Uplift history of a transform continental margin revealed by the stratigraphic record: The case of the Agulhas transform margin along the Southern African Plateau

Abstract

The south and southeast coast of southern Africa (from 28°S to 33°S) forms a highelevated transform passive margin bounded to the east by the Agulhas-Falkland Fracture Zone (AFFZ). We analysed the stratigraphic record of the Outeniqua and Durban (Thekwini) Basins, located on the African side of the AFFZ, to determine the evolution of these margins from the rifting stage to present-day. The goal was to reconstruct the strike-slip evolution of the Agulhas Margin and the uplift of the inland high-elevation South African Plateau. The Agulhas transform passive margin results from four successive stages: (1) Rifting stage, from Late Triassic to Early Cretaceous (~200?-134 Ma), punctuated by three successive rifting episodes related to the Gondwana breakup; (2) Wrench stage (134-131 Ma), evidenced by strike-and dip-slip deformations increasing toward the AFFZ; (3) Active transform margin stage (131-92 Ma), during which the Falkland/Malvinas Plateau drifts away along the AFFZ, with an uplift of the northeastern part of the Outeniqua Basin progressively migrating toward the west; (4) Thermal subsidence stage (92-0 Ma), marked by a major change in the configuration of the margin (onset of the shelf-break passive margin morphology). Two main periods of uplift were documented during the thermal subsidence stage of the Agulhas Margin: (1) a 92 Ma short-lived margin-scale uplift, followed by a second one at 76 Ma located along the Outeniqua Basin and; (2) a long-lasting uplift from 40 to 15 Ma limited to the Durban (Thekwini) Basin. This suggests that the South African Plateau is an old Upper Cretaceous relief (90-70 Ma) reactivated during Late Eocene to Early Miocene times (40-15 Ma).

Evolution of the Glorieuses seamount in the SW Indian Ocean and surrounding deep Somali Basin since the Cretaceous

“…These were initiated at ~23 Ma, most likely due to an uplift event of the North-eastern part of the South African Plateau (Said et al, 2015). Two main periods of uplift were documented during the thermal subsidence stage of the Agulhas Margin: (i) a 92-Ma short-lived margin-scale uplift, followed by a second one at 76 Ma located along the Outeniqua Basin, and; (ii) a long-lasting uplift from 40 to 15 Ma limited to the Durban (Thekwini) Basin (Baby et al, 2018 (Delaunay, 2018).…”

Drainage and Sedimentary Responses to Dynamic Topography

“…Over the last two decades, high‐resolution seismic data and thermochronology data have improved our understanding of the tectonic and geomorphic evolution of Southern Africa (Baby et al, ; Baby et al, ; Brown et al, ; Guillocheau et al, ; Said et al, ; Stanley et al, ; Tinker et al, ). For example, Hu et al () recently proposed a lithospheric delamination model for the uplift history of southern Africa based on seismic observations and geodynamic modeling.…”

The Zambezi delta (Mozambique channel, East Africa): High resolution dating combining bio- orbital and seismic stratigraphies to determine climate (palaeoprecipitation) and tectonic controls on a passive margin

“…However, the Late Cretaceous is a period of extremely long wavelength (several thousands of kilometres km) lithosphere deformation that is influenced by mantle dynamics (e.g. Nyblade and Robinson, 1994;Gurnis et al, 2000;De Wit,2007) • S7 sequence boundary (around the Cretaceous-Paleogene boundary, 67.3-63.2 Ma) is a regional-scale unconformity that extends throughout southern Africa and has a tectonic origin linked to the growth of the South African Plateau (Baby et al, 2018). However, its deformation processes are insufficiently understood.…”

“…catchment to explain the siliciclastic supply and thus the formation of the modern Zambezi Delta.• S4 sequence boundary (late Chattian, 25-23 Ma) is related to the 26 Ma unconformity thatBaby et al (2018) identified in the Durban Basin and interpreted as the paroxysmal period of the second phase of the South African Plateau uplift. This indicates a major reorganisation of drainage in the Malawi that likely extended far north before this event.…”