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

John J Armitage,David J Ferguson,Saskia Goes,James O.S Hammond,Eric Calais,Catherine A Rychert,Nicholas Harmon

doi:10.1016/j.epsl.2015.02.039

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.

Highlights

The Afar region in northern Ethiopia forms the northern tip of the largest and most active present-day continental rift, where the Main Ethiopian Rift (MER) intersects the Red Sea Rift and the Gulf of Aden (Fig. 1)
Beneath the Red Sea Rift, crust is thinned to about 15 km (Markis and Ginzburg, 1987; Hammond et al, 2011), volcanism is much more wide spread than in the MER, and much of the region has subsided below sea level in the past few million years, observations that point to the region being in transition from rifting to spreading (Markis and Ginzburg, 1987; Hayward and Ebinger, 1996; Bastow and Keir, 2011)
Afar is generally considered as a classic example of plume involvement in rifting, as it is flanked by ∼30 million year old flood basalts that erupted as rifting started in the Gulf of Aden and Red Sea (Hofmann et al, 1997)

Summary

Introduction

The Afar region in northern Ethiopia forms the northern tip of the largest and most active present-day continental rift, where the Main Ethiopian Rift (MER) intersects the Red Sea Rift and the Gulf of Aden (Fig. 1). Beneath the Red Sea Rift, crust is thinned to about 15 km (Markis and Ginzburg, 1987; Hammond et al, 2011), volcanism is much more wide spread than in the MER, and much of the region has subsided below sea level in the past few million years, observations that point to the region being in transition from rifting to spreading (Markis and Ginzburg, 1987; Hayward and Ebinger, 1996; Bastow and Keir, 2011). Afar is generally considered as a classic example of plume involvement in rifting, as it is flanked by ∼30 million year old flood basalts that erupted as rifting started in the Gulf of Aden and Red Sea (Hofmann et al, 1997). Based on seismic S-to- P receiver functions and geodynamic modelling, Rychert et al (2012) proposed that lithosphere–asthenosphere structure beneath the Afar is similar to a mid-ocean ridge system: with shallow melt generation (

Methods

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Discussion

Conclusion