Gravity tectonics affects the entire marine sedimentary sequence of the Foz do Amazonas basin, including the topmost Amazon Fan. During the various stages of the margin's evolution, the sedimentary loading and bathymetric slope induced the formation of a linked extensional-compressional system in which gravitational fold-and-thrust belts form significant structural features on the continental slope. Seismic analysis carried out in this study shows that sea-floor relief created along these thrust belts can trigger recurrent masstransport deposits in the upper-middle Amazon Fan, suggesting a long-lasting linkage between gravity tectonics and mass-transport processes throughout the sedimentary evolution of the Amazon Fan.

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ABSTRACT. Gravitational deformation affects the entire marine sequences of the Foz do Amazonas basin, including the sedimentary successions of the Amazon Fan.Associated gravity-driven structures, imaged by 2D multichannel seismic profiles, were driven by gravity in a linked extensional-contractional system gliding on weak levels, and driven by sedimentary loading and by the bathymetric slope. Sliding of the sedimentary section took place along distinct décollement surfaces and, apparently, at different stages of the margin's evolution. At least three main stratigraphic levels have acted as décollement surfaces, at either regional or local scale. Gravity-driven deep-water fold-and-thrust belts stand as the most remarkable structures along the margin. Their geometry and structural complexity vary along strike, owing to lateral changes in the development of the Amazon fan's depocenters. These structures are significantly more complex along the Northwestern Compartment, where the major margin's depocenters are located, exhibiting evidence of long-lasting deformation from multiple partially-overlapping fronts. In the South-eastern Compartment, the system is restricted to a pair of active reverse faults causing no major sea-bottom relief (inactivated belt). At local scale, antithetic normal growth faults detach on an upper décollement level and exert structural control on the segmentation of the fan's depocenters.Keywords: gravity tectonics, gravitational fold-and-thrust belts, Amazon Deep-sea Fan, Brazilian equatorial margin. RESUMO.As seqüências marinhas da Bacia Foz do Amazonas são deformadas pela tectônica gravitacional, incluindo aquelas do Leque do Amazonas. As estruturas associadas ao colapso gravitacional, imageadas por sísmica multicanal 2D, foram formadas pelo deslizamento gravitacional da cobertura sedimentar sobre níveis basais móveis, induzido pela sobrecarga sedimentar e pelo gradiente do talude. O deslizamento gravitacional ocorreu ao longo de distintas superfícies de descolamento e, aparentemente, em diferentes estágios evolutivos da margem. Ao menos três níveis estratigráficos principais atuaram como superfícies de descolamento, em escala regional ou local. Cinturões de dobramento e cavalgamento emáguas profundas são as estruturas mais notáveis deste arcabouço; sua geometria e complexidade variam ao longo da margem em repostaà variação lateral do desenvolvimento dos depocentros do Leque do Amazonas. Estas estruturas são mais complexas no Compartimento Noroeste onde os principais depocentros estão presentes, formando múltiplas frentes compressivas devido ao mais intenso encurtamento da cobertura sedimentar. No Compartimento Sudeste, o sistemaé menos desenvolvido e se limita a um cinturão parcialmente inativo, caracterizado apenas por um par de falhas reversas ativas com menor impacto morfológico. Localmente, falhas normais lístricas antitéticas se ancoram sobre a superfície de descolamento mais superior, exercendo um controle estrutural sobre a segmentação dos depocentros do próprio leque.Palavras-chav...

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