Compression of Oceanic Lithosphere: An Analysis of Intraplate Deformation in the Central Indian Basin

Intraplate compressional deformation in the Central Indian Basin is expressed as linear E-W trending topographic undulations and geoid anomlies with characteristic spacings of approximately 200 km. To elucidate the nature of this deformation, we develop models of an oceanic lithosphere in a state of horizontal compression. The lithosphere is treated as a viscous or plastic layer of uniform strength which overlies a viscous half space in which strength decreases exponentially with depth. We compare models in which deformation with a dominant wavelength develops in response to flexural buckling and the hydrodynamic growth of instabilities. For the viscous flexural buckling model, deformation is constrained to occur in the antisymmetric mode by flexural folding, which is characterized by uniform bending of the layer. In the hydrodynamic flow model a range of deformational styles are possible. In a strong viscous lithosphere, deformation of the layer occurs by flexural folding at a wavelength which agrees with that predicted from flexural buckling theory. In a lithosphere of intermediate strength, the layer deforms by folding characterized by thickening which localizes beneath topographic highs. In a relatively weak lithosphere, the layer incurs an even greater amount of localized thickening and deforms in the symmetric or pinch-and-swell mode by inverse boudinage. If the layer has a plastic rheology, deformation occurs by inverse boudinage independent of the yield stress. Results from models in which the layer folds either flexurally or with periodic thickening are consistent with the observed depth distribution of seismicity in the Central Indian Basin and with experimental rock rheological data. Because of the trade-off between buoyancy forces and layer strength in determining the dominant wavelength, a lithosphere which was overlain by weak sediments when intraplate deformation initiated can be characterized by a cOiilpressive layer strength almost a factor of 3 less than a lithosphere which was initially sediment-free. For a range of plausible models which consider both a sediment-covered and sediment-free lithosphere, the observed wavelength requires deviatoric stresses in the Indian plate of several hundreds of MPa.