CONSTRAINING THE TIMING OF CENOZOIC RIFTING AND BASIN DEVELOPMENT IN THE RUKWA RIFT BASIN OF THE EAST AFRICAN RIFT SYSTEM VIA DETRITAL ZIRCON GEOCHRONOLOGY

Preliminary zircon analyses (U/Pb ages via LA-ICPMS; n=~1,000) from a suite of samples spanning the spatial and temporal extent of the Lake Beds megasequence, the youngest sedimentary package in the Rukwa Rift Basin of the western branch of the East African Rift System in Tanzania, reveal a maximum depositional age of ~7 Ma. Our new data suggests the initiation of a late Cenozoic rifting event, further constraining the timing of rifting and basin development in the western branch. These results complement recent work that confirms active volcanism and topographic uplift in the Rukwa Rift Basin ~25 Ma, signaling the contemporaneous development of the western and eastern rift branches. The nature of sedimentary fill in a rift basin depends on climate, provenance, and subsidence rates. Since extensional events in the East African Rift System are marked by sedimentation and magmatism, the age and provenance of both detrital and tuffaceous zircons from the Rukwa Rift Basin strata provide reliable insight into the timing of rift development and erosion linked to uplift and landscape change in the western branch. Our suggested timing for basin development (~7 Ma) is consistent with demonstrated rifting in Uganda and Malawi, as well as with the initiation of volcanism in the Rungwe Volcanic Province, and the estimated development of Lake Tanganyika to the north.

The late Cenozoic rifting and sediment deposition gleaned from our analyses of rift fill strata provides important temporal context for the rich faunal record described from the East African Rift. The Rukwa Rift Basin is an important setting for the evolution of unique flora and fauna. Recovered fossils, including the oldest known members of both the ape and Old World monkey clades, suggest that climate and landscape change linked with tectonic activity may have been critical to the evolution of Africa’s unique biota. Decoding the provenance of the sediments that entomb this rich fossil record is a key to illuminating the tectonic and environmental backdrop of early hominoid evolution and later, larger-scale faunal shifts in East Africa. This work contributes to understanding the tectonic history of the East African Rift by using detrital and tuff-derived zircons to constrain Paleogene to Neogene rifting events and sediment deposition in the Rukwa Rift Basin.