Sediment provenance and silicic volcano-tectonic evolution of the northern East African Rift System from U/Pb and (U-Th)/He laser ablation double dating of detrital zircons

Detrital zircons from two major rift basins within the East African Rift System (EARS) provide a means to evaluate not only sediment provenance and landscape dynamics in sedimentary basins, but also the timing of the silicic volcano-tectonic evolution of the rift system. We sampled from drill cores collected by the Hominin Sites and Paleolakes Drilling Project (HSPDP) in Ethiopia and Kenya to study the detrital mineral records of the Northern Awash (NA; 3.3-2.9 Ma) and West Turkana (WTK; 1.9-1.4 Ma) drill cores. We performed (U-Th)/He and U/Pb analyses on detrital zircons using single crystal laser ablation double dating (LADD) techniques. Analyses of four NA samples yielded zircon Pb-206/U-238 dates younger than similar to 45 Ma, consistent with derivation from silicic volcanic rocks associated with EARS activity. Most of these samples lack zircon Pb-206/U-238 dates from similar to 22-13 Ma, due to a decrease in silicic volcanism and a watershed configuration limiting delivery of silicic source materials to the sample site. NA zircon Pb-206/U-238 dates imply a sedimentary source from the western Afar margin, with a transition to more localized sediment reworking within the Afar Depression after a major regional tectonic reorganization and formation of a disconformity at similar to 2.9 Ma. The WTK sample yielded many zircons with Cenozoic Pb-206/U-238 dates similar to those from the NA core, but the WTK sample also sources a small population of Neoproterozoic zircons associated with rocks from the Mozambique Belt and reworked sedimentary deposits. Despite being recorders of predominantly silicic activity, the detrital zircon U/Pb dates from both drill sites track the established timing of major volcanic phases in the EARS. A subset of zircons from both sites has concordant Pb-206/U-238 and (U-Th)/He dates, indicating a short duration between zircon crystallization and eruption of the host volcanic rock, but the majority of zircon (U-Th)/He dates are significantly younger than the Pb-206/U-238 dates for the same zircon. Some (U-Th)/He dates are even younger than the depositional age of the sedimentary sample from which it was collected. The observed spread in zircon (U-Th)/He dates likely reflects partial resetting associated with late mafic volcanism and/or hydrothermal activity within this dynamic rift environment. (c) 2022 Elsevier B.V. All rights reserved.

Automated summary

Detrital zircons from two major rift basins in the East African Rift System are analyzed to understand sediment provenance, landscape dynamics, and the volcanic evolution of the rift system. The results show that the sediments mainly come from silicic volcanic rocks, and the zircon ages correspond to the timing of rift activity. The discrepancy between (U-Th)/He and U/Pb ages of zircons may be related to late volcanic and/or hydrothermal activity.