Mantle Density Heterogeneities Research Articles

An analysis of core–mantle boundary Stoneley mode sensitivity and sources of uncertainty

SUMMARYStoneley modes are a special subset of normal modes whose energy is confined along the core–mantle boundary (CMB). As such, they offer a unique glimpse into Earth structure at the base of the mantle. They are often observed through coupling with mantle modes due to rotation, ellipticity and lateral heterogeneity, though they can be detected without such coupling. In this study, we explore the relative sensitivities of seismic spectra of two low-frequency Stoneley modes to several factors, taking as reference the fully coupled computation up to 3 mHz in model S20RTS. The factors considered are (i) theoretical, by exploring the extent to which various coupling approximations can accurately reproduce reference spectra and (ii) model-based, by exploring how various Earth parameters such as CMB topography, attenuation and S- and P-wave structures, and the seismic source solution may influence the spectra. We find that mode-pair coupling is insufficiently accurate, but coupling modes within a range of ±0.1 mHz produces acceptable spectra, compared to full coupling. This has important implications for splitting function measurements, which are computed under the assumption of isolated modes or at best, mode-pair or group coupling. We find that uncertainties in the P-wave velocity mantle model dominate compared to other model parameters. In addition, we also test several hypothetical models of mantle density structure against real data. These tests indicate that, with the low-frequency Stoneley mode spectral data considered here, it is difficult to make any firm statement on whether the large-low-shear-velocity-provinces are denser or lighter than their surroundings. We conclude that better constraints on long wavelength elastic mantle structure, particularly P-wave velocity, need to be obtained, before making further statements on deep mantle density heterogeneity. In particular, a dense anomaly confined to a thin layer at the base of the mantle (less than ∼100–200 km) may not be resolvable using the two Stoneley modes tested here, while the ability of higher frequency Stoneley modes to resolve it requires further investigations.

A numerical model for the gravimetric recovery of sub-lithospheric mantle structures

It is a well-known fact that the long-wavelength terrestrial geoid undulations are mainly attributed to deep mantle density heterogeneities, while more detailed features in the geoid geometry are associated with the topography and the lithospheric density structure. To enhance a gravitational signature of mantle density heterogeneities below the lithosphere, the gravitational contributions of topography and lithospheric density heterogeneities should be modelled and subsequently removed from Earth's gravity field. The refined gravity field obtained after this numerical procedure is more suitable for a recovery of a mantle density structure (below the lithosphere). Following this idea, methods for a spherical harmonic analysis and synthesis of gravity field and lithospheric density structures are presented, and a theoretical relation between gravity field and mass density structure is formulated. Since a gravimetric recovery of inner density structure has a non-unique solution, we propose an alternative method based on a conversion of seismic velocities to mass densities. A forward modelling approach is then employed to find the mantle density configuration that generates the gravitational field that best approximates the corresponding refined gravitational field obtained from observed gravity field after subtracting the gravitational signal of the lithosphere.

