Continental subductions in Central Tibet : petrological and rheologicalinheritances and the building of a Plateau

Quand et comment le Plateau Tibétain s'est édifié demeure une question complexe, aux nombreuses implications pour la compréhension du comportement des lithosphères continentales en collision. Certains modèles mettent en avant l'importance du sous-plaquage de la lithosphère indienne et d'un épaississement localisé aux limites de micro-plaques asiatiques rigides ; tandis que d'autres considèrent au contraire que la lithosphère asiatique est peu résistante et se déforme de manière distribuée. La base croissante de données de haute qualité documentant les processus de surface et les processus profonds doit à présent être intégrée afin de contraindre les différents modèles d'évolution du Plateau. Ce travail de thèse se concentre sur le nord-est du bloc du Qiangtang, au Tibet Central : alors qu'elle constitue un élément clé pour les reconstructions et les modèles, cette région demeure l'une des moins étudiées de la zone de collision. Dans une première partie, l'acquisition de nouveaux âges 40Ar/39Ar de la déformation tardi-triasique (215-200 Ma) sur la suture de Jinsha, ainsi que la datation et l'étude métamorphique de xénolites crustales à corindon d'âge Trias Inférieur (249 Ma) échantillonnées dans des laves éocènes, mettent en évidence le rôle majeur et sous-estimé des subductions mésozoïques dans l'épaississement crustal total. Les résultats suggèrent que la région avait atteint, avant le début de l'Éocène, une épaisseur crustale de l'ordre de 45 à 55 km, soit près de 80% de son épaisseur actuelle de 66 km. Cet épaississement crustal mésozoique fut probablement en grande partie la conséquence du magmatisme d'arc du NE-Qiangtang, lié à 80 Ma de subductions océaniques à ses bordures. Ces nterprétations sont cohérentes avec les données régionales d'émersion et de paléoaltitude entre le Trias et l'Éocène. Dans une seconde partie, l'étude pétrologique et géochimique de roches magmatiques d'âge Éocène des bassins de Xialaxiu et de Nangqian suggère un intense métasomatisme du manteau lithosphérique source par des fluides ou magmas riches en H2O et en CO2. Ces résultats servent de point de départ à une série de modèles rhéologiques et thermomécaniques visant à caractériser le comportement d'une telle lithosphère dans un contexte de convergence continentale. Nous montrons que le manteau lithosphérique sous notre région d'étude à l'Éocène était à la fois très peu résistant (logmin~2.3), très peu dense (~3310 kg/m3 à 2 GPa), et possédait un solidus fortement défléchi vers les basses températures aux moyennes pressions mantelliques (~930°C à 3 GPa). À partir de ces caractéristiques, les modèles thermo-mécaniques laissent envisager un régime atypique de déformation et de fusion partielle, par l'injection de manteau lithosphérique métasomatisé dans l'asthénosphère sous-jacente. Celui-ci reproduit correctement l'intervalle de temps et la distance entre les épisodes magmatiques de Xialaxiu et de Nangqian, ainsi que la magnitude de l'épaississement crustal dans notre région d'étude, sans impliquer de délamination de la racine lithosphérique adoucie. Ces résultats nous amènent à proposer un réexamen des données géophysiques disponibles sur le manteau lithosphérique tibétain, dans lequel la zone de faibles vitesses sismiques imagée à l'aplomb du Tibet Central pourrait être interprétée en terme d'anomalie chimique et minéralogique, et non d'anomalie thermique.How and when the Tibetan Plateau was built remains a complicated issue, with many implications for our understanding of the behaviour of colliding continental lithospheres. Some models highlight the importance of the underthrusting of the Indian lithosphere and of localized thickening at the edges of rigid Asian microplates ; while others consider that the Asian lithosphere is weak and deforms in a distributed manner. The growing, high-quality database documenting deep and surface processes has now to be integrated in order to constrain the different evolution models of the Plateau.This thesis work focuses on the north-eastern Qiangtang block in Central Tibet : although it is a keystone for reconstructions and models, this region remains one of the least studied of the collision zone.In a first part, new 40Ar/39Ar dating of the Late Triassic (215-200 Ma) deformation on the Jinsha suture, along with a metamorphic study of Early Triassic (249 Ma) corundum-bearing crustal xenoliths sampled in Eocene magmatic rocks, shed light on the the major, but underestimated role played by the Mesozoic subductions in bulk crustal thickening. Our results suggest that prior to the Eocene, our study area had reached a crustal thickness of 45-55 km, i.e 80% of its present-day crustal thickness of 66 km. This Mesozoic crustal thickening was likely achieved by continental arc magmatism related to the 80 Ma of continuous oceanic subductions on both edges of the NE-Qiangtang terrane.These interpretations are consistent with the regional emersion and paleo-altitude data.In a second part, a petrological and geochemical study of Eocene magmatic rocksfrom the Nangqian and Xialaxiu basins suggests an intense metasomatism of the source lithospheric mantle by H2O- and CO2-rich fluids or melts. These results are used as a starting point for a series of rheological and thermo-mechanical models, to characterize the behaviour of such lithosphere in a continental convergence context. We show that the lithospheric mantle underlying our study area in the Eocene was particularly weak (logmin~2.3) and buoyant (~3310 kg/m3 à 2 GPa), and its solidus was deflected to low temperatures at medium mantle pressures (~930°C à 3 GPa). From these characteristics, thermo-mechanical models forecast an atypical deformational and partial melting regime, through the injection of metasomatized lithospheric mantle into the asthenosphere, whichadequately reproduces the timing and location of Xialaxiu and Nangqian magmatic events, and the magnitude of crustal thickening observed in our study area, without any delamination of the weakened lithospheric root.This eventually leads us to reconsider the available geophysical data for the Tibetan lithospheric mantle : the low-velocity zone imaged beneath Central Tibet could indeed represent a geochemical, rather than thermal anomaly

