Jeroen Ritsema

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

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.

The elusive mantle plume

Abstract Mantle plumes are hypothetical hot, narrow mantle upwellings that are often invoked to explain hotspot volcanism with unusual geophysical and geochemical characteristics. The mantle plume is a well-established geological structure in computer modeling and laboratory experiments but an undisputed seismic detection of one has yet to be made. Vertically continuous low shear velocity anomalies in the upper mantle, expected for plumes, are present beneath the Afar, Bowie, Easter, Hawaii, Iceland, Louisville, McDonald, and Samoa hotspots but not beneath the other 29 hotspots in Sleep’s 1990 catalog. Whether and how plumes form remain fundamental multi-disciplinary research questions. Should they exist, detection of whole-mantle plumes will depend on deployments of dense (50–100 km station spacing), wide-aperture (>1000 km) seismic networks to maximize model resolution in the transition zone and uppermost lower mantle since plume impingement upon the 660-km phase transition leaves a unique seismic imprint.

New seismic model of the upper mantle beneath Africa

We present a seismic model of the upper 400 km of the mantle beneath Africa and surrounding regions. This model is constructed by inverse modeling of fundamental mode Rayleigh wave phase velocities (40–200 s) for about 8000 propagation paths. Among the most pronounced anomalies are high shear velocity structures (as much as 6% higher than in the Preliminary Reference Earth model) beneath the West African, Congo, and Kalahari cratons that extend to about 250 km depth. These structures have near-vertical margins across which the shear velocity changes by as much as 6% over 500 km distance. Anomalous low shear velocities (3%–4% lower than in the Preliminary Reference Earth Model) structures are observed beneath the East African, Red Sea, and Gulf of Aden rifts, and beneath the northwestern Indian Ocean. These structures extend to a depth of at least 250 km. Our model cannot be reconciled with models that invoke a large number of plumes that have impinged on the base of the lithosphere, nor does our seismic model indicate that high-temperature, low-density material beneath the lithosphere is responsible for the uplift of southern Africa.

Upper mantle seismic velocity structure beneath Tanzania, east Africa: Implications for the stability of cratonic lithosphere

The assertion of cratonic stability put forward in the model for deep continental structure can be tested by examining upper mantle structure beneath the Tanzania Craton, which lies within a tectonically active region in east Africa. Tomographic inversions of about 1200 teleseismic P and S travel times indicate that high-velocity lithosphere beneath the Tanzania Craton extends to a depth of at least 200 km and possibly to 300 or 350 km. Based on the thickness of mantle lithosphere beneath Archean cratons elsewhere, it appears that the mantle lithosphere of the Tanzania Craton has not been extensively disrupted by the Cenozoic tectonism in east Africa, thus corroborating the assertion of cratonic stability in the model for deep continental structure. The presence of thick, high-velocity structure beneath the Tanzania Craton implies relatively low temperatures within the cratonic mantle lithosphere, consistent with relatively low surface heat flow. The thick cratonic keel is surrounded by low seismic velocity regions beneath the east African rifts that extend to depths below 400 km. Our models show a shear velocity contrast between the cratonic lithosphere and the uppermost mantle beneath the eastern branch of the rift system of about 5% to 6%, but from resolution experiments we infer that this contrast could be underestimated by as much as a factor of 1.5. We attribute about half of this velocity contrast to the depleted composition of the cratonic keel and the other half to thermal alteration of upper mantle beneath the rifts. Low-density structures that may be required to provide buoyant support for the elevation of the Tanzania Craton must reside at depths greater than about 300–350 km.

Seismic evidence for a deep upper mantle thermal anomaly beneath east Africa

Upper mantle seismic velocity variations beneath northern Tanzania coupled with the structure of the 410 and 660 km discontinuities reveal a 200‐ 400-km-wide thermal anomaly extending into but not necessarily through the transition zone beneath the eastern branch of the East African rift system. This finding is not easily explained by small-scale mantle convection induced by passive stretching of the lithosphere or by a broad thermal upwelling extending from the lower mantle into the upper mantle, but it can be attributed to a mantle plume, provided that a plume head is present under the lithospheric keel of the Tanzania craton. A plume interpretation for the deep thermal anomaly is supported by evidence for mantle having the geochemical characteristics of a plume at >150 km depth beneath northern Tanzania.

Evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath Africa

We present data which indicate that the broad, low shear velocity anomaly beneath southern Africa is stronger and more extensive than previously thought. Recordings of earthquakes in the southwestern Atlantic Ocean at an array of broadband seismic stations in eastern Africa show anomalously large propagation time delays of the shear phases S, ScS, and SKS which vary rapidly with epicentral distance. By forward modeling, we estimate that the low velocity anomaly extends from the core-mantle boundary about 1500 km up into the mantle and that the average shear velocity within this structure is 3% lower than in standard models such as PREM. Strong velocity contrasts exist at its margins (2% over about 300 km). These seismic characteristic are consistent with recent numerical simulations of lower mantle mega-plume formation.