How to Calculate Bouguer Gravity Data in Planetary Studies

In terrestrial studies, Bouguer gravity data is routinely computed by adopting various numerical schemes, starting from the most basic concept of approximating the actual topography by an infinite Bouguer plate, through adding a planar terrain correction to account for a local/regional terrain geometry, to more advanced schemes that involve the computation of the topographic gravity correction by taking into consideration a gravitational contribution of the whole topography while adopting a spherical (or ellipsoidal) approximation. Moreover, the topographic density information has significantly improved the gravity forward modeling and interpretations, especially in polar regions (by accounting for a density contrast of polar glaciers) and in regions characterized by a complex geological structure. Whereas in geodetic studies (such as a gravimetric geoid modeling) only the gravitational contribution of topographic masses above the geoid is computed and subsequently removed from observed (free-air) gravity data, geophysical studies focusing on interpreting the Earth’s inner structure usually require the application of additional stripping gravity corrections that account for known anomalous density structures in order to reveal an unknown (and sought) density structure or density interface. In planetary studies, numerical schemes applied to compile Bouguer gravity maps might differ from terrestrial studies due to two reasons. While in terrestrial studies the topography is defined by physical heights above the geoid surface (i.e., the geoid-referenced topography), in planetary studies the topography is commonly described by geometric heights above the geometric reference surface (i.e., the geometric-referenced topography). Moreover, large parts of a planetary surface have negative heights. This obviously has implications on the computation of the topographic gravity correction and consequently Bouguer gravity data because in this case the application of this correction not only removes the gravitational contribution of a topographic mass surplus, but also compensates for a topographic mass deficit. In this study, we examine numerically possible options of computing the topographic gravity correction and consequently the Bouguer gravity data in planetary applications. In agreement with a theoretical definition of the Bouguer gravity correction, the Bouguer gravity maps compiled based on adopting the geoid-referenced topography are the most relevant. In this case, the application of the topographic gravity correction removes only the gravitational contribution of the topography. Alternative options based on using geometric heights, on the other hand, subtract an additional gravitational signal, spatially closely correlated with the geoidal undulations, that is often attributed to deep mantle density heterogeneities, mantle plumes or other phenomena that are not directly related to a topographic density distribution.

Insights into asthenospheric anisotropy and deformation in Mainland China

Seismic anisotropy can provide direct constraints on asthenospheric deformation which also can be induced by the inherent mantle flow within our planet. Mantle flow calculations thus have been an effective tool to probe asthenospheric anisotropy. To explore the source of seismic anisotropy, asthenospheric deformation and the effects of mantle flow on seismic anisotropy in Mainland China, mantle flow models driven by plate motion (plate-driven) and by a combination of plate motion and mantle density heterogeneity (plate-density-driven) are used to predict the fast polarization direction of shear wave splitting. Our results indicate that: (1) plate-driven or plate-density-driven mantle flow significantly affects the predicted fast polarization direction when compared with simple asthenospheric flow commonly used in interpreting the asthenospheric source of seismic anisotropy, and thus new insights are presented; (2) plate-driven flow controls the fast polarization direction while thermal mantle flow affects asthenospheric deformation rate and local deformation direction significantly; (3) asthenospheric flow is an assignable contributor to seismic anisotropy, and the asthenosphere is undergoing low, large or moderate shear deformation controlled by the strain model, the flow plane/flow direction model or both in most regions of central and eastern China; and (4) the asthenosphere is under more rapid extension deformation in eastern China than in western China.

Effect of the lithospheric thermal state on the Moho interface: A case study in South America

Gravimetric methods applied for Moho recovery in areas with sparse and irregular distribution of seismic data often assume only a constant crustal density. Results of latest studies, however, indicate that corrections for crustal density heterogeneities could improve the gravimetric result, especially in regions with a complex geologic/tectonic structure. Moreover, the isostatic mass balance reflects also the density structure within the lithosphere. The gravimetric methods should therefore incorporate an additional correction for the lithospheric mantle as well as deeper mantle density heterogeneities. Following this principle, we solve the Vening Meinesz-Moritz (VMM) inverse problem of isostasy constrained by seismic data to determine the Moho depth of the South American tectonic plate including surrounding oceans, while taking into consideration the crustal and mantle density heterogeneities. Our numerical result confirms that contribution of sediments significantly modifies the estimation of the Moho geometry especially along the continental margins with large sediment deposits. To account for the mantle density heterogeneities we develop and apply a method in order to correct the Moho geometry for the contribution of the lithospheric thermal state (i.e., the lithospheric thermal-pressure correction). In addition, the misfit between the isostatic and seismic Moho models, attributed mainly to deep mantle density heterogeneities and other geophysical phenomena, is corrected for by applying the non-isostatic correction. The results reveal that the application of the lithospheric thermal-pressure correction improves the RMS fit of the VMM gravimetric Moho solution to the CRUST1.0 (improves ∼ 1.9 km) and GEMMA (∼1.1 km) models and the point-wise seismic data (∼0.7 km) in South America.

