Julien Charreau
email: julien.charreau@univ-lorraine.fr

Jérôme Lavé

Christian France-Lanord

Nicolas Puchol

Pierre-Henri Blard

Raphaël Pik

Ananta Prasad Gajurel

Aster Team

Maurice Arnold

Didier Georges Aumaître

L Bourlès

A 6 Ma record of palaeodenudation in the central Himalayas from in situ cosmogenic 10 Be in the Surai section

Keywords: Himalaya, cosmogenic 10 Be, palaeodenudation rates, late Neogene

To better constrain late Neogene denudation of the Himalayas, we analysed in situ 10 Be concentrations in 17 Neogene sediment samples of the Surai section (central Nepal) and two modern sediment samples of the Rapti River. We first refined the depositional ages of the Surai section from 36 new paleomagnetic analyses, five 26 Al/ 10 Be burial ages, and, based on the Dynamic Time Warping algorithm, 10 4 automatically calculated likely magnetostratigraphic correlations. We also traced changing sediment sources using major element and Sr-Nd isotopic data, finding at 4-3 Ma a switch from a large, trans-Himalayan river to a river draining only the Lesser Himalaya and Siwalik piedmont, increasing the contribution of recycled sediments at that time. 10 Be concentrations in Neogene sediments range from (1.00 ± 0.36) to (5.22 ± 0.98)  10 3 at g -1 and decrease with stratigraphic age.

Based on a flood plain transport model, our refined age model, and assuming a drainage change at 4-3 Ma, we reconstructed 10 Be concentrations at the time of deposition. Assuming cosmogenic production rates similar to those of the modern basins, we calculated palaeodenudation rates of 0.9 ± 0.5 to 3.9 ± 2.7 mm a -1 from ~6 to ~3 Ma in the palaeo-Karnali basin and 0.6 ± 0.2 to 1.6 ± 0.8 mm a -1 since ~3 Ma in the palaeo-Rapti basin. Given the uncertainties and similar modern values of ~2 mm a -1 , the palaeo-Karnali denudation rates may have been steady at ~1.7 ± 0.3 mm a -1 for the last ~6 Ma. A transient acceleration of the denudation in the palaeo-Rapti basin of ~1.5 mm a -1 since ~1.5 Ma was likely due to the reworking of older, 10 Be-depleted Siwalik sediments in the foreland. If true, this steadiness of the denudation rates may suggest that Quaternary glaciations did not largely affect Himalayan denudation.

Introduction

The denudation history of the Himalayas, the largest and highest mountain range in the world, remains controversial, precluding a strong understanding of how this large mountain range grew. Sediment budget from basins surrounding the Himalayas are difficult to reconstruct because the depositional areas are very large, including the Ganga basin and the Indus and Bengal fans. Moreover, it is not straightforward to accurately assess both the complex geometry and chronology of sediment infilling in these locations. Consequently, published Cenozoic Himalayan sedimentary budgets have yielded inconsistent constrains (e.g. [START_REF] Burbank | Reduced Himalayan sediment production 8 Myr ago despite an intensified monsoon[END_REF][START_REF] Clift | Controls on the erosion of Cenozoic Asia and the flux of clastic sediment to the ocean[END_REF][START_REF] Métivier | Mass accumulation rates in Asia during the Cenozoic[END_REF]. For example, according to [START_REF] Métivier | Mass accumulation rates in Asia during the Cenozoic[END_REF] the average accumulation rates curve for Asian sedimentary basins increased quickly in an exponential manner from 0.5x10 6 km 3 Ma -1 in the Oligocene to their maximum values >1.5x10 6 km 3 Ma -1 in the Quaternary. [START_REF] Clift | Controls on the erosion of Cenozoic Asia and the flux of clastic sediment to the ocean[END_REF] reported an increase in sedimentation rate from <20 to ~100-120x10 3 km 3 .Ma -1 from 50 to ~15Ma, then a drop between ~15 and ~10 Ma followed by a new acceleration until the Quaternary where sedimentation rates reached precedent early Miocene values.

Similarly, thermochonological studies have brought contrasted constraints on the late Cenozoic evolution of Himalayan denudation. The interpretation of the in situ bedrock thermochronological ages [START_REF] Herman | Worldwide acceleration of mountain erosion under a cooling climate[END_REF][START_REF] Huntington | Climate change and Late Pliocene acceleration of erosion in the Himalaya[END_REF] suggest that the denudation rates along the Himalayan arc increased by a factor of 1 to 4 during the Quaternary, which some ascribe to an increased climatic, not tectonic, forcing. However, the denudation history is spatially variable across the range, and some regions, e.g., Bhutan, are characterized by lower denudation rates during the last 6-8 Ma [START_REF] Adams | Constraints on the tectonic and landscape evolution of the Bhutan Himalaya from thermochronometry[END_REF][START_REF] Thiede | Large spatial and temporal variations in Himalayan denudation[END_REF]. Similarly, thermochronological analyses of detrital grains in molasse sediments deposited at the Himalayan front suggest either quite stable basinaveraged denudation rates since the mid-Miocene in central Nepal (Bernet et al., 2006;[START_REF] Harrison | Isotopic Preservation of Himalya/Tibetan Uplift, Denudation and Climatic Histories of two Molasse Deposits[END_REF]Szulc et al., 2006;van der Beek et al., 2006) or a rapid acceleration of the denudation around 5Ma in the eastern Himalayan syntaxis [START_REF] Lang | Rapid exhumation of the eastern Himalayan syntaxis since the late Miocene[END_REF]. Notably, however, this thermochronological technique averages denudation rates over timescales of a few million years, possibly smoothing and offsetting rapid changes related, for example, to climate.

An alternative method to reconstruct denudation histories of a mountain range is to reconstruct basin average paleodenudation rates from the measurement of in situ cosmogenic nuclides (e.g., 10 Be) in ancient (>1 Ma) river sediments [START_REF] Charreau | Paleo-erosion rates in Central Asia since 9Ma: A transient increase at the onset of Quaternary glaciations? Earth Planet[END_REF][START_REF] Puchol | Limited impact of Quaternary glaciations on denudation rates in central Asia[END_REF]. Cosmogenic nuclide concentrations in river sands average the concentrations in all surficial rocks outcropping in the drainage basin (e.g. [START_REF] Brown | Denudation rates determined from the accumulation of in situ produced 10Be in the Luquillo experimental forest, Puerto-Rico[END_REF]. When measured in ancient sediments, cosmogenic nuclides average the palaeodenudation rates of the drainage basin at the time of deposition [START_REF] Charreau | Paleo-erosion rates in Central Asia since 9Ma: A transient increase at the onset of Quaternary glaciations? Earth Planet[END_REF][START_REF] Puchol | Limited impact of Quaternary glaciations on denudation rates in central Asia[END_REF][START_REF] Refsnider | Dramatic increase in late Cenozoic alpine erosion rates recoreded by cave sediment in the southern Rocky Moutains[END_REF][START_REF] Schaller | A 30 000 yr record of erosion rates from cosmogenic 10Be in Middle European river terraces[END_REF]. This approach is advantageous as it averages denudation rates over short characteristic timescales, typically <5 ka for denudation rates larger than 0.1 mm a -1 .

Here, to reconstruct past Himalayan denudation rates, we analysed in situ 10 Be concentrations in Neogene sediments sampled from the Surai section (Fig. 1), a well-documented stratigraphic section in the piedmont covering the last 6 Ma. The section has been dated by magnetostratigraphy (Appel et al., 1991;[START_REF] Ojha | Magnetic polarity stratigraphy of the Neogene foreland basin deposits of Nepal 61-90[END_REF]Rosler et al., 1997) and biostratigraphy [START_REF] Corvinus | Biostratigraphy and geology of the Neogene Siwalik Group of the Surai Khola and Rato Khola areas in Nepal[END_REF]. Moreover, during recent decades, it has been extensively studied through geochemical (Huyghe et al., 2001;Szulc et al., 2006), mineralogical (Szulc et al., 2006), and thermochronological studies (Bernet et al., 2006;Szulc et al., 2006;van der Beek et al., 2006). These data are key to tracing changes in sediment provenance, which are needed to estimate the cosmogenic production rates during the past and hence to reconstructing palaeodenudation rates from measured 10 Be concentrations.

Geological Setting

Geology of the Himalayas

The geology of the Himalayan arc in Nepal is traditionally divided into five main units from north to south: the Thetyan Sedimentary Series (TSS), the Higher Himalaya Crystalline unit (HHC), the Lesser Himalaya unit (LH), the Siwalik folded molasse, and the foreland basin of the Ganga plain (Fig. 1). These units are separated by four major faults [START_REF] Gansser | Geology of the Himalayas[END_REF]: from north to south, the Southern Tibetan Detachment (STD), the Main Central Thrust (MCT), the Main Boundary Thrust (MBT), and the Main Frontal Thrust (MFT). Along the Southern Tibetan plateau, the TSS comprises Mesozoic and Palaeozoic marine sediments and medium-to low-grade metamorphic rocks. The HHC comprises high-grade crystalline metamorphic units. Both formations are intruded by Miocene leucogranites and form the highest relief of the Nepalese Himalaya. To the south, the HHC units overthrust the LH at the MCT level and occur as two major klippen in western Nepal. The LH series comprises low-grade Precambrian detrital continental meta-sediments and Cretaceous to Eocene marine sediments; it represents an intermediate midland relief between the higher relief of the HHC to the north and the Siwalik Hills to the south, the first significant relief at the foot of the Himalayas (Figs. 1 and2a).

The Siwalik Hills

The Siwalik Hills, bounded to the north by the MBT and to the south by the MFT (Figs. 1, 2a and 2b), comprise successive fold and thrust belts oriented WNW-ESE along the Himalayas [START_REF] Auden | Traverses in the Himalaya[END_REF]. The Siwalik Hills do not exceed 1,000 m elevation and comprise a thick (>5 km) pile of Neogene molasse produced by several million years of intense denudation in the Himalayas [START_REF] Decelles | Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal[END_REF][START_REF] Hérail | Les Siwaliks du Népal central : Structure et géomorphologie d'un piédmont en cours de déformation (The central nepalese siwaliks : structure and geomorphology of an upheaveling piedmont)[END_REF].

How the sediments are currently deposited in the modern Ganga plain provides useful information to understand the palaeoenvironmental setting in which the Siwalik sediments formed (e.g. Sinha et al., 2005). At the very front of the Himalayas, in the Ganga plain, sediments are deposited on a series of large alluvial megafans (10 4 -10 5 km 2 ) associated with trans-Himalayan rivers that drain the Himalayas from the lowland piedmont to the high elevated regions (Fig. 1) [START_REF] Decelles | Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal[END_REF][START_REF] Gupta | Himalayan drainage patterns and the origin of fluvial megafans in the Ganges foreland basin[END_REF]. These megafans alternate with smaller interfans associated with smaller rivers originating in the Lesser Himalaya (Fig. 1) [START_REF] Decelles | Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal[END_REF][START_REF] Gupta | Himalayan drainage patterns and the origin of fluvial megafans in the Ganges foreland basin[END_REF][START_REF] Wells | Shifting of the Kosi River , northern India[END_REF]. Although the facies and origins of the Siwalik sediments vary considerably along the strike of the >2,000-km-long range, these molasse, mainly of continental origin, are usually stratigraphically divided into three litho-stratigraphic units [START_REF] Decelles | Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal[END_REF][START_REF] Dhital | Geology and structure of the Siwaliks and Lesser Himalaya in the Surai Khola-Bardanda area, Mid Western Nepal[END_REF] namely the Lower, the Middle and the Upper Siwaliks and showing an upward coarsening related to the progressive southward migration of the Himalayan thrust wedge and depositional facies (Dubille and Lavé, 2015;Lyon-Caen and Molnar, 1985).

The Surai section

The Surai section is in the southern part of western-central Nepal, ~5 km to the southeast of the town Bhalubang (Figs. 2a,b). In this area, Siwalik sediments are folded into a ~750-m-high, >100-km-long ridge developed on the hanging wall of the MFT, the so-called Dundwa ridge, which is progressively propagating to the west. A piggy-back basin has developed to the north of the Dundwa ridge, in which the West Rapti River (not to be confused with the East Rapti River, a tributary of the Narayani River) deviates to the West (Figs. 1 and2). The Surai Khola, which drains the most southern segment of the Siwaliks into the Ganga plain (Fig. 2b), has deeply incised the uplifting Dundwa ridge. Construction of the 19-km-long RH01 road between Chanauta andBhalubang between 1983 and1986 exposed a nearly continuous >5,000-m-thick section of outcrops of Siwalik sediments dipping to the north at 60-70° (Fig. 2d). This section, the 'Surai section', has enabled dense palaeomagnetic sampling for magnetostratigraphic analyses, which have roughly constrained the depositional ages of the Lower and Middle Siwaliks groups to ~14-10 Ma and 10-4 Ma, respectively (Appel et al., 1991;[START_REF] Corvinus | Biostratigraphy and geology of the Neogene Siwalik Group of the Surai Khola and Rato Khola areas in Nepal[END_REF][START_REF] Ojha | Magnetic polarity stratigraphy of the Neogene foreland basin deposits of Nepal 61-90[END_REF]Rosler et al., 1997;[START_REF] Rosler | Fidelity and time resolution of the magnetostratigraphic record in Siwalik sediments: high-resolution study of a complete polarity transition and evidence for cryptochrons in a Miocene fluviatile section[END_REF]. However, some ambiguities and uncertainties persist for the upper, coarser part of the section that is less amenable to palaeomagnetic sampling. The depositional ages of the top of the section are hence more loosely constrained between 4 and 1.5 Ma.

Building on this chronology, the Surai section has been intensively examined by palaeontological [START_REF] Corvinus | Biostratigraphy and geology of the Neogene Siwalik Group of the Surai Khola and Rato Khola areas in Nepal[END_REF], sedimentological [START_REF] Decelles | Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal[END_REF][START_REF] Dhital | Geology and structure of the Siwaliks and Lesser Himalaya in the Surai Khola-Bardanda area, Mid Western Nepal[END_REF][START_REF] Nakayama | Evolution of fluvial style in the Siwalik Group in the foothills of the Nepal Himalaya[END_REF], sediment provenance [START_REF] Decelles | Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal[END_REF]Szulc et al., 2006), geochemical (Huyghe et al., 2001;Lupker et al., 2012b;Szulc et al., 2006), and thermochronological studies (Bernet et al., 2006;Szulc et al., 2006;van der Beek et al., 2006). The section consists mainly of fluvial sediments that gradually coarsen upward, and includes the Upper, Middle, and Lower Siwaliks groups (Fig. 2), which have been subdivided from bottom to top into the Bankas, Chor Khola, Surai Khola, Dobatta, and Dan Khola local formations [START_REF] Corvinus | Biostratigraphy and geology of the Neogene Siwalik Group of the Surai Khola and Rato Khola areas in Nepal[END_REF][START_REF] Dhital | Geology and structure of the Siwaliks and Lesser Himalaya in the Surai Khola-Bardanda area, Mid Western Nepal[END_REF][START_REF] Nakayama | Evolution of fluvial style in the Siwalik Group in the foothills of the Nepal Himalaya[END_REF]. Sedimentological facies associations reveal that the depositional environmental changed from a small meandering river channel to a larger braided river channel around 10-6 Ma [START_REF] Decelles | Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal[END_REF][START_REF] Nakayama | Evolution of fluvial style in the Siwalik Group in the foothills of the Nepal Himalaya[END_REF]. Upsection, the river remained braided, but evolved from a sand-to a gravel-dominated system in the middle of the Dhan Khola formation (Fig. 4) [START_REF] Nakayama | Evolution of fluvial style in the Siwalik Group in the foothills of the Nepal Himalaya[END_REF]. Most palaeocurrents measured along the section suggest that the river flowed toward the south [START_REF] Decelles | Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal[END_REF]Szulc et al., 2006), led DeCelles et al. (1998) to conclude that the sediment source was persistently to the north; they excluded the possibility of syntectonic sedimentation by local rivers draining the uplifting Dundwa ridge itself. However, palaeocurrent directions in the Upper Siwaliks group exhibit more scatter than down section [START_REF] Decelles | Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal[END_REF], suggesting the possible contribution of local tributaries. Based on provenance data, [START_REF] Decelles | Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal[END_REF] further suggested that the river that deposited the upper and middle Surai section was similar to present trans-Himalayan rivers. It is, however, difficult from the sedimentological record alone to discriminate between a large river draining the internal part of the range and smaller interfan rivers draining only the Lesser Himalaya.

