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The East African Transform Margin – From Anza to Madgascar: A Relic and Active 4,000km Intraplate Strike Slip Fault System


Evidence of an intraplate strike slip structural system has been discovered by correlative potential fields interpretation of the East African Transform Margin. Seismic and well records have been integrated in this study that support the presence of a 4,000km strike slip fault system. The interpretation reveals typical examples of strike slip tectonics, such as transpression, transtension, duplexing, and syn-/antithetic faults. The interpretation provides a rigorous, regionally consistent framework to assess this transform margin, In consequence, this has significant and positive implications on deepwater, distal petroleum exploration in the Somali Basin.

The continental margin of East Africa is the result of several superimposed structural events, commencing episodic extensional (Karoo) rifting during the Perm0-Triassic, followed by rifting and separation of eastern Gondwana in the early Jurassic. As East Gondwana drifted south in the later Jurassic and early Cretaceous, a transform margin propagated bound by the Davie-Walu fault system with oblique rifting and oceanic spreading to the west of Davie Walu and further south. Spreading in the deep-water Somalia Basin stopped in the Aptian and the transform margin subsided during the later Cretaceous and early Cenozoic to be buried by thick deltaic to deep marine clastics, interrupted by periods of regional uplift and tilting bound by strike slip structures. Through the Tertiary, underlying Jurassic and Permo-Triassic faults of the old transform margin were reactivated to form a wider zone of secondary phase transpressional to transtensional and duplex deformation west of the bounding transform fault. The western margin is expressed as a 4,000km long dextral strike slip fault.

The latest satellite products of Sandwell’s Free Air Gravity (Sandwell, 2014) and the Enhanced Magnetic Model (Chulliat, 2015) are selected in this work. A re-assessment of earlier views is enabled utilizing (1) decompensative residual gravity and (2) reduced to pole, IGRF and amplitude gain corrected residual magnetics. No discrete modelling has been used in the interpretation, other than assuming Airy isostasy and therefore the qualitative interpretation provides an elegant solution to mapping geological structures and correlating with other scientific observations. It is concluded the margin represents a broad transform zone bound to the east by a dextral strike slip fault delimiting the Somali Basin spreading centre.