Subductions continentales au Tibet Central : héritages pétrologique, rhéologique et construction d'un plateau.

How and when the Tibetan Plateau was built remains a complicated issue, with many implications for our understanding of the behaviour of colliding continental lithospheres. Some models highlight the importance of the underthrusting of the Indian lithosphere and of localized thickening at the edges of rigid Asian microplates ; while others consider that the Asian lithosphere is weak and deforms in a distributed manner. The growing, high-quality database documenting deep and surface processes has now to be integrated in order to constrain the different evolution models of the Plateau.This thesis work focuses on the north-eastern Qiangtang block in Central Tibet : although it is a keystone for reconstructions and models, this region remains one of the least studied of the collision zone.In a first part, new 40Ar/39Ar dating of the Late Triassic (215-200 Ma) deformation on the Jinsha suture, along with a metamorphic study of Early Triassic (249 Ma) corundum-bearing crustal xenoliths sampled in Eocene magmatic rocks, shed light on the the major, but underestimated role played by the Mesozoic subductions in bulk crustal thickening. Our results suggest that prior to the Eocene, our study area had reached a crustal thickness of 45-55 km, i.e 80% of its present-day crustal thickness of 66 km. This Mesozoic crustal thickening was likely achieved by continental arc magmatism related to the 80 Ma of continuous oceanic subductions on both edges of the NE-Qiangtang terrane.These interpretations are consistent with the regional emersion and paleo-altitude data.In a second part, a petrological and geochemical study of Eocene magmatic rocksfrom the Nangqian and Xialaxiu basins suggests an intense metasomatism of the source lithospheric mantle by H2O- and CO2-rich fluids or melts. These results are used as a starting point for a series of rheological and thermo-mechanical models, to characterize the behaviour of such lithosphere in a continental convergence context. We show that the lithospheric mantle underlying our study area in the Eocene was particularly weak (logmin~2.3) and buoyant (~3310 kg/m3 à 2 GPa), and its solidus was deflected to low temperatures at medium mantle pressures (~930°C à 3 GPa). From these characteristics, thermo-mechanical models forecast an atypical deformational and partial melting regime, through the injection of metasomatized lithospheric mantle into the asthenosphere, whichadequately reproduces the timing and location of Xialaxiu and Nangqian magmatic events, and the magnitude of crustal thickening observed in our study area, without any delamination of the weakened lithospheric root.This eventually leads us to reconsider the available geophysical data for the Tibetan lithospheric mantle : the low-velocity zone imaged beneath Central Tibet could indeed represent a geochemical, rather than thermal anomaly.Quand et comment le Plateau Tibétain s'est édifié demeure une question complexe, aux nombreuses implications pour la compréhension du comportement des lithosphères continentales en collision. Certains modèles mettent en avant l'importance du sous-plaquage de la lithosphère indienne et d'un épaississement localisé aux limites de micro-plaques asiatiques