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During continental collision, considerable amounts of buoyant continental crust subduct to depth and subsequently exhume. Whether various exhumation paths contribute to contrasting styles of magmatism across modern collision zones is unclear. Here we present 2D thermomechanical models of continental collision combined with petrological databases to investigate the effect of the main contrasting buoyancy forces, in the form of continental crustal buoyancy versus oceanic slab age (i.e., its thickness). We specifically focus on the consequences for crustal exhumation mechanisms and magmatism. Results indicate that it is mainly crustal density that determines the degree of steepening of the subducting continent and separates the models' parameter space into two regimes. In the first regime, high buoyancy values (∆ρ > 500 kg/m3) steepen the slab most rapidly (to 45–58°), leading to opening of a gap in the subduction channel through which the subducted crust exhumes (“subduction channel crustal exhumation”). A shift to a second regime (“underplating”) occurs when the density contrast is reduced by 50 kg/m3. In this scenario, the slab steepens less (to 37–50°), forcing subducted crust to be placed below the overriding plate. Importantly, the magmatism changes in the two cases: Crustal exhumation through the subduction channel is mainly accompanied by a narrow band of mantle melts, while underplating leads to widespread melting of mixed sources. Finally, we suggest that the amount (or density) of subducted continental crust, and the resulting buoyancy forces, could contribute to contrasting collision styles and magmatism in the Alps and Himalayas/Tibet., Key Points We analyse buoyancy forces in 2D thermomechanical‐petrological models of continental collision and resulting postcollisional magmatismCrustal buoyancy controls whether subducting crust exhumes between plates with only mantle melts or underplates with mixed meltingTiming and distribution of crust and melting for two endmembers fits Alps and Himalaya/Tibet

Seismological constraints obtained from receiver function (RF) analysis provide important information about the crust and mantle structure. Here, we explore the utility of the free-surface multiple of the P-wave (PP) and the corresponding conversions in RF analysis. Using earthquake records, we demonstrate the efficacy of PPs-RFs before illustrating how they become especially useful when limited data is available in typical planetary missions. Using a transdimensional hierarchical Bayesian deconvolution approach, we compute robust P-to-S (Ps)- and PPs-RFs with InSight recordings of five marsquakes. Our Ps-RF results verify the direct Ps converted phases reported by previous RF analyses with increased coherence and reveal other phases including the primary multiple reverberating within the uppermost layer of the Martian crust. Unlike the Ps-RFs, our PPs-RFs lack an arrival at 7.2 s lag time. Whereas Ps-RFs on Mars could be equally well fit by a two- or three-layer crust, synthetic modeling shows that the disappearance of the 7.2 s phase requires a three-layer crust, and is highly sensitive to velocity and thickness of intra-crustal layers. We show that a three-layer crust is also preferred by S-to-P (Sp)-RFs. While the deepest interface of the three-layer crust represents the crust-mantle interface beneath the InSight landing site, the other two interfaces at shallower depths could represent a sharp transition between either fractured and unfractured materials or thick basaltic flows and pre-existing crustal materials. PPs-RFs can provide complementary constraints and maximize the extraction of information about crustal structure in data-constrained circumstances such as planetary missions., Journal of Geophysical Research: Planets, 126 (11), ISSN:0148-0227, ISSN:2169-9097

The cause for the damping of the Earth's free core nutation (FCN) and the free inner core nutation eigenmodes has been a matter of debate since the earliest reliable estimations from nutation observations were made available. Numerical studies are difficult given the extreme values of some of the parameters associated with the Earth's fluid outer core, where important energy dissipation mechanisms can take place. We present a fully 3D numerical model for the FCN capable of describing accurately viscous and Ohmic dissipation processes taking place in the bulk of the fluid core as well as in the boundary layers. We find an asymptotic regime, appropriate for Earth's parameters, where viscous and Ohmic processes contribute to the total damping, with the dissipation taking place almost exclusively in the boundary layers. By matching the observed nutational damping, we infer an enhanced effective viscosity matching and validating methods from previous studies. We suggest that turbulence caused by the Earth's precession can be a source for the enhanced viscosity., Key Points We establish asymptotic scaling laws for the free core nutation (FCN)'s Ohmic and viscous dissipation in the fluid coreMost of the energy dissipation associated with the FCN takes place in the boundary layersOur work validates inferences from previous studies that assumed the flow in the outer core as a uniform vorticity flow

