Assessment of relative active tectonics in Edea – Eseka region (SW Cameroon, Central Africa)

Journal Pre-proof Assessment of relative active tectonics in Edea – Eseka region (SW Cameroon, Central Africa) Moussa Nsangou Ngapna, Sébastien Owona, François Mvondo Owono, Christian Balla Ateba, Veronique Manga Tsimi, Joseph Mvondo Ondoa, Georges Emmanuel Ekodeck PII:

S1464-343X(20)30049-2

DOI:

https://doi.org/10.1016/j.jafrearsci.2020.103798

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AES 103798

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Journal of African Earth Sciences

Received Date: 25 May 2019 Revised Date:

12 February 2020

Accepted Date: 12 February 2020

Please cite this article as: Ngapna, M.N., Owona, Sé., Owono, Franç.Mvondo., Ateba, C.B., Tsimi, V.M., Ondoa, J.M., Ekodeck, G.E., Assessment of relative active tectonics in Edea – Eseka region (SW Cameroon, Central Africa), Journal of African Earth Sciences (2020), doi: https://doi.org/10.1016/ j.jafrearsci.2020.103798. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

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Assessment of relative active tectonics in Edea – Eseka Region (SW Cameroon, Central Africa)

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Moussa Nsangou Ngapna1, Sébastien Owona1*, François Mvondo Owono1, Christian Balla Ateba1, Veronique Manga Tsimi1,

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Joseph Mvondo Ondoa2, Georges Emmanuel Ekodeck1, 2

5 1The

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University of Douala, Faculty of Science, Department of Earth Sciences, P.O. Box: 24157, Douala–Cameroon;

The University of Yaounde, Faculty of Science, Department of Earth Sciences, P.O. Box: 812, Yaounde, Cameroon;

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Corresponding author: Prof. Dr. Sébastien OWONA; [email protected]; [email protected]; [email protected]

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Abstract

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The Edea-Eseka region (EER), a portion of West African Margin located in SW Cameroon between 3°30’—4°00’ N and 10°00’—

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10°55’ E and submitted to equatorial climate since early Miocene is used here as a proxy for climatic conditions. Geomorphological

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analysis based on morphometric parameters computed from DEM (30 m) combined with geological maps and field observations were

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used to constrain geomorphic evolution and landscape adjustment to tectonic processes in this humid zone. Three types of geomorphic

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indices reflecting specially patterns of differential uplift, those efficient in detecting tilting, and those revealing interactions between

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erosion and tectonics.

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The obtained results show that lithological boundaries and base level controls on landscape are very limited. Tectonic processes,

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peculiarly uplift induced by faults or tilting at local scale and by mantle dynamics at large scale coupled with climate condition,

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represent the main factors that drove the geomorphic evolution of this area discriminating three main morphotectonic provinces

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different by their geomorphic characteristics, uplift and degree of incision. There are, from West to East: (1) the coastal province

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(CP), deeply incised and built on Douala-Kribi/Campo basin (< 65 Ma) set up during the South Atlantic opening, recorded the very

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low uplift rate; (2) the intermediate province (IP), developed on Nyong (2400–1800 Ma) and Oubanguide (650–540 Ma)

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crystallophyllian complexes and marked by high differential erosion, includes middle to high residual reliefs; and (3) the upper

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province (UP), developed like the pervious on Nyong and Oubanguide crystallophyllian complexes is characterized exclusively by

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continental high reliefs, representing high post-Cambrian continental uplifts.

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Several geomorphic indices used to evaluate tectonic activities and their average the index of relative active tectonics (IAT) show

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that tectonic activity differs from one province to another and within each province. Generally, it increases from coast (coastal

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province) to hinterland (UP). EER are represented by classes 2 and 3 of IAT indicating moderate to high active tectonics. Class 1

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revealing the highest tectonic activity, occurs exclusively in the UP. Class 2 corresponding to high tectonic activity happens largely

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in the south of the IP along the Nyong Fault and the UP where relief are particularly high. Class 3 indicates the moderate relative

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tectonic in the IP. Class 4 mainly takes place in tectonically low to inactive Douala-Kribi/Campo basin, except along the South part

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of Sanaga Fault.

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The development and the evolution of Edea-Eseka region landscape is then acted by a dominant high-uplift rate in SW Cameroon. This

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tectonic activity apparently incompatible with the passive margin context is however realistic in this area, on one hand justified by the

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numerous reactivations over time of faults crossing the region that generate earthquakes in some places and on the other hand, by the

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activity of the Cameroon Volcanic Line, and particularly that of the Mount Cameroon located near the study area. It appears at the end

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that, due to mantle dynamics, the EER has been rejuvenated under the tectonics and climate forcings, returning to a transient stage of

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its geomorphic evolution.

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Key words: Relative Active Tectonic; Geomorphic indices; Morphotectonic provinces; Edea – Eseka Region; SW Cameroon.

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1. Introduction

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Passive margins and associated upstream reliefs display complex morphological shapes that have been abundantly described

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in arid zone such as in South African plateau (Dauteuil et al., 2015; Mvondo Owono et al., 2016) and in SE Asia (Mathew et al., 2016a,

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b; Menier et al., 2017; Ramkumar et al., 2017 and 2019) where they were well preserved. These complex topographies have been

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related to short and long wavelength deformations due to the mantle activity beneath both the South-African plateau (Guillocheau et al.,

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2015; Mvondo Owono et al., 2011, 2016) and the Indian subcontinent (Mathew et al., 2016a, b; Menier et al., 2017; Ramkumar et al.,

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2017, 2019). In the passive margins developed in humid area, such complex topographies are rare due to the fast erosion induced by

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the climate. In the zones where they are well preserved, few studies have been done. Nothing was proposed about their origin, and to

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accommodate which process: climate induced processes, lithology variations or tectonics?

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In the Central Africa plateau and South Cameroon (Figs. 1a-d), Edea–Eseka region (EER) located between latitude 3°30’–

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4°00’ N and longitude 10°00’–10°55’ E (Fig. 1e) constitutes a portion of humid passive margin where high, moderate and low

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topography coexist in the same area while the region exposed since the early Miocene to the same equatorial climate with an average

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of 4000 m of precipitations per year. In parallel, this region seems to be as a good example of a natural laboratory of tectonics cross

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cutting West African passive margin (Nsangou et al., 2019). Indeed, Champetier de Ribes et Aubague (1956), Maurizot et al. (1986) and

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Lerouge et al. (2006) have shown that the region consists of a Proterozoic basement derived from eburnean and panafrican orogenies

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and Cretaceous sedimentary cover. Nsangou et al. (2013, 2018) have later demonstrated crosschecking field data that the low

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Cameroon plateau is built on above Proterozoic basement and Cretaceous sedimentary cover. It has recorded several reactivations

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related to regional features that cross the region such as, the Sanaga Fault (SF, Dumont, 1986, Moussango Ibohn et al., 2018), the

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Central Cameroon Shear Zone (CCSZ, Ngako et al. 2003; Njonfang et al., 2008), the Cameroon Volcanic Line (CVL, Déruelle et al.,

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2007), and Benue Triple Junction (BTJ, Guiraud, 1991; Benkhelil, 1988) all being still actives and generators of earth tremors (Fig. 1c;

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Ateba et al., 1992; Ntepe et al., 2004; Nsangou et al., 2018, 2019). The cohabitation of such relief shapes, the occurrence of orogenies

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and the activity of regional geological features show that tectonics and lithologies are the main factors that drive the EER landscape

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where base level control is lesser.

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Nsangou et al. (2018) have recently shown that the lithological variation observed in this region leads to a differential erosion

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that justifies these topographies in places. However, field observations also show that the less resistant formations in places are higher

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than those deemed competent or formations of the same nature are at different altitudes while the climate has not changed. Such

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observations show that lithology is not the only factor to take into account and that its action is not dominant. Therefore, the EER,

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appears to be a good example for the assessment of the relative active tectonic in a humid passive margin. The climate being

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ubiquitous and the same in the whole study area at least since early Miocene, is used as a proxy to understand how tectonics acts on

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the evolution of the EER topography. Our hypothesis is that over this region, the activity or reactivations of the regional geological

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features have several consequences on the landscape evolution and their action is more prominent since the early Miocene when the

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climate becomes humid. This deformation coupled with climate conditions drove the geomorphic evolution of this area.

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Keller and Pinter (2002), El Hamdouni et al. (2008) and Jaberi et al. (2018) have shown that changes on landscape due to the

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deformation can be revealed through a combination of geomorphic indices. Then, a landscape quantitative analysis provides arguments

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to evaluate tectonic activities (El Hamdouni et al., 2008; Alipoor et al., 2011; Arian and Aram, 2014; Resmi et al., 2017; Eynoddin et al.,

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2017; Jaberi et al., 2018). Mathew et al. (2016a, b), Menier et al. (2017) and Ramkumar et al. (2017, 2019) established that the genesis

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and evolution of tectonic, geomorphic, drainage and sedimentary basin as coastal and continental margins of Indian subcontinent were

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influenced largely by and in part inherited from the Gondwanan signatures. Therefore, in this study, we have crossed morphometric

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indices commonly used to assess tectonic activities with field observations. These indices include hypsometric integral (HI), normalized

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steepness (Ksn), basin asymmetry factor (AF), uplift (U), stream length-gradient (SL index), transverse topographic symmetry factor (T),

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basin shape index (Bs), lineament index (Li), valley floor width to valley height ratio (Vf), concavity index (ϴ) and index of relative active

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tectonic (IAT).

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2. Geological and geomorphological frameworks

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2.1. Geological framework

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EER belongs to Cameroonian and West African passive margin and includes two complexes, Nyong and Oubanguide and, Douala-Kribi/Campo basin (Fig. 1).

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The Paleoproterozoic Nyong Complex (2400–1800 Ma, Feybesse et al., 1998; Lerouge et al., 2006), belongs to West Central

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African Fold Belt (Feybesse et al., 1998) related to the NW Congo shield reactivation during the collision between Congo and Sao

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Francisco shields and Eburnean/Tran–Amazonian orogeny (Pénaye et al., 2004; Neves et al., 2006; Loose and Schenk, 2018; Bouyo

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Houketchang et al., 2019). It consists of Tonalites-Trondhjemites-Granodiorites (TTG), anorthosites, metagabbros, charnockites,

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metapelites, metaplutonites, banded iron formations and mylonites (Maurizot et al., 1986; Lerouge et al., 2006; Nsangou, 2017). This

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complex was affected by a D1–D4 phases of deformation characterized by Nyong tectonic nappe transported top–to the East onto Congo

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shield and dissected by NW–SE shear zones (Maurizot et al., 1986; Nédélec et al., 1993; Feybesse et al., 1998; Owona et al., 2011a).

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The Meso- to Neoproterozoic Oubanguide Complex (1600–540 Ma) also called North Equatorial Fold Belt was remobilized

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during the Pan-African orogeny (650–540 Ma) which is associated with the collision between Congo, Saharan and West African shields

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(Nzenti et al., 1988; Abdelsalam et al., 2002). The Southernmost Yaounde Group investigated here (Fig. 1), includes para- and

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orthoderived rocks, tonalites and gabbros (Toteu et al., 2006). A like the Nyong Complex, this complex recorded D1–D4 polyphased

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deformation marked by Yaounde tectonic nappe emplacement, transported top– to the SSW onto the Congo shield and the Nyong

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complex (Nzenti et al., 1988; Mvondo et al., 2007; Owona et al., 2011a) and is crossed by the Central Cameroon Shear Zone, the

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Sanaga Fault, the Benue Triple Junction and the CVL (Guiraud, 1991; Toteu et al., 1994; Nzenti et al., 1988; Ngako et al., 2003; Ntepe

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et al., 2004; Déruelle et al., 2007; Mvondo et al., 2007; Owona et al., 2011a, b; Njonfang et al., 2008; Kwekam et al., 2010; Nsangou et

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al., 2018).

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The Douala–Kribi/Campo sedimentary basin (< 65 Ma), setup during the breakup of the Gondwana during the Aptian rifting,

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which conducted the opening of South Atlantic Ocean. It consists of terrigenous (conglomerates, coarse to fine sandstone, limestone,

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siltstone) and carbonates (calcareous and marl) deposits on Precambrian basement; differentiated by Nguene et al. (1992) and Tamfu

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et al. (1995) in syn-rift formations (Barremian –Cenomanian) and post-rift formations (Coniacian - Pleistocene). This region is cross cut by several major and active geological features:

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The CVL striking N30°E, is an active tectono-magmatic alignment that extends from Pagalu Island to Lake Chad (Moreau et al., 1987;

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Déruelle et al., 2007). From 30 Ma to Present, an intense volcanic activity affected the southern part of the Line and, spreading

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southwest-ward into the Gulf of Guinea (Moreau et al., 1987; Déruelle et al., 2007; Njome and de Wit, 2014, Bate Tibang et al., 2017).

