New seismic reflection profiles of ≈5370 km, running through the Ocean Drilling Program Leg 116 sites and Deep Sea Drilling Project Sites 215 and 218, were obtained to investigate the spatial extent, timing, and nature of the Tertiary deformation of the equatorial central Indian Ocean. Analyses of the data revealed basement and sedimentary structures and structural unconformities that resulted from the release of compressional forces. Long-wavelength (150--300 km) anticlinal basement structures with 1--2 km relief and tight folding and high-angle faulting (5--20 km long) of oceanic basement and overlying sediments exist between the Afanasy Nikitin seamount and the Ninetyeast Ridge. The deformation zone extends from 10 ¿S to the north up to ≈7 ¿N latitude. The changes in the regional basement trend along 87 ¿E longitude between latitudes 11 ¿S and 15 ¿N coincide with the significant tectonic events of the first major plate reorganization occurred in the eastern Indian Ocean at 90¿5 Ma and changes in the Indian plate motion at 65¿5 Ma. The basement deformation at selected places, a widespread unconformity of the upper Miocene, and subsequent younger unconformities (lower Pliocene and upper Pleistocene) indicate that the deformation activity might have begun earlier than the generally believed age of 7.5 Ma and appears to be periodic. The upper Miocene and upper Pleistocene deformational unconformities are, in general, observed south of 1 ¿S, while the basement deformation and lower Pliocene deformational unconformity are mostly present in the area north of 1 ¿S. It is surmised that the compressional stresses built up since the hard collision of India with Eurasia may have released for a short period prior to the early Miocene time and deformed the oceanic basement in the northeastern Indian Ocean. Thereafter the stress regime seems to have transferred to the north of the Indian shield and caused deformation of the continental lithosphere. Later, the activity reoccurred during the late Miocene (≈7.5 Ma), early Pliocene (≈4 Ma) and late Pleistocene (≈0.8 Ma) with a cyclicity of ≈3.5 m.y. and deformed the oceanic crust and sedimentary strata in the central Indian Ocean. ¿ 1998 American Geophysical Union