Nicolas J Bonnet
email: nicolas.bonnet.geo@gmail.com

Anicet Beauvais
email: beauvais@cerege.fr

Nicolas Olivier Arnaud

Dominique Chardon

Mudlappa J Jayananda

Cenozoic lateritic weathering and erosion history of Peninsular India from 40 Ar/ 39 Ar dating of supergene K-Mn oxides

Keywords: 40 Ar/ 39 Ar geochronology, Supergene Mn-oxides, Mn-ore deposits, Lateritic weathering, Cenozoic, India Cryptomelane, Hollandite, Goethite, Gibbsite Cryptomelane, Nsutite Cryptomelane, Pyrolusite, Hematite Cryptomelane, Pyrolusite, Goethite, Hematite Cryptomelane, Goethite, Hematite Cryptomelane, Pyrolusite Cryptomelane, Nsutite, Lithiophorite Cryptomelane, Goethite, Birnessite Cryptomelane, Pyrolusite, Lithiophorite TRI

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

Chemical rock weathering that results in the accumulation of metals such as Al, Fe or Mn, and relative depletion of silica and base elements, produce lateritic regoliths covering shields in the tropical belt. Weathering processes are intense under wet and warm climate that characterizes most tropical forest's soil environments [START_REF] Pedro | Distribution des principaux types d'altération chimiques à la surface de globe[END_REF][START_REF] Ollier | Deep Weathering, Groundwater and Climate[END_REF][START_REF] Nahon | Introduction to the petrology of soils and chemical weathering[END_REF][START_REF] Tardy | Petrology of laterites and tropical soils[END_REF]. Metals are mostly retained in duricrusts capping thick weathering profiles, which are, in turn, partly preserved from mechanical erosion. Therefore, old lateritic weathering profiles several tens of meters thick may be preserved on paleolandsurface remnants for several millions years [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] Valeton | Saprolite-bauxite facies of ferralitic duricrust on paleosurfaces off former Pangaea[END_REF]; see also [START_REF] Beauvais | Modes, tempo, and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF].

Since Deccan Traps extrusion ~ 65 Ma ago, the surface of the Indian peninsula was shaped by combined or alternating chemical weathering and mechanical erosion processes that resulted in composite landscapes made of stepped lateritic paleolandsurfaces remnants of various generations occurring on either side of the Western Ghats Escarpment (WGE) (e.g., [START_REF] Widdowson | Tertiary paleosurfaces of the SW Deccan, Western India: implication for passive margin uplift[END_REF][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]. This escarpment was carved both into Deccan Traps and Precambrian basement rocks and separates a western lowland from a dissected hinterland also known as the Mysore plateau [START_REF] Radhakrishna | Neogene uplift and geomorphic rejunevation of the Indian Peninsula[END_REF][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][START_REF] Widdowson | Tertiary palaeosurfaces and lateritization of the coastal lowlands of western peninsula India[END_REF]. Time constraints on the formation of South Indian laterites are still poorly documented [START_REF] Schmidt | Magnetic ages of some Indian laterites[END_REF]Krishna Rao et al., 1989a) although Late Paleogene 40 Ar/ 39 Ar ages of supergene K-Mn oxides have been recently obtained from the Sandur Mn-ore deposit on the highland [START_REF] Bonnet | First 40 Ar/ 39 Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India[END_REF]. Most south Indian Mn-ore deposits result from supergene weathering of late Archean supracrustal rocks, either on the highland in the Sandur and Shimoga areas [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], or in the lowland at the foot of the WGE [START_REF] Dessai | An appraisal of the manganese ore deposits of Goa, India[END_REF]Fig. 1). In both contexts, Mn-ore deposits are hosted by lateritic weathering profiles underlying remnants of several generations of paleosurfaces, which are preserved at variable elevations (Fig. 2). All these Mn-ore deposits contain K-rich Mn oxides such as cryptomelane [K x Mn 8-x IV Mn x III O 16 ], which is datable by 40 Ar/ 39 Ar geochronology.

Absolute dating of K-Mn oxides generally documents periods of intense lateritic weathering controlled by specific paleoclimatic conditions and as such may be used to reconstruct and quantify the long-term morphoclimatic evolution of tropical shield surfaces [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]Vasconcelos, 1999b;[START_REF] Vasconcelos | Geochronology of weathering and landscape evolution, Dugald River valley, NW Queensland, Australia[END_REF]. Here we report on 40 Ar/ 39 Ar geochronological data series obtained on Peninsular India that bracket three Cenozoic weathering periods and constrain the tempo of long-term South Indian morphogenesis. Our results also document divergent erosion and weathering histories across the WGE suggesting installation of a dual climatic regime on either side of the escarpment after the Eocene warming period.

Material and methods

Geological and geomorphological setting

South Indian laterites have been discriminated in two groups: highland laterites on the plateau and lowland laterites in the western coastal plain (e.g., [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]. Remnants of three main lateritic paleolandsurfaces are reported on the highland and the remnants of a lateritic pediment have been described in the lowland [START_REF] Widdowson | Tertiary paleosurfaces of the SW Deccan, Western India: implication for passive margin uplift[END_REF][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][START_REF] Widdowson | Tertiary palaeosurfaces and lateritization of the coastal lowlands of western peninsula India[END_REF]. Our field observations indicate that each highland relict paleolandsurface has a specific regolith covers. The relicts of the first, highest and oldest landsurface are capped by a weathering profile topped by an Al-Fe duricrust (Figs.

2). This duricrust is commonly economic-grade bauxite such as those preserved on the Deccan Traps [START_REF] Valeton | Saprolite-bauxite facies of ferralitic duricrust on paleosurfaces off former Pangaea[END_REF].

The bauxites are also preserved on the highest topographic massifs of the southern part of the Peninsula (e.g., Londa, Bababudan, Shevaroy, Nilgiris and Palni hills; Figs. 1 and 2). Remnants of a second younger landsurface are found downslope Al-Fe duricrust relicts. They are capped by weathering profiles topped by a Fe duricrust or by a Fe-Mn duricrust if an underlying Mn-protore is present (Fig. 2). The third and last lateritic paleolandsurface on the highland is a pediment, which is capped by reworked debris of lateritic duricrusts (Fig. 2). The debris may be cemented to form a ferricrete, or a Fe-Mn duricrust, if Mn ore debris are also included in the pediment sedimentary cover above a Mn-protore (see below). Following weathering and abandonment of the pediment, the lateritic covers of the highland have been essentially stripped and incised by rivers, leaving only sparse relicts of the three paleolandsurfaces [START_REF] Radhakrishna | Neogene uplift and geomorphic rejunevation of the Indian Peninsula[END_REF]. The lowland pediment is underlain by a weathering profile that locally hosts bauxite and Mn ore pockets and is capped by a ferricrete, which may be a Fe-Mn duricrust above Mnprotores [START_REF] Beauvais | Very long-term stability of passive margin escarpment constrained by 40 Ar/ 39 Ar dating of K-Mn oxides[END_REF].

