Variability of magmatic and cosmogenic 3 He in Ethiopian river sands of detrital pyroxenes: Impact on denudation rate determinations

International audienceIn-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

Quantifying incision rates since the early Miocene using terrestrial cosmogenic nuclides into caves: methodological issues, solutions and expectations

International audienceThe ability of discriminating the various forcing parameters controlling the growth of mountain ranges depends on the knowledge of the deformation, exhumation and sedimentation chronologies and on accurate constraints on the orogen geometry necessary to quantify accretion and erosion fluxes. The Pyrenees, an intraplate collision range, built by the shortening of continental thinned margins during the convergence between Iberia and Eurasia lasting from Late Cretaceous to Early Miocene, are a particularly well-suited study area to examine the coupling and retroactions between climate and tectonics. Indeed, consisting in a doubly vergent asymmetric orogenic wedge, the northeastern foreland basin underwent particular conditions from the Miocene to the Pliocene due to the closure of all oceanic connections resulting from the continuing convergence between Iberia and Eurasia. In addition, the orogenic growth and its sedimentary evolution occurred during the Cenozoic climatic cooling.To investigate these retroactions, alluvium-filled horizontal epiphreatic passages in limestone karstic networks supposedly recording the transient positions of former local base levels during the process of valley deepening were studied. The results obtained applying various suitable geochronological methods (26Al/10Be, 10Be/21Ne, ESR and OSL on quartz) on intrakarstic alluvial deposits of 61 caves from three valleys of the central and eastern Pyrenees, as well as on a recent analogue comprising an active branch, will be presented. In the Pyrenean context and under particular conditions, these burial duration methodologies allow deciphering the deepening history of the valleys over the long term (~ 16-14 Ma). However, in this orogenic area, some issues have been identified: the density of sub-horizontal levels on an altimetric range and their vertical connections, the low tectonic activity during the post-orogenic evolution, the Mediterranean eustatic variations, the ancient glaciation phases, or storage in the watershed may lead to networks filling stories more complex than expected. In some cases, it may be difficult to evaluate the 26Al/10Be ratio associated to the sediments entering the caves or at the time of their deposit (mixing with old deposits). We will show that most of these issues can be solved by conducting small ancillary studies and taking precautions when sampling or treating samples

An Alternative Protocol for Single Zircon Dissolution with Application to (U-Th-Sm)/He Thermochronometry

International audienceZircon (U‐Th‐Sm)/He (ZHe) thermochronometry is a powerful tool that has been widely used in geology to constrain the exhumation histories of orogens. In this study, we present an alternative protocol for dissolving zircon grains for determination of parent nuclides. This new alkali fusion procedure developed at the SARM (Service d'Analyse des Roches et des Minéraux) in Nancy, France, is fast (requiring only 2 d, including cleaning steps) and offers several advantages over conventional methods by avoiding: (i) use of HF pressure dissolution and (ii) complete removing of grains from the metal microvials. After dissolution, U, Th and Sm were measured using an ICP‐MS. We tested the new procedure on two different ZHe reference materials, the Fish Canyon Tuff and Buluk Tuff; these provided precision values for ZHe‐age estimations of 9 and 6% (1s), respectively. In addition, using this method, zircons from the Buluk Tuff are shown to be chemically more homogenous and more suitable for assessing the uncertainty of the entire integrated procedure

Rift-to-collision sediment routing in the Pyrenees: A synthesis from sedimentological, geochronological and kinematic constraints

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Progressive glacial retreat in the Southern Altiplano (Uturuncu volcano, 22°S) between 65 and 14 ka constrained by cosmogenic 3He dating

International audienceThis 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

Implementation of laser in situ (U-Th)/(He-Pb) double dating on single detrital zircon grains

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Quantifying incision rates since the early Miocene: novelties, potentialities and limitations

