MASS TRANSFER BETWEEN THE SOUTH AFRICAN PLATEAU AND THE ADJACENT ATLANTIC MARGIN (NAMIBIA - SOUTH AFRICA) SINCE THE GONDWANA BREAK-UP

International audienceThe South African Plateau (SAP) forms a large - scale topographic anomaly (×1000 km)which rises from sea level to up to 3000 m elevation. Tomographic models suggest that thehigh elevated southern African topography is the expression of a surface upwelling caused byflows in the underlying mantle. However, documenting the surface uplift due to mantle flowsis a major challenge in geology. Here we link onshore landforms (planation surfaces, incisedvalleys) of the SAP to offshore sediment accumulation along the Atlantic margin (from 18°S to38°S), using numerous seismic reflexion profiles, well data and outcrops. We attempt to relatesource and sink analysis in order to solve some first order issues relative to the timing of theexhumation and the growth of the Southern African Plateau.Offshore, we calculate the solid sediment volumes history of the margin for the last 131Ma (i.e. late Hauterivian – today) based on Guillocheau et al. [1] approach. Volumes and accumulationrates were higher during the Upper Cretaceous (335 ×103 km3 at 51.5 ×103 km3/Mafrom 100 to 93.5 Ma, 790 ×103 km3 at 63 ×103 km3/Ma from 93.5 to 81 Ma, and 395 ×103 km3 at26.3 ×103 km3/Ma from 81 to 66 Ma). Volumes and accumulation rates were lower for the LowerCretaceous (73 ×103 km3 at 4 ×103 km3/Ma from 131 to 113 Ma, and 16 ×103 km3 at 12.2 ×103km3/Ma from 113 to 100 Ma) and the Cenozoic (67 ×103 km3 at 1.6 ×103 km3/Ma from 66 to 30Ma, and 92 ×103 km3 at 6.5 ×103 km3/Ma from 30 to 11 Ma, and 35 ×103 km3 at 32 ×103 km3/Ma).Onshore, four generations of landforms were recognised and dated by geometrical relationshipswith volcanism and sediments. The successive growth of these landforms are relatedto uplifts (Guillocheau et al. [2]).• > 80 Ma: two generations of planation surfaces (at least). The oldest one forms thehighest reliefs of the study area, which are preserved as remnant plateaus (Etchplains). Its ageis not known. The younger, form a large - scale pediplain that could be related to the increaseof the sedimentary flux occurring during the Upper Cretaceous (i.e. 100 to 81 Ma), driven by aregional uplift.• 80 - 75 to 70 - 65 Ma: large scale pediplain called the Bushmanland Surface. It canbe related to a strike flexure of the margin (i.e. seaward tilting) recognised offshore from 23°Sto 36°S. Most of the present-day relief was probably created by that time. This is supported bythe decrease of the sedimentary flux which suggests a reorganisation of the interior drainagepattern.• 35 - 25 to 15 - 12 Ma: degradation by river incision of the Bushmanland Surface inresponse to a continental scale uplift (?) and/or change of the climate conditions.These results suggest that the Southern African plateau (SAP) results from a two phasesuplift history: (1) a widespread tilting of the margin during late Cretaceous, (2) continental scaledeformations during the Oligocene. Most of the relief is inherited from the Upper CretaceousReferences:[1] Guillocheau et al. (2012) Basin Res., 24(1), 3-30[2] Guillocheau et al. (2016) accepted Gondwana Res