Sediment deposited in back-arc basins preserves a record of the extensional, volcanic and tectonic history of the arc-backarc systems. Back-arc sedimentation is of particular interest as seafloor massive sulfide deposits may be preserved in back-arc basin sediments. The study of back-arc sedimentation using acoustic data, such as high-resolution sub-bottom profiling data (Parasound) and seismic reflection data, can be a much more cost effective approach than analysis of sediments recovered from drill cores. In this study, we use these two acoustic datasets to build a facies model of sedimentation in the northeast Lau Basin, an actively opening back-arc basin in the southwest Pacific Ocean. Using 830 km of Parasound and 730 km of seismic lines along 4 transects of the Lau Basin, we constructed one of the most detailed models of sedimentation in a back-arc basin to date. Parasound data show distinct echoes with sub-bottom reflections indicative of a high proportion of hemipelagic sediment, whereas the indistinct echoes with few to no sub-bottom reflections indicate a higher proportion of coarse, bedded, volcaniclastic turbidites. Hyperbolic echoes are associated with regions of rugged or uneven terrain characterized by exposed, rough basement or deposits formed by contour currents, turbidity currents, slumps or slides. These relationships form the basis of an echo-facies legend developed for typical back-arc basin sediments. The echo-facies observed in the Parasound, and confirmed by deeper-penetrating seismic reflection data, provide important insights into the sedimentary processes involved in back-arc sedimentation. We observed mass transport deposits (MTDs) in all of the sub-basins and slope deposits within and on the flanks of active rifts (e.g., the Fonualei Rift and Spreading Centre, FRSC), suggesting a direct correlation between MTDs and zones of active rifting. We observed an overall increase in sediment thickness toward the Tofua Arc which suggests it is the main sediment source, but local variations in sediment thickness suggest significant input from local intrabasinal seamounts. The uppermost echo-facies in over 60% of the sub-basins in the study area is dominated by hemipelagic material, which suggests an abrupt transition in the dominant sediment source from volcaniclastic to hemipelagic at around 0.3 Ma, when a period of volcanic quiescence from the Tofua Arc began. The study shows that a near complete record of basin evolution can be constructed using geophysical and acoustic methods and that this work may help to locate future drill sites where in situ data can be collected.

A técnica geofísica da eletrorresistividade adaptada para ambientes aquáticos, com a tecnologia que hoje temos à disposição, mostra-se bastante promissora e de grande produtividade tanto na coleta dos dados de campo como no reconhecimento de feições de interesse do substrato, possibilitando ainda a liberdade de se adquirir tais informações em perfis subaquáticos contínuos em quaisquer trechos navegáveis de rios e lagos e, eventualmente de mares. Neste contexto, esta dissertação abordou a aplicabilidade desta técnica, com a utilização de modernos equipamentos e acessórios adaptados para trabalhos em lâmina d\'água, onde foram executados e estudados os procedimentos de campo, processamentos e integração dos dados coletados, além da verificação da consistência dos resultados obtidos através da realização de linhas de estudos paralelas e também ortogonais. Os testes de aplicação da técnica foram realizados nas águas do rio São Francisco, mais precisamente em seu trecho que faz divisa dos estados da Bahia e Pernambuco, região nordeste do Brasil, local este que também tem sido objeto de estudos para o Projeto do Aproveitamento Hidrelétrico Riacho Seco, pertencentes à empresa Companhia Hidro Elétrica do São Francisco (CHESF), responsável por diversos estudos e desenvolvimentos desta região, notadamente dos recursos hídricos do baixo São Francisco. Os trabalhos de campo lançaram mão do uso de embarcação para transportar e arrastar, na superfície da água, um cabo multi-eletrodo de imageamento geoelétrico, especialmente desenvolvido para tais fins, bem como todos os equipamentos integrados a saber: equipamento GPS de navegação em ambientes aquáticos, eco-batímetro de dupla freqüência adaptado na embarcação, eletrorresistivímetro multicanal e notebook com software específico para o gerenciamento de todas as operações e dados. Os resultados mostraram que foi possível mapear a fisiografia do leito do rio bem como identificar e delimitar anomalias geofísicas compatíveis a prováveis ocorrências de depósitos de sedimentos assim como feições típicas de contatos litológicos e mesmo de estruturas do tipo alteração/fraturamento no interior dos materiais do subfundo do rio. / The electric tomography geophysical technique adapted for aquatic environment, with the modern technology available nowadays, reveals itself sufficiently promising and of great productivity both in the field data collection and in the recognition of aspects of interest of the substratum. It is possible to make the data acquiring in any navigable stretches of rivers, lakes and some seas. In this context, this paper approached the applicability of this technique, with the use of modern equipment and accessories for works in the water. The field procedures, the data processing and integration of the data collection are discussed considering the acquisition of a great amount of data both parallel and orthogonal to the river. The application tests of the technique were conducted in the river San Francisco, in the region of the border of the states of Bahia and Pernambuco, northeast region of Brazil. This area was selected because it has been subject of studies to install a hydroelectric plant (Riacho Seco), owned by the Companhia Hidro Elétrica do São Francisco (CHESF). To perform the field works a boat was used to carry and drag, in the surface of the water, a multi-electrode cable of electric tomography, especially developed for such purposes, as well as all the integrated equipment namely: GPS especially designed for navigation in aquatic environments, adapted fathometer of double frequency in the boat, multichannel eletrorresistivímeter and notebook with specific software that managed all the operations and data. The results had shown that it was possible to map the fisiography of the riverbed as well as identifying and delimiting geophysical anomalies compatible to the probable occurrences of deposits of sediments as well as typical lithologic contacts and structures like weathering and fractures in the underground materials in the bottom of the river.