rigides ; tandis que d'autres considèrent au contraire que la lithosphère asiatique est peu résistante et se déforme de manière distribuée. La base croissante de données de haute qualité documentant les processus de surface et les processus profonds doit à présent être intégrée afin de contraindre les différents modèles d'évolution du Plateau. Ce travail de thèse se concentre sur le nord-est du bloc du Qiangtang, au Tibet Central : alors qu'elle constitue un élément clé pour les reconstructions et les modèles, cette région demeure l'une des moins étudiées de la zone de collision. Dans une première partie, l'acquisition de nouveaux âges 40Ar/39Ar de la déformation tardi-triasique (215-200 Ma) sur la suture de Jinsha, ainsi que la datation et l'étude métamorphique de xénolites crustales à corindon d'âge Trias Inférieur (249 Ma) échantillonnées dans des laves éocènes, mettent en évidence le rôle majeur et sous-estimé des subductions mésozoïques dans l'épaississement crustal total. Les résultats suggèrent que la région avait atteint, avant le début de l'Éocène, une épaisseur crustale de l'ordre de 45 à 55 km, soit près de 80% de son épaisseur actuelle de 66 km. Cet épaississement crustal mésozoique fut probablement en grande partie la conséquence du magmatisme d'arc du NE-Qiangtang, lié à 80 Ma de subductions océaniques à ses bordures. Ces nterprétations sont cohérentes avec les données régionales d'émersion et de paléoaltitude entre le Trias et l'Éocène. Dans une seconde partie, l'étude pétrologique et géochimique de roches magmatiques d'âge Éocène des bassins de Xialaxiu et de Nangqian suggère un intense métasomatisme du manteau lithosphérique source par des fluides ou magmas riches en H2O et en CO2. Ces résultats servent de point de départ à une série de modèles rhéologiques et thermomécaniques visant à caractériser le comportement d'une telle lithosphère dans un contexte de convergence continentale. Nous montrons que le manteau lithosphérique sous notre région d'étude à l'Éocène était à la fois très peu résistant (logmin~2.3), très peu dense (~3310 kg/m3 à 2 GPa), et possédait un solidus fortement défléchi vers les basses températures aux moyennes pressions mantelliques (~930°C à 3 GPa). À partir de ces caractéristiques, les modèles thermo-mécaniques laissent envisager un régime atypique de déformation et de fusion partielle, par l'injection de manteau lithosphérique métasomatisé dans l'asthénosphère sous-jacente. Celui-ci reproduit correctement l'intervalle de temps et la distance entre les épisodes magmatiques de Xialaxiu et de Nangqian, ainsi que la magnitude de l'épaississement crustal dans notre région d'étude, sans impliquer de délamination de la racine lithosphérique adoucie. Ces résultats nous amènent à proposer un réexamen des données géophysiques disponibles sur le manteau lithosphérique tibétain, dans lequel la zone de faibles vitesses sismiques imagée à l'aplomb du Tibet Central pourrait être interprétée en terme d'anomalie chimique et minéralogique, et non d'anomalie thermique

Xenoliths in Eocene lavas from Central Tibet record carbonatedmetasomatism of the lithosphere

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Paleomagnetic constraints on early collisional deformation along theeastern margin of the Qiantang terrane (Tibetan plateau) at 50 and 37 Ma.

