Seismic modeling constraints on the South African super plume

Abstract

Tomographic studies of the structure of the lower mantle beneath South Africa reveal large-scale low velocities above the core-mantle boundary. Predicted SKS delay patterns (up to 3 s) for some of these models fit observations (Kaapvaal Array data) quite well except for magnitude level, explaining less than one-halfthe observed anomaly. Moreover, the sharpness in travel-time offsets and waveform complications require that nearly vertical walls separate the anomalous structure from the normal preliminary reference Earth model (PREM) mantle. We present numerous record sections along with 2D and 3D synthetics displaying multipathing of arrivals (Sd' SKS, SKKS, S, and ScS), based on a large-scale 3D structure. This kidney-shaped structure has one apex beneath the Indian Ocean (Kerguelen) and the other extending beneath the Mid-Atlantic (Cape Verde). The structure is about 1200 km wide beneath South Africa and extends upward to at least 1000 km through the lower mantle, similar to Grand's model but with an average uniform velocity decrease of about 3% relative to PREM. We have not found any evidences for ultra-low-velocity zones (ULVZ) beneath the main structure but ample evidence at some locations near the edges. We also analyzed Pd and the differentials between PcP travel times and P travel times (PcP-P) along the same great circle paths from the same events. The P-velocity is not very anomalous, perhaps -0.5%. The sharpness of the lateral boundaries (walls) and the large contrast in P and S velocities can be used in arguments for a thermochemical origin.

Origin and evolution of the deep thermochemical structure beneath Eurasia

“…2d), global seismic models have not reported shear-velocity anomalies in the Perm Anomaly that are as low as in the LLSVPs3 (Fig. 2b), although caution must be exercised as the amplitudes of seismic anomalies are often poorly constrained tomographically36. One possibility to explain the apparent smaller shear-velocity reduction in the Perm Anomaly3 (Fig.…”

Spin crossover equation of state and sound velocities of (Mg0.65Fe0.35)O ferropericlase to 140 GPa

“…Large low shear velocity provinces (LLSVPs) are arguably the largest discrete structures in the mantle. Seismic and mineral physics data suggest LLSVPs are thermochemically distinct from the surrounding material [e.g., Dziewonski et al , 2010; Helmberger and Ni , 2005; Sun et al , 2009, 2010; Wang and Wen , 2007] and geodynamic models suggest plausible scenarios for their formation [e.g., Garnero and McNamara , 2008; McNamara and Zhong , 2004; Schubert et al , 2004; Tan and Gurnis , 2005]. For example, given observed variations of δ V S and δ V P within the African LLSVP, and if the density anomaly at a given temperature is equal or greater than ambient, then K S must be larger than that of ambient mantle to maintain neutral buoyancy [e.g., Tan and Gurnis , 2005, 2007].…”

Lower mantle structure from paleogeographically constrained dynamic Earth models

“…By contrast, the more extreme basal layer model is compatible with a limited and localized mid‐mantle extent [e.g., Wen , ]. Definitive confirmation of either model is limited by sparse data coverage for azimuths other than the Drake Passage to Hindu Kush corridor [ Helmberger and Ni , ]. 3‐D multipathing along strike of the African structure [ Ni et al ., ; To et al ., ] or extreme wavespeed reductions in the basal layer can both explain delayed S ‐wave postcursors.…”