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Two end-member competing models have been proposed to describe the kinematics of the central Nepal Himalayas in the last few Myr. They differ in their interpretations of which surface breaking faults accommodate current shortening and the kinematics responsible for driving rapid exhumation in the topographic transition zone around the Main Central Thrust (MCT). These locally higher uplift and erosion rates in the High Himalaya could reflect (1) thrusting over a midcrustal ramp with the growth of a Lesser Himalaya duplex at midcrustal depth causing underplating along the Main Himalayan Thrust ramp, or (2) out‐of-sequence thrusting along the front of the High Himalaya, possibly driven by climatically controlled localized exhumation.To decipher between the two tectonic models, we compare existing low and medium-temperature thermochronometric data (40Ar/39Ar on muscovite, apatite (U-Th)/He - AHe, zircon (U-Th)/He - ZHe, apatite fission track - AFT, and zircon fission track - ZFT), extracted from the world thermochronometric data file of Herman et al. (2013), to luminescence thermochronometry data from 61 newly collected rock samples along transhimalayan rivers between the Kali Gandaki and the Trisuli. The luminescence thermochronometry data provide a new perspective on Late Pleistocene exhumation rates (timescales of 104 to 105 years) of the Nepalese Himalayas, by offering quantitative high-resolution constraints of rock cooling histories within the upper kilometres of the Earth’s crust.All of the thermochronometric data show younger ages and higher exhumation rates around the topographic transition and the MCT zone through central Nepal. For the higher temperature thermochronometers, there is a continuous trend towards younger ages from the Lesser Himalaya through the topographic transition and the MCT zone. These data suggest that the in-sequence model, with exhumation rates linked to increased erosion and the formation of a duplex below the Higher Himalayas, coincident with the MCT location in some areas, is the model that best describes the thermochronometric ages of this study area on Myr timescales. However, the luminescence thermochronometry data reveal a spatial and temporal variability of the higher exhumation rates at different timescales, suggesting an intermittency of exhumation signal due to geomorphological processes. The luminescence thermochronometry data also highlight a systematic sharp transition at the MCT, pointing to out-of-sequence activity at this tectonic boundary on 100-kyr timescales. Whether this difference in tectonic model between the two timescales is due to low resolution of the higher temperature thermochronometers, shallow isotherms deflected by fluid circulation and hot spring activity near the MCT, or to a change in tectonic regime during the last 200 kyr, out-of-sequence activity of the MCT needs to be considered in seismic hazard models as it could put the local population at risk.

Chlorine-36 (36Cl) is currently the only in situ cosmogenic nuclide applicable in carbonates, Ca- and K-rich feldspars and aphyric silicate rocks. Because the production reactions of 36Cl are more numerous and complex than those of other cosmogenic nuclides (e.g. 10Be, 3He), comprehensive and user-friendly calculators are essential for routine application of 36Cl to Earth surface research questions. However, the existing, commonly used calculators are affected by several drawbacks, such as oversimplification of the muon production model, or the absence of choice regarding production rate parameters. Here, we present the novel 36Cl exposure age calculation tool in development, “CREp 36Cl dev”, which we have implemented as a new functionality in the existing cosmic-ray-exposure program CREp for 10Be and 3He exposure ages. Taking advantage of the most recently developed scaling, muon and low-energy neutron models, “CREp 36Cl dev” allows calculating surface exposure ages, providing several choices between scaling and muon models and production parameters. “CREp 36Cl dev” can be accessed at https://crep-dev.otelo.univ-lorraine.fr/#/.

In mountain accretionary wedges, it is generally considered that the preservation of a topography in mechanical equilibrium is modulated by the activation of faults, sometimes internal to the prism, sometimes frontal. The folds of the Himalayan foothills correspond to the most frontal structures of the Himalayan prism. Understanding the timing of the initiation and the activity of these frontal folds can provide valuable information on the deformation sequences within the range (reactivation of the MCT, prograde sequences and transfer to frontal folds, ...) in response to tectonic and climatic forcing. Late Cenozoic climatic changes, including glaciations, might have impacted the denudation of the Himalayan range. The study of recent deformation rates is thus key for understanding lateral variations in deformation along the entire Himalayan arc, which will bring new constraints on the interactions between tectonics and surface processes at different scales time, as well as deepen our understanding of the seismic behaviour of the range. Here we quantify exhumation rates in the Himalayan foothills using luminescence thermochronometry, which is a recently developed very-low temperature thermochronometer applicable between tens of years and a few hundred kyr. In contrast to classical methods, it can resolve thermal histories from the upper few km of the Earth’s crust, allowing spatial variations in exhumation rates across the Himalayas to be deciphered on sub-Quaternary timescales. An extensive data set of more than 40 Siwalik rock samples, from Western Nepal to Eastern Bhutan, was measured to complement other thermochronometric data and understand the sub-Quaternary deformation on the Himalayan foreland. The results show along-strike variations in exhumation rates in the Himalayan foothills during the late Quaternary, with exhumation rates across the sub-Himalaya varying locally independently of precipitation trends and changes in the modern convergence rates. These along-strike variations may suggest that over the last 300 kyr, Himalayan shortening has not only been accommodated by the most frontal faults along the Himalayan range.

Deep-seated failures of rock slopes are partly controlled by structural, lithological and topographical factors. Among structural factors, layering, schistosity and foliation in rock material, which could be described as inherent anisotropy of the material, affect initiation and evolution of deep-seated rock slope deformation, especially in slow moving landslides.In order to document such an influence of material anisotropy on slope stability, we carry out a parametric study using discrete element modelling (DEM). After a validation exercise for fully isotropic material, where we compare our numerical approach to an analytical slope stability solution, we introduce anisotropy (transverse isotropy) in our DEM model by inserting preferentially oriented and weakened bonds between discrete elements (weakness plane) to simulate two typical transverse isotropic lithologies, claystone and gneiss respectively. Considering these two lithologies, we then explore the influence of the weakness plane’s orientation with respect to the slope angle for both ridge and valley geometries.We show that certain orientations of the weakness plane relative to the topographic slope favour deep-seated deformation. We also observe significant disparities in failure initiation, failure surface localisation, and mobilized volume depending on the weakness plane orientation. For instance, most unstable slopes occur when the weakness plane rises 10° to 30° less than the hillslope angle. These instabilities are associated with well-localized deformation at depth that when intersecting the surface mimic some of the morphological features (such as counter-slope scarps) that are commonly described along mountain ridges in association with slow-moving and deep-seated rock slope failures.Our results help explain the appearance or absence of deep-seated failure in mountainous areas and allow to better assess slope failure hazard induced by anisotropic rock strength.

The erosion and weathering of mountain ranges exert a key control on the long-term (105–106 yr) cycling of carbon between Earth’s surface and crust. The net carbon budget of a mountain range reflects the co-existence of multiple carbon sources and sinks, with corresponding fluxes remaining difficult to quantify. Uncertain responses of these carbon fluxes due to the stochastic nature of erosional processes further complicate the extrapolation of short-term observations to longer, climatically relevant timescales. Here, we quantify the evolution of the organic and inorganic carbon fluxes in response to the 2015 Gorkha earthquake (Mw 7.8) in the central Himalaya. We find that the Himalayan erosion acts as a net carbon sink due mainly to efficient biospheric organic carbon export. Our high-resolution time series encompassing four monsoon seasons before and after the Gorkha earthquake reveal that coseismic landslides did not significantly perturb large-scale Himalayan sediment and carbon fluxes. This muted response of the central Himalaya to a geologically frequent perturbation such as the Gorkha earthquake further suggests that our estimates are representative of at least interglacial timescales. Carbon fluxes in the central Himalaya did not change after the 2015 Gorkha earthquake and its accompanying landslides, according to observations of riverine sediment and carbon fluxes over four monsoon seasons spanning the event.

