This account of the geology of Western Aden Protectorate and of major basement rock groups of Eastern Aden Protectorate is based on photogeological interpretation and on data from a field survey during November 1961 to February 1962. Attention is concentrated mainly upon igneous and metamorphic rocks.An assemblage of metamorphosed sedimentary and mafic igneous rocks, to which the term "Aden Metamorphic Group" is assigned, rests upon older gneiss and is invaded by syntectonic granite with associated zones of metasomatism and migmatization. Subsequent to the main period of regional metamorphism but prior to a later period of stress, plutonic intrusions of dioritic and gabbroic rock occurred; these were followed after a short interval by intrusions of granite. Older volcanic rocks of the Eastern Aden Protectorate are regarded as approximately contemporaneous with mafic intrusions of this phase. Slightly metamorphosed sedimentary rocks of Wadi Ghabar, Eastern Protectorate, are thought to represent uppermost basement rocks. Posttectonic granites of early Paleozoic age and mafic plutonic rocks of possibly similar age are described. Rare intrusions of alkaline granite are thought to be of Tertiary age.An early phase of regional (dynamothermal) metamorphism is distinguished from a later phase of essentially dynamic type. The highest grade of metamorphism recorded is that of the almandine-amphibolite facies. The effect of syntectonic granite invasion upon metamorphic grade is considered.A synopsis of the geology of basement rocks is given, and regional correlation of basement rocks made, in which the Aden Metamorphic Group is tentatively correlated with the main part of the Medina Series, and the Ghabar Group of the Eastern Protectorate with the Fatima Series of Saudi Arabia.A section on the Aden Trap Series includes petrographic examples and a discussion of relations with surrounding rocks and of the probable history of this phase of vulcanism. The age of the series is considered to be Late Cretaceous to Tertiary.Dike rocks of three ages are present.A relatively brief account of sedimentary rocks is largely based on the work of Beydoun in Eastern Aden Protectorate. Fossils collected from these rocks are listed.An account is given of the Aden Volcanic Series, which consists mainly of basaltic lava flows of late Miocene, or Pliocene, to Recent age.The structure of the Western Protectorate is considered in conjunction with that of the Eastern Protectorate. Basement 1 Of the Overseas Geological Surveys, London. strike trends, hitherto not known in detail, are discussed and an account of folding and foliation is given. Since at least Jurassic times the region has suffered intermittent epeirogenic uplift accompanied by faulting. A major phase of uplift of the main basement block of the Western Protectorate, associated with faulting principally along Red Sea trends, dates from late Cretaceous or early Tertiary times. A dominant feature in the Eastern Protectorate is the development of the Tertiary Hadhramut arch. Faulting of fo...
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