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Upper mantle temperature and the onset of extension and break-up in Afar, Africa

Upper mantle temperature and the onset of extension and break-up in Afar, Africa
Upper mantle temperature and the onset of extension and break-up in Afar, Africa
It is debated to what extent mantle plumes play a role in continental rifting and eventual break-up. Afar lies at the northern end of the largest and most active present-day continental rift, where the East African Rift forms a triple junction with the Red Sea and Gulf of Aden rifts. It has a history of plume activity yet recent studies have reached conflicting conclusions on whether a plume still contributes to current Afar tectonics. A geochemical study concluded that Afar is a mature hot rift with 80 km thick lithosphere, while seismic data have been interpreted to reflect the structure of a young, oceanic rift basin above mantle of normal temperature. We develop a self-consistent forward model of mantle flow that incorporates melt generation and retention to test whether predictions of melt chemistry, melt volume and lithosphere–asthenosphere seismic structure can be reconciled with observations. The rare-earth element composition of mafic samples at the Erta Ale, Dabbahu and Asal magmatic segments can be used as both a thermometer and chronometer of the rifting process. Low seismic velocities require a lithosphere thinned to 50 km or less. A strong positive impedance contrast at 50 to 70 km below the rift seems linked to the melt zone, but is not reproduced by isotropic seismic velocity alone. Combined, the simplest interpretation is that mantle temperature below Afar is still elevated at 1450 °C, rifting started around 22–23 Ma, and the lithosphere has thinned from 100 to 50 km to allow significant decompressional melting.
continental break-up, melt generation, melt chemistry, seismic structure, receiver functions, East African Rift
0012-821X
78-90
Armitage, John J.
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Ferguson, David J.
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Goes, Saskia
8da1004a-3f5b-44c9-9889-046c5b6c537e
Hammond, James O.S.
14123885-010a-42c1-820b-66f5c683abf1
Calais, Eric
d62221de-f4a1-45c5-981d-5784dba704a4
Rychert, Catherine A.
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Harmon, Nicholas
10d11a16-b8b0-4132-9354-652e72d8e830
Armitage, John J.
e84ac08b-32e9-4550-b85b-779907832ee1
Ferguson, David J.
777aa9bb-9dcb-475d-a180-3b08b7fa619e
Goes, Saskia
8da1004a-3f5b-44c9-9889-046c5b6c537e
Hammond, James O.S.
14123885-010a-42c1-820b-66f5c683abf1
Calais, Eric
d62221de-f4a1-45c5-981d-5784dba704a4
Rychert, Catherine A.
70cf1e3a-58ea-455a-918a-1d570c5e53c5
Harmon, Nicholas
10d11a16-b8b0-4132-9354-652e72d8e830

Armitage, John J., Ferguson, David J., Goes, Saskia, Hammond, James O.S., Calais, Eric, Rychert, Catherine A. and Harmon, Nicholas (2015) Upper mantle temperature and the onset of extension and break-up in Afar, Africa. Earth and Planetary Science Letters, 418, 78-90. (doi:10.1016/j.epsl.2015.02.039).

Record type: Article

Abstract

It is debated to what extent mantle plumes play a role in continental rifting and eventual break-up. Afar lies at the northern end of the largest and most active present-day continental rift, where the East African Rift forms a triple junction with the Red Sea and Gulf of Aden rifts. It has a history of plume activity yet recent studies have reached conflicting conclusions on whether a plume still contributes to current Afar tectonics. A geochemical study concluded that Afar is a mature hot rift with 80 km thick lithosphere, while seismic data have been interpreted to reflect the structure of a young, oceanic rift basin above mantle of normal temperature. We develop a self-consistent forward model of mantle flow that incorporates melt generation and retention to test whether predictions of melt chemistry, melt volume and lithosphere–asthenosphere seismic structure can be reconciled with observations. The rare-earth element composition of mafic samples at the Erta Ale, Dabbahu and Asal magmatic segments can be used as both a thermometer and chronometer of the rifting process. Low seismic velocities require a lithosphere thinned to 50 km or less. A strong positive impedance contrast at 50 to 70 km below the rift seems linked to the melt zone, but is not reproduced by isotropic seismic velocity alone. Combined, the simplest interpretation is that mantle temperature below Afar is still elevated at 1450 °C, rifting started around 22–23 Ma, and the lithosphere has thinned from 100 to 50 km to allow significant decompressional melting.

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Published date: 15 May 2015
Keywords: continental break-up, melt generation, melt chemistry, seismic structure, receiver functions, East African Rift
Organisations: Geology & Geophysics

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Local EPrints ID: 375899
URI: https://eprints.soton.ac.uk/id/eprint/375899
ISSN: 0012-821X
PURE UUID: f8318ffd-ffa9-4e4b-ada6-d407f951fd3a

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Date deposited: 09 Apr 2015 14:14
Last modified: 07 Nov 2018 17:31

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