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This study was supported by the Leverhulme Trust grant 2016–21 (Nature and impacts of Middle Pleistocene volcanism in the Ethiopian Rift). KF was supported by the UK's Natural Environment Research Council (NERC) grant NE/L013932/1 (RiftVolc: The Past, Present and Future of Rift Volcanism in the Main Ethiopian Rift), a Boise Fund grant from the Department of Zoology, University of Oxford, and acknowledges Fonds de Recherche Scientifique – FNRS MIS grant F.4515.20. Tephra work on the Chew Bahir cores in the Cambridge Tephra Lab by AA, AP and CL was made possible by NERC grant NE/K014560/1. Ar–Ar dating was supported by grants NIGFSC IP-1683-1116 and IP-1680-1116. The iCRAG lab is supported by SFI 13/RC/2092. The Main Ethiopian Rift (MER) is renowned as a focus of investigations into human origins. It is also the site of many large volcanic calderas, whose eruptions have spanned the timeframe of speciation, cultural innovation, and dispersal of our species. Yet, despite their significance for dating human fossils and cultural materials, the timing and geochemical signatures of some of the largest eruptions have remained poorly constrained at best. Here, through a programme of field surveys, geochemical analysis and 40Ar/39Ar dating, we report the ages of MER ignimbrites and link them to widespread tephra layers found in sequences of archaeological and paleoenvironmental significance. We date major eruptions of Fentale (76 ± 18 ka), Shala (ca. 145–155 ka), Kone (184 ± 42 ka and ca. 200 ± 12 ka) and Gedemsa (251 ± 47 ka) volcanoes, and correlate a suite of regionally important tephra horizons. Geochemical analysis highlights the predominantly peralkaline rhyolitic melt compositions (7.5–12 wt% Na2O + K2O, 70–76 wt% SiO2) across the central MER and remarkable similarity in incompatible trace element ratios, limiting the correlation of deposits via glass composition alone. However, by integrating stratigraphic and geochronological evidence from proximal deposits, lake sediment cores and distal outcrops at archaeological sites, we have traced ash layers associated with the ca. 177 ka Corbetti, ca. 145–155 ka Shala and ca. 108 ka Bora-Baricha-Tullu-Moye eruptions across southern Ethiopia. In addition to strengthening the tephrochronological framework that supports paleoenvironmental and archaeological work in the region, our findings have wider implications for evaluating the hypothesis of a middle Pleistocene ‘ignimbrite flare-up’ in the MER, and for evaluating the impacts of these great eruptions on landscapes, hydrology, and human ecology. Publisher PDF

The Afar triple junction, where the Red Sea, Gulf of Aden and African Rift System extension zones converge, is a pivotal domain for the study of continental-to-oceanic rift evolution. The western margin of Afar forms the southernmost sector of the western margin of the Red Sea rift where that margin enters the Ethiopian flood basalt province. Tectonism and volcanism at the triple junction had commenced by similar to 31 Ma with crustal fissuring, diking and voluminous eruption of the Ethiopian-Yemen flood basalt pile. The dikes which fed the Oligocene-Quaternary lava sequence covering the western Afar rift margin provide an opportunity to probe the geochemical reservoirs associated with the evolution of a still active continental margin. Ar-40/Ar-39 geochronology reveals that the western Afar margin dikes span the entire history of rift evolution from the initial Oligocene flood basalt event to the development of focused zones of intrusion in rift marginal basins. Major element, trace element and isotopic (Sr-Nd-Pb-Hf) data demonstrate temporal geochemical heterogeneities resulting from variable contributions from the Afar plume, depleted asthenospheric mantle, and African lithosphere. The various dikes erupted between 31 Ma and 22 Ma all share isotopic signatures attesting to a contribution from the Afar plume, indicating this initial period in the evolution of the Afar margin was one of magma-assisted weakening of the lithosphere. From 22 Ma to 12 Ma, however, diffuse diking during continued evolution of the rift margin facilitated ascent of magmas in which depleted mantle and lithospheric sources predominated, though contributions from the Afar plume persisted. After 10 Ma, magmatic intrusion migrated eastwards towards the Afar rift floor, with an increasing fraction of the magmas derived from depleted mantle with less of a lithospheric signature. The dikes of the western Afar margin reveal that magma generation processes during the evolution of this continental rift margin are increasingly dominated by shallow decompressional melting of the ambient asthenosphere, the composition of which may in part be controlled by preferential channeling of plume material along the developing neo-oceanic axes of extension. (C) 2012 Elsevier Ltd. All rights reserved.

