Landscape responses to dynamic topography and climate change on the South African source-to-sink system since the Oligocene

Abstract

Abstract. The South African landscape displays important lithological and topographical heterogeneities between the eastern, western margins and the plateau. Yet the underlying mechanisms and timings responsible for this peculiar layout remain unclear. While studies have proposed a post-Gondwana uplift driver, others have related these heterogeneities to a more recent evolution induced by deep mantle flow dynamics during the last 30 million years. This theory seems supported by the rapid increase of sediment flux in the Orange basin since the Oligocene. However, the triggers and responses of the South African landscape to dynamic topography are still debated. Here we use a series of numerical simulations forced with Earth data to evaluate the contribution of dynamic topography and precipitation on the Orange river source-to-sink system since the Oligocene. We show that, if the tested uplift histories influence deposits distribution and thicknesses in the Orange sedimentary basin, they poorly affect the large-scale drainage system organisation and only strongly impact the erosion across the catchment for two of the four tested dynamic topography cases. Conversely, it appears that paleo-rainfall regimes are the major forcing mechanism that drives the recent increase of sediment flux in the Orange basin. From our simulations, we find that climate strongly smoothed the dynamic topography signal in the South African landscape and that none of the currently proposed dynamic topography scenarios produce an uplift high enough to drive the pulse of erosion and associated sedimentation observed during the Palaeocene. These findings support the hypothesis of a pre-Oligocene uplift. Our results are crucial to improve our understanding of the recent evolution of the South African landscape.

Inverting passive margin stratigraphy for marine sediment transport dynamics over geologic time

“…We choose not to incorporate these perturbations into our basement boundary condition. The magnitude and timing of uplift pulses are inconsistent-and inconsistently constrained-among the four basins for which we have data (Baby et al, 2018), and there is still debate about the existence and importance of the more recent proposed pulse (Mallard and Salles, 2021;O'Malley et al, 2021). The magnitude of these perturbations is small relative to the up to 7 km of deposits on the SAM.…”

“…We introduced a simple, nonlocal, nonlinear model for marine sediment transport and the development of marine stratigraphy over geologic time. The model builds on the concepts of sediment bypass espoused by previous authors (e.g., Syvitski et al, 1998;Ross et al, 1994;Ding et al, 2019a;Mallard and Salles, 2021) Given the general lack of terrestrial evidence for past landscape evolution dynamics, the stratigraphic record represents our best chance to learn about the erosion trajectories of landscapes long gone. We tentatively suggest that the transport dynamics encapsulated in the nonlocal, nonlinear model govern the development of passive margin stratigraphy.…”

“…If reasonably effective SFM structures and parameterizations can be identified a priori, then coupled LEM/SFMs will be more useful for inferring for example tectonic or climatic perturbations to past landscapes. The SAM is already being used as a target for such studies (e.g., Mallard and Salles, 2021) due to its enigmatic climatic and tectonic history and its well-documented offshore stratigraphy. Though our study is confined to one passive margin, it strongly favors the idea that SFMs should incorporate mechanisms for sediment bypass and long-distance transport, and that these processes cannot be adequately mimicked with parameter changes in the commonly used local, linear diffusion model for seascape evolution.…”

“…As numerical stratigraphic forward models (SFMs) became more common (e.g., Steckler et al, 1993;Syvitski and Hutton, 2001;Burgess et al, 2006), stratigraphic modelers began to use inverse techniques to extract environmental forcing information from forward simulation of the stratigraphic record (e.g., Lessenger and Cross, 1996;Cross and Lessenger, 1999;Bornholdt et al, 1999). The great potential of the stratigraphic record for revealing past landscape evolution has led to efforts to Non-peer-reviewed preprint submitted to Basin Research couple landscape evolution models (LEMs) and SFMs (e.g., Granjeon and Joseph, 1999;Salles and Hardiman, 2016;Salles et al, 2018;Ding et al, 2019a,b;Yuan et al, 2019a, Salles, 2019Zhang et al, 2020;Mallard and Salles, 2021) to build full source-to-sink numerical models, and in some cases to use large ensembles of those models to directly invert observed stratigraphy for past perturbations on eroding continents (Yuan et al, 2019a). The idea underpinning such inversions is that misfit between observed and modeled stratigraphy can be minimized to reveal best-fit values for relevant forcing parameters such as rock uplift rate, assuming that the model is an accurate representation of erosion, transport, and deposition processes integrated over geologic time.…”