The Nature and Evolution of the Ninetyeast Ridge: A Key Tectonic and Magmatic Feature of the East Indian Ocean

Abstract

The Ninetyeast Ridge is the longest linear intraplate rise in the World Ocean and the main tectonic and magmatic feature in the East Indian Ocean. Ideas about the nature and evolution of this unique aseismic ridge have changed over the more than 50 years of its research as new geological and geophysical data have been obtained. Our analytical review of available publications has shown that the current dominant hypothesis is the “hot spot trace,” which assumes the formation of the Ninetyeast Ridge under the influence of the Kerguelen mantle plume near the giant transform fault. Geophysical data indicate strong lateral and deep heterogeneity of the Ninetyeast Ridge, which is roughly divided into three segments based on its morphology, style of tectonic deformation of its sedimentary cover, and the deep structure of the crust beneath it: northern, central, and southern. Their different structures are explained by the formation at different stages of three or four phases of tectonic evolution of the East Indian Ocean, when the Kerguelen hotspot was in a different position relative to the Wharton spreading ridge. The complex structure of the central segment with low elastic thickness assumes its emplacement in the hot spreading center. The northern and southern segments with higher mechanical strength of the lithosphere were formed by intraplate volcanism on the older and cooled crust of the Indian Plate. The geochemical composition of the Ninetyeast Ridge tholeiites, enriched to varying degrees by incoherent elements, evidences its formation under the influence of the Kerguelen plume. Primary melts of magmas are of low-Ti and Si-enriched tholeiites, which were formed by high degrees of melting of the mantle protolith. That indicates particularly strong plume influence 70–50 Ma ago, which corresponds to the most complex and anomalous central segment of the Ninetyeast Ridge. The absence of typically depleted magmas and enrichment of melts in an admixture of sources with different isotopic characteristics indicates that asthenospheric magmas did not reach far up, but they were ubiquitously mixed with magmas of the Kerguelen plume.