The Droimchogaidh sill, south-central Mayo, is sheeted at high-level in the Ordovician sedimentary pile of the South Mayo Trough. This association now lies raised on the eastern fringe of the Connemara-Murrisk Tertiary uplift. The sill is basin-shaped with a maximum thickness of about 100 m. It is composed of isotropic or weakly layered olivine gabbro, and subordinate, locally-developed picritic cumulates and syenitic differentiates. The chemistry of the gabbro reveals the effects of post-emplacement hydrothermal circulation, but the magma that supplied the sill was probably similar in composition to that of the analcime-olivine dolerite dikes further west. The 40Ar/30Ar dating gives an age of 55 ± 1 Ma for the sill, consistent with a younger set of ages from the Irish/British Tertiary province.

Composite dolerite-rhyolite dikes traverse the Galway Granite batholith and its adjacent envelope. The dikes pertain to the Teach D6ite suite and were previously considered to be of Carboniferous age. New and extended examination of field relationships supports recent radiometric dating for an intrusive period that overlapped with the final consolidation of the Galway batholith. Regional crustal extension produced a complex pattern of fissuring, controlled by various preexisting structures, which permitted ascent of mantle-derived melts into and around the Galway batholith. Ponding of mafic magma at an intermediate level facilitated crustal partial melting and the generation of high-silica, high-alumina rhyolitic melts. The two contrasting magmas then rose into common or proximate dike fissures, rhyolitic injection immediately following that of dolerite. Magma storage in stratified chambers occasionally resulted in the development of a hybrid magma layer, but in all cases minor mingling and mixing beween dolerite and rhyolite magma continued up into the dikes. Rhyolite geochemistry precludes a genetic relationship with the Galway granitoids, despite a few instances where granitic material was entrained into rhyolitic magma. Introduction and setting The 400Ma Galway Granite batholith was emplaced into 470Ma island-arc orthogneisses in the Connemara sector of the Caledonides. This emplacement was followed by the intrusion of two hypabyssal suites: earlier microphyric ('porphyry') dacite dikes (Kinahan 1869; Mohr 2003) and a later complex nexus of dolerite dikes, the Teach D6ite (TD) suite (Mitchell and Mohr 1987; Fig. 1). The numerous and widespread dacite dikes have consistently been considered the youngest igneous rocks pertaining to the Galway batholith (Wager 1932; Wright 1961; Harvey 1967; Coats and Wilson 1971; Senior 1973; Leake 1974). However, new work summarised here suggests that the subsequent TD dikes were a final manifestation of the magmatic episode responsible for the batholith. The regional pattern of TD dikes comprises three major linear trends (Fig. 1). In the central part of the Galway batholith and its northern envelope, the NNE-trending Seanabhain system of dikes is intimately associated with the Shanawon Fault that separates the central and western blocks of the batholith (Feely and Madden 1988; Mohr 1993; Callaghan 1999). The na hUillinni dike system, 3.5km west of the Seanabhan system and parallel to it, projects much farther NNE into the orthogneiss envelope (Fig. 1). Secondly, a grid of ENE-trending dikes Irish Journal of Earth Sciences 22 (2004), 15-32. © Royal Irish Academy 15 This content downloaded from 207.46.13.174 on Sun, 10 Jul 2016 05:04:50 UTC All use subject to http://about.jstor.org/terms 16 Irish Journal of Earth Sciences (2004)

