The Vredefort Dome is located southwest of Johannesburg, South Africa, and represents the deeply eroded remnant of the central uplift of the world’s largest known impact structure, with an estimated diameter of ~300 km. The Vredefort impact structure is also the oldest known impact structure on Earth (~2.02 Ga). The Vredefort Dome comprises an ~40 km wide core of Archaean basement gneisses and an ~20 km wide collar of subvertical to overturned Late Archaean to Palaeoproterozoic supracrustal strata. This project presents the results of Landsat-TM and aerial photograph analysis, as well as field mapping of Witwatersrand Supergroup metasedimentary strata in the collar of the Vredefort Dome. The aim of this study was to investigate the structures (such as folds, faults, fractures), at all scales, and other deformation features (such as shatter cones and pseudotachylitic breccias) in the field area, and to establish geometric and temporal relationships between these features with regard to the impact cratering process. This study revealed a highly heterogeneous internal structure of the collar involving folds, faults, fractures and melt breccias that are interpreted as the product of shock deformation and central uplift formation during the Vredefort impact event. Broadly radially-oriented symmetric and asymmetric folds, with wavelengths from tens of metres to kilometres, and conjugate radial to oblique faults with strike-slip displacements of, typically, tens to hundreds of metres accommodated tangential shortening of the collar of the dome that decreased from ~17 %, at a radial distance from the dome centre of 21 km, to <5 % at a radial distance of 29 km. Ubiquitous shear fractures containing pseudotachylitic breccia, particularly in the metapelitic units, display variable local slip senses consistent with either tangential shortening or tangential extension; however, it is uncertain whether they formed at the same time as the larger faults during the rise of the central uplift or earlier, during the shock compression phase of cratering. Contrary to the findings about shatter cones of some earlier workers in the Vredefort structure, the Vredefort cone fractures do not show uniform apex orientations at any given outcrop, nor do small cones show a pattern consistent with the previously postulated “master cone” concept. The model of simple back-rotation of the strata to a horizontal pre-impact position also does not lead to a uniform centripetal-upward orientation of the cone apices. Striation patterns on the cone surfaces are variable, ranging from typically diverging, i.e., branching off the cone apex, to subparallel to parallel on almost flat surfaces. Striation angles on shatter cones do not increase with distance from the crater centre, as suggested previously. Instead, individual outcrops present a range of such striation angles, and a more irregular distribution of striation angle values with regard to the distance from the crater centre suggests localised controls involving the nature and shape of various heterogeneities in the target rock on this aspect of cone morphology. On the basis of the observations made during this study on small-scale structures in the collar of the Vredefort Dome, the relationship of shatter cones with curviplanar fractures (multipli-striated joint sets - MSJS) is confirmed. Pervasive, metre-scale tensile fractures crosscut shatter cones and appear to have formed after the closely-spaced MSJ-type fractures. The results of this study indicate that none of the existing models is able to explain all characteristics of shatter cones fully; therefore, a combination of aspects of the different models may currently be the best possible way to explain the formation and origin of shatter cones, and the formation of the related MSJ and their characteristic aspects (e.g., curviplanar shape, melt formation, etc.). The observed variety of shatter cone orientations, surface morphology and striation geometry in the dome concurs broadly with the results of some previous studies. The abundance of striated surfaces along closely-spaced sets of fractures (MSJ) observed in this study can be reconciled with reflection/scattering of a fast propagating wave at heterogeneities in the target rocks, as proposed by recent studies. This would mean that closely-spaced fractures and shatter cones were not formed during shock compression, as widely postulated in the past, but immediately after the passage of the shock wave, by the interference of the scattered elastic wave and the tensional hoop stress that develops behind the shock front. In addition to