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Previous U–Pb zircon studies of the Nicobar and Bengal fans indicate the Himalaya as the dominate sediment source, making these fans a major Cenozoic sink for uplifted Himalayan sediment. The fan forms part of the Bengal–Nicobar Fan System and is among the oldest and largest submarine fans on Earth. The Nicobar Fan lies within the north-eastern Indian Ocean between the Ninety-East Ridge and Sunda Arc. Plio-Pleistocene sediments are composed of clastic sediments containing fine grained volcanic detritus derived from the rapidly eroding volcanic arc on Sumatra. In the Late Miocene, outer shelf limestones correlate with a low stand of sea level. Stable convergence rates persisted through Mid-Miocene times and deposition was dominated by relatively shallow water clastic and carbonate sediments deposited on a well-developed shelf and shelf-break. Deposits include melanges (? Eocene-Oligocene) and Neogene sedimentary successions initially (Early Miocene) deposited in relatively deep water. During Oligocene and Eocene times an increase in the subduction rate led to basin inversion and uplift of the outer arc ridge. A revised stratigraphy for the outer arc part of the forearc is presented and the Tertiary evolution described. Recent fieldwork has revealed that the sedimentological and structural evolution of the northern Sumatran forearc islands can be related to the rate of plate convergence, sea-level variations and uplift of the magmatic arc on Sumatra. These islands are located along the active margin of the Sunda forearc basin and separate it from the trench slope.

Forearc islands such as Nias and Simeulue provide unique opportunities to study the active accretion of sediment in the outer part of a forearc.