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The Tibetan Plateau was formed by intense Cenozoic shortening (up to 1100 km) of a composite “proto-Tibet”, itself the product of a long Paleozoic and Mesozoic history of accretion of Gondwana-derived continental fragments and volcanic arcs against the Asian continental margin. The difficult access and the scarcity of outcrops have long limited the possibilities of studying these Mesozoic suture zones in the heart of the Plateau. In this work, we present new U-Pb and 40Ar/39Ar ages from the highly deformed units of the Yushu mélange, along the Jinsha Suture in the northeastern Qiangtang terrane. Early Triassic (c. 253 Ma) to Middle Jurassic ages (c. 165 Ma) complement the existing dataset and help to refine the chronology of the Paleo-Tethyan oceanic subductions which have structured the northeastern part of the Qiangtang terrane. The Yushu mélange records at least three successive tectono-magmatic events. The opening of a back-arc basin during the northward Paleo-Tethyan subduction along the Longmu Co-Shuanghu Suture during Early to Middle Triassic; then its closure during the southward subduction of the Songpan-Ganze Ocean along the Jinsha Suture in Late Triassic. Finally, a shortening phase related to the continental collision of the Songpan-Ganze and Qiangtang blocks from Late Triassic to Early-Middle Jurassic. No evidence for any high- or mid-temperature Cenozoic reactivation of the Jinsha suture in our study area is recorded.

International audience; Plates and the convective mantle interact with each other over geological time scales, leading to mantle flow, plate motion, and deformation along plate boundaries. At convergent boundaries undergoing subduction, the role played by mantle drag remains poorly understood despite its potential impact on subduction dynamics, and in turn on the deformation regime of the overriding plate. Previous studies were generally conducted in two dimensions, limiting their ability to faithfully reproduce processes taking place on Earth. Instead, in this study, we present 11 three-dimensional analog models of subduction at the scale of the upper mantle, including an overriding plate, and in which we control mantle drag at the base of the lower or upper plate by imposing a controlled unidirectional background mantle flow perpendicular to the trench. We systematically vary the velocity and the direction of the imposed horizontal mantle flow and quantify its impact on horizontal and vertical upper plate deformations, plate and subduction velocities, and the geometry of the slab. The geometry of the slab is only marginally affected by the velocity and direction of the mantle flow. In the absence of mantle flow, slab rolls back and deformation is accommodated by trench-orthogonal stretching in the upper plate. Instead, the addition of a background flow dragging the lower or upper plate toward the trench systematically results either in the absence of upper plate deformation, or in trench-orthogonal shortening with strain rates that increase linearly with increasing mantle flow. We show that the upper plate strain rate is primarily controlled by the velocity of the free plate in the model, which itself results from the drag exerted by the mantle at the base of the plate. Coupling between mantle and plate is larger for models with flow directed toward the upper plate, resulting in strain rates that are about three times larger than for equivalent models with flow directed toward the lower plate. This systematic study provides a better understanding of the effect of mantle drag on plate displacements and deformation along subduction zones, leading to a better understanding of the ingredients required to form Andean-type mountain ranges.

To explore the conditions that lead to mountain-building in the case of an oceanic subduction, we conduct analog experiments (with silicon putty upper and lower plates, glucose syrup upper mantle) where subduction is driven by slab pull but also by an underlying mantle flow. Here, plate displacement is not imposed as in most models, but is controlled by the overall balance of forces in the system. We simulate three scenarios: no mantle flow (slab-pull driven subduction), mantle flow directed toward the subducting plate, and mantle flow directed toward the overriding plate. In the case of this latter scenario, we also test the influence of pre-existing rheological contrasts in the upper plate to best reproduce natural cases where inheritance is common. Our experiments show that when plate convergence is also driven by a background mantle flow, the continental plate deforms with significant trench-orthogonal shortening (up to 30% after 60 Myr), generally associated with thickening. We further identify that upper plate shortening and thickening is best promoted when the mantle flow is directed toward the fixed overriding continental plate. The strength of the upper plate is also a key factor controlling the amount and rates of accommodated shortening. Deformation rates increase linearly with decreasing bulk strength of the upper plate, and deformation is mostly localized where viscosity and strength are lower. When compared to the particular natural case of the Andes, our experiments provide key insights into the geodynamic conditions that lead to the building of this Cordilleran orogen since the Late Cretaceous - Early Cenozoic.