The mineralogical, isotopic, and/or geochemical compositions of the sediments in the Surai section reveal further details of their source rocks. Several tracers suggest that the exhumation of the LH began ~12 Ma and has remained a significant source of sediment in the area [START_REF] Decelles | Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal[END_REF]Huyghe et al., 2001;Szulc et al., 2006). High-grade metamorphic minerals present in the section suggest that the palaeo-Rapti River has likely also continuously eroded HHC rocks (Szulc et al., 2006), although the exact source remains ambiguous due to the presence of the HHC klippe thrust over the midland and LH (Fig. 2). Therefore, these petrologic data only constrain that the palaeo-drainage basin has always reached, at least, the HHC klippe (Fig. 2). Importantly, it remains uncertain whether the river drained the entire Himalayas or only the Lesser Himalaya.

Methods and sampling

General approach

To document palaeodenudation rates in the Surai section, we measured the bulk 10 Be concentrations (at g -1 , see section 3.2) of Siwalik sediments exposed along the Surai section (

). The basin-averaged denudation 𝑁 total rate is calculated from by solving the equation (e.g. [START_REF] Charreau | Paleo-erosion rates in Central Asia since 9Ma: A transient increase at the onset of Quaternary glaciations? Earth Planet[END_REF][START_REF] Puchol | Limited impact of Quaternary glaciations on denudation rates in central Asia[END_REF]:

𝜀 𝑁 total (1) 𝑁 total = 𝑒 -𝜆𝑡 [ ∑ 𝑗 = 1,3 𝑃 𝑗 𝜀 𝜌 r 𝛬 𝑗 + 𝑁 fp ] + 𝑁 recent
where is the overall palaeoproduction rate of 10 Be averaged over the drainage basin at the time of erosion, which 𝑃 𝑗 accounts for all cosmogenic production mechanisms (spallation, 'n', and slow and fast muons, 'µs' and 'µf', respectively, indicated by j = 1-3), is the density of the denuded rocks (taken as 2.7 g cm -3 ), λ the radioactive 𝜌 r decay constant, the characteristic attenuation lengths of the cosmogenic production mechanisms (Λ n = 160 g 𝛬 𝑗 cm -2 , Λ µs = 1,500 g cm -2 , and Λ µf = 4,320 g cm -2 ) (Braucher et al., 2011), and t the time at which the sediments were buried (i.e., the depositional age). and are the number of 10 Be atoms (per g of quartz) accumulated

𝑁 fp 𝑁 recent
by the sediments during their transport and final deposition in the Ganga plain (i.e., until buried deep enough to be fully shielded from cosmic rays) and during their most recent exposure in outcrop, respectively.

Several corrections and assumptions are required [START_REF] Charreau | Paleo-erosion rates in Central Asia since 9Ma: A transient increase at the onset of Quaternary glaciations? Earth Planet[END_REF][START_REF] Puchol | Limited impact of Quaternary glaciations on denudation rates in central Asia[END_REF] to derive palaeodenudation rates from the measured 10 Be concentrations along the Surai section. First, we assumed 𝑁 recent to be negligible because of the recent road cut (Fig. 2) and the high erosion rates expected in the Siwalik hills (Hurtrez et al., 1999;Lavé and Avouac, 2000). Then, solving equation ( 1) requires precise knowledge of the depositional ages t, especially to correct the measured concentrations for radioactive decay. To this aim, we refined the depositional ages of the Surai section, especially in the upper part of the section, using new palaeomagnetic analyses, stochastic magnetostratigraphic correlations (Lallier et al., 2013), and 26 Al/ 10 Be cosmogenic burial dating (e.g., [START_REF] Granger | Dating sediment burial with in situ-produced cosmogenic nuclides: theory, techniques, and limitations[END_REF]; see section 3.3). We also estimated the contribution of during sediment 𝑁 fp transfer in the foreland plain and after its definitive burial in the plain, i.e. after its burial below the depth of potential sediment remobilisation induced by the migration of the channel into its floodplain (section 3.4). The cosmogenic palaeoproduction rates P at the time of deposition are mainly controlled by the distribution of elevations in the drainage basin. Therefore, we reconstructed in detail the sediment sources and their evolution through time using geochemical records, including new major and trace element and Sr-Nd isotopic analyses (section 3.5). The cosmogenic production rates were estimated using the Basinga GIS Tool (Charreau et al., 2019) (Sup.

Info.) and the topography of modern drainage basins expected to be like the past drainage basins. Indeed, H and O isotopic analyses suggest that the topography of the northern Himalayas has remained similar since the early Miocene [START_REF] Gébelin | The Miocene elevation of Mount Everest[END_REF]. Sediment provenance analyses also suggest that the drainage systems of several Himalayan rivers (Yamuna, Ganga, Tinau, Karnali) have remained steady during the late Cenozoic [START_REF] Mandal | Multiproxy Isotopic and Geochemical Analysis of the Siwalik Sediments in NW India: Implication for the Late Cenozoic Tectonic Evolution of the Himalaya[END_REF]Szulc et al., 2006). Our calculation excludes corrections for snow and glacial shielding (see Supplementary Material for justifications) meaning that the derived denudation rates must be considered as maximum values. The uncertainties on paleo-denudation rates were calculated using a Monte-Carlo simulation similar to [START_REF] Puchol | Limited impact of Quaternary glaciations on denudation rates in central Asia[END_REF]. This approach explores conservatively the range of all the input parameters (Table A) to provides an efficient and accurate determination of the uncertainty attached to the calculated paleo-denudation rates.

Cosmogenic 10 Be analyses for palaeodenudation rates

Seventeen samples of typically medium to coarse sandstone were collected from the Surai section for 10 Be analyses (Fig. 2c). Sampling locations were selected at maximum at the bottom of 10-20m high cliffs to minimize possible post burial recent exposures (Fig. 2c andd). We collected samples at maximum at the base of paleo-channel to ensure rapid burial and avoid 10 Be accumulation. However, [START_REF] Puchol | Limited impact of Quaternary glaciations on denudation rates in central Asia[END_REF] show that the 10 Be concentrations are independent of the sedimentary facies and the vertical position within each sedimentary unit. To document present denudation rates, we also sampled two river-bank deposits in the presentday West Rapti River: Rap-Cos1 was collected upstream of the Siwaliks (~5 km north of the MBT, Fig. 2a) and Rap-Cos2 was sampled north of the Dundwa ridge, just north of the Siling thrust at the Bahlubang bridge (Fig. 2a andb). Because we suspected high denudation rates (>1 mm a -1 ), and therefore very low initial 10 Be concentrations in the sampled sediments, we sampled unusually large masses of sand (sometimes >5 kg) to ensure enough pure quartz to provide detectable amounts of 10 Be. The sample treatment and 10 Be analysis procedures are detailed in the supplementary material.

Age model: magnetostratigraphy, stochastic correlations, and cosmogenic burial ages

We collected 36 new samples for palaeomagnetic analyses (Figs. 2 and3) in the upper conglomeratic part of the section. Because most of the layers were either too coarse for palaeomagnetic analysis or not amenable to drilling, we sampled only the finer silty to sandy horizons by hand by inserting quartz cylinders. We established the magnetic polarity sequence of the Surai section by incorporating our results (see the Supplementary information for more technical details) into the 436 samples analysed and interpreted by Appel et al. (1991).

Magnetic intervals were identified as at least two successive horizons with the same polarity. This revised column (see Supplementary information for the depth of the polarity intervals) was then correlated to the reference scale of Ogg (2012) to establish the depositional ages of the section. To minimize ambiguities and uncertainties related to these correlations, we used a numerical method based on the dynamic time warping (DTW) algorithm (https://www.ring-team.org/software/downloads) (Lallier et al., 2013) and automatically calculated 10,000 reasonably likely correlations. In this approach, the correlations are computed to minimize the local variation of the accumulation rate and ranked based on the calculation of a cost.

To estimate 26 Al/ 10 Be cosmogenic burial ages in the upper part of the section, (e.g., [START_REF] Granger | Dating sediment burial with in situ-produced cosmogenic nuclides: theory, techniques, and limitations[END_REF], five of the 10 Be samples were also analysed for 26 Al, and we measured both 10 Be and 26 Al in three other quartzite pebbles (Table 1). The corresponding burial ages were then calculated from the measured 10 Be and 26 Al concentrations and the mean elevation of the present drainage basin using the Matlab® code of [START_REF] Blard | Two MATLAB programs for computing paleoelevations and burial ages from paired-cosmogenic nuclides[END_REF]. [START_REF] Charreau | Paleo-erosion rates in Central Asia since 9Ma: A transient increase at the onset of Quaternary glaciations? Earth Planet[END_REF] and [START_REF] Puchol | Limited impact of Quaternary glaciations on denudation rates in central Asia[END_REF], based on on the sedimentological analysis of the foreland sediments [START_REF] Charreau | Neogene uplift of the Tian Shan Mountains observed in the magnetic record of the Jingou River section (northwest China)[END_REF], assumed that sediments in the Tianshan foreland are nearly continuously and progressively buried at a rate equal to the long-term (>100-ka) mean deposition rate, and therefore that 10 Be accumulation during transport is minimal. This implies grain transport without interaction with the plain, and deposition of thin, regular sediment layers. However, rivers draining through the Ganga plain are much larger, deeper and active than in the northern Tian Shan piedmont. They display rapid channel migration or avulsion [START_REF] Sinha | The great avulsion of Kosi on 18 August 2008[END_REF]Sinha et al., 2005), whether they are braided in their upper reach or meandering in their lower reach.

10 Be accumulation in the flood plain

In the case of the modern Karnali river for example, observations on satellite images over the last two decades indicate an average lateral migration of its channel of ~70m/a all along its course in the Ganga plain. A direct consequence of this lateral migration is bank erosion on one side and bank deposition on the other side. Sediment transfer through the floodplain is therefore better characterized by successive phases of (1) brief periods of transport, (2) temporary deposition in the floodplain, (3) floodplain and bank erosion followed by a new phase of transport. Through bank erosion and deposition, sediments are constantly exchanged between the active channel and the floodplain. For the Himalayan setting we therefore need to account for this sediment reworking and exchange to estimate the floodplain contribution (N fp ) and 10 Be accumulation during sediment transfer and final burial. Lauer and Willenbring (2010) proposed a simplified steady-state mass-balance model accounting more realistically for cosmogenic nuclide accumulation during sediment transfer within a foreland basin. In this model, already applied to the Ganga plain by Lupker et al. (2012b), floodplain sediments are regularly reworked by channel migration across the fan surface (considered as rectangular), and final sediment burial occurs through the base of the channel, i.e., when sediments are buried deeper than the mean channel reworking depth. Although this model represents a strong simplification of river dynamics, we use it but deprived of lateral supply (generally negligible along proximal parts of the Himalayan river fans) to predict the first-order evolution of 10 Be accumulation during floodplain transfer (see Supplementary information). More importantly, we hypothesize that this model, calibrated through observations on modern Karnali or Rapti rivers, can be applied to describe plain dynamic in the past. Such assumption is primarily based on the fact that external forcing like subsidence rates or incoming Himalayan sediment flux have remained roughly stable since 6Ma as described further in the text (section 4). It is also supported by sedimentologic description of the middle Siwalik facies along the Surai Khola section: this formation that presents thick, cross-bedded, multi-stored sandstones was presumably laid down by wide, braided river systems, and the presence of erosive bases attests for continuous reworking during the erosional and aggradational cycles due to stream migration [START_REF] Corvinus | Biostratigraphy and geology of the Neogene Siwalik Group of the Surai Khola and Rato Khola areas in Nepal[END_REF].

Tracing sediment sources: major element and Sr and Nd isotopic analyses

The three main Himalayan units are distinguishable by their unique Nd and 87 Sr/ 86 Sr ratios characteristics and can be tracked back in sediments [START_REF] Deniel | Isotopic study of the Manaslu granite (Himalaya, Nepal): inferences on the age and source of Himalayan leucogranites[END_REF][START_REF] France-Lanord | Evolution of the Himalaya since Miocene time: isotopic and sedimentological evidence from the Bengal Fan[END_REF][START_REF] Morin | L'érosion et l'altération en Himalaya et leur évolution depuis le tardi-pleistocène: analyse des processus d'érosion à partir de rivière actuelles et passées au Népal Central[END_REF][START_REF] Parrish | Isotopic constraints on the age and povenance of the Lesser and Greater Himalayan sequence, Nepalese Himalaya[END_REF]Robinson et al., 2001). Moreover, major element data allow the characterization of sediment sorting (for example, Al/Si and Fe/Si ratios are closely related to grain size; [START_REF] Lupker | A Rousebased method to integrate the chemical composition of river sediments : Application to the Ganga basin[END_REF] , and apparent weathering intensity (i.e., K/Al and Na/Si ratios, because Na and K are mobile elements potentially affected by weathering; Lupker et al., 2012b). Such approaches provide information critical to identifying reworking in weathered sediments or potential changes in sediment sources (e.g., [START_REF] Mandal | Multiproxy Isotopic and Geochemical Analysis of the Siwalik Sediments in NW India: Implication for the Late Cenozoic Tectonic Evolution of the Himalaya[END_REF].

We refined the existing Surai section record (Huyghe et al., 2001;Robinson et al., 2001;Szulc et al., 2006), especially in the upper part of the section where sampling has been less dense. We analysed the major element compositions of 14 new samples of silts and fine sandstones (SU-PM2 to 38), adding to the 35 samples already analysed by Lupker et al. (2012b) (samples 94-04 to 94-60 in their Table 2). We also analysed present Rapti River sands collected at the outlet of the LH (Rap-Cos1) and 200 km downstream, at the front of the MFT (Nag9, Fig. 2a).

Results and interpretations

Age model from magnetostratigraphic data and 26 Al/ 10 Be burial ages

We found a consistent demagnetization pattern in 31 of our 36 samples (Figure 3a-c) (see supplementary information for more technical details) with unambiguous and robust polarities despite low magnetic signal.

Among the 31 analysed samples, 19 can be interpreted as having primary magnetizations direction and 12 had remanence trajectories spread out about great circles (Fig. 3d) that never reached stable endpoints but had clear polarities. These 31 samples were considered to construct the magnetostratigraphic column (Fig. 4f). The resulting magnetostratigraphic column based on these new data and 436 previously analysed samples (Appel et al., 1991, with stratigraphic depths from Rosler et al., 1997) is presented in Figure 4 and compared to the previous columns of Appel et al. (1991), Rosler et al. (1997), andOjha et al. (1999). It includes 24 and 23 normal and reversed polarity intervals, respectively. To correlate this polarity column to the reference scale, the reverse interval r5, which relied on only two samples of the same polarity, was arbitrarily omitted during the calculations (see supplementary material for justification). Figure 4f shows the density plot compiling the 10,000 possible correlations to the reference scale calculated using the DTW algorithm (Lallier et al., 2013). All found correlations date the Surai section to between ~13-14 Ma and ~1-2 Ma (chrons C5A to C1), broadly similar to the previous calculations that considered all polarity intervals. However, our new accumulation rates are, on average, more steady and likely more realistic. 26 Al analyses were successful for only 7 of the 10 samples analysed, returning associated burial ages ranging from 2.4 ± 1.4 to 0.6 ± 0.8 Ma (Table 1). The 26 Al contents of the three remaining samples were below the detection limit (< ~5  10 3 at g -1 ), two of which (Cos8 and Cos16) correspond to the oldest sediments of the record. The burial ages are burdened by large error bars that can reach 100% because of high analytical uncertainties on both 10 Be and 26 Al concentrations and do not allow to constrain the burial ages better than 0.8Ma.

Though they do not provide data critical to better constraining the depositional ages in the upper part of the Surai section, these burial ages remain, at first order, consistent with the proposed magnetostratigraphic correlations (Fig. 4). Unfortunately, given the uncertainties attached to both these burial ages and their respective depositional ages, it is impossible by comparing the two set of ages (Fig. 4h) to identify neither sediments recycling nor recent re-exposure.

Major element and Sr-Nd isotopic compositions: evidence of a drainage change at 3-4 Ma?

The complete geochemical results measured along the Surai section, combining our new analyses with previously published data (Huyghe et al., 2001;Lupker et al., 2012b;Robinson et al., 2001;Szulc et al., 2006) are listed in Supplementary Table D. The main data are presented in age logs (Fig. 5) and in Na/Si vs. Al/Si and Sr vs. Nd plots (Fig. 6) with data from modern river sediments for comparison.

The Sr and Nd isotopic compositions of the silicate fractions are relatively close to the HHC and TSS endmembers of [START_REF] Morin | L'érosion et l'altération en Himalaya et leur évolution depuis le tardi-pleistocène: analyse des processus d'érosion à partir de rivière actuelles et passées au Népal Central[END_REF]: 87 Sr/ 86 Sr ratios range from 0.724 to 0.795 and Nd values from -19.2 to -14.7 (Fig. 6b). Neither isotopic composition displays any temporal variation (Fig. 5). The two modern Rapti River sediments plot in the middle of the Surai section sample array. These data suggest that the palaeodrainage basin of the Surai section has been dominated by erosion of the High Himalaya (HHC and TSS), with secondary contributions (0-30%) from the LH.

