T14B-01 INVITED 16:00h
VQM3DA Global velocity, quality factor, and anisotropy models of the lower mantle
We invert over 90,000 differential travel-time and attenuation measurements for 3D whole-mantle velocity (V) and quality factor (Q) models with identical resolution. By comparing models of velocity and quality factor obtained from measurements of SH and SV waves, we calculate velocity anisotropy and quality factor anisotropy. The resolution of each model increases with depth, making them ideal for examination of the lower mantle. High V, Q, and anisotropy are observed beneath subduction zones around the Pacific. Low V, Q, and anisotropy are observed within the lower mantle underlying the Pacific and Africa. The Pacific superplume extends from the core to the surface while the African superplume is limited to the lower mantle. The largest anomalies - both high and low - are observed within the lowermost mantle. The high correlation between upper and lower mantle anomalies suggests that D" has a strong influence on mantle convection. The observed seismic anomalies are indicative of the dominant rheology in the mantle. From strong anisotropy and low attenuation we infer that dislocation creep is the dominant deformation mechanism underlying subduction zones. The low anisotropy and high attenuation of the superplumes indicate that diffusion creep may be dominant in upwellings. The locations of hotspots are highly correlated with both low V and low Q in both the transition zone and the lowermost mantle. These anomalies are likely thermal in nature due to the correlation between V and Q. These anomalies may also have lower viscosity as inferred from the similar temperature dependencies of Q and viscosity. Likewise, the high V and Q anomalies around the Pacific are inferred to be cold, high viscosity slabs.
T14B-02 INVITED 16:15h
SEISMOLOGICAL EVIDENCE FOR GENERAL ANISOTROPY IN THE D" LAYER BENEATH THE CARIBBEAN
Current models of anisotropy in D" are based on time delays observed between the SH and the SV components of core-reflected or core-grazing phases, which can be explained by the simplest form for anisotropy, vertical transverse isotropy (VTI). In a detailed analysis of high-quality broadband S (or Sdiff) phases, we find that many recordings do not show such simple delays; rather, some coupling between the SH and SV components is apparent: SV components have a small initial pulse with the polarity opposite to that predicted by isotropic or VTI structures. We analyze deep South American events recorded by the Canadian National Seismic Network, sampling D" under the Caribbean Ocean and Central America. The data are corrected for upper mantle anisotropy and compared to full wave theory synthetic seismograms calculated from isotropic, transversely isotropic, and azimuthally anisotropic structures. While restrictions in azimuthal sampling of this region limit a complete characterization of D" anisotropy, the need for azimuthal anisotropy is robustly established. A simple form of azimuthal anisotropy is explored which entails tilting the symmetry axis of transverse isotropy away from the vertical (TTI). Anomalous data are reproduced by a 20 degree tilt, and discriminate between eastward and westward tilting orientations, as well as zero tilts (i.e., isotropy or VTI). However, we note this does not preclude other forms of azimuthal anisotropy. Out of the 80 highest quality records analyzed, 22 are uniquely explained by the eastward TTI structures (with 34 records compatible with such a structure); 16 are compatible with westward TTI, with 2 uniquely requiring such a tilt. This gives strong evidence for lateral heterogeneity in azimuthal anisotropy at a lateral scale of a several 100 km. Further interpretation of the results requires identification the physical mechanism behind seismic anisotropy in D". Together with the recent discovery of the possibly anisotropic post-perovskite phase near D", the possibility of observing general anisotropy in D" opens completely new perspectives for mapping flow in the lowermost mantle.