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The CVL remains active notably in offshore, with volcanoes such as Principe from the end of Oligocene to Early Miocene, Sao Tome

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and Bioko during the Miocene; in onshore with Mounts Rumpi, Oku and Manengouba along the Miocene. The Mount Cameroon

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peculiarly and the most important volcano, unique along the CVL that remains active since the end of Miocene, is located in

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northwestern part of the study area. These CVL activities highlighted by that of the Mount Cameroon (Déruelle et al. 1987, 2007; Moundi

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et al. 2007; Kamgang et al. 2008) and the associated seismic activities and those recorded in the Gulf of Guinea also close to the study

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area (Ateba et al., 1992; Tabod et al., 1992; Ntepe et al., 2004; Nsangou et al., 2018, 2019), favoured uplifts and tilting and induced

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various geomorphic evolutions.

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-

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amplitude and that the route is materialized by the Sanaga River (Ségalen, 1967; Dumont, 1986).

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-

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prolongation of equatorial oceanic fracture zones along which a sinistral transcurrent shear occurred (Benkhelil and Robineau, 1983).

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This trough was developed as a triple junction called Benue Triple Junction (BTJ). The fault networks that constitute this triple junction

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are inherited from the Pan-African orogeny occurred since 500 ± 100 Ma (Guiraud, 1991; Benkhelil, 1988).

The Sanaga Fault still active, crosscuts the Pan-African basement and induced the stepped strike-slip fault on the order of 4/5 km

The Benue trough formed in the lower Cretaceous during the opening of the Gulf of Guinea represents the continental

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2.2. Geomorphological framework

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EER belongs to Cameroonian Low Plateau (Ségalen, 1967; Olivry, 1986), located south to CVL which constitute the ridge of

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Centre African Plateau (Mvondo Owono, 2011). The geomorphologic evolution started during the Paleoproterozoic with the

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Eburnean/Trans-Amazonian orogeny and the emplacement of the West Central African Fold Belt (Feybesse et al., 1998), which relicts

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constitute high-relief encountered in the eastern part of the EER (Fig. 1b). The area has been reworked by another orogeny ~600 Ma

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ago, the Pan-African notable. The geomorphological evolution of the region will continuous with the Gondwana fragmentation during the

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Cretaceous that induced the monoclinal folding of the African margin. Later on, the region and the whole Centre Africa, are summit since

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the Miocene to the same equatorial climate (Bamford and de Wit, 1993; Bamford, 2000). The climate contributes to the erection of

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planation surfaces such as the late Cretaceous to early Tertiary African Surface and the late Tertiary Post-African Surface. In parallel,

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uplifts are recorded during the Miocene to the Pliocene (Mvondo Owono, 2011; Koum et al., 2013). Guillocheau et al. (2015) tied them

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to the mantle-related “swell” dynamics of the African continent. The whole region is affected by an equatorial Guinean climate since the

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early Miocene with a pluviometry exceeding 4000 mm/yr (Olivry, 1986; Ndam Ngoupayou et al., 2007; Regard et al., 2016). This climate

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influences meteoric erosion and contributes partially to the landscape development.

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3. Materials and methods To assess EER relative tectonic activity, we crossed: literature survey, geological and topographical maps, field observations, digital elevation model (DEM) and geomorphic indices within ArcGIS software.

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3.1. Fieldwork

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We applied field reconnaissance techniques to identify major landscape and landform patterns, depict lithological variations

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and knick points. According to the size of the study area (~7200 km2) and the importance of vegetal cover, we selected representatives

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out crops thanks to analysis of available geological maps, 30 m resolution DEM, and satellite images (Google Earth, 2015). Bedrock

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lithology was verified both along trunk rivers (Mbus Minkon, Pout Loloma, Nyong, and Sanaga) and carriers (Apouh, Logbadjeck,

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Pouma, Kopongo, etc…). We confirmed the locations of few knick points and determined whether they were related to lithology

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boundary or to fault. Maps combined with literature data provided additional information about the landform patterns, lithology and faults

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in the inaccessible zones.

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3.2. Geomorphic indices

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From the DEM, produced by SRTM-1 data through ArcGIS, shade and hydrographical maps were automatically generated

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and geomorphic indices extracted. The applied geomorphic indices are those which have been established as sensitive to changes in

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topography even in zones that registered low or moderate deformation rates and commonly used to assess landscape adjustments to

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tectonic forcing (Molin et al., 2004; Necea et al., 2005; Pérez‐Peña et al., 2009; Gioia et al., 2014; Menier et al., 2017; Ramkumar et al.,

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2019). The applied indices are classified into three groups according to their specificities.

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3.2.1. Indices reflecting patterns of differential tectonic uplift

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Longitudinal stream profile and knick point (Kp), stream length-gradient index (SL), normalized channel steepness index (Ksn)

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and relative tectonic uplift (U) are used because they commonly highlight differential tectonic uplift patterns (Mathew et al., 2016a, b;

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Gaidzik and Ramírez-Herrera, 2017; Menier et al., 2017; Ramkumar et al., 2017, 2019).

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Qualitative analysis of longitudinal profile was conducted in order to depict anomalies along the stream profiles and some

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spatial variations of vertical motions. The longitudinal river profile (channel gradient variations and geometry) and knick points (Fig. 3),

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express the interaction between fluvial incision, lithological patterns, active tectonic processes, sea-level and climate changes (Hack,

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1973; Snyder et al., 2000; Keller and Pinter, 2002; Molin et al., 2004; Whipple, 2004; VanLaningham et al., 2006; Kirby and Whipple,

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2012; Gürbüz et al., 2015). Longitudinal profiles were plot using the revised equation of Hack (1973) (1), ameliorated by Mvondo Owono

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(2011) (2) by introducing the logarithmic component for its applicability to any river;

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[y–y0 = ((y1–y0) / (x1–x0) * (x–x0))] (1) where, y is the elevation in normal range, and x the distance in logarithmic scale;

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AN = Am + [((AH – AL) / (Log Lm –Log LM)) * (Log Li – Log LM)] (2) where AN = equilibrium normalised altitude value to be

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calculated, AH = highest altitude, AL = lowest altitude, LM = maximum length, Lm = minimum length (with Lm = 1) and, Li = length value

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calculated for each considerate point in relation with the upstream.

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Kps were detected using longitudinal profiles and SL values. Figure 3 shows the technique of Kp measurements. Cross

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sections with longitudinal profiles were realized to discriminate Kps related to faults to those associated with lithological boundaries or

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river confluences.

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SL was developed by Hack (1973) in order to evaluate tectonic effects of river forms and drainage patterns (Keller and Pinter,

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1996, 2002; Troiani and Della–Seta, 2008), to determine local uplift and their incipient local response to regional tectonic events. We

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used Hack (1973) equation:

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SL index = (∆H / ∆L) * L (3) where ∆H/∆L is channel segment local slope and L, channel length over its midpoint reached.

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SL values were computed for each 1 km of main river length (Fig. 1) to reinforce knick point identification. El–Hamdouni et al.

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(2008) classify into three classes in respect of SL obtained values: class 1 (SL > 500), class 2 (300 ≤ SL < 500) and class 3 (SL < 300)

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for high, moderate and minor tectonic activity, respectively.

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Wobus et al. (2006), Kirby and Whipple (2012), and Whipple et al. (2013) demonstrated that Ksn helps to quantify basin

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tectonic uplift rates under homogeneous lithological and climate conditions. We calculated Ksn to determine areas where rivers respond

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to tectonic forcing by steeping their gradient and enhancing incision. This index seems to be efficient in distinguishing zones affected by

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different uplift rates (Kirby and Whipple, 2012; Castillo et al., 2013). In EER, Ksn was computed with Ɵref = 0.45 as proposed in recent

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studies (Bellin et al., 2014; Scotti et al., 2014; Azañón et al., 2015; Andreani and Gloaguen, 2016; Gaidzik and Ramirez–Herrera, 2017).

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The equation used is:

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Ksn = Ks*Acent(

ϴref–ϴ)

(4) and Acent = 10(logAmax + logAmin)/2 (5) with Ks and θ determined by regression, Amin and Amax of profile

segment analysed and its midpoint Acent. The obtained results can be divided into four classes: class 1 (Ksn ≥ 500), class 2 (300 ≤ Ksn < 500), class 3 (100 ≤ Ksn < 300) and class 4 (Ksn < 100), corresponding to highest, high, moderate and low active tectonic respectively. Finally, we computed U index (Sinha-Roy, 2002; Ajay Kumar et al., 2017) using hypsometric integral (HI). U reveals spatial variations of uplift magnitude in individual sub-catchments of the whole watershed and can be obtained by the following equation:

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U = hm + (1 – HI) (6) where, hm is the mean elevation of the sub-catchment obtained from the elevation covering 50% of the

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area and normalized against the elevation of the master drainage basin.

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U values can be grouped into three classes: class 1 (U ≥ 1.0) for very high tectonic uplift; class 2 (0.7 ≤ U < 1.0) for high tectonic uplift

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and class 3 (U < 0.7) for moderate to low tectonic uplift or mild subsidence.

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3.2.2. Geomorphic indices recognized efficient in detecting basin tilting Other indices such as basin asymmetry factor (AF), Valley floor width to valley height ratio (Vf), and transverse topographic symmetry factor (T) were computed because they are recognized to be efficient in detecting basin tilting. AF evaluates the tilting at scale of a drainage basin is applicable over a relatively large area (Hare and Gardner, 1985; Keller and Pinter, 2002). AF is obtained with the formula:  × 

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AF =  −

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The obtained values of this parameter help to discriminate four classes of relative tectonic activity; class 1: very active (AF ≥

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15); class 2: active (10 ≤ AF < 15); class 3: semi active (5 ≤ AF < 10); and class 4: inactive (AF < 5). Classes 1 and 2 reveal tilting or other

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tectonic event effects whilst classes 3 and 4 reflect little or no basin tilting.



 (7) where Ar is trunk stream right side area and At, total drainage basin area.

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Vf is defined by Bull and McFadden (1977) as:

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Vf = 2Vfw / ((Eld– Esc) + (Erd– Esc)) (8) where Vfw is the width of the valley floor, Eld, Erd are the altitudes of the left and

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right divides looking downstream; and Esc, the valley floor elevation average.

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Vf discriminates wide “U” shape valleys to narrow “V” shape types (El Hamdouni et al., 2008). Deep, V-shaped valleys

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correspond to region affected by active uplift as a result of a linear and rapid stream incision. Vf low values (< 0.5) evidence highest

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rates of uplift and incision. Contrarily, Vf high values (> 0.5) characterize flat-floored wide valleys related to tectonic quiescence (Keller

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and Pinter, 1996). El Hamdouni et al. (2008) has classified Vf into three classes; 1: Vf ≤ 0.5; 2: 0.5 < Vf < 1, and 3: Vf ≥ 1 corresponding

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to active, semi – active and inactive tectonics, respectively.

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T index, is computed in order to evaluate tilting related to active tectonics (Alipoor et al., 2011; Eynoddin et al., 2017). It is

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used to evaluate the degree and the variation of asymmetry in different segments of the valley. When a basin is symmetric, T values

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tend to 0. Otherwise, he asymmetry increases as T also rises toward 1; this implies that shift of stream channels reveals possible basin

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tilting due to slope polarity variations and the influence of bedrock is insignificant on stream migration. T is determined as follows:

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T = Da / Dd (9) where: Da is the distance basin midline to active meander belt (main active stream) and Dd, the distance from basin midline to basin divider. Three classes of T have been defined by Eynoddin et al. (2017) with as class 1 (T ≥ 0.4), class 2 (0.4 < T < 0.2) and class 3 (T ≤ 0.2) corresponding to high, moderate and low tectonic activities, respectively.

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3.2.3. Geomorphic indices detecting interactions between erosion and tectonics The third group or parameters used in this study highlight many interactions. They are: hypsometric integral (HI), basin shape index (Bs), lineament index (Li) and concavity index (ϴ).

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HI (Strahler, 1952; Schumm, 1956; Weissel et al., 1994; Hurtrez et al., 1999; Keller and Pinter, 2002; Pérez‐Peña et al., 2009)

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has capabilities to potentially reveal complex interactions between erosion and tectonics, and can be significantly correlated to uplift

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rates. Introduced by Strahler (1952), HI indicates elevation of a special area or landscape, giving uneroded area beneath hypsometric

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curve on DEM. We used the following equation of Keller and Pinter (2002):

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HI = (Hmean – Hmin) / (Hmax – Hmin) (10), where Hmin, Hmean and Hmax represent the minimum, average and maximum elevations in DEM, respectively.

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The obtained values can be divided into three classes: class 1 (HI > 0.6) for youngest basins with most of topography higher to

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the mean, class 2 (0.4 < H < 0.6) for mature basins related to extensive, long term erosion and dissected drainage basins, and class 3

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(HI < 0.4) representing oldest basins corresponding to peneplains.

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We used Bs to determine stretched or circular character of basin controlled or not by tectonic effects (Cannon, 1976; Ramirez–Herrera, 1998) determined as follows: Bs = Bl / Bw (11), where Bl represents basin length from headwaters to mouth and Bw, width at its widest point.

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Bs was computed using DEM and classified into three classes: class 1 (Bs > 4), class 2 (3 ≤ Bs ≤ 4) and class 3 (Bs < 3) representing

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elongate, semi elongate and circular basins respectively (El–Hamdouni et al., 2008).

