Large‐Scale Tectonic Forcing of the African Landscape. Issue 12 (1st December 2021)

• Record Type: Journal Article
• Authors: O'Malley, C. P. B.; White, N. J.; Stephenson, S. N.; Roberts, G. G.
• Is Part Of: Journal of geophysical research. Volume 126:Issue 12(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-12-01
• Subjects: Africa -- river profile -- neogene -- landscape evolution -- sedimentary flux -- drainage; Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2021JF006345
• Languages: English
• ISSNs: 2169-9003

Abstract

Abstract: Successful inverse modeling of observed longitudinal river profiles suggests that fluvial landscapes are responsive to continent‐wide tectonic forcing. However, inversion algorithms make simplifying assumptions about landscape erodibility and drainage planform stability that require careful justification. For example, precipitation rate and drainage catchment area are usually assumed to be invariant. Here, we exploit a closed‐loop modeling strategy by inverting drainage networks generated by dynamic landscape simulations in order to investigate the validity of these assumptions. First, we invert 4, 018 African river profiles to determine an uplift history that is independently calibrated, and subsequently validated, using separate suites of geologic observations. Second, we use this tectonic forcing to drive landscape simulations that permit divide migration, interfluvial erosion and changes in catchment size. These simulations reproduce large‐scale features of the African landscape, including growth of deltaic deposits. Third, the influence of variable precipitation is investigated by carrying out a series of increasingly severe tests. Inverse modeling of drainage inventories extracted from simulated landscapes can largely recover tectonic forcing. Our closed‐loop modeling strategy suggests that large‐scale tectonic forcing plays the primary role in landscape evolution. One corollary of the integrative solution of the stream‐power equation is that precipitation rate becomes influential only if it varies on time scales longer than ∼1 Ma. We conclude that calibrated inverse modeling of river profiles is a fruitful method for investigating landscape evolution and for testing source‐to‐sink models. Plain Language Summary: There is excellent geologic evidence that large portions of the African landscape were lifted up above sea level over the last 30 million years by upward flow of hot mantle rocks beneath the tectonic plate. The strongest evidence comes from marine deposits which contain fossil fish and sea snakes that are now perched at elevations of hundreds of meters in the middle of the North African desert. Mantle processes gave rise to an egg‐carton pattern of gigantic swells and depressions that characterizes much of the continent. As the landscape evolved, it was sculpted and eroded by the action of massive rivers such as the Niger, the Nile and the Congo. Height along the length of each of these rivers varies and appears to preserve a memory of landscape growth. In that sense, rivers appear to act as tape recorders of tectonic processes such as mantle flow. Here, we use computer simulations of an evolving landscape to test the idea that rivers contain mantle memories. These simulations, which include complexities such as variable rainfall, allow rivers to develop naturally as landscapes grow. Our results suggest that the African landscape and its drainage patterns contain valuable information about deep Earth processes. Key Points: Continent‐wide inventories of longitudinal river profiles preserve recoverable signals of regional uplift Tectonically forced landscape simulations enable testing of inverse modeling strategies Closed‐loop modeling suggests that fluvial landscapes are dominated by tectonic forcing