Four Mn-ore deposits were visited and sampled on the highland and three in the lowland (Fig. 1). Two actively operated Mn-ore deposits (Kappataswamy and Channanghi KMK-E) were sampled in the highland Sandur massif (Figs. 1 and2b; stars 1 and 2; Table 1). Lateritic Mn-ore deposits developed upon Late Archean protores younger than 2651 ± 18 Ma, as constrained by SHRIMP U-Pb dating of their stratigraphically underlying volcanics [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 protores include phyllites, argillites, arenites and Fe-Mn stromatolitic carbonates that yielded a 2475 ± 65 Ma Pb-Pb isochron age [START_REF] Chadwick | The Sandur schist belt and its adjacent plutonic rocks implications for Late Archaean crustal evolution in Karnataka[END_REF][START_REF] Russell | Whole-rock Pb/Pb isotopic ages of Late Archaean limestones, Karnataka, India[END_REF]. The open pits are weathering profiles excavated into a remnant of the second paleolandsurface capped by a Fe-Mn duricrust, at 1012-1015 m elevation (Figs. 2b;3a and 3b). A remnant of the first paleolandsurface dominates the deposit at ca. 1100 m elevation (Fig. 2b).

Highland samples were also collected closer to the WGE, in two abandoned mines of the Shimoga area (Triveni and Kumsi Mn ore open pits; Figs. 1 and 2a, stars 3 and 4, respectively, see also . In these pits, the protore is a metavolcanic rock belonging to the Shimoga greenstone belt (Rb-Sr isochron age of 2520 ± 62 Ma;

Bhaskar [START_REF] Bhaskar Rao | Rb-Sr ages of late Archean metavolcanics and granites, Dharwar craton, South India and evidence for Early Proterozoic thermotectonic event (s)[END_REF]. The protore also comprises Mn-phyllites, which weathered during the Cenozoic as constrained by palynological analyses [START_REF] Rao | Mode of occurrence and origin of manganese ores of Shimoga district, Karnataka[END_REF]. The Triveni Mn ore samples (Fig. 2a) have been collected at altitudes between 850 and 875 m (Table 1) in the Fe-Mn duricrust-capped weathering profile of the second paleolandsurface (Fig. 2a). The Kumsi pit exposes a 21 m thick lateritic profile of a relict of the pediment forming the third highland paleolandsurface (~ 710 m altitude; Figs. 2a and3d). The sampled profile consists in a ~ 6 m thick dismantled lateritic duricrust hosting a ~ 2 m thick sedimentary layer mixing decimetric ferruginous, manganiferous clasts and infracentimetric oolites (Fig. 3d) upon ~ 15 m of massive Mn-ore.

The weathering profile of the lowland pediment developed upon greenstones similar to those of the Shimoga belt [START_REF] Dessai | An appraisal of the manganese ore deposits of Goa, India[END_REF]Fig. 1) and were constrained by paleomagnetism as "late Paleogene to Neogene" in age by [START_REF] Schmidt | Magnetic ages of some Indian laterites[END_REF]. The elevation of the pediment varies from 200 m at the foot of the WGE to 50 m close to the coastline (Fig. 2c). We collected samples in four pits excavated in the pediment capping ferricrete (Caurem, star 5, Naveli 1 and Naveli 2, star 6, and Cudnem, star 7; Figs. 1 and 2c; see also Table. 1). Caurem, Naveli 1&2 and Cudnem pits have been excavated

where the ferricrete lies at 100 m, 140 m and 50 m elevation, respectively. In those pits, the weathering profile is 25 to 100 m thick and the ferricrete is made of cemented reworked lateritic clasts (Figs. 3e-f-g;[START_REF] Babu | Laterite as an unconformity plane in the evolution of the South Indian peninsulaa synthesis[END_REF][START_REF] Bonnet | Dynamique long-terme d'une marge continentale divergente (Les Ghâts Occidentaux de l'Inde péninsulaire)[END_REF].

Samples preparation, characterization and analysis

Potassium-rich end-member of coronadite group (i.e., cryptomelane) from the hollandite supergroup [START_REF] Bhaskar Rao | Rb-Sr ages of late Archean metavolcanics and granites, Dharwar craton, South India and evidence for Early Proterozoic thermotectonic event (s)[END_REF], is a common supergene Mn-oxide of the lateritic Mn-ore deposits of southern India (e.g., [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] which can be used for 40 Ar/ 39 Ar dating. The crystallographic system of this mineral allows argon gas retentiveness in a tunnel-type crystal lattice, which is characterized by a double chain of MnO 6 octahedra and K + cations in the large tunnel lacuna to insure the electronic neutrality of the lattice [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]. Tunnel oxides retained efficiently Ar and K and remain close in supergene environmental conditions (Vasconcelos, 1999b;[START_REF] Vasconcelos | Direct dating of weathering phenomena by 40 Ar/ 39 Ar and K-Ar analysis of supergene K-Mn oxides[END_REF], leading to meaningful crystallization age of the minerals. However K-rich Mn-oxides are usually mixed with other oxides, such as others Mn-oxides (e.g., pyrolusite, lithiophorite) and Fe-oxides (hematite, goethite). Therefore, careful observations and extraction techniques are required to ensure meaningful dating.

The method implemented to characterize and separate the K-rich Mn-oxides grains from field samples is summarized in figure 4. Field samples were cut with a circular saw (1.5 mm breadth) to get a section allowing accurate observations. A 200-from each sawed fragment. Polished thin sections have been studied using reflected light microscopy (Fig. 5a-g). We also used elemental cartography by X-ray microfluorescence (µ-XRF) with a XGT7000 Horiba Jobin Yvon producing a high-intensity beam with a 100 µm spot size, Rh X-ray tube, accelerating voltage of 30 kV and current of 1mA. Micro-XRF elemental maps of Fe, K and Mn are stacked together on a single image using ImageJ software by assigning a distinct color to each elemental map (Figs.

4 and 6). The resulting images are helpful to locate K-rich phases on the slab. Electron Probe Micro-chemical Analysis (EPMA) of minerals using a CAMECA SX-100 electron microprobe equipped with five wavelength-dispersive X-ray spectrometers (WDS) provided the precise micro chemical composition of K-Mn oxides.

Grains were then separated from the slabs using a micro-drill under a large magnifier (Fig. 4). Some of them were observed with a scanning electron microscope (SEM, Fig. 5h), other were crushed to produce a powder sieved at 64 µm, which was analyzed by XRD using a Panalytical X'Pert Pro MPD with a Co Kα X-ray source (λ = 1.79 Å) operating at 40kV and 40 mA. The remaining grains were ultrasonically cleaned in absolute ethanol, conditioned in aluminum foil packets and placed into a irradiation vessel along with 40 Ar/ 39 Ar dating standard Taylor Creek Rhyolite sanidine 2 (TCRs-2) monitor, dated at 28.608 ± 0.033 Ma [START_REF] Renne | Joint determination of 40 K decay constants and 40 Ar*/ 40 K for the Fish Canyon sanidine standard, and improved accuracy for 40 Ar/ 39 Ar geochronology[END_REF]. Irradiation took place in the TRIGA Mark-II reactor of Pavia University (Italy) during 50 hours. Prior experiments have shown none or very little 39 Ar recoil from those grains.

