Lionel Siame

Long-term fluvial incision rates and postglacial river relaxation time in the French Western Alps from 10Be dating of alluvial terraces with assessment of inheritance, soil development and wind ablation effects

Studying river long-profile development as a response to tectonic and climatic controls requires reliable age-dating of paleo-profile remnants preserved as river terraces. Cosmic Ray Exposure (CRE) dating often represents the only method available to date river terraces, but the interpretation of cosmogenic nuclide concentrations is complicated by pre-depositional inheritance and post-depositional disturbance of the terrace deposits through pedogenesis and surface inflation or deflation. Here, we use cosmogenic 10Be measurements to date alluvial terraces in the French western Alps, in order to estimate river incision rates and to infer river response to climatic fluctuations and tectonic forcing. We assess inheritance by constructing 10Be concentration vs. pebble depth profiles and use a Monte-Carlo technique to estimate terrace ages. We find that inheritance is negligible on all terraces, enabling us to date terraces as young as 5 ky. Terraces that predate the last glaciation experienced intense pedogenesis and wind ablation which led to significant scatter in the 10Be concentrations of surface samples. We assess these effects using a model of 10Be ingrowth and show that the oldest CRE ages of surface clasts are close to the probable terrace age. We study two catchments which have undergone varying degrees of glacial disruption. The Buech River experienced variations in runoff and sediment flux during the last glaciation, as well as occasional ice-dammings of its outlet. Its upper terraces record incision rates averaged over 190 ky of ~0.8 mm/yr, consistent with denudation rates estimated in the surrounding areas and suggesting long-term stability of river incision rates. Climatic forcing is well documented for the Drac River, which was repeatedly dammed by glaciers during cold periods. Its postglacial incision history was triggered by an 800-m drop in base level following ice-dam disappearance. Long-profile development by knickpoint propagation explains the time-lag of 2-5 ky between this base-level drop and terrace abandonment upstream, as well as subsequent peak incision rates of >6 cm/yr followed by a gradual decrease in incision rates. The present knickpoint location, ~55 km upstream from the glacial damming site, enables us to calculate a fluvial response time of 15-20 ky, controlled by knickpoint propagation rates of several meters per year, within the Drac River.

Local erosion rates versus active tectonics: cosmic ray exposure modelling in Provence (south-east France)

Over the past decade, in situ-produced cosmogenic nuclides have revolutionised the study of landscape evolution. In particular, numerous studies have demonstrated that, in active tectonic settings, cosmic ray exposure dating of deformed or displaced geomorphic features makes it possible to quantify long-term deformation rates. In western European countries, erosion due to climatically driven processes and human activities is probably the factor that most limits the accuracy of exposure ages and landscape modification rates. In this study, we present the results of a depth-profiling technique applied to alluvial terraces located along the Rhone and the Moyenne Durance rivers. The expected decrease with depth of the measured 10Be concentrations has been modelled using a χ2 inversion method in order to constrain the exposure history of the alluvial sediments. The results suggest that: (1) over the Quaternary, the local surface erosion rates including both regional uplift and climatically driven processes acting on landforms are on the order of 30 m/Myr in southeastern France, and (2) providing a fairly good bracketing of the exposure age, the modelled abandonment age of alluvial terraces affected by the Moyenne Durance Fault allows estimating incision rates, comparing the alluvial terrace elevations with topographic river profiles, and a minimum vertical slip rate value of roughly 0.02 mm/yr for the southern segment of the Moyenne Durance Fault.

Cosmogenic dating ranging from 20 to 700 ka of a series of alluvial fan surfaces affected by the El Tigre fault, Argentina

It is crucial to date continental landforms to quantify processes involved in terrestrial surface evolution, especially in regions affected by active tectonics. Andean quaternary alluvial fan surfaces affected by the El Tigre strike-slip fault have been studied using combined geomorphic and 10 Be exposure age approaches. Field observations and SPOT (French acronym for “Satellite for Observation of the Earth”) image analysis enable the identification of six alluvial fan units. Measurements of in situ–produced cosmogenic 10 Be concentrations in quartzite boulders exposed on the top of fan surfaces show that the depositional periods ended during successive major interglacial stages. The calculated minimum exposure ages date the abandonments of the alluvial fan surface from 41 000 ± 8500 yr for the youngest to 670 000 ± 140 000 yr for the oldest unit. When linked to the measured maximum cumulative right-lateral displacement of stream channels, the exposure ages yield a horizontal slip rate of about 1 mm/yr on the El Tigre fault. This study shows that for arid regions, where fan surface erosion is minimal, in situ–produced 10 Be can be used to constrain the age of stratigraphically separate alluvial fan surfaces. These fan surface exposure ages can be further used to calculate slip rates on active faults and infer depositional periods correlative with climatic events.