A técnica geofísica da eletrorresistividade adaptada para ambientes aquáticos, com a tecnologia que hoje temos à disposição, mostra-se bastante promissora e de grande produtividade tanto na coleta dos dados de campo como no reconhecimento de feições de interesse do substrato, possibilitando ainda a liberdade de se adquirir tais informações em perfis subaquáticos contínuos em quaisquer trechos navegáveis de rios e lagos e, eventualmente de mares. Neste contexto, esta dissertação abordou a aplicabilidade desta técnica, com a utilização de modernos equipamentos e acessórios adaptados para trabalhos em lâmina d\'água, onde foram executados e estudados os procedimentos de campo, processamentos e integração dos dados coletados, além da verificação da consistência dos resultados obtidos através da realização de linhas de estudos paralelas e também ortogonais. Os testes de aplicação da técnica foram realizados nas águas do rio São Francisco, mais precisamente em seu trecho que faz divisa dos estados da Bahia e Pernambuco, região nordeste do Brasil, local este que também tem sido objeto de estudos para o Projeto do Aproveitamento Hidrelétrico Riacho Seco, pertencentes à empresa Companhia Hidro Elétrica do São Francisco (CHESF), responsável por diversos estudos e desenvolvimentos desta região, notadamente dos recursos hídricos do baixo São Francisco. Os trabalhos de campo lançaram mão do uso de embarcação para transportar e arrastar, na superfície da água, um cabo multi-eletrodo de imageamento geoelétrico, especialmente desenvolvido para tais fins, bem como todos os equipamentos integrados a saber: equipamento GPS de navegação em ambientes aquáticos, eco-batímetro de dupla freqüência adaptado na embarcação, eletrorresistivímetro multicanal e notebook com software específico para o gerenciamento de todas as operações e dados. Os resultados mostraram que foi possível mapear a fisiografia do leito do rio bem como identificar e delimitar anomalias geofísicas compatíveis a prováveis ocorrências de depósitos de sedimentos assim como feições típicas de contatos litológicos e mesmo de estruturas do tipo alteração/fraturamento no interior dos materiais do subfundo do rio. / The electric tomography geophysical technique adapted for aquatic environment, with the modern technology available nowadays, reveals itself sufficiently promising and of great productivity both in the field data collection and in the recognition of aspects of interest of the substratum. It is possible to make the data acquiring in any navigable stretches of rivers, lakes and some seas. In this context, this paper approached the applicability of this technique, with the use of modern equipment and accessories for works in the water. The field procedures, the data processing and integration of the data collection are discussed considering the acquisition of a great amount of data both parallel and orthogonal to the river. The application tests of the technique were conducted in the river San Francisco, in the region of the border of the states of Bahia and Pernambuco, northeast region of Brazil. This area was selected because it has been subject of studies to install a hydroelectric plant (Riacho Seco), owned by the Companhia Hidro Elétrica do São Francisco (CHESF). To perform the field works a boat was used to carry and drag, in the surface of the water, a multi-electrode cable of electric tomography, especially developed for such purposes, as well as all the integrated equipment namely: GPS especially designed for navigation in aquatic environments, adapted fathometer of double frequency in the boat, multichannel eletrorresistivímeter and notebook with specific software that managed all the operations and data. The results had shown that it was possible to map the fisiography of the riverbed as well as identifying and delimiting geophysical anomalies compatible to the probable occurrences of deposits of sediments as well as typical lithologic contacts and structures like weathering and fractures in the underground materials in the bottom of the river.

Eastern North America experiences large intracratonic earthquakes that are not well understood but pose a risk to urban centers and other infrastructure. Compilation of regional earthquake epicentres for south-central Ontario and western Quebec demonstrate a close association with sutures and failed rifts (the St. Lawrence Rift) recording the formation and breakup respectively of successive supercontinents Rodinia and Pangea. Where seismic potential could be underestimated through lack of historical seismicity or where little is known about active faults, lake deposits can provide a valuable record of past seismic shaking events in the form of sediment deformation structures (i.e. ‘seismites’). In central Canada, the lacustrine seismographic record began approximately 10,000 years before present with the retreat of the Laurentide Ice Sheet, older records having been removed by glacial erosion. Most bedrock lake basins are structurally-controlled and underlain by the same Precambrian basement structures (shear zones, terrane boundaries and other lineaments) implicated as the source of ongoing mid-plate earthquake activity. High resolution seismo-stratigraphic data presented here supports the model that ongoing mid-plate earthquake activity is a consequence of brittle deformation of the upper crust of the North American plate. Such activity appears to have been greatest during deglaciation but continues today. The detailed geophysical and sedimentary studies, as shown here, have major societal relevance in areas of eastern North America affected by intraplate earthquakes. The recognition and mapping of earthquake related features in lakes for seismic risk analysis is a means of constraining seismic recurrence intervals and more realistically assess seismic risk across the populated area of Ontario and Quebec where events occur on time scales much longer than recorded history. / Dissertation / Doctor of Philosophy (PhD)