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Special issue Physical properties and observations of the lithosphere-asthenosphere system.-- 21 pages, 15 figures, 3 tables, 1 appendix, The topography of the Iberian Peninsula is characterized by the presence of Variscan and Alpine orogenic belts and foreland basins, but what sets it apart from the rest of Western Europe are the large elevated flat surfaces (700 m above sea-level on average) in its central parts. The origin and support of such high average topography, whether isostatic or dynamic in nature, is a matter of intense debate. To understand Iberian topography, it is key to have a reliable image of the present-day lithospheric thermochemical structure. So far, this structure remains poorly constrained, particularly at mantle level. The goal of this paper is to derive robust estimates of the thermal, compositional and density structure of the lithosphere beneath the Iberian Peninsula from an integrated geophysical-petrological probabilistic inversion of surface wave, elevation, geoid anomaly and heat flow data. Our inversion reveals an average lithospheric thickness of 80–100 km in the Iberian Peninsula with only moderate lateral variations. The most prominent lithospheric thickness change is a steep decrease from the central to the easternmost Pyrenees. The thinnest lithosphere in our models is found below the south-eastern Mediterranean margin (, This project has been funded by Spanish Ministry of Science projects CGL2012-37222 (J. Fullea) and PGC2018-095154-B-I00 (A.M Negredo). J. Fullea is supported by an Atracción Talento senior fellowship (2018-T1/AMB/11493) funded by Comunidad Autonoma de Madrid (Spain). I. Palomeras is funded by the Beatriz Galindo fellowship (BEAGAL18/00090) co-founded by the Spanish Ministry of Education and University of Salamanca (Spain). ICM-CSIC is a Centre of Excellence Severo Ochoa (Spanish Ministry of Science and Innovation, Project CEX2019-000928-S

Joint probabilistic inversions of magnetotelluric (MT) and seismic data have great potential for imaging the thermochemical structure of the lithosphere as well as mapping fluid/melt pathways and regions of mantle metasomatism. In this contribution, we present a novel probabilistic (Bayesian) joint inversion scheme for 3D MT and surface-wave dispersion data particularly designed for large-scale lithospheric studies. The approach makes use of a recently developed strategy for fast solutions of the 3D MT forward problem (Manassero et al., 2020, https://doi.org/10.1093/gji/ggaa415) and combines it with adaptive Markov chain Monte Carlo (MCMC) algorithms and parallel-in-parallel strategies to achieve extremely efficient simulations. To demonstrate the feasibility, benefits and performance of our joint inversion method for imaging the temperature and conductivity structures of the lithosphere, we apply it to two numerical examples of increasing complexity. The inversion approach presented here is timely and will be useful in the joint analysis of MT and surface wave data that are being collected in many parts of the world. This approach also opens up new avenues for the study of trans-lithospheric and trans-crustal magmatic systems, the detection of metasomatized mantle, and the incorporation of MT into multi-observable inversions for the physical state of the Earth's interior. We thank Farshad Salajegheh for providing part of his Matlab codes for plotting results 850 Special thanks to Kate Selway and Anandaroop Ray for their suggestions at different 851 stages of this work. The 3D rendering views were created using ParaView (Ahrens et al., 852 2005). 853 MCM thanks support from an International Macquarie Research Excellence Schol854 arship (iMQRES). MCM and JCA acknowledge support from ARC Grant DP160103502, 855 ARC Linkage Grant LP170100233, the ARC Centre of Excellence Core to Crust Fluids 856 Systems (http://www.ccfs.mq.edu.au) and the Centre for Earth Evolution and Dynam857 ics, Geoscience Australia and the European Space Agency via the “3D Earth - A Dy858 namic Living Planet”. FZ acknowledges support from CONICET through grant PIP 112- 859 201501-00192. SZ has been funded by the Spanish Ministry through grant DPI2017-85139- 860 C2-2-R, by Catalan government through grant 2017-SGR-1278 and by the EU’s Hori861 zon 2020 research and innovation programme under the Marie Sklodowska-Curie grant 862 agreement No 777778.

