Evidence for motion between Nubia and Somalia along the southwest Indian ridge

The East African rift marks the northern boundary of the Nubian (West Afric an) and Somalian (East African) plates, and has formed by horizontal stretc hing due to the separation of these plates(1). South of similar to 20 degre es S, any expression of deformation or seismicity due to the relative motio n of these two distinct plates vanishes, although the boundary must continu e until it intersects another plate boundary. The nearest such boundary is that of the Antarctic plate, marked by the Southwest Indian ridge. But prev ious analyses of plate-motion data have indicated no significant difference between Nubia-Antarctica and Somalia-Antarctica motion(2,3). Here we show, using: a large compilation of plate-motion data, that Nubia-Antarctica mot ion does differ from Somalia-Antarctica motion, and we determine a relative angular velocity of the two plates that has compact confidence limits. Our analysis places the pole of rotation near to the southern limit of African seismicity, implying that the southern part of the Nubian-Somalian plate b oundary is a diffuse zone of convergence (up to similar to 2 mm yr(-1)), wh ereas up to similar to 6 mm yr(-1) of separation is accommodated across the East African rift-about half the separation rate of the slowest mid-ocean ridge.