Three-dimensional structure of the African superplume from waveform modelling

Ni, Sidao; Helmberger, Donald V.; Tromp, Jeroen

doi:10.1111/j.1365-246x.2005.02508.x

Cited by 78 publications

(69 citation statements)

References 32 publications

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“…5aA); they have found both a sharp change in isotropic velocities (Ni et al 2005;To et al 2005;Sun et al 2009) as well as anisotropy (Cottaar & Romanowicz 2013). Our vote map (Fig.…”

Section: Comparison With Regional Seismic Studiessupporting

confidence: 50%

Morphology of seismically slow lower-mantle structures

Cottaar¹,

Lekić²

2016

Geophysical Journal International

137

147

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S U M M A R YLarge low shear velocity provinces (LLSVPs), whose origin and dynamic implication remain enigmatic, dominate the lowermost mantle. For decades, seismologists have created increasingly detailed pictures of the LLSVPs through tomographic models constructed with different modeling methodologies, data sets, parametrizations and regularizations. Here, we extend the cluster analysis methodology of Lekic et al., to classify seismic mantle structure in five recent global shear wave speed (V S ) tomographic models into three groups. By restricting the analysis to moving depth windows of the radial profiles of V S , we assess the vertical extent of features. We also show that three clusters are better than two (or four) when representing the entire lower mantle, as the boundaries of the three clusters more closely follow regions of high lateral V S gradients. Qualitatively, we relate the anomalously slow cluster to the LLSVPs, the anomalously fast cluster to slab material entering the lower mantle and the neutral cluster to 'background' lower mantle material. We obtain compatible results by repeating the analysis on recent global P-wave speed (V P ) models, although we find less agreement across V P models. We systematically show that the clustering results, even in detail, agree remarkably well with a wide range of local waveform studies. This suggests that the two LLSVPs consist of multiple internal anomalies with a wide variety of morphologies, including shallowly to steeply sloping, and even overhanging, boundaries. Additionally, there are indications of previously unrecognized meso-scale features, which, like the Perm anomaly, are separated from the two main LLSVPs beneath the Pacific and Africa. The observed wide variety of structure size and morphology offers a challenge to recreate in geodynamic models; potentially, the variety can result from various degrees of mixing of several compositionally distinct components. Finally, we obtain new, much larger estimates of the volume/mass occupied by LLSVPs-8.0 per cent ±0.9 (μ ± 1σ ) of whole mantle volume and 9.1 per cent ±1.0 (μ ± 1σ ) of whole mantle mass-and discuss implications for associating the LLSVPs with the hidden reservoir enriched in heat producing elements.

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“…5aA); they have found both a sharp change in isotropic velocities (Ni et al 2005;To et al 2005;Sun et al 2009) as well as anisotropy (Cottaar & Romanowicz 2013). Our vote map (Fig.…”

Section: Comparison With Regional Seismic Studiessupporting

confidence: 50%

Morphology of seismically slow lower-mantle structures

Cottaar¹,

Lekić²

2016

Geophysical Journal International

137

147

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“…Our model does not explain the edges all that well (there are some misfits between the synthetics and data); however, it does model the long-period response quite well, as discussed in Ni et al [2005], where data are compared against SEM and DWKM synthetics. Predictions from tomographic models display only about one-third of the travel-time anomaly and are too smooth to produce multipathing.…”

Section: Structural Definition Of the Eastern Provincementioning

confidence: 89%

“…However, the above 2D section discussed in detail appears well resolved and is probably representative of the Western Province. The structure beneath the Indian Ocean is less well defined, having only two good samples of thickness, namely, samples of delayed SS starting at a distance of about 125° [Ni et a!., 2005] and one sample of the SKS-S crossover [Ni and Helmberger, 2004]. The Western Province probably tilts to the east, as argued by tomographic images [Ritsema et al, 1999;Ni et al, 2003].…”