Several hypotheses on the origin of the continental Moho are still debated and multiple mechanisms may contribute to its formation. Here, we present quantitative estimation of the seismic properties and anisotropy of the crust‐mantle transition in the Western Alps where an example of newly formed (proto)‐continental Moho is unusually shallow. We make use of teleseismic P‐to‐S converted‐waves recorded by stations deployed on top of the Ivrea Body (IB), a volume of possibly serpentinized mantle peridotite below exhumed (ultra‐)high pressure crustal rocks. The IB has been mapped by gravity, magnetic, active and passive seismic surveys suggesting an extremely shallow Moho. We demonstrate that the P‐to‐S converted waves propagating through this region display coupled features: (a) they record expected presence of strong seismic velocity contrast at shallow depth as due to the lower crustal and upper mantle transition; (b) they are decomposed due to anisotropic properties of rocks involved. The proto‐continental Moho is recognized as an increase in S‐wave velocity (∼0.4–1 km/s) at shallow depths of 5–10 km. The presence of anisotropy within the IB and overlying crustal rocks is evidenced by back‐azimuthal dependence of the amplitude of P‐to‐S phases. The strength of anisotropy is ∼−14% on average pointing out the presence of metamorphosed/hydrated material (e.g., serpentinite) below the Moho. Anisotropic directions are consistent across Moho in both crust and upper mantle. The similarity of the anisotropy parameters between crust and upper mantle suggests they have been shaped by the same deformation event., Key Points Receiver Functions and harmonics decompositionContinental Moho and Ivrea BodySeismic anisotropy

Stripe‐like patterns of surface wave arrival angle deviations have been observed by several seismological studies around the world, but this phenomenon has not been explained so far. Here we test the hypothesis that systematic arrival angle deviations observed at the AlpArray broadband seismic network in Europe are interference patterns caused by diffraction of surface waves at single small‐scaled velocity anomalies. We use the observed pattern of Rayleigh waves from two earthquakes under the Southern Atlantic Ocean, and we fit this pattern with theoretical arrival angles derived by a simple modeling approach describing the interaction of a seismic wavefield with small anomalies. A grid search inversion scheme is implemented, which indicates that the anomaly is located in Central Africa, with its head under Cameroon. Moreover, the inversion enables the characterization of the anomaly: The anomaly is inferred to be between 320 and 420 km wide, matching in length the 2,500 km long upper mantle low‐velocity region under the volcano‐capped swells of the Cameroon volcanic line. We show that this approach can be generally used for studying the upper mantle anomalies worldwide., Key Points Arrival angle deviations of surface waves are explained by interference of diffracted wavefronts after passing a distant anomalyWe find a strong, elongated low‐velocity region in the upper mantle under the Cameroon volcanic lineThese findings can help for understanding the nature of small‐scale convection in the upper mantle