Isotopic analyses were then performed on irradiated K-rich Mn-oxide separates using either step heating degassing under a CO 2 laser probe coupled with an Argus VI multicollection mass spectrometer (with 4 faradays for masses 40 Ar-37 Ar and ion counting on 36 Ar) or a step-wise heating procedure in a double vacuum Staudacher-type furnace coupled with a VG3600 mass spectrometer using peak jumping and Faraday/Daly analyzer as described by [START_REF] Arnaud | Evidence for Mesozoic shear along the western Kunlun and Altyn-Tagh fault, northern Tibet (China)[END_REF]. Mass discrimination of machines and blank levels are followed daily. Isotopic ratios were corrected for irradiation interferences and air contamination using a mean air 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 40 Ar/ 39 Ar geochronology: Time for a change?[END_REF].

Ages were statistically analyzed in three ways: 39 Ar release spectra, inverse isochrons (Table . 2) and age's frequency or probability plots. Age spectra detail the homogeneity of argon released and age stability throughout the degassing process, with the prior assumption of atmospheric correction for inherited argon. When apparent ages are integrated over continuous steps overlapping at the 2σ level and releasing at least 70% 39 Ar K , the derived plateau age is statistically robust and meaningful (e.g., [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]. However, when plateau is derived from less than 70% of 39 Ar K released (e.g., Vasconcelos, 1999a;[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] Colin | First 40 Ar/ 39 Ar geochronology of lateritic manganiferous pisolites: Implications for the Palaeogene history of a West African landscape[END_REF], the critical value of 50% 39 Ar K released is accepted to calculate a "plateau age" (e.g., [START_REF] Li | Neogene weathering and supergene manganese enrichment in subtropical South China: An 40 Ar/ 39 Ar approach and paleoclimatic significance[END_REF][START_REF] Feng | Chronology of Pleistocene weathering processes, southeast Queensland, Australia[END_REF][START_REF] Vasconcelos | 40 Ar/ 39 Ar and (U-Th)/He -4 He/ 3 He geochronology of landscape evolution and channel iron deposit genesis at Lynn Peak, Western Australia[END_REF]Riffel et al., 2014;[START_REF] Bonnet | First 40 Ar/ 39 Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India[END_REF][START_REF] Deng | 40 Ar/ 39 Ar dating of supergene Mnoxides from the Zunyi Mn deposit, Guizhou Plateau, SW China: implications for chemical weathering and paleoclimatic evolution since the late Miocene[END_REF] provided that it is integrated over three or more continuous steps whose ages overlap at the 2σ 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 In: Méthodes de Datation Par Les Phénomènes Nucléaires Naturels -Applications[END_REF][START_REF] Mcdougall | Geochronology and thermochronology by the 40 Ar/ 39 Ar method[END_REF]. When more than two apparent steps overlaps at the 2 σ level but integrate only 40 to 50% of the 39 Ar K released, "pseudo plateau" are defined and considered as acceptable and meaningful [START_REF] Vasconcelos | 40 Ar/ 39 Ar and (U-Th)/He -4 He/ 3 He geochronology of landscape evolution and channel iron deposit genesis at Lynn Peak, Western Australia[END_REF]Riffel et al., 2014).

When those criteria are not satisfied but barely missed (ages do not strictly overlap at the 2 σ level) a "forced plateau" integrating more than two consecutive reasonably flat steps is calculated with a weighed mean (weighed by the error and the 39 Ar released in each step) (see [START_REF] Feng | Chronology of Pleistocene weathering processes, southeast Queensland, Australia[END_REF][START_REF] Vasconcelos | 40 Ar/ 39 Ar and (U-Th)/He -4 He/ 3 He geochronology of landscape evolution and channel iron deposit genesis at Lynn Peak, Western Australia[END_REF]Riffel et al., 2014). The 36 Ar/ 40 Ar vs. 39 Ar/ 40 Ar correlation diagrams (also called inverse isochrons) are also used to derive the best-fitted inverse isochron [START_REF] Roddick | The evolution of excess argon in alpine biotites -A 40 Ar-39 Ar analysis[END_REF] allowing to estimate a statistically robust age. The inverse isochron approach also is particularly useful to detect different contamination from various excess or inherited argon reservoirs, which may result from atmospheric argon incorporated in the less retentive site of the grain, or excess 40 Ar released from older K-bearing minerals. For some authors [START_REF] De Putter | The age of supergene manganese deposits in Katanga and its implications for the Neogene evolution of the African Great Lakes Region[END_REF], best inverse isochrons exclude the first heating steps, which are usually dominated by trapped atmospheric Ar in the less retentive crystalline sites. These ages are derived from best-fitted inverse isochrons that should have mean square weighted deviation (MSWD) as close as possible to 1 with regard to the distribution of points and their absolute error, but this value is not a limitation to derive an acceptable inverse isochron. Regressions are classically accepted as significant when MSWD is less than 2.5 [START_REF] Roddick | The evolution of excess argon in alpine biotites -A 40 Ar-39 Ar analysis[END_REF], possibly around 1.

There is therefore a complicated trade-off between the number of points used, the MSWD, and the use of the most radiogenic 40 Ar* rich steps.

Results and interpretations

Petrological and geochemical characterization

Descriptions of samples and K-Mn oxide grains are compiled in the table 1. Two main petrographic forms of manganese ore can be distinguished. Float (very porous) or platty Mn-ores show the original banding of the siliceous phyllitic protore (Figs. 3a and 5a; [START_REF] Mishra | Exploration planing for Sandur manganese ores[END_REF]; see also [START_REF] Bonnet | First 40 Ar/ 39 Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India[END_REF]. Mn-ore can also be massive podiform and botryoidal filling micro porosities (Fig. 5b and Fig. 6a-b) and larger cavities, in which geodes formed by successive overgrowth of colloidal microstructures (Fig. 5e-f and Fig. 6a) becoming massive cryptocrystalline when cavities are totally filled (Fig. 5b-c). Colloidal overgrowth around nucleus (e.g. iron oxide, clasts) can also form massive nodules where the initial protore structure is totally erased (Fig. 5d-e).

Cryptomelane EPMA data are plotted in a ternary diagram (Fig. 7; see also the data repository DR1). Cryptomelane from Shimoga and Goa is enriched in aluminum (Fig. 7). The differences in alumina result either from contrasted parent rocks composition (e.g., carbonates without alumina vs. aluminous metavolcanic phyllites) or possible intergrowth with lithiophorite (Fig. 5e-g and Fig. 6a) or even gibbsite (Fig. 5h).

These differences may also indicate a better maturation of Sandur's cryptomelane rich ores (Fig. 6), which are devoid of aluminous impurities (e.g., [START_REF] Beauvais | Pétrologie du gisement latéritique manganésifère d'Azul (Brésil)[END_REF].

Deciphering the 40 Ar/ 39 Ar age spectra

The different types of 39 Ar release spectra are shown in figures 8 and 9. Many samples yielded a regular flat age spectrum (Fig. 8a). As explained above, plateau ages are validated when at least three consecutive steps comprising up to 50% of total 39 Ar K released overlap at the 2σ confidence level.

