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, which is at the lower end of predictions from plate motions, but significant enough to recognize this area as a diffuse plate boundary. The formation of long-wavelength basement undulations and the reactivation of fracture zones and ridge-parallel fault fabrics are linked in a unified tectonic model driven by the high level of intraplate compressive stress in the area. There is little evidence from the seismic profiles for intraplate deformation starting before the widespread unconformity dated as 7 Ma.

Long-wavelength (100-300 km) folding in the central Indian Ocean associated with the diffuse plate boundary separating the Indian, Australian, and Capricorn plates is Earth's most convincing example of organized large-scale lithospheric deformation. To test the timing and mechanics of this deformation as implied by plate-kinematic and deformation models, we present a new analysis of the seismic stratigraphy of the Bengal Fan sediments. This shows that the folding of the oceanic lithosphere was multiphase, with major events in the Miocene (8.0-7.5 Ma), Pliocene (5.0-4.0 Ma) and Pleistocene (0.8 Ma). The Miocene phase was the most intense and involved deformation of an area south of 1°S, whereas in the Pliocene the activity shifted northward. In the final phase (Pleistocene) the activity was focused in the equatorial region. No evidence was found Krishna et al. 2 for deformation prior to 8.0-7.5 Ma. The spatial extent of the Pleistocene folding event overlaps the Pliocene and/or Miocene folding events and coincides with both the area of most active faulting and the zone of greatest historical seismicity. The seismic data show that the timing of reverse faulting, and thus more significant shortening of the lithosphere, generally coincided with the phases of folding, but there are examples of folding of the oceanic lithosphere without associated reverse faulting.

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Lavas from Afanasy-Nikitin Rise, possibly the Late Cretaceous product of the Crozet hotspot, cover a wide range of isotopic compositions that includes the lowest (206 Pb/ 204 Pb) t (to 16.77) and Nd (t) (to ؊8) values yet found among oceanic islands or spreading centers worldwide, as well as high (87 Sr/ 86 Sr) t (to 0.7066). In contrast, young basalts from the Crozet Archipelago exhibit a narrow range of variation around Nd ϳ ؉4, 87 Sr/ 86 Sr ϳ 0.7040, and 206 Pb/ 204 Pb ϳ 19.0, closely resembling that of shield lavas of the Réunion hotspot. The Afanasy-Nikitin rocks also have much higher Ba/Nb, Ba/Th, and Pb/Ce than modern oceanic island or ridge lavas, as well as high La/Nb. The data do not obviously support the Crozet plume model but, assuming the model to be plate tectonically correct, would indicate that the plume-source composition either changed dramatically or that Afanasy-Nikitin magmatism involved significant amounts of nonplume mantle. The low 206 Pb/ 204 Pb, low Nd lavas provide the best evidence to date of the sort of material that, by variably contaminating much of the Indian mid-ocean-ridge basalt (MORB) source asthenosphere, may be responsible for the isotopic difference between most Indian MORB and Pacific or North Atlantic MORB. The combined isotopic and trace element results suggest an ultimate origin in the continental crust or mantle lithosphere for this material, although whether it was cycled through the deep mantle or resided at shallow levels in the convecting mantle cannot currently be determined.