Long-term stratigraphic evolution of Atlantic-type passive margins: A numerical approach of interactions between surface processes, flexural isostasy and 3D thermal subsidence
Résumé
The thermal and flexural evolution of passive margins is impacted by the (un)loading effects of erosion/ sedimentation processes, which, in turn, affect their relief and sediment accumulation. These complex couplings are recorded by the stratigraphic trend of the associated sedimentary basins, which is controlled by the balance between sediment accumulation, subsidence and eustasy. Our objective is to constrain the relative contribution of the factors controlling the mechanical response of the lithosphere and the efficiency of the surface processes on parameters such as the denudation/accumulation and uplift/subsidence history and long-termstratigraphic trends. The novel aspect of our approach is to integrate the evolution of both domains in erosion and in sedimentation, using state of the art modeling of the flexure of the lithosphere including the surface processes (erosion/sedimentation) and the thermal evolution, as well as concepts in sequence stratigraphy. We investigated numerically the post-rift evolution of passive margins, testing the influence of the lithosphere's initial geometries, thermal states and stretching profiles as well as the efficiency of the surface processes. In all simulations, the initial flexural rift-shoulder is eroded away within 10 to 20 Myr. In the following stages, the sedimentary supply and the evolution of the margin are mostly controlled by the flexural response to the thermal relaxation of the isotherms by cooling and the (un)loading mass transfer at the surface. Both the sediment accumulation (controlled by relief relaxation) and the subsidence rates decrease exponentially with time. The evolution of their relative values forms a regressive/transgressive sequence. We show that variations in the efficiency of the surface processes may impact the uplift/subsidence histories and the long-term stratigraphic trend within the same range of magnitude as lithospheric parameters such as pre-rift crust thickness or depth dependency of stretching. The initial crustal thickness is then determinant in the denudation/accumulation history whereas the margin width will impact the uplift/subsidence history the second most. Depth of necking and effective elastic thickness are critical mostly during the initial phase of the margin history (syn- and immediate post-rift) whereas bulk sediment density and lithosphere thickness become critical in the late post-rift.