Assessment of the effect of three-dimensional mantle density heterogeneity on Earth rotation in tidal frequencies

In this paper, we report the assessment of the effect of the three-dimensional (3D) density heterogeneity in the mantle on Earth orientation parameters (EOP) (i.e., the polar motion, or PM, and the length of day, or LOD) in the tidal frequencies. The 3D mantle density model is estimated based upon a global S-wave velocity tomography model (S16U6L8) and the mineralogical knowledge derived from laboratory experiment. The lateral density variation is referenced against the preliminary reference earth model (PREM). Using this approach the effects of the heterogeneous mantle density variation in all three tidal frequencies (zonal long periods, tesseral diurnal, and sectorial semidiurnal) are estimated in both PM and LOD. When compared with mass or density perturbations originated on the Earth's surface such as the oceanic and barometric changes, the heterogeneous mantle contributes less than 10% of the total variation in PM and LOD in tidal frequencies. However, this is the gap that has not been explained to close the gap of the observation and modeling in PM and LOD. By computing the PM and LOD caused by 3D heterogeneity of the mantle during the period of continuous space geodetic measurement campaigns (e.g., CONT94) and the contribution from ocean tides as predicted by tide models derived from satellite altimetry observations (e.g., TOPEX/Poseidon) in the same period, we got the lump-sum values of PM and LOD. The computed total effects and the observed PM and LOD are generally agree with each other. In another word, the difference of the observed PM and LOD and the model only considering ocean tides, at all tidal frequencies (long periods, diurnals, and semidiurnals) contains the contributions of the lateral density heterogeneity of the mantle. Study of the effect of mantle density heterogeneity effect on torque-free Earth rotation may provide useful constraints to construct the reference earth model (REM), which is the next major objective in global geophysics research beyond PREM.

Theoretical deficiencies of isostatic schemes in modeling the crustal thickness along the convergent continental tectonic plate boundaries

The results of global and regional studies often show significant disagreement between the Moho depths determined using seismic and isostatic models. In this study, we estimate the differences between these two models in central Eurasia. The Vening Meinesz-Moritz (VMM) inverse problem of isostasy is utilized to determine the isostatic Moho depths. The estimated VMM Moho depths are then corrected for the sediment density contrast. The application of this correction improves the agreement between the isostatic and seismic Moho models. The existing discrepancies between the isostatic and seismic models are finally modeled by applying the non-isostatic correction, which accounts for the unmodelled mantle density heterogeneities and other geodynamic processes, which are not taken into account in classical isostatic models. Our results reveal that the non-isostatic correction still cannot fully describe mechanisms affecting the Moho geometry along the convergent continent-tocontinent tectonic plate boundaries occurring beneath Himalayas despite an overall good performance of the applied method.

Petrological-geophysical models of the internal structure of the lithospheric mantle of the Siberian Craton

Based on the simultaneous inversion of unique ultralong-range seismic profiles Craton, Kimberlite, Meteorite, and Rift, sourced by peaceful nuclear and chemical explosions, and petrological and geochemical data on the composition of xenoliths of garnet peridotite and fertile primitive mantle material, the first reconstruction was obtained for the thermal state and density of the lithospheric mantle of the Siberian craton at depths of 100–300 km accounting for the effects of phase transformation, anharmonicity, and anelasticity. The upper mantle beneath Siberia is characterized by significant variations in seismic velocities, relief of seismic boundaries, degree of layering, and distribution of temperature and density. The mapping of the present-day lateral and vertical variations in the thermal state of the mantle showed that temperatures in the central part of the craton at depths of 100–200 km are somewhat lower than those at the periphery and 300–400°C lower than the mean temperature of tectonically younger mantle surrounding the craton. The temperature profiles derived from the seismic models lie between the 32.5 and 35 mW/m2 conductive geotherms, and the mantle heat flow was estimated as 11–17 mW/m2. The depth of the base of the cratonic thermal lithosphere (thermal boundary layer) is close to the 1450 ± 100°C isotherm at 300 ± 30 km, which is consistent with published heat flow, thermobarometry, and seismic tomography data. It was shown that the density distribution in the Siberian cratonic mantle cannot be described by a single homogeneous composition, either depleted or enriched. In addition to thermal anomalies, the mantle density heterogeneities must be related to variations in chemical composition with depth. This implies significant fertilization at depths greater than 180–200 km and is compatible with the existence of chemical stratification in the lithospheric mantle of the craton. In the asthenosphere-lithosphere transition zone, the craton root material is not very different in chemical composition, thermal regime, and density from the underlying asthenosphere. It was shown that minor variations in the chemical composition of the cratonic mantle and position of chemical (petrological) boundaries and the lithosphere-asthenosphere boundary cannot be reliably determined from the interpretation of seismic velocity models only.

