The Gorringe Bank region is located at the eastern end of the Azores-Gibraltar plate boundary, which plate kinematic studies show to progressively change from extension at the Azores, through pure right lateral strike slip at the Gloria fault to compression at Gibraltar. The region is dominated by high relief (4-5 km), highly deformed, uncompensated, ENE-WSW trending seamounts and intervening abyssal plains with basin sediment thicknesses in excess of 4 km and minimal surface deformation. Gorringe Bank, which was formed by overthrusting of the African plate upon the Eurasian plate at about 10 Ma along the plate boundary, is supported in part by exure of the Eurasian plate, as indicated by pre-loading sediments and basement to the north which are tilted towards Gorringe Bank. Broken plate models show the Eurasian plate to have an elastic thickness of about 35 km which is in agreement with that expected for the crustal age (130-135 Ma) at the time of loading. Coral Patch Ridge was formed by a combination of thrust faulting and whole crustal buckling resulting from the past 20 Ma compression and was partially uplifted before deposition of an olistostrome in the Middle Miocene. Recent compressional deformation is distributed over a wide region, as indicated by the dispersed shallow seismicity and has a trend which rotates from approximately N45oE to N70oE from west to east across the region, near perpendicular to westward verging plate motion vectors. The majority of extensional and strike-slip deformation is explained by a regional strike-slip strain ellipse model, including an antithetic NNE-SSW strike slip fault between Gettysburg and Hirondelle seamounts which marks the boundary between the Eastern and Western Horseshoe Basins. Isostatic models for the Madeira-Tore Rise, which initially formed at the Mid Atlantic Ridge, give an elastic thickness of approximately 15 km indicating that significant material was added to the Rise as it moved away from the Ridge.

A Baía de Sepetiba, localizada entre a Baía de Guanabara e a Baía de Ilha Grande, Estado do Rio de Janeiro, está inserida em um cenário estratégico para o desenvolvimento econômico do Estado. Isto ocorre devido ao aumento da concentração populacional, que está diretamente relacionado com o turismo, com a presença de portos e de áreas industriais. Sendo assim, se faz necessário estudar sua estrutura geológica e dinâmica sedimentar para entender sua evolução ao longo do tempo e para uma utilização mais racional desta área. Utilizando-se da sísmica rasa de alta resolução e da sonografia de varredura lateral juntamente com dados pretéritos de amostragem superficial de sedimentos, o presente trabalho tem como objetivo principal analisar sua geologia holocênica. A investigação, em subsuperfície, da geologia estrutural e sedimentar dessa baía, através da interpretação de 09 perfis sísmicos, baseada na determinação de diferentes tipos de ecotexturas, revelou a presença de diferentes pacotes sedimentares depositados ao longo do Holoceno. Ao todo, foram encontrados 15 tipos de ecotexturas perfazendo 14 camadas sedimentares, que estão relacionados em 4 Grupos de acordo com sua distribuição. Já a investigação em superfície através dos registros sonográficos, baseada nos diferentes graus de reflexão acústica (backscattering) e parametrizada pelos dados de amostragem direta pretérita, identificou 6 padrões sonográficos distintos. Com isso foi confeccionado um novo mapa de distribuição textural dos sedimentos superficiais da Baía de Sepetiba. Com a correlação dos dados de sísmica rasa com os dados sonográficos, foi possível ainda sugerir a provável existência de neotectonismo na área de estudo. / The Sepetiba Bay, located between the Guanabara Bay and the Ilha Grande Bay, Rio de Janeiro State, is part of a strategic scenario for the economic development of the State. This is due to increased population density, which is directly related to tourism, with the presence of ports and industrial areas. Therefore, it is necessary to study its geological structure and sediment dynamics to understand its evolution over time and to a more rational use of this area. By using high resolution seismics and side scan sonar data, this study aims to investigate the holocenic geology. The subsurface investigation of its structural and sedimentary geology, by interpretation of seismic profiles, were based in the determination of the seismic facies, along the Holocene. It was found 15 seismic facies premaking 14 sedimentaries layers that were separated in 4 Groups according to their distribution. The research with the side scan sonar, was based on the different degrees of acoustic backscattering and was parameterized by data from direct sampling. It was identified six distinct sonar patterns . Thus a new textural map of the distribution of surface sediments was made. With the correlation of seismic profiles and side scan sonar data it is possible to map a shallow fault that suggests the existence of a possible neotectonism in the study area.

