EVIDENCE FOR LONG-TERM DIFFUSE DEFORMATION OF THE LITHOSPHERE OF THE EQUATORIAL INDIAN-OCEAN

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 Oce an is the locus of unusual deformation(1-8). This deformation has been interpreted as a diffuse boundary between two tectonic plates(9-13). Seismic stratigraphy and deep-sea drilling at two locations in the Ben gal fan indicate that the deformation began 7.5-8.0 Myr ago(3,14,15). Here, however, we show, using plate reconstructions, that motion acros s this diffuse oceanic plate boundary began more than 10 Myr earlier t han previously inferred and that the amount of north-south convergence across the boundary through the central Indian basin has been signifi cantly greater than the convergence estimated from seismic profiles. T he relative plate velocity accommodated across the central Indian basi n has varied with time and has been as fast as similar to 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 th an 18 Myr ago, may coincide with rapid denudation of the Tibetan plate au from similar to 21 Myr to 15-17 Myr (ref. 16). The present motion a cross 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 be gun at similar to 8 Myr, when the Tibetan plateau is thought to have a ttained its maximum elevation(16,17).