Print this pageA reappraisal of Rift Development in Western Madagascar
RAMANAMPISOA, LALANIRINA R., DAVID N. CLARK and JOELI V. LALAHARISAINA
Recent studies, based on improved seismic quality, have revealed two distinct rifting phases in Madagascar; a Permo-Triassic failed rift and a Jurassic-Cretaceous passive margin. Previous studies concluded that a single rifting phase occurred, starting in the Permian and ending in the Jurassic.
An intracontinental rift propagated through western Madagascar in Permian times. Limited sedimentation occurred in half-grabens and these were filled with fluvial and marginal sediments (Sakoa and L. Sakamena). Crustal extension continued into the early Triassic, accompanied by symmetrical rifting and deposition of marine shales (M. Sakamena) and deltaic sandstones (U. Sakamena). Thermal sag started in Late Triassic times and fluviatile sandstones (Isalo) accumulated that overstep the graben edges.
In the Late Liassic, a new rift formed to the west of the earlier failed rift. A series of half-grabens developed that filled with marine shales and sandstones (Andalia/Beronono). Madagascar started to drift southwards away from Africa in the Bajocian, and a passive margin developed along the West coast. Drifting was accompanied by thermal sag and a carbonate platform (Bemaraha) was deposited in a barrier-lagoonal environment Passive margin development continued into the Cretaceous and Tertiary, with marine shales and sandstones (e.g. Duvalia and Tsiandava) being deposited.
Renewed tectonic activity occurred in the Late Cretaceous as Madagascar separated from India. The break-up appears to have resulted from a shearing movement rather than a rift. Wrench faulting and associated folding also commonly occurred in the western basins from the Late Cretaceous onwards, although these have previously been interpreted as extensional features.