Expressions for the Global Gravimetric Moho Modeling in Spectral Domain

We apply a newly developed numerical method to improve the Moho geometry by the implementation of gravity data. This method utilizes expressions for the gravimetric forward and inverse modeling derived in a frequency domain. Methods for a spectral analysis and synthesis of the gravity field and crust density structures are applied in the gravimetric forward modeling of the consolidated crust-stripped gravity disturbances, which have a maximum correlation with the (a priori) Moho model. These gravity disturbances are obtained from the Earth’s gravity disturbances after applying the topographic and stripping gravity corrections of major known anomalous crust density structures; in the absence of a global mantle model, mantle density heterogeneities are disregarded. The isostatic scheme applied is based on a complete compensation of the crust relative to the upper mantle density. The functional relation is established between the (unknown) Moho depths and the complete crust-stripped isostatic gravity disturbances, which according to the adopted isostatic scheme have (theoretically) a minimum correlation with the Moho geometry. The system of observation equations, which describes the relation between spherical functions of the isostatic gravity field and the Moho geometry, is defined by means of a linearized Fredholm integral equation of the first kind. The Moho depths are determined based on solving the gravimetric inverse problem. The regularization is applied to stabilize the ill-posed solution. This numerical procedure is utilized to determine the Moho depths globally. The gravimetric result is presented and compared with the seismic Moho model. Our gravimetric result has a relatively good agreement with the CRUST2.0 Moho model by means of the RMS of differences (of 3.5 km). However, the gravimetric solution has a systematic bias. We explain this bias between the gravimetric and seismic Moho models by the unmodelled mantle heterogeneities and uncertainties in the CRUST2.0 global crustal model.

Paleo movement of continents since 300 Ma, mantle dynamics and large wander of the rotational pole

Apparent polar wander (APW) is known to be mainly linked to internal mass distribution changes and in particular to changes in subduction and large-scale upwellings in the mantle. We investigate plate motions during the last 410 million years in a reference frame where Africa is fixed. Indeed, Africa has remained a central plate from which most continents diverged since the break-up of Pangea. The exact amount of subduction is unknown prior to 120Ma. We propose an approach, based on one hand on the study of the past subduction volcanism to locate ancient subduction activity, and on the other hand microplate motion history in the Tethyan area derived from geology and paleomagnetism. The peri-Pacific subductions seem to be a quasi-permanent feature of the Earth's history at least since the Paleozoic, with however localized interruptions. The “Tethyan” subductions have a complex history with successive collisions of continental blocs (Hercynian, Indo-Sinian, Alpine and Himalayan) and episodical rebirth of E–W subduction trending zones. Assuming that subducted slabs sink vertically into the mantle and taking into account large-scale upwellings derived from present-day tomography and intra-plate volcanism in the past, we compute the time variation of mantle density heterogeneities since 280Ma. Due to conservation of the angular momentum of the Earth, the temporal evolution of the rotational axis is computed in a mantle reference frame where the Africa plate is fixed, and compared to the apparent polar wander (APW) observed by paleomagnetism since 280Ma. We find that a major trend of both paleomagnetic and computed APW are successive oscillatory clockwise or counter-clockwise motions, with tracks separated by abrupt cusps (around 230Ma, 190Ma and 140–110Ma). We find that cusps result from earlier major geodynamic events: the 230Ma cusp is related to the end of active subduction due to the closure of the Rheic Ocean basin after the Hercynian continental collision (340–300Ma) and to renewed subduction zone West of Laurentia, whereas the 190Ma cusp results from the Indo-Sinian collision (270–230Ma) and the subsequent end of the Neo-Tethys ocean subduction.