shatter cones, quartzite units show two other fracture types – a centimetre-spaced rhomboidal to orthogonal type that may be the product of shock-induced deformation and related to the formation of shatter cones, and later joints accomplishing tangential and radial extension. The occurrence of pseudotachylitic breccia within some of these later joints confirms the general impact timing of these features. Pseudotachylitic breccias in the collar rocks occur as up to several centimetre-wide veins with variable orientations to the bedding and as more voluminous pods and networks in zones of structural complexity, such as the hinges of large-scale folds and along large-scale faults, as well as locally, at lithological interfaces. In places, tension gash arrays along thin veins are observed indicating that movement occurred along these planes. Initial cooling calculations for pseudotachylitic breccias of different widths and compositions (metapelite or quartzite) suggest that thick veins (<10 cm) could have stayed molten over the entire duration of crater development (at least 10 minutes), making it possible for shock-induced melts to intrude dilational sites, such as fold hinges and extensional fractures, during the formation and subsequent collapse of the central uplift. Intrusion of such melts may also have lubricated movements along brittle and ductile structures. Thus, the presence of both shock- and friction-generated melts is likely in the collar of the Vredefort Dome. Based on the spatial and geometric relationship between the structures and other deformation features observed in the collar rocks of the Vredefort Dome, it is possible to establish a temporal sequence of deformation events. Shatter cones and related closely-spaced fractures were formed during the contact/compression phase of the cratering process. The formation of at least some shock-induced pseudotachylitic breccia also belongs into this phase. Large-scale folds and faults and friction-generated melts can be related to the initial formation of the central uplift and extensional joints to the subsequent collapse of the central uplift.

Geological and geophysical investigations of the Chicxulub meteorite impact structure have been ongoing since its scientific recognition in 1991 Hildebrand et al. 1991). The structure is of important significance because it is currently the only known impact crater that is linked to a global catastrophe, the Cretaceous/Tertiary boundary that occurred 65 Ma years ago. Major climatic and biological changes occurred at this interval that include the disappearance of 70% of all living species, in particular the dinosaurs. A global iridium anomaly along with the occurrence of shocked quartz grains characterize a thin clay layer this interval that led to the search for a large meteorite impact crater on continental crust. A large “volcanic” igneous province identified by oil exploration boreholes on the NW region of the Yucatán Peninsula was eventually recognized as a vast impactite deposit associated with a 180 km wide crater. Until 2002, only small grab and chip samples had been described from Chicxulub. This lack of sampling and, thus, poor understanding of the cratering conditions at Chicxulub led the International Continental Drilling Program (ICDP) to fund and drill the Yaxcopoil-1 borehole. The Yaxcopoil-1 (Yax-1) borehole was drilled 60 km south-southwest from the center of the Chicxulub meteoritic impact. It intersected 794.63 m of post-impact cover rocks, 100.31 m of impactites, and 616.03 m of Cretaceous target rocks, terminating at a final depth of 1510.97 m. The impactite interval, as well as several selected samples from the Cretaceous target rocks, is the focus of this scientific investigation. In conjunction with this work, the Yax-1 core was studied by numerous international research groups and is, thus, currently one of the best studied continuous diamond drill core from an impact crater. This petrographic and geochemical investigation provides further understanding on the primary and secondary conditions that influenced the formation of the Yax-1 impactites and selected target rocks. Five units have been recognized in the impactite interval. These subdivisions are based on macro- and microscopic observations and are complemented by geochemical characteristics. Unit 1 (795-822 m) comprises subrounded melt rock particles that are poorly sorted, yet show a progressive gradation with height, are self supported, show perlitic devitrification texture, and are generally fine-grained. Unit 2 (823-846 m) and Unit 3 (846-861 m) are relatively similar, as