Section: Distance (Deg)mentioning

confidence: 99%

Seismic modeling constraints on the South African super plume

Helmberger¹,

Ni²

2005

Earth's Deep Mantle: Structure, Composition, and Evolution

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Tomographic studies of the structure of the lower mantle beneath South Africa reveal large-scale low velocities above the core-mantle boundary. Predicted SKS delay patterns (up to 3 s) for some of these models fit observations (Kaapvaal Array data) quite well except for magnitude level, explaining less than one-halfthe observed anomaly. Moreover, the sharpness in travel-time offsets and waveform complications require that nearly vertical walls separate the anomalous structure from the normal preliminary reference Earth model (PREM) mantle. We present numerous record sections along with 2D and 3D synthetics displaying multipathing of arrivals (Sd' SKS, SKKS, S, and ScS), based on a large-scale 3D structure. This kidney-shaped structure has one apex beneath the Indian Ocean (Kerguelen) and the other extending beneath the Mid-Atlantic (Cape Verde). The structure is about 1200 km wide beneath South Africa and extends upward to at least 1000 km through the lower mantle, similar to Grand's model but with an average uniform velocity decrease of about 3% relative to PREM. We have not found any evidences for ultra-low-velocity zones (ULVZ) beneath the main structure but ample evidence at some locations near the edges. We also analyzed Pd and the differentials between PcP travel times and P travel times (PcP-P) along the same great circle paths from the same events. The P-velocity is not very anomalous, perhaps -0.5%. The sharpness of the lateral boundaries (walls) and the large contrast in P and S velocities can be used in arguments for a thermochemical origin.

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“…The differences probably arise from the fact that different frequency contents of the seismic data are used in the seismic modeling. In the 3-D models, the waveforms were band-pass-filtered to a period of 8-15 s Ni et al, 2005]. The large velocity reductions in the 2-D models were inferred from the high-frequency waveform complexities observed in the seismic data (compare broadband data in Figure 6 of Wen [2001] and their filtered components in Figure 2 of To et al [2005] and Figure 4 of Ni et al [2005] for event 97/09/04).…”

Section: The 3-d Effect Of Seismic Wave Propagationmentioning

confidence: 99%

“…In the 3-D models, the waveforms were band-pass-filtered to a period of 8-15 s Ni et al, 2005]. The large velocity reductions in the 2-D models were inferred from the high-frequency waveform complexities observed in the seismic data (compare broadband data in Figure 6 of Wen [2001] and their filtered components in Figure 2 of To et al [2005] and Figure 4 of Ni et al [2005] for event 97/09/04). The high-frequency complexities were band-pass-filtered, and synthetics fitting even to the band-passed long-period data were less satisfactory in the 3-D waveform modeling Ni et al, 2005].…”

Section: The 3-d Effect Of Seismic Wave Propagationmentioning

confidence: 99%

Geometry and P and S velocity structure of the “African Anomaly”

2007

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We constrain the geometry and P and S velocity structure of a low‐velocity anomaly in the lower mantle beneath southern Africa (we term it the “African Anomaly”) on the basis of forward traveltime and waveform modeling of seismic data sampling a great arc across the anomaly from the East Pacific Rise to the Japan Sea. Our collected data set consists of direct S, direct P, Sdiff, ScS, PcP, SKS, and SKKS phases recorded by three temporary broadband PASSCAL seismic arrays deployed in Africa between 1994 and 2002, the Tanzania seismic array (1994–1995), the Kaapvaal seismic array (1997–1999), and the Ethiopia/Kenya seismic array (2000–2002) for earthquakes occurring in the East Pacific Rise, Drake Passage, South Sandwich islands, Iran, Hindu Kush, Xinjiang, and the Japan Sea. The seismic data provide excellent sampling of the African Anomaly in the lower mantle along the specific great arc. In order to accurately account for the contributions from the African Anomaly, we relocate all the events using a global seismic shear velocity tomographic model and seismic data recorded by the Global Seismographic Network and correct for the contributions from the seismic heterogeneities outside the African Anomaly. The seismic observations suggest that the African Anomaly locally extends 1300 km above the core‐mantle boundary beneath southern Africa (around −25°N, 27°E) and exhibits a “bell‐like” geometry with both the southwestern and the northeastern flanks dipping toward its center with the lateral dimension of the anomaly increasing with depth. The base is about 4000 km wide extending broadly in both the southwestward and the northeastward directions. The seismic data can best be explained by a shear velocity structure with average velocity decreases of −5% in the base and −2% to −3% in the mid‐lower mantle above the base, and a compressional velocity structure with a uniform S to P velocity perturbation ratio of 3:1 for the entire African Anomaly. These geometric and velocity features suggest that the mid‐lower mantle portion of the African Anomaly is an integral component of the very low velocity province and the African Anomaly is compositionally distinct and geologically stable.

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Three-dimensional structure of the African superplume from waveform modelling

Cited by 78 publications

References 32 publications

Morphology of seismically slow lower-mantle structures

Morphology of seismically slow lower-mantle structures

Seismic modeling constraints on the South African super plume

Geometry and P and S velocity structure of the “African Anomaly”

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