Markov chain Monte Carlo (MCMC) methods have become standard in Bayesian inference and multi-observable inversions in almost every discipline of the Earth sciences. In the case of geodynamic and/or coupled geophysical-geodynamic inverse problems, however, the computational cost associated with the solution of large-scale 3-D Stokes forward problems has rendered probabilistic formulations impractical. Here we present a novel and extremely efficient method to produce ultrafast solutions of the 3-D Stokes problem for MCMC simulations. Our approach combines the individual benefits of Reduced Basis techniques, goal-oriented error formulations, and MCMC algorithms to produce an accurate and computationally efficient surrogate for the forward problem. Importantly, the surrogate adapts itself during the MCMC simulation according to the history of the chain and the goals of the inversion. This maximizes the efficiency of the forward problem and removes the need for preinversion off-line computations to build a surrogate. We demonstrate the benefits and limitations of the method with several numerical examples and show that in all cases the computational cost is of the order of

International audience; We reassess the mantle transition zone structure below the northeast Asia margin in the context of subduction of the Pacific plate below the Eurasian continent. We use two independent approaches of teleseismic imaging, namely, compressional-to-shear converted waves (receiver functions) and shear wave underside reflections (SS precursors), and compare them within their statistical uncertainties. We find localized complexity in the interfaces marking solid phase changes in mantle minerals, in terms of both apparent topography and reflectivity. The 660-km discontinuity is doubled, with approximate to 80-km maximum vertical distance between the interfaces, over an 890 x 350 km(2) region between 36-44 degrees N and 130-133 degrees E at the tip of the subducted Pacific plate. A similar complexity exists on the 410-km discontinuity, coinciding with the presence of a deep cluster of seismicity below the Japan Sea. Both methods suggest the presence of low-velocity zones atop the 410, within the mantle transition zone, and below the 660. This complex seismic signature is related to the Pacific plate and interpreted in light of the subduction thermal regime and phase equilibria for a pyrolitic mantle composition. Phase changes manifest themselves as broad zones of velocity gradients with localized doubled or multiple first-order discontinuities, associated with transitions in the olivine, pyroxene, and garnet systems. An average pyrolitic composition and local temperatures of 1000-1300K can explain the observed velocity gradients and multiple discontinuities. We show that the dissolution of stishovite, a high-pressure polymorph of SiO2, into the higher-pressure perovskite mineral, is a possible explanation for the low-velocity zones at the top of the lower mantle. Plain Language Summary In the early 1990s, the idea emerged that the study of seismic discontinuities near 410- and 660-km depths could inform us about the vertical circulation of material in the mantle. These two discontinuities were assigned to solid phase changes of a silicate mineral rich in magnesium (90%) and iron (10%), olivine. Advances in the field of mineralogy and high-pressure physics suggested, however, that other minerals, garnet and pyroxenes, should contribute to the general seismic signature. Due to the limited imaging capability of seismological methods and station deployments, however, a one-to-one correspondence between observed seismic structure and mineralogy has remained elusive, at best controversial. We provide here from two independent approaches compelling evidence for localized complexity in the seismic structure near 410- and 660-km depths, which we relate, through mineralogical modeling, to phase changes in olivine, garnet, pyroxene, and silica systems. This work provides the observational basis for assessing the viability of geodynamic and thermochemical models of the mantle beneath the northeast Asia margin.

A 3-D crustal and upper mantle S-wave velocity model of NE China is constructed by inversion of phase velocity dispersion curves at 6–140 s periods from ambient noise tomography and two-plane surface wave tomography. The seismic data used in this study are collected from 120 China Earthquake Administration (CEA) permanent stations and 127 portable stations of NECESSArray. We observe strong low S-wave velocity beneath the Changbaishan volcano in the upper mantle to at least 200-km depth, which is interpreted as a mantle upwelling beneath the Changbaishan volcano that is consistent with the body wave tomographic image. The Songliao Basin is dominated by a high velocity extending to at least 200-km depth. Built upon the observed velocity anomalies, we propose a sub-lithosphere mantle convection model for NE China in which the upwelling of upper mantle materials from the mantle transition zone to the Changbaishan volcano could induce a local sub-lithosphere convection in the upper mantle and the strong high velocity of the upper mantle beneath the Songliao Basin corresponds to the downwelling limb of this convection cell. The downwelling beneath the Songliao Basin could also induce secondary local convection in the asthenosphere to the west, leading to local asthenospheric upwelling beneath the Abaga and Halaha volcanoes in the Xing'an–Mongolia Orogenic Belt.

