Crustal velocity structure across the Main Ethiopian Rift: results from two-dimensional wide-angle seismic modelling

Abstract

S U M M A R YWe present the results of velocity modelling of a recently acquired wide-angle seismic reflection/refraction profile across the Main Ethiopian Rift. The models show a continental type of crust with significant asymmetry between the two sides of the rift. A 2-to 5-km-thick layer of sedimentary and volcanic sequences is modelled across the entire region. This is underlain by a 40-to 45-km-thick crust with a c. 15-km-thick high-velocity lowest crustal layer beneath the western plateau. This layer is absent from the eastern side, where the crust is 35 km thick beneath the sediments. We interpret this layer as underplated material associated with the Oligocene flood basalts of the region with possible subsequent addition by recent magmatic events. Slight crustal thinning is observed beneath the rift, where Pn velocities indicate the presence of hot mantle rocks containing partial melt. Beneath the rift axis, the velocities of the upper crustal layers are 5-10 per cent higher than outside the rift, which we interpret as resulting from mafic intrusions that can be associated with magmatic centres observed in the rift valley. Variations in seismic reflectivity suggest the presence of layering in the lower crust beneath the rift, possibly indicating the presence of sills, as well as some layering in the proposed underplated body.

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

“…Alternatively, these patterns may be caused by lateral variations in anisotropy of the uppermost few kilometers with larger upper crustal anisotropy at stations 1018, 1030 and INEE. However, controlled source seismic images of underplating [Mackenzie et al, 2005], mid-crustal conductive anomalies in MT data [Whaler and Hautot, 2005], and Quaternary eruptive centers as far north as Lake Tana all infer the presence of melt in the lower crust beneath the Ethiopian plateau. Given these independent observations, we interpret the data to show that melt induced anisotropy extends to at least 20 km subsurface.…”

“…Mackenzie et al [2005] and Keranen et al [2004] interpret cooled mafic intrusions in the mid-crust beneath these magmatic segments using models derived from wide-angle refraction data and controlled source tomography respectively. The magnitude of splitting under Boset volcano is especially pronounced, where melt-related anomalies have been interpreted in magnetotelluric data [Whaler and Hautot, 2005].…”

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

“…2) (e.g., Agostini et al, 2011a,b;Corti, 2008Corti, , 2009Hayward and Ebinger, 1996;Keir et al, 2011a). In the MER, gravity and seismic imaging show that magmatic segments are underlain by dense, high seismic velocity material, interpreted as cooled gabbroic intrusions (e.g., Beutel et al, 2010;Cornwell et al, 2006;Keranen et al, 2004;Mackenzie et al, 2005;Maguire et al, 2006). Since the voluminous intrusions into the mid-upper crust appear largely restricted to beneath the Quaternary-Recent magmatic segments, it is likely that magma intrusion has only dominated extension during the past~2 Ma (Daly et al, 2008;Keranen et al, 2004).…”

“…Thinning of the lithosphere results in adiabatic decompression of the underlying asthenosphere, which can result in the production of large volumes of molten rock if the rate and extent of thinning are sufficiently high and if the thermochemical state of the mantle is conducive to it (Bown and White, 1995;Shillington et al, 2009;White and McKenzie, 1989). Intrusion of melt into the lithosphere can also achieve extension, without marked plate thinning (e.g., Mackenzie et al, 2005;Thybo and Nielsen, 2009), but with important implications for the thermal structure and strength of the extending plate (e.g., Bialas et al, 2010;. The breakup of continents therefore occurs by the interplay between structural and rheological changes to the lithosphere due to mechanical deformation, heating, and magmatism.…”

Formation and stability of magmatic segments in the Main Ethiopian and Afar rifts

“…Crustal thickness is greatest (40-50 km) beneath the uplifted plateau flanking the rift valley, with evidence for around 10 km of magmatic underplating beneath the uplifted plateau where volcanism spans N40 m.y. (Mackenzie et al, 2005;Dugda et al, 2005;Keranen and Klemperer, 2008) (Fig. 2).…”