Constraining large-scale incision dynamics of shield and post-rift margin domains is key to understanding the sediment routing system over the overwhelming part of the continental surface. Based on dated and regionally correlated incision markers from West Africa, we reconstruct for the first time the entire paleo-long profiles of big rivers such as the Niger at ca. 24, 11 and 6 Ma, as well as the Eocene topography those rivers have dissected. The results provide boundary conditions and calibration for surface process models and paleodrainage dynamics. Though spatially and temporally variable, incision remained mostly below 10 m/my with a mean around 5 m/my. The spatial stability of both the river outlets and divides imposed maintenance or increasing concavity of the river long profiles through time, resulting from spatially contrasted adjustment of river segments bounded by recurrent lithogenic knickzones that persisted since 24 Ma. Drainages evolved preferentially by very slow slope decrease or uniform incision in between the stationary knickzones of evolving amplitude, with apparently no relation to base level change. Therefore, knickzone height or position may not simply reflect the transient response of big rivers to base level fall as predicted by stream-power incision river models. This may also challenge uplift histories of deep continental interiors retrieved from river long profiles inversion relying on such models. Very slow incision allowed amplification of the Hoggar hot spot swell and flexural uplift of the continental margin to be recorded by river long profiles, emphasizing the potential of big non-orogenic rivers as gauges of dynamic topography.
Bibliographical noteFunding Information:
This work was funded by the ANR TopoAfrica ( ANR-08-BLAN-572 0247-02 ), the CNRS and WAXI . We thank S. Carretier, S. Bonnet, P. van der Beek and G. Hérail for fruitful discussions and Ph. Dussouillez for support. We are indebted to F. Pazzaglia for very constructive reviews. The manuscript benefited from comments by K. Burke and anonymous reviewers. We acknowledge AMIRA International and the industry sponsors, including AusAid and the ARC Linkage Project LP110100667 , for their support of the WAXI project ( P934A ) as well as the Geological Surveys/Departments of Mines in West Africa as sponsors in kind of WAXI.
© 2014 Elsevier B.V.
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- Incision dynamics
- River long profiles