The Turkana Depression is a broad (∼500 km-wide), topographically-subdued (∼0.5 km), region between the elevated Ethiopian (∼2.5 km) and East African Plateaus (∼1.5 km). The Depression is site of the NW–SE-trending failed Mesozoic Anza Rift through which the near-orthogonal, N–S-trending Cenozoic East African Rift subsequently developed. How Cenozoic rifting and magmatism have developed across the previously-rifted Depression during the linkage of other comparatively narrow East African Rift zones to the north and south is poorly understood. Also uncertain is whether the Depression’s low-lying nature is a result of a thinned crust instigated by its multiple rifting phases, or due to a lack of dynamic mantle support. Utilising the region’s first broadband seismograph network—the 2019–2021 Turkana Rift Arrays Investigating Lithospheric Structure (TRAILS) project—body- and surface-wave tomography, and a joint inversion of surface-waves and receiver functions illuminate crust and mantle seismic structure. Continuous slow wavespeeds throughout the Depression’s upper mantle indicate a component of dynamic support, with markedly thinned crust (20–25 km) responsible for subdued topography. Contrasting the Main Ethiopian Rift, Miocene–Recent extension below the Depression has not been achieved principally by magma intrusion: thick basins (3–7 km), a thin crust (20–25 km), and the absence of a markedly slow wavespeed lithosphere (>4.3 km/s), attest to a predominantly mechanical style of rifting. Magma-driven axial extension likely only characterises the most recent (<1 Ma) extensional phase below Lake Turkana. Coinciding with the broadening of East African rifting into southern Ethiopia, is a NW–SE-trending ∼50 km-wide fast wavespeed zone, interpreted as refractory Proterozoic lithosphere—a rheological barrier to rift development. Thinned Mesozoic Anza Rift crust (∼22 km) is also underlain by fast wavespeed mantle lithosphere (>4.5 km/s), implying this area has resisted significant thermomechanical modification from Cenozoic rifting. Pre-existing crustal thin zones do not, therefore, necessarily represent zones of plate-weakness where subsequent phases of rifting will develop.