Continental Margins Of South-Western Indian Ocean

ABSTRACT Three multichannel seismic surveys were carried out by the French CEPM Group (Comité d'études Pétrolierès et Marines) and the IPGP (Institut de Physique du Globe de Paris) on the continental margins of the South-Western Indian Ocean. The surveys seismic sources were the Flexotir or the Flexichoc from IFP (Institut Français du pétrole). The data collected on these cruises have given a lot of new results about the structure and origin of the Mozambique Channel. The general morphological features of this area have been confirmed:The South-Western Somali basin.The deep Mozambique and Natal basins (4 500 to 5 000 m depth).The basins are bounded on the West by the Mozambique Ridge and the Beira horst and on-the East by a roughly North-South trending feature: the DAVIE Ridge.This ridge divides in fact the Mozambique Channel in two different basins : the deep Mozambique basin on the West and Western Marginal Plateau of Madagascar on the East. The new magnetic data collected on these surveys have shown the extension of oceanic crust to the North till South-East of Beira horst, with probably Jurassic anomalies of about 150 MY BP. Seismic reflexion and refraction surveys have shown an increasing of visible sediment thickness from South to North (2 sec. to 4 sec. two-way time) in the deep Mozambique Channel. The delimitation of Oceanic and Continental areas in the South-Western Somali basin and Mozambique Channel has been attempted. In conclusion position of Madagascar relative to African Mainland will be discussed. INTRODUCTION This note is intended to present the results obtained during three geophysical surveys carried out by CEPM and IPGP, and to discuss the knowledge gained by processing of these data, concerning the structure and geological history on the South-Western margin of the Indian Ocean. CEPM (Comité d'Etudes Pétrolières et Marines) is a French group made up of the oil exploration companies Elf-Aquitaine, CFP-Total and the Institut Français du pétrole. The Laboratoire de Géophysique Marine of IPGP (Institut de Physique du Globe de Paris) directed by R. Schlich was joint surveyor during these operations. GEOGRAPHICAL FRAMEWORK From North to South, the survey zone includes (Figure 2):the South-Western Somali basin,the Mozambique channel,the Natal basin (hitherto called the Mozambique basin or the Madagascar basin). The South-Western Somali basin is bounded on the West by the African coast (Tanzania, Kenya) and on the East by the structural axis of the Amirante. On the South, the boundary with the Mozambique channel is materialized by the volcanic archipelago of the Comores. The Mozambique channel lying between the island of Madagascar on the East and the coast of Mozambique on the African mainland to the West is approximately 400 to 900 km wide.