Channel profiles through the active thrust front of the southern Barbados prism.

International audienceSubmarine channels of the Orinoco River were analyzed using high-quality, dense coverage bathymetric and seismic data provided by a recent marine survey on the southern Barbados prism. Analysis of the syntectonic sediments on seismic profiles shows that the four to five frontmost structures show evidence of recent tectonic movement. Slope analysis of the major channels was performed. From their headwaters to domains of little or no active tectonics, the channels display ,0.2% slope and form levees. Slope and incision increase gradually in domains of moderate tectonics, but deep canyons with ;2% mean slope form where the channels cross the active frontal folds of the prism. Detailed correlation between the active structures, their geometry, and canyon slope suggest that systematic variations in channel gradient highlight variations in substrate uplift rate. Steep slopes induced by uplift accelerate sediment flow and enhance incision. Nonetheless, such slope analysis is subject to complications introduced by variations in sediment flux and transient erosional conditions

C and O isotope compositions of modern fresh-water mollusc shells and river waters from Himalaya and Ganga plain

The aim of this paper is to unfold the relationship between O and C isotope compositions of modern fresh-water mollusc shells and water in order to refine the basis of interpretation for paleoenvironnemental reconstruction in the sub-Himalayan river basins. Large number of mollusc shells and associated host water from both running water and closed body of water were analysed including intra-shell variability in a few cases. The O isotopic compositions of river waters in the Himalayas and Ganga plain have a large range, from -18‰ in the north of the High range up to -8 to -4 ‰ in the Ganga plain. d18O of rivers are also seasonally variable, especially in foothills rivers whereas the seasonal contrast is smoothed out for main Himalayan rivers having large catchments. O isotopic compositions of bulk shells (Aragonite) vary between -15 and -5 ‰. Average d18OAra values are consistent with precipitation at equilibrium with host waters at a temperature range of 20-25°C suggesting that shell growth may be favoured during non-monsoon conditions. Shells collected along main Himalayan rivers have d18O values uniformly distributed within -11 and -6 ‰ reflecting the minimal seasonal contrast shown by these rivers. In contrast, O isotopic compositions of shells from foothills rivers vary only by 4‰. This shows that, depending on the type of river where the molluscs grow, the information in term of d18O amplitude will be different for identical climatic conditions. In closed or pond water bodies significant enrichment in 18O due to evaporation is observed. The C isotopic compositions of river dissolved inorganic carbon (DIC) decrease downstream from 0 to -10 ‰ reflecting input of soil derived alkalinity and plant productivity in the river. d13C of shells are systematically lower than compositions calculated for equilibrium with river DIC indicating that in addition to DIC, a significant fraction of carbon is derived from metabolic sources. Intra-shell d13C are stable compared to the seasonal variability of DIC suggesting that the pool of organic carbon changes throughout year

Late Miocene – Recent exhumation of the central Himalaya and recycling in the foreland basin assessed by apatite fission-track thermochronology of Siwalik sediments, Nepal

International audienceThermochronological analysis of detrital sediments derived from the erosion of mountain belts and contained in the sedimentary basins surrounding them allows reconstructing the long-term exhumation history of the sediment source areas. The effective closure temperature of the thermochronological system analysed determines the spatial and temporal resolution of the analysis through the duration of the lag time between closure of the system during exhumation and its deposition in the sedimentary basin. Here we report apatite fission-track (AFT) data from 31 detrital samples collected from Miocene to Pliocene stratigraphic sections of the Siwalik Group in western and central Nepal, as well as three samples from modern river sediments from the same area, that complement detrital zircon fission-track (ZFT) and U-Pb data from the same samples presented in a companion paper. Samples from the upper part of the stratigraphic sections are unreset and retain a signal of source-area exhumation; they show spatial variations in source-area exhumation rates that are not picked up by the higher-temperature systems. More deeply buried samples have been partially reset within the Siwalik basin and provide constraints on the thermal and kinematic history of the fold-and-thrust belt itself. The results suggest that peak source-area exhumation rates have been constant at ~1.8 km Myr-1 over the last ~7 Ma in central Nepal, whereas they ranged between 1 and ~1.5 km Myr-1 in western Nepal over the same time interval; these spatial variations may be explained by either a tectonic or climatic control on exhumation rates, or possibly a combination of the two. Increasing lag times within the uppermost part of the sections suggest an increasing component of apatites that have been recycled within the Siwalik belt and are corroborated by AFT ages of modern river sediment downstream as well as the record of the distal Bengal Fan. The most deeply buried and most strongly annealed samples record onset of exhumation of the frontal Siwaliks along the Himalayan frontal thrust at ~2 Ma and continuous shortening at rates comparable to the present-day shortening rates from at least 0.3 Ma onward