T14B-03 16:30h
Imaging structure at and near the core mantle boundary with a generalized Radon transform of broad band ScS and SKKS coda waves
Abstract: Seismic constraints on structure at or near the core-mantle boundary (CMB) and on the nature of any nearby interfaces is of great importance for understanding the mineralogy, phase chemistry, and dynamics in Earth's lowermost mantle. Much pioneering work has been done with forward modeling of core reflected or diffracted waves, but the exponentially growing data sets available through IRIS call for powerful inversion methods. In order to produce accurate images of scatterers or interfaces near the core-mantle boundary (CMB) from seismic body waves such as ScS and SKKS - and their codas - we have developed a generalized Radon transform (GRT) from 'exact' asymptotic analysis and the theory of Fourier integral operators. We enhance the images, estimate uncertainty, and infer scaling properties of the interfaces using mixed-model statistics (i.e., properties of the GRT imaging operator are used to describe and reduce noise and data misfit). Our inverse scattering approach is set up to process and interpret tens of thousands of broad-band waveforms in order to detect (and characterize) variations in acousto-elastic properties in the so-called D" region, the CMB proper, and Earth's outermost core. We present applications of GRT imaging to a selected region of the CMB beneath Central America; all ScS rays with a midpoint in this selected region (from -115 to -65W and -10S to 40N) and generated by earthquakes with an Mb magnitude larger than 5.2 are used. We hope to constrain the lateral variation in height above the CMB (related to the Clapeyron slope in case of a phase transition) and the regularity of interfaces in the bottom 300 km or so of the mantle. A pilot study, with over 20,000 ScS records, has yielded image gathers that reveal a sharp CMB and a more gradual interface ~ 280 km above it, and there are hints of structures in between. We will present results for a great circle transect from (-105W, 0) to (-75W, 30N): some 65,000 transverse component broad band records are used to construct 40 juxtaposed radial image gathers, which in combination form a 2000 km long 2-D profile of structure at or near the CMB beneath Central America.
T14B-04 16:45h
Seismic evidence for present-day plume upwelling at the core-mantle boundary
In recent years a broad range of seismic discoveries have painted a picture of Earth's core-mantle boundary (CMB) that is far more complex than a simple one-dimensional boundary between the molten outer core and the solid silicate mantle. Over the last ten years several anomalous regions of the lowermost mantle have been detected using a multitude of seismic phases and approaches. These regions are characterized by reductions of \textit{P}-wave and \textit{S}-wave velocities by at least 10%. These ultra-low velocity zones (ULVZ) are characterized as having strong variability at short scale length (e.g. ,$\le$ 100 km) and do not appear to be a global layer. Using 300 deep focus Tonga-Fiji subduction zone earthquakes recorded at the Warramunga array in central Australia, we detect an extremely localized low-velocity feature at the CMB that lies beneath low shear wave velocities in D$^{\prime\prime}$ south of New-Caledonia. An array analysis of \textit{ScP} reveals anomalous precursors that are most robustly explained by a dense ULVZ with the following characteristics: 8.5~km thick, $\sim$50~km wide, \textit{P}- and \textit{S}-wave reductions of 10 and 25%, respectively, and a density increase of 10% ($\pm$5%). These parameters are best explained by the presence of dense partial melt in the ULVZ. A model to keep the dense lens of partially molten material from spreading out along the CMB includes the entrapment of melt by intercumulus crystal growth after drainage from the anomalously hot overlying mantle. Therefore, this region may be closely related to thermal instabilities at the thermal boundary layer of the CMB that will influence the stability, genesis and persistence of mantle plumes.
T14B-05 INVITED 17:00h
Deformation of polycrystalline Ca-Perovskite up to 50 GPa
Characterizing the lattice preferred orientations and stresses that develop in deforming deep Earth phases is crucial for understanding mantle convection and its relation to seismic anisotropy. To that extend, material having the perovskite structure are particularly important with silicate and calcium perovskites accounting for about 70% and 5% of the lower mantle, respectively. In this study, we perform an ambient temperature uniaxial deformation experiment of polycrystalline Ca-perovskite up 50 GPa in the diamond anvil cell. The state of stress and lattice preferred orientations within the sample were investigated using radial x-ray diffraction techniques on BL-10 beamline in Spring8. From the variations of the d-spacings with the diffraction angle, we deduce informations on the non-hydrostatic stress in the sample. The variation of the intensities of the diffracted peaks along the Debye-Sheme rings provides information on the lattice preferred orientations within the polycrystal. Our results show that the stress in calcium silicate perovskite under non-hydrostatic compression follows a similar trend as in previous measurements on silicate perovskite. Moreover, we were able to detect evidence of lattice preferred orientations within the sample that, in combination with polycrystal plasticity modeling, allow us to deduce the active deformation mechanisms in the experiment. These results provide new constraints for modeling and understanding anisotropic properties in the deep mantle.
