To further advance our understanding of the way in which a portion of the African superswell in eastern Africa formed, and also to draw attention to the importance of eastern Africa for the plume versus plate debate about mantle dynamics, upper-mantle structure beneath eastern Africa is reviewed by synthesizing published results from three types of analyses applied to broadband seismic data recorded in Tanzania, Kenya, and Ethiopia. (1) Joint inversions of receiver functions and surface wave dispersion measurements show that the lithospheric mantle of the Ethiopian Plateau has been significantly perturbed, much more so than the lithospheric mantle of the East African Plateau. (2) Body wave tomography reveals a broad (≥300 km wide) and deep (≥400 km) low-velocity anomaly beneath the Ethiopian Plateau and the eastern branch of the rift system in Kenya and Tanzania. (3) Receiver function stacks showing Ps conversions from the 410 km discontinuity beneath the eastern branch in Kenya and Tanzania reveal that this discontinuity is depressed by 20–40 km in the same location as the low-velocity anomaly. The coincidence of the depressed 410 km discontinuity and the low-velocity anomaly indicates that the low-velocity anomaly is caused primarily by temperatures several hundred degrees higher than ambient mantle temperatures. These findings cannot be explained easily by models invoking a plume head and tail, unless there are a sufficient number of plume tails presently under eastern Africa side-by-side to create a broad and deep thermal structure. These findings also cannot be easily explained by the plate model. In contrast, the breadth and depth of the upper-mantle thermal structure can be explained by the African superplume, which in some tomographic models extends into the upper mantle beneath eastern Africa. Consequently, a superplume origin for the anomalous topography of the African superswell in eastern Africa, in addition to the Cenozoic rifting and volcanism found there, is favored.
Abstract The May 2000 earthquake cluster, around 10° N and 41° E in southern Afar, has been studied using high quality data from 12 temporary and permanent broadband seismic stations deployed in the area. 140 earthquakes have been located using P- and S-wave arrival times, a well-constrained velocity model, and a double-difference location algorithm. Source mechanisms and moment magnitudes for the four largest events (M > 4) have been obtained from moment tensor inversion. There is no clear alignment of the epicentres along a fault zone; however, the events are clustered slightly southeast of Mount Amoissa along WNW—ESE extension of the Ayelu—Amoissa (Abida/Dabita) lineament. Focal mechanisms show fault motion along WNW—ESE to east—west striking normal faults, with extension oblique to the orientation of the Main Ethiopian Rift. The non-double-couple components of the source mechanisms range from 18–25%, suggesting that the seismic activity is of tectonic origin and not volcanic. Source depths are ≤7 km, in good agreement with estimates of the elastic thickness of the Afar lithosphere. We suggest that the Gewane earthquake swarm represents remnant strain accommodation along a previous line of weakness in southern Afar related to the separation of Arabia from Africa because the focal mechanisms show north—south extension similar to many of the events in central Afar at the triple junction where Arabia is presently rifting away from Africa.
Abstract Crustal structure beneath the GEOSCOPE station ATD in Djibouti has been investigated using H-κ stacking of receiver functions and a joint inversion of receiver functions and surface wave group velocities. We obtain consistent results from the two methods. The crust is characterized by a Moho depth of 23 ± 1.5 km, a Poisson's ratio of 0.31 ± 0.02, and a mean V p of c . 6.2 km s −1 but c . 6.9–7.0 km s −1 below a 2–5 km-thick low-velocity layer at the surface. Some previous studies of crustal structure for Djibouti placed the Moho at 8 to 10 km depth, and we attribute this difference to how the Moho is defined (an increase of V p to 7.4 km s −1 in this study vs. 6.9 km s −1 in previous studies). The crustal structure we obtained for ATD is similar to crustal structure in many other parts of central and eastern Afar. The high Poisson's ratio and V p throughout most of the crust indicate a mafic composition and are not consistent with models invoking crustal formation by stretching of pre-existing Precambrian crust. Instead, we suggest that the crust in Afar consists predominantly of new igneous rock emplaced during the late syn-rift stage where extension is accommodated within magmatic segments by dyking. Sill formation and underplating probably accompany the dyking to produce the new and largely mafic crust.
