Application of P- and S-receiver functions to investigate crustal and upper mantle structures beneath the Albertine branch of the East African Rift System
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East African Rift
Receiver function
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East African Rift
Seismic Tomography
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Abstract The northern Main Ethiopian Rift captures the crustal response to the transition from continental rifting in the East African rift to the south, to incipient seafloor spreading in the Afar depression to the north. The region has also undergone plume-related uplift and flood basalt volcanism. Receiver functions from the EAGLE broadband network have been used to determine crustal thickness and average V p / V s for the northern Main Ethiopian Rift and its flanking plateaus. On the flanks of the rift, the crust on the Somalian plate to the east is 38 to 40 km thick. On the western plateau, there is thicker crust to the NW (41–43 km) than to the SW (<40 km); the thinning taking place over an off-rift upper mantle low-velocity structure previously imaged by travel-time tomography. The crust is slightly more mafic ( V p / V s ∼ 1.85) on the western plateau on the Nubian Plate than on the Somalian Plate ( V p / V s ∼ 1.80). This could either be due to magmatic activity or different pre-rift crustal compositions. The Quaternary Butajira and Bishoftu volcanic chains, on the side of the rift, are characterized by thinned crust and a V p / V s > 2.0, indicative of partial melt within the crust. Within the rift, the V p / V s ratio increases to greater than 2.0 (Poisson’s ratio, σ > 0.33) northwards towards the Afar depression. Such high values are indicative of partial melt in the crust and corroborate other geophysical evidence for increased magmatic activity as continental rifting evolves to oceanic spreading in Afar. Along the axis of the rift, crustal thickness varies from around 38 km in the south to 30 km in the north, with most of the change in Moho depth occurring just south of the Boset magmatic segment where the rift changes orientation. Segmentation of crustal structure both between the continental and transitional part of the rift and on the western plateau may be controlled by previous structural inheritances. Both the amount of crustal thinning and the mafic composition of the crust as shown by the observed V p / V s ratio suggest that the magma-assisted rifting hypothesis is an appropriate model for this transitional rift.
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The transition between the lithosphere and the asthenosphere is subject to numerous contemporary studies as its nature is still poorly understood. The thickest lithosphere is associated with old cratons and platforms and it has been shown that seismic investigations may fail to image the lithosphere‐asthenosphere boundary in these areas. Instead, several recent studies have proposed a mid‐lithospheric discontinuity of unknown origin existing under several cratons. In this study we investigate the Tanzania craton in East Africa which is enclosed by the eastern and western branches of the East African Rift System. We present evidence from S receiver functions for two consecutive discontinuities at depths of 50–100 km and 140–200 km, which correspond to significant S wave velocity reductions under the Tanzania craton and the Albert and Edward rift segments. By comparison with synthetic waveforms we show that the lower discontinuity coincides with the LAB exhibiting velocity reductions of 6–9%. The shallower interface reveals a velocity drop that varies from 12% beneath the craton to 24% below the Albert‐Edward rift. It is interpreted as an infiltration front marking the upper boundary of altered lithosphere due to ascending asthenospheric melts. This is corroborated by computing S velocity variations based on xenolith samples which exhibit a dense system of crystallized veins acting as pathways of the infiltrating melt. Mineral assemblages in these veins are rich in phlogopite and pyroxenite which can explain the reduced shear wave velocities. Melt infiltration represents a suitable mechanism to form a mid‐lithospheric discontinuity within cratonic lithosphere that is underlain by anomalously hot mantle.
East African Rift
Asthenosphere
Xenolith
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Continental rifting is constrained by the architecture and heterogeneous composition of lithosphere within which rifting occurs. Recent studies in Ethiopia show that the Cenozoic northern Main Ethiopian Rift (NMER) has developed in a Neoproterozoic lithospheric framework modified by a Tertiary plume, magma injection having replaced mechanical failure as the main strain accommodation mechanism. A 400 km long profile of 91 broadband seismic stations striking southeast across the NMER from the uplifted Ethiopian plateau to beyond the southern margin of the rift has provided a high‐resolution P receiver function section, here interpreted in terms of crustal architecture and composition in light of independent geophysical observations. Synrift deposits are identified over a ∼110 km wide region beneath which strain was accommodated during the early stages of rifting. Major variations in crustal thickness and seismic properties along the profile divide the crust into four distinct regions. Beneath the northwestern rift flank (average crustal thickness 37.5 km and V p / V s 1.82) mafic middle and lower crustal rocks are overlain by a felsic upper crust. Here a high P wave velocity lowest crustal layer (northwestern lower crustal layer) is proposed to consist of frozen gabbroic sills and possibly some partial melt. We suggest partial melting of lower crustal rocks and/or fractional crystallization may have contributed to the bimodal prerift and synrift magmatism. Also, the presence of this layer through its effect on crustal and lithospheric strength and rift‐related diking may have controlled the location and development of the NMER in the vicinity of the profile. Beneath the rift (average crustal thickness 34.5 km and V p / V s 1.87) the crust is subdivided into a northwestern sector, with a thinned crust and strong likelihood of partially molten rocks, and a southeastern sector, where high velocity and density anomalies and the presence of a Moho “hole” in the receiver function profile constrain the limits of a well‐developed crustal magma system. To the southeast, a 35 km wide zone marks the transition from intruded and thinned (by ∼5 km) crust beneath the rift to the amagmatic, thick crust of the southeastern rift flank (average crustal thickness 39 km and V p / V s 1.77) suggested to be of felsic to intermediate composition.
