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

Titanium isotopes as a tracer for the plume or island arc affinity of felsic rocks

F.M. acknowledges funding from the European Research Council (ERC) under Horizon 2020 Framework Programme/ERC Grant Agreement 637503 (Pristine). F.M. and M.C. acknowledge the financial support of the UnivEarthS Labex Program at Sorbonne Paris Cité (Grants ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). Parts of this work were supported by IPGP Plateau d’Analyse haute Résolution (PARI) and by Region Île-de-France Sesame Grant 12015908.Indirect evidence for the presence of a felsic continental crust, such as the elevated 49Ti/47Ti ratios in Archean shales, has been used to argue for ongoing subduction at that time and therefore plate tectonics. However, rocks of intermediate to felsic compositions can be produced in both plume and island arc settings. The fact that Ti behaves differently during magma differentiation in these two geological settings might result in contrasting isotopic signatures. Here, we demonstrate that, at a given SiO2 content, evolved plume rocks (tholeiitic) are more isotopically fractionated in Ti than differentiated island arc rocks (mainly calc-alkaline). We also show that the erosion of crustal rocks from whether plumes (mafic in average) or island arcs (intermediate in average) can all produce sediments having quite constant 49Ti/47Ti ratios being 0.1–0.3 per mille heavier than that of the mantle. This suggests that Ti isotopes are not a direct tracer for the SiO2 contents of crustal rocks. Ti isotopes in crustal sediments are still a potential proxy to identify the geodynamical settings for the formation of the crust but only if combined with additional SiO2 information.PostprintPeer reviewe

Interactions entre phénomènes tectoniques et magmatiques en contexte d'extension (l'exemple du segment de rift de Dabbahu)

Dans cette thèse nous quantifions les interactions entre phénomènes tectoniques et magmatiques au niveau d'un rift proche de la rupture continentale: celui du Dabbahu / Manda Hararo en Afar (Ethiopie), qui a été le siège d'un épisode de rifting majeur en 2005. Nous apportons ici des contraintes temporelles sur ces interactions à l'échelle du millier à la dizaine de millier d'années, afin de mieux comprendre l'évolution des processus responsables de l'acquisition de la topographie. Pour cela nous avons produits des âges d'expositions de surfaces de coulées de laves et d'escarpement de failles par la technique des isotopes cosmogéniques. Ces datations ont été combinées à des travaux de cartographie et de géochimie (éléments majeurs et traces). Nous montrons que nord du segment est sous l'influence de la présence d'un volcan terminal qui impacte le développent de la topographie. Le milieu du segment permet d'évaluer l'impact des variations spatiales et temporelles de la localisation des réservoirs sur la topographie. Le long du segment, il apparaît que la topographie se développe quand les réservoirs magmatiques locaux ont une activité modérée à faible, alors que la topographie est inhibée lors des phases de magmatisme intense. Nos observations ont permis de monter que l'épisode de rifting de 2005 n'est pas représentatif de l'évolution globale de la morphologie du rift, et que ce type d'intrusion ne représente sans doute qu'un dixième des injections totales. Enfin, l'accrétion magmatique est soutenue à l'axe par différents réservoirs magmatiques qui semblent se relayer dans le temps, et dont la position peut varier de 15 kilomètres de part et d'autre de l'axeIn this thesis we quantify the interactions between tectonic and magmatic processes in the active Dabbahu rift segment, which experienced in 2005 a major rifting event. Here, we provide accurate constraints at this time scale by producing cosmogenic exposure ages of lavas surfaces and faults scarps, in order to decipher the morphological evolution of a rift depression, and to identify the processes that are responsible for the topography acquisition. The study is based on samples taken on two separate areas of the rift. The field areas were systematically mapped, and all lavas were chemically analysed in major and trace elements. The first studied focused the northern portion of the rift segment, where the lavas of the segment terminating volcano meets the neo-volcanic zone. This area allowed the influence of the presence of such volcanic edifice to be assessed. The second studied area is located at the middle of the segment, and covers the rift western margin up to the rift axis. The topography in this area is sensitive to the evolution in space and time of the location of the different axial magma reservoirs. Our results show that along the rift segment, topography develops during phases of low magmatic activity, and is inhibited during phases of lower magmatic activity. The 2005 appears to be an unusual event that represents about a tenth of the total intrusions necessary to build the depression. The maintaining of magmatic accretion at the axis seems to be the result of a relay between different magma reservoirs. The position of these reservoirs can vary between 0 and 15 km from both sides of the rift axisMETZ-SCD (574632105) / SudocNANCY1-Bib. numérique (543959902) / SudocNANCY2-Bibliotheque electronique (543959901) / SudocNANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

Détermination des paléo-taux d'érosion par l'utilisation des isotopes cosmogéniques. Cas de la transition pliocène-pleistocène

