Abstract

The northern boundary between the Nubian (West African) and Somalian (East African) plates is marked by the existence of the East African Rift, which is the locus of roughly east-west stretching due to the separation of these plates. South of ∼20°S, any expression of deformation or seismicity due to the relative motion of these two presumably distinct plates vanishes or is extremely subtle, although the boundary must continue until it intersects another plate boundary. Until recently, there were no observational limits on the location of that intersection, and the recent limits were very broad, being thousands of kilometers wide. Here, from an analysis of the locations of the old edge of magnetic anomaly 5 (which is over 11 Ma seafloor), we show that the main locus of deformation between Nubia and Somalia near the Southwest Indian Ridge over the past 11 m.y. can be no wider than a few hundred kilometers, from ∼100 km west of the Du Toit Fracture Zone to ∼50 km east of the Andrew Bain Fracture Zone complex. Deformation is most likely concentrated along a closely spaced set of several fracture zones known collectively as the Andrew Bain Fracture Zone complex, which is only ∼100 km wide. The displacement along the Nubia-Somalia boundary over the past 11 m.y. is 23 km ± 6 km (95% confidence limits), indicating a displacement rate of 2 mm/yr, much slower than typical rates of seafloor spreading and subduction, but comparable to that along the Owen fracture zone and Dalrymple trough, which separate the Arabian plate from the Indian plate, and along the Azores-Gibraltar line, which separates the Eurasian plate from the Nubian plate.

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