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

Advances in global seismic tomography have increasingly motivated identification of subducted lithosphere in Earth's deep mantle, creating novel opportunities to link plate tectonics and mantle evolution. Chief among those is the quest for a robust subduction reference frame, wherein the mantle assemblage of subducted lithosphere is used to reconstruct past surface tectonics in an absolute framework anchored in the deep Earth. However, the associations heretofore drawn between lower mantle structure and past subduction have been qualitative and conflicting, so the very assumption of a correlation has yet to be quantitatively corroborated. Here we show that a significant, time‐depth progressive correlation can be drawn between reconstructed subduction zones of the last 130 Myr and positive S wave velocity anomalies at 600–2300 km depth, but that further correlation between greater times and depths is not presently demonstrable. This correlation suggests that lower mantle slab sinking rates average between 1.1 and 1.9 cm yr−1., Key Points Significant correlation between reconstructed subduction of the last 130 Myr and mantle structure at 600 to 2300 km depthFurther correlation (beyond 2300 km/130 Ma) not presently demonstrable due to uncertainties in tomographic and paleogeographic modelsSubducted slabs sink through the lower mantle, on average, at rates between 1.1 and 1.9 cm yr−1

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

In seismic tomography we map the wave speed structure inside the Earth, but we ultimately seek to interpret those images in terms of physical parameters. This is challenging because many parameters can trade‐off with each other to produce a given wave speed. The problem is compounded by the convention of mapping seismic structures as perturbations relative to a 1‐D reference model, rather than absolute wave speeds. Using a full waveform tomography model of Europe as a case study, we quantify the extent to which thermochemical and dynamic properties can be constrained using only S wave speed, expressed in absolute values. The wave speed distributions of this tomography model are compared with 4 million thermochemical models, whose seismic properties are computed via thermodynamic modeling. These models sample the full range of realistic mantle compositions, including variable water and melt contents, and mineral intrinsic anelasticity is taken into account. Intrinsic anelasticity causes waves to travel more slowly at higher temperatures, leading to seismic attenuation, but the sensitivity of the wave speed reduction to temperature is, in turn, controlled by the wave frequency. Global studies of surface waves indicate an anticorrelation between S wave speed and attenuation. We therefore only retain thermochemical models satisfying this anticorrelation. Our study indicates that the frequency dependence of anelasticity, α, depends on temperature or rheology, with α ≈ 0.1 being most appropriate in cold or lithospheric mantle and α ≈ 0.3 in warmer regions (i.e., the asthenosphere). Additionally, the slowest regions require specific compositions and/or a velocity‐weakening mechanism, such as partial melting, elastically accommodated grain boundary sliding, or water., Key Points Knowing the S wave speed precisely with a loose constraint on mantle attenuation places strong constraints on the temperatureThe frequency dependence of anelasticity α is variable: ~0.1 in cold/lithospheric mantle and ~0.3 in warm/asthenospheric mantleEither partial melting or elastically accommodated grain boundary sliding is required to explain the slowest wave speeds at 130 km

We present SubMachine, a collection of web‐based tools for the interactive visualization, analysis, and quantitative comparison of global‐scale data sets of the Earth's interior. SubMachine focuses on making regional and global‐scale seismic tomography models easily accessible to the wider solid Earth community, in order to facilitate collaborative exploration. We have written software tools to visualize and explore over 30 tomography models—individually, side‐by‐side, or through statistical and averaging tools. SubMachine also serves various nontomographic data sets that are pertinent to the interpretation of mantle structure and complement the tomographies. These include plate reconstruction models, normal mode observations, global crustal structure, shear wave splitting, as well as geoid, marine gravity, vertical gravity gradients, and global topography in adjustable degrees of spherical harmonic resolution. By providing repository infrastructure, SubMachine encourages and supports community contributions via submission of data sets or feedback on the implemented toolkits., Key Points Web‐based tools for the interactive visualization, analysis, and quantitative comparison of global‐scale, volumetric (3‐D) data sets of the subsurfaceFocus on global seismic tomography models from body waves, surface waves, and normal modes (>30 models currently implemented)Additional tools for interacting with related data sets: plate tectonic reconstructions, topography, geoid, marine gravity, normal mode observations, etc

