Tomographic filtering of geodynamic models: Implications for model interpretation and large-scale mantle structure (original) (raw)
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A comparison of tomographic and geodynamic mantle models
2002
We conduct a comprehensive and quantitative analysis of similarities and differences between recent seismic tomography models of the Earth's mantle in an attempt to determine a benchmark for geodynamic interpretation. After a spherical harmonic expansion, we find the spectral power and radial correlation of each tomographic model as a function of depth and harmonic degree. We then calculate the correlation, at the same depths and degrees, between all possible pairs of models, to identify stable and model-dependent features (the former being usually of longer spatial wavelength than the latter). We can therefore evaluate the degree of robust structure that seismologists have mapped so far and proceed to calculate ad hoc mean reference models. Tomographic models are furthermore compared with two geodynamic subduction models that are based on plate motion reconstructions. We find systematically low intermediate-wavelength correlation between tomography and convective reconstruction models and suggest that the inadequate treatment of the details of slab advection is responsible. However, we confirm the presence of stable, slab-like fast anomalies in the mid-mantle whose geographic pattern naturally associates them with subduction. This finding, in addition to our analysis of heterogeneity spectra and the absence of strong minima in the radial correlation functions besides the one at $700 km, supports the idea of whole mantle convection with slab penetration through the 660 km phase transition, possibly accompanied by a reorganization of flow.
3SMAC: an a priori tomographic model of the upper mantle based on geophysical modeling
Physics of the Earth and Planetary Interiors, 1996
We present an a priori three-dimensional 'tomographic' model of the upper mantle. We construct this model (called 3SMAC -three-dimensional seismological model a priori constrained) in four steps: we compile information on the thickness of 'chemical' layers in the Earth (water, sediments, upper and lower crust, etc); we get a 3D temperature distribution from thermal plate models applied to the oceans and continents; we deduce the mineralogy in the mantle from pressure and temperature and we finally get a three-dimensional model of density, seismic velocities, and attenuation by introducing laboratory measurements of these quantities as a function of pressure and temperature. The model is thus consistent with various geophysical data, such as ocean bathymetry, and surface heat flux. We use this model to compute synthetic travel-times of body waves, and we compare them with observations. A similar exercise is performed for surface waves and normal modes in a companion paper Geophys. Res., in press). We find that our model predicts the bulk of the observed travel-time variations. Both the amplitude and general pattern are well recovered. The discrepancies suggest that tomography can provide useful regional information on the thermal state of the continents. In the oceans, the flattening of the sea-floor beyond 70 Ma seems difficult to reconcile with the seismic observations. Overall, our 3SMAC model is both a realistic model, which can be used to test various tomographic methods, and a model of the minimum heterogeneities to be expected from geodynamical modeling. Therefore, it should be a useful a priori model to be used in tomographic inversions, in order to retrieve reliable images of heterogeneities in the transition zone, which should, in turn, greatly improve our understanding of geodynamical processes in the deep Earth. 3SMAC and accompanying software can be retrieved by anonymous ftp at geoscope.ipgp.jussieu.fr.
Journal of Geophysical Research, 1999
Recently published images of the Earth's mantle are characterized by a nominal resolution much higher than that used in previous studies, where substantially different techniques were employed. The agreement between such "high-resolution" and "low-resolution" images often seems very poor. In an attempt to determine the reason for this discrepancy, we analyze how the choice of inversion algorithm (exact or iterative), regularization (norm or roughness minimization), and parameterization (spherical harmonics up to a variable degree, blocks) affects a tomographic model. We also investigate the effects of the varying density of the data coverage on the final solution. In our experiments we employ two seismic data sets: Rayleigh wave phase velocity at 75 s period and P wave travel times. We construct a new model of P velocity in the mantle (BDP98) based on the International Seismological Center bulletins 1964-1992. We use our findings in an evaluation of recent mantle models, including our own, focusing on similarities and discrepancies between models of different nominal resolution. In all the models the long-wavelength component is the most stable. However, consistent high-resolution details, probably corresponding to features of the real Earth, are also seen. In general, we conclude that most of the differences between existing tomographic models derive from the arbitrary choices made in the process of defining and solving the inverse problem, rather than from actual errors or approximations. scribe the three-dimensional (3-D) models by Dziewonski [1982, 1984] and Clayton and Comer [1983]. Both studies used the P wave travel time data from the bulletins of the International Seismological Centre (ISC), but where the former was aimed at extracting the very
Tomography of core-mantle boundary and lowermost mantle coupled by geodynamics
Geophysical Journal International, 2012
We conduct joint tomographic inversions of P and S travel time observations to obtain models of dy P and dy S in the entire mantle. We adopt a recently published method which takes into account the geodynamic coupling between mantle heterogeneity and core-mantle boundary (CMB) topography by viscous flow, where sensitivity of the seismic travel times to the CMB is accounted for implicitly in the inversion (i.e. the CMB topography is not explicitly inverted for). The seismic maps of the Earth's mantle and CMB topography that we derive can explain the inverted seismic data while being physically consistent with each other. The approach involved scaling P-wave velocity (more sensitive to the CMB) to density anomalies, in the assumption that mantle heterogeneity has a purely thermal origin, so that velocity and density heterogeneity are proportional to one another. On the other hand, it has sometimes been suggested that S-wave velocity might be more directly sensitive to temperature, while P heterogeneity is more strongly influenced by chemical composition. In the present study, we use only Sand nd not P-velocity, to estimate density heterogeneity through linear scaling, and hence the sensitivity of core-reflected P phases to mantle structure. Regardless of whether density is more closely related to P-or S-velocity, we think it is worthwhile to explore both scaling approaches in our efforts to explain seismic data. The similarity of the results presented in this study to those obtained by scaling P-velocity to density suggests that compositional anomaly has a limited impact on viscous flow in the deep mantle.
