Details

Autor(en) / Beteiligte

• Boyce, A.
• Bastow, I. D.
• Cottaar, S.
• Kounoudis, R.
• Guilloud De Courbeville, J.
• Caunt, E.
• Desai, S.

Titel

AFRP20: New P‐Wavespeed Model for the African Mantle Reveals Two Whole‐Mantle Plumes Below East Africa and Neoproterozoic Modification of the Tanzania Craton

Ist Teil von

• Geochemistry, geophysics, geosystems : G3, 2021-03, Vol.22 (3), p.n/a

Ort / Verlag

Washington: John Wiley & Sons, Inc

Erscheinungsjahr

2021

Link zum Volltext

Quelle

Wiley Online Library

Beschreibungen/Notizen

• Africa's Cenozoic tectonism is often attributed to mantle plumes, particularly below East Africa, but their morphology, number, location, and impact on the African lithosphere are debated. The broad slow wavespeed African Superplume, ubiquitous in large‐scale tomographic models, originates below South Africa, reaching the surface somewhere below East Africa. However, whether the diverse East African mantle geochemistry is best reconciled with one heterogeneous upwelling, or current tomographic models lack the resolution to image multiple distinct plumes, remains enigmatic. S‐wavespeed tomographic images of Africa are legion, but higher frequency P‐wavespeed whole‐mantle models possessing complementary diagnostic capabilities are comparatively lacking. This hinders attempts to disentangle the effects of Cenozoic hotspot tectonism and Pan African (and older) tectonic events on the East African lithosphere. Here we develop a continental‐scale P‐wave tomographic model capable of resolving structure from upper‐to‐lower mantle depths using a recently developed technique to extract absolute arrival‐times from noisy, temporary African seismograph deployments. Shallow‐mantle wavespeeds are δVP ≈ −4% below Ethiopia, but less anomalous (δVP ≥–2%) below other volcanic provinces. The heterogeneous African Superplume reaches the upper mantle below the Kenyan plateau. Below Ethiopia/Afar we image a second sub‐vertical slow wavespeed anomaly rooted near the core‐mantle boundary outside the African LLVP, meaning multiple disparately sourced whole‐mantle plumes may influence East African magmatism. In contrast to other African cratons, wavespeeds below Tanzania are only fast to 90–135 km depth. When interpreted alongside Lower Eocene on‐craton kimberlites, our results support pervasive metasomatic lithospheric modification caused by subduction during the Neoproterozoic Pan‐African orogeny. Plain Language Summary The African plate comprises a collage of ancient continental fragments, cratons, over 2.5 billion years old, that have remained largely unchanged since formation. In seismically and volcanically active East Africa, the continent is being broken apart (rifted). Hot mantle rock has been rising slowly from the core‐mantle boundary below Africa for millions of years in at least one mantle “plume.” Seismologists and geochemists often disagree on the number and location of mantle plumes below East Africa, perhaps because geochemical data can be interpreted in multiple ways or seismic images of the mantle are currently not sufficiently detailed to “see” more than one mantle plume. Given accurate timing of seismic energy arriving from distant earthquakes, “seismic tomography” can be used to create images of the African mantle. Old tectonic plates, which are cold, usually appear as fast seismic wavespeeds while hot plumes appear as slow wavespeeds. Our new seismic images show two mantle plumes originating in the lower mantle (>2,000 km depth) may underlie the volcanoes along the East African Rift valley. Furthermore, by unpicking the signatures of continental assembly from those of younger plumes and rifting origin, we reveal surprising evidence for modification of the ancient Tanzanian craton. Key Points AFRP20 is an absolute P‐wavespeed tomographic model for the African mantle AFRP20 reveals two whole‐mantle plumes below East Africa Slow Tanzanian craton P‐wavespeeds reveal Neoproterozoic metasomatic lithospheric modification