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The Miocene‐Recent East African Rift in Ethiopia subaerially exposes the transitional stage of rifting within a young continental flood basalt province. As such, it is an ideal study locale for continental breakup processes and hot spot tectonism. We combine teleseismic traveltime data from 108 seismic stations deployed during two spatially and temporally overlapping broadband networks to present detailed tomographic images of upper mantle P and S wave seismic velocity structure beneath Ethiopia. Tomographic images reveal a ∼500 km wide low P and S wave velocity zone at 75 to ≥400 km depth in the upper mantle that extends from close to the eastern edge of the Main Ethiopian Rift (MER) westward beneath the uplifted and flood basalt‐capped NW Plateau. We interpret this broad low‐velocity region (LVR) as the upper mantle continuation of the African Superplume. Within the broad LVR, zones of particularly low velocity are observed with absolute delay times ( δt P ∼ 4 s) that indicate the mantle beneath this region is amongst the slowest worldwide. We interpret these low velocities as evidence for partial melt beneath the MER and adjacent NW Plateau. Surprisingly, the lowest‐velocity region is not beneath Afar but beneath the central part of the study area at ∼9°N, 39°E. Whether this intense low‐velocity zone is the result of focused mantle upwelling and/or enhanced decompressional melting at this latitude is unclear. The MER is located toward the eastern edge of the broad low‐velocity structure, not above its center. This observation, along with strong correlations between low‐velocity zones and lithospheric structures, suggests that preexisting structural trends and Miocene‐to‐Recent rift tectonics play an important role in melt migration at the base of the lithosphere in this magmatic rift zone.
Flood basalt
East African Rift
Hotspot (geology)
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Abstract Narrow and wide rifts are end‐member expressions of continental extension. Within the framework of passive rifting, the transition from wide to narrow rifts requires lowering the geothermal gradient. Reconciling this view with observational evidence for narrow rift zones in regions underlain by sublithospheric hot plume material, such as the eastern branch of the East African Rift, requires invoking preexisting weak zones for strain to localize in a warm crust. Based on thermomechanical numerical models, we show that along‐rift width variations can develop spontaneously as a consequence of spatial variations of the geotherm over an evolving mantle plume impinging a lithosphere subjected to ultraslow extension. The eastern branch of the East African Rift, with a narrow Kenya segment underlain by a mantle plume head and widening to the north and south in the colder regions of the Turkana depression and North Tanzania divergence, is in agreement with this numerical prediction.
East African Rift
Mantle plume
Hotspot (geology)
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The High Atlas and the Anti Atlas are fold-belts linked to former and still ongoing continent–continent collisions. Despite their high elevation, studies indicate a lack of a deep crustal root (<40 km) while the lithosphere underneath is thinned (<100 km). Previous explanations for this thinning include asthenospheric upwelling due to small-scale convection or a small plume. We use data recorded at stations in SW Morocco to analyse teleseismic P- and S-wave receiver functions. Our study yields a crustal thickness ranging from 24 km near the Atlantic coast to 44 km beneath the High Atlas with an average crustal Vp/Vs ratio of 1.77 in the entire region. A crustal thickness of 40 km cannot entirely support the topography in this region. Furthermore, we find the lithosphere–asthenosphere boundary at ∼80 km depth. The lithosphere beneath SW Morocco is thinner than beneath northern Morocco (>150 km). This lithospheric thinning supports the theory of thermal compensation of the mountain ranges. The mantle transition zone thickness amounts to 240 ± 10 km. The transition zone seems to be slightly thinned which might indicate a higher mantle temperature in this region.
Asthenosphere
Mantle plume
Receiver function
Thinning
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