Siwalik sediments in the Surai section have Al/Si ratios ranging between 0.03 and 0.34. With a few exceptions, sediments older than ~3.5 Ma show Al/Si > 0.1, whereas several samples from the upper part of the section (i.e., younger than ~3.5 Ma) present Al/Si < 0.1, markedly below those of bed sediments or sandy banks of modern Himalayan front rivers, including the rivers draining the Siwaliks (Figs. 5 and6, Supplementary Table D). These results suggest a higher degree of sediment sorting than observed in modern main Himalayan rivers and floodplain banks. Yet, as the Himalayan range has encroached on the foreland basin, one may expect the length of transport to decrease toward the top of the section (Fig. 7), and hence the depositional area to be more proximal, resulting in decreased sorting. Consistently, to reach such low Al/Si values (<0.1) in the samples younger than ca.

3.5 Ma may require multiple episodes of erosion/transport of sediments that had already been sorted, potentially indicating the recycling of older Siwalik sediments already transported into the depositional plain.

Sediments of the Surai section have Na/Si ratios ranging from 0.001 to 0.035 that tend to decrease with decreasing depositional age, reaching values below 0.005 in the upper part of the section (i.e., younger than ~3-4 Ma). Na/Si ratios in the Surai section are far lower than those of the Ganga in Bangladesh (0.3-0.4), implying that weathering reactions affecting plagioclase were intense in the Surai section sandstones either at the time of deposition (Lupker et al., 2012b) and/or during recent exhumation of the Siwalik folds [START_REF] France-Lanord | Annual dissolved fluxes from Central Nepal rivers: Budget of chemical erosion in the Himalayas[END_REF]. Na concentrations in the Surai section are also markedly depleted compared to those in modern main Himalayan river sediments, which have Na/Si ratios ranging between 0.035 and 0.055 (Figs. 5 and6, Supplementary Table D). Only the Karnali River is characterized by lower Na/Si ratios between 0.020 and 0.035 that are comparable to those of the lower (>3-4 Ma) part of the Surai section, which has been attributed to a significant contribution from erosion of the Siwaliks (Lupker et al., 2012b). In contrast, the Na/Si ratios of sediments younger than ~3-4 Ma are significantly lower than those observed in Siwalik rivers, suggesting a higher degree of weathering for the upper conglomeratic part of the section. The increased weathering intensity in the upper part of the section is further supported by the higher degree of carbonate dissolution indicated by the lower CaO concentrations observed in sediments younger than 4 Ma (mean = 5.4 wt%,  = 6.3) compared to older sediments (mean = 11.6 wt%,  = 8.3; Supplementary Table D). This evolution (i.e., increased plagioclase weathering and carbonate dissolution upsection) is counter-intuitive, as one would expect the amplitude of weathering to decrease as deposition nears the Himalayan front and the residence time in the Ganga plain surface decreases. The most intense sediment weathering, as expressed by very low Na/Si values, therefore requires additional weathering. Such a process could occur in-situ during sediments exhumation toward the surface and could be favoured by the coarser grain size and hence the higher permeability found at the top of the section. However, first, if such process occurs, we would expect low Na/Si values at other stratigraphic level of the section where the grain size is also locally coarse (Fig. 4). Second, the sampled sediments were fresh as recently excavated by the construction of the road. Third, the grain size of the samples is similar throughout the section and fourth, the low Al/Si values found at the top request additional sorting and transport that can not be made in-situ.

Alternatively, additional weathering can be met for Siwalik sediments derived from already altered materials, for example by recycling of older Siwalik sediments. In such a case, sediments younger than ~3-4 Ma might represent sediments affected by two successive weathering cycles during the Himalayan orogeny.

Although Sr-Nd isotopic tracers reveal only that the palaeodrainage basin of the Surai section remained essentially steady, our and previous major element data indicate both (1) a strong increase in plagioclase weathering (leading to Na/Si ratios nearing zero) and ( 2) the appearance of well sorted sand since 3-4 Ma. Among the sediments exported by the modern river system in Nepal (Fig. 6a), only rivers that dominantly drain the Siwaliks carry sediments comparably well sorted (low Al/Si) and Na-depleted as those in the Surai section (Fig. 6b, Supplementary Table D). The geochemical characteristics of sediments in the Surai section are therefore consistent with an increasing proportion of sediment derived from recycling of the Siwaliks, not directly eroded from the High Himalaya. We note that such recycling is not incompatible with the stability of the Sr-Nd isotopic dataset, as recycling does not impact the original source signal (Lupker et al., 2012b;[START_REF] Morin | L'érosion et l'altération en Himalaya et leur évolution depuis le tardi-pleistocène: analyse des processus d'érosion à partir de rivière actuelles et passées au Népal Central[END_REF].

Despite very high denudation rates (>1 mm a-1 Lavé and Avouac, 2000) the small areal contribution of the Siwalik Hills to the drainage basins of most trans-Himalayan rivers prevents sediments in these large drainage basins from becoming significantly depleted in Na (or Al) upon addition of Siwalik-derived material (Fig. 6). For recycled, more mature Siwalik sediments to affect a marked change in the major element signal, as observed in the upper part of the Surai section, the relative sediment flux from terrains north of the MBT must be strongly reduced. We therefore interpret the major compositional change observed in the Surai section at ~3-4 Ma (Fig. 5) as a shift of the contributing drainage basin from a trans-Himalayan basin towards one draining the Lesser Himalaya (Fig. 7), characterized by reduced sediment flux due to a smaller basin area north of the MBT and greatly reduced erosion rates in the Lesser Himalaya compared to the High Himalayan regions [START_REF] Godard | Dominance of tectonics over climate in Himalayan denudation[END_REF]. This process does not require a reorganization of the river network across the mountain range in the Karnali region but can simply arise from steady facies migration in the Ganga plain during the growth and southward migration of the Himalayan range. As the locus of sedimentation associated with the Surai section progressively migrated towards the Himalayan front, the proportion of recycled Siwalik sediment could have suddenly increased upon crossing a boundary between two alluvial fans associated with a trans-Himalayan and a medium-sized drainage basin (Fig. 7).

Measured cosmogenic concentrations, corrections, and palaeoconcentrations

The complete results of our cosmogenic analyses are presented in Table 1 (raw data are provided in Supplementary Tables E). The measured 10 Be concentrations are on average relatively low and range between (1.00 ± 0.36) to (5.22 ± 0.98)  10 3 at g -1 . They decrease with increasing depositional age, as expected a priori from radioactive decay (Table 1). The relative uncertainties on these concentrations, which mainly arise from accelerator mass spectrometer counting statistics, are higher in the oldest strata, in which the measured concentrations are lower.

The modelled 10 Be contributions during transport in the Ganga plain are reported in Supplementary Table C. They range from (1.2 ± 0.2) to (1.6 ± 0.3)  10 3 at g -1 and from (1.3 ± 0.2) to (2.4 ± 0.7)  10 3 at g -1 considering the Karnali and Rapti fans, respectively. Although these values were calculated based on modern rivers, they may also represent the 10 Be contribution at the time of deposition. These contributions represent 4 to 20% of the total measured cosmogenic concentrations after correction for radioactive decay, irrespective of the fan model (Karnali vs. Rapti).

After correcting for radioactive decay and floodplain exposure, the 10 Be palaeoconcentrations of sediments leaving the Himalayan range that are directly related to its denudation, range between (4.9 ± 3.6) and (29.8 ± 9.8)  10 5 at g -1 (Table 1). The correction for floodplain exposure was applied consistent with a change of the contributing basin area due to facies migration at 4-3 Ma (see section 4.2): the floodplain corrections for sediments deposited before and after 4-3 Ma were based on the modern Karnali and Rapti Rivers, respectively.

Palaeodenudation rates

Again, consistent with a change of the contributing basin area due to facies migration at 4-3 Ma, we assumed that the topography of the drainage basin was similar to that of the present-day Rapti catchment during 3-0 Ma. The basin-averaged 10 Be cosmogenic surface production rates during this period were therefore estimated to be 10.95, 0.02, and 0.05 at g -1 a -1 for spallation and slow and fast muogenic processes, respectively. During 6-3 Ma, we considered that the catchment area was draining the entire Himalayas and estimated cosmogenic production rates of 39.59, 0.04, and 0.06 at g -1 a -1 for spallation and slow and fast muogenic processes, respectively.

Between 6 and 3.5 Ma, denudation rates were relatively steady at around 1.7 ± 0.3 mm a -1 in the palaeo-Karnali basin (Table 1 and Fig. 8) (except for one sample that returned a rate of ~3.9 mm a -1 , but which was affected by a large uncertainty of ±2.7 mm a -1 ). Palaeodenudation rates associated with a Rapti-type drainage basin during 3-0 Ma were lower, ranging from 0.6 ± 0.2 to 1.6 ± 1.1 mm a -1 , but denudation rates markedly increased during 1.5-0.5 Ma (Table 1 and Fig. 8). These values are higher than the modern denudation rates of 0.36 ± 0.03 and 0.33 ± 0.03 mm a -1 that we determined for present-day Rapti sediments upstream of the MBT, i.e., without any contribution from the actively uplifted and exhumed frontal fold. Only three samples (Cos14, 19, and 21; Table 1) returned palaeodenudation rates consistent (within uncertainties) with this modern rate.

Discussion

Robustness of the reconstructed 10 Be palaeoconcentrations and derived palaeodenudation rates

10 Be accumulation during sediment transport was likely of second order compared to that during denudation of the source rocks. Our sediment transport model, though simplified and strongly based on presentday basin characteristics, suggests that 10 Be concentrations increased by less than 10 and 15% of the concentrations in modern Rapti and Karnali sediments, respectively, during transport. We therefore extrapolate such moderate contributions during transport to the older deposits (though we note that this is dependent on our choice of model).

This contrasts, however, with Charreau et al.'s (2011) approach to estimate floodplain exposure by the steady sedimentation of thin sediment sheets at the long term subsidence rate (~0.5 mm/a), which predicted contributions during accumulation of ~7.3 10 3 at g -1 , or 40->600% of the total measured concentrations. These would make the correction for accumulation higher than the measured concentration and demonstrate that the approach of [START_REF] Charreau | Paleo-erosion rates in Central Asia since 9Ma: A transient increase at the onset of Quaternary glaciations? Earth Planet[END_REF] to calculate the contribution of 10 Be accumulation in the floodplain is not appropriate for the case of the Himalayan foreland.

In the lower part of the section (up to 2,500 m stratigraphic depth, reverse interval r9), our magnetostratigraphic correlations agree relatively well with previous correlations. The depositional ages and hence the derived 10 Be palaeoconcentrations in this lower part of the section are robust and reliable. Upsection, correlation of the n5-n9 sequence remains problematic but our new sampling documents the existence of several reversals and protracted periods of reversed polarities, suggesting that sediment deposition did not stop around 3 Ma (normal reference chron C2An), but lasted another several hundreds of thousands of years, probably even more up to the base of Matuyama chron (C1) around 0.7 Ma. Despite those uncertainties, the difference, for a given stratigraphic level, between the minimum and maximum ages found from all the 10000 correlations never exceeds ~1 Ma (Fig. 4). Hence and to the first order, our denudation rates and their temporal evolution are similar even when considering extreme age models (Supplementary Fig. B).

Our interpretation that the contributing drainage basin shifted at ~4-3 Ma is consistent with detrital apatite fission-track (AFT; van der Beek et al., 2006) and Ar-Ar ages (Szulc et al., 2006) that show a ~2 Ma increase in lag time (based here on our revised ages) after ~4-3 Ma (Fig. 9). Such an increased depositional lag time is evident even if we consider the magnetostratigraphic correlation that gives the oldest depositional ages in the upper part of the section (i.e., dating the top at ~2 Ma; Fig. 4). This increased lag time may reflect either (1) decreased exhumation rates across the drainage basins or (2) the reworking of older detrital grains. Indeed, the AFT age at the top of the section is older than those lower in the section (Supplementary Fig. C;van der Beek et al., 2006); as this is unrealistic in such an orogenic setting, it suggests the reworking of older grains. However, some cautions are needed because the increase in thermochronological lag-time is supported by only one AFT sample and is not seen in the ZFT data. But, similarly, provenance analyses of the Mohand-Rao sections (northwest India) evidence the recycling of Siwalik sediments, there starting at ~5.5 Ma [START_REF] Mandal | Multiproxy Isotopic and Geochemical Analysis of the Siwalik Sediments in NW India: Implication for the Late Cenozoic Tectonic Evolution of the Himalaya[END_REF]. We therefore continue our discussion under the assumption that the contributing drainage basin migrated across distinct zones of fan deposition at 3-4 Ma and subsequently recycled older Siwalik sediments.

The impact of such sediment recycling on our derived denudation rates must be evaluated before discussing the palaeo-denudation rates. The 10 Be concentrations measured at the outlet of the palaeo-Rapti basin reflect the mixing of 10 Be-enriched sediments from the Lesser Himalaya with 10 Be-deprived sediments from the rapidly exhuming frontal Siwaliks folds. However, the initial concentrations in Siwalik sediments before exhumation may not be null, and could be inherited from the original Mio-Pliocene erosion of the Himalayan ranges and vary according to the exhumation level. Most original 10 Be in the older Lower and Middle Siwaliks sediments has already radioactively decayed. The incorporation of such depleted sediments could lower the concentration measured in the river. Siwalik sediments eroded and shed to the river between the two modern samples (RapCos 1 and 2) are mainly Middle to Lower Siwalik. Yet, the 10 Be concentrations measured in the two samples are not significantly different likely because the addition in RapCos1 represents less than 10% of the drainage area of Rap-Cos2, and secondly, because this drained Siwalik part is not the most active one in terms of tectonic uplift and therefore probably in terms of erosion (Lavé and Avouac, 2000). Alternatively, in the younger Upper Siwaliks sediments the initial and inherited 10 Be concentrations has not yet radioactively decayed and remain elevated. During the past, the erosion of the most frontal and rapidly uplifting structures may have produced a significant volume of such younger enriched sediments equivalent to the Upper Siwalik and biased the cosmogenic signal. To explore further potential variations of denudation in the palaeo-Rapti basin, we therefore built a simple model of frontal fold erosion and sediment mixing with a LH-draining river such as the Rapti River (see Supplementary materials for more details). We calculated, as a function of the time since fold initiation (assumed at 3 Ma), the evolution of the 10 Be concentration expected from the denudation of the Siwalik Hills only (

) and the final concentration after mixing of recycled Siwalik sediments with sediments shed from the 𝑁 Siw upstream midland basin above the MBT (

). Consistent with modern 10 Be concentrations measured in the Rapti 𝑁 out basin upstream of the Siwaliks at the MBT, we assumed that the upstream sediments shed from the Rapti basin during Siwaliks reworking have a steady 10 Be concentration of (~20 ± 2)  10 3 at g -1 . We hence assume a steady denudation rates ~0.3 mm/a across the Rapti basin upstream of the Siwaliks at the MBT. The mean 10 Be concentration of the reworked Siwalik sediments can be inferred from the concentration measured in 7-3 Ma sediments in the Surai section, corrected for radioactive decay and transport in the floodplain (~17 ± 6)  10 3 at g -1 .

The model results are shown in Figure 10 and compared to the 10 Be palaeoconcentrations measured in 0-3 Ma sediments of the Surai section. Within errors, the measured palaeoconcentrations are consistent with the modelled values. The model also indicates that the 10 Be concentrations may vary by a factor of >2 between 𝑁 out the early and later stages of frontal fold development due to the exhumation of Siwalik sediments with variable 10 Be inheritances. Although the model results are strongly dependent on various parameters (e.g., the relative areas of the Midland and Siwalik parts of the catchment, and the erosion rate of the Midland), the model yields two important insights. First, the variations of 10 Be palaeoconcentrations measured in Surai sediments younger than ~3.5 Ma may reflect frontal fold activity, or even steady fold accretion, and not necessarily a change of erosion rate in the contributing drainage basin. Second, the low 10 Be concentrations measured in the upper Surai section, which are up to 5 times lower than present-day concentrations in Rapti River sediments above the MDT, can indeed be explained by mixing with 10 Be-depleted material shed from rapidly exhumed Siwalik sandstone in the hanging wall of the MFT. Therefore, we propose that recycling of Na-and 10 Be-depleted materials through denudation of the Siwalik belt has strongly impacted the chemistry and 10 Be concentrations of the sands exported by frontal-or midland-draining rivers and their associated fan deposits; additional measurements (major elements, 10 Be,  Nd ) of recent Rapti River sediments up-and downstream of the MFT fold should be performed to fully ascertain our assessment. To reconstruct palaeodenudation rates from in situ cosmogenic 10 Be analyses of foreland sediments, it is therefore critical to trace potentially recycled sediments. In the Himalayas, although Sr-Nd isotopes have been widely used to study sediment provenance, our results and those of [START_REF] Mandal | Multiproxy Isotopic and Geochemical Analysis of the Siwalik Sediments in NW India: Implication for the Late Cenozoic Tectonic Evolution of the Himalaya[END_REF] suggest that these geochemical proxies are insensitive to sediment recycling, which is better identified by simpler major and trace element analyses, notably Al/Si and Na/Si, which are linked to weathering and sorting processes, respectively (Lupker et al., 2012b[START_REF] Lupker | A Rousebased method to integrate the chemical composition of river sediments : Application to the Ganga basin[END_REF].