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Li, number of lineament (Nu) per total unit area (At; Gurugnanam and Kalaivanan, 2014; Luay and Ra’ad, 2016) is defined as:

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Li = Nu / At (12) where Nu is the number of lineaments and At a total area of watershed.

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The lineaments were extracted from straight portions of rivers of order 4 for at least 4 km length (Scanvic, 1983). This index is

251

used to evaluate the impact of active tectonics. Nsangou et al. (2018) have shown that Li > 0.2 suggest a structural control of

252

hydrographical patterns. The obtained values of Li can be grouped into three classes: class 1 (Li ≥ 0.5), class 2 (0.35 ≤ Li < 0.5) and

253

class 3 (0.2 ≤ Li < 0.35) equivalent to high, moderate and low impact of tectonic activity of basins.

254

ϴ (Figueroa and Knott, 2010; Ambili and Narayana, 2014) was used with the objective to compare the variation of erosion rate

255

on different rock types with uplifts of sub–basins and to reveal low- to high-concavity of equilibrium river profiles related to active tectonic

256

region. The equation to approximately calculate the index is:

257 258

ϴ = 2A / H (13) where A is difference altitude between middle profile and straight line joining two end profiles, H is difference in height between channel head and outlet.

259

When ϴ ~ 0, the shape of the profile is close to straight line due to the high incision by the river, and when ϴ > 0, the profile

260

displays L shape implying mature watersheds and low to moderate active tectonics and, ϴ < 0 indicates convexo-concave profiles

261

linked to reactivated or rejuvenated watersheds by faults or uplifts and then correspond to moderate or high active tectonics.

262

263 264 265 266

3.2.4. Index of relative active tectonics (IAT) We finally computed a single index called index of relative active tectonics (IAT; El–Hamdouni et al., 2008; Alipoor et al., 2011; Mahmood and Gloaguen, 2012) from the above used geomorphic indices (HI, SL, Af, T, Bs, Li, Vf, ϴ) using the formula: IAT = S / N (14) where S and N are sum and number of selected geomorphic indices.

267

The values of this index were grouped into four classes of different degrees of active tectonics: class 1, highest (1.0 ≤ IAT < 1.5); class

268

2, high (1.5 ≤ IAT < 2.0); class 3, moderate (2.0 ≤ IAT < 2); and class 4, low (IAT < 2.5) (El–Hamdouni et al. (2008).

269 270

4. Results

271 272

4.1. Field observations

273

We observed in the field, local evidences of uplift induced faults at local scales such as in Eseka area (Figs. 4a, b). There is

274

notably a normal fault that affects and orthogneissic basement. We also saw normal to vertical faults close to the Edea town (Figs. 4c,

275

d); the case where the water course flows along a strike slip fault within the Edea town (Fig. 4e). This fault belongs to the secondary

276

network of the main Sanaga Fault. We confirmed on the Nyong Bridge, the water courses that flows parallel to the foliation (Fig. 4f).

277

Geographical location of above field proof is shown on the landform presented in Fig. 2.

278

We verified some lithology reported on the geological maps shown in Fig. 1 (Nsangou et al., 2018). Metapelites and

279

metaplutonite from the Oubanguide Complex are strongly eroded with well-marked foliations. Plutonic rocks mainly include syenite,

280

charnockite, tonalite, trondhjemite, granodiorite and metadiorite from Nyong Complex present similar levels of erodibility (Fig. 5).

281

Limestones, sandstones and clays that belong to the Douala-Kribi/Campo sedimentary basin are easily weathered in comparison to

282

previous crystallophyllian rocks. We clearly observed amphibolites (Fig. 5a) and gneiss (Fig. 5b) of different degree of resistance

283

occurring at the same altitude in the Edea town. Contrarily, we found paragneiss at high-altitude (Fig. 5c), and quartzites both at low-

284

and at high altitude (Fig. 5d) in the Pouma area.

285 286

Also, we located in the field, two main Kps along the Sanaga Rivers related both to fault structures and changes in bedrock lithology. Both shown in Figs. 5c and 5d are observable on the Sanaga Bridge and in the North of the Edea town, respectively.

287 288

4.2. Landform patterns

289

The analysis of the EER through the DEM shows a complex topography that varies between 0 and 700 m and characterized

290

by erased, residual, and steep to high–reliefs built on Archean Congo shield, Proterozoic Nyong and Oubanguide complexes and later

291

on Cretaceous, on Douala – Kribi/Campo sedimentary cover. Three provinces or geomorphic units were spatially defined according to

292

their depth incision, elevation and lateral tilting: the coastal, intermediate and UPs (Fig. 2).

293

The CP is a large domain bordered by the Atlantic Ocean that was developed on Douala – Kribi/Campo sedimentary basin.

294

This geomorphic unit is regularly incised by a very dense dendritic drainage system evidencing a flat and monotonous relief. Only few

295

residual hills are observed in the central part of the unit where sedimentary formations crop out. The CP is crossed and incised by the

296

Sanaga River and its tributaries. Orders of streams vary from 1 to 7. Sanaga (order 7) and its tributaries (orders 1 to 4). Overall, the

297

network is dendritic with a parallel trend.

298

The IP occurs on the Nyong and Oubanguide complexes. It includes middle–to high–residual reliefs with the altitudes that vary

299

from 150 m to 400 m. This contrasted landform derived from the differential erosion facilitated by a dense fault networks that guide river

300

flows and by a variation of petrographical facies mostly occupied by para- and orthogneisses. This intermediary unit is incised by a huge

301

dendritic hydrographical pattern with rectangular tendencies.

302

The upper and hinterland geomorphic unit shows high–reliefs moderately erased. This province (400 to 700 m high) is

303

characterised by a high incision and by resistant rock types such as paragneiss, tonalite, trondhjemite and granodiorite, charnockite,

304

etc… it’s also incised by a dendritic river network made by orders varying between classes 1 and 5 after the classification of Strahler

305

(1957).

306 307 308 309

4.3. Geomorphic responses The applied geomorphic indexes permit to highlight different tectonic processes that acted for the landscape development in the Edea-Eseka region.

310 311

4.3.1. Geomorphic responses to differential uplift of Edea-Eseka region

312

Fours applied geomorphic indexes, the longitudinal profile, Kp, U and Ksn report evidences of the relative tectonic uplift.

313

From the thirty-four longitudinal profiles carried out in the three provinces independent to flow direction, local base level and

314

bedrock (Figs. 6), the curves show well graded morphologies and differ from each other as lateral variations of rock types. Depending to

315

the form of the profile and the variation of the incision, three categories of profiles are observed: double concavity, diagonal or rectilinear

316

and concave profiles (Figs. 6a-c). In the coastal province, linear profiles (50%) and those with double concavity (40%) are the most

317

important than diagonal ones (10%). These diagonal profiles exhibit concave upward portions and end with large downstream scarps

318

(e.g., Ebond River and Ossa 2 Lake, Fig. 6a). In the IP, most profiles are linear or diagonal. Some show slightly concave upstream and

319

convex downstream or display large scarps (e.g. Likouk and Mamb Rivers, Fig. 6b). There are some slightly concave or double

320

concavity profiles (20%). Contrarily, in the UP, ~80% of profiles are concavo-convexes against only 20% of linear with some scarps (e.g.

321

Lepahe River, Fig. 6c). The characteristics of these profiles are particularly important in the absence of a very important control of

322

lithology or base level.

323

We have identified 194 major Kps along all the main rivers in the EER based on longitudinal profiles , analyses of SL values

324

and field studies (Fig. 7). These major Kps mostly coincide with faults and secondary with lithological boundaries or confluences that

325

increase in drainage system in respect of differential erosion and slope as evolving away from sources (Table 1, Figs. 1, 2). In the study

326

area, any smooth profile is observed. This means that rivers are in a transient non-equilibrium state. In Kps were observed both in the

327

middle and the downstream of the Sanaga River sections. They are related to normal or oblique strike-slip faults (Figs. 7a; b). In the

328

whole study area, Kps can be categorised into mobile and anchored Kps. The mobile kps denote the transient adjustment of a fluvial

329

system occur at the same altitude for each basin or are related to confluence (Table 1). Anchored Kps are linked to lithological limits and

330

faults (Figs. 1 and 6). ~67.52% of Kps encountered in the study area are anchored types whilst ~32.47% are mobile. ~93.89% of

331

anchored kps are related to faults (Table 1, Fig. 3). Very limited number, ~6.11% Kps, are related to lithological boundaries. This great

332

number of Kps related to faults indicate a high-faulting density associated with high-uplift rates and high-erosion activity. Some rivers

333

and lake such as Sanaga and Nyong Rivers, and the Mevia Lake flow respectively along the faults named Sanaga, Nyong and Mevia

334

faults, respectively. Figure 1 shows the Kps spatial distribution and discriminates them into major, moderate to minor types in respect of

335

their elevation shown in longitudinal profile shapes (Table 1). Major and moderate Kps show up– to downstream, brutal profile drop

336

altitudes between 307–50 m whilst minor Kps range between 50–1 m linkable with slight dislevelments.

337

SL values of the drainage basins show remarkable similarities with their equivalent longitudinal profiles and Kps (Fig. 7).

338

These values that vary from one watershed to another, range between 0 and 9343. Three classes of SL are spatialized, classes 1, 2

339

and 3 (Table 1, Figs. 7, 8 and 9). Class 1 (SL ≥ 500) in ~26.47% represents high tectonic activity whilst class 2 (300 ≤ SL < 500) for

340

~11.77% corresponds to moderate tectonic activity, and class 3 (SL < 300) implies low tectonic activity in ~61.76% (Table 1). In the CP,

341

the SL mean values vary from Dipombe (85) to Ebond (4672) sub-basins. Only classes 1 (~10%) and 3 (~90%) are observed. The SL

342

mean values in the IP range between Mbandjok (40) and Mamb (8378) sub-basins. All the three classes are encountered with ~71.43%

343

for class 3, ~21.43% for class 1 and ~7.14%, for class 2 showing it low, dominant and moderate tectonic activity, respectively (Table 1).

344

The SL mean values in UP range from Djouel (74) to Ndoupe (937) sub-basins. This geomorphic unit is dominated by class 3 with

345

~55.56%, then class 2 and 1 with each ~22.22% that suggest its most important tectonic activity (Table 1). Finally, it appears that SL

346

values vary both within each province and from one province to another. Anomalously high and low SL values are observed in all the

347

basins; highlighted especially in Ebond River for the coastal province, Mamb, Likouk and Nyong Rivers for the IP, and Nwanda, Ngopi,

348

Lepahe, Ndoupe and Nyong 2 for the UP with gradient index that peaks of 1500 as in the IP, and 800 in the CP. This indicates an

349

eastward increase of tectonic activities i.e. from coastal plain to hinterland over intermediary morphometric unit (Table 1, Figs. 1, 9).

350

Major SL pics equivalent to middle- to high-values indicate Kps deriving from tectonics whilst low- to moderate-ones outline Kps

351

correlated with lithological boundaries and confluences natures (Table 1, Figs. 7, 8 and 9).

352

Ksn values vary also from one province to another and display four classes, 1 (Ksn ≥ 500), 2 (300 ≤ Ksn < 500), 3 (100 ≤ Ksn

353

< 300) and 4 (Ksn < 100) equivalent to the highest, high, moderate and low active tectonic activities (Tables 2 and 3, Fig. 10). In coastal

354

province, Ksn values belong to classes 4 and 3 with ~66.67% and ~33.33%, respectively. In intermediary province, they fit to class 4, 3

355

and 2 with ~28.57%, ~64.29% and ~7.14%, respectively. Hinterland morphometric index evidences class 1, 2 and 3 with ~55.56%,

356

~22.22% and ~22.22%, respectively. These Ksn values reveals the eastward increase of the tectonic activity in EER from the coastal to

357

hinterland morphometric units (Fig. 10).

358

Table 2 shows U values obtained in this study. U values don’t indicate absolute magnitude of tectonic uplift. Hence, the

359

distinctive uplift values observed in different sub-basins expresses spatial variation in potential tectonically controlled uplift rate (Fig. 11).

360

In the coastal province, ~77.78% basins have U < 0.7 and exhibit the dominant low tectonic uplift class (class 3), except Ngombe river

361

(~11.11%, U ϵ [0.7 – 1.0[) and Nyong river (~11.11%, U ≥ 1.0) in classes 2 and 1 which indicate high- to highest-tectonic uplift

362

respectively. For IP, U-values display ~50% (U ϵ [0.7 – 1.0[), ~42.86% (U < 0.7) and ~7.14% (U ≥ 1.0). Nevertheless, Nyong, Likouk,

363

Lepbi and Kelle basins denote the maximum range of uplift (1.13-0.84). In the UP, ~77.78% in class 2 and ~11.11% in class 1 show

364

highest-tectonic uplift and ~11.11% in class 3 representing middle subsidence. Pougue, Nwanda and Maloume sub-basins have U

365

values range from 0.83 to 1.03. The above U values evidence determined and observed uplift rates of 22.22% in coastal province,

366

57.14% in IP and 88.89% in UP; in fact ~56.25% of the strong to strongest uplift in the Edea–Eseka area (Fig. 11). These U values

367

display a gradual increase or evolution from the coast to the hinterland and may suggest an influence of uplift or tectonic activity on

368

drainage systems.

