Unbalanced sediment budgets in the catchment-alluvial fan system of the Kuitun River (northern Tian Shan, China): Implications for mass-balance estimates, denudation and sedimentation rates in orogenic systems

International audienceMass balances are often used to calculate sediment fluxes in foreland basins and denudation rates in adjacent mountain ranges on intermediate to long timescales (from a few tens of thousand to a million years). Here, we study the simple Quaternary catchment-alluvial fan system of the Kuitun River, in northern Tian Shan, to discuss some ideas about sediment storage, release, and bypass in relatively short (100 km long) sediment routing systems. This study shows that the Kuitun catchment and piedmont areas clearly present evidence of a significant and temporary storage of sediments during the Pleistocene. These sediments were then excavated and delivered farther into the foreland basin during the Holocene. The difference between the volumes of materials released from the catchment and piedmont areas (5.5 ± 1.7 km3) and the volume stored in a contemporaneous fan downstream (2.6 ± 0.6 km3) indicates that the latter did not trap the whole sediment load transported by the river. The alluvial fan was bypassed by 27 to 78% of this load toward its distal alluvial plain. If this value is well estimated, it implies a major volumetric partitioning of the deposits between the fan and the alluvial plain, with a very high sedimentation rate in the fan (1.97 ± 0.52 mm*y− 1) and a much lower one downstream (0.11 ± 0.11 mm*y− 1). However, this volumetric partitioning might only occur during periods with a very specific hydrological regime such as the Holocene deglaciation. Eventually, the peculiar sediment storage and release pattern within the Kuitun catchment and piedmont areas during the Pleistocene and Holocene complicates the calculation of mean paleodenudation rates using either sediment budgets or in situ produced cosmogenic nuclides

Late Miocene -Pleistocene evolution of India-Eurasia convergence partitioning between the Bhutan Himalaya and the Shillong plateau:new evidences from foreland basin deposits along the Dunsam Chu section, Eastern Bhutan

The Shillong plateau is a unique basement-cored uplift in the foreland of the eastern Himalaya that accommodates part of the India-Eurasia convergence since the late Miocene. It was uplifted in the late Pliocene to 1,600 metres, potentially inducing regional climatic perturbations by orographically condensing part of the Indian Summer Monsoon (ISM) precipitations along its southern flank. As such, the eastern Himalaya-Shillong plateau-ISM is suited to investigate effects of tectonics, climate and erosion in a mountain range-broken foreland system. This study focuses on a 2200 m-thick sedimentary section of the Siwalik Group strategically located in the lee of the Shillong plateau along the Dungsam Chu at the front of the eastern Bhutan Himalaya. We have performed magnetostratigraphy constrained by vitrinite reflectance and detrital apatite fission-track dating, combined with sedimentological and palynological analyses. We show that (1) the section was deposited between ~7 and 1 Ma in a marginal marine deltaic transitioning into continental environment after 5 Ma, (2) depositional environments and paleoclimate were humid with no major change during the depositional period indicating that the orographic effect of the Shillong plateau had an unexpected limited impact on the paleoclimate of the Bhutanese foothills and (3) the diminution of the flexural subsidence in the basin and/or of the detrital input from the range is attributable to a slowdown of the displacement rates along the Main Boundary Thrust in eastern Bhutan during the latest Miocene – Pleistocene, in response to increasing partitioning of the India-Eurasia convergence into the active faults bounding the Shillong plateau

Stable isotope characterization of pedogenic and lacustrine carbonates from the Chinese Tian Shan: constraints on the Mesozoic - Lower Cenozoic palaeo-environmental evolution

International audienceIn the Mesozoic–Cenozoic continental deposits of the Tian Shan area, two main levels containing pedogenic carbonates have been identified on both the southern and northern foothills of the range: one in the Upper Jurassic series and one in the Upper Cretaceous–Lower Palaeocene series. In order to reconstruct the palaeoenvironmental and palaeotopographic characteristics of the Tian Shan area during these two periods, we measured the oxygen and carbon isotope composition of these pedogenic carbonates (calcrete and nodules). The stable isotope compositions are homogeneous: most δ18O values are between 21 and 25‰ and most δ13C values are between −4 and −6‰. No distinction can be made between the calcrete and nodule isotopic compositions. The constancy of isotopic values across the Tian Shan is evidence of a development of these calcification features in similar palaeoenvironmental conditions. The main inference is that no significant relief existed in that area at the Cretaceous−Palaeogene boundary, implying that most of the present relief developed later, during the Cenozoic. In addition to the pedogenic carbonates, few beds of limestones interstratified in the Jurassic series of the southern foothills display oxygen and carbon isotope compositions typical of lacustrine carbonates, ruling out brackish water incursion at that period in the regio

