Nicolas Bonnet
email: bonnet@cerege.fr

Anicet Beauvais

Nicolas Arnaud

Dominique Chardon

Mudlappa Jayananda

First 40 Ar/ 39 Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India

Keywords: 40 Ar/ 39 Ar radiometric dating, Cryptomelane, Supergene Mn-ore deposits, Lateritic paleolandsurface, Sandur Greenstone Belt, South India

Lateritic surface processes have shaped large platform and cratons of the tropical belt. Constraining the timing of such processes is crucial to decipher their role in cratonic morphogenesis and their response to long-term climatic change and lithospheric deformation. Weathering histories have been documented for South America, Africa and Australia, but precise time constraints of the lateritic weathering processes in South India are still lacking. We present 40 Ar/ 39 Ar ages of supergene cryptomelane (K-Mn oxide) formed in the Sandur Mn ore deposits exposed on the highest lateritic paleolandsurface that once covered the Mysore plateau and the adjacent Deccan Traps. Significant 40 Ar/ 39 Ar ages are estimated between ~ 36 and ~ 26 Ma from well-defined plateaus in step heating 39 Ar release spectra and from best-fitted inverse isochrones. These ages constitute firm time constraints that document intense late Eocene-Oligocene lateritic weathering over Peninsular India under the influence of warm and wet climate comparable to that prevailing in tropical humid forests. These results imply that Southern India was weathered between ~ 36 and 26 Ma and may have been dissected mostly in the Neogene.

Introduction

Exposed laterites and lateritic duricrusts on relics of paleolandsurfaces in cratonic domains result from successive long-duration chemical weathering and erosion processes, and as such represent morphoclimatic fingerprints of tropical regions [START_REF] Ollier | Deep Weathering, Groundwater and Climate[END_REF][START_REF] Bárdossy | Lateritic bauxites[END_REF][START_REF] Thomas | Geomorphology in the tropics. A study of weathering and denudation in low latitudes[END_REF][START_REF] Tardy | Dérive des continents paleoclimats et altérations tropicales[END_REF][START_REF] Beauvais | Modes, tempo, and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. Since the first definition and description of laterites in South India [START_REF] Buchanan | A journey through the countries of Mysore, Canara and Malabar[END_REF], many studies were conducted on Indian lateritic formations [START_REF] Subramanian | Geomorphic significance of bauxite below Mio-Pliocene sediments in the West Coast[END_REF][START_REF] Babu | Laterite as an unconformity plane in the evolution of the South Indian peninsulaa synthesis[END_REF][START_REF] Sahasrabudhe | The laterites of the Maharashtra State. Lateritisation processes[END_REF]Krishna Rao et al., 1989b;[START_REF] Widdowson | Uplift and erosional history of the Deccan Traps, India: Evidence from laterites and drainage patterns of the Western Ghats and Konkan Coast[END_REF][START_REF] Widdowson | Tertiary palaeosurfaces and lateritization of the coastal lowlands of western peninsula India[END_REF], including manganiferous supergene ore deposits [START_REF] Sawkar | Geology of the manganese ore deposits of North Kanara district, Karnataka state, India[END_REF][START_REF] Rao | Mode of occurrence and origin of manganese ores of Shimoga district, Karnataka[END_REF], 1989a;[START_REF] Dessai | An appraisal of the manganese ore deposits of Goa, India[END_REF][START_REF] Roy | Environments and processes of Manganese Deposition[END_REF][START_REF] Mohapatra | Mineralogical and chemical characteristics of ferromanganese ores from Sandur, Karnataka, India[END_REF][START_REF] Sethumadhav | Late Archean manganese mineralization and younger supergene manganese ores in the Anmod-Bisgod region, Western Dharwar Craton, southern India: Geological characterisation, palaeoenvironmental history, and geomorphological setting[END_REF]. Except few geological stratigraphic constraints, radiometric dating of surface processes involved in the formation and evolution of Indian lateritic paleolandsurfaces are still lacking. Laterites are interlayered with marine sequences of late Eocene and early Miocene age [START_REF] Mcgowran | Discussion: Age and origin of laterite and silcrete duricrusts and their relationship to episodic tectonism in the mid-north of South Australia[END_REF][START_REF] Prasad | A review of the early Tertiary bauxite event in South America, Africa and India[END_REF]. Lateritisation processes have also been indirectly dated with Neogene palynofossils found in colluvial/alluvial sediments overlying in-situ formed Mn ore deposits ~ 100 km south of Belgaum (Fig. 1). These fossils constrain intense weathering that led to Mn ore concentration to be Neogene at the latest (Krishna Rao et al., 1989a). The age of lateritic weathering has also been roughly estimated on the basis of the paleomagnetic properties of iron oxides formed in laterites around 16°N [START_REF] Schmidt | Magnetic ages of some Indian laterites[END_REF], i.e., -Early Tertiary‖ (Paleogene?) for the high-level Belgaum laterites and -Late Tertiary‖ (Neogene?) for the low-level laterites found west of the Western Ghats escarpment (Fig. 1).

Chemical rock weathering commonly resulted in the accumulation of metallic elements (Al, Fe, Mn) in residual laterites and depletion of more soluble elements (Na, K, Si, Ca). Potassium, Na and/or Ba may, however, be retained by supergene Mn oxides in large crystallographic lacuna existing between the MnO 6 octahedra chains that define the typical tunnel-structure of the hollandite manganese oxide group. Cryptomelane belongs to this group with the general stoichiometric formulae K x Mn IV 8-x Mn III x O 16 , x ranging from 0.2 to 1 and K x counterbalancing Mn III substitution for Mn IV [START_REF] Burns | Manganese oxides, Marine Minerals[END_REF][START_REF] Turner | Manganese Oxide Tunnel Structures and Their Intergrowths[END_REF][START_REF] Post | Modeling tunnel-cation displacements in hollandites using structure-energy calculations[END_REF][START_REF] Pasero | A short outline of the tunnel oxides[END_REF].

Cryptomelane is a major Mn oxide of many lateritic Mn-ore deposits in South India [START_REF] Sivaprakash | Mineralogy and geochemistry of some manganeseiron deposits, Karnataka, India[END_REF][START_REF] Rao | Mode of occurrence and origin of manganese ores of Shimoga district, Karnataka[END_REF][START_REF] Mohapatra | Mineralogical and chemical characteristics of ferromanganese ores from Sandur, Karnataka, India[END_REF][START_REF] Sethumadhav | Late Archean manganese mineralization and younger supergene manganese ores in the Anmod-Bisgod region, Western Dharwar Craton, southern India: Geological characterisation, palaeoenvironmental history, and geomorphological setting[END_REF]. This mineral hosts radioactive 40 K atoms, which can decay into 40 Ar. The high radiogenic 40 Ar retentive power of cryptomelane makes this mineral a suitable chronometer for 40 Ar/ 39 Ar step heating dating of weathering processes in lateritic soils including duricrusts and manganiferous ore deposits [START_REF] Vasconcelos | Age and duration of weathering by 40 K-40 Ar and 40 Ar-39 Ar analysis of supergene potassiummanganese oxides[END_REF]Vasconcelos et al., 1994a;[START_REF] Ruffet | A geochronological 40 Ar/ 39 Ar and 87 Rb/ 87 Sr study of K-Mn oxides from the weathering sequence of Azul (Brazil)[END_REF][START_REF] Vasconcelos | K-Ar and 40 Ar/ 39 Ar geochronology of weathering processes[END_REF].

This study quantifies the radiometric ages of lateritic weathering processes that have affected the rocks of the Dharwar craton on the Mysore Plateau by 40 Ar/ 39 Ar dating of cryptomelane from supergene Mn-ore deposits. We focus on the Fe-Mn sedimentary formations belonging to the Sandur Greenstone Belt (SGB) (Figs. 1 and 2a), which hosts the biggest Indian manganese reserve [START_REF] Kameshwar | Final report on Sandur manganese exploration, Bellary district, Karnataka[END_REF]. Our results provide tangible time constraints (Late Eocene -Oligocene) on the paleoweathering processes and morphoclimatic evolution of the potentially oldest lateritic paleolandsurface preserved on the Mysore plateau and Traps in southern India.

Geological and geomorphological setting

Sandur Greenstone Belt geology

The SGB is a synform-shape greenstone belt that consists of mafic/ultramafic volcanics and sediments (Fig. 2a) having undergone low-grade regional metamorphism during latest Archean orogeny that shaped the Dharwar craton [START_REF] Roy | Metamorphic history of the Sandur schist belt, Karnataka[END_REF][START_REF] Mukhopadhyay | The structural anatomy of the Sandur schist belta greenstone belt in the Dharwar craton of South India[END_REF][START_REF] Chadwick | The Sandur schist belt and its adjacent plutonic rocks implications for Late Archaean crustal evolution in Karnataka[END_REF][START_REF] Chardon | Precambrian continental strain and shear zone patterns: South Indian case[END_REF]. The sedimentary sequences contain elements of metavolcanic rock, which are relatively rich in Ni, Cr, Co and Sc. The upper part of the greenstone stratigraphic pile includes platform-like sediments bearing stromatolites, Mn formations and carbonaceous Mn phyllites (Fig. 2a).

In the eastern part of the belt, the upper part of the sequence includes banded iron formations (BIF), felsic volcanics and terrigenous sediments (Fig. 2a). SHRIMP U-Pb dating of zircon in two interlayered felsic volcanics indicate ages ranging from 2651 ± 18 Ma to 2659 ± 40 [START_REF] Nutman | SHRIMP U/Pb zircon ages of acid volcanic rocks in the Chitradurga and Sandur Groups and granites adjacent to the Sandur Schist Belt, Karnataka[END_REF]. The BIF and Fe-Mn formations are related to marine transgression and regression cycles, respectively [START_REF] Roy | Sedimentary manganese metallogenesis in response to the evolution of the Earth system[END_REF]. These cycles implied alternating chemical and clastic sedimentation, which is also evidenced by specific iron formation facies and argillites within the BIF [START_REF] Manikyamba | Geochemical signatures of polygenetic origin of a banded iron formation (BIF) of the Archean Sandur greenstone belt (schist belt) Karnataka nucleus, India[END_REF].