369 370 371 372

4.3.2. Geomorphic response to basin tiltings in Edea-Eseka region The applied geomorphic indices such as AF, T and Vf are specifically related to the tilting of a basin due to an uplift or the (re)play of faults.

373

Values of AF in EER catchments show different responses to tectonic tilting. Four different classes are identified (Tables 2 and

374

3, Fig. 12). In the coastal plain, ~55.56% basins are semi active and belong to class 3 whilst ~11.11% and ~33.33% representing class 2

375

and class 1 correspond to active and very active sub-basins, respectively. In the IP, ~42.86% and ~21.43% belong to class 2 and 1,

376

correspondingly; and outlines active basins against ~21.43% and ~14.29% representing low to inactive basins, respectively. In the UP,

377

~44.44% and ~11.11% of sub-basins belong to class 1 and 2 and display active tectonic, respectively; on the contrary of ~11.11% and

378

~33.33% representing classes 3 and 4, evidence semi to inactive basins, correspondingly.

379

T values vary from one morphometric unit to another and display three classes of tectonic activity: 1 (T ≥ 0.4), 2 (0.2 < T < 0.4)

380

and 3 (T ≤ 0.2) outlining high-, moderate- and low-tectonic activities, respectively (Tables 2 and 3, Fig. 13). In the coastal plain,

381

~55.56% basins belong to class 2, ~33.33% and ~11.11% to class 1 and 3. In the IP, ~50% basins belong to class 1 and ~50% to class

382

2. In upper morphometric unit, class 2 represents ~66.67% of rivers and, class 1 ~33.33%. This means that the region underwent

383

various level of tectonic tilting of drainage basins in the intermediate and UPs while it is observed a lack of tilting of drainage systems in

384

CP and especially in Ossa Lake basin.

385

As concern the Vf, its values vary from one province to another and allow to spatialize the importance of the relative tectonics

386

in the study area (Tables 2 and 3, Fig. 14). The coastal plain evidences two classes of tectonic activity, notably the dominant inactive

387

tectonic class 3 with ~88.89% and the semi-active tectonic class 2 with ~11.11%; likewise, in the intermediary province, class 3

388

represents ~78.57% and leads class 2 ~21.43%. The UP exhibits the three classes: the active tectonic class 1, the inactive tectonic

389

class 2 and the semi-active tectonic class 3 with ~44.45%, ~44.45% and ~11.11%, respectively. Lowest Vf values (Vf < 1) show V-

390

shaped, deep and tight valleys characterise hinterland province. Highest Vf values (Vf > 1) indicate U-shaped or flat-floored valleys in

391

coastal plain and minor in intermediary province. These Vf values show a gradual evolution of tectonic activity from the coast to the

392

hinterland (Fig. 14).

393 394

4.3.3. Geomorphic responses erosion vs. tectonic processes

395

The BS, Li, HI and ϴ reveal the importance of erosion and its relation with tectonics.

396

The Bs values of vary from 4.63 in Sanaga River to 0.63 in Nyong River and define three classes (1, 2 and 3) in the study

397

area. Class 3 is ubiquitous in whole the EER dominates class 1 (elongate basins) and class 2 (semi elongate basins) confined in the

398

coastal and intermediates provinces (Tables 2 and 3, Fig. 15). The CP outlines Bs values that range from 4.63 in Sanaga River to 0.63

399

Nyong River and evidences the most significant class 3 than class 1. In the IP, Bs values vary from 3.31 in Mbandjok River to 1.2 Likouk

400

river and show prevalence of class 3 on class 2. In UP, Bs values range between 2.78 in Ndoupe River and 1.07 in Pougue River with

401

Bs < 3 indicating circular characters of its basins. These Bs values exhibit limited impact of brittle tectonics activities and question the

402

role that may played ductile tectonic and large-scale folds known in studied EER.

403

Li values display three classes: class 1 (Li ≥ 0.5), 2 (0.35 ≤ Li < 0.5) and 3 (0.2 ≤ Li < 0.35) equivalent to high, moderate and

404

low impact of basin tectonic activities in the whole study area and in each morphometric unit (Tables 2 and 3, Fig. 16). In coastal plain,

405

distribution of Li values consist of ~44.45% for class 2, ~44.45%) for class 3 and ~11.11% for class 1. The IP includes ~78.57% for class

406

2 and ~21.43% and for class 1 while the UP shows ~77.78% for class 2 and ~11.11% both for class 1 and 3. The mean proportions of Li

407

in the EER are ~68.74% for the class 3, ~15.63% for the class 2 and ~15.63% for the class 1 corresponding to ~84.37% moderate to

408

high tectonic activities that happened in its three morphometric units with slight ~15.63% non-tectonic zones.

409

In the EER, HI values vary from one watershed to another within the coastal, intermediate and upper morphometric provinces

410

and display various basin evolution stages; they define classes 1, 2 and 3 (Tables 2 and 3, Fig. 17). In the coastal plain, only classes 1

411

and 2 are identified with ~77.78% and ~22.28, respectively. In the IP, ~78.57% of rivers belongs to class 2 and ~21.43%, to class 1. In

412

the hinterland province, ~88.89% fits to class 1 in comparison with ~11.11% for class 2. These HI values provide a general index of

413

erosion or tectonics where class 1 (HI > 0.6) typifies convex hypsometric curves whilst ubiquitous class 2 (0.4 ≤ HI< 0.6) and class 3 (HI

414

< 0.4) show “S” intermediate form with Kps shown in Fig. 3; highlighting the simultaneously coexistence of mature and young natures

415

watersheds in the EER. The presence of those youngest watersheds, normally inconsistent within same zone exposed to same climate

416

evidence the major role of reactivations and active tectonics in addition to lithological influence on basin incisions.

417

The ϴ values typify three dimensionless graphic classes in the EER: class1 (ϴ > 0) corresponds to concave trend profiles,

418

associated with mature watersheds and low to moderate active tectonic; class 2 (ϴ ~0) for profile close to straight line related to

419

incision; and class 3 (ϴ < 0) for convexo-concave profiles linked to reactivated or rejuvenated watersheds and moderate to high-active

420

tectonic (Table 2, Fig. 18). In coastal plain, ~55.56% of profiles belong to class 1 and ~44.44% to class 3 and reveal the coexistence of

421

concaves and convexo–concave tendencies. In intermediary province, of profiles exhibit the straight line class 2 (~35.72%), class 1

422

(~35.72%) and class 3 (~28.56%) equivalent to concaves and convexo–concave types, respectively. In UP, ϴ values evidence class 3

423

(~66.67%) for convexo–concave profiles and class 1 (~33.33%) for concave ones. The mean of ϴ index in the whole EER display

424

~46.88% of convexo–concaves profiles associated with moderate to high tectonic activity, ~38.5% of concaves profiles linked to low- to

425

moderate tectonic activity and, ~14.62% of related to straight line and indicates its controls by average to high incision rates (Table 2,

426

Fig. 18). Therefore, the study area displays at ~85.38%, the active character.

427 428

4.3.4. Evaluation of index of relative active tectonics (IAT)

429

The geomorphic indices reveal the tectonics as an important component that could be considered in the development and the

430

evolution of the EER landscape. The average of eight geomorphic indices including HI, SL, AF, T, Bs, Li, Vf and Ksn was used to

431

determine the IAT spatial distribution in the study area. IAT values obtained define four classes: class 1 with 1.0 < IAT ≤ 1.5, class 2

432

with 1.5 < IAT ≤ 2.0, class 3 with 2.0 < IAT ≤ 2.5, and class 4 with IAT > 2.5; they correspond to highest, high, moderate and low

433

tectonic activity, respectively. Table 3 and Fig. 19 show the classification and distribution of the four classes per province. The coastal

434

plain highlights two classes 3 (~55.56%) and 4 (~44.44%); it is moderately affected by active tectonics. The IP displays three classes, 2

435

(~14.29%), 3 (~71.42%), and 4 (~14.29%); it is affected by a moderate- to high-tectonic activity. The UP also includes three classes: 1

436

(~22.22%), 2 (~66.67%) and 3 (~22.22%); this morphometric unit is shaped by active to very active tectonic activity. These IAT values in

437

the EER show progressive evolution of tectonic activity from the coastal plain to hinterland (Fig. 19). The simultaneous presence of all

438

the four classes, 1 (~3.12%), 2 (~25%), 3 (~53.12%), and 4 (~18.76%) indicates that this region has recorded a moderate to high

439

tectonic activity.

440 441

5. Discussion

442 443

The morphological evolution of a region can depend at least to one of the three following factors: tectonics, lithology and base

444

level changes. According to Lifton and Chase (1992), Keller and Pinter (2002), Duvall et al. (2004), VanLaningham et al. (2006), Kirby

445

and Whipple (2012), Gasparini and Whipple (2014), Gürbüz et al. (2015), Mathew et al. (2016), Menier et al. (2017) and Ramkumar et

446

al. (2017, 2019) applied morphometric parameters reflect landscape adjustments in the drainage network related either to tectonic

447

activities, climate change or both of them. In this section, we discuss the control part of each factor and the most dominant of that of

448

tectonic processes on the EER landscape development using geomorphic indices and fields controls.

449 450

5.1. Lithological control

451

Contrasted lithology that leads differences in rock erodibility affect the landscape and geomorphic parameters, particularly

452

those related to river incisions and channel gradient variations such as SL and Ksn (Whipple and Tucker, 1999; Duvall et al., 2004; Kirby

453

and Whipple, 2012; Mathew et al., 2016a, b, Menier et al., 2017; Ramkumar et al., 2017, 2019). In this case, anchored Kps as

454

demonstrated by Wobus et al. (2006) and Kirby and Whipple (2012), can be related to lithological boundaries. Correlations establish

455

between geomorphic indices, lithological map and field observations highlight the potential impact of lithology on calculated indices. It

456

appears clearly that variations in altitude along the studied area is formed independently to the lithology (Figs. 1, 2, 4 and 5). For

457

example, Figs. 1 and 2, the geological sketch correlated to the 3D model of landscape highlight different elevations of the same lithology

458

and different lithologies at the same altitude within the same morphometric unit and from one to another. At local scale, field controls

459

confirm the Eseka orthogneiss that crop out at ~305 m and ~543 m (Figs. 4a, b); as well as the amphibolite and paragneiss observed at

460

~157 m in the Buss Minkom valley at Edea latitude (Fig. 5a, b); idem with paragneiss and quartzite seen at ~208 m at the Pouma

461

latitude (Figs. 5c, d). From the CP and the IP is observed an increase in elevation (Figs. 1 and 2) that could be related to the transition

462

between sedimentary rocks (less resistant) and the crystallophyllian rocks (resistant). The latter are also encountered in the UP. Both, in

463

the IP and the UP, metamorphic rocks underlie the highest peaks (Fig. 2).

464

No real correlation was found between SL and lithology (Figs. 8 and 9). Indeed, according to Hack (1973) and Alipoor et al.

465

(2011), lithological variations affect SL values when rivers flowing over rocks of different nature that resist differently. In EER, observed

466

anomalous high SL values producing well identified largest peaks correspond to the boundary between different lithologies. (Figs. 1 and

467

9). Most of these anomalies is related to faults. This is reinforced by the fact, 93.89% of anchored kps are related to faults (Table 1, Fig.

468

6). Finally, it appears that SL anomalies strictly depend to the tectonic activity and not to the lithological variations. Also Ksn analyses

469

show that the correlation between Ksn values and lithological variations is not significant (Figs. 1, 10); the Ksn values change greatly

470

within the same rock. Sometimes, different rocks have the same values of Ksn. We therefore conclude that the different values of Ksn

471

more highlight mostly the effect of tectonic processes. Kirby et al. (2003), Ouimet et al. (2009), Kirby and Ouimet (2011), Gasparini and

472

Whipple (2014) and Chen et al. (2015) obtained similar results in regions displaying high rate of uplifts. Therefore, we propose that high

473

Ksn values obtained for Nwanda, Mandjelbe, Ngopi, Lepahe and Ndoupe rivers express the relative high tectonic uplifts of these basins.

474

Correlations between the Kp distribution and the geological map (Fig. 1) show that amount 6.11% anchored Kps are located

475

within lithological boundaries; they evidence Kps linkable to change in rock resistance (Figs. 1 and 6). All of the other anchored knick

476

points (93.89%) recorded are related to faults. This reiterate the conclusion of Nsangou et al. (2013, 2018) who have shown that the part

477

of lithological control in this landscape evolution is not significant. We also demonstrated above that the lithological variations do not

478

really explain the coexistence in the same region of the same lithology at different elevations while climate is the same.

479 480

5. 2. Climate and base-level fluctuations It has been proved by Kirby and Whipple (2012) that climate changes and peculiarly, abundance or scarcity of precipitations

481 482

have a significant incidence on SL and Ksn geomorphic indices as well as the occurrences of Kps (Whipple, 2009; Whittaker, 2012).

