During five expeditions of the Scripps Institution of Oceanography to the western Indian Ocean, more than 4,500,000 sq km of the Central Indian Ridge and its branching Southeast Indian Ridge and Southwest Indian Ridge were explored by bathymetric, magnetic, and seismic-reflection profiling. In some 2,800,000 sq km of this region, igneous rocks of the crust, lower crust, and possible upper mantle are exposed by faulting or volcanism. Fifty-six dredge hauls of these igneous rocks were obtained, largely from the major cross-fractures (transform faults) or clefts trending athwart the volcanically active ridges. From north to south, the cross-fractures most intensively sampled were the Vema Fracture Zone, which crosses the crestal area near 9°S, Argo Fracture Zone near 13°30′S, Marie Celeste Fracture Zone near 17°30′S, and the newly delineated “Melville Fracture Zone” trending north-south for more than 600 km near 60°30′E on the Southwest Indian Ridge.

Our field and laboratory studies indicate that under a capping of young flow basalt, there is a regional complex of igneous rocks produced by magma generated under the ridges, trapped and differentiated into sill-like, podiform, and larger, crudely stratified to well-stratified sheets. Rocks from the stratiform masses include abundant Iherzolite and minor harzburgite, orthopyroxenite, olivine- and two-pyroxene gabbros, Ti-ferrogabbros, norite, and anorthosite. Some associated diabase intrusions are granophyric and are cut by late-stage dikelets of quartz monzonite and Na-rich trondhjemite. Both calc-alkaline and alkalic lines of differentiation are indicated. The granitic dikelets contain clear, doubly-terminated crystals of zircon, unusual in a terrane of large-cation–depleted rocks.

The overlying basalt flows are pillowed with chemical and mineralogical characteristics typical of olivine-bearing tholeiite from the ridge-rise systems of the world oceans.

The ubiquitous nature of the crustal complex found throughout the western Indian Ocean, together with data from the Atlantic and Pacific Oceans, suggest that similar rock complexes, dominated in their lower parts by stratiform bodies, are characteristic of most of the igneous crust throughout the world oceans.

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