Implications for the late Cenozoic evolution of denudation rates in the central Himalayas

The palaeodenudation rates associated with the Karnali-type basin from ~6 to 3 Ma are close to the modern denudation values measured along the Karnali downstream of the Siwaliks (Fig. 8; Lupker et al., 2012a).

Though denudation rates since 3.5 Ma for a Karnali-type basin are missing from our record, this similarity might suggest that average denudation rates in the Karnali basin may have remained steady at around ~1.7 ± 0.3 mm a -1 since ~6 Ma. For the period 3-0 Ma and sediments associated with smaller basins like the Rapti, direct comparison of 10 Be palaeodenudation rates and observed modern denudation rates is complicated by the recycling of Siwalik sediments. If we accept that the low 10 Be concentrations measured in the upper Surai section (<3 Ma) are related to the recycling of older, 10 Be-depleted Siwalik sediments, then our denudation rates should be considered maximum values. Although it is almost impossible to reconstruct the true palaeodenudation rates in the basin upstream of the Siwaliks, our simple sediment mixing model suggests that denudation rates similar to modern rates yield theoretical 10 Be concentrations consistent with those measured downstream of the Siwaliks. Consequently, a steady denudation at around ~0.3 mm a -1 since 3 Ma in the palaeo-Rapti basin, together with recycling in the Siwalik, could produce the 10 Be concentrations that we observe in the Surai section. Although this conclusion is tentative given the over-simplifications in our recycling model, the range of expected 10 Be palaeo-(≥0.3 and << 1.6 mm a -1 ) and modern Rapti basin denudation rates (~0.3 mm a -1 , as measured above the MDT; Figs. 1, 2, and8) are consistent with denudation rates of 0.1-0.5 mm a -1 reported in other midland drainage basins of central Nepal [START_REF] Godard | Dominance of tectonics over climate in Himalayan denudation[END_REF]Scherler et al., 2014). These 10 Be data thus suggest that (1) denudation rates are higher in the HHC than in the LH and (2) denudation may have remained steady since ~6 Ma, at least in the Karnali basin.

As no in situ thermochronologically derived denudation rates are available for sites nearer the Karnali and Rapti basins, we compare our 10 Be denudation rates to in situ thermochronological data for the Sutlej region (western India, near the Nepal border) and for central Nepal (i.e., at the longitude of Kathmandu). Denudation rates in those areas range between ~0.5 and 3 mm a -1 [START_REF] Bojar | Timing and mechanisms of Central Himalayan exhumation: Discriminating between tectonic and erosion processes[END_REF][START_REF] Herman | Exhumation , crustal deformation , and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography[END_REF][START_REF] Patel | Exhumation history of the higher Himalayan Crystalline along Dhauliganga-Goriganga river valleys, NW India: New constraints from fission track analysis[END_REF][START_REF] Thiede | Large spatial and temporal variations in Himalayan denudation[END_REF]. Consistent with our findings, thermochronologically derived denudation rates tend to be lower in the LH (0.3-0.5 mm a-1) than in the HHC, where rates can reach up to ~3 mm a -1 (e.g., [START_REF] Herman | Exhumation , crustal deformation , and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography[END_REF][START_REF] Thiede | Large spatial and temporal variations in Himalayan denudation[END_REF]). High denudation rates in the HHC have also been reported from in situ thermochronological data in other regions across the Himalayas [START_REF] Blythe | Plio-Quaternary exhumation history of the central Nepalese Himalaya: 1. Apatite and zircon fission track and apatite [U-Th]/He analyses[END_REF][START_REF] Herman | Exhumation , crustal deformation , and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography[END_REF][START_REF] Huntington | Climate change and Late Pliocene acceleration of erosion in the Himalaya[END_REF][START_REF] Thiede | Large spatial and temporal variations in Himalayan denudation[END_REF], and petrological and mineralogical analyses, sediment flux monitoring [START_REF] Garzanti | Quantifying sand provenance and erosion ( Marsyandi River[END_REF], and fluvial incision rates (e.g., [START_REF] Lavé | Fluvial incision and tectonic uplift across the Himalayas of central Nepal[END_REF] further suggest that the LH denude at lower rates than the HHC. This discrepancy between the LH and HHC is likely due to crustal wedge deformation and duplexing along the Main Himalayan Thrust, which orographically triggers monsoonal precipitation, enhancing denudation in the HHC [START_REF] Herman | Exhumation , crustal deformation , and thermal structure of the Nepal Himalaya derived from the inversion of thermochronological and thermobarometric data and modeling of the topography[END_REF][START_REF] Thiede | Large spatial and temporal variations in Himalayan denudation[END_REF].

To further examine the temporal evolution of our 10 Be denudation rates, we compared with denudation histories derived from thermochronological analyses of detrital grains. In the Karnali section, assuming a geothermal gradient of 20 °C, Bernet et al. (2006) and van der Beek et al. (2006) reported exhumation rates of ~1-2 mm a -1 . In the Surai section, if we consider only the period before the recycling of Siwalik sediments (>3-4 Ma), likely also associated with the Karnali basin, revised zircon fission track (ZFT) and Ar-Ar lag times are on average ~4 and ~6 Ma, respectively, whereas AFT ages are similar to depositional ages (Bernet et al., 2006;Szulc et al., 2006;van der Beek et al., 2006) (Supplementary Fig. C). Van der Beek et al. (2006) assumed a geothermal gradient of ~20 °C and ZFT and Ar-Ar closure temperatures of 240 °C and 350 °C, respectively. This would yield respective denudation rates of ~3 and ~2.9 mm a -1 , markedly higher than 10 Be-derived denudation rates for the Karnali and Surai sections. Two effects may explain this discrepancy. First, interpretation and population modelling of detrital age distributions are not straightforward and may suffer from several biases [START_REF] Naylor | Bias in detrital fission track grainage populations: Implications for reconstructing changing erosion rates[END_REF][START_REF] Sundell | Unmixing detrital geochronology age distributions[END_REF]. Indeed, recent slow denudation rates may be unrepresented to detrital analyses because the rocks may have not yet reached the surface [START_REF] Naylor | Bias in detrital fission track grainage populations: Implications for reconstructing changing erosion rates[END_REF][START_REF] Willenbring | .1n C4An C4Ar.1n C4Ar.2 C5n.1n C5r.1n C5r.2n C5An.1n C5An.2n C5Ar.1n C5Ar.2n C5AAn C5ABn C5n.2n C3An.1n C3n.1n C3n.2n C3n.3n C2An.3n C2An.2n C2An.1n C2r.1n C2 C1r.2r-1n C1n C1r.1n C3An.2n C3n.4n Bibliography[END_REF], yielding overestimated denudation rates during the last few million years. Moreover, as detrital denudation rates are derived by comparing only the youngest peak to depositional ages, such denudation rates are maximum values that may preferentially sample the most eroding regions within a basin and, hence, may not truly average denudation throughout the basin.

Second, detrital thermochronological ages and denudation rates depend on the local geotherm, which may reach 100 °C/km in regions of rapid tectonic exhumation [START_REF] Braun | Quantitative Constraints on the Rate of Landform Thermochronology[END_REF]. Although high geothermal gradients (>50 °C/km) were observed in the Namche Barwa region [START_REF] Craw | Fluid evolution and thermal structure in the rapidly exhuming gneiss complex of Namche Barwa -Gyala Peri , eastern Himalayan syntaxis[END_REF], this region presents extreme tectonic exhumation rates and thus may not be a good analogue of the Karnali basin. Geothermal gradients of 20-50 °C/km have also been reported in central Nepal [START_REF] Bollinger | Mountain building in the Nepal Himalaya : Thermal and kinematic model[END_REF], but correspond to the LH where denudation is relatively slow (<1 mm a -1 ). Therefore, a more realistic value of the average geothermal gradient for the entire Karnali basin is likely ~50 °C/km, which would yield detrital thermochronological denudation rates of 0.6-1.2 mm a -1 in the Karnali and Surai sections. These values are more consistent with our 10 Be-derived rates for a Karnali-type drainage basin. However, these thermochronological denudation rates remain crude estimates. To provide more accurate values a careful reassessment of the detrital thermochronological data is needed including for example Monte Carlo simulations of the age population based on more realistic and spatially variable geothermal gradients (e.g. [START_REF] Brewer | Modelling detrital cooling-age populations : insights from two Himalayan catchments[END_REF].

Importantly, Lenard et al. (2020) suggest, from 10 Be analyses of the sediments of the Bay of Bengal seabed, a steady denudation across the Himalayas since 6Ma. In the Karnali section, depositional lag times determined from ZFT and AFT ages remain remarkably steady from ~14 to ~3 Ma and from ~7 to ~3 Ma, respectively (Bernet et al., 2006;van der Beek et al., 2006). In the Surai section, if we exclude the youngest part of the section where recycling likely occurred (<3-4 Ma), lag times determined from AFT, ZFT, and Ar-Ar ages are also relatively steady (Supplementary Fig. C). Even if we cannot fully exclude variable denudation rates in the Karnali basin because we lack direct records since ~3 Ma, both 10 Be data and previous detrital thermochronological data are at least consistent with steady palaeodenudation rates in the region since the late Miocene. All together, these results suggest a limited impact of Quaternary climatic oscillations on denudation rates in the Himalaya [START_REF] Lénard | Steady erosion rates in the Himalayas through late Cenozoic climatic changes[END_REF] as is the case in the Tianshan range [START_REF] Puchol | Limited impact of Quaternary glaciations on denudation rates in central Asia[END_REF] or in the southern California [START_REF] Oskin | Steady 10Be-derived paleoerosion rates across the Plio-Pleistocene climate transition, Fish Creek-Vallecito basin, California[END_REF]. In that sense, they seem to contradict thermochronological data results that document an accelerated Quaternary denudation rates over large mountain ranges [START_REF] Herman | Worldwide acceleration of mountain erosion under a cooling climate[END_REF].

Conclusions

The Surai section in the central Siwaliks is well suited to reconstruct palaeodenudation rates from in situ cosmogenic 10 Be analyses. 10 Be concentrations measured in 19 samples were transformed to palaeoconcentrations based on several corrections and to palaeodenudation rates by assuming that cosmogenic palaeoproduction rates were similar to those of the modern drainage basins and a shift from a trans-Himalayan to a midland-draining basin at 4-3 Ma as suggested by major and trace element data. Consequently, we propose that denudation rates in the palaeo-Karnali basin may have been steady around ~1.7 mm a -1 from ~6 to ~3 Ma. These denudation rates are similar to modern values observed downstream of the Siwaliks, suggesting steady denudation rates across the Karnali basin since ~6 Ma. Our data suggest that denudation rates may have accelerated from ~0.4-1.1 mm a -1 during ~3-1.5 Ma to ~1.5 mm a -1 since ~1.5 Ma in the palaeo-Rapti basin. However, based on a sediment-mixing model, we argue that this is likely only an apparent acceleration due to the reworking of older Siwalik sediments depleted in 10 Be. Consequently, denudation in the Rapti basin upstream of the MBT may have also remained steady at ~0.3 mm a -1 since ~3 Ma. These 10 Be-derived denudation rates are consistent with those derived from detrital and in situ thermochronological data. They also suggest varied denudation rates across the range that are likely controlled by deformation of the structural wedge and orographic effects. Our results argue against accelerated Quaternary denudation rates inferred from inversion of 1D thermochronological data, and instead suggest a limited impact of Quaternary climatic oscillations on Himalayan denudation rates. However, testing the impact of glaciations on Himalayan denudation at this scale requires cosmogenic data directly recording denudation during 3-0 Ma, which is complicated in the Surai section by basin reorganisation 4-3 Ma ago. Nevertheless, our study demonstrates that it is possible to reconstruct palaeodenudation rates from the in situ 10 Be concentrations of buried Siwalik sediments, as long as sediment sources and recycling are traced via several geochemical proxies. The red line in (f) shows the best-ranked minimum cost correlation that we used to calculate depositional ages (h) Cosmogenic burial ages plot against their depositional ages derived from the minimum cost magnetostratigraphic correlation. The uncertainties in depositional ages are calculated using the minimum and the maximum ages derived from the 10,000 magnetostratigraphic correlations.

Figure captions

Fig. 5:

Temporal evolution of the geochemical records measured along the Surai section. The Na/Si and Al/Si ratio can be interpreted as proxies for weathering intensity and sorting, respectively [START_REF] Lupker | A Rousebased method to integrate the chemical composition of river sediments : Application to the Ganga basin[END_REF]. The 87 Sr/ 86 Sr and  Nd isotopic ratio can be compared to the isotopic composition of the main Himalayan geological unit to track back potential changes in rock sources (e.g. [START_REF] France-Lanord | Evolution of the Himalaya since Miocene time: isotopic and sedimentological evidence from the Bengal Fan[END_REF]. Depositional ages were calculated using the best-ranked minimum cost correlation (red line in Fig. 4). Blue and purple stars indicate values measured in modern sands of the Rapti River upstream (Rap-Cos1) and downstream (Nag9) of the Siwalik Hills, respectively (see Fig. 2 for locations). Yellow and orange stars indicate values measured in modern sand of the Karnali river downstream of the Siwalik and at the MBT, respectively (Lupker et al., 2012b). plot of the Surai data compared to the values expected for the three main Himalayan units [START_REF] Deniel | Isotopic study of the Manaslu granite (Himalaya, Nepal): inferences on the age and source of Himalayan leucogranites[END_REF][START_REF] France-Lanord | Evolution of the Himalaya since Miocene time: isotopic and sedimentological evidence from the Bengal Fan[END_REF][START_REF] Morin | L'érosion et l'altération en Himalaya et leur évolution depuis le tardi-pleistocène: analyse des processus d'érosion à partir de rivière actuelles et passées au Népal Central[END_REF][START_REF] Parrish | Isotopic constraints on the age and povenance of the Lesser and Greater Himalayan sequence, Nepalese Himalaya[END_REF]Robinson et al., 2001). were calculated using ages derived from the minimum-cost magnetostratigraphic correlation (Fig. 4f). The uncertainties on denudation rates were calculated using a Monte-Carlo approach similar to [START_REF] Puchol | Limited impact of Quaternary glaciations on denudation rates in central Asia[END_REF].

Denudation rates calculated based on the minimum and maximum ages from the 10,000 stochastic magnetostratigraphic correlations are shown in Supplementary Fig. B. Brown, yellow, blue and red stars indicate denudation rates derived from the analysis of modern samples collected and analysed by Lupker et al. (2012) or in this study (RapCOS1 and 2), respectively. Fig. 9: Depositional lag times of detrital AFT (blue circles), ZFT (red circles), and Ar-Ar ages (black dots) measured along the Surai section (Bernet et al., 2006;Szulc et al., 2006;van der Beek et al., 2006). The ZFT data correspond to the moving peak age. The depositional ages were calculated using the minimum-cost correlation (red line in Fig. 4f). The continuous black line is the 1:1 line where the thermochronological ages equal the depositional ages (i.e., a lag time of zero). Dashed lines are the lag-time contours for every 2 Ma. 1) a steady denudation rates of ~0.3 mm/a in the basin upstream of the MBT, (2) a denudation of the Siwalik of 0.9 mm/a to balance the uplift rates due to tectonics and (3) an inherited 10 Be concentration of denuded Siwaliks similar to the concentration measured for 6-3 Ma old sediments ((~17 ± 6)  10 3 at g -1 ) (b) Curves of the expected mean 10 Be concentrations for sediments exported directly from the Siwalik Hills (blue, N Siw ) sediments at the outlet of the entire basin (red, N out ), including reworked Siwalik sediments. We assume that folding, denudation, and sediment reworking in the Siwaliks began at ~3 Ma. The red circles show the corrected cosmogenic concentrations measured in the upper Surai section (3-0 Ma) (see Table 1). Table 1: In situ 10 Be and 26 Al cosmogenic data and associated denudation rates with their uncertainties given at 1. Depositional ages were calculated using the minimum-cost correlation (red line, Fig. 4f); the maximum and minimum ages derived from the 10,000 correlations calculated using the DTW algorithm are also indicated. 10 Be p are palaeoconcentrations at the time of deposition, i.e., corrected for radioactive decay and transport in the floodplain. d.l.: under detection limit.

