POST-RIFT VERTICAL MOVEMENTS OF THE SOUTHERN AFRICAN MARGINS-IMPLICATIONS FOR THE SOUTH AFRICAN PLATEAU UPLIFT

Guillaume Baby (1) , François Guillocheau (1) , Cécile Robin (1) , Massimo Dall’asta (2)

1 GR - Géosciences Rennes
2 TOTAL S.A.

The South African (Kalahari) Plateau is the world’s largest non-orogenic plateau. It forms a largescale topographic anomaly (×1000 km) which rises from sea level to > 1000 m. Most mechanisms proposed to explain its elevation gain imply mantle processes. The age of the uplift and the different steps of relief growth are still debated. On one hand, a Late Cretaceous uplift is supported both by thermochronological studies and sedimentary flux quantifications. On the other hand, geomorphological studies suggest a Late Cenozoic uplift scenario. Onshore, on the mapping and chronology of all the macroforms (weathering surfaces and associated alterites, pediments and pediplains, incised rivers, wave-cut platforms) dated by intersection with the few preserved sediments and the volcanics (mainly kimberlites pipes). Offshore, on a more classical dataset of seismic lines and petroleum wells, coupled with biostratigraphic revaluations (characterization and dating of vertical movements of the marginssediment volume measurement). The main result of this study is that the South African Plateau is an old Upper Cretaceous relief (90-70 Ma) reactivated during Oligocene (30-15 Ma) times. Its evolution can be summarized as follows: • 100-70 Ma (Cenomanian to Campanian): low elevation plateau (0-500 m) with older and higher reliefs located along the Indian side, acting as a main divide between the Atlantic and the Indian Oceans. First uplift occurred in the east at ~92 Ma, with a fast flexuration of the Indian margins. This initiates a paroxysm of the erosion (90-80 Ma) with the growth of a large delta along the Atlantic margin (Orange delta). Deformation migrated progressively westward and resulted on the growth of the Atlantic marginal bulge between 81 and 70 Ma. Most of the present-day relief was probably created at this time. This is supported by the decrease of the sedimentary flux which suggests a reorganisation of the interior drainage pattern. • 70-30 Ma (Uppermost Cretaceous-Paleogene): most of the relief is fossilized and weathered relative tectonic quiescence. • 30-15 Ma (Oligocene-Early Miocene): second period of the South African Plateau uplift. Most of the deformation took place along the Indian side of the Plateau (strike flexure) feeding the Zambezi, Limpopo and Tugela deltas. • Since at least Middle Miocene times, all those reliefs have been fossilized, with very low erosion rates (x1m/Ma), in response to the major aridification of southern Africa.

Domaines: Tectonique

Guillaume Baby, François Guillocheau, Cécile Robin, Massimo Dall’asta. POST-RIFT VERTICAL MOVEMENTS OF THE SOUTHERN AFRICAN MARGINS-IMPLICATIONS FOR THE SOUTH AFRICAN PLATEAU UPLIFT. International Meeting of Sedimentology 2017, Oct 2017, Toulouse, France. pp.67. ⟨insu-01717989⟩