Sharp sides to the African superplume

Beneath southern Africa is a large structure about 1200 kilometers across and extending obliquely 1500 kilometers upward from the core-mantle boundary with a shear velocity reduction of about 3%. Using a fortuitous set of SKS phases that travel along its eastern side, we show that the boundary of the anomaly appears to be sharp, with a width less than 50 kilometers, and is tilted outward from its center. Dynamic models that fit the seismic constraints have a dense chemical layer within an upwardly flowing thermal structure. The tilt suggests that the layer is dynamically unstable on geological time scales.

Superplumes from the core-mantle boundary to the lithosphere: implications for heat flux

Three-dimensional modeling of upper-mantle anelastic structure reveals that thermal upwellings associated with the two superplumes, imaged by seismic elastic tomography at the base of the mantle, persist through the upper-mantle transition zone and are deflected horizontally beneath the lithosphere. This explains the unique transverse shear wave isotropy in the central Pacific. We infer that the two superplumes may play a major and stable role in supplying heat and horizontal flow to the low-viscosity asthenospheric channel, lubricating plate motions and feeding hot spots. We suggest that more heat may be carried through the core-mantle boundary than is accounted for by hot spot fluxes alone.

Complex Shear Wave Velocity Structure Imaged Beneath Africa and Iceland

A model of three-dimensional shear wave velocity variations in the mantle reveals a tilted low velocity anomaly extending from the core-mantle boundary (CMB) region beneath the southeastern Atlantic Ocean into the upper mantle beneath eastern Africa. This anomaly suggests that Cenozoic flood basalt volcanism in the Afar region and active rifting beneath the East African Rift is linked to an extensive thermal anomaly at the CMB more than 45 degrees away. In contrast, a low velocity anomaly beneath Iceland is confined to the upper mantle.

A correlation between ultra-Low basal velocities in the mantle and hot spots

The statistical correlation between the locations of hot spots at the surface of Earth and the distribution of ultra-low-velocity zones at the base of the mantle has about a 1 percent chance of arising randomly. This correlation is more significant than that between hot spots and negative velocity anomalies in tomographic models of deep mantle compressional and shear velocity. This correlation is consistent with the notion that many hot spots originate in a low-velocity, probably partially molten layer at the core-mantle boundary and undergo little lateral deflection on ascent.

Relation of Major Volcanic Center Concentration on Venus to Global Tectonic Patterns

Global analysis of NASA Magellan image data indicates that a major concentration of volcanic centers covering approximately 40 percent of the surface of Venus occurs between the Beta, Atla, and Themis regiones. Associated with this enhanced concentration are geological characteristics commonly interpreted as rifting and mantle upwelling. Interconnected low plains in an annulus around this concentration are characterized by crustal shortening and infrequent volcanic centers that may represent sites of mantle return flow and net down-welling. Together, these observations suggest the existence of relatively simple, largescale patterns of mantle circulation similar to those associated with concentrations of intraplate volcanism on Earth.