PERIODIC DEFORMATION OF OCEANIC-CRUST IN THE CENTRAL INDIAN-OCEAN

New seismic reflection profiles of approximate to 5370 km, running thr ough the Ocean Drilling Program Leg 116 sites and Deep Sea Drilling Pr oject Sites 215 and 218, were obtained to investigate the spatial exte nt, timing, and nature of the Tertiary deformation of the equatorial c entral Indian Ocean. Analyses of the data revealed basement and sedime ntary structures and structural unconformities that resulted from the release of compressional forces. Long-wavelength (150-300 lan) anticli nal basement structures with 1-2 lan relief and tight folding and high -angle faulting (5-20 lan long) of oceanic basement and overlying sedi ments exist between the Afanasy Nikitin seamount and the Ninetyeast Ri dge. The deformation zone extends from 10 degrees S to the north up to approximate to 7 degrees N latitude. The changes in the regional base ment trend along 87 degrees E longitude between latitudes 11 degrees S and 15 degrees 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 unconformit y of the upper Miocene, and subsequent younger unconformities (lower P liocene 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 defor mational unconformities are, in general, observed south of 1 degrees S , while the basement deformation and lower Pliocene deformational unco nformity are mostly present in the area north of 1 degrees S. It is su rmised that the compressional stresses built up since the hard collisi on of India with Eurasia may have released for a short period prior to the early Miocene time and deformed the oceanic basement in the north eastern Indian Ocean. Thereafter the stress regime seems to have trans ferred to the north of the Indian shield and caused deformation of the continental lithosphere. Later, the activity reoccurred during the la te Miocene (approximate to 7.5 Ma), early Pliocene (approximate to 4 M a) and late Pleistocene approximate to 0.8 Ma) with a cyclicity of app roximate to 3.5 m.y, and deformed the oceanic crust and sedimentary st rata in the central Indian Ocean.