Seismological constraints on the South African superplume; could be the oldest distinct structure on Earth

A recent study of the lower mantle structure beneath Africa revealed strong lateral changes in S-velocity extending upward from the core-mantle boundary to about 1500 kin. SKS travel times observed on the South African Array display jumps of about 6 s when ray paths cross these nearly vertical boundaries. Back projecting these delays onto the core mantle boundary allows a clear image of the horizontal extent of this structure starting at mid-Africa (15degreesS, 5degreesE) where it strikes roughly northwest to beyond the tip of South Africa (45degreesS, 55degreesE) where it bends toward the Indian Ocean. Waveform. sections of S, ScS, SKS, and SKKS are modeled along two corridors, one along strike and one at right angles to establish its uniformity. The structure is about 1200 km wide and has about a 3% drop in S-velocity although some small-scale features are apparent in the roof structure and midsection. If this structure is stabilized by a localized viscosity condition or a dense core as suggested by some joint inversions, gravity, and free oscillations, it may be isolated from mantle stirring and therefore very old with unique chemistry. (C) 2002 Elsevier Science B.V. All rights reserved.