MODELING THE EFFECT OF MANTLE DYNAMICS ON THE TOTAL TECTONIC SUBSIDENCE OF RIFTED PASSIVE MARGINS

The effect of mantle convection on surface topography, called dynamic topography, has been the subject of considerable interest over the past few years. Here we focus on the contribution of mantle dynamics to the total tectonic subsidence at passive margins. We use an established workflow that consists of imposing kinematics from tectonic reconstructions in global mantle convection models. However, we now use tectonic reconstructions that account for continental deformation, notably at passive margins (see for example the abstract by Flament et al., session T2D, this conference). To model lithospheric extension, we embed compositionally distinct continental crust and lithosphere within the thermal lithosphere. Two methods are commonly used to solve the compositional field in convection models (Tackley and King, G³, 2003). In the “absolute” method, the composition is proportional to the local number of tracers. The strength of this method is to minimize the number of tracers and memory. However, the method results in statistical noise that prevents the quantification of surface dynamic topography. In the “ratio” method, all the elements of the models contain a given number of “regular” or “distinct” tracers and the composition is proportional to the number of “distinct” tracers. While this method minimizes statistical noise, it requires large amounts of memory for high-resolution global models. We introduce a “hybrid” method in which the continental crust consists of positively buoyant tracers underlain by a layer of “regular” tracers. The composition is proportional to the density of “distinct” tracers where tracers are defined, and “regular” elsewhere. This method minimizes both statistical noise and computational cost. This allows us to model the evolution of tectonic subsidence at passive margins in large-scale, high-resolution models. We investigate the respective contribution of mantle flow and lithospheric stretching on the total tectonic subsidence of the South Atlantic passive margins.