Tables

Table 2: Palaeomagnetic results. N, normal polarity; R, reverse polarity; Trans., transitional direction; Unst., unstable demagnetization; GC, great circle. Values in italics are the mean values used for remanent directions that defined great-circle paths, but nonetheless provided an unambiguous polarity.

Tables Table 1: In situ 10 Be and 26 Al cosmogenic data and associated denudation rates (). Depositional ages were calculated using the minimum-cost correlation (red line, Fig. 4f); the maximum and minimum ages derived from the 10,000 correlations calculated using the DTW algorithm are also indicated. 10 Be p are palaeoconcentrations at the time of deposition, i.e., corrected for radioactive decay and transport in the floodplain.

Latitude

Longitude b. c. d. e. f. g. [START_REF] Ojha | Magnetic polarity stratigraphy of the Neogene foreland basin deposits of Nepal 61-90[END_REF] Cosmogenic burial ages Appel et al., 1991;Rosler et al., 1997 This study:

Magnetostratigraphic analyses

Sample treatments and results of the demagnetization

Bedding attitude, sun correction, and GPS coordinates were measured at each sampling location. The thickness between two successive sites was directly measured with a tape measure when possible or estimated from the GPS coordinates and elevations. All sampling sites were stratigraphically replaced according to the stratigraphic depths of Rosler et al. (1997), which were recalculated from Appel et al. (1991). To isolate the characteristic and primary magnetic remanence directions, the 36 samples were treated using 10-12 steps of alternating field (AF) demagnetization in the palaeomagnetism laboratory of the Ludwig-Maximilians University of Munich. The remanence at each step was measured using a three-axis DC superconducting quantum interference devices (Squid). From the 36 analysed samples, 19 showed a component that decayed toward the origin and could be isolated (Fig. 3a andb, Table 2). We identified a relatively stable component that usually decayed to the origin under a medium field (10-30 mT), suggesting that the remanent directions were carried by a magnetic phase of relatively low coercivity. Isothermal remanent magnetization (IRM) acquisition and thermal demagnetization of an IRM suggested the presence of hematite, magnetite, and goethite (Appel et al., 1991). As hematite dominated the magnetic remanence in most of the section, magnetite prevails in the upper part (Appel et al., 1991) and is likely the main magnetic carrier in our samples. A second component was sometimes identified under lower fields (<10 mT), and the magnetic directions were often unstable under higher fields (>30 mT). The AF demagnetization was probably unsuccessful in separating the hard magnetic hematite and goethite remanences (Appel et al., 1991), although they represent second order magnetic phases in this upper part of the section. Among these 19 samples, 10, 5, and 4 were identified as reverse, normal, and transitional directions, respectively (Table 2). Assuming a Fisherian distribution, the overall mean direction in geographic (g) and stratigraphic (s) coordinates (Fig. 3e andf) is D g = 348.2°, I g = 57.7°, k g = 22.4, a 95g = 2.5°, and D s = 356..°, I s = 25.9°, k s = 21.3, a 95s = 2.5°, respectively, where D is declination, I is inclination, k is the precision parameter, and a 95 is the 95% confidence radius. The 5 samples interpreted to have transitional directions had either E-W declination or negative inclination and a declination consistent with normal polarity. The 10 reverse directions are more reliable, having demagnetization diagrams of better quality yielding a mean of D g = 154.1°, I g = -79.9° , k g = 16.5, a 95g = 13.1, and D s = 158.8°, I s = -29.9°, k s = 16.5, a 95s = 13.1°. This mean direction is consistent with the mean value of the reverse directions that we found in the lower part of the section (D s = 178.6°, I s = -23.2°, k s = 4.9, a 95s = 4.4°), strengthening our conclusion that our new data and interpretations are reliable despite the large uncertainties in the magnetic directions and the poor quality of the demagnetization.

Of the remaining samples, 5 showed unstable directions that were useless for establishing the polarity column (Fig. 3c) and 12 had remanence trajectories spread out about great circles (Fig. 3d) that never reached stable endpoints but had clear polarities.

Correlations using all polarity intervals

When all polarity intervals are considered, all 10,000 correlations found using the DTW algorithm and the numerical approach developed by Lallier et al. (2013) date the section to between ~14-13 and ~2-1 Ma (Fig A ).

However, in that case, the correlations result in sharply reduced (by a factor of 2-3) accumulation rates above ~2 km stratigraphic depth (Fig. A). 2013) considering all polarity intervals, including the reverse interval r5 that was omitted in the final magnetostratigraphic correlation (see main text and Fig. 4).

Indeed, the eight reversals identified around 2 km depth (n5 to n9) are sandwiched between two long reverse polarity intervals (r4 and r9) and cannot be easily correlated to a sequence of the reference scale with the same number of reversals. Yet, the automatic method used here assumes that all reference chrons are known; otherwise, infinite correlations would be found. Therfore, the studied magnetostratigraphic column cannot have more polarity intervals than chrons in the reference scale (Lallier et al., 2013) and the accumulation rate must rapidly vary to account for this high number of reversals between n5 and n9. Because the Neogene has been intensively studied, especially using cyclostratigraphy, we consider the reference scale to be complete during this period (Ogg, 2012).

No major unconformity, erosional surfaces, nor strong sedimentary facies changes that could explain such a rapid drop in the accumulation rates have been identified at this depth. Therefore, an alternative solution must be considered. Local faults may yield stratigraphic repetition of the series. Furthermore, in such a fluvial environment, due to channel incision, stratigraphic horizons could have been deposited at greater depths than the previous older level. Both these effects could be hidden by surface slope deposits and/or discontinuous outcrops. If unidentified during field sampling, a given horizon could be sampled several times, yielding an extra polarity interval unknown in the reference scale. Hence, we performed a second test where the reverse interval r5, which relies on only two samples of the same polarity, was arbitrarily omitted during the calculations. This correlation is used in the final magnetostratigraphic column in the main text.

Major and trace elements and Sr/Nd isotopic analyses

Major and trace element concentrations in bulk aliquots of ~100 mg of sediment were analysed after lithium metaborate fusion [START_REF] Carignan | Routine Analyses of Trace Elements in Geological Samples using Flow Injection and Low Pressure On-Line Liquid Chromatography Coupled to ICP-MS: A Study of Geochemical Reference Materials BR, DR-N, UB-N, AN-G and GH[END_REF] by inductively coupled plasma atomic emission spectroscopy and inductively coupled plasma mass spectrometry, respectively, at the Service d'Analyses des Roches et des Minéraux (SARM-CRPG, Nancy, France). The Sr and Nd isotopic compositions of 12 new samples (ten palaeosediments along the Surai section and the two river bed samples, Rap-Cos1 and Nag9) were analysed to complete the published dataset (Huyghe et al., 2001;Robinson et al., 2001). Sr and Nd isotopic compositions were analysed at the CRPG on decarbonated fractions of sediment following the technique described in [START_REF] Galy | The strontium isotopic budget of Himalayan rivers in Nepal and Bangladesh[END_REF] and Pierson-Wickman et al. (2001).

Cosmogenic analyses and sample treatment

All cosmogenic samples collected for palaeodenudation reconstructions were crushed and sieved to isolate the 200-800 m fraction. The quartz fraction was first enriched through successive magnetic separations and flotation.

To eliminate all other mineral phases, the quartz-enriched fraction underwent successive leachings in a mixture of 4/5 H 2 SiF 6 + 1/5 HCl. The pure quartz fraction then underwent three partial dissolutions in concentrated HF, removing 30% of the quartz to eliminate atmospheric 10 Be [START_REF] Brown | Examination of surface exposure ages of Antarctic moraines using in situ produced 10Be And 26Al[END_REF]. The purified quartz fraction was then spiked with 100 μL of a carrier solution with 3,025 ppm 9 Be before being completely dissolved in HF. These house-made carrier solutions were prepared from phenakite minerals and have extremely low 10 Be/ 9 Be ratios (<4  10 -16 ), which is particularly important to accurately analyse old samples with very low 10 Be concentrations.

Following subsequent purification on chromatographic columns by anion exchange, cation exchange, alkaline precipitations, and oxidation, the purified beryllium and aluminum oxides were analysed at the French national accelerator mass spectrometer facility ASTER (Accelerator for Earth Sciences, Environment and Risks, CEREGE laboratory, Aix en Provence, France). 10 Be/ 9 Be ratios measured at ASTER are normalized using the SRM 4325 NIST reference material, considering an assigned reference 10 Be/ 9 Be ratio of (2.79 ± 0.03)  10 -11 . This standardization is equivalent to 07KNSTD within rounding errors. Total analytical 10 Be/ 9 Be blank ratios were, on average, (1.5 ± 0.8) × 10 -15 for these series (see Table E, below).

For 26 Al measurements, in samples that contained insufficient natural 27 Al for isotopic dilution to measure 26 Al, we added a commercial 1,000 ppm 27 Al standard (typically between 700 and 1,000 μL). 26 Al/ 27 Al ratios were calibrated against ASTER's in-house standard SM-Al-11, assuming a 26 Al/ 27 Al value of (7.40 ± 0.06)  10 -12 .

Analytical 26 Al/ 27 Al blank ratios were, on average, (3 ± 4) × 10 -15 for these series (see Table D,below

Cosmogenic production rates

We used the Basinga GIS tool and a cell-by-cell approach to compute the average production rates of the modern drainage basins using the Lal-Stone scheme, the ERA40 atmosphere, and the following scaling factors at sea level and high latitude (Charreau et al., 2019): S n = 4.11 ± 0.19 at g -1 yr -1 (worldwide average value from CREp; [START_REF] Martin | The CREp program and the ICE-D production rate calibration database : A fully parameterizable and updated online tool to compute cosmic-ray exposure ages[END_REF], S µs = 0.011 ± 0.001 at g -1 yr -1 , and S µf = 0.039 ± 0.004 at g -1 yr -1 (Braucher et al., 2011). These computed basin-averaged production rates exclude a correction for glacial and snow shielding. In the Himalaya snow shielding affects modern production rates by in average less than 5% (Scherler et al., 2014). Similarly, a correction of the modern glacial extent derived from the GLIMS database [START_REF] Raup | The GLIMS geospatial glacier database: a new tool for studying glacier change[END_REF] yield to cosmogenic production rates ~11% lower than the value we used. These corrections are quite low when compared to the other uncertainties associated to the method. However, neglecting the snow and glacial shielding implies that the cosmogenic production rates and, hence, the denudation rates, must be considered as maximum values.

Cosmogenic burial ages

Burial ages t burial were calculated using the Matlab code of [START_REF] Blard | Two MATLAB programs for computing paleoelevations and burial ages from paired-cosmogenic nuclides[END_REF], which solves the following equation:

P 1 N 1 e  1 .t burial  P 2 N 2 e  2 .t burial   1   2 f
where N 1 and N 2 and  1 and  2 are two radiocative nuclides (here the pair 10 Be-26 Al) and their decay constants, respectively. f is the spatial production rate scaling factor and is dimensionless. It is calculated according to the Stone model (Stone, 2000) and using the standard atmosphere. This equation has no analytical solution and is therefore numerically solved to determine t burial . Hence, the code uses a Monte Carlo approach: 5000 random raws are performed assuming 10 Be and 26 Al follow normal distributions (N 10 ,  10 ) and (N 26 ,  26 ), N 10 and N 26 (at.g -1 ) being the measured concentrations in one sample and  10 and  26 (at.g -1 ) their corresponding one sigma analytical uncertainties (see Table 1 andE).

Model of floodplain transport

We calculate the average floodplain concentration at distance x down the valley axis, N fp (x), from the sediment concentration within the channel, N c (x), according to (eq. 17 in Lauer & Willenbring, 2010):

(2)

𝑁 f (𝑥) = 1 ( 1 𝑡 ex + 1 𝑡 agg + 𝜆) ( 𝑁 c (0) 𝑡 prod + ( 1 𝑡 ex + 1 𝑡 agg ) 𝑁 c (𝑥) )
where, N c and N f are the 10 Be concentrations in sediments in the channel and the top surface of the floodplain, respectively, and t agg , t ex , and t prod are the characteristic timescales of sediment sequestration due to aggradation, lateral exchange, and 10 Be production, respectively. These timescales depend on the distance to the range front x, channel depth, sinuosity, migration rate, and mean sediment flux at the range outlet, subsidence rate, the width of the alluvial fan or floodplain, and the fractions of the analysed granulometric classes in the transported and deposited sediments. We add to this model deep (mostly muogenic) 10 Be production (following Braucher et al., 2011) once the sediment is buried below the depth of re-erosion, H, of the active channel at bankfull discharge, so that:

(3)

𝑁 fp (𝑥)≅𝑁 f (𝑥) -𝑁 c (0) + ∑ 𝑗 = 1,3 𝑃 fp 𝑗 𝑗 𝛬 𝑗 𝜎𝜌 s 𝑒 - 𝐻𝜌 s 𝛬 𝑗
where  is the mean aggradation/subsidence rate in the floodplain,  s the sediment density, and the 10 Be 𝑃 fp 𝑗 production rate (including nucleogenic and muogenic contributions) at the floodplain elevation, but neglecting radioactive decay in the initial equation (valid as long as ).

𝜎𝜌 s

𝛬 𝑗 ≫ 𝜆

This model produces a mean gain in 10 Be concentration in the plain as function of the distance to the mountain front. To apply this correction to our different samples, we must estimate their palaeoposition relative to the mountain front at the time of deposition for x. This was calculated considering the sediment depositional ages and by equating the mountain wedge-front migration rate to the mean facies migration rate of 15 ± 5 mm yr -1 for the central part of the Indo-Gangetic Plain (Lyon-Caen and Molnar, 1985).

The geomorphic characteristics and parameters describing rivers draining through the Ganga plain and used as input parameters in the model are reported in Supplementary Table A. We considered two end-members:

sediment transport and deposition either by a trans-Himalayan river such as the present Karnali River or by a medium-sized river such as the present Rapti River, which only drains the frontal and southern part (or Midlands)

of the Himalayan range. The parameters used were derived from published studies (as for sediment flux), measurements made in similar Himalayan rivers (as for grain size fractions, see Supplementary Table A), or from our own measurements (including sedimentation rates derived from our magnetostratigraphic results and lateral channel migration rates based on channel locations determined from over two decades of satellite imagery available on Google Earth). As our objective was only to provide an order-of-magnitude estimate of the 10 Be concentrations accumulated during sediment transport in the plain, we made several simplifications. As applied in Lauer and Willenbring (2010) or Lupker et al. (2012), we assumed that model parameters such as the depth and aggradation rate of the active floodplain and the sinuosity and lateral migration rate of the channel are spatially uniform from the mountain front to the plain outlet; downstream sediment fining was also omitted. Finally, to estimate the palaeolocation x, we only considered depositional ages derived from the best-ranked magnetostratigraphic correlation and not the full range of possible depositional ages provided by the DTW correlations.

Model of frontal fold erosion and sediment recycling

The 10 Be concentrations measured at the outlet of a drainage basin downstream of the Siwaliks can be calculated as:

(S1) 𝑁 𝑜𝑢𝑡 = 𝑟 𝜀 𝑀𝐵𝑇 𝑟 𝑁 𝑀𝐵𝑇 𝑟 + 𝜀 𝑆𝑖𝑤 𝑁 𝑆𝑖𝑤 𝑟 𝜀 𝑀𝐵𝑇 𝑟 + 𝜀 𝑆𝑖𝑤 where refers to the mean in situ 10 Be cosmogenic concentration measured in sediments (at g -1 ) downstream 𝑁 𝑀𝐵𝑇 𝑟 of the MBT at the time the Siwalik sediments are reworked and is the associated denudation rate (mm yr -1 ) 𝜀 𝑀𝐵𝑇 𝑟 of the same drainage basin. Similarly, and correspond to the mean 10 Be concentrations and denudation 𝑁 𝑆𝑖𝑤 𝜀 𝑆𝑖𝑤 rates measured in a basin that drains only the Siwalik Hills, respectively. is the ratio between the area of the 𝑟 drainage basin located upstream of the MBT and the area of the part draining the Siwaliks (Fig. 9).