Degasing spectra may show evidence of negligible amount of 39 Ar, low % 40 Ar* and large amount of atmospheric 40 Ar in the low energy degassing steps that generally increases the 2σ error of the first apparent ages (Fig. 8a). Another issue is the rejuvenated ages frequently observed at low temperature steps (Fig. 8b), which can be the result of a loss of 40 Ar* from less retentive and poorly crystallized sites (see (Vasconcelos, 1999b). But most of the time the apparent ages progressively reach a constant value at higher temperature steps allowing definition of a plateau, which is also well supported by an inverse isochron (see Fig. 8b). Sixteen samples show such a degasing spectrum integrating up to 90% of the total extracted signal, and excluding only the very first steps with ages lower than the plateau. These well-defined plateaus are used to calculate the absolute ages of the K-Mn oxides. Generally, both the ages estimated from spectra and inverse isochrons are equivalent (Fig. 8a-b, Table . 2).

Therefore, we choose to present our ages only with 39 Ar release spectra that clearly show a plateau, and with both spectra and inverse isochrons when no clear plateau is identified. Plateau ages (70% 39 Ar K released) and best-fitted inverse isochrons are considered as first order ages and noted (A) in figures 10, 11 and 12.

Some age spectra do not allow calculating a standard age plateau according to the definition of [START_REF] Fleck | Interpretation of discordant 40 Ar/ 39 Ar agespectra of mesozoic tholeiites from antarctica[END_REF], but only a "forced-plateau" defined from 39 Ar release spectra encompassing four steps releasing ~ 70% (Fig. 8c). In other release spectra, a probable authigenic component ages the last steps, which result in an older intermediate step with several little pseudo plateaus (10 to 30 % 39 Ar each) of similar ages (Fig. 8d).

In such cases, an estimated concordant pseudo-plateau age is often supported by a valid inverse isochron age (Table . 2; see also [START_REF] Bonnet | First 40 Ar/ 39 Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India[END_REF]. This can be a valid alternative when the total amount of integrated % 39 Ar k is higher than 40% of the total signal. But such a concordant pseudo-plateau age should also be certified by a welldefined plateau age in another sample from the same weathering profile. Plateau ages integrating less than 70% 39 Ar K released, "Forced plateau", "Pseudo plateau" and "concordant pseudo plateau" ages are considered as second-order ages noted (B) in figures 10, 11 and 12. Nevertheless, these ages are often validated by best-fitted inverse isochrons (Table 2) and are meaningful of first order weathering events.

When the age spectra present an obviously convex hump shape (Fig. 9a) the youngest and oldest apparent ages may result from a mixing between gases released from two phases (e.g., [START_REF] Ruffet | A geochronological 40 Ar 39 Ar and 87 Rb 81 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][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][START_REF] De Putter | The age of supergene manganese deposits in Katanga and its implications for the Neogene evolution of the African Great Lakes Region[END_REF]. Possible argon loss from less retentive intercrystalline site could also affect these degasing patterns (Fig. 9a). The statistical analysis of both the age spectrum and the inverse isochron may help to estimate minimum and maximum ages of mixed phases (e.g., [START_REF] Bonnet | First 40 Ar/ 39 Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India[END_REF]. In the case of sample TRI-3a the only possible inverse isochron yields a maximum estimate of the youngest phase at ~ 11 Ma (Fig. 9a). However, this age is not validated by a well-defined plateau age from another sample of the same deposit (Fig. 11), and therefore noted (C). Most spectra with a hump-shape also show a plateau integrating at least 50% of 39 Ar K in the intermediate energy levels but the derived age is a minimum estimate of the oldest phase (e.g., KUM-4oo, Fig. 11; see also NAV-3c and NAV-3b

Fig. 12). The plateaus included in these hump-shape spectra should be considered more carefully and derived ages are noted (B), but acceptable inverse isochrons are derived (Fig. 12, Table 2). Other hump shape spectra exhibit young apparent ages forming "pseudo plateaus" in low and high energy levels with concordant ages, which bracket an older plateau age in the intermediate energy level (e.g., CAU-1a, CAU-1c, CAU-3a, CAU-2, Fig. 12, Fig. 12). The correlation diagrams for these samples point to possible inverse isochrons estimating maximum and minimum age of the youngest and oldest mixed-phases respectively (Fig. 12, Table 2). For example, the oldest phases with minimum ages ~ 22 to ~ 24 Ma (Fig. 12) are probably contaminated by youngest phases with maximum age ~ 19 to 20 Ma. These ages are noted as (B) and meaningful when derived from best-fitted inverse isochrons and/or plateau integrating at least 50% 39 Ar K .

On another hand, the age difference between the youngest and oldest phases is larger (e.g., KPA-2.5, Fig. 10; see also CAU-1b, Fig. 12), ages estimations are too speculative and only the minimum age of the oldest phase can be estimated and noted with a (C) in figures 10, 11 and 12 (see also Table 2). These ages are not meaningful.

Other samples show "saddle shape" spectra (Figs. 9b-c) with a progressive decrease of the apparent ages in the intermediate energy steps and a very large increase of the ages at highest energy steps. This complex type of spectra may result from significant 39 Ar loss by recoil and/or contamination [START_REF] Turner | Possible effects of 39 Ar recoil in 40 Ar-39 Ar dating[END_REF]Vasconcelos, 1999b;[START_REF] Vasconcelos | Geochronology of weathering and landscape evolution, Dugald River valley, NW Queensland, Australia[END_REF]. The high energy steps suggest a hypogene contaminant [START_REF] Vasconcelos | Direct dating of weathering phenomena by 40 Ar/ 39 Ar and K-Ar analysis of supergene K-Mn oxides[END_REF][START_REF] Ruffet | A geochronological 40 Ar 39 Ar and 87 Rb 81 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][START_REF] Bonnet | First 40 Ar/ 39 Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India[END_REF], which can also age the other steps (e.g., Fig. 9c). Most of these spectra display overestimated ages, which are not reliable [START_REF] Vasconcelos | Geochronology of weathering and landscape evolution, Dugald River valley, NW Queensland, Australia[END_REF]. Noted that no inverse isochron is derivable, the correlation diagram showing at best a mixing of inherited contaminant and supergene phase (Fig. 9c).

The Table 2 synthetize the results and shows 24 well defined plateaus ages (> 70% 39 Ar K ) with one best-fitted inverse isochron age noted (A). In addition 18 ages noted (B) are also derived either from "plateaus" and "forced plateaus" (at least 50% 39 Ar K ) or from "concordant pseudo plateaus" ages and best-fitted inverse isochrons. All these ages are reliable and geologically significant and have been plotted against altitude, and combined with individual age probability diagram (Fig. 13a) that enhances most probable weathering age peaks (Vasconcelos, 1999b).