483

In the EER, all basins are submitted to the same climate since early Miocene (Burke and Gunnell, 2008; Dauteuil et al., 2015). Slight

484

changes related to altitude variations can be observed without any correlation with SL and Ksn values along the studied streams.

485

Hence, current climate seems to have no significant influence on morphometric parameters. According to Merrits et al. (1994), Crosby and Whipple (2006), DiBiase et al. (2015), base-level fluctuations can be recognized

486 487

in stream profile by mobile Kps. These Kps must occur at comparable altitudes. Unfortunately, this is not observed in the EER.

488

Available data in Central Africa show that the climate did not change since the early Miocene (Bamford and de Wit,

489

1993; Bamford, 2000). Climate and lithology have played a second role in this evolution. Therefore, tectonic seems to be the

490

main factor that controls the landscape evolution. This evolution may has started with the occurrence of regional features

491

such as the BTJ and CVL set up since the Cretaceous that play iteratively through time. Moreover, pulses of recent volcanic

492

activities of Mount Cameroon and earthquakes recorded along the SF are the main proofs of the neotectonic (Nsangou et

493

al., 2018, 2019).

494 495

5.3. Insights on the EER active tectonics through geomorphic investigations

496

It was established in paragraph 5.1 that the EER exhibits topographical changes as determined SL, Ksn and AF geomorphic

497

indices cannot be explained by lithology and climate affect SL values when rivers flow over rock of different resistance (Hack, 1973;

498

Ambili and Narayana, 2014; Gaidzik and Ramirez–Herrera, 2017). This is partially observed in the eastern part of EER, made of high-

499

resistant crystallophyllian rocks (gneisses, mylonites, quartzites, charnockites, syenites), outcropping in high-altitude zones where SL

500

values increase. Contrarily in central and western parts in spite of moderate (micas schists, chlorite schists, amphibolites, etc…) to low

501

(alluvial deposits, sandstones, limestones and clays) rocks encountered in low altitude zones where SL values decrease (Table 1, Figs.

502

7, 8 and 9; Nsangou et al., 2013, 2018). Correlations between lithological units and SL values (Figs. 7, 8 and 9) display anomalously

503

high SL values that do not always coincide with the lithological boundaries (Fig. 8). Observed SL anomalies are mostly related to faults,

504

corroborating the idea of Ambili and Narayana (2014), Gaidzik and Ramirez – Herrera (2017), Mukul et al. (2017), Koukouvelas et al.

505

(2018) and Jaberi et al. (2018) who think that high-SL anomalies are strictly associated with active tectonic processes.

506

EER channels well graded and the presence of major Kps associated with SL indices demonstrate that drainage network is

507

well adjusted to uplift (Bishop et al., 2005; Kirby and Whipple, 2012; Zahra et al., 2017; Baumann et al., 2018; Nsangou et al., 2018), in

508

respect of channel metric spatial variations. On the fields, the presence of the anchored Kps related to faults significantly illustrate this

509

uplift.

510

These uplifts have been evaluated through Ksn according to climate conditions and under homogenous lithological as

511

proposed Kirby and Whipple (2001), Whipple (2004), Wobus et al. (2005), Crosby and Whipple (2006). Ksn defines highest uplift

512

independently of morphometric provinces. EER Ksn trusted classes with ~15.62%, ~9.38%, ~43.75% and ~31.25% respectively in class

513

1–4 (Tables 2 and 3, Fig. 10). This proves that the study area is unstable. These instabilities can be related to uplifts, additionally to

514

EER plateau growth. Lowest Ksn values (< 100) are observed in the coastal plain (~66.67%) and partially in the IP (~28.57%)

515

highlighting unaffected fluvial rivers by tectonic uplift. Ksn values increase from West to East and confirm the importance of uplift in the

516

same direction (Wobus et al., 2006; Crosby and Whipple, 2006; Kirby and Whipple, 2012; Andreani and Gloaguen, 2016; Gaidzik and

517

Ramirez–Herrera, 2017; Baumann et al., 2018) and supported by field observations.

518

Most part of profiles shapes of the main rivers as those of their tributaries, are complex, sometimes concavo-convexes

519

particularly in the upstream of these profiles. That indicates for these profiles a very long time to be consumed to reach dynamic

520

equilibrium stages. According to the age of these rivers, these shapes were unexpected, the profiles must convey to a concave form

521

(Fig. 6). Another group of profiles show important jumps that suggest recent response to fault reactivations in absence to any relative

522

see -level control. In our case, the second case is that we observed. These profiles shapes are due to the reactivations of faults, which

523

induce uplifts at local scale. These uplifts affect each province and vary within a province from one watershed to another (Fig. 6). Kps

524

represent river evolution discontinuities (Seidl and Dietrich, 1992). They express how rivers recorded reactivations of systems partially in

525

equilibrium and define high fluvial incisions in valleys, from up- to downstream as in margin uplift zones (Ambili and Narayana, 2014;

526

Mvondo Owono et al., 2016; Gaidzik and Ramirez–Herrera, 2017; Nsangou et al., 2018). Therefore, knick points appear as good

527

revelatory of active structures as established in many active margins such as the Tibetan Plateau (Kirby et al., 2003; Kirby and Ouimet,

528

2011), Japan forearc (Hayakawa and Oguchi, 2009; Regalla et al., 2013), Himalaya (Wobus et al., 2005), etc… In the study area as we

529

said above, knick points coincide with faults (Fig. 3; e.g. Sanaga Fault, Nyong Fault, Mevia Fault, etc…). They indicate uplifts traduce by

530

up–to downstream brutal dropped of altitudes and rise of slope. Their spatial distribution reflects the kinematics of the faults and their

531

recurrence in the whole area. Then, most river profiles of the study area and their associated Kps result to the reactivations of

532

Proterozoic to Phanerozoic faults and crystallophyllian bedrock high–faulting rate (Table 1, Figs. 1, 6). The geological maps and some

533

field data, show fault strikes dominantly NE–SW in upper and IPs which correspond to Nyong Complex blastomylonitic shear zones

534

(Maurizot et al., 1986; Owona et al., 2012; Messi Ottou et al., 2014; Nsangou et al., 2013, 2018); NW–SE to N–S and NNW–SSE to W–

535

E faults which correspond to those mentioned in Oubanguide complex such as Sanaga, Kelle or Mevia faults and the Centre

536

Cameroon/Africa shear zone (Dumont, 1986; Ngako et al., 2003; Kwekam et al., 2010). To East, the main N–S fault that limits

537

intermediate and coastal provinces is associated with South Atlantic rifting (Nsangou et al., 2013, 2018) whilst active faults such as

538

Dibamba fault cross cut the Douala-Kribi/Campo sedimentary cover (Fig. 1, Nsangou et al., 2013).

539

Strahler (1952), Ohmori (1993), Cheng et al. (2012) and Baumann et al. (2018) have demonstrated that when HI tend to 0.5,

540

the topography evolves in a steady state and the geomorphological catchment presents S shape hypsometric curves. In EER, HI values

541

vary from one river to another. EER HI values vary from one river to another with stepped and uneven curves (Tables 2 and 3, Fig. 17),

542

increase considerably from coastal plain to hinterland (Fig. 17) both for mature and young watersheds (Strahler et al., 1952); exhibit

543

tectonic instabilities such as the occurrence of new faults and the reactivation of old ones and associated offsets that used to happen

544

when rivers approached close to maturity, its rejuvenation with creation of uplift and high-slopes that re-initialized high-erosion activity

545

(Ambili and Narayana, 2014; Mvondo Owono et al., 2016; Ajay Kumar et al., 2017; Jaberi et al., 2018; He et al., 2018; Nsangou et al.,

546

2018).

547

Bs values indicate limited impact of brittle tectonics activities (Tables 2 and 3, Fig. 15). The circular character of basins

548

revealed by this factor corresponds mostly to cartographic folds. These folds were described as F3 and F4 folds in Nyong and Yaounde

549

tectonic nappes (Ganwa et al., 2007; Owona et al., 2011b; Nsangou, 2017; Nsangou et al., 2013, 2018). These sub-circular character of

550

the hydrographical patterns represent a supplementary proof of the tectonic control on landscape demonstrated in SW Cameroon

551

(Mvondo Ondoa et al., 2009; Mbola Ndzana et al., 2014).

552 553

Moreover, Li values exhibit ~84.37% of moderate- to high- and ~15.63% low-EER tectonic activities (Tables 2 and 3, Fig. 16), confident to the good organization of the drainage pattern and its highest structural control by faults (Nsangou et al., 2018).

554

The tilting of the drainage basins as well as the degree of the incision due to uplift or replay of faults, have been evaluated with

555

Vf, T and AF. Tables 2 and 3 and Fig. 14 show the EER spatial distribution and variation of Vf from one morphometric unit to another. Vf

556

low values (< 1) in upper and IPs corresponds to «V» shaped valleys developed in response to active uplift rate whilst high values (Vf >

557

1) in coastal plain, indicating broad «U» shaped valleys and major lateral erosion, link to stable base level or tectonic quiescence (El

558

Hamdouni et al., 2008; Resmi et al., 2017; Ajay Kumar et al., 2017; Mukul et al., 2017; Koukouvelas et al., 2018; Jaberi et al., 2018; He

559

et al., 2018). T reveals the active character and large-scale surface tilting of the main catchments of the EER. T meanwhile indicates the

560

lateral tilting (Ribolini, 2000). In most of the EER basins, T values lower than 0.5 show that the drainage is controlled by the tilting of the

561

basin due to tectonic activity. Gentle asymmetric, moderate and strong tendencies can be explained by existence of Nyong and

562

Yaounde tectonic nappes and their associated F3-4 folds (Tables 2 and 3, Figs. 12, 13; Mvondo Ondoa et al., 2009; Owona et al.,

563

2011a); whilst symmetric catchments are confident with scarp fault reactivations and an unstable geological setting (Azañón et al., 2015;

564

Ajay Kumar et al., 2017; Koukouvelas et al., 2018; Jaberi et al., 2018; He et al., 2018).

565

Convexo–concave (ϴ < 0) and concave (ϴ > 0) profiles linked to average to high incision rates demonstrate that EER has

566

experienced one or several uplift episodes as established in other areas by such as in the Upper Saddle River in New Jersey (US) by

567

Keller and Pinter (1996), in the Massif Central (France) by Larue (2004), in the Orange River Valley (Namibia, SW Africa) by Dauteuil et

568

al. (2015) and Mvondo Owono et al. (2016) as in SW Cameroon by Nsangou et al. (2018); contrarily to linear ones (ϴ ~ 0) that

569

correspond to very low incision rates (Table 2, Fig. 18).

570 571

5.4. Origin of the Edea-Eseka region tectonics

572

We demonstrated above that the observed differences in landscape are the consequence of tectonic variations, particularly

573

relative uplift and tilting. Indeed, landscape patterns and values of applied morphometric indices suggest relatively lower uplift levels in

574

the coastal province. There is a good correlation between low reliefs, low uplifts and low incisions in this area (Fig. 11). In the UP,

575

highest reliefs have experienced highest tectonic uplifts. (Table 2). The EER IAT values in show progressive evolution from the CP to

576

UP (Fig. 19), the four classes of El Hamdouni et al. (2008) being represented with different percentages: class 1 (~3.12%), class 2

577

(~25%), class 3 (~53.12%) and class 4 (~18.76%). This imply that the region has recorded a moderate to high tectonically active

578

tectonics. These results corroborate those of Dehbozorgi et al. (2010) in the Sarvestan area (Central Zagros, Iran) and close to those of

579

Gaidzik and Ramírez-Herrera (2017) who worked in the Guerrero sector of the Mexican forearc. But the main question is: how explain

580

this tectonic activity in a context of passive margin?

581

In the most active margins such as those studied by El Hamdouni et al. (2008), Saberi et al. (2014), Gaidzik and Ramirez-

582

Herrera (2017), Resmi et al. (2017), and Jaberi et al. (2018), moderate- to high-tectonic activities are due either to (1) the variations in

583

plate-convergence rates (Clift et al., 2003; Melnick et al., 2006), which agree with the inferred decrease in relative tectonic uplift and

584

could thus contribute to observed differences in topography; (2) the fault zone cutting across the fold-thrust belt, deforming some of the

585

previous formed folds (Berberian, 1995); (3) the variations in lithology (e.g., Van Laningham et al., 2006); (4) the uplift due the mantle

586

dynamics (Guillocheau et al., 2015), etc. The EER case is exceptional, because, despite qualified as a passive margin; it is unstable.

587

Applied geomorphic indices revealed that the region has underwent several uplifts and tiltings. We think that, at local scale, cartographic

588

F3-4 folds that belong to the Nyong and Yaounde tectonic nappes or fold-thrust belts, transported top to the East and to the SSE onto

589

the Ntem and Nyong complexes, respectively (Mvondo et al., 2007; Owona et al., 2011a); the several faults observed in geological

590

maps with, some of them confirmed on field, could explain these uplifts. But at large scale or at the scale of the whole region, tectonic

591

activity may be related to several reactivations of regional faults and shear zones still active such as the Sanaga Fault (SF), the Benue

592

Triple Junction (BTJ) and more or less the Central Cameroon Shear Zone (CCSZ).