New tectonic configuration in NE Iran: Active strike-slip faulting between the Kopeh Dagh and Binalud mountains

In this paper, a combined approach of geomorphic analyses, detailed geological mapping on SPOT 5 satellite images and field surveys, complemented by radiometric dating (Ar-40/Ar-39) allows proposing a new and consistent tectonic configuration for the northeastern Arabia-Eurasia collision zone. It favors a simple strike-slip faulting mechanism in NE Iran accommodating the northward motion of central Iran with respect to Eurasia between the Binalud and Kopeh Dagh mountains. Our results demonstrate that similar to 2 mm/a (similar to 25%) of this northward motion has been transferred from the Binalud to the Kopeh Dagh through the Meshkan Transfer Zone since similar to 5 Ma. The modern state of stress deduced from fault kinematics analyses shows N30 degrees E trending maximum stress axis and confirms a dominant strike-slip faulting mechanism. Combining our geological data with other available data, we conclude that kinematics of deformation can be appropriately described by rigid block faulting in NE Iran; instead, the continuous deformation as it has been thought during the last 3 decades.

Fault growth and landscape development rates in Otago, New Zealand, using in situ cosmogenic 10Be

Abstract We use in situ cosmogenic 10 Be measurements in quartzites to examine the growth and propagation rates of a Late Quaternary anticline forming above a blind reverse fault in Central Otago, New Zealand. We obtain average uplift and propagation rates of 0.10–0.15 mm yr −1 and 1.0–2.0 mm yr −1 respectively over the last 450 000 yr, though it is probable that growth occurred episodically, with more rapid development in the periods 600–400 kyr and 200–100 kyr and relative quiescence between about 400 and 200 kyr ago. These results quantify and confirm a qualitative analysis of the fault evolution based on geomorphology and drainage patterns. We obtain minimum 10 Be exposure ages of up to 750 000 yr for the quartzites, some of the oldest ages obtained by this method outside Antarctica, which are made possible by the extremely low erosion rates of less than 0.6 mm kyr −1 in the resistant quartzites. These rocks consequently provide an opportunity to study geomorphic and tectonic processes in this region over unusually long time periods.

Active basement uplift of Sierra Pie de Palo (Northwestern Argentina): Rates and inception from10Be cosmogenic nuclide concentrations

Quaternary tectonic and denudation rates are investigated for an actively growing basement anticline: the Sierra Pie de Palo range, which belongs to the Andean foreland of Northwestern Argentina (28°S–33°S). In this study, a detailed morphometric analysis of the topography is combined with in situ-produced cosmogenic10Be concentrations measured in (1) surface boulders abandoned on alluvial terraces affected by fault activity (along the north bounding fault) and growth of the basement fold (along the southeastern border), (2) bedrock outcrops corresponding to an exhumed and folded, regional erosion surface, and (3) fluvial sediments sampled at the outlets of several watersheds. Along the eastern and northern borders of the range, incision and uplift rates have been estimated at approximately 0.5 and 1 mm/yr when integrated on Holocene and Pleistocene time scales, in close agreement with both long-term (structural and basin evolution data) and short-term (GPS-derived velocity field) analyses. Cosmogenic-derived denudation and uplift rates combined with geomorphic characteristics of watersheds and river channels allows estimating the onset of the uplift at 4–6 Ma, followed by a more recent period of topographic rejuvenation at roughly 1–2 Ma, probably synchronous with steepening of the eastern and northern flanks of the anticline.

