The structural style of intraplate deformation, central Indian Ocean Basin

• Main Author: Bull, Jonathan Mark
• Published: University of Edinburgh 1990
• Subjects: 551.8
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.642207

The intraplate deformation in the Central Indian Ocean Basin is a well known example of a deviation from an axiom of plate tectonics: that of rigid plates with deformation concentrated at plate boundaries. In this thesis a range of geophysical data collected during CHARLES DARWIN Cruise 28, together with several different geodynamic modelling techniques, is used to investigate the structural style of intraplate deformation. The deformation occurs in lithosphere of age 65-90 Ma that formed at a fast spreading-rate (im60 mm yr<SUP>-1</SUP>) at the south-east Indian Ridge. It manifests itself as a diffuse zone of compressional and strike-slip earthquakes, high localised heat-flow, geoid anomalies and tectonic deformation. The tectonic deformation can be considered to be occurring on two spatial scales: the first order is represented by long wavelength (100-300 km), large amplitude (1-2 km) undulations of oceanic basement and overlying sediments; and the second order by unusual high-angle reverse faults with associated folds in the basement cover. The reduction and inversion of disposable sonobuoy data revealed that the velocity-depth structure of the oceanic crust is not unusual. The sediment velocity increases from 1.6-1.7 km s<SUP>-1</SUP> in the near surface to 3.4-3.5 km s<SUP>-1</SUP> immediately above basement with a velocity gradient of 0.75 s<SUP>-1</SUP>. A velocity for the top of the oceanic layer 2 of 4.1 km s<SUP>-1</SUP> was identified as layer 2A. An estimate for the thickness of the crust could not be made from the sonobuoys and other refraction work is contradictory. A study of structural style from single-channel seismic reflection profiles revealed that the reverse fault fabric, which has a strike (090<SUP>0</SUP>-100<SUP>o</SUP>E) perpendicular to that of fracture zones (005<SUP>o</SUP>-010<SUP>o</SUP>E) developed in this area, resulted from the reactivation under compression of two sets of spreading-centre formed normal faults. In the survey area the first and second orders of deformation are discontinuous across fracture zones. A transpressive model is developed for the survey area to explain the discontinuity of the axes of the undulations, other basement trends, and regional seismicity studies. Previously, two hypotheses had been advanced for the formation of the long wavelength undulations: buckling and inverse boudinage of the lithosphere. This study used three modelling techniques to determine the deformation mode, two of which decisively favoured buckling. Physical modelling using the sandbox technique, in which the oceanic lithosphere was modelled with a two layer (brittle/viscous) rheology, suggested that buckling of the entire brittle lithosphere was responsible for the formation of the long wavelength undulations. Two and three-dimensional gravity modelling supports buckling of at least the oceanic crust. However, numerical modelling was unsuccessful in modelling the long wavelength undulations. The transpressive model for the Central Indian Ocean Basin, on which buckling forms within fracture zone compartments, and strike-slip motion occurs both along fracture zones (left-lateral) and along the reactivated ridge-parallel fabric, may be extended eastwards to the Wharton Basin. Compressive forces with anticlockwise rotation are seen as a consequence of a dramatic change in plate boundary activity in the north-eastern Indian Ocean from 7 Ma ago, with subduction occurring faster at the Sunda Trench than continental collision at the Himalayas.