Tomographic filtering of high-resolution mantle circulation models: Can seismic heterogeneity be explained by temperature alone?

[1] High-resolution mantle circulation models (MCMs) together with thermodynamic mineralogical models make it possible to construct 3-D elastic mantle heterogeneity based on geodynamic considerations. Recently, we have shown that in the presence of a strong thermal gradient across D 00 and corresponding large temperature variations in the lower mantle, the heterogeneity predicted from isochemical whole mantle flow agrees well with tomographic models in terms of magnitudes of S wave velocity (vs) variations. Here, we extend the comparison of geodynamic and tomographic structures by accounting explicitly for the limited resolving power of tomography. We focus on lateral variations in vs and use the resolution operator R associated with S20RTS to modify our geodynamic models so that they reflect the long-wavelength (>1000 km) nature and the effects of heterogeneous data coverage and damping inherent to the tomographic inversion. Prior to the multiplication with R, the geodynamic models need to be reparameterized onto the basis of S20RTS. The magnitude reduction introduced by this reparameterization is significant and needs careful assessment. We attempt a correction of the reparameterization effects and find that the inherent tomographic filtering alone then leads to a magnitude reduction by a factor of � 2 in the lower mantle. Our tomographically filtered models with strong core heating agree well with S20RTS, which resolves maximum negative anomalies of around � 1.5% in the lowermost mantle. Temperature variations on the order of +1000 K, corresponding to perturbations of around � 3% in vs in the unfiltered model, would be seen as � 1.5% when ‘‘imaged’’ with the data and damping of S20RTS. This supports our earlier finding that isochemical whole mantle flow with strong core heating and a pyrolite composition can be reconciled with tomography. In particular, the large lateral temperature variations associated with lower mantle plumes are able to account for the slow seismic anomalies in the large low-velocity zones under Africa and the Pacific. We also find that strong gradients in shear wave velocity of 2.25% per 50 km in our unfiltered models compare well with the sharp sides of the African superplume. Components: 5494 words, 7 figures.

Tomographic filtering of geodynamic models: Implications for model interpretation and large-scale mantle structure

[1] The resolution operator R is a critical accompaniment to tomographic models of the mantle. R facilitates the comparison between conceptual three-dimensional velocity models and tomographic models because it can filter these theoretical models to the spatial resolution of the tomographic model. We compute R for the tomographic model S20RTS (Ritsema et al., 1999, 2004) and two companion models that are based on the same data but derived with different norm damping values. The three models explain (within measurement uncertainty) S-SKS and S-SKKS travel times equally well. To demonstrate how artifacts distort tomographic images and complicate model interpretation, we apply R to (1) a thermochemical and (2) an isochemical model of convection in the mantle that feature different patterns of shear velocity heterogeneity in the deep mantle if we assume that shear velocity heterogeneity is caused by temperature variations only. R suppresses short-wavelength structures, removes strong velocity gradients, and introduces artificial stretching and tilting of velocity anomalies. Temperature anomalies in the thermochemical model resemble the spatial extent of low seismic velocity anomalies and the shear velocity spectrum in the D’’ region better than the isochemical model. However, the thermochemical model overpredicts the amplitude of shear velocity variation and places the African and Pacific anomalies imperfectly. We suspect that inaccurate velocity scaling laws and uncertain initial conditions control these mismatches. Extensive hypothesis testing is required to identify successful models.

A strongly negative shear velocity gradient and lateral variability in the lowermost mantle beneath the Pacific

A new approach for constraining the seismic shear velocity structure above the core-mantle boundary is introduced, whereby SH-SKS differential travel times, amplitude ratios of SV/SKS, and Sdiff waveshapes are simultaneously modeled. This procedure is applied to the lower mantle beneath the central Pacific using da.ta from numerous deep-focus southwest Pacific earthquakes recorded in North America. We analyze 90 broadband and 248 digitized analog recordings for this source-receiver geometry. SH-SKS times are highly variable and up to 10 s larger than standard reference model predictions, indicating the presence of laterally varying low shear velocities in the study area. The travel times, however, do not constrain the depth extent or velocity gradient of the low-velocity region. SV/SKS amplitude ratios and SH waveforms are sensitive to the radial shear velocity profile, and when analyzed simultaneously with SH-SKS times, rnveal up to 3% shear velocity reductions restricted to the lowermost 190±50 km of the mantle. Our preferred model for the central-eastern Pacific region (Ml) has a strong negative gradient (with 0.5% reduction in velocity relative to the preliminary reference Earth model (PREM) at 2700 km depth and 3% reduction at 2891 km depth) and slight velocity reductions from 2000 to 2700 km depth (0–0.5% lower than PREM). Significant small-scale (100–500 km) shear velocity heterogeneity (0.5%–1%) is required to explain scatter in the differential times and amplitude ratios.

Seismic evidence for ultralow‐velocity zones beneath Africa and eastern Atlantic

KS waveforms recorded at distances of about 110 o are extremely useful to constrain seismic velocity structure at the base of the mantle.