40 Ar/ 39 Ar geochronology of lateritic weathering in South India

Here we present 29 newly analyzed K-Mn oxide grains that are interpreted together with grains studied by [START_REF] Bonnet | First 40 Ar/ 39 Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India[END_REF] and [START_REF] Beauvais | Very long-term stability of passive margin escarpment constrained by 40 Ar/ 39 Ar dating of K-Mn oxides[END_REF] (Table 2 and Fig. 13a). The total analysis of 46 Mn-oxide grains provides 40 age spectra with significant geological meaning as "plateaus", "forced-plateaus", "pseudo plateau" "concordant pseudo-plateaus" or "hump shape". We present our results in three figures grouping age spectra obtained from K-Mn oxide grains of the Sandur (Fig. 10), Shimoga (Fig. 11) and Goa deposits (Fig. 12). All the ages are also presented in Table 2 including the best inverse isochron age estimates (See also data repository tables DR2 and DR3).

3.3.1. 40 Ar/ 39 Ar ages from highland Sandur Mn ore deposit Among all the samples dated in the Sandur Mn ore deposit, 18 samples show plateau ages noted (A). The ages range from ~ 26 Ma (KMK-3) to ~ 53 Ma (KPA-8) (Fig. 10 and Table 2). Two age groups are distinguished: ~ 53 -50 Ma and ~ 37 -26 Ma (Fig. 10). Two spectra have a "hump shape" resulting from mixed supergene phases [START_REF] Bonnet | First 40 Ar/ 39 Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India[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] Ruffet | A geochronological 40 Ar 39 Ar and 87 Rb 81 Sr study of K-Mn oxides from the weathering sequence of Azul, Brazil[END_REF][START_REF] Vasconcelos | Mechanisms and kinetics of atmospheric, radiogenic, and nucleogenic argon release from cryptomelane during 40 Ar 39 Ar analysis[END_REF]. The spectrum of sample KPA-2.5 shows a maximum apparent age ~ 46 Ma old, which is a minimum estimate of the oldest phase in the mixing. In sample KPA-12a minimum age of ~ 34 Ma noted (C) for the oldest phase is coherent with plateau ages (A) between ~ 37 Ma (KMK-3b) and ~ 32 Ma (KPA-11) (Fig. 10), which are attributed to the second weathering period. Nine grains display plateau-ages between ~ 26 and ~ 30 Ma. The high frequency of these ages is linked to the analysis of several aliquots of samples KPA-10 (4 grains) and KPA-12a (6 grains), which yielded reproducible plateaus, and also allowed quantifying the growth rate for these cryptocrystalline colloidal structures. Grains KPA-12a1 and KPA-12a5 are separated by 12.2 ± 0.1 mm (Figs. 6 and 10) and the estimated growth rate of the massive cryptocrystalline colloidal structure is 5.7 ± 3 mm. Ma -1 . A mean rate of 5.0 ± 4 mm.

Ma -1 in botryoidal overgrowth microstructure was also estimated for sample KPA-10 between grains KPA-10a and KPA-10c (Figs. 6 and 10). These rates are comparable with earlier growth rate estimates of 6.4 ± 1.2 mm. Ma -1 [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] or 1 to 5 mm. Ma -1 [START_REF] Hénocque | 40 Ar/ 39 Ar dating of West African lateritic cryptomelanes[END_REF].

3.3.2. 40 Ar/ 39 Ar ages from highland Shimoga Mn ore deposits

The obtained ages are mostly comprised between ~ 30 and 24 Ma (Fig. 11).

However, one grain (KUM-4oo) displays a spectrum with a singular hump shape, which results from a mixing of two phases (Figs. 11a-b). The "forced plateau" age at ~ 39 Ma including 58% of the total amount of 39 Ar released is the minimum estimate of the oldest phase and noted (B), which is also supported by the sole possible inverse isochron with a MSWD value less than 2.5 (Fig. 11b). The correlation diagram does not allow estimating an acceptable age for the youngest phase in this mixed grain (Table 2).

The considered grain was picked up from a lateritic clast reworked in the lateritic pediment topping the Kumsi Mn-ore deposit (site 4 on Fig. 2a). The detrital nature of this sample and its high porosity are attested by reflected light microscopy (Fig. 5d).

This suggests provenance from a lateritic Mn-duricrust previously exposed on a higher landsurface, which was dissected and eroded allowing lateritic clasts transport and deposition at lower elevations on the pediment in which the Kumsi pit is excavated.

Therefore, the Kumsi pediment must be younger than 39 Ma (minimum age of the oldest phase in KUM-4oo) but older than 26 Ma (most common age of the youngest phase in the same profile, Fig. 11a and Table 2). Sample TRI-3b (Fig. 11) shows a middle spectrum portion with ages ranging from ~ 43 to ~ 48 Ma accounting for 70% of the total 39 Ar released. An inherited phase seems to release gas especially at high energy step heating, but when this phase started to degas at lower energy, the apparent ages at intermediate heating steps are artificially aged that increases the apparent age up to ~ 48

Ma. The best age estimation of the supergene phase in this grain is most probably ~ 43

Ma but must be considered only as a maximum age estimate.

The release spectrum of samples KUM-3f and KUM-2 show weakly aged steps at the beginning of the analysis, which possibly resulted from minor 39 Ar recoil [START_REF] Turner | Possible effects of 39 Ar recoil in 40 Ar-39 Ar dating[END_REF]. However, the disturbance is very low and meaningful age at ~ 30 Ma (KUM-3f) and ~ 23 Ma (KUM-2) both by a pseudo plateau (Fig. 11a) and wellconstrained inverse isochrons (Fig. 11b; Table . 2). Several aliquots of samples TRI-3 and KUM-3a (Fig. 11) yield consistent plateau ages of 25.5 Ma and ~ 26-27.5 Ma, respectively.

3.3.3. 40 Ar/ 39 Ar ages from lowland Goa Mn ore deposits

The cryptomelane ages from samples of the lowland Mn-ore deposits are distributed in two main age groups. The first group comprises two ages at ~ 47 and 45

Ma, obtained for two aliquots of sample NAV-3, which are probably minimum estimates. The spectrum of NAV-3c allows integrating 60% of the total amount of 39 Ar released overlapping at the 2σ error [START_REF] Fleck | Interpretation of discordant 40 Ar/ 39 Ar agespectra of mesozoic tholeiites from antarctica[END_REF] that yields a plateau age (A) of 47.0 ± 0.6 Ma, which is supported by the inverse isochron age (Table 2 and Fig. 12b).

Further evidence of early weathering around ~ 45 Ma (minimum age of NAV-3b) in the lowland is supported by the spectrum and a possible inverse isochron (Fig. 12). The spectrum of sample CAU-1b (Fig. 12) also shows a maximal apparent age at ~ 45 Ma, which is a minimum age and should be noted (C). The old ages are systematically obtained from samples collected in the deepest part of the weathering profile. Younger ages between ~ 19 Ma and ~ 24 Ma (group 2) and discrete ages at ~ 8.7 Ma and 2.5 Ma are also identified (Fig. 12). All these ages are supported by inverse isochron ages (see Table . 2).

