Inception and evolution of the Turkana Depression: A review from the perspective of the longest-lived sector of the East African Rift

Abstract

The Turkana Depression comprises a highly anomalous sector of the East African Rift (EAR), distinct in its antiquity, structural complexity, geomorphology and attenuated crustal architecture. Previous studies inferred these distinctions derive from pre-existing lithospheric heterogeneities, especially Cretaceous–early Paleogene Central African Rift (CAR) tectonism. Yet, Paleogene extensional deformation is neither clearly typical CAR, nor EAR. Instead, extension gradually evolved from primarily plate-driven (slab pull), to a hybrid state where mantle plume-related (active) processes became increasingly important (particularly c. 23- 7 Ma) to one after c. 7 Ma where active rifting processes dominate. Significant extension accommodated by dyke intrusion in Turkana probably didn't begin until the Pleistocene, accompanied by basin inversion. From c. 30 Ma extension transitioned from the CAR to the Gulf of Aden-Red Sea (GARS) region, marked by Late Oligocene inversion in the CAR trend and parts of Turkana (recorded by outcrop geology and low-temperature thermochronology). Neotethys slab pull helped drive Cretaceous-Palaeogene extension, but was gradually eliminated by the Zagros collision (Early Miocene?) and diachronous opening of the GARS spreading centres (Middle Miocene). Across this transitional period, seismic reflection data indicates total upper crust Cenozoic extension is < 40 km. Conversely, Moho topography suggests >120 km extension. Given the absence of major Cretaceous basin development in western Turkana, probably pre-Cretaceous thinning and/or initially thin crust is required to explain the discrepancy. Turkana Eocene-Miocene extension rates were variable, but low (c. 1 mm/yr) before Plio-Pleistocene acceleration to Holocene-Recent rates of c. 4–5 mm/yr. The CAR, with its very limited volcanism and three phases of extension, separated by inactive periods and crustal cooling, challenges the idea that East African lithosphere is too strong to extend without a plume or magma assistance. However, mantle-weakening, magma-assisted processes accompanying early mantle plume activity probably operated at depth in the western Turkana Depression from c. 40–55 Ma onwards, and may explain the onset of Palaeogene rifting.