they both consist of a groundmass-supported breccia with melt rock particles that are angular, fluidal, and vesiculated in texture. The groundmass in both units is pervaded by numerous carbonate-veinlets and decreases in volume towards Unit 3 because of compaction. Unit 2 and Unit 1 are both altered to a predominantly green colour by the pervasive conversion of silicate phases from clay minerals. Unit 3 is of a variegated character and is suggested to be the less altered unit bove Unit 4. Unit 4 (861-885 m) comprises a massive yet brecciated microcrystalline impact melt rock. It is primarily of a silicate composition and contains only minor secondary carbonate crystals. All lithic fragments are of silicate compositions. Unit 5 (885-895 m) shows the greatest variation in the proportion of melt rock particles and lithic fragments. The melt rock particles contain numerous microlites that crystallized below the glass-transition temperature. These are suspended in a carbonate groundmass that is either of a primary impact melt origin or of a secondary nature. Units 1 and 5 both contain foraminifera fossils and greater proportions of carbonate clasts than any other units. All unit show shock metamorphic characteristics, i.e., planar deformation features, ballenquartz, and checkerboard feldspar. Geochemical results have been obtained by various analytical techniques in order to constrain cratering and alteration processes at various sampling scales. Main results reveal that samples from units 1 and 2 have been leached of their alkali elements, show negative Ce anomalies on a microscopic scale, and show less major element variation on a bulk sample scale than lower units. The groundmass in units 1 to 3 comprises a microcrystalline calcite and altered alkali element-, Ca- and Si- rich cement. In units 2, 3, and 5 melt rock particles are of a heterogeneous composition. In Unit 1, melt rock particles are highly altered, therefore volatile rich, and are of a more homogeneous composition than those of other units. On a bulk sampling scale, the silicate component for the whole impactite sequence shows remarkable homogeneity. Major and trace element compositions show that this component and Unit 4 are typical of the upper continental crust. The carbonate component is more calcite rich than dolomitic and most likely represents strong secondary alteration. No significant sulfur content was measured compared to published known target rock values. The contents of the siderophile elements, including Ni, Co, Ir, and Cr, do not indicate the presence of a significant extraterrestrial component in the Yax-1 impactites. Cretaceous rocks were also sampled in order to provide compositional constraints with the impactites and observe any shock related metamorphic features. Petrographic observations indicate that the Cretaceous rocks in the Yaxcopoil-1 drill core likely register a multistage deformation history that spans the period from pre- to post-impact. Contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins, no evidence was found from samples located between 1347–1348 m depth for the presence of melt breccia. An emplacement mechanism for the impactite sequence is proposed with regards to cratering. Unit 5 is interpreted as an early ejecta deposit that was emplaced following the passage of the initial ejecta curtain during the excavation stage of cratering. Unit 4 is an allogenic siliceous melt rock body that originated primarily from the fusion of the silicate crystalline basement. The origin of Unit 4 is based on geochemical and petrographic arguments, i.e., no carbonate component to the melt could be detected and only igneous/metamorphic mineral/rock fragments were observed in it. It is suggested Unit 4 was emplaced as an outward flow of fused crystalline basement rocks from the collapsing central uplift or it may have also been deposited from the fallback of a large melt bomb. Brecciation occurred post-deposition as fragments fit together like pieces of a jigsaw puzzle. Units 2 and 3 represent unreworked fallback suevite deposits. Vesiculated melt rock particles are a testimony of the volatile rich nature of the collapsing impact plume. Volatiles are believed to have helped disperse the suevite and inhibited the melt rock particles from undergoing compositional homogenization. Unit 1 represents a reworked fallback deposit that formed from the resurge of seawater into the impact basin. Unit 2 is the altered equivalent of Unit 3 and along with Unit 1 underwent significant post-depositional phyllosilicate alteration from circulating fluids at the top of the suevite pile.

