A 3D Model of the Upper Mantle near the Hawaiian Archipelago Based on Surface-Wave Tomography Data

The effect of the seismic velocity distribution of the upper mantle on the dispersion and amplitude of Love waves was studied in [1, 4, 7] by numerical experimentation. The main result of these investigations was the discovery of anomalous properties of the higher modes in models of the mantle with a low-velocity zone. The conclusions of [1, 4, 7] for a vertically inhomogeneous flat earth were later confirmed by calculations for a radially inhomogeneous spherical earth [3] and explained by an asymptotic theory in [2]. Similar results for dispersion curves were obtained in [9]. In the present paper, the same studies are continued for Rayleigh waves: the fundamental mode (the Rayleigh wave proper) and the higher modes (shear modes). The dispersion and amplitudes of the first three higher Rayleigh modes are calculated for a number of vertically inhomogeneous models of the earth. Combined theoretical seismograms for various models of the upper mantle and possibilities for experimental detection of waveguides from recordings of higher modes will be considered later.

Relationship between the upper mantle high velocity seismic lid and the continental lithosphere

Three‐dimensional fluid dynamic laboratory simulations are presented that investigate the subduction process in two mantle models, an upper mantle model and a deep mantle model, and for various subducting plate/mantle viscosity ratios (ηSP/ηM = 59–1375). The models investigate the mantle flow field, geometrical evolution of the slab, sinking kinematics, and relative contributions of subducting plate motion and trench migration to the total rate of subduction. All models show that the subducting plate is always moving trenchward resulting from slab pull. Furthermore, all deep mantle models show trench retreat, as do upper mantle models in the initial stage of subduction before slab tip‐transition zone interaction. Upper mantle models with a low ηSP/ηM (66, 217) continue to show trench retreat after interaction. Upper mantle models with a high ηSP/ηM (378, 709) show a period of trench advance after interaction followed by trench retreat. Upper mantle models with a very high ηSP/ηM (1375) show continued trench advance after interaction. The difference in trench migration behavior and associated slab geometries is attributed to both ηSP/ηM and the mantle depth to plate thickness ratio TM/TSP, which both affect the slab bending radius to mantle thickness ratio rB/TM. Four subduction regimes can be defined: Regime I with rB/TM ≤ ∼0.3, trench retreat, slab draping, and a concave trench; Regime II with ∼0.3 < rB/TM < ∼0.5, episodic trench migration, slab folding, and a concave trench; Regime III with rB/TM ≈ 0.5, trench advance, slab rollover geometries, and minor trench curvature; and Regime IV with rB/TM ≥ ∼0.8, trench retreat, slab draping, and a rectilinear trench. In all models, slab‐parallel downdip motion induces poloidal mantle flow structures. In addition, trench retreat and rollback motion of the slab induce quasi‐toroidal return flow around the lateral slab edges toward the mantle wedge. Rollback‐induced poloidal flow around the slab tip is not observed in any of the experiments. Finally, comparison between the slab geometries observed in the upper mantle models and slab geometries observed in nature imply that the effective viscosity ratio between slab and ambient upper mantle in nature is less than 103 and of the order 1–7 × 102, with a best estimate of 1–3 × 102.

For the velocity structure of the lithosphere of East Kamchatka, a tomographic model of high spatial resolution was constructed. Model demonstrates clear relation of velocity values in mantle with subsurface structure. The change in velocity of P-waves relative to the average 1D model reaches ±0.6 km/s. Asthenosphere, in other words the interval where velocities are lower than in solidus point, can be seen as well. Seismological model may be used to control model of deep processes in the given region. The circuit of the Alpine and recent deep processes in the crust and the upper mantle of the East Kamchatka and Cronotsky gulf is considered with use of ideas of a advection-polymorphic hypothesis. Consequences of the processes are coordinated with velocity model of the mantle and composition of magmatic rocks. The data on crustal xenoliths and the composition of igneous rocks of different ages and with different depths of centers of partial melting of mantle rocks were used. The depths of the conductors in the upper mantle are consistent with the deep asthenosphere. But the S values for 1D and 2D models are too large. When using a three-dimensional model in the southern part of Kamchatka, the S value of the electrical conductivity objects in the mantle is reduced. Thus, coordination with the thermal model seems possible. The results of construction of density models of tectonosphere along three cross-sections on the East Kamchatka and adjacent aquatorium are considered. For the model of the upper mantle a thermal model corresponding to the structure of a deep process according to the advection-polymorphous hypothesis is used. The mantle gravitational anomaly reaches a large value — more than 200 mGal. Possibility of explanation of the observed gravitational field without the selection of model parameters is shown.

This work is the mantle component of constructing the Seismological Reference Earth Model in South China (SREM-SC). Although there has been a wide range of research for imaging the upper mantle structures beneath South China, most of them focus on the large-scale features of the upper mantle, and the depth resolution is insufficient for existing surface wave tomography models to distinguish anomalies below 200 km. This study aims to develop a 3-D upper mantle Seismological Reference Earth Model in South China based on the prior tomography models. The shear wave velocity model comes from the analysis of several seismic surface wave tomography, supplemented by body wave tomography and the P-wave velocity model is constructed by the conversion from S-wave velocity. The radial anisotropy model is calculated from the SV-wave and SH-wave velocity. The Density model of the upper mantle is derived using the empirical relationship linking the density to the shear-wave velocity. The model is grid with 0.5° × 0.5° in latitude and longitude and 5 km interval in depth from 60 to 300 km. The mantle component of Seismological Reference Earth Model in South China is expected to provide a good representation of the upper mantle structures for further detailed studies. The mantle component of Seismological Reference Earth Model in South China provides new insights into upper mantle structures that should be meaningful to reveal the dynamic mechanism and tectonic evolution of South China.

SUMMARY In this study, we aim to close the gap between regional and global traveltime tomography in the contextofsurfacewavetomographyoftheuppermantleimplementingtheprincipleofadaptive parametrization. Observations of seismic surface waves are a very powerful tool to constrain the3-DstructureoftheEarth’suppermantle,includingitsanisotropy,becausetheysamplethis volumeefficientlyduetotheirsensitivityoverawidedepthrangealongtheraypath.Onaglobal scale, surface wave tomography models are often parametrized uniformly, without accounting for inhomogeneities in data coverage and, as a result, in resolution, that are caused by effective under- or overparametrization in many areas. If the local resolving power of seismic data is not taken into account when parametrizing the model, features will be smeared and distorted in tomographic maps, with subsequent misinterpretation. Parametrization density has to change locally,formodelstoberobustlyconstrainedwithoutlosinganyaccurateinformationavailable in the best sampled regions. We have implemented a new algorithm for upper mantle surface wave tomography, based on adaptive-voxel parametrization, with voxel size defined by both the ‘hit count’ (number of observations sampling the voxel) and ‘azimuthal coverage’ (how welldifferentazimuthswithrespecttothevoxelarecoveredbythesource-stationdistribution). High image resolution is achieved in regions with dense data coverage, while lower image resolution is kept in regions where data coverage is poorer. This way, parametrization is everywhere tuned to optimal resolution, minimizing both the computational costs, and the non-uniqueness of the solution. The spacing of our global grid is locally as small as ∼50 km. We apply our method to identify a new global model of vertically and horizontally polarized shear velocity, with resolution particularly enhanced in the European lithosphere and upper mantle. We find our new model to resolve lithospheric thickness and radial anisotropy better than earlier results based on the same data. Robust features of our model include, for example, the Trans-European Suture Zone, the Panonnian Basin, thinned lithosphere in the Aegean and Western Mediterranean, possible small-scale mantle upwellings under Iberia and Massif Central, subduction under the Aegean arc and a very deep cratonic root underneath southern Finland.

A thermo-chemical regime in the upper mantle in the early Earth inferred from a numerical model of magma-migration in a convecting upper mantle

We construct a new‐generation 3D density model of the upper mantle of Asia and its surrounding areas based on a joint interpretation of several data sets. A recent model of the crust combining nearly all available seismic data is employed to calculate the impact of the crust on the gravity anomalies and observed topography and to estimate the residual mantle anomalies and residual topography. These fields are jointly inverted to calculate the density variations in the lithosphere and upper mantle down to 325 km. As an initial approximation, we estimate density variations using a seismic tomography model. Seismic velocity variations are converted into temperatures and then to density variations based on mineral physics constraints. In the Occam‐type inversion, we fit both the residual mantle gravity anomalies and residual topography by finding deviations to the initial model. The obtained corrections improve the resolution of the initial model and reflect important features of the mantle structure that are not well resolved by the seismic tomography. The most significant negative corrections of the upper mantle density, found in the Siberian and East European cratons, can be associated with depleted mantle material. The most pronounced positive density anomalies are found beneath the Tarim and South Caspian basins, Barents Sea, and Bay of Bengal. We attribute these anomalies to eclogites in the uppermost mantle, which have substantially affected the evolution of the basins. Furthermore, the obtained results provide evidence for the presence of eclogites in the oceanic subducting mantle lithosphere.

Major regions of inhomogeneity are present in the mantle at depths less than 1000 km. The thermal gradient also greatly exceeds its adiabatic value at relatively shallow depths. Hence the Williamson‐Adams equation cannot be used in this part of the earth to derive the density variation from seismic data. In this paper the density in the upper mantle is obtained by explicitly introducing the constitution of the material there. In the lower mantle the extended Williamson‐Adams equation is used, and the constitution of this region is deduced from the density curve.Recent seismic results for the upper mantle, particularly those relating to low‐velocity zones, are examined. Significant regional differences are present. Beneath the oceans there is a definite LV zone forS, and possibly one forPas well. Beneath Precambrian shields the LV zone forSis less pronounced, and the LV zone forPseems to be absent. Other continental regions are intermediate between these zones. The LV zone is considered to be due to high thermal gradients with mineralogical and chemical heterogeneity superimposed. Differences between the behavior ofPandSare largely due to different temperature coefficients of the two velocities. Regional contrasts arise from regional differences in thermal gradients and mineralogy.Consideration of temperature‐depth relations leads to the conclusion that the mantle is hottest beneath the oceans because of the absence of a thick, radioactive crust, and coolest beneath Precambrian shields because of low heat flow. A self‐consistent model of the mantle requires that the thermal flux at a depth of 400 km be about 0.5 μcal/cm² sec, because geochemical evidence indicates that the K/U ratio in the upper mantle is much smaller than in chondrites. The self‐consistent model requires very high thermal conductivity at high temperatures, such as would be provided by radiative transfer or possibly by movement of material.Petrological models of the upper mantle are constructed on the assumptions of an over‐all pyrolite (ultrabasic) composition and an eclogitic composition. Densities in the mantle are then calculated from known densities, thermal expansions, and compressibilities of minerals inferred to be present. Corrections for the effect of pressure on thermal expansion and compressibility are made from results of the theory of finite strain.The transition zone, at depths between 400 and 1000 km, is the site of a series of major phase transformations leading to close‐packed structures with silicon in sixfold coordination. The density curve in this region is approximated by a linear increase in density with depth. The lower mantle, between 1000 and 2900 km, is considered to be homogeneous, and the density is computed from the Williamson‐Adams equation, modified in some cases to take account of a superadiabatic thermal gradient. The magnitude of the density increase in the transition zone is adjusted to satisfy the restrictions imposed by the total mass and moment of inertia of the earth. A complete density curve for the earth is given for each petrological model of the upper mantle. In constructing them it is assumed that the outer core is homogeneous and adiabatic, and that the inner core is of uniform density.A density curve derived from the third‐order theory of finite strain is fitted to the densities calculated in the lower mantle by least squares. The density of the lower mantle at low temperature and pressure can then be calculated. The results at 20°C and atmospheric pressure are 4.2–4.3 g/cm³ for the adiabatic pyrolite model, and 4.0–4.1 g/cm³ for the adiabatic eclogite model. These values are in good agreement with estimates for the density of the lower mantle based upon recent investigations of phase transformation in olivines and pyroxenes at very high pressures. A superadiabatic gradient of 1°C/km in the lower mantle produces results inconsistent with a plausible constitution; therefore we conclude that the thermal gradient in this part of the earth is smaller than 1°C/km.Our models imply that important differences in density persist to depths of 400 km, and it is inferred that isostatic compensation is not complete before that depth. This conclusion is consistent with gravity data, and it leads to crustal densities more plausible, in terms of observed seismic velocities, than those obtained by assuming compensation at the continental M discontinuity. Because of these deep‐seated density contrasts, the position of the present pole of rotation may well be in equilibrium with the present distribution of continents and oceans.The relatively low temperature beneath the shields could explain their behaving like rigid blocks in Paleozoic and later orogenesis.

We introduce a new method to construct integrated 3‐D models of density, temperature, and compositional variations of the crust and upper mantle based on a combined analysis of gravity, seismic, and tomography data with mineral physics constraints. The new technique is applied to North America. In the first stage, we remove the effect of the crust from the observed gravity field and topography, using a new crustal model (NACr2014). In the second step, the residual mantle gravity field and residual topography are inverted to obtain a 3‐D density model of the upper mantle. The inversion technique accounts for the notion that these fields are controlled by the same factors but in a different way, e.g., depending on depth and horizontal dimension. This enables us to locate the position of principal density anomalies in the upper mantle. Afterward, we estimate the thermal contribution to the density structure by inverting two tomography models for temperature (NA07 and SL2013sv), assuming a laterally and vertically uniform “fertile” mantle composition. Both models show the cold internal part and the hot western margin of the continent, while in some Proterozoic regions (e.g., Grenville province) NA07 at a depth of 100 km is >200°C colder than SL2013sv. After removing this effect from the total mantle anomalies, the residual “compositional” fields are obtained. Some features of the composition density distribution, which are invisible in the seismic tomography data, are detected for the first time in the upper mantle. These results serve as a basis for the second part of the study, in which we improve the thermal and compositional models by applying an iterative approach to account for the effect of composition on the thermal model.

A 3D Vs model of the upper mantle beneath Italy: Insight on the geodynamics of central Mediterranean

SUMMARY We present a new 3-D transversely isotropic shear wave velocity model of the European and Mediterraneanuppermantleobtainedbyanalysisofsurfacewaves.Datausedarefundamentalmode Rayleigh and Love group velocity measurements in the period range 35–170 s, taken on seismograms recorded by European stations for regional earthquakes. The tomographic inversion to map the 3-D earth structure is split into two steps. First, we regionalize the group velocity dispersion measurements, obtaining distinct geographical group velocity maps at different periods; then, each local dispersion curve is inverted separately to find the shear wave velocity structure at depth. The inversion benefits from using ap rioriinformation from a 3-D global mantle model (S20RTS) and a new detailed European crustal model (EPcrust) to constrain the shallower layers. The inversion scheme follows a non-linear iterative algorithm by which Rayleigh and Love group slowness are inverted simultaneously for the best-fitting isotropic Voigt shear wave speed ( 2 vSV + 1 vSH) and radial anisotropy parameter (vSH − vSV). Final merging of the vS profiles results in a new higher resolution 3-D model of European upper mantle. We find that Western Europe and Mediterranean Sea are mainly characterized by relatively low velocities, strongly contrasting with the fast roots of the Eastern European Craton. Many regional scale structures are also evident in the model, thus providing insights into the complex geodynamic framework of the European continent. Most prominent are the low-velocity West Mediterranean spreading basins and European Cenozoic rift system, and seismically fast features connected to subduction of Adria microplate, Hellenic Arc and Calabrian Arc. Radial anisotropy does not vary very significantly with respect to the PREM profile, as available data only resolve lateral variations to a limited degree due to trade-off with velocity. EPmantle has the potential to provide a reliable seismological reference for the upper-mantle structure in the broad European region.

Physical 1D-average reference models of the Earth offer valuable summaries of the radial variations in rock properties and a reference for geophysical studies. PREM, in particular, has been used widely for >40 years and comprises Vp, Vs, density, radial anisotropy and attenuation profiles, while also fitting the Earth’s mass and moment of inertia. Many of PREM’s features have proven remarkably accurate, despite the limited amount of data used to construct it, but some features are inconsistent with now available data. Also, the upper mantle structure differs so much between Earth’s different tectonic environments that a global average is not quite representative of any of them.  The recent growth in seismic station coverage yields very dense data sampling, globally and over different tectonic environments. Here, we use a large global dataset to construct ten 1D, multi-parameter, reference models of the upper mantle, for the globe and for 9 basic tectonic types: cratons; stable platforms; Phanerozoic continents with normal (<46.5 km) and thick (>46.5 km) crust; rifts and continental hotspots; old oceans; intermediate oceans; young oceans; backarcs.The dataset comprises Love and Rayleigh-wave phase velocities, measured using waveform inversion and all available data since 1990s; surface heat flow measurements; topography/bathymetry. With tomography-based tectonic regionalization, we identify areas within each tectonic environment and compute average dispersion curves in the 20-30 to 310 s period range, which constrain shear velocity and anisotropy in the entire upper mantle.We then use computational-petrology-based inversion to calculate 1D physical models for the globe and the 9 basic tectonic types. Our non-linear gradient search converges to true best-fitting models. The main unknowns in the inversion are the depth of the lithosphere-asthenosphere boundary (LAB); the geotherm from the LAB down to 400 km depth; radial anisotropy (0-800 km). The steady-state geotherm in the lithosphere is computed from the LAB depth and the radiogenic heat production and thermal conductivity profiles by solving the conductive heat transfer equation. Rock composition and the geotherm determine the density, seismic velocities and attenuation down to 400 km. Seismic velocities in the crust, transition zone (410-660 km) and shallow lower mantle can vary to fit the data. Density below 410 km and all parameters in the core and most of the lower mantle are from PREM. Like PREM, our reference models honour the Earth's mass and moment of inertia.Small phase-velocity errors and relative data-synthetic misfits (<~0.1%) are necessary to resolve radial trade-offs in the upper-mantle structure. We achieved this by obtaining very accurate dispersion curves and by meticulously tuning the inversion, its parameterisation and regularisation.The best-fitting models have slightly depleted lithospheric mantle and fertile asthenosphere for most tectonic types. In Archean and Proterozoic continents, the mantle lithosphere is more depleted. No other compositional heterogeneities are required to fit the data. Isotropic-average seismic velocities decrease monotonically from the Moho to the LAB. The geotherms follow the mantle adiabatic temperature gradient in the asthenosphere. Our results provide useful, accurate new reference models for global and regional seismic imaging and other geophysical studies. 

