analysis of asteroids and central peaks and domes inside complex craters, shows boulder size distributions consistent with our hypothesis that crater internal features are the remnants of granular impactors.; Comment: 7 pages, 10 figures. Changes: Corrected 4th author name, added more data for fig. 6, added 2 extra citations, corrected typos

Using the youngest asteroid clusters to constrain the Space Weathering and Gardening rate on S-complex asteroids

Willman, Mark; Jedicke, Robert; Moskovitz, Nicholas; Nesvorný, David; Vokrouhlický, David; Mothé-Diniz, Thais
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 21/04/2010 Português
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We have extended our earlier work on space weathering of the youngest S-complex asteroid families to include results from asteroid clusters with ages <10^6 years and to newly identified asteroid pairs with ages <5x10^5 years. We have identified three S-complex asteroid clusters with ages in the range 10^{5-6} years. The average color of the objects in these clusters agree with the prediction of Willman et al., 2008. SDSS photometry of the members of very young asteroid pairs with ages <10^5 years was used to determine their taxonomy. The average color of the S-complex pairs is PC_1=0.49+/-0.03, over 5-sigma redder than predicted by Willman et al., 2008. Therefore, the most likely pair formation mechanism is gentle separation due to YORP spin-up leaving much of the aged and reddened surface undisturbed. In this case our color measurement allows us to set an upper limit of ~64% on the disturbed surface portion. Using pre-existing color data and our new results for the youngest S-complex asteroid clusters we have extended our space weather model to explicitly include the effects of regolith gardening and fit separate weathering and gardening characteristic timescales of tau_w=960+/-160My and tau_g=2000+/-290My respectively. The first principal component color for fresh S-complex material is 0.37+/-0.01 while the maximum amount of local reddening is 0.33+/-0.06. Our first-ever determination of the gardening time is in stark contrast to our calculated gardening time of tau_g~270My based on main belt impact rates and reasonable assumptions about crater and ejecta blanket sizes. A possible resolution for the discrepancy is through a `honeycomb' mechanism in which the surface regolith structure absorbs small impactors without producing significant ejecta. This mechanism could also account for the paucity of small craters on (433) Eros.; Comment: 47 pages...

Bayesian time series analysis of terrestrial impact cratering

Bailer-Jones, C. A. L.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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Giant impacts by comets and asteroids have probably had an important influence on terrestrial biological evolution. We know of around 180 high velocity impact craters on the Earth with ages up to 2400Myr and diameters up to 300km. Some studies have identified a periodicity in their age distribution, with periods ranging from 13 to 50Myr. It has further been claimed that such periods may be causally linked to a periodic motion of the solar system through the Galactic plane. However, many of these studies suffer from methodological problems, for example misinterpretation of p-values, overestimation of significance in the periodogram or a failure to consider plausible alternative models. Here I develop a Bayesian method for this problem in which impacts are treated as a stochastic phenomenon. Models for the time variation of the impact probability are defined and the evidence for them in the geological record is compared using Bayes factors. This probabilistic approach obviates the need for ad hoc statistics, and also makes explicit use of the age uncertainties. I find strong evidence for a monotonic decrease in the recorded impact rate going back in time over the past 250Myr for craters larger than 5km. The same is found for the past 150Myr when craters with upper age limits are included. This is consistent with a crater preservation/discovery bias modulating an otherwise constant impact rate. The set of craters larger than 35km (so less affected by erosion and infilling) and younger than 400Myr are best explained by a constant impact probability model. A periodic variation in the cratering rate is strongly disfavoured in all data sets. There is also no evidence for a periodicity superimposed on a constant rate or trend...

Experimental impact shock chemistry on planetary icy bodies

Nna Mvondo, Delphine; Khare, Bishun; Ishihara, Tomoko; McKay, Christopher P.
Fonte: Elsevier Publicador: Elsevier
Tipo: Artículo Formato: 561116 bytes; application/pdf
Português
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15 pages, 14 figures.-- AGU nrs.: 5200, 5400, 5420, 5422, 5455.-- Printed version published on April 2008.; The original publication is available at http://www.sciencedirect.com/science/journal/00191035; Several icy satellites of the outer planets show impact cratering features and it is recognised that this process may have played a crucial role in the formation and evolution of icy bodies. The effect of impact by extraterrestrial objects into the surface is commonly related to physical changes. Most of the research applied to impacts on ices has been developed to study and understand the cratering formation process and their physical, geophysical characteristics. Chemical changes and synthesis occurring on icy planetary surfaces are generally explained by the influence of UV photons and high-energy charged particles on ices. Nonetheless, impact process onto ices could be a source of local or global endogenic process and could be especially advantageous as an efficient energy source for driving interesting chemistry. Impacts can ensure that icy surfaces are eventually exposed, for a limited period of time, to aqueous melt in impact craters and ejecta and one can imagine that impurities included in the ice may undergo hydrolysis and other reactions under such conditions. Upon impact...

