Karoo igneous activity, and the early stages of the break-up of Gondwanaland

The widespread early Jurassic Karoo vulcanism of southern Africa appears to have been generated from a large-scale mantle plume originating in the sub-lithospheric mantle, but with a considerable additional contribution in some areas of material from the lithosphere. Subduction-related vulcanism may also play a significant role, but the relationships of the two types of vulcanism, if indeed a clear distinction does exist, are at present uncertain. Continental break-up probably took place in two stages, the first of which, in the early Jurassic, was not accompanied by true seafloor spreading. In this stage, the reactivation of an ancient shear zone is thought to have resulted in the movement of Antarctica north-eastwards relative to Africa (in the present-day African reference frame), with the creation of a zone of thinned continental crust, or a mixed zone of oceanic and continental crust, in Mozambique. The second stage took place 10–30 Ma later as Antarctica and Madagascar moved southwards relative to Africa with the formation of the oldest-known oceanic crust of the Indian Ocean.