The Seismic Signature of Upper‐Mantle Plumes: Application to the Northern East African Rift. (20th December 2019)

• Record Type: Journal Article
• Authors: Civiero, Chiara; Armitage, John J.; Goes, Saskia; Hammond, James O. S.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 20:Number 12(2019)
• Page Start: 6106
• Page End: 6122
• Publication Date: 2019-12-20
• Subjects: mantle plumes -- Rayleigh Taylor -- thermal anomaly -- tomography -- dynamic modeling -- thermal boundary layer; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2019GC008636
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: Several seismic and numerical studies proposed that below some hotspots upper‐mantle plumelets rise from a thermal boundary layer below 660 km depth, fed by a deeper plume source. We recently found tomographic evidence of multiple upper‐mantle upwellings, spaced by several 100 km, rising through the transition zone below the northern East African Rift. To better test this interpretation, we run 3‐D numerical simulations of mantle convection for Newtonian and non‐Newtonian rheologies, for both thermal instabilities rising from a lower boundary layer, and the destabilization of a thermal anomaly placed at the base of the box (700–800 km depth). The thermal structures are converted to seismic velocities using a thermodynamic approach. Resolution tests are then conducted for the same P and S data distribution and inversion parameters as our traveltime tomography. The Rayleigh Taylor models predict simultaneous plumelets in different stages of evolution rising from a hot layer located below the transition zone, resulting in seismic structure that looks more complex than the simple vertical cylinders that are often anticipated. From the wide selection of models tested, we find that the destabilization of a 200 °C, 100 km thick thermal anomaly with a non‐Newtonian rheology, most closely matches the magnitude and the spatial and temporal distribution of the anomalies below the rift. Finally, we find that for reasonable upper‐mantle viscosities, the synthetic plume structures are similar in scale and shape to the actual low‐velocity anomalies, providing further support for the existence of upper‐mantle plumelets below the northern East African Rift. Key Points: Multiple upper‐mantle upwellings rise as Rayleigh Taylor instabilities at different evolutionary stages below Afar and Main Ethiopian Rift A 200 °C excess temperature source layer near the top of the lower mantle explains the upper‐mantle structure below the East African Rift Hotspot volcanic centers spaced by few hundred km are likely fed by hot material which has stagnated at the base of the transition zone