Miocene to Recent exhumation of the central Himalaya determined from combined detrital zircon fission-track and U/Pb analysis of Siwalik sediments, western Nepal

International audienceFission-track (FT) analysis of detrital zircon from synorogenic sediment is a well established tool to examine the cooling and exhumation history of convergent mountain belts, but has so far not been used to determine the long-term evolution of the central Himalaya. This study presents FT analysis of detrital zircon from 22 sandstone and modern sediment samples that were collected along three stratigraphic sections within the Miocene to Pliocene Siwalik Group, and from modern rivers, in western and central Nepal. The results provide evidence for widespread cooling in the Nepalese Himalaya at about 16.0 ± 1.4 Ma, and continuous exhumation at a rate of about 1.4 ± 0.2 km/Myr thereafter. The ~16 Ma cooling is likely related to a combination of tectonic and erosional activity, including movement on the Main Central thrust and Southern Tibetan Detachment system, as well as emplacement of the Ramgarh thrust on Lesser Himalayan sedimentary and meta-sedimentary units. The continuous exhumation signal following the ~16 Ma cooling event is seen in connection with ongoing tectonic uplift, river incision, and erosion of lower Lesser Himalayan rocks exposed below the MCT and Higher Himalayan rocks in the hanging wall of the MCT, controlled by orographic precipitation and crustal extrusion. Provenance analysis, to distinguish between Higher Himalayan and Lesser Himalayan zircon sources, is based on double dating of individual zircons with the FT and U/Pb methods. Zircons with pre-Himalayan FT cooling ages may be derived from either non-metamorphic parts of the Tethyan sedimentary succession or Higher Himalayan protolith that formerly covered the Dadeldhura and Ramgarh thrust sheets, but that have been removed by erosion. Both the Higher and Lesser Himalaya appear to be sources for the zircons that record either ~16 Ma cooling or the continuous exhumation afterwards

Weathering in the Himalaya, an East- West comparison:indications from major elements and clay mineralogy

Studying past weathering regimes is important for a better understanding of the influence of climate on weathering, erosion, and runoff. The Himalayan foreland basin contains a record of tectonics and paleoclimate since Miocene times. Spanning the entire mountain range, the Mio-Pliocene detrital Siwalik Group allows studies to directly compare the western and eastern Himalaya within similar sedimentary settings. In this study, we use major elements and clay mineralogy to reconstruct the weathering regime along strike since Miocene times. We studied previously dated Dharamsala (pre-Siwalik) and Siwalik sections in the western (Joginder Nagar, Jawalamukhi, and Haripur Kolar) and eastern (Kameng) Himalaya in order to constrain variations in weathering regimes along strike. The compilation of the three sections in the west makes for one of the longest continuous sedimentary records in the Himalaya, spanning over 20 My. The K/Al ratio is used as a reliable weathering proxy and shows a trend toward more intense weathering over time in both the west and the east, but with sediments in the western Himalaya generally more weathered than those in the east, despite higher precipitation in the east. Clay minerals and major elements indicate similar lateral variations in weathering. More intense weathering in the west is linked to a more seasonal climate, permitting weathering of sediments during the dry season, whereas higher runoff in the east leads to more rapid erosion and sediment transport, inhibiting extensive weathering

Weathering regime in the Eastern Himalaya since the mid-Miocene:Indications from detrital geochemistry and clay mineralogy of the Kameng River Section, Arunachal Pradesh, India

It is crucial to understand lateral differences in paleo-climate and weathering in order to fully understand the evolution of the Himalayan mountain belt. While many studies have focused on the western and central Himalaya, the eastern Himalaya remains poorly studied with regard to paleoclimate and past weathering history. Here we present a multi-proxy study on the Mio-Pliocene sedimentary foreland-basin section along the Kameng River in Arunachal Pradesh, northeast India, in order to obtain better insight in the weathering history of the eastern Himalaya. We analyzed a continuous sedimentary record over the last 13 Ma. Heavy-mineral and petrography data give insight into diagenesis and provenance, showing that the older part of the section is influenced by diagenesis and that sediments were not only deposited by a large Trans-Himalayan river and the palaeo-Kameng river, but also by smaller local tributaries. By taking into account changes in diagenesis and provenance, results of clay mineralogy and major element analysis show an overall increase in weathering intensity over time, with a remarkable change between ~10 and ~8 Ma

The tectonics and paleo-drainage of the easternmost Himalaya (Arunachal Pradesh, India) recorded in the Siwalik rocks of the foreland basin

