The development of the East African margin during Jurassic and Lower Cretaceous times: a perspective from global tectonics

摘要

The eastern margin of Africa resulted from the first successful trans-Gondwana rupture which retraced, in part, the earlier unsuccessful Karoo rift system. Widespread volcanism in southern Africa (182 Ma, Toarcian) presaged NW-SEdirected extension between East Gondwana and West Gondwana (Africa). Rifting turned progressively north-south in orientation, leading quickly to ocean growth off Somalia and off central Mozambique while, elsewhere, strike-slip within the stretched margin came to predominate. East Gondwana, including Madagascar, was demonstrably still intact at 151.4 Ma (M22, Kimmeridgian) but, as the two large continental fragments disengaged from each other, pure north-south movement became possible. After about 140 Ma (Berriasian), East Gondwana itself started to fragment off Western Australia but little separation occurred as far west as Madagascar before Aptian times (126.1 Ma). Nevertheless, the geometry of the Australia-India opening required that, in the interval 140 - 120 Ma, Madagascar-India pursued a path against Africa different from that of Antarctica. The arcuate Davie Fracture Zone, 1800 km in length, functioned as a pure strike-slip transform off the Tanzania-Mozambique coast for this fragment until the early Aptian demise of the Somali mid-ocean ridge. The active transform east of the Lebombo in southern Africa, meanwhile, relocated progressively eastwards, finally to outboard of the Mozambique Ridge at 136 Ma (Valanginian), leaving most if not all of the stretched continental crust and its volcanosedimentary load attached to Precambrian Africa.