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

Geodynamic models of mantle convection predict that Mexico and western North America share a history of dynamic support. We calculate admittance between gravity and topography, which indicates that the elastic thickness of the plate in Mexico is 11 km and in western North America it is 12 km. Admittance at wavelengths > 500 km in these regions suggests that topography is partly supported by subcrustal processes. These results corroborate estimates of residual topography from isostatic calculations and suggest that the amount of North American topography supported by the mantle may exceed 1 km. The Cenozoic history of magmatism, sedimentary flux, thermochronometric denudation estimates, and uplifted marine terraces imply that North American lithosphere was uplifted and eroded during the last 30 Ma. We jointly invert 533 Mexican and North American longitudinal river profiles to reconstruct a continent-scale rock uplift rate history. Uplift rate is permitted to vary in space and time. Erosional parameters are calibrated using incision rate data in southwest Mexico and the Colorado Plateau. Calculated rock uplift rates were 0.15-0.2 mm/yr between 25 and 10 Ma. Central Mexico experienced the highest uplift rates. Central and southern Mexico continued to uplift at 0.1 mm/yr until recent times. This uplift history is corroborated by independent constraints. We predict clastic flux to the Gulf of Mexico and compare it to independent estimates. We tentatively suggest that the loop between uplift, erosion, and deposition can be closed here. Mexico’s staged uplift history suggests that its dynamic support has changed during the last 30 Ma.

Observations of shear wave anisotropy are key for understanding the mineralogical structure and flow in the mantle. Several researchers have reported the presence of seismic anisotropy in the lowermost 150–250 km of the mantle (i.e., D ′′ layer), based on differences in the arrival times of vertically (S V) and horizontally (S H) polarized shear waves. By computing waveforms at a period > 6 s for a wide range of 1‐D and 3‐D Earth structures, we illustrate that a time shift (i.e., apparent splitting) between S V and S H may appear in purely isotropic simulations. This may be misinterpreted as shear wave anisotropy. For near‐surface earthquakes, apparent shear wave splitting can result from the interference of S with the surface reflection s S. For deep earthquakes, apparent splitting can be due to the S wave triplication in D ′′, reflections off discontinuities in the upper mantle, and 3‐D heterogeneity. The wave effects due to anomalous isotropic structure may not be easily distinguished from purely anisotropic effects if the analysis does not involve full waveform simulations., Key Points Simulations of wave propagation through the lowermost mantle display apparent S H‐S V splitting for 1‐D and 3‐D isotropic Earth modelsThe apparent splitting is due to phase interference between S and reflected wavesAnomalous Vs gradients, discontinuities, and heterogeneities may lead to misinterpretation of wave splitting

Strong, sharp, negative seismic discontinuities, velocity decreases with depth, are observed beneath the Pacific seafloor at ∼60 km depth. It has been suggested that these are caused by an increase in radial anisotropy with depth, which occurs in global surface wave models. Here we test this hypothesis in two ways. We evaluate whether an increase in surface wave radial anisotropy with depth is robust with synthetic resolution tests. We do this by fitting an example surface wave data set near the East Pacific Rise. We also estimate the apparent isotropic seismic velocity discontinuities that could be caused by changes in radial anisotropy in S‐to‐P and P‐to‐S receiver functions and SS precursors using synthetic seismograms. We test one model where radial anisotropy is caused by olivine alignment and one model where it is caused by compositional layering. The result of our surface wave inversion suggests strong shallow azimuthal anisotropy beneath 0–10 Ma seafloor, which would also have a radial anisotropy signature. An increase in radial anisotropy with depth at 60 km depth is not well‐resolved in surface wave models, and could be artificially observed. Shallow isotropy underlain by strong radial anisotropy could explain moderate apparent velocity drops (, Key Points Strong shallow azimuthal anisotropy is required for 0–10 Ma seafloor, which would also give a strong shallow radial anisotropy signatureAn increase in radial anisotropy from 0 to 100 km depth beneath the oceans could be an artificial feature of surface wave modelsNegative velocity discontinuities imaged by scattered waves cannot simply be explained by an increase in radial anisotropy with depth

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).