International audienceOngoing controversies on the timing and latitude of the India-Asia collision with associated formation of theTibetan plateau have major implications on geodynamic, climatic and biotic models. Rock paleomagnetic inclinationsand declinations enable in principle to quantify respectively paleolatitudes and tectonic rotations. However,shallow paleomagnetic inclinations observed for most of the Cenozoic rocks across the active belts of Central Asiahave been controversially interpreted as resulting from non dipolar geomagnetic fields, inclination flattening in thesedimentary data or large scale continental deformation. In addition tectonic rotations from the Eastern margin ofTibet may result from extrusion or dextral shear associated with implication on the early collision.We present new paleomagnetic results from two Cenozoic basins of the Eastern part of the Qiantang block characterizedby two short-lived volcanic fields at 37-38Ma (Nangqian area) and 49-51Ma (Xialaxiu area).In the Xialaxiu area, we sampled the volcanic field near the town of Xialaxiu and red beds filling the Sangalaxiubasin 10 to 20km farther north. Results from the red beds after tilt correction (D=328.3, I=34.3, 95=7.6) confirmthe result (D=322.0, I=32.3, 95=9.5) previously obtained by Cogne et al., (1999) but the age and natureof the characteristic magnetization are uncertain. The mean direction calculated from 21 sites in volcanic rocksprovides a more reliable paleofield (D=11.9, I=41.6, 95=8.0). Comparison with the expected direction forstable Eurasia suggest no rotation but significant post 50 Ma shortening north of the Qiantang block in agreementwith results from the Lhasa terrane at the same age (56-47 Ma) (van Hinsbergen et al., 2012).In the Nangqian basin, paleomagnetic sites have been collected in red beds sediments, sills and dikes intruding thered bed sequence and in extrusive volcanic rocks mainly found on top of the sedimentary sequence. A well-definedsecondary component of magnetization with normal polarity was recovered in the red beds in the temperature range150-600C. All sites in magmatic rocks, except one have also a normal polarity. The remagnetizations in the redbeds appear to be related to the volcanic event and confirm previous field interpretations (Spurlin et al., 2005)indicating that volcanic activity occurred at the end of a phase of deformation in the syntectonic Nangqian basin.The mean paleomagnetic inclination in volcanic rocks and in the remagnetized red beds indicate less than 5 ofshallowing in agreement with estimated shortening north of the Qiantang block and do not support the hypothesisof a large inclination anomaly at 37Ma.In summary, observed paleomagnetic inclinations are consistent with 1000 km of post 50 Ma shortening north ofQiangtang but declination are inconsistent with models of deformation implying a large component of extrusionor dextral shear despite most structures are NW-SE oriented along the eastern edge of the Qiantang block.Cogné et al., Geophys. J. Int., 192, 2103; Cogné et al., J. Geophys. Res., 104,B8, 1999; Spurlin et al. Geol. Soc.Am. Bull., 117, 2005; Van Hinsbergen et al., Proc. Nat. Acad. Sci., 109, 201

Unravelling granulitic conditions in Early Triassic crustal xenoliths from NE-Qiangtang, Tibet.

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Nature and melting processes of the lithosphere beneath the North-East Qiangqtang terrane, Central Tibet, during Eocene times.

International audienceAt the time of the collision with India (55Ma), the southern margin of Asia was a composite continental domainresulting from an already long history of successive accretions of different terranes having different rheologies.Knowledge about the structure, composition and thermal state of the Tibetan lithosphere through time is thusfundamental to understand the respective contributions of pre-Cenozoïc and Cenozoïc tectonics in the building ofthe Plateau to its present-day elevations.We focused on the boundary between the Qiangtang terrane to the south, and the Songpan-Ganze terrane to thenorth.We jointly studied deep crustal xenoliths and associated (ultra-)potassic magmatism from the Eocene basinsof Nangqian and Xialaxiu (Qinghai Province, China), north of the Qiangtang terrane. The aims were to retrievethe composition and the thermal state of the lower crust during Eocene times, to study the behavior of the lowercrust and lithospheric mantle of the Eastern Qiangtang terrane and the adjacent Songpan-Ganze terrane at the timeof the collision, and the link with the magmatic activity.Crustal xenoliths are of two types: biotite-rich, amphibole bearing metasediments; and garnet-bearing quartzofeldspathicgneisses. Such assemblages are typical of very high-grade amphibolite and granulite faciesmetamorphism; further study should allow us to quantify the pressures and temperatures those rocks experienceduntil the time they were sampled by their host lavas.Major element geochemistry places the c.a. 51-49 Ma (Spurlin et al., 2005) Xialaxiu volcanic field in a fairlydifferentiated (SiO265-70 wt%) high-K field of the calc-alcaline series. Trace element analysis suggests a strongcrustal contamination of the primary mantellic melts. C.a. 38-37 Ma (Spurlin et al., 2005) Nangqian magmaticbodies span across the alkaline series, with high to extreme (K2O6wt%) values. Complex major and traceelement patterns, coupled with high-resolution microprobe data on pyroxene xenocrysts, suggest that enrichmentoccurred at the source by metasomatism of the lithospheric mantle.Further work will precise which mechanisms could have accounted for such a metasomatism of the mantle beneaththe Northern Qiangtang terrane during Eocene, and whether the lower crust had an autochtonous or allochtonousnature

Paleomagnetic constraints on the tectonicsof the Eastern border of the Qiangtangterrane (Tibetan Plateau) during theIndia-Asia collision

