Mantle upwellings, melt migration and the rifting of Africa: insights from seismic anisotropy

Abstract

Abstract:The tiffing of continents and eventual formation of ocean basins is a fundamental component of plate tectonics, yet the mechanism for break-up is poorly understood. The East African Rift System (EARS) is an ideal place to study this process as it captures the initiation of a rift in the south through to incipient oceanic spreading in north-eastern Ethiopia. Measurements of seismic anisotropy can be used to test models of rifting. Here we summarize observations of anisotropy beneath the EARS from local and teleseismic body-waves and azimuthal variations in surfacewave velocities. Special attention is given to the Ethiopian part of the rift where the recent EAGLE project has provided a detailed image of anisotropy in the portion of the Ethiopian Rift that spans the transition from continental rifting to incipient oceanic spreading. Analyses of regional surface-waves show sub-lithospheric fast shear-waves coherently oriented in a northeastward direction from southern Kenya to the Red Sea. This parallels the trend of the deeper African superplume, which originates at the core-mantle boundary beneath southern Africa and rises towards the base of the lithosphere beneath Afar. The pattern of shear-wave anisotropy is more variable above depths of 150 km. Analyses of splitting in teleseismic phases (SKS) and local shear-waves within the rift valley consistently show rift-parallel orientations. The magnitude of the splitting correlates with the degree of magmatism and the polarizations of the shear-waves align with magmatic segmentation along the rift valley. Analysis of surface-wave propagation across the rift valley confirms that anisotropy in the uppermost 75 km is primarily due to melt alignment. Away from the rift valley, the anisotropy agrees reasonably well with the pre-existing Pan-African lithospheric fabric. An exception is the region beneath the Ethiopian plateau, where the anisotropy is variable and may correspond to pre-existing fabric and ongoing melt-migration processes. These observations support models of magma-assisted rifting, rather than those of simple mechanical stretching. Upwellings, which most probably originate from the larger superplume, thermally erode the lithosphere along sites of pre-existing weaknesses or topographic highs. Decompression leads to magmatism and dyke injection that weakens the lithosphere enough for rifting and the strain appears to be localized to plate boundaries, rather than wider zones of deformation.

Mantle upwellings and convective instabilities revealed by seismic tomography and helium isotope geochemistry beneath eastern Africa

“…At shallow depth (Figure 1, 100 km) and large depth (Figure 1, 310 km), the orientation of flow in the AfarEthiopia region is in agreement with SKS splitting measurements [Kendall et al, 2005]. In the uppermost mantle, this direction is primarily explained by oriented melt inclusions on mantle flow [Kendall et al, 2005[Kendall et al, , 2006, and at large depth, this direction reflects the average deep upper mantle flow. But the asthenospheric flow (Figure 1, 200 km) is strongly perturbed when it encounters the strong Afar plume (probably its head) and it seems to turn around it, This deflection is translated into an EW direction of anisotropy to [Dziewonski and Anderson, 1981] and plotted with the same color scale.…”

Variations in late syn‐rift melt alignment inferred from shear‐wave splitting in crustal earthquakes beneath the Ethiopian rift

“…Sv and Sh velocity models determined from inversion of surface-wave dispersion curves show faster Sv velocities than Sh velocities below 20 km along the rift axis. The results are consistent with anisotropy at 20-75 km depth due to oriented melt-filled pockets [Kendall et al, 2005b]. Bastow et al [2005] show, by comparing P-and S-wave relative arrival-time data, that upper mantle low velocity anomalies beneath the MER are likely due to hightemperatures and partial melt.…”

“…Our study shows that melt-induced anisotropy at 20-75 km depth [Bastow et al, 2005;Kendall et al, 2005aKendall et al, , 2005b continues into the uppermost crust, thereby penetrating the entire plate and facilitating continental breakup.…”

The development of extension and magmatism in the Red Sea rift of Afar

“…NE of the TGD in the Red Sea and Gulf of Aden rifts in Afar, Gao et al (2010) use the backazimuthal variations in SKS splitting to show rift parallel anisotropy throughout the lithosphere. They interpret this as strain related structures such as magmatic intrusions, a similar interpretation as by Kendall et al (2005Kendall et al ( , 2006 and Bastow et al (2010) in the MER. Below the lithosphere, anisotropic fast directions parallel the N30-40°E trend of the African Superplume and are most sensibly interpreted as related to alignment of olivine in the asthenosphere induced by subhorizontal flow (e.g., Bastow et al, 2010;Gao et al, 2010;Kendall et al, 2006;Obrebski et al, 2010).…”

“…In the MER, high-density splitting analysis of SKS phases in tandem with surface-wave studies is sensitive to a~20°change in orientation of strain fabrics from N30-40°E at the rift margins to N10°E along the rift axis. This change is interpreted as caused by the localisation of magma intrusion through the whole lithosphere toward the rift axis~2 Ma (Bastow et al, 2010;Kendall et al, 2005Kendall et al, , 2006. NE of the TGD in the Red Sea and Gulf of Aden rifts in Afar, Gao et al (2010) use the backazimuthal variations in SKS splitting to show rift parallel anisotropy throughout the lithosphere.…”