Baddeleyite (monoclinic-ZrO2) is an exceptionally common accessory phase in many of the mafic and ultra-mafic rocks prevalent throughout the Solar System. This study presents the first ground-truthing efforts in the development of this robust mineral into a diagnostic indicator, discrete barometer, and precise U-Pb geochronometer of shock metamorphism by combining electron backscatter diffraction and atom probe tomography to generate unique chemical and structural datasets. Microstructural analysis of variably shocked baddeleyite grains around the Sudbury impact structure (Ontario, Canada) highlights a series of crystallographic structures that can be correlated with discrete variations in formative pressure-temperature conditions. Decompression at high temperatures generates a series of interlocking reversion twinned structures, while quenching forms a quasi-amorphous matrix. These features are comparable to those observed in extra-terrestrial samples, where they can be directly linked with the severity and extent of lead loss and age resetting. This finding facilitates the application of baddeleyite as a shock indicator, barometer (>5 GPa) and chronometer in a wide range of planetary materials. This structural variability is also observable on the nanometre scale. Analysis of the most highly shocked Sudbury baddeleyite using atom probe tomography reveals planar and curvi-planar fractures, trace element enriched subgrain boundaries, and solid-state diffusion clusters. These micrometre and nanometre scale features encourage localised diffusion of lead, with whole-microtip U-Pb analyses yielding complex partially reset ages. The application of atom probe tomography allows these features to be spatially resolved on the nanometre scale, yielding highly accurate ages for protolith crystallization and impact metamorphism within a single grain. These results have significant implications for the isotopic analysis of baddeleyite-bearing planetary materials, where the mechanisms of U-Pb age resetting have until now been poorly understood.

The Chesapeake Bay impact structure is one of the largest and most well preserved impact structures on Earth. It has a unique morphology composed of an inner crater penetrating crystalline basement surrounded by a wider crater in the overlying sediments. In 2004, the U.S. Geological Survey conducted a seismic survey with the goals of constraining crater structure and in support of the drilling of a borehole into the deepest part of the crater. Travel-time and waveform inversion were applied to the data to produce a high-resolution velocity model of the crater. Low-fold reflection processing was also applied. Northeast of the crystalline crater, undeformed, eastward-sloping crystalline basement is ~1.5 km deep. The edge of the inner crater is at ~ 15 km radius and slopes gradually down to a depth of 1.5 - 1.8 km. A central peak of 4-5 km radius rises to a depth of ~0.8 km. Basement velocity in the crystalline crater is much lower than undeformed basement, which suggests ~10% fracturing of the crater floor, and up to 20% fracturing of the central uplift. A basement uplift and lateral change of velocity, interpreted as the edge of the transient crater, occurs at a radius of ~ 11 km. Assuming a 22 km diameter transient crater, scaling laws predict a ~30 km diameter crater and central peak diameter of 8-10 km. This indicates that post-impact collapse processes that created the ~ 30 km diameter crystalline crater were unaffected by the much weaker rheology of the overlying sediments. / Master of Science

This thesis utilizes several geophysical methods to study the Siljan Ring impact structure, focusing on the western part of the structure. This thesis, and the three papers upon which it is based, reports on attempts to delineate the Paleozoic rocks at depth within the annular ring graben and characterize their structure. In addition, the nature of the basement, which underlies these sedimentary rocks is investigated. Papers I and III focus on analysis of the down-hole logging and borehole core data. As well as the acquisition, processing and interpretation of 2D high-resolution reflection seismic data from the Mora area. The borehole log responses were compared with the core lithology from the Mora 001 borehole and information from two other cores (Mora VM 2 and Mora MV 3) in order to interpret the logs. The logs reveal significant changes in the lithology between boreholes, indicating a very high level of structural complexity, which is attributed to impact tectonics. In addition, the log data revealed a high sonic velocity contrast between the Silurian and Ordovician successions and a higher apparent temperature gradient than in the northern part of the structure. The interpretation of the high-resolution 2D seismic data suggest that the Mora area has been significantly affected by the impact. Several potential faults were identified in the area and interpreted to be post depositional and related to the impact. In paper II, a 2D seismic profile from the Orsa area (12 km) located in the northwestern part of the Siljan Ring was re-processed. To compliment this seismic line, first break traveltime tomography results, vintage seismic OPAB profiles, new and pre-existing gravity data, aeromagnetic data and the bedrock geological map were used to present a geological model along the Orsa profile. Reprocessing of the seismic data resulted in improved stacked and migrated sections and better imaging of the top of the crystalline basement than the original processing. Integrated interpretation of the seismic profiles suggests that the area has been significantly affected by faulting and that the depth to the basement varies greatly along the different profiles.

Thesis (M.S.)--Auburn University, 2008. / Abstract. Includes bibliographical references (p. 113-121).

