Evidence from cosmic ray exposure (CRE) dating for the existence of a pre-Minoan caldera on Santorini, Greece

International audience\textcopyright 2016, Springer-Verlag Berlin Heidelberg.Cosmic ray exposure (CRE) dating was performed on the caldera cliffs of Santorini with the aim of detecting cliff segments predating the Minoan eruption (17th century BCE). The methodology involved the determination of in situ-produced cosmogenic 36Cl concentration in basaltic-to-rhyodacitic whole rocks cropping out in the cliffs. After the samples were processed following the chemical protocol of 36Cl preparation for silicate rocks, 36Cl concentrations were measured by accelerator mass spectrometry (AMS). Important challenges during the implementation procedure were related to large amounts of radiogenic 36Cl, complex modeling of inherited 36Cl, and dominance of the thermal and epithermal (low-energy) neutron capture production pathway. Nevertheless, quantitative assessments on the basis of the contribution of the low-energy neutron capture pathway percent to the total production rate validated the calculated CRE dates. Current CRE ages demonstrate that an ancient caldera existed on pre-Minoan Santorini, occupying at least the northern half of the modern-day caldera

Quantifying the dynamic topography through a combination of basin-averaged erosion rates and geomorphic analysis from the Anti-Atlas and Western Peseta (Morocco) transient landscape.

International audienc

Plio-Quaternary landscape evolution in the uplifted Ardennes : new insights from Al-26/Be-10 data from cave-deposited alluvium (Meuse catchment, E. Belgium)

Despite a wealth of recent studies dealing with the evolution of the drainage network in the uplifted Ardennes massif (E. Belgium), especially from the Middle Pleistocene onwards, the Ardennian landscape evolution and long-term incision rates in the Meuse catchment remain poorly documented over the whole Plio-Quaternary. Alluvium-filled multilevel cave systems represent a relevant setting to unravel the Late Cenozoic history of re-gional river incision. We present here a dataset of Al-26/Be-10 concentration data obtained from fifteen pebble samples washed into the Chawresse system, one of the largest multi-level cave systems of Belgium, which developed in Devonian limestones of the lower Ourthe Valley, the main Ardennian tributary of the Meuse. The sample collection spans an elevation difference higher than 120 m and their depleted Al-26/Be-10 ratios yield burial ages ranging from-0.25 to 3.28 Ma. After critical assessment of our dataset for intra-karstic reworking issues, the most striking outcome of the obtained burial ages is the acceleration by a factor five of the incision rates (from-30 to-150 m/Ma) during the first half of the Middle Pleistocene. Integrating this incision peak and our pre-burial denudation rates, we then revisit the existing framework of Plio-Quaternary denudation and river incision in the Ardennian Meuse catchment. Whilst our Al-26/Be-10 concentration data shed new light on the temporal and spatial variability of the local river and hillslope system response to coupled tectonic and climatic forcings, it simultaneously highlights sampling issues and the need for further chronological data

Lithological control on topographic relief evolution in a slow tectonic setting (Anti-Atlas, Morocco)

Topographic relief results from the complex interactions between tectonics and erosional surface processes, which are primarily mediated by bedrock erodibility and climatic conditions. Ancient orogens offer a favourable setting to isolate the contribution of lithology, as tectonically driven rock uplift is typically negligible and rock strength variability can exert a critical role on the evolution of the topography. The Anti-Atlas in NW Africa is a late Paleozoic orogen comprising a well-preserved, elevated, relict landscape delimited by non-lithological knickpoints, that was uplifted during a regional late Cenozoic phase of topographic rejuvenation. Here, we combine a geomorphic analysis with 10Be-derived denudation rates to quantify bedrock erodibility and get insight into the surface evolution of the Anti-Atlas and the adjacent Siroua Massif. Specifically, we show that 10Be basin-wide denudation rates for the relict landscape are rather uniform and range from 5 to 12 m/Myr. These rates agree with long-term rates estimated from different methods suggesting that the relict topography archives erosional quasi-erosional steady-state conditions at least since the latest Cretaceous. The uniformly low 10Be denudation rates in the relict topography are consistent despite the variability in channel steepness and topographic relief that correlates with changes in rock type. The expansion of this analysis to the denudation rates of the downstream portion of the landscape, allows to demonstrate a linear relationship between denudation and channel steepness for catchments draining quartz bearing lithologies. This provides the chance to constrain a narrow range of bedrock erodibility values for different rock-types (quartzite, granitic and sedimentary rocks). These values are comparable with estimates from other slowly deforming settings. Specifically, our compilation from tectonically inactive to slow tectonic regions indicates that bedrock erodibility does not change significantly across different climatic zones and precipitation regimes. This highlights the critical role of lithology in controlling the production of topographic relief in post-orogenic/slow tectonic settings. Finally, we calculate the predicted denudation rates for the steeper portions of the landscape that adjusted to the new uplift rates based on the linear correlation between erosion rates and normalized steepness indices. These rates range from 20 to 50 m/Myr and agree with the direct measurements from two catchments