The lower part of the sequence is confined to the Deogiri region and is essentially composed of manganiferous stromatolitic limestones, dolomites and argillites (Fig. 2a) while the upper part is richer in Fe [START_REF] Roy | Late Archean initiation of manganese metallogenesis: its significance and environmental controls[END_REF]. Iron and Mn fractionation was controlled during eustatic sea-level variations by differentiated redox conditions of two stratified water masses on the shallow and deep shelf [START_REF] Roy | Environments and processes of Manganese Deposition[END_REF][START_REF] Manikyamba | Mineralogy and geochemistry of Archaean greenstone belt-hosted Mn formations and deposits of the Dharwar Craton: Redox potential of proto-oceans[END_REF]. The Fe-Mn sediments have later been affected by low-grade metamorphism by 2475 ± 65 Ma (whole rock Pb/Pb isochron dating), which is the maximal recrystallisation age of the limestones [START_REF] Russell | Whole-rock Pb/Pb isotopic ages of Late Archaean limestones, Karnataka, India[END_REF][START_REF] Chadwick | The Dharwar craton, southern India, interpreted as the result of Late Archaean oblique convergence[END_REF]. Such a metamorphism led to Mn enrichment of the sediments [START_REF] Roy | Late Archean initiation of manganese metallogenesis: its significance and environmental controls[END_REF], which are composed of Mn-carbonates, argillites, arenites and phyllites (Fig. 2a), and are the main protores of the SGB Mn-ore deposits. These deposits grade upward into supergene weathering materials [START_REF] Roy | Sedimentary manganese metallogenesis in response to the evolution of the Earth system[END_REF].

Geomorphology of southern India and Sandur Greenstone Belt

The geomorphology of southern India consists of high-elevation surfaces occurring as relics of a differentiated landscape developed on the Dharwar craton and Deccan traps (Fig. 1), and low-elevation surfaces along the western coast [START_REF] Radhakrishna | Neogene uplift and geomorphic rejuvenation of the Indian Peninsula[END_REF], both being overlain by laterites [START_REF] Widdowson | Uplift and erosional history of the Deccan Traps, India: Evidence from laterites and drainage patterns of the Western Ghats and Konkan Coast[END_REF][START_REF] Widdowson | Tertiary paleosurfaces of the SW Deccan, Western India: implication for passive margin uplift[END_REF]). The regional eastern dip of the high-elevation surfaces is interpreted to result from eastward tilting of Peninsular India due to the Seychelles rifting and Deccan Traps events (~ 65-63 Ma) [START_REF] Radhakrishna | Neogene uplift and geomorphic rejuvenation of the Indian Peninsula[END_REF]. On the Deccan Traps, bauxitic lateritic weathering of the highest preserved landsurface must be younger than 62.3 ± 0.6 Ma, which is the age of the youngest basalts [START_REF] Courtillot | Deccan flood basalts at the Cretaceous/Tertiary boundary? Earth Planet[END_REF][START_REF] Pande | Age and duration of the Deccan Traps, India: A review of radiometric and paleomagnetic constraints[END_REF]. On the Traps and the Mysore Plateau, relics of two younger surfaces step below the plateau remnants of the high bauxitic surface (900-1000 m). The three erosion surfaces remain as sparse geomorphic relics at the scale of the Mysore plateau, and have been numbered S2, S3 and S3 d , the latter being a denudation degradation landform of S3 [START_REF] Gunnell | The interaction between geological structure and global tectonics in multistoreyed landscape development: a denudation chronology of the South Indian shield[END_REF]. Spatially reduced relics of the highest massifs also overhang the Mysore plateau (Fig. 1).

Landsurfaces S2, S3 and S3 d are also identified on the SGB relief (Figs. 2b-c).

The S2 surface levels the summits of the belt over an altitude range of 900 to 1100 m and exposes deep in situ formed weathering mantles capped by Fe-Mn and Mn duricrusts at altitudes of ~ 1000-1016 m. Our study is mainly focused on the Mn ore open-pit mines carved in this surface (Figs. 2b-c). Relicts of paleo-land surface S3

extends below S2 over an altitude range of 750 to 850 m (Fig. 2c African glacis/pediments [START_REF] Beauvais | Modes, tempo, and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF], but this surface is not affected by a late ferruginous or manganiferous lateritic duricrusting.

Material and methods

Ore bodies description and sampling

We have sampled across the main Mn ore bodies of the SGB deposits, which formed within weathered meta-sedimentary phyllites at different depths below the top of landsurface S2 at 1012-1015 m altitudes (Fig. 3). Description of samples precisely located on open-pit mine benches allowed the establishment of log sections (Fig. 3b and3d). In Kappataswamy pit (N 14°59'59‖ E 76°32'42‖, Fig. 3a), the first ore body was sampled between 856 and 893 m of elevation along the E-W log section 1 on the western benches of the pit, and the second one between 904 to 935 m along the N-S log section 2 on the eastern benches (Fig. 3b). The third ore body is located in Channanghi KMK-E pit (N 14°59'50'' E 76°34'37'', Fig. 3c) at an elevation range of 926 to 994 m (log section 3, Figure 3d). The Mn-ore bodies are composed of different petrographic facies. The float and platty Mn-ores constitute the stratiform ore type, which is conformably bedded within weathered mangan-phyllites preserving this metasedimentary protore structure [START_REF] Mishra | Exploration planing for Sandur manganese ores[END_REF]. The massive Mn ore is found between ~ 850 m and 892 m elevation in the first section, between 904 m and 912 m elevation in the second ore body of the Kappataswamy pit (Fig. 3b), and between 926 and 982 m elevation in Channanghi KMK-E pit (Fig. 3d). The massive ore is a podiform type with cavity fillings (geodes) and botryoidal structures, which formed by secondary Mn supergene enrichments [START_REF] Mishra | Exploration planing for Sandur manganese ores[END_REF]. Iron-Mn duricrust of ~ 10 m thickness caps the two Mn-ore deposits immediately above a segregated/tabular ore body (Figs. 3b-d).

The samples collected in the Mn-ore deposits (Fig. 3b-d) were cut with a circular saw to get the best angle for mineralogical observations on metallographic quality polished thin sections (200-300 µm-thick) made from the sawed fragments (Fig. 4a).

Symmetrically to the thin sections, 500 µm-thick slabs are prepared for sub-sampling of the target cryptomelane grains suitable for 40 Ar/ 39 Ar dating [START_REF] Hénocque | 40 Ar/ 39 Ar dating of West African lateritic cryptomelanes[END_REF].

Analytical techniques and cryptomelane characterisation

Reflected light microscopy and X-ray micro fluorescence (-XRF) analyses allow precise characterisation of K-enriched Mn-oxide (cryptomelane) on the polished thin sections (Fig. 4b-c). Microscopic observations of thin sections were accomplished using a Leica DM-RXP microscope. The μ-XRF measurements were completed on slabs using a X-ray microscope (XGT7000, Horiba Jobin Yvon) equipped with a X-ray guide tube producing a finely focused and high-intensity beam with a 100 μm spot size, Rh X-ray tube, accelerating voltage of 30 kV and current of 1 mA. X-ray emission from the irradiated sample was detected via an energy dispersive X-ray (EDX) spectrometer equipped with a liquid-nitrogen-cooled high purity Si detector. Micro mapping of the coupled Mn-K occurrences on slabs contributes to precise identification of cryptomelane, while Fe maps document the occurrence of Fe oxihydroxides (Fig. 4c). A Scanning electron microscope (SEM) has been also used for fine-scale observations of the cryptomelane crystals (Fig. 4d). Electron probe micro chemical analysis (EPMA) of minerals on polished thin sections was completed using a CAMECA SX-100 electron microprobe equipped with five wavelength-dispersive X-ray spectrometers (WDS).

Cryptomelane grains were separated from the slabs by hand picking.

Cryptomelane separates were crushed to produce powders (< 64 m), which were analyzed by XRD using a Panalytical X'Pert Pro MPD with a Co Kα X-ray source (λ = 1.79 Å) operating at 40 kV and 40 mA. The system is equipped with a linear X'Celerator detector and a flat monochromator. The analysis range was 10-85° 2θ with a step size of 0.033° and a counting time of 10 s per step.

After ultrasonically cleaning in absolute ethanol and conditioning in aluminium foil packets, other separated grains were irradiated for 50 hours in the TRIGA Mark-II reactor of Pavia University (Italia). The standard Taylor Creek Rhyolite sanidine 2 (TCRs-2) monitor, with an age of 28.05 Ma ± 0.08 [START_REF] Baksi | Intercalibration of 40 Ar 39 Ar dating standards[END_REF], was analysed after every ten unknown samples to allow determination of the factor J. After a twomonth -cooling‖ period, the irradiated cryptomelane grains were loaded in a double vacuum Staudacher-type furnace for step heating Ar isotopes measurements. The furnace temperature was calibrated by means of a classical thermocouple, and the gas purification was accomplished using a cold trap with liquid air and Al-Zr AP10 getters (one hot, one cold) for 8 minutes before the introduction into the VG3600 mass spectrometer. One minute was allowed for equilibration before analysis. 40 Ar and 39 Ar were measured on a Faraday cup with a resistor of 10 11 ohm, while 39 Ar, 38 Ar, 37 Ar, and 36 Ar were analysed using a scintillator and photomultiplier after interaction on a Daly plate. The analytical data are reported in data repositories (Tables DR1 andDR2), and the errors are quoted at the 1σ level. Ages were analysed in two ways: plateaus and inverse isochrones. Plateaus ages were calculated from at least three consecutive steps comprising up to 50% of total 39 Ar K released and overlapping at the 2σ confidence level [START_REF] Fleck | Interpretation of discordant 40 Ar/ 39 Ar agespectra of mesozoic tholeiites from antarctica[END_REF][START_REF] Maluski | Méthode Argon 39 -Argon 40 : Principe et applications aux minéraux des roches terrestres[END_REF]. Alternatively, plateau ages encompassing two steps releasing ~ 30-40 % 39 Ar were also defined as -plateau-like‖ or -pseudo-plateau‖ (e.g., [START_REF] Feng | Chronology of Pleistocene weathering processes, southeast Queensland, Australia[END_REF]Vasconcelos et al., 2013). Ages derived from best-fit inverse isochrones in 36 Ar/ 40 Ar vs. 39 Ar/ 40 Ar correlation diagrams [START_REF] Roddick | The evolution of excess argon in alpine biotites A 40 -Ar 39 Ar analysis[END_REF] were accepted when mean square weighted deviation (MSWD) was less than 2.5 and the 40 Ar/ 36 Ar intercept within 2 from the ( 40 Ar/ 36 Ar) atm value of 298.56 ± 0.31 [START_REF] Lee | A redetermination of the isotopic abundances of atmospheric Ar[END_REF][START_REF] Renne | The isotopic composition of atmospheric argon and 40Ar/39Ar geochronology: time for a change[END_REF], since supergene cryptomelane crystallised at shallow depth, and thus incorporated excess argon, if any, in the atmospheric ratio. Age spectra also show the variations of 40 Ar* and K/Ca (derived from 39 Ar/ 37 Ar) to track the compositional effects on age.