The climatic reorganizations that occurred in the Southern and Northern hemispheres during the last deglaciation are thought to have affected the continental tropical regions. However, the respective impact of North and Southern climatic changes in the Tropics are still poorly understood. In the Norhtern Tropical Andes, moraines records indicate that the Antarctic Cold Reversal (ACR, 14.3-12.9 ka BP) stage was more represented than the Younger Dryas (12.9-11.7 ka BP) (Jomelli et al., 2014). However, further South, in the Altiplano basin (Bolivia), two cold periods of the North Hemisphere (Heinrich Stadial 1a (16.5-14.5 ka) and Younger Dryas) are synchronous with (i) major advances or stillstands of paleo-glaciers and with (ii) the highstands of the giant palaeo-lakes Tauca and Coipasa (Martin et al., 2018). Therefore, additional geochronological records of paleoglaciers fluctuations are necessary to address the respective impacts of North and South Hemisphere on the glacial dynamics in the region.We present new Cosmic Ray Exposure (CRE) ages from glacial landforms of the Bolivian Andes that extend pre-existing datasets for four different sites spreading from 16 to 21°S. We reconstruct the Equilibrium Line Altitudes (ELA) associated with each moraine with the AAR method and use them in an inverse algorithm that combines both the palaeo-glaciers and palaeo-lake budgets to derive temperature and precipitation reconstructions. Our temperature reconstruction (ΔT vs. Present) shows a consistent trend through the four glacial sites with a progressive warming from ΔT= -5°C (17 ka BP) to –2.5°C (15-14.5 ka BP, at the end of the Tauca highstand). This is followed by a return to colder conditions, around -4°C, during the ACR (15.5-12.9 ka BP). The Coipasa highstand is coeval with another warming trend followed by ΔT stabilization at the onset of the Holocene (circa 10 ka BP), around -3°C. Precipitation is mainly characterized by increases during the lake highstands, modulated by the distance from the glacial sites to the center of the paleolakes that are moisture sources (recycling processes).These new results highlight the decorrelation of the glacier dynamics to the temperature signal in regions that are characterized by high precipitation variability. They also provide a theoretical frame to explain how both regional and global forcings can imprint the paleo-glacial records. Our results strongly suggest that during the last deglaciation (20 – 10 ka BP), in the Tropical Andes, atmospheric temperatures follow the Antarctic variability, while precipitation is driven by the changes occurring in the Northern Hemisphere.ReferencesJomelli et al., Nature, 2014; Martin et al., Sc. Advances, 2018

Bedload transport modelling in rivers, which defines the threshold for pebble movement, takes into account the size and density of pebbles, but does not formally consider particle shape. The lack of analyses evaluating the influences of shape and density on particle mobility presents a major deficiency. To address this issue and to compare the relative roles of the density and shape of particles, we performed original sediment transport experiments in an annular flume using molded artificial pebbles equipped with a radio frequency identification tracking system. The particles were designed with four distinct shapes and four different densities while having the same volume, and their speeds and distances traveled under constant hydraulic conditions were analyzed. The results show that particle shape has more influence than particle density on the resting time between particle displacement and the mean traveling distance. For all densities investigated, the particle shape systematically induced differences in travel distance that were strongly correlated (R² = 0.94) with the Sneed and Folks shape index. Such shape influences, although often mentioned, are here quantified for the first time, demonstrating why and how they can be included in bedload transport models.S

International audience; The respective impacts of Northern and Southern Hemispheric climatic changes on the Tropics during the last deglaciation remain poorly understood. In the High Tropical Andes, the Antarctic Cold Reversal (ACR, 14.3-12.9 ka BP) is better represented among morainic records than the Younger Dryas (12.9-11.7 ka BP). However, in the Altiplano basin (Bolivia), two cold periods of the Northern Hemisphere (Heinrich Stadial 1a, 16.5-14.5 ka BP, and the Younger Dryas) are synchronous with (i) major advances or standstills of paleoglaciers and (ii) the highstands of giant paleolakes Tauca and Coipasa. Here, we present new cosmic ray exposure (CRE) ages from glacial landforms of the Bolivian Andes that formed during the last deglaciation (Termination 1). We reconstruct the equilibrium line altitudes (ELA) associated with each moraine and use them in an inverse algorithm combining paleoglaciers and paleolake budgets to derive temperature and precipitation during the last deglaciation. Our temperature reconstruction (ΔT relative to present day) yields a consistent regional trend of progressive warming from ΔT = −5 to −2.5°C during 17-14.5 ka BP, followed by a return to colder conditions around −4°C during the ACR (14.5-12.9 ka BP). The Coipasa highstand (12.9-11.8 ka BP) is coeval with another warming trend followed by ΔT stabilization at the onset of the Holocene (ca. 10 ka BP), around −3°C. Our results suggest that, during the last deglaciation (20-10 ka BP) atmospheric temperatures in the Tropical Andes mimicked Antarctic variability, whereas precipitation over the Altiplano was driven by changes in the Northern Hemisphere.

The isomer distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils has been shown to correlate to the local mean annual temperature. Here, we explore the use of brGDGT distributions as proxy for the elevation at which soil organic carbon is preferentially mobilized in the Central Himalaya. Soil brGDGT distributions collected along an altitudinal profile, spanning elevations from 200 to 4,450 m asl, are linearly correlated to elevation. We use this calibration to trace the provenance of soil organic matter in suspended sediments of rivers draining the Himalaya. BrGDGT distributions of fluvial sediments reflect the mean elevation of the soil cover in most catchments. Inverse modeling of the brGDGT data set suggests similar relative contribution to soil organic carbon mobilization from different land covers within a factor 2. We conclude that riverine soil organic carbon export in the Himalaya mostly occurs pervasively and is at the catchment scale insensitive to anthropogenic perturbations. ISSN:0094-8276 ISSN:1944-8007

Sediment accumulation rates and thermal trackers suggest a substantial and global increase in erosion rates over the past few million years. That increase is commonly associated with the impact of the Northern Hemisphere glaciation, but methodological biases have led researchers to debate this hypothesis. Here, we test whether Himalayan erosion rates increased by measuring beryllium-10 (10Be) in the sediment of the Bengal Bay seabed. Sediment originated from rocks that produced 10Be under the impact of cosmic rays during erosion near surface. Thus, the 10Be concentrations indicate erosion rates. The 10Be concentration of the Bengal Bay sediment depends on the contributions of the Ganga and Brahmaputra rivers. Their sediments have distinct 10Be concentrations because of distinct elevations and erosion in their drainage basins. Variable contributions could thus complicate erosion-rate calculation. We traced these contributions by a provenance study using the strontium (Sr) and neodymium (Nd) isotopic sediment compositions. Within uncertainties of ±30%, our reconstructed past erosion rates show no long-term increase for the past six million years. This stability suggests that climatic changes during the late Cenozoic have an undetectable impact on the erosion patterns in the Himalayas, at least on the ten thousand to million year timescales accounted for by our dataset. Long-term Himalayan erosion rates remained stable through the global climatic changes of the past six million years, according to the cosmogenic nuclide composition of terrestrial sediments recovered from the Bay of Bengal.

