Leg 115

Mascarene Plateau

The Indian Ocean is characterized by scattered elevated platforms and ridges which influence bottom-water circulation and inter-basin communication. The origin of these ridges has been ascribed to continental fragments, ancient island arcs, and volcanic activity associated with either fracture zones in the oceanic crust or stationary hot spots. Leg 115 investigated the nature of basement rocks underlying the Mascarene Plateau and the Chagos-Maldive-Laccadive Ridge, a system which, with Ninetyeast Ridge, were thought to have formed over hot spots now located near Reunion and Kerguelen islands, respectively. Drilling results obtained from sites along the proposed Reunion hot spot track support the hot spot model of increasing age of volcanism northward toward India. The basalts are derived from mixtures of magmas from both hot-spot and spreading-ridge melting regions. The hot spot started beneath the Indian Plate, was crossed by that spreading ridge at about 35 Ma, and has since been under the African Plate. Similarly, the Kerguelen hot spot was overrun by the Southeast Indian Ridge and separated from its volcanic trail, the Ninetyeast Ridge. Basalts at the northern end of the lineament in the Maldive archipelago are geochemically similar to Reunion basalts and were probably formed in an intraplate setting. Paleomagnetic measurements indicate paleolatitudes of ~28°S at 35 Ma, 7°S of the present-day Reunion hot spot.

All sites (Site 705 to Site 710 and Site 712 to Site 716 with the exception of the deepest site, Site 711) exhibit relatively slow carbonate deposition rates during the Oligocene, pronounced carbonate dissolution, associated in part with slump and turbidite deposition and strongly reduced sediment-accumulation rates, in the early and middle Miocene, and a sharp rise in accumulation rates from 10 Ma onward. Vigorous intermediate-depth currents flowed across the northern Mascarene Plateau and were important from the latest Oligocene to the middle Miocene. A step-wise reduction in accumulation rates, starting during the late Pliocene, is associated with the onset of large-scale glaciation of the Northern Hemisphere, indicating a direct link between climatic deterioration and increased flow rates of intermediate water masses. Low-amplitude, long-period oscillations in carbonate content occurred over the last 30 m.y. The deepest site (Site 711 at a water depth of 4.424 m) was beneath the carbonate compensation depth throughout the Neogene, except for a period during the late Plio-Pleistocene, but was above this boundary during the Oligocene.

The recovery of a complete and well-preserved middle Miocene through upper Oligocene sequence of aragonite-bearing periplatform oozes from the Maldives archipelago provides a biostratigraphic reference section for the tropical Indian Ocean.