Découverte et étude de remplissages karstiques allochtones d’âge miocène dans l’Obiou (Dévoluy, Alpes françaises) Implications géomorphologiques et paléogéographiques

International audienceAt the mountainside of Obiou (2 789 m) located in the Dévoluy massif (Southwestern Alps) a fortunate discovery has revealed the presence of allochthonous crystalline pebbles excavated from marmot holes. Successive prospecting in the surrounding caves have permitted to identifycrystalline allochthonous sediments such as conglomerates, sands and clays, which formed endokarstic deposits. According to petrographic diversity with presence of granite, orthogneiss, varia gneiss, migmatites, amphibolic gneiss, volcanic basic rocks, sandstones and even limestone with nummulites, these alluvions originate from the SW of the Ecrins massif (Champsaur and Valgaudemar), the localities which are nowadays drained by the Drac River, situated some 1 600 m below the studied caves. These alluvions were analyzed using cosmogenic nuclides (10Be, 26Al, 21Ne), in order to determine their burial ages. However, the dating was compromised due to 26Al concentrations being bellow the limit of quantification (very old burial ages, probably more than 5 Ma). Nevertheless, one sample was analyzed for 10Be/21Ne indicating 15,6 ± 3,8 Ma burial age, which corresponds to Miocene. Even though, our burial dating is quite large due to analytical limitations, the caves in Obiou should be considered as important geomorphological marker of the ancient Drac River drainage (called “paleo-Drac”), during the middle Miocene. Furthercosmogenic nuclide analyses are conducted, in order to precise the burial age of the alluvions and thus frame the incision and uplift rates of this part of the French Alps.Sur les flancs de l’Obiou (2 789 m) dans le massif du Dévoluy (Hautes-Alpes et Isère, France), une découverte fortuite a été réalisée : celle de galets cristallins affleurant, depuis un terrier de marmottes, au beau milieu d’un éboulis calcaire. La prospection de plusieurs cavités environnantes a mis au jour la présence d’alluvions cristallins allochtones sous forme de conglomérats, sables et argiles constituant une série de remplissages endokarstiques. Du point de vue pétrographique, le spectre compte notamment des granites, orthogneiss, gneiss divers, migmatites, gneiss amphiboliques, roches volcaniques basiques, mais aussi des grès, et même des calcaires gréseux à nummulites. D’après cette pétrographie, laprovenance de ces alluvions se situe dans le S-O du massif des Écrins (Champsaur et Valgaudemar), une zone des massifs cristallins externes actuellement drainée par le Drac, lequel s’écoule à environ 1 600 m en contrebas des cavités étudiées. Nous avons tenté de déterminer la durée del’enfouissement de ces alluvions en utilisant des nucléides cosmogéniques produits in situ (10Be, 26Al, 21Ne). Du fait de la faible teneur en 26Al en deçà de la limite de détection, les âges d’enfouissement seraient supérieurs à 5 Ma. Pour un échantillon dont la teneur en 21Ne a été mesurée, le rapport 10Be/21Ne permet une estimation de la durée d’enfouissement aux alentours de 15,6 ± 3,8 Ma, donnant un âge miocène.Au regard de ces âges, les cavités de l’Obiou doivent être considérées comme un niveau repère traduisant la position d’un ancien réseau hydrographique du « paléo-Drac » au Miocène moyen. Des analyses complémentaires de 21Ne couplées au 10Be sont actuellement entreprises pour préciser l’âge d’enfouissement de ces alluvions afin de parvenir à mieux contraindre les taux d’incision et de soulèvement de ce secteur des Alpes françaises

Rapid deglaciation of the La Vega gorge (Sierra de Gredos, Iberian Peninsula) at the end of the global Last Glacial Maximum

International audienceAbstract Previous studies from the Iberian Central System and other mountains of the Iberian Peninsula and Europe suggest that deglaciation in this area occurred at the end or immediately after the global Last Glacial Maximum (LGM, 26.5–19 ka). In this research, we investigate the timing and speed of deglaciation of the palaeoglacier in La Vega gorge (Iberian Central system) since the global LGM, dating the outer moraines, glacially polished bedrock and glaciofluvial deposits by means of in situ cosmogenic 10 Be and optically stimulated luminescence. The results show that one intermediate arc located at the outer frontal moraine system has an age of ~21 ka, being consistent with the global LGM. Deglaciation began at ~21–19 ka, speeding up at ~19 ka. We estimate that around 4–5 km of the palaeoglacier receded in ~1–2 kyrs since ~19 ka, leaving La Vega gorge probably ice‐free at ~18 ka. Therefore, our data confirm that deglaciation in Sierra de Gredos began at the end of the global LGM, characterised by a rapid and massive retreat of glaciers

