Cosmogenic Nuclides

Following the 2008 Wenchuan earthquake (Sichuan, China), the dextral strike-slip Longriba fault system (LFS) has been recognized as a main intracontinental structural boundary within the eastern Tibetan Plateau. While numerous studies have focused on the Longmen Shan frontal range to constrain the dynamics of the eastern Tibetan margin, little is known on the LFS, particularly on its eventual influence on the geomorphological evolution of the latter. Here, we provide a new data set of denudation rates derived from beryllium-10 concentrations in river sediments from 33 medium-sized catchments. Our sampling area covers the frontier between the dissected margin and the low relief interior plateau. Our results reveal a sharp increase of denudation across the LFS,
from <0.1 mm/y in the Ruoergai basin to 0.3 mm/y toward the Longmen Shan range. Such denudation pattern indicates a major morphotectonic control of the fault system on the eastern Tibetan margin evolution. Additional
topographic analysis confirms the role of the LFS as an important geomorphological boundary, restraining the
westward propagation of river incision into the low-relief areas, thus partly preventing the dismantling of the eastern Tibetan Plateau.

The rates and chronology of valley incision in mountain ranges have been studied in various parts of the globe, but the causes of river incision are often blurred because tectonic, climatic, and sea level-related forcing signals are difficult to distinguish from one another. The Têt River limestone gorge in the Eastern Pyrenees, which displays multiple cave levels containing datable alluvial deposits, provides an opportunity for clarifying this debate. Horizontal epiphreatic passages in limestone can be used as substitutes for fluvial terraces because they correspond to former valley floors and, therefore, also record the position of former local base levels. In the Têt canyon, the passages are filled with quartz-rich sand and gravel sequences that can be dated by 26Al/10Be burial dating. The canyon has cut into a Middle Miocene pediment system—now forming a raised plateau at 1250–1500 m—and displays nine cave levels over a vertical height of 1 km. One alluvial fill sequence in a cave
at +270 m above datum (i.e., the local river bed) yielded a weighted mean age of 5.14 ± 0.41 Ma; another, situated at +110 m above datum, yielded weighted mean ages of 2.23 ± 0.230 Ma and 1.20 ± 0.286 Ma. The data convert to a mean incision rate of ~52 m·Ma−1 since the beginning of the Pliocene, and involved an acceleration to 92 m·Ma−1 during the Quaternary. Pre-burial catchment denudation rates range from 35 to 7 m·Ma−1, and these also doubled during the early Quaternary. It is concluded that: (i) valley incision into the Miocene pediment
has been occurring since 5, probably 10 Ma; (ii) there is no evidence of a Messinian canyon in the Villefranche gorge, strongly suggesting through various additional indicators that interference of the Messinian Salinity Crisis with the canyon incision history was minimal; (iii) valley deepening was not a steady process, and recorded periods of stability around 1–2 Ma and perhaps 6–5 Ma; and (iv) the terraced network of epiphreatic cave levels is primarily explained by tectonic uplift. It follows that the elevated erosion surfaces of the Pyrenees, such as the Miocene pediment directly situated above the canyon edge, were not shaped at high elevations, e.g., by ‘altiplanation’; they formed, instead, close to base level and were uplifted in successive stages by tectonic processes. The study emphasizes the more general proposition that tectonic signals (as opposed to climatic or eustatic) in valley-incision chronologies are best singled out at locations situated among the outer ranges of mountain belts, i.e., in canyons such as the Têt, that respond immediately to base level changes relative to the adjacent foreland. In the inner ranges, fluvial incision is more likely to be affected by the interference of climatic factors (e.g., glaciers),
or to be delayed by bedrock impediments to upstream-propagating knickpoints.

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright a b s t r a c t Dartmoor, in SW England, is a classic periglaciated granite upland adorned with a population of over 150 tors. The origin of the tors has been controversial, but their emergence by differentiation after stripping of regolith during Pleistocene cold phases is accepted. However, their actual age has been unknown, with possible scenarios ranging from preservation since the early Middle Pleistocene to relatively short-lived landforms in a maritime climate with high denudation rates. The latter is now supported by 32 cosmogenic surface exposure dates from 28 tors across the whole upland. The distribution of apparent 10 Be ages peaks strongly in the Middle Devensian (36e50 ka), which with corrections for weathering and limited ice shielding could be interpreted as Early Devensian. These ages are much younger than those found for three glacially unmodified Cairngorms tors, and somewhat younger even than glacially modified Cairngorms tors. The dates show little spatial variation. Although an ice cap has now been modelled in the heart of northern Dartmoor, tors here are of median age, suggesting that ice cover sufficient to shield tors from incoming radiation was of short duration. The few younger tor ages support the idea of continuing landform instability across the landscape, with weathering flakes redeveloping soon after inferred loss of top pillows by gelifraction or gravitational toppling. The few older tor ages have no systematic explanation, and may indicate inheritance from an earlier cycle of bedrock near-exposure. Since most tors are modest in height (typically 2e5 m), volumetrically insignificant, and often in advanced stages of disintegration, the general impression is that they are evanescent features, which emerge and quickly disappear during every Pleistocene climatic downturn. Tor populations may thus flicker across the landscape rather randomly over the Quaternary. The remarkably consistent age of the present tor population could be associated with a stripping event at the start of the Devensian, but fuller analysis must await closer controls on tor denudation rates in different climatic phases, and on ice cover extent and duration. These results only date extant tor surfaces, not the landscape, but as the best available erosion pins they have evident value in exploring theories of the evolution of Dartmoor during the Quaternary.