Sample CAU-2 shows a "hump shape" explained by the probable mixing of two phases, with a minimum age of ~ 24.5 Ma for the oldest and a maximum estimate of ~ 19 Ma for the youngest. The oldest estimate cannot be validated by inverse isochrons (Fig. 12b, and Table 2). In three similar cases, secondary pseudo-plateaus at ~ 20 Ma can be detected on either side of a plateau or a "forced plateau" (e.g., CAU-2). For example, sample CAU-3a (Fig. 12) shows a plateau at ~ 24 Ma, which is a minimum estimate flanked by small concordant pseudo-plateaus at ~ 19-20 Ma in low-and highenergy steps accounting together for 32% 39 Ar degassed. Sample aliquots CAU-1a and CAU-1c (Fig. 12) also show similar "hump shape" type spectra with a plateau age flanked by concordant pseudo-plateau ages suggesting mixing of two phases of different ages [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 81 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] but close enough to be grouped in the same weathering period ca. 19 to ca. 24 Ma.

However, for these last three samples the difference between the oldest and the youngest phase (20 Ma) is small enough to allow deriving three minimum plateaus ages noted (B) at ~ 24 Ma associated with three concordant (B) maximum ages at ~ 20 Ma (Fig. 12). The youngest ages ~ 20 Ma are rather well supported by reliable inverse isochron ages (Table . 2, Fig. 12b).

Interpretation of the 40 Ar/ 39 Ar ages

The first-and second order (A and B) 40 Ar/ 39 Ar ages allow defining three main lateritic weathering periods (W1, W2a and W2b) in South India (Fig. 13a). The first weathering period W1 (ca. 53 to ca. 45 Ma) corresponds to intense chemical rock weathering upon both the highland and the lowland. Intense (bauxitic) weathering of the first paleolandsurface is interpreted as correlative to this weathering period [START_REF] Bonnet | First 40 Ar/ 39 Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India[END_REF]Krishna Rao et al., 1989b) 11b). This weathering period also led to maturation of the previously formed bauxitic weathering profiles (e.g., Krishna Rao et al., 1989b). After 26 Ma, the Sandur remnants of the two oldest paleolandsurfaces do not seem to be longer subject to weathering, while the western highland landscape (Shimoga) still weathered until ca. 23 Ma at the time the lowland weathered too. Two later minor Late Neogene weathering pulses are also recorded in the lowland (9 and 2.5 Ma).

The weathering periods would sign installation of wet and warm climate leading to thick soil development under rainforest conditions over the peninsula, whereas the time intervals between these weathering periods may be interpreted as episodes of subdued chemical weathering and correlative higher mechanical erosion and landscape dissections (e.g., [START_REF] Beauvais | Modes, tempo, and spatial variability of Cenozoic cratonic denudation: The West African example[END_REF]

Morphoclimatic implications of 40 Ar/ 39 Ar dating

Periods of intense weathering have affected the highland landscapes during the early Eocene, and from late Eocene to late Oligocene. By contrast, the lowland weathered mostly during the early Eocene and the early Miocene (Fig. 13a). The successive Eocene to early Miocene weathering periods documented by the present study coincided with the northward migration of India across the humid equatorial belt (Fig. 13b). Early Eocene bauxitic weathering also coincided with the Eocene climatic optimum (Fig. 13c), at a time of relatively high atmospheric CO 2 [START_REF] Pearson | Atmospheric carbon dioxide concentrations over the past 60 million years[END_REF], North Atlantic rifting (62 to 55 Ma) and the subduction of Tethysian carbonates [START_REF] Van Der Voo | Tethyan subducted slabs under India[END_REF]. as old as 47 Ma under the lowland pediment argue for Early Eocene installation and stabilization of the Western Ghats Escarpment with a lateritic pediment on its piedmont [START_REF] Beauvais | Very long-term stability of passive margin escarpment constrained by 40 Ar/ 39 Ar dating of K-Mn oxides[END_REF]. The old ages (ca. 53 to ca. 45 Ma) also document synchronous lateritic weathering in the lowland and the highland [START_REF] Beauvais | Very long-term stability of passive margin escarpment constrained by 40 Ar/ 39 Ar dating of K-Mn oxides[END_REF], when India drifting slowed down at the onset of the collision with Asia ~ 50 Ma ago (e.g., [START_REF] Zhu | KPA-12a4 Massive cryptocrystalline domain KPA-12a5 Massive cryptocrystalline domain KPA-12b Massive cryptocrystalline domain KPA-12b2 Massive cryptocrystalline domain KPA-13 -96 15° 0' 1" 76° 32' 37" Platy Mn-ore developed from siliceous protore Cryptomelane, Birnessite, Hematite[END_REF][START_REF] Rowley | Palaeo-altimetry of the late Eocene to Miocene Lunpola basin, central Tibet[END_REF]. In the meantime, India also entered a latitudinal range where water precipitation was higher than evaporation (Fig. 13b) propitious to the development of equatorial and/or tropical warm and humid forest [START_REF] Kent | Equatorial convergence of India and early Cenozoic climate trends[END_REF]; see also [START_REF] Patriat | India-Eurasia collision chronology has implications for crustal shortening and driving mechanism of plates[END_REF][START_REF] Manabe | CO 2 -induced change in a coupled ocean-atmosphere model and its paleoclimatic implications[END_REF][START_REF] Tardy | Dérive des continents paleoclimats et altérations tropicales[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] that further enhanced lateritic weathering. At the time of India-Asia collision, CO 2 degassing of pelagic carbonate into the atmosphere ceased with the end of the North Tethys subduction [START_REF] Caldeira | Enhanced Cenozoic chemical weathering and the subduction of pelagic carbonate[END_REF] concomitantly with intense continental (bauxitic) lateritic weathering worldwide [START_REF] Prasad | A review of the early Tertiary bauxite event in South America, Africa and India[END_REF][START_REF] Valeton | Saprolite-bauxite facies of ferralitic duricrust on paleosurfaces off former Pangaea[END_REF][START_REF] Retallack | Lateritization and Bauxitization Events[END_REF] and correlative offshore carbonate production (e.g., [START_REF] Chaubey | Analyses of multichannel seismic reflection, gravity and magnetic data along a regional profile across the central-western continental margin of India[END_REF] for the West Indian margin). Bauxitic weathering was effectively documented by 40 Ar/ 39 Ar cryptomelane ages throughout the tropical belt during the Eocene e.g., from 56 to 51 Ma in South America [START_REF] Ruffet | A geochronological 40 Ar 39 Ar and 87 Rb 81 Sr study of K-Mn oxides from the weathering sequence of Azul, Brazil[END_REF]Vasconcelos, 1999b;[START_REF] Vasconcelos | Direct dating of weathering phenomena by 40 Ar/ 39 Ar and K-Ar analysis of supergene K-Mn oxides[END_REF] and from 59 to 45 Ma in West Africa [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]. Combined together, all these concomitant phenomena consumed high quantities of atmospheric CO 2 [START_REF] Dessert | Basalt weathering laws and the impact of basalt weathering on the global carbon cycle[END_REF] that progressively cooled the climate once past the Eocene Climatic Optimum (Kent andMuttoni, 2008, 2012; see also [START_REF] Zachos | Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present[END_REF][START_REF] Zachos | An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics[END_REF]. In turn, progressive climate cooling from Mid-Eocene onward (Fig. 13c) has favoured the dissection of early Eocene bauxitic landscapes.

