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

Guillaume Baby (1) , François Guillocheau (1) , Julien Morin (1) , Massimo Dall’asta (2)

1 GR - Géosciences Rennes
2 TOTAL S.A.

The 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 the high elevated southern African topography is the expression of a surface upwelling caused by flows in the underlying mantle. However, documenting the surface uplift due to mantle flows is a major challenge in geology. Here we link onshore landforms (planation surfaces, incised valleys) of the SAP to offshore sediment accumulation along the Atlantic margin (from 18°S to 38°S), using numerous seismic reflexion profiles, well data and outcrops. We attempt to relate source and sink analysis in order to solve some first order issues relative to the timing of the exhumation and the growth of the Southern African Plateau. Offshore, we calculate the solid sediment volumes history of the margin for the last 131 Ma (i.e. late Hauterivian – today) based on Guillocheau et al. [1] approach. Volumes and accumulation rates were higher during the Upper Cretaceous (335 ×103 km3 at 51.5 ×103 km3/Ma from 100 to 93.5 Ma, 790 ×103 km3 at 63 ×103 km3/Ma from 93.5 to 81 Ma, and 395 ×103 km3 at 26.3 ×103 km3/Ma from 81 to 66 Ma). Volumes and accumulation rates were lower for the Lower Cretaceous (73 ×103 km3 at 4 ×103 km3/Ma from 131 to 113 Ma, and 16 ×103 km3 at 12.2 ×103 km3/Ma from 113 to 100 Ma) and the Cenozoic (67 ×103 km3 at 1.6 ×103 km3/Ma from 66 to 30 Ma, 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 relationships with volcanism and sediments. The successive growth of these landforms are related to uplifts (Guillocheau et al. [2]). • > 80 Ma: two generations of planation surfaces (at least). The oldest one forms the highest reliefs of the study area, which are preserved as remnant plateaus (Etchplains). Its age is not known. The younger, form a large - scale pediplain that could be related to the increase of the sedimentary flux occurring during the Upper Cretaceous (i.e. 100 to 81 Ma), driven by a regional uplift. • 80 - 75 to 70 - 65 Ma: large scale pediplain called the Bushmanland Surface. It can be related to a strike flexure of the margin (i.e. seaward tilting) recognised offshore from 23°S to 36°S. Most of the present-day relief was probably created by that time. This is supported by the decrease of the sedimentary flux which suggests a reorganisation of the interior drainage pattern. • 35 - 25 to 15 - 12 Ma: degradation by river incision of the Bushmanland Surface in response 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 phases uplift history: (1) a widespread tilting of the margin during late Cretaceous, (2) continental scale deformations during the Oligocene. Most of the relief is inherited from the Upper Cretaceous References: [1] Guillocheau et al. (2012) Basin Res., 24(1), 3-30 [2] Guillocheau et al. (2016) accepted Gondwana Res.

Domaines: Tectonique

Guillaume Baby, François Guillocheau, Julien Morin, Massimo Dall’asta. MASS TRANSFER BETWEEN THE SOUTH AFRICAN PLATEAU AND THE ADJACENT ATLANTIC MARGIN (NAMIBIA - SOUTH AFRICA) SINCE THE GONDWANA BREAK-UP. Source to Sink: a long term perspective of sediment budgets and sources characterization, Géosciences-Rennes, Nov 2016, Rennes, France. pp.33-34. ⟨insu-01406414⟩