International audience

The paleoclimatic changes that occurred in the Southern and Northern hemispheres during the last deglaciation are thought to have affected the continental tropical regions. However, the respective impact of North and Southern climatic changes in the tropics are still poorly understood. In the High Tropical Andes, the Antarctic Cold Reversal (ACR, 14.3-12.9 ka BP) was reported to be more represented than the Younger Dryas (12.9-11.7 ka BP) among morainic records. However, in the Altiplano basin (Bolivia), two cold periods of the North Hemisphere (Heinrich Stadial 1a (16.5-14.5 ka) and Younger Dryas) are synchronous with (i) major advances or stillstands of paleo-glaciers and with (ii) the highstands of the giant palaeo-lakes Tauca and Coipasa. Therefore, additional results are needed to disentangle between potential North and South Hemisphere climatic influence on the glacial dynamics in the region.We present new Cosmic Ray Exposure (CRE) ages from glacial landforms of the Bolivian Andes that extend pre-existing datasets for four different sites spreading from 16 to 21°S. We reconstruct the Equilibrium Line Altitudes (ELA) associated with each moraine with the AAR method and use them in an inverse algorithm that combines both the palaeo-glaciers and palaeo-lake budgets to derive temperature and precipitation reconstructions. Our temperature reconstruction (ΔT vs. Present) shows a consistent trend through the four glacial sites with a progressive warming from ΔT= -5°C (17 ka BP) to –2.5°C (15-14.5 ka BP, at the end of the Tauca highstand). This is followed by a return to colder conditions, around -4°C, during the ACR (15.5-12.9 ka BP). The Coipasa highstand is coeval with another warming trend followed by ΔT stabilization at the onset of the Holocene (circa 10 ka BP), around -3°C. Precipitation is mainly characterized by increases during the lake highstands, modulated by the distance from the glacial sites to the center of the paleolakes that are moisture sources (recycling processes).These new results highlight the decorrelation of the glacier dynamics to the temperature signal in regions that are characterized by high precipitation variability. They also provide a theoretical frame to explain how both regional and global forcings can imprint the paleo-glacial records. Our results strongly suggest that during the last deglaciation (20 – 10 ka BP), in the Tropical Andes, atmospheric temperatures follow the Antarctic variability, while precipitation is driven by the changes occurring in the Northern Hemisphere.

Soil erosion in high mountain ranges plays an important role in redistributing soil organic carbon across landscapes and may influence the global climate on different timescales [1, 2]. Here, we investigate the dynamics of soil organic matter export in the steep mountain belt of the Himalaya by tracing the provenance of soil-derived lipids in riverine sediments from nested catchments with areas ranging from 370 to 57700 km2. Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are a suite of lipids that occur ubiquitously in soils [3, 4]. Their isomer distribution depends on environmental parameters such as the mean annual temperature of the local environment [3]. In this study, we explore the use of brGDGT distributions as a proxy for the altitudinal provenance of soil organic matter in riverine sediments of the Central Himalaya of Nepal. BrGDGT distributions in soils collected along an altitudinal profile, spanning elevations from 200 to 4450 m asl, yield a robust calibration of soil signatures as a function of elevation. This calibration is then used to trace the provenance of soil organic matter exported from their catchments and entrained in suspended sediments of rivers draining the Central Himalaya. We show that brGDGT compositions of fluvial sediments accurately reflect the mean elevation of the soil-cover in their respective watersheds. The type of land-cover does not seem to have a significant influence on the export of organic matter at a catchment scale. We, therefore, conclude that soil organic matter mobilization in the Himalaya occurs pervasively, and is currently insensitive to anthropogenic perturbations., EGUsphere

International audience; In-situ cosmogenic 3 He is a robust tool for determining denudation rates or exposure ages of lavas bearing mafic phenocrysts. However, analyses are often complicated by the presence of several helium sources. In particular, in old magmatic rocks with high radiogenic 4 He contents, discriminating cosmogenic 3 He from magmatic 3 He is not straightforward since these varieties may vary largely between aliquots. We sampled sands from the Tekeze and Mile rivers, both draining the basaltic Ethiopian highlands, an area where erosion patterns are intimately linked to the development of the Western Afar margin and to heterogeneous monsoon precipitation. From each river we analyzed~15 aliquots of pyroxenes having variable grain sizes (0.3 mm up to N 1 mm). The total 3 He is both higher and more scattered in the bigger grains. Crushing of these largest grains and subsequent melting of the powder tends to produce more homogeneous 3 He values, suggesting that magmatic 3 He hosted in inclusions is responsible for most of the inter-aliquot variability. We also performed a Monte Carlo simulation based on a numerical denudation model of the two watersheds. The simulation confirms that cosmogenic 3 He variability cannot be responsible for the observed scatter since the cos-mogenic 3 He variability is averaged away and unobservable in aliquots of~200 grains. A compilation of previously published data also indicates that magmatic helium can be significantly variable, even between pre-crushed aliquots. Hence, magmatic helium, unlike cosmogenic 3 He, is highly variable, even in the case of aliquots of hundreds of grains. We suggest this is due to a strong nugget effect, possibly due to large fluid (or melt)-inclusions contained in phenocrysts. In addition, the fact that small and big grains have comparable radiogenic 4 He concentrations suggests that grain fragmentation during river transport is responsible for the lower magmatic helium content of the smallest grains. Therefore, one should preferably use small grain (0.2-0.5 mm) granulometry for in-situ cosmogenic 3 He analysis in mafic phenocrysts. Using the measured cosmogenic 3 He, we calculate basin-averaged denudation rates of 70 ± 20 and 57 ± 5 mm kyr −1 , for the Mile and for the Tekeze river, respectively. These values are coherent with long-term denu-dation rates previously proposed from low-temperature thermochronology.

International audience; Because of its essential role in coupling climate and tectonics, denudation is a key parameter when constraining the history of Earth’s surface. This is particularly true at the Pliocene-Pleistocene transition, and the potential impact of the onset of Quaternary glaciations remains strongly debated. In the present study, we measured in situ cosmogenic 10Be within continuous late Cenozoic sedimentary sections that had already been dated using magnetostratigraphy. The new data were obtained from four sedimentary basins in the northern and southern Tianshan range (Central Asia). We first thoroughly discuss how in situ cosmogenic 10Be concentrations can be corrected for radioactive decay and for the contribution of postdepositional cosmogenic accumulation to derive the paleo–denudation rates. Our analysis shows that, in the four sedimentary records, the potential bias remains low enough to consider the derived denudation rates reliable. The four records, although likely influenced by local particularities due to lithological heterogeneity and local tectonics, display similar trends of continuously increasing denudation between ca. 9 Ma and the present. These rates have remained relatively high but steady since 4 Ma, ∼1.5 m.y. before the onset of the Quaternary glacial cycles. Though the rejuvenation of the Tianshan range since 11 Ma may explain most of the progressive increase (×5) in denudation, our data suggest that the Quaternary glaciations had only a limited impact on denudation in the Tianshan. Our data, however, indicate an increase in the spatial and high-frequency variability (

RFID transponders, especially low-frequency Passive Integrated Transponders (PIT tags), are now commonly used for assessing bedload mobility in gravel-bed rivers. Early studies reported high PIT tag recovery rates in small streams, but recovery rates in larger systems remain low. Explanatory factors for low recovery rates have been identified, but only antenna detection ranges and clustering effects have been precisely characterized. Burial below detection limit and dispersion beyond the study site have been indirectly estimated, and PIT tag destruction is assumed to be negligible. In this study, we quantified the resistance of natural limestone pebbles equipped with PIT tags as related to tag insertion methods and distance travelled in an annular flume. We then compared the performance of these natural pebbles with synthetic pebbles made of polyurethane resin and corundum. Creation of synthetic pebbles makes it possible to test bigger transponders with a greater detection range and no clustering effects, bypassing the existing constraints of RFID tracking. We found that breakage of limestone particles leading to PIT tag destruction is far too rare to explain low recovery rates in situ. Further, breakage is more affected by initial pebble characteristics than by PIT tag insertion method. Synthetic pebbles are more sensitive to attrition, but less likely to break. Natural and synthetic pebbles show slight differences in mobility patterns, transport distances, and abrasion resistances. One of the two synthetic pebbles travelled faster than the other three indicating that mobility is not only related to density but also potentially to shape. To address clustering and detection range issues, we equipped synthetic pebbles with AIT-tags (Active Integrated Transponders). These were tested for integrity and transponder endurance and show great promise for future applications. Copyright © 2016 John Wiley & Sons, Ltd.