The Galway Granite is singular among the Irish latest Caledonian (-400Ma) batholiths for the number and variety of synand post-consolidation sheet intrusions. The most numerous and persistent are microphyric dacitic dikes, typically arranged in sets trending N-NNE, perpendicular to the long axis of the batholith. The sets cut the eastern and western blocks of the batholith, in contrast to their absence from the uplifted and eroded central block. Dacite geochemistry is similar to that of the megacrystic granodiorite that comprises the greater part of the batholith. However, the longer dike sets manifest mineralogical and chemical gradients along strike, from rhyodacite at the batholith axis to andesitic dacite outside the batholith. Hybridisation of granitoid magma with a more mafic magma is considered to have produced the dike magmas. Diking was induced by east-west stress relaxation of the batholith, coeval with a final plutonic episode involving high-level intrusion of alkali leucogranite sills. Regional ENE-WNW transcurrent faulting became active before the diking had concluded.

Dolerite dykes in Connemara and Murrisk that yield c. 315 Ma K-Ar ages vesiculated in varying degrees at the present exposure level. This incipient degassing suggests that the dyke-contemporary land surface lay not more than several hundred metres above the present-day surface. The West Connacht peneplain was cut across Dalradian and Ordovician-Silurian rocks during mid-late Devonian, following which it was buried under Devonian-Carboniferous sedimentary strata since almost entirely removed. Triassic-Jurassic rifting of the central Atlantic basin produced a strong thermal imprint on the rocks of the west of Ireland, an event assumed to have been accompanied by crustal uplift and faulting to produce horst-graben terrain. Ensuing erosion throughout Jurassic-Cretaceous time stripped off much or all of the late Palaeozoic cover. If and where the stripping penetrated into the sub-Carboniferous peneplain, then to that extent the present West Connacht peneplain has an end-Mesozoic sculpture imprinted on it. Palaeocene dolerite dykes in Connemara, Murrisk and North Mayo, associated with initiation of the North Atlantic basin, degassed to a generally greater extent than their late Palaeozoic precursors. This is consistent with geologically estimated shallower levels of emplacement into the contemporary crust, assuming similar magmatic steam overpressures at given depths. Any thickness of Chalk over West Connacht cannot have been significant in terms of overburden pressure. Late Tertiary uplift of the West Connacht peneplain produced the present Connemara-Murrisk plateau which has been deeply dissected by Pleistocene glaciation.

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.

The causes of hydrothermal alteration in dolerite dykes intruding Caledonian rocks of W Connacht are investigated using stable isotope, water content and K–Ar data for whole rocks and mineral separates. Using an isochron approach the Logmór dyke in the north is re-dated to 308±4 Ma; previously determined older whole-rock ages reflect excess 40Ar. The ∼ 305 Ma age previously proposed for the Teach Dóite suite in the south is reinforced by a 305 Ma age on a pyroxene separate, although the severe resetting of most samples is emphasised by other pyroxene and plagioclase ages of ∼210 Ma. Pyroxene δ18O values for these Upper Carboniferous dykes are mostly 5·5 to 6·1%, indicating negligible crustal contamination. Logmór whole-rock samples have water contents of 1·7–2·1 wt.%, δ5D= 59 to –47‰ and δ18O = 9·4 to 9·6‰; plagioclase shows little mineralogical alteration but its δ18O is 9·7‰. Hydrothermal alteration involving a local formation or metamorphic water took place at high fluid/rock ratios and high temperature during cooling after intrusion, most probably in a thermally-driven convection system. Teach Dóite dykes have water contents of 2·0–4·2 wt.%. δD= –58 to –38‰ and δ18O = 3·6 to 9·2‰, and were mostly altered in two stages; hydration upon intrusion to ∼ 2 wt.% water by contemporaneous meteoric water at low fluid/rock ratios was followed by extensive chemical and isotopic alteration at ∼210 Ma (Upper Triassic) by surface waters. This latter event could also have caused the extensive alteration observed in the host rocks.

