The thick late syn-to early post-rift shallow water evaporites in the most distal part of wide rifted margins is paradoxical with the deep depression at crustal breakup time predicted by isostatically compensated lithospheric thinning. Elevation of the distal margin and water depth during deposition of the late syn-rift evaporites in the central South Atlantic are not well constrained and remain to be quantified. We use forward 2-D thermo-mechanical modeling coupled with melt prediction and surface processes to assess the contribution of lithospheric and mantle processes on the distal margin topography and subsidence history during continental rifting. Models show that (a) counter-flow of depleted lower lithospheric mantle during rifting explains the magma-poor nature of these margins and (b) weak crust and syn-rift sediment control the wide crustal necking and subsidence history of the distal margin. Integration of our modeling results with quantified geophysical and geological observations suggests that (a) base level was down to 600 m below present-day global sea level (bsl) during distal margin formation in the Aptian before sag and evaporite deposition, (b) base level was about 300/ 400 m bsl at the end of evaporite deposition, and (c) scenarios with a fixed shallow base level ( 400 m bsl) or with an increasing base level from an initially deep position ( 1,600 m bsl) during evaporite deposition can both fit the observed evaporite distribution. However, erosional features along the base of evaporites suggest a deep initial base level.
Plain Language Summary
The opening of the Atlantic Ocean contributed to the fragmentation of the Pangaea supercontinent 100-200 millions years ago. Water depth in the basins formed during this process is not well constrained while it is fundamental to understand their environmental conditions. For instance, in the central South Atlantic that comprises the west African Gabon, Congo, and Angola margins and their conjugates along the Brazilian margin, a giant evaporite basin deposited during continental breakup in shallow water formed very far from the continent. In this study, we compiled data along these margins and used 2-D numerical models to show that an initially deep basin isolated from global ocean can provide the conditions necessary for this giant evaporite basin to be deposited.