Results and Discussion

Six samples (KPA-2.1, KPA-2.5, KPA-9, KPA-10, KPA-12a and KPA-12b) from the Kappataswamy pit (Fig. 3b) and one (KMK-3) from the Channanghi KMK-E pit (Fig. 3d) were analysed and dated. KPA-9 and KPA-12 show the platy preserved meta-sedimentary structure with Mn pods, while KPA-2, KPA-10 and KMK-3 are rather massive with cavity crystallisations (geode) and/or botryoidal pod concretions.

Mineralogical and geochemical characterisation of cryptomelane

Cryptomelane was identified on polished thin sections including -XRF and SEM imaging (Fig. 4), and characterized by microchemical (EPMA) and XRD analyses of separated grains (Figs. 5 and6, Table 1). Samples KPA-9, KPA-12a and KPA-12b are the richest in cryptomelane, which occurs as cryptocrystalline plasmic domains formed within preserved inherited protore microstructures (Figs. 5a-c). Appropriately, sample KPA-9 exhibits a relict of Si-Mn sedimentary protore with residual smoothed small size quartz grains preserved in acicular cryptocrystalline cryptomelane (Fig. 4d).

The two different generations of cryptomelane (Fig. 5a) have the same stoichiometric composition (Table 1). Separated grain of KPA-9 mainly contains cryptomelane, but also pyrolusite (MnO 2 ) and quartz (Fig. 6). Sample KPA-12a is composed of two cryptomelane generations, the latest one filling the micro porosity developed across the earliest generation and/or the other mineral components such as pyrolusite and Fe oxihydroxides (Fig. 5b). The stoichiometric composition is very close to that of KPA-9

(Table . 1). Sample KPA-12b is very rich in cryptomelane (Fig. 5c) with higher K content than KPA-12a (Table . 1). The two separated grains are dominantly composed of cryptomelane but pyrolusite and goethite (FeOOH) are also present (Fig. 6).

Samples KPA-2, KPA-10 and KMK-3 are composed of successive cryptomelane crystallisations organized in banded overgrowth microstructures associated with other Mn oxides (Figs. 5d-f and6). In the sample KPA-2, the first three bands are pure cryptomelane, the fourth is a cryptocrystalline mixture of nsutite and cryptomelane, and the fifth is dominantly cryptomelane, while pyrolusite crystallized in a cavity (Fig. 5).

The cryptomelane crystals analysed in the different bands have different stoichiometric compositions with K varying from 0.45 to 0.57 atoms (Table 1). Sample KPA-10 consists of a ~ 5 mm size cryptomelane concretion with a transition zone of less than 1 mm separating it from pyrolusite (Fig. 5e). Separated grain of KPA-10 is mainly composed of cryptomelane with 0.5 K atoms (Table 1). Small quantities of pyrolusite, goethite and hematite (Fe 2 O 3 ) are also present (Fig. 6). Sample KMK-3 (Fig. 5f) is a concretion of cryptomelane with less than 1.5 wt.% K (Table 1). The outer rims are composed of lithiophorite (Al,Li)MnO 2 (OH) 2 and nsutite (MnO 2 ) (Fig. 5f), which also occur as minor fractions in the XRD analysis of the separated grain (Fig. 6).

4.2. 40 Ar/ 39 Ar ages of cryptomelane Two kinds of 40 Ar/ 39 Ar age spectra are distinguished from the step heating degassing depending on their shape and interpretation. The first group displays regular 39 Ar release spectra allowing calculation of plateau ages (Fig. 7a-e), which are commonly defined at the 2σ error and accounting at least for 40% of the total 39 Ar released from the sample [START_REF] Vasconcelos | Geochronology of weathering and landscape evolution, Dugald River valley, NW Queensland, Australia[END_REF][START_REF] Feng | Chronology of Pleistocene weathering processes, southeast Queensland, Australia[END_REF][START_REF] Beauvais | Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39 Ar-40 Ar dating of supergene K-Mn oxides[END_REF]. The second group corresponds to disturbed 39 Ar release spectra (Fig. 8), which are more problematic to interpret. The analytical data are provided in the data repository tables DR1 and DR2.

Regular 40 Ar/ 39 Ar age spectra

The 40 Ar/ 39 Ar age spectra from the first group have been obtained from the first generation of cryptomelane crystallized in the meta-sedimentary protore (Figs. 5a and 5c, 7a and 7c), and for cryptomelane crystallisations in banded overgrowth microstructures of botryoids (Figs. 5f and7d) or geodes (Figs. 5d-e, 7b and 7e). In each case the calculated plateau and pseudo-plateau ages agree with statistically meaningful inverse isochron ages estimated in the correlation diagrams (Fig. 7).

However, these spectra present some anomalies at low and high temperatures steps. The age spectrum of KPA-9 (Fig. 7a) shows younger individual ages (steps 10 to 13), which may reflect the degassing of a late cryptomelane crystallisation (Figs. 4b and5a). The sample KPA-10 displays an age spectrum with a clear ascending -stair case‖ pattern at low temperature (steps 1 to 7) with young ages affected by quite a large standard deviation (Fig. 7b). The isochron suggests a moderate excess of 40 Ar, which was interpreted as the result of primary mineral weathering [START_REF] Mcdougall | Geochronology and thermochronology by the 40 Ar/ 39 Ar method[END_REF]Vasconcelos et al., 1994a), which is unlikely in this sample (Figs. 5e and7b, Table 1). The higher amounts of released 40 Ar* and 39 Ar in intermediate temperatures (step 8 to 11), however, allow the best estimation of the cryptomelane crystallisation age [START_REF] Vasconcelos | K-Ar and 40 Ar/ 39 Ar geochronology of weathering processes[END_REF][START_REF] Hautmann | 40 Ar/ 39 Ar dating of central European K-Mn oxides a chronological framework of supergene alteration processes during the Neogene[END_REF][START_REF] Li | Cenozoic continental weathering and its implications for the palaeoclimate: evidence from 40 Ar/ 39 Ar geochronology of supergene K-Mn oxides in Mt Tabor, central Queensland, Australia[END_REF][START_REF] Vasconcelos | Geochronology of weathering and landscape evolution, Dugald River valley, NW Queensland, Australia[END_REF][START_REF] Feng | Chronology of Pleistocene weathering processes, southeast Queensland, Australia[END_REF]. Sample KPA-12b is characterised by well-constrained plateau and isochron ages (Fig. 7c).

The age spectrum of KMK-3 exhibits an ascending -stair case‖ pattern at high temperature (steps 8, 9, 10, 13 and 14) with older ages than the plateau age (Fig. 7d). In such a case, it is usually admitted that radiogenic argon is released by two cryptomelane phases, the older one retaining radiogenic 40 Ar better than the younger one [START_REF] Feng | Chronology of Pleistocene weathering processes, southeast Queensland, Australia[END_REF] resulting in a maximum plateau age estimate and an older integrated age [START_REF] Li | Cenozoic continental weathering and its implications for the palaeoclimate: evidence from 40 Ar/ 39 Ar geochronology of supergene K-Mn oxides in Mt Tabor, central Queensland, Australia[END_REF]. The plateau age of KMK-3 has also quite a large error due to relatively low K content (Table 1 and Fig. 7d), and higher atmospheric correction, the isochron being less sensible and thus more precise.

The age spectrum of KPA-2.1 (Fig. 7e) is more disturbed and does not allow calculation of a standard plateau age [START_REF] Fleck | Interpretation of discordant 40 Ar/ 39 Ar agespectra of mesozoic tholeiites from antarctica[END_REF]. Pseudo-plateaus ages have been, however, previously estimated from similar spectra by considering only two concordant steps including ~ 40% of 39 Ar released (e.g., [START_REF] Feng | Chronology of Pleistocene weathering processes, southeast Queensland, Australia[END_REF]Vasconcelos et al., 2013). Two pseudo-plateaus occur in intermediate temperature steps 4-5 and 7-8, allowing for a concordant age estimate of 35.86 ± 1.10 Ma by combining the four steps at the 2 level (Fig. 7e). We note that the best-fitted isochron supports a similar age (Fig. 7e). The high temperature steps 13 to 16 suggest a hypogene contaminant (Vasconcelos et al., 1994a;[START_REF] Ruffet | A geochronological 40 Ar/ 39 Ar and 87 Rb/ 87 Sr study of K-Mn oxides from the weathering sequence of Azul (Brazil)[END_REF][START_REF] Li | Cenozoic continental weathering and its implications for the palaeoclimate: evidence from 40 Ar/ 39 Ar geochronology of supergene K-Mn oxides in Mt Tabor, central Queensland, Australia[END_REF], which could have aged the intermediate steps (e.g., the step 6). The low K content extraction in high temperature steps could have also suffered from imprecise blank corrections. Therefore, similarly to the sample KMK-3 (Fig. 7d), the best-fitted isochron might reasonably provide a more precise age (Fig. 7e).

Disturbed 40 Ar/ 39 Ar age spectra

The 39 Ar release spectra from the second group are characterized by an upward convex shape (Fig. 8) suggestive of a mixing of two or more supergene cryptomelane phases [START_REF] Vasconcelos | Mechanisms and kinetics of atmospheric, radiogenic, and nucleogenic argon release from cryptomelane during 40 Ar/ 39 Ar analysis[END_REF][START_REF] Ruffet | A geochronological 40 Ar/ 39 Ar and 87 Rb/ 87 Sr study of K-Mn oxides from the weathering sequence of Azul (Brazil)[END_REF][START_REF] Hautmann | 40 Ar/ 39 Ar dating of central European K-Mn oxides a chronological framework of supergene alteration processes during the Neogene[END_REF], which may result from the handpicking of single grains. Phase mixing is obvious in sample KPA-12a showing late cryptomelane crystallisation precipitated into the porosity (≤ 500 m) generated by the dissolution of a first plasmic cryptomelane generation (Fig. 5b). Such a process may result in remobilization of low energy sites [START_REF] Colin | First 40 Ar/ 39 Ar geochronology of lateritic manganiferous pisolites: Implications for the Palaeogene history of a West African landscape[END_REF][START_REF] Beauvais | Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39 Ar-40 Ar dating of supergene K-Mn oxides[END_REF]. Phase mixing often occurs during separation of grains from multiple overgrowth bands [START_REF] Vasconcelos | Geochronology of weathering and landscape evolution, Dugald River valley, NW Queensland, Australia[END_REF]. The grain of KPA-2.5 was tentatively separated from a thin band (~ 500 m) between a late pyrolusite crystallisation in a geodic void and a band composed of a mixing of cryptomelane and nsutite (Fig. 5d). The growth rate of cryptomelane crystals in banded overgrowth structures (botryoids or geodes) is generally very low [START_REF] Vasconcelos | Age and duration of weathering by 40 K-40 Ar and 40 Ar-39 Ar analysis of supergene potassiummanganese oxides[END_REF][START_REF] Dammer | Isotopic dating of supergene manganese oxides from the Groote Eylandt deposit, Northern territory[END_REF][START_REF] Hénocque | 40 Ar/ 39 Ar dating of West African lateritic cryptomelanes[END_REF] and may result in vey small size (≤ 1 m) intimately intergrown cryptomelane phases in a same grain (Vaconcelos et al., 1994b;[START_REF] Vasconcelos | K-Ar and 40 Ar/ 39 Ar geochronology of weathering processes[END_REF].