593

According to Guillocheau et al. (2017), the uplift of the Cameroon dome corresponding to that of the Centre Africa Plateau

594

started around 34 Ma with the stepping of the coastal surface from the African Surface and the intermediate surface. This corresponds

595

to the initiation of a mantle upwelling beneath the Cameroon dome as the magmatic activity already started 40 to 30 Ma according to the

596

emplacement of the “younger granites” in the CVL since 66 Ma (Njonfang et al. 2011). Reusch et al. (2010) through the seismic

597

tomography explained the CVL by an edge flow convecting along the northern boundary of the Congo craton lithosphere coming from

598

the East African mantle upwelling.

599

Indeed, the lowest and western CP which corresponds to the Douala–Kribi/Campo basin is affected only by active brittle

600

tectonics (Ntepe et al., 2004; Nsangou et al., 2018, 2019). In the IP and UP, orogenic imprints such as the Nyong and Yaounde tectonic

601

nappes transported top–onto the Congo craton with crustal thickening have developed both ductile tectonic (Ball et al., 1984; Nédelec et

602

al., 1986, Feybesse et al., 1998; Toteu et al., 2006; Mvondo et al., 2007; Owona et al., 2011b) and on post–orogenic faults (Ngako et al.,

603

2003; Njonfang et al., 2008). Their Phanerozoic to Recent reactivations (Njonfang et al., 2008; Moussango Ibohn et al., 2018), for

604

example the SF which severally reactivated has significantly affected the study area. Such replays are well expressed through the

605

recent earthquakes that occurred since the beginning of this century in Edea-Kribi region with the magnitude that reach 4 at the Richter

606

scale (Ambraseys and Adams, 1986; Ateba et al., 1992; Tabot et al., 1992; Ntepe et al., 2004; IRGM, 2006, Eloumala et al., 2014;

607

Elsheikh et al., 2014). The SF and the northern boundary of the Congo craton were severally reactivated as indicated by Ntepe et al.

608

(2004) and attested by the inversion of its polarity (Moussango Ibohn et al., 2018), and that of the CCSZ (Njonfang et al., 2008; Kwekam

609

et al., 2010). We can also bring out the case of the Adamawa fault, which changed polarity in the Messinian, and currently operates as a

610

dextral strike-clip fault (Ambraseys and Adams, 1986) or that of the Benue triple junction whose the network Pan-African fault been

611

reactivated several times over time. All these regional features that cross cut or are located near the study area could explain the

612

dynamic encountered in this portion of the margin and be considered as one of the possible causes of uplifts.

613

The CVL is another regional structure that borders the study area. Cretaceous to Present activity of this line has induced

614

several uplifts (Koum et al., 2013; Nsangou et al., 2018, 2019). Indeed, the CVL is active since the Cretaceous (Déruelle et al., 2007).

615

This activity became very prolific with the entrance in activity of Mount Cameroon volcano, one of the most important and active

616

volcanoes of this CVL. The latter favoured a very important uplift whose the rate evaluated at ~3.8m/Ma by Koum et al. (2013). This high

617

activity of the CVL which is a consequence of mantle dynamics (Guillocheau et al., 2015), constitute the major internal force that

618

controlled the morphology of the study area. Moderate- to highest-IAT (Fig. 19) could also be linked to the aligned high-reliefs

619

associated to continental uplift, the Nyong and Yaounde nappe inducing crustal thickening (Mvondo et al., 2007; Owona et al., 2011a).

620

The morphotectonic model proposal synthesizes interactions between regional features close to the EER and the evolution of its

621

landscape (Fig. 20).

622 623

6.

Conclusion

624 625

The Edea-Eseka region is a good example of a portion of West African margin in SW Cameroon, Central Africa presenting

626

complex morphological shapes with coexistence of high-, moderate- and low-topographies in the same geological area, under the same

627

climatic conditions at least since the early Miocene. Through geomorphic analysis carry out on topography combined with field

628

observations and literature survey, we demonstrated the contribution of tectonic processes on the landscape adjustment in this region.

629

Four results are more significant:

630

-

From West to East, three geomorphic provinces were discriminated: the CP, IP and UP. Using eight indices: SL, Af, Vf, HI, T,

631

Bs, Ksn, and Ɵ, the average of these indices, IAT, we show that tectonic activity differs from one province to another and

632

within each province, increases from coast (West) to hinterland (East). About 80% of the EER is affected by moderate to high

633

active tectonics that convey to IAT classes 2 and 3. IAT class 1 related to the most active tectonics occurs in the Lepahe sub-

634

basin, the most uplifted. Class 2 corresponding to highly active tectonics occurs mainly in the South of the IP along the Nyong

635

fault and in more than 80% of the UP where relief are particularly high. The class 4 mainly setup in tectonically inactive Douala

636

– Kribi/Campo Basin, except along a south part of SF.

637 638

-

In absence of base level and lithological controls, rivers are still adapting to tectonic processes in each province. This adjustment is clearly reveal by SL that correspond to Kps; both controlled by tectonic processes well marked in longitudinal

639

profiles. The EER landscape has experienced an active incision that developed steep and dip V-shaped valleys as tectonic

640

signatures. High- and low-relief that represent relict hills of the landscape are the witnesses of different uplift episodes that

641

occurred in the region.

642

-

During the Pliocene period, EER experienced a major uplift due to the entrance of mount Cameroon in activity that has

643

probably conducted to general uplift of the region, most significant in the IP and UP. This tectonic setting was reinforced by the

644

recent reactivations of the SF which were accompanied by tremors and hydrographic network.

645

-

It appears at the end that the EER has been rejuvenated under the dominant tectonic forcings than that of the lithology and

646

climate, returning to a transient stage of its geomorphic evolution. Such tectonic activity in this context of passive margin is

647

justified in one part by the several reactivations of regional structures crosscutting the margin and the other hand, by the

648

volcanism related to the CVL and peculiarly the activity of the Mount Cameroon; all supported by mantle dynamics.

649 650

Acknowledgments

651

Authors would like to thank Alipoor Reza and David Menier for constructive critics and suggestions.

652

653

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943

Aravalli uplift. – Journal of the Geological Society of India, 60, 7–26.

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Snyder, N.P., Whipple, K.X., Tucker, G.E. and Merritts, D.J., 2000. Interactions between onshore bedrock–channel incision and near

945

shore wave base erosion forced by eustasy and tectonics. Basin Research, 14, 105–127.

946

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947

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948

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949

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951

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957

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971

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972

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973

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974

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975

976

Table captions

977 978

Table 1: Geometrical characteristics of the Kps

979 980

Table 2: The river morphometric characteristics

981 982 983

Table 3: Values and classes of geomorphic indices and IAT (index of relative active tectonics)

984

Figure captions

985 986

Fig. 1: Location of (a) the study area in Africa DEM, (b) the Central Africa Plateau and main tectonic features, (c) the SW Cameroon

987

landform surface, (d) the main lithostructural units (Modified after Owona et al., 2012), and its (e) EER morpho-geological map showing

988

displaying correlations and controls of lithostructural on morphometric units and spatialization of minor, moderate to major knick points.

989

Note faults, lithological contrasts or confluence nature of Kps; overall E–W rise of altitudes from sedimentary cover (Extreme–West) to

990

crystallophyllian basement (East, Centre and Middle West) and associated morphometric scarp lines and zones; broad scattered fault

991

strikes that suggest multiple causes and reactivations from Proterozoic to Recent times and major N–S normal fault associated with

992

Aptian South Atlantic rifting between Proterozoic basement and Cretaceous sedimentary cover; correspondences between Kps faults

993

and lithological boundary. EG: Equatorial Guinea; CAR: Central African Republic; CCSZ: Central Cameroon shear zone; SF: Sanaga

994

Fault; TBF: Tcholiere-Banyo Fault; CVL: Cameroon Volcanic Line; NC: Ntem complex; NyC: Nyong Complex; OB: Oubanguide

995

Complex; YG: Yaounde Group; CP : Coastal province; IP : Intermediate province and UP : Upper province.

996 997

Fig. 2: Overview of UP, IP and CP morphometric provinces shown in 3D model with located of prototype of field survey photographs for

998

validation of morphometric and morphotectonic analysis. Note E-W landform, spatial distribution of relief, incision and roughness

999

variations.

1000 1001

Fig. 3: Method of knick points modified after Berlin and Anderson (2007). A stream profile at equilibrium (left) and instable (middle) and

1002

Kp determination (right).

1003 1004

Fig. 4: Field survey photographs for validation of morphometric and morphotectonic analysis (continue). Normal to vertical faults

1005

responsible of uplifts as shown at local scales by the general overview of the Eseka area (a, b) and the Edea area (c, d); the Sanaga

1006

tributary that flow along a secondary network of the Sanaga fault (e) and; the Nyong river flow guided by the foliation strike on the Nyong

1007

bridge (f).

1008 1009

Fig. 5: Field survey photographs for validation of morphometric and morphotectonic analysis (end). EER prototype different lithologies

1010

with variable resistances observed at the same altitudes, same lithology encountered at different altitudes and revealed by knick points.

1011

Amphibolite (a, low resistant) and paragneiss (b, high resistant) observed in the Buss Minkon valley in the Edea town; low resistant mica

1012

schist (c) observed at high altitude than high resistant quartzite encountered both at low- and high altitude in the Pouma area; waterfalls

1013

observed at prominent knick-zones along main channels Sanaga basin (e, f).

1014

1015

Fig. 6a-c (end): Relationships between EER main river longitudinal profiles, lithology and tectonic features in (a) coastal, (b)

1016

intermediate and (c) upper morphometric unit. Note correspondences between Kps, faults and lithological boundaries.

1017 1018

Fig. 7a-c (end): EER relationships between stream length gradient indexes and longitudinal profiles highlighting knick points and

1019

tectonic reactivations, lithological boundaries and confluences controls of river slopes in (a) coastal, (b) intermediary and (c) hinterland

1020

provinces. Note concordances between knick points and stream length gradient indexes.

1021 1022

Fig. 8: Spatial distribution of SL index anomalies and the geological strength levels. Note the predominance of SL major in the IP and

1023

UP developed on the Precambrian basement.

1024 1025

Fig. 9: Spatialization map of EER sub-basins interpolated stream length – gradient indexes. Note variation of SL indexes from one sub-

1026

basin to another in three morphometric units, dominant low tectonic activity that don’t cover active tectonics most recurrent in

1027

intermediate and UPs.

1028 1029

Fig. 10: EER spatial distribution of normalized steepness index (Ksn) values. Note overall tectonic activity revealed and it eastward rise

1030

from coastal to hinterland morphometric units.

1031 1032

Fig. 11: EER spatial distribution of the uplift (U). Note the overall progressive and eastward rise of the uplift rate from the coastal to the

1033

hinterland morphometric units.

1034 1035

Fig. 12: EER asymmetry factor (AF) map showing spatialization of inactive to very active sub-basins. Note recurrence of AF fourth

1036

classes in coastal, intermediary and UPs outlining their active character.

1037 1038

Fig. 13: EER sub-basins spatial distribution of Transverse topography symmetry factor. Note ubiquitous of class 1 and 2 within the three

1039

morphometric units and class 3 confined only in Ossa Lake within coastal province.

1040 1041

Fig. 14: EER tributary spatial distribution of Valley floor width to valley height ratio (Vf). Note coastal to hinterland rises of tectonic

1042

activities shown by Vf classes and the dominant moderate- to active tectonic character of EER basins.

1043 1044

Fig. 15: EER basin shape map in drainage network of sub–basins. Note overall recurrence of class 3 Bs that represents circular basin

1045

and confined character of elongate class1 to semi elongate class 2, indicators of active tectonics.

1046

1047

Fig. 16: EER lineament indexes showing it dominant active character. Note overall spatial distribution of class 1 and 2, equivalent to

1048

moderate to active character outlining main structural control on EER basins.

1049 1050

Fig. 17: EER hypsometric integral map of sub–basins. Note overall dominance class 2 with mature watersheds and that of class 1 with

1051

youngest basins in UP.

1052 1053

Fig. 18: Spatial distribution of concavity index for the all sub–basins in the study area.

1054 1055

Fig. 19: Spatial distribution of the index of relative tectonics activities (IAT) in the study area Rivers basins.

1056 1057

Fig. 20: Morphotectonic model of the EER (case of the SW Cameroon active margin). Note the stepped landscape from the coast to the

1058

hinterland highlighting tilting and uplifts due to fault reactivations reinforced by differential erosion processes, and the CVL volcano-

1059

seismic activity here evidenced by that of the Mt Cameroon specially.