International audience; The geometrical characteristics of pebbles are important features to study transport pathways, sedimentary history, depositional environments, abrasion processes or to target sediment sources. Both the shape and roundness of pebbles can be described by a still growing number of metrics in 2D and 3D or by visual charts. Despite new developments, existing tools remain proprietary and no pebble roundness toolbox has been available widely within the scientific community. The toolbox developed by Roussillon et al. (2009) automatically computes the size, shape and roundness indexes of pebbles from their 2D maximal projection plans. Using a digital camera, this toolbox operates using 2D pictures taken of pebbles placed on a one square meter red board, allowing data collection to be quickly and efficiently acquired at a large scale. Now that the toolbox is freely available for download,1 an evaluation is needed to help end users sample, process and interpret the data. We first assess the influence of pebble position and of the resolution and enhancement of pictures and errors associated with the sampling size. Second, we run experimental flume tests of roundness trends that confirm the great potential of the toolbox for river geomorphic diagnoses as well as for targeting sediment sources. We also validate the correlation between roundness metric trends and particle abrasion.

International audience; We document the temporal evolution of deformation in the northern Tianshan piedmont where the deformation is partitioned across several thrusts and folds. We focus on the Dushanzi anticline, where abandoned terraces and growth strata allow us to constrain the history of folding since the Miocene. Based on subsurface seismic imaging, structural measurements and morphological analysis, we show that this anticline is associated with two decollement levels. We use kink band migration in growth strata dated by paleomagnetism to constrain the shortening from the Mio-Pliocene to the Holocene. Our results show that the Dushanzi anticline has been active since at least 8 Ma and that the fold grew at a steady shortening rate of 0.6 ± 0.1 mm/yr from 8 to ~1.5 Ma with possible variations from 2.5 to 1.5 Ma. Then it accelerated rapidly to a rate of 4.3 ± 1.0 mm/yr over at least the last 100 ka. These results, together with similar temporal shortening evolutions across other structures, suggest that the deformation rate across the eastern Tianshan piedmont increased relatively recently. This may reflect either a redistribution of the deformation from the internal structures toward the borders or a general acceleration of the deformation across the entire range.

International audience; Detrital provenance analyses in orogenic settings, in which sediments are collected at the outlet of a catchment, have become an important tool to estimate how erosion varies in space and time. Here we present how Raman Spectroscopy on Carbonaceous Material (RSCM) can be used for provenance analysis. RSCM provides an estimate of the peak temperature (RSCM‐T) experienced during metamorphism. We show that we can infer modern erosion patterns in a catchment by combining new measurements on detrital sands with previously acquired bedrock data. We focus on the Whataroa catchment in the Southern Alps of New Zealand and exploit the metamorphic gradient that runs parallel to the main drainage direction. To account for potential sampling biases, we also quantify abrasion properties using flume experiments and measure the total organic carbon content in the bedrock that produced the collected sands. Finally, we integrate these parameters into a mass‐conservative model. Our results first demonstrate that RSCM‐T can be used for detrital studies. Second, we find that spatial variations in tracer concentration and erosion have a first‐order control on the RSCM‐T distributions, even though our flume experiments reveal that weak lithologies produce substantially more fine particles than do more durable lithologies. This result implies that sand specimens are good proxies for mapping spatial variations in erosion when the bedrock concentration of the target mineral is quantified. The modeling suggests that highest present‐day erosion rates (in Whataroa catchment) are not situated at the range front but around 10 km into the mountain belt.

International audience; Identifying the roles of erosional processes in the denudation of mountain ranges requires a better understanding of erosional sensitivity to climatic, topographic, or lithologic controls. We analyzed erosion in the Narayani River basin (draining central Nepal and presenting contrasted lithologic and geochemical signatures in its outcropping rocks and a wide variety of erosional processes and climatic conditions) to assess the relative contributions of erosion processes to the annual sediment export. By combining acoustic Doppler current profiler measurements with depth profiles and daily surface samplings of the suspended load, we propose a simplified model to precisely calculate sediment fluxes at the basin outlet. We estimate an equivalent erosion rate of 1:8 þ0:35 À0:2 mm/year for the year 2010, similar to the average value of 1:6 þ0:35 À0:2 mm/year estimated from 15 years of records and long-term (~ky) denudation rates of 1.7 mm/year derived from cosmogenic nuclides. The stability of erosion is attributed to efficient buffering behavior and spatial integration in the drainage system. Strong relations between rainfall events and the sediment export suggest that the system is mainly supply limited. Combining physical calculation of sediment fluxes with grain size analyses and geochemical tracers (hydroxyl isotopic compositions, carbonate contents, and total organic carbon content), we estimate that glacial and soil erosion do not contribute more than 10% and a few percentage, respectively, to the total budget and are only detectable during premonsoon and early monsoon periods. During the monsoon, erosion by landslides and mass wasting events overwhelms the sediment budget, confirming the dominant role of these erosional processes in active mountain chains.

The Tsangpo-Brahmaputra River drains the eastern part of the Himalayan range, flowing from the Tibetan Plateau through the eastern Himalayan syntaxis and downstream to the Indo-Gangetic floodplain. As such it is a unique natural laboratory to study how denudation and sediment production processes are transferred to river detrital signals. In this study, we present a new 10Be data set to constrain denudation rates across the catchment and to quantify the impact of rapid erosion within the syntaxis region on cosmogenic nuclide budgets and signals. 10Be denudation rates span around two orders of magnitude across the catchments (ranging from 0.03 mm/yr to > 4 mm/yr) and sharply increase as the Tsangpo-Brahmaputra flows across the eastern Himalaya. The increase in denudation rates however occurs ~ 150 km downstream of the Namche Barwa-Gyala Peri massif (NBGPm), an area which has been previously characterized by extremely high erosion and exhumation rates. We suggest that this downstream lag is mainly due to the physical abrasion of coarse grained, low 10Be concentration, landslide material produced within the syntaxis that dilutes the upstream high concentration 10Be flux from the Tibetan Plateau only after abrasion has transferred sediment to the studied sand fraction. A simple abrasion model produces typical lag distances of 50 to 150 km compatible with our observations. Abrasion effects reduce the spatial resolution over which denudation can be constrained in the eastern Himalayan syntaxis. In addition, we also highlight that denudation rate estimates are dependent on the sediment connectivity, storage and quartz content of the upstream Tibetan Plateau part of the catchment which tends to lead to an overestimation of downstream denudations rates. Taking these effects into account we estimate a denudation rates of ca. 2 to 5 mm/yr for the entire syntaxis and ca. 4 to 28 mm/yr for the NBGPm, which is significantly higher than other to other large catchments. Overall, 10Be concentrations measured at the outlet of the Tsangpo-Brahmaputra in Bangladesh suggest a sediment flux between 780 and 1430 Mt/yr equivalent to a denudation rate between 0.7 and 1.2 mm/yr for the entire catchment.

