New Geological – Geophysical Data on the Structure of the Ninetyeast Ridge

1208 The Ninetyeast (or East Indian) Ridge is one of the longest aseismic volcanic ridges in the World Ocean and one of the longest linear structures of the Earth. It is 200 km wide on average, and it extends in the merid ional direction via the entire eastern part of the Indian Ocean for almost 5000 km (Fig. 1). The ridge is dis tinctly expressed in the bottom topography from its intersection with the West Australian Ridge (Broken) in the 31° S area to 10° Ν, where it disappears under sediments of the Bengal fan. Farther up to 17° N, the Ninetyeast Ridge is traceable in the form of a buried anticlinal uplift of the oceanic basaltic basement. There are many hypotheses proposed for explaining the origin of this enigmatic structure, of which two are most popular at present. Many Russian and some for eign scientists believe that the Ninetyeast Ridge resulted from magmatic activity along a giant trans form fault [1, 2]. The overwhelming majority of for eign scientists and some Russian scientists consider the formation of the ridge to have resulted from hot spot magmatism in the course of the Indian–Austra lian lithospheric plate movement above the ascending mantle plume [3, 4] similar to the classical Hawaiian– Emperor volcanic chain. The main argument in favor of the last hypothesis is provided by the regular succes sive increase in basalt ages along the ridge from 43 Ma in the south (DSDP Hole 254) to 77 Ma in the north (ODP Hole 758). Despite the fact that ages of basalts from drilled holes form a linear succession, the avail able seven points are insufficient for discovery of the assumed reverse age trends in its southern part, which follow from the analysis of the anomalous magnetic field [5]. New geological–geophysical data were obtained during Cruise KNOX06RR of the R/V Roger Revelle (United States) carried out in the Ninetyeast Ridge area in 2007 [6]. Prior to this cruise, it was assumed that dredging along the ridge should yield samples for representative radiometric dating and geochemical analysis of basalts, which is necessary for testing the “hot spot” hypothesis. The second task of the cruise was a detailed geophysical site survey in areas of planned drilling on the ridge with deep (up to 200–300 m) penetration into basalts (IODP Proposal 620).