Structural style of intra-plate deformation, Central Indian Ocean Basin: evidence for the role of fracture zones

Bull, J.M., 1990. Structural style of intra-plate deformation, Central Indian Ocean Basin: evidence for the role of fracture zones. Tectanophysics, 184: 213-228. The structural style of the intraplate defo~ation developed in the Central Indian Ocean Basin was investigated in an area (7S” E-82* E, 0aS4i0S) 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 OOS* E to 010 o 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 fist and second orders of deformation, long wavelength basement undulations and high-angle reverse faults respectively, and the relationships between them. The orientations of the axes of the undulations vary from 065 o 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-angIe faults have modified the basement topography causing the a~ntuation 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 trends, and 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. Recent studies have documented the broad scale nature of the intra-plate deformation developed in the Central Indian Ocean Basin (Fig 1.) (Weissel et al., 1980; Geller et al., 1983; Neprochnov et al., 1988; Stein and Weissel, 1990), making it clear that the Indo-Australian plate is not behaving ’ Present address: Department of Geology, University of Southampton, Hi~field, South~pton SO9 SNH (United Kingdom). rigidly as required by the conventional plate tectonic theory. The observation of this deformation, together with evidence for non-closure of the plates around the Indian Ocean Triple Junction which is likely to have been caused by deformation within the Indian plate (Minster and Jordan, 19781, led to the development of the idea of a diffuse plate boundary in the Central Indian Ocean (Weins et al., 1986). Using a plate geometry with an Indo-Arabian plate separated from an Australian plate by this diffuse plate boundary, Demets et al. (1988) achieved closure around the Indian Ocean Triple Junction for the period S Ma to the present. 0040~1951/90/$03.50