UNROOFING HISTORY OF THE WESTERN ETHIOPIAN PLATEAU: INSIGHTS FROM APATITE (U-TH)/HE AND FISSION TRACK THERMOCHRONOLOGY

Ethiopian geology is dominated by a countrywide plateau that is similar in elevation to, but aerially larger than, the Colorado Plateau. Multiple large rivers have incised through the Ethiopian Plateau, exposing bedrock in deep river canyons that reach 1.5 km in depth. Due to its elevation and extent, published climate modeling has suggested that the plateau uplift could have had a first-order impact on the Cenozoic East African aridification that is linked with hominin evolution. Despite the potentially far-reaching significance of the plateau’s formation, its uplift timing is not well constrained, and several studies presented contradictory data through a variety of methods. This study aims to elucidate the uplift timing of the Western Ethiopian plateau by using river incision as a proxy for uplift. Methods used to date river incision and model the thermal history include apatite (U-Th)/He and fission track theromochronometers, SEM-BSE, and U-Pb zircon geochronology. Samples were taken in a vertical section from an incised pluton along the Didessa River canyon wall near Dabana, Oromia. Corrected cooling ages from three rock samples and 17 apatite grains range from 100-31 Ma, with crystallization ages from 797-635 Ma. Results indicate that the pluton underwent high grade metamorphism after crystallization before partially cooling during Mesozoic exhumation between 100-77 Ma. Massive outpouring of basalts during late Oligocene, which indicated the arrival of the African Superplume below Ethiopia, caused partial thermal resetting of the deepest sample. Relationships between age and eU in the deepest sample indicate that the sample sat in the partial retention zone for an extended period after the flood basalt event. Thermal modeling yields a post-Oligocene cooling history characterized by initial quiescence followed by rapid cooling from the late Miocene to present day. This thermal history points towards a recent, rapid exhumation of the Ethiopian Plateau.