The pronounced upward convex shape of 39 Ar release spectra precludes plateau age calculation, while correlation diagrams only allow estimation of minimum and maximum ages of the oldest and youngest mixing phases, respectively (Fig. 8a-b). Each step of the 39 Ar release spectra yields a mixing age of the intergrown cryptomelane generations [START_REF] Vasconcelos | K-Ar and 40 Ar/ 39 Ar geochronology of weathering processes[END_REF][START_REF] Vasconcelos | Geochronology of weathering and landscape evolution, Dugald River valley, NW Queensland, Australia[END_REF][START_REF] Feng | Chronology of Pleistocene weathering processes, southeast Queensland, Australia[END_REF], implying that the ages in low and high temperature steps are overestimates of the youngest phase, while the ages in intermediate temperature steps are underestimates of the oldest phase (see also [START_REF] Ruffet | A geochronological 40 Ar/ 39 Ar and 87 Rb/ 87 Sr study of K-Mn oxides from the weathering sequence of Azul (Brazil)[END_REF][START_REF] Beauvais | Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39 Ar-40 Ar dating of supergene K-Mn oxides[END_REF]. This interpretation is mainly based on previous experiments [START_REF] Vasconcelos | Mechanisms and kinetics of atmospheric, radiogenic, and nucleogenic argon release from cryptomelane during 40 Ar/ 39 Ar analysis[END_REF], which showed a maximal degassing of 40 Ar* and 39 Ar between 700°C and 900°C

with a peak at 800°C. In such a range of temperatures, the two isotopic components are release from the mineral structures (Vasconcelos et al., 1994b). Therefore, only maximum and minimum ages might be estimated from upward convex spectra in the temperature range specific of supergene cryptomelane degassing (Fig. 8). For KPA-2.5 (Fig. 8a), a minimum age of ~ 46 Ma (step 7) and a maximum age of ~ 30 Ma (step 10) could be at best estimated for the crystallisation of the earliest and latest cryptomelane phases, respectively. For KPA-12a the maximum and minimum ages of ~ 29 Ma (step 10) and ~ 34 Ma (step 8) are estimated for the youngest and oldest phases, respectively, in agreement with the estimated isochron age limits, although the MSWD value of the second isochron is quite large (Fig. 8b). phases of KPA-2.5 and KPA-12a cannot be precisely determined. However, the two cryptomelane phase of KPA-12a (Fig. 5b) may have crystallised between ~ 29 Ma and ~ 34 Ma (Fig. 8c) that bracket the age of KPA-12b (Fig. 7c) sampled on the same open mine bench. The maximum age (~ 30 Ma) estimated for the youngest cryptomelane of KPA-2.5 is quite realistic compared to the older cryptomelane grain (KPA-2.1) separated from the same sample (Fig. 5d and7e). Despite their limited number, our results usefully document Late Paleogene weathering history of southern India.

Cryptomelane

Potential implications for lateritic weathering and morphogenesis in southern India

The formation of cryptomelane in Mn-ore deposits is enhanced on landsurfaces covered by a humid tropical forest reflecting warm and/or at least wet climatic conditions (Vasconcelos et al., 1994a), like those prevailing for bauxite genesis in tropical cratonic regions [START_REF] Bárdossy | Lateritic bauxites[END_REF]. Such hydroclimatic conditions are required for complete weathering of K-phyllosilicates into kaolinite with release of K + in Mn 2+ -rich acidic solutions (pH ~ 4.5-5) that may ultimately lead to cryptomelane crystallisation under appropriate redox conditions [START_REF] Parc | Estimated solubility products and field of stability for cryptomelane, nsutite, birnessite and lithiophorite based on natural lateritic weathering sequences[END_REF]. Therefore, massive formation of cryptomelane in Mn ore deposits and/or lateritic weathering profiles is a good indicator of humid/warm paleoclimatic conditions (Vasconcelos et al., 1994a;[START_REF] Vasconcelos | Geochronology of weathering and landscape evolution, Dugald River valley, NW Queensland, Australia[END_REF][START_REF] Beauvais | Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39 Ar-40 Ar dating of supergene K-Mn oxides[END_REF]. Under such climatic conditions, wide areas were colonised by humid tropical forests over most of peninsular India [START_REF] Kent | Equatorial convergence of India and early Cenozoic climate trends[END_REF][START_REF] Chatterjee | The longest voyage: Tectonic, magmatic, and paleoclimatic evolution of the Indian plate during its northward flight from Gondwana to Asia[END_REF] enhancing the pervasive development of lateritic weathering profiles with differential sequestration of Al, Fe and Mn.

On the basis of global paleoclimatic reconstructions (e.g., [START_REF] Parrish | Rainfall patterns and the distribution of coals and evaporites in the Mesozoic and Cenozoic[END_REF], [START_REF] Bárdossy | Lateritic bauxites[END_REF] argued for intermittent Cenozoic bauxitic weathering in South India. Such a bauxitic weathering intermittency was not observed in West Africa, where bauxites preserved in current landscapes have been dated from Late Palaeocene to Eocene at the latest [START_REF] Prasad | A review of the early Tertiary bauxite event in South America, Africa and India[END_REF][START_REF] Colin | First 40 Ar/ 39 Ar geochronology of lateritic manganiferous pisolites: Implications for the Palaeogene history of a West African landscape[END_REF][START_REF] Beauvais | Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39 Ar-40 Ar dating of supergene K-Mn oxides[END_REF][START_REF] Beauvais | Modes, tempo, and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. Nevertheless, two stages of lateritic weathering processes have been described in the bauxitic deposits of the Boknur-Navge plateau near Belgaum (Krishna Rao et al., 1989b) (Fig. 1). The first bauxitic stage characterizes the syngenetic lateritic weathering of the Deccan basalts under the action of slightly acidic meteoric waters, whereas the second stage is a late weathering epigenetic process of the early bauxite, under the effect of highly acidic meteoric Si-rich waters (Krishna Rao et al., 1989b). The Boknur-Navge bauxitic plateau is also a remnant of paleolandsurface S2 correlated to the S2 remnant of the SGB [START_REF] Gunnell | The interaction between geological structure and global tectonics in multistoreyed landscape development: a denudation chronology of the South Indian shield[END_REF]. Syngenetic bauxitic weathering on paleolandsurface S2 was likely active in the -Early Tertiary‖ (Paleogene ?) as suggested by paleomagnetic investigations on Fe-oxihydroxides in bauxites and laterites of the nearby Belgaum area [START_REF] Schmidt | Magnetic ages of some Indian laterites[END_REF][START_REF] Kumar | Palaeolatitudes and the age of Indian laterites[END_REF]. Although no precise 40 Ar-39 Ar age correlative of syngenetic bauxitic weathering of the Traps basalts was obtained in this study, lateritic weathering up to ~ 46 Ma may not be precluded (Fig. 8a). Epigenetic lateritic weathering of paleolandsurface S2 (Krishna Rao et al., 1989b) could however be recorded by our 40 [START_REF] Campanile | Sedimentation record in the Konkan Kerala Basin: implications for the evolution of theWestern Ghats and the Western Indian passive margin[END_REF] but the links between inland weathering, erosion and offshore sedimentation are poorly understood. Our 40 Ar/ 39 Ar results might also suggest that the dissection and rejuvenation of the high-elevation lateritic surface of Peninsular India took place in the Neogene as classically inferred [START_REF] Radhakrishna | Neogene uplift and geomorphic rejuvenation of the Indian Peninsula[END_REF]. However, the relative contributions of lithospheric deformation and climate change to geomorphic rejuvenation of Peninsular India in the Neogene remain to be investigated.

Conclusion

We provide time constraints on lateritic weathering having affected Southern India by using 40 Ar/ 39 Ar radiometric dating of supergene cryptomelane in Mn ore deposits underlying a relict paleolandsurface (S2) that once covered the Mysore plateau Ma and was possibly dissected and rejuvenated afterwards, which suggests that most of the current relief of South Indian high-elevation terrains was acquired in the Neogene.

Figures and table caption

  40 Ar/39 Ar ages summary Despite mineral phase mixing of some samples, five significant plateau and isochron ages are estimated between ~ 26 and 36 Ma. The older ages (~ 33.5 and 36 Ma) are obtained either for the first cryptomelane generation in samples showing the preserved meta-sedimentary protore structure (Figs.5a-c, 7a-c) or the first crystallisation band of an overgrowth geodic microstructure (Fig.5d and 7e). The younger ages (~ 26-28 Ma) characterize the late cryptomelane crystallisations of banded overgrowth microstructures (Figs. 5e-f, 7b and 7d). The ages of the different mixing

Figure 1 .

 1 Figure 1. Simplified map of Peninsular India showing first-order geomorphic and

Figure 2 .

 2 Figure 2. Simplified geology of the Sandur Greenstone Belt adapted from Chadwick et

Figure 3 .

 3 Figure 3. North-looking Google Earth views of (a) Kappataswamy and (b) Channanghi

Figure 4 .

 4 Figure 4. Characterisation of procedure used for separating cryptomelane grains in

Figure 5 .

 5 Figure 5. Reflected-light photomicrographs of mineral assemblages of dated samples

Figure 6 .

 6 Figure 6. X-ray diagrams of separated cryptomelane sub-samples. C = cryptomelane; L

Figure 8 .