Assuming that the Siwaliks sediments are deformed and exhumed in the piedmont via a simple kink-band faultbend fold (Fig. 11), the 10 Be concentrations of the reworked Siwaliks sediments that are eroded at any point x along the structure can be estimated as:

(S2)

𝑁 𝑆𝑖𝑤 (𝑥) = (𝑁 𝑀𝐵𝑇 𝑝 + 𝑁 𝐹𝑃 (𝑥 + 𝑡𝑅 𝑓𝑚 ))𝑒 -𝜆𝑡 + ∑ 𝑗 𝑃 𝑗 𝜀 𝑆𝑖𝑤 𝜌 𝑟 𝛬 𝑗
where N MBTp is the 10 Be palaeoconcentration of sediments shed from the palaeo-drainage basin located upstream of the MBT at the time of Siwaliks deposition (Fig. 9). was estimated with the model presented in the main 𝑁 𝐹𝑃 text.

is the facies migration rate. t is the time since fold initiation during which the reworked sediments 𝑅 𝑓𝑚 𝑡 𝑓𝑖 were deposited, which can be estimated as: the Siwalik structures and found that the average elevation represents only 10% of surface uplift, suggesting that the landscape of the Siwalik Hills has rapidly reached a dynamic equilibrium and that tectonic uplift is balanced by erosion. In our simple fault-bend fold model, the erosion rate of the Siwaliks (

) is therefore equal to 𝜀 𝑆𝑖𝑤 0.9 𝑅 𝑠 sin 𝜃.

Consistent with modern 10 Be concentrations measured in the Rapti basin upstream of the Siwaliks at the MBT, we assumed a steady denudation rates of ~0.3 mm/a and hence that the upstream sediments shed from the Rapti basin during Siwaliks reworking ( have a steady 10 Be concentration of (~20 ± 2)  10 3 at g -1 . The mean 10 Be 𝑁 𝑀𝐵𝑇 𝑟 ) concentration of the reworked Siwalik sediments ( can be inferred from the concentration measured in 7-𝑁 𝑀𝐵𝑇 𝑝 ) 3 Ma sediments in the Surai section, corrected for radioactive decay and transport in the floodplain (~17 ± 6)  10 3 at g -1 . The slip rate across the frontal structure and the facies migration rate are assumed to be 21 ± 2 mm a -1 (Lavé and Avouac, 2000) and 15 ± 5 mm a -1 (Lyon-Caen and Molnar, 1985), respectively. We assumed a classic fault dip angle of 30 ± 10°. The ratio between the areas of the two parts of the basin is assumed equal to 11.3, consistent with the modern setting.

Fast muons

 fmuons 4320 950 (Braucher et al., 2003;Braucher et al., 2011;Heisinger et al., 2002b) Slow muons  smuons 1510 100 (Braucher et al., 2003;Braucher et al., 2011;Heisinger et al., 2002a) Others parameters 10 Be decay constant (yr - 1 )  4.997×10 -7 0.043×10 Sinha et al. (2005); 3, this study (sinuosity includes the average obliquity of the river relative to the radial direction); 4, Gautam and Rossler (1999); 5, [START_REF] Jain | River systems in the Gangetic plains and their comparison with the Siwaliks : A review[END_REF]; 6, Dubille and Lavé (2015); 7, [START_REF] Morin | Annual Sediment Transport Dynamics in the Narayani Basin, Central Nepal: Assessing the Impacts of Erosion Processes in the Annual Sediment Budget[END_REF].

Fig. 1 :

 1 Fig. 1: Geological map of the central Himalayas. Brown and yellow stars indicate samples collected and analysed by Lupker et al. (2012a).

Fig. 2 :

 2 Fig. 2: Geology of the Rapti River basin and the Surai section in the Siwalik Hills. (a) Geological map of the Rapti river basin; (b) geological map of the Surai region; (c) palaeomagnetic and cosmogenic sampling locations; and (d) geological cross section across the Siwalik Hills showing the projected location of the Surai section. The photo in (d) shows a typical outcrop (and associated shielding) sampled for cosmogenic analyses along the RH01 road.

Fig. 3 :

 3 Fig. 3: Representative Zijderveld (1967) diagrams of our new palaeomagnetic samples. Samples showed (a) normal polarities (b) reverse polarities, (c) unstable remanent directions, or (d) remanent directions that defined great-circle paths, but nonetheless provided an unambiguous polarity. The 19 stable remanent directions and 12 great-circle samples used to refine the magnetostratigraphic column of the upper Surai section are shown in (e) geographic and (f) stratigraphic (tilt-corrected) coordinates.

Fig. 4 :

 4 Fig. 4: Magnetostratigraphic correlation of the Surai section and cosmogenic burial ages. (a) Stratigraphic log of the Surai section (after Corvinus and Rimal, 2001); previous magnetostratigraphic columns are after (b) Appel et al. (1991) and (c)Gautam and Rossler (1999) andRosler (1997); (d) and (e) magnetic declinations and inclinations, respectively (afterGautam and Rosler, 1999), including our new analyses; (f) revised magnetostratigraphic column of the Surai section and its correlation to the reference scale using the DTW algorithm ofLallier et al. (2013), with previous correlations proposed in the literature shown as grey circles; (g) magnetostratigraphic column of[START_REF] Ojha | Magnetic polarity stratigraphy of the Neogene foreland basin deposits of Nepal 61-90[END_REF] with Ar-Ar samples ofSzulc et al. (2006) indicated by stars. The stratigraphic position ofOjha et al.'s (2009) section was set at the base of the long normal chron located at the bottom of the section.

Fig. 6 :

 6 Fig. 6: Geochemical evidence of sediment transport in the floodplain. (a) Na/Si versus Al/Si plot for the Surai data and several Himalayan rivers. We interpret that a first cycle of transport ('1') deposited newly weathered sediments and that a second cycle ('2') recycled a large proportion of previously weathered Siwalik sediments. (b) Sr vs. Nd

Fig. 7 :

 7 Fig. 7: Schematic diagram of the proposed evolution of the drainage area. Red arrows indicate finite displacement across the basin. The yellow star indicates the palaeo-(left and center) and present (right) position of the Surai section.

Fig. 8 :

 8 Fig. 8: Temporal evolution of reconstructed 10 Be palaeoconcentrations and derived palaeodenudation rates in the Surai Khola. Palaeoconcentrations (left) are corrected for radioactive decay and exposure in the floodplain (see text). Denudation rates were calculated assuming that the sediments of the Surai section were deposited by a Karnali-type trans-Himalayan river prior to ~3.5 Ma (middle), and by a Rapti-type midland-draining river since ~3.5 Ma (right). The grey arrows in the right plot indicate recycling of Siwaliks sandstone; the true denudation rates in the Rapti basin upstream of the Siwliks are necessarily lower than the plotted data. All denudation rates

Fig. 10 :

 10 Fig. 10: The modelled impact of recycling of Siwalik sediments on the observed 10 Be cosmogenic concentrations. (a) Schematic diagram explaining the sediment-mixing model used to constrain the impact of reworking. The model calculates the 10 Be concentrations downstream of the Siwalik (N out ) assuming (1) a steady denudation rates

  Fig. 2a

  Fig. 1

  Fig. 2

  Fig. 2c

  Fig. 5

Fig. A :

 A Fig. A: Revised magnetostratigraphic column of the Surai section correlated to the reference scale using the DTW algorithm of Lallier et al. (2013) considering all polarity intervals, including the reverse interval r5 that was omitted

  ). Natural 27 Al concentrations were measured by inductively coupled plasma optical emission spectrometry in the dissolved samples (after evaporation and substitution of fluorides by HNO 3 ) at the Service National d'Analyse des Roches et Minéraux (SARM, CRPG, Nancy, France).

  𝑡 = 𝑡 𝑓𝑖 + 𝑡 𝑓𝑖 𝑅 𝑠 sin 𝜃 𝐴where is the dip angle of the thrust fault, Rs (mm yr -1 ) the slip rate across the frontal fault, and A the sediment 𝜃 accumulation rate in the Siwaliks basin. Over enough time, the cumulative slip across the fold may be greater than the distance along the bedding l(x) between the point at the surface and the hinge of the fold. In that case, 𝑡 = 𝑡 𝑓𝑖 +. The mean 10 Be concentration shed from the Siwalik hills can then be estimated by averaging all individual locations x along the fold.Hurtrez et al. (1999) applied mass balance across 𝑛 𝑆𝑖𝑤(𝑥)

Table 2 :

 2 Palaeomagnetic results. N, normal polarity; R, reverse polarity; Trans., transitional direction; Unst., unstable demagnetization; GC, great circle. Values in italics are the mean values used for remanent directions that defined great-circle paths, but nonetheless provided an unambiguous polarity.

	Sample			Thickness Age	Min. Age	Max. Age	10 Be ×10 3	26 Al ×10 3	26 Al/ 10 Be age	10 Be p ×10 3		Basin
		°°m	Ma	Ma	Ma	at g -1	at g -1	Ma	at g -1	mm a-1
	Sandstone
	Cos1	27.7685	82.8378	2461	5.7	5.7	5.7	1.1±0.4			18.4±7	1.5±0.85/0.46 Karnali
	Cos2	27.7702	82.8343	2279	5.2	5.2	5.2	2.3±0.6	d.l.		29.8±9.8	0.92±0.48/0.27 Karnali
	Cos4	27.7740	82.8355	1920	4.3	4.3	4.3	1±0.5			6.9±4.9	3.89±2.75/1.45 Karnali
	Cos8	27.7770	82.8351	1629	3.6	3.5	3.9	3.3±0.4	d.l.		18.6±3.4	1.4±0.48/0.3 Karnali
	Cos10	27.7762	82.8189	1606	3.6	3.5	3.8	2.3±0.6			12.4±4.4	2.21±1.12/0.62 Karnali
	Cos13	27.7785	82.8067	1313	2.9	2.8	3.4	1.6±0.6			4.9±3.6	1.59±1.11/0.59 Rapti
	Cos14	27.7802	82.8057	1146	2.5	2.5	3.3	3.4±0.7			10.3±3.6	0.76±0.36/0.21 Rapti
	Cos16	27.7816	82.8041	1106	2.5	2.4	3.3	3±0.4	d.l.		8.6±2.2	0.89±0.34/0.21 Rapti
	Cos19	27.7841	82.8021	889	2.2	2.0	3.2	4.8±0.6 15.9±10.9	1.7±1.4	12.6±2.8	0.59±0.21/0.13 Rapti
	Cos21	27.7842	82.7999	813	2.1	1.7	3.1	4.8±0.9 16.3±10.6	1.7±1.4	12.3±3.6	0.62±0.26/0.15 Rapti
	Cos26	27.7865	82.8002	594	1.7	0.9	2.4	4.6±1 23.8±12.6	0.7±1.3	9±3.5	0.86±0.41/0.23 Rapti
	Cos27	27.7876	82.8010	524	1.5	0.9	2.2	3.1±0.6			5.1±2	1.53±0.7/0.4 Rapti
	Cos29	27.7884	82.7939	219	1.0	0.5	1.4	4.3±0.6			5.6±1.7	1.36±0.52/0.31 Rapti
	Cos32	27.7914	82.7903	0	0.6	0.3	0.9	5.2±1			5.7±2.5	1.37±0.64/0.36 Rapti
	Quartzite pebbles
	Cos18	27.7841	82.8021	889	2.2	2.0	3.2	9±0.8 21.6±13.9	2.4±1.4
	Cos20	27.7842	82.7999	813	2.1	1.7	3.1	3.9±0.7 20.6±6.6	0.6±0.8
	Cos25	27.7865	82.8002	594	1.7	0.9	2.4	3.4±0.8	d.l.
	River bed
	RAP-COS 1	27.8386	82.7628					20.2±1.5 69.1±19.4	1.5±0.7		0.36±0.03/0.03 Rapti
	RAP-COS 2	27.995	82.8218					21.9±1.6 63±11.7	1.8±0.4		0.33±0.03/0.03 Rapti

Table B :

 B -7 (Chmeleff et al., 2010; Korschinek et al., 2010) Parameters for the flood plain transfer model.

	depositional age	t (Ma) Magentostratigray	0.5	This study
	denisty of denuded rocks	 r	2.65	0.5
	Concentration accumulated during transport	N fp	See Table C	See table C	This study
	Parameters				Karnali River	Rapti River	References *
					min/max	min/max
	Sediment density				1.9/1.9	1.9/1.9
	Floodplain width (km)				50/90	15/30	3
	Bankfull channel depth (m)			5/8	4/6	5
	Fraction of sediment load (200-800 µm fraction) in the floodplain	0.3/0.4	0.3/0.5	6
	Fraction of sediment load (200-800 µm fraction) in the channel	0.15/0.25	0.15/0.25	7
	Total sediment flux at the range outlet (Mt a -1 )		100/200	20/30	1, 2
	Channel lateral migration rate (m a -1 )			50/100	50/250	3
	Sediment agradation rate in the floodplain (mm a -1 )		0.4/0.8	0.4/0.8	4
	River sinuosity				1.3/1.7	1.5/2	3

* 1, Lupker et al. (2012)

; 2,

Table C :

 C 10 Be contributions in the floodplain calculated using the floodplain transfer model.

	Cos14	2.5	1.4±0.3	1.7±0.4
	Cos16	2.5	1.4±0.3	1.7±0.4
	Cos19	2.2	1.4±0.2	1.6±0.3
	Cos21	2.1	1.4±0.2	1.6±0.3
	Cos26	1.7	1.3±0.2	1.5±0.3
	Cos27	1.5	1.3±0.2	1.5±0.3
	Cos29	1	1.3±0.2	1.4±0.3
	Cos32	0.6	1.2±0.2	1.3±0.2
	Samples	Age (Ma)	Karnali River	Rapti River
			×10 3 at g -1	×10 3 at g -1
	Cos1	5.7	1.6±0.3	2.4±0.7
	Cos2	5.2	1.6±0.3	2.3±0.7
	Cos4	4.3	1.5±0.3	2.1±0.5
	Cos8	3.6	1.5±0.3	1.9±0.5
	Cos10	3.6	1.5±0.3	1.9±0.5
	Cos13	2.9	1.4±0.3	1.8±0.4

Table D :

 D Geochemical data. Please see the excel sheet available online.

Table E1 :

 E1 Raw in situ cosmogenic 10 Be and 26 Al data of the samples.

	Sample	Total counts	10 Be/ 9 Be	Mass of quartz	Added 9 Be	10 Be	Total counts	26 Al/ 27 Al	Mass of quartz	Total 27 Al (Carrier and natural)	26 Al
		10 Be					26 Al
			(10 -15 )	(g)	(x 10 19 at)	(x10 3 at/g)		(x10 -15 )	(g)	(x10 18 at/g) (x10 4 at/g)
						Sandstone
	Cos1	16	8.7±2.4	272.4	4.1	1.1±0.4
	Cos2	13	60±16.7	528.7	2.0	2.3±0.6	0
	Cos4	5	14.9±6.7	284.7	2.1	1±0.5
	Cos8	61	86.9±11.2	533.9	2.1	3.3±0.4	2	1.4±1	533.9	4.40	0.2±1.1
	Cos10	17	25.2±6.1	214.2	2.1	2.3±0.6
	Cos13	7	19.4±7.3	235.1	2.1	1.6±0.6
	Cos14	24	20±4.1	112.8	2.1	3.4±0.7
	Cos16	47	70.2±10.3	473.9	2.1	3±0.4	0
	Cos19	62	30.4±3.9	125.4	2.0	4.8±0.6	6	4.8±2	125.4	3.70	1.6±1.1
	Cos21	33	77.3±13.5	322.9	2.1	4.8±0.9	9	4.7±1.6	322.9	3.90	1.6±1.1
	Cos26	27	53.6±11.1	229.3	2.1	4.6±1	11	6.4±2.2	229.3	4.10	2.4±1.3
	Cos27	36	19.4±3.3	122.6	2.1	3.1±0.6
	Cos29	65	26.4±3.3	121.8	2.1	4.3±0.6
	Cos32	46	23.7±4.2	88.9	2.1	5.2±1
					Quartzite pebbles
	Cos18	150	90.7±7.5 201.6191	2.1	9±0.8	8	5.7±2	201.6	4.60	2.2±1.4
	Cos20	35	59.8±10.1 300.7331	2.1	3.9±0.7	19	10.9±2.5	300.7	2.00	2.1±0.7
	Cos25	19	73.7±16.9 433.9689	2.1	3.4±0.8	3	1.7±1	434.0	2.80	0.3±0.7
	River bed		0±0
	RAP-COS 1	217	76.2±5.2 75.8601	2.1	20.2±1.5	26	26.7±6.9	75.9	2.70	6.9±1.9
	RAP-COS 2	194	102.8±7.5 95.7462	2.1	21.9±1.6	42	25.4±3.9	95.7	2.60	6.3±1.2

Table E2 :

 E2 Blank values for cosmogenic analyses.

