TECTONOMAGMATIC EVOLUTION OF THE EAST AFRICAN RIFT SYSTEM AS DOCUMENTED IN WEST TURKANA, KENYA

We present preliminary results on igneous samples collected during summer 2015 in West Turkana (northern Kenya). Samples localities span a wide area ranging north to south from the Lokitaung Gorge to Lothagam. They were collected with the aim to better constrain and explore the tectonomagmatic development of the Turkana Basin within the East African Rift System (EARS). This study seeks to combine new ⁴⁰Ar/³⁹Ar dating analyses with existing, and ultimately new, whole rock geochemistry and radiogenic isotope data.

The Turkana Basin is a unique intra-domal region of the EARS, where evidence for the earliest phases of both magmatism and extension co-occur. In fact, the onset of magmatism during the Late Eocene in the Turkana Basin shares the distinction of being the earliest magmatism with the Amaro and Gamo tholeiitic basalts in southern Ethiopia. Magmatism continued in the Turkana Basin until modern times providing the opportunity to track magma evolution throughout the history of the basin. Previous radiogenic isotopic studies in the Turkana Basin have identified one or more sub-lithospheric mantle sources as well as a plume component, with melting depths shallower than those recorded elsewhere along the rift but no samples have been analyzed from West Turkana.

The bulk of the chronological constraints available are K-Ar dates that unfortunately do not provide information to address complexities in Ar systematics involving alteration, thermal history, or trapped/excess gas components. Therefore, new high precision ⁴⁰Ar/³⁹Ar dating analyses will provide an improved characterization of the temporal evolution of this section of the rifting with several applications: (a) achieve a better understanding on what role magmatism has on rift inception and help determine the mechanism that lead to the breakage of the continental plate; (b) constrain basin development through time; (c) assist in establishing geochronology for key Miocene fossil localities. Ultimately, refining the geochronology of West Turkana volcanics will allow for more precise reconstructions of environmental variables’ respond to climate change in the East Africa region during key periods for the evolution both of the modern savanna ecosystem during the Miocene and potentially early hominins during the Pliocene as well.