SOURCE TO SINK STUDY AT CONTINENT-SCALE (AFRICA): MANTLE DYNAMICS CONTROLS AND IMPLICATIONS FOR THE SEDIMENT ROUTING SYSTEM
Résumé
A source to sink approach was performed at the scale of a continent – African the frame of the
TopoAfrica project, with three main objectives (1) the characterization of the relative importance of
deformation (uplift) and climate (precipitation) (2) the quantification of the deformation, its nature and causes,
(3) the effect of those deformations on the past African topography and on the sediment routing system. We
mainly focused on Western, Central and Austral Africa, characterized by anorogenic relief (plains and
plateaus) record of long (several 100 km) to very long (several 1000 km) wavelength deformations,
respectively of lithospheric and mantle origin.
The sink measurement was based on the seismic stratigraphic analysis of numerous regional seismic
lines (from the upstream part of the margin to the abyssal plain) merge of industrial and academic data,
calibrated in ages and lithologies on reevaluated wells to get the best possible ages. Volumes measured
between successive time-lines, were compacted for a comparison with solid eroded volumes. Uncertainties
were calculated (including ages, time-depth conversion law, porosities...) using the Volume Estimator
software.
The source study was performed using dated stepped planation surfaces (etchplains and pediplains) key
morphological features of Africa mappable at catchments-scale. During Late Paleocene to Middle Eocene
times, Africa experienced a very hot and very humid climate leading to the formation of an African-scale
weathering surface (etchplain) known as the African Surface. This surface today deformed and preserved as
large plains, domes or plateaus, can be used as (1) a marker of the very long wavelength deformations and (2)
a reference level to measure eroded volumes since 40 Ma. Some other younger planation surfaces were also
mapped of (1) Early Oligocene and (2) Late Miocene ages.
(1) Deformation (uplift) is the dominant control of the sediment budget. Climate (precipitation) changes
only enhance or inhibit a deformation-controlled flux.
(2) The sources of clastic sediments are or closed marginal bulges or far field domes due to mantle
dynamics with by-pass (transfer zones) along long-lasting polygenic surfaces located in between.
(3) Africa-scale deformations occurred during Late Cretaceous (Turonian-Coniacian) and around the
Eocene-Oligocene boundary with a break contemporaneous of intense chemical erosion from 75 Ma and
mainly from 65 to 40 Ma. Most of the African relief and topography are younger than 40 Ma. Late Cretaceous
relief are only preserved in the Guinea Rise and the Southern African Plateau.