 8 Figure 8. Disturbed 39 Ar-40 Ar age spectra with K/Ca (black) and Ar* (grey) step curves

  Figure. 6 Click here to download Figure: FIgure. 6(EPSL-D-13-00742).pdf

  Figure. 7 Click here to download Figure: Figure. 7(EPSL-D-13-00742).pdf

  Ar/ 39 Ar ages set.The obtained ages (~26, ~ 28, ~ 33.5 and ~ 36 Ma) attest for intense Late Eocene to Mid-Oligocene lateritic weathering(s) of the highest paleolandsurface (S2) preserved as sparse relics on both the Mysore plateau and the Deccan Traps. These results suggest that the high-elevation landsurface of Peninsular India evolved under a wet and warm climate over a protracted period in the Late Paleogene, essentially during the Oligocene.

	Repeated lateritic weathering between 26 and 36 Ma apparently correlates with low
	clastic sediment fluxes and dominant carbonate sedimentation on the Arabian sea
	margin

  and Deccan Traps. Well-constrained plateau ages and best-fitted inverse isochrons allow defining 40 Ar/ 39 Ar ages at ~ 26, ~ 28, ~ 33.5 and ~ 36 Ma. These results document intense lateritic weathering in Southern India during the latest Eocene and the Oligocene under the effect of warm and wet climate similar to that prevailing in humid forest tropical environments. More specifically, the highest regional lateritic paleolandsurface (S2) of Peninsular India underwent chemical weathering at least between ~ 36 and ~ 26

39 Ar/ 40 Ar 36 Ar/ 40 Ar

	10 10 K/Ca Age (Ma)	1	2	3	36.02 ± 0.87 Ma 61% 39 Ar released Integrated age = 34.30 ± 0.61 Ma 4 5 6 7 8 9 10 11	12	14 13	0 10 2 % Ar*	1	2	3	7	14	13	36.17 ± 0.38 Ma Intercept (40/36): 296 ± 2 MSWD = 0.19 No. used/No. tot: 8/14 39 Ar released: 73.29 % 5 6 9 10 8 4 11 12	0 0.001 0.002 0.003 0.004	36 Ar/ 40 Ar
		0				20	40 % 39 Ar released 60	80		a -KPA-9 0 100			0.05	0.1 39 Ar/ 40 Ar	0.15	0.2
	10 10 K/Ca Age (Ma)	1 3 2 4	5	Integrated age = 26.318 ± 0.36 Ma 28.63 ± 0.5 Ma 70% 39 Ar released 7 8 9 10 11 6	12	13 14	10 2 % Ar* 0			5	2 4	6	3	27.93 ± 0.74 Ma Intercept (40/36): 342 ± 47 MSWD: 1.57 No. used/No. tot: 7/13 39 Ar released: 81.1 % 7 11 8 10 9 12 13 14	36 Ar/ 40 Ar 0.001 0 0.002 0.003 0.004
						20	40 % 39 Ar released 60	80	b -KPA-10 0 100			0.05	0.1 39 Ar/ 40 Ar	0.15	0.2
	10 10 Age (Ma) K/Ca		2		3	4	33.54 ± 0.46 Ma 77% 39 Ar released 5 6 7	8	9 10	0 10 2 % Ar*	1	3	2		4	11	33.54 ± 0.39 Ma Intercept (40/36): 293 ± 4 MSWD = 1.01 No. used/No. tot: 7/11 39 Ar released: 73.13 % 7	0.001 0.002 0.003 0.004
																		5
					Integrated age = 32.43 ± 0.48 Ma									12	10	9	6	8	0
		0				20	40	60	80			100	0			0.05	0.1	0.15	0.2
							% 39 Ar released		c -KPA-12b
										d -KMK-3

39 Ar/ 40 Ar 36 Ar/ 40 Ar

					0.004
					0.003
					0.002
					0.001
					0
	0	0.05	0.1	0.15	0.2

25.90 ± 0.75 Ma

	10					10 2
	10
						0	Intercept (40/36): 305 ± 29
		26.33 ± 1.26 Ma 55% 39 Ar released				MSWD: 0.17 No. used/No. tot: 5/12 39 Ar released: 54.8 %
		Integrated age = 27.52 ± 0.52 Ma
	0	20	40	60	80	100
		%

39 Ar released Age (Ma) K/Ca % Ar*

							1	2
						9	13
								3
	3	4	5	6	7	8	10	14	5	6	7	4
												8
	2								13		10	9
							e -KPA-2.1
							FIG. 7

39 Ar/ 40 Ar 36 Ar/ 40 Ar Age (Ma) % 39 Ar released

	K/Ca 10 10	Concordant Age: 35.86 ± 1.1 Ma 52% 39 Ar released	10 2 % Ar* 0			35.93 ± 0.38 Ma Intercept (40/36): 302 ± 12 MSWD: 0.17 No. used/No. tot: 6/15 39 Ar released: 58.49 %	0.004 0.003 0.002
		ca. 35.94 Ma 12.8% 39 Ar released		ca. 35.84 Ma 39% 39 Ar released							0.001
		Integrated age = 35.84 ± 0.70 Ma						0
	0	20	40	60	80	100	0	0.05	0.15	0.2

Acknowledgements

We are thankful to the PhD school (ED251, AMU, OSU Pytheas), which allocated to NB a grant from the French Ministry of Research and Superior Education.

We also thank IRD and INSU for financial supports (AB). Bujkesh was our guide during a three weeks field trip. S.R. Sridhar, M.P. Deshpande and K. Raman are gratefully thanked to having allowed us the sampling of the Deogiri Mn ore deposits.

Thanks to the assistance of Prakash Babu in the mining open pits. C. Demeurie (Thin section Lab) made the polished thin sections. This work also benefited of technical helps from D. Borschneck (XRD and XRF), Y. Galy (SEM), J-P. Ambrosi (chemical supplies), D. Merlet (EPMA), P. Münch and F. Lecoeur (Argon radiochronology). Jason Price and an anonymous reviewer are also thanked for useful remarks and suggestions.

Data repositories Tables

Click here to download Supplementary material for on-line publication only: DataRepositoryTables(EPSL-D-13-00742).pdf

Mn 60.15 60.97 59.72 60.01 59.38 61.07 59.13 62.95 56.62 57.52 12.38 59.11 60.67 59.10 61.86 59.82 60.37 Al 0.15 0.30 0.12 0.30 0.22 0.33 0.20 0.00 1.41 1.24 0.37 0.20 0.29 0.45 0.11 0.18 0.37 Fe 0.13 0.13 0.12 0.13 0.13 0.13 0.13 0.14 0.58 0.51 0.17 0.05 0.13 0.81 0.14 0.17 0.85 K Ni


Jean-Louis Grimaud
email: jgrimaud@umn.edu

Dominique Chardon

Anicet Beauvais

Very long-term incision dynamics of big rivers

HAL is

Introduction

Quantifying incision dynamics of large drainage basins over geological time scales is relevant to constraining long-term landform evolution processes and the responses of the sediment routing system to lithospheric deformation and climate change. Spatial and temporal variations in incision rates are mostly investigated through the study of longitudinal profiles of rivers as gauges of transient responses of landscapes to perturbations such as base level fall due to uplift. Such responses are modeled using stream-power equations and commonly integrate the formation and headward propagation of knickpoints as a mechanism of river long profile evolution (e.g., [START_REF] Whipple | Dynamics of the stream-power river incision model; implications for height limits of mountain ranges, landscape response timescales, and research needs[END_REF][START_REF] Crosby | Knickpoint initiation and distribution within fluvial networks: 236 waterfalls in the Waipaoa River, North Island, New Zealand[END_REF][START_REF] Jeffery | Quantifying the role of paleoclimate and Andean Plateau uplift on river incision[END_REF]. Based on such models, an inversion procedure of river long profiles has been implemented to estimate surface uplift magnitudes [START_REF] Roberts | Estimating uplift rate histories from river profiles using African examples[END_REF][START_REF] Paul | The African landscape through space and time[END_REF]. But studies of incision processes are mostly based on modeling and/or focus on active tectonic settings over short time scales (10 3 -10 6 yr) and face a crucial lack of regional markers of past base levels that would allow calibrating or testing incision models on Pre-Quaternary geological time scales (10 6 -10 8 yr). This is particularly exemplified in non-orogenic settings where erosion is so slow that it may rarely be detectable by lowtemperature thermochronology methods over the Cenozoic (e.g., [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. Constraining large-scale incision dynamics in such settings is however key to understanding the long-term reactivity of the overwhelming part of the continental surface that supplied most of the world's passive margins and intracratonic sedimentary basins. In these tectonically "stable" contexts, river long profiles display series of major knickzones and are commonly convex downstream [START_REF] Summerfield | Global geomorphology; an introduction to the study of landforms[END_REF][START_REF] Summerfield | Tectonics, geology and long-term landscape development[END_REF][START_REF] Pazzaglia | Bedrock fluvial incision and longitudinal profile development over geologic time scales determined by fluvial terraces[END_REF]. These profiles pose the sizable challenge of knowing how and how fast their geometry has been acquired, how knickzones formed and evolved and what caused their convexities. If post-rift flexure of passive margins has been suggested as a cause of warping of downstream river segments [START_REF] Gilchrist | Differential denudation and flexural isostasy in formation of rifted-margin upwarps[END_REF], the contributions of climatically induced sea level or erosion process changes and lithospheric deformation to the acquisition of non-orogenic river long profiles are still unclear [START_REF] Summerfield | Global geomorphology; an introduction to the study of landforms[END_REF][START_REF] Schumm | River response to baselevel change: Implications for sequence stratigraphy[END_REF][START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF].

For the first time, we reconstruct consecutive long profiles of big rivers over their entire length since the Eocene by using the exceptional geomorphic record of successive incision stages of the West African subregion. Successive paleoprofiles allow constraining evolving rivers shapes with an emphasis on the dynamics of knickzones and are used to calibrate large-scale patterns of incision rates for three time slices over the last 24 Ma. Anomalies in river profiles are also analyzed as proxies of long wavelength lithospheric deformation.

Geomorphic setting and incision chronology

West African drainage is organized mostly around the Guinean rise and the Hoggar and Jos topographic massifs (Figure 1a). The drainage of West Africa is governed by three major river systems: the Senegal, Volta and Niger and includes shorter rivers draining the seaward slope of the Guinean rise (Figure 1a). The river long profiles are characterized by long (> 50 km), low gradient sections (i.e., 0.1 to 2 ‰) separated by knickpoints, identified as short (< 30 km) reaches of steep gradient (> 1%; Figure 1b). Two major regional knickzones made of a series of knickpoints over > 50 km long river segments are recognized on most river profiles at ca. 50-100 m and 200-250 m elevation (Figures 1a and1b).

