The Malagasy Rift, a chasm in the Tethyan margin of Gondwana

Abstract

In Permian and Triassic times plate collision and consumption along the Pacific margin of Gondwana contrasted with rifting and extensional conditions along the Tethyan rim. The latter caused the development of two major rifts, the Westralian Trough between Australia and Greater India and the Malagasy Trough between Africa-Arabia and Madagascar-India. Formation of the Malagasy Trough commenced in the Early Permain. It is evidenced by rift structures which extend from eastern Africa and Madagascar to Oman and possibly beyond. In Tanzania these early intracratonic rifts produced the mega-sequence of the Karoo Super Group. This succession which was initiated by Late Palaeozoic glacial deposits is almost entirely of terrestrial origin. However, in Madagascar and to the north thereof, marine Tethyan faunas were quick to take possession of the Malagasy Trough after deglaciation, indicating an open and unrestricted connection with the Tethys. In the same direction in which marine conditions increased, post-glacial coal deposition diminished and gave way to mildly evaporitic conditions. Various stages of rifting and disintegration are reflected by depositional sequences of east African and Madagascar Karoo Basins. These Late Palaeozoic and Triassic tectonic movements can be matched at least in parts with tectonic events in the Tethyan realm. Evidence for the development of both Malagasy and Westralian Troughs place important constraints on the positioning of continental blocks and the palaeogeography of the central Tethys in Permo-Triassic times.