Neogene uplift of the Tian Shan Mountains observed in the magnetic record of the Jingou River section (northwest China)

The Tian Shan Mountains constitute central Asia's longest and highest mountain range. Understanding their Cenozoic uplift history thus bears on mountain building processes in general, and on how deformation has occurred under the influence of the India-Asia collision in particular. In order to help decipher the uplift history of the Tian Shan, we collected 970 samples for magnetostratigraphic analysis along a 4571-m-thick section at the Jingou River (Xinjiang Province, China). Stepwise alternating field and thermal demagnetization isolate a linear magnetization component that is interpreted as primary. From this component, a magnetostratigraphic column composed of 67 polarity chrons are correlated with the reference geomagnetic polarity timescale between ∼1 Ma and ∼23.6 Ma, with some uncertainty below ∼21 Ma. This correlation places precise temporal control on the Neogene stratigraphy of the southern Junggar Basin and provides evidence for two significant stepwise increases in sediment accumulation rate at ∼16–15 Ma and ∼11–10 Ma. Rock magnetic parameters also undergo important changes at ∼16–15 Ma and ∼11–10 Ma that correlate with changes in sedimentary depositional environments. Together with previous work, we conclude that growth history of the modern Tian Shan Mountains includes two pulses of uplift and erosion at ∼16–15 Ma and ∼11–10 Ma. Middle to upper Tertiary rocks around the Tian Shan record very young (<∼5 Ma) counterclockwise paleomagnetic rotations, on the order of 15° to 20°, which are interpreted as because of strain partitioning with a component of sinistral shear that localized rotations in the piedmont

Interactions déformation - sédimentation dans les systèmes compressifs supra-crustaux. Exemples naturels et modélisation analogique

Thèse publiée dans la collection des Mémoires de Géosciences Rennes (ISSN 1240-1498) : Mémoire n° 103 (ISBN 2-914375-09-3)ForthcomingDans les systèmes compressifs, l'idée que les processus de déformation ne peuvent plus être considérés séparément des processus de surface émerge depuis une dizaine d'années. Toutefois, peu d'études ont été réalisées concernant l'effet de la sédimentation sur les structures compressives kilométriques à pluri-kilométriques. Nous avons donc tenté de comprendre comment la sédimentation agit sur la déformation à cette échelle, et comment l'enregistrement sédimentaire est influencé en retour. Des structures chevauchantes bordières de plusieurs bassins sédimentaires tertiaires de la péninsule ibérique ont tout d'abord été étudiées sur le terrain. Lorsque la sédimentation à leur mur est forte, elles présentent systématiquement plusieurs segments imbriqués en séquence arrière avec un pendage croissant. En retour, la paléogéographie et le drainage du bassin sont localement réorganisés. L'effet de la sédimentation a ensuite été testé sur des modèles analogues fragile-ductile soumis à une compression. Dans les expériences, le nombre et le pendage des failles chevauchantes augmentent avec la vitesse des dépôts syn-cinématiques à leur front. A taux de sédimentation faible ou nul, les chevauchements sont permanents et relativement plats (moins de 40°). A taux de sédimentation plus élevé, des séries de rampes chevauchantes de plus en plus redressées (jusqu'à 65°) se développent en séquence arrière. La géométrie et l'évolution des expériences dépendent donc des variations spatiales et temporelles du taux de sédimentation syn-cinématique. Elles dépendent également de la rhéologie initiale des modèles et de celle des dépôts syn-cinématiques. Pour finir, nous avons comparé des données sismiques (des Apennins et du Delta du Niger) aux données de terrain et aux résultats expérimentaux. Cette confrontation a permis de valider les lois de comportements proposées pour les structures de croissance compressives à partir des différentes approches

SEDIMENTARY LANDSCAPE AND PALEO-LANDSCAPES OF AN ACTIVEENDORHEIC FORELAND BASIN: THE JUNGGAR BASIN (XINJIANG, CHINA)