Global scale models of the mantle flow field predicted by synthetic tomography models
Physics of the Earth and Planetary Interiors, 2010
Using a multi-disciplinary technique incorporating the heterogeneous resolution of seismic tomography, geodynamical models of mantle convection, and relationships derived from mineral physics, we investigate the method of using seismic observations to derive global-scale 3D models of the mantle flow field. We investigate the influence that both the resolution of the seismic model and the relationship used to interpret wavespeed anomalies in terms of density perturbations have on the calculated flow field. We create a synthetic seismic tomography model from a 3D spherical whole mantle geodynamic convection model and compare present-day global mantle flow fields from the original convection model and from a geodynamical model which uses the buoyancy field of the synthetic tomography model as an initial condition. We find that, to first order, the global velocity field predicted by the synthetic seismic model correlates well with the flow field from the original convection model throughout most of the mantle. However, in regions where the resolving power of the seismic model is low, agreement between the models is reduced. We also note that the flow field from the synthetic seismic model is relatively independent of the density-velocity scaling ratio used.
We present a new global whole-mantle model of isotropic and radially anisotropic S velocity structure (SGLOBE-rani) based on~43,000,000 surface wave and~420,000 body wave travel time measurements, which is expanded in spherical harmonic basis functions up to degree 35. We incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure. This is possible since we utilize short-period group-velocity data with a period range down to 16 s, which are strongly sensitive to the crust. The isotropic S velocity model shares common features with previous global S velocity models and shows excellent consistency with several high-resolution upper mantle models. Our anisotropic model also agrees well with previous regional studies. Anomalous features in our anisotropic model are faster SV velocity anomalies along subduction zones at transition zone depths and faster SH velocity beneath slabs in the lower mantle. The derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts, which has been difficult to constrain due to poor access compared with continental crusts. Preliminary Reference Earth Model (PREM) [Dziewoński and Anderson, 1981], one of the most widely used 1-D reference models, incorporates radial anisotropy from the Moho (24.4 km depth) to the 220 km discontinuity. It is mainly constrained by joint analysis of Rayleigh and Love wave data. Montagner and Anderson [1989] and Montagner and Kennett [1996] modified PREM using additional normal-mode and travel time data, respectively. Moreover, Beghein et al. [2006] also modified PREM using normal-mode data in a wide frequency range. Fully 3-D radially anisotropic models for both the upper and whole mantle have been developed since the 1980s [e.g.,
Subduction to the lower mantle - a comparison between geodynamic and tomographic models
2012
It is generally believed that subduction of lithospheric slabs is a major contribution to thermal heterogeneity in Earth's entire mantle and provides a main driving force for mantle flow. Mantle structure can, on the one hand, be inferred from plate tectonic models of subduction history and geodynamic models of mantle flow. On the other hand, seismic tomography models provide important information on mantle heterogeneity. Yet, the two kinds of models are only similar on the largest (1000 s of km) scales and are quite different in their detailed structure. Here, we provide a quantitative assessment how good a fit can be currently achieved with a simple viscous flow geodynamic model. The discrepancy between geodynamic and tomography models can indicate where further model refinement could possibly yield an improved fit. Our geodynamical model is based on 300 Myr of subduction history inferred from a global plate reconstruction. Density anomalies are inserted into the upper mantle beneath subduction zones, and flow and advection of these anomalies is calculated with a spherical harmonic code for a radial viscosity structure constrained by mineral physics and surface observations. Model viscosities in the upper mantle beneath the lithosphere are ∼10 20 Pas, and viscosity increases to ∼10 23 Pas in the lower mantle above D " . Comparison with tomography models is assessed in terms of correlation, both overall and as a function of depth and spherical harmonic degree. We find