Despite decades of controversy, our understanding of the formation of the Tibetan Plateau remains limited. The role of competing mechanisms, such as distributed crustal thickening versus lateral propagation of thrust faulting at crustal or lithospheric scales, is still poorly understood. Conceptual models explaining observations at the continental scale are based on hypotheses that are hard to reconcile, on the one hand buoyancy forces dominating with low influence of upper crustal faulting, on the other hand faults dominating by favour discrete propagation of rigid upper crustal thickening since the onset of collision at ~50 Ma. However, in view of the 3D nature and temporal complexity of the involved deformation processes, no numerical model taking into account the role of strike-slip faults in accommodating stepwise evolution of thrust faulting, as well as the interaction between the deep crust and the surface, has yet been implemented. Therefore, it remains difficult to test the mechanical and rheological consistency, and the ability to explain observations, of end-member conceptual models at the scale of the Tibetan Plateau.In order to generate new insights in deformation modes in Tibet, I will present models to study the mechanical behaviour in the lower crust of the upper crustal thrust faults observed along the Tibet eastern edge, which setup is based upon recent thermo-kinematic modelling of thermochronology data (Pitard et al., 2021). During the PhD of Paul Pitard, in collaboration with Cédric Thieulot and Marie-Pierre Doin, we made schematic 2-D viscous models of thrusts embedded in the crust, to study eastern Tibet thrust activity in the building of the topography through time. We show that both the high viscosity upper crust in which the fault is embedded and more surprisingly the low viscosity lower crust with no fault, are driven toward the surface by the fault. This generates along the fault a parallel zonation of the vertical velocity field, with high velocities close to the fault, decreasing away from it, fitting well the rejuvenation of cooling ages observed toward the thrust of SE Tibet.In order to explore the influence of erosion during the building of the plateau, I will also present thermo-kinematic modelling of thermochronology data along the Mekong River at the eastern edge of Tibet, including schematic erosion process (Ou et al., 2020). During the PhD of Xiong Ou, in collaboration with Pieter van der Beek, we estimated that the Mekong River incision, locally more than 2000m, is 25-30% of the total exhumation since 10 Ma. Strong differences in elevation and relief on both sides of the Mekong River are linked to strongly differing tectonic imprint, with high elevation low relief surfaces observed when tectonic imprint is low, in part due to glacial “buzzsaw-like” processes, and high elevation high relief massif observed when tectonic imprint is high and when glacial processes are not sufficient to erase the topography created.

National audience; Unlike most "wedge" orogens, the structure and the composition of theTibetan Plateau lower crust has kept being largely inaccessible otherwisethan by geophysical studies and by the few scattered sampling ofxenoliths brought to the surface by volcanism. In the Nangqian basin (NEQiangtangterrane), Eocene syn-contractional potassic lavas carrypartially molten foliated quartzo-feldspathic xenoliths. Zircon LA-ICPMSU-Pb dating of these crustal xenoliths yields a Lower Triassic age of ca.248.5 Ma, which is interpreted as the protolith crystallization age, withno evidence for any metamorphic zircon growth or resetting. The rarerelict ferro-magnesian minerals found (clinopyroxene, amphibole,phlogopite, garnet) are in textural and chemical disequilibrium, makingconventional thermobarometry methods inoperable. Nevertheless, thexenoliths have retained unusual metamorphic features which can helpunravelling their pre-entrapment metamorphic history. Pure anorthite"patches" with magnetite inclusions are interpreted as pseudomorphsafter garnet. Since these patches are often found in contact withclinopyroxene and rutile, these minerals may together represent a formerhigh-pressure assemblage that developed at 1.2-1.5 GPa and 620-730°C.Such pressure record would indicate that 80% of the present-day crustalthickness of the Tibetan Plateau was already acquired in Eocene times.Moreover, corundum-bearing assemblages, interpreted as pseudomorphsafter phlogopite, are best explained by kinetic disequilibrium underhigh-temperature granulitic conditions, suggesting a short-livedmetamorphic event. Taken together, this set of new petrological andgeochronological data suggests that crustal granulitic metamorphism maynot b e related to the onset of a general warming of the Tibetan crustsustained until the present, but rather to the punctual injection of mantlederivedmelts in the crust before Eocene eruption.