A investigação da Terra através da gravimetria implica um grande número de informações sobre o seu interior em diferentes escalas de observação. O presente estudo considerou pesquisas anteriormente realizadas na área da Zona de Fratura São Paulo (ZFSP) e Arquipélago São Pedro e São Paulo (ASPSP), no Oceano Atlântico Equatorial, cujos dados geoquímicos e petrológicos apontam a presença de uma anomalia térmica. Nesta área alguns levantamentos geofísicos já foram realizados, mas nada que contribuísse de forma significativa através de dados gravimétricos. O objetivo principal deste trabalho é a caracterização gravimétrica da Zona de Fratura São Paulo através de dados de gravidade, anomalia ar livre, anomalia bouguer, tensor gradiente da gravidade e altura geoidal residual. Como objetivo secundário, procurou-se a expressão desta anomalia térmica no campo geopotencial. Entretanto, os resultados obtidos indicam que o efeito da batimetria sobre este dados deve ser levado em consideração. Uma vez que área de estudo está localizada na porção equatorial da dorsal mesoatlântica e o processo de compensação isostática nesta área está influenciando os valores da gravidade no local. Sendo, portanto, necessária a confluência de outros métodos geofísicos para detectar a expressão desta anomalia térmica. / The investigation of the Earth by gravity implies a lot of information about the interior at different scales of observation. This study considered research previously conducted in the area of St. Paul Fracture Zone (ZFSP) Archipelago and St Peter and St Paul (ASPSP), the Equatorial Atlantic Ocean, whose petrological and geochemical data indicate the presence of a thermal anomaly. In this area some geophysical surveys have been conducted, but nothing that would contribute significantly by gravity data. The main objective of this work is the characterization of gravimetric St. Paul Fracture Zone by gravity data, free air anomaly, Bouguer anomaly, the gravity gradient tensor and residual geoid height. As a secondary objective, have sought the expression of this thermal anomaly in the field geopotential. However, the results indicate that the effect of bathymetry on this data should be taken into consideration. Since the study area is located in the equatorial portion of the mid-ridge atlantic and the process of isostatic compensation in this area is influencing the gravity values at the site. Therefore required the confluence of other geophysical methods to detect the expression of this thermal anomaly.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences and the Dept. of Meteorology; and the Woods Hole Oceanographic Institution), 1972. / Includes bibliographical references (p. 153-159). / Free-air and simple Bouguer anomaly maps of the Venezuelan continental margin (from 60°W to 72°W and from 7°N to 13°N) are presented. The major features of the free-air map are: the large lows associated with the deep sedimentary basins, -200 mgal in the Eastern Venezuela basin and -164 mgal in the Maracaibo basin; the high of greater than 300 mgal over the Venezuelan Andes; and a belt of highs associated with the offshore islands extending from Blanquilla to Curacao and then over the Guajira peninsula, where they terminate. The Bouguer anomaly map shows a large low (-196 mgal) over the Eastern Venezuela basin and relative minimums over the coastal mountains. A minimum associated with the Venezuelan Andes is shifted to the northwest of the topographic axis and lies over the flank of the Andes and part of the Maracaibo basin. Using the gravity data, structural sections were constructed for a series of profiles across the Venezuelan Andes and Caribbean mountains. They show that there is no light.crustal root under the Andes, the relative mass excess is as much as 600 kg/cm2 , and that there is an excess of low density material under the Maracaibo basin. This appears to be caused by a combination of a southeastward dipping shear zone in the lithosphere under the basin-mountain boundary and a component of compressive stress perpendicular to this zone, both of which have resulted in the uplift of the crust under the Andes, and downwarp under the basin. The apparent flexural rigidity of the lithosphere under the Maracaibo basin is 0.6 x 10 newton-m, a normal value for lithosphere deformations of Miocene age. The Caribbean mountains have a light crustal root which has been formed by the sliding of blocks of crustal material from the north over the rocks to the south, and perhaps by the under thrusting of oceanic crust under the continental crust. This under thrusting may have been a result of the formation of a downgoing slab of lithosphere along the Venezuelan continental margin during the late Cretaceous. The downgoing slab may have existed until mid-Eocene time. The gravity minimum over the Eastern Venezuela basin is due to the down warping of lighter crustal material into the higher density mantle. This may be a result of compression from the north along a north-south direction causing down buckling buckling of the lithosphere. The present deformation along the northern boundary appears to be due to differences in relative motion between the North and South American plates. Because the Caribbean mountains are partially isostatically compensated, while the Venezuelan Andes are above isostatic equilibrium, this suggests that the relative motion of the Caribbean plate with respect to the South American plate is eastward. The compressive stress across the boundary in the region of the Venezuelan Andes is probably greater than the compressive stress across the Caribbean mountains. / by Robert Allin Folinsbee. / Ph.D.

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1981. / Microfiche copy available in Archives and Science. / Bibliography: leaves 229-245. / by Peter David Efran Goreau. / Sc.D.

Thesis. 1975. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Bibliography: leaves 198-220. / by Richard H. Burroughs, III. / Ph.D.

