Melt‐induced seismic anisotropy and magma assisted rifting in Ethiopia: Evidence from surface waves

Abstract

The East African rift in Ethiopia is unique worldwide because it captures the final stages of transition from continental rifting to seafloor spreading. A recent study there has shown that magma intrusion plays an important role during the final stages of continental breakup, but the mechanism by which it is incorporated into the extending plate remains ambiguous: wide‐angle seismic data and complementary geophysical tools such as gravity analysis are not strongly sensitive to the geometry of subsurface melt intrusions. Studies of shear wave splitting in near‐vertical SKS phases beneath the transitional Main Ethiopian Rift (MER) provide strong and consistent evidence for a rift‐parallel fast anisotropic direction. However, it is difficult to discriminate between oriented melt pocket (OMP) and lattice preferred orientation (LPO) causes of anisotropy based on SKS study alone. The speeds of horizontally propagating Love (SH) and Rayleigh (SV) waves vary in similar fashions with azimuth for LPO‐ and OMP‐induced anisotropy, but their relative change is distinctive for each mechanism. This diagnostic is exploited by studying the propagation of surface waves from a suite of azimuths across the MER. Anisotropy is roughly perpendicular to the absolute plate motion direction, thus ruling out anisotropy due to the slowly moving African Plate. Instead, three mechanisms for anisotropy act beneath the MER: periodic thin layering of seismically fast and slow material in the uppermost ∼10 km, OMP between ∼20–75 km depth, and olivine LPO in the upper mantle beneath. The results are explained best by a model in which low aspect ratio melt inclusions (dykes and veins) are being intruded into an extending plate during late stage breakup. The observations from Ethiopia join a growing body of evidence from rifts and passive margins worldwide that shows magma intrusion plays an important role in accommodating extension without marked crustal thinning.

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.…”

Melting during late-stage rifting in Afar is hot and deep

“…This discrepancy between observed and predicted crustal thickness is likely to be partly the result of 'magma-compensated' thinning 7 , whereby extensive melt addition to the crust 3,4,6 has reduced net thinning, with a possible further contribution from magmatically accommodated extension 5 . However, the response of the lower part of the Ethiopian lithosphere to extension remains unclear and it is debated whether the lower plate has been preferentially thinned 14 , effectively compensating for the modest net crustal attenuation, or whether a significant thickness of the lithospheric mantle remains intact 17 .…”

“…Investigations of a variety of continental rifts and margins worldwide have revealed that a considerable volume of melt can intrude into the crust during continental breakup [1][2][3][4][5][6][7][8] , modifying its composition and thermal structure. However, it is unclear whether the cause of voluminous melt production at volcanic rifts is primarily increased mantle temperature or plate thinning 1,2,[8][9][10][11][12] .…”

Geochemical evidence of mantle reservoir evolution during progressive rifting along the western Afar margin

“…Thus, the initial phase of rifting above a mantle plume may be marked by a pulse of widespread dike intrusion (e.g., Renne et al, 1996;Fialko and Rubin, 1999;Klausen and Larsen, 2002). During the later stages of continental rifting, extension occurs principally by dike intrusion into a thinned lithosphere (Ebinger and Casey, 2001;Keranen et al, 2004;Rooney et al, 2005;Daly et al, 2008;Keir et al, 2009;Bastow et al, 2010;Ebinger et al, 2010;Wright et al, 2012), before a final stage of plate stretching and associated decompression melting characterizing the final stages of continentocean transition (e.g., Bastow and Keir, 2011). Key unresolved questions remain concerning the geochemical signature(s) of the melt sources during the initial stages of continental rifting, and the evolution of these sources as rifting continues.…”