The Siwalik sedimentary rocks of the Himalayan foreland basin preserve a record of Himalayan orogenesis, paleo-drainage evolution, and erosion. This study focuses on the still poorly studied easternmost Himalaya Siwalik record located directly downstream of the Namche Barwa syntaxis. We use luminescence, palaeomagnetism, magnetostratigraphy, and apatite fission-track dating to constrain the depositional ages of three Siwalik sequences: the Sibo outcrop (Upper Siwalik sediments at ca. 200-800 ka), the Remi section (Middle and Upper Siwalik rocks at ca. 0.8-6.6 Ma), and the Siang section (Middle Siwalik rocks at ca. <9.3±1.5 to <13.5±1.5 Ma). Cretaceous-Paleogene detrital zircon and apatite U-Pb ages, characteristic of the Transhimalayan Gangdese Batholiths that crop out northwest of the syntaxis, are present throughout the Sibo-Remi-Siang successions, confirming the existence of a Yarlung-Brahmaputra connection since at least the Late Miocene. A ca. 500 Ma zircon population increases up section, most strikingly sometime between 3.6-6.6 Ma, at the expense of Transhimalayan grains. We consider the ca 500 Ma population to be derived from the Tethyan or Greater Himalaya, and we interpret the up-section increase to reflect progressive exhumation of the Namche Barwa syntaxis. Early Cretaceous zircon and apatite U-Pb ages are rare in the Sibo, Remi, and Siang successions, but abundant in modern Siang River sediments. Zircons of this age range are characteristic of the Transhimalayan Bomi-Chayu batholiths, which crop out east of the syntaxis and are eroded by the Parlung River, a modern tributary of the Siang River. We interpret the difference in relative abundance of Early Cretaceous zircons between the modern and ancient sediments to reflect capture of the Parlung by the Siang after 800 ka

Lateral variations in vegetation in the Himalaya since the Miocene and implications for climate evolution

The Himalaya has a major influence on global and regional climate, in particular on the Asian monsoon system. The foreland basin of the Himalaya contains a record of tectonics and paleoclimate since the Miocene. Previous work on the evolution of vegetation and climate has focused on the central and western Himalaya, where a shift from C3 to C4 vegetation has been observed at ∼7 Ma and linked to increased seasonality, but the climatic evolution of the eastern part of the orogen is less well understood. In order to track vegetation as a marker of monsoon intensity and seasonality, we analyzed δ13Cδ13C and δ18Oδ18O values of soil carbonate and associated δ13Cδ13C values of bulk organic carbon from previously dated sedimentary sections exposing the syn-orogenic detrital Dharamsala and Siwalik Groups in the west, and, for the first time, the Siwalik Group in the east of the Himalayan foreland basin. Sedimentary records span from 20 to 1 Myr in the west (Joginder Nagar, Jawalamukhi, and Haripur Kolar sections) and from 13 to 1 Myr in the east (Kameng section), respectively. The presence of soil carbonate in the west and its absence in the east is a first indication of long-term lateral climatic variation, as soil carbonate requires seasonally arid conditions to develop. δ13Cδ13C values in soil carbonate show a shift from around −10‰ to −2‰ at ∼7 Ma in the west, which is confirmed by δ13Cδ13C analyses on bulk organic carbon that show a shift from around −23‰ to −19‰ at the same time. Such a shift in isotopic values is likely to be associated with a change from C3 to C4 vegetation. In contrast, δ13Cδ13C values of bulk organic carbon remain at ∼−23‰∼−23‰ in the east. Thus, our data show that the current east–west variation in climate was established at 7 Ma. We propose that the regional change towards a more seasonal climate in the west is linked to a decrease of the influence of the Westerlies, delivering less winter precipitation to the western Himalaya, while the east remained annually humid due to its proximity to the monsoonal moisture source

Early onset and late acceleration of rapid exhumation in the Namche Barwa syntaxis, eastern Himalaya

The Himalayan syntaxes, characterized by extreme rates of rock exhumation co-located with major trans-orogenic rivers, figure prominently in the debate on tectonic versus erosional forcing of exhumation. Both the mechanism and timing of rapid exhumation of the Namche Barwa massif in the eastern syntaxis remain controversial. It has been argued that coupling between crustal rock advection and surface erosion initiated in the late Miocene (8-10 Ma). Recent studies, in contrast, suggest a Quaternary onset of rapid exhumation linked to a purely tectonic mechanism. We report new multisystem detrital thermochronology data from the most proximal Neogene clastic sediments downstream of Namche Barwa and use a thermo-kinematic model constrained by new and published data to explore its exhumation history. Modeling results show that exhumation accelerated to ~4 km/m.y. at ~8 Ma and to ~9 km/m.y. after ~2 Ma. This three-stage history reconciles apparently contradictory evidence for early and late onset of rapid exhumation, and suggests efficient coupling between tectonics and erosion since the late Miocene. Quaternary acceleration of exhumation is consistent with river-profile evolution, and may be linked to a Quaternary river-capture event

