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Several theoretical studies indicate that a substantial fraction of the measured seismic anisotropy could be interpreted as extrinsic anisotropy associated with compositional layering in rocks, reducing the significance of strain‐induced intrinsic anisotropy. Here we quantify the potential contribution of grain‐scale and rock‐scale compositional anisotropy to the observations by (i) combining effective medium theories with realistic estimates of mineral isotropic elastic properties and (ii) measuring velocities of synthetic seismic waves propagating through modeled strain‐induced microstructures. It is shown that for typical mantle and oceanic crust subsolidus compositions, rock‐scale compositional layering does not generate any substantial extrinsic anisotropy (, Key Points Rock‐scale layering in the Earth's mantle produces negligible extrinsic anisotropyRock‐scale layering cannot be detected with seismic anisotropy but mainly with seismic wave scatteringGrain‐scale layering can generate substantial extrinsic anisotropy around the transition zone

The formation of mountain belts or rift zones is commonly attributed to interactions between plates along their boundaries, but the widely distributed deformation of Asia from Himalaya to the Japan Sea and other back‐arc basins is difficult to reconcile with this notion. Through comparison of the tectonic and kinematic records of the last 50 Ma with seismic tomography and anisotropy models, we show that the closure of the former Tethys Ocean and the extensional deformation of East Asia can be best explained if the asthenospheric mantle transporting India northward, forming the Himalaya and the Tibetan Plateau, reaches East Asia where it overrides the westward flowing Pacific mantle and contributes to subduction dynamics, distributing extensional deformation over a 3,000‐km wide region. This deep asthenospheric flow partly controls the compressional stresses transmitted through the continent‐continent collision, driving crustal thickening below the Himalayas and Tibet and the propagation of strike‐slip faults across Asian lithosphere further north and east, as well as with the lithospheric and crustal flow powered by slab retreat east of the collision zone below East and SE Asia. The main shortening direction in the deforming continent between the collision zone and the Pacific subduction zones may in this case be a proxy for the direction of flow in the asthenosphere underneath, which may become a useful tool for studying mantle flow in the distant past. Our model of the India‐Asia collision emphasizes the role of asthenospheric flow underneath continents and may offer alternative ways of understanding tectonic processes., Key Points Coeval India‐Asia collision and Pacific back‐arc extension explained by asthenospheric flowMantle flow correlates with long‐term kinematics from India to Pacific borderIndian mantle reaches Pacific border and Asian lithosphere spreads above ponding slabs

Numerical simulations of mantle convection with a viscoplastic rheology usually display mobile, episodic or stagnant lid regimes. In this study, we report a new convective regime in which a ridge can form without destabilizing the surrounding lithosphere or forming subduction zones. Using simulations in 2‐D spherical annulus geometry, we show that a depth‐dependent yield stress is sufficient to reach this ridge only regime. This regime occurs when the friction coefficient is close to the critical value between mobile lid and stagnant lid regimes. Maps of convective regime as a function of the parameters friction coefficients and depth dependence of viscosity are provided for both basal heating and mixed heating situations. The ridge only regime appears for both pure basal heating and mixed heating mode. For basal heating, this regime can occur for all vertical viscosity contrasts, while for mixed heating, a highly viscous deep mantle is required., Key Points Stable ridges can appear in the lithosphere without generating subduction zonesA depth‐dependent yield stress is necessary to obtain stable ridgesThis “ridge only” regime can be found between stagnant and mobile lid regimes

International audience; The presence of melts in the first part of upper mantle is image by many geophysical studies. The nature of these melts is referring by different petrological and experimental studies that show a melt rich in volatiles, principally in CO2. To apprehend the deep carbon cycle, the first necessity is to know the mobility of these melts, like the viscosity. As a function of geodynamical context, the temperature and the pressure change, therefore different carbonated melts are produce. In this study, we investigate different SiO2/CO2/H2O content in melt to determine their influences on the viscosity. To investigate the viscosity, the experiment is making in ESRF in Grenoble. We produce in-situ measurements in Paris Edinburg cell with falling sphere viscometry experiments. Experiments are produce between 1 and 3 GPa and liquid temperature of sample. One result show a viscosity of 0.15 Pa.s for 20wt% of CO2. It is one order of magnitude more viscous than dolomite (Kono et al., 2013) and average 5 times less viscous than basalt (Sakamaki et al., 2013). Primarily results obtain in ESRF show a no linear effect of SiO2/CO2 content. Volatiles have an effect on viscosity and this has implication on melt migration in the mantle, so on the deep carbon cycle.

EGU2020: Sharing Geoscience Online, 4-8 May 2020, The modes in which the lithosphere deforms during continental collision and the mechanisms involved are not well understood. While continental subduction and mantle delamination are often invoked in tectonophysical studies, these processes are difficult to be confirmed in more complex tectonic regions such as the Gibraltar Arc. We study the present-day density and compositional structure of the lithosphere along a transect running from S Iberia to N Africa crossing the western Gibraltar Arc. This region is located in the westernmost continental segment of the African-Eurasian plates, characterized by a diffuse transpressive plate boundary. An integrated and self-consistent geophysical-petrological methodology is used to model the lithosphere structure variations and the thermophysical properties of the upper mantle. The crustal structure is mainly constrained by seismic experiments and geological data, whereas the composition of the lithospheric mantle is constrained by xenolith data. The results show large lateral variations in the topography of the lithosphere-asthenosphere boundary (LAB). We distinguish different chemical lithospheric mantle domains that reproduce the main trends of the geophysical observables and the modelled P- and S-wave seismic velocities. A sublithospheric body colder than the surrounding mantle is needed beneath the Betics-Rif to adjust the measured potential fields. We link this body to the Iberian slab localized just to the east of the profile and having some effect on the geoid and Bouguer anomalies. Local isostasy allows explaining most of the topography, but an elastic thickness higher than 10 km is needed to explain local misfits between the Atlas and the Rif Mountains., This work has been supported by Spanish Ministry by the projects MITE (CGL2014-59516) and GeoCAM (PGC2018-095154-B-100).