1060

Table 1: Geometrical characteristics of the Coastal Province Kps River

Atoume

Dipombe

Mevia lake

Ossa2 lake

Ossa1 lake

Ebond

Lembasse

Kp

H (m)

A (m)

DE (km)

DS (km)

Slope (m/km)

SL index

Lithology

Tributary confluence

Fault orientation

Kp1 Kp2 Kp3 Kp4 Kp5 Kp6

3 2 1 2 10 1

35 32 30 29 27 19

37.43 35.9 33.37 16.31 7.4 6.51

0.5 2.03 4.56 21.62 30.52 31.42

1.96 0.79 0.06 0.23 11.11 0.26

17 15 35 168 239 979

Tributary Tributary Tributary

WNW-ESE WNW-ESE

Tributary

N-S N-S

Kp7 Kp8 Kp9 Kp1 Kp2 Kp3 Kp4 Kp5 Kp6 Kp7 Kp1 Kp2 Kp3 Kp4

2 2 2 2 1 1 3 1 3 1 3 16 4 3

16 14 11 31 30 28 27 25 24 22 98 96 82 76

2.67 1.41 0.13 19.45 18.99 15.84 10.3 9.18 2.45 1.99 25.88 24.42 23.33 21.2

35.25 36.52 37.8 0.27 0.73 3.88 9.42 10.54 17.27 17.73 3.42 4.88 5.96 8.1

1.58 1.56 16.67 4.35 0.32 0.18 2.68 0.15 6.52 0.5 2.06 14.82 1.87 1.95

275 285 590 6 10 58 141 158 260 267 101 97 122 162

Kp5

25

71

19.66

9.64

42.37

296

Kp6

16

49

19.07

10.23

11.11

421

Kp7

5

33

17.62

11.67

3.4

117

Kp8

2

28

16.15

13.14

0.29

132

Kp9

12

26

9.16

20.14

7.69

215

Kp10

2

12

7.6

21.7

0.26

437

Kp1

4

15

7.92

0.08

36.36

4

Kp2

1

13

7.81

0.19

0.63

12

Kp3 Kp4 Kp5 Kp6 Kp1 Kp2 Kp3 Kp4 Kp5 Kp6 Kp7 Kp8 Kp9 Kp1 Kp2 Kp1 Kp2 Kp3

1 1 9 1 3 21 6 1 11 6 4 1 2 14 171 9 5 7

11 10 1 0 63 60 41 34 29 23 17 12 10 185 0 101 95 90

6.23 6.02 4.36 0 16.49 15.35 13.45 11.91 11.22 11.11 10.37 9.05 7.18 16.06 7 12.49 11.62 11.08

1.77 1.98 3.64 8 0.52 1.66 3.56 5.1 5.78 5.9 6.64 7.95 9.83 0.17 9.23 1.74 2.6 3.15

4.76 0.6 2.06 ? 2.63 11.05 3.9 1.47 91.67 8.11 3.05 0.53 0.28 1.58 24.43 8.91 9.09 10

65 73 1213 298 16 56 61 88 500 516 115 138 171 0 9343 33 34 62

TTG TTG TTG Alluvial deposits Alluvial deposits / TTG Alluvial deposits / TTG / Syenite Alluvial deposits / TTG Alluvial deposits / TTG Alluvial deposits / TTG Alluvial deposits Alluvial deposits Alluvial deposits Alluvial deposits Alluvial deposits Sandstones and clays Sandstones and clays TTG TTG TTG Limestone / Grt-Ky bearing gneiss / TTG Limestone / Grt-Ky bearing gneiss Limestone / Grt-Ky bearing gneiss Limestone / Grt-Ky bearing gneiss Limestone / Grt-Ky bearing gneiss Limestone / Grt-Ky bearing gneiss Limestone / Grt-Ky bearing gneiss Alluvial deposits / Limestone / Grt-Ky bearing gneiss Alluvial deposits / Grt-Ky bearing gneiss Alluvial deposits Alluvial deposits Alluvial deposits Alluvial deposits Limestone Limestone Limestone Limestone Limestone Limestone Limestone Limestone Alluvial deposits / Limestone Grt-Ky bearing gneiss / TTG TTG TTG TTG Grt-Ky bearing gneiss / TTG

Tributary

N-S N-S N-S

Tributary Tributary

Tributary Tributary

N-S Tributary

N-S

Tributary

N-S

Tributary

N-S

Tributary

N-S

Tributary

N-S

Tributary

N-S N-S

Tributary

N-S

Tributary Tributary

N-S E-W E-W E-W

Tributary Tributary Tributary Tributary Tributary

~E-W ~E-W

Tributary Tributary Tributary Tributary Tributary Tributary

N-S

Ngombe

Nyong

Sanaga

Kp4 Kp5 Kp6 Kp7 Kp8 Kp9 Kp10 Kp1 Kp2 Kp3 Kp4 Kp5 Kp1 Kp2 Kp3 Kp4 Kp5 Kp6 Kp1 Kp2 Kp3 Kp4 Kp5 Kp6 Kp7 Kp8 Kp9 Kp10

5 6 6 7 18 11 18 10 8 6 9 7 6.46 11.49 8.36 9.25 4.15 2.02 5.83 19.6 7.28 13.25 17.07 1.08 0.84 0.97 1.44 0.8

83 78 72 66 59 41 30 196 186 178 172 156 51.73 45.27 33.78 25.42 16.17 12.02 75.44 69.61 50.01 42.73 29.48 12.41 11.33 10.49 9.52 8.08

10.37 9.93 9.07 7.9 6.89 6.16 3.41 22.01 17.42 5.53 2.08 0 35.94 33.97 26.77 22.98 20.97 18.48 46.34 42.71 34.83 31.62 29.4 25.81 20.02 13.7 9.37 0.28

3.85 4.3 5.16 6.32 7.34 8.06 10.81 0.69 5.29 17.18 20.63 22.71 2.94 4.91 12.11 15.9 17.91 20.4 0.81 4.44 12.32 15.53 17.75 21.34 27.13 33.45 37.78 46.87

11.11 6.98 5.17 6.86 25 4 5.28 2.17 0.67 1.74 4.33 ? 3.28 1.6 2.21 4.6 1.67 0.11 1.61 2.79 2.27 5.97 4.76 0.19 0.13 0.22 0.16 2.86

102 85 69 85 49 111 220 8 44 73 88 680 9 236 205 106 580 6 15 66 258 532 414 136 195 144 200 426

Grt-Ky bearing gneiss Grt-Ky bearing gneiss Grt-Ky bearing gneiss Grt-Ky bearing gneiss Grt-Ky bearing gneiss Grt-Ky bearing gneiss Grt-Ky bearing gneiss Charnockites Syenite / TTG / Charnockites Grt-Ky bearing gneiss Grt-Ky bearing gneiss Grt-Ky bearing gneiss TTG TTG TTG TTG TTG Sandstone and Clays / TTG Grt-Ky bearing gneiss Grt-Ky bearing gneiss Grt-Ky bearing gneiss Grt-Ky bearing gneiss Grt-Ky bearing gneiss Grt-Ky bearing gneiss Grt-Ky bearing gneiss Alluvial deposits Alluvial deposits Alluvial deposits

Tributary Tributary Tributary Tributary Tributary Tributary Tributary Tributary Tributary Tributary Tributary Tributary

Tributary Tributary Tributary Tributary

Tributary Tributary Tributary Tributary

NNE-SSW NNE-SSW NNE-SSW NNE-SSW ~E-W ~E-W NNE-SSW NNE-SSW NNE-SSW NNE-SSW E-W E-W NNE-SSW NNE-SSW NNE-SSW NNE-SSW NE-SW NNE-SSW NNE-SSW NNE-SSW

(H = Elevation of knick point; A = Altitude; DE = Distance to exsurgence; DS = Distance to the source; TTG : Tonalitestrondhjemites-granodiorites).

Table 1 (continue): Geometrical characteristics of the Intermediate Province Kps River

Lekoung

Koumbola

Likouk Mamb

Kp

H (m)

A (m)

DE (km)

DS (km)

Slope (m/km)

SL index

Lithology

Tributary confluence

Fault orientation

Kp1 Kp2 Kp3 Kp4 Kp5 Kp6

7 7 4 4 1 12

109 102 95 91 87 86

20.44 19.23 16.72 14.79 13.07 8.35

0.65 1.87 4.38 6.3 8.02 12.75

5.74 2.79 2.08 2.33 0.21 9.3

2 39 46 56 36 58

Orthogneiss Orthogneiss / Paragneiss Paragneiss Paragneiss Paragneiss Paragneiss

Kp7

4

74

7.05

14.04

2.34

128

Paragneiss / TTG

Kp8

4

70

5.34

15.75

0.82

72

Paragneiss / Syenite

Tributary

N-S

Kp9

7

66

0.43

20.66

16.17

189

Paragneiss / Syenite

Tributary

N-S

Kp1 Kp2 Kp3 Kp4 Kp5 Kp1 Kp2 Kp3 Kp1

21 3 2 19 14 8 6 174 179

229 208 205 197 184 188 180 0 0

11.78 7.95 3.6 1.62 1.12 11.23 9.15 5.67 4.78

0.5 4.32 8.68 10.66 11.16 0.23 2.32 5.8 4.51

5.5 0.69 1.01 38 12.5 3.83 1.72 30.69 37.45

9 34 137 507 177 4 19 8370 8378

Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss

Tributary Tributary Tributary

WNW-ESE

N-S

Tributary Tributary Tributary Tributary

NE-SW NE-SW NE-SW

NE-SW ~N-S

Mbandjok

PomLep

Ngwei

Dibanga

Ndoupe

Lepbi

Loloma

Kp1 Kp2 Kp3 Kp1 Kp2 Kp3 Kp1 Kp2 Kp3 Kp4 Kp5 Kp1 Kp2 Kp3 Kp4 Kp5 Kp6 Kp7 Kp8 Kp9 Kp1 Kp2 Kp3 Kp4 Kp5 Kp6 Kp1 Kp2 Kp3 Kp4 Kp5 Kp6 Kp7 Kp1 Kp2 Kp3 Kp1 Kp2 Kp3

2 3 6 7 16 12 7 1 3 6 6 25 46 11 6 22 24 7 7 3 10 9 7 4 8 4 3 6 5 2 8 8 12 13 23 8 10 8 8

182 180 177 191 184 168 187 180 179 176 170 236 210 164 153 147 125 101 94 87 244 227 218 211 207 199 221 218 212 207 205 197 189 199 186 163 182 177 169

34.4 28.86 6.07 22.17 19.54 0.96 22.79 13.8 9.1 4.1 0.49 35.56 31.57 25.88 23.36 20 13.71 10.73 4.44 3.69 20.35 15.96 12.39 11.12 6.75 3.58 39.22 33.6 24.92 23.27 16.72 13.83 2.73 19.86 17.44 0.83 68.9 64.95 56.17

5.67 11.21 34 0.91 3.54 22.12 0.7 9.7 14.4 19.4 23.01 0.74 4.73 10.42 12.94 16.3 22.59 25.57 31.86 32.61 0.42 4.81 8.39 9.65 14.03 17.2 1.25 6.87 15.56 17.2 23.76 26.65 37.74 0.41 2.83 19.44 27.1 31.05 39.83

0.36 0.13 0.99 2.66 0.86 12.5 0.78 0.21 0.6 1.66 15 6.27 8.08 4.37 1.79 3.5 8.05 1.11 9.33 0.81 2.38 2.51 5.56 0.91 2.52 1.12 0.53 0.69 3.05 0.31 2.77 0.72 4.38 5.37 1.39 9.64 2.53 0.91 1.98

13 26 81 13 26 196 4 43 58 80 94 2 157 81 34 170 238 408 168 86 16 174 39 45 196 160 2 31 37 80 56 187 267 7 38 465 179 125 314

Kp4

28

153

52.12

43.87

4.4

513

Kp5

19

132

45.77

50.23

2.44

467

Kp6 Kp7 Kp8 Kp1 Kp2 Kp3 Kp4 Kp5 Kp6 Kp1

12 7 17 2.6 1.73 7.82 10.45 7.78 1.22 28.27

114 106 90 103.6 101 99.27 91.45 81 73.22 177.1

37.97 30.16 17.66 27.25 20.8 15.89 12.06 3.88 0.19 93.93

58.03 65.84 78.34 2.23 8.68 13.59 17.42 25.6 29.29 4

1.54 0.56 0.96 0.4 0.35 2.04 1.28 2.11 6.42 2.3

449 306 227 10 98 104 51.09 348 231 260

Kp2 Kp3 Kp4

13.81 24.89 22.74

148.83 135.02 110.13

81.61 76.77 55.68

16.32 21.16 42.25

2.85 1.18 1.86

558 12 555

Kelle

Sanaga

Nyong

Orthogneiss / TTG Orthogneiss Orthogneiss Orthogneiss / Micas schist Orthogneiss Orthogneiss Paragneiss Quartzite / Paragneiss Quartzite / Paragneiss Quartzite / Paragneiss Quartzite / Paragneiss Micas schist Micas schist / Paragneiss Paragneiss Paragneiss Micas schist / Paragneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Micas schist Micas schist Micas schist / Paragneiss Paragneiss Paragneiss Paragneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Paragneiss Quartzite Quartzite Paragneiss Paragneiss Micas schist / Paragneiss Micas schist / Banded iron formation Micas schist / Banded iron formation Paragneiss / Banded iron formation Paragneiss / Quartzite Orthogneiss / Paragneiss Orthogneiss Orthogneiss / Paragneiss Orthogneiss / Paragneiss Paragneiss Paragneiss Paragneiss Paragneiss Orthogneiss / Banded iron formation Orthogneiss Orthogneiss Orthogneiss