Length of day variations due to mantle dynamics at geological timescale

SUMMARY The geological evolution of length of day (LOD) variations is mainly controlled by the frictional tidal torque responsible for the secular slowdown of the Earth’s rotation and for the receding of the Moon. Superimposed on this variation, which has existed since the early history of the planet, there are, at shorter timescales (less than 1 Myr), LOD perturbations induced by the glaciation–deglaciation cycles. In this paper, we investigate the influence of mantle dynamics on LOD at the geological timescale. We use the complete non-linear equations to compute the influence of mantle density heterogeneities on the angular velocity of the rotation, that is to say on the LOD. We discuss the degree zero coefficient of the spherical harmonic expansion of the mantle mass anomaly, which is strongly dependent on the conservation of mass of the Earth. We first compute the effects induced by upwelling domes and subducted plates sinking into the mantle, which are known to induce geological variations in the orientation of the rotation axis with respect to a fixed terrestrial frame (the so-called True Polar Wander). We find that the time-variable mantle density heterogeneities associated with the large-scale pattern of plate tectonic motions since 120 Ma and with the upwelling domes can perturb LOD by about per year, that is, with an order of magnitude smaller than the effects induced by the last deglaciation. Superimposed on this linear trend, we show that there are fluctuations around 0.1–0.2 s Myr−1 on a timescale of a few tens of millions of years. We then combine a spherical model of mantle circulation with solutions for the equations governing the rotation of a viscous planet, to improve the constraint of mantle mass conservation. Finally, we compare our results with other effects.

Constraints on lithosphere net rotation and asthenospheric viscosity from global mantle flow models and seismic anisotropy

Although an average westward rotation of the Earth's lithosphere is indicated by global analyses of surface features tied to the deep mantle (e.g., hot spot tracks), the rate of lithospheric drift is uncertain despite its importance to global geodynamics. We use a global viscous flow model to predict asthenospheric anisotropy computed from linear combinations of mantle flow fields driven by relative plate motions, mantle density heterogeneity, and westward lithosphere rotation. By comparing predictions of lattice preferred orientation to asthenospheric anisotropy in oceanic regions inferred from SKS splitting observations and surface wave tomography, we constrain absolute upper mantle viscosity (to 0.5–1.0 × 1021 Pa s, consistent with other constraints) simultaneously with net rotation rate and the decrease in the viscosity of the asthenosphere relative to that of the upper mantle. For an asthenosphere 10 times less viscous than the upper mantle, we find that global net rotation must be <0.26°/Myr (<60% of net rotation in the HS3 (Pacific hot spot) reference frame); larger viscosity drops amplify asthenospheric shear associated with net rotation and thus require slower net rotation to fit observed anisotropy. The magnitude of westward net rotation is consistent with lithospheric drift relative to Indo‐Atlantic hot spots but is slower than drift in the Pacific hot spot frame (HS3 ≈ 0.44°/Myr). The latter may instead express net rotation relative to the deep mantle beneath the Pacific plate, which is moving rapidly eastward in our models.