International audience; For documenting recent or Late Quaternary erosion rates at the scale of a small watershed, or even an entire mountain range, the use of in-situ terrestrial cosmogenic nuclides (TCN), such as 10Be, in river sediments has become widespread over the last decade. In mountainous settings, however, landslides may induce a two-fold complication in the cosmogenic nuclide budget. First, they may episodically deliver large amounts of sediment with low TCN concentrations to the river channel. Second, they may generate a grain-size-concentration dependence in these sediments. However, studies that have explored grain-size dependence in landslide-dominated areas have reached differing conclusions and the underlying mechanisms remain poorly understood. The present study focuses on the Khudi Khola river basin, a small drainage basin in the central Himalayas. The area is characterized by steep slopes, heavy rainfall, and high sediment production rates. Importantly, the watershed contains a large area of active landsliding. In-situ 10Be was analyzed in sediments of grain sizes ranging from 75 μm to ~ 4 cm, collected from five locations: upstream, downstream, and in the small tributaries which drain the landslides. In all sampling locations, including those situated upstream of the landslides, the finest grains (75–250 μm) are two to four times more concentrated in 10Be than the largest clasts (> 4.7 mm). We developed a numerical slope model which includes a low level of background soil erosion and episodic denudation from large mass-wasting events. We then introduced a coarsening of average grain-size with soil-depth to the model. Our simulation shows that the data available for this valley are compatible with a scenario in which landslides deliver coarse grains to the channel that are less concentrated in TCN and have deeper provenance. Best-fit estimates of the model parameters suggest recurrence times of 3000 to 9000 years for a major landslide on the same slope, and mean long-term landslide denudation rates of 1.7 to 4.7 mm yr− 1. A negative correlation between TCN concentration and grain-size is also observed directly upstream and 5-km downstream of the landsliding area, suggesting that nearly all of the sediment carried by the river is derived from ancient landslides while almost no sediment is supplied by erosion of soil-mantled slopes. Despite these complications, apparent denudation rates directly derived from the TCN concentrations, and from previously published data in the region, are all on the same order of magnitude, around ~ 2 mm yr− 1. However, a factor-of-three variability remains between the different studies, depending on the location, the year of sampling and the analyzed grain size fraction. Thus, although TCN-derived average denudation rates provide a useful first order estimate, even in fast-eroding terrains dominated by landslides, caution should be applied when using quantitative results in such settings.

International audience; The Khudi river in Nepal is an example of a basin undergoing intense physical erosion by landslide under very wet monsoonal conditions. Although under such a regime, dissolved element concentrations are expected to during the monsoon, we observe marked increases in dissolved K and Ca during flood events. These peaks in K and Ca concentrations are well correlated with increases of suspended load by an order of magnitude. The data suggest that release of K and Ca is enhanced by physical erosion and rock disaggregation events during sediment transfer.

International audience; The neodymium and hafnium isotopic compositions of most crustal and mantle rocks correlate to form the “Terrestrial Array”. However, it is now well established that whereas coarse detrital sediments follow this trend, fine-grained oceanic sediments have high Hf ratios relative to their Nd isotopic ratios. It remains uncertain whether this “decoupling” of the two isotopic systems only occurs in the oceanic environment or if it is induced by sedimentary processes in continental settings. In this study, the hafnium and neodymium isotopic compositions of sediments in large rivers is expressly used to constrain the behavior of the two isotopic systems during erosion and sediment transport from continent to ocean.We report major and trace element concentrations together with Nd and Hf isotopic compositions of bedloads, suspended loads and river banks from the Ganges River and its tributaries draining the Himalayan Range i.e. the Karnali, the Narayani, the Kosi and the Marsyandi Rivers. The sample set includes sediments sampled within the Himalayan Range in Nepal, at the Himalayan mountain front, and also downstream on the floodplain and at the outflow of the Ganges in Bangladesh. Results show that hydrodynamic sorting of minerals explains the entire Hf isotopic range, i.e. more than 10 εHf units, observed in the river sediments but does not affect the Nd isotopic composition. Bedloads and bank sediments have systematically lower εHf values than suspended loads sampled at the same location. Coarse-grained sediments lie below or on the Terrestrial Array in an εHf vs. εNd diagram. In contrast, fine-grained sediments, including most of the suspended loads, deviate from the Terrestrial Array toward higher εHf relative to their εNd, as is the case for oceanic terrigenous clays. The observed Nd–Hf decoupling is explained by mineralogical sorting processes that enrich bottom sediments in coarse and dense minerals, including unradiogenic zircons, while surface sediments are enriched in fine material with radiogenic Hf signatures. The data also show that Nd–Hf isotopic decoupling increases with sediment transport in the floodplain to reach its maximum at the river mouth. This implies that the Nd–Hf isotopic decoupling observed in worldwide oceanic clays and river sediments is likely to have the same origin. Finally, we estimated the Nd–Hf isotopic composition of the present-day mantle if oceanic sediments had never been subducted and conclude that the addition of oceanic sediments with their anomalous Nd–Hf isotopic compositions has slowly shifted the composition of the Earth’s mantle towards more radiogenic Hf values through time.

International audience; The modern high topography of the Tianshan resulted from the reactivation of a Paleozoic orogenic belt by the India/Asia collision. Today, the range exhibits tectonically active forelands and intermontane basins. Based on quantitative morphotectonic observations and age constraints derived from cosmogenic 10Be dating, single-grain post-infrared infrared stimulated luminescence (p-IR IRSL) dating and modeling of fault scarp degradation, we quantify the deformation in the Nalati and Bayanbulak intermontane basins in the central Eastern Tianshan. Our results indicate that at least 1.4 mm/yr of horizontal crustal shortening is accommodated within these two basins. This shortening represents over 15% of the 8.5 ± 0.5 mm/yr total shortening rate across the entire range at this longitude. This shortening rate implies that the Eastern Central Tianshan is thickening at a mean rate of ∼1.4 mm/yr, a rate that is significantly higher than the average denudation rate of 0.14 mm/yr derived from our cosmogenic analysis. This discrepancy suggests that the Tianshan range has not yet reached a steady-state topography and remains in a transient state of topographic growth, most likely due to limited denudation rates driven by the arid climate of Central Asia.

Over the last decades, cosmogenic exposure dating has permitted major advances in many fields of Earth surface sciences and particularly in paleoglaciology. Yet, exposure age calculation remains a complicated and dense procedure. It requires numerous choices of parameterization and the use of an accurate production rate. This study describes the CREp program and the ICE-D production rate online database. This system is designed so that the CREp calculator will automatically reflect the current state of this global calibration database production rate, ICE-D. ICE-D will be regularly updated in order to incorporate new calibration data and reflect the current state of the available literature. CREp is a Octave/Matlab© online code that computes Cosmic Ray Exposure (CRE) ages for 3He and 10Be, available at crep.crpg.cnrs-nancy.fr. A stand-alone version of the CREp code is also released with the present article. Note however that only the online version is connected to the online database ICE-D. The CREp program offers the possibility to calculate ages with two scaling models: i.e. the empirical Lal-Stone time-dependent model (Balco et al., 2008; Lal, 1991; Stone, 2000) with the muon parameters of Braucher et al. (2011), and the Lifton-Sato-Dunai (LSD) theoretical model (Lifton et al., 2014). The default atmosphere model is the ERA-40 database (Uppala et al., 2005), but one may also use the standard atmosphere for comparison (N.O.A.A, 1976) to apply the atmospheric correction. To perform the time-dependent correction, users may import their own geomagnetic database for paleomagnetic corrections or opt for one of the three proposed datasets (Lifton, 2016; Lifton et al., 2014; Muscheler et al., 2005). For the important choice of the production rate, CREp is linked to a database of production rate calibration data that is part of the ICE-D (Informal Cosmogenic-nuclide Exposure-age Database) project (http://calibration.ice-d.org). This database includes published empirical calibration rate studies that are publicly available at present, comprising those of the CRONUS-Earth and CRONUS-EU projects, as well as studies from other projects. In the present study, the efficacy of the different scaling models has also been evaluated looking at the statistical dispersion of the computed Sea Level High Latitude (SLHL) production rates. Lal/Stone and LSD models have comparable efficacies, and the impact of the tested atmospheric model and the geomagnetic database is also limited. Users however have several possibilities to select the production rate: 1) using a worldwide mean value, 2) a regionally averaged value (not available in regions with no data), 3) a local unique value, which can be chosen among the existing dataset or imported by the user, or 4) any combination of single or multiple calibration data. If a global mean is chosen, the 1σ uncertainty arising from the production rate is about 5% for 10Be and 10% for 3He. If a regional production rate is picked, these uncertainties are potentially lower. CREp is able to calculate a large number of ages in a reasonable time (typically < 30 s for 50 samples). The user may export a summary table of the computed ages and the density probability function associated with each age (in the form of a spreadsheet).

