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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ABSTRACT. Analysis of 2D multi-channel seismic data provided evidences that sediment slides are recurrent elements in the sedimentary succession of the Foz do Amazonas Basin. In the Amazon Fan, a linked extensional-contractional system glides along weak levels, forming gravitational fold-and-thrust belts of regional scale. Gravity-related fault scarps and sea-floor uplifts (up to 500 m high) induced surficial sediment slides along thrust-faults (average thickness between 200-300 m), resulting in structurally-driven mass movement deposits, as large as 10 4 km 2 . We document, as well, the occurrence of two thick large-scale megaslides along the steep margin to the NW and to the SE of the fan. Megaslides can remobilize siliciclastic series downslope as allochthonous masses over areas as large as thousands of km 2 :the Pará-Maranhão Megaslide (up to 1000 m thick) extends downslope to the Pará-Maranhão Basin over an area of about 90,000 km 2 . To the Northwest of the fan, the Amapá Megaslide Complex consists of a series of recurrent megaslides (each one as thick as 300-700 m), forming a thick stratigraphic succession related to mass transport processes, across an area of about 80,000 km 2 . The instability that triggers megaslides is most likely created by pore overpressure as indicated by the seismic attributes of their respective base of slides, characterized by seismic signals of negative polarity.

ABSTRACT. The young Messinian salt basin offshore the Gulf of Lions comprises a shallow décollement layer (maximum of 3.6 km deep) that allows the seismic imaging of the subsalt relief and the correlation between the shape of subsalt relief and gravity-driven structures. The subsalt relief reveals a variable morphology below the salt layer, characterized by both convex and concave shapes, indicating the occurrence of radial gravitational gliding at the scale of the entire Gulf of Lions.Radial gravitational gliding is equally illustrated by the distribution of the Messinian salt layer. The salt isopach map shows overthickened salt mass overlying subsalt relief of concave shape, pointing to a pattern of convergent gliding, and areas of salt thinning (over convex shape of subsalt relief) indicating divergent gliding. Radial gravitational gliding is also reflected by salt-related structures. Families of normal transverse faults striking parallel to the regional dip direction attest the control exerted by components of strike-parallel extension over areas of convex shape of subsalt relief, whereas widespread buckle folds overlying concave shape of subsalt relief indicate the occurrence of a convergent pattern of salt migration and associated translational gliding. The Gulf of Lions provides then an interesting geological setting to focus on gravity tectonics as well as a model to correlate mapped salt-related structures with those predicted by analogue models.Keywords: salt tectonics, radial gravitational gliding, passive margin evolution, basin analysis, marine seismic reflection, Gulf of Lions-France. RESUMO.A bacia evaporítica Messiniana do Golfo de Lion constitui um nível basal móvel bastante raso (∼3.6 km de profundidade), o que facilita o imageamento sísmico da superfície de décollement e a correlação entre tipos de estruturas gravitacionais e a forma do relevo subsalífero. O relevo subsal revela morfologias convexas e côncavas, sugerindo direções radiais regionais de fluxo gravitacional. O deslizamento radialé ainda indicado pelas variações de espessura da camada de sal. Enquanto zonas de superespessamento do sal recobrem regiões côncavas do relevo subsalífero, e indicam padrão de deslizamento convergente; zonas de afinamento da camada de sal recobrem porções convexas da superfície de décollement , indicando um padrão de deslizamento divergente. O padrão de deslizamento radial reflete-se ainda no arcabouço estrutural pela presença de falhas normais transversais e dobramentos nucleados pelo sal. Falhas paralelasà direção de mergulho regional da bacia afetam a cobertura sedimentar sobrejacente aáreas côncavas do relevo subsalífero, atestando a intervenção de componentes de extensão paralela ao strike da margem.Além disso, uma grande concentração de estruturas compressionais afeta a cobertura sedimentar sobrejacente a porções côncavas do relevo subsalífero, atestando um padrão convergente de deslizamento translacional. O Golfo de Lion oferece, deste modo, um interessante cenário para estudos de tectônica gravitac...

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