International audience; The use of high spatial and temporal resolution data is a key condition for characterizinginterseismic deformation along major faults and understanding their roles in accommodatingthe deformation. Here we present an analysis using 4 years of Sentinel-1 InSAR data tocharacterize interseismic deformation along the Xianshuihe (XSH) fault system and theKunlun (KUN) fault in the eastern edge of the Tibetan plateau.Thanks to the NSBAS automated processing chain (Doin et al., 2011, Grandin, 2015),implemented at the CNES high-performance computing center in Toulouse as part of theFLATSIM project (ForM@Ter LArge-scale multi-Temporal Sentinel-1 Interferometryprocessing chain in Muscate), we automatically perform high-resolution time series analysisof 2014-2019 Sentinel-1 InSAR data set, acquired along 1200 km-long ascending anddescending orbits, over the study area. According to an elastic block model approach, InSARtimes series are combined to the latest published GPS velocity field and inverted using theblock modeling software TDEFNODE (McCaffrey., 2009) to model fault slip rates andcoupling distribution along these two major strike-slip faults and identify asperities andpotential creeping areas as well as analyze internal block deformation.Our models suggest the existence of a fault connecting the XSH fault and the KUN fault andaccomodating part of the deformation at a constant slip rate of 10 mm/yr. It shows that theslip rate on the XSH fault system increases from west to east, from 5 mm/yr to 14 mm/yrbetween latitudes 34.3°N and 29.3°N, while the slip rate on the Kunlun fault slightlydecreases from west to east from 10 mm/yr to 9 mm/yr. The slip deficit rate observed alongthe XSH fault system shows strong lateral variations, that we analyze with respect to theseismic history of the fault, and suggests the presence of shallow creep on the eastern part ofthe fault. The slip deficit rate observed along the Kunlun fault appears more homogeneous.However, we observe a strong deformation gradient that we interpret as a post-seismic signalat the eastern tip of the rupture of the 2001 Kokoxili earthquake and we analyze its temporalevolution.

International audience

—Due to its size and high altitude, the growth of the Tibetan Plateau remains an enigma. Based on a synthesis of anterior collisions, paleoaltimetric data, geochemistry of ultrapotassic lava and their rare mantle enclaves, combined with a reinterpretation of tomographic data, we suppose that Tibet’s growth took take place in two main stages. Initially, the accretion of Gondwana terranes to the margin of South Asia, especially during the Early Triassic–Cretaceous period, resulted in the first episode of plateau growth, which affected an area of about 2/3 of the current plateau. We suppose that during the Late Cretaceous, the Tibetan crust reached a thickness of about 50–55 km, which is equivalent to an altitude of about 2500 to 3000 m, with local landforms that could have exceeded 4000 m. Another important consequence of these successive accretions was a strong metasomatism and a softening of the upper part of the Tibetan cover. The P wave low-velocity anomaly currently observed under the central part of Tibet would correspond not to a temperature anomaly but to a composition anomaly. From 50 Ma onwards, the convergence between India and Asia, estimated at about 1000 km on the Tibetan side, led to a shortening of the plateau by about 40%. We suppose that this additional shortening, which has led to the current thickness of the Earth’s crust of about 70 km and an average altitude of 4800 m, has been compensated by the reactivation of the continental slabs along the previous sutures and by the homogeneous shortening of the crust.