Chemical production by Pulse-Laser Irradiation on Ices: Simulation of impact shock-induced chemistry

Nna Mvondo, Delphine; Khare, Bishun N.; Ishihara, Tomoko; McKay, Christopher P.
Fonte: NASA Astrobiology Institute Publicador: NASA Astrobiology Institute
Tipo: Póster Formato: 19573 bytes; application/pdf
Português
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Poster presented at: Bioastronomy 2007: Molecules, Microbes and Extraterrestrial Life (July 16-20, 2007, San Juan de Puerto Rico).; Chemical changes and synthesis occurring on icy satellites are generally explained by the influence of UV photons and high-energy charged particles on ices. Nonetheless, impact process onto ices could be especially advantageous and efficient as a local or global endogenic energy source for driving interesting surface chemistry. Impacts can ensure that ices are eventually exposed, for a limited period of time, to aqueous melt in impact craters and ejecta, allowing impurities included in the ice to undergo hydrolysis and other reactions. Upon impact, the kinetic energy of the bolide is transferred to the ground liberating a great deal of stress energy which could initiate in situ a diverse series of chemical reactions in the fracture zone beneath the crater.; In the present work, we report the experimental results of a new approach testing in laboratory the chemistry conducted by impacts into planetary ices. Mixtures of pure water ices containing impurities like CO2, Na2CO3, CH3OH and CH3OH / (NH4)2SO4 were irradiated at 77K by a pulsed Nd-YAG laser beam (1064 nm). GC-MS and FTIR analyses show that hydrogen peroxide...

Complex craters: Relationship of stratigraphy and rings to impact conditions

O'Keefe, John D.; Ahrens, Thomas J.
Fonte: American Geophysical Union Publicador: American Geophysical Union
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em 25/11/1999 Português
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One of the key issues associated with the understanding of large scale impacts is how the observable complex crater structural features (e.g., central peaks and pits, flat floors, ring shaped ridges and depressions, stratigraphic modifications, and faults) relate to the impactor's parameters (e.g., radius, velocity, and density) and the nonobservable transient crater measures (e.g., depth of penetration and diameter at maximum penetration). We have numerically modeled large-scale impacts on planets for a range of impactor parameters, gravity and planetary material strengths. From these we found that the collapse of the transient cavity results in the development of a tall, transient central peak that oscillates and drives surface waves that are arrested by the balance between gravitational forces and planetary strength to produce a wide range of the observed surface features. In addition, we found that the underlying stratigraphy is inverted outside of the transient cavity diameter (overturned flap region), but not inside. This change in stratigraphy is observable by remote sensing, drilling, seismic imaging and gravity mapping techniques. We used the above results to develop scaling laws and to make estimates of the impact parameters for the Chicxulub impact and also compared the calculated stratigraphic profile with the internal structure model developed by Hildebrand et. al. [1998]...

Planetary Cratering Mechanics

O'Keefe, John D.; Ahrens, Thomas J.
Fonte: American Geophysical Union Publicador: American Geophysical Union
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em 25/09/1993 Português
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The objective of this study was to obtain a quantitative understanding of the cratering process over a broad range of conditions. Our approach was to numerically compute the evolution of impact induced flow fields and calculate the time histories of the key measures of crater geometry (e.g. depth, diameter, lip height) for variations in planetary gravity (0 to 10^9 cm/s^2), material strength (0 to 2400 kbar), and impactor radius (0.05 to 5000 km). These results were used to establish the values of the open parameters in the scaling laws of Holsapple and Schmidt (1987). We describe the impact process in terms of four regimes: (1) penetration, (2) inertial, (3) terminal and (4) relaxation. During the penetration regime, the depth of impactor penetration grows linearly for dimensionless times τ = (Ut/a) <5.1. Here, U is projectile velocity, t is time, and a is projectile radius. In the inertial regime, τ > 5.1, the crater grows at a slower rate until it is arrested by either strength or gravitational forces. In this regime, the increase of crater depth, d, and diameter, D, normalized by projectile radius is given by d/a = 1.3 (Ut/a)^(0.36) and D/a = 2.0(Ut/a)^(0.36). For strength-dominated craters, growth stops at the end of the inertial regime...

The distribution of megablocks in the Ries crater, Germany: Remote sensing, field investigation, and statistical analyses

Sturm, Sebastian; Kenkmann, Thomas; Willmes, Malte; Posges, Gisela; Hiesinger, Harold
Fonte: University of Arkansas Publicador: University of Arkansas
Tipo: Artigo de Revista Científica
Português
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The Ries crater is a well-preserved, complex impact crater that has been extensively used in the study of impact crater formation processes across the solar system. However, its geologic structure, especially the megablock zone, still poses questions regarding crater formation mechanics. The megablock zone, located between the inner crystalline ring and outer, morphologic crater rim, consists of allochthonous crystalline and sedimentary blocks, Bunte Breccia deposits, patches of suevite, and parautochthonous sedimentary blocks that slumped into the crater during crater modification. Our remote sensing detection method in combination with a shallow drilling campaign and geoelectric measurements at two selected megablocks proved successful in finding new megablock structures (>25 m mean diameter) within the upper approximately 1.5 m of the subsurface in the megablock zone. We analyzed 1777 megablocks of the megablock zone, 81 of which are new discoveries. In our statistical analysis, we also included 2318 ejecta blocks >25 m beyond the crater rim. Parautochthonous megablocks show an increase in total area and size toward the final crater rim. The sizes of allochthonous megablocks generally decrease with increasing radial range...