4.2. Late Eocene-late Oligocene weathering W2a) This first weathering stage of period W2 is marked mostly in the highland, particularly by two prominent peaks at ~ 28 Ma and ~ 26 Ma in the Sandur massif and the Shimoga area, respectively (Fig. 13a), and more speculatively in the lowland if we consider that 24 Ma is a minimum age. This episode (37-26 Ma) is interpreted to reflect the continental weathering response to the late Oligocene warming (Fig. 13c) at a time of a marine transgression propitious to offshore carbonate production [START_REF] Biswas | Regional tectonic framework, structure and evolution of the western marginal basins of India[END_REF].

Thickening of the proto-Himalaya since ~ 40 Ma [START_REF] Aikman | Evidence for Early (> 44 Ma) Himalayan Crustal Thickening, Tethyan Himalaya, southeastern Tibet[END_REF] resulted in the installation of an orographic barrier [START_REF] Molnar | Mantle dynamics, uplift of the Tibetan Plateau, and the Indian Monsoon[END_REF][START_REF] Ramstein | Impact of the hydrological cycle on past climate changes: three illustrations at different time scales[END_REF], which redistributed the humid air masses southward [START_REF] Dupont-Nivet | Tibetan uplift prior to the Eocene-Oligocene climate transition: Evidence from pollen analysis of the Xining Basin[END_REF]. A monsoonal regime installed at that time in Southern Asia [START_REF] Licht | Asian monsoons in a late Eocene greenhouse world[END_REF] that could intensify weathering processes on the Indian peninsula. Under such climatic conditions, the lateritic weathering profiles underlying remnants of the first two paleolandsurfaces became less well drained as a result of river incision, but did not become totally inactive, as attested by younger dates in Sandur,e.g.,ca. 37 to ca. 26 Ma (Figs. 10 and 13a). During this period, the early Eocene bauxites have still evolved on the highland (Krishna Rao et al., 1989b; see also [START_REF] Bonnet | First 40 Ar/ 39 Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India[END_REF] at least until global cooling by ~ 34 Ma (Fig. 13c; [START_REF] Molnar | Late Cenozoic uplift of mountain ranges and global climate change: chicken or egg?[END_REF][START_REF] Zachos | Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present[END_REF].

Early Oligocene cooling also coincided with a sea level fall [START_REF] Chaubey | Analyses of multichannel seismic reflection, gravity and magnetic data along a regional profile across the central-western continental margin of India[END_REF], and the installation of dryer climatic conditions (see Fig. 13c), which may have resulted in the attenuation of the weathering intensity observed between 32 and 29 Ma in the highland (Fig. 13a). The highland pediment may possibly have been formed in this time interval, and subsequently weathered between 29 and 23 Ma. At that time, the lowland landscape was also possibly rejuvenated, but previously formed lateritic weathering mantles as old as ca. 47-45 Ma were preserved (Figs. 12 and 13a), attesting to a very slow denudation regime of the pediment below the WGE [START_REF] Beauvais | Very long-term stability of passive margin escarpment constrained by 40 Ar/ 39 Ar dating of K-Mn oxides[END_REF]. The contrasted weathering record of the highland and the lowland after the Mid-Eocene (Fig. 13a) suggests a spatial contrast in rainfall distribution on either side of the WGE. This contrast was mostly controlled by dominant South-East moisture fluxes during the late Eocene and the early Oligocene [START_REF] Licht | Asian monsoons in a late Eocene greenhouse world[END_REF][START_REF] Chen | 40 Ar/ 39 Ar ages and stable isotopes of supergene jarosite from the Baiyin VHMS ore field, NE Tibetan Plateau with paleoclimatic implications[END_REF] before installation of modern monsoon regimes in the early Miocene [START_REF] Clift | Correlation of Himalayan exhumation rates and Asian monsoon intensity[END_REF], which reactivated weathering preferentially in the lowland.

4.3. Late Oligocene-Early Miocene weathering W2b) This second stage of weathering period W2 is mostly marked in the western highland and the lowland. Weathering in eastern highland lateritic profiles of the Sandur massif definitively ceased at ca. 26 Ma [START_REF] Bonnet | First 40 Ar/ 39 Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India[END_REF], while the western part of the highland adjacent to the WGE (Shimoga) possibly weathered until ca. 23 Ma (Fig. 13a). Therefore, the highland lateritic pediment carrying those profiles has been incised essentially after 23 Ma at a rate < 5 m/m.y. (Figs. 2a-b). During highland incision, lateritic weathering was reactivated on the lowland pediment with a peak activity at ~ 19-21 Ma (Fig. 13a). Central Asia aridification started ~ 24 Ma ago [START_REF] Sun | Late Oligocene-Miocene mid-latitude aridification and wind patterns in the Asian interior[END_REF].

Highland weathering mitigation after ~ 23 Ma may be due to such a change in climatic regime, which increased the east-west wetness gradient over peninsular India and signed the onset of modern-like monsoonal regimes [START_REF] Clift | Correlation of Himalayan exhumation rates and Asian monsoon intensity[END_REF]; see also [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].

The lack of reliable ages between ~ 19 Ma and 9 Ma (Fig. 13a) suggests that weathering is not recorded or preserved in peninsular India during Mid-Miocene. After the early Miocene (i.e., after ca. 16 Ma), a significant change has been however observed in the clay content of sediments from the neighbouring Arabian Sea, wherein illite became dominant compared to kaolinite and smectite [START_REF] Phillips | Long-timescale variation in bulk and clay mineral composition of Indian continental margin sediments in the Bay of Bengal, Arabian Sea, and Andaman Sea[END_REF]. This, together with our results, indicates an attenuation of continental weathering and suggests dominant landscape dissection of the highland. By contrast, the lowland lateritic pediment was only slightly dissected after ~ 19 Ma [START_REF] Beauvais | Very long-term stability of passive margin escarpment constrained by 40 Ar/ 39 Ar dating of K-Mn oxides[END_REF]. The weathering pulses at ~ 9 Ma and ~ 2.5 Ma (Figs. 12 and 13a) may be linked to Late Neogene intensification of the Asian monsoon in the sub region [START_REF] Zhisheng | Evolution of Asian monsoons and phased uplift of the Himalaya-Tibetan plateau since Late Miocene times[END_REF], which may have been driven by uplift of the Tibetan plateau [START_REF] Molnar | Mantle dynamics, uplift of the Tibetan Plateau, and the Indian Monsoon[END_REF]; see also [START_REF] Clift | Correlation of Himalayan exhumation rates and Asian monsoon intensity[END_REF].

Conclusion

The Table 1. Field characteristics of samples from Sandur, Shimoga and Goa Mn ore deposits (Peninsular India) and description of cryptomelane grains extracted from these samples. Other mineral species were also identified by reflected-light microscopy, XRD and SEM.

Figures and table caption

Table 2. Synthesis of the 40 Ar/ 39 Ar ages presenting the plateau ages, the inverse isochrons ages with their properties, and the integrated ages. The results in bold are considered as the preferred ages. See the figure 10 for more information about the analytical system used.

4. 1 .

