Edinburgh University, United Kingdom

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 analysis shows that the folding of the oceanic lithosphere was multiphase, with major events occurring 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 degrees 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 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.