West Africa is characterized by an exceptional sequence of stepped lateritic paleolandsurfaces (Figure 2), whose remnants are preserved throughout the subregion.

Paleo-river long profiles were constructed by using the remnants of these paleolandsurfaces along the studied rivers (see below). In the following, the characteristics of the paleolandsurfaces are summarized after the works, among others, of [START_REF] Vogt | Aspects de l'évolution morphologique récente de l'ouest africain[END_REF], [START_REF] Eschenbrenner | La séquence de cuirasses et ses différenciations entre Agnibiléfrou et Diébougou (Haute-Volta)[END_REF], [START_REF] Burke | Geomorphology and superficial deposits related to large Quaternary climatic variations in South Western Nigeria[END_REF], [START_REF] Michel | Les bassins des fleuves Sénégal et Gambie : étude géomorphologique[END_REF], [START_REF] Grandin | Aplanissements cuirassés et enrichissement des gisements de manganèse dans quelques régions d'Afrique de l'Ouest[END_REF], [START_REF] Boulangé | Descriptions morphoscopiques, géochimiques et minéralogiques des faciès cuirassés des principaux niveaux géomorphologiques de Côte d'Ivoire[END_REF], [START_REF] Boulangé | La distribution des bauxites sur le craton ouest-africain[END_REF] and the syntheses of [START_REF] Beauvais | Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39 Ar-40 Ar dating of supergene K-Mn oxides[END_REF], [START_REF] Burke | The African Erosion Surface: A Continental-Scale Synthesis of Geomorphology, Tectonics, and Environmental Change over the Past 180 Million Years[END_REF] and [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF].

Relicts of surface 1 are preserved on West African summits and numerous mesas. It is a low-relief surface, which is the end product of enhanced chemical weathering that started in the Late Cretaceous and culminated in the Early Eocene to form bauxites (Figure 2). This bauxite-capped Eocene landscape, also called hereafter the bauxitic surface, represents today's envelope of the West African topography. The following paleolandsurfaces are stepped below the bauxite remnants and mark successive incision stages of the bauxitic surface (Figure 2). Surface 2, the so-called Intermediate surface, characterizes a differentiated remnant landscape coated by a thick ferricrete sealing an in-situ formed weathering mantle (Figure 2). As opposed to surfaces 1 and 2, the following three paleolandsurfaces in the sequence are pediments (glacis in the French literature): the so-called high (surface 3), middle (surface 4) and low glacis (Figure 2). The glacis surfaces commonly carry reworked bedrock and lateritic fragments derived from surfaces 1 and 2 and were, together with their cover, weathered to various degrees. Because the low glacis did not develop uniformly throughout West Africa or is commonly connected to local base levels, it was not used for paleo-long profiles reconstructions. Consequently, "surface 5" was considered as the current river levels (Figure 2). Each paleolandsurface (1 to 5) is taken as the end product of an incision period (I to V, respectively; Figure 2). Our study focuses on incision periods III to V and allows visualizing the amount of dissection of the Eocene topography during erosion period II.

Age constraints on the shaping, weathering and abandonment of the successive paleosurfaces in the West African sequence were obtained from radiometric 39 Ar-40 Ar dating of supergene K-Mn oxides (i.e., cryptomelane) in the weathering profiles of each paleosurface from the Tambao type locality (Figure 1a) in Burkina Faso (synthesis in [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. Surface and core samples were taken at various elevations and depths spanning the altitudinal range of the paleolandsurfaces [START_REF] Hénocque | 40 Ar/ 39 Ar dating of West African lateritic cryptomelanes[END_REF][START_REF] Colin | First 40 Ar/ 39 Ar geochronology of lateritic manganiferous pisolites: Implications for the Palaeogene history of a West African landscape[END_REF][START_REF] Beauvais | Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39 Ar-40 Ar dating of supergene K-Mn oxides[END_REF]. Paleosurfaces 1 to 4 yielded 39 Ar-40 Ar age groups of 59 -45, 29 -24, 18 -11 and 7-6 Ma, respectively, the Oligocene and Mid-Miocene weathering periods being also recorded by Ar-Ar dates of supergene jarosite and alunite in Southern Mali weathering profiles [START_REF] Vasconcelos | 40 Ar/ 39 Ar analysis of supergene jarosite and alunite: Implications to the paleoweathering history of the western USA and West Africa[END_REF]. The lower limits of the radiometric age groups date the stabilization of the weathering front established by the end of chemical weathering periods that ultimately led to duricrusting of each paleosurface in connection with their local base level [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. Therefore, ca. 45, 24, 11 and 6 Ma are interpreted as the maximum age of abandonment of each paleosurface by river incision due to climate switches from humid to seasonally dry [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]; Figure 2a). In other words, the ages of ca. 45, 24, 11 and 6 Ma are the terminal ages of paleosurfaces 1, 2, 3 and 4 respectively (Figure 2a) bracketing the incision periods separating those paleosurfaces.

Method

We construct rivers paleo-long profiles based on the identification and mapping of remnant of paleolandsurfaces S1 to S4 along the rivers courses over approximately 20 km-wide corridors. This protocol is motivated by the fact that surfaces 2 and younger systematically dip towards those rivers (e.g., Figure 3). Our database of remnant lateritic paleosurfaces is that of [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF], which was extended to encompass the selected river corridors at about 400 stations. At each station, base level elevations corresponding to each paleolandsurface were obtained by projecting or extrapolating paleosurface remnant elevation(s) onto a vertical straight line above the current river trace (Figure 3b). Surface 1-bauxitic remnants were horizontally projected (Figure 3), sometime up to 50 km from the river. Inselberg tops with higher elevation than that of surface 2 relicts provide a minimum elevation for the bauxitic surface and were also projected horizontally. Surface 2 relicts commonly encompass altitudinal ranges on the slopes of bauxitic mesas (e.g., Figure 3b). The maximal and minimal elevations of surface 2 relict(s) were horizontally projected to reflect the amplitude of its local relief at each station (Figure 3b). Glacis form as concave upstream valley sides of 0.2 to 10° in slope connected to the local base level [START_REF] Grandin | Aplanissements cuirassés et enrichissement des gisements de manganèse dans quelques régions d'Afrique de l'Ouest[END_REF][START_REF] Strudley | Emergence of pediments, tors, and piedmont junctions from a bedrock weathering-regolith thickness feedback[END_REF][START_REF] Strudley | Sensitivity analysis of pediment development through numerical simulation and selected geospatial query[END_REF]. Thanks to their duricrusted cover, remnants of glacis surfaces 3 and 4 have their original shape well preserved (e.g., Figure 3b). We therefore extrapolated the downslope shape of the glacis by picking points from their surface along a down-dip section towards the river (Figure 3b) and exponentially fitting those points using formula:

ݖ ൌ ݖ ܪ exp ିሺ௫/ఙሻ (1)
where x is the horizontal distance to the river, z is the elevation. z 0 is the minimum elevation of the considered geomorphic surface. σ is a measure of the exponential reduction of elevation, i.e. a measure of inverse concavity, and H is a constant set by the elevation amplitude of pediments surface. Base levels 3 and 4 elevation is calculated at the river. Elevation error is estimated using the derivative:

dݖ ൌ dݖ ௦௧ dݖ exp ିሺ௫/ఙ ሻ dܣ ܪ ൬ ௫ ఙ మ ൰ exp ିሺ௫/ఙ ሻ dߪ (2)
where dz srtm is the absolute error of SRTM DEM (5.6 m in Africa ; [START_REF] Farr | The Shuttle Radar Topography Mission[END_REF]. σ index represents the inverse of concavity of the exponential fit and H op and σ op are the optimum values given by the interpolation. Distribution of σ op is quasi-normal and centered on 300 (Figure 4), attesting to the repeatability of the glacis shape over the study area and justifying the use of the fit formula (equation 1). At most stations, the error induced by the extrapolation of glacis base levels is smaller than the data point size (Figure 3b and5). This, together with the little skewed distribution of σ op (Figure 4)

shows that the extrapolated paleobase levels are robust. At stations where glacis remnants are not large enough to be confidently extrapolated, the lowest elevation of the remnant the closest to the river is horizontally projected.

Given the long projection distance of some bauxite relicts and the fact that the bauxitic surface has a regional relief [START_REF] Chardon | Planation, bauxites and epeirogeny: One or two paleosurfaces on the West African margin?[END_REF], the envelope topography of the Eocene landscape was drawn passing through surface 1 elevation data points (Figure 5). The curve joining the elevation of the lowest remnants of surface 2 has been drawn

to represent the maximal elevation of rivers paleo-long profile 2 (Figure 5). Long profiles 3 and 4 were drawn from extrapolated surfaces 3 and 4 remnants or their projected lowest remnants. Modern profiles 5 were automatically extracted from the SRTM digital elevation model (Figure 5). Incision rates III, IV and V (representing time intervals 24-11, 11-6 and 6-0 Ma, respectively) were plotted along river profiles (Figure 5). Incision rates IV and V were obtained using base levels 3, 4 and 5 at each station by integrating the elevation error resulting from the extrapolation (Figure 5). Maximum values of incision rate III are derived from the difference between paleoprofiles 2 and 3.

Detection and qualification of steps on paleoprofiles depend on data resolution.

Analysis of the current and past long profiles allows defining knickzones as < 90 kmlong river segments of more than 1% slope between two longer adjoining linear segments of 0.1 to 2‰ slopes. Knickpoints, which are sharper by definition, are only rarely detectable on the reconstructed profiles. Although the river profiles have undergone long-wavelength distortion (section 5.2), their differential elevations constitute a meaningful estimate of incision. Likewise, comparing the relative geometry of short (<100 km) successive paleo-river segments is appropriate and relevant.

Results

We separate the studied river systems into four groups according to their location in the West African drainage (Figure 1). Group A includes the rivers draining the northwestern slope of the Guinean rise (Senegal and Gambia). Group B includes the short rivers draining the southern slope of the Guinean rise (Kakrima and Mano). Group C comprises the long southern rivers (Bandama, Comoé and Volta) and group D corresponds to the Niger drainage. The Niger River is divided in two segments (High and Low Niger) on both sides of the Niger inland delta (Figure 1). To facilitate the description of results, we show one representative example for each group (i.e. the Senegal, Kakrima, Volta and Niger main stream channels for groups A, B, C and D respectively) on Figure 5. The entire set of reconstructed profiles can be found in the data repositories.

