Western and southeastern Tibetan plateau - geomorphic and sedimentologic evolution through Cenozoic times

Le Tibet est le plateau le plus élevé et le plus étendu au monde. La formation de ce plateau, en arrière de l’Himalaya, résulte d’interactions complexes entre facteurs tectoniques et climatiques, ainsi que de la morphologie antérieure au soulèvement. Afin d’évaluer l’influence relative de ces différents facteurs, cette thèse s’appuie sur l’étude de l’évolution du relief des bordures du plateau en couplant analyse géomorphologique, étude de la sédimentation syn-formation du plateau et reconstitution de l’exhumation à partir de la thermochronologie de basse température.Cette approche a permis de mettre en évidence que le plateau du Tibet était déjà haut, aussi bien sur ses bordures est que ouest dès 35 Ma, soit seulement 20 Ma après la collision Inde-Asie. Il apparait donc que le plateau se serait soulevé soit en un bloc, soit de façon précoce par ses marges Ouest et Est, plutôt qu’en se propageant du sud vers le nord et vers l’est comme proposé par de nombreux modèles.Dans l’Ouest Tibet, l’existence d’un réseau de drainage anciennement connecté avec celui de l’Indus, a permis le développement précoce d’un relief significatif (supérieur à 1000 m) avant 35 Ma lors de la surrection du plateau. Ce relief est ensuite préservé dans un contexte d’érosion très faible (quelques dizaine de mètres par million d’années) associé à une évacuation des produits d’érosion vers le bassin de l’Indus. Cette connexion avec l’Indus est ensuite coupée probablement suite aux mouvements de la faille du Karakorum.A l’Est, la formation du relief est probablement plus ancienne que dans l’Ouest Tibet, car vers 35 Ma cette région, bien que déjà surélevée, est caractérisée par l’existence d’un vaste réseau fluviatile en tresse, impliquant une faible pente, ainsi qu’un relief local soumis à des précipitations plus au nord. La création du relief actuel, marqué par des rivières fortement encaissées, est probablement liée à l’évolution de la mousson sud-est asiatique ainsi qu’au fonctionnement de la faille du Fleuve rouge.Tibet is the widest and highest plateau on Earth. Tectonics, climate evolution and ante-surrection geomorphology are the main factors controlling the plateau formation. In order to assess the relative influence of these factors, we study the relief evolution on the plateau edges using geomorphic analysis, sedimentology and exhumation rates based on low-temperature thermochronometry.The results show that the western and eastern plateau edges were already at high elevation at ca 35 Ma, only 20 Ma after the India-Asia collision. This favors an “en bloc” uplift model for the plateau.In western Tibet, the hydrographic network was connected to the Indus river, allowing the early development of a >1000 m amplitude relief, probably before 35 Ma. The relief was preserved due to low erosion conditions. Western Tibet was then isolated from the Indus drainage network due to the Karakorum fault slip.The relief formation in Eastern Tibet is older than in western Tibet: at ca 35 Ma, in the Jianchuan area (northern Yunnan), which was already at high elevation, was a large braided river system. This implies a moderate regional slope. It also implies a local relief further north and significant precipitations

Évolution morphologique et sédimentologique des bordures ouest et sud-est du plateau du Tibet

