The Amazon deep-sea fan is part of the offshore Amazon Mouth basin. It comprises a thick prograding siliciclastic prism (~ 9 km thick) related to the Amazon River input since Upper Miocene (~10 Ma). Gravity-driven deep-water fold-and-thrust belts stand as the most remarkable structures along the margin, deforming both the deep-sea fan and the earlier marine sequences (Lower Cretaceous-Middle Miocene). Thrust structures, imaged by 2D multichannel seismic profiles, were driven by gravity in a linked extensional-contractional system gliding on weak overpressured shales, and driven by sedimentary loading and by the bathymetric slope. Extension is characterized by both basinward- and landward-dipping normal faults on the shelf and upper slope. Downdip contraction induced detachment folds and reverse/thrust faults leading to the formation of piggy-back basins. Sliding of the sedimentary section took place along distinct detachment surfaces and, apparently, at different stages of the margins’ evolution. At least three main stratigraphic levels may have acted as detachment surfaces, at either regional or local scale. An ancient fold-and-thrust belt (poorly imaged on seismic profiles) slides on the basal mobile section in the central fan area. The most conspicuous fold-and-thrust belts, running all along the Upper Amazon Fan (down to 3200 m), detach on an intermediate décollement surface of regional extend, although a few faults also sole onto the lowermost detachment level. The geometry and structural complexity of these thrust belts vary along strike, owing to lateral changes in the development of depocenters. Depocenters are significantly more complex to the North (where the major margins’ depocenters are located) and exhibiting evidence of long-lasting deformation from multiple partially-overlapping fronts that resulted in further shortening and its impact on the bathymetry (scarps up to 500 m high). To the South, the system is restricted to pairs of reverse faults causing no major sea-bottom relief (inactivated belt?).
Gravity sliding on the uppermost detachment surface occurs locally, to the extreme North and South of the fan, where the sedimentary cover is thinner.
Gravity structures differ broadly from features previously described, comprising families of proximal normal growth faults with no downdip toe thrusts. We also performed simple analogue experiments of gravity sliding in the presence of two stratigraphic levels weakened by fluid overpressure. Results of these models show that detachment folds evolved into ramp-and-flat thrust faults arranged in front belts. Décollement levels acted coupled or decoupled alternating in time and space, while the active front of the fold/thrust belt initially formed at a distal position, then migrated landwards. Structural differences lay in the degree of thrust-faults stacking, with models having thinner brittle layers being capable of shortening further as one single large fold-and-thrust belt.