Tributary NNE-SSW Tributary Tributary Tributary Tributary

Tributary Tributary Tributary Tributary Tributary

~NE-SW

E-W Tributary Tributary Tributary Tributary Tributary

NE-SW NW-SE NW-SE NW-SE

Tributary Tributary Tributary Tributary Tributary

~NE-SW ~NE-SW ~NE-SW ~NE-SW

Tributary Tributary NNE-SSW Tributary Tributary

NW-SE NNE-SSW

Tributary

NW-SE

Tributary

~E-W

Tributary Tributary Tributary Tributary Tributary Tributary Tributary Tributary Tributary

NNE-SSW NNE-SSW NNE-SSW NE-SW NE-SW NE-SW NE-SW NE-SW E-W ~E-W

Tributary Tributary Tributary

NNE-SSW NE-SW NW-SE

Kp5 Kp6 Kp7 Kp8 Kp9

11.13 11.61 8.07 9.21 2.66

99 87.39 60.76 70.11 54.66

49.63 43.44 37.17 27.46 1.17

48.3 54.49 79.32 70.47 43.26

1.8 1.85 0.83 0.35 2.28

804 992 648 595 2847

Orthogneiss Orthogneiss Orthogneiss Orthogneiss Paragneiss

Tributary Tributary Tributary Tributary

NW-SE NW-SE ~E-W E-W NE-SW

Tributary confluence

Fault orientation

Table 1 (end): Geometrical characteristics of the Upper Province Kps River

Pougue

Mandjelbe

Nwanda

Djouel

Maloume

Lepahe

Ngopi

Ndoupe

Nyong1

Kp

H (m)

A (m)

DE (km)

DS (km)

Slope (m/km)

SL index

Lithology

Kp1 Kp2 Kp3 Kp4 Kp5 Kp6 Kp1 Kp2 Kp3 Kp4 Kp5 Kp6 Kp7 Kp1 Kp2 Kp3 Kp4 Kp5 Kp6 Kp7 Kp1 Kp2 Kp3 Kp1 Kp2 Kp3 Kp4 Kp5 Kp1 Kp2 Kp3 Kp1 Kp2 Kp3 Kp4 Kp5 Kp6 Kp1 Kp2 Kp3 Kp4 Kp1 Kp2 Kp3 Kp4 Kp5

75 62 75 77 29 15 53 25 48 41 40 15 25 40 58 63 46 47 31 58 6 9 4 12 17 16 22 10 12 59 307 4 31 6 96 26 4 20 15 61 16 11 19 47 2 4

523 474 412 337 260 231 430 377 352 304 263 223 208 563 523 465 402 356 309 278 389 383 374 208 189 179 163 141 478 366 307 387 383 352 346 250 224 344 324 307 254 281 270 251 204 202

42.06 37.57 29.8 21.82 19.07 12.08 10.35 8.7 8.24 7.24 6.55 5.51 1.5 31.55 28.97 20.49 18.32 13.89 9.93 3.09 15.49 13.72 12.48 12.69 11.75 10.83 5.69 0.67 9.89 9.43 0.14 17.25 12.5 7.12 4.9 4.25 2.24 10.81 4.89 2.83 1.73 16.8 10.99 8.26 5.93 1.86

1.31 5.8 13.56 21.55 24.29 31.29 1.16 2.81 3.27 4.27 4.96 6 10.01 2.28 4.86 13.35 15.52 19.95 23.9 30.75 0.27 2.04 3.28 0.4 1.34 2.26 7.4 12.42 0.79 1.25 10.54 4.19 8.94 14.32 16.54 17.19 19.2 1.44 7.37 9.42 10.53 3.14 8.95 11.68 14.01 18.07

16.7 7.99 9.39 28.1 4.14 1.24 32.12 54.35 48 59.42 38.46 3.74 16.67 15.5 6.83 29.03 10.38 11.9 4.53 18.77 3.39 7.26 1.22 15 16.04 3.11 4.38 14.93 26.09 6.35 2192.86 0.84 5.76 2.7 147.69 12.94 1.79 3.37 7.32 54.96 9.29 1.89 6.96 20.17 0.49 2.62

216 154 364 912 394 423 116 359 167 658 638 1082 259 38 125 1080 275 2427 208 1635 12 150 61 6 63 32 217 643 21 442 2051 57 367 590 227 2836 892 170 712 1595 1273 92 133 347 417 538

Quartzite Quartzite / Orthogneiss Quartzite / Orthogneiss Quartzite / Paragneiss Quartzite / Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Paragneiss Quartzite / Paragneiss Quartzite / Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Micas schist Micas schist Micas schist Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss Paragneiss Paragneiss Paragneiss Paragneiss Micas schist / Paragneiss Paragneiss Paragneiss Paragneiss Paragneiss Micas schist / Quartzite Micas schist Micas schist Micas schist Orthogneiss Orthogneiss Orthogneiss Orthogneiss Orthogneiss

Tributary Tributary Tributary Tributary Tributary Tributary Tributary Tributary Tributary

NE-SW NE-SW NE-SW NW-SE ~NE-SW NW-SE ~NW-SE ~NW-SE ~NW-SE ~NW-SE ~NW-SE

Tributary Tributary Tributary Tributary

Tributary Tributary Tributary Tributary Tributary Tributary Tributary

Tributary Tributary Tributary

Tributary Tributary Tributary Tributary Tributary Tributary

NNE-SSW NNE-SSW NNE-SSW ~NNW-SSE ~NNW-SSE

NNE-SSW ~N-S E-W E-W E-W E-W E-W E-W E-W ~N-S ~E-W ~E-W NE-SW ~N-S ~N-S ~N-S ~E-W ~E-W

Nyong2

Kp6 Kp1 Kp2 Kp3 Kp4

4 28 43 59 8

192 596 568 525 466

0.34 10.73 6.51 4.38 1.68

19.6 0.64 4.86 6.98 9.69

11.76 6.64 20.28 21.77 4.76

1459 94 758 910 525

Orthogneiss Orthogneiss Orthogneiss / Amphibolite Amphibolite Orthogneiss / Amphibolite

Tributary Tributary Tributary

~E-W NE-SW NE-SW NE-SW NE-SW

Table 2: The river morphometric characteristics Drainage system

CP

Rivers

AF

TTSF

SL avg

Bs

HI

Li

ϴ

Ksn

Vf

U

Dipombe

21.44

0.73

128.53

2.17

0.59

0.2

-0.03

26.41

2.1

0.48

Atoume

6.36

0.67

289.14

1.3

0.52

0.36

-0.02

60.11

3.81

0.64

Mevia lake

5.76

0.25

209.84

2.32

0.45

0.45

-0.58

6.97

1.85

0.64

Ebond

5.26

0.3

4671.56

2.43

0.5

0.41

0.8

100.35

1.95

0.65

Lembasse

33.33

0.39

84.91

0.96

0.49

0.75

0.04

143.71

3.96

0.68

Ngombe

10.38

0.47

178.63

1.5

0.39

0.25

-0.02

129.18

0.85

0.81

Ossa1 lake

8.82

0.18

184.56

2.04

0.54

0.34

0.88

56.60

2.71

0.53

Nyong

15.93

0.27

190.23

0.63

0.3

0.22

0.72

13.84

2.74

1.77

Sanaga

5.88

0.34

238.62

4.63

0.5

0.4

0.81

64.93

19.44

0.54

Mamb

12.71

0.25

8377.97

1.51

0.54

0.45

1

70.44

1.43

0.59

Likouk

7.69

0.46

2797.68

1.2

0.51

0.41

0.44

232.07

1.13

0.84

PomLep

7.78

0.49

78.7

2.28

0.58

0.48

-0.04

219.47

0.58

0.68

Lekoung

8.62

0.45

69.5

2.07

0.53

0.43

0

112.45

4.6

0.6

Mbandjok

22.6

0.53

39.78

3.31

0.48

0.42

-0.018

90.93

1.3

0.67

Dibanga

10.79

0.64

149.38

1.53

0.53

0.42

0.22

153.84

3.67

0.77

Lepbi

10.95

0.22

94.14

1.48

0.59

0.4

0

178.04

1.55

0.9

Ngwei

12.34

0.39

55.87

1.85

0.64

0.59

0

159.61

1.62

0.62

Ndoupe

12.22

0.43

104.88

1.91

0.61

0.48

0.14

170.84

1.89

0.74

Loloma

3.33

0.25

170.14

2.43

0.51

0.46

0

257.40

1.98

0.76

Koumbola

10.2

0.56

172.69

1.99

0.6

0.52

0

342.01

1.88

0.69

Sanaga

1.18

0.34

140.36

1.38

0.5

0.63

-0.12

54.45

10.96

0.79

Kelle

21.17

0.36

322.77

1.55

0.53

0.38

-0.06

66.21

0.94

0.91

Nyong

34.72

0.39

807.8

1.35

0.5

0.42

0.28

162.48

0.95

1.13

Djouel

14.7

0.46

74.26

1.37

0.73

0.26

0.08

175.85

2.83

0.78

Pougue

16.67

0.33

410.42

1.07

0.63

0.38

0.23

219.24

0.4

0.85

Mandjelbe

16.67

0.24

468.35

1.65

0.64

0.45

-0.35

1058.39

0.8

0.79

Nwanda

1.72

0.28

826.69

1.5

0.73

0.37

-0.1

519.68

0.44

0.83

Maloume

25.41

0.38

192.19

1.61

0.54

0.4

-0.06

443.20

0.7

1.03

Ngopi

1.89

0.39

828.28

1.67

0.77

1.44

-0.32

653.92

0.61

0.57

Lepahe

15.22

0.46

838.1

1.33

0.68

0.38

-0.44

1391.19

0.49

0.78

Ndoupe

5.65

0.62

937.34

2.78

0.63

0.45

-0.21

611.78

0.82

0.7

Nyong2

2.78

0.33

571.83

2.25

0.75

0.39

-0.12

300.53

0.44

0.83

IP

UP

Table 3: Values and classes of geomorphic indices and IAT (index of relative active tectonics) Class of geomorphic indices

Drainage

IAT

IAT

Rivers Bs

HI

Li

Ksn

Vf

value

class

3

3

2

3

4

3

2.13

3

1

3

3

2

2

4

3

2.63

4

3

2

3

3

2

2

4

3

2.75

4

Ebond

3

2

1

3

2

2

3

3

2.38

3

Lembasse

1

2

3

3

2

1

3

3

2.25

3

Ngombe

2

1

3

3

3

3

3

2

2.5

3

Ossa1 lake

3

3

3

3

2

3

4

3

3

4

Nyong

1

2

3

3

3

3

4

3

2.75

4

Sanaga

3

2

3

1

2

2

4

3

2.5

3

Mamb

2

2

1

3

2

2

4

3

2.38

3

Likouk

3

1

1

3

2

2

3

3

2.25

3

PomLep

3

1

3

3

2

2

3

2

2.38

3

Lekoung

3

1

3

3

2

2

3

3

2.5

3

Mbandjok

1

1

3

2

2

2

4

3

2.25

3

Dibanga

2

1

3

3

2

2

3

3

2.38

3

Lepbi

2

2

3

3

2

2

3

3

2.5

3

Ngwei

2

2

3

3

1

1

3

3

2.25

3

Ndoupe

2

1

3

3

1

2

3

3

2.25

3

Loloma

4

2

3

3

2

2

3

3

2.75

4

Koumbola

2

1

3

3

1

1

2

3

2

2

Sanaga

4

2

3

3

2

1

4

3

2.75

4

Kelle

1

2

2

3

2

2

4

2

2.25

3

Nyong

1

2

1

3

2

2

3

2

2

2

Djouel

2

1

3

3

1

3

3

3

2.38

3

Pougue

1

2

2

3

1

2

3

1

1.88

2

Mandjelbe

1

2

2

3

1

2

1

2

1.75

2

Nwanda

4

2

1

3

1

2

1

1

1.88

2

Maloume

1

2

3

3

2

2

2

2

2.13

3

Ngopi

4

2

1

3

1

1

1

2

1.88

2

Lepahe

1

1

1

3

1

2

1

1

1.38

1

Ndoupe

3

1

1

3

1

2

1

2

1.75

2

Nyong

4

2

1

3

1

2

2

1

1.88

2

system

CP

AF

T

Dipombe

1

1

Atoume

3

Mevia lake

SL avg

IP

UP

Highlights •

Geomorphic and field data reveal uplifts in EER.

•

EER landscape is a transient response to tectonic and climate forcings.

•

Mount Cameroon activities since Pliocene period induced EER general uplift.

•

IAT evidences coastal to hinterland gradual rose of active tectonics in SW Cameroon.

•

SW Cameroon and Central African experienced relative tectonic activities.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: