Tomography of core-mantle boundary and lowermost mantle coupled by geodynamics (original) (raw)
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Geophysical Journal International, 1991
In a dynamic Earth, mantle mass heterogeneities induce gravity anomalies, surface velocities and surface topography. These lateral density heterogeneities can be estimated on the basis of seismic tomographic models. Recent papers have described a realistic circulation model that takes into account the observed plate geometry and is able to predict the rotation vectors of the present plates. The relationship between the surface observables and the heterogeneities is sensitive to the viscosity stratification of the mantle. Here we use this model, combined with a generalized least-squares method, in order to infer the viscosity profile of the Earth from the surface observations, and. to get some new insight into the 3-D density structure of the mantle. The computed radial viscosity profile presents a continuous increase of more than two orders of magnitude. The asthenosphere has a viscosity close to 2 x lo2' Pa s. No sharp discontinuity is requested at the upper-lower mantle interface. The largest viscosity 7 X 10" Pas is reached in the middle of the lower mantle. A t greater depth, approaching the core-mantle boundary, the viscosity decreases by one order of magnitude. The model suggests that the well-known degree-2 and order-2 anomaly in the transition zone of the upper mantle is merely the signature of the slabs. It also slightly increases the degree-2 and order-0 in the lower mantle and decreases it in the upper mantle. In other words the inversion requests a hotter lower mantle beneath the equator and a colder upper mantle at the same latitudes.
Geophysical Journal International, 2009
The joint interpretation of seismic and geodynamic data requires mineral physical parameters linking seismic velocity to density perturbations in the Earth's mantle. The most common approach is to link velocity and density through relative scaling or conversion factors: R ρ/s = dlnρ/dlnV S . However, the range of possible R ρ/s values remains large even when only considering thermal effects. We directly test the validity of several proposed depthdependent conversion profiles developed from mineral physics studies for thermally-varying properties of mantle materials. The tests are conducted by simultaneously inverting shear wave traveltime data and a diverse suite of convection-related constraints interpreted with viscous-flow response functions of the mantle. These geodynamic constraints are represented by surface spherical harmonic basis functions (up to harmonic degree 16) and they consist of the global free-air gravity field, tectonic plate divergences, dynamic surface topography and the excess ellipticity of the core-mantle boundary (CMB). The tests yield an optimum 1-D thermal R ρ/s profile that is generally compatible with all considered data, with the exception of the dynamic surface topography that is most sensitive to the shallow upper mantle. This result is not surprising given that cratonic roots are known to be compositionally-distinct from the surrounding ambient mantle. Moreover, it is expected that the temperature-dependence of the thermal R ρ/s in the upper mantle is significant due to the temperature-dependence of seismic attenuation or Q. Therefore, a simple 1-D density-velocity relationship is insufficient. To address this problem, we obtained independent estimates of the first-order correction factors to the selected R ρ/s profile within the cratonic roots and in the ambient (thermal) upper mantle. These correction factors, defined as ∂ R ρ/s /∂lnV S , are highly negative within the cratons signifying considerable compositional buoyancy. This result confirms that the negative buoyancy associated with the low temperatures in the cratons is significantly counteracted by the composition-induced positive buoyancy resulting in near-neutral buoyancy of the cratonic roots. Within the ambient upper mantle, the correction factors are found to be positive which is consistent with the expected behaviour of the R ρ/s relationship in thermally-varying upper-mantle material. We obtain a significantly greater reconciliation of the global gravity anomalies and dynamic surface topography when applying these laterally-varying corrections compared to a simple 1-D R ρ/s relationship. Inversion for a fully 3-D R ρ/s relationship reveals secondary effects including additional compositional variation within the cratonic roots and the deep-mantle superplume structures. We estimate the relative magnitude of the thermal and compositional (non-thermal) contributions to mantle density anomalies and conclude that thermal effects dominate shear wave and density heterogeneity throughout the non-cratonic mantle. We also demonstrate the potential pitfalls of scaling a purely seismically-derived model to obtain density rather than performing a true joint inversion to obtain velocity and density simultaneously.
Constraining mantle flow with seismic and geodynamic data: A joint approach
Earth and Planetary Science Letters, 2006
Understanding the style of convective flow occurring in the mantle is essential to understand the thermal and chemical evolution of Earth's interior as well as the forces driving plate tectonics. Models of mantle convection based on three-dimensional (3-D) seismic tomographic reconstructions have the potential to provide the most direct constraints on mantle flow. Seismic imaging of deep Earth structure has made great advances in recent years; however, it has not been possible to reach a consensus on the nature of convection in the mantle. Models of mantle flow based on tomography results have yielded variable conclusions largely because of the inherent non-uniqueness and differing degrees of resolution of seismic tomography models as well as the difficulty in determining flow directly from seismic images. Here we address this difficulty by simultaneously inverting global seismic and convection-related data sets. The seismic data consist of globally distributed shear body wave travel times including multi-bounce S-waves, shallow-turning triplicated phases, as well as core reflections and phases traversing the core (SKS and SKKS). Convection-related data sets include global free air gravity, tectonic plate divergence, and excess ellipticity of the core-mantle boundary. In addition, the convection-related constraint on dynamic surface topography is estimated on the basis of a recent global model of crustal heterogeneity. These convection-related observables are related to mantle density anomalies through instantaneous mantle flow calculations and linked to the seismic data via optimized density-velocity scaling relationships. Simultaneous inversion allows us to test various mantle flow hypotheses directly against the combined seismic and convection data sets, rather than considering flow predictions based solely on a seismically derived 3-D mantle model. In this study, we test four different mantle flow hypotheses, including whole-mantle flow and models with impenetrable flow boundaries at depths of 670 km, 1200 km, and 1800 km. This hypothesis testing shows that the combined global seismic and geodynamic data sets are best reconciled when a whole-mantle flow scenario is considered. Convection models with restrictive flow boundaries within the lower mantle provide distinctly poorer fits to these combined data sets providing evidence that the mantle flows without permanent hindrance at the boundaries considered.
Earth and Planetary Science Letters, 2012
Two large low-shear-velocity provinces (LLSVPs) in the deep mantle beneath Africa and the Pacific are generally interpreted as hot but chemically dense 'piles', which have remained isolated from mantle circulation for several hundred million years. This interpretation largely hinges on four seismic observations: (i) their shear wave velocity anomalies are considered too large for purely thermal structures; (ii) shear wave velocity gradients at their edges are sharp; (iii) their shear to compressional wave-speed anomaly ratios are high; and (iv) their shear and bulk-sound velocity anomalies appear to be anti-correlated. However, using compressible global mantle circulation models driven by 300 Myr of plate motion history and thermodynamic methods for converting from physical to seismic structure, we show that observed lower mantle shear wave velocity anomalies do not require, and are most likely incompatible with, large-scale chemical 'piles'. A prescribed core-mantle-boundary temperature of 4000 K, which is consistent with current estimates, combined with anelastic seismic sensitivity to temperature, ensures that purely thermal LLSVPs, strongly focussed beneath Africa and the Pacific by subduction history, can reconcile observed shear wave velocity anomalies and gradients. By contrast, shear wave velocity anomalies from models that include dense chemical 'piles' at the base of Earth's mantle, where 'piles' correspond to only 3% of the mantle's volume, are substantially stronger than the tomographic model S40RTS, even after accounting for limited tomographic resolution. Our results also suggest that in the presence of post-perovskite, elevated ratios between shear and compressional wavespeed anomalies and the correlation between shear and bulk-sound velocity anomalies cannot be used to discriminate between thermal and compositional heterogeneity at depth: in all calculations, an anticorrelation only occurs within the post-perovskite stability field. Taken together, this implies that although there must be considerable chemical heterogeneity within Earth's mantle, large, coherent 'piles' are not required to reconcile the seismic observations examined here. Indeed, our results suggest that if chemical heterogeneity is present in these regions, its dynamical and seismic significance is far less than has previously been inferred.
Low seismic resolution cannot explain S/P decorrelation in the lower mantle
Geophysical Research Letters, 2011
1] Inverted models of the deep mantle show a decorrelation between maps of shear V S and compressional V P wave velocities, an anti-correlation between the bulk sound velocity V and V S and a much larger variability of V S with respect to V P , expressed by large values of the ratio of their relative lateral variations. We carried out synthetic tests to verify if these features could be artifacts, explained by limits in tomographic resolution: synthetic data are calculated for an "input" model, and linearly inverted, as in tomography, to find an "output" model. Comparing the values of the aforementioned parameters for two different chemically homogeneous input models with the associated reconstructed output ones, we found that artifacts caused by realistic data noise and the nonuniform distribution of seismic sources and stations over the globe are not sufficient to introduce the features previously described. We confirm that compositional effects are required to explain them.
GyPSuM: A joint tomographic model of mantle density and seismic wave speeds
Journal of Geophysical Research, 2010
GyPSuM is a 3-D model of mantle shear-wave (S) speeds, compressional-wave (P) speeds and density. The model is developed through simultaneous inversion of seismic bodywave travel times (P and S) and geodynamic observations while using realistic mineral physics parameters linking wave speeds and density. Geodynamic observations include the global freeair gravity field, divergence of the tectonic plates, dynamic topography of the free surface, and the flow-induced excess ellipticity of the core-mantle boundary. GyPSuM is built with the philosophy that heterogeneity that most closely resembles thermal variations is the simplest possible solution. Models of the density field from Earth's free oscillations have provided great insight into the density configuration of the mantle; but are limited to very long-wavelength solutions. Alternatively, scaling higher resolution seismic images to obtain density anomalies generates density fields that do not satisfy geodynamic observations. The current study provides a 3-D density model for the mantle that directly satisfies geodynamic and seismic observations through a joint seismic-geodynamic inversion process. Notable density field observations include high-density piles at the base of superplume structures, supporting the general results of past normal mode studies. However, we find that these features are more localized and lower amplitude than past studies would suggest. When we consider both fast and slow seismic anomalies in GyPSuM, we find that P-and S-wave speeds are strongly correlated throughout the mantle. However, we find a low correlation of fast S-wave zones in the deep mantle (>1500 km depth) with the corresponding P-wave anomalies, suggesting a systematic divergence from simplified thermal effects in ancient subducted slab anomalies. The cratonic lithosphere and D"
Joint inversions of seismic and geodynamic data for models of three—dimensional mantle heterogeneity
Journal of Geophysical Research, 1994
The seismic models of three-dimensional (3-D) mantle heterogeneity may be interpreted in terms of the density perturbations which drive mantle flow and thus provide important predictions of flow-related observables, such as the nonhydrostatic geoid. The current models of global-scale shear velocity heterogeneity provide reasonably good fits to the long-wavelength nonhydrostatic geoid data (60-70% variance reductions) but rather poor fits to the corresponding free-air gravity anomahes (30-40% variance reductions). This major difference is due to two factors: (1) the very different amphtude spectrum of the gravity anomahes, which is nearly fiat for degrees oe < 8, and (2) the relatively poor match between the individual harmonic components of the predicted and observed gravity anomahes for degrees oe > 3. The largest mismatch between the pattern of predicted and observed gravity anomahes is in the southern hemisphere. This observation suggests that one reason for the poor overall match between the two fields may be that the global seismic data are not resolving sufficiently well the heterogeneity in the southern hemisphere portion of the deep mantle. In contrast, the gravity data provide accurate and geographically uniform constraints on the vertically integrated heterogeneity in the ma,ntle. We therefore perform a series of experiments in which we simultaneously invert a large set of seismic data (which includes long-period waveforms, $S-S and ScS-5' differential travel times, and normal-mode structure coefficients) and the long-wavelength gravity anomaly data. We thus determine whether it is possible to derive new 3-D heterogeneity models which satisfy both data sets. The gravity anomaly data are interpreted in the context of spherically-symmetric viscous flow models of the mantle. In these inversion experiments we test several radial viscosity and 5lnp/5lnv profiles and thereby assess their plausibihty. The joint seismic-geodynamic inversions reveal that it is indeed possible to greatly improve the fit to the free-air gravity anomahes (with variance reductions of 80-90% readily accessible) while preserving the fit to the seismic data. This improvement is achieved with some adjustment to the heterogeneity in the depth range 1500-2500 kin, where the seismic data constraints appear to be weakest. The joint inversions also reveal new structures in the southern hemisphere portion of the lower mantle which apparently are not resolved by the seismic data alone.
Mapping the lowermost mantle using core-reflected shear waves
Journal of Geophysical Research, 1994
A map of laterally varying D-double prime velocities is obtained for the region from 50 deg S to 50 deg N in latitude and 70 deg E to 190 deg E in longitude. Velocities are found using an analysis of the differential travel time residuals from 481 ScS-S and 266 sScS-sS phase pairs. The long-period data are taken from the Global Digital Seismograph Network digital waveform catalog for the time period of January 1980 to March 1987. Each differential travel time is found by a cross correlation of the S phase ground displacement, corrected to simulate differential attenuation, with all following phases. Travel times are corrected for ellipticity and mantle heterogeneity outside of their D-double prime paths, and the remaining residuals are interpreted as the result of D-double prime heterogeneity. Ray-tracing tests are made to check the validity of converting travel time residuals into velocity path anomalies. The resulting map reveals significant long-wavelength D-double prime structure including a 3% low-velocity region beneath northeastern Indonesia, surrounded by three identified high-velocity zones beneath northwestern Pacifica (+4%), Southeast Asia (+3%), and Australia (+3-5%). This structure is of continent/ocean spatial scales and is most likely created by dynamic processes dominant in the lower mantle. The low-velocity region may have both chemical and thermal origins and is very possibly the site of an incipient lower mantle plume where mature D-double prime rock which has been heated by the core has become gravitationally unstable and begun to rise. A chemical component possibly exists as a chemical boundary layer is dragged laterally toward the plume site, much the way continents are dragged toward subduction zones. The high-velocity zones possibly result from the downward convection of cold lower mantle plumes, which pond at the core-mantle boundary. These seismic anomalies may also contain a chemical signature from faster iron-poor materials brought down through the lower mantle or the additional presence of SiO2 stishovite, perhaps in its higher-pressure polymorph.
A Geodynamic Model of Mantle Density Heterogeneity
Journal of Geophysical …, 1993
Using Cenozoic and Mesozoic plate motion reconstructions, we derive a model of present-day mantle density heterogeneity under the assumption that subducted slabs sink vertically into the mantle. The thermal buoyancy of these slabs is estimated from the observed thermal subsidence (cooling) of oceanic lithosphere. Slat) velocities in the upper mantle are computed from the local convergence rate. We assume that slabs cross the upper/lower mantle interface and continue sinking into the lower mantle witIx a reduced velocity. For a velocity reduction factor between :2 and 5, our slab heterogeneity model is as correlated with current tomographic models as these models are correlated with each other. We have also computed a synthetic geoid from our density model. For a viscosity increase of about a factor of 40 from the upper to lower mantle, our model predicts the first 8 spherical harmonic degrees of the geoid witIx statistical confidence larger than 95% and explains 84% of the observed geoid assuming that the model C21 and S21 terms are absent due to a long relaxation time for Earth's rotational bulge. Otherwise, 73% of the geoid variance is explained. The viscosity increase is consistent witIx our velocity reduction factor for slabs entering the lower mantle, since downwelling velocities are expected to scale roughly as the logarithm of viscosity (loge 40 -3.7). These results show that the history of plate tectonics can explain the main features of the present-day structure of the mantle. The dynamic topography induced by this heterogeneity structure consists mainly of about 1-kin amplitude lows concentrated along the active continental margins of the Pacific basin. Our model can also be used to predict the time variation of mantle heterogeneity and the gravity field. We find that the "age" of the geoid, defined as the time in the past herore which the geoid becomes uncorrelated witIx the present geoid, is about 50 m.y. Our model for the history of the degree 2 geoid, which is equivalent to the history of the inertia tensor, should give us a tool to study the variations in Earth's rotation pole indicated in paleomagnetic studies.