Geomorphologie et origine des bancs du nord-est du canal de Mozambique — Ocean Indien occidental (geomorphology and origin of the reef-banks of the north-eastern Mozambique Channel — Western Indian Ocean)

Abstract

The Comores Archipelago is extended to the east by several emerged (Glorieuses, Geyser) and submerged (Zélée, Leven, Cordelière) reef-banks. These topographic features have been investigated by echosounding and magnetometry, and volcanic rocks have been sampled on their slopes (Geyser). All these data give some information about the origin of the banks: 1. (1) The bathymetry suggests that the banks are coral limestones and biogenic carbonate sediments capping the top of the volcanic structures. 2. (2) The magnetic anomalies reflect volcanic basement and are generally localised on the summit or upon the slopes of the banks. 3. (3) The hyaloclastic rocks sampled on the slopes of the Geyser reef-bank are related with subaerial or hydroexplosive volcanism. The Comores Archipelago and the banks of the northeastern Mozambique Channel mark the boundary between the Mesozoïc oceanic Somali Basin and the continental substratum of the Comores Basin. The basaltic magma appears to have been intruded along the north-west trending fracture zones related with southward movement of Madagascar relative to the African mainland.