Strength envelopes for deforming oceanic lithosphere with some implications for the history of lithospheric deformation of the equatorial Indian Ocean

Strength envelopes for oceanic lithosphere inferred from laboratory experiments indicate that the oceanic lithosphere in the central Indian basin is several times stronger (≈27 TN m-1) than the force available to drive deformation (9+/-2 TN m-1 [95% confidence limits]). Thus the strength of oceanic lithosphere is over-estimated. Either pore pressure is much higher than hydrostatic, or the effective coefficient of friction is much lower than found in laboratory experiments, or both. For models in which the pore pressure is nominally fixed at zero, a coefficient of friction (0.11+/-0.04 [95% confidence limits]) is needed to explain the discrepancy. Although very low, this coefficient of friction is significantly larger than the coefficient of friction inferred for the interplate thrust at most subduction zones. For models resulting in acceptable strengths from various combinations of coefficient of friction and pore pressure, the ratio of the force per unit length supported by the brittle and semi-brittle components of the lithosphere to that supported by the lower lithosphere deforming by creeping flow at a reference strain rate of 10-16 s-1 lies between limits of 3.5 and 12, which indicates that 78% to 92% of the strength of oceanic lithosphere lies in its brittle and semi-brittle portions. It follows that the onset of lithospheric folding at ≈8 Ma ago in the equatorial Indian Ocean was the result of an increase of driving force of no more than ≈28%, but a much greater increase in driving force at or before 15 to 20 Ma ago cannot be excluded.