Understanding the origin of U–Th–Ra variations in the Ganga river sediments is a prerequisite for correctly using U-series nuclides to constrain the sediment transport times in Himalayan rivers. For this purpose, U, Th, and Ra concentrations, along with 238 U– 234 U– 230 Th– 226 Ra radioactive disequilibria, were analyzed in bank, bedload and suspended sediments from the Gandak river, one of the main tributaries of the Ganga river. The data confirm that U and Th budgets of the Himalayan sediments are significantly influenced by minor resistant minerals, such as zircon, garnet and Ti-bearing minerals, the dissolution of which required the use of a high-pressure acid digestion process. Most importantly, the results indicate that the variations in ( 238 U/ 232 Th) and ( 230 Th/ 232 Th) activity ratios and 238 U– 234 U– 230 Th– 226 Ra disequilibria in sediments along the river alluvial plain mainly reflect modifications in the mineralogical and grain-size compositions rather than the degree of weathering during transport. The ( 238 U/ 232 Th) and ( 230 Th/ 232 Th) activity ratios in the bank and bed sediments are related to variations in the minor primary minerals strongly enriched in U and Th (i.e., zircon, REE-bearing minerals and Ti-bearing minerals), whereas the activity ratios in the suspended load are related to variations in the proportions of clay, Fe-oxyhydroxides and the silt-sand fraction, which contains U- and Th-bearing minor minerals. The data also indicate that 238 U– 234 U– 230 Th– 226 Ra disequilibria are strongly influenced by secondary mineral phases: the 230 Th budget is likely mainly controlled by Fe-oxyhydroxides, and the 226 Ra budget is likely mainly controlled by clay minerals. Therefore, the variations in the 238 U– 234 U– 230 Th– 232 Th system in the sediments of the Gandak river cannot simply be interpreted as the result of fractionation due to chemical transformation of the bulk sediment during its transport within the alluvial plain and/or the result of radioactive decay. Consequently, they cannot be used to infer long sediment transport times within the Gandak plain (10–100 ka), as previously proposed. Such analytical and interpretative artifacts are certainly not specific to the present study on the Gandak basin. These issues will certainly be encountered anytime this technique is applied to alluvial systems in which the U and Th budgets of the sediments are influenced by “heavy” minerals that can be sorted during the transport of sediments within the plain.

We present an extensive river sediment dataset covering the Ganga basin from the Himalayan front downstream to the Ganga mainstream in Bangladesh. These sediments were mainly collected over several monsoon seasons and include depth profiles of suspended particles in the river water column. Mineral sorting is the first order control on the chemical composition of river sediments. Taking into account this variability we show that sediments become significantly depleted in mobile elements during their transit through the floodplain. By comparing sediments sampled at the Himalayan front with sediments from the Ganga mainstream in Bangladesh it is possible to budget weathering in the floodplain. Assuming a steady state weathering regime in the floodplain, the weathering of Himalayan sediments in the Gangetic floodplain releases ca. (189 ± 92) � 10 9 and (69 ± 22) � 10 9 mol/yr of carbonate bound Ca and Mg to the dissolved load, respectively. Silicate weathering releases (53 ± 18) � 10 9 and (42 ± 13) � 10 9 mol/yr of Na and K while the release of silicate Mg and Ca is substantially lower, between ca. 0 and 20 � 10 9 mol/yr. Additionally, we show that sediment hydration, [H2O + ], is a sensitive tracer of silicate weathering that can be used in continental detrital environments, such as the Ganga basin. Both [H2O + ] content and the D/H isotopic composition of sediments increases during floodplain transfer in response to mineral hydrolysis and neoformations associated to weathering reactions. By comparing the chemical composition of river sediments across the floodplain with the composition of the eroded Himalayan source rocks, we suggest that the floodplain is the dominant location of silicate weathering for Na, K and [H2O + ]. Overall this work emphasizes the role of the Gangetic floodplain in weathering Himalayan sediments. It also demonstrates how detrital sediments can be used as weathering tracers if mineralogical and chemical sorting effects are properly taken into account. 2012 Elsevier Ltd. All rights reserved.

We present a synthesis of recently conducted tectonic, global positioning system (GPS), geomorphological and seismic studies to describe the kinematics of the Zagros mountain belt, with a special focus on the transverse right-lateral strike-slip Kazerun Fault System (KFS). Both the seismicity and present-day deformation (as observed from tectonics, geomorphology and GPS) appear to concentrate near the 1000 m elevation contour, suggesting that basement and shallow deformation are related. This observation supports a thick-skinned model of southwestward propagation of deformation, starting from the Main Zagros Reverse Fault. The KFS distributes right-lateral strike-slip motion of the Main Recent Fault onto several segments located in an en echelon system to the east. We observe a marked difference in the kinematics of the Zagros across the Kazerun Fault System. To the NW, in the North Zagros, present-day deformation is partitioned between localized strike-slip motion on the Main Recent Fault and shortening located on the deformation front. To the SE, in the Central Zagros, strike-slip motion is distributed on several branches of the KFS. The decoupling of the Hormuz Salt layer, restricted to the east of the KFS and favouring the spreading of the sedimentary cover, cannot be the only cause of this distributed mechanism because seismicity (and therefore basement deformation) is associated with all active strike-slip faults, including those to the east of the Kazerun Fault System.

International audience; Although the geology and structure of the Zagros fold-and-thrust belt (Iran) have been studied extensively, the distribution of active deformation across the belt and its seismotectonic behaviour remain controversial. We have mapped deformed fluvial terraces along the Dalaki and Mand rivers in the central Zagros, as well as marine terraces along the Persian Gulf, in order to unravel the spatial pattern of vertical displacements and to analyse active deformation and its implications for seismicity. Using appropriate fold models based on structural data allows interpreting such vertical displacements in terms of horizontal shortening across the fold structures. Obtaining well-constrained rates of deformation depends on reliably dating deformed geomorphic markers; by combining different dating techniques and correlations to sea level history, we propose an internally consistent set of ages, which allow the first geomorphic estimates of shortening rates absorbed by individual structures in the central Zagros. Our results show that shortening on Late Pleistocene timescales is concentrated in the frontal part of the belt, consistent with recent GPS data. Three or four frontal structures appear to absorb practically all of the active shortening across the Zagros, broadly consistent with a normal forward-propagating deformation sequence but with local out-of-sequence activation of structures behind the forward-most folds. The localization, rate and direction of surface shortening across individual structures appear decoupled from basement deformation, as indicated by structural cross-sections and the distribution of seismicity. Such independent behaviour suggests that the sedimentary cover of the frontal Zagros is decoupled from the basement, most probably at the level of the Hormuz Salt. This weak basal detachment level, together with several intermediate décollement levels, appears to be responsible for the overwhelmingly aseismic deformation of the Zagros sedimentary cover, and also to control the development of a large panel of fold structures, from detachment to fault-propagation folds with varying wavelength and rooting depth.