Extrusion tectonics and subduction in the eastern South Caspian region since 10 Ma: COMMENT

[Hollingsworth et al. (2008)][1] proposed a “new” kinematic model for the complicated tectonic pattern in the eastern South Caspian Sea region. Our Comment is focused on the Ashkhabad fault and the Kopeh Dagh Mountains, examining the data and hypothesis used by the authors to support their model

Valley downcutting in the Ardennes (W Europe): Interplay between tectonically triggered regressive erosion and climatic cyclicity

While climatic models of valley downcutting discuss the origin of terrace staircases in valleys of middle Europe within the frame of alternating cold and temperate periods of the Quaternary, other models, starting from a base level fall imposed by an initial tectonic signal, describe the response of the drainage network mainly as the propagation of an erosion wave from the place of base level fall (the margin of the uplifted region) toward the headwaters, the two types of model being rarely confronted. In the Ardennes (West Europe), cosmogenic 10 Be and 26 Al ages have recently been calculated for the abandonment of the Younger Main Terrace (YMT) level, a prominent feature at mid-height of the valleysides marking the starting point of the mid-Pleistocene phase of deep river incision in the massif. These ages show that the terrace has been abandoned diachronically as the result of a migrating erosion wave that started at 0.73 Ma in the Meuse catchment just north of the massif, soon entered the latter, and is still visible in the current long profiles of the Ardennian Ourthe tributaries as knickpoints disturbing their upper reaches. At first glance, these new findings are incompatible with the common belief that the terraces of the Ardennian rivers were generated by a climatically triggered stepwise general incision of the river profiles. However, several details of the terrace staircases (larger than average vertical spacing between the YMT and the next younger terrace, varying number of post-YMT terraces in trunk stream, tributaries and subtributaries) show that a combination of the climatic and tectonic models of river incision is able to satisfactorily account for all available data. The cosmogenic ages of the YMT also point out a particular behaviour of the migrating knickpoints, which apparently propagated on average more slowly in the main rivers than in the tributaries, in contradiction with the relation that makes knickpoint celerity depend directly on drainage area. We tentatively suggest a process accounting for such anomalies in migration rates.

Les nucleides cosmogeniques produits in-situ; de nouveaux outils en geomorphologie quantitative

Measurement of 10 Be and 26 Al concentrations produced by cosmic ray bombardment within the quartz mineral fraction of surficial deposits and exposed bedrocks (in situ-production) is rapidly becoming an important quantitative tool in geomorphology. Whereas conventional methods provide age control on stratigraphic profiles, surface exposure dating using in situ-produced 10 Be and 26 Al is particularly well-suited not only for continuous dating but also for quantifying spatial variations and rates of geomorphic processes.

Datation de surfaces geomorphologiques reperes par le 10 Be produit in-situ; implications tectoniques et climatiques

The evolution of continental landforms is mainly modulated by the impact of climatic and tectonic processes. Because of their distinctive morphology and the periodicity of their deposition, climatically induced landforms such as alluvial fans or terraces are well suited to infer rates of tectonic and continental climatic processes. Within tectonically active regions, an important step consists in dating displaced geomorphic features to calculate slip rates on active faults. Dating is probably the most critical tool because it is generally much more simpler to measure deformation resulting from tectonic activity than it is to accurately date when that deformation occurred. Recent advances in analytical chemistry and nuclear physics (accelerator mass spectrometry) now allow quantitative abundance measurements of the extremely rare isotopes produced by the interaction of cosmic rays with surface rocks and soils, the so-called in situ-produced cosmogenic nuclides ( 3 He, 10 Be, 21 Ne, 26 Al, 36 Cl), and allow to directly date the duration that a landform has been exposed to cosmic rays at the Earths surface [Lal, 1991; Nishiizumi et al., 1993; Cerling and Craig, 1994; Clark et al., 1995]. In fact, the abundance of these cosmonuclides is proportional to landscape stability and, under favorable circumstances, their abundance within surface rocks can be used as a proxy for erosion rate or exposure age. These cosmonuclides thus provide geomorphologists with the opportunity to constrain rates of landscape evolution. This paper presents a new approach that combines cosmic ray exposure (CRE) dating using in situ-produced 10 Be and geomorphic as well as structural analyses. This approach has been applied on two active strike-slip and reverse faults located in the Andean foreland of western Argentina. These two case studies illustrate how CRE dating using in situ-produced 10 Be is particularly well suited for geomorphic studies that aim to estimate the respective control of climate and tectonics on morphogenesis.