Master of Science / Department of Geology / Abdelmoneam Raef / Matthew W. Totten / This study examines an ovoid drainage feature southwest of Topeka, Kansas, whose discovery sparked a flurry of activity. Geomicrobial and surface gamma ray surveys indicated possible vertical migration of hydrocarbons, and a ground magnetic survey produced anomalies that resemble the profile of a crater. The area was dubbed the Echo Cliff structure and considered analogous to the Ames structure in Oklahoma, an Ordovician impact structure remarkable for significant hydrocarbon recovery. However, four wells drilled in the area were dry and abandoned. The Echo Cliff structure did yield further indications of its origins by the discovery of possible shocked quartz in drill cuttings from the Ordovician Simpson Group. Our study integrated well log analysis, geophysical modeling, and petrographic analysis to verify or refute the proposed identity of the Echo Cliff structure. Well logs from the area were used to create a structural and stratigraphic cross-section in Petrel® 2016. A gravity survey was conducted in the study area and combined with an aeromagnetic survey, donated by Applied Geophyics, Inc., to use as the basis for geophysical modeling within GM-SYS®. Finally, drill cuttings from the Simpson Group of two wells in the study area were mounted for thin sectioning. These thin sections were examined for planar deformation features, which are indicative of an impact event. The structural and stratigraphic cross sections indicated minimal variation in the subsurface, which is uncharacteristic of an impact event. The GM-SYS® geophysical models seem to indicate that variations in the topography of the Precambrian basement and faulting from the Bolivar-Mansfield Tectonic Zone are responsible for the geophysical anomalies and possibly the current drainage pattern of the study area. Finally, no planar deformation features were observed in any of the examined thin sections. Therefore, there is currently no evidence in support of the claim that the Echo Cliff structure is an impact structure.

Orientadores: Carlos Roberto de Souza Filho, Cristiano de Carvalho Lana / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Geociencias / Made available in DSpace on 2018-08-15T00:06:50Z (GMT). No. of bitstreams: 1 Machado_RogerioAmaro_M.pdf: 6890579 bytes, checksum: 4c4acd981fd71f6ac0d9f9db67e88ee3 (MD5) Previous issue date: 2009 / Resumo: Fundidos de impacto ricos em clastos e brechas de impacto pseudotaquilíticas são encontrados nos embasamentos cristalinos de muitas estruturas de impacto ao redor do mundo. São formados sob condições extremas de temperatura e pressão, geradas na colisão de um meteorito e também registram importantes pistas sobre os processos associados aos impactos em hipervelocidade e evolução planetária. Este trabalho forneceu importantes pistas sobre a produção, extração e cristalização das rochas fundidas na estrutura de impacto de Araguainha, no Brasil central. Os fundidos de impacto localizados no soerguimento central da estrutura são caracterizados por uma matriz de composição granítica rica em álcalis, envolvendo minerais e fragmentos de rochas derivados estritamente do granito impactado. Apesar da natureza mista das rochas impactadas neste evento (rochas sedimentares e graníticas) os dados de rocha total para os elementos maiores e menores (incluindo elementos do grupo da platina) indicam que o fundido foi gerado localmente, sem contato com a sequência sedimentar ou o bólido impactante. Similar a brechas pseudotaquilíticas, as rochas fundidas em Araguainha formaram-se isoladas, provavelmente abaixo da zona de fusão por impacto. A análise multispectral dos mapas químicos obtidos por raios-X e de química mineral do embasamento granítico e dos fundidos de impacto e seus clastos indicaram que o fundido originou-se por fusão seletiva do plagioclásio, feldspato potássico e biotita. O plagioclásio e o feldspato potássico fundiram-se discretamente e de forma isolada, produzindo domínios no fundido que combinam estequiometricamente com estes minerais. O fundido derivado da biotita mostrou maior mobilidade e misturou-se mais facilmente com os outros fundidos. Nenhuma evidência de fusão por fricção foi encontrada. Os fundidos em sua fase inicial migraram por uma densa rede de micro fraturas antes de serem incorporados pelos principais bolsões de rocha fundida. Evidências