Ambient noise crustal imaging of a heterogeneous rift linkage zone: Turkana Depression, East Africa

SUMMARY The role of pre-existing lithospheric heterogeneities in rifting processes remains unclear. The Eastern and Main Ethiopian rifts lie within the same geodynamic province and are kinematically connected through the Turkana Depression, but they transect heterogeneous lithosphere: Pan-African accreted terranes, failed Mesozoic-Paleogene rift systems, zones of Eocene–Oligocene flood magmatism. Rifting in these pre-extension heterogeneities offers the opportunity to evaluate their relative importance in Oligo-Miocene to Recent stretching and magmatism. We use 3-D Rayleigh shear-wave speed (Vs) models inverted from ambient noise signals recorded on a temporary seismic network to image heterogeneities in lithospheric structure, and to evaluate their influence on syn-rift faulting and magmatism. Crustal feeder zones for Eocene–Oligocene flood magmatism in southwestern Ethiopia are marked by ≤50 km-wide, 10–15 km-thick mid-lower crustal fast wave speed (Vs ≥ 3.8 km s−1) anomalies that are localized rather than widespread. Evidence for active magma intrusions only occurs beneath aligned chains of Quaternary eruptive centres in Lake Turkana and ≤1 Ma shield volcanoes east of the Turkana rift having localized low Vs (≤3.4 km s−1) at 0–20 km depth. Evidence for widespread lower crustal intrusions, however, is lacking. Pan-African oceanic accreted terranes in southern Ethiopia have high Vs anomalies of 3.6 km s−1 throughout the crust and overlay previously imaged high-wave speed lithospheric mantle that has been interpreted as cold and strong Proterozoic accreted terrane. The integrated strength of this lithospheric-scale pre-existing mechanical heterogeneity resisted Oligocene–Miocene stretching and subsequently contributed to the unusual breadth of this East African rift sector lying north of the Turkana Depression.