 1 Early Eocene lateritic weathering (ca. 53-45 Ma; W1) The 40 Ar/ 39 Ar ages indicate that lateritic weathering started at least ~ 50 Ma ago over the highland and ~ 47 Ma in the lowland (Figs. 10, 12 and 13a). Preserved laterites

  40 Ar/ 39 Ar dating of supergene cryptomelane formed in situ in lateritic weathering profiles of Peninsular India sheds light on the Cenozoic weathering and erosion history of the subcontinent's surface. Cryptomelane ages document three major periods of weathering, i.e., early Eocene (W1) both on the highland and the lowland, ~ late Eocenelate Oligocene (W2a), only in the highland, and early Miocene (W2b), mostly in lowland. The 40 Ar/ 39 Ar age series suggest subdued relief production and dominant weathering of the highland before ~ 23 Ma and its dissection and stripping after that time, while the lowland weathered in the Early Eocene and Miocene before being slightly incised after ~ 19 Ma. The oldest ages (53-45 Ma) indicate widespread Eocene lateritic weathering in South India at the time of global Eocene climatic optimum, when the peninsula crossed the equatorial belt. Our results also document the installation of a dual climatic regime across the Western Ghats escarpment after the Eocene climatic optimum, leading to divergent weathering and erosion patterns on both sides of this topographic barrier. Hence, K-Mn oxides 40 Ar/ 39 Ar age series document the tempo of South Indian morphogenesis, and as such may be viewed as a proxy for erosion and climatically driven weathering over geological time scales.

Figure 1 .

 1 Figure 1. Simplified geological map (adapted from Chardon et al., 2008) superimposed

Figure 2 .

 2 Figure 2. Synthetic topographic sections with major lateritic weathering surface relics.

Figure 3 .Figure 4 .

 34 Figure 3. Sampled lateritic weathering profiles. (a) Kappataswamy, (b) Channanghi

Figure 5 .

 5 Figure 5. Reflected-light photomicrographs of polished thin sections from samples

Figure 6 .Figure 7 .

 67 Figure 6. Procedure to separate cryptomelane grain aliquots from two sample slabs (a-

Figure 9 .

 9 Figure 9. Disturbed 40 Ar / 39 Ar age spectra of cryptomelane grains from Shimoga Mn

Figure 12 .

 12 Figure 12. (a) 40 Ar / 39 Ar age spectra and (b) inverse isochron diagrams for

Figure 13 .

 13 Figure 13. Synthesis of the 40 Ar/ 39 Ar ages results accounting for two-confidence order

  Figure 1 Click here to download high resolution image

  Figure12

(

  ¹) recalculated after[START_REF] Bonnet | First 40 Ar/ 39 Ar dating of intense Late Palaeogene lateritic weathering in Peninsular India[END_REF] (²) recalculated after[START_REF] Beauvais | Very long-term stability of passive margin escarpment constrained by 40 Ar/ 39 Ar dating of K-Mn oxides[END_REF] Locations: 1 = Kappataswamy pit; 2 = Channanghi KMK-East pit; 3 Triveni Pit; 4 = Kumsi pit; 5 = Caurem pit; 6 = Naveli pits; 7 Cudnem pit (See also Fig.1) Refer to Fig.1for localities numbers A = 1 st order age; B = 2 nd order age; C = 3 rd order age; NGS = No Geological Significance

  . The second weathering period W2 from ca. 37 to ca. 19 Ma may be divided in two stages, W2a (ca. 37-26 Ma) mostly in the highland

(Sandur,

only in the western highland

(Shimoga,

and in the lowland (Goa, ca. 24-19 Ma). The early stage (W2a) characterizes late Eocene to late Oligocene lateritic weathering of the second highland paleolandsurface, as best recorded in the eastern part (Sandur), and more discretely in Shimoga (e.g., ca. 30 Ma from KUM-3f, Fig.

  . Such climate driven erosion processes changes have resulted in the installation and preservation of the successive lateritic paleolandsurfaces over South India. The first, bauxitic paleolandsurface, was essentially shaped until a maximum age of ca. 45 Ma by intense weathering and only slightly reworked and dissected after that time during the shaping, and later weathering of the second landsurface between ca. 37 and ca. 26-23 Ma. Finally, abandonment / dissection of the highland pediment after ca. 23 Ma indicates that the thick lateritic covers of South India formed in the Eocene and Oligocene and that stripping of that material and intense dissection of the peninsula essentially took place in the Neogene (i.e., after 23

	Ma).

  The black, grey and white diamonds are samples from Sandur, Shimoga and Goa, respectively. See also the data repository table DR1.

	Figure 8. 40 Ar / 39 Ar age spectra of cryptomelane grains showing regular ages, with
	K/Ca (black) and Ar* (grey) step curves (left), and inverse isochron diagrams (right).
	(a) Plateau age for Sandur's cryptomelane grain; (b) plateau age for Shimoga's
	cryptomelane grain with small characteristic 40 Ar* losses from less retentive and poorly
	crystalline sites at the first step heating; (c) Forced-plateau age for Shimoga's sample
	grain including four apparent ages forming a reasonably flat segment; (d) Concordant
	pseudoplateaus ages for Sandur's cryptomelane grain included in the 2σ interval.
	MSWD = mean square weighted deviation of the inverse isochron. See the table 1 for
	sample description and the data repository DR2 and DR3.

table 1

 1 Figure10.40 Ar / 39 Ar age spectra of cryptomelane grains from Sandur Mn ore samples. See the table 1 for sample description, and the data repository tableDR2.40 Ar / 39 Ar age spectra and (b) inverse isochron diagrams for cryptomelane grains from Shimoga Mn ore samples. See the figure 10 for the analytical system used, age quality explanation and the table 1 for sample description. See also the data repository tables DR2 and DR3.

	for sample description and the data repository tables
	DR2 and DR3.

CAU-1c 19.6 ± 0.2 Ma (B)

	Int. (40/36): 293 ± 26
	MSWD = 0.77
	No. used/No. tot: 4/17
	39 Ar released: 45 %
	Int. (40/36): 298 ± 1
	MSWD = 1.02
	No. used/No. tot: 6/17
	39 Ar released: 22 %
	0
	FIG. 12 (2 columns)

Acknowledgements

-This work was funded by Indo-French Centre for the Promotion of Advanced Research (IFCPAR) project 5007-1, the IRD (UR 161) and the CNRS (INSU 2011-CT2). The French Ministry for the Scientific Research and Education granted N.J. Bonnet a Ph.D. scholarship (ED251, Aix Marseille Université,

Observatoire des Sciences de l'Univers Pytheas). N.J. Bonnet was also supported by a fellowship from IFCPAR. We thank L. Giosan and an anonymous referee for their comments and suggestions, and editorial handling from F. Chabaux and Michael E.

Böttcher.

Data Repository 1 EPMA Click here to download Background dataset for online publication only: Revised_DR1.xlsx Data Repository 2 ArArCalc Click here to download Background dataset for online publication only: R2_DR2.xlsx

KPA-10b

Integrated age = 27.8 ± 1.4 Ma 27.6 ± 0.7 Ma (A) (100%)

28.9 ± 0.7 Ma (A) (100%)

Integrated age = 28.9 ± 0.9 Ma

KPA-10c

Apparent age (Ma)