Thomas Theunissen

Basin evolution of the Central South Atlantic wide rifted margins, insights from 2-D geodynamic modelling

Thomas Theunissen

The thick late syn- to early post-rift shallow water evaporites in the most distal part of wide rifted margin is paradoxical with the deep depression at crustal breakup time predicted by isostatically compensated lithospheric thinning models.
Elevation of the distal margin and water depth during deposition of the late syn-rift evaporites and early post-rift carbonates in the central South Atlantic are not well constrained and remain to be quantified.
We use forward 2-D thermo-mechanical modelling 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.
Model results show that (1) counter-flow of depleted lower lithospheric mantle during rifting explains the magma-poor nature of these margins and (2) weak crust and syn-rift sediment control the crustal necking width and subsidence history of the distal margin.
Thin syn-rift sediments (~2 km) result in a deep air-loaded (-2000 +/- 200 m below global sea level (bsl)) distal margin while thick syn-rift sediments (~ 6 km) result in a shallower (-500 +/- 150 m bsl) elevation.
Integration of our modelling results with quantified geophysical and geological observations suggests that (1) base level was down to -600 m bsl during distal margin formation in the Aptian before sag and evaporite deposition, (2) base level was about -300/-400 m bsl at the end of evaporite deposition, and (3) scenarios with a fixed shallow base level (-400 m bsl) and with an increasing base level from an initially deep position (-1600 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.