Tibet is the widest and highest plateau on Earth. Tectonics, climate evolution and ante-surrection geomorphology are the main factors controlling the plateau formation. In order to assess the relative influence of these factors, we study the relief evolution on the plateau edges using geomorphic analysis, sedimentology and exhumation rates based on low-temperature thermochronometry.The results show that the western and eastern plateau edges were already at high elevation at ca 35 Ma, only 20 Ma after the India-Asia collision. This favors an “en bloc” uplift model for the plateau.In western Tibet, the hydrographic network was connected to the Indus river, allowing the early development of a >1000 m amplitude relief, probably before 35 Ma. The relief was preserved due to low erosion conditions. Western Tibet was then isolated from the Indus drainage network due to the Karakorum fault slip.The relief formation in Eastern Tibet is older than in western Tibet: at ca 35 Ma, in the Jianchuan area (northern Yunnan), which was already at high elevation, was a large braided river system. This implies a moderate regional slope. It also implies a local relief further north and significant precipitations.Le Tibet est le plateau le plus élevé et le plus étendu au monde. La formation de ce plateau, en arrière de l’Himalaya, résulte d’interactions complexes entre facteurs tectoniques et climatiques, ainsi que de la morphologie antérieure au soulèvement. Afin d’évaluer l’influence relative de ces différents facteurs, cette thèse s’appuie sur l’étude de l’évolution du relief des bordures du plateau en couplant analyse géomorphologique, étude de la sédimentation syn-formation du plateau et reconstitution de l’exhumation à partir de la thermochronologie de basse température.Cette approche a permis de mettre en évidence que le plateau du Tibet était déjà haut, aussi bien sur ses bordures est que ouest dès 35 Ma, soit seulement 20 Ma après la collision Inde-Asie. Il apparait donc que le plateau se serait soulevé soit en un bloc, soit de façon précoce par ses marges Ouest et Est, plutôt qu’en se propageant du sud vers le nord et vers l’est comme proposé par de nombreux modèles.Dans l’Ouest Tibet, l’existence d’un réseau de drainage anciennement connecté avec celui de l’Indus, a permis le développement précoce d’un relief significatif (supérieur à 1000 m) avant 35 Ma lors de la surrection du plateau. Ce relief est ensuite préservé dans un contexte d’érosion très faible (quelques dizaine de mètres par million d’années) associé à une évacuation des produits d’érosion vers le bassin de l’Indus. Cette connexion avec l’Indus est ensuite coupée probablement suite aux mouvements de la faille du Karakorum.A l’Est, la formation du relief est probablement plus ancienne que dans l’Ouest Tibet, car vers 35 Ma cette région, bien que déjà surélevée, est caractérisée par l’existence d’un vaste réseau fluviatile en tresse, impliquant une faible pente, ainsi qu’un relief local soumis à des précipitations plus au nord. La création du relief actuel, marqué par des rivières fortement encaissées, est probablement liée à l’évolution de la mousson sud-est asiatique ainsi qu’au fonctionnement de la faille du Fleuve rouge

River incision on volcanoes: Case study of La Réunion, Mauritius and Rodrigues islands, Indian Ocean

International audienc

River incision on volcanoes: Case study of La Réunion, Mauritius and Rodrigues islands, Indian Ocean

International audienc

River incision on volcanoes: Case study of La Réunion, Mauritius and Rodrigues islands, Indian Ocean

International audienc

Evolution of the Yangtze River network, southeastern Tibet: Insights from thermochronology and sedimentology

International audienceWe performed apatite and zircon (U-Th)/He dating on a granitic pluton that has been offset by ~10 km by motion on the sinistral strikeslipXiangcheng fault in SW Sichuan, SE Tibetan plateau, where the Shuoqu River incises a deep valley before joining the upper YangtzeRiver. Mean ZHe cooling ages range from 49.5 ± 2.2 Ma to 68.6 ± 6.0 Ma. Samples located above 3870 m yield mean apatite (U-Th)/Heages ranging from 30.6 ± 1.4 Ma to 40.6 ± 2.7 Ma, whereas samples at lower elevations range from 9.8 ± 1.3 Ma to 14.6 ± 2.7 Ma. In thesame region, Cenozoic continental sediments are exposed on the flanks of deep valleys. They consist of unsorted conglomerates andsandstones that partly fill a paleotopography. The sediments were deposited during an episode of rapid sedimentation, followed by incisionthat varies between 0.5 and 1.2 km. Thermal and exhumational modeling of the granite thermochronometric data indicates rapidcooling during the middle Miocene that was likely related to fluvial incision. Our findings suggest that the upper Yangtze River and itstributary (Shuoqu) were connected by the middle Miocene. Our modeling also supports the idea that the exhumation pattern during theCenozoic in the southeastern margin of the Tibetan Plateau is spatially and temporally heterogeneous

River network evolution as a major control for orogenic exhumation: Case study from the western Tibetan plateau

