EFFECT OF MANTLE CONVECTION ON THE TOTAL TECTONIC SUBSIDENCE OF SOUTH ATLANTIC MARGINS

The effect of mantle convection on surface topography, called dynamic topography, has been the subject of considerable interest over the past few years. The South Atlantic is a key area to study this phenomenon because of contrasted mantle processes beneath South America and Africa. Indeed, South America has been drifting west over subducting oceanic lithosphere for the last ~ 130 Ma whereas Africa, usually considered fixed in global reconstructions, overlies a mantle superswell.

We developed a workflow to investigate the effect of mantle dynamics on deforming continents. This workflow consists in a) a global plate reconstruction that accounts for continental deformation deduced from published geological and geophysical data; b) imposing the kinematics of such reconstructions in global mantle convection models in which compositionally distinct crust and continental lithosphere are embedded within the thermal lithosphere. For the South Atlantic, we developed a kinematic plate reconstruction that accounts for intraplate deformation in both Africa and South America, lithospheric stretching at passive margins, and mountain building along the convergent margin of South America. This allows us to investigate the surface expression of deep mantle processes in passive margins systems. In particular, the contribution of mantle flow and of lithospheric stretching can be individually quantified in the case of pure shear of the lithosphere. Our models are refined through comparison to published geological and geophysical data, which allows us to estimate the contribution of mantle dynamics to the observed asymmetry in total tectonic subsidence of the South Atlantic margins.