Mantle dynamics, geoid, inertia and TPW since 120 Myr

We investigate the effect of internal masses redistributions on the position of the Earth rotational pole for the last 120 Myr. We use a geodynamic model based on plate reconstructions that estimates the location and rate of subducted slabs under the assumption that they sink vertically into the mantle (Ricard et al., 1993b). Our model also takes into account the effect of large-scale upwellings (domes) derived from an analysis of seismic tomography. Their location is assumed to remain stable with time. We then compute the geoid associated with the time-dependent mantle density heterogeneities. In order to reconcile the computed and observed geoids, we investigate the influence of the depth down to which the subducted Pacific plates beneath the Americas present a significant density contrast with respect to the surrounding mantle on the present-day geoid, and propose a plate model in which we obtain a variance reduction greater than 0.9 for the degree 2. We show that the vertical oscillation of domes within mantle only modulates the amplitude of the associated geoid. The temporal variation of the mantle density heterogeneities is consequently essentially due to changes in the subduction history. The temporal evolution of the Principal Inertia Axis (PIA) of the Earth derived from our model (the rotational axis is aligned to the maximum PIA) is then investigated, and finally compared to estimations of TPW. Both the maximum and intermediate PIAs have moved in a plane perpendicular to Africa, along a circle corresponding to the low of geoid induced by subduction around the Pacific. The minimum PIA seems to be relatively stable since 120 Myr, and close to the maximum degree 2 geoid high under Africa. This can account for observed TPW or African APW in the last 200 Myr.

Dynamic mantle density heterogeneities and global geodetic observables

We investigate the influence of mantle dynamics on low degree deformations of the Earth at geological timescale. We first compute surface deformations, and discuss the analytical form of the tangential surface displacement induced by internal loads, in a reference frame related to the centre of mass of the planet. We use the theoretical Love numbers formalism since the Earth has a viscoelastic behaviour at geological timescale. Then we quantify degree-one and degree-two deformations induced by upwelling domes and subducted plates sinking into the mantle. We use a simple model in which the slabs are modelled as blobs diving vertically through the mantle, and in which the domes are assumed to be stable over the last 120 Ma. Their location is modelled from seismic tomography within the lower mantle. The temporal evolutions of the J2 gravitational potential coefficient and of the geocentre motion are plotted since 120 Ma. We find that: The mantle density heterogeneities within the mantle can explain the present-day non-hydrostatic flattening of the Earth. However they vary at a too slow timescale to significantly perturb the coefficient. Although there is a significant discrepancy of about a few hundred metres between the centre of figure and the centre of mass of the Earth, the secular variation of the geocentre motion is one order of magnitude smaller than the one induced by surface loads.

Global mantle flow and the development of seismic anisotropy: Differences between the oceanic and continental upper mantle

Viscous shear in the asthenosphere accommodates relative motion between Earth's surface plates and underlying mantle, generating lattice‐preferred orientation (LPO) in olivine aggregates and a seismically anisotropic fabric. Because this fabric develops with the evolving mantle flow field, observations of seismic anisotropy can constrain asthenospheric flow patterns if the contribution of fossil lithospheric anisotropy is small. We use global viscous mantle flow models to characterize the relationship between asthenospheric deformation and LPO and compare the predicted pattern of anisotropy to a global compilation of observed shear wave splitting measurements. For asthenosphere >500 km from plate boundaries, simple shear rotates the LPO toward the infinite strain axis (ISA, the LPO after infinite deformation) faster than the ISA changes along flow lines. Thus we expect the ISA to approximate LPO throughout most of the asthenosphere, greatly simplifying LPO predictions because strain integration along flow lines is unnecessary. Approximating LPO with the ISA and assuming A‐type fabric (olivine a axis parallel to ISA), we find that mantle flow driven by both plate motions and mantle density heterogeneity successfully predicts oceanic anisotropy (average misfit 13°). Continental anisotropy is less well fit (average misfit 41°), but lateral variations in lithospheric thickness improve the fit in some continental areas. This suggests that asthenospheric anisotropy contributes to shear wave splitting for both continents and oceans but is overlain by a stronger layer of lithospheric anisotropy for continents. The contribution of the oceanic lithosphere is likely smaller because it is thinner, younger, and less deformed than its continental counterpart.

Does active mantle upwelling help drive plate motions?