Lunar impact breccias: petrology, crater setting, and bombardment history of the Moon

Norman, Marc
Fonte: Blackwell Publishing Ltd Publicador: Blackwell Publishing Ltd
Tipo: Artigo de Revista Científica
Português
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Two general classes of lunar impact breccias have been recognised: fragmental breccias and melt breccias. Fragmental breccias are composed of clastic-rock debris in a finely comminuted grain-supported matrix of mineral and lithic fragments. Impact melt breccias have crystalline to glassy matrices that formed by cooling of a silicate melt. Most lunar impact breccias in our collection probably sample ejecta from large complex craters or multi-ring basins, although linking individual breccias to specific impact events has proven surprisingly difficult. A long-standing problem in lunar science has been distinguishing clast-poor impact melt breccias from igneous rocks produced by melting of the lunar interior. Concentrations and relative abundances of highly siderophile elements derived from the meteoritic impactor provide a useful discriminant, especially when combined with petrologic and geochemical evidence for mechanical mixing. Most lunar impact melt breccias have crystallisation ages of 4.0-3.8 Ga, corresponding to an episode of intensive crustal metamorphism recorded by whole-rock U - Pb isotopic compositions of lunar anorthosites. This may reflect a short-lived spike in the cratering rate, although other explanations are possible. The question of whether or not a cataclysmic bombardment struck the Earth and Moon at ca 3.9 Ga remains open and the subject of continuing investigations.

STRUCTURAL MODELING OF THE VICHADA IMPACT STRUCTURE FROM INTERPRETED GROUND GRAVITY AND MAGNETIC ANOMALIES

Hernández,Orlando; Khurama,Sait; Alexander,Gretta C.
Fonte: Boletin de Geología Publicador: Boletin de Geología
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/06/2011 Português
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A prominent positive free-air gravity anomaly mapped over a roughly 50-km diameter basin is consistent with a mascon centered on (4°30'N, -69°15'W) in the Vichada Department, Colombia, South America. Ground follow up gravity and magnetic anomalies were modeled confirming the regional free air gravity anomalies. These potential field anomalies infer a hidden complex impact basin structure filled with tertiary sedimentary rocks and recent quaternary deposits. Negative Bougueranomalies of 8 mGals to 15 mGals amplitude are associated with a concentric sedimentary basin with a varying thickness from 100m to 500 m in the outer rings to 700m to 1000m at the center of the impact crater basin. Strong positive magnetic anomalies of 100 nT to 300 nT amplitude infer the presence of a local Precambrian crystalline basement that was affected by intensive faulting producing tectonic blocks dipping to the center of the structure, showing a typical "domino structure" of impact craters such as that of Sudbury, Ontario, Canada. Basic to intermediate mineralized veins and dikes with contrasting density and magnetic susceptibility properties could be emplaced along these faulting zones, as inferred from local gravity and magnetic highs. The geologic mapping of the area is limited by the flat topography and absence of outcrops/ geomorphologic units. Nevertheless...

Fracturing and deformation in the Chicxulub crater - Complex trace analysis of instantaneous seismic attributes

Salguero-Hernández,Eduardo; Urrutia-Fucugauchi,Jaime; Ramírez-Cruz,Luis
Fonte: Instituto de Geología, UNAM Publicador: Instituto de Geología, UNAM
Tipo: Artigo de Revista Científica Formato: text/html
Publicado em 01/04/2010 Português
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Large complex impact craters form by collapse from initial excavation stage of a deep narrow bowl-shaped transient cavity. Fracturing and shattering of solids with finite tensile shear limits occur related to shock-induced damage of target material, with fracturing and fragmentation occurring during transient cavity crater collapse processes. Geophysical studies of subsurface crater structure may assist in studying shock-induced effects of deformation and fracturing of target rocks. Here we present initial results of a study of subsurface fracturing/deformation in the Chicxulub crater from seismic reflection data. The analysis is based on the instantaneous seismic attributes envelope amplitude, instantaneous frequency and Q factor, at selected sectors of the crater by looking at petrophysical properties and seismic attenuation. Shock effects with shattering and fracturing of Mesozoic target rocks show a trend to decrease away from the rim zone. Cretaceous carbonates show less attenuation inside the crater than in exterior sectors. The relative attenuation quality factor Q is lower in sections outside the crater rim as compared with higher Q values inside the rim, and particularly at depth within the Cretaceous sequence. Carbonates in the western sector are characterized by slightly larger attenuation than in the eastern sector...