[1] Traditional inversion techniques applied to the problem of characterizing the thermal and compositional structure of the upper mantle are not well suited to deal with the nonlinearity of the problem, the trade-off between temperature and compositional effects on wave velocities, the nonuniqueness of the compositional space, and the dissimilar sensitivities of physical parameters to temperature and composition. Probabilistic inversions, on the other hand, offer a powerful formalism to cope with all these difficulties, while allowing for an adequate treatment of the intrinsic uncertainties associated with both data and physical theories. This paper presents a detailed analysis of the two most important elements controlling the outputs of probabilistic (Bayesian) inversions for temperature and composition of the Earth's mantle, namely the a priori information on model parameters, ρ(m), and the likelihood function, L(m). The former is mainly controlled by our current understanding of lithosphere and mantle composition, while the latter conveys information on the observed data, their uncertainties, and the physical theories used to relate model parameters to observed data. [2] The benefits of combining specific geophysical datasets (Rayleigh and Love dispersion curves, body wave tomography, magnetotelluric, geothermal, petrological, gravity, elevation, and geoid), and their effects on L(m), are demonstrated by analyzing their individual and combined sensitivities to composition and temperature as well as their observational uncertainties. The dependence of bulk density, electrical conductivity, and seismic velocities to major-element composition is systematically explored using Monte Carlo simulations. We show that the dominant source of uncertainty in the identification of compositional anomalies within the lithosphere is the intrinsic nonuniqueness in compositional space. A general strategy for defining ρ(m) is proposed based on statistical analyses of a large database of natural mantle samples collected from different tectonic settings (xenoliths, abyssal peridotites, ophiolite samples, etc.). This strategy relaxes more typical and restrictive assumptions such as the use of local/limited xenolith data or compositional regionalizations based on age-composition relations. We demonstrate that the combination of our ρ(m) with a L(m) that exploits the differential sensitivities of specific geophysical observables provides a general and robust inference platform to address the thermochemical structure of the lithosphere and sublithospheric upper mantle. An accompanying paper deals with the integration of these two functions into a general 3-D multiobservable Bayesian inversion method and its computational implementation.

Here we present a 3-D multi-observable probabilistic inversion method, particularly designed for high-resolution (regional) thermal and compositional mapping of the lithosphere and sub-lithospheric upper mantle that circumvents the problems associated with traditional inversion methods. The key aspects of the method are as follows: (a) it exploits the increasing amount and quality of geophysical datasets; (b) it combines multiple geophysical observables (Rayleigh and Love dispersion curves, body-wave tomography, magnetotelluric, geothermal, petrological, gravity, elevation, and geoid) with different sensitivities to deep/shallow, thermal/compositional anomalies into a single thermodynamic- geophysical framework; (c) it uses a general probabilistic (Bayesian) formulation to appraise the data; (d) no initial model is needed; (e) compositional a priori information relies on robust statistical analyses of a large database of natural mantle samples; and (f) it provides a natural platform to estimate realistic uncertainties. In addition, the modular nature of the method/algorithm allows for incorporating or isolating specific forward operators according to available data. The strengths and limitations of the method are thoroughly explored with synthetic models. It is shown that the a posteriori probability density function (i.e., solution to the inverse problem) satisfactorily captures spatial variations in bulk composition and temperature with high resolution, as well as sharp discontinuities in these fields. Our results indicate that only temperature anomalies of ΔT a 150°C and large compositional anomalies of ΔMg# > 3 (or bulk ΔAl 2O3 > 1.5) can be expected to be resolved simultaneously when combining high-quality geophysical data. This resolving power is sufficient to explore some long-standing problems regarding the nature and evolution of the lithosphere (e.g., vertical stratification of cratonic mantle, compositional versus temperature signatures in seismic velocities, etc) and offers new opportunities for joint studies of the structure of the upper mantle with unprecedented resolution. © 2013. American Geophysical Union. All Rights Reserved.

We apply a novel 3-D multiobservable probabilistic tomography method that we have recently developed and benchmarked, to directly image the thermochemical structure of the Colorado Plateau and surrounding areas by jointly inverting P wave and S wave teleseismic arrival times, Rayleigh wave dispersion data, Bouguer anomalies, satellite-derived gravity gradients, geoid height, absolute (local and dynamic) elevation, and surface heat flow data. The temperature and compositional structures recovered by our inversion reveal a high level of correlation between recent basaltic magmatism and zones of high temperature and low Mg# (i.e., refertilized mantle) in the lithosphere, consistent with independent geochemical data. However, the lithospheric mantle is overall characterized by a highly heterogeneous thermochemical structure, with only some features correlating well with either Proterozoic and/or Cenozoic crustal structures. This suggests that most of the present-day deep lithospheric architecture reflects the superposition of numerous geodynamic events of different scale and nature to those that created major crustal structures. This is consistent with the complex lithosphere-asthenosphere system that we image, which exhibits a variety of multiscale feedback mechanisms (e.g., small-scale convection, magmatic intrusion, delamination, etc.) driving surface processes. Our results also suggest that most of the present-day elevation in the Colorado Plateau and surrounding regions is the result of thermochemical buoyancy sources within the lithosphere, with dynamic effects (from sublithospheric mantle flow) contributing only locally up to ∼15–35%. ©2016. American Geophysical Union. All Rights Reserved.

