STRUCTURAL STYLE OF INTRAPLATE DEFORMATION, CENTRAL INDIAN-OCEAN BASIN - EVIDENCE FOR THE ROLE OF FRACTURE-ZONES

The structural style of the intraplate deformation developed in the Central Indian Ocean Basin was investigated in an area (78°E-82°E, 0°S-6°S) to the west of the Afanasy Nikitin seamount using an integrated data set of seismic reflection profiles from Edinburgh University and Lamont-Doherty Geological Observatory. The study area contains two fracture zones, which strike ~ 005° E to 010 °E, with oceanic lithosphere (age range ~ 65-78 Ma B.P.) younging westwards across them. No evidence for recent fault activity in the oceanic basement along the fracture zones could be detected in this area, although the close association between intraplate earthquakes and fracture zones elsewhere suggests reactivation of the fracture zones at upper mantle depths in a left lateral strike-slip sense. A statistical study was carried out into the first and second orders of deformation, long wavelength basement undulations and high-angle reverse faults respectivelyand the relationships between them. The orientations of the axes of the undulations vary from 065 ° E to 085 ° E while the high-angle faults strike consistently at 090 ° E to 100 ° E. The results of this analysis suggest that the high-angle faults are the result of the reactivation of two sets of pre-existing spreading-centre normal faults, one set originally facing towards the spreading centre and the other facing away. Furthermore, although the long wavelength undulations are not fault generated, the high-angle faults have modified the basement topography causing the accentuation of some of the basement highs. The observation that the undulations are not fault-generated is consistent with them being of flexural origin (including buckling of the crust and/or lithosphere). Basement undulations are clearly discontinuous across fracture zones and the facing direction of faulting is also offset. This discontinuity, the orientation of the axes of the undulations, the presence of other strong oblique basement trendsand information from regional earthquake studies suggest that the deformation resulted from not only ~ N-S compression as a result of the continental collision between India and Asia, but also left lateral strike-slip along fracture zones caused by the difference in resistance to plate motion between the continental collision to the north and subduction at the Sunda Arc to the northeast. © 1990.