Sequences of alluvial fill-terraces record the long-term variation of fluvial transport regimes in response to Quaternary climatic changes. Through a production of increasingly precise chronologies, the broadening range of sediment dating methods in recent years has improved our understanding of correlations between alluvial dynamics and external forcing mechanisms. However, results in this field have mostly focused on periglacial and other mid-latitude settings. These may not readily apply to glaciofluvial environments, where alluvial regimes are narrowly dependent on the behavior of the adjacent ice-field. Here we constrain the ages of three glaciofluvial fill terraces based on five
vertical 10Be age profiles in the Ariège River catchment, and produce a chronofunction of related contrasts in terrace
tread topsoil characteristics. Each profile consists of 9 to 11 gravel samples collected over depths of 4.6 to 15 m. Terrace T1 (+15 m above the current riverbed) was sampled at ~4, 22 and 53 km downstream from the Last Glacial Maximum terminal moraine system of the Ariège outlet glacier. The profiles of T2 (+30 m) and T3 (+50 m) complete the data set. Beryllium-10 age profiles date the age of terrace tread abandonment as a result of fluvial incision. Results show that the incision of T1 occurred over a protracted period during the Last Termination, beginning ca. (–3.5)17.5(+2.0) ka. Prior to this, aggradation occurred from MIS 4 to the global Last Glacial Maximum (LGM), i.e., under maximum ice extent conditions, and lasted a few thousand years thereafter. The protracted influx of debris after the LGM was sustained by paraglacial storage release in the catchment and promoted by strong coupling between slope and channel processes. Fluvial incision of T1 occurred during the post-LGM cold-to-warm transition after a time (supported by paleobotanical evidence) when the vegetation cover was capable of ensuring catchment-wide slope stability. The wave of incision propagated downstream between (–3.5)17.5(+2.0) ka (proximal profile) and (–0.5)13.0(+3.5) ka (distal profiles). The exposure-age confidence intervals for T2 and T3 are much larger than for T1 (60–145 ka and 204–226 ka, respectively) but the data nonetheless allow the aggradation of T2 and T3 to be attributed to MIS 6 and MIS 8, respectively. The soil chronosequence permitted by the fill-terrace age sequence indicates that the Luvic Cambisols capping T1 formed after ~13–16 ka, i.e., during the Holocene; (ii) the Haplic Luvisols cap-
ping T2 required at least ~130 ka, i.e., two interglacials; and (iii) the deeply rubified, highly eluviated Luvisols capping T3 required at least ~250 ka, i.e., at least three interglacials.

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.

Cette  étude  présente  à  la  fois  un  intérêt  historiographique,  méthodologique  et  paléoenvironnemental.  Historiographique
d’abord, car elle fait le point sur un siècle de travaux, d’Albrecht Penck (1883) à François Taillefer (1985), sur les glaciations quater
naires dans les Pyrénées de l’Ariège. Elle montre en particulier comment le polyglacialisme alpin a cédé tardivement la place à un modèle monoglacialiste et comment ce dernier a durablement faussé les interprétations chronologiques ultérieures. Méthodologique ensuite, car elle souligne à quel point la mise en place d’une chronologie relative des séquences de formes et de dépôts glaciaires est un préalable nécessaire à toute tentative de datation absolue de ces assemblages de formes. Paléoenvironnementale enfin, puisque l’emprise spatiale et la chronologie de 6 stades d’englacement würmiens et anté-würmiens est restituée à partir d’une analyse croisée
de la répartition des dépôts de marge glaciaire et de 37 datations par le  10Be, un nucléide cosmogénique produit in situ. Les résultats montrent qu’un paléoenglacement antérieur au MIS 5e s’est avancé au-delà de l’emprise würmienne maximale ; cette dernière a été délimitée localement à partir des dépôts de marge glaciaire les plus externes attribués au cycle glaciaire würmien compte tenu de leur faible état d’altération. On montre aussi que l’emprise würmienne maximale est contemporaine du stade isotopique 4 (MIS 4), que des fluctuations plurikilométriques du front glaciaire ariégeois sont survenues durant le MIS 3 et que le Global LGM représente la dernière poussée glaciaire de grande d’ampleur bien que légèrement en retrait par rapport à l’emprise würmienne maximale. Immédiatement après le Global LGM, une phase de déglaciation majeure cantonne les fronts glaciaires aux hautes vallées. Au Dryas ancien, des glaciers de vallée de 20 et 6 kilomètres occupent respectivement la haute vallée de l’Ariège et le vallon de Suc, un affluent du Vicdessos.

The fundamentally geographic issue of the amounts and spatial patterns of erosion necessary to produce classic glacial landforms such as U-shaped valleys has been debated by scientists for over a century. Terrestrial cosmogenic nuclide (TCN) measurements in glacially abraded bedrock were used to determine patterns of glacial erosion and to quantify the amount of rock removed during the last glaciation along valley-side transects in Sinks Canyon, Wind River Range, Wyoming, and the South Yuba River, Sierra Nevada, California. Surface exposure ages from bedrock and erratic samples obtained during this study indicate last deglaciation between 13–18 ka in the South Yuba River and 15–17 ka in Sinks Canyon. These ages are in agreement with previously published glacial chronologies. In both areas, samples from valley cross sections revealed a pattern of erosion during the last glaciation that decreased toward the lateral limit of ice extent, as predicted by numerical models, while transects further upstream recorded >1.4 meters of bedrock removal throughout. The effects of varying interglacial erosion and surface exposure histories on modeled glacial erosion depths were tested, validating the methodology used. The results demonstrate that the TCN technique, applied at the valley scale, provides useful insight into the spatial pattern of glacial erosion. Extensive sampling in areas with limited erosional loss may provide detailed records of erosion patterns with which to test predictions generated by models of ice dynamics and erosion processes.