Earth's lithospheric plates are driven partly by shear tractions exerted on their base by viscous coupling to convective mantle flow. While downwelling flow associated with descent of subducted lithosphere has been well established as a driver of plate motions, the plate-driving role of upwelling flow is more controversial. We used a numerical mantle flow model to predict present-day plate motions driven by various combinations of upwelling and downwelling flow. We obtain a good fit to observed plate motions using flow driven only by downgoing mantle slabs, whose densities can be inferred either from the tectonic history of subduction or from positive velocity anomalies extracted from mantle tomography. Introducing upwelling flow (driven by negative density anomalies inferred from slow seismic velocity anomalies) to this slab-driven flow field generally degrades the fit to plate motions, and an acceptable fit that includes active upwelling requires amplitudes of mantle density heterogeneity that are at the low end of the accepted range (tomography to density conversion factor = 0.05 g cm −3 km −1 s). Because such small amplitudes lead to a deficit of slab material in the mantle, we infer that downwelling flow is a more significant driver of plate motions than upwelling flow. This conclusion can be reconciled with the presence of low-velocity anomalies in the lower mantle if upwelling flow does not couple effectively to plate motions, or if these low-velocity anomalies represent chemical differentiation of the lower mantle and thus do not drive upwelling flow.

Tales of Wander

GEOLOGY True polar wander describes relative motion between Earth's spin vector and the solid Earth. One class of this phenomenon, inertial interchange true polar wander, occurs when normal advection of mantle density heterogeneities produces changes in the relative magnitudes of the principal inertia axes, causing Earth to rotate quickly by as much as 90°, until the new major rotational axis is aligned with the spin vector. In addition to the paleomagnetic variations that would accompany such a rapid change of Earth's orientation, another observable consequence could be transient sea-level variations resulting from the differential response of the slowly re-equilibrating mantle/lithosphere and the rapidly re-equilibrating world ocean. A third potential but indirect effect, arising from sea-level change, is perturbation of the carbon cycle, as marine biological productivity is affected by water depth variations. In an investigation of possible true polar wander, Maloof et al . present paleomagnetic data from three Middle Neoproterozoic carbonate units in Svalbard, Norway, which show large shifts in paleomagnetic orientation coincident with abrupt changes in δ13C and relative sea level. They conclude that the best explanation for the data is that this area experienced rapid shifts of paleogeography during a pair of true polar wander events. Their hypothesis can be further tested by analyzing sediments of the same age from other basins for predictable related changes. — HJS Geol. Soc. Am. Bull. 118 , 1099 2006).

Detection of upper mantle flow associated with the African Superplume

A continental-scale, low seismic velocity anomaly in the mid to lower mantle beneath Africa is a robust feature of global tomographic models. Assuming the low velocities are associated with warm, less dense material, the African seismic anomaly has been ascribed to a long-lived thermal upwelling from the lower mantle. Such a large-scale upwelling should also affect the regional horizontal flow field in the upper mantle. To test this model, we compare seismic anisotropy inferred from shear-wave splitting measurements with instantaneous flow calculations that incorporate mantle density structure inferred from seismic tomography. We calculate splitting parameters at 13 ocean island stations surrounding Africa. Splitting measurements from island stations are ideal for interpreting anisotropy induced by asthenospheric flow because they lack a thick overlying lithosphere that may also contribute to the observed anisotropy. We tested for a possible lithospheric contribution by comparing the splitting measurements with the fossil spreading directions. We find that although the fossil lithospheric fabric closely matches the observed fast polarization directions at stations <500 km from a ridge axis, they are a poor fit to the data at stations located farther off-axis. Thus, we conclude that far from a ridge axis, the observed anisotropy is dominated by asthenospheric flow. To test for an active component of mantle upwelling, we considered several models with varying assumptions about the velocity at the base of the asthenosphere: that it is (1) stationary below plates moving in the no-net-rotation (NNR) or hotspot reference frames, (2) driven by plate motions at the Earth's surface, or (3) driven by a combination of plate-motion and mantle density heterogeneity inferred from either seismic tomography or the history of subduction. We find that the best-fitting flow field is generated by plate motions and density heterogeneity associated with large-scale upwelling originating in the lower mantle beneath southern Africa and is manifest as a radial pattern of flow at the base of the asthenosphere. This model provides a significantly better fit to the observed anisotropy than a model in which mantle flow is driven through a passive response to subduction. The resulting sub-asthenospheric flow field is estimated to have velocities of 0–3 cm/year, and an asthenospheric viscosity of ∼3·10 19 Pa·s is found to be most consistent with the regional anisotropy, geoid height, and dynamic topography.