	Serie	Total counts	10 Be/ 9 Be	Samples
		10 Be
			(x10 -15 )
		8	2.4±0.8	8,18,Rap1,Rap2
		4	2.1±1.1	20,25 et 26
		7	1.9±0.7	21
		5	0.9±0.4	2,16,19,25
		13	1.1±0.3	1,4,10,13,14, 27,29,32
	Serie	Total counts 26 Al	26 Al/ 27 Al (x10 -15 )	Samples
		1	0.5±0.5	2,16,19,20,21,25,26
		2	1±0.7	Rap1,Rap2,8,18

Table F -

 F Depth of the polarity intervals of the Surai magnetostratigraphic column

	Page 61 of 65						Basin Research
	CA 11.4 12.2 11.1 0.6 0.5 0.6		Narayani 4.5 4.6 5.2 0.2 0.2 0.1	0.1 0.1 0.0 8.3 8.5 5.9 Morin, 2015 Morin, 2015	27.701483 2.6 2.6 1.6 99.5 99.7 99.8	84.423417 6.0 7.5 0.3 0.0405 0.0456 0.0016	1.3 1.4 0.1 0.2178 0.2434 0.1819	59.1 2.9 3.0 2.1 0.0606 0.0647 0.0365
	CA 12.4 12.2 12.5 0.6 0.6 0.7		N17 Narayani 4.2 4.7 4.4 0.1 0.2 0.1	0.0 0.1 0.1 4.9 8.5 9.9 Morin, 2015 Morin, 2015	27.701733 2.5 2.6 2.8 99.3 100.0 100.3	84.423517 4.4 7.6 5.8 0.0469 0.0451 0.0033	1.6 1.4 0.1 0.2236 0.2437 0.2452	59.2 3.4 3.0 3.2 0.0659 0.0644 0.0678
	3893 3975 Al2O3 CA CA 11.1 8.5 12.0 12.3 9.1 2.7 TiO2 0.6 0.5 0.5 0.5 0.6 0.3	R17 N18 Narayani Narayani Fe2O3 4.0 3.5 4.4 4.6 3.7 1.7 P2O5 0.1 0.2 0.1 0.1 < L.D. 0.0 e 143 Nd	MnO 0.1 0.1 0.1 0.1 0.1 0.0 Pf 5.3 7.5 8.3 10.9 7.4 1.7 Morin, 2015 Morin, 2015 Reference Morin, 2015 Morin, 2015	27.701483 27.702383 MgO 2.3 2.0 2.4 2.6 1.5 0.2 Total 99.2 99.5 100.2 99.5 98.9 99.3	84.423317 84.424783 CaO 5.4 9.1 7.2 9.4 4.5 0.1 Na/Si 0.0481 0.0415 0.0454 0.0475 0.0021 0.0013	Na2O 1.6 1.4 1.4 1.3 0.1 0.1 Al/Si 0.1995 0.1545 0.2327 0.2660 0.1556 0.0347	60.8 54.5 K2O 2.9 2.0 2.9 3.1 2.2 0.6 K/Si 0.0555 0.0384 0.0618 0.0714 0.0406 0.0083
	CA 11.1 11.3 2.6 0.6 0.5 0.2	4537	R18 Narayani 4.0 4.3 1.3 0.1 0.1 0.1	0.1 0.1 0.0 5.3 9.6 4.2 Morin, 2015 Morin, 2015	27.702383 2.3 2.5 0.3 99.2 100.4 100.0	84.424783 5.4 8.6 4.3 0.0481 0.0454 0.0020	1.6 1.4 0.1 0.1995 0.2257 0.0353	59.2 2.9 2.7 0.7 0.0555 0.0592 0.0100
	4545 0.747931 CA 7.9 10.2 10.9 2.5 0.4 0.4 0.5 0.3	N19 Narayani 3.7 3.5 4.1 1.5 < L.D. 0.1 0.1 0.0 -16.8	0.1 0.1 0.1 0.0 6.0 4.5 8.1 0.7 This study Morin, 2015 Morin, 2015	27.702983 1.7 1.9 2.4 0.2 99.7 99.7 99.7 99.8	84.425583 5.9 5.3 7.7 0.2 0.0373 0.0452 0.0441 0.0021	1.3 1.6 1.4 0.1 0.1325 0.1725 0.2079 0.0314	61.8 1.8 2.3 2.7 0.6 0.0324 0.0414 0.0548 0.0088
	4559 0.754091 CA 8.2 9.0 10.6 6.0 0.8 0.4 0.5 0.3	R19 Narayani 3.5 2.9 3.7 2.5 0.2 0.1 0.1 0.2 -17.1	0.1 0.0 0.1 0.2 4.3 0.0 7.7 14.4 This study Morin, 2015 Morin, 2015	27.702450 1.2 2.1 2.1 1.7 99.7 0.0 100.7 99.8	84.424700 4.1 5.9 6.1 15.2 0.0387 0.0407 0.0431 0.0123	1.5 1.5 1.5 0.4 0.1317 0.1537 0.1906 0.1241	65.8 1.8 2.1 2.6 1.4 0.0305 0.0392 0.0496 0.0314
	CA 12.0 4.9 9.1 1.1 0.4 0.6	4712	N20 Narayani 6.4 2.8 4.7 0.2 0.1 0.1	27.701467 1.1 2.1 2.8 99.7 0.0 98.8 Lupker et al. (2012) 0.2 0.0 0.1 4.4 0.0 11.0 Morin, 2015 Morin, 2015	84.423550 5.1 5.7 9.4 0.0184 0.0411 0.0441	0.7 1.5 1.2 0.0769 0.1539 0.2627	53.9 0.9 2.1 3.0 0.0160 0.0394 0.0716
	CA 13.9 7.6 11.8 9.7 0.4 0.6 0.5 0.5 CA 8.4 13.9 11.4 13.4 0.5 0.7 0.5 0.7 CA 14.3 16.2 11.0 4.0 0.6 0.7 0.5 0.3 CA 12.6 12.7 12.3 15.2 0.7 0.6 0.6 0.7 CA 12.6 13.1 11.2 5.6 0.5 0.6 0.5 0.3 CA 5.1 12.2 11.5 8.0 0.3 0.6 0.5 0.4 CA 11.6 12.2 8.2 6.4 0.6 0.6 0.5 0.3 CA 6.3 11.7 9.4 11.6 0.3 0.5 0.4 0.6	4730 4851 4896 4922 4965 4994 5039 5124	R20 Narayani 4.5 5.1 4.4 3.8 0.1 0.1 0.2 0.1 N21 Narayani 4.5 5.1 4.3 5.2 0.1 0.1 0.2 0.0 R21 Narayani 5.3 6.1 4.1 2.4 0.2 0.2 0.1 0.1 N22 Narayani 5.3 4.5 4.5 5.1 0.2 0.1 0.2 0.1 R22 Narayani 5.3 4.9 4.4 2.2 0.2 0.1 0.1 0.1 N23 Narayani 2.2 4.8 4.5 2.8 0.1 0.1 0.1 0.1 R23 Narayani 4.6 4.3 3.0 2.2 0.1 0.1 0.2 0.1 N24 Narayani 2.4 4.2 3.3 3.8 0.1 0.1 0.1 0.1	27.701467 0.8 2.6 2.4 1.8 99.8 101.1 99.6 99.8 Lupker et al. (2012) 0.2 0.1 0.1 0.0 2.8 13.4 9.8 5.7 Morin, 2015 Morin, 2015 27.701617 0.1 1.0 0.1 2.3 0.1 2.4 0.0 2.1 2.3 99.7 13.6 101.5 9.5 100.1 5.4 100.0 Morin, 2015 Morin, 2015 Lupker et al. (2012) 27.701967 0.1 2.3 0.1 3.1 0.1 2.3 0.0 0.4 8.4 100.0 8.9 102.4 8.2 98.5 1.9 99.8 Morin, 2015 Morin, 2015 Lupker et al. (2012) 27.702383 0.1 2.0 0.1 2.4 0.1 2.5 0.0 2.9 7.2 100.0 9.0 100.5 10.0 100.4 7.3 99.7 Morin, 2015 Morin, 2015 Lupker et al. (2012) 27.702083 0.1 1.9 0.1 2.4 0.1 2.6 0.1 1.4 10.3 102.2 11.7 101.1 9.9 98.7 16.6 99.8 This study Morin, 2015 Morin, 2015 27.702467 0.1 1.0 0.1 2.8 0.1 2.6 0.0 1.5 2.1 102.0 9.9 101.2 9.5 100.7 6.1 99.1 This study Morin, 2015 Morin, 2015 27.696499 0.1 1.9 0.1 2.2 0.0 1.7 0.1 2.1 6.0 99.4 7.8 101.1 6.8 100.6 11.2 99.8 Lupker et al. (2012) Morin, 2015 Morin, 2015 27.699476 0.1 0.5 0.1 2.4 0.1 2.0 0.0 1.9 0.8 99.3 11.4 100.6 5.4 99.5 11.2 99.8 Lupker et al. (2012) Morin, 2015 Morin, 2015	84.423550 2.4 10.5 8.9 3.8 0.0165 0.0443 0.0436 0.0336 84.423833 0.3 12.0 8.6 0.7 0.0125 0.0522 0.0431 0.0181 84.423967 4.8 4.1 7.7 0.3 0.0219 0.0268 0.0442 0.0086 84.423567 4.2 8.0 9.2 1.4 0.0154 0.0489 0.0467 0.0278 84.423150 4.2 9.9 8.9 17.8 0.0168 0.0464 0.0437 0.0102 84.423450 1.3 7.5 8.8 4.9 0.0119 0.0418 0.0438 0.0220 84.418039 2.2 5.9 8.1 10.8 0.0281 0.0464 0.0405 0.0134 84.418964 0.5 10.6 5.4 9.5 0.0102 0.0473 0.0403 0.0272	0.7 1.1 1.3 1.2 0.1131 0.3257 0.2417 0.1631 0.5 1.3 1.3 0.6 0.1225 0.3329 0.2285 0.2301 0.7 0.8 1.4 0.4 0.2786 0.3309 0.2137 0.0531 0.5 1.5 1.4 0.9 0.2311 0.2544 0.2572 0.2912 0.6 1.3 1.3 0.3 0.2334 0.2853 0.2321 0.1235 0.5 1.3 1.3 0.8 0.0679 0.2446 0.2290 0.1324 1.0 1.5 1.4 0.4 0.1997 0.2273 0.1414 0.1176 0.5 1.4 1.5 0.8 0.0849 0.2493 0.1592 0.2423	57.3 1.5 3.4 3.0 2.4 0.0241 0.0861 0.0662 0.0444 58.9 1.9 3.4 2.9 2.6 0.0294 0.0895 0.0618 0.0493 60.6 3.2 4.6 2.7 1.0 0.0677 0.1007 0.0561 0.0149 56.5 3.1 3.1 3.2 4.0 0.0613 0.0681 0.0714 0.0825 56.9 3.1 3.2 2.8 1.3 0.0617 0.0745 0.0629 0.0318 59.0 1.7 3.0 2.9 2.0 0.0245 0.0650 0.0621 0.0357 68.7 3.1 2.9 1.9 1.8 0.0588 0.0587 0.0350 0.0354 69.8 1.3 2.9 2.3 3.1 0.0190 0.0659 0.0414 0.0711
	CA 11.3 12.2 8.9 0.7 0.6 0.5		Narayani 5.1 4.5 4.6 0.2 0.1 0.2	27.455800 2.2 2.3 2.0 99.9 100.8 99.1 Lupker et al. (2012) 0.1 0.1 0.2 6.7 13.9 5.4 Morin, 2015 Morin, 2015	83.936083 3.9 13.1 6.7 0.0116 0.0497 0.0345	0.4 1.3 1.2 0.1998 0.2891 0.1547	67.9 2.6 2.9 1.6 0.0497 0.0744 0.0294
	CA 11.6 13.3 6.9 3.9 0.8 0.6 0.5 0.4		Bheri 4.7 4.9 3.9 2.2 0.1 0.1 0.1 0.1		28.435195 2.5 2.6 2.6 0.6 99.7 99.8 100.3 99.8 Lupker et al. (2012) 0.1 0.1 0.1 0.2 6.3 12.3 6.3 18.4 Morin, 2015 Lupker et al. (2012)	81.865191 2.4 10.4 5.3 23.0 0.0143 0.0456 0.0242 0.02	0.5 1.2 0.9 0.4 0.2008 0.3074 0.1132 0.09	72.0 2.9 3.4 1.7 0.9 0.0544 0.0851 0.0301 0.02
	CA 17.2 11.3 8.5 5.5 0.7 0.5 0.4 0.3		Bheri 6.3 4.2 3.8 2.2 0.1 0.1 0.1 0.1		28.435195 2.5 2.6 3.6 0.8 100.4 100.0 100.0 99.8 Lupker et al. (2012) 0.1 0.1 0.1 0.2 8.2 10.6 8.2 10.9 Morin, 2015 Lupker et al. (2012)	81.865191 1.8 9.8 5.8 12.3 0.0203 0.0458 0.0332 0.02	0.6 1.3 1.1 0.6 0.3448 0.2338 0.1511 0.10	66.0 3.8 2.8 2.2 1.4 0.0816 0.0635 0.0429 0.03
	CA 11.5 7.0 9.6 9.0 0.4 0.5 0.5 0.5		Bheri 3.3 4.2 4.4 3.6 0.1 0.1 0.1 0.1		28.435195 1.0 2.4 4.4 1.8 99.5 100.1 100.1 99.8 Lupker et al. (2012) 0.1 0.1 0.1 0.0 2.6 13.4 10.1 11.7 Morin, 2015 Lupker et al. (2012)	81.865191 1.1 12.0 6.3 11.0 0.0135 0.0437 0.0354 0.02	0.6 1.2 1.1 0.7 0.1014 0.2620 0.1860 0.18	60.9 1.7 2.8 2.6 2.4 0.0271 0.0698 0.0535 0.05
	CA 12.2 7.2 10.0 6.1 0.5 0.5 0.5 0.3		Karnali 2.8 4.3 3.2 2.3 0.1 0.1 0.1 0.1	28.783810 1.1 2.4 1.7 1.7 98.8 100.7 99.3 99.8 Lupker et al. (2012) 0.0 0.1 0.0 0.1 2.8 8.4 5.0 10.7 Morin, 2015 Lupker et al. (2012)	81.573809 1.1 8.0 5.7 9.6 0.0144 0.0465 0.0454 0.01	0.6 1.4 1.6 0.5 0.1054 0.2386 0.1700 0.11	69.1 1.9 2.9 2.4 1.7 0.0296 0.0613 0.0448 0.03
	Depth (m) 6.9 10.9 CA 8.7 6.9 0.4 0.5 0.4 0.3 CA 6.1 12.8 12.2 9.9 0.5 0.5 0.6 0.5 372 CA 10.6 12.6 12.0 5.3 0.5 0.5 0.6 0.2 662 PB 65 0.0 13.5 16.0 4.8 0.0 0.6 0.7 0.3 701 PB 66 6.5 12.1 16.3 12.6 0.5 0.5 0.7 0.6 818 PB 67 6.8 11.9 17.1 11.8 0.5 0.6 0.8 0.6 869 PB 68 7.7 12.0 16.8 0.5 0.6 0.7 PB 69 11.7 10.3 17.5 4.0 0.5 0.5 0.7 0.2 PB 70 11.8 11.3 9.1 6.9 0.5 0.5 0.3 0.5	Karnali 3.1 4.0 3.1 2.2 0.1 0.1 0.1 0.1 Polarity Morin, 2015 0.0 0.1 0.1 0.1 6.4 10.5 4.0 24.1 Lupker et al. (2012) 28.783810 2.2 2.3 1.6 1.3 99.8 100.7 100.5 99.8 Lupker et al. (2012) Karnali 28.783810 4.1 0.1 2.3 4.5 0.1 2.2 4.5 0.1 2.5 3.1 0.1 2.2 0.1 6.6 99.7 0.1 6.9 100.7 0.2 7.0 100.2 0.1 21.9 99.7 Lupker et al. (2012) Morin, 2015 Lupker et al. (2012) interval Karnali 28.783810 3.9 0.1 3.5 4.6 0.1 2.4 4.3 0.1 2.3 1.6 0.0 1.5 0.1 10.5 100.8 0.1 10.7 100.9 0.2 6.3 100.0 0.1 11.6 100.0 Lupker et al. (2012) Morin, 2015 Lupker et al. (2012) R1 Kosi 26.847022 0.0 0.0 0.0 4.8 0.1 2.2 5.7 0.1 2.6 2.6 0.3 0.7 0.0 0.0 0.0 0.1 10.2 100.8 0.2 4.0 100.6 0.1 13.7 99.8 Lupker et al. (2012) Morin, 2015 Lupker et al. (2012) N2 Kosi 26.847022 2.6 0.1 1.4 4.5 0.1 2.6 5.8 0.1 2.7 4.0 0.0 1.6 0.1 4.6 100.5 0.1 11.8 100.9 0.2 4.2 100.8 0.1 7.8 99.8 Lupker et al. (2012) Morin, 2015 Lupker et al. (2012) R2 Kosi 26.847022 3.5 0.1 1.8 4.3 0.1 2.2 6.2 0.1 2.9 8.1 0.1 2.1 0.1 4.3 99.8 0.1 11.6 100.9 0.2 4.6 100.6 0.2 10.4 99.8 Lupker et al. (2012) Morin, 2015 Lupker et al. (2012) N3 Kosi 26.847022 2.7 0.0 1.5 4.5 0.1 2.6 6.0 0.1 3.0 0.1 