International audienceThe westernmost Tibetan plateau, despite being internally drained, has a high topographic relief. Here, using apatite (U–Th–Sm)/He and 4He/3He thermochronometry, we reconstruct the exhumation history of the Rutog batholith during the Neogene. Thermal modeling in 1D using the QTQt program indicates that relatively slow cooling occurred from 30 Ma to 19 Ma, which we interpret as an exhumation rate of ∼10 m/Ma. This was followed by two pulses of moderate cooling from 19 to 17 Ma and ∼11 to 9 Ma that correspond to a total exhumation of about 1500 m. Cooling since 9 Ma has been negligible. This differs from exhumation patterns in central Tibet but reveals timing similarities with externally drained portions of southern Tibet. We interpret our cooling constraints as recording two different transitions in western Tibet from an externally to an internally drained system since the Oligocene. External drainage allowed this part of the Tibetan plateau, unlike internally drained portions of central Tibet, to record regional-scale processes. The first cooling event, at about 20 Ma, was likely related to a major geodynamic event such as slab breakoff that induced contemporaneous potassic and ultrapotassic magmatism. The second rapid cooling pulse from ∼11 Ma to 9 Ma and subsequent negligible cooling was most likely controlled by a local factor such as Indus and Shyok river network reorganization caused by dextral motion of the Karakorum fault. We discuss these interpretations and their limitations in this contribution

Tectonic-geomorphology of the Litang fault system, SE Tibetan Plateau, and implication for regional seismic hazard

The Litang fault system (LTFS) in the eastern Tibetan Plateau has generated several large (7.5\textgreaterM\textgreater7) historical earthquakes and has exhumed granitic peaks rising \textgreater1700 m above the mean elevation of the plateau, despite being located within a tectonic block surrounded by highly active faults. We study horizontally offset moraine crests from the Cuopu basin and a vertically offset alluvio-glacial fan from the eastern Maoya basin. We determine a left-lateral rate of 0.09 +/- 0.02 mm/yr along a slowly slipping secondary fault at Cuopu, while the main active fault at present is the normal range-front N Cuopu fault, along which we determined a left-lateral rate of 2.3 +/- 0.6 mm/yr since 173 ka. At Maoya fan, matching the vertical 12 +/- 1 m cumulative offset with the 21.7 +/- 4.2 ka fan age yields a vertical (normal) rate of 0.6 +/- 0.1 mm/yr. This rate is very similar to that recently determined at the same location using low-temperature thermochronology (0.59 +/- 0.03 mm/yr since 6.6 +/- 05 Ma). Left-lateral rates along the main faults of the LTFS range between 0.9 and 23 mm/yr at all time scales from a few years to similar to 6 Ma. The facts that the LTFS is highly segmented and that at present, the Cuopu, Maoya and South Jawa segments are mostly normal (while the Litang and Dewu segments are left-lateral/normal), could prevent the occurrence of M\textgreater7.5 destructive earthquakes along the LTFS, as is generally assumed. However, motion on the normal faults appears to be linked with motion on the strike-slip faults, potentially allowing for exceptional larger earthquakes, and implying that the area is not experiencing pure similar to NS extension but rather NW-SE left-lateral transtension. (C) 2016 Elsevier B.V. All rights reserved

Western Tibet relief evolution since the Oligo-Miocene

International audienceWestern Tibet, between the Karakorum fault and the Gozha–Longmu Co fault system, is mostly internally drained and has a 1.5–2 km amplitude relief with km-large valleys. We investigate the origin of this peculiar morphology by combining a topography analysis and a study of the Cenozoic sedimentation in this area. Cenozoic continental strata correspond to a proximal, detrital fan deposition, and uncomformably rest on a palaeorelief similar to the modern one. Zircon U–Pb dating from trachytic flows interbedded within the Cenozoic continental sediments indicates that detrital sedimentation occurred at least between ca 24 and 20 Ma in the Shiquanhe basin, while K/Ar ages suggest it may have started since ~ 37 Ma in the Zapug basin. The distribution of continental deposits shows that present-day morphology features, including km-large, 1500 m-deep valleys, were already formed by Early Miocene times. We suggest that today's internally drained western Tibet was externally drained, at least during late Miocene, contemporaneously with early motion along the Karakorum Fault. Detailed study of the present day river network is compatible with a dextral offset on the Karakorum Fault of 250 km at a rate of ~ 10 ± 1 mm/yr. Displacement along the Karakorum fault possibly induced the shift from external to an internal drainage system, by damming of the Bangong Co ~ 4 Ma ago, leading to the isolation and preservation of the western Tibet relief