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Abstract(s)
A subducção é um processo chave na teoria da tectónica de placas. Todavia, apesar de todos os esforços realizados nas últimas décadas no estudo deste processo, o início da subducção ainda não é totalmente compreendido. Actualmente, não existem claras evidências da passagem de margens passivas para margens activas, muito embora as margens passivas representem os locais mais prováveis para a ocorrência de subducção. É actualmente aceite que a litosfera oceânica apresenta um valor negativo de impulsão que favorece o início da subducção, contudo o balanço entre as forças de atrito e elásticas (como forças resistivas) e as forças de abertura oceânica e gravitacionais (como forçadoras) previne o início de subducção. Desta forma, outros parâmetros devem ser tidos em conta para explicar o início da subducção, tais como a espessura da crosta continental e da litosfera continental e ainda os contrastes laterais de densidade entre as litosferas oceânica e continental. Os estudos mais recentes tentam perceber o problema através de modelos analíticos, analógicos e numéricos, todavia, todos os estudos consideram as zonas de subducção como um problema 2D, embora estas zonas tenham formas 3D na natureza. Tendo em conta as formas mais complexas das margens passivas tentamos neste trabalho, através de modelação numérica 3D, investigar os efeitos de diferentes geometrias da fronteira entre o oceano e o continente no início da subducção e testar se existem assim geometrias mais favoráveis ao início da subducção. Os resultados dos nossos testes indicam que a geometria sem variação entre as placas litosféricas é mais provável para o início da subducção, visto que nos testes com geometrias da fronteira oceano-continente a variar na terceira dimensão aumentarem o tempo para a ocorrência de subducção ou até impossibilitarem o início de subducção.
Subduction is one of the key processes in the plate tectonics theory. However, despite all efforts in the last decades studying this problem, subduction initiation at passive margins is not yet fully understood. Present-day transition from passive to active margin is not obvious, although passive margins look like the most probable places for subduction initiation to occur. Despite the favourable negative buoyancy of oceanic lithosphere, the force balance between elastic and frictional forces (as resisting forces) and ridge push and gravitational forces (as driving forces) seems to inhibit the initiation of subduction. Therefore, other parameters must be taken into account in subduction initiation, like thickness of continental crust and lithosphere, and lateral density contrasts between continental and oceanic lithospheres. The recent studies that tried to solve this enigma vary from analytical, analogue and numerical approaches; however all of them treat the subduction zone as a 2D problem, despite these zones have a 3D shape. Given the complex shape of most passive margins, we use numerical modelling in 3D to investigate the effects of different geometries of continental-oceanic boundaries on subduction initiation, and to test if one of these geometries is more prone to initiate subduction. Our results show that the most prone geometry to initiate subduction at a passive margin is the usual third dimension invariant, common of the 2D models (without any angle in the the oceanic-continental boundary respecting to third dimension). More, our results indicate that with the increasing angle in the oceanic-continental lithospheres (e.g. 20, 40 or 60 degrees) the subduction tend to be retard, and for the highest angels the subduction can be even prevented.
Subduction is one of the key processes in the plate tectonics theory. However, despite all efforts in the last decades studying this problem, subduction initiation at passive margins is not yet fully understood. Present-day transition from passive to active margin is not obvious, although passive margins look like the most probable places for subduction initiation to occur. Despite the favourable negative buoyancy of oceanic lithosphere, the force balance between elastic and frictional forces (as resisting forces) and ridge push and gravitational forces (as driving forces) seems to inhibit the initiation of subduction. Therefore, other parameters must be taken into account in subduction initiation, like thickness of continental crust and lithosphere, and lateral density contrasts between continental and oceanic lithospheres. The recent studies that tried to solve this enigma vary from analytical, analogue and numerical approaches; however all of them treat the subduction zone as a 2D problem, despite these zones have a 3D shape. Given the complex shape of most passive margins, we use numerical modelling in 3D to investigate the effects of different geometries of continental-oceanic boundaries on subduction initiation, and to test if one of these geometries is more prone to initiate subduction. Our results show that the most prone geometry to initiate subduction at a passive margin is the usual third dimension invariant, common of the 2D models (without any angle in the the oceanic-continental boundary respecting to third dimension). More, our results indicate that with the increasing angle in the oceanic-continental lithospheres (e.g. 20, 40 or 60 degrees) the subduction tend to be retard, and for the highest angels the subduction can be even prevented.
Description
Tese de mestrado em Ciências Geofísicas (Geofísica Interna), apresentada à Universidade de Lisboa, através da Faculdade de Ciências, 2012
Keywords
Subducção Modelos numéricos Geometria fronteira oceano-continente Teses de mestrado - 2012