THE MANTLE TRANSITION ZONE BENEATH THE EAST AFRICAN RIFT SYSTEM: FROM AFAR TO OKAVANGO

The topography of the 410 and 660 km discontinuities bordering the mantle transition zone (MTZ) provides critical constraints on the thermal status and water content of the MTZ. Consequently, spatial variations of the depths of the discontinuities and the resulting MTZ thickness can be used to address important geodynamic questions such as the existence of mantle upwelling from the lower to the upper mantle, and the mechanisms responsible for the initiation and development of continental rifts. P-to-S receiver functions recorded in several segments of the East African Rift System (EARS) are stacked under a non-plane wave assumption. Beneath the Afar Depression, we have found a thicker-than-normal MTZ and attributed it to the existence of anomalously high amount of water. In contrast, the MTZ beneath the Western Branch and the Tanzania Craton has a normal thickness, and that beneath the Eastern Brach is thinner than normal, which indicates a hotter than normal upper MTZ. The MTZ beneath both the young Malawi Rift and the incipient Okavango Rift is normal in thickness, ruling out the role of active mantle plumes in the initiation and early stage development of continental rifts.