Margaret H. Benoit

Upper mantle P-wave speed variations beneath Ethiopia and the origin of the Afar hotspot

The Afar hotspot has long been attributed to one or more thermal upwellings in the mantle, in particular starting thermal plumes characterized by a head that spreads laterally beneath the lithosphere, and a tail. New P-wave tomography images of the upper mantle beneath Ethiopia reveal an elongated low wave speed region that is deep (>400 km) and wide (>500 km). The location of the low wave speed anomaly aligns with the Afar Depression and Main Ethiopian Rift in the uppermost mantle, but the center of the anomaly shifts to the west with depth. The shape, depth extent, and location of the low wave speed anomaly is not consistent with a starting thermal plume presently beneath the hotspot. Instead, the anomaly suggests that the hotspot may be the surface manifestation of a broad mantle upwelling connected to the African Superplume in the lower mantle beneath southern Africa.

P and S velocity structure of the upper mantle beneath the Transantarctic Mountains, East Antarctic craton, and Ross Sea from travel time tomography

P and S wave travel times from teleseismic earthquakes recorded by the Transantarctic Mountains Seismic Experiment (TAMSEIS) have been used to tomographically image upper mantle structure beneath portions of the Transantarctic Mountains (TAM), the East Antarctic (EA) craton, and the West Antarctic rift system (WARS) in the vicinity of Ross Island, Antarctica. The TAM form a major tectonic boundary that divides the stable EA craton and the tectonically active WARS. Relative arrival times were determined using a multichannel cross-correlation technique on teleseismic P and S phases from earthquakes with mb ≥ 5.5. 3934 P waves were used from 322 events, and 2244 S waves were used from 168 events. Relative travel time residuals were inverted for upper mantle structure using VanDecars method. The P wave tomography model reveals a low-velocity anomaly in the upper mantle of approximately δVp = −1 to −1.5% in the vicinity of Ross Island extending laterally 50 to 100 km beneath the TAM from the coast, placing the contact between regions of fast and slow velocities well inland from the coast beneath the TAM. The magnitude of the low-velocity anomaly in the P wave model appears to diminish beneath the TAM to the north and south of Ross Island. The depth extent of the low-velocity anomaly is not well constrained, but it probably is confined to depths above ∼200 km. The S wave model, within resolution limits, is consistent with the P wave model. The low-velocity anomaly within the upper mantle can be attributed to a 200–300 K thermal anomaly, consistent with estimates obtained from seismic attenuation measurements. The presence of a thermal anomaly of this magnitude supports models invoking a thermal buoyancy contribution to flexurally driven TAM uplift, at least in the Ross Island region of the TAM. Because the magnitude of the anomaly to the north and south of Ross Island may diminish, the thermal contribution to the uplift of the TAM could be variable along strike, with the largest contribution in the Ross Island region. The tomography results reveal faster than average velocities beneath East Antarctica, as expected for cratonic upper mantle.

Crustal thinning between the Ethiopian and East African plateaus from modeling Rayleigh wave dispersion

The East African and Ethiopian Plateaus have long been recognized to be part of a much larger topographic anomaly on the African Plate called the African Superswell. One of the few places within the African Superswell that exhibit elevations of less than 1 km is southeastern Sudan and northern Kenya, an area containing both Mesozoic and Cenozoic rift basins. Crustal structure and uppermost mantle velocities are investigated in this area by modeling Rayleigh wave dispersion. Modeling results indicate an average crustal thickness of 25 {+-} 5 km, some 10-15 km thinner than the crust beneath the adjacent East African and Ethiopian Plateaus. The low elevations can therefore be readily attributed to an isostatic response from crustal thinning. Low Sn velocities of 4.1-4.3 km/s also characterize this region.