1. Overall profiles shape and early drainage reorganization

Past and modern long profiles of group A have a nearly straight, very low gradient slope across the Senegalo-Mauritanian basin (Figure 5a). Upstream parts of the profiles are steeper, generally stepped, with an increased slope (up to 0.7 ‰) for rivers draining the Western Guinean rise. Group B comprises the shortest rivers with paleoand modern long profiles that have the steepest mean slopes (> 2 ‰) and are the most stepped (Figure 5b). The paleo and modern profiles of the Volta (group C) and Low Niger (group D) reveal a two-stage evolution. Indeed, the envelope of surface 1 underlines a divide whose crest lies between 200 and 750 km inland (Figures 5c and5d). Groups C and D rivers have incised this divide mostly during incision period II.

This, together with the overall evolution of group A and B rivers indicates drainage reorganization during period II resulting from the dissection of an Eocene marginal upwarp that survived or was rejuvenated in the Guinean rise [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. This rearrangement involved capture of pre-45 Ma internal drainages by at least the Volta and Low Niger. More generally, depressions in the envelope of surface 1 (e.g., Figure 5c) are indicative of transverse paleovalleys before drainage rearrangement and/or second-order captures during rearrangement (see also data repositories). The fact that the post-rearrangement, 24 Ma old surface 2 systematically dips towards the current main drains warrants calibration of post 24 Ma incision.

Group C profiles 2 to 5 show a nearly parallel evolution. They are made of straight, low slope (< 0.1 -0.3 ‰) segments hundreds of kilometers long, which are separated by knickzones, notably those highlighted on Figure 1. Profiles 2 to 5 converge and merge towards the coast downstream of the lower regional knickzone (Figure 5b).

The High Niger River shows strongly concave paleo and modern profiles (Figure 5e), whereas post-surface 1 Low Niger profiles have low and slightly convex slopes diverging downstream (Figure 5d). Paleo long profiles reveal that knickzones on modern profiles have persisted since at least profile 3 (Figure 5).

Analysis of geological maps shows that those recurrent knickzones coincide with lithological contrasts expressed in the topography (Figure 5). Two main contrasts are distinguished, those related to steep contacts between greenstone belts and their adjoining granite-gneiss terrains and those formed across tabular sandstones. In the first case, the knickzone horizontally encompasses the along-river width of a greenstone belt or the lower limit of the knickzone coincides with the contact between a wide greenstone belt and granitogneisses. In the second case, the most common configuration is the lower limit of the knickzone coinciding with the basal unconformity of the sandstones.

2. Northwestern rivers (group A)

On group A rivers, post-surface 2 incision generally increases from the extremities towards the central part of the profiles resulting in an increasing concavity of the profiles through time. This is particularly the case for incision IV, and, to a lesser extent, incision III (Figure 5a). In the Guinean rise, profiles 3 and 4 show mainly convex segments, with local high incision rates (up to 20 m/my) for period IV (Figure 5a). Overall, incision is rather uniform during period III and becomes more localized afterwards though accompanied by a decrease of the incision rates. Incision V is locally enhanced (up to 10 m/my) by the formation of new knickzones on profile 5 (Figure 5a).

3. Short southern rivers (group B)

Long (> 100 km) segments of paleoprofiles 2, 3 and 4 are convex (Figure 5b).

The stepped character of the profiles is accentuated through time by the apparition of shorter convex segments, particularly during incisions IV and V. Post surface 2 incisions tend to increase from the extremities towards the central part of the profiles, with locally enhanced incision during incisions IV and V that led to the formation of the modern knickzones (Figure 5b). This attests to a heterogeneous incision pattern during these periods (2 to 25 m/my), as opposed to rather distributed incision during period III (about 10 m/my in average). Incision gradients are located in between knickzones, the highest peaks being located mainly upstream. Elevation difference between surface 1 and the modern profiles implies a maximum incision of the slope of the Guinean rise of the order of 400 m (but equivalent to only 9 m/my over the last 45 Ma) (Figure 5b).

Long southern rivers (group C)

The overall geometry of river long profiles has been acquired since the establishment of profiles 2. The two regional knickzones systematically coincide with lithological contrasts and occurred at least since profile 3 (Figure 5c). A mean cumulated incision of less than 50 m is derived from parallel and close profiles 3 to 5.

Rivers show a rather parallel evolutionary pattern of profiles 2 and 3 along the central river segment comprised between the two regional knickzones (Figure 5c). This is indicative of a rather distributed incision III of ca. 7 m/my. By contrast, lower gradient of profile 2 diverges from profile 3 downstream of the upper regional knickzone and the two profiles tend to merge towards the coast (Figure 5c). Overall, eastern rivers of group C such as the Volta become increasingly stepped with time (Figure 5c).

5. Niger River (group D)

Although the recurrence of the two regional knickzones is not readily documented along the Low Niger, the overall pattern of profiles 2 to 5 is roughly comparable to that of the Volta, with a diversion of the profiles downstream of the inland delta (e.g., comparison of Figures 5c and5d). This attests to an increase in Neogene incision(s) downstream the inland delta. The overall long wavelength convexity of profiles 3 and 4 is documented between 200 and 1200 km (Figure 5d).

Nearly parallel profiles 2 to 4 converge within the Iullemmeden basin. A single knickzone may be documented on profile 3 (ca. 450 km) and none may be detected on profile 4 (Figure 5d). But this may be due to the low resolution of our dataset downstream of the Iullemmeden basin. A two-step knickzone occurs on the modern profile between 800 and 1000 km (Figure 5d). Incision period V created high local relief compared to that of period IV, particularly downstream of the lower regional knickzone (e.g., Figure 5d). Incision V becomes limited approaching the inland delta and the coast (< 5 m/my) and increases towards the central part of the profiles, with the highest incision rates downstream of the two-step knickzone of profile 5 (ca. 10 m/my during periods II, IV and V; Figure 5d).

Paleo long river profiles of the High Niger (Figure 5e) are comparable to those of the Senegal (group A, e.g., Figure 5a). All the profiles converge downstream and merge entering the inland delta, which is preceded by a knickzone 400 km upstream (Figure 5e). Profiles 2 to 5 may be divided in two segments i.e., a lower, ca. 1000 km long and low gradient segment of parallel profiles and an upper, very steep and short segment within the Guinean rise (Figure 5e). Most of the incision along the High Niger took place during period II (locally more than 15 m/my). Incision rates III to V are remarkably low and uniform along the lower segment (< 6 m/my) and may locally attain 15 m/my in the Guinean rise (Figure 5e). The overall tendency of the upper Niger is an increase in the concavity of river profiles.

Interpretation and discussion

Incision dynamics and river profiles evolution

The present analysis shows that the overall shape of river profiles was acquired at least since ~11 Ma, and most probably ~24 Ma (Figure 6). The overall concavity of the successive river long profiles increased over the Neogene as a result of the pinning down of their two extremities (Figure 6). This attests to the horizontal stability of the divides and efficient adaptation to relative sea level change at the coast over the Neogene. This very long-term divide stability, which has also been documented for the Southeastern margin of Australia (e.g., Young, 1989) contrasts with the view of drainage networks as "permanently" reorganizing in comparable passive margin contexts (Willett et al., 2014). We suggest that following drainage rearrangement (section 4.1), local relief and regional topography of West Africa acquired by the end of erosion period II had attained a threshold preventing divide migration after the Oligocene. Neogene divide stability over West Africa implies long-term stable geometry of drainage basins, which has major implications for understanding source-tosink systems [START_REF] Grimaud | Quantifying denudation of the West African passive-transform margin: implications for Cenozoic erosion budget of cratons and source-to-sink systems[END_REF]. This stability is reflected by low incision rates through space and time with more than 75 % of the data below 10 m/my and medians and means between 3 and 15 m/my (Figure 7). The last incision period would have recorded the smallest incision rates. Highest incision rates correspond to the steepest rivers draining the Guinean rise (groups A and B; Figure 7; upper Niger; Figure 5e), suggesting a control of potentially old, large-scale reliefs on incision heterogeneities [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. To summarize, the present work provides constraints on boundary conditions and incision rates for large-scale surface process models in "stable" continental environments over geological time scales.

Our results reveal the long lasting maintenance of lithogenic knickzones along most rivers for at least ~11 my and most probably 24 my (Figure 6). Similar recurrent knickzones were documented in the comparable morphotectonic context of Eastern Australia [START_REF] Bishop | Stream profile change and longterm landscape evolution. Early Miocene and modern rivers of the East Australian highland crest, central New South Wales, Australia[END_REF][START_REF] Van Der Beek | Cenozoic river profile development in the Upper Lachlan catchment (SE Australia) as a test of quantitative fluvial incision models[END_REF]. The concavity and stepped character of West African river long profiles have been amplified since the Early Neogene by very slow, non-uniform and unsteady incision in between stationary knickzones (Figures 5 and6). Type-evolutionary patterns of rivers segments comprised between stationary knickzones are shown on Figure 8. Parallel evolution by strict uniform incision (Figure 8a) is characteristic of the High Niger River since 24 Ma (Figures 5e and6d). Such an evolution is also locally observed on segments of the Low Senegal, Volta and Low Niger (Figure 5a, 5c and 5d). In all cases, strict uniform incision is systematically of very low rate (< 5 m/my). Strict downstream rejuvenation (Figure 8b) is not recorded by any of the studied rivers (Figure 5). Instead, knickzones may be created by differential incision (Figure 8c), as exemplified by the Senegal and

Low Niger (at 1600-1800 km in Figure 5a and 400-500 km and 800-1000 km in Figure 5d). However, most knickzones in West Africa have been amplified or smoothed instead of having been created since 24 Ma (Figure 5). This resulted in slope lowering of most river segments, reflecting finite incision gradients along those segments (Figure 8d). This situation is best exemplified along the Senegal (at 1000-1200 km), Kakrima (at 100-300 km), Volta (600-800 km) and Low Niger (at 400-800 km and 1700-2000 km) (Figure 5).