The short-period seismic phase Sn has been interpreted by Stephens & Isacks as a lid wave’ in which the seismic energy is constrained to the uppermost few tens of kilometres of the mantle. We have extended their normal-mode interpretation for structures both with and without low-velocity zones (LVZ) in the upper mantle. We have used spherical, anelastic models of the Earth. For a model of an oceanic mantle with a LVZ, we agree that Sn is a lid wave for sources above 200–250 km, if only the onset of Sn is considered. The later portions of the Sn wave train sample the structure as deeply as the 420-km discontinuity. For deeper foci, the pseudo-lid wave does not appear to be generated; even the onset of Sn samples the deeper mantle structure. For a model of a continental mantle without a LVZ, in general, sources at all depths above the 420-km discontinuity appear to generate teleseismic Sn which samples the entire mantle as deeply as the discontinuity and which travels with a velocity significantly greater than the lid velocity. Thus the velocity of Sn may be an important diagnostic to determine whether or not a LVZ exists in the upper mantle.

SUMMARY We present a new, S-velocity model of the European upper mantle, constrained by inversions of seismic waveforms from broad-band stations in Europe and surrounding regions. We collected seismograms for the years 1990–2007 from all permanent stations in Europe for which data were available. In addition, we incorporated data from temporary experiments. Automated multimode inversion of surface and S-wave forms was applied to extract structural information from the seismograms, in the form of linear equations with uncorrelated uncertainties. The equations were then solved for seismic velocity perturbations in the crust and mantle with respect to a 3-D reference model with a realistic crust. We present two versions of the model: one for the entire European upper mantle and another, with the highest resolution, focused on the upper 200 km of the mantle beneath western and central Europe and the circum Mediterranean. The mantle lithosphere and asthenosphere are well resolved by both models. Major features of the lithosphere–asthenosphere system in Europe and the Mediterranean are indentified. The highest velocities in the mantle lithosphere of the East European Craton (EEC) are found at about 150 km depth. There are no indications for a deep cratonic root below about 330 km depth. Lateral variations within the cratonic mantle lithosphere are resolved as well. The locations of kimberlites correlate with reduced S-wave velocities in the shallow cratonic mantle lithosphere. This anomaly is present in regions of both Proterozoic and Archean crust, pointing to an alteration of the mantle lithosphere after the formation of the craton. Strong lateral changes in S-wave velocity are found at the northwestern margin of the EEC and may indicate erosion of cratonic mantle lithosphere beneath the Scandes by hot asthenosphere. The mantle lithosphere beneath western Europe and between the Tornquist–Teisseyre Zone and the Elbe Line shows moderately high velocities and is of an intermediate character, between cratonic lithosphere and the thin lithosphere of central Europe. In central Europe, Caledonian and Variscian sutures are not associated with strong lateral changes in the lithosphere–asthenosphere system. Cenozoic anorogenic intraplate volcanism in central Europe and the circum Mediterranean is found in regions of shallow asthenosphere and close to changes in the depth of the lithosphere–asthenosphere boundary. Very low velocities at shallow upper-mantle depths are present from eastern Turkey towards the Dead Sea transform fault system and Sinai, beneath locations of recent volcanism. Low-velocity anomalies extending vertically from shallow upper mantle down to the transition zone are found beneath the Massif Central, Sinai and the Dead Sea, the Canary Islands and Iceland.