4 Ma-long uplift cycles of the southern Peru forearc since Late Miocene

International audienceWe explore the coastal morphology along an uplifting 500 km-long coastal segment of the Central Andes, between the cities of Chala (Peru) and Arica (Chile). We use accurate DEM and field surveys to extract sequences of uplifted shorelines along the study area. In addition, we consider continental pediment surfaces that limit both the geographical and vertical extent of the marine landforms. We establish a chronology based on published dates for marine landforms and pediment surfaces. We expand this corpus with new 10Be data on uplifted shore platforms. The last 12 Ma are marked by three periods of coastal stability or subsidence dated ~12-11 Ma, ~8-7 Ma and ~5-2.5 Ma ago. The uplift that accumulated between these stability periods has been ~1000 m since 11 Ma; its rate can reach 0.25 mm/a (m/ka). For the last period of uplift only, during the last 800 ka, the forearc uplift has been accurately recorded by the carving of numerous coastal sequences. Within these sequences, we correlated the marine terraces with the sea level highstands (interglacial stages and sub-stages) up to MIS 19 (790 ka), i.e., with a resolution of ~100 ka. The uplift rate for this last period of uplift increases westward from 0.18 mm/a at the Peru-Chile border to ~0.25 mm/a in the center of the study area. It further increases northwestward, up to 0.45 mm/a, due to the influence of the Nazca Ridge. In this study, we document an unusual forearc cyclic uplift with ~4 Ma-long cycles. This periodicity corresponds to the predictions made by Menant et al. (2020) based on numerical models, and could be related to episodic tectonic underplating (subducting slab stripping) beneath the coastal forearc area

4 Ma-long uplift cycles of the southern Peru forearc since Late Miocene

International audienceWe explore the coastal morphology along an uplifting 500 km-long coastal segment of the Central Andes, between the cities of Chala (Peru) and Arica (Chile). We use accurate DEM and field surveys to extract sequences of uplifted shorelines along the study area. In addition, we consider continental pediment surfaces that limit both the geographical and vertical extent of the marine landforms. We establish a chronology based on published dates for marine landforms and pediment surfaces. We expand this corpus with new 10Be data on uplifted shore platforms. The last 12 Ma are marked by three periods of coastal stability or subsidence dated ~12-11 Ma, ~8-7 Ma and ~5-2.5 Ma ago. The uplift that accumulated between these stability periods has been ~1000 m since 11 Ma; its rate can reach 0.25 mm/a (m/ka). For the last period of uplift only, during the last 800 ka, the forearc uplift has been accurately recorded by the carving of numerous coastal sequences. Within these sequences, we correlated the marine terraces with the sea level highstands (interglacial stages and sub-stages) up to MIS 19 (790 ka), i.e., with a resolution of ~100 ka. The uplift rate for this last period of uplift increases westward from 0.18 mm/a at the Peru-Chile border to ~0.25 mm/a in the center of the study area. It further increases northwestward, up to 0.45 mm/a, due to the influence of the Nazca Ridge. In this study, we document an unusual forearc cyclic uplift with ~4 Ma-long cycles. This periodicity corresponds to the predictions made by Menant et al. (2020) based on numerical models, and could be related to episodic tectonic underplating (subducting slab stripping) beneath the coastal forearc area

Design and performance of an automated chemical extraction bench for the preparation of Be-10 and Al-26 targets to be analyzed by accelerator mass spectrometry

The growing number of studies based on data provided by the accelerator mass spectrometry (AMS) technique from increasingly diverse matrices implies that developing very effective and reproducible sample processing techniques is crucial to meet the demand generated. After being dissolved, the isolation of the cosmogenic nuclide to be measured and its chemical purification imply complex processes throughout successive stages The presented automatic system was designed to perform the final steps of the Be-10 and Al-26 chemical extraction protocol from natural matrices (sediments, pure mineral phases, ...), previously purified, decontaminated and dissolved in the case of in situ-produced cosmogenic nuclides, or leached or dissolved in the case of meteoric cosmogenic nuclides. Driven by Labview software, the developed automatic system, in addition to freeing the operator from tedious tasks, allows reproducibly processing simultaneously up to eight samples of different volumes in roughly 40 min. This article presents the design of the automated extraction bench and its performance based on the recovery efficiency of the stable element and the reproducibility of the measurements of the cosmogenic nuclide concentrations by AMS