that, compared to previous geodynamic and tomography models, correlation is improved, presumably because of advances in both plate reconstructions and mantle flow computations. However, high correlation is still limited to lowest spherical harmonic degrees. An important ingredient to achieve high correlation -in particular at spherical harmonic degree two -is a basal chemical layer. Subduction shapes this layer into two rather stable hot but chemically dense "piles", corresponding to the Pacific and African Large Low Shear Velocity Provinces. Visual comparison along cross sections indicates that sinking speeds in the geodynamic model are somewhat too fast, and should be 2 ± 0.8 cm yr −1 to achieve a better fit. Solid Earth, 3, 415-432, 2012 www.solid-earth.net/3/415/2012/ Solid Earth, 3, 415-432, 2012 www.solid-earth.net/3/415/2012/ Solid Earth, 3, 415-432, 2012 www.solid-earth.net/3/415/2012/
Journal of Geophysical Research, 1998
We investigate the very long-wavelength, global pattern of surface heat flux anomalies within the context of whole-mantle and layered-mantle anelastically compressible internal loading •heories. Since the internal loading framework does not yield a direct estimate of the geotherm, we argue that accurate predictions for the surface heat flux may nevertheless be obtained by assuming that it is linearly related to the radial component of flow velocity at shallow depth in the mantle. The mantle convective circulation is assumed to be driven by density heterogeneity inferred from global seismic tomography models. Best results for the pattern of surface heat flux anomalies are obtained for models that significantly impede the circulation at a depth of 670 km. Total variance reductions of 60-65% (degree 1-5) are obtained when the viscosity profile includes a low-viscosity asthenosphere. Within the context of our modeling assumptions, however, whole-mantle circulation models provide best descriptions of the long-wavelength nonhydrostatic gravity data. In order to resolve the gravity-heat flux impasse that is revealed herein, we consider the possibility of modifying the a priori global seismic models employed in the calculations. We show that the rigidly layered-mantle internal loading theory is equivalent to a theory in which no explicit flow-blocking boundary condition is imposed at 670 km but in which the buoyancy field inferred from the a priori tomographic model is supplemented by flow-blocking heterogeneity in the form of an appropriately constrained sheet mass load. We develop a general mathematical formalism describing how the introduction of appropriately constrained sheet mass loads allows the exact reconciliation of a number of a priori constraints or hypotheses concerning the structure of the circulation. Using this formalism, we explore the extreme nonuniqueness that not only characterizes internal loading theory inferences of the depth profile of mantle viscosity but also inferences of the radial style of the circulation. On this basis, we suggest that great caution is warranted with respect to tomography-based inferences of mantle properties. Based on a viscosity profile whose depth dependence is close to that independently inferred within the context of postglacial rebound studies, we present plausible resolutions of the gravity-heat flux impasse effected either within the framework of whole-mantle or layered-mantle circulation models. the pattern of aspherical geoid anomalies [Hager, 1984; have been invoked for this purpose. Such analyses have been performed using the internal loading formalism [e.g., Ricard et al., 1984; Richards and Hager, 1984; Forte and Peltier, 1987, 1991; Panasyuk et al., 1996] in which the internal density which constitutes the load is inferred on the basis of seismic tomography. Other convection-related fields such as the motion 23,743 23,744 PARI AND PELTIER: HEAT FLUX CONSTRAINTS ON MANTLE CONVECTION of surface tectonic plates [Hager and O'Connell, 1981; Forte and Peltier, 1987; 1991; Ricard and Vigny, 1989; Ricard et al., 1989; Forte et al., 1991; Ricard and Wuming, 1991; Richards and Engebretson, 1992; Lithgow-Bertelloni and Richards, 1998], the depth-dependent profile of radial heat advection [Hager and Clayton, 1989; Pari and Peltier, 1995; Forte et al., 1995; Forte and Woodward, 1997b], and the less well constrained dynamic core-mantle boundary topography [e.g., Forte and Peltier, 1989] and dynamic surface topography [e.g., Hager and Clayton, 1989; Forte et al., 1993b; Thoraval et al., 1995; Le Stunff and Ricard, 1995; •adek et al., 1997; Wen and Anderson, 1997] have also been employed as constraints.
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.