Determining the timing and extent of past glaciations in Tibet is essential to reconstruct regional paleoclimate and understand how atmospheric circulation varies due to the high altitude low latitude Tibetan Plateau. In SE Tibet, geomorphological field observation of glacial deposits shows two main imbricated moraines. We apply statistical analyses to a compilation of eight new 10Be cosmogenic exposure ages from two moraine crests at GMX site and 128 previously published but recalculated exposure ages from 30 additional crests in the region. The results show that ages from the sharpest inner moraines range from 14–25 ka, corresponding to the full range of Marine oxygen Isotope Stage (MIS)-2 (i.e., Last Glacial Maximum, LGM) with less than 2% of older outliers. The outer moraines have a fundamentally different distribution with scattered ages from 10 to 200 ka, obtained using the same method of sampling, dating, and age modeling proven robust for dating the LGM inner moraines, therefore excluding a methodologic artifact. This large scatter prevents the application of any statistical analysis to the age distribution. At a site with well-developed and preserved imbricated moraines (Cuopu), the outer moraine's oldest ages are MIS-6, with the oldest one being at the MIS-6/MIS-7 limit, identical to what is observed in the regional compilation. Following our observations for the LGM moraines where

The global and systematic coverage of Sentinel-1 radar images allows characterizing, by radar interferometry (InSAR), surface deformation on a continental scale.Our study focuses on the eastern part of the Tibetan plateau, where a combination of major strike-slip and thrust fault systems accommodates part of the deformation related to the collision between the Indian and Eurasian plates.We use an automated Sentinel-1 InSAR processing chain based on the NSBAS approach (Doin et al., 2011, Grandin, 2015) to measure the interseismic deformation across these fault systems. Processing is made on the CNES high-performance computer center in Toulouse in the FLATSIM project framework (ForM@Ter LArge-scale multi-Temporal Sentinel-1 Interferometric Measurement, Durand et al., 2019). We perform a time series analysis of the 2014-2020 Sentinel-1 InSAR data set, for 1200 km-long tracks (acquired along 7 ascending and 7 descending orbits), covering a 1 700 000 km2 area, with a 160 m spatial resolution. From about 130 acquisitions per track, we perform about 600 interferograms, with short, three months, and one-year temporal baselines. After inversion, we obtain time series of line-of-sight (LOS) delay maps, including residual atmospheric delay and network misclosure measurements. The time series are fitted by a seasonal signal plus a velocity trend. The velocity field on overlap areas agrees within less than 1~mm/yr.Finally, we decompose the LOS velocity maps into a vertical and a horizontal contribution.InSAR velocity maps highlight surface deformation patterns mostly localized on known major faults, short-wavelength patterns attributed to slope instabilities phenomena, and hydrological signals.The seasonal signal combines residual atmospheric phase delays and widespread hydrological phenomena in sedimentary basins, which we interpret in parallel with the regional geological map. Masking areas affected by dominant gravitational slope or hydrological deformation allows to better focus on tectonic deformation.We finally discuss slip partitioning on the various fault systems from the velocity maps and 2D profiles’ analysis.

Decoding the Tibetan plateau and its structural evolution has been a thorny issue for decades, triggering many controversial discussions between the proponents of the numerous key models. Numerical simulations of buoyancy forces associated with a thick crust and a low viscosity channel in the Tibetan crust predict continuous deformation, crustal uplift and thickening through an outward flow of partially molten middle/lower crust. Surface geological observations of fault systems, however, favor a model of localized deformation through the interaction between strike-slip and thrust faults. Here, we investigate the role of thrusting mechanisms involved in the plateau formation, which is essential in order to discuss these end-members competing models. We focus on the Muli thrust, a major Miocene thrust fault located at the eastern edge of the Tibetan Plateau, characterized by a pronounced topographic step of ~2000 m. We provide here an innovative quantitative approach combining thermo-kinematic modelling based on low-temperature thermochronology data, with conceptual 2-dimensional (2D) simulations of the crust’s mechanical behavior. Using the code PECUBE, we test different scenarios of rock cooling by forward modelling and inversion method in order to constrain the amount and timing of exhumation, as well as its simplified first-order crustal geometry. Given that low-temperature thermochronology data only provides the thermal history of the upper part of the crust (< 10 km), such thermo-kinematic modelling does not reveal any direct evidence of the potential implication of the lower crust. To overcome such limitations, we performed 2D mechanical modelling of the Muli thrust to constrain its mechanical behavior at the crustal scale to decipher its importance in the thickening of the plateau margin. We present here, how complementary numerical simulations based on in-situ geological observations on thrust faults, combined with thermochronology data, can be used to have a better understanding of the geological processes involved in the thickening of the Tibetan crust, and discuss both the strengths and weaknesses of such modelling.