Thesis. 1976. Ph.D.--Massachusetts Institute of Technology. Dept. of Earth and Planetary Sciences. / Microfiche copy available in Archives and Science. / Vita. / Includes bibliographies. / by Ken C. Macdonald. / Ph.D.

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1987. / Vita. / Includes bibliographical references. / Gravity derived from Seasat altimetry has provided a means of estimating seafloor topography and its compensation, which in turn can be used to understand the evolution of oceanic lithosphere. In the first study, the correlation between the geoid, deflection of the vertical, and seafloor topography is investigated along a section of the Southwest Indian Ridge. Geoid anomalies computed from a simple thermal model fairly accurately predict the intermediate-wavelength anomalies across the fracture zones. The shorter wavelength anomalies are consistent with those calculated from topography using elastic plate compensation. The combined effect of the thermal offset and seafloor topography produces an anomaly which has a small-amplitude, short-wavelength depression directly over the fracture zone valley. Pronounced lineations in the horizontal geoid gradient do not coincide with the valley but have trends parallel to the fracture zones. In the second study, fracture zones along the Southwest Indian Ridge are identified using altimeter profiles and bathymetry. Finite poles of rotation are determined from the fracture zone locations and magnetic anomaly lineations for anomalies 6 (20 Ma), 13 (37 Ma), and 20 (45 Ma). The new poles are in general agreement with previously published poles and describe a fairly consistent direction of relative motion between Africa and Antarctica for the past 45 Myr. A present-day pole of rotation calculated from transform fault azimuths determined primarily from their geoid anomalies, agrees with published poles based on bathymetric data. In the third study, the rate of change of the geoid with age has been estimated as a function of age from geoid offsets across the Eltanin and Udintsev fracture zones and used to constrain thermal models of lithospheric cooling. Observed trends in the geoid slope versus age plots are similar on both branches of the Eltanin and the east limb of the Udintsev fracture zone. The similarity in trends argues against the effects of isolated thermal or bathymetric anomalies and appears instead to reflect a general feature of the geoid-slope versus average age relationship across fracture zones. Although the thermal plate cooling model is successful in predicting both seafloor depths and heat flow values out to ages of at least 80 m.y. B.P., it cannot explain the observed geoid slope values for these two fracture zones. It is not clear at this point whether this is due to inadequacies in the cooling model or to peculiarities in fracture zone evolution. / by Mavis Lynn Driscoll. / Ph.D.

Investigating the archaeology of submerged landscapes beneath many metres of sea and buried under modern sands requires an understanding of the terrestrial surface as it may have been prior to the inundation. To do this, environmental evidence is required from contextualised in-situ locations and the best material evidence for preservation of archaeology, organic remains, dating proxies, pollen, diatoms, microfossils, coleoptera etc. is peat. This research supports the search for peat in submarine environments by interpreting seismic surveys of the sub-sea floor and analysing reflective signals for distinctive organic responses. By means of sedimental analysis and ground observation, the research sets out to differentiate between organic signals, to allow for the identification and location of shallow peat beds within features of a palaeolandscape. Using these results should provide an opportunity to target such peat beds in an archaeologically focused coring programme. The research also examines ways in which organic responses may be mapped over larger areas in order to integrate the results into a wider scale landscape model identifying potential peatland, marsh, valley fen and lowland areas. Finally, the research introduces an artificial intelligence neural networking technology for the identification of organic interfaces in seismic surveys, examining three different ways in which this could be accomplished using specialist computer tools and software.

Using gravity, magnetic, bathymetric and seismic refraction data, I have constructed a geophysical cross-section of the central part of the northern Gulf of California. The section exhibits a crustal thickness of 18 km and features an anomalous block of high density lower basement (3.15 g/cm³) which probably resulted from rifting processes during the opening of the Gulf. The magnetization of the upper basement ranges from 0.0005 to 0.0030 emu/cm³. Three different layers of sediments are modeled, ranging from unconsolidated (1.85 g/cm³) to compacted (2.50 g/cm³). I present a deconvolution method for automated interpretation of magnetic profiles based on Werner's (1953) simplified thin-dike assumption, leading to the linearization of complex nonlinear magnetic problems. The method is expanded by the fact that the horizontal gradient of the total field caused by the edge of a thick interface body is equivalent to the total field of a thin dike. Statistical decision making and a seven point operator are used to insure good approximations of susceptibility, dip, depth, and horizontal location of the source. After using synthetic models to test the inversion method, I applied it to the Northern Gulf of California using data collected in 1984 by the Continental Margins Study Group at Oregon State University. Fault traces, computed by the deconvolution, are plotted on a map. The faulting pattern obtained is in good agreement with that proposed by other workers using other methods. The depths to the top of the faults range from 4 to 5 km in the eastern part of the Gulf, where they may be interpreted as the top of the structural basement. Deeper estimates are obtained for the western part of the Gulf. / Graduation date: 1990