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 206 Pb/ 238 U dates younger than ∼45 Ma, consistent with derivation from silicic volcanic rocks associated with EARS activity. Most of these samples lack zircon 206 Pb/ 238 U dates from ∼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 206 Pb/ 238 U 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 ∼2.9 Ma. The WTK sample yielded many zircons with Cenozoic 206 Pb/ 238 U 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 206 Pb/ 238 U 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 206 Pb/ 238 U 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. • Detrital zircons are largely derived from Cenozoic silicic volcanic rocks. • Detrital zircon U/Pb ages track established timing of major EARS volcanic phases. • Large spread in (U-Th)/He ages is a result of hydrothermal and volcanic heating.