Geological mapping in northeastern Somalia (Midjiurtinia Region): Field evidence of the structural and paleogeographic evolution of the nothern margin of the Somalian Plate

A detailed geological investigation of the continental margins of northeastern Somalia, in land areas contiguous to major oceanic structures (e.g. the Alula-Fartaq fracture zone), was carried out from 1988 to 1991, and the results are given in the enclosed 1:200,000 geological map, derived from photointerpretation and fieldwork at the scale 1:50,000. With respect to the rift of the Gulf of Aden, the stratigraphic sequences illustrated in the map have been distinguished as pre- and syn- and post-rift sediments. The pre-rift sediments rest on the low-grade ?pre-Palaeozoic phyllites and pan-African granitic intrusions and consist of sedimentary cover of continental, lagoon and marine facies ranging in age from Dogger to Eocene. Cretaceous-Eocene sediments pass eastward into clastic and marly deposits of the pelagic domain of the Indian Ocean. The syn- and post-rift sequences ranging in age from Oligocene to Miocene crop out only in narrow "en-echelon" basins striking WNW-ESE along the coast of the Gulf of Aden. The deposits belonging to the syn- and post-rift sequences are discordant and transgressive over the Meso-Cenozoic substratum and are composed by organogenic neritic marine, lagoonal and continental deposits. The tectonic setting of the area is characterized by half-graben bordered by faults with displacements of several kilometres striking WNW-ESE. Faults bordering contiguous half-graben systems often dip in opposite directions, so that in the transition area between different ones a "transfer" or "accommodation" zone develops; the zones are on the landward projection of the oceanic fracture zones, whilst the syn-rift basins are on the landward projection of the oceanic ridge. Based on our fieldwork the following reconstruction is put forward: 1. During the Early Oligocene the Afro-Arabian plate underwent a phase of intense faulting which led to the formation of small syn-rift basins elongated in the WNW-ESE direction, separated by transfer zones and structural highs. 2. Progressive crustal extension led to the formation of spreading centres of oceanic crust corresponding to earlier syn-rift basins. The fracture zones linking spreading centres formed in alignment with continental transfer zones. © 2002 Elsevier Science Ltd. All rights reserved.

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