The late Caledonian Galway Granite batholith is bisected by a WNW-trending magma mingling and mixing zone (the MMZ). This trans-batholithic feature marks the entrainment and ascent of hydrous dioritic magmas in anatectic silicic magmas. Deep in the zone, flowage and hydraulic stress stretched out alternating mafic and hybrid granitoid sheets. At progressively shallower levels, highly elongate and deformed enclaves led up to discrete clouds of ovoid enclaves. Magmatic end-members are identified as quartz diorite and calc-alkaline granite, from which a mixing continuum was produced despite a persistent degree of immiscibility among the various hybrids. A generally west-directed flowage along the MMZ locally induced detachment and upthrusting of ductile blocks of deeper-seated mingled rock. The detailed plan of the MMZ reveals left offsets, supporting field evidence for emplacement in a broad dextral NW-SE shear zone. The Maam and Clifden fault systems provide the partners to model this emplacement within a crustal pull-apart.

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.

Synopsis The Siofra Gabbro, considered to be of Paleocene age, is exposed on the now uplifted late-Tertiary erosion surface of the West Connacht plateau. It is a vertical cylindrical body emplaced into near-vertical Ordovician slates that carry a mild thermal aurcole. The intrusion is composed of labradorite-augite-olivine-leucogabbro containing cognate mafic xenocrysts. Although ‘Mg’ values close to 60 indicate a similar degree of magmatic evolution to that of the coeval alkaline dolerite dyke systems of West Connacht, the gabbro is distinctively subalkaline. It is also relatively enriched in alumina and depleted in both compatible and incompatible trace elements. The Siofra magma appears to have rested and fractionated at the top of a zoned belljar chamber in the upper crust.

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.

Bimodal Silurian igneous activity in SW Connacht occurred during basin subsidence and sedimentation, and before basin deformation. Mafic sills averaging 3 m thickness were preferentially intruded into and supported by a thin horizon of fine-grained siliceous sediment (Tonalee Fm.). Multiple injection produced the thicker sills. A primary igneous lithology of augite–plagioclase–olivine–phyric dolerite has, in some sills, been obliterated by pervasive hydration and carbonation expressed in a biotite-calcite–quartz–albite mineralogy. This represents volatile influx into magma at a sub-intrusive level, and contrasts with syn-intrusive assimilation and post-intrusive alteration that introduced silica and potassium into, and removed magnesium from marginal dolerite. The SW Connacht magmatism occurred in an extensional tectonic setting, probably in a NNE–SSW structurally controlled basin, and was coeval with bimodal magmatism in other parts of Ireland and Britain on both sides of the Iapetus Sutures.

The Sagatu Ridge dyke swarm represents a temporary locus of crustal extension and magmatic eruption outside and east of the present Ethiopian rift margin. A bimodal association of hawaiitic and comenditic dykes comprises the swarm. Previously dated as early Pleistocene, we now present a more precise KAr isochron age of 1.97 ± 0.02 M.a. Sr-isotope data reveal the effects of sialic crustal contamination on the Sr-poor comendites. The direct correlation between 87 Sr/ 86 Sr and initial (“excess”) 40 Ar suggests that this Ar is of crustal rather than mantle origin.

Synopsis Thirty-four new K–Ar ages from West Connacht Paleocene dolerites reveal a systematic pattern of argon loss across the province. Four possible causes for this pattern are examined: thermal overprinting, nature of countryrock, depth of emplacement, and degree of microfracturing. Although some clear relationships emerge, no single cause can explain all the observed apparent ages. Nevertheless, the value of conventional K–Ar dating, as a supplement to 40 Ar/ 39 Ar dating, is emphasised as a means to obtaining regional geological insight into igneous provinces.

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.

Published major-element analyses of Ethiopian volcanic rocks have been subjected to a systematic discriminant analysis. The plateau regions can be subdivided according to the proportions of alkaline and tholeiitic basalts. In northern Ethiopia, these subprovinces show increasing basalt alkalinity with time. The most voluminous basalts have lowest magnesium values, independent of the degree of alkalinity. Rift and Afar basalt chemistry falls within the spectrum observed for the plateau basalts, with no perceptible difference resulting from lithospheric attenuation beneath Afar. However, silicic volcanics of the Rift-Afar floor differ in bulk terms from those of the plateau margins in showing a stronger bias towards peralkalinity, and having higher Na/K values. Two particularly voluminous volcanic episodes have occurred in Ethiopia, dated at ?30–19 m.y. and 4.5–0 m.y. and which link well with one model for seafloor spreading in the Red Sea and Gulf of Aden. Evidence for a mantle hotspot under Ethiopia remains ambiguous.