The Xianshuihe fault in eastern Tibet is one of the most active faults in China, with the next large earthquake most likely to occur along its SE part near Kangding. Quantifying its slip rate along...

International audience; More accurate normal fault mapping and more recently constrained extension rates in southern and central Tibet allow to better discuss the mechanical processes responsible for the distribution of extension in Tibet. First, we show that the location of the rifts in southern Tibet south of the Karakorum-Jiali fault zone (KJFZ) does not exactly correspond to that of the rigid Indian lower lithosphere flattening below southern Tibet (underthrusting) inferred by P-waves global tomography, thus suggesting an absence of mechanical coupling between the two processes. Instead, E-W extension south of the KJFZ appears primarily due to divergent, orthogonal thrusting along the curved Himalayan arc, as proposed earlier. North of the KJFZ however, lower amplitude extension, distributed on numerous scattered normal faults in the western Qiangtang terrane, absorbs distributed eastward extrusion, while eastern Qiangtang is extruded more rigidly by the Xianshuihe fault, following a slip-line resulting from in-plane forces due to the collision/indenter, visible as a major discontinuity in the GPS velocity field.

The Southeast Tibet is characterized by extensive low-relief high-elevation surfaces that have been interpreted as “relict surfaces”, where thermochronological data generally show old ages and very little exhumation during the India-Asia collision. Those relict surfaces are proposed either to be formed at low elevation and then uplifted and dissected by large rivers since middle Miocene, or to inherit a pre-existing low-relief landscape by or prior to the collision, as revealed by stable-isotope paleoaltimetry. Among these relict surfaces, the BaimaXueshan low-relief (We compile and model published thermochronometric ages for BaimaXueshan massif, east of the Mekong River, to constrain its exhumation and relief history using the thermo-kinematic code Pecube. Modelling results show regional rock uplift at a rate of 0.25 km/Myr since ~10 Ma, following slow exhumation at a rate of 0.01 km/Myr since at least 22 Ma. Estimated Mekong River incision accounts for a maximum of 30% of the total exhumation since 10 Ma. We interpret moderate exhumation of the BaimaXueshan massif since 10 Ma as a response to a regional uplift due to the continuous northward indentation of NE India in a zone around the Eastern Himalayan Syntaxis (EHS) and delimited by Longmucuo-Shuanghu suture in the north. Thus BaimaXueshan massif with significant exhumation could not be classified as “relict surface”, as proposed by previous studies and its low relief results from in part glacial “buzzsaw-like” processes at high elevation, enhancing since ~2 Ma. In contrast, modelling results for the high-relief, high-elevation Kawagebo massif to the west of the Mekong River, facing the BaimaXueshan massif, imply a similar contribution of Mekong River incision (25%) to exhumation, but much stronger local rock uplift at a rate of 0.45 km/Myr since at least 10 Ma, accelerating to 1.86 km/Myr since 1.6 Ma. We show that the thermochronometric ages are best reproduced by local rock uplift related to late Miocene reactivation of a kinked westward-dipping thrust, striking roughly parallel to the Mekong River, with a steep shallow segment flattening out at depth. Thus, the strong differences in elevation and relief that characterize both massifs are linked to variable exhumation histories due to a strongly differing tectonic imprint.

International audience