Enregistrement sédimentaire des déformations intraplaques : l'exemple de l'inversion structurale d'un bassin de la mer du Nord

Phenomena governing the evolution of sedimentary basins and basin geometry are recorded in the stratigraphic pattern. Short term fluctuations of the accomodation space are superimposed on long term phenomena that increase the basin volume (initial stretching, effects of thermal contraction of the lithosphere and sedimentary loading). Basin evolution may then be analysed by studying sedimentary sequences and especially :1) the geometry of their boundaries (faults and unconformities) , 2) their facies, 3) their age, and 4) variations of their physical properties (radio-activity, porosity .... ). Back-stripping, differential subsidence diagrams, and geometrical methods (balanced cross-sections) allow to quantify vertical and horizontal motions of the floor of the basin. These methods were applied to the Broad Fourteens Basin (Southern North Sea) using a data base composed by 3000 km of seismic reflection profiles and around fifty wells (provided by Elf-Petroland). They show both tectonic decollement related to the distribution of salt and a complex multistage evolution principally composed of: 1) Jurassic and Lower Cretaceous syn-rift stages (basin subsidence around 1000 m during the latter) and an important uplift of the margins, 2) sinistral transpression in the Upper Cretaceous involving erosion in the basin (exceeding 3000 m in the center of the basin) and a horizontal shortening of about 10 %, 3) renewed tectonic subsidence beginning in the Miocene. Sixteen sedimentary sequences were distinguished in the Cretaceous series. Their geometty was controlled by eustatic sea level changes and tectonic motions. Contemporaneous tectonic motions are recorded in other intraplate basins and could be related to the plate kinematics around NW Europe. Major plate tectonic reorganization could be responsible for horizontal stress variations involving both reactivation of pre-existing structures in the britlle crust and lithospheric buckling.Les phénomènes qui régissent l'évolution d'un bassin sédimentaire et modifient sa géométrie laissent une empreinte dans l'organisation des dépôts. Aux phénomènes qui accroissent à long terme le volume du bassin (étirement initial, rééquilibrage thermique de la lithosphère, compensation isostasique du poids des sédiments qui se déposent) se superposent des fluctuations plus rapides de l'espace disponible. L'évolution d'un bassin peut donc être décryptée en étudiant les corps sédimentaires, et plus particulièrement: 1) la géométrie des discontinuités qui les limitent (failles et discordances), 2) les faciès qui les constituent, 3) leur âge, 4) les variations de leurs propriétés physiques (radioactivité, porosité ... ). L'utilisation des méthodes de décompaction, des diagrammes de mouvements différentiels, de substitution des structures plissées/faillées (coupes équilibrées) permet de quantifier les mouvements qui affectent le substratum du bassin. L'application de ces méthodes au Broad Fourteens Basin (Sud de la Mer du Nord), à partir d'une base de données constituée par 3000 km de sismique réflexion et une cinquantaine de puits (consultée grâce à Elf-Petroland) a mis en évidence une histoire polyphasée complexe, affectant une couverture décollée de son socle. Cette histoire est composée principalement : 1) d'une extension jurassique, puis 2) d'une extension crétacé inférieur avec une subsidence dans le bassin d'environ 1000 m et une importante surrection initiale des marges, 3) d'une compression avec une composante décrochante sénestre au Crétacé supérieur le long des failles de bordure orientées N160° ; elle a induit une érosion du centre du bassin de plus de 3000 m, et un raccourcissement horizontai de l'ordre de 10 %, 4) d'une extension néogène plus limitée. Une analyse détaillée a permis de mettre en évidence seize unités sédimentaires retraçant les mouvements tectoniques et les fluctuations relatives du niveau de la mer au Crétacé dans le bassin. Des corrélations temporelles entre le Broad Fourteens Basin, d'autres bassins intraplaques et la cinématique des plaques aux pourtours de l'Europe de l'Ouest semblent envisageables. La réorganisation des plaques mondiales paraît être à l'origine de variations horizontales des contraintes intraplaques, qui induisent à la fois des réactivations d'accidents préexistants de la croûte fragile et des mouvements verticaux (flambage) de la lithosphère