Origin of the lithospheric stress field

An understanding of the tectonic stress field is geologically important because it is the agent that preserves in the crust a memory of dynamical processes. In an effort to elucidate the origin of the present state of stress of the lithosphere we use a finite element model of the Earth's lithosphere to calculate stresses induced by mantle flow, crustal heterogeneity, and topography and compare these to observations of intraplate stresses as given by the World Stress Map. We explore two models of lithospheric heterogeneity, one based directly on seismic and other observational constraints (Crust 2.0), and another that assumes isostatic compensation. Mantle tractions are computed from two models of mantle density heterogeneity: a model based on the history of subduction of the last 180 Myr, which has proved successful at accurately reproducing the present‐day geoid and Cenozoic plate velocities, and a model inferred from seismic tomography. We explore the effects of varying assumptions for the viscosity structure of the mantle, and the effects of lateral variations in viscosity in the form of weak plate boundaries. We find that a combined model that includes both mantle and lithospheric sources of stress yields the best match to the observed stress field (∼60% variance reduction), although there are many regions where agreement between observed and predicted stresses is poor. The stress field produced by mantle tractions alone shows a greater degree of long‐wavelength structure than is apparent in the stress observations but agrees very well with observations in some areas where radial mantle tractions are particularly strong such as in southeast Asia and the western Pacific. The stress field produced by lithospheric heterogeneity alone depends strongly on the assumed crustal model: Whereas the isostatically compensated model yields very poor agreement with observations, the model based on Crust 2.0 matches the observations about as well as mantle tractions alone and matches very well in certain areas where the influence of high topography is very important (e.g., Andes, East Africa). A possible interpretation of our results is that the stress field is significantly influenced by lateral variations in the viscosity of the mantle, which leads to variable amounts of decoupling between lithosphere and mantle, allowing the mantle signature to dominate in some areas and the crustal signature to dominate in others. The poor fit between the isostatically compensated crustal model and observations and the large differences between the two crustal models point toward the importance of dynamic topography and remaining uncertainties in crustal structure and rheology. We also consider the possibility that observations of stress from the shallow crust may not reflect the state of stress of the entire plate; stresses in the upper plate may be at least partially decoupled from broader‐scale plate driving forces by lateral and vertical variations in lithospheric rheology.

Seismic tomography, surface uplift, and the breakup of Gondwanaland: Integrating mantle convection backwards in time

Mantle density heterogeneities, imaged using seismic tomography, contain information about time‐dependent mantle flow and mantle structures that existed in the past. We model the history of mantle flow using a tomographic image of the mantle beneath southern Africa as an initial condition while reversing the direction of flow and analytically incorporating cooling plates as a boundary condition. If the resulting (backwards integrated) model for structures is used as a starting point for a forwards convection model, today's mantle can be adequately reconstructed if we do not integrate backwards more than than about 50–75 Ma. Flow can also be reliably reversed through the Mesozoic, but only if instability of the lower boundary layer can be suppressed. Our model predicts that the large seismically‐slow and presumably hot structure beneath southern Africa produced 500–700 m of dynamic topography throughout the Cenozoic. Since ∼30 Ma, uplift has moved from eastern to southern Africa, where uplift rates are ∼10 m/Myr, consistent with observations. During the Mesozoic, the modeled topographic high is situated near Gondwanaland rifting, raising the possibility that this buoyant structure may have been involved with this breakup.