Deep and fast wide-angle reflection arrivals observed at offsets over 180 km, and over a reduced time interval of 1–1.5 s, have been observed in a seismic experiment shot across SW Iberia as part of the IBERSEIS project. Using different modelling approaches, these reflections have been found to be consistent with a heterogeneous gradient zone located at 61–72 km depth that features an absolute P-wave velocity contrast from 8.2 to 8.3 km/s. Paradoxically, this interface has not been observed in coincident vertical incidence data, probably due to the change in the reflection coefficient with decreasing incidence angles, the lack of energy at high recording times for the near-vertical (vibroseis) data, and/or the different location of the CDPs in both experiments. Although the mantle is acknowledged to be highly heterogeneous and mantle lithologies are capable of giving impedance contrasts high enough as to be observed in seismic data, it is often seen as transparent from a seismic point of view. The short wavelength of mantle compositional heterogeneities is probably what hinders their identification with active source seismic data and only big and sharp discontinuities are imaged in vertical incidence experiments whereas regional transitional boundaries may be also observed at high incidence angles. Accordingly, we propose that deep reflectivity observed in SW Iberia must correspond to a regional–continental scale feature, not sharp enough as to be seen with vertical incidence energy. This feature, already identified in previous DSS experiments carried out in Iberia, has a depth, a P-wave velocity contrast and a transitional nature that match the characteristics proposed for the spinel-lherzolite to garnet-lherzolite phase transition, i.e. the Hales interface or gradient zone. This boundary is relatively narrow (at least 2–3 kb) in enriched mantle and appears deeper and along wider intervals when the mantle is depleted. In addition, it is a worldwide scale boundary already identified over large areas with different types of datasets. The variability in depth and sharpness of this interface, which is related to mantle chemistry, constrains the type of seismic techniques that should be used to identify it.

article i nfo We use thermodynamically self-consistent and hybrid methods to analyze the correlation of important physical parameters (e.g. bulk density, elastic moduli) with bulk Mg# and modal composition in mantle peridotites at upper mantle conditions. Temperature (anharmonic and anelastic), pressure and compositional derivatives for all these parameters are evaluated. The results show that the widely used correlations between Vp/Vs and Mg# in peridotites are strictly valid only for garnet-bearing assemblages at temperatures b900 °C. Thecorrelationbreaksdownwhen:i)spinelisthestableAl-richphaseintheassemblageandii)whenanelastic attenuation of seismic velocities becomes important (T ≳900 °C). This implies that the range of applicability of published Vp/Vs-Mg# correlations for the upper mantle is limited to a depth interval between the spinel- garnetphasetransitionandthe900 °Cisotherm.Weusenumericalsimulationstoshowthatthisdepthinterval is virtually nonexistent in lithospheres thinner than ∼140 km and can comprise up to ∼50% of the lithospheric mantle in thick (N220 km) lithospheric domains. In addition, we show that for most of the upper mantle the expected Δ(Vp/Vs) values associated with compositional variations are smaller than the resolution limit of currentseismologicalmethods.AlltheseconsiderationssuggestthattheVp/Vsratioisnotareliablemeasureof compositional variations and that for large parts of the upper mantle compositional anomalies cannot be separated from thermal anomalies on the basis of seismological studies only. We further confirm that the only reliable indicator of compositional anomalies in a peridotitic mantle is the ratio of density to shear wave velocities (ρ/Vs). Our results demonstrate that geophysical-petrological models (forward or inverse) that model these two fields (i.e. density and Vs) self-consistently within a robust thermodynamic framework are necessary for characterizing the small-scale thermal and compositional structure of the lithosphere and sublithospheric upper mantle.

