LAB depth constraints from the Turkana Depression, East African Rift: implications for rifting and magmatism development in lithospheric thin spots, from S-to-p receiver functions

The East African Rift provides a natural laboratory to study the influence of pre-existing lithospheric thin spots on the development of rifting and hotspot tectonism. Below the Ethiopian Rift and elevated Ethiopian Plateau, extensive magmatic and thermal modification due to Eocene-Oligocene flood basalt magmatism and Miocene-Recent rifting has resulted in slow lithospheric mantle velocities (< 4.1km/s; Dugda et al., 2007, JGR). In contrast, below the previously rifted, lower-lying Turkana Depression to the south, the lithospheric mantle appears relatively unmodified (4.2-4.8 km/s; Kounoudis et al., 2023, EPSL), despite being underlain by hot, mantle plume material. Important in this picture are detailed constraints on the lithosphere-asthenosphere boundary (LAB).

Why the Turkana Depression, and particularly the failed Anza Rift terranes, remained resistant to thermal and magmatic modification, is debated. Although the Turkana Depression was a lithospheric thin spot at the onset of plume magmatism, Cenozoic rifting is now circumnavigating, not exploiting, the Anza Rift terranes (Musila et al., 2023, G³). Lithospheric thin spots therefore don't necessarily mark weak zones that are exploited by subsequent rifting and magmatism. One hypothesis for the apparently refractory nature of the Anza lithosphere is that Mesozoic rifting removed easily fusible phases, suppressing subsequent melting and associated strain localisation (Kounoudis et al., 2025, Nature).

To test this geodynamic scenario, we calculated teleseismic S-to-p receiver functions and examined lithospheric thickness variations in the Turkana Depression, where the contrast between fast, relatively unmodified lithospheric mantle and slow, partially molten, plume-infiltrated asthenosphere is expected to provide impulsive S-to-p conversions at the LAB. We observe that the least impulsive and shallowest LAB conversions are associated with Miocene-Recent rift zones, and isolated shield volcanoes. Elsewhere, sharper and deeper S-to-p conversions attest to a lithosphere that has resisted thermo-mechanical modification.