Paul Mohr

The Ethiopian Flood Basalt Province

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).

Nature of the crust beneath magmatically active continental rifts

Abstract 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.

Patterns of faulting in the Ethiopian rift valley

Abstract 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.

Nature of the crust under Afar: new igneous, not thinned continental

Abstract 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.

The Morton-Black hypothesis for the thinning of continental crust—revisited in Western AFAR

Abstract 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.

Structure of Yemeni Miocene dike swarms and emplacement of coeval granite plutons

Abstract 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.

Diorite-granite magma mingling and mixing along the axis of the Galway Granite batholith, Ireland

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.

LITHOLOGY AND STRUCTURE OF THE PRECAMBRIAN ROCKS OF ERITREA

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.

Late Caledonian dolerite sills from SW Connacht, Ireland

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.

Magmatically assisted off-rift extension—The case for broadly distributed strain accommodation

Within continental rift settings, extensional strain is initially accommodated along the nascent rift margins, subsequently localizing to zones of focused magmatic intrusion. The migration of strain from rift-border faults to diking places an emphasis on constraining the magmatic plumbing system of zones of focused intrusion to resolve how extension is accommodated in the rift lithosphere. While existing rifting models concentrate on the relationship between extension and focused magmatism within the rift, there is increasing evidence of rift-related magmatic activity outside the rift valley. We examine the Galema range, an area of focused magmatic activity along the eastern margin of the Central Main Ethiopian Rift, which is morphologically similar to areas of focused magmatism within the rift. We find that whole-rock thermodynamic modeling and thermobarometric calculations on mineral-liquid pairs suggest that fractionation (and hence magma stalling depths) within the Galema range is polybaric (~7 and ~3 kbar). These results, when compared to zones of focused intrusion within the rift, indicate an incipient magmatic plumbing system. We contend that diking associated with the Galema range, which predates magmatic belts within the rift, thermomechanically modified the lithosphere along this margin. While the cessation of magmatism within the Galema range may have been precipitated by a change in magma flux, the now thermomechanically modified lithospheric mantle along this margin facilitated the subsequent development of within-rift magmatic chains. The implications of this are that off-rift magmatic activity may play an integral role in facilitating the development of rift architecture.