Résumé
The Brazilian continental margin contains natural gas hydrate (NGH) provinces known from bottom simulating reflections (BSRs). In the Amazon deep-sea fan, a BSR is recognised on the upper slope (700-2250 m depths), within a thrust-fold belt linked to gravitational collapse of the up to 10 km thick depocenter above detachment surfaces. Recurrent destabilisation of NGH in this area has been argued to account for giant landslides that extend 100s of kms seaward across the mid- to lower fan. Recent mapping using a regional 2D/3D seismic grid shows the BSR to form elongate patches aligned with the crests of thrust-fold anticlines. In places the BSR patches rise towards seafloor beneath mounds and depressions inferred to record fluid venting. Multibeam data (2016) across part of the thrust-fold belt revealed dozens of water column gas flares rising from seafloor mounds. NGHs cored at two such locations contained methane of mainly microbial origin. These findings are consistent with a structurally- controlled supply of warm, gas-rich fluids from the fan within the thrust-fold belt.The NGH-fluid-sediment system in the upper Amazon fan was investigated during the AMARYLLIS-AMAGAS I campaign in May-June 2023 using inter-disciplinary methods (water column/seafloor/subbottom acoustic imagery, cores for sedimentological, geochemical and geotechnical purposes, heat flow data). Initial results document a dynamic setting in which hemipelagic deposition is punctuated by episodes of mass failure, and by the extrusion of sediment and the venting of gas through mud volcanoes (MVs) rooted in thrust-fold anticlines. Mvs are observed at seafloor as relatively small mounds and/or depressions (10s m in relief, 10s-100s m wide), each of which yielded cores containing distinctive dark stratified deposits intrerpreted as MV outflows. Some MVs are emitting water column gas flares; cores containing platy and massive NGH were obtained at 3 sites. Analyses of the recovered sediments are underway to constrain the nature and timing of extrusion and failure; complementary analyses of geochemistry (of NGH, mudgas and pore fluids) and heat flow will provide information on rates of flux and the depths from which fluids are rising. The results will provide input to modelling of NGH dynamics and slope stability in the context of the ANR project MEGA (2023-2026).