International audience; The investigation of deformation rates on a mountain piedmont can provide key information for improving our understanding of the overall dynamics of a mountain range. Here, we estimate the shortening rate absorbed by a Quaternary emergent detachment fold on the southeastern piedmont of the Tianshan (China). Our work is primarily based on new 10Be cosmogenic exposure dating of deformed alluvial surfaces. The method we have developed combines depth profiling with sampling of surface cobbles, thereby allowing exposure time, erosion rate and inheritance to be simultaneously constrained. The exposure ages of the uppermost uplifted alluvial surfaces are around 140±17 ka140±17 ka, 130±9 ka130±9 ka and 47±9 ka47±9 ka, from west to east. A terrace lying below the 140 ka surface is dated at 65±5 ka65±5 ka. The ages of the uplifted and folded alluvial surfaces were then combined with estimates of shortening obtained using two distinct methods: (1) the excess area method, where sedimentation rates, extracted from magnetostratigraphic studies, are used to determine the amount of sedimentation after the abandonment of the river; and (2) a folding model derived from sandbox experiments. The late Pleistocene shortening rates are shown to be between 0.4±0.1 mm/yr0.4±0.1 mm/yr and 0.8±0.5 mm/yr0.8±0.5 mm/yr on the western part of the fold and 2.1±0.4 mm/yr2.1±0.4 mm/yr along its central part. The central part of the frontal Yakeng anticline therefore accommodates up to 25% of the total shortening currently absorbed across the whole Eastern Tianshan range (8 mm/yr). However, this situation seems to have prevailed for only the last 150 ka, as the shortening rate absorbed by this nascent fold was previously ten times slower. While the initiation of folding of the Yakeng anticline can be traced back to 5.5 Ma ago, the basinward migration of the active deformation front onto the Yakeng fold is a relatively recent phenomenon and appears to be diachronous from west to east, probably in relation to the tectonic activity of the folds in the hinterland.

International audience; The chronology of the seismic ruptures along the active faults of Western Nepal remains almost unconstrained despite their high seismogenic potential. We present here a slip history of one of these structures, a 120 km-long reactivated segment of the Main Boundary Thrust named the Surkhet–Gorahi fault. This slip history is based on geomorphologic and neotectonic mapping of active faults deduced from the analysis of a high resolution total station digital elevation model and 15 detrital charcoals radiocarbon ages constraining the age of deposition or abandonment of 4 alluvial terraces of the Bheri river in Botechaur. Our results show that the last two earthquakes occurred on this fault after 1860 and 640 BP, respectively, and accommodated slip greater than 8 m each, a value corresponding to the incremental vertical offset of the terraces. Such events released a significant part of the slip deficit accumulated on the Main Himalayan thrust fault. However, given the geometry of this fault system as well as the date of occurrence of the last events, the ruptures could be associated with major earthquakes also rupturing the Main Frontal Thrust, such as the great 1505 earthquake.

We use petrographic and mineralogical data on modern sediments to investigate erosion patterns in the Marsyandi basin of the central Himalaya, a privileged natural laboratory in which a series of multidisciplinary geomorphological, sedimentological, geochemical and geochronological studies have been recently carried out to unravel the interrelationships between tectonic, climatic and sedimentary processes in high-relief orogenic belts. Although relative erosion patterns are effectively constrained by analyses of replicate samples along six successive tracts of the Marsyandi River, uncertainties are caused by potential compositional variation between the monsoon and post-monsoon season. Estimates of erosion rates are significantly affected by poor knowledge of total sediment flux through the basin. Our results support focused erosion of the southern, tectonically-lower part of the Greater Himalaya in the hangingwall of the MCT Zone, where the summer monsoon reaches its peak intensity (up to 5 m/a), and sediment yields and erosion rates reach 14,100 ± 3400 t/km2 and 5.1 ± 1.2 mm/a. Erosion rates sharply decrease southward in low-relief Lesser Himalayan units (1.6 ± 0.6 mm/a), and also progressively decrease northwards in the high-altitude, tectonically-upper part of the Greater Himalaya, where rainfall decreases rapidly to b 2 m/a. Even areas of extreme topography such as the Manaslu Granite are characterized by relatively low erosion rates (2.4 ± 0.9 mm/a), because precipitations become too scarce to feed significant ice flux and glacial activity. Monsoonal rainfall decreases further to b 0.5 m/a in the Tethys Himalayan zone farther north, where erosion rates are ∼ 1 mm/a. Coupling between erosion and peak monsoonal rainfall along the southern front of the Greater Himalaya is consistent with both channel-flow models of tectonic extrusion and tectonic uplift above a mid-crustal ramp. Altitude and relief are not the principal factors controlling erosion, and the central Nepal eight-thousanders may be viewed as topographic anomalies in cold desert climate at the southern edge of the Tibetan rain shadow. © 2007 Elsevier B.V. All rights reserved.

International audience; Radical grain size changes between two main units of a sedimentary megacycle in a foreland basin are commonly interpreted to result from changes in tectonic activity or climate in the adjacent mountain range. In central Nepal, the Cenozoic Siwalik molasse deposits exposed in the frontal Himalayan folds are characterized by such a radical grain size transition. Locally gravel deposits completely replace sands in vertical succession over approximately a hundred metres, the median grain size (D50) displaying a sharp increase by a factor of ca. 100. Such a rapid gravel‐sand transition (GST) is also observed in present‐day river channels about 8–20 km downstream from the outlet of the Siwalik Range. The passage from gravel‐bed channel reaches (proximal alluvial fans) to sand‐bed channel reaches (distal alluvial fans) occurs within a few kilometres on the Gangetic Plain in central Nepal, and the D50 ratio between the two types of channels equals ca. 100. We propose that the dramatic and remarkably similar increase in grain size observed in the Neogene Siwalik series and along modern rivers in the Gangetic foreland basin, results from a similar hydraulic process, i.e. a grain sorting process during the selective deposition of the sediment load. The sudden appearance of gravels in the upper Siwalik series would be related to the crossing of this sorting transition during progressive southward migration of the gravel front, in response to continuous Himalayan orogen construction. And as a consequence, the GST would be diachronous by nature. This study demonstrates that an abrupt change in grain size does not necessarily relate to a change in tectonic or climatic forcing, but can simply arise from internal adjustment of the piedmont rivers to the deposition and run out of coarse bedload. It illustrates, in addition, the genesis of quartz‐rich conglomerates in the Himalayan foreland through gravel selective deposition associated with differential weathering, abrasion processes and sediment recycling during thrust wedge advance and shortening of the foreland basin.

Continental climate change during the late glacial period has now been widely documented thanks to Cosmic-Ray Exposure (CRE) dating of glacial features. The accuracy of these CRE ages mainly relies on a priori knowledge of the production rate of the cosmogenic nuclide that has accumulated in a specific mineral. To produce unequivocal and accurate chronologies of glacier fluctuations during the late glacial period, it is crucial that the cosmogenic nuclide production rates are better constrained, particularly in the high tropics where existing spatial and temporal scaling models show significant discrepancies. Here we report a new production rate established at low latitude (19°S) and high elevation (3800 masl) on the Challapata fan-delta, at the edge of the Paleolake Tauca, on the flank of Cerro Azanaques (Bolivia). Sedimentological evidence for synchronicity with the Tauca Lake highstand along with U–Th and 14 C measurements established that the fan-delta is 16.07 ± 0.64 kyr BP old. In situ-produced 10 Be concentrations measured in 15 boulders lying on the fan-delta yield a mean 10 Be concentration of 4.92 ± 0.05 × 10 5 at g −1 . A local in situ 10 Be production rate of 30.8 ± 1.3 at g −1 yr −1 is thus obtained at 3800 masl and 19°S. Application of the “Lal-modified” scaling scheme to this Azanaques production rate, using a standard atmosphere and the Muscheler et al. (2005) geomagnetic reconstruction, leads to a Sea Level High Latitude (SLHL) in situ 10 Be production rate of 3.76 ± 0.15 at g −1 yr −1 (1σ uncertainty). In addition, we propose a reference in situ 10 Be calibration dataset for the region that combines the production rates of this study with those of Blard et al. (2013b) and Kelly et al. (2015). This dataset of three calibration sites shows a good consistency and yields a regional in situ 10 Be production rate of 3.74 ± 0.09 at g −1 yr −1 using the same scaling.