International audienceThe rates and chronologies of valley incision are closely modulated by the tectonic uplift of active mountain ranges and were controlled by repeated climate changes during the Quaternary. The Pyrenees are a continental collision between the Iberian and Eurasian plates induced a double vergence orogen, which has been considered as a mature mountain range in spite of significant seismicity and evidence of neotectonics. Nevertheless, recent studies indicate that the range may have never reached a steady state. One option for resolving this controversy is to quantify the incision rates since the Miocene by reconstructing the vertical movement of geometric markers. However, the few available ages from the Pyrenean terrace systems do not exceed the middle Pleistocene. To enlarge the time span of this dataset, we studied alluvium-filled horizontal epiphreatic passages in limestone karstic networks, which represent former valley floors. They record the transient position of former local base levels during the process of valley deepening. We used various suitable geochronological methods (26Al/10Be, 10Be/21Ne, ESR and OSL burial durations on quartz) on intrakarstic alluvial deposits from three valleys of the central and eastern Pyrenees, as well as on a recent analogue. In the Pyrenean context, under particular conditions, these geochronometers allow us to document incision processes since ~ 16-13 Ma, and to study influences of external forcing and eustatism. In comparison with other studies, it appears that incision rates are higher in the central Pyrenees and for the Spanish slope. However, the density of horizontal levels on an altimetric range, the geodynamical and paleoclimatic contexts, the reorganization of the drainage networks can make the filling stories of the networks more complex than expected. Indeed, these radiometric approaches may be limited when some formations are reworked inside and/or outside the karst. The validity of dosimetric methods in a mountainous context, and quartz bleaching conditions at the time of deposition will also be discussed. Acknowledgments: this work was financed by the BRGM within the framework of the RGF-Pyrénées program

Morphogenetic evolution of the Têt river valley (eastern Pyrenees) using 10Be/21Ne cosmogenic burial dating

PosterInternational audienceThe rates and chronologies of valley incision are closely modulated by the tectonic uplift of active mountain ranges and were controlled by repeated climate changes during the Quaternary. The continental collision between the Iberian and Eurasian plates induced a double vergence orogen, the Pyrenees, which has been considered as a mature mountain range in spite of significant seismicity (e.g. Chevrot et al., 2011) and evidence of neotectonics (e.g. Goula et al., 1999). Nevertheless, recent studies indicate that the range may have never reached a steady state (Ford et al., in press). One option for resolving this controversy is to quantify the incision rates since the Miocene by reconstructing the vertical movement of geometric markers such as fluvial terraces. However, the few available ages from the Pyrenean terrace systems do not exceed the middle Pleistocene. Thus, to enlarge the time span of this dataset, we studied alluvium-filled horizontal epiphreatic passages in limestone karstic networks. Such landforms are used as substitutes of fluvial terraces because they represent former valley floors (e.g. Palmer, 2007; Audra et al., 2013). They record the transient position of former local base levels during the process of valley deepening. The Têt river valley (southern Pyrenees) was studied near the Villefranche-de-Conflent limestone gorge where 8 cave levels have been recognized over a vertical height of 600 meters. Given that 26Al/10Be cosmogenic burial dating in this setting was limited to the last ∼5 Ma (Calvet et al., 2015), here we used the cosmogenic 10Be/21Ne method in order to restore a more complete chronology of valley incision (e.g. Balco & Shuster, 2009; McPhilipps et al., 2016). Burial age results for alluvial deposits from 12 caves document incision rates since the Langhian (∼14 Ma). Preliminary results indicate a history of valley deepening in successive stages. The data show a regular incision rate of 70–80 mm/ka from the Langhian to the Messinian, then a decrease to a mean incision rate of ∼15 mm/ka. Together with these burial ages, pre-burial catchment denudation rates were estimated and do not exceed 21 mm/ka. Moreover, no acceleration during the Quaternary is recorded. This method also allows to infer that the Pla des Horts plateau is of Burdigalian age

Morphogenetic evolution of the Têt River valley (Eastern Pyrenees) using 10Be/21Ne cosmogenic burial dating

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Rift-to-collision transition recorded by tectono-thermal evolution of the northern Pyrenees

International audienceThe impact of rift-related processes on tectonic and thermal evolution of collisional orogens is poorly documented. Here, we study the northern Pyrenees, a region that has preserved a geological record of the transition from rifting to collision. Using modeling of new low-temperature thermochronological data, including fission track and (U-Th)/He on apatite and zircon, we propose a temporal reconstruction of the inversion of the European rifted margin. Our data confirm that rifting and related cooling started in the Late Paleozoic-Triassic. Throughout the Jurassic and Early Cretaceous the European margin recorded slow heating during postrift subdsidence. Modeling of thermochronological data allows distinguishing subsidence and denudation controlled by south dipping normal faults in granitic massifs that reflect a second episode of crustal thinning at 130–110 Ma. Following onset of convergence at 83 Ma, shortening accumulated into the weak and hot Albian-Cenomanian rift basins floored by both hyperextended continental crust and exhumed subcontinental mantle. The lack of cooling during this initial stage of convergence is explained by the persistence of a high geothermal gradient. The onset of exhumation-related cooling is recognized in the whole Pyrenean region at 50–35 Ma. This timing reveals that the main phase of mountain building started when hyperextended rift basins closed and collision between proximal domains of the rifted margin occurred