Tomographic study of upper-mantle structure of the South African continent, using wave-form inversion

Cichowicz, Artur; Green, Rod W.E.

doi:10.1016/0031-9201(92)90207-c

Cited by 27 publications

(9 citation statements)

References 6 publications

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“…Models fitting the excess topography have a linear trade‐off between thickness and density (seismic velocity) so that a 200 km, δ v s ∼ −1% anomaly would also be possible. Such velocities are neither consistent with S20RTS nor with studies using regional surface waves [ Cichowicz and Green , 1992] or regional body wave data [ Zhao et al , 1999]. Our shallow dynamic model is limited in that the buoyant mantle could have ascended earlier in the Cenozoic and moved laterally beneath the lithosphere (as suggested by Artyushkov and Hofmann [1998]).…”

Section: Discussioncontrasting

confidence: 58%

Constraining mantle density structure using geological evidence of surface uplift rates: The case of the African Superplume

Gurnis

Mitrovica

Ritsema

et al. 2000

Geochem Geophys Geosyst

279

268

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We explore the hypothesis that southern Africa is actively being uplifted by a large-scale, positively buoyant structure within the mid-lower mantle. Using a new formulation in which dynamic topography and uplift rate are jointly used, we place constraints on mantle density and viscosity. The solution of the momentum equation is coupled with the advection of the density field to solve for the surface uplift rate in both an axisymmetric and fully spherical geometry. We demonstrate how dynamic topography and its rate of change depend on density and lateral and radial variations in viscosity. In the full spherical models the geometry of mantle density is derived by scaling a tomographic shear velocity model. Using a variety of geologic observations, we estimate residual topography (i.e., the topography remaining after shallow sources of density are removed) and an average Cenozoic uplift rate to be 300± 600 m and 5 ± 30 m/Myr, respectively, for southern Africa. We are able to satisfy these constraints with a mantle model in which the mid-lower mantle beneath southern Africa is 0.2% less dense and has a viscosity of $10 22 Pa s. In addition, if the continental lithosphere is thick beneath southern Africa, as suspected from seismic inversions, and has a high effective viscosity, then we find that southern Africa can be further elevated owing to increased coupling between the deep mantle and surface. We show that recent estimates of mantle density, suggesting that the lowest parts of the African anomaly may be anomalously dense are compatible with geologic constraints. We conclude that uplift rate, when combined with estimates of present-day dynamic topography, provides a powerful tool to constrain the properties of the deep mantle.

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Section: Discussioncontrasting

confidence: 58%

Constraining mantle density structure using geological evidence of surface uplift rates: The case of the African Superplume

Gurnis

Mitrovica

Ritsema

et al. 2000

Geochem Geophys Geosyst

279

268

View full text Add to dashboard Cite

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“…The observation of low‐velocity zones beneath cratonic lithosphere is not uncommon. Seismic tomography studies for the Kaapvaal craton [ Green et al , 1992; Cichowicz and Green , 1992; Freybourger et al , 2001; Saltzer , 2002], the Canadian Shield [ Grand and Helmberger , 1984], and the Australian shield [ Simons et al , 1999] indicate negative gradients in the velocity profile associated with the transition from high‐velocity cratonic lithosphere to global average mantle at greater depths. The low‐velocity anomalies below these lithospheric roots, however, do not reach shear wave velocities below 4.4 km/s.…”

Section: Low‐velocity Anomalies Beneath Archean Lithospherementioning

confidence: 99%

Evidence for an upper mantle plume beneath the Tanzanian craton from Rayleigh wave tomography

et al. 2003

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[1] The Archean Tanzanian craton, nestled between the eastern and western branches of the East African Rift, presents a unique opportunity to study the interaction of active rifting with stable cratonic lithosphere. The high density of Rayleigh wave paths recorded in a regional seismic array yields unusually precise determinations of phase velocity within the Tanzanian craton. Shear velocities in the cratonic lithosphere are higher than a global average to a depth of 150 ± 20 km. Beginning at 140 km, shear velocity decreases sharply, reaching a minimum of 4.20 ± 0.05 km/s at depths of 200-250 km. The base of the lithosphere, identified by the depth to the center of the maximum negative velocity gradient, is similar to that found beneath other Archean lithospheres. Where Cenozoic rifting crosscuts the southern corner of the craton, velocities up to 130 km depth are reduced, indicating recent disruption of the lithosphere. The anomalously low velocities beneath the Tanzanian craton indicate high temperatures and the presence of melt, consistent with the spreading of a mantle plume head beneath the craton. Tests for the possibility of a radial pattern of azimuthal anisotropy that may indicate outward flow from a plume show that a model with average anisotropy of 0.71 ± 0.17% centered SE of Lake Victoria fits the data significantly better than a uniform, single direction of anisotropy. Thus our results agree with the suggestion that an upper mantle plume, centered beneath the Tanzanian cratonic lithosphere, provides the buoyancy required for uplift of the East African Plateau.

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“…The northern boundary of the African superswell together with Green [1992], from modeling regional waveform data, obtained a high-velocity lid extending to 100-140-km depth underlain by a LVZ. Qiu et al also investigated the S wave velocity structure of the southern African upper mantle using regional body and surface waves.…”

Section: Introductionmentioning

confidence: 99%

Upper mantle velocity structure beneath southern Africa from modeling regional seismic data

et al. 1999

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Abstract. The upper mantle seismic velocity structure beneath southern Africa is investigated using travel time and waveform data which come from a large mine tremor in South Africa (mb 5.6) recorded by the Tanzania broadband seismic experiment and by several stations in southern Africa. The waveform data show upper mantle triplications for both the 410-and 670-km discontinuities between distances of 2100 and 3000 km. Auxiliary travel time data along similar profiles obtained from other moderate events are also used. P wave travel times are inverted for velocity structure down to -800-km depth using the Wiechert-Herglotz technique, and the resulting model is evaluated by perturbing it at three depth intervals and then testing the perturbed model against the travel time and waveform data. The results indicate a typical upper mantle P wave velocity structure for a shield. P wave velocities from the top of the mantle down to 300-km depth are as much as 3% higher than the global average and are slightly slower than the global average between 300-and 420-km depth. Little evidence is found for a pronounced low-velocity zone in the upper mantle. A high-velocity gradient zone is required above the 410-km discontinuity, but both sharp and smooth 410-km discontinuities are permitted by the data. The 670-km discontinuity is characterized by high-velocity gradients over a depth range of *-80 km around 660-km depth. Limited S wave travel time data suggest fast S wave velocities above -150-km depth. These results suggest that the bouyant support for the African superswell does not reside at shallow depths in the upper mantle.

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Tomographic study of upper-mantle structure of the South African continent, using wave-form inversion

Cited by 27 publications

References 6 publications

Constraining mantle density structure using geological evidence of surface uplift rates: The case of the African Superplume

Constraining mantle density structure using geological evidence of surface uplift rates: The case of the African Superplume

Evidence for an upper mantle plume beneath the Tanzanian craton from Rayleigh wave tomography

Upper mantle velocity structure beneath southern Africa from modeling regional seismic data

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