National audienceThe sedimentary landscape and dynamics of foreland basins, which are shaped by the interplaysbetween tectonics, climate, sedimentation and erosion processes, are key elements toreconstruct the evolution of orogenic systems. Therefore, a detailed understanding of the relationshipsbetween the foreland landscapes and these processes is necessary to improve ourknowledge on the sedimentary record in compressive regions. In this study, we focus on thecharacterization of the present-day and palaeo-landscapes of an endorheic foreland basin wheredeformation, sedimentation and erosion are still active and easily observable: the Junggar Basinlocated in Central Asia. The interest of this basin resides in outstanding outcrops of its sedimentaryseries and structures, numerous surface and subsurface data (satellite images, digitaltopography, seismic profiles and drilling well data), and marked continental paleo-geographicchanges through the Cenozoic. The methodology consists in coupling different approaches (geomorphology,sedimentology, sequential stratigraphy and structural geology) to characterize thesepaleo-geographic changes across space and through time. As a starting point, we drew a morphosedimentologicalmap of the present-day landscape from the surface data to describe the currentdrainage organization and associated sedimentary environments in the basin. Then, we estab- lishedseveral paleo-geographic maps from the surface and subsurface data to reconstruct the spatiotemporallandscape evolution since 65 Ma. In the light of previous quantifications of its controlparameters (substratum deformation and sediment supplies), this evolution provides new qualitativeand quantitative constraints on landscape architecture and dynamics in continental forelandcontexts

Li and Si isotopes reveal authigenic clay formation in a palaeo-delta

International audienceMarine authigenic clay formation has long been postulated as a major process to explain the mass budgets of some elements in seawater, and might act a reverse reaction for the neutralization of atmospheric acidity by soil forming reactions on land. Nevertheless, to date, a handful of studies have directly investigated the effect of reverse weathering due to the challenges associated with sampling complexity. Deltas are thought to be one of the possible environments where reverse weathering reactions may occur because of the abundant influx of weathering derived materials and dynamic activities. In this study, we use a unique combination of three isotope systems (Si, Li, and Nd) as evidence of authigenic clay formation using sediments collected from a ∼40 Ma-old delta complex (Ainsa Basin, Spain). Sediments were collected along the land to sea depositional continuum, from alluvial and coastal plains to marine environments. Direct comparison between alluvial, coastal, and marine sediments allows for observing the potential effect of reverse weathering. Systematic differences in Si and Li isotope composition exist between marine and continental sediments, revealing the formation of an authigenic phase sequestrating light Li and Si as associated with the diagenesis of Fe in reducing deltaic environments. Finally, this study proposes a geochemical tool for helping distinguishing marine and continental origins of sedimentary rocks when lithofacies and biofacies prove to be ineffective

Impact of synkinematic sedimentationon the geometry and dynamics ofcompressive growth structures : Analoguemodelling insights

International audienceAnalogue sandbox models have been set up to study the impact of synkinematicdeposits on the geometry and evolution of single thrusts andfolds according to different sedimentation modes (a slow or rapid sedimentationrate that is constant or changing in space and time) and rheologicalprofiles (thin or thick sedimentary series, with a basal décollementlevel or not). A first series of experiments documents the influenceof synkinematic deposits according to their sedimentation rate and therheology of the prekinematic materials. A second series investigates theinfluence of changes in the sedimentation rate through time. A third oneconsiders the influence of changes in the sedimentation rate in space.All these experiments suggest that the geometry and evolution of singlecompressive growth structures vary according to the sedimentation rate.The number and dip of their frontal thrust segments change with the ratioR between the sedimentation rate (Vsed) at the footwall of the faultsand the uplift rate (Vupl) of their hanging wall. The latter is then more orless uplifted depending on the dip of the thrusts. As a result, the overallstructure has either a fault-bend fold or a fault-propagation fold geometry.Those rules are verified when the ratio R (Vsed/Vupl) changes inspace or through time. In addition, the rheological profile of the modelsalso affects the geometry and evolution of compressive growth structures.Their structural style, as well as the synsedimentary splitting andsteepening of the associated thrusts, varies according to the occurrenceand strength of the brittle and ductile layers.According to this modelling study, the ratio R (Vsed/Vupl) and itschanges in space and time, along with the rheology of the deformedmaterials, are key parameters to better understand the geometrical andkinematical complexities of natural growth thrusts and folds and to improvetheir interpretation

Mid-term erosion rate estimate of a drainage basin: eroded sediment volume approach

International audienc