The elastic properties and the coefficient of thermal expansion (CTE) of the lithospheric mantle are important parameters that affect the results of lithospheric modelling. However, there is still no consensus on which values are the most appropriate to model the lithosphere, and various average values are used for lithospheres of different age, thermal state, and composition. We present an integrated approach to calculate the elastic properties and the CTE of mantle rocks, based on the mineral physics of composites and considering the spatial heterogeneity of the lithospheric mantle. The method considers the dependence of parameters on pressure and temperature, following a procedure based on an extension of the shear-lag model and thermal expansivity systematics. Representative values are calculated for three lithospheric domains: (a) Archean lithosphere, (b) Phanerozoic continental lithosphere, and (c) oceanic lithosphere. For the case of Archean lithosphere, values of CTE between (3.04 and 3.11) × 10−5 K−1 are found to be suitable for modelling, and a constant depth-derivative for P-waves ∂Vp/∂z ∼ 2.30 × 10−3 s−1 is estimated. Results for Phanerozoic lithosphere show that no single average value of CTE can be used in modelling. Values range non-linearly between (3.25 and 3.47) × 10−5 K−1 at pressures equivalent to depths of 25 and 100 km, respectively. The P-wave velocity variation with depth exhibits a decrease in the range of 25–40 km, followed by almost a constant value of ∼8.08 km s−1 between 40 and 60 km, and a systematic increase with a depth-derivative ∂Vp/∂z ∼ 1.12 × 10−3 s−1 from 60 km downwards. The variation in the CTE is largest in oceanic lithosphere. In young plates (≲20 Ma), values of the CTE range non-linearly from (3.25 to 3.82) × 10−5 K−1 at pressures equivalent to depths of 10 and 50 km, respectively. In old oceanic lithosphere (∼100 Ma), the CTE is slightly smaller, showing values in the range of (3.0–3.7) × 10−5 K−1 at 10 and 80 km, respectively, giving a typical average value of ∼3.45 × 10−5 K−1. P-wave velocity in young oceanic lithosphere decreases from ∼8.14 to 8.0 km s−1 in the first 30 km, then follows a nearly constant path downwards. In old oceanic lithosphere, on the other hand, a systematic reduction from ∼8.2 to 8.1 km s−1 in P-wave velocities is predicted as depth increases from 10 to 80 km. The effects of heterogeneities in CTE and elastic parameters are particularly noticeable in Archean lithosphere, where difference in predicted elevations and geoid heights can reach values of ≥300 and ∼6 m, respectively, when compared to standard models. These effects are less (≲40 and 0.25 m, respectively) in Phanerozoic continental and oceanic lithospheres. Uncertainties in experimental data and geotherms indicate that compositional effects cannot be completely resolved by seismic tomography in regions with P-wave and S-wave anomalies ≲±1.5 and ±3%, respectively.

The extent of the area accommodating convergence between the African and Iberian plates, how this convergence is partitioned between crust and mantle, and the role of the plate boundary in accommodating deformation are not well-understood subjects. We calculate the structure of the lithosphere derived from its density distribution along a profile running from the Tagus Abyssal Plain to the Sahara Platform and crossing the Gorringe Bank, the NW Moroccan margin, and the Atlas Mountains. The model is based on the integration of gravity, geoid, elevation, and heat flow data and on the crustal structure across the NW Moroccan margin derived from reflection and wide-angle seismic data. The resulting mantle density anomalies suggest important variations of the lithosphere-asthenosphere boundary (LAB) topography, indicating prominent lithospheric mantle thickening beneath the margin (LAB > 200 km depth) followed by thinning beneath the Atlas Mountains (LAB similar to 90 km depth). At crustal levels the Iberia-Africa convergence is sparsely accommodated in a similar to 950 km wide area and localized in the Atlas and Gorringe regions, with an inferred shortening of similar to 50 km. In contrast, mantle thickening accommodates a 400 km wide region, thus advocating for a decoupled crustal-mantle mechanical response. A combination of mantle underthrusting due to oblique convergence, together with a viscous dripping fed by lateral mantle dragging, can explain the imaged lithospheric structure. The model is consistent with crustal shortening estimates and with the accommodation of part of the Iberia-Africa convergence farther NW of the Gorringe Bank and/or off the strike of the profile., This research was funded partly by Projects TopoMed (CGL2008‐03474‐E/BTE), ESF‐Eurocores 07‐ TOPOEUROPE‐FP006, TopoAtlas (CGL2006‐05493/BTE), SISAT (CGL2008‐01124‐E/BTE), ATIZA (CGL2009‐09662‐BTE), and Consolider‐Ingenio 2010 Topo‐Iberia (CSD2006‐00041).