Identification of a sequence of east-west trending magnetic anomalies of Mesozoic age in the western Somali Basin helps define the position of Madagascar in the Gondwana reconstruction. The anomalies are symmetric about ancient ridge segments and are flanked to the north and south by the Jurassic magnetic quiet zone. The motion of Madagascar relative to Africa was from the north and began in the middle Jurassic, about the same time as the initial breakup of Gondwanaland. Sea-floor spreading ceased when Madagascar assumed its present position in the Early Cretaceous.

Atlantic-type continental margins are formed by rifting and subsequent breakup of continents by sea-floor spreading. Large scale horizontal displacements of continental blocks on lithospheric plates are well-described by current plate tectonic theory. However, the rifting process itself entails vertical tectonics which operate prior to breakup, and such processes are not well understood within the framework of plate tectonic theory.The qualitative models, at present described in the literature involve a sequence of events illustrated by type examples: the East African Rift; the ethiopian-Afar Rift; the Red Sea Rift; and the Gulf of Aden. The thinning and subsidence of the continental crust during rifting have been considered to be the result of crustal stretching. However the writer considers the evidence ambiguous and the theory inadequate. The model presented here involves the response of the crust and the lithosphere to thermal processes implicit in plate tectonic theory. Firstly, uplift is caused by thermal expansion and phase-boundary migration in the lithosphere. which leads to crustal thinning by erosion. Secondly, subsidence is caused by metamorphism in the deep crust. These events occur out of phase and result in two cycles of uplift/erosion/and subsidence during continental margin formation.From this generalised model, theoretical time-stratigraphic and structural cross sections may be constructed to suit the different continental margins. These cross sections are composed of different litho-tectonic elements, namely: pre-rift, rift valley, and post breakup, which are in general separated by two angular unconformities. These features may be readily identified on Atlantic continental margins with only a minimum of seismic and well control.

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