Geology, geochronology, and rift basin development in the central sector of the Main Ethiopia Rift

摘要

Based on stratigraphic relationships and K/Ar dating of volcanic rocks from both of the escarpments, flanking plateaus, and from the rift floor of the central sector of the Main Ethiopian Rift, six major volcanic episodes are recognized in the rift's development over a time span from the late Oligocene to the Quaternary. Using the K/Ar data, correlation of volcanic units from the six periods of activity throughout the study area forms the basis for establishing six time-stratigraphic chronozones for the central sector that are related to volcanism in the Ethiopian Cenozoic volcanic province. The oldest basalt and rhyolite flows exposed along the rift margins of the central sector are time correlative to, or older than, those in river canyons (for example, Blue Nile) on the adjacent northwest plateau. A thinned Mesozoic stratigraphic sequence along the Guraghe western rift margin suggests that doming may have preceded volcanism and rifting of the Cenozoic. By late Miocene time, at least by 8.3 Ma and 9.7 Ma, the eastern and western faulted margins, respectively, of the rift had formed at Guraghe and at Agere Selam as indicated by containment of flows of that age within the rift wall during eruption. A paroxysm of calc-alkaline ignimbrite activity produced voluminous flows nearly fully contained within the rift during the Pliocene epoch. The Munesa Crystal Tuff (3.5 Ma), a prominent marker tuff exposed on both rift margins, is present at depth in a geothermal well beneath the rift floor and indicates a minimum of 2 km of downthrow in the central sector since its eruption. Structural and stratigraphic relationships in the central sector indicate a two- stage rift development. This is characterized by an early phase (late Oligocene or early Miocene) of a series of alternating opposed half-grabens along the rift with alternating polarity, such as that in the present Gregory and Western Rifts of East Africa and symmetrical rifts that evolved from these grabens in late Miocene or early Pliocene time. Thus, evolution from alternating half-graben to a full symmetrical graben with a medially located neovolcanic zone that is bifurcated to marginal grabens in the northern part of the study area may be a fundamental part of the rifting process. The study indicates that there are major petrologic and tectonic differences between the Main Ethiopian Rift and the Gregory (Kenyan) Rift.