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

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

Miocene to Holocene deformation rates in the southern Tianshan piedmont, China

International audienceThe Tianshan is one of the largest and highest mountain belts in central Asia and it accommodates an important part of the total India/Eurasia convergence. It is therefore critical to better quantify the deformation across this range, especially across its piedmonts that presently absorb most of the shortening. The deformation across the southern Tianshan piedmont is distributed over several thrusts and folds belts including the Qiulitagh and the Yakeng anticlines, which are identified as a fault-bend-fold and a detachment fold respectively. Across these two folds, we document the shortening rate from the Mio-Pliocene to the Pleistocene using two different approaches. First, we used a kinematic fold model based on kink band migration, in order to reproduce bed by bed the pre-growth and growth-strata of the Qiulitagh anticline. We focus our analysis on two different sections where seismic profiles and detailed surface structural data provide strong constrains on the geometry of the fold. The first section is located in the southern flank of the fold along the Kuqa River. In the bottom part of this section the depositional ages have been constrained by the magnetostratigraphic study of Sun et al. (2009), whereas the upper conglomeratic part remained undated. For this upper part, we have collected several new samples to better constrain the sediments ages using burial cosmogenic dating (10Be/26Al). The second section is located in the northern flank of the fold along the Yaha River where the depositional ages have also been constrained in detail from magnetostratigraphy (Charreau et al., 2009). Together, these two sections should provide new and high resolution (100ka) by cosmogenic (10Be) exposure method

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

International audienc

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

International audienc

Miocene to Holocene deformation rates in the southern Tianshan piedmont, China

International audienceThe Tianshan is one of the largest and highest mountain belts in central Asia and it accommodates an important part of the total India/Eurasia convergence. It is therefore critical to better quantify the deformation across this range, especially across its piedmonts that presently absorb most of the shortening. The deformation across the southern Tianshan piedmont is distributed over several thrusts and folds belts including the Qiulitagh and the Yakeng anticlines, which are identified as a fault-bend-fold and a detachment fold respectively. Across these two folds, we document the shortening rate from the Mio-Pliocene to the Pleistocene using two different approaches. First, we used a kinematic fold model based on kink band migration, in order to reproduce bed by bed the pre-growth and growth-strata of the Qiulitagh anticline. We focus our analysis on two different sections where seismic profiles and detailed surface structural data provide strong constrains on the geometry of the fold. The first section is located in the southern flank of the fold along the Kuqa River. In the bottom part of this section the depositional ages have been constrained by the magnetostratigraphic study of Sun et al. (2009), whereas the upper conglomeratic part remained undated. For this upper part, we have collected several new samples to better constrain the sediments ages using burial cosmogenic dating (10Be/26Al). The second section is located in the northern flank of the fold along the Yaha River where the depositional ages have also been constrained in detail from magnetostratigraphy (Charreau et al., 2009). Together, these two sections should provide new and high resolution (100ka) by cosmogenic (10Be) exposure method

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

Relief and paleoenvironmental conditions during the mid-late Miocene in the French Western Alps (Dévoluy Massif) revealed by Obiou cave deposits

International audienceCave levels at high elevations (2250-2370 m), hosting allochthonous sediments with clear provenance from the Pelvoux External Crystalline Massif were identified in the Obiou mountain, Dévoluy Massif (French Western Alps). These deposits result from burial of alluvial sediments of the paleo-Drac River in Miocene times (burial ages of ca. 10-15 Ma). The Drac River is currently situated 1600 m below the caves, indicating significant post-middle Miocene fluvial incision. The pollen analysis on clay cave infill indicates a humid and warm climate, typical for Western Europe in the mid-late Miocene. It also reveals the presence of dinoflagellate cysts from coastal and lagoon environments suggesting deposition of the clays in the caves when they were close to sea level. These findings indicate uplift of the caves of at least 2000 m since the mid-late Miocene. Furthermore, the pollen assemblage originated from different vegetation belts, in agreement with existing data for mid-late Miocene deposits in the western Alps. The pollen flora includes cool-temperate and boreal trees, suggesting high-elevated source areas for the deposits within the Pelvoux catchment. These data confirm the exceptional character of the Obiou cave deposits that provide new geomorphologic constraints for the evolution of the French Western Alps

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

International audienc