L'analyse dans des dépôts anciens des isotopes cosmogéniques in-situ, déjà largement employés dans les sédiments de rivières actuelles, a le potentiel de fournir des enregistrements haute résolution, sur des échelles de temps de 10 Ma, de taux de dénudations intégrés sur des bassins versant entiers. Nous avons étendu avec succès cette méthode à des sédiments des piedmonts du Tian-Shan (Chine) et de l'Himalaya (collines Siwaliks, Népal). Ces deux zones sont au coeur de débats sur l'évolution des relations climat/érosion/tectonique depuis ~10 Ma. Quatre sections au Tian-Shan, couvrant ~9 Ma, et une large étendue spatiale autour de cette chaîne, ne montrent pas de brusque accélération de l'érosion il y a ~3-5 Ma. Au contraire, nous observons à l'échelle régionale une augmentation progressive (x 4 entre 9 et 4 Ma) puis une stabilisation entre 4 Ma et l'actuel. Ces résultats suggèrent une influence limitée des cycles glaciaires quaternaires sur les taux d'érosion dans cette zone. Un enregistrement dans les sédiments himalayens ne montre également pas d'augmentation significative de l'érosion depuis ~6.5 Ma. Il est en revanche probablement le témoin de réorganisation des systèmes de drainages et d'évolutions tectoniques dans cette région. Avec ces études, nous avons ainsi pu produire pour la première fois des enregistrements haute résolution de taux d'érosion depuis ~10 Ma, grâce aux isotopes cosmogéniques. Cette méthode étant potentiellement applicable dans de nombreux contextes géologiques et climatiques, elle permettra indubitablement des avancées importantes dans la compréhension des processus de surface actuels et passésIn-situ cosmogenic nuclides analysis in ancient sediment could potentially provide high-resolution records of denudation rates, integrated over whole drainage basins, and on several million-year time scales. We successfully extended this method to sediments from the Tianshan (China), and the Himalayas (Nepal) piedmonts. These two areas are at the core of ongoing debates on climate-erosion-tectonics relationships since ~10 Ma. Four sections in the Tianshan, covering ~9 Myr and a broad spatial extent around this range, do not display a brutal increase in denudation at 3-5 Ma. On the other hand, they show at the regional scale a progressive increase (x4 between 4-9 Ma) and a subsequent stabilization since ~4 Ma. These results suggest a limited influence of Quaternary glaciations on erosion in this region. One section in the Himalayan piedmont (Siwaliks hills), neither display any significant increase since ~6.5 Ma that would be link to a major climatic shift. It has nevertheless probably recorded reorganizations of the drainage system and tectonics evolutions in this region. With these studies, we were able to produce the first cosmogenic nuclides-based high resolution records of denudation rates over 10 Myr. This method being potentially applicable in many geological and climatic settings, it will indubitably allow further advances in our understanding of past and present surface processesNANCY-INPL-Bib. électronique (545479901) / SudocSudocFranceF

Thermo-tectonic evolution of the south-central Pyrenees from rifting to orogeny: insights from detrital zircon U/Pb and (U-Th)/He thermochronometry

International audienceConstraining the thermal and denudational evolution of continental margins from extensional episodes to early orogenic stages is critical in the objective to better understand the sediment routing during the growth of orogenic topography. Here, we report 160 detrital zircon U/Pb ages and 73 (U-Th)/He ages from Albian, Upper Cretaceous and Eocene sandstones from the south-central Pyrenees. All samples show dominant zircon U/Pb age peaks at 310-320 Ma, indicating a primary contribution from Variscan granites of the central Pyrenean Axial Zone. A secondary population at 450-600 Ma documents zircon grains sourced from the eastern Pyrenees. Zircon (U-Th)/He ages recovered from older samples document, a Triassic age peak at ca. 241 Ma, corresponding to denu- dation coeval with the initiation of Atlantic rifting. An Early Cretaceous cooling event at ca. 133 Ma appears consistent with rift-related exhumation and thermal overprint on the Iberian margin. The (U-Th)/He age peaks from ca. 80 Ma to ca. 68 Ma with decreasing depositional ages are inter- preted to reflect the southward-migrating thrust-related exhumation on the pro-wedge side of the Pyrenean orogen. The increase in lag times, from ca. 15 Ma in the Tremp Formation (ca. 65 Ma) to 28 Ma in the Escanilla Formation (ca. 40 Ma), suggests decreasing exhumation rates from 0.4 km Myr-1 to 0.2 km Myr-1. The apparent inconsistency with convergence rates is used to infer that rocks cooled at 68 Ma may have resided in the crust before final exhumation to the surface. Finally, the cooling event observed at 68 Ma provides support to the inferred acceleration of conver- gence, shortening and exhumation during Late Cretaceous times