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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.

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.