Evidence for long-term diffuse deformation of the lithosphere of the equatorial Indian Ocean

Abstract

The presence of large earthquakes, east–west-striking folds and thrust faults in sediments, and east–west-striking undulations of wavelength 200 km in topography and gravity shows that the equatorial Indian Ocean is the locus of unusual deformation^{1,2,3,4,5,6,7,8}. This deformation has been interpreted as a diffuse boundary between two tectonic plates^{9,10,11,12,13}. Seismic stratigraphy and deep-sea drilling at two locations in the Bengal fan indicate that the deformation began 7.5–8.0 Myr ago³,¹⁴,¹⁵. Here, however, we show, using plate reconructions, that motion across this diffuse oceanic plate boundary began more than 10 Myr earlier than previously inferred and that the amount of north–south convergence across the boundary through the central Indian basin has been significantly greater than the convergence estimated from seismic profiles. The relative plate velocity accommodated across the central Indian basin has varied with time and has been as fast as ∼6 mm yr⁻¹ — about half the separation rate of Earth's slowest-spreading mid-ocean ridge. The earliest interval of measurable motion, which began more than 18 Myr ago, may coincide with rapid denudation of the Tibetan plateau from ∼21 Myr to 15–17 Myr ( ref. 16). The present motion across the central Indian basin began no earlier than 11 Myr — following an earlier interval of slower motion from 18 to 11 Myr — and may have begun at ∼8 Myr, when the Tibetan plateau is thought to have attained its maximum elevation¹⁶,¹⁷.