Caledonian and Hercynian faults in Iar Connacht have been examined for evidence of Quaternary movements. Dip–slip displacements are detected, both through preservation of scarps and, in areas of greater denudation, through morphological offsets. The principle sites of this phenomenon are the Maum, Clifden and Gabhla-Shannaveara faults. Tectonic scarps are distinguished from those due to differential erosion and glacial scour. Earthquake risk in Iar Connacht, though slight, is not non-existent.

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.

A general riftward younging of surficial volcanic units is a feature of the Addis Ababa sector of the Ethiopian rift valley. The narrowing of the volcanic zone with time is due to crustal necking and topographical control.

Two new seismic refraction/wide-angle reflection profiles demonstrate that the crust beneath the southern Porcupine Seabight, out to water depths in excess of 4000 m, is of continental type. They also reveal the rifted margin of the Porcupine basin on its eastern side. Crustal thickness under the Seabight, inclusive of sediments which are up to 6 km thick, decreases from 23 km in the east to about 10 km at a sharp continent-ocean transition in the west.

The Eastern Rift of Africa runs the gamut of crustal and lithospheric attenuation from undeformed shield through attenuated rift margin to active neo-oceanic spreading zones. It is therefore peculiarly well suited to an examination of relationships between volcano spacings and crust/lithosphere thickness. Although lithospheric thickness is not well known in Eastern Africa, it appears to have direct expression in the surface spacing of volcanoes for any given tectonic regime. This applies whether the volcanoes are essentially basaltic, silicic, or alkaline-carbonatitic. No evidence is found for control of volcano sites by a pre-existing fracture grid in the crust.

The most voluminous outpourings of lava on the land surface of the Earth have built imposing stacks of superimposed basalt flows, as much as several thousands of metres thick. Such stacks, or piles, were fed from linear zones of fracturing and fissuring in the crust through which basaltic magma emerged in successive pulses, flooding out over the surrounding region for distances as far as a few hundred kilometres. The fluid nature of these ‘flood basalt’ lavas resulted in the formation of a subdued or planar new topography which, when uplifted during subsequent tectonic episodes, has produced what are popularly termed ‘volcanic plateaus’.

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.

Despite a wide latitude for interpretation of previous Rb–Sr isotopic data on the Oughterard Granite the age of this intrusion has been regarded as a critical time-marker in resolving the Caledonian evolution of Connemara. New isotopic data suggest that the age of the intrusion be revised from c. 460 Ma to c. 400 Ma, thus making the Oughterard Granite one among the many Newer Caledonian Granites in Ireland. The preferred age is 407 ± 23 Ma, and the initial 87 Sr/86Sr ratio is 0·7076 ± 1. Heterogeneity within the granite is demonstrated, which explains the difficulty in obtaining reliable isotopic ages from this intrusion.

O’a is the largest of the Quaternary caldera volcanoes that punctuate the axis of the Ethiopian rift valley. The known volcanic history of O’a is brief: eruptions of restricted ash-flow tuffs and «tufolavas» were followed by extensive pumice deposition with intervening paleosols, lacustrine sediments, and flows of occasional welded tuffs and rare basalts. Ensuing caldera collapse at c. 0.24 m.y. ago was accompanied by emplacement of two massive ignimbrite flow units comprising a single cooling unit: the first was much more severely welded than the second which shows lahar characteristics. Post-caldera volcanism at O’a has been sparse compared with most other Ethiopian rift centres. O’a volcano exemplifies the common rift association of a caldera set tightly between two offset segments of the Wonji fault belt. The Wonji fault belt marks the youngest tectonism of the rift floor, and in the vicinity of O’a has been active in a major way since caldera subsidence. This faulting is clearly younger than the massive rift margin faulting, which to the northeast of O’a occurred during a tectonic climax dated at c. 1.0 m.y. ago. Radiometric analysis suggests a rather regular level of initial40Ar in O’a basalt lavas sampled near to their original vents. If this level also applies to near-vent basalts dated from other parts of the Ethiopian rift, a regional rift paroxysm of crustal extension and related silicic and basaltic volcanism is evident at c. 0.30–0.20 m.y. ago. Episodic dilatation and associated volcano-tectonism separated by long periods of quiescence appears to be a general feature of continental rift valleys.

