CONSTRAINTS ON TIMING OF FAULT INITIATION IN THE AFAR RIFT (ETHIOPIA) USING FLUVIAL INCISION AND TCN DATING

The Afar region constitutes the diffuse ridge-ridge-ridge triple junction adjoining the Red Sea, Gulf of Aden, and Main Ethiopian Rifts. Extension in the Afar is distributed between on-axis, magmatic crustal stretching spreading, such as the Tendaho Graben, and off-axis, amagmatic extension, such as the Dobe Graben. The timing of initiation and development of these amagmatic grabens is important to understanding the final stages of continental extension in the Afar region. All major graben-bounding faults cut through the Afar Stratoid Basalt series, constraining the ages of initiation to younger than 1 million years. The Afar Stratoid lavas make up the bedrock of the region, forming a pre-graben, paleo surface which includes low-relief drainage systems. The timing of a particular fault (or location along a graben-bounding fault) is expressed as a knickpoint that incises the paleo drainage in the footwalls of the grabens. The rate at which knickpoints retreat upstream permits a space-for-time approach to assessing initiation. This study aims to develop a model for longitudinal stream profile evolution in the Afar region. Remote sensing and photogrammetry are used for stream profiling and mapping, and Terrestrial Cosmogenic Nuclide (TCN) dating are used to calibrate the model based off field work done in the Dobe Graben. As the stream knickpoints retreats, fresh basalt is exposed and ³⁶Cl nuclides are used for TCN exposure dating. The exposure profile is extrapolated to provide a retreat rate of the knickpoints. The retreat rate is then used to calculate an initiation age for the incised canyon, which coincides with the initial faulting of the graben. This model can reproduced throughout the Afar and we are interested in the spatial timing of graben initiation in the region. More specifically, whether graben-bounding faults initiate simultaneously, or propagate in a certain direction, or multiple directions. Preliminary results suggest that some grabens initiate from the southeast and propagate to the northwest in the central Afar.