encontradas nesse trabalho indicam que fundidos produzidos localmente, como as brechas pseudotaquilíticas, são produzidos por fusão seletiva de minerais dentro da rocha impactada. Associou-se, desta forma, a fusão dos minerais com as temperaturas de pós-choques, que podem ter aumentado para além do ponto de fusão de feldspatos e biotita. / Abstract: Clast-bearing impact melts and pseudotachylitic breccias are found in crystalline target rocks of many impact structures around the world. They are formed under the extreme pressure-temperature conditions that accompany the meteorite collision, and record important clues to the process associated with hypervelocity impacts and planetary evolution. Here we provide important insights into the generation, extraction and crystallization of clast-laden melt rocks from the Araguainha impact structure in central Brazil. The melt bodies in the central uplift are characterized by an alkalai-rich granitic matrix embedding mineral and rock fragments derived strictly from granite target rocks. Despite the mixed nature of the target rocks - sedimentary and granitic rocks -bulk-rock major and trace element (including platinum group elements) data indicate that the granitic melts were generated locally, without direct contact with the sedimentary sequence or the projectile. Similarly to pseudotachylitic breccias, the Araguainha melt rocks formed in isolation, probably below the zone of impact melting. Multispectral analysis of X-ray maps and mineral chemistry from clasts and granitic targets indicate that the melt originated by selective melting of plagioclase, K-feldspar and biotite. Plagioclase and K-feldspar melted discretely and in isolation, producing domains in the melt which closely match the stoichiometry of these minerals. The biotite derived melt component appears to have been more mobile and mixed more readily with the other melts. No textural evidence for friction melting has been found. The initial melt phases have migrated through a dense network of microfractures before being assembled into main melt bodies. Evidence provided here indicates that locally produced melt bodies such as pseudotachylitic breccias, are produced by selective melting of minerals within the target rocks. We relate the melting of the minerals to the post-shock temperatures that may have risen beyond the melting point of biotite and feldspars. / Mestrado / Geologia e Recursos Naturais / Mestre em Geociências

Orientadores: Alvaro Penteado Crósta, Eder Cassola Molina / Acompanhado de 2 mapas (folhas soltas dobradas), acondicionados em bolso / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Geociências / Made available in DSpace on 2018-08-21T00:38:34Z (GMT). No. of bitstreams: 1 Vasconcelos_MarcosAlbertoRodrigues_D.pdf: 73486502 bytes, checksum: 543cc4aa42bc44b175427b7a391707cc (MD5) Previous issue date: 2012 / Resumo: A Bacia sedimentar do Parnaíba abriga em seus domínios diversas estruturas circulares, das quais algumas têm sua origem atribuída a impactos meteoríticos sem que, no entanto, haja evidências comprobatórias. Esta tese aborda um estudo das assinaturas geofísicas das estruturas de São Miguel do Tapuio (SMT), Santa Marta (SM), Riachão (Ria) e Serra da Cangalha (SdC) utilizando dados aerogeofísicos de baixa resolução com o intuito de compará-las com outras crateras meteoríticas similares. Aborda também estudos geofísicos, utilizando dados aéreos de alta resolução e terrestres das estruturas de SdC e Ria, além de estudos geológicos de detalhe da estrutura de SdC, duas feições circulares com diâmetros de ~13 km (SdC) e ~4 km (Ria) localizadas nos estados de Tocantins e Maranhão, respectivamente. Os resultados fornecidos pelos métodos geofísicos mostram que a estrutura de SMT exibe alto magnético e assinatura gravimétrica variável, características estas não compatíveis com o padrão de estruturas de impacto similares; SM apresenta anomalia magnética negativa e gravimétrica positiva, características compatíveis com origem por impacto; Ria exibe altos magnético e gravimétrico na região central, e anomalia gamaespectrométrica circular com abundância de K, Th e U em sua porção central. Apesar da assinatura gravimétrica de Ria não ser claramente diagnóstica, a assinatura magnética é semelhante às de crateras de impacto similares. A estrutura de SdC apresenta anomalia gravimétrica da ordem de 1 mGal e dados aeromagnéticos