The Turkana Depression in Eastern Africa separates the elevated plateaus of East Africa to the south and
Ethiopia-Yemen to the north. It remains unclear whether the Depression lacks dynamic mantle support,
or if the entire East Africa region is dynamically supported and the Depression compensated isostatically
by thinned crust. Also poorly understood is how Miocene-Recent extension has developed across the
Depression, connecting spatially separated magmatic rift zones in Ethiopia and Kenya. Receiver function
analysis is used to constrain Moho depth and bulk-crustal V P /V S ratio below new seismograph networks
in the Depression, and on the northern Tanzania craton. Crustal thickness is ∼40 km below northern
Uganda and 30–35 km below southern Ethiopia, but 20–30 km below most of the Depression, where
mass-balance calculations reveal low elevations can be explained adequately by crustal thinning alone.
Despite the fact that magmatism has occurred for 45 Ma across the Depression, more than 15 Ma before
East African Rift (EAR) extension initiated, bulk crustal V P /V S across southern Ethiopia and the Turkana
Depression (∼1.74) is similar to that observed in areas unaffected by Cenozoic rifting and magmatism.
Evidence for voluminous lower crustal intrusions and/or melt, widespread below the Ethiopian rift and
Ethiopian plateau to the north, is therefore lacking. These observations, when reviewed in light of
high stretching factors (β ≤ 2.11), suggest Cenozoic extension has been dominated until recently by
faulting and plate stretching, rather than magma intrusion, which is likely an incipient process, operating
directly below seismically-active Lake Turkana. Early-stage EAR basins to the west of Lake Turkana, with
associated stretching factors of β ≈ 2, formed in crust only moderately thinned during earlier rifting
episodes. Conversely, ∼23 km-thick crust beneath the Kino Sogo Fault Belt (KSFB) has small offset faults
and thin sedimentary strata, suggesting almost all of the observed stretching occurred in Mesozoic times.
Despite the KSFB marking the shortest path between focused extensional zones to the north and south,
seismicity and GPS data show that modern extension is localized below Lake Turkana to the west. Failed
Mesozoic rift zones, now characterized by thinned crust and relatively refractory mantle lithosphere, are
being circumnavigated, not exploited by EAR rifting.