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.

SummaryThe Ethiopian Cainozoic volcanics associated with the African rift system comprise one of the world's most voluminous alkaline igneous provinces. The Simien Mountains are the remnants of a Miocene alkali olivine-basalt volcanic centre in the north-western part of this province. The end-phase activity at Simien featured intrusion of dyke-swarms of two trends, one parallel to the rift system, the other almost perpendicular to it. Dykes of the rift trend are typically alkaline, but a dyke sampled from the other trend proves to be an olivine-tholeiite. Its presence is interpreted, along with similar rocks from the Harar region in eastern Ethiopia, in terms of upper mantle rifting extending from the Gulf of Aden and Red Sea under the continental blocks of the Ethiopian swell.

THE African rift system is usually considered to be a continental extension of the mid-oceanic ridge–rift system, a connexion being made through the Gulf of Aden and the Gulf of Tajura. The detailed connecting pattern, complicated by the convergence of the Red Sea structures, shows that in terms of seismic epicentre distribution (unpublished work of P. Gouin), volcanism1 and crustal tectonics2 the transition from oceanic to continental rift is not made as smoothly as many previous hypotheses supposed.

The discovery of extensive Pliocene pantellerite welded tuffs in southern Ethiopia necessitates a revision of the Cainozoic volcanic succession associated with the Ethiopian rift system.

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.

THE almost random occurrence of earthquakes seems to rule out any useful method of forecasting, at least until we understand and can closely monitor earth mantle processes. The probable influence of known natural factors like earth and ocean tides in triggering earthquakes is not sufficiently determining to be a forecasting aid. More hopeful are the results of ground deformation monitoring in the vicinity of such fault zones as the San Andreas, where patterns are emerging between ground deformation and ensuing local earthquakes1.

The Ethiopian flood basalt province embraces the Afar triple-rift junction, where the African Rift System meets the Red Sea and Gulf of Aden sea-floor spreading zones. Prolific eruption of basalt and subordinate other lavas during Tertiary time has built up a subaerial volcanic pile, typically 500–1500 m thick and locally attaining 3000 m. This eruptive process was particularly active during mid-Tertiary, and was renewed in Afar some 5 My ago. The bulk of the eruptions was from fissures, and was intimately associated with the development of nascent continental margins defining the western and southern limits of Afar, within a larger context of Red Sea-Gulf of Aden basin evolution (Fig. 1). The Yemen Plateau basalts were united to their Ethiopian counterparts, prior to subsidence and opening of the Red Sea basin (see Civetta et al., 1978 and Capaldi et al., 1983 for aspects of the Yemen flood basalts).

The Precambrian basement in Eritrea is composed chiefly of persistent North-South belts of Upper Proterozoic volcano-sedimentary rocks. The belts have been formed by two, possibly three, episodes of East-West compression, compression which was progressively more severe northwards. Late syn-tectonic granites occur within the synforms that characterise the belts. The final deformation was gentle folding about East-West axes, possibly synchronous with intrusion of post-tectonic granites.

Using a combination of laser ranging and GPS data acquired between 1969 and 1997 we derive a separation velocity for the Somali and Nubian plates in Ethiopia (4.5±1 mm/yr at N108±10E). This vector is orthogonal to the NNE-trending neotectonic axis (Wonji fault belt) of the Ethiopian rift axis. Current rifting is concentrated within a 33-km-wide zone that includes a 7-km-wide belt of late Quaternary faulting where maximum surface strain rates are comparable to those at active plate boundaries (0.1 μstrain/yr). The strain-field suggests that thin (

RECENT exploration and field-work in Ethiopia, in particular gravity–geology surveys made by Prof. P. Gouin, Dr. A. S. Rogers and me, have shown the Cainozoic volcanic succession in association with the Ethiopian Rift System to be more complicated than previously suspected.