East Africa's elusive LAB

A consensus has emerged over the past two decades that significant extension at crustal depths in the northern East African Rift is achieved not by ductile stretching but by magma intrusion. The implications of this for crustal structure and Moho architecture have all been the focus of intense study. East Africa's deep convecting mantle has also been the focus of intense research, with most workers now accepting of the super-plume model over traditional 'Morgan' plumes (albeit with some ongoing discussion concerning the precise internal architecture of the superplume).  In contrast, our understanding of East Africa's lithospheric mantle and, in particular, the depth to the lithosphere-asthenosphere boundary (LAB), remains remarkably poor.  For example, some studies have postulated that no lithospheric mantle exists below large parts of Afar and the Ethiopian rift where magma-assisted rifting is now underway; others have argued to the contrary, asserting that a melt-rich lithospheric mantle is essential to explain first order observations including mantle seismic anisotropy, and the depth at which melts last re-equilibrated with the mantle prior to eruption. Here we will review some of the seismological and petrological evidence that has featured in this debate, including critically assessing the efficacy of different seismological techniques for determining LAB depth in magmatic versus non-magmatic sectors of the EAR.  We show that petrology contributes strongly to the EAR LAB debate, with the added benefit that it allows the assessment of plate thickness through time.  Finally, we look to recent observations from the Turkana Depression, where a lithosphere thinned during multiple, superposed episodes of rifting, offers the chance to assess lithosphere-asthenosphere interactions in more detail than can be achieved elsewhere along the rift.