International audience; Episodic and catastrophic landslides are considered to be one of the main sources of sediment in the steep, mountainous landscapes of the Himalayas. However, the evolution of a single landslide through time and its contribution to erosional processes remain poorly constrained. In this study, we focus on a single, large (0.5 km2) landslide in a small catchment on the southern flank of the Annapurnas in Nepal (the Khudi valley) in order to quantify its importance in the overall erosion of this steep Himalayan catchment.The evolution of the Saituti landslide has been continuously monitored by remote sensing for the past 46 years. During that period, the Saituti landslide displayed sustained activity, such that the area of the landslide scar increased by a factor of 4. This retrogressive failure, a consequence of several sporadic flank and crown collapses, has not been continuous. Rather, acceleration phases have alternated with more quiescent periods. Simultaneously, the upper edge moved upward by 900 m. Based on field evidence from recent activity (such as scarps and open tension cracks above the landslide) and on an analysis of slope angles, at least the next 500 m is expected to fail.Volume losses within the landslide were estimated from differences between digital elevation models (DEMs) and from changes in landslide area, using a calibrated power law relationship between landslide area and volume. Corresponding landslide-induced erosion rates at the scale of the whole Khudi catchment were found to be 2.6 ± 0.9 mm/y for the past half-century. Those rates are similar to denudation rates obtained from sediment load measurements between 1999 and 2004. Those results, along with the lack of other major landslides in the valley for the last 46 years, suggest that the Saituti landslide plays a dominant role in the modern erosion of the High Himalayan Khudi catchment for the last years and possibly for the past few decades.We propose that continuous and sustained activity of a few major landslides over the past few decades might represent a significant contribution to the erosion of the High Himalayan range. This long-lasting, landslide-induced erosion should be taken into account when interpreting suspended load measurements, results from provenance analysis, or cosmogenic nuclides in river sand. Such processes should also be included in landscape evolution models when annual to secular problems are explored.

This work presents the first reconstruction of late Pleistocene glacier fluctuations on Uturuncu volcano, in the Southern Tropical Andes. Cosmogenic 3He dating of glacial landforms provides constraints on ancient glacier position between 65 and 14 ka. Despite important scatter in the exposure ages on the oldest moraines, probably resulting from pre-exposure, these 3He data constrain the timing of the moraine deposits and subsequent glacier recessions: the Uturuncu glacier may have reached its maximum extent much before the global LGM, maybe as early as 65 ka, with an equilibrium line altitude (ELA) at 5280 m. Then, the glacier remained close to its maximum position, with a main stillstand identified around 40 ka, and another one between 35 and 17 ka, followed by a limited recession at 17 ka. Then, another glacial stillstand is identified upstream during the late glacial period, probably between 16 and 14 ka, with an ELA standing at 5350 m. This stillstand is synchronous with the paleolake Tauca highstand. This result indicates that this regionally wet and cold episode, during the Heinrich 1 event, also impacted the Southern Altiplano. The ELA rose above 5450 m after 14 ka, synchronously with the Bolling–Allerod.

We discuss the seismic cycle in the Himalayas and its relation to mountain building on the basis of geodetic, seismological and geological data collected in the Himalaya of Nepal. On average over several seismic cycles, localized slip on a major thrust fault, the Main Himalayan Thrust fault, MHT, accommodates the ∼21 mm·yr^(−1) convergence rate between southern Tibet and India. The geodetic data show that the MHT is presently locked from the sub-Himalayas to beneath the front of the high range where it roots into a sub-horizontal ductile shear zone under southern Tibet. Aseismic slip during the interseismic period induces stress accumulation at the southern edge of this shear zone triggering intense microseismic activity and elastic straining of the upper crust at the front of the high range. This deformation is released, on the long term, by major earthquakes on the MHT. Such an event is the M_w 8.4-1934-earthquake that ruptured a 250–300-km long segment. The major seismic events along the Himalayas since the 19th century have released more than 70% of the crustal strain accumulated over that period, suggesting that, if any, aseismic slip on the MHT cannot account for more than 30% of the total slip.

Seismic anisotropy is one of the most efficient geological and geodynamical tools for understanding the dynamics of the Earth. Upper mantle anisotropy is evident in seismic data sets for the last 30 years primarily from surface wave dispersion curves and body wave SKS data. We demonstrate in this paper that surface wave and body wave derived anisotropy can be explained by the same anisotropic parameters (L, Gc, Gs) in the simplest case of a horizontal fast symmetry axis. One application of this method is to display the SKS delay time and the corresponding azimuth, which can be derived from surface wave global (with a lateral resolution of around 2000 km) and regional (lateral resolution of 350 km) anisotropy tomography. A global-scale comparison is disappointing since there are only a few areas where both data types are correctly retrieved at the same spatial scale (same lateral resolution). The anisotropy is well resolved in oceanic basins from surface waves, whereas most measurements of SKS splitting are available below continents. However, there is a good agreement between surface wave and body wave anisotropy in regions where large-scale processes (primarily tectonic) are taking place, such as the western United States and in central Asia. In particular, we show that, at high frequency (> 1Hz), the observed SKS delay time and azimuth depend on the order in which S waves propagate through the layers. A comparison between synthetic and observed SKS phases in central Asia highlights the varying sensitivity of SKS waves at depth. We can obtain a good correlation between synthetic and observed SKS by considering anisotropy only between 80 and 200 km depth in the tomographic model, rather than in the entire upper mantle. However, it is more difficult to explain the discrepancy between synthetic and observed SKS under the eastern United States, where a large-scale coherent SKS anisotropy is present. Such results call for more sophisticated SKS waveform modeling and for enhanced lateral resolution in anisotropic tomographic models.

The present study investigates correlations between an extensive range of geomorphic properties that can be estimated from a digital elevation model and the uplift rate on geological timescales. The analysis focuses on an area in the Siwalik Hills (central Nepal), where lithology and climate can be considered as uniform. This area undergoes rapid tectonic uplift at rates of up to 15 mm yr^(−1), which are derived from the geometric pattern of a fault-bend model of fold growth. The selected geomorphic properties can be divided in two categories, depending on whether or not the vertical dimension is taken into account. None of the planar properties are significantly correlated to uplift rate, unlike those that include the vertical dimension, such as the mean elevation of basins, hypsometric curve, and hypsometric integral, and relief defined by the amplitude factor of length scaling analysis. Correlation between relief and uplift rate is observed for all length scales of topography shorter than 600 m, which suggests that all orders of the streams are able to adjust to the tectonic signal. Simple mass balance considerations imply that the average elevation is only 10% of surface uplift, suggesting that a dynamic equilibrium has been reached quite rapidly. Using a simple two-process model for erosion, we find that fairly high diffusion coefficients (order of 10 m^2 yr^(−1)) and efficient transport of the material by rivers are required. This unusually high value for mass diffusivity at small length scales may be obtained by either a very efficient linear diffusion or by landsliding. Actually, both processes may be active, which appears likely given the nature of the unconsolidated substratum and the favorable climatic conditions. Local relief in the study area may therefore be used to predict either uplift or denudation, but the prediction is calibrated only for that specific climatic and lithologic conditions and cannot be systematically applied to other contexts.