T14B-06 17:15h
Dense layer entrainment and structure of mantle plumes
The entrainment of a dense layer near the bottom of the mantle by thermal plumes has important implications for the style of the mantle convection, the structure of the mantle and the conditions of melt formation below midoceanic islands. We have carried out numerical experiments in axisymmetrical, spherical shell to investigate the influence of the thickness and the excess density of the dense layer as well as the plume viscosity on the effectiveness of the dense layer entrainment, plume formation and chemical heterogeneities in the plumes. Our results show that the denser material can be entrained by the plumes and reaches the surface within a wide range of model parameters. Both the thickness and density of the dense layer and plume viscosity control the entrainment and plume structure. Temperature at the plume axis is a function of the thickness and density of the dense layer and is about 40% - 90% of maximum initial thermal perturbation when denser material is entrained. We also find that the entrainment of denser material slows the ascending plume and changes the shape of plume head. In addition, the internal structure of the plume head becomes complicated and the uneven distribution of the denser material causes strongly heterogeneities in the plume head for models with temperature-dependent rheology. Our model results imply: (1) Subducted slabs and recycling oceanic crust near the bottom of the Earth's mantle can be sampled by the plumes and preserves their distinctive nature when they reach the surface. (2) The temperature reduction of the mantle plumes due to the presence of the dense layer near the bottom of the mantle can be small which suggests that more than one mechanism is responsible for the mismatch between the plume excess temperatures inferred from petrological studies (ca. 200-300 K) and the temperature increase across D-" (ca. 1000-1300 K). (3) The geochemical heterogeneities in flood basalts (if associated with plume heads) can be greater than that in the subsequent hotspot volcanism. The large deformation near the plume axis makes accurate representation of the chemically distinct material a task. Traditional absolute tracer method may not model the chemical heterogeneities properly; it causes instabilities and generates unrealistic physical behaviors in current scenario. We will compare several methods and show that cautious usage of the numerical approaches on the thermochemical convection problems is suggested/required.
T14B-07 INVITED 17:30h
Towards a dynamical link between the formation of mantle plumes and the longevity and composition of hotspot volcanism
On Earth, a physical link has been proposed between hotspots, regions with particularly persistent, localized, and high rates of volcanism, and underlying deep mantle plumes. This plume model has provided a way to interpret observable phenomena including the volcanological, petrological and geochemical evolution of ocean island volcanoes, the relative motion of plates, continental breakup, global heat flow and the Earth's magnetic field within the broader framework of the thermal history of our planet. Despite the plume model's utility, the underlying dynamics giving rise to hotspots as long-lived stable features have remained elusive. To address the mantle dynamics underlying such hotspots we first use a synthesis of seismological, geodynamic, geomagnetic and geochemical constraints to argue that the source regions for most proposed deep mantle plumes contain dense, low viscosity material at the base of D$^{\prime \prime}$ composed of partial melt, outer core material or a mixture of both. Next, using results from laboratory experiments on thermochemical convection combined with theoretical scaling analyses, we argue that the longevity of mantle plumes in the Earth is a consequence of the interactions between core cooling and this dense layer. Our analysis explains the longevity of hotspots and mantle plumes in the Earth and leads to self-consistent scalings for the topography on the dense layer and the composition of ocean island basalts that are consistent with seismological and geochemical observations.
T14B-08 17:45h
The Influence of Mesozoic-Cenozoic Plate Motions on Thermochemical Piles
The large low-velocity seismic anomalies under the Pacific and Africa are often interpreted as being piles of more-dense material. We have performed numerical modeling of thermochemical convection in a three-dimensional spherical geometry in order to determine whether the presence of a dense chemical component can lead to the formation of two large antipodal piles in the Earth's lower mantle. We find that without imposing plate motions on the surface of the model, the dense material forms a network of linear ridges which are passively swept around by downwellings and that thermochemical structures are generally controlled by the geometry of the downwelling system. If plate motion history from 119 Ma to the present was imposed, we find that large thermochemical piles form in the lower mantle under Africa and the Pacific. Furthermore, the general shape of these structures are such that a ridge-like pile forms under Africa and a more rounded pile develops under the Pacific, consistant with seismic tomography. These general features of our models hold for a wide choice of density contrast and initial layer thickness.