The above analysis indicates that knickzones do not and did not separate downstream rejuvenated from upstream relict portions of the landscape, which is implicit in models of river long profile evolution by pure knickzone retreat, particularly that originally popularized by [START_REF] King | On the age of African land-surfaces[END_REF] in the African context. River segments bounded by recurrent lithogenic knickzones evolved apparently independently from each other by preferential slope decrease or uniform incision in between nodes made by pinned knickzones of evolving amplitude. This indicates that large non-orogenic drainage basins evolve through very slow (< 10 m/my) and non-uniform incision patterns that do not respond in a simple way to relative sea level change, to the exception of their lowermost reaches i.e., group C and D rivers during period IV (Figures 5 and6, see below). In other words, repercussion of a major relative sea level drop is not likely to be recorded at a large distance from the coast in such drainages.

This warrants caution in using knickzones' height as a gauge of discrete uplift and figuring river profile response to uplift by headward propagation of such knickzones using stream-power river incision models. Accordingly, uplift histories retrieved from river profiles inversion procedures based on such models could be challenged, particularly those obtained for reliefs of the deep interior of the African continent (see Roberts and[START_REF] Roberts | Estimating uplift rate histories from river profiles using African examples[END_REF][START_REF] Paul | The African landscape through space and time[END_REF]. As opposed to uplift, our results would argue for a dominant control of climate-driven erosion processes on incision dynamics of big non-orogenic drainages over geological time scales [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF].

Anomalies in river profiles and lithospheric deformation

Long wavelength (300 -500 km long) upward convex segments of modern river around the end of the Pliocene (Figure 9). These changes in river dynamics may result from slope decrease of those NE flowing drains. Both the upward convex river segments and internal deltas are interpreted to respond to southwestward growth of the Hoggar swell, which would be comparable to the lateral propagation of an active anticline [START_REF] Keller | Geomorphic criteria to determine the direction of lateral propagation of reverse faulting and folding[END_REF], the direction and sense of propagation being determined by the northeastward displacement of the African plate with respect to the Hoggar plume and/or to asthenospheric flow under the plate (Figure 9). Swell growth would induce slope decrease on the drains flowing towards the propagating swell to create internal deltas, whereas drains flowing across the propagating swell undergo enhanced incision to keep pace with growing uplift. The area of active uplift (Figure 9) mimics the domain of positive dynamic topography expected from mantle circulation models of [START_REF] Forte | Joint seismic-geodynamic-mineral physical modelling of African geodynamics: A reconciliation of deep-mantle convection with surface geophysical constraints[END_REF], reinforcing the causal link suggested here between mantle dynamics and river profiles evolution.

Long wavelength (500 -1000 km) convexities of the lower part of river profiles 2 to 4 are documented for drainages C and D (Figures 6c and6d). For group C, convexity seems to increase with time (from profile 3 to profile 2) and has maximum amplitude of ca. 100 m and a wavelength of ca. 500 km. This convexity is replaced on the current profile by the lower regional knick zone and a concave lowermost portion of the rivers (Figure 6c). Warping is also documented on the lower Niger particularly on nearly parallel profiles 2 and 3, with a ca. 200 m amplitude and 1000 km wavelength convexity (Figure 6d). The convexity amplitude of profile 4 is much lower, indicating that profiles 2 and 3 underwent warping mainly before 6 Ma. The last erosion period (i.e., post 6 Ma) is also characterized by the creation of the major (double) regional knickzone and the acquisition of a concave lowermost profile (Figure 6d). Warping and post-6 Ma rejuvenation by knickzone creation of the lowermost portion of group C and D rivers are interpreted to result from flexural uplift of the continental margin (e.g., [START_REF] Gilchrist | Differential denudation and flexural isostasy in formation of rifted-margin upwarps[END_REF]. Flexure was enhanced by (i) the narrowness of the two transform segments of the margin in this area (Figure 9), (ii) high and increasing Neogene clastic sedimentation rates at its foot [START_REF] Séranne | Early Oligocene stratigraphic turnover on the West Africa continental margin: a signature of the Tertiary greenhouse-to-icehouse transition?[END_REF][START_REF] Jermannaud | Plio-Pleistocene Sequence Stratigraphic architecture of the eastern Niger delta: a record of eustasy and aridification of Africa[END_REF] and (iii) onshore denudation [START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. The 500-1000 km wavelength of the flexure is too large to reflect only warping across a steep margin and is typical of asthenospheric-scale processes (e.g., [START_REF] Mckenzie | Estimates of the effective elastic thickness of the continental lithosphere from Bouguer and free air gravity anomalies[END_REF]. We suggest that flexural uplift of the margin combined with the growth of the Hoggar swell (this work) and/or with the uplift of the continent (e.g., [START_REF] Burke | The African Plate[END_REF] to produce such a wavelength.

Open questions

Creation or amplification of major knickzones achieves post 6 Ma rejuvenation of the warped lower portion of rivers flowing to the transform portions of the West African margin (Figure 6). The contribution, if any, of the Quaternary eustatic cycles to the creation of those knickzones may not be evaluated and whether these knickzones are actively migrating and/or evolve by knickpoint retreat should be the focus of future research. Knickpoint retreat is inferred to have been instrumental in the Plio-Pleistocene rejuvenation of the Appalachian landscape in a passive margin context comparable to that of West Africa [START_REF] Gallen | Miocene rejuvenation of topographic relief in the Southern Appalachians[END_REF]. Upstream West African drainage did not evolve by large wave(s) of headward migrating knickzones but our results are based on very long term measurements that may have averaged discrete periods of fast incision [START_REF] Gardner | Geomorphic and tectonic process rates: Effects of measured time interval[END_REF]. Hence, they do not allow testing whether knickpoint retreat process actually contributed to shaping individual river segments bounded by recurrent knickzones.

Likewise, documented distortion of portions of past river profiles poses the issue of whether rivers maintained their concavity against upwarping of abandoned base level markers or increased their overall concavity from more convex states. In the case of the lower reaches of group C and D rivers having undergone marginal flexural uplift, the first situation would clearly apply. The second situation could apply to the rest of the drainage with the noticeable exception of the northern slope of the Guinean rise (group A rivers; Figures 5a and6a). But in absence of independent geological constraints on the potential relative uplift of the rise during the Cenozoic, it is not possible to evaluate its impact on the acquisition of long river profiles of West Africa.

Conclusions

The spatial analysis of dated and regionally correlated incision markers allows Mission DEM to 5 km resolution. Thick grey lines labeled 1 and 2 refer to the lower and upper regional knickzones, respectively. Capital letters refer to the studied groups of rivers. A -northwestern rivers (long profiles shown in Figure 5a); B -short southern rivers (Figure 5b); C -long southern rivers (Figure 5c); D -Niger River (Figure 5d). (b)

Modern long profiles of selected rivers. The lower and upper regional knickzones' elevation ranges are indicated by dark and light grey bands numbered 1 and 2, respectively.

Figure 2. Lateritic paleolandsurface sequence and incision chronology of West Africa

(synthesized after [START_REF] Michel | Les bassins des fleuves Sénégal et Gambie : étude géomorphologique[END_REF][START_REF] Beauvais | Chemical and physical erosion rhythms of the West African Cenozoic morphogenesis: The 39 Ar-40 Ar dating of supergene K-Mn oxides[END_REF][START_REF] Beauvais | Modes, tempo and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]. Figure 1a). Spiked segments of modern profiles correspond to > 300 km long upwardconvex segments (mapped in Figure 9). Elevation ranges of the lower and upper regional knickzones are shown in grey and numbered 1 and 2, respectively.

  profiles define a large domain comprising the Upper Volta and upper part of the Low Niger drainages (Figures 6c, 6d and 9). Our field observations and digital topographic data further indicate that this area coincides with portions of the Niger drainage that have incised their lowermost terrace. The convex river segments do not exceed 50 m in amplitude and are interpreted to reflect active transient adjustment by enhanced incision. The dynamics of two other major segments of the Niger and Volta drainage have been modified in the recent geological past. Indeed, the lower portion of the High Niger and the Gondo flew to NE before becoming alluvial plains (i.e., internal deltas)

  calibrating incision dynamics of big West African rivers over the last 45 my and quantifying the evolution of their long profiles since 24 Ma. Incision rates are distributed spatially and remained below 10 m/my and mostly below 5 m/my. Spatial stability of both the outlets and divides of the rivers since 24 Ma imposed increasing overall concavity of their long profiles, which was achieved by contrasted adjustment of river segments separated by persistent lithogenic knickzones. River segments evolve by preferential slope decrease or uniform incision in between nodes defined by the pinned knickzones of evolving amplitude. Therefore big non-orogenic rivers do not respond simply to relative sea level change, which is unlikely to be recorded far inland in the form of purely retreating knickzones. Accordingly, knickzone height or position on such rivers are not obvious gauges of base level fall and caution is required in retrieving regional uplift histories of deep continental interiors from river long profiles inversion procedures. Rather uniform and slow incision of large non-orogenic drainage basins allows distortion and anomalies in river profiles to record subtle long wavelength surface uplift due to hot spot swell growth and flexure of continental margins, emphasizing their potential as gauges of dynamic topography. Willett, S.D., McCoy, S.W., Perron, T.J., Goren, L., Chen C-Y., 2014. Dynamic reorganization of river basins. Science 343, 1248765, doi:10.1126/science.1248765. Young, R.W., 1989. Crustal constraints on the evolution of the continental divide of eastern Australia. Geology 17, 528-530.

Figure captions Figure 1 .

 captions1 Figure captions

Figure 3 .

 3 Figure 3. Example of a typical field station used to estimate local paleo-base levels

Figure 4 .

 4 Figure 4. Distribution of the optimum inverse concavity σ op in the exponential fits of

Figure 5 .

 5 Figure 5. Reconstructed paleo-and modern river long profiles and corresponding

Figure 6 .

 6 Figure 6. Synthetic representation of the successive West African paleo-long river

Figure 7 .

 7 Figure 7. Statistical summary (box and whiskers plots) of incision rates over West

Figure 8 .

 8 Figure 8. Type evolutions of long river profile segments comprised between two

Figure 9 .

 9 Figure 9. Distribution of the main anomalies in the modern river long profiles of West

Acknowledgments 20

This work was funded by the ANR TopoAfrica (ANR-08-BLAN-572 0247-02), the CNRS and WAXI. We thank S. Carretier, S. Bonnet, P. van der Beek and G. Hérail for fruitful discussions and Ph. Dussouillez for support. We are indebted to F. Pazzaglia for very constructive reviews. The manuscript benefited from comments by K. Burke and anonymous reviewers. We acknowledge AMIRA International and the industry sponsors, including AusAid and the ARC Linkage Project LP110100667, for their support of the WAXI project (P934A) as well as the Geological Surveys/Departments of Mines in West Africa as sponsors in kind of WAXI.

AREA OF ACTIVE UPLIFT