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 traveltime 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.

Abstract We review previous models for the Paleogene tectonic evolution of the Arabian and Eastern Somali basins and present a model based on a new compilation of magnetic and gravity data. Using plate reconstructions, we derive a self-consistent set of isochrons for Chron 27 to Chron 21 (61-46 Ma). The new isochrons account for the development of successive ridge propagation events along the Carlsberg Ridge, leading to an important spreading asymmetry between the conjugate basins. Our model predicts the growth of the outer and inner pseudo-faults associated with the ridge propagation events. The location of outer pseudo-faults appears to remain very stable despite a drastic change in the direction of ridge propagation before Chron 24 (c. 54 Ma). The motion of the Indian plate relative to the Somalian plate is stable in direction through Paleogene time; spreading velocities decrease from 6 to 3 cm a -1 . Our reconstructions also confirm that the Arabia-India plate boundary was located west of the Owen Ridge along the Oman margin during Paleogene time; some compression is predicted at about Chron 21 (47 Ma) between the Indian and Arabian plates.

Abstract Located at the triple junction of the African, Arabian, and Somalian plates, the Afar depression has been subjected to intense and complex volcano-tectonic activity from the Miocene into the Holocene. The region also has experienced significant climatic changes. The chronology of ancient lacustrine sedimentation is based on K-Ar dating of associated volcanics, diatom biostratigraphy, and 14 C ages. Spreading of the Afar depression accounts for the migration of active sedimentary basins from the margins of the Afar triangle to the modern active segments (Lakes Asal and Afrera). Tectonic conditions have been favorable for the settlement of deep, closed lakes in central Afar since the late Pliocene (2.5 Ma). There, ancient lakes may not be related to present topography. Sediment distribution shows a southwest- to-northeast progression in age of opening of the modern basins and a change in direction of major active faults from about east-west to northwest at —1.4 Ma. Deformation of Holocene sediments and shorelines allows the direction and rate of recent vertical displacement to be estimated. Rifting also has increased elevation differences and hydraulic gradients between the depression and surrounding highlands, thereby increasing the effectiveness of both surface waters and groundwater circulation. This explains the general increase in water depth since the Miocene. The effects of climatic fluctuations have been superimposed onto the structural evolution of the depression. Because climate governs chemical weathering, it is partly responsible for the sedimentary facies, specifically whether detrital, biogenic silica, or chemical carbonate fractions dominate. At any given time, the ratio of precipitation to evaporation in the Afar and on neighboring plateaus is responsible for the presence or absence of lakes in the deepest basins. Major lacustrine stages are correlated both with evolution of other tropical lakes and with global climatic events.

Abstract Although the East African Rift (EAR) System is often cited as the archetype for models of continental rifting and break-up, its present-day kinematics remains poorly constrained. We show that the currently available GPS and earthquake slip vector data are consistent with (1) a present-day Nubia–Somalia Euler pole located between the southern tip of Africa and the Southwest Indian ridge and (2) the existence of a distinct microplate (Victoria) between the Eastern and Western rifts, rotating counter-clockwise with respect to Nubia. Geodetic and geological data also suggest the existence of a (Rovuma) microplate between the Malawi rift and the Davie ridge, possibly rotating clockwise with respect to Nubia. The data indicate that the EAR comprises at least two rigid lithospheric blocks bounded by narrow belts of seismicity (<50 km wide) marking localized deformation rather than a wide zone of quasi-continuous, pervasive deformation. On the basis of this new kinematic model and mantle flow directions interpreted from seismic anisotropy measurements, we propose that regional asthenospheric upwelling and locally focused mantle flow may influence continental deformation in East Africa.