Sites 246 and 247

Abstract

One of the characteristics of the Indian Ocean which serves to distinguish it from the Atlantic and Pacific oceans is the abundance of aseismic, relatively shallow, flattopped, often sediment-covered and steep-sided ridgeor plateau-like morphological features which have been termed "microcontinents" by Heezen and Tharp (1965) and "oceanic ridges" by Udintsev (1965). The limited geological and geophysical data available suggest that these features may represent more than one type of crustal structure and composition, and Laughton, Matthews, and Fisher (1970) have proposed the following provisional classification of the Indian Ocean aseismic ridges and plateaus. (Those marked with an asterisk are located in the western part of the Indian Ocean between Africa and the seismically active mid-ocean ridge.) Continental in origin: *Seamount chain southeast of Socotra, *north part of Mascarene Plateau, *Mozambique Ridge, *Madagascar Ridge, *Agulhas Plateau, Crozet Plateau (?), Kerguelen Plateau (?), Broken Plateau (?), Naturaliste Plateau (?), Wallaby Plateau (?). Linear volcanic features: Chagos-Laccadive Ridge, *south part of Mascarene Plateau. Uplifted oceanic crust: Ninetyeast Ridge. Several attempts have been made to drill three of these features (Leg 22: Sites 214, 216, 217; Leg 26: Sites 253, 254 on the Ninetyeast Ridge; Leg 23: Site 219 on the Laccadive-Maldive Ridge; and Leg 24: Site 237 on the Mascarene Plateau). Flexotir reflection data obtained by Schlich et al. (1971) on the Kerguelen-Heard Plateau suggest a possible continental structure for this feature. Some of the microcontinents (e.g., the Seychelles Bank at the northern end of the Mascarene Plateau) are evidently at least partly granitic, and gravity measurements show them to be in isostatic equilibrium, hence crustal thickness varies inversely with depth below sea level. It has been suggested that some microcontinents, such as Broken and Naturaliste plateaus, represent the result of crustal thinning (Francis and Raitt, 1967). More evidence relating to microcontinental basement composition and age and history of their vertical movements is clearly necessary to throw some light on the problem. The Mozambique and Madagascar ridges are obvious sites for deep-sea drilling with this major objective. Figure 1 shows that both the Madagascar and Mozambique ridges are 1000 to 2000 meters deep and trend southward continuously from Madagascar and the "Mozambique bulge" of southeastern Africa, respectively. Both are shallowest (