National audiencePaleomagnetism should be a useful tool to estimate northsouthconvergence across the Himalaya and Tibetan Plateau. Unfortunatelythe large negative inclination anomaly for most of theCenozoic rocks has been the subject of numerous controversies(non dipolar geomagnetic fields, inclination flattening in sedimentsor large scale continental deformation of Eurasia). We presentnew paleomagnetic results from two Cenozoic basins of the Easternpart of the Qiangtang terrane characterized by two short-livedvolcanic fields at ~37-38 Ma (Nangqian area) and 49-51 Ma (Xialaxiuarea). In the Nangqian basin, sites have been collected inred beds sediments, sills and dikes intruding the red beds and inextrusive volcanic rocks mainly found on top of the sedimentarysequence. A well-defined secondary component of magnetizationwith normal polarity was recovered in the red beds in the temperaturerange ~150-600C. 24 out of 25 sites in magmatic rocks,have a primary magnetization of normal polarity. The remagnetizationin the red beds is thus clearly related to the volcanic event.The paleomagnetic data confirm previous field interpretations indicatingthat volcanic activity occurred at the end of a phase ofdeformation in the syntectonic Nangqian basin. There is no inclinationanomaly in the mean paleomagnetic result (D=18.0,I=50.1 a95 =7.9) from 25 sites in volcanic rocks when comparedto the Eurasian reference pole. This paleomagnetic resultis confirmed by the remagnetization in red beds.In the Xialaxiu area, we sampled the volcanic field and red bedsfilling a basin 10 to 20 km farther north. Results from the red bedsconfirm the result (D=322.0, I=32.3, a95=9.5) previously obtainedat this location by Cogné et al. (1999). However, the meandirection from 21 sites in volcanic rocks is different (D=11.9,I=41.6, a95=8.0) from that in the red beds suggesting differentage of magnetization.In conclusion, the mean inclination in ~50 Ma volcanics at Xialaxiuis similar to that obtained by Dupont-Nivet et al. (2010) for theLhasa terrane at the same age suggesting about 1100 km ± 500N-S convergence with stable Eurasia. However, our results fromthe ~37 Ma Nangqian rocks yield a higher mean inclination implyinga paleolatitude similar to expected for Eurasia at this time.This suggests that significant N-S convergence detectable by paleomagnetismdid not occur north of the Qiangtang terrane after37 Ma. Taken at face values, our results rather suggest that mostof the ~1000 km convergence occurred north of the Qiangtangterrane before 37 Ma but we cannot exclude that this potentialinclination anomaly affected both ~50 Ma Qiangtang and Lhasaterrane results. Finally, despite most regional structures beingNW-SE oriented, there is no evidence for consistent clockwise rotationsas expected with a model of deformation implying a largecomponent of extrusion or dextral shear along the eastern marginof the Tibetan Plateau

Linking the P-T evolution of granulites andthe geochemistry of ultra-K volcanics fromBohemian Massif and Central Tibet

National audienceVariscan orogeny is traditionally compared to Himalayan-Tibetan orogeny thanks to two main reasons: 1) Variscan orogenoriginated through progressive amalgamation of Gondwanaderived blocks which formed belt that was subsequently squeezedbetween two continental plates - Laurussia and Gondwana. Similarsequence of events is characteristic for Tibetan-Himalayan orogenyas well, where previously Gondwana accreted blocks were squeezedbetween Asian continent and India. Therefore, both orogens resultfrom similar reorganization of plates in Paleozoic and Mesozoic-Cenozoic times, which initiated rapid northward drift of Gondwanaderived blocks and their final collision with northern continentalmasses. 2) This process resulted in both cases in formation orogenicroot of double-crust thickness characterized by long lasting(HP) granulite facies metamorphism in lower crust followed byisothermal decompression still at granulite facies conditions. Inaddition, the granulites in central Tibet were transported by highpotassium lavas, with geochemical and isotopical signature correspondingto Mg-K granitoids typically associated to HP granulitesin the eastern part of the Variscan belt (the Bohemian Massif inparticular). In both cases, the origin of granulitic lower crust andassociated high potassium lavas are attributed to relamination andthermal maturation of lower crustal allochthon underthrust belowupper plate crust during collision. Based on above similaritieswe argue that the Devonian - Carboniferous evolution of the BohemianMassif may be a proxy of Eocene to recent crustal dynamicsof the Tibetan system. To better quantify similarities betweenboth orogens, we present a set of numerical models which reproducethe tectonic multistage scenario proposed for the Variscantectonics of Bohemian Massif. These models are subsequentlycompared with P-T data acquired from the Himalaya and southernTibetangranulites and xenoliths from central Tibet