4.8 98.9 0.1 10.7 100.9 0.2 4.6 100.4 Lupker et al. (2012) Morin, 2015 Lupker et al. (2012) R3 Kosi 26.847022 4.4 0.1 3.5 3.7 0.1 2.3 6.0 0.1 3.8 2.6 0.1 0.7 0.1 10.0 99.7 0.1 10.1 100.5 0.2 5.8 99.9 0.1 25.7 99.9 Lupker et al. (2012) Morin, 2015 Lupker et al. (2012) N4 R4 Kosi 26.847022 4.3 0.1 3.3 4.2 0.1 2.5 3.0 0.1 1.3 3.1 0.0 0.7 0.1 9.9 98.7 0.1 9.9 100.3 0.1 1.7 100.0 0.0 2.2 99.2 Lupker et al. (2012) Morin, 2015 Lupker et al. (2012)	81.573809 5.6 9.4 4.6 29.0 0.0225 0.0444 0.0397 0.02 81.573809 5.7 4.8 4.6 23.4 0.0188 0.0410 0.0434 0.03 81.573809 8.7 8.8 4.5 12.6 0.0377 0.0447 0.0453 0.02 87.151414 0.0 8.6 2.0 16.2 0.0000 0.0425 0.0503 0.02 87.151414 4.6 11.2 1.8 5.6 0.0229 0.0481 0.0505 0.03 87.151414 4.5 10.9 1.8 6.7 0.0261 0.0476 0.0509 0.03 87.151414 4.7 9.2 2.0 0.0325 0.0444 0.0503 87.151414 6.8 9.6 2.2 29.0 0.0331 0.0450 0.0537 0.02 87.151414 6.8 8.9 1.8 0.1 0.0321 0.0468 0.0393 0.01	0.8 1.4 1.5 0.3 0.1113 0.2177 0.1368 0.25 0.7 1.4 1.4 0.5 0.0997 0.2334 0.2249 0.34 1.1 1.3 1.5 0.7 0.2111 0.2597 0.2171 0.10 0.0 1.2 1.7 0.6 0.0000 0.2811 0.2951 0.10 0.9 1.3 1.6 1.1 0.0985 0.2649 0.3043 0.24 1.0 1.4 1.6 0.9 0.1052 0.2557 0.3308 0.25 1.2 1.3 1.6 0.1238 0.2493 0.3239 1.0 1.4 1.6 0.3 0.2356 0.2021 0.3599 0.13 1.0 1.4 1.6 0.2 0.2408 0.2255 0.1360 0.10	74.6 1.7 2.6 2.0 2.4 0.0304 0.0571 0.0338 0.09 64.0 1.5 3.1 3.1 3.5 0.0266 0.0605 0.0623 0.13 65.3 2.7 2.9 3.0 1.5 0.0594 0.0648 0.0586 0.03 63.8 0.0 3.1 3.9 1.2 0.0000 0.0693 0.0787 0.03 63.3 1.6 3.0 4.0 3.3 0.0255 0.0704 0.0814 0.07 61.1 1.5 2.9 4.3 3.0 0.0249 0.0663 0.0894 0.07 61.3 1.8 3.1 4.2 0.0318 0.0686 0.0880 57.4 3.2 2.5 4.5 0.9 0.0698 0.0539 0.1005 0.03 79.1 3.1 2.8 1.9 1.5 0.0695 0.0612 0.0314 0.02
	10.4 10.9 6.5 0.5 0.5 0.3		3.8 3.9 2.1 0.1 0.1 0.0	N5	0.1 0.1 0.0 8.3 9.5 3.1 Lupker et al. (2012) Morin, 2015	3.0 2.4 0.9 99.5 100.3 99.2	6.1 9.0 1.7 0.0335 0.0450 0.01	1.1 1.4 0.4 0.1937 0.2141 0.09	2.8 2.8 1.7 0.0571 0.0586 0.03
	Surai section 9.8 12.5 4.0 0.4 0.6 0.4	3.4 4.6 1.7 0.1 0.1 0.1	R6	0.1 0.1 0.1 7.2 9.9 15.0 Lupker et al. (2012) Morin, 2015	2.5 2.4 1.4 99.1 99.5 99.8	5.4 7.8 17.1 0.0329 0.0436 0.009717034	1.1 1.3 0.3 0.1728 0.2566 0.08106069 0.026684672 2.6 3.0 1.2 0.0490 0.0663
	SU-PM38 8.4 11.5 13.1 7.6 0.5 0.5 0.7 0.4 0.770782	3.3 4.3 6.4 4.1 0.2 0.1 < L.D. 0.1 -16.8	N7	0 0.1 0.1 0.1 0.1 6.0 10.4 6.3 17.7 Morin, 2015 Morin, 2015 This study		1.8 2.4 1.4 0.9 99.8 100.3 100.2 100.0	0.6 1.5 1.2 0.2 0.2 0.1452 0.2377 0.2286 0.006373126 0.183039931 0.054005817 68.6 7.8 1.9 10.1 2.8 0.6 3.0 17.6 2.1 0.0419 0.0352 0.0424 0.0636 0.0046 0.0569
	94-60 12.3 12.0 4.2 0.6 0.5 0.3	4.6 4.3 2.0 0.2 0.1 0.1	R7	118 0.1 0.1 0.1 7.1 8.8 16.4 Morin, 2015 Morin, 2015 Lupker et al. (2012)	2.2 2.6 0.6 100.0 100.8 99.2	7.3 8.2 19.3 0.0513 0.0444 0.0028	0.8 1.6 1.4 0.1 0.2377 0.2372 0.0898	55.2 3.0 3.0 1.1 0.0633 0.0640 0.0267
	SU-PM37 9.4 12.8 7.7 13.6 0.4 0.6 0.5 0.7	3.5 5.4 3.2 4.7 0.1 0.1 < L.D. 0.1	N8	186 0.0 0.1 0.1 0.1 8.5 8.7 3.3 7.8 Morin, 2015 Morin, 2015 This study		2.1 3.1 0.7 1.2 99.8 100.2 100.0 99.7	0.9 1.4 1.4 0.2 0.3 0.1722 0.2566 0.1121 0.009250586 0.254342748 0.069611806 82.3 8.2 2.2 6.6 3.0 0.2 1.8 4.2 3.4 0.0424 0.0444 0.0450 0.0660 0.0047 0.0287
	SU-PM35 8.8 14.8 12.2 0.5 0.6 0.6 0.768205	3.4 5.2 3.9 0.1 0.1 < L.D. -17.2	R8	212 0.1 0.1 0.1 6.1 6.7 7.8 Morin, 2015 Morin, 2015 This study		2.1 2.6 1.3 99.4 99.8 99.8	7.6 4.2 3.8 0.0386 0.0426 0.0074	1.0 1.3 1.3 0.3 0.1548 0.2883 0.2184	66.8 2.1 3.7 3.0 0.0399 0.0780 0.0576
	94-59A 11.9 12.8 3.0 0.5 0.5 0.1 0.79149	4.4 4.7 1.1 0.1 0.1 0.0 -18.8	N9	257 0.1 0.1 0.0 12.0 8.1 5.6 Morin, 2015 Morin, 2015 Lupker et al. (2012)/Huygue et al. (2001) 3.1 11.7 2.8 6.9 0.6 6.2 100.4 0.0508 100.0 0.0465 99.9 0.0005	1.0 1.4 1.4 0.0 0.2701 0.2552 0.0447	81.2 3.2 3.2 2.2 0.0786 0.0691 0.0350
	94-59C 11.9 11.8 2.2 0.5 0.5 0.1 0.72404 SU-PM33 11.0 10.8 8.9 0.5 0.5 0.5 94-57 10.7 10.8 9.0 0.5 0.5 0.5 SK98 10.6 12.0 0.5 0.6 SK72 10.2 11.3 0.5 0.6 SU-PM29 8.5 10.7 10.1 0.5 0.5 0.5 SU-PM27 11.2 13.1 12.1 0.5 0.6 0.6 0.769394 SU-PM24 11.3 13.1 11.3 0.5 0.6 0.6 94-54 11.0 12.1 3.3 0.5 0.5 0.3 SK110 11.2 12.1 0.5 0.5 SU-PM17 11.1 11.7 6.2 0.5 0.5 0.4 0.764091 SU-PM16 10.8 11.7 11.8 0.5 0.5 0.6	4.4 4.4 1.2 0.1 0.1 0.0 -17.7 4.0 4.1 4.0 0.1 0.1 0.1 3.7 4.0 3.8 0.1 0.1 0.1 3.6 4.6 0.1 0.1 -16.5 3.6 4.3 0.1 0.1 -16.1 3.5 4.1 3.4 0.2 0.1 < L.D. 3.8 4.9 5.0 0.1 0.1 0.1 -17.3 3.8 4.9 4.2 0.1 0.1 0.1 3.6 4.4 2.2 0.1 0.1 0.1 3.7 4.4 0.1 0.1 -18.0 3.9 4.3 2.4 0.1 0.1 < L.D. -17.2 3.7 4.3 4.2 0.1 0.1 0.1	R9 N10 R10 N11 R11 N12 R12 N13 R13 N14 R14 N15 R15	257 0.1 0.1 0.0 11.9 9.4 16.7 Morin, 2015 Lupker et al. (2012)/Huygue et al. (2001) 3.1 11.6 2.8 7.9 0.9 20.0 100.2 0.0510 99.4 0.0477 99.9 0.0000 Morin, 2015 267 0.1 2.7 10.8 0.1 2.6 8.5 0.0 1.0 5.5 10.3 100.2 0.0501 10.1 99.4 0.0439 7.5 100.9 0.0084 Morin, 2015 This study Morin, 2015 381 0.1 2.5 10.1 0.1 2.6 8.2 0.0 1.3 6.1 9.4 99.9 0.0486 9.4 99.7 0.0440 8.5 100.0 0.0133 Morin, 2015 Lupker et al. (2012) Morin, 2015 420 0.1 2.4 9.4 0.1 2.7 8.4 8.8 100.3 0.0476 < L.D. 89.9 0.0438 Morin, 2015 Robinson et al. (2001) Morin, 2015 420 0.1 2.3 8.8 0.1 2.6 7.2 8.2 100.6 0.0450 8.4 99.5 0.0483 Morin, 2015 Robinson et al. (2001) Morin, 2015 440 0.1 2.0 9.1 0.1 2.5 8.0 0.0 1.1 2.6 7.5 99.5 0.0415 9.1 100.0 0.0429 6.3 100.4 0.0102 Morin, 2015 Morin, 2015 This study 535 0.1 2.2 11.9 0.1 2.5 7.1 0.1 1.6 5.1 11.6 98.7 0.0495 < L.D. 90.9 0.0431 8.3 100.5 0.0138 Morin, 2015 Morin, 2015 This study 574 0.1 2.1 10.9 0.1 2.5 7.1 0.0 1.4 2.9 10.1 99.9 0.0470 < L.D. 90.9 0.0431 6.2 101.0 0.0096 Morin, 2015 Morin, 2015 This study 702 0.1 2.0 9.4 0.1 2.4 8.9 0.0 0.5 12.4 8.9 99.2 0.0478 < L.D. 90.4 0.0443 11.6 99.8 0.0006 Morin, 2015 Morin, 2015 Lupker et al. (2012) 720 0.1 2.1 9.7 0.1 2.4 8.9 9.4 98.6 0.0485 < L.D. 90.4 0.0443 Morin, 2015 Morin, 2015 Robinson et al. (2001) 725 0.1 2.3 13.0 0.1 2.4 8.8 0.0 0.8 0.4 12.9 100.0 0.0444 < L.D. 91.0 0.0431 2.5 100.8 0.0020 Morin, 2015 Morin, 2015 This study 731 0.1 2.2 11.4 0.1 2.4 8.8 0.1 1.9 4.1 11.2 98.6 0.0477 < L.D. 91.0 0.0431 7.9 100.6 0.0104 Morin, 2015 Morin, 2015 This study	1.0 1.4 1.4 0.0 0.2706 0.2382 0.0445 1.1 1.5 1.3 0.3 0.2300 0.2177 0.1513 1.3 1.5 1.4 0.5 0.2151 0.2117 0.1585 1.342 1.5 1.3 0.2054 0.2464 1.342 1.5 1.5 0.1917 0.2202 1.4 1.4 1.3 0.4 0.1545 0.2064 0.1634 1.6 1.4 1.3 0.5 0.2469 0.2663 0.2249 1.6 1.4 1.3 0.3 0.2353 0.2663 0.1907 1.9 1.5 1.3 0.0 0.2173 0.2482 0.0578 1.975 1.4 1.3 0.2279 0.2482 2.0 1.2 1.3 0.1 0.2497 0.2337 0.0857 2.0 1.3 1.3 0.4 0.2317 0.2337 0.2105	58.0 3.2 2.9 0.7 0.0774 0.0640 0.0160 70.6 2.9 2.6 2.3 0.0650 0.0577 0.0422 67.9 2.8 2.7 2.3 0.0605 0.0566 0.0446 2.7 3.0 0.0576 0.0663 2.6 2.8 0.0530 0.0583 73.5 2.0 2.6 2.5 0.0384 0.0543 0.0441 64.1 2.8 3.2 3.0 0.0664 0.0707 0.0611 71.0 2.8 3.2 2.9 0.0639 0.0707 0.0523 68.6 2.7 3.0 0.9 0.0583 0.0664 0.0171 2.8 3.0 0.0615 0.0664 86.4 2.7 2.9 1.6 0.0668 0.0618 0.0233 66.9 2.7 2.9 2.8 0.0621 0.0618 0.0543
	SU-PM13 8.4 13.2 9.6 0.3 0.6 0.6	2.5 4.8 3.5 0.1 0.1 0.1	N16	769 0.0 0.1 0.0 6.4 < L.D. 6.5 Morin, 2015 Morin, 2015 This study		2.5 2.6 1.4 98.9 91.9 98.9	5.9 5.9 3.7 0.0381 0.0435 0.0142	2.1 1.4 1.3 0.5 0.1433 0.2593 0.1624	70.5 2.0 3.3 2.5 0.0363 0.0699 0.0451
	SU-PM9 10.2 13.2 11.3 0.4 0.6 0.6	3.5 4.8 3.6 0.1 0.1 < L.D.	R16	824 0.1 0.1 0.0 4.5 < L.D. 5.0 Morin, 2015 Morin, 2015 This study		1.9 2.6 1.1 99.7 91.9 99.3	5.3 5.9 0.4 0.0452 0.0435 0.0023	2.1 1.6 1.3 0.1 0.1725 0.2593 0.1810	74.8 2.3 3.3 2.4 0.0414 0.0699 0.0418
	SU-PM6 12.2 13.5 10.8 0.6 0.6 0.6 0.777707	4.6 5.1 5.3 0.1 0.2 < L.D. -15.6		889 0.1 0.1 0.1 < L.D. 10.4 4.8 Morin, 2015 Morin, 2015 This study		2.5 2.9 1.0 88.2 99.9 99.3	12.4 8.3 0.4 0.0483 0.0473 0.0019	2.2 1.3 1.3 0.1 0.2681 0.2927 0.1737	74.2 3.2 3.4 2.2 0.0758 0.0790 0.0376
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Acknowledgements

This study was funded by the ANR CALIMERO. This is CRPG contribution n° 2738. The authors have no conflict of interest to declare. The data that support the study are available in the tables of the main manuscript or in the supplementary information. We are grateful for the detailed evaluation and reviews provided by T. Schildgen G. Burch Fisher, M. Oskin and D. Scherler. Their comments and suggestions permitted to improve the manuscript and clarify some issues.

Supplementary information

"A 6 Ma record of palaeodenudation in the central Himalayas from in situ cosmogenic 10 Be in the Surai section" Julien Charreau, Jérôme Lavé, Christian France-Lanord, Nicolas Puchol, Pierre-Henri Blard, Raphaël Pik, and ASTER Team Table A -parameters used in the paleo-denudtaion rates calculation with their uncertainties and associated references. These uncertainties were used in the Monte-Carlo simulation to estimate the uncertainties on the final paleo-denduation rates. (Braucher et al., 2011), corrected from (Braucher et al., 2003) Fast muons -0.04 0.007 (Braucher et al., 2011), corrected from (Braucher et al., 2003) Cosmogenic production rates of the Rapti basin (at.g -1 .yr

Supplementary figures and tables