mostram que o embasamento está a uma profundidade média de ~1,9 km, diminuindo para cerca de 500 a 1000 metros em sua porção central. Dados gamaespectrométricos revelam altos valores de K, Th e U no núcleo soerguido e alto Th e U na região externa próximo ao limite da cratera. Do ponto de vista morfo-estrutural SdC é constituída por núcleo soerguido que tem ~5,8 km de diâmetro e que possui em seu interior proeminente colar com ~3 km de diâmetro. Dados estruturais de campo, associados com análises de imagens de sensoriamento remoto, revelam WNW-ESE como a principal direção de deformação além de camadas invertidas preferencialmente concentradas no setor noroeste do colar, o que sugere um impacto oblíquo de sul para norte. Feições de deformação por choque incluem shatter cones, feather features (FF), planar fractures (PF) e planar deformation features (PDF) formadas ao longo da direção (0001), indicando pressão de choque <10 GPa. Esse conjunto de feições foi encontrado principalmente nas brechas polimíticas e shatter cones da depressão central e comprova a origem por impacto meteorítico da estrutura de SdC. Finalmente, resultados obtidos pela modelagem numérica da formação de SdC indicam que ela foi formada por um bólido com diâmetro de 1,4 km, viajando a 12 km/s, resultando na formação de uma cratera com ~15 km de diâmetro que liberou energia da ordem de 2,74x1020 J, considerando um nível atual de erosão de aproximadamente 500 metros / Abstract: The sedimentary Parnaíba basin encompasses in its domains several circular structures, some of which have their origin attributed to meteoritic impacts without, however, diagnostic evidence. This thesis presents a study of the geophysical signatures of the São Miguel do Tapuio (SMT), Santa Marta (SM), Riachão (Ria) and Serra da Cangalha (SdC) structures using low-resolution geophysical data, in order to compare them with other similar meteorite impact structures. It also employs high-resolution aerogeophysical and ground geophysical data for the SdC and Ria structures, and detailed geological data from SdC. These are two circular features ~13 km (SdC) and ~4 km (Ria) in diameter, and which are located in the state of Tocantins and Maranhão, respectively. The results provided by geophysical methods show that SMT exhibits a high gravity and variable magnetic signature, which are not compatible with the patterns of similar sized impact structures; SM has a negative magnetic anomaly and positive gravity anomaly, which is compatible with other impact structures; Ria exhibits high magnetic and gravity in the central region, and a circular gamma-ray anomaly with high levels of K, Th and U in the center. Despite the fact that the gravity signature of Ria is not clearly comparable with that of other impact craters, the magnetic signature is similar to that of other craters. SdC shows a gravity anomaly of ~1 mGal and there is magnetic evidence that the basement rocks sit at ~1.9 km depth, decreasing to about 500 to 1000 meters depth in the central region of the structure. Gamma-ray data show high values of K, Th and U over the central uplift and high Th and U in the region near the outermost limit of the crater. Morpho-structural analysis indicates that SdC contains a central uplift ~ 5.8 km in diameter with a prominent collar ~ 3 km wide in its inner zone. Structural data associated with remote sensing images show WNW-ESE as the main direction of deformation as well as overturned layers preferentially concentrated in the northwestern sector of the collar, which suggest an oblique impact from south to north. Shock deformation features include shatter cones, feather features (FF), planar fractures (PF), and planar deformation features (PDF) formed along (0001), indicating the shock pressure experienced by the rocks of the central uplift to have been <10 GPa. These features have been found in polymict breccias and shatter coned samples from the central depression and provide definite evidence that SdC was formed by meteorite impact. Finally, numerical modeling indicates that a meteorite some 1.4 km in diameter and impacting at a velocity of 12 km/s could have formed the originally ~15 km diameter crater, releasing energy of 2.74x1020 J, assuming a current erosion level of approximately 500 meters / Doutorado / Geologia e Recursos Naturais / Doutor em Ciências

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