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The Tana basin is situated on the northwestern plateau of Ethiopia, west of the Afar depression. The basin is perched on a topographic high. New data from digital elevation modelling and satellite imagery analysis confirm the basin's location at the junction of three grabens: the Dengel Ber (buried), Gondar (exposed by erosion) and Debre Tabor (reactivated). This structural complex was notably active during the build-up of the mid-Tertiary flood basalt pile, into which the Tana basin is impressed. Fault reactivation occurred in the Late Miocene–Quaternary, accompanied locally by predominantly basaltic volcanism. Fault-slip indicators are consistent with crustal subsidence centred on the present morphologic basin. Concentric and radial dike patterns in the Tana region indicate that diking and basin formation were contemporary. Tana rifting and magmatism occurred above the inferred western side of the Afar mantle plume-head.

A terse review of the geology and geophysics of continental rift valleys associated with large-scale volcanism, points to the underlying crust being composed essentially of new igneous rock. The process of transformation from old cratonic to new igneous crust, still only sketchily known, is considered to involve two phenomena: 1. (1) Intrusion of mafic sills into the extending, thinning and diked cratonic crust, is accompanied at a deeper level by accretionary ‘underplating’. 2. (2) A steepening geothermal gradient facilitates anatexis of sill gabbro/amphibolite, a process aided by fresh batches of mafic magma passing up through the crust. Silicic magma produced from the partial melting process may mix with that fractionated from mafic magma in sill-like chambers in the middle crust. Through these processes, the cratonic component of the rift crust may ultimately become wholly subordinate to the new igneous component well before the onset of crustal separation.

Dike swarms in the Yemeni sector of the eastern margin of the Red Sea basin have been mapped and sampled. The greatest concentration of mafic dikes occurs along the Tihama (coastal) escarpment zone; less profuse swarms lace the Plateau interior. The escarpment zone also hosts microgranitic dike swarms related to a chain of closely spaced, Miocene granite plutons. Dolerite irruption overlapped in time and space with this plutonism. Southern Red Sea basin evolution commenced with flood basalt eruptions upon the late-Proterozoic Arabian craton. Subsequent continental rifting and crustal fissuring focussed dike swarms along a zone of coeval anatectic granites that largely blocked further flood basalt eruption. Immediately following this crustal fissuring/diking episode, severe crustal stretching and thinning led to block faulting and tilting. Eventually, rupture near the rift axis initiated sea-floor spreading. The resulting crust beneath the outer parts of the Red Sea basin is new igneous material that has generated two-layer neo-continental crust.

Three geodimeter networks have been established in the Ethiopian rift: at latitude 8 1 2 ° N , in 1969; at latitude 7 1 2 ° N , in 1970; and at latitude 6°N, in 1971. All three networks have been remeasured annually since their inception. Line-length changes, previously analyzed on a semi-quantitative basis, have now been obtained from a least-squares adjustment program. A rather complex but hopefully realistic weighting scheme has been applied, and the overall network adjustment sigmas (dimensionless) are close to unity. With reference to an arbitrarily selected datum for each network, station vectors have been derived, each with its appropriate foot point curve. The results confirm a significant motion of station RABBIT in the Wolenchiti quadrilateral, a region of episodic ground cracking and subsidence. Significant motions of stations in the Adama region ( 8 1 2 ° N ) form a complex pattern, but a component of longitudinal motion along the rift seems to be a common feature. In the Langana network ( 7 1 2 ° N ), motions are perpendicular to the rift faults, at rates of up to 12 mm/yr.

Mohr, P., 1983. The Morton-Black hypothesis for the thinning of continental crust-revisited in western Afar. In: P. Morgan and B.H. Baker (Editors), Processes of Continental Rifting. Tectonophysics, 94: 509–528. Geological observations along the western margin of Afar show that the Morton-Black model, in which thinning of upper continental crust at a nascent continental margin is accomplished through block faulting and tilting, is not wholly applicable. Rather, dikes are concentrated into swarms, suggestive of important direct dilatation. Faults are concentrated into sets, within which no block-tilting steeper than 45g is observed. Therefore attenuation of the Afar margins requires a significant role for processes other than block tilting, and in some margin sectors the transition from continental to neo-oceanic crust may be relatively abrupt.

Thinned continental crust is considered absent from beneath Afar, except for isolated remnants such as comprise the Danakil Block. The Ethiopian Plateau sialic crust thins abruptly across the plateau-Afar margin to abut new igneous crust under Afar, generated during the early development of the Red Sea basin. Analyses of stretching and sea-floor spreading amounts elsewhere in the Red Sea and Gulf of Aden basins are employed to support this concept. The dual layering of the Afar crust, and the similarity of P-wave velocities in these layers to velocities in sialic crust, lead to the proposal that new continental crust can be generated at magmatic rift zones.

Some typical and some unusual fault patterns are described from the Ethiopian rift valley. They include arcuate faults convex in plan towards the downthrown side; lines of obliquely tilted, faulted blocks; “Up” horsts; horizontally stratified rhomb horsts athwart tilted strata; faults whose polarity switches in conjunction with development of small horsts; intersecting fault sets; and re-entrant and semi-circular faults. Explanations remain wanting for many of these phenomena, which show that crustal extension can be complex at the meso-scale.

Five successive geodimeter surveys during a five-year period yield hints as to the manner and rate of crustal extension in the Ethiopian rift valley. The northern geodimeter network traverses an en-echelon offset in the Wonji fault belt at latitude 8°30′N; this belt comprises the youngest volcanism and faulting of the rift floor. The northern network surveys reveal progressive rift extension at mean rates of 3–5 mm/year and strain rates of 6–16 · 10−7/yr, essentially confined within the Wonji fault belt segments. Small longitudinal motions of persistent dextral sense have occurred in the intervening zone between the offset segments. It is too early to say whether these deformations are local, regional or plate-tectonic phenomena, but the present aseismicity of the rift suggests the buildup of regional strain.

The surface structural elements of Afar are indicative of crustal tension. The dominating feature is intense slicing of the Afar Neogene volcanics by early Pleistocene fault-belts, whose trends are related to those of the three rift systems converging on Afar. This faulting shows that crustal extension affected the whole width of Afar, rather than being confined to oceanic-type, axial zones. Nevertheless, the computed amounts of extension agree well with results of plate analysis of the Afar triple-junction. Refined plate analysis for this region yields data consistent with rotation of the Danakil horst, with inferred presence of cross-rift faults, with longitudinal shearing in eastern Afar, with the apparent asymmetry of spreading in the western Gulf of Aden, and with an early episode of north-south spreading in southernmost Afar. Crustal spreading in Afar seems to be a long-term stop-go phenomenon, initiated at the beginning of the Cainozoic (70 m.y. B.P.). The largest tectonic episode was probably related to the Upper Pliocene-Lower Pleistocene swell uplift of Ethiopia, and the splitting apart of the Danakil and Aisha horsts.