Morphology and long-term landscape evolution of the South African plateau in South Namibia

Evolution of the Edea‐Eseka geomorphic units: A 'natural laboratory' of a prototype active rifting cross‐cutting the Southern Cameroon (West African) passive margin

“…According to values of HI, three types of basins can be discriminated (Dehbozorgi et al, ; Pérez‐Peña, Azañón, & Azor, ; Ritter, Kochel, & Miller, ): HI > 0.6 for young basins characterised by most of topography higher to the mean; 0.3 < HI < 0.6 for mature basins related to extensive, long‐term erosion and dissected drainage basins, and HI < 0.3 for old basins corresponding to peneplains; to constrain the tilting of the basin over a relatively large area, we used: (a) the asymmetry factor A F = (Hare & Gardner, ; Keller & Pinter, ; Pérez‐Peña et al, ); (b) the transverse topography symmetry factor TTSF = Da/Dd, very efficient to evaluate if the tilting is related to by tectonic activities (Alipoor, Poorkermani, Zare, & El Hamdouni, ; Cox, ; Eynoddin, Solgi, Pourkermani, Matkan, & Arian, ); the uplift under homogeneous lithological and climate (Wobus et al, ) was evaluated thanks to the normalised steepness index (Ksn) with Ksn = K s * A cent (Θref‐Θ) and A cent = 10 (logAmax + logAmin)/2 . We used a constant concavity (Ɵ ref = 0.45) as proposed in recent studies (Andreani & Gloaguen, ; Azañón et al, ; Gaidzik & Ramirez‐Herrera, ); the drainage density Dd (Km/Km 2 ) = ΣLu/A (Horton, ) that depends of the geology (lithology and tectonics) determined in the objective to reveal uplifting areas; the watershed slope S = H /Lb (Pareta & Pareta, ) and the deviation Dv (m) of Mvondo Owono et al () that aims to compare different river profiles and to reveal their control by lithological variations or base‐level changes (Ambili & Narayana, ; Nsangou et al, ); the concavity index IC = 2A/H of river profiles calculated in order to compare quantitatively longitudinal profile forms in different rock types and sub‐basins that are undergoing differential erosion, where river tectonically active shows higher concavity indices whilst equilibrated river profiles show lower concavity index (Ambili & Narayana, ; Bull, ).…”

“…For methods of the calculation and meaning of the theoretical river profiles (Mvondo Owono et al, ; Nsangou et al, ), according to Coque (), a profile at equilibrium is balanced such that along the curve, from upstream to base level, is the current speed which transports an entire load of solid comes without digging or accumulation. River patterns and forms reveal the basin structural evolution, especially their longitudinal profiles.…”

“…We use longitudinal profiles in the goal to evaluate their geometrical forms and their capability to reveal knick points (Kp), recording methodically altitude and gradient to their origin either lithology, confluence or fault. These profiles are plot using the revised equation (1) [ y − y 0 = (( y 1 − y 0 )/( x 1 − x 0 ) * ( x − x 0 ))] of Hack (), modified by Mvondo Owono (), Mvondo Owono et al () by introducing a logarithmic component for its applicability to any river and expressed by the relation (2) A N = A m + [(( A H – A L )/(Log L m – Log L M )) * (Log L i – Log L M )], where AN = equilibrium normalised altitude value to be calculated, AH = highest altitude, AL = lowest altitude, LM = maximum length, Lm = minimum length (with Lm = 1) and, Li = length value calculated for each considerate point in relation with the upstream. It highlights (a) concave profiles, indicators of long‐term equilibrium between uplift and erosion rates; (b) convexo–concave profiles with erosion steps in the middle reaches suggesting long‐term predominance of erosional processes and (c) convex profiles, associated to uplift areas marked by active tectonics.…”

“…Pysklywec and Mitrovica () have demonstrated that these high topographies could result from an isostatic readjustment resulting from the erosion of an abnormally thick crust whilst Cox () related them to the presence of mantle activity with recurrent magmatic episodes or an uplift, even if they are not always perceptible. Recent studies have shown that the tectonic imprints and the lithological variations could also intensify the differential erosion, the incision and the topographic anomalies reinforce morphological contrasts (Mvondo Owono, Ntamak‐Nida, Dauteuil, Guillocheau, & Njom, ; Nsangou Ngapna et al, ).…”

“…For example, the channel length profiles shapes reflect interactions between the fluvial incisions, lithologies, tectonics and base‐level changes (Brocard, van der Beek, Bourlès, Siame, & Mugnier, ; Kirby & Whipple, ), as the river profile knick points indicate the uplifts, lithological contrasts, brittle tectonics and the increase of confluence shear stress downstream, where river profile in such regions provide information about tectonic evolution. Dauteuil, Bessin, and Guillocheau (), Mvondo Owono et al (), and Nsangou et al () have recently established the lithology, tectonic and climate interactions leading the landform operating processes and characteristics using the watershed morphometric parameters. We associated 12 morphometric parameters with lithology and tectonics imprints in a GIS model, to discriminate geomorphic units under the dominant structural control on morphology, and to evidence the active tectonic that cross‐cuts SW Cameroon and its passive margin.…”