BIMODAL VOLCANISM AND RIFT BASIN DEVELOPMENT IN THE MIDDLE AWASH REGION, ETHIOPIA

The Middle Awash region of Ethiopia is located along a volcanic and tectonic transition zone between the northern sector of the Main Ethiopian Rift and the Afar Rift. It is characterized by a wider rift floor (>100 km) than observed in any other part of the East African Rift south of the Afar Depression and preserves evidence of bimodal effusive and pyroclastic activity over the past 6.5 Ma. These volcanic materials are associated with sediments from numerous fluvio-lacustrine depositional systems and have been utilized to provide absolute age control on abundant vertebrate taxa, including important homind fossils such as Ardipithecus kadabba, Ardipithecus ramidus, Australopithecus afarensis, Australopithecus garhi, Homo erectus, and Homo sapiens idaltu. The combined geologic and fossil evidence suggests that the broad nature of the rift floor and episodic volcanic and tectonic activities confined to different portions of the Middle Awash region created multiple ecological sectors that were inhabited by different species of vertebrate animals during rift evolution.

The 6.5 Ma volcanic record is dominated by distal silicic fallout deposits, some widespread throughout northeast Africa and the adjoining ocean basins, with identified or postulated sources within the Ethiopian volcanic province. Local Middle Awash region sources are responsible for substantial basaltic input, including major effusive activity between 5 to 5.5 Ma and <3.9 Ma and major strombolian and phreatomagmatic activity between 4.2 to 4.4 Ma and 5.5 to 6 Ma. While many of the silicic tephra contain chemically homogenous glass, multiple glass populations suggestive of eruption from zoned magma chambers are commonly observed, particularly in the 4.5, 2.5, and <1 Ma time periods. Moreover, three very distinctive mixed magma explosive eruptions are documented at 3.85, 4.4, and 5.6 Ma. The timing of these eruptions corresponds to hightened regional/local basaltic magmatism, which may in turn be linked to episodes of increased tectonic activity and marginal basin development in the transition region between the northern Main Ethiopian and Afar rifts. We will present a chronostratigraphically controlled geochemical summary of the Middle Awash volcanic record and discuss the primary magmatic and tectonic controls on this volcanism.